Noble metal modified TiO2 microspheres: Surface properties and photocatalytic activity under UV–vis and visible light

Abstract

Composite photocatalysts that consist of TiO2 and noble metal nanostructures have been considered to be the promising and pivotal material for accessible enhancement of the efficiency in the photocatalytic process carried out in the aqueous and gas phases. In this work we fabricated porous TiO2 microspheres through a hydrothermal process followed by photochemical reduction of noble metal nanoparticles at the TiO2 surface. The morphology and structure of M-TiO2 spheres (M=Ag, Au, Pt and Pd) were studied with the use of various techniques, including transmission electron microscopy (TEM), X-ray powder diffraction analysis (XRD), photoluminescence (PL) and UV–vis diffuse-reflectance spectroscopy (DRS). The effect of metal amount (from 0.1 to 1wt.%.) on the photocatalytic activity during toluene degradation in gas phase and phenol degradation in aqueous phase was investigated. Additionally, the photocatalytic activity of the M-TiO2 samples was evaluated by measuring the formation rate of photo-induced hydroxyl radicals (OH) under UV–vis light irradiation using coumarin as a probe. The obtained results indicated that toluene could be mostly removed from the air over TiO2 microspheres modified with Ag, Au, Pt, and Pd nanoparticles. UV-mediated photoreactivity was almost similar for all samples obtained by loading metals from solutions consisting of 0.1 and 1wt.% of metal precursors. Under visible light, except for Au, in gas phase toluene oxidation, the optimized loading of the metals was 0.1wt.-% (photoreactivity changed in order: Ag-TiO2≈Pd-TiO2>Pt-TiO2>>Au-TiO2). In case of phenol degradation in the aqueous phase, in the presence of UV irradiation the highest amount of metal (1.5wt.%) was profitable, while under the Vis light reaction the medium amount of metal (0.5wt.%) was beneficial. Additionally, it was noticed that phenol was degraded not only via oxidation by OH radicals but probably also in direct reaction with the photogenerated carriers (e−/h+), particulary in the presence of TiO2 spheres loaded with Au and Ag nanoparticles.