Bridging Oxygen Rows Research Articles

Low temperature ammonia synthesis from atomic N and water on rutile TiO2(110)

We have investigated the formation of ammonia (NH3) from atomic N and water (H2O) on a rutile(R)-TiO2(110) surface using the temperature-programed desorption method. The formation of NH3 can be detected after coadsorption of atomic N and H2O on the R-TiO2(110) surface, desorbing from the 5-fold coordinated Ti4+ (Ti5c) sites at about 400 K, demonstrating that the NH3 formation on R-TiO2(110) is feasible at low surface temperature. During the process, both hydroxyl groups at the bridging oxygen rows and H2O on the Ti5c sites contribute to the formation of NH3, which are affected by H2O coverage. At low H2O coverage, the direct hopping of hydrogen atoms may be the dominant process for hydrogen transfer; while H2O-assisted hydrogen atoms diffusion may be preferred at high H2O coverage. Our result will be of significant help to get a deeper insight into the fundamental understandings of hydrogenation processes during the NH3 synthesis.

Aligned Growth of Methylene Blue Films on TiO2 Single Crystals

We found epitaxial growth of needle-like nanosized crystals of methylene blue (MB). The nanosized crystals grew spontaneously along the c-axis of the (110) surface of a TiO2 single crystal by using the dip-coating method from solution. This epitaxial growth of the nanosized crystals is induced by the bridging oxygen rows align along the crystal c-axis. We also found that the alignment direction could be controlled by rubbing the surface of the substrate prior to dye-loading. The memory of the alignment direction induced by the rubbing treatment was erased by thermal annealing at 620 K. These findings suggest that the nanostructure on the TiO2 surface plays an important role in the aligned growth of MB nanosized crystals.

Dynamic Processes of Formaldehyde at Terminal Ti Sites on the Rutile TiO2(110) Surface

We investigate the dynamic processes of formaldehyde (HCHO) molecules on 5-fold-coordinated titanium (Ti5c) sites of rutile TiO2(110) surface using scanning tunneling microscopy (STM) together with density functional theory simulations. Our results show that the adsorbed HCHO molecules at Ti5c sites are present as two types of protrusions, either centered at Ti5c rows or centered at bridging oxygen (Ob) rows in the STM images, corresponding to the monodentate adsorption configuration through a O–Ti5c bond and to the bidentate adsorption configuration through both O–Ti5c and C–Ob bonds, respectively, which can be well supported by the simulated images. It is also observed that the monodentate adsorption tends to spontaneously switch to bidentate adsorption. Our results confirm the existence of the energetically more favored bidentate adsorption for HCHO at Ti5c sites. We obtain that the energy barriers are approximately 0.28 and 0.75 eV for the adsorbed HCHO molecules switching from monodentate adsorption ...

Sensing mechanism of SnO2(1 1 0) surface to H2: Density functional theory calculations

Using density functional theory, we investigate the H2-sensing mechanism of SnO2(110) surfaces to understand the H2-sensing behaviors of SnO2 surfaces with different reduction degrees and their sensing mechanism at the atomic level. We found that oxygen concentration in the ambient atmosphere greatly affects the H2-sensing mechanism of SnO2 surface. At considerable high oxygen concentrations H2 interacts with oxygen species pre-adsorbed onto SnO2(110) surface, leading to electron release back to the semiconductor SnO2. When interacting with O2−, H2 gas dissociates with one H atom to form hydroxyl adsorbed onto Sn site and another H atom adsorbed onto the oxygen atom of pre-adsorbed O2−; when interacting with the O−, H2O molecule is formed in the production. At very low oxygen concentration, structural reconstruction is induced by the interaction between H2 and SnO2 sub-reduced surface with removed twofold-coordinated bridging oxygen rows, accompanying electron transfer from H2 to surface without H2O formation. The above-calculated results are consistent with the experimental observation.

Pt atoms adsorbed onTiO2(110)−(1×1)studied with noncontact atomic force microscopy and first-principles simulations

We have studied the local properties of single Pt atoms adsorbed on hydroxylated ${\mathrm{TiO}}_{2}(110)$-($1\ifmmode\times\else\texttimes\fi{}1$) by combining noncontact atomic force microscopy (nc-AFM) and first-principles calculations. Room-temperature high-resolution nc-AFM images for the most frequently observed contrast modes reveal bright and elongated protrusions that can be traced back to the Pt atoms, and that are centered on the fivefold coordinated titanium rows, confined between two bridging oxygen rows. These observations are in line with the theoretical results, as the lowest energy sites for the Pt atom on the ${\mathrm{TiO}}_{2}(110)$ surface are in the neighborhood of the titanium rows, and high energy barriers have to be overcome to displace the Pt atom over the bridging oxygen rows. Single Pt atoms can be distinguished from H adsorbates (OH defects) due to their characteristic shape and binding site and, because they appear as the brightest surface features in all of the contrast modes. Force spectroscopy data over the protrusion and hole imaging modes and the corresponding tip-sample forces, simulated with O and OH terminated ${\mathrm{TiO}}_{2}$ nanoclusters, provide an explanation for this puzzling result in terms of the intrinsic strength of the interaction with the Pt adatom and the adatom and tip apex relaxations induced by the tip-sample interaction. These imaging mechanisms can be extended to other electropositive metal dopants and support the use of nc-AFM not only to characterize their adsorption structure but also to directly probe their chemical reactivity.

Site-Specific Imaging of Elemental Steps in Dehydration of Diols on TiO2(110)

Scanning tunneling microscopy is employed to follow elemental steps in conversion of ethylene glycol and 1,3-propylene glycol on partially reduced TiO2(110) as a function of temperature. Mechanistic details about the observed processes are corroborated by density functional theory calculations. The use of these two diol reactants allows us to compare and contrast the chemistries of two functionally similar molecules with different steric constraints, thereby allowing us to understand how molecular geometry may influence the observed chemical reactivity. We find that both glycols initially adsorb on Ti sites, where a dynamic equilibrium between molecularly bound and deprotonated species is observed. As the diols start to diffuse along the Ti rows above 230 K, they irreversibly dissociate upon encountering bridging oxygen vacancies. Surprisingly, two dissociation pathways, one via O-H and the other via C-O bond scission, are observed. Theoretical calculations suggest that the differences in the C-O/O-H bond breaking processes are the result of steric factors enforced upon the diols by the second Ti-bound OH group. Above ∼400 K, a new stable intermediate centered on the bridging oxygen (Ob) row is observed. Combined experimental and theoretical evidence shows that this intermediate is most likely a new dioxo species. Further annealing leads to sequential C-Ob bond cleavage and alkene desorption above ∼500 K. Simulations demonstrate that the sequential C-Ob bond breaking process follows a homolytic diradical pathway, with the first C-Ob bond breaking event accompanied with a nonadiabatic electron transfer within the TiO2(110) substrate.

Photoinduced Decomposition of Formaldehyde on a TiO2(110) Surface, Assisted by Bridge-Bonded Oxygen Atoms

We have investigated the photoinduced decomposition of formaldehyde (CH2O) on TiO2(110) at 400 nm using temperature-programmed desorption. Formate (HCOO), methyl radicals (CH3), and ethylene (C2H4) have been detected, while no evidence of polymerization of CH2O was found. The initial step in the decomposition of CH2O on TiO2(110) is the formation of a dioxymethylene intermediate in which the carbonyl O atom of CH2O is bound both to a Ti atom on the five-fold-coordinated lattice site (Ti-5C) and to a nearby bridge-bonded oxygen (BBO) atom. During 400 nm irradiation, the dioxymethylene intermediate can transfer methylene to the bridging oxygen row and break the C-O bond, thus leaving the original carbonyl O atom on the Ti-5C site. After this transfer of methylene, several pathways to products are available. Thus we have found that BBO atoms are intimately involved in the photoinduced decomposition of CH2O on TiO2(110).

Water on TiO2 studied by work function change: adsorption in cycles

The nature of water adsorption on TiO2(110) rutile surface attracts a lot of attention for quite some time. In spite of the considerable experimental and theoretical efforts a lot of details remain unclear. We have been using work function study to follow the adsorption of water on TiO2 at room temperature, and interpreted the results in terms of fast dissociative adsorption on bridging oxygen vacancies (BOV) and much slower non-dissociative adsorption on Ti5f rows. Additionally, we concluded that water from Ti5f rows efficiently desorbs at room temperature which is not the case for BOV adsorption sites. Here we propose a novel experimental approach which consists of monitoring in real-time the work function change during cycles of water adsorption. Since desorption at BOVs does not take place at room temperature, this method allows us to resolve the adsorption dynamics on the two adsorption sites. The first results changed our understanding of the phenomenon: we show that both, adsorption on BOVs and Ti5f are both very fast. Additionally, slow exponential decay of the work function is observed, which is not directly related to water adsorption. The possible explanation of the third slow contribution could be related to the migration of hydrogen atoms along the bridging oxygen rows.

Interaction of CO2 with oxygen adatoms on rutile TiO2(110)

The interactions of CO2 with oxygen adatoms (Oa's) on rutile TiO2(110) surfaces have been studied using scanning tunneling microscopy. At 50 K CO2 is found to adsorb preferentially on five-coordinated Ti sites (Ti5c's) next to Oa's rather than on oxygen vacancies (VO's) (the most stable adsorption sites on reduced TiO2(110)). Temperature dependent studies show that after annealing to 100-160 K, VO's become preferentially populated indicating the presence of a kinetic barrier for CO2 adsorption onto the VO's. The difference between the CO2 binding energy on VO's and Ti5c sites next to the Oa's is found to be only 0.009-0.025 eV. The barrier for CO2 diffusion away from Oa's is estimated to be ~0.17 eV. Crescent-like features of the images of CO2 adsorbed on Ti5c's next to Oa's are interpreted as a time average of terminally bound CO2 molecules switching between the configurations that are tilted towards Oa and/or towards one of the two neighbouring bridging oxygen (Ob) rows. In the presence of VO defects, the Ti5c bound CO2 is found to tilt preferentially away from the VO containing Ob row. If another CO2 is present on the neighbouring Ti5c row, both CO2 molecules tilt towards the common Ob row that separates them.

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).

Hydrogen reactivity on highly-hydroxylated TiO2(110) surfaces prepared viacarboxylic acid adsorption and photolysis

Combined scanning tunneling microscopy, temperature programmed desorption, photo stimulated desorption, and density functional theory studies have probed the formation and reactivity of highly-hydroxylated rutile TiO(2)(110) surfaces, which were prepared via a novel, photochemical route using trimethyl acetic acid (TMAA) dissociative adsorption and subsequent photolysis at 300 K. Deprotonation of TMAA molecules upon adsorption produces both surface bridging hydroxyls (OH(b)) and bidentate trimethyl acetate (TMA) species with a saturation coverage of nearly 0.5 monolayers (ML). Ultra-violet light irradiation selectively removes TMA species, producing a highly-hydroxylated surface with up to ~0.5 ML OH(b) coverage. At high coverages, the OH(b) species typically occupy second-nearest neighbor sites along the bridging oxygen row locally forming linear (2 × 1) structures of different lengths, although the surface is less ordered on a long scale. The annealing of the highly-hydroxylated surface leads to hydroxyl recombination and H(2)O desorption with ~100% yield, thus ruling out the diffusion of H into the bulk that has been suggested in the literature. In agreement with experimental data, theoretical results show that the recombinative H(2)O desorption is preferred over both H bulk diffusion and H(2) desorption processes.

Ideal, defective, and gold-promoted rutile TiO2(110) surfaces interacting with CO, H2, and H2O: Structures, energies, thermodynamics, and dynamics from PBE+U

Extensive first-principles calculations are carried out to investigate gold-promoted ${\mathrm{TiO}}_{2}$(110) surfaces in terms of structure optimizations, electronic structure analyses, ab initio thermodynamics calculations of surface phase diagrams, and ab initio molecular dynamics simulations. All computations rely on density functional theory in the generalized gradient approximation (PBE) and account for on-site Coulomb interactions via inclusion of a Hubbard correction PBE$+U$, where $U$ is computed from linear response theory. This approach is validated by investigating the interaction between TiO${}_{2}$(110) surfaces and typical probe species ($\mathrm{H}$, ${\mathrm{H}}_{2}\mathrm{O}$, and $\mathrm{CO}$). Relaxed structures and binding energies are compared to both data from the literature and plain PBE results, thus allowing the performance of the PBE$+U$ approach for the specific purpose to be verified. The main focus of the study is on the properties of gold-promoted titania surfaces and their interactions with $\mathrm{CO}$. Both PBE$+U$ and PBE optimized structures of $\mathrm{Au}$ adatoms adsorbed on stoichiometric and reduced ${\mathrm{TiO}}_{2}$ surfaces are computed, along with their electronic structure. The charge rearrangement induced by the adsorbates at the metal (oxide) contact are also analyzed in detail and discussed. By performing PBE$+U$ ab initio molecular dynamics simulations, it is demonstrated that the diffusion of $\mathrm{Au}$ adatoms on the stoichiometric surface is highly anisotropic. The metal atoms migrate either along the top of the bridging oxygen rows or around the area between these rows, from one bridging position to the next along the [001] direction. No translational motion perpendicular to this direction is observed. Approximate ab initio thermodynamics predicts that under $\mathrm{O}$-rich conditions, structures obtained by substituting a ${\mathrm{Ti}}_{5c}$ atom with an $\mathrm{Au}$ atom are thermodynamically stable over a wide range of temperatures and pressures that are relevant to applications in the realm of catalysis. Finally, it is shown that ${\mathrm{TiO}}_{2}(110)$ surfaces containing positively charged Au ions activate molecular $\mathrm{CO}$, whereas a single negatively charged ${\mathrm{Au}}^{\ensuremath{-}\ensuremath{\delta}}$ species bound to an $\mathrm{O}$ vacancy only weakly interacts with $\mathrm{CO}$. Despite this, the calculations predict that the reactivity of gold nanoparticles nucleated at $\mathrm{O}$ vacancies can be recovered for cluster sizes as small as Au${}_{2}$.

Combined NC-AFM and DFT study of the adsorption geometry of trimesic acid on rutileTiO2(110)

The adsorption behavior of trimesic acid (TMA) on rutileTiO2(110) is studied by means of non-contact atomic force microscopy (NC-AFM) and density-functionaltheory (DFT). Upon low-coverage adsorption at room temperature, NC-AFM imagingreveals individual molecules, centered above the surface titanium rows. Based on theNC-AFM results alone it is difficult to deduce whether the molecules are lying flat orstanding upright on the surface. To elucidate the detailed adsorption geometry, we performDFT calculations, considering a large number of different adsorption positions. Our DFTcalculations suggest that single TMA molecules adsorb with the benzene ring parallel tothe surface plane. In this configuration, two carboxylic groups can anchor to the surface ina bidentate fashion with the oxygen atoms binding to surface titanium atomswhile the hydrogen atoms approach oxygen atoms within the bridging oxygenrows. The most favorable adsorption position is obtained in the presence of ahydroxyl defect, allowing for additional binding of the third carboxylic group.

Off-Normal CO2 Desorption from the Photooxidation of CO on Reduced TiO2(110)

Photoinduced reactions between O2 and CO on reduced rutile TiO2(110) are studied at low temperature (∼30 K). Photon-stimulated desorption (PSD) of O2, CO2, and CO is observed with comparable yields. Isotope labeling experiments indicate that O2 chemisorbed in a vacancy is more active for photooxidation than O2 chemisorbed on a Ti5c site. The angular distribution for the desorbing CO2 is peaked at ∼40° with respect to the surface normal in the [11̅0] azimuth (i.e., perpendicular to the bridging oxygen rows), suggesting that CO2 is produced from O2 occupying an oxygen vacancy and CO adsorbed on a Ti5c site next to it. The experimental results are consistent with CO2 being produced from a transition state that has been predicted theoretically. The CO PSD from TiO2(110) is enhanced dramatically by the presence of chemisorbed O2, suggesting that it is a byproduct of the CO photooxidation process.

Direct Visualization of Water-Induced Relocation of Au Atoms from Oxygen Vacancies on a TiO2(110) Surface

Variable-temperature scanning tunneling microscopy (STM) is used to show that Au1+ deposited onto a TiO2(110)-(1 × 1) surface under soft-landing conditions at 600 K results in a surface decorated with isolated gold atoms bound to oxygen vacancies. This result is in sharp contrast to the large, sintered islands which form from Au1+ deposited onto a hydroxylated TiO2(110)-(1 × 1) surface under soft-landing conditions at 300 K. The position of the isolated Au atoms prepared by deposition at 600 K changes from directly above the bridging oxygen rows to directly above 5c-Ti atoms when the substrate is allowed to cool from 600 to 300 K. The binding site of the Au atoms returns to directly over the bridging oxygen rows when the temperature is returned to 600 K, indicating that this process is reversible. We attribute the change in binding site to a competition between the Au atom and an adsorbed water molecule for an oxygen vacancy on the reduced TiO2 surface. Using density functional theory (DFT), we show that dissociative adsorption of water occurs at an oxygen vacancy occupied by an Au atom, displaces the Au atom, and forms a stable OH−Au−TiO 2 complex on the surface.

Surface and interstitial Ti diffusion at the rutile TiO2(110) surface

Diffusion of Ti through the TiO(2)(110) rutile surface plays a key role in the growth and reactivity of TiO(2). To understand the fundamental aspects of this important process, we present an analysis of the diffusion of Ti ad-species at the stoichiometric TiO(2)(110) surface using complementary computational methodologies of density functional theory corrected for on-site Coulomb interactions (DFT + U) and a charge equilibration (QEq) atomistic potential to identify minimum energy pathways. We find that diffusion of Ti from the surface to subsurface (and vice versa) follows an interstitialcy exchange mechanism, involving exchange of surface Ti with the 6-fold coordinated Ti below the bridging oxygen rows. Diffusion in the subsurface between layers also follows an interstitialcy mechanism. The diffusion of Ti is discussed in light of continued attempts to understand the re-oxidation of non-stoichiometric TiO(2)(110) surfaces.

Adsorption of Ag 4 cluster on stoichiometric TiO 2 (1 1 0) surface: Quantum chemical study

Titanium dioxide with silver nanoparticles deposited on the surface is known to exhibit higher photocatalytic activity than the uncovered TiO 2. A study of the electronic structure of such systems is necessary to understand this effect. A quantum chemical study of silver tetramers adsorption on stoichiometric rutile (1 1 0) surface has been carried out using periodic DFT calculations. A number of adsorption positions, which correspond to two basic adsorption sites on TiO 2 (i.e. bridging oxygen rows and hollow sites) have been considered. The hollow sites turned out to be preferable for Ag 4 adsorption. The population analysis has been performed to study mechanism of interaction between Ag 4 and TiO 2 surface. The binding has been shown to occur mainly through the interaction between cluster HOMO and rutile conduction band eigenstates. A significant overlap between Ag 4 HOMO and titanium orbitals has been found in the case of hollow site adsorption, in contrast to adsorption over bridging oxygen rows, where there is only weak binding towards oxygen atoms. The adsorbed cluster has been found to have a significant positive charge, which is a result of a charge transfer into conduction band.

First-principles study of Au nanostructures on rutileTiO2(110)

We report systematic density-functional theory calculations of the structure and energetics of ${\text{Au}}_{n}$ nanorows $(n=1--7)$ and clusters $(n=1--12)$ adsorbed on the defected (110) rutile surface. The calculations show that gold nanorows bind strongly to a missing-row defected ${\text{TiO}}_{2}$ surface with an adhesive binding energy of about 1.5 eV. The cohesive binding energy of Au atoms in a row amounts to about 2.5 eV/atom. An analysis of the gold row properties points to their metallic nature. The charge redistribution on adsorbed rows shows that all ${\text{Au}}_{n}$ rows are negatively charged compared to the free-standing structures. The adhesive bonding of gold clusters to the vacancy defected bridging oxygen row at the ${\text{TiO}}_{2}(110)$ is of covalent nature and is stronger than that of the Au rows. The cohesive energy per atom in a ${\text{Au}}_{n}$ cluster is about 2.2 eV for the $n\ensuremath{\ge}5$ clusters and is larger $(\ensuremath{\sim}2.3--2.4\text{ }\text{eV})$ for smaller ones. We found that all clusters studied are negatively charged with about 1.1 electron charge. This charging shows only a weak dependence on the odd-even number of gold atoms forming a cluster.

Imaging perylene derivatives on rutileTiO2(110)by noncontact atomic force microscopy

The adsorption of 3,4,9,10-perylene tetracarboxylic diimide derivative molecules on the rutile ${\text{TiO}}_{2}(110)$ surface was investigated by noncontact atomic force microscopy and density-functional theory (DFT) calculations. After submonolayer deposition, individual molecules are observed to adsorb with their main axis aligned along the [001] direction and centered on top of the bridging oxygen rows. Depending on the tip termination, two distinctly different molecular contrasts are achieved. In the first mode, the molecules are imaged as bright elongated features, while in another mode the molecules appear with a bright rim and a dark bow-shaped center. Comparison with the defect density on the bare ${\text{TiO}}_{2}(110)$ surface suggests that the molecules preferentially anchor to surface defects. Our DFT calculations reveal details of the molecular adsorption position, confirming the experimentally observed adsorption on top of the bridging oxygen rows. The DFT results indicate that diffusion along the rows should be quite easily possible, while diffusion perpendicular to the rows seems to be hindered by a significant energy barrier.

Growth of ordered C60 islands on TiO2(110)

Non-contact atomic force microscopy is used to studyC60 molecules depositedon the rutile TiO2(110) surface in situ at room temperature. At submonolayer coverages, moleculesadsorb preferentially at substrate step edges. Upon increasing coverage,ordered islands grow from the decorated step edges onto the lower terraces.Simultaneous imaging of bridging oxygen rows of the substrate and theC60 island structurereveals that the C60 molecules arrange themselves in a centered rectangular superstructure, with the moleculeslying centered in the troughs formed by the bridging oxygen rows. Although theTiO2(110) surface exhibits a high density of surface defects, the observedC60 islands are of high order. This indicates that theC60 intermolecular interaction dominates over the molecule–substrate interactions thatmay cause structural perturbations on a defective surface. Slightly protrudingC60 strands on the islands are attributed to anti-phase boundaries due to stacking faultsresulting from two islands growing together.