11/27/2013

1

Surface Structure of Metals

Technological applications of metals

supported metal catalysts

electrodes, mechanical fabrications

related to surface properties

Understanding of surface properties

know the amount of each type of surface

exposed

have detailed knowledge of the properties

of the surface plane, i.e. well-defined

surface or single crystal

How do the surfaces look like?

most common metallic crystal structures

Body–centered cubic (bcc)

face-centered cubic (fcc)

hexagonal close packed (hcp)

low-index surface

planes of interest & practically used e.g. (111), (110), (100)

surface vs. bulk structures

atomic arrangement

FCC and HCP Structures

both have close packed arrangement of atoms

differ in stacking arrangement

11/27/2013

2

Close Packing of Spheres

A single layer of spheres is closest-packed with a

HEXAGONAL coordination of each sphere

A second layer of spheres is placed in the indentations left by the first layer

space is trapped between the layers that is not filled by the spheres

1st layer

2nd layer

TWO different types of HOLES (so-called INTERSTITIAL sites) are left OCTAHEDRAL (O) holes with 6 nearest sphere

neighbors TETRAHEDRAL (T±) holes with 4 nearest sphere

neighbors

Tetrahedral

hole

Octahedral

hole

11/27/2013

3

atoms on the 1st layer

(yellow spheres)

atoms on the 2nd layer

(gray spheres)

atoms on the 3rd layer (red circles)

(ABA... stacking) hcp

When a third layer of spheres is placed in the

indentations of the second layer there are TWO choices

The third layer lies in indentations directly in line

(eclipsed) with the 1st layer

Layer ordering may be described as ABA

atoms on the 1st layer

atoms on the 2nd layer

atom on the 3rd layer

(ABC... stacking)

atom on the 3rd layer

(ABA... stacking)

fcc

hcp

The third layer lies in the alternative indentations

leaving it staggered with respect to both previous layers

Layer ordering may be described as ABC

3 – dimensional unit cell

face-centered cubic

(fcc)

hexagonal close packed

(hcp)

* Each atom has 12 nearest neighbors

(i.e. Coordination Number = 12)

bcc structure

not a close packed

structure

quite similar to fcc

(i.e. cubic nature of

its unit cell)

C. N. = 8

11/27/2013

4

Surface Structures of fcc

metal

fcc unit cell (100)

face (blue atoms)

(100) face

(bird’s eye view) fcc (100)-(1 x 1)

obtained by cutting the fcc metal parallel to the

front surface of the fcc cubic unit cell

atomic arrangement: 4-fold symmetry

EXERCISE: What is the coordination number of the surface layer atoms of fcc (100)?

ANS. ANSWER: CN = 8

RATIONALE: Each surface atom has four nearest neighbours in the 1st layer, and another four in the layer immediately below ; a total of 8.

This contrasts with the CN of metal atoms in the bulk of the solid which is 12 for a fcc metal.

Characteristics of fcc (100) planes

all surface atoms are equivalent

smooth surface at the atomic scale

various adsorption sites for the adsorbate molecules

On-top sites (above a single metal atom)

Bridge sites (between 2 atoms)

Hollow sites (between 4 atoms)

Depending upon the site occupied, an adsorbate species (with a single point of attachment to the surface) is therefore likely to be bonded to either one, two or four metal atoms.

11/27/2013

5

Adsorption Sites on fcc 100 Surface

On – top sites

Bridge sites

Hollow sites

18

EXERCISE: What is the coordination

number of the topmost layer atoms of fcc

(110)?

19

ANSWER: CN = 7

RATIONALE: Each surface atom has two

nearest neighbours in the 1st layer, and

another four in the layer immediately below,

and one directly below it in the third layer ;

this gives a total of 7

20

Characteristics of fcc (110) planes

first layer surface atoms are equivalent

second layer atoms also exposed

atomically rough surface with high anisotropy

various adsorption sites for the adsorbate molecules

On-top sites

Short bridge sites (between 2 atoms in a single row)

Long bridge sites (between 2 atoms in adjacent rows)

Higher coordination sites (in the troughs)

11/27/2013

6

21

Adsorption Sites on FCC surface

On–top sites

Short bridge sites

Long bridge sites

Higher coordination sites

fcc (111) surface

fcc unit cell

(111) face

(blue atoms)

(111) face

(bird’s eye view) fcc(111)-(1 x 1)

obtained by cutting the fcc metal in such a way

that the surface plane intersects the x–, y– and

z– axes at the same value

atomic arrangement: 3–fold symmetry

23

Characteristics of fcc (111) planes

all surface atoms are equivalent and have high CN

atomically smooth surface

various adsorption sites for the adsorbate molecules

On-top sites

Bridge sites (between 2 atoms)

Hollow sites (between 3 atoms)

24

Adsorption Sites on fcc 111 Surface

On – top sites

Bridge Sites

Hollow sites

11/27/2013

7

25

Intersection of surfaces in irregularly–

shaped samples

26

Close–packed plane, and one axis (the c–axis

perpendicular to these ller index notation–based on 3

axes at 120 in the planes.

Thus, the 4-index structure: hcp (0001)

3rd is redundant and sometimes left out; thus, (0001) is

reduced to (001)

resembles fcc (111)

(0001) surface plane

e.g. Ru(0001)

Surface Structures of hcp metal

hcp 001 Surface all surfaces are energetically unfavorable in

that they have a positive free energy of

formation

formation of new surfaces by cleavage of a

solid and bond-breaking between atoms on

either side of the cleavage plane in order to

split the solid and create the surfaces

Bond–breaking requires work to be done on

the system, so the surface free energy (surface

tension) contribution to the total free energy of

a system must therefore be positive.

Energetics of Surfaces

11/27/2013

8

How to minimize surface energy

1. by reducing the amount of surface area exposed

particulate metals (single-crystallites)

that of a sphere (low surface free energy; consider a droplet of water)

2. by predominantly exposing surface planes which have a low surface free energy

high surface atom density

surface atoms of high coordination number

decreasing stability (in vacuum): fcc(111) > fcc(100) > fcc(110)

3. by altering the local surface atomic geometry in a way which reduces the surface free energy

relaxation & reconstruction

Reconstruction of Surfaces

- involves change in the periodicity of the surface structure

and surface symmetry relative to the bulk

e.g. fcc (110)-(1x1)

(unreconstructed)

e.g. fcc (110)-(1x2)

(reconstructed)

31

• surface structures of single-crystals

• most frequently studied surface planes of the fcc

((100), (110), (111)) and hcp systems

• basic understanding of surface structure and

crystallography – helpful in the study of surface

chemistry of some important fcc (e.g. Pt, Rh,

Pd, Au, etc.) and hcp metals (e.g. Ru)

• Some metallic single-crystal undergo

reconstruction to minimize surface energy

Summary

32

ADSORPTION OF

MOLECULES ON SURFACES

11/27/2013

9

33

Five basic steps:

1. Diffusion of reactants to the active surface

2. Adsorption of one or more reactants onto

the surface

3. Surface reaction

4. Desorption of products from the surface

5. Diffusion of products away from the surface

Molecular Adsorption Terminologies:

• Substrate

- solid surface onto which adsorption can occur;

- e.g. Pt disc, Au(111) electrode

• Adsorbate

atomic or molecular species which are adsorbed (or are

capable of being adsorbed) onto the substrate.

inorganic and organic species

• Adsorption

process in which a molecule becomes attached onto a

surface of another phase

to be distinguished from absorption - uptake into the

bulk of a solid or liquid phase

35

- Terminologies:

• Coverage

measure of the extent of adsorption of a

species onto a surface

denoted by (“theta”)

• Exposure

measure of the amount of gas which a surface

has seen

product of pressure and time of exposure (unit:

Langmuir; where 1 L = 10-6 Torr s )

Physical Adsorption (Physisorption)

weak Van der Waals

no significant redistribution of electron density in

either the molecule or at the substrate surface

Chemical Adsorption (Chemisorption)

Involves a chemical bond

involves substantial rearrangement of electron

density, formed between the adsorbate and

substrate

between extremes of virtually complete ionic or

complete covalent character

Modes of Adsorption

11/27/2013

10

Structure of Metals & Adsorption Geometry

What happens when a molecule becomes adsorbed

onto a surface?

Why study surface chemistry?

to identify the nature of the adsorbed species

and its local adsorption geometry (i.e. its

chemical structure and co-ordination to adjacent

substrate atoms)

to determine the overall structure of the

extended adsorbate/substrate interface (i.e.

the long range ordering of the surface)

Surface Unit Cell

fcc(100) surface unit cell

4–fold rotational symmetry (unit cell: “square symmetry")

to define the unit cell shape, size, and symmetry:

2 vectors which have a common origin are used; and

2 sides of the unit cell are defined

by convention, the 2 vectors which define the fcc(100)

unit cell are called a1 & a2;

have the same length i.e. |a1| = |a2| ;

are mutually perpendicular

Surface Unit Cell

fcc(110) surface unit cell

2–fold rotational symmetry (unit cell:

“rectangular”)

by convention, |a2| > |a1|

Surface Unit Cell

fcc(111) surface unit cell

3–fold symmetry (unit cell: “hexagonal”)

2 vectors are the same i.e. | a1| = | a2|

angle between the vectors < 90(i.e. 30) or > 90

(i.e. 120)

11/27/2013

11

Wood’s Notation

simplest and most frequently used method for

describing a surface structure

only works, however, if 2 unit cells are of the

same symmetry or closely-related symmetries

(more specifically, the angle between b1 & b2

(adsorbate vectors) must be the same as that

between a1 & a2 (substrate vectors))

involves specifying the lengths of the 2 overlayer

vectors, b1 & b2 , in terms of a1 & a2 respectively

written in the format: ( |b1|/|a1| x |b2|/|a2| ) i.e. a

(2 x 2) structure has |b1| = 2|a1| and |b2| = 2|a2|

• (2 x 2) adsorbate overlayer on a fcc(100) surface adsorbate is bonded terminally on–top site of individual

atoms of the substrate

Wood’s Notation

primitive (2x2) or p(2x2)

overlayer structures

Substrate : fcc(100)

Substrate unit cell

Adsorbate unit cell

• (2 x 2) adsorbate overlayer on a fcc(100) surface

adsorbate is bonded terminally on the 4–fold hollow

site of individual atoms of the substrate

Wood’s Notation

Substrate : fcc(100)

Substrate unit cell

Adsorbate unit cell

• (2 x 2) adsorbate overlayer on a fcc(110) surface

adsorbate is bonded terminally on the hollow

sites of individual atoms of the substrate

Wood’s Notation

Substrate : fcc(110)

Substrate unit cell

Adsorbate unit cell

11/27/2013

12

Substrate : fcc(___)

__________ unit cell

__________ unit cell

Wood’s Notation

EXERCISE: Fill in the blanks by giving the Wood’s notation

of the overlayer structure and the crystallographic

orientation of the substrate.

• (_ x _) adsorbate overlayer on a fcc(___) surface

- adsorbate is bonded terminally ______ site of individual

atoms of the substrate

Substrate : fcc(111)

Substrate unit cell

Adsorbate unit cell

ANSWER:

• (2 x 2) adsorbate overlayer on a fcc(111) surface

adsorbate is bonded terminally on top site of

individual atoms of the substrate

Wood’s Notation

47

Substrate : fcc(100)

c(2 x 2)

(2 x 2)R45

- adsorbate unit cell could be:

(a) face–centered (2 x 2) or simply c(2 x 2)

(b) primitive(2 x 2)R45 or simply (2 x 2)R45

Wood’s Notation

48

• atomic / molecular adsorption on substrate

• surface unit cell of adsorbate (inorganic or

organic) species relative to the substrate (e.g.

single-crystal)

• Wood’s notation for describing surface

structures of overlayers

Summary