Surface characterization and electrochemical behavior of colloidal particles

C. PEPIN , S.H FOULGER.

E’ox (V) Eox (V) HOMO (eV) Egap (eV) LUMO (eV)

PA particles with carbazole moieties

1.15 5.55 -5.55 3.5 -2.05

PA particles with oxadiazole moieties

1.34 5.74 -5.74 3.33 -2.44

Stability and surface charge density

Zeta potential PA particles

pH

0 2 4 6 8 10 12

Zet

a P

oten

tial (

mV

)

-75

-70

-65

-60

-55

-50

-45

-40

-35

Zeta potential of PS partices vs pH

pH

0 2 4 6 8 10

Zet

a P

oten

tial (

mV

)

-60

-50

-40

-30

-20

-10

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

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

Titration of PA paticles

V(NaOH) added (µL)

0 10 20 30 40 50 60

Con

duct

ivity

(µ /

cm)

0,5

0,6

0,7

0,8

0,9

1,0

1,1

Titration of PS particles

V(NaOH) added (µL)

0 5 10 15 20 25 30 35

Co

nd

uct

ivity

(µ /

cm)

0,26

0,28

0,30

0,32

0,34

0,36

0,38

0,40

0,42

0,44

0,46

• PS particles : Charge density : 0.0225 µC/cm²

• PA particles : Charge density : 0.0218 µC/cm²

Cyclic voltammogram of PA particles

Potential (V)

-0,50,00,51,01,52,0

Cur

rent

(A

)

-2,5e-5

-2,0e-5

-1,5e-5

-1,0e-5

-5,0e-6

0,0

5,0e-6

Cyclic voltamogramm for PA/AC particles

Potential (V)

-0,50,00,51,01,52,0

Cur

rent

(A

)

-1e-4

-8e-5

-6e-5

-4e-5

-2e-5

0

2e-5

4e-5

Cyclic voltammogram of PA/AO particles

Potential (V)

-0,50,00,51,01,5

Cu

rre

nt (

A)

-2,5e-5

-2,0e-5

-1,5e-5

-1,0e-5

-5,0e-6

0,0

5,0e-6

1,0e-5

Electrochemical behavior

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

onset value of oxidation (E’ox)

Eox = E’ox +4.4 IP = -e.Eox =HOMO

Absortion carbazole molecules as a function of energy

Energy (eV)

23456

Abs

orp

tion

-0,5

0,0

0,5

1,0

1,5

2,0

2,5

Absorption of PA/AC paritcles as a function of energy

Energy (eV)

23456

Ab

sorp

tion

0,0

0,2

0,4

0,6

0,8

1,0

1,2

Absorption of oxadiazole molecules as a function of energy

Energy (eV)

23456

Ab

sorp

tion

0,0

0,2

0,4

0,6

0,8

1,0

Absorption of PA/AO particles as a function of energy

Energy (eV)

123456

Abs

orpt

ion

0,0

0,2

0,4

0,6

0,8

1,0

1,2

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

Onset of absorption= Egap

The energy profile of the material :

Ionization potentialElectron affinityBand gap energy

oxidation charge removal from the HOMO Ionization potential

reduction charge removal from the LUMO Electron affinity

Study of the redox reactions

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.

Introduction

“Particle-device” made of Poly(propargyl acrylate) with a hole transporter, an electron transporter and an emissive material

colloidcore

h+

e-

h+e-

h+

e-

h+

e-

hole transporter Oxadiazole molecule (AO)

electron transporter Carbazole molecule (AC)Poly(propargyl acrylate) (PA)

-understand the behavior of poly(propargyl acrylate) (PA) particles with surface-attached hole- and electron-transporting 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

Objective

-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

-A hole transporting group and an electron transporting group were “clicked” on the surface of a polymer particle to make an individual “particle device”.