Enhancing CO2-to-CH4 conversion efficiency of TiO2 through synergistic morphology tuning, defect engineering, and heterojunction formation

Abstract

The photocatalytic reduction of CO2 into valuable fuels represents a promising pathway toward sustainable energy solutions. In this study, the CO2-to-CH4 conversion efficiency of TiO2 is enhanced by implementing synergistic strategies, including morphology tuning, defect engineering, and composite construction. Reduced TiO2 nanosheet (2D-RT) morphology is employed to construct the ternary composite photocatalyst, Cu/reduced graphene oxide/2D-RT (Cu/G/2D-RT), which outperforms 2D-RT, P25 derived reduced TiO2 (P-RT), and Cu/G/P-RT. The CH4 production rate of Cu/G/2D-RT is nearly 62 times that of P-RT and 3.4 times that of Cu/G/P-RT. The optimal defect concentration in 2D-RT improves visible light absorption and charge separation, while the 2D structure enhances interaction with rGO, leading to better charge transport. Additionally, single-electron-trapped oxygen vacancies accelerate water oxidation, producing more protons to enhance the CO2 reduction on Cu cocatalyst. The CO2 reduction significantly improved under multi-sun illumination. However, the repeated cycling led to catalyst degradation, primarily driven by partial reduction of Cu. The in-situ diffuse reflectance infrared Fourier transform spectroscopy reveals the CO2 conversion pathway. Importantly, the results demonstrate that while a high defect concentration in TiO2 enhances visible light absorption, it does not necessarily ensure enhanced charge separation, optimal band alignment in heterojunctions, and improved CO2 reduction efficiency.