2367198x, 2022, 7, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/solr.202200132 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [18/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License RESEARCH ARTICLE www.solar-rrl.com Direct Observation of the Chemical Transformations in BiVO4 Photoanodes upon Prolonged Light-Aging Treatments Ramón Arcas, Drialys Cardenas-Morcoso, Maria Chiara Spadaro, Miguel García- Tecedor, Camilo A. Mesa,* Jordi Arbiol, Francisco Fabregat-Santiago, Sixto Giménez,* and Elena Mas-Marzá* 1. Introduction Exposing BiVO4 photoanodes to light-aging treatments is known to produce a significant photocurrent enhancement. Until now, the interpretation given to this Photoelectrocatalysis (PEC) has emerged as phenomenon is associated to the formation of oxygen vacancies and little is an attractive route to store solar energy involving photoelectro-oxidation and reported about chemical changes in the material. Herein, the chemical photoelectro-reduction processes. To date, segregation of Bi species toward the surface upon light-aging treatment is most of the PEC applications developed demonstrated, which takes place with the concomitant formation of intra- have been mainly focused either on water bandgap states associated to the oxygen vacancies. It is further demonstrated splitting to obtain molecular hydrogen[1] that these intra-bandgap states are photoactive and generate photocurrent under or on the reduction of CO2 to C1 and C2 infrared excitation. These results highlight the importance of understanding derivatives.[2] However, there is a growing interest to exploit alternative reactions to light-induced effects while employing multinary metal oxide photoelectrodes. obtain compounds with higher added value for the chemical industry.[3,4] Among the dif- ferent transformations proposed, the PEC oxidation of primary alcohols to the corresponding aldehydes or acids has attracted significant attention.[5–8] BiVO4 is a promising material for the development of PEC devices, mainly due to its suit- R. Arcas, C. A. Mesa, F. Fabregat-Santiago, S. Giménez, E. Mas-Marzá able bandgap (2.4 eV), which allows visible light absorption (up to Institute of Advanced Materials (INAM) 516 nm)[9] leading to a theoretical photocurrent of  7.5 mA cm2 Universitat Jaume I under 1 sun illumination.[10] Furthermore, its band structure 12006 Castelló, Spain E-mail: cmesa@uji.es; sjulia@uji.es; emas@uji.es (i.e., the position of the valence and conduction bands) leads to D. Cardenas-Morcoso large photovoltages for driving organic transformations.[11–13] In Material Research and Technology (MRT) Department contrast, the low electron mobility has been identified as the main Luxembourg Institute of Science and Technology (LIST) bottleneck for the PEC performance of this material.[14,15] 4422 Belvaux, Luxembourg The crystal structure of BiVO4 is composed by Bi3þ and V5þ M. C. Spadaro, J. Arbiol cations in coordination with O2.[16] During structural arrange- Catalan Institute of Nanoscience and Nanotechnology (ICN2) ment, the inherent formation of structural defects takes place CSIC and BIST 08193 Barcelona, Catalonia, Spain and their concentration can be controlled by either modifying M. García-Tecedor the synthetic conditions[17] or by post-synthetic treatments.[18,19] Unidad de Procesos Fotoactivados The most common defects in this material are oxygen vacancies Instituto IMDEA Energía (OVs), which are the result of removing O atoms from the 28935 Móstoles, Madrid, Spain lattice.[20–22] It has been shown that these OVs have a huge impact J. Arbiol on the PEC behavior of different photoanodes for the oxygen Advanced Electron Nanoscopy (GAeN) evolution reaction (OER).[23–26] Specifically, the effect of OVs ICREA 08010 Barcelona, Catalonia, Spain on BiVO4 for OER is based on the ability of these defects to increase the bulk carrier concentration and conductivity.[27] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/solr.202200132. Furthermore, an excessive density of OVs has been correlated to the decrease of performance.[28] The formation of OVs also © 2022 The Authors. Solar RRL published by Wiley-VCH GmbH. This is an creates intra-bandgap states, mainly related to V species, where open access article under the terms of the Creative Commons Attribution- NonCommercial-NoDerivs License, which permits use and distribution in the electrons are located closer to the conduction band.[29] These any medium, provided the original work is properly cited, the use is non- new electronic states lead to enhanced light absorption and have commercial and no modifications or adaptations are made. been related to the reduction of the nearby vanadium atoms DOI: 10.1002/solr.202200132 from Vov5þ to Vov4þ.[30] The attempts to directly excite these Sol. RRL 2022, 6, 2200132 2200132 (1 of 7) © 2022 The Authors. Solar RRL published by Wiley-VCH GmbH 2367198x, 2022, 7, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/solr.202200132 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [18/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.solar-rrl.com intra-bandgap states associated with oxygen vacancies in BiVO4 Supporting Information (S.I.), and were illuminated in air to elu- have been limited, and only a recent study by Selim et al. reports cidate the influence of prolonged LA treatments for the oxidation the use of a transient infrared light to modulate the electrical of benzyl alcohol. Our BiVO4 photoanodes were illuminated at response of the photoanode.[30] Consequently, infrared excitation two different light intensities (1 and 3 suns for 1LA–BiVO4 of OVs could clarify the role of these chemical defects on the PEC and 3LA–BiVO4, respectively) for 48 h, while the other films were performance of BiVO4. kept under dark conditions (Reference). All samples were tested In contrast, prolonged light-aging (LA) treatments have been using the operational conditions normally used to oxidize selec- reported to improve the PEC performance of BiVO4 toward OER. tively benzyl alcohol to benzaldehyde.[6] A 0.1 M tetrabutylammo- Trzésniewski et al. showed the enhancement of both photocur- nium perchlorate (TBAClO4) in CH3CN solution served as rent and photovoltage for undoped and uncatalyzed BiVO4 pho- nonaqueous electrolyte to avoid the competing water oxidation toanodes due to a process the authors refer as photocharging reaction. A UV filter was incorporated to the ozone-free Xe lamp (10 h under illumination at open-circuit conditions).[31] Similar to avoid the singlet oxygen generation and the non-faradaic oxi- results were reported by Li et al. after 20 h of curing under dation of benzyl alcohol, previously reported for a similar sys- ultraviolet (UV) illumination.[32] In both studies, the authors tem.[5] Figure 1 compares the photocurrent of 1LA–BiVO4 and attributed the increase in photocurrent to the reduction of sur- 3LA–BiVO4 to that for the Reference, showing a performance face recombination processes as a consequence of the decrease of enhancement after the LA treatment. Similarly, the LA treatment the defect sites at the surface.[33] The effects of the LA treatments produced films with higher photocurrents toward the OER as were also analyzed by Liu et al. using Intensity-Modulated shown in Figure S2, Supporting Information. The difference Photocurrent Spectroscopy (IMPS).[34] The authors found that in the onset potential between LA and Reference sample after 3 h of light treatment, a complex interaction between charge observed for OER and not present in alcohol (ROH) oxidation transfer and surface recombination takes place, dominating the demonstrates the higher catalytic activity of BiVO4 photoanodes suppression of surface recombination at more positive applied toward alcohol oxidation compared to OER. potentials. More recently, Feng et al. reported the beneficial To study the nature of the LA treatment on BiVO4, we first effects of a photoetching treatment, consisting of 10 min illumi- analyzed the morphology and crystalline structure of BiVO4 by nation at open-circuit conditions. This treatment revealed that scanning electron microscopy (SEM), see Figure 2a,b and S3, short illumination periods generates OVs at the surface, which Supporting Information. A clear change in the surface of the double the BiVO4 photocurrent due to the significant enhance- BiVO4 photoanodes is observed after LA. More precisely, while ment of the charge separation efficiency.[35] All these studies the Reference BiVO4 exhibits a homogenous surface (Figure 2a), demonstrated that short periods (10 min to 20 h) of LA preferen- the LA samples are characterized by the presence of several tially modify the BiVO4 surface; however, illumination periods islands (region B in Figure 2b) formed by small nanoparticles longer than 30 h have not been reported yet. surrounding the regular BiVO4 grains. Interestingly, energy dis- Few works have tried to analyze the influence of LA treatments persive X-ray spectroscopy (EDS) analysis for the different sam- on BiVO4 using in situ spectroelectrochemical measurements. ples (Figure 2c) revealed that upon LA treatment, the Particularly, Firet et al. employing a sequence of X-ray photoelec- stoichiometric Bi/V concentration ratio of the Reference tron spectroscopy (XPS), UV–vis, and X-ray raman scattering BiVO4 divides into Bi-deficient (zone A in Figure 2b) and (XRS) techniques[36] and Venugopal et al. using infrared spec- troscopy[37] revealed a dynamic nature of the light treatments on BiVO4/electrolyte interface; however, in these cases, the effect of the electrolyte could mask the true effects of light into the material during the LA process. In the present study, we report the effect of prolonged light treatments on the PEC behavior of BiVO4 photoelectrodes for the benzyl alcohol oxidation under air conditions for 48 h. We have selected this reaction as a model platform for more complex organic transformations, opening the door to more sustainable and environmentally friendly synthetic strategies. Morphological, structural, and electrical measurements allowed us to unravel the different chemical processes taking place in the material during the light treatment and correlate them with the changes observed in PEC response. Moreover, we show for the first time, to the best of our knowledge, steady-state photocur- rent generation upon continuous infrared (IR) excitation, confirm- ing the photoactivity of these light-induced electronic states. Figure 1. Cyclic voltammograms of Reference (black), 1LA–BiVO4 (red) 2. Results and Discussion and 3LA–BiVO4 (green) photoanodes measured at 20 mV s1 in 0.1 M benzyl alcohol in acetonitrile solution with 0.1 M tetrabutylammonium BiVO4 photoanodes were prepared by electrodeposition on fluo- perchlorate (TBAClO4) in the dark (solid lines) and under illumination rine doped tin oxide (FTO) substrates, as detailed in the (100 mW cm2) [dashed lines]. Sol. RRL 2022, 6, 2200132 2200132 (2 of 7) © 2022 The Authors. Solar RRL published by Wiley-VCH GmbH 2367198x, 2022, 7, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/solr.202200132 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [18/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.solar-rrl.com Figure 2. Top-view scanning electron microscope (SEM) images of a) Reference and b) 1LA–BiVO4. c) Bi/V concentration ratio for Reference and A and B zones measured by EDS in (b). Raman spectra of d) Zone A and e) Zone B. Bi-enriched domains (zone B in Figure 2b). These observations increase of Bi species on the surface of BiVO4 crystallites evidence the morphological and chemical modification of the after LA (Figure S5, Supporting Information). It is apparent BiVO4 photoanodes due to LA and are directly related to the from Figure 3 that prolonged electron-beam irradiation photocurrent increase shown in Figure 1. undergoes in a dramatic morphological transformation of our To further analyze the morphological and chemical modifica- BiVO4. Interestingly, migration of some atomic species from tions of BiVO4 photoanodes induced by LA treatment, we the bulk to form particles at the surface could be recorded during performed Raman spectroscopy on all samples in both zones the electron beam irradiation (the complete sequence can A and B in Figure 2b. As depicted in Figure 2d,e, Raman bands be visualized in the video freely available at 10.5281/ perfectly matched with the vibrational modes of BiVO4.[38,39] zenodo.5643642) and the final morphology presented the segre- Comparing the different samples, Raman bands were practically gation of a significant number of nanoparticles on top of the independent of LA in zone A. However, a significant band broad- BiVO4 grains (Figure 3c,d). ening is observed in zone B after LA, in particular, for both the To understand the nature and composition of these aggre- external and Ag modes. This has been previously attributed to the gates, we carried out a detailed high-resolution TEM (HR- formation of defects, such as oxygen vacancies or hydrogen TEM) characterization. Figure 3e shows the HR-TEM image impurities.[40,41] We note that oxygen vacancies in metal oxides of the bulk area of a BiVO4 grain. The power spectrum (fast can also be monitored by other techniques like electron paramag- Fourier transform) analysis of this area (inset in Figure 3e) con- netic resonance (EPR), [42,43] cathodoluminescence,[44] transient firms the monoclinic clinobisvanite BiVO4 phase I112/B (s.g. 15) absorption spectroscopy (TAS),[28] and spectroelectrochemis- oriented along its [011] zone axis. However, focusing the electron try.[45] In contrast, X-ray diffraction (XRD) measurements did beam on the segregated nanoparticles (Figure 3f ), the power not evidence any relevant modification of the crystalline structure spectrum analysis clearly indicates the presence of Bi species. due to LA (Figure S4, Supporting Information). The results of the power spectra analysis in different areas We move now to investigate the morphological transformation matched with two different Bi structures: metallic Bi with hex- upon LA by transmission electron microscopy (TEM). Indeed, agonal structure (R3-MH s.g. 166) oriented along [241] and compared to the homogeneous surface of the Reference [5–51] and monoclinic Bi2O3 oriented along [110] and [–114] zone BiVO4 sample (Figure 3a), 1LA-BiVO4 evidenced the develop- axis, indicating the plausible chemical composition of this seg- ment of a thin amorphous layer, as the origin of the growth regated particles (see Figure S6, Supporting Information). of small nanoparticles, as observed in Figure 3b, on the We note that the electron beam in both SEM and TEM BiVO4 crystallites, which results in a granulated surface measurements is clearly different in wavelengths and intensities (Figure 3c). Additionally, scanning TEM (STEM) combined with compared to sunlight (LA treatment). However, the morphology electron energy loss spectroscopy (STEM–EELS) proved the and presence of Bi-rich particles at the surface on the LA-treated Sol. RRL 2022, 6, 2200132 2200132 (3 of 7) © 2022 The Authors. Solar RRL published by Wiley-VCH GmbH 2367198x, 2022, 7, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/solr.202200132 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [18/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.solar-rrl.com Figure 3. Transmission electron microscope (TEM) images a,b) before and c,d) after electron-beam irradiation. a,c) correspond to the Reference and b,d) correspond to the 1LA–BiVO4, demonstrating that the BiVO4 morphology transforms during either light or electron-beam irradiation. e,f ) High- resolution TEM (HR-TEM) and power spectra images of bulk and surface BiVO4 grains, respectively, after electron-beam irradiation. sample (Figure S5, Supporting Information) suggest that the Figure 4b and S7a, Supporting Information, show the localized structural modification induced by LA is alike to that presence of two peaks at 0.35 and 0.75 V versus normal hydrogen presented after electron-beam irradiation, involving in both cases electrode (NHE), for the surface capacitance (Cs light green the segregation of Bi species. empty circles in Figure 4b) of all tested samples. The peak at Summarizing, we have shown by different spectroscopic and 0.35 V is coincident with the valley of the Rct (dark green empty microscopic techniques that LA induces OVS and a Bi segrega- circles in Figure 4b), and both features take place at the same tion process. These processes have been reported previously in potential associated to the benzyl alcohol oxidation onset (dark different studies, where BiVO4 has undergone prolonged green full cycles). A similar behavior has been already observed exposure to light, including light treatment studies at in different metal oxides, where a surface capacitance peak is open-circuit conditions and stability tests under operational observed just before the onset for OER. This behavior on hema- conditions.[18,25,31,33,35,36,46] In all cases, different morphological tite photoanodes was assigned to charge transfer controlled by and compositional processes take place at the electrode in differ- surface state charging associated to intermediate species in ent electrolytes and in different structures of BiVO4 converging the reaction process.[47] Consequently, we suggest that alcohol in a higher proportion of Bi compared to V. oxidation is controlled by a surface state, which dictates the hole Now, we turn to investigate plausible mechanistic insights of transfer process from the electrode to the substrate. We note that the effect of LA on the functional performance of the photoan- the height of this peak, associated to the Density Of electronic odes (as showed in Figure 1). To this end, we perform States (DOS) at the surface, is lower for the 1LA–BiVO4 sample Impedance Spectroscopy (IS) measurements under illumination compared to the Reference sample. A more detailed study of the to extract Nyquist plots, at different potentials, relevant for the influence of this surface capacitance on the photoelectrooxidation selective oxidation of benzyl alcohol to benzaldehyde. of benzyl alcohol remains beyond the scope of the present work. Figure 4a shows one of the experimentally obtained Nyquist plot In contrast, the broader peak at 0.75 V versus NHE either in Cs and the selected equivalent circuit (inset) used to fit the data, and Cbulk is assigned to the V4þ/V5þ redox process, which has which has been generally employed for different metal oxide previously been related to the presence of OVs in BiVO4 photoanodes.[47] This equivalent circuit is commonly used to films.[33,48,49] The presence of this peak in the reference sample separate the contribution of bulk and surface processes in suggests the intrinsic formation of these defects during the crys- the photoanodes.[33] The elements employed are Rs (series tallization step. resistance), Cbulk (bulk capacitance), Rbulk (bulk resistance), Rct Considering Cs, the DOS of these surface vancancies, g(EFn), (charge transfer resistance), and Cs (surface capacitance). The can be estimated by Equation (1) obtained capacitances and resistances as a function of the applied potential are reported in Figure S7 and S8, Supporting Information. Cs ¼ qgðE Fn Þ (1) Sol. RRL 2022, 6, 2200132 2200132 (4 of 7) © 2022 The Authors. Solar RRL published by Wiley-VCH GmbH 2367198x, 2022, 7, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/solr.202200132 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [18/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.solar-rrl.com Figure 4. a) Nyquist plot of Reference sample and selected equivalent circuit to fit the experimental data of the Reference and LA samples. b) Surface capacitance (Cs), charge transfer resistance (Rct), and photocurrent (J) of Reference BiVO4. c) Density of surface states as a function of applied potential. Reference (Black), 1LA–BiVO4 (Red) and 3LA–BiVO4 (Green). Measurements were performed under illumination, in 0.1 M benzyl alcohol oxidation conditions. where q is the elementary charge (1.6  1019 C). Figure 4c Finally, to confirm the generation of new intra-bandgap shows that the calculated DOS associated to OVs scales with electronic states associated with OVs in BiVO4, we used an IR the irradiation intensity, confirming the increase of the V4þ continuous-wave laser as light source to extract steady-state species during the LA process as mentioned previously in photocurrents for the oxidation of benzyl alcohol. Upon Raman. This result, in agreement with our Raman measure- sub-bandgap excitation, photocurrent generation can only be ments, confirms the modification of the electronic environment due to the activation of intra-bandgap transitions in the semicon- of BiVO4 during the LA. These observations are consistent with ductor material (see Figure S10, Supporting Information). This the results reported by Feng et al.[35] although, the higher con- approach, to the best of our knowledge, uses for the first-time IR centration of OVs, reflected on the bulk capacitance suggests that radiation (980 nm, 1.26 eV) to selectively excite the trapped prolonged LA affects not only the bulk but also the surface of the electrons from the intra-bandgap states to the conduction band. film. Such higher OVs concentration could also be responsible Figure 5a shows the IR-chopped chronoamperometry measure- for the slightly lower photocurrent density found for the 3LA– ments for Reference and 3LA–BiVO4 samples at 0.8 V versus BiVO4 sample in Figure 1. This is supported by the detrimental NHE. The measured photocurrent stems from the excitation role of a “too high” concentration of OVs in BiVO4 photoanodes, of the intra-bandgap states. The highest photocurrent of the as previously suggested,[23,41] being also consistent with a recent 3LA–BiVO4 sample is fully consistent with its increased density report by Corby et al. in which an excessive concentration of OVs of intra-bandgap states, associated with OVs (Figure 5b). could be negative for the PEC performance of WO3 photoano- Surprisingly, the photocurrent of the 3LA–BiVO4 sample almost des,[28] suggesting that this effect may be general for metal oxide doubled compared to that for the Reference (from 50 to photoelectrodes. Furthermore, we have performed a long LA 90 nA cm2). This is in excellent agreement with the equivalent treatment (LT) for 3 weeks under 1 sun illumination (LT– increase of the DOS (from 4 to 8  1013 cm2 eV1) associated 1LA–BiVO4, Figure S9, Supporting Information), which com- to OVs (Figure 4c). pared to 1LA–BiVO4 leads to an increased DOS of OVs, and lower We note that, although surface reduction of BiVO4[50] and the photocurrent, also in good agreement with this hypothesis. increase of surface Bi species[25,51] have been previously reported, Collectively, our observations also suggest the possibility that both effects have been characterized either after or during above certain levels, Bi segregation at the surface of the operando conditions and, consequently, they were attributed BiVO4 could impede the charge transfer of holes to the solution. to vanadium dissolution.[37] On the contrary, the LA treatments In this case, the Bi species on the surface of BiVO4 could operate and the structural characterization performed here take place in as a blocking layer for surface states as shown in Figure S7, the absence of electrolyte. Therefore, the bismuth-rich surface Supporting Information. observed is a direct consequence of the incidence of light on Sol. RRL 2022, 6, 2200132 2200132 (5 of 7) © 2022 The Authors. Solar RRL published by Wiley-VCH GmbH 2367198x, 2022, 7, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/solr.202200132 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [18/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.solar-rrl.com Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements The authors thank support from the projects ENE2017-85087-C3-1-R, PID2020-116093RB-C41, PID2020-116093RB -C43 and Network of Excellence CAT&SCALE (RED2018-102459-T), funded by MCIN/ AEI/ 10.13039/501100011033. University Jaume I (UJI-B2019-20) and Generalitat Valenciana (PROMETEO/2020/028) are also acknowledged for financial support. Serveis Centrals d’Instrumentació Científica from UJI are also acknowledged for SEM, TEM, Raman, and XRD measure- ments. M.C.S. and J.A. acknowledge funding from Generalitat de Catalunya 2017 SGR 327. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant no. SEV-2017-0706) and is funded by the CERCA Programme/Generalitat de Catalunya. M.C.S. has received funding from the European Union's Horizon 2020 research and innovation pro- gramme under the Marie Skłodowska-Curie Grant Agreement no. 754510 (PROBIST) and the Severo Ochoa programme. C.A.M. acknowl- edges the University Jaume I for the postdoc fellowship POSDOC/ 2019/20 and Generalitat Valenciana for the APOSTD/2021/251 fellowship. Dr. Beatriz Julián-López and Laura Montañés are also acknowledged for their help with the measurements with the infrared laser beam and some electrodes preparation. Figure 5. a) Infrared (IR)-chopped chronoamperometry measurements of Conflict of Interest Reference (Black) and 3LA-BiVO4 (green). b) Band diagrams representa- The authors declare no conflict of interest. tion of the fresh and light-aged-treated BiVO4 samples. Data Availability Statement BiVO4. Moreover, we demonstrate the correlation between the The data that support the findings of this study are available from the segregation of small Bi-rich nanoparticles and the formation corresponding author upon reasonable request. of new intra-bandgap states associated to oxygen vacancies. The photoactivity of these intra-bandgap states observed under IR illumination opens new perspectives for the development of competitive BiVO4 photoanodes. Keywords Bi segregation, BiVO4, light-aging treatments, oxygen vacancies Received: February 11, 2022 3. Conclusion Revised: March 7, 2022 Published online: March 20, 2022 Herein, we have investigated the effeect of prolonged light-aging (LA) on BiVO4 photoanodes, providing a direct observation of the chemical and structural modifications after LA treatments. We [1] M. Grätzel, Nature 2001, 414, 338. showed that light-assisted treatments under air conditions led [2] B. Kumar, M. Llorente, J. Froehlich, T. Dang, A. Sathrum, C. P. Kubiak, to the chemical transformation of the material, which depended Annu. Rev. Phys. Chem. 2012, 63, 541. on the light intensity used during the LA. Electrical, PEC and [3] K. Sayama, ACS Energy Lett. 2018, 3, 1093. HR-TEM characterizations allowed assigning the chemical trans- [4] C. R. Lhermitte, K. Sivula, ACS Catal. 2019, 9, 2007. formations observed to the segregation of Bi species on top of [5] R. Arcas, E. Peris, E. Mas-Marzá, F. Fabregat-Santiago, Sustain. Energy BiVO4 grains, leading to the formation of new intra-bandgap Fuels 2021, 5, 956. states associated to oxygen vacancies. Moreover, effective photo- [6] T. Li, T. Kasahara, J. He, K. E. Dettelbach, G. M. Sammis, C. P. Berlinguette, Nat. Commun. 2017, 8, 1. current generation from these intra-bandgap states was demon- [7] H. G. Cha, K. S. Choi, Nat. Chem. 2015, 7, 328. strated by IR light excitation, confirming the formation of oxygen [8] D. Liu, J. C. Liu, W. Cai, J. Ma, H. Bin Yang, H. Xiao, J. Li, Y. Xiong, vacancies during the LA treatment and their implications on the Y. Huang, B. Liu, Nat. Commun. 2019, 10, 1779. generation of IR photocurrent. These results highlight the [9] M. Long, W. Cai, H. Kisch, J. Phys. Chem. C 2008, 112, 548. importance of understanding the light-induced effects while [10] J. H. Kim, J. S. Lee, Adv. Mater. 2019, 31, 1806938. employing multinary metal oxide photoelectrodes either for [11] J. K. Cooper, S. Gul, F. M. Toma, L. 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