Photoanodes For Water Splitting Research Articles

Modulating light absorption and charge recombination in photoelectrochemical water oxidation with spin-coated MoS2 co-catalyst on ZnO nanorods

Efficient oxygen evolution reaction (OER) on a photoanode is until today considered a major challenge. In the case of ZnO, one of the most investigated semiconductors, the limitation is due to its low visible light absorption and severe charge recombination. Combining ZnO with catalytically active 2D materials such as MoS2 has been proven to increase its water oxidation performance, but they are still often produced using complex methods. Here, we present a simple and low-cost method of spin-coating to fabricate ZnO nanorods/MoS2 nanosheets (ZRM) heterostructures for OER photoanode. Using this method, we successfully produce heterostructures with improved performance, that is, measured photocurrent density (Jph) of 0.61 mA cm−2 at 1.23 V vs. RHE, about threefold than that of pure ZnO, with excellent photostability, which is on par with similar heterostructures synthesized using more complex methods. Beyond benchmarking the performance, we also unravel the mechanism by which MoS2 improves the water oxidation process, that is, through the efficient light-harvesting (ηLHE), charge separation (ηsep), and charge injection (ηinj). Our findings provide a straightforward route to produce metal oxide/2D materials heterostructure-based photoanodes for photoelectrochemical water splitting.

Chemical Bond-Modulated Interfacial Energy Band Structures of Heterojunctions for Efficient Photoelectrochemical Water Splitting

Understanding the interfacial interaction of a nanostructure heterojunction and improving the efficiency of photoanodes are of great significance to develop photoelectrochemical (PEC) water splitting. Herein, taking BiVO4 and Bi2S3 as model materials, we investigate the modulation effect of a chemical bond at the heterojunction interface on the energy band structure. A BiVO4/Bi2S3 heterojunction is favorably constructed by a convenient chemical technique of successive ionic layer absorption and reaction (SILAR) method. We find that a Bi–O chemical bond is reasonably introduced at the BiVO4 and Bi2S3 interface, which is different from the physical contact heterojunction of BiVO4/Bi2S3(DC). Experimental and theoretical studies reveal that the Bi–O bond at the heterojunction interface distinctly downshifts the Fermi level of the BiVO4 surface and reverses the bending direction of the interfacial band from the former type II structure to a direct Z-scheme structure. Due to the excellent charge separation efficiency and high redox potential, the heterojunction of BiVO4/Bi2S3 (SILAR) exhibits a significantly raised photocurrent density of 2.71 mA cm–2 at 1.23 VRHE, 11.29 times higher than that of BiVO4/Bi2S3(DC). This study emphasizes the modulation effect of interfacial chemical bonds in the fabrication of heterojunctions and provides a reference to construct high-activity photoanodes for PEC water splitting.

Sol-Gel-Derived Ordered Mesoporous High Entropy Spinel Ferrites and Assessment of Their Photoelectrochemical and Electrocatalytic Water Splitting Performance.

The novel material class of high entropy oxides with their unique and unexpected physicochemical properties is a candidate for energy applications. Herein, it is reported for the first time about the physico- and (photo-) electrochemical properties of ordered mesoporous (CoNiCuZnMg)Fe2 O4 thin films synthesized by a soft-templating and dip-coating approach. The A-site high entropy ferrites (HEF) are composed of periodically ordered mesopores building a highly accessible inorganic nanoarchitecture with large specific surface areas. The mesoporous spinel HEF thin films are found to be phase-pure and crack-free on the meso- and macroscale. The formation of the spinel structure hosting six distinct cations is verified by X-ray-based characterization techniques. Photoelectron spectroscopy gives insight into the chemical state of the implemented transition metals supporting the structural characterization data. Applied as photoanode for photoelectrochemical water splitting, the HEFs are photostable over several hours but show only low photoconductivity owing to fast surface recombination, as evidenced by intensity-modulated photocurrent spectroscopy. When applied as oxygen evolution reaction electrocatalyst, the HEF thin films possess overpotentials of 420mV at 10mA cm-2 in 1m KOH. The results imply that the increase of the compositional disorder enhances the electronic transport properties, which are beneficial for both energy applications.

Fabrication of Homogeneous Nanoporous Structure on 4H-/6H-SiC Wafer Surface via Efficient and Eco-Friendly Electrolytic Plasma-Assisted Chemical Etching.

Nanoporous single-crystal silicon carbide (SiC) is widely used in various applications such as protein dialysis, as a catalyst support, and in photoanodes for photoelectrochemical water splitting. However, the fabrication of nano-structured SiC is challenging owing to its extreme chemical and mechanical stability. This study demonstrates a highly-efficient, open-circuit electrolytic plasma-assisted chemical etching (EPACE) method without aggressive fluorine-containing reactants. The EPACE method enables the nano-structuring of SiC via a plasma-enveloped microtool traversing over the target material in an electrolyte bath. Through process design, EPACE readily produces a uniform nanoporous layer on a 4H-SiC wafer in KOH aqueous solution, with adjustable pore diameters in the range 40-130nm. Plasma diagnosis by optical emission spectrometry (OES) and surface microanalysis reveal that EPACE realizes a nanoporous structure by electrolytic plasma-assisted oxidation and subsequent thermochemical reduction of an oxide. An increase in voltage or a decrease in etch gap intensifies the plasma and improves the etching efficiency. The maximum etch rate and depth reach 540nm min-1 and 10µm, respectively, demonstrating the significant potential of the approach as a time-saving and sustainable nanofabrication method for industrial applications. Further, the effectiveness of the fabricated SiC nanoporous structure for application in photoelectrochemical water splitting is demonstrated.

Photoelectrochemical performance of bismuth vanadate photoanode for water splitting under concentrated light irradiation

Photoelectrochemical (PEC) water splitting is a promising way to convert solar energy into hydrogen energy. It is typically carried out at room temperature (RT) and 1 sun illumination. The PEC water splitting under concentrated light is expected to be an effective route to improve PEC performance, but there are few studies on it. Herein, CoPi/Mo:BiVO4 photoanode was selected to investigate the effect of concentrated light and the reaction temperature on its PEC performance. It was revealed that CoPi/Mo:BiVO4 showed enhanced PEC performance under concentrated light. The photocurrent density was enhanced with increased light intensity and increased reaction temperature. At a high temperature (60 °C), the normalized photocurrent density (3.31 mA cm−2 at 1.23 V vs. RHE) was found to be optimal at 4 suns, which was attributed to the synergistic effect of concentrating light and heating. It is proved that concentrated light can effectively improve the PEC performance, which has important guiding significance to realize the low-cost and efficient PEC water splitting.

Co-dependency of TiO2 underlayer and ZrO2 top layer in sandwiched microwave-assisted Zr-Fe2O3 photoanodes for photoelectrochemical water splitting

We report the synergistic effect of ZrO2 top and TiO2 under layers on the microwave-assisted Zr-Fe2O3 photoanodes. The optimum TZF2ZQ exhibited 144.5 and 71.3 μmol of H2 and O2 evolution at 1.23 V vs. RHE.

Optimizing the performance of Auy/Nix/TiO2NTs photoanodes for photoelectrochemical water splitting.

Water splitting using photoelectrochemical (PEC) techniques is thought to be a potential method for creating green hydrogen as a sustainable energy source. How to create extremely effective electrode materials is a pressing concern in this area. In this work, a series of Nix/TiO2 anodized nanotubes (NTs) and Auy/Nix/TiO2NTs photoanodes were prepared by electrodeposition via cyclic voltammetry and UV-photoreduction, respectively. The photoanodes were characterized by several structural, morphological, and optical techniques and their performance in PEC water-splitting for oxygen evolution reaction (OER) under simulated solar light was investigated. The obtained results revealed the nanotubular structure of TiO2NTs was preserved after deposition of NiO and Au nanoparticles while the band gap energy was reduced allowing for effective utilization of solar light with lower charge recombination rate. The PEC performance was monitored and it was found that the photocurrent densities of Ni20/TiO2NTs and Au30/Ni20/TiO2NTs were 1.75-fold and 3.25-fold that of pristine TiO2NTs, respectively. It was confirmed that the performance of the photoanodes depends on the number of electrodeposition cycles and duration of photoreduction of gold salt solution. The observed enhanced OER activity of Au30/Ni20/TiO2NTs could be attributed to the synergism between the local surface plasmon resonance (LSPR) effect of nanometric gold which increased solar light harvesting and the p-n heterojunction formed at the NiO/TiO2 interface which led to better charge separation and transportation suggesting its potential application as an efficient and stable photoanode in PEC water splitting for H2 production.

Durability of SrTiO3-TiO2 eutectic composite as a photoanode for photoelectrochemical water splitting.

The idea of employing sunlight - a virtually inexhaustible source of energy - to catalyze various chemical reactions or generate electrical current is intensively studied nowadays. Here, we describe a method for testing photoelectrochemical (PEC) stability developed using the example of photoanodes from an SrTiO3-TiO2 eutectic composite. Eutectic composite stability measurements were carried out in long-term cycles: 0.5, 1, 2, 5, 10, 20 and 50 h of constant electrode operation (total of 88.5 h). After each cycle, cyclic voltammetry, electrochemical impedance spectroscopy, reflectance, roughness, SEM/EDS microstructure analysis and the content of Sr and Ti ions in the applied electrolyte solution were examined. The initial value of the photocurrent density was 1.95 mA cm-2 at a potential of 1.5 V vs. Ag/AgCl in a pH 2 electrolyte environment and under 6 suns of illumination it increased almost four times, reaching 7.22 mA cm-2 after a total of 88.5 h of PEC stability cycles. Due to the better catalytic properties of TiO2, this phase degrades faster, causing an increase in the roughness of the electrode surface. At the same time, reflectance of the photoanode active layer dropped from around 35% to 15%. The investigated method of PEC material testing can be applied in areas beyond photoelectrochemical water splitting, such as chemistry, photovoltaics, sensing and others.

Recent progress and perspectives on heteroatom doping of hematite photoanodes for photoelectrochemical water splitting

Over the past few decades, extensive research on photoelectrochemical (PEC) water splitting has been conducted as a promising solution to meet the increasing demand for cleaner and renewable energy in a sustainable manner.

Effect of Sr and Ti substitutions on optical and photocatalytic properties of Bi1-x Sr x Fe1-x Ti x O3 nanomaterials.

The potential use of down-sized BFO-xSTO systems (x ≤ 25%) as highly efficient photoanodes for photocatalytic water splitting is investigated. BFO-xSTO is prepared by a solid-state method and subsequently deposited by spray coating. The compounds possess rhombohedral symmetry for x ≤ 15% and phase coexistence for x > 15%, as demonstrated by Raman spectroscopy and transmission electron microscopy. Our findings revealed a drastic grain size decrease with increasing STO content, namely 260 nm for BFO to 50 nm for BFO with 25% STO. Moreover, BFO-xSTO, x > 10% exhibited high optical absorption (> 80%) in the full spectrum and interestingly a very promising band alignment with water redox potentials. Moreover, the photochemical measurements revealed a photocurrent density of ∼0.17 μA cm-2 achieved for x = 15% at 0 bias. Using DFT calculations, the substitution effects on the electronic, optical, and photocatalytic performances of the BFO system were investigated and quantified. Surprisingly, a high hydrogen yield (∼191 μmol g-1) was achieved by BFO-12.5%STO compared to 1 μmol g-1 and 57 μmol g-1 for BFO and STO, respectively. This result highlights the beneficial effects of both the downsizing and substitution of BFO on the photocatalytic water splitting and hydrogen production performances of Bi1-x Sr x Fe1-x Ti x O3 systems.

Core–shell InN/PM6 Z-scheme heterojunction photoanodes for efficient and stable photoelectrochemical water splitting

This Z-scheme heterostructure forms strong In–O bonds under light, driving the electrons of InN to combine with the holes of PM6, thus inhibiting charge recombination. The optimized device exhibits excellent PEC performance and photochemical stability.

Photochemical and electrochemical co-regulation of the BiVO4 photoanode for water splitting.

A novel pretreatment strategy that can regulate the amount of oxygen vacancies (Ovac) across the wormlike-BiVO4 photoanode by photochemical and electrochemical co-processing. Upon decorating NiFeOx as an oxygen evolution cocatalyst for promoting the surface oxidation kinetics, a record-high photocurrent density of 6.42 mA cm-2 is obtained at 1.23 vs. RHE (100 mW cm-2).

From doping to composites: zirconia (ZrO2) modified hematite photoanodes for water splitting.

Herein, a ZrO2 added α-Fe2O3 photoanode that can split water at low applied potential is reported. First, the pristine hematite α-Fe2O3 photoanode was synthesized using an aerosol-assisted chemical vapour deposition (AACVD) method followed by modification with various amounts of ZrO2 (2 to 40%) in the form of thin films on conducting glass substrate. The XRD, Raman spectroscopy and scanning electron microscopy (SEM) analyses confirmed the presence of the monoclinic phase of ZrO2 in the composites with multifaceted particles of compact morphology. The optical analysis showed an increase in the absorbance and variation in band gap of the composites ascribed to the heterogeneity of the material. The photoelectrochemical studies gave a photocurrent density of 1.23 mA cm-2 at 1.23 V vs. RHE for the pristine hematite and remarkably higher value of 3.06 mA cm-2 for the optimized amount of ZrO2 in the modified α-Fe2O3 photoanode. To the best of our knowledge, this is the highest photocurrent reported for a ZrO2 containing photoanode. The optimized composite electrode produced nine times more oxygen than that produced by pristine hematite.

Fabrication of a stable Au/Ta2O5 plasmonic photoanode for water splitting under visible light irradiation

Plasmonic water splitting to H2 and O2 with the ratio of 2 : 1 occurred continuously under irradiation of visible light when an Au/Ta2O5/FTO photoanode was used under an externally applied potential of 0.8 V vs. RHE to a Pt counter electrode.

Meso-pore generating P doping for efficient photoelectrochemical water splitting

Hematite (Fe2O3) has been widely used as a photoanode in photoelectrochemical water splitting (PEC) for green hydrogen production. Here, for the first time, we investigate how a simple in-situ phosphorus (P) doping strategy improves the overall PEC performance of hematite with a systematic analysis of the various effects on the PEC performance. By introducing enriched FePO4 regions on the Ti-doped FeOOH surface and subsequent high-temperature annealing via P-doping, meso-porous P,Ti co-doped Fe2O3 (P,Ti-Fe2O3) nanorods were fabricated. P,Ti-Fe2O3 exhibited four-fold and two-fold increased BET surface area and electrical active area, respectively, compared to that of Ti-Fe2O3. Benefiting from the nano-structuring and efficient P doping effects [e.g., increased carrier density (Nd=3.48168 ×1020 cm−3), enhanced charge separation (ηbulk= 38.7% and ηsurface= 79.1%), and steeper band bending (Wd=3.910 nm)], the resulting P,Ti-Fe2O3 photoanode exhibited 94% improved photocurrent density of 2.50 mA cm−2 compared to that of Ti-Fe2O3 (@ 1.23 VRHE) under 1 sun illumination. With the deposition of the NiFeOx cocatalyst, the NiFeOx/P,Ti-Fe2O3 photoanode exhibited excellent photocurrent density of 3.54 mA cm−2 (@ 1.23 VRHE) with a remarkable cathodic shift (180 mV) of the onset potential, marking the highest value among P doped hematite studies. This study suggests a new paradigm of P doped hematite with mesopores and gradient doping properties affordable in a cost-efficient way, achieving an excellent PEC water splitting performance.

A TiO2@MWCNTs nanocomposite photoanode for solar-driven water splitting.

A TiO2@MWCNTs (multi-wall carbon nanotubes) nanocomposite photoanode is prepared for photoelectrochemical water splitting in this study. The physical and photoelectrochemical properties of the photoanode are characterized using field emission-scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and linear sweep voltammetry. The results show that the TiO2@MWCNTs nanocomposite has an optical bandgap of 2.5 eV, which is a significant improvement in visible-light absorption capability compared to TiO2 (3.14 eV). The cyclic voltammograms show that incorporating TiO2 with the MWCNTs leads to a decrease in the electrical double layer, thereby facilitating the electron transfer rate in the TiO2@MWCNTs electrode. Moreover, the current density of the photoelectrochemical electrode formed by TiO2@MWCNTs under solar irradiation is significantly higher than that prepared by TiO2 (vs Ag/AgCl). The low charge capacity of the TiO2@MWCNTs electrode-electrolyte interface hinders the recombination of the photogenerated electrons and holes, which contributes to the enhancement of the solar-to-hydrogen (STH) conversion efficiency. The average STH conversion efficiency of the TiO2@MWCNTs electrode under solar exposure from 6 AM to 5 PM is 11.1%, 8.88 times higher than that of a TiO2 electrode. The findings suggested TiO2@MWCNTs is a feasible nanomaterial to fabricate the photoanode using photoelectrochemical water splitting under solar irradiation.

Effect of reaction temperature on morphology and photoelectrochemical performance of titanium dioxide

• Nanorod and nanowire mixed-phase of TiO 2 is synthesized via hydrothermal method. • Effect of reaction temperature on photoelectrochemical performance is investigated. • A superior photoelectrochemical water splitting performance was obtained. Titanium dioxide (TiO 2 ) is best photoanode for photoelectrochemical water splitting. Here, the effect of reaction temperature on titanium dioxide nanorods is studied via a hydrothermal synthesis method. The results reveal the reaction temperature has high impact on morphology, bandgap and photoelectrochemical properties. The photoelectrochemical performance of TiO 2 is enhanced as reaction temperature increase. When reaction temperature increases from 130℃ to 180℃, the photocurrent density and hydrogen generation increase from 0.65 mA/cm 2 , 72.49 μmol/cm 2 to 1.84 mA/cm 2 , 126.86 μmol/cm 2 respectively These results suggest the potential for the synthesis and usage of transition metal oxides/sulfides for efficient photoelectrochemical hydrogen production.

Enabling high low-bias performance of Fe2O3 photoanode for photoelectrochemical water splitting

Fe2O3 is a promising photoanode material used for photoelectrochemical water splitting due to its narrow bandgap and excellent stability in solution. However, because the nanorods shrink and coalesce when annealed under high temperatures, the charge separation and injection efficiencies are suppressed in the conventional nanocoral Fe2O3, resulting in its high bias potential and low photocurrent density. Herein, by improving the radial growth of FeOOH precursor, highly dispersed Fe2O3 nanorods could be prepared. It enabled them to have sufficient light-harvesting and short charge transport distance, high light absorption and charge separation/injection efficiencies, increased photocurrent density and reduced onset potential Von. The optimized Fe2O3 photoanodes obtained a remarkable low-bias photocurrent density of 0.84 mA cm−2 at 1.0 V versus reversible hydrogen electrode (vs. RHE). It was further improved to 1.36 mA cm−2 at 1.0 V vs. RHE with the Von reduced to 0.50 V vs. RHE when post-treated with a solvothermal method and loaded with NiOOH/FeOOH cocatalysts. The applied bias photo-to-current conversion efficiency was maximized to 0.45% at 0.84 V vs. RHE.

Tuning the Carrier Transfer Behavior of Coaxial ZnO/ZnS/ZnIn2 S4 Nanorods with a Coherent Lattice Heterojunction Structure for Photoelectrochemical Water Oxidation.

Invited for this month's cover is the group of Feng Li at the Ningxia University. The image shows how the coherent lattice heterojunction interface can play a role in the efficient separation of photogenerated carriers of ZnO-based photoanode for photoelectrochemical water splitting. The Research Article itself is available at 10.1002/cssc.202201469.

Synergistic effects of nano-structured WO3-Se heterojunction decorated by organic Nafion layer on improving photoelectrochemical performance

Exploration of high-performance photoanodes is considered as an essential challenge in photoelectrochemical (PEC) water splitting due to the complex four-electron reaction in water oxidation. Herein, the nano-structured WO3-Se heterojunction decorated by organic Nafion layer is designed. The optimized WO3-Se200-0.05Nafion photoanode shows a remarkable photocurrent of 1.40 mA cm−2 at 1.23 V versus reversible hydrogen electrode, which is 2.5-fold higher than that of pure WO3 nanosheets (WO3 NS) photoelectrode. Remarkably, the photocurrent increments of WO3-Se200-0.05Nafion is larger than the increment sum of WO3-Se200 and WO3-0.05Nafion, which affirming the synergistic effect of Se nanospheres and Nafion layer. The improved PEC performances are attributed to the quick charge separation and transfer, the increased electric conductivity, and the excellent kinetics of oxygen evolution, which is derived from the strong interaction among WO3, Se and Nafion. Meanwhile, the better visible-light harvesting from Se nanospheres as photosensitizer and the induction of transparent Nafion as a passivation layer can explain this synergy. It hopes this heterostructure design with organic Nafion decoration can inspire to exploit outstanding performance photoanodes for PEC water splitting.