Synthesis and characterization of monodisperse hollow SnO<Subscript>2</Subscript> microspheres and their enhanced sensing properties to ethanol

The monodisperse hollow SnO₂ (H-SnO₂) microspheres were successfully synthesized by the ion exchange method using sulfonated PS microspheres as a template. The structure and morphology were characterized by X-ray diffraction, transmission electron microscopy and high-resolution transmission electron microscopy, which confirms the hollow structure of the products. The H-SnO₂ microspheres are composed of numerous SnO₂ nanoparticles with a shell thickness of about 13 nm. The monodisperse H-SnO₂ microspheres have a high specific surface area of 55.54 m²/g, which improves the gas sensing properties toward ethanol. Gas-sensing measurement results indicate that H-SnO₂ microspheres exhibit an excellent sensitivity (103.1) toward 200 ppm ethanol at 260 °C, which is much higher than that (65.8) of SnO₂ nanoparticles.     

Synthesis of layered hierarchical porous SnO<Subscript>2</Subscript> for enhancing gas sensing performance

Layered hierarchical porous SnO₂ (LHP-SnO₂) have been synthesized by a two-step method, in which pure SnO₂ nanoparticles(NPs) with the diameter about 3.2 nm were prepared firstly through a hydro-thermal method, and then LHP-SnO₂ were prepared by utilizing polystyrene (PS) microspheres as a template and SnO₂ NPs as a precursor. The as-prepared sample consisted of porous SnO₂ layers, in which each layer presents a three-dimensional random arrangement of macropores with average pore diameter of about 260 nm. The Nitrogen adsorption–desorption analysis implied that the sample was characterized with large surface area of 140.67 m²/g and extensive micropores and mesopores structure. Compared with pure SnO₂ NPs, the LHP-SnO₂ exhibited an obvious improvement in gas sensing properties. These results indicate that the layered hierarchical porous structure possess potential application in sensing materials.     

Enhanced gas sensing properties of hierarchical SnO2 nanoflower assembled from nanorods via a one-pot template-free hydrothermal method

Well-defined three-dimensional (3D) hierarchical tin dioxide (SnO₂) nanoflowers with the size of about 200 nm were successfully synthesized by a simple template-free hydrothermal method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and N₂ adsorption-desorption analyses were used to characterize the structure and morphology of the products. The as-synthesized full crystalline and large specific surface area SnO₂ nanoflowers were assembled by one-dimensional (1D) SnO₂ nanorods with sharp tips. A possible self-assembly mechanism for the formation the SnO₂ nanoflowers was speculated. Moreover, gas sensing investigation showed the sensor based on SnO₂ nanoflowers to exhibit high response and fast response-recovery ability to detect acetone and ethanol at an operating temperature lower than 200 °C. The enhancement of gas sensing properties was attributed to their 3D hierarchical nanostructure, large specific surface area, and small size of the secondary SnO₂ nanorods.     

Polyaniline–13X zeolite composite‐supported platinum electrocatalysts for direct methanol fuel cell applications

We successfully synthesized 13X zeolite using a hydrothermal method. Then, composites of polyaniline (PANI) with 13X zeolite and PANI–13X with platinum were prepared by chemical oxidative polymerization and chemical reduction, respectively. Field emission scanning electron microscopy, X‐ray diffraction, Raman spectroscopy and Brunauer–Emmett–Teller techniques were used to characterize the PANI–Pt and PANI–Pt–13X composites. Further, the electrocatalytic activity towards methanol oxidation of the synthesized catalysts was explored using cyclic voltammetry in 1 mol L^?1 CH₃OH + 0.5 mol L^?1 H₂SO₄ solution. From the obtained results, PANI–Pt–13X shows superior performance compared to PANI–Pt towards methanol oxidation and electrical conductivity. Hence, the 13X zeolite‐incorporated PANI–Pt composite could be an efficient catalyst for direct methanol fuel cell applications. © 2019 Society of Chemical Industry     

Effect of synthesis condition and annealing on the sensitivity and stability of gas sensors made of Zn-doped <Emphasis Type="Italic">γ</Emphasis>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> particles

Gas sensors made of flame-synthesized Zn-doped γ-Fe₂O₃ nanoparticles were found to have high sensitivity and high aging resistance. Zinc-doped γ-Fe₂O₃ nanoparticles and microparticles were synthesized by flame spray pyrolysis (FSP). Gas sensors were fabricated with as-synthesized particles, and with particles that had been annealed. The sensors’ response to acetone vapor and H₂ was measured as fabricated, and measured again after the sensors were aged for three days. The sensors made from as-synthesized particles showed a gas sensing sensitivity 20 times higher than the literature value. However, sensors made of microparticles lost their sensing ability after three days of aging; sensors made of nanoparticles retained their gas sensing capability after aging. Sensors made of annealed particles did not have significant gas sensing capabilities. Analysis using the William and Hall method showed that the microstrains decreased significantly in both H₂/O₂ and H₂/Air flame synthesized particles after annealing. The results showed that sensors made of flame-synthesized particles have much higher sensitivity than sensors made of particles previously reported. Especially, sensors made of flame-synthesized nanoparticles are resistant towards aging. This aging resistance may be attributed to the particles’ ability to retain their microstrains.     

Activity,selectivity, and methanol tolerance of novel carbon-supported Pt and Pt<Subscript>3</Subscript>Me (Me = Ni,Co) cathode catalysts

The activity, selectivity, and methanol tolerance of novel, carbon supported high-metal loading (40 wt.%) Pt/C and Pt₃Me/C (Me = Ni, Co) catalysts for the O₂ reduction reaction (ORR) were evaluated in model studies under defined mass transport and diffusion conditions, by rotating (ring) disk and by differential electrochemical mass spectrometry. The catalysts were synthesized by the organometallic route, via deposition of pre-formed Pt and Pt₃Me pre-cursors followed by their decomposition into metal nanoparticles. Characteristic properties such as particle sizes, particle composition and phase formation, and active surface area, were determined by transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. For comparison, commercial Pt/C catalysts (20 and 40 wt.%, E-Tek, Somerset, NJ, USA) were investigated as well, allowing to evaluate Pt loading effects and, by comparison with the pre-cursor-based catalyst with their much smaller particle sizes (1.7 nm diameter), also particle size effects. Kinetic parameters for the ORR were evaluated; the ORR activities of the bimetallic catalysts and of the synthesized Pt/C catalyst were comparable and similar to that of the high-loading commercial Pt/C catalyst; at typical cathode operation potentials H₂O₂ formation is negligible for the synthesized catalysts. Due to their lower methanol oxidation activity the bimetallic catalysts show an improved methanol tolerance compared to the commercial Pt/C catalysts. The results indicate that the use of very small particle sizes is a possible way to achieve reasonably good ORR activities at an improved methanol tolerance at DMFC cathode relevant conditions.     

Facile hydrothermal synthesis of manganese-metal organic framework nanostructures in the presence of various organic ligands for SO<Subscript>2</Subscript> and CO<Subscript>2</Subscript> gas adsorption

Herein, we successfully synthesized the manganese-metal organic framework via hydrothermal route using three kinds of carboxylates as organic ligands and Mn(NO₃)₂ as a manganese precursor. The morphology and microstructure of the obtained products were characterized by X-ray diffraction, Fourier transformed infrared spectrum, scanning electron microscopy, transition electron microscopy, thermo-gravimetric analysis, differential scanning calorimetry and nitrogen adsorption [i.e. Brunauer–Emmett–Teller (BET) surface area analysis] techniques. Moreover, the effect of organic ligand type and preparation route on morphology and textural properties were investigated. The results showed that each type of used ligands leads to formation of products with specific morphologies and textural properties. Also, the hydrothermal route has significant effect on microstructures and textural properties of obtained products. The SO₂ and CO₂ gas uptake investigation of synthesized Mn-MOFs exhibited that the products obtained via hydrothermal route have high SO₂ and CO₂ gas uptake compared to method without using hydrothermal route.     

CO tolerance and catalytic activity of Pt/Sn/SnO2 nanowires loaded on a carbon paper

Electrochemical activities and structural features of Pt/Sn catalysts supported by hydrogen-reduced SnO₂ nanowires (SnO₂NW) are studied, using cyclic voltammetry, CO stripping voltammetry, scanning electron microscopy, and X-ray diffraction analysis. The SnO₂NW supports have been grown on a carbon paper which is commercially available for gas diffusion purposes. Partial reduction of SnO₂NW raises the CO tolerance of the Pt/Sn catalyst considerably. The zero-valence tin plays a significant role in lowering the oxidation potential of CO_ads. For a carbon paper electrode loaded with 0.1 mg cm⁻² Pt and 0.4 mg cm⁻² SnO₂NW, a conversion of 54% SnO₂NW into Sn metal (0.17 mg cm⁻²) initiates the CO_ads oxidation reaction at 0.08 V (vs. Ag/AgCl), shifts the peak position by 0.21 V, and maximizes the CO tolerance. Further reduction damages the support structure, reduces the surface area, and deteriorates the catalytic activity. The presence of Sn metal enhances the activities of both methanol and ethanol oxidation, with a more pronounced effect on the oxidation current of ethanol whose optimal value is analogous to those of PtSn/C catalysts reported in literature. In comparison with a commercial PtRu/C catalyst, the optimal Pt/Sn/SnO₂NW/CP exhibits a somewhat inferior activity toward methanol, and a superior activity toward ethanol oxidation.     

Preparation of meso-porous SnO2 fibers with enhanced sensitivity for n-butanol

Meso-porous SnO₂ fibers were synthesized using a solvothermal method with metaplexis fruit as the bio-template. The products were characterized by powder X-ray diffraction, high resolution scanning electron microscopy, transmission electron microscopy and nitrogen adsorption/desorption measurements. Results show that SnO₂ fibers present a high specific surface area of 73.665 m²/g and a meso-porous structure with the pore size of 7.821 nm, and the crystal size of SnO₂ is about 6.5 ± 0.5 nm. The gas sensing performance of the prepared SnO₂ fibers toward several volatile organic compounds was investigated. The results show that the meso-porous SnO₂ fibers were highly sensitive and selective to n-butanol.     

Solvothermal preparation and gas sensing properties of hierarchical pore structure SnO2 produced using grapefruit peel as a bio-template

Hierarchical pore structure nano-sized SnO₂ was synthesized using a solvothermal method with SnCl₄ as the raw material and grapefruit peel as the bio-template. The products were characterized by powder X-ray diffraction, high resolution scanning electron microscopy, transmission electron microscopy and nitrogen adsorption/desorption measurements. The results show that the SnO₂ prepared from the grapefruit peel bio-template consists of many large size (5–20 µm) interconnected pores with a honeycomb structure and nanosized pores (9.46 nm) on the walls of the large pores. The as-prepared SnO₂ presented a high specific surface area of 42.98 m²/g and the average crystallite size was about 10±0.5 nm. The gas sensing performance of the prepared material toward several volatile organic compounds was investigated. The results show that the hierarchical pore structure nano-sized SnO₂ was highly sensitive and selective to n-butanol, indicating that this material may be a promising candidate for future development as a n-butanol gas sensor.     

Low temperature operated highly sensitive,selective and stable NO2 gas sensors using N-doped SnO2-rGO nanohybrids

In this study, we report synthesis of SnO₂-rGO and N-doped SnO₂-rGO nanohybrids by facile hydrothermal method. The XRD analysis of the synthesized nanohybrids revealed a tetragonal rutile structure of SnO₂ lattice.Further structural, chemical, morphological and optical properties of SnO₂-rGO and SnO₂-NrGO nanohybrids were investigated by Raman, X-Ray Photoelectron spectroscopy, Transmission Electron Microscopy, UV–Vis spectroscopy and Photoluminescence spectroscopy. Furthermore, the gas sensing properties of the synthesized nanohybrids were studied in detail. It was observed that SR2 and SRN2 nanohybrids exhibited superior NO₂ sensing response (55.2 and 84.5% respectively) at low operating temperature (120 °C) and low gas concentration (0.5 ppm). Moreover, SR2 and SRN2 also exhibited excellent selectively towards NO₂ along with remarkable stability upto 90% over 30 days. This improved performance can be attributed to the synergetic effect of small particle size, high defect concentration and high surface area due to incorporation of SnO₂ along with N doping in rGO. Therefore, SR2 and SRN2 can be utilized as effective NO₂ gas sensors.     

Improving the Stability and Ethanol Electro‐Oxidation Activity of Pt Catalysts by Selectively Anchoring Pt Particles on Carbon‐Nanotubes‐Supported‐SnO2

To improve the stability and activity of Pt catalysts for ethanol electro‐oxidation, Pt nanoparticles were selectively deposited on carbon‐nanotubes (CNTs)‐supported‐SnO₂ to prepare Pt/SnO₂/CNTs and Pt/CNTs was prepared by impregnation method for reference study. X‐ray diffraction (XRD) was used to confirm the crystalline structures of Pt/SnO₂/CNTs and Pt/CNTs. The stabilities of Pt/SnO₂/CNTs and Pt/CNTs were compared by analyzing the Pt size increase amplitude using transmission electron microscopy (TEM) images recorded before and after cyclic voltammetry (CV) sweeping. The results showed that the Pt size increase amplitude is evidently smaller for Pt/SnO₂/CNTs, indicating the higher stability of Pt/SnO₂/CNTs. Although both catalysts exhibit degradation of electrochemical active surface area (EAS) after CV sweeping, the EAS degradation for the former is lower, further confirming the higher stability of Pt/SnO₂/CNTs. CV and potentiostatic current–time curves were recorded for ethanol electro‐oxidation on both catalysts before and after CV sweeping and the results showed that the mass specific activity of Pt/CNTs increases more than that of Pt/SnO₂/CNTs, indicating that Pt/CNTs experiences more severe evolution and is less stable. The calculated area specific activity of Pt/SnO₂/CNTs is larger than that of Pt/CNTs, indicating SnO₂ can co‐catalyze Pt due to plenty of interfaces between SnO₂ and Pt.     

Polyaniline‐supported platinum‐hydrous ruthenium oxide nanocomposite as electrocatalyst for methanol oxidation

A novel composite electrode is fabricated through the electrodeposition of hydrous ruthenium oxide (RuO₂·xH₂O) and platinum (Pt) particles into the matrix of polyaniline (PANI). Scanning electron microscopy reveals that RuO₂·xH₂O and Pt particles are homogeneously distributed into the matrix of PANI. A comparison of the sizes of Pt and RuO₂·xH₂O particles incorporated into the PANI film reveals that Pt particles are smaller in sizes as compared with the sizes of RuO₂·xH₂O particles. The catalytic activity of composite electrodes was evaluated for the oxidation of methanol by using cyclic voltammetry and chronoamperometry. A relatively high catalytic current was noticed for the oxidation of methanol (2.37 mA/cm²) at PANI‐Pt‐RuO₂·xH₂O electrode (+0.6 V (V vs. Ag/AgCl) in comparison to oxidation current at PAN‐Pt (1.27 mA/cm²) electrode. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

SnO2-coated multiwall carbon nanotube composite anode materials for rechargeable lithium-ion batteries

SnO₂-coated multiwall carbon nanotube (MWCNT) nanocomposites were synthesized by a facile hydrothermal method. The as-prepared nanocomposites were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The SnO₂/MWCNT composites, when combined with carboxymethyl cellulose (CMC) as a binder, show excellent cyclic retention, with the high specific capacity of 473 mAh g⁻¹ beyond 100 cycles, much greater than that of the bare SnO₂ which was also prepared by the hydrothermal method in the absence of MWCNTs. The enhanced capacity retention could be mainly attributed to good dispersion of the tin dioxide particles in the matrix of MWCNTs, which protected the particles from agglomeration during the cycling process. Furthermore, the usage of CMC as a binder is responsible for the low cost and environmental friendliness of the whole electrode fabrication process.     

Mesoporous TiO<Subscript>2</Subscript> microspheres synthesized via a facile hydrothermal method for dye sensitized solar cell applications

Mesoporous TiO₂ microspheres were successfully synthesized by a facile hydrothermal process and the obtained product was sintered at 450 °C. The sintered TiO₂ powder was characterised by powder X-ray diffraction pattern and the result shows pure anatase phase with good crystalline nature. The morphological image of field emission scanning electron microscopy and high resolution transmission electron microscopy shows spherical shape and size of the particles is around 100 to 300 nm. The Brunauer–Emmett–Teller surface area of synthesized TiO₂ material was 56.32 m² g^?1 and average pore width of synthesized materials was 7.1 and 9.3 nm. Bimodal pore structure of TiO₂ microspheres has been very effective for electrolyte diffusion into photoanode in dye sensitized solar cells. The synthesized anatase TiO₂ microsphere based dye sensitized solar cells have high surface area with light scattering effect to enhance the photocurrent and conversion efficiency than the commercial P25 photoanode material. The power conversion efficiency of synthesized mesoporous TiO₂ microspheres and commercial P25 material is 4.2 and 2.7 % respectively. Therefore bimodal mesoporous anatase TiO₂ microsphere appears to be a promising and potential candidate for dye sensitized solar cells (DSSC) application.     

Fabrication of uniform SnO2–SiO2–Pt composite nanofibres via co-electrospinning

We successfully fabricated uniform SnO₂–SiO₂–Pt composite nanofibres (NFs) by using a co-electrospinning technique, in which we set up two coaxial capillaries. Morphology control of NFs was investigated, along with their structural properties and chemical compositions. Furthermore, to systematically investigate the morphological changes in SnO₂–SiO₂–Pt composite NFs, the relative weight ratios of the Sn precursor to the Si precursor including the 4 wt% Pt precursor were controlled at 3:1, 1:1, and 1:3. To demonstrate the formation mechanism of the composite NFs, the precursor positions of the shell section and the core section in co-electrospinning were reversed. The resultant composite NFs were investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). These results showed that in the case of the optimum weight ratio (1:1) of the Sn precursor in the shell section to the Si precursor including the 4 wt% Pt precursor in the core section, SnO₂ and Pt nanoparticles were uniformly grown on SiO₂ NFs, implying the successful formation of uniform SnO₂–SiO₂–Pt composite NFs.     

Realization of low-temperature and selective NO2 sensing of SnO2 nanowires via synergistic effects of Pt decoration and Bi2O3 branching

Metal oxide semiconductors with branched structures, such as branched nanowires (b-NWs), have promising properties for being used in gas sensors. In this work, we synthesized Pt-decorated Bi₂O₃-branched SnO₂ nanowires (NWs). NO₂ sensing studies revealed the superior capacity of a Pt-decorated Bi₂O₃-branched SnO₂ NWs gas sensor relative to pristine and branched SnO₂ gas sensors, and it worked at near room temperature (50 °C). The increased sensing capacity was related to the synergistic effects of Pt decoration and Bi₂O₃ branching, particularly the morphology of the gas sensor with branched structures, the promising effects of Pt as a noble metal with good catalytic activity, and the generation of homo- and heterojunctions in the Pt-decorated Bi₂O₃-branched SnO₂ NWs gas sensor. The results obtained in this work are useful for design and development of NO₂ gas sensors using a simple strategy, which can be easily extended to various metal oxides.     

Electronic Metal–Support Interactions and the Production of Hydrogen Through the Water-Gas Shift Reaction and Ethanol Steam Reforming: Fundamental Studies with Well-Defined Model Catalysts

The electronic properties of Ni and Pt nanoparticles deposited on CeO₂(111) have been examined using core and valence photoemission. The results of valence photoemission point to a new type of metal–support interaction which produces large electronic perturbations for small Ni and Pt particles in contact with ceria. The Ni/CeO₂(111) and Pt/CeO₂(111) systems exhibited a density of metal d states near the Fermi level that was much smaller than that expected for bulk metallic Ni or Pt. The electronic perturbations induced by ceria on Ni made this metal a very poor catalyst for CO methanation, but transformed Ni into an excellent catalyst for the production of hydrogen through the water-gas shift and the steam reforming of ethanol. Furthermore, the large electronic perturbations seen for small Pt particles in contact with ceria significantly enhanced the ability of the admetal to adsorb and dissociate water made it a highly active catalyst for the water-gas shift. The behaviour seen for the Ni/CeO₂(111) and Pt/CeO₂(111) systems illustrates the positive effects derived from electronic metal–support interactions and points to a promising approach for improving or optimizing the performance of metal/oxide catalysts.     

Monodispersed hard carbon spherules as a catalyst support for the electrooxidation of methanol

Hard carbon spherules (HCS) were used as support of Pt nanoparticles as electrocatalyst for direct methanol fuel cells (DMFCs). Scanning electron microscopy (SEM) images show that the size of the Pt particles on HCS by reduction of K₂PtCl₆ with ethylene glycol is 4-5 nm. High-resolution transmission electron microscopy (HRTEM) study reveals that the Pt particles on the HCS surface have faceted crystalline structures. The size and aggregation of the Pt particles depend on the surface properties of the carbon support and the medium of the reduction reaction. Cyclic voltammetry and galvanostatic polarization experiments show that the Pt/HCS catalyst exhibits a higher catalytic activity in the electrooxidation of methanol than either the Pt/MCMB or the commercial Pt/Vulcan XC-72 catalyst does.     

Synthesis of Novel Submicrometer‐Scale Flat Carbon Fibers and Application in the Electrooxidation of Methanol

Novel submicrometer‐scale flat carbon fibers (SFCF) have been synthesized by catalytic chemical vapor deposition of acetylene over an Ni‐Al layered double hydroxide (NiAl‐LDH) compound, and the electrochemical activity of Pt supported on as‐synthesized SFCF for methanol oxidation has been investigated. The materials were characterized by power X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectra, and cyclic voltammetry tests. The results reveal that the active crystal facets of the NiAl₂O₄ spinel phase derived from NiAl‐LDH can deposit carbon atoms to grow SFCF, and that the co‐growing Ni nanoparticles are not catalytically active for the formation of SFCF. Furthermore, after support with Pt, the resultant Pt/SFCF electrocatalyst shows much higher activity for methanol oxidation than the Pt/C one in both acid and alkaline media, which is attributed to the combined beneficial effects of the microstructure of the SFCF support, improved electrical conductivity originating from the NiAl₂O₄ spinel catalyst embedded in SFCF, and improved dispersion of Pt particles through exposed Ni nanoparticles adhering intimately to the SFCF.