Nils Hebestreit

Physical and electrochemical characterization of nanocomposites formed from polythiophene and titaniumdioxide

Abstract A simple method for covering titanium dioxide particles with a polythiophene film by chemical preparation was developed. The resulting nanocomposites consisted of a titanium dioxide core with a grain size of 25–250 nm and a polythiophene shell between 1 and 2 nm thickness. The composites were characterized by scanning electron microscopy, thermogravimetry, X-ray photoelectron spectroscopy, cyclovoltammetry, impedance spectroscopy and photocurrent spectroscopy. The content of polythiophene in the composite (determined by thermogravimetry), was between 2% and 5%. Disk-like electrodes were prepared by pressing and then characterized by various electrochemical methods. A reversible redox potential of the polythiophene of +1.0 V (NHE) was determined by cyclic voltammetry. The reduced form of polythiophene behaved as a p-type semiconductor so that the composite with n-type TiO 2 contained the properties of a p/n-junction. In the photocurrent spectra (depending on the applied potential), the characteristic anodic peaks of the TiO 2 at λ =320 nm and cathodic peaks of the polythiophene around λ =500 nm were found. A new cathodic peak observed at 370 nm was explained as a new feature of the pn interface.

Investigation of polybithiophene/n-TiO2 bilayers by electrochemical impedance spectroscopy and photoelectrochemistry

Extremely adhesive polybithiophene (PBT) films were grown on TiO2 in a new, two-step procedure using a silyl substituted thiophene derivative as a surface coupling agent and a subsequent chemical polymerization process. Electrochemical impedance and photocurrent measurements were carried out in order to achieve information on the semiconducting properties of both materials. When a PBT film deposited onto TiO2 is reduced it can be described as a p-type semiconductor with a flatband potential of about 0.65 V (vs. SCE), whereas the n-type semiconductor TiO2 gives a flatband potential of about −0.4 V (vs. SCE). The photocurrent measurements show an anodic photocurrent at wavelength of between 300 and 400 nm and a cathodic photocurrent of between 400 and 600–700 nm, confirming the impedance measurements. The results of the impedance and photocurrent measurements are summarized in a band structure model which explains the behaviour of the semiconductor electrode Ti/n-TiO2/p-PBT.

Adhesion promoters for polythiophene

Polythiophene layers have been grafted onto silicon, iron, platinum and titanium for applications as organic semi-conducting films on solid surfaces. In general, the adhesion between the polymer films and substrates is insufficient for technical and microelectronic applications. The use of adhesion promoters between substrate and polymer and enables the formation of a strongly bonded composite. The polymers were synthesized by chemical or electrochemical surface polymerization.