2, Center for Condensed Matter Sciences, National Taiwan University, Taipei, , Taiwan
The development of solar fuels has been limited by the low conversion efficiency and lack of product selectivity of the current available photocatalysts. Recently, defective electro-catalysts have been successfully demonstrated to enhance hydrogen evolution reaction (HER) performance in liquid-phase reaction. However, gas-phase reaction for CO2 conversion has yet to achieve the same status. Here, we present defect engineering in 2D materials with multiscale active sites as a promising photocatalysts for CO2 reduction reaction (CO2RR). Specifically, two types of defects have been investigated.
For the first type of zero-dimensional defect in photocatalyst, continuous MoS2 thin films (MoS2 TFs) with typical layer thickness of 5~10 nm and large area of 2x2 cm2 were directly synthesized on SiO2/Si substrates by chemical vapour phase reaction of MoO3 and S powders. The MoS2 TFs exhibited Raman peak frequency difference between A1g and E2g modes (Δ) of ~24 cm-1. Subsequently, the films were post-treated with hydrogen plasma in order to create around 20% sulfur vacancies (S-V) with the resultant stoichiometry ratio of Mo/S confirmed by X-ray photoelectron spectroscopy. The MoS2 TFs with S-V exhibited enhanced CO2RR performance compared with pristine MoS2 TFs and showed selective photoreduction of CO2 to multi-hydrocarbon products such as acetaldehyde, acetone, methanol and ethanol.
For the second type of two-dimensional defect in photocatalyst, discontinuous MoS2 TFs with different layer thickness of 1~5 nm were directly synthesized on multilayer graphene/SiO2/Si substrates by chemical vapor deposition. The MoS2 TFs grown on graphene-template exhibited different value of Δ (23~26 cm-1), and also exhibited significantly enhanced CO2RR performance compared with only pristine MoS2 TFs. Plausible mechanisms of enhanced CO2RR activity by defect engineering in 2D materials were further investigated by in situ near ambient pressure X-ray photoelectron spectroscopy experiments to reveal the essential steps for activating CO2 on a MoS2 TFs surface, and will be presented in the meeting.
This defective photocatalysts can serves as an initial platform to understand the basic working principle of active site from atomic to nano scale and open new opportunity to develop higher-efficiency photocatalysts for solar fuels application.