TY - BOOK

T1 - An iron-based photoelectrode

AU - Granone, Luis Ignacio

PY - 2019

Y1 - 2019

N2 - 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.

AB - 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.

U2 - 10.15488/5374

DO - 10.15488/5374

M3 - Doctoral thesis

CY - Hannover

ER -