J.R. Ramos-Barrado

Preparation and characterization of transparent ZnO thin films obtained by spray pyrolysis

b ´´ Abstract Structural, optical, chemical and electrical properties of thin films of ZnO obtained by spray pyrolysis over Pt or silica substrates are determined at temperature ranges between 223 and 373 K. The thin films are pure ZnO with a preferred crystalline orientation of ( 002 ). Grain-boundary barriers are created by the band bending and the mobility of charge carriers is limited by the thermal field emission of electrons at the grain-boundary barriers. The density of ionized acceptor atoms and the width of the space charge, both obtained from the capacitance-voltage (C-V) method, are consistent with the theory of the depletion layer in the Schottky barrier device. The density of states of defect, obtained from admittance spectroscopy, only presents a maximum at approximately 0.44 eV, whose position in the band gap does not change under bias. 2002 Elsevier Science B.V. All rights reserved.

Growth of pure ZnO thin films prepared by chemical spray pyrolysis on silicon

Structural, morphological, optical and electrical properties of ZnO thin films prepared by chemical spray pyrolysis fromzinc acetate (Zn(CH 3COO)2 2H2O) aqueous solutions, on polished Si(1 0 0), and fused silica substrates for optical characterization, have been studied in terms of deposition time and substrate temperature. The growth of the films present three regimes depending on the substrate temperature, with increasing, constant and decreasing growth rates at lower, middle, and higher-temperature ranges, respectively. Growth rate higher than 15 nm min � 1 can be achieved at Ts ¼ 543 K. ZnO film morphological and electrical properties have been related to these growth regimes. The films have been characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.15.Rs

Electrical conductivity in lithium orthophosphates

The electrical conductivity of lithium orthophosphates was investigated as a function of the nature of the transition-metal cation. LiMPO4 (M � /Mn, Co, Ni) compounds and their derivatives were investigated. The enhancement in ionic conductivity appeared for compounds containing manganese cations. A relationship between crystal structure and ionic conductivity was established and discussed.

Electrodeposition of ZnO thin films on n-Si(1 0 0)

In this study, ZnO thin films have been deposited onto monocrystalline n-type Si(1 0 0) by electrodeposition at different applied potentials. XRD shows a preferential orientation (0 0 0 2) that increases when the applied cathodic potential increases. The XPS analysis presents a Zn/O composition close to stoichiometric. SEM micrographs show a compact structure withlocalized platelets witha grain size of about 10 mm. However, crystallite size determined by the Scherrer method shows a size close to 2.50 � 10 � 2 mm, then the grains can be considered as clusters of crystallites. Optical measurements were made on samples deposited on ITO/glass through the same procedures giving a band gap of 3.3 eV in agreement withth e reported values for ZnO at room temperature. r 2001 Elsevier Science B.V. All rights reserved.

Electrochemical Growth of Diverse Iron Oxide (Fe3O4, α-FeOOH , and γ-FeOOH) Thin Films by Electrodeposition Potential Tuning

Magnetite, goethite, and lepidocrocite thin films have been electrochemically grown on titanium substrates by the anodic oxidation of ferrous ions in a 0.01 M FeSO 4 (NH 4 ) 2 SO 4 ·6H 2 O + 0.04 M CH 3 COOK, pH 6.0, aqueous solution. It is demonstrated that the deposition potential can be used as a tool to tune the obtainment of the different pure phases of the iron oxide-oxyhydroxides thin films. Results of an exhaustive structural characterization, a morphological study, and X-ray photoelectron spectroscopy characterization are presented.

XPS, SEM, EDX and EIS study of an electrochemically modified electrode surface of natural chalcocite (Cu2S)

Abstract An electrode surface of a natural chalcocite mineral obtained from the ‘Chuquicamata’ mine in Chile has been studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy including microanalysis (SEM/EDX). Different potentials were applied via CV to the electrode surface in a specially designed preparation chamber of inert (Ar gas) atmosphere. This chamber was coupled, in the case of XPS analysis, to the X-ray photoelectron spectrometer and, in the case of SEM/EDX analysis, to the electron microscope in order to study quasi-in-situ changes in the chemical composition and morphology at the electrode surface induced by the electrochemical treatment and to relate them to the EIS results obtained for the same applied potential. CV was performed in an aqueous 0.05 M borax electrolyte solution of pH 9.2 at 300 K (cycles: 0→−850 mV (cathodic sense: reduction)→+200 mV (anodic sense: oxidation)→−400 mV, all vs. SCE). Along the cyclic voltammogram, which shows two anodic peaks, ten points of applied potential (for 1 or 400 s) have been studied by XPS. Partial oxidation from Cu(I) to Cu(II) is observed at +100 mV versus SCE applied for 400 s which is assigned to the formation of CuO, Cu(OH) 2 and possibly Cu 3 (SO 4 )(OH) 4 . For comparison, the fractured mineral surface was also studied by XPS. The measurements of SEM and EDX did not show any relevant alterations, except for the potential of +100 mV versus SCE in the positive-going potential, in which the formation of protrusions with a high concentration of oxygen (44%) was detected. The modifications detected by XPS, SEM and EDX reflect large changes in the electrochemical parameters obtained using EIS and are characteristic of a partially covered electrode; the charge-transfer resistance across the irregularly formed layer on the surface increases for the point +100 mV versus SCE in the positive-going potential and its capacitance is reduced.

SEM, EDX and EIS study of an electrochemically modified electrode surface of natural enargite (Cu3AsS4)

Abstract Enargite electrodes, made from mineral originating in the ‘Salvador’ mine (Chanaral, Chile), have been studied using SEM, EDX, CV and EIS in an alkaline solution for a range of different potentials in the positive sweep direction (PSD) and negative sweep direction (NSD). The results of SEM and EDX have shown that the principal changes in morphology and chemical composition take place on the surface of the electrode for oxidation potentials close to the value that corresponds to the most intense peak in the PSD, where there is clearly formation of a layer of oxidised material with areas of morphology which are irregular and high in oxygen content. These results are in accordance with the findings from XPS studies published previously. The charge transfer resistance and the resistance of the anodic layer of the electrode were determined by EIS measurements; the values depend heavily on the applied potential in the PSD or NSD. These changes are related to the changes on the surface of the electrode studied by SEM and EDX

Proton conductivity of mesoporous MCM type of zirconium and titanium phosphates

Abstract A family of novel mesoporous zirconium(IV) and titanium(IV) phosphate materials has been prepared by sol-gel synthesis in the presence of cetyltrimethylammonium bromide, a structure-directing cationic surfactant. The solids obtained after removal of the surfactant by acid-ethanol extraction or by calcination have high BET surface areas of 240–330 m2g−1 and a narrow pore size distribution in the mesopore range. AC conductivity measurements of these solids were carried out by impedance spectroscopy. The data indicate that two electric relaxation processes are present; one at low frequency, associated with the grain boundary and one at higher frequency related to a relaxation within the pores. Although a partial contribution of electronic conduction cannot be excluded, the major contribution comes from proton mobility along the channels.

A study of the electrical behaviour of isolated tomato cuticular membranes and cutin by impedance spectroscopy measurements

Abstract Different isolated tomato fruit cuticular membranes (ripe and green tomato cuticles and the cutin of these membranes) were studied by impedance spectroscopy measurements when the membranes were in contact with NaCl solutions at different concentrations. Remarkable differences in the impedance plots and the equivalent circuits associated to each membrane sample were obtained: the ripe tomato cuticle and the cutin, only present a relaxation process, but for the green tomato cuticle two relaxation processes were obtained. Using the equivalent circuits as models, electrical and electrochemical parameters for each membrane were determined. These results permit us to assign the relaxation processes to the different components of the tomato membrane (polyester matrix, carbohydrates and pigments), obtaining in this way a detailed picture of the different environments of the plant interface. Variation with NaCl concentration for the different electrical parameters was also studied, and the electrical resistance of the biopolymer matrix was obtained.

Compositional, structural and electrical characterization of barium titanate thin films prepared on fused silica and Si(111) by spray pyrolysis

Barium titanate thin films have been prepared by chemical spray pyrolysis. Aqueous spray solutions of titanium isopropoxide and barium acetate, equimolar with respect to titanium and barium, were used as precursors. Deposition was carried out at a substrate temperature of 250°C. As-deposited films are amorphous. Films of BaTiO 3 (perovskite) were obtained by calcination in air to 600°C for various hours. Perovskite films are compact and smooth, according to their morphological aspect studied by scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) combined with 4 keV Ar + depth profiling has shown that the as-deposited film corresponds to BaTiO 3 and that its composition stays constant through the whole film thickness. Residual carbon (in the form of BaCO 3 ) coming from the organic precursor solutions is only detected at the surface of the film, limited to the first atomic monolayers as revealed by depth profiling. Calcination in air at 600°C decreases this amount but BaCO 3 is still present at a percentage of 25% with respect to the whole amount of Ba in the film surface. Electrical characterization of a calcined BaTiO 3 film deposited on conductive n-type Si(111) was carried out with impedance spectroscopy in a frequency range of 10 Hz to 1 MHz. Assuming a film thickness of 100 nm as deduced by XPS depth profiling, the relative dielectric permittivity of our films can be estimated to be 80 at a frequency of 1 kHz.