Yanbai Shen

Room temperature gas sensing of p-type TeO2 nanowires

Tellurium dioxide (TeO2) nanowires with a tetragonal structure have been grown by thermally evaporating tellurium metal at 400°C in air. The nanowires produced have diameters ranging from 30to200nm and have lengths of several tens of micrometers. Gas sensors were fabricated using the obtained TeO2 nanowires. The sensing behavior to NO2, NH3, and H2S gases at room temperature showed typical characteristics of a p-type semiconductor. The results demonstrate the potential to develop TeO2 nanowire based gas sensors with low power consumption.

Proton conduction in metal pyrophosphates (MP2O7) at intermediate temperatures

Intermediate-temperature fuel cells have received much recent attention as next generation energy sources. In particular, current efforts are devoted to developing proton conductors that operate at 120 °C or more and at low relative humidity. Proton conduction in several metal pyrophosphates (MP2O7, M = Sn, Ti, Si, Ge, Ce, and Zr) that have the potential to meet the demands for intermediate-temperature fuel cell applications are reviewed with an emphasis on the material aspects.

Synthesis and Characterization of TeO2 Nanowires

TeO2 nanowires were successfully synthesized by a simple reactive thermal evaporation method using pure Te metal as the source material. The study on synthesis process indicates an optimal synthesis temperature of 400 °C. Structural characterization using X-ray diffraction and transmission electron microscopy shows that the TeO2 nanowires have a single phase tetragonal structure. Scanning electron microscopy observation demonstrates that each TeO2 nanowire starts to grow from a TeO2 nanopartice, which suggests a self-catalytic growth mechanism. The Fourier transform infrared spectrum of TeO2 nanowires shows a blue shift of 20 cm-1 in the axial stretching bands. The gas sensing measurements indicate that TeO2 nanowires have a p-type electrical conduction and can reversibly response to NO2 gas at room temperature.

Synthesis and characterization of dense SnP2O7–SnO2 composite ceramics as intermediate-temperature proton conductors

Dense SnP2O7–SnO2 composite ceramics were prepared by reacting a porous SnO2 substrate with an 85% H3PO4 solution at elevated temperatures. At 300 °C and higher, SnO2 reacted with H3PO4 to form an SnP2O7 layer on exterior and interior surfaces in the substrate, the growth rate of which increased with increasing reaction temperature. Finally, at 600 °C, the pores of this composite ceramic were perfectly closed and its electrical conductivity became several orders of magnitude higher than that of the SnO2 substrate alone. Proton conduction was demonstrated in this composite ceramic using electrochemical measurements and various analytical techniques. Comparison of the observed electromotive force with the theoretical value in two gas concentration cells demonstrated that this composite ceramic is a pure ion conductor, wherein the predominant ion species are protons. Fourier transform infrared (FT-IR) and proton magic angle spinning (MAS) nuclear magnetic resonance (NMR) analyses revealed that the protons interacted with lattice oxide ions in the SnP2O7 layer to form hydrogen bonds. An H/D isotope effect suggested that proton conduction in this composite ceramic was based on a proton-hopping mechanism. The proton conductivity in this material reached ∼10−2 S cm−1 in the temperature range of 250–600 °C.

Hydroxide Ion Conducting Antimony(V)‐Doped Tin Pyrophosphate Electrolyte for Intermediate‐Temperature Alkaline Fuel Cells

Alkaline fuel cells have received significant interest in recent years relative to acid fuel cells, because of advantages when operating under alkaline conditions, which include enhancement of the electrode reaction kinetics, especially at the cathode, as the cathode catalyst is not subjected to corrosion. Consequently, non-noble metals or inexpensive metal oxides can be used as catalysts. In addition, high energy density liquids and gases such as ethanol, hydrazine, and ammonia can be adopted as fuels. Anion exchange polymers are widely viewed as promising candidates for electrolyte membranes; however, a major challenge in the development of such polymers is their stability at high pH, because both the main chain and functional groups are easily degraded by hydroxide ion attacks. In addition, the poor chemical stability of current anion exchange polymers means that the operating temperature is limited to 80 8C or less; therefore, this type fuel cell is typically operated between 50 and 60 8C. Operating a fuel cell at elevated temperatures provides the anode catalyst with high tolerance to CO, which is useful for both acid and alkaline fuel cells. Additional benefits include small polarization loss, good drainage at the anode, and effective heat dissipation from the fuel cell system. A few anion-conducting electrolyte materials capable of operating at intermediate temperatures (100–200 8C) have been reported, such as KOH-doped polybenzimidazole (PBI) and hydroxide ion-intercalated Mg–Al layered double hydroxide (LDH). However, the reaction of KOH in the former electrolyte with CO2 present in the air to form K2CO3 is a possibility, and the hydroxide ion conductivity of the latter (0.03 S cm 1 at 200 8C) is not sufficiently high to achieve satisfactory cell performance. Further increases in chemical stability and conductivity would enhance the position of intermediate-temperature anion exchange membranes as the preferred electrolyte material for practical alkaline fuel cells. In this study, metal pyrophosphates (MP2O7) were studied as anion-conducting electrolytes for intermediate temperature applications. The metal pyrophosphate structure can be described as a network of MO6 octahedra sharing corners with P2O7 units, characterized by the presence of intersecting zigzag tunnels delimited by pentagonal windows. This unique crystalline structure provides many ion exchange sites and transport pathways. To date, proton exchange capability is typically introduced into the bulk of SnP2O7 by the partial substitution of Sn cations with low-valency cations, such as In, Al, Mg, Sb, Sc, Ga, and Zn. The resultant proton conductivity reaches approximately 0.1 Scm 1 at 200 8C. An opposite effect is expected by the partial substitution of Sn cations with high-valency cations, which would result in hydroxide ion exchange capability because of charge compensation for the high-valency cations. We demonstrate the hydroxide ion conduction of pentavalent cation-doped SnP2O7 at intermediate temperatures using electrochemical measurements, including complex impedance, gas concentration cells, and H/D isotope replacement methods. Moreover, Sn0.92Sb0.08P2O7, which exhibits the highest hydroxide ion conductivity among the tested compounds, is characterized with respect to the hydroxide ion environment and the basicity of the compound. In addition, fuel cell tests with the Sn0.92Sb0.08P2O7 electrolyte were conducted in the temperature range of 50–200 8C. The crystalline structure of Sb-doped SnP2O7 as an example of Sn1–xAxP2O7 (A V = pentavalent cation dopants) was identified using X-ray diffraction (XRD) measurement (see Figure S1 in the Supporting Information). The XRD pattern of the nondoped sample was assigned to SnP2O7 (72 mol %) and SnO2 (28 mol%). Tao interpreted the presence of the SnO2 phase to be due to the use of large SnO2 particles as a raw material, which causes the formation of a core–shell structure, although such a structure could not be determined from transmission electron microscopy (TEM) measurements (see Figure S2 in the Supporting Information). (The content of SnO2, which has a negative effect on ionic conduction, can be reduced when smaller SnO2 particles are used as a raw material.) Sn1–xSbxP2O7 with an Sb 5+ content of not more than 8 mol% had the same pattern as that of the nondoped sample. The peaks attributable to SnP2O7 were shifted slightly toward higher Bragg angles by an increase in the Sb content, which is expected with substitution of Sn (0.69 ) with Sb (0.60 ) of a smaller ionic radius. As a consequence, the lattice constant of SnP2O7 decreased from 7.925 to 7.916 with increasing Sb content from zero to 8 mol%. In contrast, Sn1–xSbxP2O7 with an Sb 5+ content of 10 mol% or higher contained some peaks attributable to Sn2.5P3O12, which indicates that 8 mol% Sb 5+ is the substitution limit. The temperature dependency of the electrical conductivity for Sn1–xSbxP2O7 was measured in air saturated with H2O vapor at 50 8C (Figure 1) and revealed two common features. [*] Prof. Dr. T. Hibino, Dr. Y. Shen, Dr. M. Nagao Graduate School of Environmental Studies, Nagoya University Nagoya 464-8601 (Japan) E-mail: hibino@urban.env.nagoya-u.ac.jp

Nitrogen dioxide sensing using tungsten oxide microspheres with hierarchical nanorod-assembled architectures by a complexing surfactant-mediated hydrothermal route

WO3 microspheres with hierarchical nanorod-assembled architectures were successfully synthesized by a complexing surfactant-mediated hydrothermal method in the presence of K2SO4 and H2C2O4 with a molar ratio of 1 : 1. Microstructural characterization by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy showed that WO3 microspheres with diameters ranging from 3 to 5 μm were assembled by 90 nm diameter nanorods and had a single crystal hexagonal structure. The analysis results of the elemental composition and chemical state demonstrated that the obtained WO3 microspheres were nearly stoichiometric. Based on the experimental results, a possible growth mechanism consisting of nucleation, Ostwald ripening, and self-assembly of WO3 crystals was proposed. Gas sensing properties demonstrated that WO3 microspheres exhibited not only a high response and excellent reversibility to NO2, but also a good linear relationship between the response and NO2 concentration in the range of 1 to 10 ppm. The response and recovery times significantly decreased as the operating temperature increased gradually. The highest response of 790 to 20 ppm NO2 was obtained at a relatively low operating temperature of 100 °C, which revealed that WO3 microspheres were very promising for fabricating low-consumption chemical gas sensors. The electron depletion theory was used for explaining the gas sensing mechanism by the chemical adsorption and reaction of NO2 gas molecules on the surface of WO3 microspheres.

Effective Surface Area of SnO2-Sputtered Films Evaluated by Measurement of Physical Adsorption Isotherms

The microstructure of SnO2 sputtered films was investigated by field emission electron microscopy (FE-SEM), and by the measurement of film density and physical adsorption isotherms. FE-SEM shows that the SnO2-sputtered films are composed of columnar grains with thicknesses of about 10–45 nm. The density of the film deposited at a high temperature of 573 K and a low pressure of 0.4 Pa was 6.38×103 kg/m3. On the other hand, the density of the film deposited at room temperature and a high pressure of 12 Pa was 3.82×103 kg/m3. This value is much lower than the bulk density 6.95×103 kg/m3, indicating that the film is very porous. According to the results of the physical adsorption isotherms for Kr gas, the effective surface area of this porous film was 91.0 times larger than the apparent surface area, indicating that the columnar grains are mostly isolated by gaps. Furthermore, the density is discussed in relation to the total pore volume determined from pore size distributions.

Proton conduction in AIII0.5BV0.5P2O7 compounds at intermediate temperatures

Proton conductors capable of operation between 100 and 400 °C are attractive electrochemical materials due to their high utility value in energy and environmental applications. However, such proton conductors have not yet made headway in the marketplace, due to insufficient proton conductivity. Here, we present new types of metal pyrophosphates as promising candidates for intermediate temperature proton conductors. A series of AIII0.5BV0.5P2O7 (AIIIBV = InSb, SbSb, FeSb, GaNb, FeNb, YNb, GaTa, AlTa, FeTa, YTa, BiTa, and SmTa) compounds were synthesized, of which In0.5Sb0.5P2O7, Fe0.5Nb0.5P2O7, and Fe0.5Ta0.5P2O7 exhibited the highest proton conductivities in the temperature ranges of 50–100 °C (0.045 S cm−1@100 °C), 100–200 °C (0.12 S cm−1@150 °C), and 200–400 °C (0.18 S cm−1@250 °C), respectively, in unhumidified air. The proton conductivity of these three compounds was further enhanced by the introduction of AIII or BV deficiency into the bulk. Consequently, Fe0.4Ta0.5P2O7 exhibited the highest proton conductivity of 0.27 S cm−1 at 300 °C in unhumidified air. Such high proton conductivity values were also observed under fuel cell operating conditions. The environments of protons in the bulk of these compounds were monitored using Fourier transform infrared (FT-IR) spectroscopy, temperature-programmed desorption (TPD), and proton magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Protons were incorporated into the compounds for charge-compensation of the deficient AIII or BV cations, which results in an increase in the quantity of protons. More importantly, the mobility of the protons was also enhanced. Various electrochemical measurements demonstrate that proton conduction is dominant in these compounds, where the protons migrate according to a hopping mechanism.

Assembly of 3D flower-like NiO hierarchical architectures by 2D nanosheets: synthesis and their sensing properties to formaldehyde

Flower-like NiO hierarchical architectures were synthesized by a solvothermal process without using any other surfactant. Absolute ethanol and distilled water were adopted as solvent, and nickel nitrate hexahydrate was employed as the nickel source. The morphology and crystal structure were mainly investigated. Through annealing the as-obtained products, flower-like NiO hierarchical architectures with a cubic structure were synthesized, which were assembled by a number of thin nanosheets with a thickness of about 30 nm. The formaldehyde gas sensing measurements showed that well-defined NiO flower-like structures with large surface area exhibited higher responses compared with microsheets/nanosheets at a relatively lower operating temperature of 200 °C. Moreover, a reversible and fast response to formaldehyde gas at various gas concentrations, good selectivity and stability were obtained. The results indicated that the flower-like NiO hierarchical architectures are promising materials for gas sensors.

Selective depression effect in flotation separation of copper–molybdenum sulfides using 2,3-disulfanylbutanedioic acid

Abstract 2,3-disulfanylbutanedioic acid (DMSA) was found to be a selective depressant in the flotation separation of copper– molybdenum sulfides. The flotation results suggest that a low dosage of DMSA has a strong depression effect on chalcopyrite in the pH range between 4 and 12. At pH 6, the recoveries of molybdenum are up to 85%, 75%, and 80% while those of chalcopyrite are 15%, 5%, and 20% respectively when flotation tests are carried out with single minerals, mixed minerals and molybdenum-bearing copper concentrates. Adsorption isotherms measurement indicates that DMSA adsorbs more strongly on chalcopyrite than on molybdenite. The frontier orbital calculation reveals that the two S atoms of DMSA molecule are active centers for the adsorption of the DMSA molecule on chalcopyrite surface. Fermi level calculation shows that chalcopyrite can obtain electrons from the DMSA molecule while molybdenite cannot.