Current Nanoscience - Volume 15, Issue 1, 2019

Anodization of valve metal such as those of Al, Ti, and W among others have been extensively studied largely because of their unique morphology and extensive applications including gas and bio-sensing. While large volumes of published materials are available on the oxide of each metal, a concise review of previous works on these anodic oxides is timely. Herein, we present an overview of the formation process and applications (with emphasis to gas and bio-sensing) of anodic metal oxides that have been extensively researched. While porous and nonporous metal oxides have been produced and applied, the former has been given much attention as it provides more reactive surface area making it sine-qua-non in nanoscience and nanotechnology. The large effective surface area enables their applications as templates for the fabrication of periodic arrays of nanostructures, e.g., nanowires, nanodots, and nanotubes for various sensing technologies. Porous structures with different shape and size can be obtained by modulating the anodization conditions such as current, time, voltage, purity of metal, doping element, type and concentration of the electrolyte, electrolyte temperature and the pre-treatment of the metal substrate. The fabrication procedure, characterization and applications of each anodic metal oxide are presented in this review.

Self-ordering nanostructured anodic films fabricated onto the surface of iron and ironbased alloys opened new horizons for their application in the recent energy harvesting and storage devices and catalysis. The anodic passivity of iron in the aqueous solutions is known since the beginning of the past century due to formation of extremely thin iron oxides or salt layer. Ten years ago the discovery of the formation of thick and self-ordered nanoporous and nanotubular iron anodic films in organic solutions containing some fluorides and water opened a new pathway for investigation and application of these materials and stimulated a still growing interest. Therefore, the purpose of this review paper is to provide a better insight in the processes of iron anodizing for nanoporous and nanotubular film formation, their composition, and possible application trends in the view of the latest and our advances in this field. Apart from the formation of nanostructured films in well-known ethylene glycol electrolytes, the peculiarities of iron anodizing in other organic electrolytes, such as dimethyl sulfoxide, are presented herein. Since earlier published papers are almost forgotten, but they could give the basic knowledge on nanostructured anodic film formation in other electrolytes, we briefly introduce the behavior of iron in aqueous solutions resulted from the active dissolution of iron and polarization-dependent the passive film formation. To reveal the composition of as-grown anodic films on iron, X-ray diffraction, X-ray photoelectron, X-ray energy dispersive and Mössbauer spectroscopies are used and the reactions of passive films formation are discussed.

Background: This mini-review paper is focused on the anodic formation of selforganized nanotubular oxide layers on niobium as a nontoxic and allergy-free metallic biomaterial. Objective: The main purpose of the work was to outline the research activities being undertaken on the electrochemical modification of niobium to obtain its porous oxide with enhanced biocompatibility. Method: The effect of various parameters, such as concentration of fluoride anions in electrolyte, pH of electrolyte, and current-voltage-time conditions on the anodic formation of Nb2O5 nanotubes, was summarized. Results: Thirty-seven references were included in this mini-review and they were divided into main five parts. First part outlined the electrochemical formation of self-organized nanotubular oxide layers on niobium via anodic oxidation. The mechanism of the electrochemical formation of niobium oxide nanotubes was discussed. Second part presented the influence of the electrolyte type used for anodic oxidation of niobium and the fluoride ion concentration in the electrolyte on the type and dimensions of the obtained oxide niobium nanotubes. The influence of the applied voltage during anodic oxidation of niobium on the morphological parameters of the formed nanotube arrays was described in third part. The importance of the selection of the electrolyte pH for tailoring the length of niobium oxide nanotubes was demonstrated in fourth part. The last part outlined the structure of niobium oxide nanotubes. Conclusion: Key results were extracted and reviewed from publications of research activity in anodic oxidation of niobium introduced basics of that process and the current trends in electrochemical improvement of biocompatibility of metallic nanobiomaterials were presented.

Background: Nanoporous anodic aluminium oxide (NAAO) prepared by self-ordering anodization is fascinating and versatile nanostructured material predestined for a variety of applications. Objective: NAAO possesses remarkable properties with a highly ordered array of cylindrical pores which can be produced with tunable pore diameters and inter-pore spacing. For the past few decades, different approaches have been introduced to improve the surface properties of NAAO, however, though the various approached established, surface modification of NAAO is still a significant challenge. In this review, we highlight the current state of research on NAAO addresses the formation, properties and numbers of applications are further demonstrated for the application of catalytic, biosensing and nanostructure templates synthesis. Method: We systematically introduce the concept of material fabrication of the NAAO based on inexpensive electrochemical anodization with the self-ordering process of nanopores and the outcome of the process are entirely ordered and size-controlled nanopores with distinctive pore geometries. Also, we described the recent advances approaches such as chemical vapor deposition (CVD), spin coating, electrodeposition, electroless deposition, impregnation, etc. for structural engineering of the NAAO with targeting applications. Results: The combination of unique properties with tunable pore diameter and surface functionality made these NAAO materials very attractive for a wide range of application such as chemo & biosensors, catalytic and used as a template in the fabrication of various nanostructured materials such as nanorods, nanotubes and nanowires. Conclusion: The present review addresses the formation, properties, surface functionalities and several applications of NAAO.

Implementation of nanostructures into the organic solar cell (OSC) architecture has a great influence on the device performance. Nanostructuring the active layer increases the interfacial area between donor and acceptor, which enhances the probability of exciton dissociation. Introduction of nanostructures into the active layer and nanopatterning of the electrodes leads to increased light absorption due to light scattering and plasmonic effects, and nanostructured antireflection coatings constrict light within the device. The appealing features of Anodic Aluminum Oxide (AAO), mainly: scalability, low fabrication cost and easy control over its nano-scale morphology, make AAO patterning methods an intriguing candidate for nanopatterning. Hence, in this work, we present a review on the fabrication techniques and on nanostructures from Anodic Aluminum Oxide (AAO) for OSC applications. The versatility of such patterning technique is shown by pointing out the possibility of using an AAO template for the fabrication of nanowires by wetting, nanodots by evaporation, nanostructures by imprinting resists, organic layers and much more.

The review summarizes the recent results on the influence of various modifiers on the anodic aluminum oxide (AAO) growth and morphological parameters. It is demonstrated that the modifiers can play an important role in the formation and self-ordering of AAO. The intrinsic function of a modifier seems to depend on other operating conditions, such as a type of electrolyte used, applied anodizing voltage, the chemical structure of the modifier and its stability in the electric field, or a complex interplay between physical and chemical variables (electrical conductivity, viscosity, dielectric constant, pH, etc.). The function can also vary depending on whether the anodization is carried out under mild (MA) or hard (HA) conditions. Although there is still no coherent description of the role of modifiers in the aluminum anodization, the review shows a potential for prospective research and indicates a possibility to control the AAO formation by application of a given modifier.

Background: Quasi-one dimensional nanostructures: nanowires, nanotubes, nanorods, nanobelts/nanoribbons and complex “nanowire-nanoparticle” composites have been synthesized over the years. These nanostructures are particularly appealing due to their specific properties defined by their high aspect ratio: two dimensions are in the nanoscale range and one dimension is in the microscale. Methods: One of the well-designed approaches for the synthesis of such nanostructured materials is template-assisted fabrication combined with electrodeposition. The fabrication approaches for the growth of iron-group alloy nanostructures inside nanoporous oxide membranes by means of different electrodeposition techniques, and the resulting unique properties and potential applications of this type of materials are reviewed. Results: Arrays of nanostructures can be obtained by filling a porous oxide template that contains a large number of straight cylindrical, nano-sized diameter holes. Generalities of metals electrodeposition into nanoporous oxide membranes are discussed. Measures to minimize the nonuniformity of deposits inside pores need to be addressed to thin the barrier layer, to control hydrogen evolution and to improve mass transport inside the pores. Examples of binary and ternary iron group alloys grown inside nanoporous oxide templates are provided. Catalytic hydrogen evolution and methanol oxidation on the nanowires arrays are described. The “sample size effect” on the magnetic properties of materials and the electrodeposition of multilayered structures necessary for giant magnetoresistance (GMR) are discussed in details. Conclusion: Electrodeposition of binary and ternary iron-group alloys confirm that controlling alloy composition inside nanopores is still a challenge.

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.