Patrizia Bocchetta

Asymmetric alumina membranes electrochemically formed in oxalic acid solution

Alumina membranes were fabricated by anodizing aluminium metal in 0.15 M oxalic acid. The growth kinetics of the porous layer were investigated in the temperature range −1 to 16 °C using linear potential scans up to 70 V. The faradaic efficiencies of metal oxidation and of porous layer formation, determined by applying Faradays law, were found to be independent of both temperature and electrical charge. SEM analysis of the metal-side and solution-side surfaces revealed different morphologies. After dissolution of the barrier layer in phosphoric acid, the metal-side surface showed circular pores whose size of about 90 nm was found to be uniform and independent of temperature. The pore population was also practically independent of temperature and a value of about 4 × 1013 pores m−2 was determined. On the solution-side surface the presence of a deposit partially occluding the mouths of pores was observed. This coating could be removed by chemical etching in NaOH or thermal treatment at 870 °C, where decomposition of oxalate occurs. This supports the hypothesis that the deposit consists of an aluminium salt containing oxalate anions precipitated from the solution. The results show that it is possible to control the morphological characteristics of the anodic alumina membranes by careful choice of experimental conditions.

Fabrication of a Sealed Electrochemical Microcell for in Situ Soft X-ray Microspectroscopy and Testing with in Situ Co-Polypyrrole Composite Electrodeposition for Pt-Free Oxygen Electrocatalysis

In this paper we report on the fabrication and testing of a novel concept of sealed electrochemical microcell for in situ soft X-ray microspectroscopy in transmission, dedicated for nonvacuum compatible electrolytes. The microcell, fabricated using ultraviolet lithography, at variance with previous versions of electrochemical wet cells, that featured an optical window glued on top of the electrode system and a very limited electrolyte volume, the device presented here is a single solid block based around a microfabricated channel with fixed optical windows and apt for microfluidic work. Moreover, this cell allows to employ an advanced electrodic geometry developed in our group - so far used only in open electrochemical cells for work with vacuum-compatible electrolytes - also with low-vapor pressure liquids, possibly saturated with the required gases. The cell optimal electrode design allows three-electrode electrochemical control typical of traditional electrochemical experiments. The first electrochemical experiments with this new cell explore the electrochemical growth of a Co-polypyrrole, a composite electrocatalyst material with promising performance to replace the expensive Pt catalyst in fuel-cell oxygen electrodes. Morphological and chemical-state distributions of Co codeposited with polypyrrole has been followed as a function of time and position, yielding unprecedented information on the processes relevant to the synthesis of this catalyst.

Electrochemical Fabrication of Metal/Oxide/Conducting Polymer Junction

After discovery of conducting polymers and the possibility to modify their electrical properties from insulating to metallic like behavior by doping and a careful choice of the processing conditions, a large amount of research effort has been devoted to the theoretical understanding of their solid state properties as well as to exploit the possible application of conducting polymers in many technological fields including large area organic electronics, polymer photovoltaic cell, and sensors. 1-4 Organic thin film transistors appear very promising devices for the development of low cost, flexible, and disposable plastic electronics. In order to reduce the operating voltage it has been suggested in the literature to use mixed inorganic–organic thin film transistors by assembling a structure formed by metal bottom contact/dielectric layer gate/organic semiconductor/top contact source/drain. According to this, a wet electrochemical route appears to be very promising in terms of cost, at least for the preparation of thin thickness 10 nm or thick thickness 10 nm oxide films by anodizing in aqueous electrolytes. Moreover, by taking into account the possibility to grow semiconducting polymers on wide bandgap dielectric oxide Ta2O5 by a photoelectrochemical route, which has been shown recently, 5,6 it seems very appealing to exploit an integral electrochemical route to fabricate advanced inorganic/organic hybrid structure which could be used as a building block for a field effect transistor FET junction. In this paper, we describe and discuss the electrochemical fabrication of a hybrid structure to be used in the production of an inorganic–organic field effect transistor IOFET using 3,4polyethylenedioxythiophene PEDOT as a semiconducting polymer and anodic films grown on the Ti-10 atom % Zr alloy as dielectrics. The choice of the oxide is based on its low dark current value and quite high photocurrent intensity, under monochromatic light, at not too high anodic potential and photon energy, 7 as well as on its high dielectric permittivity 45, according to Ref. 8. The metal/oxide/ polymer junctions are investigated by photocurrent spectroscopy PCS and scanning electron microscopy SEM. Finally, output transistor characteristics are recorded in order to test the performance of the junctions in the IOFET structure. Experimental Ti-10 atom % Zr alloys were prepared by dc magnetron sputtering. Targets consisted of 99.9% zirconium disk, of 100 mm diameter, with an appropriate number of 99.9% titanium disks, of 20 mm diameter, located symmetrically on the erosion region for the preparation of the alloys. Substrates used were glass plates. 8 The composition of the alloy was determined by Rutherford backscattering

Quasi-in-situ single-grain photoelectron microspectroscopy of Co/PPy nanocomposites under oxygen reduction reaction.

This paper reports an investigation into the aging of pyrolyzed cobalt/polypyrrole (Co/PPy) oxygen reduction reaction (ORR) electrocatalysts, based on quasi-in-situ photoelectron microspectroscopy. The catalyst precursor was prepared by potentiostatic reverse-pulse coelectrodeposition from an acetonitrile solution on graphite. Accelerated aging was obtained by quasi-in-situ voltammetric cycling in an acidic electrolyte. Using photoelectron imaging and microspectroscopy of single Co/PPy grains at a resolution of 100 nm, we tracked the ORR-induced changes in the morphology and chemical state of the pristine material, consisting of uniformly distributed ∼20 nm nanoparticles, initially consisting of a mixture of Co(II) and Co(III) oxidation states in almost equal amounts. The evolution of the Co 2p, O 1s, and N 1s spectra revealed that the main effects of aging are a gradual loss of the Co present at the surface and the reduction of Co(III) to Co(II), accompanied by the emergence and growth of a N 1s signal, corresponding to electrocatalytically active C-N sites.

Electrosynthesis of Ce–Co Mixed Oxide Nanotubes with High Aspect Ratio and Tunable Composition

Cerium oxide and cobalt oxides have attracted the interest of several researchers due to their potential application in several technological fields electrochromism, lithium batteries, catalysis, etc.. Ceria has been used as a promoter in the so-called “three-way catalyst” for the control of toxic emission from automobile exhaust. The promotion consists of the enhancement of the noble metal dispersion, as well as stabilization of the supporting medium toward thermal sintering. 1,2 A direct catalytic effect of CeO2 in chemical processes such as water–gas shift reaction or NOx decomposition has been also evidenced. 3,4

Photoelectrochemical Synthesis of Polypyrrole on Anodic Ta2O5 Films

Polypyrrole film was photoelectrochemically grown on insulating Ta 2 O 5 anodic film in acetonitrile solution. A characterization by photocurrent spectroscopy (PCS) of metal/oxide/polypyrrole interface was carried out. The PCS results suggest that a metallic-like PPy is formed under illumination at constant anodic potential. By polarizing the polypyrrole at cathodic potentials a photocurrent spectrum typical of p-type semiconducting film was recorded. A scanning electron microscopy study of PPy surfaces solution side and oxide side allowed us to obtain information on the morphology of the polymer as well as a rough estimate of the film thickness and of the diameter of PPy globules at the two interfaces.

In situ soft x-ray fluorescence and absorption microspectroscopy: A study of Mn-Co/polypyrrole electrodeposition

This paper reports the development and application of a novel thin-layer electrochemical microcell for in situ soft x-ray fluorescence and absorption microspectroscopy. The microcell, fabricated using ultraviolet lithography, is an improved version of concepts previously developed in our group, featuring a wide optical window that allows the extension of the range of accessible in situ microspectroscopy and imaging methods, including those requiring small emission take-off angles. The three-electrode design implemented in the cell enables optimal electrochemical control. The first in situ experiment employing this new cell explores the electrochemical growth of a novel Mn-Co/polypyrrole composite that is a prospective electrocatalyst for Pt replacement in air cathodes. Morphological, compositional, and chemical-state distributions of Mn and Co codeposited with polypyrrole are subsequently performed in situ as a function of time and position, yielding otherwise unachievable information regarding the electr...

Cerium Oxyhydroxide Nanowire Growth via Electrogeneration of Base in Nonaqueous Electrolytes

The preparation of compact cerium oxyhydroxide nanowires into anodic alumina membranes from cerium chloride in ethanol solution via electrogeneration of base is reported. Scanning electron microscopy analyses indicate that dense, well-aligned, and highly ordered nanowires can be formed in a wide range of applied potentials and current densities in alcoholic solution. The employment of water brings to a hybrid nanotube/nanowire structure, suggesting a key role of the electrolyte nature in determining the morphology of the deposit. Electrochemical findings and X-ray diffraction analysis have shown that nanowires are constituted by a Ce(III)/Ce(IV) oxyhydroxide that can be completely oxidized into CeO 2 by thermal treatment.

Electrodeposition and pyrolysis of Mn/polypyrrole nanocomposites: a study based on soft X-ray absorption, fluorescence and photoelectron microspectroscopies

Electrodeposition of manganese/polypyrrole (Mn/PPy) nanocomposites has been recently shown to be a technologically relevant synthesis method for the fabrication of Oxygen Reduction Reaction (ORR) electrocatalysts. In this study we have grown such composites with a potentiostatic anodic/cathodic pulse-plating procedure and characterised them by a multi-technique approach, combining a suite of in situ and ex situ spectroscopic methods with electrochemical measurements. We have thus achieved a sound degree of molecular-level understanding of the hybrid co-electrodeposition process consisting of electropolymerisation of polypyrrole with incorporation of Mn. By in situ Raman spectroscopy we followed the formation of MnOx and the polymer by monitoring the build-up and development of the relevant vibrational bands. The compositional and chemical-state distribution of the as-deposited material has been investigated ex situ by soft X-ray fluorescence (XRF) mapping and micro-absorption spectroscopy (micro-XAS). XRF shows that the spatial distribution of Mn is consistent in a rather wide range of current densities (c.d.s), while micro-XAS reveals a mixture of Mn valencies, with higher oxidation states prevailing at higher c.d.s. Pyrolysis of electrodeposits, desirable for obtaining more durable and active catalysts, has been followed in situ by photoelectron microspectroscopy, allowing to assess the evolution of: (i) the electrodeposit morphology, resulting in a uniform distribution of nanoparticles; (ii) the chemical state of manganese, changing from a mixture of valences to a final state consisting of Mn(III) and Mn(IV) oxides and (iii) the bonding nature of nitrogen, from initially N-pyrrolic to a combination of pyridinic and Mn–N/graphitic.

In Situ and Ex Situ X-Ray Microspectroelectrochemical Methods for the Study of Zinc–Air Batteries

Electrical energy storage based on Zn–air concepts is experiencing increasing interest for applications ranging from consumer electronics to automotive and grid storage, owing to their high energy density, intrinsic safety, environmental friendliness, and low cost. Their implementation is nevertheless daunted by several materials science riddles, affecting the actually available power density and durability. In this scenario, in operando dynamic physicochemical information at length scales between mesoscopic and nanometric is highly desirable for knowledge-based advancements. This overview summarizes recent contributions of in situ and quasi- in situ X-ray methods – absorption and fluorescence microspectroscopies and microtomography – to studies of cathodes, anodes, and model cells.