Grundlagenstudien zur photoelektrochemischen Wasseroxidation an Hämatit-Nanostrukturen

authored by: Imme Elisabeth Kretschmer
supervised by: Detlef Bahnemann
Abstract: During the last decade, the synthesis of new materials for photoelectrochemical water splitting has gained increased attention. Understanding on the fundamental reaction mechanisms is of high importance for efﬁciency enhancement and design of such materials. Hematite is among the most well known materials for photoelectrochemical water splitting and its reaction mechanisms are well researched. However, there are still open questions concerning the nature of the charge carriers created upon irradiation and its recombination kinetics. Transient absorption spectroscopy via laser ﬂash photolysis is a well known and widely used technique to investigate fundamental charge carrier dynamics. In the present work, the nature of photogenerated charge carriers in commercial and self prepared hematite powders was investigated via Nanosecond Laser Flash Photolysis Spectroscopy in diffuse reﬂectance mode. The kinetic dependency of the transients from oxygen concentration on the surface was studied via variation of surrounding atmosphere (argon, nitrogen, air, oxygen, ozone). Increasing oxygen concentration was found to lead to shorter lifetimes of the trapped char- ge carriers. A comparison with thermally pre-treated samples revealed a correlation of oxygen concentration and the number of defects on the cristallite surface. A reconstruction of the transient absorption spectrum allowed to assign signals in the wavelength region of 550-850 nm to trapped holes of different species. With high output energy during laser irradiation, irreversible structural changes could be observed on powdered samples. Characterization via X-ray diffractometry reveals phase change of hematite to magnetite. This change could be described as structural change caused by an excess of defects and resulting lattice distortion. Transient absorption spectra recorded from magnetite powdered samples conﬁrmed the role of defects created by laser exposure. Finally, two different ways of charge transfer, dependent on oxygen concentration were observed and could be assigned to signals at speciﬁc wavelengths via mathematical deconvolution of the absorption spectrum. These correspond to hole trapping at iron-kations and oxygen-anions, respectively. A third charge transfer signal corresponds to oxygen dependent defects.
Organisation(s): Institute of Technical Chemistry
Type: Doctoral thesis
No. of pages: 102
Publication date: 2023
Publication status: Published
Sustainable Development Goals: SDG 7 - Affordable and Clean Energy
Electronic version(s): https://doi.org/10.15488/13198 (Access: Open)