Selective formation of VO2(A) or VO2(R) polymorph by controlling the hydrothermal pressure

Missing VO₂(A) usually occurs during the preparation of VO₂ polymorphs. This leads to an ambiguous understanding of the transformation between VO₂ polymorphs. The calculation of the ground state energies for different VO₂ polymorphs indicated that there is only a small energy gap between VO₂(A) and VO₂(R), which destined that the transformation from VO₂(A) to VO₂(R) should be pressure sensitive. This hypothesis was verified during the synthesizing of VO₂ polymorphs by reducing V₂O₅ with oxalic acid through hydrothermal treatment process. Selective formation of pure phase VO₂(A) or VO₂(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 VO₂(R) while a reduced filling ratio to 0.4 or lower results in the formation of VO₂(A). Based on our experiments, VO₂(B) nanobelts were always first formed and then it transformed to VO₂(A) by assembling process at increased temperature or extended reaction time. Under further higher pressure, the VO₂(A) transformed spontaneously to VO₂(R) initialized from the volume shrinkage due to the formation of denser VO₂(R).