Photocatalytic hydrogen generation from water–methanol mixtures using halogenated reconstituted graphenes

Abstract

Graphene oxide (GO) obtained by Hummers oxidation of graphite and subsequent exfoliation was modified by treatment with aqueous solutions of HX at 150 °C for 30 h. This procedure introduces a percentage of the corresponding halogen, accompanied by reconstitution of G. The samples were characterized by spectroscopic techniques, including: (i) IR spectroscopy that shows the reconstitution of G, (ii) Raman spectroscopy that shows a high density of defects in the G layer, (iii) microscopy showing the typical 2D morphology, and (iv) XPS measurements to quantify the percentage of halogen in the samples. Graphene-based materials show promise as solar light photocatalysts for hydrogen generation from water, the target being improvement of the efficiency of the process. The halogenated reconstituted Gs exhibit upon 355 nm irradiation higher photocatalytic activity for hydrogen generation than GO, the most efficient sample in the series being the chlorinated one that is about seven times more active than conventional GO. This enhanced efficiency is probably the result of the influence of the nature and percentage of the halogen present in the G layer. The materials maintained the morphology after irradiation, with some variations in Raman spectroscopy compatible with reduction of the (X)G sheet as indicated by the appearance of C–H vibration peaks at 2950 cm⁻¹. Time resolved transient spectroscopy allows monitoring of a transient signal decaying in the microsecond time scale, attributable to electrons in the conduction band of the semiconductor as supported by quenching with oxygen and signal enhancement and prolongation of lifetime by methanol.