- Email: [email protected]
Growth of uniform tungsten oxide nanowires with small diameter via a two-step heating process
ARTICLE IN PRESS
Journal of Crystal Growth 306 (2007) 395–399 www.elsevier.com/locate/jcrysgro
Growth of uniform tungsten oxide nanowires with small diameter via a two-step heating process Rong Hu, Huasheng Wu, Kunquan Hong Department of Physics and the HKU-CAS Joint Laboratory on New Materials, The University of Hong Kong, Pokfulam Road, Hong Kong, PR China Received 3 November 2006; received in revised form 28 February 2007; accepted 5 May 2007 Communicated by J.M. Redwing Available online 22 May 2007
Abstract Uniform tungsten oxide nanowires with small diameter have been fabricated with potassium hydroxide as catalyst on a tungsten plate and by the method of thermal evaporation via a unique two-step heating process. Typical temperatures of this two-step process are 390 and 610 1C, respectively. The first-step heating was found crucial for the growth of small and uniform nanowires because it made the potassium hydroxide solution more uniformly distributed on the tungsten plate surface. The structure and composition of the grown nanowires were characterized by various methods. The diameter of the nanowires ranges from 30–200 nm and the length is up to several tens of micrometers. The structure of the nanowires is found to be orthorhombic tungsten oxide (W3O8), with cell parameters a ¼ 1.035 nm, b ¼ 1.399 nm, c ¼ 0.378 nm. r 2007 Elsevier B.V. All rights reserved. PACS: 81.07.Bc; 81.16.c; 82.33.Ya; 68.37.Hk; 61.10.Nz Keywords: A1. SEM; A1. XRD; A3. Thermal evaporation; B1. Tungsten oxide
1. Introduction Among the numerous nano-scaled metal oxides, transition metal oxide has attracted great interests in physics, chemistry and materials areas over the last decades, due to their distinctive properties: the electrochromic [1], optochromic [2], gaschromic [3] and magnetic properties [4]. Especially, tungsten oxide nanowires have been paid considerable attention for their unique applications: flat panel displays, photoelectrochromic ‘‘smart’’ windows, optical modulation devices, writing–reading–erasing optical devices, gas sensors, humidity and temperature sensors [1,5–7] and so forth. Many synthetic methodologies have been devoted to the growth of nano-scaled tungsten oxide [8–12]. A primary method is to heat a tungsten foil at a high temperature with different chemicals as catalysts, for example at 1600 1C with SiO2 plate in an argon atmosphere [8], at 1600 1C with Corresponding author. Tel.: +852 28592362; fax: +852 25599152.
E-mail address: [email protected] (H. Wu). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.05.007
B2O3 powders in a nitrogen atmosphere [9] and at 1400 1C with WS2 powders in an argon (490 Torr) and oxygen (10 Torr) atmosphere [10]. Due to the use of high temperature, the synthesis of tungsten oxide nanostructures is much more difficult than that for other nanomaterials. Later, a simpler method for synthesizing tungsten oxide nanowires was reported that uses potassium halide powder or solution as catalyst resulted in a growth temperature as low as 650 1C. However, the size of the resultant nanowires with this method is quite large with an average diameter of more than 400 nm, which is too large for most of the applications of nanowires. In this paper, we report an improved simple strategy to synthesize tungsten oxide nanowires with small diameters by using potassium hydroxide solution as catalyst directly on tungsten plates via a unique two-step heating process. The average diameter of the nanowires is less than 200 nm and the length is up to tens of micrometers with aspect ratios higher than 50. The properties of the samples have been characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX),
ARTICLE IN PRESS 396
R. Hu et al. / Journal of Crystal Growth 306 (2007) 395–399
X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). This improved strategy may serve as a general method to synthesize one-dimensional tungsten oxide nanowires with small diameters.
angle range used in the measurement was from 151 to 801 with a step of 0.051. HRTEM images and selected area electron diffraction (SAED) patterns were obtained with a model TECNAI 20 with an accelerating voltage of 200 kV.
2. Experiments
3. Results and discussion
The experiment was carried out in a horizontal quartz tube, which was mounted in a conventional horizontal high-temperature furnace. A tungsten plate of size 5 mm 5 mm 0.1 mm with a quoted purity of 99.95% from Goodfellow Cambridge Limited was cleaned consecutively in an ultrasonic bath with hydrochloric acid, ethanol and de-ionized water. Then, 0.25 ml of 10 wt% potassium hydroxide solution was dropped on the plate. When the solvent was vaporized, tiny potassium hydroxide seeds were precipitated from the droplets on the surface of the plate. After the plate was put in a quartz boat in the uniform temperature zone of the furnace, its temperature was raised from the room temperature up to 390 1C at a ramping rate of 30 1C min1. After keeping at this temperature for half an hour (dwell heating), the temperature was then raised up to 610 1C at a ramping rate of 30 1C min1. After maintained at this temperature for 2 h (reaction heating), the furnace was let to cool down to the room temperature freely. During the experiment, the quartz tube was kept to the atmospheric pressure. After the sample was taken out of the furnace, it was rinsed softly with de-ionized water, and then characterized by SEM, XRD and HRTEM. SEM images and EDX spectra were taken with a LEO 1530 field emission SEM at 5.00 kV. XRD was performed on a Siemens D5000-advance X-ray diffractometer. The 2y
Fig. 1(a–c) show a set of SEM images, with different magnifications, of a typical sample of the tungsten oxide nanowires synthesized with our two-step heating process. It can be seen that the diameters of the nanowires are largely around 200 nm with the smallest diameter being less than 30 nm resulted in an average diameter much smaller than that synthesized using other catalysts, such as KI by Qi et.al. [11], which is larger than 400 nm. With the lengths up to several tens of micrometers, the aspect ratios are higher than 50. It can also be seen that all nanowires are well oriented and their diameters are quite uniform along their growth direction. As a comparison, tungsten oxide nanowires grown without the dwell heating stage at 390 1C is shown in Fig. 1(d), which are not as uniform and as small as the previous ones. The chemical composition of the tungsten oxide nanowires was characterized by the EDX spectrum shown in Fig. 2(a). It indicates clearly that besides signals from the constituent elements oxygen and tungsten, there exists weaker signal from potassium. The origin of the potassium must be from the potassium hydroxide as a catalyst. The potassium signal can be significantly reduced by rinsing the sample with de-ionized water. A typical XRD spectrum of the tungsten oxide nanowires is shown in Fig. 3. The diffraction peaks can be indexed to the known structure of orthorhombic
Fig. 1. Typical SEM images at different magnifications: (a–c) the nanowires grown via a two-step heating process. The diameter ranges from 30 to 200 nm and lengths are up to several tens of micrometers; (d) the nanowires grown via a one-step heating process.
ARTICLE IN PRESS R. Hu et al. / Journal of Crystal Growth 306 (2007) 395–399
tungsten oxide (W3O8), whose cell parameters are a ¼ 1.035 nm, b ¼ 1.399 nm, c ¼ 0.378 nm. [JCPS Card no: 81–2263]. It shows clearly that the growth is
397
preferentially along the c-axis, the [0 0 1] orientation. The values of the full-width at half maximum of (0 0 1) and (0 0 2) diffraction peaks were determined to be 0.151 and 0.111, respectively, from the XRD spectrum, which illustrates the tungsten oxide nanowires have a good crystalline structure. Further HRTEM and corresponding SAED investigations provided more detailed information about the structure of the tungsten oxide nanowires. The HRTEM image shown in Fig. 4 displayed the single-crystal structure of an individual nanowire, in which the [0 0 1] direction is the growth direction of the nanowire. The layer spaces along the two mutually orthogonal directions [0 0 1] and [1 4 0] were determined to be 0.38 and 0.33 nm, respectively, well agree with layer spaces along the two directions in the suggested orthorhombic tungsten oxide (W3O8). A corresponding SAED image in the inset of Fig. 4 also supports that the nanowire is single crystalline with [0 0 1] as the growth direction. The lengths of the reciprocal basic vectors along [0 0 1] and [1 4 0] directions obtained from the inset of Fig. 4 agree well with the layer spaces mentioned above. There were some periodic weaker spots in the middle of the bright ones in the reciprocal space. They are resulted from the formation of larger lattices each composed of two or more unite cells. All the data from EDX, XRD, TEM and SAED analyses together shows that the products are large quantities of uniform and singlecrystalline tungsten oxide nanowires. Several models were proposed to explain the growth mechanism of tungsten nanowires, including dislocation, vapor–liquid–solid (VLS), vapor–solid (VS) and oxide assist growth (OAG) mechanism. Based on the SEM
Fig. 2. EDX spectra of a nanowire: (a) from the middle of the wire and (b) from the tip of the wire. The relative potassium content on the tip is much more than that in the middle of the wire.
Fig. 3. XRD profile of the synthesized tungsten oxide nanowires. All the peaks correspond to W3O8.
Fig. 4. HRTEM of the tungsten oxide nanowires. The inset shows the SAED pattern of the nanowires.
ARTICLE IN PRESS 398
R. Hu et al. / Journal of Crystal Growth 306 (2007) 395–399
measurement shown in Fig. 5 of a tungsten oxide nanowire with a diameter of about 35 nm, there was no evidence of existence of dislocations. So the dislocation growth mechanism does not seem to be suitable. Furthermore, the VS growth mechanism does not seem to apply either, since the tungsten plate was placed at the high-temperature zone so the saturated vapor cannot be deposited back on the substrate. According to the experiment conditions, there was no oxide used as the reagent in the production process, thus the OAG mechanism cannot account for the formation of the tungsten oxide nanowires, either. Taking all these factors into account, the VLS growth mechanism, proposed by Wagner and Ellis [13] in 1964, is likely to be the dominant mechanism and agrees well with our experimental observations. It has been suggested that the role played by potassium halide as a catalyst in the synthesis of tungsten oxide nanowires is to form low melting point compound containing K, W and O [10]. Potassium hydroxide solution as catalyst in our experiment looks to have played the same role as can be seen in Fig. 5, where nanoscale liquid droplets containing K, W and O appears on the surface of the tungsten plate. Amorphous tungsten oxide, produced by the reaction between the tungsten metal and oxygen in the tube, evaporated and then was dissolved in the droplet. Crystalline tungsten oxide precipitated from the supersaturated droplets to form the nanowires. Continuous dissolution and precipitation guarantee the continuous growth of the nanowires. Certainly, the diameter of the nanowires was directly related to and often smaller than the diameter of the initial liquid droplets [13], which can be seen clearly in Fig. 5. The hemispheric tips are suspected to be formed during the cooling down process to room temperature. The relative potassium content on the tip is much more than that in the wire, which can be clearly seen in the EDX spectrum shown in Fig. 2.
To demonstrate the role of the catalyst, potassium hydroxide, used in this work, additional experiments were performed with the same condition as previously described except that no potassium hydroxide was used or the potassium hydroxide powder was used instead of the potassium hydroxide solution. In the case of no potassium hydroxide on the plate surface, no nanowires were grown. With potassium hydroxide powder as catalyst, the diameter of the tungsten oxide nanowires was larger than that with potassium hydroxide solution. It indicates that the use of potassium hydroxide solution as catalyst is a crucial factor for the growth of the nanowires in small diameters. This is because the solution was more uniformly distributed on the surface of tungsten substrate than the powders, so does the tiny potassium hydroxide particles after the water evaporates. The necessity of the dwell heating at 390 1C is demonstrated in Fig. 1(d), where the nanowires were grown with the same condition for that shown in Fig. 1(b) except that only one-stage heating (reaction heating) was used. It is obvious that the nanowires grown without the dwell heating are larger and much less uniform. The reason is the following. When heated at 390 1C for certain amount of time, the potassium hydroxide with melting temperature of 380 1C were kept at liquid phase and thus distributed even more uniformly on the surface of tungsten substrate resulting in more uniform and smaller nanowires. The influence of reaction temperature on the formation of tungsten oxide nanowires was also studied with our twostage heating process. It was confirmed that no tungsten nanowires were formed at reaction temperatures below 550 1C, which is in agreement with the previous report with K2SO4 as catalyst [12]. At the high temperature end, we found that the nanowires can be synthesized at 790 1C, which is much higher than the upper limit of 690 1C reported. This difference must be due to the employment of potassium hydroxide solution and extra stage of dwell heating at 390 1C. 4. Conclusions In conclusion, uniform tungsten oxide nanowires with small diameter were synthesized by two-stage heating and with potassium hydroxide solution as catalyst on tungsten plates. The diameters of the nanowires range from 30 to 200 nm and the lengths are up to tens of micrometers. The nanowires were characterized for their morphology, chemical composition and structure (W3O8). The VLS growth mechanism is applied for the formation of the tungsten oxide nanowires. The influences of the dwell heating stage and the other factors of the experiment such as reaction temperature are also discussed. Acknowledgements
Fig. 5. High-magnification SEM image of a nanowire with a diameter of 35 nm. The hemispheric tip is the evidence of VLS growth mechanism.
This work is supported by RGC grant No. HKU704605.
ARTICLE IN PRESS R. Hu et al. / Journal of Crystal Growth 306 (2007) 395–399
References [1] C. Santato, M. Odziemkowski, M. Ulmann, J. Augustynski, J. Am. Chem. Soc. 123 (2001) 10639. [2] K. Sayama, K. Mukasa, R. Abe, Y. Abe, H. Arakawa, Chem. Commun. (2001) 2416. [3] J.L. Solis, S. Saukko, L. Kish, C.G. Granqvist, V. Lantto, Thin Solid Films 391 (2001) 255. [4] A. Shengelaya, S. Reich, Y. Tsabba, K.A. Muller, Eur. Phys. J. B 12 (1998) 13. [5] S.H. Baeck, T. Jaramillo, G.D. Stucky, E.W. McFarland, Nano Lett. 2 (2002) 831. [6] E. Ozkan, S.H. Lee, P. Liu, C.E. Tracy, F.Z. Tepehan, J.R. Pitts, S.K. Deb, Solid State Ion 149 (2002) 139.
399
[7] J. Shieh, H.M. Feng, M.H. Hon, H.Y. Juang, Sensors Actuators B 86 (2002) 75. [8] Y.Q. Zhu, W.B. Hu, W.K. Hsu, M. Terrones, N. Grobert, J.P. Hare, H.W. Kroto, D.R.M. Walton, H. Terrones, Chem. Phys. Lett. 309 (1999) 327. [9] Z.W. Liu, Y. Bando, C.C. Tang, Chem. Phys. Lett. 372 (2003) 179. [10] W.B. Hu, Y.Q. Zhu, W.K. Hsu, B.H. Chang, M. Terrones, N. Grobert, H. Terrones, J.P. Hare, H.W. Kroto, D.R.M. Walton, Appl. Phys. A 70 (2000) 231. [11] H. Qi, C. Wang, J. Liu, Adv. Mater. 15 (2003) 411. [12] B. Zhang, C.B. Cao, H.L. Qiu, Y.J. Xu, Y.C. Wang, H.S. Zhu, Chem. Lett. 34 (2005) 154. [13] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89.
Journal of Crystal Growth 306 (2007) 395–399 www.elsevier.com/locate/jcrysgro
Growth of uniform tungsten oxide nanowires with small diameter via a two-step heating process Rong Hu, Huasheng Wu, Kunquan Hong Department of Physics and the HKU-CAS Joint Laboratory on New Materials, The University of Hong Kong, Pokfulam Road, Hong Kong, PR China Received 3 November 2006; received in revised form 28 February 2007; accepted 5 May 2007 Communicated by J.M. Redwing Available online 22 May 2007
Abstract Uniform tungsten oxide nanowires with small diameter have been fabricated with potassium hydroxide as catalyst on a tungsten plate and by the method of thermal evaporation via a unique two-step heating process. Typical temperatures of this two-step process are 390 and 610 1C, respectively. The first-step heating was found crucial for the growth of small and uniform nanowires because it made the potassium hydroxide solution more uniformly distributed on the tungsten plate surface. The structure and composition of the grown nanowires were characterized by various methods. The diameter of the nanowires ranges from 30–200 nm and the length is up to several tens of micrometers. The structure of the nanowires is found to be orthorhombic tungsten oxide (W3O8), with cell parameters a ¼ 1.035 nm, b ¼ 1.399 nm, c ¼ 0.378 nm. r 2007 Elsevier B.V. All rights reserved. PACS: 81.07.Bc; 81.16.c; 82.33.Ya; 68.37.Hk; 61.10.Nz Keywords: A1. SEM; A1. XRD; A3. Thermal evaporation; B1. Tungsten oxide
1. Introduction Among the numerous nano-scaled metal oxides, transition metal oxide has attracted great interests in physics, chemistry and materials areas over the last decades, due to their distinctive properties: the electrochromic [1], optochromic [2], gaschromic [3] and magnetic properties [4]. Especially, tungsten oxide nanowires have been paid considerable attention for their unique applications: flat panel displays, photoelectrochromic ‘‘smart’’ windows, optical modulation devices, writing–reading–erasing optical devices, gas sensors, humidity and temperature sensors [1,5–7] and so forth. Many synthetic methodologies have been devoted to the growth of nano-scaled tungsten oxide [8–12]. A primary method is to heat a tungsten foil at a high temperature with different chemicals as catalysts, for example at 1600 1C with SiO2 plate in an argon atmosphere [8], at 1600 1C with Corresponding author. Tel.: +852 28592362; fax: +852 25599152.
E-mail address: [email protected] (H. Wu). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.05.007
B2O3 powders in a nitrogen atmosphere [9] and at 1400 1C with WS2 powders in an argon (490 Torr) and oxygen (10 Torr) atmosphere [10]. Due to the use of high temperature, the synthesis of tungsten oxide nanostructures is much more difficult than that for other nanomaterials. Later, a simpler method for synthesizing tungsten oxide nanowires was reported that uses potassium halide powder or solution as catalyst resulted in a growth temperature as low as 650 1C. However, the size of the resultant nanowires with this method is quite large with an average diameter of more than 400 nm, which is too large for most of the applications of nanowires. In this paper, we report an improved simple strategy to synthesize tungsten oxide nanowires with small diameters by using potassium hydroxide solution as catalyst directly on tungsten plates via a unique two-step heating process. The average diameter of the nanowires is less than 200 nm and the length is up to tens of micrometers with aspect ratios higher than 50. The properties of the samples have been characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX),
ARTICLE IN PRESS 396
R. Hu et al. / Journal of Crystal Growth 306 (2007) 395–399
X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). This improved strategy may serve as a general method to synthesize one-dimensional tungsten oxide nanowires with small diameters.
angle range used in the measurement was from 151 to 801 with a step of 0.051. HRTEM images and selected area electron diffraction (SAED) patterns were obtained with a model TECNAI 20 with an accelerating voltage of 200 kV.
2. Experiments
3. Results and discussion
The experiment was carried out in a horizontal quartz tube, which was mounted in a conventional horizontal high-temperature furnace. A tungsten plate of size 5 mm 5 mm 0.1 mm with a quoted purity of 99.95% from Goodfellow Cambridge Limited was cleaned consecutively in an ultrasonic bath with hydrochloric acid, ethanol and de-ionized water. Then, 0.25 ml of 10 wt% potassium hydroxide solution was dropped on the plate. When the solvent was vaporized, tiny potassium hydroxide seeds were precipitated from the droplets on the surface of the plate. After the plate was put in a quartz boat in the uniform temperature zone of the furnace, its temperature was raised from the room temperature up to 390 1C at a ramping rate of 30 1C min1. After keeping at this temperature for half an hour (dwell heating), the temperature was then raised up to 610 1C at a ramping rate of 30 1C min1. After maintained at this temperature for 2 h (reaction heating), the furnace was let to cool down to the room temperature freely. During the experiment, the quartz tube was kept to the atmospheric pressure. After the sample was taken out of the furnace, it was rinsed softly with de-ionized water, and then characterized by SEM, XRD and HRTEM. SEM images and EDX spectra were taken with a LEO 1530 field emission SEM at 5.00 kV. XRD was performed on a Siemens D5000-advance X-ray diffractometer. The 2y
Fig. 1(a–c) show a set of SEM images, with different magnifications, of a typical sample of the tungsten oxide nanowires synthesized with our two-step heating process. It can be seen that the diameters of the nanowires are largely around 200 nm with the smallest diameter being less than 30 nm resulted in an average diameter much smaller than that synthesized using other catalysts, such as KI by Qi et.al. [11], which is larger than 400 nm. With the lengths up to several tens of micrometers, the aspect ratios are higher than 50. It can also be seen that all nanowires are well oriented and their diameters are quite uniform along their growth direction. As a comparison, tungsten oxide nanowires grown without the dwell heating stage at 390 1C is shown in Fig. 1(d), which are not as uniform and as small as the previous ones. The chemical composition of the tungsten oxide nanowires was characterized by the EDX spectrum shown in Fig. 2(a). It indicates clearly that besides signals from the constituent elements oxygen and tungsten, there exists weaker signal from potassium. The origin of the potassium must be from the potassium hydroxide as a catalyst. The potassium signal can be significantly reduced by rinsing the sample with de-ionized water. A typical XRD spectrum of the tungsten oxide nanowires is shown in Fig. 3. The diffraction peaks can be indexed to the known structure of orthorhombic
Fig. 1. Typical SEM images at different magnifications: (a–c) the nanowires grown via a two-step heating process. The diameter ranges from 30 to 200 nm and lengths are up to several tens of micrometers; (d) the nanowires grown via a one-step heating process.
ARTICLE IN PRESS R. Hu et al. / Journal of Crystal Growth 306 (2007) 395–399
tungsten oxide (W3O8), whose cell parameters are a ¼ 1.035 nm, b ¼ 1.399 nm, c ¼ 0.378 nm. [JCPS Card no: 81–2263]. It shows clearly that the growth is
397
preferentially along the c-axis, the [0 0 1] orientation. The values of the full-width at half maximum of (0 0 1) and (0 0 2) diffraction peaks were determined to be 0.151 and 0.111, respectively, from the XRD spectrum, which illustrates the tungsten oxide nanowires have a good crystalline structure. Further HRTEM and corresponding SAED investigations provided more detailed information about the structure of the tungsten oxide nanowires. The HRTEM image shown in Fig. 4 displayed the single-crystal structure of an individual nanowire, in which the [0 0 1] direction is the growth direction of the nanowire. The layer spaces along the two mutually orthogonal directions [0 0 1] and [1 4 0] were determined to be 0.38 and 0.33 nm, respectively, well agree with layer spaces along the two directions in the suggested orthorhombic tungsten oxide (W3O8). A corresponding SAED image in the inset of Fig. 4 also supports that the nanowire is single crystalline with [0 0 1] as the growth direction. The lengths of the reciprocal basic vectors along [0 0 1] and [1 4 0] directions obtained from the inset of Fig. 4 agree well with the layer spaces mentioned above. There were some periodic weaker spots in the middle of the bright ones in the reciprocal space. They are resulted from the formation of larger lattices each composed of two or more unite cells. All the data from EDX, XRD, TEM and SAED analyses together shows that the products are large quantities of uniform and singlecrystalline tungsten oxide nanowires. Several models were proposed to explain the growth mechanism of tungsten nanowires, including dislocation, vapor–liquid–solid (VLS), vapor–solid (VS) and oxide assist growth (OAG) mechanism. Based on the SEM
Fig. 2. EDX spectra of a nanowire: (a) from the middle of the wire and (b) from the tip of the wire. The relative potassium content on the tip is much more than that in the middle of the wire.
Fig. 3. XRD profile of the synthesized tungsten oxide nanowires. All the peaks correspond to W3O8.
Fig. 4. HRTEM of the tungsten oxide nanowires. The inset shows the SAED pattern of the nanowires.
ARTICLE IN PRESS 398
R. Hu et al. / Journal of Crystal Growth 306 (2007) 395–399
measurement shown in Fig. 5 of a tungsten oxide nanowire with a diameter of about 35 nm, there was no evidence of existence of dislocations. So the dislocation growth mechanism does not seem to be suitable. Furthermore, the VS growth mechanism does not seem to apply either, since the tungsten plate was placed at the high-temperature zone so the saturated vapor cannot be deposited back on the substrate. According to the experiment conditions, there was no oxide used as the reagent in the production process, thus the OAG mechanism cannot account for the formation of the tungsten oxide nanowires, either. Taking all these factors into account, the VLS growth mechanism, proposed by Wagner and Ellis [13] in 1964, is likely to be the dominant mechanism and agrees well with our experimental observations. It has been suggested that the role played by potassium halide as a catalyst in the synthesis of tungsten oxide nanowires is to form low melting point compound containing K, W and O [10]. Potassium hydroxide solution as catalyst in our experiment looks to have played the same role as can be seen in Fig. 5, where nanoscale liquid droplets containing K, W and O appears on the surface of the tungsten plate. Amorphous tungsten oxide, produced by the reaction between the tungsten metal and oxygen in the tube, evaporated and then was dissolved in the droplet. Crystalline tungsten oxide precipitated from the supersaturated droplets to form the nanowires. Continuous dissolution and precipitation guarantee the continuous growth of the nanowires. Certainly, the diameter of the nanowires was directly related to and often smaller than the diameter of the initial liquid droplets [13], which can be seen clearly in Fig. 5. The hemispheric tips are suspected to be formed during the cooling down process to room temperature. The relative potassium content on the tip is much more than that in the wire, which can be clearly seen in the EDX spectrum shown in Fig. 2.
To demonstrate the role of the catalyst, potassium hydroxide, used in this work, additional experiments were performed with the same condition as previously described except that no potassium hydroxide was used or the potassium hydroxide powder was used instead of the potassium hydroxide solution. In the case of no potassium hydroxide on the plate surface, no nanowires were grown. With potassium hydroxide powder as catalyst, the diameter of the tungsten oxide nanowires was larger than that with potassium hydroxide solution. It indicates that the use of potassium hydroxide solution as catalyst is a crucial factor for the growth of the nanowires in small diameters. This is because the solution was more uniformly distributed on the surface of tungsten substrate than the powders, so does the tiny potassium hydroxide particles after the water evaporates. The necessity of the dwell heating at 390 1C is demonstrated in Fig. 1(d), where the nanowires were grown with the same condition for that shown in Fig. 1(b) except that only one-stage heating (reaction heating) was used. It is obvious that the nanowires grown without the dwell heating are larger and much less uniform. The reason is the following. When heated at 390 1C for certain amount of time, the potassium hydroxide with melting temperature of 380 1C were kept at liquid phase and thus distributed even more uniformly on the surface of tungsten substrate resulting in more uniform and smaller nanowires. The influence of reaction temperature on the formation of tungsten oxide nanowires was also studied with our twostage heating process. It was confirmed that no tungsten nanowires were formed at reaction temperatures below 550 1C, which is in agreement with the previous report with K2SO4 as catalyst [12]. At the high temperature end, we found that the nanowires can be synthesized at 790 1C, which is much higher than the upper limit of 690 1C reported. This difference must be due to the employment of potassium hydroxide solution and extra stage of dwell heating at 390 1C. 4. Conclusions In conclusion, uniform tungsten oxide nanowires with small diameter were synthesized by two-stage heating and with potassium hydroxide solution as catalyst on tungsten plates. The diameters of the nanowires range from 30 to 200 nm and the lengths are up to tens of micrometers. The nanowires were characterized for their morphology, chemical composition and structure (W3O8). The VLS growth mechanism is applied for the formation of the tungsten oxide nanowires. The influences of the dwell heating stage and the other factors of the experiment such as reaction temperature are also discussed. Acknowledgements
Fig. 5. High-magnification SEM image of a nanowire with a diameter of 35 nm. The hemispheric tip is the evidence of VLS growth mechanism.
This work is supported by RGC grant No. HKU704605.
ARTICLE IN PRESS R. Hu et al. / Journal of Crystal Growth 306 (2007) 395–399
References [1] C. Santato, M. Odziemkowski, M. Ulmann, J. Augustynski, J. Am. Chem. Soc. 123 (2001) 10639. [2] K. Sayama, K. Mukasa, R. Abe, Y. Abe, H. Arakawa, Chem. Commun. (2001) 2416. [3] J.L. Solis, S. Saukko, L. Kish, C.G. Granqvist, V. Lantto, Thin Solid Films 391 (2001) 255. [4] A. Shengelaya, S. Reich, Y. Tsabba, K.A. Muller, Eur. Phys. J. B 12 (1998) 13. [5] S.H. Baeck, T. Jaramillo, G.D. Stucky, E.W. McFarland, Nano Lett. 2 (2002) 831. [6] E. Ozkan, S.H. Lee, P. Liu, C.E. Tracy, F.Z. Tepehan, J.R. Pitts, S.K. Deb, Solid State Ion 149 (2002) 139.
399
[7] J. Shieh, H.M. Feng, M.H. Hon, H.Y. Juang, Sensors Actuators B 86 (2002) 75. [8] Y.Q. Zhu, W.B. Hu, W.K. Hsu, M. Terrones, N. Grobert, J.P. Hare, H.W. Kroto, D.R.M. Walton, H. Terrones, Chem. Phys. Lett. 309 (1999) 327. [9] Z.W. Liu, Y. Bando, C.C. Tang, Chem. Phys. Lett. 372 (2003) 179. [10] W.B. Hu, Y.Q. Zhu, W.K. Hsu, B.H. Chang, M. Terrones, N. Grobert, H. Terrones, J.P. Hare, H.W. Kroto, D.R.M. Walton, Appl. Phys. A 70 (2000) 231. [11] H. Qi, C. Wang, J. Liu, Adv. Mater. 15 (2003) 411. [12] B. Zhang, C.B. Cao, H.L. Qiu, Y.J. Xu, Y.C. Wang, H.S. Zhu, Chem. Lett. 34 (2005) 154. [13] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89.