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Needle-like ZnO nanostructure synthesized by organic-free hydrothermal process
ARTICLE IN PRESS
Physica E 40 (2008) 660–663 www.elsevier.com/locate/physe
Needle-like ZnO nanostructure synthesized by organic-free hydrothermal process Raghvendra S. Yadav, Avinash C. Pandey Nanophosphor Application Centre, University of Allahabad, Allahabad 211002, India Received 6 October 2006; accepted 30 August 2007 Available online 14 September 2007
Abstract In present work, needle-like ZnO nanostructures were synthesized by an organic-free hydrothermal process. The hydrothermal synthesis was performed at temperature 330 K. The products were characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The XRD pattern indicated that the needle-like ZnO nanostructures were hexagonal. Morphology, diameter and length of needles were investigated. Finally, the mechanism for organic-free hydrothermal synthesis of the needle-like ZnO nanostructure is discussed. r 2007 Elsevier B.V. All rights reserved. PACS: 61.46 Keywords: ZnO; Organic-free hydrothermal process
1. Introduction In the recent years, nanostructures have attracted much attention owing to their great potential for application in nanodevices; since, in a nanostructure, the density of states of the carrier is concentrated at some specific energy levels. The wurtzite nanostructure family has a few important members such as ZnO, GaN, AlN, ZnS and CdSe. ZnO is one of the most important nanomaterials of this family because it has three key properties. First, it is semiconductor, with a direct wide band gap of 3.37 eV and a large excitation binding energy (60 meV) [1]. It exhibits nearultraviolet emission and transparent conductivity. Secondly, because of its non-central symmetry, ZnO is piezoelectric, which is a key property in building electromechanical-coupled sensors and transducers [2]. Finally ZnO is bio-safe and biocompatible, and can be used for biomedical applications. The current initiatives in nanoscience and nanotechnology have led to the fabrication of a variety of nanostructures Corresponding author. Tel.: +91 532 2460675.
E-mail address: [email protected] (R.S. Yadav). 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.08.087
of ZnO such as nanowires, nanorods, nanotubes, nanobelts, nanocombs, nanosaws, nanosprings, nanospirals, nanorings, etc. Many interesting nanostructures of ZnO including nanobelts, nanobridges, nanonails, and nanoribbons have been fabricated by thermal evaporation of oxide powders [3–6]. Recently, chemical solution routes including solvothermal, hydrothermal, self-assembly and a template assisted sol–gel process have been employed to synthesize ZnO nanowires and nanorods [7–17]. The synthesis of ZnO nanowires and nanorods by a solvothermal process has also been reported. It is inevitable to use toxic, dangerous, and expensive solvents such as amine in solvothermal process. Nowadays, one of the methods of ‘‘soft chemistry’’, namely hydrothermal technique is widely utilized for the preparation of nanocrystalline oxide material such as ZnO. It is important that hydrothermally obtained powders could be produced with different nanostructures by varying parameters such as temperature, pressure, duration of process, concentration of chemical species and pH of solution. Herein, we have synthesized needle-like ZnO nanostructures by organic-free hydrothermal process. The tip of ZnO nanoneedles offer potential application as AFM probing tips, which have
ARTICLE IN PRESS R.S. Yadav, A.C. Pandey / Physica E 40 (2008) 660–663
10000
2. Experimental section
The needle-like ZnO nanostructures were prepared in the following process: 1 g of zinc acetate dihydrate was put into 105 mL of distilled water under vigorous stirring. After 10 min stirring, 15 mL of 2 M NaOH aqueous solution was introduced into the above aqueous solution, resulting in a white aqueous solution (pH value was equal to 12), which was then transferred into stainless steel autoclaves, sealed, and maintained at temperature 330 K for 20 h. The precipitate in the autoclave was taken out and washed repeatedly with distilled water and ethanol to remove the ions possibly remaining in the final products. Then a white powder was obtained after drying at 60 1C in air for 2 h. A part of the powder was put into a vacuum oven and tube furnace kept at 200 1C for 4 h.
(112) (200)
2000
2.2. Sample preparation
(201)
(103)
4000
(110)
(100) (002)
6000
(102)
The zinc acetate dihydrate (98%) Zn(CH3COO)2.2H2O, sodium hydroxide NaOH was from E. Merck (India) Limited, Mumbai 400018, India. These chemicals were directly used without special treatment.
8000 Intensity (a.u.)
2.1. Chemicals
(101)
high spatial resolution in both vertical and horizontal dimensions [18,19].
661
0 30
40
50
60
70
2-theta (degree)
Fig. 1. XRD pattern of needle-like ZnO nanostructure synthesized by organic-free hydrothermal process at low pressure 20 kPa.
2.3. Apparatus The ultimate products were characterized by X-ray powder diffraction (XRD) using a Rigaku X-ray diffractometer with Cu Ka radiation (l=1.54178 A˚). XRD data were collected over the range 20–801 with a step interval of 0.021 at room temperature. Transmission electron microscopy (TEM) observation was performed with a Philips CM 12 microscope operated at 110 kV. Photoluminescence studies were performed using He–Cd laser (KIMMON) and Mechelle900 spectrograph working in 200–1100 nm wavelength region. The PL set up has a cooled CCD arraybased detection system. The laser light is incident on the sample at 45 1C and the luminescence light is collected using a collector assembly and transmitted to the spectrograph through optical fiber for detection and analysis. The PL studies were performed using excitation by 325 nm and the data were recorded for 30 s on each sample. 3. Results and discussion Fig. 1 shows powder X-ray diffraction pattern of sample prepared at pressure 20 kPa in the solution of pH 12 using NaOH as mineralizer indicate that sample is single phase with a wurtzite structure. Furthermore, it can be seen that the diffraction peaks are higher and narrower, implying that the ZnO crystallizes well.
Fig. 2. TEM image of needle-like ZnO nanostructure synthesized by organic-free hydrothermal process.
Fig. 2 shows the TEM images of the ZnO nanostructures prepared by the hydrothermal process at pressure 20 kPa and pH values 12 using NaOH as the mineralizer. When pressure was 20 kPa, ZnO nanostructures consisting of needle-like nanorods 20–30 nm in diameter and 200–250 nm in length are obtained. Yang et al. [20] reported that OH is first introduced into Zn2+ aqueous solution, and then Zn(OH)2 colloids form, according to Zn2þ þ 2OH 2ZnðOHÞ2 :
(1)
When the pH value in the aqueous solution is about 12, Zn(OH)2 is the main composition. During hydrothermal process, part of the Zn(OH)2 colloids dissolves into Zn2+ and OH according to reaction (2): ZnðOHÞ2 ! Zn2þ þ 2 OH :
(2)
ARTICLE IN PRESS R.S. Yadav, A.C. Pandey / Physica E 40 (2008) 660–663
662
When the concentration of Zn2+ and OH reaches the super saturation degree of ZnO, ZnO nuclei will form according to reaction (3):
3000
unannealed vacuum annealed furnace annealed
2500
Zn2þ þ 2 OH 2ZnO þ H2 O:
(3)
ZnðOHÞ2 þ 2 OH 2½ZnðOHÞ4 :
2
(4)
This indicates that Zn(OH)2 not only contributes to ZnO nuclei but also transforms into [Zn(OH)4]2. It is reported that there are different parameters— temperature, time, pH value and capping agent; that can influence the growth pattern of nanocrystal under nonequilibrium kinetic growth conditions in the solution-based approach [21]. In our experiment temperature, pH value and time are the key parameters. Based on the well-known behavior of zinc in alkaline media, we propose that when temperature is 330 K and pH value is equal to 12, a larger quantity of zinc hydroxide and a sufficient quantity of growth unit are obtained. Furthermore, at temperature 330 K, active sites can generate around circumference of ZnO nuclei, so that ZnO will preferentially grow on the active sites. Therefore, needlelike ZnO nanostructures can be obtained. It is well known that ZnO is a polar crystal whose positive polar plane is Zn rich and the negative polar plane is rich in O. In the hydrothermal process, the growth unit of ZnO is [Zn(OH)4]2, which leads to the different ¯ growth rates of planes as follows: V ð0 0 0 1Þ4V ð 1¯ 0 1 1Þ4 ¯ ¯ ¯ V ð 1 0 1 0Þ4V ð 1 0 1 1Þ4V ð0 0 0 1Þ. In principle, the more rapid the growth rate, the quicker the disappearance of the plane. Therefore, the (0 0 0 1) plane of ZnO, the most rapid growth rate plane, disappears in the hydrothermal process, which leads to the tip at the (0 0 0 1) end of the c-axis ¯ plane, the slowest growth rate [22,23], while the ð0 0 0 1Þ plane, is maintained in the hydrothermal process, leading ¯ end of the c-axis. Therefore, needle-like ZnO to the ð0 0 0 1Þ nanorods can be achieved (Fig. 2). Fig. 3 shows the PL spectrum of needle-like ZnO nanostructure synthesized by organic-free hydrothermal process, under various conditions that include unannealed samples, samples annealed at 200 1C in vacuum oven and furnace for 4 h. In general, the room temperature PL spectra of all the un-annealed and annealed samples show same features. Here, all the samples have emission peak at 580 nm without changing its position. Though vacuum annealed sample show higher intensity while furnace annealed sample show degradation in intensity, indicating decrease and increase in defects responsible for nonradiative transitions on vacuum and furnace annealing, respectively. The excitation wavelength was 325 nm from He–Cd laser. The UV emission at 380 nm was not observed in hydrothermally synthesized ZnO nanoneedles. Generally, in ZnO green emission (520 nm) resulted from the oxygen vacancies, although origion of orange emission
2000 Intensity (a.u.)
Thus, the growth units of [Zn(OH)4]2 form according to reaction (4):
1500 1000 500 0 -500 200
400
600
800
1000
1200
wavelength (nm)
Fig. 3. Room temperature PL spectrum of needle-like ZnO nanostructure synthesized by organic-free hydrothermal process, also annealing it at 200 1C in vacuum oven and in furnace for 4 h.
(580 nm) is not fully understood, seems to involve interstitial oxygen ions (Oi) [24]. 4. Conclusions In conclusion, we have successfully prepared needle-like ZnO nanostructure with a diameter of about 20–30 and 200–250 nm in length by organic-free hydrothermal process at low temperature 330 K. Investigation of X-ray diffraction demonstrates that ZnO nanoneedles are single phase with hexagonal structure. Due to high mechanical stability of ZnO, sharp needle shaped nanostructure would still be a better option for the tip of the AFM probe. This method also promises for production of material at mass scale. Acknowledgments We acknowledge the support obtained from various institutions like IUAC (Delhi) with special thanks to Manavendra Kumar for PL measurements and BHU (varanasi) for TEM measurements. References [1] D.C. Reynolds, D.C. Look, B. Jogai, J.E. Hoelscher, R.E. Sheriff, M.T. Harris, M.J. Callahan, J. Appl. Phys. 88 (2000) 2152. [2] M. Hiramatsu, K. Imaeda, N. Horio, M. Nawata, J. Vac. Sci. Technol. A 16 (1998) 669. [3] W.Z. Pan, R.Z. Dai, Z.L. Wang, Science 291 (2001) 1947. [4] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Lett. 3 (2003) 235. [5] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Lett. 2 (2003) 1287. [6] C. Ye, G. Meng, Y. Wang, Z. Jiang, Z.L. Wang, J. Phys. Chem. B 106 (2002). [7] L. Vayssieres, Adv. Mater. 15 (2003) 464.
ARTICLE IN PRESS R.S. Yadav, A.C. Pandey / Physica E 40 (2008) 660–663 [8] L. Vayssieres, K. Keis, S.-E. Lindquist, A. Hagfeldt, J. Phys. Chem. B 105 (2001) 3350. [9] L. Vayssieres, K. Keis, A. Hagfeldt, S.-E. Lindquist, Chem. Mater. 13 (2001) 4395. [10] L.E. Greene, M. Law, J. Goldberger, F. Kim, J.C. Johnson, Y. Zhang, R.J. Saykally, P. Yang, Angew. Chem. Int. Ed. Engl. 42 (2003) 3031. [11] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 125 (2003) 4430. [12] K. Govender, D.S. Boyle, P. O’Brien, D. Binks, D. West, D. Coleman, Adv. Mater. 14 (2002) 1221. [13] C. Pacholski, A. Kornowski, H. Weller, Angew. Chem. Int. Ed. Engl. 41 (2002) 1188. [14] B.B. Lakshmi, P.K. Dorhout, C.R. Martin, Chem. Mater. 9 (1997) 857. [15] B.B. Lakshmi, C.J. Patrissi, C.R. Martin, Chem. Mater. 9 (1997) 2544.
663
[16] H. Zhang, X.Y. Ma, J. Xu, J.J. Niu, D.R. Yang, Nanotechnology 14 (2003) 423. [17] J. Zhang, L.D. Sun, J.L. Yin, H.L. Su, C.S. Liao, C.H. Yan, Chem. Mater. 14 (2002) 4172. [18] W.I. Park, G.C. Yi, M. Kim, S.J. Pennycook, Adv. Mater. 14 (2002) 1841. [19] J.L. Yang, S.J. An, W.I. Park, G.C. Yi, W. Choi, Adv. Mater. 16 (2004) 1661. [20] H. Zhang, D. Yang, Y. Ji, X. Ma, J. Xu, D. Que, J. Phys. Chem. B 13 (2004) 108. [21] S.M. Lee, S.N. Cho, J.W. Cheon, Adv. Mater. 15 (5) (2003) 441. [22] N.D. Lioudmila, V.K. Dmitriy, Ann. Chim. Sci. Mat. 26 (2001) 193. [23] W.J. Li, E.W. Shi, W.Z. Zhong, Z.W. Yin, J. Synth. Cryst. 28 (2) (1999) 117. [24] M. Liu, A.H. Kitai, P. Mascher, J. Lumin. 54 (1992) 35.
Physica E 40 (2008) 660–663 www.elsevier.com/locate/physe
Needle-like ZnO nanostructure synthesized by organic-free hydrothermal process Raghvendra S. Yadav, Avinash C. Pandey Nanophosphor Application Centre, University of Allahabad, Allahabad 211002, India Received 6 October 2006; accepted 30 August 2007 Available online 14 September 2007
Abstract In present work, needle-like ZnO nanostructures were synthesized by an organic-free hydrothermal process. The hydrothermal synthesis was performed at temperature 330 K. The products were characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The XRD pattern indicated that the needle-like ZnO nanostructures were hexagonal. Morphology, diameter and length of needles were investigated. Finally, the mechanism for organic-free hydrothermal synthesis of the needle-like ZnO nanostructure is discussed. r 2007 Elsevier B.V. All rights reserved. PACS: 61.46 Keywords: ZnO; Organic-free hydrothermal process
1. Introduction In the recent years, nanostructures have attracted much attention owing to their great potential for application in nanodevices; since, in a nanostructure, the density of states of the carrier is concentrated at some specific energy levels. The wurtzite nanostructure family has a few important members such as ZnO, GaN, AlN, ZnS and CdSe. ZnO is one of the most important nanomaterials of this family because it has three key properties. First, it is semiconductor, with a direct wide band gap of 3.37 eV and a large excitation binding energy (60 meV) [1]. It exhibits nearultraviolet emission and transparent conductivity. Secondly, because of its non-central symmetry, ZnO is piezoelectric, which is a key property in building electromechanical-coupled sensors and transducers [2]. Finally ZnO is bio-safe and biocompatible, and can be used for biomedical applications. The current initiatives in nanoscience and nanotechnology have led to the fabrication of a variety of nanostructures Corresponding author. Tel.: +91 532 2460675.
E-mail address: [email protected] (R.S. Yadav). 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.08.087
of ZnO such as nanowires, nanorods, nanotubes, nanobelts, nanocombs, nanosaws, nanosprings, nanospirals, nanorings, etc. Many interesting nanostructures of ZnO including nanobelts, nanobridges, nanonails, and nanoribbons have been fabricated by thermal evaporation of oxide powders [3–6]. Recently, chemical solution routes including solvothermal, hydrothermal, self-assembly and a template assisted sol–gel process have been employed to synthesize ZnO nanowires and nanorods [7–17]. The synthesis of ZnO nanowires and nanorods by a solvothermal process has also been reported. It is inevitable to use toxic, dangerous, and expensive solvents such as amine in solvothermal process. Nowadays, one of the methods of ‘‘soft chemistry’’, namely hydrothermal technique is widely utilized for the preparation of nanocrystalline oxide material such as ZnO. It is important that hydrothermally obtained powders could be produced with different nanostructures by varying parameters such as temperature, pressure, duration of process, concentration of chemical species and pH of solution. Herein, we have synthesized needle-like ZnO nanostructures by organic-free hydrothermal process. The tip of ZnO nanoneedles offer potential application as AFM probing tips, which have
ARTICLE IN PRESS R.S. Yadav, A.C. Pandey / Physica E 40 (2008) 660–663
10000
2. Experimental section
The needle-like ZnO nanostructures were prepared in the following process: 1 g of zinc acetate dihydrate was put into 105 mL of distilled water under vigorous stirring. After 10 min stirring, 15 mL of 2 M NaOH aqueous solution was introduced into the above aqueous solution, resulting in a white aqueous solution (pH value was equal to 12), which was then transferred into stainless steel autoclaves, sealed, and maintained at temperature 330 K for 20 h. The precipitate in the autoclave was taken out and washed repeatedly with distilled water and ethanol to remove the ions possibly remaining in the final products. Then a white powder was obtained after drying at 60 1C in air for 2 h. A part of the powder was put into a vacuum oven and tube furnace kept at 200 1C for 4 h.
(112) (200)
2000
2.2. Sample preparation
(201)
(103)
4000
(110)
(100) (002)
6000
(102)
The zinc acetate dihydrate (98%) Zn(CH3COO)2.2H2O, sodium hydroxide NaOH was from E. Merck (India) Limited, Mumbai 400018, India. These chemicals were directly used without special treatment.
8000 Intensity (a.u.)
2.1. Chemicals
(101)
high spatial resolution in both vertical and horizontal dimensions [18,19].
661
0 30
40
50
60
70
2-theta (degree)
Fig. 1. XRD pattern of needle-like ZnO nanostructure synthesized by organic-free hydrothermal process at low pressure 20 kPa.
2.3. Apparatus The ultimate products were characterized by X-ray powder diffraction (XRD) using a Rigaku X-ray diffractometer with Cu Ka radiation (l=1.54178 A˚). XRD data were collected over the range 20–801 with a step interval of 0.021 at room temperature. Transmission electron microscopy (TEM) observation was performed with a Philips CM 12 microscope operated at 110 kV. Photoluminescence studies were performed using He–Cd laser (KIMMON) and Mechelle900 spectrograph working in 200–1100 nm wavelength region. The PL set up has a cooled CCD arraybased detection system. The laser light is incident on the sample at 45 1C and the luminescence light is collected using a collector assembly and transmitted to the spectrograph through optical fiber for detection and analysis. The PL studies were performed using excitation by 325 nm and the data were recorded for 30 s on each sample. 3. Results and discussion Fig. 1 shows powder X-ray diffraction pattern of sample prepared at pressure 20 kPa in the solution of pH 12 using NaOH as mineralizer indicate that sample is single phase with a wurtzite structure. Furthermore, it can be seen that the diffraction peaks are higher and narrower, implying that the ZnO crystallizes well.
Fig. 2. TEM image of needle-like ZnO nanostructure synthesized by organic-free hydrothermal process.
Fig. 2 shows the TEM images of the ZnO nanostructures prepared by the hydrothermal process at pressure 20 kPa and pH values 12 using NaOH as the mineralizer. When pressure was 20 kPa, ZnO nanostructures consisting of needle-like nanorods 20–30 nm in diameter and 200–250 nm in length are obtained. Yang et al. [20] reported that OH is first introduced into Zn2+ aqueous solution, and then Zn(OH)2 colloids form, according to Zn2þ þ 2OH 2ZnðOHÞ2 :
(1)
When the pH value in the aqueous solution is about 12, Zn(OH)2 is the main composition. During hydrothermal process, part of the Zn(OH)2 colloids dissolves into Zn2+ and OH according to reaction (2): ZnðOHÞ2 ! Zn2þ þ 2 OH :
(2)
ARTICLE IN PRESS R.S. Yadav, A.C. Pandey / Physica E 40 (2008) 660–663
662
When the concentration of Zn2+ and OH reaches the super saturation degree of ZnO, ZnO nuclei will form according to reaction (3):
3000
unannealed vacuum annealed furnace annealed
2500
Zn2þ þ 2 OH 2ZnO þ H2 O:
(3)
ZnðOHÞ2 þ 2 OH 2½ZnðOHÞ4 :
2
(4)
This indicates that Zn(OH)2 not only contributes to ZnO nuclei but also transforms into [Zn(OH)4]2. It is reported that there are different parameters— temperature, time, pH value and capping agent; that can influence the growth pattern of nanocrystal under nonequilibrium kinetic growth conditions in the solution-based approach [21]. In our experiment temperature, pH value and time are the key parameters. Based on the well-known behavior of zinc in alkaline media, we propose that when temperature is 330 K and pH value is equal to 12, a larger quantity of zinc hydroxide and a sufficient quantity of growth unit are obtained. Furthermore, at temperature 330 K, active sites can generate around circumference of ZnO nuclei, so that ZnO will preferentially grow on the active sites. Therefore, needlelike ZnO nanostructures can be obtained. It is well known that ZnO is a polar crystal whose positive polar plane is Zn rich and the negative polar plane is rich in O. In the hydrothermal process, the growth unit of ZnO is [Zn(OH)4]2, which leads to the different ¯ growth rates of planes as follows: V ð0 0 0 1Þ4V ð 1¯ 0 1 1Þ4 ¯ ¯ ¯ V ð 1 0 1 0Þ4V ð 1 0 1 1Þ4V ð0 0 0 1Þ. In principle, the more rapid the growth rate, the quicker the disappearance of the plane. Therefore, the (0 0 0 1) plane of ZnO, the most rapid growth rate plane, disappears in the hydrothermal process, which leads to the tip at the (0 0 0 1) end of the c-axis ¯ plane, the slowest growth rate [22,23], while the ð0 0 0 1Þ plane, is maintained in the hydrothermal process, leading ¯ end of the c-axis. Therefore, needle-like ZnO to the ð0 0 0 1Þ nanorods can be achieved (Fig. 2). Fig. 3 shows the PL spectrum of needle-like ZnO nanostructure synthesized by organic-free hydrothermal process, under various conditions that include unannealed samples, samples annealed at 200 1C in vacuum oven and furnace for 4 h. In general, the room temperature PL spectra of all the un-annealed and annealed samples show same features. Here, all the samples have emission peak at 580 nm without changing its position. Though vacuum annealed sample show higher intensity while furnace annealed sample show degradation in intensity, indicating decrease and increase in defects responsible for nonradiative transitions on vacuum and furnace annealing, respectively. The excitation wavelength was 325 nm from He–Cd laser. The UV emission at 380 nm was not observed in hydrothermally synthesized ZnO nanoneedles. Generally, in ZnO green emission (520 nm) resulted from the oxygen vacancies, although origion of orange emission
2000 Intensity (a.u.)
Thus, the growth units of [Zn(OH)4]2 form according to reaction (4):
1500 1000 500 0 -500 200
400
600
800
1000
1200
wavelength (nm)
Fig. 3. Room temperature PL spectrum of needle-like ZnO nanostructure synthesized by organic-free hydrothermal process, also annealing it at 200 1C in vacuum oven and in furnace for 4 h.
(580 nm) is not fully understood, seems to involve interstitial oxygen ions (Oi) [24]. 4. Conclusions In conclusion, we have successfully prepared needle-like ZnO nanostructure with a diameter of about 20–30 and 200–250 nm in length by organic-free hydrothermal process at low temperature 330 K. Investigation of X-ray diffraction demonstrates that ZnO nanoneedles are single phase with hexagonal structure. Due to high mechanical stability of ZnO, sharp needle shaped nanostructure would still be a better option for the tip of the AFM probe. This method also promises for production of material at mass scale. Acknowledgments We acknowledge the support obtained from various institutions like IUAC (Delhi) with special thanks to Manavendra Kumar for PL measurements and BHU (varanasi) for TEM measurements. References [1] D.C. Reynolds, D.C. Look, B. Jogai, J.E. Hoelscher, R.E. Sheriff, M.T. Harris, M.J. Callahan, J. Appl. Phys. 88 (2000) 2152. [2] M. Hiramatsu, K. Imaeda, N. Horio, M. Nawata, J. Vac. Sci. Technol. A 16 (1998) 669. [3] W.Z. Pan, R.Z. Dai, Z.L. Wang, Science 291 (2001) 1947. [4] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Lett. 3 (2003) 235. [5] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Lett. 2 (2003) 1287. [6] C. Ye, G. Meng, Y. Wang, Z. Jiang, Z.L. Wang, J. Phys. Chem. B 106 (2002). [7] L. Vayssieres, Adv. Mater. 15 (2003) 464.
ARTICLE IN PRESS R.S. Yadav, A.C. Pandey / Physica E 40 (2008) 660–663 [8] L. Vayssieres, K. Keis, S.-E. Lindquist, A. Hagfeldt, J. Phys. Chem. B 105 (2001) 3350. [9] L. Vayssieres, K. Keis, A. Hagfeldt, S.-E. Lindquist, Chem. Mater. 13 (2001) 4395. [10] L.E. Greene, M. Law, J. Goldberger, F. Kim, J.C. Johnson, Y. Zhang, R.J. Saykally, P. Yang, Angew. Chem. Int. Ed. Engl. 42 (2003) 3031. [11] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 125 (2003) 4430. [12] K. Govender, D.S. Boyle, P. O’Brien, D. Binks, D. West, D. Coleman, Adv. Mater. 14 (2002) 1221. [13] C. Pacholski, A. Kornowski, H. Weller, Angew. Chem. Int. Ed. Engl. 41 (2002) 1188. [14] B.B. Lakshmi, P.K. Dorhout, C.R. Martin, Chem. Mater. 9 (1997) 857. [15] B.B. Lakshmi, C.J. Patrissi, C.R. Martin, Chem. Mater. 9 (1997) 2544.
663
[16] H. Zhang, X.Y. Ma, J. Xu, J.J. Niu, D.R. Yang, Nanotechnology 14 (2003) 423. [17] J. Zhang, L.D. Sun, J.L. Yin, H.L. Su, C.S. Liao, C.H. Yan, Chem. Mater. 14 (2002) 4172. [18] W.I. Park, G.C. Yi, M. Kim, S.J. Pennycook, Adv. Mater. 14 (2002) 1841. [19] J.L. Yang, S.J. An, W.I. Park, G.C. Yi, W. Choi, Adv. Mater. 16 (2004) 1661. [20] H. Zhang, D. Yang, Y. Ji, X. Ma, J. Xu, D. Que, J. Phys. Chem. B 13 (2004) 108. [21] S.M. Lee, S.N. Cho, J.W. Cheon, Adv. Mater. 15 (5) (2003) 441. [22] N.D. Lioudmila, V.K. Dmitriy, Ann. Chim. Sci. Mat. 26 (2001) 193. [23] W.J. Li, E.W. Shi, W.Z. Zhong, Z.W. Yin, J. Synth. Cryst. 28 (2) (1999) 117. [24] M. Liu, A.H. Kitai, P. Mascher, J. Lumin. 54 (1992) 35.