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Novel Iron-Based Cocatalysts for Photocatalytic Water Splitting and Nitrogen Reduction

Many state-of-the-art photocatalyst systems suffer from high charge recombination rates that limit the number of charge carriers available for photocatalytic reactions on the surface. Additionally, the overpotential for the desired conversion reactions is often relatively high, likewise having an impeding effect on the activity. To overcome these limitations, the catalyst surface can be decorated with a cocatalyst that improves charge separation and lowers the overpotential. Noble metals have been especially promising in this regard.[1] However, the use of noble metals significantly increases the material costs and requires rare resources, thus compromising their sustainability. Molecular cocatalysts can be used alternatively, but suffer from stability issues, especially in aqueous media.[2]
 
 Recently, earth-abundant cocatalyst systems have emerged as alternative materials to improve photocatalytic hydrogen or oxygen generation.[3] Among the elements used, iron is one of the most abundant elements in the earth crust, and non-toxic. The use of iron-based cocatalysts can therefore significantly decrease photocatalyst production costs and minimize the creation of hazardous waste. Furthermore, iron is a core constituent of many active centers in enzymes, such as hydrogenase, nitrogenase or CO dehydrogenase, that are active in natural reduction and oxidation reactions.[4] This implies, that the development of efficient iron-based cocatalysts should generally be achievable. Interestingly, iron-based materials have only rarely been used as cocatalysts in photocatalysis. However, very prominent examples for photoelectrochemistry are FeOOH or Fe-based layered double hydroxides (LDH).[5]
 [6]
 
 On the other hand, ternary iron oxides and sulphides are promising electrocatalysts for water splitting and CO2 reduction, exhibiting low overpotentials for the respective reaction.[7]
 [8] E.g. we recently showed the first microwave-synthesis of phase-pure violarite Ni2FeS4 in only 30 minutes, and its application for electrocatalytic CO2 reduction and hydrogen generation.[8] The observed activity underlines the potential of these materials for the application in photocatalysis as well. Additionally, some of the known iron-based chalcogenides possess near metallic conductivity that should facilitate charge transfer. However, apart from the reduction of the overpotential, an efficient charge transfer from photocatalyst to cocatalyst is required, which necessitates a suitable band alignment, something, that is frequently overlooked in literature.
 In this contribution, novel ternary iron-based chalcogenides will be presented as efficient cocatalysts on TiO2 and C3N4, leading to a significant activity increase in hydrogen evolution, oxygen evolution and nitrogen reduction for both absorbers, even upon addition of relatively low amounts of 1 wt.-%. The influence of cocatalyst loading will be presented, and the activity will be compared to noble metal cocatalysts such as platinum. Additionally, electronic interactions between absorber and cocatalyst have been explored.
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