Control of the Photocatalytic Activity of Metastable Layered Oxynitride K₂LaTa₂O₆N through Topochemical Transformation of Tuned Oxide Precursors

Abstract

In the development of oxynitride photocatalysts, thermal ammonolysis of a metal oxide precursor has often been conducted by varying the reaction conditions (e.g., temperatures, reaction times, and ammonia gas flow rates) to obtain high-quality oxynitride particles that efficiently function as photocatalysts. However, this approach may suffer from undesirable changes in the physicochemical properties of the resulting oxynitride, leading to the lowering of the photocatalytic activity. Here, we show that it is possible to control the photocatalytic activity of Ruddlesden–Popper metastable layered oxynitride K2LaTa2O6N, obtained from the Dion–Jacobson phase KLaTa2O7 through a topochemical ammonolysis reaction, by controlling the quality of the KLaTa2O7 template. During the ammonolysis of KLaTa2O7, in the presence of K2CO3, to K2LaTa2O6N, the structural properties (e.g., degree of crystallinity and particle size) of the oxide precursor were replicated in the resulting oxynitride. Namely, the use of KLaTa2O7, possessing a higher degree of crystallinity, led to larger K2LaTa2O6N particles being formed. By increasing the crystallinity of KLaTa2O7, the photocatalytic activity of the resulting K2LaTa2O6N for H2 evolution was improved for reaction in aqueous NaI solution under visible light irradiation. This improvement in performance was due to the longer lifetime of the photogenerated mobile electrons in high-crystallinity K2LaTa2O6N compared with that in the low-crystallinity analogue, as confirmed by femtosecond transient absorption spectroscopy. However, the photocatalytic activity of K2LaTa2O6N derived from well-grown larger KLaTa2O7 particles was an order of magnitude lower than that of the best-performing material. Physicochemical measurements revealed that the large K2LaTa2O6N particles contained a relatively high density of anionic defects on the surface, which shortened the lifetime of the photogenerated charge carriers, leading to lower photocatalytic activity.

Interlayer modification and single-layer exfoliation of the Ruddlesden–Popper perovskite oxynitride K2LaTa2O6N to improve photocatalytic H2 evolution activity

“…In our previous work, lower-valence Ta species (specifically Ta 3+ ) that were associated with the formation of anionic defects were found to lower the photocatalytic H 2 evolution activity of H x K 2− x LaTa 2 O 6 N by shortening the lifetimes of photogenerated free electrons. 26 As shown in Fig. 5, the valence state of the Ta in the EA/H x K 2− x LaTa 2 O 6 N, which was the most active material, was similar to those in the H x K 2− x LaTa 2 O 6 N and R–K 2 LaTa 2 O 6 N. All three specimens exhibited features related almost entirely to Ta 5+ .…”

“…The synthesis was done according to a previously reported procedure with minor modications. 23,26 Briey, CsLaTa 2 O 7 was rst synthesized using a polymerized complex method employing Cs 2 CO 3 ($99.5%; Kanto Chemicals Co.), La(NO 3 ) 3 $6H 2 O ($99.0%; Kanto Chemicals Co.) and TaCl 5 ($99.9%; Mitsuwa Chemicals Co.) with calcination in air at 1273 K for 2 h. The resulting material was then subjected to a topochemical cation exchange reaction in molten KNO 3 ($99.0%; Wako Pure Chemicals Co.) at 673 K for 48 h to obtain KLaTa 2 O 7 . K 2 LaTa 2 O 6 N was produced by heating the as-prepared KLaTa 2 O 7 together with 75 mol% K 2 CO 3 ($99.5%; Kanto Chemicals Co.) in a tube furnace at 1173 K for 3 h under a 50 mL min −1 ow of gaseous ammonia ($99.9995%; Sumitomo Seika Chemicals Co.).…”

“…S3 † for details), as discussed in a previous paper by our group. 26 Following protonexchange, the Ta 4f peak became sharper and showed features typical of Ta 5+ . A loss of the lattice nitrogen content from the surface was also observed aer the proton-exchange (Table 1).…”

“…K 2 LaTa 2 O 6 N, which has a structure comprising two-layer perovskite blocks, undergoes H + /K + exchange in water while maintaining a suitable degree of visible light absorption. 23,26 This material also shows much higher photocatalytic activity than the three-layer analogue because of a longer photogenerated free electron lifetime. 24 Therefore, the application of K 2 LaTa 2 O 6 N to photocatalytic reactions aer suitable modication is of interest.…”

Orange Zn–Al–O–N Powder Active for Photocatalytic Water Reduction and Oxidation Synthesized from a Zn–Al Layered Double Hydroxide Using Urea as the Nitriding Agent

“…Oxynitrides, including N-doped oxides, have been attracting attention as functional materials. , Oxynitrides are typically synthesized via thermal ammonolysis of an oxide precursor at high temperatures (>1073 K). , The successful synthesis of a desired oxynitride is strongly dependent on the choice of the precursor and the synthesis conditions. Because the diffusion of N 3– ions in the crystal lattice is slow, the use of smaller-sized oxide precursors leads to more effective conversion to the corresponding oxynitrides. − Thermal ammonolysis requires a continuous flow of toxic ammonia gas; however, a solid-state nitriding agent such as urea or carbon nitride, both of which are much less toxic than ammonia, can be used as an alternative to thermal ammonolysis. − …”