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J. Phys. Chem. B 2006, 110, 20182-20188
Polymer-Assisted Growth of Molybdenum Oxide Whiskers via a Sonochemical Process Chirakkal V. Krishnan, Jinglu Chen, Christian Burger, and Benjamin Chu* Department of Chemistry, State UniVersity of New York at Stony Brook, Stony Brook, New York 11794-3400 ReceiVed: May 23, 2006; In Final Form: July 12, 2006
Whiskers of molybdenum oxides with high aspect ratios were synthesized from peroxomolybdate precursor solutions in the presence of small amounts of poly(ethylene glycol) (PEG) via a sonochemical process at temperatures of 25-70 °C. Irradiation with ultrasound reduces the time needed for the growth of micrometersized whiskers from weeks to a few hours. The simplicity of the sonochemical approach also compares favorably to a hydrothermal/solvothermal process. The morphology, crystal structure, and other characteristics of the whiskers were characterized by scanning electron microscopy, transmission electron microscopy, selective area electron diffraction, energy-dispersive X-ray spectroscopy, wide-angle X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and the Brunauer-EmmettTeller method. The surface area of the calcified molybdenum oxide whiskers (55.4 m2/g) was found to be much higher than those of molybdenum oxide nanofibers (35 m2/g)5 or nanorods (13.4 m2/g).6 The growth rate of various crystal faces could be postulated to be controlled by the binding of peroxomolybdate ions to pseudo-crown ether cavities formed by PEG. The reduction of molybdenum oxide to produce mixed-valent oxides and their growth could also be controlled by the reducing ability of PEG. The aspect ratio of the molybdenum oxide whiskers increased with decreasing concentration in the initial peroxomolybdate precursor solution. Whether the precursor solution species was H2Mo2O3(O2)4(H2O)2, H2MoO2(O2)2, or MoO2(OH)(OOH), the peroxide group in all the species disproportionates to give the final product MoO3 by a catalytic process. On the basis of experimental evidence of the dual role of glycols, a mechanism for the growth of the molybdenum oxide whiskers is proposed.
Introduction In recent years, many ingenious schemes and pathways have been developed for the fabrication of nanostructured materials. These materials have been the focus of intense research because of their potential applications in various fields of technology. The role of polymeric systems as templates for nanofabrication has also been reviewed recently.1 Polymers provide templates with different morphologies and tunable sizes. They can also be modified to enhance interactions and can be removed after the reactions, either by dissolution with a solvent or by calcification. A large polyoxomolybdate cluster that resembled a zeolite structure with a very large length scale (about 5 nm, instead of a typical 1 nm lattice constant for zeolites) has been synthesized from aqueous solution of peroxomolybdate and a triblock copolymer, E45B14E45, or simply by using a homogeneous long chain polyoxyethylene.2 A smaller polyoxomolybdate cluster with a diameter of 2.5 nm and having 132 Mo (60 MoV and 72 MoVI) atoms has been synthesized from solutions of ammonium molybdate, ammonium acetate, acetic acid, and hydrazine sulfate.3 These studies clearly demonstrate the dominant role of the mixed valence of molybdenum in forming nanostructures and/or supramolecular assemblies. Among the important transition-metal oxides, MoO3 and its derivatives are especially interesting because of their catalytic, electrochromic, and photochromic properties.4 Molybdenum trioxide is known to exist in a variety of oxidation and hydration states, such as oxides, suboxides, hydroxides, and crystalline * To whom correspondence
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hydrates. Three different crystalline polymorphs of MoO3, i.e., stable orthorhombic R-MoO3, metastable β-MoO3, and the metastable high-pressure phase MoO3-II, have been reported.5 The MoO3 hydrates include monoclinic dihydrate (MoO3‚2H2O), monoclinic monohydrate (MoO3‚H2O), triclinic monohydrate (MoO3‚H2O), monoclinic hemihydrate (MoO3‚1/2H2O), and orthorhombic MoO3‚1/3H2O.5 Recently, remarkable progress has been achieved concerning the preparation of molybdenum oxide nanoparticles and its derivatives. Starting from the structure-directing organic template, R-MoO3‚H2O with fibrous structures has been prepared using a lamellar molybdenum oxide-amine (with long alkyl chains) composite and hydrothermal reaction followed by nitric acid treatment.6 One-dimensional R-MoO3 nanoribbons or nanorods were also synthesized via acidification and aging (about 1 month) of ammonium heptamolybdate tetrahydrate solutions under hydrothermal conditions.7 The as-grown R-MoO3 crystals were further used as metal oxide precursors for the preparation of hexagonal MoS2 nanorods under H2S/H2-reduction.7 Very recently, Patzke et al. developed a flexible one-step solvothermal procedure for the synthesis of MoO3 nanorods using MoO3‚2H2O as the starting material in both neutral and acidic media.8 The rods have typical diameters of around 100 nm and microscale lengths of 3-8 µm. However, all these hydrothermal or solvothermal routes for the formation of MoO3 nanorods require high pressure with the use of stainless steel autoclaves, high temperatures (120-200 °C) and long reaction times (20 h to 7 days). In the present work, we report the rapid synthesis of molybdenum oxide whiskers with high aspect ratios via an ultrasonic irradiation procedure. A trace amount of polymer
10.1021/jp063156f CCC: $33.50 © 2006 American Chemical Society Published on Web 09/26/2006
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