Bismuth-metal oxide band-gap tailored visible light photocatalysts for wastewater purification

Water pollution is a growing environmental problem. To combat this, semiconductor
photocatalysts are considered one of the most promising tools for decomposing hazardous
pollutants. Among these, mullite-type phases of the general formula Bi2M4O9 (M: metal)have
been investigated for some time as promising materials for catalytic reactions using light in
the visible range. Accordingly, in this dissertation, mullite-like solids of the composition
Bi2(Al1-xFex)4O9 and Bi2(Ga1-xFex)4O9 (x = 0-1 in 0.1 steps) in four different average crystallite
sizes (ACSs) were synthesized in order to investigate their photocatalytic activity under
visible light. The smallest quantum crystalline (qc) materials (1-3 nm) mainly represent
nucleation seeds of the mullite-like structure. Further heat treatment then produced the target
phases, mainly nanocrystalline mullite type (ncM) phases with average crystallite sizes of
about 20-160 nm. In the synthesized ncM samples, strong correlations were found between
the micro-stress of the crystallites and both the composition as well as the ACS. EPR
measurements show the incorporation of Fe3+ into the distorted octahedral and tetrahedral
positions in agreement with the respective FTIR and XRPD findings. The combination of the
investigations showed that the band gap energy Eg is influenced by both the composition and
the ACS. However, depending on the composition, opposite trends in the Eg development
with respect to the ACS were observed, as there are several structural effects that can
influence each other as well as the respective band gap energy.
In order to measure the photocatalytic activity of the synthesized samples, a modular
standardized setup for simultaneous photocatalytic measurement experiments with
interchangeable light sources was constructed and described in the second part of this work. It
was evaluated and an analytical methodology established by degrading common dyes –
methylene blue (MB), methyl orange (MO) and ortho-nitrophenol (o-NPh) – under UV and
visible light with TiO2 compounds as reference catalysts.
Subsequently, several synthesized mullite-type Bi2(Al1-xFex)4O9 and Bi2(Ga1-xFex)4O9 (x = 0.0,
0.1 & 1.0) phases, as well as additionally produced perovskite-type BiFeO3 and
BiFeO3/Bi2Fe4O9 phases, were tested for their photocatalytic activity. Contrary to the
descriptions in the literature, they showed no direct photocatalytic activity. This was only
observed after adding H2O2 to the solutions. This showed that the degradation measured in
these cases was caused either by the self-degradation of H2O2 or by Fenton or photo-Fenton
reactions. The absence of measurable photocatalytic activity of bismuth metal oxides without
the addition of an oxidizing agent can be explained by the fact that their valence band position
is too high to allow the formation of OH● from water.
One possible way to still use the manufactured semiconductors as photocatalysts is to couple
them via a heterojunction with another suitable semiconductor that ensures an S-scheme
transfer mechanism between the compounds. This mechanism is most likely to prove
indispensable for photocatalytic water purification under visible light in general.