(Invited) High-Rate Solar-Light Photoconversion of CO2 to Fuel: Synthesis and Characterization

Abstract

The production of solar fuels offers a viable pathway for reducing atmospheric CO2 concentrations and the storage and transport of solar energy. While photoconversion of CO2 into C1 hydrocarbon products, notably methane (CH4), is known, the ability to directly achieve significant quantities of higher-order hydrocarbons represents an important step towards practical implementation of solar fuel technologies. We describe an efficient, stable, and readily synthesized CO2-reduction photocatalyst, Pt-sensitized graphene-wrapped defect-induced blue-coloured titania, that produces a record high combined photocatalytic yield of ethane (C2H6) and methane. For the first time, a systematic ultraviolet photoelectron spectroscopy study on the mechanism underlying ethane formation indicates that the process is dependent upon upward band bending at the reduced blue-titania/graphene interface. Furthermore, transient absorption spectroscopy indicates photogenerated holes move into the graphene while electrons accumulate on the Ti3+ sites, a phenomenon contradicting prior assumptions that graphene acts as an electron extractor. We find that both mechanisms serve to enhance multielectron transfer processes that generate ·CH3. Utilizing a continuous flow-through (CO2,H 2O) photoreactor, over the course of multiple 7 h runs approximate totals of 77 mmol g-1 C2H6 and 259 mmol g-1 CH4 are obtained under one sun AM 1.5G illumination. The photocatalyst exhibits an apparent quantum yield of 7.9%, 5.2% CH4 and 2.7% C2H6, and stable photocatalytic performance over the test duration of 42 h. The carbon source for both products is verified using 13CO2 isotopic experiments.