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Modeling of high resolution nitrogen adsorption isotherms by the Grand Canonical Monte Carlo (GCMC) method for zirconia-pillared clays containing 20 wt.% of ZrO2 revealed that the pillars are comprised of separate Zr4(μ-OH)8(OH)m (H2O)n units (Zr4 tetramers) and their dimers (Zr8 complexes) as sheets or loose 3D structures. This agrees with the results of modeling of the condensation process in solution using the Density Functional Theory approach (DFT) and the semiempirical PM3 method. Bridging and terminal hydroxyls strongly bound with Zr cations are involved in anchoring supported Cu cations and Pt. This nanostructure of the active component as well as the developed micro- and mesoporosity appear to be responsible for the high performance of Pt+Cu/ZrPILC catalysts in the NOx selective reduction by propylene and decane in realistic feeds.
Thermally-carbonized porous silicon humidity sensor showed ageing affecting electrical characteristics. During the first month the variations in electrical characteristics were very distinctive. The decline in the sensitivity of the sensor after three months storage was found to be 37%, however, the sensitivity was still over 200%. When aged, the sensor stabilized and only minor variations on capacitance were noticeable. Sensitivity, on other hand, remains nearly constant. The hysteresis of the sensor did not change remarkably during storage. The variations on capacitance values at different relative humidities during storage were measured as a function of detection frequency. This showed an interesting feature of the ageing of the sensor, which is also discussed.
Our direct density function-based simulations of Ru-, Pt- and mixed Ru-Pt clusters on carbon-based supports reveal that substrates can mediate the PtRu5 particles . Oblate structure of PtRu5 on C has been found . Nevertheless, the cluster-substrate interface interactions are still unknown. In this work, we present the applications of combinations of quantitative z-contrast imaging and high resolution electron microscopy in investigating the effect of different substrates and ligand shells on metal particles. Specifically, we developed a relatively new and powerful method to determine numbers of atoms in a nanoparticle as well as three-dimensional structures of particles including size and shape of particles on the substrates by very high angle (~96mrad) annular dark-field (HAADF) imaging [2-4] techniques. Recently, we successfully synthesize icosahedra Au13 clusters with mixed ligands and cuboctahedral Au13 cores with thiol ligands, which have been shown by TEM to be of sub-nanometer size (0.84nm) and highly monodisperse narrow distribution. X-ray absorption and UV-visible spectra indicate many differences between icosahedra and cuboctahedral Au13 cores. Particles with different ligands show different emissions and higher quantum efficiency has been found in Au11 (PPH3) SC12)2C12. We plan to deposit those ligands-protected gold clusters onto different substrates, such as, TiO2 and graphite, etc. Aforementioned analysis procedure will be performed for those particles on the substrates and results will be correlated with that of our simulations and activity properties. This approach will lead to an understanding of the cluster-substrates relationship for consideration in real applications.
A supported Si3N4 membrane was prepared via a non-aqueous sol-gel process using tris(dimethylamino)silylamine, H2NSi(NMe2)3, as a starting material. The SEM image indicated the formation of the Si3N4 membrane with thickness about 2.3 μ m on the surface of the α-Al2O3 support. Nitrogen adsorption analysis indicated that, although most of the pores were similar to those of the α-Al2O3 disk, a new pore size distribution of 20–50 Å for the Si3N4 membrane was observed. The new membrane demonstrates high selective absorption of NO2, suggesting a potential application as a selective filter for gas sensors.
A novel concept of solid-state chemical sensors for neutral radical detection in gas-phase and related technique are proposed based on chemiluminescence of sensing materials excited by heterogeneous chemical reactions of radicals on sensor surface. The radical species of interest include H, O, N, O2, CO, SO, NO and others. Surface activated phosphors, nano-phosphors and semiconductor films are good candidates for sensors. The advantages of these sensors are the enhanced sensitivity (~105at/cm-3or better), real-time response, reliability, proximate analysis, ability to be fabricated in combination with up-to-date nanotechnologies.
Polymer-inorganic hybrid materials composed of polymethyl methacrylate (PMMA) and zinc compounds were prepared by sol-gel in-situ transition polymerization of zinc complex in PMMA matrix. Zinc acetate dihydrate dissolved in ethanol was used as the inorganic precursor. Monoethanolamine (MEA) acted as a complexing agent to control the hydrolysis of zinc acetate to produce a zinc compound network, and then PMMA, formed in-situ through a radical polymerization, were chemically bonded to the forming zinc compound network to realize a hybrid material. Transparent homogenous hybrid materials with slight colours from pink to yellow were fabricated by varying the composition. TEM, FT-IR were employed to investigate structural and physical properties. The UV-shielding effect was evaluated by UV-VIS. The low content of zinc (around 0.02 wt%) and the fine particle size rendered it visibly transparent and capable of greatly attenuating UV radiation in the full UV range.
Changes in the electronic structure in superficial space-charge regions may substantially affect the properties of metals near their surface. In materials with a nanoscale porosity and with a high surface to volume ratio, changes in the properties of even a thin surface layer may have a noticeable effect on the properties of the entire material. In nanoporous metals immersed in an electrolyte, the space charge can be induced as a part of the electric double layer at the metal-electrolyte interface. Here we present first experiments on the effect of surface charging in a nanoporous metal on the magnetism. We report reversible changes in the magnetic moment in Ni-Pd alloys. As possible origins of the dependency of the magnetization on the surface charge density we discuss band filling and, alternatively, magnetostriction. X-ray diffraction and dilatometry reveal a considerable strain amplitude, about of 6×10-4, resulting from surface charging; this corresponds to a surface-induced pressure in the crystal lattice, in the order of 0.3 GPa.
Pores in single crystalline semiconductors can be produced in a wide range of geometries and morphologies, including the “nano” regime. Porous semiconductors may have properties completely different from the bulk, and metamaterials with e.g. optical properties not encountered in natural materials are emerging. Possible applications of porous semiconductors include various novel sensors, but also more “exotic” uses as, e.g. high explosives or electrodes for micro fuel cells. The paper briefly reviews pore formation (including more applied aspects of large area etching), properties of porous semiconductors and emerging applications.
Nanoporous carbon films were deposited by 248 nm pulsed laser ablation of a graphite target in different background pressures of argon (PAr). The morphology changed from smooth, high-density amorphous carbon films at PAr = 20 mTorr to ultra-low density nanoporous material at PAr = 380 mTorr. Subsequently, the nanostructural, chemical and electrical properties of metal containing nanoporous carbon samples were investigated by ablating graphite targets containing known contents of Ni and Co. We demonstrate how the ablation plume dynamics affect both the nanostructure of the material and the incorporation of metal atoms. The suitability of these functionalised ultra-low density materials for gas sensing applications is discussed.
Atomic layer deposition (ALD) is ideal for applying precise, conformal coatings over nanoporous materials. We have recently used ALD to coat two nanoporous solids: anodic aluminum oxide (AAO) and silica aerogels. AAO possesses hexagonally ordered pores with diameters d∼10 nm and thicknesses L∼70 microns. The AAO membranes were coated by ALD with successive layers of A1203, TiO2 and V2O5 to fabricate catalytic membranes. SEM, TEM and EDAX analysis of the membranes demonstrate that the ALD layers uniformly coat the extremely high aspect ratio (L/d∼104) AAO pores. These catalytic membranes show remarkable selectivity in the oxidative dehydrogenation of cyclohexane. Additional AAO membranes coated with ALD Pd films show promise as hydrogen sensors. Silica aerogels have the lowest density and highest surface area of any solid material. Consequently, these materials serve as an excellent substrate to fabricate novel catalytic materials and gas sensors by ALD. In this study, both thin film and monolithic aerogels were coated by ZnO ALD and the properties of the aerogels were investigated as a function of the coating thickness.
Some cobaltites with perovskite-type structure exhibit outstanding transport properties and high chemical activity, which make these materials suitable for applications in areas of gas sensors, heterogeneous catalysis, gas separation membranes and cathodes for solid oxide fuel cells. In this work, polycrystalline samples of Sm1-xBaxCoO3 (x = 0, 0.1) were prepared by an aqueous solution method using the corresponding nitrates. X-ray diffraction patterns of calcined samples showed that single-phase SmCoO3 was obtained at 900°C, whereas Sm0.9Ba0.1CoO3 was formed at 700°C. Electron microscopy images revealed that micron-sized particles were obtained for SmCoO3, whereas a nanostructured and nanoporous material wasobserved for Sm0.9Ba0.1CoO3. Electrical measurements made on thick films of the oxides revealed a semiconductor behavior in both phases, however Sm0.9Ba0.1CoO3 samples showed a larger conductivity compared with SmCoO3; dynamic response of resistance experiments made in air and CO2 revealed that Sm0.9Ba0.1CoO3 is selective to CO2.
High surface area cerium oxide has been prepared using a carbon templating method. Impregnation of a highly mesoporous activated carbon (Darco KB-B) with an aqueous solution of cerium nitrate, followed by carbon burn off, afforded ceria with surface area of up to 148 m2/g. According to thermogravimetric studies, ceria formation proceeds via decomposition of cerium nitrate at ca. 410 K; oxidation of the carbon template commences at the same temperature, being facilitated by the release of NO2 from the Ce compound. Use of activated carbon fibers (ACFs) as template was found to provide a simple route to fibrous cerium oxide. The lower surface areas (3 - 59 m2/g) of the resulting ceria fibers reflect the largely microporous nature of the ACFs; evidently the Ce nitrate solution is unable to penetrate their micropores. Consequently, the surface area of the ceria product is found to increase with increasing mesoporosity of the ACF template. Electron microscopy reveals that the ceria fibers are composed of highly crystalline primary particles of 5-10 nm diameter; further, the fibers display a number of interesting morphological features at the macro- and nano-scales.
Long and fine Zn1-xCdxSe pseudo-binary alloy nanowires of various compositions x covering the entire range were grown by metalorganic chemical vapor deposition, using diethlyzinc, dimethylcadmium and diisopropylselenide as precursors, on Si (100) and GaAs (100) substrates; sputtered gold was used as a catalyst to promote nanowire formation. By controlling the ratio of the flows of the precursors, the temperature and the pressure during growth, we obtained nanowires of desired compositions. The morphology, structure and optical properties of the nanowires were studied by various techniques, including secondary electron microscopy, atomic force microscopy, transmission electron microscopy, X-ray diffraction, photoluminescence, and Raman scattering. Depending on the substrate, composition and conditions of growth, either the zincblende or wurtzite nanowires were obtained. At compositions where the stable form would have been normally wurtzite, the zincblende form could be obtained under certain growth conditions. From the orientations of the ordered nanowires on the substrate surface, their directions of growth were deduced and confirmed by high resolution lattice imaging. The relationship between the band gap and the composition of the nanowires were measured and found to deviate from that of bulk alloys and epilayers. The interplay between the growth conditions and compositions and morphology of the nanowires are discussed.
Present work shows that simple, standard methods of metal addition, without the need for ion implantation or other complex and expensive processes, can dramatically improve the performance of titania based structures compared to P25 for (i.e. hydrocarbon oxidation) photocatalytic reactions. In this work, Au and Pt were incorporated into titania nanotubes, and their photocatalytic activities were investigated in detail. The samples were analyzed using a JEOL FEG-2010F field emission gun scanning transmission electron microscopy (STEM) with attached Oxford Instruments' X-ray energy-dispersive spectroscopy (EDS) system and Gatan imaging filtering (GIF) system. Both high-resolution TEM (HRTEM) images and high angle annular dark-field (HAAD) images were recorded for the specimens. The performance of the samples was tested for the oxidation of acetaldehyde using a continuous flow reactor. The pure nanotube is more photoreactive than commercial P25 titania. Both Au and Pt treated nanotube samples increased the photo reactivity. The most significant result of this work is that the activity of Pt (< 1 nm) containing nanotube is more than 10 times the rate of P25, and more than 6 times the rate of the pure nanotube. However, sizes of the Au and Pt nanoparticles on the nanotube surfaces likely affected the photo-reactivity. Large size of the Au and Pt particles decreased the photo-reactivity. Specifically, the addition of platinum without formation of obvious nanoparticles on the nanotube surfaces increased the maximum activity significantly, and increased the total yield.
Silica-supported iron oxide nanoparticles are prepared by precipitation within the pores of amine-functionalized SBA-15 silica. The loading of the iron oxide possible by this method is at least 11 wt%. STEM and TEM images show that the supported particles have a uniform diameter (average ∼ 4.0 nm) and are well dispersed. The supported iron oxide nanoparticles are amorphous after calcination at 300°C and, consistent with their nanoscale dimensions, are superparamagnetic at room temperature.
One promising route to integrate nanostructured titania (NST) into nano/micro electrical mechanical systems (N/MEMS) devices is by reacting Ti films with aqueous hydrogen peroxide (aq. H2O2) solution. However, little is known about the reaction kinetics between aq. H2O2 and Ti thin films. Here, the effect of Ti microstructure and film thickness on kinetics of reaction was investigated. For films less than 50 nm thick, the kinetics is interface-reaction controlled. For thicker films, the reaction is controlled by diffusion through a hydrated titania gel layer. Activation energies of these kinetics were extracted. Pore size of NST is affected by thickness of parent Ti films. Depending on thickness of parent Ti films, NST with average pore sizes ranging from 15 nm to 150 nm was formed. The ability to form integrated porous NST features with controllable pore sizes may have implications on the development of devices for drug delivery and macromolecular separation.
The surface modification of nanoporous tin dioxide materials was achieved with trialkynylorganotin(IV) (C4H9-C=C)3Sn-(CH2)n(C10H20) (n = 4,6)1 endowed with a perylene dye to yield powders containing up to 0.13 mmol.g-1 of perylene unit or dye-modified thin films. Irreversible chemisorption occurred in solution at room temperature to give perylene dye grafted at the oxide surface via the cleavage of the three tin-alkynide bonds of the precursor and the formation of Snbulk-O-Sn-Cdye linkages. The photoelectrochemical cells made with the 1-modified films demonstrated maximum incident photon to current efficiency (IPCE) as high as 18% at 430 nm under white light illumination. The performances of the cells were interpreted in terms of the intrinsic properties of tin dioxide and aggregation of the perylene dye.
A novel approach is presented to synergistically enhance photocatalytic activity in a nanocomposite using the high aspect ratio of carbon nanotube (CNT) and their unique electrical properties. Composite nanoparticles were synthesized with sol-gel nano-coating on multi-walled carbon nanotubes (MWNTs). The nanostructure was characterized using SEM, TEM, XRD, Raman, FTIR and UV-VIS spectroscopies. Photocatalytic efficiencies of commercial photocatalysts (Degussa P25) and TiO2 nano-coated MWNTs were evaluated by Azo dye degradation tests. Superior photocatalytic activity was observed for the nanocomposite with UV-A and with visible light only irradiation.
We present a method for controlled deposition of polyaniline from a colloidal suspension. Stable suspensions of polyaniline colloids were formed by dispersing polyaniline/formic-acid solutions into acetonitrile. It is demonstrated that the positively charged polyaniline colloids can be electrophoretically patterned onto microfabricated device features with great precision and control. Futhermore, the electrophoretically deposited polyaniline films reveal a nanoporous morphology, which greatly enhances the gas diffusion that leads to improved sensor performance.
Reductive removal by hydrogeneration using supported Pd/M (M= Cu, Pt, Ag, Co, Fe, Mo, Ni, Rh, Ir, Mn and Cr) bimetallic catalysts has emerged as a promising alternative for nitrate removal in drinking water . Fundamental understanding how the atomic arrangement of Pd and a second element, such as Cu, affect the activity nitrite reduction and selectivity of dinitrogen will be accomplished by coordinated synthesis (Shapley), activity/selectivity/efficiency measurements (Werth) and nanostructure determination (Yang & Xu). In this paper, we report a systematic study of novel polyvinylpyrrolidone (PVP) stabilized nanoscale Pd-Cu colloids, with homogeneous and narrow size distribution, with Pd: Cu ratios varying from 50:50 to 90:10. Initial measurements on catalytic activity for nitrate reduction demonstrated a dependence on the relative composition. Electron microscopy studies, including Z-contrast imaging , energy-dispersive X-ray emission (EDX), electron diffraction and high-resolution electron microscopy (HREM), revealed a surprising change in structure at the 80:20 Pd-Cu composition, where, with less than 80% Pd,the nanoparticle forms a core-shell structure but for nanoparticles containing 80% or more Pd, it is homogeneous. We are at the pivotal point of directly correlating these nano-structures with the catalytic activity. Such an understanding is essential for the efficient development of catalysts for the purification of drinking water.