Photoassisted reduction of metal ions and organic dye by titanium dioxide nanoparticles in aqueous solution under anoxic conditions

The photoassisted reduction of metal ions and organic dye by metal-deposited Degussa P25 TiO2 nanoparticles was investigated. Copper and silver ions were selected as the target metal ions to modify the surface properties of TiO2 and to enhance the photocatalytic activity of TiO2 towards methylene blue (MB) degradation. X-ray powder diffraction (XRPD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) were used to characterize the crystallinity, chemical species and morphology of metal-deposited TiO2, respectively. Results showed that the particle size of metal-deposited TiO2 was larger than that of Degussa P25 TiO2. Based on XRPD patterns and XPS spectra, it was observed that the addition of formate promoted the photoreduction of metal ion by lowering its oxidation number, and subsequently enhancing the photodegradation efficiency and rate of MB. The pseudo-first-order rate constant (kobs) for MB photodegradation by Degussa P25 TiO2 was 3.94 × 10− 2 min− 1 and increased by 1.4–1.7 times in kobs with metal-deposited TiO2 for MB photodegradation compared to simple Degussa P25 TiO2. The increase in mass loading of metal ions significantly enhanced the photodegradation efficiency of MB; the kobs for MB degradation increased from 3.94 × 10− 2 min− 1 in the absence of metal ion to 4.64–7.28 × 10− 2 min− 1 for Ag/TiO2 and to 5.14–7.61 × 10− 2 min− 1 for Cu/TiO2. In addition, the electrons generated from TiO2 can effectively reduce metal ions and MB simultaneously under anoxic conditions. However, metal ions and organic dye would compete for electrons from the illuminated TiO2.