Effect of nanostructure on the surface dipole moment of photoreversibly tunable superhydrophobic surfaces

Article abstract of DOI:10.1039/c0cc00323a

Fluorinated azobenzene-modified nanoporous substrates were fabricated such that the surface properties were photoreversibly converted between superhydrophobicity and superhydrophilicity as a result of UV irradiation. This result was attributed to the enha

Full text of DOI:10.1039/c0cc00323a

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Effect of nanostructure on the surface dipole moment of photoreversibly
tunable superhydrophobic surfacesw
Ho Sun Lim, Wi Hyoung Lee, Seung Goo Lee, Dongkyu Lee, Sangmin Jeon and
Kilwon Cho*
Received 11th March 2010, Accepted 14th April 2010
First published as an Advance Article on the web 12th May 2010
DOI: 10.1039/c0cc00323a
Fluorinated azobenzene-modified nanoporous substrates were
fabricated such that the surface properties were photoreversibly
converted between superhydrophobicity and superhydrophilicity
as a result of UV irradiation. This result was attributed to the
enhanced surface roughness of the nanostructured surface, which
supplied sufficient free space within the monolayer for the
tethered azobenzenes to facilely isomerize.
and superhydrophilicity upon UV or visible exposure, respectively.
By measuring the difference between the surface dipole
moments of the trans and cis states, we quantitatively elucidated
that the extreme wettability change was intimately related to
an enhancement in the surface dipole moment due to surface
nanostructures.
Photoswitchable azobenzene derivative, 7-[(trifluoromethyl
phenylazo)phenoxy]pentanoic acid was synthesized via the
diazo-coupling of 4-(trifluoromethyl)aniline with phenol.6
The procedure for preparation of the phototunable nano-
structured substrates is described in Fig. 1a. Building on
previous work, porous surfaces were fabricated on negatively
charged silicon wafers using a layer-by-layer deposition of ten
cycles with the poly(allylamine hydrochloride) (PAH) and
silica (SiO2) nanoparticles, respectively.4 Fig. 1b shows a
scanning electron microscopy image of an azobenzene-modified
(SiO2/PAH)10 polyelectrolyte multilayer, with a nanoporous
film, showing the increased surface roughness compared to the
smooth substrate. The multi-level structures created hierarchically
on the substrate, featured a rough inner structure with many
pores, formed by the complicated interconnections between
silica nanoparticles. Finally, the introduction of azobenzene
molecules to the substrate produced the surface covered with
string-like fluorinated azobenzene derivatives on the topmost
layer of the nanostructured substrate.
Smart materials that change their physicochemical properties
and function in response to external-stimuli have received
special attention in interdisciplinary studies due to their
promise for applications in artificial responsive devices.1
Environmental stimuli such as heat, light, pH, and electric
field have been used to trigger responsive behaviors in particular
materials.2 For example, photosensitive azobenzene molecules
undergo reversible trans-cis isomerization, causing large
changes in the molecular geometry as a result of UV/visible
irradiation.3 When the azobenzene derivatives are covalently
attached to a surface, the wettability of the modified surface
can be controlled. We have recently exploited photoswitchable
smart surfaces with special wettability and rewritable patterns
that could be reversibly controlled between superhydrophobic
and superhydrophilic states as a result of UV/visible irradation.4
These phenomena were attributed to a large photoinduced
change in the surface dipole moment of tethered azobenzenes
as the molecules converted between trans and cis isomers. The
trans isomer of azobenzene has a very small nearly zero, dipole
moment, whereas the dipole moment of the cis isomer is
around 3.0 Debye.3 Moreover, the surface roughness induced
the wettability difference to a significant enhancement.5
However, no clear explanation for the relationship between
surface dipole moment and wettability of photoresponsive
nanotextured substrates, which yield large changes in water
attracting/repelling behavior upon UV light irradiation, has
yet been presented.
The azobenzene-modified smooth surface underwent limited
photoinduced changes in water contact angle (CA) of 6 Æ 11,
reduced from 76 Æ 21 to 70 Æ 11 by irradiation with UV light
(Fig. 2). The conformation (cis or trans) of the azobenzene
isomers attached to the surface did not significantly affect the
hydrophobicity of the functionalized smooth surface. However,
the hydrophobic–hydrophilic switching effect was pronounced
in the presence of rough underlying nanostructured layers.5
UV irradiation of the multilayered films promoted a dramatic
change in the wetting properties of the substrate, from CAs of
151 Æ 21 to below 51. These observations can only result from
a combination of azobenzene trans-cis isomerization on the
top surface and the underlying nanostructured layers. In the
trans state, azobenzenes containing CF3 moieties ordinarily
align in an array of upright chains, yielding a hydrophobic
surface. The air trapped in the porous structures of the rough
substrates prevents water droplets from penetrating the surface,
thus, water droplets on the surface tend to be spherical. UV
illumination induced isomerization of the trans azobenzene
molecules to yield the cis one. The bent cis isomers expose
nitrogen atoms at the top layer, which induces a large change
in surface dipole moment. The cis-modified surface is more
To investigate this behavior in more detail, we focus on the
influence of surface dipole moment on the photoswitchable
wettability of azobenzene-modified rough surfaces. We fabricated
a photoresponsive superhydrophobic surface with a wettability
that could be reversibly switched between superhydrophobicity
Department of Chemical Engineering, Polymer Research Institute,
Pohang University of Science and Technology (POSTECH), Pohang,
790-784, Korea. E-mail: kwcho@postech.ac.kr;
Fax: +82-54-279-8298
w Electronic supplementary information (ESI) available: Experimental
procedures and additional data. See DOI: 10.1039/c0cc00323a
ꢀc
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photoisomerization caused large changes in the surface dipole
moment of the azobenzene monolayer, producing a four-fold
increase in kinetic energy relative to the smooth film. These
results are well consistent with the change of the water CA,
shown in Fig. 2.
The enhancement of the photoinduced surface dipole
moment changes, induced by the presence of surface nano-
structures, was investigated using a series of model monolayer
surfaces. Self-assembled monolayers (SAMs) of several
o-terminated alkanethiols (CH3, OH, and CF3) with a variety
of surface dipole moments were prepared on flat or nano-
structured substrates.8 As shown in Fig. 3a, the surface
potentials of the OH- and CF3-functionalized smooth surfaces
were higher than that of the CH3-functionalized smooth
surface by 0.64 and 1.84 eV, respectively. These differences
arose from the surface dipole character of each SAM.
Interestingly, we observed the same results for the rough
substrate, i.e., the nanostructured surfaces yielded nearly the
same kinetic energies as the smooth surfaces (Fig. 3b). These
results indicated that the surface potential of the thin film did
not depend at all on the nanostructure of the substrates. The
results shown in Fig. 3 are surprising, because the nano-
structured surface was capable of adsorbing a larger quantity
of alkanethiols than the smooth substrate.
Fig. 1 (a) Fabrication and reversible photoisomerization of a photo-
reversibly tunable superhydrophobic surface. (b) SEM images of a
smooth (left) surface and a rough (right) substrate produced by ten
nanoparticle deposition cycles.
hydrophilic than the trans-coated surface. Moreover, the
porous structures of the rough substrates allow water to
penetrate the nanopores by 3D capillary action.
This unusual effect was explored by measuring the adsorption
behavior of alkanethiols on flat and nanostructured substrates
using a quartz crystal analyzer (QCA). Since the surface area
of a rough substrate is much larger than that of a smooth
substrate, more alkanethiols were expected to react with the
nanostructured surface, that is, the number of the adsorbed
alkanethiols was expected to be larger on the rough than on
the smooth surfaces. Fig. 3c shows the change in resonant
frequency as a function of time during the experiments in
To seek quantitative understanding of the macroscopic
nature between these transitions, in particular, the degree of
trans-cis photoisomerization and the effect of this switching
behavior on surface wettability were studied by measuring a
surface potential using secondary electron emission spectra
(Fig. 2). The surface potential is closely related to the surface
dipole moment oriented normal to the plane of the monolayer
of molecules and strongly depends on the areal density of
molecules.7 The onset of secondary electron emission, which
corresponded to a surface potential change, was determined by
the intersection of the linearly extrapolated background and
straight onset in each spectrum. In the trans state, the onset of
secondary electron emission occurred at the same kinetic
energy, 19.4 eV, for both the flat and nanostructured surfaces.
However, when exposed to UV light, the onset for the smooth
substrate shifted by 0.1 eV and the onset for the rough surface
shifted by 0.4 eV, towards higher kinetic energies, relative to
the trans state surface. In rough nanostructured substrates,
Fig. 3 Secondary electron emission spectra of three films; CH3, OH,
and CF3-functionalized (a) smooth and (b) rough substrates. (c) QCA
resonant frequency as a function of time during the adsorption of
1-dodecanethiol onto smooth (black) or rough (nanostructured, red)
quartz crystals.
Fig. 2 Water droplet profiles and secondary electron emission spectra
on (a) smooth and (b) rough substrates before and after UV exposure.
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This journal is The Royal Society of Chemistry 2010
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apparently sterically hindered the isomerization reaction.1b
On the other hand, nanoporous structures provided sufficient
free volume for photoisomerization, which allowed the switching
to take place. Therefore, when azobenzene molecules isomerized,
the fraction of successful trans-cis isomerization events on the
nanostructured substrate was expected to be larger than on the
flat substrate, such that the actual surface energy of the rough
surface tended to increase relative to the smooth substrate.
Consequently, on the nanostructured surface azobenzene
molecules have a greater ability to reversibly switch between
two states than does on the flat surface, leading to a larger
variation of the surface dipole moment and water CA upon
UV/vis irradiation.
Fig. 4 X-ray photoelectron spectroscopy spectra of the (a) S 2p and
(b) C 1s levels on the CH3-functionalized smooth and rough surfaces.
which 1-dodecanethiol solution was exposed to the flat and
nanostructured substrates. The frequency change for the flat
substrate, Df ( = f À f0) was À4 Hz after injecting a 2 mM
1-dodecanethiol ethanolic solution for 15 min. In contrast, the
value of Df for the nanostructured surface was À30 Hz, which
constitutes an increase of 7.5 times the frequency change for
the flat substrate. This result suggests that the nanostructured
surface contained a greater number of 1-dodecanethiols than
the flat substrate due to the increased surface area, as expected.
The amount of adsorbed 1-dodecanethiol was estimated by
measuring the frequency shift of a quartz crystal using
Sauerbrey’s equation.9 The quantity of 1-dodecanethiols
adsorbed onto the flat gold-coated quartz crystals was calculated
to be B20 ng cmÀ2, whereas the amount adsorbed onto the
Here, we report a photoreversible surface potential switching
effect resulting from the photoinduced trans-cis isomerization
of azobenzene molecules in a monolayer formed on either a
smooth or a nanoporous substrate. The presence of surface
nanostructures strongly enhanced the magnitude of the
wettability changes resulting from isomerization. This effect
was attributed to an increase in available space and a reduction
in steric hindrance, for molecules in the monolayer formed on a
nanostructured surface. The additional available space allowed
for efficient isomerization of tethered azobenzenes, which
yielded large changes in the surface dipole moment. This
explanation is in good agreement with the extremely tunable
wetting properties seen on rough substrates.
nanostructured quartz crystals was estimated to be B146 ng cmÀ2
,
an increase by a factor of 7.3 relative to the adsorption quantity
of the smooth one. These results agreed well with the changes
in the resonant frequency for both flat and nanostructured
surfaces.
We gratefully acknowledge the support of Nano R&D
program through the National Research Foundation of Korea
(NRF) (Code No. 2009-0083218) and a grant from the
Fundamental R&D Program for Core Technology of Materials
funded by the Ministry of Knowledge Economy, and Pohang
Accelerator Laboratory for providing the 2B1 and 4B1 beam
lines used in this study.
In addition, for comparing the grafting densities of SAMs
on the smooth and nanostructured substrates, the S 2p and C
1s peaks of the CH3-functionalized surfaces were investigated
by X-ray photoemission spectroscopy (XPS). The integrated
areas of the surfaces’ S 2p and C 1s peaks, observed at 160.8 eV
and 285.0 eV, respectively, confirmed the quantity of sulfur
and carbon atoms per unit area due to the coordination of
alkanethiols to the Au layer (Fig. 4). In contrast to the QCA
results, the S 2p and C 1s peaks revealed no large differences
between the alkanethiol densities present on either substrate,
indicating that the alkanethiol adsorbed to the Au surface with
a fixed density, irrespective of the surface nanoporosity, even
though nanostructured substrates reacted with a larger total
quantity of alkanethiols than the flat surface. Therefore, XPS
measurements provide evidence that the grafting areal density
of alkanethiols adsorbed onto a rough surface is similar to that
of a flat surface. From these results, we could infer that the
surface potential in the o-terminated alkanethiol-modified
surfaces did not significantly change with the presence of
nanostructure, as was shown in Fig. 3.
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By measuring the surface dipole moment of both the trans-
and cis-presenting surfaces, however, we found that UV
irradiation induced large changes in the molecular dipole
moment of the azobenzene-modified nanoporous substrate,
although the surface potential did not vary with surface
nanostructures. Azobenzene molecules require space for the
trans to cis isomerization. The more ordered and densely
packed structure of the monolayer on the smooth surface
ꢀc
This journal is The Royal Society of Chemistry 2010
4338 | Chem. Commun., 2010, 46, 4336–4338