Controlling the size and optical properties of ZnO nanoparticles by capping with SiO2

Abstract

The size and shape of the ZnO nanoparticles synthesized through sol–gel method were controlled by capping with SiO2. X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM) and High Resolution Transmission Electron Microscope (HR-TEM) results demonstrated that the particle growth of the ZnO nanoparticles has been restricted to 5 nm with SiO2 capping. As a result, the absorption spectra of ZnO nanoparticles capped with SiO2 got blue shifted (toward lower wavelength side) due to strong quantum confinement effects. BET (Brunauer–Emmet–Teller) surface area pore size analyzer results showed that surface area of samples increased monotonously with increase of SiO2 concentration. It was observed that the absorption spectra of ZnO capped with SiO2 broadened with increase of SiO2 concentration. Absorption and photoluminescence excitation results (PLE) confirmed that this broadening is due to the absorption of non-bridging oxygen hole centers (NBOHC) of SiO2. These results also indicated that ZnO nanoparticles capped with SiO2 are insensitive to Raman scattering. Maximum UV emission intensity was achieved with 353 nm excitation wavelength compared to 320 nm in ZnO as well as in SiO2 capped ZnO nanoparticles. Furthermore, there is an enhancement in the intensities of emission peaks related to oxygen vacancies and interstitials with SiO2 capping. The enhancement in the UV intensity is attributed to the surface passivation of ZnO nanoparticles and excitation processes in SiO2.