Now showing 1 - 10 of 40
  • Publication
    Ab Initio Molecular Dynamics Studies of the Effect of Solvation by Room-Temperature Ionic Liquids on the Vibrational Properties of a N719-Chromophore/Titania Interface
    The accurate ab initio modeling of prototypical and well-representative photoactive interfaces for candidate dye-sensitized solar cells (DSCs) is a perennial problem in physical chemistry. To this end, the use of ab initio density functional theory-based molecular dynamics (AIMD) has been studied here to investigate the effect the choice of functional has on a system mimicking the essential workings of a DSC: the energetic properties of a [bmim]+[NTf2]- room-temperature ionic liquid (RTIL) solvating an N719-sensitizing dye adsorbed onto an anatase-titania (101) surface were scrutinized. In so doing, we glean important insights into how using an RTIL as electrolytic hole acceptor alters and modulates the dynamical properties of the widely used N719 dye. A fully crossed study has been carried out comparing the Becke-Lee-Yang-Parr (BLYP) and Perdew-Burke-Ernzerhof (PBE) functionals, both unsolvated and solvated by the RTIL, both with and without Grimme D3 dispersion corrections. Also, vibrational spectra for the photoactive interface in the DSC configuration were calculated by means of Fourier-transforming atomic mass-weighted velocity autocorrelation functions. The ab initio vibrational spectra were compared to high-quality experimental data and against each other; the effects of various methodological choices on the vibrational spectra were also studied, with PBE generally performing best in producing spectra, which matched the experimental frequency modes typically expected.
      345Scopus© Citations 3
  • 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.
      423Scopus© Citations 20
  • Publication
    First-principles calculation of nitrogen-tungsten codoping effects on the band structure of anatase-titania
    (American Institute of Physics, 2009-04-02) ;
    The electronic properties and photocatalytic activity of nitrogen (N) and/or tungsten (W)-doped anatase are calculated using density functional theory. For N-doping, isolated N 2p states above the top of the valence band are responsible for experimentally observed redshifts in the optical absorption edge. For W-doping, W 5d states below the conduction band lead to band gap narrowing; the transition energy is reduced by 0.2 eV. Addition of W to the N-doped system yields significant band gap narrowing gap by 0.5 eV. This rationalizes recent experimental data which showed that N/W-doped titania exhibits higher visible-light photocatalytic efficiency than either N- or W-doping alone.
      1390Scopus© Citations 117
  • Publication
    Influence of doping on the photoactive properties of magnetron-sputtered titania coatings: Experimental and theoretical study
    (American Physical Society, 2012-09) ; ; ;
    Titanium dioxide (TiO2) thin films, doped with chromium (Cr) and codoped with chromium-carbon (Cr, C) and chromium-nitrogen (Cr, N) of various concentrations, were deposited using magnetron sputtering. Postdeposition thermal treatments were carried out at 450 ◦C for 5 h to change the as-deposited amorphous coatings to their crystalline form. The crystalline phase was found to be dependent on the amount and type of dopant present. Ultraviolet-visible (UV-Vis) absorption data and band gap energies calculated by spectroscopic ellipsometry showed that, on increasing Cr concentration, a shift of the absorption edge towards visible light and a reduction of the band gap occurred. This was further improved by codoping of Cr with either N or C, with the latter case exhibiting more photo-activity towards visible light. In addition, hybrid density functional theory (DFT) calculations were performed for Cr-, N-. and C-monodoping, together with Cr-C and Cr-N codoping, in both rutile and anatase phases of crystalline titania. Using this method, absorption coefficients and band gaps were determined to explore photo-activity. Very good, semiquantitative agreement was found between the DFT and experimental approaches for these quantities, underlining the key role of state-of-the-art quantum calculations in interpreting and guiding experimental studies of doping in metal oxides.
      568Scopus© Citations 22
  • 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.
      315Scopus© Citations 5
  • Publication
    Towards the design of novel boron- and nitrogen-substituted ammonia-borane and bifunctional arene ruthenium catalysts for hydrogen storage
    Electronic-structure density functional theory calculations have been performed to construct the potential energy surface for H2 release from ammonia-borane, with a novel bifunctional cationic ruthenium catalyst based on the sterically bulky β-diketiminato ligand (Schreiber et al., ACS Catal. 2012, 2, 2505). The focus is on identifying both a suitable substitution pattern for ammonia-borane optimized for chemical hydrogen storage and allowing for low-energy dehydrogenation. The interaction of ammonia-borane, and related substituted ammonia-boranes, with a bifunctional η6-arene ruthenium catalyst and associated variants is investigated for dehydrogenation. Interestingly, in a number of cases, hydride-proton transfer from the substituted ammonia-borane to the catalyst undergoes a barrier-less process in the gas phase, with rapid formation of hydrogenated catalyst in the gas phase. Amongst the catalysts considered, N,N-difluoro ammonia-borane and N-phenyl ammonia-borane systems resulted in negative activation energy barriers. However, these types of ammonia-boranes are inherently thermodynamically unstable and undergo barrierless decay in the gas phase. Apart from N,N-difluoro ammonia-borane, the interaction between different types of catalyst and ammonia borane was modeled in the solvent phase, revealing free-energy barriers slightly higher than those in the gas phase. Amongst the various potential candidate Ru-complexes screened, few are found to differ in terms of efficiency for the dehydrogenation (rate-limiting) step. To model dehydrogenation more accurately, a selection of explicit protic solvent molecules was considered, with the goal of lowering energy barriers for H-H recombination. It was found that primary (1°), 2°, and 3° alcohols are the most suitable to enhance reaction rate. © 2014 Wiley Periodicals, Inc.
      447Scopus© Citations 6
  • 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.
      973Scopus© Citations 21
  • Publication
    Functional Assessment for Predicting Charge-Transfer Excitations of Dyes in Complexed State: A Study of Triphenylamine-Donor Dyes on Titania for Dye-Sensitized Solar Cells
    (American Chemical Society, 2012-12-13) ; ;
    Time-dependent density functional theory (TD-DFT) was employed to calculate the UV/vis spectra for three of the triphenylamine (TPA)-donor dyes, TC1, L1, and LJ1, in isolation as well as when complexed with a titania nanoparticle. TPA-donor dyes are a class of promising organic dyes for use in dye-sensitized solar cells (DSSCs). The three dyes studied here are among the smallest of these molecules and provide important insight into the entire series of TPA dyes that are being explored as possible sensitizers in titania-based DSSCs. An attempt to calculate the optical spectra for these dyes within the B3LYP approximation to the exchange correlation functional produces erroneous results. However, Coulomb attenuated approximation (CAM-B3LYP) captures the correct photophysics of the dyes and produces more accurate charge-transfer excitation energies of their complexes with titania. This work shows that the extent to which a given approximation fails or succeeds to correctly predict the charge-transfer excitation energies in the isolated dyes is propagated in that it fails (or succeeds) to correctly predict the values of the excitation energies for the complexes. It is, therefore, important to determine the most appropriate functional for a dye before considering it in more complicated structures such as dye-titania complexes.
      568Scopus© Citations 50
  • 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.
      1087Scopus© Citations 72
  • Publication
    Electronic properties of F/Zr co-doped anatase TiO2 photocatalysts from GGA + U calculations
    (Elsevier, 2010-10-08) ;
    The energetic and electronic properties of F and/or Zr-doped anatase TiO2 are investigated by first-principles calculations. For F-doping, reduced Ti3+ ions are formed and Ti orbitals lie slightly below the conduction band, leading to band gap narrowing. For Zr-doping, Zr 4d orbitals reside well into the conduction band, with essentially no band gap change. For F/Zr–codoping, the electronic structure is similar to that for F–monodoping, where Ti3+ gap states are induced by both the oxygen vacancy and F dopant. The influence of oxygen vacancies indicates that interplay between dopants and oxygen vacancies is key for improvement of photocatalytic activity. The theoretical findings present a reasonable explanation of recent experimental results.
      3610Scopus© Citations 27