Carbon coated SnO₂: synthesis, characterization, and photocatalytic performance

Abstract

A carbon coated SnO₂ photocatalyst was prepared by using sucrose as a carbon source and the microwave hydrothermal method at the temperature as low as 180 °C. The resulting carbon coated SnO₂ was characterized by XRD, Raman, TEM, TG, UV-Vis adsorption and XPS. The carbon layer was found to have multiple functions, increase of the adsorption capability for organic dyes, enhancement of visible light absorption, and facilitating the separation of photogenerated charges. The prepared carbon coated SnO₂ exhibits high and stable photocatalytic activity for the degradation of rhodamine B (RhB) under UV-Vis irradiation. Moreover, hydroxyl radicals (˙OH) are found to be the main active species generated in the oxidation reaction of RhB over the carbon coated SnO₂ photocatalyst. It is believed that this synthesis method can be extended to prepare a wide variety of functional nanohybrids for different applications.

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1. Introduction

Over the past few decades, photocatalysis, as a “green” technique, has attracted much attention because various kinds of organic pollutant can be decomposed completely by photocatalytically active semiconductors under light irradiation.^1,2 Among various photocatalysts, SnO₂ has been demonstrated to be an efficient photocatalyst because of its excellent transparency, high photosensitivity, low cost and environmental friendliness.^3,4 However, the universal use of SnO₂ is still limited due to its wide band gap (E_g ≈ 3.6 eV), which means that it can only utilize UV light.

Recently, several methods, doping or coating, for example, have been developed to increase the visible light photocatalytic activity of nanosized SnO₂. Doping is an approach which constructs a hybrid orbital just above the valence band of SnO₂ and extends its optical absorption to the visible spectral region, thus enhancing its photocatalytic performance. As far as we know, different metal and nonmetal elements have been doped to convert SnO₂ into a visible-light photocatalyst.^5–9 However, the facts that the hybrid orbital introduced acts as electron–hole pairs combining centers as well as the thermal instability associated with doped materials give rise to some doubts about the doping method.^10–13 Another promising route to increase the photocatalytic efficiency is coating, in which the electronic interaction and interfacial charge transfer process between SnO₂ and coating materials play an important role in realizing visible light photocatalytic activity.^14–18 Among the different coating materials, nonmetal has been proved to be more effective than metal, because metal and semiconductor materials are thermally unstable, tend to form charge carrier combination centers, and have no noticeable change in the band gap of SnO₂.^19,20 Recently, carbon is reported to be a potential coating material and it has been found that carbon could effectively change the band gap of SnO₂ (ref. 21) and TiO₂.²²

Based on above analysis, it is reasonable to construct a carbon coated SnO₂ structure to extend its optical absorption to the visible spectral region. Until now, various approaches like high-temperature calcinations,^21,22 CVD or pyrolysis,^23,24 and solution-phase strategies^25–27 have been used to prepare carbon coated semiconductors. Most of the methods mentioned above require high-temperature treatment, which will increase particle size and decrease surface area, and most of the synthetic processes are based on the tedious sacrificial template methods. Herein, we present a simple template-free and temperature-low method to synthesize SnO₂ nanospheres with homogenous carbon coating. Results demonstrate that suitable carbon coating is an effective strategy to improve the photocatalytic activity of metal oxide in photodegradation of organic pollutant.

2. Experimental section

2.1 Preparation of carbon coated SnO₂ nanospheres

Analytical grade tin dichloride dehydrate (SnCl₂·2H₂O), sodium citrate (Na₃C₆H₅O₇·2H₂O, Beijing), sucrose (C₁₂H₂₂O₁₁), sodium dodecylbenzenesulfonate (C₁₈H₂₉NaO₃S), and sodium hydroxide (NaOH) were purchased from Shanghai Chemical Co. and used as received. Deionized water was purified using distillation equipment to prepare an aqueous solution. In a typical experiment, SnCl₂·2H₂O (0.90 g, 3.9 mmol), C₁₂H₂₂O₁₁ (3.0 g, 9.0 mmol), Na₃C₆H₅O₇·2H₂O (8.82 g, 30 mmol), and sodium dodecylbenzenesulfonate (SDBS, 0.03 g) were successively dissolved in distilled water (30 mL) and stirred for 10 min. NaOH (0.39 M, 30 mL) was added to the above solution with continuous stirring to form a homogeneous solution. After stirring for 30 min, the suspension was transferred to a Teflon sealed can and heated in a microwave oven at 180 °C, 400 W for 90 min. After naturally cooling down to room temperature, the brown precipitate was washed and dried at 80 °C for several hours.

2.2 Characterization

The phase structure of the as-synthesized samples was examined by X-ray diffraction (XRD; D/Max2550VB+/PC, Rigaku, Tokyo, Japan) with Cu-Kα radiation (λ = 1.5406 Å). The morphologies of the samples were examined using a transmission electron microscopy (TEM; JEM-3010, Questar, New Hope, USA) equipped with energy-dispersive X-ray spectroscopy (EDX; FeatureMax, Oxford Instruments, Oxfordshire, UK), which was performed with an acceleration voltage of 300 kV. Thermogravimetric analysis (TG; Q600 SDT, TA, New Castle, USA) were carried out at a heating rate of 10 °C min⁻¹ under air using α-Al₂O₃ as the standard material. Raman spectra (inVia, Renishaw, London, UK), using the 514.5 nm line of an Ar laser as the excitation source, were employed to analyze the nature of the carbon present in the products. The ultraviolet-visible (UV-Vis) absorption spectra were recorded by spectrophotometer (UV3150; Shimadzu Corporation, Kyoto, Japan). X-ray photoelectron spectroscopy (XPS) measurement was performed on a PHI-5400 spectrometer using Al Kα (E = 1486.6 eV) radiation.

2.3 Photocatalytic measurement

The photocatalytic reaction suspension was prepared by adding the sample or P25 (40 mg) to 40 mL of rhodamine B (RhB) solution with a concentration of 10 mg L⁻¹. The suspension was treated in an ultrasonic bath for 10 min and then stirred in the dark for 30 min to ensure an adsorption/desorption equilibrium prior to light irradiation. The suspension was then irradiated with a 500 W high-pressure xenon lamps under continuous stirring. Analytical solution were taken from the reaction suspension after various reaction times and centrifuged at 6500 rpm for 3 min to remove the photocatalysts for analysis.

Photoelectrochemical measurements were carried out in a conventional three-electrode, single-compartment glass cell, fitted with a synthesized quartz window, using a potentiostat. Samples deposited on ITO conducting glass were served as the working electrode. The counter and the reference electrodes were platinum black wire and saturated calomel electrode (SCE), respectively. 500 W high-pressure xenon lamps were used as the excitation light source.

3. Results and discussion

3.1 Phase structure and morphology observations

Fig. 1(a) shows the XRD pattern of the as-prepared composites obtained by microwave hydrothermal treatment. The diffraction peaks at 26.588, 33.876, 37.955 and 38.979°, corresponding to the (110), (101), (200) and (111) face, are in good agreement with the tetragonal rutile SnO₂ structure (JCPDS no: 88-0287, space group: P42/mnm, a₀ = b₀ = 4.737 Å, c₀ = 3.186 Å). The broaden peaks indicate that the sample has poor crystallinity, which is because the sample is prepared at lower temperature. No peaks corresponding to SnO or elemental Sn are present in the pattern indicating pure phase of the prepared materials. The peak of carbon is not observed, suggesting that the carbon remains amorphous.

Fig. 1 (a) XRD patterns and (b) Raman spectra of the carbon coated SnO₂ by microwave hydrothermal treatment at 180 °C for 90 min.

The Raman spectrum of the as-prepared composites is shown in Fig. 1(b). As shown, there is a very broad peak around 600 cm⁻¹, which is attributed to the A1g vibration modes of SnO₂.²⁸ And the two peaks at 1367 and 1582 cm⁻¹ are assigned to the disordered (band D) band and grapheme band (G band), respectively.²⁹ The relative intensity ratio of the D and G bands depends on the perfection of the graphite layer structure. Larger area D band confirms a typical disordered amorphous carbon.

Fig. 2 illustrates the morphology and composition of as-prepared samples. As shown in Fig. 2(a), the synthesized SnO₂/carbon composite are of nanospheres morphology and is composed with inner SnO₂ core (Region II) and outer layer (Region I) of polymer, which is proved to be carbonaceous compounds in the latter analysis. The diameter of each inner SnO₂ core is estimated to be 125 nm, while the outer layer of polymer has a thickness of about 20 nm. The enlarged TEM image shown in Fig. 2(b) clearly reveals that the SnO₂ core is comprised by numerous fine SnO₂ nanocrystals (black region) with a diameter of around 3 nm, while polymer is mostly coated on the outer surface of SnO₂ core and few is dispersed between SnO₂ nanocrystals. Clearly, sucrose serves not only as a carbon precursor for the carbon layer, but also decreases the growth rate of SnO₂ nanoparticles. Clear lattice fringes of the small SnO₂ nanocrystals can also be observed in Fig. 2(c), and interplanar spacings are measured to be 0.34 nm, which agrees with the d value of the (110) plane of the rutile SnO₂. The SAED in Fig. 2(d) indicates that the nanospheres are composed by amorphous carbon and polycrystalline SnO₂ due to the spot and ring patterns. Fig. 2(e) and (f) show the EDX spectra of the outer layer (Region I) and inner core (Region II) of the sample respectively. As observed, the strong peaks for C, Sn, and O elements are detected and C is mainly located in the outer part, namely the surface of SnO₂. The rich O in outer layer indicates that the outer layer of polymer is composed by carbonaceous compounds, rather than just C element. Some amount of Cu, which comes from the grid, has also been identified.

Fig. 2 (a and b) TEM, (c) HRTEM and (d) SEAD images of carbon coated SnO₂. EDX of (e) Region I and (f) Region II labelled in (a).

3.2 Content and chemical state of carbon

To determine the content and chemical states of carbon in the carbon coated SnO₂ nanospheres, TG analysis and XPS were carried out. As shown in Fig. 3, two main weight loss events are observed in the TG curve. The one that occurs between room temperature and 236 °C with a weight loss of about 6.1% can be attributed to desorption of physically adsorbed water from the SnO₂ nanospheres. A second weight loss of 46.7%, which occurs between 236 and 780 °C might be due to the removal of carbon layer, namely the combustion of the carbon layer in air. According to the TG curves, it is estimated that the carbon content in the product is about 46.7%.

Fig. 3 TG curves of the carbon coated SnO₂ in flowing air.

The surface chemical composition and chemical states of the carbon coated SnO₂ studies were investigated by XPS. All binding energies are referenced to the C 1s neutral carbon peak, which is assigned the value of 284.6 eV to compensate for surface charge effects. Fig. 4(a) shows the fully scanned spectrum of carbon coated SnO₂ nanospheres in the range of 0–1350 eV. The overview spectra demonstrate that only O, Sn, and C are present in the sample. Fig. 4(b) depicts XPS core level of Sn 3d spectrum which exhibits the characteristics of doublets at 486.1 and 496.1 eV. The peak centered at 486.1 eV corresponds to the Sn–O bond⁶ and the other one centered at 496.1 eV is assigned to Sn 3d_3/2.³ The wide and asymmetric XPS spectrum of O 1s in Fig. 4(c) can be fitted into four peaks. One peak centered at 530.8 eV corresponds to SnO₂ (Sn–O),³⁰ two peaks centered at 531.4 eV and 533.5 eV are attributed to oxidized carbon C–O and C=O, and the peak centered at 535.1 eV shows the formation of oxygen vacancies. The C 1s peak in Fig. 4(d) can be fitted into three peaks centered at 284.6, 286.3 and 288.6 eV. The C 1s peak at 284.6 is dominated by elemental carbon, while two shoulders located at 286.1 and 287.9 eV are ascribed to the existence of C–O (or C–O–Sn) and C=O (or O–C=O) bonds, respectively.³¹ No C 1s peak for Sn–C bond is observed suggesting the chemical environments for Sn and O are not changed either. Moreover, in the XRD spectrum, SnO₂ powders containing carbon element present only as the rutile phase and their lattice parameters are hardly changed compared with those uncoated SnO₂ in the HRTEM image. Hence it can be speculated that the carbon does not enter into the SnO₂ phase or substitute Sn/O atom. On the other side, carbon coated SnO₂ prepared via the direct hydrothermal synthesis is brown, while a mechanical mixture of hydrothermal carbon and SnO₂ is black. Therefore, based on above analysis, it is concluded that an electronic interaction and chemical bond (probably C–O–Sn) form between the coated layer of carbon and SnO₂ nanospheres. And this carbon coated SnO₂ structure has great advantage for photocatalytic applications, because the contaminant molecules have to be adsorbed into the carbon layer, and then penetrate the carbon layer to reach the active surface. Therefore, a coupling between photoactivity and adsorptivity is achieved in a single process. Meanwhile, this carbon layer is conducive and favorable to charge transfer upon light excitation which is not expected as many conventional carbon coated SnO₂ synthesized at temperatures higher than 600 °C.^32,33

Fig. 4 XPS spectra of (a) survey spectrum, (b) Sn 3d, (c) O 1s and (d) C for carbon coated SnO₂.

The observed UV-Vis absorption spectra of carbon coated SnO₂ nanospheres compared with commercial SnO₂ are shown in Fig. 5. As expected, commercial SnO₂ shows the characteristic spectrum of SnO₂ with its fundamental absorption sharp edge rising at 355 nm (3.50 eV). As shown, a noticeable shift of absorption edge to the visible light region is observed for the carbon coated SnO₂ in comparison with commercial SnO₂, indicating the carbonaceous layer can photosensitize SnO₂ and extend the absorption to visible light range.

Fig. 5 UV-Vis spectra of commercial and carbon coated SnO₂.

3.3 Photocatalytic activity

Due to the unique structure and photosensitization, it is expected that carbon coated SnO₂ nanospheres have good photocatalytic performances, and this was confirmed by the photocatalytic degradation of RhB under UV-Vis irradiation. The TiO₂ (P25) was used as a photocatalytic reference. The temporal adsorption spectral changes of RhB solution during the test process over the carbon coated SnO₂ nanospheres and P25 are illustrated in Fig. 6. At the initial stage of photocatalysis, the UV-Vis absorption spectrum of the RhB solution exhibits a characteristic peak at 552 nm, which is associated with molecules in the intense chromophoric azo double bond in the molecule and the large conjugated system composed by the entire dye molecule. With the elapsed time of photocatalysis, the maximum absorption peaks of RhB diminish rapidly and nearly disappear after 100 min in the presence of the carbon coated SnO₂ nanocatalyst, without any new absorption peak appears concomitantly, which confirms the photoinduced cycloreversion of RhB (Fig. 6(c)). As a comparison, the absorption peak of RhB solution with P25 has a weak decrease even after 120 min (Fig. 6(b)), suggesting that P25 can't be activated by the irradiation of xenon lamps. And the absorption peak of RhB solution without catalysts has an unnoticeable decrease even after 120 min (Fig. 6(a)), which means that RhB is fairly stable and dye sensitization is not the main contribution in the photocatalytic process.

Fig. 6 Absorbance changes of RhB solution after different visible light irradiation times (a) without any photocatalysts, in the presence of (b) P25 and (c) carbon coated SnO₂. (d) Corresponding kinetic curves of the degradation of RhB solution. (e) Cyclic runs in the photodegradation of RhB in the presence of carbon coated SnO₂.

Based on the Langmuir–Hinshelwood model, we have derived the apparent reaction rate constants for the photocatalysis of RhB from the linear slope of the relationship between ln(c_t/c₀) and k_t (Fig. 6(d)), where c₀ and c_t are the concentrations of initial solution and the concentrations at the irradiation time t (min), respectively. Obviously, the photocatalytic degradation of SnO₂ is remarkably enhanced through carbon coating and is superior to its P25 counterpart.

In order to evaluate the photostability, the cyclic photodegradation of RhB over carbon coated SnO₂ nanospheres was examined. As shown in Fig. 6(d), the carbon coated SnO₂ sample exhibits excellent photostability with a trivial loss of photocatalytic activity towards the degradation of RhB after three cycles, which is a key requirement for a possible industrial application.

The stable and enhanced photocatalytic activity of carbon coated SnO₂ nanospheres can be explained as follow. Firstly, the amorphous carbonaceous layer can enhance the adsorption capability for organic dyes, which can enrich the dye molecules on the surface of photoactive SnO₂ particles, thus resulting in the acceleration of photocatalytic reactions.³⁴ Secondly, the carbon coating on the SnO₂ particles can effectively enhance visible light absorption. Thirdly, the electronic contact between SnO₂ and carbon will increase surface electronic transfer and hence results in a lower possibility of the recombination of photo-induced carriers.^32,35 Similar results are also reported by Sakthivel and Kisch.^36,37 Meanwhile, sucrose not only serves as a carbon precursor for the carbonaceous layer, but also retard the growth rate of SnO₂ nanoparticles, and small particle size favors the faster transportation of photo-induced carriers.

The radicals, electron and holes trapping experiments were designed to elucidate the photocatalytic degradation process of carbon coated SnO₂ (Fig. 7(a) and (b)). As can be seen, under UV-Vis light irradiation, the photodegradation of RhB is slightly suppressed by the addition of hole scavenger, EDTA, while it is obviously inhibited when an electron and hydroxyl radical scavenger, tBuOH, is added.³⁸ This indicates that hydroxyl radicals (˙OH) are the main active species that can oxidize the adsorbed organic pollutants, which is also reported in other literatures.³⁹ As known, ˙OH radicals are the main active species that can oxidize the adsorbed organic pollutants, and they can be formed either through the hole oxidation of OH⁻/H₂O or the electron reduction of O₂ (reactions (1)–(3)). According to our results, the hole scavenger EDTA could just exert limited influence on the photodegradation of RhB, meaning that ˙OH in our system is mainly formed through action of electron reduction, which further prove the deduction of carbon sensitization and surface charge transfer.

h⁺ + OH⁻ → ˙OH (1)

O₂ + 2h⁺ + 2e⁻ → H₂O₂ (2)

H₂O₂ + e⁻ → ˙OH + OH⁻ (3)

Fig. 7 Absorbance changes of RhB solution after different visible light irradiation times in the presence of (a) carbon coated SnO₂ + 1 mM EDTA and (b) carbon coated SnO₂ + 1 mM tBuOH. (c) Corresponding kinetic curves of the degradation of RhB solution.

To further confirm the separation of photogenerated charges after carbon coating in SnO₂, the transient photocurrent responses of pure commercial SnO₂ and carbon coated SnO₂ were measured to reflect the number of photogenerated electrons and holes. As shown in Fig. 8, fast and uniform photocurrent responses to the transient light were detected. In addition, the photocurrent of the carbon coated SnO₂ is almost four times as high as that of the commercial SnO₂, indicating enhanced photoinduced electrons and holes separation.

Fig. 8 Transient photocurrent responses for commercial SnO₂ and carbon coated SnO₂. Electrolyte: 0.1 M Na₂SO₄.

4. Conclusions

Carbon coated SnO₂ nanospheres were synthesized through a simple one-step microwave hydrothermal method. A carbonaceous out layer is formed on the SnO₂ surface during the hydrothermal process via the dehydration of sucrose. The prepared carbon coated SnO₂ shows high and stable photocatalytic activity for the degradation of RhB under UV-Vis irradiation. On the one hand, the coated carbonaceous layer can promote the adsorption of organic pollutants on the photocatalyst, extend the absorption of light to the visible region, and reduce the possibility of the recombination of photo-induced carriers; on the other hand, the presence of carbonaceous layer inhibits the growth rate of SnO₂ nanoparticles thus favoring the photocatalytic degradation. Moreover, photocatalytic studies reveal that the ˙OH are the main active species generated in the photocatalysis process and is mainly formed through action of electron reduction. In particular, given the synthetic ease, scalability for mass production, and excellent photocatalytic activities, we believe our method for carbon coated SnO₂ nanospheres will have wide applications in catalysis, separation technology, biomedical engineering and nanotechnology.

Acknowledgements

This work was supported by the National Natural Science Foundation (51172187), the SPDRF (20116102130002,20116102120016) and 111 Program (B08040) of MOE, and Xi'an Science and Technology Foundation (CX1261-2, CX1261-3, CX12174), and Shaanxi Province Science Foundation (2013KW12-02), and the NPU Fundamental Research Foundation (NPU-FRF-JC201232) of China.

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