In-situ Raman study of relation between microstructure and photoactivity of CdS@TiO2 core-shell nanostructures

Abstract

In the present study, the CdS@TiO2 core-shell nanostructure was fabricated by the interface assembly of TiO2 nanoparticles on the surface of CdS. All the samples were characterized by XRD, SEM, TEM and BET. Hydrogen production demonstrates that CdS@TiO2 core-shell nanocomposites exhibited much higher photocatalytic activity than bare CdS. Raman and in-situ Raman techniques were used to identify the presence of TiO2 ultrathin shell on the surface of CdS. It was found that the lattice strain of CdS@TiO2 and CdS nanospheres increased linearly with the time in photocatalytic reaction. Except for the innitial point at t = 0, the ratio of the peak intensity for 2LO and 1LO (I2LO/I1LO) for CdS@TiO2 is always lower than that of CdS. For CdS@TiO2 composites, the increasement for the I2LO/I1LO ratio with time is basically unchanged, while that of the CdS nanospheres shows a general increase, with a significant fluctuation at t = 30min. It was also found that the electron phonon coupling for the cubic phase of CdS is much lower than the hexagonal CdS, which is mainly due to the difference of 1LO phonon symmetry. The above results suggested that the interaction between electron and phonon is a function of reaction time, which can be a good indicator of the status of the photocatalyst, even for such complex core-shell nanostrucures, during the reaction process. The in-situ Raman results directly proves that the ultra-thin TiO2 shell can effectively protect the CdS core and avoid to be photocorrosion. Ultrathin TiO2 film absorbs ultraviolet light, while it is transparent for visible light, the CdS can respond to the visible light, in this way the CdS@TiO2 can achieve the efficient utilization for the solar energy.