Solar-assisted water splitting using photoelectrochemical cells (PECs) is one of the promising pathways for the production of hydrogen for renewable energy storage. The nature of the semiconductor material is the primary factor that controls the overall energy conversion efficiency. Finding semiconductor materials with appropriate semiconducting properties (stability, efficient charge separation and transport, abundant, visible light absorption) is still a challenge for developing materials for solar water splitting. Owing to the suitable bandgap for visible light harvesting and the abundance of iron-based oxide semiconductors, they are promising candidates for PECs and have received much research attention. Spinel ferrites are subclasses of iron oxides derived from the classical magnetite (FeIIFe2IIIO4) in which the FeII is replaced by one (some cases two) additional divalent metals. They are generally denoted as MxFe3−xO4 (M=Ca, Mg, Zn, Co, Ni, Mn, and so on) and mostly crystallize in spinel or inverse spinel structures. In this mini review, we present the current state of research in spinel ferrites as photoelectrode materials for PECs application. Strategies to improve energy conversion efficiency (nanostructuring, surface modification, and heterostructuring) will be presented. Furthermore, theoretical findings related to the electronic structure, bandgap, and magnetic properties will be presented and compared with experimental results.
The term photocatalysis is used to describe a photon-driven catalytic process. Titanium dioxide is a well-known
photocatalyst in such fields as self-cleaning material and anti-microbial effects. Besides these photocatalytic applications,
TiO2 is a widely-used high index material for optical thin films.
In the present investigation, the photocatalytic activity of transparent TiO2 thin films was optimized to achieve
multifunctional high precision optical coatings. The films have been deposited by ion assisted deposition (IAD), applying
a Leybold APS plasma source as well as a Denton CC-105 ion source. The cause-and-effect chain between the use of
different parameters in the IAD process and optical properties of the TiO2 layers as well as their photocatalytic activities
are described.
As test reaction for the determination of the photocatalytic properties, the degradation of methylene blue (MB) was
chosen. The used setup based on a high precision two-path laser measurement system was developed by the LZH in
order to determine the kinetic performance of TiO2 catalysts under well-defined UV illumination conditions. Photonic
efficiencies of the TiO2 thin films were calculated from the obtained data. Additionally, crystal structure analysis has
been investigated for the identification of anatase and rutile modifications.
The comparison of the results shows that ion assisted deposition is an appropriate technology for the preparation of
photocatalytic active thin films for optical applications.
Molecular sieves, such as nanoporous AlPO4-5, can host a wide variety of laser-active dyes. Slim dyes like Coumarin40 or Oxazine 1, fitting into the channel pores, as well as bulky dyes like rhodamines and Oxazine 750, being located in defect sites of the molecular sieve structure, are embedded during microwave-assisted crystallization. The fast microwave-assisted crystallization offers a new procedure for the stable and monomeric encapsulation of organic dyes into molecular sieves without any degradation of the chromophores and enables the control of the morphology of the host material. Based on this class of materials a new form of microlasers has been created. Their properties depend on size and shape of the molecular sieve crystals. The microcrystals act as hexagonal ring resonators (whispering gallery mode). In dependence on the morphology of the crystals different laser properties have been observed in agreement with theoretical predictions. Large hexagonal crystals (diameter of the hexagonal plane > 7 micrometers ) revealed multiline laser emission, while smaller crystals (diameter about 5 micrometers ) oscillate on one single line. The laser threshold power density decreases with decreasing diameter of the crystals. In terms of pumping needed to reach lasing molecular sieve microlasers are comparable to quantum dot semiconductor lasers.
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