Ni(OH)2 modified CdS nanorods for highly efficient visible-light-driven photocatalytic H2 generation

Abstract

Conventional titania photocatalysts possess good activity and stability, but require near-ultraviolet (UV) irradiation (about 4% of the solar spectrum) for effective photocatalysis, which severely limits their practical applications. Here we report the synthesis of new visible-light-driven Ni(OH)₂-modified CdS nanorod-type composite photocatalysts by a simple precipitation method using CdS nanorods as a support and Ni(NO₃)₂ as a nickel hydroxide precursor. A special emphasis is placed on the effect of Ni(OH)₂ content in this composite photocatalyst on the photocatalytic rate of H₂ production in triethanolamine aqueous solutions under visible light irradiation. This study shows that the photocatalytic H₂-production activity of CdS nanorods is significantly enhanced by modifying them with Ni(OH)₂. The optimal Ni(OH)₂ loading was found to be 23 mol%, which resulted in a visible-light H₂-production rate of 5085 μmol h⁻¹ g⁻¹ corresponding to 28% quantum efficiency (QE) at 420 nm, exceeding that of pure CdS and 1wt% Pt loaded CdS nanorods by a factor of 145 and 1.3 times, respectively. This enhanced photocatalytic H₂-production activity was achieved because the potential of Ni²⁺/Ni (Ni²⁺ + 2e⁻ = Ni, E° = −0.23 V) is less negative than the conduction band (CB) (−0.7 V) of CdS, meanwhile this potential is more negative than the reduction potential of H⁺/H₂ (2H⁺ + 2e⁻ = H₂, E° = 0.0 V), which favors the transfer of CB electrons from CdS to Ni(OH)₂ and the reduction of Ni²⁺ to Ni⁰. The role of Ni⁰ is to help the charge separation and to act as a co-catalyst for water reduction, thus enhancing the photocatalytic H₂-production activity. This work not only shows a possibility for the utilization of low cost Ni(OH)₂ as a substitute for noble metal Pt in photocatalytic H₂-production but also demonstrates its capability for the consumption of the pollutant triethanolamine, used as a sacrificial reagent in this process.