Low-cost and highly efficient composite visible light-driven Ag–AgBr/γ-Al₂O₃ plasmonic photocatalyst for degrading organic pollutants

Abstract

Visible light-driven photocatalysts have attracted a great deal of attention for their direct use of solar energy. In this paper, an efficient and low-cost visible light-driven composite photocatalyst, comprising of Ag–AgBr/γ-Al₂O₃ nanostructures, has been facilely prepared by a surfactant-controlled precipitation–deposition and subsequent light-driven route. Hollow flower-like γ-Al₂O₃ nanostructures was chosen as the support. CTAB controlled the size of the deposited nanoparticles in both the Br source and the surfactant. The microstructures, surface chemical composition, visible light-driven photocatalytic performance and mechanism have been investigated systematically by SEM, TEM, XPS and so forth. The Ag–AgBr nanoparticles were distributed uniformly on the surface of the hollow flower-like γ-Al₂O₃ nanostructures, whose specific surface area value reached 116.5 m² g⁻¹. The Ag–AgBr/γ-Al₂O₃ composite nanostructures exhibit photocatalytic advantages over pure Ag–AgBr nanoparticles, such as decomposing aromatic compounds and organic dyes within several minutes of visible light irradiation. They also have good stabilities meaning the photocatalytic activity seldom decreases even when used 5 times under visible light irradiation. Meanwhile, the amount of Ag–AgBr needed and the cost decreases significantly. Thus, the as-prepared composite photocatalyst, consisting of Ag–AgBr/γ-Al₂O₃ nanostructures, will be an efficient and low-cost material for the removal of organic pollutants under visible light irradiation.

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1. Introduction

Many kinds of aromatic compounds and organic dyes, produced from industry, agriculture and waste processes, have had a negative impact on humans and the environment. A photocatalytic route towards removing these pollutants is widely accepted because it is an economic and clear approach.^1–4 Many studies concentrated on TiO₂ due to its stability, non-toxicity and low-cost.^5–7 However, the low utilization of visible light limits its practical applications although tremendous attention has been paid towards extending its absorption into the visible light region.^8–11 Hence, it is important and exigent to find new materials with a lower band energy that can be directly and efficiently used under sunlight irradiation.

Silver halides, as new efficient visible light photocatalysts, have drawn the attention of many researchers recently.^12–18 Their excellent visible light photocatalytic performance was a surprise, especially as their high stability seems inconsistent with the fact that silver halides are very unstable under sunlight irradiation. Almost all the superior photocatalytic activity and stability is contributed to the surface plasmonic resonance (SPR) of the silver nanoparticles produced on the surface of the silver halide.^19–21 Photoelectrons can be quickly brought out through the silver nanoparticles to prevent them from combining with the photogenerated hole, which markedly enhances the stability of the catalyst. However, silver is one of the noble metals and as such the cost of silver halide, as a material which removes pollutants, is much higher than that of TiO₂-based photocatalytic materials. Hence, efforts are increasingly focused on decreasing the cost of silver halide materials to make them viable photocatalysts.

Based on the points discussed above, subsequent work has been carried out to improve their performance and reduce the cost. Examples include controlling the morphology,²² building composite materials based on silver halides²³ and looking for good supports for loading silver halides.^24–26 Interestingly, a cheap and stable support is a direct approach to both improving their performance and reducing the cost. On the one hand, the catalysts could be distributed uniformly on the surface which significantly reduces the conglomeration of the nanoparticles. On the other hand, the amount needed and the cost of the catalysts can also be decreased significantly. Hence, it is desirable to load silver halide nanoparticles onto a suitable support.

Herein, we chose hollow flower-like γ-Al₂O₃ nanostructures as the support for AgBr, which has been recognized as a highly active visible light photocatalyst. Firstly, γ-Al₂O₃ is nontoxic to the environment and very stable under natural conditions. Secondly, it is much cheaper than Ag-based photocatalysts. Finally, the hollow flower-like nanostructures have a higher specific surface area than other nanostructures. The selective hollow flower-like γ-Al₂O₃ nanostructures are synthesized according to a previous report,²⁵ and the composite Ag–AgBr/γ-Al₂O₃ photocatalysts are prepared by a facile surfactant-controlled precipitation–deposition and subsequent light-driven method. Typically, CTAB and AgNO₃ are introduced into the γ-Al₂O₃ colloids in turn to prepare the AgBr/γ-Al₂O₃ nanostructures. Then the Ag–AgBr/γ-Al₂O₃ can be obtained after light irradiation. CTAB can control the size of the deposited nanoparticles in both the Br source and the surfactant. As a result, the photocatalytic nanoparticles are distributed uniformly onto the surface of the hollow flower-like γ-Al₂O₃ nanostructures. Moreover, the small size (about 20 nm) of the Ag–AgBr nanoparticles is very beneficial for SPR absorption in the visible light region. In addition, the as-designed route is suitable for the synthesis of other silver halide/γ-Al₂O₃ composite nanostructures.

In this work, the photocatalytic activity and stability of the as-prepared catalysts is evaluated by degrading the organic pollutants of p-nitrophenol (PNP) and methyl orange (MO) under visible light irradiation. PNP is a typical aromatic compound and can be difficult to decompose under sunlight irradiation. Therefore, the removal of PNP is very effective for evaluating a photocatalyst and is important for humans and the environment. The photocatalytic performance is in good agreement with the design. The organic pollutants can be decomposed within several minutes under visible light irradiation, even with very low amounts of Ag–AgBr. Moreover, the as-prepared Ag–AgBr/γ-Al₂O₃ photocatalysts exhibit good stabilities meaning that the activity seldom decreases even after it has been used 5 times. The catalytic activity of the supported composite catalyst is much higher than that of the unsupported pure Ag–AgBr nanoparticles, meaning the performance is significantly enhanced and the cost is decreased. Therefore, the as-prepared composite Ag–AgBr/γ-Al₂O₃ photocatalyst will be a practical, low-cost and highly active visible light-driven material for degrading organic pollutants.

2. Experimental

All the chemicals were analytic grade except for AgNO₃, which was chemical grade. They were obtained from Shanghai Chemicals Company (China) without any purification before being used. Additionally, the Xe lamp (PLS-SXE300) was bought from Beijing Perfect Light Limited Corporation (China) with a UV cut-off filter (λ > 400 nm).

2.1 Preparation of Ag–AgBr/γ-Al₂O₃

Generally, the photocatalyst was prepared using the precipitation–deposition and subsequent photoreduction method. The details are described as follows.

2.1.1 Preparation of hollow γ-AlOOH. The hollow AlOOH nanostructures were fabricated according the method reported elsewhere.²⁷ In a typical process, 0.83 g of KAl(SO₄)₂·12H₂O and 0.21 g of urea were dissolved in 35 mL of deionized water and stirred vigorously. The solution was then transferred into a 70 mL Teflon-lined stainless-steel autoclave and maintained at 180 °C for 5 h. After cooling to room temperature, the product was collected and washed with both deionized water and absolute ethanol 2–3 times and then dried in a vacuum at 60 °C for 12 h.

2.1.2 Preparation of hollow γ-Al₂O₃. The hollow AlOOH nanostructures fabricated above were calcined at 400 °C for 2 h and hollow γ-Al₂O₃ nanostructures were obtained.

2.1.3 Preparation of AgBr/γ-Al₂O₃. This step is the so-called precipitation-deposition method. Typically, 20 mg of the prepared Al₂O₃ was dispersed in 10 mL of a 0.01 M CTAB aqueous solution with vigorous stirring for 20 min and then 10 mL of a 0.01 M silver nitric aqueous solution was added drop by drop. The mixture was continually stirred for 3 h, collected and washed with deionized water and absolute ethanol 2–3 times.

2.1.4 Preparation of Ag–AgBr/γ-Al₂O₃. The AgBr/γ-Al₂O₃ nanostructures were redispersed in deionized water and irradiated under sunlight for 2 h. The color of the product changed from light yellow to light grey, which indicated that silver nanoparticles were produced.

2.2 Characterization of catalysts

The as-prepared samples were characterized by scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM; Tecnai G2 20 S-TWIN, Holland), X-ray diffraction (XRD, Philips X'Pert with Cu–Kα₁ radiation (λ = 0.154056 nm), X-ray photoelectron spectroscopy (XPS, ESCALAB 250 with monochromatized Al–Kα X-ray as the source), and room temperature UV-visible diffuse reflectance spectroscopy (UV-4100, SHIMADZU). Specific surface areas were computed from the results of the N₂ physisorption at 77 K (model: BECKMAN SA3100 COULTER) using the BET (Brunauer–Emmet–Teller) formalism.

2.3 The degradation of p-nitrophenol (PNP)

2.3.1 The photocatalytic PNP activity of the as-prepared photocatalyst. The photocatalytic activity of Ag–AgBr/γ-Al₂O₃ catalyst was evaluated through photodegradation of PNP. In a typical experiment, 20 mg of the as-prepared photocatalyst was dispersed in 20 mL of a PNP aqueous solution (10 mg L⁻¹) and stirred for 30 min in the dark to reach adsorption–desorption equilibrium at room temperature. The experiments were carried out under visible light irradiation supplied by a 300 W Xe lamp with a UV cut-off filter (λ > 400 nm). About 3 mL of the solution was obtained at 3 min intervals under visible light irradiation, was separated by centrifugation at 7500 rpm to remove the photocatalyst and the degradation results were monitored by UV-visible spectra (U-4100 spectrophotometer).

2.3.2 The photocatalytic stability of the as-prepared photocatalyst. The cycling degradation experiments were carried out to evaluate the photocatalytic stability of the as-prepared photocatalyst. The operation and parameters of the cycling degradation experiments were same as that mentioned above.

2.4 The degradation of methyl orange (MO) dye

The degradation of MO (0.05 mmol L⁻¹) was nearly the same as the procedure for the degradation of PNP. The difference between these two experiments is the sampling. About 3 mL of the MO solution was obtained at 30 s intervals under visible light irradiation.

3. Results and discussion

Fig. 1a and b are typical SEM and TEM images of the as-prepared hollow γ-Al₂O₃ nanostructures, prepared according to a previous report.²⁵ As is shown, the supported γ-Al₂O₃ material had a flower-like morphology with a diameter of 2–3 μm. We found they were typical hollow nanostructures according to the TEM image. As shown in the high-magnification SEM and TEM images (Fig. 1c and d), the final products were assembled by many lamellas, which contributed to an increase in the specific surface. Fig. 1e and f are the SEM images of the as-prepared Ag–AgBr/γ-Al₂O₃ nanostructures. As is clearly shown, the hollow flower-like nanostructures were not destroyed after loading. Compared with the SEM images of pure γ-Al₂O₃, we can see that the Ag–AgBr catalysts were successfully deposited onto the surface of the γ-Al₂O₃ nanostructures with a uniform distribution (as shown in the high-magnification SEM image (Fig. 1f). The size of Ag–AgBr nanoparticles was 10–20 nm.

Fig. 1 The low (a) and high (c) magnification SEM image, low (b) and high (d) magnification TEM images of pure γ-Al₂O₃, and low (e) and high (f) magnification SEM images of the as-prepared Ag–AgBr/γ-Al₂O₃ catalyst.

Fig. 2a shows the N₂ adsorption–desorption isotherm of the obtained γ-Al₂O₃ hollow flower-like microspheres. The specific surface area value of 116.5 m² g⁻¹ was determined according to the computer calculations. Such a high specific surface area supplies sufficient space for the CTAB molecule to be adsorbed onto the surface and into the pore, resulting in the small mean particle size (about 20 nm) of the supported materials and uniform distribution. The procedure is shown in Scheme 1. Firstly, CTAB was adsorbed onto the surface and into the pore of γ-Al₂O₃. CTAB acts as the source of both Br⁻ and Ag⁺, which can easily react with CTAB to form the AgBr nanoparticles because of the electrostatic attraction. Finally, the composite Ag–AgBr/γ-Al₂O₃ nanostructures were obtained after the photo-induced production of Ag⁰. This method prevents conglomeration among the AgBr nanoparticles, leading to a uniform distribution on the surface and in the pores. These nanostructures ensure the Ag–AgBr nanoparticle surfaces are sufficiently exposed, increasing the active area and efficiency.

Fig. 2 Nitrogen adsorption–desorption isotherm (a) and the corresponding pore-size distribution of the γ-Al₂O₃ hollow microspheres (b).

Scheme 1 The proposed formation mechanism of the as-prepared Ag–AgBr/γ-Al₂O₃ nanostructures.

Fig. 3a shows the XRD patterns of pure γ-Al₂O₃ and the Ag–AgBr/γ-Al₂O₃ nanostructures, further confirming the composition of the nanostructures. Compared with γ-Al₂O₃ (red line), we found that the six additional peaks (shown in Fig. 3a, black line), could be indexed to AgBr. Because the Ag⁰ content is a fraction of the composite, the weak peaks are covered by the impure peaks. However, the existence of Ag⁰ can be clearly demonstrated by the XPS spectrum (as shown in Fig. 3d). It shows peaks located at 368.2 eV and 374.2 eV, which belong to the Ag 3d orbital. As shown in Table 1, the content of Br is less than that of Ag, indicating Ag⁰ is produced. This result makes up for the deficiency in the XRD measurement. In addition, the overview XPS spectrum is displayed in Fig. 3b. The O and Al are from Al₂O₃, and the Ag and Br are from Ag–AgBr. Furthermore, the C is from the adsorbed CTAB or CO₂. Fig. 3c shows the peaks of Al and Br in the XPS spectrum. Thus, all the characterizations indicate that the obtained hollow flower-like nanostructures were Ag–AgBr/γ-Al₂O₃.

Fig. 3 (a) XRD patterns of γ-Al₂O₃ (red line) and the as-prepared Ag–AgBr/γ-Al₂O₃ nanostructures (black line). The whole (b), Al 3p, Br 3d (c) and Ag 3d (d) XPS spectra.

Table 1 The binding energy of the Ag⁰ 3d peak and atomic number ratio of Ag, Br and Al from the samples before and after photocatalysis, based on the XPS data

Sample	Binding energy of Ag^0 3d peak	The atomic number ratio of Ag	The atomic number ratio of Br	The atomic number ratio of Al
before photocatalysis	368.2, 374.2	1.47	1.32	53.03
after photocatalysis	368.3, 374.3	1.03	0.86	48.98

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Fig. 4 shows the UV-vis diffuse reflectance spectra of pure γ-Al₂O₃ and the as-prepared Ag–AgBr/γ-Al₂O₃ composite plasmonic photocatalysts. As is clearly shown, the Ag–AgBr/γ-Al₂O₃ catalyst has a strong absorption throughout the whole UV-vis light region (200–800 nm) while pure γ-Al₂O₃ has almost no absorption at all. The strong absorption in the visible light region is indexed to the strong surface plasmonic resonance (SPR) (see Fig. S1, ESI†) of the light-induced formation of small Ag nanoparticles.^19–21 As a consequence, the composite materials can make use of the whole visible light region, making them a practical material under visible light irradiation.

Fig. 4 UV-vis diffuse reflectance spectra of γ-Al₂O₃ (red line) and the as-prepared Ag–AgBr/γ-Al₂O₃ nanostructures (black line).

In this work, we investigated the visible light-driven photocatalytic decomposition of p-nitrophenol (PNP) as a way of evaluating the advantages of the as-prepared Ag–AgBr/γ-Al₂O₃ nanostructures. As is known, PNP is a toxic organic pollutant to both human beings and the environment and can be difficult to decompose under solar light due to its chemical and biological stability.^28,29 As shown in the UV-vis absorption spectra (see Fig. S2, ESI†), the peak located at 400 nm, belongs to the nitro group and declines quickly under visible light irradiation, revealing that PNP is quickly removed. As is shown in Fig. 5a, PNP can be decomposed by up to 80% within 3 min of visible light irradiation with the as-prepared catalysts, and can be almost completely decomposed after 12 min of irradiation. However, pure Ag–AgBr nanoparticles exhibit much lower photocatalytic activity, resulting in only about 50% of the PNP being decomposed after 12 min of irradiation. Furthermore, the PNP kept almost the original concentration when no photocatalysts were present. In addition, in the presence of pure Al₂O₃, the concentration of PNP declined significantly as a result of the adsorption of the hollow flower-like γ-Al₂O₃ structure with a high specific surface area. Interestingly, we found that the concentration of PNP increased when the system was under visible light irradiation, which is due to the desorption of PNP with increasing temperature during visible light irradiation.

Fig. 5 (a) The time-dependent photodecomposition curve of PNP. C₀ is the original concentration (10 mg L⁻¹) and C is the concentration of the remaining PNP at time t. (b) Histogram of the cycling photodecomposition of PNP. C is the concentration of PNP after 6 min of irradiation, and C₀ is the original concentration of PNP.

The photocatalytic stability is also an important factor. Herein, the cycling degradation experiments (after 6 min of visible light irradiation) were carried out to evaluate the photocatalytic stability under the same conditions. As shown in Fig. 5b, it can be clearly seen that the Ag–AgBr/γ-Al₂O₃ structures have a high stability, resulting in a high decomposition ratio even after 5 cycles with 6 min of visible light irradiation every time. Furthermore, based on the XPS data (Table 1), the atomic ratio of silver and bromide was 1.11 and 1.20 before and after the photocatalytic reactions, respectively. This means Ag (I) is seldom reduced into Ag⁰ during the photocatalysis. In addition, according to Table 1, the Ag, Br, and Al content was reduced while the O content was increased during the photocatalytic process. One reason may be due to the adsorption of organic O onto the photocatalyst. Another may be that some Ag⁺ ions are transformed into Ag₂O, leading to instability in the Ag–AgBr/γ-Al₂O₃ photocatalyst, which can be seen in Fig. 5b and 6b in which a slight reduction in the photocatalytic activity appears.

The degradation of PNP has been extensively studied.^30–34 Based on previous reports and our photocatalytic experiments, we proposed the possible degradation processes of PNP and the corresponding products from the as-prepared Ag–AgBr/γ-Al₂O₃ photocatalysis, which is illustrated in Scheme 2. The photocatalytic mechanism for AgX (X = Cl, Br and I)-based plasmonic photocatalysts has been confirmed by Hu et al.^16,17 In their work, an ESR spin-trap technique (with DMPO) confirmed the existence of OH˙ and O₂˙ radicals during the visible light irradiation, and the effect of various radical scavengers further revealed that the OH˙ and O₂˙ radicals were the main active species. Generally, the combination of photo-induced OH˙/O₂˙ radicals (the formation mechanism will be discussed later in detail) with PNP decides the degradation products. If OH˙ attacks the 3 position of PNP, 4-nitrocatechol (product I) can be obtained. If OH˙ attacks the 1 position of PNP, p-benzenediol (product II) is produced. In addition, p-benzenediol can further react with photo-induced O₂˙ or OH˙ and so benzoquinone (product III) and hydroxyquinol (product IV) appear, respectively. In summary, four main products may be produced during the photodecomposition of PNP. Furthermore, PNP can also be reduced to p-amino phenol (product V) by electrons.²⁹

Scheme 2 The possible degradation process of PNP and the corresponding products from the as-prepared Ag–AgBr/γ-Al₂O₃ photocatalysis.

To further investigate the advantages of the as-prepared Ag–AgBr/γ-Al₂O₃ photocatalysts, we carried out photodegradation reactions for MO under visible light irradiation. As shown in Fig. S3 (ESI†), the peak located at 463 nm, belongs to the azo group, which quickly declines under visible light irradiation with the as-prepared catalysts. According to Fig. 6a, we can see that the Ag–AgBr/γ-Al₂O₃ nanostructures display excellent photocatalytic activities. The MO dye can be decomposed by 82% within 1 min of visible light irradiation, and can be almost completely decomposed within 3 min. Similar to the results for the degradation of PNP, the pure Ag–AgBr nanoparticles display much less activity, meaning that less than 50% is decomposed after 3 min irradiation. In addition, we evaluated the photocatalytic stability of the structures by cycling degradation experiments under the same conditions. As shown in Fig. 6b, the stability of the photocatalyst is very good. The photocatalytic activity of the Ag–AgBr/γ-Al₂O₃ photocatalysts was seldom reduced even after 5 cycles. Therefore, the stability of the photocatalysts, comprised of Ag–AgBr/γ-Al₂O₃ nanostructures, is significantly enhanced and they are potential visible light-driven catalysts for the degradation of organic pollutants.

Fig. 6 (a) The photodecomposition curve of MO. C₀ is the original concentration of MO (0.05 mmol L⁻¹), and C is the concentration of the remaining MO at time t. (b) Histogram of the cycling photodecomposition of MO. C is the final concentration (3 min) of degraded MO, and C₀ is the original concentration of MO.

Based on these results, we can see that the prepared Ag–AgBr/γ-Al₂O₃ photocatalyst shows a faster degradation rate for the MO dye than that of PNP. To further investigate the photocatalytic performance, we investigated the kinetic rate constant value. As shown in Fig. 7, the normalized concentration of PNP (a) and MO (b) show a linear relationship. In other words, the photocatalytic reactions have first-order kinetics (−dc/dt = kt). In accordance with Fig. 7a and b, we can calculate the kinetic rate constant value for PNP and MO and k_PNP is 0.1515 min⁻¹ and k_MO is 0.1972 min⁻¹, respectively. Based on the kinetic rate constant, the degradation rate for MO is faster. On the one hand, MO is a dye, so MO may produce a sensitizer effect under visible light irradiation. On the other hand, the structure of MO may be destroyed more easily than that of PNP.

Fig. 7 The normalized concentration of PNP (a) and MO (b) as a function of reaction time in a logarithmic scale. C₀ is the concentration after adsorption, and C is concentration at time t.

According to our results, the excellent visible light-driven photocatalytic performance of the as-prepared Ag–AgBr/γ-Al₂O₃ nanostructures may be due to the following 3 reasons. Firstly, the high visible light catalytic performance of Ag–AgBr ensures the basic photocatalytic ability of the Ag–AgBr/γ-Al₂O₃ composite nanostructures. The proposed photocatalytic mechanism is illustrated in Scheme 3. It has been confirmed that the Ag nanoparticles contribute to the high visible light photocatalytic activity and stability due to their SPR produced by the collective oscillations of the surface electrons.^19–21 In addition, Ag is one of the best conductive metals meaning light-induced electrons can be quickly transferred outside the structure and therefore avoid recombination with the light-induced hole (h⁺). As a result, the electrons can combine with O₂ to form active O₂˙ radicals. H⁺ can also be captured by H₂O and produce OH˙ radicals.^35–37 OH˙/O₂˙ act on the pollutants and decompose them efficiently. Secondly, the electrons can be translated quickly from AgBr because of the excellent conductivity of the Ag nanoparticles, resulting in Ag⁺ seldom capturing the electrons and the stability ensured.^12,13 Finally, but no less importantly, the hollow flower-like γ-Al₂O₃ support ensures that the Ag–AgBr photocatalyst can be distributed well, which optimizes the photocatalytic performance of the Ag–AgBr nanoparticles. As shown in the BET measurements (Fig. 2), the hollow flower-like γ-Al₂O₃ nanostructures have a large specific surface area of 116.5 m² g⁻¹, which supplies a sufficient surface and pores for the loading of the Ag–AgBr nanoparticles. In addition, with the help of CTAB, the size of the deposited Ag–AgBr nanoparticles are so small and uniform that the utilization of the Ag–AgBr nanoparticles is significantly increased, which has been revealed by the photocatalytic results. In other words, the photocatalytic performance is enhanced significantly while the cost greatly decreased.

Scheme 3 The proposed photocatalytic mechanism of the Ag–AgBr/γ-Al₂O₃ nanostructures.

4. Conclusions

In summary, a very efficient and low-cost visible light-driven composite photocatalyst, comprised of Ag–AgBr/γ-Al₂O₃ nanostructures, has been facilely prepared. Their microstructures, surface chemical composition, visible light-driven photocatalytic performance and mechanism have been investigated systematically. The Ag–AgBr/γ-Al₂O₃ composite nanostructures exhibit photocatalytic advantages over pure Ag–AgBr nanoparticles as they can decompose aromatic compounds and organic dyes within several minutes of visible light irradiation. Meanwhile, the amount of Ag–AgBr needed and the cost is decreased significantly. Thus, the as-prepared composite Ag–AgBr/γ-Al₂O₃ nanostructures will be an efficient and low-cost material for the removal of organic pollutants under visible light irradiation.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (20671003 and 20971003), the Key Project of Chinese Ministry of Education (209060), the Program for New Century Excellent Talents in University (NCET 11-0888), the Science and Technological Fund of Anhui Province for Outstanding Youth (10040606Y32), the Foundation of Key Project of Natural Science of Anhui Education Committee (KJ2012A143) and the Program for Innovative Research Team at Anhui Normal University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional figures and figure captions. See DOI: 10.1039/c2cy20074k

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