Herein we report on the synthesis and the effects of gradual loading of TiO2 nanotube array layers with ZnO upon surface wettability. Two-step preparation was chosen, where TiO2 nanotube layers, grown in a first instance by anodization of a Ti foil, were gradually loaded with controlled amounts of ZnO using the reactive RF magnetron sputtering. After crystallization annealing, the formerly amorphous TiO2 nanotubes were converted to predominantly anatase crystalline phase, as detected by XRD measurements. The as-prepared nanotubes exhibited a well-aligned columnar structure, 1.6 μm long and 88 nm in diameter, and a small concentration of oxygen vacancies. Ti2+ and Ti3+ occur along with the Ti4+ state upon sputter-cleaning the layer surfaces from contaminants. The Ti2+ and Ti3+ signals diminish with gradual ZnO loading. As demonstrated by the VB-XPS data, the ZnO loading is accompanied by a slight narrowing of the band gap of the materials. A combined effect of material modification and surface roughness was taken into consideration to explain the evolution of surface super-hydrophilicity of the materials under UV irradiation. The loading process resulted in increasing surface wettability with approx. 33%, and in a drastic extension of activation decay, which clearly points out to the effect of ZnO-TiO2 heterojunctions.

A discharge plasma is created by simultaneously biasing two concentric spherical grids with axisymmetric orifices. In this geometry, space charge structures in the form of multiple quasi-spherical luminous plasma bodies appear simultaneously inside and around the cathodes. The plasma formations are highly interdependent supplying each other with the particle flow and current closure necessary for the maintenance of the discharge. To diagnose these structures, space-resolved cold Langmuir probe measurements and optical emission spectroscopy investigations were performed in the axial direction allowing for the mapping of the axial profiles of plasma potential, electron temperature and density, ion density and optical emission. The existence of an accelerating double layer in the vicinity of the holes has been confirmed here, and in previous research (Teodorescu-Soare C T et al 2016 Phys. Scr. 91 034002; Schrittwieser R W et al 2017 Phys. Scr. 92 044001; Teodorescu-Soare C T et al 2019 Int. J. Mass Spectrom. 436 83). Besides the assessment of the relationship between discharge conditions and plasma parameters in the novel cathode system, the importance of a multiple concentric cathode discharge configuration is revealed for deposition applications.

We propose an approach for computer-aided morphological analysis of titania nanotube arrays by using high-resolution scanning electron microscopy images. Based on top-view images and several preset parameters, our experimental MATLAB® routine finds possible candidates for nanotube openings and validates them by size, solidity, and eccentricity. Centers and inner diameters are computed followed by a local validation of pixels belonging to nanotube openings and walls. To achieve this goal, input gray level images are repeatedly converted to black and white using the entire range of thresholds from 254 to 1 to reveal black patches as nanotube openings. Nanotube locations are identified level after level in a systematic manner, taking into account the local gray level distribution. This approach allowed to identify most nanotubes in a reasonable computation time. When the nanotube detection is completed, the routine computes the nanotube surface density, the average nanotube diameter, the percentage of area occupied in the input image by nanotubes and unreacted titania, and the distribution of nanotubes based on their diameter, solidity and eccentricity. Our method provides a reliable and automated quality assessment of titania nanotube array layers used in sensor-based and photocatalytic applications. The total number of the detected nanotubes in the SEM images is dramatically increased by 30% to 40% when compared to previous methods. The proposed procedure can be easily adjusted for morphological analysis of any other materials structured as nanotube layers.

Extensive experimental investigations have been carried out on the interaction between three plasma fireballs in low-temperature plasma. The particle dynamics within the plasma space charge formations and the exchange between them have been investigated. Cold Langmuir probe measurements were taken showing the influence of the fireballs on the background plasma parameters. The current oscillations show specific frequencies for the dynamic states of the fireballs, where periodic expulsion and backflow of ions occur near the surrounding double layer. Interaction maxima or resonance phenomena are revealed at specific distances between the electrodes.