D. Leinen

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

Electrochemical properties of lead oxide films obtained by spray pyrolysis as negative electrodes for lithium secondary batteries

Lead(II) oxides in bulk and thin film form were assessed as electrodes for lithium rechargeable batteries. Films were prepared by spray pyrolysis of aqueous solutions of Pb(CH3–COO)2·2H2O and deposited onto lead substrates at 175°C. Films heated at 250°C were found to consist of well-crystallized tetragonal PbO and evolve to the orthorhombic polymorph with prolonged heating. Cycling of the cells at a current density of 0.25 mA/cm2 over the range 1.0–0.0 V led to the formation of various LiyPb alloys. Cells made from bulk oxides, whether tetragonal or orthorhombic, were found to exhibit poor performance (their capacity rapidly faded with cycling). By contrast, PbO film electrodes exhibited reversible capacity above 500 mA h/g beyond 40 cycles. The lead substrate must thus appreciably influence the electrochemical properties of the cell by facilitating adhesion of LiyPb microcrystals to its surface, thereby favoring alloying/de-alloying processes.

ZnO nanorod/CdS nanocrystal core/shell-type heterostructures for solar cell applications

ZnO/CdS core/shell nanorod arrays were fabricated by a two-step method. Single-crystalline ZnO nanorod arrays were first electrochemically grown on SnO(2):F (FTO) glass substrates. Then, CdS nanocrystals were deposited onto the ZnO nanorods, using the successive ion layer adsorption and reaction (SILAR) technique, to form core/shell nanocable architectures. Structural, morphological and optical properties of the nanorod heterojunctions were investigated. The results indicate that CdS single-crystalline domains with a mean diameter of about 7 nm are uniformly and conformally covered on the surface of the single-crystalline ZnO nanorods. ZnO absorption with a bandgap energy value of 3.30 ± 0.02 eV is present in all optical transmittance spectra. Another absorption edge close to 500 nm corresponding to CdS with bandgap energy values between 2.43 and 2.59 eV is observed. The dispersion in this value may originate in quantum confinement inside the nanocrystalline material. The appearance of both edges corresponds with the separation of ZnO and CdS phases and reveals the absorption increase due to CdS sensitizer. The photovoltaic performance of the resulting ZnO/CdS core/shell nanorod arrays has been investigated as solar cell photoanodes in a photoelectrochemical cell under white illumination. In comparison with bare ZnO nanorod arrays, a 13-fold enhancement in photoactivity was observed using the ZnO/CdS coaxial heterostructures.

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.

An XPS study of the mixing effects induced by ion bombardment in composite oxides

Abstract XPS has been used to study MxOy-TiO2 (MxOy: CeO2 and Al2O3) composite systems in powder form submitted to bombardment with ions (Ar+ and O+2) of 3.5 keV kinetic energy. In CeO2-TiO2 submitted to Ar+ bombardment it is found that cerium is reduced to Ce3+. This species remains stable against re-oxidation by exposure to O2, which contrasts with the behaviour of Ce3+ in partially reduced CeO2 where under similar conditions cerium is oxidized to Ce4+. Reduction to Ce3+ is also observed after O+2 bombardment which points to the fact that the cerium ions are mixed within the titanium oxide lattice and this is the reason for the stabilization of the Ce3+ state. In TiO2-Al2O3 the existence of mixing of the two components upon ion bombardment is shown by an accurate assessment of the photoelectron and Auger peaks of the spectra and the evaluation of the Auger parameters of the different elements. The use of XPS as a tool to show the existence of mixing in composite oxides after ion bombardment is discussed.

Single-Crystalline Silicon Nanowire Array-Based Photoelectrochemical Cells

Single-crystalline n-type silicon nanowire (n-SiNW) arrays have been synthesized by electroless metal deposition on a silicon wafer chip in an ionic Ag/HF solution through selective etching. The photoelectrochemical properties of the SiNW arrays were studied (under 40 mW cm -2 white illumination conditions) in an aqueous electrolyte [with Fe(CN) 4- 6 /Fe(CN) 3- 6 redox couple] and compared to those of its untreated bare polished Si sample counterpart (n-Si). The open-circuit photovoltage was lower, on average, for SiNWs array photoelectrodes, while short-circuit current density, fill factor, and overall energy conversion efficiency of the SiNWs array photoelectrodes were generally superior to those of the n-Si junction devices.

Electrodeposited Nanostructured α-Fe2O3 Photoanodes for Solar Water Splitting: Effect of Surface Co-Modification on Photoelectrochemical Performance

Photoelectrochemical response of electrodeposited hematite (a-Fe2O3) thin films, whose surface has been modified by a catalytic cobalt layer, has been studied. Structural and morphological characteristics of the resulting nanostructured hematite films have been studied by X-ray diffraction and scanning electron microscopy techniques. Surface characterization of the Co-modified hematite films has been carried out by X-ray photoelectron spectroscopy in order to study the chemical nature of Co onto these films. The hematite films exhibited an n-type behavior, and a donor carrier concentration, ND ¼3 � 10 17 and 10 18 cm � 3 for the un-modified and Co-modified ones, respectively. The photoelectrochemical performance of the un-modified and Co-modified electrochemically grown hematite thin films has been explored using a liquid-junction in a photoelectrochemical system in a 0.1 M NaOH electrolytic solution, under monochromatic illumination of wavelengths over the range 350‐600 nm. Significant performance gaining can be seen upon Co-modification throughout the illumination wavelengths. Cobalt modification enhances the IPCE by a factor of 1.1‐2.0 (370‐450 nm range), and by a factor of 2.0 (at 350 nm). With an IPCE of 46% at 370 nm at 1.43 V vs. RHE, this electrode is much more efficient in water photo-oxidation than the best published Fe2O3 single-crystal specimen.