Efficient photocatalytic reduction of chromium (VI) using photoreduced graphene oxide as photocatalyst under visible light irradiation

Graphene oxide (GO), a new and promising material, has been widely used as a co-catalyst in photocatalytic reactions; however, its capacity as a sole photocatalyst has rarely been investigated. In this study, ultraviolet (UV) light irradiation was used as a modification method to obtain reduced GO (rGO) samples. The samples were used as photocatalysts to examine their visible light photocatalytic activity toward hexavalent chromium (Cr(VI)) removal. Atomic force microscopy (AFM), X-ray diffraction (XRD), UV–vis spectrophotometry, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron spin resonance (ESR) spectroscopy were applied to interpret the surface and structure changes with UV irradiation. The oxygen-containing functional groups (OFGs) on the GO surface were reduced to defective carbons and π-conjugated C=C (sp2 domains) under UV light; this led to a decrease in the interlayer distance between GO sheets, GO fragmentation, and increased disorder on the GO surface. The restoration of sp2 domains led to a narrower band gap of GO, which favored the rGO excitation by visible light to generate electron-hole pairs. The rGO pre-irradiated with UV for 1h (rGO-1), possessing the highest defect density and electron generation efficiency, exhibited the best Cr(VI) reduction efficiency, which was about three times that of the GO sample; moreover, it outperformed most of the reported GO-based nanomaterials. In addition, low pH and the addition of citric acid as a hole scavenger could further improve the photocatalytic activity. This study proves that GO or rGO can be used as a sole photocatalyst under visible light to remove environmental pollutants such as heavy-metal ions, and it paves the way for the development of this kind of material and its UV-irradiation modification for further applications.