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 (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, N2 adsorption analysis, ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of as-prepared N-graphene/CdS for hydrogen production from water under visible light irradiation atλ≥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-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 N-graphene/CdS > graphene/CdS > GO/CdS > CdS. Furthermore, the N-graphene/CdS photocatalyst does not show deactivation for H2 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 H2 production.