Centimeter-long V2O5 nanowires: from synthesis to field-emission, electrochemical, electrical transport, and photoconductive properties

Adv. Mater. 2010, 22, 2547–2552 2010 WILEY-VCH Verlag G One-dimensional nanostructures have attracted considerable attention due to their importance in basic scientific research and potential technologic applications. Among them, vanadium pentoxide (V2O5) nanowires have been extensively studied in recent years because of their prospective applications in chemical sensors, field-emitters, catalysts, lithium-ion batteries, actuators, and electrochromic or other nanodevices. Several different approaches have been explored for the synthesis of V2O5 nanowires, such as thermal evaporation methods, hydrothermal/solvothermal syntheses, sol–gel techniques, and electrodeposition. However, the nanowires synthesized by these methods have typical lengths in the micrometer range (most of them are shorter than 10mm);moreover, if one canmake centimeter-long V2O5 nanowires, which should be much more useful compared to short wires for some specific purposes, such as field-emission (FE), device interconnects, and reinforcing fibers in composites. Herein, we fabricated high-quality single-crystalline centimeter-long V2O5 nanowires ( 80–120 nm in diameter, several centimeters in length; aspect ratio >10–10) using an environmental friendly hydrothermal approach without dangerous reagents, harmful solvents, and surfactants. The FE, electrochemical and electrical transport, and photoconductive properties of the synthesized V2O5 nanowires were then investigated in detail. Our results suggest a high potential of utilizing these novel nanowires in field-emitters, lithium-ion batteries, interconnects, and optoelectronic devices. The representative morphologies of the V2O5 nanowires were investigated by FE scanning electron microscopy (SEM), as shown in Figure 1a. Other SEM images (see the Supporting Information, Fig. S1) also confirm the high-yield fabrication of smooth and straight nanowires of 80–120 nm in diameter. Large portions of the nanowires are usually several millimeters or even up to several centimeters in length (inset of Fig. 1a), resulting in an aspect ratio of 10–10. To the best of our knowledge, this is the first time that such ultra-long V2O5 nanowires have been obtained. An X-ray diffraction (XRD) pattern of the sample is shown in Figure 1b. All the diffraction peaks can be indexed to an orthorhombic V2O5 phase with the lattice parameters of a1⁄4 11.54 A, b1⁄4 3.571 A, and c1⁄4 4.383 A, in good agreement with the literature values (Joint Committee on Powder Diffraction Standards (JCPDS) Card, no. 89-0612). No characteristic peaks of any impurities are detected in this pattern. Figure S2 (Supplementary Information) depicts a room temperature micro-Raman spectrum of the ultralong V2O5 nanowires. The peaks, located at 145, 197, 285, 305, 407, 480, 525, 694, and 990 cm , can be assigned to the Raman signature of V2O5. [18,19] A predominant low-wavelength peak at 145 cm 1 is attributed to the skeleton bent vibration (B3g mode), while the peaks at 197 and 285 cm 1 derive from the bending vibrations of OC V OB bond (Ag and B2g modes). The bending vibration of V OC (Ag mode), the bending vibration of V OB V bond (Ag mode), the stretching vibration of V OB V bond (Ag mode), and the stretching vibration of V OC bond (B2g mode) are regarded at about 305, 407, 525, and 694 cm , respectively. The layered structure of V2O5 is stacked up from distorted trigonal bipyramidal atoms that share edges to form (V2O4)n zigzag double chains along the [001] direction and are cross-linked along the [100] direction through the shared corners. The mode of a skeleton bent, corresponding to the peak at 145 cm , provides an evidence for the layered structure of V2O5. Furthermore, the narrow peak centered at 990 cm , corresponding to the stretching of vanadium atoms connected to oxygen atoms through double bonds (V1⁄4O), is also an additional clue to the layer-type structure of V2O5. [22,23] The detailed microstructures of V2O5 nanowires were further studied by transmission electron microscopy (TEM). Figure 2a shows a TEM image of V2O5 nanowires, which demonstrates that the V2O5 nanowires have uniform diameters throughout their entire lengths. An X-ray energy-dispersive spectrum (EDS) acquired from an individual nanowire exhibits strong V and O peaks. The atomic ratio of V and O is close to the 2:5