ISRN NanotechnologyVolume 2012 (2012), Article ID 379510, 6 pagesdoi:10.5402/2012/379510

Research Article

Liquid Phase Morphology Control of ZnO Nanowires, Ellipse Particles, Hexagonal Rods, and Particle in Aqueous Solutions

Yoshitake Masuda,1,2 Naoto Kinoshita,² and Kunihito Koumoto²

¹Advanced Manufacturing Research Institute (AMRI), National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan²Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan

Received 31 August 2011; Accepted 18 October 2011

Academic Editors: C. A. Charitidis, H.-C. Hsu, and B. R. Kimball

Copyright © 2012 Yoshitake Masuda et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Liquid phase morphology control of ZnO crystals was realized with simple aqueous solution system. ZnO nanowires were successfully fabricated at 50∘C. They were over 50 μm in length and about 100 nm in width. Aspect ratio was estimated to be over 500. They had no branches and were obtained without aggregations. Curved nano-wires clearly indicated high flexibility and high mechanical strength. Additionally, ellipse particles, hexagonal rods and particles were fabricated in the solutions. Morphology, crystal growth speed, and preferred growth faces were well-controlled by precise adjustment of growth conditions.

1. Introduction

Metal oxides have been used for electronic devices, optoelectronic device, and so forth. They will further expand beyond the present borders of research area. Especially, expectations for metal oxide nanostructures have increased recently for future applications. Syntheses of novel metal oxide nanostructures and development of their applications are under intense investigation [1–8]. Special issues [9], reviews [10–13], and books [14] on applications of metal oxide nanostructures have received a great response.

ZnO has attracted much attention as varistors [15], piezoelectric devices [16], electroacoustic transducers [17], and highly transparent conducting windows for solar cells, displays [18], vacuum fluorescent displays (VFDs) [19], field emission displays (FEDs) [20], electroluminescent displays (ELDs) [21], UV light-emitting diodes (LEDs), laser diodes [22], and gas sensors [23, 24], dye-sensitized solar cells [25–29], and molecular sensors [30].

Novel ZnO nanostructures are eagerly anticipated for the applications [31–34]. ZnO nanowires especially are required for luminescent devices, dye-sensitized solar cells and high sensitive sensors. The nanowires have high electrical conductivity along the longer direction. They have large surface area due to high aspect ratio of the shape. High flexibility and high mechanical strength are required to apply for the devices.

Recently, several ZnO nano-/microstructures were prepared in the solutions [34–39]. Patterning of them was realized on self-assembled monolayers (SAMs) [35]. Hydrophobic surface of SAMs accelerated deposition of ZnO nanostructures rather than hydrophilic silanol group surfaces [35]. These studies contributed development of future ZnO devices.

In this study, ZnO nanowires were developed in aqueous solutions. Anisotropic crystal growth of ZnO was well-controlled by precise adjustment of solution conditions. Additionally, ellipse particles, hexagonal rods, and particles were fabricated.

2. Experimental Details

Octadecyltrichlorosilane (OTS, C₁₈H₃₇SiCl₃, Acros Organics), anhydrous toluene (Aldrich Chemical Co., Inc.), zinc acetate (Zn(CH₃COO)₂, Kishida Chemical Co., Ltd.), ammonia (28% solution, Kishida), NaOH, HCI, ethanol, and acetone were used as received. Distilled water was used as a solvent.

An Si wafer (p-type Si , NK Platz Co., Ltd.) was sonicated for 10 min in water, ethanol, or acetone, in that order. It was exposed for 15 min to UV light (184.9 nm and 253.7 nm) (low-pressure mercury lamp 200 W, PL21-200, 15 mW/cm² for 254 nm, SEN Lights Co.) to clean the surface. UV light which has been used in this study (PL21-200) had stronger power than that we used in former studies (NL-UV253, Nippon Laser and Electronics Lab.) [40–44]. SAMs of octadecyltrichlorosilane (OTS, C₁₈H₃₇SiCl₃) were prepared by immersing Si substrates in anhydrous toluene (Aldrich Chemical Co., Inc.) solutions containing 1 vol% OTS (Acros Organics) for 15 min under an N₂ atmosphere. The substrates with the SAMs were baked at 120°C for 5 min to remove residual solvents and promote chemisorption of the SAMs.

Morphology of ZnO was evaluated with a scanning electron microscope (SEM; S-3000N, Hitachi, Ltd.). Crystal structure of ZnO was evaluated with an X-ray diffractometer (XRD; RINT-2100, Rigaku, in-plane X-ray diffraction profiles) with CuK radiation (40 kV, 30 mA) and Ni filter plus a graphite monochromator.

3. Results and Discussion

3.1. Synthesis of ZnO in Aqueous Solutions

(1) WiresZinc acetate (Zn(CH₃COO)₂, 1.375 g) was dissolved into water of 100 mL at 50°C. Ammonia (28% solution, Kishida, 910 mL) was then added as complexing agent with stirring to be [NH₃]/[Zn] = 2.0. The solutions showed pH = 6.99 at 1 min after addition of ammonia. The substrates were immersed in the solutions at bottom up for 3 h. The pH of the solutions was still pH = 6.99 at 3 h after immersion of the substrates.

(2) Ellipse Particles The solutions were diluted by 4% (concentration: 1/25) for fabrication of ellipse particles.

(3) Hexagonal RodsThe solutions were diluted by 20% (concentration: 1/5) for fabrication of hexagonal rods. HCI was added to control pH.

(4) ParticlesThe solutions were diluted by 20% (concentration: 1/5) for fabrication of particles. NaOH was added to control pH.ZnO was formed in the aqueous solutions as follows: Zinc acetate solutions were transparent. The solutions became slightly white after addition of ammonia. High ion concentration proceeded formation of ZnO. The particles were formed in the solutions by consumption of ions. Supersaturation degree decreased with the decrease of ion concentrations. The solutions became transparent after a few hours.

(1) WiresThe substrates were rinsed with distilled water and dried in air after immersion. They were evaluated with SEM and XRD. XRD showed that precipitated nano-wires were single phase of ZnO without additional phases. Many ZnO nano-wires were observed on the substrates (Figures 1(a)–1(d)). Length was over 50 μm. Width was about 100 nm. Aspect ratio was estimated to be over 500 (= 50 μm/100 nm). Some nanowires curved in top right region of Figure 1(a). Magnified image was shown as Figure 1(b). Radius of the curves was about 1.77 μm (Figure 1(b)). It indicated high flexibility and high mechanical strength of the nanowires. They grew along c-axis of wurtzite crystal structure. High crystallinity allowed forming 1D structure with uniform width, long length, high flexibility, and high strength. Morphology and size in different areas were shown in Figure 1(c). Small amount of the nanowires deposited on a substrate (Figure 1(d)).This image indicated that the nano-wires were not aggregated and had no branches. They were obtained as separated nanowires. For comparison, ZnO 1D structures were prepared on fluorine doped tin oxide substrates in aqueous solutions. The solutions contained zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 15 mM) and ethylenediamine (H₂NCH₂CH₂NH₂, 15 mM). They were kept at 60°C for 6 h (Figure 2) [45]. Electron diffraction pattern indicated that the structure was a single crystal of ZnO. Lengthwise direction was parallel to c-axis of ZnO. Anisotropic crystal growth of ZnO along c-axis caused anisotropic 1D structure.

(a) ZnO nano-wires

(b) ZnO nano-wires

(c) ZnO nano-wires

(d) ZnO nano-wires

Figure 1: SEM micrographs of ZnO nanowires. (a) Some nanowires in top right region curved. (b,c) Magnified area of (a) showing morphology of the wires. (d) Separated straight wires on a substrate. This image indicated that the nano-wires were not aggregated and had no branches.

Figure 2: TEM micrograph and electron diffraction pattern of ZnO 1D structure.

(2) Ellipse ParticlesZinc acetate (0.055 g) was dissolved into water of 100 mL at 50°C. Ammonia (36.4 mL) was then added as complexing agent with stirring to be [NH₃]/[Zn] = 2.0. Concentrations of zinc acetate and ammonia were 1/25 of the solution for nanowires. The solutions had low ion concentrations and low supersaturation degree. These induced slow crystallization speed. The solutions showed pH = 7.49 at 1 min after addition of ammonia. The substrates were immersed in the solutions at bottom up for 3 h. The pH of the solutions was pH = 6.87 at 3 h after immersion of the substrates. XRD showed that precipitated ellipse particles were single phase of ZnO. Any additional phase was not detected. Many ZnO ellipse particles were observed on the substrates (Figure 3(a)). They were about 800 nm in length and 400 nm in width. Aspect ratio was estimated to be 2 (= 800 nm/400 nm). The ellipse particle had two sharp tips and rounded body. For comparison, similar ellipse particles were obtained in our previous study [35]. The previous ellipse particles had more rounded tips.

(a) ZnO ellipse particles

(b) ZnO hexagonal rods

(c) ZnO particles

Figure 3: SEM micrographs of (a) ZnO ellipse particles, (b) ZnO hexagonal rods, and (c) ZnO particles.

(3) Hexagonal RodsHCI (1.0 mL) was added into water of 100 mL at 50°C. Zinc acetate (0.275 g) and ammonia (182 mL) were then added with stirring to be [NH₃]/[Zn] = 2.0. Concentrations of zinc acetate and ammonia were 1/5 of the solution for nanowires. The solutions showed pH = 6.87 at 1 min after addition of ammonia. The substrates were immersed in the solutions at bottom up for 3 h. The pH of the solutions was pH = 6.72 at 3 h after immersion of the substrates.XRD showed that precipitated hexagonal rods were single phase of ZnO without additional phases. Many ZnO hexagonal rods were observed on the substrates (Figure 3(b)). They were about 500 nm in length and 200 nm in width. Aspect ratio was estimated to be 2.5 (= 500 nm/200 nm). For comparison, hexagonal cylinder particles were also prepared in our previous studies [35, 36]. The aspect ratio and sizes were slightly different due to difference in synthesis conditions.

(4) ParticlesNaOH (1.0  mL) was added into water of 100 mL at 50°C. Zinc acetate (0.275 g) and ammonia (182 mL) were then added with stirring to be [NH₃]/[Zn] = 2.0. Concentrations of zinc acetate and ammonia were same to those of the solution for hexagonal rods. These solutions showed pH = 7.39 at 1 min after addition of ammonia. The substrates were immersed in the solutions at bottom up for 3 h. The pH of the solutions was pH = 6.91 at 3 h after immersion of the substrates. The pH of the solutions was higher than that of the solution for hexagonal rods due to the addition of NaOH. XRD showed that precipitated particles were single phase of ZnO without additional phases. Many ZnO particles were observed on the substrates (Figure 3(c)). They were about 20 nm in size. High pH of the solutions would decrease aspect ratio of hexagonal rods to form particles.

4. Conclusions

ZnO nanowires were successfully fabricated in aqueous solutions at ordinary temperature. They were over 50 μm in length and about 100 nm in width. Aspect ratio was estimated to be over 500. They had no branches and were obtained without aggregations. Curved nanowires clearly indicated high flexibility and high mechanical strength. They can be applied to next-generation ZnO devices. Additionally, ellipse particles, hexagonal rods, and particles were obtained. The solution conditions had large affects to crystal growth of ZnO. The knowledge obtained in this study will contribute to the development of morphology control of metal oxides.

References

L. Tang, M. Salamon, and M. R. D. Guire, “Cerium oxide thin films on solid oxide fuel cell anodes,” Science of Advanced Materials, vol. 2, no. 1, pp. 79–89, 2010.

D. Chu, Y.-P. Zeng, D. Jiang, and Y. Masuda, “Room temperature ferromagnetism in transition metal doped TiO₂ nanowires,” Science of Advanced Materials, vol. 1, no. 3, pp. 227–229, 2009.

T. Kawai, H. Takahashi, Y. Matsushima, T. Ogata, and H. Unuma, “Evaluation of photocatalytic activity of TiO₂ thin films by spin-trap ESR spectroscopy,” Science of Advanced Materials, vol. 2, no. 1, pp. 74–78, 2010.

Y. Masuda, T. Ohji, and K. Kato, “Multineedle TiO₂ nanostructures, self-assembled surface coatings, and their novel properties,” Crystal Growth and Design, vol. 10, no. 2, pp. 913–922, 2010.

Y. Masudaa and K. Katoa, “Tin oxide coating on polytetrafluoroethylene films in aqueous solutions,” Polymers for Advanced Technologies, vol. 21, no. 3, pp. 211–215, 2010.

M. Inada, K. Mizue, N. Enomoto, and J. Hojo, “Thermal stability of rutile TiO₂ with high specific surface area synthesized by self-hydrolysis process,” Science of Advanced Materials, vol. 2, no. 1, pp. 102–106, 2010.

Y. Tokunaga, H. Uchiyama, Y. Oaki, and H. Imai, “Specific photocatalytic performance of nanostructured rutile-type Tio₂: selective oxidation of thiazin dye with a bundled architecture consisting of oriented nanoneedles,” Science of Advanced Materials, vol. 2, no. 1, pp. 69–73, 2010.

Y. Masuda, Y. Jinbo, and K. Koumoto, “Room temperature CVD of TiO₂ thin films and their electronic properties,” Science of Advanced Materials, vol. 1, no. 2, p. 138, 2009.

Y. Masuda, “A special issue on applications of metal oxide nanostructures,” Science of Advanced Materials, vol. 2, no. 1, pp. 1–2, 2010.

Y. Masuda, “Liquid phase patterning of ceramics,” Journal of the Ceramic Society of Japan, vol. 115, no. 1338, pp. 101–109, 2007.

Y. Masuda, Y. F. Gao, P. X. Zhu, et al., “Site-selective deposition of ceramic thin films using self-assembled monolayers,” Journal of the Ceramic Society of Japan, vol. 112, no. 5, p. 1495, 2004.

A. D. Handoko and G. K. L. Goh, “One-dimensional perovskite nanostructures,” Science of Advanced Materials, vol. 2, no. 1, p. 16, 2010.

G. Neri, “Non-conventional sol-gel routes to nanosized metal oxides for gas sensing: from materials to applications,” Science of Advanced Materials, vol. 2, no. 1, pp. 3–15, 2010.

A. Umar and Y.-B. Hahn, Metal Oxide Nanostructures and Their Applications, American Scientific Publishers, Calif, USA, 2009.

Y. Lin, Z. Zhang, Z. Tang, F. Yuan, and J. Li, “Characterisation of ZnO-based varistors prepared from nanometre precursor powders,” Advanced Materials for Optics and Electronics, vol. 9, no. 5, pp. 205–209, 1999.

G. Agarwal and R. F. Speyer, “Current change method of reducing gas sensing using ZnO varistors,” Journal of the Electrochemical Society, vol. 145, no. 8, pp. 2920–2925, 1998.

F. Quaranta, A. Valentini, F. R. Rizzi, and G. Casamassima, “Dual-ion-beam sputter deposition of ZnO films,” Journal of Applied Physics, vol. 74, no. 1, pp. 244–248, 1993.

T. Pauporté and D. Lincot, “Electrodeposition of semiconductors for optoelectronic devices: results on zinc oxide,” Electrochimica Acta, vol. 45, no. 20, pp. 3345–3353, 2000.

S. Ruan, “Color picture element tube for large-screen display,” in Proceedings of the SPIE, vol. 262, p. 2892, 1996.

Y. Nakanishi, A. Miyake, H. Kominami, T. Aoki, Y. Hatanaka, and G. Shimaoka, “Preparation of ZnO thin films for high-resolution field emission display by electron beam evaporation,” Applied Surface Science, vol. 142, no. 1–4, pp. 233–236, 1999.

L. Yi, Y. Hou, H. Zhao et al., “Photo- and electro-luminescence properties of ZnO: Zn thin film,” Displays, vol. 21, no. 4, pp. 147–149, 2000.

R. F. Service, “Will UV lasers beat the blues?” Science, vol. 276, no. 5314, p. 895, 1997.

H. Gong, J. Q. Hu, J. H. Wang, C. H. Ong, and F. R. Zhu, “Nano-crystalline Cu-doped ZnO thin film gas sensor for CO,” Sensors and Actuators, B: Chemical, vol. 115, no. 1, pp. 247–251, 2006.

W. J. Moon, J. H. Yu, and G. M. Choi, “The CO and H-2 gas selectivity of CuO-doped SnO₂-ZnO composite gas sensor,” Sensors and Actuators, B: Chemical, vol. 87, no. 3, pp. 464–470, 2002.

M. Law, L. E. Greene, J. C. Johnson, R. Saykally, and P. Yang, “Nanowire dye-sensitized solar cells,” Nature Materials, vol. 4, no. 6, pp. 455–459, 2005.

J. B. Baxter and E. S. Aydil, “Nanowire-based dye-sensitized solar cells,” Applied Physics Letters, vol. 86, no. 5, Article ID 053114, pp. 1–3, 2005.

R. Katoh, A. Furube, K. Hara et al., “Efficiencies of electron injection from excited sensitizer dyes to nanocrystalline ZnO films as studied by near-IR optical absorption of injected electrons,” Journal of Physical Chemistry B, vol. 106, no. 50, pp. 12957–12964, 2002.

S. Karuppuchamy, K. Nonomura, T. Yoshida, T. Sugiura, and H. Minoura, “Cathodic electrodeposition of oxide semiconductor thin films and their application to dye-sensitized solar cells,” Solid State Ionics, vol. 151, no. 1–4, pp. 19–27, 2002.

K. Keis, C. Bauer, G. Boschloo et al., “Nanostructured ZnO electrodes for dye-sensitized solar cell applications,” Journal of Photochemistry and Photobiology A: Chemistry, vol. 148, no. 1–3, pp. 57–64, 2002.

H. Tokudome, Y. Yamada, S. Sonezaki et al., “Photoelectrochemical deoxyribonucleic acid sensing on a nanostructured TiO₂ electrode,” Applied Physics Letters, vol. 87, no. 21, Article ID 213901, pp. 1–3, 2005.

R. Wahab, Y.-S. Kim, D. S. Lee, J.-M. Seo, and H.-S. Shin, “Controlled synthesis of zinc oxide nanoneedles and their transformation to microflowers,” Science of Advanced Materials, vol. 2, no. 1, pp. 35–42, 2010.

S. K. Mohanta, D. C. Kim, B. H. Kong, H. K. Cho, W. Liu, and S. Tripathy, “Optical properties of ZnO nanorods and hybrid structures grown on p-type GaN/sapphire and silicon-on-insulator substrates,” Science of Advanced Materials, vol. 2, no. 1, pp. 64–68, 2010.

D. W. Chu, Y. Masuda, T. Ohji, and K. Kato, “Formation and photocatalytic application of ZnO nanotubes using aqueous solution,” Langmuir, vol. 26, no. 4, pp. 2811–2815, 2010.

X. L. Hu, Y. Masuda, T. Ohji, and K. Kato, “Micropatterning of ZnO nanoarrays by forced hydrolysis of anhydrous zinc acetate,” Langmuir, vol. 24, no. 14, pp. 7614–7617, 2008.

Y. Masuda, N. Kinoshita, F. Sato, and K. Koumoto, “Site-selective deposition and morphology control of UV- and visible-light-emitting ZnO crystals,” Crystal Growth and Design, vol. 6, no. 1, pp. 75–78, 2006.

Y. Masuda, N. Kinoshita, and K. Koumoto, “Hexagonal symmetry radial whiskers of ZNO crystallized in aqueous solution,” Journal of Nanoscience and Nanotechnology, vol. 9, no. 1, pp. 522–526, 2009.

Y. Masuda and K. Kato, “Morphology control of zinc oxide particles at low temperature,” Crystal Growth and Design, vol. 8, no. 8, pp. 2633–2637, 2008.

Y. Masuda and K. Kato, “High c-axis oriented stand-alone ZnO self-assembled film,” Crystal Growth and Design, vol. 8, no. 1, pp. 275–279, 2008.

Y. Masuda and K. Kato, “Rapid growth of thick particulate film of crystalline ZnO in an aqueous solution,” Thin Solid Films, vol. 516, no. 9, pp. 2474–2477, 2008.

Y. Masuda, T. Sugiyama, H. Lin, W. S. Seo, and K. Koumoto, “Selective deposition and micropatterning of titanium dioxide thin film on self-assembled monolayers,” Thin Solid Films, vol. 382, no. 1-2, pp. 153–157, 2001.

Y. Masuda, Y. Jinbo, T. Yonezawa, and K. Koumoto, “Templated site-selective deposition of titanium dioxide on self-assembled monolayers,” Chemistry of Materials, vol. 14, no. 3, pp. 1236–1241, 2002.

Y. Masuda, T. Sugiyama, and K. Koumoto, “Micropatterning of anatase TiO₂ thin films from an aqueous solution by a site-selective immersion method,” Journal of Materials Chemistry, vol. 12, no. 9, pp. 2643–2647, 2002.

Y. Masuda, S. Ieda, and K. Koumoto, “Site-selective deposition of anatase TiO₂ in an aqueous solution using a seed layer,” Langmuir, vol. 19, no. 10, pp. 4415–4419, 2003.

Y. Masuda, N. Saito, R. Hoffmann, M. R. de Guire, and K. Koumoto, “Nano/micro-patterning of anatase TiO₂ thin film from an aqueous solution by site-selective elimination method,” Science and Technology of Advanced Materials, vol. 4, no. 5, pp. 461–467, 2003.

Y. Masuda and K. Kato, “Aqueous synthesis of ZnO rod arrays for molecular sensor,” Crystal Growth and Design, vol. 9, no. 7, pp. 3083–3088, 2009.