Now showing 1 - 10 of 27
  • Publication
    Molecular dynamics study of water in contact with the TiO2 rutile-110, 100, 101, 001 and anatase-101, 001 surface
    We have carried out classical molecular dynamics of various surfaces of TiO2 with its interface with water. We report the geometrical features of the first and second monolayers of water using a Matsui Akaogi (MA) force field for the TiO2 surface and a flexible single point charge model for the water molecules. We show that the MA force field can be applied to surfaces other than rutile (110). It was found that water OH bond lengths, H–O–H bond angles and dipole moments do not vary due to the nature of the surface. However, their orientation within the first and second monolayers suggest that planar rutile (001) and anatase (001) surfaces may play an important role in not hindering removal of the products formed on these surfaces. Also, we discuss the effect of surface termination in order to explain the layering of water molecules throughout the simulation box.
      441Scopus© Citations 82
  • Publication
    Serendipity following attempts to prepare C-doped rutile TiO2
    Attempts to mimic the band gap narrowing seen in anatase TiO2 following C-doping of the lattice where the C arose from a melamine borate precursor were made in situations where the sol-gel mixture was directed towards rutile formation. The formed materials were characterised using XRD, BET, UV-Vis spectroscopy, XPS and TEM and their activities in promoting the photo-degradation of 4-chlorophenol were analysed. It was found that carbon was not doped into the lattice (in contrast to the situations where the sol-gel mixture was directed towards the precipitation of anatase TiO2). In spite of how common reports of the preparation of C-doped TiO2 using sol-gel processes have been, the presence of carbon dopant precursors in a crystallising sol does not necessarily result in the incorporation of C dopants within the final crystalline material, i.e. the nature of the condensing sol is also important. The presence of melamine borate did however increase the proportion of rutile in the final mixture (indeed in the presence of melamine borate the pure rutile phase was formed) and also resulted in materials with higher surface areas (as measured using BET). Furthermore, TEM has shown that rutile TiO2 condensed in the presence of melamine borate had a much more distinct rod-like shape than that condensed in its absence (the latter being more spherical in shape). These materials, notwithstanding the absence of any dopant effect, demonstrated enhanced photocatalytic activity when compared with analogous materials prepared in the absence of melamine borate and this effect is ascribed to both their relatively larger surface areas and their specific shape. Therefore, we have serendipitously come across a method for improving the performance of rutile photocatalysts while searching for a method to generate C-doped rutile TiO2.
      552Scopus© Citations 13
  • Publication
    The influence of Ti and Si doping on the structure, morphology and photo-response properties of α-Fe2O3 for efficient water splitting: experiment and first-principle calculations
    Ti- and Si- doping effects on morphology, structure, optical and photo-response of α-Fe2O3 nanoscale coatings from atmospheric-pressure chemical vapour deposition (APCVD) have been studied. Si- and Ti-doping led to larger clusters with finer grains and smaller clusters with larger grains, respectively. Photocurrent performance was increased remarkably by doping, especially Si. Excellent agreement was found for band gaps and optical properties compared to hybrid-Density Functional Theory. Substitutional replacement of Fe by Si shrinks the volume more than Ti-doping; it is conjectured that this affects hopping probability of localised charge-carriers more and leads to enhanced photocurrent activity for Si-doping, supported by experiment.
      598Scopus© Citations 18
  • Publication
    Mechanisms for thermal conduction in hydrogen hydrate
    (American Institute of Physics, 2012-01-23) ; ;
    Extensive equilibrium molecular dynamics (MD) simulations have been performed to investigate thermal conduction mechanisms via the Green-Kubo approach for (type II) hydrogen hydrate, at 0.05 kbar and between 30 and 250 K, for both lightly-filled H2 hydrates (1s4l) and for more densely-filled H2 systems (2s4l), in which four H2 molecules are present in the large cavities, with respective single- and double-occupation of the small cages. The TIP4P water model was used in conjunction with a fully atomistic hydrogen potential along with long-range Ewald electrostatics. It was found that substantially less damping in guest-host energy transfer is present in hydrogen hydrate as is observed in common type I clathrates (e.g., methane hydrate), but more akin in to previous results for type II and H methane hydrate polymorphs. This gives rise to larger thermal conductivities relative to common type I hydrates, and also larger than type II and H methane hydrate polymorphs, and a more crystal-like temperature dependence of the thermal conductivity.
      698Scopus© Citations 23
  • Publication
    Implicit and explicit solvent models for modelling a bifunctional arene ruthenium hydrogen-storage catalyst: a classical and ab initio molecular simulation study
    Classical and ab initio, density functional theory- and semiempirical-based molecular simulation, including molecular dynamics, have been carried out to compare and contrast the effect of explicit and implicit solvation representation of tetrahydrofuran (THF) solvent on the structural, energetic, and dynamical properties of a novel bifunctional arene ruthenium catalyst embedded therein. Particular scrutiny was afforded to hydrogen-bonding and energetic interactions with the THF liquid. It was found that the presence of explicit THF solvent molecules is required to capture an accurate picture of the catalyst's structural properties, particularly in view of the importance of hydrogen bonding with the surrounding THF molecules. This has implications for accurate modeling of the reactivity of the catalyst.
      366Scopus© Citations 6
  • Publication
    Diffusive hydrogen inter-cage migration in hydrogen and hydrogen-tetrahydrofuran clathrate hydrates
    (American Institute of Physics, 2013-03-07) ; ;
    Classical equilibrium molecular dynamics (MD) simulations have been performed to investigate the diffusive properties of inter-cage hydrogen migration in both pure hydrogen and mixed hydrogen-tetrahydrofuran sII hydrates at 0.05 kbar from 200 K and up to 250-260 K. For mixed H2- THF systems in which there is single H2 occupation of the small cage (labelled ‘1SC 1LC’), we found that no H2 migration occurs. However, for more densely-filled H2-THF and pure- H2 systems, in which there is more than single H2 occupation in the small cage, there is an onset of inter-cage H2 migration events from the small cages to neighbouring cavities at around 200 K. The mean square displacements of the hydrogen molecules were fitted to a mathematical model consisting of an anomalous term and a Fickian component, and non-linear regression fitting was conducted to estimate long-time (inter-cage) diffusivities. An approximate Arrhenius temperature relationship for the diffusion coefficient was examined and a rough estimation of the hydrogen hopping energy barrier was calculated for each system.
      429Scopus© Citations 47
  • Publication
    Electrophoretic deposition of poly(3-decylthiophene) onto gold-mounted cadmium selenide nanorods
    Molecular mechanisms of electrophoretic deposition (EPD) of poly(3-decylthiophene) (P3DT) molecules onto vertically aligned cadmium selenide arrays have been studied using large-scale, nonequilibrium molecular dynamics (MD), in the absence and presence of static external electric fields. The field application and larger polymer charges accelerated EPD. Placement of multiple polymers at the same lateral displacement from the surface reduced average deposition times due to “crowding”, giving monolayer coverage. These findings were used to develop and validate Brownian dynamics simulations of multilayer polymer EPD in scaled-up systems with larger inter-rod spacings, presenting a generalized picture in qualitative agreement with random sequential adsorption.
      648Scopus© Citations 6
  • Publication
    A TD-DFT study of the effects of structural variations on the photochemistry of polyene dyes
    We report a TD-DFT study of three polyene dyes namely: NKX-2553, NKX-2554 and NKX-2569 in isolation as well as upon their adsorption on TiO2 nanoparticles. By choosing closely related dyes we wish to focus on the effects of structural variations on the absorption and charge-transfer properties of these systems. These three dyes show a non-intuitive trend in their respective efficiencies and therefore, were chosen to shed light on the structural components that contribute to this behaviour. Although, NKX-2554 has an additional donor group, it is less efficient compared to the simpler NKX-2553 dye that contains only one donor group. When NKX-2554 structure is slightly modified by lengthening the linker-group, one obtains the most efficient dye among this set, namely, NKX-2569. In this work, we show that the changes in the donor moiety has very little or no effect on the efficiency of these dyes as can be seen in the case of NKX-2553 and NKX-2554. On the other hand, the improved performance of NKX-2569-titania complex can be understood to be a result of the longer linker group. A better understanding of these properties within different dye-titania complexes is important for the continual improvement of DSSCs. In this regards, this study will serve to provide guidelines to improve efficiencies of novel organic dyes.
    Scopus© Citations 39  909
  • Publication
    Routes to visible light active C-doped TiO2 photocatalysts using carbon atoms from the Ti precursors
    In this work, different thermal treatments of titanium isopropoxide-derived photo-catalyst precursors, designed with the purpose of generating C-doped TiO2 photo-catalysts using carbon atoms present in the TiOx gel precursors as dopants, are presented. Specifically, these look at varying the standard calcination techniques using heat treatments in He (rather than calcination in air) and lower temperature calcinations (200 °C rather than 500 °C). The formed materials are characterised using N2 physisorption, XRD, UV Visible spectroscopy and XPS and their activities in promoting the oxidation of 4-chlorophenol under visible-light-only conditions were analysed. The nature of carbon remaining on the (or in the) material is discussed found to be both surface graphitic carbon and carbon present in anionic dopant positions. The different contributions of each type of carbon to the catalytic photo-activity under visible light are discussed.
      479Scopus© Citations 25
  • Publication
    Dynamical cage behaviour and hydrogen migration in hydrogen and hydrogen-tetrahydrofuran clathrate hydrates
    (American Institute of Physics, 2012-01-24) ; ;
    Classical equilibrium molecular dynamics(MD) simulations have been performed to investigate dynamical properties of cage radial breathing modes and intra- and inter-cage hydrogen migration in both pure hydrogen and mixed hydrogen-tetrahydrofuran sII hydrates at 0.05 kbar and up to 250K. For the mixed H2-THF system in which there is single H2 occupation of the small cage (labelled ‘1SC 1LC’), we find that no H2 migration occurs, and this is also the case for pure H2 hydrate with single small-cavity occupation and quadruple occupancy for large cages (dubbed ‘1SC 4LC’). However, for the more densely-filled H2-THF and pure- H2 systems, in which there is double H2 occupation in the small cage (dubbed ‘2SC 1LC’ and ‘2SC 4LC’, respectively), there is an onset of inter-cage H2 migration events from the small cages to neighbouring cavities at around 200 K, with an approximate Arrhenius temperature-dependence for the migration rate from 200 to 250 K. It was found that these ‘cage hopping’ events are facilitated by temporary openings of pentagonal small-cage faces with the relaxation and reformation of key stabilising hydrogen bonds during and following passage. The cages remain essentially intact up to 250 K, save for transient hydrogen bond weakening and reformation during and after inter-cage hydrogen diffusion events in the 200 to 250 K range. The ‘breathing modes’, or underlying frequencies governing the variation in the cavities’ radii, exhibit a certain overlap with THF rattling motion in the case of large cavities, while a there is some overlap of small cages’ radial breathing modes with lattice acoustic modes.
    Scopus© Citations 33  577