Dumitru Luca

Synthesis and hydrophilic properties of Mo doped TiO2 thin films

Amorphous undoped TiO2 and Mo-doped TiO2 thin films were obtained by spray pyrolysis. By heat treatment, they became polycrystalline consisting in anatase, or mixed anatase/rutile phases, if deposited on glass or silicon, respectively. Mo enters in the TiO2 matrix as Mo5+, determining a slight red shift of the absorption edge. Based on Fourier transmission infrared analysis, performed on the studied films, before and after irradiation, providing information on the relationship between hydrophilicity and the amount of the adsorbed hydroxyl groups, we certify once again that the anatase phase (present in proportion of 100% in the films deposited on glass) is superior concerning the hydrophilic properties. Mo enhances wettability for the films deposited on silicon, compared to the undoped one, even if determines a rise in the rutile weight percentage, as a combined effect with the substrate nature. The decomposition of the oleic acid confirms that titania films with higher hydrophilic performances are also g...

Gas sensing materials based on TiO2 thin films

TiO2 thin films were prepared by spray pyrolysis using a solution of titanium tetrachloride and ethyl alcohol. The deposition was performed onto different substrates (silicon, quartz, glass) maintained at the same temperature, 270°C. After annealing, a predominant rutile structure is obtained for films deposited onto silicon and quartz substrates, as revealed by x-ray diffraction patterns. The TiO2 films were exposed to different gases, at different temperatures, in order to evaluate their gas sensitivity. The optimum operating temperatures, showing the highest gas sensitivity, were determined for some gases (acetone, ethanol, methane, and liquefied petroleum gas).

Nanostructured thin layers of vanadium oxides doped with cobalt, prepared by pulsed laser ablation: chemistry, local atomic structure, morphology and magnetism

Cobalt-doped vanadium oxide thin layers prepared by pulsed laser ablation are investigated from the following points of view: (1) the chemical states by X-ray photoelectron spectroscopy (XPS), (2) the local atomic order by X-ray absorption fine structure at both vanadium and cobalt K-edges, (3) the morphology of the films by atomic force microscopy (AFM) and (4) the magnetic properties by magneto-optical Kerr effect (MOKE). The chemical composition of the host matrix was found to be close to VO2 at the sample surface, with V2O3 in the bulk. Co ions are found near the surface in high ionisation states Co4+ (for the samples synthesised in a high vacuum condition, denoted by VO1), or with Co(4− δ )+ (for the samples synthesised in an oxygen atmosphere, denoted by VO2), whereas in the bulk, Co1.5+ is obtained for VO1 and Co2+ is obtained for VO2. The AFM revealed nanoparticles with sizes 10–25 nm for VO1 samples, whereas a few bigger nanoparticles are observed for VO2 samples. The VO1 samples presented high coercitive fields with a relatively low saturation magnetisation at room temperature, superposed with a superparamagnetic component attributed to the observed nanoparticles, whereas the VO2 samples presented double-loop hysteresis curves, indicating the co-existence of two kinds of magnetic moieties with antiparallel coupling at zero applied field. The proposed two magnetic phases are Co-doped V2O3 and VO2.

Pulsed Regime of a Hollow-Cathode Discharge Used in a Sputter Source

The pulsed operation of a cavity hollow-cathode post-discharge sputtering source was proposed to facilitate the sputtering of low-conductivity materials. Self-oscillating and externally driven pulsed regimes were investigated. The plasma density, temperature and relative density of atomic species along the discharge axis were determined by optical emission spectroscopy and Langmuir probe diagnostics. The temporal evolution of these parameters and dynamic current–voltage characteristics of the discharge were recorded in both regimes as functions of gas pressure and discharge current. The results bring more insight into the utilization of the above-mentioned device as a small sputtering source.

Optical Emission Spectroscopy Diagnostic of Discharge Plasma in a Hollow-Cathode Sputtering Source

Experimental results of the characterization of a cavity hollow-cathode post-discharge sputtering source by optical emission spectroscopy are presented. The discharge parameters are similar to those currently used in sputter-deposited ferromagnetic (Ni, Fe) and nonferromagnetic targets (Cu, Ti): Ar pressures, 6×10-2–1×10-1 mbar and discharge currents, 5–60 mA. The spatial distribution of the relative electron density in front of the cathode nozzle was determined from the radial distribution of light intensity by applying the Abel transformation. Therefore, the optical data could be correlated with the electrical diagnostic results obtained under low-density plasma conditions. The radial and axial distributions of the emission spectra were also mapped in the 340 to 650 nm wavelength range for Ar and metal (Ni, Ti, Cu) species.

Mass spectrometric study of Ar/NH3 surface wave plasma utilized for surface functionalization of ZnO nanoparticles

Owing to its low toxicity toward living organisms and specific optical properties, we promote the use of zinc oxide (ZnO) as an alternative to existing semiconductor-based materials for developing new bioimaging techniques. ZnO nanoparticles (NPs) were prepared using the laser ablation technique in oxygen reactive atmosphere at room temperature by ablating a commercial high-purity ZnO target. The surface functionalization of ZnO NPs was successfully achieved using a dry chemical reactor with ammonia/argon mixture plasma. The roles of various plasma ions in the surface interaction with ZnO NPs were investigated to understand the mechanism of functionalization by quadrupole mass spectrometry.

Cavity-hollow cathode-sputtering source for titanium films

A cavity-hollow cathode was investigated as low-cost sputtering source for titanium. An argon discharge is produced inside a hollow cathode consisting of two specifically formed disks of titanium. An additional cavity further enhances the pendulum effect of the electrons. Measurements with small Langmuir probes yielded evidence for the formation of a space charge double layer above the cathode. The sputtered atoms form negatively charged clusters. After further acceleration by the double layer the clusters impinge on the substrates. Titanium thin films were produced on highly oriented pyrolytic graphite. The films were investigated by a scanning tunnel microscope and X-ray photoelectron spectroscopy.

Platinum role in hydrophilicity enhancement of Cr-doped TiO2 thin films

Abstract In this paper, we have investigated the hydrophilic properties of the titania films doped with increasing chromium percentages (from 2.1 at.% till 4.0 at.%). Cr-doping induces an increase in the rutile weight %, a more compact structure, and a significant red shift of the TiO2 absorption edge, the last property being very important in the self cleaning applications. For the chosen Cr concentrations, the films did not show promising hydrophilic properties. To improve them, we have applied a novel surface modification method, reported in literature mainly for powders, namely, surface metallisation. We have observed that, by depositing Pt islands on the film with the highest Cr content, its hydrophilic properties improve for a certain metal coverage area. The explanation was based on FT-IR and X-ray photoelectron spectroscopy analysis, performed on the UV irradiated and non-irradiated films, which gives information on the relationship between hydrophilicity and the amount of the adsorbed hydroxyl groups.

Nb-doped TiO2 thin films as photocatalytic materials

Amorphous undoped and Nb-doped films were obtained by the spin coating method. The films have a compact structure, as revealed by scanning electron microscopy, and are very thin, with thickness values under 100 nm. The photocatalytic activity of the films was evaluated by observing the decomposition of an oleic acid solution under UV irradiation, and by studying the change in the optical transmittance of an aqueous solution containing methylene blue, in the presence of the UV-irradiated films. More than 30 h, depending on doping, are needed to recover their initial contact angles before applying oleic acid. The increase of the optical transmittance of the methylene blue solution confirms the photocatalytic degradation of methylene blue on the Nb-doped TiO2 films. X-ray photoelectron spectroscopy studies, performed to detect the presence of the carbon on the irradiated surface of the films, drive to the conclusion that at the surface of the films, even for contact angles close to 0 ∘, the presence of carbon still can be detected, which demonstrates that hydrophilicity is ruled by a different mechanism than photocatalysis.

PHOTOINDUCED WETTABILITY OF TITANIUM OXIDE THIN FILMS

We present here the results of a systematic investigation of the influence of the main deposition parameters (substrate temperature, deposition time, reactive gas composition, etc.) on the hydrophilic properties of DC sputtered titania films, grown on heated glass substrates (463−573 K), using water vapors as the reactive gas. Reference samples prepared in standard oxygen-based deposition conditions were used to compare the results. All the investigated samples were polycrystalline, with either pure anatase or mixed anatase-rutile nano-phase mixtures, depending on substrate temperature and film thickness. The surface wettability, evaluated from contact angle data during UV irradiation and in the back-reaction conditions, is discussed in terms of the synergic effects of materials structure, surface morphology, elemental composition, and electronic properties.