Low‐Temperature Miniemulsion‐Based Routes for Synthesis of Metal Oxides

Abstract The use of miniemulsions containing chemical precursors in the disperse phase is a versatile method to produce nanoparticles and nanostructures of different chemical nature, including not only polymers, but also a variety of inorganic materials. This Minireview focuses on materials in which nanostructures of metal oxides are synthesized in processes that involve the miniemulsion technique in any of the steps. This includes in the first place those approaches in which the spaces provided by nanodroplets are directly used to confine precipitation reactions that lead eventually to oxides. On the other hand, miniemulsions can also be used to form functionalized polymer nanoparticles that can serve either as supports or as controlling agents for the synthesis of metal oxides. Herein, the description of essential aspects of the methods is combined with the most representative examples reported in the last years for each strategy.


Introduction
Dropletso faliquid dispersed in another immiscible liquid form colloidal systems commonly known as emulsions. If such droplets contain chemical reagents, they are able to confine spaces in which chemical processes can occur,s erving as soft templates for the formation of nanostructures. [1] In an ideal case, if coalescence of the droplets and Ostwaldr ipening are suppressed-or at least minimized-eachd roplet can act as an independent "nanoreactor". [2,3] This situation takes place, indeed, in so-called miniemulsions, which have been widely used for the preparation of polymer particles, [3,5] buta lso for the formation of hybrida nd inorganic materials. [6,9] The confinement can occur not only within droplets, but also at the liquid-liquid interface, [10] which is interesting for preparing nanocapsules.
Before enteringi nt he detailso fs ynthesis of specific materials, it is necessary to clarify the concept of miniemulsion,b ecause the terminology used for emulsion systems is not especially logical, and may be confusing for an outsider of the field. Following ac ommonly used classification, [11] based on the size of the droplets and the stabilityi nt ime, emulsions can be classified in macroemulsions (with droplets typically larger than 1 mma nd short stability up to minutes or hours), miniemulsions (with droplets between 50 and 500 nm and stability from days to months), and microemulsions( with droplets below 100 nm and thermodynamically stable). Af irst confusing issue may be the fact that, in spiteo fw hat the name could suggest, microemulsions have sizes clearly below the micron size. The term miniemulsion has its origins in the polymer community, in the contexto ft he miniemulsion polymerization, contrasting to conventional emulsion polymerization, and it has been typically linked not only to droplets ize, but also to mechanistic aspects, such as the use of high-shear forces for kinetic stabilization and the important presence of osmotic pressure agentsi n the system. To complicate more the terminology,t he label nanoemulsion (intentionally hyphenated asn ano-emulsion by some authors [12,13] )i sa lso found in literature, mostlya sa "quasi-synonym" of miniemulsion, but apparently free of the mechanistic implications imposed to the definition of the latter.T he term nanoemulsion, clearly more recent than miniemulsion, has become relatively frequent in the pharmaceutical field and food science in the last years. [14][15][16] This name seems to be purely based on size, so that, in ab road sense, any emulsion with droplets in the nanometric scale could be theoretically considered as ananoemulsion. In practice, however,t he most common is to keep microemulsions (thermodynamically stable systems) as adifferentiated case in any classification andr eserve the term nanoemulsion for metastable systems. In our case, following the tradition of our own school, we speak here about miniemulsions,t he most established term from ah istoric point of view.N evertheless,i nm osto ft he situations, the termm iniemulsionc an be exchanged here with nanoemulsion.
In this Minireview,w ef ocus very specifically on the use of miniemulsion-baseds ystems for the preparationo fm etal oxide materials, including first those nanoparticles that are strictly prepared within the confinement of nanodroplets, but also extending the overview to metal oxides supported on other materials prepared by miniemulsion (i.e.,m etal oxides crystallized on the surfaceo fp olymer nanoparticles resulting from miniemulsion polymerization). Finally, because of the affinity with the topic, we will also shortly refer to the use of miniemulsion-basedm aterials, namely functionalized latex particles, as additives or controlling agentsi nt he synthesis of metal oxides.

Miniemulsions for the Synthesis of Metal Oxide Nanoparticles
The formation of inorganic nanoparticles starting from miniemulsions of precursors was reviewed in detail af ew years ago. [6] In the meantime, other works have also partially ad- [a] Dr.R.MuÇoz-Espí dressedt he topic. [1,[17][18][19] In this section, we aim to focus specifically on the use of miniemulsion droplets as "nanoreactors" for the confinement of precipitation reactions leading to the formation of crystallinem etal oxides.
When speaking about the use of water-in-oil dropletsf or inorganic synthesis, the work of Pileni and her team is ac ompulsory reference, although they focused almoste xclusively on thermodynamically stable systems, that is, on microemulsions. [20][21][22] Nevertheless, in spite of the differences between microemulsions and miniemulsions, the idea behind in terms of confinement is essentially the same:ametal precursor is contained in droplets dispersed in an immiscible continuous phase. At least theoretically,c hemical systems prepared in microemulsion can be analogously prepared in miniemulsion. However,i ti si mportant to note that in miniemulsions the identityo ft he droplets does not change witht ime. Therefore, ar eaction within the droplet is indeedc onfined to the same droplet. Ar eactionb etween droplets is only possible if the droplets are forced to combine by applyings hear.I nm icroemulsions, the identity of droplets is not maintained, leading to af ast interchange of materials between droplets. Although the use of water-soluble precursors in the disperse phase of inverse emulsions is the most common, oil-soluble precursors can also be applied in direct (oil-in-water) systems. [23][24][25][26] For the works dealing with microemulsions, the readeri sr eferred to specific reviews on the topic. [24,[27][28][29][30] For miniemulsions dealing with metal oxide and oxidic-related materials, we have compiled the most relevant works of the last years in chronological order in Ta ble 1.
Some of the systems (e.g.,F e 2 O 3 ,C eO 2 ,C uO) can be obtained in the form of crystalline metal oxides already at low temperatures close to room temperature, [43,45] while others ystems, typicallyo btainedt hrough conventional sol-gel routes (e.g.,T iO 2 ,Z rO 2 ,H fO 2 ), require calcination steps to reach crystals from the amorphous hydroxogels obtained in miniemulsion. [32,36] The high temperature requirement is intrinsic to these sol-gels ystems and it is not exclusive to miniemulsion. Any methodd ealingw ith these systemsw ill face the same temperature needs to reach crystalline materials.
Calcination steps have also been applieds ometimes for particles containing inorganic metal precursors (e.g.,c erium(III) nitrate to obtain CeO 2 [34] )o re ncapsulating organometallic compounds (e.g.,o rganotin compounds to obtain SnO 2 [49] ). We   mention these examples here, since they are prepared in miniemulsion, but we do not enter in furtherd etails, as they cannotbec onsidered "low-temperature" routes. Typical apolar solvents in inverse miniemulsion are cyclohexane (b.p.:8 0.7 8C, which allows for easy evaporation after preparation of the oxidic particles), toluene (b.p.:1 10.6 8C) or oils such as n-decane and Isopar M( b.p. > 150 8C, which allows for higher synthesis temperatures, but it makes the removal of the solventa fterwards difficult). The stabilization of inverse (waterin-oil) systems is alwaysm ore challenging than the stabilization of direct (oil-in-water) ones. As surfactants, compounds with al ow hydrophilic-lipophilic balance (HLB) are typically required (values < 7a re common), although exceptions to this general rule can sometimes be found. The chemical structures of some commons urfactants for inverse miniemulsions are presented in Figure 1. Block copolymers such as poly(styreneblock-ethylene oxide) (P(S/EO)) or poly(ethylene-co-butylene)block-poly(ethyleneo xide) (P(E/B-b-EO)) have been efficiently used (see Ta ble1). However,t hese typeso fc opolymers are synthetically complex and usually not easily available from commercial sources. In this sense, commerciala lternatives, such as polyisobutylenes uccinimide pentamine( PIBSP) and polyglycerol polyricinoloeate (PGPR), are quite convenient.I t should be mentioned though that the amine groupso fP IBSP can strongly interact with certain ions such as Cu 2 + , [43] which may or may not lead to desirable effects. On the other hand, PGPR, an emulsifier used in food technology,i su nfortunately not alwaysa ble to efficiently stabilize somes ystems containing metal ions.
The first example of the use of miniemulsions for inorganic synthesis, including the case of Fe 2 O 3 ,w as reported by Willert et al. [31] The initial approachw as to use molten salts dispersed in an immiscible solvent. The molten salts recrystallize when the temperature is decreased. This strategy,h owever, cannot be directly used for the case of metal oxides, since the melting point of oxidesi sm uch above the operating temperature of any organic solvent. Therefore, ap recipitation reaction or a sol-gelp rocess is alwaysi nvolved in the formation of metal oxides in miniemulsions. In an attempto fs ystematization, we classify the existing possibilities in three different approaches, which are schematically depicted in Figure 2: I) "two-miniemulsion methods", II) external addition of ap recipitating agent, and III) combination of precursors in the disperse phase or,a lternatively,use of single-source precursors.
I. "Two-miniemulsion methods":I nt his strategy,t wo independentm iniemulsions containing the metal precursor (first component) and the precipitating agent (second component) are mixed,f orcing the droplets of both components to coalesce and react, so that metal hydroxide speciesp recipitate, turning eventually into the oxide.T his methodh as been used, for instance, to prepareZ nO and doped-ZnO nanoparticles by mixing miniemulsions containing az inc salt and ab ase, such as NaOH or ammonia. [38,39,42] In comparison, in microemulsions, the reactions between the different speciest ake place instantaneously and less controlled without applying extra shear. [1] II. External addition of ap recipitating agent to an emulsion of the metal precursor:The precipitating agent (normally ab ase that generates OH À )c an be water-soluble or oil-soluble. If workingw ith inverse systems (water-in-oil), which is the most common for metal oxides, the addition of ab ase soluble in the continuous phase, such as triethylamine, can lead to the formation of hollow nanostructures. This effect, involving an interfacial precipitation/crystallization,h as been reported for sol-gel systemss uch as zirconia and hafnia. [36] Zirconium or hafnium oxychloride is dissolved in water and dispersed in an oil phase. Afterward, triethylamine (Et 3 N) is added to the system.I nc ontact with the water of the droplets, Et 3 Ng enerates hydroxide ions [Equation (1)]: The oxychloride precursor (MOCl 2 ·n H 2 O) reacts with the hydroxide ions to form hydrous zirconia or hafnia (MO(OH) 2 ·n H 2 O, M = Zr,H f) [Equation (2)]: The presence of hydroxide ions catalyzes the condensation reactiono ft he metal hydroxo species, as occurs in as ol-gel process. As above indicated, the final oxide formation requires ac alcination step. This methodologyh as certain parallelism to the one reported in other works for preparing silica capsules in inverse miniemulsions, [50][51][52][53] in which an alkoxysilane is added through the continuous phase to an inverse miniemulsion. When the alkoxysilane enters in contact with the water of the droplets, it hydrolyzes andstarts ac ondensation process.
In as imilar fashion, but with conventional precipitation reactions withouts ol-gel, the preparation of CuO, [43] ZnO, [37,44] CeO 2 , [45] and Fe 2 O 3 [35,45] was also reported. In all thesec ases, oil-soluble amines (i.e.,t riethylamine-which is also partially soluble in water-or the more apolaro leylamine) wereu sed as ap recipitating base. The contact of the OH À ions with the metal ions contained in the droplets starts at the interface, which may lead to capsular morphologies. The shell formation may be assisted by the presence of surfactants at the interface such as poly(styrene-block-acrylic acid) [45] or polyisobutylene succinimidep entamine (PIBSP), [43] able to complex metal ions and act as structuring agents. Interestingly,f or these systems, the materials are already crystalline when the synthesisi sc arried out at temperatures as low as room temperature. The combination of different metal precursors during the synthesis in defined amountsa llows for obtaining ternary or doped oxides (e.g.,YCrO 3 or Ce 1Àx Cu x O 2 ). [40,46] If the added base is water-soluble (e.g.,N aOH, KOH, or ammonia)a nd added as an aqueous solution,i ti sn ot miscible with the continuous phase, and an additional homogenization step (strong stirring or af urtheru ltrasonication step) is requiredt oi nduce the contact between the metal precursor and the precipitating agent. [47,48] III. Mixture of precursors in the disperse phase or use of single-source precursors:When the reaction between the precursor and the precipitating agent is not very fast or requires some stimulus, such as temperature, they can be combined in the disperse phase of as ingle miniemulsion, which is subsequently left to react. This situation is the case of sol-gel systems, such as TiO 2 .R ossmanith et al. [32] reported how inverse miniemulsions containing at itanium alkoxide in aqueous solution in the disperse phase led under acidic catalysis to TiO 2 nanoparticles. Zirconium-doped anatase (Zr x Ti 1Àx O 2 )c ould be prepared in an analogous way. [33] For some systems, it is possible to obtain the oxide starting from one single precursor,aso-called single-sourcep recursor. One example of this case is the formation of molybdic acid (hydrated forms of molybdenum trioxide, MoO 3 ·n H 2 O) from peroxo-complexes [Equations (3)(4)(5)]: [54] Such peroxo-complexes can be obtained by reacting molybdenum metal with hydrogen peroxide, [55] and have also been investigated in miniemulsion to obtain molybdic acid and, in an analogous way,t ungstic acid and mixtures of these hydrated oxides. [56] Another example of the use of ac omplex single-source precursor in miniemulsion was reportedb yH eutz et al., [41] who prepared Au/TiO 2 materials starting with ag oldcontaining titaniump eroxo-complex with structure AuCl 4 (NH 4 ) 7 [Ti 4 (O 2 ) 4 (cit)(Hcit) 2 ] 2 ·12 H 2 O.
In general, the miniemulsion technique mainly allows us to reach spherical shapes.H owever,t his does not always apply to inorganic materials, since the droplets ize in the precursor emulsion is typicallyl arger than the final inorganic particles obtained, so that the shape is not templated "one-to-one", and other morphologies are possible. In addition, non-spherical shapes are sometimes reachable under certain conditions, such as higherc oncentration of surfactant, which may lead to cylindrical micelles in the initial emulsion and, consequently,t o rod-like morphologies of the metal oxides. [56] So far, most of thei nvestigationsa bout theu se of miniemulsionsf or confiningc rystallization processesh aveb eenc arried outu nder ambientp ressure. Thec ombination of hydrothermal conditions (i.e., noto nlyt emperatures abover oomt emperatures, buta lsop ressures abovea mbient conditions)w itht he miniemulsion techniqueh aveavery wide ande xcitingr ange of possibilitiesf or them orphosynthetic controlo fa morphous and crystallinem aterials.I nrecent work,t he synergyo fh ydrothermala nd miniemulsion conditions wasi nvestigatedf or thep reparationo fd ifferent nanostructured metalf errites. [47] Different spinel ferrites,i ncluding Fe 3 MnO 8 ,C oFe 2 O 4 ,C uFe 2 O 4 ,N iFe 2 O 4 , andZ nFe 2 O 4 were prepared by adding ab ase( NaOH)t oa ni nverseemulsionofthe metalprecursors(metalsalts)and sonicated againt oa llow coalescencea nd precipitationa ccording to method II.Thisworkdemonstrated that forsome of thesystems (namely, thez incf errites),t he analogousm aterials obtained with miniemulsion at ambientp ressurea nd under bulk conditions either at ambientp ressure or unders olvothermal conditions didn ot result in comparativelyh ighlyc rystalline ferrites, thus outliningt he relevanceo ft he combined synthetic strategy.

Miniemulsions for the Synthesis of Supporting Materials for Metal Oxides
The synthesis of metal oxide nanostructures can be confined not only within droplets, as described in the previouss ection, but also on the nanometric surfaces of nanoparticles of another material. In this context,f unctionalized polymer nanoparticles have been widely used fors upporting the in situ growth of different inorganic crystals, including metal oxides (see ad etailed overview in Ref. [1]). Herein, we refer briefly to the examples dealing exclusively with polymer particles preparedb y miniemulsion polymerization. Thes teps of the approach, togetherw ith some representative micrographs of the materials obtained, are presentedi nF igure 3. Carboxylates, phosphates, and phosphonate groups have been shownt ob ee ffective nucleating agentsf or the subsequent precipitation of metal oxide nanoparticles. The introductiono ft hese functional groups on thes urface of polymer nanoparticles can be achieved by copolymerization in miniemulsion of as upporting structuralm onomer (such as styrene or methylm ethacrylate) with functional monomers (typically hydrophilic) that contain the desired metal complexing group. To avoid the use of surfactants, which may involve undesired effectsi ns ome cases, the use of so-called surfmers (i.e.,p olymerizables urfactants or surface active monomers)h ave been proposed. [57] Am etal precursor( usually aw ater-solublem etal salt) is added to ad ispersion of the surface-functionalized polymer nanoparticles. The surface should have the ability to bind the metal cations, creating centers in which the nucleation of the metal oxide starts upon addition of the precipitating agent. Them ethod is versatile for any metal oxide involving precipitation by addition of a base. Reported examples are CeO 2 ,F e 2 O 3 ,F e 3 O 4 ,a nd ZnO. The support with this strategy of catalytic oxides, such as CeO 2 , leads to highly efficient and easily separableh eterogeneous catalysts. [57,58] If the supporting material possesses ac apsular morphology, which is also possible by employing miniemulsion polymerization, the resulting materialc an encapsulate further substances, in addition to the metal oxide on the surface.T his methodology has been recently used to prepare hybrid nanocapsules containing CeO 2 on the surface and organic fluorophores in the core. [59,60] 4. Miniemulsions for the Synthesis of Controlling Agents for the Crystallizationo fM etal Oxides strategyi nspired by biomineralization processes in nature. Althoughm etal oxides are rather unusual in biomineralization, there are af ew examples of iron and manganese oxides in bacteria. [61] In materials science,t he use of polymers in crystallization has been often termed as "polymer-controlledc rystallization", [62] and it may be linked to so-called "non-classical crystallization" processes involvingt he formation of mesocrystals. [63,64] Ar eview of the topic is clearly beyondt he scope of this paper,b ut we consider appropriate at this position to refer to the few specifice xamples of miniemulsionp olymers used to control crystallization of metal oxides.
In as eminal work by Wegner and co-workers, [65] zinc oxide was synthesized in the presence of poly(styrene/acrylica cid) latex particles prepared by miniemulsionc opolymerization. It was found that the polymer nanoparticles became incorporated into the crystals, leading to what the authors named a "Swiss cheese morphology". [66] In the following years, am ore detailedi nvestigation of the mechanism of these systems followed ( Figure 4). [67,68] Zinc oxide,w ith only one crystal phase under normalc onditions (namely,z incite), was found to be a very suitable model for studying the change in morphology as ar esult of the presence of additives.T he ability of miniemulsion copolymerization to deliver particlesw ith different functional groups at the surface and with controlled surface charge density was further exploited for the zinc oxide system.N egatively charged polymer particles were found to adsorb preferentiallyo nt he {0 01}f acets, retarding the growth in the perpendicular direction[ 100].I nt he presenceo fa ni ncreasing amount of acrylic-acid-functionalized polystyrene particles, the crystalsb ecame shorter and wider.T his decreasei nt he aspect ratio wasc orrelated with the adsorption process of the polymer on the growing sites of the zinc oxide crystals, whichw as found to follow aL angmuir isotherm model at lower concentrations. The functionalization of the latex particles with groups showingastronga ffinity towards the {0 01}f acets of the growing crystals, such as carboxylates and phosphates,r esulted in very peculiar nanostructures as ac onsequence, in a limiting case, of the blocking of the growth in the [1 00]d irection. The polymer could be effectively removed by calcination. The crystallite size obtained with the Scherrer formula for the different zinc oxide morphologies ranged from 40 to 100 nm. [67,69] Interestingly,w hile the presence of latex particles affected significantly the spectroscopic features of the materials-mainly by reducing the defect-related visible photoluminescence, [70] the long range crystalline order remained essentially undisturbed. [69]

Conclusions and Outlook
Nanodroplets in am iniemulsions ystem are able to confine spaces in whichchemical processes can takeplace. This Minireview has revisedt he advancements on the use of both miniemulsion themselves and materials prepared in miniemulsion to confinea nd control crystallization processes of metal oxides.
The different approaches for preparingmetal oxide nanoparticles from miniemulsions of metal precursors have been classified in three groups:I )"two-emulsion methods", based on the coalescence between droplets containing the metal precursor and the precipitating agents; II) externaladdition of the precipitating agent to an emulsion of the metal precursor;a nd III) emulsions in which both metal precursor and precipitating agent are simultaneously containedi nt he dispersed phase. The latter case also includes the so-called single-sourcep recursors, which are compounds that are sources of both the metal and the oxygen. Peroxo-complexes are typical examples of single-sourcep recursors.
The article has also reviewed the confinement of the formation of metal oxide nanocrystals on the surface of polymer particles, which are previously prepared by miniemulsion polymerization.N otably,t he obtained materials showp otentiality as heterogeneous catalysts and can also encapsulate further components in the core. Finally,t he last section hasd emonstrated that polymer particles prepared by miniemulsion polymerization can also be used as controlling agentsf or the formation of nanostructured metal oxides.
Among the significant advantages of the miniemulsiont echnique, we find its versatility to produce av ariety of systems of different-and even hybrid-nature, and to allow the synthesis of certain phases under milder conditions than required by other techniques (lowert emperature and lower pressure). This latter point, the focus of current investigations,i sm ainly ar esulto f the droplet confinement. However,a sw ith all techniques, miniemulsion has also limitations. One point to take into account is the limited control of shapeb yt his method. Another not negligible limitation, which can be of relevance for certain applications, especially where high purity of the inorganic phase is relevant (e.g.,e lectronic applications), is the presence of surfactant in the final material. The removal of organic matter by calcination may let behind ac arbonaceous residue, even under an oxidative atmosphere,a nd removal with organic solvents is not always completely efficient. Accordingly,i n the context of metal oxide synthesis, miniemulsion is rather suited for applications in fields in which the presence of remaining surfactant or organic matter is not decisive. Nevertheless, it needs to be mentioned that this apparent disadvantage can turn into an advantage when dispersion in an organic mediumi su seful (e.g.,i ns ome cases of heterogeneous catalysis, in cosmetics, in industrial pigments, etc.), because metal oxides prepared in miniemulsion are typically" born" in an organic solvent, so that further functionalization becomes unnecessary or can by directly implementedd uring the miniemulsion steps.
Currently,t he most promisingd irectionf or on-going and future work appearst ob et he combination of miniemulsions with other techniques, such as hydrothermal synthesis. Exposing miniemulsion droplets to pressure mayo pen new possibilities in the preparation of crystalline phases not reachable by conventional miniemulsions.