Visualization of Water-Induced Surface ... - ACS Publications

Aug 6, 2018 - Water-oxide surfaces are ubiquitous in nature and of widespread importance to phenomena like corrosion as well as contemporary industria...
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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Visualization of Water-induced Surface Segregation of Polarons on Rutile TiO(110) 2

Chi Ming Yim, Ji Chen, Yu Zhang, Bobbie-Jean Shaw, Chi Pang, David C. Grinter, Hendrik Bluhm, Miquel Salmeron, Christopher Andrew Muryn, Angelos Michaelides, and Geoff Thornton J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01904 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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The Journal of Physical Chemistry Letters

Visualization of Water-induced Surface Segregation of Polarons on Rutile TiO2(110)

Chi M. Yim, †,∇ Ji Chen, ‡,∇ Yu Zhang,† Bobbie-Jean Shaw,† Chi L. Pang,† David C. Grinter,† Hendrik Bluhm,§,∥ Miquel Salmeron,⊥ Christopher A. Muryn,# Angelos Michaelides,‡ and Geoff Thornton*,†



Department of Chemistry and London Centre for Nanotechnology, University

College London, 20 Gordon Street, London WC1H 0AJ, UK ‡

Department of Physics and Astronomy, London Centre for Nanotechnology and

Thomas Young Centre, University College London, London WC1E 6BT, UK §

Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley,

California 94720, USA ∥

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley,

California 94720, USA ⊥

Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley,

California 94720, USA #

School of Chemistry, The University of Manchester, Manchester, M13 9PL, UK

*Corresponding author: [email protected]

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ABSTRACT Water-oxide surfaces are ubiquitous in nature and of widespread importance to phenomena like corrosion as well as contemporary industrial challenges such as energy production through water splitting. So far a reasonably robust understanding of the structure of such interfaces under certain conditions has been obtained. Considerably less is known about how overlayer water modifies the inherent reactivity of oxide surfaces. Here we address this issue experimentally for rutile TiO2(110)

using

scanning

tunneling microscopy

and photoemission,

with

complementary density functional theory calculations. Through detailed studies of adsorbed water nanoclusters and continuous water overlayers, we determine that excess electrons in TiO2 are attracted to the top surface layer by water molecules. Measurements on methanol show similar behavior. Our results suggest that adsorbateinduced surface segregation of polarons could be a general phenomenon for technologically relevant oxide materials, with consequences for surface chemistry and the associated catalytic activity.

Table of Contents (TOC) Graphic

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The Journal of Physical Chemistry Letters

All real materials contain defects, ranging from atomic vacancies, to interstitials, to impurities, to excess electrons, and more. Defects greatly affect the physical and optical properties of materials and when present at the surfaces of materials can significantly alter the chemical activity of a material. Indeed, a large body of work has shown that defects at the surfaces of materials act as preferential sites for adsorbate binding; sites at which adsorbates are pinned or bonded more strongly and sites with altered catalytic properties.1-3 However, considerably less is known about the other side of this interplay between adsorbates and defects, namely how adsorbates alter the distribution of defects at the surfaces of materials.4,5 This is the issue we address in the current study through a detailed set of experiments and simulations of water on rutile TiO2, in which we focus on the interplay between water and excess electron defects. The water/rutile TiO2(110) interface has been selected for this study because of its importance to catalysis and photocatalysis and because it is one of the most widely studied and most well understood of all model oxide surface systems.3,6-8 Thanks to this earlier work we now have a broad understanding of the behavior of water on TiO2(110) at a range of temperatures and water coverages. It is known, for example, that water dissociates at defects on TiO2(110). 9-13 Higher exposure to water at