Abstract As-grown and heat-treated vapour grown car- bon fibres (VGCF) in the as-prepared state, washed in HCl/H 2 O, and treated in O 2 plasma for different periods have been investigated by means of XPS and scanning electron microscopy (SEM). The surface energy of the carbon fibres before and after plasma treatment was deter- mined from the wetting contact angle. Washing intro- duced hydroxyl, carbonyl and carboxyl groups onto the fi- bre surfaces and oxygen plasma treatment increases the total atomic concentration of oxygen up to 17%. This is in good agreement with the value of the polar component of the surface energy. Plasma treatment also enhanced the fi- bre surface porosity (by etching). Keywords Vapour grown carbon fibre (VGCF) · X-ray photoelectron spectroscopy · Plasma treatment · Surface analysis Introduction Due to improvements in electrical conductivity and me- chanical stiffness, applications of carbon fibre composites have increased significantly during the last decade partic- ularly in the automotive and aerospace industries. The suitability of vapour-grown carbon fibres (VGCF) in such composites depends essentially on their surface proper- ties, by which the adhesion between fibres and surround- ing matrix is determined. Untreated VGCF have a non- polar nature and a low surface energy and, therefore, are unable to form strong adhesive bonds with the matrix and show insufficient wettability. In order to improve the overall adhesion, various surface treatments of the fibres have been applied with the aim of enhancing their surface energy and wettability by forming polar groups at the fi- bre surface and by modifying the surface morphology [1, 2, 3, 4]. One procedure to increase the degree of adhesion of the fibres to the polymer matrix, consists in a plasma treatment of the fibres. In this way functional groups (hy- droxyl, carbonyl, carboxyl groups etc) can be formed on the fibre surface, which react with the polymer matrix en- hancing the adhesion to the surrounding polymer. The aim of the present work was to determine the changes of the fibre surface caused by oxygen plasma treatment and to investigate the influence of this treatment on wetta- bility, mechanical interlocking and chemical adhesion. Experimental The fibres studied in these experiments were Pyrograph I from Ap- plied Sciences (USA). Two kinds of carbon fibre were examined: as-grown (VGCF-AG) and heat-treated (VGCF-HT) vapour- grown carbon fibres. The as-prepared fibres were washed for 3 h in concentrated hydrochloric acid before plasma exposure. The wash- ing procedure was applied in order to eliminate metallic nano-par- ticles that might remain on the fibre surface from the growth process [5]. The VGCF samples spread on ceramic plates which were sput- tered beforehand with gold were exposed to a cold oxygen plasma working under the following conditions: 13.56 MHz radio fre- quency, 50 W power, 6.6 Pa pressure, 100 scm 3 min –1 . The time of exposure was varied up to 90 min in order to obtain different oxi- dation degrees of the fibres. It has been shown that this plasma technique provides increased levels of surface activity with func- tionalisation of the basal plane sites at the fibre surface [6]. The morphology of the VGCF before and after the plasma treatment was studied by SEM (Philips ESEM XL 30TMP) under the following operating conditions: accelerating voltage 10 kV, working distance 8–10 mm, tilted angle between 10° and 40° with respect to the normal of the sample holder. The concentrations of the oxygen-containing groups were determined by X-ray photo- electron spectroscopy (XPS). Various kinds of sample preparation were tested for XPS mea- surements. The most successful method proved to be one in which a bunch of carbon fibres was deposited on a gold foil that had been pre-cleaned by sputter etching until carbon and oxygen could no H. Bubert · W. Brandl · S. Kittel · G. Marginean · D. Toma Analytical investigation of plasma-treated carbon fibres Anal Bioanal Chem (2002) 374 : 1237–1241 DOI 10.1007/s00216-002-1640-0 Received: 21 February 2002 / Revised: 11 October 2002 / Accepted: 11 October 2002 / Published online: 12 November 2002 ORIGINAL PAPER H. Bubert (✉) · S. Kittel Institut für Spektrochemie und Angewandte Spektroskopie (ISAS), Bunsen-Kirchhoff-Straße 11, 44139 Dortmund, Germany e-mail: bubert@isas-dortmund.de W. Brandl · G. Marginean · D. Toma Fachbereich Maschinenbau, Fachhochschule Gelsenkirchen, Neidenburger Straße 10, 45877 Gelsenkirchen, Germany © Springer-Verlag 2002