Formation of self-assembled monolayers of π-conjugated molecules on TiO2 surfaces by thermal grafting of aryl and benzyl halides

Article abstract of DOI:10.1021/la300271h

We demonstrate the formation of molecular monolayers of π-conjugated organic molecules on nanocrystalline TiO₂ surfaces through the thermal grafting of benzyl and aryl halides. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy were used to characterize the reactivity of aryl and benzyl chlorides, bromides, and iodides with TiO ₂ surfaces, along with controls consisting of nonhalogenated compounds. Our results show that benzyl and aryl halides follow a similar reactivity trend (I > Br > Cl ? H). While the ability to graft benzyl halides is consistent with the well-known Williamson ether synthesis, the grafting of aryl halides has no similar precedent. The unique reactivity of the TiO₂ surface is demonstrated using nuclear magnetic resonance spectroscopy to compare the surface reactions with the liquid-phase interactions of benzyl and aryl iodides with tert-butanol and -butoxide anion. While the aryl iodides show no detectable reactivity with a tert-butanol/tert-butoxide mixture, they react with TiO₂ within 2 h at 50 °C. Atomic force microscopy studies show that grafting of 4-iodo-1-(trifluoromethyl)benzene onto the rutile TiO₂(110) surface leads to a very uniform, homogeneous molecular layer with a thickness of ～0.45 nm, demonstrating formation of a self-terminating molecular monolayer. Thermal grafting of aryl iodides provides a facile route to link π-conjugated molecules to TiO₂ surfaces with the shortest possible linkage between the conjugated electron system and the TiO₂.

Full text of DOI:10.1021/la300271h

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Article
pubs.acs.org/Langmuir
Formation of Self-Assembled Monolayers of π-Conjugated Molecules
on TiO2 Surfaces by Thermal Grafting of Aryl and Benzyl Halides
Caroline R. English, Lee M. Bishop, Jixin Chen, and Robert J. Hamers*
Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States
S
* Supporting Information
ABSTRACT: We demonstrate the formation of molecular monolayers of π-
conjugated organic molecules on nanocrystalline TiO2 surfaces through the
thermal grafting of benzyl and aryl halides. X-ray photoelectron spectros-
copy and Fourier-transform infrared spectroscopy were used to characterize
the reactivity of aryl and benzyl chlorides, bromides, and iodides with TiO2
surfaces, along with controls consisting of nonhalogenated compounds. Our
results show that benzyl and aryl halides follow a similar reactivity trend (I >
Br > Cl ≫ H). While the ability to graft benzyl halides is consistent with the
well-known Williamson ether synthesis, the grafting of aryl halides has no
similar precedent. The unique reactivity of the TiO2 surface is demonstrated using nuclear magnetic resonance spectroscopy to
compare the surface reactions with the liquid-phase interactions of benzyl and aryl iodides with tert-butanol and -butoxide anion.
While the aryl iodides show no detectable reactivity with a tert-butanol/tert-butoxide mixture, they react with TiO2 within 2 h at
50 °C. Atomic force microscopy studies show that grafting of 4-iodo-1-(trifluoromethyl)benzene onto the rutile TiO2(110)
surface leads to a very uniform, homogeneous molecular layer with a thickness of ∼0.45 nm, demonstrating formation of a self-
terminating molecular monolayer. Thermal grafting of aryl iodides provides a facile route to link π-conjugated molecules to TiO2
surfaces with the shortest possible linkage between the conjugated electron system and the TiO2.
“bridging” hydroxyl groups, as depicted in Figure 1.24,25 The
former arises from −OH groups binding to undercoordinated
INTRODUCTION
■
The integration of π-conjugated molecules with metal oxide
surfaces provides a way to combine the tunable electronic and
chemical properties of organic molecules with the mechanical
and electronic properties of inorganic semiconductors, as
demonstrated by recent advances in dye-sensitized solar
cells.2−4 The overall properties of such hybrid systems are
often dominated by the interfacial chemistry that connects the
organic molecules with the inorganic substrates. While a
number of different molecule-to-surface linkages have been
investigated,5,6 most research on TiO2 makes use of simple
carboxylate linkages, linking the molecular systems to the
surface via the electrostatic binding between the carboxylate
groups and the exposed, coordinatively unsaturated Ti4+ atoms
present at the surface.7−9 Other approaches to functionalization
of TiO2 have included use of silanes,10−16 phosphonic acids,
and phosphonates,16−20 formation of ether linkages from
alcohols,21 and photochemical grafting of alkenes.22,23 The
desire to improve the overall stability and electron-transfer
properties of molecules at surfaces leads to continued interest
in the development of new approaches for linking π-conjugated
molecules to TiO2 and other inorganic semiconductors.
Figure 1. Reaction of aryl halides (top) and benzyl halides (bottom)
with hydroxyl groups on the TiO2 surface. Both terminal and bridge-
bonded hydroxyl groups1 are shown.
Surfaces of TiO2 in both anatase and rutile phases expose Ti
and O atoms having coordination numbers N lower than the
bulk values of N = 6 for Ti and N = 3 for oxygen. Under
ambient conditions, water dissociates on the surface, with the
H+ ions protonating exposed O atoms and OH− groups
bonding to exposed Ti ions. TiO2 surfaces have at least two
distinct types of −OH groups, referred to as “terminal” and
Received: January 18, 2012
Revised: March 14, 2012
Published: March 26, 2012
© 2012 American Chemical Society
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Ti ions, while the latter arises from H+ binding to surface
oxygen atoms that are bound to two Ti ions (instead of 3 as in
the bulk). The net result is a surface partially covered with
−OH groups. The number, type, and protonation state of the
hydroxyl groups can be further manipulated through processes
such as exposure to acidic or basic conditions or by heat
treatments. Under ambient conditions on microcrystalline
samples, the density is typically 3−4 −OH groups/nm2,26,27
covering roughly 20−40% of the available surface sites.
Instead of using electrostatic bonding of carboxylate groups
to the surface Ti ions, an alternative approach to binding
molecules to TiO2 surfaces under benchtop (nonvacuum)
conditions is to develop new surface linkages based on the
chemistry of the surface hydroxyl groups.28,29 Previous work
showed that exposing TiO2 samples in vacuum to ethyl iodide
vapor followed by heating to >100 °C yielded surface ethoxy
groups, and subsequent work showed that exposing TiO2 to
iodobenzene vapor in vacuum followed by heating to >150 °C
for 2 h produced surface phenoxy groups.30 Recently, we
showed29 that liquid alkyl iodides form monolayers on
hydroxylated TiO2 surfaces under near-ambient conditions
(mild heating), as a convenient route to functionalized TiO2
surfaces via simple benchtop chemistry.
surface bond. Because interfacial charge transfer is largely
controlled by the degree of overlap of the wave functions of the
molecule and substrate,9,37,38 this approach may provide a
convenient route to improved TiO2−molecule interfaces with
facile charge-transfer properties.
EXPERIMENTAL SECTION
■
Preparation of TiO2 Nanocrystalline Thin Films. Nanocrystal-
line TiO2 films were made using a commercially available paste (Ti-
Nanoxide T20/SP, Solaronix, Switzerland) containing 20 nm diameter
anatase TiO2 nanoparticles. The TiO2 paste was screen-printed onto
substrates consisting of single-crystal Si(001) wafers with a thin
evaporated Ti film to provide good adhesion to the TiO2. Substrates
used for XPS experiments used a Ti film 100 nm thick on low-
resistivity (<0.1 Ω cm) wafers to provide good electrical conductivity.
For infrared studies, we used high-resistivity, float-zone-refined
Si(001) wafer and a thinner (20 nm) Ti film; the thin Ti film
provides good adhesion of the nanoparticle film to the Si substrate
while also reducing optical interference effects that otherwise arise
from reflections at the TiO2−Ti interface. On all samples, the TiO2
film was prepared by screen-printing. The resulting nanoparticle film
was immediately dried at 110 °C and then annealed by gradually
heating to 500 °C over 30 min, followed by an additional 15 min at
500 °C. Immediately before use, the TiO2 nanoparticle films received
either an acid-piranha treatment (Caution! Piranha solution is
extremely dangerous, and proper safety precautions must be followed)
or a NaOH (pH = 11) treatment for 20−30 min. The piranha solution
consisted of a mixture of 98% concentrated H2SO4 and 30% hydrogen
peroxide in a 3:1 volumetric ratio. Samples were then rinsed a
minimum of three times with isopropanol and dried with N2 after each
rinse.
The interaction of organic halides with surface hydroxyl
groups appears formally equivalent to the well-known
Williamson ether synthesis of organic chemistry.31 In the
Williamson ether synthesis, an organic alcohol R−OH (usually
deprotonated to form the alkoxides, R−O−) reacts with an alkyl
halide R′−X to form the ether linkage while eliminating HX:
Preparation of Single-Crystal TiO2 Samples. Single-crystal
rutile TiO2 (110) (Crystek) samples were ultrasonically agitated in
methanol for 10 min and then placed below a low-pressure Hg lamp in
air for 30 min; 185 nm light from the lamp generates a small amount
of ozone that removes traces of organic contamination. Afterward, the
samples received an HF etch (48 wt % in water) for 30 min before
being annealed for 2 h at 900 °C. Directly before use, samples received
either an acid-piranha treatment or a base (pH = 11) treatment for
20−30 min. Samples were then rinsed a minimum of three times with
isopropanol and dried with N2 after each rinse.
Thermal Grafting of Halogenated Organic Compounds to
TiO2 Surface. Grafting reactions were performed by immersing TiO2
samples into neat organic compound and heating in an oven; exposure
to light was minimized to avoid photochemical reactions. All grafting
compounds were used as received from Sigma-Aldrich except for
iodomethyl-TFMB, which is not commercially available and was
therefore synthesized and purified as described in the Supporting
Information. A range of reaction times was explored; unless otherwise
specified, data reported here used a reaction time of 24 h. Grafting
experiments were conducted at a temperature of 50 °C for all reactions
except those involving iodomethyl-TFMB. Because the melting point
of this compound is higher than 50 °C, all reactions using it were
conducted at 80 °C. Upon reaction completion, all samples were
rinsed a minimum of three times with isopropanol and dried with N2
after each rinse.
Reactivity of Iodo Compounds toward tert-Butanol. Reac-
tions of 1-(iodomethyl)-4-(trifluoromethyl)benzene and 1-iodo-4-
(trifluoromethyl)benzene with tert-butanol were performed under
the same reaction conditions as those used for the TiO2 samples. tert-
Butanol was chosen because its tertiary structure mimics the geometry
of TiO2 surface hydroxyl groups. In some experiments, sodium tert-
butoxide (Aldrich, 97%) was used to explore the reactivity of the
deprotonated alcohol. Reaction progress was monitored by NMR
spectroscopic analysis.
X-ray Photoelectron Spectroscopy. X-ray photoelectron spec-
troscopy (XPS) was performed using a custom-built XPS system
(Physical Electronics Inc., Eden Prairie, MN) consisting of a model 10-
610 Kα X-ray source (1486.6 eV photon energy) with a model 10-420
R−OH + R′−X→R−O−R′ + HX
The Williamson ether synthesis readily takes place with
benzyl and primary alkyl halides via a bimolecular nucleophilic
substitution (SN2) mechanism.32 While this reaction is facile at
the sp3-hybridized carbon atoms of benzyl and primary alkyl
halides, similar etherification reactions with aromatic halides
such as aryl halides (in which a halide is directly bonded to a
benzene ring) are much less effective and typically proceed by
nucleophilic aromatic substitution or radical reaction mecha-
nisms, if they proceed at all.32−36 Despite these difficulties, the
possibility of using aromatic halides as a pathway to molecular
layers on surfaces such as TiO2 is of significant interest because
the resulting structure would have a direct linkage between the
O atoms of a (hydroxylated) TiO2 surface and a C atom whose
electrons are part of the π-conjugated electron system of the
molecule. Figure 1 illustrates the binding configurations on a
TiO2 surface that would result from surface hydroxyl groups
reacting with aryl halides and benzyl halides.
Here, we report an investigation of the interactions of aryl
and benzyl halides with TiO2 surfaces using X-ray photo-
electron spectroscopy (XPS), Fourier-transform infrared
(FTIR) spectroscopy, and atomic force microscopy (AFM).
We use the compounds 4-halo-1-(trifluoromethyl)benzene and
4-halomethyl-1-(trifluoromethyl)benzene, depicted in Figure 1,
as model systems. Our results show that aryl and benzyl iodides
will readily graft to TiO2 with cleavage of the C−I bond with
only mild heating, leading to phenyl-terminated molecular
layers. AFM images of the sample reacted with the aryl iodide
show formation of smooth, homogeneous molecular layers.
Some reactivity is also observed for the bromo-compounds,
while the chloro-compounds show little or no reactivity. The
results demonstrate the ability to form well-defined molecular
layers in which π conjugation extends directly to the molecule-
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toroidal monochromator and a model 10-360 hemispherical analyzer
with a 16-channel detector array; electrons were typically collected at
an emission angle of 45° from the surface normal. The analyzer was
typically operated with a 58.7 eV pass energy that yielded an analyzer
energy resolution of 0.88 eV. Procedures for analyzing XPS data and
converting to coverages are described in the Supporting Information.
Fourier-Transform Infrared (FTIR) Spectroscopy. Infrared
spectroscopy was performed using a Bruker Vertex 70 FTIR
spectrometer with a Veemax II variable-angle single-bounce reflection
accessory. A liquid nitrogen cooled HgCdTe detector was used.
Measurements were made with a resolution of 4 cm−1 at a 50° angle of
incidence from the surface normal using p-polarized light. Reference
(background) samples consisted of Si samples with an evaporated 20
nm Ti film that received the same heat treatment as the actual samples.
Absorbance spectra were measured before and after reaction and then
subtracted to eliminate absorption features of the TiO2 film itself and
bring out the changes induced by the surface reactions. In some cases,
broad areas of the spectra were fit to sixth degree polynomials (using
data points only in areas where no absorption features were observed)
to reduce remaining curved backgrounds.
Atomic Force Microscopy. Atomic force microscopy (AFM)
experiments were performed in contact mode using a Multimode
Nanoscope IV instrument. Measurements of the molecular layer
thickness were performed at higher sample-tip forces to scratch away
the molecular layer in a square 1 × 1 μm region followed by imaging a
larger 4 × 4 μm region centered at the same spot. Experiments were
conducted using a range of forces to determine conditions under
which the molecular layers could be reliably removed without
scratching the underlying TiO2 surface. Diamond-like carbon
(DLC)-coated Si AFM tapping probes (Tap300DLC, Budget Sensors)
with a force constant of 40 N/m were used to perform the surface
scratching and imaging.
1
Nuclear Magnetic Resonance (NMR). H NMR spectra were
obtained using a Varian Mercury-Plus 300 (300 MHz) instrument
using CDCl3 as the solvent. Chemical shifts (δ) are reported in parts
per million (ppm) with the CDCl3 reference peak set to 7.24 ppm for
all spectra. Hexamethylbenzene was added to some samples as an
internal standard for quantification.
Mass Spectrometry. Mass spectrometry was performed on NMR
samples to confirm NMR findings. A Waters (Micromass) AutoSpec
was used to collect spectra, using electron impact ionization with 70
eV electrons. Perfluorokerosene was used to calibrate the AutoSpec.
Figure 2. F(1s) XPS data from nanocrystalline TiO2 films after
reaction with aryl halide compounds for 12 h at 50 °C. The bottom
trace shows data for methyl-TFMB as a control having a nonreactive
group bound to the aromatic ring in place of a halogen.
TFMB show no F, Cl, Br, or I signal. The trend in reactivity
observed in the F(1s) spectrum is also reflected in the C(1s)
spectra shown in the Supporting Information, although
interpretation of C(1s) spectra is complicated by small amounts
of contamination that is unavoidable in benchtop chemistry;
thus, the F(1s) intensity from the CF3 group serves as a more
robust measure of the extent of adsorption. The Ti(2p) and
O(1s) spectra are nearly identical for all four samples; these
spectra reflect primarily the underlying substrate and can be
used as an internal standard for quantitative analysis.
RESULTS
■
Reaction of Aryl Halide with TiO2 Surfaces. As depicted
in Figure 1, we investigated aryl compounds of the form 4-halo-
1-(trifluoromethyl)benzene, which we refer to as “iodo-TFMB”,
“bromo-TFMB”, and “chloro-TFMB”, along with the 4-methyl-
1-(trifluoromethyl)benzene (methyl-TFMB). The trifluoro
compounds were chosen because the −CF3 group serves as a
good marker in XPS and is itself unreactive toward TiO2 (vide
infra).39 Figure 2 shows XPS data for the F(1s) region for all
three aryl halides and methyl-TFMB, which we use as a control
sample. XPS data are also shown for the I(3d5/2), Br(3d), and
Cl(2p) regions for the halogenated compounds and for methyl-
TFMB. Spectra of the Ti(2p), O(1s), and C(1s), regions are
shown in the Supporting Information. In Figure 2, the F(1s)
spectrum of the sample exposed to iodo-TFMB shows a single,
sharp F(1s) feature at 689.4 eV. The sample exposed to bromo-
TFMB also shows some F(1s) intensity near at 689.2 eV, but it
is significantly smaller than that observed for the sample
exposed to iodo-TFMB, while TiO2 samples exposed to chloro-
TFMB or to methyl-TFMB show no significant F(1s) intensity.
The I(3d5/2) spectrum from iodo-TFMB shows only a very
small peak, while the Cl(2p) spectrum from chloro-TFMB and
the Br(3d) spectrum from bromo-TFMB show no detectable
intensity above the noise level. Samples exposed to methyl-
The data show that iodo-TFMB and, to a lesser extent,
bromo-TFMB react with the TiO2 surface under the relatively
mild conditions used here, while chloro-TFMB and methyl-
TFMB show no detectable reaction. Quantitative analysis of the
XPS data using the Ti(2p) peak as an internal standard yield
molecular coverages of 0.8
0.2 molecules/nm2 for bromo-
TFMB, and 2.0 0.1 molecules/nm2 for iodo-TFMB. This is
slightly lower than the values obtained for densely packed
phenyl rings on Si(111) surfaces,40 but still reflects a
comparatively dense layer. Analysis of the Br(3d) and I(3d)
regions shows that bromo-TFMB leaves no detectable bromine
on the surface and iodo-TFMB shows only a trace amount of
iodine, with quantitative analysis yielding a coverage of 0.17
iodine atoms/nm2. On the basis of the I:F ratio (after
correction for relative sensitivity factors and the I:F ratio of
1:3 for iodo-TFMB), this corresponds to approximately 0.1
iodine atom on the surface per iodo-TFMB molecule adsorbed
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onto the surface. The absence of any detectable F(1s) intensity
after exposure to methyl- or chloro-TFMB shows that the
binding of iodo-TFMB and bromo-TFMB is a direct result of
the presence of the Br or I groups and does not arise from the
aromatic ring or the −CF3 group. However, the low
concentration of Br and I atoms on the surface demonstrates
that adsorption of iodo-TFMB and bromo-TFMB onto the
surface involves breaking C−I or C−Br bonds, and that the
halogen byproducts of the reaction are rinsed away.
These results are consistent with what would be expected
from the reaction depicted in Figure 1a, in which the incoming
molecules interact with surface Ti−OH groups to form Ti−O−
C bonds and eliminate HBr or HI. Despite the apparent
simplicity of this reaction on paper, it is well-known in organic
chemistry32 that aryl halides like those used here typically show
very little reactivity toward alcohols in the absence of a catalyst
due to both the steric and the electronic effects that greatly
inhibit the most common nucleophilic substitution (SN2)
reaction pathway. For example, previous studies of aryl
bromides and iodides showed almost no reaction with liquid-
phase tert-butanol in the absence of a catalyst.34,36
methyl-TFMB. As with the aryl halides, the absence of
detectable intensity from the displaced halogen atoms indicates
that molecular adsorption is accompanied by C−X bond
cleavage, with the HX byproducts rinsed away from the surface.
The maximum surface coverage obtained for iodomethyl-
TFMB is slightly smaller than that observed with iodo-TFMB
(shown in Figure 2). This suggests that the additional CH2
group may introduce additional steric interactions that limit the
molecular density. While a direct comparison of reactivity is
complicated by the fact that the iodomethyl-TFMB experi-
ments were performed at a higher temperature (80 °C) as
compared to the other compounds investigated (50 °C for all
others), the higher surface coverage of iodomethyl-TFMB as
compared to bromomethyl-TFMB or chloromethyl-TFMB is
consistent with the reactivity trend (I > Br > Cl > F) typically
found with the Williamson ether synthesis.
Comparison of Base- versus Acid-Treated Samples
toward Aryl Iodide. The most likely structure for binding aryl
and benzyl halides to the TiO2 surface is via the surface
hydroxyl group, releasing HX and forming a Ti−O−C bond at
the interface as depicted in Figure 1. To facilitate understanding
of this reaction, we investigated whether manipulating the
concentration of surface hydroxyl groups and/or their
protonation state affected the grafting of the aryl iodide to
TiO2 surfaces. Because the isoelectric point of TiO2 in water is
approximately 5.9,41 treatment with base is expected to increase
the concentration of deprotonated Ti−O− groups, while the
acidic piranha solution is expected to yield protonated Ti−OH
groups. To this end, we compared grafting on samples
immersed in the piranha solution with grafting on samples
treated with NaOH solution using procedures described in the
Experimental Section. Figure 4 compares the C(1s) and F(1s)
data from piranha-treated and base-treated TiO2 samples,
showing spectra both before and after exposure to iodo-TFMB
(50 °C, 24 h). Similar data are shown for the Ti(2p), O(1s),
and I(3d) peaks in the Supporting Information. The F(1s) data
Comparison with Benzyl Halides. To understand how
differences in molecular structure impact reactivity on TiO2
surfaces, we also investigated the bonding of benzyl halides with
TiO2. In this case, the nonaromatic nature of the pendant
CH2X group (where X = Cl, Br, I) is expected to yield reactivity
more like that of the classical Williamson ether synthesis. Figure
3 shows F(1s) XPS data for nanocrystalline TiO2 films that
Figure 3. F(1s) XPS data for nanocrystalline TiO2 films after reaction
with benzyl halide compounds for 12 h at 50 °C (X = Cl, Br, H).
Because of its higher melting point, a temperature of 80 °C was used
for 4-iodomethyl-TFMB (X = I).
were exposed to 4-(halomethyl)-1-(trifluoromethyl)benzene
derivatives (which we refer to as “chloromethyl-TFMB,”,
“bromomethyl-TFMB”, and “iodomethyl-TFMB”). XPS data
for the Ti(2p), O(1s), C(1s), Cl(2p), Br(3d), and I(3d)
regions are shown in the Supporting Information. In Figure 3,
all three halides show some F(1s) intensity at 689.5 eV, with
the iodomethyl-TFMB clearly showing the highest intensity.
Using the Ti(2p) peaks as an internal standard for quantitative
analysis yields molecular coverages of 1.1 molecules/nm2 for
iodomethyl-TFMB, as compared to only 0.37 molecules/nm2
for chloromethyl-TFMB, and 0.31 molecules/nm2 for bromo-
Figure 4. F(1s) and C(1s) XPS data from nanocrystalline TiO2
samples prepared using piranha treatment and base treatment. Data
are shown on freshly treated samples before (- - - -) and after (−)
exposure to iodo-TFMB.
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