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.
Highlights
Nanoporous alumina structures (NPASs) obtained by electrochemical anodization of aluminum foils according to the two-step anodization method [1,2] are of great interest due to their application as nanofilters, drug deliverers, templates for nanoparticles, nanotubes, nanowires, platforms for sensors, and medical devices [3,4,5,6,7,8,9]
The presence of non-characteristic sample elements, such as nitrogen and carbon, is associated to both environmental [44] and fabrication process contaminations, with the latter being the reason for sulphur and phosphorus presence; small percentages (≤0.6%) of other non-characteristic elements (N, Cr, . . . )
The analysis of optical and electrochemical measurements carried out with three nanoporous alumina structures (NPASs) with different average pore size, porosity, and symmetric/asymmetric structure (NPAS-Sf: 12 nm, 12%, symmetric; NPAS-Ph: 90 nm, 12%, symmetric; NPAS-And: 10 nm, 23% side (A) and 100 nm 30% size (B), asymmetric) allowed us to establish the effect of sample geometry on characteristic parameters such as the refraction index, extinction coefficient, ion transport number, and permselectivity, as well as interfacial effects, which are of interest in different nanostructure applications
Summary
Nanoporous alumina structures (NPASs) obtained by electrochemical anodization of aluminum foils according to the two-step anodization method [1,2] are of great interest due to their application as nanofilters, drug deliverers, templates for nanoparticles, nanotubes, nanowires, platforms for sensors, and medical devices [3,4,5,6,7,8,9]. Other applications of NPASs such as spintronics, solar cells, light emitting diodes, and photonic crystals have been studied [10,11,12,13,14,15,16]. Most of these applications are related to the high structural regularity of NPAS, which exhibit almost ideal cylindrical pores with narrow pore radius distribution and without tortuosity, but their thermal and chemical resistance are of great interest when used as nanofilters or membranes due to their stability under cleaning protocols commonly used to reduce fouling (adsorption/deposition of transported molecules or particles), which is. The two-step anodization method creates samples with a wide range of pore radii (rp ranging between 10 nm and 200 nm) and interpore distances
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