Synthesis, Structure And Properties Of One-Dimensional MoO₃ Nanomaterials

One-dimensional (1-D) nanomaterials including nanotubes, naorods, naowires and nanobelts, have been significantly attracted attentions due to their unique electrical, optical, magnetic and mechanical properties. It is important to control the geometry, dimension, composing, crystal structure and properties of the 1-D nanomaterias and investigate the relationship between structure and properties by studying their growth mechanism, which is useful to achieve the goal of synthesizing functional materials by our willing. Orthorhombic molybdenum trioxide have been widely applied in many fields, such as lithium ion secondary batteris, catalysts, sensors, electrochromic and optical materials because of their typical two-dimensional layered structure.In this dissertation, one-dimensional MoO₃ nanomaterials were chosen as the object of study. Modern testing methods were used to study the preparation, structure and properties of 1-D MoO₃ naomaterials, and MoO₃ nanobelts modified by some polymers and metal cations were also investigated. The obtained main results are as follows:(1) MoO₃ nanobelts were synthesized via the simple hydrothermal reaction with no template or catalyst by using stable MoO₃ sols, which were fabricated by three different methods. All of the as-synthesized samples consisted majority of orthorhombic MoO₃ nanobelts with width of 150～400 nm, average thickness of 70 nm, length of 4～15μm. The single nanobelt was structurely uniform single crystal without any dislocation and the surface of nanobelt was clean. The MoO₃ naobelts tended to grow together to form nanoblets bundle.(2) The electrochemical resistance firstly increased and then decreased with the discharge process because of the Li⁺ enrichment on the cathode surface. The Li⁺ ions were inserted not only into [MoO₆] interlayers space but also into intralayers along the b axis, and the Li⁺ ions could merely reversibly intercalte/deintercalate between the interlayer, however, the Li⁺ accommodated in the intralayers tended to trigger an uncoverable structural transformation of MoO₃, causing irreversible capacity loss. The first discharge capacity of the MoO₃ nanobelts was 301 mAh/g which was higher than that of bulk MoO₃ (249 mAh/g) and the nanobelts cell exhibited a capacity loss of only 12% after 5 cycles. The better electrochemical performance of MoO₃ nanobelts was attributed to their special structure. The single MoO₃ nanobelt showed nonlinear current/voltage (I/V) characteristics. The photoluminescence bands of the MoO₃ nanobelts were at 402 and 475 nm, and there was no obvious PL band in bulk MOO₃. MoO₃ could effectively decolorate four dyes. Contrasting with the bulk MOO₃, nanobelts had better photocatalytic activity because the specific surface area of MoO₃ nanobelts was larger, and the oxidation and reduction ability of the photo-generated electrons and holes of the nanobelts increased and the recombination rate of photo-generated electrons and holes decreased.(3) The growth mechanism of the MoO₃ nanbelts prepared via different process were investigated by studying the influence of hydrothermal temperature and reaction time on the structure and morphology of the products. The MoO₃ sols obtained by ion exchage method were firstly tansformed into metastable h-MoO₃ microrods, and subsequently stable orthorhombic MoO₃ nanobelts were formed with the reaction temperature increasing and time prolonging. With respect to the latter two MoO₃ sols, fabricated by peroxide oxidation and dispersion, the unstable O-O bonds in the MoO₃·pH₂O·qH₂O were broken up at the low hydrothermal temperature or for short reaction time to form the hydrated molybdenum trioxide particles and plates. When the temperature and time increasing, the H₂O molecule disappeared to form the orthorhombic MoO₃ nanobelts. The axis direction of the nanobelts was [001] direction.(4) PEO and PANI were introduced between the [MoO₆] interlayers in MoO₃ nanobelts, which led to the increasement of the interlayer distance, to form the (PEO)_0.1MoO₃, (PANI)_0.02MoO₃ and (PANI)_0.05MoO₃ nanobelts, respectively. Because of the existing of polymers, the nanobelts were easier to aggregate to form the flower-like morphology. The electrochemical properties of the nanobelts were improved with the intercalation of PEO and PANI into the MoO₃ nanobelts. The hydrothermally synthesizedα-MoO₃ nanobelts were lithiated by a sencondary reaction while preserving crystal structure and surface morphology. The single lithiated MoO₃ nanobelt showed linear current/voltage (I/V) characteristics and the electrical conductivity became higher due to the introduction of Li⁺ ions, and the electrochemical property was enchanted. In addition, MoO₃ nanobelts were doped by other metal ions, such as Cr⁶⁺, V⁵⁺ and Ag⁺, and different cations had different influence on the structure and morphology of the products.(5) Large-area ordered arrays of MoO₃ nanobelt were obtain on silicon substrate based on the hydrothermal process. The width range of nanobelts became smaller (width of 150～250 nm) because they grew tightly on the width direction. The improved electrochemical property was attributed to the alignment of the nanobelts. The field emission turn-on field (E_to) was 12.8 V/μm and the largest current density was 0.4 mA/cm², and the MoO₃ nanobelt arrays followed F-N theory. In addition, ordered amorphous molybdenum oxide nanorod arrays were synthesized in the pores of PC template by sol-gel method. MoO₃·2H₂O nanorods arrays were transformed after the heattreatment at 390℃for 1 hour. And their cycling property and field emission property were better.