Tuning bandgap and surface wettability of NiFe 2 O 4 driven by phase transition

• Published in: Scientific reports 2018-01, Vol.8 (1), p.1338
• Main Authors: Tong, Sheng-Kai, Chi, Po-Wei, Kung, Shu-Hsiang, Wei, Da-Hua
• Format: Article
• Language: English
• ISSN: 2045-2322

Stress variation induced bandgap tuning and surface wettability switching of spinel nickel ferrite (NiFe O , 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 (M ) values of NFO films at room temperature increased up to 273 emu/cm 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.