Precursor Concentration Engineering of ZnO Nanorod Array Photoanodes for Enhanced Photoelectrochemical Performance

Zinc oxide (ZnO) is a promising photoanode material for photoelectrochemical (PEC) water splitting owing to its high electron mobility and low cost, yet its practical performance is often limited by severe charge recombination and suboptimal nanostructural organization. In this work, we systematically investigate how precursor concentration regulates the evolution of ZnO nanorod-array architecture and its associated semiconductor/electrolyte interfacial charge-transfer behavior. As the Zn2+ concentration increases from 0.01 to 0.15 M, the ZnO architecture evolves from disordered nanorods to highly aligned vertical arrays (0.05 M) and then to coarse, compact polyhedral aggregates. The optimized ZnO-0.05 M photoanode delivers a photocurrent density of 2.0 mA·cm–2 at 1.23 V vs RHE under AM 1.5G illumination with an applied bias photon-to-current efficiency of 0.52%. Comprehensive electrochemical analyses reveal that this enhanced performance originates from a synergistic modulation of solid–liquid interfacial properties: the well-aligned nanorod architecture with open cross-sectional channels exhibits the lowest charge-transfer resistance (1944.6 Ω), the longest Bode-derived characteristic time constant (16.11 ms), the highest apparent donor density (3.78 × 1020 cm–3), and the largest double-layer capacitance (Cdl) (49.61 μF·cm–2). These results indicate that the vertically aligned and open nanorod architecture provides favorable electron-transport pathways while improving electrolyte accessibility to the ZnO surface. In contrast, insufficient array development at low precursor concentration and crystal coalescence at high concentration result in less favorable interfacial charge-transfer behavior. This work, therefore, establishes a concentration–structural evolution–interfacial kinetics–PEC performance relationship and provides insight into how solution-phase growth conditions regulate the architecture and solid–liquid interfacial behavior of metal-oxide photoelectrodes.