Tuning the electronic band structure of microporous titanates with the hollandite structure

Abstract

We present the electronic band structures of microporous titanates with the hollandite-type structure. These titanium-based oxides have the general formula of K_x[Ti, M]₈O₁₆ with M = Sc to Ni and can readily accommodate lower valent transition metals into the titanate framework due to charge compensation from the K⁺ cations in the micropores. This chemical control allows for facile tuning of the electronic band structure, which we explored through X-ray photoelectron spectroscopy, UV-vis spectroscopy, and density functional theory (DFT) studies. In these titanates, the band-gap can be tuned between 4.1 eV and 3.0 eV by varying the M cation from Sc to Ni. Furthermore, the formation of interband states within the band gap for M = V to Ni allow for lower-energy transitions in the visible range. By combining the results from the photoelectron and UV-vis spectra, we construct a master band structure diagram for the whole series and discuss their potential in photoelectrochemical applications.