22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
From hydrophobic to superhydrophobic: influence of SiO 2 and TiO 2
nanoparticles on fluorocarbon films synthesized by atmospheric plasma
J. Mertens, J. Hubert, N. Vandencasteele and F. Reniers
CHANI, Université Libre de Bruxelles, Brussels, Belgium
Abstract: The influence of SiO 2 and TiO 2 nanoparticles on the atmospheric pressure
PECVD of C 6 F 12 precursor is investigated. Chemical and hydrophobic properties of the
films synthesized by atmospheric plasma are studied by XPS, WCA and profilometry. The
roughness of the samples appears as a crucial parameter since its increase promotes the
creation of superhydrophobic coatings.
Keywords:
atmospheric
superhydrophobicity
plasma,
DBD,
1. Introduction
Superhydrophobic surfaces have been widely studied
because of their large range of applications: antireflective
[1], antibacterial [2], low adhesion [3] and bio-fluid
transportation [4]. A surface can be considered as
superhydrophobic when the water contact angle value is
higher than 150°.
This particular property is a
combination of two main factors: roughness and low
surface energy. In the present study, the roughness is
induced by the presence of SiO 2 or TiO 2 nanoparticles.
A low surface energy film is then synthesized by PECVD
of a liquid fluorinated precursor (the perfluoro-2-methyl2-pentene, C 6 F 12 ) in argon or helium using a dielectric
barrier discharge (DBD). Indeed, fluorinated coatings are
well known to create low energy [5], biocompatible [6] or
relatively inert surfaces with ambient air [7].
2. Content
In the present work, 80 nm SiO 2 and 15 nm TiO 2
nanoparticles were deposited on silicon wafer substrates
by the solvent evaporation method, using dispersions of
nanoparticles in water of 0.05 g/L. No additional
surfactant was used to prevent any contamination of the
surface. The low surface energy coating was synthesized
by atmospheric plasma using the liquid fluorinated
precursor. The experimental scheme of the study is
represented in Fig. 1. First, a residual vacuum as low as
2 Torr is achieved to limit atmospheric contaminations.
The atmospheric pressure is raised by filling up the
reactor with the carrier gas used for the plasma deposition
fluorocarbon
coating,
nanoparticles,
(argon or helium). The influence of the nanoparticles on
the chemical, textural and hydrophobic properties was
investigated by X-ray photoelectron spectroscopy (XPS),
profilometry and drop shape analysis of water contact
angles (WCA), respectively.
The first result showed a difference between the
chemical composition of the films synthesized in argon
and helium plasma.
These dissimilarities can be
explained by the differences in electron density and
amount of discharge streamers between the filamentary
argon and the more homogeneous helium discharges.
This discharges characteristics have an impact on the
fragmentation of the precursor which is higher when
argon is used as carrier gas. On the contrary, the more
homogeneous helium discharge allows a better
conservation of the precursor’s initial structure.
Furthermore, a difference in the deposition rate was also
observed between the two carrier gases. The argon
plasma induces a deposition rate of 225 nm/min while in
helium, it only reaches 22 nm/min. This can also be
explained by the higher electron density and
fragmentation of the precursor in the argon plasma.
The additional roughness created by the presence of the
nanoparticles under the plasma polymerized fluorocarbon
film presents a significant impact on the hydrophobic
properties of the film. The water contact angle value
increases from 111° to a value of 126° when helium is
used as carrier gas. This corresponds to a 12% increase,
while only a 5% increase (129° to 137°) is observed for
argon. This difference can be explained by the variation
Fig. 1. Graphical representation of the experiment.
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in the roughness of the synthesized films when
nanoparticles are present or not. Indeed, the films
synthesized in argon and helium show a respective R RMS
of 244 nm and 25 nm when no nanoparticles are
previously deposited on the substrate.
This R RMS
increases up to 1250 nm with the addition of
nanoparticles, no matter which carrier gas is used.
However, no significant difference in the hydrophobic
and roughness was observed when TiO 2 or SiO 2
nanoparticles were used. This can be explained by the
great agglomeration of the nanoparticles into the
dispersions because of the absence of surfactant.
The impact of the nanoparticles on the chemical
composition of the film was investigated by XPS. No
significant influence of the nanoparticles, SiO 2 or TiO 2 ,
was observed on the chemical composition of the
“freshly” synthesized films.
Nevertheless, aging
measurements achieved by XPS revealed that the films
covering TiO 2 nanoparticles presented a chemical
modification over time. Indeed, the largely accepted XPS
high resolution C1s curve deconvolution for fluorocarbon
films (CF 3 , CF 2 , CF, CCF and CC) does not allow a good
fitting for the samples older than 15 days. In fact, a sixth
component (COH) is necessary for an optimal fitting of
the curve. As shown in Fig. 2, this component, centred
around 286.5 eV, takes a more and more important part of
the curve when the sample is aging. This increase in
COH component is offset by a decrease in the
hydrophobic components, CF 3 and CF 2 , centred around
294.4 eV and 292.2 eV, respectively. The XPS survey
analysis of these samples also revealed a degradation of
the film by the appearance of the Ti 2p and O 1s signal
for the films older than 15 days. This chemical
composition modification was not observed when no
nanoparticles or when SiO 2 nanoparticles were used.
Fig. 2. Evolution of the XPS C 1s High resolution spectrum of a coating covering TiO2 nanoparticles over time.
This influence of the presence of TiO 2 or SiO 2
nanoparticles was also noticed by WCA measurements
over time. The water contact angle of 137° in argon and
126° in helium remained constant over time when SiO 2
nanoparticles were used. On the contrary, coatings
covering TiO 2 nanoparticles presented a decrease in
contact angle, reaching a value of 120° and of almost 75°
when the film was respectively synthesized in argon and
helium plasma after 70 days. This difference in the
degradation rate is mostly related to the important
difference of deposition rate of the precursor into the two
gases.
Furthermore,
superhydrophobic
coatings
were
synthesized by increasing the concentration in SiO 2
nanoparticles of the dispersions deposited by solvent
2
evaporation on the silicon substrates. The influence of the
concentration of nanoparticles was investigated by using
0.05, 0.5, 1, 2, 5 g/L dispersions. WCA and roughness
were measured before and after a two minutes plasma
deposition of C 6 F 12 in argon at 50 W. WCAs of 24.8°
and 11.3° were respectively measured for the samples
containing nanoparticles deposited with the 0.05 and the
0.5 g/L dispersion before plasma treatment.
Superhydrophilic properties were observed for the
samples of 1, 2 and 5 g/L before plasma treatment. This
very high affinity with water hinders the water contact
angle measurement because of a total staggering of the
droplet on the surface. The roughness of the samples
increases with the concentration of nanoparticles in the
dispersions. For instance, a 1250 nm and a 25000 nm
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R RMS were measured for the 0.05 g/L and 5 g/L
dispersions. However, the roughness before and after
plasma treatment is not significantly modified. This
indicates that the nanoparticles are mainly responsible for
the total roughness. The 0.05 g/L dispersion permitted
the observation of a WCA value of 137° and was
therefore used for the experiments previously described.
Superhydrophobic coatings were synthesized when 0.5, 1,
2 and 5 g/L dispersions were used to create the additional
roughness. This property is detected by the nonadherence of the water droplet on the surface as shown in
Fig. 3. This is explained by the obtainment of a sufficient
double-scale roughness (nanometric and micrometric).
On one hand, the size of the nanoparticles and the natural
film roughness are responsible for the nanometric scale
roughness. On the other hand, the agglomeration of the
nanoparticles is responsible for the micrometric scale
roughness as it was measured by profilometry.
Fig. 1.
Non-adherent water droplet on a superhydrophobic coating.
3. References
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Q. Zhang and Q. Fu. Soft Matter, 7, 6435-6443
(2010)
[2] H. Yang and P. Jiang. Langmuir, 26, 12598-12604
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[3] Y.C. Jung. Natural and Biomimetic Artificial
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[4] P. Bayiati, A. Tserepi, P.S. Petrou, K. Misiakos,
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[5] T. Nishino, M. Meguro, K. Nakamae, M. Matsushita
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