Metal Oxide Nanosheets as 2D Building Blocks for the Design of Novel Materials

Abstract Research into 2‐dimensional materials has soared during the last couple of years. Next to van der Waals type 2D materials such as graphene and h‐BN, less well‐known oxidic 2D equivalents also exist. Most 2D oxide nanosheets are derived from layered metal oxide phases, although few 2D oxide phases can be also made by bottom‐up solution syntheses. Owing to the strong electrostatic interactions within layered metal oxide crystals, a chemical process is usually needed to delaminate them into their 2D constituents. This Review article provides an overview of the synthesis of oxide nanosheets, and methods to assemble them into nanocomposites, mono‐ or multilayer films. In particular, the use of Langmuir–Blodgett methods to form monolayer films over large surface areas, and the emerging use of ink jet printing to form patterned functional films is emphasized. The utilization of nanosheets in various areas of technology, for example, electronics, energy storage and tribology, is illustrated, with special focus on their use as seed layers for epitaxial growth of thin films, and as electrochemically active electrodes for supercapacitors and Li ion batteries.


Introduction
2D materials,o rn anosheets, are ac lass of nanomaterials that draws more and more attention since graphene was discovered. [1] These 2D materials exhibit as heet-like structure, hence the name nanosheets, with laterald imensionso ft ens to hundreds of nanometers to even micrometers,a nd thicknesses no more than 5nm. [2] Owing to the effect of spatial confinement in one dimension, nanosheets exhibit av ariety of electronic, chemicala nd opticalp roperties that are not presenti nt heir layered bulk counterparts. [3] Ac ouple of advantages arises from the two-dimensional nature of nanosheets. Firstly,t he electrons are confinedi na thin (nanoscale)r egion. [3] Their electrons are thus confined to a 2-dimensional lattice plane,w hich providesa ni deal model system for fundamental studies in condensed matter physics, but also for development of small (opto-)electronic devices. Since nanosheets have strong in-plane bonds and the sheet is atomicallyt hin, they tend to show ac ombination of high mechanical strength, flexibility and optical transparency,w hich are all highly desirable properties for utilization in various types of devices. [4] Their atomict hicknesses results in very specific surfacea reas, [5] which is av ery important property for applications in which the surface area is relevant,s uch as catalysts and supercapacitors. [6] Furthermore, the aqueouss olutionbased dispersions of nanosheets are suitablep recursors for the fabrication of nanosheet-based films using simple methods like spin-coating and ink jet printing, usable in applications such as solar cells. [7] And finally,t he fact that all atoms are surface atoms providesahandle to regulate the properties and functionalities of nanosheets by meanso fs urfacem odification and functionalization, for example with graphene oxide, substitutional elementdoping, or strain and phase engineering. [8] Over the last 10 years, we have been synthesizing and using ar ange of 2D transition metal oxide nanosheets and have been developing methods for their assembly into monolayer films, multilayer heterostructures and hybrid nanocomposites. [9] This review providesaconcise overview of the synthesis and processing of 2D metal oxide nanosheets into functional films, and gives some examples of their application in technological realmsa sd iversea sn anoelectronics, [10] energy storage [11] and tribology, [12] with emphasis on butn ot limited to research done in our research group. First, the synthesis of oxide nanosheetsb yt op-down andb ottom-up strategies is summarized. Then, we discussm ethods for making morphologically distinct types of nanosheet films by methods such as Langmuir-Blodgett deposition and ink jet printing. In the final sections some examples of functional device components based on or constructed from oxide nanosheets illustratet heir wide range of application.I np articular,t he use of monolayer nanosheets films as seed layers to guidee pitaxial growth of functional thin films, for example, ferroelectric, ferromagnetic and memristive oxides, and the application of oxide nanosheets as electrochemically active electrodes for (flexible) supercapacitors and Li ion batteries are emphasized.

Synthesis of 2-Dimensional Oxide Nanosheets
Top-down synthesis:exfoliation strategies Various layered materials have been exfoliated into 2D nanosheets. In thesel ayers, the in-plane atoms are connected to each other via strongc hemical bonds, whereas the individual layers stick together by relatively weaker van der Waals interactions. [2,13] The best-known example of al ayered material is graphite, which consists of as tack of 2D graphene layers that are held together by such weak secondary forces. Other examples of layered materials include hexagonal boron nitride (h-BN), transition metal dichalcogenides like MoS 2 ,M Xenes, layered metal oxidesa nd perovskites. [2] Typically,i nt hese cases the layers are held togetherb ys tronger Coulombic and polar interactions.
Graphene is the mostw ell-known 2D material (see Figure 1a). It is as ingle atom thick form of graphite. The carbon atoms are arranged in a2 Dh oneycombs tructure (see Figure 1a), and all of them are covalently bound to three neighbouring carbon atoms through a s-bond. [15] The distance between neighbouringc arbon atoms in ag raphene nanosheet is 1.42 .I nn on-exfoliated graphene, that is, graphite, the individual graphene layersa re held together by van der Waals forces, where the distance between the layers is approximately 3.35 . [16] Metal trioxides with the general formula MO 3 (M = Mo, Ta, etc.) are also known to have al ayered structure. [2] For example, MoO 3 can be thought of as being constructed from distorted MoO 6 octahedra that sharetheir edges with their neighbouring octahedra, thus forming 2D layers. [17] The bulk MoO 3 crystal is essentially as tack of such 2D layerst hat are held together along the z-axis via van der Waals forces. Other layered oxides that are discussed in more detail below include the layered lepidocrocite-type compound K 0. 8  MXenesa re 2-dimensional sheetso ft ransition metal carbides and/orn itrides that can be derived from so-calledM AX phases with the general formula M n + 1 AX n (n = 1, 2o r3 ). Here Mi satransition metal (e.g.,T i, V, Nb), Ai sa ne lement from group IIIA or IVA( Si, Al, Sn, In), andXis carbon and/or nitrogen. [18] MAX phases have al ayered structure in which the M layers are hexagonallyc lose-packed and the Xa toms fill the octahedral sites. [2,18] The element Af orms 2D planes that separate the M n + 1 X n layers. The Al ayer can be etched away selectively from the MAX phase using strong acid solutions, for example HF solution.M Xenes sheetsw ith three different structures can thus be formed:M 2 X, M 3 X 2 ,o rM 4 X 3 .S ee Figure1c for the structure of the MXenetitanium carbide.
The layered nature of bulk layeredc ompounds makes them potentially suitable to be exfoliated into their 2D constituents by some top-down exfoliation process, such as mechanical or ion intercalation exfoliation or one of the methods mentioned below.
The synthesis of 2D nanomaterials with targeted composition, sheet size, layer thickness, crystal phase, defect concentration, ands urface chemistry is important for further exploration of their physical and chemical properties, as well as for the development of new applications made using nanosheets as building blocks. [2] Severals ynthetic strategies have been developed to make aw ide range of nanosheets. These methods include mechanical cleavage, mechanicalf orce-assisted exfoliation, ion intercalation-assisted exfoliation, oxidation-assisted exfoliation, selective etching-assisted exfoliation, chemical vapourd eposition (CVD), and wet-chemical syntheses. [2] All methods all fall into one of two categories, namely top-down and bottom-up methodologies. CVD and the wet-chemical syntheses routes for2 DM nO 2 described in section2.2 are exampleso fb ottom-up routes. They are typically based on  (d) the proposed intercalation and exfoliation mechanism for layered metal oxides. Reproduced with permission. [14] Copyright 2015, Wiley-VCH. chemicalr eactions of specific molecular precursors leading to larger 2D structures. The top-downr outes are limited to the availability of layered parent compounds, but they have the advantage that nanosheets are derived frompreformed crystalline parent materials, hence the resulting sheets are also highly crystalline themselves.
The scotch-tape methodt oo btain graphene nanosheets from graphite is an example of mechanical exfoliation, but mechanical exfoliation hasl ow yields. [14] Ultrasonication-assisted exfoliation can damage the morphology of nanosheets. [19] On the other hand, ion intercalation exfoliation is normally driven by ac hemical reactiona nd is generally considered to present am uch milder synthesis route. More than 40 two-dimensional compounds made by exfoliating their parent bulk layered compounds have been reported till date. This route can yield large quantities of dispersed nanosheets and is potentially the most promising for large-scale production processes among the above-mentioned exfoliation methods. [13] Considering the exfoliation of layered transition metal oxidesu sing organic quarternarya mmonium hydroxidesa se xfoliation agents( e.g. tetrabutyla mmonium hydroxide, TBAOH), it was generally thought until recently that the structural evolution of the layered parent oxide into the final exfoliated state proceeds via an intermediate "swollen"s tate in which steric crowding of bulky tetrabutyl ammonium (TBA + )i ons present between the negativelyc harged oxide layers initiates the separation of the layers. Such an exfoliation mechanism should involve ar elatively slow diffusion-controlled ion exchange process between bulky TBA + prior to exfoliation. [10a] However,i th as been found that the ion intercalation process is actuallyd riven by av ery fast acid-base reactiont hat can happen even within seconds once the reactants are mixed. [14] The exfoliation of one of the most well-known layered metal oxides, lepidocrocite-type titanateH 1 [14]). The potassium ions reside between the Ti 1.73 Li 0.27 O 4 layers, to charge-stabilize the layered bulk compound.S ince these titanatel ayerso ught to be separated, the interlayer forces should be as weak as possible. The interlayer force can be reduced by exchanging the potassium ions by protons via an acide xchanger eaction. The bulk compound KLTO is therefore treated in nitric acid solution for three days. All potassium and lithium ions are leached out in this process and the HTO phase is formed. Now,p rotons reside between the layers, and with the inclusion of the cations' solvatation shells,t he distance between the Ti 1.73 O 4 layers increases, thereby weakening the Coulombic interactions between the titanate layers and the interlayer cations,thus making it easier to exfoliate the layered compound into nanosheets. The exfoliation process itself is triggered by mixing the protonated layered HTO material with TBAOH. Due to their Brønsted acidity,t he hydroxyl groups attack the interlayer protons and trigger an acid-base reaction, forming H 2 O. As the protons are removed from the layers, the negatively charged titanaten anosheets become stabilized by the positively charged TBA + ions. Since TBA + ions are amphiphilic molecules that want to be at the air-wateri nterface, they are able to transfer nanosheets to the air-wateri nterface, where they can be utilized using deposition techniques such as the Langmuir-Blodgett method. The same colloidal suspensions can be used in other deposition techniques as well, like ink-jet printing. Typically,s uch suspensions contain about 1-5 go fn anosheets per litre water.T he TBAOH/HTO molar ratio influences the concentration of nanosheetsi nt he suspension. Ar atio of at least 1:8i sn ecessary to form an oticeable concentration of nanosheets in the suspension. At higher ratios, up to 2:1, exfoliationh appens. At even higherr atios, restacking tends to become dominant. Highly crystalline nanosheets of varying composition, with lateral sizes ranging from hundreds of nanometers to tens of micrometersh ave been realized with this method.I th as been shown that by controlling the grain size of the layered parent compound, the laterals ize of nanosheets can be tunedw ithin certainl imits. [20] Bottom-up synthesis:self-assembly of 2D MnO 2 In contrast to exfoliation strategies that need high temperatures to synthesize solid state parentc ompounds, soft-chemical bottom-up approaches to prepare MnO 2 nanosheets under very mild conditions involves chemical oxidation of aqueous Mn 2 + cations or reduction of Mn 7 + cations and oxidation of template materials such as graphene oxide.U nilamellar MnO 2 nanosheets can be prepared by oxidizingM nCl 2 ·4 H 2 Ow ith H 2 O 2 in the presenceo fT BAOH [21] or tetramethylammonium hydroxide (TMAOH). [22] Both TBA + and tetramethyl ammonium (TMA + )a ct as charge-compensating cationic species and soft Yang Wang is aP hD candidate under Dr.P rofessor J. E. ten Elshof's supervision in the MESA + Institute for Nanotechnology at the University of Twente, the Netherlands.H ei s currentlyf ocusing on 2D materials and printing 2D materials for flexible energys torage devices.
Dr.Johan E. ten Elshofisp rofessor of Inorganic and Hybrid Nanomaterials Chemistrya tt he MESA + Institute for Nanotechnologyo ft he University of Twente in Enschede,N etherlands. His research focuses on novel functional metal oxide and organic-inorganic nanomaterials, nanopatterns and nanostructures, with specifice mphasis on low-dimensional structures like flexible nanofibers, nanosheets and nanowires. Main application areas of these materials are in the areas of energy materials and nanoelectronics.
templates for the formation of MnO 2 nanosheets. The AFM (Figure 2a)a nd TEM (Figure2b) images shows the ultrathin structure of MnO 2 nanosheets with at hickness of around 1nm. [21] This value is higher than the crystallographic thickness of monolayer MnO 2 nanosheets of 0.52 nm, because of the hydration and the presence of organic ions, that is, TBA + or TMA + on the surfaceo fM nO 2 nanosheets. [21,22] MnO 2 nanosheets made by this methodh ave laterals izes of 50-150 nm, and an average lateral size of 89 nm which makes them usable for inkjet printing.
An alternative chemical reduction process to synthesize sizetuneable MnO 2 nanosheets has also been demonstrated. [23] As shown in Figure 2c,K MnO 4 aqueous solution added into isooctane wasr educed by sodium bis(2-ethylhexyl) sulfosuccinate (Na(AOT)). TheM nO 2 nanosheet sizes could be tuned by varying the water-surfactant molar ratio (W = [H 2 O]/[AOT]). Their thickness is 2nmw hich equals the thickness of three or four monolayers. By increasing the water content, the lateral sizes of nanosheets increasedw hile the thickness was not affected.
The mechanism of another methodologyt os ynthesize MnO 2 nanosheets using graphene oxide templates is illustrated in Figure 2d. [24] First, CÀCb onds are broken andCa toms are oxidizedt of orm CO 2 or CO 3 2À .T he MnO 4 À is then reduced to [MnO 6 ], an octahedral structure. To keep energy stabilization, neighbouring [MnO 6 ]o ctahedral share their edges. [25] In short, these MnO 2 nanosheets result from in situ replacemento fC atoms of graphene oxide template by edge-shared [MnO 6 ]o ctahedrat hrough redox reaction.

Electrical Properties of Nanosheets
For applicationsi nw hich electronic conductivity plays ac rucial role, such as in the supercapacitors that are discussed in sec-tion 5.2, the electronic structure of nanosheets, their charge transfer kinetics and the availability of mobile charge carriers are of utmost importance. [26] Most nanosheet compositions that have been realized so far are oxides of Nb, Ta and/or Ti,a nd these are typically wide band gap n-type semiconductors (E g > 3eV). The main contribution to the valence bands are Ti 3d-O 2p bonding interactions (or Nb 4d or Ta 5d), and the conduction bands consist primarily of Ti 3d states that are antibonding with O2 p. [27] One of the few exceptions is d-MnO 2 ,w hich has ab and gap of only 2.23 eV in its pristine (defect-free) state. [26] Owing to the anisotropyo fn anosheets, it is important to distinguish electron transport in the in-plane and out-of-plane directions. In-planee lectron transport is basically ac onfined 2D conductivity that is proportionalw ith the charge carrier concentrationa nd carrier mobility within the sheets.O nt he other hand, electron transport in the out-of-plane direction is only possible via surface redox reactions (electron transfer). Furthermore, since all atoms are directly at or very close to the surface, all electronic properties are heavily influenced by the naturea nd permittivity of the immediate surroundings of the nanosheets. Adsorbed speciesm ay thusa ct as dopantsa nd influence the concentration and/or mobility of charge carriers, and Coulombic charges and local electrical fields that are present within the 2D oxide lattice will be screenedb yc ounter ions and the permittivity of the surrounding medium. For example,i onic attachment of C n H 2n + 1 -NH 3 + ions (n = 14,18) onto Ti 1Àx O 2 nanosheets increased the band gap from 3.84 to 4.06 eV. [28] Other routes to modulate the conductivity of nanosheets and their charget ransfer kinetics are defect engineering and aliovalent doping. [26] The importance of defect engineering to influence the electronic and redox properties of d-MnO 2 nanosheets has been illustrated clearly in recent studies. [29] The presence of Mn vacancies in the 2D MnO 2 lattice leads to new electronic states in the originalb and gap, thereby reducing the gap effectively from 2.23 to 1.2-1.7 eV,a nd making the nanosheet more conductive. [29b] Such defects have also been shownt ob ev ery important for improving the specific capacitance of d-MnO 2 -based pseudocapacitors, [29a] see also section5. Another way to improvec onductivity is to introduce aliovalent substitutional dopants, for example, Co 2 + and/orF e 3 + ions into Ti 1Àx O 2 ,o rR ui nto MnO 2 .T his leads to additional mobile chargec arriers in the lattice, andt he formation of new energy states in the band gap. [30] Both phenomenam ay facilitate an increaseo fc onductivity.

Langmuir-Blodgettf ilms
In order to deposit metal oxide nanosheets on solid substrates, several deposition techniques are developed in the decade, such as Langmuir-Blodgett( LB) deposition [31] and ink-jet printing. [32] LB deposition is am onolayerd epositionm ethod, which allows detection of the intrinsic properties of densely packed monolayer films of single (layer) nanosheets.I nk-jet printing is am ore mature deposition process to make devices from nanosheet solution. Compared with LB deposition, ink-jet printing is much faster and it is much easier to vary the thickness of the nanosheet stack,w hich also makes it suitable for deviced evelopment. The deposition process,l imitations and suitable patterns of LB deposition and ink-jet printing are discussed in this section and section 4.3, respectively.
Normally,a fter the exfoliation or bottom-up synthesis process, metal oxide nanosheets are homogeneously dispersed in the solution. The surfaces of the nanosheets are covered with hydrophobic organic groups,f or example, TBA + ,t hat are anchored to the sheetsvia electrostatic interactions. These hydrophobic organic groups will help to transport nanosheets to the liquid-air interface of the solution and form ar elativelyd ense nanosheet monolayer at liquid-air interface.
Langmuir-Blodgett deposition is an effective technique to transfer such Langmuir monolayersf rom the air-solution interface to as olid substrate. Langmuir monolayers can be formed by organic molecules, [34] polymers, [35] and nanosheets. [11,36] In this review, the discussionw ill be limited to using Langmuir-Blodgett deposition for metal oxide nanosheet thin films.
LB deposition starts with the formationo fadense nanosheet monolayer,a ss hown in Figure 3a (steps 1-3). Firstly,t he nanosheet solution is transferred in as hallow trough (Langmuir trough) to allow more nanosheets to move to the liquidair interface. Then, the barriers compress the liquid-air interface to increaset he nanosheet concentration at the liquid-air interface and form ad ense nanosheet monolayer.I nt his stage, the change of the surface pressure at the liquid-air interface is monitored in situ by as urface balance, which results in as urface pressure-surface area isotherm as shown in Figure 3a. [19] The surface pressure usually increases with the decrease of surface area until ap lateau is reached, whichi st aken as evidence of the successful formation of ad ense nanosheet monolayer. Yuan et al. showed that the degree of monolayerc overage can be controlledb yt uning the surface pressured uringt he deposition process. However,t he bulk concentration of nanosheets in the solution has am uch smaller impact on the coverage. [9d] Thus, amphiphiles are almost always used as surfactants to increase the nanosheet concentration at the liquid-air interface, [37] althoughM uramatsu et al. showedt hat it is also possible to form ad ense nanosheet monolayer film without amphiphiles. [38] The Langmuir monolayer transfer processi ss hown in Figure 3b.T he vertical pull-up methodi )isu sed for hydrophilic substrates (such as Si wafer,S iO 2 etc.) and the vertical pushdown method ii)isu sed for hydrophobic substrates (see Figure 3b). The vertical pull-up methodi st he most commonly used method. Using ah ydrophilic substrate as an example, the substrate is firstly immersed in the solutionb efore the lift-up process is started. Duringt he deposition process, the surface pressure is kept constant by further compressing the monolayer when needed. In this stage, the transfer ratio, which is defined as the ratio between the decrease of area at the liquidair interfacea nd the covered area on the substrate, can be recorded to show the quality and quantity of the Langmuir monolayer.T hese two vertical methods are massively used for nanosheet layer deposition. Langmuir-Blodgettd eposition is not limited to the vertical method. There are several modified versions of this transfer technique, such as the horizontal lifting method,a lso called the Langmuir-Schaefer method( LS, in Figure 3c)a nd the horizontal pull-up method (in Figure 3d). [31] These methods are rarely used to develop nanosheet monolayers, but rather for molecular thin films. The LB process for monolayer andm ultilayer films is schematically depicted in Figure 3e.E xamples of ad enselyp acked (97 %) monolayer film and ap artially covered (63 %) monolayer film of titanate nanosheetsa re shown in Figure 3f,g.

LB multilayer films and heterostructures
Langmuir-Blodgett-based nanosheet monolayers are usually made to measure the intrinsic properties of single nanosheets or as ingle layer of nanosheets. Nanosheet multilayers are more relevant for devices.N anosheet monolayer films usually contain as mall percentage of overlapping areas and uncovered gaps. By repeating the LB process several times in al ayerby-layer (LbL) fashion, multilayer nanosheet films can be made to prevent such issues. In 2010, Sasakie tal. [33] reported ah ighkd ielectric multilayer Ca 2 Nb 3 O 10 nanosheet thin film made by Reproduced with permission. [19] Copyright2 016, American Chemical Society; (b) Vertical pull-up (i)and push-down(ii)methodo fL Bdeposition. (c) Horizontal lifting methodo fL Bdeposition. (d) Horizontal pull up methodofL B deposition. Reproduced with permission. [31] Copyright 2013, Wiley-VCH; (e) Fabrication procedurefor multilayer films usingt he LB method. Reproducedw ith permission. [33] Copyright 2010,American Chemical Society.( f) LBderived titanate film with 97 %c overage (suspension concentration 30 mg L À1 ,surface pressure 20 mN m À1 ); (g) LB titanate film with 63 %c overage (suspension concentration 20 mg L À1 ,surface pressure 5mNm À1 ). Reproducedw ith permission. [9d] Copyright2 014,American Chemical Society. such ap rocess. In order to decompose the TBA + ions still present on the surface of the nanosheet after LB deposition, the films were irradiated by UV white light after every deposition step, as illustrated in Figure 3e.T he leakage currentd ecreased upon increase of the numberso fC a 2 Nb 3 O 10 (CNO) monolayers in the device structure. Similarly,K ang et al. [39] reported aC dS thin-film transistor using a1 0-layer CNO nanosheet-based thin film as ag ate insulator.T he same type of CNO nanosheetf ilms are also used in organic light-emitting devices (OLED). [40] Yoo et al. [41] reported Ag-doped RuO 2 nanosheet multilayersf or flexible transparent electrodes, and noted that the electrical conductivity increased with the number of layers.S imilarly,t he LbL method can also be applied to fabricate heterostructure thin films, for example, aL aNb 2 O 7 /Ca 2 Nb 3 O 10 perovskite nanosheet superlattice thin film. [42] This heterostructure thin film showeds uperior ferroelectricity and potential for application in nanodevices. The same group also developedaTi 0.8 Co 0.2 O 2 / Ca 2 Nb 3 O 10 nanosheet heterostructure thin film using the same methoda nd discovered that ferromagnetism and ferroelectricity coexistint his system. [43] Next to the LbL approach in combination with LB deposition, the sequential stacking of oppositely chargen anosheets is also an applicable method to synthesize nanosheet heterostructuret hin films. Li et al. [44] reported Ti 0.91 O 2 /Mg 2/3 Al 1/3 (OH) 2 and Ca 2 Nb 3 O 10 /Mg 2/3 Al 1/3 (OH) 2 nanosheet heterostructure thin films synthesized in this way.M g 2/3 Al 1/3 (OH) 2 is al ayered double hydroxide (LDH) nanosheet that carries ap ositive surface charge in colloidal state, while Ti 0.91 O 2 and Ca 2 Nb 3 O 10 metal oxide nanosheets are negatively charged. The process is carried out by immersing as ubstrate in an LDH nanosheet/formamide solution,f ollowed by immersion in am etal oxide nanosheet/formamide solution, with ac leaningp rocess (immersioni np ure formamide)i nb etween. The process can be repeated multiple times. Unfortunately,o nly al imited number of cationic nanosheet compositions is available, whichl imits the method to as mall range of layeredn anocomposite films. The method is not abletof abricate fully covered layers,that is, it does not provide the same degree of film qualitya sL Bd eposition does. The methodi sn owadays mostly used fors ynthesizing superlattice micro/nanocrystals. [45] It would be desirable to develop this methodf or heterostructure nanosheet thin films further, because combinationso fd ifferent layers may even show emergent properties that cannot be found in the individual layers. [46] Inkjet printing of nanosheets As ad igital, non-contact and high-resolution deposition technique, inkjet printing attracted great attention in the field of flexible electronics. [47] Due to the mask-free feature of inkjet printing, inksc an be deposited onto variouss ubstrates, such as rigids ubstrates, paper and flexible polymer substrates. The two main inkjet printing modes are continuous inkjet and drop-on-demand inkjet printing. Drop-on-demand piezoelectric type inkjet printers have been widely used for lab research. To jet the ink from the nozzle, an electric field is applied to the piezoelectric materiali no rder to create af orce that pushes the ink through the nozzle onto the substrate (Figure 4a).
The crucial issues for successful inkjet printing are ink formulation and optimization of printed patterns. In order to form high quality patterns, droplet formation needs to be optimized to prevent undesired satellite droplets or tails. It is possible to optimize droplet formation by optimizing the firing waveform or the voltage of the nozzles of the inkjet printer.L ee et al. reported that double waveformsw ith two square pulses could overcome the problems associated with the use of low viscosity inks in inkjet printing. [48] Inks for inkjet printing typicallyconsist of small entities dispersed in solvents to form homogeneous colloidal suspensions, optionally also containing certain additives.Ah igh concentrationo fs olids and high ink stability are the two main issues for 2D material-based preparation of inks. Additives like surfactants can be used to improvet he dispersion of 2D materials in the inks. Solvents play an important role in the inkjet printing process. Various solvents like water and organics olvents are often used for inkjet printing. However,w ater is not an ideal solventf or inkjet printing due to its high surface tension (around 70 mN m À1 )a nd low viscosity (around1mPa s). [21] To evaluate ink printability,t he inverse Ohnesorge number Z which contains the physical properties of functional ink, is defined as [Eq. (1)]: The scale bar is 100 mm. [21] (i)High angle annular dark-field-scanning transmission electron microscopy cross-sectionalo fp rinted heterostructures. Reproduced from ref. [51].
in which a is the nozzle diameter, 1 is the density, g is the surface tension, and h is the viscosity of the ink. To preparep rintable inks, the lateral dimensions of functional materials dispersed in the inkshould be less than 1/20 of the nozzle diameter to avoid nozzle blocking during printing. [49] The ink viscosity should also be optimized to meet the requirements of the printer.T he ideal viscosity range for inkjet printingi sb etween 2-25 mPa s. [50] The surface tension of the ink also plays an important role in ink formulation engineering. Low surfacet ensions lead to spontaneousd ripping of droplets whileh igh surface tensionsmake it difficult to print. Our group reported the printable MnO 2 nanosheets ink by adding additive to modify the fluidic properties. Triton X-100 was used to not only decrease the surfacet ension of water to 46 mN m À1 but also to keep the electrostatic stabilization of MnO 2 nanosheets. Propylene glycol was added to increase the viscosity of water to 1.71 mPa s. The concentration of Triton X-100 must be carefullyo ptimizedt og et ah igh-quality printed film. As shown in Figure4b, excess Triton X-100 can lead to non-uniform distribution of MnO 2 nanosheets, as shown by the atomicf orce microscopy (AFM) image in Figure 4d.A no p-timizedT riton X-100 concentration led to uniform deposition of MnO 2 nanosheets (Figure 4c), as illustrated by the AFM image in Figure 4e.T he optimized Triton X-100 concentration was able to balance the outflow force and weak Marangoni flow,t hereby reducing the "coffee-ring" effect. [52] The Z-value based on above values is about 19 fort he water-basedM nO 2 nanosheets ink. Theo ptimal Z-value suggested by Jang et al. is between 4a nd 14. [53] However,o ur findings hows that ink with Z-value outsideo fo ptimal range can also be printed smoothly (Figure 4f).
Interfacee ngineering is another keyi ssue for high quality inkjet printing. The wetting process is defined by Young's equation[Eq. (2)]: in which g sv is the solid (s)-vapor (v) surface energy, g si the solid-liquid (i)s urface energy and g iv liquid-vapors urface energy.T he q is the contact angle ( Figure 4g). To print continuous lines or films, the contact angle q should be less than 908,i ndicating good wetting of printed patterns. Large q (> 908)m eansp oorw etting, resulting in discontinuous printed patterns. Pre-treatmento fs ubstrates by plasma is ac ommon strategyt oc ontrol the surfacee nergy of the substrates. Tuning the droplets pacing is also an effective methodt op rint continuousp atterns.A ss hown in Figure 4h,t he printed line becameb ulged with 15 mmd roplets pacing because the droplets overlapped significantly with each other.T he lines became uniform when the droplet spacing was in the range of 20 to 40 mm. Furtheri ncreasing the droplet spacing would result in discontinuous lines since the droplets were too far apart to merge with each other. [21] The interface plays an important role in the performance of devices.I t's challenging to achieves harp and controllable interfaces between printed complex heterostructures, due to the remixing of different 2D materials at the interface. However,a binder can be employed to modify the formulation and to minimize re-dispersion of nanosheets. Recently,C asiraghi et al. demonstrated that water-soluble and environmentally friendly polysaccharide xanthan gum binder successfully changed the chemistry of printed 2D crystal inks to allow fabrication of controlled interfaces in all-printed heterostructures. [54] The underlying principle is that the viscosity of the ink increases after printing, resulting in minimized remixing of different 2D inks. To keep the 2D crystal ink's electrical properties, as mall amount of binder that could have al arge rheological effect on the ink was needed. Formulation engineering can also be used to control the interface of printed heterostructures. To rrisi et al. reported that low boiling point solvents ( 100 8C) with fast evaporation at room temperature could reduce the materials transport and re-dispersiono fm aterials at the heterostructures interface (Figure 4i). [51] Applications of Oxide Nanosheets Nanosheets as templates for epitaxial thinfilm growth Oxide-based thin-film technology is the key to fabricating modern devices with practical applications,s uch as transistors, solar cells and energy storage. In order to fully utilize devices functions, epitaxial growth playsa ne ssential role to precisely control oxide thin films' crystal quality in the early stage of fabrication.
However,t he integration of oxide-based devices with the currentS i-based technology has been challenging. The constrainto flattice-matching on Si or native amorphous SiO 2 limits the number of oxidest hat can be grown on Si and the ability to control the properties of their thin films. Moreover, even for non-Si based technology,t he commercialization of oxide-based devices has been hampered by the high cost and the limited size of single crystal substrates. Low cost and ubiquitousa vailability of large area glass and plastic satisfy industrial requirements. However,t hese are usually amorphous substrates, and the fabricated oxide thin films growno nto them are polycrystalline with many orientationsa nd ah igh concentrationo fgrain boundaries.A sar esult,t hey exhibit poor physical properties and thus, devices made from them show poor performance.
In order to realize epitaxial growth of oxideso nS io ra morphouss ubstrates, the introduction of ab uffer layer is necessary.O xide nanosheetf ilms are one of the candidates to bridge the gap betweens ubstrates and epitaxiallyg rown functional oxide thin films. They have as inglew ell-defined type of surface termination and the 2D unit cell parameters of many nanosheets are in closea greement with the lattice parameters of aw ide range of functionalo xides, for example, perovskitetype oxides. Recently,v ariouso xide films have been epitaxially growno nn anosheet buffer layers on Si and amorphous substrates. [55] Nanosheets may be considered as micro-single crystals, with al arge library of crystal lattices and symmetric twodimensional crystal structures.T he crystal orientation of thin films can controlt heir transport, magnetic, and ferroelectric properties. For instance, in the case of La 0.7 Sr 0.33 MnO 3 films with thickness below 12 nm on LaAlO 3 ,t he film was insulating with (001) orientation,w hile it behaved like am etal with (110) orientation. [56] The use of oxide nanosheets as seed layersm ay provide new possibilities to control growth orientation, and thus, to tune the properties of functional oxide films. The use of nanosheet seed layers also enables the free choice of substrate, which can be exploited both for practical applications and in fundamental studies.
Epitaxialg rowth via lattice matching can be realized under strict conditions in terms of structurals imilarities between a grown oxide crystal and as ingle crystal substrate. However, dangling bondsa re always present on the top-most layer of the substrate and these restrict the mobility of adatoms at the beginningo fe pitaxial growth due to their anisotropic nature. This causest he self-organization of adatoms into ac rystal lattice not being favourable in terms of energy.A saresult,e pitaxy is possible when there are small differences betweena grown layer and as ingle crystal substrate in term of thermal expansion coefficients, crystal symmetry and lattice parameters along as pecific orientation. Therefore, coherente pitaxial growth on as ingle crystal substrate requires al attice mismatch of less than 8%. [57] Vand er Waals epitaxy was proposed to lessen the lattice mismatch prerequisite by utilizing cleaved surfaces of substrates, such as mica, metal dichalcogenides, and so on. [58] These materials have clean surfaces without dangling bonds, on which epitaxial growth of thin films proceeds by exploiting relatively weak and isotropic van der Waals forces. Good epitaxial growth is still possible with al attice mismatch as large as 20 %. [58a] Even for large lattice mismatchingb etween at hin film and as ubstrate, Narayan and Larson suggested that thin film epitaxy on as ingle crystal substrate is still feasible via domain matching. [57] AlN film over (111)Si substrate orientation has al attice mismatch of about 19 %. Via domain matching epitaxy,amismatch between 5 AlN(2-1-10)p lanes and 4 Si(220) planes of less than 1% still makes it possible for AlN to grow epitaxially on Si(111). [57] However,i ti se nergetically favourable for the film to have dislocations above ac ritical thickness, which depends on the type of materialg rown, and consequently,t he grownf ilm may have ah igh density of dislocation and other defects.
The well-defined surfaces tructure of nanosheets is quite similar to those of mica and metal dichalcogenides, and nanosheetf ilms can thus be utilized as a seed layer to grow oriented functional oxide thin films. However, it is hard to say if the epitaxial growth on individual oxide nanosheets is "pure" van der Waals or lattice/domain matching epitaxy.F irstly, al ocalized high charge density compared with van der Waals materials is expected in oxide nanosheets. This may influence adatoms' interactions in the early stages of growth. Secondly,i nv an der Waals epitaxy, the grown films have similarl attice constantsa st he bulk form even when the films are only one unit cell thick. [58a] There is evidence that anatase TiO 2 ,a st he grown film, and Ca 2 Nb 3 O 10 À (CNO) nanosheet, as the bufferl ayer,h ave lattice constantsi nrestraint with each. [59] Recently,o ur report on VO 2 films grown by pulsed laser deposition( PLD) on Ti 0.87 O 2 dÀ (TO) and NbWO 6 À (NWO) nanosheets showed that the metal-insulator transition temperature of VO 2 films on nanosheetsw as shifted from its bulk value, which may be the result of the differencei nc-axis lattice constant between the nanosheet-buffered films and the bulk VO 2 rutile phase (Figure 5a). [55e] Even thought he lattice or domain matching can be dominant in such epitaxial growth on oxide nanosheets, furthers tudy is necessary to verify the growth mechanism on oxide nanosheets.
Although SrRuO 3 (SRO) is generally ferromagnetic around 160 K, it is one of the most important conducting perovskites because of its common application as an electrode in oxide heterostructures at room temperature. [61] Controlling the prop- erties of SRO on Si substrates is ag ood illustration of the ability of integration of oxide based devices with Si based technology.O ur group has shown that SRO grows epitaxially in (001) pc and (011) pc orientations on CNO and TO nanosheets, respectively,f or which" pc" refers to ap seudocubic crystal symmetry. [9c, 55c] However,d irect growth of SRO on nanosheets results in minor orientations of (011) pc and (001) pc on CNO and TO. [9c] The introduction of af ew unit cells of SrTiO 3 prior to the deposition of SRO completely suppresses these minor orientations on these two types of nanosheets. [55c] Even thought he SRO films have grain boundaries and random in-plane orientations due to the micrometer dimensions and random distribution of oxide nanosheets on Si, their resistivity is comparable with those on single crystal STO substrates. [55c] In addition, the magnetic easy axis ("easy" refers to the direction within ac rystal structurei nw hich magnetization can be alteredm ost easily)o f SRO, whichc an be parallel or perpendicular to the surfaceo f the film, can be easily tuned by choosing properly buffered CNO or TO nanosheets, [55c] independentlyf rom Si substrate.
Pb 1Àx RE x (Zr y Ti 1Ày )O 3 materials, with RE representing ar are earth element, are one of the mosti mportantp erovskite family of ferroelectrics, relaxor ferroelectrics, antiferroelectrics and piezoelectrics for various applications, such as microelectromechanical systemsa nd energy storaged evices. [55d, 60, 62] The deposition of this family of materials on Si substrates is the next step in realizing oxide-based devices. Ferroelectric PbZr 0.52 Ti 0.48 O 3 (PZT), which is knownt oh ave the largest dielectric constant andp iezoelectric coefficients in its family,w as depositedo no xide nanosheets on Si with SRO as electrodes. [55d] PZT films have main (001) and (011) orientations, the same as SRO films on CNO and TO nanosheets,r espectively, but minor orientations are presenti nP ZT films (011) on CNO and (001) on TO nanosheets as well.
[55d] They can be suppressed by introducing an STOl ayer before the deposition of the whole stack of SRO/PZT/SRO. It is worth mentioning that (001) oriented PZT films grown on CNO nanosheets on an amorphous (glass) substrate demonstrates the best piezoelectric coefficient (490 pm V À1 )i np iezoelectric films, thanks to columnar microstructurea nd strong orientation of the films along the c-axis (Figure 5b). [60] In addition to PZT composition,relaxor ferroelectric Pb 0.9 La 0.1 Zr 0.52 Ti 0.48 O 3 anda ntiferroelectric PbZrO 3 have also been drawing attentionf or energy storage devices. [62b-d] The films of these two compositions can be oriented in the same mannera sP ZT films by using oxide nanosheets. The performance of these devices scales with the recoverable energy storaged ensity (U reco ), which depends on the critical electric breakdown field (E BD ), and the maximum and remanentp olarizations. Differentk inds of nanosheets can be used to tune the microstructure of films. While Pb 0.9 La 0.1 Zr 0.52 Ti 0.48 O 3 film has a dense microstructure on CNO nanosheets, it has ac olumnar one on TO nanosheets. [62c] As ar esult, E BD increases for the dense film, leading to ahigher U reco value. [62c] Vanadium dioxide VO 2 is one of the leadingc andidates for electronic oxide devices forl ow powero peration in field-effect type or neuromorphic electronic devices. [ As we mentioned above, the former film has am etal-insulator transition temperature (T MIT )a t3 32 K, lower than the bulk value of 341 K, whereas the latter has T MIT at 347 K, highert han the bulk value. [55e] It is knownt hat the compressives train on the rutile c-axis of VO 2 leads to ad ecrease of T MIT ,w hilet he tensile strain on it increases T MIT . [64] We suggested that the same phenomena happent oV O 2 films on NWO and TO nanosheets. [55e] In order to highlight the additive potential to the free choiceo fs ubstrates andd emonstrate how well oxide nanosheetsc an be controlled on the micrometer-scale, our group showed that two differentorientationsoffunctional oxidescan be realized on as ingle substratei nadefined pattern by utilizing lithography. [55c, e] For example, the alternative line pattern between (À402) M1 and (011) M1 (whereM 1r efers to the monoclinic phase) VO 2 is achievedb yg rowingo nt he same pattern of NWO and TO nanosheets (Figure 5c). [55e] Although the minimum feature size of the patterns may be limited by the size of nanosheets, either top-down etching of nanosheets, or sizecontrolling nanosheets before or after exfoliation is feasible to achieve smaller feature sizes. The ability to micropattern the orientations of functional oxideso nt he micrometer-scale may open new functionalities for devices that single crystal substrate cannotp rovide. Next to micrometer-scale patterning, a methodology to transfer pre-grown nanosheet-seeded thin films from as acrificial mica substrate to any arbitrary substrate, for example, thermally sensitive flexible plastic substrates, has also been developed. [65] In addition to potential applications, oxide nanosheets can facilitatet he fundamentals tudy of various materials, in which mechanisms or structural changes usually happen alongaspecific crystal axis. Advanced characterization techniques, such as soft X-ray absorption spectroscopy in transmission mode and transmission electron microscopy (TEM), also need the film thickness to be in the range to facilitate electron or X-ray transparency.B oth requirements are hardly achieved on single crystal substrates or amorphous X-ray transparent TEM grids at the same time. Remarkably,o xide nanosheets are transparent to electrons and X-rays thanks to their thickness of only af ew atomicl ayers.W eg ave ap roof of principle for the case of VO 2 in soft X-ray absorption spectroscopy (Figure 5d). [55e] Nanosheets in supercapacitors and batteries Due to their unique atomically thin structurea nd large surface area, 2D nanosheets have recentlya ttracted considerable attention for their potential application in the field of energy storage. [11,66] Micro-supercapacitors (MSCs)w hich have interdigitated electrode structures showg reat potentialf or integration with chips or flexible electronics. [67] As shown in Figure6a, the in-plane interdigitated configurationofMSCs using stacked nanosheet electrodes in the same plane,offers fast ionic movement in horizontal directions and an increaseo ft he accessibility of the electrodes. Inkjet printing, ad igital printingt echnique, has been widely used for the fabrication of in-plane interdigitated MSCs. [68] Graphene,w hich was exfoliated in 2004, shows promise as electrode materials for supercapacitors be-cause of its high electrical conductivity,e xcellent stability and mechanical flexibility andl arge specific surface area. [69] Li et al. reporteds calable fabrication of, fully inkjet-printed graphene MSCs. [70] Graphene wasp rinted as electrode on flexible polyimide substrate followed by printing poly(4-styrenesulfonic acid) as electrolyte on top of electrodes. These fully printed MSCs exhibited areal capacitances of around0 .7 mF cm À2 .F urthermore, large scale MSC arrays whichw ere printed on both silicon wafersa nd polyimide can be charged to voltages as high as 12 V. Beyond graphene, aw ide range of other novel 2D nanosheets have been proposed to serve as electrode materials for supercapacitors, such as MoS 2 , [71] black phosphorus, [72] MXene [73] and transition metal oxides. [11,74] Recently, Zhang et al. reported the additive-free MXenei nk for printed MSCs. [75] To fabricate all-MXene MSCs, MXenei nk was first printed on substrates followed by coating with H 2 SO 4 -poly (vinyl alcohol, PVA) as ag el electrolyte on top of the device. The printed MSCs exhibited volumetric capacitances up to 562 Fcm À3 and energy densities of 0.32 mWh cm À2 . MnO 2 nanosheets, which exhibit al arge theoretical capacitance (1233 Fg À1 ), have been reported as electrodes for printed supercapacitors. [32,74] However,t he electronic conductivity of MnO 2 is low (10 À5 to 10 À6 Scm À1 for Na-birnessite). [78] One strategyt oi mprovet he electronic conductivity of MnO 2 is by defect engineering. [79] An alternative strategy is to combine MnO 2 with highly electron conductive materials such as graphene. [80] The charge storage mechanism of MnO 2 nanosheets occurs by the reduction of Mn 4 + to Mn 3 + ,e.g.: in which Ai sa na lkali ion or ap roton. As shown in Figure 6b, MnO 2 electrode show fast, reversible surface redox reactions leadingt op seudocapacitive charge storage in mild aqueous electrolytes. [76] The cyclic voltammograms of MnO 2 electrode in Figure 6b is closetoelectrochemical double layer capacity.
Our group demonstrated all-solid-state MSCs by inkjet printing MnO 2 nanosheets as active materials. [21] MnO 2 nanosheets were synthesized following the bottom-up synthesis strategy outlined in section2.2. To prepare water-based printable MnO 2 inks, Triton X-100 and propylene glycol were added into the MnO 2 solutiont om odify the surface tension and viscosity of the solution, respectively. [54] By formulation engineering of MnO 2 nanosheets ink, it can be printed on arbitrary substrates without the "coffee-ring" effect. To fabricate asymmetrical MSC on flexible polyimide substrate, MnO 2 nanosheets ink was firstly printed on polyimide with in-planei nterdigitated configuration followed by thermala nnealing. Highlyc onductive poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:P SS) ink was printed on top of MnO 2 to serve as current collectors. To complete the solid state MSC device fabrication process, PVA/LiCl as gel electrolyte was dropped on top of the device and dried at room temperature. The MSC exhibited high volumetric capacitances of 2.4Fcm À3 and high energy densities of 1.8 10 À4 Wh cm À3 at power densities of 0.018 Wcm À3 (Figure 6c). To improvet he electrochemical performance of MnO 2 micro-supercapacitor,5%s ubstitutionali ron doping was applied and the resulting Fe-doped MnO 2 nanosheets were inkjet-printed as activem aterials with interdigitated structures for MSC. [77] Cobalt and nickel were also active dopantsb ut to am uch lesser extent than Fe. The Fe-doped MnO 2 MSC exhibited av olumetric energy density of 1.13 10 À3 Wh cm À3 at volumetric powerd ensity of 0.11Wcm À3 ,b oth more than six times higher than the MSC based on undoped MnO 2 nanosheets (Figure 6c). Furthermore, the Fe-doped MnO 2 MSC showed good mechanical flexibility and cycling stability with ac apacitance retention of 78.7 %a fter 5200 cycles of charge/discharge (Figure 6d). These MnO 2 based printed MSCs show promise as energy storage units for utilization in flexible electronics. These performance data confirm that inkjet printingi sapromising method for manufacturing flexible energy storagedevices like supercapacitorsa nd batteries.
Nanosheets have also gatheredinterest for utilization in lithium ion batteries. When using nanosheets as as olid-state electrolyte,b atteries can be made thinner.U sage of 2D materials leads to shorter pathways for ions, increasing the rate capacity of the battery.M oreover,l ess packaging compared to liquid electrolytes is neededd ue to the absence of possible leakages, which makes that these types of batteries are intrinsically safer. The lifetime of solid-state electrolytes is superior to liquid electrolytes, because they do not degrade easily compared to using liquid electrolytes. The ionic conductivity,h owever,i s thus far lower than in liquid electrolytes. Different 2D materials have already been explored for the different components of a battery.For example, graphene is ac ommonly used anode material for lithium ion batteries, because of its capability for reversible lithium ion intercalation in the layered crystals. The structurals imilarities of graphene nanosheets to graphite may provide another type of intercalation anode compound. [81] MXenesh ave also been attracting ag reat deal of attentiona s emerging low-cost and high energy-density anodes for batteries, not only for lithium, but also for non-lithium batteries. [82] LiMPO 4 (M = Fe, Mn, Co, Ni)h as become of great interesta s cathodes for next-generation high-power lithium ionb atteries. This is an olivine-type 2D material, which can be optimized by as olvothermal lithiation process to increaset he lithium diffusion. [83] And leaf-like V 2 O 5 nanosheets can be fabricated via a facile green approach as high energy cathode material. [84] Since nanosheets have al arge surface to volumer atio, it is possible to attach functional groups on the nanosheet surface or dope the 2D sheets with foreign elements. For example, titanate nanosheets have been used as electrode in batteries, but their electronic conductivity is rather low.B yd oping titanate with an element like niobium, [30a] the conductivitym ay be increased to facilitateits use as an electrode.

Catalyst design using nanosheetsa st emplates
The concept of employing nanosheetsa st emplates to control the oriented growth of PLD-derived films can also be applied to wet-chemical depositionprocesses. The deposition and crystallization steps are typicallys eparated in aw et-chemical process:f irst, an amorphousp recursor phase is deposited onto a nanosheet. In the subsequent step the precursor is thermally annealeda nd crystallized. Sol-gel-derived anatase films have been grown from (NH 4 ) 2 TiF 6 andH 3 BO 3 on Ca 2 Nb 3 O 10 and Ti 0.87 O 2 and it was shown that the nanosheets could survive both the conditions during the wet-chemical deposition process, and the subsequent thermal annealing step at 450 8C. [85] It was found that {100} and af ew {001}f acets were exposed at the surface of the anatase film grown on Ti 0.87 O 2 .T he anatase film grown on Ca 2 Nb 3 O 10 exposed mainly its {001}f acets. The differences between the two forms of anatase becamep rominent in photocatalytic hydrogen formation experiments involving these films. [85] Nanosheet-based softhybrid materials Althought he emphasis in this review is on the synthesis and applicationso ft he 2-dimensional forms of the oxides, the parentlayered oxidesmay also be chemically modified with organic intercalants to render al ayered hybrid material. Such a materiali sr elativelye asy to deform under shear force, and might thusf ind applicationw here that property is exploited, for example, asasolid lubricant. [12,86] n-Alkylamines, with alkyl chains having 3t o1 2c arbon atoms, can easily intercalate into protonated layered oxides, such as H 1.07 Ti 1.73 O 4 (HTO). [12] The intercalation process is driven by the acid-baser eactionb etween the intercalating amines and the interlayer protons. The nanosheet layer distance in the hybrid can be controlled by the length of the n-alkylamine chain and the amine/HTO ratio. In another study,t he zwitterionic amino acid 11-aminoundecanoic acid (AUA) was intercalated into HTO through ion exchange, see Figure 7a and 7b. [86] The amino acid molecules formed an ordered paraffinic bilayer in the gallery region of the layered host and replaced H + at pH below the isoelectric point (IEP) of AUA (pH 7.85). The nanocomposite can be exfoliated into a disordered structure at pH @ IEP,w here the amino acid becomesn egatively charged and destabilizes the ordered layered structure. Above 180-200 8C, the amino acid polymerized into nylon-11 confined between crystalline titanatem onolayers. In both studies, the intercalation process followed first-order Langmuir-type kinetics, that is, random irreversible adsorption with an adsorption rate proportionalt ot he number of available adsorption sites. Both hybridss howed good performance as high temperature solid lubricants (Figure 7c), showing friction coefficients similart ot hat of the state-of-the-artm aterial graphite (Figure 7d).

Conclusions and Outlook
Ad iverse range of nanosheet compositions derived from layered metal oxidesh as been reported. Mostn anosheets are made via soft chemical exfoliation routes, but af ew compositions, such as d-MnO 2, can also be synthesized directly by as olution process involving self-assembly of molecular precursors in water.T he compositionand doping levels in nanosheets can be modulated relatively easily via modificationo ft he composition and grain size of the layered parentc ompounds. This versatile and generic approach provides various easy handles to introduce and optimize the functional properties and dimensions of oxide nanosheets. As is illustrated in this review, by utilizing 2D nanosheets as elementary building blocks for further materials design,awide range of functional layered hybridn anocomposites, nanosheet-based monolayersa nd micropatterned films with diverse properties can be generated for emerging applications in energy conversion and storage, (nano)electronics,c atalysis and even mechanical engineering.