Magnetic Hollow Spheres Assembled from Graphene-Encapsulated Nickel Nanoparticles for Efficient Photocatalytic CO₂ Reduction

The exploitation of efficient, robust, and easily recyclable catalysts is highly desirable for photochemical CO₂ reduction to produce fuels and chemicals. Herein, we demonstrate the preparation of Ni@GC magnetic hollow spheres composed of metallic Ni nanoparticles surrounded by few-layered graphitic carbon (GC) for photocatalytic CO₂ reduction with high efficiency. The Ni@GC hollow spheres were prepared by thermal annealing a Ni-containing metal–organic framework (Ni-MOF) under N₂ atmosphere. A series of physiochemical characterizations reveal that the Ni@GC hollow spheres are successfully synthesized with large surface area and highly porous structure. In the presence of Ni–C bonding, the porous Ni@GC material can efficiently accelerate the separation and transportation of photoexcited charges, as well as improve CO₂ adsorption. With the cooperation of a ruthenium photosensitizer under visible light irradiation, the Ni@GC catalyst exhibits a high CO₂-to-CO conversion activity, giving a superior CO-production rate of 27 μmol h^–1 (e.g., 9.0 mmol h^–1 g^–1). Moreover, the Ni@GC photocatalyst is highly stable and can be separated easily by a magnetic field for reuse. The possible photosensitized CO₂ conversion mechanism is also proposed based on the relative energy levels of the Ni@GC catalyst and the ruthenium photosensitizer.