Surface characterization and electrochemical behavior
of colloidal particles
C. PEPIN , S.H FOULGER.
Introduction
Electrochemical behavior
Stability and surface charge density
-Ongoing interest in research focused on developing
polymeric organic-light-emitting-devices (OLED)
-OLEDs could present better properties such as the
ease of device fabrication, low material cost, low
environmental impact, facile synthesis routes and
high rates of improvement in luminous efficiency
Zeta potential:
-electrokinetic potential in colloidal system related to the
stability of the colloidal dispersion
-calculated at the surface of the shear
- calculated from the electrophoretic mobility of the particles
Cyclic Voltammetry:
-important technique that can be used to determine the energy
profile of an organic material  HOMO
-cycling a potential across a sample and measuring the resulting
current
Cyclic voltamogramm for PA/AC particles
Cyclic voltammogram of PA particles
Zeta potential PA particles
Zeta potential of PS partices vs pH
4e-5
5,0e-6
1,0e-5
2e-5
0,0
-35
-10
Cyclic voltammogram of PA/AO particles
5,0e-6
0
0,0
-30
-40
-45
-1,0e-5
Current (A)
Zeta Potential (mV)
-20
Current (A)
-40
Zeta Potential (mV)
-A hole transporting group and an electron
transporting group were “clicked” on the surface of a
polymer particle to make an individual “particle
device”.
Current (A)
-5,0e-6
-2e-5
-4e-5
-50
-2,0e-5
-55
-6e-5
-1,5e-5
-8e-5
-2,0e-5
-1e-4
-2,5e-5
2,0
1,5
1,0
0,5
0,0
-2,5e-5
2,0
-0,5
1,5
h+
colloid
core
e-
h+
h+
e-
6
8
0
10
-0,5
1,5
1,0
0,5
0,0
-0,5
Potential (V)
Eox = E’ox +4.4  IP = -e.Eox =HOMO
2
4
6
8
10
12
pH
pH
UV-VIS Spectroscopy:
-use to determine optically the energy band gap
-onset of absorption correspond to the energy needed to promote
an electron from the HOMO to the LUMO
Surface charge density:
-determine the number of charge at the surface of the
particles
-use of conductometric titration
-calculation of the surface charge density in µC/cm²
Absorption of PA/AC paritcles as a function of energy
Absortion carbazole molecules as a function of energy
2,5
1,2
Titration of PS particles
hole transporter
Oxadiazole molecule (AO)
2,0
Titration of PA paticles
1,0
0,8
1,5
0,46
1,1
0,44
0,6
0,4
0,2
Conductivity (µ/cm)
Conductivity (µ/cm)
1,0
0,5
1,0
0,42
Absorption
h+
e-
4
0,0
onset value of oxidation (E’ox)
Absorption
e-
2
0,5
Potential (V)
-65
-75
0
1,0
Potential (V)
-60
-70
“Particle-device” made of Poly(propargyl acrylate)
with a hole transporter, an electron transporter and
an emissive material
-1,0e-5
-1,5e-5
-50
-60
-5,0e-6
0,40
0,38
0,36
0,34
0,32
0,0
0,0
0,9
-0,5
6
5
4
3
2
6
5
4
3
2
Energy (eV)
Energy (eV)
Absorption of oxadiazole molecules as a function of energy
Absorption of PA/AO particles as a function of energy
0,8
Onset of absorption= Egap
0,7
1,0
1,2
0,30
0,6
1,0
0,8
0,28
0,8
5
10
15
20
V(NaOH) added (µL)
25
30
35
0,5
0
10
20
30
40
V(NaOH) added (µL)
50
60
Absorption
Poly(propargyl acrylate) (PA)
electron transporter
Carbazole molecule (AC)
0
Absorption
0,6
0,26
0,4
0,6
0,4
0,2
0,2
0,0
• PS particles : Charge density : 0.0225 µC/cm²
0,0
6
5
4
3
2
6
Energy (eV)
Objective
5
4
3
2
1
Energy (eV)
• PA particles : Charge density : 0.0218 µC/cm²
-understand the behavior of poly(propargyl acrylate)
(PA) particles with surface-attached hole- and electrontransporting moieties
-characterize their colloidal stability: Zeta potential and
surface charge density
-characterize their electrochemical behavior with the
determination of the HOMO energy level, LUMO
energy level and the energy band gap: Cyclic
voltammetry and UV-VIS spectroscopy
E’ox (V) Eox (V) HOMO (eV) Egap (eV) LUMO (eV)
The energy profile of the material :
Ionization potential
Electron affinity
Band gap energy
Study of the redox reactions
oxidation  charge removal from the HOMO
 Ionization potential
reduction  charge removal from the LUMO
 Electron affinity
PA particles with
carbazole moieties
PA particles with
oxadiazole moieties
1.15
5.55
-5.55
3.5
-2.05
1.34
5.74
-5.74
3.33
-2.44
Conclusion
-study of the zeta potential of PA particles; the results show their
stability over a range of pH from 9 to 2
- Determination of HOMO, LUMO and energy band gap for PA/AC
and PA/AO particles
Acknowledgements: Prof. S.H. Foulger, Foulger’s group (Michael, Ali , Ryan and Parul), Prof. K.Richardson and Prof. E. Fargin for the MILMI Master.