Electrostatic adsorption of ultra-small silver nanoclusters on titanium dioxide modified mesoporous MCM-41as a high-performance photocatalyst for wastewater treatment

Abstract

For the first time, monodispersed bare silver nanoclusters with an average particle size of 1.2 nm were synthesized without protecting ligand and deposited inside the pores of a titanium dioxide modified mesoporous MCM-41 utilizing the novel strong electrostatic adsorption (SEA) technique. The pH of the MCM-41 and TiO2/MCM-41 surfaces was adjusted at 11 to optimize the electrostatic adsorption of the silver metal complexes and MB dye. Where, the pH point of zero charges (pHPZC) of the MCM-41 and TiO2/MCM-41 are 4 and 4.9, respectively. For comparison, microwave assisted sol–gel and hydrothermal methods were used to load silver nanoparticles on a titanium dioxide modified mesoporous MCM-41, where, the average particle size of the silver nanoparticles is 2–5 nm and 4–5.5 nm, respectively. The texture properties of the synthesized photocatalysts were investigated using the liquid nitrogen sorption isotherms at _196 °C. The crystallinity of the photocatalysts was characterized by X-ray diffraction (XRD). The charge of the loaded silver nanoclusters and silver nanoparticles was investigated by X-ray photoelectron spectroscopy (XPS). The silver content was around 1.0 wt% over the TiO2/MCM-41 and MCM-41 supports that was measured by the XPS. The performance of the synthesized photocatalysts was tested by photocatalytic degradation of MB dye under visible light irradiation. Around 100 % decolorization of 50 ppm MB solution was archived after two hours of irradiation over Ag/TiO2/MCM-41 (SEA) photocatalyst. This catalyst shows stable efficiency for at least five cycles without significant leaching or loss of catalytic activity. The surface morphology of the Ag/TiO2/MCM-41 (SEA) photocatalyst was investigated using scanning electron microscopy (SEM). The excellent performance of the SEA catalyst is attributed to ultra-small Ag nanoclusters that uniformly dispersed in the mesopores of MCM-41 so that more Ag surface sites are available for the catalytic reaction. In conclusion, the SEA is an excellent synthetic strategy for the deposition of ultra-small bare metal nanoclusters on porous materials.