- Email: [email protected]
Controllable synthesis of ZnO microstructures with morphologies from rods to disks
Materials Letters 92 (2013) 154–156
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Controllable synthesis of ZnO microstructures with morphologies from rods to disks Minhong He a,b, Hongfang Jiu a, Yaqing Liu a,b,n, Ye Tian a,b, Donglin Li a,b, Youyi Sun a,b, Guizhe Zhao a,b a b
Research Center for Engineering Technology of Polymeric Composites of Shanxi Province, North University of China, Taiyuan 030051, PR China College of Material Science and Engineering, North University of China, Taiyuan 030051, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 October 2012 Accepted 12 October 2012 Available online 22 October 2012
A simple controllable chemical bath deposition method was successfully developed to prepare ZnO rods and disks at low-temperature. The crystalline structure and morphologies of the prepared ZnO were characterized with X-ray diffraction (XRD) and scanning electronic microscopy (SEM), respectively. The XRD results proved that the ZnO belongs to wurtzite structure. SEM images showed that the morphology of ZnO was evolved from rod shape to disk shape just by changing the precursor concentration. The aspect ratio (length/diameter) of ZnO crystal decreases from 9.7 to 0.15. And the formation mechanism of these morphologies was discussed. Besides, the photoluminescence (PL) properties of ZnO architectures were investigated at room temperature, indicating that ZnO obtained were highly crystallized. & 2012 Elsevier B.V. All rights reserved.
Keywords: Controllable synthesis Crystal growth Microstructure ZnO Photoluminescence
1. Introduction Among the metal oxides, Zinc oxide is a direct band gap semiconducting and piezoelectric material with a large band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature. Previous reports have described a diverse range of ZnO nano/microstructures, such as gear-shaped microwires, belts, disks, tubes, flowers, and hollow spheres etc. [1–5]. And the nano/microstructures often have interesting physical or chemical property for room-temperature UV lasers, light-emitting diodes, solar cells, catalytic, or sensing applications, which strongly depend on its particular size, shape, and composition [6–10]. So, in recent years, one of the most important techniques for controlling their physical and chemical property is to control the size and shape of materials. Along with the development of science and technology, lots of methods to control morphology of ZnO have been developed quickly in recent years, such as chemical vapor deposition (CVD) [11], and hydrothermal route [12]. However, the growth temperature in the above methods is relatively high. On the other side, it is highly desirable to develop an effective and simple procedure to synthesize nano/microstructures of ZnO because of their unique properties and potential applications [13–15]. Recently, a chemical bath deposition (CBD) has the advantages of low-cost, simple, low-temperature
n Corresponding author at: Research Center for Engineering Technology of Polymeric Composites of Shanxi Province, North University of China, Taiyuan 030051, PR China. Tel./fax: þ86 351 3559669. E-mail addresses: [email protected], zffl[email protected] (Y. Liu).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.049
operation. And it can generate different controllable morphology and final size of the as-synthesized samples, which depend on the precise conditions of deposition, such as pH value, stabilizer, and so on [16]. Herein, we report a controllable synthesis of ZnO microstructures with morphologies from rods to disks. The present method shows a simple and easy processing to control final morphology and size of ZnO by adjusting the concentration of zinc acetate dihydrate additive in the growth solution. The mechanism of forming these morphologies is also discussed. And the rods and disks can be attained easily on a large scale.
2. Experimental All the reagents used in the experiments were analytical grade and used without further purification, including zinc acetate dihydrate (Zn(Ac)2 2H2O) (Tianjin Kermel Chemical Reagent Development Center), polyvinylpyrrolidone (PVP) (Mn¼30000, K30, Tianjin Dengfeng Chemical Reagent Co., Ltd), ethanol (Tianjin Guangfu Technology Development Co., Ltd). ZnO particles were synthesized via chemical bath deposition method. A typical synthesis procedure is the following: a specific amount of Zn(Ac)2 2H2O and 0.25 g PVP were dissolved in 40 ml deionized water with stirring at room temperature. Then, the reaction continued to the stationary state at 80 1C for 6 h. The resulting product was washed with water and ethanol several times, consecutively. Finally, the product was dried under a dynamic vacuum at room temperature for 24 h before characterization.
M. He et al. / Materials Letters 92 (2013) 154–156
155
Fig. 1. SEM images of the as-prepared ZnO at different Zn(Ac)2 2H2O concentrations of (a) 0.01, (b) 0.05, (c) 0.1 and (d) 0.2 M.
X-ray diffraction patterns (XRD) were obtained with a Rigaku D/max-rB diffractometer using Cu Ka radiation. Scanning electron microscopy (SEM) measurements were performed on a SHIMADZU SU-1500 microscope. The photoluminescence (PL) spectra were recorded on a Hitachi fluorescence spectrometer with a Xe lamp at room temperature.
Table 1 The dimension and aspect ratio of the ZnO obtained at varied precursor solutions of Zn(Ac)2 2H2O concentration. Zn(Ac)2 2H2O/M Average diameter (mm) Average length (mm) Aspect ratio
0.01 0.6 5.8 9.7
0.05 1.2 3.5 2.9
0.1 3.6 5.2 1.4
0.2 6.2 0.9 0.15
(103)
a
(200) (112) (201)
(102)
(110)
(101) (002)
Intensity(a.u.)
The morphologies of the as-prepared ZnO crystals were analyzed by SEM. Fig. 1 displays typical morphologies of ZnO synthesized at different Zn(Ac)2 2H2O concentrations. The morphology of ZnO was changed from rod shape to disk shape by increasing Zn(Ac)2 2H2O concentration. When a very small quantity of Zn(Ac)2 2H2O was added, ZnO rods could be obtained (Fig. 1a and b). And a great quantity of Zn(Ac)2 2H2O caused the formation of disks, and the hexagonal particles became thinner (Fig. 1d). At the same time, these results indicate that the morphology of ZnO is easily controlled by the Zn(Ac)2 2H2O concentration. The average diameter and length of ZnO crystals are shown in Table 1. It can be observed that with the increase of Zn(Ac)2 2H2O concentration, the crystal diameter increased from about 0.6–6.2 mm and the average length also changed greatly, leading to the decrease of aspect ratio of ZnO crystal from 9.7 to 0.15. It also suggests that the size of ZnO crystal strongly depends on the Zn(Ac)2 2H2O concentration. ZnO is a promising material for spintronics since it can possess the ferromagnetic properties. And it has been demonstrated recently that the ferromagnetic properties of fine-grained ZnO strongly depend on the presence of defects like grain boundaries [17] and on the presence of amorphous surficial and intergranular layers [18]. It can be also shown in Fig. 1 that the obtained ZnO samples are fine-grained and contain the very developed free surfaces as well as grain boundaries and interfaces. So, the obtained ZnO is valuable for the potential application in spintronics. The typical XRD patterns of ZnO rods and disks prepared at 0.01 M and 0.2 M Zn(Ac)2 2H2O concentration respectively are illustrated in Fig. 2. The diffraction peaks were observed, in good
(100)
3. Results and discussion
b
20
30
40
50
60
70
80
2 theta (degree) Fig. 2. Typical XRD pattern of the ZnO particles: (a) rods, (b) disks.
agreement with the wurtzitic form of ZnO (hexagonal phase, space group P63mc, ICDD no. 36-1451). And no characteristic peaks from other impurities are detected, indicating high purity of ZnO. Fig. 2b showed that the (002) peak of ZnO disks was much stronger than that of the rods (Fig. 2a), suggesting the evolution of the morphology from rod to disk. As we know, ZnO with a wurtzite structure has the stacking sequences of Zn2 þ and O2 that make it one kind of polar crystal. The oppositely charged ions produce positively Zn2 þ -terminated
156
M. He et al. / Materials Letters 92 (2013) 154–156
4. Conclusion
PL Intensity (a.u.)
In summary, a simple controllable chemical bath deposition method was successfully developed to prepare ZnO rods and disks at low-temperature. The present method shows a simple and easy processing to control the final morphology and size of ZnO. The mechanism of forming these morphologies was discussed also. And the ZnO rods and disks are valuable for potential applications in spintronics. This work may represent an effective, controllable and facile strategy for the creation of highly structured nano/ micromaterials, and the strategy reported here might be used for the controlled synthesis of other materials. b a
Acknowledgments 300
400
500
600
700
Wavelength (nm) Fig. 3. Typical PL spectra of ZnO with morphologies of rods (a) (sample illustrated in Fig. 1a) and disks (b) (sample illustrated in Fig. 1d).
(0001) and negatively O2 -terminated (000–1) polar surfaces, respectively, which possess an intrinsic dipole moment along the c-axis [19]. When the Zn(Ac)2 2H2O concentration is low, the OH ions adsorb on the (0001) face and combine with Zn2 þ ions to form Zn(OH)2, then the Zn(OH)2 dehydrates to ZnO, and the polar surface transforms into O2 -terminated (000–1) surface. After that the Zn2 þ ions adsorb on the (000–1) face and immediately turn into (0001) surface. This repeated fast growth along the c-axis causes the formation of ZnO rods, as shown in Fig. 1a. At a higher concentration of Zn(Ac)2 2H2O, the hydroxylation occurs at a much faster rate before the Zn(OH)2 completely converts into ZnO, and that will lead to the OH ions combine with Zn(OH)2 to form [Zn(OH)4]2 on the (0001) surface, which will prevent new OH ions from incorporating effectively into the as-formed ZnO crystallites along the c-axis direction [20]. So, the crystal growth along the c-axis is considerably restrained and the growths of other directions are therefore improved greatly, which causes the formation of thinner ZnO disks, as shown in Fig. 1d. Fig. 3 shows the room-temperature PL spectra of prepared ZnO rods (a) and disks (b) with a Xe lamp as excitation source at 336 nm. It is clear that the spectra consists of a strong emission peak located at 392 nm, and no green-yellow emission peak which can be attributed to the recombination of photogenerated hole with electrons in singly occupied oxygen vacancies is observed. So, the results of PL indicate that our method can produce a low concentration of oxygen vacancies of ZnO crystal [21]. The dominant ultraviolet (UV) peaks at 392 nm correspond to recombination of free excitons, namely the near band edge emission (NBE) procedure [22]. Generally speaking, the high crystal quality of ZnO nanostructures is likely to lead to strong UV emission peaks [23]. As a result, the PL data presented here indicate that our samples are in high quality.
This work was supported by the National Natural Science Foundation of China (50025309, and 90201016), University’s Science and technology exploiture of Shanxi Province (20080320ZX) and Youthful Science Foundation of North University of China.
References [1] Qiu Y, Hu LZ, Yu DQ, Zhang HQ, Sun JC, Wang B, et al. Micro Nano Lett 2010;5:251–3. [2] Xia YN, Yang PD, Sun YG, Wu YY, Mayers B, Gates B, et al. Adv Mater 2003;15:353–89. [3] Zhang H, Yang D, Ji Y, Ma X, Xu J, Que DJ. Phys Chem B 2004;108:3955–8. [4] Choi K, Kang T, Oh SG. Mater Lett 2012;75:240–3. [5] Gao SY, Zhang HJ, Wang XM, Deng RP, Sun DH, Zheng GLJ. Phys Chem B 2006;110:15847–52. [6] Yang PD, Yan HQ, Mao S, Russo R, Johnson J, Saykally R, et al. Adv Funct Mater 2002;12:323–31. [7] Zhang JH, Liu HY, Wang ZL, Ming NB, Li ZR, Biris AS. Adv Funct Mater 2007;17:3897–905. ¨ H, Sodergren ¨ [8] Rensmo H, Keis K, Lindstrom S, Solbrand A, Hagfeldt A, et al. J Phys Chem B 1997;101:2598–601. [9] Sun S, Murray CB, Weller D, Folks L, Moser A. Science 2000;287:1989–92. [10] Li F, He J, Zhou W, Wiley JBJ. Am Chem Soc 2003;125:16166–7. [11] Liao X, Zhang XJ. Phys Chem C 2007;111:9081–5. [12] Jiang H, Hu JQ, Gu F, Li CZJ. Phys Chem C 2008;112:12138–41. [13] Huang LS, Wright S, Yang SG, Shen DZ, Gu BX, Du YWJ. Phys Chem B 2004;108:19901–3. [14] Law M, Greene LE, Johnson JC, Saykally R, Yang P. Nat Mater 2005;4:455–9. [15] Song J, Zhou J, Wang ZL. Nano Lett 2006;6:1656–62. [16] Liu XH, Afzaal M, Ramasamy K, O’Brien P, Akhtar JJ. Am Chem Soc 2009;131:15106–7. [17] Straumal BB, Mazilkin AA, Protasova SG, Myatiev AA, Straumal PB, Goering E, et al. Thin Solid Films 2011;520:1192–4. [18] Straumal BB, Protasova SG, Mazilkin AA, Baretzky B, Myatiev AA, Straumal PB, et al. Mater Lett 2012;71:21–4. [19] Wang ZL. J. Phys: Condens Matter 2004;16:829–58. [20] Bai X, Yi L, Liu DL, Nie EY, Sun CL, Feng HH, et al. Appl Surf Sci 2011;257:10317–21. [21] Du GX, Zhang LD, Feng Y, Xu YY, Sun YX, Ding B, et al. Mater Lett 2012;73:86–8. [22] Yu QJ, Fu WY, Yu CL, Yang HB, Wei RH, Li MHJ. Phys Chem C 2007;111:17521–6. [23] Yu JX, Huang BB, Qin XY, Zhang XY, Wang ZY, Liu HX, et al. Appl Surf Sci 2011;257:5563–5.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Controllable synthesis of ZnO microstructures with morphologies from rods to disks Minhong He a,b, Hongfang Jiu a, Yaqing Liu a,b,n, Ye Tian a,b, Donglin Li a,b, Youyi Sun a,b, Guizhe Zhao a,b a b
Research Center for Engineering Technology of Polymeric Composites of Shanxi Province, North University of China, Taiyuan 030051, PR China College of Material Science and Engineering, North University of China, Taiyuan 030051, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 October 2012 Accepted 12 October 2012 Available online 22 October 2012
A simple controllable chemical bath deposition method was successfully developed to prepare ZnO rods and disks at low-temperature. The crystalline structure and morphologies of the prepared ZnO were characterized with X-ray diffraction (XRD) and scanning electronic microscopy (SEM), respectively. The XRD results proved that the ZnO belongs to wurtzite structure. SEM images showed that the morphology of ZnO was evolved from rod shape to disk shape just by changing the precursor concentration. The aspect ratio (length/diameter) of ZnO crystal decreases from 9.7 to 0.15. And the formation mechanism of these morphologies was discussed. Besides, the photoluminescence (PL) properties of ZnO architectures were investigated at room temperature, indicating that ZnO obtained were highly crystallized. & 2012 Elsevier B.V. All rights reserved.
Keywords: Controllable synthesis Crystal growth Microstructure ZnO Photoluminescence
1. Introduction Among the metal oxides, Zinc oxide is a direct band gap semiconducting and piezoelectric material with a large band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature. Previous reports have described a diverse range of ZnO nano/microstructures, such as gear-shaped microwires, belts, disks, tubes, flowers, and hollow spheres etc. [1–5]. And the nano/microstructures often have interesting physical or chemical property for room-temperature UV lasers, light-emitting diodes, solar cells, catalytic, or sensing applications, which strongly depend on its particular size, shape, and composition [6–10]. So, in recent years, one of the most important techniques for controlling their physical and chemical property is to control the size and shape of materials. Along with the development of science and technology, lots of methods to control morphology of ZnO have been developed quickly in recent years, such as chemical vapor deposition (CVD) [11], and hydrothermal route [12]. However, the growth temperature in the above methods is relatively high. On the other side, it is highly desirable to develop an effective and simple procedure to synthesize nano/microstructures of ZnO because of their unique properties and potential applications [13–15]. Recently, a chemical bath deposition (CBD) has the advantages of low-cost, simple, low-temperature
n Corresponding author at: Research Center for Engineering Technology of Polymeric Composites of Shanxi Province, North University of China, Taiyuan 030051, PR China. Tel./fax: þ86 351 3559669. E-mail addresses: [email protected], zffl[email protected] (Y. Liu).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.049
operation. And it can generate different controllable morphology and final size of the as-synthesized samples, which depend on the precise conditions of deposition, such as pH value, stabilizer, and so on [16]. Herein, we report a controllable synthesis of ZnO microstructures with morphologies from rods to disks. The present method shows a simple and easy processing to control final morphology and size of ZnO by adjusting the concentration of zinc acetate dihydrate additive in the growth solution. The mechanism of forming these morphologies is also discussed. And the rods and disks can be attained easily on a large scale.
2. Experimental All the reagents used in the experiments were analytical grade and used without further purification, including zinc acetate dihydrate (Zn(Ac)2 2H2O) (Tianjin Kermel Chemical Reagent Development Center), polyvinylpyrrolidone (PVP) (Mn¼30000, K30, Tianjin Dengfeng Chemical Reagent Co., Ltd), ethanol (Tianjin Guangfu Technology Development Co., Ltd). ZnO particles were synthesized via chemical bath deposition method. A typical synthesis procedure is the following: a specific amount of Zn(Ac)2 2H2O and 0.25 g PVP were dissolved in 40 ml deionized water with stirring at room temperature. Then, the reaction continued to the stationary state at 80 1C for 6 h. The resulting product was washed with water and ethanol several times, consecutively. Finally, the product was dried under a dynamic vacuum at room temperature for 24 h before characterization.
M. He et al. / Materials Letters 92 (2013) 154–156
155
Fig. 1. SEM images of the as-prepared ZnO at different Zn(Ac)2 2H2O concentrations of (a) 0.01, (b) 0.05, (c) 0.1 and (d) 0.2 M.
X-ray diffraction patterns (XRD) were obtained with a Rigaku D/max-rB diffractometer using Cu Ka radiation. Scanning electron microscopy (SEM) measurements were performed on a SHIMADZU SU-1500 microscope. The photoluminescence (PL) spectra were recorded on a Hitachi fluorescence spectrometer with a Xe lamp at room temperature.
Table 1 The dimension and aspect ratio of the ZnO obtained at varied precursor solutions of Zn(Ac)2 2H2O concentration. Zn(Ac)2 2H2O/M Average diameter (mm) Average length (mm) Aspect ratio
0.01 0.6 5.8 9.7
0.05 1.2 3.5 2.9
0.1 3.6 5.2 1.4
0.2 6.2 0.9 0.15
(103)
a
(200) (112) (201)
(102)
(110)
(101) (002)
Intensity(a.u.)
The morphologies of the as-prepared ZnO crystals were analyzed by SEM. Fig. 1 displays typical morphologies of ZnO synthesized at different Zn(Ac)2 2H2O concentrations. The morphology of ZnO was changed from rod shape to disk shape by increasing Zn(Ac)2 2H2O concentration. When a very small quantity of Zn(Ac)2 2H2O was added, ZnO rods could be obtained (Fig. 1a and b). And a great quantity of Zn(Ac)2 2H2O caused the formation of disks, and the hexagonal particles became thinner (Fig. 1d). At the same time, these results indicate that the morphology of ZnO is easily controlled by the Zn(Ac)2 2H2O concentration. The average diameter and length of ZnO crystals are shown in Table 1. It can be observed that with the increase of Zn(Ac)2 2H2O concentration, the crystal diameter increased from about 0.6–6.2 mm and the average length also changed greatly, leading to the decrease of aspect ratio of ZnO crystal from 9.7 to 0.15. It also suggests that the size of ZnO crystal strongly depends on the Zn(Ac)2 2H2O concentration. ZnO is a promising material for spintronics since it can possess the ferromagnetic properties. And it has been demonstrated recently that the ferromagnetic properties of fine-grained ZnO strongly depend on the presence of defects like grain boundaries [17] and on the presence of amorphous surficial and intergranular layers [18]. It can be also shown in Fig. 1 that the obtained ZnO samples are fine-grained and contain the very developed free surfaces as well as grain boundaries and interfaces. So, the obtained ZnO is valuable for the potential application in spintronics. The typical XRD patterns of ZnO rods and disks prepared at 0.01 M and 0.2 M Zn(Ac)2 2H2O concentration respectively are illustrated in Fig. 2. The diffraction peaks were observed, in good
(100)
3. Results and discussion
b
20
30
40
50
60
70
80
2 theta (degree) Fig. 2. Typical XRD pattern of the ZnO particles: (a) rods, (b) disks.
agreement with the wurtzitic form of ZnO (hexagonal phase, space group P63mc, ICDD no. 36-1451). And no characteristic peaks from other impurities are detected, indicating high purity of ZnO. Fig. 2b showed that the (002) peak of ZnO disks was much stronger than that of the rods (Fig. 2a), suggesting the evolution of the morphology from rod to disk. As we know, ZnO with a wurtzite structure has the stacking sequences of Zn2 þ and O2 that make it one kind of polar crystal. The oppositely charged ions produce positively Zn2 þ -terminated
156
M. He et al. / Materials Letters 92 (2013) 154–156
4. Conclusion
PL Intensity (a.u.)
In summary, a simple controllable chemical bath deposition method was successfully developed to prepare ZnO rods and disks at low-temperature. The present method shows a simple and easy processing to control the final morphology and size of ZnO. The mechanism of forming these morphologies was discussed also. And the ZnO rods and disks are valuable for potential applications in spintronics. This work may represent an effective, controllable and facile strategy for the creation of highly structured nano/ micromaterials, and the strategy reported here might be used for the controlled synthesis of other materials. b a
Acknowledgments 300
400
500
600
700
Wavelength (nm) Fig. 3. Typical PL spectra of ZnO with morphologies of rods (a) (sample illustrated in Fig. 1a) and disks (b) (sample illustrated in Fig. 1d).
(0001) and negatively O2 -terminated (000–1) polar surfaces, respectively, which possess an intrinsic dipole moment along the c-axis [19]. When the Zn(Ac)2 2H2O concentration is low, the OH ions adsorb on the (0001) face and combine with Zn2 þ ions to form Zn(OH)2, then the Zn(OH)2 dehydrates to ZnO, and the polar surface transforms into O2 -terminated (000–1) surface. After that the Zn2 þ ions adsorb on the (000–1) face and immediately turn into (0001) surface. This repeated fast growth along the c-axis causes the formation of ZnO rods, as shown in Fig. 1a. At a higher concentration of Zn(Ac)2 2H2O, the hydroxylation occurs at a much faster rate before the Zn(OH)2 completely converts into ZnO, and that will lead to the OH ions combine with Zn(OH)2 to form [Zn(OH)4]2 on the (0001) surface, which will prevent new OH ions from incorporating effectively into the as-formed ZnO crystallites along the c-axis direction [20]. So, the crystal growth along the c-axis is considerably restrained and the growths of other directions are therefore improved greatly, which causes the formation of thinner ZnO disks, as shown in Fig. 1d. Fig. 3 shows the room-temperature PL spectra of prepared ZnO rods (a) and disks (b) with a Xe lamp as excitation source at 336 nm. It is clear that the spectra consists of a strong emission peak located at 392 nm, and no green-yellow emission peak which can be attributed to the recombination of photogenerated hole with electrons in singly occupied oxygen vacancies is observed. So, the results of PL indicate that our method can produce a low concentration of oxygen vacancies of ZnO crystal [21]. The dominant ultraviolet (UV) peaks at 392 nm correspond to recombination of free excitons, namely the near band edge emission (NBE) procedure [22]. Generally speaking, the high crystal quality of ZnO nanostructures is likely to lead to strong UV emission peaks [23]. As a result, the PL data presented here indicate that our samples are in high quality.
This work was supported by the National Natural Science Foundation of China (50025309, and 90201016), University’s Science and technology exploiture of Shanxi Province (20080320ZX) and Youthful Science Foundation of North University of China.
References [1] Qiu Y, Hu LZ, Yu DQ, Zhang HQ, Sun JC, Wang B, et al. Micro Nano Lett 2010;5:251–3. [2] Xia YN, Yang PD, Sun YG, Wu YY, Mayers B, Gates B, et al. Adv Mater 2003;15:353–89. [3] Zhang H, Yang D, Ji Y, Ma X, Xu J, Que DJ. Phys Chem B 2004;108:3955–8. [4] Choi K, Kang T, Oh SG. Mater Lett 2012;75:240–3. [5] Gao SY, Zhang HJ, Wang XM, Deng RP, Sun DH, Zheng GLJ. Phys Chem B 2006;110:15847–52. [6] Yang PD, Yan HQ, Mao S, Russo R, Johnson J, Saykally R, et al. Adv Funct Mater 2002;12:323–31. [7] Zhang JH, Liu HY, Wang ZL, Ming NB, Li ZR, Biris AS. Adv Funct Mater 2007;17:3897–905. ¨ H, Sodergren ¨ [8] Rensmo H, Keis K, Lindstrom S, Solbrand A, Hagfeldt A, et al. J Phys Chem B 1997;101:2598–601. [9] Sun S, Murray CB, Weller D, Folks L, Moser A. Science 2000;287:1989–92. [10] Li F, He J, Zhou W, Wiley JBJ. Am Chem Soc 2003;125:16166–7. [11] Liao X, Zhang XJ. Phys Chem C 2007;111:9081–5. [12] Jiang H, Hu JQ, Gu F, Li CZJ. Phys Chem C 2008;112:12138–41. [13] Huang LS, Wright S, Yang SG, Shen DZ, Gu BX, Du YWJ. Phys Chem B 2004;108:19901–3. [14] Law M, Greene LE, Johnson JC, Saykally R, Yang P. Nat Mater 2005;4:455–9. [15] Song J, Zhou J, Wang ZL. Nano Lett 2006;6:1656–62. [16] Liu XH, Afzaal M, Ramasamy K, O’Brien P, Akhtar JJ. Am Chem Soc 2009;131:15106–7. [17] Straumal BB, Mazilkin AA, Protasova SG, Myatiev AA, Straumal PB, Goering E, et al. Thin Solid Films 2011;520:1192–4. [18] Straumal BB, Protasova SG, Mazilkin AA, Baretzky B, Myatiev AA, Straumal PB, et al. Mater Lett 2012;71:21–4. [19] Wang ZL. J. Phys: Condens Matter 2004;16:829–58. [20] Bai X, Yi L, Liu DL, Nie EY, Sun CL, Feng HH, et al. Appl Surf Sci 2011;257:10317–21. [21] Du GX, Zhang LD, Feng Y, Xu YY, Sun YX, Ding B, et al. Mater Lett 2012;73:86–8. [22] Yu QJ, Fu WY, Yu CL, Yang HB, Wei RH, Li MHJ. Phys Chem C 2007;111:17521–6. [23] Yu JX, Huang BB, Qin XY, Zhang XY, Wang ZY, Liu HX, et al. Appl Surf Sci 2011;257:5563–5.