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Construction and photoluminescence of In2O3 nanotube array by CVD-template method
ARTICLE IN PRESS
Journal of Crystal Growth 276 (2005) 471–477 www.elsevier.com/locate/jcrysgro
Construction and photoluminescence of In2O3 nanotube array by CVD-template method Xiao-Ping Shena,b, Hong-Jiang Liua, Xin Fana, Yuan Jianga, Jian-Ming Hongc, Zheng Xua, a
State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, P.R. China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212003, P.R. China c Center for Materials Analysis, Nanjing University, Nanjing 210093, P.R. China
b
Received 30 May 2004; accepted 14 November 2004 Communicated by J.M. Redwing Available online 28 December 2004
Abstract Using Indium acetylacetonate as single-source precursor, In2O3 nanotube arrays have been synthesized within the pores of the porous anodic alumina (PAA) membranes by chemical vapor deposition (CVD) at low temperature. The nanotubes with an outer diameter of about 100–300 nm and a length of up to tens of microns were characterized using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopic analyzer (EDS), X-ray diffractometer (XRD) and transmission electron microscopy (TEM), respectively. A novel Y-branched structure and the quasihemispherical ends were also observed. A blue-green emission band centered at 475 nm was observed in the photoluminescence spectrum of the In2O3 nanotube arrays, which could be ascribed to the existence of oxygen vacancies. r 2004 Elsevier B.V. All rights reserved. PACS: 68. 65.+g; 81. 05.Hd; 81. 15.Gh Keywords: A1. Nanostructures; A3. Chemical vapor deposition; B1. Nanotubes; B1. Oxides; B2. Semiconducting materials
1. Introduction During the past decade, the synthesis and functionalization of one-dimensional (1D) nanomaCorresponding
author. Tel.: +862583593133; +862583314502. E-mail address: [email protected] (Z. Xu).
fax:
terials has become one of the most highly energized research areas [1–5]. These materials are of fundamental importance, e.g., nanotubes and nanowires have great potential for testing and understanding fundamental concepts about the effect of dimensionality and size on physical and chemical properties, and they hold potential applications in numerous
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.394
ARTICLE IN PRESS 472
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areas such as nanoscale electronics and photonics due to their special properties distinctive from conventional bulk materials. One successful route leading to one-dimensional nanostructures is template-mediated growth using zeolites, membranes or nanotubes. Porous anodic alumina (PAA) membrane has been considered as one of the most suitable templates for synthesis of 1D nanostructures due to its tunable pore dimensions, narrow pore size distribution, good mechanical and thermal stability [6–13]. The In2O3, a wide-bandgap transparent semiconductor (with a direct bandgap around 3.6 eV and an indirect bandgap around 2.5 eV), has been widely used in window heaters, solar cells, organic light-emitting diodes and liquid crystal devices, etc. [14–17]. The current researches on In2O3 1D nanostructures mainly focus on the preparation and properties of In2O3 nanowires and nanobelts [9,18–22]. In2O3 nanowires have been demonstrated to work as ultra-sensitive chemical sensors for NO2 and NH3, which exhibited significantly improved chemical sensing performance compared to existing thin-film-based sensors due to the enhanced surface-to-volume ratio [23]. In contrast, the investigations on nanotubular In2O3 are quite limited. Only two successful examples of synthesis of In2O3 nanotubes have been reported so far. One used the sol–gel porous alumina templating method and obtained polycrystalline product [24], the other was evaporation of a mixture of In/In2O3 under vacuum and obtained singlecrystalline nanotubes filled with metallic indium [25]. Both the methods were carried out at high temperatures above 700 1C. In order to apply the In2O3 nanostructures to many nanoscale devices, it is very desirable to synthesize high-density and wellaligned In2O3 nanotubes at lower temperatures. In this paper, we report an efficient route for the preparation of In2O3 nanotube arrays in PAA membranes at much lower temperature by metalorganic chemical vapor deposition (CVD) using indium acetylacetonate as a single-source precursor.
2. Experimental procedure Indium acetylacetonate [In(acac)3] was synthesized according to a method in the literature [26].
The commercially available anodic alumina membranes (Whatman Ltd., Anodisc 13) with a nominal pore diameter of 100 nm and thickness of 60-mm were used as templates. The equipment used for synthesis of In2O3 nanotubes is a tube furnace and similar to that reported previously [27]. In a typical experiment, the organometallic precursor (300 mg) in a ceramic boat was placed at upstream end and the PAA template was placed vertically at the central high-temperature zone of the reaction system. O2 was used as carrier and reactive gas for both the oxide synthesis and the removal of organic ligands as oxidized volatile byproducts. After the system was evacuated to 100 Pa, O2 was kept flowing via the precursor area toward the template to carry precursor into reaction zone. The gas flow rate was 20 sccm (standard cubic centimeters per minute) and the pressure of the system was maintained at ca. 2 kPa. The system was heated to 350 1C at the central reaction region at a rate of 10 1C min1 and kept at this temperature for 1 h for CVD. The precursor was situated in advance according to the temperature gradient from the center to the ends of the tube furnace so that its temperature could be held at about 150 1C. After deposition, the temperature of the template was raised to 500 1C at a rate of 5 1C min1 and maintained for 4 h for annealing. The resulting template was collected after the furnace was cooled to room temperature. The cover layer on the surface of the resulting templates was removed by polishing with 1500 grid sand paper for further characterization and detection. The as-deposited products with PAA membranes support were characterized by a D/MaxRA X-ray diffractometer (XRD) with Cu Ka radiation. The morphology and the composition of the products were examined by scanning electron microscopy (SEM, JSM-840A) equipped with an energy-dispersive spectrometry (EDS). The structure and diffraction pattern of the nanotubes were investigated by transmission electron microscopy (TEM, JEM-200CX and JEM2000EX). Photoluminescence (PL) studies were conducted using an Aminco, Bowman Series 2 fluorescence spectrophotometer with a Xe lamp at room temperature. The excitation wavelength
ARTICLE IN PRESS X.-P. Shen et al. / Journal of Crystal Growth 276 (2005) 471–477
was 250 nm. Specimens for SEM were prepared as follows: the sample was fixed at a piece of copper tape and immersed in 3 mol L1 NaOH aqueous solution for ca.20 min to remove the alumina template. After careful rinsing with deionized water and drying, the tape was attached to an SEM sample stub and was sputtered with a thin layer of gold. Samples for TEM/HRTEM were prepared by placing a piece of the resulting membrane in 3 mol L1 NaOH aqueous solution for ca. 40 min to dissolve the alumina. The solution was removed carefully via syringe and the sample was rinsed with distilled water more than twice. The sample was collected on a carboncoated copper grid and allowed to air-dry before measurement.
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3. Results and discussion Fig. 1 shows SEM images and an EDS spectrum of the as-prepared In2O3 nanotubes. Fig. 1a indicates the brush-like morphologies of the In2O3 nanotube arrays after partly removing PPA templates. The surface layer was not completely removed, which resulted in the In2O3 nanotubes sticking together. The lengths of the nanotubes are up to several tens of microns. From Fig. 1b and c, it can be seen that the nanotubes are arranged in a parallel and well-ordered way, and some nanotubes broke during the SEM sample preparation, which reveals hollow structures. The nanotubes are straight and have uniform diameter over their whole length. The diameters of the
Fig. 1. SEM images and a EDS spectrum of the as-prepared In2O3 nanotubes. (a), (b) Large area of the aligned nanotubes shown at different magnifications. (c) Local view of the nanotubes. (d) EDS spectrum of the nanotubes.
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nanotubes are in the range of 100–300 nm, which correspond to the pore diameter of PPA template. The energy-dispersive X-ray spectroscopies (EDS) of the nanotubes shown in Fig. 1d confirmed that the nanotubes were composed of Indium and oxygen elements, in which the gold originates from the gold-sputtered sample for SEM measurement. The result indicates that the tubular In2O3 nanostructures are highly pure and there is no contaminant of carbon, which could be attributed to the oxidation reaction with oxygen. XRD measurement was performed to probe the crystal structure and phase purity of the nanotubes. Fig. 2 shows a typical XRD pattern of the In2O3 nanotube arrays in PAA template. All the relatively sharp diffraction peaks can be indexed to a body-centered cubic (bcc) structure with ( which is consislattice constant of a ¼ 10:118 A; tent with the standard value for bulk cubic-In2O3 (JCPDS 6-0416). However, comparing the intensities of the (4 0 0) and (2 2 2) peaks of the nanotubes with those of the standard bulk bccIn2O3, it was found that the relatively intensity of (4 0 0) peak has been dramatically improved, indicating that the nanoparticles composing nanotubes may preferentially grow along the [1 0 0] direction [22]. The morphology and structure of individual In2O3 nanotubes have been characterized in further detail using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected-area
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Fig. 2. XRD pattern of the In2O3 nanotube arrays in PAA membrane.
electron diffraction (SAED), which are shown in Fig. 3. Fig. 3a shows a typical TEM image of the In2O3 nanotubes. It can be seen that the nanotubes are hollow throughout their entire length with uniform wall thickness of about 20–40 nm. In addition, we also observed a few nanotubes with quasi-hemispherical ends and the branched structures (Fig. 3b and c). This indicated that the nanotubes could grow along and adhering to the inner wall of the template channels and the channel morphology of the template could be transferred into that of the nanotubes, which is similar to the previous report [28]. Furthermore, the HRTEM image (Fig. 3d) of the wall of a single nanotube and corresponding SAED pattern (Fig. 3d inset) show that the In2O3 nanotubes are polycrystalline and are composed of nanocrystals with sizes ca. 5–15 nm. The lattice fringes of a nanoparticle, with a d-spacing of 0.292 nm, as shown in Fig. 3d, are consistent with that of the (2 2 2) planes of cubic In2O3. The diffraction rings observed in the SAED pattern can be indexed as the (2 2 2), (4 0 0), (4 4 0) and (6 2 2) lattice planes of cubic In2O3. The PL of the In2O3 nanotubes was investigated at room temperature and the PL spectrum of the In2O3 nanotubes embedded in alumina template under excitation at 250 nm was shown in Fig. 4. It can be seen that a PL emission appears in the bluegreen region with its maximum intensity centered at 475 nm. Since alumina has a band gap around 8.8 eV, the possibility that the observed PL was emitted from the coexisting alumina template could be excluded [29]. The PL peak position of the present In2O3 nanotubes is similar to that of In2O3 nanowires reported by Liang et al. [22], in which PL emission is due to oxygen vacancies. Up to now, there are several papers in the literature, that have reported the PL emission of the In2O3 resulting from oxygen deficiencies, such as at 430, 480 and 520 nm from In2O3 nanoparticles [30], at 637 nm from In2O3 films [31], at 570 nm from In2O3 nanobelts [21], at 465 nm [19] as well as 416 and 435 nm [20] from In2O3 nanowires, and at 442 nm [32] and 398 nm [9] from In2O3 nanowires embedded in anodic alumina membranes. We consider that the PL emission in our work can mainly be attributed to the effect of the oxygen
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Fig. 3. TEM/HRTEM images and SAED pattern of the In2O3 nanotubes. (a) Typical TEM image of the In2O3 nanotubes. (b) TEM image of an In2O3 nanotube with quasi-hemispherical end, inset in (b) is its magnified view. (c) TEM image showing the branched structure. (d) HRTEM image showing the microstructure of the In2O3 nanotubes, inset is SAED pattern.
PL intensity (a.u.)
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Wavelength(nm) Fig. 4. Room temperature PL spectrum of the In2O3 nanotubes embedded in alumina template under excitation at 250 nm.
deficiencies. Some factors such as reduced pressure reaction condition, high aspect ratio and peculiar morphologies of the nanotubes could generate oxygen vacancies in the as-synthesized In2O3 nanotubes [22]. These oxygen vacancies generally act as deep defect donors in semiconductors and would induce the formation of new energy levels in the band gap. The emission thus results from the radioactive recombination of a photo-excited hole with an electron occupying oxygen vacancies. For deeper energy levels, the emission would move to a longer wavelength, so blue-green light emission was also observed. Finally, based on the space limit within the nanochannels and the catalytic function of the
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pore wall of PAA templates [28], a primary formation mechanism of the In2O3 nanotubes could be proposed (Fig. 5). At the low CVD process temperature (350 1C), the decomposition reaction of the precursor molecules, rather than their diffusing into the pores of the template, is the rate-limiting step of the overall heterogeneous process. Because the rate of catalytic crack on the pore wall of PAA is faster than that of noncatalytic pyrolysis in the pore space, the precursor molecules reacted mainly on the pore wall of the PAA, and the formed In2O3 nanoparticles deposited on the pore wall to form a nanotube. As we know, the catalytic cracking reaction also took place on the outside surfaces of the PAA membrane and finally formed cover layers (Fig. 5), which blocked the ends of the pores and terminated the growth of In2O3 nanotubes within the template. Therefore, the final products were nanotubes rather than nanowires. Our experimental results also indicated that the successful synthesis of In2O3 nanotubes by CVD requires a low deposition temperature. In fact, when deposition temperature was above 600 1C, the rate of deposition reaction was very fast and the reaction would mainly take place on the outside surfaces of the PAA membrane to form cover layers, which prevented the precursor molecules from entering the pores of the PAA; therefore, In2O3 nanotubes could not be formed in the channels of the PAA. It is consistent with the above proposed mechanism.
Fig. 5. Schematic diagram of the formation process of the In2O3 nanotubes.
4. Conclusion The present work demonstrates the CVD growth of well-aligned In2O3 nanotubes within the pores of the PAA template at a rather low temperature. The In2O3 nanotubes have diameters in range of 100–300 nm and lengths up to tens of microns. The PL spectrum of the In2O3 nanotube arrays at room temperature shows light emission at 475 nm, which may be attributed to the existence of oxygen vacancies. The present method can be used to fabricate nanotubes of other various species and such nanotubes grown at low temperature without catalyst can promise potential applications to nanoscale devices.
Acknowledgements Thanks for financial support from the National Natural Science Foundation of China (No. 20371026).
References [1] S. Iijima, Nature 354 (1991) 56. [2] J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. [3] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [4] G.R. Patzke, F. Krumeich, R. Nesper, Angew. Chem. Int. Ed. 41 (2002) 2447. [5] C.N.R. Rao, M. Nath, Dalton Trans. (2003) 1. [6] C.J. Brumlik, C.R. Martin, J. Am. Chem. Soc. 113 (1991) 3174. [7] J. Bao, C. Tie, Z. Xu, Q. Zhou, D. Shen, Q. Ma, Adv. Mater. 13 (2001) 1631. [8] T. Peng, H. Yang, K. Dai, X. Pu, K. Hirao, Chem. Phys. Lett. 379 (2003) 432. [9] H.Q. Cao, X.Q. Qiu, Y. Liang, Q.M. Zhu, Appl. Phys. Lett. 83 (2003) 761. [10] C.R. Martin, Chem. Mater. 8 (1998) 1739. [11] J. Bao, C. Tie, Z. Xu, Q. Ma, J. Hong, H. Sang, D. Sheng, Adv. Mater. 14 (2002) 44. [12] M. Lahav, T. Sehayek, A. Vaskevich, I. Rubinstein, Angew. Chem. Int. Ed. 42 (2003) 5576. [13] C. Liu, J.A. Zapien, Y. Yao, X. Meng, C.S. Lee, S. Fan, Y. Lifshitz, S.T. Lee, Adv. Mater. 15 (2003) 838. [14] C.G. Granqvist, Appl. Phys. A: Mater. Sci. Process 57 (1993) 19. [15] I. Hamburg, C.G. Granqvist, J. Appl. Phys. 60 (1986) R123.
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[25] Y. Li, Y. Bando, D. Golberg, Adv. Mater. 15 (2003) 581. [26] G.T. Morgan, H.D.K. Drew, J. Chem. Soc. 119 (1921) 1058. [27] C.C. Chen, C.C. Yeh, C.H. Chen, M.Y. Yu, H.L. Liu, J.J. Wu, K.H. Chen, L.C. Chen, J.Y. Peng, Y.F. Chen, J. Am. Chem. Soc. 123 (2001) 2791. [28] Y. Yang, Z. Hu, Q. Wu, Y.N. Lu, X.Z. Wang, Y. Chen, Chem. Phys. Lett. 373 (2003) 580. [29] R.H. French, J. Am. Ceram. Soc. 73 (1990) 477. [30] H.J. Zhou, W.P. Cai, L.D. Zhang, Appl. Phys. Lett. 75 (1999) 495. [31] M.S. Lee, W.C. Choi, E.K. Kim, C.K. Kim, S.K. Min, Thin Solid Films 279 (1996) 1. [32] M.J. Zheng, L.D. Zhang, G.H. Li, X.Y. Zhang, X.F. Wang, Appl. Phys. Lett. 79 (2001) 839.
Journal of Crystal Growth 276 (2005) 471–477 www.elsevier.com/locate/jcrysgro
Construction and photoluminescence of In2O3 nanotube array by CVD-template method Xiao-Ping Shena,b, Hong-Jiang Liua, Xin Fana, Yuan Jianga, Jian-Ming Hongc, Zheng Xua, a
State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, P.R. China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212003, P.R. China c Center for Materials Analysis, Nanjing University, Nanjing 210093, P.R. China
b
Received 30 May 2004; accepted 14 November 2004 Communicated by J.M. Redwing Available online 28 December 2004
Abstract Using Indium acetylacetonate as single-source precursor, In2O3 nanotube arrays have been synthesized within the pores of the porous anodic alumina (PAA) membranes by chemical vapor deposition (CVD) at low temperature. The nanotubes with an outer diameter of about 100–300 nm and a length of up to tens of microns were characterized using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopic analyzer (EDS), X-ray diffractometer (XRD) and transmission electron microscopy (TEM), respectively. A novel Y-branched structure and the quasihemispherical ends were also observed. A blue-green emission band centered at 475 nm was observed in the photoluminescence spectrum of the In2O3 nanotube arrays, which could be ascribed to the existence of oxygen vacancies. r 2004 Elsevier B.V. All rights reserved. PACS: 68. 65.+g; 81. 05.Hd; 81. 15.Gh Keywords: A1. Nanostructures; A3. Chemical vapor deposition; B1. Nanotubes; B1. Oxides; B2. Semiconducting materials
1. Introduction During the past decade, the synthesis and functionalization of one-dimensional (1D) nanomaCorresponding
author. Tel.: +862583593133; +862583314502. E-mail address: [email protected] (Z. Xu).
fax:
terials has become one of the most highly energized research areas [1–5]. These materials are of fundamental importance, e.g., nanotubes and nanowires have great potential for testing and understanding fundamental concepts about the effect of dimensionality and size on physical and chemical properties, and they hold potential applications in numerous
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.394
ARTICLE IN PRESS 472
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areas such as nanoscale electronics and photonics due to their special properties distinctive from conventional bulk materials. One successful route leading to one-dimensional nanostructures is template-mediated growth using zeolites, membranes or nanotubes. Porous anodic alumina (PAA) membrane has been considered as one of the most suitable templates for synthesis of 1D nanostructures due to its tunable pore dimensions, narrow pore size distribution, good mechanical and thermal stability [6–13]. The In2O3, a wide-bandgap transparent semiconductor (with a direct bandgap around 3.6 eV and an indirect bandgap around 2.5 eV), has been widely used in window heaters, solar cells, organic light-emitting diodes and liquid crystal devices, etc. [14–17]. The current researches on In2O3 1D nanostructures mainly focus on the preparation and properties of In2O3 nanowires and nanobelts [9,18–22]. In2O3 nanowires have been demonstrated to work as ultra-sensitive chemical sensors for NO2 and NH3, which exhibited significantly improved chemical sensing performance compared to existing thin-film-based sensors due to the enhanced surface-to-volume ratio [23]. In contrast, the investigations on nanotubular In2O3 are quite limited. Only two successful examples of synthesis of In2O3 nanotubes have been reported so far. One used the sol–gel porous alumina templating method and obtained polycrystalline product [24], the other was evaporation of a mixture of In/In2O3 under vacuum and obtained singlecrystalline nanotubes filled with metallic indium [25]. Both the methods were carried out at high temperatures above 700 1C. In order to apply the In2O3 nanostructures to many nanoscale devices, it is very desirable to synthesize high-density and wellaligned In2O3 nanotubes at lower temperatures. In this paper, we report an efficient route for the preparation of In2O3 nanotube arrays in PAA membranes at much lower temperature by metalorganic chemical vapor deposition (CVD) using indium acetylacetonate as a single-source precursor.
2. Experimental procedure Indium acetylacetonate [In(acac)3] was synthesized according to a method in the literature [26].
The commercially available anodic alumina membranes (Whatman Ltd., Anodisc 13) with a nominal pore diameter of 100 nm and thickness of 60-mm were used as templates. The equipment used for synthesis of In2O3 nanotubes is a tube furnace and similar to that reported previously [27]. In a typical experiment, the organometallic precursor (300 mg) in a ceramic boat was placed at upstream end and the PAA template was placed vertically at the central high-temperature zone of the reaction system. O2 was used as carrier and reactive gas for both the oxide synthesis and the removal of organic ligands as oxidized volatile byproducts. After the system was evacuated to 100 Pa, O2 was kept flowing via the precursor area toward the template to carry precursor into reaction zone. The gas flow rate was 20 sccm (standard cubic centimeters per minute) and the pressure of the system was maintained at ca. 2 kPa. The system was heated to 350 1C at the central reaction region at a rate of 10 1C min1 and kept at this temperature for 1 h for CVD. The precursor was situated in advance according to the temperature gradient from the center to the ends of the tube furnace so that its temperature could be held at about 150 1C. After deposition, the temperature of the template was raised to 500 1C at a rate of 5 1C min1 and maintained for 4 h for annealing. The resulting template was collected after the furnace was cooled to room temperature. The cover layer on the surface of the resulting templates was removed by polishing with 1500 grid sand paper for further characterization and detection. The as-deposited products with PAA membranes support were characterized by a D/MaxRA X-ray diffractometer (XRD) with Cu Ka radiation. The morphology and the composition of the products were examined by scanning electron microscopy (SEM, JSM-840A) equipped with an energy-dispersive spectrometry (EDS). The structure and diffraction pattern of the nanotubes were investigated by transmission electron microscopy (TEM, JEM-200CX and JEM2000EX). Photoluminescence (PL) studies were conducted using an Aminco, Bowman Series 2 fluorescence spectrophotometer with a Xe lamp at room temperature. The excitation wavelength
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was 250 nm. Specimens for SEM were prepared as follows: the sample was fixed at a piece of copper tape and immersed in 3 mol L1 NaOH aqueous solution for ca.20 min to remove the alumina template. After careful rinsing with deionized water and drying, the tape was attached to an SEM sample stub and was sputtered with a thin layer of gold. Samples for TEM/HRTEM were prepared by placing a piece of the resulting membrane in 3 mol L1 NaOH aqueous solution for ca. 40 min to dissolve the alumina. The solution was removed carefully via syringe and the sample was rinsed with distilled water more than twice. The sample was collected on a carboncoated copper grid and allowed to air-dry before measurement.
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3. Results and discussion Fig. 1 shows SEM images and an EDS spectrum of the as-prepared In2O3 nanotubes. Fig. 1a indicates the brush-like morphologies of the In2O3 nanotube arrays after partly removing PPA templates. The surface layer was not completely removed, which resulted in the In2O3 nanotubes sticking together. The lengths of the nanotubes are up to several tens of microns. From Fig. 1b and c, it can be seen that the nanotubes are arranged in a parallel and well-ordered way, and some nanotubes broke during the SEM sample preparation, which reveals hollow structures. The nanotubes are straight and have uniform diameter over their whole length. The diameters of the
Fig. 1. SEM images and a EDS spectrum of the as-prepared In2O3 nanotubes. (a), (b) Large area of the aligned nanotubes shown at different magnifications. (c) Local view of the nanotubes. (d) EDS spectrum of the nanotubes.
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(622) (611)
(431)
(332)
(211)
(411)
(440)
(222)
Intensity (a.u.)
(400)
nanotubes are in the range of 100–300 nm, which correspond to the pore diameter of PPA template. The energy-dispersive X-ray spectroscopies (EDS) of the nanotubes shown in Fig. 1d confirmed that the nanotubes were composed of Indium and oxygen elements, in which the gold originates from the gold-sputtered sample for SEM measurement. The result indicates that the tubular In2O3 nanostructures are highly pure and there is no contaminant of carbon, which could be attributed to the oxidation reaction with oxygen. XRD measurement was performed to probe the crystal structure and phase purity of the nanotubes. Fig. 2 shows a typical XRD pattern of the In2O3 nanotube arrays in PAA template. All the relatively sharp diffraction peaks can be indexed to a body-centered cubic (bcc) structure with ( which is consislattice constant of a ¼ 10:118 A; tent with the standard value for bulk cubic-In2O3 (JCPDS 6-0416). However, comparing the intensities of the (4 0 0) and (2 2 2) peaks of the nanotubes with those of the standard bulk bccIn2O3, it was found that the relatively intensity of (4 0 0) peak has been dramatically improved, indicating that the nanoparticles composing nanotubes may preferentially grow along the [1 0 0] direction [22]. The morphology and structure of individual In2O3 nanotubes have been characterized in further detail using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected-area
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2 (degree)
Fig. 2. XRD pattern of the In2O3 nanotube arrays in PAA membrane.
electron diffraction (SAED), which are shown in Fig. 3. Fig. 3a shows a typical TEM image of the In2O3 nanotubes. It can be seen that the nanotubes are hollow throughout their entire length with uniform wall thickness of about 20–40 nm. In addition, we also observed a few nanotubes with quasi-hemispherical ends and the branched structures (Fig. 3b and c). This indicated that the nanotubes could grow along and adhering to the inner wall of the template channels and the channel morphology of the template could be transferred into that of the nanotubes, which is similar to the previous report [28]. Furthermore, the HRTEM image (Fig. 3d) of the wall of a single nanotube and corresponding SAED pattern (Fig. 3d inset) show that the In2O3 nanotubes are polycrystalline and are composed of nanocrystals with sizes ca. 5–15 nm. The lattice fringes of a nanoparticle, with a d-spacing of 0.292 nm, as shown in Fig. 3d, are consistent with that of the (2 2 2) planes of cubic In2O3. The diffraction rings observed in the SAED pattern can be indexed as the (2 2 2), (4 0 0), (4 4 0) and (6 2 2) lattice planes of cubic In2O3. The PL of the In2O3 nanotubes was investigated at room temperature and the PL spectrum of the In2O3 nanotubes embedded in alumina template under excitation at 250 nm was shown in Fig. 4. It can be seen that a PL emission appears in the bluegreen region with its maximum intensity centered at 475 nm. Since alumina has a band gap around 8.8 eV, the possibility that the observed PL was emitted from the coexisting alumina template could be excluded [29]. The PL peak position of the present In2O3 nanotubes is similar to that of In2O3 nanowires reported by Liang et al. [22], in which PL emission is due to oxygen vacancies. Up to now, there are several papers in the literature, that have reported the PL emission of the In2O3 resulting from oxygen deficiencies, such as at 430, 480 and 520 nm from In2O3 nanoparticles [30], at 637 nm from In2O3 films [31], at 570 nm from In2O3 nanobelts [21], at 465 nm [19] as well as 416 and 435 nm [20] from In2O3 nanowires, and at 442 nm [32] and 398 nm [9] from In2O3 nanowires embedded in anodic alumina membranes. We consider that the PL emission in our work can mainly be attributed to the effect of the oxygen
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Fig. 3. TEM/HRTEM images and SAED pattern of the In2O3 nanotubes. (a) Typical TEM image of the In2O3 nanotubes. (b) TEM image of an In2O3 nanotube with quasi-hemispherical end, inset in (b) is its magnified view. (c) TEM image showing the branched structure. (d) HRTEM image showing the microstructure of the In2O3 nanotubes, inset is SAED pattern.
PL intensity (a.u.)
475 nm
400
450
500
550
600
Wavelength(nm) Fig. 4. Room temperature PL spectrum of the In2O3 nanotubes embedded in alumina template under excitation at 250 nm.
deficiencies. Some factors such as reduced pressure reaction condition, high aspect ratio and peculiar morphologies of the nanotubes could generate oxygen vacancies in the as-synthesized In2O3 nanotubes [22]. These oxygen vacancies generally act as deep defect donors in semiconductors and would induce the formation of new energy levels in the band gap. The emission thus results from the radioactive recombination of a photo-excited hole with an electron occupying oxygen vacancies. For deeper energy levels, the emission would move to a longer wavelength, so blue-green light emission was also observed. Finally, based on the space limit within the nanochannels and the catalytic function of the
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pore wall of PAA templates [28], a primary formation mechanism of the In2O3 nanotubes could be proposed (Fig. 5). At the low CVD process temperature (350 1C), the decomposition reaction of the precursor molecules, rather than their diffusing into the pores of the template, is the rate-limiting step of the overall heterogeneous process. Because the rate of catalytic crack on the pore wall of PAA is faster than that of noncatalytic pyrolysis in the pore space, the precursor molecules reacted mainly on the pore wall of the PAA, and the formed In2O3 nanoparticles deposited on the pore wall to form a nanotube. As we know, the catalytic cracking reaction also took place on the outside surfaces of the PAA membrane and finally formed cover layers (Fig. 5), which blocked the ends of the pores and terminated the growth of In2O3 nanotubes within the template. Therefore, the final products were nanotubes rather than nanowires. Our experimental results also indicated that the successful synthesis of In2O3 nanotubes by CVD requires a low deposition temperature. In fact, when deposition temperature was above 600 1C, the rate of deposition reaction was very fast and the reaction would mainly take place on the outside surfaces of the PAA membrane to form cover layers, which prevented the precursor molecules from entering the pores of the PAA; therefore, In2O3 nanotubes could not be formed in the channels of the PAA. It is consistent with the above proposed mechanism.
Fig. 5. Schematic diagram of the formation process of the In2O3 nanotubes.
4. Conclusion The present work demonstrates the CVD growth of well-aligned In2O3 nanotubes within the pores of the PAA template at a rather low temperature. The In2O3 nanotubes have diameters in range of 100–300 nm and lengths up to tens of microns. The PL spectrum of the In2O3 nanotube arrays at room temperature shows light emission at 475 nm, which may be attributed to the existence of oxygen vacancies. The present method can be used to fabricate nanotubes of other various species and such nanotubes grown at low temperature without catalyst can promise potential applications to nanoscale devices.
Acknowledgements Thanks for financial support from the National Natural Science Foundation of China (No. 20371026).
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