Journal of Industrial and Engineering Chemistry 18 (2012) 1441–1445
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry
journal homepage: www.elsevier.com/locate/jiec
Improved hydrophilicity of zinc oxide-incorporated layer-by-layer
polyelectrolyte film fabricated by dip coating method
T. Charinpanitkul a,*, W. Suthabanditpong a,b, H. Watanabe b, T. Shirai b, K. Faungnawakij c,
N. Viriya-empikul c, M. Fuji b
a
b
c
Center of Excellence in Particle Technology, Faculty of Engineering, Chulalongkorn University, Payathai, Patumwan, Bangkok, Thailand
Ceramics Research Laboratory, Nagoya Institute of Technology, Nagoya, Japan
National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency, Thailand Science Park, Patumtani, Thailand
A R T I C L E I N F O
Article history:
Received 20 July 2011
Accepted 6 February 2012
Available online 13 February 2012
Keywords:
ZnO
Polyelectrolyte
Layer-by-layer
Hydrophilicity
Water contact angle
A B S T R A C T
ZnO nanoparticles suspended in poly(acrylic acid) (PAA) were deposited onto layer-by-layer (LBL)
polyelectrolyte (PET) films fabricated from poly(allylamine hydrochloride) (PAH) and PAA by dip coating
method. Effect of etching time and concentration of ZnO suspension on hydrophilicity of the LBL-PET
films before and after UV irradiation was examined using water contact angle measurement. 2.0 M PAH/
PAA solutions with a dipping speed of 3.0 cm/min provided stable LBL-PET films with thickness sufficient
for HCl etching. Glass substrates with the etched LBL-PET film dipped into 0.2 wt.% ZnO suspension
exhibited the contact angle of 108 after irradiated by UV for 60 min.
ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights
reserved.
1. Introduction
Hydrophilic property is one of the key features essential for self
cleaning surface, which is required for various applications, such as
anti-fog window, dirt-free wall [1–4]. There are many attempts to
improve hydrophilicity of various surfaces using polarized
molecules or some photocatalytic nanoparticles [2–5]. Recently,
it has been found that polyelectrolyte compounds, which contain
an organic chain and a polarized end, could hold strongly on
surface of some specific materials, especially glass surface which
would potentially provide functionalized characteristics. The
polarized end of polyelectrolyte compound has hydrophilic
property that could break down water droplets to spread over
the coated surface and help rinse away loose dirt attached to the
coated surface. Meanwhile, it is also reported that hydrophilic
property of a coated surface containing photocatalytic particles can
be triggered by the ultraviolet (UV) irradiation [6,7]. In general, the
hydrophilicity can simply be evaluated by mean of water contact
angle measurement. With the water contact angle lower than 58,
the coated surface would be recognized as superhydrophilic
characteristics [2,3]. On the other hand, if the water contact angle
is greater than 1508, the coated surface is classified as superhydrophobic.
* Corresponding author. Tel.: +66 2 218 6481; fax: +66 2 218 6480.
E-mail address: [email protected] (T. Charinpanitkul).
Among various semiconductive materials, zinc oxide (ZnO)
with a wide band gap of approximately 3.37 eV can be activated by
UV irradiation to exhibit hydrophilicity so that it is recognized as
an excellent surface functionalizing agent [4–6]. Accordingly, ZnO
is recognized as a promising compound in self-cleaning surface,
optical transmission, solar cell, varistor and chemical sensing
applications [5–10]. Meanwhile, a variety of synthetic polymeric
films have been studied to mimic the natural surface by enhancing
the surface roughness of the underlying organic film. It has been
recognized that a systematic coating of polyelectrolyte compounds, such as in poly(acrylic acid) (PAA) and poly(allylamine
hydrochloride) (PAH), in a fashion of layer by layer would provide a
functionalized surface which is known as layer-by-layer (LBL) film
[5]. Chemical etching of LBL films followed by deposition of
nanoparticles and organic molecules has been employed for
preparing modified LBL films with finer-scaled roughness and
lower surface energy [6–8].
However, investigation on incorporating the advantages of ZnO
and polyelectrolyte compounds for a control of hydrophilic
property of LBL films coated on substrates is still radical. A
combination of ZnO and polyelectrolyte would potentially provide
an alternative for the controllable functionalization of coated
substrates. In this study, we investigate the hydrophilic properties
of LBL-PET film incorporated with ZnO deposited onto glass
substrates under the effect of UV irradiation. The chemical etching
and loading of ZnO into the LBL-PET films prepared by a dip coating
method was experimentally examined and discussed.
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.jiec.2012.02.003
T. Charinpanitkul et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1441–1445
1442
2. Experimental
ZnOin PAA
PAA
2.1. Preparation of LBL-PET film coated on glass substrate
For the preparation of LBL-PET film, poly(allylamine hydrochloride) (PAH with molecular weight of 56,000) and poly(acrylic
acid) (PAA with molecular weight of 100,000) purchased from
Aldrich Chemicals were used as polycation and polyanion,
respectively. As schematically shown in Fig. 1, PAH is a cationic
polyelectrolyte prepared by the polymerization of allylamine with
hydrochloride. In general, PAH could be combined with any anionic
polyelectrolyte to form a layer-by-layer polymeric film. Meanwhile, PAA is a type of anionic polymer which is water-soluble at
neutral pH. It is well known that the side chains of PAA would lose
their protons and acquire negative ions. Therefore, it is recognized
as anionic polyelectrolyte which is applicable for dispersion of
nanoparticles. Difference in electrolytic properties of both polyelectrolytes could be supposed to provide surface improvement of
glass substrate and to accommodate nanoparticles in their layerby-layer structure [7,10].
Before dip coating, glass substrates were ultrasonically cleaned
for 30 min in de-ionized water (DI-water), ethanol, and acetone
consecutively. LBL-PET film could be fabricated by the exchange of
substrate dipping in polycation and polyanion solutions using a dip
coater. After cleaning, glass substrates were dipped into 2.0 M PAH
aqueous solutions with a dipping speed of 3.0 cm/min for 15 min.
Then the naturally dried glass substrates, which contained a singlelayer PAH film was further coated with polyanion by dipping into
the PAA aqueous solution with a concentration of 2.0 M. With this
one cycle, a bilayer of LBL-PET film could be prepared as shown
schematically in Fig. 2(a). For multilayer LBL-PET film fabrication,
the above mentioned procedures were repeated consecutively. The
LBL-PET film was subject to chemical etching with hydrochloric
(37 wt.%, analytical reagent A.R., Lab-Scan, Japan) for enhancing its
surface roughness. Etching time was varied in a range of 15–
90 min. In order to remove residual solvents, the etched PET film
on glass substrate was dried in the oven at a temperature of 180 8C
for 2 h. The etched film deposited on glass substrate is
schematically shown in Fig. 2(b), which reveals a denser composite
film with higher stability but thinner thickness when compared
with its untreated condition.
2.2. Deposition of ZnO onto LBL-PET film
Commercial ZnO powder with average particle size of 0.02 mm
was deposited into the etched LBL-PET film by dipping the coated
substrate into the suspension of ZnO in PAA solution. A
concentration of the ZnO suspension was varied between 0.1
and 0.2 wt.% with a purpose to investigate its effect on physical and
hydrophilic properties of the fabricated LBL-PET film. Schematic
appearance of LBL-PET film containing ZnO is also shown in
Fig. 2(c). Similarly, the ZnO-incorporated LBL-PET film exhibited
good stability and transparency, which could be observed by water
permeability test. Then, the samples were dried (180 8C), stored in
dark place for 24 h and activated by UV irradiation with a UV-A
NH2
O
x HCl
n
PAH
C
C
C
H
OH
H
PAA
Fig. 1. Chemical structure of PAH and PAA.
H
n
PAA
PAA
PAH
PAH
PAH
Substrate
Substrate
Substrate
(a) Original bilayerfilm
(b) Etched bilayerfilm
(c) ZnOincorporated film
Fig. 2. Schematic images of fabricated LBL-PET films.
lamp (Saphit A.J.L., Supplies, Thailand) which has an emission
wavelength in a range of 300–460 nm and output of 20 W.
Wettability of the fabricated LBL-PET films on glass substrates was
investigated by measuring the contact angle of water droplets with
a contact angle meter. Water droplets with a consistent volume of
0.012 103 m3 were placed at five different positions on the LBLPET films for investigating the uniformity of the hydrophilicity of
the fabricated films. All typical coated glass substrates were also
examined by an atomic force microscope (AFM; Veeco, Japan), a
confocal laser scanning microscope (CLSM; LEXT 3D Measuring
Laser Microscope CLS-4000; Olympus, Japan), a field emission
scanning electron microscope ((FESEM; JSM5410V; JEOL, Japan)
equipped with SEI and COMPO detectors, a Fourier transform
infrared spectroscope (FTIR; FT/IR6200; JASCO, Japan) and a water
contact angle meter (WCA, Japan).
3. Results and discussion
3.1. Effect of etching time on hydrophilicity of LBL-PET films
In prior to fabrication of layer-by-layer film of PET on glass
substrates, single-layer polycation (PAH) and polyanion (PAA)
films were separately examined. Cleaned glass substrates were
dipped into either PAH or PAA solution with a concentration
ranging from 0.5 to 3.0 M with a dipping speed of 3.0 cm/min. It
was found that 2.0 M PAH and PAA solutions could provide a stable
coating film, which was thick enough for etching by HCl solution.
These results are consistent with previous reports, such as
Numpud et al., and Nimittrakoolchai and Supothina [4,5].
Therefore, PAH and PAA solutions with a concentration of 2.0 M
were selected as a basis for fabrication of LBL-PET films. Fig. 3
illustrates AFM images revealing topological characteristics of
typical LBL-PET film surface etched by HCl solution at pH of 1.1
with different etching time of 15, 30, 60 and 90 min. Normally, bare
glass substrate would exhibit a root-mean-square surface roughness of 8–10 nm [11]. The untreated LBL-PET films subject to AFM
analyses would possess a relatively smaller surface roughness of
5–8 nm (not shown here). After the chemical etching treatment, it
could be clearly observed that the etching process could provide
the fabricated LBL-PET films with a different degree of surface
roughness in a range of 4–6 nm. With the shortest etching time of
15 min, a number of mountains and valleys with an average
diameter smaller than 12 nm could be observed thorough the
etched LBL-PET film surface (Fig. 3(a)). Etching time of 30 min
could provide rather uniform spike-like mountains on the etched
LBL-PET film as could be observed in Fig. 3(b). However, with a
further increase in the etching time to 60 and 90 min, the spike-like
mountains were transformed into round-edge ones with some
agglomerations (Fig. 3(c) and (d)). Typical 2D analyses of the
surface topology of the etched LBL-PET film could also provide a
consistent result that the LBL-PET film etched for 30 min exhibited
spikes with rather uniform height and depth of 4–6 nm. These
results would be attributed to the enhanced hydrolysis of PAH or
PAA by H+ ions, which would be affected by the longer etching time
T. Charinpanitkul et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1441–1445
1443
Fig. 3. 3D and 2D topographical images of LBL-PET film etched with different etching time. (a) 15 min, (b) 30 min, (c) 60 min, and (d) 90 min.
[7]. H+ ions would diffuse to the sharp edge of spike surface and
hydrolyze polymeric chains, resulting in gradual change of surface
roughness with respect to the etching time.
Surface roughness of the fabricated LBL-PET film was also
quantitatively verified using the CLSM. Root mean square (RMS)
values of the surface roughness of each fabricated film were
determined based on Eq. (1) whereas Zi is height of each spike and
summarized in Table 1.
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u P
2
u
N
t
i¼1 ðZ i Z avg Þ
(1)
RMS ¼
N
The wettability in term of water contact angle of each fabricated
film was also summarized in Table 1 for comparison. The LBL-PET
film etched for 30 min exhibited the average surface roughness of
1.9 nm, leading to the lowest contact angle at 22.48. This result is
consistent with the Wenzel model, which postulates that the
degree of surface roughness could affect the surface hydrophilicity
due to the capillary effect [3]. The existence of higher mountains or
deeper valleys on the surface could entrap lower volume of gas that
could resist the spreading of liquid water, resulting in a dominating
effect of capillary force which consequently leads to a lower water
Table 1
Summary of water contact angle and surface roughness of each coating film.
Sample
RMS roughness (nm)
Contact angle (8)
PAH
PAA
PAH/PAA (LBL-PET)
LBL-PET etched for 15 min
LBL-PET etched for 30 min
LBL-PET etched for 60 min
LBL-PET etched for 90 min
0.7
0.2
0.3
1.8
1.9
1.6
1.7
naa
69.6 3
49.5 3
27.2 4
22.4 2
29.7 3
33.1 2
a
na = not available because PAH film was dissolved in water.
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T. Charinpanitkul et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1441–1445
Fig. 4. Schematic diagram of ZnO distribution in LBL-PET film (a) and typical FESEM images in SEI and COMPO modes of ZnO in LBL-PET film (b).
contact angle [12]. Based on these comparative results, the LBL-PET
films etched for 30 min was selected for ZnO incorporation with an
expectation of further improvement in its hydrophilicity.
3.2. Improved hydrophilicity of ZnO-incorporated LBL-PET films
Etched LBL-PET films were immersed into a ZnO suspension in
PAA for 15 min with a purpose of deposition of ZnO onto the base
LBL-PET film. Instead of PAH, the PAA solution was selected for ZnO
suspension because PAA film was more stable when it was subject
to water. However, it should be noted that PAH film was rapidly
dissolved when water was applied onto it. Non-uniformity of PAH
film deposited on glass substrates would be rather difficult to
confirm by AFM or CLSM analyses but it could be verified by HPLC
analysis of liquid solution, which revealed that there was some
contents of PAH dissolved in the final liquid solution. As a result, it
is important to apply another layer of PAA onto the layer of PAH as
schematically shown in Fig. 2. Consistent results were also
reported by some previous works [5]. After applying ZnO onto
the base LBL-PET films coated glass substrates, distribution of ZnO
in the films was also analyzed by FESEM as depicted schematically
in Fig. 4(a). It should be noted that the COMPO mode makes use of
the back-scattered electrons, which are a reflection from the
sample due to elastic scattering [8]. The intensity of back-scattered
electrons signal is strongly related to the atomic number (Z) of the
specimen. As could be confirmed in Fig. 4(b), the LBL-PET surface
with atomic number lower than that of ZnO exhibited dark
background emission. In addition, SET mode analysis revealed that
two-dimensional growth mode could be retained about 18 nm.
Larger clusters of ZnO exist in the LBL-PET film would be ascribed
to the agglomeration of ZnO nanoparticle, which would inevitably
take place due to the strong cohesive interaction of ZnO
nanoparticles suspended in the PAA.
Typical infrared spectra of all representative films coated onto
the glass substrates are illustrated in Fig. 5. Fig. 5(a) and (b) is
respective spectra obtained from single-layer film fabricated by
dipping glass substrates into PAH and PAA solution at pH of 1.0 and
incubated by hot air at 180 8C for 2 h. A common absorption band
at 3700 cm1 represents the existence of the stretch O–H in both
PAH and PAA films [13]. However, an absorption band at
Fig. 5. FTIR spectra of PAH film (a), PAA film (b), LBL-PET film (c), and ZnO-incorporated LBL-PET film (d).
T. Charinpanitkul et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1441–1445
2990 cm1 would reveal the presence of CH2 group on the PAH film
surface where primary amine group (stretch N–H) could be
confirmed by peaks at 3100–3500 cm1. Meanwhile, the carboxylic group representing by peaks at 2400–3400 cm1 was observed
in PAA film only. Fig. 5(c) and (d) shows the UV spectra of LBL-PET
and ZnO-incorporated LBL-PET films fabricated by the same
procedure. A combination of some functional groups, which were
CH2 group at 2990 cm1, primary amine at 3100–3500 cm1 and
carboxylic group at 2400–3400 cm1, was observed in the LBL-PET
film surface. The existence of such combination is attributed to the
interaction between PAA and PAH film in the bilayer PET film [10].
However, an unique peak of O–H group at 3700 cm1 became
dominating when the ZnO-incorporated LBL-PET film was analyzed. This result is attributed to the rather uniform dispersion of
ZnO nanoparticles, which is consistent with the FESEM micrograph
shown in Fig. 4.
Furthermore, based on the water contact angle measurement,
the hydrophilicity of ZnO-incorporated LBL-PET films with two
different ZnO loading was significantly improved after the UV
irradiation as shown in Fig. 6. Before UV treatment, the LBL-PET
films which were subject to 0.2 wt.% ZnO in PAA exhibited a
contact angle of approximately 36.98. The contact angle significantly decreased to 13.78 and 10.18 after the fabricated LBL-PET
films were irradiated by UV light for 30 and 60 min. It should be
noted that the average water contact angle of the LBL-PET films
treated with 0.2 wt.% ZnO suspension (&) was lower than that of
film treated with 0.1 wt.% ZnO suspension (~). This result would
be attributed that the higher concentration of ZnO in LBL-PET films
could induce the higher polarity on the film surface [14]. In
addition, the contact angle of ZnO-incorporated LBL-PET film was
significantly lower than that of typical LBL-PET films because of the
ZnO nanoparticles photosensitive behavior resulted from the
photo-induced defective site on the ZnO surface. These results
would suggest us that the improvement of hydrophilicity of ZnOincorporated LBL-PET films coated on glass substrates after
irradiated with UV light would be dependent upon the formation
of active site on the ZnO surface embedding in the LBL-PET film.
Lowman and Buratto reported that the surface hydrophilicity of
treated PAA coated glass substrates would be dependent on the
initial charge density and surface roughness of the glass substrate
[15]. They found that the charge density on surface of a bare glass
substrate was rather low and not sensitive to UV irradiation.
However, PAA-coated glass substrate would exhibit an improved
hydrophilicity after UV irradiation because of an increase in anions
emitted from the PAA film. Meanwhile, ZnO thin film would also
provide excited electrons which could result in lower electrical
resistivity when exposed to UV irradiation [16]. Therefore, the
integrated results of the increasing polarity on the surface of the
LBL-PET film coated on the glass substrate would reasonably
enhance its hydrophilicity. Our experimental results would also
confirm that the hydrophilicity of LBL-PET films coated in glass
substrates could be improved by incorporating commercial ZnO
nanoparticles into the LBL-PET films fabricated by dip coating
method. Investigation on hindering ZnO agglomeration within the
LBL-PET films would be further required to realize the superhydrophilic coating film.
1445
Fig. 6. Effect of UV irradiation time on water contact angle of ZnO-incorporated LBLPET films ((~) 0.1 wt.% ZnO loading in LBL-PET film, and (&) 0.2 wt.% ZnO loading in
LBL-PET film).
4. Conclusion
LBL-PET films incorporated with ZnO nanoparticles were
simply prepared by dipping glass substrates into PAH and PAA
solutions. Etching LBL-PET films by HCl solution would give rise to
an improved hydrophilic property because of the capillary effect
due to surface roughness. The etching time of 30 min could provide
the LBL-PET film with the spiky surface roughness of 4–6 nm. The
fabricated ZnO LBL-PET film exhibited a significant difference in
the presence of O–H group on its surface. In addition, after
irradiated by UV light for 60 min LBL-PET films dipped into 0.2 wt.%
ZnO suspension in PAA exhibited significant improvement in its
hydrophilicity represented by a water contact angle of 108.
Acknowledgments
Financial supports from the Centenary Fund of CU and Japan
Student Service Organization (JASSO), JAPAN are gratefully
acknowledged.
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