Fundamentals and challenges of ultrathin 2D photocatalysts in boosting CO2 photoreduction

Carbon dioxide photoreduction currently suffers from low photoconversion efficiency and poor product selectivity. Ultrathin two-dimensional materials, which possess highly active sites with high density and high uniformity, can serve as ideal models to tailor three crucial parameters that determine the carbon dioxide photoconversion efficiency and product selectivity. In this review, we summarize the extended absorption spectrum range enabled by ultrathin two-dimensional semiconductors with defect levels and intermediate bands, as well as conductors with special partially occupied bands. Moreover, we overview the boosted carrier separation efficiency aroused by ultrathin two-dimensional semiconductors with defect states, surface polarization states and built-in electric fields. We also review the accelerated redox reaction kinetics induced by ultrathin two-dimensional semiconductors with in-plane heterostructures, isolated single atoms and abundant low-coordinated dual-metal sites. Finally, we end this review with an outlook on unsolved issues concerning highly selective and efficient photo-conversion of carbon dioxide into C2+ products by ultrathin two-dimensional materials with dual or multiple active sites.