Visible-light-driven CO2 reduction to CO boosted by hydrogenated sphalerite-type CoO atomic layers with exposed polar {111} surfaces and the photocatalytic mechanism

In order to explain the universality of charge separation driven by spontaneous electric field (Es) between polar crystalline planes, we synthesizd the atomically thick sub-stable sphalerite-type CoO nanosheets with exposed {111} polar crystal faces using a salt template method. Photocatalytic activity of the CoO ultrathin nanosheets for the reduction of CO2 to CO is enhanced by removing O-H groups and Cl- anions located on Co-(1 1 1) face through hydrogenation. The CO production rate is 5576.5 mu mol g-1 h-1. The apparent quantum yield for CO production is as high as 4.11 %, which is obviusly higher than that of most photocatalysts reported so far. The CoO are thus expected to be applied in the preparation of CO by CO2 reduction. The atomic arrangement of the exposed {111} polar crystalline planes of sphalerite-type CoO is studied by density functional theory calculations. An Es is generated in the CoO ultrathin nanosheets via spontaneous polarization. The enhancement of the photocatalytic activity of the hydrogenated CoO ultrathin nanosheets is primarily attributed to an increase in the Es in CoO ultrathin nanosheets. We put forward thus a photocatalytic mechanism of charge separation induced by Es between Co-(1 1 1) and O-(1 1 1) polar crystalline planes. The finding indicates that the charge separation model driven by Es between the polar surfaces is a general photocatalytic mechanism. The photocatalytic model can contribute to comprehending the morphology and crystal plane-dependent photocatalytic activities and guide the development of high-performance-photocatalysts and optoelectronic devices.