fenómenos de superficie

JChemEd.chem.wisc.edu • Vol. 75 No. 2 February 1998 • Journal of Chemical Education 161

The Flexible Surface: 162Molecular Studies Explain theExtraordinary Diversity of SurfaceChemical Properties

OutlineHistorical PerspectiveExternal Surfaces

Surface ConcentrationClusters and Small ParticlesThin Films

Internal Surfaces—Microporous SolidsClean SurfacesInterfaces

Adsorption

Techniques of Surface SciencePhenomena Discovered by MolecularSurface Chemistry

Surface Structure Is Different from Bulk StructureAdsorbate-Induced Restructuring of SurfacesRough Surfaces Do ChemistryClusterlike Bonding of Adsorbed MoleculesThe Flexible Surface

Technological Impact of Molecular SurfaceChemistryFuture Directions in Surface Chemistry

Coadsorption on SurfacesHigh-Pressure Surface ScienceMonitoring Surface Chemistry at Ever-Improving

Spatial Resolution and Time ResolutionStudies of the Buried interfaces, Solid–Liquid

and Solid–SolidAchieving 100% Selectivity in Surface

Reactions—the Environmental ImperativeNanoparticles: Surfaces in Three Dimensions

Viewpoints: Chemists on Chemistry

The Flexible SurfaceGabor A. Somorjai and Günther Rupprechter

About Viewpoints

Viewpoints is a major feature of the celebration of the Journalof Chemical Education’s 75th year. It is being supported by TheCamille and Henry Dreyfus Foundation, Inc., which recently cel-ebrated its own 50th anniversary. Each paper in the Viewpoints se-ries will be written by a chemist or group of chemists with specialexpertise in a particular field, with the aim of providing an overviewof that field’s accomplishments, importance, and prospects. The goalis to reflect on developments during the past 50 years and to predicthow each field will evolve over the next 25 years. The total perspec-tive encompassed by Viewpoints corresponds with the 75 years ofthis Journal’s lifetime and reflects its comprehensive interest in all ofchemistry. The 50-year retrospective view of each field correspondswith the period during which the Camille and Henry Dreyfus Foun-dation has been supporting the chemical sciences.

Authors of Viewpoints papers will provide perspectives on whatchemistry has done during the lifetime of the Dreyfus Foundationand to set the stage for what chemistry will become well into thenext century. The papers will be written at a level appropriate forupper-division undergraduate chemistry students and will extend andenhance the Journal’s role as, in the words of an early editor, “a liv-ing textbook of chemistry”. In addition, they will be published inelectronic format via JCE Online (whose founding was also supportedby the Dreyfus Foundation). In the Journal, Viewpoints papers willtake full advantage of color graphics, which will also appear in theelectronic version. In JCE Online there also will be links to the au-thors’ and other related Web sites, and video and animations whenrelevant.

The Viewpoints series begins this month with “The Flexible Sur-face”, a paper from the laboratories of Gabor A. Somorjai of the Uni-versity of California, Berkeley. Somorjai and his co-author, GüntherRupprechter, discuss concepts related to external and internal sur-faces and interfaces and the dynamic nature of surfaces during chemi-cal processes. A broad overview of the most frequently used surfacescience techniques is also included, along with references for eachtechnique. Somorjai and Rupprechter close with a discussion of thepresent and future technological impact of surface chemistry on ca-talysis and semiconductor devices, and the chemist’s role in surfacescience in the coming years.

Glenn T. Seaborg, Chair of the Viewpoints Editorial Board

Other Material onSurface Chemistry in This Issue

On the Surface 176AMini-Activities Exploring SurfacePhenomena: JCE Classroom Activity #6

Flying over Atoms CD-ROM 247JCE Software Special Issue 19,by John R. Markham

162 Journal of Chemical Education • Vol. 75 No. 2 February 1998 • JChemEd.chem.wisc.edu

Viewpoints

Historical Perspective

Surface science in general and surface chemistry in par-ticular have a long and distinguished history. The spontane-ous spreading of oil on water was described in ancient timesand was studied by Benjamin Franklin. The use of surface-chemical processes on a large industrial scale began in theearly part of the 19th century. The application of catalysisstarted with the discovery of the platinum-surface-catalyzedreaction of H2 and O2 in 1823 by Döbereiner. Döbereinerused this reaction in his portable “flame source”, of which hesold a large number. By 1835 the discovery of heterogeneouscatalysis was complete, thanks to the studies of Kirchhoff,Davy, Henry, Philips, Faraday, and Berzelius (1). It was aboutthis time that the Daguerre process was introduced for photog-raphy. The study of tribology, which includes friction, lubrica-tion, and adhesion, also started around this time; this coin-cides with the industrial revolution, as machinery with mov-ing parts became prevalent (although some level of under-standing of friction appears in the work of Leonardo da Vinci).Surface-catalyzed-chemistry-based technologies first appearedin the period of 1860 to 1912, starting with the Deacon pro-cess (2HCl + 1/2 O2 → H2O + Cl2), SO2 oxidation to SO3(Messel, 1875), the reaction of methane with steam to produceCO and H2 (Mond, 1888), ammonia oxidation (Ostwald,1901), ethylene hydrogenation (Sabatier, 1902), and ammoniasynthesis (Haber, Mittasch, 1905–1912). Surface tensionmeasurements and recognition of equilibrium constraints onsurface chemical processes led to the development of the thermo-dynamics of surface phases by Gibbs (1877).

The existence of polyatomic or polymolecular aggregatesthat lack crystallinity and diffuse slowly (gelatine and albu-min, for example) was described in 1861 by Graham, whocalled these systems “colloids”. Polymolecular aggregates thatexhibit internal structure were called “micelles” by Nageli,and stable metal colloids were prepared by Faraday. The colloidsubfield of surface chemistry gained prominence in the be-ginning of the 20th century with the rise of the paint industryand the preparation of artificial rubbers. Studies of high-surface-area gas-absorber materials for gas masks and other gas-separationtechnologies and investigations of the lifetime of the light bulbfilament led to the determination of the dissociation prob-ability and adsorption probability of many diatomic moleculeson surfaces as a function of gas pressure (adsorption isotherm)and temperature (Langmuir 1915). The properties of chemi-sorbed and physisorbed monolayers, adsorption isotherms,dissociative adsorption, energy exchange, and sticking upongas–surface collisions were studied. Studies of electrode sur-faces in electrochemistry led to detection of the surface spacecharge (2). Surface diffraction of low-energy electrons wasdiscovered by Davisson and Germer in 1927, and atom dif-fraction (helium) from surfaces, somewhat later. Major aca-

demic and industrial laboratories focusing on surface studieshave been formed in Germany (Haber, Polanyi, Farkas,Bonhoefer), the United Kingdom (Rideal, Roberts, Bowden),the United States (Langmuir, Emmet, Harkins, Taylor, Ipatief,Adams), and many other countries. They have helped to bringsurface chemistry into the center of development of chemistry,both because of the intellectual challenge to understand therich diversity of surface phenomena and because of its im-portance in chemical and energy conversion technologies.

Up to the 1950s, studies of surfaces were mostly on themacroscopic scale. Then the rise of the solid-state-device-based electronics industry and the availability of economicalultra-high-vacuum systems—developed by research in spacesciences—provided surface chemistry with new challengesand opportunities and resulted in explosive growth of the dis-cipline. Clean surfaces of single crystals could be studied forthe first time and the development of a large number of newtechniques (cf. section on Techniques of Surface Science) fromthe 1960s onwards made possible the investigation of surfacesat atomic and molecular levels.

As a result, macroscopic surface phenomena (adsorption,bonding, catalysis, oxidation, and other surface reactions; dif-fusion, desorption, melting, and other phase transformations;growth, nucleation, charge transport; atom, ion, and elec-tron scattering; friction, hardness, lubrication) are being re-examined on the molecular scale. This has led to a remark-able growth of surface chemistry that has continued unin-terrupted up to the present. The discipline has again becomeone of the frontier areas of chemistry. The newly gainedknowledge of the molecular ingredients of surface phenomenahas given birth to a steady stream of high technology prod-ucts, including new hard coatings that passivate surfaces;chemically treated glass, semiconductor, metal, and polymersurfaces to which the treatment imparts unique surface proper-ties; newly designed catalysts, chemical sensors, and carbonfiber composites; surface-space-charge-based copying; andnew methods of electrical, magnetic, and optical signal pro-cessing and storage. Molecular surface chemistry is being uti-lized increasingly in biological sciences.

External Surfaces

Surface ConcentrationThe concentration of atoms or molecules at the surface

of a solid or liquid can be estimated from the bulk density.For a bulk density of 1 g/cm3 (such as ice or water), the mo-lecular density ρ, in units of molecules per cubic centimeter,is ≈ 5 × 1022. The surface concentration of molecules σ (mol-ecules/cm2) is proportional to ρ2/3, assuming cubelike pack-ing, and is thus on the order of 1015 molecules/cm2. Becausethe densities of most solids or liquids are all within a factorof 10 or so of each other, 1015 molecules/cm2 is a good order-

The Flexible Surface: Molecular Studies Explainthe Extraordinary Diversity of Surface Chemical Properties

Gabor A. Somorjai and Günther RupprechterDepartment of Chemistry, University of California at Berkeley, and Materials Sciences Division, E. O. Lawrence BerkeleyNational Laboratory, Berkeley, CA 94720; http://www.cchem.berkeley.edu/~gasgrp


The Flexible Surface

of-magnitude estimate of the surface concentration of atomsor molecules for most solids or liquids. Of course, surfaceatom concentration of crystalline solids may vary by a factorof two or three, depending on the type of packing of atomsat a particular crystal face.

Clusters and Small ParticlesAll atoms in a three- or four-atom cluster are by necessity

“surface atoms”. As a cluster grows in size, some atoms maybecome completely surrounded by neighboring atoms andare thus no longer on the “surface” (Fig. 1). We frequentlydescribe a particle of finite size by its dispersion D, where Dis the ratio of the number of surface atoms to the total numberof atoms:

D = number of surface atomstotal number of atoms

For very small particles, D is unity. As the particle growsand some atoms become surrounded by their neighbors, thedispersion decreases (Fig. 1). Of course, D also depends some-what on the shape of the particles and how the atoms arepacked. The dispersion is already as low as 10�3 for particles of10-nm (100-Å) radius.

Many chemical reactions are facilitated by surface atomsof heterogeneous catalysts. These catalysts increase the rate offormation of product molecules and modify the relative dis-

tribution of products. Most catalysts are in small-particleform, including those used to produce fuels and chemicalsranging from high-octane gasoline to polyethylene.

Thin FilmsConsider a monolayer of gold atoms (a layer of gold atoms

one atom thick) deposited on iron (Fig. 2). This film has adispersion of unity, since all the atoms are on the surface. About50 layers of gold atoms (D = 1/50) are needed to obtain theoptical properties that impart the familiar yellow color char-acteristic of bulk gold.

Thin films are of great importance to many real-worldproblems. Their material costs are very little compared to costof the bulk material, and they perform the same functionwhen it comes to surface processes. For example, a monolayerof rhodium (a very expensive metal), which contains onlyabout 1015 metal atoms per square centimeter, can catalyzethe reduction of NO to N2 by its reaction with CO in thecatalytic converter of an automobile, or it can catalyze theconversion of methanol to acetic acid by the insertion of aCO molecule.

Thin ordered silicon layers optimize electron transportin integrated electronic circuits and thin films of organicmolecules lubricate our skin or the moving parts of internalcombustion engines. A green leaf is a high-surface-area systemdesigned to maximize the absorption of sunlight in order tocarry out chlorophyll-catalyzed photosynthesis at optimum rates.

Often the surface of a thin film is roughened deliber-ately. Automobile brake pads are designed to optimize thedesired mechanical properties of surfaces in this way, as isthe corrugated design of rubber soles of tennis shoes. Thelarge number of folds of the human brain helps to maximizethe number of surface sites, which also facilitate charge trans-port and transport of molecules. These are some examplesthat show how external surfaces are used in nature. Externalsurfaces are a key element of technology, ranging from cata-lysts and passivating coatings to computer-integrated circuitryand the storage and retrieval of information.

Figure 1. Clusters of atoms with cubic packing having 8, 27, 64,125, and 216 atoms. While in an 8-atom cluster all of the atomsare on the surface, the dispersion rapidly declines with increasingcluster size, as shown in the lower part of the figure.

Figure 2. An iron particle with one surface covered with a mono-layer of gold atoms. When it comes to surface properties such asadsorption or catalysis, one monolayer of atoms is all that is neededto carry out the necessary chemistry.


Viewpoints

Internal Surfaces—Microporous Solids

Microporous solids are materials that are full of pores ofmolecular dimensions or larger. These materials have verylarge internal surface areas. Many clays have layer structuresthat can accommodate molecules between the layers by a pro-cess called intercalation. Graphite will swell with water vaporto several times its original thickness as water molecules be-come incorporated between the graphitic layers. Crystallinealumina silicates, often called zeolites, have ordered cages ofmolecular dimensions (3), where molecules can adsorb orundergo chemical reactions (Fig. 3). These materials are alsocalled molecular sieves, because they may preferentially adsorbcertain molecules according to their size or polarizability. Thisproperty is of great commercial importance and may be usedto separate mixtures of gases (air) or liquids or to carry outselective chemical reactions. Bones of mammals are made outof calcium apatite, which has a highly porous structure, withpores on the order of 10 nm (100 Å) in diameter. Coal andchar have porous structures, with pore diameters on the orderof 102–103 nm (103–104 Å). These materials have very largeinternal surface areas, in the range of 100–400 m2 per gramof solid. As this short survey shows, nature has provided uswith many useful microporous materials; and many syntheticmicroporous substances are used in technology, both to sepa-rate gas and liquid mixtures by selective adsorption and tocarry out surface reactions selectively in their pores, whichare often of molecular dimensions. Because surface reactionrate (product molecules formed per second) is proportionalto surface area, materials with high internal surface areas carryout surface reactions at very high rates.

Clean Surfaces

To study atomically clean surfaces, we must work underso-called ultrahigh vacuum (UHV) conditions (4, 5), as the fol-lowing rough calculation shows. We know that the concentra-tion of atoms on the surface of a solid is on the order of 1015

cm�2. To keep the surface clean for 1 s or for 1 h, the flux ofmolecules incident on the initially clean surface must thereforebe less than ≈ 1015 molecules/cm2/s or ≈ 1012 molecules/cm2/s,respectively. From the kinetic theory of gases (6), the flux, F, ofmolecules striking the surface of unit area at a given ambientpressure P is

F =NAP

2πMRTor

F (atoms/ cm2 ⋅ s) =

2.63 × 1020 ⋅ P (Pa)

M (g / mol) T

or

F (atoms/ cm2 ⋅ s) =

3.51 × 1022 ⋅ P (torr)

M (g / mol) T

where M is the average molar weight of the gaseous species,T is the temperature (in Kelvin), R is the gas constant, andNA is Avogadro’s number. Substituting P = 4 × 10�4 Pa (3 ×

10�6 torr) and using the values M = 28 g/mol (for N2) and T= 300 K, we obtain F ≈ 1015 molecules/cm2/s. Thus, at thispressure the surface is covered with a monolayer of gas withinseconds, assuming that each incident gas molecule “sticks”.For this reason the unit of gas exposure is 1.33 × 10�4 Pa-s(10�6 torr-s), which is called the Langmuir (L). Thus, a 1-Lexposure will cover a surface with a monolayer amount ofgas molecules, assuming a sticking coefficient of unity. Atpressures on the order of 1.33 × 10�7 Pa (10�9 torr), it maytake 103 s before a surface is covered completely.

In practice, one usually wants to study a surface with-out worrying about contamination from ambient gases. Cur-rent surface science techniques can easily detect contamina-tion on the order of 1% of a monolayer. This then will beour operational definition of “clean”. Thus, ultrahigh vacuumconditions (< 1.33 × 10�7 Pa = 10�9 torr) are required to main-tain a clean surface for about 1 h, the time usually needed toperform experiments on clean surfaces.

Interfaces

In most circumstances, however, and certainly in Earth’senvironment, surfaces are continually exposed to gases or liq-uids or placed in contact with other solids. As a result, weend up investigating the properties of interfaces—that is, be-tween solids and gases, between solids and liquids, betweensolids and solids, and even between two immiscible liquids.Thus, unless specifically prepared otherwise, surfaces are al-ways covered with a layer of atoms or molecules from theenvironment.

Figure 3. Alumina silicates with pores of molecular dimensions(zeolites) are used as selective absorbers of gases or liquids (mo-lecular sieves) and as catalysts in chemical and petroleum tech-nologies. The figure shows a synthetic zeolite, zeolite A. Thered spheres represent oxygen atoms and the yellow spheres rep-resent either silicon or aluminum atoms. For each aluminum thereis a corresponding Na+ ion somewhere in one of the open chan-nels. The molecular formula of this molecular s ieve isNa12(Al12Si12O48) �27H2O.



AdsorptionOn approaching the surface, each atom or molecule en-

counters an attractive potential that ultimately will bind itto the surface under proper circumstances. The process thatinvolves trapping of atoms or molecules incident on the sur-face is called adsorption. It is always an exothermic process.For historical reasons, the heat of adsorption ∆Hads is alwaysdenoted as having a positive sign—unlike the enthalpy ∆H,which for an exothermic process would be negative accordingto the usual thermodynamic convention. The residence timeτ of an adsorbed atom (7) is given by

τ = τ0 exp(∆Hads/RT )

where τ0 is correlated with the surface-atom vibration times(it is frequently on the order of 10�12 s), T is the tempera-ture, and R is the gas constant. The value of τ can be 1 s orlonger at 300 K for ∆Hads > 69 kJ/mol (16.5 kcal/mol). Thesurface concentration σ (in molecules/cm2) of adsorbed mol-ecules on an initially clean surface is given by the product ofthe incident flux F and the residence time τ:

σ = Fτ

The surface of the material on which adsorption occursis often called the substrate. Substrate-adsorbate bonds areusually stronger than the bonds between adsorbed molecules.As a result, the monolayer of adsorbate bonded to the sub-strate is held most tenaciously and is difficult to remove.Therefore, the properties of real surfaces are usually deter-mined in the presence of an adsorbed monolayer.

Techniques of Surface Science

Over the last three decades, a large number of techniqueshave been developed to study various surface properties, in-cluding structure, composition, oxidation states, and changesof chemical, electronic, and mechanical properties. The em-phasis has been on surface probes that monitor propertieson the molecular level and are sensitive enough to detect ever-smaller numbers of surface atoms. The frontiers of surfaceinstrumentation are constantly being pushed toward detec-tion of finer detail: atomic spatial resolution, ever-smallerenergy resolution, and shorter time scales. Because no singletechnique provides all necessary information about surfaceatoms, the tendency is to use a combination of techniques.The most commonly used techniques (Table 1) involve thescattering, absorption, or emission of photons, electrons, at-oms and ions, although some important surface-analysis tech-niques cannot be classified this way.

Most surface probes require high vacuum during theirapplication, which prevents their use during high-pressurestudies. To circumvent this restriction, UHV-compatiblehigh-pressure cells (“environmental cells”) were developed (8).The sample to be analyzed is first subjected to the usual high-pressure and/or high-temperature conditions encounteredduring reactions in the environmental cell. Afterwards thesample is transferred, without exposure to air, into the evacu-ated UHV chamber where the surface probe is located forsubsequent surface analysis (generally the sample surfaceshould be characterized before and after any treatment).

During the past five years, two new surface science tech-niques in particular proved capable of obtaining molecular-level surface information during chemical change at both lowand high ambient pressures (9, 10): scanning tunneling micros-copy (STM) and infrared–visible sum frequency generation(SFG) surface vibrational spectroscopy. Both of these techniquescan operate within a 14-order-of-magnitude pressure range(10�10–104 torr) without significant change in signal qualityin terms of spatial or energy resolution. Using these two tech-niques, we can monitor both substrate and adsorbate structuresduring reactions at high pressures. One such apparatus,designed for in situ STM, is shown in Figure 4.

Sample preparation is always an important part of surfacestudies. Single crystals are oriented by X-ray back-diffraction,cut, and polished. They are then ion-bombarded or chemi-cally treated to remove undesirable impurities from their sur-faces. Thin films are deposited from vapor by sublimation,sputtering, or the use of plasma-assisted chemical vapor depo-sition. Materials of high internal surface area are preparedfrom a sol-gel or by calcination at high temperatures. Thegenesis and environmental history of the surface is primarilyresponsible for its structure and composition and must al-ways be carefully monitored.

Table 1 lists a selection of the surface-science techniquesused most frequently in recent years to learn about the inter-face on the atomic scale. The names of the techniques, theiracronyms, and brief descriptions are provided (along withsome references [11–39], if a more detailed study of the ca-pabilities and limitations of a particular technique is desired).We also indicate the primary surface information that canbe obtained by the application of each technique.

Figure 4. Schematic diagram of a scanning tunneling microscopecapable of operating in a pressure range from UHV to 1 atmo-sphere. The STM is located inside a high-pressure chemical reac-tor, which is attached to a UHV surface characterization chamber.The two sections are separated by a gate valve. For surface clean-ing and analysis, a transfer system is used to move the samplefrom the high-pressure cell to the UHV part of the apparatus (andvice versa).


Viewpoints

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Viewpoints

Phenomena Discovered by Molecular Surface Chemistry

Surface Structure Is Different from Bulk StructureTwo dominant phenomena occurring at clean surfaces

of materials distinguish their atomic structure from that inthe bulk: relaxation and reconstruction (40). Upon relaxationof metal surfaces, the first layer of atoms moves inward, andthis contraction leads to a much shortened interlayer spac-ing between the first and second layer of the surface. Themore open (“rougher”) the surface, the larger the relaxation.Often, but not always, the contraction in the first layer isfollowed by a small expansion in the second layer. At roughedges, such as at stepped surfaces, the atoms at the step relaxby a large amount in order to smooth the surface irregular-ity. This is shown schematically in Figure 5.

At ionic surfaces, the nature of surface relaxation is verydifferent. Figure 6 shows what happens at iron oxide surfaces(41). Iron oxide, in its bulk structure, shows alternating layersof oxygen ions and iron ions, where the iron ions are in tetra-hedral or octahedral positions. At the surface, the two ions

move in such a way that the positive and negative ions arealmost coplanar. Presumably because of the necessary conditionof charge neutrality, this type of surface structure is thermo-dynamically more stable than having alternating oxygen-ionand iron-ion layers. Such an expansion at the surface is clearlya property of ionic solids, and future studies will prove howgeneral this type of relaxation is.

Because of directionality of bonding in most solids, suchcontraction or relaxation at the surface moves atoms from theirposition of optimum bonding. As a result, the atoms not onlymove perpendicular to the surface, but also parallel to the sur-face. This leads to the formation of new surface unit cells, aphenomenon called surface reconstruction. Perhaps the most cel-ebrated example is the (7×7) surface structure that forms on theSi(111) crystal face. Figure 7a shows the low-energy electron dif-fraction (LEED) pattern from this surface. The complex unitcell has 49 different locations of surface atoms that are distin-guishable. The Si(100) surface shows a (2×1) reconstruction (Fig.7b). It shows the formation of staggered dimers differing fromthe arrangement of Si atoms in the bulk near the surface.

Figure 5. Restructuring at a step site on a clean surface. Each atomattempts to optimize its coordination and “cracks” open to closethe step edges.

Figure 6. Side (a) and top (b) views on the Fe3O4(111) surfacestructure with the spacing relaxations shown. The correspondingbulk values are ∆ = 0.63 Å, and d12 = d23 = 1.19Å. The A and Blayers are strongly expanded by ~ 0.46Å.

Figure 7. Left: Low-energy electron diffraction (LEED) pattern of the reconstructed Si(111) crystal face, exhibiting a (7×7) surface structure.Right: The reconstructed Si(100) crystal face as obtained by LEED surface crystallography. Note that surface relaxation extends to threeatomic layers into the bulk.



Pt, Au, and Ir (100) surfaces that should have squareunit cells reconstruct to form hexagonal surface unit cells (40,42). This is shown in Figure 8. STM studies have imagedboth reconstructed and unreconstructed surfaces (some domainsmay be unreconstructed because of contamination by adsor-bates of various types).

The water molecules at the ice surface vibrate with a muchlarger amplitude (0.24 Å) than molecules in bulk ice (0.1 Å)(43). This motion causes surface “softness”, which is likelyresponsible for the anomalously low friction coefficient of ice(i.e., it makes ice so slippery).

Molecules at the liquid–vapor interface also show restruc-turing, as shown in Figure 9 for the arrangement of moleculesat liquid alcohol surfaces (44). The alcohol molecules are ori-ented with their O–H bonds pointing inward, presumablyfor optimum hydrogen bonding. The alkane chains stick outfrom the surface. This orientation has been readily detectableby nonlinear laser optics sum frequency generation (SFG).

Adsorbate-Induced Restructuring of SurfacesWhen the clean surfaces are covered with a near mono-

layer of chemisorbed molecules, the structure of the surfaceundergoes profound alterations. This is perhaps best shownin the field ion microscopy (FIM) studies carried out by Kruseand coworkers (45) with rhodium field emission tips (Fig. 10).When carbon monoxide is chemisorbed on these tips, everycrystal face restructures as shown by the figure. This massiverestructuring is reversible if CO is removed when the surfaceis heated in vacuum.

Our new STM system (Fig. 4) is placed in an environ-mental cell that can be pressurized and heated to elevatedtemperatures. It shows surface restructuring of the Pt(110)surface when this surface is exposed to atmospheric pressuresof hydrogen, then oxygen, and then carbon monoxide (46).When these surfaces are heated, the surface restructures froma reconstructed ordered structure (exhibiting the so-called“missing row” reconstruction) in the presence of hydrogen,

to large (111) orientation facets in the presence of oxygen,and then again to smooth (110) unreconstructed surfaces inthe presence of carbon monoxide (Fig. 11).

Low-energy electron diffraction surface crystallographystudies indicate the detailed atomic level nature of such re-constructions (42). When carbon is adsorbed on the Ni(100)surface, it occupies fourfold hollow sites (Fig. 12). As a re-sult of the formation of the carbon–metal chemisorptionbonds, the surface metal atoms move away from the adsorptionsite, presumably to give more space to the carbon atom so itcan sink deeper into the surface, thereby forming bonds withsecond-layer nickel atoms underneath (47). This expansionaround the chemisorption site induces strain, which is relievedby rotation of the surface unit cell as shown in Figure 12.

When sulfur is chemisorbed on the Fe(110) crystal face(48), the S atom pulls the neighboring Fe atoms into equaldistances from the chemisorption site to form four equal Fe–Sbonds. The strength of these bonds pays for the weakening ofthe metal–metal bonds as a result of the restructuring. WhenNO is adsorbed on the Ni(111) surface, the molecule occupiesa threefold hollow site, a so-called hcp (hexagonal close-packed)hollow site. This means there is a metal atom directly under-neath the chemisorption site in the second metal layer.Chemisorption induces an upward movement of this metalatom in the second layer, and rumpling of the metal surface.When ethylene adsorbs on the Rh(111) surface (49), it re-arranges and occupies a hollow site (in this case, again, anhcp hollow site). The rearranged ethylene (which has lost ahydrogen) is called ethylidyne. This is shown in Figure 13.The metal atoms move away from the carbon atom boundto the hollow site to allow the carbon to bond to the Rh atomdirectly underneath the carbon in the second layer. On thePt(111) surface (50, 51), ethylene also forms an ethylidynemolecule—again in a threefold hollow site, but in this caseit is an fcc hollow site. That is, there is no metal atom di-rectly underneath the carbon in the second metal layer. Inthis circumstance, the surface metal atoms move inward topresumably provide as strong a bond as possible to the carbon,and metal–metal distances are altered on the surface as well.The second metal atom next to the chemisorption bondmoves downward to produce a corrugated surface. It appearsthat surface bonding is clusterlike, where nearest-neighbormetal atoms that surround the adsorbate move to optimizethe surface chemical bond. The heat of adsorption, which isalways exothermic, pays for the weakening of the next nearestneighbor metal–metal bonds, which are altered as a result ofthe movement of the metal atoms nearest to the chemisorp-tion site.

Figure 8. Top and side view of the Ir(100)-(1×5) surface reconstruc-tion. The more open square (100) lattice is reconstructed into aclose-packed hexagonal overlayer, with a slight buckling as shownin the side view.

Figure 9. The normal alcohols show substantial ordering at the liq-uid–vapor interface. The OH groups of the alcohols extend intothe liquid forming a hydrogen-bonded network, while the alkylchains are oriented away from the liquid.


Viewpoints

When two molecules are coadsorbed on the surface,adsorbate-induced reconstruction may be very different fromwhen only one or the other molecule is chemisorbed. This isshown for benzene and CO coadsorption on the Rh(111)surface (Fig. 14). When these molecules form a mixed unitcell, under the benzene the metal atoms are closer to theirbulklike configuration than under the CO molecule (52).There is rumpling of the surface that occurs because of thedifferences in chemical bonding of the coadsorbed species tothe substrate metal atoms. When benzene adsorbs alone it isbent. Four of the C atoms are in one type of surface site whiletwo of the others are in different types of surface sites. Whencoadsorption occurs, the benzene molecule is flattened out.

Rough Surfaces Do ChemistrySurface irregularities, steps, and kinks are very effective

for breaking adsorbate chemical bonds and in catalysis as well.This is best known by the temperature-programmed desorption(TPD) of H2 from flat, stepped, and kinked surfaces of Pt(Fig. 15). H2 desorbs at maximum rates at the highest tempera-ture from kink sites, then at somewhat lower temperatures fromstep sites, and at even lower temperatures from flat (111) ter-races (53). This indicates higher heats of adsorption of the H

atom at these defect sites. Thus, the thermodynamic drivingforce for dissociation is certainly greater at these sites, whichcan explain their enhanced bond-breaking activity.

It is difficult to understand, however, that these samestrongly adsorbing sites are also very active sites for catalysis.This is shown in Table 2. The reaction probability of H2/D2exchange on stepped surfaces is near unity at low pressureson a single scattering event, whereas it is below the detectionlimit (< 10�3) on the flat (111) crystal face, as shown by mo-lecular beam surface scattering studies (54). How is it possiblethat the strongly adsorbing step sites, where H has a longresidence time because of its high binding energy, are alsothe sites of rapid reaction turnover?

One possible explanation is that the strongly adsorbedhydrogen restructures the surface near the step, thereby creat-

Figure 10. Field ion micro-graphs (image gas: Ne; T= 85 K) of a (001)-orientedRh tip (top left) before and(bottom left) after reactionwith 10�4 Pa CO for 30 minat 420 K . The stereo-graphic projections at theright demonstrate thechange in morphology fromnearly hemispherical to po-lygonal. The scheme at thebottom right indicates thecoarsening of the crystaland the dissolution of anumber of crystallographicplanes due to the reactionwith CO.

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ing the active site for the catalytic exchange process. At thelow pressures of these molecular beam scattering experiments,the low coverages keep the structure of the flat part of thesurface unaltered.

Similarly, stepped Ni surfaces dehydrogenate C2H4 atmuch lower temperatures (< 150 K) than the (111) face ofNi (~230 K). The more open (111) and (211) crystal facesof Fe (Fig. 16) are several orders of magnitude more activefor NH3 synthesis than the close-packed Fe (110) crystal face,which showed no detectable reaction rate (55).

Clusterlike Bonding of Adsorbed MoleculesWhen ethylene chemisorbs at ~ 300 K on the (111) crystal

faces of various transition metals (Pt, Rh, Pd), it chemicallyrearranges to form the molecule-surface compound shownin Figure 13. Its structure as determined by LEED-surfacecrystallography is very similar to multinuclear organometal-lic complexes such as Os3 CCH3 or Co3 (CO)9 CCH3. Therearranged ethylene, which has also lost a hydrogen, is calledethylidyne and belongs to the alkylidyne group (species of theformula CnH2n�1), a common substituent in surface chemistry

and organometallic chemistry. The vibrational spectrum ofchemisorbed ethylidyne is nearly identical to that of the organo-metallic cluster Os3 CCH3, which contains three metal atoms.The C–C bond distance (1.45 Å) is slightly less than thesingle carbon–carbon bond length of 1.54 Å (0.154 nm), asin cluster compounds. Thus, the surface chemical bond ofchemisorbed ethylene can, as a first approximation, be viewedas a clusterlike bond that contains at least three metal atoms(56). The C–C bond order present in gaseous ethylene isreduced from two to nearly one upon chemisorption. Thisreduction in bond order of alkenes and alkynes upon chemi-sorption on metal surfaces is commonly observed, indicatingcharge transfer from the molecules into the metal.

Benzene usually chemisorbs on metals with its ring parallelto the surface (although it may adsorb in a different configura-tion when it loses hydrogen). Because of charge transfer to themetal, C–C bond elongations occur with respect to the sym-metry of the adsorption site. The ring may even bend, withtwo of the opposing carbon atoms closer to the metal surfacethan the other four carbon atoms. Distortions and elonga-tions of C–C bonds are also found when benzene is boundto clusters of metal atoms in organometallic complexes. Thusthe clusterlike bonding model appears to be valid for chemi-sorbed benzene as well.

The bonding picture of adsorbed molecules becomesmore complicated if there are more bonding sites availableon the same molecule. For example, pyridine (C5H5N) maybind through the lone electron pair of its nitrogen or throughthe π-electrons of the carbon ring. Thus, depending on themetal, the binding geometry of the substrate, the temperature,or the adsorbate coverage, the molecule may be tilted withrespect to the substrate surface, its ring may be parallel with it,or it may be upright with bonding solely through the nitrogen.

It is too simplistic to consider that only the nearest-neighbor metal atoms of the substrate participate in the bond-ing. There is evidence that the atoms at next-nearest-neigh-bor sites change their location when chemisorption occurs,moving either closer or further away from the chemisorp-tion bonds.

Figure 11. In situ high-pressure STMpictures showing adsorbate-inducedsurface reconstructions of Pt(110) un-der atmospheric pressures: Top: Topo-graphic image of the surface in hydro-gen after heating to 425 K for 5hours, showing (n×1) missing-row re-construction randomly nested. Verticalrange: ∆z = 10 Å. Center: Topo-graphic image of the surface in oxy-gen after heating to 425 K for 5hours; ∆z = 25 Å. Bottom: Topo-graphic image of the surface in car-bon monoxide after heating to 425 Kfor 4 hours; ∆z = 42 Å.

Figure 12. Carbon-chemisorption-induced restructuring of theNi(100) surface.


Viewpoints

The Flexible SurfaceAs a result of all these studies that indicate adsorbate-

induced restructuring and clusterlike bonding, the new modelof the surface which has been adopted is the so-called “flex-ible surface” (53). In the past it was assumed that the metalatoms at the surface are rigid and occupy equilibrium sitesdictated by the bulk unit cell. On adsorption, their locationwould not be altered. Instead, the flexible surface is one wherethe metal atoms move into new sites, dictated by the chemi-sorption bond so as to optimize that bond: upon adsorptionthe surface restructures, thereby creating the active sites forsurface chemical processes.

The flexible-surface model explains why rough surfacesor defect sites at surfaces are so active in surface chemistry.Bond breaking and catalysis most frequently occur at lowcoordination sites such as steps and kinks or at defect sitessuch as oxygen vacancies in oxide surfaces. The lower the co-ordination of metal atoms (the fewer nearest neighbors), themore easily they restructure to optimize the surface adsorp-tion bond. Thus, rough surfaces or atoms at steps move morereadily, and of course small clusters of atoms where the co-ordination is much reduced are the most flexible. It is notsurprising therefore that we use nanoclusters in the field ofcatalysis (and in many instances chemisorption) to optimizechemical effects, such as chemical reactions or adsorption.

The extraordinary diversity of surface chemistry is dueto the chameleon-like change of surface structure and bond-ing as the chemical environment of the surface is altered.Platinum is an excellent combustion catalyst operating in anoxidizing atmosphere and is a primary ingredient of the auto-mobile catalytic converter. Platinum is also an excellent catalystfor hydrocarbon conversion under reducing conditions toproduce high-octane gasoline (aromatic molecules andbranched isomers) from straight chain alkanes (for examplefrom n-hexane and n-heptane, which have octane numbersnear zero). The platinum surface structure and thus its bondingbehavior is completely different under the different reactionconditions, thereby mediating dramatically different catalyticsurface chemistry.

Technological Impact of Molecular Surface Chemistry:Selected Examples

Molecular surface chemistry contributed to the devel-opment and improvement of a wide range of technologies(57). A complete description is far beyond the limits of thisarticle. In this section we will discuss only its present andfuture impact on catalysis and semiconductor devices.

Since the early 1970s, molecular surface chemistry hasmade significant contributions to our understanding of catalyst-

Figure 13. The structure ofethylidyne on Rh(111). Ethylidyneis bonded on the hcp 3-fold hollowsite. This site has a metal atom rightunderneath the carbon bonding sitein the second layer. The adsorption-induced distortion in the top metallayers pulls the nearest neighbormetal atoms up out of the surfaceplane.

Figure 14. The coadsorbed surface structure of benzene and car-bon monoxide on the Rh(111) crystal face as obtained by low-energy electron diffraction surface crystallography.



based chemical and petroleum technologies. The hydrogenationof carbon monoxide yields methane or methanol exclusively,oxygenated molecules containing several carbon atoms or liquid,and high-molecular-weight hydrocarbon products (dependingon the type of catalyst employed) (40). The dissociation ofCO was found to be the dominant reaction step in producingmethane, followed by stepwise hydrogenation of the surfacecarbon over several transition metal surfaces. Potassium wasfound to be an outstanding “promoter” of CO dissociationthrough weakening of the C–O bond by charge transfer.Methanol production was found to occur through the hydro-genation of undissociated CO2 or CO (which reaction domi-nates depends on catalyst formulation). Higher-molecular-weighthydrocarbons are produced by secondary polymerizationreactions.

The reforming of naphtha over platinum was found tobe a structure-sensitive reaction. By altering the surface struc-ture of platinum particles [(111) or (100) orientation] theproduct distribution could be altered. Bimetallic platinum-based catalysts (Pt–Re, Pt–Ir, and Pt–Sn) have also been in-vestigated by surface-science studies. These studies have con-tributed greatly to their optimization in this important high-octane fuel producing technology.

The same is true for the iron-based catalyst that producesammonia from N2 and H2. The structure sensitivity of thisreaction was uncovered, implicating the (111) and (211) crys-tal faces of iron as the most active (58, 59) (Fig. 16). Thesurface structures that are more open and contain sites thatare surrounded by seven iron neighbor atoms (C7 sites) arethe most active. These are the (111) and (211) crystal faces.The structure of the (110) crystal face does not allow theadsorbed nitrogen species to bind with second- and third-layeratoms (55), and probably for this reason the rate for the syn-thesis of ammonia is some 500 times lower than on the (111)

Figure 15. Thermal desorption spectra of hydrogen from a flat(111), a stepped (557), and a kinked (12,9,8) surface of plati-num.

Figure 16. Schematic representations of the idealized surface struc-tures of the (111), (211), (100), (210), and (110) orientation ofiron single crystals (the coordination of each surface atom is indi-cated). The bar diagram shows the corresponding activity of thesingle crystal surfaces in ammonia synthesis.


Viewpoints

surface structure. The role of potassium promoters as bond-ing modifiers in aiding the dissociation of N2, as well as weak-ening the bonding of ammonia to the iron surface to inhibitproduct poisoning, has been uncovered. The role of aluminaas a structure modifier, aiding the restructuring of iron par-ticles to possess crystal faces most active for ammonia synthe-sis [(111) and (211)], has been proven by surface-science stud-ies. As a result, a new generation of catalysts with superior ac-tivity could be prepared for this important industrial process.

Environmentally important catalytic processes have be-come the focus of rapid development in recent years. Noneof them is more important than the 3-way catalytic converterutilized to clean automobile exhaust. It utilizes Pt, Pd, Rh,and cerium oxide as an important promoter (60). The chal-lenge is to fully oxidize unburned hydrocarbons and CO toCO2 while reducing NOx to N2 under all conditions of en-gine use: cold start, steady-state operation, and using a broadrange of air and fuel mixtures. This is achieved with the helpof one of the most successful chemical sensors, the oxygendetector (λ-probe) on automobiles. It helps to adjust the air-to-fuel ratio of the mixture entering the internal combustionengine and to optimize the efficiency of the 3-way catalyticconverter. This technology works well on the present-dayautomobile. The development of lean-burning, more fuel-efficient cars presents new challenges to surface chemistry andthe technology used to clean automobile exhaust.

Semiconductor-based technologies are at the heart of com-puter manufacturing. The fabrication of microelectronic circuitsoften involves layer-by-layer deposition of semiconductor (Si,GaAs, etc.), metal (Al, Cu, etc.), and insulator (SiO2, polymer)thin films, in various configurations. The film thickness ofeach of these materials is presently in the 103–104-Å range,and these layers alternate in both two and three dimensions.Fabrication of these layers is carried out by surface processesusing chemical vapor deposition, sublimation, or sputterdeposition from a radio-frequency plasma. Nucleation andgrowth mechanisms are monitored by surface-science techniquessuch as reflection high-energy electron diffraction (RHEED)and electron microscopy. We shall look at two problems ofsemiconductor device technology that are currently the fo-cus of intense surface-science studies.

The first is that insulating gate oxides for “metal oxidesemiconductor field effect transistors” (MOSFET) are producedby oxidizing silicon to SiOx. Both the oxygen-to-silicon ratioand the thickness of the oxide are important process vari-ables, as they control the device’s performance. The gate oxidesmust become thinner, their surfaces or interfaces must beatomically smooth, and their impurity concentrations mustbe minimized in order to increase the speed of electron trans-port and device reliability.

The second problem is that the chemical and mechanicalintegrity of the metal-insulator interfaces can be compromisedby water vapor or by the chemical attack of impurities segre-gating at the interface (e.g., alkali atoms, carbon, oxygen).When this happens, the adhesion of the insulator oxide tothe metal is altered and delamination occurs. “Trap” statesthat arise at the Si–SiO2 interface from Si atoms with coordina-tion numbers other than 4 are another problem because theycan trap charge at the interface. All these changes of chemicaland mechanical properties at the interface can have very del-eterious effects on the electrical properties. It is essential thatwe learn how to fabricate chemically stable insulator–metal

interfaces that maintain adhesion under changing ambientconditions (temperature, humidity, etc.). As the insulatingoxide is replaced by a polymer with a smaller dielectric con-stant, the study of metal–polymer interfaces becomes a fron-tier area of the science of semiconductor surface technology.

Other important surface technologies developed in recentyears with the help of molecular surface chemistry shouldalso be mentioned. Air separation to oxygen and nitrogen wasaccomplished with the help of molecular sieves because oftheir higher heat of adsorption for N2 (~ 7 kcal/mol) thanfor O2 (~ 3 kcal/mol), thereby preferentially releasing oxygen.Conversely, microporous carbon engineered to have bimodaldistribution of pores adsorbs the smaller O2 (3.46 Å) in itssmall pores in preference to N2 (3.64 Å) and preferentiallyreleases nitrogen. The magnetic disc drive provides informationstorage in computers by nanoscale tracking of a magnetic thinfilm that is coated by an atomically smooth carbon depositand lubricated by a drop of fluoro-ether lubricant. The opticalfiber operates on total internal reflection in glass by thecladding that is made by another glass of different compositionand with a smaller refractive index. Such glass structures areproduced using chemical vapor deposition of oxides in anappropriate sequence. Diamond films are produced by appli-cation of a high-energy plasma of methane and hydrogen orby chemical vapor deposition to provide a chemically inert,extremely hard coating with high thermal conductivity, thatwithstands chemical and mechanical attacks superbly (61).Adhesives and the contact lens are important surface technolo-gies using polymers that provide controlled adhesion andsome degree of biocompatibility, respectively.

Future Directions in Surface Chemistry

Coadsorption on SurfacesMost studies of modern surface chemistry focus on single

adsorbate systems, atoms, or molecules and investigate structureand bonding, adsorption, diffusion, and desorption dynamicsas a function of temperature. In most chemical reactions, how-ever, two or more reactants are utilized. Oxygen, hydrogen,and water are the coadsorbates present most frequently, althoughmixtures of organic molecules are adsorbed during hydro-carbon conversion reactions. Catalytic systems always useadditives that accelerate the reaction rate or improve selec-tivity. Potassium, which is an electron donor to iron, weakensthe bonding of the product molecule, ammonia (NH3),thereby accelerating its desorption from the surface. Con-versely, potassium increases the bonding of carbon monoxide(an electron acceptor) to iron and other transition metals,leading to the dissociation of the molecule (62). As a result,the rate of production of hydrocarbons by the hydrogenationof CO is greatly accelerated. In the future, more studies involv-ing coadsorbed atoms and molecules must be pursued to un-cover their influence on surface chemistry as a consequence oftheir interactions, adsorbate–adsorbate and adsorbate-substrate.

High-Pressure Surface ScienceIncreasing the coverage of adsorbates often enhances restruc-

turing of the substrate and changes the surface chemistry. Theeasiest way to increase coverage is to increase the reactant pres-sure, since pressure is proportional to coverage (adsorption iso-therm). When CO adsorbs on platinum surfaces it occupiesmostly top and bridge sites, with its C=O bond perpendicular



to the metal surface (63). As the pressure is increased to above10 torr a high coverage incommensurate overlayer forms, andat even higher pressures (≥ 100 torr) the formation of platinumcarbonyl clusters with CO-to-Pt-ratio greater than one can bedetected. This pressure-dependent change in surface chemistryis driven by the formation of strong CO–Pt bonds that in-duce the weakening and then the breaking of metal–metalbonds to produce a thermodynamically more stable surfacestructure. Newly available techniques permit atomic and mo-lecular studies of external surfaces in the presence of high-pressure gas or a liquid at the interface. These include thescanning tunneling and atomic force microscopes (STM andAFM) and sum frequency generation (SFG)–surface vibra-tional spectroscopy (10, 46). In the future these techniquesand others will be used to carry out in situ studies of mo-lecular surface chemistry at high reactant pressures and at hightemperatures.

Monitoring Surface Chemistry at Ever-ImprovingSpatial Resolution and Time Resolution

STM and AFM provide spatial resolution of surfaces andsurface species on the nanometer scale. There is a continuingneed to develop spectroscopic techniques that have the sameresolution because they will provide means to study and ma-nipulate surfaces on that spatial scale. Increased time resolu-tion will permit us to monitor the motion of surface atoms andmolecules, their diffusion, rotation, vibrational and electronicexcitation, and their reaction dynamics.

Studies of the Buried Interfaces, Solid–Liquid andSolid–Solid

Techniques that open up high-pressure surface chemis-try also permit molecular studies of the buried interfaces. Thiswill result in rapid developments in molecular phenomenaat solid–liquid (64) and solid–solid interfaces, including elec-trochemistry, biology, and tribology (friction, lubrication,wear) (65).

Achieving 100% Selectivity in Surface Reactions—the Environmental Imperative

In most surface catalyzed reactions we desire to obtainonly one product, although the formation of other chemicals isalso thermodynamically allowed. We need to understand cata-lytic selectivity, how to obtain 100% selectivity to avoid theformation of undesirable molecules that often lead to sepa-ration problems, pollution, or catalytic deactivation.

Nanoparticles: Surfaces in Three DimensionsParticles with dispersions between 1 and 0.1 represent

transition between single atoms and molecules and the bulksolid. They have many surprising properties as their electronicstructure, atomic structure, and phase diagram—and as a con-sequence, their surface chemistry—change with particle size.The fabrication of nanoparticles of uniform size (66, 67) andtheir study is one of the intellectual frontiers of modern sur-face chemistry.

Molecular surface chemistry is one of the most rapidlydeveloping branches of chemistry. It is an intellectual frontierof the discipline with enormous potential to develop surfacetechnologies that improve our quality of life, create employ-ment, and create wealth. It will remain a rapidly advancingfrontier area for many years to come.

Acknowledgment

This work was supported by the Director, Office of En-ergy Research, Office of Basic Energy Sciences, Materials Sci-ences Division, of the U.S. Department of Energy underContract No. DE-AC03-76SF00098.

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Gabor A. SomorjaiUniversity of California at BerkeleyDepartment of Chemistry andMaterial Sciences Division ofBerkeley National Laboratory

Ph.D., Chemistry, 1960, Universityof California at BerkeleyB.S., Chemical Engineering,1956, Technical University,Budapest, Hungary

Günther RupprechterUniversity of California at Berkeley

Department of Chemistry andMaterial Sciences Division ofBerkeley National Laboratory

Ph.D., Chemistry, 1996, LeopoldFranzens University, Innsbruck

B.S., Chemistry, 1992, LeopoldFranzens University, Innsbruck

Gabor A. Somorjai is one of the most influential scientistsin the area of surface chemistry today. He has received nu-merous awards, including the 1997 Von Hippel Award whichhe was awarded on December 3, 1997. His influence on sur-face chemistry can be seen in more than 750 papers pub-lished on surface science, heterogeneous catalysis, and solidstate chemistry. Somorjai has educated more than 90 Ph.D.students and collaborated more than 110 postdoctoral sci-entist of whom Günther Rupprechter is one. The researchinterests of his group include molecular studies of the struc-ture and bonding of surfaces, surface science of heterogenouscatalysis, and molecular studies of polymer surfaces and po-lymerization. During the past 30 years Somorjai has madesignificant contributions in surface science, new surface in-strumentation, and catalysis.

Günther Rupprechter received his Ph.D. in 1996 under thesupervision of Konrad Hayek on “Microstructural and mor-phological changes on epitaxially grown noble metal catalystparticles upon oxidation and reductive activation.” At presenthe is a postdoctoral fellow at the University of California atBerkeley working with Gabor A. Somorjai. Rupprechter’scurrent research interests include structure–activity correla-tions in heterogeneous catalysis, fabrication of well-facetedpolyhedral nanocrystals on oxidic supports by epitaxialgrowth and electron beam lithography, and analysis of sur-face structure and surface composition. He also has receiveda number of awards and fellowships.

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