Abstract
Stress variation induced bandgap tuning and surface wettability switching of spinel nickel ferrite (NiFe2O4, NFO) films were demonstrated and directly driven by phase transition via a post-annealing process. Firstly, the as-deposited NFO films showed hydrophilic surface with water contact angle (CA) value of 80 ± 1°. After post-annealing with designed temperatures ranged from 400 to 700 °C in air ambience for 1 hour, we observed that the crystal structure was clearly improved from amorphous-like/ nanocrystalline to polycrystalline with increasing post-annealing temperature and this phenomenon is attributed to the improved crystallinity combined with relaxation of internal stress. Moreover, super-hydrophilic surface (CA = 14 ± 1°) was occurred due to the remarkable grain structure transition. The surface wettability could be adjusted from hydrophilicity to super-hydrophilicity by controlling grain morphology of NFO films. Simultaneously, the saturation magnetization (Ms) values of NFO films at room temperature increased up to 273 emu/cm3 accompanied with transitions of the phase and grain structure. We also observed an exceptionally tunable bandgap of NFO in the range between 1.78 and 2.72 eV under phase transition driving. Meanwhile, our work demonstrates that direct grain morphology combined with the stress tuning can strongly modulate the optical, surface and magnetic characteristics in multifunctional NFO films.
Highlights
The heading of wetting characteristics of solid materials surface has been received much attention due to the demonstrating fundamental materials and many promising applications such as green device of environmental cleanup[1,2] and photonic or optoelectronic devices[3,4,5]
It can be clearly seen that the contact angle (CA) for the samples A400, A500 and annealing temperature up to 700 °C (A700) dramatically decrease by 8%, 45% and 83% comparing to that of the as-deposited sample A0, respectively
The surface wettability of NFO nanostructured films could be tuned from hydrophilic state (80 ± 1° and surface free energy (SFE) = 25.1 mJ m−2) to super-hydrophilic state (14 ± 1° and SFE = 70.6 mJ m−2) through the remarkable transition of grain morphology
Summary
The heading of wetting characteristics of solid materials surface has been received much attention due to the demonstrating fundamental materials and many promising applications such as green device of environmental cleanup[1,2] and photonic or optoelectronic devices[3,4,5]. Among of all polymetallic oxides, the nanocrystalline nickel ferrite has unique magnetic properties including superparamagnetism and quantum tunneling of magnetization. It has been regarded as an excellent candidate in numerous application fields including catalysis[11,12,13], sensor technology[14,15], electromagnetic shielding[16,17,18,19], water treatment[20,21,22,23], and biomedical and biotechnology[24,25,26,27]. The work focused on the fundamental surface wetting characteristics of nanocrystalline spinel nickel ferrites films is still lacking.
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