Fabrication of porous TiO2-SiO2 multifunctional anti-reflection coatings by sol-gel spin coating method

Article abstract of DOI:10.1039/c4ra10424b

In this study, we report that an antireflective coating with multifunctional properties, including high transparency, self-cleaning and abrasion resistance, was prepared by a facile sol-gel spin-coating method. The average transmittance of single coated g

Full text of DOI:10.1039/c4ra10424b

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Fabrication of porous TiO2–SiO2 multifunctional
anti-reflection coatings by sol–gel spin coating
method†
Cite this: RSC Adv., 2014, 4, 58101
a
ab
Keng Xua and Changsheng Xieb
*
Qiangqiang Mao, Dawen Zeng,
In this study, we report that an antireflective coating with multifunctional properties, including high
transparency, self-cleaning and abrasion resistance, was prepared by a facile sol–gel spin-coating
method. The average transmittance of single coated glass substrates was greater than 93% in the
wavelength range of 300–1000 nm compared to 90.1% for bare glass substrate. The photocatalytic
activity of the coatings showed a close dependence on the content of TiO2 nanoparticles. The coatings
showed the ability to decompose air borne organic contaminants (gaseous benzene) and applied
contaminants (fingerprints) under UV irradiation when the content of TiO2 was greater than 10%. After
illumination by UV light, the TiO2–SiO2 coatings showed hydrophilic properties with a water contact
angle of about 1ꢀ, which favored greatly the self-cleaning function of the coatings. When the content of
TiO2 was 10%, the critical load of the coating increased up to 305 mN as compared to that of the base-
catalyzed SiO2 coating (198 mN), which indicated the abrasion resistance property of this coating. This
approach suggested novel opportunities to a variety of practical applications, including windows of high
rise buildings, solar thermal collectors and solar cell cover glasses, etc.
Received 14th September 2014
Accepted 29th October 2014
DOI: 10.1039/c4ra10424b
www.rsc.org/advances
hydrophilicity.17–20 TiO2 could decompose the organic
pollutant absorbed on the surface of lm under UV light irra-
Introduction
diation. And dirt particles could be washed off from the surface
when water droplet rolled over the surface of the lm.2,19,20
In the past few years, two main methods have been used to
fabricate AR-SC coatings on glass substrate.7,9,10,12–14 In the rst
method, SiO2 and TiO2 particles were deposited on substrate by
various techniques, such as dip-coating, spin-coating and spray
pyrolysis techniques, to obtain double layer coating.9,10,13,14 The
bottom SiO2 and the top TiO2 lms were prepared to provide anti-
reection and self-cleaning properties, respectively. As the
thickness of TiO2 layer increased, the transmittance of the
coating decreased sharply with the self-cleaning capacity
increased.10 In the second method, the co-deposition of TiO2 and
SiO2 particles to form a single layer ARCs.7,12 However, the
refractive index of TiO2 was high (n > 1.45), so the content of TiO2
and the structure of the coating has to be controlled.2 According
to previous reports,2,6,7 the index of the coating could be lowered
by introducing porosity and reducing the particle size of TiO2.
Meanwhile, the more photo-active of TiO2 nanoparticle was due
to the organic molecules were decomposed at or near the TiO2
surface and its wider band gap.18 So a coatings with a high
porosity and TiO2 nanoparticle will maintain both the high
transmittance and self-cleaning property for a long time, which
was important when a coating was used in practical applications.
But only a few research works has focused on the preparation of
AR-SC coatings with TiO2 nanoparticles to maintain high anti-
reective and photocatalytic self-cleaning properties.21
Anti-reection coatings (ARCs) have been widely used in energy-
related applications including solar thermal collectors,
windows of high rise buildings and covers of solar cells.1–5 Up to
now, many research works have been conducted to improve the
transmittance property of the ARCs.6–8 But, a coating with only
anti-reection capacity will not satisfy the increasing demands
of more competitive technologies and practical applications.9
More and more recent research works consolidate the interest
and feasibility of elaboration of multifunctional coatings, such
as anti-fogging, anti-abrasion and self-cleaning coatings.7,10,11
For outdoor and solar energy-related applications, anti-
reection and self-cleaning (AR-SC) are two practical proper-
ties, which have been achieved on glass substrate by SiO2 and
TiO2 lms, respectively.7,9,10,12–14 On the one hand, porous SiO2
was considered to achieved high anti-reection property due to
its tunable refractive index properties.6,7,15,16 On the other hand,
TiO2 lm showed self-cleaning property because of its two
photo-induced phenomena: photocatalysis and photo-induced
aState Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong
University of Science and Technology (HUST), No. 1037, Luoyu Road, Wuhan 430074,
P. R. China. E-mail: dwzeng@mail.hust.edu.cn
bNanomaterials and Smart Sensors Research Lab (NSSRL), Department of Materials
Science and Engineering, HUST, No. 1037, Luoyu Road, Wuhan 430074, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra10424b
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Recently, there were mainly two approaches to investigate spin coating (8000 rpm, 20 s). The coated glass samples were
the self-cleaning property of these AR-SC coatings. Some dried in air at room temperature and were calcined at 500 ꢀC for
authors investigated the self-clean capacity of coatings by 30 min and cooled down to room temperature in the closed
measuring the degradation of organic lms deposited on the furnace.
surface of the coatings throughout water contact angle
measurement13,22 and IR-spectroscopy.7,12 Degradation of some
Photocatalytic activity
liquid pollutants, including rhodamine B14 and methylene
Photocatalytic performance of the coatings was evaluated by
blue,9,10 was also used to measure the self-cleaning property of
degrading gaseous benzene under irradiation of UV light at
the coatings. On the other hand, during practical application
ambient temperature in a 2 L cubic reactor, before which the air
process, the surface of the anti-reection coating was easily
tightness of the reactor was tested. The UV light was generated
polluted by various pollutants in the air and the applied
from a Xenon lamp (UV300, Aulight of China Co., Ltd) with a
contaminants. Those pollutants would dramatically deteriorate
wavelength range of 200–400 nm. Aer six pieces of the coated
the long term transmittance property of the anti-reection
glass substrates (3 ꢁ 3 cm, each piece) were placed into the
coatings. However, few researchers measured the self-cleaning
reactor, an amount of aqueous benzene was also injected within
property of AR-SC coating by the degradation of gaseous
by a syringe. The bottom of the reactor was slightly heated
pollutants and the applied contaminants.7 But, during practical
(below 40 ꢀC) to ensure benzene all evaporated. The analysis of
application process, those contaminants would dramatically
the concentration of benzene and CO2 was performed with a gas
deteriorate the long term transmittance property of the AR-SC
chromatography (GC9560, Shanghai Huaai Co., Ltd). Each set of
coatings. Therefore, it is an interesting and important work to
experiments was monitored for 3 h.
fabricate a coating with self-cleaning property to maintain the
high anti-reection property for a long time.
Characterization
In this study, TiO2–SiO2 anti-reection coating with multi-
The transmission spectra of the coated and uncoated glasses
were measured by using UV-vis transmittance spectra (Unico
UV-2012PC) in the wavelength range of 200–1000 nm. The
surface morphology of the coatings was characterized by scan-
ning electron microscopy (SEM, Sirion 200). Transmission
electron microscope (TEM) and high-resolution transmission
electron microscope (HRTEM) characterizations were
performed with a FEI Tecnai G220 microscope working at 200 kV.
Chemical composition of the coating was dened by X-ray
photoelectron spectroscopy (XPS, AXIS-ULTRA DLD-600W).
The surface wettability of the coatings was studied by a
contact angle/interface system (SL-200) with ꢂ1ꢀ accuracy in
atmospheric air at room temperature. Water droplets of 1 mL
were dropped carefully onto the coating surface. Nanoscratch
test was carried out using the Tribolndenter system (Hysitron
TI750) with a spherical diamond indenter tip.
functional characteristics, including high transmittance, self-
cleaning and abrasion resistance properties, was prepared by
spin-coating method. The effect of content of TiO2
nanoparticles on light transmittance, photocatalytic activity,
hydrophilicity and abrasion resistance properties of the coat-
ings was explored respectively. The combination of those
properties provided promising outdoor uses, such as window of
high rise buildings and solar thermal collectors.
Experimental
Materials
Tetraethyl orthosilicate (TEOS), ammonium hydroxide
(NH4OH), hydrochloric acid (HCl, 37%) and ethanol (EtOH)
were purchased from Sinopharm Chemical Reagent. Titanium
n-butoxide (n-Buti, 99%) was obtained from Alfa Aesar. All
chemicals were used as received.
Results and discussion
Preparation of sol–gel spin coatings
Anti-reection property
The base-catalyzed SiO2 sol was prepared by mixing TEOS, The effect of the content of TiO2 on the transmittance of coat-
EtOH, H2O and NH4OH with a molar ratio of 1 : 37 : 2 : 0.8. This ings was shown in Fig. 1. According to the UV-vis spectra, the
solution was stirred for 4 h and aged at room temperature for coated glass substrates showed high anti-reection. As shown
3 days. And then it was reuxed at 80 ꢀC until the pH of the in Fig. 1a, in contrast to the transmittance of the base-catalyzed
solution was about 6. TiO2 sol was prepared by mixing n-Buti SiO2 coating, the transmittance of the coatings decreased
and EtOH, followed by the dropwise addition of an aqueous gradually in the range of 400–1000 nm with the content of TiO2
solution of HCl. The chemicals were mixed at a molar ratio increased from 2 to 20 wt%. The average transmittance of the
n-Buti : EtOH : H2O : HCl of 1 : 60 : 4 : 0.3. This sol was stirred coated glass substrate was decreased from 93.43% to 92.3%
for 4 h and aged at room temperature for 1 day. Both precursor with the content of TiO2 increased from 0% to 20% in the
solutions were mixed together to obtain sols with different TiO2 wavelength range 300–1000 nm compared to that (90%) of bare
concentrations.
glass substrate, as shown in Fig. 1b.
The glass substrates (1 mm thick, 30 ꢁ 30 mm soda-lime
The refractive index of coating (nc) was one parameter to its
glass) were cleaned with acetone, ethanol and distilled water ideal anti-reection property. According to the relevant
by ultrasonic cleaning method, and then dried at 80 ꢀC. The research,23 the refractive index of porous coating can be calcu-
coatings were single deposited onto pre-cleaned glass slides by lated. The transmittance of the bare glass substrate is 90.6% at
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the TiO2 content was no more than 10% (Fig. 2a–d), the size of
most pores were less than 80 nm. The porosity of the coatings
was helpful to trap and restrain the scattered and reected
light.15 As a result; more light could transmit through the
coating. Then, the coatings showed high transmittance property
in the wavelength range (400–1000 nm). But, when the TiO2
content was 20%, there were some large pores (about 100 nm)
on the surface of the coating (Fig. 2e). The pore size was about
10% of the incident light wavelength, which could lead to
surface scattering.16 This was maybe the main reason for the
decrease of transmittance of this coating.
The thickness of TiO2–SiO2 coatings were also observed by
cross section morphology of the coatings from FSEM images
(Fig. 2). The thickness of the coatings was about 200 nm. On the
other hand, the thickness of the coating was corresponding to
the maximum transmittance of those coatings.16 As shown in
Fig. 1, the wavelength of the maximum transmittance of those
coating has not changed signicantly. This result also indicated
the uniform thickness of those coatings.
Chemical composition
Fig. 1 (a) Transmittance spectra and (b) average transmittance of the
coatings with different contents of TiO2.
XPS was used to determine the chemical composition of the
surface of 10% TiO2–SiO2 coating, as shown in Fig. 3. The
appearance of the Ti2p peak in the 10% TiO2–SiO2 coating
conrmed the introduction of TiO2 (Fig. 3b). The two peaks at
460.3 and 465.2 eV were the 2p3/2 orbital and the 2p1/2 orbital
of the Ti atom,24,25 respectively. The reported binding energy of
Ti2p3/2 for pure TiO2 coating was 458.2 eV.24,25 But, in this
article, the Ti2p3/2 of the 10% TiO2–SiO2 coating was 460.3 eV.
It showed a 1.1 eV of chemical shis to the pure TiO2 coating. In
the similar research work,24 the Ti2p3/2 binding energy of
the wavelength of 701 nm, and the refractive index was
estimated to be 1.55. The transmittance of the different coatings
(0%, 2%, 5%, 10% and 20% TiO2–SiO2) at 701 nm were 94.6%,
94.2%, 94.0%, 93.5% and 93.1%, respectively. Then, the nc were
estimated to be 1.47, 1.48, 1.49, 1.50 and 1.51, respectively.
Therefore, according to the results of the experiments, the
coatings could maintain the high transmittance when the
content of TiO2 was no more than 20%.
Surface morphology
FESEM images of the base-catalyzed SiO2 and TiO2–SiO2 coat-
ings were shown in Fig. 2. As shown in Fig. 2a–e, porous
structures were generated on the surface of the coatings and the
surface was changed with the increase in content of TiO2. When
Fig.
2 Scanning electron microscope micrographs for different
coatings: (a) base-catalyzed SiO2 coating; (b) 2% TiO2–SiO2 coating;
(c) 5% TiO2–SiO2 coating; (d) 10% TiO2–SiO2 coating; (e) 20%
TiO2–SiO2 coating. The insets are the cross-sectional electron Fig. 3 XPS spectra of 0% and 10% TiO2–SiO2 coating: (a) survey
micrographs of each coating. The scale bars represent 200 nm.
spectra, (b) Ti2p.
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(TiO2)10(SiO2)90 coating shied 1.25 eV as compared to that of
pure TiO2 coating. The chemical shis of Ti2p gave an evidence
of the formation of interaction between SiO2 and TiO2 particles.
To get further indication of TiO2 nanoparticles, the micro-
structures and crystalline of 10% TiO2–SiO2 powder were
studied. TEM and HRTEM micrographs were shown in Fig. 4. It
was obvious that the sample was consisting of good crystal TiO2
nanoparticles, which were dispersed in amorphous SiO2 parti-
cles, as shown in Fig. 4a. The average particle sizes of TiO2
estimated from HRTEM micrographs were about 5 nm (Fig. 4b).
The spacing d ¼ 0.232 nm of 10% TiO2–SiO2 powders
corresponded to the (004) plane of anatase TiO2 phase.26,27 The
nano-crystalline TiO2 particles in this coating provided high
photocatalytic property (Fig. 5 and 6).25 And this size of TiO2
nanoparticles could be made optically clear enough for
commercial window use.18
Self-cleaning properties
In this study, we investigated the self-cleaning property of
coating by the degradation of air borne organic contaminants
(gaseous benzene) and applied contaminants (ngerprints)
under UV irradiation.
Fig. 5 The curves of (a) conversion of C6H6 and (b) the amount of
produced CO2 of different coatings under UV light irradiation.
On the one hand, photocatalytic activities of different coat-
ings for degradation of gaseous benzene under UV irradiation at
ambient temperature were investigated. The conversion of
benzene and the production of CO2 were not observed when
these samples were in dark. Fig. 5 showed the photocatalytic
activities of coatings with different concentrations of TiO2
nanoparticles under UV light irradiation. As shown in Fig. 5a,
the amount of benzene still decreased when there was only glass
substrate and SiO2 coating in the reactor. This was due to the
adsorption of gaseous benzene. But the amount of adsorption is
limit. On the other hand, the coatings in this article were porous
coating, which can be seen in the SEM images (Fig. 2). Thus, as
shown in Fig. 5a, all porous coatings have the good absorption
effect of gaseous benzene. But the conversion amount of
benzene was different. Then the conversion of benzene was not
enough to evaluate the photocatalytic activity of the coatings. So
the curves of the production of CO2 of different coatings were
also shown (Fig. 5b). The production of CO2 of bare glass and
SiO2 coating were merely about 15 ppm which was almost same
as that of the blank test (no glass substrate, only with benzene
and UV-light irradiation), as shown in ESI (Fig. 10†). This
increasing may be due to the increasing temperature of the
reactor which was caused by the UV-light irradiation during the
ꢀ
test (about from 25 C to 36 ꢀC). And the concentration of CO2
Fig. 6 Photo-degradation of fingerprints applied on 10% TiO2–SiO2
coating surface upon exposure to simulated UV light. (a) After appli-
cation of the fingerprint; (b) after 60 min; (c) after 120 min; (d) after 150
Fig. 4 (a) TEM micrograph of 10% TiO2–SiO2 powders and (b) HRTEM min. The samples were briefly rinsed with water after 60 min to
micrograph of TiO2 nanoparticles.
simulate the effect of water.
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Table 1 The conversion of C6H6 and the amount of produced CO2 of
different coatings under UV light irradiation
Glass
0%
2%
5%
10%
20%
DC6H6/ppm
DCO2/ppm
DCO2/DC6H6
32
16
0.5
60
15
0.25
61
19
0.31
64
30
0.46
68
63
0.93
76
121
1.59
was affected by the temperature of the gas when it was
measured with a gas chromatography. This result also proved
that the decrease of benzene of bare glass and SiO2 coating was
due to the absorption effect. As shown in Table 1, the produc-
tion of CO2 increased remarkably from 19 ppm to 121 ppm
(Fig. 5b and Table 1) and the contrast of generated carbon
dioxide to the amount of degradation of benzene increased
from 0.31 to 1.59, when the content of TiO2 nanoparticles
increased from 2% to 20%. Those results shown the amount of
production of CO2 was correlated to the total amount of TiO2.
The photocatalytic activity of the coatings was corresponding to
the formation of anatase TiO2 nanoparticles (Fig. 4b).26 From
the above discussions, as the content of TiO2 nanoparticles
increased in the coatings, their photocatalytic activities gradu-
ally increased.
On the other hand, photocatalytic activities of 10%
TiO2–SiO2 coating for decomposing of ngerprint under UV
irradiation at ambient temperature was investigated. During
practical application, ngerprints will inevitably stick on the
surface of coating. Then the anti-reection property of the
coating will be seriously affected. As shown in Fig. 6, the
ngerprint disappeared completely from the surface of the
coating aer irradiation with UV light for 150 min. This result
indicated that the self-cleaning property of this coating was
useful to maintain high transmittance for a long time.
Fig. 7 Loading-time curves of (a) the base-catalyzed SiO2 coating and
(b) the 10% TiO2–SiO2 coating.
in Table 2. On the other hand, we measured the duration of the
photo-induced hydrophilicity of those TiO2–SiO2 coatings. The
change of the contact angle in dark was shown in ESI
(Fig. 11†). It seemed that the photo-induced hydrophilicity of
those coatings was not permanent. The highly hydrophilic
surface produced by UV light irradiation gradually converted
to less hydrophilic state in several days. But the hydrophilicity
state can easily be recovered by exposing the coatings to UV-
light.
Adhesion property
During practical application, the coated glass will be cleaned
usually, thus the coating should have good abrasion resistance
property. In this study, nanoscratch was performed to evaluate
Surface wettability
Water droplets could spread out to form a thin water lm on a
hydrophilic surface and then, dusts on the surface could be
taken away easily, showing the self-cleaning ability.2,19,20 Table
2 showed the contact angle of 1 mL water on the coatings with
different content of TiO2 (0%, 2%, 5%, 10%, 20% and 100%),
respectively. The water contact angle of the SiO2 coating was
about 1ꢀ. The increase of the contact angle was proportional to
the increase of the content of TiO2 nanoparticles in the coat-
ings. However, the wettability of the coatings became almost
same aer UV irradiation. This was due to the special photo-
induced hydrophilicity of TiO2.13,19,22 All coatings showed
hydrophilic property with contact angle less than 1ꢀ, as shown
Table 2 Water contact angles (CA) of coatings with different content
of TiO2 (0%, 2%, 5%, 10%, 20% and 100%) before and after UV irradi-
ation for 90 min
TiO2%
Glass
0%
2%
5%
10%
20%
100%
CA (before)
CA (aer)
38ꢀ
37ꢀ
<1ꢀ
<1ꢀ
<1ꢀ
<1ꢀ
9ꢀ
15ꢀ
<1ꢀ
21ꢀ
<1ꢀ
51ꢀ
<1ꢀ
Fig. 8 AFM images of scratch on the surface of (a) the base-catalyzed
SiO2 coating and (b) the 10% TiO2–SiO2 coating.
<1ꢀ
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Fig. 9 Schematic showing the preparation and the self-cleaning phenomena of the anti-reflection self-cleaning coating on glass substrate.
the abrasion resistance of the coating (10% TiO2–SiO2 coating), chemically bound to each other and to the surface through
as shown in Fig. 7 and 8. The critical load, Pc, is dened as the Ti–O–Si bonds (Fig. 3b and 9).2 As shown in Fig. 9, in particle
lateral load when the lateral load uctuation occurs in the application, organic pollutants and dirt particles will be stuck
scratch test. It is a comprehensive index that it represents the on the surface of coatings inevitably. The dirt particles will be
carrying capacity of the coating/substrate.28 It was mainly easily aggregated in the organic pollutants, which may affect
affected by the adhesive and hardness of the coating and the anti-reection effect of the coating. But, aer UV irradia-
substrate, the structure and thickness of the lm. Fig. 7 showed tion, the TiO2 nanoparticles in the coating could exhibit the
the load-distance curves of nanoscratch test of base-catalyzed desired simultaneous property of photocatalysis and photo-
SiO2 coating and 10% TiO2–SiO2 coating. The Pc of the base- induced hydrophilic. Then, the absorbed organic contami-
catalyzed SiO2 coating and 10% TiO2–SiO2 coating were 198 nants could be decomposed and the dust on the surface could
mN and 305 mN, respectively. This result pointed to excellent be removed easily by the rainwater.
abrasion resistance property of this coating. It also could be
identied in the AFM images of the scratches on the surface of
Conclusions
two coatings (Fig. 8). It was obviously that the 10% TiO2–SiO2
coating exhibited better scratch resistance property.
The preparation and the self-cleaning phenomena of the
AR-SC coating were presented in Fig. 9. The base-catalyzed
In this study, glass substrates were coated with TiO2 and SiO2
nanoparticles to achieve coating with both anti-reection, self-
cleaning and abrasion resistance properties by spin-coating
SiO2 particles were negatively charged and those particles
method. The coated glass substrate exhibit high trans-
repelled each other to form particulate sol.8,16 In the base-
mittance even the concentration of TiO2 was up to 20%. The
catalyzed SiO2 coating, individual SiO2 particles kept intact
average transmittance of the coating (10% TiO2–SiO2) increased
only by point contact forces,16 pointing to the poor mechanical
robustness. On the other hand, the negatively charged SiO2
more than 2.3% as compared to uncoated glass substrate.
Moreover, the photocatalytic activity of this coatings will lead to
particles in silica sol could selectively adsorb positive charged
the decomposition of contaminants (gaseous benzene and
TiO2 nanoparticles from the surrounding solution.29 TiO2
ngerprints) keeping the surface clean and maintaining the
nanoparticles seemed to act as viscous glue between the silica
high transmittance property. The photocatalytic and hydro-
particles. During the heat treatment process, ethanol solvent
philic properties of coating provided the self-cleaning function.
was evaporated and alkanol was burned; then pores with
different sizes were appeared. In order to enhance the
mechanical property, the neighboring particles were
Further, this coating exhibited abrasion resistance property.
The Pc of the coatings enhanced from 198 mN (0%) to 305 mN
(10%), pointing to excellent abrasion resistance of this coating.
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The combined anti-reection, self-cleaning and abrasion resis- 13 Z. Y. Liu, X. T. Zhang, T. Murakami and A. Fujishima, Sol.
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This work was supported by the National Basic Research
Program of China (Project no. 2009CB939705 and
2009CB939702). Also, we thank the technology support by the
Analytic Testing Center of HUST for carrying out FESEM and
XPS analyses.
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