A novel approach to low-temperature synthesis of cubic HfO2 nanostructures and their cytotoxicity

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.

Figure 1

HR-TEM micrographs of the as-prepared c-HfO₂ nanoparticles (a,c); (b) FFTs show a crystalline ring pattern of the selected area in image (a); (d) SAED pattern of c-HfO₂; (e) d-spacing calculation using the average of 10 fringes; (f) energy-dispersive X-ray (EDX) spectrum; (g) scanning tunnelling electron microscopy (STEM) EDX mapping images, exhibiting the homogeneous distribution of hafnium and oxygen.

Figure 2

TEM micrographs of c-HfO₂ nanostructures obtained after treatment in hydrothermal autoclave with PEG (a,b) and FU (c,d).

Figure 3

Schematic of the formation of HfO₂ nanostructures in the presence of PEG and FU (MW represents microwave heating).

Figure 4

XRD patterns obtained with varied concentrations of Na₂S·xH₂O (0, 1, 3, and 5 mmol); without washing after reaction completion (using 3 mmol of Na₂S·xH₂O); reaction in aqueous media (used 3 mmol Na₂S·xH₂O, and 15 mL water).

Figure 5

(a) XRD patterns show the cubic-to-monoclinic conversion of HfO₂ with calcination; (b) crystallographic structural transformation from cubic-to-monoclinic HfO₂ upon calcination at temperatures greater than 500 °C (red and olive-green spheres represent O and Hf atoms, respectively).

Figure 6

XPS binding energy survey spectrum with marked corresponding peaks (a); high-resolution XPS spectra of Hf (b) and O (c) for synthesised c-HfO₂.

Figure 7

(a) XRD patterns, and (b) Raman spectra of c-HfO₂ nanostructures; (c) UV-vis absorption spectrum with inset depicting Tauc plots for band gaps; (d) thermogravimetric analysis (TGA) curves for c-HfO₂ nanostructures.

Figure 8

Structural evolution of HfO₂ when synthesised using varying concentrations of NaOH (4, 6, and 8 mmol). XRD patterns of as-prepared HfO₂ (a–c) and after calcining at 550 °C for 1 h (d–e) with various NaOH concentrations (4, 6, and 8 mmol, respectively).

Figure 9

Morphological changes of breast cancer cells (MCF-7) after treatment with HfO₂, PEG-HfO₂, and FU-HfO₂ nanostructures (scale bar for all images is 50 µm; arrows indicate cell death).

Figure 10

Effect of HfO₂, PEG-HfO₂, and FU-HfO₂ nanostructures on: (a) LDH cytotoxicity, (b) ATP proliferation, and (c) Caspase-3/7 activities. Statistical significance values between the control and treated cells are shown as ^(*) P < 0.05 ^(**) P < 0.01 and ^(***) P < 0.001.