Fabrication of nanoporous TiO2 by electrochemical anodization

The formation of self-organized porous titania is achieved by electrochemical anodization under a potentiostatic regime. Anodic titanium oxide (ATO) was fabricated by a three-step self-organized anodization of the Ti foil in an ethylene glycol electrolyte containing 0.38 wt% of NH₄F and 1.79 wt% of H₂O. Anodizing was carried out at the constant cell potential ranging from 30 to 70 V at the temperature of 20 °C. It was found that nanoporous TiO₂ arrays can be obtain only after a short duration of the third step (10 min). The influence of anodizing potential on the structural parameters of porous anodic titania including pore diameter, interpore distance, wall thickness, porosity and pore density was extensively studied. The linear dependencies between interpore distance, pore diameter and wall thickness upon the anodizing potential were found. The regularity of pore arrangement was monitored qualitatively by fast Fourier transforms (FFTs) of top-view FE-SEM images. It was found that the best arrangement of nanopores is observed at 40 V. This finding was confirmed by the analysis of pore circularity. The highest circularity of pores was observed once again at 40 V.     

Electrochemical Fabrication of Metallic Nanowires and Metal Oxide Nanopores

A nuclear track etched polycarbonate membrane filter with numerous cylindrical nanopores was applied as a nanoporous template for growing metallic nanowires. Nickel, cobalt, and iron nanowires were electrodeposited into the cylindrical nanopores. Cathodic polarization curves were measured to determine an optimum condition for growing nanowires. The shape of nanowires was observed using scanning electron microscope (SEM) and the crystal structure was analyzed using transmission electron microscopy (TEM). Diameter and length of nanowires corresponded to those of nanopores and each nanowire was composed of a single crystal. Anodized aluminum oxide films were also fabricated as a novel nanoporous template. The pore length and diameter was controlled changing anodizing conditions. Ordering behavior of nanopores array in an anodized aluminum oxide film was also investigated to make a novel nanoporous template with a highly ordered honeycomb array of nanopores.     

Photoelectric effects of nanostructured amorphous carbon films

In this work, the photoelectric properties of nanostructured amorphous hydrogenated carbon films was studied; these films are a variation of diamond-like carbon films. For this study, carbon films with different compositions were deposited and their electrical and optical proprieties were analyzed. These films were deposited by reactive magnetron sputtering using a pure graphite target and methane, nitrogen, carbon tetrafluorine and argon as processing gases. The amorphous carbon films are naturally nanostructured and show amount of optical proprieties as photoemission in the visible range and photoelectric effects. To enhance these effects in this work, the selective etching of the carbon films was promoted, and a nanopores and nanoholes was obtained. These defects show characteristics of potential gaps and modify the carbon films properties. Plasma etching performs the generation of the nanopores and nanoholes in the carbon films. The characteristics of these films were observed by photoluminescence analyze, optical absorbance, electro-optic analyzes. In this study, the development of the new structure for optical analyses of the nanoporous carbon films was necessary. For this, an auto-aligned test structure based in interdigital electrodes was developed.     

One-Dimensional Phonon State of 4He Films Adsorbed in Straight Nanopores

We have studied heat capacities of ⁴He adsorbed in straight nanopores 1.8, 2.2, and 2.8 nm in diameter. Heat capacities of the ⁴He fluid film on the solid layer at 0.08–0.4 K show power laws close to T in 1.8 nm pores, close to T ² in 2.8 nm pores, and a crossover from T to T ² with increasing temperature in 2.2 nm pores. These heat capacities are explained by a model assuming a phonon dispersion with continuous one-dimensional (1D) states in the axial direction and discrete energy levels in the azimuthal direction. By fitting experimental data to the model, the phonon velocity along the pore axis and the energy gap for propagation in the cross section are derived. At temperatures sufficiently lower than the energy gap, where the thermal wave length of phonons is much longer than the effective pore diameter, the ⁴He fluid films show a T-linear heat capacity of 1D phonons propagating only along the pore axis. At higher temperatures, a 1D-2D crossover of phonons occurs.      

Electrochemical production of nanopore arrays in a nickel aluminium alloy

Directional solidification of a eutectic is a novel route for the production of nanostructures. This method was applied to the quasibinary NiAl-Re system. Re as the minor phase forms fibers, which are parallel aligned in the NiAl matrix. At a temperature of 1690 ± 10 °C using a thermal gradient of 40 °C cm⁻¹ and a growth rate of 30 mm h⁻¹, the fibers formed had a diameter of about 400 nm. An electrochemical method is presented here that simultaneously passivates the NiAl matrix and selectively electrodissolves the Re. In this manner, it was possible to form an array of nanopores each with the same diameter of 400 nm. The mechanisms behind this procedure, as well as the potential of this method for the production of nanoelectrode arrays or nanofilters are discussed.     

Molecular dynamics simulation of water transport through graphene-based nanopores: Flow behavior and structure characteristics

The flowbehavior of pressure-drivenwater infiltration through graphene-based slit nanopores has been studied by molecular simulation. The simulated flow rate is close to the experimental values, which demonstrates the reasonability of simulation results. Water molecules can spontaneously infiltrate into the nanopores, but an external driving force is generally required to pass through the whole pores. The exit of nanopore has a large obstruction on the water effusion. The flow velocity within the graphene nanochannels does not display monotonous dependence upon the porewidth, indicating that the flowis related to the microscopic structures ofwater confined in the nanopores. Extensive structures of confined water are characterized in order to understand the flow behavior. This simulation improves the understanding of graphene-based nanofluidics, which helps in developing a new type of membrane separation technique.     

First step of anodization influences the final nanopore arrangement in anodized alumina

Anodic porous alumina is a well-known template material for nanofabrication. To obtain highly ordered nanoporous array, sophisticated and expensive methods are applied. On the other hand there is two-step self organized anodization, which is much cheaper, but obtained alumina arrangement is not ideal. In our paper, influence of the first step of anodization on the final AAO structure arrangement is studied in details. Anodizations were carried out in 0.3 M oxalic acid at 35 °C at various potentials and durations of the first step of anodization. Second step of the anodization was constant for all the experiments and was 15 min long. Oxide layer thickness formed during the first step of the anodization was 50, 100, 150 and 200 μm and after oxide removal, remaining concaves were serving as a mask for further growth during second step of the anodization. The longer first step of anodization, the higher regularity ratio and circularity, and lower percentage of defects in the nanoporous array. Anodic porous alumina was formed at four various potentials: 30, 40, 50, 60 V. The best arrangement of nanopores was found for samples anodized at 40 V. Maximum of regularity ratio was corresponding to the minimum of defect content in the AAO array. Long enough first step of two-step self-organized anodization in 0.3 M oxalic acid can provide high-ordered nanoporous template for fabrication of various nanomaterials with new, unique properties.     

Temperature influence on well-ordered nanopore structures grown by anodization of aluminium in sulphuric acid

The nanostructure dimensions and regularity of the hexagonal arrangement of nanopores formed by self-organized anodization of aluminium in a 20 wt.% sulphuric acid was investigated at various cell potentials and temperatures. The quantitative analyses of defects and Fourier transforms (FFT) performed from SEM images showed that regularity of nanopores arrangement can be improved by increasing anodizing potential, independently of the anodizing temperature. The best result in controlled anodization of aluminium can be obtained at 25 V and the temperature of 1 °C. The pore size and interpore distance distribution diagrams constructed for 1000 independent measurements showed that increasing uniformity of pore diameter and interpore distance is directly responsible for improvement of the regularity of hexagonal arrangement of nanopores observed with increasing anodizing potential at temperatures of −8 or 1 °C. At 25 V and independently of the anodizing temperature, the reduced number of generated defects is predominant factor improving regularity of the nanopore arrangement. The temperature influence on the lattice data, porosity of the nanostructure and real current density at the bottom of nanopores have been demonstrated.     

Surface morphology control on porous anodic alumina in phosphoric acid

We report the detailed surface morphology control of porous anodic aluminas (PAAs) fabricated in phosphoric acid. The surface defects can be dramatically lessened by employing the second anodization process, but there are still distortions on the surface. In the second anodizing process, the electric field distribution is not regular at the beginning due to the various pore shape and size, which leads to PAA surface distortion. One way to eliminate the surface morphology distortion is to burn the surface defects under higher temperature and higher anodizing voltage, and another way is to have well ordered nanopore array in oxalic acid before anodizing in phosphoric acid to restrict the electric field.