ZnO submicron structures of controlled morphology
synthesized in zinc-hexamethylenetetramine-ethylenediamine
aqueous system
Xiang-Dong Gaoa) and Xiao-Min Li
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai
Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China
Sam Zhangb)
School of Mechanical and Aerospace Engineering, Nanyang Technological University,
Singapore 639798
Wei-Dong Yu and Ji-Jun Qiu
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai
Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China
(Received 31 October 2006; accepted 1 February 2007)
The morphology of ZnO submicron crystals formed in a weak alkaline environment
(pH value less than 11.0) was systematically studied for the first time. ZnO submicron
particles with different morphologies (flowers, rod, and wire) were synthesized from
an aqueous solution by adopting ethylenediamine as the source of hydroxyl group,
hexamethylenetetramine (HMT) as the additive, and potassium chloride (KCl) as the
background electrolyte. The effects of primary experimental parameters such as HMT
and KCl addition, precursor concentration, and reaction temperature on the
microstructure, crystallinity of the resultant particles, and their distribution on substrate
are discussed in this paper. In the flowerlike structure, the particle size is more
controlled by the precursor concentration, and the microstructure is modulated by
increasing the concentration of HMT and the reaction temperature. The introduction
of ZnO seed layer on substrate promotes even distribution of ZnO flowers. High
concentration KCl electrolyte inhibits formation of the flowerlike structure and
promotes the growth of submicron ZnO crystals in rod or wire shape. Mechanism
studies indicate that the degree of supersaturation of Zn(OH)2 and the adsorption of
organic/inorganic species on the surface of ZnO are the prime factors influencing the
nucleation, growth rate, and eventual morphology.
I. INTRODUCTION
ZnO has been one of the most intensively investigated
functional oxides in recent years due to its versatile
properties such as stability at high temperature, good
biological compatibility, near-ultraviolet (UV) emission,1 transparent conductivity,2 piezoelectricity,3 and
high gas sensitivity.4 Because of the successful demonstration of room-temperature UV lasing by Yang’s
group5 and piezoelectric nanogenerators by Wang et al.6
a)
Address all correspondence to this author.
e-mail: [email protected]
b)
This author was an editor of this journal during the review and
decision stage. For the JMR policy on review and publication of
manuscripts authored by editors, please refer to http://www.mrs.
org/jmr_policy
DOI: 10.1557/JMR.2007.0250
J. Mater. Res., Vol. 22, No. 7, Jul 2007
recently based on ZnO nanorod arrays, morphology control of ZnO nano- and submicron structure has received
much attention in the fundamental study of structure–
property relations as well as technological potential in
catalysts, selective separations, sensor arrays, wave-guides,
drug carriers, biomedical implants with macroporosity,
and photonic crystals. Up to now, ZnO nanostructures
with an abundant variety of shapes have been demonstrated, such as nanorods or nanowires, nanoneedles,7
nanocables and nanotubes,8 nanowalls,9 nanobridges and
nanonails,10 nanosprings and nanorings,11,12 and tube-,
tower-, and flowerlike structures.13,14
Solution-based synthesis of ZnO nanostructures is an
attractive approach compared to physical vapor techniques because of its simplicity, low cost, good reproducibility, suitability for large-scale production, and
most importantly, the versatile possibility to tune the size
and morphology of ZnO by using organic functional
© 2007 Materials Research Society
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X-D. Gao et al.: ZnO submicron structures of controlled morphology synthesized in weak alkaline aqueous system
groups.15–17 Especially of interest is that various shapes
of ZnO have been fabricated from aqueous solutions.
These shapes include nanowire array,18 oriented helical
structures,19 and tube-, tower-, teardrop-, and flowerlike
structures.13,14,20–23 However, due to the complexity of
chemical reactions in solution and the functional diversity of organic additives, the precise control of ZnO
nanostructures with complex morphology is still far from
reality.
It is known that pH value is the determinative
factor influencing the structure of ZnO obtained in
aqueous solution. While the synthesis of ZnO in a faintly
acidic starting environment (pH of 5–6) and strong
basic environment (pH > 13) has been widely investigated,18–22 reports on ZnO structures formed from
a weak alkaline starting environment (pH < 11) are
scarce. 24 The weak alkaline environment provided
by the hydrolysis of ammonia or organic amine is expected to have a different mechanism from both faint
acidic and strong alkaline precursors, thus providing
more possibility to tune the morphology of ZnO particles.
Herein, we have adopted ethylenediamine (EN) as the
source of a hydroxyl group and the complexing reagent
of zinc ions at the same time, and we systematically
investigated the effects of typical experimental parameters, hexamethylenetetramine (HMT), a highly watersoluble tetradentate cyclic tertiary amine, and the background electrolyte of KCl. The morphology, crystallinity,
and particle distribution on the substrate of the resultant
ZnO structures were studied to explore the underlying
principle of the morphological evolution of ZnO particles
in the weak alkaline environment, thus opening up a new
dimension in morphology control of ZnO deposition
from aqueous solution.
II. EXPERIMENTAL
A. Substrate preparation
Three types of substrate were selected to collect ZnO
particles, i.e., a silicon wafer with (111) orientation, an
ordinary glass slide, and a ZnO seed layer coated on
glass. Prior to use, the Si (111) wafer was rinsed by dilute
hydrofluoric acid (1:1 by volume) to remove the natural
oxide layer on the surface first, and then it was ultrasonically rinsed in ethanol, acetone, and deionized water in
succession. For the slide glass substrate, an additional
boiling procedure in dilute sulfuric acid (1:10 by volume)
for 30 min was performed before the rinsing process
in ethanol, acetone, and deionized water. For the
ZnO-coated glass substrate, the ZnO seed layer was prepared by the ultrasonic irradiation-assisted successive
ionic layer adsorption and reaction (SILAR) method with
5 deposition cycles; details can be found elsewhere.25
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B. Chemicals
All of the chemicals were analytic grade reagents purchased from China National Medicine Group (Shanghai,
China); they were used without further purification. The
aqueous precursors were prepared by mixing a suitable
amount of zinc sulfate, EN, HMT, or KCl in deionized
water. The precursor was filled into a laboratory Pyrex
glass bottle with polypropylene autoclavable screw caps
with a filling ratio of 80%. The substrate was placed at
the bottom of the bottle to collect the deposits. The prepared precursor was heated from room temperature and
maintained at a constant temperature of 95 °C for 1 h in
a regular laboratory oven. Subsequently, the obtained
white layer was thoroughly washed with water to eliminate the residual of salts and organic substance and dried
in air for subsequent measurements.
C. Parametric studies
To investigate the effects of typical experimental parameters (Zn2+ concentration, reaction temperature),
HMT, KCl, and ZnO seed layer on the morphology of
ZnO, five series of samples were performed. The experimental details are listed in Table I. In brief, three [Zn2+]
(0.1, 0.001, 0.0002 mol/l), two temperature (95 and
105 °C), two HMT-Zn molar ratios (1:1 and 10:1), two
concentrations of KCl (0 and 1 mol/L), and three types of
substrate were selected. For all samples, the molar ratio
of Zn2+ to EN was maintained at 1:3.
D. Particle characterization
The crystalline and morphological characterization of
as-deposited sample was examined by D/max 2550V diffractometer (Rigaku Ltd., Tokyo, Japan) and JSM 6700F
scanning electron microscope (SEM) (JEOL, Tokyo, Japan), respectively. For SEM characterization, a thin Pt
film was evaporated to enhance the conductivity of surface.
III. RESULTS AND DISCUSSION
A. Effect of HMT
Figures 1 and 2 illustrate the morphology of the ZnO
layer obtained in the precursor without and with the addition of HMT (i.e., samples 1 and 2), respectively.
Clearly, the simple Zn-EN system gives rise to both ZnO
nanorods (∼200 nm in diameter) and flowers. While the
nanorods possess the obvious hexagonal shape as shown
in Fig. 1(c), the petals of ZnO flowers exhibit a cone
shape, possessing a thick bottom and a sharp tip
[Fig. 1(b)]. The overall size of a flower ranges from 1 to
2 ␮m. When HMT is added in the precursor, as shown in
Fig. 2(a), mostly ZnO flowers are deposited, and very
few ZnO nanorods are observed. In addition, the flower
petals also exhibit different structures. (i) The surface of
J. Mater. Res., Vol. 22, No. 7, Jul 2007
X-D. Gao et al.: ZnO submicron structures of controlled morphology synthesized in weak alkaline aqueous system
TABLE I. Sample numbers, reaction conditions, and obtained morphologies.
Sample
number
Reaction media
and molar ratio
[Zn2+]
(mol/L)
Reaction
temperature (°C)
pH
value
Substrate
1
2
3
Zn:EN ⳱ 1:3, without HMT
Zn:EN:HMT ⳱ 1:3:1
Zn:EN:HMT ⳱ 1:3:10
0.001
0.001
0.001
95
95
95
10.0
10.0
10.0
Si(111)
Si(111)
Si(111)
4
5
6
Zn:EN:HMT ⳱ 1:3:1
Zn:EN:HMT ⳱ 1:3:1
Zn:EN:HMT ⳱ 1:3:1
0.0002
0.01
0.001
95
95
105
10.0
10.0
10.0
Si(111)
Si(111)
Si(111)
7
Zn:EN:HMT ⳱ 1:3:1
0.001
95
10.0
8
Zn:EN:HMT ⳱ 1:3:1,
KCl ⳱ 1 mol/L
Zn:EN:HMT ⳱ 1:3:1,
KCl ⳱ 1 mol/L
Zn:EN:HMT ⳱ 1:3:1,
KCl ⳱ 1 mol/L
0.1
95
0.01
0.001
9
10
Morphology
Studied parameter
Control sample
10.9
ZnO seed
layer
Glass
Flower + rod
Flower
Flower, good
crystallinity
Flower, low yield
Large size flower
Flower, good
crystallinity
Flower, even
distribution
Rod, with sharp tip
95
10.2
Glass
Rod, with round tip
KCl buffer
95
10.0
Glass
Spindle and cobweb
HMT concentration
Zn2+ concentration
Temperature
Substrate
FIG. 1. Field-emission SEM (FE-SEM) images of ZnO layer deposited from Zn-EN system (sample 1) with the molar ratio of Zn2+ to EN of 1:3:
(a) overall morphology at low resolution (3000×), (b) typical ZnO flower, and (c) typical short ZnO nanorods.
FIG. 2. FE-SEM images of ZnO layer deposited from the Zn-EN-HMT system (sample 2), with Zn2+-EN-HMT molar ratio of 1:3:1: (a) cluster
of ZnO flowers and their distribution on Si(111) substrate and (b) a typical ZnO flower.
the petal exhibits a granular morphology other than
smooth surface, indicating that the petal is made up of
numerous well-aligned nanorods.23 (ii) The interface between neighboring petals can be clearly detected for almost all the investigated flowers in SEM analysis, as
indicated by the arrow in Fig. 2(b).
Figure 3 shows SEM images of ZnO flowers in sample
3, in which the content of HMT is ten times higher than
that in sample 2. Though the overall distribution of ZnO
flowers is similar to sample 2 (not shown here), the
flower structure is quite different. (i) Obviously there are
more petals for each flower. (ii) The petal is rodlike with
J. Mater. Res., Vol. 22, No. 7, Jul 2007
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X-D. Gao et al.: ZnO submicron structures of controlled morphology synthesized in weak alkaline aqueous system
FIG. 3. FE-SEM images of ZnO layer deposited from the Zn-EN-HMT system (sample 3), with high concentration of HMT (Zn2+-EN-HMT ratio
of 1:3:10): (a) several closely accumulated ZnO flowers and (b) enlarged view of a typical flower.
a sharp tip rather than the cone shape. Many petals exhibit obvious hexagonal features [Fig. 3(b)]. (iii) The
diameter of each petal (∼100–300 nm) is much smaller
than that in sample 2 (300–600 nm), though the overall
size of each flower does not change.
To further explore the cause of morphological change
in the Zn-EN-HMT system, we carried out x-ray diffraction (XRD) analysis for a series of samples with different
HMT content, as shown in Fig. 4. In the simple Zn-HMT
system where no EN is added as the complexing reagent
of Zn2+, zinc hydroxysulfate [(Zn(OH)2)3⭈(ZnSO4)⭈
(H2O)5] (ZHS) was obtained, not ZnO [Fig. 4(a)]. In the
simple Zn-EN system where no HMT was present, only
pure ZnO crystals were obtained [Fig. 4(b)]. In the composite Zn-EN-HMT system, the majority of products
were ZnO, with some presence of ZHS. When the concentration of HMT increased by 10 or 100 times, only
ZHS crystals were obtained, and ZnO vanished. These
results indicate that the morphological variation of flow-
erlike particles with HMT concentration is deeply rooted
in the change of crystalline structure, as well.
The formation mechanism of flowerlike particles has
been discussed in detail in another paper.23 Therefore, we
focus our attention on the effects of HMT on the morphology. In the Zn-EN-HMT aqueous system, the majority of Zn2+ is chelated by EN, thus maintaining the
low concentration of Zn2+ in the precursor. As the precursor undergoes continuous heating, the Zn-EN complex and HMT decompose to release free Zn2+ and OH−
[Eqs. (1) and (2)].26 Because of the high stability of the
Zn-EN complex (Kdec ⳱ 7.76 × 10−15), decomposition of
HMT occurs first. This is also supported by the fact that
the precipitation occurs in the Zn-HMT system soon after
the temperature exceeds ∼60 °C, while the precipitation
in the Zn-EN or Zn-EN-HMT system occurs only 10–
30 min (dependent on the concentration of precursor)
after the temperature reaches 95 °C. Thus the presence of
HMT in the Zn-EN system results in a high concentration
of OH− group prior to formation of the white precipitates
[i.e., ZnO/Zn(OH)2 particles]. It is this high [OH−] that
affects the nucleation and crystal growth of ZnO. The
high [OH−] will result in a higher supersaturation degree,
which can inhibit the heterogeneous precipitation of
ZnO/Zn(OH)2 and promote the homogeneous precipitation in the bulk solution because of the increased solubility product (i.e., [Zn2+]⭈[OH−]2).27 Therefore, ZnO
nanorods resulting primarily from the heterogeneous precipitation are eliminated in the aqueous system where
HMT is added (Fig. 2):
关Zn共EN兲3兴2+ ↔ Zn2+ + 3EN
,
C6H12N4 + 10H2O → 4NH3⭈H2O + 6HCHO .
FIG. 4. XRD patterns of obtained samples deposited from the Zn-ENHMT system on Si(111) wafer: (a) Zn2+-HMT ⳱ 1:1, without addition
of EN, (b) Zn2+-EN ⳱ 1:3, without addition of HMT, (c) Zn2+-ENHMT ⳱ 1:3:1, (d) Zn2+-EN-HMT ⳱ 1:3:10, and (e) Zn2+-ENHMT ⳱ 1:3:100.
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(1)
(2)
When the concentration of HMT is significantly increased (as in the case of sample 3), the concentration of
OH− is also increased accordingly due to the thermolysis
of HMT prior to the formation of ZnO/Zn(OH)2 particles, rendering a higher degree of supersaturation or
high driving force for nucleation. As a result, more nuclei
J. Mater. Res., Vol. 22, No. 7, Jul 2007
X-D. Gao et al.: ZnO submicron structures of controlled morphology synthesized in weak alkaline aqueous system
and petals form at higher HMT concentration. In addition, higher [OH−] is also beneficial to the formation of
thinner and well-grown crystals.
Another point worth mentioning is that although the
product obtained in high concentration of HMT is not
directly ZnO, it can be converted to ZnO through heat
treatment while the original morphologies are maintained.
B. Effect of Zn2+ concentration
Of the three [Zn2+] concentrations (i.e., 0.0002, 0.001,
0.01 mol/L), the two lower concentrations give rise to
similar flowers structure, differing only in yield of flowers. The highest concentration (0.01 mol/L), however,
results in not only greatly increased yield but also different microstructures (compared with Fig. 2). It can be
seen from Fig. 5(a) that each ZnO petal in the flower
possesses a columnlike shape, hexagonal structure, flat
tip, and longer size (∼5 ␮m) than those in the sample
with lower [Zn2+] (0.001 mol/L).
Though the precursor concentration is one of the most
studied factors, its effect on chemical kinetics and microstructure of the resultant crystals remains complicated
and mostly unclear. When the molar ratio of Zn2+, EN,
and HMT is unchanged, an increase of [Zn2+] brings
about an increase in the concentration of EN and HMT.
Because all of the Zn2+ ions are chelated by EN, the
concentration of free Zn2+ changes little. This indicates
that the reaction mode in high precursor concentration is
similar to that in the low concentration, and the formation
of flowerlike structure is expected. Meanwhile, the significant increase of [OH−] resulting from the thermolysis
of HMT and hydrolysis of EN, together with the continuously increased [Zn2+] resulted from the decomposition of Zn-EN complex, greatly improves the supersaturation and promotes the formation of ZnO with a high
degree of crystallization. Because the supply of Zn2+ and
OH− is much more plentiful than the case with low precursor concentration, ZnO petals with much longer
length and much higher crystallinity are obtained. The
fairly flat end of each petal may indicate that the free
Zn2+ in the precursor has not been completely consumed,
thus possessing the potential to grow further.
C. Effect of reaction temperature
Figure 5(b) illustrates SEM images of typical ZnO
flowers deposited at 105 °C (sample 6), 10 °C higher
than sample 2 and 5 °C higher than the boiling point of
solvent. It can be seen that the petals exhibit a smoother
surface and sharper end than sample 2 deposited at 95 °C
(see Fig. 2). In addition, both the yield of flowers and the
quantity of flower petals (not shown here) are obviously
lower than that in sample 2.
The effect of the reaction temperature is readily understood through the Zn2+ release rate. Higher temperature brings about faster release of Zn2+ and improves the
crystallinity. Therefore the deposition process of ZnO at
higher temperature is characterized by higher supersaturation and faster decreasing rate of [Zn2+] when compared with the case at lower temperature. The high supersaturation in the deposition process may promote the
crystallization of ZnO, but it may inhibit the nucleation at
the interface of neighboring particles. Therefore the
flowers formed at 105 °C exhibit fewer petals but better
crystallization (smooth surface), as seen in Fig. 6. In
addition, the sharp tip of the flower petal is believed to
come from the much higher consuming rate of Zn2+ and
the lower [Zn2+] at the later stage of deposition.
D. Effect of substrate
Though the type of bare substrates [Si(111) and glass]
have a negligible effect on the morphology of ZnO flowers, the ZnO seed layer does have an obvious influence
on the distribution of ZnO flowers. As seen from Fig. 6,
the distribution of ZnO flowers is fairly uniform compared with ZnO deposited on bare glass (sample 2), and
the aggregation of neighboring flowers is prevented to a
great extent. Figure 6(b) illustrates that the flower petals
possess distinctive hexagonal feature (as indicated by the
arrows), indicating good crystallinity. In addition, many
ZnO short rods are deposited on substrate apart from the
flowers, just like the case in pure Zn-EN system (Fig. 1).
FIG. 5. FE-SEM images of ZnO flowers deposited from the Zn-EN-HMT system: (a) high concentration of precursor ([Zn2+] ⳱ 0.01 mol/L)
(sample 5) and (b) high reaction temperature (105 °C) (sample 6).
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X-D. Gao et al.: ZnO submicron structures of controlled morphology synthesized in weak alkaline aqueous system
FIG. 6. FE-SEM images of ZnO layer deposited from the Zn-EN-HMT system (sample 7) on Si (111) wafer coated with ZnO seed layer: (a)
overall morphology at low resolution (3000×), illustrating the even distribution of ZnO flowers on substrate and (b) enlarged view of a typical
ZnO flower.
The ZnO seed layer can affect the nucleation process
of ZnO on substrate significantly. In this study, the ultrasonic irradiation assisted SILAR method was used to
prepare the seed layer, which is constituted by numerous
sparsely and evenly distributed ZnO nanoparticles
(∼100–300 nm). In addition, the ultrasonic irradiation can
also produce numerous active nucleation sites on substrate.28 Both the nanoparticles and active nucleation
sites on substrate can trigger the nucleation and crystal
growth of ZnO. While the ZnO nanoparticles may induce
the formation of flowerlike structures due to the multiple
nucleation at the particle interface, the active nucleation
site may promote the growth of ZnO nanorods. It is the
evenly distributed nature of precoated ZnO nanoparticles that determines the arrangement of ZnO flowers
on substrates. In addition, the ZnO seed layer is also
beneficial to the crystallinity improvement of ZnO particles [as indicated in Fig. 6(b)]. It is known that the
nucleation on bare and smooth substrates requires relatively higher energy (or supersaturation degree) than the
crystal growth. The presence of precoated ZnO nanoparticles and active nucleation sites can eliminate this process, thus maintaining higher supersaturation degree during the crystal growth. Hence obtained ZnO particles
exhibit higher crystallinity.
E. Effect of KCl background electrolyte
Figure 7 shows SEM images of ZnO layer deposited
from the Zn-EN-HMT system with KCl of 1 mol/L and
[Zn2+] of 0.1, 0.01, and 0.001 mol/L, respectively. When
[Zn2+] is 0.1 mol/L, a large quantity of ZnO rods other
than flowerlike structures are deposited, as seen from
Figs. 7(a) and 7(b). In addition, all rods exhibit excellent
hexagonal structure and a sharp end, as shown in the
inset of Fig. 7(b). When [Zn2+] is lowered to 0.01 mol/L,
the ZnO crystal also possesses a rodlike structure, but
with slightly smaller diameter (∼700–800 nm), lower
yield [Fig. 7(c)], and a round tip [Fig. 7(d)]. When [Zn2+]
is 0.001 mol/L, both ZnO particles with the spindle shape
and ZnO cobweb structure are obtained [Figs. 7(e) and
1820
7(f)]. The spindles are 300–500 nm in diameter and 1–
2 ␮m in length, and the cobweb is constituted by numerous interlinked ZnO nanowires (∼100 nm in diameter).
The crystallinity of the obtained layer shown in Fig. 7
was characterized by XRD, and the typical spectrum is
illustrated in Fig. 8. Results show that the crystallization
degree of ZnO particles formed with a high concentration
of KCl electrolyte was significantly higher than that
without KCl, and preferential orientation along (100)
plane was detected, which resulted from the voluminous
presence of horizontally laid ZnO rods.
For the indissoluble metallic oxide hydrooxide, such
as ZnO or Zn(OH)2 in the present study, higher ionic
strength means higher solubility of the solid phase.26 In
the weak alkaline environment where almost all Zn2+ is
chelated by EN, the increase of the solubility of ZnO/
Zn(OH)2 indicates that it is more difficult for ZnO to
nucleate, and the nucleation process requires much
higher degree of supersaturation. Therefore, when the
activation energy (or the degree of supersaturation) required by the nucleation at the interface of particles is
higher than that required by the crystal growth on already
formed ZnO crystals, the flowerlike structure will not
develop. As for the difference in the tip shape of ZnO rod
observed in Figs. 8(b) and 8(d), it may reflect the morphology of ZnO rods grown at different stages. For the
sample with [Zn2+] of 0.1 mol/L [Fig. 8(b)], the free Zn2+
in the precursor is not completely consumed by the end
of the experiment. In this situation, the (0001) plane of
ZnO possesses the highest growth rate compared with
(1011), (1010), or (1011) plane. So the sharp tip structure
indeed reflects the difference in the growth rate of different planes of ZnO crystals in aqueous solution. For the
sample with [Zn2+] of 0.01 mol/L [Fig. 8(d)], all free
Zn2+ in the precursor will be consumed at the end of the
experiment, and the dissolution–reprecipitation mechanism prevails, which indicates that the sharp tip of ZnO
rod may be dissolved, and the growth along negative
planes is facilitated. As a result, a round-shaped tip structure will develop.
J. Mater. Res., Vol. 22, No. 7, Jul 2007
X-D. Gao et al.: ZnO submicron structures of controlled morphology synthesized in weak alkaline aqueous system
FIG. 7. FE-SEM images of ZnO layer deposited from the Zn-EN-HMT system (samples 8–10) with the addition of 1mol/l KCl buffer. For (a)
and (b), [Zn2+] ⳱ 0.1 mol/L; for (c) and (d), [Zn2+] ⳱ 0.01 mol/L; for (e) and (f), [Zn2+] ⳱ 0.001 mol/L. (a) Overall distribution of ZnO rods
on substrate for sample 8. (b) Morphology of typical ZnO rods, with the inset showing the sharp end of a typical rod. (c) Overall distribution of
ZnO nanorods on substrate for sample 9. (d) Enlarged view of ZnO rod clusters, with each rod possessing a round end. (e) ZnO spindle and cobweb
for sample 10. (f) Enlarged view of the ZnO cobweb structure.
The formation of ZnO spindles in precursor with
[Zn2+] of 0.001 mol/L is closely related to the lower
content of Zn2+ in the precursor and the corresponding
lower degree of supersaturation in the crystal growth.
While the former factor leads to the smaller size of obtained spindles, the latter results in the lower crystallinity. The formation of cobweb structure may result from
the dissolution and recrystallization of already formed
ZnO spindles on the substrate, which should be closely
related to the low precursor concentration. Previously,
Vayssieres fabricated ZnO nanowire in the Zn-HMT system with [Zn2+] of 0.001 mol/L.18 This has also been
supported by other researchers who adopted a lower reaction temperature (85 °C) and longer reaction period
(more than 20 h) for the Zn-HMT system.29 However,
the exact growth mechanism of this cobweb ZnO structures can not be understood at this stage.
FIG. 8. XRD patterns of ZnO layer deposited from the Zn-EN-HMT
system on the glass substrate with the addition of 1 mol/L KCl buffer
and [Zn2+] of 0.01mol/L (sample 9).
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X-D. Gao et al.: ZnO submicron structures of controlled morphology synthesized in weak alkaline aqueous system
FIG. 9. FE-SEM images of typical ZnO particles other than the flowerlike shape formed in the Zn-EN-HMT system: (a) ZnO spindles observed
in sample 2 (Zn2+-EN-HMT molar ratio of 1:3:1) and (b) ZnO nanorods observed in high concentration of precursor ([Zn2+] ⳱ 0.01 mol/L)
(sample 5).
F. Mechanism of morphology variation
The above results have demonstrated that factors such
as organic amine additive, precursor concentration, reaction temperature, substrate, and ionic strength have significant influences on the morphology and crystallinity
of obtained ZnO-based particles. Then what are the underlying factors that govern the morphology of the particles?
For the growth of ZnO from aqueous solution, three
zones exist: the bulk solution, the solid ZnO particle, and
the thin layer near the solid surface (interfacial zone).
The nucleation and growth of ZnO are greatly affected by
chemical species in the interfacial zone. In particular, the
degree of supersaturation of Zn(OH)2 in the interfacial
zone and the adsorption of organic/inorganic species on
the surface of ZnO are two determinative factors influencing the nucleation and growth of ZnO.
The degree of supersaturation (DSS), defined as the
ratio of [Zn2+]⭈[OH−]2 to KSPZn(OH)2, together with its
variation with time, is the critical factor reflecting the
driving force of nucleation and crystal growth in aqueous
solution. The higher the DSS becomes, the more the
nucleation, and the better the crystallinity. All of the
experimental parameters will affect the magnitude of
DSS and its variation with the time. For example, the
addition of HMT will improve the concentration of OH−
immediately prior to the first nucleation. The reaction
temperature can change the decomposition rate of HMT
and Zn-EN complex, thus modifying the variation of
DSS with time. The increase of the precursor concentration (i.e., the total content of Zn2+) will greatly improve
the DSS value in the initial nucleation stage and prolong
the sustaining time of high DSS in the growth stage of
ZnO, and so on. Therefore, the absolute magnitude of
DSS and, in particular, its changing behavior with the
time are fundamental factors governing the morphology
of ZnO particles
Apart from DSS, the adsorption of organic/inorganic
species on the surface of ZnO is also very important to
1822
the growth behavior of ZnO crystals. With the adsorption
of formaldehyde (decomposition product of HMT), EN
molecules, or aquo-Cl− ions on the side surface of the
ZnO crystal, the crystal’s growth on the side direction
will be inhabited, and one-dimensional structures tend to
develop. However, effects of the adsorption of organic/
inorganic species are complicated,23 and further studies
on this aspect are required.
It can be seen from the above analysis that, although
no direct relations between the chemical surroundings in
the bulk solution and the morphology of ZnO particles
can be established based on the present data, the morphology of ZnO in Zn-EN-HMT system can be tuned
reasonably to a certain extent by adjusting the degree of
supersaturation and the adsorption of organic/inorganic
species on the surface of ZnO crystals.
As for the yield of flowerlike ZnO particles in the
Zn-EN-HMT system, it was low when no EN was added
[sample 1, Fig. 1(a)] and when ZnO seed layer was used
as the substrate [sample 7, Fig. 6(a)] because of the deposition of many of nanorods. For all other cases, the flowerlike structure is dominant, although the other shapes of
ZnO particles such as spindles or nanorods were also
observed, as shown in Figs. 9(a) and 9(b). The ZnO
spindle observed in sample 2 (Zn2+-EN-HMT molar ratio
of 1:3:1) may be an intermediate product of the flowerlike ZnO particle, the growth of which was not completed at the end of experiment.23 The ZnO nanorod obtained sample 5 (high [Zn2+]) may be the result of the
heterogeneous nucleation of ZnO on the substrate at the
later stage of ZnO growth, when the degree of supersaturation in the bulk solution became low and the heterogeneous precipitation was preferred.
As-prepared ZnO submicron particles deposited on
Si-wafer possess a high stoichiometric ratio and chemical purity, large specific surface area (compared with
traditional powder), and refined nanostructures. They
could find wide application in ultraviolet photoluminescence, electrode, gas sensor, catalyst support, or other
areas.
J. Mater. Res., Vol. 22, No. 7, Jul 2007
X-D. Gao et al.: ZnO submicron structures of controlled morphology synthesized in weak alkaline aqueous system
IV. CONCLUSION
ZnO submicron particles with flower, rod, or cobweb
structures were synthesized from the zinc-EN-HMT
aqueous system by varying the concentration of HMT
and zinc source, reaction temperature, ionic strength, and
substrate. Results show that not only the size and shape
of ZnO particles but also the crystallinity and the particle
distribution nature can be tuned. Mechanism analyses
show that the degree of supersaturation and the adsorption of organic/inorganic species on the surface of ZnO
are critical factors influencing the crystalline morphology of ZnO in the weak alkaline environment.
This work represents the first attempt to investigate the
morphology of ZnO submicron particles formed in the
weak alkaline environment, valuable for controlled synthesis of ZnO crystals in mild aqueous solutions.
ACKNOWLEDGMENTS
This work was supported by National Natural Science
Foundation of China (50502038), and Shanghai Natural
Science Foundation (05ZR14132), Shanghai-Applied
Materials Research and Development Fund (06SA07),
and the State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL200505SIC).
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