Photoelectrochemistry of Ferrites: Theoretical Predictions vs. Experimental Results

Abstract

This paper gives an overview about recent theoretical and experimental work on electronic and optical properties of spinel ferrites MFe2O4. These compounds have come into focus of research due to their possible application as photocatalyst material for photoelectrochemical water splitting. The theoretical background of state-of-the-art quantum-chemical approaches applied for predicting electronic and optical band gaps, absolute band positions, optical absorption spectra, dielectric functions and Raman spectra, is briefly reviewed. Recent applications of first-principles methods on magnetic and electronic properties of ferrites with M = Mg and the first row of subgroup elements Sc to Zn are presented, where it is shown that the fundamental band gap is strongly dependent on the spin state and the degree of inversion of the spinel structure. The observed variation of electronic properties may serve as an explanation for the large scattering of experimental results. The exchange of M and Fe cations has also a pronounced effect on the Raman spectra of ferrites, which is analyzed at atomic scale from first principles. Calculated optical absorption spectra of ferrites are compared to experimental spectra. The electronic nature of the first excitations and the role of oxygen vacancies are discussed. For the calculation of absolute band positions, which have a significant impact on the photoelectrochemical activity of the ferrites, models of the most stable ferrite surfaces are developed that take into account their polar nature and the interaction with the solvent. Theoretically predicted valence and conduction band edges are compared to results from electrochemical measurements. The role of cation exchange on the surface electronic structure is investigated both theoretically and experimentally.

Highly improved supercapacitance properties of MnFe2O4 nanoparticles by MoS2 nanosheets

“…3 B), the E g band (189 cm −1 ) is due to the asymmetric and symmetric bending of O with respect to Fe, the F 2g (1) band (107 cm −1 ) is due to the translational movement of the whole tetrahedron (FeO 4 ), and the A 1g band (630 cm −1 ) is due to the symmetric stretching of oxygen atoms along Fe–O (or Mn–O) tetrahedral bonds 14 . The bands confirm the inverse spinel structure of the MnFe 2 O 4 43 . The Raman spectrum of the MnFe 2 O 4 /MoS 2 nanocomposite coated on a nickel foam shows the peaks due to MnFe 2 O 4 and also the F 2g (2) band, which is due to the asymmetric stretching of Fe/Mn–O bonds, as well as the E 2 g band of few-layer MoS 2 nanosheets, confirming the presence of both MnFe 2 O 4 and MoS 2 in the nanocomposite.…”

Observation of dielectric dispersion and relaxation behavior in Ni2+-substituted cobalt ferrite nanoparticles

“…Ulpe and co-workers investigated in depth the electronic structure of several spinel materials, confirming that the band gap description is challenging for DFT calculations. 52 Herein, the calculated values at PBE0 allowed us to relate the increased band gap value to the variation in the Ni amount, which is in agreement with previous experimental data. 54,55 Another important aspect to consider about the electronic structure of CNF nanoparticles is the atomic state localization along the valence/conduction bands.…”

“…The calculated values showed excellent agreement with previous theoretical and experimental data. 25,51,52 In particular, it was observed that the calculated dielectric constant was reduced as the Ni amount was increased. However, all values were calculated in the range of 7.1-7.9, indicating that all models exhibit good electrical properties for energy-harvesting applications.…”

“…Similar results were calculated for CoFe 2 O 4 and NiFe 2 O 4 with the PW1PW exchange-correlation functional, confirming that the PBE0 results are in accordance with experimental and theoretical predictions. 25,52,53 Another important aspect about CNF nanoparticles is their electronic structural analysis. Herein, the band structure and atom-resolved density of states profiles were considered, as presented in Fig.…”

Highly improved supercapacitance properties of MnFe2O4 nanoparticles by MoS2 nanosheets

“…In the Raman spectrum of MnFe2O4 nanoparticles, the Eg band (189 cm -1 ) is due to the asymmetric and symmetric bending of O with respect to Fe, the F2g(1) band (107 cm -1 ) is due to the translational movement of the whole tetrahedron (FeO4), and the A1g band (630cm -1 ) is due to the symmetric stretching of oxygen atoms along Fe-O (or Mn-O) tetrahedral bonds13 . The bands confirm the inverse spinel structure of the MnFe2O434 . The Raman spectrum of the MnFe2O4/MoS2 nanocomposite shows the peaks due to MnFe2O4 and also the F2g(2) band that is due to the asymmetric stretching of Fe/Mn-O bonds as well as the E2g band of few-layer MoS2 nanosheets, confirming the presence of both MnFe2O4 and MoS2 in the nanocomposite.…”