i'm working on counter electrodes for DSSC. VO2 shows good thermochromic property. Are there other transitional metal oxides which show the same effect? What is the physics behind this significant property of VO2?
VO2 is a thermochromic material as it changes its color with fluctuations in temperature. Zinc oxide and sulfide behave in similar fashion. The behavior May be attributed to the changes in lattice patterns with temperature and thereby resulting into color change. Behavior similar to phenomenon of polymorphism. Excitation of electrons followed by deexcitation with release of energy in the form of photons may also be responsible for color change with temperature.
The thermochromic properties of VO2 stem from a phase transition from monoclinic (semiconductor like and thus optically transparent) to tetragonal (metallic and thus optically reflecting). Phase transition occurs at 68°C.
There are numerous other thermochromic materials, e.g. mercury iodides, liquid crystals, inks and dyes.
Yes, it exist lots of other transition metal oxide exhibiting thermochromism. In case of VO2, it is a metal-insulator transition, which is widely studied.
You have similar mechanisms in cobaltite and nickelate perovskite (RENiO3 and RECoO3; with RE= La, Pr, Nd and Sm).
You can have a look on these paper:
doi.org/10.1021/jp4111597
doi.org/10.1063/1.4821884
doi.org/10.1016/0022-4596(91)90077-U
Lacorre and Torrance published a lot on this topic in 90s-00s.
The reason, in case of perovskite material, is due to the bandgap overlapping of the 2p O orbitals and 3d metal orbitals by changing the temperature. In ABO3 perovskite, you have BO6 octahedra (metal atom surrounded by oxygen octahedra cage). The angle B-O-B between two octahedra is responsible of the bandgap energy.
The transition has been described as the ZSA model (Zaanen, Allen, Sawatzky):
At low temperature, there is no overlapping, insulator behavior
At high temperature, lattice distortion and bandgap narrowing, that can go up to the overlapping of 2p O and 3d metal orbital: metallic behavior.
The angle between the B-O-B is directly dependent of the RE size (it is 180° for cubic and decrease as the RE size decrease).
Regards,
Emile HAYE
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