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

A series of N-doped graphene (N-graphene)/CdS nanocomposites were synthesized by calcination and characterized by X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, N-2 adsorption analysis, ultraviolet-visible diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy. The photocatalytic activity of as-prepared N-graphene/CdS for hydrogen production from water under visible light irradiation at lambda >= 420 nm was investigated. The results show that N-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 N-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 N-graphene/CdS photocatalysts. The amount of N-graphene is an important factor affecting photocatalytic activity of N-graphene/CdS nanocomposites; the optimum amount of N-graphene is ca. 2 wt%, at which the N-grapherie/CdS sample displays the highest reactivity. Photocatalytic activity of graphene/CdS and GO/CdS composites for H-2 production from water under visible light irradiation was also measured. The relative order of reactivity for the synthesized catalysts was found to be N-graphene/CdS > graphene/CdS > GO/CdS > CdS. Furthermore, the N-graphene/CdS photocatalyst does not show deactivation for H-2 evolution for longer than 30 h, indicating that the cocatalyst of N-graphene as a protective layer can prevent CdS from photocorrosion under light irradiation. Our findings demonstrate that N-graphene as a cocatalyst is a more promising candidate for development of high-performance photocatalysts in the photocatalytic H-2 production.