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: andrew.g.thomas@manchester.ac.uk

b School of Chemistry, The University of Nottingham,University Park, Nottingham, NG7 2RD.E-mail: karen.syres@nottingham.ac.uk

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.

References

1 Q. Xie, Y. Y. Zhao, X. Chen, H. M. Liu, D. G. Evans andW. S. Yang, Biomaterials, 2011, 32, 6588–6594.

2 M. A. Henderson, Surf. Sci. Rep., 2011, 66, 185–297.3 B. O’ Regan and M. Gratzel, Nature, 1991, 353, 737–740.4 B. Kasemo and J. Lausmaa, Crc Critical Reviews in Biocompatibility,1986, 2, 335–380.

5 U. Diebold, Surf. Sci. Rep., 2003, 48, 53–229.6 C. L. Pang, R. Lindsay and G. Thornton, Chem. Soc. Rev., 2008,37, 2328–2353.

7 M. B. Smith and J. Michl, Chem. Rev., 2010, 110, 6891–6936.8 K. Kalyanasundaram and M. Gratzel, Curr. Opin. Biotechnol.,2010, 21, 298–310.

9 T. Kotsokechagia, F. Cellesi, A. Thomas, M. Niederberger andN. Tirelli, Langmuir, 2008, 24, 6988–6997.

10 T. Rajh, Z. Saponjic, J. Q. Liu, N. M. Dimitrijevic, N. F. Scherer,M. Vega-Arroyo, P. Zapol, L. A. Curtiss and M. C. Thurnauer,Nano Lett., 2004, 4, 1017–1023.

11 S. Koppen, O. Bronkalla and W. Langel, J. Phys. Chem. C, 2008,112, 13600–13606.

12 X. Q. Gong, A. Selloni, M. Batzill and U. Diebold, Nat. Mater.,2006, 5, 665–670.

13 M. Lazzeri, A. Vittadini and A. Selloni, Physical Review B, 2002,65, 1.

14 W. Hebenstreit, N. Ruzycki, G. S. Herman, Y. Gao and U. Diebold,Phys. Rev. B: Condens. Matter, 2000, 62, R16334–R16336.

15 G. S. Herman, Z. Dohnalek, N. Ruzycki and U. Diebold, J. Phys.Chem. B, 2003, 107, 2788–2795.

16 R. Hengerer, B. Bolliger, M. Erbudak and M. Gratzel, Surf. Sci.,2000, 460, 162–169.

17 A. G. Thomas, W. R. Flavell, A. K. Mallick, A. R. Kumarasinghe,D. Tsoutsou, N. Khan, C. Chatwin, S. Rayner, G. C. Smith,R. L. Stockbauer, S. Warren, T. K. Johal, S. Patel, D. Holland,A. Taleb and F. Wiame, Physical Review B, 2007, 75.

18 S. C. Li, Y. Losovyj, V. K. Paliwal and U. Diebold, J. Phys. Chem.C, 2011, 115, 10173–10179.

19 L. Patthey, H. Rensmo, P. Persson, K. Westermark, L. Vayssieres,A. Stashans, A. Petersson, P. A. Bruhwiler, H. Siegbahn, S. Lunelland N. Martensson, J. Chem. Phys., 1999, 110, 5913–5918.

20 A. G. Thomas, W. R. Flavell, C. Chatwin, S. Rayner, D. Tsoutsou,A. R. Kumarasinghe, D. Brete, T. K. Johal, S. Patel and J. Purton,Surf. Sci., 2005, 592, 159–168.

Fig. 8 Schematic reaction model of azobenzene and aniline on TiO2

determined from STM and photoelectron spectroscopy. Adapted from

ref. 84 with permission of the author. Black spheres represent carbon

atoms, purple spheres are nitrogen atoms and white spheres represent

hydrogen atoms.

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9/07

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3 15

:22:

01.

View Article Online

This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 4207–4217 4217

21 P. Kruger, S. Bourgeois, B. Domenichini, H.Magnan, D. Chandesris,P. Le Fevre, A. M. Flank, J. Jupille, L. Floreano, A. Cossaro,A. Verdini and A. Morgante, Phys. Rev. Lett., 2008, 100, 055501.

22 T. J. Beck, A. Klust, M. Batzill, U. Diebold, C. Di Valentin andA. Selloni, Phys. Rev. Lett., 2004, 93, 4.

23 J. G. Tao, T. Luttrell, J. Bylsma and M. Batzill, J. Phys. Chem. C,2011, 115, 3434–3442.

24 Y. B. He, O. Dulub, H. Z. Cheng, A. Selloni and U. Diebold,Physical Review Letters, 2009, 102, 4.

25 M. Lazzeri and A. Selloni, Phys. Rev. Lett., 2001, 87.26 R. E. Tanner, A. Sasahara, Y. Liang, E. I. Altman and H. Onishi,

J. Phys. Chem. B, 2002, 106, 8211–8222.27 G. S. Herman, M. R. Sievers and Y. Gao, Phys. Rev. Lett., 2000,

84, 3354–3357.28 Y. Liang, S. P. Gan, S. A. Chambers and E. I. Altman, Phys. Rev. B:

Condens. Matter, 2001, 63.29 R. E. Tanner, Y. Liang and E. I. Altman, Surf. Sci., 2002, 506,

251–271.30 M. Aizawa, Y. Morikawa, Y. Namai, H. Morikawa and

Y. Iwasawa, J. Phys. Chem. B, 2005, 109, 18831–18838.31 A. Gutierrez-Sosa, P. Martinez-Escolano, H. Raza, R. Lindsay,

P. L. Wincott and G. Thornton, Surf. Sci., 2001, 471, 163–169.32 Q. Guo, I. Cocks and E. M. Williams, Surf. Sci., 1997, 393, 1–11.33 Q. Guo and E. M. Williams, Surf. Sci., 1999, 433–435, 322–326.34 J. Schnadt, J. N. O’Shea, L. Patthey, J. Schiessling, J. Krempasky,

M. Shi, N. Martensson and P. A. Bruhwiler, Surf. Sci., 2003, 544,74–86.

35 J. Schnadt, J. Schiessling, J. N. O’Shea, S. M. Gray, L. Patthey,M. K. J. Johansson, M. Shi, J. Krempasky, J. Ahlund,P. G. Karlsson, P. Persson, N. Martensson and P. A. Bruhwiler,Surf. Sci., 2003, 540, 39–54.

36 J. N. O’Shea, Y. Luo, J. Schnadt, L. Patthey, H. Hillesheimer,J. Krempasky, D. Nordlund, M. Nagasono, P. A. Bruhwiler andN. Martensson, Surf. Sci., 2001, 486, 157–166.

37 P. R. McGill and H. Idriss, Surf. Sci., 2008, 602, 3688–3695.38 E. L. Quah, J. N. Wilson and H. Idriss, Langmuir, 2010, 26,

6411–6417.39 G. J. Fleming, K. Adib, J. A. Rodriguez, M. A. Barteau and

H. Idriss, Surf. Sci., 2007, 601, 5726–5731.40 A. Fahmi, C. Minot, P. Fourre and P. Nortier, Surf. Sci., 1995,

343, 261–272.41 I. Dolamic and T. Burgi, J. Phys. Chem. B, 2006, 110, 14898–14904.42 H. Ashima, W. J. Chun and K. Asakura, Surf. Sci., 2007, 601,

1822–1830.43 S. P. Bates, G. Kresse and M. J. Gillan, Surf. Sci., 1998, 409,

336–349.44 J. N. Wilson, D. J. Titheridge, L. Kieu and H. Idriss, J. Vac. Sci.

Technol., A, 2000, 18, 1887–1892.45 E. M. J. Johansson, S. Plogmaker, L. E. Walle, R. Scholin,

A. Borg, A. Sandell and H. Rensmo, J. Phys. Chem. C, 2010,114, 15015–15020.

46 L. C. Mayor, J. Ben Taylor, G. Magnano, A. Rienzo,C. J. Satterley, J. N. O’Shea and J. Schnadt, J. Chem. Phys.,2008, 129.

47 A. Sasahara, C. L. Pang and H. Onishi, J. Phys. Chem. B, 2006,110, 4751–4755.

48 K. L. Syres, A. G. Thomas, D. J. H. Cant, S. J. O. Hardman andA. Preobrajenski, Surf. Sci., 2011, 606, 273–277.

49 M. Haukka and P. Hirva, Surf. Sci., 2002, 511, 373–378.50 J. Schnadt, P. A. Bruhwiler, L. Patthey, J. N. O’Shea,

S. Sodergren, M. Odelius, R. Ahuja, O. Karis, M. Bassler,P. Persson, H. Siegbahn, S. Lunell and N. Martensson, Nature,2002, 418, 620–623.

51 K. Kanie and T. Sugimoto, Chem. Commun., 2004, 1584–1585.52 E. Soria, I. Colera, E. Roman, E. M. Williams and J. L. de Segovia,

Surf. Sci., 2000, 451, 188–196.53 E. Soria, E. Roman, E. M. Williams and J. L. de Segovia, Surf.

Sci., 1999, 433, 543–548.54 G. J. Fleming, K. Adib, J. A. Rodriguez, M. A. Barteau,

J. M. White and H. Idriss, Surf. Sci., 2008, 602, 2029–2038.55 G. J. Fleming and H. Idriss, Langmuir, 2004, 20, 7540–7546.56 T. Z. Qiu and M. A. Barteau, J. Colloid Interface Sci., 2006, 303,

229–235.57 J. N. Wilson, R. M. Dowler and H. Idriss, Surf. Sci., 2011, 605,

206–213.58 R. Tonner, ChemPhysChem, 2010, 11, 1053–1061.59 D. Szieberth, A. Maria Ferrari and X. Dong, Phys. Chem. Chem.

Phys., 2010, 12, 11033–11040.60 A. G. Thomas, W. R. Flavell, C. P. Chatwin, A. R. Kumarasinghe,

S. M. Rayner, P. F. Kirkham, D. Tsoutsou, T. K. Johal andS. Patel, Surf. Sci., 2007, 601, 3828–3832.

61 K. Onda, B. Li, J. Zhao and H. Petek, Surf. Sci., 2005, 593, 32–37.62 P. Persson, R. Bergstrom and S. Lunell, J. Phys. Chem. B, 2000,

104, 10348–10351.63 J. Q. Liu, L. de la Garza, L. G. Zhang, N. M. Dimitrijevic,

X. B. Zuo, D. M. Tiede and T. Rajh, Chem. Phys., 2007, 339,154–163.

64 W. R. Duncan and O. V. Prezhdo, J. Phys. Chem. B, 2005, 109,365–373.

65 P. C. Redfern, P. Zapol, L. A. Curtiss, T. Rajh and M. C.Thurnauer, J. Phys. Chem. B, 2003, 107, 11419–11427.

66 S. C. Li, J. G. Wang, P. Jacobson, X. Q. Gong, A. Selloni andU. Diebold, J. Am. Chem. Soc., 2009, 131, 980–984.

67 S. C. Li, L. N. Chu, X. Q. Gong and U. Diebold, Science, 2010,328, 882–884.

68 S. C. Li, Y. Losovyj and U. Diebold, Langmuir, 2011, 27,8600–8604.

69 N. M. Dimitrijevic, E. Rozhkova and T. Rajh, J. Am. Chem. Soc.,2009, 131, 2893–2899.

70 N. M. Dimitrijevic, Z. V. Saponjic, D. M. Bartels, M. C.Thurnauer, D. M. Tiede and T. Rajh, J. Phys. Chem. B, 2003,107, 7368–7375.

71 M. Vega-Arroyo, P. R. LeBreton, T. Rajh, P. Zapol andL. A. Curtiss, Chem. Phys. Lett., 2005, 406, 306–311.

72 K. Syres, A. Thomas, F. Bondino, M. Malvestuto and M. Gratzel,Langmuir, 2010, 26, 14548–14555.

73 E. Farfan-Arribas and R. J. Madix, J. Phys. Chem. B, 2003, 107,3225–3233.

74 S. Suzuki, Y. Yamaguchi, H. Onishi, K. Fukui, T. Sasaki andY. Iwasawa, Catal. Lett., 1998, 50, 117–123.

75 S. Suzuki, Y. Yamaguchi, H. Onishi, T. Sasaki, K. Fukui andY. Iwasawa, J. Chem. Soc., Faraday Trans., 1998, 94, 161–166.

76 S. C. Li and U. Diebold, J. Am. Chem. Soc., 2010, 132, 64–+.77 A. Grirrane, A. Corma and H. Garcia, Science, 2008, 322,

1661–1664.78 J. P. Prates Ramalho and F. Illas, Chem. Phys. Lett., 501, 379–384.

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