Ethylene glycol assisted hydrothermal synthesis of flower like ZnO architecturesS.. But in the present study,flower like ZnO architectures have been prepared by thermal decomposition of zi
Trang 1Ethylene glycol assisted hydrothermal synthesis of flower like ZnO architectures
S Ashokaa, G Nagarajua, C.N Tharamanib, G.T Chandrappaa,⁎
a
Department of Chemistry, Central College Campus, Bangalore University, Bangalore-560001, India
b
Department of Chemistry, Ruhr Universität Bochum, Bochum, Germany
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 16 September 2008
Accepted 13 January 2009
Available online 26 January 2009
Keywords:
Hydrothermal
Ethylene glycol
ZnO flowers
Nanorods
Wurtzite structure
We describe a simple route toflower like ZnO architectures, based on the decomposition of zinc acetate precursor in water-ethylene glycol solution at 140–160 °C for 1d through hydrothermal method The PXRD pattern reveals that the ZnO crystals are of hexagonal wurtzite structure Ethylene glycol plays a key role on the morphology control of ZnO crystals The SEM images of ZnO products prepared at 140 °C and 160 °C mainly exhibitflower like architecture composed of many rods Whereas, the product prepared at 180 °C shows bunches accompanying a few number of free rods TEM results reveal that the rods resemble swords with decrease in size from one end to another From Raman spectrum, the peaks at 437 cm− 1, 382 cm− 1and
411 cm− 1correspond to E2(high), A1(TO) and E1(TO) of ZnO crystals respectively The photoluminescence spectrum exhibits strong UV emission at ~ 397 nm, which comes from recombination of exciton The possible mechanism for the formation offlower like ZnO architecture is proposed
© 2009 Elsevier B.V All rights reserved
1 Introduction
The controlling syntheses of inorganic materials with unusual and
complex characters are of considerable interest in materialsfields due
to the properties of these materials that depend mainly on their
shapes, sizes and structures[1] ZnO is one of the most important
multifunctional semiconductors with its wide direct energy band gap
of 3.37 eV and its large exciton binding energy (about 60 meV)
Out of several nanomaterials studied so far carbon nanotubes[2,3]and
ZnO[4,5]exhibit the widest variety of nanostructures ZnO is presently
hotly pursued material in regard to the formation of myriad nano/
microstructures[6,7] Even though many methods have been reported in
the literature, the interest in thefield of genesis of ZnO
micro/nanos-tructure has not been diminished Various syntheses methods capable of
formation of different types of ZnO nanostructures have been
demon-strated [8,9] Up to now, well-defined ZnO nanostructures with an
abundant variety of shapes, such as nanoneedles, nanocables and
nano-tubes[10], nanowalls[11], nanobridges and nanonails[12], nanohelixes,
nanosprings, nanorings [13,14], hierarchical nanostructures [15], and
mesoporous polyhedral cages and shells[16]have been achieved through
vapor-based techniques Via the chemical solution route, tube-, tower-,
andflowerlike ZnO nanostructures [17–19] and oriented helical ZnO
nanorod arrays [20] have also been realized very recently However,
despite great progress in thisfield, the shape-controlled synthesis of ZnO
nanocrystals, especially regarding control over the complex structure, still
remains a remarkable challenge The simple solution synthesis, by
thermal treatment of the reactant in different solvents, may be the most
simple and effective way to prepare sufficiently crystallized materials at relatively low temperatures, while exempted from further calcination Besides this, the benefits of a utilizing solution-based method have also involved the considerable influence of reaction species on the final size and morphology of the as-prepared samples In this aspect, most of the previous investigations on flower like ZnO prepared mainly utilized NaOH/NH4OH/ethylenediamine as hydroxide source as well as complex-ing agents[21–23] The NaOH/NH4OH/ethylenediamine forms complex and these complexes are subjected to hydrothermal treatment in presence of water/alcohol to obtain nano/microstructures[21–23] But
in the present study,flower like ZnO architectures have been prepared by thermal decomposition of zinc acetate in presence of water-ethylene glycol mixture under hydrothermal condition The possible reaction mechanism for the formation offlower like ZnO architecture is proposed
2 Experimental Zinc acetate dihydrate (98%) and ethylene glycol (99%) were pur-chased form Merck Limited and used without further purification Double distilled water was used throughout the experiments
In a typical hydrothermal process [24], 0.545 g zinc acetate (2.4 mmol) was dissolved in 10 ml distilled water To this, 20 ml ethylene glycol was introduced resulting in the formation of clear solution The clear solution was stirred for 15 min on a magnetic stirrer and transferred into Teflon-lined stainless steel autoclaves with a capacity of 50 ml, sealed and maintained at different tem-peratures (140–180 °C) for 1d under autogenously pressure The resulting white solid products were retrieved from the solution by centrifugation, washed with distilled water followed by ethanol to remove ions possibly in the end product andfinally dried in air
⁎ Corresponding author Tel.: +91 80 22961350.
E-mail address: gtchandrappa@yahoo.co.in (G.T Chandrappa).
0167-577X/$ – see front matter © 2009 Elsevier B.V All rights reserved.
Contents lists available atScienceDirect Materials Letters
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / m a t l e t
Trang 2Powder X-ray diffraction (PXRD) data were recorded on Philips X'pert PRO X-ray diffractometer with graphite monochromatized Cu
Kα radiation (λ = 1.541 Å) The Fourier transform infrared spectrum
of the sample was collected using Thermo Nicollet FTIR spectro-meter Scanning electron micrograph images were taken with JEOL (JSM-840 A) scanning electron microscope (SEM) The transmission electron microscopy (TEM) was performed with a Hitachi-H-8100 instrument (accelerating voltage up to 200 kV, LaB6 filament) equipped with EDS (Kevex Sigma TM Quasar, USA) Raman spectrum was recorded at room temperature with a confocal laser micro-Raman spectrometer (LABRM-HR) Photoluminescence studies were carried out on a Perkin–Elmer LS-55 luminescence spectrometer using Xe lamp with an excitation wavelength of 325 nm at room temperature
3 Results and discussion
Fig 1shows the PXRD pattern of theflower like ZnO prepared at
160 °C for 1 day All the diffraction peaks in the PXRD pattern can be Fig 1 PXRD pattern of the ZnO powder prepared at 140 °C for 1 day.
Fig 2 SEM images of the ZnO crystals prepared using (i) 20 ml ethylene glycol for 1 day at (a) 140 °C, (b) 160 °C (low magnified), (c) 160 °C (high magnified), (d) 180 °C and (ii) at
Trang 3Fig 3 TEM images of the ZnO rod at (a) low magnification, (b) high magnification and (c) EDS spectrum of the ZnO.
Fig 4 Growth schematic diagram of flower like ZnO architecture prepared (a) absence of ethylene glycol and (b) presence of ethylene glycol.
Trang 4indexed as the hexagonal phase of ZnO (wurtzite structure) without
contamination from other crystallite phases[25]
It has been reported that anhydrous zinc acetate, which was used as a
precursor in this work, can undergo decomposition in water-ethylene
glycol mixture and forms ZnO nuclei without forming any intermediates
[26] The possible reaction for the formation of ZnO is as follows
ZnðCH3COOÞ2→ZnO þ CH3COOCOCH3
The formation of ZnO was also confirmed by FTIR spectrum (not
shown) The broad band around 417 cm− 1 associated with the
characteristic vibrational mode of Zn\O bonding[27]
Fig 2shows the SEM images of ZnO products prepared at 140 °C,
160 °C and 180 °C for 1 day.Fig 2a exhibits well orderedflowers with
good symmetry Theseflowers made of 1D rods with typical diameter
of about 300–700 nm From theFig 2b–c one can observe the existence
of bothflowers as well as twinned crystals These twinned crystals are
the intermediates for the formation offlowers.Fig 2d reveals that the
bulk quantity of ZnO bunches exist Every bunch is composed of closely
packed submicrometer-sized rods with diameters of 350–600 nm and
lengths of 4–5 µm and forms radiating structures In addition to
bunches, we can also observe some submicrometer- sized rods with
sharp tips Flower like ZnO architectures were not obtained when the
quantity of ethylene glycol is reduced to less than 20 ml On the other
hand, the morphology of the ZnO is changed to hexagonal rods (Fig 2e
and f) when the quantity of ethylene glycol is reduced to 5–10 ml while
carrying out the experiment at 160 °C for 1 day
Fig 3a and b shows the TEM images of the single rod taken at
different magnification The sword like rod with size decreases from
one end to another exhibit rough surface as observed The diameter of
the rod is found to be in the range of 40–70 nm.Fig 3c presents the
EDX spectrum of the ZnO rod The quantitative data indicates that the
flowers are made of Zn and O with stoichiometric ratio of ~1:1 within
the experimental error and no other elements are found This result is
in good agreement with Raman spectroscopic and PL studies
In the present synthetic route, ethylene glycol is a key factor to
prepare ZnO flower like architecture, which is confirmed by the
results of experiments that did not use an ethylene glycol assisted
hydrothermal process
As a wurtzite-structured metal oxide, zinc oxide belongs to the p63mc
space group The (0001) and (000-1) planes of ZnO crystals are rich in Zn
(positive polar plane) and O (negative polar plane) respectively The
hydroxyl groups of ethylene glycol can adsorb on the positive polar plane
of ZnO by coulomb interaction The adsorption retards the growth along
(0001) plane resulting in the formation of polyhedral crystals[25] The
two polyhedral crystals are joined each other by the action of ethylene
glycol, forms twinned crystals Similar observation was made by Zang et al
in presence of polyvinyl alcohol[23] These twinned crystals having high
surface energy along negative polar plane Zinc acetate molecules will
adsorb on the negative polar plane resulting in a decrease of the surface
energy and further nucleation takes place along this direction The
adsorbed zinc acetate molecules will trigger the nucleation and promoting
the formation of rods around the twinned crystals The preparedflower
like architecture is different from the previous reports [22,23,28]
Randomly distributed ZnO rods are formed in the absence of ethylene
glycol The possible growth schematic diagram for the formation offlower
like architecture is shown inFig 4 In general, slow crystallization is
required to form products with a thermodynamically stable structure
because the crystallizing partners have time to recognize each other and
follow the lowest-energy path However, at 180 °C, the temperature is
high and fast crystallization takes place This fast crystallization often leads
to kinetically controlled products, such as unstable or metastable crystal
structures, and even defects can be formed during fast nucleation[29]
Hence, we believe that theflowers like ZnO architectures are preferably
formed from slow nucleation and growth Whereas, the ZnO bunches
is obtained from fast nucleation and growth process
Raman spectroscopy was carried out to study the vibrational properties of the ZnO crystals.Fig 5represents the Raman spectrum of the samples at the range of 250 to 600 cm− 1 The peaks at 437 cm− 1,
382 cm− 1and 411 cm− 1corresponds to E2(high), A1(TO) and E1(TO) of ZnO crystals respectively[30] The absence of E1(LO) peak (583 cm− 1) indicating good quality of the as prepared samples since the E1(LO) mode is associated with defects such as oxygen vacancy, zinc interstitials, or their complexes[31]
ZnO exhibited a wide band gap at room temperature with a large exciton binding energy, which is suitable for effective UV emission However, due to the poor crystal quality of the nanomaterials, i.e., high density of structural defects, the UV emission of nanoscaled ZnO is liable to be quenched and only defect emission in visible region is detected[32] This deficiency hinders progress for the applications of ZnO in optoelectronic and lasing devices The photoluminescence of the obtained ZnO crystals (Fig 6) exhibits strong UV emission at
~397 nm, which comes from recombination of exciton and no defect emission is detected[17]
4 Conclusions Flower like ZnO architectures were fabricated by hydrothermal method using zinc acetate in water-ethylene glycol solution at 140–
160 °C for 1 day Ethylene glycol is a key factor for the preparation of flower like architectures The ZnO products prepared by this method having good quality i.e., free from defects sites The synthesis method discussed in the present work opens a new approach for the fabri-cation of other metal oxides nanostructure
Acknowledgements The author G.T Chandrappa is thankful to the Department of Science and Technology, NSTI Phase-IV, New Delhi, Government of India forfinancial support to carryout the research We also thank Prof Sarala Upadhya the Department of Mechanical Engineering, UVCE, Bangalore, for recording SEM images
References [1] Taubert S Angew Chem Int Ed Engl 2004;43:5380.
[2] Iijima S Nature 1991;354:56.
[3] Srivastava A, Srivastava ON, Talapatra S, Vajtai R, Ajayan PM Nat Matters 2004;3:610 [4] Wang ZL J Phys Condens Matter 2004;16:R829.
[5] Yan CH, Zhang J, Sun LD In: Nalwa HS, editor Encyclopedia of Nanoscience and Nanotechnology, vol 10 Los Angeles: American Scientific Publishers; 2004 p 767 [6] Lao JY, Huang JY, Wang DZ, Ren ZF Nano Lett 2003;3:235.
[7] Zhang H, Yang D, Li D, Ma X, Li S, Que D Cryst Growth Des 2005;5:547 [8] Yao BD, Chan YF, Wang N Appl Phys Lett 2002;81:757.
[9] Ghosh M, Seshadri R, Rao CNR J Nanosci Nanotech 2004;4:136.
[10] Hu JQ, Li Q, Meng XM, Lee CS, Lee ST Chem Mater 2003;15:305.
[11] Lao JY, Huang JY, Wang DZ, Ren ZF, Steeves D, Kimball B, et al Appl Phys A 2004;78:539 [12] Lao JY, Huang JY, Wang DZ, Ren ZF Nano Lett 2003;3:235.
[13] Kong XY, Wang ZL Nano Lett 2003;3:1625.
[14] Kong XY, Ding Y, Yang R, Wang ZL Science 2004;303:1348.
[15] Lao JY, Wen JG, Ren ZF Nano Lett 2002;2:1287.
[16] Gao PX, Wang ZL J Am Chem Soc 2003;125:1299.
[17] Zhang J, Sun L, Yin J, Su H, Liao C, Yan C Chem Mater 2002;14:4172.
[18] Zhang H, Yang D, Ji Y, Ma X, Xu J, Que D J Phys Chem B 2004;108:3955 [19] Wang Z, Qian XF, Yin J, Zhu ZK Langmuir 2004;20:3441.
[20] Tian ZR, Voigt JA, Liu J, McKenzie B, McDermott MJ J Am Chem Soc 2002;124:12954 [21] Zhang J, Sun L, Yin J, Su H, Liao C, Yan C Chem Mater 2002;14:4172.
[22] Liu B, Zeng HC J Am Chem Soc 2004;126:16744.
[23] Zhang H, Yang D, Li D, Ma X, Li S, Que D Cryst Growth Des 2005;5:547 [24] Nagaraju G, Tharamani CN, Chandrappa GT, Livage J Nanosci Res Lett 2007;2:461 [25] Kunjara S, Ayudhya N, Tonto P, Mekasuwandumrong O, Pavarajarn V, Praserthdam
P Cryst Growth Des 2006;6:2446.
[26] Arii T, Kishi A Thermochim Acta 2003;400:175.
[27] Gu F, Wang SF, Lu MK, Zhou GJ, Xu D, Yuan DR Langmuir 2004;20:3528 [28] Gao X, Li X, Yu W J Phys Chem B 2005;109:1155.
[29] Yang J, Qin Y, Yang S Cryst Growth Des 2007;7:2561.
[30] Peng W, Qu S, Cong G, Wang Z Cryst Growth Des 2006;6:1518.
[31] Pradhan AK, Zhang K, Loutts GB, Roy UN, Cui Y, Burger A J Phys Condens Matter 2004;16:7123.