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Synthesis and implementation of highly active, stable, and affordable electrocatalysts for the oxygen evolution reaction (OER) is a major challenge in developing energy efficient and economically viable energy conversion devices such as electrolyzers, rechargeable metal-air batteries, and regenerative fuel cells. The current benchmark electrocatalyst for OER is based on iridium oxide (IrOx) due to its superior performance and excellent stability. However, large scale applications using IrOx are impractical due to its low abundance and high cost. Herein, we report a highly active hafnium-modified iridium oxide (IrHfxOy) electrocatalyst for OER. The IrHfxOy electrocatalyst demonstrated ten times higher activity in alkaline conditions (pH = 11) and four times higher activity in acid conditions (pH = 1) than a IrOx electrocatalyst. The highest intrinsic mass activity of the IrHfxOy catalyst in acid conditions was calculated as 6950 A gIrOx-1 at an overpotential (η) of 0.3 V. Combined studies utilizing operando surface enhanced Raman spectroscopy (SERS) and DFT calculations revealed that the active sites for OER are the Ir-O species for both IrOx and IrHfxOy catalysts. The presence of Hf sites leads to more negative charge states on nearby O sites, shortening of the bond lengths of Ir-O, and lowers free energies for OER intermediates that accelerate the OER process.

Sodium tantalate, NaTaO3, has attracted interest as one of the few photocatalysts capable to perform overall water splitting, that is, simultaneously produce oxygen and hydrogen from water. In particular, an interesting and not fully understood observation is that the efficiency of NaTaO3 increases dramatically in the presence of cation doping. To obtain better insight into the origin of this effect, we use first-principles calculations to investigate the fundamental structural, electronical, and chemical properties of pristine and Sr-doped NaTaO3, a system for which several experimental studies have recently become available. Our results show that Sr donor-acceptor codoping at Na and Ta sites significantly reduces the formation energy of the Sr dopants. Further study of the energetics of the oxygen evolution reaction (OER) shows a substantial reduction of the OER overpotential for the codoped material, consistent with recent suggestions that codoping is crucial for increasing NaTaO3's efficiency. The detailed insights provided by our work could benefit the design and preparation of new efficient catalysts based on NaTaO3

We investigate the mechanism of H-2 activation on Ag-modified cerium oxide surfaces, of interest for different catalytic applications. The study is performed on thin epitaxial cerium oxide films, investigated by X-ray photoemission spectroscopy to assess the changes of both the Ag oxidation state and the concentration of Ce3+ ions, O vacancies, and hydroxyl groups on the surface during thermal reduction cycles in vacuum and under hydrogen exposure. The results are interpreted using density functional theory calculations to model pristine and Ag-modified ceria surfaces. Although the reactivity of ceria toward H-2 oxidation improves when a fraction of Ce cations is substituted with Ag, the concentration of reduced Ce3+ ions in Ag-modified ceria is found to be lower than in pure ceria under the same conditions. This behavior is observed even though the number of surface oxygen vacancies caused by the thermal treatment under hydrogen exposure is larger for the Ag-modified surface. These results are explained in terms of a change of the oxidation state of the surface Ag, which is able to acquire some of the extra surface electrons created by the oxygen vacancies and the adsorbed hydrogen atoms. Our findings provide new insights into the reactivity of Ag-modified ceria, which has been proposed as a promising alternative to platinum electrodes in electrochemical devices.

Reducing the temperature for methane activation is an important objective that would benefit many technological applications. In this work, we explore the possibility to achieve this goal using single-atom catalysts (SACs) on ceria surfaces. We focus on Ag SACs, which have recently been suggested to be promising catalysts for both H2 and CH4 oxidation. Using first-principles calculations, we investigate methane activation on CeO2(111) and CeO2(100), two frequently exposed surfaces on ceria nanoparticles. The presence of Ag is found to reduce the activation energy for methane dissociation on both surfaces. On Ag-doped CeO2(111), the formation of methanol via the Mars-van Krevelen mechanism has a slightly lower energy barrier than the dissociation to CH3 + H, suggesting that methanol is a likely product of methane activation on this surface. A novel aspect of this work is the focus on stable surface structures where each Ag dopant substituting Ce forms a complex with a charge-compensating surface oxygen vacancy. These complexes are found to play a critical role and accounting for their presence is essential for a proper description of the surface reactivity.

We investigate the effect of adsorbates on the structure and photoabsorption of reduced TiO2 by first-principles calculations of rutile TiO2(110) in the presence of both titanium interstitials (Tiint’s) and adsorbed water or methanol. Our results show that while Tiint’s prefer to reside in deep inner layers when the surface is clean, they tend to diffuse toward the surface in the presence of water or methanol. This migration is due to the mutual stabilization of the adsorbates and Tiint defects in the near-surface region. We also find that adsorbed water/methanol changes the orbital character and localization sites of the excess electrons associated with the Tiint. These results can explain why the adsorption of water and methanol enhances the photoabsorption of the reduced TiO2(110) surface.

The excellent photocatalytic properties of titanium oxide (TiO2) under ultraviolet light have long motivated the search for doping strategies capable of extending its photoactivity to the visible part of the spectrum. One approach is high-pressure and high-temperature hydrogenation, which results in reduced ‘black TiO2’ nanoparticles with a crystalline core and a disordered shell that absorbs visible light. Here we elucidate the formation mechanism and structural features of black TiO2 using first-principles-validated reactive force field molecular dynamics simulations of anatase TiO2 surfaces and nanoparticles at high temperature and under high hydrogen pressures. Simulations reveal that surface oxygen vacancies created upon reaction of H2 with surface oxygen atoms diffuse towards the bulk material but encounter a high barrier for subsurface migration on {001} facets of the nanoparticles, which initiates surface disordering. Besides confirming that the hydrogenated amorphous shell has a key role in the photoactivity of black TiO2, our results provide insight into the properties of the disordered surface layers that are observed on regular anatase nanocrystals under photocatalytic water-splitting conditions. The nature of structural disorder in photocatalytic black TiO2 is not known. Here, using simulations, it is shown that water formation drives surface reduction, with slowly migrating oxygen vacancies at the {001} facets resulting in the nucleation of disorder.

As one of the most important photocatalysts, TiO2 has triggered broad interest and intensive studies for decades. Observation of the interfacial reactions between water and TiO2 at microscopic scale can provide key insight into the mechanisms of photocatalytic processes. Currently, experimental methodologies for characterizing photocatalytic reactions of anatase TiO2 are mostly confined to water vapor or single molecule chemistry. Here, we investigate the photocatalytic reaction of anatase TiO2 nanoparticles in water using liquid environmental transmission electron microscopy. A self-hydrogenated shell is observed on the TiO2 surface before the generation of hydrogen bubbles. First-principles calculations suggest that this shell is formed through subsurface diffusion of photo-reduced water protons generated at the aqueous TiO2 interface, which promotes photocatalytic hydrogen evolution by reducing the activation barrier for H2 (H–H bond) formation. Experiments confirm that the self-hydrogenated shell contains reduced titanium ions, and its thickness can increase to several nanometers with increasing UV illuminance.

Bismuth vanadate (BiVO4) has emerged as one of the most promising photoanode materials for solar fuel production. Oxygen vacancies play a pivotal role in the photoelectrochemical efficiency, yet their electronic nature and contribution to n-type conductivity are still under debate. Using first-principles calculations, we show that oxygen vacancies in BiVO4 have two distinguishable geometric configurations characterized by either undercoordinated, reduced VIVO3 and BiIIO7 subunits or a VIV−O−VIV/V bridge (split vacancy), quenching the oxygen vacancy site. While both configurations have similar energies in the bulk, the (001) subsurface acts like an energetic sink that stabilizes the split oxygen vacancy by ∼1 eV. The barrierless creation of a bridging V2O7 unit allows for partial electron delocalization throughout the near-surface region, consistent with recent experimental observations indicating that BiVO4(001) is an electron-rich surface.

Titanium oxide structures featuring highly reduced TiOx films on top of nearly stoichiometric TiO2 hold promise for applications ranging from photocatalysis to resistance switching devices. Here, we focus on titanium monoxide (TiO) layers on anatase TiO2 (001) and use first principles calculations to investigate their structure and properties as well as their interface with liquid water. We find that only a single TiO layer can grow epitaxially on anatase (001) and subsequent growth leads to the formation of TiO islands. The TiO layers decrease the work function and enhance the surface conductivity in comparison to pure anatase, two features that can improve the TiO2 performance in photocatalytic hydrogen evolution but are thermodynamically unstable relative to pure TiO2 in humid/aqueous environment. Furthermore, first principles molecular dynamics simulations of the TiO (001)-water and anatase (001)-water interfaces show that unlike the multilayer structure of interfacial water on the anatase surface, a very dense and tightly packed first water layer is present on the surface of TiO, which could represent the first stage of partial surface reoxidation.

Oxide Chemistry Below the Surface Although metal oxides, such as titanium dioxide (TiO 2 ), are used for catalytic oxidation reactions and photocatalysis, the O 2 does not react directly with substrates. Vacancies in the surface region of the TiO 2 rutile phase can transfer a negative charge to adsorbed O 2 to create more reactive species. By contrast, in anatase—the phase associated with nanoscale TiO 2 particles—subsurface vacancies form. Setvin et al. (p. 988 ) used a scanning tunneling microscopy tip to pull these vacancies to the surface in a niobiumdoped anatase crystal and followed the transformation of adsorbed O 2 − into a peroxo species and a bridging O 2 dimer.

Nickel ferrite, NiFe2O4, is a material with interesting physical properties and useful technological applications. We have used Density-functional theory with on-site Coulomb repulsion U term to study the structure, electronic properties, and energetics of the NiFe2O4(001) surface and its interaction with water both in the absence and in the presence of surface oxygen vacancies. In a humid environment, water adsorbs dissociatively on the surface oxygen vacancies leading to the formation of surface hydroxyls. At high temperature, water desorbs leaving a surface containing oxygen vacancies. These defects could represent useful reactive sites for various catalytic reactions.

Defect states in reduced and n-type doped titania are of fundamental importance in several technologically important applications. Still, the exact nature of these states, often referred to as "Ti3+ centers", is largely unclear and a matter of debate. The problem is complicated by the fact that electronic structure calculations based on density functional theory (DFT) in the local density approximation (LDA) or semilocal generalized gradient approximation (GGA) provide results that do not account for many of the experimentally observed fingerprints of the formation of Ti 3+ centers in reduced TiO2. Here, we investigate the properties of at least four different types of Ti3+ centers in bulk anatase, (1) 6-fold-coordinated Ti6c3+ ions introduced by F- or Nb-doping, (2) Ti6c3+-OH species associated with H-doping, (3) undercoordinated Ti5c3+ species associated with oxygen vacancies, and (4) interstitial Ti5c3+ species. The characterization of these different kinds of Ti3+ centers is based on DFT+U and/or hybrid functional calculations, which are known to (partially) correct the self-interaction error of local and semilocal DFT functionals. We found that strongly localized solutions where an excess electron is on a single Ti3+ ion are very close in energy and sometimes degenerate with partly or highly delocalized solutions where the extra charge is distributed over several Ti ions. The defect states corresponding to these different solutions lie at different energies in the band gap of the material. This has important implications for the conductivity mechanism in reduced or n-type doped titania and suggests a significant role of temperature in determining the degree of localization of the trapped charge. © 2009 American Chemical Society.

Density functional theory (DFT) calculations with on-site Coulomb repulsion are carried out to study the relative stabilities of crystalline cobalt oxides and hydroxides—CoO, Co(OH)2, Co3O4, CoO(OH), and CoO2—in electrochemical environment. Co(OH)2 is the thermodynamic ground state under reducing conditions, i.e., at voltages V < 0 relative to the standard hydrogen electrode (SHE) potential in acidic solution, whereas CoO(OH) and CoO2 are stable under oxidizing conditions, i.e., at external voltage larger than 1.23 eV vs SHE in basic solution. These results, combined with surface structure studies of the (0001) natural cleavage surface of CoO(OH), show that a CoO2x– (x = 0–0.5) layer is present when the surface is exposed to solution under oxidizing conditions, in agreement with recent experimental findings. Study of the energetics of water oxidation at regular surface sites of CoO(OH)(0001) indicates however that water deprotonation to form a surface OH species is energetically very costly. Different act...

We carried out density functional theory calculations with on-site Coulomb repulsion U terms to study the interaction of water with the (110) surface of the spinel cobalt oxide, Co3O4, a widely used oxidation catalyst. This surface has two different terminations, one positively (A) and the other negatively charged (B). Dissociative water adsorption is preferred from low up to one monolayer coverage on the A termination and up to half monolayer on the B termination. On the latter, a mixed molecular and dissociated monolayer is more stable at full coverage. The computed structures are used to investigate the free-energy changes during water oxidation on both surface terminations. We find that the most difficult step of the oxygen evolution reaction is the second deprotonation to form an adsorbed O species (O*). Moreover, the A-terminated surface is more active than the B-terminated surface. Analysis of the surface electronic structure shows a larger density of cobalt states near the Fermi energy on the A te...

Bacterial di-iron hydrogenases produce hydrogen efficiently from water. Accordingly, we have studied by first-principles molecular-dynamics simulations (FPMD) electrocatalytic hydrogen production from acidified water by their common active site, the [FeFe]H cluster, extracted from the enzyme and linked directly to the (100) surface of a pyrite electrode. We found that the cluster could not be attached stably to the surface via a thiol link analogous to that which attaches it to the rest of the enzyme, despite the similarity of the (100) pyrite surface to the Fe4S4 cubane to which it is linked in the enzyme. We report here a systematic sequence of modifications of the structure and composition of the cluster devised to maintain the structural stability of the pyrite/cluster complex in water throughout its hydrogen production cycle, an example of the molecular design of a complex system by FPMD.

Hydrogen incorporation into well-defined nanocrystals of anatase titanium dioxide (TiO2) has been investigated by a combination of experimental studies and density functional theory (DFT) calculations. The hydrogenation of TiO2 nanocrystals was determined at 450 °C with an initial hydrogen pressure of 7.0 MP, and storage capacities of 1.0 wt % and 1.4 wt % were achieved for nanocrystals with predominant (001) and (101) surface terminations, respectively. X-ray diffraction and Raman spectroscopy measurements indicate that the TiO2 crystal structure is very well preserved during the hydrogenation. DFT calculations show that hydrogen occupies the interstitial sites between titanium–oxygen octahedra and the energy barrier for hydrogen incorporation through the anatase (101) surface is lower than that through (001).

We introduce a novel theoretical approach for determining oxidation states (OS) from quantum-mechanical calculations. For a transition-metal ion, for example, the metal-ligand orbital mixing contribution to the charge allocated to the ion is separated from that due to the actual occupation of the d-orbitals from which OS can then be inferred. We report the application of this approach to different transition-metal systems: molecular complexes, ruthenium-dye molecules, ruthenium complexes with noninnocent ligands, and bulk semiconductors. The computations were carried out using density-functional theory with a Hubbard U correction. The oxidation states were determined without ambiguity.

The interaction of water with the fluorine-covered (001) surface of anatase titanium dioxide (TiO2) has been studied within the framework of density functional theory (DFT). Our results show that water dissociation is unfavorable due to repulsive interactions between surface fluorine and oxygen. We also found that the reaction of hydrofluoric acid with a surface hydroxyl group to form a surface Ti–F bond is exothermic, while the removal of fluorine from the surface needs additional energy of about half an eV. Therefore, water molecules are predicted to remain intact at the interface with the F-terminated anatase (001).

Sustainable economic production of hydrogen from water and sunlight is an attractive goal. It requires an active electrocatalyst comprised of earth-abundant elements. Such a catalyst exists in nature, the [FeFe]H cluster in the active site of the di-iron hydrogenase enzymes. To reach the required specific activity within an actual solar hydrogen-production system, the catalytically active site must, figuratively, be stripped from the enzyme, attached to a cathode or photocathode, and immersed in water. Thus modifications of the composition and structure of the cluster are to be found which allow for stable attachment to the electrode surface and for maintenance of its integrity and activity throughout the H2-producing cycle in an environment drastically different from that in the enzyme. We have addressed that problem by simulating the behavior of model clusters by first-principles electronic-structure and molecular-dynamics simulations. We review our studies, first of the [FeFe]H cluster in vacuum; next of the [FeFe]H cluster in water; then of a systematic sequence of modifications which culminates with the design of the successful phosphorous-substituted [FeFe]P cluster; and, finally, an investigation of the H2 producing cycle of [FeFe]P. We then discuss the limitations of our results and conclude with a brief consideration of future directions.

The possibility of using the active site, the [FeFe](H) cluster, of the bacterial di-iron hydrogenases as a catalyst for hydrogen production from water by electro- or photocatalysis is of current scientific and technological interest. We present here a theoretical study of hydrogen production by a modified [FeFe](H) cluster stably linked to a pyrite electrode immersed in acidified water. We employed state-of-the-art electronic-structure and first-principles molecular-dynamics methods. We found that a stable sulfur link of the cluster to the surface analogous to that linking the cluster to its enzyme environment cannot be made. However, we have discovered a modification of the cluster which does form a stable, tridentate link to the surface. The pyrite electrode readily produces hydrogen from acidified water when functionalized with the modified cluster, which remains stable throughout the hydrogen production cycle.

The adsorption of water on a reduced TiO2 anatase (101) surface is investigated with scanning tunneling microscopy and density functional theory calculations. The presence of subsurface defects, which are prevalent on reduced anatase (101), leads to a higher desorption temperature of adsorbed water, indicating an enhanced binding due to the defects. Theoretical calculations of water adsorption on anatase (101) surfaces containing subsurface oxygen vacancies or titanium interstitials show a strong selectivity for water binding to sites in the vicinity of the subsurface defects. Moreover, the water adsorption energy at these sites is considerably higher than that on the stoichiometric surface, thus giving an explanation for the experimental observations. The calculations also predict facile water dissociation at these sites, confirming the important role of defects in the surface chemistry of TiO2.

An extensive search for possible structural models of the (2 × 1)-reconstructed rutile TiO 2 (0 1 1) surface was carried out by means of density functional theory (DFT) calculations. A number of models were identified that have much lower surface energies than the previously-proposed ‘titanyl’ and ‘microfaceting’ models. These new structures were tested with surface X-ray diffraction (SXRD) and voltage-dependent STM measurements. The model that is (by far) energetically most stable shows also the best agreement with SXRD data. Calculated STM images agree with the experimental ones for appropriate tunneling conditions. In contrast to previously-proposed models, this structure is not of missing-row type; because of its similarity to the fully optimized brookite TiO 2 (0 0 1) surface, we call it the ‘brookite (0 0 1)-like’ model. The new surface structure exhibits two different types of undercoordinated oxygen and titanium atoms, and is, in its stoichiometric form, predicted to be rather inert towards the adsorption of probe molecules.

We present a density-functional study of the adsorption and reactions of oxygen and water with the (100) surface of pyrite. We find that dissociative adsorption is energetically favorable for oxygen, forming ferryl-oxo, Fe(4+)═O(2-), species. These transform easily to ferric-hydroxy, Fe(3+)-OH(-), in the presence of coadsorbed water, and the latter fully covers the surface under room conditions. A mechanism for surface oxidation is identified, which involves successive reactions with molecular oxygen and water, and leads to the complete oxidation of a surface sulfur to SO4(2-). The crucial recurring process is the surface O(2-) and OH(-) species acting as proton acceptors for incoming water molecules. Using a recently proposed method, we examine the oxidation state changes of the surface ions and the electron flow during the adsorption and oxidation processes. The oxidation mechanism is consistent with isotopic labeling experiments, suggesting that the oxygens in SO4(2-) from gas-phase oxidation are derived from water.

ConspectusThe interaction of molecular oxygen with titanium dioxide (TiO2) surfaces plays a key role in many technologically important processes such as catalytic oxidation reactions, chemical sensing, and photocatalysis. While O2 interacts weakly with fully oxidized TiO2, excess electrons are often present in TiO2 samples. These excess electrons originate from intrinsic reducing defects (oxygen vacancies and titanium interstitials), doping, or photoexcitation and form polaronic Ti3+ states in the band gap near the bottom of the conduction band. Oxygen adsorption involves the transfer of one or more of these excess electrons to an O2 molecule at the TiO2 surface. This results in an adsorbed superoxo (O2–) or peroxo (O22–) species or in molecular dissociation and formation of two oxygen adatoms (2 × O2–). Oxygen adsorption is also the first step toward oxygen incorporation, a fundamental reaction that strongly affects the chemical properties and charge-carrier densities; for instance, it can transform the ...

The titanium dioxide rutile (0 1 1) (equivalent to (1 0 1)) surface reconstructs to a stable (2 × 1) structure upon sputtering and annealing in ultrahigh vacuum. A previously proposed model (T.J. Beck, A. Klust, M. Batzill, U. Diebold, C. Di Valentin, A. Selloni, Phys. Rev. Lett. 93 (2004) 036104/1) containing onefold coordinated oxygen atoms (titanyl groups, Ti O) is supported by Scanning Tunneling Microscopy (STM) measurements. These Ti O sites are imaged bright in empty-states STM. A few percent of these terminal oxygen atoms are missing at vacuum-annealed surfaces of bulk-reduced samples. These O vacancies are imaged as dark spots. Their number density depends on the reduction state of the bulk. Double vacancies are the most commonly observed defect configuration; single vacancies and vacancies involving several O atoms are present as well. Formation of oxygen vacancies can be suppressed by annealing a sputtered surface first in vacuum and then in oxygen; annealing a sputtered surface in oxygen results in surface restructuring and a (3 × 1) phase. Anti-phase domain boundaries in the (2 × 1) structure are active adsorption sites. Segregation of calcium impurities from the bulk results in an ordered overlayer that exhibits domains with a centered (2 × 1) periodicity in STM.

We report a combined experimental and computational study of several ruthenium(II) sensitizers originated from the [Ru(dcbpyH(2))(2)(NCS)(2)], N3, and [Ru(dcbpyH(2))(tdbpy)(NCS)(2)], N621, (dcbpyH(2) = 4,4'-dicarboxy-2,2'-bipyridine, tdbpy = 4,4'-tridecyl-2,2'-bipyridine) complexes. A purification procedure was developed to obtain pure N-bonded isomers of both types of sensitizers. The photovoltaic data of the purified N3 and N621 sensitizers adsorbed on TiO(2) films in their monoprotonated and diprotonated state, exhibited remarkable power conversion efficiency at 1 sun, 11.18 and 9.57%, respectively. An extensive Density Functional Theory (DFT)-Time Dependent DFT study of these sensitizers in solution was performed, investigating the effect of protonation of the terminal carboxylic groups and of the counterions on the electronic structure and optical properties of the dyes. The calculated absorption spectra are in good agreement with the experiment, thus allowing a detailed assignment of the UV-vis spectral features of the two types of dyes. The computed alignments of the molecular orbitals of the different complexes with the band edges of a model TiO(2) nanoparticle provide additional insights into the electronic factors governing the efficiency of dye-sensitized solar cell devices.

Recently several theoretical studies have examined oxygen adsorption on the clean, reduced TiO2(110) surface. However the photocatalytic behavior of TiO2 and the scavenging ability of oxygen are known to be influenced by the presence of surface hydroxyls. In this paper the chemistry of O2 on the hydroxylated TiO2 surface is investigated by means of first-principles total energy calculations and molecular dynamics (MD) simulations. The MD trajectories show a direct, spontaneous reaction between O2 and the surface hydroxyls, thus supporting the experimental hypothesis that the reaction does not necessarily pass through a chemisorbed O2 state. Following this reaction, the most stable chemisorbed intermediates are found to be peroxide species HO2 and H2O2. Although these intermediates are very stable on the short time scale of MD simulations, the energetics suggests that their further transformation is connected to a new 300 K feature observed in the experimental water temperature programmed desorption (TPD) spectrum. The participation of two less stable intermediate states, involving terminal hydroxyls and/or chemisorbed water plus oxygen adatoms, to the desorption process, is not supported by the total energy calculations. Analysis of the projected density of states, however, suggests the possibility that these intermediates have a role in completing the surface oxidation immediately before desorption.

Recent experimental work has shown that the addition of styrene molecules to hydrogen-terminated Si(001) surfaces leads to the formation of one-dimensional molecular structures through a radical-initiated surface chain reaction mechanism. These nanometric structures are observed to be directed parallel to the dimer rows on the H-Si(001)-(2 x 1) surface and perpendicular to the same rows on H-Si(001)-(3 x 1). Using periodic density functional theory (DFT) calculations, we have studied the initial steps of the radical chain mechanism on the H-Si(001)-(3 x 1) surface and compared them to analogous results for H-Si(001)-(2 x 1). On the H-Si(001)-(3 x 1) surface, one of the crucial steps of the surface chain reaction, namely, the abstraction of a H atom from a nearby surface hydride unit, is found to have a somewhat smaller activation energy in the direction perpendicular to the dimer rows (H abstraction from the nearest dihydride site) than along the rows (H abstraction from a neighboring dimer). Additionally, due to the steric repulsion between the styrene molecules and the SiH2 subunits, growth along the dimer rows is not thermodynamically favorable on the (3 x 1) surface. On the other hand, due to the absence of the SiH2 subunits, growth parallel to the Si dimer rows becomes favored on the H-Si(001)-(2 x 1) surface.

Most alkali-group-IV liquid alloys exhibit strong chemical effects leading to well-defined equiatomic compounds. This is generally explained by the survival of particular clusters of lead-''Zintl'' ions-in the liquid state. We report on an ab initio simulation of liquid CsPb, for which the chemical ordering is believed to be particularly strong. Good agreement with experiment is obtained, provided the valence states of cesium include the 5s and 5p states. The liquid contains a much richer variety of Pb conformations than isolated Pb4 units only. This is in accordance with the expectation that with increasing temperature the isolated ''Zintl'' ions break down rapidly.

The adsorption and reactivity of methanol on TiO2 anatase(101) surfaces are studied through first-principles total energy calculations and Car−Parrinello molecular dynamics simulations. The effects of different methanol coverages up to one monolayer and different surface oxidation states are investigated. The most stable monolayer state involves two different molecular adsorption states, with specific vibrational and electronic properties. Molecular adsorption is always favored on the stoichiometric surface, even though dissociation becomes increasingly more favorable with increasing coverage. Oxygen vacancies shift the energy balance in favor of the dissociated state, whereas dissociative and molecular adsorption appear competitive on the hydroxylated surface. On the latter, a specific molecular adsorption mode, not available on the clean surface, turns out to be stable. The vibrational and electronic properties of adsorbed methanol and methoxy are determined and compared to available experimental data.

We present a theoretical study of the influence of silicon ad-dimers on carbon incorporation in the Si (0 0 1) surface. We consider configurations involving either ad-dimers located directly above the dimer rows of the defectless Si (0 0 1) surface, or ad-dimers located between the dimer rows. In both cases, the presence of ad-dimers makes carbon penetration in the subsurface layers easier. The energetic barrier at the crossing of the second layer substantially decreases or vanishes, and the third layer is always favoured. Carbon adsorption is more likely in α than in β type sites, but the difference can be less important in the presence of ad-dimers. The effects of carbon–defect interaction and local geometry (Si–C distances) are also investigated. Our results suggest that acting on the surface defects might improve carbon penetration in Si (0 0 1) and allow a better control of the carbon atoms positions in the subsurface layers.

Understanding how defects in solids interact with external electric fields is important for technological applications such as memristor devices. Using Density Functional Theory (DFT) calculations, we have studied the influence of an external electric field on the formation energies and diffusion barriers of surface and subsurface oxygen vacancies at the (101) surface of anatase TiO2. DFT in the generalized gradient approximation as well as DFT+U methods with different U values have been utilized, with the electric field treated self-consistently by adding a sawtooth-like potential to the bare ionic potential. Our results show that the direction and strength of the applied field can have a significant influence on the relative stabilities of surface and subsurface defects, whereas the effect on the subsurface-to-surface defect migration is found to be relatively minor.

In this project, we have successfully designed and characterized a promising biomimetic catalyst/electrode complex, [FeFe]P/FeS2 for producing hydrogen from water. It is comprised of earth-abundant materials and, with a diffusion-limited rate in acidified water, is efficient as well as oxygen tolerant. The theoretical techniques we have developed and the experience we have gained are broadly applicable for the design and analysis of biomimetic electrochemically active catalysts.

Recent experiments have indicated that titanium dioxide (TiO(2)) codoped with nitrogen and fluorine may show enhanced photocatalytic activity in the visible region with respect to TiO(2) doped only with nitrogen. Prompted by these findings, we have investigated N-F codoped TiO(2) through a combined theoretical and experimental study. Density functional theory (DFT) calculations have been carried out both within the generalized gradient approximation (GGA) and using hybrid functionals to accurately describe the electronic structure; substitutional as well as interstitial locations of nitrogen in the TiO(2) lattice were considered. From these calculations we infer that N-F codoping reduces the. energy cost of doping and also the amount of defects (number of oxygen vacancies) in the lattice, as a consequence of the charge compensation between the nitrogen (p-dopant) and the fluorine (n-dopant) impurities. The UV-visible spectra of the sol-gel prepared TiO(2) powders confirm the synergistic effect of N-F codoping: more impurities are introduced in the lattice with an increased optical absorption in the visible. EPR spectroscopy measurements on the codoped samples identify two paramagnetic species which are associated to bulk N impurities (N(b)(center dot)) and Ti(3+) ions. Preliminary photocatalytic tests also indicate an enhanced activity under vis-light irradiation toward degradation of methylene blue for the codoped system with respect to N-doped TiO(2).

The adsorption of formic acid and sodium formate on the stoichiometric anatase (101) surface has been studied by means of density functional calculations with a slab geometry. On the clean surface, the most stable adsorption structure for HCOOH is a molecular monodentate configuration, hydrogen bonded to a surface bridging oxygen, while for HCOONa a dissociated bridging bidentate geometry is preferred. The bidentate chelating structure is energetically unstable for both the acid and the salt. On the hydrated surface, both HCOOH and HCOONa preferentially form an inner-sphere adsorption complex. HCOOH maintains a monodentate coordination, but, due to the interaction with a nearby water molecule, it becomes dissociated, while HCOONa again prefers a bridging bidentate structure. The energies for adsorption from an aqueous solution are estimated to be 0.30 and 0.79 eV for HCOOH and HCOONa, respectively.

The incorporation of nitrogen impurities in the anatase TiO2(101) surface is investigated by first principles density functional theory calculations. Several substitutional and interstitial configurations and different concentrations for the N impurities in the surface and subsurface layers are considered, as well as their interactions with oxygen vacancies in the TiO2 lattice. The stability of the various investigated systems is compared on the basis of their formation energy as a function of the oxygen chemical potential, which determines whether the system is in an oxidizing or reducing environment. Under oxygen-rich conditions, N bound to a surface O is preferred, whereas, under oxygen-poor conditions, substitutional N together with oxygen vacancies is favored, as previously found for bulk TiO2. The cost of formation of a surface oxygen vacancy is almost cancelled in the presence of N impurities in subsurface layers. The incorporation of nitrogen in the lattice modifies the electronic structure by introducing localized states in the band gap, consistent with the experimentally observed absorption of N-doped anatase samples in the visible region. Also, these N impurity states are excellent traps for the Ti3+ electrons deriving from surface oxygens vacancies. STM images for the various N-doped anatase (101) surface models have been computed to provide a reference to future experimental studies of this surface

Fluorine-containing species can cause titania to crystallize with an unusually large fraction of reactive {001} facets.

Recent experimental studies have determined that carbon doping dramatically improves the photocatalytic activity of TiO2 in the visible-light region. Using density functional theory (DFT) calculations within the generalized gradient corrected approximation, we investigate various structural models of carbon impurities in both the anatase and rutile polymorphs of TiO2 and analyze the associated modifications of the electronic band structure. We compare the stability of all these diverse species on the basis of their energy of formation as a function of the oxygen chemical potential, which determines whether the system is in an oxidizing or reducing environment. At low carbon concentrations, we find that, under oxygen-poor conditions, substitutional (to oxygen) carbon and oxygen vacancies are favored, whereas, under oxygenrich conditions, interstitial and substitutional (to Ti) C atoms are preferred. Higher carbon concentrations undergo an unexpected stabilization caused by multidoping effects, interpreted as inter-species redox processes. Carbon impurities result in modest variations of the band gap but induce several localized occupied states in the gap, which may account for the experimentally observed red shift of the absorption edge toward the visible. Our results also indicate that carbon doping may favor the formation of oxygen vacancies in bulk TiO2

First-principles local-density-functional calculations on the interaction of chlorine with the $\mathrm{Si}(100)\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}1$ surface are presented. ${\mathrm{Cl}}_{2}$ adsorption and a number of reactions relevant to etching via desorption of ${\mathrm{SiCl}}_{2}$ species are considered, both at the defect-free surface, and in the presence of small surface defects. The formation of ${\mathrm{SiCl}}_{2}$ species via an isomerization mechanism, their stabilization and desorption are studied. Whereas this is a likely route towards spontaneous etching of the surface in the limit of low Cl coverage $({\ensuremath{\theta}}_{\mathrm{Cl}}),$ at ${\ensuremath{\theta}}_{\mathrm{Cl}}\ensuremath{\sim}1$ it is energetically very costly. In the high coverage limit the formation of ${\mathrm{SiCl}}_{2}$ by either a Cl atom reacting with a monochloride surface species or by dissociative chemisorption of hyperthermal ${\mathrm{Cl}}_{2}$ molecules is investigated. For the latter, a barrier for dimer-bond breaking of 0.5--1 eV is estimated. Our results show that both the ${\mathrm{Cl}}_{2}$ adsorption energy and the ${\mathrm{SiCl}}_{2}$ desorption energy decrease significantly with increasing the local chlorine concentration above one monolayer.

The interaction of H2 with the Si(111)-(7 × 7) surface is investigated by means of density functional slab calculations on a (4 × 2) reconstructed model surface. A viable mechanism for β1 desorption is identified, which involves thermally activated SiH2 units at adatom sites. This mechanism leads to adsorption and desorption barriers of 1.0 and 2.4 eV, respectively, in agreement with experiment. An explanation for the two components observed in the β1 peak of temperature-programmed desorption spectra is proposed. The lattice deformation energy at the transition state for desorption is large (not, vert, similar0.6 eV). If we assume that this remains in the surface after H2 desorption, the low translational energy of desorbing molecules which has been observed experimentally can in part be explained.

We present first principle total energy calculations of carbon incorporation in the Si(100) surface in the presence of silicon ad-dimers either parallel or perpendicular to the dimer rows of the clean surface. By modifying the local stress, the ad-dimers are found to make the carbon penetration in the subsurface layers easier. At low coverages (thetasles0.125), carbon atoms are preferentially adsorbed in the third layer directly below an ad-dimer, and the barrier at the crossing of the second layer on the clean surface is strongly reduced or completely vanishes. At higher coverage (0.125lesthetasles0.25), configurations with part of the carbons at the surface and part in the third layer are favored. These tendencies are enhanced with increasing ad-dimer density

Certain bacterial enzymes, the diiron hydrogenases, have turnover numbers for hydrogen production from water as large as 10 4 /s. Their much smaller common active site, composed of earth-abundant materials, has a structure that is an attractive starting point for the design of a practical catalyst for electrocatalytic or solar photocatalytic hydrogen production from water. In earlier work, our group has reported the computational design of [FeFe] P /FeS 2 , a hydrogenase-inspired catalyst/electrode complex, which is efficient and stable throughout the production cycle. However, the diiron hydrogenases are highly sensitive to ambient oxygen by a mechanism not yet understood in detail. An issue critical for practical use of [FeFe] P /FeS 2 is whether this catalyst/electrode complex is tolerant to the ambient oxygen. We report demonstration by ab initio simulations that the complex is indeed tolerant to dissolved oxygen over timescales long enough for practical application, reducing it efficiently. This promising hydrogen-producing catalyst, composed of earth-abundant materials and with a diffusion-limited rate in acidified water, is efficient as well as oxygen tolerant.

We use first principles calculations to investigate the adsorption and incorporation of nonmetal impurities (N, C) at the anatase ${\mathrm{TiO}}_{2}(001)\mathrm{\text{\ensuremath{-}}}(1\ifmmode\times\else\texttimes\fi{}4)$ reconstructed surface. We analyze in detail the influence of the surface structure and local strain on the impurity binding sites and incorporation pathways and identify important intermediates that facilitate impurity incorporation. We find various subsurface interstitial binding sites and corresponding surface $\ensuremath{\rightarrow}$ subsurface penetration pathways on the reconstructed surface. This surface also favors the presence of subsurface oxygen vacancies, to which adsorbed species can migrate to form substitutional impurities. Most notably, we show that the nonexposed oxygen sites just below the surface have a key role in the incorporation of nitrogen and carbon in ${\mathrm{TiO}}_{2}(001)$.

We present density functional calculations of the bonding structures and diffusion barriers for a Cl adatom on Si(001)-(2×1). Besides the stable adsorption site at the dangling bond (DB), a metastable bridge-bonded state breaking a surface dimer bond and ~1.1 eV higher in energy than the DB is found. The calculated properties of this state agree with recent ESDIAD and HREELS observations. This bridge-bonded site is not along the Cl intradimer diffusion pathway of lowest energy. For this path a transition state also having a bridging structure (but not breaking the dimer bond) and rather low in energy ( ~0.6 eV with respect to the DB) is determined. The low intradimer barrier is consistent with the facile switching of Cl recently observed in scanning tunneling microscopy experiments.

We present first-principle total-energy calculations of carbon incorporation in the hydrogen-saturated Si( 100)2H-1 X I surface. A large variety of atomic configurations involving one, two, and four substitional C impurities in surface and subsurface sites is considered. While carbon incorporation in the surface layer is energetically favored, C-C interactions play an important role and can stabilize configurations that would be otherwise unlikely, particularly configurations with carbon atoms in the 5th layer, as observed in recent x-ray photoelectron-spectroscopy experiments.

The burning rate of the monopropellant nitromethane (NM) has been observed to increase by adding and dispersing small amounts of functionalized graphene sheets (FGSs) in liquid NM. Until now, no plausible mechanisms for FGSs acting as combustion catalysts have been presented. Here, we report ab initio molecular dynamics simulations showing that carbon vacancy defects within the plane of the FGSs, functionalized with oxygen-containing groups, greatly accelerate the thermal decomposition of NM and its derivatives. This occurs through reaction pathways involving the exchange of protons or oxygens between the oxygen-containing functional groups and NM and its derivatives. FGS initiates and promotes the decomposition of the monopropellant and its derivatives, ultimately forming H(2)O, CO(2), and N(2). Concomitantly, oxygen-containing functional groups on the FGSs are consumed and regenerated without significantly changing the FGSs in accordance with experiments indicating that the FGSs are not consumed during combustion.

Anatase is a metastable polymorph of TiO$_2$. In contrast to the more widely-studied TiO$_2$ rutile, O vacancies (V$_\mathrm O$'s) are not stable at the anatase (101) surface. Low-temperature STM shows that surface V$_\mathrm O$'s, created by electron bombardment at 105 K, start migrating to subsurface sites at temperatures $\geq$ 200 K. After an initial decrease of the V$_\mathrm O$ density, a temperature-dependent dynamic equilibrium is established where V$_\mathrm O$'s move to subsurface sites and back again, as seen in time-lapse STM images. We estimate that activation energies for subsurface migration lie between 0.6 and 1.2 eV; in comparison, DFT calculations predict a barrier of ca. 0.75 eV. The wide scatter of the experimental values might be attributed to inhomogeneously-distributed subsurface defects in the reduced sample.

The interaction of atomic hydrogen with the majority (101) surface of anatase TiO(2) is studied using density functional theory calculations both with a standard semi-local functional and with the inclusion of on-site Coulomb repulsion terms. We investigate the energetics of different adsorption configurations at surface and subsurface sites and different coverages, from low to one monolayer, as well as diffusion pathways among the different sites and recombinative H(2) desorption barriers. While H(2) desorption is the energetically most favorable process, the diffusion of H into the subsurface is found to be at least equally favorable kinetically. It is further shown that subsurface oxygen vacancies on reduced anatase are favorable adsorption sites for hydrogen atoms.