Liwei Lin,
Masayoshi
Hideo Watanabe, Takashi Shirai and Minoru Takahashi
セラミックス基盤工学研究センター年報(2008).
Vol. 8,Fuji,
17-22

Review

Potential Applications and Synthesis of Zinc Oxide Tubes
Liwei Lin, Masayoshi Fuji*, Hideo Watanabe, Takashi Shirai and Minoru Takahashi
Ceramics Research Laboratory, Nagoya Institute of Technology
Asahigaoka 10-6-29, Tajimi, Gifu 507-0071, JAPAN*fuji@nitech.ac.jp
Abstract
Zinc oxide is a unique material that exhibits optical, semiconducing, piezoelectric, and magnetic properties. In the past decade,zinc oxide (ZnO) tubes are a newly-developing morphology with the special properties of low densities, thermal insulation, andhigh surface permeability. In light of wide potential applications of zinc oxide tubes, the applications and potential applications
were reviewed in this paper. We have proposed a new and facile method to synthesize ZnO tubes in a large scale, which introduce
ammonia water into zinc chloride aqueous solution. The advantages of our utilizing-aqueous solution-based method are templatefree, surfactant-free, low temperature and normal pressure. The growth mechanism of ZnO microtubes have been proposed basedon the experimental results and ZnO special polar property.

2. Properties and potential applications of ZnO
particles

1. Introduction
Zinc oxide (ZnO) is a distinguished material with somespecial properties, which has attracted intensive research
efforts for its unique properties and versatile applications
in transparent electronics, ultraviolet (UV) light emitters,piezoelectric devices, chemical sensors and spinelectronics in the last few decades. [1-12] ZnO is a wideband-gap (3.37 eV) compound semiconductor that issuitable for short wavelength optoelectronic application.
The high exciton binding energy (60 mV) in ZnO crystal
can ensure efficient excitonic emission at room
temperature and room temperature ultraviolet (UV)luminescence has been reported in disordered particles.
The lack of a center of symmetry in wurtzite, combinedwith large electromechanical coupling, results in strong
piezoelectric and pyroelectric properties and theconsequent use of ZnO in mechanical actuators andpiezoelectric sensors. Based on these remarkable physicalproperties and the motivation of device miniaturization,
large effort has been focused on the synthesis,characterization and device application of ZnO nano- andmicro-materials.
This review paper describes traditional and novel
synthesis techniques of tubular ZnO particles on the
recently published works. The potential applications of
ZnO particles in the various fields are presented.

a) Luminescent property
ZnO exhibits a direct band-gap of 3.37 eV at room
temperature with a large exciton of 60 mV. The strong
exciton binding energy, which is much larger than thatof GaN (25 mV), and the thermal energy at room
temperature (26 mV) can ensure an efficient exciton
emission at room temperature under low excitation energy.
As a consequence, ZnO is recognized as a promising
photonic material in the blue-UV region.
b) Gas and chemical sensors
Conductometric metal oxide semiconductor thin films
are the most promising among solid state chemical sensors,
due to their small dimension, low cost, low power
consumption, on-line operation and high compatibility
with microelectronic processing. The fundamental sensing
mechanism of metal oxide based gas sensors relies on achange in electrical conductivity due to the process ofinteraction between the surface complexes, such as O-,
O2-, H+ and OH- reactive chemical species and the gas
molecules to be detected. Oxygen vacancies on metaloxide surface are electrically and chemically active. These
vacancies function as n-type donors, often significantly
increase the conductivity of oxide. Upon adsorption of
charge accepting molecules at the vacancy sites, such as
NO2 and O2, electrons are effectively depleted from theconduction band, leading to a reduced conductivity of
the n-type oxide. On the other hand, molecules, such as― 17 ―

Potential Applications and Synthesis of Zinc Oxide Tubes

CO and H2, would react with surface adsorbed oxygenand consequently remove it, leading to an increase of
conductivity. As one of the major materials for solid stategas sensor, bulk and thin films of ZnO have been proposed
for CO, NH3, alcohol and H2. [13-16]c) Piezoelectric effect and polar surfaces
As one of the important properties of ZnO, itspiezoelectricity has been extensively studied for various
applications in force sensing, acoustic wave resonator,
acousto-optic modulator, etc. [17-19] Piezoelectricity is due
to atomic scale polarization. To illustrate thepiezoelectricity, one considers an atom with positive
charge that is surrounded tetrahedrally by anions, as shownin Fig.1. [20] The center of gravity of the negative charges
is at the centre of the tetrahedron. On exerting a pressure
on the crystal along the cornering direction of thetetrahedron, the tetrahedron will experience a distortionand the center of gravity of the negative charges will nolonger coincide with the position of the positive central
atom; an electric dipole is generated. If all of the tetrahedralin the crystal have the same orientation or some othermutual orientation that does not allow for a cancellation
among the dipoles, the crystal will have a macroscopic
dipole. The two opposite faces of the crystal have opposite
electric charges.

is displaced from that of negative charge and the directionof displacement is also [0001]. The net result of this
spontaneous polarization is a charged (0001) surface.
3. Synthesis methods of tubular ZnO particles
Besides nanowires, nanobelts and nanorods, othercomplex ZnO nanostructures, such as tubular ZnO
particles [23, 24] also attract considerable research interests.
Since the discovery of carbon nanotube [25], several
methods for fabrication of ZnO particles with tubular
structure have been reported, such as vapor phase
depositions, thermal oxidation, a template-assisted
method and a hydrothermal process. A few of them aredescribed in the followings:
A) The most common method synthesize ZnOnanostructure utilizes a vapor transport process. [26, 27] Insuch a process, zinc or zinc oxide and oxygen or oxygen
mixture vapor are transported and react with each other,
forming ZnO nanostructure. According to the difference
on the formation mechanisms, the extensively used vapor
transport process can be categorized into the catalyst freevapor-solid (VS) process [28, 29] and catalyst assisted vaporliquid-solid (VLS) process [30]. The typical process is
usually carried out in a horizontal tube furnace, as shownin Fig.2 [20] , which is composed of a horizontal tube furnace,
an alumina tube, a rotary pump system and a gas supply
and control system. R.M. Wang has synthesized ZnOnanotubes through heating the mixture of Zn and ZnO
powder at 1300˚C in Ar flow. [31]Figure 2 Schematic diagram of the experimental apparatus by
Figure 1 Schematic diagrams showing the piezoelectric effectthe vapor transport process

in a tetrahedrally coordinated cation-anion unit

Another interesting result of the non-centrosymmetric
ZnO crystal structure is its spontaneous polarization andpolar face dominated nanostructure. [21, 22] The crystal
structure of ZnO can be visualized in a way that oxygen
atoms and zinc atoms are tetrahedrally bonded. These
tetrahedrons stack along [0001] direction. Due tospontaneous polarization, the position of positive chargeB) Another evaporating synthesis method to synthesize
ZnO particles is metal-organic chemical vapor deposition.
[32]
The typical diagrammatic sketch of metal-organic
chemical vapor deposition (MOCVD) device system isshown in Fig.3. [33] In a typical CVD process, the substrateis exposed to the volatile precursor, which reacts ordecomposes on the substrate surface to produce the highpurity, high performance film. Frequently, volatile by-― 18 ―

Liwei Lin, Masayoshi Fuji, Hideo Watanabe, Takashi Shirai and Minoru Takahashi

products are also produced, which are removed by gas
flow through the reaction chamber. Through CVD method,
the steams of volatile metal compounds or metal organic
compounds is used as raw materials, and through chemical
reaction made into the material needed. Then it condenses
rapidly under protection of inert gases for preparation ofparticles. The ZnO particles prepared by CVD possess highpurity, uniform particle size which can be accurately
controlled. B. P. Zhang et al [34] have reported that ZnO
tubes were epitaxially grown on sapphire (0001) substrateby MOCVD. The growth pressure and temperature were0.3-3 Torr and 475˚C, separately. Oxygen gas and diethyl
zinc were used as precursors and nitrogen gas was usedas the carrier gas for DEZn by-product.

Figure 3 Diagrammatic sketch of atmospheric metal-organic
chemical vapor deposition (MOCVD) device system

C) Other synthesis methods
Although the vapor transport process and hydrothermal
method are the dominant synthesis processes for synthesis
ZnO particles with tubular structure. Other growthmethods such as hydrothermal methods [35, 36], replicationand template techniques [37, 38] and laser patterning [39] havebeen developed in parallel. These methods provide thepossibility of forming ZnO tubes at low temperature. For
example, via a hydrothermal method [36], Vayssieres etal. have reported the synthesis of ZnO microtubes, on a
range of substrates, immersed in a bottle with anautoclavable screw cap containing equimolar amounts ofzinc nitrate and hexamethylenetetramine and maintained
at 90˚C for 2 days. Z. Wang and H. L. Li [38] havesynthesized ZnO nanotubules via a sol-gel process within
the pores of an anodic aluminum oxide (AAO) template.
4. The developed aqueous solution method
Rigid experimental conditions (such as hightemperature), sophisticated equipments (such as
autoclave), and complex procedure were usually employedin the above mentioned methods in order to obtain singlecrystalline and high purity ZnO tubes. Furthermore, mostof the reported approaches produced ZnO tubes in small
quantities and high cost, restricting their commercial
applications. So it is necessary to develop a template-freeand surfactant-free method at low temperature, low costand normal pressure.
In this paper, we proposed a new and facile method tosynthesize the hexagonal ZnO microtubes in a large
scale.[40-42] In comparison with other previous studies, we
did not introduce any other chemicals except reactantsin the reaction solution. Besides this, the advantages of
our utilizing-aqueous solution-based method are templatefree, surfactant-free, low temperature and normal pressure.
Figure 4 is the experimental procedure. The typicalexperimental procedures were as follows: the conical glass
flask with the aqueous solution (400 ml) of zinc chloride(purity of 98 % from Wako, Japan), of which zinc ionicconcentration is 0.5 M, was heated in the oil bath understirring. Aqueous ammonia water (25 wt%) was added
dropwise into the reaction solution with stirring,
immediately resulting in the formation of white
precipitates. When the pH of the reaction solution reached7.5, the introduction of ammonia water was stopped. The
precipitate was kept at reaction solution for different time,then filtered and dried.

Figure 4 Experimental procedure

Figure 5 shows XRD patterns of wet samples withdifferent aging time. The precipitates were filtrated andthen analyzed using XRD. The products without aging
are the hydrate of zinc oxide precursor. With the aging
time increasing, the precursors turn into zinc oxide. Until
the aging time is 8 h, the products still contain some
precursor except zinc oxide.

― 19 ―

Potential Applications and Synthesis of Zinc Oxide Tubes

show the morphologies of ZnO samples obtained after
3 days aging duration, the hexagonal shell peel with thecolumn-like particles inside, which is the evidence ofintermediate process in the second stage. The preferential
chemical dissolution of the metastable (001)-Zn faces ofthe microrods shall lead to the tubular ZnO particles, as
shown in Fig.6 (E) and Fig.6 (F). Figure 6 (F) shows themagnified images of ZnO tubes. The aging duration isthat the systems will tend to reach their thermodynamic
stability and may therefore undergo variations ofmorphology, size and structure properties.

Figure 5 XRD patterns of wet samples with different aging time

Figure 7 TEM images of ZnO microtubes

Figure 6 SEM and images of ZnO microtubes at different aging
time

Figure 6 reveals the morphological evolution of ZnO
particles from layer-like precursor to tube-like zinc oxideby adjusting the aging time. It can be clearly seen thatlayer-like particles with 10~12 mm in the length of layerside were obtained without aging duration, shown in Fig.6(A), which is the white precipitate directly filtrated after
the reaction. While aging duration was 2 h, as shown in
Fig.6 (B), layer-like particles began to decomposing into
rod-like particles. In the decomposing layer structure, it
is likely that some of rod-like just grow up. Fig.6 (C) shows
SEM image of well hexagonal ZnO microrods formed
when the aging duration for 6 h. Some of hexagonal rods
assemble together and form flower-like structure. Fig.6 (D)Figure 7 shows the further characterization of the
prepared tubes using TEM. Figure 7 (A) is the TEM imagesof a horizontally cut tube. The dark center and brightedge indicates the presence of hollow structure inside of
the tube. The small particles observed inside of the tube
are believed to be the result of cutting. The inset of Fig.7(A) is an electron diffraction (ED) pattern selected fromthe wall of tube, which reveals that the tube was single
crystalline. The inset of Fig.7 (B) is an ED pattern selectedfrom the hexagonal opening, which indicates that the tube
is single crystalline and grew along [0001] direction (caxis).
In the progress of the formation of ZnO, the growth
units [Zn(NH3)4]2+ and [Zn(OH)4]2- encapsulated by water
were formed first. These growth units are easy to attachon the surface of precipitate. The formation of hexagonal
ZnO microtubes is attributed to the different growth rateof the various crystal facets and the polar (0001) surface.
Generally, the growth rate of ZnO crystal in the aqueous
solution in the [0001] direction is about twice as fast asthat in the <0110> direction. [43] That is due to (0001)
surface has roughly a 60 % higher cleavage energy than
the nonpolar {0110} facets [44], which results in the higher
growth rate along the [0001] direction with the formationof unstable surface. According to classical crystal growththeory, a crystal facet grows rapidly, and easily disappears
from the final crystal morphology. Finally ZnO microtubes― 20 ―

Liwei Lin, Masayoshi Fuji, Hideo Watanabe, Takashi Shirai and Minoru Takahashi

were formed by the dissolution of unstable (0001) surface.
Figure 8 shows schematic illustration of possible growthmechanism for ZnO microtubes.

thin-film heterojunction”, Sensor Actuat. B, 17 (1994), 221226.
[8] H. Harima, “Raman studies on spintronics materials basedon wide bandgap semiconductors”, J. Phys.: Condens. Matter
16 (2004), S5653-S5660.
[9] S. J. Pearton, W. H. Heo, M. Ivill, D. P. Norton, and T. Steiner,“Dilute Magnetic Semiconducting Oxides”, Semicond. Sci.
Technol., 19 (2004), R59-74.
[10] J. N ishii, F.M. Hossain, S. Takagi, T. Aita, K. Saikusa, Y.
Ohmaki, I. Ohkubo, S. Kishimoto, A. Ohtomo, T. Fukumura,
F. Matsukura, Y. Ohno, H. Koinuma, H. Ohno, and M.

Figure 8 Growth mechanism of ZnO micro-tubes

Kawasaki, “High Mobility Thin Film Transistors withTransparent ZnO Channels”, Jpn. J. Appl. Phys., 42 (2003),

4. Summary

L347-349.
[11] F. M. Hossain, J. Nishii, S. Takagi, T. Sugihara, A. Ohtomo,This paper reviews the potential application of zinc oxide
particles in the semiconducting, piezoelectric and opticalfields and discusses until now the traditional techniques forthe synthesis of zinc oxide tube. These methods involve
rigid conditions, sophisticated equipments and complex
procedure. Our developed aqueous solution method is a
facile and new process to synthesize ZnO tubes with thecharacteristic of low cost and high production.

T. Fukumura, H. Koinuma, H. Ohno, M. Kawasaki,“Modeling of grain boundary barrier modulation in ZnO
invisible thin film transistors”, Physica E, 21 (2004), 911-915.
[12] Q. H. Li, Y. X. Liang, Q. Wan, T. H. Wang, “Oxygen sensing
characteristics of individual ZnO nanowire transistors”, Appl.
Phys. Lett., 85 (2004), 6389-6391.
[13] J. X. Wang, X. W. Sun, H. Huang, Y. C. Lee, O. K. Tan,
M. B. Yu, G. Q. Lo, D. L. Kwong, “A two-step hydrothermallygrown ZnO microtube array for CO gas sensing”, Appl. Phys.

Reference

A, 88 [4] (2007), 611-615.

[1] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M. Hirano, and[14] G. Sberveglieri, “Recent developments in semiconducting

H. Hosono, “Thin-film transistor fabricated in single-

thin-film gas sensors”, Sens. Actuator B, 23 (1995), 103-109.

crystalline transparent oxide semiconductor”, Science, 300[15] G. S. Trivikrama Rao, D. Tarakarama Rao, “Gas sensor(2003) 1269-1272.

response of ZnO based thick film sensor to NH3 at room[2] H. Rensmo, K. Keis, H. Lindstro1m, S. Sö1dergren, A.

temperature”, Sens. Actuator B, 55 (1999), 166-169.

Solbrand, A. Hagfeldt, and S.-E. Lindquist, “ High Light-[16] X. L. Cheng, H. Zhao, L. H. Huo, S. Gao, J. G. Zhao, “ZnO

to-Energy Conversion Efficiencies for Solar Cells Basednanoparticulate thin film: preparation, characterization andon Nanostructured ZnO Electrodes”, J. Phys. Chem. B, 101

gas-sensing property”, Sens. Actuator B, 102 (2004), 248-

(1997), 2598-2601.

252.

[3] S. Y. Lee, E. S. Shim, H. S. Kang, S. S. Pang, and J. S.

[17] C. R. Wuethrich, C. A. P. Muller, G. R. Fox, and H. G.

Kang, “Fabrication of ZnO thin film diode using laser

Limberger, “All-fiber acousto-optic modulator using ZnO

annealing”, Thin Solid Films, 437 [1] (2005), 31-34.

piezoelectric actuators”, Sensor Actuat. A, 66 (1998), 114-

[4] R. Könenkamp, R. C. Word, and C. Schlegel, “Verticalnanowire light-emitting diode”, Appl. Phys. Lett., 85 (2004),

117.
[18] T. Itoh, and T. Suga, “Force sensing microcantilever using

6004-6006.

sputtered zinc oxide thin film”, Appl. Phys. Lett., 64 (1994),[5] Z. L. Wang, X. Y. Kong, Y. Ding, P. Gao, W. L. Hughes,

37-39.

R. Yang, and Y. Zhang, “Semiconducting and Piezoelectric[19] R. Paneva, G. Temmel, E. Burte, and H. Ryssel,

Oxide Nanostructure Induced by Polar Surface”, Adv. Funct.

“Micromechanical ultrasonic liquid nebulizer”, Sensor

Mater., 14 (2004), 943-956.

Actuat. A, 62 (1997), 765-767.

[6] M. S. Wagh, L. A. Patil, T. Seth, and D. P. Amalnerkar, “Surface[20] Z. L. Wang, “Zinc oxide nanostructures: growth, properties

cupricated SnO2-ZnO thick films as a H2S gas sensor”, Mater.

and applications”, J. Phys. Condens. Matter, 16 (2004), 829-

Chem. Phys., 84 (2004), 228-233.

858.

[7] Y. Ushio, M. Miyayama, and H. Yanagida, “Effects of[21] X. Y. Kong, Z. L. Wang, “Spontaneous polarization-induced

interface states on gas- sensing properties of a CuO/ZnO

nanohelixes, nanosprings, and nanorings of piezoelectric― 21 ―

Potential Applications and Synthesis of Zinc Oxide Tubes

nanobelts”, Nano. Lett., 3 (2003), 1625-1631.

[33] B. Yang, “Film Science and Technology”, (2006), 82.

[22] X. Y. Kong, Y. Ding, R. Yang, and Z. L. Wang, “ Single-crystal[34] B. P. Znang, N. T. Binh, K. Wakatsuki, Y. Segawa, Y. Yamada,nanorings formed by epitaxial self-coiling of polar-

N. Usami, M. Kawasaki, H. Koinuma, “Formation of highly

nanobelts”, Science, 303 (2004), 1348-1351.

aligned ZnO tubes on sapphire (0001) substrates”, Appl. Phys.

[23] J. Zhou, Z. D. Wang, L. Wang, M. Wu, S. X. Ouyang, E.

Lett., 84 (2004), 4098-4100.

Gu, “ Synthesis of ZnO hexagonal tubes by a microwave[35] Y. Y. Ding, Z. Gui, J. X. Zhu, S. S. Yan, J. Liu, Y. Hu, Z.

heating method”, Superlattice Microst., 39 (2006), 314-318.

Z. Wang, “Fabrication of tubular ZnO by vesicle-template[24] L. G. Yu, G. M. Zhang, S. Q. Li, Z. H. Xi, D. Z. Guo,fusion”, Mater. Lett., 61 (2007), 2195-2199.

“Fabrication of arrays of zinc oxide nanorods and nanotubes[36] L. Vayssieres, K. Keis, A. Hagfeldt, S. E. Lindquist, “Three-in aqueous solution under an external voltage”, J. Cryst.

dimensional array of highly oriented crystalline ZnO

Growth, 299 (2007), 184-188.

microtubes”, Chem. Mater., 13 (2001), 4395-4398.

[25] S. Iijima, “Helical microtubules of graphitic carbon”, Nature,[37] B.I. Seo, U.A Shaislamov, M.H. Ha, S.-W. Kim, H.-K. Kim

354 (1991), 56-58.

and Beelyong Yang, “ZnO nanotubes by template wetting[26] P. Chang, Z. Fan, W. Tseng, D. Wang, W. Chiou, J. Hong,process”, Physica E, Proceedings of the E-MRS 2006

J. G. Lu, “ZnO Nanowires Synthesized by Vapor Trapping

Symposium E: Science and Technology of Nanotubes and

CVD Method”, Chem. Mater., 16 [24] (2004), 5133-5137.

Nanowires, 37 [1-2] (2007), 241-244.

[27] B. P. Zhang, N. T. Binh, K. Wakatsuki, Y. Segawa, Y.

[38] Z. Wang, H. L. Li, “Highly ordered zinc oxide nanotubules

Kashiwaba, and K. Haga, “Synthesis and optical property

synthesized within the anodic aluminum oxide template”,

of single crystal ZnO nanorods”, Nanotechnology, 15 (2004),
382-388.

Appl. Phys. A, 74 (2002), 201-203.
[39] Nanai L., George T.F., “Laser-assisted formation of metallic[28] H. J. Fan, F. Bertram, A. Dadgar, J. Christen, A. Krost, andoxide microtubes”, J. Mater. Res., 12 [1] 1997, 283-284.

M. Zacharias, “Self-assembly of ZnO nanowires and the[40] L. Lin, Y. Han, M. Fuji, T. Endo, X. Wang and M. Takahashi,

spatial resolved characterization of their luminescence”,“Synthesis of Hexagonal ZnO Microtubes by a Simple Soft

Nanotechnology, 15 (2004), 1401-1404.

Aqueous Solution Method”, J.Ceram. Soc. JPN, 116[2][29] Y. K. Tseng, C. T. Chia, C. Y. Tsay, L. J. Lin, H. M. Cheng,(2008), 198-200.

C. Y. Kwo, and I. C. Chen, “Growth of epitaxial needle-like[41] Y. Han, L. Lin, M. Fuji, and M. Takahashi, “A Novel One-ZnO nanowires on GaN films”, J. Electrochem. Soc., 152

step Solution Approach to Synthesize Tubular ZnO(2005), 95-98.

Nanostructures”, Chem. Lett., 36[8] (2007), 1002-1003.

[30] H. Chik, J. Liang, S. G. Cloutier, N. Kouklin, J. M. Xu,[42] L.Lin, H. Watanabe, M. Fuji and M. Takahashi,“Periodic array of uniform ZnO nanorods by second-order“Morphological Control of ZnO particles synthesized via

self-assembly”, Appl. Phys. Lett., 84 (2004), 3376-3378.

a New and Facile Aqueous Solution Route”, Adv. Powder[31] R. M. Wang, Y. J. Xing, J. Xu and D. P. Yu, “Fabricationand microstructure analysis on zinc oxide nanotubes”, New

Tech. (In press)[43] R.A. Laudise and A.A. Ballman, “Hydrothermal Synthesis

J. Phys., 5 (2003), 115.1-115.7.

of Zinc Oxide and Zinc Sulfide”, J. Phys. Chem., 64 (1960)[32] X.L. Yuana, B.P. Zhangb, J. Niitsumaa, T. Sekiguchia,

688-691.

“Cathodoluminescence characterization of ZnO nanotubes[44] B.Meyer and D. Marx, “Density-functional Study of thegrown by MOCVD on sallire substrate”, Mat. Sci. Semicon.

Structure and Stability of ZnO Surface”, Phys. Rev. B, 67

Proc., 9 (2006), 146-150.

(2003) 035403.

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