Mesoporous Anodic Alumina Research Articles

Micro- and nanoparticles of mesoporous anodic alumina: Morphological and physicochemical properties

In current biotechnological research, nanostructured particles in the micrometric and nanometric size range are of great importance for the development of new platforms for biological track and image applications. Although mesoporous anodic alumina is a well-known material that is widely used in optoelectronics, it has been used so little in research into the development of particles that it can be considered a new material in this area. In this paper we discuss the steps that need to be taken to obtain MAA particles with different morphologies in the nanometric and micrometric size range and make a systematic study and reference analysis of their physicochemical and optical properties as micrometric and nanometric elements. Particles are characterized by transmission and scanning electron microscopy, dynamic light scattering, zeta potential measurements, X-ray diffraction, energy dispersive X-ray analysis, photoluminescence spectroscopy and optical microscopy. The characterization is designed to provide valuable information about the biotechnological use of micro and nanoparticles of MAA. The versatility and capability of the particles and the relation of the properties studied to several possible applications is assessed.

Multifunctional nanotube-like Fe3O4/PANI/CDs/Ag hybrids: An efficient SERS substrate and nanocatalyst

In this paper, the stable and environment-friendly Fe3O4 nanotubes with polyaniline (Fe3O4 NTs/PANI hybrids) have been prepared via mesoporous anodic alumina oxide (AAO) template, sol–gel method and in-situ polymerization. Then multifunctional Fe3O4 NTs/PANI/Ag hybrids have been obtained by decorating Ag nanoparticles by glucose reduction on surface of Fe3O4 NTs/PANI hybrids. The morphologies and structures of these hybrids were subsequently investigated by SEM, XRD, TEM and XPS measurements. The Fe3O4 NTs/PANI/Ag hybrids presented high catalytic activity due to the template-assisted presence, preventing Ag particulate agglomeration. Importantly, the Fe3O4 NTs/PANI/Ag hybrids achieve sensitive surface-enhanced Raman scattering (SERS) signals. Furthermore, the introduction of carbon dots (CDs) endows these hybrids good dispersion and stable photoluminescence (PL). Therefore, the obtained hybrids may have potential applications in waste water treatment, biomedicine, photocatalyst, and environmental analysis.

Adsorption of argon on mesoporous anodic alumina

We have studied the adsorption of Ar on regular, highly-ordered alumina membranes made by anodization. The straight, non-interconnected pores have nominal diameters of 31 and 83 nm, with a relative dispersion better than 5 % in the pore size. Adsorption isotherms taken on bare membranes with pores of 83 nm present two distinct hysteresis loops. This is found to be a consequence of the fabrication procedure that yields a central circular region formed by open pores surrounded by an outer ring with closed bottom pores of smaller size, about 40 nm. For the membrane with pores of 31 nm, the difference between these pores is much smaller, about 2 nm, and this explains why the isotherms on these membranes show a single hysteresis loop as expected. Detailed real space analysis of the membranes by electron microscopy confirms the adsorption conclusions.

Nanostructural characterizations of hydrogen-permselective Si–Co–O membranes by transmission electron microscopy

Nanostructural characterizations of liquid metal–organic precursors-derived cobalt-doped amorphous silica (Si–Co–O) membranes supported on a mesoporous anodic alumina capillary (MAAC) tube were performed to study their unique high-temperature hydrogen gas permeation properties. Cross-sectional scanning transmission electron microscopy images and selected-area electron diffraction patterns indicated that the metal cobalt and the different oxidation states of cobalt oxides (CoO and Co3O4) nanocrystallites having a size range of 5–20 nm were in situ formed in the mesopore channels of the MAAC tube. In addition, high-resolution transmission electron microscopy micrographs and electron energy loss spectroscopy elemental mapping images indicated that the highly dense Co-doped amorphous Si–O formed within the mesopore channels of the MAAC tube. These nanostructural features could contribute to the hydrogen-selective permeation properties observed for the membranes.

Atomic layer deposition of palladium films on Al2O3 surfaces

We examined the atomic layer deposition (ALD) of Pd films using sequential exposures of Pd(II) hexafluoroacetylacetonate (Pd(hfac)2) and formalin and discovered that formalin enables the efficient nucleation of Pd ALD on Al2O3. In situ quartz crystal microbalance measurements revealed that the Pd nucleation is hampered by the relatively slow reaction of the adsorbed Pd(hfac)2 species, but is accelerated using larger initial Pd(hfac)2 and formalin exposures. Pd nucleation proceeds via coalescence of islands and leaves hfac contamination at the Al2O3 interface. Pd films were deposited on the thermal oxide of silicon, glass and mesoporous anodic alumina following the ALD of a thin Al2O3 seed layer and analyzed using a variety of techniques. We measured a Pd ALD growth rate of 0.2 Å/cycle following a nucleation period of slower growth. The deposited films are cubic Pd with a roughness of 4.2 nm and a resistivity of 11 μΩ cm at 42 nm thickness. Using this method, Pd deposits conformally on the inside of mesoporous anodic alumina membranes with aspect ratio ∼1500 yielding promising hydrogen sensors.

TEM observation of hydrogen permeable Si-M-O (M = Ni or Sc) membranes synthesized on mesoporous anodic alumina capillary tubes

Microstructures of Si-M-O (M = Ni or Sc; M/Si = 0.5) membranes synthesized on a mesoporous anodic alumina capillary (MAAC) tube using chemical processing techniques were characterized by transmission electron microscopy. The chemical composition of the membranes was analyzed by energy-dispersive X-ray spectroscopy. An as-synthesized Si-Ni-O membrane was found to be formed as a thin amorphous layer with a thickness of 400 nm on the MAAC tube, and in the mesopore channels of the MAAC tube, amorphous Si-Ni-O was also formed. The EDS analysis indicated a compositional gradient of Ni/Si for the Si-Ni-O membrane. The ratio of Ni/Si increased the further in depth from the surface of the membrane. After heat treatment in a hydrogen flow, a multilayered structure was formed, and nickel and nickel oxides were precipitated in the membrane. In the Si-Sc-O membrane, neither precipitation nor compositional gradient of Sc/Si were observed. The compositional change and precipitating behaviors of the Si-Ni-O and Si-Sc-O membrane are discussed in terms of the stability of the silica-dopant metal network and the hydrogen permselectivity of the membranes.

Lanthanide‐doped sol‐gel‐derived films fabricated in mesoporous anodic alumina

Abstract— Porous anodic alumina films with controllable morphology were fabricated under varied anodizing conditions and examined by scanning electron microscopy and photoluminescence analysis. Erbium‐, terbium‐, and europium‐doped xerogel films were formed inside the porous layer. Strong visible and infrared photoluminescence from the erbium‐doped xerogel/porous anodic alumina structure was observed. An electroluminescent cell based on xerogel/porous anodic alumina structure was also developed, with terbium‐ and europium‐related electroluminescence investigated.

Meso-Porous Alumina Capillary Tube as a Support for High-Temperature Gas Separation Membranes by Novel Pulse Sequential Anodic Oxidation Technique

A meso-porous anodic alumina capillary tube (MAAC) having highly oriented radial meso-pore channels with a minimum diameter of 3 nm has been successfully synthesized using a novel pulse sequential anodic oxidation technique at 100 Hz of pulse frequency. A value resulting in a high channel-pore formation rate at 1 V of the pulse sequential voltage was determined to be the optimum pulse frequency for the anodization. Transmission electron microscopy observation and N2 sorption analysis revealed that controlling the minimum pore channel diameter at 3 nm was possible by the voltage of 1 V. The gas permeance according to Knudsen’s diffusion mechanism was demonstrated at 500 °C, by evaluating gas permeation properties through the meso-porous anodic alumina capillary tube with radial meso-pore channels with minimum diameter of 3 nm, achieving hydrogen permeance of 1.8 × 10−6 mol/m2 s Pa.

Luminescence from Sol-Gel-Derived Europium-Doped Films Confined in Mesoporous Anodic Alumina

Photoluminescence (PL) properties of the structure porous anodic alumina (PAA)/europium-doped xerogel film have been investigated with respect to the xerogel content its transmittance and PL excitation wavelength. Three mechanisms of lanthanide optical excitation in the structure are proposed: (i) excitation through the PAA matrix; excitation through the xerogel matrix that depends on its transparency and, in turn, on xerogel content and thickness within the porous morphology; and direct excitation of a lanthanide ion due to the relative transparency of both the PAA and xerogel films. It is speculated that exploiting PAA allows significant change in the excitation conditions of lanthanides, probably due to multiple scattering of the exciting light, which is the main reason for the reported PL enhancement of lanthanides from the structure compared with similar PAA/lanthanide-doped xerogel films fabricated on flat substrates. © 2003 The Electrochemical Society. All rights reserved.

High‐efficiency luminescent sources fabricated in mesoporous anodic alumina by sol‐gel synthesis

Abstract— This paper summarizes our recent results on the synthesis and investigation of photoluminescence (PL) from lanthanide‐doped microporous xerogel solids mesoscopically confined in porous anodic alumina (PAA). It was demonstrated, for Tb‐doped samples, that the PL intensity is strongly enhanced in comparison to thin xerogel films processed onto flat surfaces and increased with the thickness of the PAA layer. It was revealed for both Tb‐ and Eu‐doped PAA‐based structures that maximum emission is achieved at a excitation wavelength near 285 nm for the employed TiO2 and Al2O3 xerogels. Strong Eu‐ and Tb‐related PL visible to the naked eye was demonstrated, and a method for the fabrication of luminescent images based on anodizing, photolithography, and sol‐gel processes is proposed.

Enhancement of Green Terbium-Related Photoluminescence from Highly Doped Microporous Alumina Xerogels in Mesoporous Anodic Alumina

Strong enhancement of green photoluminescence (PL) from microporous alumina xerogels, highly doped with terbium (from 30 to 60 wt % as is reported. The regular structure of a 30 μm thick, mesoporous anodic alumina layer was exploited for spin-on deposition of the alumina xerogel in a single step. The green PL, associated with predominant transitions, along with 4, 5, 6 transitions of was found to increase with terbium concentration in the alumina xerogel. This effect is attributed to cross-relaxation. The thermal quenching of the green terbium-related emission does not exceed a factor of two within a temperature range from 10 to 300 K for any of the alumina xerogels confined in anodic alumina. Further, such quenching is much reduced with the rise of temperature compared with Tb-doped titania xerogel, (ii) Tb-implanted thermally grown silicon dioxide film, and (iii) Tb-doped alumina xerogels fabricated onto monocrystalline silicon. Thus, the terbium-doped structure comprising alumina xerogel/anodic alumina is proposed as a basis for green room-temperature luminescent images. © 2001 The Electrochemical Society. All rights reserved.

Terbium photoluminescence in polysiloxane films

In the present communication we report on luminescence from terbium doped polysiloxanes SR-350, prepared from a liquid precursor, fabricated onto monocrystalline silicon and mesoporous anodic alumina. The optical bands ranging from 380 to 630 nm associated with 5D 4→ 7F j ( j=3, 4, 5, 6) transitions with a predominant band at 545 nm have been investigated over the temperature range of 10–300 K. Strong enhancement of terbium luminescence from the spin-on SR-350 films processed on porous anodic alumina is reported.

Strongly enhanced Tb luminescence from titania xerogel solids mesoscopically confined in porous anodic alumina

We report strongly enhanced Tb photoluminescence (PL) from a doped titania xerogel deposited on mesoporous anodic alumina. Transmission electron microscopy has confirmed the presence of the xerogel inside the pores of alumina films of up to several microns thickness. The optical bands attributed to D35→Fi7 (i=3,4,5), D45→Fj7 (j=3,4,5,6) transitions of Tb3+, with a predominant band at 2.28 eV, are investigated between 6 and 300 K. At all temperatures, the Tb PL increases and the bands become narrower with an increase in the Tb concentration in the xerogel, the number of deposited layers employed and the thickness of the porous alumina film.