Site competition during coadsorption of acetone with methanol and water on TiO2(110)

The competitive interaction between acetone and two solvent molecules (methanol and water) for surface sites on rutile TiO(2)(110) was studied using temperature-programmed desorption (TPD). On a vacuum-annealed TiO(2)(110) surface, which possessed ~5% oxygen vacancy sites, excess methanol displaced preadsorbed acetone molecules to weakly bound and physisorbed desorption states below 200 K. In contrast, acetone molecules were stabilized on an oxidized surface against displacement by methanol through formation of acetone diolate species. The behavior of acetone with methanol differs from the interactions between acetone and water which are less competitive. Examination of acetone + methanol and acetone + water multilayer combinations shows that acetone is more compatible in water-ice films than in methanol-ice films, presumably because water has greater potential as a hydrogen-bond donor than does methanol. Acetone molecules displaced from the TiO(2)(110) surface by water are more likely to be retained in the near-surface region, in turn having a greater opportunity to revisit the surface, than when methanol is used as a coadsorbate.     

Interaction of organic molecules with the TiO2 (110) surface: ab inito calculations and classical force fields

We have studied the adsorption of a number of organic molecules consisting of methyl, benzyl, and carboxylic groups on the rutile TiO2 (110) surface using both ab initio and atomistic simulation techniques. We have tested the applicability of a simple embedded cluster model to studying the adsorption of small organic molecules on the perfect rutile TiO2 (110) surface, and used this model to develop a classical force field for the interactions of a wide class of organic molecules consisting of these groups with the rutile TiO2 (110) surface. The force field accounts for physisorption and ionic bonding of organic molecules at the surface. It allows the reproduction of adsorption energies and of geometries of organic molecules on the rutile surface. It should be useful for studying diffusion of these molecules and their manipulation with use of AFM and STM tips.     

Charge transfer dynamics of model charge transfer centers of a multicenter water splitting dye complex on rutile TiO2(110)

Charge transfer dynamics between an adsorbed molecule and a rutile TiO(2)(110) surface have been investigated in three organometallic dyes related to multicenter water splitting dye complexes: Ru 535 (cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(II)), Ru 455 (cis-bis(2,2'-bipyridyl)-(2,2'-bipyridyl-4,4'-dicarboxylic acid)-ruthenium(II)), and Ru 470 (tris(2,2'-bipyridyl-4,4'-dicarboxylic acid)-ruthenium(II)). The adsorption of the dye molecules on the rutile TiO(2)(110) surface has been studied using core-level and valence photoemission. Dye molecules were deposited in situ using ultrahigh vacuum electrospray deposition. Core-level photoemission spectra reveal that each complex bonds to the surface via deprotonation of two carboxylic groups. All three dye complexes show evidence of ultrafast charge transfer to the TiO(2) substrate using the core-hole clock implementation of resonant photoemission spectroscopy.     

Comparisons of multilayer H2O adsorption onto the (110) surfaces of alpha-TiO2 and SnO2 as calculated with density functional theory

Mono- and bilayer adsorption of H2O molecules on TiO2 and SnO 2 (110) surfaces has been investigated using static planewave density functional theory (PW DFT) simulations. Potential energies and structures were calculated for the associative, mixed, and dissociative adsorption states. The DOS of the bare and hydrated surfaces has been used for the analysis of the difference between the H2O interaction with TiO2 and SnO 2 surfaces. The important role of the bridging oxygen in the H2O dissociation process is discussed. The influence of the second layer of H2O molecules on relaxation of the surface atoms was estimated.     

A single centre water splitting dye complex adsorbed on rutile TiO2(110): photoemission, x-ray absorption, and optical spectroscopy

A single centre water splitting dye complex (aqua(2,2'-bipyridyl-4,4'-dicarboxylic acid)-(2,2':6',6'-terpyridine)Ruthenium(II)), along with a related complex ((2,2'-bipyridyl-4,4'-dicarboxylic acid)-(2,2':6',6'-terpyridine)chloride Ruthenium(II)), has been investigated using photoemission and compared to molecules with similar structures. Dye molecules were deposited in situ using ultra-high vacuum electrospray deposition, which allows for the deposition of thermally labile molecules, such as these dye molecules. Adsorption of the dye molecules on the rutile TiO(2)(110) surface has been studied using core-level and valence photoemission. Core-level photoemission spectra reveal that each complex bonds to the surface via deprotonation of its carboxylic acid groups. A consideration of the energy level alignments reveals that both complexes are capable of charge transfer from the adsorbed molecules to the conduction band of the rutile TiO(2) substrate.     

NC-AFM observation of atomic scale structure of rutile-type TiO2(110) surface prepared by wet chemical process

We succeeded in observing the atomic scale structure of a rutile-type TiO2(110) single-crystal surface prepared by the wet chemical method of chemical etching in an acid solution and surface annealing in air. Ultrahigh vacuum noncontact atomic force microscopy (UHV-NC-AFM) was used for observing the atomic scale structures of the surface. The UHV-NC-AFM measurements at 450 K, which is above a desorption temperature of molecularly adsorbed water on the TiO2(110) surface, enabled us to observe the atomic scale structure of the TiO2(110) surface prepared by the wet chemical method. In the UHV-NC-AFM measurements at room temperature (RT), however, the atomic scale structure of the TiO2(110) surface was not observed. The TiO2(110) surface may be covered with molecularly adsorbed water after the surface was prepared by the wet chemical method. The structure of the TiO2(110) surface that was prepared by the wet chemical method was consistent with the (1 x 1) bulk-terminated model of the TiO2(110) surface.     

Weakly interacting molecular layer of spinning C60 molecules on TiO2 (110) surfaces

The adsorption of C(60), a typical acceptor organic molecule, on a TiO(2) (110) surface has been investigated by a multitechnique combination, including van der Waals density functional calculations. It is shown that the adsorbed molecules form a weakly interacting molecular layer, which sits on the fivefold-coordinated Ti that is confined between the prominent bridging oxygen rows (see figure).     

Theoretical Study of Cr Doped into TiO2（110） Surface

1 INTRODUCTION TiO2 is an excellent photocatalyst for decom- posing organic pollution[1]. TiO2 can only adsorb the short wavelengh light (λ < 387 nm), namely, ultra- violet radiation because its band-gap is 3.12 eV. Since ultraviolet radiation only occupies 4～6 percent of solar light, the efficient of solar energy is poor. Therefore, the study is now focused on photocata- lysis in visual light. Anpo’s[2] experimental result indicates that doping Cr and V into TiO2 can pro- mote its…     

Adsorption and assembly of copper phthalocyanine on cross-linked TiO2(110)-(1 x 2) and TiO2(210)

Copper phthalocyanine (CuPc) on reconstructed rutile TiO(2) was studied with ultrahigh vacuum variable temperature scanning tunneling microscopy. On cross-linked TiO(2)(110)-(1 x 2), the CuPc molecules at low coverages sparsely lay flat at the link sites and tilted in troughs between [001] rows. Increase of the CuPc coverage led to the trapping of the CuPc molecules by the rectangular surface cells fenced by the oxygen columns along the [001] direction and the cross-link rows. Each cell could trap one CuPc molecule at intermediate coverages and two CuPc molecules at higher coverages. On TiO(2)(210), the CuPc molecules tilted in defect-free areas and lay at defect sites with their molecular planes parallel to the substrate surface. Further increase of the CuPc coverage induced the formation of one- and two-dimensional assemblies on TiO(2)(210).     

Peptide/TiO2 surface interaction: a theoretical and experimental study on the structure of adsorbed ALA-GLU and ALA-LYS

The adsorption on the TiO(2) surface of two dipeptides AE (L-alanine-L-glutamic acid) and AK (L-alanine-L-lysine), that are "building blocks" of the more complex oligopeptide EAK16, has been investigated both theoretically and experimentally. Classical molecular dynamics simulations have been used to study the adsorption of H-Ala-Glu-NH(2) and H-Ala-Lys-NH(2) dipeptides onto a rutile TiO(2) (110) surface in water solution. Several peptide conformers have been considered simultaneously upon the surface. The most probable contact points between the molecules and the surface have been identified. Carbonyl oxygens as well as nitrogen atoms are possible Ti coordination points. Local effects are responsible for adsorption and desorption events. Self-interaction effects can induce molecular reorientations giving less strongly adsorbed species. The chemical structure and composition of thin films of the two dipeptides AE and AK on TiO(2) were investigated by XPS (X-ray photoelectron spectroscopy) measurements at both O and N K-edges. Theoretical ab initio calculations (DeltaSCF) were also performed to simulate the spectra, allowing for a direct comparison between experiment and theory.     

Computational studies on the behavior of sodium dodecyl sulfate (SDS) at TiO2(rutile)/water interfaces

Molecular dynamics simulations to study the behavior of an anionic surfactant close to TiO(2) surfaces were carried out where each surface was modeled using three different crystallographic orientations of TiO(2) (rutile), (001), (100) and (110). Even though all three surfaces were made with the same atoms the orientation was a key to determine adsorption since surfactant molecules aggregated in different ways. For instance, simulations on the surface (100) showed that the surfactant molecules formed a hemicylinder structure whereas the molecules on the surface (110) were attached to the solid by forming a hemisphere-like structure. Structure of the aggregated molecules and surfactant adsorption on the surfaces were studied in terms of tails and headgroups density profiles as well as surface coverage. From density profiles and angular distributions of the hydrocarbon chains it was possible to determine the influence of the solid surface. For instance, on surfaces (100) and (001) the surfactant molecules formed molecular layers parallel to the surface. Finally, it was found that in the solids (100) and (110) where there are oxygen atoms exposed on the surface the surfactant molecules were attached to the surfaces along the sites between the lines of these oxygen atoms.     

Quantum molecular dynamics study of water on TiO2(110) surface

The adsorption of water on perfect TiO(2)(110) surface is studied by quantum molecular dynamics simulation adopting a periodic model formed by five water molecules on a (5 x 1) surface unit cell of a five layer slab of TiO(2). The total simulation time is 3.2 ps. At about 1.3 ps, one water molecule dissociates with the help of other adsorbed waters and surface bridging oxygens. During the remaining 1.9 ps, the waters and OH groups vibrate, but no more dissociation or recombination is observed. By comparing recent experimental O1s photoemission (x-ray photoelectron spectroscopy) spectra of H(2)O/TiO(2)(110) to the computed spectrum of the adsorbate in the configurations supplied by the molecular dynamics simulation, the observed peaks can be attributed to different oxygen species. The proposed assignment of the main spectral features supports the occurrence of partial water dissociation (approximately 20%) also on a perfect TiO(2) surface.     

Directed adhesion and patterning by ultraviolet irradiation of TiO2(110)

Forces of adhesion between a hydroxylated silicon oxide tip and a TiO(2)(110) surface, before and after irradiation of the surface with 254 nm light, were measured using atomic force microscopy. The work of adhesion before and after irradiation was 32 and 166 mJ/m(2), respectively, but a difference was observed only if ultraviolet light exposure was used in the presence of oxygen. The change in adhesion correlated strongly with decreasing water contact angle, which changed from ca. 70 to 0° because of irradiation. The contrast in adhesion between irradiated and nonirradiated regions of the surface makes possible a simple method of patterning molecules with micrometer, and potentially nanoscale, resolution. As an example, fluorescein was selectively adsorbed onto hydrophilic regions of the surface by spin coating an ethanolic fluorescein solution onto TiO(2)(110) that had been irradiated through a photomask.     

SO2 on TiO2(110) and Ti2O3(102) nonpolar surfaces: a DFT study

Density functional molecular cluster calculations have been used to investigate the interaction of SO(2) with defect-free TiO(2)(110) and Ti(2)O(3)(102) surfaces. Adsorbate geometries and chemisorption enthalpies have been computed and discussed. Several local minima have been found for TiO(2)(110), but only one seems to be relevant for the catalytic conversion of SO(2) to S. In agreement with experiment, the bonding of SO(2) to Ti(2)O(3)(102) is much stronger than that on TiO(2)(110). Moreover, our results are consistent with the surface oxidation and the formation of strong Ti-O and Ti-S bonds. On both substrates, the bonding is characterized by a two-way electron flow involving a donation from the SO(2) HOMO into virtual orbitals of surface Lewis acid sites (), assisted by a back-donation from surface states into the SO(2) LUMO. However, the localization of surface states and the strength of back-donation are very different on the two surfaces. On TiO(2)(110), back-donation is weaker, and it involves unsaturated bridging O atoms, while on Ti(2)O(3)(102), it implies the -based valence band maximum and significantly weakens the S-O bond.     

Chain structures of surface hydroxyl groups formed via line oxygen vacancies on TiO2(110) surfaces studied using noncontact atomic force microscopy

Structures of surface hydroxyl groups arranged on a reduced TiO2(110) surface that had line oxygen vacancies were studied using noncontact atomic force microscopy (NC-AFM). NC-AFM results revealed that by increasing the density of oxygen vacancies on the TiO2(110) surface, line oxygen vacancies were formed by removal of oxygen atoms in a bridge oxygen row on the TiO2(110) surface. After the TiO2(110) surface with the line oxygen vacancies was exposed to water, the surface showed hydroxyl chain structures that were composed of hydroxyl groups linearly arranged in a form of two rows on the line oxygen vacancies and on a neighboring bridge oxygen row. In-situ NC-AFM measurements of these surfaces exposed to water at room temperature revealed that hydroxyl chain structures were formed at the line oxygen vacancy. Annealing above 500 K was sufficient to remove the hydroxyl chain structures on the TiO2(110) surface and allowed line oxygen vacancies to reappear on the surface. The line oxygen vacancies are active sites for water dissociation. In conclusion, the formation of the hydroxyl chain structure suggests that the surface hydroxyl groups on a TiO2(110) surface can be controlled by preparing oxygen vacancy structures on the surface.     

Density functional theory study on the structural and electronic properties of low index rutile surfaces for TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 composite systems

The present study is concerned with the structural and electronic properties of the TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 composite systems. Periodic quantum mechanical method with density functional theory at the B3LYP level has been carried out. Relaxed surface energies, structural characteristics and electronic properties of the (110), (010), (101) and (00) low-index rutile surfaces for TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 models are studied. For comparison purposes, the bare rutile TiO2 and SnO2 structures are also analyzed and compared with previous theoretical and experimental data. The calculated surface energy for both rutile TiO2 and SnO2 surfaces follows the sequence (110) < (010) < (101) < (001) and the energy increases as (010) < (101) < (110) < (001) and (010) approximately = (110) < (101) < (001) for SnO2/TiO2/SnO2 and TiO2/SnO2/TiO2 composite systems, respectively. SnO2/TiO2/SnO2 presents larger values of surface energy than the individual SnO2 and TiO2 metal oxides and the TiO2/SnO2/TiO2 system renders surface energy values of the same order that the TiO2 and lower than the SnO2. An analysis of the electronic structure of the TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 systems shows that the main characteristics of the upper part of the valence bands for all the studied surfaces are dominated by the external layers, i.e., by the TiO2 and the SnO2, respectively, and the topology of the lower part of the conduction bands looks like the core layers. There is an energy stabilization of both valence band top and conduction band bottom for (110) and (010) surfaces of the SnO2/TiO2/SnO2 composite system in relation to their core TiO2, whereas an opposite trend is found for the same surfaces of the TiO2/SnO2/TiO2 composite system in relation to the bare SnO2. The present theoretical results may explain the growth of TiO2@SnO2 bimorph composite nanotape.     

STM study of glycine on TiO2(110) single crystal surfaces

The adsorption of glycine (NH2CH2COOH) was examined by scanning tunneling microscopy (STM) on TiO2(110) surfaces at room temperature. A (2x1) ordered overlayer was observed on the TiO2(110)-(1x1) surface. The adsorption of acetic acid and propanoic acid was also investigated on this surface and their STM images were quite similar to that of glycine. Since acetate and propanoate are formed by dissociative adsorption of these acids on TiO2(110), it is proposed that glycine adsorbs in the same way to form a glycinate. The amino group in the glycinate adlayer structurally analogous to those formed from aliphatic carboxylic acids would be extended away from the surface and potentially free to participate in additional reactions. The underlying structure of the TiO2 surface is important in determining the structure of the glycinate adlayer; no ordering of these adsorbates was observed on the TiO2(110)-(1x2) surface.     

Enhanced adsorption energy of Au1 and O2 on the stoichiometric TiO2(110) surface by coadsorption with other molecules

During heterogeneous catalysis the surface is simultaneously covered by several adsorbed molecules. The manner in which the presence of one kind of molecule affects the adsorption of a molecule of another kind has been of interest for a long time. In most cases the presence of one adsorbate does not change substantially the binding energy of another adsorbate. The calculations presented here show that the stoichiometric rutile TiO(2)(110) surface, on which one of the compounds -OH, Au(3), Au(5), Au(7), Na, K, or Cs or two different gold strips was preadsorbed, behaves differently: the binding energy of Au(1) or O(2) to such a surface is much stronger than the binding to the clean stoichiometric TiO(2)(110) surface. Moreover, the binding energy of Au(1) or O(2) and the amount of charge they take from the surface when they adsorb are the same, regardless of which of the above species is preadsorbed. The preadsorbed species donate electrons to the conduction band of the oxide, and these electrons are used by Au(1) or O(2) to make stronger bonds with the surface. This suggests that adding an electron to the conduction band of the clean stoichiometric TiO(2)(110) slab used in the calculation will affect similarly the adsorption energy of Au(1) or O(2). Our calculations show that it does. We have also studied how the preadsorption of Au(4) or Au(6) affects the binding of Au(1) or O(2) to the surface. These two gold clusters do not donate electrons to the surface when they bind to it and therefore should not influence substantially the binding energy of Au(1) or O(2) to the surface. However, adsorbing O(2) or Au(1) on the surface forces the clusters to change their structure into that of isomers that donate charge to the oxide. This charge is used by Au(1) or O(2) to bind to the surface and the energy of this bond exceeds the isomerization energy. As a result the surface with the isomerized cluster is the lowest energy state of the system. We believe that these results can be generalized as follows. The molecules that we coadsorbed with Au(1) or O(2) donate electrons to the oxide and are Lewis bases. By giving the surface high energy electrons, they turn it into a Lewis base and this increases its ability to bind strong Lewis acids such as Au(1) and O(2). We speculate that this kind of interaction is general and may be observed for other oxides and for other coadsorbed Lewis base-Lewis acid pairs.     

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

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