A simple and green approach for preparation of ZnO2 and ZnO under sunlight irradiation

Article abstract of DOI:10.1016/j.cplett.2007.06.098

A green approach is developed for sol-gel synthesis of ZnO and ZnO₂ through photochemical reaction under sunlight irradiation. The component and structure of the sol and powders are systematically analyzed, and the reaction equations are deduced and the fabrication mechanisms are discussed. It is found that the ZnO₂ powder is obtained when sintering temperature is below 200 °C, while ZnO powder is obtained as sintering temperature increases from 200 to 800 °C. All ZnO and ZnO₂ prepared by this method are with crystal structure and high purity. Our fabrication process has low energy consumption and it brings no environment pollution. This simple and green approach is considered of great potential applications in industry.

Full text of DOI:10.1016/j.cplett.2007.06.098

Chemical Physics Letters 443 (2007) 342–346
www.elsevier.com/locate/cplett
A simple and green approach for preparation of ZnO2 and ZnO
under sunlight irradiation
a,
a
a
Meng Sun a,b, Weichang Hao *, Chunzhong Wang , Tianmin Wang
a
Center of Materials Physics and Chemistry, School of Science, Beijing University of Aeronautics and Astronautics, Beijing 100083, China
b
Department of Technology and Physics, Zhengzhou University of Light Industry, Zhengzhou 450002, China
Received 18 January 2007; in final form 16 April 2007
Available online 28 June 2007
Abstract
A green approach is developed for sol–gel synthesis of ZnO and ZnO2 through photochemical reaction under sunlight irradiation. The
component and structure of the sol and powders are systematically analyzed, and the reaction equations are deduced and the fabrication
mechanisms are discussed. It is found that the ZnO2 powder is obtained when sintering temperature is below 200 °C, while ZnO powder
is obtained as sintering temperature increases from 200 to 800 °C. All ZnO and ZnO2 prepared by this method are with crystal structure
and high purity. Our fabrication process has low energy consumption and it brings no environment pollution. This simple and green
approach is considered of great potential applications in industry.
Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction
to H2O2 solution [16]. These ZnO and ZnO2 fabrication
processes are complicated; they not only have huge energy
consumption, but also bring serious environmental pollu-
tions. In this Letter, we have developed a simple and green
method for synthesis of ZnO and ZnO2 using sunlight irra-
diation. We have analyzed systematically the reaction
process and the composition of product with UV–Vis
absorption spectra, XRD, DSC/TG, TEM, Raman spectra
measurements. It is shown that during the ZnO2 decompo-
sition into ZnO at sintering temperature higher than
200 °C, the by-product in the preparation process is O2,
which makes no pollution to environment. The ZnO and
ZnO2 powders prepared by this method are with crystal
structure and high purity. This green technology without
any pollution is simple, low energy consumption.
Recently, there has been increasing interest in zinc oxide
(ZnO) and zinc peroxide (ZnO2) semiconductor materials.
The direct wide band gap (Eg is ꢀ3.3 eV at 300 K) makes
ZnO one of the most promising candidates for application
in ultraviolet optoelectronics devices [1–4], such as gas sen-
sor [5], solar cells [6], surface acoustic wave devices [7], light
emitting diodes (LEDs) [8] and transistor [9], etc. ZnO2
powder is a useful padding widely used in industry; it has
been extensively used as blowing composition in preparing
a foamed product of high-melting synthetic resin [10]. It
has also been applied in accelerated vulcanization of
nitril-carboxyl rubber to produce an attrition resistant rub-
ber [11,12]. Furthermore, ZnO2 has been used an oxidant
and oxygen donor in compositions or mixtures containing
explosive materials [13].
2. Experiments
Normally, ZnO powder is prepared in industry by oxi-
dation of Zn atom evaporated from Zn ingot (indirect pro-
cess) [14,15], and ZnO2 powder is prepared by adding ZnO
ZnO and ZnO2 are prepared by sol–gel technique. A
35 W Xenon lamp is used as the light source in photochem-
ical reaction process because the energy distribution of the
Xe lamp is identical to sunlight [17]. A 5 ml 30% hydrogen
peroxide (H2O2) is first diluted with 50 ml deionized water,
*
Corresponding author. Fax: +86 10 82315351.
E-mail address: whao@buaa.edu.cn (W. Hao).
0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2007.06.098

M. Sun et al. / Chemical Physics Letters 443 (2007) 342–346
343
then 1 g of zinc acetate dihydrate (Zn(CH3COO)2 Æ 2H2O)
is dissolved in the diluted H2O2 reagent. All the reagents
are of analytical grade and the deionized water’s resistivity
is higher than 18.2 MX cm. The solution is irradiated by
the 35 W Xenon lamp for 6 h with continuous stirring to
get the homogeneous white ZnO2 precursor sol. After
being aged for two days, the ZnO2 precursor sol is baked
at 40 °C overnight. Finally, the dried gel is crushed to be
fine ZnO2 powders. The ZnO2 are sintered in air at different
temperatures ranging from 100 to 800 °C with 100 °C steps,
respectively. The crystal structure and phase purity of
obtained ZnO2 and ZnO powders are characterized. The
preparation of ZnO2 sol is also performed under sunlight
irradiation, and the result is identical to that under Xenon
lamp.
the UV–Vis absorption spectra of ZnO2 colloid with differ-
ent photochemical reaction time, and the inset of Fig. 1
shows the variation of absorbance at 350 nm with different
reaction times. It is seen that the absorption onset shifts
towards long wavelength as the reaction time increasing,
which means the ZnO2 sol particles size increases with
the reaction time. The absorbance of ZnO2 colloid
increases linearly with reaction time in the region of 0.5–
5 h, but it saturates as the reaction time is longer than
5 h. These results indicate clearly that photochemical reac-
tions finish after 5 h, and the corresponding average parti-
cle size of ZnO2 sol reaches about 68 nm.
The XRD spectra of gel powders with different sintering
temperatures ranging from room temperature to 800 °C are
shown in Fig. 2. When the sintering temperature is 100 °C,
the gel powder’s XRD spectrum is identical to that of
ZnO2, which indicates the ingredient of gel powder is
purely cubic ZnO2. When the sintering temperature is equal
or higher than 200 °C, powder’s XRD spectra are the same
as that of hexagonal wurtzite ZnO, which indicates that the
ZnO2 is totally transformed into single-crystal ZnO. All
ZnO2 and ZnO powders are found to be highly crystalline
without any impurity or second phase. With the sintering
temperature rising from 200 to 800 °C, the peak intensities
of all the XRD spectra increase, and the full-width at half-
maximum (FWHM) values decrease. These results indicate
that crystalline quality of the gel powders improves with
the sintering temperature increasing.
The XRD spectra are measured in the 2h range of 10–
80° with a Rigaku D/max 2200PC using Cu Ka radiation
˚
(1.5418 A) at 40 kV. The optical absorption spectra of sol
in the 200–600 nm range are obtained using a HITACHI
U-3010 UV–Vis spectrophotometer. DSC and TG analysis
are carried out in argon atmosphere with a heating rate of
20 °C/min in the temperature region from 300 to 600 K
using a NETZSCH STA449C thermal analyzer. The parti-
cle sizes of ZnO2 sol are determined by a laser particle size
analyzer (CILAS 1064L). Transmission electron micro-
scope (TEM) images and selected area electron diffraction
(SAED) images are recorded with a JEM-2100F transmis-
sion electron microscope operating at 200 kV.
The formation of ZnO2 results from the photochemical
reaction of H2O2 and Zn(CH3COO2)2, and the correspond-
ing process is considered as following:
3. Results and discussions
ZnO2 colloid is monitored during the preparation pro-
cess using an UV–Vis spectrophotometer. Fig. 1 shows
sunlight
ZnðCH3COO2Þ2 þ H2O2 ! ZnO2 þ 2HCH3COO2 ð1Þ
Fig. 1. The evolution of UV–Vis absorbance spectrum with the reaction time of ZnO2 sol. Inset: the relationship of the absorbance at 350 nm vs. the
reaction time.

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M. Sun et al. / Chemical Physics Letters 443 (2007) 342–346
Fig. 2. The XRD patterns of the gel powders as-prepared and sintered at different temperatures.
The ZnO2 decomposes into ZnO when the sintering tem-
loss takes place at three stages. The first weight loss of 1.2%
is observed below 200 °C, which is considered to be due to
the evaporation of water. The second weight loss is
observed at about 200–250 °C, which is caused by the
decomposition of ZnO2, as the observed weight loss of
16.4% is nearly the same as the theoretical value of
15.83%. The third weak weight loss takes place from 300
to 500 °C, which is considered to be due to decomposition
of slightly residual untreated Zn(CH3COO2)2. In addition,
two exothermic reactions are, respectively, observed in
DSC curve in the temperature range of 200–250 °C and
400–500 °C during the heat treatment of ZnO2 gel powder.
The sharp exothermic peak at around 230 °C is due to
decomposition of ZnO2, which is in good consistent with
perature is above 200 °C, which agrees well with the theo-
retical decomposition temperature of zinc peroxide (at
about 182.2 °C) [18]. The decomposition ZnO2 is believed
to be:
2ZnO2 ! 2ZnO + O2
ð2Þ
It is clear from Eq. (2) that the by-product in this prep-
aration process is O2, thus it causes no environmental
pollution.
Fig. 3 shows the TG and DSC curves of ZnO2 xerogel.
The exothermic and weight loss temperature corresponding
to ZnO2 decomposition is found at about 200–250 °C from
DSC and TG. It is seen from the TG curve that the weight
Fig. 3. The (a) DSC and (b) TGA curves of gel powders.

M. Sun et al. / Chemical Physics Letters 443 (2007) 342–346
345
our TG result. The flat exothermic plateau ranging from
400 to 500 °C is considered to be due to continuous
improvement of crystallization quality with temperature
rising, in good agreement with our XRD results as shown
above in Fig. 2.
due to the 2-E2(M) mode of ZnO, which is related to mul-
tiphonon process [19]. These results indicate that the gel
powder after sintering at 500 °C for 1 h have been com-
pletely transformed into ZnO. In contrary, the Raman
spectra of gel powders after sintering at 100 °C for 1 h is
much different from that after annealing at 500 °C or that
of ZnO bulk. According to our XRD results, we tentatively
attribute the observed Raman spectrum to ZnO2. As no
Raman spectrum of ZnO2 is available, theoretical calcula-
tions are expected to further verification of our assignment.
Fig. 4a and b show transmission electron micrographs
of ZnO2 and ZnO nano-powders and the inset of Fig. 4b
shows the selected area electron diffraction pattern of the
ZnO powder. The ZnO particles have hexagonal shapes
resulting from their wurtzite structure. The average particle
size is estimated to be about 100 nm for ZnO2 and 70 nm
for ZnO, respectively. The selected area electron diffraction
pattern is consisting of a series of diffused diffraction spots
4. Conclusions
ꢀ ꢀ
with zone axis along ½1211ꢁ direction, which indicates
We have developed a simple green method for sol–gel
synthesis of ZnO2 and ZnO through photochemical reac-
tion. ZnO2 nano-powders have been prepared through
the reaction of Zn(Ac)2 and H2O2 under sunlight radia-
tions. It is found that the obtained product is pure ZnO2
nano-particles with an average size of about 100 nm when
the sintering temperature is under 200 °C. When the sinter-
ing temperature is above 200 °C, the product is trans-
formed into pure ZnO with an average size of about
70 nm. Both the ZnO2 and ZnO have crystal structure
and high purity. This synthesis approach utilizes sunlight
as the energy resource to promote reaction process, and
the by-product during the preparation process is O2, which
causes no environmental pollution. This fabrication pro-
cess is considered to be superior to the other widely used
approaches and it is of great potential applications in
industry.
clearly the single-crystal feature of our ZnO powders.
The single-crystal nature of nano ZnO particles is further
confirmed by the clear lattice fringes, as shown in Fig. 4c.
The lattice spacing is of about 0.195 nm, which is the dis-
ꢀ
tance of ð1012Þ planes of hexagonal ZnO.
Fig. 5 shows the Raman spectra of gel powders after sin-
tering at 500 and 100 °C for 1 h, respectively. It is noted
that the Raman spectrum of gel powders after sintering
at 500 °C for 1 h is completely the same as that of bulk
ZnO sample. There are five peaks observed at 98, 331,
438, 385 and 578 cmꢂ1, respectively. The peaks at about
98 and 438 cmꢂ1 are assigned to the two nonpolar Raman
active E2 vibrational modes of ZnO. They are associated to
the E2-low mode and the E2-high mode, respectively. The
peaks at around 385 and 578 cmꢂ1 are attributed to the
A1(TO) and A1(LO) modes, and the band of 331 cmꢂ1 is
Fig. 4. The TEM images of (a) ZnO2 nano-particles and (b) ZnO nano-crystal. Inset: selected area electron diffraction (SAED) pattern. (c) HRTEM image
of the ZnO single-crystal.

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M. Sun et al. / Chemical Physics Letters 443 (2007) 342–346
Fig. 5. The Raman spectra of (a) ZnO and (b) ZnO2.
[7] B.A. Buchine, W.L. Hughes, F.L. Degertekin, Z.L. Wang, Nano Lett.
Acknowledgements
6 (2006) 1155.
[8] R. Konenkamp, R.C. Word, M. Godinez, Nano Lett. 5 (2005) 2005.
[9] E.M.C. Fortunato, P.M.C. Barquinha, A.C.M.B.G. Pimentel, et al.,
Adv. Mater. 17 (2005) 590.
[10] S. Ohno, N. Aburatani, N. Veda, Patent US4247651-A, 1980.
[11] L. Ibarra, M. Alaorriz, J Appl. Polym. Sci. 84 (2002) 605.
This work was financially supported by Young
Researcher Project of Beihang University, Engineering
Research Institute, Peking University (ERIPU) under
Grant No. 204031 and Key Lab Project of Lanzhou Insti-
tute of Physics.
´
[12] L. Ibarra, A. Marcos-Fernandez, M. Alaorriz, Polymer 43 (2002)
1649.
[13] R. Hagel, K. Redecher, Patent US4363679-A, 1981.
[14] H. Tao, L. Zheng, S. Chi, Patent CN1443705-A, 2003.
[15] K.R. Han, H.J. Jang, C.S. Kim, J Ceram. Soc. Jpn. 113 (2005) 337.
[16] T. Yu, Patent CN1224696-A, 1999.
[17] Y.Q. Wang, X.J. Yu, H.F. Yang, Q. Ming, D.Z. Sun, Chin. J. Inorg.
Chem. 22 (2006) 771.
References
[1] D.P. Norton, Y.W. Heo, M.P. Ivill, et al., Mater. Today 7 (2004) 34.
[2] Y.W. Heo, D.P. Norton, L.C. Tien, et al., Mat. Sci. Eng. R 47 (2004)
1.
[3] U. Ozgur, Ya.I. Alivov, C. Liu, et al., J. Appl. Phys. 98 (2005)
041301.
[18] B.D. Cullity, Elements of X-ray Diffractions, Addison-Wesley, MA,
1978.
[4] Z.L. Wang, J. Mater. Chem. 15 (2005) 1021.
[5] H.J. Lim, D.Y. Lee, Y.J. Oh, Sensor. Actuat. A 125 (2006) 405.
[6] W.J.E. Beek, M.M. Wienk, R.A.J. Janssen, Adv. Funct. Mater. 16
(2006) 1112.
[19] N. Ashkenov, B.N. Mbenkum, C. Bundesmann, et al., J. Appl. Phys.
93 (2003) 126.