Photoanodes For Photoelectrochemical Water Splitting Research Articles

Anion exchange and successive ionic layer adsorption and reaction-assisted coating of BiVO4 with Bi2S3 to produce nanostructured photoanode for enhanced photoelectrochemical water splitting

Photoelectrochemical water splitting is an environmentally benign way to store solar energy. Properties such as fast charge recombination and poor charge transport rate severely restrict the use of BiVO4 as a photoanode for photoelectrochemical water splitting and many attempts were made to improve the current performance limit of the photoanode. To address these disadvantages, a highly efficient BiVO4/Bi2S3 heterojunction was fabricated applying facial anion-exchange (AE) and successive ionic layer adsorption and reaction (SILAR). The deposition of Bi2S3 on BiVO4 nanoworms by both AE and SILAR was confirmed through morphological, structural, and optical analyses. The morphological analysis indicated that Bi2S3 grown through SILAR has relatively more crystalline-amorphous phase boundaries than Bi2S3 generated using the anion-exchange method. The highest photocurrent density was observed for the SILAR-coated Bi2S3 on BiVO4, which is three times the value of the pristine BiVO4 measured under 1 sun illumination (100 mW cm−2 with Air mass (AM) 1.5 filter) in a 0.5 M Na2SO4 electrolyte at 1.6 V vs. RHE. In addition, the deposition of Bi2S3 through AE results in a twofold higher photocurrent density compared to uncoated BiVO4. The comparison of the two cost-effective AE and SILAR methods to deposit Bi2S3 on BiVO4 showed a negative shift in the flat band Mott-Schottky values, which coincides with the drifted onset potential values of the current density-voltage (J–V) curve. Furthermore, photoelectrochemical impedance spectroscopy (PEIS) analyses and band alignment studies revealed that SILAR-grown Bi2S3 creates an effective heterojunction with BiVO4, which leads to an efficient charge transfer.

CdS Sensitized Solvothermal Phase Change Assisted Synthesis of CoTiO3 @ TiO2 Nanorods As a Ternary Heterojunction Photoanode for Photoelectrochemical Water Splitting

Development of efficient semiconductor heterostructures with adequate band alignment for enhanced charge separation and increased light absorbance has been at the forefront of research on photoelectrodes. In this work we demonstrate the fabrication of CdS sensitized CoTiO3 @ TiO2 photoanodes for photoelectrochemical water splitting. TiO2 nanorods were synthesized by a hydrothermal method followed by surfactant assisted solvothermal impregnation of Co2+ ions into the TiO2 lattice resulting in development of a core-shell CoTiO3 @ TiO2 p-n junction heterostructure. The photoanodes were further sensitized by CdS nanoparticles by a facile SILAR method not only to increase light absorbance in the visible region but also to form a ternary heterojunction photoanode with cascade type electron transfer. The fabricated photoanodes showed an ABPE % of 0.43% at 0.6 V vs Ag/AgCl reference electrode using 0.5 M Na2S and 0.5M Na2SO3 electrolyte under A.M 1.5 G simulated solar illumination. Optical properties were analyzed using UV-Vis Absorbance and Photoluminescence Spectroscopy. Transient photocurrent, EIS and Mott-Schottky analysis were carried out to analyze charge transport processes occuring at the semiconductor / electrolyte interface. Structure and morphological analysis was carried out using XRD and FESEM. Band structure and DOS was analyzed using DFT calculations.This work hopes to inspire new research in development of ternary heterojunctions for photoelectrochemical water splitting.

Two‐Dimentional Nanostructured Metal Oxide/Sulfide–Based Photoanode for Photoelectrochemical Water Splitting

The utilization of solar energy plays a vital role in relieving energy crisis and associated environmental problems. Photoelectrochemical (PEC) water splitting, which can directly transform solar energy into clean hydrogen energy, is one of the most promising strategies to utilize solar energy. 2D nanostructured metal oxides/sulfides have been widely applied as photoanodes in PEC cells due to their unique configuration and features, which can efficiently improve light absorption, enlarge electrode/electrolyte contacting interfaces, and promote carrier transfer and injection. This article begins with the introduction of performance parameters, carrier dynamics measurement, and mechanism exploration techniques for photoanodes, which is followed by an overview of research progress in using 2D nanostructure metal oxides/sulfides to fabricate efficient photoanodes for PEC water splitting. Subsequently, the recent representative strategies to optimize the PEC performance of 2D metal oxides/sulfides photoanodes, including structure engineering, defect engineering, element doping, heterojunction construction, and surface modification, are summarized. Finally, the exciting advances and future research directions for designing highly efficient 2D photoanodes are provided.

Efficient Improved Charge Separation of FeP Decorated Worm-Like Nanoporous BiVO4 Photoanodes for Solar-Driven Water Splitting

Photoelectrochemical (PEC) water splitting using BiVO4 semiconductor photoanodes have been appealed to plenty of attentions in the past few decades. In this study, FeP was used as an active precious-metal-free cocatalyst for the first time to enhance the performance of worm-like nanoporous BiVO4 photoanodes for PEC water splitting. Characterization results demonstrate that FeP nanoparticles were successfully deposited on surface of the pritine BiVO4 photoanodes, and served as an noble metal-free cocatalyst for solar-driven PEC water splitting. Furthermore, a maximum photocurrent density of ~ 3.05 mA/cm2 at 1.23 V vs RHE could be achieved by loading moderate amount of FeP on the surface of BiVO4, which is about 1.6 times higher than that of the unmodified BiVO4. Meanwhile, the efficient suppression of surface recombination by FeP was also confirmed by the PEC measurements in Na2SO3 solution. The maximum ABPE of FeP/BiVO4 photoanodes was ~ 0.53% at 0.60 V vs RHE, indicating the potential application for overall solar-driven water splitting.

Engineering of a high-efficiency water splitting photoanode by synergistic effects of doping, compositing, and coupling on TiO2 nanoparticles

Highly efficient photoelectrochemical (PEC) water splitting is achieved by Ag doped hierarchical porous TiO2 nanoparticles in compositing with GQDs (TiO2:Ag-GQDs). The unique optical properties of VO2 (m), monoclinic phase of VO2, is a motivation to use it in PEC processes. To further improve the efficiency of TiO2:Ag-GQDs photoanode, it is coupled with undoped and doped VO2 (m) with earth alkali metals. This coupling strategy is used to engineer the interfacial and optical properties and charge separation/transport on the photoanodes for PEC water splitting. Hence, the efficiency of the photoanodes with structures of TiO2:Ag-GQDs/VO2 or (Mg, Ca, Sr, or Ba)-doped VO2 in PEC water splitting process is investigated. A remarkable photocurrent density of 1.5 mA/cm2 at 1.23 V vs. RHE is achieved for FLGs/TiO2:Ag-GQDs/VO2:Ba photoanode. This value is 20 times higher than that of reported previously for pristine TiO2 photoanode (0.07 mA/cm2 at 1.23 V vs RHE).

Morphology control of the hematite photoanodes for photoelectrochemical water splitting

The morphology of hematite photoanode is a significant relevant factor in its photoelectrochemical (PEC) performance. Hematite nanowires and nanocubes as well as nanorods with intentional Sn doping were prepared by hydrothermal processes containing disparate additives. The band-gap decreases in the sequence of nanowires, nanorods and nanocubes. Compared with nanorods, nanowires show higher carrier density but a lower light absorbance. With both inhibited bulk and surface charge recombination, nanowires achieve an enhanced photocurrent. Meanwhile, it is more complicated for the charge conversion in the hematite nanocubes. Light absorption is limited due to the compact arrangement of nanocubes. Besides, nanocubes show a highly oriented (104) plane which is unfavorable to the charge conductivity. Despite the negative factors hindering its PEC performance, the extremely high carrier density in the nanocubes benefits to the distinctly enhanced photocurrent collected from the hematite samples annealed at 550 and 650 °C respectively. However, the superiority of hematite nanocubes annealed under 800 °C is restricted by the high onset potential. Still, attributed to the high surface charge transfer efficiency, the hematite nanocubes achieve the highest photocurrent among the samples at biases above 1.3 V. Electrodes made of hematite nanorods, nanowires and nanocubes annealed at 800 °C achieve a photocurrent of 1.01, 1.30 and 1.40 mA cm−2 at 1.6 V vs. RHE, respectively.

Zinc Oxide Nanostructured Thin Film as an Efficient Photoanode for Photoelectrochemical Water Oxidation

Synthesizing nanostructured thin films of oxide semiconductors is a promising approach to fabricate highly efficient photoelectrodes for hydrogen production via photoelectrochemical (PEC) water splitting. In this work, we investigate the feasibility as an efficient photoanode for PEC water oxidation of zinc oxide (ZnO) nanostructured thin films synthesized via a simple method combined with sputtering Zn metallic films on a fluorine-doped tin oxide (FTO) coated glass substrate and subsequent thermal oxidation of the sputtered Zn metallic films in dry air. Characterization of the structural, optical, and PEC properties of the ZnO nanostructured thin film synthesized at varying Zn sputtering powers reveals that we can obtain an optimum ZnO nanostructured thin film as PEC photoanode at a sputtering power of 40 W. The photocurrent density and optimal photocurrent conversion efficiency for the optimum ZnO nanostructured thin film photoanode are found to be 0.1 mA/cm2 and 0.51 %, respectively, at a potential of 0.72 V vs. RHE. Our results illustrate that the ZnO nanostructured thin film has promising potential as an efficient photoanode for PEC water splitting.

Reduced graphene oxide decorated SnO2/BiVO4 photoanode for photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting technology offers a sound strategy for the production of chemical energy using abundant solar energy. Herein, a ternary photoanode of SnO2/BiVO4/rGO was fabricated by plain chemical vapor deposition (CVD) and metal-organic decomposition followed by spin-coated rGO on the SnO2/BiVO4 junction. The ternary photoanode yields the highest photocurrent density of 2.05 mA cm−2 at 1.23 V vs. RHE, which is 3.73 times of the BiVO4 photoanode (0.55 mA cm−2). The incident photon-to-electron conversion efficiency (IPCE) of the ternary photoanode is 2.47 times that of the BiVO4 photoanode at 400 nm, and the onset potential exhibits a cathodic shift of ∼300 mV. This enhancement can be attributed to the formation of n-n heterojunctions between the SnO2 and BiVO4, and decoration of rGO on said heterojunctions because they synergistically improve the absorption of visible light, enhance the efficiency of charge separation, and accelerate electron transfer at the electrode/electrolyte interface.

Expanded light-absorption and efficient charge-separation: bilayered thin film nano-hetero-structures, CuO/Cu–ZnO, make efficient photoanode in photoelectrochemical water splitting

High efficiency photoelectrochemical water splitting is achieved, using uniquely evolved bi-layered nano-hetero-structured (BNHS) thin films, CuO/Cu–ZnO, grown over ITO (In:SnO2) glass substrate by spray-pyrolysis and sol–gel spin-coating. Films were characterized by X-ray diffractometry, scanning electron microscopy, atomic force microscopy, UV–visible spectrometry, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. Significant gain in photocurrent and applied bias photon-to-current efficiency, with 3% Cu incorporated BNHS films yielding maximum photocurrent ~ 2.98 mA cm−2, is attributable to favourable changes in material microstructure and electrical properties. CuO nanparticles existing as dispersed phase in ZnO overlayer and tendering a possible mechanism for the transfer of photogenerated holes from the underneath CuO layer to electrolyte is a highlighting proposition of this report.

Oxygen vacancies engineering in TiO2 homojunction/ZnFe-LDH for enhanced photoelectrochemical water oxidation

The high-efficiency separation and transfer of photogenerated electrons and holes are significant to achieve highly photoelectrochemical water redox efficiency. Here, we integrate anatase TiO2 with moderate oxygen vacancies and ZnFe-LDH cocatalyst on 3D homojunction r-TiO2(rutile TiO2)/a-TiO2(anatase TiO2) photoanode to promote the separation and transfer efficiency of carriers for a high photoelectrochemical activity. The as-prepared r-TiO2/a-TiO2/ZnFe-LDH photoanode demonstrates an enhanced photocurrent density of 1.86 mA cm−2 at 1.23 V vs. RHE and the onset potential of 0.26 V vs. RHE, 2.55 times and 100 mV cathodic shift by contrast with those of bare r-TiO2. The experimental and calculated results suggest the improved electron transport by oxygen vacancies and r-TiO2/a-TiO2 with matched energy bands, along with the accelerated hole capture and facilitated surface oxygen production reaction kinetics by cocatalyst ZnFe-LDH. The compare researches for the sample without the oxygen vacancies are introduced to further illustrate the increased carrier concentration and conductivity caused by oxygen vacancies. This work manifests the effective use of oxygen vacancies in regulating carriers transport and provides a novel strategy for manufacturing high-performance photoanodes in photoelectrochemical water splitting.

Enhanced photoelectrochemical performance of plasmonic Ag nanoparticles grafted ternary Ag/PaNi/NaNbO3 nanocomposite photoanode for photoelectrochemical water splitting

In this work, we have successfully synthesized silver nanoparticles (Ag-NPs) decorated binary PaNi/NBO nanocomposite by facile chemisorption route and demonstrated enhanced photoelectrochemical (PEC) water splitting activity. The ternary Ag/PaNi/NBO nanocomposite photoanode exhibits ∼5 fold higher current density than pristine NaNbO3 nanofibers (NBO-NFs). The ternary Ag/PaNi/NBO photoanode exhibits a current density of 5.93 mA/cm2 at 0.9 V with respect to Ag/AgCl, whereas the pristine NBO-NFs photoanode exhibits only 0.85 mA/cm2 at 0.9 V. The improved performance is correlated to the high surface area, formation of the type-II heterojunction among PaNi and NBO-NFs and the introduction of the plasmonic behaviour of Ag-NPs as it expands the absorption of the visible part of the solar spectrum, which together decreases the recombination of charges with strong redox capability. Furthermore, the ternary Ag/PaNi/NBO photoanode showed lower charge transfer resistance (Rct) in the EIS measurements and improved incident photon to current efficiency (IPCE) of ∼31.3% which is ∼23% higher to the IPCE of bare NBO-NFs photoanode. Thus, these results suggest that the ternary Ag/PaNi/NBO nanocomposite is a promising material for PEC activity and wide practical applications.

Direct Z‐Scheme Janus‐Shaped Heterojunction of TiO2‐Ti2O3(H2O)2(C2O4)⋅H2O: A Novel Photocatalyst or Photoanode

AbstractThe novel n‐n type heterojunction of TiO2‐Ti2O3(H2O)2(C2O4)⋅H2O, realizing a direct Z‐scheme of the charge transfer mechanism, preparing by in‐situ growth of titanium oxalate through one‐step hydrothermal method, possessing janus‐shaped structure, is a photocatalyst for wastewater treatment or photoanode for photoelectrochemical water splitting.

Pulsed Laser Deposition of Bismuth Vanadate Thin Films-The Effect of Oxygen Pressure on the Morphology, Composition, and Photoelectrochemical Performance.

Thin layers of bismuth vanadate were deposited using the pulsed laser deposition technique on commercially available FTO (fluorine-doped tin oxide) substrates. Films were sputtered from a sintered, monoclinic BiVO4 pellet, acting as the target, under various oxygen pressures (from 0.1 to 2 mbar), while the laser beam was perpendicular to the target surface and parallel to the FTO substrate. The oxygen pressure strongly affects the morphology and the composition of films observed as a Bi:V ratio gradient along the layer deposited on the substrate. Despite BiVO4, two other phases were detected using XRD (X-ray diffraction) and Raman spectroscopy—V2O5 and Bi4V2O11. The V-rich region of the samples deposited under low and intermediate oxygen pressures was covered by V2O5 longitudinal structures protruding from BiVO4 film. Higher oxygen pressure leads to the formation of Bi4V2O11@BiVO4 bulk heterojunction. The presented results suggest that the ablation of the target leads to the plasma formation, where Bi and V containing ions can be spatially separated due to the interactions with oxygen molecules. In order to study the phenomenon more thoroughly, laser-induced breakdown spectroscopy measurements were performed. Then, obtained electrodes were used as photoanodes for photoelectrochemical water splitting. The highest photocurrent was achieved for films deposited under 1 mbar O2 pressure and reached 1 mA cm−2 at about 0.8 V vs Ag/AgCl (3 M KCl). It was shown that V2O5 on the top of BiVO4 decreases its photoactivity, while the presence of a bulk Bi4V2O11@BiVO4 heterojunction is beneficial in water photooxidation.

Nanoporous WO3 films synthesized by tuning anodization conditions for photoelectrochemical water oxidation

Morphology engineering is of prime significance in designing semiconductor photoelectrodes for efficient photoelectrochemical (PEC) fuel generation. Herein, we report a strategy to fabricate nanostructured tungsten oxide films using anodic oxidation of tungsten and their performance as photoanodes for PEC water splitting. A solution of an organic compound, dimethyl sulfoxide, with hydrofluoric acid was used as the electrolyte. The anodization parameters such as voltage and electrolyte temperature were found to have a critical influence in determining the porous nature and dimensions of the resulting oxide film. Anodization conducted at a voltage ~10 V and a temperature ~40 °C led to the formation of compact tungsten oxide films due to the low strength of the electric field partially responsible for the porous morphology development. The surface of the film became porous at elevated temperatures. At higher voltages, a thin nanoporous film atop a thicker compact layer was formed; however, the structure became more disordered and less porous when the electrolyte temperature was increased. The study demonstrated that by adjusting the voltage and temperature, the oxide film morphology could be varied from a compact layer to a porous structure resembling nanotubes. The films heat treated at 500 °C attained the monoclinic phase of WO3. The PEC performance was influenced strongly by the morphology of the films. The films with nanotube like morphology yielded incident photon to current conversion efficiency values of over 40%. This study would provide a practical basis for developing optimal WO3 nanostructures for efficient PEC fuel generation.

Supersonically sprayed Zn2SnO4/SnO2/carbon nanotube films for high-efficiency water splitting photoanodes

We demonstrate the supersonic cold-spray deposition of a Zn2SnO4/SnO2/carbon nanotube (CNT) composite as an exceptionally high-performance photoanode for photoelectrochemical water splitting. We optimized the photoanode thickness (number of deposition passes) and composition to achieve a maximum photocurrent density of 17.2 mA cm−2 at 1.78 V vs. reversible hydrogen electrode, with no additional catalysts or additives. The morphology of the deposited films, as characterized by scanning and transmission electron microscopy, comprises CNTs intermingled with Zn2SnO4 and SnO2 nanoparticles. The high electron concentration and charge transfer rate achieved by adding CNTs are demonstrated through Mott–Schottky and Nyquist analyses of electrochemical impedance spectroscopy (EIS) data. While we find no prior reports of Zn2SnO4/SnO2/CNT nanocomposites for water splitting, we demonstrate that this combination of ball-milled Zn2SnO4/SnO2 nanoparticles mixed with CNTs is a promising candidate for high-performance solar water splitting with dramatically enhanced performance compared to previously-reported ZnO- and SnO-based photoanodes.

Plasmonic nanometal decorated photoanodes for efficient photoelectrochemical water splitting

Plasmonic metal nanoparticles containing photoanodes are known to exhibit stable photoelectrochemical (PEC) performance due to their optical and electronic properties. In this work, we report the application of plasmonic Bi nanoparticles supported over a g-C3N4/Bi2S3 photoanode for PEC water splitting. Typical results indicated that g-C3N4/Bi2S3/BiNPs ternary composite photoanode showed a high photo-current density of 7.11 mA cm−2 at 1.23 V under solar irradiation, which was ∼ 5 and 10 times higher than g-C3N4/Bi2S3 and g-C3N4 photoanodes, respectively. Further, the composite electrode also demonstrated superior solar to hydrogen efficiency and long-term stability. It was concluded that Bi nanoparticles play a major role in enhancing the PEC performance for hydrogen evolution reaction. Thus, g-C3N4/Bi2S3/BiNPs has superior PEC performance and proved to work as an alternative to noble metal based photo-electrodes for solar-water splitting reactions.

Visible-light activation of low-cost rutile TiO2 photoanodes for photoelectrochemical water splitting

Structure and photoelectrochemical properties of rutile-based TiO2 films formed by high-temperature oxidation of titanium metal are described. It is shown that a chemical etching treatment leads to the activation of initially almost inert film allowing its operation as visible light responsive water photo-oxidation photoanode. Additional use of a polyoxometalate electrocatalyst in the supporting electrolyte results in a 0.9 mA cm-2 photocurrent under simulated 1 sun irradiation at 1.23 V vs. RHE.

G-C3N4/TiO2 composite microspheres: in situ growth and high visible light catalytic activity

The g-C3N4/TiO2 heterojunction, possessing a 3D flower-like self-assembled morphology, realizing excellent visible-light photocatalytic properties, is a photocatalyst for wastewater treatment or a photoanode for photoelectrochemical water splitting.

The effect of the photochemical environment on photoanodes for photoelectrochemical water splitting.

Besides photoelectrode materials, realizing the synergy of the photochemical environment and photoelectrodes for high charge carrier utilization is crucial for enhancing the performance of photoelectrochemical (PEC) water splitting systems. However, few researchers have focused on this important aspect. Herein, the effect of the photochemical environment on photoanodes in PEC water splitting, including the redox potential of electrolytes and light direction, is rationally discussed. A combined study of the potential distribution and electrochemical impedance spectroscopy reveals that the low redox potential of electrolytes facilitates the interior charge transfer and surface charge utilization by enlarging the depletion layer. In addition, it is found that the optimum thickness of semiconductors in photoelectrodes is the length of the depletion layer plus diffusion layer.

Rational Design of 3D Hierarchical Ternary SnO2/TiO2/BiVO4 Arrays Photoanode toward Efficient Photoelectrochemical Performance.

BiVO4 as a promising semiconductor absorber is widely investigated as photoanode in photoelectrochemical water splitting. Herein, the rational design of 3D hierarchical ternary SnO2/TiO2/BiVO4 arrays is reported as photoanode for photoelectrochemical application, in which the SnO2 hierarchically hollow microspheres core/nanosheets shell arrays act as conductive skeletons, while the sandwiched TiO2 and surface BiVO4 are working as hole blocking layer and light absorber, respectively. Arising to the hierarchically ordered structure and synergistic effect between each component in the composite, the ternary SnO2/TiO2/BiVO4 photoanode enables high light harvesting efficiency as well as enhanced charge transport and separation efficiency, yielding a maximum photocurrent density of ≈5.03 mA cm−2 for sulfite oxidation and ≈3.1 mA cm−2 for water oxidation, respectively, measured at 1.23 V versus reversible hydrogen electrode under simulated air mass (AM) 1.5 solar light illumination. The results reveal that electrode design and interface engineering play important roles on the overall PEC performance.