Features of the porous morphology of anodic alumina films at the initial stage of disordered growth

A characteristic feature of disordered porous anodic film growth at the initial stage of aluminum anodizing was revealed by varying the electrolyte type and anodizing voltage. The samples were obtained by the electrochemical oxidation of thin aluminum films (100 nm thick) on SiO2/Si substrates in a 0.3 M oxalic acid at 10–50 V and were studied by SEM. The ImageJ analysis of the images revealed the simultaneous development of two large groups of pores: major pores with a large diameter and minor pores with a smaller diameter. When anodizing in oxalic acid at 10–50 V, it has been shown that the ratio of the diameters of the major and minor pores remains constant and is about 1.17. Using a geometric model, we demonstrated that the centers of the minor pores are located inside the elementary hexagonal cell formed by the centers of the major pores. Moreover, our results are very close to the theoretical value of 2/√3. At the initial stage of disordered pore growth, the development of minor pores rather than major pores is not a random process and is determined by energy-efficient conditions for the development of pores inside the hexagonal cells formed by the major pores. The increase in compressive mechanical stress in the anodic film leads to an interruption in the development of such pores

Effect of anodic oxygen evolution on cell morphology of sulfuric acid anodic alumina films

The purpose of this work was to study and analyze the effect of electrolyte temperature and anodization voltage on cell morphology of thin films of sulfuric acid anodic alumina formed on substrates of different nature, such as SiO2/Si, glass-ceramic, glass substrates, and polished aluminum. The data obtained demonstrated that the thermal conductivity of the substrate in the voltage range from 12 to 14 V affected a pore diameter (dpore) in anodic films. Depending on the substrate type, dpore increased in the following order: glass > glass- ceramic > SiO2/Si > aluminum. It was found that the anodizing voltage (Ua) of 16 V was a turning point for anodic films obtained in sulfuric acid after which the slope of the lines for both dpore and Dinter (interpore distance) vs. Ua changed. This behavior might be explained by the occurrence of the overpotential enough for the beginning of the oxygen evolution reaction. We assumed that the oxygen evolution on aluminum oxide surface at the pore bottom at Ua> 16 V results in an increase in acid concentration in the solution and, consequently, in rise in acidic nature of the electrolyte and increase in the dissolution rate of the oxide layer of pore walls

Stored charge and its influence on properties of anodic alumina films

In porous and barrier-type anodic alumina films, the stored charge has electronic nature and it plays a significant role in the process of aluminum anodizing. The charge stored can modify the distribution of local field generated by a voltage applied and thus it can affect the oxide growth. The method for the investigation of thermally activated defects in anodic alumina films by reanodizing technique was also described. It was applied for computation of activation energy of electron traps in barrier layer for sulfuric and oxalic acid alumina films and concentration of the traps

Modification of alumina matrices through chemical etching and electroless deposition of nano-Au array for amperometric sensing

Simple nanoporous alumina matrix modification procedure, in which the electrically highly insulating alumina barrier layer at the bottom of the pores is replaced with the conductive layer of the gold beds, was described. This modification makes possible the direct electron exchange between the underlying aluminum support and the redox species encapsulated in the alumina pores, thus, providing the generic platform for the nanoporous alumina sensors (biosensors) with the direct amperometric signal readout fabrication

Electrical behaviour, characteristics and properties of anodic aluminium oxide films coloured by nickel electrodeposition

Porous anodic films on 1050 aluminium substrate were coloured by AC electrodeposition of nickel. Several experiments were performed at different deposition voltages and nickel concentrations in the electrolyte in order to correlate the applied electrical power to the electrical behaviour, as well as the characteristics and properties of the coatings. The content of nickel inside the coatings reached 1.67 g/m2, depending on the experimental conditions. According to the applied AC voltage in comparison with the threshold voltage Ut, the coating either acted only as a capacitor when U\Ut and, when U[Ut, the behaviour during the anodic and cathodic parts of the power sine wave was different. In particular, due to the semi-conducting characteristics of the barrier layer, additional oxidation of the aluminium substrate occurred during the anodic part of the electrical signal, whilst metal deposition (and solvent reduction) occurred during the cathodic part; these mechanisms correspond to the blocked and pass directions of the barrier layer/electrolyte junction, respectively

Copper Selenide Nanosnakes: Bovine Serum Albumin-Assisted Room Temperature Controllable Synthesis and Characterization

Herein we firstly reported a simple, environment-friendly, controllable synthetic method of CuSe nanosnakes at room temperature using copper salts and sodium selenosulfate as the reactants, and bovine serum albumin (BSA) as foaming agent. As the amounts of selenide ions (Se2−) released from Na2SeSO3 in the solution increased, the cubic and snake-like CuSe nanostructures were formed gradually, the cubic nanostructures were captured by the CuSe nanosnakes, the CuSe nanosnakes grew wider and longer as the reaction time increased. Finally, the cubic CuSe nanostructures were completely replaced by BSA–CuSe nanosnakes. The prepared BSA–CuSe nanosnakes exhibited enhanced biocompatibility than the CuSe nanocrystals, which highly suggest that as-prepared BSA–CuSe nanosnakes have great potentials in applications such as biomedical engineering

Structural and Electrical Characterization of Bi2Se3 Nanostructures Grown by Metalorganic Chemical Vapor Deposition

We characterize nanostructures of Bi2Se3 that are grown via metalorganic chemical vapor deposition using the precursors diethyl selenium and trimethyl bismuth. By adjusting growth parameters, we obtain either single-crystalline ribbons up to 10 microns long or thin micron-sized platelets. Four-terminal resistance measurements yield a sample resistivity of 4 mOhm-cm. We observe weak anti-localization and extract a phase coherence length l_phi = 178 nm and spin-orbit length l_so = 93 nm at T = 0.29 K. Our results are consistent with previous measurements on exfoliated samples and samples grown via physical vapor deposition.Comment: Related papers at http://pettagroup.princeton.ed

Interfacial Chemistry in Al/CuO Reactive Nanomaterial and Its Role in Exothermic Reaction.

Interface layers between reactive and energetic materials in nanolaminates or nanoenergetic materials are believed to play a crucial role in the properties of nanoenergetic systems. Typically, in the case of Metastable Interstitial Composite nanolaminates, the interface layer between the metal and oxide controls the onset reaction temperature, reaction kinetics, and stability at low temperature. So far, the formation of these interfacial layers is not well understood for lack of in situ characterization, leading to a poor control of important properties. We have combined in situ infrared spectroscopy and ex situ X-ray photoelectron spectroscopy, differential scanning calorimetry, and high resolution transmission electron microscopy, in conjunction with firstprinciples calculations to identify the stable configurations that can occur at the interface and determine the kinetic barriers for their formation. We find that (i) an interface layer formed during physical deposition of aluminum is composed of a mixture of Cu, O, and Al through Al penetration into CuO and constitutes a poor diffusion barrier (i.e., with spurious exothermic reactions at lower temperature), and in contrast, (ii) atomic layer deposition (ALD) of alumina layers using trimethylaluminum (TMA)produces a conformal coating that effectively prevents Al diffusion even for ultrathin layer thicknesses (∼0.5 nm), resulting in better stability at low temperature and reduced reactivity. Importantly, the initial reaction of TMA with CuO leads to the extraction of oxygen from CuO to form an amorphous interfacial layer that is an important component for superior protection properties of the interface and is responsible for the high system stability. Thus, while Al e-beam evaporation and ALD growth of an alumina layer on CuO both lead to CuO reduction, the mechanism for oxygen removal is different, directly affecting the resistance to Al diffusion. This work reveals that it is the nature of the monolayer interface between CuO and alumina/Al rather than the thickness of the alumina layer that controls the kinetics of Al diffusion, underscoring the importance of the chemical bonding at the interface in these energetic materials