Wettability control and water droplet dynamics on SiC-SiO2 core-shell nanowires.
ABSTRACT We present a simple method for fabricating superhydrophobic SiC-SiO(2) core-shell nanowire surfaces via the facile dip-coating of alkyltrichlorosilanes. Water droplets displayed a variety of shapes with varying surface energies on the nanowire surfaces, which could be modified through chemisorption of alkyltrichlorosilanes with variable carbon chain length. The effects of UV irradiation on the superhydrophobic nanowire arrays were also investigated. UV light efficiently decomposed the chemisorbed molecules, and the superhydrophobic surface gradually converted into a hydrophilic surface with increasing UV exposure. The water droplet impact behavior on the modified surfaces was studied to test the stability of the superhydrophobicity under dynamic conditions.
Langmuir 2010, 26(14), 12273–12277 Published on Web 05/28/2010
©2010 American Chemical Society
Wettability Control and Water Droplet Dynamics on SiC-SiO2
Geunjae Kwak, Mikyung Lee, Karuppanan Senthil, and Kijung Yong*
Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang
University of Science and Technology (POSTECH), Pohang 790-784, Korea
Received March 29, 2010. Revised Manuscript Received May 20, 2010
We present a simple method for fabricating superhydrophobic SiC-SiO2core-shell nanowire surfaces via the facile
dip-coating of alkyltrichlorosilanes. Water droplets displayed a variety of shapes with varying surface energies on the
nanowire surfaces, which could be modified through chemisorption of alkyltrichlorosilanes with variable carbon chain
length. The effects of UV irradiation on the superhydrophobic nanowire arrays were also investigated. UV light
efficiently decomposed the chemisorbed molecules, and the superhydrophobic surface gradually converted into a
hydrophilic surface with increasing UV exposure. The water droplet impact behavior on the modified surfaces was
studied to test the stability of the superhydrophobicity under dynamic conditions.
Lotus leaves exhibit extreme water-repellent and self-cleaning
properties.1Nature achieves such fascinating superhydrophobic
properties through a combination of reduced surface energy via
chemical coating and enhanced surface roughness. Considerable
effort has been applied toward the fabrication of biomimetic
materials, such as ZnO,2-7TiO2,8-10WOxnanowires,11,12and
carbon nanotubes,13-17have been widely tested for fabricating
and unique properties. Nanostructures with low surface energies
reduce water droplet contact area and prevent penetration of
water into the interstices between the nanostructures, leading to a
large contact angle (CA) and a small CA hysteresis (CAH).
The interplay between controllable parameters, such as nano-
structure spacing, diameter, height, and surface energy, controls
the wettability transition criteria. Reduced surface energy and
hydrophobicity. Previous studies18-21havedemonstratedchanges
in the superhydrophobic behavior that result from variations in
geometric parameters (nanopost spacing, diameter, and height).
However, experimental control over geometric parameters gener-
ally requires strict conditions and complex processing techniques.
Hydrophobic chemical coatings have been tested as an approach
to the fabrication of artificial superhydrophobic surfaces. Low-
ering the surface energy through chemical modification can
enhance the hydrophobicity of a surface with fixed geometric
parameters. Simple chemical coating methods have been used to
fabricate superhydrophobic surfaces composed of nanomaterials.
However, such studies22have not extensively investigated the
tunable parameters that may alter surface energy and wettability.
an efficient means for control over surface wettability. Also, in
practical application, although superhydrophobicity should be
stable under dynamic rather than stationary droplet conditions,
the stability of artificial superhydrophobic surfaces with different
surface energies have not been studied under dynamic conditions.
SiC nanowires are wide-band-gap semiconductors and have
shown excellent stability under harshconditionsdue to their high
mechanical strength, high chemical stability, and low induced
To date, few studies have described the fabrication of super-
hydrophobic SiC nanostructures with CAs as large as 160?.27,28
In this paper, we report a novel and simple method for
fabricating superhydrophobic core-shell SiC-SiO2nanowire
*Corresponding author: e-mail email@example.com.
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DOI: 10.1021/la101234p Langmuir 2010, 26(14), 12273–12277
ArticleKwak et al.
arrays through the chemisorption of an octadecyltrichlorosilane
(OTS) self-assembled monolayer (SAM). A wettability control
lengths. The effects of UV irradiation on the superhydrophobic
SiC-SiO2nanowire arrays were investigated because the photo-
decomposition of OTS molecules adsorbed onto the surface was
found to locallyalterthesurfaceenergy.Moreover, thedynamics
of water droplet impact on superhydrophobic surfaces were
investigated to characterize the influence of surface energy on
the drop impact transition. Practical applications require that
superhydrophobicity be maintained under dynamic conditions.
2. Experimental Section
A. Fabrication of Core-Shell SiC-SiO2Nanowire Ar-
rays and Chemisorption of Alkyltrichlorosilanes on Nano-
wire Substrates. Core-shell SiC-SiO2nanowires were grown
were immersed in the Ni(NO3)2/ethanol solution (0.01 M). After
drying in air, growth was carried out at 1100 ?C for 2 h under a
the silicon substrate was observed to be covered with a white
deposit. After growth, the substrate was rinsed with deionized
water then dried under N2flow. Alkyltrichlorosilane was depos-
in 3 mmol of toluene solutions of hexyltrichlorosilane (HTS,
C = 6), dodecyltrichlorosilane (DTS, C = 12), or octadecyltri-
chlorosilane(C =18) for3h at4?C.At low temperatures(4?C),
the alkyltrichlorosilane molecules display well-ordered mono-
with ethanol to remove excess reactants thendried under N2flow
for further analysis. The water contact angle and intensities of
function of the SAM carbon chain length.
B. UV-Stimulated Conversion of Wettability and Selec-
tive Wetting. The wettability ofthe preparedsurfaceswastuned
by exposure to UV light: OTS-modified SiC-SiO2nanowire
samples were placed under a 300 W mercury lamp, which yielded
185 and 254 nm illumination. The light intensity was maintained
at 1 W/cm2. The OTS-coated SiC-SiO2nanowire samples were
directly illuminated at a working distance of 10 cm. The UV-
stimulated conversion of wettability was investigated by measur-
ing the water contact angle and XPS peaks as a function of UV
irradiation time, from 0 to 60 min.
C. Characterization. The surface morphology, structure,
and chemical states of the nanowire samples were examined by
field-emission scanning electron microscopy (FESEM, JEOL,
JSM 330F) and XPS. The water CAs were measured using 5 μL
droplets of deionized water using a contact angle measurement
system (Kr€ uss, DSA-10) under ambient atmospheric conditions.
Advancing and receding contact angles were measured with the
dispensing needle embedded in the sessile drop. The dynamic
impact behavior of droplets and the sliding motion of the water
droplets were recorded using a high-speed camera (Fastcam,
Ultima 512) operated at 2000 and 250 frames/s, respectively.
3. Results and Discussion
A large quantity of nanowire products was obtained by the
simple heating method. The as-grown sample was composed of
uniform nanowires 20-50 nm in diameter and several tens of
micrometers in length (Figure 1a). The surface showed an open
mesh of nanowires lying parallel on a substrate with a high
roughness, as shown in Figure 1. According the previous study,31
this open mesh structure showed the most superhydrophobic
behavior in various surface structures, including smooth surface,
assembly of spherical nanoparticles, vertically aligned nanowire
array, and mesh structure of nanofibers. The internal structure of
the nanowires was investigated by transmission electron micro-
scopy (TEM). The TEM image shown in Figure 1b showed that
the nanowires were composed of core SiC and shell SiO2.
Typically, the thickness of the shell layer was 20 nm, and the
diameter of the core fell in the range 10-30 nm. The selected area
electron diffraction (SAED) spectrum from the core SiC was
indexed as a cubic β-SiC structure, whereas no diffractionpattern
was observed from the amorphous shell SiO2layers (Figure 1b,
insets). The surface morphology and density of SiC-SiO2nano-
wires depended on the growth parameters, such as NiO catalyst
concentration, growth temperature, and time.29
The outer nanowire SiO2shell layer acted as a protective layer
for the core SiC nanowires and assisted the chemical binding of
chlorosilane surface modifiers were deposited on the as-prepared
Figure 1. (a) SEM image of core-shell SiC-SiO2nanowires. (b) TEM image of a core-shell SiC-SiO2nanowire. The crystalline SiC core
appears darker than the amorphous SiO2sheath in this imaging mode. The selected area electron diffraction (SAED) pattern (SiC core)
corresponds to a cubic SiC crystal structure. No diffraction pattern was observed from the amorphous SiO2shell layer.
(29) Senthil, K.; Yong, K. Mater. Chem. Phys. 2008, 112, 88.
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Langmuir 2010, 26(14), 12273–12277
Kwak et al. Article
were conductedonthe modified SiC-SiO2nanowire surfacesas
a function of SAM carbon chain length. As shown in Figure 2,
the water CAs (static CA, advancing and receding CAs) of
SAM-treated nanowire surfaces increased monotonically with
increasing SAM alkyl chain length. The HTS treated surface
showed 89.5? and 54? for advancing and receding CAs, respec-
tively. The contact angle hysteresis (CAH), which is the differ-
ence between the advancing and receding contact angles, was
35.5?. Increasing the alkyl chain length caused an increase in
CAH were observed with increased alkyl chain length (the inset
of Figure 2). The DTS-modified surface showed advancing/
modified surface both advancing (163.5?) and receding (162?)
water CAs were almost the same with a CAH of 1.3?. The
exceedingly low CAH and large receding CA caused the non-
adhesion behavior between solid/liquid interfaces. These results
indicate that longer alkyl chain lengths decrease the surface
energy of the modified nanowire surfaces. The OH group that
was inherently present on the shell SiO2layer of the nanowires
imparted a superhydrophilic state with a very low CA on the as-
grown nanowire surfaces (C = 0). The OTS-modified surface
(C = 18) supported a water droplet that was nearly spherical,
with a static CA of 164?. This superhydrophobicity could be
explained by the Cassie model. The contact area between the
water droplet and the sample surface consisted of two parts: the
OTS-coated SiC-SiO2nanowire surface and air pockets. Im-
pact dynamic studies supported this model, which will be
discussed later in the article.
According to the Cassie model,32water CAs are directly
influenced by the surface fraction of solid ( f1) versus air pockets
( f2), where the sum of these two parameters is 1:
cos θ?¼ f1cos θ-f2
The decreased surface energy produced by the OTS coating
indicated a low solid surface fraction ( f1) and a high air surface
fraction ( f2), yielding a high water CA. In this Cassie state,
capillary effects and the air pockets supported the droplet and
prevent the droplet from penetrating the nanostructure.33,34The
stability ofthesuperhydrophobicstatewas also investigated. The
phobicity, in which no apparent changes in CA were observed
after 1 month under ambient atmospheric conditions with ex-
posure to sunlight (data not shown).
The chemical binding states of the modified nanowire surfaces
Supporting Information, Figure S1). Figure S1a shows the C 1s
XPS spectra for the various alkyltrichlorosilanemolecules tested.
The C 1s peaks at 284.8 eV were ascribed to the carbon atoms in
the aliphatic chain (C-C), consistent with the data presented in a
In contrast, the XPS intensities of both the O 1s (532.6 eV) andSi
2p (102.8 eV) peaks assigned to the outer SiO2shell of the
nanowire decreased gradually with the increased chain length
(Figures S1b and S1c). These results showed that alkyltrichloro-
silane molecules with longer carbon chains shielded the nano-
structures from exposure to the exterior.
Interestingly, the coating molecules could be effectively re-
moved by irradiation with intense UV light (300 W). We mea-
sured the changes in CA and XPS spectra to confirm the UV-
enhanced wettability transformation of OTS-coated SiC-SiO2
nanowires as a function of UV irradiation time under ambient
from 164? to 0? with increasing UV illumination time. After UV
irradiation in excess of 45 min, the CA approached 0?, similar to
Figure 2. Watercontactangles(CAs)onchemicallymodifiednanowiresurfacesasafunctionofthealkylchainlengthofSAMmolecules:(0)
static contact angle, (b) advancing contact angle, and (O) receding contact angle. Inset: contact angle hysteresis (CAH) as a function of the
alkyl chain length.
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DOI: 10.1021/la101234pLangmuir 2010, 26(14), 12273–12277
ArticleKwak et al.
the CA of the as-grown SiC-SiO2 nanowires. The gradual
conversion into a hydrophilic surface indicated that intensive
UV light effectively decomposed the OTS molecules adsorbed
produce UV-assisted decomposition ofthe OTS alkyl chains.36,37
The OTS alkyl chains decomposed through attack from the OH
radicals and atomic oxygen produced by the UV-catalyzed
dissociation of ozone, which was photogenerated in air. The
water CA, both advancing and receding CAs, decreased mono-
tion time. This increase in hysteresis may result from an increase
in interaction of the water droplet with the surface due to the
photodecomposition of the adsorbed OTS molecules.
The UV-enhanced decomposition of the alkyl chains was
characterized by XPS analysis. Figure S2a (see Supporting
Information) shows XPS spectra of C 1s as a function of UV
This result suggests that UV irradiation gradually reduced the
alkyl chain length of the OTS molecules chemisorbed on the
nanowire surfaces. After termination of the photochemical reac-
tion, the alkyl chains were found to be completely decomposed,
and only residual carbon remained on the surfaces. Figures S2b
and S2c show respectively the Si 2p and O 1s XPS spectra as a
2pand O 1speaks increased uponUV irradiation.These changes
indicatethat the nanowire surface covered withthe SAM became
containingsilicate material will possiblybeformed onthesurface
during photodecomposition of OTS molecules. However, in the
current study, there are no peaks observed for C 1s and Si 2p
corresponding to Si-C bonds (283.5 and 100.3 eV, respectively),
and C 1s and Si 2p peaks are nearly symmetric centered at 284.6
and 102.8 eV corresponding to C-C bonds (aliphatic chain) and
Si-O bonds (outer shell ofthenanowire),respectively(FigureS2
in Supporting Information). We believe that this is because
the SiO2shell layer is too thick to detect the core SiC in XPS.
Also, these results indicate that the C signal originating from
carbon-silicate material is too weak to detect, if any.
stationery states, the stability of the superhydrophobicity under
dynamic conditions (e.g., impinging water droplets) is a key
consideration for practical applications. In real applications,
superhydrophobicity must persist under dynamic, not only sta-
tionary,droplet conditions.Whendroplets impingeonatextured
surface, the transition criteria of wetting states depend on the
balance of wetting pressure (Pw) and antiwetting pressure
(Pa).12,18,19,38,39When Pwwas larger than Pa, the droplet struck
the surface in a wetting state. The Pwis given by
where F and V are the density and the velocity of the imping-
ing liquid, respectively. For SiC-SiO2nanostructured surface
with a maximum spacing between nanowires (D, ∼300 nm), the
maximum value of Pais calculated as the Laplace pressure of
the maximum deformation of the water-air interface between
Pa ¼ -2γLVcos θA=D
where γLVis the surface energy of the water at the liquid-vapor
interface (0.073 N/m) and θAis the advancing CA of the water
droplet on the flat surface. For a flat OTS-modified and UV-
irradiated silica surface, the measured θAwere ∼120? and 12?,
We studied the impact dynamics of droplets impinging on the
OTS-modified and UV-irradiated nanowire surfaces to under-
stand how the superhydrophobic coating influenced the dynamic
of water droplets with diameters of 1 mm and velocities of 0.52
m/s impinging on the OTS-modified surfaces, recorded using a
high-speed camera operated at 2000 frames/s. The calculated Pa
value of 243.13 kPa from eq 3 was significantly greater than the
calculated Pw(135.2 Pa) from eq 2. The droplet clearly bounced
off the OTS-modified substrate without penetrating the nano-
structure. The air pockets and capillary forces supported the
droplet throughout the impact event. Finally, the droplet rested
on the surface and maintained a high contact angle without
undergoing a transition to a wetted state, which suggested the
formation of a solid-air-liquid interface. However, all super-
hydrophobic surfaces do not completely perform the same
behavior of bouncing off surface. When the surface does not
have sufficiently low surface energy or procure enough geome-
trical spacing between the nanoscale structures, the wetting state
of a droplet can be partially pinned at the contact area.12,19In
contrast, on UV-irradiated nanowire surfaces, the calculated Pa
of UV irradiated one has a negative value and acts as a wetting
pressure. In this case, two wetting pressures (Pw, Pa) caused a
droplet to wet the surface. Thus, the water droplet appeared to
soak into the texture without bouncing or vibrating and subse-
quently spread out within a few milliseconds on the surface
(Figure 4b). These results indicate that restoration of the super-
hydrophilic surface favored penetration of the water droplet into
the surface structure by 3D capillary effects.40,41
Sliding of a water droplet on the chemically modified/UV-
treated surfaces was directly observed by dropping the water
droplet ontoslightlytiltedsubstratesplacedina row (tiltangleof
5?). Photographs of the sliding droplets were recorded using a
Figure 3. Water CAs of the OTS-treated nanowire surfaces vs.
exposure time to 185 and 254 nm UV light at 1 mW/cm2.
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Langmuir 2010, 26(14), 12273–12277
Kwak et al. Article
high-speed camera at 250 frames/s. All three samples were OTS-
treated. The left-most sample was extensively UV-irradiated to
very easily slid off the two right-hand OTS-modified surfaces,
the left-most surface as soon as it reached the UV exposed area
(Figure 5) (see Supporting Information video for details). This
extreme wettability conversion of the nanostructured surface
suggests many potential applications in biological, chemical, and
In summary, we havepresented a facile methodfor fabricating
superhydrophobic SiC-SiO2nanowire surfaces with a static
water CA of 164?. The high roughness and low surface energy
provided by the particular geometry of the nanostructure and
alkyltrichlorosilane coating, respectively, contributed to the
superhydrophobicity. The surface energy of the nanowire
substrates, which determined the wetting state of the water
droplet, could be systematically modified through chemisorp-
tion of alkyltrichlorosilanes with varying carbon chain lengths
and by UV-assisted photodecomposition of the SAM mole-
cules on the nanowire surfaces. Also, we studied the stability of
the fabricated superhydrophobic surface under dynamic con-
ditions for practical applications and characterized the influ-
ence of surface energy on the wetting transition through
measurements of water droplet impact dynamics. A droplet
on the OTS-modified surface bounced cleanly on the surface,
nanostructure surfaces. This study presents guidelines for the
design of stable nanostructured surfaces that are antiwetting
with respect to water and are self-cleaning.
01-04 from the Regional Technology Innovation Program of the
Ministry of Commerce, Industry, and Energy (MOCIE), and
Korean Research Foundation Grants funded by the Korean
Government (MOEHRD) (KRF-2008-005-J00501).
Supporting Information Available: XPS spectra from the
OTS-modified nanowires as a function of the SAM alkyl
chain length and XPS spectra for OTS-modified nanowires
after UV irradiation during various times; video of sliding
water droplet on the patterned SiC-SiO2nanowire surface.
This material is available free of charge via the Internet at
Figure 4. Photographs of water droplets impinging on modified and irradiated to 185 and 254 nm UV light at 1 mW/cm2nanowire arrays.
Figure 5. Sequential images of a water droplet sliding on the
patterned tilted substrates arranged in a row. The two right-most
the left surface was a UV-exposed surface with both 185 and 254
nm wavelengths and 1 mW/cm2.