Synthesis and characterization of F-doped zinc oxides, Al-doped zinch hydroxyfluoride, and oxide—fluoride composites for transparent visible/infrared absorbers
Résumé
Among the most promising transparent conductive oxides, F-doped SnO2 can be prepared, for instance, by spray pyrolysis or hydrothermal routes and can exhibit rather good transparency in visible range and high infrared absorption associated to its electronic conductivity due to n charge carriers. F-doping in ZnO is more difficult to control, and the n-type conductivity is lower than that of F-doped SnO2 or M3+ (M = Al, Ga)-doped ZnO. However, the F concentration remains very low in these oxygenated networks and does not contribute significantly to reduce the refractive index related to the transparency in the visible range. Then, the exploration of other systems such as zinc hydroxyfluoride or even pure fluoride should allow tuning the transparency in the visible range. Various Zn(OH,F)2 hydroxyfluoride can be prepared by coprecipitation route at various pH in fluorinated medium, and the fluorine content varies with the pH. X-ray diffraction and 19F magic angle spinning–nuclear magnetic resonnance (MAS-NMR) analyses contribute to identify the F/OH atom distributions and locations in this network. Al3+ doping into Zn(OH,F)2 matrix leads to slightly increase the infrared absorption of Zn(OH,F)2 compound which remains very low in comparison with Al/Ga-doped Zn oxides. No Al3+ ions can be incorporated into ZnF2 obtained by annealing under anhydrous-HF of previous Al-doped Zn(OH,F)2. In order to improve the transparency in visible range, the zinc oxide–fluoride core–shell seems to be an alternative interesting solution. This composite material can be prepared from the fluoride sol filtration through a ZnO powder bed followed by an annealing stage. Then, the ZnO@MgF2 composite coatings with 99%:1% compositions exhibit 80% average transparency in visible range whereas the pure ZnO films have an average transparency around 50%.