Download Semiconductor/Electrolyte Interface

Semiconductor/Electrolyte
Interface
Dr. Katarzyna Skorupska
Electrical double layer
•
•
•
IHL (Inner Helmholtz Layer) electrical
centers of the specifically adsorbed ions.
Contains
solvent
molecules
and
sometimes other species (ions or
molecules) that are said to be specifically
adsorbed (Stern layer). In order of few
Angstroms.
OHL (Outer Helmholtz Layer) centers of
nearest solvated ions. long-range electrostatic forces, independent of the
chemical properties of the ions
diffuse layer, extends from the OHP into
the bulk of the solution
Space charge layer
0.1Å
semiconductor
metal
in nm scale
The charge is distributed over certain distance
below the surface
Semiconductor surface
• Specific adsorption at the surface by chemical bond:
-OH-F• Physical adsorption by electrostatic forces
Processes at the electrode – solution interface
Faradaic
(the amount of chemical reaction caused by the flow of current is
proportional to the amount of electricity passed))
- electron transfer (ox : red)
non-Faradaic
• adsorption and desorption at electrode
surface
• charge does not cross the interface,
external currents can flow when the
potential, electrode area, or solution
composition changes
• Mass transfer (e.g., from the bulk solution to the electrode surface).
• Electron transfer at the electrode surface.
• Chemical reactions preceding or following the electron transfer.
• homogeneous processes (e.g., protonation or dimerization)
• heterogeneous ones (e.g., catalytic decomposition) on the electrode surface.
• Other surface reactions,
• adsorption,
• desorption,
• crystallization (electrodeposition).
When a steady-state current is obtained, the rates of all reaction steps in a series are the same.
semiconductor - electrolyte
Characterized by:
- energy bands (ECB, EVB, EF)
The electrochemical potential of electrons is
given by Nernst equation
In the bulk it depends on doping:
Cox, Cred – concentration of oxidized and
reduced components
theoretical absolute scale with vacuum level as given in relation to standard hydrogen
electrode – reference electrode
a reference point
Reference electrodes
* Standard hydrogen electrode (SHE) (E=0.000 V) activity of H+=1
* Normal hydrogen electrode (NHE) (E ≈ 0.000 V) concentration H+=1
The platinized platinum electrode is dipped in an acidic solution and pure hydrogen gas (1 bar) is bubbled through it.
* Saturated calomel electrode (SCE) (E=+0.244 V saturated)
The reaction between elemental mercury and mercury(I) chloride (Hg2Cl2) in saturated solution of KCl in water
* Copper-copper(II) sulfate electrode (E=+0.314 V)
* Silver chloride electrode (E=+0.197 V saturated)
The redox electrode based on silver metal (Ag) and its salt — silver chloride (AgCl, also called silver(I) chloride).
* pH-electrode (in case of pH buffered solutions, see buffer solution)
The Gerischer Model
Charge transfer at SC electrolyte interface
Frank-Condon principle
• e- transfer 10-15s
• relaxation time for solvent 10-9-10-13s
λ- reorganization energy (~1eV)
energy of product with respect to its equilibrium state when its solvent coordinate is still the same as that of reactant state
R. Memming, “Semiconductor Electrochemistry”, WILEY-VCH 2001
Model based on:
• Electronic state in solid
• Energy level in solution
Electronic energies in electron energy diagram
(simplified model, no motion of the solvent)
Distribution function
(depending on electronic energy)
Wox(E), Wred(E)
Wox, Wred- probabilities of finding the empty and occupied
electronic state at energy E
only single electronic state of redox system is considered
DOX
E0ox
E0F, redox
DRED
E0red
W0-constant
distance x
density of states
Dox, Dred- distribution function
E0ox, E0red- electron energy of redox system (empty and occupied states)
E0F, red- standard Fermi level of a redox system
Distribution of electronic levels of the redox system
DOX
E0ox
E0F, redox
Wox- distribution of empty electronic levels
Wred- distribution of occupied electronic levels
DRED
Density of states DOS is proportional to concentration of species
distance x
E0red
density of states
R. Memming, “Semiconductor Electrochemistry”, WILEY-VCH 2001
DOX
ΔE1/2
E0ox
E0F, redox
E0red
DRED
distance x
density of states
as the λ- reorganization energy (~1eV)
ΔE1/2 ~ 0.5 eV
Negative bias applied to the SC
DOX
E0ox
E0F, redox
DRED
distance x
E0red
density of states
- Flattening of the bands
- Increase concentration of e- at the surface
- Continuous increase of cathodic bias –
accumulation situation at the surface
n-Si : electrolyte at the equilibrium
for three redox couples
- before contact
- after contact formation
b) SC in depletion
c) High barier height
a) SC in accumulation
electrons available at the surface deficit of electrons at the surface exchange by EVB
above the EF
below the EF
in the middle of the gap
below the EF
close to EVB
b) SC in depletion
c) SC in inversion (pa) SC in accumulation
type at the surface
electrons available at the surface deficit of electrons at the surface
- via ECB less probable
high barrier height
e- available at the surface
via ECB excluded
e- exchange via ECB
exchange by EVB
Charge transfer via surface states
- empty above EF
- occupied below EF
n-Si : redox couple
in equilibrium and
under illuminations
Vph
Illumination
- Excess carriers are generated
VPh = VOC
kT  jL 
=
ln + 1
q  js

Decomposition of SC
thermodynamic
Decomposition potentials
for reduction of semiconductor
for oxidation of semiconductor
F – Faraday constant
ΔG – known change in free Gibbs energy for each reaction
favorable
decomposition
stable
against
cathodic
reaction
stable
against
anodic
reaction
PROTECTION
In flat band situation by introducing into electrolyte a redox couple
- below nEdecomp
- above pEdecomp
Appling an external potential in order to create depletion situation at the surface
During illumination
- minority carriers will participate in the decomposition process
Stability rules:
system has not enough energy to perform the decomposition to noticeable rate
Photoexited charges can be used by:
- Photoelectrocatalytic process
- Corrosion of photoelectrode
hν
electrolyte
p-Si
Si + 2H2O + 4h+ => SiO2 +4H+
DESIRED PROCESS
competition
2H2O + 4h+ => O2 + 4H+
ELECTRODE CORROSION
R. Memming, “Semiconductor Electrochemistry”, WILEY-VCH 2001