Surface science

Surface science is the study of physical and chemical phenomena that occur at the interface of two phases, including solid-liquid, solid-gas, liquid-gas, and solid-vacuum interfaces. It includes the fields of surface chemistry and surface physics. The science and technology of interacting surfaces in relative motion is known as tribology. Some related practical applications are grouped together as surface engineering.

Surface science deals with phenomena such as adhesion, adsorption, friction, lubrication, and heterogeneous catalysis. In addition, it is important for the production of semiconductor devices, fuel cells, self-assembled monolayers, biomaterials, and pharmaceuticals.

Contents

Surface science is closely related to interface and colloid science. Interfacial chemistry and physics are common subjects for both, but the methods are different. In addition, interface and colloid science involves the study of macroscopic phenomena that occur in heterogeneous systems as a result of the peculiarities of interfaces.

History

The field of surface chemistry started with heterogeneous catalysis pioneered by Paul Sabatier for hydrogenation reactions and Fritz Haber for the Haber process for the synthesis of ammonia.[1] Irving Langmuir was also one of the founders of this field, and a scientific journal on surface science, Langmuir was named after him. The Langmuir adsorption equation is used to model monolayer adsorption where all surface adsorption sites have the same affinity for the adsorbing species.

Gerhard Ertl in 1974 described for the first time the adsorption of hydrogen on a palladium surface using a novel technique called LEED.[2] Similar studies with platinum,[3] nickel[4][5], and iron[6] followed. Gerhard Ertl was awarded the 2007 Nobel Prize for Chemistry for his studies in surface chemistry, specifically his investigation of the interactions between carbon monoxide molecules and platinum surfaces.

Surface chemistry

Surface chemistry can be roughly defined as the study of chemical reactions at interfaces. It is closely related to surface functionalization, which aims at modifying the chemical composition of a surface by incorporation of selected elements or functional groups that produce various desired effects or improvements in the properties of the surface or interface. Surface chemistry also overlaps with electrochemistry. Surface science is of particular importance to the field of heterogeneous catalysis.

The adhesion of gas or liquid molecules to the surface is known as adsorption. This can be due to either chemisorption or by physisorption. These too are included in surface chemistry.

The behavior of a solution based interface is affected by the surface charge, dipoles, energies and their distribution within the electrical double layer.

Surface physics

Surface physics can be roughly defined as the study of physical changes that occur at interfaces. It overlaps with surface chemistry. Some of the things investigated by surface physics include surface diffusion, surface reconstruction, surface phonons and plasmons, epitaxy and Surface enhanced Raman scattering, the emission and tunneling of electrons, spintronics, and the self-assembly of nanostructures on surfaces.

Analysis techniques

The study and analysis of surfaces involves both physical and chemical analysis techniques.

Several modern methods probe the uppermost 1-10 nanometers (nm) of the surface exposed to vacuum. They include X-ray photoelectron spectroscopy, Auger electron spectroscopy, low-energy electron diffraction, electron energy loss spectroscopy, thermal desorption spectroscopy, ion scattering spectroscopy, secondary ion mass spectrometry, and other surface analysis methods. Many of these techniques require vacuum as they rely on the detection of electrons or ions emitted from the surface under study.

Purely optical techniques can be used to study interfaces under a wide variety of conditions. Reflection-Absorption Infrared, Surface Enhanced Raman and Sum Frequency Generation spectroscopies can be used to probe solid-vacuum as well as solid-gas, solid-liquid, and liquid-gas surfaces.

Modern physical analysis methods include scanning tunneling microscopy (STM) and a family of methods descended from it, such as atomic force microscopy (AFM) and scanning probe microscopy (SPM). These techniques have considerably heightened the interest and ability of surface scientists to measure the physical structures of many surfaces. This interest is also related to a more general interest in nanotechnology.

Adhesion

The strength of attachment between an adhesive and its substrate depends on many factors, including the mechanism by which this occurs and the surface area over which the two materials contact each other. Materials that wet each other tend to have a larger contact area than those that don't. Five mechanisms have been proposed to explain adhesion.

  • Mechanical Adhesion: Two materials may be mechanically interlocked, as when the adhesive works its way into small pores of the materials.
  • Chemical Adhesion: Two materials may form a compound at the join.
  • Dispersive Adhesion: In dispersive adhesion (also known as adsorption), two materials are held together by what are known as "van der Waals forces." These are weak (but numerous) interactions between molecules of the materials, arising by electron movements or displacements within the molecules.
  • Electrostatic Adhesion: Some conducting materials may pass electrons to form a difference in electrical charge at the join. This gives rise to a structure similar to a capacitor and creates an attractive electrostatic force between the materials.
  • Diffusive Adhesion: Some materials may merge at the joint by diffusion. This may occur when the molecules of both materials are mobile and soluble in each other.

Adsorption

Adsorption is a process by which a gas, liquid, or solute (substance in solution) binds to the surface of a solid or liquid (called the adsorbent), forming a film of molecules or atoms (called the adsorbate).[7]

Adsorption has been found to occur in many natural physical, biological, and chemical systems. It is a consequence of attractive interactions between the surface of the adsorbent and the species being adsorbed. In the bulk of an adsorbent, all the bonding requirements (be they ionic, covalent, or metallic) of the constituent atoms of the material are fulfilled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract molecules of the adsorbate. The exact nature of the bonding depends on details of the species involved.

The adsorption process is generally classified as either physisorption (physical adsorption) or chemisorption (chemical adsorption). Weak interactions, characteristic of weak van der Waals forces, lead to physisorption; strong interactions, characteristic of covalent bonding, lead to chemisorption. In the former case, adsorbed molecules remain unbroken; in the latter case, the adsorbed molecules may or may not be broken up.

The degree of adsorption is influenced by certain properties of the adsorbent and adsorbate, and conditions such as temperature and pressure. Generally, adsorption is enhanced at low temperatures and high pressures. In addition, it is favored by larger surface areas of the adsorbent and by greater attractive forces between the adsorbent's surface and the adsorbate.

Adsorption, ion exchange, and chromatography are sorption processes in which certain adsorbates are selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column.

Tribology

Tribology deals with the interactions of surfaces in relative motion. It includes the study and application of the principles of friction, lubrication, and wear. Any product in which one material slides over or rubs against another is affected by complex tribological interactions.

The study of tribology is commonly applied in the design of mechanical bearings, but it extends to such products as hip implants, hair conditioners, lipstick, powders, and lipgloss.

In high temperature sliding wear in which conventional lubricants can not be used but in which the formation of compacted oxide layer glazes have been observed to protect against wear.

Tribology plays an important role in manufacturing. In metal-forming operations, friction increases tool wear and the power required to work a piece. This results in increased costs due to more frequent tool replacement, loss of tolerance as tool dimensions shift, and greater forces are required to shape a piece. A layer of lubricant which eliminates surface contact virtually eliminates tool wear and decreases needed power by one-third.

See also

Notes

  1. Håkan Wennerström and Sven Lidin, October 10, 2007. Scientific Background on the Nobel Prize in Chemistry 2007: Chemical Processes on Solid Surfaces. The Royal Swedish Academy of Sciences. Retrieved December 1, 2008.
  2. H. Conrad, G. Ertl, and E. E. Latta. Feb. 1974. Adsorption of hydrogen on palladium single crystal surfaces. Surface Science 41(2): 435-446. DOI:10.1016/0039-6028(74)90060-0
  3. K. Christmann, G. Ertl, and T. Pignet. Feb. 1976. Adsorption of hydrogen on a Pt(111) surface. Surface Science 54(2): 365-392. DOI:doi:10.1016/0039-6028(76)90232-6
  4. K. Christmann, O. Schober, G. Ertl, and M. Neumann. June 1, 1974. Adsorption of hydrogen on nickel single crystal surfaces. The Journal of Chemical Physics 60(11): 4528-4540. DOI:10.1063/1.1680935
  5. K. Christmann et al. May 1, 1979. Chemisorption geometry of hydrogen on Ni(111): Order and disorder The Journal of Chemical Physics 70(9): 4168-4184. DOI:10.1063/1.438041
  6. R. Imbihl, R.J. Behm, K. Christmann, G. Ertl, and T. Matsushima. 1982. Phase transitions of a two-dimensional chemisorbed system-H on Fe(110). Surface Science 117:257-266. DOI:doi:10.1016/0039-6028(82)90506-4
  7. Adsorption differs from absorption in that the latter is a process by which a substance diffuses into (or permeates) the solid or liquid absorbing medium. The term sorption encompasses both processes, and desorption is the reverse of either of the two processes.

References

  • Christmann, K., G. Ertl, and T. Pignet. Feb. 1976. Adsorption of hydrogen on a Pt(111) surface. Surface Science 54(2): 365-392.
  • Christmann, K., O. Schober, G. Ertl, and M. Neumann. June 1, 1974. Adsorption of hydrogen on nickel single crystal surfaces. The Journal of Chemical Physics 60(11): 4528-4540.
  • Ibach, Harald. 2006. Physics of Surfaces and Interfaces. Berlin: Springer. ISBN 978-3540347095.
  • Imbihl, R., R.J. Behm, K. Christmann, G. Ertl, and T. Matsushima. 1982. Phase transitions of a two-dimensional chemisorbed system-H on Fe(110). Surface Science 117:257-266.
  • Kolasinski, Kurt W. 2002. Surface Science: Foundations of Catalysis and Nanoscience. Chichester: Wiley. ISBN 0471492450.
  • McCash, Elaine M. 2004. Surface Chemistry. Oxford, UK: Oxford Univ. Press. ISBN 0198503288.
  • Prutton, Martin. 1994. Introduction to Surface Physics. Oxford, UK: Oxford University Press. ISBN 0198534760.
  • Somorjai, Gabor A. 1994. Introduction to Surface Chemistry and Catalysis. New York: Wiley. ISBN 0471031925.
  • Woodruff, D. P., and T. A. Delchar. 1994. Modern Techniques of Surface Science, 2nd ed. Cambridge Solid State Science Series. Cambridge, UK: Cambridge University Press. ISBN 0521424984.

External links

All links retrieved November 5, 2015.

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