Conventional photoanode using a singular semiconductor material is not technically viable for photoelectrochemical (PEC) water oxidation owing to the properties relating to its wide band gap, sluggish charge mobility, as well as poor separation and rapid recombination of photogenerated charge carriers. The main aim of this study was to fabricate an n-n heterojunction photoanode of V2O5/BiVO4 via a facile electrodeposition synthesis method in order to overcome the technical bottlenecks encountered in conventional singular photoanode structures. Additionally, the synergistic effect of band potentials matching and conductivity difference between BiVO4 and V2O5 were studied using LSV, IMPS, EIS, HR-TEM, XRD, XPS, Raman and ultraviolet-visible spectroscopies. This was followed by the performance evaluation of the light-induced water splitting using a standard three-electrode assembly PEC cell under 1.5 AM solar simulator. Results showed that the V2O5/BiVO4 heterojunction photoanode achieved a significantly improved photocurrent density of 1.53 mA/cm 2 at 1.5 V vs Ag/AgCl, which was a 6.9-fold and a 7.3-fold improvement over the individual pristine BiVO4 (0.22 mA/cm 2) and V2O5 (0.21 mA/cm 2), respectively. The improvement was attributed to the lower charge resistances at the FTO/semiconductor, semiconductor/FTO and semiconductor/electrolyte interfaces as well as the fast transit time () of 6.4 millisecond for photo-injected electrons in the V2O5/BiVO4 heterojunction photoanode. Finally, the experimental results were used to reconstruct a theoretical band diagram in validating the heterojunction alignment between V2O5 and BiVO4 as well as in elucidating the photogenerated charge carriers transfer mechanism in the V2O5/BiVO4 heterojunction photoanode.

Herein, we report the rational design of a novel hierarchical V2O5/BiVO4 heterojunction photoanode structure with rGO interlayer that functionalises as photogenerated electron collector, and dual electrocatalyst thin films of FeOOH and NiOOH as photogenerated hole extractors for solar-driven PEC water splitting. Results showed that the novel hierarchical FTO/V2O5/rGO/BiVO4/FeOOH/NiOOH photoanode exhibited an unprecedented and stable photocurrent density of 3.06 mA/cm 2 at 1.5 V vs Ag/AgCl, and an apparent cathodic onset potential shift down to 0.2 V under AM 1.5 G simulated solar light illumination. The significant enhancement in PEC performance is ascribed to band potentials matching between V2O5 and BiVO4 in forming a Type II staggered heterojunction alignment, and further coupling with rGO interlayer and dual-electrocatalyst thin films as photogenerated electron collector and photogenerated hole extractors, respectively. Three different configurations of the novel hierarchical FTO/V2O5/rGO/BiVO4 photoanodes without electrocatalyst, with mono-and dualelectrocatalyst thin films were systematically examined. It was proven though EIS and IMPS measurements that the dual-electrocatalyst configuration photoanode exhibited the shortest transit time (τ) of 31.8 ms for the diffusion of photogenerated electrons to the counter electrode, and the lowest charge transfer resistance across the interface of electrode/electrolyte as estimated using the Randles-Ershel model. We believe that the proof-of-principle work described here not only provides an in-depth understanding on the roles of electrocatalyst thin films but also provides a design guide over the incorporation of electrocatalyst materials for further improving the photogenerated charge carrier dynamics in photoanodes used in solardriven PEC water splitting.

Abstract.The main aim of this study was to investigate the effect of deposition time on the physicochemical and photoelectrochemical properties of cupric oxide (CuO) thin films synthesized via electrodeposition method. Firstly, the electrodeposition of amorphous CuO films on fluorine-doped tin oxide (FTO) working electrodes with varying deposition time between 5 and 30 min was carried out, followed by annealing treatment at 500 ℃. Resultant nanocrystalline CuO thin films were characterised using field emission-scanning electron microscopy (FE-SEM), photocurrent density, and photoluminescence measurements. Through FE-SEM analysis, it was observed that the surface of thin films was composed of irregular-sized CuO nanocrystals. A smaller CuO nanocrystals size will lead to a higher photoactivity due to the increase in overall catalytic surface area. In addition, the smaller CuO nanocrystals size will prolongs the electron-hole recombination rate due to the increase in copious amount of surface defects. From this study, it was revealed that the relationship between deposition time and CuO film thickness was non-linear. This could be due to the detachment of CuO thin films from the FTO surface at an increasing amount of CuO mass being deposited. It was observed that the amount of light absorbed by CuO thin films increased with film thickness until a certain extent whereby, any further increase in the film thickness will result in a reduction of light photon penetration. Therefore, the CuO nanocrystals size and film thickness have to be compromised in order to yield a higher catalytic surface area and a lower rate of surface charge recombination. Finally, it was found that the deposition time of 15 min resulted in an average CuO nanocrystals size of 73.7 nm, optimum film thickness of 0.73 μm, and corresponding photocurrent density of 0.23 mA/cm 2 at the potential bias of -0.3 V (versus Ag/AgCl). The PL spectra for the deposition time of 15 min has the lowest rate of recombination of photogenerated electron-hole pairs by referring to its lowest PL intensity.

Although bismuth vanadate (BiVO4) has shown excellent photoelectrochemical (PEC) properties and is a good candidate of photoanode materials, the solar-driven PEC water splitting performance is still remained below its full potential due to the fast recombination and sluggish charge mobility of photogenerated charge carriers. Previously, we have communicated a proven Type II staggered vanadium pentoxide (V2O5)/BiVO4 heterojunction photoanode that could improve the photocurrent density. This study aimed to examine the effect of introducing an rGO thin film as an efficient electron conductive interlayer in a proven V2O5/BiVO4 heterojunction photoanode, and subsequently tuning the rGO film thickness in achieving the optimum PEC performance. The resultant ternary photoanode structure of V2O5/rGO/BiVO4 was characterised by using field emission-scanning electron microscopy (FE-SEM), high resolution-transmission electron microscopy (HR-TEM), UV-vis spectroscopy, X-ray diffractometer (XRD), Raman spectroscopy and photoluminescence (PL) measurements. Results showed that the interlayer rGO thin film arising from the sequential drop cast and electrochemical reduction of 320 μL ultrasonicated GO solution resulted in the optimal photocurrent density of 2.1 mA/cm 2 at 1.5 V vs. Ag/AgCl. Furthermore, the chemical physics surrounding the photogenerated charge carrier transfer for heterojunction V2O5/BiVO4 was validated for the structure with and without the rGO interlayer. In particular, the electrochemical impedance spectroscopy (EIS) was used to measure multiple resistances at the FTO/semiconductor, semiconductor/semiconductor and semiconductor/electrolyte interfaces. Additionally, the charge transfer (Kt) and recombination (Kr) rate constants for the heterojunction V2O5/BiVO4 with the rGO interlayer were quantified using intensity modulated photocurrent spectroscopy (IMPS). Finally, the PEC H2 evolution rate from the ternary V2O5/rGO/BiVO4 photoanode was measured to be 32.7 μmol/hr, which was about 3-fold higher than the bare V2O5/BiVO4 heterojunction photoanode.

The main aim of this study was to electrochemically synthesize and characterise bismuth vanadate (BiVO4) photoelectrodes for photoelectrochemical (PEC) water splitting. The influence of annealing temperature on the nanostructured semiconductor BiVO4 thin film structure was studied systematically. This was followed by advanced characterisation of the BiVO4 photoelectrodes by using field emission-scanning electron microscopy (FE-SEM), Raman spectroscopy, photoluminescence and PEC properties measurements. When the electrochemically synthesized BiVO4 thin films were subjected to different annealing temperatures, phase transitions occurred for tetragonal BiVO4 at 300 °C and monoclinic BiVO4 at 400 °C. Through this study, it was found that the annealing treatment at 400 °C resulted in the highest photocurrent density (i.e. photoactivity) of 1.23 mA/cm2 at 0.6 V vs. Ag/AgCl. Finally, the BiVO4/CuO heterojunction photoelectrode was also fabricated in order to further enhance its photoactivity under visible light irradiation.