Control of composite thin film made in an Ar/isopropanol/TiO2 nanoparticles dielectric barrier discharge by the excitation frequency

The synthesis of composites thin films made by injecting an aerosol suspension of 20nm-size TiO2 nanoparticles (NPs) and isopropanol (IPA) in a filamentary argon Dielectric Barrier Discharge (DBD) is studied as a function of the DBD frequency from 1 to 50kHz. The plasma is modulated to get homogeneous coatings. The deposition rate and morphology of the composite thin films are determined from SEM images of both surface and cross section. Their chemical composition is investigated by XPS, Raman spectroscopy and FTIR measurements. The structural composition of the NPs is examined by XRD. All the deposited composites show the chemical signature of the NPs as well as of the polymer-like coating resulting from the plasma polymerization of IPA. No mixed phase is observed and the sizes of the NPs as well as of their aggregates are not affected by the plasma. With this method aerosol droplets are evaporated before entering the plasma and the NPs inside a same droplet are aggregated. Results show that the DBD frequency controls the composite composition by independently influencing the NPs transport and the matrix growth rate. At 1kHz, the coating is essentially made of NPs with a low carbon coating. From 1 to 50kHz, the Ti/C ratio is divided by two orders of magnitude. As the frequency increases the quantity of NPs decreases and since 10kHz the matrix thickness increases. The decrease of the NPs is explained by the numerical modeling of the NPs trajectory. It is found that from 10 to 1kHz, the lower is the frequency, the higher is the transport of the NPs to the surface due to the electrostatic force. On the other hand the matrix growth rate increases from almost zero at 10kHz up to 19nmmin(-1) at 50kHz because of the linear increases of the DBD power with the frequency.