Flower shaped homocentric pencil like ZnO nanorod bundles: Synthesis, characterisation and study of their photocatalytic activity

Article abstract of DOI:10.1039/c3ra45818k

Flower shaped homocentric pencil like ZnO nanorod bundles are synthesized by a hydrothermal method. The PXRD pattern corresponds to the typical wurtzite structures of ZnO, which exhibits excellent activity as a photocatalyst for the degradation of malachite green under solar light irradiation.

Full text of DOI:10.1039/c3ra45818k

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Flower shaped homocentric pencil like ZnO
nanorod bundles: synthesis, characterisation and
Cite this: RSC Adv., 2014, 4, 8256
study of their photocatalytic activity†
a
a
b
a
Received 14th October 2013
Accepted 2nd December 2013
*
Diganta Bhuyan, Banajit Malakar, Sudhir S. Arbuj and Lakshi Saikia
DOI: 10.1039/c3ra45818k
www.rsc.org/advances
Flower shaped homocentric pencil like ZnO nanorod bundles are temperature or accurate gas concentration, ow rate or costly
synthesized by a hydrothermal method. The PXRD pattern corre- raw materials, or a complex process, and so on. It is therefore
sponds to the typical wurtzite structures of ZnO, which exhibits important to have a simple, cost effective and low temperature
excellent activity as a photocatalyst for the degradation of malachite method for the synthesis of ZnO nanomaterials.
green under solar light irradiation.
Herein, the ower shaped homocentric pencil like ZnO
nanorod bundles were synthesized using the procedure
employed for the synthesis of nanotubes, taking PEG-4000 as a
template at a pH of about 9.7. The hydrothermal reaction was
carried out at 110 C in a Teon lined steel autoclave for 16 h.
The PXRD pattern of the ZnO owers is shown in Fig. 1. All
diffraction peaks can be indexed as a wurtzite structure of ZnO
Control of the morphology and size of particulate materials have
received more attention recently due to the fact that they play
very important roles in determining magnetic, optical, electrical
and other properties.1 One-dimensional nanostructures such as
nanowires, nanotubes, nanobelts, and nanorods have attracted
great interest because of their novel physical properties as well
as their potential applications in constructing nanoscale elec-
tronic and optoelectronic devices.2 The wide direct band gap
(ꢀ3.37 eV) and large exciton binding energy (ꢀ60 meV) make
ZnO an excellent optoelectronic material.3 ZnO has also been
conrmed as a promising functional material in other nano-
devices such as eld emitters4 and gas sensors.5 The synthesis of
ZnO nanostructures is therefore currently attracting intense
worldwide interest. Numerous ZnO nanostructures have been
demonstrated, for example, nanowires,6 nanotubes,7 nanobelts,8
nanopropellers,9 and nanocages.10 A variety of methods have
been employed for the synthesis of ZnO nanomaterials such as
high-temperature metalorganic chemical vapor deposition
(MOCVD) and thermal evaporation processes.11 These processes
require either a high vacuum and temperature or some catalysts.
Other methods generally applied to synthesize ZnO nano-
structures include electrochemical deposition techniques,12
hydrothermal methods,13 sputter deposition techniques14 and
carbothermal reduction methods.15 However, these methods
generally require harsh reaction conditions, such as high
ꢁ
˚
˚
(JCPDS 36-1451, a ¼ 3.24982 A, c ¼ 5.20661 A). No peak attrib-
utable to possible impurities is observed. The sharp diffraction
peaks indicate that the as-synthesized ZnO nanostructure has
high crystallinity. The intensity of the ZnO (002) peak is much
stronger than that of bulk ZnO, revealing [0001] growth orien-
tation of the ZnO owers and indicating that the synthesized
ZnO is a 1D structure.
The wurtzite-structured ZnO crystal is described schemati-
cally as a number of alternating planes composed of tetrahe-
drally coordinated O2ꢂ and Zn2+ ions, stacked alternately along
the c-axis. The oppositely charged ions produce positively
ꢀ
charged (0001)-Zn and negatively charged (0001)-O polar
aMaterials Science Division CSIR-North East Institute of Science and Technology,
Jorhat-785006, Assam, India. E-mail: lakshi_saikia@yahoo.com
bCentre for Materials for Electronics Technology, Off Pashan Road, Panchwati, Pune-
411008, India
† Electronic supplementary information (ESI) available: Synthesis procedure, PL,
FT-IR and UV-Vis spectra. See DOI: 10.1039/c3ra45818k
Fig. 1 PXRD pattern of the as-synthesized ZnO flowers.
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surfaces,16 and the Zn-terminated Zn-(0001) polar surface is
ꢀ
chemically active whether the oxygen-terminated (0001) polar
surface is inert or not.17 In this report, the formation of nano-
rods may be due to the seed growth of ZnO nuclei on the rod
shaped PEG micelle, and the sharp pencil like tip was developed
because PEG micelles were attached to the nonpolar faces of the
hexagonal ZnO nanocrystal, which allowed growth of the
nanorod only along the [0001] direction (Scheme S2 & S3, ESI†).
Subsequently, as time goes on, the non polar–non polar inter-
action dominates between the surface attached PEG, so that the
ZnO nanorods are aggregated and form nanorod bundles.
These ZnO nanorod bundles then grow in several [0001] direc-
tions forming a homocentric ower like structure aer 16 h of
hydrothermal treatment.
Fig. 2A–C show the FESEM images of the synthesized ZnO.
Flower-like clusters grow in several directions and have
symmetrical arms in level directions. Every arm consists of a few
nanorods with almost uniform diameters and an average length
Fig. 3 Photodegradation of malachite green using ZnO flowers as a
photocatalyst.
of about 3 mm. Fig. 2B shows that the nanorod bundles are structure PEG-4000 in this report. The ZnO nuclei are obtained
2ꢂ
2+
highly disperse within the space, without any aggregation by the dehydration of either Zn(OH)4 or Zn(NH3)4 in the
between them, and have approximately uniform morphologies presence of NH3 (Scheme S1, ESI†). On the other hand, in the
2+
with diameters of 250–300 nm and lengths up to 2 mm.
presence of PEG-4000, the growth units Zn(OH)42ꢂ or Zn(NH3)4
Fig. 2C shows that the ZnO ower is composed of symmet- are easily adsorbed by the O atom in the C–O–C chain, so that
2ꢂ
2+
rical nanorod bundles extending radially from the centre. Every Zn(OH)4 or Zn(NH3)4 can be carried and transformed into
rod has a sharp pencil like tip, in comparison with its base, and ZnO crystalline particles and grow on active sites around the
ꢁ
a length of up to 2 mm. Fig. 2D shows the TEM image of ZnO in surface of the ZnO nuclei. At a temperature of about 80 C, a
an individual nanorod bundle, which suggests that the inner greater amount of NH3 is produced and Zn2+ is transformed into
part of the rods are not hollow and that the tip is sharp. Fig. 2A & ZnO nuclei at a very fast crystal growth rate. Thus ZnO nuclei
B shows the FESEM images of ZnO nanorod bundle growth aer aggregate spontaneously with hexagonal symmetry, which
8 h hydrothermal treatment and Fig. 2C shows the FESEM provides a chance for the formation of ZnO nanorod bundles.
image of the ZnO nanorod bundles arranged in a owery The formation of ZnO is also dependent on pH. When the
pattern aer 16 h of hydrothermal treatment. It can therefore be concentration of ammonia in the system is low (less than
observed, from the FESEM images, that 16 h of hydrothermal pH ¼ 9.7), more Zn(NH3)42+ are easily transformed into ZnO in
treatment is the optimal time required for the growth of ZnO the function of weak OHꢂ. Therefore, abundant ZnO nuclei are
nanorod bundles to arrange in a owery pattern.
formed in the initial stages, which is a possible reason for the
The formation of ZnO nanorod bundles is mainly affected by formation of some shorter nanorods. There are more EO groups
2ꢂ
2+
the particular hydrothermal temperature, basic pH and micelle in PEG-4000 than PEG-2000, so more Zn(OH)4 or Zn(NH3)4
growth units were adsorbed by the O atom in the C–O–C chain of
PEG-4000 and the fast crystal growth rate of ZnO resulted in the
formation of ZnO nanorod bundles rather than ZnO nanotubes.
PEG-4000 is therefore used as a template for the synthesis of ZnO
nanorod bundles; it is shown in Fig. S3 of ESI† that PEG micelles
are attached to the non-polar facet of the ZnO crystal which
allows for the growth of individual ZnO nanorods in the (0001)
polar direction. Due to the hydrophobic interaction existing
between these surface attached PEG micelles, the individual ZnO
nanorods aggregate and form nanorod bundles aer 8 h of
hydrothermal treatment. Aer 16 h of hydrothermal treatment,
these ZnO nanorod bundles aggregate at the bottom in a ower
like pattern. This is due to the hydrophobic interactions between
the surface attached PEG in individual ZnO nanorod bundles,
which arranges in a ower like shape due to the same hydro-
phobic interactions. The growth is therefore unidirectional and
attachment occurs at the bottom.
The room temperature photoluminescence spectra have a
strong UV emission of 390 nm, a weak blue emission of 468 nm
and a broad green-yellow emission of 550 nm (Fig. S1, ESI†). The
Fig. 2 (A & B) Growth of ZnO flowers (8 h), (C) ZnO flowers (16 h) and
(D) TEM image of ZnO in an individual nanorod bundle.
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green emission at 550 nm originates from the deep level (DL) material may also be applied for the degradation of other
defect emission associated with oxygen vacancies in ZnO hazardous dyes found in industrial waste and dye contaminated
lattices.18 The blue emission may be from zinc ion vacancies and water. Further modications, like doping noble metals, which
interstitial zinc defects.19 The green-yellow emission is stronger may improve its photocatalytic activity, require further research.
in comparison with the blue emission. FTIR spectra of the as-
We would like to thank the Department of Science and
synthesized ZnO owers have broad bands around 3429 cmꢂ1 Technology, New Delhi for the nancial support (GPP-0267). We
and 1589 cmꢂ1, which are due to the surface adsorbed O–H are also grateful to Dr R. C. Boruah, Acting Director, Dr D. K.
stretching and bending mode of the vibration respectively Dutta, Chief Scientist, Dr Pinaki Sengupta, Head and Chief
(Fig. S2, ESI†). The sharp band at 419 cmꢂ1 is a characteristic Scientist, Materials Science Division and Mr Ananta Kr Sarma,
vibration mode of Zn–O bonding. The UV-Visible spectra of the Biotechnology Division, CSIR-North East Institute of Science
ZnO owers have a UV absorption peak at 388 nm and a very and Technology, Jorhat for their support.
broad visible absorption peak (Fig. S3, ESI†). The Brunauer–
Emmett–Teller (BET) specic surface area of the material
measured by a N2 adsorption–desorption technique is 26.94 m2
Notes and references
gꢂ1, with a hysteresis loop of type IV isotherm (Fig. S4, ESI†)
revealing the existence of mesopores in the architecture. The
corresponding pore diameter distribution curves (inset in
Fig. S4†) show that the size of the mesopores is not uniform but
hierarchically distributed, and we mainly attribute the larger size
of the mesopores to the explosive release of CO2 in the conned
space of the smaller mesopores during the thermal process. It is
reasonable that the release of CO2 and the collapse of the
interconnected mesopores enlarge and change the smaller
mesopores into the irregularly shaped larger mesopores.20
The synthesized ZnO owers were efficiently tested as an
excellent photocatalyst for the self sensitized photodegradation
of a cationic triphenylmethane dye, Malachite Green (MG),
under solar light. The amount of degradation of MG was almost
94% of the initial dye concentration of 0.2 g Lꢂ1 and the initial
catalyst amount was 0.02 g in 100 mL solution as shown in
Fig. 3.
The possible reason for its excellent photocatalytic activity
may be the high surface oxygen vacancies in the ZnO ower
conrmed from its room temperature photoluminescence
spectra. These high surface oxygen vacancies may allow it to
adsorb more O2 from the water resulting in the formation of a
greater number of reactive radicals like O2c, OHc which
contribute to its excellent photocatalytic activity.
Photodegradation of the dye leads to complete mineraliza-
tion under solar light which was conrmed by qualitatively
measuring the CO2 production using Warburg apparatus. The
Warburg manometric method is generally used for biological
samples to measure gas absorption as well as gas production.21
We took 4 mL of 0.2 g Lꢂ1 MG dye and 0.002 g catalyst initially
for the experiment. The evolved gas was conrmed to be CO2 as
a BaCO3 precipitate was formed by adding BaCl2 solution to the
KOH solution, removed from the centre wells of the Warburg
vessels, using CO2 free water.
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