The Flexible Surface: Molecular Studies Explain the Extraordinary

Viewpoints: Chemists on Chemistry

The Flexible Surface The Flexible Surface: Molecular Studies Explain the Extraordinary Diversity of Surface Chemical Properties

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Outline

Historical Perspective External Surfaces Surface Concentration Clusters and Small Particles Thin Films

Internal Surfaces—Microporous Solids Clean Surfaces Interfaces Adsorption

Techniques of Surface Science Phenomena Discovered by Molecular Surface Chemistry Surface Structure Is Different from Bulk Structure Adsorbate-Induced Restructuring of Surfaces Rough Surfaces Do Chemistry Clusterlike Bonding of Adsorbed Molecules The Flexible Surface

Technological Impact of Molecular Surface Chemistry Future Directions in Surface Chemistry Coadsorption on Surfaces High-Pressure Surface Science Monitoring Surface Chemistry at Ever-Improving Spatial Resolution and Time Resolution Studies of the Buried interfaces, Solid–Liquid and Solid–Solid Achieving 100% Selectivity in Surface Reactions—the Environmental Imperative Nanoparticles: Surfaces in Three Dimensions

Other Material on Surface Chemistry in This Issue On the Surface Mini-Activities Exploring Surface Phenomena: JCE Classroom Activity #6 Flying over Atoms CD-ROM JCE Software Special Issue 19, by John R. Markham

Gabor A. Somorjai and Günther Rupprechter

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iewpoints is a major feature of the celebration of the Journal of Chemical Education’s 75th year. It is being supported by The Camille and Henry Dreyfus Foundation, Inc., which recently celebrated its own 50th anniversary. Each paper in the Viewpoints series will be written by a chemist or group of chemists with special expertise in a particular field, with the aim of providing an overview of that field’s accomplishments, importance, and prospects. The goal is to reflect on developments during the past 50 years and to predict how each field will evolve over the next 25 years. The total perspective encompassed by Viewpoints corresponds with the 75 years of this Journal’s lifetime and reflects its comprehensive interest in all of chemistry. The 50-year retrospective view of each field corresponds with the period during which the Camille and Henry Dreyfus Foundation has been supporting the chemical sciences. Authors of Viewpoints papers will provide perspectives on what chemistry has done during the lifetime of the Dreyfus Foundation and to set the stage for what chemistry will become well into the next century. The papers will be written at a level appropriate for upper-division undergraduate chemistry students and will extend and enhance the Journal’s role as, in the words of an early editor, “a living textbook of chemistry”. In addition, they will be published in electronic format via JCE Online (whose founding was also supported by the Dreyfus Foundation). In the Journal, Viewpoints papers will take full advantage of color graphics, which will also appear in the electronic version. In JCE Online there also will be links to the authors’ and other related Web sites, and video and animations when relevant. The Viewpoints series begins this month with “The Flexible Surface”, a paper from the laboratories of Gabor A. Somorjai of the University of California, Berkeley. Somorjai and his co-author, Günther Rupprechter, discuss concepts related to external and internal surfaces and interfaces and the dynamic nature of surfaces during chemical processes. A broad overview of the most frequently used surface science techniques is also included, along with references for each technique. Somorjai and Rupprechter close with a discussion of the present and future technological impact of surface chemistry on catalysis and semiconductor devices, and the chemist’s role in surface science in the coming years. Glenn T. Seaborg, Chair of the Viewpoints Editorial Board

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

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The Flexible Surface: Molecular Studies Explain the Extraordinary Diversity of Surface Chemical Properties Gabor A. Somorjai and Günther Rupprechter Department of Chemistry, University of California at Berkeley, and Materials Sciences Division, E. O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720; http://www.cchem.berkeley.edu/~gasgrp

Historical Perspective Surface science in general and surface chemistry in particular have a long and distinguished history. The spontaneous spreading of oil on water was described in ancient times and was studied by Benjamin Franklin. The use of surfacechemical processes on a large industrial scale began in the early part of the 19th century. The application of catalysis started with the discovery of the platinum-surface-catalyzed reaction of H2 and O2 in 1823 by Döbereiner. Döbereiner used this reaction in his portable “flame source”, of which he sold a large number. By 1835 the discovery of heterogeneous catalysis was complete, thanks to the studies of Kirchhoff, Davy, Henry, Philips, Faraday, and Berzelius (1). It was about this time that the Daguerre process was introduced for photography. The study of tribology, which includes friction, lubrication, and adhesion, also started around this time; this coincides with the industrial revolution, as machinery with moving parts became prevalent (although some level of understanding of friction appears in the work of Leonardo da Vinci). Surface-catalyzed-chemistry-based technologies first appeared in the period of 1860 to 1912, starting with the Deacon process (2HCl + 1/2 O2 → H2O + Cl2), SO2 oxidation to SO3 (Messel, 1875), the reaction of methane with steam to produce CO and H2 (Mond, 1888), ammonia oxidation (Ostwald, 1901), ethylene hydrogenation (Sabatier, 1902), and ammonia synthesis (Haber, Mittasch, 1905–1912). Surface tension measurements and recognition of equilibrium constraints on surface chemical processes led to the development of the thermodynamics of surface phases by Gibbs (1877). The existence of polyatomic or polymolecular aggregates that lack crystallinity and diffuse slowly (gelatine and albumin, for example) was described in 1861 by Graham, who called these systems “colloids”. Polymolecular aggregates that exhibit internal structure were called “micelles” by Nageli, and stable metal colloids were prepared by Faraday. The colloid subfield of surface chemistry gained prominence in the beginning of the 20th century with the rise of the paint industry and the preparation of artificial rubbers. Studies of high-surfacearea gas-absorber materials for gas masks and other gas-separation technologies and investigations of the lifetime of the light bulb filament led to the determination of the dissociation probability and adsorption probability of many diatomic molecules on surfaces as a function of gas pressure (adsorption isotherm) and temperature (Langmuir 1915). The properties of chemisorbed and physisorbed monolayers, adsorption isotherms, dissociative adsorption, energy exchange, and sticking upon gas–surface collisions were studied. Studies of electrode surfaces in electrochemistry led to detection of the surface space charge (2). Surface diffraction of low-energy electrons was discovered by Davisson and Germer in 1927, and atom diffraction (helium) from surfaces, somewhat later. Major aca162

demic and industrial laboratories focusing on surface studies have 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 bring surface chemistry into the center of development of chemistry, both because of the intellectual challenge to understand the rich diversity of surface phenomena and because of its importance in chemical and energy conversion technologies. Up to the 1950s, studies of surfaces were mostly on the macroscopic scale. Then the rise of the solid-state-devicebased electronics industry and the availability of economical ultra-high-vacuum systems—developed by research in space sciences—provided surface chemistry with new challenges and opportunities and resulted in explosive growth of the discipline. Clean surfaces of single crystals could be studied for the first time and the development of a large number of new techniques (cf. section on Techniques of Surface Science) from the 1960s onwards made possible the investigation of surfaces at atomic and molecular levels. As a result, macroscopic surface phenomena (adsorption, bonding, catalysis, oxidation, and other surface reactions; diffusion, desorption, melting, and other phase transformations; growth, nucleation, charge transport; atom, ion, and electron scattering; friction, hardness, lubrication) are being reexamined on the molecular scale. This has led to a remarkable growth of surface chemistry that has continued uninterrupted up to the present. The discipline has again become one of the frontier areas of chemistry. The newly gained knowledge of the molecular ingredients of surface phenomena has given birth to a steady stream of high technology products, including new hard coatings that passivate surfaces; chemically treated glass, semiconductor, metal, and polymer surfaces to which the treatment imparts unique surface properties; newly designed catalysts, chemical sensors, and carbon fiber composites; surface-space-charge-based copying; and new methods of electrical, magnetic, and optical signal processing and storage. Molecular surface chemistry is being utilized increasingly in biological sciences. External Surfaces

Surface Concentration The 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 molecular density ρ, in units of molecules per cubic centimeter, is ≈ 5 × 1022. The surface concentration of molecules σ (molecules/cm2) is proportional to ρ2/3, assuming cubelike packing, and is thus on the order of 1015 molecules/cm2. Because the densities of most solids or liquids are all within a factor of 10 or so of each other, 1015 molecules/cm2 is a good order-

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

The Flexible Surface

of-magnitude estimate of the surface concentration of atoms or molecules for most solids or liquids. Of course, surface atom concentration of crystalline solids may vary by a factor of two or three, depending on the type of packing of atoms at a particular crystal face.

Clusters and Small Particles All atoms in a three- or four-atom cluster are by necessity “surface atoms”. As a cluster grows in size, some atoms may become completely surrounded by neighboring atoms and are thus no longer on the “surface” (Fig. 1). We frequently describe a particle of finite size by its dispersion D, where D is the ratio of the number of surface atoms to the total number of atoms: D = number of surface atoms total number of atoms

For very small particles, D is unity. As the particle grows and some atoms become surrounded by their neighbors, the dispersion decreases (Fig. 1). Of course, D also depends somewhat on the shape of the particles and how the atoms are packed. The dispersion is already as low as 10᎑3 for particles of 10-nm (100-Å) radius. Many chemical reactions are facilitated by surface atoms of heterogeneous catalysts. These catalysts increase the rate of formation of product molecules and modify the relative dis-

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 atoms are on the surface, the dispersion rapidly declines with increasing cluster size, as shown in the lower part of the figure.

tribution of products. Most catalysts are in small-particle form, including those used to produce fuels and chemicals ranging from high-octane gasoline to polyethylene.

Thin Films Consider a monolayer of gold atoms (a layer of gold atoms one atom thick) deposited on iron (Fig. 2). This film has a dispersion of unity, since all the atoms are on the surface. About 50 layers of gold atoms (D = 1/50) are needed to obtain the optical properties that impart the familiar yellow color characteristic of bulk gold. Thin films are of great importance to many real-world problems. Their material costs are very little compared to cost of the bulk material, and they perform the same function when it comes to surface processes. For example, a monolayer of rhodium (a very expensive metal), which contains only about 1015 metal atoms per square centimeter, can catalyze the reduction of NO to N2 by its reaction with CO in the catalytic converter of an automobile, or it can catalyze the conversion of methanol to acetic acid by the insertion of a CO molecule. Thin ordered silicon layers optimize electron transport in integrated electronic circuits and thin films of organic molecules lubricate our skin or the moving parts of internal combustion engines. A green leaf is a high-surface-area system designed to maximize the absorption of sunlight in order to carry out chlorophyll-catalyzed photosynthesis at optimum rates. Often the surface of a thin film is roughened deliberately. Automobile brake pads are designed to optimize the desired mechanical properties of surfaces in this way, as is the corrugated design of rubber soles of tennis shoes. The large number of folds of the human brain helps to maximize the number of surface sites, which also facilitate charge transport and transport of molecules. These are some examples that show how external surfaces are used in nature. External surfaces are a key element of technology, ranging from catalysts and passivating coatings to computer-integrated circuitry and the storage and retrieval of information.

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


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Internal Surfaces—Microporous Solids Microporous solids are materials that are full of pores of molecular dimensions or larger. These materials have very large internal surface areas. Many clays have layer structures that can accommodate molecules between the layers by a process called intercalation. Graphite will swell with water vapor to several times its original thickness as water molecules become incorporated between the graphitic layers. Crystalline alumina silicates, often called zeolites, have ordered cages of molecular dimensions (3), where molecules can adsorb or undergo chemical reactions (Fig. 3). These materials are also called molecular sieves, because they may preferentially adsorb certain molecules according to their size or polarizability. This property is of great commercial importance and may be used to separate mixtures of gases (air) or liquids or to carry out selective chemical reactions. Bones of mammals are made out of calcium apatite, which has a highly porous structure, with pores on the order of 10 nm (100 Å) in diameter. Coal and char have porous structures, with pore diameters on the order of 102–103 nm (103–104 Å). These materials have very large internal surface areas, in the range of 100–400 m2 per gram of solid. As this short survey shows, nature has provided us with many useful microporous materials; and many synthetic microporous substances are used in technology, both to separate gas and liquid mixtures by selective adsorption and to carry out surface reactions selectively in their pores, which are often of molecular dimensions. Because surface reaction rate (product molecules formed per second) is proportional to surface area, materials with high internal surface areas carry out surface reactions at very high rates. Clean Surfaces To study atomically clean surfaces, we must work under so-called ultrahigh vacuum (UHV) conditions (4, 5), as the following rough calculation shows. We know that the concentration 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 of molecules incident on the initially clean surface must therefore be less than ≈ 1015 molecules/cm2/s or ≈ 1012 molecules/cm2/s, respectively. From the kinetic theory of gases (6), the flux, F, of molecules striking the surface of unit area at a given ambient pressure P is N AP F= 2πMRT or 20

F (atoms/ cm2 ⋅ s) =

2.63 × 10

⋅ P (Pa)

M (g / mol) T

or 22

2

F (atoms/ cm ⋅ s) =

3.51 × 10

⋅ 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, and NA is Avogadro’s number. Substituting P = 4 × 10᎑4 Pa (3 ×

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Figure 3. Alumina silicates with pores of molecular dimensions (zeolites) are used as selective absorbers of gases or liquids (molecular sieves) and as catalysts in chemical and petroleum technologies. The figure shows a synthetic zeolite, zeolite A. The red spheres represent oxygen atoms and the yellow spheres represent either silicon or aluminum atoms. For each aluminum there is a corresponding Na+ ion somewhere in one of the open channels. The molecular formula of this molecular sieve is Na12(Al 12Si 12O 48) ⴢ27H 2O.

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 this pressure the surface is covered with a monolayer of gas within seconds, 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-L exposure will cover a surface with a monolayer amount of gas molecules, assuming a sticking coefficient of unity. At pressures on the order of 1.33 × 10᎑7 Pa (10᎑9 torr), it may take 103 s before a surface is covered completely. In practice, one usually wants to study a surface without worrying about contamination from ambient gases. Current surface science techniques can easily detect contamination on the order of 1% of a monolayer. This then will be our operational definition of “clean”. Thus, ultrahigh vacuum conditions (< 1.33 × 10᎑7 Pa = 10᎑9 torr) are required to maintain a clean surface for about 1 h, the time usually needed to perform experiments on clean surfaces. Interfaces In most circumstances, however, and certainly in Earth’s environment, surfaces are continually exposed to gases or liquids or placed in contact with other solids. As a result, we end up investigating the properties of interfaces—that is, between solids and gases, between solids and liquids, between solids and solids, and even between two immiscible liquids. Thus, unless specifically prepared otherwise, surfaces are always covered with a layer of atoms or molecules from the environment.



Adsorption On approaching the surface, each atom or molecule encounters an attractive potential that ultimately will bind it to the surface under proper circumstances. The process that involves trapping of atoms or molecules incident on the surface is called adsorption. It is always an exothermic process. For historical reasons, the heat of adsorption ∆Hads is always denoted as having a positive sign—unlike the enthalpy ∆H, which for an exothermic process would be negative according to the usual thermodynamic convention. The residence time τ 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 temperature, and R is the gas constant. The value of τ can be 1 s or longer at 300 K for ∆Hads > 69 k J/mol (16.5 kcal/mol). The surface concentration σ (in molecules/cm2) of adsorbed molecules on an initially clean surface is given by the product of the incident flux F and the residence time τ: σ = Fτ

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

Figure 4. Schematic diagram of a scanning tunneling microscope capable of operating in a pressure range from UHV to 1 atmosphere. The STM is located inside a high-pressure chemical reactor, which is attached to a UHV surface characterization chamber. The two sections are separated by a gate valve. For surface cleaning and analysis, a transfer system is used to move the sample from the high-pressure cell to the UHV part of the apparatus (and vice versa).

Techniques of Surface Science Over the last three decades, a large number of techniques have been developed to study various surface properties, including structure, composition, oxidation states, and changes of chemical, electronic, and mechanical properties. The emphasis has been on surface probes that monitor properties on the molecular level and are sensitive enough to detect eversmaller numbers of surface atoms. The frontiers of surface instrumentation are constantly being pushed toward detection of finer detail: atomic spatial resolution, ever-smaller energy resolution, and shorter time scales. Because no single technique provides all necessary information about surface atoms, the tendency is to use a combination of techniques. The most commonly used techniques (Table 1) involve the scattering, absorption, or emission of photons, electrons, atoms and ions, although some important surface-analysis techniques cannot be classified this way. Most surface probes require high vacuum during their application, which prevents their use during high-pressure studies. To circumvent this restriction, UHV-compatible high-pressure cells (“environmental cells”) were developed (8). The sample to be analyzed is first subjected to the usual highpressure and/or high-temperature conditions encountered during reactions in the environmental cell. Afterwards the sample is transferred, without exposure to air, into the evacuated UHV chamber where the surface probe is located for subsequent surface analysis (generally the sample surface should be characterized before and after any treatment). During the past five years, two new surface science techniques in particular proved capable of obtaining molecularlevel surface information during chemical change at both low and high ambient pressures (9, 10): scanning tunneling microscopy (STM) and infrared–visible sum frequency generation (SFG) surface vibrational spectroscopy. Both of these techniques can operate within a 14-order-of-magnitude pressure range (10᎑10–104 torr) without significant change in signal quality in terms of spatial or energy resolution. Using these two techniques, we can monitor both substrate and adsorbate structures during 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 surface studies. Single crystals are oriented by X-ray back-diffraction, cut, and polished. They are then ion-bombarded or chemically treated to remove undesirable impurities from their surfaces. Thin films are deposited from vapor by sublimation, sputtering, or the use of plasma-assisted chemical vapor deposition. Materials of high internal surface area are prepared from a sol-gel or by calcination at high temperatures. The genesis and environmental history of the surface is primarily responsible for its structure and composition and must always be carefully monitored. Table 1 lists a selection of the surface-science techniques used most frequently in recent years to learn about the interface on the atomic scale. The names of the techniques, their acronyms, and brief descriptions are provided (along with some references [11–39], if a more detailed study of the capabilities and limitations of a particular technique is desired). We also indicate the primary surface information that can be obtained by the application of each technique.


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Table 1. Selection of Most Frequently Used Surface Science Techniques Primary Surface Information

Acronym

Name (References)

Description

—

Adsorption or selective chemisorption (11)

Atoms or molecules are physisorbed; the amount of gas adsorbed is a measure of total surface area. Chemisorption of atoms or molecules yields surface concentration of selected elements or adsorption sites.

Surface area, adsorption site concentration

AD

Atom or helium diffraction (12 )

Monoenergetic beams of atoms are scattered from ordered surfaces and detected as a function of scattering angle, giving structural information on the outermost layer of the surface. Method is extremely sensitive to surface ordering and defects.

Surface structure

AES

Auger electron spectroscopy (12– 14 , 23, 24, 26 )

Core-hole excitations are created, usually by 1– 10-keV incident electrons, and Auger electrons of characteristic energies are emitted through a 2-electron process as excited atoms decay to their ground state. AES gives information on the near-surface chemical composition.

Chemical composition

AFM

Atomic force microscopy (15, 16 )

Similar to STM. An extremely delicate mechanical probe is used to scan the topography of a surface by measuring forces exerted by surface atoms. Light interference is used to measure the deflection of the mechanical surface probe. This is designed to provide STM-type images of insulating surfaces or to detect mechanical properties at the molecular level.

Surface structure

ELS or EELS

Electron energy loss spectroscopy (14, 17– 19 )

Monoenergetic electrons ~5– 50 eV are scattered off a surface and the energy losses are measured. This gives information on the electronic excitations of the surface and the adsorbed molecules (see HREELS).

Electronic structure, surface structure

ESCA

Electron spectroscopy for chemical analysis (12, 20 )

Now generally called XPS (see XPS).

Composition, oxidation state

EXAFS

Extended X-ray absorption fine structure (21, 22 )

Monoenergetic photons excite a core hole. Modulation of the absorption cross-section with energy at 100– 500 eV above the excitation threshold yields information on radial distances to neighboring atoms. The cross section can be monitored by fluorescence as core holes decay or by the attenuation of the transmitted photon beam. EXAFS is one of the many "finestructure" techniques.

Local surface structure and coordination numbers

FEM

Field emission microscopy (12, 20, 21 )

A strong electric field (on the order of V/Å ) is applied to the tip of a sharp, single-crystal wire. Electrons tunnel into the vacuum and are accelerated along radial trajectories by Coulomb repulsion. When the electrons impinge on a fluorescent screen, variations of the electric field strength across the surface of the tip are displayed.

Surface structure

FIM

Field ionization microscopy (12, 20, 21, 23 )

A strong electric field (on the order of V/Å ) is created at the tip of a sharp, single-crystal wire. Gas atoms, usually He, are polarized and attracted to the tip by the strong electric field, then ionized by electrons tunneling from the gas atoms into the tip. These ions, accelerated along radial trajectories by Coulomb repulsion, map variations in the electric field strength across the surface, showing the surface topography with atomic resolution.

Surface structure and surface diffusion

FTIR

Fourier transform infrared spectroscopy (13, 24 )

Broad-band IRAS experiments are performed, and the IR absorption spectrum is deconvoluted by using a Doppler-shifted source and Fourier analysis of the data. This technique is not restricted to surfaces.

Bonding geometry and strength

HREELS

High-resolution electron energy loss spectroscopy (12, 21, 23 )

A monoenergetic electron beam, usually ~2– 10 eV, is scattered off a surface, and the energy losses below ~0.5 eV to bulk and surface phonons and vibrational excitations of adsorbates are measured as a function of angle and energy (also called EELS).

Bonding geometry, surface atom vibrations

IRAS

Infrared reflection absorption spectroscopy (21, 25 )

IR photons are reflected off a surface and the attenuation of IR intensity is measured as a function of frequency yielding a spectrum of the vibrational excitations of adsorbed molecules. Recent improvements in sensitivity allow IRAS measurements to be made on single-crystal surfaces.

Molecular structure

ISS

Ion scattering spectroscopy (12, 13, 21, 23, 24, 26 )

Ions are inelastically scattered from a surface, and the chemical composition of the surface is determined from the momentum transfer to surface atoms. The energy range is ~1 keV to 10 MeV, and the lower energies are more surface sensitive. At higher energies this technique is also known as Rutherford back-scattering (RBS).

Surface structure, composition

LEED

Low-energy electron diffraction (12– 14, 20, 21, 23, 24, 26 )

Monoenergetic electrons below ~500 eV are elastically back-scattered from a surface and detected as a function of energy and angle. This gives information on the structure of the near surface region.

Atomic and molecular surface structure Continued on next page

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Table 1. Selection of Most Frequently Used Surface Science Techniques —Continued Primary Surface Information

Acronym

Name (References)

Description

—

Neutron diffraction (27 )

Neutron diffraction is not an explicitly surface-sensitive technique, but experiments on large surface-area samples have provided important structural information on adsorbed molecules and surface phase transitions.

NMR

Nuclear magnetic resonance (28 )

NMR is not an explicitly surface-sensitive technique, but NMR data on large Chemical state s u r f a c e - a r e a s a m p l e s ( ⱖ 1 m 2) h a v e p r o v i d e d u s e f u l i n f o r m a t i o n o n molecular adsorption geometries. The nucleus magnetic moment interacts with an externally applied magnetic field and provides spectra highly dependent on the nuclear environment of the sample. The signal intensity is directly proportional to the concentration of the active species. This method is limited to the analysis of magnetically active nuclei.

RHEED

Reflection high-energy electron diffraction (13, 20, 21, 23, 24 )

Monoenergetic electrons of ~1– 20 keV are elastically scattered from a surface at glancing incidence and detected as a function of angle and energy for small forward-scattering angles. Backscattering is less important at high energies and glancing incidence is used to enhance surface sensitivity.

Surface structure, structure of thin films

SFG

Sum frequency generation (29, 30 )

Similar to SHG, but the laser output is split into a visible laser beam and a tunable IR beam. The two beams meet at the surface. The sum frequency signal is monitored as a function of IR frequency. In this way the vibrational spectrum of adsorbed molecules is obtained.

Molecular structure

SHG

Second harmonic generation (29, 31 )

A surface is illuminated with a high-intensity laser, and photons are generated at the second harmonic frequency through nonlinear optical processes. For many materials only the surface region has the appropriate symmetry to produce an SHG signal. The nonlinear polarizability tensor depends on the nature and geometry of adsorbed atoms and molecules.

Electronic structure, molecular orientation

SIMS

Secondary ion mass spectrometry (12, 13, 21, 23, 24, 32 )

Ions and ionized clusters ejected from a surface during ion bombardment are detected with a mass spectrometer. Surface chemical composition and some information on bonding can be extracted from SIMS ion fragment distributions.

Surface composition

STM

Scanning tunneling microscopy (12, 13, 16, 24, 33 )

The topography of a surface is measured by mechanically scanning a probe over a surface. The distance from the probe to the surface is measured by the probe-surface tunneling current. Angstrom resolution of surface features is routinely obtained.

Atomic surface structure

TEM

Transmission electron microscopy (13, 24, 34, 35 )

TEM can provide surface information for carefully prepared and oriented bulk samples. Real images have been formed of the edges of crystals where surface planes and surface diffusions have been observed. Diffraction patterns of reconstructed surfaces, superimposed on the bulk diffraction pattern, have also provided surface structural information.

Surface structure

TDS

Thermal desorption spectroscopy (21, 23, 36, 37 )

An adsorbate-covered surface is heated, usually at a linear rate, and the desorbing atoms or molecules are detected with a mass spectrometer. This gives information on the nature of adsorbate species and some information on adsorption energies and the surface structure.

Composition, heat of adsorption, surface structure

TPD

Temperature programmed desorption (21, 36, 37 )

Similar to TDS, except the surface may be heated at a nonuniform rate to obtain more selective information on adsorption energies.

Composition, heat of adsorption, surface structure

UPS

Ultraviolet photoemission spectroscopy (12, 14, 20, 21, 23 )

Electrons photoemitted from valence and conduction bands are detected as a function of energy to measure the electronic density of states near the surface. This gives information on the bonding of adsorbates to the surface.

Valence band structure

XPS

X-ray photoemission spectroscopy (12, 13, 20, 21, 23, 24, 26, 38 )

Electrons photoemitted from atomic core levels are detected as a function of energy. Shifts of core-level energies give information on the chemical environment of the atoms.

Composition, oxidation state

XRD

X-ray diffraction (39 )

X-ray diffraction has been carried out at extreme glancing angles of incidence where total reflection ensures surface sensitivity. This provides structural information that can be interpreted by well-known methods. An extremely high X-ray flux is required to obtain useful data from single-crystal surfaces. Bulk X-ray diffraction is used to determine the structure of organometallic clusters, which provide comparisons to molecules adsorbed on surfaces. X-ray diffraction has also given structural information on large surface-area samples.

Surface structure

Molecular structure


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Figure 5. Restructuring at a step site on a clean surface. Each atom attempts to optimize its coordination and “cracks” open to close the step edges.

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

Phenomena Discovered by Molecular Surface Chemistry

move in such a way that the positive and negative ions are almost coplanar. Presumably because of the necessary condition of charge neutrality, this type of surface structure is thermodynamically more stable than having alternating oxygen-ion and iron-ion layers. Such an expansion at the surface is clearly a property of ionic solids, and future studies will prove how general this type of relaxation is. Because of directionality of bonding in most solids, such contraction or relaxation at the surface moves atoms from their position of optimum bonding. As a result, the atoms not only move perpendicular to the surface, but also parallel to the surface. This leads to the formation of new surface unit cells, a phenomenon called surface reconstruction. Perhaps the most celebrated example is the (7×7) surface structure that forms on the Si(111) crystal face. Figure 7a shows the low-energy electron diffraction (LEED) pattern from this surface. The complex unit cell has 49 different locations of surface atoms that are distinguishable. The Si(100) surface shows a (2×1) reconstruction (Fig. 7b). It shows the formation of staggered dimers differing from the arrangement of Si atoms in the bulk near the surface.

Surface Structure Is Different from Bulk Structure Two dominant phenomena occurring at clean surfaces of materials distinguish their atomic structure from that in the bulk: relaxation and reconstruction (40). Upon relaxation of metal surfaces, the first layer of atoms moves inward, and this contraction leads to a much shortened interlayer spacing between the first and second layer of the surface. The more open (“rougher”) the surface, the larger the relaxation. Often, but not always, the contraction in the first layer is followed by a small expansion in the second layer. At rough edges, such as at stepped surfaces, the atoms at the step relax by a large amount in order to smooth the surface irregularity. This is shown schematically in Figure 5. At ionic surfaces, the nature of surface relaxation is very different. Figure 6 shows what happens at iron oxide surfaces (41). Iron oxide, in its bulk structure, shows alternating layers of oxygen ions and iron ions, where the iron ions are in tetrahedral or octahedral positions. At the surface, the two ions

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 three atomic layers into the bulk.

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Pt, Au, and Ir (100) surfaces that should have square unit cells reconstruct to form hexagonal surface unit cells (40, 42). This is shown in Figure 8. STM studies have imaged both reconstructed and unreconstructed surfaces (some domains may be unreconstructed because of contamination by adsorbates of various types). The water molecules at the ice surface vibrate with a much larger amplitude (0.24 Å) than molecules in bulk ice (0.1 Å) (43). This motion causes surface “softness”, which is likely responsible for the anomalously low friction coefficient of ice (i.e., it makes ice so slippery). Molecules at the liquid–vapor interface also show restructuring, as shown in Figure 9 for the arrangement of molecules at liquid alcohol surfaces (44). The alcohol molecules are oriented with their O–H bonds pointing inward, presumably for optimum hydrogen bonding. The alkane chains stick out from the surface. This orientation has been readily detectable by nonlinear laser optics sum frequency generation (SFG).

Adsorbate-Induced Restructuring of Surfaces When the clean surfaces are covered with a near monolayer of chemisorbed molecules, the structure of the surface undergoes profound alterations. This is perhaps best shown in the field ion microscopy (FIM) studies carried out by Kruse and coworkers (45) with rhodium field emission tips (Fig. 10). When carbon monoxide is chemisorbed on these tips, every crystal face restructures as shown by the figure. This massive restructuring is reversible if CO is removed when the surface is heated in vacuum. Our new STM system (Fig. 4) is placed in an environmental cell that can be pressurized and heated to elevated temperatures. It shows surface restructuring of the Pt(110) surface when this surface is exposed to atmospheric pressures of hydrogen, then oxygen, and then carbon monoxide (46). When these surfaces are heated, the surface restructures from a reconstructed ordered structure (exhibiting the so-called “missing row” reconstruction) in the presence of hydrogen,

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

to large (111) orientation facets in the presence of oxygen, and then again to smooth (110) unreconstructed surfaces in the presence of carbon monoxide (Fig. 11). Low-energy electron diffraction surface crystallography studies indicate the detailed atomic level nature of such reconstructions (42). When carbon is adsorbed on the Ni(100) surface, it occupies fourfold hollow sites (Fig. 12). As a result of the formation of the carbon–metal chemisorption bonds, the surface metal atoms move away from the adsorption site, presumably to give more space to the carbon atom so it can sink deeper into the surface, thereby forming bonds with second-layer nickel atoms underneath (47). This expansion around the chemisorption site induces strain, which is relieved by 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 equal distances from the chemisorption site to form four equal Fe–S bonds. The strength of these bonds pays for the weakening of the metal–metal bonds as a result of the restructuring. When NO is adsorbed on the Ni(111) surface, the molecule occupies a threefold hollow site, a so-called hcp (hexagonal close-packed) hollow site. This means there is a metal atom directly underneath the chemisorption site in the second metal layer. Chemisorption induces an upward movement of this metal atom in the second layer, and rumpling of the metal surface. When ethylene adsorbs on the Rh(111) surface (49), it rearranges and occupies a hollow site (in this case, again, an hcp hollow site). The rearranged ethylene (which has lost a hydrogen) is called ethylidyne. This is shown in Figure 13. The metal atoms move away from the carbon atom bound to the hollow site to allow the carbon to bond to the Rh atom directly underneath the carbon in the second layer. On the Pt(111) surface (50, 51), ethylene also forms an ethylidyne molecule—again in a threefold hollow site, but in this case it is an fcc hollow site. That is, there is no metal atom directly underneath the carbon in the second metal layer. In this circumstance, the surface metal atoms move inward to presumably 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 bond moves downward to produce a corrugated surface. It appears that surface bonding is clusterlike, where nearest-neighbor metal atoms that surround the adsorbate move to optimize the surface chemical bond. The heat of adsorption, which is always exothermic, pays for the weakening of the next nearest neighbor metal–metal bonds, which are altered as a result of the movement of the metal atoms nearest to the chemisorption site.

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


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When two molecules are coadsorbed on the surface, adsorbate-induced reconstruction may be very different from when only one or the other molecule is chemisorbed. This is shown for benzene and CO coadsorption on the Rh(111) surface (Fig. 14). When these molecules form a mixed unit cell, under the benzene the metal atoms are closer to their bulklike configuration than under the CO molecule (52). There is rumpling of the surface that occurs because of the differences in chemical bonding of the coadsorbed species to the substrate metal atoms. When benzene adsorbs alone it is bent. Four of the C atoms are in one type of surface site while two of the others are in different types of surface sites. When coadsorption occurs, the benzene molecule is flattened out.

Rough Surfaces Do Chemistry Surface 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 temperature from kink sites, then at somewhat lower temperatures from step sites, and at even lower temperatures from flat (111) terraces (53). This indicates higher heats of adsorption of the H

Table 2. Structure Sensitivity of H2/D2 Exchange at Low Pressures (~10 ᎑ 6 torr) Surface

Reaction Probability

Stepped Pt(332)

0.9

Flat Pt(111)

~10 ᎑1

Defect-free Pt(111)

ⱕ10 ᎑3

atom at these defect sites. Thus, the thermodynamic driving force for dissociation is certainly greater at these sites, which can explain their enhanced bond-breaking activity. It is difficult to understand, however, that these same strongly adsorbing sites are also very active sites for catalysis. This is shown in Table 2. The reaction probability of H2/D2 exchange on stepped surfaces is near unity at low pressures on a single scattering event, whereas it is below the detection limit (< 10᎑3) on the flat (111) crystal face, as shown by molecular beam surface scattering studies (54). How is it possible that the strongly adsorbing step sites, where H has a long residence time because of its high binding energy, are also the sites of rapid reaction turnover? One possible explanation is that the strongly adsorbed hydrogen restructures the surface near the step, thereby creat-

Figure 10. Field ion micrographs (image gas: Ne; T = 85 K) of a (001)-oriented Rh tip (top left ) before and (bottom left ) after reaction with 10᎑4 Pa CO for 30 min at 420 K . The stereographic projections at the right demonstrate the change in morphology from nearly hemispherical to polygonal. The scheme at the bottom right indicates the coarsening of the crystal and the dissolution of a number of crystallographic planes due to the reaction with CO.

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Figure 11. In situ high-pressure STM pictures showing adsorbate-induced surface reconstructions of Pt(110) under atmospheric pressures: Top: Topographic image of the surface in hydrogen after heating to 425 K for 5 hours, showing (n×1) missing-row reconstruction randomly nested. Vertical range: ∆ z = 10 Å . Center: Topographic image of the surface in oxygen after heating to 425 K for 5 hours; ∆ z = 25 Å . Bottom: Topographic image of the surface in carbon monoxide after heating to 425 K for 4 hours; ∆z = 42 Å.

ing the active site for the catalytic exchange process. At the low pressures of these molecular beam scattering experiments, the low coverages keep the structure of the flat part of the surface unaltered. Similarly, stepped Ni surfaces dehydrogenate C2H4 at much lower temperatures (< 150 K) than the (111) face of Ni (~230 K). The more open (111) and (211) crystal faces of Fe (Fig. 16) are several orders of magnitude more active for NH3 synthesis than the close-packed Fe (110) crystal face, which showed no detectable reaction rate ( 55).

Clusterlike Bonding of Adsorbed Molecules When ethylene chemisorbs at ~ 300 K on the (111) crystal faces of various transition metals (Pt, Rh, Pd), it chemically rearranges to form the molecule-surface compound shown in Figure 13. Its structure as determined by LEED-surface crystallography is very similar to multinuclear organometallic complexes such as Os3 CCH3 or Co3 (CO)9 CCH3. The rearranged ethylene, which has also lost a hydrogen, is called ethylidyne and belongs to the alkylidyne group (species of the formula CnH2n᎑1), a common substituent in surface chemistry

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

and organometallic chemistry. The vibrational spectrum of chemisorbed ethylidyne is nearly identical to that of the organometallic cluster Os3 CCH3 , which contains three metal atoms. The C–C bond distance (1.45 Å) is slightly less than the single carbon–carbon bond length of 1.54 Å (0.154 nm), as in cluster compounds. Thus, the surface chemical bond of chemisorbed ethylene can, as a first approximation, be viewed as a clusterlike bond that contains at least three metal atoms (56). The C–C bond order present in gaseous ethylene is reduced from two to nearly one upon chemisorption. This reduction in bond order of alkenes and alkynes upon chemisorption on metal surfaces is commonly observed, indicating charge transfer from the molecules into the metal. Benzene usually chemisorbs on metals with its ring parallel to the surface (although it may adsorb in a different configuration when it loses hydrogen). Because of charge transfer to the metal, C–C bond elongations occur with respect to the symmetry of the adsorption site. The ring may even bend, with two of the opposing carbon atoms closer to the metal surface than the other four carbon atoms. Distortions and elongations of C–C bonds are also found when benzene is bound to clusters of metal atoms in organometallic complexes. Thus the clusterlike bonding model appears to be valid for chemisorbed benzene as well. The bonding picture of adsorbed molecules becomes more complicated if there are more bonding sites available on the same molecule. For example, pyridine (C5H5N) may bind through the lone electron pair of its nitrogen or through the π-electrons of the carbon ring. Thus, depending on the metal, the binding geometry of the substrate, the temperature, or the adsorbate coverage, the molecule may be tilted with respect 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 nearestneighbor metal atoms of the substrate participate in the bonding. There is evidence that the atoms at next-nearest-neighbor sites change their location when chemisorption occurs, moving either closer or further away from the chemisorption bonds.


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Figure 13. The structure of ethylidyne on Rh(111). Ethylidyne is bonded on the hcp 3-fold hollow site. This site has a metal atom right underneath the carbon bonding site in the second layer. The adsorptioninduced distortion in the top metal layers pulls the nearest neighbor metal atoms up out of the surface plane.

The Flexible Surface As a result of all these studies that indicate adsorbateinduced restructuring and clusterlike bonding, the new model of the surface which has been adopted is the so-called “flexible surface” (53). In the past it was assumed that the metal atoms at the surface are rigid and occupy equilibrium sites dictated by the bulk unit cell. On adsorption, their location would not be altered. Instead, the flexible surface is one where the metal atoms move into new sites, dictated by the chemisorption bond so as to optimize that bond: upon adsorption the surface restructures, thereby creating the active sites for surface chemical processes.

The flexible-surface model explains why rough surfaces or defect sites at surfaces are so active in surface chemistry. Bond breaking and catalysis most frequently occur at low coordination sites such as steps and kinks or at defect sites such as oxygen vacancies in oxide surfaces. The lower the coordination of metal atoms (the fewer nearest neighbors), the more easily they restructure to optimize the surface adsorption bond. Thus, rough surfaces or atoms at steps move more readily, and of course small clusters of atoms where the coordination is much reduced are the most flexible. It is not surprising therefore that we use nanoclusters in the field of catalysis (and in many instances chemisorption) to optimize chemical effects, such as chemical reactions or adsorption. The extraordinary diversity of surface chemistry is due to the chameleon-like change of surface structure and bonding as the chemical environment of the surface is altered. Platinum is an excellent combustion catalyst operating in an oxidizing atmosphere and is a primary ingredient of the automobile catalytic converter. Platinum is also an excellent catalyst for hydrocarbon conversion under reducing conditions to produce high-octane gasoline (aromatic molecules and branched isomers) from straight chain alkanes (for example from n-hexane and n-heptane, which have octane numbers near zero). The platinum surface structure and thus its bonding behavior is completely different under the different reaction conditions, thereby mediating dramatically different catalytic surface chemistry. Technological Impact of Molecular Surface Chemistry: Selected Examples

Figure 14. The coadsorbed surface structure of benzene and carbon monoxide on the Rh(111) crystal face as obtained by lowenergy electron diffraction surface crystallography.

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Molecular surface chemistry contributed to the development and improvement of a wide range of technologies (57). A complete description is far beyond the limits of this article. In this section we will discuss only its present and future impact on catalysis and semiconductor devices. Since the early 1970s, molecular surface chemistry has made significant contributions to our understanding of catalyst-



based chemical and petroleum technologies. The hydrogenation of carbon monoxide yields methane or methanol exclusively, oxygenated molecules containing several carbon atoms or liquid, and high-molecular-weight hydrocarbon products (depending on the type of catalyst employed) (40). The dissociation of CO was found to be the dominant reaction step in producing methane, followed by stepwise hydrogenation of the surface carbon over several transition metal surfaces. Potassium was found to be an outstanding “promoter” of CO dissociation through weakening of the C–O bond by charge transfer. Methanol production was found to occur through the hydrogenation of undissociated CO2 or CO (which reaction dominates depends on catalyst formulation). Higher-molecular-weight hydrocarbons are produced by secondary polymerization reactions. The reforming of naphtha over platinum was found to be a structure-sensitive reaction. By altering the surface structure of platinum particles [(111) or (100) orientation] the product distribution could be altered. Bimetallic platinumbased catalysts (Pt–Re, Pt–Ir, and Pt–Sn) have also been investigated by surface-science studies. These studies have contributed greatly to their optimization in this important highoctane fuel producing technology. The same is true for the iron-based catalyst that produces ammonia from N2 and H2. The structure sensitivity of this reaction was uncovered, implicating the (111) and (211) crystal faces of iron as the most active (58, 59) (Fig. 16). The surface structures that are more open and contain sites that are surrounded by seven iron neighbor atoms (C7 sites) are the most active. These are the (111) and (211) crystal faces. The structure of the (110) crystal face does not allow the adsorbed nitrogen species to bind with second- and third-layer atoms (55), and probably for this reason the rate for the synthesis 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 platinum.

Figure 16. Schematic representations of the idealized surface structures of the (111), (211), (100), (210), and (110) orientation of iron single crystals (the coordination of each surface atom is indicated). The bar diagram shows the corresponding activity of the single crystal surfaces in ammonia synthesis.


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surface structure. The role of potassium promoters as bonding modifiers in aiding the dissociation of N2, as well as weakening the bonding of ammonia to the iron surface to inhibit product poisoning, has been uncovered. The role of alumina as a structure modifier, aiding the restructuring of iron particles to possess crystal faces most active for ammonia synthesis [(111) and (211)], has been proven by surface-science studies. As a result, a new generation of catalysts with superior activity could be prepared for this important industrial process. Environmentally important catalytic processes have become the focus of rapid development in recent years. None of them is more important than the 3-way catalytic converter utilized to clean automobile exhaust. It utilizes Pt, Pd, Rh, and cerium oxide as an important promoter (60). The challenge is to fully oxidize unburned hydrocarbons and CO to CO2 while reducing NOx to N2 under all conditions of engine use: cold start, steady-state operation, and using a broad range of air and fuel mixtures. This is achieved with the help of one of the most successful chemical sensors, the oxygen detector ( λ-probe) on automobiles. It helps to adjust the airto-fuel ratio of the mixture entering the internal combustion engine and to optimize the efficiency of the 3-way catalytic converter. This technology works well on the present-day automobile. The development of lean-burning, more fuelefficient cars presents new challenges to surface chemistry and the technology used to clean automobile exhaust. Semiconductor-based technologies are at the heart of computer manufacturing. The fabrication of microelectronic circuits often 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 of each 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 processes using chemical vapor deposition, sublimation, or sputter deposition from a radio-frequency plasma. Nucleation and growth mechanisms are monitored by surface-science techniques such as reflection high-energy electron diffraction (RHEED) and electron microscopy. We shall look at two problems of semiconductor device technology that are currently the focus of intense surface-science studies. The first is that insulating gate oxides for “metal oxide semiconductor field effect transistors” (MOSFET) are produced by oxidizing silicon to SiOx. Both the oxygen-to-silicon ratio and the thickness of the oxide are important process variables, as they control the device’s performance. The gate oxides must become thinner, their surfaces or interfaces must be atomically smooth, and their impurity concentrations must be minimized in order to increase the speed of electron transport and device reliability. The second problem is that the chemical and mechanical integrity of the metal-insulator interfaces can be compromised by water vapor or by the chemical attack of impurities segregating at the interface (e.g., alkali atoms, carbon, oxygen). When this happens, the adhesion of the insulator oxide to the metal is altered and delamination occurs. “Trap” states that arise at the Si–SiO2 interface from Si atoms with coordination numbers other than 4 are another problem because they can trap charge at the interface. All these changes of chemical and mechanical properties at the interface can have very deleterious effects on the electrical properties. It is essential that we learn how to fabricate chemically stable insulator–metal 174

interfaces that maintain adhesion under changing ambient conditions (temperature, humidity, etc.). As the insulating oxide is replaced by a polymer with a smaller dielectric constant, the study of metal–polymer interfaces becomes a frontier area of the science of semiconductor surface technology. Other important surface technologies developed in recent years with the help of molecular surface chemistry should also be mentioned. Air separation to oxygen and nitrogen was accomplished with the help of molecular sieves because of their higher heat of adsorption for N2 (~ 7 kcal/mol) than for O2 (~ 3 kcal/mol), thereby preferentially releasing oxygen. Conversely, microporous carbon engineered to have bimodal distribution of pores adsorbs the smaller O2 (3.46 Å) in its small pores in preference to N2 (3.64 Å) and preferentially releases nitrogen. The magnetic disc drive provides information storage in computers by nanoscale tracking of a magnetic thin film that is coated by an atomically smooth carbon deposit and lubricated by a drop of fluoro-ether lubricant. The optical fiber operates on total internal reflection in glass by the cladding that is made by another glass of different composition and with a smaller refractive index. Such glass structures are produced using chemical vapor deposition of oxides in an appropriate sequence. Diamond films are produced by application of a high-energy plasma of methane and hydrogen or by chemical vapor deposition to provide a chemically inert, extremely hard coating with high thermal conductivity, that withstands chemical and mechanical attacks superbly (61). Adhesives and the contact lens are important surface technologies using polymers that provide controlled adhesion and some degree of biocompatibility, respectively. Future Directions in Surface Chemistry

Coadsorption on Surfaces Most studies of modern surface chemistry focus on single adsorbate systems, atoms, or molecules and investigate structure and bonding, adsorption, diffusion, and desorption dynamics as a function of temperature. In most chemical reactions, however, two or more reactants are utilized. Oxygen, hydrogen, and water are the coadsorbates present most frequently, although mixtures of organic molecules are adsorbed during hydrocarbon conversion reactions. Catalytic systems always use additives that accelerate the reaction rate or improve selectivity. Potassium, which is an electron donor to iron, weakens the bonding of the product molecule, ammonia (NH3 ), thereby accelerating its desorption from the surface. Conversely, 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 hydrogenation of CO is greatly accelerated. In the future, more studies involving coadsorbed atoms and molecules must be pursued to uncover their influence on surface chemistry as a consequence of their interactions, adsorbate–adsorbate and adsorbate-substrate.

High-Pressure Surface Science Increasing the coverage of adsorbates often enhances restructuring of the substrate and changes the surface chemistry. The easiest way to increase coverage is to increase the reactant pressure, since pressure is proportional to coverage (adsorption isotherm). When CO adsorbs on platinum surfaces it occupies mostly top and bridge sites, with its C=O bond perpendicular



to the metal surface (63). As the pressure is increased to above 10 torr a high coverage incommensurate overlayer forms, and at even higher pressures (≥ 100 torr) the formation of platinum carbonyl clusters with CO-to-Pt-ratio greater than one can be detected. This pressure-dependent change in surface chemistry is driven by the formation of strong CO–Pt bonds that induce the weakening and then the breaking of metal–metal bonds to produce a thermodynamically more stable surface structure. Newly available techniques permit atomic and molecular studies of external surfaces in the presence of highpressure gas or a liquid at the interface. These include the scanning tunneling and atomic force microscopes (STM and AFM) and sum frequency generation (SFG)–surface vibrational spectroscopy (10, 46). In the future these techniques and others will be used to carry out in situ studies of molecular surface chemistry at high reactant pressures and at high temperatures.

Monitoring Surface Chemistry at Ever-Improving Spatial Resolution and Time Resolution STM and AFM provide spatial resolution of surfaces and surface species on the nanometer scale. There is a continuing need to develop spectroscopic techniques that have the same resolution because they will provide means to study and manipulate surfaces on that spatial scale. Increased time resolution will permit us to monitor the motion of surface atoms and molecules, their diffusion, rotation, vibrational and electronic excitation, and their reaction dynamics.

Studies of the Buried Interfaces, Solid–Liquid and Solid–Solid Techniques that open up high-pressure surface chemistry also permit molecular studies of the buried interfaces. This will result in rapid developments in molecular phenomena at solid–liquid (64) and solid–solid interfaces, including electrochemistry, biology, and tribology (friction, lubrication, wear) (65).

Achieving 100% Selectivity in Surface Reactions— the Environmental Imperative In most surface catalyzed reactions we desire to obtain only one product, although the formation of other chemicals is also thermodynamically allowed. We need to understand catalytic selectivity, how to obtain 100% selectivity to avoid the formation of undesirable molecules that often lead to separation problems, pollution, or catalytic deactivation.

Nanoparticles: Surfaces in Three Dimensions Particles with dispersions between 1 and 0.1 represent transition between single atoms and molecules and the bulk solid. They have many surprising properties as their electronic structure, atomic structure, and phase diagram—and as a consequence, their surface chemistry—change with particle size. The fabrication of nanoparticles of uniform size (66, 67) and their study is one of the intellectual frontiers of modern surface chemistry. Molecular surface chemistry is one of the most rapidly developing branches of chemistry. It is an intellectual frontier of the discipline with enormous potential to develop surface technologies that improve our quality of life, create employment, and create wealth. It will remain a rapidly advancing frontier area for many years to come.

Acknowledgment This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. Literature Cited 1. Berzelius, J. Jahres-Bericht über die Fortschritte der Physischen Wissenschaften (Tübingen), 1836, 15. 2. Thomas, J. M. Michael Faraday and the Royal Institution; IOP: Bristol, 1991. 3. Csicsery, S. M. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monographs, Vol. 171; American Chemical Society: Washington, DC, 1976; Chapter 1. 4. O’Hanlon, J. F. A User’s Guide to Vacuum Technique, 2nd ed.; Wiley: New York, 1989. 5. Klauber, C. In Surface Analysis Methods in Materials Science; Springer Series in Surface Sciences, Vol. 23; O’Connor, D. J.; Sexton, B. A.; Smart, R. S. C., Eds.; Springer: Berlin, 1992; pp 67–76. 6. de Boer, J. H. The Dynamical Character of Adsorption; Oxford University Press: New York, 1968. 7. Tompkins, F. C. Chemisorption of Gases on Metals; Academic: New York, 1978. 8. Cabrera, A. L.; Spencer, N. D.; Kozak, E.; Davies, P. W.; Somorjai, G. A. Rev. Sci. Instrum. 1982, 53, 1888. 9. Somorjai, G. A.; Rupprechter, G. In Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis; Studies in Surface Science and Catalysis Series, Vol. 109; Froment, G. F.; Waugh, K. C., Eds.; Elsevier: Amsterdam, 1997; p 35. 10. Su, X., et al. Faraday Discuss. 1996, 105, 263. 11. Greg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity; Academic: New York, 1967. 12. Hudson, J. B. Surface Science: An Introduction; ButterworthHeinemann: Boston, 1992. 13. MacDonald, R. J.; King, B. V. In Surface Analysis Methods in Materials Science; Springer Series in Surface Sciences, Vol. 23; O’Connor, D. J.; Sexton, B. A.; Smart, R. S. C., Eds.; Springer: Berlin, 1992; Chapter 5. Also see Chapters 3, 6, 10–14 in this volume. 14. Ertl, G.; Küppers, J. Low Energy Electrons and Surface Chemistry; VCR: Weinheim, 1985. 15. Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930. 16. Hansma, P. K.; Elings, V. B.; Marti, O.; Bracker, C. E. Science 1988, 242, 157. 17. Gasser, R. P. H. An Introduction to Chemisorption and Catalysis by Metals; Oxford University Press: New York, 1985. 18. Richardson, N. V.; Bradshaw, A. M. In Electron Spectroscopy: Theory, Techniques and Applications, Vol. 4; Brundle, C. R.; Baker, A. D., Eds.; Academic: New York, 1981; pp 153–193. Also see Chapters 1, 2, and 5. 19. Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibration; Academic: New York, 1982. 20. Prutton, M. Surface Physics; Oxford University Press: New York, 1975. 21. Woodruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science; Cambridge Solid State Science Series; Cambridge University Press: New York, 1986. 22. Heald, S. M. In X-ray Absorption; Chemical Analysis, Vol. 92; Koningsberger, D. C.; Prins, R., Eds.; Wiley: New York, 1988. 23. Somorjai, G. A.; Van Hove, M. A. In Absorbed Monolayers on Solid Surfaces ; Structure and Bonding Series, Vol. 38; Dunitz, J. D.; Goodenough, J. B.; Hemmerich, P.; Ibers, J. A.; Jorgensen, C. K.; / Neilands, J. B.; Reinen, D.; Williams, R. J. P., Eds.; Springer: Berlin, 1979; Chapter 4. 24. Roberts, N. K. In Surface Analysis Methods in Materials Science; Springer Series in Surface Sciences, Vol. 23; O’Connor, D. J.; Sexton, B. A.; Smart, R. S. C., Eds.; Springer: Berlin, 1992; Chapter 8, pp 187–201. 25. Willis, R. F.; Lucas, A. A.; Mahan, G. D. In Adsorption at Solid State Surfaces. The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 2; King, D. A.; Woodruff, D. P., Eds.; Elsevier: New York, 1983. 26. Somorjai, G. A.; Van Hove, M. A. In Investigations of Interfaces and Surfaces and Interfaces, Part B, Vol. IXB; Rossiter, B. W.; Baetzold, R. C., Eds.; Wiley-Interscience: New York, 1993; Chapter 1.


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Viewpoints 27. Lechner, R. E.; Riekel, C. In Neutron Scattering and Muon Spin Rotation; Springer Tracts in Modern Physics, Vol. 101; G. Höhler, Ed.; Springer: Berlin, 1983; pp 1–84. 28. Barrie, P. J.; Klinowski, J. Prog. Nucl. Magn. Reson. 1992, 24, 91. 29. Shen, Y. R. Nature 1989, 337, 519. 30. Shen, Y. R. Annu. Rev. Phys. Chem. 1989, 40, 327. 31. Richmond, G. L.; Robinson, J. M.; Shannon, V. L. Prog. Surf. Sci. 1988, 28, 1. 32. MacDonald, R. J.; King, B. V. In Surface Analysis Methods in Materials Science; Springer Series in Surface Sciences, Vol. 23; O’Connor, D. J.; Sexton, B. A.; Smart, R. S. C., Eds.; Springer: Berlin, 1992. 33. Binnig, G.; Rohrer, H. Rev. Mod. Phys. 1987, 59, 615. 34. Cowley, J. M. Prog. Surf. Sci. 1986, 21, 209. 35. Thomas, G. Ultramicroscopy 1986, 20, 239. 36. Yates, J. T. In Solid State Physics: Surfaces; Methods of Experimental Physics Series, Vol. 22; Park, R. L.; Lagally, M. G., Eds.; Academic: New York, 1985; Chapter 8, pp 425–464. 37. Madix, R. J. In Chemistry and Physics of Solid Surfaces, Vol. 2; Vanselow, R., Ed.; CRC: Boca Raton, FL, 1979; pp 63–72. 38. Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-Ray Photoelectron Spectroscopy; Perkin Elmer: Eden Prairie, MN, 1992. 39. Robinson, I. K.; Tweet, D. J. Rep. Prog. Phys. 1992, 55, 599. 40. Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; Wiley: New York, 1994. 41. Barbieri, A.; Weiss, W.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1994, 302, 259. 42. Starke, U.; Van Hove, M. A.; Somorjai, G. A. Prog. Surf. Sci. 1994, 46, 305. 43. Materer, N.; Starke, U.; Barbieri, A.; Hove, M. A. V.; Somorjai, G. A. Surf. Sci. 1997, 381, 190. 44. Stanners, C. D.; Du, Q.; Chin, R. P.; Cremer, P.; Somorjai, G. A.; Shen, Y.-R. Chem. Phys. Lett. 1995, 232, 407. 45. Kruse, N.; Gaussman, H. Surf. Sci. 1992, 266, 51. 46. McIntyre, B. J.; Salmeron, M. B.; Somorjai, G. A. Rev. Sci. Intrum. 1993, 64, 687.

Gabor A. Somorjai

Günther Rupprechter

University of California at Berkeley Department of Chemistry and Material Sciences Division of Berkeley National Laboratory

University of California at Berkeley Department of Chemistry and Material Sciences Division of Berkeley National Laboratory

Ph.D., Chemistry, 1960, University of California at Berkeley B.S., Chemical Engineering, 1956, Technical University, Budapest, Hungary

Ph.D., Chemistry, 1996, Leopold Franzens University, Innsbruck B.S., Chemistry, 1992, Leopold Franzens University, Innsbruck

Gabor A. Somorjai is one of the most influential scientists in the area of surface chemistry today. He has received numerous awards, including the 1997 Von Hippel Award which he was awarded on December 3, 1997. His influence on surface chemistry can be seen in more than 750 papers published on surface science, heterogeneous catalysis, and solid state chemistry. Somorjai has educated more than 90 Ph.D. students and collaborated more than 110 postdoctoral scientist of whom Günther Rupprechter is one. The research interests of his group include molecular studies of the structure and bonding of surfaces, surface science of heterogenous catalysis, and molecular studies of polymer surfaces and polymerization. During the past 30 years Somorjai has made significant contributions in surface science, new surface instrumentation, and catalysis.

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47. Gauthier, Y.; Baudoing-Savois, R.; Heinz, K.; Landskron, H. Surf. Sci. 1991, 251, 493. 48. Shih, H. E.; Jona, F.; Marcus, P. M. Phys. Rev. Lett. 1981, 46, 731. 49. Wander, A.; Van Hove, M. A.; Somorjai, G. A. Phys. Rev. Lett. 1991, 67, 626. 50. Cremer, P. S.; Stanners, C.; Niemantsverdriet, J. W.; Shen, Y. R.; Somorjai, G. A. Surf. Sci. 1995, 328, 111. 51. Cremer, P. S.; Su, X.; Shen, Y. R.; Somorjai, G. A. J. Am. Chem. Soc. 1996, 118, 2942. 52. Van Hove, M. A.; Lin, R. F.; Somorjai, G. A. J. Am. Chem. Soc. 1986, 108, 2532. 53. Somorjai, G. A. Langmuir 1991, 7, 3176. 54. Salmeron, M.; Gale, R. J.; Somerjai, G. A. J. Chem. Phys. 1979, 70, 2807. 55. Somorjai, G. A.; Materer, N. Top. Catal. 1994, 1, 215. 56. Van Hove, M. A.; Bent, B.; Somorjai, G. A. J. Phys. Chem. 1988, 92, 973. 57. Somorjai, G. A. Chem. Rev. 1996, 96, 1223. 58. Strongin, D. R.; Carrazza, J.; Bare, S. R.; Somorjai, G. A. J. Catal. 1987, 103, 213. 59. Spencer, N. D.; Schoonmaker, R. C.; Somorjai, G. A. J. Catal. 1982, 74, 129. 60. Taylor, K. C. Catal. Rev.—Sci. Eng. 1993, 35, 457. 61. Perry, S. S.; Somorjai, G. A. J. Vac. Sci. Technol. A 1994, 12, 1513. 62. Crowell, J. E.; Tysoe, W. T.; Somorjai, G. A. J. Phys. Chem. 1986, 89, 1598. 63. Su, X.; Cremer, P. S.; Shen, Y. R.; Somorjai, G. A. Phys. Rev. Lett. 1996, 77, 3858. 64. Somorjai, G. A. Surf. Sci. 1995, 335, 10. 65. Perry, S. S.; Somorjai, G. A.; Mate, C. M.; White, R. L. Tribol. Lett. 1995, 1, 233. 66. Jacobs, P. W.; Wind, S. J.; Ribeiro, F. H. Somorjai, G. A. Surf. Sci. 1997, 372, L249. 67. Eppler, A.; Rupprechter, G.; Guczi, L.; Somorjai, G. A. J. Phys. Chem. B 1997, 101, 9973–9977.

Günther Rupprechter received his Ph.D. in 1996 under the supervision of Konrad Hayek on “Microstructural and morphological changes on epitaxially grown noble metal catalyst particles upon oxidation and reductive activation.” At present he is a postdoctoral fellow at the University of California at Berkeley working with Gabor A. Somorjai. Rupprechter’s current research interests include structure–activity correlations in heterogeneous catalysis, fabrication of well-faceted polyhedral nanocrystals on oxidic supports by epitaxial growth and electron beam lithography, and analysis of surface structure and surface composition. He also has received a number of awards and fellowships.


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