ZnO 1− x/Carbon Dots Composite Hollow Spheres: Facile Aerosol Synthesis and Superior CO 2 Photoreduction under UV, Visible and Near-Infrared Irradiation

For the first time, ZnO1−x/carbon dots composite hollow spheres (denoted ZnO1−x/C) have been synthesized via a single-step aerosol process and employed for CO2 photoreduction over the whole UV–vis-NIR spectrum. The effects of the precursor component ratio and synthesis temperature on the physicochemical properties of the composites are systematically investigated to maximize CO2 conversion efficiency. Under UV–vis–NIR light, the best performing sample had an average CO production rate of 60.77 μmol g−1 h−1, which is about 54.7 times higher than that of pristine ZnO, and 11.5 times higher than that of commercial TiO2 (Degussa-P25). More importantly, whereas ZnO and Degussa-P25 are photocatalytically inactive, the photocatalytic response of the ZnO1−x/C composite was successfully achieved under NIR illumination alone, with an average CO production rate of 15.98 μmol g−1 h−1. The realization of NIR-driven CO2 photoreduction with enhanced photocatalytic activity benefits from 1) the hollow structure, which allows multiple internal reflections of light for enhanced light absorption; 2) the oxygen deficiency of ZnO1−x and the deposited carbon, which enable efficient charge carrier transfer and improved CO2 adsorption; and 3) the strong NIR absorption of ZnO1−x/C, in which ZnO1−x is excited by absoring the up-converted photoluminescence emissions (410–560 nm) of the carbon dots.