Improving the Photocatalytic activity for Hydrogen Evolution of LaBaInO4 via Topochemical Fluorination to LaBaInO3F2

In this paper, we report on a non-oxidative topochemical route for the synthesis of a novel indate-based oxyfluoride, LaBaInO3F2 using a low-temperature reaction of Ruddlesden-Popper-type LaBaInO4 with polyvinylidene difluoride as a fluorinating agent. The reaction involves the insertion and substitution of oxide ions by fluoride ions. From the characterization via powder X-ray diffraction and Rietveld analysis as well as attenuated electron diffraction tomography it could be followed that the fluorination results in a symmetry lowering from I4/mmm (139) to monoclinic C2/c (15) with an expansion perpendicular to the perovskite layers and a strong tilting of the octahedra along the ab plane. While an ordering of vacancies on the interstitial sites is observed, disorder of anions on the three anion sites seems be favored. The most stable configuration for the anion ordering is estimated based on an evaluation of bond distances from ADT measurements via bond valence sums. The observed disordering of the anions in the oxyfluoride consequently resulted in changes of the bandgap, interestingly leading to an enhanced photocatalytic activity for the hydrogen (H2) evolution as compared to the pure oxide. Thus, we show that topochemical anion modification can present a viable route to obtain improved photocatalysts.