Porous Anodic Oxide Research Articles

Formation process of zirconia nanotubes and porous structures and model of oxygen bubble growth

Preparation and growth mechanism of anodization of Ti and Al has been widely concerned for two decades, but the research on anodic ZrO2 is relatively lacking. In this paper, anodic TiO2 and ZrO2 nanotubes were prepared in glycerol electrolyte containing 0.35 M NH4F and 4 vol% H2O under different anodizing voltages. We had successfully prepared the anodic ZrO2 nanotubes (AZNTs) with a complete top and a “bulb” at the bottom under 60 V, and with the increase of the applied anodizing voltage, the “bulb” cavity also increased. However, under the same anodizing conditions, the surface of anodic TiO2 nanotubes (ATNTs) is a cluster of nano-tip morphology, and the bottom of the ATNTs is a conventional hemisphere shape. In addition, both AZNTs and porous anodic zirconia (PAZ) were found to coexist in the anodic ZrO2 layer prepared at 60 V. Here, we used the oxygen bubble model and ionic current and electronic current theories to analyze the reason of the special morphology. It is confirmed that the porous anodic oxides are actually evolved from nanotubes. In other words, the structure is essentially the same.

A review: research progress on the formation mechanism of porous anodic oxides.

Owing to the great development potential of porous anodic oxides (PAO) in many fields, research on their formation mechanisms, fabrication processes and applications has a history of more than ten years. Although compared with research on the fabrication processes and applications of PAO, research on their formation mechanisms started late, several mainstream theories have been formed in the academic community, including the field-assisted dissolution (FAD) theory, the field-assisted ejection (FAE) theory, the self-organization theory, the ionic and electronic current theory and the oxygen bubble mould effect. This review will focus on summarizing the core views of the mainstream mechanisms mentioned above and comparing the explanations for some of their classical experimental phenomena.

Effect of Heat Treatment on the Structure and Optical Properties of Porous Anodic Titanium Oxide Films

Effect of Heat Treatment on the Structure and Optical Properties of Porous Anodic Titanium Oxide Films

The Mechanism Regulating Film Thickness During Porous Anodic Oxide Growth on Aluminium: Transition from Homogeneously Thick Films During Soft Anodizing to Nodules Formation During Hard Anodizing

The Mechanism Regulating Film Thickness During Porous Anodic Oxide Growth on Aluminium: Transition from Homogeneously Thick Films During Soft Anodizing to Nodules Formation During Hard Anodizing

The growth rate of nanotubes and the quantity of charge during anodization

Porous anodic oxides have attracted great attention because of their unique morphology and porous structure. At the same time, the study of the mechanism has become a hot topic. Here, in order to study the formation mechanism, the growth of porous oxides of Ti and Zr at different voltages has been studied. The results show that the growth rate of ZrO2 nanotube is much higher than that of TiO2 nanotube, and the length of ZrO2 nanotube is much longer than that of TiO2 nanotube. By comparing the current–time curves, it is found that the difference of growth rate of nanotube is related to the quantity of charge, that is, integral of current–time curve (Q=∫I dt). At the same time, it is also found that the growth rate of ZrO2 nanotubes is higher than that of TiO2 nanotubes. For example, the average growth rate of ZrO2 nanotubes is 0.422 μm/min, while the growth rate of TiO2 nanotubes is only 0.131 μm/min at 20 V. These interesting findings negate the idea of fluoride ion dissolution in the classical field-assisted dissolution theory. The oxygen bubble model and electronic current theory can well explain this conclusion.

Porous Surface State Analysis of Anodized Titanium for Biomedical Applications

In this work, pure Ti was anodically oxidized using 1M H3PO4 + 2–10 pct HF electrolytes at 10 V vs open circuit potential (OCP). The structure of the anodized samples’ surfaces was investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The results show the presence of titanium dioxide (TiO2) and titanium hydrogen phosphate (Ti(HPO4)2) in the surface layer. Due to the chemical composition and properties that the materials have, they do not pose a threat and support the osseointegration process. The rough and porous anodic oxide morphology shown by the AFM is useful for tissue growth, as well as for improving corrosion resistance. The XPS confirms that the short anodization results in thin anatase-type TiO2, which is a candidate for hard tissue implant applications. The corrosion is improved after oxidation in 1M H3PO4 + 2 pct HF electrolyte for 15 minutes. The wetting analysis shows hydrophilic properties and suggests good bioadhesion of the anodically oxidized surface. The surface supports osteoblast cell proliferation, indicating that the material has a surface state appropriate for hard tissue implant applications.

Template directed perovskite X-ray detectors towards low ionic migration and low interpixel cross talking

The metal halide perovskites exhibit excellent performance as the direct X-ray detectors owing to their strong absorption capability, long carrier lifetime and diffusion length, radiation ruggedness, etc. For imaging applications, the ionic migration of perovskites and charge sharing effect between the adjacent pixels have a significantly negative impact on the spatial resolution. Herein, for the first time, the porous anodic aluminum oxides (AAO) have been used as a template to grow the CsPbBr2I thick film for the direct X-ray detection. Benefiting from the oxygen passivation effect, the activation energy for ionic migration has been observed to increase to 0.701 eV, whereas the dark current drift (1.01 × 10−5 nA cm−1s−1V−1) is one to two orders of magnitude lower than the other lead halide perovskite single crystals and films. Moreover, the AAO insulating wall effectively blocks the charge diffusion effect across a pixel pitch of 10 μm. Overall, the findings reported in this study open a new route for reducing the ionic migration and pixel crosstalk, thus, bringing the perovskite X-ray detectors close to the practical applications.

Nanoporous nickel thin film anode optimization for low-temperature solid oxide fuel cells

Thin film solid oxide fuel cells (TF–SOFCs) having anode–substrate nanostructure that was optimized for the low-temperature operation were fabricated. Nickel thin film anodes with four different anode thicknesses were deposited on anodic aluminum oxide templates, nanoporous substrates having two different pore sizes, by the sputtering method. Subsequently, a yttria-stabilized zirconia (YSZ) electrolyte and platinum cathode were deposited on them, which completed the entire fuel cell structure. The anode nanostructure of fuel cells in six combinations was analyzed by the cross-sectional view, surface microscopy method, and three-dimensional morphology observation. Those investigations enabled the anode nanostructure to be identified, such as the anode porosity and the roughness of the interface between anodes and electrolytes. Then, the six TF–SOFCs were electrochemically characterized in a 500 °C operating environment. The maximum power densities were obtained through the i–V–P curves, and the highest performance of 294.1 mW/cm2 was measured in the cell having a combination of 200 nm–sized porous aluminum anodic oxide (AAO) and 1200 nm–thick Ni anode. This showed up to 20.1% improvement over the other cells. EIS analysis showed that the optimized ohmic and faradaic resistance originated from each part of the unique TF–SOFC structure.

Fast fabrication of small pore anodic titania nanotube arrays under high voltage

Due to their unique structures and physicochemical properties, porous anodic metal oxides including Al2O3, Fe2O3, TiO2, Ta2O5 and so on, prepared by anodization of corresponding metal substrates in appropriate electrolytes have found applications in various fields. Pore diameter plays a significant role in applications of these anodic metal oxides. It is a common knowledge that the pore diameter of porous metal oxides increases with formation voltage despite of a vast variation of anodization parameters and electrolytes used. Here, we demonstrate that the pore diameter and cell size of anodic TiO2 nanotube arrays can decrease dramatically under high voltage accompanied by high current density. Impact factors determining the formation of small pore TiO2 nanotubes under high voltage were discussed. Typically, with the same electrolyte used, the average pore diameter of TiO2 nanotube arrays changes from ca 26 nm at a formation voltage of 10 V to ca 13 nm at 200 V. It is also demonstrated that at high voltage, small pore TiO2 nanotubes could be grown with a rate orders of magnitude faster than those grown in a classic way with low voltage and current density.

Electrochemical Synthesis of Crystalline Niobium Oxide

Features of creation of porous nanostructured oxides of transition materials on an example of niobium are considered. It has been experimentally shown that variation in anodizing modes makes it possible to obtain non-porous and porous amorphous anodic oxide films (AOF) and films of the crystalline type. It is determined that the process of AOF formation on niobium, as well as its structure and properties depend on such parameters as the type of electrolyte, anodizing voltage, activator concentration, the duration of the process. It is confirmed that the presence of an activator in the electrolyte is a necessary and decisive factor in the process of forming a nanostructured anode oxide layer. To obtain a nanostructured surface of niobium oxide, a necessary condition is the introduction of fluoride into the electrolyte, but also an important task is to determine the type of compound with which F– ions are introduced into the electrolyte. It has been experimentally determined that the optimal solution for the rapid growth of porous crystalline oxide is a solution consisting of a background electrolyte in the form of 1M H2SO4 with the addition of a fluoride ion activator in the form of 0.5M NaF. The increase in the activator accelerates the formation of the crystal structure on the surface of niobium. It is shown that higher voltage and longer anodizing time leads to an increase in the size of microcones and their number on the surface of niobium. Optimal for the formation of porous crystalline oxide is a voltage of 60 V in the electrolyte 1M H2SO4 + 0.5M NaF for 2 hours.

Comparative study on the anodizing process of Ti and Zr and oxide morphology

Metal anodizations have attracted much attention, but the formation mechanisms of the porous anodic oxides are still unclear. Compared with the Ti anodization, the anodization of Zr is relatively less studied. In order to compare the anodizing processes of Ti and Zr, a comparative study of Ti and Zr was carried out under the different voltages of anodizing process in the same mixed electrolyte of ammonium fluoride and phosphate acid. It was found that the structure and morphology of their nanotubes were very different. The anodizing curve of Ti showed a state of constant current. However, the anodizing curve of Zr did not show the state of constant current and ZrO2 nanotubes were still formed. This indicates that there is no equilibrium between oxide growth and dissolution in the anodization. TiO2 nanotubes show an orderly array of nanotubes, but ZrO2 nanotubes have three kinds of composite structures: nanoflower, nanotube and dense films. By comparing the great difference of current-time curves between the two anodizing processes, the composite structure of ZrO2 nanotubes are interpreted reasonably by combining electronic current theory and oxygen bubble mold.

Electrocatalytic Properties of Co Nanoconical Structured Electrodes Produced by a One-Step or Two-Step Method

One-dimensional (1D) nanostructures, such as nanotubes, nanopores, nanodots and nanocones, are characterized by better catalytic properties than bulk material due to their large active surface area and small geometrical size. These structures can be produced by several methods of synthesis including the one- and two-step methods. In the one-step method, a crystal modifier is added to the solution in order to limit the horizontal direction of structures growing during electrodeposition. In this work, NH4Cl was used as a crystal modifier. Another way of production of 1D nanocones is the electrodeposition of metal in porous anodic alumina oxide (AAO) templates, called the two-step method. In this case, the AAO template was obtained using a two-step anodization process. Nanocones obtained by the two-step method show smaller geometrical size. In this work, cobalt nanoconical structures were obtained from an electrolyte containing CoCl2 and H3BO3. The electrocatalytic properties of materials fabricated by one-step and two-step methods were measured in 1 M NaOH and compared with bulk material electrodeposited from the same electrolyte. There were several microshell structures in the case of Co deposits obtained by the one-step method. To solve this problem, different conditions of synthesis Co cones by the one-step method were applied. The electrocatalytic activity of these samples was checked as well.

Synthesis of Co-Fe 1D Nanocone Array Electrodes Using Aluminum Oxide Template.

Porous anodic alumina oxide (AAO) obtained via two-step anodization is a material commonly used as a template for fabricating 1D nanostructures. In this work, copper and cobalt-iron 1D nanocones were obtained by an electrodeposition method using AAO templates. The templates were produced using two-step anodization in H2C2O4. The Co–Fe nanostructures are characterized by homogeneous pore distribution. The electrocatalytic activity of the produced nanomaterials was determined in 1 M NaOH using the linear sweep voltammetry (LSV) and chronopotentiometry (CP) methods. These materials can be used as catalysts in the water-splitting reaction. The sample’s active surface area was calculated and compared with bulk materials.

Comparison of Formation Mechanism between Porous Anodic Sn Oxide and Anodic TiO2 Nanotubes

Porous anodic Sn oxide (PASO) and other porous anodic oxides have aroused worldwide scientific interest. However, the relationship between the complex porous structure and the current-time curve cannot be interpreted by the traditional dissolution theory. In particular, with the change of anodizing conditions of Sn, the current-time curve sometimes shows three stages, sometimes five stages. In order to study the relationship between current-time curve and the growth model of complex PASO structure during Sn anodization, this study compares the anodizing processes of Sn and Ti. Under normal circumstances, the current-time curve of Ti in anodizing process is three stages, which can be interpreted reasonably by electronic current and oxygen bubble model. By adjusting the anodizing processes of both metals, their current-time curves would be changed into five stages, which means that the formation mechanism of the two metal oxide porous structures may be inherently connected. Here, the current-time curves of three stages and five stages are carefully compared. The internal relationship between the structure of anodic TiO2 nanotubes and the complex porous structure of PASO is found, revealing that the complex porous structure of PASO is caused by excessive gas evolution which is the result of electronic current.

Evidence of oxygen bubbles forming nanotube embryos in porous anodic oxides.

Anodic TiO2 nanotubes have been studied widely for two decades because of their regular tubular structures and extensive applications. However, the formation mechanism of anodic TiO2 nanotubes remains unclear, because it is difficult to find convincing evidence for popular field-assisted dissolution or field-assisted injection theories and the oxygen bubble model. Here, in a bid to find direct evidence that oxygen bubbles form nanotube embryos, a new method is applied to handle this challenge. Before nanotube formation, a dense cover layer was formed to make nanotubes grow more slowly. Many completely enclosed nanotube embryos formed by oxygen bubbles were found beneath the dense cover layer for the first time. The formation of these enclosed and hollow gourd-shaped embryos is convincing enough to prove that the nanotubes are formed by the oxygen bubble mold, similar to inflating a football, rather than by field-assisted dissolution. Based on the ‘oxygen bubble model’ and ionic current and electronic current theories, the formation and growth process of nanotube embryos is explained clearly for the first time. These interesting findings indicate that the ‘oxygen bubble model’ and ionic current and electronic current theories also apply to anodization of other metals.

Nanoporous fluorescent sensor based on upconversion nanoparticles for the detection of dichloromethane with high sensitivity.

A sensor with high sensitivity and response rate is still lacking in the detection of poisonous and volatile chemicals. Here, we report a highly sensitive nanoporous fluorescence sensor based on core@shell upconversion nanoparticles (UCNPs) for the detection of dichloromethane. UCNPs were deposited on porous anodic alumina oxide (AAO) templates supported by glass slides to form a thin film-like gas sensor in which UCNPs with active shells exhibit intense background-free fluorescence and simultaneously high optical sensitivity, while an AAO template acts as a porous substrate for UCNPs to increase the absorption capacity for molecules to be tested. A detection limit of 2.91 ppm was obtained for dichloromethane based on this sensor at room temperature. The involved response mechanism was attributed to lowered surface fluorescence quenching and scattering of UCNPs by dichloromethane.

(Invited) Growth of Self-Ordered Porous Anodic Oxide Nanomaterials By Coupled Ion Migration and Viscous Flow

Porous anodic oxide (PAO) films are self-organized porous oxide nanomaterials formed by electrochemical oxidation. PAO includes porous anodic alumina and TiO2 nanotube layers. These films eventually evolve into a hexagonally-ordered array of parallel cylindrical pores or nanotubes of order 100 nm diameter, with a characteristic interpore spacing that scales directly with the cell voltage.We explore the roles of mechanical stress and oxide flow in self-ordering, through a modeling analysis of stress-driven viscous flow of oxide (1). The model is based on coupled viscous flow of oxide and stress- and electric field-driven migration of metal and oxygen ions (2). Stress at the film interfaces is generated by ion-transfer processes and relaxed by viscous flow, resulting in layers with elevated compressive or tensile stress within a few nm of the interfaces (3,4). Experimental evidence for these layers has been found from in situ stress measurements (5,6). We present linear stability analysis incorporating the surface stress boundary conditions in the coupled oxide flow-electrical migration model of Houser and Hebert (2).For porous alumina, the calculations reveal self-organized pattern formation with steady-state pore spacing-voltage scaling ratios that compare favorably to those found experimentally (1). The scaling ratios are independent of oxide viscosity or uncertainties in the values of any other model parameters. The stress driving oxide flow arises at the oxide-solution interface due to strong electrolyte anion adsorption, which blocks direct oxide deposition on the surface itself. New results for TiO2 nanotube layers are presented. Near-surface stress is not present in TiO2 nanotube layers (6); instead, compressive stress driving flow arises at the metal-oxide interface from the volume change upon conversion of metal to oxide.REFERENCES P. Mishra and K. R. Hebert, Electrochim. Acta, 340, 135879 (2020).J. E. Houser and K. R. Hebert, Nature Mater., 8, 415 (2009).K. R. Hebert, P. Mishra, J. Electrochem. Soc., 165, E737 (2018).K. R. Hebert, P. Mishra, J. Electrochem. Soc., 165, E744 (2018).Ö. Ö. Çapraz, P. Shrotriya, P. Skeldon, G. E. Thompson, K. R. Hebert, Electrochim. Acta, 167, 404 (2015).Q. Dou, P. Shrotriya, W. F. Li, K. R. Hebert, Electrochim. Acta, 292, 676 (2018).Q. Dou, P. Shrotriya, W. F. Li, K. R. Hebert, Electrochim. Acta, 295, 418 (2019).

(Invited) Multiscale Modelling of Titanium Oxide Growth and Dissolution in Fluoride-Containing Electrolytes

Nanostructured valve metal oxides have been extensively investigated due to the variety of applications related to their valuable chemical and physical properties, including mechanical and thermal resistance, catalytic and photocatalytic activity, and biocompatibility. The technology to form well defined porous anodic oxides on titanium will strongly benefit from an optimization of the process parameters based on deterministic modelling. The relatively large number of adjustable parameters often precludes the unambiguous interpretation of steady-state and transient electrochemical data in terms of a unique kinetic model.An approach to overcome this ambiguity by parameterization of the models of barrier film growth [1, 2] on Ti and its alloys (Figure 1) using a combination of in-situ EIS in a large frequency range, dynamic high-frequency impedance and photocurrent measurements, combined with ex-situ characterization of the oxides by X-ray photoelectron spectroscopy (XPS) is presented. Additional estimates of kinetic parameters are derived from density functional theory (DFT) modelling of adsorption of water and fluoride on Ti and TiO2 [3, 4]. Ways to extract information on the rate-limiting steps of the process of passive film formation, growth and restructuring are described and the significance of the obtained parameters for the kinetics of barrier film growth and dissolution is outlined. Stancheva, M. Bojinov, Electrochim. Acta 78, 65 (2012).Bojinov, I. Betova, V. Karastoyanov, Electrochim. Acta 344, 136137 (2020).Stancheva, B. Diawara, F. Lebreau, M. Bojinov, J. Electrochem. Soc. 161, E3188 (2014).Betova, M. Bojinov, V. Karastoyanov, M. Stancheva, Computational Mater. Sci.171, 109260 (2020). Acknowledgments: The financial support of the National Science Fund of Bulgaria under contract ДН19/3-2017 „Modeling, optimization and modification of nanoporous anodic oxide films as catalysts for water photoelectrolysis“is gratefully acknowledged. Figure 1

The influence of water-induced crystallization on the photoelectrochemical properties of porous anodic tin oxide films

Nanoporous SnOx films with ultra-small channels (<20nm in diameter) were synthesized by one-step anodic oxidation of Sn foil in 1M sodium hydroxide. Then, as-prepared materials were immersed in distilled water for various durations (2–120h) to find out whether a simple soaking process can affect the photoelectrochemical performance of such kind of photoanodes. It was confirmed that during exposure to water, spontaneous crystallization of the amorphous SnOx matrix into rutile-type SnO2 occurs via a dissolution–redeposition process, which results in a gradual loss of the initial porous morphology and thinning of the anodic film. Moreover, due to the formation of less defective, more stoichiometric, and crystalline SnO2-x, gradual widening of the semiconductor band gap, the cathodic shift of the flat band and onset potentials, as well as decrease in the donor density are observed. All these factors are the reason for the significant deterioration of the photoelectrochemical performance of SnOx photoanodes during their exposure to water. Therefore, it was confirmed that even slight differences in the experimental procedure, especially sample washing, drying, and storage, can result in meaningful changes in the composition, and the photoelectrochemical properties of anodically generated tin oxide films.

Compositional Variations in Porous Anodic Oxides Formed on Fe(100), Fe(110) and Fe(111) Single Crystals

Anodizing is a high voltage electrochemical conversion process that forms barrier-type oxide layers or self-organized nanoporous/nanotubular structures. So far, alumina-like nanopores and titania-like nanotubes could be successfully synthesized on many valve metals and alloys. Recently, anodizing become possible for base metals such as polycrystalline iron and its alloys. The challenge in anodizing of iron lies in the proper control of corrosion/passivity, which can be successfully realized in organic electrolytes containing small amount of water.1-3 Anodic nanotubes and anodic nanopores may be formed on polycrystalline iron depending on electrochemical conditions adjusted in anodizing protocol. The polycrystalline iron is typically composed of the grains having the size of 2-10 µm. The kinetics of metal oxidation as well as oxygen evolution reaction, which is typically observed upon iron anodizing, may depend on the iron facet exposed to the electrolyte. In order to evaluate the dependence of the crystallographic orientation on the formed anodic film structure we investigated anodizing process on iron single crystals, namely Fe(100), Fe(110) and Fe(111). The compositional differences for porous films formed on single crystals were analyzed by means of by high resolution electron microscopy EDS elemental mapping.