Editorial pubs.acs.org/CR
Introduction: Surface Chemistry of Oxides
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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 3859
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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 3860
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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]. 3861
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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 oxidesStructure, 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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