Introduction to Surface/ Interface Chemistry

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Introduction to Surface/

Interface Chemistry

Patrick Theato office: 625

phone: 02-880-7071

email: theato@uni-mainz.de

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Expectation from the class?

topics:

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syllabus

Forces & Adsorption

characterization methods imaging methods

surface chemistry (SAMs) grafting polymers

micropatterning

biological interfaces

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What is a surface/interface?

boundary between two different phases of solid matter:

liquid - liquid

liquid - solid

solid - solid

solid - vapor

liquid - vapor

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What is a surface/interface?

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What is a surface/interface?

common sense: A surface is the shell of a macroscopic object in contact with its environment. An interface is the boundary between two phases.

The surface of an object determines its optical appearance, stickiness, wetting behavior, frictional behavior, and chemical reactivity, etc.

* in large objects with small surface area A to volume V ratio (A/V) the physical and chemical properties are primarily defined by the bulk (inside)

* in small objects with a large A/V-ratio the properties are strongly influenced by the surface

E.g. silver (Ag):

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example: gold nanoparticles

Gold nanoparticles are composed of small crystalline Au clusters with dimensions usually around 2 nm in diameter.

example for material with high A/V ratio, quantum structures (size effects)

surface coating with molecular layer to prevent aggregation (alkylthiols or PPh3)

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example: teflon frying pan

Polytetrafluoroethylene (PTFE, Teflon) is obtained by free radical polymerization of tetrafluoroethylene.

"How does Teflon stick to the frying pan when nothing sticks to Teflon???"

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example: lotus leaf effect

The surface of plant leafs, especially of the lotus flower, can show extreme hydrophobicity to water (large water contact angles > 150°C).

explanation:

- hydrophobic material

- surface structure (20-100µm)

Such hydrophobic surfaces also show a remarkable self-cleaning effect.

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example: antireflecting surfaces

Structured surfaces with feature sizes below the wavelength of visible light (ca. <

200 nm ) show antireflecting behavior.

surface roughness from sol-gel cold-spraying process

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surface roughness by lithographic patterning and etching

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example: biochips

immobilized biomolecules for sensors and diagnostics:

immunoassay in microfluidics

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the “chemical nose” DNA chips

specific surface modification for site-selective adsorption / growth of cells:

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• surface material (chemical composition):

-> determines chemical behavior (reactive / inert) -> physical materials behavior (conducting / insulating) -> polarity (hydrophilic / hydrophobic)

-> surface structure on a molecular scale ...

• topography ("valleys and hills"):

-> determines chemical "reactivity" / kinetics (fast / slow reaction, reactive sites) -> physical surface behavior (reflectivity, effective wetability)

-> tribological behavior (friction on "rough" or "smooth" surface) -> surface morphology on a microscopic to macroscopic scale

...

important key features of surfaces

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• understanding and design of catalysts (industrial production of NH3 e.g., car exhaust)

• understanding and inhibition of corrosion (ships, cars, buildings)

• modification of surface properties like:

- friction (tires, bearings)

- wear (polymer lenses of glasses → ormocers) - stickiness (frying pan, adhesive tape)

- wetting, condensation (scuba diving goggles, outdoor gear, inkjet printing) - anti-reflection (picture frames, displays)

- color (paint)

• chip manufacturing / microelectronics

• hard disks (anti-friction, ultra-smooth,...)

• biological surfaces (biocompatibility, patterned cell growth)

• sensors (chemical, biological)

• microfluidics ...

application examples of surface science

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schematic representation of a surface

Translational symmetry of structure elements in an ordered surface: idealized crystal lattice

In three dimensions the structure extends to one side of the surface into the bulk.

FN is the normal force acting onto a surface element towards the bulk, due to missing elements at the exterior.

WS is the work to transfer a surface element into the gas phase.

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What is the structure of a surface?

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The morphology of a surface is a macroscopic or ensemble property that defines its form and shape.

example: grainy structure of wooden table top

The structure of a surface is given by the atomic and molecular composition and arrangement of the atoms in space.

Geometry of AgBr(111)-(2x1) ->

The topography of a surface is its profile determined by "valley", "planes" and "hills".

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What is an Interface?

adhesion: attractive

interactions between two different media

An interface is the separating layer between two condensed phases (usually molecular dimensions).

cohesion: attractive

interactions within a phase (solid or liquid)

surrounding medium interfacial region

substrate (solid or liquid)

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At the border of a solid or liquid in contact with vapor there is usually no abrupt change in density, but a more or less continuous transition from high density to low density.

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An interphase is a chemical compound composed of the two surrounding phases.

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different surface & interface scenarios

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• liquid highly mobile & disordered

• constant evaporation and recondensation at surface

• solid highly immobile

• crystalline solids highly ordered / structured

• usually no evaporation of surface atoms & molecules, only lateral

diffusion (depends on T)

(18)

different surface & interface scenarios 2

solid / liquid liquid1 / liquid2 solid1 / solid2

• liquid can dissolve surface atoms → may lead to

surface charges

• liquid molecules at the interface can be much

higher ordered than in the bulk

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• both phases highly mobile

→ shape of interface is controlled by surface tension

• depending on solubility molecules will migrate from one phase to the other → controlled by chemical

potential (partition coeff.)

• if two crystalline solids are in atomic contact the

different lattice constants will generate strain @ interface

• if both materials react together new compound will be formed in contact region

→ interphase

• at high T interdiffusion possible (e.g. Cr & Au)

(19)

The planes of ideal crystals (a cut through the lattice) are closely related to ideal crystal surfaces. To specify a particular plane most commonly Miller indices are used.

crystal planes - Miller indices

Procedure:

1) Identify intercepts of plane on the x−, y−, and z−axes. 1 x a, ∞ x b, ∞ x c (example: cubic lattice a=b=c, α=β=γ=90°)

2) Specify intercepts in fractional coordinates of unit cell parameters a, b, c.

(1 x a) / a, (∞ x b) / b, (∞ x c) / c → 1, ∞, ∞ 3) Take reciprocal of fractional intercepts, clear fractions.

1 / (1, ∞, ∞) = (1 0 0) → bar for negative values (h,k,l)

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(20)

translation vectors, unit cell vectors: a,b

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example: Au (100) surface

(21)

low index surfaces:

ideal crystal surfaces

high index surfaces:

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(22)

relaxation:

real crystal surfaces

reconstruction:

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(23)

→ surface defects are important for crystal growth:

• traps for newly adsorbed atoms / molecules

• restructuring of surfaces happens at defects first

kink−, step−, and terrace−atoms have large equilibrium concentrations on real surfaces

(hard to get perfect surface...)

isolated adatoms ("adsorbed atom") and vacancies are important for atomistic transport (restructuring), but equilibrium concentration is low

(< 1% of monolayer even at Tmelt)

surface defects and structures

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(24)

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(25)

Wood’s Notation and Matrix Notation

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Description of the ordered adlayer (adsorbed atoms and molecules) in terms of the relationship to the underlying ideal crystal plane.

Introduction to Surface/ Interface Chemistry