In this presentation I will give a brief overview of the research activities in the surface science group at USF and then focus on molecular scale investigations of titania surfaces. Titania remains one of the most studied transition metal oxides. Its surfaces are model systems for understanding surface (defect) structure and chemical properties of metal oxides in general. Here I present investigations of surface modifications of pure TiO2 for gaining a better fundamental understanding of the role of surface structure in (photo)chemical applications and to propose methods for enhancing the photochemical properties of TiO2.
Minority sites at surfaces are often reactive sites in heterogeneous (photo)catalysis. One stable defect-site that may have special properties are atomic-height step-edges. We developed an approach, using glancing ion beam irradiation, to prepare ‘nano-rippled' and highly stepped TiO2(110) surfaces with defined step edge orientations . I will discuss the ripple-formation mechanism and characterize the structure and (thermal)stability of meta-stable step edges which forms the basis for future characterization of photochemical properties. In addition to (structurally)modified (110) surfaces, the surface properties of the rarely studied, but highly relevant, rutile-(011) surface are investigated. Our studies have already revealed quite surprising difference in the physical and chemical properties of the (011) vs. (110) rutile surfaces and thus illustrates that there is still much to be learned about metal oxide surfaces from titania. The (011) surface forms a 2x1 reconstruction that saturates ‘broken' surface bonds and consequently makes this surface much less reactive compared to a bulk truncation. We find that even strong adsorbates, like carboxylic acids, cannot form covalent bonds to the vacuum truncated surface. Instead the surface restructures, breaking sub-surface O-Ti bonds to enable bonding to the adsorbate. Oxide surfaces are often regarded as ‘rigid', thus our finding of a bond- and atom re-arrangement in the substrate upon molecular adsorption illustrates that some oxide surfaces are more ‘mobile' than previously thought. Structural anisotropy in the surface re-arrangement results in the formation of quasi-1D adsorbate clusters and thus indicates a potential approach for formation of nanostructures at this surface. Finally, we discuss the recent discovery of a new meta-stable surface phase of titania that can be grown homoepitaxially on the rutile (011) surface . This surface structure exhibits a reduced band gap, close to ~2eV, and thus illustrates that there may be TiO2 phases that can address one of the main challenges in TiO2-photocatalysis, i.e. absorption of visible light.
"New directions for atomic steps: step alignment by grazing incident ion beams on TiO2(110)" T. Luttrell, W.K. Li, X.Q. Gong, M. Batzill Phys. Rev. Lett. 102, 166103 (2009); "Nano-ripple formation on TiO2(110) by low-energy grazing incidence ion sputtering" T. Luttrell, M. Batzill Phys. Rev. B 82, 035408 (2010).
"Role of Surface Structure on the Charge Trapping in TiO2 Photocatalysts" J.G. Tao, M. Batzill J. Phys. Chem. Lett. 1, 3200 (2010).
"Adsorption of Acetic Acid on Rutile TiO(2)(110) vs (011)-2 x 1 Surfaces" J. Tao, T. Luttrell, J. Bylsma, M. Batzill J. Phys. Chem. C 115, 3434-3442 (2011).
"A two dimensional phase of TiO2 with a reduced band gap" J.G. Tao, T. Luttrell, M. Batzill Nature Chemistry 3, 296-300 (2011).