Scanning tunneling microscopy study of water adsorption on anatase TiO2(101)

COLL 353

Yunbin He, yunbinhe@tulane.edu1, Olga Dulub, odulub@tulane.edu1, Linghang Ying1, Ulrike Diebold, diebold@tulane.edu1, Cristiana Di Valentin2, Annabella Selloni, aselloni@princeton.edu3, and Antonio Tilocca4. (1) Department of Physics, Tulane University, 6400 Freret St, 2001 Percival Stern Hall, New Orleans, LA 70118, (2) Dipartimento di Scienza dei Materiali, Universita degli Studi di Milano-Bicocca, 20125 Milano, Italy, (3) Department of Chemistry, Princeton University, Washington Road, Princeton, NJ 08544, (4) Department of Chemistry, University College London, Christopher Ingold Building-G19B, 20 Gordon Street, London WC1H 0AJ, UK
The discovery of photochemical water splitting on TiO2, which holds great potentials in solar energy conversion, has triggered extensive studies of water on the surfaces of TiO2, one of the most important materials for photocatalysis and in photoelectrochemical cells. Whereas the majority of the studies has so far focused on the (110) surface of the most stable rutile polymorph, anatase, which is more frequently used and more efficient in photocatalytic applications, is rarely explored. Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) have previously been used to study the interaction of water with anatase TiO2(101), the most stable surface of anatase TiO2, which reveals that water adsorbs molecularly on the surface in accordance with the theoretical predictions. However, a real space picture revealing the atomic adsorption geometry by, e.g., scanning tunneling microscopy (STM) is yet not available. In the present study, we have employed low temperature STM to study water adsorption on anatase-TiO2 (101). We dose various amounts of water (from 0.2 to 1.8 Langmuir) at a sample temperature around 130 K. It is found by STM that water favors molecular adsorption rather than dissociation on the anatase (101) surface, in agreement with the experimental results from TPD and XPS as well as theoretical predictions. Atomically resolved STM images further revealed that, at low coverages, water molecules adsorb at the Ti5c site forming preferentially one-dimensional chains along the [010]. Surprisingly, we observe locally a doubling periodicity along [010]. When the sample temperature becomes higher (near room temperature), the water molecules become somewhat mobile, hopping between the Ti5c sites. The observed doubling in periodicity of the adsorbate overlayercould either be due to half-monolayer coverage, or be caused by different orientations of water molecules. First-principles calculations using Density Functional Theory are currently underway to validate one or the other scenario.
 

The Physical Chemistry of Environmental Interfaces
8:30 AM-12:40 PM, Wednesday, April 9, 2008 Morial Convention Center -- Rm. 225, Oral

Division of Colloid & Surface Chemistry

The 235th ACS National Meeting, New Orleans, LA, April 6-10, 2008