Theoretical investigation of the uranyl ion sorption on the rutile TiO2(110) face

Canister integrity and radionuclide retention is of first importance for assessing the long-term safety of nuclear waste stored in engineered geologic depositories. Uranyl ion sorption on the TiO(2) rutile (110) face is investigated using periodic density functional theory (DFT) calculations. From experimental observations, only two uranyl surface complexes are observed and characterized. When the pH increases (from 1.5 to 4.5), the relative ratios of these two surface complexes are modified. From a crystallographic point of view, three sorption sites can be considered and have been studied with different protonation states of the surface to account for very acidic and low acidic conditions. The two surface complexes experimentally observed were calculated as the most stable ones, while the evolution of their sorption energies agrees with experimental data.     

CO2 adsorption on TiO2(110) rutile: insight from dispersion-corrected density functional theory calculations and scanning tunneling microscopy experiments

Adsorption of CO(2) on the rutile(110) surface was investigated using dispersion-corrected density functional theory and scanning tunneling microscopy (STM). On the oxidized surface the CO(2) molecules are found to bind most strongly at the five-fold coordinated Ti sites adopting tilted or flat configurations. The presence of bridging oxygen defects introduces two new adsorption structures, the most stable of which involves CO(2) molecules bound in tilted configurations at the defect sites. Inclusion of dispersion corrections in the density functional theory calculations leads to large increases in the calculated adsorption energies bringing these quantities into good agreement with experimental data. The STM measurements confirm two of the calculated adsorption configurations.     

Chemical reactions on rutile TiO2(110)

Understanding the surface chemistry of TiO2 is key to the development and optimisation of many technologies, such as solar power, catalysis, gas sensing, medical implantation, and corrosion protection. In order to address this, considerable research effort has been directed at model single crystal surfaces of TiO2. Particular attention has been given to the rutile TiO2(110) surface because it is the most stable face of TiO2. In this critical review, we discuss the chemical reactivity of TiO2(110), focusing in detail on four molecules/classes of molecules. The selected molecules are water, oxygen, carboxylic acids, and alcohols-all of which have importance not only to industry but also in nature (173 references).     

A density functional theory study of the coadsorption of water and oxygen on TiO2(110)

The behavior of adsorbed water on oxides is of fundamental interest in many areas. Despite considerable attention received recently, our understanding of water chemistry is still short of needs and expectations, particularly on the topic of the coadsorption of water and other species. In this study we carry out density functional theory calculations to investigate the coadsorption of water and oxygen on the TiO(2)(110) surface. We show that oxygen exerts profound influences on the water adsorption, altering the mechanism of water dissociation. On the one hand, the possible dissociation route along [-110] is prohibited due to the weakening of the H bond between water and the lattice bridging oxygen in the presence of the coadsorbed oxygen, and on the other hand the coadsorbed oxygen induces dissociation along [001]. These results lead to a consistent interpretation of experiments. Furthermore, several possible final states and the related formation mechanisms are discussed in detail.     

Intrinsic diffusion of hydrogen on rutile TiO2(110)

The combined experimental and theoretical study of intrinsic hydrogen diffusion on bridge-bonded oxygen (BBO) rows of TiO 2(110) is presented. Sequences of isothermal scanning tunneling microscopy images demonstrate a complex behavior of hydrogen formed by water dissociation on BBO vacancies. Different diffusion rates are observed for the two hydrogens in the original geminate OH pair suggesting the presence of a long-lived polaronic state. For the case of separated hydroxyls, both theory and experiment yield comparable temperature-dependent diffusion rates. Density functional theory calculations show that there are two comparable low energy diffusion pathways for hydrogen motion along the BBO from one BBO to its neighbor, one by a direct hop and the other by an intermediate minimum at a terrace O. The values of kinetic parameters (prefactors and diffusion barriers) determined experimentally and theoretically are significantly different and indicate the presence of a more complex diffusion mechanism. We speculate that the hydrogen diffusion proceeds via a two-step mechanism: the initial diffusion of localized charge, followed by the diffusion of hydrogen. Both experiment and theory show the presence of repulsive OH-OH interactions.     

Prediction of tetraoxygen formation on rutile TiO2(110)

In this paper, we propose a new adsorption model for molecular oxygen on reduced TiO2(110), based on extensive first principles density functional calculations. For the first time, our calculations predict formation of tetraoxygen (O4) anchored at the vacancy site, which in turn allows adsorption of three O2 molecules per vacancy in saturation coverage. We present the structure, bonding, and energetics of adsorbed oxygen species by changing the number of adsorbed oxygen molecules per vacancy. We also find that thermally activated O2 desorption may take place via two channels that require overcoming barriers of 0.41 and 1.25 eV, respectively. In addition, our study provides strong theoretical evidence for the change in O2 reactivity with O2 coverage. Our findings associated with tetraoxygen complexes are consistent with existing experimental results.     

Theoretical investigation on RuO2 nanoclusters adsorbed on TiO2 rutile (110) and anatase (101) surfaces

Minimizing the energy of an $N$ -electron system as a functional of a two-electron reduced density matrix (2-RDM), constrained by necessary $N$ -representability conditions (conditions for the 2-RDM to represent an ensemble $N$ -electron quantum system), yields a rigorous lower bound to the ground-state energy in contrast to variational wave function methods. We characterize the performance of two sets of approximate constraints, (2,2)-positivity (DQG) and approximate (2,3)-positivity (DQGT) conditions, at capturing correlation in one-dimensional and quasi-one-dimensional (ladder) Hubbard models. We find that, while both the DQG and DQGT conditions capture both the weak and strong correlation limits, the more stringent DQGT conditions improve the ground-state energies, the natural occupation numbers, the pair correlation function, the effective hopping, and the connected (cumulant) part of the 2-RDM. We observe that the DQGT conditions are effective at capturing strong electron correlation effects in both one- and quasi-one-dimensional lattices for both half filling and less-than-half filling.     

Theoretical investigation of the energies and geometries of photoexcited uranyl(VI) ion: a comparison between wave-function theory and density functional theory

In order to assess the accuracy of wave-function and density functional theory (DFT) based methods for excited states of the uranyl(VI) UO2(2+) molecule excitation energies and geometries of states originating from excitation from the sigma(u), sigma(g), pi(u), and pi(g) orbitals to the nonbonding 5f(delta) and 5f(phi) have been calculated with different methods. The investigation included linear-response CCSD (LR-CCSD), multiconfigurational perturbation theory (CASSCFCASPT2), size-extensivity corrected multireference configuration interaction (MRCI) and AQCC, and the DFT based methods time-dependent density functional theory (TD-DFT) with different functionals and the hybrid DFTMRCI method. Excellent agreement between all nonperturbative wave-function based methods was obtained. CASPT2 does not give energies in agreement with the nonperturbative wave-function based methods, and neither does TD-DFT, in particular, for the higher excitations. The CAM-B3LYP functional, which has a corrected asymptotic behavior, improves the accuracy especially in the higher region of the electronic spectrum. The hybrid DFTMRCI method performs better than TD-DFT, again compared to the nonperturbative wave-function based results. However, TD-DFT, with common functionals such as B3LYP, yields acceptable geometries and relaxation energies for all excited states compared to LR-CCSD. The structure of excited states corresponding to excitation out of the highest occupied sigma(u) orbital are symmetric while that arising from excitations out of the pi(u) orbitals have asymmetric structures. The distant oxygen atom acquires a radical character and likely becomes a strong proton acceptor. These electronic states may play an important role in photoinduced proton exchange with a water molecule of the aqueous environment.     

Thermal and photochemistry of tert-butyl iodide on rutile TiO2(110)

The thermal and photochemistry of tert-butyl iodide (t-buI) dosed at 100 K on rutile TiO2(110) has been studied using isothermal and temperature programmed desorption mass spectrometry. Nondissociative adsorption and desorption dominate the thermal behavior with dose-dependent t-buI desorption peaks at nominally 220 and 150 K. Ultraviolet photochemistry occurs readily, but the behavior of submonolayers and multilayers differ qualitatively. Ejection of t-buI and i-C4H8 dominate during submonolayer photolysis at 100 K. Multilayer photolysis results are also dominated by ejection during irradiation, but the t-buI component is strongly suppressed, and the maximum rates of i-C4H8 and HI ejection did not occur at the outset. A mechanistic model capturing the observations involves both direct and substrate-mediated electronic excitation of t-buI. According to this model, ejection of t-buI occurs only from transient substrate-mediated formation of anionic t-buI. For either excitation path, the C-I dissociation probability is significant, and the excited tert-butyl product rearranges readily to form i-C4H8 that is ejected. For any local region where there is multilayer coverage, products formed at the adsorbate-vacuum interface are ejected promptly, but products formed within the multilayer are trapped. Thus, ejection of t-buI is suppressed, and trapped primary photodissociation products, tert-butyl and I, react to either reform t-buI or rearrange to i-C4H8 and HI. The latter two products remain trapped and are subsequently induced to desorb by acquisition of momentum from collisions with subsequently formed translationally excited photodissociation products.     

First-principles calculations of hydrogen diffusion on rutile TiO2(110) surfaces

Density functional calculations are performed to study the H-atom diffusion on titanium dioxide (110) surface in the cases of water-molecule dissociation and splitting of the adjacent hydroxyl OH pair. It is shown that, when a water molecule is adsorbed at a surface oxygen-vacancy site, a fragment H atom of the water molecule tends to diffuse toward the nearest-neighboring bridging-oxygen sites by using a straight-line or relay-point path. As the result, a pair of surface hydroxyl OH is formed on the same oxygen row. In a thermal process, on the other hand, such OH pair favorably splits only by using a relay-point path, i.e., by transferring one H atom from a bridging-oxygen site to a next-neighboring one along the same oxygen row by way of another in-plane oxygen site. We found that the latter splitting reaction is activated around room temperature.     

Adsorption properties versus oxidation states of rutile TiO2(110)

Using density functional theory we have studied the adsorption properties of different atoms and molecules deposited on a stoichiometric, reduced, and oxidized rutile TiO(2)(110) surface. Depending on the oxidation state of the surface, electrons can flow from or to the substrate and, therefore, negatively or positively charged species are expected. In particular, we have found that a charge transfer process from or to the surface always occurs for highly electronegative or highly electropositive species, respectively. For atoms or molecules with intermediate electron affinity, the direction of the charge flow depends on the oxidation state of the rutile surface and on the adsorption site. Generally, the charging effect leads to more stable complexes. However, the increase in the binding energy of the adsorbates is highly dependent on the electronic states of the surface prior to the adsorption event. In this work we have analyzed in details these mechanisms and we have also established a direct correlation between the enhanced binding energy of the adsorbates and the induced gap states.     

Ethanol photo-oxidation on a rutile TiO2(110) single crystal surface

The reaction of ethanol has been studied on the surface of rutile TiO(2)(110) by Temperature Programmed Desorption (TPD), online mass spectrometry under UV excitation and photoelectron spectroscopy while the adsorption energies of the molecular and dissociative modes of ethanol were computed using the DFT/GGA method. The most stable configuration is the dissociative adsorption in line with experimental results at room temperature. At 0.5 ML coverage the adsorption energy was found equal to 80 kJ mol(-1) for the dissociative mode (ethoxide, CH(3)CH(2)O(a) + H(a)) followed by the molecular mode (67 kJ mol(-1)). The orientation of the ethoxides along the [001] or [110] direction had minor effect on the adsorption energy although affected differently the Ti and O surface atomic positions. TPD after ethanol adsorption at 300 K indicated two main reactions: dehydration to ethylene and dehydrogenation to acetaldehyde. Pre-dosing the surface with ethanol at 300 K followed by exposure to UV resulted in the formation of acetaldehyde and hydrogen. The amount of acetaldehyde could be directly linked to the presence of gas phase O(2) in the vacuum chamber. The order of this photo-catalytic reaction with respect to O(2) was found to be 0.5. Part of acetaldehyde further reacted with O(2) under UV excitation to give surface acetate species. Because the rate of photo-oxidation of acetates (acetic acid) was slower than that of ethoxides (ethanol), the surface ended up by being covered with large amounts of acetates. A reaction mechanism for acetaldehyde, hydrogen and acetate formation under UV excitation is proposed.     

First-row transition metal atoms adsorption on rutile TiO2(110) surface

We performed periodic DFT calculations for adsorption of metal atoms on a perfect rutile TiO₂(110) surface (at low coverage, ???=?1/3) to investigate the interaction of an individual metal atom with TiO₂ and to compare it with a study previously done on MgO(100). We considered partial period of Mendeleev??s table from K to Zn. The overall evolution of the adsorption energies shows two maxima as for MgO(100). Two main differences, however, exist: the adsorption energy is much stronger and the first maximum is enhanced relative to the second one. This is attributed to the reducibility of the surface titanium cation. When the adsorbed metal is electropositive, it is oxidized under adsorption transferring electrons to titanium cations. We present the effect of introducing a Hubbard term to the gradient-corrected approximation band-structure Hamiltonian (GGA?+?U). The introduction of a reasonable Hubbard correction preserves the trends and allows localizing the electron of the reduction on Ti atoms in the near surface region. Finally, our results conclude that for heavier M atoms of the period, insertion is energetically favored relative to adsorption.     

Anisotropy in the electrical conductivity of rutile TiO(2) in the (110) plane

The bulk electrical conductivity of a TiO(2)(110) single crystal has been measured in all azimuths parallel to the (110) plane using a four-point probe. A distinct anisotropy in the conductivity has been found, with the highest bulk conductivity direction being parallel to the 001 direction.     

Reactivity of sub 1 nm supported clusters: (TiO2)n clusters supported on rutile TiO2 (110)

Metal oxide clusters of sub-nm dimensions dispersed on a metal oxide support are an important class of catalytic materials for a number of key chemical reactions, showing enhanced reactivity over the corresponding bulk oxide. In this paper we present the results of a density functional theory study of small sub-nm TiO(2) clusters, Ti(2)O(4), Ti(3)O(6) and Ti(4)O(8) supported on the rutile (110) surface. We find that all three clusters adsorb strongly with adsorption energies ranging from -3 eV to -4.5 eV. The more stable adsorption structures show a larger number of new Ti-O bonds formed between the cluster and the surface. These new bonds increase the coordination of cluster Ti and O as well as surface oxygen, so that each has more neighbours. The electronic structure shows that the top of the valence band is made up of cluster derived states, while the conduction band is made up of Ti 3d states from the surface, resulting in a reduction of the effective band gap and spatial separation of electrons and holes after photon absorption, which shows their potential utility in photocatalysis. To examine reactivity, we study the formation of oxygen vacancies in the cluster-support system. The most stable oxygen vacancy sites on the cluster show formation energies that are significantly lower than in bulk TiO(2), demonstrating the usefulness of this composite system for redox catalysis.     

O deficiency in the rutile TiO2 (110) surface: Ab initio quantum chemical investigation of the electronic properties

The (110) surface of rutile TiO₂ (110) has been modeled using a density functional theory (DFT) plane‐wave pseudo‐potential method (CASTEP). In this study, 6 and 9 atomic‐layer slabs have been examined. The stoichiometric surface converges to a low‐spin solution in both cases with a density of states (DOS) similar to that for the bulk. O deficiencies are introduced by the removal of neutral O atoms thus leaving a neutral model with a surfeit of 2 e^? per vacancy. This results in the partial filling of the Ti t_2g conduction band orbitals and a compensatory shift in the Fermi level. The reduced surface converges to a high‐spin solution in all cases, with the excess spin located within the previously unoccupied Ti t_2g orbitals. Removal of the bridging surface O atoms results in an excess spin of 2 electrons per unit cell with approximately one‐half that for removal of in‐plane surface O atoms and subsurface O atoms. The removal of O atoms from the surface leads to an increase of the band gap, with the largest increase due to the removal of in‐plane 3‐fold coordinated surface O atoms, and the smallest one due to the removal of subsurface O atoms. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Transient resonance Raman and density functional theory investigation of the 2-fluorenylnitrenium ion.

We report a transient resonance Raman spectrum for the 2-fluorenylnitrenium ion obtained after photolysis of 2-azidofluorene. The 10 experimental Raman band frequencies of the transient spectrum show very good agreement with the computed frequencies from BPW91/cc-PVDZ density functional theory calculations for the 2-fluorenylnitrenium ion. Our results confirm the assignment of the approximately 460 nm transient absorption band formed after photolysis of 2-azidofluorene in water/acetonitrile or water solution to the singlet ground electronic state 2-fluorenylnitrenium ion. Our study indicates the 2-fluorenylnitrenium has a large degree of iminocyclohexadienyl cation character with significant delocalization of the charge over both phenyl rings of the fluorene moiety. We compare our results for the 2-fluoreneylnitrenium ion to those previously reported for several other arylnitrenium ions.     

Vibrational spectroscopy investigation and density functional theory calculations on N-benzoylhydrazine ligand and the corresponding uranyl complex

Infrared spectra of N-benzoylhydrazine (BHZ) phCONHNH2 and its uranyl complex have been studied in the 4000-50 cm(-1) frequency range. Complete equilibrium geometry of the ligand molecule have been determined by DFT and BLYP/6-31G* force field calculations. Theoretical calculations reveal the existence of a keto tautomer. No enol form is present in the molecule in the solid. A complete vibrational assignment of the solid state IR and Raman spectra of BHZ was performed on the basis of normal coordinate analysis of a single molecule. The coordination of oxygen and nitrogen centers of BHZ to UO2(II) has been confirmed by study of the IR spectra of UO2 (phCONHNH2)2 complexes.     

Spectroscopy of ultrathin epitaxial rutile TiO(2)(110) films grown on W(100)

Epitaxial ultrathin titanium dioxide films of 0.3 to approximately 7 nm thickness on a metal single crystal substrate have been investigated by high resolution vibrational and electron spectroscopies. The data complement previous morphological data provided by scanned probe microscopy and low energy electron diffraction to provide very complete characterization of this system. The thicker films display electronic structure consistent with a stoichiometric TiO(2) phase. The thinner films appear nonstoichiometric due to band bending and charge transfer from the metal substrate, while work function measurements also show a marked thickness dependence. The vibrational spectroscopy shows three clear phonon bands at 368, 438, and 829 cm(-1) (at 273 K), which confirms a rutile structure. The phonon band intensity scales linearly with film thickness and shift slightly to lower frequencies with increasing temperature, in accord with results for single crystals.     

Li ion scattering from Au nanoclusters on TiO2(110)

The neutralization of low energy 7Li+ scattered from Au nanoclusters deposited on TiO2(110) was measured with time-of-flight spectroscopy as a function of cluster size, emission angle, and ion energy. The neutralization shows maxima for cluster diameters approximately 3 nm, and again for thick Au films. The data are compared to previous experiments with Na projectiles. Possible explanations of the observed effects are discussed.