Microwave assisted chemical bath deposition of vertically aligned ZnO nanorods on a variety of substrates seeded by PVA–Zn(OH)2nanocomposites

Abstract

Applied Surface Science 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 / a p s u s c Microwave assisted chemical bath deposition of vertically aligned ZnO nanorods on a variety of substrates seeded by PVA–Zn(OH) 2 nanocomposites Keywords: ZnO nanorods PVA–Zn(OH)2 nanocomposites MA–CBD Photoluminescence XRD FESEM a b s t r a c t Vertically aligned zinc oxide (ZnO) nanorods were synthesized successfully on p-type GaN, c-lattice Al 2 O 3 , ITO glass, and quartz single crystal substrates using the microwave-assisted chemical bath deposition method. All substrates were seeded with a PVA–Zn(OH) 2 nanocomposites layer prior to nanorods growth. The effect of substrate type on the vertically alignment and morphology of the zinc oxide nanorods was studied. The diameter of the grown ZnO nanorods ranged from 30 to 170 nm. Their structural quality and morphology were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), which revealed hexagonal wurtzite structures perpendicular to the substrate along the z-axis in the direction of (0 0 2) plane. Photoluminescence (PL) measurements of the grown ZnO nanorods on all substrates exhib-ited high UV peak intensity compared to broad visible peak. Raman scattering studies were conducted to estimate the lattice vibration modes.

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Applied Surface Science 258 (2012) 4467–4472
Contents lists available at SciVerse ScienceDirect
Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
Microwave assisted chemical bath deposition of vertically aligned ZnO nanorods
on a variety of substrates seeded by PVA–Zn(OH)2nanocomposites
J.J. Hassana,b,∗, M.A. Mahdia,b, C.W. China, Z. Hassana, H. Abu-Hassana
aNano-Optoelectronics Research and Technology Laboratory (N.O.R.), School of Physics Universiti Sains Malaysia, Penang 11800, Malaysia
bDepartment of Physics, College of Science, University of Basrah, Basrah, Iraq
a r t i c l e i n f o
Article history:
Received 7 September 2011
Received in revised form
31 December 2011
Accepted 1 January 2012
Available online 16 January 2012
Keywords:
ZnO nanorods
PVA–Zn(OH)2nanocomposites
MA–CBD
Photoluminescence
XRD
FESEM
a b s t r a c t
Verticallyalignedzincoxide(ZnO)nanorodsweresynthesizedsuccessfullyonp-typeGaN,c-latticeAl2O3,
ITO glass, and quartz single crystal substrates using the microwave-assisted chemical bath deposition
method.AllsubstrateswereseededwithaPVA–Zn(OH)2nanocompositeslayerpriortonanorodsgrowth.
The effect of substrate type on the vertically alignment and morphology of the zinc oxide nanorods was
studied. The diameter of the grown ZnO nanorods ranged from 30 to 170nm. Their structural quality and
morphologyweredeterminedbyX-raydiffraction(XRD)andscanningelectronmicroscopy(SEM),which
revealed hexagonal wurtzite structures perpendicular to the substrate along the z-axis in the direction of
(002) plane. Photoluminescence (PL) measurements of the grown ZnO nanorods on all substrates exhib-
ited high UV peak intensity compared to broad visible peak. Raman scattering studies were conducted
to estimate the lattice vibration modes.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Research on nanomaterials which involves the synthesis and
characterization of diverse nanostructure shapes revealed novel
properties that differ from their bulk nature. Nanorods are one
of the most important 1-dimensional nanomaterials due to their
massive surface area to a volume ratio which results in a number
of unique properties [1]. Zinc oxide (ZnO) is a versatile wide band
gap material (3.37eV) with high binding energy (60meV) and can
grow easily as nanorods at a low-temperature [2]. ZnO nanorods
show several excellent properties like high sensitivity to adsorbed
oxygen at the surface, excellent electric transport, optical wave
guiding, and large surface area to volume ratio [3]. These unique
properties of ZnO nanostructure make it an ideal candidate for
a variety of applications as short-wavelength nanolasers [4],
field-effect transistors [5], nanosized gas sensors [6], UV sensors
[7], blue electroluminescent devices [8,9], high heterojunction
area solar cells [10], fast data-storage [11] and field emitters [12].
To facilitate these applications, a number of methods are used to
synthesize ZnO nanorods depending on the requirement of the
∗Correspondingauthorat:Nano-OptoelectronicsResearchandTechnologyLabo-
ratory (N.O.R.), School of Physics Universiti Sains Malaysia, Penang 11800, Malaysia.
Tel.: +60 174497314; fax: +60 46579150.
E-mail address: j1j2h72@yahoo.com (J.J. Hassan).
device property. These synthesis methods include chemical vapor
deposition (CVD) [13,14], metal organic chemical vapour deposi-
tion (MOCVD) [15,16], thermal evaporation [17], electrodeposition
[18,19], and chemical bath deposition (CBD) [20–22]. In addition,
different types of ZnO seed layer have been used to grow epitaxial
ZnO nanostructures such as spin-coated ZnO nanoparticles [23],
sol–gel layers and RF magnetron sputtered ZnO [24]. The quality
andthekindofZnOseedlayerinadditiontothesubstratetypeplay
a significant role in the vertically aligned growth of ZnO nanorods
[25]. Moreover, the structure type and crystal orientation of the
substrate and their lattice mismatch with an epitaxial layer are the
most significant parameters which control the nature of the pro-
duced nanorods. Consequently, there have been more studies on
epitaxial growth by chemical solution of ZnO nanorods with seed-
ing and without seeding layer [23,26]. In the present study, CBD
method assisted with microwave heating was used to grow ZnO
nanorods. The microwave oven provides even distribution of the
temperature for the reaction vessels through the direct interaction
of electromagnetic radiation with polar and ionic medium [27].
Thus, the chemical reactions of the precursor under microwave
heating take place in short time comparing to the conventional
heatingmethods[28].Ontheotherhand,allsubstrateswereseeded
with a new nanocomposites seed layer which was synthesized by
a complex of zinc ions with the hydroxyl groups of Poly(vinyl alco-
hol) (PVA). Here, the PVA works as a stable host material for ZnO
nanocrystals because the PVA has excellent distribution of ligand
0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2012.01.007

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J.J. Hassan et al. / Applied Surface Science 258 (2012) 4467–4472
radicals on the side-chain with winding mesh structure, which
confines the ZnO nanocrystals growth [29]. The effects of substrate
type on nanorods properties are correspondingly studied.
2. Experimental
All chemicals used were of analytical grade and used with-
out further purification. Prior to the epitaxial growth of the ZnO
nanorods, the substrates were cleaned using the suitable proce-
dure. The experimental setup and the mechanism of the growth
which were used to grow the nanorods were the same as that in
our previous work [2]. In brief, 0.1mol/L aqueous solution of zinc
chloride was vigorously stirred at 70◦C for 10min. Similarly, 1.5g
aqueous solution of PVA was stirred at 80◦C for 30min. These solu-
tions were mixed together via high-speed stirring and placed on
a hot plate at 70◦C for 2h. Then, the solution was transferred to
a microwave oven for 15min at 80◦C to facilitate complexation
of zinc ions with PVA. After that, ammonia solution was added
to the mixture until the pH reached 8.3 and the nanocomposites
PVA–Zn(OH)2solution was synthesized. Next, the nanocomposites
PVA–Zn(OH)2solution was spin-coated on all substrates (p-type
GaN, c-plane sapphire, quartz single crystal, and ITO glass) as a
seed layer. Then these substrates were annealed at 210◦C for 1h
to decompose Zn(OH)2to ZnO; the temperature was then ele-
vated to 380◦C for 2h. After annealing, these substrates were
inserted vertically in a beaker containing 0.1mol/L of zinc nitrate
hexahydrate Zn(NO3)2·6H2O and an equal molar concentration
of hexamethylenetetramine (C6H12N4) dissolved in DI water. The
beakerwasthenplacedinsideamicrowaveoven(2.4GHz)for2hat
90◦C.Finally,thegrownnanorodsonthesesubstrateswerewashed
with hot ethanol to remove the remaining salt. Transmission elec-
tron microscopy (EFTEM Libra 120-Carl Zeiss) was used to analyse
the nanocomposites solution. The topography of nanocrystals seed
was determined by atomic force microscopy (AFM) (Dimension
edge, Bruker) with tapping operation mode. Field emission scan-
ning electron microscopy (FESEM) (model Leo-Supra 50VP, Carl
Zeiss, Germany) determined the surface morphology of the ZnO
nanorods. X-ray diffraction (PANalytical X’Pert PRO MRD PW3040,
Almelo, The Netherlands) was used to determine the structural
properties of ZnO nanorods. Optical properties were measured at
room temperature by photoluminescence (PL) and Raman spec-
troscopy (Jobin Yvon HR 800 UV, Edison, NJ, USA).
3. Results and discussion
3.1. Formation of ZnO nanocrystals seed
The synthesized nanocomposites PVA–ZnO(OH)2solution was
shown in TEM image in Fig. 1. The image displays the Zn(OH)2
nanoparticles distributed in PVA polymers with diameters ranged
from 15 to 40nm. After spin-coating these nanocomposites on the
Fig. 1. Transmission electron microscope image of poly(vinyl alcohol)–Zn(OH)2
nanocomposites synthesis by complex of Zn ion with OH groups of PVA.
substrates, the Zn(OH)2nanoparticles will evenly be distributed on
the substrates as well as PVA polymer. With raising the annealing
temperature to 210◦C, the Zn(OH)2nanoparticles decomposed to
ZnO nanoparticles. Here, the PVA will confine the growth of these
nanoparticles with the temperature processes on the substrates.
Once the temperature of the substrate reaches around 380◦C, the
PVA polymer begins the carbonization process to create carbon
grids, which will surrounding ZnO nanocrystals. In the same time,
ZnO nanocrystals begin to grow from the substrate to form small
individual nanorods as shown in AFM image in Fig. 2. One can see
that ZnO nanocrystals were grown as small nanorods directly from
the surface of the substrate in the absence of forming thin film seed
layer. The direct evidence was shown also in the SEM cross section
image in Fig. 3 which displays the growth of ZnO nanorods directly
from p-type GaN substrate without any interface layer between
the ZnO nanorods and GaN substrate. Accordingly, the properties
of ZnO nanorods are related to the nature of the substrate.
3.2. Structural analysis
TheX-raydiffractionpatternsofZnOnanorodsgrownonp-type
GaN, c-plane sapphire, ITO glass, and quartz single crystal sub-
strates are displayed in Fig. 4. As seen from the figure, the ZnO
nanorods that grew on all substrates have a hexagonal wurtzite
Fig. 2. Atomic force microscope topography (2D and 3D) of PVA–Zn(OH)2spin-coated on the substrate after annealing processing at 380◦C for 2h.

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Fig. 3. Cross-section scanning electron microscope image of ZnO nanorods grown
on p-type GaN substrate which showed the absence of seed layer at the bottom of
ZnO nanorods.
structure with a strong diffraction intensity peak related to the
(002) plane, indicating that they have a c-axis orientation per-
pendicular to the substrates. Fig. 4a shows the diffraction pattern
of the ZnO nanorods grown on p-type GaN substrate and the
inset shows two peaks diffracted from the (002) plane of the ZnO
nanorods (2? =34.425◦) and the GaN substrate (2? =34.52◦). The
strong diffraction intensity of these two peaks revealed that the
ZnO nanorods and GaN substrate have excellent quality structure
andsuperiororientationtowardthe(002)plane.Fig.4bshowsthat
Fig. 4. XRD patterns of ZnO nanorods grown by CBD on a variety of substrates (a)
p-type GaN (b) sapphire (c) ITO glass and (d) quartz single crystal substrates respec-
tively. The inset in (a) shows the two peaks of ZnO nanorods and GaN substrate. The
inset in (b) shows the ZnO nanorods peaks and sapphire peak.
the XRD pattern of the ZnO nanorods grown on sapphire substrate
exhibited a high diffraction peak at 2? =34.477◦which is shifted
from the standard position (34.42◦). This shift is due to the large
value of lattice mismatch (16.9%) between the d-spacing of the
(002)planeofZnO(2.602˚A)andthed-spacingofthe(001)planeof
sapphire (2.16˚A) [30,31]. The other weak diffraction peak of (101)
was also noted in the XRD pattern (Fig. 4b) which originated from
some growth of (002) plane for the ZnO nanorods parallel with the
(10¯14) plane of sapphire. This condition causes the ZnO nanorods
to be inclined at 51.8◦with the sapphire substrate as can be seen
in the FESEM image (Fig. 5b) [23]. For ITO glass, the XRD pattern
in Fig. 4c shows several peaks which are related to the diffraction
from multiple faces of the hexagonal structure. In spite of the low
lattice mismatch (3%) between ZnO nanorods and ITO [32], the ZnO
nanorods do not grow vertically well-aligned due to the polycrys-
talline nature of the ITO glass as seen in the FESEM image (Fig. 5c).
In the case of the quartz substrate, the XRD pattern in Fig. 4d dis-
plays the strong (002) peak with weak peaks for other faces of the
hexagonal structure. This finding indicates that most of the ZnO
nanorodsgrowverticallyonthequartzsinglecrystalsubstratewith
lattice mismatche of 33.8%. On the other hand, the texture coeffi-
cient(TC)expressesthetextureoftheparticularplane,deviationof
which from unity indicates the preferred growth [33]. The TC val-
ues corresponding to the (002)-orientation of ZnO nanorods for all
samples were calculated using the relation of [34]:
TC(0 02) =
I(002)/I0(002)
?I(hkl)/I0(hkl)
1
n
(1)
where I(hkl) is the observed intensity of ZnO nanorods and I0(hkl)
corresponds to values of standard PDF data (04-006-1673) mea-
sured from randomly oriented powder samples, n is the number
of diffraction lines. The TC values of ZnO nanorods grown on GaN,
sapphire, ITO glass, and quartz substrates were 5.99, 3.97, 2.11 and
4.09, respectively. It is clear that ZnO nanorods grown on GaN have
high TC value compared to other substrates, which indicated the
well aligned ZnO nanorods in the (002) direction.
3.3. FESEM observation
The FESEM images and the diameters distribution of the ZnO
nanorods grown on the GaN, sapphire, quartz, and ITO glass are
shown in Fig. 5. The hexagonal shape of vertically aligned ZnO
nanorods is clearly noted. The ZnO nanorods grown on the p-type
GaN substrate (Fig. 5a) have better morphology and vertical well-
alignment on the substrate due to the close matching in the lattice
parameters with the low lattice mismatch (1.9%). On the other
hand, the diameters distribution of the nanorods on the surface of
GaN is in the range of 80–170nm. Fig. 5b shows the ZnO nanorods
grown on the c-plane sapphire substrate. In this case, most of the
ZnOnanorodswithadiameterfrom30to90nmareverticallywell-
aligned, and a small number of ZnO nanorods made an angle of
51.8◦with the sapphire substrate as explained in the XRD section.
The morphology of the ZnO nanorods grown on ITO glass as seen in
Fig. 5c is not vertically well-aligned on the substrate compared to
other substrates. This may be due to the polycrystalline nature of
the ITO glass. On the other hand, the diameter distribution of ZnO
nanorods on ITO glass ranged from 40 to 100nm. In the case of ZnO
nanorods grown on quartz substrates (Fig. 5d), the nanorods are
vertically well-aligned on the substrate, with size diameter distri-
bution ranged from 30 to 90nm.
3.4. Photoluminescence characteristics
The optical properties of the ZnO nanorods’ epitaxial growth on
various substrates were examined at room temperature by pho-
toluminescence spectroscopy, where the PL spectra of all samples

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J.J. Hassan et al. / Applied Surface Science 258 (2012) 4467–4472
Fig. 5. Field emission scanning electron microscope (FE-SEM) images of ZnO nanorods and their diameter distributions grown by CBD on various substrates: (a) p-type GaN,
(b) c-plane sapphire, (c) ITO glass, and (d) quartz single crystal substrates.
were excited by a He–Cd laser (?ex=325nm). Fig. 6 shows the PL
spectra of the nanorods grown on GaN, sapphire, ITO glass, and
quartz substrates. The dominant peaks observed in the UV region
are attributed to the near band edge UV emission (NBE) of the
wide bandgap of ZnO, which results from the recombination of free
excitons [35]. Moreover, the broad visible peaks in the wavelength
range460–700nmandcenteredaround600nminFig.6arerelated
to the deep energy levels’ emission (DE) of ZnO, which is due to the
intrinsic defects in ZnO nanorods [36]. The highest intensity of UV
peak compared with weak spectral bands in the visible region in all
substrates indicated that the ZnO nanorods have a good hexagonal
structure with low structural and surface defects. In other words,
the high ratio of the UV intensity peak to the visible intensity peak
(IUV/Ivis) is the main identifying feature of high-quality ZnO. The

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Fig. 6. Room temperature photoluminescence spectra of ZnO nanorods grown by
CBD on (a) p-type GaN, (b) c-plane sapphire, (c) quartz single crystal, and (d) ITO
glass substrates, respectively.
small peaks appearing at 760nm were connected to second order
diffractionofthehighintensityofUVemission(NBE)peaks[37–39].
The strong PL intensity for the ZnO nanorods grown on the p-type
GaN substrate compared with that grown on the other substrates
was a result of the enhancement of the optical characteristic of the
vertically well-aligned ZnO nanorods [40].
3.5. Raman spectra
The optical phonon modes of wurtzite ZnO structure with C6?4
space group consist of 1A1+2B1+1E1+2E2 modes [41]. This is
due to the perpendicular incident radiation of the backscattering
geometry used to collect Raman scattering, to the c-axis of the ZnO
nanorods. Therefore, Raman selection rules allow only E2 (high)
and A1 (LO) modes [42]. E2 (high) is associated with the vibration
of the oxygen sublattice in the ZnO hexagonal structure. This peak
(E2 (high)) shown in Fig. 7 for all substrates is a unique property
of the Raman active mode of wurtzite hexagonal ZnO and is very
sensitive to the strain of this material [43].
4. Conclusions
VerticallyalignedZnOnanorodsperpendiculartothesubstrates
surfaces were grown on p-type GaN, c-plane sapphire, ITO glass,
and quartz single crystal substrates, which were seeded with
ZnO–PVA nanocomposites. The PVA backbone chain carbon-grids
play a significant role in the alignment of the ZnO nanorods by
surrounding the ZnO nanocrystalline seed particles during anneal-
ing treatments. Annealing process for seed layer induces ZnO
nanocrystals to growth into small nanorods, which represents
the foundation bases for growth of ZnO nanorods in solution
Fig. 7. Raman spectra of ZnO nanorods grown by CBD on various substrates (a) p-
type GaN (b) c-plane sapphire (c) ITO glass and (d) quartz single crystal substrates
respectively.
deposition. The MA–CBD method was used to grow ZnO nanorods
on the seeded substrates. The microwave heating used provided
excellent distribution of heat inside the reactor vessel and supplied
the kinetic energy directly to chemical reactions. The difference
in lattice mismatch, structural type and orientation between the
substrates and the epitaxially grown ZnO nanorods effect on the
vertical alignment and nanostructure morphologies of the grown
ZnO nanorods. High TC value for ZnO nanorods grown on GaN sub-
strate was due to structural similarity between ZnO and GaN. The
highest PL UV intensity revealed the high-quality of ZnO nanos-
tructures with low defects.
Acknowledgments
The authors gratefully acknowledge support from a Research
University (RU) grant and Universiti Sains Malaysia.
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