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Core 9: Compounds and materials > Processes at solid surfaces > Flashcards

Processes at solid surfaces Flashcards

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1
Q

What is the Bragg equation for constructive interference?

A

2 d sinθ= n λ

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2
Q

What is the problem with X-rays (diffraction) in regards to crystal penetration?

A

X-rays penetrate the bulk of the crystal- they are not sensitive to (relatively few) surface atom layers.

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3
Q

How can you alter the angle of incidence to affect the sensitivity to surface atoms?

A

Shallow angle of incidence enhances sensitivity (glancing/grazing angle) to surface atoms.

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4
Q

Are there more bulk atoms or surface atoms?

A

More bulk atoms.

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5
Q

What type of electrons offer surface selectivity?

A

Low energy electrons (LEE).

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6
Q

What surface technique is LEED and what principle does this rely on?

A

Low energy electron diffraction; wave particle duality (electrons have wave properties–> diffraction.

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7
Q

What are the key features for the LEED experimental setup?

A

Monochromatic electron beam, elastic back-scattered electrons are detected but only for conducting surfaces.

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8
Q

How is the LEED pattern spacing affected by interatomi separation and electron energy?

A

Pattern spacing decreases with increasing interatomic separation and electron energy- it is inversely proportional (Bragg equation).

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9
Q

What LEED pattern does constructive interference yield?

A

Parallel lines pattern.

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10
Q

What information can we get from an LEED diffraction pattern and what can be worked out from computer analysis?

A

Get a regular 2D pattern- evidence of regular surface layer, spacing gives atom density. Need computer analysis for bond lengths, angles etc.

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11
Q

What do we use to label planes in a solid?

A

Miller indices.

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12
Q

What close-packed structure is a more realistic example of a crystal in solids than a simple cubic lattice?

A

Face-centered cubic (FCC): in the bulk each atom is surrounded by 12 nearest neighbours (stable and strong.

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13
Q

What do the ‘neighbours’ in a FCC structure govern and why? How does this link to surface atoms?

A

Neighbours govern ‘surface energy’, because if you remove a layer of the nearest neighbours, you alter the stability hence the reactivity of the atoms.
All surface atoms have far fewer near neighbours (hence lower coordination number) than bulk atoms. Fewer nearest neighbours= more exposed (reactive) atoms—> high SURFACE ENERGY. The reactivity of a surface plane depends on the number of nearest neighbours.
(110) > (100) > (111)

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14
Q

Describe and explain 3 features of a good surface experiment

A
  1. Surface selective: sensitive to surface, not to bulk atoms- often use low energy electrons (LEE).
  2. Sensitive: few atoms are on the surface- use shallow glancing angle.
  3. Avoid contamination: use an ultra high vacuum (UHV)- avoids contamination and probe beam is unperturbed by gases.
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15
Q

What is surface reconstruction?

A

Spontaneous rearrangement of atoms to lower ‘surface energy’- no change in the number of surface atoms. Most likely for high surface energy planes.

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16
Q

What is the evidence for surface reconstruction?

A

Evidence from LEED: pattern spacing decreases with increasing interatomic separation (so here the 2D pattern of spots gets closer together).

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17
Q

How can LEED results be used to distinguish FCC from BCC structures?

A

Spots converge as eV increases (Bragg equation) hence FCC (111) can be distinguished from BCC (110).

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18
Q

What LEED results provide evidence of surface ‘relaxation’?

A

Intensities change, as spots fade away and new spots appwar- ie 3D info for 1st/2nd/3rd/4th layers.

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19
Q

What is the difference between the surface/bulk atomic distances in unrelaxed Vs relaxed surfaces?

A

Unrelaxed: d(surface) = d(bulk).
Realxed: d(surface) < d(bulk).

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20
Q

How does surface relaxation affect the surface energy?

A

Relaxed surface leads to a reduction in surface energy (greatest for high energy surfaces eg FCC (110) as most relaxation can occur).

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21
Q

Does relaxation just affect the surface layers?

A

No- perturbs the first few layers.

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22
Q

What is an adsorbate and an adsorbent?

A

Adsorbate= gas phase molecule that binds to the surface (adsorbent) in the process of adsorption.

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23
Q

Is adsorption an endothermic or exothermic process?

A

Always exothermic (think about entropy and Gibbs free energy equation).

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24
Q

In the case of adsorption for crystal growth, what is the adsorbate and adsorbent and what does the speed of growth depend on? What dominates the crystal appearance?

A

Adsorbate (gas) and adsorbent (seed crystal) are identical. Speed of growth depends on surface energy of the crustal plane; high surface energy faces grow the fastest. Slowest growing faces dominate the crystal appearance.

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25
Q

What desorption and what must be overcome for it to occur?

A

Desorption is the reverse process to adsorption. To desorb, molecule must overcome attractive forces hence is endothermic reaction with Arrhenius-like kinetics.

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26
Q

Name and describe a desorption experiment

A

Temperature programmed desorption (TPD) experiments:

  1. Adsorb molecules to the surface.
  2. Increase the temperature.
  3. Monitor desorption ie the gas evolved.

Peak position links to activation energy, number of peaks= number of binding sites. Peak areas are proportional to the amount of gas evolved.

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27
Q

Describe the differences between physisorption and chemisorption

A

Physisoption= physical adsorption, chemisorption= chemical adsorption.
Chemisorption: large variations between different materials, marked variations between different surface planes of same material, wide temperature range for adsorption (generally), wide range for adsorption enthalpy (typically 40-50 kJ/mol), adsorption may be dissociative and irreversible, monolayer uptake and wide range of kinetics (often activated processes).
Physisorption: only slight variations between chemically different substrates, nearly no variations between different surface planes of the same material, temperature range for adsorption is near or below condensation point of the gas, adsorption enthalpy is generally lower than for chemisorption (5-40 kJ/mol), adsorption is non-dissociative and reversible, multilayers possible in uptake and the kinetics are generally fast (non-activated process).

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28
Q

What variables does surface coverage depend on?

A

The characteristics of the gas-phase adsorbate and the adsorbent surface, adsorbate conc or pressure, temperature.

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29
Q

What do isotherms plot?

A

Plot of surface coverage as a function of gas pressure.

30
Q

What are the key assumptions made in the Langmuir isotherm?

A
  1. Fixed number of identical sites (–> monolayer only).
  2. ΔHads independent of coverage.
  3. Adsorbates do not interact.
31
Q

Name 2 ways of probing surface coverage and give examples of each

A
  1. By surface analysis: traditional methods eg mass and radioactivity, modern techniques eg microscopy and spectroscopy.
    By looking at change in gas phase: volume, pressure, radioactivity, mass spec, spectroscopy.
32
Q

Why is the Langmuir isotherm not always appropriate?

A

Can’t always deal with real-life problems: dissociative adsorption, competitive adsorption, adsorbate-adsorbate interactions, multilayers (important for most realistic scenarios).

33
Q

What is the BET isotherm used for and what does it assume?

A

Assumes random site distribution (ie empty or monolayer or 2 layers…), for multilayers, widely used in industry to estimate surface area of solids and powders.
ie is used for more complex, realistic multilayer systems.

34
Q

What does STM stand for and what is it used for?

A

Scanning tunnelling microscope (tunnelling- electrons tunnel from surface to probe tip), allows magnification x10^8 so for direct observations of surface atoms and features.

35
Q

What is the tunnelling current related to in STM?

A

Separation d–> atomic resolution.

36
Q

What do surface atoms look like on an STM image?

A

Surface atoms appear as brighter regions.

37
Q

Describe the 2 modes of operation of STM

A
  1. Constant distant (and voltage): map current I (nA) as surface is scanned. Not good for rough surfaces as atoms will get in the way of the tip.
  2. Constant current mode: more common and safe for rough surfaces. Scan tip over surface, adjust probe height to maintain constant current, map V applied to piezotube as surface is scanned.
38
Q

What are defects (steps/kinks/vacancies) on an STM image a feature of?

A

Defects= features of high surface energy.

39
Q

Discuss the advantages and disadvantages of STM

A

Advantages: amazing surface detail (individual atoms, surface structure and density, defects, adsorbates and contaminants, can use in vacuum/air/liquid.
Disadvantages: technically demanding (sharp tips, vibrational control, hi-tech electronics), only for conducting surfaces.

40
Q

Discuss AFM

A

AFM= atomic force microscopy. Works by measuring atomic deflections of the tip from surface atoms, laser amplifies signal, can map out atoms/steps. Can use for anything (in vacuum and air), not just for conducting surfaces.

41
Q

Which has greater surface energy, defects or terraces? Hence what dominates the crystal appearance?

A

Defects&raquo_space; terraces

Defects rapidly filled in (high surface energy= fast growth) hence terraces dominate crystal appearance.

42
Q

What does XPS stand for and what effect does it use?

A

X-ray photoelectron spectrometry; uses the photoelectric effect.
NB: also known as XPES/PES/ESCA.q

43
Q

Give some details for XPS experiments

A

Conducted under UHV, sensitive (X-rays at shallow grazing angle), surface selective as only consider LEE.

44
Q

Is XPS qualitative or quantitative?

A

Quantitative: peak areas are related to % composition.

45
Q

Name and describe an experiment which is closely related to XPS

A

Auger electron spectroscopy (AES): also conducted under UHV, sensitive (X-rays or electron beam at shallow grazing angle- electron beam allows 20pm spatial resolution), surface selective.

46
Q

Describe the Auger process

A
  1. X-ray (or electron beam) induced ionisation.
  2. Electron drops down to fill the hole (as initial excited state is unstable).
  3. Relaxation energy transferred to higher energy electron—> this 2nd ionisation is the AUGER EMISSION.
47
Q

Describe some similarities and differences of XPS and AES

A

Both are: conducted under UHV, sensitive (grazing angle) to &laquo_space;monolayer, surface selective (if consider only LEE).
XPS and AES data provide: elemental composition, some chemical environment info (chemical shifts), quantitative data (signal size is proportional to % composition).

Differences: XPS emission energy (eV) shifts with hv (1 electron process with poor spatial resolution), AES emission energy (eV) is independent of hv (multiple electron process, decent spatial resolution).

48
Q

How can we learn more about adsorption thermodynamics?

A

From experiments at different temperatures (Clausius-Clapeyron equations).

49
Q

What does isosteric mean?

A

Same size or density.

50
Q

Name a special case of chemisorption

A

Dissociative adsorption (Langmuir-like assumptions and derivation).

51
Q

At equilibrium, how are the rates of adsorption/desorption related?

A

They’re equal.

52
Q

Name 3 pieces of evidence for dissociative adsorption

A

Thermochemistry, coverage data (constant T) and desorption kinetics.

53
Q

What is RAIRS?

A

Reflection adsorption infra-red spectroscopy= adaption of IR for surfaces.

54
Q

How is sensitivity and surface selectivity achieved in RAIRS?

A

Sensitivity- shallow glancing angle.

Surface selectivity- reflections monitored.

55
Q

Describe some limitations of RAIRS

A
  • Highly reflective (metal crystal) surface only.
  • Different spectra needed (surface, adsorbent, gas phase).
  • No info at less than 600 cm^-1 - ie strong absorbers only.
56
Q

What is the selection rule for RAIRS?

A

Oscillating dipole must be perpendicular to surface (some modes inactive- RAIRS absence not proof of dissociation).

57
Q

Name 4 types of IR surface techniques

A

Transmission (TIR), diffuse-reflectance (DRIFTS), attenuated total reflection (ATR), reflection-absorption (RAIRS).

58
Q

How do we know if adsorption is dissociative?

A

From experimental evidence: thermochemistry, desorption kinetics, coverage (isotherm), spectroscopy, subsequent chemistry.

59
Q

List the 5 steps of a surface reaction

A
  1. Diffusion of reactants onto the reaction surface.
  2. Adsorption of greater than or equal to 1 reactant onto the surface.
  3. Surface reaction- rate.
  4. Desorption of product.
60
Q

What does Z and s stand for in relation to the rate of adsorption?

A 
Z= collision frequency.
s= sticking probability.
61
Q

What are the 2 basic reaction mechanisms for looking at the kinetics of surface reactions?

A

Langmuir-Hinshelwood and Eley-Rideal.

62
Q

Describe the Eley-Rideal mechanism

A

Only A adsorbs and B reacts from the gas phase.

63
Q

Describe the Langmuir-Hinshelwood mechanism

A

Both reactants (A and B) must adsorb onto the surface, rate is maximised when the surface coverage of A is equal to the surface coverage of B. A and B are competing for surface sites so rate eventually decreases.

64
Q

Why are laser properties (pulsed) ideal for realistic surface kinetic studies?

A
  1. Fast: short pulse duration (MHz). Nb- use light (not LEE)- no need for UHV, can work in air.
  2. Sensitivity- highly collimated light source (not divergent)–> intense beam at very shallow glancing angle.
65
Q

What is the selection rule with pulsed lasers for SHG (second harmonic generation) and how does this allow surface selectivity and enhanced sensitivity?

A

Required high intensity for frequency doubling, selection rule: asymmetric medium required for ‘doubling’= surface (not bulk) can double- surface selective. Pulses of ‘doubled’ light readily distinguished from probe beam- enhanced selectivity.

66
Q

What changes is SHG sensitive to?

A

Surface coverage, composition, binding site, adsorbate orientation- is a very versatile technique.

67
Q

List some properties of an efficient catalyst

A

Large surface area, separable from reactants (ie don’t bind too strongly), enhances reaction rate.

68
Q

What is the consequence in catalysis if a surface binds too strongly?

A

Slow desorption- inefficient catalysis.

69
Q

Order these defects in terms of surface energy: kinks, vacancies, steps

A

Vacancies > kinks > steps

70
Q

What defect is the exception to the rule regarding surface energy and growth?

A

Screw dislocation.

Core 9: Compounds and materials (4 decks)

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