A Novel and Facile Method to Fabricate SiO
2
Nano-pillar
Arrays on Glass Surface
H B Xu
1
, J Zhang
1
, Y Huang
1, 2
, Z H Zhou
1, 2, *
and S Shen
3
1
College of Materials, Xiamen University, Xiamen, Fujian, China 361005
2
Fujian Key Laboratory of Advanced Materials, Xiamen, Fujian, China 361005
3
CSIRO Manufacturing, Clayton, VIC 3168, Australia
Abstract. SiO
2
regular nano-pillar arrays have been successfully fabricated on glass
substrates by imprinting anodized nanoporous aluminum oxide (AAO) templates on inorganic
SiO
2
sol coatings. The prepared SiO
2
nano-pillar array by using a template with the pore
diameter of 43.01 nm shows an average pillar diameter of 48.78 nm, a surface roughness of
33.1 nm and a static water contact angle of 150.47°. Surface roughness of SiO
2
nano-pillar
arrays can be adjusted by changing pore size of AAO templates and increases with the
decrease of pore diameter of AAO templates. Transparent superhydrophobic glass is obtained
when the prepared nano-pillar array is modified by fluoroalkylsilane, presenting a new way of
preparing superhydrophobic glass.
1. Introduction
In recent years, transparent superhydrophobic surfaces have received extensive attention [1-4] due to
their transparent, self-cleaning and anti-icing properties, and their potential applications on
automotive, building and solar cell glass. Two requirements for superhydrophobic surface are surface
roughness and low surface energy [5]. Rough surface structures are usually constructed with
nanoparticles [6,7], nanopits [8], nanofibers [9], nanopillars [10,11], nanocones [12] and the like.
Nanopillars and nanocones with the effect of anti-reflection and increasing visible light transmittance
have great advantages in the preparation of transparent superhydrophobic glass.
There are two methods reported for preparing superhydrophobic nano-pillar arrays on glass
substrates, one of which is patterned etching of glass surface to prepare glass nano-pillar arrays. Kim
et al. [10] used nano-imprint lithography technology with anodized nanoporous alumina oxide (AAO)
as template to form a Cr mask pattern on glass and then used inductively coupled plasma dry etching
process to produce transparent superhydrophobic glass composed of nano-pillar array. Son et al. [13]
produced an AAO mask pattern on glass by E-beam evaporation and anodizing technologies, and
then prepared nano-pillar array using a dry etching process. However, patterned etching process on
glass surface is complicated and requires expensive instruments, which limits its practical
applications. Another method is to prepare nanopillars by embossing AAO templates on organic
coatings on glass surface. Cho et al. [11] prepared superhydrophobic glass consisting of
polydimethylsiloxane nano-pillar array with contact angle as high as 163.4° by using PDMS as
coating material and AAO as template; Liu et al. [14] successfully prepared polyimide nano-pillar
Xu, H., Zhang, J., Huang, Y., Zhou, Z. and Shen, S.
A Novel and Facile Method to Fabricate SiO2 Nano-pillar Arrays on Glass Surface.
In Proceedings of the International Workshop on Materials, Chemistry and Engineering (IWMCE 2018), pages 493-499
ISBN: 978-989-758-346-9
Copyright © 2018 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
493

structural superhydrophobic glass with PI coating and AAO template; Lee et al. [15] produced
transparent superhydrophobic glass consisting of polystyrene nano-pillar array with PS as coating
material by hot-pressing process with AAO template. This method is relatively simple, but organic
nanopillars have various weaknesses, such as poor adhesion on glass substrate, and degradation
during long-term use. As far as our knowledge, there are few reports about SiO
2
nano-pillar arrays
prepared by AAO template imprinting method.
In this work, we report a novel and facile method to fabricate SiO
2
nano-pillar arrays on glass
surface by using acidic SiO
2
sol as coating material and AAO as template. By changing pore size of
AAO templates, a series of SiO
2
nanopillar structural rough surfaces with controllable morphologies
can be obtained.
2. Experimental
Single-pass nanoporous alumina oxide templates (Shenzhen Topology Technology Co., Ltd.) were
used as recieved. The template includes three-layer structure consisting of a regular AAO porous
layer (effective AAO layer), an intermediate Al base and a back random AAO layer. Acidic SiO
2
sol,
which preparation details had been described in our previous report [16], was obtained by acid
hydrolysis of tetraethyl orthosilicate in a water-ethanol solution containing nitric acid and a coupling
agent (KH560) was added as a binder.
The SiO
2
sol was spin-coated on a clean glass substrate (3.2 mm float green glass) at 380 rpm for
10 sec, and after aging in air for 6-8 min, an AAO template was placed on the coating and slowly
pressed to perform embossing, as shown in Figure 1. Subsequently, the glass covered by AAO
template was heated to 80 °C for 10 min and then kept at 180 °C 60 min for solidification. After
cooled to room temperature, the glass was successively placed in 5 wt% H
3
PO
4
solution at 45 °C for
1 h to remove the back AAO layer, 23 wt% CuCl
2
+ 8.5 wt% HNO
3
aqueous solution at room
temperature for 5 min to remove the intermediate Al base, and again 45 °C, 5 wt% H
3
PO
4
solution
for 1 h to remove the effective AAO layer. Finally, SiO
2
nano-pillar array on glass surface was
obtained.
Fluoroalkylsilane (FAS) modification was implemented by chemical vapor deposition according
to reference [17], using 97% perfluorodecyltrichlorosilane as FAS reagent. Before FAS modification,
UV-O
3
irradiation was conducted as pretreatment.
Figure 1. AAO imprinting process.
Contact angle meter (DGD-ADR, 8 μL water droplet, tangent method, room temperature) was
used to measure static water contact angle (WCA) and roll angle (RA) of as-prepared SiO
2
nano-pillar structural surface. Scanning electron microscope (SEM, Hitachi SU-70, 5 kV) was used
for morphology and structure characterization after gold coating (approximately 10 nm thickness).
Visible light transmittance spectra were recorded by UV-Vis-NIR spectrometer (Lambda 750, Perkin
Elmer) to characterize optical properties. In addition, atomic force microscope (AFM, nanoscope
multimode VIII, DNP-10 tips, tapping mode) was used to measure surface roughness.
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494

3. Results and discussion
Anodized nanoporous alumina oxide (AAO) with honeycomb structure consists of closely packed
hexagonal cylinders of alumina cells, each with a circular pore in the middle. The model of
“sp-100-40-150” AAO template represents that AAO layer contained regularly arranged nanopores
with pore spacing of 100 nm, pore diameter of 40 nm, and pore depth of 150 nm. Figure 2. shows
SEM micrographs of AAO template sp-100-40-150. From SEM top view (Figure 2a), the measured
average pore spacing is 101.94 nm and average pore diameter is 43.01 nm, and from SEM
cross-section view (Figure 2b), the pore depth is 150 nm, which is basically the same as the model.
Figure 2. SEM micrographs of AAO template of sp100-40-150, (a) top view of
effective AAO layer; (b) cross-section view of effective AAO layer.
According to Choi [18], longer nanopillars (aspect ratio >10) tend to clump together and form
collapsed aggregation after AAO templates are chemically etched. In this work, AAO templates with
a small pore depth (150 nm) were selected in order to prevent collapse of nanopillars and ensure
visible light transmission of rough surface. Under the condition of same pore depth of 150 nm, AAO
templates with different pore diameters were used, and a series of vertical nano-pillar arrays were
obtained. SEM morphologies of SiO
2
nano-pillar arrays obtained by templates with the pore
diameters of 40 nm (template “sp 100-40-150”), 60 nm (template “sp 125-60-150”), 80 nm (template
“sp125-80-150”) and 100 nm (template “sp125-100-150”) are shown in Figure 3(a)(aʹ), (b)(bʹ), (c)(cʹ)
and (d)(dʹ), respectively. The as-prepared SiO
2
nano-pillars are regular arrays, as shown in Figure 3.,
and average diameters of SiO
2
nanopillars are measured to be 48.78 nm, 72.73 nm, 87.98 nm and
104.90 nm, respectively, which correspond to pore sizes of the templates used.
Cassie and Baxter give a defined equation of apparent contact angle with a droplet on a composite
surface [19], as shown below:
2211
coscoscos 

ff 
(1)
Here, θ
ω
is apparent contact angle of a composite surface; ƒ
1
and ƒ
2
are area fractions of two
media on the contact surface respectively, and ƒ
1
+ ƒ
2
= 1; θ
1
and θ
2
are intrinsic contact angles on the
two media respectively. When one of the media is air, the gas-liquid contact angle is 180°(θ
2
= 180°),
and the above equation becomes:
1)cos1(cos
11
 

f
(2)
A Novel and Facile Method to Fabricate SiO2 Nano-pillar Arrays on Glass Surface
495

Figure 3. SEM micrographs of AAO – imprinted SiO
2
nanopillar arrays by different pore
size templates: (a)(aʹ) sp100-40-150, (b)(bʹ) sp125-60-150, (c)(cʹ) sp125-80-150, (d)(dʹ)
sp125-100-150. (a)(b)(c)(d): 30K magnification, (aʹ)(bʹ)(cʹ)(dʹ): 100K magnification.
Wherein, ƒ
1
is area fraction of solid surface in contact with water and always less than 1; θ
1
is
intrinsic angle on the solid surface. According to equation (2), when θ
1
> 90°, the smaller the value of
ƒ
1
, the larger the apparent contact angle θ
ω
is.
The theoretical apparent contact angle (θ
ω
), measured static water contact angle (measured WCA),
measured rolling angle (measured RA) and water droplet state diagram are listed in Table 1. Here, the
ƒ
1
in Table 1. is calculated according to measured average diameters of SiO
2
nanopillars. In Table 1.,
sample "flat coating" is FAS-modified SiO
2
sol coating without AAO template embossing, which
measured WCA (107°) can be used for the intrinsic contact angle θ
1
of SiO
2
nano-pillar arrays. Table
1. shows that: (1) the SiO
2
nano-pillar array by a template with the pore diameter of 43.01 nm has a
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496

measured WCA of 150.47° and a measured RA < 5°, showing superhydrophobicity; (2) with the
increase of average diameter of SiO
2
nanopillars, the static water contact angle gradually decreases,
and the dynamic wetting property also gradually deteriorates; (3) measured WCA is basically
consistent with theoretical θ
ω
, and hydrophobicity of SiO
2
nano-pillar arrays can be controlled by
changing pore diameter of AAO templates.
Table 1. Comparisons between theoretical θ
ω
and measured WCA of SiO
2
nano-pillar array.
Samples
Pore
spacing/nm
Pore
diameter/nm
ƒ
1
θ
ω
Measured
WCA
Measured
RA
Water
droplet state
Flat coating
\
\
\
\
107°
85°
100-40-150
101.45
48.78
19.97%
149.17°
150.47°
<5°
125-60-150
130.43
72.73
28.20%
143.17°
145.38°
20°
125-80-150
131.97
87.98
41.26%
135.08°
137.11°
46°
125-100-150
131.12
104.90
58.66%
125.80°
128.35°
55°
AFM image of SiO
2
nano-pillar array of sample 100-40-150 is shown in Figure 4. Nanopillars are
regular arrays, which is consistent with SEM result (Figure 3.). In 1 μm × 1 μm scan size, surface
roughness (Ra) is 33.1 nm and maximum fluctuation (Rmax) is 179 nm. AFM results of surface
roughness and maximum fluctuation are listed in Table 2. Figure 5. shows the relationship of Ra and
Measured WCA, θ
ω
. With the increase of pore size of AAO templates, the roughness (Ra) of
nano-pillar arrays decreases, which results in the decrease of measured WCA and θ
ω
.
Table 2. AFM test results of samples.
Samples
Ra\nm
Rmax\nm
100-40-150
33.1
179
125-60-150
17.2
164
125-80-150
13.5
146
125-100-150
7.0
56
The transmittance spectrum of sample 100-40-150 is shown in Figure 6., comparing with blank
glass and glass with SiO
2
flat coating. The VIS transmittances (TL) of blank glass, glass with SiO
2
flat coating and glass with SiO
2
nano-pillar array (sample 100-40-150) are 74.91%, 75.50% and
78.55%, respectively. TL of sample 100-40-150 increases by 3.64% compared to the blank glass,
because that the sub-wavelength-sized cylindrical structure with a feature size smaller than VIS
wavelength can produce an effective refractive index gradient between air and nano-pillar array,
resulting in reflection reduce and transmittance increase [12].
A Novel and Facile Method to Fabricate SiO2 Nano-pillar Arrays on Glass Surface
497

35 30 25 20 15 10 5
125
130
135
140
145
150
155
Contact angle/°
Ra/nm
Measured WCA


Figure 4. AFM image of the sample
100-40-150.
Figure 5. The relationship of Ra and Measured
WCA, θ
ω
.
300 400 500 600 700 800
30
40
50
60
70
80
90
Transmittance%
Wavelength\nm
glass with nano-pillar array
glass with flat coating
blank glass
Figure 6. Transmittance spectra of blank glass, glass with SiO
2
flat
coating and glass with SiO
2
nano-pillar array (sample 100-40-150).
4. Conclusions
In this work, inorganic SiO
2
nano-pillar arrays were prepared on glass surface by template imprinting
method using AAO as template and SiO
2
sol as coating material. The diameter and surface roughness
of SiO
2
nanopillars can be effectively controlled by changing pore size of template. This is a novel
and facile method to prepare SiO
2
nano-pillar structural surface on glass substrate, presenting a new
way to prepare super-hydrophobic self-cleaning glass.
Acknowledgement
This work is supported by the Science and Technology Major Program of Fujian Province
(2014HZ0005), China.
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