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Water oxidation is the key kinetic bottleneck of photoelectrochemical devices for fuel synthesis. Despite advances in the identification of intermediates, elucidating the catalytic mechanism of this multi-redox reaction on metal–oxide photoanodes remains a significant experimental and theoretical challenge. Here, we report an experimental analysis of water oxidation kinetics on four widely studied metal oxides, focusing particularly on haematite. We observe that haematite is able to access a reaction mechanism that is third order in surface-hole density, which is assigned to equilibration between three surface holes and M(OH)–O–M(OH) sites. This reaction exhibits low activation energy (Ea ≈ 60 meV). Density functional theory is used to determine the energetics of charge accumulation and O–O bond formation on a model haematite (110) surface. The proposed mechanism shows parallels with the function of the oxygen evolving complex of photosystem II, and provides new insights into the mechanism of heterogeneous water oxidation on a metal oxide surface.

The exploration of photoanode materials with high efficiency and stability is the eternal pursuit for the realization of practically solar-driven photoelectrochemical (PEC) water splitting. Here we develop a deficient ternary metal sulfide (CdIn 2 S 4 ) photoanode, and its PEC performance is significantly enhanced by introducing surface sulfur vacancies, achieving a photocurrent density of 5.73 mA cm −2 at 1.23 V vs. RHE and 1 Sun with an applied bias photon-to-current efficiency of 2.49% at 0.477 V vs. RHE. The experimental characterizations and theoretical calculations highlight the enhanced effect of surface sulfur vacancies on the interfacial charge separation and transfer kinetics, which also demonstrate the restrained surface states distribution and the transformation of active sites after introducing surface sulfur vacancies. This work may inspire more excellent work on developing sulfide-based photoanodes. While water splitting could provide a means to utilize solar energy, identifying sufficiently stable and active semiconductors is challenging. Here, authors report a deficient CdIn 2 S 4 photoanode with improved water splitting performances due to efficient charge separation/transfer kinetics.

Developing low-cost and highly efficient catalysts toward the efficient oxygen evolution reaction (OER) is highly desirable for photoelectrochemical (PEC) water splitting. Herein, we demonstrated that N-incorporation could efficiently activate NiFeO x catalysts for significantly enhancing the oxygen evolution activity and stability of BiVO 4 photoanodes, and the photocurrent density has been achieved up to 6.4 mA cm −2 at 1.23 V (vs. reversible hydrogen electrode (RHE), AM 1.5 G). Systematic studies indicate that the partial substitution of O sites in NiFeO x catalysts by low electronegative N atoms enriched the electron densities in both Fe and Ni sites. The electron-enriched Ni sites conversely donated electrons to V sites of BiVO 4 for restraining V 5+ dissolution and improving the PEC stability, while the enhanced hole-attracting ability of Fe sites significantly promotes the oxygen-evolution activity. This work provides a promising strategy for optimizing OER catalysts to construct highly efficient and stable PEC water splitting devices. While solar-to-fuel conversion offers a renewable energy production strategy, photoelectrochemical water splitting requires further optimization. Here, authors examined nitrogen-doped NiFeO x catalysts to improve the oxygen evolution activity and stability of BiVO 4 photoanodes.

Understanding the function of surface states on photoanodes is crucial for unraveling the underlying reaction mechanisms of water oxidation. For hematite photoanodes, only one type of surface states with higher oxidative energy (S1) has been proposed and verified as reaction intermediate, while the other surface state located at lower potentials (S2) was assigned to inactive or recombination sites. Through employing rate law analyses and systematical (photo)electrochemical characterizations, here we show that S2 is an active reaction intermediate for water oxidation as well. Furthermore, we demonstrate that the reaction kinetics and dynamic interactions of both S1 and S2 depend significantly on operational parameters, such as illumination intensity, nature of the electrolyte, and applied potential. These insights into the individual reaction kinetics and the interplay of both surface states are decisive for designing efficient photoanodes. Detailed understanding of the reaction kinetics of the surface states of hematite photoanodes is crucial to improving their water oxidation performance. Here the authors reveal two dynamically interacting states which play a key part in the structure properties of these anodes.

Light absorption in strongly correlated electron materials can excite electrons and holes into a variety of different states. Some of these excitations yield mobile charge carriers, whereas others result in localized states that cannot contribute to photocurrent. The photogeneration yield spectrum, ξ ( λ ), represents the wavelength-dependent ratio between the contributing absorption that ultimately generates mobile charge carriers and the overall absorption. Despite being a vital material property, it is not trivial to characterize. Here, we present an empirical method to extract ξ ( λ ) through optical and external quantum efficiency measurements of ultrathin films. We applied this method to haematite photoanodes for water photo-oxidation, and observed that it is self-consistent for different illumination conditions and applied potentials. We found agreement between the extracted ξ ( λ ) spectrum and the photoconductivity spectrum measured by time-resolved microwave conductivity. These measurements revealed that mobile charge carrier generation increases with increasing energy across haematite’s absorption spectrum. Low-energy non-contributing absorption fundamentally limits the photoconversion efficiency of haematite photoanodes and provides an upper limit to the achievable photocurrent that is substantially lower than that predicted based solely on absorption above the bandgap. We extended our analysis to TiO 2 and BiVO 4 photoanodes, demonstrating the broader utility of the method for determining ξ ( λ ). Although the photogeneration yield spectrum is a key property for photoabsorbers in photovoltaic and photoelectrochemical cells, its characterization remains challenging. An empirical method to extract this parameter through quantum efficiency measurements of ultrathin films is proposed.

Addressing the intrinsic charge transport limitation of metal oxides has been of significance for pursuing viable PEC water splitting photoelectrodes. Growing a photoelectrode with conductive nanoobjects embedded in the matrix is promising for enhanced charge transport but remains a challenge technically. We herein show a strategy of embedding laser generated nanocrystals in BiVO 4 photoanode matrix, which achieves photocurrent densities of up to 5.15 mA cm −2 at 1.23 V RHE (from original 4.01 mA cm −2 ) for a single photoanode configuration, and 6.22 mA cm −2 at 1.23 V RHE for a dual configuration. The enhanced performance by such embedding is found universal owing to the typical features of laser synthesis and processing of colloids (LSPC) for producing ligand free nanocrystals in desired solvents. This study provides an alternative to address the slow bulk charge transport that bothers most metal oxides, and thus is significant for boosting their PEC water splitting performance. While photoelectrochemical water splitting offers a low-cost, integrated means to generate fuel from light, poor charge carrier transport limits performances. Here, authors embed laser-synthesized colloids in bismuth vanadate photoanodes to boost charge carrier mobilities and enhance photocurrents.

In this study, the potential of zinc oxide (ZnO), tungsten oxide (WO3), and their composites (ZnO–WO3) as photoanodes for photoelectrochemical (PEC) water splitting was investigated. ZnO–WO3 nanocomposites (NCs) were deposited on fluorine-doped tin oxide substrates at room temperature using a one-step dry coating process, the nanoparticle deposition system, with no post-processes. Different compositions of ZnO–WO3 NCs were optimized to enhance the kinetics of the PEC water-splitting reaction. Surface morphology analysis revealed the transformation of microsized particle nanosheets (NS) powder into nanosized particle nanosheets (NS) across all photoanodes. The optical characteristics of ZnO–WO3 photoanodes were scrutinized using diffuse reflectance and photoluminescence emission spectroscopy. Of all the hybrid photoanodes tested, the photoanode containing 10 wt.% WO3 exhibited the lowest bandgap of 3.20 eV and the lowest emission intensity, indicating an enhanced separation of photogenerated carriers and solar energy capture. The photoelectrochemical results showed a 10% increase in the photocurrent with increasing WO3 content in ZnO–WO3 NCs, which is attributed to improved charge transfer kinetics and carrier segregation. The maximum photocurrent for a NC, i.e., 10 wt.% WO3, was recorded at 0.133 mA·cm−2 at 1.23V vs. a reversible hydrogen electrode (RHE). The observed improvement in photocurrent was nearly 22 times higher than pure WO3 nanosheets and 7.3 times more than that of pure ZnO nanosheets, indicating the composition-dependence of PEC performance, where the synergy requirement strongly relies on utilizing the optimal ZnO–WO3 ratio in the hybrid NCs.

Dihydroxyacetone is the most desired product in glycerol oxidation reaction because of its highest added value and large market demand among all possible oxidation products. However, selectively oxidative secondary hydroxyl groups of glycerol for highly efficient dihydroxyacetone production still poses a challenge. In this study, we engineer the surface of BiVO 4 by introducing bismuth-rich domains and oxygen vacancies (Bi-rich BiVO 4-x ) to systematically modulate the surface adsorption of secondary hydroxyl groups and enhance photo-induced charge separation for photoelectrochemical glycerol oxidation into dihydroxyacetone conversion. As a result, the Bi-rich BiVO 4-x increases the glycerol oxidation photocurrent density of BiVO 4 from 1.42 to 4.26 mA cm −2 at 1.23 V vs. reversible hydrogen electrode under AM 1.5 G illumination, as well as the dihydroxyacetone selectivity from 54.0% to 80.3%, finally achieving a dihydroxyacetone production rate of 361.9 mmol m −2 h −1 that outperforms all reported values. The surface atom customization opens a way to regulate the solar-driven organic transformation pathway toward a carbon chain-balanced product. Photoelectrochemical oxidation of glycerol to produce dihydroxyacetone is limited by its low selectivity. Here, a bismuth vanadate photoanode enriched with a bismuth-rich surface and containing oxygen vacancies was utilized to overcome this dilemma.

Interface engineering is a proven strategy to improve the efficiency of thin film semiconductor based solar energy conversion devices. Ta 3 N 5 thin film photoanode is a promising candidate for photoelectrochemical (PEC) water splitting. Yet, a concerted effort to engineer both the bottom and top interfaces of Ta 3 N 5 thin film photoanode is still lacking. Here, we employ n-type In:GaN and p-type Mg:GaN to modify the bottom and top interfaces of Ta 3 N 5 thin film photoanode, respectively. The obtained In:GaN/Ta 3 N 5 /Mg:GaN heterojunction photoanode shows enhanced bulk carrier separation capability and better injection efficiency at photoanode/electrolyte interface, which lead to a record-high applied bias photon-to-current efficiency of 3.46% for Ta 3 N 5 -based photoanode. Furthermore, the roles of the In:GaN and Mg:GaN layers are distinguished through mechanistic studies. While the In:GaN layer contributes mainly to the enhanced bulk charge separation efficiency, the Mg:GaN layer improves the surface charge inject efficiency. This work demonstrates the crucial role of proper interface engineering for thin film-based photoanode in achieving efficient PEC water splitting. Solar-to-fuel energy conversion requires well-designed materials properties to ensure favorable charge carrier movement. Here, authors employ interface engineering of Ta 3 N 5 thin film to enhance bulk carrier separation and interface carrier injection to improve the water-splitting efficiency.

Photoelectrochemical cells are emerging as powerful tools for organic synthesis. However, they have rarely been explored for C–H halogenation to produce organic halides of industrial and medicinal importance. Here we report a photoelectrocatalytic strategy for C–H halogenation using an oxygen-vacancy-rich TiO 2 photoanode with NaX (X=Cl − , Br − , I − ). Under illumination, the photogenerated holes in TiO 2 oxidize the halide ions to corresponding radicals or X 2 , which then react with the substrates to yield organic halides. The PEC C–H halogenation strategy exhibits broad substrate scope, including arenes, heteroarenes, nonpolar cycloalkanes, and aliphatic hydrocarbons. Experimental and theoretical data reveal that the oxygen vacancy on TiO 2 facilitates the photo-induced carriers separation efficiency and more importantly, promotes halide ions adsorption with intermediary strength and hence increases the activity. Moreover, we designed a self-powered PEC system and directly utilised seawater as both the electrolyte and chloride ions source, attaining chlorocyclohexane productivity of 412 µmol h −1 coupled with H 2 productivity of 9.2 mL h −1 , thus achieving a promising way to use solar for upcycling halogen in ocean resource into valuable organic halides. Photoelectrochemical cells are promising tools for C–H functionalisation coupled with H2 production. In this work, Duan et. al., reported the photoelectrocatalytic C–H halogenation to produce organic halides of industrial and medicinal importance with promoted H 2 production.