Photothermal effect: an important aspect for the enhancement of photocatalytic activity under illumination by NIR radiation

A key auxiliary role of the photothermal effect (PTE) in the enhancement of photocatalytic activity under illumination by near-infrared (NIR) radiation has been experimentally revealed. It was found that the photoexcitation of electrons, required for the process of photocatalysis, does not occur in the presence of NIR radiation alone without the support of both UV and visible radiation. Herein, a binary heterogeneous nanophotocatalyst, GR-ZnO, was fabricated by a novel approach using a hydrothermal process, in which ZnO nanotrapezoids were deposited over the surfaces of graphene nanosheets. In the unique approach reported, the reduction of graphene oxide to graphene has been accomplished using water by an entirely alternative mechanism compared to traditional reduction processes which require hazardous reducing agents. The produced GR-ZnO photocatalyst exhibited excellent performance in the rapid degradation of a perilous dye, methyl orange voluntarily used in industry. It has been found that the PTE is mainly responsible for the rapid degradation of methyl orange, which occurs under NIR laser irradiation. Furthermore, it has been established that the PTE could not emerge with NIR radiation alone, without the support of both UV and visible radiation. Upon considering its importance, the PTE of the GR-ZnO nanocatalyst has been revealed and compared with that of its individual components, viz., graphene and ZnO. Moreover, the photothermal conversion efficiency of the GR-ZnO nanocatalyst was evaluated. Overall, the excellent catalytic activity found in the GR-ZnO nanocatalyst was accounted for by its decreased band gap, which arose because the hybridization of ZnO with graphene could efficiently prevent the recombination of photo-generated charge carriers, resulting in enhanced catalytic activity of GR-ZnO nanocatalyst was attributed to improved optical absorption and enhancement in the adsorption affinity to methyl orange molecules.