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Water Adsorption on a-Fe2O3(0001) at near Ambient Conditions
Susumu Yamamoto,,' Tom Kendelewicz,t John T. Newberg, Guido Ketteler,"'
David E. Starr,"' Erin R. Mysak, Klas J. Andersson,t,1 Hirohito Ogasawara,t
Hendrik Bluhm, Miquel Salmeron, # Gordon E. Brown, Jr.,t,* and Anders Nilsson*,t,',V
Stanford Synchrotron Radiation Lightsource, 2575 Sand Hill Road, Menlo Park, California 94025, Surface and
Aqueous Geochemistry Group, Department of Geological and Environmental Sciences, Stanford University,
Stanford, California 94305-2115, Chemical Sciences Division, Lawrence Berkeley National Laboratory,
Berkeley, California 94720, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley,
California 94720, FYSIKUM, Stockholm University, AlbaNova University Center, SE-10691 Stockholm,
Sweden, Department of Materials Science and Engineering, University of California-Berkeley, Berkeley,
California 94720, and Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator
Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025
Received: October 14, 2009; Revised Manuscript Received: November 23, 2009
We have investigated hydroxylation and water adsorption on a-Fe2O3(0001) at water vapor pressures up to
2 Torr and temperatures ranging from 277 to 647 K (relative humidity (RH) S 34%) using ambient-pressure
X-ray photoelectron spectroscopy (XPS). Hydroxylation occurs at the very low RH of 1 x 10-7 % and precedes
the adsorption of molecular water. With increasing RH, the OH coverage increases up to one monolayer
(ML) without any distinct threshold pressure. Depth profiling measurements showed that hydroxylation occurs
only at the topmost surface under our experimental conditions. The onset of molecular water adsorption
varies from -2 x 10-5 to -4 x 10-2 % RH depending on sample temperature and water vapor pressure. The
coverage of water reaches 1 ML at -15% RH and increases to 1.5 ML at 34% RH.
Iron oxides are central components of many environmental,
geological, planetary, and technological processes.",2 In natural
environments, iron oxides are products of chemical weathering
and bacterial processes and are important constituents of rocks
and soils. In addition, iron oxides are among the most important
environmental sorbents and as such play important roles in
determining the composition and quality of natural waters, the
mobility of inorganic and organic pollutants, and the availability
of plant nutrients in soils.3 For example, iron oxides play a
critical role in the sequestration and release of arsenic in deltaic
sediments in Southeast Asia, which is responsible for widespread
arsenic pollution of drinking water in this region and impacts
the health of 60 million people who live there.45 Iron oxides
are also used in a wide range of industrial applications, including
heterogeneous catalysis,6,7 pigments,8 gas sensors,9 electrodes
for photoelectrochemistry,'0" and magnetic materials in data
storage devices.12 A major question concerning the reactivity
* Corresponding author. Tel: +1-650-926-2233. Fax: +1-650-926-4100.
Stanford Synchrotron Radiation Lightsource.
* Stanford University.
Chemical Sciences Division, Lawrence Berkeley National Laboratory.
Materials Sciences Division, Lawrence Berkeley National Laboratory.
#University of California-Berkeley.
V SLAC National Accelerator Laboratory.
o Present address: FOM Institute for Atomic and Molecular Physics
(AMOLF), Science Park 113, 1098 XG, Amsterdam, The Netherlands.
* Present address: Department of Applied Physics, Chalmers University
of Technology, SE-412 96 G6teborg, Sweden.
I Present address: Center for Functional Nanomaterials, Brookhaven
National Laboratory, Upton, New York 11973.
+ Present address: Center for Individual Nanoparticle Functionality
(CINF), Department of Physics, Technical University of Denmark, Fysikvej
312, DK-2800 Kgs. Lyngby, Denmark.
of iron oxides under humid ambient conditions, where these
important environmental processes and technological applica-
tions usually occur, is how their interaction with water modifies
their surface structure and composition. Most metal oxide
surfaces react with water and become partially covered with
molecular H20 and/or its dissociated species OH.'3,4 It is well-
known that the presence of water and hydroxyl species on
surfaces has a significant influence on the mechanisms and
kinetics of surface chemical reactions.15 Detailed information
on in situ surface structure and composition under ambient
conditions is therefore essential for a thorough understanding
of important chemical reactions on iron oxides in environmental
processes and technological applications.
Hematite (a-Fe203) is the most thermodynamically stable and
often the most abundant iron oxide in soils and sediments among
a number of otherpolymorphs of iron oxides and oxyhydroxides.2,6
Hematite has the corundum crystal structure, with layers of
distorted hexagonally close-packed oxygen atoms separated by
an iron double layer with Fe 3 occupying two-thirds of the
octahedral sites in a stacking sequence of -(Fe-03-Fe)- along
the c axis.'7 The (0001) hematite surface is one of the
predominant growth faces,2 and therefore the structure of the
a-Fe203(0001) surface and its interaction with water have been
the subject of extensive experimental'1--43 and theoretical4044-59
studies [see refs 60 and 61 for reviews on the surface structure
of a-Fe203(000 1)]
Three different ideal terminations of the a-Fe203(0001)
surface are known to exist: a single Fe termination
(Fe-03-Fe-R-), a double Fe termination (Fe-Fe-03-R-),
and an O termination (03-Fe-Fe-R-), where R represents
the remaining atomic layers with the bulk stacking sequence
(see Figure la-c). The most stable surface configuration under
ultrahigh vacuum (UHV) conditions has been proposed to be
Work supported in part by US Department of Energy contract DE-AC02-76SF00515.
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Yamamoto, Susumu. Water Adsorption on a-Fe2O3(0001) at Near Ambient Conditions, article, August 19, 2011; United States. (digital.library.unt.edu/ark:/67531/metadc831486/m1/1/: accessed February 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.