Hydrothermal synthesis of CuAlO2 with the delafossite structure in supercritical water

Hydrothermal synthesis of transparent conducting p-type delafossite CuAlO2 crystals from Cu(NO3)2 and Al(NO3)3 aqueous solution has been carried out at 400 °C and 30 MPa for reaction times of 10–30 min with a titanium alloy autoclave. NaOH and HCOOH were used to control metal oxide solubility and oxidation state of Cu, respectively. The method uses the specific property that supercritical water forms a homogeneous mixture with H2 that is produced by hydrothermal decomposition of HCOOH. Phase and size of the obtained crystals were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. Molality of remaining metal ions was measured by atomic absorption spectrometry (AA). Effect of reaction time and molality of NaOH and HCOOH on the crystal phase was examined. Single-phase CuAlO2 crystal with the rhombohedral R3m structure was successfully synthesized at a yield of 99% from 0.1 mol/kg Cu(NO3)2, 0.1 mol/kg Al(NO3)3, 1.0 mol/kg HCOOH and 0.2 mol/kg NaOH aqueous solution for 30 min.