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Monodispersed synthesis of hierarchical wurtzite ZnS nanostructures and its functional properties
Materials Letters 81 (2012) 209–211
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Monodispersed synthesis of hierarchical wurtzite ZnS nanostructures and its functional properties A. Silambarasan a, Helen P. Kavitha a,⁎, S. Ponnusamy b, M. Navaneethan c, Y. Hayakawa c a b c
Department of Chemistry, SRM University, Ramapuram, Chennai, 600089, Tamil Nadu, India Centre for Nanoscience and Nanotechnology, Department of Physics, SRM University, Kattankulathur, Chennai, 603203, Tamil Nadu, India Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432–8011, Japan
a r t i c l e
i n f o
Article history: Received 3 April 2012 Accepted 2 May 2012 Available online 9 May 2012 Keywords: Colloidal process Hierarchical nanostructures Wurtzite ZnS Morphology directing agent Optical properties
a b s t r a c t Wurtzite type ZnS with flower-like hierarchical morphology has been synthesised at low temperature by wet chemical route using L-threonine as a surface modifying and morphology directing agent. X-ray diffraction studies indicate the formation of wurtzite crystal structure of ZnS. Transmission electron microscopy analysis indicates the self-assembly of small ZnS nanoparticles of size 5–8 nm to form flower like nanostructures of diameter 300 nm. Fourier transform infrared spectrum confirms the presence of L-threonine. Optical absorption studies reveal the presence of coupling effect in addition to the quantum confinement effect in these hierarchical nanostructures. Photoluminescence spectrum on deconvolution exhibits two Gaussian peaks centred at 486 nm and 554 nm indicating the presence of defects. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In the past few years, extensive research has been carried out on the synthesis of novel structures of inorganic materials ranging from microscale to nanoscale. The unique structure induced optical, electrical and surface properties of these materials bring a series of opportunities for their potential applications in photoelectric devices, drug delivery, sensors, filters, coatings and chemical catalysis [1–4]. Many efforts have been devoted to the controlled synthesis of nanostructures of various forms including nanoparticles, nanorods, nanobelts, nanotubes, nanosheets, etc. [5]. The shape dependent properties of nanomaterials bring about a prospect to utilise hierarchical structures to improve the physical and chemical properties with simple configurations [6]. Zinc sulphide (ZnS) is among the II–VI semiconductors possessing such an excellent shape and size dependent property at nanoscale. Moreover, ZnS is an excellent phosphor material and shows a variety of applications in the field of flat panel displays, sensors, and lasers [7] and also as an effective agent for pollutant treatment [8]. Most of the recent works are focussed on the synthesis of hierarchical structures of zinc sulphide nanoparticles. For example, Guozhen Shen et al. [9] have synthesised hierarchical saw-like ZnO nanobelt/ZnS nanowire heterostructures by thermal evaporation. Zhang and Li [10] reported the synthesis of fluffy ZnS hollow spheres from zinc nitrate and thioacetamide in aqueous ammonium sulphate and ammonium
⁎ Corresponding author. Tel.: + 91 44 30603038; fax: + 91 44 3060 3072. E-mail address: [email protected] (H.P. Kavitha). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.05.005
hydroxide buffer solution at a temperature of 4 °C and 60 °C. Yao et al. [11] fabricated ZnS with a hierarchical architecture composed of nanorod arrays with branched nanosheets and nanowires on a gold coated silicon substrate by CVD method. In order to control the shape and size, various methods have been employed like embedding ZnS in polymer [12], zeolite [13] or by arrested precipitation using organic molecules such as alkyl thiols, phosphines, phosphine oxide, amines, carboxylic acids and nitrogen containing aromatics [14]. The use of biomolecules such as amino acids, antibodies, peptides, etc., for the surface modification of semiconductor and metal nanoparticles has attracted researchers for the reasons that they are non-toxic unlike amines and thiols and find wide biological applications. In this paper, we report the synthesis of hierarchical ZnS with wurtzite phase at lower temperature using L-threonine, a biomolecule as a morphology directing agent. 2. Materials and methods All the reagents were of analytical grade and used without further purification. In a typical experiment, 5 mmol zinc nitrate and 5 mmol L-threonine were dissolved in 50 ml of deionised water and subjected to mild stirring. To the above solution, 5 mmol of thioacetamide dissolved in 25 ml of deionised water was added in drops. The final volume was made up to 100 ml using deionised water. Stirring was continued for 5 h at a temperature of 70 °C. The precipitate was centrifuged and washed thoroughly with deionised water and ethanol to remove any residues. The collected precipitates were dried at 60 °C in a hot air oven. The obtained product was characterised using X'per PRO (PANalytical) advanced X-ray Diffractometer with CuKα radiation
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A. Silambarasan et al. / Materials Letters 81 (2012) 209–211
(λ = 1.5406 Å), with 2θ ranging between 20° and 80° at the scanning rate of 0.025° per second to determine the crystalline structure. Optical absorption measurement was performed using Varian Cary 5E UV–vis NIR spectrophotometer in the range of 200–800 nm. Photoluminescence spectrum was obtained using He–Cd LASER with the excitation wavelength of 325 nm. Microstructure of the products was observed by QUANTA FEG field-emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM JEOL JEM 2100F) at an accelerating voltage of 200 kV. For TEM analysis, the samples were dispersed in ethanol and this dispersion was drop casted on a copper-grid. 3. Results and discussion The crystallinity and crystal structure of the ZnS hierarchical nanostructure was analysed by XRD pattern shown in Fig. 1a. The diffraction peaks are indexed to primitive wurtzite structure and are in accordance with the JCPDS card no. 89‐2345. According to literature, ZnS forms cubic phase at lower temperatures [15]. In the present work, wurtzite crystal structure was obtained at 70 °C. This is because of the smaller size of the nanoparticles, which changes the surface energy and thereby reduces the phase transformation temperature. On the basis of molecular dynamics simulations and thermodynamic analysis, the phase transition temperature of smaller sized ZnS (≈7 nm) was shown to be as low as 25 °C [16]. It was reported that, the crystal structure of ZnS is sensitive to organic molecules which act as a surface modifying agent [17]. Here, the particle size is around 8 nm and L-threonine was used as surface modifying agent, which adsorbs on the surface of ZnS and effectively decreases the phase transformation temperature. The sharp peaks indicate that product is well crystalline. No other peaks related to impurities were detected within the detection limit. Fig. 1b shows the FTIR spectrum of L-threonine capped flowerlike ZnS nanostructures. In this spectrum, the peaks observed at 884 cm − 1, 1041 cm − 1, 1093 cm − 1, 1385 cm − 1, 2888 cm − 1 and 2971 cm − 1 correspond to C–CN stretching, C–N stretching, C–CN asymmetric stretching, CH3 bending, C–H stretching and NH3+ asymmetric stretching of L-threonine respectively [18]. The above peaks confirm the presence of L-threonine in the synthesised product. The peak at 640 cm − 1 and 1107 cm − 1 corresponds to Zn–S vibrations [19]. Fig. 2 shows the absorption and photoluminescence spectra of ZnS nanostructures. The absorption peak is observed at 265 nm and is blue shifted compared to bulk ZnS (340 nm). This blue shift is quite larger when compared to the particle size. Hence, the large blue shift is expected to occur due to coupling effect in addition to quantum confinement effect. Coupling effect arises because of strong self-organisation of nanoparticles. Similar results were observed by Liang et al. for CdS nanoparticles [20].
Fig. 2. Optical absorption and photoluminescence spectra of ZnS nanostructures.
The PL band is asymmetric and hence it is deconvoluted. The deconvoluted PL contains two Gaussian peaks centred at 486 nm and 554 nm. The high intensity peak centred at 486 nm related to sulphur vacancies generated by incomplete sulphidation and crystalline nature of flower-like hierarchical morphology. In addition to that, the broadening of the peak is much higher and this may be related to transfer of electron from sulphur to zinc defect which is similar to that observed by Sharma et al. [21]. The green emission peak at 554 nm may arise due to self-activated centres, vacancy states element sulphur species on the surface or interstitial states associated with the peculiar nanostructures as shown in a previous report [22]. The morphology and size of the as synthesised nanostructures were observed by FESEM and TEM. Fig. 3a, b and c shows the FESEM image of flowerlike hierarchical ZnS nanostructures having the mean diameter of 300 nm. Further investigation using TEM (Fig. 3d, e and f) shows that these nanostructures have been formed by self assembly of nanoparticles with a mean diameter of 8 nm. These nanoparticles possess amino acids on the surface which are in the form of zwitterions. The zwitterions contain positive ion (+NH3) and negative ion (COO−). These functional groups interact with one another due to electrostatic attraction. Further +NH3 has the ability to form N–H····O hydrogen bonds with oxygen atom of carboxyl functional group. Moreover, L-threonine contains one –OH functional group which also has tendency to form hydrogen bonds. The electrostatic attraction and hydrogen bonds act as a linker
Fig. 1. (a) XRD pattern and (b) FTIR spectrum of hierarchical ZnS nanostructures.
A. Silambarasan et al. / Materials Letters 81 (2012) 209–211
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Fig. 3. FESEM (a, b, c) and TEM (d, e, f) images of hierarchical ZnS nanostructures.
between different nanoparticles and thus result in the formation of hierarchical structures [23]. 4. Conclusion ZnS hierarchical flower like nanostructure has been synthesised by simple wet chemical method using L-threonine as a morphology directing agent. The addition of L-threonine has found to influence greatly the nucleation rate and hence the growth of the flower-like structures. The zwitterion nature of L-threonine leads to interaction between nanoparticles of diameter 8 nm, which aggregates to form flower-like structures. XRD result reveals the formation of wurtzite phase of ZnS nanostructures. The presence of L-threonine has been confirmed from the FTIR studies. PL studies reveal the presence of defect level emission at 486 nm and 554 nm. References [1] Caruso F. Adv Mater 2001;13:11–22. [2] Dinsmore AD, Hsu MF, Nikolaides MG, Marquez M, Bausch AR, Weitz DA. Science 2002;298:1006–9. [3] Wang Z, Daemen LL, Zhao Y, Zha CS, Downs RT, Wang X, et al. Nat Mater 2005;4: 922–7. [4] Xu A, Yu Q, Dong W, Antonietti M, Colfen H. Adv Mater 2005;17:2217–21.
[5] Tong H, Zhu YJ, Yang LX, Li L, Zhang L, Chang J, et al. J Phys Chem C 2007;111: 3893–900. [6] Wang WS, Hu YX, Goebl J, Lu ZD, Zhen L, Yin YD. J Phys Chem C 2009;113: 16414–23. [7] Panda SK, Datta A, Chaudhuri S. Chem Phys Lett 2007;440:235–8. [8] Franco A, Neves MC. Ribeiro Carrott MML, Mendonca MH, Pereira MI, Monteiro OC. J Hazard Mater 2009;161:545–50. [9] Shen G, Chen D, Lee C. J Phys Chem B 2006;110:15689–93. [10] Zhang H, Qi L. Nanotechnology 2006;17:3984–8. [11] Yao L, Zheng M, He S, Ma L, Li M, Shen W. Appl Surf Sci 2011;257:2955–9. [12] Wozniak ME, Sen A. Chem Mater 1992;4:753–5. [13] Wang Y, Herron N. J Phys Chem 1988;92:4988–94. [14] Suryajaya, Nabok A, Davis F, Hassan A, Higson SPJ, Evans-Freeman. J Appl Sur Sci 2008;254:4891–8. [15] Zhao Y, Zhang Y, Zhu H, Hadjipanayis GC, Xiao JQ. J Am Chem Soc 2004;126: 6874–5. [16] Zhang HZ, Huang F, Gilbert B, Banfield JF. J Phys Chem B 2003;107:13051–60. [17] Murakoshi K, Hosokawa H, Tanaka N, Saito M, Wada Y, Sakata T, et al. Chem Commun 1998:321–2. [18] Silva BL, Freire PTC, Melo FEA, Guedes I, Araujo Silva MA, Mendes filho J, et al. Braz J Phys 1998;28:19–24. [19] Mohagheghpour E, Rabiee M, Moztarzadeh F, Tahriri M, Jafarbeglou M, Bizari D, et al. Mat Sci Eng C 2009;29:1842–8. [20] Liang HC, Xiang H, Rong MZ, Zhang MQ, Zeng HM, Wang SP. Chin Chem Lett 2002;19:871–8. [21] Sharma M, Singh S, Pandey OP. J Appl Phys 2010;107:104319/1–8. [22] Shen G, Bando Y, Golberg D. Appl Phys Lett 2006;88:123107/1–3. [23] Choi K, Lee JH. Sci Adv Mat Lett 2011;3:811–20.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Monodispersed synthesis of hierarchical wurtzite ZnS nanostructures and its functional properties A. Silambarasan a, Helen P. Kavitha a,⁎, S. Ponnusamy b, M. Navaneethan c, Y. Hayakawa c a b c
Department of Chemistry, SRM University, Ramapuram, Chennai, 600089, Tamil Nadu, India Centre for Nanoscience and Nanotechnology, Department of Physics, SRM University, Kattankulathur, Chennai, 603203, Tamil Nadu, India Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432–8011, Japan
a r t i c l e
i n f o
Article history: Received 3 April 2012 Accepted 2 May 2012 Available online 9 May 2012 Keywords: Colloidal process Hierarchical nanostructures Wurtzite ZnS Morphology directing agent Optical properties
a b s t r a c t Wurtzite type ZnS with flower-like hierarchical morphology has been synthesised at low temperature by wet chemical route using L-threonine as a surface modifying and morphology directing agent. X-ray diffraction studies indicate the formation of wurtzite crystal structure of ZnS. Transmission electron microscopy analysis indicates the self-assembly of small ZnS nanoparticles of size 5–8 nm to form flower like nanostructures of diameter 300 nm. Fourier transform infrared spectrum confirms the presence of L-threonine. Optical absorption studies reveal the presence of coupling effect in addition to the quantum confinement effect in these hierarchical nanostructures. Photoluminescence spectrum on deconvolution exhibits two Gaussian peaks centred at 486 nm and 554 nm indicating the presence of defects. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In the past few years, extensive research has been carried out on the synthesis of novel structures of inorganic materials ranging from microscale to nanoscale. The unique structure induced optical, electrical and surface properties of these materials bring a series of opportunities for their potential applications in photoelectric devices, drug delivery, sensors, filters, coatings and chemical catalysis [1–4]. Many efforts have been devoted to the controlled synthesis of nanostructures of various forms including nanoparticles, nanorods, nanobelts, nanotubes, nanosheets, etc. [5]. The shape dependent properties of nanomaterials bring about a prospect to utilise hierarchical structures to improve the physical and chemical properties with simple configurations [6]. Zinc sulphide (ZnS) is among the II–VI semiconductors possessing such an excellent shape and size dependent property at nanoscale. Moreover, ZnS is an excellent phosphor material and shows a variety of applications in the field of flat panel displays, sensors, and lasers [7] and also as an effective agent for pollutant treatment [8]. Most of the recent works are focussed on the synthesis of hierarchical structures of zinc sulphide nanoparticles. For example, Guozhen Shen et al. [9] have synthesised hierarchical saw-like ZnO nanobelt/ZnS nanowire heterostructures by thermal evaporation. Zhang and Li [10] reported the synthesis of fluffy ZnS hollow spheres from zinc nitrate and thioacetamide in aqueous ammonium sulphate and ammonium
⁎ Corresponding author. Tel.: + 91 44 30603038; fax: + 91 44 3060 3072. E-mail address: [email protected] (H.P. Kavitha). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.05.005
hydroxide buffer solution at a temperature of 4 °C and 60 °C. Yao et al. [11] fabricated ZnS with a hierarchical architecture composed of nanorod arrays with branched nanosheets and nanowires on a gold coated silicon substrate by CVD method. In order to control the shape and size, various methods have been employed like embedding ZnS in polymer [12], zeolite [13] or by arrested precipitation using organic molecules such as alkyl thiols, phosphines, phosphine oxide, amines, carboxylic acids and nitrogen containing aromatics [14]. The use of biomolecules such as amino acids, antibodies, peptides, etc., for the surface modification of semiconductor and metal nanoparticles has attracted researchers for the reasons that they are non-toxic unlike amines and thiols and find wide biological applications. In this paper, we report the synthesis of hierarchical ZnS with wurtzite phase at lower temperature using L-threonine, a biomolecule as a morphology directing agent. 2. Materials and methods All the reagents were of analytical grade and used without further purification. In a typical experiment, 5 mmol zinc nitrate and 5 mmol L-threonine were dissolved in 50 ml of deionised water and subjected to mild stirring. To the above solution, 5 mmol of thioacetamide dissolved in 25 ml of deionised water was added in drops. The final volume was made up to 100 ml using deionised water. Stirring was continued for 5 h at a temperature of 70 °C. The precipitate was centrifuged and washed thoroughly with deionised water and ethanol to remove any residues. The collected precipitates were dried at 60 °C in a hot air oven. The obtained product was characterised using X'per PRO (PANalytical) advanced X-ray Diffractometer with CuKα radiation
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A. Silambarasan et al. / Materials Letters 81 (2012) 209–211
(λ = 1.5406 Å), with 2θ ranging between 20° and 80° at the scanning rate of 0.025° per second to determine the crystalline structure. Optical absorption measurement was performed using Varian Cary 5E UV–vis NIR spectrophotometer in the range of 200–800 nm. Photoluminescence spectrum was obtained using He–Cd LASER with the excitation wavelength of 325 nm. Microstructure of the products was observed by QUANTA FEG field-emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM JEOL JEM 2100F) at an accelerating voltage of 200 kV. For TEM analysis, the samples were dispersed in ethanol and this dispersion was drop casted on a copper-grid. 3. Results and discussion The crystallinity and crystal structure of the ZnS hierarchical nanostructure was analysed by XRD pattern shown in Fig. 1a. The diffraction peaks are indexed to primitive wurtzite structure and are in accordance with the JCPDS card no. 89‐2345. According to literature, ZnS forms cubic phase at lower temperatures [15]. In the present work, wurtzite crystal structure was obtained at 70 °C. This is because of the smaller size of the nanoparticles, which changes the surface energy and thereby reduces the phase transformation temperature. On the basis of molecular dynamics simulations and thermodynamic analysis, the phase transition temperature of smaller sized ZnS (≈7 nm) was shown to be as low as 25 °C [16]. It was reported that, the crystal structure of ZnS is sensitive to organic molecules which act as a surface modifying agent [17]. Here, the particle size is around 8 nm and L-threonine was used as surface modifying agent, which adsorbs on the surface of ZnS and effectively decreases the phase transformation temperature. The sharp peaks indicate that product is well crystalline. No other peaks related to impurities were detected within the detection limit. Fig. 1b shows the FTIR spectrum of L-threonine capped flowerlike ZnS nanostructures. In this spectrum, the peaks observed at 884 cm − 1, 1041 cm − 1, 1093 cm − 1, 1385 cm − 1, 2888 cm − 1 and 2971 cm − 1 correspond to C–CN stretching, C–N stretching, C–CN asymmetric stretching, CH3 bending, C–H stretching and NH3+ asymmetric stretching of L-threonine respectively [18]. The above peaks confirm the presence of L-threonine in the synthesised product. The peak at 640 cm − 1 and 1107 cm − 1 corresponds to Zn–S vibrations [19]. Fig. 2 shows the absorption and photoluminescence spectra of ZnS nanostructures. The absorption peak is observed at 265 nm and is blue shifted compared to bulk ZnS (340 nm). This blue shift is quite larger when compared to the particle size. Hence, the large blue shift is expected to occur due to coupling effect in addition to quantum confinement effect. Coupling effect arises because of strong self-organisation of nanoparticles. Similar results were observed by Liang et al. for CdS nanoparticles [20].
Fig. 2. Optical absorption and photoluminescence spectra of ZnS nanostructures.
The PL band is asymmetric and hence it is deconvoluted. The deconvoluted PL contains two Gaussian peaks centred at 486 nm and 554 nm. The high intensity peak centred at 486 nm related to sulphur vacancies generated by incomplete sulphidation and crystalline nature of flower-like hierarchical morphology. In addition to that, the broadening of the peak is much higher and this may be related to transfer of electron from sulphur to zinc defect which is similar to that observed by Sharma et al. [21]. The green emission peak at 554 nm may arise due to self-activated centres, vacancy states element sulphur species on the surface or interstitial states associated with the peculiar nanostructures as shown in a previous report [22]. The morphology and size of the as synthesised nanostructures were observed by FESEM and TEM. Fig. 3a, b and c shows the FESEM image of flowerlike hierarchical ZnS nanostructures having the mean diameter of 300 nm. Further investigation using TEM (Fig. 3d, e and f) shows that these nanostructures have been formed by self assembly of nanoparticles with a mean diameter of 8 nm. These nanoparticles possess amino acids on the surface which are in the form of zwitterions. The zwitterions contain positive ion (+NH3) and negative ion (COO−). These functional groups interact with one another due to electrostatic attraction. Further +NH3 has the ability to form N–H····O hydrogen bonds with oxygen atom of carboxyl functional group. Moreover, L-threonine contains one –OH functional group which also has tendency to form hydrogen bonds. The electrostatic attraction and hydrogen bonds act as a linker
Fig. 1. (a) XRD pattern and (b) FTIR spectrum of hierarchical ZnS nanostructures.
A. Silambarasan et al. / Materials Letters 81 (2012) 209–211
211
Fig. 3. FESEM (a, b, c) and TEM (d, e, f) images of hierarchical ZnS nanostructures.
between different nanoparticles and thus result in the formation of hierarchical structures [23]. 4. Conclusion ZnS hierarchical flower like nanostructure has been synthesised by simple wet chemical method using L-threonine as a morphology directing agent. The addition of L-threonine has found to influence greatly the nucleation rate and hence the growth of the flower-like structures. The zwitterion nature of L-threonine leads to interaction between nanoparticles of diameter 8 nm, which aggregates to form flower-like structures. XRD result reveals the formation of wurtzite phase of ZnS nanostructures. The presence of L-threonine has been confirmed from the FTIR studies. PL studies reveal the presence of defect level emission at 486 nm and 554 nm. References [1] Caruso F. Adv Mater 2001;13:11–22. [2] Dinsmore AD, Hsu MF, Nikolaides MG, Marquez M, Bausch AR, Weitz DA. Science 2002;298:1006–9. [3] Wang Z, Daemen LL, Zhao Y, Zha CS, Downs RT, Wang X, et al. Nat Mater 2005;4: 922–7. [4] Xu A, Yu Q, Dong W, Antonietti M, Colfen H. Adv Mater 2005;17:2217–21.
[5] Tong H, Zhu YJ, Yang LX, Li L, Zhang L, Chang J, et al. J Phys Chem C 2007;111: 3893–900. [6] Wang WS, Hu YX, Goebl J, Lu ZD, Zhen L, Yin YD. J Phys Chem C 2009;113: 16414–23. [7] Panda SK, Datta A, Chaudhuri S. Chem Phys Lett 2007;440:235–8. [8] Franco A, Neves MC. Ribeiro Carrott MML, Mendonca MH, Pereira MI, Monteiro OC. J Hazard Mater 2009;161:545–50. [9] Shen G, Chen D, Lee C. J Phys Chem B 2006;110:15689–93. [10] Zhang H, Qi L. Nanotechnology 2006;17:3984–8. [11] Yao L, Zheng M, He S, Ma L, Li M, Shen W. Appl Surf Sci 2011;257:2955–9. [12] Wozniak ME, Sen A. Chem Mater 1992;4:753–5. [13] Wang Y, Herron N. J Phys Chem 1988;92:4988–94. [14] Suryajaya, Nabok A, Davis F, Hassan A, Higson SPJ, Evans-Freeman. J Appl Sur Sci 2008;254:4891–8. [15] Zhao Y, Zhang Y, Zhu H, Hadjipanayis GC, Xiao JQ. J Am Chem Soc 2004;126: 6874–5. [16] Zhang HZ, Huang F, Gilbert B, Banfield JF. J Phys Chem B 2003;107:13051–60. [17] Murakoshi K, Hosokawa H, Tanaka N, Saito M, Wada Y, Sakata T, et al. Chem Commun 1998:321–2. [18] Silva BL, Freire PTC, Melo FEA, Guedes I, Araujo Silva MA, Mendes filho J, et al. Braz J Phys 1998;28:19–24. [19] Mohagheghpour E, Rabiee M, Moztarzadeh F, Tahriri M, Jafarbeglou M, Bizari D, et al. Mat Sci Eng C 2009;29:1842–8. [20] Liang HC, Xiang H, Rong MZ, Zhang MQ, Zeng HM, Wang SP. Chin Chem Lett 2002;19:871–8. [21] Sharma M, Singh S, Pandey OP. J Appl Phys 2010;107:104319/1–8. [22] Shen G, Bando Y, Golberg D. Appl Phys Lett 2006;88:123107/1–3. [23] Choi K, Lee JH. Sci Adv Mat Lett 2011;3:811–20.