Tuning bandgap and surface wettability of NiFe2O4 driven by phase transition

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.

Dependence of Structural, Morphological and Magnetic Properties of Manganese Ferrite on Ni-Mn Substitution

“…The obtained NiFe 2 O 4 particle size ( Figure 2 m–o) of 22 nm at 400 °C, 30 nm at 800 °C, and 58 nm at 1200 °C are in good agreement with the data reported by Tong et al [ 37 ] and Ashiq et al [ 38 ] for nanoparticles obtained by reverse micelle technique. The progressive replacement of the Mn 2+ ions by Ni 2+ ions has direct consequences on the particle size ( Figure 2 d–l), which progressively increases with the increase of the Ni content and the annealing temperature.…”

Microwave Catalytic Degradation of Antibiotic Molecules by 2D Sheets of Spinel Nickel Ferrite

“…A higher heat diffusion helps in the uniform crystallization of the SNFO sheets after increasing the calcination temperature up to 700 °C. The band gap energy of SNFO employing Tauc’s plot is found to be 1.78 eV (see Figure b). − PL spectra of SNFO under an excitation wavelength of 600 nm at room temperature (see Figure c) reveal the luminescence peak at ∼660 nm for SNFO which seems to arise from the band edge excitation. To investigate the binding in the molecule especially the valence state, the XPS analysis was carried out for the SNFO sheets (see Figure d–g).…”

“…The XRD pattern of SNFO shown in Figure a demonstrates a spinel cubic structure of the prepared SNFO sheets. The characteristic peaks at 2θ = 18.4°, 30.3°, 35.7°, 43.3°, 53.8°, 57.3°, 63.0°, and 75.5° are assigned to (111), (220), (311), (400), (422), (511), (440), and (622) planes of SNFO, respectively . A higher heat diffusion helps in the uniform crystallization of the SNFO sheets after increasing the calcination temperature up to 700 °C.…”

Tuning Surface Plasmonic Resonance and Surface Wettability of Au/CrN Films Using Nitrogen-Containing Gas

“…This phenomenon could be attributed to an air pocket forming and existing between the grains and against the grain boundaries. In other words, air pockets play an important role in supporting the water droplets in the case of the penetration, and they lead to higher water-contact angles [ 57 , 58 , 59 , 60 , 61 ], hence the similar, corresponding tendency of the SEM images with calculated average grain sizes performed via histograms, as shown in Figure 2 a. Moreover, the histograms of the calculated average grain sizes show the same tendency, with the gradually increasing values of the water-contact angles.…”