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 andπ-conjugated C=C(sp^(2) 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 sp^(2) 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 1 h(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.