ZnO_1−x/carbon dots composite hollow spheres: Facile aerosol synthesis and superior CO₂ photoreduction under UV, visible and near-infrared irradiation

For the first time, ZnO_1−x/carbon dots composite hollow spheres (denoted ZnO_1−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 ZnO_1−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 ZnO_1−x and the deposited carbon, which enable efficient charge carrier transfer and improved CO₂ adsorption; and 3) the strong NIR absorption of ZnO_1−x/C, in which ZnO_1−x is excited by absoring the up-converted photoluminescence emissions (410–560 nm) of the carbon dots.