Fabrication of one-dimensional heterostructured TiO₂@SnO₂ with enhanced photocatalytic activity

Abstract

TiO₂@SnO₂ nanosheets@nanotubes heterostructures were successfully prepared by a facile two-step method: prefabricated SnO₂@PNT coaxial nanocables based on the in situ growth of SnO₂ in the sulfonated gel matrix of polymeric nanotubes, and then the assembly of TiO₂ nanoclusters that consist of ultrathin nanosheets through a solvothermal process. These heterostructures were characterized for the morphological, structural and optical properties by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-visible (UV-vis) and diffuse reflectance spectroscopy (DRS). The photocatalytic investigations showed that the TiO₂@SnO₂ heterostructures possessed enhanced photocatalytic efficiency in the photodegradation of Rhodamine B (RhB) and photocatalytic H₂ evolution from water splitting under ultraviolet (UV) light irradiation, compared with the pristine TiO₂ nanosheets, SnO₂ nanotubes, the mechanically mixed two samples and P25. The enhanced photocatalytic performance can be ascribed to the beneficial microstructure and synergistic effects of coupled TiO₂@SnO₂ nanosheets@nanotubes heterostructures.

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1. Introduction

Photocatalysis has attracted a great deal of attention as a promising candidate for the elimination of organic contaminants in water and air, since the pioneering work by Honda and Fujishima.¹ Titanium dioxide (TiO₂) has and continues to be one of the most widely studied semiconductors for photocatalytic applications due to its advantages, such as a suitable band gap for redox reactions, long-term stability, low cost, non-toxicity and so on.^2,3 However, as a single component semiconductor photocatalyst, TiO₂ has an inherent drawback that the photogenerated electron/hole (e⁻/h⁺) pairs recombine fast (∼10 ns)⁴ and hence only a fraction of e⁻/h⁺ pairs are available for the photoreaction. To date, this disadvantage can be overcome by means of semiconductor coupling.⁵ With this approach, the assembled semiconductors are selected suitably so that efficient charge transfer occurs between them. This condition spatially separates the photoexcited electrons and holes onto the different constituents and reduces the rate of e⁻/h⁺ pair recombination.⁶

In recent years, various coupled semiconductor systems based on TiO₂ have been developed, such as SnO₂/TiO₂,^7–10 ZnO/TiO₂,¹¹ WO₃/TiO₂,¹² CdS/TiO₂,¹³ Fe₂O₃/ZnO¹⁴ and so on. In particular, the SnO₂/TiO₂ system with high photocatalytic activity has attracted extensive interest. SnO₂ possesses a high electron mobility (∼100–200 cm² V⁻¹ s⁻¹),¹⁵ indicating a faster transport of photoexcited electrons.¹⁶ Moreover, the conduction-band edge of SnO₂ is more positive than that of TiO₂, leading to the migration of photoexcited electrons from the conduction band of TiO₂ to that of SnO₂,^17–20 consequently the recombination of charge carriers can be suppressed, thereby resulting in a higher photocatalytic performance.^21–23 Aerosol assisted chemical vapour deposition was also introduced to synthesize a TiO₂–SnO₂ composite film, and the as-prepared TiO₂–SnO₂ film exhibited enhanced photocatalytic activity over the degradation of dye compared to the TiO₂ film and SnO₂ film.²² Gao et al. demonstrated that the power conversion efficiency value of the SnO₂–TiO₂ core–shell structured DSSCs is above five times higher than that of SnO₂ nanotubes.²⁴ Zhang et al. reported a novel-structured TiO₂/SnO₂ hybrid nanofiber photocatalyst with enhanced photocatalytic activity prepared via the electrospinning method, the heterostructures promote the photogenerated e⁻/h⁺ pairs separation.²⁵

To date, numerous types of coupled SnO₂/TiO₂ systems, such as particle,²⁶ composite film,²⁷ stacks,¹⁵ core–shell heterostructures²⁸ and so on, have been successfully obtained. However the particle-like morphology photocatalyst is very inconvenient for recycling due to its extreme dispersive property and it may cause secondary pollution. On the other hand, the film-type photocatalyst increases the recovery efficiency at the expense of the specific surface area, resulting in a low photocatalytic activity. One-dimensional (1D) structures, such as nanowires, nanobelts and nanotubes, have attracted tremendous attention within the last decade. One promising approach to overcoming these drawbacks simultaneously is to use 1D photocatalysts, which provide a relatively large specific surface area as compared with nanoparticles, and are recycled easily due to the large length to diameter ratio. Among the huge variety of 1D nanostructures, semiconducting nanotubes are particularly interesting, not only for fundamental research due to the unique hollow structural and physical properties relative to their bulk counterparts, but also for their fascinating potential in optoelectronic and electronic devices. More importantly, it has been proven that the 1D nanomaterials effectively support the direct growth of secondary nanostructures by heterogeneous nucleation because of their porous surface and large surface area.^29–31

In this work, we employ polymeric nanotubes (PNTs) with uniform size, synthesized by the cationic polymerization of divinylbenzene using immiscible initiator nanodroplets of boron trifluoride etherate complex as hard templates after sulfonation.³² The synthetic process is illustrated in Scheme 1. Firstly, we prefabricated SnO₂@PNT coaxial nanocables based on the in situ growth of SnO₂ in the sulfonated gel matrix of polymeric nanotubes.^33–37 Then the SnO₂@PNT coaxial nanocables were assembled by TiO₂ nanoclusters that consist of ultrathin nanosheets after a solvothermal process. After being calcined in air, the hierarchically structured TiO₂@SnO₂ composite was successfully obtained. Moreover, the uniform PNTs used in this work have three main advantages as templates: (i) the positively charged precursor ions can be easily adsorbed to the surface of the sulfonated PNTs due to the electrostatic interaction with the negative functional groups (–SO₃); (ii) PNTs can be easily removed through calcination at a lower temperature, by which the nanostructure of the metal oxide can be well preserved; (iii) the produced gas and osmotic pressure during the calcination can be released through the inner channels and open ends of the PNTs.³³

Scheme 1 Schematic illustration of the synthetic procedure of the TiO₂@SnO₂ composite.

2. Experimental

2.1 Material synthesis

2.1.1 Sulfonated PNTs. Polymeric nanotubes (PNTs) were prepared according to a previously reported method.³² PNTs (3 g) were added to concentrated sulfuric acid (polymeric nanotubes: H₂SO₄ = 1 : 30, w/w) and the mixture was ultrasonicated for 10 min to disperse them. After stirring for 24 h at 40 °C the yellow precipitate was collected by centrifugation and washed thoroughly with ethanol.

2.1.2 SnO₂@PNT. Acid-treated polymeric nanotubes (100 mg) were dispersed into a 20 mM mercaptoacetic acid solution (40 mL) by sonication for 10 minutes, followed by the addition of SnCl₂ (100 mg), urea (0.5 g), and HCl solution (37 wt%, 0.5 mL). The reaction mixture was then transferred into a 100 mL round bottom flask. After stirring for 6 h at 60 °C, the flask was left to cool to room temperature. The precipitate was collected by centrifugation, washed thoroughly with ethanol, and dried at 60 °C overnight.

2.1.3 TiO₂@SnO₂@PNT. 200 mg of the SnO₂@PNT synthesized using the previous method was dispersed in 40 mL of isopropyl alcohol (IPA) by sonication for 10 min, followed by the addition of 0.03 mL of DETA. After gentle stirring for 2 min, 1.5 mL of TIP was added. The solution was then transferred into a 60 mL Teflon-lined stainless steel autoclave and kept in an electric oven at 200 °C for 24 h. The autoclave was then taken out of the oven and left to cool down to room temperature. The precipitate was collected by centrifugation, washed thoroughly with ethanol, and dried at 60 °C overnight.

2.1.4 TiO₂@SnO₂. To obtain the TiO₂@SnO₂, the as-prepared composite was subjected to calcination at 450 °C for 2 h to remove the PNT template and obtain the hierarchically structured TiO₂@SnO₂ composite.

2.2 Characterization

The product morphology was examined using field-emission scanning electron microscopy (FESEM; HITACHI, su-8010) and transmission electron microscopy (TEM; JEOL, JEM-2100). Crystallographic information of the samples was collected using powder X-ray diffraction (XRD; SHIMADZU, Lab X XRD-6000). The chemical states of the products were studied using the X-ray photoelectron spectroscopy (XPS) measurement performed on an Axis Ultra, Kratos (UK) at monochromatic Al Kα radiation (150 W, 15 kV and 1486.6 eV). The vacuum in the spectrometer was 10–9 Torr. Binding energies were calibrated relative to the C 1s peak (284.6 eV). Thermogravimetric analysis (Perkin-Elmer TGA 7) was carried out under a flow of air with a temperature ramp of 20 °C min⁻¹ from room temperature to 600 °C. The specific surface area and pore size distribution of the products were measured using a BET analyzer (ASAP 2020M) at 77 K.

2.3 Photocatalytic tests

The photocatalytic performance of the as-fabricated products was studied by degrading Rhodamine B (RhB) in simulated wastewater under UV irradiation. The photocatalytic experiments were conducted in a homemade quartz photochemical reactor with a quartz jacket and cooled off by circulating water. In the experiment, a 500 W xenon lamp with a cutoff filter (λ < 420 nm) was employed as the ultraviolet light source; the distance between the xenon lamp and the front surface of the reactor was fixed to be 8 cm. For comparison, the mechanically mixed SnO₂ nanotubes with TiO₂ nanosheets (the weight ratio of SnO₂: TiO₂ = 1:4, which is similar to that of TiO₂@SnO₂ composite), and which is labeled as SnO₂ + TiO₂, was studied as a reference.

2.3.1 Photocatalytic RhB degradation. Prior to illumination, 50 mL of RhB aqueous solutions (10 mg L⁻¹) containing 0.05 g samples were magnetically stirred in the dark for 30 min to achieve adsorption/desorption equilibrium and good dispersion. During irradiation, about 3.0 mL of the reaction solution was withdrawn from the reactor at intervals of 10 mins and centrifuged to separate the photocatalyst. The supernatant solution was analyzed by an Agilent 8430 UV-vis spectrophotometer at λ = 554 nm, which is the maximum absorption wavelength of RhB. The remaining RhB concentration (%) after various intervals of time could be estimated using the following equation:

%RhB concentration = C/C₀ × 100%

where C₀ is the initial concentration of RhB aqueous solution (that is, after the dark adsorption equilibrium), while C is the concentration at the illumination time t min. The control experiment indicates that RhB can be slightly degraded under UV light irradiation without a catalyst, suggesting that the photocatalytic process was primarily responsible for the RhB degradation.

2.3.2 Photocatalytic H₂ evolution from water splitting. In each experiment, 0.05 g of photocatalyst was dispersed in a 50 mL aqueous solution containing 0.25 M Na₂S and 0.35 M Na₂SO₃ which acted as sacrificial agents to protect photocatalysts from the photocorrosion process. The light intensity reaching the surface of the reaction solution was tested to be 145 mW cm⁻². Before irradiation, the system was bubbled with nitrogen for 30 min to remove the air inside and to ensure that the reaction system is under anaerobic conditions. The amount of H₂ evolution was determined on a gas chromatograph (Agilent Technologies: 6890N) with TCD (Thermal Conductivity Detector) equipped with a 5 Å molecular sieve using N₂ as the carrier gas. Blank experiments revealed that no hydrogen was produced without the addition of the catalysts or light irradiation.

3. Results and discussion

Fig. 1 shows the morphology of the acid-treated PNTs and SnO₂@PNT composite. It can be seen from the scanning electron microscope (SEM) image of the acid-treated PNTs that these templates are about 100–250 nm in diameter (Fig. 1A). The transmission electron microscopy (TEM) image indicates that the acid-treated PNTs possess an extremely smooth surface (Fig. 1B). The morphology of the SnO₂@PNT is shown in Fig. 1C and D. It can be observed that the diameter of the acid-treated PNTs increases to approximately 125–275 nm (Fig. 1C), the surface of the SnO₂@PNT composite becomes much rougher than the PNTs before coating with SnO₂, and the thickness of the SnO₂ layer is approximately 15–25 nm (Fig. 1D).

Fig. 1 (A) SEM image and (B) TEM image of the sulfonated PNTs; (C) SEM image and (D) TEM image of the SnO₂@PNT composite.

Fig. 2 shows the SEM and TEM images of the TiO₂@SnO₂@PNT and TiO₂@SnO₂ composites. The TiO₂@SnO₂@PNT composite displays a tubular structure with a diameter of around 500–600 nm (Fig. 2A). The TEM image shows that the SnO₂@PNT nanotubes are uniformly assembled by TiO₂ nanoclusters that consist of ultrathin nanosheets (Fig. 2B), and the heights of these TiO₂ nanosheets are about 225–475 nm through calculation. A hierarchical structure of the TiO₂@SnO₂@PNT composite's surface can be clearly revealed from the TEM image (Fig. 2C). The formation of the TiO₂ nanosheets could be explained by the dual function of diethylenetriamine (DETA): catalyzing the hydrolysis of titanium (IV) is opropoxide (TIP) to form TiO₂ nanosheets and assisting the self-assembly of the formed TiO₂ nanostructures on the surface of SnO₂@PNT, possibly through electrostatic interactions.^38,39Fig. 2D shows the SEM image of the TiO₂@SnO₂ composite obtained after calcining the TiO₂@SnO₂@PNT composite at 450 °C for 2 h. The diameter of the hierarchically structured TiO₂@SnO₂ composite is approximately 450–550 nm, smaller than the TiO₂@SnO₂@PNT composite, possibly because of the partial collapse of the hollowed TiO₂@SnO₂ structure during the annealing process. The calcined TiO₂@SnO₂ composite maintains its tubular and sheet-like structures after removing the PNT templates. The TEM image of the calcined TiO₂@SnO₂ composite displays that the sample has an extra porous structure of the inside nanotubes, and the thickness of the TiO₂ nanosheets is approximately 10 nm (Fig. 2E and F). It is noteworthy that the ultrasonic process before TEM observation was unable to cause TiO₂ nanosheets to fall off from the SnO₂ nanotubes, which may suggest that a compact interface was established between the TiO₂ and SnO₂. As shown in the HRTEM image of Fig. S1,† we can see the clear interface between the SnO₂ and TiO₂. Moreover, the XPS data further confirmed the composition of the heterostructured TiO₂@SnO₂ composite (Fig. S2†).

Fig. 2 (A) SEM image; (B and C) TEM images of TiO₂@SnO₂@PNT composite; (D) SEM image; (E and F) TEM images of TiO₂@SnO₂ composite after calcining the TiO₂@SnO₂@PNT composite at 450 °C for 2 h.

The chemical composition of the different samples was characterized by using X-ray diffraction (XRD; Fig. 3A). It is clear from the patterns of curve I and curve II that pure TiO₂ nanosheets and pristine SnO₂ nanotubes can be assigned to anatase TiO₂ (JCPDS card no. 21-1272) and rutile SnO₂ (JCPDS card no. 41-1445). All the diffraction peaks of curve I and curve II can be discovered from curve III clearly, indicating that SnO₂ and TiO₂ successfully formed on the surface of acid-treated PNTs. The weight fraction of SnO₂ and TiO₂ in the hybrid material was determined by thermogravimetric analysis (TGA), with the results shown in Fig. 3B. The SnO₂@PNT and TiO₂@SnO₂@PNT composites show significant weight loss at about 450 °C, which is attributed to the combustion of the acid-treated PNT templates. After reaching 600 °C, the SnO₂@PNT and TiO₂@SnO₂@PNT composites show a total weight loss of 79.3% and 44.0%, respectively. The mass ratio of SnO₂ and TiO₂ calculated from the two curves is about 1:4.

Fig. 3 (A) XRD patterns of the TiO₂ nanosheets (I), SnO₂ nanotubes (II) and TiO₂@SnO₂ composite and (B) TGA analysis of the pure sulfonated PNTs (I), SnO₂@PNT composite (II) and TiO₂@SnO₂@PNT composite (III).

Fig. 4A shows the solid diffuse reflection UV-vis spectrum (DRS) of the SnO₂ nanotubes, TiO₂ nanosheets and TiO₂@SnO₂ composite. A strong absorption edge at ca. 395 nm appears in the absorption spectrum of the TiO₂@SnO₂ composite, while the onset of the absorption of the TiO₂ nanosheets and SnO₂ nanotubes were 385 nm and 375 nm. According to the formula of the band gap (E_g = 1240/λ),¹³ the band-gap values of TiO₂@SnO₂ composite, TiO₂ nanosheets and SnO₂ nanotubes were calculated to be 3.15 eV, 3.21 eV and 3.42 eV. The absorption edge of the TiO₂@SnO₂ composite was red-shifted compared to the TiO₂ nanosheets and SnO₂ nanotubes, which reflects that the electronic structure and optical properties of the bare TiO₂ or SnO₂ have been modified. This is presumably because the TiO₂ was doped by a fraction of the Sn on the interface between the TiO₂ and SnO₂ during the final annealing process,⁴⁰ or the effective coupling of SnO₂ and TiO₂,¹¹ and results in the red-shifting of the absorption edge in the TiO₂@SnO₂ composite.

Fig. 4 (A) DRS of the SnO₂ nanotubes, TiO₂ nanosheets and TiO₂@SnO₂ composite, (B) degradation profiles of RhB over different samples: SnO₂ nanotubes (□), TiO₂ nanosheets (○), SnO₂ + TiO₂ mixture (■), P25 (●), TiO₂@SnO₂ composite (*), without photocatalyst (◑) and without UV light (◐).

Fig. 4B shows the degradation profiles of the Rhodamine B (RhB) wastewater under ultraviolet light (UV) with various photocatalysts. As one can see, the degradation of RhB by the pure SnO₂ nanotubes is the least significant among all the catalysts, and only about 82.0% of RhB was degraded in 60 min. This is primarily ascribed to the wide band gap of SnO₂, leading to a limited absorption of photons. In comparison, the samples of TiO₂ nanosheets and mechanically mixed SnO₂ nanotubes with TiO₂ nanosheets exhibit higher activities, with a degradation rate of 88.9% and 90.3% for RhB, respectively. The improved activity can be assigned to the narrow band gap of the TiO₂. The degradation efficiency of P25 reaches 94.5%, which is higher than the TiO₂ nanosheets due to the coexistence and optimal ratio of rutile and anatase TiO₂. The TiO₂@SnO₂ composite exhibits the best performance of the photodegradation of RhB among the as-prepared samples, and almost 100% of the RhB molecules decomposed in 60 min. To investigate the degradation kinetics of RhB in solution and quantitatively compare the photocatalytic performances of these samples, the pseudo first-order equation was adopted to describe the experimental data because of the low initial concentrations of the substrates as follows: ln(C₀/C) = kt, where k is a reaction rate constant. As can be seen in Fig. S3,† the order of the k values, which is derived from the plots of ln(C₀/C) versus irradiation time (t), is summarized as follows: TiO₂@SnO₂ composite > P25 > TiO₂ + SnO₂ mixture > TiO₂ nanosheets > SnO₂ nanotubes. This is in good agreement with the results presented in Fig. 4B. In addition to organic pollutants removal, photocatalytic H₂ generation was also observed over the SnO₂@TiO₂ heterostructures. As illustrated in Fig. 5, the H₂ evolution over the SnO₂@TiO₂ heterostructures was significantly higher than that of the bare SnO₂ nanotubes, pristine TiO₂ nanosheets, the mechanically mixed two samples and P25 throughout the 6 h of UV light irradiation, which is similar to the photocatalytic efficiency. These outcomes demonstrated that the SnO₂@TiO₂ heterostructures obtained in this work possess excellent properties in both photo-oxidation and photo–reduction reactions.

Fig. 5 Photocatalytic H₂ production efficiency of samples TiO₂@SnO₂, TiO₂ + SnO₂, TiO₂ nanosheets, SnO₂ nanotubes and P25.

The reasons for the high photocatalytic performance of the sample TiO₂@SnO₂ could be explained in detail as follows: (i) SnO₂@TiO₂ heterostructures have a large specific surface area (124 m² g⁻¹) compared with SnO₂ nanotubes (102 m² g⁻¹).³³ As is known, nanomaterials with larger specific surface areas could offer more active sites and absorb more oxygen, which leads to a higher photocatalytic performance.⁹ (ii) The band alignment between SnO₂ and TiO₂ is generally classified as a type II structure (staggered band alignment between these two different materials).⁴¹ In this band-gap configuration, when photogenerated e⁻/h⁺ pairs are generated in SnO₂ nanotubes and TiO₂ nanosheets, the electrons on the TiO₂ particle surface transfer swiftly to the conduction band of SnO₂via interfaces; similarly, the holes on the SnO₂ surface migrate to TiO₂ owing to the different valence band edge. The probability of the recombination of the photogenerated e⁻/h⁺ pairs is retarded at the same time. When the heterojunctions between SnO₂ and TiO₂ are formed, the Fermi energies in the boundary between the SnO₂ and TiO₂ phases should be matched.⁴² This leads to the potential of the conduction band and valence band of SnO₂ to be more negative.^43,44 (see Fig. S2 E in the ESI†) Therefore, the conduction band position of SnO₂ will meet the requirements of the hydrogen evolution reaction.⁴⁵ Moreover, the nanosheets/nanotubes-type structure possesses a large interfacial region, which can improve the charge carrier separation to a large extent.⁴⁶ (iii) The nanosheets-nanotubes structures could improve the photon utilization efficiency. When the surfaces of the TiO₂@SnO₂ composite was irradiated with light, some of the incident photons were directly absorbed by SnO₂ and TiO₂, some were reflected and scattered, multiplied by the nanotubes and nanosheets. Hence, along with the increase in effective optical path length, the photon–matter interaction length multiplies, leading to a higher light harvesting based on the Beer–Lambert Law.^47–49 (iv) The unique 1D TiO₂@SnO₂ heterostructures obtained in this work could prevent the photo induced agglomeration and result in low boundary density,⁵⁰ thereby leading to the decrease of the recombination centre. This could be proven by the SEM images of the sample after catalytic reaction (Fig. S4†). (v) The TiO₂@SnO₂ composite has an obvious red shift of ca. 10–20 nm in the absorption edge, compared with the pure SnO₂ nanotubes and TiO₂ nanosheets. This larger absorption edge would lead to the enhancement of the photocatalytic performance of the TiO₂@SnO₂ composite.⁵¹

From the viewpoint of practical application, it is crucial that the as-prepared photocatalyst maintains a high activity and stability for long-term use. As Fig. 6 illustrates, the TiO₂@SnO₂ composite photocatalyst possessed a sustainable photocatalytic performance in H₂ generation after 4 cycles. There was insignificant loss of the H₂ evolution rate, which indicates that the TiO₂@SnO₂ heterostructures were highly stable and reusable. The SEM images of the TiO₂@SnO₂ after the photocatalytic process display that the TiO₂@SnO₂ still retain the original morphology even after four recycles without any detected agglomeration or damage (Fig. S4†). Moreover, at the end of the reaction when the mixing was stopped, the TiO₂@SnO₂ composite photocatalyst can be easily separated from the solution by sedimentation, thus indicating it has good separation characteristics, which may be due to the large length to diameter ratio of the TiO₂@SnO₂ lD heterostructures.

Fig. 6 Reuse cycles of photocatalytic H₂ generation over the sample TiO₂@SnO₂ composite; every 6 h the reaction system was bubbled with N₂ for 30 min to remove the H₂ inside.

4. Conclusions

In summary, a TiO₂@SnO₂ composite has been successfully prepared by a facile two-step method. SEM, TEM, XRD and XPS analysis revealed the successful coating of TiO₂ nanosheets on the surface of SnO₂ nanotubes. In comparison with the uncoated SnO₂ nanotubes, pure TiO₂ nanosheets, the mechanically mixed two samples and P25, the TiO₂@SnO₂ composite exhibited the best photocatalytic performance for the decomposition of RhB and photocatalytic H₂ production over water splitting under UV light irradiation. In particular, 1D TiO₂@SnO₂ composites processed several excellent properties including a relatively large specific surface area for supplying abundant active sites, suitable band engineering for charge carriers separation and unique 1D heterostructures for resistance to photo induced agglomeration. We assumed that the synergistic effects of the aforementioned factors contributed to the enhance photocatalytic performance of the 1D TiO₂@SnO₂ heterostructures.

Acknowledgements

This research was supported partially by the National Natural Science Foundation of China (no. 51273158, 21303131); Natural Science Basis Research Plan in Shaanxi Province of China (no. 2012JQ6003); Fundamental Research Funds for the Central Universities of China (2011JDGZ15, xjj2012092, 2012jdhz34) and Natural science fund of Jiangsu Province, China (SBK201022919).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ta12863f

‡ Xin Xu and Guorui Yang contributed equally to this work.

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