Steering the Charge Kinetics in Dual-Functional Photocatalysis by Surface Dipole Moments and Band Edge Modulation: A Defect Study in TiO2-ZnS-rGO Composites

Developing an efficient photocatalyst for concurrent hydrogen production and environmental remediation by using solar energy is a challenge. Defect engineering, although it offers a strategical promise to enhance the photocatalytic performance, has limitations that come from the ambiguity surrounding its role. In the current work, a comprehensive study on defects in promoting the charge transfer, band edge modulation, and surface reaction was carried out. The excess electrons springing from defects act like donor states and cause band bending at the junction interface. Characterization techniques such as X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, electron spin resonance, and photoluminescence were employed to investigate defect functionality, and its ultimate effect on photocatalytic performance was studied by simultaneous H2 production and methylene blue degradation. The role of graphene in optoelectronics and defect formation in the composite catalysts was explored. In addition, efforts have been made to unveil the reaction pathway for hydrogen evolution reaction and oxygen evolution reaction where excess defect density greatly hampered the quantum yield of the process. Results suggest that maintaining optimal defect concentration aborts the undesired thermodynamically favored back reactions. The conduction band and valence band values of the catalysts indicate that the photocatalytic mechanism was dominated by the electron pathway. Graphene acted as an effective electron sink when its concentration was around 2.5-3%. The superior activity of TiO2-ZnS-rGO was attributed to the narrow bandgap, rapid separation of photo-excited charge carriers, and favorable conduction band position for photocatalytic reactions. This work may assist in exploring the fundamental role of defects in driving the photocatalytic reactions and improve the selectivity in heterogeneous photocatalysis.