adsorption of organic molecules on rutile tio2 and anatase tio2 single crystal surfaces
TRANSCRIPT
This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 4207–4217 4207
Cite this: Chem. Soc. Rev., 2012, 41, 4207–4217
Adsorption of organic molecules on rutile TiO2 and anatase TiO2 single
crystal surfaces
Andrew G. Thomas*aand Karen L. Syres
b
Received 29th February 2012
DOI: 10.1039/c2cs35057b
The interaction of organic molecules with titanium dioxide surfaces has been the subject of many
studies over the last few decades. Numerous surface science techniques have been utilised to
understand the often complex nature of these systems. The reasons for studying these systems
are hugely diverse given that titanium dioxide has many technological and medical applications.
Although surface science experiments investigating the adsorption of organic molecules on
titanium dioxide surfaces is not a new area of research, the field continues to change and
evolve as new potential applications are discovered and new techniques to study the systems
are developed. This tutorial review aims to update previous reviews on the subject. It describes
experimental and theoretical work on the adsorption of carboxylic acids, dye molecules,
amino acids, alcohols, catechols and nitrogen containing compounds on single crystal
TiO2 surfaces.
1. Introduction
Titanium dioxide is utilised in a range of technological applica-
tions including as a pigment, a biosensor support,1 photocatalyst2
and in the Gratzel photovoltaic cell3 to name but a few. In
addition, it is also found at the surface of Ti and Ti alloy
biomaterials where its presence is thought to give rise to
the excellent osseointegration properties of these materials.4
The surface properties and interactions of TiO2, particularly
with small molecules such as water, oxygen, formate and
methanol for example,5,6 have been widely studied for over
thirty years. In many of the applications described above TiO2
is either intentionally functionalised or expected to interact
with organic molecules. In the case of dye-sensitised solar cells
(DSSCs) the titania surface is coated with a dye molecule, the
most efficient of which to date is the N3-dye. However, new dyes
which utilise the process of singlet fission (SF), whereby two charge
carriers are produced following absorption of a single photon,
a School of Physics and Astronomy and the Photon Science Institute,The University of Manchester, Oxford Road, Manchester M13 9PL.E-mail: [email protected]
b School of Chemistry, The University of Nottingham,University Park, Nottingham, NG7 2RD.E-mail: [email protected]
Andrew G. Thomas
Andrew Thomas received hisBSc in Chemistry from theUniversity of Manchester andhis MSc in instrumentationand Analytical Science fromUMIST, before obtaining hisPhD from the University ofLiverpool. Following this heheld three research appointmentsat UMIST before becomingan Experimental Officer inthe Department of Physics.He was made a ResearchFellow in Physics at UMISTin 2001 and moved to thePhoton Science Institute at
The University of Manchester following the merger of UMISTand The Victoria University of Manchester in 2004.
Karen L. Syres
Karen Syres obtained herMPhys degree from theUniversity of Manchesterbefore obtaining her PhDunder the supervision ofAndrew Thomas and WendyFlavell. Following this she helda one-year PhD plus scholar-ship before moving to TheSchool of Chemistry at theUniversity of Nottingham whereshe is currently a Post-doctoralResearch Fellow.
Chem Soc Rev Dynamic Article Links
www.rsc.org/csr TUTORIAL REVIEW
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4208 Chem. Soc. Rev., 2012, 41, 4207–4217 This journal is c The Royal Society of Chemistry 2012
are now being sought.7 In order to determine suitable SF dyes
the TiO2-organic bonding, energy level alignment and charge
injection rates must be fully characterised. Whilst dye sensi-
tised solar cells are already commercially available the electri-
city can not be stored. In order to overcome the energy storage
problem TiO2 is also being investigated as a substrate for
artificial photosynthesis. Here light energy is used to drive
photocatalytic redox reactions to produce hydrogen, or other
fuels from sunlight.8
Targeted biomaterials based upon TiO2 nanoparticles,
which are designed to cluster at the site of tumours, have
been functionalised with polyethylene glycol to evade the
body’s immune system. Here molecules such as dopamine or
dihydroxy phenylalanine (DOPA) have been used as the
anchor groups.9 Furthermore, studies have shown dopamine
to be effective both as an anchor molecule to bind DNA to
titania nanoparticles and to enhance charge separation.10
DOPA and dopamine are members of the catechol group of
chemicals and along with the simplest catechol, pyrocatechol,
have been shown to shift the optical absorption spectrum of
TiO2 from the ultraviolet (UV) to the visible part of the
electromagnetic spectrum. The interactions of TiO2 at the
surface of biomaterials are numerous and extremely complex.
As the molecules become larger their interactions also become
more complex as the number of potential bonding groups on
the molecule increases.11
Due to its ready availability, the rutile TiO2 (110) surface is
the most widely studied of the three structural phases of TiO2
(anatase, rutile and brookite). Many technological applica-
tions consist of TiO2 in the anatase form since this is the phase
adopted by nanoparticulate TiO2.12 This is thought to be due
to the fact that the anatase TiO2 (101) surface has the lowest
surface energy.13 Rutile TiO2 (110) in its (1 � 1) termination is
considered a prototypical metal oxide surface and much is now
understood about this surface. There have also been a number
of studies of the (100), (001) and (011) rutile TiO2 surfaces.
Anatase surfaces have been less widely studied as high quality
single crystals have been difficult to obtain. However over the
past ten years or so the quality of anatase single crystals and
growth of thin films has improved.14–16 This has allowed
comparisons, particularly between the crystallographically
equivalent rutile TiO2 (110) and anatase TiO2 (101) surfaces
to be made.17 Subtle differences in both the surface electronic
structure and adsorption strengths have been observed,
although the adsorption geometries of the molecules that have
been studied on both surfaces are similar.18–20
This review seeks to give a summary of adsorption of
organic molecules on TiO2 from both an experimental and
theoretical perspective. It will concentrate mainly on the
adsorption on highly-idealised vacuum-prepared single crystal
surfaces. We will also try to avoid repeating work described in
the comprehensive review by Diebold5 and the more recent
critical review by Pang et al.6 Rather, we hope to bring those
articles up to date as well as describing why this area is so
heavily researched. In order to fully appreciate the adsorption
and interaction of organic molecules with the various surfaces
we shall begin with a very brief overview of the geometric and
electronic structure of the most widely studied rutile and
anatase TiO2 surfaces.
2. Clean surfaces
The reviews by Diebold5 and Pang et al.6 describe the
structures of clean TiO2 surfaces in some detail. Here we
wish only to give a basic understanding of the structure. It
is well established that TiO2 single crystal surfaces can be
routinely prepared in vacuum by Ar+ ion sputter/anneal
cycles. The age and history of TiO2 single crystals has a large
effect on the surface. The rutile TiO2 (110) 1 � 1 surface
consists of rows of bridging oxygen ions (Obr) with 5 fold
Ti4+ ions (Ti5c) and in plane oxygen ions. This structure has
been widely confirmed experimentally using various diffrac-
tion and scanning probe microscopy (SPM) techniques.6 With
reference to the surface chemistry and electronic structure
some of the most interesting information obtained concerns
surface oxygen vacancies.5 It has long been known that
O-vacancies at the TiO2 (110) surface lead to the formation
of a band gap state around 1 eV below the Fermi level.17
Although this peak is not removed by water adsorption it is
removed by treatment with molecular oxygen. SPM has
indicated that the peak is associated with O-vacancies in the
bridging oxygen rows. However, the charge is distributed over
several Ti atoms and only a small amount remains on the
topmost surface atoms.21
Apart from the (110) surface the three most commonly
investigated rutile TiO2 surfaces are the (100), (001) and
(011) surfaces. The (100) surface forms a corrugated (1 � 1)
structure with rows of oxygens at the outermost surface.5
Annealing at higher temperatures leads to a (1 � 3) recon-
struction thought to consist of (110) microfacets.5 The (001)
surface is inherently unstable due to the large number of
broken bonds at the surface. Low energy electron diffraction
(LEED) data suggests the surface forms (011) facets at the
surface to minimise the surface energy. The TiO2 (011) surface
undergoes a (2 � 1) reconstruction which exhibits Ti5c and
two-fold coordinated oxygen atoms at the surface.22 The two-
fold coordinated oxygen atoms partially block the Ti5c sites
potentially blocking adsorption at these sites from gas phase
molecules.23 In addition, this surface seems resilient to the
formation of surface oxygen vacancies.
The anatase TiO2 (101) surface is the most stable and most
frequently observed surface of anatase TiO2. It has a sawtooth
structure with fully co-ordinated six-fold and under co-ordinated
five-fold Ti atoms. As with the rutile phase, surface preparation
in vacuum by Ar+ ion etching and annealing to 700 1C leads
to a well-ordered surface. Again, similarly to the rutile (110)
surface, photoemission spectra of the valence band region
often show a feature in the band gap region at a binding
energy of 1 eV. However, unlike rutile this peak is thought to
arise from subsurface O-vacancies.24 The anatase TiO2 (001)
surface is known to undergo a (1 � 4)(4 � 1) reconstruction
when prepared in vacuum. A number of different models have
been suggested for this structure including the added molecule
model (ADM),25,26 added and missing row models, which
result in formation of planes in the [103]27 or [014]20,28 direc-
tions, and a model based on [101] microfacets.20,29
We will turn now to the main focus of this tutorial
review, namely the adsorption of organic molecules on TiO2
surfaces.
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3. Carboxylic acids, dicarboxylic acids and
anhydrides
Simple carboxylic acid adsorption on TiO2 surfaces has been
widely studied. The motivation for these studies lies in techno-
logical applications of TiO2, such as DSSCs, and as model
molecules for understanding the (photo)catalytic activity of
TiO2 surfaces. We shall discuss photosensitising dyes and their
ligands and amino acids separately in sections 4 and 5. We will
concentrate here on simple carboxylic acids, anhydrides and
di-carboxylic acids.
3.1 Monocarboxylic acids
3.1.1 Adsorption on the rutile TiO2 (110) surface. The
review by Pang et al.6 discussed the adsorption of formic acid
on rutile TiO2 (110) thoroughly thus we shall summarise only
the main points here. Suffice to say, that formic acid adsorbs at
room temperature on all TiO2 surfaces which have been
studied. The adsorption occurs dissociatively as formate and
is considered to be a model for all monocarboxylic acids
adsorbed on TiO2. In addition, formate, acetate, propionate
and trimethyl acetate (TMA) form (2 � 1) overlayers at
saturation coverage.6 Chemical-state specific scanned-energy
mode photoelectron diffraction (PhD) carried out by Sayago
et al. concluded that the formate is bound to two surface Ti5catoms in a bridging geometry. This geometry (A) is shown in
Fig. 1.6 Although other experiments and theoretical treatments
support this geometry there are also results which suggest
alternative geometries. One of these involves filling of an Obr
site by one of the carboxyl oxygen atoms with the other bound
to Ti5c (B in Fig. 1)6 and another involves bonding through
only one formate oxygen atom (site C in Fig. 1).30
It has been suggested that experimental conditions such as
the surface preparation or history of the rutile TiO2 (110)
crystal, and the sample temperature during dosing may all play
a part in determining the adsorption geometry.6 A near edge
absorption fine structure spectroscopy (NEXAFS) study of
formate, acetate and propionate supported the existence of
minority adsorption sites involving bridging-oxygen vacancies.
The molecules were found to exhibit different twist angles
relative to the [001] azimuth. This observation was ascribed to
a majority of molecules being adsorbed to Ti5c along the [001]
azimuth and a smaller proportion to Obr vacancy sites, roughly
perpendicular to the [001] azimuth (B in Fig. 1). The molecules
were also found to be roughly perpendicular to the TiO2 (110)
surface.31 The proton lost from from formic acid upon adsorp-
tion is thought to bind to bridging oxygen atoms to form a
bridging hydroxyl. However, Lyubinetsky et al. suggested that
for TMA overlayers the proton is only weakly bound to the
bridging oxygens and may oscillate between two oxygen atoms.6
Benzoic acid adsorption on the rutile TiO2 (110) surface has
been studied by LEED, NEXAFS, electron stimulated
desorption ion angular distribution (ESDIAD), X-ray photo-
electron spectroscopy (XPS) and scanning tunneling micro-
scopy (STM).32–35 Like the aliphatic acids it is found to adsorb
dissociatively in a bridging bidentate geometry. However,
unlike the simple aliphatic acids benzoic acid is found to form
dimers by rotation of the phenyl ring. This then allows the
formation of bonds between the hydrogen atoms of one ring
and the p-system of its neighbour along the direction.32,33 This
geometry is also thought to allow interaction of another
hydrogen atom in the molecule with the bridging oxygen rows,
via hydrogen bonds. Pyridine carboxylic acids are found to
adsorb to the rutile TiO2 (110) surface in a similar manner to
benzoic acid.36 A detailed study of the adsorption of picolinic
acid, nicotinic acid and isonicotinic acid showed that slow
deposition of these molecules onto the (110) surface led to the
formation of monolayer dimer structures. These are formed by
interaction between the nitrogen lone pair and hydrogen
atoms of nearest neighbours. It was found that this interaction
was strongest for isonicotinic acid where tilting of the molecule
led to increased interaction between the molecules. Interest-
ingly this work showed that the rate of molecular deposition
onto the surface was critical to the degree of ordering.34,35
With regard to thermal and photodissociation of small
carboxylic acids on the rutile TiO2 (110) surface, there have
been many studies. These are discussed in depth in the reviews
of Diebold,5 Pang et al.6 and Henderson2 and the reader is
referred to these works for more details.
3.1.2 Other rutile surfaces. Adsorption of small carboxylic
acids, on the rutile TiO2 (100) (1 � 3) and TiO2 (100) (1 � 1)
show the adsorption is similar to that seen for the rutile (110)
surface, i.e. the acid adsorbs dissociatively. Similar results were
obtained for adsorption on the (001) surface.5 More recently
there have been a few experimental and theoretical studies
of carboxylic acid adsorption on the TiO2 (011) (2 � 1)
surface23,37,38 which suggest adsorption in a bridging bidentate
mode.38 This bonding mode is favoured despite the larger
Ti–Ti distances found on the (011) surface relative to the (110)
surface.37 Temperature programmed desorption (TPD) measure-
ments suggest the molecule decomposes above 500 1C with
ketene, CO and CH4 being the main decomposition products.
At temperatures lower than 500 1C the acetate recombines
with an adsorbed proton and desorbs intact.38
Quah et al. studied the photoexcited decomposition of acetic
acid on the rutile TiO2 (011) surface using XPS and TPD.38
UV illumination in the absence of oxygen gas reproduced the
results found for the TiO2 (110) surface, i.e. C 1s XPS spectra
Fig. 1 The three adsorption geometries deduced for formate on the
rutile TiO2 (110) surface. A is the geometry thought to be adopted by
the majority of adsorbed formate, deduced from PhD and NEXAFS.6
B is a minority species inferred fromNEXAFS and STMmeasurements
and C from STM.6
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showed no change. However, in the presence of O2 gas, C 1s
peaks due to adsorbed acetate were found to decrease in
intensity, indicating loss of acetate from the surface. The cross
section for acetate removal was shown to decrease with
decreasing oxygen partial pressure. The main decomposition
products were found to be ethane and methane. Ethane
production was reduced if the background oxygen partial
pressure was not replenished. It was suggested that this
occurred because surface oxygen was used up in the reaction
and replaced by gas phase oxygen. The production of ethane could
be restored by the introduction of more oxygen. Preparation
of a reduced TiO2 (011) surface by electron bombardment
showed no difference in photocatalytic activity towards
adsorbed acetate.
3.1.3 Single crystal anatase TiO2 surfaces. Tanner et al.
have studied adsorption of acetic and formic acids on the
anatase TiO2 (001) (1 � 4)(4 � 1) reconstructed surface
epitaxially grown on a (100) Nb-doped SrTiO3 substrate.26,29
STM showed that the acids adsorb on the fully oxidised
surface.26 Neither acetate nor formate species could be desorbed
by heating up to 700 1C. Above 700 1C, CO and CO2 were
observed in TPD experiments. Both acids were found to be
adsorbed dissociatively to form a (4 � 2) structure at satura-
tion coverage. This structure was thought to arise due to
adsorption of the deprotonated carboxyl groups to under-
coordinated Ti atoms at the surface. Flashing the surfaces to
around 850 1C gave different results for formate and acetate.
Formate appeared particularly resistant to annealing and
formed strongly bound patches. As the temperature was
increased above 850 1C it was found that a new phase was
formed which the authors describe as a disordered (4 � 4)
overlayer.29 Acetate, on the other hand, was simply lost from
the surface at 850 1C, resulting in a small coverage of acetate
and decomposition products.
TPD measurements from anatase TiO2 (001) which had
been Ar+ ion etched prior to exposure to formic or acetic acid
showed desorption of a number of species between 300 and
750 1C. Formate decomposed to form CO, formaldehyde
(H2CO), CO2 and water. Acetate gave rise to CO, ketene
(CH2CO) and smaller amounts of water and acetic acid,29
similar to the rutile surfaces described above.
3.2 Dicarboxylic acids
The interest in dicarboxylic acids lies in the fact that bonding
via two acid groups to the TiO2 surface may lead to more
efficient charge transfer between an adsorbed molecule and the
oxide surface. In addition, similar molecules are of interest in
preventing crystal growth of particular surface planes, or
limiting the size of crystals in the preparation of nanoparticles.
This is achieved if the bonding to the surface is sufficiently
strong to prevent further reaction of the crystal with the
growth medium. Fig. 2 shows the various ways in which a
dicarboxylic acid may be expected to adsorb on a TiO2
surface. As in the case of monocarboxylic acids, a chelating
mode is unlikely at a Ti5c site since this would lead to 7-fold
coordinated Ti ions.39 To the best of the authors’ knowledge
the only experimental studies of dicarboxylic acid adsorption
on single crystal surfaces are those of dyes and dye ligands
used in DSSC systems which are discussed below. The adsorp-
tion of oxalic acid40 on TiO2 has been the subject of some
theoretical work. The surfaces were modelled with simple TiO2
polymers to represent rutile and anatase surfaces. The calcula-
tions suggest the most stable adsorption geometry (i.e. the
structure which gives the highest adsorption energy) involves
deprotonation of both carboxyl groups. The adsorption then
takes place through two oxygen atoms in a bridging mode.
Adsorption was found to be stronger on the anatase surface
than the rutile surface.40 Malonic acid adsorbed on P25
degussa particulate TiO2 has been studied using attenuated
total reflection infrared spectroscopy. This work suggested
adsorption of malonic acid on the TiO2 surface occurred via
one bridging bidentate and one monodentate carboxylate
group as shown in Fig. 2j. Illumination of the surface with
UV showed decomposition of the malonic acid first to oxalic
acid and eventually to CO and H2O.41
3.3 Anhydrides
The adsorption of acetic anhydride on the rutile TiO2 (110)
surface at room temperature has been studied by XPS,
LEED and high resolution electron energy loss spectroscopy
(HREELS).42 Unlike acetic acid, the anhydride does not have a
terminal proton but instead has two carbonyl groups in an almost
co-planar structure. The distance between the two carbonyl groups
Fig. 2 Possible binding modes of a dicarboxylic acid to a TiO2 surface.
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is 0.265 nm which is slightly smaller than the separation
between Ti5c atoms in the (110) surface (B0.296 nm). A direct
XPS comparison of adsorption of acetic anhydride and acetic
acid showed that 12% more carbon was present on the
anhydride dosed surface relative to the acetic acid dosed
surface. Theoretical work has suggested that the strength of
carboxylic acid adsorption was partly due to the formation of
bridging hydroxyl groups (OHbr) stabilising the carboxylate
moiety.43 However, the results for anhydride adsorption suggest
the presence of the proton has little effect on the stability.
LEED and HREELS spectra indicated that acetic anhydride
adsorbs dissociatively on the surface to form acetate ions.
For an intact acetic anhydride molecule one would expect a
p(3 � 1) LEED pattern,42 however a p(2 � 1) LEED pattern
was observed. This is also the LEED pattern one obtains at
saturation coverage of acetic acid.32 The dissociative adsorp-
tion process involves a surface bridging oxygen species as
shown in Scheme 1a. The authors rationalise that the part of
the anhydride which attaches to the surface bridging oxygen
acts in a similar manner to the proton in the acetic acid, i.e. to
reduce the negative charge of the bridging oxygen rows.42 In
doing so it allows the negatively charged acetate moiety to
approach the Ti5c atom. However, attempts to confirm the
requirement to neutralise the bridging oxygen atoms using
methyl acetate indicated that methyl acetate did not adsorb on
the surface. This is despite the fact that the methyl fragment
should be able to neutralise Obr as shown in Scheme 1b. The
authors concluded that it was the requirement for the bridging
geometry which governed the adsorption mechanism, resulting
in two acetate adsorption. The authors also suggest some of
the acetate species which had formed on Obr could convert to
occupy two Ti5c sites resulting in a bridging oxygen vacancy.
This vacancy site is usually associated with the presence of
Ti3+ at the surface. However, here no redox reaction occurs
since the surface oxygen binds to the oxygen deficient acetate
moiety (Scheme 1b) thus the Ti atom retains its +4 oxidation
state. This, they argue leads to an excess positive charge on the
bridging oxygen row which stabilises the bridging adsorption
geometry in a similar way to the presence of OHbr. Wilson
et al. have studied adsorption of maleic anhydride (MA) on
the rutile TiO2 (001) surface.44 The motivation lies in its wide
use in chemical synthesis and also more recently as a potential
dye anchor system for DSSCs. It was suggested from the
desorption products observed in TPD that adsorption on this
surface occurred via ring cleavage at the central oxygen atom.
This results in a bidentate chelating adsorption mode involving
a surface oxygen atom. TPD showed decomposition products
which varied as a function of the surface stoichiometry with
CO, CO2 and ketene dominating the TPD spectra for both
surface treatments. Acetylene was also found to be desorbed
from the surface.
MA adsorption on the (101), (001) and (100) anatase TiO2
surfaces has been studied by Johansson et al.45 Adsorption on
the (101) and (100) surfaces also occurs via ring opening and
attachment through the three oxygen atoms of the molecule
and a surface oxygen atom.45 However, in this case the
adsorption mode was deduced to be a bridging geometry.
Adsorption on the TiO2 (101) surface involves three surface
titanium atoms as shown in Fig. 3. On the anatase TiO2 (001)
surface, O 1s XPS spectra showed two peaks arising from
MA which is indicative of oxygen in two different chemical
environments. Although the chemical shifts of these two peaks
were different from that of a multilayer of MA, the authors
suggest that the ring opening process does not occur on the
(001) surface.45
4. Photosensitising dye molecules
The adsorption of dye-sensitising molecules and charge transfer
between these molecules and titania surfaces is of great
fundamental and technological interest from the point of view
of DSSCs. The most widely used dye for these TiO2 nano-
particle cells is the so-called N3 dye, ruthenium di-2,20-bipyridyl-
4,40-dicarboxylic acid diisocyanate. This large molecule is
labile so that the normal method of adsorption onto clean
surfaces in ultra-high vacuum (evaporation by heating in
vacuum) is not possible. Instead the molecule has been depo-
sited by electrospray deposition46 in UHV or by removing
vacuum prepared crystals capped with small molecules and
Scheme 1
Fig. 3 Proposed adsorption geometry for maleic anhydride on the
anatase TiO2 (101) surface determined from photoemission and
NEXAFS spectroscopy. The arrow indicates the surface bridging
oxygen to which the molecule binds, resulting in a doubly-bidentate
bridging geometry. Adapted from ref. 45 with permission of the
author.
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dropping the dye from ethanolic solution.47,48 Electrospray
experiments suggests the dye bonds via one of the bi-isonicotinic
acid (BINA) ligands in a bidentate geometry with further bonding
through the sulfur atom of one of the thiocyanate groups as
shown in Fig. 4. A density functional theory (DFT) study of a
simpler dye molecule (cis(CO)-trans(I)-Ru-(4,40-dicarboxylate-
2,20-bipyridine)(CO)2I2 on the anatase TiO2 (101) surface also
suggests bidentate bonding via the BINA ligand is the most
stable configuration.49
A number of studies have looked at the BINA ligand adsorbed
on the rutile TiO2 (110) surface19,34,35 and the anatase TiO2 (101)
and (001) surfaces.20 Regarding the adsorption geometry of the
ligand the mode of adsorption is the same on both the rutile and
anatase surfaces studies, i.e. both acid groups become deproto-
nated and the molecule adsorbs in a doubly bidentate geometry.
There were some slight differences found in the adsorption angle
with respect to the surface normal. On the rutile TiO2 (110)
surface the tilt angle was found to be 251 from the surface normal
with an azimuthal twist of 441 relative to the [001] crystallo-
graphic direction. On anatase TiO2 (101) the tilt angle was
around 201 from the surface normal with a twist of around 401
from the [010] azimuth.20 On the anatase (001) surface the tilt
angle was much larger at 531 but the authors pointed out that
this may be due to the surface reconstruction. Charge transfer
from the BINA ligand to the TiO2 (110) surface was found to
occur in o 3 fs.50 For the complete dye molecule deposited by
the electrospray method, electrons were found to be injected
from the third lowest unoccupied molecular orbital (LUMO+3)
to the surface in less than 16 fs.46
5. Amino acids
The adsorption of amino acids on titania surfaces is of interest
from the point of view of biomaterials and biosensors. Amino
acids have also been investigated to control the size and shape
of TiO2 nanoparticles.51 The success of Ti based biomaterials
is attributed to the layer of passivating oxide at the surface and
it is this surface which will be exposed to the biological
environment and govern the success or failure of the implant.
Functional biomaterials based upon TiO2 nanoparticles are
also being investigated. Amino acid adsorption is also funda-
mentally interesting since TiO2 is an amphoteric oxide. Amino
acids, of course, have an acidic carboxyl group and a basic
amine group. One may therefore expect there to be some
competition as to which group will bind to the surface. There
are some difficulties in using photoelectron spectroscopy to
investigate amino acid adsorption due to the instability of the
molecules under high intensity radiation. One of the earliest
studies of glycine adsorption on rutile TiO2 (110) showed that
the molecule dissociated on the surface under synchrotron
radiation.52,53 It has been known for some time that pyridine
does not form a strong bond to the TiO2 surface (see below).
Experimentally, this behaviour is echoed in amino acid adsorp-
tion where binding on TiO2 single crystal surfaces in vacuum
occurs in a similar manner to that seen for carboxylic acids.39,54–57
Tonner has carried out DFT calculations of proline and
glycine adsorbed on the rutile TiO2 (110) surface. The results
showed adsorption through the carboxyl group, with proton
transfer to the surface.58 They also showed hydrogen bonding
via the amine group which led to further stabilisation of the
adsorption as shown in Fig. 5. Szieberth et al. performed DFT
calculations of glycine adsorption on the anatase (101) surface.59
The highest adsorption energy was obtained for a model where
the carbonyl oxygen of glycine bonds to a Ti5c site. In this
model the hydroxyl group forms a hydrogen bond to a twofold
coordinated oxygen ion and the amine group is bonded to Ti5cvia the nitrogen lone pair. However, the authors point out that
the energy difference between this model and one involving
adsorption via the deprotonated carboxyl group, or indeed
solely via the amine group, is very small thus it is possible all
three modes of adsorption may be present.
Fig. 4 Proposed adsorption geometry of the N3 dye on the rutile
TiO2 (110) surface deposited by the electrospray method. Figure
adapted from ref. 48 with permission of the author.
Fig. 5 Proposed adsorption geometry for glycine on the rutile TiO2
(110) surface determined from DFT calculations.58 Hydrogen bonding
occurs between the nitrogen atom and the proton lost (marked with an
arrow) from the carboxylic acid group.
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Fleming and co-workers have carried out studies of glycine
and proline adsorbed on rutile TiO2 (110) single crystal
surfaces. Proline is found to adsorb in a bidentate geometry
via the carboxyl group. They observed the presence of NH and
NH2+ in proline suggesting two adsorption structures – a
deprotonated anionic form and a zwitterionic form. Upon
heating the zwitterion was lost from the surface due to loss of
the additional proton in the amine. Glycine adsorption on the
rutile TiO2 (110) has been studied by STM.56 Again it was
found that the molecule adsorbed preferentially via deproto-
nation of the carboxyl group and formed a (2 � 1) overlayer
similar to acetic and formic acids. Glycine adsorption on the
TiO2 (011) surface studied using XPS found similar results. As
for the case of proline, there was evidence of coexistence of
anionic and zwitterionic glycine. The latter was again found to
be lost upon heating due to conversion to the anion.
Adsorption of phenylalanine on the rutile TiO2 (110) surface
suggests the molecule adsorbs in a bidentate geometry following
deprotonation of the acid group.60 There was little evidence for
formation of the zwitterionic state in agreement with DFT
calculations for glycine and proline adsorbed on this surface.58
The authors also observed the possible formation of hydrogen
bonds between the amine group of the first layer and the
carboxyl group of a second layer.
Finally, in this section we point out that for peptides it is
likely to be the side groups of the amino acids which control
the overall adsorption mechanism. This is because the terminal
carboxyl and amine groups will form a small proportion of the
overall molecule.11
6. Alcohols
The decomposition of adsorbed alcohol molecules has been
the subject of many studies aiming to understand the nature of
photochemical reactions on the TiO2 surface. In addition,
methanol and ethanol production from photocatalytic reactions
are of particular interest for use as fuel. Since the adsorption of
small alcohols (methanol, ethanol and propanol) adsorbed on
rutile TiO2 surfaces is covered comprehensively elsewhere,5,6 we
shall mention only the main points and more recent findings.
6.1 Adsorption on the rutile TiO2 (110) surface
Henderson et al.5 carried out an extensive study of methanol
on a TiO2 (110) surface using TPD, HREELS, static secondary
ion mass spectroscopy (SSIMS) and LEED. The majority of
methanol molecules were found to adsorb molecularly to
the surface, with evidence of some dissociative adsorption
(creating methoxy), particularly at oxygen vacancy sites. It
was found that exposing the TiO2 surface to O2 resulted in
more methoxy groups on the surface due to cleavage of the
CH3O–H bond. It was also found that co-adsorbed water had
little effect on methanol at the surface. An earlier study by
Henderson et al., reported a high cross-section for electron-
stimulated decomposition of methanol-related adsorbed species
on TiO2 (110).5 Therefore, methods such as LEED, where low-
energy electrons are fired at the surface, prove difficult in
characterising the methanol–TiO2 interface.
More recently, a TPD study of photochemical hole scaven-
ging reactions of methanol adsorbed on a rutile TiO2 (110)
single crystal5 found, like Henderson et al., that molecular
methanol was the majority surface species. It was found that
under UV light methoxy is more reactive than molecularly
adsorbed methanol for hole-mediated photo-oxidation. UV
light was found to lead to decomposition of methanol to
formaldehyde and a surface OH group.
Onda et al. studied the adsorption of methanol onto a rutile
TiO2 (110) surface using two-photon photoemission (2PPE).61
Following adsorption of methanol onto the surface they
observed a resonance which they assign to a charge transfer
from the reduced Ti5c+4-d ions to the free H atoms on surface
hydroxyl groups. They believe that the charge-transfer state is
stabilised by the presence of the methanol molecules at the
Ti5c+4-d sites. Gamble et al. used TPD and XPS to study the
decomposition of ethoxy groups on a TiO2 (110) surface in
the presence water or hydroxyl groups.5 Deuterated ethanol
was found to adsorb dissociatively on the surface to create
ethoxy groups (ethoxy groups were also formed by adsorption
of tetraethoxysilane). They found that ethoxy groups bound to
surface Ti atoms can be readily removed from the surface
by combination with surface hydroxyl groups. They were
desorbed from the surface as ethanol gas (B250–400 K).
However, ethoxy groups bound to bridging oxygen vacancies
at the surface cannot react with water or hydroxyl groups on
the surface (below B450 K).
Jayaweera et al. used XPS to investigate the photoreaction
under UV light of ethanol adsorbed on a rutile TiO2 (110)
single crystal.5 They found that ethanol adsorbs dissociatively
through its oxygen atom to one titanium atom on the surface.
They suggest that due to steric effects and repulsion between
neighbouring molecules that ethanol saturates at 0.5 molecules
of ethanol to every one Ti atom, which also agreed with their
estimation based on the attenuation of the Ti 2p signal. Under
UV irradiation and an O2 atmosphere they observed a
decrease in the peaks associated with ethanol adsorption and
the rise of a peak due to CH3COO and HCOO. It is under-
stood these species are formed by chemical reactions triggered
by the photo-excited electron.
6.2 Alcohols on other TiO2 single crystal surfaces
Kim and Barteau reported that on the TiO2 (001) surface
methanol adsorbs molecularly and dissociatively at 200 K.
Below room temperature molecular methanol desorbs from
the surface.5 Roman et al. found that methanol coverage
on the (100) and (110) surfaces increased with the number
of defects created on the surface by electron or Ar+
bombardment.5
Kim and Barteau have also studied the adsorption and
decomposition of ethanol, n-propanol and isopropanol on a
TiO2 (001) surface using TPD and XPS. They found the
alcohols adsorbed molecularly and dissociatively at 200 K but
only dissociatively at room temperature.5 Methanol adsorption
on anatase TiO2 (101) has been studied by Diebold’s group
using TPD and XPS.15 It was found that methanol adsorbed
molecularly on this surface with no signs of methoxy formation.
The difference to adsorption on the rutile surface was attributed
to the difficulty in forming surface oxygen vacancies on the
anatase TiO2 (101) surface.
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7. Catechols
Catechols (benzenediols) such as pyrocatechol and dopamine
adsorbed on TiO2 surfaces are being studied for many appli-
cations. Adsorbing pyrocatechol onto TiO2 nanoparticles is
thought to result in interesting charge transfer processes62 and
dopamine is widely used as a bridging molecule which facilitates
electron/hole transfer between TiO2 nanoparticles and a bio-
logical system.63 In addition to their charge transfer properties,
catechols are used in many systems because they form strong
bonds with TiO2. Dopamine and L-dihydroxyphenylalanine
(L-DOPA) are being used to attach molecules, which would
not normally adsorb strongly on TiO2. This is done by grafting
the molecule via the amine side group resulting in a strong
bridge between the TiO2 and the molecule.
7.1 Pyrocatechol
The simplest catechol is pyrocatechol (1,2-benzene-diol)
shown in Fig. 6. The pyrocatechol-TiO2 system is of interest
since it has potential applications in solar cells.64 Pyrocatechol
does not absorb light below 4.2 eV (300 nm) which is much
larger than the 3.2 eV (370 nm) band gap of TiO2. However, in
pyrocatechol-sensitised TiO2 nanoparticles there is an absorp-
tion shift to 3 eV (420 nm). It has been proposed by Persson
et al.62 that instead of the electron being excited in the
adsorbate and then being transferred into the TiO2 conduction
band, there is a direct pyrocatechol-to-TiO2 charge transfer.
This means the electron is directly photoinjected from the
pyrocatechol into the conduction band of the TiO2 without the
participation of excited states in the pyrocatechol.62 This
direct charge transfer is believed to be an excitation from the
p orbital in the pyrocatechol to the Ti 3d levels at the bottom
of the conduction band of the TiO2.
The three possible ways a catechol can adsorb on a TiO2
surface are shown in Fig. 6. In the bidentate chelating structure,
both the oxygen atoms are bonded to the same titanium atom.
In the bridging bidentate structure, each oxygen atom is bonded
to a different titanium atom on the surface. In a monodentate
structure, only one of the oxygen atoms is bonded to a
titanium atom.
Redfern et al.65 have shown in theoretical calculations that
the pyrocatechol molecule should adsorb on the anatase (101)
surface in a bidentate bridging structure. They also showed
that on a defect site (common in nanoparticles) that it would
adsorb in a chelating bidentate structure.
In a combined theoretical and experimental study, Li et al.
investigated the correlation between bonding geometry and
band gap states for pyrocatechol adsorbed on rutile TiO2
(110).66 They used STM to elucidate the bonding geometry
and UV photoemission to investigate the presence of band gap
states in the TiO2. In conjunction with DFT calculations they
were able to interpret their experimental results. They propose
that pyrocatechol adsorbs on the rutile TiO2 (110) surface in a
mixed monolayer coverage of both monodentate and bridging
bidentate structures and that the two can easily convert from
one structure to another via proton exchange between the
pyrocatechol and the surface. They also proposed that only
pyrocatechol adsorbed in a bridging bidentate geometry intro-
duces states into the band gap of the TiO2. Fig. 7 shows the
calculated adsorption geometry of pyrocatechol on rutile TiO2
(110) proposed by Li et al. This arrangement shows adjacent
catechol molecules tilted in opposite directions in a mixture of
bridging bidentate and monodentate adsorption geometries.
Using angle-resolved UPS they calculated that the pyrocatechol
molecules adsorbed in a bridging bidentate geometry (which
give rise to the band gap state) are tilted by �15–301 from the
surface normal.
Liu et al.66 studied the organization of pyrocatechol on an
anatase TiO2 (101) surface using STM and DFT calculations.
They found that isolated pyrocatechol molecules prefer to
adsorb at step defects on the surface. On terraces they found
both monodentate and bidentate structures present, with mono-
dentate favoured at low-coverage and bidenate favoured at
higher coverages. They propose that monodentate pyrocatechol
is mobile at room temperature and can move to preferential
adsorption sites, but bidentate pyrocatechol is much less
mobile. In addition, they observed the formation of one-
dimensional islands that change shape over time without breaking
up. This is caused by individual molecules ‘hopping’ to the next
site along, followed shortly after by neighbouring molecules.67
Fig. 6 Possible bonding modes of pyrocatechol to a TiO2 surface.
A. Bidentate chelating, B. Bidentate bridging, C. Monodentate.
Fig. 7 Calculated adsorption geometry of pyrocatechol on rutile TiO2
(110) suggested by STMmeasurements. Adapted with permission of the
author from ref. 74.
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In a further investigation, the same group deduced that these
pyrocatechol islands are responsible for a band-gap state
found in the valence band spectra of the pyrocatechol-dosed
TiO2 surface.68
7.2 Dopamine
Dopamine adsorbed on TiO2 has been widely studied for
biological and environmental applications such as photo-
degradation of bacteria, targeted biomaterials, anti-fouling
materials and bioelectronics.9,10,63,69 In these applications,
dopamine is often used to anchor other molecules, such as
polymer chains, to the surface of TiO2 nanoparticles. In many
of these applications, dopamine facilitates electron/hole transfer
across the interface between the TiO2 and the biological system.
Dopamine has been employed as an anchoring molecule
between DNA and TiO2 nanoparticles with an aim towards
DNA-sequence recognition.10,63 The system creates a light-
induced charge separation capable of carrying information
about the electronic properties of the biomolecules. The
dopamine molecules in this system allow charge separation
across the interface. Dopamine-modified TiO2 was found to be
more efficient at charge separation than carboxyl-group-modified
TiO2. Dopamine-modified TiO2 was found to be more photo-
active because of the higher reducing power of the delocalized
electrons and/or an increased absorption of visible light due
to a shift of the absorption edge in the dopamine modified
particles.70 In addition, using dopamine to anchor carboxyethyl-
b-cyclodextrin resulted in very efficient charge separation. This
is interesting for applications of dye-sensitised solar cells
where dye molecules are often attached to TiO2 via carboxyl
groups.
As with pyrocatechol, there are few studies of dopamine on
model single crystal anatase surfaces. Vega-Arroyo et al.71
carried out a theoretical study of the TiO2/dopamine-DNA
system. They found that the dopamine molecules provide a
strong covalent bond to the TiO2 surface and they facilitate
charge separation. The dopamine/TiO2 interface also provides
an electronic transition in a suitable range for photoexcitation.
Calculations of dopamine on undercoordinated sites (defect
sites) on the anatase TiO2 (101) surface carried out by this
group,71 concluded that the dopamine molecule would adsorb
in a chelating bidentate structure following deprotonation.
This is in agreement with results for the pyrocatechol molecule
on a defect site.65
Vega-Arroyo et al. also calculated that dissociative chelating
bidentate adsorption of dopamine on corner/defects sites was
more favourable than dissociative monodentate adsorption or
molecular adsorption. On an anatase TiO2 (101) surface with
few defects calculations suggested dopamine adsorbs in a
bridging bidentate geometry.71
The adsorption of dopamine on an anatase TiO2 (101) single
crystal was studied by Syres et al.72 using photoemission and
NEXAFS. It was found that dopamine adsorbs in a bridging
or chelating bidentate geometry. Valence band photoemission
spectra indicated that the adsorption of dopamine removes the
band gap state at the surface present on the clean TiO2 surface.
Carbon K edge NEXAFS spectra indicated that the dopamine
molecules orientate themselves with their phenyl rings normal
to the surface (i.e. ‘standing up’ on the surface). Experimental
and computational results indicated the appearance of new
unoccupied states upon adsorption of dopamine, which could
be due to hybridisation between the dopamine and the TiO2
surface.
8. Nitrogen containing compounds
Nitrogen containing compounds are of interest due to their
many industrial uses in the manufacture of dyes, explosives
and polymers. In addition, knowledge of how these materials
interact with particular materials is also of interest in the
development of sensors for detection of trace amounts of
explosives. We have discussed amino acids and pyridine
carboxylic acids above and here will concentrate on other
nitrogen containing compounds.
8.1 Aliphatic amines
Farfan-Arribas and Madix used amine adsorption to deter-
mine the Lewis acidity of surface Ti4+ ions in rutile TiO2 (110)
surfaces.73 Ethylamine and diethylamine were both found to
adsorb on the stoichiometric surface through formation of an
N–Ti bond. On defected surfaces bonding also occurred at
oxygen-vacancy (VO) sites. In both cases the amines remained
intact but on the defected surface less amine was adsorbed.
The authors suggested that this may be due to adsorption at
the defect sites blocking adsorption at more than one neigh-
bouring Ti5c site. They also found that the desorption activation
energy decreased in the order diethylamine > ethylamine 4ammonia which is also the order of decreasing Lewis basicity.
This, the authors suggested, was evidence of adsorption driven
by a Lewis acid–base interaction with the Ti4+ ions behaving
as Lewis acids.
8.2 Pyridine
Suzuki et al.74,75 have studied the adsorption of pyridine and
2,6-dimethylpyridine (2,6-DMP) on the rutile TiO2 (110) surface.
Unlike the aliphatic amines described above, neither pyridine
nor 2,6-DMP form a strong bond to Ti5c on terraces of the
TiO2 (110) surface but are rather weakly bound with the plane
of the ring parallel to the surface.75 However, an STM study of
a rutile TiO2 (110) surface with single-atom height steps found
that pyridine would attach to four-fold coordinated Ti atoms
at the step edges. Pyridine molecules at the step edges were
much less mobile than those on the terraces. The adsorbed
pyridine molecules were able to exchange between these step
adsorption sites and the terraces.74
8.3 Other nitroaromatics
Li et al. used STM, XPS and LEED to study the conversion of
azobenzene to aniline at anatase (101) and rutile (110) TiO2
surfaces.76 The interaction of these molecules with TiO2
surfaces is driven by work which showed that TiO2 supported
Au nanoparticles act as a high yield catalyst for synthesis of
azobenzene via oxidation of anilines and reduction of nitro-
aromatic compounds to aniline.77 Li et al. compared azobenzene
and aniline adsorption on TiO2 and all three techniques gave
almost identical results. They propose that the NQN bond in
azobenzene is cleaved by the TiO2 surface resulting in a phenyl
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4216 Chem. Soc. Rev., 2012, 41, 4207–4217 This journal is c The Royal Society of Chemistry 2012
imide species adsorbed to the surface. This can then be
converted to aniline on reaction with hydrogen. Conversely,
when aniline is evaporated onto the surface it loses one or
more hydrogen atoms, and also adsorbs as a phenyl imide
species (see Fig. 8). Hence, azobenzene and aniline give almost
identical results, forming ordered superstructures of phenyl
imide on the TiO2 surfaces studied.
As further proof of this mechanism the authors carried out a
further study using ultraviolet photoemission spectroscopy
(UPS) to compare the electronic structure of azobenzene
and aniline adsorbed on anatase (101) and rutile (110) TiO2
surfaces.18 They found that at saturation coverage the UPS
spectra of the two adsorbed molecules are identical, proving
that the NQN bond in azobenzene is cleaved by the TiO2
surface, resulting in an adsorbed phenyl imide species. In
addition, they discovered that at low coverages of azobenzene
adsorbed on anatase, photon irradiation converted azobenzene
from a flat-lying molecule to two upright phenyl imide species.
They propose that the NQN bond cleavage is facilitated by a
photon-induced trans–cis isomerization.
Ramalho et al. studied the adsorption of cis- and trans-
isomers of azobenzene on the rutile TiO2 (110) surface using
DFT based methods and a periodic model.78 They found that
the cis- conformation is the most stable when adsorbed. In
addition, they found that the TiO2 surface is important in
reducing the endothermic character of the reaction.
9. Conclusion and outlook
We hope that the preceding review has given the reader a
flavour of the numerous technologically important molecules
adsorbed on TiO2 single crystal surfaces which have been
studied.
As outlined in the introduction TiO2 is still being heavily
researched for novel applications involving photochemical
reactions, which will require functionalisation by new dyes
or biologically active species. In biomaterials, a fundamental
understanding of the success of Ti based implants can only be
obtained by studying how molecules interact with TiO2 surfaces
in a similar way to that achieved in catalysis. The advent of
new techniques for deposition of molecules, such as electro-
spray methods, will allow the study of more complex molecules
adsorbed on TiO2 surfaces. In addition, novel methods for
preparation of TiO2 single crystal surfaces under ambient
conditions are being investigated. Coupled with the availability
of environmental SPM, sum frequency spectroscopy and high
pressure XPS adsorption of molecules under more techno-
logically ‘realistic’ conditions can be studied. For many processes
this will allow the study of adsorption in the presence of solvents,
and in particular water which is likely to play a major role in
adsorption in technological applications since it will interact both
with the surface and the molecules of interest. Finally, the use of
ultrafast lasers and X-ray and UV lasers are allowing us to probe
surface chemical reactions and electron dynamics on the fs time
scale. These techniques offer the opportunity to monitor adsorp-
tion and surface reactions in real time as well as monitoring
charge transfer between molecular overlayers and the substrate.
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