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Hydrothermal growth of monodispersed rutile TiO2 nanorods and functional properties
Materials Letters 98 (2013) 38–41
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hydrothermal growth of monodispersed rutile TiO2 nanorods and functional properties J. Archana a,b, M. Navaneethan b, Y. Hayakawa a,b,n a b
Graduate School of Science and Technology, Shizuoka University, Japan 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
a b s t r a c t
Article history: Received 21 November 2012 Accepted 7 February 2013 Available online 17 February 2013
Monodispersed rutile TiO2 nanorods have been synthesized by hydrothermal method. Citric acid is used as a capping agent to prevent agglomeration. Crystal structure, morphology and phase formation were characterized by X-ray diffraction (XRD), ultraviolet visible spectroscopy (UV), Raman spectroscopy, fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). XRD pattern revealed the formation of rutile phased TiO2. The prominent UV absorption was detected and the band gap was found to be 3.22 eV. Spectroscopic studies evidenced the presence of inorganic and organic compounds. FESEM and TEM images illustrated the formation of monodispersed TiO2 nanorods with 5–7 mm in length and of 20–30 nm in thickness. & 2013 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Structural Electron microscopy Raman XPS FTIR
1. Introduction For the past decades, dye sensitized solar cells (DSSCs) have attracted a great interest due to the conversion of light to electrical energy [1–5]. Titanium (TiO2) has been considered as a promising semiconductor material for sensors, photo catalytic and photo voltaic applications due to the wide band gap [6–8]. Hierarchical one dimensional nanostructures of TiO2 received much attention due to the enhanced properties compared to that of the bulk TiO2. The one dimensional structures have high surface to volume ratio and the unidirectional channels possess a direct pathways which increases the electron mobility and enhances the performance of the DSSCs. The TiO2 has three crystalline structures such as anatase, rutile, and brookite. It is reported that the rutile phase has a good physical properties and is used in various applications such as lithium ion batteries, DSSCs etc [9,10]. The rutile nanorods are more stable at high temperatures when compared to brookite and anatase phases. However, synthesis of the well aligned rutile TiO2 nanorods is difficult due to the high hydrolysis rate. There are several methods to prepare the well aligned TiO2 nanorods such as sol gel, chemical vapor deposition, hydrothermal, electro spinning methods and so on [11,12]. Jian shi et al. had synthesized rutile TiO2 nanorods by pulsed chemical vapor deposition and studied the effects of purging time coating of Au
and the temperature on the product. The obtained morphologies were nanorods, nanowires, nano flakes and nanoparticles [13]. Wenxi Guo et al. had prepared the rectangular branched rutile TiO2 nanorods arrays by a new technique called dissolve, grow and etch grow method [14]. They studied the photovoltaic measurement for the prepared nanorods and obtained the efficiency of 1.68%. M.Ge et al. had synthesized the rutile phased 3D TiO2 hierarchical structures by one step template free hydrothermal method and obtained excellent photo catalytic performance [15]. Tahir et al. had reported the hydrothermal growth of rutile TiO2 nanorods using 3-hydrosytytramine as the functionalization agent [16]. Hydrothermal method is simple and inexpensive to extend for large scale production when compared to sol–gel, chemical vapor deposition and electro spinning technique. However, the monodispersed synthesis is a challenging task due to the ripening and agglomeration processes. The organic capping molecules such as triethylamine, ethylenediaminetetraacetic acid, N-methylaniline were effectively used to synthesis the monodispersed semiconducting nanostructures [17–19]. In this work, we report the synthesis of rutile TiO2 nanorods by simple hydrothermal method using citric acid as a capping agent. The role of capping agent for the formation of TiO2 nanorods and the detailed functional properties are investigated.
2. Experimental details n
Corresponding author at: Research Institute of Electronics, Shizuoka University 3-5-1 Johoku, Naka-Ku, Hamamatsu, Shizuoka 432-8011, Japan. Tel./fax: þ81 534 781 338. E-mail address: [email protected] (Y. Hayakawa). 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.02.031
All the chemicals were purchased from WAKO chemicals, Japan and used without further purification. Synthesis of the TiO2 nanorods is as follows: 1 mL of titanium trichloride was
J. Archana et al. / Materials Letters 98 (2013) 38–41
dissolved in the mixture of 10 mL of de-ionized water and 10 mL of hydrochloric acid under vigorous magnetic stirring of 450 rpm at room temperature. 1 mg of citric acid was added to the above
39
solution. The reaction was continued for 8 h, and then the solution was transferred to the Teflon-lined stainless steel autoclave and hydrothermal growth was carried out at 180 1C for 24 h.
Fig. 1. (a) XRD pattern, (b) UV visible absorption (inset: band gap plot), (c) Raman spectrum and (d) FTIR spectrum of rutile TiO2 nanorods.
Fig. 2. (a and b) FESEM images at various magnifications, (c) TEM image, and (d) HRTEM image of rutile TiO2 nanorods.
40
J. Archana et al. / Materials Letters 98 (2013) 38–41
After the growth, the precipitates were collected and annealed at 200 1C for 3 h. The XRD pattern was recorded using a Rigaku (Japan) X-ray diffractometer (RINT-2200, Cu Ka radiation) at 0.021/sec as the step interval. UV–visible absorption analysis was performed by a Shimadzu (Japan) 3100 PC spectrophotometer using ethanol as a dispersing medium. The field emission scanning electron microscope (FESEM) images were recorded using a JSM 7100 microscope. The transmission electron microscope (TEM) images were recorded by a JEOL JEM 2100 F microscope at an accelerating voltage of 200 kV.
3. Results and discussion Fig. 1(a) depicts the XRD pattern of TiO2 nanorods. All the diffraction peaks indicated the formation of rutile phase of the crystal structure. It was good agreement with the standard JCPDS card no: 89–0554. No other diffraction peaks of other phases such as anatase and brookite were observed [20]. XRD patterns for uncapped and various concentrations of citric acid capped TiO2 show rutile phase (See Fig. S1). The optical absorption spectrum of the rutile TiO2 nanorods is shown in Fig. 1(b). The bandgap was calculated as 3.22 eV using the band gap plot (hu Vs. (ahu)2 as shown
in the inset of Fig. 1(b). (c) shows the Raman spectrum of TiO2 nanorods. The two major peaks located at 447 and 612 cm 1 represented the Eg and A1g modes of the rutile phase, respectively. The peak observed at 237 cm 1 (Eg) was weak and broaden which was due to the phonon confinement effect [21]. Fig. 1(d) represents the FTIR measurement of the TiO2 nanorods. The peaks at 620 and 1200 cm 1 indicated the symmetric stretching vibration of Ti-O-Ti and the asymmetric stretching vibrations of Ti-O, respectively. The peak at 1440 cm 1 corresponded to the –OH bending. The peaks at 1600 and 1750 cm 1 corresponded to CQO stretching vibrational modes for the presence of the carboxylic group. It indicated the presence of the citric acid [22,23]. The peak at 2400 cm 1 corresponded to the atmospheric CO2. The morphological studies are described in Fig. 2. FESEM images of rutile TiO2 nanorods at lower magnifications are provided in Fig. 2(a). It revealed the formation of monodispersed nanorods with the length of 5–7 mm. It is seen that the branched structure was composed of rod - like array geometry. Fig. 2(b) revealed that the nanorods were uniformly aligned with the smooth surface at the side walls of the entire length. The inset figure represents that they had the square facets at the top surface which were the growth habit for the tetragonal crystal structure. The corresponding TEM image shown in the Fig. 2(c) confirms the thickness of the nanorods was 20–30 nm. The HRTEM image as shown in Fig. 2(d) indicates
Fig. 3. XPS spectra (a) Ti 2p3/2 state and (b) O 1s state of rutile TiO2 nanorods.
Fig. 4. Formation mechanism of rutile TiO2 nanorods.
J. Archana et al. / Materials Letters 98 (2013) 38–41
that the nanorods were well crystalline. Moreover, the morphology of uncapped and various concentrations of citric acid capped TiO2 nanostructures are studied (see Figs S2–S5 and Table S1). The results show that the concentration of citric acid has significant role to obtain the monodispersed and well-aligned TiO2 nanorods. Fig. 3(a) and (b) present the core level spectra of Ti 2P and O 1s of the rutile nanorods. Since the binding energies of Ti 2p3/2 and Ti 2p1/2 are 459.5 and 464.9 eV, the peaks at 459.5 and 464.9 eV attributed to the Ti3 þ and Ti4 þ ions, respectively [24]. The O 1s peak showed an asymmetric shape and was deconvoluted into two peaks using the Gaussian fitting curve. The main peak located at 530.6 eV was produced by the signature of the lattice oxygen O1s in the Ti–O–Ti bonds. Whereas, the peak at 531.9 eV corresponded to the defect level oxygen in the TiO2 nanorods [25,26]. Fig. 4 describes the growth mechanism of TiO2 nanorods. The hydrolysis of TiCl3 resulted in the formation of TiO2. Usually a rapid hydrolysis occurs during the hydrothermal growth of TiO2. When the citric acid is added, it increases the ionic strength of the solution and slows down the hydrolysis rate of the solution which promotes the establishment of smaller crystals by electrostatic screening [27]. Moreover, the interaction between the Ti4 þ and carboxylated group of citric acid controls the chemical kinetics of the hydrothermal method.
4. Conclusion The rutile TiO2 nanorods with the thickness of about 20–30 nm and length of 5–7 mm have been successfully synthesized by facile one-step hydrothermal method. The addition of citric acid to the solution retards the hydrolysis and favors the formation of one dimensional structure with monodispersed size distribution. The obtained TiO2 nanorods would be the affirmative material for the dye adsorption in the dye-sensitized solar cell.
Acknowledgments J. Archana would like to thank MEXT, Japan, for the award of a research fellowship.
41
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.02.031.
References [1] Peng X, Manna L, Yang W, Wickham J, Scher E, Kadavanich A, et al. Nature 2000;404:59–61. [2] Colvin VL, Schlamp MC, Alivisatos AP. Nature 1994;370:354–7. [3] Liu B, Aydil ES. J Am Chem Soc 2009;131(11):3985–90. [4] Armstrong AR, Armstrong G, Canales J, Garcia F, Bruce PG. Adv Mater 2005;17(7):862–5. [5] Hwang YJ, Boukai A, Yang PD. Nano Lett 2009;9(1):410–5. [6] Ramakrishna G, Ghosh HN. J Phys Chem B 2001;105:7000–8. [7] Regan BO, Gratzel M. Nature 1991;353:737–40. [8] Asahi R, Morikawa T, Ohwaki T, Aoki K, Taga Y. Science 2001;293:269–71. [9] Kim KJ, Benksten KD, Van de Lagemaat J, Frank A. J Chem Mater 2002;14: 1042–7. [10] Zhang H, Li GR, An LP, Yan TY, Gao XP, Zhu HY. J Phys Chem C 2007;111: 6143–8. [11] Miao Z, Xu DS, Ouyang JH, Guo GL, Zhao XS, Tang YQ. Nano Lett 2002;2(7):717–20. [12] Formo E, Lee E, Campbell D, Xia YN. Nano Lett 2008;8(2):668–72. [13] Shi J, Wang X. Cryst Growth Des 2011;11:949–54. [14] Guo W, Xu C, Wang X, Wang S, Pan C, Lin C, et al. J Am Chem Soc 2012;134:4437–41. [15] Ge M, Li JW, Liu L, Zhou Z. Ind Eng Chem Res 2011;50:6681–7. [16] Tahir MN, Theato P, Oberle P, Melnyk G, Faiss S, Kolb U, et al. Langmuir 2006;22:5209–12. [17] Navaneethan M, Nisha KD, Ponnusamy S, Muthamizhchelvan C. Mater Chem Phys 2009;117:443–7. [18] Navaneethan M, Archana J, Arivanandhan M, Hayakawa Y. Phys Status Solidi RRL 2012;6:120–2. [19] Navaneethan M, Archana J, Nisha KD, Hayakawa Y, Ponnusamy S, Muthamizhchelvan C. Mater Lett 2012;68:78–81. [20] Nguyen Phan TD, Pham HD, Cuong TV, Kim EJ, Kim S, Shin EW. J Cryst Growth 2009;312:79–85. [21] Tompsett GA, B GA, Cooney RP, Metson JB, Rodgers KA, Seakins JM. J Raman spectrosc 1995;26:57–62. [22] Li G, Li D, Zhang, Zhai J, Wang E. Chemistry 2009;15:9868–73. [23] Thistlethwaite PJ, Hook MS. Langmuir 2000;16:4993–8. [24] Zhang YG, Ma L, Li JL, Yu Y. Environ Sci Tech 2007;41(17):6264–9. [25] Shamaila S, Sajjad A K L, Feng C, Jinlong Z. Chem Eur J 2010;16:13795–804. [26] Li J, Wang D, Liu H, He Z, Zhu Z. Appl Surf Sci 2011;257:5879–84. [27] Feng J, Zeng HC. Chem Mater 2003;15:2829–35.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hydrothermal growth of monodispersed rutile TiO2 nanorods and functional properties J. Archana a,b, M. Navaneethan b, Y. Hayakawa a,b,n a b
Graduate School of Science and Technology, Shizuoka University, Japan 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
a b s t r a c t
Article history: Received 21 November 2012 Accepted 7 February 2013 Available online 17 February 2013
Monodispersed rutile TiO2 nanorods have been synthesized by hydrothermal method. Citric acid is used as a capping agent to prevent agglomeration. Crystal structure, morphology and phase formation were characterized by X-ray diffraction (XRD), ultraviolet visible spectroscopy (UV), Raman spectroscopy, fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). XRD pattern revealed the formation of rutile phased TiO2. The prominent UV absorption was detected and the band gap was found to be 3.22 eV. Spectroscopic studies evidenced the presence of inorganic and organic compounds. FESEM and TEM images illustrated the formation of monodispersed TiO2 nanorods with 5–7 mm in length and of 20–30 nm in thickness. & 2013 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Structural Electron microscopy Raman XPS FTIR
1. Introduction For the past decades, dye sensitized solar cells (DSSCs) have attracted a great interest due to the conversion of light to electrical energy [1–5]. Titanium (TiO2) has been considered as a promising semiconductor material for sensors, photo catalytic and photo voltaic applications due to the wide band gap [6–8]. Hierarchical one dimensional nanostructures of TiO2 received much attention due to the enhanced properties compared to that of the bulk TiO2. The one dimensional structures have high surface to volume ratio and the unidirectional channels possess a direct pathways which increases the electron mobility and enhances the performance of the DSSCs. The TiO2 has three crystalline structures such as anatase, rutile, and brookite. It is reported that the rutile phase has a good physical properties and is used in various applications such as lithium ion batteries, DSSCs etc [9,10]. The rutile nanorods are more stable at high temperatures when compared to brookite and anatase phases. However, synthesis of the well aligned rutile TiO2 nanorods is difficult due to the high hydrolysis rate. There are several methods to prepare the well aligned TiO2 nanorods such as sol gel, chemical vapor deposition, hydrothermal, electro spinning methods and so on [11,12]. Jian shi et al. had synthesized rutile TiO2 nanorods by pulsed chemical vapor deposition and studied the effects of purging time coating of Au
and the temperature on the product. The obtained morphologies were nanorods, nanowires, nano flakes and nanoparticles [13]. Wenxi Guo et al. had prepared the rectangular branched rutile TiO2 nanorods arrays by a new technique called dissolve, grow and etch grow method [14]. They studied the photovoltaic measurement for the prepared nanorods and obtained the efficiency of 1.68%. M.Ge et al. had synthesized the rutile phased 3D TiO2 hierarchical structures by one step template free hydrothermal method and obtained excellent photo catalytic performance [15]. Tahir et al. had reported the hydrothermal growth of rutile TiO2 nanorods using 3-hydrosytytramine as the functionalization agent [16]. Hydrothermal method is simple and inexpensive to extend for large scale production when compared to sol–gel, chemical vapor deposition and electro spinning technique. However, the monodispersed synthesis is a challenging task due to the ripening and agglomeration processes. The organic capping molecules such as triethylamine, ethylenediaminetetraacetic acid, N-methylaniline were effectively used to synthesis the monodispersed semiconducting nanostructures [17–19]. In this work, we report the synthesis of rutile TiO2 nanorods by simple hydrothermal method using citric acid as a capping agent. The role of capping agent for the formation of TiO2 nanorods and the detailed functional properties are investigated.
2. Experimental details n
Corresponding author at: Research Institute of Electronics, Shizuoka University 3-5-1 Johoku, Naka-Ku, Hamamatsu, Shizuoka 432-8011, Japan. Tel./fax: þ81 534 781 338. E-mail address: [email protected] (Y. Hayakawa). 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.02.031
All the chemicals were purchased from WAKO chemicals, Japan and used without further purification. Synthesis of the TiO2 nanorods is as follows: 1 mL of titanium trichloride was
J. Archana et al. / Materials Letters 98 (2013) 38–41
dissolved in the mixture of 10 mL of de-ionized water and 10 mL of hydrochloric acid under vigorous magnetic stirring of 450 rpm at room temperature. 1 mg of citric acid was added to the above
39
solution. The reaction was continued for 8 h, and then the solution was transferred to the Teflon-lined stainless steel autoclave and hydrothermal growth was carried out at 180 1C for 24 h.
Fig. 1. (a) XRD pattern, (b) UV visible absorption (inset: band gap plot), (c) Raman spectrum and (d) FTIR spectrum of rutile TiO2 nanorods.
Fig. 2. (a and b) FESEM images at various magnifications, (c) TEM image, and (d) HRTEM image of rutile TiO2 nanorods.
40
J. Archana et al. / Materials Letters 98 (2013) 38–41
After the growth, the precipitates were collected and annealed at 200 1C for 3 h. The XRD pattern was recorded using a Rigaku (Japan) X-ray diffractometer (RINT-2200, Cu Ka radiation) at 0.021/sec as the step interval. UV–visible absorption analysis was performed by a Shimadzu (Japan) 3100 PC spectrophotometer using ethanol as a dispersing medium. The field emission scanning electron microscope (FESEM) images were recorded using a JSM 7100 microscope. The transmission electron microscope (TEM) images were recorded by a JEOL JEM 2100 F microscope at an accelerating voltage of 200 kV.
3. Results and discussion Fig. 1(a) depicts the XRD pattern of TiO2 nanorods. All the diffraction peaks indicated the formation of rutile phase of the crystal structure. It was good agreement with the standard JCPDS card no: 89–0554. No other diffraction peaks of other phases such as anatase and brookite were observed [20]. XRD patterns for uncapped and various concentrations of citric acid capped TiO2 show rutile phase (See Fig. S1). The optical absorption spectrum of the rutile TiO2 nanorods is shown in Fig. 1(b). The bandgap was calculated as 3.22 eV using the band gap plot (hu Vs. (ahu)2 as shown
in the inset of Fig. 1(b). (c) shows the Raman spectrum of TiO2 nanorods. The two major peaks located at 447 and 612 cm 1 represented the Eg and A1g modes of the rutile phase, respectively. The peak observed at 237 cm 1 (Eg) was weak and broaden which was due to the phonon confinement effect [21]. Fig. 1(d) represents the FTIR measurement of the TiO2 nanorods. The peaks at 620 and 1200 cm 1 indicated the symmetric stretching vibration of Ti-O-Ti and the asymmetric stretching vibrations of Ti-O, respectively. The peak at 1440 cm 1 corresponded to the –OH bending. The peaks at 1600 and 1750 cm 1 corresponded to CQO stretching vibrational modes for the presence of the carboxylic group. It indicated the presence of the citric acid [22,23]. The peak at 2400 cm 1 corresponded to the atmospheric CO2. The morphological studies are described in Fig. 2. FESEM images of rutile TiO2 nanorods at lower magnifications are provided in Fig. 2(a). It revealed the formation of monodispersed nanorods with the length of 5–7 mm. It is seen that the branched structure was composed of rod - like array geometry. Fig. 2(b) revealed that the nanorods were uniformly aligned with the smooth surface at the side walls of the entire length. The inset figure represents that they had the square facets at the top surface which were the growth habit for the tetragonal crystal structure. The corresponding TEM image shown in the Fig. 2(c) confirms the thickness of the nanorods was 20–30 nm. The HRTEM image as shown in Fig. 2(d) indicates
Fig. 3. XPS spectra (a) Ti 2p3/2 state and (b) O 1s state of rutile TiO2 nanorods.
Fig. 4. Formation mechanism of rutile TiO2 nanorods.
J. Archana et al. / Materials Letters 98 (2013) 38–41
that the nanorods were well crystalline. Moreover, the morphology of uncapped and various concentrations of citric acid capped TiO2 nanostructures are studied (see Figs S2–S5 and Table S1). The results show that the concentration of citric acid has significant role to obtain the monodispersed and well-aligned TiO2 nanorods. Fig. 3(a) and (b) present the core level spectra of Ti 2P and O 1s of the rutile nanorods. Since the binding energies of Ti 2p3/2 and Ti 2p1/2 are 459.5 and 464.9 eV, the peaks at 459.5 and 464.9 eV attributed to the Ti3 þ and Ti4 þ ions, respectively [24]. The O 1s peak showed an asymmetric shape and was deconvoluted into two peaks using the Gaussian fitting curve. The main peak located at 530.6 eV was produced by the signature of the lattice oxygen O1s in the Ti–O–Ti bonds. Whereas, the peak at 531.9 eV corresponded to the defect level oxygen in the TiO2 nanorods [25,26]. Fig. 4 describes the growth mechanism of TiO2 nanorods. The hydrolysis of TiCl3 resulted in the formation of TiO2. Usually a rapid hydrolysis occurs during the hydrothermal growth of TiO2. When the citric acid is added, it increases the ionic strength of the solution and slows down the hydrolysis rate of the solution which promotes the establishment of smaller crystals by electrostatic screening [27]. Moreover, the interaction between the Ti4 þ and carboxylated group of citric acid controls the chemical kinetics of the hydrothermal method.
4. Conclusion The rutile TiO2 nanorods with the thickness of about 20–30 nm and length of 5–7 mm have been successfully synthesized by facile one-step hydrothermal method. The addition of citric acid to the solution retards the hydrolysis and favors the formation of one dimensional structure with monodispersed size distribution. The obtained TiO2 nanorods would be the affirmative material for the dye adsorption in the dye-sensitized solar cell.
Acknowledgments J. Archana would like to thank MEXT, Japan, for the award of a research fellowship.
41
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.02.031.
References [1] Peng X, Manna L, Yang W, Wickham J, Scher E, Kadavanich A, et al. Nature 2000;404:59–61. [2] Colvin VL, Schlamp MC, Alivisatos AP. Nature 1994;370:354–7. [3] Liu B, Aydil ES. J Am Chem Soc 2009;131(11):3985–90. [4] Armstrong AR, Armstrong G, Canales J, Garcia F, Bruce PG. Adv Mater 2005;17(7):862–5. [5] Hwang YJ, Boukai A, Yang PD. Nano Lett 2009;9(1):410–5. [6] Ramakrishna G, Ghosh HN. J Phys Chem B 2001;105:7000–8. [7] Regan BO, Gratzel M. Nature 1991;353:737–40. [8] Asahi R, Morikawa T, Ohwaki T, Aoki K, Taga Y. Science 2001;293:269–71. [9] Kim KJ, Benksten KD, Van de Lagemaat J, Frank A. J Chem Mater 2002;14: 1042–7. [10] Zhang H, Li GR, An LP, Yan TY, Gao XP, Zhu HY. J Phys Chem C 2007;111: 6143–8. [11] Miao Z, Xu DS, Ouyang JH, Guo GL, Zhao XS, Tang YQ. Nano Lett 2002;2(7):717–20. [12] Formo E, Lee E, Campbell D, Xia YN. Nano Lett 2008;8(2):668–72. [13] Shi J, Wang X. Cryst Growth Des 2011;11:949–54. [14] Guo W, Xu C, Wang X, Wang S, Pan C, Lin C, et al. J Am Chem Soc 2012;134:4437–41. [15] Ge M, Li JW, Liu L, Zhou Z. Ind Eng Chem Res 2011;50:6681–7. [16] Tahir MN, Theato P, Oberle P, Melnyk G, Faiss S, Kolb U, et al. Langmuir 2006;22:5209–12. [17] Navaneethan M, Nisha KD, Ponnusamy S, Muthamizhchelvan C. Mater Chem Phys 2009;117:443–7. [18] Navaneethan M, Archana J, Arivanandhan M, Hayakawa Y. Phys Status Solidi RRL 2012;6:120–2. [19] Navaneethan M, Archana J, Nisha KD, Hayakawa Y, Ponnusamy S, Muthamizhchelvan C. Mater Lett 2012;68:78–81. [20] Nguyen Phan TD, Pham HD, Cuong TV, Kim EJ, Kim S, Shin EW. J Cryst Growth 2009;312:79–85. [21] Tompsett GA, B GA, Cooney RP, Metson JB, Rodgers KA, Seakins JM. J Raman spectrosc 1995;26:57–62. [22] Li G, Li D, Zhang, Zhai J, Wang E. Chemistry 2009;15:9868–73. [23] Thistlethwaite PJ, Hook MS. Langmuir 2000;16:4993–8. [24] Zhang YG, Ma L, Li JL, Yu Y. Environ Sci Tech 2007;41(17):6264–9. [25] Shamaila S, Sajjad A K L, Feng C, Jinlong Z. Chem Eur J 2010;16:13795–804. [26] Li J, Wang D, Liu H, He Z, Zhu Z. Appl Surf Sci 2011;257:5879–84. [27] Feng J, Zeng HC. Chem Mater 2003;15:2829–35.