Darren J. Simpson

A first-principles study of the dielectric properties of TiO2 polymorphs

We present an analysis of the dielectric properties of the three polymorphs of TiO₂ (rutile, anatase and brookite phases), using ab initio time-dependent density functional perturbation theory based on the Vignale-Kohn functional. We implement this functional, which incorporates many-body effects, using the periodic program BAND. The improved result for the density of states spectra for brookite is suggestive of increased titanium ion Jahn-Teller effects for this phase. The imaginary and real components of the frequency-dependent dielectric functions show notable dielectric anisotropies, with implications for excitonic interactions, for all three common phases of TiO₂. Comparison of the electron energy-loss spectrum for undoped and doped rutile and anatase reveals the critical role of collective charge excitations in photocatalytic mechanisms. The correlation between plasmon peaks present at lower energies and decreased photocatalytic activity due to substitutional aluminum doping in combination with oxygen vacancies in rutile and anatase is highlighted. Moreover, there is clear correlation between dielectric properties and the microstructure of the TiO₂ polymorphs as suggested via the framework of the Born effective charge and Hirshfeld charge distribution schemes.

Quantum chemical modelling case studies relevant to metal oxide dissolution and catalysis

Over the past 20 years quantum-chemical methods have been developed sufficiently so that they can now be applied to the elucidation of the complex mechanistic processes that occur during metal oxide dissolution and catalysis reactions. Many of the reactions occurring during these processes are not directly accessible to experimental techniques and therefore quantum-chemical modelling can be applied to probe the individual reaction steps involved in the overall mechanism.Quantum chemistry provides the means of calculating the electronic properties of solids (e.g. band structures) structural properties of solids and surfaces (for instance surface relaxation and rumpling) heats of formation and reaction, activation energies, spectroscopic excitation energies and vibrational frequencies.Three case studies are described, which have been chosen to cover a range of quantum chemical applications and methodologies. These case studies are a) the dissolution mechanism of MgO, b) the parameterisation of titanium dioxide for the determination of electronic properties and c) the mechanism and energetics of adsorption of Pd onto rutile. These case studies utilise Hartree-Fock semiempirical andab initio quantum-chemical methods as well as density functional methodologies. A range of model types are used, namely cluster models embedded in pseudo-atoms, 3-dimensional periodic models and 2-dimensional periodic surface models.

The effect of iron and copper impurities on the wettability of sphalerite (110) surface

The effect of impurities in the zinc sulfide mineral sphalerite on surface wettability has been investigated theoretically to shed light on previously reported conflicting results on sphalerite flotation. The effect of iron and copper impurities on the sphalerite (110) surface energy and on the water adsorption energy was calculated with the semi‐empirical method modified symmetrically orthogonalized intermediate neglect of differential overlap (MSINDO) using the cyclic cluster model. The effect of impurities or dopants on surface energies is small but significant. The surface energy increases with increasing surface iron concentration while the opposite effect is reported for increasing copper concentration. The effect on adsorption energies is much more pronounced with water clearly preferring to adsorb on an iron site followed by a zinc site, and copper site least favorable. The theoretical results indicate that a sphalerite (110) surface containing iron is more hydrophilic than the undoped zinc sulfide surface. In agreement with the literature, the surface containing copper (either naturally or by activation) is more hydrophobic than the undoped surface.

SiO2 coated pure and doped titania pigments: low temperature CVD deposition and quantum chemical study

We report the in-flight CVD coating with smooth 1-2 nm thick SiO(2) of pure and doped rutile particles via the oxidation of SiCl(4) vapour introduced in the high temperature zone of a purpose built thermal reactor. The effectiveness of the coatings was determined by a combination of electron microscopy, surface analysis and photocatalytic measurements. No excess Cl was detected on the coated pigment particles indicating the complete oxidation of the SiCl(4) precursor. In conjunction with the experimental outcomes of this optimised deposition process, we use first-principles density functional and semi-empirical quantum chemical calculations to examine the underlying electronic processes which determined the morphology and photocatalytic properties of the coated titania. We highlight the presence of low lying valence electronic states which reduce photocatalytic activity, and as a consequence decrease the population of photo-excited titania electrons which transfer to the surrounding matrix. Born-Oppenheimer molecular dynamics (MD) simulations indicate that the coating process is completed within the order of 10 ps.

MSINDO study of acid promoted dissolution of planar MgO and NiO surfaces

Despite the structural similarity, MgO is fast dissolving in low pH solution, whereas NiO is slow dissolving under the same conditions. In addition, the planar MgO (100) surface immediately reconstructs to form pits and protrusions, whereas this behavior has not been observed with NiO (100). Our previous study, using the semiempirical self‐consistent field molecular orbital (SCFMO) method MSINDO showed that it was possible for MgO dissolution but not NiO dissolution to occur via the migration of a metal‐oxygen pair to an ex situ position. However, we have now found a more energetically favorable and realistic dissolution mechanism involving the dissociation of a water molecule (adsorbed on a metal site) prior to migration. Products from this dissociation (H and OH) weaken adjacent metal‐oxygen bonds. Dissociation of a second adsorbed water molecule is required to complete the process. For both oxides, the energy barrier determined from the energy profile of the metal‐oxygen pair migration was found to be lower than the activation energy of water dissociation at the planar surface as reported in previous study. This would suggest that the dissociation of water molecules at the planar surface is ratedetermining in the surface restructuring step of dissolution. It was demonstrated that surface restructuring and dissolution of MgO is possible whereas highly improbable for NiO, in agreement with experimental observations.

A first-principles study of the dielectric properties of TiO₂ polymorphs.

We present an analysis of the dielectric properties of the three polymorphs of TiO₂ (rutile, anatase and brookite phases), using ab initio time-dependent density functional perturbation theory based on the Vignale-Kohn functional. We implement this functional, which incorporates many-body effects, using the periodic program BAND. The improved result for the density of states spectra for brookite is suggestive of increased titanium ion Jahn-Teller effects for this phase. The imaginary and real components of the frequency-dependent dielectric functions show notable dielectric anisotropies, with implications for excitonic interactions, for all three common phases of TiO₂. Comparison of the electron energy-loss spectrum for undoped and doped rutile and anatase reveals the critical role of collective charge excitations in photocatalytic mechanisms. The correlation between plasmon peaks present at lower energies and decreased photocatalytic activity due to substitutional aluminum doping in combination with oxygen vacancies in rutile and anatase is highlighted. Moreover, there is clear correlation between dielectric properties and the microstructure of the TiO₂ polymorphs as suggested via the framework of the Born effective charge and Hirshfeld charge distribution schemes.

A first-principles study of the dielectric properties of TiO2polymorphs

We present an analysis of the dielectric properties of the three polymorphs of TiO₂ (rutile, anatase and brookite phases), using ab initio time-dependent density functional perturbation theory based on the Vignale-Kohn functional. We implement this functional, which incorporates many-body effects, using the periodic program BAND. The improved result for the density of states spectra for brookite is suggestive of increased titanium ion Jahn-Teller effects for this phase. The imaginary and real components of the frequency-dependent dielectric functions show notable dielectric anisotropies, with implications for excitonic interactions, for all three common phases of TiO₂. Comparison of the electron energy-loss spectrum for undoped and doped rutile and anatase reveals the critical role of collective charge excitations in photocatalytic mechanisms. The correlation between plasmon peaks present at lower energies and decreased photocatalytic activity due to substitutional aluminum doping in combination with oxygen vacancies in rutile and anatase is highlighted. Moreover, there is clear correlation between dielectric properties and the microstructure of the TiO₂ polymorphs as suggested via the framework of the Born effective charge and Hirshfeld charge distribution schemes.