Plasma-assisted doping of nitrogen into cobalt sulfide for loading cadmium sulfide: A direct Z-scheme heterojunction for efficiently photocatalytic Cr(VI) reduction under visible light

Abstract

• Doping nitrogen into CoS x polyhedron via N 2 plasma treatment to yield N-CoS x . • CdS was controllably loaded on N-CoS x forming CdS/N-CoS x photocatalyst. • Surface wettability, conductivity and Fermi level of CoS x were tuned by plasma treatment. • Visible-light photocatalysis for Cr(VI) reduction was achieved with efficiency up to 100%. • A photocatalytic mechanism of direct Z-scheme heterojunction was proposed. A direct Z-scheme heterojunction has been constructed by loading cadmium sulfide (CdS) on cobalt sulfide doped with nitrogen (N-CoS x ) by N 2 plasma treatment for efficient photocatalytic Cr(VI) reduction under visible light. It was discovered that the resulting CdS/N-CoS x nanocomposites could present different photocatalytic performances controllably depending on the plasma treatment time and CdS-to-N-CoS x ratios. Especially, the one with 5-min plasma treatment and 8/1 M ratio could exhibit the highest Cr(VI) photoreduction efficiency, which is over four and two folds higher than those of pristine N-CoS x and CdS, respectively. Herein, the plasma-assisted N doping could not only increase the surface wettability and conductivity of CoS x but also decrease its Fermi level for improving the photocatalytic Cr(VI) reduction. More importantly, a direct Z-scheme heterojunction could be constructed by the combination of CdS and N-CoS x with suitable band structures, which could further booster the charge separation and transferring, without sacrificing the redox capacity of photogenerated carriers. The enhanced photocatalytic activity and stability of CdS/N-CoS x nanocomposites could thereby be expected for Cr(VI) reduction, with the efficiency up to 100% within 25 min. Such a direct Z-scheme heterojunction created by the plasma treatment route may promise for the large-scale applications for the removal of various toxic pollutants in the environmental remediation field.