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Structural engineering of thin films of vertically aligned TiO2 nanorods
Materials Letters 64 (2010) 1614–1617
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Structural engineering of thin films of vertically aligned TiO2 nanorods Y. Zhang a, Y. Gao a, X.H. Xia a, Q.R. Deng a,b, M.L. Guo a, L. Wan a, G. Shao b,⁎ a b
Faculty of Physics and Electronic Technology, Hubei University, Wuhan, PR China Institute for Materials Research and Innovation, University of Bolton, Bolton BL3 5AB, UK
a r t i c l e
i n f o
Article history: Received 25 March 2010 Accepted 22 April 2010 Available online 28 April 2010 Keywords: TiO2 Nanorod Nano-wire Hydrothermal synthesis Oxide semiconductor Thin film
a b s t r a c t Self-assembled and vertically aligned rutile titania nanorods and thin films with a preferred [002] axial orientation were grown on substrates of fluorine-doped tin dioxide, using a hydrothermal method. Each nanorod was made of a bundle of densely packed and ultra fine nano-fibers growing along the [002] direction. The results show that ethanol substitution of water as solvent is highly effective in promoting the one-dimensional growth of the rutile nanorods and increasing their packing density in the thin films, which offers a simple-but-effectual leverage to monitor the nanorod structures for varied applications. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Thin films made of self-assembled and vertically aligned TiO2 nanorods or nano-wires, which provide un-interrupted electrical pathways for photogenerated charge carriers along the growth axis, have attracted great attention for their potential application in dye- and quantum-dot-sensitized solar cells [1–3], or as photo-catalysts [4–6]. To date, most of the reported one-dimensional TiO2 nanostructures were synthesized in the form of oriented or disoriented TiO2 nanorods/wires on nontransparent or nonconductive substrates using various synthesis techniques [7–13]. It was until last year that hydrothermal methods began to be developed to fabricate heterogeneous growth of oriented single-crystalline TiO2 nanorods/wires on substrates of transparent conducting (TCO) glass or without supporting substrate [3,14–16]. The hydrothermal route is promising for low-cost and high-throughput synthesis of well-aligned TiO2 nano-wire/rod arrays, since low temperature is employed and little investment is required for processing facilities. In general, no toxic chemicals are involved and the process does not raise environmental concerns. It was demonstrated that the use of well-aligned TiO2 single crystal nanorods resulted in significantly improved energy conversion efficiency of dye-sensitized TiO2 solar cells [3,14], and rutile TiO2 nanorods were shown to have enhanced photocatalytic activity [15]. All reported TiO2 nanorod thin films contained numerous gaps or pores between the independently nucleated and grown nanorods. While such loosely packed nanorods are useful for some applications
⁎ Corresponding author. Tel.: + 44 (0)1204 903592. E-mail address: [email protected] (G. Shao). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.04.048
such as dye-sensitized solar cells, photo-catalysts and sensing, one still desires fully dense thin films of well controlled orientation for the application of TiO2 as metal oxide semiconductors for most electronic devices including solid-state photovoltaic cells and spintronic/piezoelectric devices. It has been predicted theoretically that rutile TiO2 is a direct gap semiconductor which tends to be n-type due to oxygen deficiency [17,18]. The electron mobility in n-type rutile is reasonably high, being in the range of 20–100 cm2 V− 1 s− 1, depending on microstructures [19], making crystalline rutile TiO2 an attractive wide gap semiconductor. It is thus envisaged that the hydrothermal method could be of great potential for the fabrication of high quality thin films of TiO2, should the growth orientation and packing density be controlled so well that a pore-free and highly textured thin film of nanorods could be fabricated using such an economical technology. It is thus essential to identify additional processing factors for the fabrication of TiO2 nanorods or nano-wires with appropriate microstructures for various applications. In this work, we have studied the growth of thin films of self-assembled TiO2 nanorods on fluorine-doped tin dioxide (FTO) substrates using a simple and efficient hydrothermal method. We have established a reliable way to control the packing density, growth speed and orientation of nanorods, by introducing absolute ethanol in the deionized water with various ethanol/water ratios. 2. Experimental The material synthesis procedure began at cleaning the substrates of the FTO coated glass (F:SnO2, Tec15, 10 Ω/square) with acetone, absolute ethanol and deionized water in sequence. The substrates were then dried under ambient condition. The chemical precursor solutions were obtained by dissolving 30 ml of concentrated hydrochloric acid
Y. Zhang et al. / Materials Letters 64 (2010) 1614–1617
(36.5%–38% by weight) in a 30 ml solution of mixed deionized water and absolute ethanol with various volumetric ethanol/water ratios. The chosen HCl concentration was based on previous work of Liu and Aydil for optimum restraint of oxide precipitation and solution stability [3]. The mixture was stirred at ambient condition for 5 min before adding 1 ml of titanium butoxide, Ti(OB)4. After stirring for another 5 min, two pieces of substrates were place at an angle against the wall of a Teflonlined steel autoclave (180 ml volume) with the conducting side of the FTO facing downward. The hydrothermal synthesis was conducted at 150 °C for several hours. After synthesis, the autoclaves were cooled to room temperature under flowing water for about 15 min. The samples were then taken out and immersed into deionized water for 3 h, and then rinsed extensively and dried in ambient air. The planar and cross-sectional morphology of the TiO2 nanorod thin films was characterized using field-emission-gun scanning electron microscopy (FE-SEM, Philips FEI XL30) at an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) was carried out using a FEI Tecnai G2 microscope operated at 200 kV. X-ray diffraction (XRD) was performed with a Bruker D8 X-ray diffractometer using the Cu Kα radiation (λ = 1.5418 Å). 3. Results and discussion Firstly, a nanorod thin film was grown on FTO substrates at 150 °C for 20 h, using pure water as solvent. Fig. 1(a,b) shows the typical surface morphology and corresponding cross-section view. The film is largely
1615
made of vertically oriented nanorods, together with some mis-oriented rods. It is apparent from the SEM image (Fig. 1(a)) that each nanorod is made of a bundle of densely packed nano-fibers. Being consistent with Liu and Aydil [3], the separately grown nanorods are loosely populated on the FTO substrate, and the macroscopic morphology of the nanorods is characteristic of a tetragonal compound phase such as the rutile TiO2. We have discovered for the first time in this work that the introduction of ethanol into the precursor solution has remarkable effect on the structural evolution of the TiO2 nanorods. Here we demonstrate the ethanol effect, using ethanol to substitute some water in the solvent and allowing the reaction for a much shorter period of time (only 4 h). Fig. 1(c,d) gives two examples of SEM morphologies of the films formed with different volumetric ethanol/water ratios, with the cross-sectional views being shown as insets of the top view images. As is shown in Fig. 1(c), with a moderate 5 ml ethanol substitution of water as solvent, the morphology of the thin film was similar to that synthesized using only water as solvent, with nanorods being roughly perpendicular to the substrate. It was shocking to notice that the total length of the nanorods reached 2 μm within only 4 h, suggesting that the average growth rate was 5 times of those grown using pure water as solvent. As the ethanol volume was increased to 10 ml (30% of solvent volume), the packing density of the nanorods was also remarkably increased and there were few gaps between the nanorods. Completely dense thin films were achieved when the ethanol volume reached 15 ml (50% replacement of water), with the longitudinal growth rate being enhanced further to result in an average nanorod length of 3.8 μm in 4 h.
Fig. 1. SEM images of (a) surface morphology and (b) cross-section of the samples prepared with pure water as solvent for 20 h. Surface morphology and cross-section SEM images of the samples prepared at 150 °C for 4 h with ethanol/water volume ratios of (c) 5:25 and (d) 30:0.
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Y. Zhang et al. / Materials Letters 64 (2010) 1614–1617
Replacement of 75% of water by ethanol delivered a film thickness of 4.1 μm in 4 h and total replacement of water by ethanol led to an average nanorod length of 4.7 μm (Fig. 1(d)). Fig. 2(a) presents the corresponding XRD spectra of thin films, together with the spectrum of the FTO substrate as a reference. There were only two peaks owing to the (101) and (002) planes of rutile TiO2 and all other diffraction peaks of TiO2 were missing. Owing to the ethanol induced enlargement of the film thickness, the intensity of the FTO peaks decreased with respect to increased amount of ethanol substitution of water. The intensity of the rutile (002) diffraction peak, R(002), increased with increasing ethanol concentration, in contrast to reduced intensities for the (101) peak of rutile TiO2. This suggests that the presence of ethanol in the solvent enhanced the preferential growth in the [002] direction. The ratio of the peak intensity between the R(002) and R(101) peaks is derived from Fig. 2(a) and shown in Fig. 2(b). It is interesting to observe that the ratio is drastically decreased with increasing ethanol substitution of water in the solvent. The existence of the diffraction peak R(101), which corresponded to the strongest peak in the standard powder diffractogram, indicates the occurrence of some degree of random growth off the [002] normal growth direction. Such random growth mainly occurred at the initial stage of nucleation, and the nucleation density is the main factor to affect the population of random crystals [3]. The observed reduction of peak ratio suggests that the
random growth was suppressed as the ethanol concentration increased, with the ethanol induced higher nucleation rate being advantageous to offer adequate population of the preferred [002] growth direction. TEM revealed the fine structure within each nanorod, and Fig. 3(a) shows typical diffraction contrast within each rod at the [1̄20] zone axis of rutile. The corresponding selected area diffraction (SAD) pattern from the rod corresponds to a single crystal diffraction pattern (Fig. 3(b)). The longitudinal direction of the nanorod is [002], which is consistent with the XRD results that the [002] direction was the preferred growth direction perpendicular to the substrate plane. The fibrous diffraction contrast within the rod is attributed to sub-grain boundaries between the nano-fibers, which were formed largely by slight rotation around the [002] growth axis. Fig. 3(c) is a corresponding high resolution TEM image (HRTEM) to show the refined structural features between the three nano-fibers. This confirms that each nanorod is indeed made of slightly mis-oriented nano-fibers growing along the [002] direction, with an average fiber diameter of about 5 nm to form a collective columnar nanorod. Most of the rods were oriented close to the normal direction of the substrate, leading to drastically enhanced (002) diffraction peak in the XRD spectra. The results of this work suggest that the ethanol substitution of water as solvent not only assisted the preferred growth along the [002] direction, but also enhanced the nucleation density and anisotropic growth rate. The chemical process for the synthesis of TiO2 is through a hydrolysis-condensation process of the titanium alkoxides Ti(OR)4, i.e. Ti(OB)4 in this work. The source of –OH for the hydrolysis stage can be provided either by water or ethanol. The condensation process is carried
Fig. 2. (a) XRD spectra of samples with various ethanol/water volume ratios in the solvent. XRD spectrum of the FTO substrate is included for reference. (b) The intensity ratio of rutile (101) to (200) peaks against ethanol volumes in the solvent, showing remarkable enhancement of the (200) fibrous texture due to ethanol substitution of water as solvent.
Fig. 3. (a) Bright-field TEM image of, and (b) the corresponding SAD pattern from, a typical rutile nanorod, showing fibrous diffraction contrast of sub-grains. (c) Corresponding HRTEM image to show the interfaces between the fibrous sub-grains, with the inset being the corresponding fast Fourier transformation (FFT) of the image.
Y. Zhang et al. / Materials Letters 64 (2010) 1614–1617
out by dehydration to form the –[Ti–O–Ti]– chains in the TiO2 structure, and the replacement of water in the precursor solution thus enhances the condensation kinetics to promote nucleation and growth, leading to higher packing density and increased film thickness. A higher nucleation rate also offers higher population of crystal seeds with their c-axis in the normal direction of the substrate plane, with their preferential growth leading to better alignment of rods in the direction of the c-axis of the tetragonal lattice of the rutile phase. 4. Conclusion Rutile TiO2 nanorod films with preferred [002] growth orientation were successfully grown on FTO substrates in the presence of HCl and absolute ethanol and water by a hydrothermal process. This study demonstrated that ethanol substitution of water as solvent is highly effective in promoting the directional growth of the rutile phase and increasing the packing density of the nanorods. This approach offers a simple but efficient way to synthesize thin films of aligned TiO2 nanorods with controlled orientation and packing density for various applications as a multi-functional semiconductor material. Acknowledgements This work was partially supported by the NSFC (No: 50702019), the Educational Commission of Hubei Province of China (No: D20071006),
1617
the Joule Centre (Manchester, UK), and the Technology Strategy Board of UK (TP11/LCE/6/IAE142J).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Kuang D, Brillet J, Chen P, Takata M, Uchida S, Miura H, et al. ACS Nano 2008;2:1113. Chuangchote S, Sagawa T, Yoshikawa S. Appl Phys Lett 2008;93:033310. Liu B, Aydil ES. J Am Chem Soc 2009;131:3985. Jiang Z, Yang F, Luo NJ, Chu BTT, Sun DY, Shi HH, et al. Chem Commun 2008;47: 6372–4. Hwang YJ, Boukai A, Yang PD. Nano Lett 2009;9:410. Wu GS, Nishikawa T, Ohtani B, Chen A. Chem Mater 2007;19:4530. Bae CD, Yoo HJ, Kim S, Lee K, Kim J, Sung MM, et al. Chem Mater 2008;20:756. Kang TS, Smith AP, Taylor BE, Durstock MF. Nano Lett 2009;9:601. Liu ZF, Yamazaki T, Shen YB, Meng D, Kikuta T, Nakatani N. J Phys Chem C 2008;112:4545. Amin SS, Nicholls AW, Xu TT. Nanotechnology 2007;18:445609. Yu J, Chen Y, Glushenkov AM. Cryst Growth Des 2009;9:1240. Xiang B, Zhang Y, Wang Z, Luo XH, Zhu YW, Zhang HZ, et al. J Phys D Appl Phys 2005;38:1152. Guo YG, Hu JS, Liang HP, Wan LJ, Bai CL. Adv Funct Mater 2005;15:196. Feng XJ, Shankar K, Varghese OK, Paulose M, Latempa TJ, Grimes CA. Nano Lett 2008;8:3781. Wang CH, Shao CG, Liu YC, Li XH. Inorg Chem 2009;48:1105. Han YG, Wu G, Wang M, Chen HZ. Nanotechnology 2009;20:235605. Shao G. J Phys Chem C 2008;112(2008):18677–85. Shao G. J Phys Chem C 2009;113:6800–8. Pearton SJ, Abernathy CR, Overberg ME, Thaler GT, Norton DP, Park YD, et al. J Appl Phys 2003;93:1.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Structural engineering of thin films of vertically aligned TiO2 nanorods Y. Zhang a, Y. Gao a, X.H. Xia a, Q.R. Deng a,b, M.L. Guo a, L. Wan a, G. Shao b,⁎ a b
Faculty of Physics and Electronic Technology, Hubei University, Wuhan, PR China Institute for Materials Research and Innovation, University of Bolton, Bolton BL3 5AB, UK
a r t i c l e
i n f o
Article history: Received 25 March 2010 Accepted 22 April 2010 Available online 28 April 2010 Keywords: TiO2 Nanorod Nano-wire Hydrothermal synthesis Oxide semiconductor Thin film
a b s t r a c t Self-assembled and vertically aligned rutile titania nanorods and thin films with a preferred [002] axial orientation were grown on substrates of fluorine-doped tin dioxide, using a hydrothermal method. Each nanorod was made of a bundle of densely packed and ultra fine nano-fibers growing along the [002] direction. The results show that ethanol substitution of water as solvent is highly effective in promoting the one-dimensional growth of the rutile nanorods and increasing their packing density in the thin films, which offers a simple-but-effectual leverage to monitor the nanorod structures for varied applications. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Thin films made of self-assembled and vertically aligned TiO2 nanorods or nano-wires, which provide un-interrupted electrical pathways for photogenerated charge carriers along the growth axis, have attracted great attention for their potential application in dye- and quantum-dot-sensitized solar cells [1–3], or as photo-catalysts [4–6]. To date, most of the reported one-dimensional TiO2 nanostructures were synthesized in the form of oriented or disoriented TiO2 nanorods/wires on nontransparent or nonconductive substrates using various synthesis techniques [7–13]. It was until last year that hydrothermal methods began to be developed to fabricate heterogeneous growth of oriented single-crystalline TiO2 nanorods/wires on substrates of transparent conducting (TCO) glass or without supporting substrate [3,14–16]. The hydrothermal route is promising for low-cost and high-throughput synthesis of well-aligned TiO2 nano-wire/rod arrays, since low temperature is employed and little investment is required for processing facilities. In general, no toxic chemicals are involved and the process does not raise environmental concerns. It was demonstrated that the use of well-aligned TiO2 single crystal nanorods resulted in significantly improved energy conversion efficiency of dye-sensitized TiO2 solar cells [3,14], and rutile TiO2 nanorods were shown to have enhanced photocatalytic activity [15]. All reported TiO2 nanorod thin films contained numerous gaps or pores between the independently nucleated and grown nanorods. While such loosely packed nanorods are useful for some applications
⁎ Corresponding author. Tel.: + 44 (0)1204 903592. E-mail address: [email protected] (G. Shao). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.04.048
such as dye-sensitized solar cells, photo-catalysts and sensing, one still desires fully dense thin films of well controlled orientation for the application of TiO2 as metal oxide semiconductors for most electronic devices including solid-state photovoltaic cells and spintronic/piezoelectric devices. It has been predicted theoretically that rutile TiO2 is a direct gap semiconductor which tends to be n-type due to oxygen deficiency [17,18]. The electron mobility in n-type rutile is reasonably high, being in the range of 20–100 cm2 V− 1 s− 1, depending on microstructures [19], making crystalline rutile TiO2 an attractive wide gap semiconductor. It is thus envisaged that the hydrothermal method could be of great potential for the fabrication of high quality thin films of TiO2, should the growth orientation and packing density be controlled so well that a pore-free and highly textured thin film of nanorods could be fabricated using such an economical technology. It is thus essential to identify additional processing factors for the fabrication of TiO2 nanorods or nano-wires with appropriate microstructures for various applications. In this work, we have studied the growth of thin films of self-assembled TiO2 nanorods on fluorine-doped tin dioxide (FTO) substrates using a simple and efficient hydrothermal method. We have established a reliable way to control the packing density, growth speed and orientation of nanorods, by introducing absolute ethanol in the deionized water with various ethanol/water ratios. 2. Experimental The material synthesis procedure began at cleaning the substrates of the FTO coated glass (F:SnO2, Tec15, 10 Ω/square) with acetone, absolute ethanol and deionized water in sequence. The substrates were then dried under ambient condition. The chemical precursor solutions were obtained by dissolving 30 ml of concentrated hydrochloric acid
Y. Zhang et al. / Materials Letters 64 (2010) 1614–1617
(36.5%–38% by weight) in a 30 ml solution of mixed deionized water and absolute ethanol with various volumetric ethanol/water ratios. The chosen HCl concentration was based on previous work of Liu and Aydil for optimum restraint of oxide precipitation and solution stability [3]. The mixture was stirred at ambient condition for 5 min before adding 1 ml of titanium butoxide, Ti(OB)4. After stirring for another 5 min, two pieces of substrates were place at an angle against the wall of a Teflonlined steel autoclave (180 ml volume) with the conducting side of the FTO facing downward. The hydrothermal synthesis was conducted at 150 °C for several hours. After synthesis, the autoclaves were cooled to room temperature under flowing water for about 15 min. The samples were then taken out and immersed into deionized water for 3 h, and then rinsed extensively and dried in ambient air. The planar and cross-sectional morphology of the TiO2 nanorod thin films was characterized using field-emission-gun scanning electron microscopy (FE-SEM, Philips FEI XL30) at an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) was carried out using a FEI Tecnai G2 microscope operated at 200 kV. X-ray diffraction (XRD) was performed with a Bruker D8 X-ray diffractometer using the Cu Kα radiation (λ = 1.5418 Å). 3. Results and discussion Firstly, a nanorod thin film was grown on FTO substrates at 150 °C for 20 h, using pure water as solvent. Fig. 1(a,b) shows the typical surface morphology and corresponding cross-section view. The film is largely
1615
made of vertically oriented nanorods, together with some mis-oriented rods. It is apparent from the SEM image (Fig. 1(a)) that each nanorod is made of a bundle of densely packed nano-fibers. Being consistent with Liu and Aydil [3], the separately grown nanorods are loosely populated on the FTO substrate, and the macroscopic morphology of the nanorods is characteristic of a tetragonal compound phase such as the rutile TiO2. We have discovered for the first time in this work that the introduction of ethanol into the precursor solution has remarkable effect on the structural evolution of the TiO2 nanorods. Here we demonstrate the ethanol effect, using ethanol to substitute some water in the solvent and allowing the reaction for a much shorter period of time (only 4 h). Fig. 1(c,d) gives two examples of SEM morphologies of the films formed with different volumetric ethanol/water ratios, with the cross-sectional views being shown as insets of the top view images. As is shown in Fig. 1(c), with a moderate 5 ml ethanol substitution of water as solvent, the morphology of the thin film was similar to that synthesized using only water as solvent, with nanorods being roughly perpendicular to the substrate. It was shocking to notice that the total length of the nanorods reached 2 μm within only 4 h, suggesting that the average growth rate was 5 times of those grown using pure water as solvent. As the ethanol volume was increased to 10 ml (30% of solvent volume), the packing density of the nanorods was also remarkably increased and there were few gaps between the nanorods. Completely dense thin films were achieved when the ethanol volume reached 15 ml (50% replacement of water), with the longitudinal growth rate being enhanced further to result in an average nanorod length of 3.8 μm in 4 h.
Fig. 1. SEM images of (a) surface morphology and (b) cross-section of the samples prepared with pure water as solvent for 20 h. Surface morphology and cross-section SEM images of the samples prepared at 150 °C for 4 h with ethanol/water volume ratios of (c) 5:25 and (d) 30:0.
1616
Y. Zhang et al. / Materials Letters 64 (2010) 1614–1617
Replacement of 75% of water by ethanol delivered a film thickness of 4.1 μm in 4 h and total replacement of water by ethanol led to an average nanorod length of 4.7 μm (Fig. 1(d)). Fig. 2(a) presents the corresponding XRD spectra of thin films, together with the spectrum of the FTO substrate as a reference. There were only two peaks owing to the (101) and (002) planes of rutile TiO2 and all other diffraction peaks of TiO2 were missing. Owing to the ethanol induced enlargement of the film thickness, the intensity of the FTO peaks decreased with respect to increased amount of ethanol substitution of water. The intensity of the rutile (002) diffraction peak, R(002), increased with increasing ethanol concentration, in contrast to reduced intensities for the (101) peak of rutile TiO2. This suggests that the presence of ethanol in the solvent enhanced the preferential growth in the [002] direction. The ratio of the peak intensity between the R(002) and R(101) peaks is derived from Fig. 2(a) and shown in Fig. 2(b). It is interesting to observe that the ratio is drastically decreased with increasing ethanol substitution of water in the solvent. The existence of the diffraction peak R(101), which corresponded to the strongest peak in the standard powder diffractogram, indicates the occurrence of some degree of random growth off the [002] normal growth direction. Such random growth mainly occurred at the initial stage of nucleation, and the nucleation density is the main factor to affect the population of random crystals [3]. The observed reduction of peak ratio suggests that the
random growth was suppressed as the ethanol concentration increased, with the ethanol induced higher nucleation rate being advantageous to offer adequate population of the preferred [002] growth direction. TEM revealed the fine structure within each nanorod, and Fig. 3(a) shows typical diffraction contrast within each rod at the [1̄20] zone axis of rutile. The corresponding selected area diffraction (SAD) pattern from the rod corresponds to a single crystal diffraction pattern (Fig. 3(b)). The longitudinal direction of the nanorod is [002], which is consistent with the XRD results that the [002] direction was the preferred growth direction perpendicular to the substrate plane. The fibrous diffraction contrast within the rod is attributed to sub-grain boundaries between the nano-fibers, which were formed largely by slight rotation around the [002] growth axis. Fig. 3(c) is a corresponding high resolution TEM image (HRTEM) to show the refined structural features between the three nano-fibers. This confirms that each nanorod is indeed made of slightly mis-oriented nano-fibers growing along the [002] direction, with an average fiber diameter of about 5 nm to form a collective columnar nanorod. Most of the rods were oriented close to the normal direction of the substrate, leading to drastically enhanced (002) diffraction peak in the XRD spectra. The results of this work suggest that the ethanol substitution of water as solvent not only assisted the preferred growth along the [002] direction, but also enhanced the nucleation density and anisotropic growth rate. The chemical process for the synthesis of TiO2 is through a hydrolysis-condensation process of the titanium alkoxides Ti(OR)4, i.e. Ti(OB)4 in this work. The source of –OH for the hydrolysis stage can be provided either by water or ethanol. The condensation process is carried
Fig. 2. (a) XRD spectra of samples with various ethanol/water volume ratios in the solvent. XRD spectrum of the FTO substrate is included for reference. (b) The intensity ratio of rutile (101) to (200) peaks against ethanol volumes in the solvent, showing remarkable enhancement of the (200) fibrous texture due to ethanol substitution of water as solvent.
Fig. 3. (a) Bright-field TEM image of, and (b) the corresponding SAD pattern from, a typical rutile nanorod, showing fibrous diffraction contrast of sub-grains. (c) Corresponding HRTEM image to show the interfaces between the fibrous sub-grains, with the inset being the corresponding fast Fourier transformation (FFT) of the image.
Y. Zhang et al. / Materials Letters 64 (2010) 1614–1617
out by dehydration to form the –[Ti–O–Ti]– chains in the TiO2 structure, and the replacement of water in the precursor solution thus enhances the condensation kinetics to promote nucleation and growth, leading to higher packing density and increased film thickness. A higher nucleation rate also offers higher population of crystal seeds with their c-axis in the normal direction of the substrate plane, with their preferential growth leading to better alignment of rods in the direction of the c-axis of the tetragonal lattice of the rutile phase. 4. Conclusion Rutile TiO2 nanorod films with preferred [002] growth orientation were successfully grown on FTO substrates in the presence of HCl and absolute ethanol and water by a hydrothermal process. This study demonstrated that ethanol substitution of water as solvent is highly effective in promoting the directional growth of the rutile phase and increasing the packing density of the nanorods. This approach offers a simple but efficient way to synthesize thin films of aligned TiO2 nanorods with controlled orientation and packing density for various applications as a multi-functional semiconductor material. Acknowledgements This work was partially supported by the NSFC (No: 50702019), the Educational Commission of Hubei Province of China (No: D20071006),
1617
the Joule Centre (Manchester, UK), and the Technology Strategy Board of UK (TP11/LCE/6/IAE142J).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Kuang D, Brillet J, Chen P, Takata M, Uchida S, Miura H, et al. ACS Nano 2008;2:1113. Chuangchote S, Sagawa T, Yoshikawa S. Appl Phys Lett 2008;93:033310. Liu B, Aydil ES. J Am Chem Soc 2009;131:3985. Jiang Z, Yang F, Luo NJ, Chu BTT, Sun DY, Shi HH, et al. Chem Commun 2008;47: 6372–4. Hwang YJ, Boukai A, Yang PD. Nano Lett 2009;9:410. Wu GS, Nishikawa T, Ohtani B, Chen A. Chem Mater 2007;19:4530. Bae CD, Yoo HJ, Kim S, Lee K, Kim J, Sung MM, et al. Chem Mater 2008;20:756. Kang TS, Smith AP, Taylor BE, Durstock MF. Nano Lett 2009;9:601. Liu ZF, Yamazaki T, Shen YB, Meng D, Kikuta T, Nakatani N. J Phys Chem C 2008;112:4545. Amin SS, Nicholls AW, Xu TT. Nanotechnology 2007;18:445609. Yu J, Chen Y, Glushenkov AM. Cryst Growth Des 2009;9:1240. Xiang B, Zhang Y, Wang Z, Luo XH, Zhu YW, Zhang HZ, et al. J Phys D Appl Phys 2005;38:1152. Guo YG, Hu JS, Liang HP, Wan LJ, Bai CL. Adv Funct Mater 2005;15:196. Feng XJ, Shankar K, Varghese OK, Paulose M, Latempa TJ, Grimes CA. Nano Lett 2008;8:3781. Wang CH, Shao CG, Liu YC, Li XH. Inorg Chem 2009;48:1105. Han YG, Wu G, Wang M, Chen HZ. Nanotechnology 2009;20:235605. Shao G. J Phys Chem C 2008;112(2008):18677–85. Shao G. J Phys Chem C 2009;113:6800–8. Pearton SJ, Abernathy CR, Overberg ME, Thaler GT, Norton DP, Park YD, et al. J Appl Phys 2003;93:1.