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Synthesis and enhanced photocatalytic performance of WO3 nanorods @ graphene nanocomposites
Materials Letters 89 (2012) 258–261
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and enhanced photocatalytic performance of WO3 nanorods @ graphene nanocomposites Minjie Zhou a,b, Jianhui Yan a,n, Peng Cui b,nn a b
School of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Hunan 414006, PR China School of Chemical Engineering, Hefei University of Technology, Anhui 230009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2012 Accepted 19 August 2012 Available online 31 August 2012
WO3 nanorods @ graphene (WO3@GE) nanocomposites were successfully synthesized via a photoreduction method. The obtained samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), Raman spectroscopy, and ultraviolet-visible diffuse reflectance spectroscopy (DRS) techniques. The functional groups present in graphene oxide (GO) such as carboxyl (C ¼O) were mostly reduced in the WO3@GE nanocomposites. The photocatalytic activity of the samples was evaluated by the degradation of methyl orange (MO). The results showed that the WO3@GE nanocomposites exhibited higher photocatalytic activity compared to bare WO3 nanorods. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.
Keywords: Carbon materials Solar energy materials Nanocomposites
1. Introduction Nanostructured tungsten oxide (WO3), as an important n type semiconductor with a band gap of 2.8 eV, has attracted considerable interest in photocatalysis due to its promising physical and chemical properties [1,2]. However, WO3 nanomaterials are usually not efficient photocatalysts because of the high electron–hole recombination rate [3]. Recently, many attempts have been made to improve the efficiency of electron–hole pair separation in WO3 nanomaterials, such as doping [4] and composites [5]. Recently, graphene-based composites have received considerable attention due to their potential application in the photocatalytic field [6–9]. In comparison with bare photocatalysts, graphene-based composites exhibit higher photocatalytic property because graphene has perfect two-dimensional carbon structure with better conductivity and larger surface area [6]. In general, graphene-based composites are prepared from the reduction of graphene oxide (GO)-based composites through the chemical reduction method, the flash reduction method or the solvothermal method, etc. However, photoreduction of GO-based composites has recently proven to be an effective method to produce graphene-based composites [10,12]. In this work, WO3@GE nanocomposites were synthesized via a photoreduction
n
Corresponding author. Tel./fax: þ 86 0730 8640122. Corresponding author. Tel./fax: þ 86 0551 2901450. E-mail addresses: [email protected] (J. Yan), [email protected] (P. Cui).
nn
method. The photocatalytic activity of the WO3@GE nanocomposites was evaluated by the degradation of methyl orange (MO).
2. Experimental details GO was synthesized from natural graphite powder based on modified Hummers method [7]. WO3 nanorods were synthesized via a hydrothermal method [5]. The WO3@GE nanocomposites were synthesized via a photoreduction method. Typically, 1 g of WO3 nanorods was dispersed in deionized water. 0.02 g of GO powders was dispersed rapidly in ethanol with ultrasonic. Then, the two solutions were mixed to yield a homogenous suspension. After that, the suspension was irradiated by a 150 W Xenon lamp for 4 h at room temperature. The final sample was centrifuged and dried in air. The XRD patterns obtained on a D/max-V X-ray diffractometer were used to determine phase structure. Fourier transform infrared (FT-IR) spectroscopy was recorded on a Bruker VECTOR22 FTIR spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed in a PHI 5000C ESCA System. Raman spectra were recorded on a microscopic confocal Raman spectrometer (Renishaw 1000 NR). The morphologies of the samples were evaluated by scanning electron microscope (SEM, Nova Nano230). The UV–vis spectra were obtained via UVvisible spectrophotometer (Shimadzu UV-2500, Japan). The photocatalytic activity of the sample was evaluated by degradation of MO under Xenon lamp (150 W) irradiation. The initial concentrations of MO and photocatalyst powders were 0.025 and 1 g/L, respectively. Prior to irradiation, the suspension
0167-577X/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.081
M. Zhou et al. / Materials Letters 89 (2012) 258–261
10
20
30
40
50
60
GO
WO3
70
80
617.38
Absorbance/a.u.
WO3@GE
3430
(401)
(002) (202) (220) (221) (400)
(201)
(200) (111)
(110) (101)
(100)
(001)
Intensity (a.u.)
WO3-GE
1601.98 1401.85
Fig. 1(a) shows the XRD patterns of WO3 nanorods and WO3@GE nanocomposites, both of which exhibited characteristic peaks of the tungsten oxide (WO3) (JCPDS card no. 33–1387) [5], indicating that WO3 nanorods did not change during the photoreduction of GO. Any peak of GE diffraction peaks was not observed in the nanocomposites due to extremely small amount of GE used. Fig. 1(b) shows the FTIR spectra of WO3 nanorods and WO3@GE nanocomposites. The characteristic absorption bands of GO were observed at 984.21 cm 1 (epoxy stretching),
1723.76 1624.76 1401.85 1224.89 1122.56 1045.43 984.21
3. Results and discussion
1045.43 cm 1 (alkoxy C–O stretching), 1224.89 cm 1 (phenolic C–OH stretching), 1401.85 cm 1 (carboxyl O¼C–OH stretching), and 1723.76 cm 1 (C¼O stretching vibrations of carboxyl or carbonyl groups) [9]. As compared to the peaks of the functional groups of GO, the broad absorption band of the WO3@GE nanocomposites at low frequencies was ascribed to the vibration of W–O–W bond. Moreover, the peaks at 1723.76 cm 1, 1224.89 cm 1, 1624.76 cm 1, 1045.43 cm 1 in the WO3@GE nanocomposites were not observed, indicating that the oxygen functional groups in GO were removed. In addition, the photocatalytic reduction of GO was also confirmed by the corresponding Raman spectra shown in Fig. 1(c). The Raman spectrum displayed two prominent peaks of the WO3@GE nanocomposites at around 1601 cm 1 and 1357 cm 1, which corresponded to the well-documented G and D bands, suggesting that the structure of GE was maintained in the nanocomposites [10]. The C1s XPS spectra of GO and the WO3@GE nanocomposites were shown in Fig. 2, respectively. In Fig. 2(a), the peak with a binding energy of 284.6 eV could be attributed to the C–C and
2350.20
was stirred in the dark to establish adsorption–desorption equilibrium. Once the concentration of MO had stabilized, the solution was exposed to Xenon lamp irradiation. Then 3 mL MO solution was extracted each 20 min for UV-visible spectrophotometer measurement at 465 nm.
259
4000 3500 3000 2500 2000 1500 1000 500 Wavenumber/cm-1
Intensity/a.u.
2θ( ° )
WO3@GE
WO3@GO
1000
1200
1400
1600
1800
2000
Raman Shift/cm-1 Fig. 1. (a) XRD patterns of WO3 and WO3@GE; (b) FTIR spectra of GO and WO3@GE; (c) Raman spectra of WO3@GO and WO3@GE.
C=C-C
C=O
C-O O-C=O
280 282 284 286 288 290 292 294 Binding Energy/eV
Intensity/a.u.
Intensity/a.u.
C=C-C
C-O C=O O-C=O
280 282 284 286 288 290 292 294 Binding Energy/eV
Fig. 2. The C1s XPS spectra of (a) GO and (b) WO3@GE.
260
M. Zhou et al. / Materials Letters 89 (2012) 258–261
2
1-WO3 2-WO3@GE
1
200 300 400 500 600 700 800 900 Wavelength (nm)
photocatalytic rate( C/C0)
Absorbance(a.u.)
Fig. 3. (a) SEM image WO3 nanorods and (b) WO3@GE.
1.0 WO3 WO3@GE
0.8 0.6 0.4 0.2 0.0
0
20
40
60 80 Time (min)
100 120
Fig. 4. (a) UV–vis diffuse reflectance spectra of WO3 and WO3@GE; (b) Photocatalytic degradation rate of WO3 and WO3@GE.
C–H bonds, while the other three peaks centered at the binding energies of 286.3, 287.7, and 289.2 eV could be assigned to the C–O, C¼O and O¼C–OH functional groups, respectively [11–12]. The C1s XPS spectra of the WO3@GE nanocomposites (Fig. 2(b)) also exhibited the same oxygen functionalities, while the peaks at 287.7 eV decreased remarkably suggesting that considerable the C¼O groups were removed. The SEM image of WO3 nanorods and WO3@GE nanocomposites was shown in Fig. 3. The SEM image shown in Fig. 3(a) exhibited that the product was mainly composed of numerous rod-like structures. Several thinner nanorods assembled together along the axis direction formed nanorod bundles. The WO3 nanorods and GE nanosheets in the WO3@GE nanocomposites could be clearly seen in Fig. 3(b). The UV–vis diffuse reflectance spectra for the WO3 nanorods and WO3@GE nanocomposites were shown in Fig. 4(a). The WO3 nanorods showed a sharp edge at about 465 nm, whereas the WO3@GE nanocomposites displayed a little red shift in the absorption edge. Moreover, the absorption intensity of the WO3@GE nanocomposites in visible region was higher than that of the WO3 nanorods. Therefore, we could infer that the introduction of GE in WO3 nanorods was effective for the visible light response of the photocatalyst. The photocatalytic activities of the WO3 nanorods and WO3@GE nanocomposites were evaluated by the degradation of MO in aqueous solution, and the results were shown in Fig. 4(b). For bare WO3 nanorods, a relatively low photocatalytic degradation rate was observed as expected due to the rapid recombination of conduction band (CB) electrons and valence band (VB) holes [13]. Compared with bare WO3 nanorods, the photocatalytic activity of the WO3@GE nanocomposites was enhanced, and the
photocatalytic degradation rate was 92.7% under Xenon lamp irradiation within 120 min. It was attributed to GE served as an acceptor of the electrons generated in the WO3 and effectively decreased the recombination probability of the photogenerated electron–hole pairs.
4. Conclusions In summary, WO3@GE nanocomposites were synthesized via a photoreduction method. The functional groups present in GO such as carboxyl (C¼O) were mostly reduced in the WO3@GE nanocomposites. The WO3@GE nanocomposites exhibited higher photocatalytic activity compared to bare WO3 nanorods.
Acknowledgment This work was supported by the natural science foundation of China (Grant No. 21171175).This work was also financially supported by the department of education of Hunan province, China (Grant No. 11B054). References [1] Sun SM, Wang WZ, Zeng SZ, Shang M, Zhang L. J Hazard Mater 2010; 178:427–33. [2] Chang XT, Sun SB, Xu X, Li ZJ. Mater Lett 2011;65:1710-1712. [3] Cheng P, Yang Z, Wang H, Cheng W, Chen MX, Shangguan WF, et al. Int J Hydrogen Energy 2012;37:2224–30. [4] An XQ, Yu JC, Wang Y, Hu YM, Yu XL, Zhang GJ. J Mater Chem 2012; 22:8525–31.
M. Zhou et al. / Materials Letters 89 (2012) 258–261
[5] Song XC, Yang E, Liu G, Zhang Y, Liu ZS, Chen HF, et al. J Nanopart Res 2010; 12:2813–9. [6] Liu JC, Bai HW, Wang YJ, Liu ZY, Zhang XW, Sun DL. Adv Funct Mater 2010; 20:4175–81. [7] Ying H, Wang ZY, Guo ZD, Shi ZJ, Yang SF. Acta Phys Chim Sin 2011; 27:1482–6. [8] Wang F, Zhang K, Mol J. Catal A Chem 2011;345:101–7.
261
[9] Li Q, Guo BD, Yu JG, Ran JR, Zhang BH, Yan HJ, et al. J Am Chem Soc 2011;133:10878–84. [10] Qin JW, Cao MH, Li N, Hu CW. J Mater Chem 2011;21:17167–74. [11] Wu TS, Liu S, Luo YL, Lu WB, Wang L, Sun XP. Nanoscale 2011;3:2142–4. [12] Chandra V, Park J, Chun Y, Lee JW, Hwang IC, Kim KS. ACS Nano 2010;4: 3979–86. [13] Xie YP, Liu G, Yin LC, Cheng HM. J Mater Chem 2012;22:6746–51.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and enhanced photocatalytic performance of WO3 nanorods @ graphene nanocomposites Minjie Zhou a,b, Jianhui Yan a,n, Peng Cui b,nn a b
School of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Hunan 414006, PR China School of Chemical Engineering, Hefei University of Technology, Anhui 230009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2012 Accepted 19 August 2012 Available online 31 August 2012
WO3 nanorods @ graphene (WO3@GE) nanocomposites were successfully synthesized via a photoreduction method. The obtained samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), Raman spectroscopy, and ultraviolet-visible diffuse reflectance spectroscopy (DRS) techniques. The functional groups present in graphene oxide (GO) such as carboxyl (C ¼O) were mostly reduced in the WO3@GE nanocomposites. The photocatalytic activity of the samples was evaluated by the degradation of methyl orange (MO). The results showed that the WO3@GE nanocomposites exhibited higher photocatalytic activity compared to bare WO3 nanorods. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.
Keywords: Carbon materials Solar energy materials Nanocomposites
1. Introduction Nanostructured tungsten oxide (WO3), as an important n type semiconductor with a band gap of 2.8 eV, has attracted considerable interest in photocatalysis due to its promising physical and chemical properties [1,2]. However, WO3 nanomaterials are usually not efficient photocatalysts because of the high electron–hole recombination rate [3]. Recently, many attempts have been made to improve the efficiency of electron–hole pair separation in WO3 nanomaterials, such as doping [4] and composites [5]. Recently, graphene-based composites have received considerable attention due to their potential application in the photocatalytic field [6–9]. In comparison with bare photocatalysts, graphene-based composites exhibit higher photocatalytic property because graphene has perfect two-dimensional carbon structure with better conductivity and larger surface area [6]. In general, graphene-based composites are prepared from the reduction of graphene oxide (GO)-based composites through the chemical reduction method, the flash reduction method or the solvothermal method, etc. However, photoreduction of GO-based composites has recently proven to be an effective method to produce graphene-based composites [10,12]. In this work, WO3@GE nanocomposites were synthesized via a photoreduction
n
Corresponding author. Tel./fax: þ 86 0730 8640122. Corresponding author. Tel./fax: þ 86 0551 2901450. E-mail addresses: [email protected] (J. Yan), [email protected] (P. Cui).
nn
method. The photocatalytic activity of the WO3@GE nanocomposites was evaluated by the degradation of methyl orange (MO).
2. Experimental details GO was synthesized from natural graphite powder based on modified Hummers method [7]. WO3 nanorods were synthesized via a hydrothermal method [5]. The WO3@GE nanocomposites were synthesized via a photoreduction method. Typically, 1 g of WO3 nanorods was dispersed in deionized water. 0.02 g of GO powders was dispersed rapidly in ethanol with ultrasonic. Then, the two solutions were mixed to yield a homogenous suspension. After that, the suspension was irradiated by a 150 W Xenon lamp for 4 h at room temperature. The final sample was centrifuged and dried in air. The XRD patterns obtained on a D/max-V X-ray diffractometer were used to determine phase structure. Fourier transform infrared (FT-IR) spectroscopy was recorded on a Bruker VECTOR22 FTIR spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed in a PHI 5000C ESCA System. Raman spectra were recorded on a microscopic confocal Raman spectrometer (Renishaw 1000 NR). The morphologies of the samples were evaluated by scanning electron microscope (SEM, Nova Nano230). The UV–vis spectra were obtained via UVvisible spectrophotometer (Shimadzu UV-2500, Japan). The photocatalytic activity of the sample was evaluated by degradation of MO under Xenon lamp (150 W) irradiation. The initial concentrations of MO and photocatalyst powders were 0.025 and 1 g/L, respectively. Prior to irradiation, the suspension
0167-577X/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.081
M. Zhou et al. / Materials Letters 89 (2012) 258–261
10
20
30
40
50
60
GO
WO3
70
80
617.38
Absorbance/a.u.
WO3@GE
3430
(401)
(002) (202) (220) (221) (400)
(201)
(200) (111)
(110) (101)
(100)
(001)
Intensity (a.u.)
WO3-GE
1601.98 1401.85
Fig. 1(a) shows the XRD patterns of WO3 nanorods and WO3@GE nanocomposites, both of which exhibited characteristic peaks of the tungsten oxide (WO3) (JCPDS card no. 33–1387) [5], indicating that WO3 nanorods did not change during the photoreduction of GO. Any peak of GE diffraction peaks was not observed in the nanocomposites due to extremely small amount of GE used. Fig. 1(b) shows the FTIR spectra of WO3 nanorods and WO3@GE nanocomposites. The characteristic absorption bands of GO were observed at 984.21 cm 1 (epoxy stretching),
1723.76 1624.76 1401.85 1224.89 1122.56 1045.43 984.21
3. Results and discussion
1045.43 cm 1 (alkoxy C–O stretching), 1224.89 cm 1 (phenolic C–OH stretching), 1401.85 cm 1 (carboxyl O¼C–OH stretching), and 1723.76 cm 1 (C¼O stretching vibrations of carboxyl or carbonyl groups) [9]. As compared to the peaks of the functional groups of GO, the broad absorption band of the WO3@GE nanocomposites at low frequencies was ascribed to the vibration of W–O–W bond. Moreover, the peaks at 1723.76 cm 1, 1224.89 cm 1, 1624.76 cm 1, 1045.43 cm 1 in the WO3@GE nanocomposites were not observed, indicating that the oxygen functional groups in GO were removed. In addition, the photocatalytic reduction of GO was also confirmed by the corresponding Raman spectra shown in Fig. 1(c). The Raman spectrum displayed two prominent peaks of the WO3@GE nanocomposites at around 1601 cm 1 and 1357 cm 1, which corresponded to the well-documented G and D bands, suggesting that the structure of GE was maintained in the nanocomposites [10]. The C1s XPS spectra of GO and the WO3@GE nanocomposites were shown in Fig. 2, respectively. In Fig. 2(a), the peak with a binding energy of 284.6 eV could be attributed to the C–C and
2350.20
was stirred in the dark to establish adsorption–desorption equilibrium. Once the concentration of MO had stabilized, the solution was exposed to Xenon lamp irradiation. Then 3 mL MO solution was extracted each 20 min for UV-visible spectrophotometer measurement at 465 nm.
259
4000 3500 3000 2500 2000 1500 1000 500 Wavenumber/cm-1
Intensity/a.u.
2θ( ° )
WO3@GE
WO3@GO
1000
1200
1400
1600
1800
2000
Raman Shift/cm-1 Fig. 1. (a) XRD patterns of WO3 and WO3@GE; (b) FTIR spectra of GO and WO3@GE; (c) Raman spectra of WO3@GO and WO3@GE.
C=C-C
C=O
C-O O-C=O
280 282 284 286 288 290 292 294 Binding Energy/eV
Intensity/a.u.
Intensity/a.u.
C=C-C
C-O C=O O-C=O
280 282 284 286 288 290 292 294 Binding Energy/eV
Fig. 2. The C1s XPS spectra of (a) GO and (b) WO3@GE.
260
M. Zhou et al. / Materials Letters 89 (2012) 258–261
2
1-WO3 2-WO3@GE
1
200 300 400 500 600 700 800 900 Wavelength (nm)
photocatalytic rate( C/C0)
Absorbance(a.u.)
Fig. 3. (a) SEM image WO3 nanorods and (b) WO3@GE.
1.0 WO3 WO3@GE
0.8 0.6 0.4 0.2 0.0
0
20
40
60 80 Time (min)
100 120
Fig. 4. (a) UV–vis diffuse reflectance spectra of WO3 and WO3@GE; (b) Photocatalytic degradation rate of WO3 and WO3@GE.
C–H bonds, while the other three peaks centered at the binding energies of 286.3, 287.7, and 289.2 eV could be assigned to the C–O, C¼O and O¼C–OH functional groups, respectively [11–12]. The C1s XPS spectra of the WO3@GE nanocomposites (Fig. 2(b)) also exhibited the same oxygen functionalities, while the peaks at 287.7 eV decreased remarkably suggesting that considerable the C¼O groups were removed. The SEM image of WO3 nanorods and WO3@GE nanocomposites was shown in Fig. 3. The SEM image shown in Fig. 3(a) exhibited that the product was mainly composed of numerous rod-like structures. Several thinner nanorods assembled together along the axis direction formed nanorod bundles. The WO3 nanorods and GE nanosheets in the WO3@GE nanocomposites could be clearly seen in Fig. 3(b). The UV–vis diffuse reflectance spectra for the WO3 nanorods and WO3@GE nanocomposites were shown in Fig. 4(a). The WO3 nanorods showed a sharp edge at about 465 nm, whereas the WO3@GE nanocomposites displayed a little red shift in the absorption edge. Moreover, the absorption intensity of the WO3@GE nanocomposites in visible region was higher than that of the WO3 nanorods. Therefore, we could infer that the introduction of GE in WO3 nanorods was effective for the visible light response of the photocatalyst. The photocatalytic activities of the WO3 nanorods and WO3@GE nanocomposites were evaluated by the degradation of MO in aqueous solution, and the results were shown in Fig. 4(b). For bare WO3 nanorods, a relatively low photocatalytic degradation rate was observed as expected due to the rapid recombination of conduction band (CB) electrons and valence band (VB) holes [13]. Compared with bare WO3 nanorods, the photocatalytic activity of the WO3@GE nanocomposites was enhanced, and the
photocatalytic degradation rate was 92.7% under Xenon lamp irradiation within 120 min. It was attributed to GE served as an acceptor of the electrons generated in the WO3 and effectively decreased the recombination probability of the photogenerated electron–hole pairs.
4. Conclusions In summary, WO3@GE nanocomposites were synthesized via a photoreduction method. The functional groups present in GO such as carboxyl (C¼O) were mostly reduced in the WO3@GE nanocomposites. The WO3@GE nanocomposites exhibited higher photocatalytic activity compared to bare WO3 nanorods.
Acknowledgment This work was supported by the natural science foundation of China (Grant No. 21171175).This work was also financially supported by the department of education of Hunan province, China (Grant No. 11B054). References [1] Sun SM, Wang WZ, Zeng SZ, Shang M, Zhang L. J Hazard Mater 2010; 178:427–33. [2] Chang XT, Sun SB, Xu X, Li ZJ. Mater Lett 2011;65:1710-1712. [3] Cheng P, Yang Z, Wang H, Cheng W, Chen MX, Shangguan WF, et al. Int J Hydrogen Energy 2012;37:2224–30. [4] An XQ, Yu JC, Wang Y, Hu YM, Yu XL, Zhang GJ. J Mater Chem 2012; 22:8525–31.
M. Zhou et al. / Materials Letters 89 (2012) 258–261
[5] Song XC, Yang E, Liu G, Zhang Y, Liu ZS, Chen HF, et al. J Nanopart Res 2010; 12:2813–9. [6] Liu JC, Bai HW, Wang YJ, Liu ZY, Zhang XW, Sun DL. Adv Funct Mater 2010; 20:4175–81. [7] Ying H, Wang ZY, Guo ZD, Shi ZJ, Yang SF. Acta Phys Chim Sin 2011; 27:1482–6. [8] Wang F, Zhang K, Mol J. Catal A Chem 2011;345:101–7.
261
[9] Li Q, Guo BD, Yu JG, Ran JR, Zhang BH, Yan HJ, et al. J Am Chem Soc 2011;133:10878–84. [10] Qin JW, Cao MH, Li N, Hu CW. J Mater Chem 2011;21:17167–74. [11] Wu TS, Liu S, Luo YL, Lu WB, Wang L, Sun XP. Nanoscale 2011;3:2142–4. [12] Chandra V, Park J, Chun Y, Lee JW, Hwang IC, Kim KS. ACS Nano 2010;4: 3979–86. [13] Xie YP, Liu G, Yin LC, Cheng HM. J Mater Chem 2012;22:6746–51.