The exploration of novel photocatalytic active materials is the key for critically advancing solar hydrogen production. The ideal size and position of the band gap, a low electron recombination rate of the materials and corrosion resistance are main parameters. Binary and ternary mixed-metal oxides systems offer a wide range of band gap sizes and positions compared to traditional single semiconductor materials such as TiO₂ or Fe₂O₃. The systematic combination of two mixed-metal oxide materials in a heterojunctions has additional advantages: Firstly, the light spectrum, which can be utilized for the electron excitation, is increased and secondly, the probability of electron-hole recombination is reduced by charge separation.

However, the production methods for this mixed-metal oxide materials and the design of tailored interfaces between two different crystalline semiconductors with unequal band gaps are often limited regarding size and composition control and associated with several production steps. Flame Spray Pyrolysis is a high temperature aerosol technique for synthesizing nano-sized metal oxide catalysts [1] and also offers the possibility for synthesis of meta-stable phases like In₄Sn₃O₁₂ through its high temperature gradients [2]. In the double Flame Spray Pyrolysis process individual mixed-metal oxide particles are synthesized in two opposing flames, intersecting at a defined temperature range. The exclusive deposition of noble metals such as platinum or ruthenium as co-catalysts on the individual compounds can further be realized in this single step synthesis technique. Properties such as particle size, shape, and specific surface area can be tuned for each compound individually by adjusting the flames separately.

In the resent study, material characteristics including crystal and band structure of promising iron based mixed-metal oxides (e.g. Fe₂ZnO₄) are intensively investigated (BET, UV-Vis and XRD) and the experimental results are validated and completed with DFT studies. The composition and the interface of the synthesized heterojunctions are quantified with (S)TEM analysis. The photocatalytic activities of the materials are measured photochemically and in a water splitting reactor.

[1] W.Y. Teoh, R. Amal, L. Mädler, Nanoscale, 2, 1324 (2010)

[2] J. A Kemmler, S. Pokhrel, et al. Sensors and Actuators B: Chemical 161(1): 740 (2012).