Synthesis and Characterisation of Perovskite-type Oxynitrides and Hybride Materials

Tailor-made perovskite-type materials can enable better future technologies or improve existing devices. Cationic and anionic substitutions in perovskites are pursued to improve the properties of a given perovskite-type phase. Particularly the exchange of the oxide ions for halide or nitride ions can have a substantial influence on the structural and physical characteristics of perovskites.

The introduction of nitrogen into the anionic sublattice under formation of oxynitrides has a direct influence on the band structure of the respective compounds. Owing a lower electronegativity nitrogen’s 2p orbitals are higher in energy than oxygen 2p orbitals resulting in a hybridisation of both bands forming a new valance band higher in energy. While cationic substitution leads to a decrease of the conduction band in energy. In both cases the band gap of the compound is reduced. Therefore, an adjustment of the band gap is possible by a variation of the N/O ratio, which might be accompanied by an additional cationic substitution to keep charge balance and to avoid the formation of vacancies. A tailoring of the band gap is strongly recommended to shift the absorption edge of the compound from UV to visible light allowing a significant increase in efficiency for the splitting of water into hydrogen and oxygen. A further option to avoid the formation of anionic vacancies is the simultaneous substitution of oxygen by nitrogen and halogenides.

Perovskite hybrid materials have been intensively developed for solar applications recently. Especially, organometal halide perovskites were reported as a promising material with outstanding electric properties in the 1990s. The structure of organometal halide perovskite could be represented as ABX3, where B is metal cation (e.g. Pb2+) and A is an organic cation to neutralize the total charge. The organic cation must fit into a strong cuboctahedral frame. However, the structure would be distorted if the organic cation is too large. Here we study the organic or inorganic atoms substitution. On the other hand, the environmental safety of the material is another problem of this new material due to the toxic properties of Lead. Although it is claimed that the amount of lead for perovskite-based PV will only account for 0.1 %, it is still important to explore Pb-free materials. Therefore, we will focus on the way of tuning the polarizability by replacing Pb with other metal cations (e.g. Sn, Cu, Co, Ag, Bi, or Sb). To synthesize high quality perovskite materials, we adopt two-step sequential deposition by using the metal halide MI2 and organic ammonium iodide solution, which could obtain better morphology and grain size of the materials.

Current research topics include:

  • Perovskite-type oxynitrides for solar water splitting photoelectrochemical cells
  • High temperature perovskite-type thermoelectrics
  • Perovskite-type materials for solar CO2 conversion
  • Perovskite-type membranes for gas separation
  • Luminescent materials
  • Battery materials
  • Memristor Materials
  • Porous Materials as Substrates for Catalytic Converters
  • Noble Metal Free Materials for Catalytical Exhaust Gas Treatment
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