Introduction: Surface Chemistry of Oxides

Understanding the surface chemistry of oxide materialsholds 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 thepreparation and operation of catalysts for the production ofclean fuels and in their efficient and pollution-free use duringcombustion. Oxide surface chemistry is also crucial for makingand using catalysts for the manufacture of chemicals and forpollution 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 everythingfrom microelectronic devices, computer chips, some types ofsolar cells, chemical and biochemical sensors, prostheticmedical devices, reflective and protective coatings, optical,electro-optic and opto-electric devices, and adhesives. Thesynthesis, stabilization, and utilization of oxide-based compositematerials and nanomaterials also involve a great deal of surfacechemistry. With the many advances that have been made withinthe past decade in our fundamental understanding of thesurface chemistry of oxides, we are well poised to achieve greatprogress in these technologies through the control of oxidesurface chemistry. The purpose of this issue is to review thestate of our fundamental and predictive understanding of thischemistry.Optimization of our ability to improve or enable the

technologies outlined above requires detailed knowledge ofthe relationships between surface atomic- and electronic-levelstructure and chemical reactivity or device-related function,such as interfacial charge transfer or energy transfer, opticalproperties, photocurrent generation, or photon emission. Thus,we focus in this issue on systems for which much structuralinformation is available. This naturally means that most of theexperimental work being reviewed here has been performed onclean and well-ordered surfaces of single crystals withcontrolled surface defects, since it is only through their usethat one can measure the atomic-level structural detailsrequired, and correlate functional properties with them. Onefortunate aspect of surface chemistry research is that the highestquality theoretical descriptions of surfaces are usually alsoperformed on single crystals, since periodic boundaryconditions are needed to increase computational efficienciessufficiently to address the inherently many-body problem ofchemical bonding at solid surfaces. Thus, most of thetheoretical research discussed here is based on single crystalmodels, too, although some of these also address the veryimportant role of quantum size effects.The issue starts with a review by Woodruff of quantitative

structural characterizations of oxide surfaces using a variety ofpowerful techniques that mainly involve X-ray probes andanalysis of the angular distributions of the scattered X-rays orphotoemitted electrons. Such quantitative information aboutthe locations of the surface atoms on clean oxide surfaces asbeautifully demonstrated here is a necessary prerequisite forany understanding of the reactivity of that surface. This paperfocuses on the surfaces of oxides having the corundum and rock

salt bulk structures that comprise the two largest groups ofmetal oxide (MnOm) systems that have been studied, with theexception of TiO2, whose surface structure in its rutile form isreviewed 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 enoughto be studied with all the powerful tools of surface science; (2)it was perhaps the first oxide surface to yield itself to high-quality imaging with atomic resolution using scanningtunneling microscopy (STM); and (3) it is an importantingredient in many catalysts and biocompatible materials andhas considerable promise in solar energy conversionapplications. Thus, rutile TiO2 has served as an importantparadigm which has enabled the evolution in our understandingof how to think about oxide surfaces at the atomic level andhow 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 agreat overview of the generic types of behavior one can expectat oxide surfaces, in terms of clean surface structural properties,surface reduction and reoxidation behavior, chemisorptionproperties toward small molecules, and interactions with vapor-deposited metals. Metals whose most stable oxide has a heat offormation more exothermic than the energy cost of reducingTiO2 to Ti2O3 generally get oxidized upon adsorption, whereasother metals (e.g., late transition metals of the type used ascatalysts) stay generally neutral and usually make metallicclusters. Thus, metals on the left of the periodic table makemixed-metal oxides with titanium, which are addressed in detailin the papers described below. This review also serves tointroduce and highlight the powerful capabilities of STM forcharacterizing chemistry at the surfaces of semiconductingoxides.After titania, ceria is one of the next most studied of oxide

surfaces, partially due to its importance as a component incurrent industrial catalysts and in many promising materials forfuture applications in energy and environmental technology. Aswith titania, ceria is conductive enough to study with STM andthe other surface science probes, and oxygen vacancies play amajor role in its surface chemistry. Paier, Penschke, and Sauerreview the current state of knowledge of the surfaces of ceriaand doped ceria and of the interaction of small molecules withthem, with emphasis on the in-depth insights that can beprovided by state-of-the-art theoretical studies and their abilityto help with the interpretation of the sometimes complexexperimental results.To overcome the limitations of many other oxides which are

not conductive enough to study with STM and other surfacescience techniques, Freund’s group has pioneered the use ofhighly ordered oxide thin films grown on metal single crystalsfor surface chemical studies. The paper by Kuhlenbeck,

Special Issue: 2013 Surface Chemistry of Oxides

Published: June 12, 2013

Editorial

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Shaikhutdinov, and Freund presents several case studies of thistype which provide an in-depth review of the surface structureand adsorption properties of several very important well-ordered 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 onCeO2(111), a catalytically important system that is alsoreviewed 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 begrown in thin film form and studied with STM, but alsobecause its bulk crystals cleave beautifully and because there hasbeen decades-long experience with preparing clean and highlyordered surfaces of MgO(100) microcubes (also called“MgO(100) smoke”), which is a high surface area materialwhose 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 thisoxide surface, the role of defects in such charge transfer, and theeffect of MgO film thickness on its chemical behavior when inthe form of an ultrathin film supported on a metal. Thisincludes some excellent insights that result from electronparamagnetic resonance (EPR), a technique that has rarelybeen used on single crystal surfaces before, and the theoreticalmodeling of EPR spectra on MgO(100).Many oxides can have polar surfaces, which offer a wide

variety of exciting potential applications, especially if control-lable at the nanoscale. Noguera and Goniakowski examinefundamental issues regarding polar oxide surfaces and thespecial properties they adopt when present on nanoscalecrystals, with a review of recent experimental and theoreticaladvances. They first examine theoretically the electrostaticcharacteristics of semi-infinite polar oxide surfaces and thenvarious polar oxide nano-objects to obtain a first insight intothe manifestations of polarity; then they successively review andreanalyze the physics of polarity in ultrathin films, three-dimensional clusters, two-dimensional nanoribbons, andislands.Adsorption at an oxide surface is an essential step in any

catalytic, electrocatalytic, or photocatalytic application ofoxides, and in their use as sorbents. Among the most importantproperties one must know about adsorption are its fundamentalthermodynamic parameters: the enthalpy and entropy ofadsorption. Campbell and Sellers comprehensively reviewexperimental measurements of the heats and entropies ofadsorption on well-defined oxides surfaces, including thermo-dynamic studies of the adsorption of a variety of smallmolecules and metal atoms on the full range of oxides surfaces.These data can serve as important benchmarks for validatingnew computational methods that are very actively beingdeveloped today to provide better energy accuracy in modelingthe surface chemistry of oxides.Oxide surfaces are used in countless catalytic applications,

but their catalytic reactions involving oxygenates are one oftheir most important application areas. Vohs reviews thecurrent state of knowledge regarding the site requirements forthe adsorption and reaction of oxygenates on metal oxidesurfaces. He shows that the experimental observations cangenerally be described in terms of acid−base or redox typemechanisms and that both Brønsted and Lewis acid−baseformalisms are important to consider in this respect. Hisinterpretations of these studies serve as a great paradigm for

understanding acid−base properties of oxide surfaces and theirmanifestation in catalytic reaction mechanisms and oxidesurface phenomena in general.The very late transition metals, such as Pd, Pt, Rh, Ir, and Ru,

are catalytically active for many reactions. They are usuallydispersed across the surface of some oxide or carbon support asnanoparticles. Because the oxides of these metals have verysmall heats of formation, they are very easily reducible and themetals are usually considered in their elemental state. However,recent evidence suggests that they are also active in their oxideform, and the surface chemical properties of their oxides areoften important in the preparation, activation, and reactivationof catalysts. Weaver reviews the surface chemistry of well-ordered oxide surfaces of Pd, Pt, Rh, Ir, and Ru, with emphasison their preparation, oxygen desorption energetics and kinetics,and chemisorption properties toward small molecules. Theseoxides show some surprising behavior. For example, PdO(101)can adsorb short alkanes much more strongly than most oxidesand even more strongly than the metallic surfaces of Pd, and asa consequence, PdO(101) is exceptionally reactive in cleavingthe C−H bond in small alkanes.While the majority of this issue focuses on surface chemistry

involving the interactions of very small molecules with oxidesurfaces, where in-depth insight at the atomic scale can beobtained experimentally, the interactions of biopolymers withthe oxide surface are extremely important for understanding awide variety of areas, including material biocompatibility,biomineralization, bioanalytical chemistry and biomoleculesensing, biofouling, and drug delivery. To give an insight intothe fundamental surface chemical issues associated with thisfield, the review by Ugliengo, Rimola, Sodupe, Dominique, andLambert of the adsorption of biomolecules on silica surfacesfocuses on both computational modeling and experimentalstudies of this complex topic. An example of the importance ofbiomolecule interactions with silica surfaces is highlighted bythe recent discovery by Jeffrey Brinker’s group at SandiaNational Laboratories that biological organisms can beeffectively frozen in a rigid structure by coating with a thinsilica film, possibly removing the necessity for freeze-fracturingfor microscopic investigation of subcellular biological nano-structure (Kaehr, B.; Townson, J. L.; Kalinich, R. M.; Awad, Y.H.; Swartzentruber, B. S.; Dunphy, D. R.; Brinker, C. J. Cellularcomplexity 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 newproperties. The review by Netzer, Surnev, and Fortunelliaddresses ultrathin films of metal-supported oxides in theextreme nanoscale regime. They discuss how nanostructures ofthe supported oxide are electronically and elastically coupled tothe underlying metal surface and how this can lead to theemergence of novel properties when the oxide film is in thetwo-dimensional (2-D) thin film limit of 1−5 atomic layers orwhen present as one-dimensional (1-D) oxide line structuresand (quasi)zero-dimensional (0-D) oxide clusters or nanodots.These emergent properties result from the hybrid character andthe low dimensionality of these oxide nanostructures.As mentioned above, mixed-metal oxides offer exciting

possibilities as catalytic, electrocatalyic, photocatalytic, andenergy storage materials. The review by Stacchiola, Senanayake,Liu, and Rodriguez shows that when an oxide surface is coveredwith a second oxide at coverages below one monolayer, newsynergistic catalytic effects can be obtained. They generated and

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studied novel structures such as monomers of vanadia, 1Dstrips of ruthenia, dimers of ceria, and (WO3)3 clusters onTiO2(110). Some of these have a strong influence on theactivity of the material as a catalyst for the selective oxidation ofalkanes and the dehydrogenation of alcohols. These studieshelp provide a conceptual framework for controlling thechemical properties of mixed-metal oxides that can help uslearn how to engineer new catalysts.Metiu and McFarland review the closely related subject of

catalysis by doped oxides (mainly with substitutional metals asthe dopants), with emphasis on an exciting new modeldeveloped by Metiu that can explain much of their behaviorobserved in computational “DFT experiments” and verified inreal experiments. This model analyzes the interactions betweenthe dopant and the host oxide as well as the interactions ofadsorbates with both types of centers in terms of acid−baseinteractions, and it provides several simple but very useful rulesfor interpreting and even predicting these interactions.The Holy Grail in oxide surface chemistry is to use

photocatalysis, photoelectrocatalysis, and/or dye-sensitizedsolar cells to help harvest the sun’s energy to make renewableclean fuels with no environmental impact and thus a sustainablefuels future. Henderson and Lyubinetsky review the deepmolecular-level insights into photocatalysis that have beenprovided by scanning probe microscopy studies of photo-chemical reactions of small molecules on rutile TiO2(110)surfaces, which again serve as a prototype system forunderstanding photochemical events at oxide surfaces. Tounderstand photocatalysis, photoelectrocatalysis, and dye-sensitized solar cells, one must have a theoretical descriptionof the excited states of the oxide surfaces involved, both asextended oxide surfaces and in nanoparticle form, includingboth pure and doped oxide materials. Sousa, Tosoni, and Illasreview the present state of the art in the theoretical descriptionof excited states at such oxide surfaces. The review by Akimov,Neukirch, and Prezhdo summarizes the deep insights intophotocatalysis and charge transfer at oxide surfaces that havebeen achieved through theoretical studies of these highlycomplex phenomena using a variety of well-chosen computa-tional approaches.A very important area which we have not included in this

issue is the study of adsorption onto single-crystalline oxidesurfaces from liquid solutions. An extensive overview of thebeautiful 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 oxidesurface chemistry, it does provide a very broad and deepsummary of this field that we hope will be useful both as anintroductory text for new students in this field as well as avaluable reference for use by experienced researchers.

Charles T. Campbell*University of Washington, SeattleJoachim SauerHumboldt University, Berlin

AUTHOR INFORMATION

Corresponding Author

*E-mail: campbell@chem.washington.edu.

Biographies

Charles T. Campbell is a Professor and the B. Seymour RabinovitchEndowed Chair in Chemistry, and an Adjunct Professor of bothChemical 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 SurfaceScience for 11 years. He is an elected Fellow of both the AmericanChemical Society (ACS) and the American Association for theAdvancement of Science. He received the Arthur W. Adamson Awardof the ACS and the ACS Award for Colloid or Surface Chemistry, theGerhard Ertl Lecture Award, the Ipatieff Lectureship at NorthwesternUniversity, and an Alexander von Humboldt Research Award. Heserved as Chair, Chair-Elect, Vice-Chair, and Treasurer of the Colloidand Surface Chemistry Division of the ACS. He was the founding Co-Director and Director of the University of Washington’s Center forNano Technology, and helped develop the USA’s first Ph.D. programin Nanotechnology there. He received his B.S. in ChemicalEngineering (1975) and his Ph.D. in Physical Chemistry (1979,under J. M. White) from the University of Texas at Austin, and thendid research in Germany under Gerhard Ertl (2007 Nobel PrizeWinner) through 1980.

Joachim Sauer received the Dr. rer. nat. degree in Chemistry fromHumboldt University in Berlin in 1974, and the Dr. sc. nat. degreefrom the Academy of Sciences in (East-)Berlin in 1985. Since 1993 heis Professor of Theoretical Chemistry at the Humboldt University inBerlin, 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 NationalAcademy Leopoldina, and the Academia Europaea. His research hasexplored 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 areaof modeling the structure and reactivity of transition metal oxide

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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 ResearchCenter of the German Research Foundation (DFG) “Aggregates oftransition metal oxidesStructure, dynamics, reactivity” and he iscofounder and principal investigator of the DFG-funded Cluster ofExcellence UNICAT in Berlin.

ACKNOWLEDGMENTSC.T.C. gratefully acknowledges support for this work by theU.S. Department of Energy, Office of Basic Energy Sciences,Chemical Sciences Division under Grant #DE-FG02-96ER14630. J.S. acknowledges support from the GermanResearch Society (DFG) within the CRC 546 “Transitionmetal oxides” and the Cluster of Excellence UNICAT.

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