Characterization of Iron-Doped Titanium Dioxide by Electron Microscopy Techniques
Date
2022-10-18
Authors
Parisi Couri, Atieh
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Abstract
Access to clean water is essential for human health and dignity. The increasingly rapid population growth, combined with the emergence of resistant chemical compounds and more concentrated toxic residues in the effluent streams of treatment plants, point towards a decline in freshwater resources resulting in a global water crisis in the next decades. Current wastewater treatment plants rely on Advanced Oxidation Processes (AOPs) for the tertiary (or advanced) step of the treatment. Photocatalysis is one of such processes, by which semiconductors are exposed to radiation of specific wavelengths (traditionally UV) to generate Reactive Oxygen Species (ROS) that can degrade organic molecules through a chain of radical oxidation reactions.
Anatase titania (TiO2) has been used for many decades as a photocatalyst. Its electronic band structure has a band gap of 3.2 eV, requiring radiation in the UV range to trigger its photocatalytic properties. One way to reduce the band gap energy and shift the absorption peak wavelength to the visible part of the spectrum (thus reducing operation costs) is by doping the photocatalyst particles with transition metal atoms. Iron (III) is a great candidate due to the placement of its conduction/valence bands within titania’s band gap, its atomic radius similar to titanium (IV) and its variety of oxidation states. However, iron-doped anatase titania synthesized by ordinary sol-gel methods shows a photodegradation efficiency that is much lower than undoped anatase. Previous studies have shown that this is caused by an inconspicuous iron oxide layer on the surface of the catalyst particle, forming a physical barrier to the mobility of charge carriers that trigger the formation of ROS radicals. Small changes to the synthesis protocol, namely slowing down the hydrolysis of the Ti precursor by lowering the solution’s pH and acid-washing the final product, have been shown to result in particles that are photoactive under visible radiation and boast an unobstructed reactive surface.
In this work, the novel Fe-TiO2 photocatalyst is studied and characterized in terms of its particle size distribution, inner structure and composition using electron microscopy techniques. It is important to know the particle size profile arising from this novel synthetic method, as the presence of nanoparticles could pose a health risk whereas an abundance of oversized particles is undesirable from the perspective of chemical reaction engineering (low surface-to-volume ratio). Inner structure/composition analyses could reveal whether the iron content inside the photocatalyst segregates into iron oxides, which would hinder reaction rates by behaving as a recombination center for charge carriers. As well, gathering more information about the inner structure of the catalyst (such as degree of crystallinity) is desirable as that could help fine-tune the synthesis protocol in order to obtain optimal photocatalytic activity.
The particle size distribution studies using scanning electron microscopy revealed that the catalyst samples contain a significant fraction of nanoparticles (31.55% smaller than 100 nm), even though those particles represent a very small fraction of total sample volume (0.00015%) and reactive area (0.03%). Moreover, oversized particles (bigger than 5 m) account for the biggest fraction of sample volume and reactive surface. It was suggested that the size distribution of the catalyst be shifted to intermediate particle sizes by introducing additional grinding and separation steps into the synthesis protocol.
The inner structure studies were carried using a combination of scanning, transmission and scanning-transmission electron microscopy, as well as spectroscopy methods such as EDX and EELS to map composition. It was found that the original anatase lattice structure remained unchanged in terms of interplanar spacings and crystallographic orientations, indicating that the addition of iron impurities at the small concentrations used here (0.5at%) did not result in discernible changes to the lattice. The monocrystalline units of Fe-TiO2 (termed crystallites) often appear to be bound together by amorphous material. No segregation of Fe was observed inside the particles at this concentration, as shown by the apparent homogenous composition of the catalyst across crystalline and amorphous regions. The external iron oxide contamination layer observed in previous studies was theorized to form during the later steps of the sol-gel process due to the precipitation of the iron content in solution that failed to be incorporated into the TiO2 gel network. More in-depth studies must be carried to assess whether preferential segregation of iron within the catalyst could occur at higher dopant concentrations.
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Keywords
photocatalysis, titanium dioxide, electron microscopy, water treatment