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Nadheer Jassim Mohammed et al./ Elixir Nanotechnology 81 (2015) 31835-31839
31835
Introduction
Zinc oxide (ZnO), a II-VI compound oxide semiconductor
with a direct band gap of 3.37 eV and a high exciton binding
energy of 60 meV at room temperature, is an important kind of
technological semiconductor due to its distinguished optical,
electrical, and piezoelectrical properties, which can be widely
used in optoelectronic and photovoltaic devices [1, 2]. Various
methods, such as precipitation , sol–gel, vapor–liquid-solid
(VLS) growth, chemical vapor deposition (CVD), thermal
decomposition, metal organic vapor-phase epitaxy, have been
developed for controlling ZnO structures, since its various
properties strongly depend on its structures including the crystal
size, orientation, morphology, aspect ratio and even crystalline
density. Currently, many interesting ZnO nanostructures
including nanorods,nanowires, tetrapods, nanocombs, nanotubes,
nanopencils and star-like have been successfully synthesized [3-
7]. Metal oxide semiconductors such as ZnO, SnO
2
, TiO
2
, Fe
2
O
3
,
NiO, WO
3
, In
2
O
3
etc., have been widely used for gas sensors.
Among these sensing materials, ZnO has attracted increasing
attention and been proven to be a highly useful sensing material
for detecting both oxidizing and reducing gases. In recent years,
great efforts have been made to fabricate low dimensional ZnO
nanostructures, since their gas sensing properties can be
efficiently improved in this way. Taking advantage of their small
and uniform particle size, high surface-to-volume ratio, specific
pore structure, anti-aggregation properties and so on, these low-
dimensional nanostructures may exhibit better sensing properties
than those of traditional nanoparticles and thin films. Hitherto,
low-dimensional ZnO nanostructures with different
morphologies including nanobelts, nanotubes, nanorods,
nanowires, nanofibers, nanodisks, nanospindles and
nanoneedles, have been successfully developed, and many
exhibit interesting gas sensing performances towards H
2
, CO,
NO
2
, H
2
S, SO
2
and some volatile organic compounds (VOCs).
Pawar et al.obtained interesting morphological transformations
from rod-to-disk-to-spindle-toflower merely by varying the pH
of the growth solution. Pawar and co-workers also synthesized
vertically aligned ZnO nanorods, hexagonal nanorods, faceted
microrod rods, nanoneedles and nanotowers assisted with
different surfactants (polyetherimide PEI, polyacrylic acid PAA,
diammonium phosphate DAP and DAP-PAA). Chai et al.
reported the synthesis of functionalized individual ZnO
microwires prepared by a carbothermal reduction vapor phase
transport method and their gas sensing properties for natural
gases such as H
2
, O
2
, CO
2
, CO, CH
4
and C
2
H
5
OH. Hamedani et
al. applied a fast and facile microwave assisted method to
prepare various ZnO nanocrystal morphologies and investigated
their response and selectivity for CO, CH
4
and C
2
H
5
OH [8].
Experimental Method
Zinc acetate [(CH3COO)2Zn.H2O] and sodium dodecyl
sulfate (C12H25NaO3S) were dissolved in deionized water at
0.2 mol/L concentration, respectively
Figure 1. Pulsed laser deposition technique (PLD)
Synthesis of Nanostructure Zinc Oxide Formation from Zinc Acetate and
Deposited on Sapphire Substrate using Pulsed Laser Deposition for NO
2
Gas
Sensor
Nadheer Jassim Mohammed, Marwa Abdul Muhsien Hassan, Ibrahim R. Agool and Nisreen Zaid
Department of Physics, College of Science, Al-Mustansiryah University, Baghdad, Iraq.
AB S T RA C T
Zinc oxide nanostructure were successfully synthesized by chemical method and deposited
on Al
2
O
3
substrate using PLD. XRD analysis demonstrated that the ZnO nanostructure has a
wurtzite structure with orientation of (002). SEM results indicated that by increasing the
calcined temperature, the dimension of the ZnO nanostructure increases. The optimum
temperature for synthesizing high density ZnO nanostructure was determined to be 1250 K.
Room temperature PL spectra of the ZnO nanostructure showed a strong UV emission peak
located at around 380 nm and a relatively weak green emission at around 540 nm,
confirming that the as-grown nanorods possess good optical properties. The sensitivity of
zinc oxide NRs films to 50 ppm vapor NO
2
gas as a function of working temperature with
different doping.
© 2015 Elixir All rights reserved.
A R T I C LE IN F O
Ar t i cl e h is t o r y:
Received: 8 February 2015;
Received in revised form:
28 March 2015;
Accepted: 13 April 2015;
Ke y w or d s
ZnO Nanostructure,
Gas Sensor,
FESEM,
PLD.
Elixir Nanotechnology 81 (2015) 31835-31839
Nanotechnology
Available online at www.elixirpublishers.com
(Elixir International Journal)
Tele:
E-mail addresses:
marwa_alganaby@yahoo.com
© 2015 Elixir All rights reserved
Nadheer Jassim Mohammed et al./ Elixir Nanotechnology 81 (2015) 31835-31839
31836
Then, certain volume zinc acetate solution was slowly added
to sodium dodecyl sulfate solution under vigorous stirring at
room temperature for 25 min. The samples were filtrated and
washed with distilled water several times. Finally, the samples
were dried in air at 378 K for 4 h. The ZnO samples were
obtained by calcined at (1150, 1200 and 1250 ) K for 6 h in tube
furnace.
Zinc oxide and different doping noble metal (Ag and Ni)
with high purity (99.999%) at concentrations (4%) mixed with
corresponding concentrations in methanol by magnetic blender
for 2 hour. After the liquid was dry out, the mixed powder was
blended mechanically again so that the mixture is uniformly
distributed. The resultant powder was ground again and was
pressed under 5 ton to form a target with 2.5 cm diameter and
0.4 cm thickness. The target should be as dense and homogenous
as possible to ensure a good quality of the deposit. Thin films
from the prepared target were deposited on (α-Al
2
O
3
(006))
single crystal sapphire substrate by pulsed laser deposition
technique (PLD). The pulsed laser deposition experiment is
carried out inside a vacuum chamber generally in (10
-3
Torr)
vacuum conditions, at low pressure of a background gas for
specific cases of oxides and nitrides. A schematic diagram of the
set-up of laser deposition chamber, given in figure (1), shows the
arrangement of the target and substrate holders inside the
chamber with respect to the laser beam. The focused Nd:YAG
SHG Q-switching laser beam coming through a window is
incident on the target surface making an angle of 45° with it. The
substrate is placed in front of the target with its surface parallel
to that of the target. Sufficient gap is kept between the target and
the substrate so that the substrate holder does not obstruct the
incident laser beam. Modification of the deposition technique is
done by many investigators from time to time with the aim of
obtaining better quality films by this process. These include
rotation of the target, heating the substrate, positioning of the
substrate with respect to the target etc. The oxygen background
pressure 5×10
-2
mbar. ZnO pellets were ablated by a Q-switched
Nd: YAG Laser Second Harmonic Generation (SHG) (Huafei
Tongda Technology—DIAMOND-288 pattern EPLS, λ = 532
nm, 5 Hz and 10 ns pulse duration) with a fluence of 2 J/cm
2
.
The substrate temperature is maintained at ~450˚C.
Results and Discussion
Figure 2. XRD of ZnO powder calcined at (a) 1150 K, (b)
1200 K and (c) 1250 K for 6 h.
Structural Characterization.XRD pattern of ZnO powder
nanorods obtained via the chemical method only consist of a
pure phase of ZnO nanomaterial is shown in figure (2) (a, b, c).
The unit cell of the ZnO crystal was found to be hexagonal
structure with the presence of the peak (002) plane, compaerd
with the card number (JCPDS 36-1451) and measured lattice
constants of a and c of 3.25 and 5.21 Å (c/a ) 1.60), respectively.
The crystallite size was calculated using the Scherer´s formula at
highest intensity (002) peak was analysed and considered it to be
Gaussian. The crystallite size was found to be 35 nm. Besides,
no impurity peaks were detected which indicates that the
construction ZnO powder is highly pure nanomaterials. EDX
spectrum curve of figure (3) (a, b) shows that only O and Zn
elements, the atomratio of Zn to O is quantitatively calculated
found to be 70:30 besides the carbon.
Figure 3. EDX of ZnO powder calcined at (a) 1200 K and
(b) 1250 K for 6 h
Figure (4) show FESEM images with different
magnifications of the as-prepared zinc oxide powder calcined at
1200 and 1250 K for 6 h. The ZnO surface morphology
nanostructures are randomly distributed in the powdered ZnO
sample. The ZnO powder contains nanoneedles and nanorods
with avrege diameter found to be ~35-60 nm and length ~ (250-
300) nm. The software used in calculation of this work was
MBF_Image J. program. We have taken the FESEM images
from different point region of the distributed powder sample.
The surface nanorod was characterized using AFM
micrographs. It shows a change in roughness of the oxide
surface with calcined temperature as shown in figure (5). It is
known that the increase in surface roughness may cause
deterioration of the electrical and optical properties.
Nadheer Jassim Mohammed et al./ Elixir Nanotechnology 81 (2015) 31835-31839
31837
Figure 4. FESEM of ZnO powder calcined at (a) 1200
K and (b) 1250 K for 6 h
Figure 5. AFM f ZnO powder calcined at (a) 1150 K, (b)
1200 K and (c) 1250 K for 6 h.
Figure 6. Transmittance with wavelength f ZnO powder
calcined at (a) 1150 K, (b) 1200 K and (c) 1250 K for 6 h.
Figure 6 shows the transmittance spectra curve of ZnO
powder calcined at (a) 1150 K, (b) 1200 K and (c) 1250 K for 6
h in the wavelengths range of 300-800 nm at room temperature.
As can be seen, the average optical transmittance of the ZnO
samples in the visible range is amount 80%, leading to a good
optical quality of the produced ZnO nano-materials. The direct
Nadheer Jassim Mohammed et al./ Elixir Nanotechnology 81 (2015) 31835-31839
31838
allowed band gap semiconductors calculated using the following
equation (1) for n = 1/2 [4].
(1)
The absorption coefficient α could be calculated from the
following equation [20]:
(2)
where T is the transmittance and d is the thickness of the film.
The plot of the graph (αhυ)
2
vs hυ (see inset Figure 6) by using
equation (1). The optical band gap value of the ZnO powder,
determined by the optical method, is obtained by extrapolating
the linear portion of this graph to (αhυ)
2
= 0 and optical band gap
is found equal range to be E
g
=3.2-3.3 eV.
Figure 7. PL with wavelength f ZnO powder calcined at
(a) 1150 K, (b) 1200 K and (c) 1250 K for 6 h.
The PL spectrum is recognized of an ultraviolet (UV)
emission located at about 380 nm and a broad green emission
position at about 540 nm. The UV emission band can be
explained by the near band-edge transition of the wide band gap
ZnO nanorods, the recombination of free excitons through an
exciton-exciton collision process, whereas the peak at 545 nm is
due to the deep-level emission (DLE) related to the defects such
as oxygen vacancies and Zn interstitials. It has been suggested
that the DLE corresponds to the singly ionized oxygen vacancy
in ZnO and results from the recombination of a photo-generated
hole with the singly ionized charge state of this defect. Strong
UV emission and relatively weak green emission from the ZnO
nanorods confirm that the grown nanorods posses good optical
properties with less structural defects and impurities [9]. NO
2
gas was prepared in laboratory by adding three gm of potassium
nitrate ( into (100 ml) of dilute sulfuric acid (
(3)
Metal oxides semiconductors, such as ZnO, SnO
2
, In
2
O
3,
CdO
and TiO
2
, can be exercised to gases sensors which are mainly
based on the current change responses to the target gases. The
sensing mechanism of metal oxide semiconductor gas sensors
based ultimately on trapping of electrons at adsorbed molecules
and band bending induced by these charged molecules are
answerable for a change in conductivity.
The current of the film was measured before and after
exposure to gas.
(4)
where Ia is the current in air and Igas is the current in a sample
gas. In general, intrinsic ZnO nanorod behaved as an N-type
semiconductor and has many oxygen vacancies, thus, its gas
sensitive effect is obvious and is generally considered a surface
adsorption-controlled mechanism as shown in figure (8). Its
response to the measured gas is caused by the chemisorption
reaction between oxygen in the air and the ZnO NRs sensor
Figure 8. Relation between current and time of zinc
oxide NRs film deposited on Al
2
O
3
with different doping.
surface. Oxygen ions exist in the grain boundaries between
grains, thereby causing the grain boundary barrier to become
higher, thus, the resistance of the ZnO NRs sensor increases,
blocking the transfer of the carriers. When meeting the reducing
gas or the electron supply gas, an oxidation reduction occurs
between the surface adsorbed oxygen ions and the reducing gas.
The number of adsorbed oxygen ions decreases sharply, the
sensor surface potential barrier is reduced, carrier shifting is
promoted, ZnO resistance is reduced, and the gas sensing
response is finally achieved [10, 11]. The interpretation of metal
oxide semiconductor gas-sensitive materials is extremely
influenced by the working temperature. Figure (9) shows the
response curve (sensitivity) of ZnO nanorod gas sensor
semiconductor at different working temperatures (i.e., surface
temperature) at 50 ppm vapor NO
2
gas. As obvious, the
sensitivity increases with the temperature and reaches a
maximum value in identification of work temperature T= 200-
300 °C. If the temperature increases again, the sensitivity
decreases.
Figure 9. Relation between sensitivity and temperature
of zinc oxide NRs film on Al
2
O
3
with different doping
Conclusion
High quality zinc oxide nanostructure were successfully
synthesized by chemical method and deposited on Al
2
O
3
substrate using PLD. XRD analysis demonstrated that the ZnO
nanostructure has a wurtzite structure with orientation of (002).
SEM results indicated that by increasing the calcined
temperature, the dimension of the ZnO nanostructure increases.
The optimum temperature for synthesizing high density ZnO
Nadheer Jassim Mohammed et al./ Elixir Nanotechnology 81 (2015) 31835-31839
31839
nanostructure was determined to be 1250 K. Room
temperature PL spectra of the ZnO nanostructure showed a
strong UV emission peak located at around 380 nm and a
relatively weak green emission at around 540 nm, confirming
that the as-grown nanorods possess good optical properties. the
sensitivity of zinc oxide NRs films to 50 ppm vapor NO
2
gas as
a function of working temperature. As evident, the sensitivity
increases with the temperature and reaches a maximum value in
correspondence of T = 200-300 °C. If the temperature increases
again, the sensitivity decreases.
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