An electrochemical etching based on oxalic acid was developed for use in the chemical lift-off of GaN epitaxial structures. It was shown that only the Si-doped n-GaN layer was etched away, while the p-type and undoped GaN layers were not etched at all. The etch rate and the remaining structure were analyzed for various doping concentrations and etching voltages. A lateral etch rate of 12 μm/min was achieved under 60 V for n-type doping concentration of 8×1018 cm−3. This doping selective etching was used to lift-off a GaN epitaxial layer patterned into 300×300 μm2 squares.

BiVO₄ is ubiquitously known for its potential use as photoanode for PEC-WS due to its well-suited band structure; nevertheless, it suffers from the major drawback of a slow electron hole separation and transportation. We have demonstrated the one-pot synthesis of BiVO₄/Ag/rGO hybrid photoanodes on a fluorine-doped tin oxide (FTO)-coated glass substrate using a facile and cost-effective hydrothermal method. The structural, morphological, and optical properties were extensively examined, confirming the formation of hybrid heterostructures. Ternary BiVO₄/Ag/rGO hybrid photoanode electrode showed enhanced PEC performance with photocurrent densities (J _ph ) of ~2.25 and 5 mA/cm² for the water and sulfate oxidation, respectively. In addition, the BiVO₄/Ag/rGO hybrid photoanode can convert up to 3.5% of the illuminating light into photocurrent, and exhibits a 0.9% solar-to-hydrogen conversion efficiency. Similarly, the photocatalytic methylene blue (MB) degradation afforded the highest degradation rate constant value (k = 1.03 × 10⁻² min⁻¹) for the BiVO₄/Ag/rGO hybrid sample. It is noteworthy that the PEC/photocatalytic performance of BiVO₄/Ag/rGO hybrid architectures is markedly more significant than that of the pristine BiVO₄ sample. The enhanced PEC/photocatalytic performance of the synthesized BiVO₄/Ag/rGO hybrid sample can be attributed to the combined effects of strong visible light absorption, improved charge separation-transportation and excellent surface properties.

We report a systematic and scalable single-step growth of three-dimensional hierarchical ZnO/ZnS heterojunction branched nanowires (3D ZnO/ZnS h-b-NWs) on a Si substrate using MOCVD for the first time and their application as photoanodes for photoelectrochemical water splitting (PEC-WS). Vertically well-aligned main trunk ZnO nanowires (MTr-ZnO NWs) were grown on a low-temperature ZnO buffer layer, which was followed by ZnS deposition. Subsequently, hierarchically branched ZnO nanowires (h-b-ZnO NWs) were grown on the side wall of the core–shell heterostructure, which was further modified with a thin layer of ZnS for the growth of multiple ZnO NW branches. The PEC-WS results revealed that the photo-conversion efficiency achieved using MTr-ZnO NWs and 3D ZnO/ZnS h-b-NWs was 0.03% and 0.46% at 0.5 V vs. RHE, respectively, which corresponds to 16-fold enhancement. The achieved improvements can be attributed to the enhanced photoabsorption, which is directly related to the augmented surface area. In addition, ZnS plays a significant role in promoting faster photogenerated carrier transport with high efficiency and visible light absorption by type-II transition at the ZnO/ZnS interface. The novel growth approach that we followed to fabricate the photoanode opens a new avenue for the synthesis of multiple-level hierarchical heterostructures that are beneficial for PEC-WS devices.