Engineering Coexposed {001} and {101} Facets in Oxygen-Deficient TiO₂ Nanocrystals for Enhanced CO₂ Photoreduction under Visible Light

This work for the first time reports engineered oxygen-deficient, blue TiO₂ nanocrystals with coexposed {101}-{001} facets (TiO_2–<i>x</i>{001}-{101}) to enhance CO₂ photoreduction under visible light. The TiO_2–<i>x</i>{001}-{101} material demonstrated a relatively high quantum yield (0.31% under UV–vis light and 0.134% under visible light) for CO₂ reduction to CO by water vapor and more than 4 times higher visible light activity in comparison with TiO₂ with a single {001} plane or {101} plane and TiO₂(P25). Possible reasons are the exposure of more active sites (e.g., undercoordinated Ti atoms and oxygen vacancies), the facilitated electron transfer between {001} and {101} planes, and the formation of a new energy state (Ti³⁺) within the TiO₂ band gap to extend the visible light response. An in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study was applied to understand the roles of coexposed {001}-{101} facets and Ti³⁺ sites in activating surface intermediates. The in situ DRIFTS analysis suggested that the coexposed {001}-{101} facets increased the capacity of reversible CO₂ adsorption and that the combination of {001}-{101} and Ti³⁺ enhanced the activation and conversion kinetics of adsorbed species. The visible light responsive TiO_2–<i>x</i>{001}-{101} material is not oxidized after long-term exposure to an air environment. This work is a significant contribution to the design of efficient and stable solar fuel catalysts