Nanotubular structures of zinc oxide

Y.J. Xinga,b, Z.H. Xia, X.D. Zhanga, J.H. Songa, R.M. Wangb, J. Xub, Z.Q. Xuea,D.P. Yub,*

aDepartment of Electronics, Peking University, Beijing 100871, People’s Republic of ChinabSchool of Physics, Electron Microscopy Laboratory, and State Key Laboratory for Mesoscopic Physics, Peking University, Beijing 100871,

People’s Republic of China

Received 10 July 2003; received in revised form 7 September 2003; accepted 10 November 2003 by Z.Z. Gan

Abstract

ZnO nanotubes with a regular polyhedral shape, hollow core, and wall thickness as small as 4 nm, have been prepared in

large-area substrate by vapor phase growth. The nanotubes can be classified into two groups consisting of either polycrystalline

or straight single crystal. The formation of the ZnO nanotubes was found closely related to the hexagonal structure of the ZnO

crystal and the peculiar growth conditions used.

q 2003 Elsevier Ltd. All rights reserved.

PACS: 61.46. þ w; 81.16. 2 c; 81.07.De; 81.05.Hd

Keywords: A. Nanostructures; A. Semiconductors; C. Scanning and transmission electron microscopy

1. Introduction

Nano-sized tubular structures have stimulated intensive

research interests [1–6], because these materials have

enabled studies on fundamental physical properties [7,8],

and served as building blocks to construct nanoscale devices

[9,10]. However, nanotubular structures have only been

grown for few materials, which have bulk lamellar

structures, such as graphite or graphite-like structures

(BN, BCN, WS2, MoS2). Little is known for direct growth

of nanotubular structures from 3-dimensional materials to

date [11], though complex lithography was used to engineer

Ge–Si thin films into nanotubes [12]. The nanophased ZnO

compound was widely studied since this material exhibits a

number of interesting fundamental properties and possible

applications, including ZnO films [13], disordered nano-

particles [14], nanobelts [15], and nanowires [16] in which

very intense stimulated UV lasing action was observed at

room temperature. Recently, the tubular structures of ZnO

with the diameters of 30/350 nm were observed in the

heterostructures of Zn core/ZnO sheath nanocables and in

Zn(NH3)42þprecursor solutions [17,18]. However, these

products have a low yield of the production and poor

morphology and crystal quality. We demonstrate, in this

communication, crystalline ZnO nanotubes with a regular

polyhedral shape, hollow core, and wall thickness of 4–

20 nm, were prepared in large-area substrate by vapor phase

growth.

2. Experimental

The ZnO nanotubes were grown using thermal evapor-

ation of Zn/ZnO powder mixture. Powders of zinc oxide

(1.0 g) and zinc (0.3 g) were well mixed and put into the

central zone of alumina tube in a tube furnace. The whole

system was evacuated to about 300 Torr, and filled with

Argon. The furnace was heated to about 1300 8C under

flowing Ar atmosphere (40 sccm) for 1 h. Si substrates

(5 £ 20 mm) were placed downstream inside the tube

(temperature 700 8C) in sequence to collect the products.

It is noted that proper amount of water was held in a glass

vessel upstream the alumina tube to keep a wet oxidation

0038-1098/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ssc.2003.11.049

Solid State Communications 129 (2004) 671–675

www.elsevier.com/locate/ssc

* Corresponding author. Tel./fax: þ8610-62759474.

E-mail address: yudp@pku.edu.cn (D.P. Yu).

ambient during the growth of the ZnO nanotubes. The

morphology of the as-grown product was analyzed using a

Dual Beam-235 focused ion beam (FIB) system. A Tecnai

F30 transmission electron microscope (TEM) equipped with

nano-beam energy dispersive spectroscopy (EDS) was used

to characterize the microstructure and chemical composition

of the nanowires.

3. Results and discussion

X-ray diffraction analysis of the nanotubes revealed a

hexagonal wurtzite ZnO phase. SEM analysis revealed that

the whole Si substrate (5 £ 20 mm) was covered with

nanoscale tubes. The amount of the tubular structures is

estimated more than 90 wt%, as shown in Fig. 1A. The ZnO

nanotubes have a diameter distribution between 30 and

100 nm, with a mean value of ,60 nm. Their length can be

over a few tens micrometers. Detailed SEM analysis shown

in Fig. 1B revealed that the ZnO nanotubes have distinctive

hollow cores, which appear apparently transparent to other

underlying nanotubes. An extremely surprising feature is

that many ZnO nanotubes have well defined polyhedral

shapes. The nanotube marked with ‘1’ has one open end

from which its hollow core is visible, and possesses a

rectangular shape in cross sectional view (further magnified

in the left inset, and marked with arrow). Another nanotube

marked with ‘2’ shows a very beautiful hexagonal shape, as

shown in the right inset. It is well known that the

macroscopic morphology usually reflects the microscopic

nature of a faceted crystal. Accordingly, the hexagon

faceted morphology of the nanotube ‘2’ provides strong

evidence that those nanotubes grow along the [001]

direction (the c-axis). The very bright contrast at the tip of

the nanotube (marked with arrow head) evidenced the

existence of zinc nanoparticle with the sheath of ZnO at the

end of the nanotube. A collection of different nanotubes

with broken ends (Fig. 1C–E) demonstrates that the

nanotubes can have diverse morphologies from a regular

hexagonal shape (C, E), a square shape (D), to an irregular

shape (E).

TEM studies provide further insight into the microstruc-

ture of ZnO nanotubes. As revealed in the TEM investi-

gation in Fig. 2, all ZnO nanotubes manifest their hollow

core in contrast. The side walls (parallel to the incident

electron beam) of the ZnO nanotubes appear much dark in

the TEM contrast due to the relative larger thickness than

the other part (perpendicular to the electron beam) of the

nanotubes, which is independent of the focus degree. Based

on the difference in morphology and microstructure, the

ZnO nanotubes can be classified into two groups. As

revealed from the TEM image in Fig. 2A, the first group

appears ‘regular’, and usually has a straight morphology,

smooth surface, and a homogenous wall contrast. The inset

corresponds to single pattern of electron diffraction along

the [011] zone axis, revealing that the regular straight

nanotubes are of a single crystal.

Detailed TEM analysis also revealed that many ZnO

nanotubes have longitudinal axis parallel to the [001] zone

axis. By setting the focus point near to the Gaussian value,

both the top and bottom walls are visible at the broken end

of a single nanotube (indicated by two white arrows in the

HREM image in Fig. 2B). The lattice fringes at the side wall

(dark contrasted) corresponding to the (100) planes are well

resolved (magnified and shown in inset), revealing that the

walls of the ZnO nanotubes are the same 3-D solid as the

bulk, which is distinct to the multi-walled graphite-like

nanotubes (C, BN, BCN, and WS2), in which there exist

spaces between wall sheets with interlayer van der Waals

Fig. 1. (A) SEM image revealing abundance of hollow ZnO nanotubes on Si substrate. (B) Magnified SEM image of the ZnO nanotubes. The

broken end of the tube ‘1’ reveals the hollow core with a rectangular shape (left inset). The tube ‘2’ has a well-defined hexagonal shape (right

inset), and appears transparent to other underlying nanotubes. The very bright-contrast at the tip of the nanotube (marked with arrow head)

shows the existence of zinc-rich nanoparticle at the ends of the nanotubes. (C–E) A collection of the SEM images revealing the open tips of the

ZnO nanotubes, showing that many of the ZnO nanotubes have a regular polyhedral shape.

Y.J. Xing et al. / Solid State Communications 129 (2004) 671–675672

interaction. The EDS spectrum on a single nanotube shown

in Fig. 2C revealed that chemical compositions of the ZnO

nanotubes are zinc and oxygen with Zn/O , 1. It is worth

noting that the single crystalline nanotubes have a very fine

wall thickness as small as 4 nm. Such a wall thickness is

unique for the ZnO nanotubes compared to ZnO nanowires,

and makes them a quantum-confined structure. This unique

feature is important and enables to evaluate the dimension-

ality-related properties such as electron transport, optical

behavior, and to explore novel nanodevices in which the

quantum mechanics dominates.

The second group of ZnO nanotubes usually has an

irregular morphology. Compared to the ‘regular’ single

crystalline ZnO nanotubes, this kind of nanotubes is

polycrystalline, and has a thicker wall (.20 nm), as well

as inhomogeneous wall thickness, as shown in Fig. 2D. The

variable bright/dark contrast of the grains on the nanotube

walls revealed that the nanotubes is polycrystalline. The

textured nanotubes can have very curved morphology (Fig.

2E).

In the detailed TEM observations, we found that some

ZnO nanotubes had solid ends. Fig. 3 showed the TEM

image of the heterostructure composed of a ZnO nanotube

and a solid nanorod, in which the distinct contrast of the left

and right part of the heterostructure clearly revealed the

solid and hollow structure. It can be seen that the wall of

nanotube was grown directly from the outer layer of the

nanorod. The chemical composition of the nanotube and

nanorod of the heterostructure was characterized by EDS

analysis. The EDS spectrum shown in inset was taken from

the nanorod, in which Zn, O, C, Cu and Si elements were

marked. The relative intensive Zn peak compared with that

of the tubular part reveals that the solid end of nanotube (left

part) is actually a Zn rich ZnO nanorod. The EDS analysis

from the nanotubule part revealed that the composition of

the nanotube was ZnO.

Fig. 2. (A) HREM image of a single ZnO nanotube with wall thickness around 8 nm. The electron diffraction in inset reveals a single crystalline

nature of the regular ZnO nanotubes. (B) Magnified HREM image showing the details of the open end of a single ZnO nanotube. The top and

bottom walls of the nanotube are well resolved and marked with two arrows. The (100) lattice plane image of the wall is shown in inset. (C) EDS

analysis showing that the nanotubes consist of zinc and oxygen. (D) Low magnified TEM image of the irregular nanotube morphology. (E) TEM

image of highly curved ZnO nanotubes.

Y.J. Xing et al. / Solid State Communications 129 (2004) 671–675 673

The formation of the ZnO nanotubular structures is

extremely interesting, however, the growth mechanism is

yet unclear. Our growth conditions distinguished others

methods of ZnO nanowire/nanobelt growth at least in two

aspects [15,16]: the mixing of metallic zinc with zinc oxide,

and a weak wet-oxidation atmosphere. The growth of the

ZnO nanotubes can be understood on basis of the peculiar

growth conditions and the particular microstructure of the

ZnO crystal. Two important factors are responsible for the

growth of the ZnO nanotubes: the formation of the nanotube

embryos at proper nucleation sites, and unidirectional

growth of the nanotube embryos. Based on our experimental

evidences, we proposed the following model to depict the

growth of the peculiar ZnO nanotubes. The metallic zinc has

a very low melting point (419 8C) compared to zinc oxide, a

oxygen deficient vapor phase was evaporated from the

source materials at a higher temperature, forming nano-

sized metastable zinc-rich rod-like oxide (ZnOx, x , 1)

structures at the nucleation sites on the substrate. Those

metastable rods are zinc-rich, and they can serve as the

embryos (nuclei) of the nanotube growth. Once formed, the

metastable nuclei rod will absorb continuously the incoming

zinc oxide species to form a stable outer shell sheathing it.

The wet-oxidation ambient used here provides a steady

zinc-rich oxide source to support the continuous growth of

the nanotubular embryos. On the other hand, the metastable

zinc-rich embryos will decompose at 1300 8C that makes the

outer shell stable at the expense of embryos itself. The stable

outer shells will growth continuously to form ZnO

nanotubules, while the nuclei themselves will vanish.

Because k001l direction is fastest growth face of ZnO

crystal, the ZnO crystals at the nanotube walls tend to grow

along the c-axis and the (100) facet is mostly exposed [19].

The whole growth process is depicted schematically in Fig.

4. Above arguments were first confirmed by observation of

remaining zinc-rich particles wrapped inside the ZnO tubes

both in SEM and TEM analysis. Besides, a similar

metastable phase was proposed in formation of micro-

sized ZnO nanotubes [19]. It is noted that the weak wet-

oxidation conditions plays a central role in the formation of

the ZnO nanotubes embryos. In fact ZnO nanowires instead

of tubular nanostructures were observed if no water was

introduced into the growth furnace.

4. Conclusions

Nanotubular structures of zinc oxide were prepared by

physical evaporation of ZnO/Zn powder mixtures under a

weak wet oxidation atmosphere. The ZnO nanotubes had a

Fig. 3. TEM image of a short ZnO nanotube with a solid end, the white hexagon outlines the hexagonal shape of the nanotube tip. The inset is the

EDS spectrum of solid end.

Fig. 4. Schematic depiction of the formation of the ZnO nanotube:

(A) Formation of a metastable Zn-rich embryo; (B) A stable ZnO

sheathing layer was formed on the surface of the embryo; (C) The

ZnO nanotubes start to grown from the sheath layer while the

metastable embryo was decomposed and vanished.

Y.J. Xing et al. / Solid State Communications 129 (2004) 671–675674

regular polyhedral shape and a distinct hollow core. The

nanotubes can be classified into two groups consisting of

either polycrystalline or straight single crystal. The

formation of the ZnO nanotubes was found closely related

to the hexagonal structure of the ZnO crystal and the

peculiar growth conditions used.

Acknowledgements

This project was financially supported by the national

Natural Science Foundation of China (Grant No. 60071015,

60261010, 50025206, 19834080), and the Research Fund

for the Doctoral Program of Higher Education (RFDP),

China.

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