Photocatalysis deconstructed: design of a new selective catalyst for artificial photosynthesis

A rapid increase in anthropogenic emission of greenhouse gases, mainly carbon dioxide, has been a growing cause for concern. While photocatalytic reduction of carbon dioxide (CO2) into solar fuels can provide a solution, lack of insight into energetic pathways governing photocatalysis has impeded study. Here, we utilize measurements of electronic density of states (DOS), using scanning tunneling microscopy/spectroscopy (STM/STS), to identify energy levels responsible for photocatalytic reduction of CO2-water in an artificial photosynthetic process. We introduce desired states in titanium dioxide (TiO2) nanoparticles, using metal dopants or semiconductor nanocrystals, and the designed catalysts were used for selective reduction of CO2 into hydrocarbons, alcohols, and aldehydes. Using a simple model, we provide insights into the photophysics governing this multielectron reduction and design a new composite photocatalyst based on overlapping energy states of TiO2 and copper indium sulfide (CIS) nanocrystals. These nanoparticles demonstrate the highest selectivity for ethane (>70%) and a higher efficiency of converting ultraviolet radiation into fuels (4.3%) using concentrated sunlight (>4 Sun illumination), compared with platinum-doped TiO2 nanoparticles (2.1%), and utilize hot electrons to tune the solar fuel from alkanes to aldehydes. These results can have important implications for the development of new inexpensive photocatalysts with tuned activity and selectivity.