U Introduction: Surface Chemistry of Oxides

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Introduction: Surface Chemistry of Oxides
U
salt bulk structures that comprise the two largest groups of
metal oxide (MnOm) systems that have been studied, with the
exception of TiO2, whose surface structure in its rutile form is
reviewed in the second paper, by Thornton, Lindsay, and Pang.
Rutile TiO2 is probably the most widely studied oxide surface,
at least using the single crystal approach, for several reasons:
(1) it is a semiconductor that can be made conductive enough
to be studied with all the powerful tools of surface science; (2)
it was perhaps the first oxide surface to yield itself to highquality imaging with atomic resolution using scanning
tunneling microscopy (STM); and (3) it is an important
ingredient in many catalysts and biocompatible materials and
has considerable promise in solar energy conversion
applications. Thus, rutile TiO2 has served as an important
paradigm which has enabled the evolution in our understanding
of how to think about oxide surfaces at the atomic level and
how to measure a variety of important phenomena at them.
The comprehensive review of this prototype oxide’s surface
chemistry by Thornton, Lindsay, and Pang thus provides a
great overview of the generic types of behavior one can expect
at oxide surfaces, in terms of clean surface structural properties,
surface reduction and reoxidation behavior, chemisorption
properties toward small molecules, and interactions with vapordeposited metals. Metals whose most stable oxide has a heat of
formation more exothermic than the energy cost of reducing
TiO2 to Ti2O3 generally get oxidized upon adsorption, whereas
other metals (e.g., late transition metals of the type used as
catalysts) stay generally neutral and usually make metallic
clusters. Thus, metals on the left of the periodic table make
mixed-metal oxides with titanium, which are addressed in detail
in the papers described below. This review also serves to
introduce and highlight the powerful capabilities of STM for
characterizing chemistry at the surfaces of semiconducting
oxides.
After titania, ceria is one of the next most studied of oxide
surfaces, partially due to its importance as a component in
current industrial catalysts and in many promising materials for
future applications in energy and environmental technology. As
with titania, ceria is conductive enough to study with STM and
the other surface science probes, and oxygen vacancies play a
major role in its surface chemistry. Paier, Penschke, and Sauer
review the current state of knowledge of the surfaces of ceria
and doped ceria and of the interaction of small molecules with
them, with emphasis on the in-depth insights that can be
provided by state-of-the-art theoretical studies and their ability
to help with the interpretation of the sometimes complex
experimental results.
To overcome the limitations of many other oxides which are
not conductive enough to study with STM and other surface
science techniques, Freund’s group has pioneered the use of
highly ordered oxide thin films grown on metal single crystals
for surface chemical studies. The paper by Kuhlenbeck,
nderstanding the surface chemistry of oxide materials
holds great promise for impacting countless technologies
that will be critical for our energy and environmental future.
Chemical reactions at the surfaces of oxides are central to the
preparation and operation of catalysts for the production of
clean fuels and in their efficient and pollution-free use during
combustion. Oxide surface chemistry is also crucial for making
and using catalysts for the manufacture of chemicals and for
pollution cleanup, and for the production and use of fuel cells,
solar fuel photocatalysts, batteries, sorbents, and solid reactants.
Thin films of oxides must be designed and grown for everything
from microelectronic devices, computer chips, some types of
solar cells, chemical and biochemical sensors, prosthetic
medical devices, reflective and protective coatings, optical,
electro-optic and opto-electric devices, and adhesives. The
synthesis, stabilization, and utilization of oxide-based composite
materials and nanomaterials also involve a great deal of surface
chemistry. With the many advances that have been made within
the past decade in our fundamental understanding of the
surface chemistry of oxides, we are well poised to achieve great
progress in these technologies through the control of oxide
surface chemistry. The purpose of this issue is to review the
state of our fundamental and predictive understanding of this
chemistry.
Optimization of our ability to improve or enable the
technologies outlined above requires detailed knowledge of
the relationships between surface atomic- and electronic-level
structure and chemical reactivity or device-related function,
such as interfacial charge transfer or energy transfer, optical
properties, photocurrent generation, or photon emission. Thus,
we focus in this issue on systems for which much structural
information is available. This naturally means that most of the
experimental work being reviewed here has been performed on
clean and well-ordered surfaces of single crystals with
controlled surface defects, since it is only through their use
that one can measure the atomic-level structural details
required, and correlate functional properties with them. One
fortunate aspect of surface chemistry research is that the highest
quality theoretical descriptions of surfaces are usually also
performed on single crystals, since periodic boundary
conditions are needed to increase computational efficiencies
sufficiently to address the inherently many-body problem of
chemical bonding at solid surfaces. Thus, most of the
theoretical research discussed here is based on single crystal
models, too, although some of these also address the very
important role of quantum size effects.
The issue starts with a review by Woodruff of quantitative
structural characterizations of oxide surfaces using a variety of
powerful techniques that mainly involve X-ray probes and
analysis of the angular distributions of the scattered X-rays or
photoemitted electrons. Such quantitative information about
the locations of the surface atoms on clean oxide surfaces as
beautifully demonstrated here is a necessary prerequisite for
any understanding of the reactivity of that surface. This paper
focuses on the surfaces of oxides having the corundum and rock
© 2013 American Chemical Society
Special Issue: 2013 Surface Chemistry of Oxides
Published: June 12, 2013
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understanding acid−base properties of oxide surfaces and their
manifestation in catalytic reaction mechanisms and oxide
surface phenomena in general.
The very late transition metals, such as Pd, Pt, Rh, Ir, and Ru,
are catalytically active for many reactions. They are usually
dispersed across the surface of some oxide or carbon support as
nanoparticles. Because the oxides of these metals have very
small heats of formation, they are very easily reducible and the
metals are usually considered in their elemental state. However,
recent evidence suggests that they are also active in their oxide
form, and the surface chemical properties of their oxides are
often important in the preparation, activation, and reactivation
of catalysts. Weaver reviews the surface chemistry of wellordered oxide surfaces of Pd, Pt, Rh, Ir, and Ru, with emphasis
on their preparation, oxygen desorption energetics and kinetics,
and chemisorption properties toward small molecules. These
oxides show some surprising behavior. For example, PdO(101)
can adsorb short alkanes much more strongly than most oxides
and even more strongly than the metallic surfaces of Pd, and as
a consequence, PdO(101) is exceptionally reactive in cleaving
the C−H bond in small alkanes.
While the majority of this issue focuses on surface chemistry
involving the interactions of very small molecules with oxide
surfaces, where in-depth insight at the atomic scale can be
obtained experimentally, the interactions of biopolymers with
the oxide surface are extremely important for understanding a
wide variety of areas, including material biocompatibility,
biomineralization, bioanalytical chemistry and biomolecule
sensing, biofouling, and drug delivery. To give an insight into
the fundamental surface chemical issues associated with this
field, the review by Ugliengo, Rimola, Sodupe, Dominique, and
Lambert of the adsorption of biomolecules on silica surfaces
focuses on both computational modeling and experimental
studies of this complex topic. An example of the importance of
biomolecule interactions with silica surfaces is highlighted by
the recent discovery by Jeffrey Brinker’s group at Sandia
National Laboratories that biological organisms can be
effectively frozen in a rigid structure by coating with a thin
silica film, possibly removing the necessity for freeze-fracturing
for microscopic investigation of subcellular biological nanostructure (Kaehr, B.; Townson, J. L.; Kalinich, R. M.; Awad, Y.
H.; Swartzentruber, B. S.; Dunphy, D. R.; Brinker, C. J. Cellular
complexity captured in durable silica biocomposites. Proc. Natl.
Acad. Sci. U.S.A. 2012, 109, 17336−17341.).
When oxides are supported on metals and have dimensions
that are only a few atomic layers thick, they take on unique new
properties. The review by Netzer, Surnev, and Fortunelli
addresses ultrathin films of metal-supported oxides in the
extreme nanoscale regime. They discuss how nanostructures of
the supported oxide are electronically and elastically coupled to
the underlying metal surface and how this can lead to the
emergence of novel properties when the oxide film is in the
two-dimensional (2-D) thin film limit of 1−5 atomic layers or
when present as one-dimensional (1-D) oxide line structures
and (quasi)zero-dimensional (0-D) oxide clusters or nanodots.
These emergent properties result from the hybrid character and
the low dimensionality of these oxide nanostructures.
As mentioned above, mixed-metal oxides offer exciting
possibilities as catalytic, electrocatalyic, photocatalytic, and
energy storage materials. The review by Stacchiola, Senanayake,
Liu, and Rodriguez shows that when an oxide surface is covered
with a second oxide at coverages below one monolayer, new
synergistic catalytic effects can be obtained. They generated and
Shaikhutdinov, and Freund presents several case studies of this
type which provide an in-depth review of the surface structure
and adsorption properties of several very important wellordered oxide surfaces: NiO(100) and (111), M2O3(0001) (M
= Cr, V, Fe), V2O5(001), Fe3O4(111), RuO2(110), CeO2(111).
They also describe the chemistry of VOn clusters on
CeO2(111), a catalytically important system that is also
reviewed from the theoretical point of view by Paier, Penschke,
and Sauer.
MgO(100) is probably the most studied example of an
insulating oxide surface. This is partially because it can be
grown in thin film form and studied with STM, but also
because its bulk crystals cleave beautifully and because there has
been decades-long experience with preparing clean and highly
ordered surfaces of MgO(100) microcubes (also called
“MgO(100) smoke”), which is a high surface area material
whose surfaces are dominated by MgO(100) terraces.
Pacchioni and Freund review the chemistry of MgO(100),
with emphasis on electron transfer to, from, and through this
oxide surface, the role of defects in such charge transfer, and the
effect of MgO film thickness on its chemical behavior when in
the form of an ultrathin film supported on a metal. This
includes some excellent insights that result from electron
paramagnetic resonance (EPR), a technique that has rarely
been used on single crystal surfaces before, and the theoretical
modeling of EPR spectra on MgO(100).
Many oxides can have polar surfaces, which offer a wide
variety of exciting potential applications, especially if controllable at the nanoscale. Noguera and Goniakowski examine
fundamental issues regarding polar oxide surfaces and the
special properties they adopt when present on nanoscale
crystals, with a review of recent experimental and theoretical
advances. They first examine theoretically the electrostatic
characteristics of semi-infinite polar oxide surfaces and then
various polar oxide nano-objects to obtain a first insight into
the manifestations of polarity; then they successively review and
reanalyze the physics of polarity in ultrathin films, threedimensional clusters, two-dimensional nanoribbons, and
islands.
Adsorption at an oxide surface is an essential step in any
catalytic, electrocatalytic, or photocatalytic application of
oxides, and in their use as sorbents. Among the most important
properties one must know about adsorption are its fundamental
thermodynamic parameters: the enthalpy and entropy of
adsorption. Campbell and Sellers comprehensively review
experimental measurements of the heats and entropies of
adsorption on well-defined oxides surfaces, including thermodynamic studies of the adsorption of a variety of small
molecules and metal atoms on the full range of oxides surfaces.
These data can serve as important benchmarks for validating
new computational methods that are very actively being
developed today to provide better energy accuracy in modeling
the surface chemistry of oxides.
Oxide surfaces are used in countless catalytic applications,
but their catalytic reactions involving oxygenates are one of
their most important application areas. Vohs reviews the
current state of knowledge regarding the site requirements for
the adsorption and reaction of oxygenates on metal oxide
surfaces. He shows that the experimental observations can
generally be described in terms of acid−base or redox type
mechanisms and that both Brønsted and Lewis acid−base
formalisms are important to consider in this respect. His
interpretations of these studies serve as a great paradigm for
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Biographies
studied novel structures such as monomers of vanadia, 1D
strips of ruthenia, dimers of ceria, and (WO3)3 clusters on
TiO2(110). Some of these have a strong influence on the
activity of the material as a catalyst for the selective oxidation of
alkanes and the dehydrogenation of alcohols. These studies
help provide a conceptual framework for controlling the
chemical properties of mixed-metal oxides that can help us
learn how to engineer new catalysts.
Metiu and McFarland review the closely related subject of
catalysis by doped oxides (mainly with substitutional metals as
the dopants), with emphasis on an exciting new model
developed by Metiu that can explain much of their behavior
observed in computational “DFT experiments” and verified in
real experiments. This model analyzes the interactions between
the dopant and the host oxide as well as the interactions of
adsorbates with both types of centers in terms of acid−base
interactions, and it provides several simple but very useful rules
for interpreting and even predicting these interactions.
The Holy Grail in oxide surface chemistry is to use
photocatalysis, photoelectrocatalysis, and/or dye-sensitized
solar cells to help harvest the sun’s energy to make renewable
clean fuels with no environmental impact and thus a sustainable
fuels future. Henderson and Lyubinetsky review the deep
molecular-level insights into photocatalysis that have been
provided by scanning probe microscopy studies of photochemical reactions of small molecules on rutile TiO2(110)
surfaces, which again serve as a prototype system for
understanding photochemical events at oxide surfaces. To
understand photocatalysis, photoelectrocatalysis, and dyesensitized solar cells, one must have a theoretical description
of the excited states of the oxide surfaces involved, both as
extended oxide surfaces and in nanoparticle form, including
both pure and doped oxide materials. Sousa, Tosoni, and Illas
review the present state of the art in the theoretical description
of excited states at such oxide surfaces. The review by Akimov,
Neukirch, and Prezhdo summarizes the deep insights into
photocatalysis and charge transfer at oxide surfaces that have
been achieved through theoretical studies of these highly
complex phenomena using a variety of well-chosen computational approaches.
A very important area which we have not included in this
issue is the study of adsorption onto single-crystalline oxide
surfaces from liquid solutions. An extensive overview of the
beautiful work in this area has recently appeared (Brown, G. E.,
Jr.; Calas, G. Geochemical Perspectives 2012, 1 (4−5), 483−742
(DOI: 10.7185/geochempersp.1.4)).
In summary, while this issue does not give anywhere near full
coverage to all the very exciting areas of research in oxide
surface chemistry, it does provide a very broad and deep
summary of this field that we hope will be useful both as an
introductory text for new students in this field as well as a
valuable reference for use by experienced researchers.
Charles T. Campbell is a Professor and the B. Seymour Rabinovitch
Endowed Chair in Chemistry, and an Adjunct Professor of both
Chemical Engineering and Physics, at the University of Washington.
He is the author of over 270 publications on surface chemistry,
catalysis, and biosensing. He was Editor-in-Chief of the journal Surface
Science for 11 years. He is an elected Fellow of both the American
Chemical Society (ACS) and the American Association for the
Advancement of Science. He received the Arthur W. Adamson Award
of the ACS and the ACS Award for Colloid or Surface Chemistry, the
Gerhard Ertl Lecture Award, the Ipatieff Lectureship at Northwestern
University, and an Alexander von Humboldt Research Award. He
served as Chair, Chair-Elect, Vice-Chair, and Treasurer of the Colloid
and Surface Chemistry Division of the ACS. He was the founding CoDirector and Director of the University of Washington’s Center for
Nano Technology, and helped develop the USA’s first Ph.D. program
in Nanotechnology there. He received his B.S. in Chemical
Engineering (1975) and his Ph.D. in Physical Chemistry (1979,
under J. M. White) from the University of Texas at Austin, and then
did research in Germany under Gerhard Ertl (2007 Nobel Prize
Winner) through 1980.
Joachim Sauer received the Dr. rer. nat. degree in Chemistry from
Humboldt University in Berlin in 1974, and the Dr. sc. nat. degree
from the Academy of Sciences in (East-)Berlin in 1985. Since 1993 he
is Professor of Theoretical Chemistry at the Humboldt University in
Berlin, and since 2006 an external member of the Fritz Haber Institute
(Max Planck Society). He is a member of the Berlin-Brandenburg
(formerly Prussian) Academy of Sciences, the German National
Academy Leopoldina, and the Academia Europaea. His research has
explored the application of quantum chemical methods in chemistry,
with emphasis on surface science, particularly adsorption and catalysis.
He has published more than 300 research papers, notably in the area
of modeling the structure and reactivity of transition metal oxide
Charles T. Campbell*
University of Washington, Seattle
Joachim Sauer
Humboldt University, Berlin
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]
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catalysts and zeolites, and he has given more than 330 invited lectures.
From 1999 to 2011 he was chairman of the Collaborative Research
Center of the German Research Foundation (DFG) “Aggregates of
transition metal oxidesStructure, dynamics, reactivity” and he is
cofounder and principal investigator of the DFG-funded Cluster of
Excellence UNICAT in Berlin.
ACKNOWLEDGMENTS
C.T.C. gratefully acknowledges support for this work by the
U.S. Department of Energy, Office of Basic Energy Sciences,
Chemical Sciences Division under Grant #DE-FG0296ER14630. J.S. acknowledges support from the German
Research Society (DFG) within the CRC 546 “Transition
metal oxides” and the Cluster of Excellence UNICAT.
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