Photocatalytic CO₂ reduction of C/ZnO nanofibers enhanced by an Ni-NiS cocatalyst

The photocatalytic reduction of CO₂ into valuable hydrocarbon fuels via solar energy is a promising strategy for carbon utilization. In the present paper, a hierarchical Ni-NiS/C/ZnO photocatalyst was prepared via the in situ photodeposition of compact Ni-NiS nanosheets onto C/ZnO electrospun nanofibers. The existence of metallic Ni and NiS was confirmed by X-ray photoelectron spectroscopy. Photoluminescence (PL) and time-resolved PL spectra revealed that the cocatalyst Ni-NiS enhanced the charge separation efficiency of the C/ZnO nanofibers. The as-prepared Ni-NiS/C/ZnO showed enhanced CO₂ reduction activity, with CO and CH₄ production rates 10 and 15 times greater than those of pristine C/ZnO under 350 W visible light illumination. The intermediates of CH₃O⁻, HCHO, and HCOO− were detected by in situ Fourier transform infrared spectroscopy, confirming that CO₂ reduction is a complex reaction with multiple steps. The 13C isotopic tracer method proved that CH₄ and CO were obtained from the reduction of CO₂ rather than from other carbon species in the environment. The amorphous carbon in C/ZnO could promote optical absorption, improve conductivity and reduce the interfacial charge transport resistance. Ni-NiS improved the electron–hole-pair separation of the C/ZnO nanofibers. The observed enhancement in photocatalytic activity was largely attributed to higher light utilization and effective electron–hole separation. This work proves that Ni-NiS is a promising cocatalyst to ZnO for photocatalytic CO₂ reduction. Copyright © 2020 The Royal Society of Chemistry.