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Journal of Sol-Gel Science and Technology
Superhydrophobic ZnO thin film modified by stearic acid on copper
substrate for corrosion and fouling protections
Milad Abdolahzadeh Saffar1 Akbar Eshaghi
Mohammad Reza Dehnavi1
Received: 23 November 2021 / Accepted: 18 February 2022
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2022
In the present study, a two-stage process was used to create hydrophobic and anti-fouling thin films on copper substrates.
Initially, zinc oxide (ZnO) thin film was deposited on the copper substrate via a sol-gel method. Then, the film was modified
with stearic acid. The structure and morphology of the thin films were characterized through X-ray diffraction (XRD), field
emission scanning electron microscopy (FE-SEM), and Fourier transform infrared (FTIR) spectroscopy. The water contact
angle on the film surface was investigated by a water contact angle analyzer. Potentiodynamic polarization and salt spray
tests were used to study the corrosion behavior of the copper and coated copper samples. Also, the antifouling properties of
the thin films were investigated. Based on the FE-SEM result, the zinc oxide nanoparticle size was obtained as 60 nm. The
water contact angle on the copper and coated copper samples increased from 39° to 155°. The corrosion current densities of
the copper and coated copper samples were reduced from 1.31 μA/cm2 to 2.7 × 10−3 μA/cm2. In addition, the thin film
showed the antifouling effect.
* Akbar Eshaghi
Department of Materials Engineering, Malek Ashtar University of
Technology, Shiraz, Iran
Journal of Sol-Gel Science and Technology
Graphical abstract
Zinc oxide Superhydrophobic Anti-fouling Corrosion Sol–gel Stearic acid
1 Introduction
When unprotected materials are immersed in a water environment, seawater, or freshwater, unwanted biological deposits,
containing algae and marine animals, are formed on their
surfaces [1, 2]. Accumulation and growth of these biological
deposits on the hull surface increase the surface roughness,
friction, and corrosion rate in marine systems thus incurring
unnecessary costs [3]. Also, with microorganisms attaching
onto the ship hull, these organisms may be transferred to
another area which they do not belong [4]. This leads to
ecological damage and, thus endangers the ecosystem balance
[5, 6]. To prevent this problem, the marine structure surface
could be covered with wax, bitumen, and lead sheets [7].
Antifouling paints were developed in the late of eighteenth and
early nineteenth centuries [8]. Antifouling paints have always
been considered as a significant solution to prevent marine
fouling [9]. The use of tributyltin began in the middle of the
twentieth century [10]; however, it has detrimental effects on
moss larvae, causing the larvae to detach from the hull surface
[11]. In addition, the toxicity of tributyltin on marine animals
has been investigated [12]. These studies have reported the
presence of tributyltin in the bodies of most marine animals
including fish, seabirds, and whales [10]. Then, researchers
began to replace the tributyltin with new materials [13–15].
The use of nanotechnology, or in other words, surface engineering technology at the atomic and molecular scales, provides unique opportunities for establishing antifouling effects
[16–20]. Eshaghi et al. [21] developed superhydrophobic
nano-coatings with antifouling properties. The superhydrophobic coatings prevents marine organism adhesion on
the marine structures and equipment [22–24]. In this study,
zinc oxide (ZnO) layer was deposited on the copper substrate.
Then, to reduce the surface energy, the coated samples were
deposited by stearic acid (SA). Superhydrophobic, antifouling,
and corrosion behavior of the pieces were investigated.
Journal of Sol-Gel Science and Technology
2 Experimental procedure
2.1 Materials
High purity and laboratory-grade materials, including
ethanolamine (C2H7NO, Merck, 99%), acetic acid
(CH3COOH, Merck, 96%), stearic acid (C18H36O2, Merck,
98%), and diethanolamine (C4H11NO2, Merck, 97%) were
used to synthesis the solutions. Sulfuric acid (H2SO4,
Merck, 60%) and hydrochloric acid (HCl, Merck, 37%)
were also applied to wash and prepare the substrates.
2.2 Preparation of the samples
The adhesion of the ZnO film on the copper substrate is
critical. Before ZnO film deposition, Copper substrates were
polished through mechanical polishing (SiC sandpaper) to
create suitable surface roughness. The adhesion is obtained
by mechanical interlocking. The samples were also washed
with soap and water to remove primary contaminants and
soaked in 60% sulfuric acid for 15 min to remove residual
grease. In the next step, the samples were immersed in a
hydrochloric acid solution (50 wt.%) for 5 min. Copper
substrates were then placed for 10 min in acetone in an
ultrasonic bath for final cleaning. Finally, the samples were
washed with deionized water for 2 min and dried in the
oven at 50 °C for 60 min.
2.3 Preparation of the zinc oxide (ZnO) film
To prepare 0.2 M zinc oxide solution, first 4.38 g of the
zinc acetate was added to 50 ml solution of water and
Fig. 1 Schematic diagram of the thin film fabrication process
ethanol (water to ethanol ratio 3): (1) under magnetic
stirring for 30 min. To obtain a clear and stable solution, a
few drops of the acetic acid were added. The volume of
the solution increased to 100 ml by adding ethanol. It was
then placed inside a water bath at 60 °C and stirred for
60 min. The solution was then slowly cooled to an
ambient temperature, whereby finally zinc oxide sol was
obtained. The ZnO thin film was deposited on the copper
substrate via a dip-coating method. The prepared film was
calcinated at 350 °C for 5 min (heating rate of 4 °C/min).
The thickness of the ZnO films was increased by repeating
the cycles from withdrawing to heating. This procedure
was repeated 20 times. The thickness of the ZnO film was
about 500 nm.
2.4 Surface modification
After deposition of the zinc oxide thin film to obtain an
appropriate surface morphology, the film surface should be
chemically modified with a low surface energy material.
Stearic acid was used for this purpose. The stearic acid
powder was dissolved in 100 ml of acetone and subjected to
magnetic stirring at 40 °C for 120 min. For the stability of
the solution, 3 ml of diethanolamine was added to the
solution and putted in the ultrasonic bath at 50 kHz for
10 min. The spray coating method was employed to deposit
the solution on the zinc oxide film surface (air pressure was
adjusted to 1.5 bar, and the distance was 10 cm). Next, the
samples were dried in an oven at 80 °C (2 °C/min) for
120 min. The step of the modified zinc oxide thin film
deposition on the copper substrate is schematically shown
in Fig. 1.
Journal of Sol-Gel Science and Technology
2.5 Thin film characterization
The morphology of the film surface was investigated using a
field emission scanning electron microscope (FE-SEM, voltage of 15 kV). Surface and chemical analyses of the film
surface were performed via Fourier transform infrared spectroscopy (FTIR) and EDS analysis methods. The crystalline
structure of the thin film was investigated through the grazing
X-ray diffraction method. Water contact angles (WCAs) were
measured using a video and photo-based contact angle measurement system (Dino-lite, AM-4515T8-Edge). To evaluate
the corrosion behavior of the thin film, potentiodynamic
polarization tests were performed using a galvanostat/potentiostat (EG&G, A273) in a standard corrosion cell with Pt
electrode and an Ag/AgCl reference electrode saturated with
KCl (SSE). NaCl solution (3.5 wt%) was used as a corrosive
medium. The potentiodynamic polarization curve was
obtained from −50 to + 250 mV relative to the open-circuit
potential at a sweep rate of 1 mV/s. A salt spray test was also
utilized to measure the corrosion resistance of the samples.
The samples were placed in a salt spray chamber for 120 h at
Fig. 2 FE-SEM images of the
uncoated copper surface (a),
ZnO film with ×50,000
magnification with water droplet
placement (b), ZnO film with
×100,000 magnification (c), and
SA film with ×100,000
magnification with water droplet
40 ± 5 °C (sodium chloride solution 5 wt%, pH = 6.5–7.2).
Finally, the antifouling test was performed in a glass container
with a dimension of 30 × 30 cm2. An electric motor was used
to supply air. Seawater containing fouling was obtained from
the Caspian Sea, and high-power LED lamps were used to
accelerate the fouling growth.
3 Results and discussion
FE-SEM was used to study the nanoparticle size distribution and surface morphology. According to Fig. 2a, the
surface of the copper substrate is completely heterogeneous
and porous. Figure 2b, c displays the surface morphology of
the zinc oxide thin film. As can be seen, the shape of the
particles is quasi-spherical, and the average size is 60 nm.
Figure 2d reveals the morphology of the modified surface of
the zinc oxide film with stearic acid.
As can be seen from Fig. 3, micro-nano structures have
been formed on the surface of the film surface. The existence
of such structures is one of the main factors for the formation
Journal of Sol-Gel Science and Technology
Fig. 3 Schematic diagram of the
thin film structure and
Cassie model
of superhydrophobicity [25]. Indeed, according to the Cassie
model [26], these structures reduce the contact surface of the
droplet on the solid surface through trapping air packages, thus
facilitating the slip of the droplet from the surface and
increasing the WCA.
EDS elemental analysis was performed to confirm the
successful formation of the ZnO and SA films. As displayed in
Fig. 4a, the elements Cu, O, and Zn are present within the film
structure, confirming the formation of the ZnO film due to
strong Zn and O peaks. The strong Cu peak is also related to
the copper substrate. The EDS results indicate the successful
formation of the ZnO film on the copper substrate. According
to Fig. 4b, the presence of the two strong peaks, O and C
elements, confirms the formation of the stearic acid film.
Figure 5 indicates the X-ray diffraction pattern of the ZnO
thin film. Considering this pattern and comparing it with
standard zinc oxide patterns (JCPDS card number, 1451-351),
it was found that the sample has a hexagonal wurtzite crystal
structure. It can be seen that the X-ray diffraction angles
associated with extreme peaks are slightly increased compared
to the diffraction angles of the standard JCPDS card (145136). This is due to the slight deviation of the hexagonal
wurtzite lattice from its equilibrium structure due to inherent
point defects within the zinc oxide lattice, including oxygen
voids or interstitial Zn2 + ions. It can also be seen that the three
peaks of (100), (002), and (101) hexagonal wurtzite crystals
are more intense than the other peaks. The intensity of these
three peaks is close to each other. Thus, the crystallites in all
three samples have been oriented in three main directions
<100>, <002>, and <101>. Indeed, their orientation has been
random, though the intensity of the (101) peak has been more
intense than that of the other two peaks. This result is consistent with other results produced via a sol–gel method [27].
Figure 6 depicts the FTIR spectrum of the stearic acidcoated samples. In this spectrum, two strong peaks are
observed at the wavenumbers of 2912 and 2847 cm−1,
which correspond to the symmetric and asymmetric
stretching vibrations of the CH in the –CH2 groups [28]. As
also, the peak corresponding to 2959 cm−1 is assigned to the
CH3 group [29]. The absorption band at 1697 cm−1 is
associated with the carbonyl group (C=O) [28], while the
absorption bands at 1296 and 1296 cm−1 are related to the
unstable vibration of the CO and OH groups in the carboxyl
group (–COOH) [30]. Also, the peaks at 1469 and 563 cm−1
belong to the C–H bending vibration and C–C carbon chain
[31]. The presence of the CH3, –CH2, and COO bonds in
the stearic acid can undoubtedly reduce the surface energy
and enhance the hydrophobic property [29].
Figure 7 shows the static contact angle of the water
droplets on the pristine copper surface, copper surface
prepared with sandpaper (numbers 600 and 1200), ZnO
coated copper surface, and ZnO coated copper surface
modified with different concentrations of the stearic acid.
It is well known that to obtain a superhydrophobic surface,
both roughness and low surface energy parameters need to be
present at the same time [32]. It is observed that the contact
angle obtained on the ZnO coated copper surface is 74 ± 3°
and the optimal concentration of the stearic acid for the highest
contact angle (155 ± 3°) is 6 g/l. Stearic acid creates the
minimum surface energy on the surface due to the formation
of the CH3 groups. In this way, water and fouling repellent
properties can be achieved. When stearic acid is applied to the
surface, it is created a dense outer CH3 surface for the maximum repellent effect. The proposed mechanism of the zinc
oxide with an applied stearic acid coating is illustrated in
Fig. 8.
The potentiodynamic polarization diagrams of the samples after 0.5 h immersion in 3.5 wt% sodium chloride
solution are shown in Fig. 9. In the bare copper diagram, a
decline in the current density at a potential of 60 mV is
observed in the anode branch. At this potential, as the
current density diminishes, the diagram enters the quasiinactive region, which is similar to the results obtained by
other researchers [33, 34]. Indeed, the reduction in the
current density in the anodic branch is due to forming a
CuCl2 film on the copper surface. The film is formed
through an anodic dissolution reaction of the copper in the
active region of the diagram [35]. The corrosion current
Journal of Sol-Gel Science and Technology
Fig. 4 EDS spectra of the ZnO
(a) and SA (b) films
Fig. 5 X-ray diffraction pattern
of the ZnO thin film
Journal of Sol-Gel Science and Technology
Fig. 6 FTIR spectrum of the
stearic acid-coated sample
Fig. 7 Water contact angle
images on the uncoated copper
surface (a), after sandpaper
preparation 600 (b) and 1200 (c),
ZnO coating (d), after
modification by stearic acid at
different concentrations; 5 g/l (e),
5.5 g/l (f), 6 g/l (g) and 6.5 g/l (h)
Fig. 8 Reaction mechanism for
ZnO and SA [36]
Journal of Sol-Gel Science and Technology
density (icorr) and the corrosion potential (Ecorr) in Fig. 9
have been obtained using the extrapolation method with the
values reported in Table 1.
Fig. 9 Potentiodynamic polarization diagram of the samples
Table 1 Corrosion
characteristics of the samples
IE (%) Rp (KΩ cm2) βα (mV dec−1) βc (mV dec−1) icorr(μA cm−2)
Ecorr (mV)
13.9 ± 1.15
1.31 ± 0.11
−203 ± 7
Cu + SA
13.8 ± 21
1.405 ± 0.015
−298 ± 25
577.3 ± 196
Cu + ZnO/SA 98
Fig. 10 Light microscopic
images of the samples under salt
spray test; copper sample before
(a) and after (b) salt spray,
coated copper sample before (c)
and after (d) salt spray
Table 1 shows that the SA coating cannot protect the
copper substrate from corrosion in the presence of chloride
ions. The corrosion potential of this coating is almost equal
to that of the uncoated copper. This is due to the lack of
proper adhesion to the copper substrate. As outlined in
Table 1, the ZnO/SA coating showed better corrosion protection than the SA coating and copper samples. This
coating reduced the icorr in the anodic and cathodic branches. The effect on both anodic and cathodic branches
indicates that the anodic (dissolution of the copper substrate
in the anodic regions) and cathodic reactions are limited by
the coating. This can be attributed to the protective properties of the coating. ZnO coating can be adhesive on the
copper substrate through a Cu-Zn bonding. Indeed, this
coating prevents the penetration of aggressive agents (such
as chloride ions and oxygen molecules) from reaching the
copper surface, thus reducing the corrosion reaction rates.
Figure 10 indicates the salt spray test results of the uncoated
and coated copper samples. According to Fig. 10a, b, the bare
2.71 × 10−3 ± 0.18 −154 ± 15
Journal of Sol-Gel Science and Technology
copper has been completely attacked by corrosive ions, and
corrosion effects can be seen on its entire surface. In contrast,
corrosion is not visible on the coated surface and corrosion
signs are only observed around the scratched areas in the
coated copper substrate (Fig. 10c, d). As a result, there is no
Fig. 11 Variation of the static contact angle on the coated copper
substrate in the salt spray exposure
Fig. 12 Schematic design of the
anti-fouling performance
Fig. 13 Fouling test on copper
and coated copper surface after
1 day (a) and 30 days (b)
sign of blistering, separation of the coating, or accumulation of
particles in the coated sample. It indicates the appropriate
resistance of the coating to the penetration of corrosive ions,
and hence, its high corrosion resistance.
Figure 11 reveals the WCA of the coated copper surface
during the salt spray test. The WCA decreased from 162° to
114° after 60 h exposed to the salt spray test, which indicates
the conversion of the superhydrophobic to the hydrophobic
state. The result suggests the appropriate stability of the
coating under marine environments. It can be used as a protective corrosion-resistant coating in marine environments.
The simulated environment was used to evaluate the antifouling properties of the samples. The superhydrophobicity of
the ZnO/SA coating prevents fouling from adhering onto the
coating surface by reducing the surface energy and trapping air
bubbles in the hierarchical structure [18]. A schematic of this
process is shown in Fig. 12.
Figures 13 and 14 depicted the fouling formation on
the copper and coated copper surface. As displayed in
Fig. 13a, after 30 days, fouling begins to form on the bare
copper surface. It is also observed that fouling occurs less
Journal of Sol-Gel Science and Technology
Compliance with ethical standards
Conflict of interest Akbar Eshaghi have affiliations with organizations
with direct or indirect financial interest in the subject matter discussed
in the manuscript.
Publisher’s note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional affiliations.
Fig. 14 Fouling test on copper and coated copper surfaces after
90 days
on the coated copper than on the bare copper surface
(Fig. 13b).
Also, according to Fig. 14, after 90 days, the amount
of fouling on the bare copper surface has increased in
comparison to the coated surface. The mechanism of the
fouling growth is such that initially, the primary fibers
adhere to the surface, after which the process of growth
and reproduction begins [21]. As a result, the coated
surface has far less fouling after 90 days (approximately
60%). It indicates that the ZnO/SA superhydrophobic
coating reduces the fouling formation on the solid surface
and can be used as an antifouling coating in marine
4 Conclusion
In this study, ZnO/ SA superhydrophobic nano-coating was
created on the copper substrate. The results showed:
1. The optimal concentration of the stearic acid to obtain
the maximum WCA was 6 g/l.
2. The size of the ZnO nanoparticles was observed as
60 nm.
3. The corrosion current densities (icorr) for bare copper
and ZnO/SA coated copper were 1.31 ± 0.11 and
2.71 × 10−3 ± 0.18 mV dec−1, respectively.
4. ZnO/SA nano-coating increased the WCA of the
copper substrate from 39° to 155°.
5. According to the antifouling results, the ZnO/ SA
coating would reduce the accumulation rate of the
fouling on the copper surface.
Thus, it can be concluded that ZnO/SA nano-coatings
can be used as an antifouling coating for marine equipment.
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