Highly Durable N-Doped Graphene/CdS Nanocomposites with Enhanced Photocatalytic Hydrogen Evolution from Water under Visible Light Irradiation

Abstract

A series of N-doped graphene (<i>N</i>-graphene)/CdS nanocomposites were synthesized by calcination and characterized by X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, N₂ adsorption analysis, ultraviolet–visible diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy. The photocatalytic activity of as-prepared <i>N</i>-graphene/CdS for hydrogen production from water under visible light irradiation at λ ≥ 420 nm was investigated. The results show that <i>N</i>-graphene/CdS nanocomposites have a higher photocatalytic activity than pure CdS. Transient photocurrents measured with a photoelectrochemical test device show that the photocurrent of the <i>N</i>-graphene/CdS sample is much increased as compared to the sole CdS. This enhanced photoresponse indicates that the photoinduced electrons in the CdS prefer separately transferring to the N-doped graphene. As a consequence, the radiative recombination of the electron–hole pairs is hampered and the photocatalytic activity is significantly enhanced for the <i>N</i>-graphene/CdS photocatalysts. The amount of <i>N</i>-graphene is an important factor affecting photocatalytic activity of <i>N</i>-graphene/CdS nanocomposites; the optimum amount of <i>N</i>-graphene is ca. 2 wt %, at which the <i>N</i>-graphene/CdS sample displays the highest reactivity. Photocatalytic activity of graphene/CdS and GO/CdS composites for H₂ production from water under visible light irradiation was also measured. The relative order of reactivity for the synthesized catalysts was found to be <i>N</i>-graphene/CdS > graphene/CdS > GO/CdS > CdS. Furthermore, the <i>N</i>-graphene/CdS photocatalyst does not show deactivation for H₂ evolution for longer than 30 h, indicating that the cocatalyst of <i>N</i>-graphene as a protective layer can prevent CdS from photocorrosion under light irradiation. Our findings demonstrate that <i>N</i>-graphene as a cocatalyst is a more promising candidate for development of high-performance photocatalysts in the photocatalytic H₂ production.