Fabrication Of Porous Anodic Alumina Research Articles

Nanopore/Nanosphere‐Induced Optical Enhancement of Monolayer MoS2

AbstractHeterostructured transition metal dichalcogenides (TMDs) have received much attention due to their tunable optical and electronic properties. Here, a strategy for facile fabrication of porous anodic alumina (PAA) membrane‐supported monolayer molybdenum disulfide (MoS2) is described. PAA is either empty or filled with gold nanoparticles (AuNPs). Such nanopore/nanosphere‐supported MoS2 (i.e., PAA‐MoS2 and PAA‐Au‐MoS2) exhibits greatly enhanced optical performances compared with those of silicon wafer‐supported MoS2 (i.e., Si/SiO2‐MoS2). Notably, 11‐fold Raman enhancement, 13‐fold photoluminescence enhancement, and 419‐fold second‐harmonic generation enhancement are achieved in PAA‐Au‐MoS2. Moreover, the optical enhancement mechanism of this unique structure is proposed. This work provides a platform for the construction of curved/heterostructured TMDs and exploration of their strong light‐matter interactions.

Influence of Ethanol on Porous Anodic Alumina Growth in Etidronic Acid Solutions at Various Temperatures.

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.

Bipolar Electrochemical Anodization Route for the Fabrication of Porous Anodic Alumina with Nanopore Gradients.

Porous anodic alumina (PAA) films with homogeneous nanopores are achieved by traditional anodization. Here, we present a unique anodization technique based on bipolar electrochemistry to fabricate PAA films with nanopore gradients. In an oxalic acid solution dissolved in ethylene glycol, a stable bipolar anodization process is realized. The PAA film prepared at 280 V exhibits a continuous change in interpore distance from ∼171 to ∼83 nm over a range of only 5 mm on the aluminum sheet. Higher driving voltages lead to larger interpore distances and steeper nanopore gradients. Further, no direct electrical connection is required for this bipolar anodization.

Fabrication of Porous Anodic Alumina (PAA) by High-Temperature Pulse-Anodization: Tuning the Optical Characteristics of PAA-Based DBR in the NIR-MIR Region.

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 (iamax) after application of subsequent UH pulses started to stabilize upon decreasing high (UH) and low (UL) voltage pulses, which was reflected in a smaller difference between initial and final thickness of alternating dH and dL segments (formed under UH and UL, respectively) and a better DBR performance. Shortening UH 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 dH+dL 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.

Continuous Photocatalytic Reduction of CO₂ Using Nanoporous Reduced Graphene Oxide (RGO)/Cadmium Sulfide (CdS) as Catalyst on Porous Anodic Alumina (PAA)/Aluminum Support.

Solar conversion of CO₂, using a photocatalyst in the presence of water, has attained considerable attention due to added value of the process in terms of both reduction of an environmental pollutant and production of valuable synthetic chemicals. Room temperature fabrication of porous anodic alumina (PAA), for sufficiently low time, facilitates the synthesis of self-withstanding PAA with a middle layer of aluminum. Nanoporous reduced graphene oxide (RGO), deposited on the pore walls of PAA, with subsequent deposition of cadmium sulfide (CdS) as photocatalyst over it, and efficiently enhances the photocatalytic reduction of CO₂. Morphological, structural and optical characterizations of the catalyst are executed using field emission scanning electron microscopy (FE-SEM), X-ray powder diffraction (XRD), electron dispersive X-spectroscopy (EDX) and UV-Vis absorption spectroscopy methods. Continuous photocatalytic reduction of CO₂ was carried out using flat sheet reactors and a compound parabola as the solar reflector. Semiconducting CdS nanorods, grown over PAA support with conducting RGO, show enhanced photocatalytic reduction of CO₂ to 153.8 μmol/g/hr of CH₃OH with higher photocatalytic stability than CdS alone.

Influence of anodizing voltage and electrolyte concentration on Al-1 wt% Si thin films anodized in H2SO4

Porous anodic alumina (PAA) templates have been extensively studied due to their unique morphology and extensive applications. The fabrication of PAA can be modulated to achieve a self-ordered pore structure with desired pore size and interpore separation. PAA thin film templates were fabricated from silicon doped aluminum film, Al-1 wt% Si, based on one-step anodization method at 22 °C. Effects of anodizing potential (10–20 V) as well as sulfuric acid concentrations (0.6–1.8 M) on current density, interpore distance, anodization rate, and volume expansion were evaluated. The results indicated that current density increased largely exponentially with a concentration of the electrolyte at a given anodizing voltage. In addition, distinct and well-ordered pores were obtained for anodization conducted at 15 V and 20 V. The volume expansion factor is proportional to the applied voltage. At 1.8 M sulfuric acid, the expansion factor increases with pore regularity with 1.4 considered as the transition point. The minimum current density (2.1 ± 0.15 mA cm−2) was observed at the minimum anodizing condition (0.6 M, 10 V). Also, maximum anodizing condition (1.8 M, 20 V) resulted in the highest current density of 34 ± 4 mA cm−2. As expected, anodization time decreases with an increase in both anodization voltage and electrolyte concentration.

Fabrication of porous anodic alumina (PAA) templates with straight pores and with hierarchical structures through exponential voltage decrease technique

The present work aims to become a standard step for the fabrication of the next generation porous anodic alumina (PAA) templates based devices and to provide new insights on the mechanism involved in the porous anodic alumina formation. The oxide barrier layer at the bottom of the pores has been successfully thinned by applying an exponential voltage decrease process followed by a wet chemical etching. The impact of the potential drop on the PAA structure has been deeply investigated, as well as the electrolyte temperature, the number of potential steps and the exponential decay rate. The presented results underline that a smart adjustment between the anodization conditions and exponential voltage decay parameters can simultaneously give PAA structures with straight pores and remove the dielectric layer in spite of applying the exponential voltage decay step. Additionally, the PAA structure has been tuned to fabricate hierarchically nanoporous templates with secondary pores ranging from 2 up to 10 branches.

Effect of ethanol on the fabrication of porous anodic alumina in sulfuric acid

Porous anodic alumina (PAA) films with small pore diameters and interpore distances are significantly important in many practical applications such as isolation of pathogens, catalysis and molecular sieving. This kind of PAA films is generally prepared in diluted sulfuric acid, and the film growth is very slow due to the low anodizing potential and current density. Many efforts were done to enhance the PAA formation, and the majorities focused on increasing growth rate by increasing the anodizing potential, which also increases the pore diameter and interpore distance of the obtained PAA. In our paper, the influence of ethanol addition in electrolyte of sulfuric acid was studied in details. Results show that the addition of ethanol can maintain the low self-ordering potentials in sulfuric acid and at the same time significantly increase the growth rate of PAA. With a series of ethanol addition, it was found that with the 10% ethanol addition, the growth rate increases by 5 times from 1.6μm/h to 8μm/h at 2°C and 25V, and there is a nonlinear relationship between ethanol addition amount and PAA growth rate. These findings are of great significance for further investigation on PAAs with ultrasmall nanopores, as well as for PAA commercialization.

Effect of Temperature of Oxalic Acid on the Fabrication of Porous Anodic Alumina from Al‐Mn Alloys

The influence of temperature of oxalic acid on the formation of well‐ordered porous anodic alumina on Al‐0.5 wt% Mn alloys was studied. Porous anodic alumina has been produced on Al‐0.5 wt% Mn substrate by single‐step anodising at 50 V in 0.5 M oxalic acid at temperature ranged from 5°C to 25°C for 60 minutes. The steady‐state current density increased accordingly with the temperature of oxalic acid. Hexagonal pore arrangement was formed on porous anodic alumina that was formed in oxalic acid of 5, 10 and 15°C while disordered porous anodic alumina was formed in oxalic acid of 20 and 25°C. The temperature of oxalic acid did not affect the pore diameter and interpore distance of porous anodic alumina. Both rate of increase of thickness and oxide mass increased steadily with increasing temperature of oxalic acid, but the current efficiency decreased as the temperature of oxalic acid increased due to enhanced oxide dissolution from pore wall.

Reduction of nanoparticle deposition during fabrication of porous anodic alumina

In this paper, we present evidences of nanoparticle deposition on porous anodic alumina (PAA) surface under different anodizing conditions, and discuss the formation mechanism of precipitations. Low-temperature anodization, vigorous stirring and ultrasound-assisted anodization are proposed to reduce the hydrolysis product or hinder its deposition on the surface as well as inside nanopores of PAA. It is discovered that the ultrasound is very efficient to prevent the deposition of Al3+ hydrolysis product on PAA. This study is of great value for realizing ordered PAAs with clean surface.

Effect of Manganese Content on the Fabrication of Porous Anodic Alumina

The influence of manganese content on the formation of well‐ordered porous anodic alumina was studied. Porous anodic alumina has been produced on aluminium substrate of different manganese content by single‐step anodizing at 50 V in 0.3 M oxalic acid at 15°C for 60 minutes. The well‐ordered pore and cell structure was revealed by subjecting the porous anodic alumina to oxide dissolution treatment in a mixture of chromic acid and phosphoric acid. It was found that the manganese content above 1 wt% impaired the regularity of the cell and pore structure significantly, which can be attributed to the presence of secondary phases in the starting material with manganese content above 1 wt%. The pore diameter and interpore distance decreased with the addition of manganese into the substrates. The time variation of current density and the thickness of porous anodic alumina also decreased as a function of the manganese content in the substrates.

Fabrication of Porous Anodic Alumina by Single-Step Anodization: Influence of the Molar Concentration and effect of the Chemical Etching

Porous anodic alumina (PAA) films have been fabricated by single-step anodization for different concentrations of oxalic acid. The influence of the molar concentration on their average pore diameter was investigated and chemical etching processes for different times on the as-etched layer was performed in order to study its effect on the formation of self-organized nanopores. The experimental results show that controlling both, molar concentration and chemical etching times after anodization pores with different diameters can be easily prepared. This procedure leads to form porous alumina layers with high-quality two-dimensional hexagonal lattices and provide an ideal template for preparation of many one-dimensional nanomaterials.

Thermal Driving Fast Fabrication of Porous Anodic Alumina

We present detailed information on thermal driving anodization (TDA) process for fast fabrication of porous anodic alumina in both oxalic and phosphoric acids. The TDA process was realized under high temperature of 30–70°C. Fast fabrication with high growth rate of 15–90 μm/h was achieved under 30–50°C in oxalic acid. In phosphoric acid, the growth rate of 1 μm/h at 0°C can be improved to 37.0 μm/h at 50°C. The morphology transformations, as well as the current-time evolutions, under higher temperature of 50–70°C were also investigated in oxalic acid and phosphoric acid, respectively.

Fabrication of porous anodic alumina with ultrasmall nanopores.

Anodization of Al foil under low voltages of 1–10 V was conducted to obtain porous anodic aluminas (PAAs) with ultrasmall nanopores. Regular nanopore arrays with pore diameter 6–10 nm were realized in four different electrolytes under 0–30°C according to the AFM, FESEM, TEM images and current evolution curves. It is found that the pore diameter and interpore distance, as well as the barrier layer thickness, are not sensitive to the applied potentials and electrolytes, which is totally different from the rules of general PAA fabrication. The brand-new formation mechanism has been revealed by the AFM study on the samples anodized for very short durations of 2–60 s. It is discovered for the first time that the regular nanoparticles come into being under 1–10 V at the beginning of the anodization and then serve as a template layer dominating the formation of ultrasmall nanopores. Under higher potentials from 10 to 40 V, the surface nanoparticles will be less and less and nanopores transform into general PAAs.

Uniform and Reproducible Barrier Layer Removal of Porous Anodic Alumina Membrane

A method for the fabrication of porous anodic alumina (PAA) membrane without bottom barrier layer on Al substrate is described. In this method, two-step anodizing followed by a barrier thinning process at the end of the second anodization was used to prepare wide range highly-ordered PAA membrane with a thin barrier layer. Finally, cathodic polarization and pore widening processes were combined to remove the barrier layer completely between the oxide film and Al substrate. Scanning electron microscope (SEM) was used to characterize the morphology and structure of the PAA membrane. From the SEM images, the PAA membrane prepared with the assistance of cathodic polarization showed more homogeneous pore diameters and pore wall quality than that made by pore widening only. In addition, the barrier layer was removed completely with 7.5 min of cathodic polarization and 70 min of pore widening without corrosion in the Al substrate. It was possible to control the pore diameter without any damage to the PAA template from 70 to 90 nm. The fabricated PAA template can be applied to the growth of ordered nanowires, nanorods, nanotubes, and similar structures.

The fabrication of ordered nanoporous metal films based on high field anodic alumina andtheir selected transmission enhancement

A two-step high field anodization and a controllable barrier layer removing process havebeen used for the fabrication of porous anodic alumina (PAA) with different morphologies.Based on the PAAs, porous noble metal films with widely tunable pore size and inter-poredistance have been realized by a simple sputtering method. Their morphology and opticalproperties were studied with a field-emission scanning electron microscope, and ultravioletand ultraviolet–visible spectrophotometers. An enhanced light transmission of thenanoporous metal films was detected. The transmissivity of a normal incidence light can beenhanced over ten times within a certain selected wavelength range. The intensity andposition of the transmission peak depend on the morphology and porosity of the metalfilms.

Fabrication and Characterization of Porous Anodic Alumina Films from Impure Aluminum Foils

We report here on the fabrication of porous anodic alumina (PAA) films from commercially available impure aluminum foil. While the expensive ultrapure PAA films are restricted to potential applications in nanoelectrophotonics, the impure PAA films are more suitable for large-scale applications, such as in catalysis and filtration. The anodization current behavior and chemical composition of the resulting PAA films from impure and ultrapure foils were found to be similar for the same set of anodizing conditions. However, the PAA films from impure aluminum foil contained pore arrays of much smaller size and less consistently sized pores than those of PAA from ultrapure foils. We find that these qualities are improved by either annealing or electropolishing the aluminum foil prior to anodization, although not to the degree of PAA produced from ultrapure foils. Greater improvement is found for annealed foils compared to electropolished foils.