Hydrothermally Controlled ZnO Nanorod Dimensions in ZnO/CdS Photoanodes for Enhanced PEC Water Splitting

Zinc oxide (ZnO) has garnered significant attention as a promising photoanode material for photoelectrochemical (PEC) water splitting. However, its photoelectrocatalytic performance is hindered by a wide bandgap and fast recombination of photogenerated electron–hole pairs. In this study, ZnO/CdS heterostructured photoanodes were fabricated using a combination of atomic layer deposition, hydrothermal synthesis, and successive ionic layer adsorption and reaction (SILAR) techniques. Compared with the conventional single‐step hydrothermal method, a secondary hydrothermal treatment significantly improved the PEC performance of the ZnO/CdS photoanodes. Under standard simulated solar irradiation (AM 1.5 G, 100 mW/cm 2 ), the optimized ZnO/CdS photoanode achieved a remarkable photocurrent density of 9.46 mA/cm 2 (at 1.23 V vs. RHE) and a photoconversion efficiency of 4.61%, which is among the highest values reported for pristine ZnO/CdS‐based photoanodes. Comprehensive microstructural characterization and electrochemical analyses revealed that the charge carrier migration mechanism in the as‐prepared ZnO/CdS system follows a Z‐scheme pathway. Furthermore, the secondary hydrothermal process facilitated the controlled growth of secondary ZnO NRs, which increased the electrochemically active surface area and promoted efficient charge transport. These structural and electronic improvements collectively led to enhanced photocurrent density and photoconversion efficiency. This study presents a novel and effective approach for enhancing the PEC performance of semiconductor‐based photoanodes.