L3 Exams Pt2 > Hard Matter > Flashcards

Hard Matter Flashcards

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1
Q

What are the electrical resistivity bands?

A

Insulator
Eg > 4 eV

Semiconductor
0 eV < Eg < 4 eV

Conductor
Eg = 0 eV

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2
Q

What causes coloured diamonds?

A

Diamond rarely perfect
most have defects
accumulated during crystal growth

Defects modify way light passes through diamond

so crystal selectively transmits some wavelengths & selectively absorbs others
∴ determines colour

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3
Q

How can graphene be fabricated?

A

Fabrication 2D material can use mechanical exfoliation

does not need heat treatment
just use of adhesive/scotch tape
to detach single layer C atoms from bulk graphite

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4
Q

What is the reaction equation for the fabrication of Si?

A

C + SiO2 –Δ–> Si + CO2

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5
Q

Why should H2 be used for fuel?

A

Clean

Abundant

Efficient

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6
Q

Why use H2O for H2 fuel production?

A

Clean

Abundant

Simple

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7
Q

What is the linear combination of molecular orbitals?

A

Pairing electrons in bonding orbital
overall energy of system lower than single atom
molecule forms

Electrons in anti-bonding orbitals
energy of system greater than single atom
molecule doesn’t form

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8
Q

How do bonds leave to bands?

A

Each combination creates MO

As no. atoms increases
no. overlapping MOs increase
E between neighbouring allowed orbitals
systematically reduced

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9
Q

Why do Group 1 alkali metals conduct?

A

Availability empty orbitals
in half filled s-band permits
easy movement electrons

almost no E needed
to promote valence electrons
to open unoccupied level

Leads to delocalisation of
electron energy over crystal

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10
Q

How does conduction occur in group 2?

A

Group 2 s-orbital is full

E separation of constituent
higher E s & p orbitals
close enough in solid state
that bands overlap
so partially filled band

Overlapping MO level
enables metal

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11
Q

What is the bandgap?

A

Forbidden gap
where no MOs allowed

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12
Q

What is the valence and conduction bands?

A

Highest E band
containing electrons

Lowest E level
above valence band

Separation dictates electronic properties

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13
Q

What is the effect of the band gap?

A

Large energy gap (Eg)
between VB & CB
is insulator
large E gap for electron movement

If Eg not large
electrons can move from VB to CB
from certain disturbances
eg. light or thermal treatment
semiconductor

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14
Q

What is the equation for resistance?

A

R = 𝜌L / A

𝜌 = resistivity
L = length
A = cross sectional area

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15
Q

What is an alloy?

A

“Composition knob”
Mixture of metallic elements

More components
more possible
atomic environments

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16
Q

What are the types of alloys?

A

Pure metal

Binary alloy
Low-entropy alloy

Ternary & quaternary
Medium-entropy alloy

> 5
High-entropy alloy

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17
Q

What is the configurational entropy?

A

Entropy purely from
how many different ways
atoms can be arranged on lattice site

S(conf) ⍺ kB x ln(W)

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18
Q

What are the different oxide structures?

A

Rocksalt - MO
simple fcc lattice

Rutile - MO2
chain of octahedra

Corundum
M2O£
Close-packed O
octa site

Monoclinic oxides
low-symmetry structures

Spinel - AB2O4
normal - A tetra, B octa
inverse - B tetra, A/B octa

Perovskite - ABO3
cube corner - A
octa corner - B

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19
Q

What are multinary semiconductors?

A

Tuned by composition

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20
Q

What are 2D layers?

A

Strong in-plane bonds
Weak interlayer coupling

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21
Q

How does XRD show phase transition?

A

No visible splitting =/= no phase transition

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22
Q

What is the universal link?

A

High-symmetry phase
eg. cubic
many different hkl equivalent
one peak

Symmetry lowers
eg. cumin -> tetragonal
equivalence lost
peak splitting

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23
Q

How to distinguish polymorphs using XRD?

A

Peak presence/absence

Peak position
d-spacing
Compare 2θ/d with reference

Relative intensities

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24
Q

What causes varying conductivity in the same element?

A

eg. group 14 elements
lower E with hybridising AOs

Carbon:
Diamond insulting
Graphene conductive
∵ bonding, hybridisation & band structure

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25
What is a Wigner-Seitz cell?
Constructed primitive cell
26
What is the first Brillouin Zone?
Analogue for Wigner-Seitz cell in real space
27
What is the equation for a complex number?
z = a + ib a = real no. ib = imaginary part
28
What is the complex conjugate?
z* = a - ib |z|^2 = z x z*
29
What is a way to determine the energy of the band gap?
From gaps in the reduced BZ
30
What is an indirect gap semiconductor?
Uppermost VB & lowest CB E occur at different locations in k-space eg. Ge
31
What is the Fermi surface?
Interface in reciprocal space which separates occupied from unoccupied electronic states (MOs) at 0 K
32
What is the Fermi level?
Highest occupied crystal orbital cutting through a band to produce a partially filled band. Semiconductor or insulator lies in middle of forbidden band f(E) = 1/2
33
What is the Fermi-Dirac function?
Gives fraction of allowed electron states f(E) at an E level that is populated at a given temperature (T > 0K)
34
What is the typical band gap for a semiconductor?
Gap between filled VB & empty CB 0.5 - 3 eV At low temp. not enough energy to overcome band gap As temp. increases gains thermal E & can jump gap
35
What affects the conductivity of metals?
Reduces as temp. increases ∵ increased electron scattering ∵ photons/defects reduces mobility of electrons
36
What affects the conductivity of semiconductors?
Increases from exponential increase in no. electrons & holes on heating
37
What are intrinsic semiconductors?
Band gap small Thermal E promote electrons from VB to CB Conductivity increases w/ temp. As electrons promoted holes created in VB allowing conductivity eg. Si & Ge
38
What is semiconductor doping?
Changes intrinsic semiconductor to extrinsic semiconductor
39
What are n-type conductors?
Electron donor dopant releases mobile conduction electron into crystal lattice Majority charge carries negatively-charged electrons
40
What are p-type semiconductors?
Creates vacancy which can move through crystal like positively-charged carrier
41
Why is TiO2 typically n-type?
Often contains donor-like defects these introduce extra electrons donor states lie near CB so electrons easily available Ef shifts towards conduction band minimum n-type behaviour
42
Why is CuO typically p-type?
Commonly forms acceptor-like defects creates holes as majority carriers shifts Ef towards valence band minimum p-type behaviour
43
How are p-n junctions formed?
When p-type & n-type semiconductors contact carriers diffuse depletion region & built-in filed form at interface At eqmb. built-in field stops further diffusion Ef becomes constant throughout Causes CB & VB to bend
44
What is the Anderson herterojunction model?
Ideal band alignment Assumes: E(vac) continuous access interface Atomically sharp interface (no interface dipoles/states)
45
What are type I heterostructures?
Both CB & VB of 1 semiconductor lie within band gap of other Electrons & holes relax into narrow-gap region Strong carrier confinement Enhanced radiative recombination
46
What are type II heterostructures?
E(CBM) & E(VBM) belong to different semiconductors Electrons relax to lower CBM Holes relax to higher VBM Spatial seperation reduces recombination
47
What are type III heterostructures?
E(c) of 1 semiconductor lies below E(v) of other (broken-gap) overlap Tunneling-enabled interfacial transport can dominate Ultrafast transport & tunneling-based device functions
48
What is a Z-scheme?
Charge-transfer / recombination pathway Selective recombination removes low-E carriers Remaining carriers retain strong reduction (high CB) & strong oxidation (low VB)
49
What are the implementation routes for Z-scheme architecture?
Redox-mediated Z-scheme heterojunction Direct Z-scheme All-solid-state Z-scheme
50
What are redox-mediated Z-schemes?
2 semiconductor photoabsorbers coupled by soluble redox mediator Mediator shuttles charge between 2 components +) enables flexible pairing of 2 photoabsorbers -) back reactions light shielding stability issues
51
What are direct Z-schemes?
No redox mediator Z-scheme from intimate interfacial contact Selective interfacial recombination removes low-E carriers strong redox electron/proton remain +) strong redox without mediator simpler & typically more stable -) needs good band matching & clean interfaces defect sensitive
52
What are all-solid-state Z-schemes?
2 photoabsorbers bridged by solid electron mediator (eg. Au/Ag/graphene) no liquid redox shuttle +) higher charge-transfer efficiency better stability that solution mediator -) possible Schottky barriers unfavourable energetics Mediator placement & interfaces hard to control
53
What is the key idea of electrocatalytic water splitting?
Electricity drives non-spontaneous reaction to store E in chemical bonds eg. H2
54
What are the reaction directions for water cycling?
Electrical E to chemical E: hydrogen evolution reaction (HER) oxygen evolution reaction (OER) Chemical E to electricity: hydrogen oxidation reaction (HOR) oxygen reduction reaction (ORR)
55
What is the Nernst equation?
E = E° - [RF/nF] lnQ
56
What is overpotential?
𝜂 Extra potential beyond eqmb potential needed to deliver target current 𝜂 = E(applied) - E(eq)
57
Why do electrocatalysts matter?
Reduce overpotential achieve same current @ lower applied potential Enhance stability Interface matters performance depends on surface adsorption & electrode-electrode interface
58
What are the steps of the HER mechanism?
1. Volmer step water dissociation 2. Tafel step 3. Heyrovsky step
59
What is the benchmark for HER catalysts?
High activity Low overpotential accelerating H* formation / desorption at active site
60
What is the OER mechanism?
Acid: 2H2O -> )2 + 4H+ + 4e- Alkaline: 4OH- -> O2 + 2H2O + 4e- AEM Multi-step 4e- PCET + O-O formation slow kinetics
61
What is AEM?
Adsorbate Evolution Mechanism on active sites OH* -> O* + OOH* -> O2 via 4 proton-coupled electron transfer (PCET)
62
What is LOM?
Lattice Oxygen Mechanism OER descriptor Some oxides lattice O participates in O-O/O2 formation partially bypassing AEM scaling O vacancies/O-redox-active lattice can promote LOM & facilitate O2 desorption
63
What are example OER catalysts?
Acid/PEM IrO2 RuO2 Alkaline NiFe LDH NiFe (oxy)hydroxides
64
What is the main difference between AWE & PEM?
Ion conductor Separator/electrolyte environment Different catalysts/materials
65
What is photochemical water splitting?
Light absorber generates e-/H+ Surface HER/OER proceeds w/out external circuit +) Simple Low cost Bias free operation -) H2/O2 hard to separate Recombination/back reaction
66
What is photoelectrochemical water splitting?
Light generates carriers Built in potential separates them External circuit closes loop w/ some bias to drive HER/OER +) Controllable reaction - via bias/current H2/O2 separable -) Higher system complexity Cost
67
What is methanol photoreforming?
Use light-assisted methanol oxidation supply H+/e- for efficient H2 evolution MeOH + H2O -> CO2 + 3H2
68
What is CO2RR?
CO2 reduction reaction Convert CO2 & renewable E into fuel/chemicals Outputs: CO/C2+/alcohols Mainly HER
69
What is syngas?
CO + H2
70
What are the challenges in electrochemical CO2RR?
High overpotential Low E efficiency Low sensitivity Stability issues
71
What are the pros & cons of H-type electrolyser?
+) Easy to assemble & operate Affordable Fundamental studies -) Large ohm resistance due to large distance Limited current density due to finite CO2 solubility in aqueous electrolyte
72
What is the set up of MEA?
Membrane electrode assembly electrolyser Cathode GDE catalyst layer + microporous layer + carbon substrate Membrane ion conductor Anode OER catalyst + porous transport layer (PTL)
73
What is microfluidic electrolyser?
Uses thin flowing electrolyte layer between electrodes lower ohmic loss than H-type Higher mass transport than H-type can reach moderate-high w/out full MEA complexity
74
What is GDE?
Gas diffusion electrode Delivers CO2 directly to catalyst breaks aqueous solubility limit Creates triple-phase boundary high rate & unable selectivity Challenge keeping right wetting state
75
What are the advantage and challenge of photoelectrochemical CO2RR?
+) Combines high-rate GDE CO2RR w/ solar-assisted E saving -) Same issues as GDE
76
What are the difference GC detectors?
Thermal Conductivity Measures difference in TC between carrier gas & carrier gas + components Flame Ionisation Organic components w. C-H bonds combusted in air/H flame to produce ions
77
How are liquid products identified?
NMR High performance liquid chromatography (HPLC)
L3 Exams Pt2
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