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Indian Journal of Engineering & Materials Sciences
Vol. 24, December 2017, pp. 469-476
Contact angle hysteresis, wettability and optical studies of sputtered zinc oxide
nanostructured thin films
Kartik H Patel & Sushant K Rawal*
CHAMOS Matrusanstha Department of Mechanical Engineering,
Chandubhai S Patel Institute of Technology, Charotar University of Science and Technology,
Changa 388 421, India
Received 22 January 2016; accepted 24 April 2017
Zinc oxide (ZnO) nanostructured thin films are deposited by RF magnetron sputtering on corning glass substrates. The
effects of RF power and deposition temperature on ZnO nanostructured thin films are investigated. The structural
characterization is done by X-ray diffraction; the deposited ZnO nanostructured thin film is amorphous at 30W RF power.
The increase of RF power to 90 W and 150 W leads to evolution of (100), (002) and (101) textures of ZnO nanostructured
thin films. A well intense (002) peak of ZnO nanostructured thin films is evolved and (100) peak diminishes with increase in
deposition temperature from 200ºC to 600ºC. The wettability studies of ethylene glycol are rarely done, so we have
investigated contact angle hysteresis and wettability properties of two liquids; water and ethylene glycol on deposited ZnO
nanostructured thin films measured by contact angle goniometer. The motivation of this research work is to explore the
wettability studies specifically for ethylene glycol on zinc oxide nanostructured thin films as it is used as antifreeze agent
and coolant in industry and commercial applications. The contact angle formed by water and ethylene glycol varies as a
function of RF power and deposition temperature. The optical properties were measured by UV-Vis-NIR spectrophotometer.
Keywords: Zinc oxide, Sputtering, Contact angle hysteresis, Wettability, Ethylene glycol, Optical properties
The requirements for existing thin film techniques
coatings have encouraged the improvement of various
deposition techniques. These makes achievable to
control the chemical and phase composition as well as
microstructure of thin film, thereby observing their
performance and properties. Zinc oxide (ZnO) has
fascinated a widespread research interest for use in
mechanical, optical, electrical and biomedical devices
as a result of its adaptable characteristics. It has been
reported that the properties of ZnO are diligently reliant
on their crystalline density crystal size, orientation,
dimensions, morphologies and aspect ratio1,2
.
Zinc oxide is a very expedient material for
electronic and photonic application and is mainly
auspicious in nanodevice applications because of its
inclusive direct band gap of 3.37 eV and large exciton
binding energy allow to different fields like photo-
detectors, thin film gas sensors and light emitting
diodes especially for UV region3,4
.
Wettability has substantiated to be an important
property of solid surfaces and has subsequently
growing research interest in the last few years.
Wetting properties can be modified by deploying the
morphology and chemistry of any substrate. By
controlling the wettability of surface is very useful for
many applications it would be constructive to be able
to modify between hydrophilicity and
hydrophobicity5. Hydrophobicity and transparency are
complicated properties that are inversely proportional
to each other. Translucent hydrophobic coatings may
be used in several industrial applications such as anti-
rusting, anti-wetting, anti-fogging, anti-ice adherence,
and moderated friction resistance coatings6. Ethylene
glycol is used as a medium for convective heat
transfer in automobiles7.
The studies of wettability property of ethylene
glycol on ZnO nanostructured thin films are limited in
literatures. This paper aims to explore specifically the
wettability properties of ZnO nanostructured thin films
with water and ethylene glycol. The objective of the
current work is to improve transparent hydrophobic
zinc oxide nanostructured thin films by reactive RF
magnetron sputtering using argon as inert gas. Zinc
oxide nanostructured thin films were deposited on
corning glass substrate at different RF power and
deposition temperature; their effect on structural,
wettability and optical properties of deposited films
have been investigated in this present work. __________
*Corresponding author (E-mail: kartik511@gmail.com)
INDIAN J. ENG. MATER. SCI., DECEMBER 2017
470
Experimental Procedure Zinc oxide nanostructured thin films were
deposited on corning glass substrate by RF magnetron
sputtering in custom designed 16” diameter × 14”
cylindrical vacuum chamber (Excel Instruments,
India) as shown in Fig. 1. ZnO target of 2” diameter
was kept at a distance of 50 mm from substrate and
argon was used as inert gas to deposit zinc oxide
nanostructured thin films. The flow of argon was kept
constant at 10 sccm which was measured and
controlled using mass flow controller (Alicat, USA).
During each sputtering experiment, the mass flow rate
of inert gas and working pressure inside the chamber
was kept constant and cautiously observed since the
sputtering current is very sensitive to the pressure of
the sputtering gas. The deposition was carried out for
60 min at working pressure of 2.0 Pa. Zinc oxide
nanostructured thin films were deposited at RF power
of 30 W, 90 W and 150 W at deposition temperature
of 300 ºC; the sample names for these coatings are
30 W, 90 W and 150 W, respectively. The second set
was deposited at temperature of 200ºC, 450ºC and
600ºC at constant RF power of 90 W; the sample
names for these coatings are 200T, 450T and
600T, respectively.
The structural properties of zinc oxide
nanostructured thin films were characterized by X-ray
diffractometer (Bruker, Model D2 Phaser). The
surface topography was studied by atomic force
microscopy (Nanosurf easyscan2). The wettability
properties of zinc oxide nanostructured thin films
were done by contact angle measuring system
(Ramehart, Model 290). The optical properties of zinc
oxide nanostructured thin films were recorded by
UV-vis-NIR spectrophotometer (Shimadzu, Model
UV-3600 plus).
Results and Discussion The XRD graphs of ZnO nanostructured thin films
prepared at various RF powers of 30 W, 90 W and
150 W are shown in Fig. 2(a). Figure 2(b) shows the
XRD graphs of ZnO nanostructured thin films
deposited at temperature of 200ºC, 450ºC and 600ºC
at a constant RF power of 90 W.
The XRD pattern of ZnO nanostructured thin films
deposited at RF power of 30 W does not show any
peak of ZnO. Huang et al.8 deposited ZnO thin films
at different RF powers of 50 W, 100 W, 130 W,
160 W and 190 W for a fixed deposition time of
15 min. They hardly observed ZnO (0 0 2) peak at
50 W but it was seen in all other ZnO thin films
deposited at higher powers. So the amorphous ZnO
nanostructures films observed in our case at RF power
of 30 W is in agreement with the literature. When the
sputtering power is increased to 90 W; (100), (002),
Fig. 2 — XRD patterns of the ZnO films deposited at different (a) RF power and (b) temperature.
Fig. 1 — Experiment set-up
PATEL & RAWAL : SPUTTERED ZINC OXIDE NANOSTRUCTURED THIN FILMS
471
(101) and (110) peaks of ZnO are observed. We have
kept the deposition time of 60 min so even at low RF
power of 90 W the evolution of (100), (002), (101)
and (110) peaks of ZnO is observed whereas Huang
et al.8 had reported evolution of only (002) peak of
ZnO at RF power of 100 W. The intensity of (100),
(002), (101) and (110) peaks rises when the sputtering
power is increased to 150 W. This indicates that
increase of the RF power enhances crystallization of
ZnO nanostructured thin films thereby resulting in
formation of ZnO thin films having different
orientations. The deposition time is 60 min, so with
increase in RF power from 30 W to 150 W, the
proportion of ZnO atoms in the chamber increases
which will have high kinetic energy thereby leading
to evolution of various textures of ZnO.
At temperature of 200ºC; (100), (002) and (101)
peaks of ZnO are observed, but its intensity is very
low. When temperature is increased up to 600ºC only
(002) peak grows whereas (100) and (101) peaks
diminishes gradually. Shaginyan et al.9 reported that
evolution of nanostructure in deposited materials
depends on temperature which effects diffusion and
mobility of atoms during film growth. The amount of
potential phase separation in material and the rate of
surface reactions in deposition process are influenced
by temperature as reported in literature10-13
. Palmero
et al.13
examined deposition rate of different metals
such as Si, Ge, Al, Cr, V, W, and Ta by fitting
experimental results found in the literature in
transport theory equation. They found that the
temperature of cathode shows an increase with
deposition power.
When the deposition temperature is gradually
increased from 200ºC, 450ºC and 600ºC at constant
RF power of 90 W, it may lead to mobility of ZnO
atoms in the reaction zone with increase in deposition
temperature. ZnO atoms may be free to move at
higher deposition temperatures of 450ºC and 600ºC
aligning themselves in the direction of preferred (002)
orientation for ZnO peak. Hence, higher deposition
temperature (450ºC or more) may have led to
separation of orientation along (100) and (101) peaks
of ZnO resulting in preferred orientation along (002)
peak for ZnO. The average crystallite size of ZnO
nanostructured thin films as calculated by Scherrer
formula14
is given in Table 1. It increases from 15 nm
to 19 nm when RF power is increased from 30 W to
150 W and from 16 nm to 20 nm when deposition
temperature is raised from 200ºC to 600ºC.
Normally to form exceptional hydrophobic
surfaces, the surfaces with nanotextures and
microtextures or their combination are desirable. The
smoother the surface, the smaller will be the contact
angle and more the nanotextured surfaces larger will
be the contact angle15
. Wenzel16
and Cassie–Baxter17
suggested two mathematical models to describe the
wetting phenomena on rough surfaces. Contact angle
of and surface roughness are correlated by Wenzel's
equation16
by:
cos θw = A cos θ … (1)
where A is the proportion of the real and apparent
surface areas, known as a roughness factor and θw is a
contact angle of water for a rough thin film surface
and θ the distinguishing contact angle of water
contingent on the interfacial energy between the three
phases at the area of contact. If θw is less than 90° then
surface is known as hydrophilic surface and if θw is
more than 90° then it’s called a hydrophobic surface.
Contact angle θ corresponds to the flat surface value.
Roughness can increase or decrease the apparent
contact angle of a rough solid surface depending on θ.
If the surface of a substrate is rough, then the
actual surface area is greater than the plan surface
area and thus for a given drop volume, the total
liquid–solid interaction is greater on the rough
surface than on a flat surface. The presence of
surface roughness increases θ angle still further,
Wenzel, in 1936, assumed that the drop liquid fills
up the grooves on a rough surface and related the
surface roughness with the contact angle by a
simple expression.
Table 1 — Calculated parameters of zinc oxide thin films.
Sample name RF power (W) Temperature (°C) Avg d(XRD) (nm) Band gap (eV) Refractive
index (n)
Thickness (nm)
by %T data
30 W 30 300 15 3.29 1.49 889
90 W 90 300 17 3.26 1.50 927
150 W 150 300 19 3.22 1.52 1110
200 T 90 200 16 3.27 1.50 1068
450 T 90 450 17 3.24 1.51 1128
600 T 90 600 20 3.21 1.53 1224
INDIAN J. ENG. MATER. SCI., DECEMBER 2017
472
cos
cos
r r
eW s s
e
Ar
A
θ
θ= =
... (2)
Where rw is the ratio of the actual surface area, A
r, to
the apparent, macroscopic plan area, As, cos r
eθ cosθ is
the equilibrium contact angle of the real solid, and
cos s
eθ is the equilibrium contact angle on a flat,
smooth surface. Due to that we got change in
advancing and receding contact angle.
The AFM micrographs of zinc oxide nanostructured
thin films deposited at different RF power and
deposition temperature are shown in Fig. 3. The average
crystalline size increases with increase in the power and
temperature which is visible from AFM micrographs
thereby confirming XRD results.
Ethylene glycol is widely used in many
commercial and industrial applications as antifreeze
agent and coolant. Ethylene glycol helps keeping
car’s engine from sub-zero in the winter and
performances as a coolant to decrease stickiness in the
summer. The ethylene glycol used as heat transfer
fluids in many industrial system for ventilating, gas
compressors, heating, air-conditioning systems and
thermal solar energy systems motivated us to explore
the wettability properties of it with the deposited zinc
oxide nanostructured thin films7,18,19
. Higher contact
angle of ethylene glycol is also very useful in
corrosion inhibitor during cleaning after metal
chemical mechanical polishing.
The contact angle values for two liquids: water and
ethylene glycol with respect to surface roughness of
zinc oxide nanostructured thin films is shown in
Fig. 4. The contact angle was measured by sessile
drop technique with accuracy of ± 2o. When power is
varied from 30 W to 150 W the surface roughness of
zinc oxide nanostructured thin films is increased from
5.3 nm to 12.9 nm; the contact angle of water is
Fig. 3 — AFM images of the ZnO films deposited at different (a)
RF power and (b) temperature
Fig. 4 — Contact angle and surface roughness of ZnO films
Table 2 — Static and dynamic contact angle and contact angle hysteresis (CAH)
Static angle (in deg.) Dynamic angle (in deg.)
Sample
Roughness,
nm
Water EG Water EG CAH
θA θR θA θR Water EG
30W 5.3 45.4 36.4 46.2 34.4 37.2 22.8 11.8 14.4
90W 8.7 65.3 60.4 70.11 61.94 63.7 51.6 8.17 12.1
150W 12.9 97.6 75.6 99.7 93.5 79.3 69.9 6.2 9.4
200T 7.3 52.3 40.8 55.6 40.1 43.3 26.7 15.5 16.6
450T 9.2 68.6 61.5 73.6 64.3 66.4 53.4 9.3 13
600T 14.7 99.2 79.1 106.2 100.7 87.6 79.4 5.5 8.2
PATEL & RAWAL : SPUTTERED ZINC OXIDE NANOSTRUCTURED THIN FILMS
473
increased from 45.4° ± 2o to 97.6°± 2
o and of
ethylene glycol is improved from 36.4° ± 2o to
75.6° ± 2o. When the deposition temperature is
increased from 200°C to 600°C the surface
roughness is raised from 7.3 nm to 14.7 nm which
leads to higher contact angle of water from 52.3° to
99.2° whereas for ethylene glycol contact angle
varies from 40.8° to 79.1°. The zinc oxide
nanostructured thin films shows increase in their
surface roughness values with an increase in RF
power and deposition temperature. The deposited
zinc oxide nanostructured thin films shows an
increase in contact angle for water and ethylene
glycol with increase in surface roughness as contact
angle is directly proportionate to surface roughness,
that is consistent with literatures20-22
.
To characterize wettability properties of a thin film,
it is not sufficient to find out only the static contact
angle. Therefore, the dynamic contact-angle of water
and ethylene glycol (EG) was measured to study the
wetting behavior of the ZnO thin film surface. Due to
the expansion and contraction of the liquid the
advancing contact angle (θA) and receding contact
angle (θR) are formed; Contact angle hysteresis
(CAH) is the variance between these two angles. The
contact angle hysteresis is related to surface
roughness and adhesion of droplet to the surface21
.
The advancing contact angle (θA), receding contact
angle (θR) and CAH values measured for water and
ethylene glycol are listed in Table 2. When RF power
of zinc oxide films is increased from 30 W to 150 W,
CAH of water decreases from 11.8°± 2o to 6.2°± 2
o
and for ethylene glycol decline of values from
14.4°± 2o to 9.4°± 2
o is observed. CAH for water
declines from 15.5°± 2o to 5.5°± 2
o and for ethylene
glycol from 16.6°± 2o to 8.2°± 2
o when temperature of
zinc oxide films is raised from 200°C to 600°C.
Brassard et al.20
examined variation of contact angle
and CAH of 0-60 wt%, stearic acid (SA) functionalized
ZnO nanoparticles. They measured maximum CAH
value of 20° ± 5° at 7.6 ± 1.3 µm surface roughness for
0 wt% SA functionalized ZnO nanoparticles. The
maximum roughness value of 13.8 ± 1.7 µm was
observed at 60 wt% SA functionalized ZnO
nanoparticles with CAH value of 5°± 2°. We were able
to achieve lowest CAH values of 5.5° and 8.2° for
water and ethylene glycol respectively at maximum
surface roughness value of 14.7 nm at temperature of
600°C for deposited zinc oxide films. When CAH
decreases the drop of liquid gets easily rolled on that
surface when it’s slightly tilted from horizontal level.
This behavior is very useful for glasses which are used
in multi storage building and vehicle. We found that
the magnitude of CAH decreased with increasing RF
power and deposition temperature, which may be due
to decreasing interaction of water and ethylene glycol
droplet with nanostructured zinc oxide films surface.
Lower CAH values specifically for ethylene glycol can
be useful for its application as a corrosion inhibitor.
Surface energy for a film surface can be
personalized via two challenging processes; namely,
varying the surface chemical composition and the
surface morphology. Contact angle that depend on
surface roughness which differs inversely with
surface energy for a thin film23
. Surface energy of
ZnO nanostructured thin films calculated by Owens–
Wendt6,24
and Wu method is shown in Figs 5(a) and
5(b), respectively. The surface energy of ZnO
nanostructured thin films found by both methods
decreases when RF power is increased from 30 W to
Fig. 5 — Surface energies of ZnO films calculated by (a) OW method and (b) Wu method.
INDIAN J. ENG. MATER. SCI., DECEMBER 2017
474
150 W and temperature is increased from
200°C to 600°C. The total surface energy which is
sum of the polar and dispersion components
found by two methods are in good agreement with
each other. The highest contact angle of water and
ethylene glycol for ZnO nanostructured thin films is
obtained for samples 150 W and 600T, respectively.
So we have demonstrated the development of
repellent ZnO nanostructured thin films that can be
tailor made as per the requirement of specific
applications involving water and ethylene glycol. It
can have possible uses as wear and erosion resistant
defensive coatings.
To measure absorbance and transmittance spectra
used UV-vis-NIR spectrophotometer for zinc oxide
nanostructured thin films. The optical transmittance of
the film was measured by UV-visible spectrometer in
the range from 350 to 800 nm. The transmission
curves for zinc oxide nanostructured thin films
deposited at different RF power and deposition
temperature are shown in Figs 6(a) and 6(b),
respectively. It is experimental that the transmittance
of the films decreases with increase in the RF power
and deposition temperature. The thickness and grain
size affects the transmission and optical band gap
values. The thickness of the deposited films as
calculated from the transmission data25,26
are given in
Table 1. The thickness of ZnO thin films increases
with increase in RF power from 30 W to 150 W.
When deposition temperature increases from 200°C to
600°C thickness of ZnO coating increases, thereby
leading to greater surface roughness values and
decline in transmission for both cases.
It is clearly observed from Figs 6(a) and 6(b) that
with increase in the RF power and deposition
temperature the transmission values of ZnO
nanostructured thin films decreases. The thickness
and average crystallite size of ZnO nanostructured
thin films increases with increase in RF power and
deposition temperature. Larger crystallite size
collective with high surface roughness will lead to
more electrons scattering when increasing the RF
power and deposition temperature. This results in
decline of transmission values of ZnO nanostructured
thin films. The model projected by Manifacier et al.27
is used to obtain refractive index of ZnO
nanostructured thin films from its transmission data as
given in Table 1. It’s clear that the refractive index ‘n’
is in the range of 1.49 to 1.52 for variation of RF
power and 1.50 to 1.53 for deposition temperature
variation. The value of refractive index increases with
increase of the RF power and deposition temperature.
To measure the optical band gap of zinc oxide
films, the absorption spectra of the films were noted
as a function of the wavelength. Using the Tauc
relation, find out the optical band gap (Eg) of films
from the absorption coefficient (α)28
. As reported in
the literatures zinc oxide is direct band gap
semiconductor29,30
. Figures 7(a) and 7(b) show the
plot of (αhυ)2 on the y-axis versus photon energy hυ
on the x-axis for the zinc oxide films, an
extrapolation of the linear region of a plot indicate
approximation of the optical band-gap Eg since
Eg=hυ when (αhυ)2 = 0, an energy as per Tauc
relation. The optical band gap value of zinc oxide is
around 3.2 eV as reported in literature30
. The
calculated Eg value for zinc oxide films varies from
3.29 eV to 3.22 eV for RF power variation from 30
W to 150 W and from 3.27 eV to 3.21 eV for
deposition temperature variation from 200°C to
600°C. The observed band gap values of ZnO
Fig. 6 — Optical transmission curves of ZnO films deposited at different (a) RF power and (b) temperature
PATEL & RAWAL : SPUTTERED ZINC OXIDE NANOSTRUCTURED THIN FILMS
475
nanostructured thin films deposited at various
sputtering conditions are in good indenture with
literatures31,32
.
Conclusions ZnO nanostructured thin films were deposited at
various RF power and deposition temperature. The
(0 0 2) peak of ZnO thin films improves, and the grain
size develops larger with increasing sputtering power
and deposition temperature. The maximum surface
roughness of 12.9 nm, 97.6° contact angle for water
and 75.6° for ethylene glycol is observed at RF power
of 150 W. At deposition temperature of 600°C, ZnO
nanostructured thin films have contact angle values of
99.2° and 79.1° for water and ethylene glycol,
respectively. The value of contact angle hysteresis
(CAH) decreases with increase in RF power and
deposition temperature for deposited zinc oxide films.
These films can have potential use as water repellent
protective coatings. The optical energy band gap
decreases while the refractive index increases as the
RF power and deposition temperature of ZnO
nanostructured thin films is increased.
Acknowledgement
This work has been supported by AICTE grant
number 20/AICTE/RIFD/RPS (POLICY-III)
24/2012-13 sanctioned under Research Promotion
Scheme (RPS). We are thankful to President and
Provost of CHARUSAT for supporting this research
work. We are thankful to Dr Jaymin Ray and Dr T K
Chaudhuri, Professor and Head, Dr K C Patel,
Research and Development Centre (KRADLE)
affiliated to Charotar University of Science and
Technology (CHARUSAT), India, for granting
permission to use various equipment’s available in
their characterization laboratory.
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