Enhancing the PEC water splitting performance of In2O3 nanorods by a wet chemical reduction

Generating oxygen vacancies is an effective way to improve photoelectrochemical (PEC) water splitting performance of semiconductor materials owing to the formation of shallow donor level and the supply of additional electron donor. Herein, oxygen vacancies were introduced into In₂O₃ nanorods by a hydrothermal reduction method using NaBH₄ solution as the reductant, and the effects of hydrothermal reduction time on the oxygen vacancy concentration, optoelectronic property and PEC water splitting activity over In₂O₃ nanorods were systematically investigated. The results of LSV, EIS and MS showed that the reduced samples displayed superior PEC performance and In₂O_3-x-1 exhibited a highest photocurrent density of 0.97 mA cm^?2 at 1.23 V vs. RHE under the irradiation of visible light, which was roughly 4 times of bare In₂O₃. The remarkable performance of In₂O_3-x-1 is mainly ascribed to the introduction of oxygen vacancies, which leads to better light absorption capacity, higher carrier concentration, and increased electron transport efficiency at the electrode/electrolyte interface.     

Recent trends in development of hematite (α-Fe2O3) as an efficient photoanode for enhancement of photoelectrochemical hydrogen production by solar water splitting

Solar-assisted water splitting using photoelectrochemical (PEC) cell is an environmentally benign technology for the generation of hydrogen fuel. However, several limitations of the materials used in fabrication of PEC cell have considerably hindered its efficiency. Extensive efforts have been made to enhance the efficiency and reduce the hydrogen generation cost using PEC cells. Photoelectrodes that are stable, efficient and made of cost-effective materials with simple synthesizing methods are essential for commercially viable solar water splitting through PEC technology. To this end, hematite (α-Fe₂O₃) has been explored as an excellent photoanode material to be used in the application of PEC water oxidation owing to its suitable bandgap of 2.1 eV that can utilize almost 40% of the visible light. In this study, we have summarized the recent progress of α-Fe₂O₃ nanostructured thin films for improving the water oxidation. Strategic modifications of α-Fe₂O₃ photoanodes comprising nanostructuring, heterojunctions, surface treatment, elemental doping, and nanocomposites are highlighted and discussed. Some prospects related to the challenges and research in this innovative research area are also provided as a guiding layout in building design principles for the improvement of α-Fe₂O₃ photoanodes in photoelectrochemical water oxidation to solve the increasing environmental issues and energy crises.     

Three-dimensional plasmonic photoanode of Co3O4 nanosheets coated onto TiO2 nanorod arrays for visible-light-driven water splitting

In this work, we report for the first time a plasmonic photoanode by decorating Au nanoparticles (NPs) onto two-dimensional (2D) Co₃O₄ nanosheets (NSs)/one-dimensional (1D) TiO₂ nanorod arrays (NRAs) (Au/Co₃O₄/TiO₂-NRAs) for enhanced visible-light photoelectrochemical (PEC) water splitting. In this plasmonic photoanode, TiO₂ NRAs act as an electron acceptor, plasmonic Au NPs and hierarchical Co₃O₄ NSs serve as visible-light harvesters. Light absorption shows that Au/Co₃O₄/TiO₂-NRAs heterojunction architectures exhibit greatly improved ability to harvest visible light due to the surface plasmon resonance (SPR) absorption of Au NPs and visible light harvesting ability of Co₃O₄ NSs. Spectroscopic measurements demonstrate that a type II band alignment is formed between Co₃O₄ and TiO₂. Benefiting from the SPR effect, type II band alignment and novel hierarchical architecture, plasmonic Au/Co₃O₄/TiO₂-NRAs photoanode shows remarkably enhanced visible-light PEC water splitting activity compared with Co₃O₄/TiO₂-NRAs and pristine TiO₂-NRAs photoanodes. Photocurrent density achieved by plasmonic photoanode is 37 and 1.2 times higher than those of TiO₂-NRAs and Co₃O₄/TiO₂-NRAs photoanodes, respectively. This work provides a promising strategy to highly enhance visible-light PEC water splitting activity of wide band-gap semiconductor-based photoelectrode materials.     

Construction of CdSe@TiO2 core‐shell nanorod arrays by electrochemical deposition for efficient visible light photoelectrochemical performance

The CdSe@TiO₂ core‐shell nanorod arrays for photoelectrochemical (PEC) application were designed and constructed by a facile electrochemical deposition strategy. The CdSe@TiO₂ photoanodes exhibit highly efficient PEC performance under visible light irradiation, among which the CdSe shell layer thickness can be precisely adjusted by different electrodeposition time. In comparison with nude TiO₂ nanorods, the optimized CdSe@TiO₂ photoanode (TC‐500) shows a significant saturated photocurrent density of 2.1 mA/cm² at 0 V (vs Ag/AgCl), which is attributed to the good distribution of CdSe nanoparticles on TiO₂ nanorod arrays, the favorable band alignment, and the intimate interfacial interaction between CdSe nanoparticles and TiO₂ nanorods. The introduction of CdSe shell layer does not only improve light absorption ability but also enhances photogenerated charge carrier's transfer and separation. This current work systematically studies the accurate adjustment of CdSe shell layer thickness on TiO₂ nanorod arrays by electrochemical deposition strategy and provides a paradigm to design and fabricate heterostructure composite for PEC application.     

In2O3:Sn/TiO2/CdS heterojunction nanowire array photoanode in photoelectrochemical cells

The design of photoanode with highly efficient light harvesting and charge collection properties is important in photoelectrochemical (PEC) cell performance for hydrogen production. Here, we report the hierarchical In₂O₃:Sn/TiO₂/CdS heterojunction nanowire array photoanode (ITO/TiO₂/CdS-nanowire array photoanode) as it provides a short travel distance for charge carrier and long light absorption pathway by scattering effect. In addition, optical properties and device performance of the ITO/TiO₂/CdS-nanowire array photoanode were compared with the TiO₂ nanoparticle/CdS photoanode. The photocatalytic properties for water splitting were measured in the presence of sacrificial agent such as SO₃²⁻ and S²⁻ ions. Under illumination (AM 1.5G, 100 mW/cm²), ITO/TiO₂/CdS-nanowire array photoanode exhibits a photocurrent density of 8.36 mA/cm² at 0 V versus Ag/AgCl, which is four times higher than the TiO₂ nanoparticle/CdS photoanode. The maximum applied bias photon-to-current efficiency for the ITO/TiO₂/CdS-nanowire array and the TiO₂ nanoparticle/CdS photoanode were 3.33% and 2.09%, respectively. The improved light harvesting and the charge collection properties due to the increased light absorption pathway and reduced electron travel distance by ITO nanowire lead to enhancement of PEC performance.     

Ag grid induced photocurrent enhancement in WO3 photoanodes and their scale-up performance toward photoelectrochemical H2 generation

The hydrogen generation from photoelectrochemical (PEC) water splitting under visible light was investigated using large area tungsten oxide (WO₃) photoanodes. The photoanodes for PEC hydrogen generation were prepared by screen printing WO₃ films having typical active areas of 0.36, 4.8 and 130 cm² onto the conducting fluorine-doped tin oxide (FTO) substrates with and without embedded inter-connected Ag grid lines. TiO₂ based dye-sensitized solar cell was also fabricated to provide the required external bias to the photoanodes for water splitting. The structural and morphological properties of the WO₃ films were studied before scaling up the area of photoanodes. The screen printed WO₃ film sintered at 500 °C for 30 min crystallized in a monoclinic crystal structure, which is the most useful phase for water splitting. Such WO₃ film revealed nanocrystalline and porous morphology with grain size of ∼70-90 nm. WO₃ photoanode coated on Ag grid embedded FTO substrate exhibited almost two-fold degree of photocurrent density enhancement than that on bare FTO substrate under 1 SUN illumination in 0.5 M H₂SO₄ electrolyte. With such enhancement, the calculated solar-to-hydrogen conversion efficiencies under 1 SUN were 3.24% and ∼2% at 1.23 V for small (0.36 cm²) and large (4.8 cm²) area WO₃ photoanodes, respectively. The rate of hydrogen generation for large area photoanode (130.56 cm²) was 3 mL/min.     

Surface engineering of Ba-doped TiO2 nanorods by Bi2O3 passivation and (NiFe)OOH Co-catalyst layers for efficient and stable solar water oxidation

The rational design of heterostructures as an ideal photoelectrode system for H₂ and O₂ conversion in photoelectrochemical (PEC) system has been regarded as an essential key to boost PEC performance. In this work, to demonstrate the energetic photoanode cell, deposition of a thin layer of Bi₂O₃ is utilized to hybridize with the 5 wt% Ba-doped TiO₂ nanorod heterostructure under the cascading band diagram, where Ba-doping can enhance the charge transport/separation rate in bulk phase, in terms of increasing the donor density, enhancing the bulk electronic conductivity, and increasing the band bending. Furthermore, with optimizing the thickness (～15 nm) of Bi₂O₃, the (NiFe)OOH as a cocatalyst was adapted to improve the interfacial charge transfer rate in the PEC cell, reaching the high photocurrent density (J) of ～4.1 mA/cm² at 1.23 V (vs. Reversible Hydrogen Electrode) and stability retention of 100%, even after 15 h at 1 M NaOH under 1 Sun illumination condition. The improvement mainly comes from the extended absorption of visible light from the thin Bi₂O₃ layer, effective transfer/separation of photogenerated charge carriers, and acceleration of water oxidizing reaction, caused by the narrowed band gap and the favorable charge transfer under the cascading band alignment built by the heterojunction, as well as electrocatalyst, offering the timely consumption of photogenerated holes accumulated at the electrode surface.     

Nickel-doped TiO2 and thiophene-naphthalenediimide copolymer based inorganic/organic nano-heterostructure for the enhanced photoelectrochemical urea oxidation reaction

To overcome the global challenges of energy crises and environmental threats, urea oxidation is a hopeful route to utilize urea-rich wastewater as an energy source for hydrogen production. Herein, we report an inorganic/organic type of nano-heterostructure (NHs–Ni-TiO₂/p-NDIHBT) as a photoanode with excellent urea oxidation efficiency driven by visible light. This heterostructured photoanode consists of nickel (Ni)-doped TiO₂ nanorods (NRs) arrays as an inorganic part and a D-A-D type organic polymer i.e p-NDIHBT as an organic part. The as-prepared photoanode was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The morphological studies of TEM confirm the coating of p-NDIHBT on Ni–TiO₂ NPs (～1 μm). The consequence of heterostructure formation on optical and photoelectrochemical (PEC) properties of photoanode were explored through photoelectrochemical responses under visible light irradiation. The photoelectrochemical activity of Ni–TiO₂ and Ni–TiO₂/p-NDIHBT photoanode from linear sweep voltammetry (LSV) shows the ultrahigh photocurrent density of 0.36 mA/cm² and 2.21 mA/cm², respectively measured at 1.965 V_RHE. Electrochemical impedance spectroscopy (EIS) of both photoanodes shows a highly sensitive nature toward the urea oxidation reaction. The hybrid photoanode also exhibits high photostability, good solar-to-hydrogen conversion efficiency, and high faradaic efficiency for urea oxidation.     

Bi2S3 entrenched BiVO4/WO3 multidimensional triadic photoanode for enhanced photoelectrochemical hydrogen evolution applications

The catalytic reactivity and photoactivity of WO₃ and BiVO₄ oxide semiconductors have general obstacles as electrodes in emergent photo-electrochemical (PEC) hydrogen evolution applications. The present work comprises the integration of photocatalyst with wide visible photon absorption material which is vital for hydrogen evolution in photo-electrocatalytic water splitting. Herein, the 1D WO₃ NWs have been integrated with stable water oxidation photocatalysts of BiVO₄ and Bi₂S₃ as a photoanode (Bi₂S₃/BiVO₄/WO₃) for photoelectrochemical hydrogen evolution reactions. The morphological variations in the Bi₂S₃/BiVO₄/WO₃ heterostructure manifest catalytic activity and rapid charge transfer characteristics owing to band alignment and a wide range of visible photon absorption. The optimized Bi₂S₃/BiVO₄/WO₃ multidimensional photoanode accomplishes a superior photocurrent density of 1.52 mA/cm², a seven-fold higher than pristine WO₃ photoanode counterpart (0.2 mA/cm²) at 1 V vs. RHE. A prodigious lowest onset potential of ?0.01 V vs. RHE) has been achieved which enables very high solar to hydrogen conversion. The photoelectrode with entangled morphology such as nanosheets, nanocrystals and nanorods expanded their surface to volume ratio having enhanced catalytic performance. The hybrid photoanodes have demonstrated the lowest charge transfer resistance of 360 Ohm/cm² with a 7-fold rise in hydrogen evolution performance. The resultant triadic Bi₂S₃/BiVO₄/WO₃ heterostructure appeared to be an emerging stable photo-electro catalyst for hydrogen evolution applications.     

Novel synthesis of graphene oxide/In2S3/TiO2 NRs heterojunction photoanode for enhanced photoelectrochemical (PEC) performance

Novel heterostructure photocatalyst built from titanium dioxide (TiO₂), graphene oxide (GO) and indium sulfide (In₂S₃) has been successfully prepared for photoelectrochemical hydrogen production. The stepwise introduction of three materials on conductive glass substrate has been realized through hydrothermal, electrochimical and spin coating deposition methods, respectively. The structure, morphology, composition, optical and photoelectrochemical properties of the resultant photoanodes were investigated in detail. The presence of GO in the heterostructure film was confirmed by Raman analysis with D and G band intensity. Surface morphology analysis of the GO/In₂S₃/TiO₂ NRs structure reveal the homogenous distribution of graphene oxide on In₂S₃/TiO₂ NRs surface. From UV–Vis analysis, band gap energy of the samples decreases gradually from 3.34 eV (TiO₂ NRs) to 3.12 eV, with In₂S₃ and GO addition. The electrochemical impedance spectroscopy (EIS) further confirmed that GO/In₂S₃/TiO₂NRs heterostructure possessed the lowest charge-transfer resistance, revealing that In₂S₃ and GO could significantly accelerate the electron mobility compared with bare TiO₂. From Mott-Schottky plots, several parameters such as flat-band potential and free carrier concentration were determined. Next, The GO/In₂S₃/TiO₂ NRs electrode achieved remarkably improved current density (0.45 mAcm² at 0.8 V vs Ag/AgCl) compared to pure TiO₂ NRs or In₂S₃/TiO₂ NRs electrodes, which attributes to the uniform structure and excellent electrical conductivity of GO, which could reduce the combination rate of the photo electron-hole pairs. These results reveal that GO/In₂S₃/TiO₂ NRs possesses great potential toward the development of newly synthesizable catalysts in the field of photoelectrochemical water splitting.     

Enhanced photoelectrocatalytic activity of Bi2S3–TiO2 nanotube arrays hetero-structure under visible light irradiation

Constructing heterosystems by sensitizing a wide band gap semiconductor with a narrow band gap semiconductor is an effective way to improve photocatalytic performance. Bismuth sulfide (Bi₂S₃) has a direct band gap of 1.38 eV and shows great potential in capturing visible light, which makes it a good candidate for photocatatlytic applications. In this work, Bi₂S₃ nanoparticles were efficiently deposited on TiO₂ nanotube arrays (Bi₂S₃-TNTAs) by sequential chemical bath deposition (CBD) method to enhance visible light response of the photocatalytic system. Notably, a high-throughput screening method of scanning photoelectrochemical microscopy (SPECM) was exploited to evaluate photoelectrochemical response of the as-prepared composites and to find out the optimized photocatatlytic system. The effects of Bi₂S₃ nanoparticles on visible light absorption and photoelectrocatalytic hydrogen production rate of the TiO₂-system were investigated in detail. When adopted as photoanode, the optimized heteroelectrode exhibited a more than 13-fold enhancement in hydrogen production rate. The result of electrochemical impedance spectroscopy (EIS) and photoluminescence (PL) shows that photo-generated charges excited under visible light in Bi₂S₃-TNTAs composites are efficiently separated, which gives rise to the superior photoelectrocatalytic performance of the Bi₂S₃-TNTAs photoanodes.     

High-efficiency photoelectrochemical water splitting with heterojunction photoanode of In2O3-x nanorods/black Ti–Si–O nanotubes

Constructing heterojunction was an efficient way to promote photoelectrochemical (PEC) water splitting performance of TiO₂-based nano-photoanode. In this work, we demonstrated the feasible preparation of oxygen vacancies-induced In₂O₃ (In₂O_3-x) nanorods/black Si-doped TiO₂ (Ti–Si–O) nanotubes heterojunction photoanode for enhanced PEC water splitting. Black Ti–Si–O nanotubes were fabricated through Zn reduction of the as-annealed Ti–Si–O nanotubes, followed by In₂O_3-x nanorods coupling by a facile electrodepositing and Ar heat treatment. Solar to hydrogen conversion efficiency of the heterojunction photoanode reached as high as 1.96%, which was almost 10 times that of undoped TiO₂. The improved PEC properties were mainly attributed to co-doping effects of Si and Ti³⁺/oxygen vacancy as well as In₂O_3-x decoration, which resulted in enhanced optical absorption and facilitated separation-transport process of photogenerated charge carriers. Charge transfer process in the composite system and hydrogen production mechanism were proposed. This work will facilitate designing TiO₂-based nano-photoanodes for promoting water splitting by integrating with elements doping, oxygen vacancies self-doping and semiconductors coupling.     

Semiconductor-sensitized solar cells based on nanocrystalline In2S3/In2O3 thin film electrodes

A possibility of semiconductor-sensitized thin film solar cells have been proposed. Nanocrystalline In₂S₃-modified In₂O₃ electrodes were prepared with sulfidation of In₂O₃ thin film electrodes under H₂S atmosphere. The band gap (E_g) of In₂S₃ estimated from the onset of the absorption spectrum was approximately 2.0 eV. The photovoltaic properties of a photoelectrochemical solar cell based on In₂S₃/In₂O₃ thin film electrodes and I⁻/I₃⁻ redox electrolytes were investigated. This photoelectrochemical cell could convert visible light of 400–700 nm to electron. A highly efficient incident photon-to-electron conversion efficiency (IPCE) of 33% was obtained at 410 nm. The solar energy conversion efficiency, η, under AM 1.5 (100 mW cm⁻²) was 0.31% with a short-circuit photocurrent density (J_sc) of 3.10 mA cm⁻², a open-circuit photovoltage (V_oc) of 0.26 V, and a fill factor ( ff ) of 0.38.     

Hydrothermal synthesis of pyramid-like In2S3 film for efficient photoelectrochemical hydrogen generation

Herein, the pyramid-like In₂S₃ film was synthesized for photoelectrochemical (PEC) hydrogen generation using a facile hydrothermal route for the first time. The hydrothermal time was crucial to determine the morphology and composition of the as-prepared films. When the hydrothermal time lasted for 24 h, the pyramid-like In₂S₃ film could be achieved. With the increased hydrothermal time, the photocurrent first increased and then decreased, and the pyramid-like In₂S₃ film prepared for 24 h showed the highest photocurrent. After an annealing treatment, the photocurrent could be improved to 2.7 mA cm^?2 at 0.78 V vs. RHE, and a substantial hydrogen generation was observed. Compared to previously reported In₂S₃ films with different morphologies, the pyramid-like In₂S₃ film exhibited better PEC property. It was revealed that the remaining In₂O₃ after the synthesis of In₂S₃ had a great influence on the PEC performance. The pyramid-like In₂S₃ film with a moderate amount of In₂O₃ induced an efficient charge separation and transfer, as well as good light-absorption ability, which led to the superior PEC performance. This work has demonstrated great potentials of the pyramid-like In₂S₃ film for PEC hydrogen generation and opened a promising avenue towards the design and fabrication of novel pyramid-like nanostructures.     

Molybdenum doped bilayer photoanode nanotubes for enhanced photoelectrochemical water splitting

Conversion of solar energy into hydrogen energy via photoelectrochemical (PEC) water splitting is one of the most promising approaches for generation of clean and sustainable hydrogen energy in order to address the alarming global energy crisis and environmental problems. To achieve superior PEC performance and solar to hydrogen efficiency (STH), identification, synthesis, and development of efficient photoelectrocatalysts with suitable band gap and optoelectronic properties along with high PEC activity and durability is highly imperative. With the aim of improving the performance of our previously reported bilayer photoanode of WO₃ and Nb and N co-doped SnO₂ nanotubes i.e. WO₃-(Sn_0.95Nb_0.05)O₂:N NTs, herein, we report a simple and efficient strategy of molybdenum (Mo) doping into the WO₃ lattice to tailor the optoelectronic properties such as band gap, charge transfer resistance, and carrier density, etc. The Mo doped bilayer i.e. (W_0.98Mo_0.02)O_3-(Sn_0.95Nb_0.05)O₂:N revealed a higher light absorption ability with reduced band gap (1.88 eV) in comparison to that of the undoped bilayer (1.94 eV). In addition, Mo incorporation offered improvements in charge carrier density, photocurrent density, with reduction in charge transfer resistance, contributing to a STH (～3.12%), an applied bias photon-to-current efficiency (ABPE ～ 8% at 0.4 V), including a carrier density (N_d ～ 7.26 × 10²² cm^?3) superior to that of the undoped bilayer photoanode (STH ～2%, ABPE ～ 5.76%, and N_d ～5.11 × 10²² cm^?3, respectively). The substitution of Mo⁶⁺ for W⁶⁺ in the monoclinic lattice, forming the W–O–Mo bonds altered the band structure, realizing further enchantments in the PEC reaction and charge transfer kinetics. Additionally, doped bilayer photoanode revealed excellent long term PEC stability under illumination, suggesting its robustness for PEC water splitting. The present work herein provides a simple and effective Mo doping approach for generation of high performance photoanodes for PEC water splitting.     

Enhanced photoelectrochemical performance of an α-Fe2O3 nanorods photoanode with embedded nanocavities formed by helium ions implantation

Hematite is a prospective semiconductor in photoelectrochemical (PEC) water oxidation field due to its suitable bandgap for the solar spectrum absorption. Nevertheless, the low transfer and separation efficiency of the charge carriers are restricted by its diffusion length of hole which is 2–4 nm and further reduce the PEC performance. Here, we report an innovative method, by introducing nanocavities into the α-Fe₂O₃ nanorod arrays photoanodes through helium ions implantation, to improve the charge carriers' transfer and separation efficiency and further to enhance water oxidation performance. The result indicates that, the photocurrent density of nanocavities embedded α-Fe₂O₃ photoanode (S2-A sample) reaches 1.270 mA/cm² at 1.6 V vs. RHE which is 1-fold higher than that of the pristine α-Fe₂O₃ (0.688 mA/cm²) and the photocurrent density of S2-A sample reaches 0.652 mA/cm² at 1.23 V vs. RHE. In this work, the ion implantation combined with post annealing method is found to be a valid method to improve the photoelectrochemical performance, and it also can be further used to modify the other semiconductor photoelectrodes materials.     

Ultrathin hematite films deposited layer-by-layer on a TiO2 underlayer for efficient water splitting under visible light

Ultrathin hematite (α-Fe₂O₃) film deposited on a TiO₂ underlayer as a photoanode for photoelectrochemical water splitting was described. The TiO₂ underlayer was coated on conductive fluorine-doped tin oxide (FTO) glass by spin coating. The hematite films were formed layer-by-layer by repeating the separated two-phase hydrolysis-solvothermal reaction of iron(III) acetylacetonate and aqueous ammonia. A photocurrent density of 0.683 mA cm⁻² at +1.5 V vs. RHE (reversible hydrogen electrode) was obtained under visible light (>420 nm, 100 mW cm⁻²) illumination. The TiO₂ underlayer plays an important role in the formation of hematite film, acting as an intermediary to alleviate the dead layer effect and as a support of large surface areas to coat greater amounts of Fe₂O₃. The as-prepared photoanodes are notably stable and highly efficient for photoelectrochemical water splitting under visible light. This study provides a facile synthesis process for the controlled production of highly active ultrathin hematite film and a simple route for photocurrent enhancement using several photoanodes in tandem.     

2D/3D WO3/BiVO4 heterostructures for efficient photoelectrocatalytic water splitting

Developing novel photoanodes with high efficiency for photoelectrochemical (PEC) water splitting has become the key to solar energy conversion and storage realm. Herein, 3D worm-like bismuth vanadate (BiVO₄) is grafted on 2D thin tungsten trioxide (WO₃) underlayer by electrodeposition to form mixed–dimensional structured photoanode, resulting in significant improvement of the photocatalytic performance and the charge separation efficiency. Characterization results prove that the mixed–dimensional structured can boost the photocatalytic activity by suppressing back reaction and charge recombination of the bulk BiVO₄. Simultaneously, the electrical conductivity of photoanode can be increased by W⁶⁺ doping. Furthermore, a robust catalyst NiCo₂O_x is coated onto the surface of WO₃/BiVO₄ photoanode, exhibiting a desirable photocurrent of 3.85 mA cm^?2 at 1.23 V vs. RHE and an excellent stability over 3 h. Both the excellent photocurrent density and great operational stability of this 2D/3D WO₃/BiVO₄ photoanode make it a promising material for practical applications.     

Oxygen evolution NiOOH catalyst assisted V2O5@BiVO4 inverse opal hetero-structure for solar water oxidation

Using vanadium oxide (V₂O₅) inverse opal (IO) as a three-dimensional (3D) electron transporting tunnel, bismuth vanadate (BiVO₄) as a light harvester, and Amorphous Nickel Hydroxide (NiOOH) as an oxygen evolution co-catalyst, a V₂O₅@BiVO₄@NiOOH IO architecture was fabricated as an efficient photoanode on a conductive fluorine doped tin oxide (FTO) substrate for photoelectrochemical (PEC) water oxidation. V₂O₅ is the visible light absorbing photoanodes for water oxidation; however, the efficiency of this compound remains low (∼0.08 mA/cm² at 1.23 V vs. reversible hydrogen electrode (V_RHE)) and the unfavorable surface trap states limits the activity of V₂O₅ photoelectrodes in a PEC system. We found that the photoactive thin conductive BiVO₄ (∼12 nm) in the V₂O₅ IO greatly enhanced the charge separation efficiency to achieve better PEC water oxidation through modification of the surface states. The subsequent addition of NiOOH as an effective Oxygen evolution catalyst subsequently reduces the large overpotential and generates the photocurrent density of 1.14 mA/cm² at 1.23 V_RHE. Electrochemical impedance spectroscopy (EIS) evidenced that NiOOH deposition can substantially lower the charge transfer resistance (R_ct) at the semiconductor interface. Specifically, the consecutive and ordered morphology renders direct conduction pathways for the extraction of photogenerated electron/hole pairs and the convenient structure to penetrate the photogenerated carriers toward the semiconductor surface over the electrolyte. It is expected that the uninterrupted pathways will improve the electron transportation and thus the charge collection properties.     

TiO2 nanoparticles modified with ZnIn2S4 nanosheets and Co-Pi groups: Type II heterojunction and cocatalysts coexisted photoanode for efficient photoelectrochemical water splitting

The optimization of photoelectrode is the key issue for the efficient photoelectrochemical water splitting process. In this work, the TiO₂ photoanode is synthesized and modified with ZnIn₂S₄ nanosheets and Co-Pi cocatalyst (TiO₂/ZnIn₂S₄/Co-Pi) for a favorable photoelectrochemical performance. The synthesis and modification process of the TiO₂ photoanode are optimized. The physical and chemical characterizations indicate that the TiO₂ has a nano-cauliflower-like structure and rutile crystal form modified with a network hexagonal ZnIn₂S₄ nanosheets and amorphous Co-Pi groups. After optimization of the hydrothermal and annealing process, the optimized TiO₂ photoanode manifests a photocurrent density of 1.82 mA cm^?2, 1.73-fold of the pristine TiO₂ photoanode (1.05 mA cm^?2). With the surficial ZnIn₂S₄ and Co-Pi modification, the photocurrent density of the TiO₂/ZnIn₂S₄/Co-Pi photoanode is raised to 5.05 mA cm^?2, 5.32-fold of the optimized TiO₂ photoanode (1.82 mA cm^?2). The applied bias photon-to-current efficiency, the charge separation and injection efficiencies of the TiO₂/ZnIn₂S₄/Co-Pi photoanodes are 8.79, 3.40, and 1.64-folds of the optimized TiO₂ photoanode. Combined the Tauc plot, valence band XPS spectra, EIS and Mott-Schottky analysis, the PEC water splitting mechanism could be that: (i) the type II heterojunction formed by the TiO₂ and ZnIn₂S₄ semiconductors improves the charge separation/injection efficiencies; (ii) the Co-Pi groups facilitate the oxygen evolution kinetics; (iii) the Co-Pi groups and 2D ZnIn₂S₄ nanosheets synergistically enhance the charge separation efficiency. This investigation could offer a prospect of practical implementation for photoelectrochemical water splitting.