Hydrothermal Synthesis of Doped VO2(M) and Its Application to Thermochromic Thin Film

Abstract

The doped VO2(M)s were synthesized by direct hydrothermal and hydrothermal/post-thermal methods from V2O5-oxalic acid(H2C2O4)-H2O system and their thin films on polyethylene terephtalate (PET) substrate were prepared by wet-coating method using commercial polymer dispersing agent and mixed organic solvents. Their temperature-dependent phase transition characteristics were comparatively investigated. The direct hydrothermal synthetic method of VO2(M) in V2O5-H2C2O4-H2O system was strongly dependent upon many control parameters such as reactants molar ratio, reaction temperature and time, reactant concentration and doping materials. In this study the direct hydrothermal method was investigated under the condition of 1:2 molar ratio of V2O5 and H2C2O4 and 250 oC. Pure phase of VO2(M) was formed through the mixed phases with VO2(B) as the reaction time increased. However, VO2(M) was converted to other phases of VO2 when the reaction time became longer than a certain time depending on the precursor concentration. W-doped VO2(M)s with 0.5 – 1.25 at.% of W could be easily obtained simply by adding Na2WO4 into the reactant mixture of 0.4 M V2O5 and 0.8 M H2C2O4 by hydrothermal treatment for 4 days. The phase transition of these W-doped VO2(M) to R-phase occurred in the broader temperature range than that of the un-doped one although the transition temperature was lowered to 45 oC with 1.25 at.% of W. Those W-doped samples did not show the systematic reduction of transition temperature (Tc) on the fed amount of W. The thin film of these W-doped VO2(M) on PET showed very poor NIR switching efficiencies (ΔT) regardless of the feeding amount of W. The hydrothermal treatment of the same reactants at 200 oC for 1 day yielded pure VO2(B) instead of VO2(M) whether doping materials such as Na2WO4 and Mg(NO3)2 were included or not. VO2(B) that was prepared in this hydrothermal condition, whether it was doped or not, was easily transformed to VO2(M) by thermal treatment at 500 oC for 2 hours under Ar atmosphere. W-doped and W/Mg-codoped VO2(M) were synthesized in this way with 0.2 -2.0 at.% of W and 1.5 – 6.0 at.% of Mg and their thin films on PET were also prepared by wet-coating method. These W-doped VO2(M) had much better crystallinity and homogeneity than those prepared by direct hydrothermal method. Tc also systematically decreased up to 32 oC as the amount of W increased. The resistivity changes by phase transition were higher in one order of magnitude than those from direct hydrothermal method. ΔT of these thin films were more than twice better than those from direct hydrothermal method regardless of the amount of W. W/Mg-codoped VO2(M) that were prepared by this method with 1.5 at.% of W and 1.5 – 6 at.% of Mg had slightly better crystallinity and higher Tc than only W-doped ones. Codoping of Mg into W-doped VO2(M) increased the magnitude of resistivity and the change of resistivity by phase transition. Tc of W/Mg codoped ones increased initially and then decreased as the amount of Mg increased. ΔT of the thin films of W/Mg-codoped ones were noticeably improved with 4.5 at.% of Mg and the visible transmittance was also increased, even though very slight, in all used amounts of Mg.