Role of ZrO2 in TiO2 composites with rGO as an electron mediator to enhance the photocatalytic activity for the photodegradation of methylene blue

Abstract

The development of effective methods to overcome the limitations associated with recombination of photocatalysts used in the treatment of dye effluent are important. In this paper we describe a synthetic pathway to produce reduced graphene oxide (rGO)/TiO₂–ZrO₂ (GTZ-X) composites via an ex situ solution mixing method without using strong reducing agents. These composites were then compared with rGO/TiO₂ (GT) composite, where rGO is used as a suitable support and mediator for the stabilization of semiconductors with a view to overcoming the above limitations. The structural, morphological, optical, and photocatalytic properties of GT/GTZ-X were studied using several techniques. X-ray diffraction patterns suggested that a significant amount of GO has been reduced to rGO during the synthesis of composites. The surface analysis data confirms that the pore diameters of the composites are between 3–3.5 nm and can be categorised as mesoporous solids, which positively reflects on their photocatalytic activity. The characterization data confirmed that the metal oxides were successfully incorporated into the rGO sheets while SEM and TEM images display uniformly dispersed spherical morphology of TiO₂ and ZrO₂ exposing a high number of active sites for enhanced photodegradation. Furthermore, the presence of a Ti–C bond in the deconvoluted C 1s XPS spectrum of the composite confirms bond formation, further supported by Raman and FT-IR spectral data. The photocatalytic performance of the GT/GTZ-X composites was evaluated using methylene blue. Compared to the control, the composite with 20 wt% ZrO₂ (GTZ-20) showed the highest photocatalytic activity with 95% degradation in 100 minutes with a rate constant of 0.0414 min⁻¹, while higher amounts of ZrO₂ in composites lowered its photocatalytic activity. Hence, the synthesis of rGO/TiO₂–ZrO₂ by converting GO to rGO without using strong reducing agents appears to be a promising strategy for preparing mixed metal oxide composite photocatalysts, with enhanced photocatalytic activity.