One-pot synthesis of monoclinic ZrO2 nanocrystals under subcritical hydrothermal conditions

•ZrO2 nanocrystals from a ZrO(NO3)2 precursor were synthesized by a one-pot hydrothermal method utilizing subcritical water.•The reaction temperature and time strongly affected the crystalline phase of ZrO2 products.•A single-phase monoclinic ZrO2 product was synthesized at 250°C for the reaction time exceeded 12h.•The crystallite size of the single-phase monoclinic ZrO2 product was approximately 20nm.•The nanocrystals had chemisorbed water, terminated hydroxides, and nitro groups onto its surface. This study reports a one-pot synthesis technique for the preparation of single-phase monoclinic zirconium oxide (ZrO2) nanocrystals. The products were synthesized from only zirconium oxynitrate (ZrO(NO3)2) as the precursor under hydrothermal conditions using subcritical water. The precursor was heat-treated in a batch-type reactor at a reaction temperature of 250°C for 24h to obtain pure monoclinic-structured ZrO2 nanocrystals. The crystallization temperature of the ZrO2 phase was also greater than 200°C. However, the products of reactions conducted at 200°C for 24h were mixtures of the tetragonal and monoclinic structures. At a reaction temperature of 250°C, the volume fraction of the monoclinic phase increased; however, the reaction time was also important. The heat-treatment was performed for more than 12h in order to obtain single-phase monoclinic ZrO2 nanocrystals. The crystallite size of this product was approximately 20nm, and water, hydroxide groups, and nitro groups were chemisorbed on its surface.