Characteristics of molybdenum trioxide nanobelts prepared by thermal evaporation technique

Abstract

Single-crystalline nanobelts of molybdenum trioxides (MoO₃) were grown by a thermal evaporation method of molybdenum metal pellets at ambient pressure in a flow of O₂. The chemical composition, crystalline structure and optical properties of the nanobelts were investigated by various characterization techniques such as scanning electron microscopy, transmission electron microscopy, Raman-scattering, energy dispersive X-ray spectroscopy and UV–vis-NIR spectroscopy. The samples were nanobelts with a width up to 50 μm, about 85 nm in thickness and from tens to several hundred micrometers in length. The analysis indicated that as-synthesized samples were orthorhombic structured MoO₃ grown with [0 0 1] preferred orientation. The fundamental optical absorption edge corresponds to direct allowed transitions with an energy gap located at about 3.01 eV.

Introduction

Micro and nanomaterials with various structures such as tubes, wires, belts and rods have attracted increasing attention in recent years because of their unique electrical, optical and mechanical properties [1]. A material of particular interest is molybdenum trioxide (MoO₃) which is a layered n-type semiconductor with various advanced applications as catalysts [2], gas sensors [3], [4], [5], batteries [6], [7], lubricants [8], memory materials [9] and electrochromic devices [10], [11]. Hence, molybdenum oxides in the form of micro and nanostructures are promising candidates for electronic and optoelectronic microdevices.

According to literature, molybdenum trioxide nanostructures have been synthesized using various techniques. Li and Bando [12] prepared single-crystal MoO₃ nanotubes on a Ta substrate by a simple CVD process in a high vacuum infrared irradiation-heating furnace. Ding et al. [13] fabricated MoO₃ nanobelts and microballs by oxidization of molybdenum metal in an ambient atmosphere. Chu et al. [14] reported single-crystalline nanobelts of MoO₃ synthesized by direct thermal oxidization evaporation of metal molybdenum foils. Li et al. [15] grew large amounts of single-crystal MoO₃ nanobelts based on the preparation of MoO₃·H₂O solution, and subsequent treatment of a hydrothermal reaction. Wang et al. [16] synthesized single-crystalline MoO₃ nanobelts through homogeneous precipitation and subsequent hydrothermal treatment. Fang et al. [17] presented a hydrothermal synthetic method for the preparation of anisotropic uniform single-crystal MoO₃ nanostructures via decomposition and condensation of peroxomolybdic acid. Song et al. [18] obtained one-dimensional MoO₃ nanostructures directly from commercial bulk MoO₃ crystals by a surfactant-templated hydrothermal process. Zhao et al. [19] reported the synthesis of nanostructures of molybdenum trioxides by directly oxidizing a spiral coil of molybdenum, at ambient atmosphere, by passing a current through the coil. Gouma et al. [20] synthesized single-crystal MoO₃ nanowires by means of the electrospinning method.

In this work, we report the synthesis of molybdenum trioxide nanobelts produced by a simple thermal evaporation of Mo pellets at ambient pressure in a flow of O₂. The samples were analyzed with scanning electron microscopy, X-ray diffraction, transmission electron microscopy and Raman-scattering. The optical absorption coefficient was calculated for all samples. From these results, using the interband absorption theory, the optical gap was deduced.