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

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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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