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

p-n heterojunction constructed by γ-Fe2O3 covering CuO with CuFe2O4 interface for visible-light-driven photoelectrochemical water oxidation

Fe₂O₃ is a promising n-type semiconductor as the photoanode of photoelectrochemical water-splitting method due to its abundance, low cost, environment-friendly, and high chemical stability. However, the recombination of photogenerated holes and electrons leads to low solar-to-hydrogen efficiency. In this work, to overcome the recombination issue, a p-type semiconductor, CuO, is introduced underneath the γ-Fe₂O₃ to synthesize γ-Fe₂O₃/CuO on the FTO substrate. Along with the formation of p-n heterojunction, CuFe₂O₄ is in situ generated at the interface of γ-Fe₂O₃ and CuO. The existence of Cu₂O in CuO and CuFe₂O₄ promotes the charge transfer from CuO to γ-Fe₂O₃ and within CuFe₂O₄, respectively, resulting in creating an internal electric field in γ-Fe₂O₃/CuO and leading to the conduction band of CuO bending up and γ-Fe₂O₃ bending down. Additionally, Cu(II) in CuFe₂O₄ contributes to fast electron capture. Consequently, the charge transfer efficiency and charge separation efficiency of photo-generated holes are promoted. Hence, γ-Fe₂O₃/CuO exhibits an enhanced photocurrent density of 13.40 mA cm^-2 (1.9 times higher than γ-Fe₂O₃). The photo corrosion resistance of CuO is dramatically increased with the protection of CuFe₂O₄, resulting in superior high chemical stability, i.e. 85% of the initial activity remains after a long-term test.

Hole-Storage Enhanced a-Si Photocathodes for Efficient Hydrogen Production

Ferrihydrite (Fh) has been demonstrated as an effective interfacial layer for photoanodes to achieve outstanding photoelectrochemical (PEC) performance for water oxidation reaction owing to its unique hole-storage function. However, it is unknown whether such a hole-storage layer can be used to construct highly efficient photocathodes for hydrogen evolution reaction (HER). In this work, we report Fh interfacial engineering of amorphous silicon photocathode (with nickel as HER cocatalyst) achieving a photocurrent density of 15.6 mA cm^-2 at 0 V vs. the reversible hydrogen electrode and a half-cell energy conversion efficiency of 4.08 % in alkaline solution, outperforming most of reported a-Si based photocathodes including multi-junction configurations integrated with noble metal cocatalysts in acid solution. Besides, the photocurrent density is maintained above 14 mA cm^-2 for 175 min with 100 % Faradaic efficiency for HER in alkaline solution. Our results demonstrate a feasible approach to construct efficient photocathodes via the application of a hole-storage layer.

Tuning Structural Properties of WO3 Thin Films for Photoelectrocatalytic Water Oxidation

The preparation of tungsten oxide (WO3) thin film by direct current (DC) reactive sputtering magnetron method and its photoelectrocatalytic properties for water oxidation reaction are investigated using ultraviolet-visible radiation. The structural, morphological, and compositional properties of WO3 are fine-tuned by controlling thin film deposition time, and post-annealing temperature and environment. The findings suggest that the band gap of WO3 can be controlled by adjusting the post-annealing temperature; the band gap decreased from 3.2 to 2.7 eV by increasing the annealing temperature from 100 to 600 °C. The theoretical calculations of the WO3 bandgap and the density of state are performed by density functional theory (DFT). Following the band gap modification, the photoelectrocatalytic activity increased and the maximum photocurrent (0.9 mA/cm2 at 0.6 VSCE) is recorded with WO3 film heated at 500 °C. The WO3 film heated under air exhibits much better performance in photoelectrochemical water oxidation process than that of annealed under inert atmosphere, due to its structural variation. The change in sputtering time leads to the formation of WO3 with varying film thickness, and the maximum photocurrent is observed when the film thickness is approximately 150 nm. The electrical conductivity and charge transfer resistance are measured and correlated to the properties and the performance of the WO3 photoelectrodes. In addition, the WO3 photoelectrode exhibits excellent photoelectrochemical stability.