CdS nanoparticles incorporated onion-like mesoporous silica films: Ageing-induced large stokes shifted intense PL emission

Abstract

CdS nanoparticles (NPs) were generated in onion-like ordered mesoporous SiO2 films through a modified sol–gel process using P123 as a structure directing agent. Initially Cd2+ doped (12 equivalent mol% with respect to the SiO2) mesoporous SiO2 films were prepared on glass substrate. These films after heat-treatment at 350°C in air yielded transparent mesoporous SiO2 films having hexagonally ordered onion-like pore channels embedded with uniformly dispersed CdO NPs. The generated CdO NPs were transformed into CdS NPs after exposing the films in H2S gas at 200°C for 2h. The as-prepared CdS NPs incorporated mesoporous SiO2 films (transparent and bright yellow in color) showed a band-edge emission at 485nm and a weak surface defect related emission at 530nm. During ageing of the films in ambient condition the band-edge emission gradually weakened with time and almost disappeared after about 15days with concomitant increase of defect related strong surface state emission band near 615nm. This transformation was related to the decay of initially formed well crystalline CdS to relatively smaller and weakly crystalline CdS NPs with surface defects due to gradual oxidation of surface sulfide. At this condition the embedded CdS NPs show large Stokes shifted (∼180nm) intense broad emission which could be useful for luminescent solar concentrators. The detailed process was monitored by UV–Visible, FTIR and Raman spectroscopy, XPS, XRD and TEM studies. The evolution of photoluminescence (PL) and life times of CdS/SiO2 films were monitored with respect to the ageing time.