Now showing 1 - 10 of 40
  • Publication
    Fabrication of nano-structured TiO2 coatings using a microblast deposition technique
    Micron thick titanium dioxide (TiO2) coatings exhibiting a nano-structured, anatase, meso-porous structure were successfully deposited across a range of polymer, conductive glass and metallic substrates at low velocities using a microblasting technique. This process was conducted at atmospheric pressure using compressed air as the carrier gas and commercially available agglomerated nano particles of TiO2 as the feedstock. An examination of the effect of impact kinetics on the agglomerated powder before and after deposition was undertaken. A further examination of the coating microstructure along with photocurrent density measurements before and after thermal treatments was explored. Owing to the low temperature and velocity of the powder during deposition no change in phase of the powder or damage to the substrate was observed. The resulting TiO2 coatings exhibited relatively good adhesion on both titanium and FTO coated glass substrates with coating thickness of approximately 1.5 μm. Photo-catalytic performance was measured under solar simulator illumination using a photo-electrochemical cell (PEC) with a 5-fold increase in performance observed after thermal treatment of the TiO2 coated substrates. Microblasting was demonstrated to be a rapid and cost effective method for the deposition of nano-structured, photo-catalytic, anatase TiO2 coatings.
    Scopus© Citations 10  1631
  • 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
    First-principles study of S doping at the rutile TiO2 (110) surface
    (ACS Publications, 2009-09-15) ; ;
    The structural, energetic and electronic properties of various S doping configurations by substitution and adsorption at the rutile TiO2 (110) surface have been investigated by first-principles density functional theory (DFT) calculations. The stability of these configurations has been compared on the basis of the calculated formation and adsorption energies. Our results indicate that S dopants replace surface O atoms or bind to Ti atoms preferentially. Moreover, implantation of S dopants into the rutile lattice favored the formation of oxygen vacancies, which promotes further S incorporation. Doping of single S atoms into Ti sites (S-cation doping) led to relatively small reductions of the photon transition energy, while S-substitution of O atoms (S-anion doping) and adsorption on the surface (S-cation/anion doping) resulted in significant red-shifts of the optical absorption edge. Our results suggest that the interplay between S impurities and oxygen vacancies does not enhance visible light absorption in an obvious way, and helps to rationalise recent experimental studies.
    Scopus© Citations 22  1073
  • 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
    Band gap engineering of (N, Ta)-codoped TiO2 : a first-principles calculation
    (Elsevier, 2009-07-29) ;
    The electronic properties and photocatalytic activity of X (N, C) / transition metal (TM=Ta, Hf, Fe) – codoped anatase TiO2 have been investigated using density functional theory. It was found that only the (N, Ta)-codoping case narrows the band gap significantly by about 0.48 eV, driven by the continuum-like p-d hybridized states above the top of valence band and d states at the bottom of conduction band. The calculated energy results suggest that codoping of Ta with N can increase the N concentration in N-doped TiO2 based on energy results.
    Scopus© Citations 96  1948
  • Publication
    Magnetic properties of first-row element-doped ZnS semiconductors : a density functional theory investigation
    (American Physical Society, 2009-09-22) ;
    Based on first-principles calculations, we have investigated the magnetic properties of the first-row element-doped ZnS semiconductors. Calculations reveal that Be, B, and C dopants can induce magnetic, while N cannot lead to spin polarization in ZnS. A possible explanation was rationalized from the elements’ electronegativity and interaction between dopant atoms and host atoms. The total magnetic moments are 2.00, 3.16, and 2.38 μB per 2 x 2 x 2 supercell for Be, B, and C doping, respectively, and ferromagnetic coupling is generally observed in these cases. The ferromagnetism of Be-, B-, and C-doped ZnS can be explained by hole-mediated s-p or p-p interactions’ coupling mechanisms. However, the clustering effect was found to be in Be-, B-, and C-doped ZnS but the degree is more obvious in the former two cases than in latter case. Analysis revealed that C-doped ZnS displays better potential ferromagnetic behavior than Be- and B-doped ZnS due to its half-metallic characteristic.
      1167Scopus© Citations 78
  • 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
    Energetic and electronic properties of P Doping at the rutile TiO2 (110) Surface from First Principles
    (ACS Publications, 2009-04-20) ;
    The energetic and electronic properties of various P doping configurations at the rutile TiO2 (110) surface are investigated by first-principles density functional theory (DFT) calculations. Several substitution and adsorption configurations for P impurities at the surface and the subsurface are considered. The stability of the P-doped systems is compared on the basis of the calculated formation energy and adsorption energy. Our calculated results indicate that the P impurities replace surface Ti atoms preferentially under O-rich growth conditions,and surface O atoms under Ti-rich conditions. In addition, it was found that the creation of oxygen vacancies favors P incorporation at substitution sites but not at adsorption sites. Doping with a single P atom into an O site may lead to either anionic or cationic states in the dopant. This causes either band-to-band transitions or introduces gap states to band transitions, with the former corresponding to a small band gap narrowing or broadening and the latter resulting in obvious reductions of photon transition energy. Substitutional replacement of Ti atoms by P atoms and adsorption on the surface (P-cation doping) results in either a small band reduction or a slight band gap enlargement, depending on the doping sites. It is speculated that the interaction between P impurities and surface oxygen vacancies will lead to further enhanced photocatalytic activity in the visible light region.
    Scopus© Citations 19  834