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

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 (I_2LO/I_1LO) for CdS@TiO₂ is always lower than that of CdS. For CdS@TiO₂ composites, the increasement for the I_2LO/I_1LO 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.