Defect-Mediated Charge Transfer in Y–Cd-Codoped ZnO: Enhanced Hydroxyl Radical Generation and Solar Photocatalytic Carbaryl Degradation

Abstract In this study, yttrium–cadmium codoped ZnO (YCdZnO) photocatalysts were synthesized and investigated for the sunlight-excited photocatalytic degradation of carbaryl (CBR). The photocatalytic degradation assays showed a pseudo-first-order degradation rate constant of 12.38 × 10–3 min–1, approximately 2.08 times that of bare ZnO (5.95 × 10–3 min–1). The photocatalysts also retained over 48% degradation efficiency (DE) after four cycles. Scavenger tests indicated that hydroxyl radicals were the dominant reactive species during CBR degradation. Characterization of YCdZnO, including vibrational and structural analyses, revealed the formation of a hexagonal wurtzite structure without any impurity phases. This indicates the successful substitution of Y3+ and Cd2+ into the ZnO matrix. The Williamson–Hall plot indicated an increase in crystallinity. This was evidenced by a positive correlation between crystallite size and microstrain, attributed to ion substitution during the doping process. However, the smaller particle size with mesoporous features and increased surface area of YCdZnO can provide shorter carrier migration paths and more active sites. Optical investigations revealed an increase in visible-light absorption and a marginal narrowing of the band gap from 3.24 eV (ZnO) to 3.21 eV (YCdZnO). X-ray photoelectron spectroscopy (XPS) confirmed the presence of Zn2+, Y3+, Cd2+, and defect-related states in YCdZnO, which served as electron traps to retard electron–hole recombination and promote hydroxyl radical generation. Furthermore, fluorescence spectra of YCdZnO present comparatively lower intensity for shallow Zn-related and surface oxygen-related defect states, implying enhanced carrier trapping and charge transfer. Therefore, the synergy of defect-related states, high surface areas, and increased solar-light absorption may favor the practical application of YCdZnO for sunlight-facilitated degradation of CBR.