Insight into Charge Separation in WO3/BiVO4 Heterojunction for Solar Water Splitting

Promoting the Photoelectrochemical Properties of BiVO4 Photoanode via Dual Modification with CdS Nanoparticles and NiFe-LDH Nanosheets

Bismuth vanadate (BiVO4) has long been considered a promising photoanode material for photoelectrochemical (PEC) water splitting. Despite its potential, significant challenges such as slow surface water evolution reaction (OER) kinetics, poor carrier mobility, and rapid charge recombination limit its application. To address these issues, a triadic photoanode has been fabricated by sequentially depositing CdS nanoparticles and NiFe-layered double hydroxide (NiFe-LDH) nanosheets onto BiVO4, creating a NiFe-LDH/CdS/BiVO4 composite. This newly engineered photoanode demonstrates a photocurrent density of 3.1 mA cm-2 at 1.23 V vs. RHE in 0.1 M KOH under AM 1.5 G illumination, outperforming the singular BiVO4 photoanode by a factor of 5.8 and the binary CdS/BiVO4 and NiFe-LDH/BiVO4 photoanodes by factors of 4.9 and 4.3, respectively. Furthermore, it exhibits significantly higher applied bias photon-to-current efficiency (ABPE) and incident photon-to-current efficiency (ICPE) compared to pristine BiVO4 and its binary counterparts. This enhancement in PEC performance is ascribed to the formation of a CdS/BiVO4 heterojunction and the presence of a NiFe-LDH OER co-catalyst, which synergistically facilitate charge separation and transfer efficiencies. The findings suggest that dual modification of BiVO4 with CdS and NiFe-LDH is a promising approach to enhance the efficiency of photoanodes for PEC water splitting.

Improved Photoelectrochemical Performance of WO3/BiVO4 Heterojunction Photoanodes via WO3 Nanostructuring

WO3/BiVO4 heterojunction photoanodes can be efficiently employed in photoelectrochemical (PEC) cells for the conversion of water into molecular oxygen, the kinetic bottleneck of water splitting. Composite WO3/BiVO4 photoelectrodes possessing a nanoflake-like morphology have been synthesized through a multistep process and their PEC performance was investigated in comparison to that of WO3/BiVO4 photoelectrodes displaying a planar surface morphology and similar absorption properties and thickness. PEC tests, also in the presence of a sacrificial hole scavenger, electrochemical impedance analysis under simulated solar irradiation, and incident photon to current efficiency measurements highlighted that charge transport and charge recombination issues affecting the performance of the planar composite can be successfully overcome by nanostructuring the WO3 underlayer in nanoflake-like WO3/BiVO4 heterojunction electrodes.

Regulating excitonic effects in non-oxide based XPSe3 (X = Cd, Zn) monolayers towards enhanced photocatalysis for overall water splitting

The non-oxide 2D materials have garnered considerable interest due to their potential utilization as photocatalysts, which offer a superior substitute to metal-oxide-based photocatalysts. This study investigates the impact of the dielectric environment on the size and binding energy of excitons in atomically thin, experimentally synthesized semiconducting monolayers [XPSe₃, X = (Cd, Zn)] to address the critical problem of electron-hole recombination, which significantly hinders the efficiency of most photocatalysts. We employ a precise non-hydrogenic model surpassing the hydrogenic-based Mott-Wannier model. Our findings are among the first few demonstrations of an increase in exciton size (and decrease in exciton binding energy) as environmental screening increases. These findings have implications for photocatalytic water splitting and are not limited to metal phosphorus trichalcogenides, but can be applied to other classes of 2D materials as well. This work also compares metal-oxide photocatalysts, which have been the focus of much research over the past five decades, to non-oxide-based metal phosphorus trichalcogenide photocatalysts, which offer a superior alternative due to their ability to address issues such as light-harvesting inability in the visible spectrum and unwanted charge recombination centres. Furthermore, the implications of this study extend beyond photocatalysts and are significant for the design and development of next-generation optoelectronic devices that incorporate excitonic processes, such as solar cells, photodetectors, LEDs, etc.

Dual functional WO3/BiVO4 heterostructures for efficient photoelectrochemical water splitting and glycerol degradation

Dual functional heterojunctions of tungsten oxide and bismuth vanadate (WO₃/BiVO₄) photoanodes are developed and their applications in photoelectrochemical (PEC) water splitting and mineralization of glycerol are demonstrated. The thin-film WO₃/BiVO₄ photoelectrode was fabricated by a facile hydrothermal method. The morphology, chemical composition, crystalline structure, chemical state, and optical absorption properties of the WO₃/BiVO₄ photoelectrodes were characterized systematically. The WO₃/BiVO₄ photoelectrode exhibits a good distribution of elements and a well-crystalline monoclinic WO₃ and monoclinic scheelite BiVO₄. The light-absorption spectrum of the WO₃/BiVO₄ photoelectrodes reveals a broad absorption band in the visible light region with a maximum absorption of around 520 nm. The dual functional WO₃/BiVO₄ photoelectrodes achieved a high photocurrent density of 6.85 mA cm^-2, which is 2.8 times higher than that of the pristine WO₃ photoelectrode in the presence of a mixture of 0.5 M Na₂SO₄ and 0.5 M glycerol electrolyte under AM 1.5 G (100 mW cm^-2) illumination. The superior PEC performance of the WO₃/BiVO₄ photoelectrode was attributed to the synergistic effects of the superior crystal structure, light absorption, and efficient charge separation. Simultaneously, glycerol plays an essential role in increasing the efficiency of hydrogen production by suppressing charge recombination in the water redox reaction. Moreover, the WO₃/BiVO₄ photoelectrode shows the total organic carbon (TOC) removal efficiency of glycerol at about 82% at 120 min. Notably, the WO₃/BiVO₄ photoelectrode can be a promising photoelectrode for simultaneous hydrogen production and mineralization of glycerol with a simple, economical, and environmentally friendly approach.

WO3 Nanorods Decorated with Very Small Amount of Pt for Effective Hydrogen Evolution Reaction

The electrochemical hydrogen evolution reaction (HER) is one of the most promising green methods for the efficient production of renewable and sustainable H₂, for which platinum possesses the highest catalytic activity. Cost-effective alternatives can be obtained by reducing the Pt amount and still preserving its activity. The Pt nanoparticle decoration of suitable current collectors can be effectively realized by using transition metal oxide (TMO) nanostructures. Among them, WO₃ nanorods are the most eligible option, thanks to their high stability in acidic environments, and large availability. Herein, a simple and affordable hydrothermal route is used for the synthesis of hexagonal WO₃ nanorods (average length and diameter of 400 and 50 nm, respectively), whose crystal structure is modified after annealing at 400 °C for 60 min, to obtain a mixed hexagonal/monoclinic crystal structure. These nanostructures were investigated as support for the ultra-low-Pt nanoparticles (0.2-1.13 μg/cm²): decoration occurs by drop casting some drops of a Pt nanoparticle aqueous solution and the electrodes were tested for the HER in acidic environment. Pt-decorated WO₃ nanorods were characterized by performing scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), Rutherford backscattering spectrometry (RBS), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and chronopotentiometry. HER catalytic activity is studied as a function of the total Pt nanoparticle loading, thus obtaining an outstanding overpotential of 32 mV at 10 mA/cm², a Tafel slope of 31 mV/dec, a turn-over frequency of 5 Hz at -15 mV, and a mass activity of 9 A/mg at 10 mA/cm² for the sample decorated with the highest Pt amount (1.13 μg/cm²). These data show that WO₃ nanorods act as excellent supports for the development of an ultra-low-Pt-amount-based cathode for efficient and low-cost electrochemical HER.

Bismuth-based nanostructured photocatalysts for the remediation of antibiotics and organic dyes

A serious threat to human health and the environment worldwide, in addition to the global energy crisis, is the increasing water pollution caused by micropollutants such as antibiotics and persistent organic dyes. Nanostructured semiconductors in advanced oxidation processes using photocatalysis have recently attracted a lot of interest as a promising green and sustainable wastewater treatment method for a cleaner environment. Due to their narrow bandgaps, distinctive layered structures, plasmonic, piezoelectric and ferroelectric properties, and desirable physicochemical features, bismuth-based nanostructure photocatalysts have emerged as one of the most prominent study topics compared to the commonly used semiconductors (TiO₂ and ZnO). In this review, the most recent developments in the use of photocatalysts based on bismuth (e.g., BiFeO₃, Bi₂MoO₆, BiVO₄, Bi₂WO₆, Bi₂S₃) to remove dyes and antibiotics from wastewater are thoroughly covered. The creation of Z-schemes, Schottky junctions, and heterojunctions, as well as morphological modifications, doping, and other processes are highlighted regarding the fabrication of bismuth-based photocatalysts with improved photocatalytic capabilities. A discussion of general photocatalytic mechanisms is included, along with potential antibiotic and dye degradation pathways in wastewater. Finally, areas that require additional study and attention regarding the usage of photocatalysts based on bismuth for removing pharmaceuticals and textile dyes from wastewater, particularly for real-world applications, are addressed.

Improving Photoelectrochemical Activity of Magnetron-Sputtered Double-Layer Tungsten Trioxide Photoanodes by Irradiation with Intense Pulsed Ion Beams

The photoelectrochemical (PEC) activity of metal oxide photoelectrodes for water-splitting applications can be boosted in several different ways. In this study, we showed that PEC activity can be significantly improved with a double-layer (crystalline-amorphous) configuration of WO₃ thin films irradiated with intense pulsed ion beams (IPIB) of a nanosecond duration. It was found that IPIB irradiation promotes the formation of crystalline and sponge-like WO₃ structures on the surface. Due to an increase in the active surface and light scattering in irradiated samples, photocurrent generation increased by ~80% at 1.23 reversible hydrogen electrodes (RHE).

Emerging Surface, Bulk, and Interface Engineering Strategies on BiVO4 for Photoelectrochemical Water Splitting

The photoelectrochemical (PEC) cell that collects and stores abundant sunlight to hydrogen fuel promises a clean and renewable pathway for future energy needs and challenges. Monoclinic bismuth vanadate (BiVO₄ ), having an earth-abundancy, nontoxicity, suitable optical absorption, and an ideal n-type band position, has been in the limelight for decades. BiVO₄ is a potential photoanode candidate due to its favorable outstanding features like moderate bandgap, visible light activity, better chemical stability, and cost-effective synthesis methods. However, BiVO₄ suffers from rapid recombination of photogenerated charge carriers that have impeded further improvements of its PEC performances and stability. This review presents a close look at the emerging surface, bulk, and interface engineering strategies on BiVO₄ photoanode. First, an effective approach of surface functionalization via different cocatalysts to improve the surface kinetics of BiVO₄ is discussed. Second, state-of-the-art methodologies such as nanostructuring, defect engineering, and doping to further enhance light absorption and photogenerated charge transport in bulk BiVO₄ are reviewed. Third, interface engineering via heterostructuring to improve charge separation is introduced. Lastly, perspectives on the foremost challenges and some motivating outlooks to encourage the future research progress in this emerging frontier are offered.

Recent advances in degradation of organic pollutant in aqueous solutions using bismuth based photocatalysts: A review

Today, a major concern associated with the environment is the water pollution occurred due to the introduction of variety of persistent organic pollutants and residual dyes from different sources (e.g., dye and dye intermediates industries, paper and pulp industries, textile industries, tannery and craft bleaching industries, pharmaceutical industries, etc.) into our natural water resources. Recently, advanced oxidation processes (AOPs) by photocatalyst have garnered great attention as a new frontier promising eco-friendly and sustainable wastewater treatment technology. Utilization of the photocatalytic technology efficiently is significant for cleaner environment. Bismuth based photocatalyst have aroused widespread attention as a visible light responsive photocatalyst for waste water treatment due to their non-toxicity, low cost, modifiable morphology, and outstanding optical and chemical properties. In this review, we have dealt with the research progress on bismuth-based photocatalysts for waste water treatment. However, it seems to give limitation over pristine photocatalysts such as slow migration of charge carriers, charge carrier recombination, low visible light absorption, etc., Various bismuth based photocatalyst and its modifications via doping, heterojunction, Z-scheme etc., are discussed in detail. Further, the strategies adopted to improve the photocatalytic activity of bismuth based photocatalyst to improve the waste water treatment (mostly drugs and dyes) are critically reviewed. Also, we have discussed the bacterial inactivation by bismuth based photocatalyst. Finally, the challenges and future aspects against bismuth based photocatalyst are explored for further research.

Temperature Effect on Photoelectrochemical Water Splitting: A Model Study Based on BiVO4 Photoanodes

Photoelectrochemical (PEC) water splitting is typically studied at room temperature. In this work, the temperature effect on PEC water splitting is studied using crystalline BiVO₄ thin film photoanode as a model system. Systematic temperature-dependent electrochemical study demonstrates that the PEC activity is boosted at elevated electrolyte temperatures and indicates that thermal energy plays a main role in improving charge carrier transport in the bulk of BiVO₄. Irreversible surface reconstruction is observed after PEC reactions at elevated temperature in the presence of hole scavengers, with regularly spaced stripes emerging on BiVO₄ grains. The surface-reconstructed photoanode exhibits up to 40% improvement in photocurrent densities and ∼ 0.25 V shift of photocurrent onset to favorable direction. Detailed investigation shows the formation of an amorphous layer without stoichiometric change at the reconstructed surface. This work provides insights of the temperature effect on the photoelectrode in solar water splitting and reveals the non-negligible effect of hole scavengers in photoelectrochemical measurement.

Preparation of multilayer periodic nanopatterned WO3-based photoanode by reverse nanoimprinting for water splitting

Photoelectrochemical (PEC) water splitting has been studied extensively as an environmentally friendly technology for hydrogen production using solar energy. WO₃is considered a promising semiconducting material for photoanodes due to its high electron mobility, good hole diffusion length, and chemical stability. Periodic nanostructures of WO₃have been investigated for enhancing the PEC performance of WO₃-based photoanodes. In this study, facile fabrication of periodic nanostructures of WO₃was achieved using reverse nanoimprint lithography, and the multilayer stacking of nanopatterned WO₃film was also confirmed. The multilayer nanopatterned WO₃films were used as photoanodes for PEC water splitting. The performance of the fabricated photoanode in PEC was 2 times higher than that of planar WO₃film due to its higher light absorbance and lower charge transfer resistance.

Characterizing photocatalysts for water splitting: from atoms to bulk and from slow to ultrafast processes

This review provides a comprehensive overview on characterisation techniques for light-driven redox-catalysts highlighting spectroscopic, microscopic, electrochemical and spectroelectrochemical approaches.

p-CuInS2 /n-Polymer Semiconductor Heterojunction for Photoelectrochemical Hydrogen Evolution

An inorganic p-type CuInS₂ semiconductor was combined with the semiconducting polymer of PNDI3OT-Se1 and PNDI3OT-Se2 with different HOMO/LUMO levels for photoelectrochemical hydrogen production. Charge transfer behaviors at polymer/CuInS₂ junctions were investigated by electrochemical impedance spectroscopy. The heterojunction of p-CuInS₂ and n-type polymer (both PNDI3OT-Se1 and Se2) successfully made p-n junctions and showed improved charge transfer. However, we found that higher HOMO levels of polymer than valence band maximum (VBM) of CuInS₂ spurred charge recombination at interfaces. As a result, CuInS₂ /PNDI3OT-Se1/TiO₂ /Pt, which has suitable energy levels matched between PNDI3OT-Se1 and CuInS₂ , shows photocurrent (-15.67 mA cm^-2 ) improved concretely when compared to a CuInS₂ /TiO₂ /Pt photoelectrode (-7.11 mA cm^-2 ) at 0.0 V vs. RHE applied potential. Additionally, the photoelectrochemical stability of CuInS₂ /PNDI3OT-Se1/TiO₂ /Pt photoelectrode was also investigated.

Regulation of Ferroelectric Polarization to Achieve Efficient Charge Separation and Transfer in Particulate RuO2 /BiFeO3 for High Photocatalytic Water Oxidation Activity

Exploiting spontaneous polarization of ferroelectric materials to achieve high charge separation efficiency is an intriguing but challenging research topic in solar energy conversion. This work shows that loading high work function RuO₂ cocatalyst on BiFeO₃ (BFO) nanoparticles enhances the intrinsic ferroelectric polarization by efficient screening of charges to RuO₂ via RuO₂ /BFO heterojunction. This leads to enhancement of the surface photovoltage of RuO₂ /BFO single nanoparticles nearly 3 times, the driving force for charge separation and transfer in photocatalytic reactions. Consequently, efficient photocatalytic water oxidation is achieved with quantum efficiency as high as 5.36 % at 560 nm, the highest activity reported so far for ferroelectric materials. This work demonstrates that, unlike low photocurrent density in film-based ferroelectric devices, high photocatalytic activity could be achieved by regulating the ferroelectric spontaneous polarization using appropriate cocatalyst to enhance driving force for efficient separation and transfer of photogenerated charges in particulate ferroelectric semiconductor materials.

Photoactive Tungsten-Oxide Nanomaterials for Water-Splitting

This review focuses on tungsten oxide (WO3) and its nanocomposites as photoactive nanomaterials for photoelectrochemical cell (PEC) applications since it possesses exceptional properties such as photostability, high electron mobility (~12 cm2 V-1 s-1) and a long hole-diffusion length (~150 nm). Although WO3 has demonstrated oxygen-evolution capability in PEC, further increase of its PEC efficiency is limited by high recombination rate of photogenerated electron/hole carriers and slow charge transfer at the liquid-solid interface. To further increase the PEC efficiency of the WO3 photocatalyst, designing WO3 nanocomposites via surface-interface engineering and doping would be a great strategy to enhance the PEC performance via improving charge separation. This review starts with the basic principle of water-splitting and physical chemistry properties of WO3, that extends to various strategies to produce binary/ternary nanocomposites for PEC, particulate photocatalysts, Z-schemes and tandem-cell applications. The effect of PEC crystalline structure and nanomorphologies on efficiency are included. For both binary and ternary WO3 nanocomposite systems, the PEC performance under different conditions-including synthesis approaches, various electrolytes, morphologies and applied bias-are summarized. At the end of the review, a conclusion and outlook section concluded the WO3 photocatalyst-based system with an overview of WO3 and their nanocomposites for photocatalytic applications and provided the readers with potential research directions.

Photocatalytic Degradation of Chlorpyrifos with Mn-WO3/SnS2 Heterostructure

Tungsten trioxide (WO3) is a photocatalyst that has gained interest amongst researchers because of its non-toxicity, narrow band gap and superior charge transport. Due to its fast charge recombination, modification is vital to counteract this limitation. In this paper, we report on the fabrication of Mn-doped WO3/SnS2 nanoparticles, which were synthesised with the aim of minimising the recombination rates of the photogenerated species. The nanomaterials were characterised using spectroscopic techniques (UV-Vis-diffuse reflectance spectroscopy (DRS), Raman, XRD, photoluminescence (PL) and electrochemical impedance spectroscopy (EIS)) together with microscopic techniques (FESEM-EDS and high resolution transmission electron microscopy selected area electron diffraction (HRTEM-SAED)) to confirm the successful formation of Mn-WO3/SnS2 nanoparticles. The Mn-doped WO3/SnS2 composite was a mixture of monoclinic and hexagonal phases, confirmed by XRD and Raman analysis. The Mn-WO3/SnS2 heterojunction showed enhanced optical properties compared to those of the un-doped WO3/SnS2 nanoparticles, which confirms the successful charge separation. The Brunauer–Emmett–Teller (BET) analysis indicated that the nanoparticles were mesoporous as they exhibited a Type IV isotherm. These nanomaterials appeared as a mixture of rectangular rods and sheet-like shapes with an increased surface area (77.14 m2/g) and pore volume (0.0641 cm3/g). The electrochemical measurements indicated a high current density (0.030 mA/cm2) and low charge transfer resistance (157.16 Ω) of the Mn-WO3/SnS2 heterojunction, which infers a high charge separation, also complemented by photoluminescence with low emission peak intensity. The Mott–Schottky (M-S) plot indicated a positive slope characteristic of an n–n heterojunction semiconductor, indicating that electrons are the major charge carriers. Thus, the efficiency of Mn-WO3/SnS2 heterojunction photocatalyst was monitored for the degradation of chlorpyrifos. The effects of pH (3–9), catalyst loading (0.1–2 g) and initial chlorpyrifos concentration (100 ppb–20 ppm) were studied. It was observed that the degradation was purely due to photocatalysis, as no loss of chlorpyrifos was observed within 30 min in the dark. Chlorpyrifos removal using Mn-WO3/SnS2 was performed at the optimum conditions of pH = 7, catalyst loading = 1 g and chlorpyrifos concentration = 1000 ppb in 90 min. The complete degradation of chlorpyrifos and its major degradation by-product 3,5,6-trichloropyridin-2-ol (TCP) was achieved. Kinetic studies deduced a second order reaction at 209 × 10−3 M−1s−1.

Engineering the Morphology and Crystal Phase of 3 D Hierarchical TiO2 with Excellent Photochemical and Photoelectrochemical Solar Water Splitting

Owing to their unique characteristics, hierarchical TiO₂ nanostructures have several advantages in solar-fuel production. In this work, a single-step approach has been developed to control both the crystal phase and morphology of TiO₂ with 3 D urchin-like structure via a surfactant-free solvothermal route. The growth of 3 D hierarchical structure with phase-engineered band alignment, the role of the H₂ O/HCl ratio, and fine-tuning of the reaction parameters are investigated systematically. An optimum ratio of anatase (41 %) to rutile (59 %) in the mixed-phase TiO₂ (AR-2) results in excellent photocatalytic H₂ generation activity of 5753 μmol g^-1 after 5 h of irradiation with apparent quantum yields of 20.9 % at 366 nm and 4.5 % at 420 nm. The superior performance of AR-2, attributed to efficient separation of charge carriers through the phase junction, is apparent from the transient photocurrent response and photoluminescence studies. The 3 D urchin-like pure rutile TiO₂ (R-1) composed of nanorods shows enhanced photocatalytic activity compared with pure anatase and pure rutile TiO₂ nanoparticles, and this demonstrates the role of morphology. The best-performing mixed-phase 3 D TiO₂ shows excellent durability up to 25 h and is shown to produce 3522 μmol g^-1 of H₂ under natural sunlight, which highlights its potential for long-term application.

Surface Engineering of WO3/BiVO4 to Boost Solar Water-Splitting

Single-phase photoanodes often suffer inferior charge transport, which can be mitigated by constructing efficient heterojunctions. Thus, we have fabricated a fluorine-doped tin oxide (FTO)/WO3/BiVO4 heterojunction using hydrothermal and spin-coating methods. Surface engineering was exploited to further accelerate the reaction kinetics, which was achieved via post-modification with NaOH solution. This treatment alters the surface chemical state of the BiVO4 nanoparticles, leading to enhanced charge transport and surface water oxidation processes. As a result, the optimized sample can produce a photocurrent more than two times that of WO3. The simple post-treatment provides a viable and cost-effective strategy for promoting the photoelectric properties of photoanodes.

The optimization of surface morphology of Au nanoparticles on WO3 nanoflakes for plasmonic photoanode

Among many candidates for photoanode materials of photoelectrochemical (PEC) cell, nanostructured tungsten trioxide (WO₃) is regarded as one of the most promising materials due to its superior electrical properties and adequate bandgap (∼2.8 eV) and band edge position. WO₃ nanoflakes (WO₃ NFs), which have merits on its high surface area and crystallinity, have been actively studied for this manner but solar-to-hydrogen efficiency of WO₃ NFs based photoanode is still not sufficient both in light absorption and charge separation. Plasmon-induced enhancement using Au nanoparticles is excellent approach for both the efficiency of light absorption and charge separation of WO₃. However, it still needs optimization on its amount, shape, coverage, and etc. Here, we synthesized WO₃ NFs by solvothermal growth and decorated gold nanoparticles on these nanoflakes by e-beam evaporation and rapid thermal annealing process in a row. By this process, a large-area AuNPs/WO₃ nanocomposite structure with various size, interparticle distance, and coverage of AuNPs were fabricated. These AuNPs/WO₃ NFs type photoanode achieve high light absorption both in UV and visible range and consequently higher photocurrent density. The optimized AuNPs/WO₃ nanocomposite photoanode exhibits 1.01 mA cm^-2 of photocurrent density, which is increased to 19.8% compared with bare WO₃ nanoflakes. Field emission-scanning electron microscope, x-ray diffraction, UV-vis spectrometer analysis were measured to analyze the morphology and crystallinity and relationship between structure and PEC performance.

Multilayer WO3/BiVO4 Photoanodes for Solar-Driven Water Splitting Prepared by RF-Plasma Sputtering

A series of WO3, BiVO4 and WO3/BiVO4 heterojunction coatings were deposited on fluorine-doped tin oxide (FTO), by means of reactive radio frequency (RF) plasma (co)sputtering, and tested as photoanodes for water splitting under simulated AM 1.5 G solar light in a three-electrode photoelectrochemical (PEC) cell in a 0.5 M NaSO4 electrolyte solution. The PEC performance and time stability of the heterojunction increases with an increase of the WO3 innermost layer up to 1000 nm. A two-step calcination treatment (600 °C after WO3 deposition followed by 400 °C after BiVO4 deposition) led to a most performing photoanode under back-side irradiation, generating a photocurrent density of 1.7 mA cm−2 at 1.4 V vs. SCE (i.e., two-fold and five-fold higher than that generated by individual WO3 and BiVO4 photoanodes, respectively). The incident photon to current efficiency (IPCE) measurements reveal the presence of two activity regions over the heterojunction with respect to WO3 alone: The PEC efficiency increases due to improved charge carrier separation above 450 nm (i.e., below the WO3 excitation energy), while it decreases below 450 nm (i.e., when both semiconductors are excited) due to electron–hole recombination at the interface of the two semiconductors.

First-principles investigation of the electronic properties of the Bi2O4(101)/BiVO4(010) heterojunction towards more efficient solar water splitting

First-principles calculations based on density functional theory were carried out to explore the geometric structure, light absorption, charge separation, over-potential and stability of Bi₂O₄ (101)/BiVO₄ (010) heterojunction. The results show that the formed heterojunction can improve visible light utilization and promote transfer of photo-generated holes from BiVO₄ to Bi₂O₄. Furthermore, the Bi⁵⁺ site in the Bi₂O₄(101) surface is energetically more favorable as the photoanode for the oxygen evolution reaction (OER) than the Bi³⁺ sites in Bi₂O₄(101) and BiVO₄(010). At the same time, it is also found that the Bi⁵⁺ in Bi₂O₄(101) are more stable than the Bi³⁺ due to the lower surface energy and stronger bond energy with neighbors. Therefore, forming the Bi₂O₄/BiVO₄ heterojunction can effectively improve the activity and stability of BiVO₄ for water splitting reactions.

Photoinduced electron transfer in WO3/BiVO4 heterojunction photoanodes: effects of the WO3 layer thickness

The PEC performance of WO₃/BiVO₄ heterojunction photoanodes with a fixed BiVO₄ thick top layer and different WO₃ layer thicknesses was investigated under backside irradiation, in comparison with the performance of the same electrodes without a top BiVO₄ layer. While the performance of these latter increase with increasing WO₃ thickness, the presence of a BiVO₄ layer, besides leading to an effective sensitization up to 520 nm, leads to a decrease of incident photon to current efficiency in the short wavelength's range. After having excluded major WO₃ filter effects, this has been attributed to charge carrier recombination effects occurring when both oxides get excited and becoming more relevant with increasing WO₃ thickness and decreasing excitation wavelength.

Tungsten oxide nanostructures and nanocomposites for photoelectrochemical water splitting

Hydrogen production from photoelectrochemical (PEC) water splitting using semiconductor photocatalysts has attracted great attention to realize clean and renewable energy from solar energy. The visible light response of WO3 with a long hole diffusion length (∼150 nm) and good electron mobility (∼12 cm2 V-1 s-1) makes it suitable as the photoanode. However, WO3 suffers from issues including rapid recombination of photoexcited electron-hole pairs, photo-corrosion during the photocatalytic process due to the formation of peroxo-species, sluggish kinetics of photogenerated holes, and slow charge transfer at the semiconductor/electrolyte interface. This work highlights the approaches to overcome these drawbacks of WO3 photoanodes, including: (i) the manipulation of nanostructured WO3 photoanodes to decrease the nanoparticle size to promote hole migration to the WO3/electrolyte interface which benefits the charge separation; (ii) doping or introducing oxygen vacancies to improve electrical conductivity; exposing high energy crystal surfaces to promote the consumption of photogenerated holes on the high-active crystal face, thereby suppressing the recombination of photogenerated electrons and holes; (iii) decorating with co-catalysts to reduce the overpotential which inhibits the formation of peroxo-species; (iv) other methods such as coupling with narrow band semiconductors to accelerate the charge separation and controlling the crystal phase via annealing to reduce defects. These approaches are reviewed with detailed examples.

All-Solution-Processed WO3/BiVO4 Core-Shell Nanorod Arrays for Highly Stable Photoanodes

Tungsten oxide (WO₃) and bismuth vanadate (BiVO₄) are one of the most attractive combinations to construct an efficient heterojunction for photoelectrochemical (PEC) applications. Here, we report an all-solution-processed WO₃/BiVO₄ heteronanostructure photoanode with highly enhanced photoactivity and stability for sustainable energy production. The vertically aligned WO₃ nanorods were synthesized on a fluorine-doped tin oxide/glass substrate by the hydrothermal method without a seed layer and BiVO₄ was deposited by pulsed electrodeposition for conformal coating. Owing to the long diffusion lengths of charge carriers in the WO₃ nanorods, the ability to absorb the wider range of wavelengths, and appropriate band-edge positions of the WO₃/BiVO₄ heterojunction for spontaneous PEC reaction, the optimum WO₃/BiVO₄ photoanode has a photocurrent density of 4.15 mA/cm² at 1.23 V versus RHE and an incident-photon-to-current efficiency of 75.9% at 430 nm under front illumination, which are a double and quadruple those of pristine WO₃ nanorod arrays, respectively. Our work suggests an environment-friendly and low-cost all-solution process route to synthesize high-quality photoelectrodes.

Enhanced Photoelectrochemical Water Splitting with Er- and W-Codoped Bismuth Vanadate with WO3 Heterojunction-Based Two-Dimensional Photoelectrode

A novel two-dimensional (2D) heterojunction photoelectrode composed of WO₃ and (Er,W):BiVO₄ is proposed for water oxidation with efficient photoinduced charge carrier separation and transfer. Er stoichiometric along with W nonstoichiometric codoping was introduced to simultaneously manage vacancy creation during substitutional doping, enhance light absorption, and reduce overall impedance. It was found that Er³⁺ is substituted at the Bi³⁺ sites in the BiVO₄ lattice to provide expanded light absorption from 400 to 680 nm. The fabricated WO₃/(Er,W):BiVO₄ electrode shows photocurrent densities of 4.1 and 7.2 mA cm^-2 at 1.23 and 2.3 V (vs reversible hydrogen electrode, RHE), respectively, under a 1 sun illumination in K₂HPO₄ electrolyte. This electrode has shown remarkably high charge separation efficiency of 93% at 1.23 V (vs RHE). With the addition of a standard surface catalyst (i.e., Co-Pi), the WO₃/(Er,W):BiVO₄/Co-Pi electrode exhibits the highest photocurrent of 5.6 ± 0.3 mA cm^-2 at 1.23 V (vs RHE), nearing the theoretical limit (i.e., 7.5 mA cm^-2) while retaining 98% of the photoelectrochemical cell performance after 3 h. By concomitantly doping the Bi³⁺ and V⁵⁺ sites to enhance absorption, this study demonstrates for the first time a planar WO₃/BiVO₄ heterojunction that reaches 88% of the record-high performance of its nanostructured counterpart. Through a detailed characterization of the electrodes, it is concluded that the stoichiometric Er and nonstoichiometric W codoping extend light absorption region and improve charge separation efficiency by reducing bulk resistance. The photoactive materials with 2D morphology were synthesized using a facile ultrasonic spray-coating technique without any complex process steps and thus it can be scaled for commercial development.

Novel multilayer TiO2 heterojunction decorated by low g-C3N4 content and its enhanced photocatalytic activity under UV, visible and solar light irradiation

In this paper, we used a facile ball milling, microwave radiation and heating treatment method to achieve the surface modification of TiO2 with low g-C3N4 concentration, and a multilayer heterojunction composite with TiO2 as the main part and g-C3N4 as the modification agent was obtained. The obtained materials were analyzed by several characterizations to get information on their chemical composition, crystalline structure, vibrational features and optical properties. The photocatalytic performance was evaluated by degradation of rhodamine B (RhB) and methylene blue (MB) under UV, visible and direct solar light irradiation. Its photocatalytic activity was enhanced depended on the novel structure of g-C3N4/TiO2 hybrid and the special Z-scheme electron-hole transfer model of multilayer heterointerfaces. The material preparation and structural features could be useful for the design and development of other photocatalysts with high photocatalytic activity.

Ultrathin TiO2/BiVO4 nanosheet heterojunction arrays modified with NiFe-LDH nanoparticles for enhanced photoelectrochemical oxidation of water

Herein, we have constructed NiFe-LDH nanoparticles modified ultrathin TiO₂/BiVO₄ nanosheet heterojunction arrays and explored the effects of different NiFe-LDH loading amount on photoelectrochemical water oxidation performance. The photocurrent of as-prepared TiO₂/BiVO₄/NiFe-LDH photoanode is about 2.5 times than that of TiO₂/BiVO₄, which is ascribed to the synergistic effect of heterojunction and co-catalyst. The heterojunction between TiO₂ and BiVO₄ suppresses the recombination of photogenerated electron-hole pairs effectively and the co-catalyst of NiFe-LDH accelerates the surface water oxidation reaction kinetics.

Conformal BiVO4-Layer/WO3-Nanoplate-Array Heterojunction Photoanode Modified with Cobalt Phosphate Cocatalyst for Significantly Enhanced Photoelectrochemical Performances

Constructing semiconductor heterojunctions via surface/interface engineering is an effective way to enhance the charge carrier separation/transport ability and thus the photoelectrochemical (PEC) properties of a photoelectrode. Herein, we report a conformal BiVO₄-layer/WO₃-nanoplate-array heterojunction photoanode modified with cobalt phosphate (Co-Pi) as oxygen evolution cocatalyst (OEC) for significant enhancement in PEC performances. The BiVO₄/WO₃ nanocomposite is fabricated by coating a thin conformal BiVO₄ layer on the surface of presynthesized WO₃ nanoplate arrays (NPAs) via stepwise spin-coating, and the decoration of Co-Pi OEC is realized by photoassisted electrodeposition method. The optimized Co-Pi@BiVO₄/WO₃ heterojunction photoanode shows a maximum photocurrent of 1.8 mA/cm² at 1.23 V vs RHE in a phosphate buffer electrolyte under an AM1.5G solar simulator, which is 5 and 12 times higher than those of bare WO₃ and BiVO₄ photoanode, respectively. Measurements of UV-vis absorption spectra, electrochemical impedance spectra (EIS) and photoluminescence (PL) spectra reveal that the enhanced PEC performances can be attributed to the increased charge carrier separation/transport benefited from the type II nature of BiVO₄/WO₃ heterojunction and the promoted water oxidation kinetics and photostability owing to the decoration of Co-Pi cocatalyst.

Activation of Water on MnOx-Nanocluster-Modified Rutile (110) and Anatase (101) TiO2 and the Role of Cation Reduction

Surface modification of titania surfaces with dispersed metal oxide nanoclusters has the potential to enhance photocatalytic activity. These modifications can induce visible light absorption and suppress charge carrier recombination which are vital in improving the efficiency. We have studied heterostructures of Mn4O6 nanoclusters modifying the TiO2 rutile (110) and anatase (101) surfaces using density functional theory (DFT) corrected for on-site Coulomb interactions (DFT + U). Such studies typically focus on the pristine surface, free of the point defects and surface hydroxyls present in real surfaces. In our study we have considered partial hydroxylation of the rutile and anatase surfaces and the role of cation reduction, via oxygen vacancy formation, and how this impacts on a variety of properties governing the photocatalytic performance such as nanocluster adsorption, light absorption, charge separation, and reducibility. Our results indicate that the modifiers adsorb strongly at the surface and that modification extends light absorption into the visible range. MnOx-modified titania can show an off-stoichiometric ground state, through oxygen vacancy formation and cation reduction spontaneously, and both modified rutile and anatase are highly reducible with moderate energy costs. Manganese ions are therefore present in a mixture of oxidation states. Photoexcited electrons and holes localize at cluster metal and oxygen sites, respectively. The interaction of water at the modified surfaces depends on the stoichiometry and spontaneous dissociation to surface bound hydroxyls is favored in the presence of oxygen vacancies and reduced metal cations. Comparisons with bare TiO2 and other TiO2-based photocatalyst materials are presented throughout.

WO3/BiVO4 Type-II Heterojunction Arrays Decorated with Oxygen-Deficient ZnO Passivation Layer: A Highly Efficient and Stable Photoanode

In the present work, we report a ternary WO₃/BiVO₄/ZnO photoanode with boosted PEC efficiency and stability toward highly efficient water splitting. The type-II WO₃/BiVO₄ heterojunction arrays are firstly prepared by hydrothermal growth of WO₃ nanoplate arrays onto the substrates of fluorine-doped tin oxide (FTO)-coated glass, followed by spin-coating of BiVO₄ layers onto the WO₃ nanoplate surfaces. After that, thin ZnO layers are further introduced onto the WO₃/BiVO₄ heterojunction arrays via atomic layer deposition (ALD), leading to the construction of ternary WO₃/BiVO₄/ZnO photoanodes. It is verified that the ZnO thin layer in the WO₃/BiVO₄/ZnO photoanode contains abundant oxygen vacancies, which could act as an effective passivation layer to enhance the charge separation and surface water oxidation kinetics of photogenerated carriers. The as-prepared WO₃/BiVO₄/ZnO photoanode produces a photocurrent of 2.96 mA cm^-2 under simulated sunlight with an incident photon-to-current conversion efficiency (IPCE) of ∼72.8% at 380 nm at a potential of 1.23 V versus RHE without cocatalysts, both of which are comparable to the state-of-the-art WO₃/BiVO₄ counterparts. Moreover, the photocurrent of the WO₃/BiVO₄/ZnO photoanode shows only 9% decay after 6 h, suggesting its high photoelectrochemical (PEC) stability.