Link between Structural and Optical Properties of Co_xFe_3–xO₄ Nanoparticles and Thin Films with Different Co/Fe Ratios

Abstract

Co x Fe 3−x O 4 nanoparticles (x = 0.4 to x = 2.5) and thin films (x = 0.9 to x = 2.2) are analyzed by Raman, absorption, and photoluminescence spectroscopy to link structural and optical properties to different cobalt to iron (Co/Fe) ratios. Raman spectroscopy shows that with decreasing Co content, the crystal structure changes from a predominantly normal cubic spinel phase to a mixed inverse spinel phase. This finding is supported by absorption spectroscopy that points out that inter valence charge transfer (IVCT) processes between octahedrally coordinated Co 2+ and Fe 3+ cations become more prominent with increasing Fe content. Independent of the Co/Fe ratio, Co x Fe 3−x O 4 nanoparticles show a broad photoluminescence (PL) band with a maximum at around 510 nm. Time-resolved photoluminescence spectroscopy shows subnanosecond lifetimes and temperatureresolved photoluminescence experiments reveal that the green PL increases with decreasing temperature (300 to 10 K) while showing no temperature-dependent shift in energy. It is proposed that this green PL originates from OH-groups on the particles' surface.

Engineering of Cation Occupancy of CoFe2O4 Oxidation Catalysts by Nanosecond, Single‐Pulse Laser Excitation in Water

“…The laser treatment was conducted with CoFe 2 O 4 -NPs from three different sources: a commercial CoFe 2 O 4 , a wet-chemically synthesized CoFe 2 O 4 , [27,30,40] and CoFe 2 O 4 -NPs obtained in a carbon templating process [5,21,41] providing three different initial defect densities. The high thermal stability of CoFe 2 O 4 allowed maintaining the particle size and phase purity of the materials.…”

Limitations of the Tauc Plot Method

“…Copyright 2014, American Chemical Society). b) Influence of a change of the Co/Fe ratio on the optical absorption (left panel) and the crystal structure (middle and right panels) of Co x Fe 3−x O 4 thin films with x = 0.9 and 2.2 (Adapted with permission [77]. Copyright 2021, American Chemical Society).…”

Atmosphere-sensitive photoluminescence of CoxFe3−xO4metal oxide nanoparticles

“…Using these methods, it is determined that the crystal structure converts from a predominantly normal spinel for Co-rich nanoparticles to a more mixed inverse spinel for Fe-rich nanoparticles. 69 This in turn affects which metal cations occupy the octahedral and tetrahedral sites, respectively. Raman spectroscopy indicates that in Co-rich nanoparticles, Co 2+ ions are mainly present on the tetrahedral sites (Co 2+ tet ) and Co 3+ as well as Fe 3+ ions occupy mainly octahedral sites (Co 3+ oct or Fe 3+ oct ).…”