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Hydrofluoric acid (HF) etching to dissolve SiO2 cores. The SiO2@TiO2 spheres calcined at 700 ℃ revealed fine photocatalytic activity. Interestingly, most of samples transformed into TiO2 nanocaps via HF etching, and TiO2 nanocaps prepared using optimal conditions exhibited quick degradation (k is 0.052 min-1) compared with commercial P25 (k is 0.030 min-1) and the TiO2 nanostructures etched by a NaOH solution. The excellent photocatalytic performance is attributed to its unique hollow hemispherical nanocaps structure, which is in favor of making full use of incident light. The photocatalysis phenomenon in visible light was also observed after depositing Au nanoparticles on anantase TiO2 nanocaps. 

We successfully developed a feasible approach to synthesize anatase TiO2 nanocaps by using hydrofluoric acid (HF) as etching agent. HF-assisted chemical etching of SiO2 templates extremely improved the photocatalytic behavior of anatase TiO2 nanocaps and provided a new idea for the template synthesis of hollow structures. We also explored the effect of calcination temperature and HF amounts on the photocatalytic properties of SiO2@TiO2 (ST) samples. Etched sample using 0.2 mL HF solution (EST-3) exhibited the best ability of photocatalytic degradation of Rhodamine B (RhB) under UV light. In order to improve its photocatalytic performance in visible light, Au nanoparticles (NPs) were deposited on the sample of EST-3. The result indicated that the deposition of noble metal is effective in improving photocatalysis under visible light.

The SiO2 cores were dissolved with different amounts of Hydrofluoric acid solution at room temperature. 30 mg of sample ST-700 ℃ was dispersed in 2 mL of DI water, ultrasound for 10 min. 0.05, 0.1, 0.2, and 0.3 mL of diluted HF solution was then added with stirring, respectively. The obtained samples were denoted as EST-1, EST-2, EST-3 and EST-4, respectively. Characterization: the morphology and size of samples can be observed by a field emission scanning electron microscope and a transmission electron microscope. A high resolution transmission electron microscope was used to get TEM images with high resolution. A powder X-ray diffractometer was used to confirm the phase structure of samples. Nitrogen adsorption isotherms were obtained using a multi-function adsorption instrument. Before measurement, the samples were firstly degassed under vacuum at 110 ℃ for 2.5 h. Fourier transform infrared spectroscopy (FTIR) spectra were collected on a Fourier infrared spectrum instrument (Nicolet 380). Diffuse reflectance spectra (UV–vis) and absorption spectra of samples were measured using a traditional UV–vis spectrometer (Hitachi U-4100).