Photoelectrochemical Water Oxidation Research Articles

Tailored BiVO4 Photoanode Hydrophobic Microenvironment Enables Water Oxidative H2O2 Accumulation

Direct photoelectrochemical 2-electron water oxidation to renewable H2 O2 production on an anode increases the value of solar water splitting. BiVO4 has a theoretical thermodynamic activity trend toward highly selective water oxidation H2 O2 formation, but the challenges of competing 4-electron O2 evolution and H2 O2 decomposition reaction need to overcome. The influence of surface microenvironment has never been considered as a possible activity loss factor in the BiVO4 -based system. Herein, it is theoretically and experimentally demonstrated that the situ confined O2 , where coating BiVO4 with hydrophobic polymers, can regulate the thermodynamic activity aiming for water oxidation H2 O2 . Also, the hydrophobicity is responsible for the H2 O2 production and decomposition process kinetically. Therefore, after the addition of hydrophobic polytetrafluoroethylene on BiVO4 surface, it achieves an average Faradaic efficiency (FE) of 81.6% in a wide applied bias region (0.6-2.1V vs RHE) with the best FE of 85%, which is 4-time higher than BiVO4 photoanode. The accumulated H2 O2 concentration can reach 150µm at 1.23V versus RHE under AM 1.5 illumination in 2h. This concept of modifying the catalyst surface microenvironment via stable polymers provides a new approach to tune the multiple-electrons competitive reactions in aqueous solution.

Photosensitization Efficiency Modulation of Atomically Precise Silver Nanoclusters for Photoelectrocatalysis.

Atomically precise metal nanoclusters (NCs) have emerged as feasible alternatives to traditional photosensitizers in solar energy conversion due to the unique atomic stacking mode, quantum size effect, and abundant active sites. Despite the sporadic advancement in fabricating metal NC-based photosystems, most of which are predominantly centered on Au NCs, unleashing atomically precise silver nanoclusters as light-harvesting antennas has still been in the infant stage, with the charge transfer mechanism remaining elusive. Herein, we comprehensively demonstrate the photosensitization effect of Ag NCs in the photoelectrochemical (PEC) water-splitting reaction and strictly evaluate the correlation of photosensitization efficiency with atomic architecture. To these ends, tailor-made negatively charged l-glutathione (GSH)-capped Ag NCs [Agx, Ag9(GSH)6, Ag16(GSH)9, Ag31(GSH)19] as building blocks are controllably deposited on the metal oxide (MOs = TiO2, WO3, Fe2O3) substrate by a facile self-assembly strategy. Benefiting from the highly efficient photosensitization effect of atomically precise Ag NCs, these self-assembled MOs/Ag NC heterostructured photoanodes with an elegant charge transfer interface demonstrate significantly enhanced photoelectrochemical water oxidation performances under visible-light irradiation on account of efficient charge transport from Ag NCs to the MO substrate, substantially prolonging the charge lifetime of Ag NCs. Our work would significantly inspire ongoing interest in unlocking the generic photosensitization capability of atomically precise metal NCs for solar energy conversion.

NdCo3 Molecular Catalyst Coupled with a BiVO4 Photoanode for Photoelectrochemical Water Splitting

Photoelectrochemical water splitting is a promising strategy for harvesting and converting solar energy to green hydrogen energy. However, the current inferior performance restricts further improvement of the solar-to-hydrogen efficiency. In this work, a molecular catalyst [NdCo3(btp-3H)2(Ac)2(NO3)2] (NO3)·2H2O (referred to as NdCo3 herein) was deposited onto a porous BiVO4 photoanode using a drop-casting method, and the molecular catalyst was held in place on the BiVO4 surface via intermolecular forces. The photoelectrochemical water oxidation performance of the BiVO4/NdCo3 photoanode reached 2.25 mA cm–2 at 1.23 V vs RHE under AM 1.5G illumination (100 mW cm–2), which was much higher than the pristine BiVO4 photoanode (1.49 mA cm–2). The enhanced performance could be attributed to the improvement of the charge carrier transfer efficiency, resulting in the acceleration of the water oxidation kinetics and inhibiting charge carrier recombination. In addition, the electrocatalytic properties of the homogeneous system were also studied. It was found that a heterogeneous catalytic film was formed due to the water solubility of NdCo3, which enabled a long electrolysis process to be maintained. The electrocatalytic performance of a homogeneous system reached 1 mA cm–2 at 2.31 V vs RHE and was different from the heterogeneous catalytic film (reached 1 mA cm–2 at 2.10 V vs RHE). This integrated system showed that the combination of a molecular catalyst with a photoelectrode helped to promote charge-carrier transport and separation, reducing the amount of charge-carrier recombination. Our approach may be applicable to other materials, helping to provide ideas for developing material combinations capable of achieving greater solar-to-hydrogen efficiencies.

Controllable Synthesis of N2-Intercalated WO3 Nanorod Photoanode Harvesting a Wide Range of Visible Light for Photoelectrochemical Water Oxidation.

A highly efficient visible-light-driven photoanode, N2-intercalated tungsten trioxide (WO3) nanorod, has been controllably synthesized by using the dual role of hydrazine (N2H4), which functioned simultaneously as a structure directing agent and as a nitrogen source for N2 intercalation. The SEM results indicated that the controllable formation of WO3 nanorod by changing the amount of N2H4. The β values of lattice parameters of the monoclinic phase and the lattice volume changed significantly with the nW: nN2H4 ratio. This is consistent with the addition of N2H4 dependence of the N content, clarifying the intercalation of N2 in the WO3 lattice. The UV-visible diffuse reflectance spectra (DRS) of N2-intercalated exhibited a significant redshift in the absorption edge with new shoulders appearing at 470-600 nm, which became more intense as the nW:nN2H4 ratio increased from 1:1.2 and then decreased up to 1:5 through the maximum at 1:2.5. This addition of N2H4 dependence is consistent with the case of the N contents. This suggests that N2 intercalating into the WO3 lattice is responsible for the considerable red shift in the absorption edge, with a new shoulder appearing at 470-600 nm owing to formation of an intra-bandgap above the VB edges and a dopant energy level below the CB of WO3. The N2 intercalated WO3 photoanode generated a photoanodic current under visible light irradiation below 530 nm due to the photoelectrochemical (PEC) water oxidation, compared with pure WO3 doing so below 470 nm. The high incident photon-to-current conversion efficiency (IPCE) of the WO3-2.5 photoanode is due to efficient electron transport through the WO3 nanorod film.

ZnO/BiOI heterojunction photoanodes with enhanced photoelectrochemical water oxidation activity

ZnO/BiOI heterojunction photoanode thin films were prepared by aerosol-assisted chemical vapour deposition, and the impact of growth temperature and film thickness on the water oxidation functionality was systematically investigated. A top ZnO layer with a thickness of 120 nm (deposited at 350 °C) and a 390 nm thick BiOI layer (deposited at 300 °C) were found to achieve the best photoelectrochemical performance of the heterojunction. The ZnO/BiOI heterojunction exhibited a significant increase in photoelectrochemical activity, with a photocurrent of 0.27 mA·cm−2 observed at 1.1 VRHE (350 nm, 2.58 mW·cm−2), which is ~ 2.2 times higher than that of single-layer ZnO and far higher than that of BiOI. Photoluminescence spectroscopy and transient absorption spectroscopy measurements showed that there was effective charge transfer across the heterojunction which spatially separated charge carriers and increased their lifetime and ability to drive photoelectrochemical water oxidation.

Significantly boosted photoelectrochemical water splitting performance by plasmonic enhanced Hematite@MOF composite photoelectrodes

Hematite as a catalyst for photoelectrochemical water splitting offers huge potential, due to its high chemical stability, great abundance, and low cost. However, the low water oxidation kinetics and poor charge transportation have hindered progress towards the manufacture of practical water splitting devices. To tackle these problems, a visible light responsive metal-organic framework (MOF) polyhedral zeolitic imidazolate (ZIF-67), and optimised plasmonic Ag nanorods were incorporated into hematite nanostructures to form a three-component heterojunction photoelectrode. The designed photoanode showed dramatically improved light harvesting in the visible range and enhanced charge transport. A mechanistic investigation allowed the deconvolution of the enhanced performance pathways. First, the Hematite@ZIF-67 core-shell p-n junction enables facile charge carrier transfer between ZIF-67 and hematite. In addition, ZIF-67 also provides active sites for water oxidation and boosts surface oxygen evolution reaction (OER) kinetics. Guided by finite-difference time-domain (FDTD) modelling, Ag nanorods with optimised aspect ratio were incorporated between ZIF-67 and hematite. The Ag nanorods facilitate broadband light absorption and surface charge injection, induced by near-field excitation enhancement and plasmonic resonance energy transfer (PRET) pathways. The design and addition of ZIF-67 and Ag nanorods result in superior performance for a hematite-based photoanode for photoelectrochemical (PEC) water oxidation.

Charge separation at BiVO4/Co3O4 and BiVO4/CoOOH interfaces: Differences between dense and permeable cocatalysts

A CoOOH cocatalyst overlayer is electrochemically deposited on the BiVO4 surface, and thermal annealing converted CoOOH to Co3O4 in situ. The primary difference between the two cocatalysts lies in their electrolyte permeabilities, resulting in significant differences in their thermodynamic effects on enhancing the photoelectrochemical water oxidation performance. We proved that the CoOOH cocatalyst is unable to achieve true chemical passivation of the surface states of the BiVO4 photoanode, whereas the Co3O4 cocatalyst can chemically passivate the surface states. Nevertheless, the CoOOH cocatalyst still exhibits better water oxidation performance than Co3O4, which is attributed to the fact that the permeable CoOOH cocatalyst can accumulate more photogenerated holes in the bulk phase and provide more active sites. However, the CoOOH cocatalyst still exhibits better water oxidation performance compared to Co3O4, which is attributed to the permeable CoOOH cocatalyst can accumulate more photogenerated holes in its bulk phase and offer more active sites.

Photoelectrochemical water oxidation and methylene blue degradation enhanced by Nb doping and CoPi modification for hematite photoanodes

Nanostructured α-Fe2O3 films doped with niobium and supported with CoPi cocatalyst (Nb-α-Fe2O3 @CoPi) are prepared by hydrothermal growth together with electrochemical deposition. The effect of Nb doping and CoPi cocatalyst modification on the performance of photoelectrochemical water oxidation and methylene blue degradation of α-Fe2O3 is studied in detail. It shows that Nb doping and CoPi modification toward α-Fe2O3 not only enhances its light absorption, but also improves its electrical properties, including conductivity, bulk charge separation efficiency and charge transfer efficiency at the electrode/electrolyte interface. When used as photoanode for photoelectrochemical water oxidation, the Nb-α-Fe2O3 @CoPi photoanodes show a negative shift of 540 mV in the onset potential in comparison with the pristine α-Fe2O3 films. At the same time, the photocurrent density (at 1.6 V, vs. RHE) of Nb-α-Fe2O3@CoPi is more than three times that of the pristine α-Fe2O3, reaching 1.44 mA·cm−2. In addition, the obtained Nb-α-Fe2O3 @CoPi photoanodes also show excellent performance in visible light-driven photoelectrochemical degradation of methylene blue, a common pollutant in wastewater. For example, a degradation rate of 98% is achieved in 2 h by the Nb-α-Fe2O3 @CoPi, which is 3.6 times that of the pristine α-Fe2O3 (26%).

Mesoporous CuFe2 O4 Photoanodes for Solar Water Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties.

Metal oxide-based photoelectrodes for solar water splitting often utilize nanostructures to increase the solid-liquid interface area. This reduces charge transport distances and increases the photocurrent for materials with short minority charge carrier diffusion lengths. While the merits of nanostructuring are well established, the effect of surface order on the photocurrent and carrier recombination has not yet received much attention in the literature. To evaluate the impact of pore ordering on the photoelectrochemical properties, mesoporous CuFe2 O4 (CFO) thin film photoanodes were prepared by dip-coating and soft-templating. Here, the pore order and geometry can be controlled by addition of copolymer surfactants poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127), polyisobutylene-block-poly(ethylene oxide) (PIB-PEO) and poly(ethylene-co-butylene)-block-poly(ethylene oxide) (Kraton liquid™-PEO, KLE). The non-ordered CFO showed the highest photocurrent density of 0.2 mA/cm2 at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density for water oxidation. Conversely, the ordered CFO presented the best photoelectrochemical water oxidation performance. These differences can be understood on the basis of the high surface area, which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron-hole recombination at the defective surface. This interpretation is confirmed by intensity-modulated photocurrent (IMPS) and vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPS). The lowest surface recombination rate was observed for the ordered KLE-based mesoporous CFO, which retains spherical pore shapes at the surface resulting in fewer surface defects. Overall, this work shows that the photoelectrochemical energy conversion efficiency of copper ferrite thin films is not just controlled by the surface area, but also by surface order.

Temperature of photoanode for photoelectrochemical water oxidation

Solar-light-driven photoelectrochemical that can utilise water for high-performance artificial photosynthesis for low-cost green hydrogen fuel production. However, the interaction of photoanode with water is crucial to its heterogeneous water oxidation for sustainable fuel production for the replacement of fossil fuels. Here, we report experiment investigations of photoelectrode with temperature for solar energy conversion. These measurements revealed that a rise in temperature affects the catalytic generation of hydrogen in three routes, viz., minimising the required energy for water splitting, reducing bandgap energy and mitigating the resistance at the interface.

Thin-layer metal bismuth inserted into Bi2S3/C, N co-doped α-Fe2O3 achieving efficient photoelectrochemical water oxidation

α-Fe2O3 has been considered a promising candidate for photoelectrochemical water splitting. Unluckily, the activity of α-Fe2O3 is significantly lower than the theoretical value due to the severe carrier recombination in the bulk phase and the slow kinetics of surface water oxidation. Herein, we developed a novel photoanode of Bi2S3/Bi/C, N co-doped α-Fe2O3. Compared with the reversible hydrogen electrode, this photoanode had a photocurrent density of 10.78 mA cm−2 at 1.23 V, which was about 3.44 times that of the pristine α-Fe2O3. Metallic Bi as an intermediate conducting layer effectively reduced the charge transport barrier between Bi2S3 and C, N co-doped α-Fe2O3. The Bi2S3 as a cover layer reduced the surface defect state of Bi2S3/Bi/C, N co-doped α-Fe2O3. The C, N co-doped α-Fe2O3 combined with Bi2S3/Bi forming a type-II heterojunction, which provided a sustained and powerful driving force for efficient carrier separation.

Unraveling Photothermal-Enhanced bulk charge transport and surface oxygen reactions in TiO2 photoanodes for highly efficient photoelectrochemical water oxidation

Titanium dioxide (TiO2) has been extensively used as an important photoanode material for photoelectrochemical (PEC) water splitting, but its activity is greatly hindered by the low charge transport and sluggish oxygen evolution reaction (OER) kinetics. Herein, we demonstrate spinel oxide NiCo2O4 (NCO) acted as both photothermal material and oxygen evolution cocatalyst could significantly promote the PEC performance of TiO2 photoanodes, and a high photocurrent of 2.34 mA cm−2 at 1.23 V vs. reversible hydrogen electrode has been achieved by NCO/TiO2 photoanodes assisted by the photothermal effect. Importantly, a mechanism is proposed to explain the enhanced bulk charge transport and surface OER activity by an integrated various operando characterizations and density functional theory (DFT) study. Intensity-modulated photocurrent spectroscopy (IMPS) analysis and DFT calculations imply the relatively high temperature induced by the photothermal effect could accelerate small polaron hopping in the bulk of TiO2, which is verified by the electron diffusion coefficient variation of TiO2 with oxygen vacancies at different temperatures. Operando cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) reveal that photothermal conversion facilitates the oxidation of Ni2+ at lower potential and promotes the adsorption of *OH and deprotonation of *OOH intermediates. This work provides deep insight into the photothermal-enhanced PEC performance by observing the bulk small polaron hopping and surface dynamic evolution during the PEC process, which may underpin future advances in promoting PEC and photocatalytic performances of small-polaron-type semiconductors.

Polypyrrole modification on BiVO4 for photothermal-assisted photoelectrochemical water oxidation.

The bismuth vanadate (BiVO4) photoanode receives extensive attention in photoelectrochemical (PEC) water splitting. However, the high charge recombination rate, low electronic conductivity, and sluggish electrode kinetics have inhibited the PEC performance. Increasing the reaction temperature for water oxidation is an effective way to enhance the carrier kinetics of BiVO4. Herein, a polypyrrole (PPy) layer was coated on the BiVO4 film. The PPy layer could harvest the near-infrared light to elevate the temperature of the BiVO4 photoelectrode and further improve charge separation and injection efficiencies. In addition, the conductive polymer PPy layer acted as an effective charge transfer channel to facilitate photogenerated holes moving from BiVO4 to the electrode/electrolyte interface. Therefore, PPy modification led to a significantly improved water oxidation property. After loading the cobalt-phosphate co-catalyst, the photocurrent density reached 3.64 mA cm-2 at 1.23V vs the reversible hydrogen electrode, corresponding to an incident photon-to-current conversion efficiency of 63% at 430nm. This work provided an effective strategy for designing a photothermal material assisted photoelectrode for efficient water splitting.

Fully Dispersed IrOx Atomic Clusters Enable Record Photoelectrochemical Water Oxidation of Hematite in Acidic Media.

Ir-based materials are still the benchmark catalysts for various reactions in acidic environment, but the high loading and low atom utilization limit their large-scale deployment. Herein, we report an effective strategy for implanting fully dispersed iridium-oxide atomic clusters onto hematite for boosting photoelectrochemical water oxidation in acidic solution. The resulting photoanode achieves a record-high photocurrent of 1.35 mA cm-2 at 1.23 V, corresponding to a mass activity of 172.70 A g-1 (3 times higher than electrodeposited control sample) and demonstrating the merits from the high atomic utilization of Ir. The systematically experimental and theoretical results reveal that the performance improvement correlates with the modulated electronic structure including the adjusted Fermi level and d-band center, which significantly enhances charge separation efficiency and promotes the conversion from intermediate *O into *OOH.

Enhancement in Photoelectrochemical Efficiency and Modulation of Surface States in BiVO4 through the TiO2 Outer Layer Using the Atomic Layer Deposition Technique

Owing to several important properties, BiVO4 is the promising photoanode material for the photoelectrochemical (PEC) water oxidation reaction; however, the poor charge transfer and transport and slow surface catalytic activity limit to achieve the expected high theoretical efficiency. In the present investigation, thin films of TiO2 have been formed over the BiVO4 photoanode surface using the atomic layer deposition technique (ALD). Films are formed at 5, 50, and 150 nm thicknesses, and the PEC performances have been evaluated. Enhancement in the performance has been observed upon inclusion of the ALD/TiO2 films over BiVO4. The performance has been observed to be higher in the case of 50 nm ALD/TiO2 films. The ALD/TiO2 films have several important roles in the enhancement in the PEC efficiency other than only the enhancement of stability through retardation of the photocorrosion process. The intensity modulated photocurrent spectroscopy measurements have been carried out for the selective evaluation of the different modes of charge transfer processes upon excitation of the photoanode materials on photoexcitation. The enhancement in the charge transfer rate constant (kt) over the relative retardation of the charge recombination rate constant (kr) indicated the specific roles of modulation of the surface states. The investigation revealed the significant scopes of the modifications of surfaces using ALD techniques to selectively modulate the surface electronic states of materials in enhancing catalytic efficiency.

Heterostructured 2D/2D ZnIn2S4/g-C3N4 nanohybrids for photocatalytic degradation of antibiotic sulfamethoxazole and photoelectrochemical properties

In recent years, antibiotic drugs have been extensively used owing to increased industrial growth, and this has created issues related to drinking water and a green environment. Different techniques have been used to resolve these issues, among which heterogeneous photocatalysis has been widely explored for the elimination of toxic compounds from wastewater resources. In this study, ZnIn2S4, g-C3N4, and ZnIn2S4/g-C3N4 hybrid heterostructured composites are synthesized via hydrothermal method and used these (i) for the removal of antibiotic sulfamethoxazole pollutant and (ii) photoelectrochemical water oxidation. The nanomaterials were characterized using X-ray diffraction, Scanning electron microscopy, transmission electron microscopy, and UV–vis spectroscopy. The developed hybrid heterostructured composites were able to degrade sulfamethoxazole pollutants as well as offer improved photoelectrochemical properties compared to pristine samples. The catalytic performance of the materials developed under visible light irradiation was greatly improved for the degradation of the antibiotic drug up to 89.4% in 2 h. Moreover, the hybrid heterostructured photoelectrode showed a better photocurrent density (8.68 mA/cm2) and exhibited ∼19.2 and 29.9 times greater photocurrent density than the pristine photoelectrodes. Such a considerably increased catalytic activity was attributed to the active separation of charge carriers and transmission. The study offers an innovative approach to develop effective catalysts, and for the degradation of sulfamethoxazole as well as the PEC properties for hydrogen production.

Novel 2D sulfur-doped V2O5 flakes and their applications in photoelectrochemical water oxidation and high-performance energy storage supercapacitors

Development of high-performance energy storage devices with exceptional and consistent performance is of immense significance due to ecological concerns to meet the growing demands for high-performance energy conversion and storage. V2O5 materials have attracted particular attention for energy storage owing to their excellent Faradaic performance, several oxidation valences, inexpensive, ease of synthesis, and non-toxic, all of which make them favorable materials for both energy conversion and storage applications. In this investigation, an efforts has been made to synthesize the novel 2D S-doped V2O5 flakes via a facile hydrothermal procedure. The materials developed were utilized in photoelectrochemical (PEC) water oxidation as well as electrochemical energy storage supercapacitors. XRD analysis confirmed that V2O5 crystal structure is orthorhombic, and doping of the sulfur ions into V2O5 lattice, which has considerably narrowed the band gap from 2.17 eV to 2.02 eV. Nyquist plots revealed that the doped electrode showed reduced inner resistance (RS) and charge transmission resistance (RCT) compared to that of naked electrode. Density functional theory (DFT) calculations illustrate that sulfur-ion doping in the two-dimensional V2O5 monolayer remarkably enhances the electrical properties of the materials. Furthermore, the doping of S into the host matrix causes a positive influence on its structure, electrical conductivity and ion diffusion. Doped V2O5 2D flakes exhibited a specific capacitance of 749F/g at 1 A/g with excellent stability after 3000 cycles at 2.5 A/g compared to the undoped sample. Furthermore, 2D S-doped V2O5 flakes showed ∼ 4.7 folds improvement in photocurrent density and reduced charge resistance than the undoped sample, indicating that doped V2O5 has excellent photoelectrochemical (PEC) water oxidation properties. The fabricated 2D nanostructured materials can be the potential materials for constructing inexpensive and high-performance energy conversion (e.g., hydrogen production, green energy generation), and energy storage devices.

Roles of Surface States in Cocatalyst-Loaded Hematite Photoanodes for Water Oxidation

Photoelectrochemical water oxidation over bare hematite (α-Fe2O3) involved physical–chemical processes through surface states. It is found that the surface state with a higher oxidative energy (S1) served as a reaction intermediate for water oxidation, while the other surface state with a lower oxidative energy (S2) was basically catalytically inactive and induced charge recombination. Since a cocatalyst is commonly loaded on α-Fe2O3 to enhance water oxidation, the physical–chemical processes between the surface states and the cocatalyst are supposedly important but remain unclear. In the present study, we elucidated such processes by employing photoelectrochemical impedance spectroscopy. We found that, in a CoPi-loaded α-Fe2O3, S1 no longer served as an intermediate for water oxidation and was mostly passivated. At the same time, S2 became the key intermediate for water oxidation by serving as a hole reservoir, prolonging the hole lifetime, and finally transferring holes to CoPi. This study reveals the surface chemistry for optimizing cocatalyst-loaded α-Fe2O3, which is different from that for bare α-Fe2O3.

P-n heterojunction constructed by γ-Fe2O3 covering CuO with CuFe2O4 interface for visible-light-driven photoelectrochemical water oxidation

Fe2O3 is a promising n-type semiconductor as the photoanode of photoelectrochemical water-splitting method due to its abundance, low cost, environment-friendly, and high chemical stability. However, the recombination of photogenerated holes and electrons leads to low solar-to-hydrogen efficiency. In this work, to overcome the recombination issue, a p-type semiconductor, CuO, is introduced underneath the γ-Fe2O3 to synthesize γ-Fe2O3/CuO on the FTO substrate. Along with the formation of p-n heterojunction, CuFe2O4 is in situ generated at the interface of γ-Fe2O3 and CuO. The existence of Cu2O in CuO and CuFe2O4 promotes the charge transfer from CuO to γ-Fe2O3 and within CuFe2O4, respectively, resulting in creating an internal electric field in γ-Fe2O3/CuO and leading to the conduction band of CuO bending up and γ-Fe2O3 bending down. Additionally, Cu(II) in CuFe2O4 contributes to fast electron capture. Consequently, the charge transfer efficiency and charge separation efficiency of photo-generated holes are promoted. Hence, γ-Fe2O3/CuO exhibits an enhanced photocurrent density of 13.40 mA cm−2 (1.9 times higher than γ-Fe2O3). The photo corrosion resistance of CuO is dramatically increased with the protection of CuFe2O4, resulting in superior high chemical stability, i.e. 85% of the initial activity remains after a long-term test.

Promoting photoelectrochemical water oxidation of BiVO4 photoanode via Co-MOF-derived heterostructural cocatalyst

The sluggish surface reaction kinetic is the bottleneck limiting the photoelectrochemical (PEC) water oxidation of BiVO4 photoanode, especially for the porous structured BiVO4 with high surface area. Here, Co-based metal–organic frameworks (MOFs) were modified on porous BiVO4 photoanode followed by calcining in the N2 atmosphere, forming BiVO4-MOF-N2 photoanode. The MOF-derived heterostructural cocatalysts significantly enhanced the photocurrent density of BiVO4-MOF-N2, reaching 2.32 mA·cm−2 (1.23 V vs RHE), about 1.15 times that of BiVO4-MOF and 2.64 times that of pristine BiVO4. The comparative study of BiVO4-MOF-N2, BiVO4-MOF, and pristine BiVO4 photoanodes suggested that the Co-MOF-derived heterostructural cocatalyst had a much stronger effect on promoting the surface reactions than pure MOF. The BiVO4-MOF-N2 photoanode had the highest charge transfer efficiency of 63.1 % at 1.3 V vs RHE and the lowest charge transfer resistivity compared with BiVO4 and BiVO4-MOF. Both Co-active centers and conductive carbon matrix contributed to the enhanced charge transfer of BiVO4-MOF-N2 photoanode. A MOF-loading-time dependent PEC performance of BiVO4 photoanode also indicated that the intermediate charge transfer process from BiVO4 to active centers strongly affected the final charge transfer and PEC performances. This work provides new insights into the design of high-performance photoelectrochemical electrodes based on heterostructural cocatalysts.