Nanostructural zinc oxide hollow spheres: A facile synthesis and catalytic properties

Article abstract of DOI:10.1016/j.ica.2010.06.048

The development of reproducible procedures for the synthesis and organization of nanostructured metal oxides is important in order to exploit the unique properties of these materials for practical applications. The present work describes the transformation of Zn(NH₃)₄] ²⁺ into hollow structured ZnO materials through solvothermal decomposition. An increase in ammonia concentration in the reaction medium, significantly changes the morphology of ZnO from spheres made of nanoparticles (20-30 nm) to hollow spheres composed of nanorods (200-350 nm) or to free microrods as evidenced from scanning and transmission electron micrographs (SEM/TEM). The powder X-ray diffraction (XRD) pattern of ZnO confirms formation of the wurtzite structure. Raman and Energy-dispersive spectroscopic (EDS) studies indicate the presence of oxygen deficiency in ZnO. The investigation on the catalytic behavior of ZnO in the synthesis of (4-methoxyphenyl)(phenyl) methanone (MPPM) by Friedel-Crafts acylation of anisole with benzoyl chloride has also been carried out. The results reveal that the prepared ZnO could produce ～98% of yield compared to 41% produced by commercial ZnO.

Full text of DOI:10.1016/j.ica.2010.06.048

Inorganica Chimica Acta 363 (2010) 3442–3447
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Nanostructural zinc oxide hollow spheres: A facile synthesis and catalytic properties
Siddaramanna Ashoka a, Pallellappa Chithaiah a, Kumarappa Veerappa Thipperudraiah b,
Gujjarahalli Thimmanna Chandrappa a,
*
a Department of Chemistry, Bangalore University, Bangalore 560 001, India
b Department of Physics, Ambedkar First Grade College, Bangalore 560 008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The development of reproducible procedures for the synthesis and organization of nanostructured metal
oxides is important in order to exploit the unique properties of these materials for practical applications.
The present work describes the transformation of Zn(NH3)4]2+ into hollow structured ZnO materials
through solvothermal decomposition. An increase in ammonia concentration in the reaction medium,
significantly changes the morphology of ZnO from spheres made of nanoparticles (20–30 nm) to hollow
spheres composed of nanorods (200–350 nm) or to free microrods as evidenced from scanning and trans-
mission electron micrographs (SEM/TEM). The powder X-ray diffraction (XRD) pattern of ZnO confirms
formation of the wurtzite structure. Raman and Energy-dispersive spectroscopic (EDS) studies indicate
the presence of oxygen deficiency in ZnO. The investigation on the catalytic behavior of ZnO in the syn-
thesis of (4-methoxyphenyl)(phenyl) methanone (MPPM) by Friedel–Crafts acylation of anisole with ben-
zoyl chloride has also been carried out. The results reveal that the prepared ZnO could produce ꢀ98% of
yield compared to 41% produced by commercial ZnO.
Received 1 January 2010
Received in revised form 25 June 2010
Accepted 28 June 2010
Available online 31 July 2010
Keywords:
Chemical synthesis
Ethylene glycol
ZnO
Hollow spheres
Catalysis
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
duced shells remain more or less accidental, require high energies
or complex set-ups. The self-assembly of ZnO nanoparticles into
ZnO, a polar inorganic material has been known for its many
interesting physical and chemical properties, and for its potential
applications in different areas including solar cells, sensors, varis-
tors, UV lasers, surface acoustic wave devices, catalysis, and
adsorption [1–7]. As a consequence, ZnO nanostructures with dif-
ferent morphologies, such as nanobelts, nanocombs, nanohelix,
nanocables, hollow urchins, nanoplatelets, nanorings, nanosprings,
and hierarchical nanostructures have been successfully fabricated
with the aim of expanding the scope of its applications [8]. There
is however, another important class of zinc oxide materials with
interesting functionalities which merits special attention hollow
spheres. In recent times, attention has been focused on the synthe-
sis of ZnO hollow spheres in view of their potential for applications
in the delivery of drugs, catalysis, chemical storage, microcapsule
reactors and photoelectric materials [9]. Several methods have
been developed for the fabrication of sub-micrometer hollow
spheres of ZnO materials. For instance, ZnO hollow structures can
be prepared by oxidation of vapor-deposited zinc, laser-assisted
growth on the surface of ethanol droplets as templates, chemical
vapor deposition on carbon nanotubes and pyrolysis [10–14]. All
these processes, in which the shape, size and quality of the pro-
hollow structures is also made by solvothermal method [15] using
coordination polymer, [Zn(4,41-bipy)(NCS)2]n. However, this meth-
od requires prior synthesis of coordination polymer by a separate
process. Recently, ZnO hollow spheres prepared using the sacrifi-
cial template route [16–19] has been reported. This method ap-
pears to be complicated since, first, a coating of the preferred
shell material is formed on a template of the desired shape, and
subsequently the template is subjected to heat treatment or dis-
solves in a suitable solvent to make the template-free hollow
ZnO material.
In wet chemical synthesis, water is generally employed as a
reaction medium to facilitate the production of ZnO in various
morphologies. In conventional hydrothermal synthesis, water can
adversely affect synthesis of the desired compounds due to the
sensitivity of some precursor to hydrolysis [20]. The solvothermal
method therefore, attracts many researchers for the preparation of
various nano/micro structural materials by virtue of its unique
reaction environment [7]. Organic solvents used in the solvother-
mal process, can effectively prevent the agglomeration and control
the morphology development of the crystals. Crystal growth is sig-
nificantly modified by physico-chemical properties of the solvent
such as polarity, viscosity, softness and dielectric constant [21].
In this paper, we report a facile synthesis of tunable ZnO hollow
spheres with the assistance of ethylene glycol and ammonia under
* Corresponding author. Tel.: +91 80 22961350.
E-mail address: gtchandrappa@yahoo.co.in (G.T. Chandrappa).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.06.048

S. Ashoka et al. / Inorganica Chimica Acta 363 (2010) 3442–3447
3443
mild conditions. The catalytic activity of the resulting ZnO hollow
spheres for Friedel–Crafts acylation of anisole with benzoyl chlo-
ride has also been carried out.
2500
2000
1500
1000
500
b
2. Experimental
2.1. Materials
Zinc acetate dihydrate [Zn(CH3COO)2ꢁ2H2O], ethylene glycol
(C2H6O2), ammonia solution (NH4OH, 35.5%) and zinc oxide
(ZnO) were purchased from Merck Limited and used without fur-
ther purification. Double distilled water was used throughout the
experiments.
0
500
400
300
200
100
0
a
2.2. Synthesis of ZnO materials
As in a typical solvothermal synthesis, 0.5 g zinc acetate
(2.27 mmol) was dissolved in 13 ml ethylene glycol. To this,
ammonia solution (0.5–2 ml) was added in drops resulting in the
formation of [Zn(NH3)4]2+ clear solution. The clear solution was
stirred for 10 min on a magnetic stirrer and transferred into Tef-
lon-lined stainless steel autoclaves with a capacity of 20 ml. Auto-
claves were sealed and kept at 160 °C for 1 day under autogenous
pressure. The resulting yellowish white solid was retrieved by cen-
trifugation. The solid product was washed with distilled water fol-
lowed by ethanol and finally dried at 70 °C for 2 h in an electric
oven.
10
20
30
40
50
60
70
2 Theta (degree)
Fig. 1. PXRD pattern of the (a) hollow structured ZnO and (b) commercial ZnO
powder.
broadening, which occurs when a sample is made up of very small
crystallites [22].
4.2. X-ray photoelectron spectroscopy
2.3. Catalytic property
The Zn 2p XPS spectra shown in Fig. 2a consist of Zn 2p1/2 and
2p3/2 doublet peak. The Zn 2p3/2 line of the zinc oxides is generally
observed at a lower binding energy of 1022.3 eV and the Zn 2p1/2
peak at 1045.7 eV. The O1s spectrum (Fig. 2b) of the zinc oxide
contains four peaks originating from oxide, hydroxides and ad-
sorbed water. A low intense peak at 528.6 eV along with a high in-
tense peak at 530.4 eV can be attributed to O2ꢂ type of species
associated with oxide of Zn, which is in the form of ZnO. The peaks
at 531.8 and 533.2 eV arise mainly from the hydroxide groups of
Zn(OH)2 and adsorbed water, respectively. A comparison of the rel-
ative intensities of the four O(1s) peaks shows that the amount of
oxide associated with Zn is more. Taking this fact into consider-
ation along with the PXRD observation, the most probable species
could be the ZnO [23].
Benzoyl chloride (0.1 mmol) was added to a mixture of solvo-
thermally derived ZnO and anisole (0.1 mmol) taken in dichloro
methane at room temperature and stirred for 20 min on a magnetic
stirrer in order to complete the reaction. After separation of the
catalyst by filtration, the dichloromethane containing the product
was then washed with an aqueous solution of sodium bicarbonate
and dried over anhydrous sodium sulfate. The product MPPM was
obtained on evaporation of the solvent. 1H NMR and LCMS spectral
studies were carried out on the product for identification.
3. Materials characterization
Powder XRD data were recorded on Philips X’pert PRO X-ray
diffractometer with graphite monochromatized Cu K
a radiation
(k = 1.541 Å). The X-ray photoelectron spectroscopy (XPS) of ZnO
was examined using ultra-high vacuum set-up equipped with a
high resolution Gammadata-Scienta SES 2002 analyzer. A mono-
4.3. Mechanism and morphology of ZnO
Most of the transition metals form complexes with ammonia in
aqueous solution [24]. The [Zn(NH3)4]2+ complex is formed when
ammonia solution is added to zinc acetate–ethylene glycol solution
at room temperature (Eq. (1)). During solvothermal treatment,
[Zn(NH3)4]2+ complex undergoes dissociation to ZnO [25] as desig-
nated in Eq. (2)
chromatic Al K
a X-ray source (1486.3 eV; anode operating at
14.5 kV and 50 mA) was used as incident radiation and pass energy
of 200 eV was chosen. The morphology of the ZnO was examined
by JEOL-JSM–6490 LV scanning electron microscope and CM12
Philips transmission electron microscope equipped with EDS (Ke-
vex Sigma TM Quasar, USA). Raman spectrum was recorded at
room temperature with a confocal laser micro-Raman spectrome-
ter (LABRMHR).
2þ
Zn2þ þ 4NH3 ꢀ ½ZnðNH3Þ ꢃ
ð1Þ
ð2Þ
4
½ZnðNH3Þ ꢃ2þ þ 2OHꢂ ꢀ ZnO þ 4NH3 þ H2O
4
4. Results and discussion
The growth of ZnO depends mainly on its internal structure as
well as external parameters such as temperature, complexing
agents, capping agents and pH of the solution [26]. Regarding the
formation of ZnO hollow structures, micro-bubbles of NH3 gas pro-
duced in the reaction system with the participation of NH4OH must
have played a crucial role, since no other templates, surfactants or
emulsion were used. NH4OH can be easily dissolved in ethylene
glycol to give a basic environment (Eq. (3)), while NH3 can also
be removed from water–ethylene glycol solution on boiling due
4.1. Powder X-ray diffraction
Fig. 1a and b shows the PXRD patterns of the ZnO hollow
spheres and commercial ZnO, respectively. All the diffraction peaks
can be indexed as the hexagonal phase of pure wurtzite ZnO struc-
ture. The ZnO hollow spheres have much broader and less intense
peaks compared to the commercial ZnO powder due to particle size

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S. Ashoka et al. / Inorganica Chimica Acta 363 (2010) 3442–3447
a
2p
O1s
Zn 2p
900
800
700
600
500
400
3/2
b
400
2p
1/2
360
320
280
240
1010
1020
1030
1040
1050
1060
538
536
534
532
530
528
526
Binding energy / eV
Binding energy / eV
Fig. 2. XPS spectra of (a) Zn (2p) and (b) O (1s).
to the difference in boiling point. Hence, there exists equilibrium
(Eq. (4)) when NH4OH is maintained at 160 °C in an airtight auto-
clave [15].
belongs to the C6V (P63mc) space group [35]. According to group
theory, Uopt = A1 + 2B1 + E1+2E2 vibrational modes are possible
[36,37]. Among these vibrational modes, A1, E1, and E2 are Raman
active and B1 is Raman forbidden. In addition, A1 and E1 are in-
fra-red active and can split into longitudinal and transverse optical
components. The sharp band at 432 cmꢂ1 is attributed to the ZnO
nonpolar optical phonons high E2 vibration mode, which corre-
sponds to the characteristic band of the wurtzite phase. The band
at 383 cmꢂ1 corresponds to A1 symmetry with the TO model. The
bands at 208 and 333 cmꢂ1 observed in the spectrum are due to
the second modes. In addition, a sharp and intensive band centered
at 98 cmꢂ1, can be assigned to the E2 low vibration mode [37]. The
vibration mode at 570 cmꢂ1 is LO modes with E1 symmetry of ZnO.
It is believed that the appearance of the E1(LO) mode generally re-
sults from impurities and structural defects [38], which is in accor-
dance with the results obtained from EDS.
NH4OH ꢀ NH4þ þ OHꢂ
NH4OH ꢀ NH3 ðgÞ þ H2OðlÞ
ð3Þ
ð4Þ
The presence of ethylene glycol has a significant influence on
the morphology of the ZnO product. Here, ethylene glycol serves
two purposes: advocating the formation of ZnO crystals and
adsorbing on the polar surface of ZnO [27]. At a low concentration
of ammonia (0.5 ml), the two hydroxyl groups of ethylene glycol
could effectively attach to the (0 0 0 1) surface of ZnO crystals
due to the slightly positive charged Zn surface. This interaction
could inhibit crystal growth along (0 0 0 1) direction and lead to
the formation of ZnO nanoparticles (Fig. 3a). The nanoparticles
aggregated into spheres to minimize the interfacial energy
[28,29] resulting in the formation of densely packed spherical
4.5. Energy-dispersive spectroscopy
structures of diameter 4–5 lm. As the concentration of ammonia
increases to 1 ml, one can clearly observe the existence of a hole.
The ZnO nuclei aggregate together around the gas–liquid interface,
and finally hollow nanorod-based spheres formed [15]. The size of
The chemical purity of ZnO as well as its stoichiometry was
investigated by EDS analysis. The EDS spectrum (Fig. 6) clearly
indicates that the hollow spheres are composed of only Zn, and
O, in addition to the Cu signal arising from the copper TEM grid.
Elemental analysis reveals that the atomic ratio of Zn:O is
55.3:44.7, indicating the presence of oxygen deficiency, which
might have been caused during the rapid formation of ZnO under
solvothermal conditions. These crystal defects are important
parameters for the materials to exhibit a good catalytic activity.
the hole is found to be in the range of 0.8–1.2 lm. The ZnO nano-
rods (200–350 nm) are assembled around the hole (Fig. 3b). During
this process, new nuclei can be continually produced and adhere to
(0 0 0 1) surface of ZnO particles because of limited diffusion in the
presence of ammonia [30]. This favors secondary growth along
(0 0 0 1) plane of ZnO and results in the formation of nanorods.
On further increase in ammonia concentration to 1.5 ml, one can
observe the large number of hollow spheres made of well-arranged
nanorods (Fig. 3c–e). These hollow spheres resemble pot morphol-
ogy (Fig. 3d). The ZnO hollow structures have many advantages
over the previous reports in terms of product quality (better mor-
phology control, good monodispersity and high yield) and reaction
conditions (lower temperatures and pressures), and formation
mechanism [31–34]. Hollow spheres disappear and free nanorods
appear when the quantity of ammonia is increased to 2 ml (Fig. 3f).
Fig. 4 shows the TEM images and corresponding selected-area
electron diffraction (SAED) pattern of ZnO spheres prepared at
160 °C for 1d using 0.5 ml ammonia solution. The ZnO spheres
were made up of nanoparticles with a diameter of 20–30 nm which
can be clearly observed in Fig. 4a and b. All of these nanoparticles
are single crystalline in nature and can be indexed as the hexagonal
ZnO phase as evidenced from the SAED pattern (Fig. 4c). This result
is consistent with the XRD results.
4.6. Catalytic property
Friedel–Crafts alkylation and acylation of aromatic hydrocar-
bons have been studied extensively using Lewis acid catalysts,
such as, BF3, AlCl3, FeCl3, TiCl4 and protonic acid such as HF,
H2SO4 [39]. But, the use of conventional Lewis and mineral acid
catalysts has led to environmental problems, especially in large-
scale production sites. Hence, there is a need to find newer catalyst
systems to replace conventional acids for the Friedel–Crafts reac-
tions. Several solid materials such as alumina, silica gels and clays
have commonly been used in surface organic chemistry for the
preparation of various useful organic compounds [40].
Nanomaterials are expected to have significant differences from
their bulk counterparts in both chemical reactivity and physical
properties. In order to understand the catalytic properties of the
ZnO materials, Friedel–Crafts acylation of anisole and benzoyl
chloride (Fig. 7) was carried out and the results are summarized
in Table 1. The conversion of the ZnO hollow spheres is ꢀ98% in
20 min as compared to the commercial ZnO (ꢀ 41%) and the results
are reported by Du et al. [41]. It is also interesting to note that the
conversion rate of regenerated ZnO hollow sphere is ꢀ95%. The
4.4. Raman spectroscopy
Raman spectroscopy was carried out to study the vibrational
properties of the ZnO crystals. Fig. 5 represents the Raman spec-
trum of the samples in the range of 80–1000 cmꢂ1. Wurtzite ZnO

S. Ashoka et al. / Inorganica Chimica Acta 363 (2010) 3442–3447
3445
Fig. 3. SEM images of the ZnO products prepared using (a) 0.5, (b) 1, (c–e) 1.5 and (f) 2 ml.
Fig. 4. (a and b) TEM images of the hollow structured ZnO and (c) SAED pattern of the ZnO.

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S. Ashoka et al. / Inorganica Chimica Acta 363 (2010) 3442–3447
using any templates. In this method, ethylene glycol serves only
E2(low)
as a capping agent but ammonia acts as both complexing and cap-
ping agent. The preliminary measurement of the catalytic proper-
ties suggests that the obtained ZnO materials could have
potentially valuable applications in Friedel–Crafts acylation for
the synthesis of industrially important and useful organic
compounds.
E2(high)
Acknowledgements
A1(TO)
2ndorader
The authors acknowledge the financial support from the
Department of Science and Technology, NSTI Phase-IV, New Delhi,
Government of India. We also thank Prof. Sarala Upadhya, Depart-
ment of Mechanical Engineering, UVCE, Bangalore, for recording
the SEM images.
E1(LO)
200
400
600
800
1000
Raman shift (cm-1)
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Fig. 5. Raman spectrum of hollow structured ZnO.
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