A Capacitive Liquid-Phase Sensor and Its Sensing Mechanism Using Nanoporous Anodic Aluminum Oxide

Nanoporous anodic aluminum oxide (AAO) is an important template for 1D nanomaterial synthesis. It is used as an etching template for nanopattern transfer in a variety of contexts, including nanostructured material synthesis, electrical sensors, optical sensors, photonic and electronic devices, photocatalysis, and hardness and anticorrosion improvement. In this review, we focus on various fabrication methods, pore geometry modification, and recent advances of AAO, as well as sensor applications linked to our environment, daily life, and safety. Pore geometry is concerned with the material composition, applied voltage mold, electrolyte type, temperature, and anodizing time during the fabrication of AAOs and for adjusting their pore size and profile. The applied voltage can be divided into four types: direct current anodization (DCA), reverse pulse anodization, pulse anodization (PA), and hybrid pulse anodization (HPA). Conventional AAOs are fabricated using DCA and mild anodization (MA) at a relatively low temperature (-5~15 °C) to reduce the Joule heating effect. Moreover, the issues of costly high-purity aluminum and a long processing time can be improved using HPA to diminish the Joule heating effect at relatively high temperatures of 20-30 °C with cheap low-purity (≤99%) aluminum. The AAO-based sensors discussed here are primarily divided into electrical sensors and optical sensors; the performance of both sensors is affected by the sensing material and pore geometry. The electrical sensor is usually used for humidity or gas measurement applications and has a thin metal film on the surface as an electrode. On the contrary, the AAO optical sensor is a well-known sensor for detecting various substances with four kinds of mechanisms: interference, photoluminescence, surface plasma resonance, and surface-enhanced Raman scattering (SERS). Especially for SERS mechanisms, AAO can be used either as a solid support for coating metal nanoparticles or a template for depositing the metal content through the nanopores to form the nanodots or nanowires for detecting substances. High-performance sensors will play a crucial role in our living environments and promote our quality of life in the future.

Anodic Aluminum Oxide (AAO) has a variety of applications depending on its pore diameter, uniformity and porosity. Here, we investigate a new finding of the widened pore diameter of AAO greatly linked to the oxalic acid anions contaminated in the AAO. As AAO was prepared by hybrid-pulse anodization in oxalic acid at low-to-high concentrations (0.3∼0.9 M) and room temperature (25°C) instead of conventional low temperature (0∼10°C) and low concentration (0.3 M), the average pore diameter roughly increased with concentration from 42.8 nm to 46.5 nm; while it inversely decreased with concentration from 85.6 nm to 80.0 nm after widening to enhance all pore diameters. High oxalic acid concentration resulted in AAO walls containing more anions that further decreased etching rate during widening. This new phenomenon became more pronounced by direct-current anodization than by the hybrid-pulse anodization. The possible mechanism of evolution of AAO pore diameter during pore widening in the phosphoric acid was proposed and supported by the plane-view and cross-section SEM results and photoluminescence spectra.

Anodic aluminum oxide (AAO) has been widely applied for the surface protection of electronic component packaging through a pore-sealing process, with the enhanced hardness value reaching around 400 Vickers hardness (HV). However, the traditional AAO fabrication at 0~10 °C for surface protection takes at least 3-6 h for the reaction or other complicated methods used for the pore-sealing process, including boiling-water sealing, oil sealing, or salt-compound sealing. With the increasing development of nanostructured AAO, there is a growing interest in improving hardness without pore sealing, in order to leverage the characteristics of porous AAO and surface protection properties simultaneously. Here, we investigate the effect of voltage on hardness under the same AAO thickness conditions in oxalic acid at room temperature from a normal level of 40 V to a high level of 100 V and found a positive correlation between surface hardness and voltage. The surface hardness values of AAO formed at 100 V reach about 423 HV without pore sealing in 30 min. By employing a hybrid pulse anodization (HPA) method, we are able to prevent the high-voltage burning effect and complete the anodization process at room temperature. The mechanism behind this can be explained by the porosity and photoluminescence (PL) intensity of AAO. For the same thickness of AAO from 40~100 V, increasing the anodizing voltage decreases both the porosity and PL intensity, indicating a reduction in pores, as well as anion and oxygen vacancy defects, due to rapid AAO growth. This reduction in defects in the AAO film leads to an increase in hardness, allowing us to significantly enhance AAO hardness without a pore-sealing process. This offers an effective hardness enhancement in AAO under economically feasible conditions for the application of hard coatings and protective films.

A high-and-rapid-response capacitive humidity sensor of nanoporous anodic alumina by one-step anodizing commercial 1050 aluminum alloy and its enhancement mechanism

The effects of pore widening and calcination on anodized aluminum oxide prepared from Al6082

Effects of temperature and voltage mode on nanoporous anodic aluminum oxide films by one-step anodization

Keynote 1Prof. Ke Wang, East China University of Technology, Nanchang, ChinaSpeech Title:Magnetic and Perpendicular Exchange Coupling Properties of Rare-earth Transition Metal Alloy Films:In this talk, we will present some work we are carrying out on perpendicularly magnetized rare-earth transition metal alloy films (e.g., TbFeCo or Gd FeCo), which are currently attracting more attention for applications in low-power spintronic devices, ultrafast switching and logic memories. The perpendicular exchange coupling properties of TbFeCo/GdFeCo hard/soft bilayers are systematically investigated and some new results will be presented.Keynote 2Prof. Rangsu Liu, Hunan University, ChinaSpeech Title:Visualizing Study for Formation, Evolution and Hereditary Mechanisms of Nano-Clusters during Solidification Processes of Liquid Alloy under High Pressures:In recent years, we have deeply investigated the solidification processes of liquid metals of Al, Ag, Cu, Mg Na, Pb, Zn and alloys of Mg-Zn, Mg-Y, Al-Mg, Al-Zn, Ca-Zn by molecular dynamics simulation for the systems with different sizes of 100,000, 1,000,000 and 10.000,000 atoms, respectively. By using the visualizing method, the formation, evolution and hereditary mechanisms of nano-clusters, especially, under high pressures the large-scale microstructurel evolution, the crystallization and the hereditary mechanisms during their solidification processes have been clearly analyzed and described by the cluster type index method (CTIM) proposed by authors. Recently, many important results obtained. Highly interesting, for liquid alloy Mg-Y, it is found that all the simulation systems are directly solidified into crystal structures, and the crystallization temperature Tc is enhanced almost linearly with the increase of pressure. Highly interesting, there is an important phase transformation point from FCC to BCC structures between 15 ~ 20GPa during the solidification processes. And the structures formed at 20GPa possess the admirable microstructure configurations and excellent mechanical properties of materials.Keynote 3Prof. Chen Hsu, Lungwha University of Science and Technology, TaiwanSpeech Title:Small Molecular Sensing with Graphene, Zinc oxide, and Zinc Oxalate:Small molecules such as hydrogen and methane are able to be sensible using graphene, zinc oxide (ZnO), and zinc oxalate (ZnC2O4) via Van der Waals force. First of all, graphene is generally regarded as to have the function of adsorbing hydrogen molecules, but in really the adsorbing effect is not fit the demand. Graphene does not have the function of attracting hydrogen molecules. Graphene does not have the function of bullying hydrogen molecules. The main reason is that its plane is fully occupied by functional group. The graphene reduction exhibits many defect structures and residual oxidized groups, so even if many reduction methods are proposed, it is impossible to obtain a structure close to the perfect lattice of graphene. The basal plane and the edge of the reduced graphene oxide will contain a large number of oxygen functional groups; epoxy (C=O) and hydroxyl (hydroxyl, C)-OH is formed on the base surface, and carboxyl groups (COOH) and carbonyl groups (CO, CO) are distributed at the boundary.Keynote 4Prof. Chen-Kuei Chung, Department of Mechanical Engineering, National Cheng Kung University, TaiwanSpeech Title:Fabrication and Characteristics of Nanoporous Anodic Aluminum Oxide at High Temperature:The nanoporous anodic aluminum oxide (AAO) is one important nano-template used for synthesis of nanocomposite materials or as an etching mask for pattern transfer in a variety of applications. Conventional AAO templates were synthesized using two-step potentiostatic method of direct current anodization (DCA) at low temperature (0-10°C) to avoid dissolution effect. In this talk, an effective method of hybrid pulse anodization (HPA) with normal-positive and small-negative voltages has been proposed for AAO synthesis at relatively high temperature of 15-25°C for enhancing performance of AAO structure for both the cheap low-purity (99%) and costly high-purity (99.997%) aluminum foils. The pore size distribution and circularity of AAO by HPA is much better than DCA due to its effective cooling at relatively high temperature. The HPA not only merits manufacturing convenience and cost reduction but also promotes pore characteristics of AAO at severe conditions of cheap low-purity Al foils and high temperature. Moreover, the pore diameter can be enlarged by wet etching. A new finding of AAO pore diameter highly linked to the electrolyte anions contaminated AAO prepared by HPA at low-to-high concentrations (0.3~0.9 M) of oxalic acid at 25°C. The increase of widened diameter decreased with concentration due to AAO walls containing more anions high oxalic acid concentration to decrease etching rate during widening. Some application in humidity sensing and SERS will be presented.

The nanoporous anodic aluminum oxide (AAO) is one important nano-template used for synthesis of nanocomposite materials or as an etching mask for pattern transfer in a variety of applications. Conventional AAO templates were synthesized using two-step potentiostatic method of direct current anodization (DCA) at low temperature (0–10 °C) to avoid dissolution effect. In this chapter, an effective method of hybrid pulse anodization (HPA) with normal positive and small negative voltages has been proposed for AAO synthesis at a relatively high temperature of 15–25°C for enhancing performance of AAO structure from the cheap low-purity (99%), costly high-purity (99.997%) aluminum foils and the aluminum (Al) films on Si substrates. The pore size distribution and circularity of AAO by HPA is much better than DCA due to its effective cooling at relatively high temperature. The HPA not only merits manufacturing convenience and cost reduction but also promotes pore characteristics of AAO at severe conditions of cheap low-purity Al foils and high temperature. Moreover, the pore diameter can be enlarged by wet etching. The etching rate of AAO pore diameter is greatly affected by the electrolyte anions contaminated AAO prepared by HPA at low-to-high concentrations (0.3~0.9 M) of oxalic acid at 25°C. Increasing oxalic acid concentration decreases the widening rate of pore diameter because more anions in the AAO would decrease etching rate during widening. The applications to effective nanoporous membranes fabrication and photocatalysis are presented.

Dual effects of ultrasound on fabrication of anodic aluminum oxide

In this study, we have developed a swift and well-ordered growth of a nanoporous anodic aluminum oxide (AAO) structure using two-step high temperature anodization of the pure aluminum substrate. The pre-anodization surface treatment of the aluminum substrate assisted in the formation of well-organized nanoporous structures. The two-step anodization process was performed in 0.3 mol/L of oxalic acid at 20 °C with 40 and 45 V to obtain tunable pore diameters. The high temperature of the electrolyte solution facilitated the rapid growth of the nanoporous AAO structure. The top surface image of AAO shows a well-ordered nanoporous structure with an average pore diameter of 70 nm at 40 V and 100 nm at 45 V. The SEM cross-sectional view also illustrates the well-ordered nano-channel and the elemental mapping elaborates the presence of aluminum and oxygen. The thickness of the nanoporous AAO structure was determined using SEM for three anodization time spans (20, 24, and 28 h), in which an increasing trend was observed. The fabricated AAO has a higher thickness and a well-ordered nanoporous structure, which shows that it can be used as a template for fabricating nanostructured materials.

Many anodic aluminum oxide (AAO) templates were traditionally performed by potentiostatic anodization at 0---10 °C to inhibit the Joule's heat enhanced dissolution in aluminum oxide for ordered AAO configuration. In this article, the hybrid pulse anodization has been performed for offering the effective suppression of Joule's heat generation in high-aspect-ratio AAO formation at room temperature. The effect of pulse period, duty ratio, potential and electrolytic concentration on the evolution of pore characteristics was investigated using Taguchi method. The AAO morphology was captured by scanning electron microscope, and analyzed by gray-scale imaging in order to identify the pore size distribution and AAO thickness for Taguchi analysis. Short pulse-off period and low current density improved the uniformity of pore distribution. Moreover, high current density and intermediate pulse period/duty ratio can enhance the reaction rate and resulted in thick AAO. The evolution of AAO configuration and thickness were further correlated and discussed with pulse modulations and electrolytic concentrations.

The role of metal interlayers in maintaining adhesion during the direct fabrication of anodic aluminum oxide (AAO) with high-aspect ratio pores on tin-doped indium oxide (ITO) is studied. Chromium and titanium interlayers can maintain adhesion while anodization is conducted in either sulfuric, oxalic, or phosphoric acid solutions. However, the ability to form high-aspect ratio pores is dependent on the interlayer and aluminum thickness, the method of aluminum deposition, and possibly the cleaning/surface treatment of ITO prior to aluminum deposition. AAO films approximately thick were prepared in oxalic and phosphoric acids, yielding high-aspect ratio pores with length to diameter ratios of 47 and 14, respectively. The distinct stages of pore formation are also correlated with the time-resolved current response of the anodization cell, which provides in situ information about the anodization process so that adhesion can be maintained throughout pore formation. The direct fabrication of AAO on ITO/glass substrates from a single-step evaporation of thick aluminum films enables the formation of smooth and continuous gold nanowires, which have potential applications in photonics.

A simple and new relative humidity (RH) capacitive sensor with a two-layer planar structure based on anodic aluminium oxide (AAO) film serving as a sensing layer has been fabricated. A pair of separated tantalum thin films under the AAO layer served as two coplanar electrodes. AAO film was fabricated by anodising deposited aluminium thin film in oxalic acid. The influence of the pore size and thickness of the AAO film, and the electrode spacing, on the performance of the sensor was investigated. The RH sensor with a thicker AAO film and narrower electrode spacing was more sensitive. By pore widening, the sensitivity of the sensor can be improved. The achieved average sensitivity as high as ∼3.57 pF/RH% was obtained for sensors with an AAO layer of 2 µm thickness and 80 nm pore size. These results were explained using the partial capacitance method and parallel-serial model. In addition, under the humidity range of 20–80%, sensors with 70 nm pores had the highest sensitivity, which was interpreted in terms of effective permittivity and anion concentration. This study provides an easy method to fabricate high-performance humidity sensors with very simple structures for practical applications.

Fabrication of anodic aluminum oxide (AAO) on 4 in. glass substrate was demonstrated. Using unique structure with quadruple contacts as electrode support during anodization, we reported gradual transformation from opaque aluminum to transparent AAO over the entire substrate. In contrast, partial transformation was observed with typical single contact approach where growth of insulating AAO inhibited subsequent anodization by interrupting current flow from the contact point. Results from optical spectroscope revealed moderate reduction in transmittance after AAO formation. Successful development of large-area AAO on glass substrate opens up opportunities for AAO templated devices that require a large footprint.

Fabrication of enhanced anodic aluminum oxide performance at room temperatures using hybrid pulse anodization with effective cooling