The influences of the reaction temperature, the OH/Zn2þ mol ratio and the reaction time on the morphologies of the ZnO powders were discussed.. However, when the reaction temperature was
Trang 1Xiaoyi Shen, Yuan Liang, Yuchun Zhai*, Zhiqiang Ning
School of Materials and Metallurgy, Northeastern University, Shenyang 110819, China
[Manuscript received January 17, 2012, in revised form May 24, 2012, Available online 24 December 2012]
By employing zinc acetate and sodium hydroxide as raw materials, ultrafine ZnO powders with different morphologies were successfully synthesized through hydrothermal method The influences of the reaction temperature, the OH/Zn2þ mol ratio and the reaction time on the morphologies of the ZnO powders were discussed The reaction conditions were obtained, under which the ZnO of flower-like particles, micro-rods and flake particles was synthesized, respectively The crystal structures and morphologies of those ZnO particles were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) The ZnO with flower-like structures was composed of lots of micro-rods with hexagon morphology The XRD patterns indicated that the ZnO powders were hexagonal wurtzite structures with high purity Finally, the growth mechanism of the ZnO particles was discussed
KEY WORDS: ZnO particles; Hexagonal wurtzite structure; Hydrothermal method; Growth mechanism
1 Introduction
Micro/nano-materials have stimulated great interest due to
their special characteristic in optical, catalyst, thermal properties
that differ from those of bulk materials and their importance in
basic scientific research and potential technology application[1,2]
The ultrafine crystalline of ZnO is a good example, which is an
attractive material due to its wide variety of application including
pigments, catalysts and phosphors[3,4] There are lots of ways for
synthesis of micro/nano ZnO powder, among which the wet
chemical processes are of special interest due to their simplicity
and low temperature[5,6] As it is known, the so-called
hydro-thermal method is a relatively simple wet chemical method to
prepare micro/nano ZnO particles with different morphology,
such as prism-like,flower-like, rods and spherical particles And
the controllable hydrothermal method has its unique advantages
of simplicity, low temperature and high yield[7,8] In addition to
these, the hydrothermal method can produce micro/nano
parti-cles with different morphology and better crystal quality, without
using metal catalyst or template, and it is an easy and economical
process[9,10] The ZnO withflower-like structures was obtained
in previous work[11e13], but detailed discussion was notfinished
In this study, the influencing factors were investigated in more detail and the ultrafine ZnO powders of flower-like particles, rods and flake particles were successfully synthesized, respec-tively The influences of the reaction temperature, the OH/Zn2 þ
mol ratio and the reaction time on the morphologies of ZnO powders were discussed in detail The crystal structures and morphologies of the ZnO powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) Finally, the growth mechanism was discussed
2 Experimental 2.1 Materials Zinc acetate and sodium hydroxide with analytic grade were used as raw materials Polyethylene glycol (PEG20000) with analytic grade was used as dispersant agent Distilled water was obtained in laboratory
2.2 Preparation The preparation of the ultrafine ZnO powder involved several steps First, 0.5 mol L1 zinc acetate solution and 1, 5,
10 mol L1 sodium hydroxide solutions were formulated, respectively Second, according to the mol ratio of OH/Zn2þ, zinc acetate and sodium hydroxide solutions were added into
a reaction kettle with lining Teflon, and then the PEG20000 was added Afterward the solution was stirred with a magnetic agitator to homogeneous Third, the reaction kettle was placed in
* Corresponding author Prof., Ph.D.; Tel./Fax: þ86 24 83687731;
E-mail address: zhaiyc@smm.neu.edu.cn (Y Zhai).
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Trang 2an oven at a desired temperature for several hours When the
reaction ended, the reaction kettle was cooled down to room
temperature naturally The product was centrifugated and washed
by using distilled water repeatedly and using absolute ethanol
twice Finally, the ultrafine ZnO powder was obtained by drying
in an oven
2.3 Characterization
The structures of the ZnO powders were measured with
a Japan Rigaku X-ray diffractometer The morphologies and
sizes of the ZnO powders were observed by using a Japan
shi-madzu SSX-550 scanning electron microscope
3 Discussion
3.1 Influence of reaction temperature on the morphologies of
ZnO powders
The influence of the reaction temperature on the morphologies
of ZnO powders was studied, and the results were shown in
Fig 1.Fig 1(a)e(c) was corresponding to 120, 150 and 180C
for 10 h, respectively as the mol ratio of OH/Zn2þwas 5 The
ZnO withflower-like structures was observed both inFig 1(a)
and (b), but the flower-like structures were more regular in
Fig 1(b) than those inFig 1(a) Theflower-like structures were
composed of micro-rods with 4mm in length, which aligned in
a radical way from a center and grew longer and bigger with
increasing temperature However, when the reaction temperature
was up to 180 C, the flower-like structures of ZnO were
destroyed and regular ZnO micro-rods with hexagonfigure and
flat top were obtained, as shown inFig 1(c) The ZnO
micro-rods were hexagon with perfect morphology and smooth
surface, of which the diameter and the length were about 1mm
and more than 20 mm, respectively That is to say, with
increasing temperature, the flower-like structures of ZnO grew
more regular and uniform However, when the temperature was
too high, the structures were broken down and yielded out micro-rods
3.2 Influence of the OH/Zn2þmol ratio on the morphologies of
ZnO powders The influence of the OH/Zn2 þmol ratio on the morphologies
of the ZnO particles was also investigated, as shown inFig 2
Fig 2(a)e(c) was corresponding to the OH/Zn2 þmol ratio 2,
5 and 10 at 150 C for 10 h, respectively In Fig 2(a), the ZnO powder looked like flake In Fig 2(b), the ZnO with flower-like structures were observed with regular figure and uniform micro-rods, and the micro-rods aligned in a radical way from a center The micro-rods of the ZnOflower-like structures grew longer and bigger with increasing the OH/Zn2þmol ratio When the OH/Zn2þmol ratio was 10, the micro-rods grew so big that theflower-like structures were damaged and the micro-rods were yielded out, as shown in Fig 2(c) This could be confirmed by the residual flower-like structure in Fig 2(c), from the center of which the residual of the stripping of those micro-rods could be found The micro-rods were hexagon and were more than 20mm in length with smooth surface and regular tip
From the above, the influence of the OH/Zn2þmol ratio on
the morphologies of ZnO particles was significant The morphologies of ZnO particles transited fromflake to flower-like
to micro-rods with increasing the OH/Zn2þmol ratio This is because the NaOH solution was alkaline with strong polarity, and the polarity of the solution was enhanced when the OH/
Zn2þmol ratio increased It was evident that the ZnO particles grew with self-organizing
3.3 Influence of the reaction time on the morphologies of ZnO powders
The influence of the reaction time on the morphologies of ZnO powders was also discussed, as shown inFig 3.Fig 3(a) and (b)
Fig 1 SEM images of ZnO powder prepared at: (a) 120C; (b) 150C; (c) 180C.
Fig 2 SEM images of ZnO particles synthesized at: (a) OH/Zn2þ¼ 2; (b) OH /Zn2þ¼ 5; (c) OH /Zn2þ¼ 10.
Trang 3was corresponding to reaction time for 4 and 10 h at 150C as
the OH/Zn2þ mol ratio was 5, respectively The ZnO with
flower-like structures were observed in bothFig 3(a) and (b)
However, the flower-like structures in Fig 3(a) were still in
growth stage with irregularfigures The flower-like structures,
shown inFig 3(b), had enough time to grow by extending time
to 10 h, and the micro-rods of theflower-like structures grew
more regular
In summary, the influences of the reaction temperature, the
OH/Zn2þmol ratio and the reaction time on the morphologies
of the ultrafine ZnO powders were notable, among which the
OH/Zn2þ mol ratio was the most remarkable It could be
concluded that the appropriate reaction conditions for obtaining
ZnO with flower-like structures were reaction temperature
150C, OH/Zn2þmol ratio 5 and reaction time 10 h
3.4 XRD patterns of ZnO powders and EDS pattern of flower-like structure
The XRD patterns of ZnO offlower-like particles, rods and flake, and the EDS pattern of the ZnO with flower-like structures are shown in Fig 4 The XRD patterns were obtained by employing CuKa radiation with a voltage of 40 kV, at
a scanning rate of 6 deg/min with 2q ranging from 20 to
80 deg The crystal planes of (100), (002), (101) etc belonged
to the hexagonal system, and the crystal parameters were
a ¼ 0.3249 nm and c ¼ 0.5206 nm All diffraction data of
Fig 4(a)e(c) were in good agreement with JCPDS files No
361451 (a ¼ 0.324982 nm and c ¼ 0.520661 nm) No other phases were detected, and the reflections were sharp, which indicated that the ZnO powders with flower-like, rods and
Fig 3 SEM images of ZnO with flower-like structures reaction for: (a) 4 h, (b) 10 h.
Fig 4 XRD patterns of ZnO powders of (a) flower-like, (b) rods and (c) flake shape and (d) EDS pattern of ZnO powder with flower-like structures.
Trang 4flake morphologies were hexagonal wurtzite structure with
regular crystal form and high purity
The EDS pattern was performed on one of the ZnO
flower-like structures, and the composition analysis of the powder
showed the existence of Zn and O (from the sample), as well as
Au (from the coating sprayed before testing) This provided an
additional evidence that the ZnO withflower-like structures was
pure
3.5 Growth mechanism
As it is well known, the crystal formation in solution can
be divided into two steps: nucleation and growth The
ob-tained particles morphology depends on both the nucleation
and crystal growth rate[7,14,15] Based on the experimental
results, the growth mechanism of ZnO particles could be
represented by the following chemical reactions, and Fig 5
shows the fundamental schematic growth diagram of ZnO
particles The similar results were also reported in
literature[16e18]
ZnðCH3COOÞ2/ Zn2þþ 2CH3COO (1)
ZnðOHÞ2 þ 2OH/ ZnðOHÞ24 (3)
ZnðOHÞ2
In the process of preparing ZnO with flower-like structures,
both nucleation and crystal growth were fast At the initial stage,
large amounts of ZnO nuclei were formed, and some ZnO nuclei
agglomerated together and then became small crystals by driving
force of the surface energy and electrostatic effect This was
a spontaneous process that occurred, since small crystals were
more active than larger ones Therefore, small particles nucleated
in supersaturated solution kinetically favored to grow larger
particles The growth of ZnO with flower-like structures was
controlled by nucleation and growth process in aqueous solution
In hexagonal system, the crystal plane of (001) is close-over
plane[19] The energy is minimum when the crystal grows along with (001) plane Based on surface energy minimization, ZnO crystals developed along the c-axis from ZnðOHÞ24 supersaturation solution[7,16], and then micro-rod crystals were formed, which were branches of ZnO with the flower-like structures
ZnO is a polar crystal, whose positive polar plane is rich in
Zn2þand the negative plane is rich in O2[15,16,20] The Zn and
O atoms were arranged alternatively along the c-axis, and the top surfaces were Zn terminated (001) Therefore, the top surfaces were energetically active, which made the micro-rods growing in radical way once the nuclei were formed[15,16] The reason was that the Zn (001) surface with positive charge was chemically active and the O (0 01) surface with negative charge was inert However, when the reaction equilibrium was broken down, theflower-like structures were destroyed to form micro-rods
4 Conclusions (1) The reaction temperature, OH/Zn2+mol ratio and reaction time all had significant influences on the morphologies of the ZnO powders, among which the OH/Zn2+ mol ratio was the most notable one The micro-rods of the flower-like structures grew larger as the reaction temperature or the OH/Zn2+ mol ratio increased When the reaction temperature or the mol ratio was increased to a certain degree, the flower-like structures would be destroyed to get micro-rods
The appropriate reaction conditions for preparing ZnO with flower-like structures were reaction temperature 150C, OH/Zn2+mol ratio 5 and reaction time 10 h
(2) The synthesized ZnO powders of different morphologies were hexagon wurtzite structure with high purity and regular crystal form The ZnO with flower-like structures was regular with uniform micro-rods that were hexagon with smooth surface
(3) The growth mechanism of ZnO withflower-like structures, rods and flake morphologies was proposed, which was
a complex process including: the formation of ZnðOHÞ24 from Zn2+, the dehydration of ZnðOHÞ2
4 , the appearance
of ZnO nuclei and the growth of ZnO crystals with different morphologies
Fig 5 Schematic growth diagram of ZnO particles.
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