S. WEI et al.: FABRICATION AND OPTIMUM CONDITIONS OF A SUPERHYDROPHOBIC SURFACE ...
651–656
FABRICATION AND OPTIMUM CONDITIONS OF A
SUPERHYDROPHOBIC SURFACE USING A FACILE REDOX
REACTION AND A SOLUTION-IMMERSION METHOD ON ZINC
SUBSTRATES
IZDELAVA IN OPTIMALNI POGOJI ZA SUPERHIDROFOBNO
POVR[INO Z UPORABO REDOKS REAKCIJE IN Z METODO
POTOPITVE V RAZTOPINO CINKOVIH SUBSTRATOV
Shichao Wei1,2, Fumin Ma1, Wen Li1, Hong Li1,2, Min Ruan1, Zhanlong Yu1,
Wei Feng1
1Hubei Polytechnic University, Hubei Key Laboratory of Mine Environmental Pollution Control & Remediation, and School of Chemical and
Materials Engineering, Huangshi 435003, China
2School of Physics and Electronic Science, Hubei Normal University, Huangshi 435003, China
mahongbin12345@sina.com
Prejem rokopisa – received: 2016-08-05; sprejem za objavo – accepted for publication: 2016-11-16
doi:10.17222/mit.2016.243
A superhydrophobic surface on a zinc substrate was prepared with a simple and economic redox reaction and a solution-
immersion method, and the fabrication procedures and conditions were optimized. The effects of the reactant concentrations,
reaction time, modifier types, modifier concentrations and modification time on the fabrication of the superhydrophobic surface
were systematically studied by analyzing the water contact angle (CA) and SEM results. The results show that an excellent
superhydrophobic surface was prepared when zinc plates were immersed in a 0.01-M CuSO4 solution for 1 h and then modified
with 5 % of mass fraction of lauric acid for 1 h. The water CA of the prepared surface was as high as 155.7°. The surface
structure and composition of the superhydrophobic surfaces were characterized with SEM and FT-IR. The results show that the
surface structure consists of a small number of particles and petal-like structures, and the product exhibits a long alkyl chain
with a low surface energy, which contributes to the formation of the surface superhydrophobicity.
Keywords: superhydrophobic surface, cupric sulfate, redox reaction, solution immersion, optimum condition
Superhidrofobna povr{ina na substratu iz cinka je bila pripravljena z enostavno in ekonomi~no redoks reakcijo in z metodo
potopitve v raztopino ter z optimiziranjem postopkov in pogojev izdelave. U~inki koncentracij reagenta, reakcijski ~as,
modifikator u~inka, njegove koncentracije in ~as modifikacije pri izdelavi hidrofobne povr{ine, so bili sistemati~no preu~evani z
analizo stika kota z vodo (CA) in s SEM- rezultati. Rezultat je pokazal, da je bila superhidrofobna povr{ina odli~no pripravljena,
ko so bile cinkove plasti potopljene v 0,01-M CuSO4 raztopino za 1 h in nato 1 h modificirane s 5 % lavrinske kisline. Kontaktni
vodni kot pripravljene povr{ine je 155,7°. Struktura povr{ine in sestava superhidrofobne povr{ine sta bili preiskovani s SEM in
FT-IR analizo. Rezultati so pokazali, da je struktura povr{ine sestavljena iz majhnega {tevila delcev in ima listno strukturo.
Produkt ima dolgo alkilno verigo z nizko energijo povr{ine, ki pripomore k formaciji superhidrofobnosti povr{ine.
Klju~ne besede: superhidrofobna povr{ina, bakrov sulfat, redoks reakcija, potopitev v raztopino, optimalno stanje
1 INTRODUCTION
Surface wetting is an important property, which is
characterized by how a liquid makes contact with a solid
surface. It depends upon the combined effects of the
surface chemistry and surface morphology.1–3 In nature,
various biological surfaces, such as lotus leaves and rice
leaves, exhibit surface-wetting properties, which are
beneficial for their subsistence.4–9 Superhydrophobicity
and self-cleaning of lotus leaves were found to be a
result of a waxy hierarchical surface structure.10 Rice
leaves exhibit anisotropic dewetting behavior due to
anisotropic hierarchical structures with ordered arrange-
ments.11 Superhydrophobic surfaces, which exhibit
self-cleaning and non-wetting properties with a water
contact angle (CA) larger than 150°, have drawn much
attention both in scientific research and applications
based on their unique properties,12,13 such as anti-fogg-
ing,14 anti-icing,15,16 and self-cleaning.17,18 These
properties are desirable for many industrial and biologi-
cal applications such as anti-biofouling paints for boats,
anti-sticking of snow for antennas and windows, self-
cleaning wind shields for automobiles, metal refining,
stain-resistant textiles, and anti-soiling architectural
coatings.17
Inspired by nature, a large variety of methods have
been developed to synthesize a superhydrophobic surface
by constructing rough micro-nanostructures and
modifying the surface with chemical materials with a
low surface free energy, such as the sol-gel method,19
electrohydrodynamics method,20 plasma fluorination
method,21 laser etching,22 chemical vapor deposition23
and so on. Most of the methods need sophisticated and
expensive equipment, so it is necessary to use a simple
Materiali in tehnologije / Materials and technology 51 (2017) 4, 651–656 651
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 62-4-023.7:66.094.1 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(4)651(2017)
method to fabricate a surperhydrophobic surface. It is
well known that solution immersion is a simple method
to prepare a surperhydrophobic surface. D. K. Sarkar and
N. Saleema24 prepared a leaf-like Ag micro-nanostruc-
ture on copper substrates by means of adjusting the
concentration of the aqueous silver-nitrate solution and
the reaction time. The water contact angle (CA) they got
was about 162°. T. Ning, W. Xu and S. Lu25 prepared a
Pt micro-nanostructure on zinc substrates using solution
immersion. They simply prepared a superhydrophobic
surface using the replacement-deposition process. X.
Hou, F. Zhou, B. Yu, W. Liu26 prepared a superhydro-
phobic surface with hierarchical nanorods on zinc sub-
strates by means of differential etching and hydrophobic
modification. S. Wang, L. Feng, L. Jiang27 fabricated a
novel superhydrophobic surface merely by immersing a
copper sheet into a solution of fatty acid at ambient tem-
perature.
As a popular engineering material, zinc is widely
used in industrial fields. Many ways have been used to
fabricate a superhydrophobic surface on zinc substrates.
H. Liu, S. Szunerits, W. Xu, R. Boukherroub28 prepared a
superhydrophobic surface on zinc substrates with a con-
tact angle of 151° using a simple immersion technique.
T. Ning, W. Xu, S. Lu25 also introduced a simple way for
constructing superhydrophobic surfaces with hierarchical
flower-like micro-nanostructures using the deposition
process.25 The maximum CA value they got was about
171°. They also tested the stability of the prepared
surface and proved that a pure platinum surface on a zinc
substrate is stable. Nevertheless, the materials they used
are not economic. It is necessary to find an economic
material to fabricate a micro-nanostructure on a zinc
substrate. In addition, economic surface modifiers with a
lower surface energy can be used to promote the super-
hydrophobicity, such as lauric acid, myristic acid,
palmitic acid and octadecanoic acid.
In this work, we tried to fabricate a superhydrophobic
surface on a zinc substrate using a simple redox reaction
and surface modification. The micro-nanostructure on
the zinc substrates was created by immersing zinc plates
in aqueous solutions of copper sulfate with different con-
centrations and for different times. The superhydro-
phobic surface was prepared by modifying the surface
with different concentrations of the modifier and for
different times. The conditions for preparing superhydro-
phobic surfaces were further optimized. The procedure is
environmentally friendly, economic and easy to control.
2 EXPERIMENTAL PART
2.1 Materials
All the reagents used were of the analytical-reagent
(AR) grade and used as received without further purifi-
cation. A zinc substrate (10 × 10 × 0.2) mm with a purity
of 99.9 % was obtained from Tianjin Damao Chemical
Reagent Co. Ltd., China. Copper sulfate was purchased
from Tianjin Kaitong Chemical Reagent Co. Ltd., China.
Lauric acid was purchased from Tianjin Tailande Che-
mical Regent Co. Ltd., China. The other experimental
chemicals of the analytical-reagent grade were purchased
from Shanghai Chemical Regent Co. Ltd., China.
2.2 Preparation of a micro-nanostructure on zinc
substrates
Before the experiment, the surface of zinc was po-
lished with abrasive papers to remove the oxides and
then cleaned ultrasonically in sequence with alcohol and
deionized water, each time for 10 min to remove the
impurities of the zinc surface. After being dried at am-
bient temperature, dried zinc specimens were immersed
in a CuSO4 solution (a series of CuSO4 solutions with the
Cu2+ concentration ranging from 0.002 to 0.05 mol/L)
for a few minutes at room temperature. Then the
immersed specimens were rinsed with distilled water and
finally dried at 120 °C in air for 20 min. The plate with
the highest CA was considered for the optimized reac-
tion time and reactant concentration.
2.3 Modification
The plates were etched using the optimized reaction
time and solution concentration and then they were
modified with different modifiers for 1h, such as lauric
acid, myristic acid, palmitic acid and octadecanoic acid
with a concentration of 5 % mass fraction in ethanol.
The plate with the highest CA was considered for the
optimized modifier. Then the plates were modified with
the optimized modifier at different concentrations of (1,
2, 5, 10, 15, 20, 30 and 40) % mass fractions, and then
the optimized concentration could be found.
2.4 Characterizations
Both CA values were measured with a contact-angle
goniometer (FTA 1000, First Ten Ångstroms Inc., USA)
following the standard procedures. Water droplets were
placed at four positions on one sample, and the averaged
value was adopted as the contact angle. The volume of
an individual water droplet used for the static CA
measurements was about 5 μL. All Fourier transform
infrared (FT-IR) spectra were obtained with a FT-IR
spectrometer (Bruker Tensor 27, Germany). The micro-
nanostructure of the superhydrophobic surfaces was
observed with a scanning electron microscope (SEM,
HITACHI, S-3400, Japan).
3 RESULTS AND DISCUSSION
3.1 Influence of the reactant concentration and time
The effects of the CuSO4 concentration and reaction
time on the surface wettability of the obtained surfaces
were meticulously studied using CA measurements
(Figure 1). The zinc plates were immersed in the solu-
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652 Materiali in tehnologije / Materials and technology 51 (2017) 4, 651–656
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tions with different CuSO4 concentrations ranging from
0.002 to 0.05 M for different times. Then the reaction
products were modified with 5 % mass fraction of lauric
acid-ethanol solution for 1 h. As shown in Figure 1, the
water CAs of the surfaces varied significantly depending
upon the concentration of the CuSO4 solution under the
condition of a constant reaction time and the reaction
time under the condition of a constant CuSO4 concen-
tration.
With an increase in the CuSO4 concentration, the
values of the CA increase at first. When the CuSO4
concentration is 0.01 M, the CAs obtain their maximum
values. As the CuSO4 concentration continues to grow,
the CAs decrease. In addition, the CAs reach their maxi-
mum values when the reaction time is 1 h for different
concentration curves. It therefore indicates that 0.01 M is
the optimum concentration and 1 h is the optimum
reaction time. The change trend of the CAs may be a
result of an increase of the amount of the deposition
product formed on the surface and the resulting changes
in the surface roughness. With an increase in the CuSO4
concentration, the quantity of deposition increases under
the constant reaction time. As the reaction time in-
creases, the quantity of the product also increases under
the constant CuSO4 concentration. In fact, M. Miwa, A.
Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe29
found that surfaces with a water contact angle of 150°
could be developed by introducing a proper roughness to
the materials with low surface energies. Thus, when the
product quantity reaches a certain value, the value of the
surface roughness is proper, resulting in the maximum
CA.
3.2 Influence of the kind of modifier
Figure 2 shows the water CA for the zinc surfaces,
reacted for 1 h and modified with different modifiers
(lauric acid, myristic acid, palmitic acid and octadeca-
noic acid) for 1 h. The CA is 155.7°, 149.3°, 148.6°, and
144.4° when the surface is modified with lauric acid,
myristic acid, palmitic acid and octadecanoic acid, res-
pectively. The cause of this phenomenon is the same
mass fraction because lauric acid’s molecular weight is
minimum, and the numbers of myristic acid’s or palmitic
acid’s molecules in the unit volume are lower than the
number of molecules of lauric acid. It can be concluded
that among these modifiers, lauric acid is the most
powerful for achieving the best superhydrophobicity.
3.3 Influence of modifier concentrations
Figure 3 shows the water CA for the zinc surfaces
reacted for 1 h and modified with a lauric acid–ethanol
solution with concentrations of (1, 2, 4, 5, 10, 15, 20 and
25) % for 1 h. The CA is 144.5°, 146.4°, 150.6°, 155.7°,
145.9°, 144.0°, 142.8° and 141.0°. The CAs of the sur-
faces vary significantly depending upon the concentra-
tion of the lauric acid solution under the conditions of
constant reactant concentrations and reaction time
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Figure 2: Curves of the contact angle versus the modifier types. The
modifiers are lauric acid, myristic acid, palmitic acid and octadecanoic
acid
Figure 1: Curves of the water contact angle of the prepared samples
as a function of the concentration of the CuSO4 solution and reaction
time
Figure 3: Curves of the contact angle versus the lauric-acid concen-
tration
(0.01M CuSO4 concentration and 1 h reaction time).
When the concentration of lauric acid is 5 % mass
fraction, the water CA reaches the maximum value of
about 155.7°. This indicates that for the lauric-acid mo-
difier, 5 % of mass fraction is the optimum concen-
tration. When the concentration of lauric acid is less than
5 % of mass fraction, the CAs of the surfaces increase
with the concentration of lauric acid. When the
lauric-acid concentration is increased, there are more
deposit products on the surface structure, which may
damage the prepared micro-nanostructure on the zinc
plate so that the deep-seated rough structure obtained
with a redox reaction cannot play an effective role in the
preparation of the final surfaces, which results in a
decrease in the water CA.
3.4 Influence of the modification time
Figure 4 shows the water CAs for the zinc surfaces
reacted for 1 h and modified with 5 % mass fraction of
lauric acid for different modification times of (0.5, 1,
1.5, 2, 3, 4) h and 5 h. The CA is 147.3°, 155.7°, 147.7°,
145.3°, 145.2°, 145.2° and 144.6°, respectively. It is
clear that 1 h is a suitable modification time. The reason
for this is the fact that a chemical reaction taking 0.5 h is
not completed and the surface modification is not com-
pleted either. On the other hand, if the chemical-reaction
time is greater than 1 h, a layer of lauric-acid self-assem-
bled molecular film forms on the micro-nanostructure
surface, covering the original micro-nanostructure.
3.5 Surface micro-nanostructure and composition of
superhydrophobic surfaces
Figure 5 shows digital images and the wetting pro-
perty of water droplets on the untreated surface and the
prepared superhydrophobic surface using water-contact-
angle measurements, demonstrating the superhydro-
phobicity of the prepared surface. The water contact
angle of the untreated zinc substrate is 64.1° (the inset in
Figure 5a) while the treated zinc substrate surface
exhibits a water-contact-angle value of 155.7° (the inset
in Figure 5b). A smooth zinc surface is hydrophilic, but
a rough modified zinc surface becomes superhydro-
phobic. It is well known that the roughness structure and
the chemical composition of a surface result in the
wettability of a solid surface. A successful fabrication of
a superhydrophobic surface is believed to be a cooper-
ative effect of a surface structure prepared with a redox
reaction and a surface modification with low-surface-
energy materials.
The surface micro-nanostructure was further studied
with scanning electronic microscopy (SEM) as shown in
Figure 6. There are obvious differences as the reaction
time changes from 15 min to 105 min. Figures 6a and
6b are SEM images of a 15-min reaction at 2000× and
6000× magnifications, respectively. The results show
that a small amount of particles and petal-like structures
formed on the zinc substrate. The average diameter of
the particles is about 400 nm, as shown in Figure 6b;
that is, a certain micro-nanostructure is formed, which
makes the contact angle to be about 150°. Figures 6c
and 6d are SEM images of a 1-h reaction at 2000× and
5000× magnifications, respectively. Compared with
S. WEI et al.: FABRICATION AND OPTIMUM CONDITIONS OF A SUPERHYDROPHOBIC SURFACE ...
654 Materiali in tehnologije / Materials and technology 51 (2017) 4, 651–656
Figure 4: Water contact angles of the prepared samples for different
modification times ranging from 0.5 h to 5.0 h
Figure 5: Photographs of a water droplet on the surfaces of: a) un-
treated zinc substrate and b) zinc substrate treated under the optimum
condition. The insets correspond to the water contact angles
Figures 6a and 6b, the petal-like structures become
dense and the average diameter of the particles increases
to about 1 μm. A certain degree of hierarchical structure
is formed when the reaction time rises to 1 h. This kind
of hierarchical structure will keep the air in the inter-
spaces when a droplet drops on the surface structure.
Combined with the surface modification with a layer of
low-surface-energy materials, the contact angle reaches
about 155.7°. However, when extending the reaction
time to 105 min, the petal-like micro-nanostructure
becomes bigger as shown in Figures 6e and 6f. The
contact angle decreases to 148.5°.
Figure 7 shows the change in the surface com-
position of the prepared superhydrophobic surfaces on
the zinc substrates. The two peaks at 599.8 cm–1 and
617.2 cm–1 that appeared in the FT-IR spectra of the
reacted zinc surface without a modification are the Cu-O
stretching.30,31 Compared with the reacted zinc surface
without a modification, there are two new peaks at 2849
cm–1 and 2918 cm–1 in the FT-IR spectra of the super-
hydrophobic surface, which were identified as the
symmetric and asymmetric vibrations of -CH2 and -CH3
groups of lauric acid, respectively. What is more, the
stretching vibration of the free C=O band from lauric
acid at 1701 cm–1 disappeared and a new band appeared
at 1539 cm–1. This goes to prove the C=O stretching
vibration of coordinated COO– moieties to Cu2+ ions.
Thus, the results demonstrate that CH3(CH2)10COO– and
Cu2+ formed a bond of Cu(CH3(CH2)10COO)2 on the zinc
surface. The combination of the product with a low
surface energy and the micro-nanostructure allows the
prepared surface to have superhydrophobic properties.
4 CONCLUSIONS
In summary, we have prepared surperhydrophoic
surfaces on zinc substrates using a simple redox reaction
and the solution-immersion method. This fabrication
strategy is based upon the replacement process and
modification. In addition, we also investigated the opti-
mum conditions. The results confirmed that the surface
could be prepared by immerging zinc plates in a 0.01 M
CuSO4 solution for 1 h, using 5 % mass fraction of lauric
acid-ethanol solutions to modify the surface for 1 h. The
corresponding water CA of the surface was as high as
155.7°. The present work provides a simple and eco-
nomic method for preparing commendable superhydro-
phobic zinc surfaces, which holds bright prospects in our
daily life due to its simple and economic process.
Acknowledgements
The financial assistance of the National Nature
Science Foundation of China (51202082), the Natural
Science Foundation of Hubei Province (2010CDA026
and 2012FFB01001), the Key Program of Hubei
Provincial Department of Education (Z20104401), the
Outstanding Scientific and Technological Innovation
Team Project of Universities in Hubei Province
(T201423) and the Talent Program of Hubei Polytechnic
University (11yjz04R) is acknowledged.
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 7: FT-IR spectra of lauric acid (dotted line), the zinc surface
reacted for 105 min without a modification (solid line), and the
surperhydrophobic surface reacted for 105 min and modified with 5 %
mass fraction of lauric acid-ethanol solution for 1 h (thick line)
Figure 6: SEM images of the superhydrophobic surfaces on zinc
substrates reacted in a 0.01 M CuSO4 solution for different times and
modified with 5 % of lauric acid-ethanol solution for 1 h: a) 15 min
reaction, 2000×; b) magnified part of a), 6000×; c) 1 h reaction,
2000×; d) magnified part of c), 5000×; e) 105 min reaction, 2000×; f)
magnified part of e), 5000×
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