Ultrathin porous NiO nanosheets with oxygen vacancies boost photocatalytic CO2 reduction

The rapid increase in global industrialization over the past few centuries has resulted in the disposal of excessive CO₂ into the atmosphere, leading to serious environmental issues such as the greenhouse effect, rising sea levels, and an increase in extreme weather events. Photocatalytic CO₂ reduction, which converts CO₂ into valuable fuels and chemicals using light as an energy source, is considered a promising approach to mitigate these problems. However, the low concentration of CO₂ in the atmosphere (~416 ppm) poses a significant challenge, necessitating the development of efficient catalysts with both high CO₂ enrichment capacity and excellent charge separation performance.

Recent advancements in addressing the global challenge of excessive CO₂ emissions have led to innovative solutions in photocatalytic CO₂ reduction. A research team from Lanzhou University and Inner Mongolia University, led by Professor Yong Ding and Professor Jiangwei Zhang have designed and synthesized a novel catalyst that demonstrates exceptional performance in this field. Their findings have been published in the Chinese Journal of Catalysis (DOI: 10.1016/S1872-2067(25)64687-0).

In this study, the research team successfully synthesized a series of ultrathin porous NiO nanosheets with abundant oxygen vacancies through a Maillard reaction process. These nanosheets, denoted as NiO-1 to NiO-9, were prepared by varying the amount of nickel nitrate precursor and the calcination temperature. Among them, NiO-1, with the highest oxygen vacancy content, exhibited the best photocatalytic performance.

Under a pure CO₂ atmosphere and in the presence of the photosensitizer [Ru(bpy)₃]Cl₂, NiO-1 achieved a CO evolution rate of 16.8 μmol/h with a selectivity of 96%. The introduction of oxygen vacancies in NiO was found to significantly enhance the CO₂ adsorption capacity and photogenerated charge separation efficiency of the catalyst. Characterization techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, electron paramagnetic resonance, and extended X-ray absorption fine structure spectroscopy, combined with density functional theory calculations, revealed that the oxygen vacancies induced lattice distortion in NiO, forming an internal electric field that facilitated the separation and transport of photogenerated carriers.

Interestingly, when the photocatalytic CO₂ reduction reaction was conducted under atmospheric conditions (containing 21% O₂), the product distribution changed dramatically, with the formation of both CO and CH₄. Further investigation revealed that the presence of O₂ promoted water splitting to generate abundant protons, which altered the reaction pathway and led to the formation of CH₄. The team conducted a series of control experiments and in situ Fourier-transform infrared spectroscopy to elucidate the mechanism behind this phenomenon.

This research not only provides a new strategy for the efficient reduction of low-concentration carbon dioxide but also reveals the influence of oxygen on the selectivity of the products in the photocatalytic carbon dioxide reduction reaction. The findings have important implications for the design and development of advanced photocatalysts for CO₂ conversion technologies.