Promoting charge carriers transfer of Co–S bonds in Co/CdS for enhanced photocatalytic CO 2 reduction

Converting greenhouse gas of carbon dioxide (CO2) into hydrocarbon fuels using solar energy has been generally considered to be possible to solve the ever-growing energy crisis and environmental pollution, simultaneously. As one promising method, photocatalytic CO2 reduction into useful chemicals is receiving considerable attention around the world. However, the efficiency of photocatalytic CO2 reduction is far beyond satisfactory due to the low catalytic performance and poor stability. In general, the whole photocatalytic CO2 reduction process involves three steps to achieve: (1) Generation of electron-hole pairs resulting from light absorption of a semiconductor, (2) separation and transfer of photoexcited electron-hole pairs to the surface of the semiconductor, and (3) surface reactions of CO2 with H2O. The kinetic process of charge separation/transfer and surface reactions involved in the second and third steps are believed to be the two key steps to limit the efficiency of photocatalytic CO2 reduction. As one of the promising photocatalysts for CO2 reduction, cadmium sulfide (CdS) has attracted extensive attention due to its relatively suitable bandgap (2.4 eV) to utilize visible light and sufficiently negative potential of the conduction band edge to reduce CO2 to other carbon sources. However, the photocatalytic activity of pure CdS for CO2 reduction is very low due to the fast charge carrier recombination as well as photocorrosion during the photocatalytic process. Both disadvantages of CdS can be overcome after introducing noble metal cocatalysts onto CdS, which can not only realize the suppressing of charge carrier recombination but also afford reaction active sites for proton-reduction, and finally boost photocatalytic efficiency for CO2 activation. Nevertheless, the scarcity and high costs of noble metals inspired the researchers to explore earth abundant metals as alternative cocatalysts. The recent investigation showed that cobalt metal (Co) has loosely bonded d-electrons as high electrical conductivity. More importantly, partial oxidation of Co to CoO x can improve the CO2 adsorption capacity. Because of these merits, Co is expected to be a suitable cocatalyst for photocatalytic CO2 reduction. In this work, highly dispersed Co/CdS photocatalyst was synthesized via a two-step method. Firstly, the hexagonal phase CdS was prepared by a hydrothermal method. The Co metal was deposited on the surface of CdS using NaBH4 as the reductant. The results of energy dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) suggested that highly dispersed Co2+ was anchored on CdS surface to form Co–S bond. Combined with the photoluminescence (PL), transient photocurrent ( I-t ) and electrochemical impedance spectroscopy (EIS) analysis, the Co–S bonds acted as charge transfer channels were revealed to effectively enhance the separation and migration of photogenerated carriers from Co/CdS. Theoretical calculation using DFT demonstrated that the introducing Co onto CdS improved the adsorption ability of CO2 and CO, and promoted the formation of CH4, achieving 64.9% selectivity toward CH4 evolution. As a result, the formation rates of CO (3.23 μmol g–1 h–1) and CH4 (1.86 μmol g–1 h–1) were 2.5 and 7.5 folds higher than that of bare CdS, respectively. In addition, the photocatalytic activity remained CO (3.07 μmol g–1 h–1) and CH4(2.01 μmol g–1 h–1) after 5 recycle tests. Therefore, this work provides an attractive strategy for designing efficient and stable CdS photocatalyst for CO2 reduction.