Understanding the light-induced oxygen vacancy in the photochemical conversion

The formation of light-induced oxygen vacancy (VO) is detected and confirmed on the surface of various metal-oxide-based semiconductors under mild reaction conditions with low cost energy source (sunlight). This self-structural transformation of the materials can bring about new characteristics and functionalities, which has inspired many researchers to explore the applications of light-induced VO in the photochemical conversion. In this perspective, generating and maintaining the light-induced VO are discussed based on some of the important work in the field of photochemical conversion. The effects and utilizations of the light-induced VO are revealed including the models proposed to explain mechanism. Then, the electric current measurements and key challenges of the light-induced VO are also summarized in a comprehensive introduction. Finally, some important aspects and questions in terms of the future research of light-induced VO are emphasized via discussing the potential contribution and development. And the schematic of future developments for light-induced VO is provided based on loop-locked materials design, light engineering and mechanism investigation.


Introduction
Metal oxides based photo-responsive materials are the key components of photocatalytic, photoelectrochemical and photo-thermochemical systems that generate useful chemicals from wastes, carbon dioxide and water by using sunlight [1][2][3][4]. The in depth research has found that the absorbers are not perfectly crystalline during photochemical conversion. The irradiation treatment could bring about self-structural transformation of metal-oxide-based semiconductors [5][6][7]. The formation of light-induced oxygen vacancy (V O ) is detected and confirmed on the surface of materials such as In 2 O 3 , perovskite and TiO 2 in many previous studies [8][9][10]. These findings may raise three relevant questions, i.e. how could the light-induced V O form on the surface? Does the formed Vo has any positive effects on the photochemical conversion? If yes, how can we measure and evaluate these effects? In recent years, the research has been focused on finding answers for these questions; the light-induced V O is being widely studied and becomes more attractive for the photochemical conversion and in the fields of nanomaterial science, catalysis and energy conversion [11][12][13].
The formation of light-induced V O , same as other techniques of V O formation, will release separate educts and leave the defective substrates behind [14][15][16][17]. However, light should be absorbed to generate electron/hole pairs (EHPs) by electron transition in the semiconductors or localized surface plasmons resonance (LSPR) in the noble metal nanoparticles (NPs) to drive the light-induced V O formation on the absorbers at room temperature [12,18,19]. Thus, the absorbers as well as the substrates should normally be metal-oxide-based semiconductors and metal oxides decorated by NPs for the light-induced structural transformation to occur, which is different from those V O formations usually driven by thermochemical and/or electrochemical processes. As a non-destructive method, light irradiation can in-situ produce V O during photochemical conversion, and compared to the formation of thermal or electric V O , light-induced V O generation could be driven by solar energy at room temperature without the need of high temperature and other energy input. Vacuum [5,8,9,12,20,21], inert atmosphere [7,10,13,[17][18][19][22][23][24][25][26][27][28][29] or reducing agents [11, could be provided to promote exclusion of oxygen and metal ions in the metal-oxide semiconductors and meanwhile, the V O would be created on the surface of metal oxides (see figures 1(a) and (b)). Since light energy is usually weaker than thermal or electric energy, it may not be able to entirely exclude metal ions from the semiconductors. Thus, in most cases, the ions from exsolution are only oxygen ions. The metal ions on the surface of metal oxides could not be totally reduced to metal. For example, Ti 4+ ions are usually reduced to Ti 3+ [13, 17-19, 24, 27].
The light-induced V O is confirmed to play a very important role in the photochemical conversion based on many studies, which is mostly used in useful fuel and chemicals production. It not only can provide a defective site as well as reaction center for adsorption and activation on the surface of photo-absorbing materials in the process of catalysis, but also can serve as a reactant for subsequent consumption in the cyclic reactions (see figures 1(c) and (d)) [8,10,12,28,33,35]. Additionally, the light-induced formation of V O brings about structural transformation, spectral and electromagnetism improvement, so it is widely used for improving performance of light-induced carriers including their lifetime, separation and mobility [21,32,37]. It is noted that this function is also utilized in surface detection research such as surfaceenhanced Raman spectroscopy [11,42,[51][52][53]. However, since this is not a photochemical conversion process, it will not be discussed here.
In this article, we will discuss some of the important works devoted to generating and maintaining the light-induced V O in the photochemical conversion. After a brief description, we will focus on some key researches which have revealed important effects of the light-induced V O on the surface of metal oxide, and present the models proposed to explain its mechanism. Then, we will summarize the measurements and current challenges of the light-induced V O . Finally, we will discuss the potential contribution and development of light-induced V O with emphasis on some important aspects and questions for future research in this field.

Generation of light-induced V O
According to the previous reports, the light-induced V O is mostly found on different kinds of metal-oxide-based semiconductors. Depending on the circumstance of its generation, it could be divided into three main approaches. Firstly, to avoid reoxidation, the vacuum system should be a natural choice for    [5]. Although it is difficult to apply vacuum in the photo-chemical process, some researchers still used vacuum to pre-treated metal-oxide-based semiconductors to generate light-induced V O on the surface for subsequent photo-chemical catalysis. For example, Zu et al fabricated the defective Bi 2 O 2 CO 3 nanosheets, in which the light-induced V O needed to be regenerated under UV light irradiation in the vacuum in every circle [12].
Secondly, inert atmosphere without reducing agent could be employed to achieve the similar effect as using vacuum. Inspired by heat-induced V O , Zhang et al extensively studied the light-induced V O in the inert atmosphere such as Ar gas [10, 13, 17-19, 22-24, 28] and have published their research outputs for CO 2 reduction and H 2 O splitting by using pure TiO 2 and various decorated TiO 2 . Since formation energy could be reduced and absorbance of solar light could be expanded from UV to visible light, doping and loading co-catalysts (e.g. Au, Pd, Cu) on the TiO 2 have been proven to be the efficient methods for improving generation of light-induced V O on the material surface used in their work. As shown in figure 2(c), Xu et al introduced a LSPR effect by loading Pd NPs on the TiO 2 [18]. Since light absorption was enhanced in vis-NIR spectrum, more available charge carriers were generated to induce more V O s on TiO 2 . Besides TiO 2 , various metal-oxide-based semiconductors were also found to be good substrates for generating light-induced V O . For instance, amorphous zinc germanate (α-Zn-Ge-O) semiconductor with weak lattice constraint was used by Wang et al to promote the formation of light-induced V O [8]; they also conducted generation and consumption of V O simultaneously to reduce CO 2 to C in the CO 2 atmosphere (see figure 2(d)).
Thirdly, the light-induced V O could be produced in the solution under some additional conditions such as biased voltages and reducing agents. These additional conditions can create reductive circumstance to promote O exsolution on the surface of metal-oxide-based semiconductors. Normally, the V O could only be maintained when additional conditions were applied. Recently, Sun et al fabricated light-induced V O on the photoanodes (e.g. BiVO 4 and WO 3 ) in a neutral solution by applying a reduced potential under AM 1.5 sunlight (see figure 2(e)). They demonstrated that light-induced V O could remarkably enhance the photoelectrochemical performance [37]. Furthermore, during some photocatalytic processes including photocatalytic degradation, dehalogenation and water treatment, it is easy for metal-oxide-based semiconductors (e.g. In 2 O 3 , BiOCl crystals) to produce light-induced V O with organic reactants such as rhodamine B under UV light irradiation [36,40,41,54]. Additionally, to promote fabrication of light-induced V O , the Raman reporter such as mercaptobenzoic acid is usually utilized for enhancing signal on the surface in the photo-induced enhanced Raman spectroscopy [51,53].
Generally, the generation of light-induced V O proceeds with the following steps [8,17,27]: in reaction (1) as seen below, the light-induced EHPs are produced as a result of illumination on the M x O y surface. In reaction (2), the photo-generated excitons are reduced on the M x O y surface, causing the conversion of M ( 2y x +) sites to M ( 2y−2 x +) sites. In reactions (3) and (4), the excited holes are trapped in the lattice O ions to produce free oxygen radicals (O · ) and generate V O on the nonstoichiometric surface. In reaction (5), the two adjacent O · ions combine to form an oxygen molecule in the vacuum and inert atmosphere. If reducing agents or protons are added, the O · ions will combine with reducing agents or protons. reaction (6) shows the overall reaction of light-induced O exsolution [from reactions (3) to (5)]. It should be noted that the A(B) means item A is adsorbed on the item B in the bracket as in the following reactions.
Obviously, the key points are the excitation and separation of EHPs, decreasing formation energy of V O and maintaining V O under the required conditions, which leads to the requirements for both photoabsorbers and circumstances during the generation of light-induced V O . In general, all of these three conditions, vacuum, inert atmosphere and solution, are available to create V O . The light-induced V O generated in vacuum and inert atmosphere is usually consumed in the cycle system since it could not be well maintained when vacuum and inert atmosphere are replaced by reactants. These circumstances are created to avoid re-oxidation which normally reduces the number of V O . Because additional conditions are usually applied to keep reduction, the generated V O could be easily created and maintained in solution and utilized in the continuous system. Researchers should create suitable circumstances in different applications to avoid re-oxidation or keep reduction. While most of works could use UV light to generate V O [27,[55][56][57], the photoabsorbers need to be further modified for using visible light. Expanding the absorbance of light could provide more available EHPs to drive V O generation. Meanwhile, recombination of EHPs has been found to be the most energy-consumption step in the light conversion [4,56,58,59]. To depress the recombination of EHPs, co-catalysts that could create electron or hole traps and prolong lifetime of carriers will play an important role [4,60,61]. But the co-catalysts could also be a recombination center [17,62], which should be further studied and need delicate adjustment in the fabrication of photoabsorbers. Based on density functional theory (DFT) calculations, Xu et al reported that both interstitial doping and replaced doping are demonstrated to be effective to decrease the formation energy of V O (see figure 2(f)), which could greatly increase the number of V O [19]. However, the easier the generation of V O is, the harder the V O is maintained and used. Thus, the balance between the difficulty of V O formation and utilization needs to be fine-tuned in the experiments. This issue is important but is often neglected by researchers. It should also be noted that the valence of metal could play an important role in the formation of V O . On one hand, because the capacity of losing and obtaining electrons could be much different between the metal ions with single oxidized valence and the metal ions with multiple oxidized valences (e.g. Zn and Ti ions), it leads to different abilities of oxygen evolution in different metal oxides. On the other hand, the valence of metal could also be important even in the same metal oxide. Generating V O is always harder in the metal oxide with low-valence metal ions than that with high-valence metal ions. For example, the formation of V O on the CeO 2 could be much easier than on the Ce 2 O 3 . Besides, Qi et al also found that the enlarged surface area allows more exposed oxygen atoms on the surface, which is favorable for the generation of V O under the irradiation of light [46]. Constructing efficient nanostructure via light engineering on the photoabsorber to achieve multiple reflection and absorption should be a promising way to increase light-induced V O formation.

The way to benefit from light-induced V O
Currently, the productions of useful fuels and chemicals such as CO 2 reduction [18,19,23], H 2 O splitting [13,22,24], nitrogen fixation [63][64][65], reverse water gas conversion [33,35,46,47] and dry reforming of methane (DRM) [25,26,47,56] are the most studied applications of the light-induced V O for the photochemical conversion. This light-induced V O formation is a unique process whereby the exsolved atoms are initial part of the support crystal lattice of semiconductors, this phenomenon forms the base for the two functionally useful ways used in the photochemical conversion. On one hand, the remaining V O on the non-stoichiometric semiconductor could be utilized as a reactant as well as consumables under a specified condition and regenerated under irradiation. As a result, a completed cycle of V O consumption and regeneration could be established. Taking CO 2 reduction and H 2 O splitting as the examples, they could be used to produce CO and H 2 by capturing the oxygen in the CO 2 and H 2 O. Then, the dissociation of CO 2 and H 2 O could be completed by forming a photochemical looping via combining reaction 7 or 8 (see figure 1). Meanwhile, the metal ion M ( 2y−2 x +) will be oxidized to M ( 2y x +) in reaction (9).
Zhang et al and Yoon et al have replaced the heat-induced V O with light-induced V O to lower the reaction temperature and improve solar-to-fuel efficiency for solar CO 2 reduction and H 2 O splitting by photo-thermochemical cycles [10,13,[17][18][19][22][23][24][25][26][27][28]. The mechanism of photo-thermochemical cycle over TiO 2 , which achieved CO 2 reduction by generating and consuming light-induced V O at a temperature of less than 500 • C, is shown in figure 3(a). As a comparison, the heat-induced V O needs a temperature of higher than 1000 • C [56]. In the CO 2 photoreduction work by Zu et al (as shown in figure 3(b)), V O s are generated under fast UV light irradiation on the Bi 2 O 2 CO 3 nanosheets in near vacuum and reproduced after every 24 h cycling test [12]. Due to light-induced V O , the formation energy of COOH * intermediate can be decreased from 1.64 to 1.13 eV, which was the rate-limiting step in the photochemical CO 2 conversion. Finally, a high CO evolution rate of 275 mmol g −1 h −1 using Bi 2 O 2 CO 3 with V O , which was 120 times higher than that without V O , was achieved for the visible-light-driven CO 2 photoreduction during 110 cycling tests. The light-induced V O could also be utilized in photoelectrochemical reactions. As shown in figure 3(c), Sun et al conducted a photocharge/discharge strategy to initiate WO 3 photoanode by generating light-induced V O [37]. There was no significant decay in more than 25 cycles with 50 h durability with the photocharged WO 3 surrounded by a 8-10 nm overlayer and V O s. It was attributed to the prolonged charge carrier lifetime caused by light-induced V O .
On the other hand, the light-induced V O could act as the activated sites for photochemical conversion on the surface [6, 30, 31, 33, 35, 38, 40-42, 44-46, 48, 66, 67]. Different from the cyclic utilization, all the steps are completed under the same condition. Taking H 2 production for example, the mechanism could be shown as follows [6,17,56,67]: According to the electronegativity of ions, H + and OH − could be generated and adsorbed on the reduced metal ions (M ( 2y x −1) ) and V O in the water ionization, respectively (reaction 10). The V O and metal ions could be the activated sites for further reactions of O 2− and H 2 (reactions 11 and 12). The O 2− could react with holes and V O could be unoccupied in the O 2 production (reaction 13). Different from the application in the photochemical looping, the light-induced V O will not be consumed, but act as an activation center in the photocatalysis. Zhang et al found that light-induced V O could be sustainable on the BiOBr nanoflower [30]. And reaction rates were found to be a greatly increased which was attributed to the V O -induced improvement for transfer efficiency of photoinduced carriers and the light absorption capacity in photocatalytic water splitting. In addition, Qi et al fabricated a large quantity of dual-functional light-induced V O on the 2D In 2 O 3 nanostructures, where local heat was more efficiently produced to achieve a high photothermal conversion and provide active sites for the photothermal CO 2 reduction reaction simultaneously (see figures 3(d) and (e)) [46]. A great CO production rate of 103.21 mmol g cat  light-induced V O on CeO 2 by photoinduced electrons, which could avoid deactivation to stabilize the thermochemical DRM process. It has also been found that CO disproportionation as the major side-reaction could be restrained with the light-induced V O , therefore the rate of carbon deposition was decreased during the process of DRM. Interestingly, Lee et al generated light-induced V O in 2D BaSnO 3 epitaxial films to reversibly control photocurrent responsivity and persistent photoconductivity (see figure 3(h)). It indicates that light-induced V O could be applied to develop even 2D optoelectronic devices by optically controllable manipulation of surface defect states, which should be another interesting application for light-induced V O [9]. Additionally, the stability of light-induced V O needs to be considered in two cases according to its application. The first one is the application of V O consumption for reduction reaction, which requires that oxygen vacancies could be steadily decreased and easily participate in the reduction reaction. In the second case, the environment where the oxygen vacancies perform catalytic reactions mostly exists in the presence of oxygen, so how to maintain V O is challenging. Especially, light-induced V O s are mostly formed on the surface, which are less stable than those in the bulk. Some modification methods have been employed, such as nitrogen doping and nanostructure design to stabilize valance state of metal ions to keep V O s stable [68,69]. Besides, starting from the reaction circumstances, utilizing some reaction conditions to maintain a reducing environment, such as reducing gas, sacrificial agent and biased voltages, can also maintain oxygen vacancies. Both ways of utilizing light-induced V O have been demonstrated to be efficient in improving the photochemical conversion. The production and utilization of light-induced V O are simple, precise, nondestructive and easy to be expanded. It is foreseeable that more and more photochemical conversions including solar-driven water splitting, CO 2 reduction, synthesis gas conversion, seawater desalinization and organic pollutant disposal could benefit from the activation and catalysis of light-induced V O .

Measurements of light-induced V O
Revealing the relationship between the light-induced V O formation and activities is very important for designing high-performance defective catalysts. Therefore, measurements and evaluations of light-induced V O need to be carefully handled by using various characterizations. In that perspective, the measurements could be generally divided into electron microscopy techniques and spectroscopy techniques.
The atomic structure of imaging materials could be shown by electron microscopy technology directly. Due to the low resolution, some normal microscopy techniques, including scanning electron microscopy and transmission electron microscopy, are not able to accurately measure V O [33,41,43,54]. As mentioned before, STM could directly observe and demonstrate the V O formation (see figures 1(a) and (b)) [5]. And scanning transmission electron microscopy (STEM) which owns a resolution of atomic level is a very useful tool for observing the V O on the surface of materials [9,15,40,54,63,65,70]. Light-induced V O was well confirmed by Qi et al in the magnified image obtained by an annular bright-field scanning transmission electron microscopy (ABF-STEM), which could show the atom arrangements of the 2D In 2 O 3 − x layers (see figure 4(a)) [46]. Furthermore, as shown in figure 4(b), Lee et al found that it was better to use the low-angle annular dark-field (LAADF) signal of STEM, which was more sensitive to the strain field from point defects (i.e. V O ), than the high-angle annular dark field signal to measure light-induced V O [9]. It shows that LAADF-STEM is a good direct way to observe light-induced V O optically. Besides, Dagdeviren et al employed a time-resolved atomic force microscopy with fast-detection electronics and high-frequency cantilevers to demonstrate the effect of the light-induced surface V O on charge carrier dynamics (see figure 4(c)) [21]. They measured the resonance frequency shift (∆f 0 ) of the oscillating cantilever to assess the time-dependent variation in the tip-sample interaction force, which could further obtain the effective activation energy related to the migration of holes, E a * , and the migration barrier for a single hole motion, E a . The E a in dark and that under light could be measured and detected to uncover the change of hole migration dynamics by this method. It has been proven to be effective to study the deep mechanism in the formation of light-induced V O .
In regards to the x-ray spectroscopy techniques, resolution of x-ray diffraction is usually too low to detect light-induced V O , but x-ray photoelectron spectroscopy (XPS) [12, 13, 22-24, 30-34, 36-42, 71-73] and x-ray absorption spectroscopy (XAS) [1,2,24,27,33,46,50,54,60,62,63,65,71,74] are effective. XAS, including x-ray absorption near-edge structure spectroscopy and extended x-ray absorption fine structure, could assess oxidation state in bulk and average coordination environment of elements in materials. As shown in figure 4(d), Qi et al studied the In K-edge of In 2 O 3 − x nanosheets and standard In 2 O 3 and found the existence of lower average oxidation states of In species in In 2 O 3 − x nanosheets [46]. According to results in the R space (see figure 4(e)), the light-induced V O led to the decrease of the In-O distance, which caused a split between 1 and 2 Å of In 2 O 3 − x nanosheets. XAS could show ample information of the defects in bulk, however, the light-induced V O is normally generated on the surface only. Thus, use of XPS gains more popularity than XAS in the study of light-induced V O , since the former only detects the surface defects. Especially, time-resolved in-situ XPS could be more powerful to clarify the formation and function of light-induced V O . As shown in figure 4(f), the process of light-induced V O formation could be easily observed in Lee et al's study [9]. To reveal the interaction between light-induced V O and metal ions, Zu et al employed a synchrotron-radiation quasi in-situ XPS to study the Bi ions near the V O in figures 4(g) and (h) [12]. Their XPS results indicated that the Bi 3+ sites could easily receive the photoexcited electrons with the presence of light-induced V O and then adsorb CO 2 molecules, i.e. not only CO 2 was efficiently activated, the partially reduced Bi sites were returned into the original Bi 3+ sites as well.
As an effective method for investigating unpaired electrons in materials, electron paramagnetic resonance (EPR) is also often used for qualitative and quantitative characterizations of light-induced V O   [13, 24, 30, 32, 34, 35, 37-39, 41, 42, 71, 75]. As shown in figure [4, 9, 11, 22-24, 33, 36-38, 41, 43, 44, 46, 48-51, 54, 56]. Nonetheless, the accuracy of results highly depends on the V O concentration, materials property and conditions of measurements, making these methods not very universal. For example, the changes in the results of UV-vis DRS when producing light-induced V O on the TiO 2 in Ar gas were much smaller than those on the α-Zn-Ge-O in vacuum (figures 4(j) and (k)) [8,13]. In addition, the production in the V O formation (such as O 2 ) could also be analyzed by using gas chromatography and traced by mass spectrum with isotopic gas [13,24,27,35,60,63,67]. Firstly, in order to enable light-induced V O formation, the semiconductor support should meet two conditions: (a) the bandgap of semiconductor should be suitable to generate enough EHPs, (b) the formation energy of V O should be low enough to be driven by EHPs. In the previous studies, many approaches have been taken to adjusting the bandgap and V O formation energy with some promising results, including doping with transition metal ions and loading noble metals [13, 18, 22-24, 51, 56, 76, 77]. However, more efficient semiconductor supports should be developed and fabricated to produce more light-induced V O . Besides, many studies have shown that the formation of V O could also be the recombination of EHPs [9,21,30,39,56]. Although it is important and should be well adjusted in the fabrication, no detailed information is available and no discussion and research have been conducted yet.
Secondly, it is difficult to evaluate the performance of light-induced V O without the accurate information of the numbers of V O . However, on the one hand, due to the formation of light-induced V O occurs on the surface of metal oxide and its concentration is quite low in terms of the whole material, making it difficult to detect and measure it accurately. On the other hand, since the light-induced V O is usually not stable in the atmosphere and tends to be transient during the photochemical conversion, critical conditions should be applied to be able to detect and evaluate it [39,54,56]. Therefore, in-situ techniques may need to be combined with above measuring techniques to make a detailed and real-time evaluation of light-induced V O during photochemical conversion. In-situ tests combined with STM, STEM, EPR and XPS could be employed to observe the light-induced V O formation directly. However, the very strict requirements of these tests are hardly to be practical, and the complete quantitative V O measurements also remain challenging.
Thirdly, although the mechanism of the effects of light-induced V O has been extensively investigated and many possible reasons have been proposed, there is lack of a clear picture in regards to the contributions of each effect, e.g. the synergetic effect and the effect of interaction between V O and reduced metal ions in the process of photochemical conversion. Additionally, in addition to the number of exsolved sites, the qualities of V O and metal ions are also important since some low-quality V O may not be the activated sites and would not participate in the reaction. It is worth noting that some recent calculation work has noticed this point. The DFT calculations of Li et al indicate that the balance between the formation and reactivity of light-induced V O is the key factor for the pathway preference for water splitting, which should be taken into consideration for catalyst design and screening [77,78]. Currently, there is still no reported experimental work focusing on this. The three issues mentioned above could greatly affect the final efficiency of reactions which would dictate the future development and potential applications of the light-induced V O for the photochemical conversion.

Future perspectives
According to the particular requirements of light-induced V O , the exciting light is the driven force but, despite its importance, remains much less studied so far. The light-induced V O could be greatly enhanced by increasing the power density of incident light. Furthermore, if the photons with different frequencies could be applied to the processes of light-induced V O formation and the subsequent photochemical conversions, the whole light energy will be fully utilized. To adjust the wavelength and power density of the incident light, specific light equipment including mirrors, beam splitter and filters needs to be designed based on the latest techniques developed in field of advanced optic devices manufacture.
To fabricate suitable semiconductors, the bandgap and V O formation energy are critical since they are the preconditions for absorbing light energy and generating exsolution, respectively. The ideal bandgap and V O formation energy of semiconductors could be obtained by calculating the density of states and Gibbs free energy based on DFT before the experiments (see figure 2(f)), but the screening speed and range are low and narrow. The recent developments in the field of machine learning (ML) have further broadened the range of theoretical calculations to efficiently screen suitable semiconductor supports [79]. The much larger screening range of ML can greatly narrow down the ranges of DFT calculations and experimental verifications, thus forming an effective iterative looping to design efficient semiconductor supports. The ML applicable fabrication methods include doping, loading and compositing materials.
Besides light energy absorbance and final conversion, the energy transfer (including carrier's mobility rate and transfer direction) is also important to reduce the energy dissipation in the bulk of semiconductors, but it is difficult to control and study this process due to its very short time span (usually from nanoseconds to microseconds). The mobility of energy carrier could be slowed down at ultra-low temperature, thus to enlarge the time scale and magnify the signals of energy transfer by combining specially-made exsolution cells, spectroscopy instruments and cryogenic equipment in the light-induced V O formation. Accordingly, the mechanism of energy transfer could be clarified to guide the design of semiconductors. In addition, some ingenious methods based on the optic characterizations of certain materials could be applied to control and smooth the energy transfer process. For example, the light-induced hot electrons generated by the LSPR of some special metal particles (e.g. Ag, Cu, Pd…) could exceed the Schottky barrier (see figure 2(c)), in which the direction of energy transfer could be reversed to promote the exsolution [4,18,56].
In terms of the measurement of V O , a range of material spectroscopy techniques, especially the techniques at synchrotron, would be highly desirable. The customized in-situ cell could be designed to cooperate with the XPS, diffraction, absorption spectroscopy. Because the irradiation of synchrotron is not in the absorbance range of semiconductor, these spectroscopy techniques would not affect the generation of light-induced V O . Thus, time-resolved, even space-resolved measurements could be achieved to study the generation, disappearance and distribution of V O and exsolved sites in the semiconductor supports. Besides the in-situ techniques of materials structure, the in-situ tests for measuring chemical groups could be conducted to investigate the mechanism of the effects of light-induced V O on the photochemical conversion. In-situ Fourier transform infrared spectroscopy and EPR are among the good choices [12,35,36,44,50,60]. Different from the measurements of materials structure, the intermediate chemical groups are easily to be affected by the conditions of measurements. Therefore, a stable and precise in-situ reaction cell is more important in this regard. Lastly, the theoretical calculations could also be employed as a powerful tool in support of the in-situ measurement results and provide deeper insights into the reaction mechanism [76][77][78]80].
Additionally, there are a lot of other methods for V O formation in the metal oxide including hydrogenation, hydride reaction, mixed gas reduction, high-energy particle reaction, metal reaction, organic reactant, electrochemical reduction and so on [68,69]. Although these approaches are different from light-induced V O in the step of V O formation, the applications of V O are similar, which could be well studied to inspire researchers to expand the utilization of light-induced V O . Light-induced V O is a unique method for photochemical conversion and serves as an alternative way for material design, catalysis and energy conversion. With the mild reaction conditions, low cost energy source (sunlight) and the benefits of photo reactions, the light-induced V O is suitable for a wide range of applications and shows a great potential in the photochemical conversion. Figure 5 shows the schematic of potential future developments for light-induced V O . To tackle the current challenges of (a) suitable semiconductor support, (b) the measurement of V O , (b) the mechanism of the effects of light-induced V O , the novel research approaches and creative ways of light engineering, materials design, coupled with the advanced theoretical calculations and in-situ spectroscopy techniques, would bring breakthroughs in the field of light-induced V O for the photochemical conversion in the foreseeable future.

Data availability statement
No new data were created or analyzed in this study.