The photocatalytic oxidation of water to molecular oxygen is a key step toward the conversion of solar energy to fuels. Understanding the detailed mechanism and kinetics of this reaction is important for the development of robust catalysts with improved efficiency. TiO2 is one of the best-known photocatalysts as well as a model system for the study of the oxygen evolution reaction (OER). Here we use hybrid density functional based energetic calculations and first-principles molecular dynamics simulations to investigate the pathway and kinetics of the OER on the majority (101) surface of anatase TiO2 in a water environment. Our results show that terminal Ti–OH groups are stable intermediates at the aqueous (101) interface, in accord with the experimental observation that OH radicals are efficiently produced on anatase. Oxidation of Ti–OH gives rise to a second stable intermediate, a surface-bridging peroxo dimer ((O22–)br) composed of one water and one surface lattice oxygen atom, consistent with the surface peroxo intermediates revealed by “in situ” measurements on rutile. Our calculations further predict that molecular oxygen evolves directly from (O22–)br through a concerted two-electron transfer, thus leading to oxygen exchange between TiO2 and the adsorbed species. Oxygen exchange is found to be negligible on rutile, so that different OER pathways are likely to be operative on the two main TiO2 polymorphs. This difference could explain the observed lower OER activity of anatase relative to rutile.Keywords: activation energy; density functional theory; oxygen exchange; photocatalysis; titanium dioxide; water oxidation

We investigate the absorption spectra of photoexcited carriers in a prototypical anatase TiO2 nanoparticle using hybrid time dependent density functional theory calculations in water solution. Our results agree well with experimental transient absorption spectroscopy data and shed light on the character of the transitions. The trapped state is always involved, so that the SOMO/SUMO is the initial/final state for the photoexcited electron/hole absorption. For a trapped electron, final states in the low energy tail of the conduction band correspond to optical transitions in the IR, while final states at higher energy correspond to optical transitions in the visible. For a trapped hole, the absorption band is slightly blue-shifted and narrower in comparison to that of the electron, consistent with its deeper energy level in the band gap. Our calculations also show that electrons in shallow traps exhibit a broad absorption in the IR, resembling the feature attributed to conductive electrons in experimental spectra.

In reduced TiO2, electronic transitions originating from the Ti3+-induced states in the band gap are known to contribute to the photoabsorption, being in fact responsible for the material’s blue color, but the excited states accessed by these transitions have not been characterized in detail. In this work we investigate the excited state electronic structure of the prototypical rutile TiO2(110) surface using two-photon photoemission spectroscopy (2PPE) and density functional theory (DFT) calculations. Using 2PPE, an excited resonant state derived from Ti3+ species is identified at 2.5 ± 0.2 eV above the Fermi level (EF) on both the reduced and hydroxylated surfaces. DFT calculations reveal that this excited state is closely related to the gap state at ∼1.0 eV below EF, as they both result from the Jahn–Teller induced splitting of the 3d orbitals of Ti3+ ions in reduced TiO2. Localized excitation of Ti3+ ions via 3d → 3d transitions from the gap state to this empty resonant state significantly increases the TiO2 photoabsorption and extends the absorbance to the visible region, consistent with the observed enhancement of the visible light induced photocatalytic activity of TiO2 through Ti3+ self-doping. Our work reveals the physical origin of the Ti3+ related photoabsorption and visible light photocatalytic activity in prototypical TiO2 and also paves the way for the investigation of the electronic structure and photoabsorption of other metal oxides.

The (110) surface of tricobalt tetraoxide, Co3O4(110), has attracted considerable interest because of its high catalytic activity, especially for CO oxidation. However, understanding of its surface structure and reactivity under relevant experimental conditions remains limited. Here, we use density functional theory with the on-site Coulomb U term to study the structure and stability of the two possible truncations of Co3O4(110) in the presence of oxygen gas and water vapor. We examine the effects of U on the stability diagram by considering three representative U values often used in previous studies, notably U = 0, 3.0, and 5.9 eV. For all U values, the hydrated B surface, exposing only octahedral Co ions, is predicted to be the thermodynamically stable termination under ambient conditions and at low temperatures. In other situations, the predicted stability generally depends on U with smaller (larger) U values favoring the B (A) termination. By combining our results with those of previous studies, we conclude that U = 3.0 eV provides a better overall description of the electronic structure and surface reactivity, whereas U = 5.9 eV is better suited for description of the magnetic properties.

Infrared spectroscopy in combination with density functional theory calculations has been widely used to characterize the structure of graphene oxide and its reduced forms. Yet, the synergistic effects of different functional groups, lattice defects, and edges on the vibrational spectra are not well understood. Here, we report first-principles calculations of the infrared spectra of graphene oxide performed on realistic, thermally equilibrated, structural models that incorporate lattice vacancies and edges along with various oxygen-containing functional groups. Models including adsorbed water are examined as well. Our results show that lattice vacancies lead to important blue and red shifts in the OH stretching and bending bands, respectively, whereas the presence of adsorbed water leaves these shifts largely unaffected. We also find unique infrared features for edge carboxyls resulting from interactions with both nearby functional groups and the graphene lattice. Comparison of the computed vibrational properties to our experiments clarifies the origin of several observed features and provides evidence that defects and edges are essential for characterizing and interpreting the infrared spectrum of graphene oxide.

The 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 material from an n-type semiconductor to a poor electronic conductor.In this Account, we present an overview of recent theoretical work on O2 adsorption and reactions on the reduced anatase (101) surface. Anatase is the TiO2 polymorph that is generally considered most active in photocatalysis. Experiments on anatase powders have shown that the properties of photoexcited electrons are similar to those of excess electrons from reducing defects, and therefore, oxygen on reduced anatase is also a model system for studying the role of O2 in photocatalysis. Experimentally, the characteristic Ti3+ defect states disappear after adsorption of molecular oxygen, which indicates that the excess electrons are indeed trapped by O2. Moreover, superoxide surface species associated with two different cation surface sites, possibly a regular cation site and a cation close to an anion vacancy, were identified by electron paramagnetic resonance spectroscopy. On the theoretical side, however, density functional theory studies have consistently found that it is energetically more favorable for O2 to adsorb in the peroxo form rather than the superoxo form. As a result, obtaining a detailed understanding of the nature of the observed superoxide species has proven difficult for many years.On reduced anatase (101), both oxygen vacancies and Ti interstitials have been shown to reside exclusively in the susbsurface. We discuss how reaction of O2 with a subsurface O vacancy heals the vacancy while leading to the formation of a surface bridging dimer defect. Similarly, the interaction of O2 with a Ti interstitial causes migration of this defect to the surface and the formation of a surface TiO2 cluster. Finally, we analyze the peroxo and superoxo states of the adsorbed molecule. On the basis of periodic hybrid functional calculations of interfacial electron transfer between reduced anatase and O2, we show that the peroxide form, while energetically more stable, is kinetically less favorable than the superoxide form. The existence of a kinetic barrier between the superoxo and peroxo states is essential for explaining a variety of experimental observations.

Mixed nickel–iron oxides have recently emerged as promising electrocatalysts for water oxidation because of their low cost and high activity, but the composition and structure of the catalyst’s active phase under working conditions are not yet fully established. We present here density functional theory calculations with on-site Coulomb repulsion of the energetics of the oxygen evolution reaction (OER) on selected surfaces of pure and mixed Ni–Fe oxides that are possible candidates for the catalyst’s active phase. The investigated surfaces are pure β-NiOOH(011̅5) and γ-NiOOH(101), Fe-doped β-NiOOH(011̅5) and γ-NiOOH(101), NiFe2O4(001), and Fe3O4(001). We find that Fe-doped β-NiOOH(011̅5) has by far the lowest overpotential (η = 0.26 V), followed by NiFe2O4(001) (η = 0.42 V). Our results indicate that Fe-doped β-NiOOH and, to a lesser extent, NiFe2O4 could be the phases responsible for the enhanced OER activity of NiOx when it is doped with Fe.Keywords: active phases; DFT; electrocatalysis; mixed Ni−Fe oxides; water oxidation

Fe-doped NiOx has recently emerged as a promising anode material for the oxygen evolution reaction, but the origin of the high activity is still unclear, due largely to the structural uncertainty of the active phase of NiOx. Here, we report a theoretical study of the structure of β-NiOOH, one of the active components of NiOx. Using a genetic algorithm search of crystal structures combined with dispersion-corrected hybrid density functional theory calculations, we identify two groups of favorable structures: (i) layered structures with alternate Ni(OH)2 and NiO2 layers, consistent with the doubling of the c axis observed in high resolution transmission electron microscopy (TEM) measurements, and (ii) tunnel structures isostructural with MnO2 polymorphs, which can provide a rationale for the mosaic textures observed in TEM. Analysis of the Ni ions oxidation state further indicates a disproportionation of half of the Ni3+ cations to Ni2+/Ni4+ pairs. Hybrid density functionals are found essential for a correct description of the electronic structure of β-NiOOH.Keywords: battery; crystal structure; DFT; genetic algorithm; mosaic texture;

We study the electron transfer from a reduced TiO2 surface to an approaching O2 molecule using periodic hybrid density functional calculations. We find that the formation of an adsorbed superoxo species, *O2–, via the reaction O2(gas) + e– → *O2–, is barrierless, whereas the transfer of another electron to transform the superoxo into an adsorbed peroxide, i.e. *O2– + e– → *O22–, is nonadiabatic and has a barrier of 0.3 eV. The origin of this nonadiabaticity is attributed to the instability of an intermediate where the second electron is localized at the superoxo adsorption site. These results can explain the experimental finding that O2 is not an efficient electron scavenger in photocatalysis.

Hydrofluoric acid (HF)-assisted hydrothermal/solvothermal methods are widely used to synthesize anatase TiO2 single crystals with a high percentage of {001} facets, which are generally considered to be highly reactive. We have used Density Functional Theory calculations and first principles molecular dynamics simulations to investigate the structure of these facets, which is not yet well understood. Our results suggest that (001) surfaces exhibit the bulk-terminated structure when in contact with concentrated HF solutions. However, (1 × 4)-reconstructed surfaces, as observed in UHV, become always more stable at the typical temperatures, 400–600 °C, used to clean the as-prepared crystals in experiments. Since the (1 × 4)-reconstructed surfaces are only weakly reactive, our results predict that synthetic anatase crystals with dominant {001} facets should not exhibit enhanced photocatalytic activity, consistent with recent experimental observations.

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 H2O, CO2, and N2. 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.

The performance of dye-sensitized solar cells (DSSCs) depends significantly on the adsorption geometry of the dye on the semiconductor surface. In turn, the stability and geometry of the adsorbed molecules is influenced by the chemical environment at the electrolyte/dye/TiO2 interface. To gain insight into the effect of the solvent on the adsorption geometries and electronic properties of dye-sensitized TiO2 interfaces, we carried out first-principles calculations on organic dyes and solvent (water or acetonitrile) molecules coadsorbed on the (101) surface of anatase TiO2. Solvent molecules introduce important modifications on the dye adsorption geometry with respect to the geometry calculated in vacuo. In particular, the bonding distance of the dye from the Ti anchoring atoms increases, the adsorption energy decreases, and the two C–O bonds in the carboxylic moieties become more symmetric than in vacuo. Moreover, the adsorbed solvent induces the deprotonation of the dye due to the changing the acid/base properties of the system. Analysis of the electronic structure for the dye-sensitized TiO2 structures in the presence of coadsorbed solvent molecules shows an upward shift in the TiO2 conduction band of 0.2 to 0.5 eV (0.5 to 0.8 eV) in water (acetonitrile). A similar shift is calculated for a solvent monolayer on unsensitized TiO2. The overall picture extracted from our calculations is consistent with an upshift of the conduction band in acetonitrile (2.04 eV vs SCE) relative to water (0.82 eV vs SCE, pH 7), as reported in previous studies on TiO2 flatband potential (Redmond, G.; Fitzmaurice, D. J. Phys. Chem.1993, 97, 1426–1430) and suggests a relevant role of the solvent in determining the dye–semiconductor interaction and electronic coupling.

Anatase TiO2 is a widely used photocatalytic material, and catechol (1,2-benzendiol) is a model organic sensitizer for dye-sensitized solar cells. The growth and the organization of a catecholate monolayer on the anatase (101) surface were investigated with scanning tunneling microscopy and density functional theory calculations. Isolated molecules adsorb preferentially at steps. On anatase terraces, monodentate (‘D1’) and bidentate (‘D2’) conformations are both present in the dilute limit, and frequent interconversions can take place between these two species. A D1 catechol is mobile at room temperature and can explore the most favorable surface adsorption sites, whereas D2 is essentially immobile. When a D1 molecule arrives in proximity of another adsorbed catechol in an adjacent row, it is energetically convenient for them to pair up in nearest-neighbor positions taking a D2–D2 or D2–D1 configuration. This intermolecular interaction, which is largely substrate mediated, causes the formation of one-dimensional catecholate islands that can change in shape but are stable to break-up. The change between D1 and D2 conformations drives both the dynamics and the energetics of this model system and is possibly of importance in the functionalization of dye-sensitized solar cells.

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 interaction between water (H2O) and titanium dioxide (TiO2) has a central role in many environment- and energy-related applications, such as the photodecomposition of organic pollutants, solar cells, and solar-hydrogen production. The importance of these applications has motivated strong interest and intensive experimental and theoretical studies of H2O adsorption on TiO2 surfaces for decades. This review attempts to summarize the major theoretical outcomes on this topic in the last twenty years, ranging from low coverages of adsorbed water molecules up to water multilayers on various TiO2 surfaces. Theoretical/computational methods as well as structural models are discussed and a detailed comparison of the results from various computational settings is presented. The interaction of water with photoexcited TiO2 surfaces is a challenging but very interesting subject for future studies.

Adsorbate-induced band gap states in semiconductors are of particular interest due to the potential of increased light absorption and photoreactivity. A combined theoretical and experimental (STM, photoemission) study of the molecular-scale factors involved in the formation of gap states in TiO2 is presented. Using the organic catechol on rutile TiO2(110) as a model system, it is found that the bonding geometry strongly affects the molecular electronic structure. At saturation catechol forms an ordered 4 × 1 overlayer. This structure is attributed to catechol adsorbed on rows of surface Ti atoms with the molecular plane tilted from the surface normal in an alternating fashion. In the computed lowest-energy structure, one of the two terminal OH groups at each catechol dissociates and the O binds to a surface Ti atom in a monodentate configuration, whereas the other OH group forms an H-bond to the next catechol neighbor. Through proton exchange with the surface, this structure can easily transform into one where both OH groups dissociate and the catechol is bound to two surface Ti in a bidentate configuration. Only bidendate catechol introduces states in the band gap of TiO2.

First principle density functional theory (DFT) calculations are carried out to study fatty (carboxylic) acid molecules adsorbed on Au(111) via their COOH functional group. Focusing on model systems consisting of monolayers of COOHC6H12X molecules with different terminal groups X (X = COOH, SH, and CH3), we examine various properties including the following: the adsorption structure and the corresponding energetics, the bond dipole and charge transfer at the carboxyl−gold interface and their dependence on the monolayer density, the modification of the Au work function induced by the adsorbed monolayer, the voltage-dependent tunneling current through the monolayer (I−V characteristics) in a molecular junction with a single chemical contact, and the electronic density of states. Our calculations predict that the carboxyl−gold bond dipole is large, ∼1 Debye in the case of dense monolayers, and gives rise to a substantial (∼1.2 eV) electrostatic potential energy drop at the interface. This is at variance with the case of alkanethiol monolayers, for which the S−Au interface bond dipole is very small. This difference between thiol and carboxylic acid monolayers leads to different alignments of the molecular energy levels relative to the Fermi energy of the Au(111) surface, and affects both the computed work function modifications and the I−V characteristics.

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 Ti3+ 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.

To explore the possibility that the active center of the di-iron hydrogenases, the [FeFe]H subcluster, can serve by itself as an efficient hydrogen-producing catalyst, we perform comprehensive calculations of the catalytic properties of the subcluster in vacuo using first principles density functional theory. For completeness, we examine all nine possible geometrical isomers of the Fe(II)Fe(I) active-ready state and report in detail on the relevant ones that lead to the production of H2. These calculations, carried out at the generalized gradient approximation level, indicate that the most efficient catalytic site in the isolated [FeFe]H subcluster is the Fed center distal (d) to the [4Fe−4S]H cluster; the other iron center site, the proximal Fep, also considered in this study, has much higher energy barriers. The pathways with the most favorable kinetics (lowest energy barrier to reaction) proceed along configurations with a CO ligand in a bridging position. The most favorable of these CO-bridging pathways start from isomers where the distal CN− ligand is in up position, the vacancy V in down position, and the remaining distal CO is either cis or trans with respect to the proximal CO. These isomers, not observed in the available enzyme X-ray structures, are only marginally less stable than the most stable nonbridging Fed−CO-terminal isomer. Our calculations indicate that this CO-bridging CN-up isomer has a small barrier to production of H2 that is compatible with the observed rate for the enzyme. These results suggest that catalysis of H2 production could proceed on this stereochemically modified [FeFe]H subcluster alone, thus offering a promising target for functional bioinspired catalyst design.

A combined theoretical and experimental approach is used to study water on the TiO2(0 1 1)-(2 × 1) surface. Based on simple proximity arguments dissociative adsorption is expected. Density functional theory and photoemission spectroscopy show that, at low temperatures, a mixed molecular/dissociated water monolayer is stabilized by a H-bonding network. Scanning tunneling microscopy and molecular dynamics simulations provide evidence of a dissociated layer with a preferential non-uniform arrangement of the adsorbates at room temperature.

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 interaction between water (H2O) and titanium dioxide (TiO2) has a central role in many environment- and energy-related applications, such as the photodecomposition of organic pollutants, solar cells, and solar-hydrogen production. The importance of these applications has motivated strong interest and intensive experimental and theoretical studies of H2O adsorption on TiO2 surfaces for decades. This review attempts to summarize the major theoretical outcomes on this topic in the last twenty years, ranging from low coverages of adsorbed water molecules up to water multilayers on various TiO2 surfaces. Theoretical/computational methods as well as structural models are discussed and a detailed comparison of the results from various computational settings is presented. The interaction of water with photoexcited TiO2 surfaces is a challenging but very interesting subject for future studies.