Mesoporous ZnO nanocrystals: preparation and functionalization by hybrid organic-inorganic perovskites

Zinc oxide (ZnO) is considered a very interesting material owing to its direct wide band gap [3.37 eV at room temperature (RT)] and high exciton binding energy (60 meV). Recently, attention has been devoted to the fabrication of porous ZnO nanostructures since large surface areas and a high surface-to-volume ratio make porous materials interesting for a variety of applications such as catalysts, gas sensors, and solar cells. In particular, attention could be devoted to the possibility of fabricating solid Dye-Sensitized Solar Cells (s-DSSC) based on porous ZnO combined with self-assembled hybrid organometal halide perovskites CH3NH3PbX3 (X = Cl, Br, I) that have been recently used as sunlight absorbers in TiO2-based s-DSSCs achieving remarkable power conversion efficiencies [1]. In this communication we report about the processes that allow monocrystalline porous ZnO nanosheets and nanobelts to be obtained from the ZnS(en)0.5 (en=ethylenediamine) hybrid organic-inorganic precursor synthesized by a solvothermal route. In particular it is shown that porous ZnO nanostructures are obtained through the topotactic transformation ZnS(en)0.5?ZnS?ZnO induced by proper thermal treatments. The properties of ZnO porous nanostructures are thoroughly investigated and their potential for photovoltaic applications is envisaged. To this aim we also discuss about ZnO nanostructure sensitization by hybrid organometal halide perovskites. In particular, we report about the preparation and characterization (structural and optical) of CH3NH3PbI3 and CH3NH3PbI3-xXx (X=Cl, Br) films obtained from percursor solution as well as their deposition on porous ZnO nanosheets. The expected advantages deriving from using perovskite-functionalized porous ZnO nanostructures are discussed considering the combination of their single crystal nature with high surface-to-volume ratio.