On-surface polymerization on a semiconducting oxide: Aryl halide coupling controlled by surface hydroxyl groups on rutile TiO2(011)

Article abstract of DOI:10.1039/c5cc02989a

Based on scanning tunneling microscopy experiments, we show that the covalent coupling of aryl halide monomers on the rutile TiO₂(011)-(2 × 1) surface is controlled by the density of surface hydroxyl groups. The efficiency of the polymerization reaction depends on the level of surface hydroxylation, but the presence of hydroxyl groups is also essential for the reaction to occur.

Full text of DOI:10.1039/c5cc02989a

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On-surface Polymerization on a Semiconducting
Oxide: Aryl Halide Coupling Controlled by Surface
Hydroxyl Groups on Rutile TiO2(011)†
Cite this: DOI: 10.1039/x0xx00000x
Marek Kolmer,*a Rafal Zuzak,a Amir A. Ahmad Zebari,§a Szymon Godlewski,a
Jakub S. Prauzner-Bechcicki,a Witold Piskorz,b Filip Zasada,b Zbigniew Sojka,b
David Bléger,c Stefan Hechtc and Marek Szymonskia
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
dioxide is a profitable inorganic semiconductor, and particularly the
surface of rutile TiO2(011) attracts much attention.17, 18
Based on scanning tunneling microscopy experiments we
show that the covalent coupling of aryl halide monomers on
Following our latest report on the feasibility of the on-surface
covalent coupling of aryl halide precursors on the rutile TiO2(011)
surface,11 herein we investigate the role of surface hydroxyl groups
and demonstrate univocally that they are essential for the reaction to
occur. We show that the polymerization proceeds most effectively
when the reduced TiO2(011) sample is prepared with a moderate
density of surface hydroxyls, leading to formation of long molecular
oligomers. Increasing the density of the surface hydroxyls by surface
exposure to atomic hydrogen results in formation of shorter
oligomers, whereas the hydroxyl-free surface suppresses the
polymerization reaction completely. The control over the on-surface
polymerization by managing the coverage of hydroxyls, exemplified
herein by aryl halide coupling on the rutile TiO2 surface, can most
likely be extended to the entire class of transition metal oxides
surfaces where acidic hydroxyl groups are present.19-21 In this regard,
our results open an easy, yet powerful route for formation of
conjugated organic semiconducting polymers on semiconducting
inorganic oxide surfaces.
The (2×1) reconstruction of TiO2(011) surface (Fig. 1a) consists
of two-fold coordinated terminal oxygen and five-fold coordinated
terminal titanium atoms forming a characteristic zigzag pattern on
the rows running along the [01-1] direction.22 Hydrogen atom
adsorption on the exposed terminal oxygen atoms leads to direct
formation of surface hydroxyl groups.23 In the empty state STM
images these surface hydroxyl groups are observed as bright
protrusions located non-symmetrically on top of the zigzag pattern
(inset in Fig. 1c). To study the role of the surface hydroxyl groups in
the on-surface polymerization we prepared a reduced r-TiO2(011)-
(2×1) surface containing hydroxyl groups with the less than 0.5%
coverage18 (Fig. 1b), where the coverage is relative to the maximum
the rutile TiO2(011)-(2×1) is controlled by the density of
surface hydroxyl groups. The efficiency of the polymerization
reaction not only depends on the level of surface
hydroxylation, but also the presence of hydroxyl groups is
essential for the reaction to occur.
On-surface synthesis of covalently bound molecular assemblies in a
bottom-up approach has recently become a very attractive field of
nanoscience.1-3 Such strategy allows formation of large variety of
well-defined nanostructures including molecular wires,4-6 2D
molecular networks,3, 7, 8 or confined graphene nanostructures.2, 9, 10
On-surface synthesis relies on proper functionalization of the
precursor molecules with specific linking sites, which are activated
after deposition of the respective monomer building blocks on the
surface. To date, surfaces of the coinage metals (Cu, Ag, and Au)
were mostly used as substrates, mainly because they essentially
facilitate reaction analysis by scanning tunneling microscopy (STM).
However, for the sake of practical applications of the on-surface
covalent coupling, highly ordered semiconductor11 or insulator10, 12
surfaces clearly represent much more attractive platforms. In the
case of insulators, covalently bound molecular assemblies are
electronically decoupled from the substrate, and hence may serve as
13
independent molecular devices.5,
On the other hand, properly
functionalized surfaces of semiconductors, in particular transition
metal oxides, offer advantageous optical as well as photo- and
electrochemical properties useful in photonics, photocatalysis or gas-
and bio- sensing,14-16 which may benefit from the on-surface
polymerized organic semiconductor layer. In this context, titanium
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Fig. 1 Rutile surfaces with various hydroxyl group coverage. (a) Model of the TiO2(011)-(2×1) surface representing substrates with different
densities of surface hydroxyls (light gray, dark gray and yellow balls represent Ti, O, and H atoms, respectively). Bottom: STM images of (b)
reduced r-TiO2(011), (c) moderately hydroxylated 5%H-TiO2(011), and (d) fully hydroxylated 20%H-TiO2(011). Inset in (c): high resolution
STM image showing surface hydroxyl groups (+1.3 V, 10 pA, 7.5×7.5 nm2). For all large scale STM images the parameters are +2 V, 10 pA.
number of the available two-fold coordinated terminal oxygen protrusions corresponding to the three dimethyl groups and two
atoms, i.e. the full coverage corresponds to 8.0 × 1014 cm-2. The r-
smaller lobes at both termini of the admolecules that are attributed to
the iodine atoms (cf. a terfluorene molecule in Fig S9, ESI†).
TiO2(011) substrate was then used to prepare two other types of
well-defined surfaces under ultra-high vacuum conditions. First, we
exposed the sample to atomic hydrogen and produced the
hydroxylated surfaces. For the purpose of this study, we used a
reduced rutile surface with the moderate coverage of 5 ± 1% (5%H-
TiO2(011)), shown in Fig. 1c, and with a saturation coverage of 20 ±
1% (20%H-TiO2(011), Fig. 1d). The oxidized o-TiO2(011) surface,
in turn, was obtained by annealing the hydroxylated H-TiO2(011)
surface in an O2 atmosphere. This process led to formation of the
well-defined surface with low density of the surface hydroxyl groups
amounting again to less than 0.5%. As a result, such protocols allow
for an effective control over the density of the surface hydroxyl
groups and the oxidation state of the substrate as well.
To study the influence of the density of the surface hydroxyl
groups on the polymerization process, ~0.3 ML of DITF monomers
was evaporated on the substrates of different hydroxylation extent,
all kept at 260 °C (for details see sections 1 and 3 in ESI†). The
STM images of the resulting molecular structures (Fig. 3) reveal that
on the r-TiO2(011) surface (Fig. 3a and Fig. S5a, ESI†) and the o-
TiO2(011) surface (Fig. S5b, ESI†) oligomer formation is quenched.
Both substrates have almost no surface hydroxyl groups, and in such
a case the DITF monomers are found only on the terrace edges and
domain boundaries, which are chemically more active than the bare
terraces. In particular, the molecules trapped on the domain
We used the diiodoterfluorene (DITF) molecule as a monomer
for the on-surface polymerization. Similar monomers bearing two
terminal bromine atoms (DBTF) have recently been reported to form
6, 24
π-conjugated polymer chains on gold substrates,4,
and
investigated with regard to their single molecule charge transport
properties. The choice of DITF instead of the DBTF was motivated
by desorption of iodine-related reaction byproducts at elevated
temperatures (for details see section 3 in ESI†). Deposition of ~0.3
monolayer (ML) of DITF on the 5%H-TiO2(011) substrate kept at
room temperature gives rise to randomly distributed monomers (Fig.
2a). Comparison of the high resolution experimental STM image
(Fig. 2c) with the simulated STM image (Fig. 2d), based on density
functional theory (DFT) computational modeling of the DITF
admolecule structure (Fig. 2b and Fig. S8, ESI†), clearly indicates
that the precursor species still preserve their terminal iodine atoms
after deposition on the rutile substrate. Indeed, both experimental
and simulated STM images exhibit characteristic three central
Fig. 2 Depositing the DITF monomer on a rutile surface. (a) STM
image of DITF molecules deposited on 5%H-TiO2(011) at room
temperature. (b) Model of DITF molecule. Iodine atoms are
represented by purple balls. (c,d) High resolution experimental (c)
and simulated (d) STM images of a single DITF molecule on
TiO2(011) (dashed circle in (a)). Black and green dots indicate
dimethyl groups and iodine atoms, respectively. For the
experimental STM images the parameters are +2 V, 10 pA.
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Fig. 3 Polymerizing the DITF monomers on rutile surfaces with varying hydroxyl groups coverage. (a) STM image of DITF molecules
deposited on reduced r-TiO2(011) kept at 260 °C. (b) STM image of DITF molecules deposited on moderately hydroxylated 5%H-TiO2(011)
kept at 260 °C. Inset: high resolution STM image showing a dimer (7×7 nm2). (c) STM image of DITF molecules deposited on fully
hydroxylated 20%H-TiO2(011) kept at 260 °C. For all STM images the parameters are +2 V, 10 pA. Arrows in (a) point to DITF monomers
trapped on domain boundaries (blue) and DITF forming supramolecular aggregates (white).
boundaries are clearly intact and rather well separated (blue arrows polymerization to the total number of reactive groups (C-I bonds)
in Fig. 3a), proving that under these conditions the polymerization initially present. The conversion (evaluated using the total number of
reaction did not occur. The remaining admolecules form counted DITF monomers N = 901) drops from 55.1 ± 2.5% for
supramolecular aggregates, which are also clearly seen in STM 5%H-TiO2(011) to 40.1 ± 2.8% for 20%H-TiO2(011) (N = 521). The
images (white arrows in Fig. 3a). In dramatic variance to these lower efficiency of the reaction leading to the shorter oligomers that
results, evaporation of the DITF monomers on the hydroxylated take place on the highly hydroxylated substrate, strongly points to
5%H-TiO2(011) and 20%H-TiO2(011) substrates leads to formation the surface hydroxyl groups serving as the key species controlling
of well-developed oligomeric species (Fig. 3b and 3c), which are the polymerization process.
preferentially oriented along the surface reconstruction rows.
We have recently proposed a tentative multistep proton-assisted
Analysis of the intramolecular STM contrast of the dimer structure mechanism that might be operating during aryl halide coupling
(inset in Fig 3b) proves clearly a covalent coupling between the leading to polymerization on the hydroxylated TiO2(011) surface.11
monomer building blocks. The alternating protrusions along the [01- In this process, proton transfer from the surface hydroxyl group to an
1] direction, which correspond to the differently oriented dimethyl adsorbed precursor molecule leads to an activated monomer, which
groups in the polyfluorene chain, all occur with the same separation may couple with another, non-activated molecule. After C-C bond
formation the proton could remain on the molecule and the process
is repeated leading to the oligomerization. All our experimental
findings presented here support this proton-induced polymerization
distance of 0.84 ± 0.01 nm (Fig. S7, ESI†). This value is in
agreement with the expected length of a single fluorene unit
(0.845 nm).
In the STM images (Fig. 3b and 3c) we also observe a decrease
of the density of the surface hydroxyl groups as compared to the
substrate state prior to the polymerization. This observation is more
evident for the 5%H-TiO2(011) surface, where almost no hydroxyl
groups remain on the surface, whereas in the case of the 20%H-
TiO2(011) surface their number is largely reduced, yet they do not
vanish completely. It is worth noting here, that simple annealing of
the rutile TiO2(011)-(2×1) at these temperatures does not lead to
elimination of the surface-bound hydrogen.23 Therefore, the
spontaneous removal of the surface hydroxyl groups during the
course of the polymerization may suggest a potential involvement of
the hydrogen atoms in the polymerization process. For example,
hydrogen could be involved in the formation of hydrogen iodide as a
byproduct of the DITF deiodination activating step. Moreover, the
oligomers grown on the 5%H-TiO2(011) substrate (Fig. 3b) exhibit
increased chain lengths as compared to those grown on the 20%H-
TiO2(011) substrate (Fig. 3c). This qualitative observation is
substantiated by statistical analysis of the polymerization reaction
products from the detailed inspection of several high resolution STM
images (Fig. 4). It proves indeed, that for the 5%H-TiO2(011)
substrate the oligomers are longer than for 20%H-TiO2(011), where
the mere dimer structures dominate. Then, the efficiency of the
DIFT coupling reaction can be described by a conversion parameter,
expressed as the ratio (percentage) of the number of monomer
connections (C-C bonds) successfully made during the
Fig. 4 Oligomer chain length obtained on rutile surfaces with
varying hydroxyl groups coverage. Histogram presenting oligomer
length on TiO2(011) surfaces prepared with different densities of
hydroxyls after deposition of DITF on substrates kept at 260°C. The
histogram was obtained from N=198 molecules for r-TiO2(011),
N=901 for 5%H-TiO2(011) and N=521 for 20%H-TiO2(011). The
dotted lines are plotted for eye guidance.
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mechanism. It nicely explains the observed optimal surface hydroxyl
group coverage for the best polymerization outcome, since for
coupling to occur the activated, protonated monomers (and growing
oligomers) may only react with the intact monomers (due to charge
repulsion between the protonated species). Thus the surface
hydroxyl coverage, and hence the proton availability, controls the
ratio of the activated to non-activated monomers, and thereby the
entire polymerization process.
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This research was supported by the 7th Framework Programme
of the European Union Collaborative Project ICT (Information and
Communication Technologies) “Atomic Scale and Single Molecule
Logic Gate Technologies” (AtMol), contract no. FP7-270028, the
Polish Ministry for Science and Higher Education, contract no.
0322/IP3/2013/72, as well as by the German Research Foundation
(DFG via SFB 951). The experiment was carried out using
equipment purchased with financial support from the European
Regional Development Fund within the framework of the Polish
Innovation Economy Operational Program (contract no.
POIG.02.01.00-12-023/08).
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†
Electronic Supplementary Information (ESI) available. See
DOI: 10.1039/c000000x/
Present address: Salahaddin University, College of Science,
§
Department of Physics, 44001 Erbil, Kurdistan, Iraq
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