Photochemistry in the Low‐Temperature Processing of Metal Oxide Thin Films by Solution Methods

Abstract Photochemistry has emerged in the last few years as a powerful tool for the low‐temperature processing of metal oxide thin films prepared by solution methods. Today, its implementation into the fabrication procedure makes possible the integration of amorphous semiconductors or functional crystalline oxides into flexible electronic systems at temperatures below 350 °C. In this review, the effects of UV irradiation at the different stages of the chemical solution deposition of metal oxide thin films are presented. These stages include from the synthesis of the precursor solution to the formation of the amorphous metal‐oxygen network in the film and its subsequent crystallization into the oxide phase. Photochemical reactions that can be induced in both the solution deposited layer and the irradiation atmosphere are first described, highlighting the role of the potential reactive chemical species formed in the system under irradiation, such as free radicals or oxidizing compounds. Then, the photochemical effects of continuous UV light on the film are shown, focusing on the decomposition of the metal precursors, the condensation and densification of the metal‐oxygen network, and the nucleation and growth of the crystalline oxide. All these processes are demonstrated to advance the formation and crystallization of the metal oxide thin film to an earlier stage, which is ultimately translated into a lower temperature range of fabrication. The reduced energy consumption of the process upon decreasing the processing temperature, and the prospect of using light instead of heat in the synthesis of inorganic materials, make photochemistry as a promising technique for a sustainable future ever more needed in our life.


Introduction
Photochemistry is defined as the branch of chemistry concerned with the chemical effects of light (ultraviolet, visible, or infrared radiation). Since light is actually af orm of radiant energy,acompound can be promoted from its ground state to an excited state of energy upon light absorption. This may produce severalc hemical (also physical) processes according to the Grotthuss-Draper law,t hat is, the first law of photochemistry.T hus, the excited-state energy can induce photochemicalr eactions such as elimination, cleavage, rearrangement, isomerization,c yclization,a ddition, or electron transfer. [1] Such reactions constitute the basis of organic photochemistry, which contributed to accelerate the synthetic chemistry methodology as it is known today. [2] Organicp hotochemistry enables unique pathways fort he synthesis of compounds not thermodynamically favored nor easily accessible by other methods. [3] However,d uring the last few years photochemistry has gained attention in the field of inorganic chemistry,p articularly in the low-temperature processing of metal oxide thin films. [4][5][6][7] To understand how this discipline has evolved up to reaching the former point,w em ust first consider the eruption of flexi-ble electronics at the beginningo ft his century (2000s). [8] This emerging technology, often referred to as the next ubiquitous platform of our lives, involves the fabrication of large-area electronic devices (e.g.,v isual displays, solar cells, smart textiles, or electronic skin) integrated on lightweight and low-costf lexible systemsb ased on plastic, rubber,o re ven paper. [9] Polymer materials sucha sp olyimide( PI), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polycarbonate (PC) are the most widely used substrates by this technology.H owever, their relatively low thermals tability( thermald egradation or glass transition severalh undreds of degrees Celsius below for example, rigid glass or silicon wafers) drastically limits the temperaturea tw hich the functional layer deposited onto them can be furthera nnealed-usually not beyond 350 8C. Due to their low processing temperatures and "soft" nature, organic materials (carbon based polymers or organic molecules) were soon considered the ideal candidate materials for their growth on this type of substrates giving rise to the so-calledo rganic electronics. In spite of the higher performance and stability of their inorganic (mostly oxide based) counterparts, the relatively high processing temperatures of mostm etal oxide layers prevented their direct integration into flexible electronic systems at first instance.
In 2004, high-performancet hin film transistors (TFTs) were demonstrated on amorphous InGaZnO (a-IGZO) semiconductor oxides. [10] The studys uggested that high field effect mobility and high-level uniformity could be in principle obtained in metal oxide semiconductors processed at low temperatures despite reaching an amorphous state in the material. This significantly boosted the research on amorphous semiconductors for applicationsi nf lexible electronics, since processing temperatures compatible with the polymer substrates could now be applied. Due to their outstanding advantages such as relatively low-cost,scalability and high deposition rate, solution methods were preferred over vacuum techniques to grow metal oxides on flexible substrates. However,s olution-processed layers require critical processing steps to removet he organic species from the system and to promote the formation of ad efect-free, highly densified metal-oxygen network. Thermala nnealing is in most cases insufficientd ue to the temperature constraintsi mposedb yt he polymer substrate, and complementary strategies to induce the formation of the metal oxide in the film are therefore needed. [6] Figure 1s hows the number of publications on low-temperature processing of metal oxide thin films reported in the last ten years, together with those of them where photochemistry is addressed to induce either the formationo rt he crystallization of the metal oxide.A lthough the number of papers on this last topic is still moderate, there is an exponentially growing interesto nt he use of light irradiation to attain metal oxide thin films at low temperatures.T he pioneer works on amorphousm etal oxide semiconductors (2012) and multifunctional crystalline metal oxides (2014) directly grown on flexible plastic by photochemical methods will probablyp ave the way fort he large number of contributions to this field that are yet to come. [11,12] Therefore, we foresee an encouraging prospectf or this techniquei nt he coming years for the fabrication of next-generation flexible systems based on new electronic metal oxide materials. [13] The reduced energy consumption of the process upon decreasing the fabrication temperature, together with the potential sustainability of using light as an alternative energy source instead of thermal heating, make photochemical methods one of the basic alternatives to be considered today for ar eally green synthesis of materials. [14,15] But, how can photochemistry aid to the low-temperature processing of solution-derived metal oxide thin films?T he answer to this question is basically by breakingc hemical bonds and facilitating the formation of new ones. Energy is always required to break ab ond in am olecule, whichi s known as bond energy.I fc hemical bonds are cleaved by light, the decomposition of metal precursorsi nt he system would be favored without the need of thermala nnealing. If molecules are cleaved into smaller molecules or individual atoms, new entitiess uch as reactive oxygen species (e.g.,r adicals) could also be formed upon (photo)chemical reaction. These species can furthera ssist to the elimination of organic residualsf rom the system, besides promoting condensation and densification of the amorphousm etal-oxygen network. If subatomic parti- M. Lourdes Calzada is Full Professor at the Institute of Materials Science of Madrid (ICMM-CSIC). Her projectsa re focused on the "Development of low-temperature sol-gel synthesis strategies to attain metal oxidem aterials" and on the "Integrationo ff unctional thin films with semiconductor and flexible substrates (Si-technology and Flexible Electronics)". Her group is pioneer in the "Low-temperature solution processing of ferroelectric and multiferroic complex oxides for flexible electronicd evices". She has supervised six Ph.D Theses, published 190 papersa nd presented 15 invited talks in International Conferences, 10 of which in the field of the low-temperature processing of films. cles are generated by light irradiation (e.g,. photogenerated electrons or holes), the acceleration of photoreactions would be induced by photocatalysis, or even the crystallization rate of the amorphous system could be increased.
This review presents the photochemical effects of continuous light irradiation at different stages of the chemical solution deposition process of metal oxide thin films. The most recent advances showing how photochemistry can induce the formation of ad efect-free, highly densified metal-oxygen network first, and its crystallization into the final metal oxide phase afterwardsw ill be described. Although equally valid in the context of low-temperature crystallization of metal oxide layers, studies on localized heatingb yt he use of pulsed laser irradiation have not been included due to the absence of ac hemical reactioni nt he system.T he primary aim of this review is to provide the fundamental insights of photochemistry that, ultimately,d rive the low-temperature crystallization of solutionprocessedm etal oxide thin films.

Photochemical Reactions Induced by Irradiation with UV Light
Penetration of UV light is limited to the surface region of condensed matter,t ypically below 200 nm. Hence, it can be a powerful tool to induce chemical and physical changes in thin film materials. [16] Irradiation with intense UV light such as that used by pulsedl asers (coherent UV radiation) mainly produces the increaseo ft he local temperature at the film surface, that is, a thermal excitation. In contrast, electronic excitation is dominant when using continuous irradiation lamps (incoherent UV radiation).D espite these lamps having much lower power than lasers, they can irradiateo ver large areas under controlled atmospheres thus minimizing thermal excitation and enhancing chemicalr eactions that are the basis of photochemistry. Figure 2s hows the emission wavelengths of commercial UV sourcesa nd the type of chemical bonds that can be excited. To day,w ec an find commercial excimer lamps with wavelengthsi nt he range of chemical bond energies of many functional groups. [17] However,p hotochemical reactions induced by UV irradiation of solution-deposited layers can take place in two different media:d irectly in the thin film and in the irradiation atmosphere surroundingt he film.

Photochemical reactions in the thin film
Solution (sol-gel)d erived metal oxide layers can absorb the UV light emitted by the aforementioned excimer lamps because they contain molecules that are excited by the energetic photons coming from theses ources. [18][19][20] These photons penetrate into the layer and are consumedb yt he cleavage of chemical bonds and/or by charge transfer within/between the molecules, and among ions. Therefore, the most common photoreactions that can be produced by UV-irradiationi nt he as-depositedfilmare photolysis and photoinduced charge transfer.
Breaking or cleavage of chemical bonds by UV irradiation (generally referredt oa sp hotolysis, photodissociation or photodecomposition) easily occurs in solution-derived layers, since they are actually formed by organic compounds based on metal alkoxides (metal complexes containing organic ligands) and ionic compounds such as organic or inorganic metal salts. Typical bonds presentinthe former molecular species are summarized in Table 1. It is clear that the energy supplied by excimer UV lamps ( Figure 2) is sufficient to produce the dissociation of thesec hemical bonds. Furthermore, the resultingb yproducts can induces ubsequent reactions depending on the chemicaln ature of the photolyzed compounds.P hotolysis of organic compounds with long carbon chains results in compounds with shorter carbon chains that would be easily removed from the film by pyrolysis at moderate temperatures. Additionally,d uring the decompositiono fs ome metal acetates (the most used organic salts in these systems), [21] the photolysis of intermediate carbonate compounds can result in the formation of free carbonate radicals (CO 3 C À )that would assist in the elimination of organic speciesf rom the film due to their strong oxidative character [Eq. (1)]: [22] Metal nitratesa re typical inorganic salts used in the synthesis of precursor solutions of metal oxides, either by alkoxide or aqueous routes. [23] Photolysis of nitrate ions is the mechanism whereby nitrate compounds are decomposed under UV irradiation, which is developed throught he photoisomerization of nitrate (NO 3 À )t op eroxonitrate (ONOO À )i ons followed by the formation of reactive NO 2 Cand OHC radicals after the photodecomposition of the protonated form of the peroxonitratei on (ONOOH)[ Eq. (2) and (3)]: NO 2 C and OHC radicals are known to be highly reactive to hydrogen capturef ollowedb yasubsequent MÀOH activation for the efficient polycondensation among the metal precursors leadingt oametal-oxygen network in the film with ah igh degree of condensation and densification. [24] Besides chemical bond cleavage (photolysis), excited species generated in the film upon light absorptionc an also transfer their energy to ground state species by different processes. One of them involves the photoinduced charget ransfer from activatedo xygen species (e.g.,o xygen with photogenerated electrons)t om etal cations (M x + ), thus inducing the photochemicalr eduction (photoreduction)o ft he metallic center. [25,26] The reduced metal atomsc an migrate in the amorphous matrix and re-oxidize with the reactive oxygen species, forming the crystalline oxide (M 2 O x ).Another process is based on the photoinduced charge transfer developed in somem etal complexes. [27] Irradiation of these compounds with UV light may induce intra-and inter-molecular processes based on the shift of the electronic distribution in the molecule after light absorption. These charge transfer transitions may result in different photoreactionst hat have been of great usefulness in the synthesis of inorganic materials and also in the low-temperatureprocessing of crystallinem etal oxide thin films. [28][29][30]

Photochemical reactions in the irradiation atmosphere
Photochemical reactions are also produced in the gas atmosphere where the thin film sample is subjected to UV irradiation. Here, the primary process is based on the photolysis of gas molecules into reactive chemical species (mainly,f ree radicals) usually followed by secondary reactions. These may involve the attack of free radicals to the molecules present in the gas atmosphere formingn ew radicals and new molecular species.A ll these chemically reactive compounds can react with the thin film materialand contribute to accelerate its crystallization by different mechanisms.
The most conventionally used atmospheresi nt he UV irradiation of metal oxide thin films are pure oxygen (O 2 ), air (O 2 ,N 2 and other minor gases) and inert argon (Ar), where dissociation of oxygen and nitrogen typicallyo ccurs under UV light.
Photolysis of oxygen produces free oxygen radicals (OC)t hat can react with molecular oxygen forming ozone (O 3 ), whose photodissociation yields oxygen and OC radicals again[ Eq. (4-6)]: The strong oxidant character of ozone would enhancet he decomposition of organic compounds presenti nt he film by ozonolysis (oxidation), whereas the presenceo fO C radicals would compensate for the charge defects of the crystal lattice, thus improving the oxide stoichiometry.A dditionally,w hen water vapor (H 2 O) is also present in an oxygen atmosphere, the reactionb etween OC radicals and aqueous vapor can result in the formation of free hydroxyl radicals (OHC)[ Eq. (7)]: These highly reactive OHC species would facilitatet he hydrolysis and condensation of the metal-oxygen network, and also contributet ot he formation of hydroxyl anions (OH À )t hat can promote the incipientc rystallization of metal oxide thin films at low temperatures. [31] On the other hand, photolysis of nitrogen resultsini ts dissociationi nto free nitrogen radicals(NC)that can react with OC radicals (if oxygen is present in the irradiation atmosphere) beginning new cycles of events [Eq. (8-10)]: Summarizing, energetic photons coming from UV lamps can excite many types of molecules present both in the thin film and in the irradiation atmosphere.T his electronic excitation can induce molecular dissociations, formation of reactive species or structural changes in the metal-oxygen network. All these phenomena push the chemicals ystem far from equilibrium, making possible processes that are not achieved by means of conventionalthermal treatments.

Photochemical Solution Deposition of Metal Oxide Thin Films
The first publications reporting the preparation of crystalline metal oxide thin films by chemical solution deposition (CSD) date from the 1980s. [32,33] Since then, many functional oxide thin films are fabricated by this techniqued ue to particular advantages such as low investment costs, large surface coating, homogeneity and high throughput fabrication. [34] The method consists first in the synthesis of as table precursor solutiont hat is deposited on as ubstrate by ac oating technique (e.g.,s pincoating,d ip-coating,spray-coating). Organicc ompounds in the resultingl ayer are then eliminated from the system by thermal treatment (e.g.,e vaporation, thermolysis, pyrolysis), leading to the decomposition of the respective metal precursors. During this stage, condensation among the metal reagents and densification of the amorphous metal-oxygen network obtained are gradually developed in the thin film. Crystallizationofthe functional metal oxide layer is finally carried out by annealing at relativelyh igh temperatures (conventionally between 600-700 8C). However,t he versatility of the CSD method has made possible along the years the implementation of complementa-ry techniques in the process, such as the irradiation of the system with UV lamps (see Figure 3). Born in the 2000s, the photochemical solution deposition (PCSD)m ethod is originally based on the electronic excitation of the photoactive species present in sol-gel thin film materials upon the irradiation with light of adequate energy. [16,35] This can be used for different processes in solution derived films such as patterning, reduction, condensation, densification, or crystallization. However,i n the last decadet he PCSD method has gained significant relevance in the low-temperature processing of metal oxide thin films. This is because photochemical reactions induced in the system by light makes possible to attain an appreciable reduction in the processing temperature of the film. In this section, the effects of UV irradiation at each of the different stages (I-IV) of the thin-film fabrication process by PCSD will be presented. The key processes responsible for promoting both the formationo fa na morphous metal-oxygen network andt he further crystallization of the metal oxide thin film down to a lower temperature range will be described.

Photoinduced synthesis of low-temperature liquid precursors
Most of the works reporting the low-temperature processing of metal oxide thin films by ap hotochemical methoda re actually focusedo nt he effects induced by UV light after the irradiationo ft he corresponding solution-derived layers. [4,6,7] However,f ew years ago (2015) it was shown that low-temperature precursors of metal oxidesc an be directly synthesized in liquid media after the irradiation of the respective precursor solutions containing photocatalytic nanoparticles. [36] Once these nanoparticlesa re removed from the solution afterwards, the resulting low-temperature liquid precursors yield metal oxide thin films with al ower crystallization temperature comparedt ot he films derived from the original (initial) solution followingt he standards teps of aC SD process (deposition, pyrolysis, and crystallization). The proposed concept was developed merging two rather distant fields in materials science;h eterogeneous photocatalysis and the low-temperature processing of metal oxide thin films. The interest in the first scientific discipline accelerateds ignificantly in 1972, when the phenomenon of photocatalytic splitting of water was discovered. [37] Sincet hen, titanium dioxide (TiO 2 )f eatures predominantly the work on semiconductor photocatalysis with applications addressing many environmental and pollution challenges. [38] Particularly,h undreds of organic compounds are readily photodegradedt oday by TiO 2 photocatalysis (e.g.,p ollutants, bacteria, tumor cells, etc.).Thus, the novel application of this effect to precursor solutions of metal oxidesd emonstrated that this photocatalysis assisted method led to the partial decompositiono ft he organic moieties typically constituento fm etal precursors and subsequent polycondensation among metal precursorsb ya na dvancedo xidation process carriedo ut at room temperature in liquid media. The investigation was conducted on precursor solutionso ff erroelectric Pb(Zr 0.3 Ti 0.7 )O 3 and multiferroic BiFeO 3 oxides, to which nanoparticles of TiO 2 were introduced and the resultings uspensionsi lluminated with as olarl amp (Osram Ultra-Vitalux 300 W) to induce the photocatalytic effect. Figure 4s hows the evolution with photocatalysis time of the integrated areas calculated in two representative absorption bands measured by Fourier-transform infrared spectroscopy (FTIR) at 1734 cm À1 [n(C=O)] and 368 cm À1 [n(MÀO)].The intensity decrease of the first mode is associated with the partial decomposition of organic compounds-in this case, acetatesfrom the system (Figure 4a), whereas the substantial rise in the intensity of the second mode accountsf or the formation of a metal-oxygen network that resultsf rom the polycondensation among the metal precursors (Figure 4b).
The photocatalytic activity of the TiO 2 nanoparticles was also investigated in aq uasi-static scenario such as av iscoelastic solid supported on as ubstrate. Thus, xerogel layersd eposited from as uspension of Pb(Zr 0.3 Ti 0.7 )O 3 containing TiO 2 nanoparticles weres ubjected to UV irradiationf or 1hat room temperature. Figure 5s hows the corresponding micrographs obtained by scanning electron microscopy (SEM) in non-irradiated (Figure 5a)a nd irradiated (Figure 5b)r egions of the sample, together with the respective EDS (energy dispersive spectroscopy) analyses. In general, the surface morphology reveals the presence of agglomerated TiO 2 particles( typically % 0.1 mmi n diameter) within ah omogeneous matrix of amorphous matter. To quantify the photo-oxidation of the residual organic species, the relative amounto fc arbon surrounding aT iO 2 agglomerate (expressed as Ct oT ir atio relative to the size of the agglomerate) wasr egistered in both irradiated and non-irradiated regions of the sample. An appreciable decreaseo f% 45 %i so btained in the former that accounts for the advanced oxidation of organic speciesc lose to the semiconductor surface upon the absorption of UV light. The relatively high degree of decomposition and subsequentp olycondensation of metal precursors reached in the solution by this photocatalysis-assisted process developed at room temperature would be the main feature of this system with respect to the non-photocatalyzed one. Lower crystallization temperatures are induced in the metal oxide thin films prepared from these low-temperature liquid precursors (350 and 325 8Cf or the Pb(Zr 0.3 Ti 0.7 )O 3 and BiFeO 3 perovskite systems, respectively) in contrast to the amorphous structures obtained using the same processing conditions for the counterpart films derived from the respective initial solutions ( Figure 6).

Photoinduced decomposition of metalp recursors
Precursor solutionsc ontaining photosensitives peciesm ay lead to photoactivated thin films once deposited on as ubstrate and subjected to irradiation with light of adequate energy.T o this, metallicc enters must react first with organicm olecules forming photosensitive coordinationc omplexest hat are stable in the solution.Irradiation of these metal complexes in the corresponding gel layers results in intra-and inter-molecular processesu pon light absorption involving different electronic transitions such as metal centered (MC), ligand to metal charget ransfer (LMCT), metal to ligand charge transfer (MLCT), and intra-ligand or ligand centered (LC). [27] Figure 7s hows the molecular orbitald iagram for at ransition metal complex with the different electronic transitions arisen from light absorption, together with the chemical structure of some photosensitive metal complexes and their ultraviolet-visible (UV/Vis) spectra measured in solution. [29,35] The excited states induced in the metal complexes would be responsible for their chemical cleavage into smaller molecular units,a ccording to ac hemical process generally knowna sp hotolysis.A si tw as explained in previousS ection2,c omplementary to the former process the reactive oxygen species generated in the system under UV irradiation (atomico xygen, ozone,r adicals,e tc.) can also assist the decomposition of molecular species into smallg aseous molecules by rapid radical-mediated reactions.
The removal of water molecules and nitrate ligandsi nt hin films derived from ap recursors olution of Al 2 O 3 as af unction of the exposure time to UV irradiation at 150 8Ccan be inferred from the FTIR spectra of Figure 8. [24] Concerning the broad absorptionb ands at % 3500 cm À1 [n(OÀH)] and % 1650 cm À1 [d(H-O-H)] ascribed to water (Figure 8a); note how these rapidly decrease in the photoactivated film after irradiationf or 5min reachings imilar values to those measured in af ilm thermally annealed at 350 8Cf or 1h.I nt he case of the absorptionb ands centered at % 1400 cm À1 and % 1430 cm À1 [n(NÀH)] that corre-spond to nitrate ligands (Figure 8a), they practically disappear after the same exposure time to UV irradiation( 5min). In addition, time-of-flight secondary ion mass spectrometry (TOF-SIMS) 3D mapping of residual carbon in these films (Figure 8b) revealed that the content of this elementw as dramatically reduced in the entire film thickness upon photoactivation, whereas as ubstantial amount of carbon was still confined inside the thermally annealed film without photoactivation. Both results reported in this work confirm that deep ultraviolet (DUV) irradiation effectively decomposes the nitrate ligands and solvent moleculesp resenti nt he as-depositedf ilms into smaller and diffusible molecules due to direct photodecomposition (photolysis) or complementary radical-assisted photoreactions.I namore recent study (2020), [39] ap hotosensitive metal complex formed between either bismuth or iron with Nmethydiethanolamine (MDEA) has been used to induce the crystallization of BiFeO 3 perovskite thin films at relatively low temperatures (325-350 8C) by UV irradiation. Figure 9d epicts the evolution with irradiation time of the integrated areas corresponding to the CÀHv ibration bands measured by FTIR spectroscopy in the respective as-deposited Bi-MDEA and Fe-MDEA thin films annealed at 150 8C. The photochemical cleavage of the CÀHb ond presenti no rganic compounds such as alkanesc an be easily deduced from the clear decrease observed in the calculateda reas. This result supports the effect of UV light on the decomposition of the organic species present in the system (alkanes). Note that when the film is not photoactivated, the content of organic compounds remains practically constant.

Photoinduced condensation and densification of amorphousmetal-oxygen network
The application of UV light to improve the condensation among the metal precursors and the densification of the resulting metal-oxygen network probablyh ad its origins in the early studies of mesoporous thin film materials based on SiO 2 . Such materialsa re typically formed using as elf-assembled organic phaset hat templates the formation of inorganic silica by at emplated sol-gel synthesis process.T his method requires the selective removal of the organic phase (surfactant)f rom the mesostructured thin film, which is usually accomplished by calcination at relativelyh igh temperature ( 450 8C). To avoid the drawbacks associatedw ith this step, such as the collapse of the mesostructured framework or the damage of atemperature-sensitive metal substrate, an ominally room-temperature photochemical methodw as originally proposed. [40] It consists of aU V/ozone treatment on the thin film that effectively removes the organic template phase besides strengthening the silicate skeleton through increased silica condensation. This last effect is demonstrated in Figure 10, where grazing incidence FTIR spectra are shown for an as-deposited templated nanocomposite (Brij56/TEOS) thin film subjected to UV/ozone treatment with al ow-pressurem ercury lamp (l = 184-257 nm). [41] The increase in the intensity of the Si-O-Sim ode upon irradiation (Figure 10 a) would be related to the formation of more metal-oxygenbonds within the inorganic network by promoting the condensation among silanol groups.N ote how the intensity of this mode gradually increases with the time of exposure (Figure 10 b), thus confirming the silica condensation induced by this process at room temperature.
Whereas the initial studies on mesoporous silica thin films shownb efore were mainly focusedo np reserving the structural morphology and surfacec haracteristics of the material, the significant enhancemento fp olycondensation upon UV irradiation drew the attention of the semiconductor community a few years ago. As stated in the Introduction section, the performance of amorphous metal oxide semiconductors is strongly dependento nt he formationo fad efect-free,h ighlyd ensified metal-oxygen network in the thin film. [10] Irradiation with UV light would not only promote the condensation among metal precursors, but it would also enable the use of flexible polymerics ubstrates due to the lowt emperature associated with the film processing (near room-temperature). High-resolu-   Figure 11 shows the Gaussian curve fits of the O1ss ignal measured by XPS in solution-processed Al 2 O 3 thin films treated under different conditions. [24] In all cases the O1ss ignal is very broad, which suggestsm ultiple oxygen environments in the system resultingi nd ifferent chemical shifts. Thus, the oxygen peak can be fitted to as uperposition of two Gaussian components with positions centered around5 31.0 and 532.3 eV that reflect two different oxygen environments ascribed to M-O-M and MÀ OH species, respectively.T he results clearly show that the film dried at 100 8Cf or 5min without UV irradiation (withoutD UV) containsa na ppreciable concentration of MÀOH speciesv ery similar to that observed in the as-deposited film (as spun). In contrast, the filmc ounterpart subjected to UV irradiation (with DUV) denotes am uch larger contribution from the M-O-M signal, showing anX PS profilep ractically identical to at hermally annealed film at 350 8C( high temperature). From this analysisi tc an be easily deduced that the condensation among the metal precursors in the as-depositedl ayer is significantly enhanced upon DUV irradiation.
Besidesp romoting the condensation among the metal precursors,f urtheri rradiation of the metal-oxygen network can also improvet he densification of the metal oxide thin film. This effect occurs in ap rocess parallel to that of polycondensation, with both process developing gradually and concomitantly with increasing the exposure to UV light. Figure 12 shows the thickness measured in severalA l 2 O 3 thin films annealed at differentt emperatures with and withoutU Vi rradiation. [24] Thus, DUV exposure reduced the final thickness of oxide films annealeda t1 50 8Cf rom 16 nm to 11 nm (non-irradiated). This result is confirmed by many other works, [42][43][44] suggestingt hat irradiation with UV light inducest he tight packingo ft he initially coarse M-O-M network leadingt od ense films with comparatively lowt hicknesses. As it was described in the previous section, exposure to UV light enhances the removal of organic speciesf rom the systemi nalow-temperature regime that results in the more uniform drying of the as-depositedf ilm with respectt ot hermal treatments at highert emperatures (e.g.,p yrolysis). Here, the contraction of the gel network after solvent evaporation (due to capillaryt ension)c ritically determines the final pore volume in the thin film. Densification of the metaloxygen network can be therefore promotedb yf urtherc ondensation reactions among the chemical precursors that are broughtn ow into closer contact after ac ontrolled drying process inducedb yU Vl ight.I na ddition to this structurale ffect, photogenerated speciess uch as atomico xygen can also react with suboxides ando xygen vacancies (V O CC)p resent in the thin film thus improving the metal oxide stoichiometry and decreasingt he density of crystal defects by compensation. [45]

Photoinduced nucleation and growth of crystalline metal oxides
Whereas the effects of UV irradiation on the low-temperature processing of metal oxide thin films are primarily focused on the stages of the PCSD process described in the previous sections (decomposition of the metal precursors and condensation-densification of the metal-oxygen network), as mall number of recent studies have demonstrated the validity of this approach to crystallize dissimilari norganic materials such as zeolites or metal oxide nanostructures. In spiteo ft heir different composition or morphology,m etal oxide thin films could be affected by the same mechanisms for promoting the nucleation and growth events once the following approaches were optimized and extrapolated to the corresponding metal oxide layers.
In 2016, it was reported that hydroxyl free radicals (OHC)generated by UV irradiation can accelerate the crystallization of zeolites( microporous crystalline aluminosilicates) under hydro-thermalconditions. [46] Such radicals are originated upon irradiation of water in the media, and these speciesw ould be responsible for accelerating the kinetics of the nucleation stage.  Figure 13. Whereas 60 hours were required to obtain ah ighly crystalline zeolite under dark conditions, this time was significantly reduced to 32, 20, and 16 hours for irradiances of 2.0, 4.0, and 8.0 mW cm À2 ,r espectively.C onsidering am echanism for zeolite crystallization based in two steps (depolymerization of the gel by breakingS i-O-Si bonds and subsequent formationo fn ew Si-O-Si bonds), theoretical calculations yield lower activation energies for both processes when OHC radicals (instead of only OHhydroxyl groups)a re present in the media. Particularly,a n enhanced positive effect of OHC againstO H À was observed in the dissociation of the Si-O-Si bonds (4 versus 29 kcal mol À1 ) and formation of new Si-O-Si bonds (8 versus 17 kcal mol À1 )when using the mono-deprotonated [Si(OH) 2 -O-Si(OH) 3 ]Nam odel of the gel. Although not explored yet, the proposed mechanism for zeolite crystallization could be potentially transferred to metal oxide thin films prepared under similar conditions (e.g.,h ydrothermals ynthesis) after ap roperi mplementation.
The photoinduceds ynthesis of nanostructured metal oxides has been reported in the last few years using energy-efficient processes at room temperature. For example, crystallization of TiO 2 anatasef rom amorphousTiO 2 can be significantly accelerated by storing in the material photogenerated electrons from UV irradiation. [26] As uspensiono fa morphous TiO 2 powder in Figure 11. Deconvoluted O1sX PS spectrao fAlOx thin films prepared under different conditions: A) the as-spun film, afilm treated at 100 8Cf or 5min B) without and C) with DUV irradiation,a nd D) af ilm thermally annealed at 350 8Cf or 60 min. Reprinted with permission from ref. [24].Copyright 2015 Wiley-VCH. aqueous methanol, which works as as acrificial agent to capture the photogenerated holes, was subjected to UV light for 30 min generating abundant electrons on the surface of the titania powders.A fter maintaining the photoactivated suspension for severald ays, as olid product corresponding to crystalline TiO 2 anatase was collected after drying and washing of the corresponding dispersion. Figure14s hows the X-ray diffraction (XRD) patterns of different TiO 2 samples obtained by this method, together with as cheme depicting the process. Note that amorphousT iO 2 requires 80 days to convert into crystalline TiO 2 anatasea tr oom temperature (Figure 14 b), whereas this time is abruptly reduced down to 2days after storing photogenerated electrons in the precursor (photoactivated suspension). The underlying mechanism is explained in the following terms (Figure 14 a): (I) first, the storageo fe lectrons reduces some Ti 4 + to Ti 3 + weakening the TiÀOb ond between titania and the residual solvent( ethylene glycol), which is easily removed from the TiO 6 octahedra leaving hydroxyl groups exposed on the titania surface;( II) then,aprotonc an be attached to the surface oxygen after the combination with an electron stored in the neighboring Ti 3 + .T hese protonated surfaces would easily interactw ith the hydroxyl groups of other TiO 6 octahedra releasing aH 2 Om olecule and forming ab ridging oxygen bond (Ti-O-Ti) between the two neighboring octahedra.
Very recently (2019), irradiation with UV light has also been demonstrated to promote the crystallization of a-Bi 2 O 3 nanotubes from amorphous bismuth hydroxide at room temperature. [25] Ultraviolet treatments were conducted on an amorphous bismuth hydroxide precursor dispersed in aqueous media with irradiation times of 1h (Xe lamp, 420 nm cutoff filter). Figure 15 shows how the XRD pattern of the product remains amorphous under dark conditions (Figure 15 a), whereas under UV conditions using light irradiances of 50 and 200 mW yields a-Bi 2 O 3 nanotubes and bulk a-Bi 2 O 3 accordingt ot heir respective TEM images (Figure 15 b). In this work, UV irradia-tion can not only induce the crystallization of bismuth oxide at room temperature, buti ta lso controls the morphology of a-Bi 2 O 3 by means of the intensity of the UV light employed. Similar to the previous example on the crystallization of TiO 2 anatase, the electronic excitation of the BiÀOb ond present in the amorphous bismuth hydroxide( whereby ap hotoinduced charget ransfer of electrons from oxygen to bismuth is produced)w ould facilitate the formation of Bi-O-Bi bridges after the elimination of aw ater molecule. Thus, UV irradiation promotest he dehydration and condensation of amorphous bismuth hydroxide to crystalline a-Bi 2 O 3 . Figure 13. Crystallization curves of az eolite without and with UV irradiation under different irradiance conditions. Reprintedw ith permission from ref. [46].Copyright 2016 ScienceMag. Figure 14. a) Schematicrepresentation of the accelerated room-temperature crystallizationf rom amorphous to anatase TiO 2 .b)Crystallization time of TiO 2 after 80 days (grey) andafter storinge lectronsf or 2days (red). Inset showst he corresponding XRD patterns of the respective TiO 2 -80d and TiO 2 (e À )-2dsamples. Reprinted with permission from ref. [26].Copyright 2017 The RoyalS ocietyofC hemistry.

Summary and Outlook
In this review,weh ave shown the chemical effects of light irradiation (mainly,ultraviolet) at the different stages of the chemical solution deposition of metal oxide thin films. Illumination with solar or UV lamps can be carried out in both the precursor solutionand the solution deposited layer.This makes possible to induce photochemical reactions in the chemical system (solution,t hin film and atmosphere)leading to 1) the synthesis of low-temperature liquid precursors, 2) an enhanced decompositiono fm etal precursors and elimination of organic residuals, 3) ah ighd egree of condensation andd ensification of the metal-oxygen network, and 4) ap rompt nucleation and growth of the crystalline oxide phase. The formation of a close-packedm etal-oxygen network in the amorphous film and its subsequentcrystallization would be therefore advanced to al ower temperature range(< 350 8C), enabling its directi ntegrationa sa na ctive layer into high-performancef lexible electronic systems besides opening the door to aplethora of applications for metal oxide thin films today hindered by their relatively high crystallization temperatures. In addition to this technological advantage, the reduction of the processing temperature would also provide environmental and economic benefits derived from the reduction of the energy consumption of the whole manufacturing process.
The types of chemical reactions induced by light in organic compounds are well describedi nt he literature, [2] in contrastt o those that can be produced in inorganic materials. This is because the range of photochemical reactions is scarcer in the latter systems. Actually,i rradiation with UV light began to be used in the fabrication of solution-derived metal oxide thin films due to the large amount of organic species present in the as-deposited layers.S ince light penetration in solids is limited to af ew hundred nanometers, reactions induced by light are alwaysc onfined to the surfaceo ft he material. This phenomenology therefore entails ag reat challenge as well as an opportunity for using photochemistry in the low-temperature processing of solutiond erived metal oxide thin films. Thus, the absorption of light in the as-depositedf ilm results in photochemicalr eactions such as photolysis and photoinduced charge transfer,a nd the generation of reactive species (e.g., active radicals)t hat overall lead to the cleavage of chemical bonds, condensation and densification of the system and even the formation of the first crystal nuclei. The extent of the crystallization throughout the film needs however the rearrangement of the metal and oxygen atoms. This involves diffusion processes that are typicallys low in solids and require very long times to attain full crystallization in the film using only photochemistry.C onsequently,asmalla mount of energy has to be supplied to the system that is usually provided by ag entle heatinga tt emperatures wellb elow the values conventionally required in the absence of any photochemical assisted method. The prospect of as ustainable fabrication process for metal oxide thin films with practically null heating demands may look distantt oday,b ut we believe that the shorter road to it goes unavoidably through photochemistry.
To conclude,w ith this review we aim to convey the great potentialo fp hotochemistry for the low-temperature processing of metal oxide thin films by solution methods. Photochemical methods open novel pathways to synthesize thermodynamically disfavored chemical species and to overcome energy barriers otherwise not accessible with low thermalb udgets. This can push the chemical system far from its equilibrium making possible the fabrication of metal oxide thin film at temperatures well below the valuest ypically applied by conventional thermaltreatments.