VO2(A) nanorods: One-pot synthesis, formation mechanism and thermal transformation to VO2(M)

The monoclinic VO2(M) has promising applications in intelligent devices but its preparation still requires improvement to permit cost-effective mass production. In this work, we report a 2-stage approach for producing VO2(M) nanorods by (1) hydrothermal reduction of vanadium pentoxide by sodium bisulfate at 220 °C to form VO2(A), and (2) subsequent thermal activated phase transformation of VO2(A) to VO2(M) at 350-450 °C in vacuum. The obtained VO2(M) nanorods showed a reversible phase transition temperature at about 62.5 °C and a narrow thermal hysteresis width of 10 °C. The mechanism of the hydrothermal reduction was studied by combined ex situ microscopic and diffraction characterization of cooled samples as well as in situ PXRD experiments, in which the hydrothermal synthesis was monitored in real time by time-resolved diffraction datasets. It was found that the hydrothermal synthesis of VO2(A) is a 4-step process: (1) reduction of V2O5 to form VO2(B) nanoparticles, (2) oriented attachment of VO2(B) nanoparticles along the [110] direction, (3) formation of VO2(B) nanorods as a results of oriented attachments, and (4) hydrothermal transformation of the metastable intermediate VO2(B) nanorods to VO2(A) nanorods. This clear understanding of the mechanism will help the further optimization of synthesis temperature and time for preparing VO2(A). This method provides a low temperature thermal treatment alternative and hence helps the reduction of cost for the production of VO2(M), bring the mass application of VO2(M) one step closer.