Scott M. Woodley

Band alignment of rutile and anatase TiO2

The most widely used oxide for photocatalytic applications owing to its low cost and high activity is TiO₂. The discovery of the photolysis of water on the surface of TiO₂ in 1972 launched four decades of intensive research into the underlying chemical and physical processes involved. Despite much collected evidence, a thoroughly convincing explanation of why mixed-phase samples of anatase and rutile outperform the individual polymorphs has remained elusive. One long-standing controversy is the energetic alignment of the band edges of the rutile and anatase polymorphs of TiO₂ (ref. ). We demonstrate, through a combination of state-of-the-art materials simulation techniques and X-ray photoemission experiments, that a type-II, staggered, band alignment of ~ 0.4 eV exists between anatase and rutile with anatase possessing the higher electron affinity, or work function. Our results help to explain the robust separation of photoexcited charge carriers between the two phases and highlight a route to improved photocatalysts.

Crystal structure prediction from first principles

The prediction of structure at the atomic level is one of the most fundamental challenges in condensed matter science. Here we survey the current status of the field and consider recent developments in methodology, paying particular attention to approaches for surveying energy landscapes. We illustrate the current state of the art in this field with topical applications to inorganic, especially microporous solids, and to molecular crystals; we also look at applications to nanoparticulate structures. Finally, we consider future directions and challenges in the field.

The prediction of inorganic crystal structures using a genetic algorithm and energy minimisation

A genetic algorithm has been used to generate plausible crystal structures from the knowledge of only the unit cell dimensions and constituent elements. We successfully generate 38 known binary oxides and various known ternary oxides with the Perovskite, Pyrochlore and Spinel structures, from starting configurations which include no knowledge of the atomic arrangement in the unit cell. The quality of the structures is initially assessed using a cost function which is based on the bond valence model with a number of refinements. The lattice energy, based on the Born model of a solid, is minimised using a local optimiser for the more plausible candidate structures. The method has been implemented within the computational package GULP. An extensive collection of Buckingham potential parameters for use in such simulations on metal oxides is also tabulated.

Advances in computational studies of energy materials

We review recent developments and applications of computational modelling techniques in the field of materials for energy technologies including hydrogen production and storage, energy storage and conversion, and light absorption and emission. In addition, we present new work on an Sn2TiO4 photocatalyst containing an Sn(II) lone pair, new interatomic potential models for SrTiO3 and GaN, an exploration of defects in the kesterite/stannite-structured solar cell absorber Cu2ZnSnS4, and report details of the incorporation of hydrogen into Ag2O and Cu2O. Special attention is paid to the modelling of nanostructured systems, including ceria (CeO2, mixed CexOy and Ce2O3) and group 13 sesquioxides. We consider applications based on both interatomic potential and electronic structure methodologies; and we illustrate the increasingly quantitative and predictive nature of modelling in this field.

Zinc oxide: A case study in contemporary computational solid state chemistry

Computational techniques have been applied to study a broad range of chemical and physical properties of zinc oxide. Both interatomic‐potential and density functional theory methods are used to investigate structural, thermodynamic, surface, and defect properties. We survey the structures and energies of nano‐particulate zinc oxide.

Properties of small TiO2, ZrO2 and HfO2 nanoparticles

Ground state structures and energies are predicted for (MO2)n, where n = 1 to 8 and M is one of three isovalent cations, titanium(IV), zirconium(IV) and hafnium(IV), with the minimised binding energies calculated using Density Functional Theory. An increased number of single coordinated oxygen atoms, as opposed to more densely packed configurations, were found for the titania ground state clusters n = 5 and 7. We present, as a function of n, calculated nucleation energies, the energies of the respective highest occupied and lowest unoccupied molecular orbitals, harmonic frequencies of vibration and infrared spectra for all three compounds. Similarities and differences in the data produced for the three oxides are considered. Calculations were performed using the GAMESS-UK software on HPCx (phase 2a); aspects of the computational procedures are discussed.

Sonic bands, bandgaps, and defect states in layered structures—Theory and experiment

The propagation of sound through a one‐dimensional periodic array of water and perspex plates is studied theoretically and experimentally. It is shown that the passbands and stop bands of a scatterer with a finite number of layers correspond to the bands and bandgaps of an infinite ‘‘sonic bandgap crystal.’’ The transmission coefficient of various finite structures is computed and measured as a function of frequency. The analogy with the electronic bandstructure of crystals, and the photonic bandstructure of macroscopic periodic dielectric structures, is found to be a close one. It is shown that the position and width of passbands can easily be engineered. Results are included for a finite ‘‘crystal’’ with a vacancy defect, in which a narrow passband appears in each of the stop bands.

Experimental and computational studies of ZnS nanostructures

We review the experimental and computational studies of nanoparticulate ZnS, a system that has received much attention recently. We describe in detail how the nanoparticle structures evolve with increasing size. The results of the computational studies reveal intriguing families of structures based on spheroids, which have the greater stability for clusters with less than 50 ZnS pairs. More complex structures are predicted for larger systems, such as double bubbles, BCT nanoparticles and nanotubes.

Evolutionary structure prediction and electronic properties of indium oxide nanoclusters

Indium sesquioxide is widely used as a transparent conducting oxide in modern optoelectronic devices; the rising cost of indium has generated interest in the nanoscale properties of In(2)O(3), and questions arise as to the nature of its physicochemical properties below the bulk regime. We report the stable and metastable stoichiometric clusters of (In(2)O(3))(n), where n = 1-10, as predicted from an evolutionary search within the classical interatomic potential and quantum density functional energy landscapes. In contrast to the paradigm set by ZnO, which favours high symmetry bubble-like structures, the In(2)O(3) nanoclusters are found to tend towards dense, low symmetry structures approaching the bulk system at remarkably small molecular masses. Electronic characterisation is performed at the hybrid density functional and many-body GW levels to obtain accurate predictions of the spectroscopic properties, with mean values of the ionisation potentials and electron affinities calculated as 7.7 and 1.7 eV, respectively.

Modeling Excited States in TiO2 Nanoparticles: On the Accuracy of a TD-DFT Based Description.

We have investigated the suitability of Time-Dependent Density Functional Theory (TD-DFT) to describe vertical low-energy excitations in naked and hydrated titanium dioxide nanoparticles. Specifically, we compared TD-DFT results obtained using different exchange-correlation (XC) potentials with those calculated using Equation-of-Motion Coupled Cluster (EOM-CC) quantum chemistry methods. We demonstrate that TD-DFT calculations with commonly used XC potentials (e.g., B3LYP) and EOM-CC methods give qualitatively similar results for most TiO2 nanoparticles investigated. More importantly, however, we also show that, for a significant subset of structures, TD-DFT gives qualitatively different results depending upon the XC potential used and that only TD-CAM-B3LYP and TD-BHLYP calculations yield results that are consistent with those obtained using EOM-CC theory. Moreover, we demonstrate that the discrepancies for such structures originate from a particular combination of defects that give rise to charge-transfer excitations, which are poorly described by XC potentials that do not contain sufficient Hartree–Fock like exchange. Finally, we consider that such defects are readily healed in the presence of ubiquitously present water and that, as a result, the description of vertical low-energy excitations for hydrated TiO2 nanoparticles is nonproblematic.