# Growth of ZnO Nanowires in Aqueous Solution by a Dissolution-Growth Mechanism

Authors: Shaojing Bu; Chunxiang Cui; Qingzhou Wang; Ling Bai
Journal: Journal of Nanomaterials
 (2008-01-01)

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## Abstract

A novel methodology based on the dissolution-growth mechanism was developed to prepare ZnO nanowires films. The film morphology and structure were investigated by using field emission scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction analysis methods. The results show that the ZnO nanowires are single crystallinec-oriented wurtzite. The ZnO rod crystals were eroded to provide the growth primitive of ZnO nanowires, which formed on top of the rod crystals when the erosion reaction got the equilibrium. The length of the resultant nanowires is rather large because the successive erosion of the rod crystals maintains the low concentration of Zn2O(OH)2n−2 in the aqueous solution.


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## Body

## 1. Introduction

One-dimensional zinc oxide represents an important basic material due to
its low-cost, large-band [1], and luminescent properties and has wide
applications in photocatalyst [2], gas sensor [3], varistor [4], transparent
conductive coating [5], and solar cells [6]. The
fabrication of one-dimensional zinc
oxide has been widely studied by different methods. For example, vapor-liquid-solid
epitaxial (VLSE) mechanism [7], thermal evaporation [8], hydrothermal methods [9],
template-based growth [10], chemical vapor deposition [11], and pulsed laser
deposition [12] have been successful in
creating 2D-oriented arrays of ZnO rods. Among these methods, solution process
has been demonstrated to be a facile
method for synthesizing ZnO due to its own advantages such as simplicity,
reproducibility, non-hazardousness, cost
effectiveness, being suitable for producing large-area thin films. Various morphologies
have been achieved by aqueous solution method, for
example, rod-like [13], tube-like, flower-like crystals [14], and so on.But up
to now, growth of nanowire with large length has not been achieved by aqueous
solution method. The diameter of ZnO nanocrystals is dependent on the
concentration of the solution [13, 15]; nanorods can be obtained when the concentration
is relatively low, but nanowires cannot because the concentration of the
solution will decrease greatly with the crystal growth. So it is essential to maintain
the concentration of the solution during the whole growth process to get
nanowires with larger length. In this paper, a novel method was
developed to obtain large-scale ZnO nanowires based on the basic
dissolution-growth mechanism. The successive erosion of rod crystals was used
to maintain the zinc concentration in the solution.


## 2. Experimental

ZnO
rod films were fabricated in aqueous solution using the method in [13, 16]. To
prepare ZnO nanowires, 1 M NH3·H2O aqueous solution was added in a 25 mL beaker, and
stirred for several minutes. Then ZnO rod films (2×1cm) were inserted in the
solution and the beaker was sealed. After the beakers were kept in an oven at
80°C for 2‐6 hours, the
films were removed from the solution and dried under vacuum at 80°C. The pH value of the initial solution was 11.5, and it
decreased slightly to 11.2 after the erosion for 6 hours.The ZnO nanowires were
characterized by using field emission scanning electron microscopy (FESEM, JEOL 6700, 200 kV), high-resolution
transmission electron microscopy (HRTEM, PHILIPS TECNAI G2 F20, 300 kV). X-ray diffraction was recorded by Rigaku D/max-2500 X-ray diffractometer (30 kV, 20 mA) with copper targets(λ=0.154059nm).


## 3. Results and Discussion

Morphology of ZnO films obtained in ammonia
solutions with different reaction times is shown in Figure1. Figure 1(a) illustrates the FESEM image of the as-grown ZnO rod films, from which it
can be seen that the substrate was covered with hexagonal rod crystals with the
diameter of 0.2‐0.6 μm. The morphology
evolution during the erosion process is shown in Figures 1(b), 1(c), and 1(d).
As depicted in Figure 1(b), after 2 hours of reaction, hexagonal pits with the
wall thickness of ∼30 nm
formed on top of the rod crystals. This process is similar to the rod crystals growth,
which we have found previously [15], namely, both erosion and growth begin from the center at (0001) plane of the rod crystals.
As the period of the process was prolonged to 4 hours, nanowires appeared on
surface of the films, of which the diameter was about 25 ~ 40 nm (as seen in Figure
1(c)). The nanowires became longer and thicker when the reaction time was
increased to 6 hours (Figure 1(d)).FESEM photographs of ZnO films eroded for different periods: (a) 0 hour, (b)
2 hours, (c) 4 hours, and (d) 6 hours.
(a)(b)(c)(d)Figure2 shows the XRD pattern of the ZnO film illustrated in
Figure 1(d). The high (002) peak is indexed as the wurtzite structure of ZnO,
indicating that the nanowires are highly c-oriented. An
energy dispersive spectroscopic (EDS) analysis of the film (Figure 3) shows
that the products are mainly composed of Zn and O elements, consistent with the
result of XRD.Figure 2
XRD pattern of ZnO film obtained after erosion for 6 hours in the ammonia
solution.Figure 3
EDS picture of the ZnO film
obtained after erosion for 6 hours.Figure4 shows the HRTEM image of a part of a typical ZnO
nanowire. The diameter and length of the nanowire are 30 nm and more than 5μm,
respectively. So the aspect ratio of the nanowire is more than 160, which is
the typical characteristic of nanowire.Figure 4
HRTEM photograph of a typical ZnO nanowire.Figure5 shows HRTEM and TEM (inset) images of a ZnO nanowire, demonstrating the
single-crystalline structure of the nanowire. The lattice  spacing of 0.28 nm
shown in the HRTEM image of Figure 5 corresponds to a d-spacing of  (1¯00)
crystal planes, confirming that the nanowire is c-oriented.Figure 5
TEM (inset) and HRTEM
photograph of a ZnO nanowire.The growth of ZnO nanowire can be described based on the chemical
equilibrium of dissolution-regrowth
in the solution. Yamabi and Imai [17] and Peterson and Gregg [18] found that
ZnO(s) only formed at small region in the phase diagram, thus suitable pH value
and Zn species concentration is necessarily required. In aqueous
solutions at pH > 9, Zn (II) soluble species in the form of hydroxyl complexes
such as Zn(OH)2(aq) and Zn(OH)42− increase [16], the
thermodynamic equilibrium for ZnO(s)‐H2O system can be represented by
(1)ZnO(s)+(n−1)H2O⟷Zn(OH)n(aq)2−n+(n−2)H+.As is
well known, supersaturation is required for crystal growth in solution. In
our system, the erosion should stop when the process achieves the equilibrium
according to (1). However,
the growth of nanowire indicates that the solution is supersaturated, which
suggests that the erosion and growth proceed simultaneously when the reaction get
the equilibrium.WhenZnO(s)  is
introduced into the basic solution, the equilibrium in (1) moves to the right
because of the low Zn(OH)n(aq)2−n concentration and H+ consumption. Thus the ZnO(s)   will be eroded. As the erosion time increases, the concentration of  Zn(OH)n(aq)2−n increases and
approaches to the critical supersaturation. Peterson and Gregg [18] and Yamabi
and Imai [19] have found that Zn(OH)n(aq)2−n can form polyhydroxyl zinc complex, which was
represented by
(2)[Zn(OH)n]2−n+[Zn(OH)n]2−n→[Zn2O(OH)2n−2]4−2n+H2O,where n=2 or 4, Zn2O(OH)2n−2   supplies the source
of the heterogeneous nucleation and growth of ZnO nanowires. The concentration of Zn(OH)n(aq)2−n and  Zn2O(OH)2n−2
increases
with the erosion of the rod crystals and then the reaction achieves the
equilibrium. The erosion process  continues, then new ZnO nuclei form on top of
the ZnO rod because the solution is supersaturated. In previous work, we have found
that the diameter of ZnO rods, which were fabricated by aqueous solution method
decreased with the reduction of the zinc precursor concentration, due to the
shift of chemical equilibrium in the solution [15]. As a result of the low zinc
concentration in the solution, the diameter of the crystals obtained in this
work is in nanoscale [13]. The nanocrystals transform to nanowires with the
increase of growth time  for the successive erosion of ZnO rod, which maintains
the rather low concentration of Zn2O(OH)2n−2. Figure 6 gives the sketch of the growth mechanism.Figure 6
Nucleation and growth of the ZnO nanowire by dissolution-growth mechanism.In
addition, the growth manner of the nanowires can be analyzed by the aid of TEM
and HRTEM characterization. Three typical fractions of the ZnO nanowire were
observed in the film eroded for 6 hours (Figure7(a)). For the first fraction (as
shown in Figure 7(b)), nanowire was composed of nanoparticles with the
diameter of 5∼8 nm, between which there are many mesopores. No crystalline
fringe can be detected in these nanoparticles, suggesting the amorphous nature
of the nanowire. As for the
second fraction (Figure 7(c)), it can be seen that almost all of the nanoparticles
are crystalline grains, and many of them begin to attach with each other along c-axis. The densification of the
nanowire can be inferred according to disappearance of the pores. But the
interfaces among the nanoparticle packing are still visible. A similar oriented
attachment has also been found by Searson et al. [20] when preparing TiO2 nanoparticles under hydrothermal condition. The driving force for   the disorder-order
assembly process is the reduction of surface energy, and this mechanism has
been reported previously [21, 22]. The third kind of nanowire fraction is very
smooth and compact (Figure 7(d) (inset)). Its HRTEM image shows that a large
quantity of small grain domains has transformed into a single crystalline
nanowire. According to these observed results, the proposed growth mechanism of
ZnO nanowire can be illustrated in Figure 8. Based on the growth period, there
are three different fractions in one nanowire, corresponding to three growing
stages of the nanowire. Step (a) is nanoparticle packing
process. Step (b) is the process of crystallization and oriented attachment. In
Step (c), the single crystalline nanowire forms.(a)
A typical TEM image of ZnO
nanowire fractions, three kinds of fractions with different crystallinity were
shown. (b) A typical HRTEM image of a
ZnO nanowire fraction in the early growth stage and its TEM image (inset). (c) A typical HRTEM image of a
ZnO nanowire fraction in the medium growth stage and its TEM image (inset), the
arrowhead indicates the oriented attachment. (d) The HRTEM image of a typical
crystallized ZnO nanowire fraction and its TEM image (inset).
(a)(b)(c)(d)Figure 8
Schematic disorder-order growth
process of ZnO nanowire: (a) nanoparticle packing, (b) crystallization and oriented attachment
of small grain domains, and (c) formation of single crystalline ZnO nanowire.


## 4. Conclusions

ZnO
nanowires were synthesized in aqueous solution by a dissolution-growth
mechanism. The nanowires are single crystallinec-oriented  wurtzite. The ZnO rod crystals were eroded to provide
the growth primitive of ZnO nanowire. The nanowire formed on top of the rod
crystals when the erosion reaction achieved the equilibrium. The growth manner
of nanowires has been discussed based on the TEM and HRTEM results. The length
of the nanowire is rather large because the successive erosion of the rod
crystals maintains the low concentration of Zn2O(OH)2n−2.As we
all know, ZnO nanowires have great application potential in the photoelectric field, such as luminescence,
window and electrode material for solar cells, phosphors, piezoelectric
transducers and actuators, surface acoustic coatings, varistors, and sensors. This
strategy can provide a novel and simple route to obtain ZnO nanowires, even ultralong ZnO nanowires which
may improve the properties of nanowire-based devices.



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*Source: 610541-2008-03-16.xml*