Impact of surface chemistryGabor A. Somorjai1 and Yimin LiDepartment of Chemistry and Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720

Edited by John T. Yates, University of Virginia, Charlottesville, VA, and approved September 1, 2010 (received for review June 30, 2010)

The applications of molecular surface chemistry in heterogeneous catalyst technology, semiconductor-based technology, medical tech-nology, anticorrosion and lubricant technology, and nanotechnology are highlighted in this perspective. The evolution of surface chemistryat the molecular level is reviewed, and the key roles of surface instrumentation developments for in situ studies of the gas–solid, liquid–solid, and solid–solid interfaces under reaction conditions are emphasized.

surface science | nanotechnology | heterogeneous catalysis | in situ techniques | technological application

Surfaces and interfaces define aboundary between a material and itssurrounding environment and in-fluence interactions with that envi-

ronment. At the molecular level, thesurface atoms have a different chemicalenvironment, that is, fewer nearest neigh-bors, from that in the bulk. As a conse-quence, these surface atoms with changedatomic and electronic structures exhibithigh chemical reactivity. This propertymakes surfaces and interfaces a favoredmedium for chemical and biologicalprocesses in nature and in technologicalapplications (1).Modern surface chemistry is about mo-

lecular-level understanding and the controlof surface chemical reactions. Over thepast decades, various surface sciencetechniques have been developed and a vastamount of knowledge about surfacechemistry has been accumulated (1–6).The fundamental knowledge that is accu-mulated provides a foundation for thedevelopment of many industrial technolo-gies that produce items including chem-icals and fuels (7), semiconductor devices(8), and biomedical devices (9). Techno-logical advances, in turn, necessitate thefurther development of new surface char-acterization techniques with higherspatial, temporal, and energy resolution.This perspective begins with an overview

of several applications of surface chemistrythat have an enormous economic impacton our society. Advances in the funda-mental understanding of molecular pro-cesses on surfaces over the past decades arethen reviewed with an emphasis on thedevelopment of themolecular-level surfacetechniques. We highlight new scientificinsights provided by in situ experimentaland theoretical techniques capable ofcharacterizing surfaces and interfacesunder working conditions.

Selected Technological Applications ofSurface ChemistryThroughout modern industrialization,surface chemistry plays an indispensablerole in various industrial technologiesfor chemical and energy conversion, in-

formation processing, heath care, andmaterial and environmental protection.The paramount importance of surfacechemistry is reflected in the tremendouseconomic impact made by these technol-ogies. The roles of surface chemistry inseveral major industrial technologies arediscussed here (Table S1). The majorchallenges for surface chemistry infurther technological developments arehighlighted.

Heterogeneous Catalyst Technology. A cata-lyst is an entity that accelerates a chemicalreaction without being consumed in theprocess. This ability is usually referred to asthe activity of a catalyst. For a chemicalreaction with multiple possible products,a catalyst may promote the productionof a specific product, which is referred toas the selectivity of a catalyst. Heteroge-neous catalytic reactions occur on thesurface of solid catalysts and involve ele-mentary surface chemical processes such asadsorption of reactants from a reactionmixture, surface diffusion and reaction ofadsorbed species, and desorption of re-action products. The acceleration ofa chemical reaction is due to the highreactivity of surface atoms that facilitatesbond breaking and bond rearrangementof adsorbed molecules.In the late 1800s, the large-scale

synthesis of simple but important bulkchemicals, including sulfuric acid, ammo-nia, and nitric acid, were among the firstindustrial processes based on heteroge-neous catalysis (7). After these early suc-cesses, many complex catalytic processesevolved in the chemical industry. Theseprocesses include the synthesis of metha-nol from carbon monoxide and hydrogen,the oxidation of ethylene to ethylene oxidefor the manufacture of ethylene glycolantifreeze, the dehydrogenation of butaneto butadiene for the production of syn-thetic rubber, and the polymerizationof ethylene and propylene to produceplastics (7).In the middle of the 1900s, the large-

scale industrial applications of heteroge-neous catalysis emerged in the petroleum

industry with the development of catalyticcracking processes for naphtha refining.Since then, the applications of zeolites ascracking catalysts, the supported metalclusters as reforming catalysts, in the pe-troleum industry have allowed large-scaleproduction of high quality fuels andbulk chemicals (7). These developmentshave helped to lay out the foundation ofour current fast-growing, petroleum-basedeconomy (10).The development of catalytic processes

for pollution control and preventionalso accompanied the fast-growing econ-omy. One of many successful examplesof pollution control is the three-way cata-lytic converter that reduces the pollutantemission from automobiles (11, 12). Fig. 1shows that, despite a twofold increase inthe number of automobiles, mobile pol-lutant emissions in the Los Angelesarea have continuously decreased sincethe three-way catalytic converter wasintroduced in 1975.In the 21st century, emphasis shifted to

catalytic selectivity. It is desired to produceone specific product out of many otherthermodynamically possible compoundsthat are undesirable waste byproducts(green chemistry) (13, 14). To identify themolecular factors that control selectivity, itis required to monitor surface chemicalprocesses under reaction conditions (15).Innovations in catalytic processes areplaying a key role in green chemistry byreducing or eliminating the use and gen-eration of hazardous substances in chem-ical conversion processes and in thedevelopment of renewable energy andchemical feedstocks (16, 17). An exampleis the commercialization of the hydrogenperoxide to propylene oxide process for

Author contributions: G.A.S. designed research; Y.L. per-formed research; and G.A.S. and Y.L. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.1To whom correspondence should be addressed. E-mail:somorjai@berkeley.edu.

This article contains supporting information online atwww.pnas.org/lookup/suppl/doi:10.1073/pnas.1006669107/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1006669107 PNAS | January 18, 2011 | vol. 108 | no. 3 | 917–924

SPECIA

LFEATURE:PERSP

ECTIV

ED

ownl

oade

d by

gue

st o

n M

ay 1

5, 2

020

making propylene oxide, a polyurethaneraw material (18). In this process, pro-pylene is oxidized by hydrogen peroxideover a Ti-silicalite catalyst. The use oftoxic reagents, such as chlorine, and theproduction of coproducts, such as chlor–alkali complexes, tert-butyl alcohol, orstyrene, are avoided.Many challenging catalytic processes

are under intensive research and de-velopment for: (i) efficient water and CO2splitting to produce fuels by photocatalysts(19–23), (ii) efficient conversion of fuels toelectricity by electrochemical fuel cells(24, 25), (iii) selective conversion of bio-mass to fuels and bulk chemicals (26–31),(iv) selective production of fine chemicalsand pharmaceutical compounds by het-erogeneous catalysts (32), and (v) green-house gas (e.g., CO2 and CH4) reductionand utilization in fuel and chemical syn-thesis (32, 33). To achieve the optimalcatalytic properties (efficient sunlight andelectrical energy conversion, and high ac-tivity and selectivity for chemical conver-sion), much of this research is heavilyrelying on controlling the structure andcomposition of catalyst systems ona nanometer or molecular scale (nano-catalysis) (34). For example, the catalyticactivity and selectivity of a transitionmetal nanocatalyst can be tuned by con-trolling the size and shape of the nano-catalyst (35–38).

Semiconductor-Based Technology. The tran-sistor and the integrated circuit shaped thesemiconductor industry and the moderninformation-driven world. A transistor isa semiconductor device used to amplify orswitch an electronic signal, and an in-tegrated circuit contains millions of tran-sistors etched out of a small semiconductor

surface. Integrated circuits are in manymodern electronic devices such as com-puters and cell phones.Extensive research on the effect of

electron surface states led to the creationof the first point-contact germaniumtransistor in 1947 (39, 40). After the in-vention of transistors, advances in epitax-ial growth of silicon thin films, chemicalvapor deposition, and surface etchingtechniques made the industrial large-scalemanufacture of silicon-based integratedcircuits possible (41, 42). Since then, thenumber of transistor that can be placedinexpensively on an integrated circuit hasdoubled approximately every 2 y, improv-ing the performance and lowering themanufacturing cost of integrated circuits(43). New 18-nm manufacturing processesare expected to arrive in 2018. Theseprocesses include the following: thegrowth and deposition of sub–10-nm thinfilms of low-k insulating materials (forexample, SiOC) and high-k gate dielectricmaterials (for example, SiOxNy); the con-formal etching of deposited thin films withhigh aspect ratios; high-spatial-resolutionpatterning with new lithography techni-ques; the doping of the sub–10-nmsilicon channels; and the deposition ofnew wiring materials with low resistanceand high reliability (for example, Cu)(8, 44–46). The development of thesenew manufacturing processes leads toa broad range of surface and interfacechemistry issues.Unlike the silicon semiconductor, com-

pound semiconductors such as GaAs, GaN,and InGaN are optically active materials.Their tunable optical response throughcomposition control makes them extremelyuseful for optoelectronics applications(47). Other advantages of compound

semiconductors over silicon include thefollowing: higher electron mobility andelectron velocity, low noise, and higherbreakdown voltage, which make themideal for radio frequency communicationdevices such as cell phones and wirelesslocal area network cards (48). However,their compound character often in-troduces formidable surface chemistrychallenges in large-scale manufacturingprocesses (8, 49). For example, afternearly 30 y of intensive research, the dis-covery of a metal–organic chemical vapordeposition process (MOCVD) for growingdevice-quality GaN thin films finally en-abled the commercialization of the GaN-based technology (50). In this hetero-epitaxial growth process, a thin AlN bufferlayer is deposited on the substrate beforethe GaN growth. The essential role ofthis buffer layer is to serve as a templatefor nucleation and to promote lateralgrowth of the GaN film due to the de-crease in interfacial free energy betweenthe film and the substrate.

Medical Technology. In the late 1940s, alongwith the development of biomaterials formedical implants such as intraocularlenses, hip joint replacement, and bloodcontact devices, researchers began ex-ploring biological reactions at surfaces(51). It was indicated by many ob-servations that the biocompatibility ofa material is correlated with the surfaceproperties of the material (52–55). Forexample, when a foreign material comesinto contact with blood, it will rapidlyadsorb various proteins onto its surface.This process is usually referred to asnonspecific protein adsorption. The ad-sorbed protein layer determines sub-sequent platelet adhesion, which playsa major role in thrombogenesis on for-eign surfaces (56). To prevent thrombo-genesis, many surface modificationstrategies to achieve protein-resistant(antifouling) surface have been de-veloped (57, 58).Surface chemistry processes for pro-

ducing antifouling coatings and immobili-zation of biological recognition elementsare pervasive in many commercializedmedical applications, including implantbiomaterials, chromatography separations,and biosensors (59–60). For example, theglucose biosensor is one of the mostcommercialized biosensor technologies(9). It is at the heart of the hand-helddevices used by millions of diabetic pa-tients for monitoring glucose levels. Ina glucose biosensor, an enzyme layer ofglucose oxidase immobilized on an oxygenelectrode selectively catalyzes the oxida-tion of glucose in a blood sample (61).Then, the concentration of the reactionproduct, H2O2, is monitored by either anelectrochemical or an optical transducer

10

12

14Carbon monoxideNitrogen OxidesHydrocarbonto

ns p

er d

ay)

4

6

8

mis

sion

(103

×t

0

2

1975 1980 1985 1990 1995 2000 2005 2008

Mob

ile E

m

Year

Fig. 1. Annual average of toxic mobile emissions in Los Angeles County from 1975 to 2008. (Source:California Environmental Protection Agency, Air Resources Board. http://www.arb.ca.gov/app/emsinv/emssumcat.php, last date access: 08/10/2010.).

918 | www.pnas.org/cgi/doi/10.1073/pnas.1006669107 Somorjai and Li

Dow

nloa

ded

by g

uest

on

May

15,

202

0

to detect the glucose concentration in theblood sample (60, 62). One of majorchallenges in the development of thenew generation of enzyme-based bio-sensors is to find new enzyme immobili-zation approaches to maintain the activityof the immobilized enzyme for a longtime period.At the more fundamental level, a de-

tailed understanding of how surfaceproperties control the biological activityat biointerfaces is the key to engineeringnew biomaterials for advanced biologicalapplications. Advances in surface analysismake it possible to characterize the surfacechemistry and structure of increasinglycomplex biointerfaces (51, 63, 64). Forexample, the PEG-terminated self-assem-bly monolayers have been studied by usingsurface-specific optical spectroscopy tech-niques to unveil the molecular structuralorigin associated with their antifoulingproperties (65, 66).

Anticorrosion Technology. In many practicalapplications, high reactivity of surfacesmay cause undesirable chemical (corro-sion) or mechanical (wear and friction)processes, which lead to material and en-ergy waste. Depending on the materials(metal, oxide, and polymer) and the sur-rounding environment (gas, corrosive liq-uid, temperature, and radiation), there aremany surface chemical processes that canlead to the progressive corrosion of ma-terial surfaces. On a nonnoble metal sur-face in an oxidative moist environment,severe corrosion usually occurs through anelectrochemical process (67).Anticorrosion technology uses various

surface chemical processes to coat a ma-terial with a protective layer. Most of theprotective layers consist of metal oxides(e.g., chromium oxide on stainless steels)or polymers (e.g., Teflon on pans) that areinert in the environment of their specificapplications. A protective layer usuallyserves as a kinetic barrier to slow downcorrosion process (68–70). The long-termstability of the protective layers is themajor concern in many applications. Forexample, due to corrosion, the aluminized-Teflon coating on the Hubble SpaceTelescope has been severely degraded af-ter 19 y of exposure to atomic radicals andUV radiation in low Earth orbit (70).The design of protective layers with

enhanced stability requires surface chem-istry investigations on the structures,growth, and breakdown mechanisms of theprotective layers. Many surface analysistools are used to elucidate the chemicalcomposition and morphology of the pro-tective layers, as well as the kinetics of theirgrowth and breakdown in various corrosiveenvironments (68, 69, 71, 72). These re-search findings have provided useful in-formation for designing new protective

layers with enhanced stability. Severalcurrent challenges in this field include thefollowing: (i) the need for detection tech-niques with high spatial resolution to in-vestigate protective layers that are a fewnanometers thick and the corrosion pro-cesses that occur locally on the defect siteson the thin layer; and (ii) the developmentof model systems that simulate real cor-rosion processes where the corrosive en-vironment may evolve as the corrosionprogresses, or where the harsh corrosiveenvironments are hard to reproduce inlaboratories (73, 74).

Lubricant Technology. A lubricant is amaterial confined between two hard sur-faces in relative motion. Its existence helpsreduce the friction and wear at the inter-faces (75). Most of the fluid-base lu-bricants consist of viscous base stocks (e.g.,paraffinic-, aromatic-, and naphthenic-hy-drocarbon–type molecules with carbonnumber ranging from 14 to 40+) and ad-ditives to enhance performance underheavy-duty and high temperature con-ditions (76). This type of lubricant iswidely used in internal combustion en-gines, vehicle and industrial gear boxes,compressors, turbines, and hydraulic sys-tems. Solid lubricants are layered materi-als such as graphite, tungsten disulfide,and molybdenum disulfide (77). Thesesolids are used in high-temperature gasturbines and space applications wherefluid-based lubricants are not stable be-cause of vaporization. Other lubricants ofgreat technological importance includeultra-thin lubricant films coated on a com-puter hard disk surface to prevent wear bythe disk head, biocompatible lubricantsused in artificial joints, and organicmonolayer lubricants in microelectro–mechanical systems (78–81).The terminological introduction of tri-

bology, the science of friction, wear, andlubrication, in the late 1960s reflected theurgent need for scientific focus on molec-ular mechanisms of lubrication for thedevelopment of lubricant technology. Inpast decades, many tribological studieshave focused on properties of thin lubri-cant layers that hold two sliding surfacesapart (82–86). Now, it is known that, underlow sliding speeds and/or high loadingconditions, a “boundary lubrication” pro-cess occurs. In this process, an active lu-bricant is a thin film only several molecularlayers thick. A strongly adsorbed and ori-ented monolayer is formed on each of theslide surfaces, which protects the surfacesagainst wear. The shear plane resideswithin the thin lubricant film, and the lowshear strength between lubricant molecu-lar layers reduces the friction.The development of environmentally

benign lubricants is a major driving forcefor advances in tribology (85). A fluid-

based lubricant is a complex mixture. De-tailed understanding of how various com-ponents function in the lubrication processmay assist the discovery of more benignalternatives. Solid lubricants are usuallymore environmentally friendly than fluid-based lubricants. However, severe perfor-mance degradation in the presence ofhumidity or oxygen limits the applicationsfor using solid lubricants exposed to theEarth’s atmosphere (87, 88). The surfacechemistry of their degradation is an im-portant research subject for improvingtheir chemical stability.Natural joints in humans are the most

complex and sophisticated lubricationsystems. Their performance is far superiorto any current artificial joint (89). Theirworking mechanism is poorly understood.Further tribological investigations on themolecular level are needed to improve theperformance of artificial joints.

Surface Chemistry at the MolecularLevelAs many technologies develop, there isconstant calling for molecular-levelunderstandings of surface chemical pro-cesses in complex environments. It tookalmost 50 y for surface chemistry to de-velop into a molecular-level science, whichis now capable of in situ characterization ofa variety of surface properties and pro-viding fundamental understanding to aid inthe design and engineering of large-scalechemical processes in technological appli-cations (90, 91).The revolutionary development of sur-

face science at the molecular level began inthe 1960s when techniques for preparationof clean single crystal surfaces and char-acterization of surface structure andchemical compositions in ultrahigh vac-uum, such as low energy electron diffrac-tion (LEED), Auger electron spectroscopy(AES), and X-ray photoelectron spec-troscopy (XPS), became available to theresearch community (5). Throughoutthe 1960s to the early 1990s, thousandsof surface structures were studied. Adynamic view of surfaces emerged fromthe many observations of thermal- andchemisorptions-activated surface recon-structions. In many cases, massive re-construction on clean surfaces can leadto a surface structure totally different fromthat projected from bulk structure (Forexample, a reconstructed Ir(100) (92, 93)is shown in Fig. 2A). Chemisorption canlead to not only the restructuring of ad-sorbates but also a significant reconstruc-tion of single crystal surfaces [an exampleof ethylidyne-chemisorption–induced re-construction of Rh(111) (94) is shownin Fig. 2B].The next big step taken after surface

structural techniques became widelyavailable was the establishment of

Somorjai and Li PNAS | January 18, 2011 | vol. 108 | no. 3 | 919

Dow

nloa

ded

by g

uest

on

May

15,

202

0

correlations between surface structure andchemical properties of technological im-portance. Early examples are the studies ofammonia synthesis over various Fe crystalsurfaces by an experimental apparatuscapable of isolating a single crystal from anultra-high vacuum system after the surfaceis characterized by surface structuraltechniques (95). Catalytic activity meas-urements of the isolated crystal surfacesat high pressures allow the determinationof structure–activity correlations. It wasfound that certain Fe crystal faces havesignificantly higher catalytic activity forammonia synthesis than other Fe crystalfaces (Fig. 3) (96). The high catalytic ac-tivity was attributed to the existence ofsurface active sites with specific atomic

arrangements (96). These findings, theinvestigations of elementary reaction steps(97), and the effects of catalyst supportsand additives (98, 99) provided the mo-lecular-level understandings for improvingindustrial ammonia synthesis processes.More importantly, this example high-lighted that molecular-level studies on wellcontrolled model systems (the surfacescience approach; refs. 1 and 2) are capa-ble of extracting the essence of complexsurface chemical processes in industries,which were previously believed impossibleto obtain.The surface structural studies were fol-

lowed by extensive experimental and the-oretical investigations of the nature ofchemical bonding at surfaces and inter-

faces (6). Chemsorption bonding trendsacross the periodic table were recognizedby using the accumulation of data fromadsorption energy measurements (100).Detailed electronic structures of surfacechemical bonds were obtained by spec-troscopic techniques such as ultravioletphotoelectron spectroscopy, X-ray photo-electron spectroscopy, and X-ray emissionspectroscopy (5). Theoretical techniques,especially the density functional theory(DFT)-based computational simulations,were developed to rationalize the experi-mental findings and to provide a consis-tent framework for understanding sur-face chemical bonding (101–103). Forexample, in the d-band model, the surfacebonding strength on transition metal sur-faces has been correlated with the localelectron density in the d-band of the bondsite (104). Along with the experimentaldata, the d-band model provides answersto fundamental questions such as whybulk gold is noble while small gold clustersare not (105, 106) and why surfacedefect sites (steps and kinks) are morechemically active (107).The detailed molecular-level studies of

surface elementary processes (adsorption,diffusion, and reaction of adsorbed spe-cies) were enabled by molecular beamtechnique in the early 1970s (108, 109).These studies have yielded informationabout energy transfer in gas-surface colli-sion processes (110) and helped discernthe activities of various surface sites fora given reaction (111). For some simplesurface reactions, it was even possible toobtain the kinetic parameters (e.g., stick-ing probabilities and activation energies)of elementary surface reaction steps (112).The invention of scanning tunneling mi-croscopy in 1981 and its later developmenttransformed surface chemistry into a trulyatomic-level science (113, 114). For ex-ample, by monitoring an individual ad-sorbate in motion, the surface diffusionrate of a single molecule or a cluster wasobtained based on statistics of single mo-lecular events (115). By in situ imaging ofthe evolution of coadsorbed surface in-termediates under reaction conditions, theactive sites for many catalytic processeswere unveiled (116).At present, a vast number of experi-

mental and theoretical techniques havebeen developed for surface chemistry re-search and are serving as the research anddevelopment and quality-control work-horses in industry. The in situ techniquesare specifically of great importance and areunder intensive development. These tech-niques (Table 1) feature the capability toaccess molecular information at complexgas–solid, liquid–solid, and solid–solid in-terfaces under practical working con-ditions. Three of these in situ techniquesdeveloped in Berkeley are briefly

H

H

H

Side view

Top view

Reconstructed Pt(100) Ethylidyne - chemisorp�on - inducedrestructuring of Rh111)

A B

Side view

Top view

Fig. 2. (A) The structure of the reconstructed Ir(100) crystal face obtained by low energy electron dif-fraction (LEED). The surface layer assumes hexagonal packing that induces bucking (the 5x1 unit cell isindicated by a red box). The second layer retains its bulk-terminated square unit cell indicated by a lightblue box. (B) Ethylidyne-chemisorption–induced restructuring of Rh (111). The red arrows indicate theexpansion of metal atoms around the adsorption sites and the bulking up of metal atoms in the secondlayer underneath the adsorption sites.

0

2

4

6

8

10

12

14

(111) (211) (100) (210) (110)Surface Orienta�on

T=400 oC

20 atm 3:1 H2:N2

loM

HN

3mc/

2ces

01x-9

The 1st layer The 2nd layer The 3rd layer

Fe(111)

c4c7

c7

Fe(210)

c4c8

c5

Fe(110)

c6

Fe(211)

c5

c7

Fe(100)

c4c8

Fig. 3. The activities of ammonia synthesis over different Fe crystal faces (Right) and the structures ofcorresponding crystal faces (Left). It was suggested that the surface atoms with high coordinationnumber (C7 and C8) are the active sites for ammonia synthesis.

920 | www.pnas.org/cgi/doi/10.1073/pnas.1006669107 Somorjai and Li

Dow

nloa

ded

by g

uest

on

May

15,

202

0

introduced here along with scientific in-sights recently obtained from their appli-cation in heterogeneous catalysis.

Ambient Pressure X-Ray Photoelectron Spec-troscopy (AP-XPS). AP-XPS (117, 118) canbe operated at total reactant pressure upto 10 torr. The key component that makesthis technique different from conventionalXPS is the differentially pumped electro-static lens system (Fig. S1) that refocusesthe photoelectrons from the sample sur-face into the object plane of a 1 standardelectron energy analyzer working inhigh-vacuum. The kinetic energy of thephotoelectrons can be tuned by varyingthe energy of the X-ray source. By tuningthe kinetic energy of the photoelectronsto an appropriate value, the electronmean free path can be minimized fora given sample surface so that the oxida-tion state and the composition of thesurface layer with a thickness of ≈1 nmcan be determined.AP-XPS has been applied to studying the

oxidation states of Rh nanoparticles withsizes ranging from 2 to 11 nm during theCO oxidation reaction (119). It was foundthat the thickness of the surface oxide onthe Rh nanparticles increases with sizereduction, which is correlated with an in-crease of the reaction turnover.The active site on the bimetallic catalyst

surfaces usually consists of an atomic ar-rangement with a specific composition. Itis of great importance to determine thesurface composition of bimetallic nano-particle catalysts under reaction con-ditions. A recent AP-XPS study (120) ofthe surface composition of Rh/Pd andPd/Pt bimetallic nanoparticles demon-strated that the surface composition ofnanoparticles are extremely sensitive tothe ambient chemical environment. Forexample, under oxidizing conditions ofNO, the Rh atoms are pulled out tothe surface of the Rh0.5Pd0.5 nanoparticles

(Fig. 4). Under reducing conditions ofa mixture of CO and NO, the Pd surfaceconcentration increases. The surfacecomposition variation of a given metalis up to 20% at 300 °C. These resultsindicate that the surface active sites onbimetallic nanoparticle catalysts maymarkedly change under varying reactionconditions.

High Pressure Scanning Tunneling Microscopy(HP-STM). Scanning tunneling microscopyoperating under high pressure and hightemperature conditions offers a promisingway to monitor the structure of surfacesand adsorbates on the molecular levelduring surface reactions. Since the firstsystem (121) was demonstrated in 1992,several HP-STM systems have been de-signed and applied to in situ catalytic re-action studies (122–125). A recently de-veloped system (126) is shown in Fig. S2.By integrating a small, high pressure re-actor into the UHV chamber, the newsystem is capable of imaging surfaces withatomic resolution under a wide range ofpressures (from 10−13 bar to several bars)and temperatures (from 300 to 700 K).The adsorbate mobility, an important

factor in hydrocarbon conversion reac-tions, has been studied by HP-STM. Dur-ing the hydrogenation/dehydrogenation ofcyclohexene over the Pt(111) surface, nodistinguishable feature of the surfacecan be resolved by STM (Fig. S3) (127).Given the limited scanning speed of theSTM tip (typically, 10 nm/ms), the fea-tureless STM image implies that the ad-sorbates move at a speed much higherthan that of the STM tip. After poisoningthe reaction by a small amount of CO, thereaction turnover stops. Meanwhile, theordered structures emerge on the surfacesince the coadsorption of strongly bondedCO limits the mobility of hydrocarbonadsorbates. These results and similar ob-servations during ethylene hydrogenation

(128) suggest that, on the catalyst surfacessaturated by various hydrocarbon adsor-bates under high pressures, adsorbatemobility is crucial for the active sites oc-cupied by inactive species to be continu-ously released to allow the reactive speciesaccess to the active sites.The HP-STM studies on catalyst surfa-

ces unveiled many phenomena that werenot seen under ultrahigh vacuum con-ditions. For example, HP-STM, AP-XPS,and DFT simulations were combined ina recent study of CO chemisorption-in-duced reconstruction of stepped Pt surfa-ces at elevated pressures. It was found thatstepped Pt surfaces can undergo extensiveand reversible restructuring when exposedto carbon monoxide (CO) at pressures>0.1 torr (Fig. 5 A–C). Scanning tunnelingmicroscopy and photoelectron spectros-copy studies under gaseous environmentsnear ambient pressure at room tempera-ture revealed that as the CO surface cov-erage approaches 100% (Fig. 5D) under≈0.5 torr of CO, the originally flat terracesof (557) oriented Pt crystals break upinto nanometer-sized clusters and revertto the initial morphology after pumping

Table 1. Commonly used in situ techniques for the molecular-level surface characterization

Techniques Properties characterized

Transmission electron microscopy (TEM) Surface/interface structure, the size, shape, and crystallinityof nanocrystals

X-ray diffraction (XRD) Structure of micro- and mesoporous materialsSurface X-ray diffraction (SXRD) Surface structureAmbient pressure X-ray photoelectron spectroscopy (AP-XPS) Surface chemical compositionSmall Angle X-ray scattering (SAXS) Characteristic distances of ordered nanomaterialsX-ray emission spectroscopy (XES) Electronic structure of surfacesNear-edge X-ray absorption fine structure (NEXAFS) Surface chemical compositionExtended X-ray Absorption fine structure (EXAFS) Chemical composition and bonding environment of nanoparticlesPolarization-modulated reflection–absorption infraredspectroscopy (PM–RAIRS)

Surface reaction intermediates

Surface enhanced Raman spectroscopy (SERS) Surface reaction intermediatesSum frequency generation spectroscopy (SFG) Surface reaction intermediatesHigh pressure scanning tunneling microscopy (HP-STM) Surface morphology and electronic structureAtomic force microscopy (AFM) Surface morphology, tribological properties, and work function

Fig. 4. Chemical environment effects on surfacesegregation of the Rh/Pd nanoparticles detectedby AP-XPS. In the experiment, the partial pressureof NO or CO is ≈100 mtorr.

Somorjai and Li PNAS | January 18, 2011 | vol. 108 | no. 3 | 921

Dow

nloa

ded

by g

uest

on

May

15,

202

0

out the CO gas. Density functional theorycalculations provide a rationale of theobservations whereby the creation of in-creased concentrations of low-coordinationPt edge sites in the formed nanoclustersrelieves the strong CO–CO repulsion inthe highly compressed adsorbate film(Fig. 5E). This example and many othershighlight the indispensible role of in situtechniques in surface chemistry research.

Sum-Frequency Generation (SFG) VibrationalSpectroscopy. SFG vibrational spectroscopy(129) is a nonlinear spectroscopy tech-nique in which two high-energy pulsedlaser beams are overlapped spacially andtemporally on an interface of interest.Because of the properties of the nonlinearsusceptibility tensor, media with inversionsymmetry such as isotropic gases or bulkmetal crystals cannot generate a SFG sig-nal. Thus, the entire signal is generatedat the interface. The infrared visible SFGprocess (Fig. S4) can be thought of as aninfrared excitation followed by an anti-Stokes Raman relaxation process, the re-sult of which is emitted radiation at thesum of the two incoming frequencies. Thistechnique has been applied in our labo-ratory to detect surface intermediates ofmany catalytic reactions over single crystaland nanoparticle model catalysts at highpressures (130, 131).This technique helps us not only to de-

termine the major reaction intermediateson the catalyst surfaces but also to un-derstand the competition between inter-mediates for active sites. For example,Fig. 6 shows the SFG spectra of thePt(111) and Pt(100) surfaces during pyr-role hydrogenation (132, 133). In these

spectra, the C–Hx vibrational modes below≈3,000 cm−1 can be attributed to the ad-sorbed n-butylamine. On the Pt(111) sur-face, the C–H aromatic stretching modeat 3,105 cm−1 indicates that an intactaromatic ring is adsorbed on the surface at298 K. Thus, at 298 K, n-butylamine andcertain aromatic species are coadsorbedon the Pt(111) surface. The disappearanceof the aromatic stretching mode onthe Pt(100) surface at 363 K indicatesn-butylamine is bonded more strongly tothe surface than the aromatic species,which blocks the sites for the adsorptionof the aromatic species on the Pt(100)surface. These findings can be applied tounderstand the shape dependence of se-lectivity over the nanoparticle catalyst.

Summary and OutlookThe huge economic impact of surfacechemistry is manifested by its pervasiveapplications in modern industries thatproduce chemicals, fuels, microelectronicand optical devices, and medical devices.The challenges associated with high-impactindustrial applications provide a majorthrust to the progressive advances in sur-face chemistry. At present, there are manytechnological challenges in surface chem-istry applications. A few examples includethe development of energy-efficient andenvironmentally benign processes forchemical and energy conversion, furthersize reduction of transistors down to sub-10nm, the development of biocompatibleimplant materials with long-term stabilityin the human body, and the development ofbiosensors with fast detection time andmultiple-analyte response capabilities.Surfaces are involved in many impor-

tant chemical and biological processes innature and in industry largely becauseof their high chemical reactivity. Un-derstanding surface processes at the mo-lecular level, however, has proven to bechallenging because spatial resolutiondown to a few nanometers is a prerequisitefor any useful surface science technique.The advancement of various surface-specific techniques is the main theme in thedevelopment of modern surface chemistry.These techniques have been used to in-vestigate surfaces at an unprecedentedspatial, time, and chemical resolution.A recent trend in surface chemistry is to

study surface reactions at the molecularlevel under conditions close to those of thereal applications by using experimentaltechniques combined with theoretical sim-ulations (134). This type of research high-lights the importance of developing new insitu techniques and is expected to directly

Fig. 5. STM images of Pt(557) in ultrahigh vacuum (A) and under 1 torr of CO (B). (Scale bars: 5 nm.) (C)An enlarged view of B showing the roughly triangular shape of the nanoclusters formed at 1 torr. (D)Coverage of CO on Pt(557) under different pressures as determined by AM-XPS measurements. (E) Therelatively stable triangular-shaped nanoclusters predicated by DFT calculations, which provides an ex-planation to the formation of triangular nanoclusters roughly observed in the STM image shown in C.(Reproduced from ref. 142)

Pyrrole

+ 2H2

Pyrrolidine

N +NH3

H

N

H

+ H2

n-butylamine Butane and ammonia

H2N + H2

A

B C

Fig. 6. (A) Pyrrole hydrogenation. The major products are the aromatic molecule, pyrrolidine, and thering-opening product, n-butylamine. (B) The SFG spectra of the Pt(111) surface during the pyrrole hy-drogenation reaction at 298 K and 363 K. (C) The SFG spectrum of the Pt(100) surface during the pyrrolehydrogenation at 298 K. Pyrrole (3 torr) and 30 torr of hydrogen are used in these studies. Note that theC–H aromatic peak at 3,105 cm−1 (the red color dash line) shows up on the Pt(111) surface at 298 K, andthat it is missing on the Pt(111) surface at high temperature (363 K) or on the Pt(100) surface at 298 K.

922 | www.pnas.org/cgi/doi/10.1073/pnas.1006669107 Somorjai and Li

Dow

nloa

ded

by g

uest

on

May

15,

202

0

aid in the rational design of surface prop-erties for technological applications (135).During the past decade, studying nano-

materials has become a thrust in surfacechemistry. The high surface-volume ratiomakes surface properties dominant innanomaterials. Because of classical andquantum confinement effects, a nano-material may exhibit novel physical andchemical properties not seen in its bulkform. These properties can be tailored foradvanced technological applicationsthrough contolling the size, shape, compo-siton, and assembly structure of nano-materials. Surface science principles andtechniques have been extensively applied insize- and shape-controlled nanoparticle

synthesis, nanostructure assembly, andmaterial property characterization (136–140). Advanced applications of nanomate-rails in medicine, electronics, biomaterials,chemicals, and energy production are un-der intensive investigations. For example,the catalytic activity and selectivity ofa transition metal nanocatalyst can betuned by controlling the size and shape ofthe nanocatalyst (35–38). A solid lubricantmade of hollow MoS2 nanoparticles ex-hibits superior frictional properties andchemical stability over the thin filmMoS2 (88).In many potential technological appli-

cations of nanomaterials, the develop-ment of large-scale yet well-controlled

synthesis and assembly approaches isa challenging task. Nanomaterials areusually more susceptible to environmentalchanges than the bulk material. Thus,their thermal and chemical stabilities aremajor concerns in many applications (141).Given the rapid molecular-level

advances in surface chemistry and theirintimate roles in the development ofmodern technologies, molecular surfacechemistry has a bright future in the21th century.

ACKNOWLEDGMENTS. This work was supportedby the Director, Office of Science, Office of BasicEnergy Sciences, of the US Department of Energyunder Contract DE-AC02-05CH11231.

1. Somorjai GA, Li Y (2010) Introduction to SurfaceChemistry and Catalysis (Wiley, Hoboken, NJ) 2nd Ed.

2. Ertl G (2009) Reactions at Solid Surfaces (Wiley,Hoboken, NJ).

3. Adamson AW, Gast AP (1997) Physical Chemistry ofSurfaces (Wiley, New York), 6th Ed.

4. Hiemenz PC, Rajagopalan R (1997) Principles ofColloid and Surface Chemistry (Marcel Dekker, NewYork), 3rd Ed.

5. Woodruff DP, Delchar TA (1994) Modern Techniques ofSurfaceScience (CambridgeUnivPress,NewYork),2ndEd.

6. Nilsson A, Pettersson LGM, Norskov JK, eds (2008)Chemical Bonding at Surfaces and Interfaces (ElsevierScience & Technology, Amsterdam).

7. Sinfelt JH (2002) Role of surface science in catalysis.Surf Sci 500:923–946.

8. Ekerdt JG, Sun YM, Szabo A, Szulczewski GJ, White JM(1996) Role of surface chemistry in semiconductor thinfilm processing. Chem Rev 96:1499–1518.

9. D’Orazio P (2003) Biosensors in clinical chemistry. ClinChim Acta 334:41–69.

10. Bartholomew CH, Farrauto RJ (2006) Fundamentalsof Industrial Catalytic Processes (Wiley, Hoboken, NJ),2nd Ed.

11. Taylor KC (1984) Automobile catalytic converters.Catalysis: Science and Technology, eds Anderson JR,Boudart M (Springer, Berlin), Vol 5, pp 120–170.

12. Bagot PAJ (2004) Fundamental surface science studiesof automobile exhaust catalysis.Mater Sci Technol 20:679–694.

13. Anastas PT, Kirchhoff MM, Williamson TC (2001)Catalysis as a foundational pillar of green chemistry.Appl Catal A Gen 221:3–13.

14. Anastas PT, Kirchhoff MM (2002) Origins, currentstatus, and future challenges of green chemistry. AccChem Res 35:686–694.

15. SomorjaiGA,ParkJY(2008)Molecular factorsof catalyticselectivity. Angew Chem Int Ed Engl 47:9212–9228.

16. Taniewski M (2008) Sustainable chemical tech-nologies in production of clean fuels from fossil fuels.Clean-Soil Air Water 36:393–398.

17. Taniewski M (2006) Sustainable chemical technologies—Development trends and tools. Chem Eng Technol 29:1397–1403.

18. Tullo AH, Short PL (2006) Propylene oxide routes takeoff. Chem Eng News 84:22.

19. Balzani V, Credi A, Venturi M (2008) Photochemicalconversion of solar energy. ChemSusChem 1:26–58.

20. Grätzel M (2001) Photoelectrochemical cells. Nature414:338–344.

21. Bak T, Nowotny J, Rekas M, Sorrell CC (2002) Photo-electrochemical hydrogen generation from waterusing solar energy. Materials-related aspects. Int JHydrogen Energ 27:991–1022.

22. Roy SC, Varghese OK, Paulose M, Grimes CA (2010)Toward solar fuels: Photocatalytic conversion of car-bon dioxide to hydrocarbons. ACS Nano 4:1259–1278.

23. Centi G, Perathoner S (2010) Towards solar fuels fromwater and CO2. ChemSusChem 3:195–208.

24. Steele BCH, Heinzel A (2001) Materials for fuel-celltechnologies. Nature 414:345–352.

25. Winter M, Brodd RJ (2004) What are batteries, fuelcells, and supercapacitors? Chem Rev 104:4245–4269.

26. Carlini C, Patrono P, Galletti AMR, Sbrana G, Zima V(2005) Selective oxidation of 5-hydroxymethyl-2-furaldehyde to furan-2,5-dicarboxaldehyde by catalyticsystems based on vanadyl phosphate. Appl Catal A Gen289:197–204.

27. Bianchi CL, Canton P, Dimitratos N, Porta F, Prati L(2005) Selective oxidation of glycerol with oxygenusing mono and bimetallic catalysts based on Au, Pdand Pt metals. Catal Today 102:203–212.

28. Crossley S, Faria J, Shen M, Resasco DE (2010) Solidnanoparticles that catalyze biofuel upgrade reactionsat the water/oil interface. Science 327:68–72.

29. Hara M (2010) Biomass conversion by a solid acidcatalyst. Energy Environ Sci 3:601–607.

30. Casanova O, Iborra S, Corma A (2009) Biomass intochemicals: Aerobic oxidation of 5-hydroxymethyl-2-furfural into 2,5-furandicarboxylic acid with goldnanoparticle catalysts. ChemSusChem 2:1138–1144.

31. Casanova O, Iborra S, Corma A (2009) Biomass intochemicals: One pot-base free oxidative esterificationof 5-hydroxymethyl-2-furfural into 2,5-dimethylfuroatewith gold on nanoparticulated ceria. J Catal 265:109–116.

32. Sheldon RA, Downing RS (1999) Heterogeneouscatalytic transformations for environmentally friendlyproduction. Appl Catal A Gen 189:163–183.

33. Arakawa H, et al. (2001) Catalysis research of relevanceto carbon management: Progress, challenges, andopportunities. Chem Rev 101:953–996.

34. Li Y, Somorjai GA (2010) Nanoscale advances in catalysisand energy applications. Nano Lett 10:2289–2295.

35. Witham CA, et al. (2010) Converting homogeneous toheterogeneous in electrophilic catalysis using mono-disperse metal nanoparticles. Nat Chem 2:36–41.

36. Somorjai GA, Frei H, Park JY (2009) Advancing thefrontiers in nanocatalysis, biointerfaces, and re-newable energy conversion by innovations of surfacetechniques. J Am Chem Soc 131:16589–16605.

37. Burda C, Chen XB, Narayanan R, El-Sayed MA (2005)Chemistry and properties of nanocrystals of differentshapes. Chem Rev 105:1025–1102.

38. Bond GC, Thompson DT (1999) Catalysis by gold. CatalRev Sci Eng 41:319–388.

39. Brattain WH (1957) Surface properties ofsemiconductors. Science 126:151–153.

40. Bardeen J (1956) Nobel Lecture, Physics 1942-1962(Elsevier, Amsterdam).

41. Duke CB (2003) The birth and evolution of surfacescience: Child of the union of science and technology.Proc Natl Acad Sci USA 100:3858–3864.

42. Bent SF (2008) Semiconductor surface chemistry.Chemical Bonding at Surfaces and Interfaces, edsNilsson A, Pettersson LGM, Norskov JK (ElsevierScience & Technology, Amsterdam), pp 323–395.

43. Keyes RW (2008) Moore’s Law today. IEEE Circuits andSystems Magazine 8:53–54.

44. Gates SM (1996) Surface chemistry in the chemicalvapor deposition of electronic materials. Chem Rev96:1519–1532.

45. Belau L, Park JY, Liang T, Seo H, Somorjai GA (2009)Chemical effect of dry and wet cleaning of the Ruprotective layer of the extreme ultraviolet lithographyreflector. J Vac Sci Technol B 27:1919–1925.

46. Wilk GD, Wallace RM, Anthony JM (2001) High-kappagate dielectrics: Current status and materialsproperties considerations. J Appl Phys 89:5243–5275.

47. Mokkapati S, Jagadish C (2009) III-V compound SC foroptoelectronic devices. Mater Today 12:22–32.

48. Feng M, Shyh-Chiang S, Caruth DC, Huang JJ (2004)Device technologies for RF front-end circuits in next-generation wireless communications. Proc IEEE 92:354–375.

49. Pearton SJ, Ren F, Zhang AP, Lee KP (2000)Fabrication and performance of GaN electronicdevices. Mater Sci Eng Rep 30:55–212.

50. Amano H, Sawaki N, Akasaki I, Toyoda Y (1986)Metalorganic vapor-phase epitaxial-growth of a high-quality GaN film using an AlN buffer layer. Appl PhysLett 48:353–355.

51. Castner DG, Ratner BD (2002) Biomedical surfacescience: Foundations to frontiers. Surf Sci 500:28–60.

52. Baier RE, Meyer AE, Natiella JR, Natiella RR, Carter JM(1984) Surface properties determine bioadhesiveoutcomes: Methods and results. J Biomed Mater Res18:327–355.

53. Hoffman AS (1974) Principles governing biomoleculeinteractions at foreign interfaces. J Biomed Mater Res8:77–83.

54. Vroman L, Adams AL (1969) Findings with therecording ellipsometer suggesting rapid exchange ofspecific plasma proteins at liquid/solid interfaces. SurfSci 16:438–446.

55. Vroman L (1964) Effects of hydrophobic surfaces uponblood coagulation. Thromb Diath Haemorrh 10:455–493.

56. Tanaka M, et al. (2000) Blood compatible aspects ofpoly(2-methoxyethylacrylate) (PMEA)—relationshipbetween protein adsorption and platelet adhesion onPMEA surface. Biomaterials 21:1471–1481.

57. Chen H, Yuan L, Song W, Wu Z, Li D (2008)Biocompatible polymer materials: Role of protein-surface interactions. Prog Polym Sci 33:1059–1087.

58. Wang YX, Robertson JL, Spillman WB, Jr, Claus RO(2004) Effects of the chemical structure and thesurface properties of polymeric biomaterials on theirbiocompatibility. Pharm Res 21:1362–1373.

59. Luong JHT, Male KB, Glennon JD (2008) Biosensortechnology: Technology push versus market pull.Biotechnol Adv 26:492–500.

60. BoschME, SánchezAJR, Rojas FS, Ojeda CB (2007)Opticalchemical biosensors for high-throughput screening ofdrugs. Comb ChemHigh Throughput Screen 10:413–432.

61. Grieshaber D, MacKenzie R, Voros J, Reimhult E(2008) Electrochemical biosensors—Sensor principlesand architectures. Sensors 8:1400–1458.

62. Wang Y, Xu H, Zhang JM, Li G (2008) Electrochemicalsensors for clinic analysis. Sensors 8:2043–2081.

63. KimJ,KoffasTS,LawrenceCC,SomorjaiGA(2004)Surfacestructural characterization of protein- and polymer-modified polystyrene microspheres by infrared-visible

Somorjai and Li PNAS | January 18, 2011 | vol. 108 | no. 3 | 923

Dow

nloa

ded

by g

uest

on

May

15,

202

0

sum frequency generation vibrational spectroscopy andscanning force microscopy. Langmuir 20:4640–4646.

64. Koffas TS, Amitay-Sadovsky E, Kim J, Somorjai GA (2004)Molecular composition and mechanical properties ofbiopolymer interfaces studied by sum frequencygeneration vibrational spectroscopy and atomic forcemicroscopy. J Biomater Sci Polym Ed 15:475–509.

65. Harder P, Grunze M, Dahint R, Whitesides GM,Laibinis PE (1998) Molecular conformation in oligo(ethylene glycol)-terminated self-assembledmonolayerson gold and silver surfaces determines their ability toresist protein adsorption. J Phys Chem B 102:426–436.

66. Zolk M, et al. (2000) Solvation of oligo(ethyleneglycol)-terminated self-assembled monolayers studiedby vibrational sum frequency spectroscopy. Langmuir16:5849–5852.

67. Roberge PR (2006) Corrosion Basics: An Introduction(NACE Intl, Houston), 2nd Ed.

68. Schmuki P (2002) From Bacon to barriers: A review onthe passivity of metals and alloys. J Solid StateElectrochem 6:145–164.

69. Olsson COA, Landolt D (2003) Passive films onstainless steels—Chemistry, structure and growth.Electrochim Acta 48:1093–1104.

70. Yang JC, de Groh KK (2010) Materials issues in thespace environment. MRS Bull 35:12–16.

71. Williams DE, Newman RC, Song Q, Kelly RG (1991)Passivity breakdown and pitting corrosion of binaryalloys. Nature 350:216–219.

72. Kirchheim R, et al. (1989) The passivity of iron-chromium alloys. Corros Sci 29:899–917.

73. Payer JH, Scully JR (2005) Research opportunities incorrosion science for long termprediciton of materialsperformance. J Corros Sci Eng 7:Preprint 15.

74. Reddy MR (1995) Effect of low-earth orbit atomicoxygen on spacecraft materials. J Mater Sci 30:281–307.

75. Bhushan B (2000) Tribology: Friction, Wear, andLubrication. The Engineering Handbook, ed Dorf RC(CRC, Boca Raton, FL).

76. Booser ER (1983) American Society of LubricationEngineers, Society of Tribologists and LubricationEngineers (1983) CRC Handbook of Lubrication: Theoryand Practice of Tribology (CRC, Boca Raton, FL).

77. Campbell ME (1972) Solid Lubricants: A Survey (NatlAeronaut Space Admin, Washington, DC).

78. Freeman ME, Furey MJ, Love BJ, Hampton JM (2000)Friction, wear, and lubrication of hydrogels assynthetic articular cartilage. Wear 241:129–135.

79. Gellman AJ (1998) Lubricants and overcoats formagnetic storage media. Curr Opin Colloid InterfaceSci 3:368–372.

80. Mate CM (2005) Preface: Friction, lubrication, andadhesion inmicro-andnano-scaledevices.TribolLett19:1.

81. Mori S (1998) Present and future of lubricants formagnetic disks. J Japanese Soc Tribol 43:388–393.

82. Israelachvili JN, McGuiggan PM, Homola AM (1988)Dynamic properties of molecularly thin liquid films.Science 240:189–191.

83. Bowden FP, Tabor D (2001) The friction andlubrication of solids (Oxford Univ Press, New York).

84. Allen CM, Drauglis E (1969) Boundary layer lubrication:Monolayer or multilayer.Wear 14:363–384.

85. Gellman AJ, Spencer ND (2002) Surface chemistry intribology. Proc Inst Mech Eng, Part J 216:443–461.

86. Spikes HA (1996) Direct observation of boundarylayers. Langmuir 12:4567–4573.

87. Rapoport L, et al. (1997) Hollow nanoparticles of WS2as potential solid-state lubricants. Nature 387:791–793.

88. Chhowalla M, Amaratunga GAJ (2000) Thin films offullerene-like MoS2 nanoparticles with ultra-lowfriction and wear. Nature 407:164–167.

89. Berrien LSJ, FureyMJ, Veit HP (2000) Tribological study ofjoint pathology. Crit Rev Biomed Eng 28:103–108.

90. Somorjai GA (2000) The development of molecularsurface science and the surface science of catalysis:Berkeley contribution. J Phys Chem B 104:2969–2979.

91. Somorjai GA, Park JY (2009) Concepts, instruments,and model systems that enabled the rapid evolutionof surface science. Surf Sci 603:1293–1300.

92. Van Hove MA, et al. (1981) The surface reconstruc-tions of the (100) crystal faces of iridium, platinumand gold. 1. Experimental-observations and possiblestructural models. Surf Sci 103:189–217.

93. Lang E, et al. (1983) Leed intensity analysis of the(1×5) reconstruction of Ir(100). Surf Sci 127:347–365.

94. Wander A, Somorjai GA, Somorjai GA (1991) VanHove MA (1991) Molecule-induced displacive recon-struction in a substrate surface: Ethylidyne adsorbedon Rh(111) studied by low-energy-electron diffrac-tion. Phys Rev Lett 67:626–628.

95. Blakely DW, Kozak EI, Sexton BA, Somorjai GA (1976)New instrumentation and techniques to monitorchemical surface-reactions on single-crystals overa wide pressure range (10−8-105 torr) in sameapparatus. J Vac Sci Technol 13:1091–1096.

96. Strongin DR, Carrazza J, Bare SR, Somorjai GA (1987) Theimportance of C7 sites and surface roughness in theammonia-synthesis reactionover iron. JCatal103:213–215.

97. Ertl G (1983) Primary steps in catalytic synthesis ofammonia. J Vac Sci Technol A 1:1247–1253.

98. Strongin DR, Bare SR, Somorjai GA (1987) The effectsof aluminum-oxide in restructuring iron single-crystalsurfaces for ammonia-synthesis. J Catal 103:289–301.

99. Strongin DR, Somorjai GA (1988) The effects ofpotassium on ammonia-synthesis over iron single-crystal surfaces. J Catal 109:51–60.

100. Toyoshima I, Somorjai GA (1979) Heats of chem-isorption of O2, H2, CO, CO2 and N2 on polycrystallineand single-crystal transition-metal surfaces. Catal RevSci Eng 19:105–159.

101. Hammer B, Norskov JK (2000) Theoretical surfacescience and catalysis - Calculations and concepts. AdvCatal 45:71–129.

102. Norskov JK (1990) Chemisorption on metal surfaces.Rep Prog Phys 53:1253–1295.

103. Norskov JK, Scheffler M, Toulhoat H (2006) Densityfunctional theory in surface science and hetero-geneous catalysis. MRS Bull 31:669–674.

104. Bligaard T, Norskov JK (2008) Heterogeneous catalysis.Chemical Bonding at Surfaces and Interfaces, edsNilsson A, Pettersson LGM, Norskov JK (Elsevier Science& Technology, Amsterdam), pp 255–321.

105. Hammer B, Norskov JK (1995) Why gold is the noblestof all the metals. Nature 376:238–240.

106. Lopez N, Nørskov JK (2002) Catalytic CO oxidation bya gold nanoparticle: A density functional study. J AmChem Soc 124:11262–11263.

107. Nørskov JK, et al. (2008) The nature of the active sitein heterogeneous metal catalysis. Chem Soc Rev 37:2163–2171.

108. West LA, Kozak EI, Somorjai GA (1971) Molecularbeam scattering from single crystal surfaces underultrahigh vacuum conditions. J Vac Sci Technol 8:430.

109. Engel T, Ertl G (1978) Molecular-beam investigationof catalytic oxidation of CO on Pd(111). J Chem Phys69:1267–1281.

110. Jones RH, Olander DR, Schwarz JA, Siekhaus WJ(1972) Investigation of gas-solid reactions bymodulated molecular-beam mass-spectrometry. J VacSci Technol 9:1429.

111. Salmeron M, Gale RJ, Somorjai GA (1977) Molecularbeam study of H2-D2 exchange reaction on steppedplatinum crystal surfaces: Dependence on reactantangle of incidence. J Chem Phys 67:5324–5334.

112. Tully JC (1981) Dynamics of chemical processes atsurfaces. Acc Chem Res 14:188–194.

113. BinnigG, RohrerH (1987) Scanning tunnelingmicroscopy- from birth to adolescence. RevMod Phys 59:615–625.

114. Besenbacher F, et al. (2009) Atomic-scale surfacescience phenomena studied by scanning tunnelingmicroscopy. Surf Sci 603:1315–1327.

115. Briner BG, Doering M, Rust HP, Bradshaw AM (1997)Microscopic molecular diffusion enhanced byadsorbate interactions. Science 278:257–260.

116. Wintterlin J, Volkening S, Janssens TVW, Zambelli T,Ertl G (1997) Atomic and macroscopic reaction rates ofa surface-catalyzed reaction. Science 278:1931–1934.

117. Salmeron M, Schlogl R (2008) Ambient pressurephotoelectron spectroscopy: A new tool for surfacescience and nanotechnology. Surf Sci Rep 63:169–199.

118. Bluhm H, et al. (2007) In situ x-ray photoelectronspectroscopy studies of gas-solid interfaces at near-ambient conditions. MRS Bull 32:1022–1030.

119. Grass ME, et al. (2008) A reactive oxide overlayer onrhodium nanoparticles during CO oxidation and itssize dependence studied by in situ ambient-pressureX-ray photoelectron spectroscopy. Angew Chem IntEd Engl 47:8893–8896.

120. TaoF, et al. (2008)Reaction-driven restructuringofRh-Pdand Pt-Pd core-shell nanoparticles. Science 322:932–934.

121. McIntyre BJ, Salmeron MB, Somorjai GA (1992) Ascanning tunneling microscope that operates at highpressures and high temperatures (430-K) and duringcatalytic reactions. Catal Lett 14:263–269.

122. Rasmussen PB, Hendriksen BLM, Zeijlemaker H,Ficke HG, Frenken JWM (1998) The “reactor STM”: Ascanning tunneling microscope for investigation ofcatalytic surfaces at semi-industrial reaction con-ditions. Rev Sci Instrum 69:3879–3884.

123. Petersen L, et al. (2001) A fast-scanning, low- andvariable-temperature scanning tunneling microscope.Rev Sci Instrum 72:1438–1444.

124. Kolmakov A, Goodman DW (2003) In situ scanningtunneling microscopy of individual supported metalclusters at reactive gas pressures from 10(-8) to 10(4)Pa. Rev Sci Instrum 74:2444–2450.

125. Rossler M, Geng P, Wintterlin J (2005) A high-pressurescanning tunneling microscope for studying hetero-geneous catalysis. Rev Sci Instrum 76:023705.

126. Tao F, Tang D, Salmeron M, Somorjai GA (2008) Anew scanning tunneling microscope reactor used forhigh-pressure and high-temperature catalysis studies.Rev Sci Instrum 79:084101.

127. Montano M, Salmeron M, Somorjai GA (2006) STMstudies of cyclohexene hydrogenation/dehydrogenationand its poisoning by carbon monoxide on Pt(111).Surf Sci 600:1809–1816.

128. Grunes J, Zhu J, Yang MC, Somorjai GA (2003) COpoisoning of ethylene hydrogenation over Ptcatalysts: A comparison of Pt(111) single crystal and Ptnanoparticle activities. Catal Lett 86:157–161.

129. Shen YR (1989) Surface properties probed by second-harmonic and sum-frequency generation. Nature 337:519–525.

130. Cremer PS, Su XC, Somorjai GA, Shen YR (1998) Highpressurecatalyticprocesses studiedby infrared-visible sumfrequency generation. J Mol Catal A Chem 131:225–241.

131. Somorjai GA, Kliewer CJ (2009) Reaction selectivity inheterogeneous catalysis. React Kinet Catal Lett 96:191–208.

132. Tsung CK, et al. (2009) Sub-10 nm platinumnanocrystals with size and shape control: Catalyticstudy for ethylene and pyrrole hydrogenation. J AmChem Soc 131:5816–5822.

133. Kliewer CJ, Bieri M, Somorjai GA (2008) Pyrrolehydrogenation over Rh(111) and Pt(111) single-crystalsurfaces and hydrogenation promotion mediated by1-methylpyrrole: A kinetic and sum-frequencygeneration vibrational spectroscopy study. J PhysChem C 112:11373–11378.

134. Somorjai GA, Li YM (2010) Major successes of theory-and-experiment-combined studies in surface chemistryand heterogeneous catalysis. Top Catal 53:311–325.

135. Norskov JK, Bligaard T, Rossmeisl J, Christensen CH(2009) Towards the computational design of solidcatalysts. Nat Chem 1:37–46.

136. Yin Y, Alivisatos AP (2005) Colloidal nanocrystalsynthesis and the organic-inorganic interface. Nature437:664–670.

137. Somorjai GA, Park JY (2008) Colloid science of metalnanoparticle catalysts in 2D and 3D structures.Challenges of nucleation, growth, composition, par-ticle shape, size control and their influence on activityand selectivity. Top Catal 49:126–135.

138. Tao AR, Habas S, Yang P (2008) Shape control ofcolloidal metal nanocrystals. Small 4:310–325.

139. Zhao DY, Yang PD, Huo QS, Chmelka BF, Stucky GD(1998) Topological construction of mesoporous ma-terials. Curr Opin Solid State Mater Sci 3:111–121.

140. Murray CB, Kagan CR, Bawendi MG (2000) Synthesisand characterization of monodisperse nanocrystalsand close-packed nanocrystal assemblies. Annu RevMater Sci 30:545–610.

141. Campbell CT, Parker SC, Starr DE (2002) The effect ofsize-dependent nanoparticle energetics on catalystsintering. Science 298:811–814.

142. Tao F, et al. (2010) Break-up of stepped platinum cat-alyst surfaces of high CO coverage. Science 327:850–853.

143. Bylinsky G (May 27, 1985) The magic of designercatalysts. Fortune, pp 82–88.

144. Deutschmann O, Knozinger H Kochloefel, Turek T(2009) Heterogeneous catalysis and solid catalysts.Ullmann’s Encyclopedia of Industrial Chemistry(Wiley, Wiley-VCH Verlag, Weinheim), ElectronicRelease, 7th Ed.

924 | www.pnas.org/cgi/doi/10.1073/pnas.1006669107 Somorjai and Li

Dow

nloa

ded

by g

uest

on

May

15,

202

0