Abstract: | |
Spinel ZnFe2O4 (ZFO) is a widely studied iron-based semiconductor for
application as photoanode material in photoelectrochemical water splitting tandem
cells. However, the current benchmark efficiency reported for photoelectrochemical
water oxidation at ZFO photoanodes is approximately one order of
magnitude smaller than the predicted theoretical maximum. In addition, a large
dispersion between the efficiencies reported for ZFO photoanodes prepared by
different synthetic approaches, as well as poor reproducibility, become obvious
from published data. It has been recently reported that the cation distribution, i.e.,
the ordering of the Fe3+ and Zn2+ cations within the oxygen lattice, has an impact on
the photoelectrochemical activity of the semiconductor. However, the impact of the
cation distribution on physicochemical properties directly related to the
photoelectrochemical activity was poorly understood. The parameter employed to
characterize the cation distribution is the degree of inversion, x, defined as
T[Zn1-xFex]O[ZnxFe2-x]O4, with 0 ≤ x ≤ 1 (the superscripts T and O denote tetrahedral
and octahedral sites, respectively).
In this work, highly pure ZFO samples exhibiting degrees of inversion ranging
from x ≈ 0.07 to x ≈ 0.20 were synthesized. The samples exhibited, within the limit
of the experimental determination, equal particle size, crystallite size, and
crystallinity, as was confirmed by XRD plus Rietveld refinement, Mössbauer
spectroscopy, Raman spectroscopy, scanning electron microscopy, and elemental
analysis. Oxygen vacancies were not detected. Therefore, the degree of inversion is
assumed to be the only independent variable between the different samples. The
light absorption, charge carrier transport, and electronic properties were
investigated by UV-Vis-NIR reflectivity, impedance spectroscopy, and time-averaged
as well as transient photoluminescence spectroscopy, respectively. The
photoelectrochemical efficiency for the methanol oxidation reaction under
simulated solar irradiation was determined in order to compare the activity of the
ZFO samples having different degrees of inversion.
VI
It was found that the cation distribution does not affect the band gap energy
of ZFO but has a large impact on the charge carrier transport and the electronic
properties. An increase in the photoelectrochemical activity was observed by
increasing the degree of inversion. This impact was mainly ascribed to the enhanced
charge carrier transport properties of the samples having higher degrees of
inversion. In addition, changes in the probability of the photoinduced electronic
transitions of ZFO produced by increasing the degree of inversion were found to
additionally contribute, to a lesser extent, to the observed enhancement in the
photoelectrochemical activity.
This thesis provides a fundamental insight concerning the impact of the
degree of inversion on the photoelectrochemical activity of ZFO. Furthermore, the
results presented herein contribute to the understanding of some factors limiting
the efficiency of ZFO photoanodes.
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License of this version: | Es gilt deutsches Urheberrecht. Das Dokument darf zum eigenen Gebrauch kostenfrei genutzt, aber nicht im Internet bereitgestellt oder an Außenstehende weitergegeben werden. |
Publication type: | DoctoralThesis |
Publishing status: | publishedVersion |
Publication date: | 2019 |
Keywords german: | Spinell ZnFe2O4, Inversionsgrad, Kationenverteilung, Photoelektrochemie, Photoanode |
Keywords english: | degree of inversion, cation distribution, photoelectrochemistry |
DDC: | 660 | Technische Chemie |