Search Results (13)

This study reports the production of mid-infrared (MIR) porous anodic alumina (PAA)-based microcavities with tunable optical quality. The spectral position of the cavity resonance peak (λ_C), along with its intensity (I_R) and Q-factor, varies depending on the geometric positioning of the cavity layer within the multilayer stack of alternating low- and high-porosity layers, as well as the type of cavity produced—either by high voltage (Cv_H-type) or low voltage (Cv_L-type) pulses. In most cases, PAA microcavities with Cv_H-type cavity layers exhibited superior light confinement properties compared to those with Cv_L-type cavities. Additionally, shifting the cavity layer from the center toward the edges of the multilayer stack enhanced the intensity of the resonance peak. For PAA microcavities with Cv_H-type cavity layers, the highest intensity (I_R = 53%) and the largest Q-factor (Q = 31) were recorded at λ_C of around 5.1 µm. The anodization approach used in this study demonstrates significant potential for designing PAA-based microcavities with high optical performance in the MIR spectral region, especially with further refinement of electrochemical parameters. These findings pave the way for the development of new photonic materials specifically tailored for the MIR spectral range, broadening their applications in various optoelectronic and sensing technologies. Full article

The fabrication technology of surface nanocomposites based on hexagonally ordered gold nanoparticle (AuNP) layers (quasi-arrays) and their possible application as surface-enhanced Raman spectroscopy (SERS) substrates are presented in this paper. The nanoparticle layers are prepared using a nanotextured template formed by porous anodic alumina (PAA) and combined with gold thin-film deposition and subsequent solid-state dewetting. Three types of hexagonal arrangements were prepared with different D/D₀ values (where D is the interparticle gap, and D₀ is the diameter of the ellipsoidal particles) on a large surface area (~cm² range), namely, 0.65 ± 0.12, 0.33 ± 0.10 and 0.21 ± 0.09. The transfer of the particle arrangements to transparent substrates was optimized through three generations, and the advantages and disadvantages of each transfer technology are discussed in detail. Such densely packed nanoparticle arrangements with high hot-spot density and tunable interparticle gaps are very beneficial for SERS applications, as demonstrated with two practical examples. The substrate-based enhancement factor of the nanocomposites was determined experimentally using a DNA monolayer and was found to be between 4 × 10⁴ and 2 × 10⁶ for the different particle arrangements. We also determined the sensing characteristics of a small dye molecule, rhodamine 6G (R6G). By optimizing the experimental conditions (e.g., optimizing the laser power and the refractive index of the measurement medium with an ethylene-glycol/water mixture), concentrations as low as 10⁻¹⁶ M could be detected at 633 nm excitation. Full article

The fabrication and design of carbon-based hierarchical structures with tailored nano-architectures have attracted the enormous attention of the materials science community due to their exceptional chemical and physical properties. The collective control of nano-objects, in terms of their dimensionality, orientation and size, is of paramount importance to expand the implementation of carbon nanomaterials across a large variety of applications. In this context, porous anodic alumina (PAA) has become an attractive template where the pore morphologies can be straightforwardly modulated. The synthesis of diverse carbon nanomaterials can be performed using PAA templates, such as carbon nanotubes (CNTs), carbon nanofibers (CNFs), and nanodiamonds, or can act as support for other carbon allotropes such as graphene and other carbon nanoforms. However, the successful growth of carbon nanomaterials within ordered PAA templates typically requires a series of stages involving the template fabrication, nanostructure growth and finally an etching or electrode metallization steps, which all encounter different challenges towards a nanodevice fabrication. The present review article describes the advantages and challenges associated with the fabrication of carbon materials in PAA based materials and aims to give a renewed momentum to this topic within the materials science community by providing an exhaustive overview of the current synthesis approaches and the most relevant applications based on PAA/Carbon nanostructures materials. Finally, the perspective and opportunities in the field are presented. Full article

Etidronic acid, used in aluminum anodization, has a great potential for the fabrication of porous anodic alumina (PAA) with large cell sizes (>540 nm). PAAs are particularly suited to applications in optics and photonics where large-scale periodicity corresponding to visible or infrared light is needed. Additionally, such PAAs should be characterized by long-range pore ordering. However, to obtain regular pore arrangement in an etidronic electrolyte, the anodization should be performed at high electric fields using relatively high temperatures, which makes the process challenging in terms of its stability. To stabilize the process, the electrolyte can be modified with ethanol. In this work, the impact of ethanol on pore geometry and a level of pore ordering is systematically analyzed. It is shown that the additive tends to reduce pore ordering. Moreover, by changing the anodizing temperature and the amount of ethanol, it is possible to tune the porosity of the PAA template. At 20 °C, porosity drops from 14% in PAA grown in a pure water-based electrolyte to ca. 8% in PAA fabricated in the 1:3 v/v EtOH:H₂O electrolyte. The larger PAA thickness obtained for the same charge density strongly suggests that PAA formation efficiency increases in the 1:3 v/v EtOH:H₂O mixture. Full article

The results of studies on the wettability properties and preparation of porous anodic alumina (PAA) membranes with a 3.3 ± 0.2 μm thickness and a variety of pore sizes are presented in this article. The wettability feature results, as well as the fabrication processing characteristics and morphology, are presented. The microstructure effect of these surfaces on wettability properties is analyzed in comparison to outer PAA surfaces. The interfacial contact angle was measured for amorphous PAA membranes as-fabricated and after a modification technique (pore widening), with pore sizes ranging from 20 to 130 nm. Different surface morphologies of such alumina can be obtained by adjusting synthesis conditions, which allows the surface properties to change from hydrophilic (contact angle is approximately 13°) to hydrophobic (contact angle is 100°). This research could propose a new method for designing functional surfaces with tunable wettability. The potential applications of ordinary alumina as multifunctional films are demonstrated. Full article

The great success of anodic alumina membranes is due to their morphological features coupled to both thermal and chemical stability. The electrochemical fabrication allows accurate control of the porous structure: in fact, the membrane morphological characteristics (pore length, pore diameter and cell density) can be controlled by adjusting the anodizing parameters (bath, temperature, voltage and time). This article deals with both the fabrication and use of anodic alumina membranes. In particular, we will show the specific role of the addition of aluminum ions to phosphoric acid-based anodizing solution in modifying the morphology of anodic alumina membranes. Anodic alumina membranes were obtained at −1 °C in aqueous solutions of 0.4 M H₃PO₄ added with different amounts of Al(OH)₃. For sake of completeness, the formation of PAA in pure 0.4 M H₃PO₄ in otherwise identical conditions was also investigated. We found that the presence of Al(OH)₃ in solution highly affects the morphology of the porous layer. In particular, at high Al(OH)₃ concentration (close to saturation) more compact porous layers were formed with narrow pores separated by thick oxide. The increase in the electric charge from 20 to 160 C cm⁻² also contributes to modifying the morphology of porous oxide. The obtained anodic alumina membranes were used as a template to fabricate a regular array of PdCo alloy nanowires that is a valid alternative to Pt for hydrogen evolution reaction. The PdCo alloy was obtained by electrodeposition and we found that the composition of the nanowires depends on the concentration of two metals in the deposition solution. Full article

This work provides a conceptually new way of thinking about the light-absorbing mechanism in additive-free black porous anodic alumina (black PAA, or b-PAA) layers obtained via “burning” anodizing regime. The new insight into the controllable photonic effects in PAA allows the implementation of the optical blackening method based on the deliberate randomization of the initially well-ordered nanopore arrangement. The proposed black coloration mechanism rests solely on the destructive interference of light after its multiple scattering. Similar effects have been earlier considered for some natural or artificially created biomimetic structures (e.g., the so-called “moth eye effect”, or the coloration mechanism in the Neurothemis tullia dragonfly wings). Comprehensive analysis confirmed that the chemical composition of b-PAA has only a minor influence on the color changes and the optical density increase, and that the light-absorbing properties most likely result from the structural effects. The new functional 2D materials exhibit strong adhesion to aluminum surface, are cost-effective and suitable for application under harsh thermal or UV-light conditions. They are potentially useful for manufacturing of optical devices or heat-resistant coatings in aerospace technologies, as well as solid supports for biological filtration and fluorescence imaging. Full article

Over the past few years, researchers have made numerous breakthroughs in the field of aluminum anodizing and faced the problem of the lack of adequate theoretical models for the interpretation of some new experimental findings. For instance, spontaneously formed anodic alumina nanofibers and petal-like patterns, flower-like structures observed under AC anodizing conditions, and hierarchical pores whose diameters range from several nanometers to sub-millimeters could be explained neither by the classical field-assisted dissolution theory nor by the plastic flow model. In addition, difficulties arose in explaining the basic indicators of porous film growth, such as the nonlinear current–voltage characteristics of electrochemical cells or the evolution of hexagonal pore patterns at the early stages of anodizing experiments. Such a conceptual crisis resulted in new multidisciplinary investigations and the development of novel theoretical models, whose evolution is discussed at length in this review work. The particular focus of this paper is on the recently developed electroconvection-based theories that allowed making truly remarkable advances in understanding the porous anodic alumina formation process in the last 15 years. Some explanation of the synergy between electrode reactions and transport processes leading to self-organization is provided. Finally, future prospects for the synthesis of novel anodic architectures are discussed. Full article

A robust and reliable method for fabricating porous anodic alumina (PAA)-based distributed Bragg reflectors (DBRs), operating in mid-infrared (MIR) spectral region, is presented. The method relies on application of high (U_H) and low (U_L) voltage pulse sequence repeated in cycles. PAA-based DBR consists of alternating high-(d_H) and low-porosity (d_L) layers translated directly into periodically varied refractive index. Two anodization modes were used: time- and charge density-controlled mode. The former generated d_H + d_L pairs with non-uniform thickness (∆d) and effective refractive index (∆n_eff). It is supposed, that owing to a compensation effect between the ∆d and ∆n_eff, the photonic stopbands (PSBs) were symmetrical and intensive (transmittance close to zero). Under the charge density-controlled mode d_H + d_L pairs of uniform thickness were formed. However, the remaining ∆n_eff provided an asymmetrical broadening of PSBs. Furthermore, it is demonstrated that the spectral position of the PSBs can be precisely tuned in the 3500–5500 nm range by changing duration of voltage pulses, the amount of charge passing under subsequent U_H and U_L pulses, and by pore broadening after the electrochemical synthesis. The material can be considered to be used as one-dimensional transparent photonic crystal heat mirrors for solar thermal applications. Full article

The treatment of organic dye contaminants in wastewaters has now becoming more imperative. Fenton-like degradation of methylene blue (MB) and methyl orange (MO) in aqueous solution was investigated by using a nanostructure that a layer of CuCl₂ nanoflake film grown on the top surface of nanoporus anodic alumina substrate (nano-PAA-CuCl₂) as catalyst. The new nano-PAA-CuCl₂ composite was fabricated with self-assembly approach, that is, a network porous structure film composed of CuCl₂ nanoflake grown on the upper surface of nanoporous anodic alumina substrate, and the physical and chemical properties are characterized systematically with the X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), and high-resolution transmission electron microscopy (HRTEM), Energy Dispersive Spectrometer (EDS), X-ray photoelectron spectroscopy (XPS). The experimental results showed that the nano-PAA-CuCl₂ catalyst presented excellent properties for the degradation of two typical organic pollutants such as MB and MO, which were almost completely degraded with 8 × 10⁻⁴mol/L nano-PAA-CuCl₂ catalyst after 46 min and 60 min at reaction conditions of H₂O₂ 18 mM and 23 mM, respectively. The effects of different reaction parameters such as initial pH, H₂O₂ concentration, catalyst morphology and temperature were attentively studied. And more, the stability and reusability of nano-PAA-CuCl₂ were examined. Finally, the mechanism of MB and MO degradation by the nano-PAA-CuCl₂/H₂O₂ system was proposed, based on the experimental data of the BCA and the temperature-programmed reduction (H₂-TPR) and theoretical analysis, the reaction kinetics belonged to the pseudo-first-order equation. This new nanoporous composite material and preparation technology, as well as its application in Fenton-like reaction, provide an effective alternative method with practical application significance for wastewater treatment. Full article

In this work, the influence of various electrochemical parameters on the production of porous anodic alumina (PAA)-based DBRs (distributed Bragg reflector) during high-temperature-pulse-anodization was studied. It was observed that lowering the temperature from 30 to 27 °C brings about radical changes in the optical performance of the DBRs. The multilayered PAA fabricated at 27 °C did not show optical characteristics typical for DBR. The DBR performance was further tuned at 30 °C. The current recovery (i_a^max) after application of subsequent U_H pulses started to stabilize upon decreasing high (U_H) and low (U_L) voltage pulses, which was reflected in a smaller difference between initial and final thickness of alternating d_H and d_L segments (formed under U_H and U_L, respectively) and a better DBR performance. Shortening U_H pulse duration resulted in a progressive shift of photonic stopbands (PSBs) towards the blue part of the spectrum while keeping intensive and symmetric PSBs in the NIR-MIR range. Despite the obvious improvement of the DBR performance by modulation of electrochemical parameters, the problem regarding full control over the homogeneous formation of d_H+d_L pairs remains. Solving this problem will certainly lead to the production of affordable and efficient PAA-based photonic crystals with tunable photonic properties in the NIR-MIR region. Full article

In this work, the influence of a wide range anodizing temperature (5–30 °C) on the growth and optical properties of PAA-based distributed Bragg reflector (DBR) was studied. It was demonstrated that above 10 °C both structural and photonic properties of the DBRs strongly deteriorates: the photonic stop bands (PSBs) decay, broaden, and split, which is accompanied by the red shift of the PSBs. However, at 30 °C, new bands in transmission spectra appear including one strong and symmetric peak in the mid-infrared (MIR) spectral region. The PSB in the MIR region is further improved by a small modification of the pulse sequence which smoothen and sharpen the interfaces between consecutive low and high refractive index layers. This is a first report on PAA-based DBR with a good quality PSB in MIR. Moreover, it was shown that in designing good quality DBRs a steady current recovery after subsequent application of high potential (U_H) pulses is more important than large contrast between low and high potential pulses (U_H-U_L contrast). Smaller U_H-U_L contrast helps to better control the current evolution during pulse anodization. Furthermore, the lower PSB intensity owing to the smaller U_H-U_L contrast can be partially compensated by the higher anodizing temperature. Full article

An ionic polymer microactuator driven by an organic photoelectric proton pump transducer is described in this paper. The light responsive transducer is fabricated by using molecular self-assembly to immobilize oriented bacteriorhodopsin purple membrane (PM) patches on a bio-functionalized porous anodic alumina (PAA) substrate. When exposed to visible light, the PM proton pumps produce a unidirectional flow of ions through the structure’s nano-pores and alter the pH of the working solution in a microfluidic device. The change in pH is sufficient to generate an osmotic pressure difference across a hydroxyethyl methacrylate-acrylic acid (HEMA-AA) actuator shell and induce volume expansion or contraction. Experiments show that the transducer can generate an ionic gradient of 2.5 μM and ionic potential of 25 mV, producing a pH increase of 0.42 in the working solution. The ΔpH is sufficient to increase the volume of the HEMA-AA microactuator by 80%. The volumetric transformation of the hydrogel can be used as a valve to close a fluid transport micro-channel or apply minute force to a mechanically flexible microcantilever beam. Full article