Controlled Synthesis of Self-Assembled Metal Oxide Hollow Spheres Via Tuning Redox Potentials: Versatile Nanostructured Cobalt Oxides |
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A general method to produce nanostructured self-assembled metal oxides with hollow cores was successfully developed by controlling the net redox potentials. Cobalt and cerium oxides are demonstrated in this study. The nanostructured CoOOH hollow spheres are versatile precursors for various cobalt oxide datives (eg. Co3O4 , LiCoO2), and also possess excellent catalytic activity.
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[Applied Catalysis A: General 2008, 335, 187–195] |
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Behavior of H2 chemisorption on Ru/TiO2 surface and its application in evaluation of Ru particle sizes compared with TEM and XRD analyses |
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Hydrogen adsorption on Ru surfaces of Ru/TiO2 catalysts has been investigated in a gas adsorption apparatus at temperatures from 35 to 150
°C,
H2 partial pressures from 0.05 to 700 Torr, and adsorption equilibration time at each pressure from 3 to 60 min. Results showed that adsorption of
H2 on the TiO2 support are negligible. TEM measurements suggested that there was no strong metal–support interaction (SMSI) on the synthesized
Ru/TiO2 samples to inhibit H2 adsorption. A transition pressure point of 60 Torr was observed for H2 adsorption on Ru/TiO2. Most of the strong chemisorption occurs before this transition pressure and weak chemisorption happens thereafter. H2 adsorption increases with temperatures from 35 to 75 °C indicating an activation energy present for H2 chemisorption on Ru. Above 75 °C, the adsorption slightly decreases with further increase of temperature up to 150 °C. Monolayer coverage was attained at 75 °C for 30 min equilibration time with a H2 pressure higher than 300 Torr. Monolayer H2 chemisorption was used to determine Ru metal particle size in Ru/TiO2 systems compared with XRD and TEM analyses. Selected area electron diffraction (SAD) indicated that there was no preferred crystallographic orientation of the TiO2 supported Ru. Therefore, exposed Ru atoms equally contribute from the three low-index planes [(0 0 1), (1 0 0), and (1 1 0)] with the highest atomic density instead of only the (1 0 0) plane for conventional treatments.With this assumption, the average Ru particle size calculated from H2 chemisorption (4.6 nm) agrees with the TEM measurements (4.1 nm). |
[Applied Catalysis A: General 2008, 335, 187–195] |
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Tuning of Texture and Structure of Copper-Containing Nanocomposite Oxide Materials |
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For studies of textural tuning, structural tuning, or materials sintering, copper/aluminum and copper/zinc
nanocomposite materials were prepared. Resistance to sintering of different phases was investigated. Thermal
analysis methods were used to design feasible thermal treatment methods that can avoid destructive damages
to gels. X-ray diffraction and nitrogen sorption measurements were used for structural and textural analysis. Compared with the wide distributions of pore sizes and low surface areas of the products prepared via conventional coprecipitation methods, a novel urea-gelation/thermal-modification method was developed to produce CuO/Al2O3 nanocomposites with narrow distributions of pore sizes and high surface areas. In comparison with the products of conventional coprecipitation methods, this novel urea-gelation/thermalmodification method produces copper/aluminum nanocomposites with significantly higher specific copper loading, which should be valuable in apparatus that have space limitations, such as vehicle fuel cell systems. Stepwise reduction and reoxidation were studied for the structural tuning and purification of Cu-Al-O spinel phases with isotropic and gradual unit-cell contractions. The textural and structural features of some copper/ aluminum nanocomposite materials were observed by electron microscopy methods, that is, field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and convergent beam electron diffraction (CBED). |
[ Journal of Physical Chemistry C 2008, 112(5), 1446-1454 ] |
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Ultrasonic Nozzle Spray in Situ Mixing and Microwave-Assisted Preparation of |
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Nanocrystalline spinel nickel ferrite and zinc aluminate particles (6-20 nm) can be prepared by a recently
developed continuous flow method that combines microwave heating and in situ ultrasonic nozzle spray
mixing. The preparations were carried out at ambient pressure (1 atm), microwave power (0-600 W), and
ultrasonic nozzle with resonant frequency of 48 or 120 kHz. The products were characterized by X-ray
diffraction, field emission scanning electron microscopy, transmission electron microcopy, X-ray electron
dispersive spectroscopy, Brunauer-Emmett-Teller spectroscopy, Raman spectroscopy, Fourier transform
infrared spectroscopy, thermogravimetric analysis, and temperature-programmed desorption mass spectrometry.
The results showed that the ultrasonic nozzle and microwave irradiation complement each other, with respect
to the purity of the products. The specific advantages of INM method for the preparing nickel ferrites are that
pure materials with high surface areas and tunable particle sizes are produced, and this process is continuous. |
[J. Phys. Chem. C, 2008, early view] |
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Inorganic synthesis for the stabilization of nanoparticles: application to Cu/Al2O3 nanocomposite materials |
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A series of copper/aluminum nanocomposites with different mixing homogeneities and different texture
features were prepared via various inorganic synthesis methods including coprecipitation, gelation, and
stepwise thermal modification. Nitrogen sorption measurements and X-ray diffraction were used for textural
and structural analysis, respectively. Solid-solid reaction analysis and differential scanning calorimetry
(DSC) analysis were developed for the determination of the mixing homogeneities of copper-based
nanocomposite materials. A sintering experiment at 250-600 °C for 350 h under methanol-steam reforming
conditions was carried out to compare the stability of supported Cu0 nanoparticles. Although a large
initial size of supported nanoparticles is not favorable, those supported nanoparticles with a small initial
size cannot ensure good thermal stability. The mixing homogeneities of CuO/Al2O3 mixed metal oxide
(MMO) nanocomposites significantly affected the thermal stability of their reduced Cu0 crystallites. Besides
homogeneity control, creation of narrow distributions of pore sizes with small major pore diameters
(e.g., around 3.5 nm) can also be used for the stabilization of supported Cu0 nanoparticles. We found that mixing homogeneity of a nanocomposite is likely the major factor in the stabilization of nanoparticles, whereas a narrow distribution of pore sizes might confine the growth of nanoparticles, possibly via space limitations. Field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and convergent beam electron diffraction (CBED) were used for the observation or structure determination of the copper/aluminum nanocomposites. This paper also provides information on the deactivation of copper catalysts via thermal sintering under methanol-steam reforming conditions. |
[Chemistry of Materials, 2007, 19(19), 4820-4826] |
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New Synthetic Route , Characterization, and Electrocatalytic Activity of Nanosized Manganite |
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Nanosized g-MnOOH (manganite) has been synthesized by a new route via the reduction of KMnO4 with sucrose and MnSO4 in acidic medium under refluxing conditions for 4 and 6 h. Different characterization techniques of these manganite materials were used XRD, FESEM, TEM, TGA, and IR. Two new synthetic methods were developed, one involving addition of KMnO4 into a solution of both sucrose and MnSO4 while the other involved addition of KMnO 4 solution into sucrose only followed by addition of MnSO4 (s). The latter method yielded smaller particles (up to 30 nm) than the former method (up to 80 nm) and the conventionally prepared manganite (up to 50 nm). The synthesized manganite materials exhibited promising characteristics when tested as electrocatalysts in the reduction of O2.
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[Chem. Mater., 2007 , 19, 1832-1839] |
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Hydrothermal Growth of Manganese Dioxide into Three-Dimensional Hierarchical Nanoarchitectures |
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Novel three-dimensional (3D) hierarchical nanoarchitectures of e-MnO2 have been synthesized by a simple chemical route without the addition of any surfactants or organic templates. The self-organized crystals consist of a major e-MnO2 dipyramidal single crystal axis and six secondary branches, which are arrays of single-crystal e-MnO2 nanobelts. The growth directions of the nanobelts are perpendicular to the central dipyramidal axis, which shows sixfold symmetry. The shape of the e-MnO2 assembly can be controlled by the reaction temperature. The morphology of e-MnO2 changes from a six-branched star-like shape to a hexagonal dipyramidal morphology when the temperature is increased from 160 to 180 °C. A possible growth mechanism is proposed. The synthesized e-MnO2 shows both semiconducting and magnetic properties. These materials exhibit ferromagnetic behavior below 25 K and paramagnetic behavior above 25 K. The e-MnO2 system may have potential applications in areas such as fabrication of nanoscale spintronic materials, catalysis, and sensors.
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[Adv. Funct. Mater. 2006 , 16 , 549–555] |
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Shape-Controlled Synthesis of Manganese Oxide Octahedral Molecular Sieve Three-Dimensional Nanostructures |
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We successfully synthesized novel OMS-2 nanostructures under mild and organic template-free conditions. The shape evolution of OMS-2 can be performed via simply varying temperature. The hierarchically ordered nanoscale architectures of OMS-2 originated from primary microporous structures of cryptomelane-type manganese oxides, secondary structures of uniform nanoparticles, and tertiary architectures of microscopic arrays. These OMS-2 nanoparticles with specific structures may find potential applications in sensors, catalysis, biomarkers, microelectronics, and energy storage. |
[Journal of the American Chemical Society, 2005, 127(41), 14184-14185. ] |
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Synthesis and Characterization of Silicon Nanowires on Mesophase Carbon Microbead Substrates by Chemical Vapor Deposition |
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Silicon nanowires (SiNWs) have been fabricated by chemical vapor deposition at ambient pressure using SiCl4 as a silicon source and mesophase carbon microbead powder as a substrate without any templates and/or metal catalysts. The SiNWs have a crystalline core with a very thin amorphous SiOX sheath. The obtained SiNWs are homogeneous with average diameters below 50 nm and lengths up to micrometers. Temperature and time effects on the growth of SiNWs were systematically studied. Higher reaction temperatures and longer reaction times resulted in larger diameters and higher yields of SiNWs. SiNWs with a better crystallinity can be obtained at higher temperatures and longer reaction times. The obtained SiNWs were characterized by field-emission scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and transmission electron microscopy.
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[J. Phys. Chem. B, 2005, 109 ( 8 ), 3291 - 3297 |
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Synthesis of mordenite nanocrystals
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Mordenite with crystal diameter <100 nm has been prepared by the modification of different synthetic parameters such as the source of aluminum, the presence of seeds, the use of low temps., longer crystalization times, and different silica to alumina ratios. The decrease in the crystal diameter of the prepared mordenite was monitored by the application of the Scherrer equation that relates the broadness of the x-ray diffraction peaks to crystal sizes.
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[Microporous and Mesoporous Materials, 2004, 67, 19-26] |
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Self-Assembly of OMS Hexagonal Flakes into Mesoporous Hollow Nanospheres
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Manganese oxide hollow nanospheres were prepared using a straightforward, template-free synthesis. The resulting material was mesoporous, crystalline, and of uniform diameter. The nanospheres were characterized by XRD, HR-SEM, and HR-TEM, and pore size distributions were calcined from nitrogen desorption. Unlike previous synthesis methods that use an inorganic template, this procedure requires no separation after synthesis to remove the template. The nanospheres are composed of hexagonal g-manganese oxide flakes and are approx. 400 nm in diameter g-MnO2 is composed of a ramsdellite matrix (1 x 2 tunnels) with randomly distributed microdomains of pyrolusite (1 x 1 tunnels). These materials could have applications as cathodic battery materials, oxidation.catalysts, catalyst supports, and adsorbents for pollutants.
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[Journal of the American Chemical Society, 2003, 125, 4966-4967] |
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Dynamic organization of inorganic nanoparticles into periodic micrometer-scale patterns
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We report the study of oscillatory phenomena, in which inorg. manganese oxide nanoparticles are organized with a high degree of periodicity with multiscale ordering.
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[Angewandte Chemie International Ed., 2003, 42, 2905-9] |
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Magnesium Manganese Oxide Nanoribbons
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A todorokite-type magnesium manganese oxide molecular sieve material with a morphology of nanoribbons has been synthesized using a combination of techniques consisting of a sol-gel process to synthesize a nanosized tetraethylammonium manganese oxide layered material, an ion-exchange method to prep. a thin-film Mg-buserite (Mg-OL-1) precursor, and a hydrothermal reaction to transform the Mg-buserite to nanoribbon-like Mg-todorokite (Mg-OMS-1) material. The transformation of the Mg-buserite thin films to Mg-todorokite nanoribbons has been studied using XRD and TEM. XRD data indicate that the structural transformation of Mg-OL-1 to Mg-OMS-1 is completed after hydrothermal heating at 150 °C for about 40 h and at 200 °C for less than 8 h. TEM data show that the nanoribbons form at an early stage of the hydrothermal treatment, followed by the appearance of lattice fringes. TEM data also indicate that the well-formed nanoribbon-like crystals have a uniform tunnel dimension along the a axis and a long-range-ordered 3 ´ 3 tunnel structure along the b axis. FESEM data show that the lengths of the nanoribbons range from a few micrometers to tens of micrometers and that the widths range from 20 to 100 nm. Elemental and average oxidation state analyses indicate a formula for the nanoribbon of Mg2.11Mn5.46O12xH2O. According to TGA and TPD-MS data, these nanoribbons are thermally stable up to 500 °C . The BET surface area is 81 m2/g, which is higher than that of bulk todorokite synthesized using conventional methods. The catalytic performance of nanoribbon-like Mg-todorokite materials on converting benzyl alcohol to benzaldehyde was improved greatly as compared to that of bulk Mg-todorokite materials. |
[Journal of Physical Chemistry B, 2002, 106, 9761-9768] |
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Colloids, helices, and patterned films made from heme proteins and manganese oxide
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New colloidal materials with enzyme-like peroxidase activity were made from octahedral layered manganese oxide and myoglobin and Hb, and were converted to macroscopic helixes and patterned films.
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[Chemical Communications, 2002, 19, 2254-2255] |
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Synthesis and Characterization of Nanofibrous Sodium OMS
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Na Mn oxide octahedral mol. sieves with a 2 x 4 tunnel structure (Na-2 x 4) were hydrothermally synthesized from Na-birnessite materials at low temperatures and pressures. The synthetic template materials, the pH value of the medium, and the autoclaved temperature are critical in the synthesis. Na salts, such as NaCl, NaNO3, and Na2SO4, are good templates for Na-2 x 4. In strong basic solution or <160 °C, Na-birnessite does not transform to the Na-2 ´ 4 structure. TEM images show the synthesized Na-2 x 4 is made up of thin slab-like single crystals elongated along the b axis. The formula of Na-2 x 4 can be written as Na0.33Mn3+0.33Mn4+0.67O2 × xH2O, and it is monoclinic with space group C2/m. The unit cell parameters (a, b, c, and b ) for Na-2 x 4 are 14.434(5) .ANG., 2.849(7) .ANG., 23.976(6) .ANG., and 98.18 ° , respectively. These data for Na-2 ´ 4 are slightly different from the data for Rb-2 ´ 4 synthesized under high pressure and high temp., which are reported by Rziha et al. (Eur. J. Miner. 1996, 8, 1155-1161). The surface area of Na-2 x 4 is .apprx.57 m2/g. Na-2 x 4 materials are thermally stable up to 450 °C as indicated by TGA and TPD data.
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[Chemistry of Materials, 2001, 13, 1585-1592] |
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A review of zeolite-like porous materials
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A review with 50 references on the most recent reports of zeolite-like microporous and mesoporous materials and in particular their synthesis. We focus on porous transition metal oxides of various types, also including porous pillared interlayer materials that have either pos. or neg. charges. This review covers the most recent papers published from about Jan. 1995 to the present. A discussion of new transition metal oxides, porous pillared materials, sulfide layered materials and porous phosphates is included. The section on porous transition metal oxides focuses on both microporous and mesoporous transition metal oxides and, in particular, synthetic strategies used to produce new materials. The next section concerns pillared materials, including layers that are anionic as well as cationic. Sulfide systems discussed in a further section are primarily layered phases. The final section concerns porous phosphate materials. |
[Microporous and Mesoporous Materials, 2000, 37, 243-252] |
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Spontaneous formation of inorganic helices
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The authors show that stable helixes of porous manganese oxide materials can be formed spontaneously from uniform sols of tetra-Me ammonium (TMA) permanganate and that they are excellent semiconductors. These inorganic helixes contain micropores, can be converted into other structures, and their compn. can be varied. Ion exchange of TMA helix with K+ leads to crystalline helixes with composition K+0.93Mn4+2.1Mn3+1.9O7(OH)1.03x2.7H2O. Thermal treatment of the crystalline helixes leads to formation of an octahedral mol. sieve (OMS) tunnel structure of synthetic cryptomelane (OMS-2), and a composition of K+1.86Mn4+4.7Mn3+3.3O14.57(OH)1.43 × 0.7H2O (K-OMS-2) with an av. oxidation state for Mn of 3.58. The microstructure of K-OMS-2 helixes obtained by scanning and transmission electron microscopy indicates microporosity of the 2 ´ 2 crystal structure of OMS-2, mesoporosity in between the colloidal crystallites, and a net alignment and connectivity of the microporous network along the length of the helix. This alignment leads to conduction along the helix and, as a result of the porosity, rate processes such as ion exchange are enhanced owing to fast diffusion. The anisotropic DC-4 probe condition of 4.2 ´ 10-1 W-1cm-1 at 21 ° for the K-OMS-2 system is roughly an order of magnitude more conductive than most well-formed single crystals of semiconducting cryptomelane-like materials. |
[Nature, 2000, 405, 38]
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Last update: 01/25/2008 by Anais Espinal |