Now showing 1 - 10 of 27
  • Publication
    Evaluation of microwave plasma oxidation treatments for the fabrication of photoactive un-doped and carbon-doped TiO2 coatings
    The photoactivity of both un-doped and carbon-doped titanium dioxide (TiO2) coatings has been widely reported. In this paper, the use of a microwave plasma as a novel oxidation treatment for the fabrication of these coatings is evaluated. The photoactivity performance of the microwave plasma-formed coatings is benchmarked against those fabricated through air furnace oxidation as well as those deposited using reactive magnetron sputtering. The un-doped and carbon-doped TiO2 coatings were prepared respectively by microwave plasma-oxidizing titanium metal sheets and sputter deposited titanium carbide thin films. The resulting oxides were characterized using XPS, XRD, FEG-SEM, and optical profilometry. The oxide layer thicknesses achieved over the 15 to 45 minute oxidation times were in the range of 0.15 to 3.44 µm. These coatings were considerably thicker than those obtained by air furnace oxidation. The microwave plasma-formed oxides also exhibited significantly higher surface roughness values compared with the magnetron-sputtered coatings. The photoactivity performance of both un-doped and carbon-doped coatings was assessed using photocurrent density measurements. Comparing the un-doped TiO2 coatings, it was observed that those obtained using the microwave plasma oxidation route yielded photocurrent density measurements that were 4.3 times higher than the TiO2 coatings of the same thickness that were deposited by sputtering. The microwave plasma-oxidized titanium carbide coatings did not perform as well as the un-doped TiO2 probably due to the presence of un-oxidized carbide in the coatings, which reduced their photoactivity.
      1501Scopus© Citations 37
  • Publication
    First-principles study of the excited-state properties of coumarin-derived dyes in dye-sensitized solar cells
    Using Time-Dependent Density Functional Theory (TD-DFT), we have investigated the optical properties of dye-sensitized solar cells (DSSCs) comprised of TiO2 nanoparticle sensitized with two coumarins, namely, NKX-2311 and NKX-2593. The two sensitizers (dyes) differ only in their linker moieties and are shown to have different absorption spectra when adsorbed on to the TiO2 surface. Knowledge of different light absorption and charge transfer (CT) behavior within these complexes is useful for further improving the photo-dynamics of newer organic dyes presently being designed and investigated worldwide. Moreover, we have also investigated the effect of deprotonation of the sensitizers' carboxylic groups during adsorption on the titania surface and the excited state electronic properties of the resulting species.
      1448Scopus© Citations 53
  • 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.
    Scopus© Citations 53  445
  • Publication
    Study of translational, librational and intra-molecular motion of adsorbed liquid water monolayers at various TiO2 interfaces
    Equilibrium classical molecular dynamics (MD) simulations have been performed to investigate the vibrational motion of water in contact with rutile-(110), rutile-(100), rutile-(001), anatase-(101) and anatase-(001) surfaces at room temperature (300 K). The vibrational density of states (VDOS) of the first adsorbed monolayer of liquid water has been analysed for each surface. These have been compared with reported experimental INS values involving rutile and anatase polymorph surfaces, along with ab initio MD results. It is observed that good qualitative agreement is obtained for the rutile-(110) and the anatase-(101) surfaces with the corresponding experimental VDOS. A significant contribution from librational dynamics is found for planar rutile surfaces, but no such demarcation is seen in the experimental VDOS.
    Scopus© Citations 27  769
  • Publication
    Hydrogen bond dynamical properties of adsorbed liquid water monolayers with various TiO2 interfaces
    Equilibrium classical molecular dynamics (MD) simulations have been performed to investigate the hydrogen bonding kinetics of water in contact with rutile-(110), rutile-(101), rutile-(100), and anatase-(101) surfaces at room temperature (300 K). It was observed that anatase-(101) exhibits the longest-lived hydrogen bonds in terms of overall persistence, followed closely by rutile-(110). The relaxation times, defined as the integral of the autocorrelation of the hydrogen bond persistence function, were also larger for these two cases, while decay of autocorrelation function was slower. The increased number and overall persistence of hydrogen bonds in the adsorbed water monolayers at these surfaces, particularly for anatase-(101), may serve to promote possible water photolysis activity thereon.
    Scopus© Citations 25  659
  • Publication
    Dynamical and energetic properties of hydrogen and hydrogen–tetrahydrofuran clathrate hydrates
    Classical equilibrium molecular dynamics (MD) simulations have been performed to investigate the dynamical and energetic properties in hydrogen and mixed hydrogen-tetrahydrofuran sII hydrates at 30 and 200K and 0.05 kbar, and also at intermediate temperatures, using SPC/E and TIP4P-2005 water models. The potential model is found to have a large impact on overall density, with the TIP4P-2005 systems being on average 1 % more dense than their SPC/E counterparts, due to the greater guest-host interaction energy. For the lightly-filled mixed H2-THF system, in which there is single H2 occupation of the small cage (1s1l), we find that the largest contribution to the interaction energy of both types of guest is the van der Waals component with the surrounding water molecules in the constituent cavities. For the more densely-filled mixed H2-THF system, in which there is double H2 occupation in the small cage (2s1l), we find that there is no dominant component (i.e., van der Waals or Coulombic) in the H2 interaction energy with the rest of the system, but for the THF molecules, the dominant contribution is again the van der Waals interaction with the surrounding cage-water molecules; again, the Coulombic component increases in importance with increasing temperature. The lightly-filled pure H2 hydrate (1s4l) system exhibits a similar pattern vis-à-vis the H2 interaction energy as for the lightly-filled mixed H2-THF system, and for the more densely-filled pure H2 system (2s4l), there is no dominant component of interaction energy, due to the multiple occupancy of the cavities. By consideration of Kubic harmonics, there is some evidence of preferential alignment of the THF molecules, particularly at 200 K; this was found to arise at higher temperatures due to transient hydrogen bonding of the oxygen atom in THF molecules with the surrounding cage-water molecules.
      519Scopus© Citations 21
  • Publication
    Photo-active and dynamical properties of hematite (Fe2O3)-water interfaces: An experimental and theoretical study
    The dynamical properties of physically and chemically adsorbed water molecules at pristine hematite-(001) surfaces have been studied by means of equilibrium Born–Oppenheimer molecular dynamics (BOMD) in the NVT ensemble at 298 K. The dissociation of water molecules to form chemically adsorbed species was scrutinised, in addition to ‘hopping’ or swapping events of protons between water molecules. Particular foci have been dynamical properties of the adsorbed water molecules and OH− and H3O+ ions, the hydrogen bonds between protons in water molecules and the bridging oxygen atoms at the hematite surface, as well as the interactions between oxygen atoms in adsorbed water molecules and iron atoms at the hematite surface. Experimental results for photoelectrical current generation complement simulation findings of water dissociation.
    Scopus© Citations 26  539
  • Publication
    Activation of hematite nanorod arrays for photoelectrochemical water splitting
    Hematite nanorod arrays were activated through proper control of annealing conditions. The 100-fold improvement in photocurrent was correlated with increased absorption and Sn doping from the tin oxide coated glass substrate. The low onset potential is attributed to a reduction in surface defects, while the morphology is credited for promoting tin diffusion and facilitating electron transport.
      2211Scopus© Citations 154
  • 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.
    Scopus© Citations 6  666
  • 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.
    Scopus© Citations 6  394