Fabrication and photocatalytic activities in visible and UV light regions of Ag@TiO2 and NiAg@TiO2 nanoparticles

Completely discrete Ag@TiO2 and NiAg@TiO2 nanoparticles were prepared by the hydrazine reduction of Ag+/Ni2+ ions and the subsequent sol-gel coating of TiO2 in an aqueous solution of CTAB. TEM analysis revealed that their core diameters were 6.55 +/- 1.20 and 7.57 +/- 1.33 nm, respectively, and their shell thicknesses were 2.59 and 2.80 nm, respectively. By the analyses of EDX, UV-vis absorption spectra, FTIR spectra, and zeta potential, their core-shell structure, crystal structure, optical properties, and surface state were demonstrated. In addition to exhibiting significant absorption in the visible light region, it was noted that they had lower zeta potentials than TiO2 nanoparticles, which favored the adsorption of positively charged organic compounds on the particle surface and thereby increased the photocatalytic reaction rate. By measuring the photocatalytic degradation rate of rhodamine B, Ag@TiO2 and NiAg@TiO2 nanoparticles were demonstrated to possess significantly higher photocatalytic activities than TiO2 nanoparticles in the visible light region because of the formation of Schottky barrier banding at the core-shell interface as well as the excitation of photogenerated electrons from the surface of Ag or NiAg cores to the conduction band of TiO2 shells. Although NiAg@TiO2 nanoparticles had lower photocatalytic activity than Ag@TiO2 nanoparticles owing to weaker surface plasmon resonance, they could be recovered magnetically from the treated solutions. Under UV light illumination, the photocatalytic activities of Ag@TiO2 and NiAg@TiO2 nanoparticles were lower than that of TiO2 nanoparticles because of the lower TiO2 content and the transfer of photogenerated electrons from TiO2 shells to Ag or NiAg cores, which also acted as the new recombination centers of photoinduced electrons and holes and hence led to a decrease in the photocatalytic activity.