Raman studies of cation distribution and thermal stability of epitaxial spinel NiCo2O4 films

Abstract

Epitaxial thin films of spinel NiCo2O4 (NCO) grown on MgAl2O4 (001) substrates are reported to exhibit dramatic changes in the magnetic and transport properties with deposition temperature. While films grown at lower temperatures (<450°C) are ferrimagnetic with metallic characteristics, those grown at higher temperatures are non-magnetic and insulating. Detailed polarized Raman spectroscopy studies indicate that the higher temperature films have close to the ideal inverse spinel cation distribution, Co3+[Ni2+Co3+]O42−, whereas those deposited at lower temperature are characterized by mixed cation/charge distribution at both the tetragonal (A) and octahedral (B) sites. Additionally, temperature-dependent Raman studies demonstrate that, unlike bulk polycrystalline samples, all the NCO films are robust against thermal treatment with full reversibility after annealing at 600°C in oxygen and air. However, partial decomposition is observed after annealing in vacuum.

Atomic layer deposition of nickel–cobalt spinel thin films

“…44 In order to demonstrate the presence of weak Raman peaks in the stoichiometric (Co 1−x Ni x ) 3 O 4 films with x=0.33, an inset with the Raman spectrum of a thicker film (∼ 192 nm) is included in Figure 5. Iliev et al 42 also observed weak and broad peaks for their metallic films grown at low temperatures but obtained sharp peaks for their insulating films grown at high temperatures during their epitaxy studies supporting the idea that ionic disorder is responsible for the low resistivity.…”

“…It is understood that the cation distribution can differ significantly from the ideal inverse spinel and varies with the preparation conditions leading to a broad variation of electric and magnetic properties. For example, while Iliev et al 42 obtained a distribution close to that of the ideal inverse spinel for their insulating (Co 2/3 Ni 1/3 ) 3 O 4 films from Raman analysis, the results for the conducting, metal-like films suggested a distribution differing from that. Bitla et al 19 who investigated similar films (epitaxial, on MgAl 2 O 4 ) carried out extensive investigations of the cation distribution by X-ray absorption spectroscopy (XAS) and obtained (Co 43 Another F 2g would be expected at about 522 cm −1 but cannot be identified here due to the interference with a signal from Si.…”

Low temperature propane oxidation over Co3O4 based nano-array catalysts: Ni dopant effect, reaction mechanism and structural stability

“…However, Ni x Co 3−x O 4 has been reported to not be stable at high temperature (>400 • C) especially when the Ni concentration is high [41,42]. Raman scattering was applied to study the structural change of Ni doped Co 3 O 4 at different temperatures.…”

Nickel Cobaltite Nanostructures for Photoelectric and Catalytic Applications

“…[ 26 ] Iliev and co-workers have reported that epitaxial NiCo 2 O 4 fi lms deposited at temperatures lower than 450 °C are ferrimagnetic, and those grown at elevated temperatures are nonmagnetic. [ 5 ] The bulk NiCo 2 O 4 , nanorod NiCo 2 O 4 , and 3 wt% NiO/97 wt% NiCo 2 O 4 nanocomposite have been found to exhibit strong magnetization with a high saturation magnetization (M s ) of 29, ≈26, and 24 emu g −1 , respectively. [25][26][27] On account of these intrinsic properties and other advantages such as their low cost, rich abundance, and environmental friendliness, NiCo 2 O 4 materials have been extensively used in the fi elds of photodetection, [ 28 ] photocatalysis, [ 18 ] supercapacitors, [ 29 ] Li-ion batteries [ 30 ] and metal-air batteries, [ 31 ] fuel cells, [ 32 ] magnetic devices, [ 33 ] fl at-panel displays, [ 12 ] ferrofl uid technologies, [ 34 ] drug delivery, [ 35 ] optical limiters and switches, [ 36 ] infrared transparent conducting materials, [ 24 ] heterogeneous catalysis, [ 16 ] and various electrochemical reactions, for example: Cl 2 evolution, [ 37 ] O 2 evolution (OER), [ 38 ] O 2 reduction reaction (ORR), [ 39 ] H 2 O 2 reduction, [ 40 ] methanol oxidation, [ 41 ] and electrochemical sensors.…”