Optical and Electrochemical Characterization of Nanoporous Alumina Structures: Pore Size, Porosity, and Structure Effect

Abstract

Three nanoporous alumina structures (NPASs) obtained by the two-step anodization method were optically and electrochemically characterized. Two of the structures were symmetric (NPAS-Sf and NPAS-Ph) and one was asymmetric (NPAS-And); pore size ranged from 10 nm to 100 nm and porosity was 12% in the case of the symmetrical NPAS and 23% and 30% for each surface of the asymmetric structure NPAS-And(A) and (B), respectively. Optical parameters of the studied samples (refraction index and extinction coefficient) were obtained from ellypsometric spectroscopy measurements carried out for wavelengths ranging between 250 nm and 1700 nm (visible and near infrared regions), with the total average refraction indices being 1.54, 1.52, 1.14, and 1.05 for NPAS-Sf, NPAS-Ph, NPAS-And(A), and NPAS-And(B), respectively, which indicates porosity control of refraction index values. Electrochemical characterizations (concentration potential and impedance spectroscopy measurements) were performed with NaCl solutions, and they allowed us to estimate samples of effective fixed charge concentration (1.22 × 10−2 M, 1.13 × 10−3 M, and 1.15 × 10−3 M), ion transport numbers, permselectivity (33.0%, 3.1%, and 9.6%), and the electrical resistance of each solution/sample system as well as the interfacial effects associated to solution concentration–polarization, which seems to be mainly controlled by pore size and sample symmetry.

Nanoporous Alumina Support Covered by Imidazole Moiety–Based Ionic Liquids: Optical Characterization and Application

“…Average values of the refractive index determined for the three IL/NPAS for both optical regions (visible and nir) are indicated in Table 4 . As it can be observed, the values of the refractive index obtained for the IL/NPAS samples do not differ significantly from the tabulated values previously indicated in Table 1 (4% increase considering the visible region) and following the same sequential order; however, these values are significantly higher (~28%) than those determined for the NPAS support (<n visible > = 1.277 ± 0.03 and <n nir > = 0.82 ± 0.23 [ 39 ]), which can be taken as a confirmation of adequate surface coverage by the corresponding IL. In this context, it should be indicated that the tabulated values of the ILs refractive index are usually determined at a given wavelength (or discrete number of wavelengths) for the visible region, covering these results a wider range of wavelengths.…”

“…Information on optical parameters of the studied samples was obtained from spectroscopic ellipsometry (SE) and light transmission/reflection measurements, which are two non-destructive techniques used commonly for the characterization of layered and/or modified thin films [ 63 , 64 ]. Figure 4 shows the wavelength dependence of the refractive index (n) determined for the IL/NPAS samples, which were obtained from spectroscopic ellipsometry (SE) experimental parameters (angles Ψ and Δ) taking into account the ellipsometer software and the variation of tanΨ and cosΔ with wavelength for the three IL/NPAS is presented as Supplementary Information (Figure S4) ; for comparison, n values for the NPAS [ 39 ] are also indicated in Figure 4 . Average values of the refractive index determined for the three IL/NPAS for both optical regions (visible and nir) are indicated in Table 4 .…”

“…A commercial asymmetric nanoporous alumina structure (NPAS) from Whatman (Anodisc TM , Sigma-Aldrich, Barcelona, Spain) was used as support. Characteristic NPAS geometrical parameters are 60 μm thickness, 10 nm nominal pore radius for one surface (surface A) and 100 nm nominal pore radius for the other surface (surface B), and porosity around 23% (surface A) or 35% (surface B) [ 39 ]. A certain amount of each IL was plugged on the denser surface (A) of the NPAS for its coverage and these samples are hereafter named as BMIMPF 6 /NPAS, OMIMPF 6 /NPAS, and EMIMBF 4 /NPAS.…”

Surface Modification of Nanoporous Anodic Alumina during Self-Catalytic Atomic Layer Deposition of Silicon Dioxide from (3-Aminopropyl)Triethoxysilane

“…Optical techniques are of great interest in the characterization of materials due to their non-invasive and non-destructive character. In particular, spectroscopic ellipsometry has already demonstrated their suitability for characterization of nanoporous alumina membranes with different pore size, porosity or structure [ 43 ], but also for biosensors characterization [ 44 ], while light transmission spectra allows estimation of differences in both geometrical parameters (pore-size/porosity) and surface material of analyzed samples [ 8 , 12 ]. Moreover, a particular property of nanoporous alumina structures (NPASs) obtained by the two-step anodization method such as their potoluminescence (PL) character, which is associated to the ionized oxygen vacancies and the carboxylate from the electrolyte solution incorporated into the NPAS during the fabrication process has also been reported [ 45 , 46 , 47 ].…”

New, Fully Implantable Device for Selective Clearance of CSF-Target Molecules: Proof of Concept in a Murine Model of Alzheimer’s Disease

“…The device was miniaturized to adapt it to the size of mice and to manufacture in biocompatible materials. It consists of two main components, a subcutaneous reservoir and an apheresis module endowed with tailored NPMBs [ 33 , 34 ], which were encapsulated together in a capsule made by Neuroscience Innovative Technologies ( Figure 1 A, part A). For this study, NPMBs have a pore size of 9 ± 2 nm.…”