Authors: Shuanglin Hu, Zhuo Wang, Andreas Mattsson, Lars Österlund, and Kersti Hermansson
We explore a method that can simulate infrared reflection–absorption spectroscopy (IRRAS) spectra for molecules adsorbed on semiconductor surfaces. The method makes it possible to directly correlate experimental spectra with possible adsorbate structures. Our example in this paper is CO adsorbed on rutile TiO2(110). We present simulated IRRAS spectra for coverages in the range from 0.125 to 1.5 monolayer (ML). An explanation is provided for the apparent inconsistency in the literature concerning the tilting geometry of 1 ML CO on this surface. We find that a tilted structure (which is also the lowest-energy configuration) generates IRRAS spectra in excellent agreement with the experimental spectra. Furthermore, we predict the adsorption structure for 1.5 ML CO coverage over TiO2(110), which consists of very weakly bound CO molecules on top of the monolayer. In all cases, our simulation method, which is based on density functional theory (DFT) vibrational calculations, produces s- and p-polarized IRRAS spectra in excellent agreement with the experimental spectra.
J. Phys. Chem. C, 2015, 119 (10), pp 5403–5411
Authors: Shuanglin Hu, Phillipe A. Bopp, Lars Österlund, Peter Broqvist, and Kersti Hermansson
The adsorption and dissociation of a formic acid molecule (HCOO) on a partially reduced rutile TiO2–x (110) surface and the subsequent transformations of the adsorbed fragments are studied via quantumechanical molecular dnamics simulations and climbing-image nudged elastic band (CI-NEB) calculations. The electronic structure methods used are self-consistent-charge density functional tight binding (SCC-DFTB) and DFT+U calculations. We address the apparent lack of consensus in the literature regarding the formic acid adsorbate species that heal the O vacancies, where different experiments have suggested the occurrence of one, two, or no such species types. From our calculations, we propose that the formic acid molecule quickly dissociates on the surface into a formate ion and a proton. If no mechanism exists by which the dissociation products can migrate away from each other, three formate species will coexist on the partially reduced TiO2 surface: one majority species bound to the Ti rows and two minority species healing the O vacancies. However, if such a diffusion mechanism does exist, our barrier calculations show that one of the minority species will transform into the other, and only two adsorbate types can be expected on the surface. We also identify a new adsorbate configuration (which we denote C′), where the formate is located on the row of two-coordinated oxygen atoms, healing an O vacancy and accepting an H-bond from the detached H atom.
J. Phys. Chem. C 118, 14876 (2014).
Authors: A. Mattsson, Shuanglin Hu, L. Österlund, and K. Hermansson
Formic acid (HCOOH) adsorption on rutile TiO2 (110) has been studied by s- and p-polarized infrared reflection-absorption spectroscopy (IRRAS) and spin-polarized density functional theory together with Hubbard U contributions (DFT+U) calculations. To compare with IRRAS spectra, the results from the DFT+U calculations were used to simulate IR spectra by employing a threelayer model, where the adsorbate layer was modelled using Lorentz oscillators with calculated dielectric constants. To account for the experimental observations, four possible formate adsorption geometries were calculated, describing both the perfect (110) surface, and surfaces with defects; either O vacancies or hydroxyls. The majority species seen in IRRAS was confirmed to be the bridging bidentate formate species with associated symmetric and asymmetric frequencies of the ν(OCO) modes measured to be at 1359 cm−1 and 1534 cm−1, respectively. The in-plane δ(C–H) wagging mode of this species couples to both the tangential and the normal component of the incident p-polarized light, which results in absorption and emission bands at 1374 cm−1 and 1388 cm−1. IRRAS spectra measured on surfaces prepared to be either reduced, stoichiometric, or to contain surplus O adatoms, were found to be very similar. By comparisons with computed spectra, it is proposed that in our experiments, formate binds as a minority species to an in-plane Ti5c atom and a hydroxyl, rather than to O vacancy sites, the latter to a large extent being healed even at our UHV conditions. Excellent agreement between calculated and experimental IRRAS spectra is obtained. The results emphasize the importance of protonation and reactive surface hydroxyls – even under UHV conditions – as reactive sites in e.g., catalytic applications.
The Journal of Chemical Physics 140, 034705 (2014)