Room‐Temperature Laser Synthesis in Liquid of Oxide, Metal‐Oxide Core‐Shells, and Doped Oxide Nanoparticles

Abstract Although oxide nanoparticles are ubiquitous in science and technology, a multitude of compositions, phases, structures, and doping levels exist, each one requiring a variety of conditions for their synthesis and modification. Besides, experimental procedures are frequently dominated by high temperatures or pressures and by chemical contaminants or waste. In recent years, laser synthesis of colloids emerged as a versatile approach to access a library of clean oxide nanoparticles relying on only four main strategies running at room temperature and ambient pressure: laser ablation in liquid, laser fragmentation in liquid, laser melting in liquid and laser defect‐engineering in liquid. Here, established laser‐based methodologies are reviewed through the presentation of a panorama of oxide nanoparticles which include pure oxidic phases, as well as unconventional structures like defective or doped oxides, non‐equilibrium compounds, metal‐oxide core–shells and other anisotropic morphologies. So far, these materials showed several useful properties that are discussed with special emphasis on catalytic, biomedical and optical application. Yet, given the endless number of mixed compounds accessible by the laser‐assisted methodologies, there is still a lot of room to expand the library of nano‐crystals and to refine the control over products as well as to improve the understanding of the whole process of nanoparticle formation. To that end, this review aims to identify the perspectives and unique opportunities of laser‐based synthesis and processing of colloids for future studies of oxide nanomaterial‐oriented sciences.


Introduction
Oxide nanoparticles (NPs) are largely exploited for av ariety of purposes,w hich embraces fields as different as, for instance, heterogeneous catalysis, biotechnology,m edicine, photonics, solar energy conversion,m icroelectronics, automotive industry, pharmaceutics, and food additives. [1,2] This variety of applications also comes with av ast number of distinct compounds belonging to the class of oxide nanomaterials. The synthesis of oxides with tailored properties requires am ultitude of different synthetic procedures, including for instancet he hydrothermal, calcination, mini-emulsion, spray pyrolysis, plasma-assisted and inert-atmosphere growth methods. [1][2][3] Usually,t hese procedures allow highp roductivity but requires ophisticated setups (autoclave reactors for the hydrothermalm ethods, furnaces for calcination, vacuum systems for plasma-assisted and inert atmosphere methods, pressure-and precursor-flow-controlled flame synthesis). Also tailored experimental conditions including high temperature and pressure (hydrothermal, calcination, spray pyrolysis) and chemical precursors and/or additives that potentially persist as contaminants in the final products (calcination, mini-emulsions,p lasma-assisted chemical vapor deposition) are needed. The required precursors and their synthesis, as well as the post-treatment,o ften leads to toxic or pollutant waste, which poses the problem of their disposal. [3] Moreover, for some oxide cations, no precursors are available at all, limiting their flame spray synthesis. In the framework of the global efforts towards ac irculara nd sustainable economy,i ti st herefore of utmost importance to develop synthesis routes running at room temperature and ambient pressure, which allows the cost-effectivea nd green development of nanotechnologies based on oxides.
To this end, the laser-assisted synthesis of colloidal NPs, that is, the use of laser beams to generate ad ispersion of NPs in a liquid environment, is emerging as ap romising approach. [4][5][6][7] The oldest examples are dated back around 1991-1993, [8,9] and are based on laser ablation in liquid (LAL), [10] where al aser beam is directed on as olid target immersed in al iquid solution, to generateacolloid through the ablationo ft he surface of the solid ( Figure 1A). In most cases, LAL is performed with pulsed lasers, and it is also called pulsed-LAL (PLAL). The synthesis of nanomaterials by LAL is sometimes called laser ablation synthesis in solution (LASiS). [11] In 1997, [12] av ariant of LAL appeared, where the laser beam is focused inside al iquid dispersion of micrometric or nanometric powders, to obtaint heir photo-fragmentation into smallerN Ps, in ap rocess knowna s laser fragmentationi nl iquid (LFL, Figure 1B). [10,13,14,243] Soon, LFL turned out to be also an effective way for the reductiono fs ize and polydispersityo ft he colloids obtainedb y LAL. While performing LFL with laser pulse energyl ower than the fragmentation threshold, it was observed that it is possible to obtain the photothermal fusion of aggregates of NPs into larger nanospheres, [11,15] or the photothermal meltinga nd vaporization of micrometric powders into submicron spheres, [16] through ap rocess knowna sl aser melting in liquid (LML) or pulsed-LML (PLML), Figure 1C. [10,16] LML was also applied to the size increase of the colloidso btained by LAL. In fact, LAL, LFL, and LML can be combined for the production and the subsequent sizec ontrol (reduction or increase) of colloidal NPs, as showna lready in 2007. [17] Using milder fluence regimes to irradiate ac olloid with the intention mainly to change the atomic SalvatoreS cir is an AssociateP rofessor of In-dustrialC hemistry at the Department of Chemical Sciences of Catania University (Italy). His research activity is focused on heterogeneousc atalysis, with special interest in oxide-supported mono and bimetallic catalysts, and more recently to the application of photocatalysis to environmental protection and energy production. His activityi sd ocumentedb ya bout 100 papersi ni nternational journals and books, 1p atent, and over 110 contributions in scientific meetings.
Giuseppe Compagnini is Full Professor of Physical Chemistry at the University of Catania. His research group focuses on fundamental and applied aspectso fn anocomposites, including laser ablation, micro-and nanojoining,a nd vibrational spectroscopy.H eis head of the Thin films and Nanostructures laboratory at the Department of Chemical Sciences and Head of the PhD Schoolo fM aterials Science and Nanotechnology.H ei s author of about 160 paperso ni nternational peer reviewed journals (ISI) and 10 invited reviews, receiving more than 5000 citations. structureo fN Ps by introducing defects, whilek eeping the size unchanged, is called laser defect-engineering in liquid (LDL, Figure 1D). Inspired by the first seminal reports in the field, a growingi nternational community of scientists pursued the study of physicala nd chemical processes involved in laser synthesis of colloids. [6,10] They demonstrated severala dvantages of the method ( Figure 1E)a nd that colloidsc an be generated with peculiar features (e.g.,m etastable phases and doped nano-crystals)n ot present in NPs obtained by other procedures, as will be described in the followingt ext. [6,10,18] 1) First of all, laser-generated colloids are highlyp ure, ligands-free and expose an uncoated surface, because no chemical precursors, chelating agents or coordinating molecules are required in the majority of cases. [11,19,20] Often, the absence of pollutant waste and the use of raw materials make laser synthesis compatible with the 12 principles of green chemistry, [11] offering new opportunities for the development of ag reen and sustainable nanotechnology,and for the integrationo fcolloidal NPs in acirculareconomy.
2) The achievemento fo xide NPsa sacolloid does not expose the operator to airborne particle inhalation risks, thus allowing occupational safe and easym anipulation of the products, comparedt od ry nano-powders. [21] Besides,t he effective interaction of NPs in as table colloid with solid substrates and matrixes is facilitated by impregnationo rm ixing with the liquid phase. [10,19] 3) Another relevant advantage of laser-assisted synthesis methodsi st he access to aw ide range of oxide (and nonoxide)n anoparticles in similar experimental conditions, all at room temperature andp ressure, [18] as shown in Figure 1F on the basis of al iterature overview.T oc larify why laser synthesis falls under the "room-temperature and pressure synthesis" classification,o nt he one hand, the mechanistically relevant local (temporal and spatial) temperature andp ressure profiles and on the other hand, the practically relevant global-extrinsicp arametersh ave to be differentiated. It is worth specifying that locally and temporally,a tt he level of the matter interacting with the laser beam, extremelyh igh temperatures and pressures are reached,b ut this is self-confined in al imited region of space coincident with the laser spot and the early explosive boiling volume. Molecular dynamic simulations coupled with the two-temperature-model werer ecently extended from the ultrashort-pulsed to nanosecond-pulsed laser ablation regime, [22] where after short non-equilibrium phase the majority of nanoparticles are in thermodynamic equilibrium with their local environment at the end of the simulation (a few nanoseconds).
These predictions are backed by experimental findings on binary,p artly immiscible nanoparticles ystems, where both a kinetic control and thermodynamic contribution of the particle formationd ynamics are concluded. [23][24][25] Hence, on the one hand, highly non-equilibrium conditions are pointing at ak inetic controlo ft he synthesis at the very early,sub-nanosecond formation time regime. [26] Later,t he whole system quickly reachesa nd remains in equilibrium with ambient conditions. Globally,t here is no need for strategies for heat or pressure regulation, which is ab ig advantage compared to gas-phase (pressure), hydrothermal( pressure, temperature), or sol-gel (temperature) synthesis, andw et chemical co-precipitation or reduction (temperature). Even with high repetition rate lasers (> kHz), liquid flux simultaneously works for draining the col-loid and coolingt he synthesis environment macroscopically keepingasteady state of temperature and pressure.
This meanst hat laser synthesis does not require unit operations for in-process-heating/cooling or pressure control,w hich makes laser synthesis systems easily implemented in laboratories. Overall, laser synthesis mechanistically benefits from accessibility to metastable nanoparticle crystal structures or compositions via temporal, pulsed, non-equilibrium condition that is confined in am icroscopic volume, at the same time macroscopically continuously operating in steady state, atr oom temperature and pressure. 4) By using the same equipment, it is possible to synthesize NPs in af ew minutes and to switch from one type of nanoparticle to another with a"plug-and-play"a pproach. [21] 5) The experimental configuration can be tailored to the desired quantity of NPs, going from batch to flow cells ( Figure 2). [10] Flow cells have the advantage of limiting or avoidingt he absorption of the laser beam by the just-formed NPs and persistent microbubbles, [27] is especially relevant at visiblea nd UV wavelengths. [10,18] 6) The rich library of oxide nanoparticles obtainedb yl aser synthesis in liquid also includes non-equilibrium phases and complex morphologies such as core-shell NPs, dendrites, spindles and, in specific cases,a lso nanowires, nanoflakes, nanoflowers, urchins, rods, sheets,a nd hollow spheres. [18,35] As explained in the following paragraphs, particlef ormation after each single laser pulse occurs on at imescale of 10 À6 s, [26] Figure 2. Sketch of basic and advanced set-upsf or the generationo fcolloids by LAL, LFL,o rL ML. Upper panel (from left to right, reprinted withp ermission from ref. [10],C opyright 2017,American Chemical Society): batch (ablation in beakerwithout stirring; reprinted with permission from ref. [28],Copyright 2015, American Chemical Society and ablation in beaker with stirring;reprinted with permission from ref. [29],Copyright 2015,Royal Society of Chemistry), semi-batch (reprinted with permission from ref. [30],Copyright 2014,American Chemical Society), flow (reproduced with permission from ref. [31],Copyright 2013, Royal SocietyofC hemistry) and flow-jet (Adapted with permission under CC BY 4.0 from ref. [32],Copyright 2016, Springer Nature Ltd.). The latterh as been reported only for LFL andLML, see the following paragraphs. Lower panel:high throughput LAL set up with productivity up to > 1gh À1 (reprinted with permission from ref. [33],Copyright2 016, The Optical Society), remote-controlled flow LAL set up with embedded stop-flow optical control (reprinted withp ermission from ref. [34],Copyright2 019, AIP Publishing). making possible the freezing of non-equilibrium phases or highly defective structureso therwise difficult to achieve. [19,[36][37][38] 7) Af eature making laser synthesis very appealing for research and industriale xploitation is that the methodi si ntrinsically self-standinga nd can be implementedw ith minimal manualo peration. For instance, it was recently demonstrated, that LASiS of variousN Ps is possible by controlling the equipment remotelyw ith aP Co rasmartphone. [34] This is useful for an even more economic synthesis also for safer synthesis conditions as it minimizes operatorp ermanencei nn earby of the laser beam, flammable solvents or harmful compounds such as radioactivee lements or volatile organic molecules, even when dealingwithnon-toxic oxide NPs. [34] Further developments in the automation of laser synthesis have been recently reported by real-time correlation of NP productivity with acoustic emission energy, [39] as well as temperature in the ablation chamber. [40] 8) Laser synthesis in liquid is al inearly scalablem ethod (productivity linearly scales with both laser power and time, at liquid flow operation), not yet demonstrated for productiono f kilogramso fN Ps or more. [10,18] Nonetheless,t he use of ah ighpower ultrafast laser with MHz repetition rate coupled with a polygon scanner (that achieves bypassing cavitation bubbles by supersonic lateralb eam displacement) and ag alvanometric mirror led to several grams/hour productivity of NPs by LAL, in as elf-standingc ontinuous flow set-up. [33,41] 9) Laser synthesis is generally considered an economically viable approach in the case of NPs involving precious metals or expensive compounds. [42] Its economic viabilityi ss trongly dependento nm aterialt ype. As ar ule of thumb,t he LAL productivity scales with the material density, [43] makingt he lighter oxides less productive than, for example, the noble metals.O f course,a lso the ablationt hreshold fluence (often higherf or oxides), the electron-phonon coupling as well as the (temperature-dependent) target reflectivity contributet ol aser ablation efficiency and thereby the power-specific productivity.B ut more material-specific factors contributet ot he overall cost, including raw materials (bulk solids, solvents),h ourly labour costs (manpower), the depreciationo ft he investment (equipment and its maintenance), the rental cost of the facilities, storage of products and the quality control of the whole procedure. The bottleneck for economic scale-up can only be identified for ap articular business case, but laser synthesis access to hundreds of different materials, making the detailedc omparison of economic viability for each of them out of the scope of this review.T he commercial interest in laser-generated colloids is demonstrated by the existence of well-established companies in Germany,I srael, and the U.S.,c ommercializing this type of product for more than ad ecade. [44][45][46] Besides, af ew studies specifically addressed the case of laser-generated colloids and afforded some of the parameters in the previous list. Benchmarkingb etween hydrothermalp rocess,p hotochemistry and laser ablation in liquids has been conducted for low-priced oxide NPs (cerium oxide)d edicated to organophosphorus degradation. [47] On the one hand, LAL was the most expensive methodi nt his study on oxide nanoparticles, because of the usage of an ot state-of-the-art laser setup with limited produc-tivity of only 21 mg h À1 and the relatively high depreciationo f the investment (equipment). On the other hand, LAL-generated CeO 2 NPs exhibited the best degradation activity because of the minor surface contamination inherent to the LAL-generated NPs and were the only one amenable to in situ production without the need for high-temperature ovens. Conversely, for gold colloids, it has been calculated that the break-even point where laser synthesis beats chemical synthesis in the costs versus the mass of produced NPs plot already happens at tens of grams. [42] Thisi sp ossible for the limitedm anual operation, absence of expensive chemical precursors, and continuous advancementi ns ynthesis scale-up. Interestingly,t he lower cost of waste managementw as not even considered in the study.I nb oth cases, the main source of cost in laser synthesis is connected to the laser equipment, that in the last 15 years showed ac ontinuous growth of average power available on the market, and ap arallel decay of the equipment cost per watt, [10] suggesting af avourable prospectf or further reduction of production costs in case of laser-assisted synthesis of colloids.
In this review article, we provide the workingp rinciples behindL AL, LFL, LML, and LDL focusing on oxide nanostructures, with an overview of nanomaterials produced so far by this method,a nd of their functional properties and reported applications. So far key reviewsh ave mainly focused on metals, alloys or processing variants during LSPC, whileo xides were mentioned only peripherally. [5,10,19,36,48,49] By definition, an oxide nanomaterial consists of an anoscale objecti ncluding oxygen atoms in its chemicalf ormula. Generally,i nt his review, oxide materials are defineda si norganic compounds obtained by reactiono fo xygen with an element with low reductionp otential. The redox potential changes with environmental parameters such as solvent, solute (concentration), temperature and pressure and, in fact, also noble metals can be surface-oxidized in some laser synthesis conditions. [19] However,t his is usually limited to am inority of the atoms in the NPs, therefore the chemical compositiono ft he resulting material only contains arelatively small amount of oxygen. Hence, laser-generated and laser-processed nanomaterials that only express limited oxidation are excluded by this review,w hich deals only with the compounds where the content of oxygen is comparable to that of the other main elements.
It is expected that the research on laser synthesis for the preparation of oxide nanoparticles in liquids will continue to grow in the near future, especially if one considers the variety in terms of compositiona nd structures that are achievable, the interesti nb etter control of the composition,s ize, morphology, and phase, and the need for improving the understandingo f NP formationm echanism.T os upport this development, an onprofit conference series on AdvancedN anoparticle Generation and Excitation by Lasers in liquids( ANGEL) [50] exists since more than ad ecade, and handbooks about laser synthesisa nd processing of colloidsa re availablet hrough open access for beginners, [21] in addition to multiple specific and advanced review articles that appeared in recent years. [10, 11,16, 18-20, 26, 35, 36 48, 52-54] In the following, the review will introduce the fundamental concepts of LAL and give an overview and discussion of the work and perspectives of laser-baseds ynthesis of conventional nano-oxides, core-shell NPs, defect-engineeredo xide NPs, and ligand-stabilized oxide NPs. The discussion will include also the synthesis of multicomponent oxide nanostructures by sequential LAL and reactive LAL, highlighting the issues encountered with the compositional homogeneity of the products. Subsequently,t he fundamentals and an overview of oxide NPs obtained by LFL and LMLa sw ell as upscaling considerations will be treated. Progress in the recently established laser-based defect engineering in liquid (LDL) will be also discussed.
Similar to LFL and LML, the LDL methodt reats dispersed particles, with comparable mild excitation conditions, but not primarily intendingt od ownsize particles (like LFL) or total particle melting (like LML). Instead, LDLa ims at defect introduction, also relevant for the preparation of (surface-)doped oxide NPs. The review is concluded by an overview of recent, most relevant applications of laser-generated oxide nanomaterials, with special emphasis on photo-catalysis, oxidation-catalysis, bio-applications, and photonics.

LAL LAL fundamentals
Before enteringamore insightful description of the ablation mechanisms, it is useful to consider first the major steps of the most prominent laser synthesis method, the LAL( Figure 3). Startingf rom the general LAL configurationw here al aser beam is focused on ab ulk target immersed in al iquid,f irst of all, the laser beam should travel through al ow-absorbing liquid layer,w hich meanst hat the liquid must be transparent at the chosen laser wavelength, and liquid breakdown must be avoideda tt he fluence selected for the experiment. [10,18] This issue is commont oL AL, LFL and LML as well, and it becomes especially challenging when using ultrashort pulses, due to the self-focusing and filamentationeffects. [55][56][57][58][59] Assuming the requisite of liquid transparency,the interaction of the laser beam with the bulk target results in the formation of ap lasma in af ew hundreds of picoseconds. [60][61][62][63][64][65] The plasma is initially made of the target material.B ecause of the fast volumee xpansion( appearing already after about 10-100 ps), [57,66] am echanical shockwave is released both in the target and in the liquid (Figure 3). [67][68][69][70][71] Such as hockwave can lead to phase transition at the target and the pressure at the focal pointc an reach af ew gigapascals. [60] On the other hand, the interaction of the plasma with the liquid leads to the fast vaporization of the liquid, which is observed already at the shortestg ating times (few ns) of CCD cameras commonly used to observe plasma dynamics. Ac avitation bubble is initiated and appears mainly composed of the vaporized solvent. [61,[71][72][73] As depicted in Figure 3, the cavitation bubble grows and collapses after ac haracteristic time depending on the pulse energy and the pulse duration (typicallyahundred microseconds for nanosecondp ulses of af ew mJ). When the cavitation bubble collapses,t he NPs are released in the liquid. [74] However,c rystalline particles have been observed also outside the expanding bubble, preceding the bubble'se xpansion front, by small-angle X-ray scattering (SAXS) experiments, [75] confirming the theoretical predictionsm ade by Zhigilei et al. [76] Due to the high amount of energy accumulated in the point of bubble collapse, rebound cavitation bubbles can grow and collapse again, depending on system parameters such as liquid mechanical properties and initial bubble energy.T he Rayleigh-Plessetequation is only suitablet odescribe the first oscillation, while the Gilmore model including the liquid compressibility is required for modelling the subsequent oscillations (rebounds). [77] Note that even Gilmore model cannot adequately describe bubble dynamics with broken symmetry.I nd etail, the LAL bubble neither has as ap erfect hemisphere aspect ratio (whichd ynamically strongly deviates from 0.5, in particular at expansion and collapse phase) nor has ac ircular contour.T he bubble contour'sc ircularity is broken at the interface layer directly on top of the target, so aL AL bubble geometry can- Figure 3. The timeline in LAL for ultrashort pulses shows the successive steps that occur duringL AL synthesis, from the laserp ulse interaction with the target to the release of the as-produced NPs in the solution. On top, the characterization techniquesa re displayed according to their temporal resolution. At the bottom,f rom left to right:electron-phonon coupling scheme, phase transition snapshot from molecular dynamics simulation, optically active plasma and released shockwave, bubble dynamics and produced colloidal solution. Reprintedw ith permission from ref. [60],Copyright 2017,Elsevier. simplified-be divided into the root part on which aq uasihemispherical cap sits. These effects become quite obvious at smaller bubble sizes (i.e.,s maller LAL pulse energies) and are particularly expressed in high viscosityl iquids. [78] After bubble collapse and NP release in the liquid, NPs mayf urthero xidize [79] and slowly grow on ar elativelyl imited extent or may undergo agglomeration if they have limited colloidal stability. [10,18] Importantly,t he requisite of transparency at laser wavelength holds also fort he laser-generated NPs, that may reduce productivity per pulse due to absorption and scattering (and by strongly reducing the Kerr limit for opticalb reakdown). In fact, the ablation rate is higher when using low wavelengths as long as no NPs synthesized by previous pulsesa re present ( Figure 4A -C). [10,80] Besides, it was demonstrated that productivity is affecteda lso by persistent microbubbles stemming from vaporization or even degradation of the vapour layer at the boundary with the plasma plume, [27] so that huge amount of permanent gases are formed proportional to the redox potential of the target. [81] It is worth noting that laser beam absorptionb yN Ps (known asb eam self-absorption) may induce structural and chemical changes to the NPs (LFL or LML like). Structurala nd chemical changes are associated with broadening and polymodalityo ft he size distribution, as well as to phase heterogeneity of products. [10,18] To avoid or limit the selfabsorption of the primary beam, infrared or near-infrared wavelengths, andL AL with ac ontinuous flow set up (see Figure 2), are usually preferred.
Besides, when the productivityn eeds to be pushed on the gram scale, sufficiently high laser fluence ( Figure 4D-F), low pulse duration (Figure4G-I) and high repetition rate lasers (> kHz, that is, inter-pulse delay of < 1ms) have shownt ob e most successful up until now. [10,80] But, it is worth to stress that limitation to the general trends reported in Figure 4n eeds to be considered. For instance, the dependence of productivity versusf luence ( Figure 4F)d epicts the regime where the fluence is below % 7.4 times (e 2 )t he ablation threshold found (f th )t ob ev alid for fs-, ps-and nslaser pulses. [33,82,83] When the fluence reaches f th ·e 2 the productivity wasf ound to reach am aximum. [33,82] Here, the former was initially predicted by the theoretical model of Neuenschwandere tal. linking the increase of ablationr ate with the increasingo pticalp enetration depth into the target( with rising laser fluence). [84] At laser fluence above f th ·e 2 the opticalp enetration depth is limited [84] andh ence (further increasing) only leads to as tagnating or even decreasing productivity. [33,82] In this context,s elf-focusing and filamentatione ffects in the liquid medium, occurring especially for high laser intensities (> 10 13 Wcm À2 , [56] usually only reached with ultra-short pulses such as fs up to several ps), may furtherd ecrease the productivity at high laser fluence as significantly less energy reaches the target in this case. [82] In general, the laser fluence can be increased by varying the pulse energy of the laser ( Figure 4D)o rd ecreasing the beam spot size by optimizingt he workingd istance between the targeta nd focusing lens ( Figure 4E). Again note that, if the spot size becomes too small, the productivity will drop, [10,80] since the penetration depth of light into the matter is limited by the material's absorptionp roperties (temperature-dependent reflectivity). In other words, al inear increase in fluence is not compensated by al inear increase in the ablation efficiency. [10,85] Consequently,i ncreasing the laser fluence by lowering the spot size from the mm scale to the micron scale will lead to as maller ablation rate ( Figure 4F). [10,80] Additionally,e specially from the work of Kauteka nd co-workers, [86] the beneficial effect of material defects in lowering the ablation threshold is known for decades. With as maller spot size, generally lessd efects would interact with the laser pulse such that ah igher ablation threshold (and thereby al ower productivity) can be expected with decreasing spot size. [86] In case of pulse duration,t wo processes need to be considered for productivity:1 )thermal loss, which is highesti nc ase of longerp ulse duration (> 10 ps, see Figure 4G)a nd 2) plasma shielding( Figure 4H)w hich is especially pronounced for ns laser pulse duration and above (in line with plasma dynamics, compare Figure 4I and Figure 2). [10,80] Hence, highest ablatione fficiency is usually predicted in case of fs-and pslaser ablation as depicted in Figure 4I. [10,80] While cavitation bubble shielding was neglected by employing sufficientlyo ptimized scanning speed in both cases, the productivity,t herefore, seems not only to decreasew ith increasing pulse duration but it appears that there exists an additional sweet spot of pulse duration,materialand pulse energy.
Hence, att he same nominal fluence (the effective fluence decreases with increased pulse duration) there will be amaterial-specific optimal laser pulse duration that also avoidsl osses by NP absorption, for example, 2psf or LAL of gold. [87] Above the kHz frequency threshold, the lifetimeo ft he cavitation bubble exceeds the inter-pulse delay ( Figure 4J-L), so that the laser beam propagates through ah ighly scatteringl iquid/gas interface that is responsible for as ensible reduction in the ablation efficiency. When the inter-pulse distance is too short, the subsequent laser pulse andt he cavitation bubble generated from the previous pulse overlap in time ( Figure 4J). In this case, extensive scattering of the laser pulse occursa nd, hence, the productivity decreases ( Figure 4L). [10,80] These issues were solved by Barcikowski'sg roup using ap olygon scanner( compare Figure 2) coupled with as ingle galvanometric mirror,a llowing the laser beam to spatially bypasst he cavitation bubble ( Figure 4K)a tM Hz repetition rate and laser power % 500 W( ps-laser). The polygon scanner allows supersonic scan-rates and ablates ad ifferent position of the target with each pulse. [33,41] Ablation rates of several grams per hour were reachedw ith this high-end class laser type. [83,88] Interestingly, Dittrich et al. have shown that the ablation efficiency (but not the absolutep roductivity) of ultra-low-power and comparably cheap compact ns-lasers is af actor of 8h igherc ompared to the high-end class ps-laser. [83]

Mechanistic insights
The processes involved in laser ablation of solids in liquid environmenth ave been the subject of extensive studies, [86] with a special focus on non-thermalp rocesses and the early stages, [26,89] usually with metallict argets and ultrashort pulses. Many efforts were made to develop in situ characterization methodsa nd numerical simulations, leading to thec onclusion that the processes occurringa tt he earlyt imes cales after laser energy deposition are critical in the definitiono ft he final product. [26] Figure 3a lso lists the differentc haracterization methods reported in the literature with the reachable time scale for each of them. These measurements gave evidenceo nt he early generation of the NPs. Light scattering experiments, [90,91] as well as in situ time-resolved small-angle X-ray scattering (SAXS), [75,92] not only showed that the NPs are confined inside the vapor bubble buta lso showed that NPs are present earlier than bubble formation. In particular, light scattering experiments suggest that NPs are presenta fter af ew hundreds of nanoseconds. [90,91] This is consistent with the fast cooling of the plasma reported from plasma spectroscopy (10 Kns À1 ) [60] or depicted in modelling. [22,93,94] The fast cooling of alaser-generated hot gas or plasma commonly leads to nucleation and growth of particles. [53,95,96] The standard pathway to the formation of nanoparticles generally includes three stages( Figure5A): nucleation,e volution of nuclei into seeds, and seed growth into final nanocrystals. Althoughageneral pictureo fh ow these steps evolve in LAL is still under construction, Zhang and Liang argued that the time required to reach the critical concentrationf or nucleationi n LAL is much shorter than that of the wet-chemistry synthetic routes, due to the fast ejection dynamics of the "precursors", [53] and relatedl arge temperature gradients of up to 10 12 Ks À1 . Moreover,a salarge part of the process takesp lace in the gas phase (cavitation bubble), or at relativelyl ow concentration once in the liquid phase, where particles have slow mobility, particleg rowth by coalescence and ripening may last for a longer time than in the conventional La Mer mechanism used for wet-chemistry methods. [53] Some relevant open points for oxide nanoparticle formation concern the chemistry that is discussed in the next paragraphs. This echoest he questions of the physicochemical interaction whichm ust be addressed to understand the fast vaporizationo ft he solvent, as well as the parameters favouring its decomposition and its reactivity.
The scenario of Figure 5A is supported by molecular dynamics simulations developed by Zhigilei's group. [22,76,93,94] They have developeda tomistic simulations of the laser ablation of metal targets in water,c ombining ac oarse-grained representation of the liquid environment and an atomistic description of the laser interaction with metal targets and for pulse durations from fs up to few ns. [22,76,93,94] For ultrashortl aser pulse durations ab imodal size distribution is predicted, as frequently reportedf rom transmission electron microscopy, [97] and also from SAXS measurements. [74,75,92] In the case of hundreds of ps to ns pulses,t he thermala nd stress confinement characteristico fu ltrashort pulse laser ablation is not observed. [22] This is associated with different ablation dynamics and partly explains why the nanoparticles ize distribution tends to be broad in ns LAL but not bimodal as in ultrashort-pulsed LAL, as described in Figure 5B. [22] At the same effective fluence, independento ft he pulse duration (ps or ns), three NP formation pathways are predicted by the works of the Zhigilei group, which are linked to the regions where they originate from, summarized as follows alongt hree regional sections, from the target towards the liquid:1 )The nanoparticles mostly emerge from the spinodal decomposition of ap art of the ablated materiall ocated between the target surfacea nd the transienti nterfacial metal layer.H igh density of ablatedm atter (and low amount of supercriticalw ater) is characteristic for this region. High temperature causes the seeds (1-4 nm) to be thermodynamically unstable and therefore evaporate unless their rapid collision and coalescence leads to small (about 5n m) nanoparticles, while the larger nanoparticles in this region continues to grow.These processes result in thermodynamically stable, mostly larger nanoparticles of around 5-10 nm, although some smaller ones survived the growth process. 2) The decomposition of the thin transient metal layer at the interface between the ablationp lume and water environment causes the generation of large (> 10 nm) molten nanoparticles, only slowly cooled by surrounding supercritical water.3 )A tt he very fronto ft he emerging cavitation bubble, evaporation from the hot interfacial layer into the supercritical water causes the formation of very small (< 5nm) nanoparticles through the nucleation and growth from the vapor-phase metal atoms. These small nanoparticles solidify in nanoseconds, likely in defect-rich nanocrystals, whereas particles stemming from pathways 1) and 2) are stilli nm olten stage after af ew nanoseconds.I nF igure 5B), one obvious differencethat shorterp ulse duration causes is jetting of particles stemming from the pathway 2) as ar esult of am ore vigorous ablation process, which includes the ejectiono fm etal droplets directly into the high-density water region. Note that the intriguing sketches in Figure 5B)are intended to explain the mechanism [22] but they are not to scale at all. In reality,a fter some ns the height of this earlyf ormation volume is only hundreds of nm, whereas the width is the laser spot size (tens of microns),s ot he virtualp ictureo ne should have in mind is av ery flat objectwith an aspect ratio of about 100.
Noteworthy,t here are severala dditional complexitiesw hen considering oxide targets instead of metal ones. First, there is al ack of optimized empirical potentials for metal oxidesw hich could be effectively used in am olecular dynamics simulation. [98] The reasons are the increased complexity of the interatomic potentialsw hen severalc hemical elements are involved (at least the oxygen andam etallic element), and the polymorphism issue since the metal oxides usually form various stoichiometries and crystal structures. Second, whilei nm etals there is ad irect laser-heating of the free electrons in the conductionb and, in metal oxides electrons mustb ep romoted acrosst he band gap by the laser excitation before their heating (as discussed below). The modelling of the laser-target interaction is then more challenging than for metals. Third, molecular dynamics are not suited yet to catch the chemistry in the early ablationp hase leadingt om etal oxide NPs. Fourth, numerical simulation assuming ultrashortp ulse duration cannotc atch the whole complexity of the processes resulting from the plasma-laser interaction which occurs for (several) nanosecondp ulse duration, althoughr ecently ac onvincing modelling approachint hat direction has been presented. [22] Indeed, for nanosecondp ulses,s hielding of the target by the plasma occurs, with an increased effect with the decreasing wavelength. [99] In addition, the surfaces tructure such as its roughnessc hanges significantly also on the ns time scale, which is expected to locally modify the opticalp roperties of the target and, thus, its light absorptionp roperties. [22] There- Blue:liquid;grey:metal target;light grey:ablation plume; light blue:cavitationbubble precursor;dark blue:ejected materials. For description seetext.Republished with permission from ref. [22],C opyright 2020, Royal SocietyofC hemistry.(C) Timescalesofv arious electron and lattice processesi nlaser-excited solids. [89] Each bar representsana pproximate range of characteristictimes consistent for carrier densities from 10 17 to 10 22 cm À3 .( D) Sketch of main differences in lasera blation of metallic and oxide targets. fore, the energy deposited on the target, as well as plasma warming are difficult to account quantitatively.
Despite the apparent complexity of the processes, some evidence comesf rom the experimental investigation of the ablation processes, including ablation of semiconductors and dielectrics. Figure 5C shows the timescales of various electron and lattice processes in laser-excited solids for an ultrashort pulse (femtosecond). The subsequentt ypical pathways of energy dissipation and phase transformationsf ollowing the excitation by an ultrashort pulse are also displayed in Figure 5C. As anticipated above, the main difference between metals and metal oxidesl ies in the mechanismso fe lectron excitation due to the large energy band gap of the latter,l eading to several differences in linear and nonlineara bsorption (especially at NIR wavelength), as wella si ne lectron dynamics. Overall, the ablation of metal targets could appear more convenient, which justifies why most works dealingw ith the synthesis of metal oxides are based on the oxidative laser ablation of bulk metals (see next paragraph).A nother reason for this choice is related to the mechanical properties of the materials, since pure metals are commonly more ductilet han their oxides. Brittle metal oxidesa re more subject to shockwave-induced damage, which leads to target-crushing and target break-up (Figure 5D). As ac onsequence, the mass of NPs produced by laser ablation of bulk oxide targets can strongly differ from the mass removal from the target, requiring additional purification steps to remove the unwantedt arget pieces. For instance, the ablation of aGd 2 O 3 target with an Nd:YAG laser source (500 ps, 2mJ/pulse, 1064 nm, 2 10 11 Wcm À2 )l ed to the production of NPs (diameter < 100 nm) of 2.00 AE 0.18 ng/pulse, which however corresponds to only 13 %o ft he removed materialf rom the target ( % 15 ng/pulse). [100] In this context,t he porosityo fp ressed YIG (yttriumi ron garnet)m icro-powder targets was shown to be detrimental, since ah igh porosity leads to al arge fraction of microparticles detachingf rom the target and being present in the colloid. [101] Conversely, the ablation of ah ighdensity powder target( > 99 %) led to the formation of % 3nm YIG NPs similar to the ablation of abulkYIG target. [101] Concerning the ablationm echanism of oxide targets with commonl aser sourcesu sed in LAL, such as the 1064 nm nanosecond pulses of Nd:YAG laser in fundamentalm ode, the photon energy( 1.17 eV) is lower than the band gap energy of most oxides. As an example, the energy of 8p hotonsi s neededt oc ross the Al 2 O 3 band gap with a1 064 nm laser source.T he promotion of the electrons from the valence band thus involves photoionization processes preceding avalanche ionization. Photoionization acts as an initial step in the laser energy deposition and subsequentm aterial modifications, leadingt ot he optical breakdown of the solid. [102] There are two different processes of photoionization,multiphoton ionization and tunnelling ionization. [103] The multiphoton excitation involves the simultaneous absorption of several photons. The ionization by electron tunnelling is induced by ad istortion of the potentialb arrierf or large laser fields, and it is relevant for high electric field intensities, as those reached with ultrashort laser pulses.
Ab etter understanding of the ablation conditions is provided by the value of the parameter g,w hich gives the balance between the multiphoton ionization regime andt he tunnelling ionization regime and is defined as [Eq. (1)]: where l is the laser wavelength, c light speed, m the reduced mass of the electron, e the electron charge, D the energy band gap of the material, and E the magnitude of the laser electric field, which scales with ffiffiffiffiffiffiffi ffi 1=t p ,w here t is the pulse duration. [104] When is decreased below 1, the optical breakdown is governed by the tunnelling ionization regime, [105] that is regarded as a" deterministic breakdown regime", that is, with high precisioni nt he spatial delivery of pulse energy to the target. [103,106] According to Equation (1), decreasing g to reach ad eterministic breakdownr egime can be achievedb y increasing the wavelength and decreasing the pulse duration.
Once electrons are promoted to the conduction band, they can absorb laser energy throughi nverse bremsstrahlung (electron acceleration). For ns pulses at ordinary fluencies, electron acceleration is equilibrated by electron-phonon scattering. Conversely,f or ultrashort pulses, electrons are accelerated up to the threshold for achieving avalanchemultiplication by electron-electron scattering, thus furtheri ncreasing the electron density in the conduction band.
For semiconductors, one can imagine that the initial presence of free carriers (doping) could help to decrease the ablation threshold. For dielectrics, opticald efects could also help to decrease the ablation threshold. However, Leyder et al. have comparedt he non-linear absorption inside silicon for samples with different initial free-carrier densities, [107] that is, for doping concentrations from 10 13 cm À3 to 10 18 cm À3 .F or a1 30 fs pulse at 1.3 mm, their results demonstratet hat the laser energy deposition does not depend on the doping concentration, and thus the avalanchei sn ot efficiently triggered even up to a 10 18 cm À3 free electron density.T he multiphoton excitation is a nonlinearp rocess highly sensitivet ot he laser intensity I,l eading to as calingl aw I N for the absorptionw ith N the number of photonsi nvolved. [108] The ablation threshold is decreased at a shorter wavelength, with ac onsequenti ncreasei nt he ablation rate. [108][109][110] To increase LAL efficiency with oxide targets, it could appear convenient to use short wavelengths in the near UV,b ut with the drawbacko fs elf-absorption of the laser beam by the produced NPs (see Figure 3B-D). Another approach to improve the ablation efficiency relies on promoting materialb reakdown in the tunnelling ionization (deterministic) regime instead of the multiphoton one.
On the other hand, the decrease in pulse duration is associated with the issue of self-focusing and liquid breakdown or filamentation for such high fluences (fori nstance the threshold of optical breakdown in water is 1.11 10 13 Wcm À2 ). [111] Recently,D oÇate-Buendíae tal. have elegantly overcome the issue of the filamentationa nd non-linear energy losses in the water when femtosecond laser sources are used in LAL. [55] They have appliedt he simultaneous spatiala nd temporal focusing (SSTF) of femtosecond pulses configuration, that avoids the unwanted non-linear effects, and ensures at high controlo ft he ablation spot for af emtosecond laser source (45 fs pulse duration, 800 nm, 1kHz, 200 mJ/pulse, 7 10 13 Wcm À2 at the focal point in water). [55] Though promising, the best productivities to date remain those obtained with high repetition rate ns and ps laser sources coupled with scanning optics to bypass the cavitation bubble. [33,112] Overview of oxide NPs obtained by LAL Conventional nano-oxides:r ole of bulk target.
The workingp rinciple of LAL seems to suggest that the easiest way to produce oxide NPs is startingf rom ab ulk oxide target. However,t his is the less frequentc ase found in the literature, where the majority of works report the laser ablation synthesis in liquid solutiono fo xide NPs startingf rom ab ulk targeto f pure metal. In fact, it is worth noting that in LAL, the matter extracted fromt he solid target ineluctably encounters the molecules of the liquid solution, in three different conditions which are (Figure6A): 1) interface between the ablation plume and the surrounding liquid, 2) the interior of the cavitation bubble, in the gas phase and3 )the liquid at ambient temperature and pressure after the collapse of the cavitation bubble. The formation of radical speciesd uring laser-inducedb reakdown of solvent molecules in the plasma at target surfaceh as been intensively investigated in the last years, [27,[113][114][115] showing that these radicals may react formingp ersistent microbubbles consisting amongst others of H 2 ,O 2 , [27,115] and H 2 O 2 [114] when LAL is performed in water.E xcited oxygen speciesh ave been observed in real-time inside the plasma plume in aqueous environment, up to hundreds of nanoseconds after pulse absorption, [116] and also in ambient air duringa blation of oxide targets. [117] Hence,o xygen comingf rom the molecules of liquid (e.g.,H 2 O) or additives (e.g.,H 2 O 2 or atmosphericO 2 )w ill react with the ablated target species, and the extent of the oxidation reaction will depend on the type and concentration of reactive oxygen species and on the redox potential of the metal. [81] This is the source of persistent microbubbles affecting the ablationr ate. [27] For the ablation of 7d ifferentm etals, Kalus et al. observed that the developed gas volume is directly correlated with the respective redox potential of the metal. [81] Ap ossible correlation of (temperature-dependent) redox potentiala nd oxidation state during LAL has been discussed recently in literature. [19] All this makes the choice of the bulk target crucial for the achievement of the desired oxide NPs. The case of iron nanooxidesi su seful to exemplify this aspect, given the variety of possible iron compoundsa nd the relativelys imple characterization. [118,119] It has been reported that ns-laser ablation of bulk metal Fe target in water gives ap revalenceo fm agnetite Fe 3 O 4 NPs, with am inority of hematite a-Fe 2 O 3 ,w ustite FeO and even some traces of metal Fe, likely present as ac ore inside a protecting oxide shell. [118] However,w hen ns-laser ablationi s performed with ah ematite target in water,a morphous Fe 2 O 3 particles arec ollected. [120] Conversely,m aghemite (g-Fe 2 O 3 ) nano-oxides are obtained from the hematite target in ethanol or acetone,w hich are known to have ap artially reducing effect on the ablated material duringL AL. [69,121] This is in agreement with computer simulations and experiments performed on alumina, indicatingt hat as light excess of oxygen is required to achieve oxide NPs with the same stoichiometry of the target when ablated material is still in the gas phase of the cavitation bubble. [117] This suggests that LAL of oxide targets in organic solvents such as alcohols or acetoneg ive slightly oxidized NPs, while oxidation is promoted in water eventually forming amorphous and hydroxylated compounds. Nonetheless,c rystalline oxide NPs can be obtained by laser ablation of crystalline oxide targets in water,a sd emonstrated with 1064 nm (13 ns) pulses and a9 8wt.% ZnO:2wt.% Al 2 O 3 target in MilliQ water. [122] Figure 6. A) SketchofL AL highlighting the three environmentswhere the target speciesencounter oxygen species:i nthe plasma plume(1), in the cavitation bubble (2) and in the liquid at ambientconditions (3). B) By changing the LAL parameters listed in (A), it is possible to switch from AgOmicrocubes to metal Ag (adapted with permission from ref. [123],Copyright2 011, American Chemical Society,from Fe 3 O 4 (adapted with permission from ref. [119],Copyright 2011, Royal Societyo fC hemistry), to Fe-Fe 3 O 4 (reprintedwith permission from ref. [157],Copyright2 011, American Chemical Society), from CuO/Cu 2 O/Cu (reprinted with permission from ref. [180]),t oC u-CuO (reprintedw ith permission from ref. [173],Copyright 2019, Elsevier), or from ZnO (adapted with permission from ref. [144], Copyright 2005,American Chemical Society) to Zn-ZnO NPs (adaptedw ith permission from ref. [142],Copyright 2005, American Chemical Society).
Conversely,L AL of metal targetsi na queous environments gives oxides, sub-stoichiometric-oxides or hybrid metal-oxide structures. These cases are discussed in better detail and with the help of specific examples in the next paragraphs.

Conventional nano-oxides:r ole of oxide type and LAL parameters
In several cases, LAL allows tuning the compositiona nd the structureo fo xide NPs, rangingf rom compounds where the metal has the largestp ossible oxidation state, to core-shell structures where only an externals hell of oxide is formed aroundacore of pure metal ( Figure 6B). The presence of oxygen atoms in the plasma plume is associated to highly oxidative conditions, such that even noble metal (Ag, [123] Au, [124,125] Pt, [126] Pd, [127] Rh [128,129] )N Ps with af raction of oxidised atoms have been observed. Oxidation is promoted by the use of ns laser pulses, UV wavelengtha nd ion stabilizinga dditives, compared to pulses with ashorter duration,use of NIR wavelength, or presence of additives not interacting with metal ions. For instance,A g 2 On anocubes were obtainedw ith 248 nm (30 ns) LAL of bulk Ag in aqueous solutions of polysorbate, while metal Ag NPs with limited surfaceo xidation are obtained in similar conditions withouta dding polysorbate, [123] or at 1064 and 532 nm in aqueous solutionso fs odium dodecyls ulphate. [130] UV light is re-absorbed by the NPs, giving as imultaneous process of LAL and LFL, with the result of increasing the chance of reactionb etween metal and oxygen atoms. The role of the ion stabilizing additive is that of slowing down the rate of electron transfer from reducing species to coordinated metal cations in solution, thus increasing the probability of reaction with oxygen. [123] Chemical oxidants can be added to the solution to promote the reactiono ft arget atoms with oxygen,a ss hown by LAL of bulk Cu in pure water anda queous solutionso fH 2 O 2 (1-5 vol.%) with 532 nm (5 ns) laser pulses, obtaining, respectively Cu 2 Oo rC uO nanocrystals. [131] LAL with 355 nm ns pulses in 3v ol.%H 2 O 2 has been reported also with aN it arget to achieve NiO NPs. [132] In anotherr eport, gallium oxide Ga 2 O 3 has been found after LAL of pure GaAs in acetone with 532 nm (7 ns), while non-oxidisedG aAs NPsw ere achieved when using 250 fs pulses in the same conditions. [133] LAL with 1064 nm (10 ns) pulses of aG aN target in water also originated GaNO NPs. [134] Regarding the pulse duration,t he different resultsm ay be ascribed to the longer lifetime of the plasma plume when longer laser pulses are used. This is associated with am ore extended mixingo ft arget and solution speciesi nt he highly reactive plasma conditions, [10,18,26] as wella si nalonger lifetime of the cavitation bubble and potentially prolonged persistence of NPs in the gaseous phase at temperature @ than room temperature. [26,77,135,136] In fact, there are many reports where LAL with ns pulses of metal targets in water produced oxide NPs. For instance, g-Al 2 O 3 nanocrystals co-doped with H + and Al 2 + , [137,138] Co 3 O 4 , [139] Fe 3 O 4 , [118,119] TiO 2 [29] and MoO 3 [140] NPs were produced by 1064 nm ns pulses startingf rom at arget of, respectively,m etal Al, Co, Fe or Mo in water.W urtzite ZnO NPs were produced with 1064 or 532 nm ns pulses startingf rom a Zn target in water. [141][142][143][144] Photoluminescence measurements showedt hat ZnO particles obtained by LAL can be rich with oxygen vacancies. [145] ZnO particles co-doped with Al 2 O 3 have been synthesized starting from ab ulk ZnO target doped with 2wt. %o fa lumina, dipped in water,a nd using 1064nmn s pulses. [122] Recently,n on-oxidizedZ na toms from aZ nt arget ablated in water with 7ns-1064 nm pulses were detected inside the cavitation bubble still after tens of microseconds by in situ x-ray absorption spectroscopy,a nd the metal signature prevailed even for milliseconds (i.e. after bubble collapse). [51] SnO 2 was produced by 355 nm (10 ns) LAL of Sn in water. [146] Analogous results werer eportedw hen using longerl aser pulseso f6msa t1 064 nm and at arget of Gd in diethylene glycol, which produced Gd 2 O 3 NPs by the reaction of ablated Gd atoms with atmospherico xygen dissolved in the liquid and oxygen comingf rom solvent pyrolysis. [147] The use of laser pulses with longerd uration is likely to further extend the plasma lifetime and volumec ompared to ns-pulses,f acilitating the ionization of solvent molecules and the reaction of target and solution species in the plasma or nearly plasma conditions. [26,77,135] Interestingly,t he average size of Gd 2 O 3 NPsi s 4nm, significantly lower than the average size of oxide NPs obtained by LAL in water (at the same pulse duration of 6 ms), that ranged between 10 and 30 nm. [118,119,[137][138][139] Thisi sa ttributed to ethylene glycolp roperties, such as adsorption on the surfaceo fG do xide clusters and high viscosity hindering cluster coalescence. [18,147] Oxidation of target species occurs also during LAL of organic materials like coal in ethanol and 355 nm (10 ns) pulses, giving graphene oxide quantum dots. [148] When LAL is appliedt oo xide target, the surrounding liquid may influence the crystallinity of final products.T his has been showne specially for the LASiS of rare-earth-doped oxides, such as YVO 4 :Eu 3 + NPs obtained with 532 nm (10 ns) pulses in water,e thanol or mixtures of the two liquids. [149] The different interaction of solvent molecules with the surfaceo fY VO 4 :Eu 3 + NPs was evident from the achievement of crystalline ovoidal particles in water versus spherical and partially amorphous particles in presence of ethanol, [149] or spherical crystalline particles in an aqueous solutiono fS DS. [150] This suggested that YVO 4 :Eu 3 + nanocrystals are stabilized more effectively by ethanol molecules than by aqueous surfactants like SDS. Indeed, the reactivity of organic solvents with the ablated species needs to be considered for each specific material and set of synthetic parameters.F or instance, LASiS of GaO colloids has been reported with 1064 nm (10 ns) pulses and aG aO target in ethanol, [151] or defective CeO 2 nanocrystals were produced by 1064 nm (10 ns) pulses and aC eO 2 target in water. [152] Instead, in the case of Ti target and 1064 or 532 nm ns pulses, the oxide phases or rutile and anataseT iO 2 were achieved only in water,w hile TiCw as found in alcohols. [153][154][155][156] Also in the case of Fe targetsa nd 1064 nm (10 ns) pulses, magnetite (Fe 3 O 4 )w as found in solvents like acetonitrile and dimethylformamide, while am ixture of magnetite and iron carbide( Fe 3 C) was obtained in ethanol. [157] Analogous resultsw ere found in LAL of Fe with fs pulses. [158,159] In some cases, the reactive oxygen speciesg eneratedi nt he plasma plume from water molecules may interact strongly with some metals. For instance, Wp oly-oxo-clusters wereo btained with 1064 nm (10 ns) pulses and Wt arget in water.T he stabilization of poly-oxo-clusters is possible also by the addition of strongly interacting ligandst ot he liquid solution, as demonstrated with 1064 nm (6 ns) LAL of iron in aqueous solution of N-(phosphonomethyl) iminodiacetic acid (PMIDA), which generated nanoaggregates of iron poly-oxo-clusters. [160] Core-shell NPs When LAL is performed in organic solvents under inert (Ar,N 2 ) atmosphere,e ven the elements with the highest tendencyt o oxidation andf eaturing an on-passivating oxide (such as Fe) may be obtained stable in liquid with ap urem etal core protected by an oxide layer. [157,161] Such metal-oxide core-shell NPs are formed when the concentration of reactive oxygen speciesi nthe plasma plumei sl ow enought oa void the complete oxidation of the metallic core. Such ac ondition occurs in organic solvents withouto xygen in their chemical formula. Nonetheless, dissolved oxygen is present in the liquid at equilibrium with the ambient atmosphere, then oxygen atoms are presenti nside the cavitation bubblea nd in the liquid phase, causing to the oxidation of metal atoms in the outer shell of the laser-generated NPs ( Figure 6A). The formation of coreshell NPs is observeda lso in solvents containing oxygen in their chemical formula,l ike alcohols or tetrahydrofuran, [121] because oxygen atoms are sequestrated by oxygen-scavenging speciesf rom the degradation of solvent molecules in the plasma plume, reducing the overall amount of oxygen.T he amount of oxygen can be controlled also introducing scavenging species in the target,s uch as as acrificial layer of iron in an iron-gold thin bilayer film. [113] There is ac onspicuousl ist of core-shell NPs obtained in one step by LAL. For instance, LASiS of NPs composed of aF ec ore and an iron oxide shell has been demonstrated in tetrahydrofuran, [157] acetone, [162] and alcohols. [163,164] Particles of metal Ni core with nickel oxide shell have been produced by LAL of Ni target in water or alcohols. [28,165] Zn-ZnO core-shell NPs were obtainedb y1 064 nm LAL in an aqueous solution of SDS. [142] LAL with 1064 nm (7 ns [166] or 30 ps [167] )p ulses and Mo target in water formed NPs with am etal Mo core surrounded by an oxide-hydroxide Mo shell, with shell thickness increasing with the ageingt ime of the solution. Si NPs passivatedb yaSiO 2 shell were produced by LAL of Si in water or organic solvents. [168,169] Fe-FeMn 2 O 4 NPs were obtained by laser ablation of aF eMn target in ethanol with 1064 nm (10 ps)p ulses. [170] In the case of aF eNi target in acetone and ps pulses, NPs with a core of mixed Fe-Ni carbide andashell of iron oxide were obtained. [171] Formation of iron oxide shell was observed also during LAL with 1064nm( 6ns) pulses of Au-Fe alloys [161] and Au/Fe bilayer thin film targets in water. [113] Cu-Cu 2 Oc ore-shell NPs have been obtained in methanol and 2-propanol, with shell thickness increasing with ageing time. [172] In some cases, Cu-Cu 2 Oc ore-shell NPs have been observed also after LAL of Cu in water with 532 or 1064 nm ns pulses. [173][174][175] Hollow cobalto xide nanospheresw ere observed during LAL of Co in water,d ue to the Kirkendall effect. [132,176] In the case of 1064 nm ns LAL of Ta in ethanol, NPs resulted in ac ore of partially oxidised Ta coated with as hell of Ta 2 O 5 . [177] Also, oxocarbon-encapsulated metal [178] or oxide NPs [179] have been obtained by LAL, exploiting the pyrolysis of organic solvents inside the plasma plume.

Ligand-stabilizedoxide NPs
Additiono fl igandso ro ther solutes to the liquid before LAL can help to achieve better control of the colloidal stability,a s well downsizing the produced NPs. Organic ligands or inorganic compounds (e.g.,s alts) are usually used. [10] The aggregation of the NPs is avoided due to steric or electro-steric hindrance when using organic ligands, while salts improvedt he electrostatic repulsion by increasing the surface charge of the NPs. Such ap ositive effect of the ligands has been reported for both laser ablation in liquids( LAL) and laser fragmentation in liquids (LFL). [10] Althought hese methods are commonly used in colloidalc hemistry,t he first systematic use of ligands in the framework of LAL has been reported in the seminals tudies of MafunØ et al. for noble metal particles. [130,181] They demonstrated as hifto ft he NPs size distribution towards maller sizes when the concentration of the ligand is increased. The authors used an aqueous solution of sodium dodecyls ulfate (SDS), an anionic surfactant, for the narrowed-size synthesis of silver NPs [130,181] and gold NPs. [182] Thereafter,t he same method has been appliedf or the preparation of metal oxide nanomaterials, such as SnO 2 ,T iO 2 ,a nd YVO 4 :Eu 3 + in SDS aqueous solutions. [146,150] Moreover,t he crystallinity of the NPs and their abundance seem to strongly depend on the SDSc oncentration. Usuie tal. succeeded in preparingZ nO NPs by laser ablation of az inc metal plate in solutions of deionized water with different surfactants like lauryl dimethylamino acetic acid betaine (LDA), cetyltrimethylammoniumb romide (CTAB), and octaethylene glycolm onododecyle ther (OGM). [144] Figure 7A shows that different size distributions of the ZnO NPs are found by varying the type and concentration of the three surfactants. The carboxylate (R-COO À )g roup and phosphonates are also well known to react with mineral surfaces, with variousc oordination modes to metal ions. The average size of Y 2 O 3 NPs dramatically shifts from above 6.4 to 1.9 nm in as olution of 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (Figure 7B). [63] The same ligand has been used to stabilize lanthanide sesquioxides and yttrium aluminium garnet (YAG), [63] but also upconversion NPs NaYF 4 :Yb,Er. [183] N-(phosphonomethyl) iminodiacetic acid (PMIDA)h as been used to stabilize nanoaggregates of iron poly-oxo-clusters (Figure7C). [160] In LAL, the drastic size quenching observed with ligands [63,160] (see Figures 6B-C) suggestsa ne arly quenching of the particleg rowth, and thus an earlyp enetration of the ligands into the environment where NPs form. This echoest he questionso nt he process leading to thef ast vaporization of the solvent, [71] to its decomposition, and then to its reactivity. [184] On the one hand, there is no doubt that solvated species penetrate the plasma.I ndeed, the transfer of solvatedi ons (Na + ,L i + )h as been reported using plasma spectroscopy. [64] Laser ablationo fagadolinium oxide in ae uropium chloride solution leads to europium doped gadolinium oxide NPs, which demonstrates that impurities from the solvent can effectively penetrate the core of the NPs as they grow. [100] SAXS measurements performed during laser ablationo fg oldi na NaCl solution show that the size quenching usually reported during laser synthesis of metallic particles in as olution of low salinity happens already in the vapor phase of the cavitation bubble. [185] This has recently been confirmed for organic li-gands. [186] However,s uch penetration of solvated ions or organic molecules is unexpected fors tandard vaporization.I th as been suggested that the water experiences explosive boiling for nanosecond laser pulses. On the other hand, one could expect ad ecomposition of the organic ligands in contact with the plasma,s ince the decomposition of the solventi tselfh as been reported. [187] It would suggest that the plasma is quickly quenched by the solvent vaporization, leadingt oaf ast decreaseo ft he kinetic temperature and then to the nucleation of the particles, while their subsequentg rowth is quenched by the ligandss upply by the solventv aporization. Then organic ligands act in the vapor phase, probably early after the plasma quenching, in am ore friendly environment.
The use of polymers, biopolymers, gelatin, albumin, starch, and chitosan during LAL has been also explored, although mainly on metallic particles. [10] In the case of ZnO NPs obtained by laser ablationo faZnO plate in starch solutions, [188] the starch acted as ac omplexing template that prevented both aggregation and crystal growth through steric hindrance.

Multicomponent oxide nanostructuresb ys equential LAL or reactive LAL
Sequential LAL (S-LAL, Figure 8A)h as been frequently used to produce heterostructures of oxides with other nanomaterials, typicallym etals like Au or Pt. In S-LAL, the laser ablation is performed sequentially on the same solution but with different targets.F or instance, ZnO-Au, [141] and NiO 2 -Au [189] heterostructures were obtained by S-LAL with Zn or Ni and Au targets. Another alternative, consisting of as ingle LAL step, is the addition of reactive solutes in the liquid environment before or after the laser synthesis, and it is known as reactive LAL (R-LAL, Figure 8B). Pt/SnO 2 ,P tCo/CoO x ,A u/TiO 2 ,A g/TiO 2 and Pt/FeO x have been obtained by adding metal salts (Na 2 PtCl 4 ,K 2 PtCl 4 , HAuCl 4 ,A gNO 3 )d uring or after the LAL of Sn, Co, Ti or Fe targets in water. [190][191][192] Self-assembled oxide nanostructures obtained by LAL and externalelectric fields or ageing In somec ases, the LAL is performed simultaneously to the application of externale lectric or magnetic fields. [35] Thee lectric field can be appliedb yp lacing two electrodes in the liquid layer above the target, or using the metal target as one of the electrodes. In the first configuration, laser ablation is performed with 532 nm (10 ns) on aG eo rC ut arget in water to give, respectively,G eO 2 or CuO nanocrystals, [193,194] where the externale lectric field promoted the assembly of laser-generated NPs in anisotropic spindle structures ( Figure 8C). The same configuration has been appliedt ot he generation of copper or silver vanadate microstructures, using 532 nm (10 ns) pulses, a Vt arget in aqueous solutions and, respectively,C uo rA ge lectrodes. [195,196] In these cases,t he metal anodes are electrolyzed by the applied electric field at as uitable voltage, generating metal ions (Cu 2 + or Ag + )t hat dissolve into the liquid environment and form hydrated species that react with laser-generated Vt of orm mixed metal oxides. [195,196] Therefore, the process has been defined as "electrochemical LAL". [35] Electrochemical LAL has been used also fort he production of copper molybdate polyoxometalates( POM)i natwo-step procedure, consisting in the laser ablation of Mo in water with 532 nm (10 ns) pulses and Cu electrodes to achieve crystalline lindgrenite (Cu 3 (OH) 2 (MoO 4 ) 2 ), followed by annealing of the products at 500 8Cf or 5h to obtain the final POM structure. [197] Although the preparation of copper molybdate POMs required heating at high temperature, post-LALa geing of products at room temperature has been reported as as trategy to achieve the self-assembly of NPs into submicron structures such as wires, leaves and flakes. [35] For instance, Mn oxide nanocubes were formed by the ageing of Mn 2 O 3 NPs in water,w hereas nanofibers and nanosheets wereo bserved startingf rom Fe-Mn oxides. [170] This is facilitated by the absenceo fs tabilizing ligands or other chemical additives which typically passivate the surfaceo fn anocrystals generated by chemical routes.I no ther reports, ZnO nanorods and nanoflowers were obtained instead of nanospheres by LAL of aZ np late in water and ageing. [141,198,199] The resultsw ere attributed to the heatingo f the liquid solution due to the long pulse duration (in case of ms pulses) [198] and to NPs re-irradiation (in case of 532 nm pulses). [141,199] Even when the electric potential is applied to the ablated target, the chemical composition of products can be modified, due to the alterationo fr edox processes between ablated species ands olution species. For instance, laser ablation of Al in water with 1064 nm (10 ns) pulses generated crystalline boehmite (AlOOH)N Ps when the target was biased with an egative potentialo fÀ10 Vv ersus the counter electrode in solution, while the fraction of crystalline hydroxide is much lower when LAL is performed withoutany electric field. [200] Issues related to compositional homogeneity In am inority of cases, crystalline hydroxides were found togetherw ith oxide phases in laser-generated NPs. For instance, ablation with 800 nm (200 ps) pulses of In target in water originated am ixture of In(OH) 3 and In 2 O 3 NPs, [201] although only In 2 O 3 was observed by laser ablation of In in water with 532 nm (5 ns) pulses. [202] The structurald ifferences may be explained by the use of 200 ps instead of 5nsp ulses,b ecause the use of ps pulses is associated with as horter plasma lifetime and ac older plasma compared to ns pulses. Hydroxides are less stable than oxides at the high temperatures reached inside the plasma plume. [201,202] In fact, it has been suggested that hydroxide formation occurs in the last stage of LAL, or even after ageing in water of the NPs. [170,201] In general, the formation of hydroxidesd epends on the reactivity of the ablated materiala nd solventm olecules in the specific synthetic conditions. For instance, bruciteZ n(OH) 2 platelets were obtained by LAL with 355 nm (10 ns) pulses of Zn in water, [203] while ZnO is typicallya chieved with 532 and 1064 nm pulses. [141][142][143] Gd(OH) 3 was obtained by high energy 1064nmp ulses of an sl aser in aqueous environment. [204] The relative yield of oxide versus hydroxide phases is influenced also by dissolved oxygen molecules in the liquid sinceL AL of Co in water or N 2 purged water resulted, respectively,i nC oO and b-Co(OH) 2 NPs, that spontaneously evolved into Co 3 O 4 by ageing. [205] Also, spindle-like GaOOH particles slowly grew in as olution of CTAB during a few days after the ablation of aGat arget. [206] Different types of surfactants can be used to promote or prevent the formation of metal hydroxides, since lamellar b-Zn(OH) 2 platelets were obtainedi nS DS (anionic surfactant) solutions, [203] while ZnO NPs wereo bserved in aqueous solutions Figure 8. A) SketchofS -LAL with the targets of two distinct materials "A" and "B" to give multicomponent "A" + "B" heterostructures like Au/ZnO.Adapted with permission underC CB Y4.0 from ref. [141], Copyright 2016,EDP Sciences). B) Sketch of R-LAL with at arget of am aterial "A" to give NPs that interact with ac hemicallyreactive precursor of "B"togive am ulti-component "A/B" system like Pt/SnO 2 (adapted with permission from ref. [190],Copyright2 019, Elsevier). C) Sketch of E-LAL with at arget of am aterial "A" in the presenceo fa nelectric potential applied by an electrode of amaterial "B" to give elongated nanostructuresl ike CuO nanospindles, or multi-component "A" + "B" oxides( reprintedwith permission from ref. [193], Copyright2 009, American Chemical Society).
of amphoteric (LDA), cationic (CTAB) and nonionic (OGM) surfactants (see Figure 7A). [144] Similarly,b ruciteM g(OH) 2 nanostructures were obtained by laser ablation with 1064 nm (5 ns) pulses of aM gt arget in water or aqueous SDS solutions, while MgO NPs weref ound in organic solvents (acetone and 2-propanol). [207] Besides, the morphology of Mg(OH) 2 nanostructures depended on the SDS concentration:u ltrafine tubular-like fibres were obtained at low concentrations of SDS solution, while stripe-like rods and large plateletsg rew preferentially with increasing surfactant concentration. [207] Ions are also important in determining the composition of the final products,a ss hown by LALe xperiments at 355 nm (8 ns) of Cu or Zn targets in water and aqueous solutionso f the corresponding metal salt (Cu 2 + or Zn 2 + ), but coupled with differentc ounterions (CuCl 2 ,C u(NO 3 ) 2 + NH 4 OH, ZnCl 2 + NH 4 OH, Zn(NO 3 ) 2 ). LAL in neat water generated mixtures of metal and metal oxide NPs, whereas nano-paratacamite Cu 2 Cl(OH) 3 was produced in aqueous CuCl 2 solution,n ano-rouaite Cu 2 (NO 3 )(OH) 3 was formed in aqueous Cu(NO 3 ) 2 + NH 4 OH solution, nano-simonkolleite Zn 5 (OH) 8 Cl 2 ·H 2 Of ormed in ZnCl 2 + NH 4 OH solution, and layered zinc hydroxiden itrate Zn 5 (OH) 8 (NO 3 ) 2 ·2H 2 ON Ps formed in Zn(NO 3 ) 2 solution. [208] Overall, it should be noted that ag eneral problem encountered in the LASiS of oxide nanostructures may be the presence of non-crystalline by-products due to hydroxides or pyrolyzed solvent residuals, andt he coexistence of multiple phases. Although amorphous phases can be detected by TEM, X-ray diffractiono rs pectroscopic( FTIR, Raman, optical) analysis, the evaluation of the extent of non-crystalline versusc rystalline fractionsi sg enerally not possible or inaccurate by these techniques. LAL of Cu in water is such an example, since am ixture of CuO and Cu 2 ON Ps has been frequently observed, and with ap roportion changingw ith ageing time in favour of the more oxidizedC up hase. [209] Another paradigmatic case is the LAL of Fe, since non-crystalline iron hydroxidesw ere evidenced by LAL in water,a nd there is av ariety of iron oxides (wustite, maghemite, magnetite, hematite), which were observed in the same samples, even if as pecific oxide phase (Fe 3 O 4 )w as dominant. [118,119] Besides, amorphous carbonb y-products were found after LAL of Fe in variouso rganic solvents, as evidenced by transmission electron microscopy and Raman analysis. [118,119,[157][158][159] This phenomenonw as exploited for the generation of graphitic carbon-encapsulated MnO, ZnO or Fe 3 O 4 NPs by LAL in acetone, [210] graphitic carbon-encapsulated LiNbO 3 NPs in toluene as wella sc hloroform [211] and carbon-encapsulated TiO 2 after performing LAL in gaseous, liquid and supercritical CO 2 . [179] To avoid the problem of non-crystalline hydroxides in lasergenerated iron oxides, as imple etching procedure has been proposed, that is based on treatment with Ethylenediaminetetraacetic acid (EDTA) or diluted HCl, [118,119] which are easily washable compounds and allow to preserve the purity of the crystalline product at the end of the procedure. In this case, the NPs are collected by centrifugation, because the additives compromise the colloidal stabilityo ft he system.
In general, oxide nanostructures show limited colloidal stability at high concentration (% mg mL À1 )o rl ong ageing times (typicallyd ays), unless stabilizing moleculesa re added during the synthesis, or the pH of the liquid is tuned far from the isoelectric point of the material. The decrease in colloidal stability over time can be due also to chemical modifications such as the reaction with atmospherico xygen or the release of ions. The collection of laser-generated oxidesb yc entrifugation is easy in case of unstable colloids, and this electrostatic instability can be obtained by pH tuning towards the isoelectric point.

LFL LFL fundamentals
In contrast to LAL, which requires bulk targets, LFL starting materials are micro-or nanoparticles dispersed in liquids. This prerequisite of LFL perfectly matches with the fact that basically all oxidesa re available in powder form. Generally,L FL has provent ob ea ne ffective method to gain small (~10 nm) and ultra-small (< 3nm) NPs, starting from an educt of larger NPs or microparticles dispersed in al iquid. LFL can be performed on ligand-free educt particles, [114,[212][213][214][215][216][217][218][219][220] or in the presence of stabilizing molecules, [152,221,222] or other types of supports such as oxides. [152,[223][224][225][226][227][228][229][230][231] Hence, the technique is generally applicable to colloidal NPsr egardless of the respective synthesis method used. However, it is worth noting that the use of LAL-generated educt NPs allows performing LFL in ac hemical-freew ay,t o obtainc lean size-selectedp roducts. [17,119] During LFL, laser pulses with duration ranging from fs to ns and wavelength from visible to ultraviolet are applied to the liquid solutiona t laser fluences high enough to match the photo-fragmentation threshold of the "educt" particles (typically in the 1-100 mJ cm À2 range). [10] In fact, contrary to LAL being performed with near-infrared pulses in the majority of cases, LFL requires ac areful selection of the appropriate wavelength, fluence, and pulse duration based on the band gap and size of the starting oxide particles, while avoiding as olvent breakdown.
The majorityo fL FL experimentsa re performedw ith ab atch setup [10, 114, 225-232, 152, 212, 213, 218-222] employing ad ispersion of the precursor NPs in as tatic, constantly stirred, or af luxed cell with the laser beam being either unfocused or focused inside the cell. In the latter case, the focal plane is usually behind the cell to avoid liquid breakdowno re ntrance window damage.
By controlling the duration of laser irradiation, the number of pulses applied to each particle statistically depends on how often the NPs pass through the laser beam. Consequently,t he average energy and number of laser pulses applied to the educt particles, as wella st he related standard deviation, change if the liquid is stirred, fluxed, or is static and subjected only to convective mixing (non-optimal condition). Indeed, only in af ew cases in the literature, [214-217, 223, 224] the liquid is fluxed by ap ump throughout the cell or at ransparent tube, openingt he questiono nh ow much the gradientw ith respect to the number of laser pulses appliedt oe ach nanoparticle affected the reports appeared so far. To ensure that all particles are irradiated with as imilar number of laser pulses,L au et al. performed LFL in af ree liquid jet (Figure 9). [223] The jet is obtained from ac apillary located at the bottom of al iquid reservoir containing the dispersion of educt particles, and the pulsed laser beam with defined laser fluences is applied at 908 to the liquid jet direction. In this way,t he number of laser pulses applied to each volume of liquid passing through the capillary can be controlled by tuning the particle's residence time (inversely proportionalt o the appliedf low rate through the illuminated part) and repetition rate of the laser. [37,214,223] Finally,t oa ccount for fragmentation efficiency,t he authors introduced the term "mass-specific energy input", describing the total amount of energy applied to the educt particlem ass. [223] This mass-specifice nergy input is directlyc ontrolled by the number of laser pulses per liquid volumee lement. At given particlem ass-concentration this gives the number of laser pulses per educt-particle. The latter can be controlled by the flow rate (or residence time of the educt-particle inside the irradiated volume element) or the repetition rate of the laser.W hen using the passage reactor setup shown in Figure 9c onstant number of laser pulses is applied to each nanoparticle with each passage through the irradiated volumee lement. With each passage, the number of laser pulses and hence mass-specific energy input increases by the same value and, hence,i ti sp recisely controlled. [214,215,223] This approacha llows ad irect( mostly linear)t unability of particle size, [214,223] and particle properties (e.g.,o pticalp roperties [224] and band gap, [37,223] surface chemistry, [214,234] crystal phase [37,234] or catalytic activity [37,224,234] ).
The basic steps leadingt oL FL are the following (Figure 9): light absorptionb yt he initial NPs, heat transfer to the surrounding liquid (with plasma formation at highest irradiation fluence), phase transition and fragmentation, particle nucleation, growth and ripening. The individual fragmentation processes ands teps ( Figure 9) generallyo ccur on an ultra-short time scale ranging from ps (for heat transfer processes inside the solid) to ns and ms( for particleg rowth), although subsequent secondary ripeningp rocesses may last from seconds to hours and days. [235] Hence, coalescence and re-growth until reachingt he thermodynamic equilibrium at ambient conditions are additional processes that need to be taken into account. [21,212] By adding ions (salts) [215] or organic steric ligands [222,236] that coordinate the photo-fragmented nanocrystals, regrowthc an be limited in situ, resulting in smaller sizes compared to the ligand-free particles. [236] So far,t he majority of mechanistic studies were focusedo nL FL of gold NPs as model material, [212-216, 218, 232, 237] but LFL was employed for ab road range of particles (metals, [219,220] oxides, [152,223,[225][226][227][228][229][230] nitrides, [227] iodides, [231] sulfides, [238] semiconductors, [152,223,225,226,[228][229][230] organic crystals, [239] hybrid perovskites [226,236] ). Fragmentation mechanisms are still under debate in literature but beyond the scope of this review.T he consensus is thatn s-lasersm ainly initiate photothermal evaporation processes and meltingw hile ultrashort laser pulses (! 10 ps) mainlyl ead to electron dynamicmediated mechanisms. [212,213,223,232,[240][241][242][243] Most earlierm echanistic studies focused on post-mortema nalysisw here only the final particles ize after LFL (after ripening processes occurred) was investigated. [212,213,223,232,235,[240][241][242][243] New studies start to refine recentf ragmentation mechanisms. Concerning the employed laser parameters shown in Figure 9, in situ methods like ultrashort X-ray and electron diffraction [233] allow to directly observe the fragmentation mechanism with high temporal resolution. Additionally,n umerical studies provide more and more insight into the fragmentation mechanisms when using different laser parameters. For the case of fs-400 nm-laser pulses, Delfour and Itina found that metal NPs of % 30 nm have the lowest fluence threshold for fragmentation. [244] Similar size dependence of the threshold fluence was observed by Ziefuß et al. for ns-532 nm laser pulses but discussed in terms of photothermal mechanism. [214] Here, quantitative (i.e.,n early 100 %) fragmentation of Au NP was observedf or educt particles ize > 40 nm. Experimentsw ere conducted with single-pulse per particleconditions. [214] Interestingly,a bove the size threshold of % 40 nm for educt particles,t he quantitative formationo ff ragments with peak size located at 3nmr egardless if ns-or pslaser pulses were used. In the case of smaller educt particles (< 40 nm) or lower laser intensities (< 1.6 10 12 Wm À2 @5 3nm educt-particle size) am ixture of % 3nma nd % 13-20nmN Ps were observed. Formation of similar product particles ize was also found for ns-532 nm-LFL of CoFe 2 O 4 and BiFeO 3 indicating as imilar fragmentation mechanism. [224,245] Although LFL has been studied extensively in recent years, the effective nanoparticle productivity gained by LFL has barely been addressed in literatures ofar.
In general,t he main parameters influencing LFL may be summarized as shown in Figure 10: A) the educt particles ize, with an optimal range of 30-60 nm [214,244,246,247] but well above 10 nm, [214] because the Rayleigh scattering of incident light by the colloid scales with the 6 th powero ft he particle diameter,w hile the absorption coefficient scales with the 3 rd poweroft he size; [248] B) laser wavelength, because it affects the absorption and is usually larger at shorter wavelength; C) the laser fluence, that is the driving force of the fragmentation process; D) laser pulsed uration,w hich sets the timescale of energy absorption versus heat loss dynamics. [80] High productivity is possible with low nanoparticle concentration( Figure 10  All this was combined in the work of Ziefuße tal. with single-pulseper-particle conditions. [214] In addition to the determinantsA )-D), the repetition is an important factor,e ven at constant laser power. The laser pulsing rate shouldb eh igh enough to reach all particles within the irradiatedv olume, so that higherc olloidal quality (narrowers ize distributions) is achieved using higherr epetition rates,w ith the mechanismi nabatch chamber being relatedt oc onvective-diffusion phenomena from and into the irradiated volume. [249] In liquid flow,t he repetition rate should be matched to the volumee xchange rate within the intersection of laser beam and liquid flow channel, so that one volumee lementi sh it at least once by al aser pulse. On the other hand, there must be an upper repetition rate limit, as the fragmentation event needs to at least proceed so far that the mother particles have enough time to be fragmented. Also, the cavitation bubble emerging during LFL should be temporally bypassed. Recently,t emporally resolved, X-raybased structuralk inetic studies during LFL pointed at aL FLcavitation bubble lifetime of 50 to 100 ns, and the fragmenta-tion product still growing after this period, on ah undreds of nanoseconds (to afew microseconds)s cale. [233] Accordingly,ifa subsequentp ulse is intended to be applied only after the fragmentation process causedb yt he previous pulse is terminated, the upper repetition rate limit of efficient LFL could be expected in the (few) MHz regime.
With the synthesis parameters given in ref. [214]( yielding % 2mgh À1 W À1 )i tc an be estimated that al aser power of about 500 Wi sr equired to achievep roductivities of % 1gh À1 , that are enough for use in several "real world"t echnological applications. [42] Interestingly,t his power regime appearst ob e independento ft he pulse duration used. In the case of LAL, Streubel et al. achieved about 4t imes higher productivity with the same laser power. [33] Yet, for LAL, the NPsb ear am uch broader particles ize distribution and potentially requirep osttreatment, [43] while NP after LFL are < 3nmi ns ize. [42] While this is only af irst rough estimation, ac ritical comparison of the economic performance of LAL versusL FL needs to be conducted in future studies.

Overview of oxide NPs obtained by LFL
As previously discussed, LFL is an effective approach for size reduction and concomitantr efinement of the size distribution of educt particles. Although the majority of publications on LFL (especially mechanistically ones) focus on noble metal NPs (mainly AuNP), [10,212] LFL of oxide-based NPs has especially been used in application-oriented publications. Several studies in literature showed that LFL is ap owerful method for the introduction of defects in nanocrystalline oxides, that have found relevant applicationsi nc atalysis. For instance, 3nm CeO 2 particles obtained by LFL showed as ignificantly in-creasedC e 3 + content of % 40 %c ompared to the initial value of 7.5 %. [152] The increased defect content was discussed in terms of ah igher density of edges due to the small particle size after LFL, favouring the formation of stable Ce 3 + defects of potentiali nterestf or catalytic studies. [250] Lau et al. performed the LFL of ZnO in al iquid jet, showing the linear change of particles ize and opticalp roperties with mass-specific energy input (Figure 11 A). [223,239] This was attributed to a shock-wave mediated disaggregationa nd partial vaporization of the particles, which resulted to be several orders of magnitudes more effective than performing an ultrasonic treatment. [223] Employing LFL of submicron-sized cerium oxide particles using ns 1064 nm laser pulses with high energy (350 mJ), Ta kaedae tal. observed the formation of 3nmC eO 2 particles (Figure 11 B) that obtained high-density oxygen vacancies and hence Ce 3 + defects. [152] Wang et al. performed LFL of La:BaSnO 3 perovskite NPs with an initial size of 40 nm using ns 355 nm laser pulses at an intermediate laser fluence of % 400 mJ cm À2 . [226] The authorso bserved an efficient reduction of the particles ize down to < 10 nm while retaining the phase pure La:BaSnO 3 perovskite crystal structure ( Figure 11 C). The fragmented NPs were subsequently embedded as co-catalysts into Mo-doped BiVO 4 at gradually changing mass-loadings, to achieve highly active photo-anodes. [226] The usage of La:BaS-nO 3 after LFL allowed to improvet he photocurrent density by about 50 %w ithoutu sing hole scavenger,u pt oalevel that is more than 80 %o ft he theoreticallyp ossible value. [226] Consequently,o xide NPs after LFL do not only suit as ideal catalysts themselves, but also as co-catalysta nd components for the synthesis of supported catalysts. Though the choice of the appropriate precursor NPs is crucial,a ss hown by Schaumberg et al. at the example of laser-based coppern anoparticle synthesis. [227] Through LFL of CuO in acetyl acetate, the authors reportedt he formation of Cu/Cu 2 Oc ore-shell NPs, while pure Cu NP formed on LFL of Cu 3 NN Ps in the same liquid. [227] Since the authors observedacarbon shell covering the NPs in the case of Cu 3 N, the occurrence of redoxr eactions during LFL appears to follow similarc orrelations as in the case of LAL, [180] in agreement with other recent reports aboutmetal alloys. [121] LDL While LFL reduces the particles ize, especially fundamentalc atalytic studies require ac onstant particles ize to study structure-activity correlations.H ence, we proposet he termL aser Defect-engineering in Liquid (LDL, Figure 1D)t os ummarize all studies that aim to alter materials properties and defectd ensities without any significant change in particles ize. In LDL, defects must be introducedi nt he final product, that are not present in the pristine particles. Therefore, there must be some experimentale vidence about the change in defect density as the main effect of the laser treatment, to verify the occurrence of LDL. When considering the laser-induceds ize-tailoring methods (LFL, LML), one can identify three different situations: 1) structural changes do not occur during laser-induceds ize tailoring; 2) structural changes occur during size tailoring as the effect of the laser treatment but are connected to phase or morphological modifications, not to defectengineering; 3) defect engineering occurs simultaneously to size-tailoring, thus the process falls in an intermediate case between LFL/ LML and LDL.
Zuniga-Ibarrae tal. performed LDLo fT iO 2 NPs using high intensity 532 nm, ns-laser pulseso bserving the transformation of anatase to rutile accompanied by ar eduction of band gap from 3.2 eV down to 1.8 eV and the formationo fa morphous TiO 2 ,l ikely located near the surface of TiO 2 particles, [228] similar to other studies using UV-laser pulses. [37,229] So far the formation of black TiO 2 has only been reportedu nder hydrogen treatment and explained by hydrogen-induced n-doping of the semiconductor,l eadingt os ignificantly increased conductivity,d efect-related tailoring in the conduction and valence band, [228,251] as well as broadbanda bsorption and improved photocatalytic activity. [228,250] Since the educt TiO 2 NPs were polydisperse, Zuniga-Ibarra et al. observed simultaneous LFL of larger particles and LML of smaller ones. This highlightsthe importanceo fu sing educt particles with narrow size distribution and avoid laser fluence gradients for effective LDL. The negative effect of such fluence gradients was indicated by Waag et al.,w ho assigned occurringp hase segregation processes to the former after performing laser beam ray-tracing calculations. [224] Further,i th as been highlighted before,t hat using a cylindrical lens in case of the liquid jet setupi sh elpful to achieve better illumination, as demonstrated for the laser-based tuning of ITOo ptical absorbance. [252] Consequently,s haping the naturally circular liquid jet cross-section into am ore flat geometry using an elliptical nozzle has recentlyb een reported to improve the energy inputd uring laser irradiation of oxide particles. [245] Af urthers tudy focusedo nt he effect of laser treatment of TiO 2 and ZnO NPs to tune their defectd ensity and materials properties. As final readout,t he exhibited photocurrents were measured for samples treated with ad ifferent number of ns-UV-o rp s-VIS-laser pulses per particle. [37] Interestingly,f or ns-UV-laser pulses the authors observed ad ecreasing photocurrent in all cases. Here the smallest number of pulsest hat were used in the study was % 70 pulses per particle.F rom an extensive analysis, the formation of bulk defects due to thermally initiated isochoric melting (typical of ns-pulses) was proposed for all cases. When the same educt materialw as treated with a small number of ps-VIS-pulses( < 150 pulses per particle), the performances/photocurrents improved by af actor of two. This was attributedt op referentiala bsorption of the laser pulse at pre-existing defects during LDL( since, wavelength:5 32 nm! 2.3 eV < E BG (TiO 2 ) = 3.2eV) improving excitong eneration during measurement of photocurrents. For al arger number of applied ps-VIS-laser pulses (> 150 pulses per particle), the photolumi-nescencea nd photocurrents decreased as it was observed in all experiments using the ns-UV-laser.H ere, segregation and movement of the defects to the surfacew ere discussed as a reasonf or the decreasing photocurrents at ah ighern umber of laser pulses. [37] Note that, in the above studies, the change of colour in TiO 2 has alwaysb een accompaniedb yr utile formation and has only been observed when ns-laser pulses have been used, regardless of its wavelength. [37,228,229] For this reason,i sochoric meltinga lteringc rystal structure and colour of TiO 2 was discussed for ns-laserp ulses. [37] Yet, irradiation with ps-laser pulsesa lso led to enhanced photocatalytic properties, though without any colourc hange. [37] Therefore, the fundamentalp rocessesb ehind defect formation still deserve further investigation.
As previously indicated by the LFL experimentso fC uO in acetylacetate, the high temperature reached by the eductN Ps also promotes solvent degradationa nd redox reactions. Depending on the nature of the involved chemical substances, the reduction or oxidization may be favoured. [19] This process can be seen as the nanoscale version of what happens microscopically in the ablation plume during LAL. The origin of such redox reactions in LFL has been discussed in the literature, [253] but it has not yet been addressed in as ystematic study,e specially not in terms of LDL. Since LDL aims to maintain the particle size, defectf ormationc an only be introduced by photo-induced redox reactions. Understanding and controlling these is hence of utmost importance for efficient LDL. Af irst indication of the underlying mechanisms can be drawn from the recent studies of Waag et al. The authors fragmented CoFe 2 O 4 NPs using ps 532 nm laser pulses andt he liquid jet setup ( Figure 9). [224] The particle size of CoFe 2 O 4 was nearly constant (slightlyr educed)b ut with clear morphologyc hanges from ratherc ubic to spherical shapea fter LFL. The homogenouse lemental distribution has been maintained (Figure 11 D). Yet, the optical properties were tuned and electrocatalytic activity significantly enhanced. That was attributed to the formation of CoO and layered Co di-hydride during LFL, the latter being associated with the sheet-like background observed in TEM analysis of the samples after LFL. [224] Hence, reduction processes must have taken place. They discussed that the decomposition may have been induced by self-focusing of the laser when being refracted at the liquid jet. [224] The authors recentlyp erformed LFL of BiFeO 3 in water and propylene carbonate using an elliptical liquid jet (instead of as pherical one) to improve the homogeneity of irradiation.T he authors successfully reduced the particle size from initially 450 nm down to 10 nm but still observed partial decomposition of the BiFeO 3 forming additional iron oxides regardless of the solvent used. [245] Hence, the origin of phase decomposition remains still an open question.
Although oxidation is more frequently observed in literature during laser treatment of metal NPs in water,t wo other independent studies of LFL of Co 3 O 4 also observed the reduction of Co-ions from Co 3 + to Co 2 + accompanied by the formation of oxygen vacancies [254] and CoO, [234] Hence, one may conclude that laser processing of oxide NPs is ar eductivep rocess. In the study of the Tüysüzg roup, [234] the authors also studied CoO particles apart from the Co 3 O 4 spinel mentioned before.I nterestingly, after the laser processing of CoO, af raction of Co 3 O 4 spinel was observed in the products,n ow indicating oxidation of Co 2 + ions to Co 3 + .C onsequently, for the same element the occurrence of oxidation or reduction during LDL in water appears to depend on the initial state of oxidization of the educt particles. Note that the educt material of the Tüysüzg roup is derived from templating chemistry of coffee waste, connecting laser synthesis even more tightly with green chemistry and circular economy.
The interplay of initial oxidization state, redoxp otentials and laser parameters are still unclear. However,s eminall aser studies about laser irradiation of colloidsa ppeared, where the laser fluence is chosen to be low enough to maintain the initial oxide particle size but high enough to induce redox reaction between liquid media and NPs. [19,37] In these studies, the band gap, photoluminescence,a nd photocatalytic activity of TiO 2 and ZnO NPs were laser-modified independent of the particle size, due to the alteration of location and density of defects. [19,37] It is expected that LDL will not only significantly impact studies of structure/defect-activity independent of particle size but also be an efficient tool to study fundamental redox-based processes occurring during laser excitation of oxides.

LML Mechanism and processing conditions
Pulsed laser melting in liquid is at echnique in which laser irradiation of as tarting particulate materiald ispersed in al iquid is utilized to induce at hermal process, particularly melting, to fabricate new particles. This melting can be either isochoric (through re-shaping particles like rods, polygonso rf usion or pre-existing aggregates) or cause size increase (through fusion of aggregates forming during LML) referred to the starting particle diameter.L ML is also known as laser-inducedr eshaping, and it differs from the LAL process because the fluence used for LML is one to two orders of magnitude lower than that of LAL. Suspended particles are generally used as raw materials for LML, so the laser beam is directed (usually unfocused and occasionally focused depending on the fluence required) onto al iquid dispersion of the raw particles. The shape of the particles is modified by the laser-induced thermal process. Thus, plasma emission and the generation of shockwaves commonly observed in LAL are not evidentd uring LML.
As chematic illustration of the basic LML process is shown in Figure 12 A. When particles absorb light energy from the pulsed laser,o nly the temperature of the particlesi ncreases rapidly.T his is due to the transparency of the surrounding liquid and the small heat capacity of the raw particles. If the temperature of the particles exceeds their melting point, they form transient molten droplets. Then, in the interval between different pulses,t hey are quenched by the surroundingl iquid mediumand form solid spherical particles.
The formation of sub-micrometre spherical oxide particles was initially reported for TiO 2 , [255] although with very low yield. In subsequent studies, similar submicrometric particles were occasionally found in LAL products as minor unwanted byproducts. [256,257] Theiro rigin has not yet been uniquelyi dentified, being possibly formed by irradiation of particles agglomerates or large target fragments at the laser beam edge, usually with aG aussian profile, which might be in an appropriate fluence range for LML. They might also come from the ejection of meltedt arget droplets during target ablation (i.e.,a th igh effective laser fluences during LAL [22] ). The term sub-micrometre particles refers to hundreds of nanometre particle diameter, which is at ypical size range obtainedb yL ML. The intentional formation of sub-micrometre spherical particles as main products, by the LML technique, wasr eported only ad ecadea go for carbides, [258] and metals, [17] and later achieved with various particleso btained( C) with a3 55 nm laser at af luence of 133 and 400 mJ pulse À1 cm À2 ,and (D) with a5 32 nm laser at af luence of 500 and 700 mJ pulse À1 cm À2 .( E) Size distributions of spherical TiO 2 particles irradiatedw ith a3 55 nm lasera t200 mJ pulse À1 cm À2 ,a nd 66 mJ pulse À1 cm À2 .Size-fluence diagram of TiO 2 underi rradiation by a3 55 nm lasera fter correctingp article absorbancec hanges by laser irradiation. Particlesize ranges of the experimentally obtained data are showninb lue crossbars for 200 mJ pulse À1 cm À2 ,a nd ag reen crossbar for 66 mJ pulse À1 cm À2 .Correspondings chematics are illustrated on the rights ide. oxide materials, [252,[259][260][261][262][263][264] even in the size range of tens of nm, [119] or for the incorporationo fA uN Ps into ZnO starting from AuNP supported on ZnO. [262] The terms "sub-micrometre particle" or sub-micrometre sphere (SMS) refer to hundreds of nanometre particled iameter,w hich is at ypical size range obtained by LML, where in most cases perfectly spherical and (poly)crystalline particles are produced.
The pulsed lasers required for LML have pulsew idths on the order of nanoseconds or picoseconds, and their fluences range from 10 to 200 mJ pulse À1 cm À2 depending on the material. Due to heat dissipation, ah igher fluence is needed to fabricate sub-micrometre spherical particles using long-pulse lasers with pulse widths of severalt ens of nanoseconds. [265] If picosecond pulses with the same wavelength and energy are used for LML of low-thermal-conductivity ceramic materials, the obtained sub-micrometre spherical particles will be smaller than with nanosecondp ulses. [266] The reason is that, with ps pulses, heating occurs in ap article on as horter time scale than heat dissipation to the surrounding, which usually requires ns. Thus, ps pulses are associated with ah igher heating efficiencyo fp ristine agglomerates of NPs, such that the meltingt emperature is reacheda tasmaller size compared to ns pulse irradiation with the same wavelength and energy.
Another laser parameter relevant for the final particle size is the laser fluence. [259] No obvious morphological changes are observeda talower laser fluence, while higherl aser fluence will induce the formation of NPs from the raw particles through vaporization,s etting intermediate conditions between LML and LFL. This stepwise size change from raw particle size to the sub-micrometre scale, then down to nanometres with the laser fluence increase, well corresponded to the phase change from as olid to al iquid and finally to av apor by particle temperature increase. Therefore, appropriate fluence control is necessary to selectively obtain sub-micrometre spherical particles and stay in the LML range without falling in intermediate LML/LFL operating conditions.
The fluence threshold to fabricate sub-micrometre spherical particles via LML process is approximately calculated based on the opticala nd thermal properties of the materials and the assumption of an adiabatic process. [16,267] That is, the laser energy supplied to the particles is exclusively used to heat the particles from room temperature to their melting point without dissipationo fh eat to the surroundings. The laser fluence, (mJ cm À2 ), required to start the melting of as ingle spherical particlecan be calculated using Equation (2): where 1 p (g cm À3 )i st he particled ensity, DH (kJ g À1 )i st he enthalpy from room temperature to the melting point, d (nm) is the diameter of the particle, and Q l abs ðdÞ is the absorption efficiency (absorption cross-section divided by geometrical crosssection) of the particle-based on the Mie theory.Asize-fluence diagram can be then obtained to visualize the relationship between the laser fluence (J)u sed to fabricate the spherical particles and the resultant particle size (d). [267] Size-fluence diagrams for the starto fT iO 2 melting with the commonly used 355 nm (wavelength) Nd-YAG lasers are shown in Figure 12 B. The 355 nm laser can melt TiO 2 particles and yield sub-micrometre spherical particles at al ower fluence than in the visible range. This is due to the relatively higher Q l abs ðdÞ at 355 nm wavelength comparedt ot he visible light range,w hich derives from the difference between the complex refractive indiceso fT iO 2 and reflects the relationship between photon energy and the TiO 2 band gap. [261,268] The gradual fluence increase with increasingp article size in Figure 12 B, that is apparent for sizes above 300 nm for 355 nm wavelength, is due to the nearly constant absorption efficiency Q l abs ðdÞ in the large particles ize range, where geometric optics becomes dominant. In contrast, ad rastic increase of fluence curvesf or particles smaller than 100 nm is attributed to the steep decrease in Q l abs ðdÞ when particles ize is much smaller than the laser wavelength.
This trend is commona mong most insulating or semiconductingo xides, and the fluence minimum for meltingw ith a 355 nm laser is usuallyf ound at diameters of 100 to 200 nm. Particles in this size range are thus easily melted by applying a fluence that exceeds the calculated minimum value, and the diameters of particleso btained via LML are often in this range. Scanning electron microscope (SEM) images of TiO 2 particles obtained at different fluences by 355 nm and 532 nm laser irradiation are shown in Figure 12 Ca nd D. As expected,t he images indicatethe formation of sub-micrometre spherical particles at 355 nm in this size range andt he increase of size with increasing fluence due to the repetitive meltinga nd fusing. At 532 nm, due to the lower optical absorption, af airly large fluence is required for the formation of spherical particles. Therefore, suitable laser wavelengths mustb es elected to fabricate sub-micrometre spherical particles at lower laser fluences.
The size-fluence diagram is also closely relatedt ot he obtained particle size distribution. [268] The size distributionso f sub-micrometre spherical TiO 2 particles obtained with a 355 nm laser at fluences of 200 and 66 mJ pulse À1 cm À2 are shown in Figure12E.N early monodisperse sub-micrometre spherical particlesw ere obtained at 66 mJ pulse À1 cm À2 .I nc ontrast, laser irradiation at 200 mJ pulse À1 cm À2 yieldedq uite large spherical particles( 400-600 nm) along with small particles % 100 nm in diameter.P articles with diameters of % 200 nm were absent.S ize-fluence curves showingt he start meltinga nd start vaporization of TiO 2 under irradiation with a 355 nm laser after correction for changes in absorbance by reductiono fT iO 2 during irradiation are shown in Figure 12 E. The start vaporization curve was obtained by substituting DH for the total enthalpy from room temperature to the boiling temperature including the latent heat of melting. The blue and green crossbarsi nF igure 12 Ei ndicatet he presence of spherical particles and nearly correspond to the melt-phase formation range in the size-fluence diagram. The absence of 200 nm particles after irradiation at 200 mJ pulse À1 cm À2 can be attributed to the size-selective vaporizationo fp articles into NPs. The relationship between the size of fabricated particles and laser fluence can thus be explained semi-quantitively using size-fluence curves, thought his assumes an adiabaticp rocess

Chemistry-A European Journal
Review doi.org/10.1002/chem.202000686 and the dependence of 1 p and Q l abs ðdÞ on temperature is not considered. Heat dissipation can be largely ignored in oxide particles smaller than 100 nm in diameter due to their low optical absorption. Therefore, the conditions for sub-micrometre spherical oxide particle formation can be effectively estimated using the size-fluence curvesobtainedw ith Equation (2).
The discussion so far has considered only photo absorption by single particles and the melting of individual particles under laser irradiation. However,l aser irradiation in the LML processc learly increases the size of "educt" particles from nanometres to the sub-micrometre range.I ft he particles in the liquid medium are aggregated, some of the constituent particles are melteda nd merget of orm larger particles under laser irradiation. Thus,t he aggregation of raw particles in the dispersion medium has an important role in the formation of larger sub-micrometre spherical particles. [269,270] Photo-absorption-assisted LML Sub-micrometre spherical oxide particles can thus be fabricated through LML using materials that are capable of sufficient opticala bsorption andt hat do not sublimate but melt. An appropriate selection of laser wavelength and fluence is important for sub-micrometre spherical oxide particlef abrication. SEM images of typicalo xide particles fabricated via LML are shown in Figure 13 A, including ZnO, [271] Fe 3 O 4 , [272] and the complexo xide CuFe 2 O 4 . [273] As describeda bove,e nough opticala bsorption is required to fabricate sub-micrometres pherical particles. However,v arious insulating oxidesh ave band gaps larger than 5eV, and in these cases,s horter laser wavelengths are required to ensure opticala bsorption at the level required for melting. Unfortunately,o pticala bsorption in the liquid phase cannotb ei gnored at shorterw avelengths, often making impossible the selective heatingo ft he raw particles.C onversely,i fL ML is attempted on materials with large band gaps using a3 55 nm or 532 nm laser,t he fluence required to melt the particles would exceed several Jpulse À1 cm À2 .T he fluence would approach the typical ablation threshold, even if al aser with an anosecond pulse width is used. Thus, ad ifferent approachi sn eeded to enhancee ffective photoabsorption, consisting of LML assisted by close contact with photo absorber particles. SEM images that illustrate the fabrication of sub-micrometre spherical Al 2 O 3 particles with the assistance of photo-absorptive carbon NPs are shown in Figure 13 B. [274] Close contact between the photo absorption-assistingc arbon particles and the non-photo-absorptive raw particles is achieved by co-milling.
This contact is needed to indirectly heatt he raw material to obtains ub-micrometre spherical particles by LML. This technique can be applied to other materials with large band gaps, like ZrO 2 , [274,275] MgO, [274,275] and yttria-doped ZrO 2 . [276] By extending this idea, sub-micrometre spherical particles of complex oxide MgTi 2 O 5 can be reactively fabricated,m imicking conventional ceramic processing. [277] Photo-absorptive TiO 2 and less photo-absorptive MgO were mixed and milled together to ensure close contact,a nd the mixed powderw as irradiated with a3 55 nm laser.T he X-ray diffraction (XRD) pattern of MgTi 2 O 5 waso bserved in the pattern of the LML product (Figure 13 C).

Upscaling LML
Most LMLe xperimentsa re performed by irradiating ac olloidal suspension of raw particles in ab atch cell with ap ulsed laser while agitating the suspension with am agnetic stirrer or an ultrasonicator.I rradiation is typicallyp erformed for 5-30 min using a1 0-30Hzl aser with an unfocused beam, and sub-micrometre spherical particlesc an be obtainedi no ver 90 %y ield at production rates of severalmgh À1 .L ML can also be performed by irradiating free-fall liquid flow with ah igh-repetition laser with ap icosecond pulse width. [32] This idea was recently extended to slit-flow irradiation, and sub-micrometre spherical particles were obtained in over 90 %y ield after just one passage. [278] The number of irradiating pulses can be precisely controlled in this process, and the flow system has been applied to analyse the initial stage of particle formation. [272] Applications of Laser-Generated Oxide NPs Catalysis

Metal oxidesi nphotocatalysis
Metal oxides( MOs) are widely employed as catalystf or oxidation catalysis, [19,279,280] electrocatalytic, [224,[281][282][283] and photocatalytic pollutant degradation [284][285][286] in our living environment. As ap rominent example, av ariety of MOs (e.g.,T iO 2 ,W O 3 ,N iO) in the pure and mixed forms have been utilized in the heterogeneous photocatalytic depollution of air and water.L ight-induced generation of electron/hole pairs, migration of the carrier at nanostructure surface,a nd the reactive eventsa tt he interfaces,a re the key steps for ap hotocatalytic process.F or this reason,t he photocatalytic efficiency is generally dictated by the light-absorption efficiency,w hich determinest he number of photo-generated charge carriers and their separation, while electron-hole recombination is ac ompeting process, decreasing the carrierm obility and preventing carriers from reaching the surface.
In this scenario, laser-assisted synthesis of oxidesi nl iquids can be considered ah igh-quality strategy to generate clean oxide NPs, to stabilizet hese particles and to obtain heterogeneous catalysts thus improving the catalytic performances of colloidal samples prepared by classical chemical methods.
Generally,m etal oxide films and nanostructures prepared by wet-chemical processes requireamultistep approach involving high temperature or high-pressurec onditions to achieve the optimal catalytic behaviour.T hese treatments address several features, among them the removal of solvents andb inding agents, phase and microstructure changes, sintering and densification,t he introduction of defects ando xygen vacancies. Regardinga ll photocatalytic processes, defects and band gap engineering is crucial for tailoring the specific features of the semiconductor oxide with the radiation whiche nhances the catalytic activity and the adsorption/desorption of reactants. [229,250,284,285,287] In this respect, LAL has the potential to provide easy andg reen ways to achieveo ptimal results with minimal efforts. As an example, LAL-generated ZnO NPs exhibited interstitial sites and oxygen vacancies (Frenkel/Schottky and ionization reactions), [288] which were evidenced through green and blue photoluminescence effects.
The use of laser ablation was also exploited by Kohsakowski et al. [289] for the preparation of TiO 2 -CuO x andT iO 2 -FeO x composites with photocatalytic rates in the degradationo f2 ,4-dichlorophenoxyacetic acide nhanced by af actor of % 1.5 under solar irradiation. The authors generated "naked" (ligand-free) NPs of CuO x or FeO x by LAL of metal oxide targets in water,followed by colloidal deposition of CuO x and FeO x NPs onto anatase TiO 2 ,b ya djusting the pH to establish electrostatic attraction between the colloidsand the substrate.
Among the binary oxides( e.g.,T iO 2 ,Z nO, SnO 2 ), TiO 2 NPs are the most investigated photocatalysts due to their low cost, inert nature,a nd photostability. [285] The overall photocatalytic activity of TiO 2 is determined by its crystalline structure, surface area, density of surface hydroxyl groupsa nd adsorption/desorptionc haracteristics. Anyway,i ts main drawback is the fact that its efficiency under visible irradiation is low due to its large band gap, and it can absorb only the UV radiation, which is about the 5% of solar radiation, thus its use on large scale is limited. Moreover,t he fast recombination of photo-produced electron-hole pairs leads to low photo-efficiency.P rocessing such as doping and self-doping (reduction) through hydrogenation or addition of sacrificial agents can improve the catalytic performances, even thoughsuch classical approaches relying on the use of gaseous H 2 are dangerous and difficultt oc ontrol. For theser easons, TiO 2 has been treated by lasers directly in the colloidal form. [37] This produces defective blue titania, [37,229] which contains surface defect states as under-coordinated Ti 3 + sites (Figure 14 A) and oxygen vacancies which play ac rucial role in the electron injectiona nd recombination dynamics resulting in increased photocatalytic activity. [290] Chen et al. [229] have demonstrated that laser irradiated colloidal TiO 2 NPs provide enhanced photocatalytic activity towards the degradationofr hodamine B, am odel dye molecule for this kindo f studies. [291] The curvesr eported in Figure 14 Ar epresent the photocatalytic degradation of rhodamine-containing water solution under green irradiation (LED, centre wavelength 520 nm) for P25 (a mixture of rutile and anatase crystal phases, curve (a)), raw TiO 2 (curve b), TiO 2 laser-treatedf or 60 min (curve c), and TiO 2 laser-treatedf or 120 min (curve d). Similarly Filice et al. [287] have shown that the same treatment fort itania increases thep hotocatalytic water splitting performances of TiO 2 under UV action. As reported in Figure 14 B, the results pointed to an enhancement of up to af actor of three for hydrogen production under UV irradiation and twofold under visible irradiation, compared to the non-irradiated titania, althought he total amount of hydrogen production with visible light is too low for "real-world" applications.A lso, catalyst durability (recyclability) studies are neededt ov alidate itsa pplication potential. The same authors observed that the modifications induced by laser irradiation, that are responsible for the increased photocatalytic activity,d epend on the laser process parameters. At ag iven irradiation wavelength andf or nanosecond pulses,alaser fluence increasea cts linearly by increasing the amount of hydrogen produced per unit time. Such ab ehaviour hasb een correlatedt ot he parallel increaseo fu ndercoordinated Ti ions which have consequences in the modification of the electronic structure. [287] Moreover,f undamental roles of the liquid in which the catalyst is embedded have been recently evidenced by as eries of experimental findings, supported by ab initio molecular dynamicss imulations which involvew ater and ethanol. Ethanolm olecules strongly passivate surfaceo xygen vacancies while water only weakly interacts with this surface. Then ac orrect balanceb etween the two liquids should be properly considered either for the laser modification of the materials and for the catalytic experiments. [292] LFL has also been used to produce materials suitable for plasmon-enhanced photocatalysis, which is typically investigated by using plasmonic noble metal NPs loaded onto semiconductor oxides. [37,[293][294][295] Besides, Lin et al. were able to form 2D nanosheets by LFL of NiO NPs in amethanol/water solution. [253] LFL of NiO in pure water or an aqueous solutiono fe thanol always originated amorphous 2D sheets with plasmonic behaviour and superior photocatalytic activity and stability compared to the not irradiatedm aterial, boosting quantume fficiency of H 2 evolution by solar irradiation (Figure14C). [253] Indeed, the reducing environment created by ethanol promotes the formation of Ni 0 -like defects with plasmonic properties by hydrogen n-type doping of the nanosheets, thereby raising the Fermi level above the conduction band edge, thus leadingt ot he appearance of surfacep lasmonsa tv isible light frequencies. [253] On the contrary,L FL in pure water led to an arrowing of band gap due to additional defect formationb ut was not able to overcome the intrinsic p-doping of NiO. [253] It is therefore evident how the choice of the liquid during LFL is of utmost importance, although it was less investigated than for LAL. [19] Supported redox heterogeneous catalysts Laser processes in liquids are particularly efficient to load transition metal NPs or clusters onto oxide NPs. [19] Either the supporting species( oxide) and/ort he metallic particle can be separately generated by LAL and then mixed under to obtain the final heterogeneousc atalyst. Metal oxide NPs, as well as other forms of oxidess uch as graphene oxide, are by far the most prominents upports for (co-)catalytic species( mainly metal NPs or other oxides/semiconductors). Supports are known to not only improvet ransferability and manageability of the nanomaterials concerningt he practical catalytic process itself but in the ideal case also to improvethe overall catalyst performance, for example, due to nanoparticle-support interactions or an individual catalytic activity of the support whichi sf urther mediated by the NPsa cting as co-catalyst. [19,296,297] To yield high mass-based activities it is however generally mandatory to maintain ah igh dispersion and stabilization of the active species [297,298] as well as electrolyte accessibility and better conductivity in case of electrocatalysis. [299] Especially the reducible oxides like TiO 2 ,CeO 2 ,V 2 O 5 ,N b 2 O 5 are beneficialt oa lso strongly interactw ith metal species, leading to Strong Metal Support Interaction (SMSI), resultingi ns pecial properties and altered or even improved catalytic performance. [300] Indeed, the control of the interaction degree between the support and the active metal depends on pre-andp ost-treatments toa chieve the desired loading, size, electronic andb ulk structure of the metal. Further,t he search for suitable combinations of co-catalyst and support is ak ey issue for many thermo-and electro-catalytic processes. [301] Oxygen reduction reactions (ORR), oxygen evolution reactions (OER) and water splitting processes, in general, require efficient and low-costc atalysts as ap art of the overall performance of fuel cells and batteries from the commercial point of view. [301] Laser-induced transformationo fp articles openst he way to increase the abundance of sites with tuned surface energy through the control of metastable and defective NPs. So far,p ure iron oxides, as well as Co 3 O 4 NPs produced by LAL, have been tested successfully when supported ontoi ndium tin oxide (ITO) electrodes [302] and unsupported. [303] Besides, Hu et al. [191] measured ac ombined ORR and OER overpotential of 756 mV versus RHE for PtCo/CoO x catalystsp repared by laser ablation in tandemw ith galvanic replacement reaction (Figure 15 A), whichi so ne of the highest values reported using carbon black as the supporting material. Very good catalytic performances in ORR were also demonstrated with perovskite LaMnO 3 + d [304] and Co 3 O 4 [305] NPs synthesized by LAL. In this context,o xygen vacancies introduced in CoOOH by LAL and LFL can strongly improveo xygen evolution reaction intermediates, showing al ow overpotential of 330 mV at ac urrent density of 10 mA cm À2 ,asmallT afel slope of 63.2 mV dec À1 ,a nd high catalytic durability,w hich all exceed those of commercial RuO 2 . [254,306] Similar defect formation scenarios and improved catalytic activity have also been observed apart from size reduction during LFL using ps-532 nm-laser pulses of CoO and Co 3 O 4 [234] as well as CoFe 2 O 4 . [224] Although the detailed defectf ormationm echanisms are not yet fully understood, these examples represent ap romisingb asis for popularizingt he use of laser technology in the designo fd efective oxide nanomaterials, for example, for application as efficient catalysts, [37,224,234,254,306,307] or plasmonic materials. [253] More complex oxidesand hydroxide nanostructures catalysts have been tested so far for similar applications.F or instance, layered double hydroxides have shown to be active for water oxidation. [282,308] These compounds are based on mineral structures and can be easily found in nature. [282] It is possible to obtain [NiFe]-layeredd ouble hydroxides (LDH) with variousi ntercalated anions such as Ti 4 + or La 3 + by using LAL, featuring very low overpotentials in the electrocatalytic water oxidation, as demonstrated by Hunter et al. [309,310] Another example is the synthesis of an oble-metalf ree photocatalyst, namely CdS-NiFe layered double hydroxide nanocomposite by using LAL. [311] This nanocomposite exhibited am orphology of 2D-NiFe LDH nanosheets on 1D-CdS nanorods, providing an interfacial contact of heterostructures which allowed the efficient carrier transporta nd migration and as ignificant reactivity in the hydrogen evolution reaction. Such 2D oxide nanosheets are frequently used in catalysis. Ar ecent example has been given by Thiagoe tal. [312] in which graphene oxide (GO) and itsr educed form act as ap latform for covalently bonded sulfonic groups which enhancea cid-catalysed trans-esterification reactions. GO also has excellent properties such as ah igh surface to mass ratio (1000 m 2 g À1 )a nd good solubility in water,t hen the chemicaln ature of functional groups and their concentration onto the graphene plane can be directly tuned in solution by laser irradiation. [313] Among other 2D materials used in catalysis, it is noteworthy to mention bismuth sub-carbonate (Bi 2 O 2 CO 3 ). It is am ember of Aurivilliusr elatedo xide family and has the interesting property to form 2D layers composed of orthogo- Figure 15. (A)PtCo/CoO x catalysts prepared by laser ablation in tandem with galvanic replacement reaction for ORR and OER (reprintedw ith permission from ref. [191], Copyright 2016, Elsevier). (B) HER performances of Rh 2 O 3 -coated metalRhNPs in an acidic environment(E/Rh/C and W/Rh/C stand for NPs obtained, respectively in ethanolorw ater and supported on carbon electrodes;reprintedwith permission from ref. [128], Copyright 2019,R oyal Societyo fChemistry). (C) CO conversion ratio as af unctiono ftemperature under the catalysiso fA u/TiO 2 heterostructures, where the curves a, b, and cc orrespond to one, two, and three cycles of catalysis, respectively. Inset shows aT EM image of Au NPs decorated iron oxide nanosphere and the sketch of the heterostructures obtained by this R-LAL route (reprinted with permission from ref. [192], nally interleaved Bi 2 O 2 (2 +) layers with CO 3 (2À) groups, held together by Vand er Waals interactions alongt he c-axis. In recent work, D'Angeloe tal. [314] have demonstrated the formation of Bi 2 O 2 CO 3 nano-sheets by UV laser irradiation of beta-Bi 2 O 3 in water,w hilei ne thanolt he process is accompaniedb yap artial reduction of the bismuth oxide, even containing zero-valent Bi. Again, this highlights the importance of understanding the role of redox chemistry happening duringlaser-based materials synthesis and processing. For example, the reactive oxygen speciesg enerated during LAL in water allows the achievement of oxidized noble metals like Rh 2 O 3 coated metal Rh NPs, which exhibited high performances in thee lectrocatalytic hydrogen evolution( HER, Figure 15 B). [128] Another interesting approach is the synthesis of catalytically active Au NPs/silica using ao ne-step femtosecond-reactive laser ablation in liquid (fs-RLAL) technique. [315] In this case, the use of fs laser pulses onto as ilicon wafer immersed in an aqueousK AuCl 4 solution allowed to obtain Au NPs with significantly smaller sizes than in previously reported RLAL studies, which were found to be highly active in the catalytic reduction of para-nitrophenol. LAL synthesized NPs of various elements (Si, Ge, TiO x ,S nO x , and MnO x )w ereu sed also as doping precursors for hydrothermal synthesis of hematite with improved photoelectrochemical performances. [316,317] In this frame,w el ike to recall the pioneering work by Lin et al., [192] one of the first attempts to use reactive laser synthesis in the liquid phase to obtain nanomaterials for catalytic application.I nj ust one step, laser irradiationi nl iquid solutions can generate hybrid catalyst/support NPs in am etal-core/ oxide-shell fashion like that reported in Figure 15 C. Such an "in situ" reactive loading permits the formation of Ag/TiO 2 ,A u/ FeO x, and Pt/FeO x particlesw hichh aveb een successfully tested for carbon monoxide oxidation reactions (Figure 15 C). A crucial parameter,a part from the particle size, is the mass-load of NPso nt heir support. For example, in case of the selective oxidization of ethanol using Au/TiO 2 catalysts, an optimum of the absolute yield of acetic acid with the AuNP load has been found by Dong et al. which was hypothesized to be correlated with the coverage of all surfaced efects on TiO 2 (e.g.,o xygen vacancies) with laser-generated AuNP. [318] As imilarc onclusion was drawn by Jovic et al. in case of Au/TiO 2 used for photocatalytic H 2 evolution who correlatedt he optimum of catalytic activity and AuNP loading with the coverage of all "hot spots" with NPs. [319] In other examples, Pt loaded SnO x has been proposedf or the hydrogenation of 4-nitrophenol (4-NP). [190] LAL of am etallic Sn target in aP tCl 4 À solution generatesahighly reactivee nvironmentw ith long-living radicals, even after the ablation, which favourst he formationo fP tc lusters and the reactive loading of the catalyst onto the oxide support (Figure 15 D).
UV laser irradiation has been also used to build PdO/Pd/ CNTsh eterojunctions, [322] with ac ontrollable reduction degree and steerableP dO-Pdi nterface. The nanostructures have been tested for the electrocatalytic N 2 reduction reaction, where Pd absorbsN 2 to form chemisorbed Pd-N 2 and PdO transmits protons to form a-PdH, thus breaking NNt riple bonds. The net result of the PdO/Pdi nterface enhanced the catalytic per-formance, coupling also low cost and an eco-friendly wayf or ammonia synthesis under ambient conditions. Moussa et al. proposed af amilyo fh ighly activeP dn anoparticle catalysts supported on partially reduced graphene oxide nanosheets for carbon-carbon cross-coupling reactions. [320] There, pulsed laser irradiation of aqueous solutionso fg raphene oxide and palladium ions provides an excellent environment for anchoring the Pd NPs, thus hindering the migration of the particles and increasingt he catalyst-support interaction. With these catalysts, Suzuki, Heck, andS onogashira cross-coupling reactions have been tested with excellent results ( Figure 15 E), with at urn over number (TON) of 7,800 and ar emarkable turnover frequency( TOF)o f2 30 000 h À1 at 120 8Cu nder microwaveh eating and good recyclability forSuzukicoupling. [320] The electrostatici nteraction of the catalysts with the 2D nano-beds can be improved by external physicals timuli such as ultrasounds as reported by Liu et al. [321] in the case of rhodium NPs (average size of 1.8 AE 0.4 nm) decorated on graphene oxide nanosheets.T he catalytic performance of Rh/GO on the reduction of 4-nitrophenol is reported in Figure15F.R ate constants (k)f or laser-generated Rh/GO composites are 27 times higher than commercialR h/C catalyst.C atalytic GO composites have been fabricated and tested also with Co 3 O 4 NPs, [323] TiO 2 [324] and other NPs. Laser reduced graphene oxide has been also tested for direct dye removal from water solution and antibacterial activity. [325][326][327] For instance, Russo et al. [325] demonstrated that laser-treated GO shows an excellent methylene blue (MB) adsorption capacity in water.C ontrolling the laser irradiation time between 10 and 250 minutes permitted to obtain GO sheets with different amountsofoxygen functionalities and, therefore, different degrees of reduction,a ffecting the hydrophilicity and the removal of MB. Further,c ompared to commercial catalysts, Haxhiaj and co-workerso bserved as ignificantly highert olerance of Pt@rGO (reduced GO) catalysts towardsC Op oisoning when the Pt NP were supported in situ during laser ablation of Pt in graphene nanosheet dispersion. The observation was discussed in terms of an enhanced interaction betweenr GO and Pt NPs due to ar eaction of nascent and defect-rich Pt NP with rGO surfaced irectly after ablation. [328] In conclusion, the synthesis of composites by laser-assisted approaches in liquidsa ppears ap romising strategy to obtain active and stable speciesf or as eries of catalytic and photocatalytic applications,w here NPs with defective structure and special surfacec haracteristics are required to provide highly active and selective catalytic sites. The tuning of these properties, by controlling the parameters of the employed laser techniques, will be the main aim of the future research in this field, to expanda nd to optimize the performance of the synthesized systems.

Bio-applications
Oxide NPsh ave appealing properties for bio-applications, thankst ot he physicala nd chemical properties arising only at the nanoscale. The most common oxide nanostructure forb iomedicalp urposes are iron oxides, and ultrapure non-toxic laser-generated NPs have been used form agnetophoretic sorting of cells and fluorescent cell labelling (Figure 16 A), [118] or for doping of hydroxyapatite coatings of interest for biomedical devices. [329] Phosphonate-coated poly-oxo-clusters showed properties as T 1 contrasta gents for magnetic resonance imaging (MRI). [160] Gd [147] and Dy [330] oxideshave been also extensively investigated as T 1 or T 2 MRI contrasta gentsw ith promising performances (Figure 16 B). Doping of laser-generated Gd 2 O 3 oxide NPs with Eu 3 + conferred photoluminescence properties in addition to theM RI contrast ability. [331] Laser-synthesized Si nanoparticles with variable level of surface oxidation showed appealing properties for biomedical applicationst hanks to their high purity. [332] Graphene oxide quantum dots exhibitedphotoluminescence performances useful for bio-imaging (Figure 16 C), [148,333] as demonstrated with long-term trackingofcells in in vivo experiments. [333] Besides,m odification of graphene oxide by laser irradiationh as been shown to enhancei ts antibacterial activity. [327] NiO nanostructures have been also studied for antibacterial purposes. [334] Of remarkablei nterest, ar ecent study showedt he suitability of laser synthesis for implementation in ac irculare conomy,b y using waste battery cell powders to generate MnO 2 NPs with excellent antibacterial properties. [335] Opticsa nd photonics In addition to photocatalysis and biophotonics, the optical properties of oxide nanostructures accessedb yt he laser-assisted synthesis in liquid have stimulated the investigation and the application in different fields relatedt oo ptics and photonics. For instance, photoluminescence properties of core-shell Si-SiO 2 NPsp roduced by pulsed laser ablation in aqueous solution were studied in detail, evidencing the presence of non-radiatived efects located in the suboxide interlayer between Si and SiO 2 . [336] Core-shell NPs made of LiNbO 3 coated with a carbon shell exhibitedb lue-green luminescence similart o carbon dots. [211] Luminescence denoting quantum sizer ange properties was reported in ultrafine ZnO nanocrystalso btained by LFL. [337,338] Strong second harmonic generation signals were measured in ZnO NPso btained by the simple LAL procedure. [339] Intense multi-peak photoemission in the blue-range was observed in Zn(OH) x -DS layered composites obtained by LAL of Zn in aqueous SDS solutions. This behaviour is originated by the inorganic-organic lamellar structure, as demonstrated also by scanning near-field optical microscopy (SNOM) on a single Zn(OH) x -DS nanosheet. [340] Rare-earth doped oxidesh ave been synthesized for their emissionp roperties, [341,342] including co-doped Gd 2 O 3 phos- phors for upconversion, namely Gd 2 O 3 :Er,Yb [343] and Gd 2 O 3 :Yb,Tm. [344] Further,l aser fragmentation has efficiently been employed to synthesize YVO 4 :Eu 3 + which is another typical oxide relevant for upconversion. [345] Nanocomposites of iron oxide and silver,o btained by S-LAL, showeds imultaneous properties of magnetism for magnetic attraction and plasmon resonances for spectroscopic detection of analytes by surface-enhanced Raman scattering. [346] Yttrium iron garnet NPs which are relevantf or opto-magnetic applications (e.g.,m agnetic circulard ichroism) were gained with unexpectedly high magnetic coercivity behaviour by LAL. [101] IrO 2 nanocrystals,o btained by LAL in ethanol and subsequently supported on graphene oxide, exhibited peroxidase-like behaviour useful for the colorimetric determination of ascorbic acid. [347] Size-specific optical applications for oxide particles fabricated by LML have been examined, including random laser using ZnO (Figure 16 D) [348,349] and back reflectors of solar cells using TiO 2 (Figure 16 E), [261] thanks to their appreciable sub-micrometre size range and monodispersity.T hese applications require the strong scatteringp roperties of dense sub-micrometre particles having high refractive indices. Al 2 O 3 NPs, generated by LAL in ethanol, were recently tested as ap assivating and antireflection coating for silicon photodiodes. [350] Generation of TiO 2 NPs by LAL wasa lso exploited to promote their functionalization with water-soluble poly[3-(potassium-6-hexanoate)thiophene-2,5-diyl] (P3P6T), to easily achieve aqueous processed hybrid solar cells (Figure 16 E). [29] LASiS of MoO 3 NPs in water without undesired chemicals allowed easy spray coating deposition for the realizationo fs olar cells. [140] Piezo-magneto-plasmonicp roperties were reported for lasergenerated multi-component PbZrTiO 3 /Au/Co nanostructures. [351] While these are only as everale xamples of the application of LAL in optics andp hotonicst he high potential of LAL in this field is evident.

Other applications
Oxide nanostructures are ubiquitous in science and technology,a nd as eries of specific applicationsh ave been proposed in the literature for laser-generated oxide NPs, taking advantage of their purity,l ow production costs and biocompatible synthetic procedure.I nt he following, interesting examples are mentioned that go beyond catalysis, bio-applications, and photonics.
The rising of additivem anufacturing has benefited from depositingl aser-generated Y 2 O 3 NPs on iron-chromium powders to obtain oxide dispersed strengthened alloy parts by laser powderb ed fusion (often called selectivel aser meltingS LM) (Figure 16 F). [361] Summary and Outlook Awide variety of oxide nanostructures exist, each one with important properties for scientific and technological applications. Therefore, it is of utmost importance to realize the synthesis of oxide NPs by environmentally friendly,e nergy-saving, simple and effective routes.Inthis review,weshowed that laser-assisted synthesis in liquid environment is av ersatile approachr unning at room temperature and pressure, with highly encouraging resultsi nt erms of purity of products and absence of undesired chemicalc ompounds or pollutant wastes. Laser-assisted synthesis allows productivities up to the g/h-scale of NPs, while the synthesis of low-priced oxide materials and all those cases demanding > kg/h-scales today are sometimes better achieved by other routes such as gas-phase synthesis, at least if aggregation (that is inherent to gas phase synthesis) is not hindering application.G ram scale productivities are more difficult to reachc ompared to (noble)m etal ablation as mass productivitya lso scales with the density of the material. [43] Also, with only few exceptions,o xidesg enerally show weakera bsorbance in the infrared range, so shorter wavelengths and/or pulse durations are required for efficient LAL, LFL, LML, and by that more expensivel aser systems are required for up-scaled laser synthesis of oxides. Conversely,l aser approaches were shown to be economically preferable over conventional synthesis protocols for high value-added nanomaterials and are worthy of consideration for advanced fundamental studies or for benefits that go beyond the economic balance. For instance,t he integration of laser-assisted synthesis of oxidesi na circulare conomy has been also envisaged in the literature. [234,335] Yet, with the laser-based productivity of NPs having been scaled of more than 3-orders of magnitude (from less than mg h À1 to gh À1 )w ithin the last decade, af urther increase is foreseeable. In the future, this increase may be achieved by smarterb eam handlings trategies such as parallel processing, automation measures for target feeding (as target feeding on the multi g/h-scale still presents ab ottleneck)a nd control systems, [34,39,40] as well as commercial high-power,n sa nd ps pulsedl aser systems steadily are getting cheaper year by year. Note that this upper-end productivity limit values are not hinderinga pplication or Te chnology Readiness Level of the method in general. For example, bio-application mass demand in diagnosis or therapy( e.g.,w ithl aser-generated bioconju-gates or SiO 2 )i sm et already with today's throughput level. But not only the upper-end productivity value is a( commercialization) limitation of laser synthesis in liquid,c ompared with the conventional routes for the synthesiso fo xides. On the one hand, laser synthesis has the advantage to provide aggregation-free colloidal nanoparticles, different from gas-phase synthesis or hydrothermals ynthesis. On the other hand, the produced colloid often has ac omparable low concentration (some 100 mg l À1 instead of several gl À1 )t oa void beam attenuation effects. This naturally lower concentrationl imit of LAL and LFL (that may be higherf or LML and LDL) requires more post-processing efforts as the liquid has to be removed for the final products.A nother limitation is the understanding to synthesize multi-element oxide materials with better control in stoichiometry,l ike in hydrothermals ynthesis of dopedo r multi-element oxides. Laser synthesis community is quite active in the fundamental research level as outlined in the related chapters above, but in the literature the yield of as pecific doped/multi-element oxide (referred to by-products with different stoichiometry) is often not described, same for the particle size dependency of the oxide nanoparticle composition.
Here size quenching methods that do not rely on organic ligands (that would compromiseo ne of the keya dvantages of laser synthesis) are required that reachahigh yield of for doped and multi-element oxides with monodisperse size.
Only two experimental set-ups( laser ablation of solids and laser irradiation of colloids) allowed the access to al ibrary of oxide NPs (spheres, rods, flowers) in one step, which also includes metal-oxide core-shells or heterostructures. As reviewed in this paper,t he largep art of oxide nanostructures can be obtained all by LAL with near-infrared( e.g.,1 064 nm) ns laser pulses in pure liquids, that makes very easy to switch from one type of material( also non-oxide) to another in a "plug-and-play" procedure. On the other hand, the choice of the main parameters, such as the synthetic approach( LAL, LFL, LML, LDL), the composition( oxide,m etal, alloys), and type (plate, powder,c olloid) of the starting material, the liquid type, the solutes, the laser parameters (wavelength,p ulse duration, fluence) can be used to control the size, composition, phase and sometimes morphologyo ft he products. Aiming at finetuning oxide properties and defectd oping, the emerging field of laser defect engineering in liquid (LDL) is expected to provide new insights on structure-activity correlations in catalysis research.
Although the LAL, LFL, LML, and LDL procedures ande ffectivenessh aveb een sensibly developed in the past few years, the fundamental processes are still under debate, which is leadingt oaclose collaboration of experimentalists andt heoreticianst or each the same level of understanding and control as already achieved by the chemical nanoparticle synthesis routes. The theoretical and experimental understanding of the whole synthetic process is still undergoing, especially for what concerns the challenging and elusive steps of high-energy laser pulses-matter interaction, plasma/liquid interactions, and chemicalr eactiono fa blated matter with solution species, as well as the gradient of thermodynamic parameters from the centre of the laser spot to its boundaries.
Going deeper into our understanding is crucial to improve the control over the particle size distribution obtained by LAL, that is more difficult than with chemical routes unlessw hen they are post-processed with LFL or LML. For example, the LAL synthesis of quinary high extropya lloy at the gram scale has recently been shown with ad ominant small particle size fraction, but still, large particles appear that compromise the mass yield andc atalytic performance. [362] Thei dentification of synthesis conditions that avoid size polydispersity,n on-crystalline products and coexistence of different phases is important also to unify the multiple laser synthesis parameters among the variety of solvents, target materials, laser pulse wavelength, duration, fluence andr epetition rate. To date, this variety was usefult os how all the potentialitieso fl aser-assisted synthesis but also delayed the worldwide diffusion of the best production protocols. An effort in that direction has recently been made by publishing recipes of best practice in laser synthesis. [21] The solution to these problemsw ill likely lead laser-generated oxide nanostructures to real products in the market, given the overall series of advantages inherently associated with these synthetic approaches.