# 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 .:

• 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 :

## 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.

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 "For his discoveries and research in the field of surface chemistry"
1937
physics
Clinton Davisson and
George Paget Thomson
United States United Kingdom

"For their experimental discovery of the diffraction of electrons by crystals"
1981
physics
Kai Manne Siegbahn  Sweden "For his contribution to the development of high-resolution electron spectroscopy "
1986
physics
Gerd Binnig and
Heinrich Rohrer
Federal Republic of Germany Switzerland

"For your construction of the scanning tunneling microscope "
2007
chemistry
Gerhard Ertl  Germany "For his studies of chemical processes on solid surfaces"
2007
physics
Albert Fert and
Peter Grünberg
France Germany

"For the discovery of giant magnetoresistance (GMR)"

## literature

Individual evidence

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)

Books

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, , 1993, pp. 248-255.

Magazines