Subsequent Thermal Reduction Research Articles

Vertical ZnO nanowires/graphene hybrids for transparent and flexible field emission

We present a transparent and flexible optoelectronic material composed of vertically aligned ZnO NWs grown on reduced graphene/PDMS substrates. Large-area reduced graphene films were prepared on PDMS substrates by chemical exfoliation from natural graphitevia oxidative aqueous dispersion and subsequent thermal reduction. ZnO NWs were hydrothermally grown on the reduced graphene film substrate and maintained their structural uniformity even in highly deformed states. The electrical contact between semiconducting ZnO NWs and the metallic graphene film was straightforwardly measured by electric force microscopy (EFM). It shows a typical metal–semiconductor ohmic contact without a contact barrier. Owing to the mechanical flexibility, transparency, and low contact barrier, the ZnO NWs/graphene hybrids show excellent field emission properties. Low turn-on field values of 2.0 V μm−1, 2.4 V μm−1, and 2.8 V μm−1 were measured for convex, flat, and concave deformations, respectively. Such variation of field emission properties were attributed to the modification of ZnO NWs emitter density upon mechanical deformation.

ALD-grown seed layers for electrochemical copper deposition integrated with different diffusion barrier systems

The deposition of Cu seed layers for electrochemical Cu deposition (ECD) via atomic layer deposition (ALD) of copper oxide and subsequent thermal reduction at temperatures between 110 and 120 °C was studied on different diffusion barrier systems. While optimization of the process is required on TaN with respect to reduction and plating, promising results were obtained on blanket PVD Ru. The plating results on layers of ALD Cu with underlying Ru even outperformed the ones achieved on PVD Cu seed layers with respect to morphology and resistivity. Applying the processes to via and line patterns gave similar results, suggesting that a combination of ALD Cu with PVD or ALD-grown Ru could significantly improve the ECD Cu growth.

Ultra-large single-layer graphene obtained from solution chemical reduction and its electrical properties

Graphene is a promising candidate for making next-generation nanoelectronic devices. Developing methods to produce large sized graphene with high yield is the key for graphene applications. Here, we report a simple method for large-scale production of ultra-large single-layer graphene sheet (up to 50 microm) reduced from graphene oxides by hydrazine in the presence of aromatic tetrasodium 1,3,6,8-pyrenetetrasulfonic acid (TPA) which efficiently disperse the resulting graphene sheet in aqueous solutions. Field-effect transistors can be readily fabricated using such large reduced graphene oxide sheets. It was found that the mobility of the reduced graphene oxide increases with the temperature of subsequent thermal reduction and reaches 3.5 cm(2) V(-1) s(-1) after reduction at 1000 degrees C. Such solution-processable method is of great potential in printable fabrication of graphene-based devices.

Deposition and electrochemical activity of Pt-based bimetallic nanocatalysts on carbon nanotube electrodes

This article reports an approach to prepare bimetallic Pt–M (M = Fe, Co, and Ni) nanoparticles as electrocatalysts and examines their electrochemical activities in 1 M sulfuric acid. The approach consists of chemical oxidation of carbon nanotubes (CNTs), two-step refluxing, and subsequent thermal reduction in hydrogen atmosphere. Three bimetallic pairs of Pt–M catalysts are found to deposit well onto CNT surface, forming Pt–M/CNT composites. The electrochemical behavior of Pt–M/CNT electrodes was investigated in 1 M H 2SO 4 using cyclic voltammetry (CV) and ac electrochemical impedance spectroscopy. The active surface coverage (=electrochemical surface area/geometric surface area) of Pt–M catalysts is significantly enhanced, i.e., Pt–Co (85.1%) > Pt–Ni (80.4%) > Pt–Fe (76.2%) > Pt (26.3%). This enhancement of electrochemical activity can be attributed to the fact that the introduction of Co and Ni may reduce the required potential for water electrolysis and thus the associated carbon oxidation, thereby contributing to hydrogen adsorption. Equivalent circuit analysis indicates that charge transfer resistance accounts for (i) the major proportion of the equivalent serial resistance of Pt–M/CNT electrodes, and (ii) Pt–Co and Pt–Ni catalysts not only improves the electrochemical capacitance but also lowers the equivalent serial resistance. The results shed some light on how use of Pt–M/CNT composite would be a promising electrocatalyst for high-performance fuel cell applications.

Efficient fabrication of transparent antimicrobial poly(vinyl alcohol) thin films

We have explored in situ synthesis of Ag nanoparticles in transparent PVA films in view of increasing areas of application of those films. The two-step procedure consists of ion incorporation in the matrix and subsequent thermal reduction. Smooth and transparent PVA films containing Ag nanoparticles of 5–20 nm were fabricated by this approach. The optical property of the films and the size of metal nanoparticles could be controlled by changing the reaction conditions. By increasing heating temperature, the absorbance and wavelength of surface plasmon resonance (SPR) of the composite film increased, and nanoparticles with larger particle sizes and broader size distributions were obtained. In the temperature range of 130–170 °C, the wavelength of SPR increased with increasing the AgNO3 concentration. At 190 °C, however, the wavelenght of SPR blue-shifted initially when the AgNO3 concentration increased from 10 to 80 mmol/L, and red-shifted thereafter. The composite films showed excellent antimicrobial performance toward bacteria such as Escherchia coli. Such hybrids afford very effective and environment-friendly antimicrobial surface coatings.

Preparation of Platinum−Nafion−Carbon Black Nanocomposites via a Supercritical Fluid Route as Electrocatalysts for Proton Exchange Membrane Fuel Cells

Novel platinum−Nafion−carbon black nanocomposites were synthesized using supercritical deposition. The method consists of preparation of a Nafion-carbon black composite followed by impregnation of a platinum precursor (PtMe2COD) into the composite via adsorption from a solution of supercritical carbon dioxide. Subsequently, the impregnated composite is subjected to thermal treatment resulting in a platinum−Nafion−carbon black composite. The platinum−Nafion−carbon black composite was characterized by hydrogen chemisorption, and the average size of the platinum metal particles was found to be around 2.6 nm. A preliminary cyclic voltammetry test demonstrated that the composite displayed promising electroactivity as an electrocatalyst for proton exchange membrane fuel cells. To investigate the effects of the impregnation under supercritical conditions at high pressures and subsequent thermal reduction at elevated temperatures on the Nafion structure, composite films of platinum−Nafion 112 were prepared by the same supercritical deposition method. The infrared spectra of the films indicated that the structure of Nafion 112 did not change significantly during processing.

Metallized poly(etherether ketone) films achieved by supercritical fluid impregnation of a silver precursor followed by thermal curing

Reflective PEEK films with silver surfaces were prepared by supercritical fluid impregnation with CO 2 as the carrier and subsequent thermal reduction of (1,5-cyclooctadiene-1,1,1,5,5,5-hexafluoroacetylacetonato)silver(I) dimer, [Ag · (COD)HFA] 2. No chemical reducing agent was needed to effect silver reduction. Reflectivity of the surface was dependent on the impregnation and cure conditions. At constant CO 2 pressure and impregnation time, the reflectivity of the surface increased as the impregnation temperature increased. At constant temperature and CO 2 pressure, the reflectivity increased as the impregnation time increased. Thermal cure parameters at fixed impregnation conditions suggested that lower cure temperatures required longer cure times in order to obtain high reflectivity. Silver particles <10 nm in diameter appeared in the film bulk near the surface. Different sized silver clusters on the surface could be grown by controlling the impregnation and cure conditions. The silver on the surface strongly adhered to the polymer surface.

New Chelate Complexes of Copper and Iron: Synthesis and Impregnation into a Polymer Matrix from Solution in Supercritical Carbon Dioxide

New chelate complexes of copper diiminate and iron diiminate were synthesized. Their physical characteristics have been studied. The diffusion of supercritical carbon dioxide (scCO2) into polyarylate (PAR) films and their impregnation with diiminates were investigated. The equilibrium degree of PAR swelling in scCO2 is about 10%. The conditions of impregnation (45 °C, 8 MPa, 1.5 h) were determined. The impregnation was confirmed by the FT-IR spectroscopic data. Thermal reduction of metal ions was investigated. Transformations of the chelate complexes on the polymer-film surface were studied by ESCA. The value of the Auger factor indicates that, after thermal reduction, the copper ion is in a nearly univalent state. The copper content in films is as large as 6.3 wt %; the iron content is 4.5 wt %. The FT-IR and ESCA spectroscopic studies showed that, in the course of impregnation, chelate complexes interact with functional groups of the polymer. Subsequent thermal reduction of metal in air results in diiminate evaporation and thermooxidative degradation with ligand decomposition and enrichment of the surface with metal atoms. According to the SAXS data, the size distribution of metal-containing particles ranges from 20 to 60 nm with a maximum at 34 nm.

Electronic structure of anatase TiO2 oxide

Photoemission spectromicroscopy was used to investigate the electronic structure of TiO2 anatase single crystals and polycrystalline thin films. The stoichiometry and the degree of oxidation of as-grown crystals, as-deposited films, as well as of thermally annealed samples in different atmospheres, were analyzed, based on the Ti 2p and O 1s core levels, with an energy resolution of 0.4 eV. The experimental density of states (DOS) was found to be in agreement with the theoretical DOS reported in the literature for anatase crystals, and shows some characteristics similar to the experimental DOS reported for rutile crystals. In reduced samples, the experimental DOS is characterized by intense emission in the region of O 2p bonding orbitals, and does not exhibit an appreciable density of states in the band gap. As-grown crystals exhibit small band gap emission (a few percent of the valence band VB signal) at about 0.8 eV, which is attributed to Ti3+ (3d) defect states. Annealing the crystals at high temperatures in O2 or subsequent thermal reduction in an Ar–H2 mixture (95%–5%) produces nearly stoichiometric surfaces with smaller or undetectable density of Ti3+ states. In addition, some redistribution of the spectral weight is observed in the VB spectra.