Co-Assembly of Nanoparticles in Evaporating Aerosol Droplets: Preparation of Nanoporous Pt/TiO2 Composite Particles

Nanoporous Pt/TiO₂ micro-particles were synthesized via an aerosol assisted co-assembly (AACA) route. Aerosol droplets were produced from a colloidal mixture of 5 nm Pt and 20 nm TiO₂ nanoparticles, which formed spherical micro-aggregates of Pt and TiO₂ with average diameter of around 1.2 μm. The resulting composite micro-particles have very open structure with pore sizes ranging from 20 to 200 nm. Pt nanoparticles were found to be well dispersed on the surface of the supporting TiO₂ particles. Electrocatalytic application of the nanoporous Pt/TiO₂ composites was examined through methanol oxidation reaction. The performance of 20 wt% Pt/TiO₂ particles was found to be comparable to that of commercial 20 wt% Pt/carbon black catalyst.     

Structural and electrochemical features of 3D nanoporous platinum electrodes

The morphologies, roughness factors, and thicknesses of 3D nanoporous Pt (3D-npPt) films were investigated in terms of electroplating conditions. The electrochemical behaviors of 3D-npPt films with regard to electrochemical glucose oxidation, O₂ reduction, and H₂O₂ reduction were investigated as a function of roughness factors (Rf). Close comparison of glucose oxidation on 3D-npPt and 1D nanoporous Pt (1D-npPt) showed that the overall electrode activity of 3D-npPt is significantly higher than that of 1D-npPt. Electrochemical impedance analysis based on transmission line theory confirmed a substantially low pore resistance of 3D-npPt, which may account for the superior electrode response of this material.     

Finely dispersed Pt nanoparticles in conducting poly(o-anisidine) nanofibrillar matrix as electrocatalytic material

A new approach based on stepwise oxidation of o-anisidine is explored for generating nanoporous films of poly(o-anisidine), POA and loading of Pt nanoparticles that are subsequently used for electrocatalysis of methanol oxidation are presented and compared with bulk Pt. POA film can easily be prepared by stepwise electro-oxidation procedure with very high porosity consisting of nanofibrillar structure using without templates. Controlled sizes of Pt nanoparticles were entrapped into POA matrix by a two-step process in which first PtCl₆²⁻ ions are sorbed into the pores of polymer matrix followed by electroreduction at −0.55 V in a 0.5 M H₂SO₄ solution. Loading of Pt nanoparticles (10-200 μg/cm²) onto POA matrix were accomplished by varying the concentration (2-10 mM) of the sorbate, i.e., H₂PtCl₆. The sizes of the Pt nanoparticles were determined from TEM analysis and Pt particles were found to be in the range of 10-20 nm. The crystallite phase of Pt particles in POA was examined from XRD pattern. AFM image further supports Pt particles embedded in POA matrix.     

Conductometric discrimination of electro-inactive metal ions using nanoporous electrodes

We investigated conductometric analysis of electro-inactive metal ions at high concentration based on nanoporous electrodes by electrochemical impedance spectroscopy (EIS). The three dimensional interconnected nanoporous Pt (L₂-ePt) was found to enable significantly sensitive and selective conductometric detection of alkali and alkaline earth metal ions of high concentration at low frequency without any additional surface modification, which can be hardly done by flat Pt. The extremely large surface area of L₂-ePt remarkably suppressed the electrode impedance and the pore effect was additional positive contribution to selective ion sensing by conductometry at low frequency. Importantly, L₂-ePt allowed recognition of fractional ratio reversal of Na⁺ to K⁺ ions in a mixed solution at physiological concentration maintaining the constant total ionic strength. The results suggest the possibility of real time extracellular monitoring of instantaneous ion exchange near ion channels of a cell membrane such as action potential propagation along axons in a neuronal system.     

Formation of nanoporous oxide layer over a binary β-phase titanium in simulated body fluid

In the present investigation, the surface of Ti-15Mo (??-Ti) alloy was oxidized using hydrogen peroxide (H₂O₂) and the layer was densified by thermal treatment. Morphological characterization of treated surface by Field Emission Scanning Electron Microscope (FE-SEM) revealed the formation of nanoporous layer. Electrochemical studies of H₂O₂ treated specimen exhibited higher corrosion resistance in simulated body fluid (SBF) solution compared to untreated ??-Ti alloy. Further, the formation of nanoporous layer and their electrical components were evinced from impedance studies by fitting it to an circuit model.     

Electrochemical preparation of oligo(azulene) on nanoporous TiO<Subscript>2</Subscript> and characterization of the composite layer

A nanoporous oligo(azulene)-TiO₂ (OAz-TiO₂) composite layer was formed by electrochemical oxidation of azulene (Az) on a nanoporous ITO/TiO₂ electrode. Polymerization was performed in tetrabutylammonium hexafluorophosphate electrolyte salt dissolved in acetonitrile. The electrochemical and optical properties of the composite layer were studied by cyclic voltammetry (CV) and in situ UV–vis spectroelectrochemistry. The chemical and crystalline structure of the layer was studied by FTIR and X-ray diffraction (XRD) spectroscopy techniques and the morphology by Scanning Electron Microscopy. The TiO₂ layer was found to have a catalytic activity on the polymerization of Az. The CV experiments in monomer-free electrolyte solution demonstrated the electron donor property of OAz by its p-doping and the electron accepting property of TiO₂ by the large reduction current in the negative potential region. The FTIR and XRD spectroscopic measurements showed the well-defined anatase structure of TiO₂ with inclusion of OAz. A composite layer was formed rather than a bilayer structure. The in situ UV–vis spectroelectrochemical measurements gave evidence of a higher delocalization and easier movement of the π-electrons in the composite layer than in pure poly (azulene).     

Phase and Texture Evolution in Chemically Derived PZT Thin Films on Pt Substrates

The crystallization of lead zirconate titanate (PZT) thin films was evaluated on two different platinum‐coated Si substrates. One substrate consisted of a Pt coating on a Ti adhesion layer, whereas the other consisted of a Pt coating on a TiO₂ adhesion layer. The Pt deposited on TiO₂ exhibited a higher degree of preferred orientation than the Pt deposited on Ti (as measured by the Full Width at Half Maximum of the 111 peak about the sample normal). PZT thin films with a nominal Zr/Ti ratio of 52/48 were deposited on the substrates using the inverted mixing order (IMO) route. Phase and texture evolution of the thin films were monitored during crystallization using in situ X‐ray diffraction at a synchrotron source. The intensity of the Pt₃Pb phase indicated that deposition on a highly oriented Pt/TiO₂ substrate resulted in less diffusion of Pb into the substrate relative to films deposited on Pt/Ti. There was also no evidence of the pyrochlore phase influencing texture evolution. The results suggest that PZT nucleates directly on Pt, which explains the observation of a more highly oriented 111 texture of PZT on the Pt/TiO₂ substrate than on the Pt/Ti substrate.     

Can a Single Atom Serve as the Active Site in Some Heterogeneous Catalysts?

We examine a number of distinct situations relating to heterogeneous catalysts where either a single atom (or ion), or a very small cluster of atoms functions as the locus of chemical turnover in various distinct kinds of conversion. There is little doubt that individual ions at certain crystallographic sites in nanoporous solids can indeed act as single-site catalysts. The situation concerning nanoclusters of pure metal (or bimetallic entities) is rather more ambiguous. What was hitherto thought to be an effective catalyst made up of a small cluster of Pt supported on ??-Al₂O₃ (for hydrogenation) now seems to be a single atom of Pt attached to a 5-coordinated Al^III ion. And in the case of Au or Pt on other supports, there is evidence that a single Pt atom, positively charged, but surrounded by alkali-metal ions, is a powerful catalyst for the water?Cgas shift (CO + H₂O ?? H₂ + CO₂) reaction. We also report interesting results concerning the mobility of CeO₂ support material.     

Activity of oxygen reduction reaction on small amount of amorphous CeOx promoted Pt cathode for fuel cell application

Pt on ceria (CeO_x) particles supported on carbon black (CB) were synthesized using the combined process of hot precipitation and impregnation methods. During 30 cycles of cyclic voltammetry pre-treatment in the potential ranging from −0.2 to 1.3 V (V vs. Ag/AgCl), it was observed that a small amount of CeO_x, which consisted of the interface region between Pt and CeO_x, remained on Pt particles. Other free CeO_x particles were dissolved into H₂SO₄ aqueous solution. To develop the Pt-CeO_x/CB catalyst, the surface chemical states, the net chemical composition, morphology and electrochemical behavior in H₂SO₄ aqueous solution were characterized. Our microanalysis and electrochemical analysis indicate that the active CeO₂ with high specific surface area provides the continuous amorphous cerium oxide (Ce³⁺, Ce⁴⁺) layer with pores on the surface of Pt particles. It is concluded that the amorphous cerium oxide layer on Pt inhibits the oxidation of Pt surface and contributes to enhancement of the activity on Pt cathode. The single cell performance was also improved using the Pt-CeO_x/CB cathode. Based on all data, it is expected that the design based on characterization of the interface between Pt and small amount of amorphous cerium oxide layer could help in preparation of more active Pt catalyst.     

Use of Double Wash-Coatings of Platinum and Zeolite Catalysts to Improve Selective Reduction of NOx by Hydrocarbons

The selective catalytic reduction by hydrocarbons (HC-SCR) of NO _x under lean conditions has been improved by the use of double-layered catalysts with a lower layer of Pt/SiO₂ and an upper layer of a zeolite such as H-, Ce-, and Cu-ferrierite (-FER). H-FER wash-coated over Pt/SiO₂ (H-FER//Pt/SiO₂) performed best among the samples examined. The promotional effect was attributed to the synergy of the oxidation catalyst (Pt/SiO₂) in converting NO into NO₂, which is more reactive to C₃H₆, and the HC-SCR catalyst (H-FER). Cu-FER//Pt/SiO₂ was also effective at widening the temperature window, but with this combination the performance was attributed to a simple summation of the activity of two HC-SCR catalysts that were active at different temperatures.     

The photoelectrochemical properties of N3 sensitized CaTiO3 modified TiO2 nanocrystalline electrodes

The preparation of nanoporous TiO₂ electrodes modified with CaTiO₃ layers and their application in dye-sensitized solar cells (DSSCs) were reported. The as-prepared TiO₂/CaTiO₃ electrodes were characterized by XPS and XRD, indicating that a thin CaTiO₃ layer was formed on the surface of nanoporous TiO₂ electrodes. Compared with bare TiO₂ electrodes, CaTiO₃ modified TiO₂ electrodes presented more dye adsorption. Moreover spectroelectrochemical studies showed that the concentration of free electrons in the conduction band of TiO₂ was remarkably increased after surface modification. As a result, the photocurrent and photoelectric conversion efficiency of the modified electrodes were increased. The influence of the thickness of CaTiO₃ layer on the photoelectrochemical properties of the modified electrodes was investigated. Experiment results showed that proper thickness of the modification layer is crucial to the photoelectrochemical properties of modified electrodes. The highest conversion efficiency reaches 9.23% under irradiation of 100 mW cm⁻² white light, obtained with the electrode TiO₂/CaTiO₃(45 min), a 34% increase than that of bare TiO₂ electrodes.     

Kinetic analysis of the hydrogen oxidation reaction at Nafion film covered Pt-black rotating disk electrodes

The kinetics of the H₂ oxidation reaction at Nafion film covered Pt-black rotating disk electrodes (RDEs) in 0.5 M H₂SO₄ at 298 K was investigated by varying the Pt loading, Nafion film thickness, and rotating rate. The equation describing the H₂ oxidation kinetics at an RDE with a Nafion film covered porous Pt layer was derived, assuming a Tafel-Volmer mechanism and taking into account the mass transfer resistances in the aqueous electrolyte, Nafion film, and Pt layer. The H₂ oxidation reaction at the Pt layer was proved to be reversible and the measurable current density was determined entirely by the mass transfer of H₂ in the aqueous electrolyte and the Nafion film; the apparent kinetic current density measured was due to the experimental error. More accurate results of kinetic analysis were obtained in this work than our results reported previously.     

Effects of Bismuth Oxide Buffer Layer on BiFeO3 Thin Film

Bismuth ferrite (BiFeO₃) thin films with Bi₂O₃ buffer layers were prepared on Si/SiO₂/TiO₂/Pt substrates by sol–gel‐derived spin‐coating method. The structural and electrical properties of BiFeO₃ was effectively improved by adding a Bi₂O₃ buffer layers either at Pt/BiFeO₃ interface or on BiFeO₃ surface, also strongly depending on the positions and the annealing conditions of buffer layers. A 500°C‐annealed Bi₂O₃ buffer layer could act as a Bi source for compensating Bi volatilization and a diffusion barrier for species from BiFeO₃. A near stoichiometric BiFeO₃ with less defects and substrate contamination was obtained by employing a 500°C‐annealed Bi₂O₃ buffer layer in between Pt substrate and BiFeO₃. The structure change in BiFeO₃ led by such a buffer layer should result from the interfacial constraint between buffer layer and BiFeO₃. Furthermore, this crystalline BiFeO₃ specimen exhibited a highly (100)‐textured, where this preferred orientation was attributed to the accumulation of Bi at Pt/BFO interface. Therefore, the Pt/500°C‐annealed Bi₂O₃/BiFeO₃/Pt thin film exhibited the good ferroelectric and magnetic properties. As compared to the usual method for controlling BiFeO₃ composition by adding excess Bi, this study indicates the more advantages using a Bi₂O₃ buffer layer.     

Pt-decorated nanoporous gold for glucose electrooxidation in neutral and alkaline solutions

Exploiting electrocatalysts with high activity for glucose oxidation is of central importance for practical applications such as glucose fuel cell. Pt-decorated nanoporous gold (NPG-Pt), created by depositing a thin layer of Pt on NPG surface, was proposed as an active electrode for glucose electrooxidation in neutral and alkaline solutions. The structure and surface properties of NPG-Pt were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and cyclic voltammetry (CV). The electrocatalytic activity toward glucose oxidation in neutral and alkaline solutions was evaluated, which was found to depend strongly on the surface structure of NPG-Pt. A direct glucose fuel cell (DGFC) was performed based on the novel membrane electrode materials. With a low precious metal load of less than 0.3 mg cm^-2 Au and 60 μg cm^-2 Pt in anode and commercial Pt/C in cathode, the performance of DGFC in alkaline is much better than that in neutral condition.     

Metal-assisted etching of p-type silicon under anodic polarization in HF solution with and without H2O2

Pore formation under anodic polarization of a lightly doped p-type Si wafer previously loaded with Pt, Pd and Ag nanoparticles was investigated in HF solution with and without H₂O₂. In HF solution without H₂O₂, a microporous layer was formed in p-Si loaded with Pt or Pd. However, Ag metal nanoparticles yielded pores due to their intrusion in the Si wafer. The addition of H₂O₂ to the etching solution leads to different pore morphologies depending on the metals. Particles of Ag were found at the bottom of most pores. In the presence of Pt nanoparticles, cone-shaped macropores were produced, and the pore depth and diameter increased with increasing H₂O₂ content. Current density influenced the pore morphology. For a sample loaded with Pt or Ag, an increase in applied current density widened the pore diameter. The mechanism of the metal-assisted pore formation was discussed by considering a competitive process between the formation of a microporous layer under polarization and metal-assisted chemical oxidation of the microporous layer by a dissolved oxidizing agent.     

CO oxidation from syngas (CO and H<Subscript>2</Subscript>) using nanoporous Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst

The concept of “waste-to-wealth” is spreading awareness to prevent global warming and recycle the restrictive resources. To contribute towards sustainable development, hydrogen energy is obtained from syngas (CO and H₂) generated from waste gasification, followed by CO oxidation and CO₂ removal. In H₂ generation, it is key to produce more purified H₂ from syngas using heterogeneous catalysts. In this respect, we prepared Pt/Al₂O₃ catalyst with nanoporous structure using precipitation method, and compared its catalytic activity with commercial alumina (Degussa). Based on the results of XRD and TEM, it was found that metal particles did not aggregate on the alumina surface and showed high dispersion. Optimum condition for CO conversion was 1.5 wt% Pt loaded on Al₂O₃ support, and pure hydrogen was obtained after removal of CO₂ gas.     

Electrodeposition and characterization of Bi2Se3 thin films by electrochemical atomic layer epitaxy (ECALE)

Bismuth selenide thin films were grown on Pt substrate via the route of electrochemical atomic layer epitaxy (ECALE) in this work for the first time. The electrochemical behaviors of Bi and Se on bare Pt, Se on Bi-covered Pt, and Bi on Se-covered Pt were studied by cyclic voltammetry and coulometry. A steady deposition of Bi₂Se₃ could be attained after negatively stepped adjusting of underpotential deposition (UPD) potentials of Bi and Se on Pt in the first 40 deposition cycles. X-ray diffraction (XRD) analysis indicated that the films were single phase Bi₂Se₃ compound with orthorhombic structure, and the 2:3 stoichiometric ratio of Bi to Se was verified by EDX quantitative analysis. The optical band gap of the as-deposited Bi₂Se₃ film was determined as 0.35 eV by Fourier transform infrared spectroscopy (FTIR), which is consistent with that of bulk Bi₂Se₃ compound.     

Apparent electrocatalysis on 3D nanoporous platinum film electroplated from hexagonal lyotropic liquid crystalline phase of Triton X-100

Electrocatalytic effect of nanoporous interface between platinum (Pt) and electrolyte was investigated in terms of direct electrochemical oxidation of glucose. A Pt film with 3D nanopores was electroplated from the hexagonal (H₁) lyotropic liquid crystal (LLC) phase of t-octylphenoxypolyethoxyethanol (Triton X-100), a nonionic surfactant with a phenyl group in a hydrophobic tail. This Pt film, which was electroplated from H₁-LLC of Triton X-100 (ePt-H₁-TX100), comprised closely stacked crystalline (face centered cubic) nanoparticles (diameter, 4-7 nm) with 3D nanopores (width, 1-2 nm) developed among the particles. The ePt-H₁-TX100 showed high surface roughness and selective enhancement of a sluggish redox reaction, that is, kinetic-controlled reaction such as glucose oxidation. Nonenzymatic glucose sensing can be achieved by using discriminatively enhanced sensitivity to glucose over redox active interfering molecules.     

Synthesis and Catalytic Properties of the Electrochemical NO x Reduction System

Catalytic and electrical properties of an electrochemical NO_x reduction system were investigated. This system had a laminated structure composed of BaCo(Al,Ga)₁₁O₁₉-based catalyst layer on a Pt/YSZ/Pt sheet. The stacked catalyst system can directly reduce more than 65% of NO_x to N₂ under an external bias above 2.5 V at 650 °C. In this system, oxygen existing around the catalyst layer was removed by O²⁻ transportation through the YSZ layer.     

Electrochemical studies of ceramic carbon electrodes for fuel cell systems: A catalyst layer without sulfonic acid groups

Ceramic carbon electrodes (CCEs) have been produced via the sol-gel process using 20% Pt on Vulcan XC72 carbon black and tetra ethyl orthosilicate (TEOS) as the organosilane precursor. This process produces a homogenous distribution of SiO₂ and carbon supported Pt catalyst. Electrochemical experiments (cyclic voltammetry, electrochemical impedance spectroscopy) were performed to determine the effect of SiO₂ loading on the active area of Pt in the catalyst layer. A volcano-type dependence was observed with the maximum active area of Pt occurring with an SiO₂ loading of 45% by mass. Pt utilization was lower than that achieved with Nafion-based catalyst layers and was explained in terms of the lower proton conductivity of SiO₂ compared to Nafion. These CCE structures may be useful for high temperature fuel cell systems.