Highly crystallized monoclinic vanadium dioxide, VO (M), is successfully synthesized by a two-step thermal treatment: thermolysis of vanadyl ethylene glycolate (VEG) and postannealing of the poorly crystallized VO powder. In the first thermolysis step, the decomposition of VEG at 300 °C is investigated by X-ray diffraction and scanning electron microscopy (SEM). A poorly crystallized VO powder is obtained at a strict time of 3 min, and it is found that the residual carbon content in the powder played a critical role in the post crystallization of VO (M). After postannealing at 500 and 700 °C in an oxygen-free atmosphere, VO particles of various morphologies, of which the crystallite size increases with increasing temperature, are observed by SEM and transmission electron microscopy. The weight percent of crystalline VO, calculated using the Fullprof program, increases from 44% to 79% and 100% after postannealing. The improved crystallinity leads to an improvement in metal-insulator transition behaviors demonstrated by sharper and more intense differential scanning calorimetry peaks. Moreover, VO and VO with novel and particular microstructures are also successfully prepared with a similar two-step method using postannealing treatment under reductive or oxidizing atmospheres, respectively.

A wearable piezoelectric membrane device of excellent flexibility, lightweight and air breathability is firstly intergrated by one step continuous electrospinning method. Piezoelectric membrane device displays a three-layer structure that PVDF nanofiber membrane is sandwiched between two PVDF-rGO electrode membranes. Since the whole piezoelectric device is prepared by elelctrospinning, both of active layer and electrode layer have high content of electroactive β-phase crystalline. The as-prepared device demonstrates high electric output. Open-circuit voltage, short-circuit current and power density can reach up to 46 V, 18 μA and 18.1 μW cm -2 , respectively. The electric output is remarkably affected by frequency and magnitude of exerted excitation force, especially the output present a linear relationship versus excitation force. Moreover, the piezoelelctric membrane device demonstrateds long time working stability of over 50, 000 circles and is capable to light LEDs directly. Fig. 2 Morphologies of PVDF-rGO electrode membrane (a, b) and PVDF nanofiber membrane (c, d); Schematic of piezoelectric device (e); Cross-sections of piezoelectric device assembled by continuous electrospinning (f).The piezoelectric membrane device integrated by one-step electrospinning has porous and uniform structure.