n–n ZnO–Ag₂CrO₄ heterojunction photoelectrodes with enhanced visible-light photoelectrochemical properties

Abstract

In this study, ZnO nanorods (NRs) were hydrothermally grown on an Au-coated glass substrate at a relatively low temperature (90 °C), followed by the deposition of Ag₂CrO₄ particles <i>via</i> a successive ionic layer adsorption and reaction (SILAR) route. The content of the Ag₂CrO₄ particles on ZnO NRs was controlled by changing the number of SILAR cycles. The fabricated ZnO-Ag₂CrO₄ heterojunction photoelectrodes were subjected to morphological, structural, compositional, and optical property analyses; their photoelectrochemical (PEC) properties were investigated under simulated solar light illumination. The photocurrent responses confirmed that the ability of the ZnO-Ag₂CrO₄ heterojunction photoelectrodes to separate the photo-generated electron-hole pairs is stronger than that of bare ZnO NRs. Impressively, the maximum photocurrent density of about 2.51 mA cm^-2 at 1.23 V (<i>vs.</i> Ag/AgCl) was measured for the prepared ZnO-Ag₂CrO₄ photoelectrode with 8 SILAR cycles (denoted as ZnO-Ag₂CrO₄-8), which exhibited about 3-fold photo-enhancement in the current density as compared to bare ZnO NRs (0.87 mA cm^-2) under similar conditions. The improvement in photoactivity was attributed to the ideal band gap and high absorption coefficient of the Ag₂CrO₄ particles, which resulted in improved solar light absorption properties. Furthermore, an appropriate annealing treatment was proven to be an efficient process to increase the crystallinity of Ag₂CrO₄ particles deposited on ZnO NRs, which improved the charge transport characteristics of the ZnO-Ag₂CrO₄-8 photoelectrode annealed at 200 °C and increased the performance of the photoelectrode. The results achieved in the present work present new insights for designing n-n heterojunction photoelectrodes for efficient and cost-effective PEC applications and solar-to-fuel energy conversions.