Abstract
The development of a strategy to stabilise the cubic phase of HfO2 at lower temperatures is necessary for the emergence of unique properties that are not realised in the thermodynamically stable monoclinic phase. A very high temperature (>2600 °C) is required to produce the cubic phase of HfO2, whereas the monoclinic phase is stable at low temperature. Here, a novel rapid synthesis strategy was designed to develop highly crystalline, pure cubic-phase HfO2 nanoparticles (size <10 nm) using microwave irradiation. Furthermore, the as-prepared nanoparticles were converted to different morphologies (spherical nanoparticles and nanoplates) without compromising the cubic phase by employing a post-hydrothermal treatment in the presence of surface modifiers. The cytotoxicities and proliferative profiles of the synthesised cubic HfO2 nanostructures were investigated over the MCF-7 breast cancer cell line, along with caspase-3/7 activities. The low-temperature phase stabilisation was significantly attributed to surface imperfections (defects and deformations) induced in the crystal lattice by the desirable presence of Na2S·xH2O and NaOH. Our work provides unprecedented insight into the stabilisation of nanoscale cubic-phase HfO2 in ambient environments; the method could be extended to other challenging phases of nanomaterials.
Highlights
Hafnium oxide or hafnia (HfO2), a group IV-b metal oxide, has emerged as a leading technological material because it shows outstanding physicochemical properties
The selected-area electron diffraction (SAED) pattern confirms the polycrystalline nature of the c-HfO2, with clear visible spots corresponding to the four main reflection planes of (111), (200), (220), and (311), as expected from the c-HfO2 via X-ray diffraction (XRD)[3, 24]
A strategy is presented to realise the goal of achieving high-quality cubic HfO2 nanoparticles with excellent properties in an ambient environment by employing microwave heating
Summary
Hafnium oxide or hafnia (HfO2), a group IV-b metal oxide, has emerged as a leading technological material because it shows outstanding physicochemical properties. HfO2 has a high melting point (~2780 °C), excellent mechanical and corrosion resistance, high neutron absorption coefficient, high density (9.6 g/cm3), and low thermal conductivity These properties permit its use as a refractive protective coating for thermocouples in nuclear applications and as a thermal barrier coating in engines and chemical manufacturing equipment to allow operation at high temperatures or under harsh conditions. It remains imperative to develop a novel facile route to prepare cubic- and tetragonal-phase HfO2 nanostructures at low temperatures and to study the unique shapes and size-dependent properties of these nanostructures, which may permit novel applications
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