Experimental and Theoretical Evidences of p-Type Conductivity in Nickel Carbodiimide Nanoparticles with a Delafossite...

Nickel carbodiimide (NiCN₂) was synthesized using a two-step precipitation-decomposition route leading to a brown powder with gypsum-flower-like morphology and a large specific surface area (75 m²/g). This layered material crystallizes in the 2H structure type of delafossite (space group P6₃/mmc), which is built upon infinite ²/_∞[NiN₂] layers connected by linear carbodiimide ([N═C═N]^2-) bridges. An X-ray diffraction Rietveld refinement and thermal analyses pointed out some nickel deficiencies in the material, and band structure calculations carried out on the defect compound predicted p-type conductivity in relation to a slight amount of N^2-. This p-type conductivity was demonstrated by electrochemical impedance spectroscopy measurements, and a flat band potential of 0.90 V vs SCE at pH 9.4 was measured. This value, which is more positive than those of CuGaO₂ and CuCrO₂ delafossite oxides and NiO, prompted us to test NiCN₂ nanoparticles as a photocathode in p-type dye-sensitized solar cells.