Surface Structure and Reactivity of Anatase TiO₂ Crystals with Dominant {001} Facets

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

Water Adsorption and Oxidation at the Co₃O₄ (110) Surface

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 termination, which stabilizes the O* species and thus reduces the overpotential

Mosaic Texture and Double <i>c</i>‑Axis Periodicity of β‑NiOOH: Insights from First-Principles and Genetic Algorithm Calculations

Fe-doped NiO_<i>x</i> 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 NiO_<i>x</i>. Here, we report a theoretical study of the structure of β-NiOOH, one of the active components of NiO_<i>x</i>. 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 <i>c</i> 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

Theoretical Study of Interfacial Electron Transfer from Reduced Anatase TiO₂(101) to Adsorbed O₂

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^– → *O₂^2–, 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

Pathway of Photocatalytic Oxygen Evolution on Aqueous TiO₂ Anatase and Insights into the Different Activities of Anatase and Rutile

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 ((O₂^2–)_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 (O₂^2–)_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

Mechanism and Activity of Water Oxidation on Selected Surfaces of Pure and Fe-Doped NiO_<i>x</i>

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 NiO_<i>x</i> when it is doped with Fe

Bulk and Surface Polarons in Photoexcited Anatase TiO₂

Using hybrid functional electronic structure calculations, we have investigated the structure and energetics of photogenerated electrons and holes in the bulk and at the (101) surface of anatase TiO₂. Excitons formed upon UV irradiation are found to become self-trapped, consistent with the observation of temperature-dependent Urbach tails in the absorption spectrum and a large Stokes shift in the photoluminescence band of anatase. Electron and hole polarons are localized at Ti³⁺ and O^– lattice sites, respectively. At the surface, the trapping sites generally correspond to undercoordinated Ti³⁺_5c and O^–_2c surface atoms or to isolated OH species in the case of a hydroxylated surface. The polaron trapping energy is considerably larger at the surface than in the bulk, indicating that it is energetically favorable for the polarons to travel from the bulk to the surface. Computed one-electron energy levels in the gap and hyperfine coupling constants compare favorably with oxidation potential and EPR measurements

Solvent Effects on the Adsorption Geometry and Electronic Structure of Dye-Sensitized TiO₂: A First-Principles Investigation

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. <i>J. Phys. Chem.</i> <b>1993</b>, <i>97</i>, 1426–1430) and suggests a relevant role of the solvent in determining the dye–semiconductor interaction and electronic coupling

Ab Initio Simulation of the Absorption Spectra of Photoexcited Carriers in TiO₂ Nanoparticles

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

Formation, Electronic Structure, and Defects of Ni Substituted Spinel Cobalt Oxide: a DFT+U Study

Nickel substituted spinel cobalt oxide is a promising technological material with complex electronic and magnetic structures. Understanding these structures is important for improving the material’s performance in various applications. We have carried out first-principles calculations on the formation, electronic properties, and defects of bulk NiCo₂O₄ using density functional theory (DFT) with on-site Hubbard U terms on the transition metal d states. Analysis of the electronic structure of Ni_<i>x</i>Co_3‑<i>x</i>O₄ as a function of <i>x</i> = 0–1 shows that Ni acts as a p-type dopant in Co₃O₄, gradually transforming the minority spin channel from insulating to conducting. As a result, the inverse spinel NiCo₂O₄ (NCO) is found to have a ferrimagnetic half-metallic ground state with fractional valence on Ni and Co cations at tetrahedral sites (Td), in agreement with experimental observations. Projected densities of states confirm that the states around the Fermi energy originate from Ni and Co­(Td) 3d states hybridized with oxygen 2p orbitals. The influence of two common defects, Ni ↔ Co­(Td) exchanges and oxygen vacancies, on the structural and electronic properties has been also investigated. Our results are consistent with the experimental observation that intermediate structures between inverse spinel and normal spinel occur frequently in NCO. Oxygen vacancies are predicted to occur more frequently at sites coordinated to a larger number of Ni ions and found to have only minor effects on the conductivity and magnetic structure