REPOSITORY > RESULTS

Abstract

In this doctoral thesis, we present the superhydrophilic effect on surfaces of three different materials, i.e. highly oriented pyrolytic graphite, graphite foil and graphite grain in polymer composite. In order to induce this complicated effect on the surface we used reactive oxygen plasma treatment. The plasma was created in glass chamber using an inductively coupled radiofrequency generator operating at 13.56 MHz, in pure oxygen. Plasma parameters were evaluated by means of catalytic probes and optical emission spectroscopy (OES). At the generator output power of 500 W, the discharge reaches its optimal inductively coupled capacity which means that free electrons are accelerated in the induced electric field. The degree of dissociation in the plasma can reach up to 100 % because of the large density of charged particles and very low probability for heterogeneous surface recombination of neutral oxygen atoms on the surface of our plasma chamber. The plasma in our system is thermodynamically non-equilibrium as the temperature of electrons is significantly higher than the temperature of other plasma species. Plasmas with such characteristics are very efficient sources of chemically active species like ionized atoms, ground state neutral oxygen atoms and metastable states with high excitation energies.
The samples were subject to plasma treatment at the constant generator output power of 700 W, and at source gas pressures ranging from 1 Pa to 50 Pa. The treatments lasted from 1s to 500 s. Some of the samples were treated in the plasma chamber and some were treated in the early plasma afterglow, where the density of ionized species is negligible and the density of atoms is similar to that in plasma chamber. Our experimental setup allows measuring the temperature of the sample surfaces by means of an infrared pyrometer. After treatment in the plasma or its afterglow, the samples were analysed using several techniques, such as: contact angle measurements, atomic force microscopy (AFM), scanning and transmission electron microscopy (SEM and TEM), laser Raman and X-ray photoelectron spectroscopy (LRS and XPS) or energy dispersive spectrometry (EDS).
To eliminate the process of heating as the cause of observed changes in the samples, we exposed samples of all three materials to high temperatures in a controlled heating chamber. We have found that the increased temperature alone does not change any properties of the HOPG and the graphite foil, whereas the mass of the graphite-polymer composite was significantly decreased. We attribute this to thermal degradation and evaporation of the polymer matrix. Surface properties, such as hydrophilicity and morphology remained unchanged in samples exposed to temperatures up to 700 ºC.
The surface energy of the plasma treated samples was observed by means of water contact angle measurements. Using this method, we found different types of behaviour in different sample materials. The contact angle of the HOPG rapidly decreases and reaches a minimum of 48º at samples treated for 3 seconds. At longer treatment times, it increases until it reaches a maximum at 5 seconds. Then, it decreases again until it reaches its second minimum of 22º at 30 seconds. After that, it continues to increase with longer treatment times. We attribute the first minimum exclusively to functionalisation of the surface, whereas the second minimum is a consequence of the combined effect of an
increase in the surface roughness and the presence of functional groups. The increased surface roughness is caused by spontaneous growth of oriented nano-cones, which has been confirmed both by SEM and AFM analyses. Increasing the plasma treatment time leads to an increase of the sample temperature, which in turn causes the elimination of the nano-cones as well as the spontaneous decomposition of the functional groups. For that reason, samples treated for around 40 s exhibit the same contact angle as untreated samples. The results obtained in the afterglow chamber, where the temperature of the processed samples never exceeded 300 ºC, were completely different. Nano-cones were found on surfaces of samples that were treated for longer times. We have also observed the effect of superhidrophilicity at samples treated for several hundreds of seconds. The moderate temperature of the samples and a simultaneous presence of plasma radical therefore allows for the appearance of functional groups and nano-roughness, which in turn causes the superhidrophilicity of the material.
In the case of the graphite foil, we have observed a similar behaviour of the water drop contact angle as in the case of the HOPG, with the difference that at large treatment times, the contact angle dropped below our level of detection. The appearance of superhidrophilicity depends on the oxygen pressure in the experimental reactor, where the effect was most easily obtained at higher pressures. The samples retain their hidrophilicity even after being exposed to the ambient atmosphere for a period of one week. Analyses by XPS showed an increase of the oxygen concentration on the sample surfaces by more than an order of magnitude. We have also observed measurable concentrations of microelements which are present in un-measurable concentrations in the untreated material. We attribute this phenomenon to selective plasma etching: oxygen particles from the plasma react with carbon atoms so that the majority of the material is eroded, whereas impurities such as silicon and phosphorus, which can’t be removed by oxygen plasma, remain on the surface. The appearance of superhidrophilicity was observed also in samples treated in the afterglow chamber; however the rate of appearance of this state is lower. Graphite foil treated in the afterglow chamber also retains its superhidrophilicity after being exposed to ambient atmosphere for one week. Results of the OES measurements revealed an important difference between the HOPG and the graphite foil: in the case of HOPG processing, hydrogen was present in the plasma only for the first second of the process, whereas during processing of the graphite foil, we observed a well pronounced spike in the intensity of the Hαline, appearing at approximately 3 seconds into the treatment. We attribute the appearance of the extreme to a considerable content of water vapours in the foil, which is a consequence of poor orientation of the material.
In the case of samples of graphite-polymer composite, we also observe the appearance of superhidrophilicity even after a few seconds of plasma treatment. The phenomenon is especially well observable at higher pressures of oxygen in the reactor. Increasing the treatment time does not produce a local maximum in the contact angle. The contact angle monotonously increases with the treatment time and can even exceed the initial values of an untreated sample. We attribute this behaviour to the diffusion of the polymer from the bulk to the surface, where it forms a thin hydrophobic layer. The graphite-polymer composite samples did not retain their superhidrophilicity after one week exposure to ambient atmosphere. Because the appearance of superhidrophilicity was observed at samples which were subject to very short plasma treatments, where erosion does not become an important effect, we assume that the observed aging of the samples exposed to ambient atmosphere is a consequence of the spontaneous decomposition of oxygen-rich functional groups which are formed on the polymer surface during the short plasma treatment. Superhidrophilicity was not observed on composite samples treated in the afterglow chamber, regardless of the discharge parameters. The water drop contact angle initially dropped to around 40º and remained constant regardless of treatment time.