N/S-doped graphene derivatives and TiO2 for catalytic ozonation and photocatalysis of water pollutants

Abstract

Graphene oxide (GO) was synthesized by two methods (Hummers and Brodie), N and S-doping of reduced graphene oxide (rGO) being achieved by thermal treatment. These materials were tested as ozonation catalysts, the N-doped (3.3 at.%) material prepared by Brodie’s method accomplishing complete removal of oxalic acid, while this compound is refractory to non-catalytic ozonation. Titanium dioxide (TiO2) nanoparticles were then introduced as own catalytic phase and protection component to avoid the strong erosive effect of ozone over the carbon phase (GO, rGO, N- and S-doped rGO, obtained by both methods). Among the resulting set of eight TiO2-based composites, the highest catalytic activity in ozonation was obtained with the composite containing N-doped reduced graphene oxide derived from Hummers’ method (rGOT-H-N, kapp = 9.87 × 10−3 min−1). This was attributed to the high content of oxygen in the graphene oxide precursor, the oxygen surface groups acting as reactive sites for doping of N-functionalities and mediating the efficient and uniform assembly of TiO2 nanoparticles on graphene oxide sheets. The same composites, plus two prepared by simple mixing and sonication, were then tested in the photocatalytic degradation of diphenhydramine under near-UV/Vis radiation. Those with graphene oxide (GOT-H and GOT-B) exhibited the highest photocatalytic activity (kapp = 52.5 × 10−3 and 51.3 × 10−3 min−1, respectively), ascribed to the largest blue shift of the band in the UV range and lowest photoluminescence intensity in the visible range. Thus, reduction of graphene oxide has a negative effect in the case of photocatalysis.