Surface chemistry

Surface chemistry ( English surface chemistry, surface science ) is a branch of physical chemistry , in which the chemical and structural processes are investigated that take place at interfaces , mostly solid / gaseous. Surface-sensitive analytical methods are used for which several Nobel Prizes have been awarded in the last few decades . Since the structures examined are in the nanometer range, surface chemistry is one of the nanosciences .

Basics

The area of a solid body is defined as the surface in which the physical and chemical properties (e.g. structure, electronic properties ) differ from the rest ( bulk ), with the deviation from the volume properties i. a. decays exponentially with distance from the surface (proportional to ). The ideal image of a surface is analogous to the ideal solid body, a strictly periodic, infinite arrangement of atoms or molecules in two spatial directions . ${\ displaystyle f (x)}$ ${\ displaystyle x}$ ${\ displaystyle f (x) = \ exp (-x)}$

Bravais grid

A periodic arrangement of atoms or molecules on a surface can be described in two dimensions with a Bravais lattice , analogous to a solid . There are five Bravais grids in two dimensions, the square , the rectangular , the rectangular body-centered, the diamond-shaped and the hexagonal structure, whereby the hexagonal or rectangular body-centered structures can be viewed as special cases of the diamond-shaped structure with certain angles.

Unit cell

Scanning tunnel microscope

A unit cell reflects the symmetry of the Bravais lattice, it has the same elements of symmetry . Due to the periodicity of the grating, the unit cells can be mapped onto one another using a translation vector. The unit cells themselves are spanned by linearly independent unit vectors and . The following applies: ${\ displaystyle {\ vec {R}}}$ ${\ displaystyle {\ vec {a}} _ {1}}$ ${\ displaystyle {\ vec {a}} _ {2}}$

${\ displaystyle {\ vec {R}} = {\ vec {a}} _ {1} + {\ vec {a}} _ {2}}$

One can the grid in a different room with other basis vectors and transform. Do you work z. B. with diffraction methods, one measures the unit cell in reciprocal space , also called k-space. ${\ displaystyle {\ vec {a}} _ {1} ^ {*}}$ ${\ displaystyle {\ vec {a}} _ {2} ^ {*}}$

The vectors of the unit cell in spatial space can u. U. can be determined by means of scanning tunneling microscopy . The average size of the unit cell in reciprocal space is obtained, for example, from the diffraction of slow electrons (LEED) on the surface.

A special type of unit cell is the Wigner-Seitz cell . It corresponds to the first order Brillouin zone in k-space.

LEED diffraction pattern in k-space

Points and lines in the grid

A point in the grid is described by a vector from the origin to the point. A straight line is described with a vector that is parallel to the grid line. ${\ displaystyle P (x, y, z)}$ ${\ displaystyle {\ vec {P}} (x, y, z)}$

Lattice planes

When a single crystal breaks, it often happens along the lattice plane . This creates surfaces that differ in their 2-dimensional surface structure depending on the 3-dimensional crystal structure and cutting direction. The cutting planes can be described by the points of intersection of the plane with the axes of the coordinate system . The more common notation, however, is the specification of the Miller indices , which are the integer multiple of the reciprocal axis intercepts. z. B. (111), (110), (100) ${\ displaystyle a_ {1}, a_ {2}, a_ {3}}$ ${\ displaystyle h, k, l}$

Superstructures

Superstructures are additional, larger structures that are formed by rearrangement or adsorption on the surface. They can be described with vectors and as multiples of the basic vectors and , by Wood's nomenclature or by matrix representation . ${\ displaystyle a_ {2}}$ ${\ displaystyle b_ {2}}$ ${\ displaystyle a_ {1}}$ ${\ displaystyle b_ {1}}$

Surface preparation

Before a surface can be reproducibly analyzed on a microscopic scale , it has to be freed from contamination. To protect it from further contamination, it is handled in an ultra-high vacuum (UHV) ( ). This reduces the surface impact rate of impacting molecules from the gas phase. This is for a gas particle of the type ${\ displaystyle p <10 ^ {- 10} \, \ mathrm {hPa}}$ ${\ displaystyle n_ {i}}$ ${\ displaystyle i}$

{\ displaystyle n_ {i} \ approx 2 {,} 6 \ cdot 10 ^ {22} {\ frac {1} {\ mathrm {mbar}}} {\ frac {p} {\ sqrt {M_ {i} \ cdot T}}}}

In a study with an organic molecule layer adsorbed on Ag (111), a reaction with oxygen gas could be made visible directly in the local area using scanning tunneling microscopy .

Possible causes for surface contamination include: B .:

Adsorption of air molecules
dust
Migration of particles from inside the sample to the surface

Surface defects

Typical nanoscale defects on single crystal surfaces [e.g. B. the Ag (111) surface] are steps, kinks as well as atoms released from terraces. These can be made visible on an atomic scale using scanning tunneling microscopy and are generally more reactive than atomically smooth terraces.

Surface cleaning methods

After machining (e.g. grinding, turning), workpieces generally have residues such as oils, dust, abrasion or abrasives. These residues usually have a negative effect on the processing steps and must therefore be removed. Typical procedures are:

Oxidation or reduction of the surface: converting the impurities into volatile compounds. Oxidation can lead to the chemical conversion of adsorbates, which are then more easily desorbed. For example, CO strongly bound to a surface can be oxidized to CO ₂ , which is only weakly bound due to its chemical structure.
Sputtering with argon ions : During sputtering, the sample is bombarded with ions that are accelerated in an electric field. However, more or less large “craters” form on the substrate. B. can be smoothed by heating the sample.
Tempering (heating the sample): When the sample is heated to a certain temperature (approx. 1000 K), the thermodynamic equilibrium can be established, the surface is minimized, which corresponds to a reduction in surface energy. In this way, temperature-dependent reconstructions or structures can form. These can exist in domains with different orientations. In addition, desorption of adsorbates can occur during tempering.

Techniques for applying additional layers

Further layers of atoms or molecules can be applied to a surface in order to modify the properties of the interface. This allows z. B. accommodate semiconductor components in three-dimensional form in an integrated circuit (IC) because they are separated by the layers. An important tool in basic research is the chemisorption of probe molecules whose vibration properties are e.g. B. give information about the surface. The layers are applied i. a. with one of the following methods of thin film technology :

Chemical vapor deposition (CVD)
Plasma assisted chemical vapor deposition (PECVD)
Physical vapor deposition (PVD)
Molecular Beam Epitaxy (MBE)
galvanic separation
Oxidation of the surface with oxygen
Sol-gel process

Examples of questions

Examples of questions in surface chemistry are: the elemental composition of surfaces, the concentration of elements in the surface area, the distribution of elements in the depth profile of the surface and the chemical bonding of adsorbates . Also the study of the kinetics of adsorption , the adsorption and desorption kinetics , and the (e) structure at the interface and the vibration characteristics are functions of the surface chemistry. Furthermore, surface chemistry deals with reaction mechanisms of heterogeneously catalyzed reactions, creates models for catalytic reactions for the development of industrial catalysts and investigates the diffusion of adsorbates on surfaces (surface dynamics ) as well as the oxidation state of surface atoms.

Surface coordination chemistry

outer-sphere complex of the anion
[Cr (CN) ₅ NO] ³⁻ on a metal oxide-hydroxide surface

The coordination chemistry on metal oxide surfaces has many parallels to complex chemistry in solutions. Oxide ions and in particular hydroxide groups , which are formed by dissociative adsorption of water molecules on the metal oxide surface, serve as ligands for metal ions or metal ion complexes from an adjacent phase. Here metal complexes can be bound by weak interactions (outer-sphere complexes) or the binding takes place via exchange reactions of ligands (inner-sphere complexes). Example of an inner-sphere complex formation:

{\ displaystyle \ mathrm {Metal-OH + Fe (H_ {2} O) _ {6} ^ {2 +} \ longrightarrow Metal-OFe (H_ {2} O) _ {5} ^ {+} + H_ { 3} O ^ {+}}}

The production of surface complexes is of great importance for heterogeneous catalysts .

Acid-base reactions in particular also take place on the surface . The hydroxide groups can react either as a Brönsted acid or a Brönsted base. Depending on the metal, the Brönsted acid has a different acidity . Such surfaces play an important role as catalysts for acid-catalyzed reactions in non-aqueous solvents and in the gas phase. Centers on metal oxide surfaces that can react as Lewis acids also play a role in catalysis . The number of metal cations and thus the Lewis acidity increase, especially at higher temperatures.

Surface sensitive methods

Atomic force microscope image of the data layer of a compact disc .

Surface analytical methods are used in industry and in basic research.

Heterogeneous catalysis (e.g. Haber-Bosch process for the production of ammonia )

Semiconductor technology

Fuel cell research

Processes on electrodes during electrochemical reactions

Nanoelectronics , d. H. Manufacture of electronic components on the nanometer scale

High density information store
Adhesives
Resistant coatings of surfaces (e.g. corrosion protection )
Medical technology applications
Materials research , e.g. B. atomic composition of surface alloys
Certain biological issues

In order to be able to investigate the processes at interfaces, methods must be used that only “see” processes in the area of a sample that differs in its properties from the rest of the solid . For this purpose, the interactions of the following waves / particles with matter are used:

Radiation / particle	mean free path in the solid / gas	Examples
Electrons	small ( Coulomb interaction ), dependent on kinetic energy, see universal curve
Photons	large (no Coulomb interaction)	UV radiation , infrared radiation , X-rays
neutral thermal atoms and molecules	none, turning point before surface	Helium atoms, hydrogen molecules
Ions	small (Coulomb interaction)
magnetic fields	big
warmth	big

The mean free paths of charged particles are due to Coulomb interactions i. a. much smaller than that of neutrals. Another strong influence is the kinetic energy of the particles; In certain energy ranges, processes can be stimulated, which reduces the mean free path. It is decisive for the surface sensitivity of a method that either the particle or wave that interacts with the sample or the detected particle or wave has a short mean free path in the matter. That is why an ultra-high vacuum is necessary for many methods . The method chosen depends on the question. The following overview is only intended to give an overview. There are also different spatially resolving techniques for several methods. For further description see their article. Each of the methods has advantages and disadvantages that must be taken into account in the experiment.

microscopy

The first scanning tunneling microscope from Rohrer and Binnig

The surface of sodium chloride imaged with an atomic force microscope in non-contact mode, with the individual atoms being recognizable as elevations or depressions.

STM image of a graphite surface in atomic resolution.

STM measurement of the reconstruction of the (100) face of a Au - monocrystal

Scanning probe microscopy
method	Information received	inserted particle / wave	detected size / particle / wave	exploited effect
Scanning tunnel microscope (STM)	Electronic density of states (LDOS) and topography on the surface in local space , superstructures	Electrons	Tunnel current / z-position of the tip	Tunnel effect
Atomic Force Microscope (AFM)	Topography on the surface in the local area	Swinging tip ( cantilever )	Deflection of a laser beam ( frequency , phase and amplitude change )	Force between AFM cantilever and surface ( Pauli repulsion , van der Waals interaction )
Near field microscopy (SNOM)
Chemical force microscope (CFM)
Magnetic Force Microscope (MFM)

Photoresist in the electron microscope

Electron microscopy
method	Information received	inserted particle / wave	detected size / particle / wave	exploited effect
Transmission Electron Microscopy (TEM)	Surface structure in the local space, sliding planes of crystallites on the surface	Electrons	Electrons	Transmission of electrons through a thin sample
Scanning Electron Microscopy (SEM)	Surface structure in the local space, sliding planes of crystallites on the surface	Electrons	Electrons	Scanning the sample with an electron beam
Scanning Transmission Electron Microscopy (STEM)	Surface structure in the local space, sliding planes of crystallites on the surface	Electrons	Electrons	Combination of TEM and SEM
X-ray microanalysis (XRMA)
Photoemission electron microscopy (PEEM)	Magnetic domain structure in local space	Circularly polarized X-ray photons	Photoelectrons	Photoelectric effect , enlarged representation of the emitted photoelectrons on a fluorescent screen

FIM image of a tungsten tip in (110) orientation at 11 kV. The ring structure results from the arrangement of the atoms in a krz lattice. Individual bright points can be interpreted as individual atoms.

Field-induced microscopy
method	Information received	inserted particle / wave	detected size / particle / wave	exploited effect
Field emission microscopy (FEM)	Illustration of the structure of peaks, no atomic resolution	electric field ionizes tip atoms	emitted electrons from the tip on fluorescent screen	Ionization, tunnel effect
Field Ion Microscopy (FIM)	Illustration of the structure of spikes, atomic resolution	electric field, image gas	Image gas with fluorescent screen	Ionization of the image gas, tunnel effect
Field desorption / field evaporation	Illustration of the structure of spikes	electric field	Adatoms / tip atoms	Desorption of adatoms from the tip / evaporation of tip material
Field ion mass spectrometry	Composition of lace	electric field, image gas	Molar mass of tip atoms by time-of-flight mass spectrometer (TOF)	Desorption of atoms of the tip, different flight times with different masses in the TOF

Spectroscopy

Example of an XPS spectrum

Typical XPS system with hemisphere analyzer , X-ray tubes and various preparation methods

In the spectroscopy is generally a method in which a spectrum is produced, d. That is, an intensity is plotted against a quantity equivalent to the energy, e.g. B. Frequency . In electron spectroscopy , the energy of electrons is the quantity that is plotted against the intensity. There are the following methods:

Electron spectroscopy
method	Information received	inserted particle / wave	detected size / particle / wave	exploited effect
X-ray Photoelectron Spectroscopy (XPS)	Oxidation state and concentration of elements in the surface area	X-ray photons	Photo electrons	Photoelectric effect
Auger Electron Spectroscopy (AES)	Oxidation state and concentration of elements in the surface area	X-ray photons or electrons	Auger electrons	Auger effect
Ultraviolet Photoelectron Spectroscopy (UPS)	Electronic structure	Photons in the UV range	Photo electrons	Photoelectric effect
Metastable Impact Electron Spectroscopy (MIES)	Electronic structure	Metastable helium atoms	Auger electrons	De-excitation of the metastable atoms on the surface; Auger effect

Rotational vibration spectrum of gaseous hydrogen chloride at room temperature.

Vibrational Spectroscopy
method	Information received	inserted particle / wave	detected size / particle / wave	exploited effect
Infrared Spectroscopy (IR)	Spectrum, oscillation modes of adsorbates (often carbon monoxide as a probe)	Infrared photons	Infrared photons	Vibrational excitation of IR-active bands
Raman spectroscopy	Spectrum, vibration modes of adsorbates	VIS, NIR lasers	Rayleigh / Raman scattering (VIS, NIR)	Vibrational excitation of raman-active bands
Electron Energy Loss Spectroscopy (EELS)	spectrum	Electrons	Electrons	Excitation of processes in the solid body: phonon excitation , plasmon excitation , ionization

Ion spectroscopy
method	Information received	inserted particle / wave	detected size / particle / wave	exploited effect
Ion scattering spectroscopy (ISS = LEIS)	Molar mass of the surface atoms on the outermost layer (qualitative)	low-energy ions (often positive noble gas or alkali metal ions )	scattered ions with a mass spectrometer	Elastic scattering of ions on the surface, conservation of energy and momentum
Secondary ion mass spectrometry (SIMS)	Molar mass of the atoms in the depth profile of the surface (quantitative)	Ions (often positive noble gas or metal ions )	Clusters and fragments of the surface, scattered ions with a mass spectrometer	Surface sputtering
Rutherford Backscattering Spectrometry (RBS)	Composition of the surface	high-energy helium ions
Nuclear Response Analysis (NRA)	Composition of the surface	high energy ions or neutrons	Decay products of nuclear reactions	Nuclear reactions
Secondary neutral particle mass spectrometry (SNMS)

X-ray absorption spectrum in the area of an absorption edge (schematic). The edge is marked with an arrow and the energy range examined by EXAFS is highlighted in light blue.

X-ray absorption spectroscopy (XAS)
method	Information received	inserted particle / wave	detected size / particle / wave	exploited effect
(Surface) Extended X-Ray absorption Fine Structure ((S) EXAFS = XANES)	Information about local order , bond lengths, coordination number	tunable X-ray photons ( synchrotron radiation )	X-ray photons	Interference from original photoelectrons and photoelectrons scattered at neighboring atoms lead to different likelihood of photoelectric effect
X-ray absorption near edge structure (XANES = NEXAFS)	Information on local order , electronic structure, oxidation state	tunable X-ray photons ( synchrotron radiation )	X-ray photons	like EXAFS but more precise resolution of the one near the absorption edge
Mössbauer spectroscopy	Composition, structural information, oxidation states, particle size	Gamma radiation (mostly off ) ${\ displaystyle {} ^ {57} \ mathrm {Co}}$	Gamma radiation	Mössbauer effect , Doppler effect

Other types of spectroscopy
method	Information received	inserted particle / wave	detected size / particle / wave	exploited effect
Scanning Tunnel Spectroscopy (STS)	Density of states of the surface region in local space	Electrons, variation of location and tunnel voltage	Tunnel current	Tunnel effect

diffraction

method	Information received	inserted particle / wave	detected size / particle / wave	exploited effect
Diffraction of low energy electrons (LEED)	Surface structure in reciprocal space , superstructures , 2D long-range order must be present	low energy electrons	diffracted electrons	diffraction
X-ray diffraction (XRD)	Lattice structure of the entire solid in reciprocal space , 3D long-range order must be present	X-ray photons	diffracted x-rays	diffraction
MEED	Monolayer growth as a function of time, long-range order with full monolayer must be present	Electrons	diffracted electrons	diffraction
Reflection high energy electron diffraction (RHEED)	In-situ structural analysis during deposition, long-range order must be present	Electrons	Electrons	Diffraction with a small glancing angle

Kinetic Methods

method	Information received	inserted particle / wave	detected size / particle / wave	exploited effect
Temperature-programmed desorption (TPD)	Order of desorption kinetics , number of particles per monolayer	warmth	Desorbed surface particles	Desorption when the temperature rises

Sorptive methods

method	Information received	inserted particle / wave	detected size / particle / wave	exploited effect
BET measurement	Size of surfaces	nitrogen	adsorption	Adsorption / desorption when the temperature rises
Chemisorption	active centers	Hydrogen, oxygen, carbon monoxide	Chemisorption, adsorption	Chemisorption, desorption

Combinations

Certain types of radiation can stimulate several processes, which can have advantages and disadvantages for the respective method. For example, during ionization by X-rays, Auger electrons and photoelectrons can be generated at the same time, which may overlap in the spectrum and thus make evaluation more difficult. On the other hand, with TEM, additional information about the sample is obtained in an apparatus through the additional emission of Auger and photoelectrons, backscattered electrons, emitted particles and EELS.

The "Big Four"

The XPS, AES, SIMS and ISS measurement methods are referred to as the “Big Four”.

Nobel Prizes for developments in surface chemistry and surface physics

The Nobel laureate Gerhard Ertl is regarded as the founders of modern surface chemistry

Year / subject	person	nationality	Reason for awarding the prize
1932 chemistry	Irving Langmuir	United States 48 United States	"For his discoveries and research in the field of surface chemistry"
1937 physics	Clinton Davisson and George Paget Thomson	United States 48 United States United Kingdom United Kingdom	"For their experimental discovery of the diffraction of electrons by crystals"
1981 physics	Kai Manne Siegbahn	Sweden Sweden	"For his contribution to the development of high-resolution electron spectroscopy "
1986 physics	Gerd Binnig and Heinrich Rohrer	Germany Federal Republic Federal Republic of Germany Switzerland Switzerland	"For your construction of the scanning tunneling microscope "
2007 chemistry	Gerhard Ertl	Germany Germany	"For his studies of chemical processes on solid surfaces"
2007 physics	Albert Fert and Peter Grünberg	France France Germany Germany	"For the discovery of giant magnetoresistance (GMR)"

literature

Individual evidence

↑ Thomas Waldmann, Daniela Künzel, Harry E. Hoster, Axel Groß, R. Jürgen Behm: Oxidation of an Organic Adlayer: A Bird's Eye View . In: Journal of the American Chemical Society . tape 134 , no. 21 , May 30, 2012, p. 8817-8822 , doi : 10.1021 / ja302593v .
↑ Surface physics of the solid (page 101)

Books

G. Ertl , J. Küppers: Low Energy Electrons and Surface Chemistry . 2nd Edition. Verlag Chemie, Weinheim 1985, ISBN 3-527-26056-0 .
G. Ertl: Reactions at Solid Surfaces . 1st edition. Wiley, New Jersey 2009, ISBN 978-0-470-26101-9 .
Gábor A. Somorjai : Introduction to Surface Chemistry and Catalysis . Wiley, New York 1994, ISBN 0-471-03192-5 (English).

items

Gerhard Ertl: Reactions on surfaces: from atomic to complex (Nobel Lecture) . In: Angewandte Chemie . tape 120 , no. 19 , 2008, p. 3578-3590 , doi : 10.1002 / anie.200800480 .
K. Köhler, CW Schläpfer: Coordination chemistry on oxide surfaces . In: Chemistry in Our Time. 27, No. 5, ISSN 0009-2851 , 1993, pp. 248-255.

Magazines

Surface Science, Elsevier , ISSN 0039-6028
Surface Science Letters, Elsevier, ISSN 0167-2584
Surface Science Reports, Elsevier, ISSN 0167-5729
Surface Science Spectra, Elsevier, ISSN 1055-5269
Applied Surface Science, Elsevier, ISSN 0169-4332
Progress in Surface Science, Elsevier, ISSN 0079-6816
Applications of Surface Science, Elsevier, ISSN 0378-5963
ChemPhysChem 11, Special Issue on Surface Phenomena, 2010

Web links

Introduction to Surface Chemistry (Queen Mary University of London)
Video dossier of Swiss television on the subject of nanotechnology ( memento from September 10, 2010 in the Internet Archive )
Surface chemistry in its purest form. tagesschau.de, archived from the original on February 12, 2013 ; Retrieved April 11, 2015 .
Richard Feynman : There's Plenty of Room at the Bottom Lecture at Caltech, 1959.

[1] Thomas Waldmann, Daniela Künzel, Harry E. Hoster, Axel Groß, R. Jürgen Behm: Oxidation of an Organic Adlayer: A Bird's Eye View . In: Journal of the American Chemical Society . tape 134 , no. 21 , May 30, 2012, p. 8817-8822 , doi : 10.1021 / ja302593v .

[2] Surface physics of the solid (page 101)