Cristiana Di Valentin

Rational Band Gap Engineering of WO3 Photocatalyst for Visible light Water Splitting

Owing to its optical, chemical and electronic properties, tungsten oxide (WO3) has direct applications in catalysis, sensing, electroor photo-chromic devices, photocatalysis, etc. Photoelectrochemical water splitting is obtained by using WO3 as a component in Z-Scheme systems devoted to O2 evolution. A co-catalyst (commonly Pt) improves the efficiency of such systems. The system works under irradiation with l= 450 nm visible (vis) light thanks to the band gap of WO3, which is smaller than 3.0 eV. The main drawback is that WO3 is inactive as regards H2 evolution, since the photoexcited electrons do not have the right potential required to reduce protons to hydrogen. Indeed, the bottom of the conduction band (CB) is too low with respect to the H/H2 redox potential. If this limitation could be removed, WO3 would become an ideal vis-light photocatalyst for water splitting, eventually in combination with a co-catalyst. The real challenge is to find a way to upshift both the bottom of the CB and the top of the valence band (VB), so as to keep the band gap essentially unchanged for vis-light activity. Band-gap engineering is possible in oxides via doping or nanostructuring. Sometimes doping results in localized states in the gap rather than in shifts of band edges, with negative consequences on the photocatalytic activity, which can be attributed to the occurrence of recombination processes. In these cases, the formation of localized impurity states can be avoided with relatively high dopant concentrations (e.g. 10 %). Electronic structure theory provides a useful complement to experiments for the design of photoactive materials. Ideally, one would like to identify a single dopant, which, once incorporated in WO3, can induce the desired upward shifts of both VB and CB edges. Starting from simple concepts corroborated by DFT calculations based on hybrid functionals, we have found that Hf substituting W atoms in the g-monoclinic room temperature WO3 phase at concentrations of 12 % can induce the desired effects. Despite several attempts to improve the photoactivity of WO3 by selective doping, [5–12] we are not aware of any report on successful doping by Hf. We have identified the main factors controlling the electronic structure of the material and we give a rational of the doping effect, thus providing a useful conceptual framework to direct the synthesis of new WO3-based photocatalysts. We will show in particular that the anisotropic structural properties of g-monoclinic WO3 play an important role in determining the response of the material to structural and electronic modifications. The g-monoclinic structure of WO3 results from a slight distortion of the cubic structure, with the corner-sharing WO6 octahedra tilted, as shown in Figure 1 a. One can look at it as a 2 2 2 superstructure based on an idealized cubic unit cell, or, alternatively as consisting of pseudo-one-dimensional weakly interacting -W-O-Wchains. The computed direct band gap is 3.10 eV; experimentally, values in the range 2.6–3.0 eV have been reported. The upper part of the VB is mainly derived from the O 2p states. The bottom of the CB is mainly composed of W 5d states partly mixed in with O 2p orbitals. Owing to a distorted-octahedral coordination, the W 5d states split into t2g and eg-like components (see Figure 2). As the W O bond lengths are asymmetric along both y and z directions while are nearly symmetric along the x direction, the lower energy t2g states further split into dyz and dxz/dxy components. Thus, it can be expected that a distortion of the WO6 octahe-

Spectroscopic Properties of Doped and Defective Semiconducting Oxides from Hybrid Density Functional Calculations

CONSPECTUS: Very rarely do researchers use metal oxides in their pure and fully stoichiometric form. In most of the countless applications of these compounds, ranging from catalysis to electronic devices, metal oxides are either doped or defective because the most interesting chemical, electronic, optical, and magnetic properties arise when foreign components or defects are introduced in the lattice. Similarly, many metal oxides are diamagnetic materials and do not show a response to specific spectroscopies such as electron paramagnetic resonance (EPR) spectroscopy. However, doped or defective oxides may exhibit an interesting and informative paramagnetic behavior. Doped and defective metal oxides offer an expanding range of applications in contemporary condensed matter science; therefore researchers have devoted enormous effort to the understanding their physical and chemical properties. The interplay between experiment and computation is particularly useful in this field, and contemporary simulation techniques have achieved high accuracies with these materials. In this Account, we show how the direct comparison between spectroscopic experimental and computational data for some selected and relevant materials provides ways to understand and control these complex systems. We focus on the EPR properties and electronic transitions that arise from the presence of dopants and defects in bulk metal oxide materials. We analyze and compare the effect of nitrogen doping in TiO2 and ZnO (two semiconducting oxides) and MgO (a wide gap insulator) and examine the effect of oxygen deficiency in the semiconducting properties of TiO2-x, ZnO1-x, and WO3-x materials. We chose these systems because of their relevance in applications including photocatalysis, touch screens, electrodes in magnetic random access memories, and smart glasses. Density functional theory (DFT) provides the general computational framework used to illustrate the electronic structure of these systems. However, for a more accurate description of the oxide band gap and of the electron localization of the impurity states associated with dopants or defects, we resorted to the use of hybrid functionals (B3LYP), where a portion of exact exchange in the exchange-correlation functional partly corrects for the self-interaction error inherent in DFT. In many cases, the self-interaction correction is very important, and these results can lead to a completely different physical picture than that obtained using local or semilocal functionals. We analyzed the electronic transitions in terms of their transition energy levels, which provided a more accurate comparison with experimental spectroscopic data than Kohn-Sham eigenvalues. The effects of N-doping were similar among the three oxides that we considered. The nature of the impurity state is always localized at the dopant site, which may limit their application in photocatalytic processes. Photocatalytic systems require highly delocalized photoexcited carriers within the material to effectively trigger redox processes at the surface. The nature of the electronic states associated with the oxygen deficiency differed widely in the three investigated oxides. In ZnO1-x and WO3-x the electronic states resemble the typical F-centers in insulating oxides or halides, with the excess electron density localized at the vacancy site. However, TiO2 acts as a reducible oxide, and the removal of neutral oxygen atoms reduced Ti(4+) to Ti(3+).

Transition levels of defect centers in ZnO by hybrid functionals and localized basis set approach.

A hybrid density functional study based on a periodic approach with localized atomic orbital basis functions has been performed in order to compute the optical and thermodynamic transition levels between different charge states of defect impurities in bulk ZnO. The theoretical approach presented allows the accurate computation of transition levels starting from single particle Kohn-Sham eigenvalues. The results are compared to previous theoretical findings and with available experimental data for a variety of defects ranging from oxygen vacancies, zinc interstitials, and hydrogen and nitrogen impurities. We find that H and Zn impurities give rise to shallow levels; the oxygen vacancy is stable only in the neutral V(O) and doubly charged V(O) (2+) variants, while N-dopants act as deep acceptor levels.

Electronic structure and phase stability of oxide semiconductors: Performance of dielectric-dependent hybrid functional DFT, benchmarked against GW band structure calculations and experiments

We investigate band gaps, equilibrium structures, and phase stabilities of several bulk polymorphs of wide-gap oxide semiconductors ZnO, TiO2,ZrO2, and WO3. We are particularly concerned with assessing the performance of hybrid functionals built with the fraction of Hartree-Fock exact exchange obtained from the computed electronic dielectric constant of the material. We provide comparison with more standard density-functional theory and GW methods. We finally analyze the chemical reduction of TiO2 into Ti2O3, involving a change in oxide stoichiometry. We show that the dielectric-dependent hybrid functional is generally good at reproducing both ground-state (lattice constants, phase stability sequences, and reaction energies) and excited-state (photoemission gaps) properties within a single, fully ab initio framework.

Cerium-Doped Zirconium Dioxide, a Visible-Light-Sensitive Photoactive Material of Third Generation

The dispersion of small amounts of Ce(4+) ions in the bulk of ZrO2 leads to a photoactive material sensitive to visible light. This is shown by monitoring with EPR the formation and the reactivity of photogenerated (λ > 420 nm) charge carriers. The effect, as confirmed by DFT calculations, is due to the presence in the solid of empty 4f Ce states at the mid gap, which act as intermediate levels in a double excitation mechanism. This solid can be considered an example of a third-generation photoactive material.

Tungsten Oxide in Catalysis and Photocatalysis: Hints from DFT

Despite the importance of tungsten oxide in various areas of materials science including catalysis and photocatalysis, relatively few systematic theoretical studies have been devoted to this system. In this review we report the results of first principle density functional theory calculations based on a hybrid functional that properly reproduces the band gap and other fundamental properties of WO3. We briefly describe the dependence of the band gap on the crystalline phase of WO3. Then, we address the nature of defects and dopants in bulk WO3. As WO3 can be easily reduced to WO3−x, we first discuss the nature of isolated O vacancies showing that three different situations arise from the removal of one O atom along each of the three crystallographic directions of RT monoclinic WO3. The data provide insight into the origin of electrochromism of this material. Then we discuss the role doping of WO3 with substitutional atoms in order to increase the activity for water splitting and we show that Hf is a promising dopant. The redox properties of WO3 are discussed also in relation to H2 adsorption on the WO3(001) surface. Finally, the role of nanostructuring is analyzed by studying the properties of (WO3)3 cyclic clusters deposited on the rutile TiO2(110) surface. Charge transfers at the (WO3)3/TiO2 interface and their role on the activity of this heterogeneous catalyst are discussed.

Bonding of NO to NiO(100) and NixMg1−xO(100) surfaces: A challenge for theory

The NO/NiO(100) system represents an excellent test case for the theory of surface chemical bond since accurate information about geometry, adsorption strength, and spin properties is available from experiments performed on NiO and Ni-doped MgO powders, single crystals, and thin films. We used cluster models to describe the NO/NiO interaction in combination with density functional theory (DFT) and wave function-based methods. We have identified four major aspects of the interaction: (1) the bonding cannot be described by a single determinant; (2) a spin-polarized DF-B3LYP approach gives reasonable adsorption properties at the price of a physically incorrect spin distribution; (3) a key ingredient of the interaction is the Coulomb repulsion within the Ni 3d shell; since this term is described very differently depending on the exchange-correlation functional it can result in overbound generalized gradient approach or Becke, Lee, Yang, and Parr or in strongly unbound (HFLYP) systems depending on the DFT appr...

A route toward the generation of thermally stable Au cluster anions supported on the MgO surface

On the basis of experimental evidence and DFT calculations, we propose a simple yet viable way to stabilize and chemically activate gold nanoclusters on MgO. First the MgO surface is functionalized by creation of trapped electrons, (H (+))(e (-)) centers (exposure to atomic H or to H 2 under UV light, deposition of low amounts of alkali metals on partially hydroxylated surfaces, etc.); the second step consists in the self-aggregation of gold clusters deposited from the gas phase. The calculations show that the (H (+))(e (-)) centers act both as nucleation and activation sites. The process can lead to thermally stable gold cluster anions whose catalytic activity is enhanced by the presence of an excess electron.

Copper impurities in bulk ZnO: A hybrid density functional study

Transition metal doping of ZnO is considered as a promising way to obtain a diluted magnetic semiconducting oxide. In this work we investigate copper doping of ZnO by means of density functional theory, using a hybrid exchange-correlation functional and a periodic approach with localized atomic basis functions. Isolated copper species, such as copper substitutional to zinc, Cu(s), and Cu interstitial, Cu(i), are analyzed in terms of transition energy levels and hyperfine coupling constants with reference to available spectroscopic data. We also examine the potential magnetic interaction between copper species, their interaction with oxygen vacancies, and the possibility of copper clustering. The relative stability of the various copper impurities considered in this study is finally compared on the basis of their formation energy at different oxygen chemical potentials and Fermi level values.

Hydration structure of the Ti(III) cation as revealed by pulse EPR and DFT studies: new insights into a textbook case.

The (17)O and (1)H hyperfine interactions of water ligands in the Ti(III) aquo complex in a frozen solution were determined using Hyperfine Sublevel Correlation (HYSCORE) and Pulse Electron Nuclear Double Resonance (ENDOR) spectroscopies at 9.5 GHz. The isotropic hyperfine interaction (hfi) constant of the water ligand (17)O was found to be about 7.5 MHz. (1)H Single Matched Resonance Transfer (SMART) HYSCORE spectra allowed resolution of the hfi interactions of the two inequivalent water ligand protons and the relative orientations of their hfi tensors. The magnetic and geometrical parameters extracted from the experiments were compared with the results of DFT computations for different geometrical arrangements of the water ligands around the cation. The theoretical observable properties (g tensor (1)H and (17)O hfi tensors and their orientations) of the [Ti(H(2)O)(6)](3+) complex are in quantitative agreement with the experiments for two slightly different geometrical arrangements associated with D(3d) and C(i) symmetries.