This paper investigates the formation mechanism of ribs on the outer wall of anodized TiO 2 nanotubes (NTs) prepared in a NaF/Glycerol electrolyte containing 2wt% of water. The effect of potential and time on the morphology of the NTs is evaluated along with growth efficiency (% of total charge at the electrode used to form the oxide) and X-ray Photoelectron Spectroscopy (XPS) measurements, providing an insight into the mechanism of formation of ribs. XPS analysis confirms the presence of fluorine, as TiF 6 2-, and carbon as impurities in the anodic film. The growth efficiency of the process decreases from ~70% at 10V to 55-58% at 20-30V and 14% at 40V. Similarly, the anodic growth factor (migration of ions expressed as the maximum radius of the "oxide cell" per applied potential, nm V-1) decreases at higher potentials, due to oxygen bubbles evolving at the anode at 20-40V and disrupting the anodizing process. The formation of gas bubbles also affects the morphology of the NTs; while NTs are smooth at 10V, oxide rings appear over the range 20-40V. Partial dissolution of the oxide rings due to fluorine ions eventually reshapes the NTs forming ribs, whereas excessive dissolution over extended anodizing times tends to smoothen the NTs and eventually leads to collapse of the NTs. On the basis of these observations, we suggest oxygen evolution (requiring a minimum amount of water in the electrolyte and a sufficiently high potential 20-40V) plays a primary role on the formation of ribs on anodized TiO 2 NTs. Ribs are also observed on NTs grown in aqueous electrolytes, although since dissolution is more difficult to control the resulting structure is more irregular than in organic media.
Surface plasmon resonance of gold and silver nanoparticle (NP) layers is investigated by the experiment as well as simulations. Although the good agreement was found for gold NP film, a significant mismatch in the resonance energy for silver NP film was observed. The deviation was assigned to the presence of silver oxide (Ag2O) in silver NPs. As an alternative to the NP size-dependent Drude model, the analysis based on effective medium approximation for refractive index of Ag-Ag2O material system is carried out and compared with the core-shell model. Both Mie's model and numerical simulation results illustrate shift of the surface plasmon resonance due to silver NP surface oxidation.
Evaporation of colloidal nanoparticle solutions is known to produce ordered monolayers of nanoparticles, self-assembled arrays of magnetic nanoparticles being of special importance for applications. The in situ time-resolved grazing-incidence small-angle x-ray scattering with the temporal resolution down to 100 ms was employed to study the self-assembling process of iron oxide nanoparticles after a colloidal drop was applied on a silicon substrate. The x-ray scattering contributions from the evaporating drop volume, drop surface, and substrate surface were monitored and separated. The x-ray scattering from the drop for the distances from the substrate surface larger than Ϸ80 m shows the absence of self-assembled clusters in the drop volume or self-assembled domains on the drop surface. These results indicate that the nanoparticle self-assembling occurs in the vicinity of the three-phase drop contact line. The ordered nanoparticle monolayer exhibits hexagonal close-packed arrangement.
An in situ small-angle x-ray scattering study of the nanoparticle displacement in a self-assembled monolayer as a function of a supporting membrane strain is presented. The average nanoparticle spacing is 6.7 nm in the unstrained state and increases in the applied force direction, following linearly the membrane strain which reaches the maximum value of 11%. The experimental results suggest a continuous mutual shift of the nanoparticles and their gradual separation with the growing stress rather than nanoparticle islands formation. No measurable shift of the nanoparticles was observed in the direction perpendicular to the applied stress.
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