The field of transition metal oxides represents an exciting and rapidly expanding research area that spans the border between the physical and engineering sciences^{[1], [2], [3] and [4]}. Molybdenum oxides (MoO_x) are one of the most attractive metal oxides due to their special structural characteristics. MoO_x comprises two simple binary oxides, namely, MoO₃ and MoO₂. MoO₃ has several polymorphs, such as the thermodynamically stable α-MoO₃ (space group Pnma), metastable β-MoO₃ (P2₁/c), -MoO₃ (P2₁/m), and hexagonal metastable h-MoO₃ (P6₃/m)⁵. MoO₂, with its distorted rutile structure, is an unusual but interesting transition metal oxide because of its low metallic electrical resistivity (8.8 × 10⁻⁵Ω·cm at 300 K in bulk samples), high melting point, and high chemical stability⁶. MoO₂ has been used as a catalyst for alkane isomerization^{[7], [8], [9], [10] and [11]}, oxidation reactions¹², and as a gas sensor¹³. It is also a promising anode material for Li-ion batteries14 N. Sharma et al., Chem Mater 16 (3) (2003), p. 504.^{[14], [15], [16], [17] and [18]}.

As nanotechnology has developed, nanostuctrues have received significant attention. Interesting physical phenomena appear as the scale of the building blocks approaches the nanoscale, such as the size effect, quantum conductance, and coulomb blockades. Nanowires are one of these building blocks that possess several distinct, practical properties, such as well-controlled dimensional composition, electronic radial transport, and crystallinity; this helps organize the nanoscale building blocks into assemblies and, ultimately, useful systems. Although research on molybdenum oxide (MoO_x) nanowires started relatively late, MoO_x nanowires are now showing potential for both fundamental research and applications in industry. MoO_x nanowires represent attractive building blocks for active nanodevices. By controlling the growth and organization, they can be used to produce a number of novel, highly-efficient, robust, integrated nanoscale devices, including field emission devices (FED) and photodetectors.

Nanowires and nanorods are typically cylindrical, hexagonal, square, or triangular in cross-section. Nanobelts are typically rectangular in cross-section, with highly anisotropic dimensions. In this article, nanobelts and nanorods are regarded as special kinds of nanowire. Nowadays, the bottom-up technique is usually used for the assembly of MoO_x nanowires and devices. A bottom-up design necessitates building with precisely controlled nanomaterial parameters (including chemical composition and structure), which determine the final performance of the device^{[19] and [20]}. Fig. 1 shows the bottom-up procedure for MoO_x nanowire device fabrication. To date, there has been no review that systematically summarizes the recent advances in MoO_x nanowires from synthesis to devices and their properties. In this article, we focus on recent advances on MoO_x nanowires based on our group's work.