Fast and reversible photo-responsive wettability on TiO2 based hybrid surfaces

Article abstract of DOI:10.1039/c5ta01710f

A hybrid surface exhibiting a fast and reversible switch between hydrophobic and hydrophilic states was prepared by spin-coating a porous TiO₂ layer by a mixture of TiO₂ nanoparticles with 11-(4-phenylazo)phenoxy undecanoic acid (den

Full text of DOI:10.1039/c5ta01710f

Journal of
Materials Chemistry A
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PAPER
Fast and reversible photo-responsive wettability on
TiO based hybrid surfaces†
2
Cite this: J. Mater. Chem. A, 2015, 3,
1533
1
Gwendoline Petroffe, Chao Wang, Xavier Sallenave, Gjergji Sini, Fabrice Goubard
and Sebastien Peralta*
´
´
A hybrid surface exhibiting a fast and reversible switch between hydrophobic and hydrophilic states was
prepared by spin-coating a porous TiO layer by a mixture of TiO nanoparticles with 11-(4-phenylazo)
phenoxy undecanoic acid (denoted as the AzoC11 acid). The nanocomposite film corresponding to the
2
2
1
0% AzoC11 acid/TiO
2
mass fraction exhibited both a very important and fast variation of the contact
ꢀ
angle (by more than 120 ), along with an excellent reversibility. The wettability switching is explained by
the trans / cis / trans isomerization of the AzoC11 acid under light irradiation or heating. Upon trans
/
cis isomerization, optical absorption bands were found to exhibit drastic evolution, the origin of
which is discussed in detail by means of a time-dependent density functional theory (TDDFT) study.
Subsequent to these isomerization processes, various mechanisms explaining the wettability variations
were investigated by comparing the results obtained by DFT calculations, wetting measurements for
different surface ratios of the AzoC11 acid under UV irradiation, and by analyzing the molecular structure
and molecular surfaces of trans and cis-isomers of the AzoC11 acid.
Received 6th March 2015
Accepted 24th April 2015
DOI: 10.1039/c5ta01710f
www.rsc.org/MaterialsA
A wide number of inorganic materials are used for stimuli-
responsive surfaces. Semiconductor oxides are well-known as
Introduction
In recent years, superhydrophobic and superhydrophilic stable, nontoxic and highly reactive materials which are able to
ꢀ
surfaces with a water contact angle greater than 150 and less induce a high wettability conversion between superhydrophobic
ꢀ
than 10 , respectively, are of great scientic interest due to their and superhydrophilic states. In previous studies, surface
1
2
potential applications in self-cleaning, anti-fogging or antire- wettability of light-responsive inorganic oxides under UV irra-
3
4–6
19
20–22
23

ective surfaces and microuid manipulations.
Further- diation, such as ZnO, TiO
2
or V
2
O
5
has been reported.
more, the rapid and reversible wetting transition between However, the main problem with these materials is the too long
superhydrophilic and superhydrophobic states is one of the hot time needed for the kinetic switching (several days for one
22,24
topics in research on developing functional materials in the last cycle).
decade. Many investigations have shown that besides the
As compared to inorganic materials, organic materials have
chemical nature of surfaces, the roughness and morphology many advantages in terms of the number of adaptable species,
7,8
also affect their wetting behaviour.
the potential for chemical modications, and the reaction
The reversible wetting variation can be induced by the diversity. Such light-sensible materials are able to alter their
surface energy variations or/and the morphology modications. properties reversibly, especially their wetting behaviour upon
9
,10
In general, external stimuli, such as temperature,
treatment, electrical potential, pH
chemical irradiation. These changes are oen based on chromophoric
11
12
13–17
18
4,25 7,26
or light can change groups of organic dyes such as azobenzene,
spiropyran,
27
28
the surface properties and the surface behavior. The use of coumarin or cinnamate which undergo a reversible isomer-
materials with light-tunable wettability has promising applica- ization under illumination. The wetting of azobenzene surfaces,
tions notably because of avoiding the contact with materials for example, was tuned by alternation of UV and visible illu-
29
which minimizes contaminations.
mination for a few minutes. Unfortunately, the water contact
ꢀ
angle variations (CAV) are very small (10 for a smooth
2
9,30
surface).
It is possible to increase the CAV using other
Laboratoire de Physicochimie des Polym `e res et des Interfaces (LPPI, EA 2528), Institut
des Mat ´e riaux, Universit ´e de Cergy-Pontoise, 5 mail Gay-Lussac Neuville/Oise, 95000
Cergy-Pontoise Cedex, France. E-mail: sebastien.peralta@u-cergy.fr
31
32
solvents such as diiodomethane or benzonitrile instead of
water.
Few authors have associated an organic layer with a semi-
conductor oxide. Cho et al. have elaborated a rough substrate
†
Electronic supplementary information (ESI) available: Theoretical absorption
spectra of trans- and cis-AzoC11 acid in THF, absorption onset on the UV-vis
spectra of AzoC11 acid solution, theoretical estimation of molecular-areas, based on layer-by-layer deposition of poly(allylamine hydro-
expression of surface ratio, values of the wetting study, comparison of contact
33
chloride) and SiO
2
nanoparticles. This lm was functionalized
angle variations versus the irradiation conditions. See DOI: 10.1039/c5ta01710f
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Water used for the synthesis and contact angle measure-
ments was puried by a Milli-Q system (resistance 18 MU).
Fig. 1 11-(4-Phenylazo)phenoxy undecanoic acid (AzoC11 acid).
Techniques of analysis
1
13
H and C NMR spectra were recorded on a Bruker Avance DPX
50 MHz spectrometer. Chemical shis are given in ppm using
2
with a photoswitchable azobenzene molecule, the 7-[(tri-
the residual solvent signal (CDCl ) as an internal reference.

uoromethoxyphenylazo)phenoxy]pentanoic acid (CF AZO).
3
3
Fourier transform infrared (FTIR) spectra were recorded using a
Bruker Tensor 27 spectrometer. The sample was pressed into
KBr pellets. UV-visible measurements of the azobenzene deriv-
ative were performed at room temperature in spectrometric
grade tetrahydrofuran solutions using a Jasco V570 spectrom-
eter and an analysis cell with an optical path length of 10 mm.
Photoisomerizations trans / cis and cis / trans were real-
This system can be switched between superhydrophobicity and
superhydrophilicity with 20 min of UV irradiation and can be
reversible within 3 h of visible light irradiation. Huang et al.
have reported an original hybrid material based on a lter paper
34
in cellulose pre-coated by a titanium ultrathin lm. With the
CF AZO monolayer self-assembled onto the titanium lm, the
3
ꢀ
ꢀ
water contact angle varied between 145.2 and 24.2 aer 13 h of
UV irradiation. The original high hydrophobicity was recovered
aer storage in the dark for one week. More recently, Wu et al.
have presented a facile method of fabricating the surface with
reversibly tunable wettability. A mixture of poly(styrene-butyla-
ised from a UV lamp (15 W, l
¼ 365 nm, intensity at 2.0 cm ¼
max
ꢁ
2
1.5 mW cm , Vilber Lourmat VL-215.LC) and blue light (150 W,
ꢁ2
l
¼ 410 nm, intensity at 2.0 cm ¼ 21.5 mW cm , Fort GLI
56), respectively. In all cases, the samples were placed at 2.0 cm.
The atomic force microscopy (AFM) experiments were per-
max
1
crylate-acrylic acid) with TiO
2
nanoparticles in tetrahydrofuran
14
formed in tapping mode with a Nanoscope V controller coupled
with an Icon microscope from Bruker. Measurements were
carried out in air at room temperature. For each sample, AFM
images were recorded on at least four samples and in several
places in order to check the reproducibility of the displayed
images.
(THF) was cast onto the glass substrate. The water contact
ꢀ
ꢀ
angle of the lm changed from the original 156 to about 0 ,
aer irradiation with UV light for 2 h. The initial state was
ꢀ
recovered aer heating this superhydrophilic lm at 150
C
for 1 h.
These three examples show the strong potential of hybrid
materials to obtain signicant variations in wetting. However,
Contact angles (CA) were determined using a DSA10-Mk2
(
Kr u¨ ss, Germany) in air at room temperature with a 15 mL
although the switching times are much faster compared to TiO
materials, they are too slow to be considered for use in the eld
2
ultrapure water drop. The contact angles of the super-
ꢀ
35
hydrophilic surface are arbitrarily set at 10 because it is very
of microuidics.
difficult to obtain a reliable measurement of the super-
hydrophilic surface. For each sample, contact angles were
measured on about three samples; ve drops per sample were
analyzed. The reported contact angle values correspond to the
average of all measurements with an error bar corresponding to
the standard deviation.
Aiming to prepare functional hybrid materials with both the
advantages of inorganic nanoporous TiO lms and photosen-
2
sitive moieties, we synthesized azobenzene functionalized tita-
nium oxide derivatives with different degrees of coverage with
azobenzene moieties. Initially, an organic photoswitchable
molecule (Fig. 1), the 11-(4-phenylazo)phenoxy undecanoic acid
The molecular areas are estimated by extrapolation of the
surface pressure isotherm obtained on a Langmuir trough
(
denoted as the AzoC11 acid) was easily synthesized. Then, the
photosensitivity of the AzoC11 acid in both solution and
deposited on a roughness controlled TiO lm was investigated.
The azobenzene moieties of the AzoC11 acid/TiO2 nano-
composite exhibited a reversible trans / cis isomerization
transition upon irradiation with UV, leading to a change of
surface wettability. Thanks to the combination of this chro-
mophore with a controlled roughness of TiO
wettability switch is obtained as compared to the previous
studies in this domain.
(
model 611 from Nima). The spreading solution was the AzoC11
2
ꢁ1
acid dissolved in chloroform at a concentration of 0.4 mg mL
.
To obtain the molecular area of the cis form of the AzoC11 acid,
the spreading solution was previously irradiated with UV light
for 10 min. The monolayer was compressed with a barrier speed
ꢁ1
of 20 mm min and the subphase was the ultrapure water at 20
2
, a faster reversible
ꢀ
ꢂ 1 C. The molecular areas of the AzoC11 acid correspond to
14,32,34
the average of 5 extrapolations of the surface pressure isotherm.
The layer thicknesses are the average of 10 measurements on
two samples. The measurements are obtained by using prol-
ometry (Dektak 150 form Veeco).
Experimental section
Materials
Synthesis of 11-(4-phenylazo)phenoxy undecanoic acid
4
-Phenylazophenol (98%), Triton X-100, acetyl acetone from
36,37
(AzoC11 acid)
Sigma-Aldrich, 11-bromoundecanoic acid (98%), potassium
carbonate, titanium isopropoxide from Fisher Scientic, and To a solution of 4-phenylazophenol (1.12 g, 5.6 mmol) and 11-
poly ethylene glycol from Merck were all used as received. bromoundecanoic acid (3 g, 11.2 mmol) in 150 mL of absolute
Titanium nanoparticles (TiO P25) were provided from Degussa. ethanol was added dry potassium carbonate (1.58 g, 16.8
2
Tetrahydrofuran (THF) was dried by distillation from sodium mmol). The mixture was stirred under reux for 48 h. Aer
˚
metal/benzophenone and stored over 4 A molecular sieves.
cooling, the ethanol was evaporated and the residue was
11534 | J. Mater. Chem. A, 2015, 3, 11533–11542
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ꢀ
dissolved in chloroform (200 mL). The organic layer was washed 550 C, for over 30 min. A 1000 nm thick lm was obtained, as
with HCl (2 M) solution (2 ꢃ 50 mL), then water (3 ꢃ 50 mL), measured by using a prolometer. These titanium layers allow
dried over MgSO and concentrated. The product was puried roughness to be induced on the substrates and promote adhe-
4
by recrystallization from acetonitrile to give an orange solid sion of the active layer during the spin coating.
1
(
7
1.15 g, 54%). H NMR (CDCl
3
) d ¼ 7.94–7.87 (m, 4H, Ar-N]),
.54–7.43 (m, 3H, Ar), 7.01 (dd, J ¼ 6.9 Hz et J ¼ 2.1 Hz, 2H, Ar-
Preparation of AzoC11 acid/TiO
The nanocomposite lm is composed of a mixture of the
AzoC11 acid and TiO nanoparticles (P25) in THF. 100 mg of
TiO nanoparticles were dispersed in 2 mL of THF by sonication
for 10 min. 5, 8, 10, 15 or 25 mg of AzoC11 acid was added to this
mixture. The solution was denoted as x% AzoC11 acid/TiO
2
nanocomposite lms
O), 4.04 (t, J ¼ 6.5 Hz, 2H, CH
2
O), 2.30 (t, J ¼ 7.3 Hz, 2H, CH
2
CO),
CO),
). C NMR
1
1
.85–1.76 (m, 2H, CH
2
CH
2
O), 1.67–1.61 (m, 2H, CH CH
2
1
2
3
2
.50–1.42 (m, 2H, CH
2
CH CH
2
2
O), 1.31 (m, 10H, CH
2
2
(
1
(
1
CDCl ) d ¼ 179.2, 161.8, 152.6, 146.7, 130.3, 129.0, 124.9, 122.5,
3
14.7, 68.4, 34.4, 33.9, 29.4, 29.3, 29.2, 29.1, 29.0, 26.0, 24.6. IR
2
.
ꢁ1
cm ): 2936, 2921, 2849, 1709, 1606, 1586, 1499, 1479, 1435,
309, 1281, 1249, 1221, 1146, 1019, 845. Anal. calcd for
: C, 71.94; H, 7.90; N, 7.34. Found: C, 72.22; H, 7.91;
Aer 24 h of vigorous stirring, the mixture was a uniform milky
liquid, slightly orange in colour. The AzoC11 acid/TiO solu-
2
ꢁ
23 30 2 3
C H N O
N, 7.32.
1
tions were spin coated on the substrates at 2800 tr min for
0 s. Visually, the samples which are initially whitish become
4
slightly orange. This nanocomposite lm provides easily a
structured surface.
Preparation of substrates
The substrates are composed of a glass slide with a coating of a
dense and nanoporous TiO lm. All microscope slides from
2
38
Computational methodology
39
40,41
42
RS France were subsequently cleaned in a beaker of diluate DFT calculations employing the B3LYP
and uB97XD
ꢀ
43
Hellmanex solution (5%) at least for 30 min at 50 C, rinsed with functionals were performed with a Gaussian 09 program. The
ultrapure water, sonicated for 10 min in ethanol, before being geometry optimizations for all the molecules were carried out
treated for 10 min by UV–ozone treatment (UVO cleaner 42 without symmetry constraints and were followed by frequency
Jeligth). The glass substrates were used immediately.
calculations. Given that several properties of this compound
A dense layer of TiO2 is applied by spin coating from a depend principally on the pi-conjugated backbone, a limited
precursor of TiO solution. For the preparation of the solution alkyl chain without the carboxylic group was considered for the
2
of the TiO
2
precursor, 7 drops of nitric acid (70% v/v) and 500 mL calculation of the optical properties and for the solvation energy
of water were added to 20 mL of pure ethanol. The solution was of the cis-isomer. The spectroscopic properties of the molecules
stirred for 30 min. 1.5 mL of titanium isopropoxide was added were calculated by mean of a time dependent density functional
4
4–48
to this solution. The solution was mixed and kept for 24 h theory method (TDDFT)
without light. The TiO
solution was deposited by spin-coating excited states were calculated and the theoretical absorption
on the microscope slides at 3000 tr min for 60 s. The layers bands were obtained by considering a band half-width at half-
with the 6-31G(d) basis set. Up to 20
2
ꢁ
1
ꢀ
49
were then annealed at 500 C for 15 min. The thickness maximum of 0.2 eV. The interaction energies of the model
measured by using the prolometer was about 80 nm.
water-complexes were calculated with respect to the isolated
A nanoporous layer of TiO
is applied by spin coating with a molecules at the uB97XD/6-311+G(d) level in gas phase, and
TiO paste. For the preparation of the TiO paste, 100 mg of TiO
were corrected for the basis set superposition error (BSSE) by the
nanoparticles was dispersed in 10 mL of ultrapure water by counterpoise correction method of Boys and Bernardi. The
sonication for 10 min. 100 mL of acetyl acetone, 120 mL of acetic zero point energy (ZPE) corrections were also taken into account.
acid and 100 mL of Triton X-100 were added. The mixture was
2
2
2
2
50
vigorously stirred to get a colloidal state. 120 mg of poly ethylene
Results and discussion
Geometry and molecular orbitals (MO) of trans- and cis-
ꢁ1
glycol (PEG, Mw ¼ 4000 g mol ) was employed as the blowing
agent. With magnetic bar stirring, this colloidal TiO was thor-
2
isomers
oughly mixed for 24 h followed by spin coating deposition on the
2
microscope slides covered with the dense TiO layer at 2800 tr The geometrical differences between the two isomers trans- and
ꢁ
1
min for 40 s. Finally, the substrate was gradually annealed at cis-AzoC11 acid are shown in Fig. 2. While the trans-isomer
Fig. 2 Geometries of the trans- and cis-azoC11 acid as optimized at the B3LYP/6-31G* level by taking into account the solvent effect (THF).
Some dihedral angles around the N]N bond in the cis-azoC11 acid are given.
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exhibits a perfectly planar p-system backbone, a C–N]N–C Absorption spectra of trans- and cis isomers in solution
ꢀ
dihedral angle of ꢁ10.6 is found in the case of the cis-isomer,
The experimental UV-vis spectra of the AzoC11 acid in THF,
before and aer 40 s of UV irradiation, are given in Fig. 4a,
whereas the corresponding theoretical absorption spectra of the
trans- and cis-AzoC11 acid in THF are given in Fig. 4b. Both
due to the steric hindrance between the phenyl rings.
Inspection of Fig. 3 indicates two important differences
between trans- and cis MOs: (i) increase of the n–p mixture
appears in the MO plots from HOMOꢁ1 to HOMO in the case of
trans- and cis-AzoC11 acid spectra present one low-energy band
the cis-isomer and (ii) due to the non-zero C–N]N–C dihedral
centered around 450 nm, and a higher-energy one peaking
around 320 nm. Theoretical absorption spectra indicate that
ꢀ
angle (ꢁ10.6 ) in the cis isomer, the p-conjugation efficiency
decreases upon trans / cis isomerization. The energy order
each band is due to a single excitation (Fig. S1a of the ESI†),
between the p-type (HOMO) and the n-type (HOMOꢁ1) orbitals
principally of the n / p* and p / p* character for the low-
and high energy bands respectively (Fig. 4b). In the case of the
in the trans-isomer is reversed in the case of the cis-isomer
(
dominant n-character in HOMO, dominant p-character in
trans-isomer, the theoretical n / p* transition (HOMOꢁ1 /
LUMO) is symmetry forbidden (zero oscillator strength, Fig. 4b),
due to the zero-overlap of the corresponding MOs (Fig. 3). The
non-zero experimental value for this band may be due to the
geometrical deformations of the trans-isomer in solution, which
are absent in the theoretical results. Indeed, test calculations
considering small deviations from the planarity around the
HOMOꢁ1), which will be seen to impact the positioning and
intensity of the absorption spectra. As for the LUMOs, they
remain dominantly of N]N p-contribution for both isomers.
ꢀ
ꢀ
ꢀ
ꢀ
N]N bond (2 , 5 , 8 , and 10 ) result in a nonzero oscillator
strength of 0.002, 0.003, 0.022, and 0.033 respectively for the n
/
p* transition (Fig. S1b of the ESI†).
Upon UV-irradiation of the trans-isomer, the intensity of the
p–p* transition band (ꢄ350 nm) decreases signicantly and the
maximum position is blue shied to 315 nm. These important
modications are principally due to two effects: (i) because of
the n–p nature mismatch between HOMOꢁ1 and LUMO at the
N]N moiety of the cis-isomer, the HOMOꢁ1–LUMO overlap
(impacting the transition intensity) is reduced signicantly, and
Fig. 3 Selected set of MO plots corresponding to the trans- and cis- (ii) in the case of the cis-isomer, the p–p* excitation principally
azoC11 acid. The alkyl chains are hidden for the sake of clarity. The corresponds to the HOMOꢁ1 / LUMO transition (77%),
encircled parts (red circles) in each MO correspond to the contribution
of the N–N fragment.
involving a larger gap as compared to the HOMO–LUMO gap for
ꢁ4
ꢁ1
Fig. 4 (a) UV-vis spectra of the AzoC11 acid in THF solution (C ¼ 10 mol L ) before (red curve), during (blue curve) and after (blue bold curve)
UV irradiation and after the blue irradiation (black dotted curve). The highlighted discontinuity of the high-energy band of the cis-isomer is
supposed to be due to the presence of some trans-isomer. Inset (b) theoretical absorption spectra obtained at the TDB3LYP/6-31G* level. The
dominant transitions are also indicated for each band.
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the trans-isomer, consequently resulting in the observed blue- illumination, cis / trans isomerisation occurs with very high
shi. Similar arguments apply to the n–p* transition band at yield, as the spectrum of the trans-azobenzene is totally resti-
4
42 nm: due to the n–p mixing, the n–p* transition in the case tuted. The restituted spectrum (black dotted line) is superposed
of the cis-isomer becomes less forbidden, resulting in a slight on the initial spectrum (red solid line) in Fig. 4a.
increase in the intensity of the low-energy band. Upon trans / Despite the presence of some mixture of cis- and trans-
cis isomerisation of the AzoC11 acid in THF solution, the isomers in solution, Fig. 4a indicates that the intensity of the
absorption spectra exhibit important changes in the intensity high-energy absorption band of the AzoC11 acid upon UV and
and position of the bands. The UV-vis spectroscopy presents visible light irradiation exhibits important variations and is
consequently an easy method to follow the trans / cis almost totally reversible. Fig. 5 shows the normalized absorp-
isomerization.
tion intensities of the high-energy band of the AzoC11-acid for
The experimental UV-vis spectra of the AzoC11 acid in THF several UV 4 vis irradiation cycles.
during UV irradiation (Fig. 4a) suggest that some amount of the
The absorption maxima and minima at 348 nm correspond
trans-isomer is still present in solution. Indeed, aer 40 s of UV respectively to predominantly the trans- and cis-AzoC11 acid.
irradiation (blue bold curve), the high-energy band exhibits a The trans / cis and cis / trans isomerization of a solution of
discontinuity of the right-hand part (highlighted by an arrow, the AzoC11 acid is fast (less than 4 min for a full cycle) and
Fig. 4a), the position of which ts perfectly with the maximum completely reversible, making the AzoC11 acid a good candi-
position of the trans-isomer. This conclusion is supported by date for developing photosensitive surfaces.
the similar positions of the absorption onset (Fig. S2 of the ESI†)
between the two bands in the experimental spectra, both
features (discontinuity and identical onset) being absent in the
theoretical absorption spectra (Fig. 4b). A possible explanation
Morphology characterization of nanocomposite lms
The nanocomposite lm is composed of a mixture of the
AzoC11 acid and TiO nanoparticles (P25) in THF spin coated
2
for this mixture could be given in terms of the relative stability
ꢁ1
on the titanium substrate. Optimization of a large and repro-
ducible photoresponsive switch is based on the control of
wettability of the lm, which is generally inuenced by the
morphology and surface free energy. The former is oen
exhibited as the surface roughness, and the latter is usually
displayed as the change in the dipole moment.
of the trans-and cis-isomers, the latter one being ꢄ11 kcal mol
higher in energy as compared to the trans-isomer (uB97XD/6-
11+G** level in THF). Finally, aer subsequent blue
3
Firstly, the roughness of the lm was estimated from AFM
height images before and aer the AzoC11 acid/TiO
Fig. 6).
The AFM images of the substrate reveal the nanoporous
structure of the TiO layer. The pores are formed by calcination
2
deposition
(
2
ꢁ
1
of the blowing agent (PEG, M ¼ 4000 g mol ). The average size
w
of grains is 300 nm. These grains are therefore the result of the
aggregation of nanoparticles whose initial diameter is 20 nm.
2 2
The images of the pure TiO lm and the AzoC11 acid/TiO layer
Fig. 5 Reversible trans–cis isomerisation of the AzoC11 acid in THF (C
are very similar. However the slight orange color of the sample
shows that the organic layer is present. No additional aggre-
gates were observed between the image in Fig. 6(a) and the
image in Fig. 6(b), and the AzoC11 acid forms a homogeneous
ꢁ
5
ꢁ1
¼
5 ꢃ 10 mol L ) under 2 min UV irradiation and 2 min blue light
irradiation. The location of the markers indicate the normalized
absorbance of the high-energy absorption band (at 348 nm) of the
AzoC11 acid upon UV and visible light irradiation.
2 2
Fig. 6 AFM images of the substrate: dense + nanoporous TiO layers (a) and the mixture of 10% wt AzoC11 acid/TiO on dense + nanoporous
TiO
2
layers (b) (2 mm ꢃ 2 mm).
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layer on the surface. The active layer thus preserves the nano-
porous nature of the substrate. Furthermore, the AzoC11 acid to correspond to roughly 111% of TiO surface coverage with the
Note that an AzoC11 acid/TiO
2
weight ratio of ꢄ10% seems
2
appears to form a homogeneous layer on the nanoparticle cis-AzoC11 acid, but only 77% with the trans-AzoC11 acid.
surface without lling the nanopores. Indeed, the root-mean-
square roughness (Rrms) calculated on 5 images of 4 mm were following, unless otherwise specied, the surface coverage
Effect of surface-coverage on the contact angles. In the
2
very close, respectively 49 ꢂ 4 nm and 64 ꢂ 3 nm for pure TiO
and the AzoC11 acid/TiO layer on the substrate.
2
ratios considered during the discussions will correspond to the
surface ratios of the trans-AzoC11 acid (Table 1). The contact
angles, corresponding to nanocomposite lms with different
2
AzoC11-acid/TiO
coverage on the contact angles
Surface coverage ratios. The wettability of the nano-
composite lms on nanoporous TiO layers is followed by
measuring the contact angle. The inuence of surface-coverage
on the contact angles was studied by considering the AzoC11-
2
nanocomposite lms: effect of surface-
AzoC11 acid/TiO
Fig. 7a.
Initial contact angles (ICA). In the absence of the AzoC11 acid,
2
surface ratios, were measured and shown in
the TiO surface is superhydrophilic (the contact angle is less
2
ꢀ
2
than 10 ). Fig. 7a suggests clearly that the increase of the ICA
with increasing surface ratios between 0 and 193% is simply
due to the increase in surface coverage with the AzoC11 acid
2
acid/TiO weight ratios of 0, 5, 8, 10, 15 and 25%. In order to
correlate these weight ratios with the individual surface ratios of
(Table 1). The ICA increase rapidly when the AzoC11/TiO
2
surface ratio is smaller than 62%, but remain stable (close to
the trans- and cis-AzoC11 acid on the TiO nanoparticles, the
2
ꢀ
1
50 ) for larger surface coverage values. Modulating a surface
molecular areas of the trans- and cis-AzoC11 acid were esti-
state from a superhydrophilic to a superhydrophobic state can
thus be easily achieved by varying the amount of the AzoC11
acid in the mixture.
Contact angles aer UV irradiation. Contact angles aer UV
irradiation (CAAUV) are lower than the initial contact angles for
mated by extrapolating surface pressure isotherms. The
2
2
molecular areas of 0.27 ꢂ 0.02 nm and 0.39 ꢂ 0.08 nm for the
trans- and cis-isomers respectively were found, and the results
2
were also supported by the theoretical estimations of 0.26 nm
2
and 0.41–0.42 nm (see the ESI: annexe III†). The value of the
53
all surface ratios, which could be explained by a competition
between different processes: (i) trans / cis isomerization of the
azobenzene function upon UV irradiation induces a variation in
the dipole moment, which has been considered as the domi-
nant factor inuencing the surface energy upon isomeriza-
trans AzoC11 acid is comforted by the molecular area of the
trans 4-octyl-[phenylazo-phenoxy] pentanoic acid estimated by
2
51
Zhang (0.28 nm ).
Based on these molecular areas and the specic area of TiO
nanoparticles P25 (55 m g ), the surface ratio of the AzoC11
acid on the nanoparticles can be estimated (see the ESI: annexe
IV†). The surface ratios for the different mass fractions of the
AzoC11 acid are presented in Table 1.
2
2
ꢁ1
54,55
tion.
Indeed, the theoretical dipole moments of the isolated
trans- and cis-AzoC11 acid ranges are, respectively, from 2.2–2.9
D and 3.6–6.1 D (B3LYP/6-31G** level “in THF”). While the
Table 1 Surface ratios (in %) calculated as a function of the mass fraction of the AzoC11 acid
Mass fraction of AzoC11 acid/TiO
Surface ratios with trans AzoC11 acid
Surface ratios with cis AzoC11 acid
2
5%
8%
10%
77 ꢂ 6%
111 ꢂ 23%
15%
25%
39 ꢂ 3%
62 ꢂ 5%
116 ꢂ 9
193 ꢂ 14% (ref. 52)
56 ꢂ 12%
89 ꢂ 19%
167 ꢂ 34%
279 ꢂ 57%
Fig. 7 (a) Initial water contact-angle for different surface ratios of the AzoC11 acid (full blue bars), and contact angle after 12 min of UV irradiation
(
patterned red bars). (b) The variations of the contact angles between before and after the UV irradiation are shown by the diamond markers (A).
ꢀ
The authors recall that the angles of superhydrophilic surfaces are arbitrarily set at 10 .
11538 | J. Mater. Chem. A, 2015, 3, 11533–11542
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Journal of Materials Chemistry A
absolute values of the dipole moments of the AzoC11 acid on CAAUV in the presence of the AzoC11 acid can be due to the fact
the TiO surface may be different as compared to the isolated that the trans / cis isomerization is not complete even aer 40
2
molecules, their trend between trans- and cis-isomers is expec- min of UV irradiation (Fig. 4). The CAAUV remains however less
ꢀ
ted to remain identical. The polarity of the AzoC11 acid thus than 40 for surface ratios from zero to 77%, which is consistent
increases aer UV irradiation, in turn increasing the surface with both the 77% of the surface coverage ratio for the trans-
hydrophilicity and decreasing the contact angle. (ii) Trans / cis isomer and the 111% of the surface coverage ratio for the cis-
isomerization additionally allows for the establishment of isomer. Indeed, due to important geometrical modications
hydrogen bonds between the nitrogen lone pairs and water during the trans / cis isomerization (increase in the molecular
2
2
molecules, thus increasing the surface hydrophilicity. Indeed, areas from 0.27 nm to 0.39 nm ), free room is needed around
59
the HOMO plot of the cis-isomer (Fig. 8a) shows both nitrogen the trans-azobenzene moieties. The 77% surface ratio for the
lone pairs oriented outside with respect to the surface. Our test trans-isomer (10% of weight ratio) thus corresponds to enough
calculations on the model cis-AzoC11 acid–2H O complex free room around the trans-azobenzene groups for the trans /
2
clearly indicate the establishment of efficient hydrogen bonds, cis isomerization to be sterically unhindered. As expected, the
ꢁ1
with a net energy stabilization of 5.4 kcal mol , consistent with CAAUV rises dramatically when the surface ratio for the trans-
ꢁ1
a typical value of ꢄ3 kcal mol per individual H-bond. In isomer is greater at 100%, suggesting a small degree of trans /
contrast, the nitrogen lone pairs are little visible when the cis isomerisation. This last effect is consistent with the insuffi-
AzoC11 acid is in the trans form, as also suggested by the shape cient free space around trans-azobenzene groups for a sterically
of HOMOꢁ1 (Fig. 3). (iii) Upon UV irradiation, photogenerated unhindered trans / cis isomerization.
holes diffuse to the TiO surface and react with lattice oxygen of
Contact angle variations. The contact angle variations (calcu-
2
TiO itself, resulting in the creation of oxygen vacancies at the lated as ICA-CAAUV, Fig. 7b) depend consequently on the
2
56
2
surface. These oxygen vacancies are presumably favorable for composition of the AzoC11 acid/TiO mixture. In line with the
57
water adsorption. Water molecules are both adsorbed on evolution of ICA and CAAUV, the contact angle variations
defect sites and dissociated, resulting in the generation of two initially increase due to the sharp increase in ICA whereas
20,34,58
hydroxyl groups on each defect
(Fig. 8b). Increased CAAUV remains small due to efficient trans / cis isomeriza-
hydrophilicity character of the TiO
consequently occur.
2
(and global) surface will tion. The contact angle variation (ICA-CAAUV) reaches the
ꢀ
maximum value of 124.5 when the AzoC11 acid/TiO2 mass
The change in the contact angles aer UV irradiation can ratio reaches 10%, which corresponds to the ideal value of
thus result from a competition between these three factors. roughly 100% for the surface ratio (coverage) with the cis-
However, hydrophobicity-to-hydrophilicity switching times are isomer. For surface ratios larger than this ideal value, the trans-
much larger for the pristine titania surface as compared to the AzoC11 acid layer becomes very compact and the ICA continues
TiO surface covered with the AzoC11 acid, suggesting that the to remain large, but the azobenzene groups are less and less
2
third factor should be of minor inuence in the above compe- able to undergo trans / cis isomerization because of the
tition. This conclusion is also supported by the observation that increased steric hindrance, thus resulting in much larger
decreasing the free surface of TiO by increasing the surface CAAUV and smaller contact angle variation.
2
coverage with the AzoC11 acid, results in an increased contact
angle variation (Fig. 8b).
These results show that there is a good correlation between
the variations of the contact angle and the AzoC11 acid/TiO2
Concerning the evolution of the CAAUV with the increasing surface ratios. The mixture at 10% wt of the AzoC11 acid,
surface ratio (Fig. 7b), the smallest CAAUV corresponds to the resulting in trans- and cis-surface ratios of roughly 77% and
pristine TiO surface (0% surface ratio, Fig. 8a). The increase in 111% respectively, is a good compromise between the amount
2
Fig. 8 (a) Model cis-AzoC11 acid–2H
water). The indicated distances are in A˚ . (b) The different effects of UV irradiation on AzoC11 acid/TiO
photo-generation (in red).
2
O complex as calculated at the wB97XD/6-311+G(d,p) level by taking into account the effect of the solvent
(
2
: azobenzene isomerization and hydroxyl
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2
of the AzoC11 acid present at the surface of TiO and its ability
to isomerize. In addition, for this fraction, the contact angle
ꢀ
variation is very important and fast: over 124 aer 12 min
under UV irradiation.
Reversibility study
Fig. 9 shows the evolution of water contact angles of the 10% wt
AzoC11 acid/TiO
2
lm as a function of irradiation time under
ꢀ
UV light (15 W, lmax ¼ 365 nm) and heating at 150 C to induce a
60
return to the hydrophobic state.
Upon UV irradiation, the azobenzene-groups in the lm
isomerize from the trans- to the cis-conformation, resulting in a
decrease in the contact-angle and lm switching from the
superhydrophobic to superhydrophilic state. Subsequent heat-
Fig. 10 Reversible wettability transition of the 10% wt AzoC11 acid/
TiO
2
film after several cycles of 12 min UV irradiation followed by 20
ing of AzoC11 TiO lms induces reverse cis / trans isomeri-
ꢀ
2
min at 150 C.
zation, and the lm hydrophobicity is accordingly recovered.
The minimum of contact angle is obtained aer 12 min of UV
ꢀ
irradiation. Aer 20 min at 150 C, the contact angle recovers
the cis form, the molecules may be very close against each other.
almost its initial value. The variation state is reversible and the
full cycle is only for 32 min. This is to our knowledge, both the
most rapid kinetics encountered in the literature and with large
variation as well.
The reversibility of the photoresponsive switch was esti-
mated by following the evolution of the water contact angle of
61,62
Some intermolecular interactions can thus be formed
and
can limit the cis / trans isomerization of the AzoC11 acid. To
check this hypothesis, the isomerizations, trans / cis and cis
/
trans, of the 10% wt AzoC11 acid/TiO lm were followed by
2
UV-vis spectroscopy in reectance mode. The absorbance values
at 438 nm (in order to avoid the TiO absorption at 348 nm) of
lm are reported in Table 2 aer
2
2
the 10% wt AzoC11 acid/TiO lm during several cycles (Fig. 10).
Each cycle corresponds to 12 min of UV irradiation followed by
the 10% wt AzoC11 acid/TiO
each step.
2
ꢀ
2
0 min of heating at 150 C.
The absorption at 438 nm is characteristic of the n–p*
transition. Under UV irradiation, the absorbance at 438 nm
increases, as the AzoC11 acid isomerizes from the trans state to
the cis state. Also aer the heating step, the absorbance at 438
nm decreases, and the AzoC11 acid returns to the trans state.
Finally, the absorbance value aer the rst cycle (0.073) is very
close to its initial value (0.074). It seems that the reversibility of
the isomerization process in the lm is satisfying, thus sug-
gesting that the decrease of the contact angle aer the rst cycle
2
The water contact angle of the 10% wt AzoC11 acid/TiO lm
decreased from 144 to 16 aer 12 min of UV irradiation. The
quasi original high hydrophobicity was recovered aer heating
ꢀ
ꢀ
ꢀ
the sample at 150 C for 20 min. The process of reversible
switching of the surface wettability was carried out over 3 cycles
of UV irradiation and heating which indicates that the lm has
reproducible reversible wettability. While the contact angle
aer the rst cycle is less than the initial contact angle (118 and
44 respectively), it remains however constant for the next
cycles.
Different hypotheses can be proposed to explain the differ-
ence between the initial contact angle and the contact angle
aer the rst cycle. The rst hypothesis is based on the partial
reversibility of cis / trans isomerization. Unlike in solution, the
mobility of AzoC11 acids is limited. When AzoC11 acids are in
ꢀ
ꢀ
1
ꢀ ꢀ
from 144 to 118 ) could not stem from uncompleted cis /
(
trans isomerisation.
The second hypothesis is based on the process related to the
nature of the substrate. A decrease in contact angle aer the

rst cycle has been already observed in the case of substrates
14,34,63
based on TiO
over the cycles in the case of silica-based substrates.
2
,
whereas contact angles appear to be stable
64–66
In the
previous section we showed that the photo-generated hydroxyl
groups was not the dominant process in transitions between
hydrophobic and hydrophilic states. However, this process may
explain the differences of the initial contact angle and the
contact angle aer the rst cycle. Indeed, while the
Table 2 Evolution of absorbance at 438 nm of the 10% wt AzoC11
acid/TiO
min at 150 C
2
film after one cycle of 12 min UV irradiation followed by 20
ꢀ
Aer 12 min under
UV irradiation
Aer 20 min
at 150 C
ꢀ
Initial
0.074
2
Fig. 9 Water contact angle of the 10% wt of the AzoC11 acid/TiO film
as a function of time of the UV irradiation (C in red) and time of Absorbance at 438 nm
0.138
0.073
ꢀ
heating at 150 C (- in blue).
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isomerization process is fast (few minutes, Fig. 9) relative to the
desorption of hydroxyl groups photo-generated by UV, the
return to the initial density of hydroxyl groups was shown to be
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