Porosity of anodic alumina membranes from electrochemical measurements

A procedure based on the high-field mechanism of the growth of anodic oxides was developed in order to evaluate the morphological features of porous layers. Since the thickness of the barrier film, separating the porous layer from the metal, does not change during the steady-state growth of an anodic porous layer, the rate of displacement of the metal-oxide interface to the metal direction must be equal to the rate of displacement of the pore base to the oxide direction. As a consequence, porosity can be expressed in terms of the ratio i _diss/i _ion, where i _diss is the dissolution current density at the pore base, and i _ion is the ionic current density at the metal-oxide interface. Pore diameter can be determined from geometrical considerations, while average pore population can be obtained from the ratio of porosity to the average surface area of a single pore. This procedure was checked by comparison with experimental results relative to membranes prepared in various conditions. The satisfactory agreement between theoretical and experimental findings indicates that porosity can be evaluated by current density data and vice-versa. Therefore, anodic alumina membranes may be tailored for different applications by choosing operative conditions giving the desired value of i _diss/i _ion.     

Interface physicochemical processes controlling sulphate anion incorporation in porous anodic alumina and their dependence on the thermodynamic and transport properties of cations

Aluminium was anodised in H₂SO₄, LiHSO₄, NaHSO₄, KHSO₄, Mg(HSO₄)₂ and Al(HSO₄)₃ electrolytes. The kinetics of growth of porous anodic alumina films and of the pore wall oxide dissolution during anodisation was studied. Based on the derived kinetic parameters, suitable physicochemical processes in the barrier layer electrolyte interface controlling the anion incorporation in the barrier layer were suggested and relevant models were formulated. According to these processes Al³⁺ and H⁺ ions are rejected from the pore base surface in the attached double layer, where Al³⁺ ions are solvated, and are transferred to the pore filling solution. The strongly different mobilities of Al³⁺ and H⁺ and the necessary space negative charge density distribution in the double layer result in similar concentration distributions of Al³⁺ and anions inside it, which differ strongly from that of H⁺. These Al³⁺ and anion concentrations increase with decreasing mobility of the main cations in the solution which depends on their hydration enthalpy and transport mechanism. The concentration of incorporated anions inside both a thin surface layer of the barrier layer and the double layer vary similarly. For identical surface density and base diameter of pores the decrease of the above mobility reinforces anion incorporation.     

Investigation of the incorporation of electrolyte anions in porous anodic Al2O3 films by employing a suitable probe catalytic reaction

A new method has been developed capable of describing the incorporation of electrolyte anions along the pore wall surface and across both the barrier layer and the pore wall oxide after the establishment of the steady state of growth of porous anodic Al₂O₃ where other methods cannot be applied to obtain reliable results. The knowledge of the nature/composition of anodic oxides as regards the incorporation of species like electrolyte anions is of specific importance for both the understanding of the electrochemical mechanism of oxide production and growth and the scientific and technological applications of porous anodic Al₂O₃ films. The method consists of the selection and use of a suitable catalytic probe reaction on porous anodic oxides at thicknesses varying from a value near zero up to the maximum limiting thickness and the treatment of the experimental reaction rate results by a properly developed mathematical formalism. This method was employed in anodic Al₂O₃ films prepared in H₂SO₄ anodizing electrolyte at a constant bath temperature and different current densities using as a probe reaction the decomposition of HCOOH on these oxides, which is almost exclusively a dehydration reaction, at relatively high reaction temperatures, 350 °C and 390 °C, where the effect of other species except SO₄ ²⁻ incorporated in the oxide on the reaction rate is eliminated. It has been shown that the fraction of the intercrystallite surfaces occupied by SO₄ ²⁻ follows a parabola-like distribution. It has a significant value at the pore base surface, depending on the current density, then it passes through a maximum along the pore wall surface and across both the barrier layer and the pore walls near the pore bases at positions depending on the current density and then becomes almost zero at the mouths of the pores of the oxide with the maximum limiting thickness and at both the Al₂O₃/Al interface and cell boundaries. The maximum value of the surface coverage is almost independent of the current density and is always near 1, showing an almost complete saturation of intercrystalline surfaces at these positions. The above distribution of surface coverage predicts a qualitatively similar distribution of the SO₄ ²⁻ bulk concentration across both the barrier layer and pore wall oxide around the pore bases. The method may be improved and developed further either for a more detailed investigation of the above films or to investigate films prepared in other pore-forming electrolytes. Received: 30 July 1998 / Accepted: 30 September 1998     

Adsorption of potential-determining ions on porous glasses of different compositions

The adsorption characteristics are studied for nano- and ultraporous glasses (PGs) produced from sodium borosilicate glasses and a glass containing small amounts of fluoride ions and phosphorus oxide by acid (HCl) leaching and additional alkaline (KOH) and thermal treatment. The surface charges σ₀ of PGs are determined by continuous potentiometric titration in 10⁻³−1 M NaCl, KCl, and (C₂H₅)₄NCl solutions. Only negative surface charges of PGs are observed for all investigated systems. The |σ₀| value is predetermined by the following factors: the composition of PG, the pore radius in the nanometer region (r ≤ 13 nm), the specificity of counterions, the content of secondary silica in the pore space, and the temperature of the additional thermal treatment of the membranes. The introduction of fluoride ions and phosphorus oxide into sodium borosilicate glass, an increase in the pore sizes and the amount of the secondary silica in PGs, and a rise in the specificity of counterions enhance the |σ₀| values, which decrease with a rise in the temperature of the thermal treatment due to the surface dehydration and dehydroxylation. For ultraporous glasses (r > 13 nm), the surface charge is almost independent of the pore radius.     

Fabrication of anodic photocurrent generation systems by use of 6-O-dihydrophytylcellulose as a matrix or a scaffold of porphyrins

Langmuir-Blodgett (LB) films containing porphyrin molecules were fabricated by use of 6-O-dihydrophytylcellulose (DHPC) toward anodic photocurrent generation systems. To suppress the porphyrin aggregation, two different approaches were applied: (1) mixing a low-molecular-weight porphyrin having a diterpenoid carbon skeleton (DPor) with DHPC as a matrix (matrix fabrication) and (2) bonding porphyrin molecules to the hydroxyl groups of DHPC covalently, converting into 6-O-dihydrophytyl-2,3-di-O-[p-(10,15,20-triphenyl-5-porphyrinyl)-benzoyl]cellulose as a scaffold (scaffold fabrication). The structure and film properties of the monolayers and the LB films were investigated by the surface pressure (π)–area (A) isotherm measurements, atomic force microscopy, UV–Vis spectroscopy, and absorption dichroism measurements. The porphyrin aggregation in the LB film could be well suppressed only by the scaffold fabrication, leading to the improvement of the photocurrent quantum yields. The efficient photocurrent performance can be demonstrated by the isolation and the parallel orientation of porphyrin moieties due to the cellulose rigid scaffold. This paper was the subject of the Best Poster Award of the 235th edition of the ACS National Meeting, Cellulose and Renewable Materials.     

Low-voltage anodized TiO<Subscript>2</Subscript> nanostructures studied by alternate current electrochemical microscopy and photoelectrochemical measurements

This paper presents the characterization of TiO₂ nanostructures obtained by low-voltage anodization using alternate current electrochemical microscopy (AC-SECM) and photoelectrochemical (PEC) measurements. TiO₂ nanostructures were obtained from the exposure of titanium foils to several aqueous acidic solutions of hydrofluoric acid + phosphoric acid at potentials of 1 to 3 V. Scanning electron microscopy, X ray diffraction, and atomic force microscopy studies evidence the formation of a thin porous amorphous layer (<600 nm) with pore size in the range of 200–1,000 nm. By AC-SECM studies at different bias, we were able to confirm the unambiguous semiconducting properties of as-obtained porous titania films, as well as differences in surface roughness and conductivity in specimens obtained at both potentials. The difference in conductivity persists in air annealed samples, as demonstrated by electrochemical impedance spectroscopy and PEC measurements. Specimens obtained at 3 V show lower photocurrent and dark current than those obtained at 1 V, regardless of their larger conductivity, and we proposed it is due to differences on the oxide layer formed at the pore bottom.     

Fabrication of ideally ordered anodic porous TiO2 by anodization of pretextured two-layered metals

Ideally ordered anodic porous TiO₂ was fabricated by anodizing an Al/Ti layered specimen. A two-layered specimen composed of an Al top layer and a Ti underlying layer was prepared and then processed by nanoimprinting. The Al top layer was easily pretextured by nanoimprinting owing to its softness and it was straightforward to introduce an ideally ordered pore arrangement by anodization. This pore arrangement was transferred to the underlying Ti layer, resulting in ideally ordered porous structures in TiO₂. This process can be applied to the high-throughput fabrication of ideally ordered anodic porous oxides other than TiO₂ and also to other metals with high hardness.     

Study of the electrosynthesis of poly(3-butylthiophene) and its semiconductor properties

The mechanism of the electrosynthesis of poly(3-butylthiophene) (PBuT) was studied by cyclic voltammetry and potential step methods in comparison with polybithienyl. The anodic oxidation polymerization of the 3-butylthiophene underwent two steps: oligomer formation and further polymerization to form the polymer. The doping level of the PBuT increases with the cycle number of the potential sweeps during polymerization. The current responses to the potential steps indicate a nucleation and nuclei growth process which is repeated layer to layer. The differential capacity (C _d) and photocurrent were measured at the PBuT films in the aqueous electrolyte solution. The C _d ⁻² vs. E plot shows two regions of linearity, one with a negative slope and the other with a positive slope in different potential regions, which give the same flat-band potential. This indicates that the PBuT film exhibits both p-type and n-type features of a semiconductor at differrent potential regions. The cathodic photocurrent spectrum was analysed by the (j _ph hν)^{2/ n} vs. hν plots, giving band gap energies of 2.41 eV for n=1 and and 2.01 eV for n=4. Received: 29 July 1999 / Accepted: 15 November 1999     

Lithium Incorporation into Titanium Dioxide Films from a Propylene Carbonate Solution: A Photoelectrochemical Study

Photoelectrochemical behavior of thin-film TiO₂ electrodes produced by chemical and electrochemical oxidation of a titanium substrate is studied at potentials corresponding to the domain of active incorporation/extraction of lithium in an LiClO₄ solution in propylene carbonate (1.0 V <; E <; 2.5 V vs. Li/Li⁺) and the inert domain (2.6 V <; E <; 3.6 V). Spectral and current–voltage characteristics are obtained for the photocurrent of TiO₂- and Li _x TiO₂-electrodes, which show n-semiconductor properties, in particular, generate an anodic (hole) photocurrent under illumination. The disappearance of the anodic photocurrent and the onset of a small cathodic photocurrent correlate with the beginning of active incorporation of lithium into the source oxide. The photosensitivity of intercalate Li _x TiO₂ in the studied domain of spectrum is low and decreases with increasing lithium concentration. A photoeffect is discovered in the long-wave domain of spectrum beyond the limits of intrinsic absorption of a solid. The photocurrent is described by a characteristic exponential spectral curve.     

Photophysical and Photoelectric Properties of 2—{[4—（N—Hexadecyl—N—methylamino）phenyl]methylene}—propanedinitrile

2-{[4-(N-Hexadecyl-N-methylamino)phenyl]methylene}-propanedinitrile(HMAPN) with typical donor-π-acceptor(D-π-A）structure was synthesized.It could be easily assembled into stable films by LB technique.The photophysical properties of HMAPN were investigated in solution and on LB films.The photoelectric properties of HMAPN were examined and the anodic photocurrent of the ITO electrode modified by the monolayer LB film of HMAPN was measured as 835 nA/cm^2 under the white light of 218.2 mW/cm^2 without bias voltage.The effects of light intensity,bias voltage on the photocurrent were discussed.The possible mechanism of the photocurrent formation was given.     

Effect of Sb on the properties of anodic Pb(Ⅱ) oxide film formed in alkaline solution

The composition and properties of the anodic films formed on Pb and Pb-3at.%Sb alloy at -0.10 V (vs. Hg/HgO) for 2.5 h in 0.1 mol.dm-3 NaOH solution (25℃) were investigated by cyclic voltammetry, linear sweep voltammetry, open circuit decay curve, photocurrent technique, X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the anodic film formed oh Pb mainly consists of t-PbO, while that on Pb-3at.%Sb consists of o-PbO, t-PbO and a small amount of orthorhombic Sb2O3. The dominant component of the film anodically grown on Pb-3at.%Sb for less than 5 min is o-PbO, however, t-PbO is the major component of the anodic film formed for 1 h or longer. It is established that Sb suppresses the growth of t-PbO. The anodic film formed on Pb-3at.%Sb is less porous than that on Pb. The bandgap energies of t-PbO and o-PbO in the films were determined by photocurrent measurements to be 1.83-1.84 eV and 2.60 eV, respectively.     

Discovery by kinetic studies of the latent physicochemical processes and their mechanisms during the growth of porous anodic alumina films in sulfate electrolytes

The kinetics of growth of porous anodic alumina films in pure H₂SO₄, in mixtures of H₂SO₄ and Al₂(SO₄)₃ and in Al(HSO₄)₃, NaHSO₄ and KHSO₄ electrolytes were studied. The latent physicochemical processes at the pore base surface/electrolyte interface, across the barrier layer, inside the metal/oxide interface and at the pore wall surface/electrolyte interface and their mechanisms were revealed. High field strength equations were formulated describing the ionic migrations from the pore base surface. These showed that, at constant current density and temperature, the inverse of the pore base square diameter depends linearly on the inverse of the H⁺ activity in the anodizing solution and that this diameter increases with H⁺ activity, in agreement with the experimental results. The mechanism of electrolyte anion incorporation inside the barrier layer and the real distribution of the anion concentration across both the barrier layer and pore walls were deduced. The effects of the different kinds and concentrations of the electrolyte anions and cations on both the above processes and their mechanisms were also examined. Electronic Publication     

Solid surface and field catalysed interface formation of colloidal Al<Subscript>2</Subscript>(SO<Subscript>4</Subscript>)<Subscript>3</Subscript> during Al anodizing affecting the kinetics and mechanism of development and structure of porous oxides

Overall kinetic and potentiometric studies of the growth of porous anodic alumina films in saturated H₂SO₄+Al₂(SO₄)₃ electrolyte showed non-saturation conditions inside the pores and supersaturation conditions at the pore surface/electrolyte interface where the field and the solid surface catalyse the formation of colloidal Al₂(SO₄)₃ micelles. Suitable high-strength field thermodynamically sustained electrochemical and chemical kinetic equations were formulated. It was shown that the diameter and surface fraction of charge exchange at the pore bases, the real pore wall surface fraction where oxide dissolution occurs, and its rate are strongly affected by the conditions. The mechanism of growth and structure of the films are quite different from those in H₂SO₄. A mechanism of regular film growth is imposed and the critical current density, above which pitting appears, strongly increases. The formulated theory may predict improved or new Al anodizing technologies. Electronic Publication     

Deposition and characterisation of a porous Sn(IV) semiconductor nanofilm on boron-doped diamond

Nanofilm deposits of a porous Sn(IV) oxide are formed by anodic electrodeposition on a polished boron-doped diamond electrode immersed in an aqueous Sn²⁺ solution. Mechanically and electrochemically stable deposits of 10–15 nm thickness are formed irrespective of the Sn²⁺ concentration and mass-transport enhancement by power ultrasound. Atomic force microscopy images indicate the presence of a smooth and noncrystalline film, which is stable under ambient conditions. n-type semiconducting characteristics are observed for the aqueous solution redox couples Fe(CN)₆ ^3–/4– and Ru(NH₃)₆ ^3+/2+. However, preliminary results from voltammetric experiments indicate that the small and neutral organic molecule N,N,N′,N′-tetramethylphenylenediamine is able to diffuse through the porous film to undergo oxidation directly at the surface of the boron-doped diamond electrode. Electronic Publication     

Porous structure of the catalyst layers of electrodes in a proton-exchange membrane fuel cell: A stage-by-stage study

Porous structure is studied by standard contact porosimetry after each stage in the preparation of a catalyst layer, which contains a carbon substrate (CS), an ionomer in the form of Nafion resin, and a platinum catalyst. The influence of the ionomer on the porous structure of ten different CS is investigated. The structure of these samples is studied over the maximum range of their pore radii r ∼ 0.3–10⁵ nm. Pores of main volume within particles of the CS under investigation are mainly distributed over the maximum range of their radii from r ≤ 1 to ∼ 50 nm. Ionomer introduction into all the CS under investigation leads to an increase in the integral porosity due to the porosity of the intergranular structure. The change in porosity of the intragranular structure is caused by ionomer blocking small pores in the CS. In most CS, ionomer blocks pores of different sizes, from micropores with radii r ≤ 1 nm and up to r ∼ 1000 nm. It is concluded that the extent of blockage of CS pores is largely determined by the surface properties of the CS and Nafion resin and, more precisely, by the difference in resin adhesion to the CS surface because of the presence of different surface groups on the CS surface. When platinum is applied to CS, this leads to an increase in the specific volume of the micropores. The smallest dimensions of platinum particles are estimated to be on the order of 1 nm.     

Hydrophobic and Physical Properties of the Two Step Processed Ambient Pressure Dried Silica Aerogels with Various Exchanging Solvents

The experimental results by using various exchanging solvents in the preparation of two step (acid and base) processed ambient pressure dried hydrophobic silica aerogels, are reported. Silica alcogels were prepared by hydrolysis with oxalic acid and condensation with NH₄OH of ethanol diluted tetraethylorthosilicate (TEOS) precursor and hexamethyldisilazane(HMDZ) methylating agent. The exchanging solvents used were: hexane, cyclohexane, heptane, benzene, toluene and xylene. The physical properties such as % of volume shrinkage, density, pore volume, % of porosity, thermal conductivity, % of optical transmission, surface area, pore size distribution and contact angle (θ) of the silica aerogels with water, were measured as a function of EtOH/TEOS molar ratios (R) for all the exchanging solvents. It was found that the physical and hydrophobic properties of the silica aerogels strongly depend on the nature of the solvent and R. Heptane solvent resulted in highly transparent (≈90% optical transmission at 700 nm for 1 cm thick sample), low density (≈0.060 g/cm³), low thermal conductive (≈0.070 W/m·K), high % of porosity (97%), high surface area (750 m²/g), uniform porosity and hydrophobic (θ ≈ 160°) aerogels compared to other solvents. On the otherhand, xylene resulted in aerogels with higher hydrophobicity (θ ≈ 172°) among other solvents.     

Formation and characterization of phosphate-modified silicate materials derived from sol–gel process

Phosphate-containing silicate materials prepared using sol–gel method from Si(OC₂H₅) were investigated at the variation of the amount of phosphate modifier from 5 to 50 wt% in term of P₂O₅. Chemical composition, textural and structural properties of these materials were characterized by FTIR-spectroscopy, TEM, X-ray diffraction and nitrogen adsorption. It was shown that the materials posse monomodal pore size distribution of 5–20 nm for the samples dried at 100 °C and 40–60 nm for the specimens calcined at 600 °C. The mean pore size and surface area depended on the amount of phosphoric acid. Before the stage of high temperature treatment phosphoric acid, introduced into the structure of the materials as a modifying agent, was uniformly distributed inside a porous space of the material and was not chemically bonded with silicate. After high temperature treatment both chemical interaction of silicate with phosphate, providing the formation of silicate-phosphate structures, as well as redistribution of free modifier from the bulk of granules to their surface took place. The polyphosphate layer is formed on the material surface closing the internal porous space. However, in this case a part of the phosphate modifier remains chemically unbound to SiO₂ structure.     

Solubilization of styrene in the catanionic system dodecyltrimethylammonium hydroxide–n′-dodecanephosphonic acid

 The solubilization of styrene in micelles of the catanionic surfactant dodecyltrimethylammonium hydroxide (DTAOH)–n-dodecane-phosphonic acid (DPA) was studied by UV–Vis. spectrometry, as a function of the DTAOH:DPA proportion in the surfactant mixture. The styrene molecules are adsorbed at the surface of the micelles, with the vinyl group closer to the hydrocarbon core than the aromatic ring, which is oriented to the water. In micelles with an excess of DTAOH, the dielectric constant of the water surrounding the micelles was strongly affected by the non-neutralized –N(CH₃)⁺ ₃ groups at the Stem layer. In micelles with an excess of DPA, the –PO₃H₂ groups which are not neutralized by –N(CH₃)⁺ ₃, remain almost unionized and hydrogen-bonded. The effect of the micellar surface on the surrounding water dielectric constant dropped sharply. The dielectric constant in the hydrogen-bonded polar layer is ∼65, rising to the value of pure water very close to the micellar surface. Received: 2 September 1997 Accepted: 20 October 1997     

Formation process of indium oxide transparent conducting films via thermal decomposition

Indium 2-ethylhexanoate monohydroxide, In(OH)(O₂CCH(CH₂CH₃)(CH₂)₃CH₃)₂, is a precursory material to fabricate In₂O₃-based transparent conducting films by dip-coating process. Formation process of indium oxide transparent conducting films was investigated using an ultra-low acceleration voltage FE-SEM. The nanostructure change of the precursory layer was observed during the electron beam irradiation in vacuum. A flat and homogeneous surface of the as-coated layer changed to porous and net-work like nanostructure after 80 s; the pore diameter increased and the pore distance decreased although the number of pores remained unchanged. These processes were interpreted as the preliminary step to form porous films composed of nm-sized inter-linked oxide particles as reported in the previous papers by the authors.