Selective formation of VO 2(A) or VO 2(R) polymorph by controlling the hydrothermal pressure

Abstract

Missing VO 2(A) usually occurs during the preparation of VO 2 polymorphs. This leads to an ambiguous understanding of the transformation between VO 2 polymorphs. The calculation of the ground state energies for different VO 2 polymorphs indicated that there is only a small energy gap between VO 2(A) and VO 2(R), which destined that the transformation from VO 2(A) to VO 2(R) should be pressure sensitive. This hypothesis was verified during the synthesizing of VO 2 polymorphs by reducing V 2O 5 with oxalic acid through hydrothermal treatment process. Selective formation of pure phase VO 2(A) or VO 2(R) was achieved by controlling the hydrothermal pressure through varying the filling ratio at 270 °C. It was found that a filling ratio over 0.5 favors the formation of pure VO 2(R) while a reduced filling ratio to 0.4 or lower results in the formation of VO 2(A). Based on our experiments, VO 2(B) nanobelts were always first formed and then it transformed to VO 2(A) by assembling process at increased temperature or extended reaction time. Under further higher pressure, the VO 2(A) transformed spontaneously to VO 2(R) initialized from the volume shrinkage due to the formation of denser VO 2(R).