Abstract:

Artificial photosynthesis is an ideal method for solar-to-chemical energy conversion, wherein solar energy is stored in the form of chemical bonds of solar fuels. In particular, the photocatalytic reduction of CO₂ has attracted considerable attention due to its dual benefits of fossil fuel production and CO₂ pollution reduction. However, CO₂ is a comparatively stable molecule and its photoreduction is thermodynamically and kinetically challenging. Thus, the photocatalytic efficiency of CO₂ reduction is far below the level of industrial applications. Therefore, development of low-cost cocatalysts is crucial for significantly decreasing the activation energy of CO₂ to achieving efficient photocatalytic CO₂ reduction. Herein, we have reported the use of a Ni₂P material that can serve as a robust cocatalyst by cooperating with a photosensitizer for the photoconversion of CO₂. An effective strategy for engineering Ni₂P in an ultrathin layered structure has been proposed to improve the CO₂ adsorption capability and decrease the CO₂ activation energy, resulting in efficient CO₂ reduction. A series of physicochemical characterizations including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and atomic force microscopy (AFM) were used to demonstrate the successful preparation of ultrathin Ni₂P nanosheets. The XRD and XPS results confirm the successful synthesis of Ni₂P from Ni(OH)₂ by a low temperature phosphidation process. According to the TEM images, the prepared Ni₂P nanosheets exhibit a 2D and near-transparent sheet-like structure, suggesting their ultrathin thickness. The AFM images further demonstrated this result and also showed that the height of the Ni₂P nanosheets is ca 1.5 nm. The photoluminescence (PL) spectroscopy results revealed that the Ni₂P material could efficiently promote the separation of the photogenerated electrons and holes in [Ru(bpy)₃]Cl₂·6H₂O. More importantly, the Ni₂P nanosheets could more efficiently promote the charge transfer and charge separation rate of [Ru(bpy)₃]Cl₂·6H₂O compared with the Ni₂P particles. In addition, the electrochemical experiments revealed that the Ni₂P nanosheets, with their high active surface area and charge conductivity, can provide more active centers for CO₂ conversion and accelerate the interfacial reaction dynamics. These results strongly suggest that the Ni₂P nanosheets are a promising material for photocatalytic CO₂ reduction, and can achieve a CO generation rate of 64.8 μmol·h^-1, which is 4.4 times higher than that of the Ni₂P particles. In addition, the XRD and XPS measurements of the used Ni₂P nanosheets after the six cycles of the photocatalytic CO₂ reduction reaction demonstrated their high stability. Overall, this study offers a new function for the 2D transition-metal phosphide catalysts in photocatalytic CO₂ reduction.

Key words: Photocatalysis, Nanosheet, Ni₂P, CO₂ reduction, CO