Preparation and Photocatalytic Activity of SnO2@TiO2 Core–Shell Composites Modified by Ag

Photocatalytic degradation is an important method to mediate organic pollution in the environment. This article reports Ag-modified SnO2@TiO2 core–shell composite photocatalysts prepared via a hydrothermal method. The Ag modification and core structure in the composite enhanced the photocatalytic activity and stability of TiO2 in Rhodamine B degradation under visible light irradiation. The composite modified in 0.15 M AgNO3 showed an optimal level of photocatalytic activity, as it degraded 99.14 % Rhodamine B in 60 min while pure TiO2 only degraded 45.7 % during the same time.

Modifying TiO 2 with semiconductors such as SnO 2 has proven to be an effective way to improve the photocatalytic activity, by using the transport and separation of photoproduction carriers between two kinds of semiconductors with different energy gaps. Separately, Ag or other noble metals deposited on the surface of TiO 2 form a short-circuit battery with TiO 2 , which leads to the effective separation of the photogenerated electrons/holes and lower potential in the reduction reaction, thus greatly improving the photocatalytic activity.
In this paper, Ag-modified SnO 2 @TiO 2 core-shell composites were fabricated in two steps: first the synthesis of SnO 2 @TiO 2 composite using a hydrothermal method, and then surface modification of the composite with Ag. The as-prepared samples demonstrated excellent photocatalytic activity and cycle stability under visible light. Tetrabutyl titanate, acetic acid, and anhydrous alcohol were mixed together, and then alcohol-water solution was slowly added to the mixture and stirred for 30 min. SnO 2 was then added, followed by another 30 min of stirring. The obtained solution was transferred to a 100 mL Teflonlined autoclave and kept in an oven at 150°C for 4 h. After cooling to room temperature, the precipitation was collected by centrifugation, washed with anhydrous alcohol, dried at 60°C overnight. The obtained SnO 2 @TiO 2 sample is designated as ST.

Preparation of Ag-Modified SnO 2 @TiO 2 Core-Shell Composites
The ST composite was added to AgNO 3 solutions of different concentrations and irradiated under a 500 W mercury lamp. The precipitates were collected by centrifugation and dried at 60°C overnight to obtain Agmodified SnO 2 @TiO 2 core-shell composites. The samples are designated as AgST-M, where M is the molar concentration of AgNO 3 solution used. For comparison, pure TiO 2 and Ag modified TiO 2 were synthesized separately, and labeled as PT and AgT.

Characterization of the Samples
The chemical composition of the samples was analyzed by energy dispersive X-ray spectroscopy (EDX, INCA). Transmission electron microscopy (TEM, JEM-2100, JEOL) was used for morphology characterization. The crystalline structures of the samples were determined by X-ray diffraction (XRD-6000, Japan Shimadzu) using Cu Ka radiation (k = 0.154 nm) at 40 kV, 250 mA under room temperature. An X-ray photoelectron spectrometer (XPS, Thermo ESCALAB 250, Waltham, MA) was used to analyze the elemental composition and valence states. UVvis diffuse reflectance spectra (DRS) of the samples was recorded in the range of 200-800 nm using a Shimadzu UV240 spectrophotometer (Kyoto, Japan) equipped with an integrating sphere, and BaSO 4 was used as the reference. Thermogravimetry and differential thermal analysis (TG-DTA) were performed by a Pyris Diamond 851e analyzer (PerkinElmer) at a heating rate of 20°C min -1 under N 2 environment with a flow rate = 50 mL min -1 .

Photocatalytic Activity
The photocatalytic activities of the samples were evaluated by the degradation of Rhodamine B in a 500 mL jacketed beaker, and a 500 W mercury lamp was selected as the visible light source. In a typical experiment, 0.05 g of the as-prepared sample was dispersed in the Rhodamine B solution (40 mL, 5 mg L -1 ). The solution was then maintained in the dark for 30 min prior to irradiation, in order to reach the adsorption-desorption balance. During irradiation, 4 mL of the sample solution was taken out every 10 min and analyzed by UV-vis spectroscopy. The degradation rate of Rhodamine B can be calculated via the formula: g ¼ ð1 À A=A 0 Þ Â 100%; where A 0 and A are the absorbance values of the original Rhodamine B solution and irradiated solution.
3 Results and Discussion

Phase Structure and Morphology Characterization
The XRD spectra of Ag-modified SnO 2 @TiO 2 and comparison samples are shown in Fig. 1. Figure [12][13][14]. Figure 2 shows the typical TEM images of AgST-0.15, showing that the as-prepared sample consists of many uniform and decentralized nanoparticles with diameters of 30-50 nm. The core-shell nanostructures can be observed in Fig. 2c, as the surface of SnO 2 nanoparticles is coated with a layer of TiO 2 . Figure 3a shows the TEM images of SnO 2 nanoparticles and Fig. 3b shows the TEM images of TiO 2 nanoparticles, it is showed the diameters of SnO 2 are 20-30 nm and the diameters of TiO 2 nano hybrids are about 400 nm, but these TiO 2 nano hybrids can disperse and cover SnO 2

Thermal Analysis-TG
TG-DTA curves of the Ag-modified SnO 2 @TiO 2 coreshell nanoparticles are exhibited in Fig. 4. The results show that the nanoparticles undergo a multistep decomposition process in the temperature range of 25-800°C. The endothermic peak in the temperature range of 25-90°C is related to the desorption of physically adsorbed free water and residual ethanol, with about 8 % weight loss. The sharp exothermic peak in the temperature range of 200-300°C is due to the combustion of organics on the surface of the sample. During this stage the weight loss percentage is about 5 %. In the temperature range of 300-400°C there is another exothermic peak owing to further carbonization of organics and removal of constitution water. These results indicate that the as-prepared Agmodified SnO 2 @TiO 2 core-shell composite exhibits high thermal stability.

Qualitative Elementary Analysis
AgST-0.15 was further analyzed by XPS to determine the main elements and their chemicals states on the TiO 2 surface. The survey spectrum is shown in Fig. 5a. The binding energies obtained in the analysis were corrected for   Preparation and Photocatalytic Activity of SnO 2 @TiO 2 Core-Shell Composites Modified by Ag 169 specimen charging by referencing C ls to 284.8 eV [15].
The results indicate the presence of five elements C, Ti, O, Sn, and Ag. Figure 5b shows the Sn 3d XPS spectrum. The two peaks centered at 495.2 and 486.9 eV correspond to the Sn 3d 3/2 and Sn 3d 5/2 peaks, respectively. The binding energy of Sn 3d 5/2 (486.9 eV) matches the typical values for SnO 2 [16,17], indicating that the Sn 4? dopant was incorporated into TiO 2 to form SnO 2 . Figure 5c shows the XPS spectrum of Ag 3d. The binding energies of 368.2 and 374.2 eV were, respectively attributed to Ag 3d 5/2 and Ag 3d 3/2 . The 6.0 eV slitting of the 3d doublet of Ag indicates the formation of metallic Ag on the surface of TiO 2 [18,19]. The Ag content of AgST-0.3 estimated from XPS was 3.76 %. The atomic ratio of O/Ti determined from XPS is 2.82:1, which suggests the existence of oxygen vacancies on the surface of TiO 2 . The bandgap of composite SnO 2 @TiO 2 became smaller compared with TiO 2 is because of the atomic orbital of Ti is 3d 2 ,4s 2 , and the atomic orbital of Sn is 5s 2 ,5p 2 , the four electrons of outer orbital of Ti will migrate to the outer orbital of Sn after they are all lost four electrons to form composite SnO 2 @TiO 2 , it can achieve the lowest energy for composite SnO 2 @TiO 2 . The deposition of Ag particles greatly increased the activity, the main reason is that the binding energies of Ag is lower than Sn and Ti(368.2 and 374.2 eV were, respectively attributed to Ag 3d 5/2 and Ag 3d 3/2 ), of course the activity depends on the loading amount of Ag, the energy is lower, the electronic migration is easier, the catalytic performance is better.

UV-vis DRS Analysis
The activity of a photocatalyst mainly depends on the width of its band gap and the recombining rate of electronhole pairs. From the UV-vis DRS results in Fig. 6, the band gaps for PT, AgT, ST, and AgST-0.3 are estimated to be 3.17, 3.06, 3.02, and 2.85 eV, respectively. Compared to PT, both AT and ST exhibited enhanced absorption extending towards the visible region. AgST-0.3 exhibited significant absorption of both ultraviolet and visible regions, revealing the synergistic effect of SnO 2 addition and Ag modification in narrowing the band gap of TiO 2 . It can be ascribed to the following reasons: (1) the chargetransfer transitions between SnO 2 electrons and the TiO 2 conduction band [20,21], (2) the incorporated metallic Ag serving as a delocalization energy level in the energy gap, which would extend the absorbance of TiO 2 -based photocatalyst to the visible region [22].

Photocatalytic Activity in Rhodamine B Degradation
The photocatalytic activities of the samples were evaluated by the degradation of Rhodamine B under visible light irradiation, and the results are shown in Fig. 7. Compared with PT, all modified TiO 2 samples show improved photocatalytic performance, especially the Ag-modified SnO 2 @TiO 2 core-shell composites, which remarkably accelerated the photodegradation. Importantly, the photocatalytic efficiency initially increased with the concentration of AgNO 3 used for treatment, from 0 to 0.1 and then 0.15 M. Afterwards, the efficiency decreased with AgNO 3 concentration up to 0.3 M. The AgST-0.15 sample exhibited the best catalytic efficiency, which degraded 99.14 % of the Rhodamine B in 60 min, while PT degraded only 45.7 % during the same time.