Oxidation of Thin Titanium Films: Determination of the Chemical Composition of the Oxide and the Oxygen Diffusion Factor

Anisotropic Stick-Slip Frictional Surfaces via Titania Nanorod Patterning

Nanoscale or microscale surface texturing is an effective technique to tailor the tribological properties between two surfaces that are rubbed against each other. In order to achieve the desired frictional properties by a patterned surface, one needs an in-depth understanding of the underlying mechanisms. Here, we demonstrate anisotropic stick-slip friction achieved via a nanotextured surface of tilted titania nanorods (TiNRs). The surface was developed by using the glancing angle deposition (GLAD) technique, and exhibited load-dependent variations in stick-slip friction as well as frictional anisotropy in different sliding directions. For studying the frictional properties of the newly developed surface, lateral force microscopy (LFM) was performed in three different reciprocal orientations (0° rotated, 45° rotated, 90° rotated) using a custom-made colloidal alumina atomic force microscopy (AFM) probe. The frictional behavior was found to vary significantly with the orientation. At 0° rotated position) a prominent "stick-slip" was observed when scanning opposite to the tilt direction, whereas the phenomenon reduced significantly when the nanotextured surface was scanned along the tilt direction or rotated to different angles (45 and 90°) with respect to the sliding direction of the AFM cantilever supporting the probe. The experimental findings were interpreted based on the classical solution for large deflections of tilted elastic rods. Overall, the textured surface, LFM-based frictional measurement, and the quantitative analysis presented here provide a fundamental understanding of how friction can be significantly varied on a surface patterned with tilted TiNRs at a length scale of about 1 μm, which can be comprehensively applied to nanorod patterns of other materials on different substrates.

The Phase Evolution and Photocatalytic Properties of a Ti-TiO2 Bilayer Thin Film Prepared Using Thermal Oxidation

Ti-TiO2 bilayer thin films were successfully prepared onto a glass substrate using magnetron sputtering with different TiO2 bottom layer conditions. These represent a lack of (as-deposited) and full oxygen content (annealed). Single-layer Ti was additionally used as a control. The influence of oxygen diffusion phenomena of the bottom layer of TiO2 to the upper layer of Ti thin films at different oxidation temperatures on structural, optical, and photocatalytic performance was investigated. X-ray diffraction (XRD) results confirmed that the crystalline phases coexisting on thin-film samples oxidized at 450 °C were TiO, TiO1.4, (bilayer, as-deposited TiO2), anatase (bilayer, annealed TiO2), and rutile (single and bilayer). This finding showed that the film’s phase structure evolution is significantly affected by oxygen diffusion from the bottom layer. Further increasing the thermal oxidation temperature caused a notable decline in the amorphous zone in bilayer thin films based on TEM analysis. Bilayer thin films lead to higher degradation of methylene blue under UV light radiation (63%) than single-layer films (45%) oxidized at 450 °C. High photocatalytic activity performance was found in the bilayer annealed TiO2-Ti thin-film sample. This study demonstrates that the bilayer modification strategy promotes the oxygen-induced bottom layer of TiO2 bilayer thin films.

Generation of Charged Ti Nanoparticles and Their Deposition Behavior with a Substrate Bias during RF Magnetron Sputtering

This study is based on the film growth by non-classical crystallization, where charged nanoparticles (NPs) are the building block of film deposition. Extensive studies about the generation of charged NPs and their contribution to film deposition have been made in the chemical vapor deposition (CVD) process. However, only a few studies have been made in the physical vapor deposition (PVD) process. Here, the possibility for Ti films to grow by charged Ti NPs was studied during radio frequency (RF) sputtering using Ti target. After the generation of charged Ti NPs was confirmed, their influence on the film quality was investigated. Charged Ti NPs were captured on amorphous carbon membranes with the electric bias of −70 V, 0 V, +5 V, +15 V and +30 V and examined by transmission electron microscopy (TEM). The number density of the Ti NPs decreased with increasing positive bias, which showed that some of Ti NPs were positively charged and repelled by the positively biased TEM membrane. Ti films were deposited on Si substrates with the bias of −70 V, 0 V and +30 V and analyzed by TEM, field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and X-ray reflectivity (XRR). The film deposited at −70 V had the highest thickness of 180 nm, calculated density of 4.974 g/cm3 and crystallinity, whereas the film deposited at +30 V had the lowest thickness of 92 nm, calculated density of 3.499 g/cm3 and crystallinity. This was attributed to the attraction of positively charged Ti NPs to the substrate at −70 V and to the landing of only small-sized neutral Ti NPs on the substrate at +30 V. These results indicate that the control of charged NPs is necessary to obtain a high quality thin film at room temperature.