Universität zu Köln

Bialuschewski, Danny (2020). Laser-assisted Modification of Metals and Metal Oxide Semiconductors as Photoactive Materials. PhD thesis, Universität zu Köln.

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Abstract

Laser-induced order and disorder on both chemical and structural level through absorption of femtosecond (fs) laser pulse was investigated on different metal (e.g. Al or Ti) and metal oxide systems (e.g. FexOy or TiOx) in order to modulate their surface chemistry and topography. For example, semiconductor materials were engineered towards their optical, electrical and electronic properties by femtosecond laser treatment to improve their water splitting properties. Thin metal and metal oxide films with tunable photophysical properties can be used for new solar harvesting systems, which might fulfill the demand for renewable energy and reduce CO2 emission, which poses one of the biggest challenges in the current era. The combination of chemistry (precursor synthesis), material science (thin film fabrication and device engineering) and physics (femtosecond laser treatment) is rarely performed due to the sheer complexity of the different subjects, but has been proven to be invaluable in this work to develop a fundamental understanding of the relationship of the surface treatment and the functional properties. The first step was to standardize the laser structuring parameters to enable reproducible processes. In order to reduce random absorption and scattering of the laser beam on interfaces like grain boundaries, metal films were introduced to efficiently absorb the photon energy due to their high free electron density. For this reason, either metal substrate (Al or Ti) or a sputtered metal film (Al) were used. The metal oxide thin films were grown by plasma-enhanced chemical vapor deposition (PE-CVD) techniques. With reproducible laser processing more complex systems could be investigated, for example the effects of gradual changes in laser fluence on films of titanium oxide in combination with iron oxide, or bimetallic oxides (MAl2O4, M = Ni, Co) were studied. Possible structures ranging from shallow to deep ripples to particle formation were observed. Surface roughness, ripple depth, hydrophobicity and absorption was found to improve with increasing laser power. Additionally, laser treated titanium oxide layers revealed a gas sensing response towards reductive gas (H2) at room temperature. Through precise parameter tuning, the titanium oxide thin films could be crystallized by laser-assisted patterning. Laser-induced texturing of metal substrates has shown to improve two-fold, the photoelectrochemical performance of titanium oxide (from 0.031 to 0.061 mA/cm²) and iron oxide absorber films (from 0.028 to 0.057 mA/cm²) due to increased light trapping, and availability of a higher number of surface states. The photoelectrodes patterned by fs laser pulses were investigated towards their light absorption, conversion efficiency, morphology, crystallographic structure and composition, to further improve the understanding of periodically patterned materials by laser treatment. Moreover, transition metal spinel films (MAl2O4, M = Ni, Co) were decomposed by laser treatment with the intention of embedding transition metal clusters into an aluminum oxide matrix, forming so called CERMETs (ceramic-metal composite materials). These show unique optical and catalytic properties as supported by photocatalytic degradation of methylene blue. It could be observed that laser treated NiAl2O4 samples showed photocatalytic performance, while compositional studies have shown that laser treatment reduce the divalent transition metal centers to elemental state producing Ni/ or Co/Al2O3 nanocomposites. In summary, this research improved the understanding and possibilities of ultrashort laser pulses to structurally and compositionally modify metal and metal oxides systems, and enhance their properties for many different applications, like photoelectrochemical water splitting, gas sensing or photocatalysis.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Bialuschewski, Danny d.bialuschewski@gmx.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-106943
Series Name:	Anorganische Chemie
Date:	27 January 2020
Publisher:	Dr. Hut Verlag
Place of Publication:	München
ISBN:	978-3-8439-4346-8
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Chemistry > Institute of Inorganic Chemistry
Subjects:	Chemistry and allied sciences
Uncontrolled Keywords:	Keywords Language femtosecond laser English thin films English photoactive materials English
Date of oral exam:	25 November 2019
Referee:	Name Academic Title Mathur, Sanjay Prof. Dr. Lindfors, Klas Prof. Dr.
Funders:	DFG SPP 1839
Projects:	Tailored Disorder - A science- and engineering-based approach to materials design for advanced photonic applications
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/10694