In March 2018, we tried to grow some vanadium oxide nanorods on the surface of a V foil via the thermal oxidation route following an article in Nanotechnology . We did not control the growth parameters such as peak temperature, heating rate, cooling rate, etc in the first several fabrication runs. When we examined the morphology of one of the obtained products using scanning electron microscopy (SEM), we were surprised to see these unprecedented hollow, rod-like and rectangular cross-sectional microtubes, as shown in the SEM image below. Thereafter, we did many experiments for finding the key growth conditions of the microtubes. We soon realized that the microtubes can always be obtained by gradually heating a V foil in air to incandescent temperature and then rapid cooling by turning off the heating direct current. However, we did not know what the microtubes were at that time.
Thus, we examined the composition and microstructure of the microtubes using X-ray energy dispersive spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron microscopy. These examinations confirmed that the microtubes are single crystal VO2. Next, we carried out the differential scanning calorimetry (DSC) experiments for examining the phase transition property of our single crystal VO2 microtubes. The DSC results were perfect because the sharp metal-insulator transition with excellent thermal cycle stability was clearly revealed. And, an in situ optical microscope observation for an individual microtube directly confirmed the metal-insulator transition. At this point, we finally knew that we had discovered a completely new form of single crystal VO2, which we named it as microtube arrays. What's even more exciting is that this new product can be easily fabricated in just tens of seconds with a very facile procedure. We have presented all fabrication details in our paper and we will be very glad if you are willing to use this method to fabricate single crystal VO2 microtube arrays and apply it to a variety of advanced devices.