Carbon-mediated fabrication of core–shell structured SnO₂@TiO₂ nanocomposites with excellent photocatalytic performance

Abstract

In this work, a facile and carbon-mediated hydrothermal route has been developed for fabrication of core–shell structured SnO₂@TiO₂ nanocomposites with excellent photocatalytic performance for the degradation of the organic dye rhodamine B (RhB). The heterostructures were formed by using SnO₂–C nanospheres as the core and TiO₂ as the outer shell, followed by calcination. The as-synthesized SnO₂@TiO₂ products have uniformly spherical morphology with an average diameter of 64 nm and the thickness of TiO₂ shell was about 4 nm. Additionally, the structure and properties of the products were greatly affected by the calcination temperatures and the carbon content. In comparison to the SnO₂@TiO₂ synthesized in the absence of carbon, the SnO₂@TiO₂ composite exhibited a better photocatalytic performance for the decomposition of RhB. In particular, the carbon-mediated SnO₂@TiO₂-550 product showed superior properties including a large specific surface area for supplying abundant active sites and unique heterojunctions for helping to facilitate the charge separation. As a result, the photocatalytic performance was notably enhanced owing to the above synergistic effect. We expect that the present method based on a carbon-mediated process may be used for the preparation of other nanocomposites.

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1. Introduction

In recent years, semiconducting metal oxides such as SnO₂, TiO₂, and ZnO, etc. have presented themselves as some of the most important materials owing to their outstanding properties and numerous potential applications in gas sensing,^1,2 lithium-ion batteries (LIBs),^3,4 solar cells,^5,6 electrochemical,^7,8 supercapacitors,⁹ photoelectrochemical,¹⁰ and catalysis.^11–13 Among them, TiO₂ has been extensively investigated due to its strong UV light absorption, low cost, high stability, and excellent photocatalytic activity.^14,15 However, the photocatalytic activities of TiO₂ encounter an intrinsic drawback because of the fast recombination of photo-generated electron/hole (e⁻/h⁺) pairs, resulting in a low photocatalytic efficiency. Therefore, the study of how to improve the photocatalytic efficiency of TiO₂ attracts much attention, including the tailor of the morphology,¹⁶ the noble metal doping^17,18 and the design of nanocomposities.^19,20 Particularly, the construction of semiconductor/TiO₂ heterostructure is an effective and feasible method because the special band alignment could improve the photon absorption and promote the separation of the e⁻/h⁺ pairs. For example, TiO₂–ZnO core–shell structured heterojunction nanofibers with good photocatalytic activities had been obtained via electrospinning and atomic layer deposition.²¹ Hybrid multifunctional material of Fe₃O₄ nanoparticles coated by TiO₂ was found to be a highly efficient catalyst for the removal of 4-MMA.²² Moreover, SnO₂/TiO₂ nanocomposites were suggested as efficient photocatalyst because of the suppression of photogenerated carriers recombined between the SnO₂ and TiO₂ interface.²³

On the other hand, carbon materials have drawn much attention in virtue of their low cost,²⁴ high surface areas,²⁵ simple synthesis and good electrical conductivity.^26,27 A lot of studies have demonstrated that the growth of metal oxides on carbon support could be effectively promoted and the electrical conductivity of the composites could also be greatly improved.²⁸ Meanwhile, carbon layer may enhance the contact areas between the catalyst and substrate molecules and improve the structural stability of the materials during the reaction process.²⁹ Additionally, the introduction of carbon may also adjust the morphology and structures of TiO₂.^30,31 For example, mesoporous TiO₂–Sn@C core–shell microspheres with Sn encapsulated into a TiO₂ mesoporous microsphere matrix was synthesized with well-coated of carbon shell.³²

Stimulated by these studies, we demonstrate a facile hydrothermal route for preparation of the core–shell structured SnO₂@TiO₂ nanocomposites based on a carbon-mediated method. The heterostructure was obtained by using SnO₂–C nanospheres as the core and TiO₂ as the outer shell, followed by thermal treatment. The structure and the morphology of SnO₂@TiO₂ nanostructures as well as the role of carbon layer were thoroughly investigated by various techniques including XRD, SEM, HRTEM, BET, XPS and TG analysis. The photocatalytic performance of the products was further studied for degradation of organic dye rhodamine B (RhB) under UV light irradiation. More importantly, the effect of the carbon layer and the thermal treatment temperature on the photocatalytic properties of SnO₂@TiO₂ nanocomposites was also discussed in detail.

2. Experimental section

2.1 Materials

Stannous sulphate (SnSO₄), trisodium citrate dihydrate (Na₃C₅H₆O₇·2H₂O), glucose (C₆H₁₂O₆), rhodamine B (RhB), anhydrous ethanol were purchased from Shanghai Chemical Industrial Co. Ltd. (Shanghai, China). Titanyl sulfate (TiOSO₄) was purchased from Sigma Limited Company. All chemical reagents were used without further purification. Distilled water was used throughout the experiments.

2.2 Synthesis of SnO₂@TiO₂ nanocomposites

In a typical procedure, 4 mmol of Na₃C₅H₆O₇·2H₂O and 10 mmol of glucose were dissolved in 80 ml of mixed solvents containing ethanol (24 ml) and H₂O (56 ml) under vigorous stirring for 30 min to form a clear solution. Then, 1 mmol of SnSO₄ was added into the above solution. After the solution was stirred for 1 h, the resultant mixture was transferred into a Teflon-lined stainless steel autoclave and kept at 180 °C for 12 h. The SnO₂–C spheres were obtained after centrifuged and washed for several times. And the resultant SnO₂–C was dispersed in 80 ml of distilled water with 0.5 mmol of TiOSO₄ and stirred for 10 min. After that, the resultant mixture was transferred into a Teflon-lined stainless steel autoclave at 180 °C for 1 h. Subsequently, the precipitation was centrifuged and washed with ethanol and distilled water in sequence several times and dried at 60 °C overnight. Finally, the SnO₂@TiO₂ nanocomposites were calcined at 350 to 750 °C in air for 2 h. The as-synthesized and calcined samples are denoted as precursor and SnO₂@TiO₂-T (T represents the calcination temperature), respectively.

For comparsion, the SnO₂@TiO₂ nanospheres (named as SnO₂@TiO₂–N) without the addition of glucose and pure SnO₂ nanoparticles were also prepared under similar conditions, and calcined at 550 °C in air for 2 h.

2.3 Characterizations

The samples were studied by transmission electron microscopy (TEM, JEOL 200CX), scanning electron microscopy (SEM, JEOL JSM-6700F), and high resolution transmission electron microscopy (HRTEM, JSM-2010F). Elemental qualitative analysis was conducted by the energy-dispersive X-ray spectroscopy (EDX, OXFORD INCA) which was mounted in the JSM-6700F. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250Xi (Thermofisher, England) using monochromatic AlKα X-ray source operating at a vacuum better than 10⁷ Pa. X-ray powder diffraction (XRD) patterns were obtained on the Japan Rigakul D/max-2550 instrument operating at 40 kV and 40 mA using CuKα radiation (λ = 0.154 nm). N₂ adsorption–desorption isotherms were recorded on a QUADRASORB SI surface area & pore size analyzer at 77 K. Brunauer–Emmett–Teller (BET) specific surface area was calculated by using the desorption data. Thermal analysis (TG, STA409, Netzsch, Germany) was performed at a heating rate of 10 °C min⁻¹ in a flow of air. UV-vis spectra were recorded on a HITACHI U-3010 UV-vis spectrophotometer.

2.4 Photodegradation measurements

The photocatalytic activity of the products was tested in a SGY-IB photochemical reactor using rhodamine B (RhB) degradation in aqueous solution under UV light irradiation (λ < 400 nm). Typically, 10 mg of SnO₂@TiO₂ catalysts was added to 50 ml aqueous solution containing 10 mg L⁻¹ RhB in a 100 ml beaker. The mixture was then placed an ultrasonic water bath for 20 min, followed by stirring in the dark at ambient temperature for 60 min to achieve adsorption–desorption equilibrium. Upon equilibrium, 3–5 ml of the suspension was extracted out to determine the initial concentration of RhB solution, which was recorded as the base concentration C₀. The remaining mixture was transferred into a 50 ml quartz tube and illuminated with a 300 W Hg lamp. In the following 60 min, 3–5 ml of the suspension was extracted out every 5 min. Extracted suspensions were centrifuged immediately to separate any suspended solid. UV-Vis spectra of the upper solution were measured using a Hitachi U-3010 UV-Vis spectrophotometer to determine the concentration of remaining RhB over time, which was recorded as C_t.

3. Results and discussion

A schematic representation for the formation process of SnO₂@TiO₂ nanocomposites was briefly illustrated in Scheme 1. Firstly, the decomposition of sodium citrate could produce OH⁻ anions, which would further react with the Sn²⁺ ions, released by SnSO₄ from the solution, resulting in the formation of SnO₂ nanoparticles (Nps). At the same time, glucose was carbonized, which would lead to the generation of SnO₂–C spheres. With the increasing of glucose added, the produced carbon layer was covered outside the SnO₂–C spheres. Subsequently, small TiO₂ nanoparticles were deposited onto the surface of the SnO₂–C spheres by the hydrolysis of TiOSO₄. Finally, a series of SnO₂@TiO₂ nanocomposites were successfully obtained after annealed at the range of temperature from 350 to 750 °C in air at 2 h.

Scheme 1 Schematic illustration for the growth process of SnO₂@TiO₂ nanocomposites.

3.1 Structure and morphology

The morphology and the structure of SnO₂@TiO₂ nanocomposites were examined by electron microscopy techniques. Fig. 1a and d show the typical TEM images of SnO₂@TiO₂ nanocomposites before and after annealing. As was observed, the composites had uniform morphology and good monodispersity. The noticeable contrast between the black core and the grey shell of the spheres confirmed the core–shell nanostructure of products. Moreover, the products possessed rough surface, in which a lot of small TiO₂ nanoparticles uniformly grew onto the surface of the SnO₂ nanospheres. The thickness of the TiO₂ shell was about 4 nm. Seen from magnified TEM image inset in Fig. 1d, the individual SnO₂@TiO₂ nanoparticle further revealed the hierarchical nanostructure of the composites. Before and after calcination, the spherical morphology still kept well, suggesting a good stability. By close observation, the particle size had a little decrease after annealing. The particle size distribution diagram of based on the statistical analysis of 100 nanoparticles was presented in Fig. 1c and f, further revealing the fact. The average particle size of SnO₂@TiO₂ nanospheres calcined at 550 °C for 2 h was about 48 nm (Fig. 1f), smaller than that of SnO₂@TiO₂ nanospheres without calcination (64 nm, Fig. 1c). The phenomenon was mainly attributed to the shrinkage and removal of carbon layer during the thermal treatment process, resulting in smaller SnO₂@TiO₂ nanoparticles. SEM images of SnO₂@TiO₂ nanospheres before and after annealing were shown in Fig. 1b and e, respectively. On a large scale, the nanospheres still remained well-defined morphology and good dispersity, in line with the TEM images. The inset of Fig. 1e is the corresponding EDS spectrum of SnO₂@TiO₂-550, confirming the product was composed of only Sn, Ti and O elements, while the presence of Cu peak was derived from the support of the samples in observations. Consequently, the above results indicated that the TiO₂ nanoparticles were successfully deposited onto the SnO₂.

Fig. 1 TEM images (a and d), SEM images (b and e), particle size distributions (c and f) of SnO₂@TiO₂ nanocomposite before (a–c), and (d–f) after calcined at 550 °C for 2 h. Inset of (e) is the corresponding EDX pattern.

What's more, the orientations and compositions of SnO₂@TiO₂-550 were clearly observed by an intact nanosphere, as shown in Fig. 2a and b. HRTEM images confirmed the prepared nanospheres had a high crystalline nature with a lattice spacing of 0.335 nm, corresponding to the (110) planes of tetragonal SnO₂, and a lattice spacing of 0.352 nm, corresponding to the (101) planes of anatase TiO₂, suggesting the coexistence of SnO₂ and TiO₂. N₂ adsorption–desorption measurements were performed to estimate the textural properties of SnO₂@TiO₂ nanocomposites. As shown in Fig. 2c, the SnO₂@TiO₂-550 sample exhibited a hysteresis loop at the relatively high pressure p/p₀ = 0.60–0.98, corresponding to the type IV isotherm, revealing its porous structure. In addition, the sample showed broad pore size distribution, suggesting the hierarchical nanostructure of SnO₂@TiO₂ nanocomposites (Fig. 2d). The pores in materials were mainly originated from the aggregation of among the nanoparticles. The BET surface area of the SnO₂@TiO₂-550 was calculated to be 105.9 m² g⁻¹ and the average pore size was 3.7 nm. In addition, the surface area of as-prepared SnO₂@TiO₂ (153.8 m² g⁻¹), SnO₂@TiO₂-350 (163.8 m² g⁻¹), and SnO₂@TiO₂-450 (121.7 m² g⁻¹) was also measured, respectively, in which the relatively high surface area may be attributed to the existence of the carbon.

Fig. 2 (a and b) HRTEM image, (c) N₂ adsorption–desorption isotherm, and (d) the corresponding BJH pore size distribution of SnO₂@TiO₂-550 product.

XPS measurements were carried out to further investigate the surface composition and chemical state of SnO₂@TiO₂-550. The typical high-resolution XPS spectral of Ti 2p, Sn 3d and O 1s were presented in Fig. 3. XPS spectra of Sn 3d region were given in Fig. 3b. The peak position corresponding to Sn (3d5/2, 3/2) doublet was observed at about 486.6 and 494.8 eV, which was assigned to Sn⁴⁺ species. The Ti 2p XPS spectra depicted in Fig. 3c showed two peaks at about 458.0 and 463.9 eV, which can be assigned to Ti⁴⁺ 2p3/2 and Ti⁴⁺ 2p1/2, respectively.³³ The O 1s XPS spectral (Fig. 3d) were wide and asymmetric, demonstrating that there were at least two kinds of O chemical states. As shown in Fig. 3d, O 1s XPS peak can be divided into two peaks, which may be attributed to Sn–O (531.8 eV)³⁴ and Ti–O (533.2 eV).³⁵

Fig. 3 XPS spectra of SnO₂@TiO₂-550 product.

To insight into the role of carbon, the concentration-dependent experiments were performed, while keeping other conditions constant. The morphology evolution of the products was investigated by TEM technique. At the absence of glucose, the SnO₂ product was uniform flower-like morphology, which was composed of a lot of very small nanoparticles (Fig. 4a). Furthermore, plenty spaces of among the nanoparticles could be observed. However, when the glucose/SnSO₄ mole ratio increased to 5, the void of among the nanoparticles was indistinct, suggesting the carbon filling (Fig. 4b). While more glucose was added in the reaction system (the glucose/SnSO₄ mole ratio increasing from 5 to 10 and 15), the surface of the nanospheres was smoother and thicker, in which the coating carbon layer was 4 nm and 10 nm, respectively. That may be attributed to the pyrolysis of the much glucose during the hydrothermal process. By comparison, we chose the 10 of glucose/SnSO₄ mole ratio as the best rate, since the SnO₂–C spheres were more dispersed than other products.

Fig. 4 TEM images of as-synthesized SnO₂–C products with different glucose/SnSO₄ mole ratio: (a) without glucose, (b) 5, (c) 10, and (d) 15. Scale bars: 50 nm.

As reported previously, the calcination temperature had a profound effect on the structure and morphology of the nanomaterials.^2,13,36 A series of SnO₂@TiO₂ nanocomposites were obtained after annealing at the range of temperature from 350 to 750 °C, as shown in Fig. 5. It can be seen in Fig. 5a that once calcined at 350 °C, the small particles were generated and adhered to the spheres compared with the precursor (Fig. 1a). What's more, as increasing the calcination temperature, the amount of the small TiO₂ particles became less and less. Upon calcined at 750 °C, the spheres further aggregated and the surface was smoother.

Fig. 5 TEM images of: (a) SnO₂@TiO₂-350, (b) SnO₂@TiO₂-450, (c) SnO₂@TiO₂-650, and (d) SnO₂@TiO₂-750. Scale bars: 50 nm.

The crystal phase of SnO₂@TiO₂ nanocomposites was examined by XRD technique. Seen from Fig. 6, the diffraction speaks of all samples were perfectly indexed to the pure tetragonal phase of SnO₂ (JCPDS card no. 41-1445). No anatase peaks appeared in the precursor, which may be ascribed to the poor crystallinity of TiO₂. Moreover, the intensity of the peaks corresponding to the anatase phase of TiO₂ (JCPDS card no. 21-1272) increased with increasing of calcination temperature from 350 to 650 °C, meaning the enhancement of crystallinity in comparsion to the uncalcined sample. However, when the calcination temperature was 750 °C, the crystal phase began to transform from anatase to rutile (JCPDS card no. 21-1276), as marked by asterisk. The phenomenon was common, as reported in previous work.³⁶ Consequently, it could be deduced that the SnO₂ tetragonal phase and TiO₂ anatase phase coexisted in these nanocomposites, in good agreement with the HRTEM results.

Fig. 6 XRD patterns of various SnO₂@TiO₂ nanocomposites.

3.2 Photocatalytic properties

As we known, the performance of materials was greatly affected by their intrinsic physicochemical properties including the surface area, crystal phase, particle size, crystallinity, as well as the morphology.³⁷ Considering that, the photocatalytic performances of a series of SnO₂@TiO₂ samples synthesized at different calcination temperatures were evaluated by the photodegradation of rhodamine B (RhB). Fig. 7 shows the degradation profiles of RhB under UV light over various photocatalysts. The results demonstrated that all SnO₂@TiO₂ heterostructures calcined possessed excellent properties in photodegradation reactions compared with the uncalcined sample (precursor) owing to the improvement of crystallinity. It was obvious that SnO₂@TiO₂-550 and SnO₂@TiO₂-350 catalysts both showed superior photocatalytic performance, the 99.57% and 99.29% of RhB could be degradated within 15 min, respectively. For comparsion, the photocatalytic performance of Degussa P25 was also tested under same conditions, the 99.87% of RhB could be degradated within 15 min, only slightly better than those of SnO₂@TiO₂ composites, as shown in Fig. 8. The photocatalytic degradation rate could be sequenced as SnO₂@TiO₂-550 > SnO₂@TiO₂-350 > SnO₂@TiO₂-450 > precursor > SnO₂@TiO₂-650 > SnO₂@TiO₂-750. The possible reasons will be discussed in subsequent part in detail. In addition, the inset digital images showed the color changes of the resultant liquid from purple to completely transparent colorless with the extension of time, which directly tracked the reaction process.

Fig. 7 Time-dependent UV-vis spectral changes of RhB aqueous solution in the presence of SnO₂@TiO₂ nanocomposites: (a) precursor, (b) SnO₂@TiO₂-350, (c) SnO₂@TiO₂-450, (d) SnO₂@TiO₂-550, (e) SnO₂@TiO₂-650, and (f) SnO₂@TiO₂-750.

Fig. 8 The comparison of photocatalytic activities among various SnO₂@TiO₂ catalysts obtained from different calcination temperatures (a), the cycling stability of SnO₂@TiO₂-550, and TEM (c) images of SnO₂@TiO₂-550 after cycling 5 times.

More research has down on the difference of photodegradation performance among these products. As described in the experimental part, the remnants RhB concentration (%) after being exposed to light for a specific time was estimated by the following equation:

%RhB concentration = (1 − C_t/C) × 100%

where C_t is the intensity of the characteristic peak of RhB at the illumination time t min and C is the initial intensity (that is, before the dark adsorption equilibrium). Fig. 8a shows the comparsion of the photocatalytic activities among the different products. Among the samples, the initial concentrations C₀ (that is, after the dark adsorption equilibrium, t = 0) were quite different from each other, which may mostly relate to the carbon adsorption. We suspected that the amount of carbon in the sample played a key role in photocatalytic reaction. The adsorption of the precursor was extremely high of 91.7%, which was almost 10 times that of the SnO₂@TiO₂-350. Furthermore, it was also more than 20 times of those calcined samples including the SnO₂@TiO₂-450, SnO₂@TiO₂-550, SnO₂@TiO₂-650, and SnO₂@TiO₂-750 samples. TG analysis was used to investigate the thermal decomposition behavior of the as-synthesized SnO₂@TiO₂ product. As shown in Fig. 9, it can be seen that the precursor took a weight loss of 29.5% between 25 and 480 °C (predominantly DTA peak at around 395 °C), which was mainly attributed to the removal of the water weakly adsorbed on the surface of the products, and especially the carbon layer. Besides that, no notable weight loss was observed at temperatures higher than 480 °C. Therefore, the residuals and other organic components could be completely removed by annealing at 550 °C, and pure SnO₂@TiO₂ product can be obtained upon thermal treatment. We further studied the thermal decomposition of precursor, SnO₂@TiO₂-350 and SnO₂@TiO₂-450. The weight loss between 100 to 500 °C corresponding to the removal of carbon was 29.2%, 6.1% and 3.9%, respectively. The color change from the carbon amount in these samples can also be seen in the digital pictures, as displayed in Fig. 9b. The TG result of the carbon content was agreed with the adsorption results, which were all decreased fast at first and then tended to be constant. This phenomenon was line with our assumption that the carbon content played an important part in the absorption efficiency. Except the precursor, the photodegradation process of other samples can be mainly attributed to the SnO₂@TiO₂ photocatalysts. By contrast, we found that SnO₂@TiO₂-350 and SnO₂@TiO₂-550 both had similarly photodegradation efficiency. This may on account of the coefficient of carbon content, crystallinity and the specific surface area.

Fig. 9 TG curve of the precursor of SnO₂@TiO₂ nanocomposite (a), and the corresponding digital photos of various products calcined at different temperature (b).

The effect of the carbon from glucose was as the media for the construction of unique core–shell nanostructured SnO₂@TiO₂ nanocomposites. In addition, the carbon of the SnO₂@TiO₂-350 and SnO₂@TiO₂-450 was not completely been removed. Hence, the existence of carbon may improve the separation efficiency of photogenerated electron–hole pairs. As a result, the photocatalytic activity of SnO₂@TiO₂-350 could be improved. That's why SnO₂@TiO₂-350 performs better than SnO₂@TiO₂-450, even though the crystalline improved from 350 to 450 °C. The anatase TiO₂ of SnO₂@TiO₂-550 product has much higher crystallinity than the SnO₂@TiO₂-350, resulting in better performance in spite of no carbon in the sample. More importantly, the SnO₂@TiO₂-550 photocatalyst exhibited a good cycling stability, the degradation rates were scarcely changed after 5 cycles, as displayed in Fig. 8b. As was observed in Fig. 8c, the morphology of the product still remained well after 5 cycles. Additionally, the low performance of the SnO₂@TiO₂-650 and SnO₂@TiO₂-750 product was related to crystal transformation at 750 °C and the reduction of the specific surface area, which were only 55.4 and 40.0 m² g⁻¹, respectively.

In order to explore the function of the carbon in the preparation process of the SnO₂@TiO₂-550 photocatalyst, the photocatalytic properties of pure SnO₂ nanoparticles and SnO₂@TiO₂–N were also measured under same conditions (Fig. 10). As shown in Fig. 10a, the degradation of RhB by the SnO₂@TiO₂–N was about 40 min, which was much longer than the carbon-mediated SnO₂@TiO₂-550 of 15 min. And the pure SnO₂ nanoparticle was less significant than the other catalysts, and only 68.1% of RhB was degraded after being illuminated for 20 min (Fig. 10c). On one hand, pure SnO₂ product has a low excitation efficiency of e⁻/h⁺ pairs due to the wide band gap (∼3.6 eV) of SnO₂, which leads to a limited absorption of photons. On the other hand, pure SnO₂ product has a poor quantum efficiency because of the recombination of photo-generated e⁻/h⁺ pairs. For the SnO₂@TiO₂–N, only 78.3% of RhB was degraded after being illuminated for 20 min. Compared with the SnO₂@TiO₂–N, the SnO₂@TiO₂-550 product exhibits higher photocatalytic activity, with a degradation rate of 99.6% for 15 min. As a result, we suspect that the carbon layer may play three roles in the formation of SnO₂@TiO₂ nanostructures as follows: (I) as morphology controller, (II) improvement the electrical conductivity, (III) as growth-promoting agent so as to construct the heterostructures of SnO₂ (tetragonal phase) and TiO₂ (anatase phase). The TiO₂ Nps with narrow band gap (∼3.2 eV) enhanced the photon absorption and the SnO₂ spheres provided a continuous path for the faster transfer of the carrier.

Fig. 10 Time-dependent UV-vis spectral changes of RhB aqueous solution in the presence of SnO₂@TiO₂ without carbon-mediated (a), the comparison of photocatalytic activities among various catalysts (c); TEM images of SnO₂@TiO₂–N (b), and SnO₂ nanoparticle (d).

Based on the above results, the possible reasons for the superior photocatalytic performance of SnO₂@TiO₂-550 nanostructures could be explained in detail as follows: (I) the larger specific surface areas (∼105.9 m² g⁻¹) offer more active sites and absorb more substrates, leading to a higher photocatalytic performance. Moreover, the porous nanostructure and large surface area of the SnO₂@TiO₂ enhance the mass transfer of RhB and the independence of TiO₂ Nps impedes the recombination of photo-generated electrons and holes at the same time.³⁸ (II) SnO₂@TiO₂ nanocomposites improve the photon utilization efficiency due to their heterostructures. In this band-gap configuration, when photo-generated e⁻/h⁺ pairs are generated in SnO₂ nanospheres and TiO₂ nanoparticles, the electrons on the TiO₂ nanoparticles surface transfer swiftly to the conduction band of SnO₂ via interfaces; similarly, the holes on the SnO₂ surface migrate to TiO₂ owing to the different valence band edge (Scheme 2).³⁹ (III) As an unique advantage of our synthetic method, the carbon as the mediate could provide support of the unique structure and conduct the electron, thus enhancing the photocatalytic performance.⁴⁰

Scheme 2 Schematic diagram for the photodegradation towards RhB over SnO₂@TiO₂ nanocomposites under UV irradiation.

4. Conclusions

In summary, we present a carbon-mediated approach for preparing the SnO₂@TiO₂ nanostructures by using SnO₂–C nanosphere as the core and TiO₂ as the shell, and a subsequent calcination process. After annealing, the TiO₂ thin shell was uniformly coated onto the surface of the SnO₂ nanoparticles. The property and structure of SnO₂@TiO₂ nanocomposites were greatly affected by the calcination temperature and the carbon contents. In comparison to the SnO₂@TiO₂ synthesized without carbon, the SnO₂@TiO₂ composite exhibited better photocatalytic performance for the decomposition of RhB under UV light irradiation. In particular, carbon-mediated SnO₂@TiO₂-550 composite processed some excellent properties including a relatively large specific surface area for supplying abundant active sites and unique heterojunctions for charge carriers separation. Consequently, the notable enhancement of photocatalytic performance of the SnO₂@TiO₂ heterostructures can be attributed to the synergistic effects. We hope that the method of carbon mediation and construction of the heterostructures can be utilized for more potential applications such as battery electrodes, dye sensitization and so on.

Acknowledgements

We are grateful to Prof. Shuai Yuan from Shanghai University for kind help. The work is supported by the National Natural Science Foundation of China (11275121, 21471096, 61174011, 21371116), and Program for Innovative Research Team in University (IRT13078).

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