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).