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Growth of porous ZnO nanosheets by electrodeposition with the addition of KBr in nitrate electrolyte
Materials Letters 89 (2012) 283–286
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Growth of porous ZnO nanosheets by electrodeposition with the addition of KBr in nitrate electrolyte Qin Hou, Liqun Zhu, Haining Chen, Huicong Liu, Weiping Li n Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e i n f o
abstract
Article history: Received 22 April 2012 Accepted 29 August 2012 Available online 8 September 2012
Porous ZnO nanosheets show much prospect in many applications. In this letter, highly porous ZnO nanosheets with new and interesting morphology were synthesized by electrodeposition followed by annealing process. The electrodeposition of the nanosheets was conducted by introducing 1 M KBr into 0.1 M Zn(NO3)2 electrolyte. The obtained porous nanosheet is polycrystalline structure composed of ZnO nanoparticles with the grain size about tens of nanometers. The porous ZnO nanosheets were measured to be an n-type semiconductor with a band gap of 3.2 eV. It is expected that the porous ZnO nanosheets could exhibit a good performance in photocatalysis, sensors and photoelectrochemical cells. & 2012 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline materials Porous materials Semiconductors Electrodeposition
1. Introduction Porous ZnO nanosheet has attracted much interest because it shows much prospect in applications in photocatalysis [1], sensors [2,3] and photoelectrochemical cells [4,5] due to its unique advantages, such as high porosity, large surface area and enhanced light scattering capacity [6,7]. Many methods have been used to prepare porous ZnO nanosheet including chemical bath deposition (CBD) [8,9], hydrothermal synthesis [3,7] and electrodeposition [6]. For example, Lai et al. [8] synthesized porous ZnO nanosheet via CBD method and dyesensitized solar cells based on this structure achieved high conversion efficiency. Jing and Zhan [10] obtained porous ZnO nanosheet using hydrothermal method which exhibited good performance in gas sensor. Liu et al. [7] reported on the hydrothermal preparation of porous ZnO nanosheet using Zn5(CO3)2(OH)6 as precursor and its good photocatalytic activities. Compared with the two methods mentioned above, electrodeposition has recently triggered considerable interest owing to its intrinsic advantages, such as the precise control over the film morphology and thickness, high deposition rate, direct growth on substrate with good adhesion, arbitrary substrate shapes, mass production, low cost and so on [5,11,12]. In our previous work [6], porous ZnO nanosheets have been achieved by electrodeposition by adding KCl into Zn(NO3)2 electrolyte. And due to porous and multi-layer structure, the porous ZnO nanosheets exhibited large surface area and enhanced light scattering capacity. As a result,
n
Corresponding author. Tel./fax: þ 86 1082317113. E-mail addresses: [email protected], [email protected] (W. Li). 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.137
the porous ZnO nanosheets performed really well as the photoelectrode of quantum dots-sensitized solar cells. Based on the above discussion, the porous ZnO nanosheets prepared by electrodeposition have shown much promise. However, so far few studies on this topic have been reported and the morphology is too single. Therefore, it is meaningful to further explore and study the porous ZnO nanosheets with new and wellcontrolled morphology by electrodeposition. In this letter, we report on preparing and studying the highly porous ZnO nanosheets with new and interesting morphology using electrodeposition by introducing KBr into Zn(NO3)2 electrolyte followed by annealing process. The highly porous ZnO nanosheets with new and interesting morphology are expected to be widely applied in photocatalysis, sensors and photoelectrochemical cells.
2. Experimental The as-deposited nanosheets were in situ grown on ITO conducting glass by electrodeposition in the electrolyte containing 0.1 M Zn(NO3)2 and 1 M KBr. The electrodeposition experiment was carried out in a two-electrode cell system with a plate of pure zinc as counter electrode, and the temperature was controlled at 75 1C by immersing the cell into a water bath. The deposition potential was fixed at 0.8 V (vs. saturated calomel electrode (SCE)) and the electrodeposition duration is 30 min. The as-deposited nanosheets were annealed at 250 1C for 30 min in air atmosphere to obtain porous ZnO nanosheets. The field-emission gun scanning electron microscope (FESEM, Apollo 300) was used to evaluate the morphology of the samples. The powder X-ray diffractometry (Rigaku D/MAX-RB) with CuKa ˚ was used to indentify the crystal phase radiation (Ka ¼1.5418 A)
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of the sample. The transmission electron microscopy (TEM, JEM2100 F) was also applied to characterize the sample. The ultraviolet–visible (UV–vis) absorption spectrum was recorded on a GBC spectrometer (Cintra 10e) equipped with an integrating sphere attachment in the wavelength range of 200–800 nm. The CHI 600 A electrochemical analyzer was employed to record Mott–Shocktty plot and a typical three-electrode system was used in this measurement, in which the porous ZnO nanosheets, platinum foil and SCE served as working electrode, counter electrode and reference electrode, respectively. The electrolyte was the propylene carbonate solution of 0.1 M lithium perchlorate and the frequency was set at 1000 Hz.
3. Results and discussion Fig. 1(A) presents the surface morphology of as-deposited nanosheets, clearly showing the compact sheet-like nanostructure. And the cross-section view presented in the inset also reveals the nanosheet structure of the as-deposited film with a thickness in the range of 30–40 mm. Besides, some novel and regular structures such as cones and flowers can also be observed, as shown in Fig. 1(A1)– (A3). These novel structures are about tens of micrometers in width and consist of stacked and overlapped nanosheets that show distinct edges and hexagonal shape and are about tens of nanometers in thickness. On further detailed observation, it is easy to be found that the surfaces of the nanosheets are very smooth. After heat treatment, the overall morphology of nanosheets (Fig. 1(B)) is similar to that of as-deposited nanosheets. And the novel structures composed of nanosheets are well maintained, as shown in Fig. 1(C). However, the high magnification SEM image in Fig. 1(C) reveals that the surfaces of the nanosheets become rough and some pores can be observed on the nanosheets. The higher magnification SEM image in Fig. 1(D) clearly exhibits the highly porous structure. However, the dimension of the pores is still too small to be determined in the SEM image. In comparison to our previous work [6], the porous ZnO nanosheets obtained here possess a different morphology with new and diverse structures, and the nanosheets are more regular and
thinner than those in Ref. [6]. Compared with Lai’s [8] and Liu’s reports [7] in which ZnO nanosheets were synthesized by CBD and hydrothermal method, respectively, the porous nanosheets here are more regular in shape with distinct edges and defined corners. Moreover, electrodeposition exhibits much higher deposition rate than CBD method, with a film thickness of about 30–40 mm in 30 min for the former in comparision to about 18–33 mm in 4 h for the latter [8]. Fig.2(A) presents the XRD pattern of the annealed nanosheets. The peaks at 31.781, 34.421, 36.281, 47.521, 56.521, 62.821, 66.341, 67.921 and 69.081 in the XRD pattern can be all indexed as hexagonal ZnO (JCPDS card no. 36-1451), indicating the annealed nanosheets are mainly composed of hexagonal ZnO. Further structure analysis of the annealed nanosheets was investigated by TEM. Fig. 2(B) shows the TEM image of a single nanosheet, indicating that the nanosheet is highly porous and composed of small nanoparticles with the grain size of about tens of nanometers. And the distribution of pores in ZnO nanosheets obtained here is more uniform and regular than that in Liu’s report [7]. The electron diffraction (ED) pattern in the inset presents diffraction rings, suggesting the polycrystalline nature of the porous nanosheet. A high-resolution TEM (HRTEM) image taken from the edge of the porous nanosheet is shown in Fig. 2(C). The lattice spacing is measured to be 0.281 nm that is well consistent with the d-spacing of the (100) planes of ZnO. ZnO is a common and important semiconductor widely applied in various fields. To further study the light absorption and semiconductor behavior, the UV–vis diffuse reflectance spectrum and Mott–Schoktty plot of the obtained porous ZnO nanosheets were recorded. The UV–vis diffuse reflectance data were converted by instrument software to absorbance values, F(R), based on the Kubelka–Munk theory [13], as shown in Fig. 3A. An obvious enhancement in the absorption at wavelengths less than 380 nm can be assigned to the intrinsic band gap absorption of ZnO due to the electronic transitions from the valence to the conduction band (O2p/Zn3d) [7]. And based on the relationship of the absorption coefficient with incident photon energy [13] a(hn)¼c(hn-Eg)n, where a is the absorption coefficient, hn is the
Fig. 1. SEM images of (A) as-deposited nanosheets and (B, C and D) the nanosheets after annealing. The inset in (A) is the corresponding cross-section view. (A1), (A2) and (A3) are the high magnification images of three particular regions in (A).
Q. Hou et al. / Materials Letters 89 (2012) 283–286
285
Fig. 2. (A) XRD pattern and (B) TEM image of the annealed nanosheets, (C) HRTEM image of the edge at annealed nanosheets. The inset in (B) is the corresponding ED pattern.
Fig. 3. (A) UV–vis absorption spectrum and (B) Mott-Schottky plot of porous ZnO nanosheets. The inset in (A) is (F(R)hv)2 vs. hv plot of porous ZnO nanosheets.
energy of the incident photon, Eg is the band gap of semiconductor, n is 0.5 for a direct transition semiconductor, e.g. ZnO. Since a is proportional to F(R), namely, F(R)¼ a/s (s, the scattering coefficient), the energy intercept of the curve of (F(R)hn)2 vs. hn gives Eg when the linear region is extrapolated to the zero ordinate, the band gap is further calculated to be 3.2 eV as shown in the inset (Fig. 3(A)). Fig. 3(B) presents the Mott–Schoktty plot measured at the interface between the electrolyte and the obtained porous ZnO nanosheets. A visible onset on the Mott–Schottky plot in Fig. 3(B) can be observed and according to Mott–Schottky equation [14] 1=C 2 ¼ 7
2 eee0 A2 ND=A
ðVV FB
kT Þ e
(7: ‘‘þ’’ represents n-type and ‘‘’’ represents p-type, where C is the capacitance of the space charge region, e is the electron charge, e is the dielectric constant of the semiconductor, e0 is the permittivity
of vacuum, A is the working electrode area, ND/A is the carrier (donor or acceptor) concentration, V is the applied potential, and VFB is the flatband potential), the positive slope displayed in Fig. 3(B) indicates the porous ZnO nanosheets are n-type semiconducting.
4. Conclusions Highly porous ZnO nanosheets with new and interesting morphology have been prepared through electrodeposition by introducing KBr into Zn(NO3)2 electrolyte followed by annealing process. The obtained porous nanosheets were composed of small ZnO nanoparticles with tens of nanometers in size. The light absorption and semiconductor behavior of the porous ZnO nanosheet were investigated and it was measured to be n-type semiconducting with a band gap of 3.2 eV. And the highly porous ZnO nanosheets are expected to perform well in the applications such as photocatalysis, sensor, photoelectrochemical cells and so on.
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References [1] Lei A, Qu B, Zhou W, Wang Y, Zhang Q, Zou B. Mater Lett 2012;66:72–5. [2] Li J, Fan H, Jia X. J Phys Chem C 2010;114:14684–91. [3] Gu C, Huang J, Wu Y, Zhai M, Sun Y, Liu J. J Alloys Compd 2011;509: 4499–504. [4] Hosono E, Fujihara S, Honma I, Zhou H. Adv Mater 2005;17:2091–4. [5] Zhang Q, Dandeneau CS, Zhou X, Cao G. Adv Mater 2009;21:4087–108. [6] Chen H, Li W, Liu H, Zhu L. Electrochem Commun 2011;13:331–4. [7] Liu S, Li C, Yu J, Xiang Q. CrystEngComm 2011;13:2533–41.
[8] Lai Y, Lin C, Chen H, Chen J, Kung C, Vittal R, et al. J Mater Chem 2010;20:9379–85. [9] Huang XH, Xia XH, Yuan YF, Zhou F. Electrochim Acta 2011;56:4960–5. [10] Jing Z, Zhan J. Adv Mater 2008;20:4547–51. [11] Xu F, Sun L. Energy Environ Sci 2011;4:818–41. [12] Xu L, Guo Y, Liao Q, Zhang J, Xu D. J Phys Chem B 2005;109:13519–22. [13] Chen H, Li W, Liu H, Zhu L. Microporous Mesoporous Mater 2011;138:235–8. [14] Yang X, Wolcott A, Wang G, Sobo A, Fitzmorris RC, Qian F, et al. Nano Lett 2009;9:2331–6.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Growth of porous ZnO nanosheets by electrodeposition with the addition of KBr in nitrate electrolyte Qin Hou, Liqun Zhu, Haining Chen, Huicong Liu, Weiping Li n Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e i n f o
abstract
Article history: Received 22 April 2012 Accepted 29 August 2012 Available online 8 September 2012
Porous ZnO nanosheets show much prospect in many applications. In this letter, highly porous ZnO nanosheets with new and interesting morphology were synthesized by electrodeposition followed by annealing process. The electrodeposition of the nanosheets was conducted by introducing 1 M KBr into 0.1 M Zn(NO3)2 electrolyte. The obtained porous nanosheet is polycrystalline structure composed of ZnO nanoparticles with the grain size about tens of nanometers. The porous ZnO nanosheets were measured to be an n-type semiconductor with a band gap of 3.2 eV. It is expected that the porous ZnO nanosheets could exhibit a good performance in photocatalysis, sensors and photoelectrochemical cells. & 2012 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline materials Porous materials Semiconductors Electrodeposition
1. Introduction Porous ZnO nanosheet has attracted much interest because it shows much prospect in applications in photocatalysis [1], sensors [2,3] and photoelectrochemical cells [4,5] due to its unique advantages, such as high porosity, large surface area and enhanced light scattering capacity [6,7]. Many methods have been used to prepare porous ZnO nanosheet including chemical bath deposition (CBD) [8,9], hydrothermal synthesis [3,7] and electrodeposition [6]. For example, Lai et al. [8] synthesized porous ZnO nanosheet via CBD method and dyesensitized solar cells based on this structure achieved high conversion efficiency. Jing and Zhan [10] obtained porous ZnO nanosheet using hydrothermal method which exhibited good performance in gas sensor. Liu et al. [7] reported on the hydrothermal preparation of porous ZnO nanosheet using Zn5(CO3)2(OH)6 as precursor and its good photocatalytic activities. Compared with the two methods mentioned above, electrodeposition has recently triggered considerable interest owing to its intrinsic advantages, such as the precise control over the film morphology and thickness, high deposition rate, direct growth on substrate with good adhesion, arbitrary substrate shapes, mass production, low cost and so on [5,11,12]. In our previous work [6], porous ZnO nanosheets have been achieved by electrodeposition by adding KCl into Zn(NO3)2 electrolyte. And due to porous and multi-layer structure, the porous ZnO nanosheets exhibited large surface area and enhanced light scattering capacity. As a result,
n
Corresponding author. Tel./fax: þ 86 1082317113. E-mail addresses: [email protected], [email protected] (W. Li). 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.137
the porous ZnO nanosheets performed really well as the photoelectrode of quantum dots-sensitized solar cells. Based on the above discussion, the porous ZnO nanosheets prepared by electrodeposition have shown much promise. However, so far few studies on this topic have been reported and the morphology is too single. Therefore, it is meaningful to further explore and study the porous ZnO nanosheets with new and wellcontrolled morphology by electrodeposition. In this letter, we report on preparing and studying the highly porous ZnO nanosheets with new and interesting morphology using electrodeposition by introducing KBr into Zn(NO3)2 electrolyte followed by annealing process. The highly porous ZnO nanosheets with new and interesting morphology are expected to be widely applied in photocatalysis, sensors and photoelectrochemical cells.
2. Experimental The as-deposited nanosheets were in situ grown on ITO conducting glass by electrodeposition in the electrolyte containing 0.1 M Zn(NO3)2 and 1 M KBr. The electrodeposition experiment was carried out in a two-electrode cell system with a plate of pure zinc as counter electrode, and the temperature was controlled at 75 1C by immersing the cell into a water bath. The deposition potential was fixed at 0.8 V (vs. saturated calomel electrode (SCE)) and the electrodeposition duration is 30 min. The as-deposited nanosheets were annealed at 250 1C for 30 min in air atmosphere to obtain porous ZnO nanosheets. The field-emission gun scanning electron microscope (FESEM, Apollo 300) was used to evaluate the morphology of the samples. The powder X-ray diffractometry (Rigaku D/MAX-RB) with CuKa ˚ was used to indentify the crystal phase radiation (Ka ¼1.5418 A)
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Q. Hou et al. / Materials Letters 89 (2012) 283–286
of the sample. The transmission electron microscopy (TEM, JEM2100 F) was also applied to characterize the sample. The ultraviolet–visible (UV–vis) absorption spectrum was recorded on a GBC spectrometer (Cintra 10e) equipped with an integrating sphere attachment in the wavelength range of 200–800 nm. The CHI 600 A electrochemical analyzer was employed to record Mott–Shocktty plot and a typical three-electrode system was used in this measurement, in which the porous ZnO nanosheets, platinum foil and SCE served as working electrode, counter electrode and reference electrode, respectively. The electrolyte was the propylene carbonate solution of 0.1 M lithium perchlorate and the frequency was set at 1000 Hz.
3. Results and discussion Fig. 1(A) presents the surface morphology of as-deposited nanosheets, clearly showing the compact sheet-like nanostructure. And the cross-section view presented in the inset also reveals the nanosheet structure of the as-deposited film with a thickness in the range of 30–40 mm. Besides, some novel and regular structures such as cones and flowers can also be observed, as shown in Fig. 1(A1)– (A3). These novel structures are about tens of micrometers in width and consist of stacked and overlapped nanosheets that show distinct edges and hexagonal shape and are about tens of nanometers in thickness. On further detailed observation, it is easy to be found that the surfaces of the nanosheets are very smooth. After heat treatment, the overall morphology of nanosheets (Fig. 1(B)) is similar to that of as-deposited nanosheets. And the novel structures composed of nanosheets are well maintained, as shown in Fig. 1(C). However, the high magnification SEM image in Fig. 1(C) reveals that the surfaces of the nanosheets become rough and some pores can be observed on the nanosheets. The higher magnification SEM image in Fig. 1(D) clearly exhibits the highly porous structure. However, the dimension of the pores is still too small to be determined in the SEM image. In comparison to our previous work [6], the porous ZnO nanosheets obtained here possess a different morphology with new and diverse structures, and the nanosheets are more regular and
thinner than those in Ref. [6]. Compared with Lai’s [8] and Liu’s reports [7] in which ZnO nanosheets were synthesized by CBD and hydrothermal method, respectively, the porous nanosheets here are more regular in shape with distinct edges and defined corners. Moreover, electrodeposition exhibits much higher deposition rate than CBD method, with a film thickness of about 30–40 mm in 30 min for the former in comparision to about 18–33 mm in 4 h for the latter [8]. Fig.2(A) presents the XRD pattern of the annealed nanosheets. The peaks at 31.781, 34.421, 36.281, 47.521, 56.521, 62.821, 66.341, 67.921 and 69.081 in the XRD pattern can be all indexed as hexagonal ZnO (JCPDS card no. 36-1451), indicating the annealed nanosheets are mainly composed of hexagonal ZnO. Further structure analysis of the annealed nanosheets was investigated by TEM. Fig. 2(B) shows the TEM image of a single nanosheet, indicating that the nanosheet is highly porous and composed of small nanoparticles with the grain size of about tens of nanometers. And the distribution of pores in ZnO nanosheets obtained here is more uniform and regular than that in Liu’s report [7]. The electron diffraction (ED) pattern in the inset presents diffraction rings, suggesting the polycrystalline nature of the porous nanosheet. A high-resolution TEM (HRTEM) image taken from the edge of the porous nanosheet is shown in Fig. 2(C). The lattice spacing is measured to be 0.281 nm that is well consistent with the d-spacing of the (100) planes of ZnO. ZnO is a common and important semiconductor widely applied in various fields. To further study the light absorption and semiconductor behavior, the UV–vis diffuse reflectance spectrum and Mott–Schoktty plot of the obtained porous ZnO nanosheets were recorded. The UV–vis diffuse reflectance data were converted by instrument software to absorbance values, F(R), based on the Kubelka–Munk theory [13], as shown in Fig. 3A. An obvious enhancement in the absorption at wavelengths less than 380 nm can be assigned to the intrinsic band gap absorption of ZnO due to the electronic transitions from the valence to the conduction band (O2p/Zn3d) [7]. And based on the relationship of the absorption coefficient with incident photon energy [13] a(hn)¼c(hn-Eg)n, where a is the absorption coefficient, hn is the
Fig. 1. SEM images of (A) as-deposited nanosheets and (B, C and D) the nanosheets after annealing. The inset in (A) is the corresponding cross-section view. (A1), (A2) and (A3) are the high magnification images of three particular regions in (A).
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Fig. 2. (A) XRD pattern and (B) TEM image of the annealed nanosheets, (C) HRTEM image of the edge at annealed nanosheets. The inset in (B) is the corresponding ED pattern.
Fig. 3. (A) UV–vis absorption spectrum and (B) Mott-Schottky plot of porous ZnO nanosheets. The inset in (A) is (F(R)hv)2 vs. hv plot of porous ZnO nanosheets.
energy of the incident photon, Eg is the band gap of semiconductor, n is 0.5 for a direct transition semiconductor, e.g. ZnO. Since a is proportional to F(R), namely, F(R)¼ a/s (s, the scattering coefficient), the energy intercept of the curve of (F(R)hn)2 vs. hn gives Eg when the linear region is extrapolated to the zero ordinate, the band gap is further calculated to be 3.2 eV as shown in the inset (Fig. 3(A)). Fig. 3(B) presents the Mott–Schoktty plot measured at the interface between the electrolyte and the obtained porous ZnO nanosheets. A visible onset on the Mott–Schottky plot in Fig. 3(B) can be observed and according to Mott–Schottky equation [14] 1=C 2 ¼ 7
2 eee0 A2 ND=A
ðVV FB
kT Þ e
(7: ‘‘þ’’ represents n-type and ‘‘’’ represents p-type, where C is the capacitance of the space charge region, e is the electron charge, e is the dielectric constant of the semiconductor, e0 is the permittivity
of vacuum, A is the working electrode area, ND/A is the carrier (donor or acceptor) concentration, V is the applied potential, and VFB is the flatband potential), the positive slope displayed in Fig. 3(B) indicates the porous ZnO nanosheets are n-type semiconducting.
4. Conclusions Highly porous ZnO nanosheets with new and interesting morphology have been prepared through electrodeposition by introducing KBr into Zn(NO3)2 electrolyte followed by annealing process. The obtained porous nanosheets were composed of small ZnO nanoparticles with tens of nanometers in size. The light absorption and semiconductor behavior of the porous ZnO nanosheet were investigated and it was measured to be n-type semiconducting with a band gap of 3.2 eV. And the highly porous ZnO nanosheets are expected to perform well in the applications such as photocatalysis, sensor, photoelectrochemical cells and so on.
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Q. Hou et al. / Materials Letters 89 (2012) 283–286
References [1] Lei A, Qu B, Zhou W, Wang Y, Zhang Q, Zou B. Mater Lett 2012;66:72–5. [2] Li J, Fan H, Jia X. J Phys Chem C 2010;114:14684–91. [3] Gu C, Huang J, Wu Y, Zhai M, Sun Y, Liu J. J Alloys Compd 2011;509: 4499–504. [4] Hosono E, Fujihara S, Honma I, Zhou H. Adv Mater 2005;17:2091–4. [5] Zhang Q, Dandeneau CS, Zhou X, Cao G. Adv Mater 2009;21:4087–108. [6] Chen H, Li W, Liu H, Zhu L. Electrochem Commun 2011;13:331–4. [7] Liu S, Li C, Yu J, Xiang Q. CrystEngComm 2011;13:2533–41.
[8] Lai Y, Lin C, Chen H, Chen J, Kung C, Vittal R, et al. J Mater Chem 2010;20:9379–85. [9] Huang XH, Xia XH, Yuan YF, Zhou F. Electrochim Acta 2011;56:4960–5. [10] Jing Z, Zhan J. Adv Mater 2008;20:4547–51. [11] Xu F, Sun L. Energy Environ Sci 2011;4:818–41. [12] Xu L, Guo Y, Liao Q, Zhang J, Xu D. J Phys Chem B 2005;109:13519–22. [13] Chen H, Li W, Liu H, Zhu L. Microporous Mesoporous Mater 2011;138:235–8. [14] Yang X, Wolcott A, Wang G, Sobo A, Fitzmorris RC, Qian F, et al. Nano Lett 2009;9:2331–6.