Enhancement of photocatalytic H2 production activity of CdS nanorods by cobalt-based cocatalyst modification

Abstract

Decreasing the recombination of photogenerated charge carriers in photocatalysts is critical to enhancing the efficiency of photocatalytic H₂ production by water splitting. In this paper, we report the enhancement of visible-light photocatalytic H₂ production activity by introducing various earth-abundant cobalt species onto CdS nanorods to facilitate charge transport and separation. The modification of Co(OH)₂, Co₃O₄, CoS, CoO and Co²⁺ as a cocatalyst enhanced the photocatalytic H₂ production activity of CdS nanorods under visible light irradiation. Among them, the as-prepared CdS–Co₃O₄ sample exhibited the highest photocatalytic H₂ production rate of 150.7 μmol h⁻¹ with a quantum efficiency of 9.7% at 420 nm, even without Pt as a cocatalyst. This outstanding activity arises from the formation of p–n heterojunctions at the interface of the n-type CdS nanorods and p-type Co₃O₄ nanoparticles, which can facilitate the charge separation and transport due to the effect of utilizing the internal electric field. Electrochemical measurement results further proved that the observed superiority of the CdS–Co₃O₄ arises from the effect of the p–n heterojunction including enhancement of light-harvesting capacity, and improvement of charge transfer and separation. To our surprise, the loading of various Co species as a cocatalyst onto Degussa P25 TiO₂ did not show an enhanced photocatalytic H₂ production activity due to the unsuitable positions of the band-gap edge of Co-based TiO₂ heterojunctions in photocatalytic H₂ production. This work demonstrated for the first time that the low-cost Co₃O₄ nanocluster is the most suitable Co-based cocatalyst to replace noble metals for photocatalytic H₂ production in the CdS system.