The electronic structure and charge carrier driven chemistry on solid surfaces
EMSL Project ID
46393
Abstract
The experimental and theoretical understanding of photocatalysis is of vital importance for the solar-to-chemical energy conversion and for environmental remediation. Photocatalytic conversion of light into chemical energy involves a series of complex photophysical and photochemical steps whereby light excites carriers in a semiconductor and the carriers in turn drive chemical transformations. Our research groups combine time-resolved-two-photon photoemission spectroscopy [TR-2PP; University of Pittsburgh (Pitt)], scanning tunneling microscopy [STM; University of Science and Technology of China (USTC)], and ab initio electronic structure theory (Pitt and USTC), with the goal of elucidating the role of surface electronic structure, photon and electron induced surface dynamics, and light and electron induced chemistry in photocatalysis on TiO2. The primary focus of the research will be on the electron and hole mediated chemical processes on a variety of TiO2 surfaces. In the past, we have discovered by experiment and theory the wet electron states on protic solvent covered rutile TiO2(110) surfaces. In addition to electron driven processes, it is well known that the valance band holes in TiO2 are a potent oxidizing agents. Holes, however, are more difficult to study by experimental methods. With a newly acquired broadly tunable femtosecond laser source, soon to be available at Pitt, it should be possible to study the ultrafast hole dynamics using time-resolved photoemission techniques. We have strong experimental evidence from TR-2PP and STM that the band gap excitation of holes induces the deprotonation of methanol on TiO2. Therefore our theoretical methods will focus on the valence band electronic structure of protic solvent covered methanol surfaces. Furthermore, TiO2 is an intriguing material for the reduction of CO2 to hydrocarbon fuels. Despite the potential benefits of photocatalytic conversion of CO2, very little is known about the unoccupied electronic structure of CO2 on metal and metal oxide surfaces. We will use TR-2PP spectroscopy to characterize the electronic structure of CO2 on TiO2 with and without coadsorbed molecules. We plan to study by experiment and theory the stabilization of the unoccupied CO2 anion states for different surfaces, CO2 coverages, and coadsorption with other molecules, such as H2O. Ongoing STM measurements are helping to identify the reactive sites and activation energies for electron induced processes of CO2 on rutile TiO2(110) surface. In addition to studies on photocatalysis, we will investigate the electronic structure of other novel molecular systems. In particular, by STM and theory we have discovered the atom-like superatom molecular orbitals (SAMO) of hollow molecules such as fullerenes. These states are remarkable because they exhibit metal-like intermolecular hybridization leading to formation of nearly free-electron bands. We plan to study such states in a variety of molecular materials by both experiment and theory in order to explore their application in molecular electronics and as molecular machines.
The Chinook supercomputing facilities at EMSL are essential for the large scale computing that is necessary to describe surfaces with large unit cells, the diffuse, unoccupied sates on molecule covered surfaces, and GW calculations of excitations at surfaces.
Project Details
Project type
Exploratory Research
Start Date
2011-11-01
End Date
2012-11-04
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Feng M, P Cabrera-Sanfelix, C Lin, A Arnau, D Sanchez-Portal, J Zhao, PM Echenique, and H Petek. 2011. "Orthogonal Interactions of CO Molecules on a One-Dimensional Substrate." ACS Nano 5(11):8877-8883. doi:10.1021/nn203041c
Feng M, Y Shi, C Lin, J Zhao, F Liu, S Yang, and H Petek. 2013. "Energy Stabilization of the S-Symmetry Superatom Molecular Orbital by Endohedral Doping of C82 Fullerene with a Lanthanum Atom." Physical Review. B, Condensed Matter 88:075417. doi:10.1103/PhysRevB.88.075417
Huang T, J Zhao, M Feng, AA Popov, S Yang, L Dunsch, and H Petek. 2011. "A Molecular Switch Based on Current-Driven Rotation of an Encapsulated Cluster within a Fullerene Cage." Nano Letters 11(12):5327-5332. doi:10.1021/nl2028409
Huang T, J Zhao, M Feng, AA Popov, S Yang, L Dunsch, and H Petek. 2012. "A Multi-State Single-Molecule Switch Actuated by Rotation of an Encapsulated Cluster within a Fullerene Cage." Chemical Physics Letters 552:1-12. doi:10.1016/j.cplett.2012.09.064
Lin C, M Feng, J Zhao, P Cabrera-Sanfelix, A Arnau, D Sanchez-Portal, and H Petek. 2011. "Theory of orthogonal interactions of CO molecules on a one-dimensional substrate." Physical Review. B, Condensed Matter and Materials Physics 85(12):125426. doi:10.1103/PhysRevB.85.125426
M. Feng, C. Lin, J. Zhao, and H. Petek, “Orthogonal intermolecular interactions of CO molecules on a one-dimensional substrate,” Ann. Rev. Phys. Chem. 63 (2012).
DOI: 10.1146/annurev-physchem-032210-103353
Zhao J, Q Zheng, H Petek, and J Yang. 2014. "Nonnuclear Nearly Free Electron Conduction Channels Induced by Doping Charge in Nanotube–Molecular Sheet Composites." Journal of Physical Chemistry A. doi:10.1021/jp410460m