The search for a sustainable energy source to satisfy the rising energy demand of an increas-ing world population is one of the most important tasks of contemporary research. Although solar energy has enough potential to satisfy the demand many times over, its intermittency is a serious bottleneck. Photocatalysis is a very promising way to convert solar energy into chemical fuels, which can be stored, transported and used on demand.
However, the most commonly used metal oxide semiconductors – such as TiO2 – have large band gaps, which seriously restricts their application in solar light conversion. Herein, we have used defect-pyrochlores AMxW2-xO6 (A: K, Cs; M: transition metal) as model systems to achieve visible light absorption by changing the crystal structure composition.
The defect-pyrochlore crystal structure consists of corner-sharing MO6 and WO6 octahedra forming hexagonal channels, in which the loosely bound A ions are located (Figure 1). Ion exchange of these ions with Sn2+ results in the formation of a new valence band with Sn 5s2 orbitals, which decreases the band gap by 1.4 eV down to 2.2 eV.
Another advantage of defect-pyrochlores is their high elemental diversity: since Mn+n+ and W6+ are located on the same site, it is possibly to incorporate Mn+ cations of different oxidation states by changing the M/W ration, thus retaining charge balance. By incorporating Mn+ cations with partially occupied d levels, it is possible to change the band structure and thus the band gap of these materials.
 M. Weiss, T. Bredow, R. Marschall, Chem. Eur. J., 2018, 24, 69, 18535-18543, 10.1002/chem.201803276.