Inorganic core-shell particles bearing titanium dioxide for enhanced hydrophilicity and photoluminescence

Abstract

Core-shell structure of the nanometer- or micrometer-sized particles has been a major consideration for materials having desired physical properties. The structure, thickness, and composition of these shells can be easily altered in a controllable way to tailor the magnetic, optical, mechanical, thermal, electrical, electro-optical, and catalytic properties of core materials. Although they are very effective in improving optical properties, several previous studies were particle-like or did not uniformly cover the surface of the inorganic particles. In this study, inorganic core-shell particles bearing titanium dioxide for enhanced hydrophilicity and photoluminescence were synthesized by sol-gel process using surface-activated core particles to form uniform and thin shells. Surface-activated core particles with ammonium hydroxide (NH3�H2O) exhibited a potential ability to induce a Ti-O-M chemical linkage at core-shell interface as well as a catalyst with controlled hydrolysis. In Chapter 2, TiO2-SiO2 particles with uniform and thin SiO2 layer were prepared by sol-gel process using surface-activated TiO2. TiO2-SiO2 particles were characterized by HR-TEM, FT-IR, XPS, and VB-XPS. A uniform and thin SiO2 shell with ca. 4 nm over TiO2 core suppressed the photocatalytic activity by broadening the band gap because of the existence of Ti-O-Si chemical bonds at the interface of TiO2 and SiO2. The water contact angle of TiO2-SiO2�coated film was lower than that of pure TiO2-coated film (41o and 5.4o in case of pure TiO2 and TiO2-SiO2 nanoparticles, respectively). From a practical point of view, the increase in hydrophilicity of the TiO2-SiO2�coated films observed was a permanent property and did not require UV activation. In Chapter 3, Ba2+Mg2+ co-doped Sr2SiO4:Eu phosphor particles were coated with the thin and uniform TiO2 shell by sol-gel process using surface-activated phosphors. The TiO2 shell with 20 nm was uniformly coated over the phosphor surface. The photoluminescence (PL) properties of the TiO2-coated phosphors showed improved yellow-emission intensity compared to that of the pristine phosphors. The temperature dependence of photoluminescence was measured from 25 to 150 oC. The TiO2-coated phosphors showed superior thermal quenching property. It could be explained that the thin and uniform TiO2 shells on the phosphor particles are an effective way to improve the phosphor efficiency and the thermal quenching stability.