Improved visible-light activity of nitrogen-doped layered niobate photocatalysts by NH3-nitridation with KCl flux

Abstract

In the present study, nitrogen doping into layered perovskite niobites, KCa2Nan–3NbnO3n+1 (n = 3 or 4), was attempted to enhance their response to visible light during photocatalytic water reduction and oxidation. Although conventional nitridation, i.e., heating in NH3 stream at a high temperature, produced black-colored samples due to the undesirable reduction of Nb5+ to Nb4+ (and/or Nb3+), the use of KCl flux with NH3 stream effectively prevented the reduction of Nb5+, resulting in yellow-colored samples that had a much lower concentration of reduced Nb species. The samples prepared with KCl flux exhibited stronger absorption (at λ = 350–550 nm) when compared to those prepared without KCl flux. This was due to the higher concentration of N3– anions, as confirmed by elemental analysis and the observed peak shift in XRD patterns. On the other hand, the absorption band with λ > 550 nm, which indicated the presence of reduced species or anion vacancies, were astonishingly smaller for the samples prepared with KCl than for those without KCl. This indicated that the formation of reduced Nb species was inhibited by KCl. XPS analysis also supported this effect. The use of KCl flux also prevented the problematic decrease in the concentration of potassium cations in the interlayer space during high-temperature treatment in NH3 flow. The samples prepared with KCl flux exhibited much higher O2 and H2 evolution rates under visible-light irradiation in the presence of an electron acceptor and a donor, respectively as compared to those prepared without KCl. This clear improvement in O2 and H2 evolution was probably due to the larger amount of doped nitrogen introduced and/or the suppressed formation of reduced Nb species.