Efficient Visible Light-Driven Photocatalytic Degradation of

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Efficient Visible Light-Driven Photocatalytic Degradation of Pentachlorophenol with Bi2O3/TiO2−xBx Ke Su, Zhihui Ai,* and Lizhi Zhang Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, People’s Republic of China S Supporting Information *

ABSTRACT: In this study, a new TiO2-based photocatalyst with both B doping and Bi2O3 coupling (Bi2O3/TiO2−xBx) was synthesized to degrade pentachlorophenol under visible light (λ > 420 nm) irradiation. The resulting Bi2O3/TiO2−xBx sample exhibited much higher photocatalytic performance than the counterparts with only B doping or Bi2O3 coupling or pure TiO2. This is because B doping could result in more visible light absorption to produce more photogenerated electron−hole pairs, while Bi2O3 coupling could favor the separation and transfer of photoinduced charge carriers to inhibit their recombination. We interestingly found that the visible light-driven degradation of pentachlorophenol was mainly attributed to photogenerated holes and ·O2− other than ·OH as reported previously because the hybridization of B 2p orbital and O 2p orbital could elevate the VB edge of Bi2O3/TiO2−xBx as compared to that of pure TiO2 and thus lower the oxidation ability of photogenerated holes, blocking the pathway of photogenerated holes induced oxidation of surface OH− and water to generate ·OH. The intermediates during the PCP photodegradation were systematically analyzed, ruling out the existence of high toxic polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans. These results reveal that the visible light-driven photocatalytic degradation of PCP over Bi2O3/TiO2−xBx is an effective and green method to remove highly toxic halogenated aromatic compounds.

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INTRODUCTION

TiO2 is the most promising semiconductor photocatalyst for application because of its strong oxidizing power, costeffectiveness, and long-term stability against photocorrosion and chemical corrosion.9,10 It is known that anatase is the most photoactive form of TiO2.11 However, the large band gap (3.2 eV) of anatase makes it only active under UV light with wavelength less than 387.5 nm, limiting its practical environmental application by using solar light with more than 43% of visible light. Therefore, scientists are seeking various methods to develop titania-based photocatalysts highly active under visible light. Among these methods, ion doping has been widely adopted to adjust the position of conduction band (CB) or valence band (VB) of TiO2, which could make the electrons excitable under visible light irradiation to produce the photoelectron−hole pair.12−15 It is usually thought that nonmetal ions doping is more promising than metal ions doping in most cases because metal ions doping would

Pentachlorophenol (PCP) is commonly regarded as a highly toxic halogenated aromatic compound because of its five chlorine substitutions. It exists in soil and water, which could migrate to groundwater, thereby bringing about a potential risk to human and animals.1,2 Biological degradation of PCP is very slow and ineffective at high concentrations,3,4 accompanied by the production of more toxic compounds like polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans.5 Fortunately, recent studies reveal semiconductor-mediated photodegradation of PCP is more attractive. For instance, Yu et al. found 90% of PCP could be degraded over the NiO/TiO2 composite in 3 h under 365 nm LED light irradiation.6 Zhang and his co-workers reported their synthesized Bi2WO6 could degrade 10% of PCP in 1 h under visible light irradiation from a 300 W xenon lamp.7 Yin et al. investigated the degradation of PCP over Ti-doped β-Bi2O3 at various pH values under the visible light irradiation of a 1000 W xenon lamp with a 420 nm cutoff filter and found that PCP could be completely degraded in 1 h at the optimum pH value of 11.2.8 © 2012 American Chemical Society

Received: June 3, 2012 Revised: July 18, 2012 Published: July 19, 2012 17118

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were washed with deionized water and ethanol several times and then calcined at 600 °C with a heating rate of 3 °C/min for 3 h in a muffle furnace to obtain the final product, which was denoted as Bi2O3/TiO2−xBx. For the comparison, other samples were prepared with Bi(NO3)3·5H2O (0.1 mmol) and TiCl4 (10 mmol) in the absence of H3BO3; H3BO3 (5 mmol) and TiCl4 (10 mmol); TiCl4 (10 mmol); and Bi(NO3)3·5H2O (0.1 mmol) via the same method, which were denoted as Bi2O3/ TiO2, TiO2−xBx, pure TiO2, and Bi2O3, respectively. Characterization. The powder X-ray diffraction (XRD) measurements were carried out by using a Rigaku D/MAX-RB diffractometer with monochromatized Cu Kα radiation (λ = 0.15418 nm). The Brunauer−Emmett−Teller (BET) surface areas of the powder samples were determined by nitrogen adsorption−desorption isotherm measurements at 77 K with a Micromeritics Tristar-3000 nitrogen adsorption apparatus. The UV−vis diffuse reflectance spectrum (DRS) was measured with a UV−vis spectrometer (UV-2550, Shimadzu) with BaSO4 as a reference. X-ray photoelectron spectroscopy (XPS) measurements were performed in a VG Scientific ESCALAB Mark II spectrophotometer equipped with two ultrahigh vacuum (UHV) chambers. All binding energies were referenced to the C 1s peak at 284.6 eV of the surface adventitious carbon. Photocatalytic PCP Removal Experiments. The photocatalytic removal of PCP over the prepared catalysts was carried out with a visible light source consisting of a 500 W tungsten halogen lamp from the bottom. The lamp was set inside a cylindrical vessel surrounded by a circulating water jacket for cooling, and the distance between the lamp and the PCP solution was about 15 cm. The UV light with wavelength shorter than 420 nm was removed with a glass filter. A 0.05 g amount of photocatalyst was suspended in 50 mL of 10 mg/L PCP. Prior to irradiation, the suspension was sonicated in the dark for 5 min and then allowed to reach an adsorption− desorption equilibrium for 2 h. The photocatalytic reaction was stopped by removing the photocatalysts from the suspension through centrifuging and filtering with a Millipore membrane (220 nm), and then the supernatant solution was analyzed with high-performance liquid chromatography (HPLC, LC-20AT, Shimadzu). Each set of experiment was performed for 5 h. For the analysis of PCP, a C-18 reverse phase column was equipped for separation. The mobile phase was composed of methanol and 0.15% of acetic acid aqueous solution (V:V = 4:1) with a flow rate of 0.8 mL min−1. The detection wavelength was set at 254 nm. Intermediates identification was performed with a typical procedure as follows. 50 mL of the degraded solution was taken out and filtered, and then the sample was concentrated to 2 mL by a rotary evaporator for the subsequent analysis on a HPLC-MS (API2000, AB SCIEX). Ion chromatography system (Metrohm 861 Advanced Compact IC) was used to detect released chloride ions. The mobile phase was an aqueous 1.7 mmol of NaHCO3−1.8 mmol of Na2CO3 solution with a flow rate of 1.0 mL min−1.

introduce undesirable defects as the recombination center of the photoelectron−hole pair, and thus reduce the pollutant degradation efficiency.16,17 Among the previous studies on nonmetal ions doping, B doping attracts more and more attention. Theory calculations revealed that the substitution of O with B in TiO2 possessed lower energy than all other possible cases and resulted in the band gap narrowing because of the hybridization of B 2p and O 2p orbitals.18 Gopal and his co-workers attributed the absorption red shift of their B-doped TiO2 nanoparticles to the substitution of boron at an oxygen site.19 In et al. found that the conversion ratio of methyl tertiary butyl ether over B-doped TiO2 under visible light strongly depended on the proportion of “active B” to total B.20 Liu and his co-workers thought the O−Ti−B structure contributed to the additional visible light absorption, whereas the Ti−O−B structure caused a blue shift of the absorption edge.21 Although the hybridization of B 2p and O 2p orbitals could theoretically elevate the position of CB of TiO2, several groups found the degradation efficiency of B-doped TiO2 did not increase significantly as compared to the undoped sample under visible light irradiation.18−21 These unexpected results might result from the fast photoelectron−hole pair recombination accompanied by the band gap narrowing. Therefore, it is vital to develop effective ways to inhibit the photoelectron−hole pair recombination of B-doped TiO2. It is well-known that the recombination of photoelectron− hole pairs could be effectively inhibited by constructing heterostructures between TiO2 and other semiconductors with proper band potentials. For instance, the SnO2/TiO2 composite possessed a higher photocatalytic activity on the degradation of rhodamine B than the bare TiO2 under UV light irradiation.22 Zhao and his co-workers deposited Ni2O3 on TiO2−xBx to improve the activity on the removal of toxic organic pollutants under visible light irradiation.18 Because Bi2O3 is a fairly good electron-conducting material with the CB of Bi 6p for the electrons transfer and its potentials of CB and VB match with the TiO2 well,23 Bi2O3/TiO2 was found to exhibit enhanced photocatalytic activity on the decomposition of p-chlorophenol under visible illumination (λ > 420 nm) because of the photosensitization of Bi2O3 and the effective inhabitation of photoelectron−hole pair recombination.24 In this study, we employ a facile method to realize both B doping in TiO2 and Bi2O3 coupling with B-doped TiO2 (Bi2O3/ TiO2−xBx) for the first time. The resulting new photocatalyst Bi2O3/TiO2−xBx is used to remove PCP under visible light (λ > 420 nm) irradiation. We systematically investigate the visible light-driven degradation pathway of PCP over this new photocatalyst with high-performance liquid chromatography− mass spectrometry (HPLC−MS) and ion chromatography (IC).

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EXPERIMENTAL SECTION Sample Preparation. Bi(NO3)3·5H2O and TiCl4 were obtained from National Medicines Corporation Ltd., China. H3BO3 was purchased from Shanghai Chemical Reagent Limited Co. All of the chemicals were of analytical grade and used as received without further purification. In a typical synthesis, Bi(NO3)3·5H2O (0.1 mmol) and H3BO3 (5 mmol) were dissolved in an 60 mL of HNO3 aqueous solution (1 mol/ L) under vigorous stirring, and then TiCl4 (10 mmol) was added dropwise into the ice-cooled solution. The resulting solution was stirred for 30 min and heated at 80 °C for 12 h under autogenous pressure in an oven. The white precipitates

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RESULTS AND DISCUSSION X-ray diffraction was used to characterize the phase composition of the resulting samples (Figure 1). Only anatase TiO2 (JCPDS card no. 71-1167, space group: I41/amd) was found in the two samples synthesized in the presence of bismuth precursor (Bi2O3/TiO2 and Bi2O3/TiO2−xBx). However, both anatase TiO2 (JCPDS card no. 71-1167, space group: I41/amd) and rutile TiO2 (JCPDS card no. 4-551, space group: P42/mnm) existed in the other two samples synthesized in the 17119

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Figure 1. XRD patterns of the as-prepared samples: (a) pure TiO2, (b) TiO2−xBx, (c) Bi2O3/TiO2, and (d) Bi2O3/TiO2−xBx.

absence of bismuth precursor (TiO2−xBx and pure TiO2). These differences suggest that the presence of bismuth precursor could prevent the transformation of anatase to rutile, which is consistent with the reports that Bi2O3 species could prevent the TiO2 nanoparticles from agglomerating and rearranging during calcination.25,26 However, no obvious diffraction peaks of the bismuth oxide phase were observed, indicative of an amorphous state or a poor crystallinity of bismuth oxide. It is interesting to notice that the (101) plane of anatase patterns of TiO2−xBx and Bi2O3/TiO2−xBx shifted slightly to higher diffraction angles as compared to those of pure TiO2 and Bi2O3/TiO2 (inset of Figure 1). This shift should be attributed to the incorporation of boron atoms into the framework of TiO2 because of the smaller ion radius of B3+ (0.023 nm) than that of Ti4+ (0.064 nm) and O2− (0.132 nm).27 On the contrary, the (101) plane of anatase patterns of Bi2O3/TiO2 did not shift as compared to pure TiO2, revealing Bi element was not incorporated into the lattice of TiO2. Figure 2 shows the TEM and HRTEM images of TiO2−xBx and Bi2O3/TiO2−xBx. The surface of TiO2−xBx nanoparticles (Figure 2a) was smooth as compared to that of Bi2O3/ TiO2−xBx (Figure 2b). The HRTEM image of Bi2O3/TiO2−xBx clearly reveals that plenty of tiny particles of about 2 nm in size are highly dispersed on TiO2 nanoparticles (Figure 2c). Apart from the lattice planes from anatase TiO2, we could not observe other lattice fringes from these 2 nm-sized particles and therefore assigned these tiny particles to amorphous Bi2O3 in view of the previous XRD results. A similar phenomenon was observed on the Bi2O3/TiO2 composite photocatalyst.24 XPS analysis was performed to investigate the surface chemical states of the resulting samples (Figure S1, Supporting Information). The Bi2O3/TiO2−xBx mainly contains Ti, O, C, B, and Bi elements according to the survey spectrum. Two peaks were observed at binding energies of about 458.0 eV (Ti 2p3/2) and 463.7 eV (Ti 2p1/2) in the high-resolution Ti 2p spectrum of pure TiO2. Two strong peaks at 159.1 and 164.4 eV in the high-resolution Bi 4f spectrum correspond to the signals from the two doublets of Bi 4f5/2 and Bi 4f7/2 in the trivalent oxidation state.28,29 For B 1s, a peak at binding energy of about 192.0 eV revealed that boron atoms were incorporated into TiO2 with a state of B−Ti−O, rather than B−Ti−B or B− O,18,19,21 or a separate phase of B2O3 (193.1 eV) or Ti−B

Figure 2. TEM and HRTEM images of (a) TiO2−xBx, and (b,c) Bi2O3/TiO2−xBx.

species (188.2 eV). The Ti 2p peaks of Bi2O3/TiO2−xBx and TiO2‑xBx showed a positive shift of approximately 0.3 eV as compared to those of pure TiO2, confirming that the formation of the B−Ti−O band,21 whereas the presence of Bi2O3 did not affect the XPS spectra of the Ti 2p, confirming that Bi was not incorporated into the lattice of TiO2, but existed as the separated Bi2O3 nanoparticles.24 These results are in good agreement with the aforementioned XRD and TEM results. The nitrogen adsorption−desorption isotherm curves of the resulting samples (Figure S3, Supporting Information) revealed that all of the samples exhibited a Type IV adsorption− desorption isotherm. The Bi2O3/TiO2 possessed the highest surface area (33.9 m2/g), larger than that (15.4 m2/g) of TiO2−xBx, and more than 4 times that of pure TiO2 (7.4 m2/g). However, the surface area of Bi2O3/TiO2−xBx decreased to 27.7 m2/g after simultaneous B doping and Bi2O3 coupling. The as-prepared samples were used to degrade PCP under visible light irradiation (Figure 3). Both self-degradation of PCP in the absence of photocatalyst and the degradation of PCP in the presence of pure TiO2 under visible light irradiation were negligible. In contrast, Bi2O3/TiO2−xBx could degrade 85% of PCP within 5 h under visible light irradiation and exhibited a much higher photocatalytic activity than TiO2−xBx and Bi2O3/ TiO2, which could only remove 40% and 10% of PCP in 5 h, respectively. The pseudo-first-order degradation constant of Bi2O3/TiO2−xBx was calculated to be 0.172, about 2 times that (0.083) of TiO2−xBx and 7 times that (0.025) of Bi2O3/TiO2, respectively. These data reveal that simultaneous B doping and Bi2O3 coupling could greatly enhance the visible light-driven photocatalytic activity of TiO2 on PCP removal. It is well-known that the semiconductor photocatalysis involves the light absorption, the semiconductor photoexcitation, the separation and transfer of photoinduced charge carriers, the recombination of photoinduced charge carriers, the generation of active species, and the subsequent chemical reactions.30 During these processes, a high surface area could enhance the photocatalytic activity by facilitating the mass 17120

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degradation to eliminate dissolved oxygen. We found that the addition of sodium oxalate or superoxide dismutase could completely depress the degradation of PCP, suggesting photogenerated holes and ·O2− are the major species for the photodegradation of PCP under visible light irradiation. When N2 was bubbled to purge out the oxygen in the solution, the degradation of PCP was also totally inhibited (Figure 4). This

Figure 3. Comparison of the photodegradation of PCP over the assynthesized samples under visible light (λ > 420 nm) irradiation.

transfer of reactants or reaction intermediates and promoting the surface chemical reactions. Therefore, we first normalized the photocatalytic degradation rates of the photocatalysts with their surface areas to check the effect of surface areas on their photocatalytic activities (Table S1, Supporting Information) and found that the order of the normalized rates was the same as that of the original ones. Therefore, we concluded that surface area plays a minor role in PCP photodegradation. The semiconductor photoexcitation strongly depends on the light absorption and the band gap of the semiconductor. A semiconductor could only absorb the light with energy equal to or greater than its band gap for excitation of electrons from the valence band to the conduction band. We therefore used UV− vis diffuse reflectance spectroscopy (DRS) to characterize the light absorption and the band gaps of the as-prepared samples (Figure S2, Supporting Information). The photoabsorption of both Bi2O3/TiO2 and Bi2O3/TiO2−xBx significantly increased at wavelengths ranging from 400 to 600 nm, and an obvious red shift in the band gap transition appeared for TiO2−xBx Bi2O3/ TiO2 and Bi2O3/TiO2−xBx as compared to pure TiO2, which are consistent with their own color as shown in the inset of Figure S2. The plot of transformed Kubelka−Munk function versus the energy of light affords band gap energies of 3.03, 2.94, 2.79, and 2.85 eV for pure TiO2, TiO2−xBx, Bi2O3/TiO2, and Bi2O3/TiO2−xBx, respectively. Bi2O3/TiO2‑xBx with a larger band gap exhibited much higher visible light-driven PCP removal efficiency than Bi2O3/TiO2 with a narrower band gap, suggesting that B doping could result in more visible light absorption to produce more photogenerated electron−hole pairs. Although the band gap of Bi2O3/TiO2−xBx is larger than that of TiO2−xBx, Bi2O3/TiO2−xBx possessed a higher visible light photocatalytic activity than the latter, revealing that the coupling of Bi2O3 nanoparticles with TiO2−xBx could favor the separation and transfer of photoinduced charge carriers to inhibit their recombination. Active species trapping experiments were further carried out to determine the major reactive species accounting for the photocatalytic degradation of PCP over Bi2O3/TiO2−xBx, including tert-butylalcohol (10 mM) for hydroxyl radicals,31 sodium oxalate (10 mM) for holes,32 and superoxide dismutase (66.7 mg/L) for superoxide radicals.33 Additionally, N2 was bubbled into the PCP solution during the photocatalytic

Figure 4. Photodegradation curves of PCP with N2 bubbling or in the presence of different scavengers of tert-butylalcohol, sodium oxalate, and superoxide dismutase over Bi2O3/TiO2−xBx under visible light (λ > 420 nm) irradiation.

phenomenon reveals that the trapping of photoelectrons with molecular oxygen is vital for the photocatalytic degradation of PCP over Bi2O3/TiO2−xBx, which could not only prevent the recombination of photogenerated electron−hole pairs for the generation of more holes, but also produce ·O2− for the subsequent formation of H2O2 and ·OH to degrade PCP. However, the addition of tert-butylalcohol did not significantly inhibit the photodegradation of PCP, indicating that the amount of ·OH produced during the photocatalysis of Bi2O3/ TiO2−xBx was very small and could not contribute much to the photodegradation of PCP in this study. This result is reasonable because the hybridization of B 2p orbital and O 2p orbital in Bi2O3/TiO2−xBx would elevate the VB edge as compared to that of pure TiO2 and thus reduce the oxidation ability of photogenerated holes, blocking the pathway of photogenerated holes induced oxidation of surface OH− and water to generate ·OH. The small amount of ·OH could only be produced via the O2 → ·O2− → ··· → ·OH route. Although direct reductive dechlorination of electrons was also reported to eliminate PCP,8 the more negative reduction potential of PCP (−0.54 V vs NHE) than the conduction band potential (0.50 V vs NHE) of TiO2 restricts the reduction of PCP by photogenerated electrons in this study. To elucidate the photocatalytic degradation pathway of PCP over Bi2O3/TiO2−xBx, HPLC−MS analysis was used to identify the intermediates produced during the photocatalysis (Figure 5 and Figure S5). The detected intermediates include 3,5dichlorophenol and 2,3,5-trichloro-1,4-hydroquinone (3,5,6trichloro-1,2-pyrocatechol or 3,4,5-trichloro-1,2-pyrocatechol). On the basis of these intermediates and active species trapping experimental results, we proposed a possible degradation pathway of PCP (Figure 6). In the solution, PCP 17121

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previously reported in the literature,37,38 where pentachlorophenoxy radicals were mainly generated by the attack of ·OH. We also employed ion chromatography to detect chlorine ions produced during the photodegradation of PCP. Figure 7

Figure 5. The HPLC spectrum of solution after 2 h of degradation over Bi2O3/TiO2−xBx under visible light (λ > 420 nm) irradiation: (a) PCP, (b) 2,3,5-trichloro-1,4-hydroquinone (3,5,6-trichloro-1,2-pyrocatechol or 3,4,5-trichloro-1,2-pyrocatechol), and (c) 3,5-dichlorophenol.

Figure 7. The photodegradation efficiency and variation of dechlorination during PCP photodegradation over Bi2O3/TiO2−xBx under visible light (λ > 420 nm) irradiation. [Cl−] is the concentration of chloride ions by ion chromatography, while [Cl0] is the initial stoichiometric concentration of organic chlorine.

molecules mainly exist in the form of pentachlorophenol negative ions, which could mainly be oxidized by photogenerated holes into pentachlorophenoxy radicals, resulting in a facile oxidative degradation by active species.34 Considering the resonance effect and electron negativity, the ortho- and parapositions are more apt to be attacked due to more electron cloud density distribution,35 and thus suffer from dechlorination by ·O2−.36 Besides the major degradation pathway, the small amount of ·OH might also attack the pentachlorophenol negative ions and/or the partially dechlorinated phenols in this system. Finally, the double bonds in the benzene ring were broken to form smaller intermediates of carboxylic acid substances, which could not be detected by HPLC−MS because of their low concentration and instability in the solution. Obviously, the visible light-driven degradation pathway of PCP over Bi2O3/TiO2−xBx is different from those

shows that the chlorine ion concentration increased along with the degradation of PCP under visible light (λ > 420 nm) irradiation, confirming the photocatalytic dechlorination of PCP. The dechlorination ratio was calculated to be about 20% by dividing the total chlorine ions amount in the solution with the initial amount of chlorine in PCP, much less than the total PCP removal ratio (85%), suggesting the formation of some chlorinated intermediates during the PCP photodegradation process. This phenomenon is consistent with HPLC−MS analysis results. Fortunately, the absence of high toxic polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans in the intermediates reveals that this visible lightdriven photocatalytic degradation of PCP over Bi2O3/TiO2−xBx

Figure 6. The possible degradation pathway of PCP over Bi2O3/TiO2−xBx under visible light (λ > 420 nm) irradiation. 17122

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is an effective and green method to remove highly toxic halogenated aromatic compounds.

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CONCLUSIONS In summary, we have prepared a new TiO2-based photocatalyst with both B doping and Bi2O3 coupling (Bi2O3/TiO2−xBx) for degradation of PCP under visible light (λ > 420 nm) irradiation. The Bi2O3/TiO2−xBx photocatalysts exhibited much higher photocatalytic performance than the counterparts with only B doping or Bi2O3 coupling or pure TiO2. We attributed the visible light-driven degradation of pentachlorophenol to photogenerated holes and ·O2− other than ·OH. The intermediates during the PCP photodegradation were systematically analyzed, ruling out the existence of high toxic polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans. This study suggests that the visible light-driven photocatalytic degradation of PCP over Bi2O3/TiO2−xBx is an effective and green method to remove highly toxic halogenated aromatic compounds.

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ASSOCIATED CONTENT

S Supporting Information *

XPS spectra of the resulting samples; UV−vis diffuse reflectance spectra (inset) and plots of the (ahν)1/2 versus photon energy (hν) of the as-prepared samples; nitrogen adsorption−desorption isotherm of the resulting samples; photodegradation of PCP in the presence of different scavengers of tert-butylalcohol, sodium oxalate over Bi2O3 under visible light irradiation; positive ion mass spectra in the photodegradation of PCP intermediate products in the presence of Bi2O3/TiO2−xBx under visible light; and results of the high-resolution XPS spectra for the Bi 4f and B 1s regions, surface area, degradation efficiencies (K), degradation efficiencies normalized with the surface areas (K′), and band gap energy of various photocatalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone/fax: +86-27-6786 7535. E-mail: [email protected]. edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grants 20977039, 21073069, 91023010, 21173093, and 21177048), Program for Changjiang Scholars and Innovative Research Team in University (Grant IRT0953), and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei province (Grants CHCL11001).

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REFERENCES

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dx.doi.org/10.1021/jp305432g | J. Phys. Chem. C 2012, 116, 17118−17123