Surface Ti Atoms Research Articles

Inherent electronic trap states in TiO2 nanocrystals: effect of saturation and sintering

We report a quantum mechanical investigation on the nature of electronic trap states in realistic models of individual and sintered anatase TiO2 nanocrystals (NCs) of ca. 3 nm diameter. We find unoccupied electronic states of lowest energy to be localized within the central part of the NCs, and to originate from under-coordinated surface Ti atoms lying mainly at the edges between the (100) and (101) facets. These localized states are found at about 0.3–0.4 eV below the fully delocalized conduction band states, in good agreement with both electrochemical and spectro-electrochemical results. The overall Density-Of-States (DOS) below the conduction band (CB) can be accurately fitted to an exponential distribution of states, in agreement with capacitance data. Water molecules adsorbed on the NC surface raise the energy and reduce the number of localized states, thus modifying the DOS. As a possible origin of additional trap states, we further investigated the oriented attachment of two TiO2 NCs at various possible interfaces. For the considered models, we found only minor differences between the DOS of two interacting NCs and those of the individual constituent NCs. Our results point at the presence of inherent trap states even in perfectly stoichiometric and crystalline TiO2 NCs due to the unavoidable presence of under-coordinated surface Ti(IV) ions at the (100) facets.

Effects of Adsorbed Pyridine Derivatives and Ultrathin Atomic-Layer-Deposited Alumina Coatings on the Conduction Band-Edge Energy of TiO2 and on Redox-Shuttle-Derived Dark Currents

Both the adsorption of t-butylpyridine and the atomic-layer deposition of ultrathin conformal coatings of insulators (such as alumina) are known to boost open-circuit photovoltages substantially for dye-sensitized solar cells. One attractive interpretation is that these modifiers significantly shift the conduction-edge energy of the electrode, thereby shifting the onset potential for dark current arising from the interception of injected electrons by solution-phase redox shuttle components such as Co(phenanthroline)(3)(3+) and triiodide. For standard, high-area, nanoporous photoelectrodes, band-edge energies are difficult to measure directly. In contrast, for flat electrodes they are readily accessible from Mott-Schottky analyses of impedance data. Using such electrodes (specifically TiO(2)), we find that neither organic nor inorganic electrode-surface modifiers shift the conduction-band-edge energy sufficiently to account fully for the beneficial effects on electrode behavior (i.e., the suppression of dark current). Additional experiments reveal that the efficacy of ultrathin coatings of Al(2)O(3) arises chiefly from the passivation of redox-catalytic surface states. In contrast, adsorbed t-butylpyridine appears to suppress dark currents mainly by physically blocking access of shuttle molecules to the electrode surface. Studies with other derivatives of pyridine, including sterically and/or electronically diverse derivatives, show that heterocycle adsorption and the concomitant suppression of dark current does not require the coordination of surface Ti(IV) or Al(III) atoms. Notably, the favorable (i.e., negative) shifts in onset potential for the flow of dark current engendered by organic and inorganic surface modifiers are additive. Furthermore, they appear to be largely insensitive to the identity of shuttle molecules.

Chemistry and Atomic Distortion at the Surface of an Epitaxial BaTiO3 Thin Film after Dissociative Adsorption of Water

We present a study of the atomic and chemical structure of the surface of a fully strained, TiO2-terminated, ferroelectric BaTiO3 (BTO) (001) epitaxial film on a SrTiO3 substrate after controlled exposure to water. The epitaxial quality was checked by atomic force microscopy and X-ray diffraction. Quantitative low-energy electron diffraction compared with multiple scattering simulations was used to measure the structure of the first few atomic layers of BTO surface. The surface chemistry was investigated using high-resolution X-ray photoelectron spectroscopy. Finally, temperature-programmed desorption measured the desorption energies. We find that water undergoes mainly dissociative adsorption on the polarized BTO(001) surface. There are two competing sites for dissociative adsorption: oxygen vacancies and on-top Ti surface lattice atoms. The Ti on-top site is the dominant site for OH– chemisorption. One fifth of the surface Ti atoms bind to OH–. The concentration of surface oxygen vacancies acts mainly to favor initial physisorption. Before exposure to water, the outward pointing polarization in the BTO film is stabilized by atomic rumpling in the TiO2 termination layer. After exposure to water, the chemisorbed OH– species provide the screening, inverting the surface dipole layer and stabilizing the bulk polarization. Molecular adsorption is observed only for high water coverage.

Influence of surface modified TiO2 nanoparticles by gallates on the properties of PMMA/TiO2 nanocomposites

Nanocomposites based on poly(methyl methacrylate) (PMMA) and TiO2 nanoparticles were synthesized by in situ radical polymerization of MMA in solution. The surface of TiO2 nanoparticles was modified with four gallic acid esters (octyl, decyl, lauryl and cetyl gallate). The content of gallates present on the surface of TiO2 was calculated from the TGA results. The influence of length of hydrophobic tail of amphiphilic alkyl gallates on dispersability of surface modified TiO2 nanoparticles in PMMA matrix, the molecular weight and glass transition temperature of PMMA, as well as the thermal stability of the prepared PMMA/TiO2 nanocomposites in nitrogen and air was investigated. The influence of content of TiO2 nanoparticles on the properties of these nanocomposites was also examined. The formation of a charge transfer complex between the surface Ti atoms and the gallates was confirmed by FTIR and UV spectroscopy. TEM micrographs of the PMMA/TiO2 nanocomposites revealed that degree of TiO2 aggregation can be significantly lowered by increasing the length of aliphatic part of the used gallates. The molecular weight of PMMA slightly decreases with the increase of TiO2 content, indicating that used TiO2 nanoparticles act as radical scavengers during the polymerization of MMA. The presence of surface modified TiO2 nanoparticles do not have an influence on the mobility of PMMA chain segments leading to the same values of glass transition temperature for all investigated samples. Thermal and thermo-oxidative stability of the PMMA matrix are improved by introducing TiO2 nanoparticles modified with gallates.

First-Principles Study of Formaldehyde Adsorption on TiO2 Rutile (110) and Anatase (001) Surfaces

This study investigated adsorption and reactions of formaldehyde (HCHO) on TiO2 rutile (110) and anatase (001) surfaces by first-principles calculation. The structure, vibrational frequencies, and electronic properties of the interaction system are studied to investigate the adsorption mechanisms of HCHO on TiO2 surfaces. It is found that HCHO can chemically adsorb on all surfaces to form into a dioxymethylene structure with O of HCHO bonding to a coordinatively unsaturated surface Ti atom (Ti4C or Ti5C) and C bonding to a surface O2C. The anatase (001) surface is found to be more active in HCHO adsorption with lower adsorption energy and larger charge transfer. In addition, the (1 × 4) reconstructed anatase (001) surfaces are found to have higher adsorption ability and more stable surface properties than that on (1 × 1) unreconstructed ones. These findings indicate that the (001) surface holds the potential for the improvement of sensitivity to reductive HCHO gas, in which the (1 × 4) reconstructed surface may play an important role for further improving gas-sensing properties of TiO2-based sensors while keeping the stability of them.

Dissociation of Single 2-Chloroanthracene Molecules by STM-Tip Electron Injection

We have studied the adsorption and tip-induced chemistry of 2-chloroanthracene on TiO2(110). STM images show that at 135 K and low coverage, i.e., ∼0.1 ML, these molecules are physisorbed along the five-coordinated titanium rows on the rutile(110) surface as a result of electrostatic interaction. Applying electric pulses >2.5 V from the STM tip to individual molecules causes either desorption or dissociation of the molecules, as indicated by the changes in the STM images. We have observed dissociative electron capture of a single 2-chloroanthracene molecule, which leaves behind a surface chlorine atom adsorbed in the on-top configuration on a surface Ti atom. The threshold energy required for the dissociation was found to be ∼2.7 eV.

Ethanol Diffusion on Rutile TiO2(110) Mediated by H Adatoms.

We have studied the diffusion of ethanol on rutile TiO2(110)-(1 × 1) by high-resolution scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations. Time-lapsed STM images recorded at ∼200 K revealed the diffusion of ethanol molecules both parallel and perpendicular to the rows of surface Ti atoms. The diffusion of ethanol molecules perpendicular to the rows of surface Ti atoms was found to be mediated by H adatoms in the rows of bridge-bonded O (Obr) atoms similarly to previous results obtained for water monomers. In contrast, the diffusion of H adatoms across the Ti rows, mediated by ethanol molecules, was observed only very rarely and exclusively on fully hydrogenated TiO2(110) surfaces. Possible reasons why the diffusion of H adatoms across the Ti rows mediated by ethanol molecules occurs less frequently than the cross-row diffusion of ethanol molecules mediated by H adatoms are discussed.

Surface modification of anatase nanoparticles with fused ring catecholate type ligands: a combined DFT and experimental study of optical properties

Surface modification of nanocrystalline TiO(2) particles (45 Å) with catecholate-type ligands consisting of an extended aromatic ring system, i.e., 2,3-dihydroxynaphthalene and anthrarobin, was found to alter the optical properties of the nanoparticles in a similar way to modification with catechol. The formation of inner-sphere charge-transfer (CT) complexes results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites and the reduction of the band gap upon the increase of the electron delocalization on the inclusion of additional rings. The binding structures were investigated by FTIR spectroscopy. The investigated ligands have the optimal geometry for binding to surface Ti atoms, resulting in ring coordination complexes of catecholate type (binuclear bidentate binding-bridging) thus restoring the six-coordinated octahedral geometry of surface Ti atoms. From the Benesi-Hildebrand plot, stability constants in methanol/water = 90/10 solutions at pH 2 of the order 10(3) M(-1) have been determined. Quantum chemical calculations on model systems using density functional theory (DFT) were performed to obtain vibrational frequencies of charge transfer complexes, and the calculated values were compared with the experimental data.

Computational study of ethanol adsorption and reaction over rutile TiO2 (110) surfaces

Studies of the modes of adsorption and the associated changes in electronic structures of renewable organic compounds are needed in order to understand the fundamentals behind surface reactions of catalysts for future energies. Using planewave density functional theory (DFT) calculations, the adsorption of ethanol on perfect and O-defected TiO(2) rutile (110) surfaces was examined. On both surfaces the dissociative adsorption mode on five-fold coordinated Ti cations (Ti(4+)(5c)) was found to be more favourable than the molecular adsorption mode. On the stoichiometric surface E(ads) was found to be equal to 0.85 eV for the ethoxide mode and equal to 0.76 eV for the molecular mode. These energies slightly increased when adsorption occurred on the Ti(4+)(5c) closest to the O-defected site. However, both considerably increased when adsorption occurred at the removed bridging surface O; interacting with Ti(3+) cations. In this case the dissociative adsorption becomes strongly favoured (E(ads) = 1.28 eV for molecular adsorption and 2.27 eV for dissociative adsorption). Geometry and electronic structures of adsorbed ethanol were analysed in detail on the stoichiometric surface. Ethanol does not undergo major changes in its structure upon adsorption with its C-O bond rotating nearly freely on the surface. Bonding to surface Ti atoms is a σ type transfer from the O2p of the ethanol-ethoxide species. Both ethanol and ethoxide present potential hole traps on O lone pairs. Charge density and work function analyses also suggest charge transfer from the adsorbate to the surface, in which the dissociative adsorptions show a larger charge transfer than the molecular adsorption mode.

First Principles Analysis of H2O Adsorption on the (110) Surfaces of SnO2, TiO2 and Their Solid Solutions

Both associative and dissociative H(2)O adsorption on SnO(2)(110), TiO(2)(110), and Ti-enriched Sn(1-x)Ti(x)O(2)(110) surfaces have been investigated at low ((1)/(12) monolayer (ML)) and high coverage (1 ML) by density functional theory calculations using the Gaussian and plane waves formalism. The use of a large supercell allowed the simulation at low symmetry levels. On SnO(2)(110), dissociative adsorption was favored at all coverages and was accompanied by stable associative H(2)O configurations. Increasing the coverage from (1)/(12) to 1 ML stabilized the (associatively or dissociatively) adsorbed H(2)O on SnO(2)(110) because of the formation of intermolecular H bonds. In contrast, on TiO(2)(110), the adsorption of isolated H(2)O groups ((1)/(12) ML) was more stable than at high coverage, and the favored adsorption changed from dissociative to associative with increasing coverage. For dissociative H(2)O adsorption on Ti-enriched Sn(1-x)Ti(x)O(2)(110) surfaces with Ti atoms preferably located on 6-fold-coordinated surface sites, the analysis of the Wannier centers showed a polarization of electrons surrounding bridging O atoms that were bound simultaneously to 6-fold-coordinated Sn and Ti surface atoms. This polarization suggested the formation of an additional bond between the 6-fold-coordinated Ti(6c) and bridging O atoms that had to be broken upon H(2)O adsorption. As a result, the H(2)O adsorption energy initially decreased, with increasing surface Ti content reaching a minimum at 25% Ti for (1)/(12) ML. This behavior was even more accentuated at high H(2)O coverage (1 ML) with the adsorption energy decreasing rapidly from 145.2 to 101.6 kJ/mol with the surface Ti content increasing from 0 to 33%. A global minimum of binding energies at both low and high coverage was found between 25 and 33% surface Ti content, which may explain the minimal cross-sensitivity to humidity previously reported for Sn(1-x)Ti(x)O(2) gas sensors. Above 12.5% surface Ti content, the binding energy decreased with increasing coverage, suggesting that the partial desorption of H(2)O is facilitated at a high fractional coverage.

Size-dependent electronic structure of rutile TiO 2 quantum dots

We present results of our theoretical calculation on structural and electronic properties of passivated rutile (TiO 2) n quantum dots as a function of the size of the dots. Structural properties viz. geometry, bond lengths, binding energies and electronic properties such as HOMO–LUMO gap, density of states and HOMO, LUMO densities have been analyzed. The passivation of surface Ti with –OH groups and surface O atoms with –H atoms increases the HOMO–LUMO energy gap of the clusters. Our study reveals that the HOMO densities for rutile quantum dots are delocalized throughout the whole cluster while that of LUMO are strongly localized on a few surface Ti atoms.

CO2 adsorption on TiO2(101) anatase: A dispersion-corrected density functional theory study

Adsorption, diffusion, and dissociation of CO(2) on the anatase (101) surface were investigated using dispersion-corrected density functional theory. On the oxidized surface several different local minima were identified of which the most stable corresponds to a CO(2) molecule adsorbed at a five-fold coordinated Ti site in a tilted configuration. Surface diffusion is characterized by relatively small activation barriers. Preferential diffusion takes place along Ti rows and involves a cartwheel type of motion. The presence of a bridging oxygen defect or a surface interstitial Ti atom allows creation of several new strong binding configurations the most stable of which have bent CO(2) structures with simultaneous bonding to two surface Ti atoms. Subsurface oxygen vacancy or interstitial Ti defects are found to enhance the bonding of CO(2) molecules to the surface. CO(2) dissociation from these defect sites is calculated to be exothermic with barriers less than 21 kcal/mol. The use of such defects for catalytic activation of CO(2) on anatase (101) surface would require a mechanism for their regeneration.

DFT Study of the Adsorption of Aspartic Acid on Pure, N-Doped, and Ca-Doped Rutile (110) Surfaces

Understanding the interaction mechanism between titanium oxide surfaces and proteins/peptides/amino acids is crucial to the success of Ti implants. Aspartic acid (abbreviated as Asp or D) is one of the most abundant amino acid in nature. In this study, Dmol3, a quantum mechanics first-principles density functional theory code, was employed to investigate the interaction of Asp with pure, nitrogen-doped, and calcium-doped rutile (R(110)) surfaces. The effect of water on the interaction was also studied. The adsorption energy analysis demonstrated that the strongest adsorption happened when both the amino and carboxyl groups of Asp approached the R(110) surfaces and formed a bidentate coordination to two surface Ti atoms. Hydrogen bonds from the H atoms of Asp and bridging-O atoms on the surface also contributed to the adsorption. Water hindered the Asp adsorption. N-doping and Ca-doping were not beneficial to Asp adsorption. The results imply that we may realize selective protein/peptide/amino acid adsorption on materials and determine the adsorption of specific biomolecules by an elaborately designed ion doping process. Our results could have potential impact on the design of effective material surface treatments for biomedical applications.

Oxidation of HCl over TiO2-Supported RuO2: A Density Functional Theory Study

With density functional theory (DFT) calculations, we studied the oxidation of HCl with oxygen producing Cl2 and water on the TiO2(110)-supported RuO2(110). This so-called Sumitomo—a novel Deacon—process proceeds via a one-dimensional Langmuir−Hinshelwood mechanism, in which the recombination of two adjacent chlorine atoms on the surface of the catalyst constitutes the rate-determining step. Very important for industrial application is that substantial Ru resources can be saved in the production of the Sumitomo catalyst. According to our DFT calculations already 1 ML of RuO2(110) supported on TiO2(110) suffices to maintain practically the full activity of bulk-RuO2 in the HCl oxidation reaction. The calculated electron density differences of the TiO2(110)-supported 1 ML RuO2 system in comparison with bulk RuO2(110) are localized at the internal interface, leaving the electronic structure of the topmost undercoordinated Ru sites (active sites) unaffected by the support. This explains naturally the invariant activity of supported 1 ML RuO2(110) in comparison with bulk RuO2(110). The stoichiometric TiO2(110) is not active at all in the HCl oxidation reaction. However, if the undercoordinated Ti surface atoms are substituted by Ru then the resulting 1/2 ML RuO2−TiO2(110) catalyst is active with an activation barrier that is 58 kJ/mol higher than for bulk-RuO2(110).

A density functional study on the electronic structures of TiN solid

The electronic structures of TIN bulk have been studied by using different theoretical formalisms, and the DFT method, especially the BLYP method can produce reasonable results. The band structure of TIN(00l) surface is also investigated and two σ type surface states are presented in our results. The state located at 2.9 eV below EF in angle resolved photoemission in (ARPES) is well reproduced in this work, which consists essentially of 2pz orbital of surface N atom. Another surface state is associated with the bands originated from 3d orbital of surface Ti atom. Furthermore, the elastic constants of TiN are also calculated by using BLYP method.

Adsorption of benzene over the rutile TiO2 (110) surfaces: A theoretical study

AbstractIn this work, we present our theoretical results for the relaxation of the rutile TiO2 (110), as well as for the adsorption of benzene over this surface. Our results are in good agreement with both the available theoretical and experimental data, whenever these comparisons were possible. Based on our obtained results, we show that the interaction between the adsorbate and the TiO2 (110) surface shows a van der Waalslike character, with the calculated adsorption energies at the order of 43 to 173 meV. In this adsorption process, the H atoms of the benzene prefer to bind with the surface Ti atoms. As a result, two occupied electronic levels appear at the surface bandgap, which are responsible for the optical absorption peaks at the blue‐violet region of the electromagnetic spectra. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Interaction induced transition in the nanoporous TiO2/Pd‐porphyrin system

AbstractMolecules of Pd(II) 5‐(4‐carboxyphenyl)‐10,15,20‐tris(4‐methylphenyl) porphyrin were adsorbed from highly diluted dichloromethane solution on ultra‐thin layers of nanoporous TiO2 and investigated by modulated surface photovoltage (SPV) spectroscopy. An additional transition with a maximum at 2.03 eV appeared in the SPV spectra after moderate annealing in vacuum irrespective to the amount of adsorbed molecules. The new transition has been ascribed to the formation of a charge transfer complex due to an interaction between the Pd centred porphyrin and Ti surface atoms. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

STM imaging of a model surface of Ru(4,4′-dicarboxy-2,2′-bipyridine) 2(NCS) 2 dye-sensitized TiO 2 photoelectrodes

A TiO 2(1 1 0)-(1 × 1) surface was prepared in an ultra-high vacuum, transported in laboratory air, and observed with a scanning tunneling microscope (STM) operated in a vacuum of 10 −4 Pa. Empty state images showed atomically flat terraces separated by single-height steps, on which 5-fold-coordinated surface Ti atoms were observed as spots arranged in a rectangular lattice. The Ru(4,4′-dicarboxy-2,2′-bipyridine) 2(NCS) 2 (N3) dye was adsorbed on the TiO 2 surface by immersing the TiO 2 wafer into an acetonitrile solution of the dye. In the empty state images, individual N3 molecules were observed as oval particles protruding by 0.6 nm from the TiO 2 surface. The oval shape elongated to the [1 1 ¯ 0] directions was attributed to electron tunneling from tip to unoccupied states localized at the two carboxyl groups bound to the TiO 2 surface.

New Hybrid Properties of TiO2 Nanoparticles Surface Modified With Catecholate Type Ligands

Surface modification of nanocrystalline TiO2 particles (45 Å) with bidentate benzene derivatives (catechol, pyrogallol, and gallic acid) was found to alter optical properties of nanoparticles. The formation of the inner-sphere charge–transfer complexes results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites. The binding structures were investigated by using FTIR spectroscopy. The investigated ligands have the optimal geometry for chelating surface Ti atoms, resulting in ring coordination complexes (catecholate type of binuclear bidentate binding–bridging) thus restoring in six-coordinated octahedral geometry of surface Ti atoms. From the Benesi–Hildebrand plot, the stability constants at pH 2 of the order 103 M−1 have been determined.

Surface segregation of Ti atoms during NiTi alloy sputtering

The paper addresses NiTi alloy sputtering by 9 keV He and Ar ions and discusses the experiment performed by V.S. Chernysh et al. about 10 years ago. The binary collision simulation has been applied to extract the concentrations of surface Ni and Ti atoms from the experimental data. The results of simulations favor segregation of Ti for both He and Ar ion bombardment. The effect of non-symmetric surface collisions (Ti on Ni and Ni on Ti) was found to be negligible. A pronounced effect of the interatomic (target–target) potential is noted.