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Nickel element cannot exist stably on its own, but with an bared of an NiO shell, nickel element can realize desirable stable existence. Herein, Ni2+ was photoreduced to Ni particles and deposited on graphite carbon nitride (g-C3N4) under vacuum. The surface of Ni particles was oxidized to form an NiO shell after the samples were exposed to air, and elemental Ni was protected by the encapsulation of the NiO shell to realize the coexistence of Ni and NiO. The chainmail co-catalyst of NiO shell-encapsulated Ni over g-C3N4 (Ni/NiO/g-C3N4) was prepared and applied in the photocatalytic reduction of CO2 and showed the best photocatalytic activity when the Ni/NiO loading was 3 wt%. After illumination for 2 h, the CO generation rate reached a value of 27.9 μmol g−1 h−1, which was about nine times that of pure g-C3N4. The steady-state surface photovoltage spectrum (SPV), ultrafast spectrum of transient photovoltage (TPV) and time-resolved photoluminescence (PL) spectra as well as electrochemical tests showed that Ni/NiO/g-C3N4 has a low recombination rate of photogenerated electron–hole pairs. In situ Fourier transform infrared spectroscopy (FT-IR) and DFT calculations were used to explain the photocatalytic mechanism at the molecular level.

Photocatalytic conversion of CO2 to solar fuels is considered an alternative approach for simultaneously mitigating the greenhouse effect and solving energy shortage. The efficient light harvesting and the thermochemical conversion have been demanding quests in photocatalysis due to the relatively low solar energy utilization efficiency. In this work, oxygen vacancies are induced in MoO3 for improving photo-thermal CO2 reduction efficiency by capturing near-infrared (NIR) photons. The localized surface plasmon resonance (LSPR) of MoO3−x triggered by oxygen vacancies enables the efficient capture of NIR photons. Additionally, oxygen vacancies can promote the carrier separation, improve CO2 adsorption on the defective surface and lower the barrier of CO2 hydrogenation during the conversion process. As a result, MoO3−x displayed dramatically enhanced photo-thermal synergistic CO2 reduction under simulated sunlight (UV-Vis-IR) irradiation than MoO3. The amount of CO produced by MoO3−x can reach 10.3 μmol g−1 h−1, which is 20 times higher than that of MoO3 (0.52 μmol g−1 h−1). And the CH4 production of MoO3−x can reach 2.08 μmol g−1 h−1, which is 52 times higher than that of MoO3 (0.04 μmol g−1 h−1). In situ FT-IR and theoretical calculations also proved the enhanced activity of MoO3−x. This work highlights the significance of defect engineering for improving the photo-thermal catalytic conversion of CO2.

Layered bismuth oxychloride (BOC) exhibits highly efficient activity for photocatalytic environmental remediation due to the confinement effect induced excitonic photocatalytic process. However, the strong excitonic process suppresses catalytic reactions with photo-induced electrons, such as hydrogen generation, CO2 conversion and nitrogen fixation. Moreover, the wide band gap of BiOCl limits its application under visible light. In this study, flexible BiOCl nanosheets with oxygen vacancies (BOC-OV) were successfully prepared. Molecular oxygen activation, electronic spin resonance (ESR), transient photocurrent, transient absorption spectroscopy, and transient fluorescence spectroscopy indicated that oxygen vacancies induced exciton dissociation of flexible BiOCl nanosheets. Moreover, oxygen vacancies induced wide spectrum (UV-Vis) absorption. The enhanced exciton dissociation resulted in the superior CO2 conversion of BOC-OV under UV-Vis light irradiation, where the light to carbon monoxide (LTCO) conversion efficiency reached up to 26.5 × 10−6. Theoretical calculations and in situ Fourier transform infrared spectrometry (FT-IR) analysis revealed that the mechanism of oxygen vacancies improves the photocatalytic CO2 conversion with BOC-OV via the CO2 hydrogenation pathway. This study indicates that oxygen vacancies have a great influence on photocatalytic CO2 reduction due to their special surface and electron structure properties.