Photocatalyst separation from aqueous dispersion using graphene oxide/TiO2 nanocomposites

A series of nanocomposite photocatalysts was prepared from aqueous dispersions of Degussa P25 TiO2 and exfoliated graphite oxide (GO) by heterocoagulation. The structure and catalytic activity of samples in the photooxidation of phenol were characterized in line with their separability by controlled settling experiments at different pHs, irradiation times and mechanical exposure. It was found that single or few-layer graphene oxide sheets were dispersed uniformly in the matrix of titanium dioxide aggregates and were partially reduced to oxygen-containing graphene-like carbon. Although the presence of GO was associated with a loss of photocatalytic efficiency, it has induced an accelerated sedimentation of catalyst slurry as compared to pure TiO2, resulting in the complete separation of photocatalyst from the treated water in a highly reduced time scale. The loose physical network of weakly adhered particles can be destroyed by sonication or other strong mechanical impacts, but spontaneous aggregation occurs upon mild redispersion. Therefore, heterocoagulation is reversibly induced and can be used to collect the finely suspended catalyst particles worn off from the composite. Extensive reuse of the catalyst without its abrasion and the concomitant need of its post-treatment separation is thus possible. Along with the beneficial sedimentation properties of the composites, this facilitates their use for large scale treatment of waste waters.