Synthesis and thermochromic property studies on W doped VO2 films fabricated by sol-gel method

Abstract

Tungsten-doped VO2 thin films have been synthesized by a modified sol–gel process and followed by a post annealing. Vanadium pentoxide and tungstic acid as raw materials with the addition of hydrogen peroxide, concentrated hydrochloric acid (catalyst) and oxalic acid used as reducing agent were reacted in isobutanol. Finally, the uniform sol of vanadyl oxalate in isobutanol solvent was obtained as precursor. Detailed study suggested that W doped in VO2 introduces additional electron carriers and induces the formation of V3+. Post annealing under vacuum promotes the releasing of chemical stress and generates oxygen vacancies in the samples. Temperature dependent transmittance study revealed that the releasing of chemical stress and deliberately introducing oxygen vacancies in W-doped VO2 films have positive effects on enhancing its switching ability in the infrared transmittance as the metal-insulator transition (MIT) occurs. The largest switching of transmittance was obtained about 48% in the infrared range at 43 °C in 1.5%W doped VO2 films, which is significantly larger than the reported ones. The findings in this work open a new way to synthesize the novel and thermochromic W doped VO2 films with facility and low cost. Therefore, it has extensive application to construct smart windows and electronic devices.

VO2/TiO2bilayer films for energy efficient windows with multifunctional properties

“…12,13 Much research effort has gone into lowering the T c value of VO 2 films, for example through doping with high-valence transition metal cations, such as Nb 5+ , Ta 5+ , Mo 6+ , W 6+ and Ru 4+ . [14][15][16][17][18][19] Even though doping has been found to be effective in lowering T c , it is still difficult to both lower T c and increase visible light transmittance, as well as reducing thermal hysteresis. 20 Porosity has been demonstrated to have a positive effect on visible light transmittance in tungsten doped VO 2 films with low T c values.…”

An Invariable Temperature during the Phase Transition of W Doped VO2 Film

“…The change of the hysteresis gap causes various selections of transition temperature points of the impurity‐doped VO 2 . In turn, this becomes the main reason for the disagreements in transition temperature between reports . Therefore, we used T CE as a variable temperature to estimate the W doping (and other extrinsic stress) effect on the transition temperature of VO 2 .…”

“…As shown in Figure 4(c), the transmittance gap between the two phases for the lightly W doped films ( x < 0.005) is pretty large (~40%), but that for the heavily W doped films ( x = 0.04) is small (~5%) as also reported by others . The transition hysteresis gap (temperature gap) between heating and cooling cycles, decreased from ~20 to ~4 K by the doping of 4% of W in VO 2 ( x = 0.04).…”

“…The T CE depression is expected ~9.5 K/mol % W doping in the whole range of x, <~4%. This is somewhat different from other reports that show the linear relationship between T C ( i.e ., peak position in DSC curve) and the x value, but a broken line in the T C vs. x plot—high value (up to 40 K/x) in x < 0.015 and low value (< 9 K/x) in 0.015 < x < 0.036 . The hysteresis gap (T g : the temperature gap at the same resistivity in the middle of the thermal cycles as marked in Figure 5(b)) is drastically reduced by the slight level of W doping.…”

Structural, morphological, and optical properties of W‐doped VO2 thin films prepared by sol‐gel spin coating method

“…In the present research, VO 2 sol was coated on soda‐lime glass substrate via a low‐cost sol‐gel spin coating method followed by gelation during thin film drying process. Sol‐gel process has been widely used in the fabrication of thermochromic thin films, however, thermochromic coating on soda‐lime glass by sol‐gel spin coating process was less investigated. Since tungsten is the most common dopant for reducing transition temperature of VO 2 , the main purpose of this research is to profoundly study the changes in VO 2 electrical and optical properties by W‐doping which was done by adding a less investigated dopant precursor (tungstic acid) to the VO 2 precursor solution.…”