In photoelectrochemical (PEC) water splitting, heterojunction electrodes consisting of two or more dissimilar semiconductors offer more advantages over those made from single semiconductors. Pairing tungsten trioxide (WO₃) with bismuth vanadate (BiVO₄) is a promising strategy that helps to achieve better charge separation and improved overall performance in PEC water splitting. In this work, we have demonstrated a WO₃/BiVO₄/TiO₂ core–shell nanostructured photoanode for efficient water splitting and stability. A detailed valence band (VB) edge analysis of WO₃/BiVO₄ was performed with X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy, and it was revealed that due to electronic equilibration between WO₃ with BiVO₄, Fermi energy level shifting occurs, forming a p–n junction at the interface that allows electrons from the photoexcited BiVO₄ to be transferred efficiently to WO₃, achieving better charge separation and higher efficiency. This electrode exhibited significantly higher photocurrent density than the individual WO₃ and BiVO₄ electrodes, producing an unprecedented photocurrent of 4.2 mA cm⁻² at 1.23 V versus a reversible hydrogen electrode under simulated sunlight without an added catalyst. A thin layer of photocatalytically inactive amorphous TiO₂ was deposited onto WO₃/BiVO₄ to react with surface defects, deactivating them toward surface charge-carrier recombination, and to increase the stability of photoanodes.

We demonstrate the morphological control method of ZnO nanostructures by atomic layer deposition (ALD) on an Al₂O₃/ZnO seed layer surface and the application of a hierarchical ZnO nanostructure for a photodetector. Two layers of ZnO and Al₂O₃ prepared using ALD with different pH values in solution coexisted on the alloy film surface, leading to deactivation of the surface hydroxyl groups. This surface complex decreased the ZnO nucleation on the seed layer surface, and thereby effectively screened the inherent surface polarity of ZnO. As a result, a 2-D zinc hydroxyl compound nanosheet was produced. With increasing ALD cycles of ZnO in the seed layer, the nanostructure morphology changes from 2-D nanosheet to 1-D nanorod due to the recovery of the natural crystallinity and polarity of ZnO. The thin ALD ZnO seed layer conformally covers the complex nanosheet structure to produce a nanorod, then a 3-D, hierarchical ZnO nanostructure was synthesized using a combined hydrothermal and ALD method. During the deposition of the ALD ZnO seed layer, the zinc hydroxyl compound nanosheets underwent a self-annealing process at 150 °C, resulting in structural transformation to pure ZnO 3-D nanosheets without collapse of the intrinsic morphology. The investigation on band electronic properties of ZnO 2-D nanosheet and 3-D hierarchical structure revealed noticeable variations depending on the richness of Zn-OH in each morphology. The improved visible and ultraviolet photocurrent characteristics of a photodetector with the active region using 3-D hierarchical structure against those of 2-D nanosheet structure were achieved.