Yttrium barium copper oxide thin films on yttria-stabilized zirconia: growth and properties

Y Ba Cu oxide thin films were grown epitaxially on single cryst. yttria-stabilized zirconia substrates by laser deposition. [on SciFinder(R)]

Microstructure of epitaxial YBa2Cu3O7-δ thin films on (001)-tilt Y-ZrO2 bicrystals

The microstructures of the grain boundaries in epitaxial YBa2Cu3O7-δ thin films grown on [001]-tilt yttria-stabilized ZrO2 bicrystal substrates were characterized by TEM and at. force microscopy. The exact boundary plane geometries of the bicrystal substrates were not transferred to the films which instead had wiggling grain boundaries. [on SciFinder(R)]

Very thin yttrium barium copper oxide films made by coevaporation

Superconducting YBa2Cu3O7 thin films with various thicknesses from 100 Å to 5000 Å were deposited on (100) SrTiO3 substrates with std. BaF2 coevaporation process. The films had crit. temps. of up to 93 K. The best crit. currents were 1 × 106 A/cm2 at 77 K and 3 × 107 A/cm2 at 4.2 K. The crit. current was generally higher for thinner films. Two different etching methods were used to pattern the films for jc measurements: Ar ion etching and EDTA wet etching. The wet etching was found to work well for thicker films (>1000 Å). For the thinner films, the ion etching process was preferred because of the reduced film surface degrdn. [on SciFinder(R)]

Formation of polyaniline/Pt nanoparticle composite films and their electrocatalytic properties

Polyaniline (PANI) thin films modified with platinum nanoparticles have been prepared by several methods, characterised and assessed in terms of electrocatalytic properties. These composite materials have been prepared by the in situ reduction of a platinum salt (K2PtCl4) by PANI, in a variety of solvents, resulting in the formation of platinum nanoparticles and clusters of different sizes. The further deposition of platinum clusters at spin cast thin films of PANI/Pt composites from a neutral aqueous solution of K2PtCl4 has also been demonstrated. Thin-film electrodes prepared from these materials have been investigated for their electrocatalytic activity by studying hydrazine oxidation and dichromate reduction. The properties of the composite materials have been determined using UV–visible spectroscopy, atomic force microscopy and transmission electron microscopy. The nature of the material formed is strongly dependent on the solvent used to dissolve PANI, the method of preparation of the PANI/Pt solution and the composition of the spin cast thin film before subsequent deposition of platinum from the aqueous solution of K2PtCl4.

Shape dependent electrocatalytic behaviour of silver nanoparticles

The electrochemical and electrocatalytic behaviour of silver nanoprisms, nanospheres and nanocubes of comparable size in an alkaline medium have been investigated to ascertain the shape dependent behaviour of silver nanoparticles, which are an extensively studied nanomaterial. The nanomaterials were synthesised using chemical methods and characterised with UV-visible spectroscopy, transmission electron microscopy and X-ray diffraction. The nanomaterials were immobilised on a substrate glassy carbon electrode and characterised by cyclic voltammetry for their surface oxide electrochemistry. The electrocatalytic oxidation of hydrazine and formaldehyde and the reduction of hydrogen peroxide were studied by performing cyclic voltammetric and chronoamperometric experiments for both the nanomaterials and a smooth polycrystalline macrosized silver electrode. In all cases the nanomaterials showed enhanced electrocatalytic activity over the macro-silver electrode. Significantly, the silver nanoprisms that are rich in hcp lamellar defects showed greater activity than nanospheres and nanocubes for all reactions studied.

Bacterial kinetics-controlled shape-directed biosynthesis of silver nanoplates using Morganella psychrotolerans

We show for the first time that by controlling the growth kinetics of Morganella psychrotolerans, a silver-resistant psychrophilic bacterium, the shape anisotropy of silver nanoparticles can be achieved. This is particularly important considering that there has been no report that demonstrates a control over shape of Ag nanoparticles by controlling the growth kinetics of bacteria during biological synthesis. Additionally, we have for the first time performed electrochemistry experiments on bacterial cells after exposing them to Ag(+) ions, which provide significant new insights about mechanistic aspects of Ag reduction by bacteria. The possibility to achieve nanoparticle shape control by using a "green" biosynthesis approach is expected to open up new exciting avenues for eco-friendly, large-scale, and economically viable shape-controlled synthesis of nanomaterials.

An investigation of silver electrodeposition from ionic liquids : influence of atmospheric water uptake on the silver electrodeposition mechanism and film morphology

The electrodeposition of silver from two ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]) and N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide ([C4mPyr][TFSI]), and an aqueous KNO3 solution on a glassy carbon electrode was undertaken. It was found by cyclic voltammetry that the electrodeposition of silver proceeds through nucleation–growth kinetics. Analysis of chronoamperometric data indicated that the nucleation–growth mechanism is instantaneous at all potentials in the case of [BMIm][BF4] and [C4mPyr][TFSI], and instantaneous at low overpotentials tending to progressive at high overpotentials for KNO3. Significantly, under ambient conditions, the silver electrodeposition mechanism changes to progressive nucleation and growth in [C4mPyr][TFSI], which is attributed to the uptake of atmospheric water in the IL. It was found that these differences in the growth mechanism impact significantly on the morphology of the resultant electrodeposit which is characterised ex situ by scanning electron microscopy and X-ray diffraction.

The active site behaviour of electrochemically synthesised gold nanomaterials

Even though gold is the noblest of metals, a weak chemisorber and is regarded as being quite inert, it demonstrates significant electrocatalytic activity in its nanostructured form. It is demonstrated here that nanostructured and even evaporated thin films of gold are covered with active sites which are responsible for such activity. The identification of these sites is demonstrated with conventional electrochemical techniques such as cyclic voltammetry as well as a large amplitude Fourier transformed alternating current (FT-ac) method under acidic and alkaline conditions. The latter technique is beneficial in determining if an electrode process is either Faradaic or capacitive in nature. The observed behaviour is analogous to that observed for activated gold electrodes whose surfaces have been severely disrupted by cathodic polarisation in the hydrogen evolution region. It is shown that significant electrochemical oxidation responses occur at discrete potential values well below that for the formation of the compact monolayer oxide of bulk gold and are attributed to the facile oxidation of surface active sites. Several electrocatalytic reactions are explored in which the onset potential is determined by the presence of such sites on the surface. Significantly, the facile oxidation of active sites is used to drive the electroless deposition of metals such as platinum, palladium and silver from their aqueous salts on the surface of gold nanostructures. The resultant surface decoration of gold with secondary metal nanoparticles not only indicates regions on the surface which are rich in active sites but also provides a method to form interesting bimetallic surfaces.

The facile formation of silver dendritic structures in the absence of surfactants and their electrochemical and SERS properties

In this work a simple approach to the creation of highly dispersed electrocatalytically active silver microstructured dendrites on indium tin oxide in the absence of any surface modification or surfactant is presented. It is found that the addition of low concentrations of supporting electrolyte to the AgNO3 solution dramatically influences the morphology of electrodeposited silver which is independent of both the anion and the cation employed. The silver dendrites are characterized by SEM, XRD, XPS as well as by cyclic voltammetry under alkaline conditions. It is found that the surface oxide formation and removal processes are significantly influenced by the microstructured morphology of the silver electrodeposits compared to a smooth macrosized silver electrode. The facile formation of dendritic silver microstructures is also shown to be beneficial for the electrocatalytic oxidation of both formaldehyde and hydrazine and oxygen reduction. The formation of a continuous film of dendritic silver is also investigated for its SERS activity where the connectivity between the individual dendrites is found to be particularly important.

Electrodeposited α- and β-phase MoO3 films and investigation of their gasochromic properties

α- and β-Phase MoO3 are synthesized using an electrodeposition method on fluorine-doped tin oxide (FTO) glass substrates from sodium-molybdate (Na2MoO4) solutions. We show that it is possible to obtain both α- and β-MoO3 by manipulating the cyclic voltammetry (CV) parameters during electrodeposition. Raman spectroscopy, X-ray diffraction, and scanning electron microscopy indicate that the applied potential range and sweep rate are strongly influential on the phase obtained and the surface morphology of the electrodeposited thin films. Gasochromic measurements were carried out on the annealed samples by exposing them to H2 gas. It was revealed that α-MoO3 thin films provided better response to H2 interaction than β-MoO3 films did. Additionally, porous films provided significantly larger responses than smooth films.

A study of the galvanic replacement reaction at surfaces and the role of lateral charge propagation

The galvanic replacement of isolated nanostructures of copper and silver on conducting supports as well as continuous films of copper with gold is reported. The surface morphology was characterized by scanning electron microscopy and the replacement with gold was confirmed by EDX analysis. It was found that lateral charge propagation during the replacement reaction had a significant effect in all cases. For the isolated nanostructures the deposition of gold was observed not only at the sacrificial template but also at the surrounding unmodified areas of the conducting substrate. In the case of copper films the role of lateral charge propagation was also confirmed by connecting it to an ITO electrode through an external circuit upon which gold deposition was also observed to occur. Interestingly, by inhibiting the rate of charge propagation, through the introduction of a series resistor, the morphology of gold on the copper substrate could be changed from discrete surface decoration with cube like nanoparticles to a more porous rough surface.

Reusable surface confined semi-conducting metal-TCNQ and metal-TCNQF4 catalysts for electron transfer reactions

The synthesis of organic semiconducting materials based on silver and copper-TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) and their fluorinated analogues has received a significant amount of attention due to their potential use in organic electronic applications. However, there is a scarcity in the identification of different applications for which these interesting materials may be suitable candidates. In this work, we address this by investigating the catalytic properties of such materials for the electron transfer reaction between ferricyanide and thiosulphate ions in aqueous solution, which to date has been almost solely limited to metallic nanomaterials. Significantly it was found that all the materials investigated, namely CuTCNQ, AgTCNQ, CuTCNQF4 and AgTCNQF4, were catalytically active and, interestingly, the fluorinated analogues were superior. AgTCNQF4 demonstrated the highest activity and was tested for its stability and re-usability for up to 50 cycles without degradation in performance. The catalytic reaction was monitored via UV-vis spectroscopy and open circuit potential versus time measurements, as well as an investigation of the transport properties of the films via electrochemical impedance spectroscopy. It is suggested that morphology and bulk conductivity are not the limiting factors, but rather the balance between the accumulated surface charge from electron injection via thiosulphate ions on the catalyst surface and transfer to the ferricyanide ions which controls the reaction rate. The facile fabrication of re-usable surface confined organic materials that are catalytically active may have important uses for many more electron transfer reactions.

Engineering electrodeposited ZnO films and their memristive switching performance

We report the influence of zinc oxide (ZnO) seed layers on the performance of ZnO-based memristive devices fabricated using an electrodeposition approach. The memristive element is based on a sandwich structure using Ag and Pt electrodes. The ZnO seed layer is employed to tune the morphology of the electrodeposited ZnO films in order to increase the grain boundary density as well as construct highly ordered arrangements of grain boundaries. Additionally, the seed layer also assists in optimizing the concentration of oxygen vacancies in the films. The fabricated devices exhibit memristive switching behaviour with symmetrical and asymmetrical hysteresis loops in the absence and presence of ZnO seed layers, respectively. A modest concentration of oxygen vacancy in electrodeposited ZnO films as well as an increase in the ordered arrangement of grain boundaries leads to higher switching ratios in Ag/ZnO/Pt devices.