Synthesis of Three-Layer Perovskite Oxynitride K₂Ca₂Ta₃O₉N·2H₂O and Photocatalytic Activity for H₂ Evolution under Visible Light

Abstract

Substitution of oxide anions (O2–) in a metal oxide for nitrogen (N3–) results in reduction of the band gap, which is attractive in heterogeneous photocatalysis; however, only a handful of two-dimensional layered perovskite oxynitrides have been reported, and thus, the structural effects of layered oxynitrides on photocatalytic activity have not been sufficiently examined. This study reports the synthesis of a Ruddlesden–Popper phase three-layer oxynitride perovskite of K2Ca2Ta3O9N·2H2O, and the photocatalytic activity is compared with an analogous two-layer perovskite, K2LaTa2O6N·1.6H2O. Topochemical ammonolysis reaction of a Dion–Jacobson phase oxide KCa2Ta3O10 at 1173 K in the presence of K2CO3 resulted in a single-phase layered perovskite, K2Ca2Ta3O9N·2H2O, which belongs to the tetragonal P4/mmm space group, as demonstrated by synchrotron X-ray diffraction, scanning transmission electron microscopy measurements, and elemental analysis. The synthesized K2Ca2Ta3O9N·2H2O has an absorption edge at around 460 nm, with an estimated band gap of ca. 2.7 eV. K2Ca2Ta3O9N·2H2O modified with a Pt cocatalyst generated H2 from an aqueous solution containing a dissolved NaI as a reversible electron donor under visible light (λ > 400 nm) with no noticeable change in the crystal structure and light absorption properties. However, the H2 evolution activity of K2Ca2Ta3O9N·2H2O was an order of magnitude lower than that of K2LaTa2O6N·1.6H2O. Femtosecond transient absorption spectroscopy revealed that the lifetime of photogenerated mobile electrons in K2Ca2Ta3O9N·2H2O was shorter than that in K2LaTa2O6N·1.6H2O, which could explain the low photocatalytic activity of K2Ca2Ta3O9N·2H2O.

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

“…The decay kinetics of the photogenerated free and/or shallowly trapped electrons, which are more influential in surface reactions, 24 in the H + /Ir/K 2 LaTa 2 O 6 N-T samples were further investigated by femtosecond transient absorption measurements. The time-dependent changes in transient absorption intensity at 2000 cm −1 after photoexcitation are shown in Figure 9.…”

“…Transient absorption measurements were conducted using a purpose-built spectrometer described previously . The experimental details are given elsewhere . Briefly, photocatalyst samples coated on a CaF 2 plate at a density of 1.5 mg cm –2 were photoexcited under N 2 (20 Torr) or air using 420 nm pulses from a Nd:YAG laser (Continuum Surelite; duration, 6 ns; power, 5 mJ; repetition rate, 5 Hz).…”

“…Similar D–J to R–P topochemical conversions have been reported using KCa 2 Nb 3 O 10 and RbNdNb 2 O 7 , which were converted into K 1+ x Ca 2 Nb 3 O 10– x N x and Rb 2 NdNb 2 O 6 N, respectively. Recently, this topochemical method has been applied to the synthesis of Ta 5+ -based layered oxynitrides. , For example, K 2 LaTa 2 O 6 N·1.6H 2 O is an R–P phase layered oxynitride that functions as a visible-light-driven photocatalyst for H 2 evolution . Loading an Ir cocatalyst onto K 2 LaTa 2 O 6 N and subsequent proton exchange improved the photocatalytic activity, outperforming Pt-loaded ZrO 2 /TaON − and Rh-doped SrTiO 3 , which are representative 3D oxynitride and oxide photocatalysts for the H 2 evolution reaction under visible light irradiation, respectively.…”

Recent Progress in Mixed‐Anion Materials for Solar Fuel Production

“…Finally, it should be mentioned that the development of new mixed‐anion compounds with photocatalytic activity remains important both for practical applications and for basic science. Recent advances in inorganic solid state chemistry allow the synthesis of meta‐stable phases of 2D‐layered oxynitrides, [ 130,131 ] some of which exhibit higher performance for H 2 evolution than the benchmark ZrO 2 /TaON. The photocatalytic properties of a MXene‐related material, a new class of 2D material showing potential for many applications, [ 132 ] were also reported very recently.…”

Preparation of Multifunctional Metal Oxynitride 2D Crystals and Oriented Transparent Free-Standing Oxynitride Films

“…This can possibly be ascribed to the lower likelihood that the delamination agent will be intercalated between the layers of the oxynitride as well as the low chemical stability of layered oxynitrides. The Ruddlesden–Popper phase layered oxynitrides Rb 1.8 Ca 2 Nb 3 O 9.2 N 0.8 ·0.6H 2 O and Rb 1.8 LaNb 2 O 6.3 N 0.7 ·1.0H 2 O have been reported to be unstable in water, although, more recently, layered oxynitrides such as Rb 2 NdNb 2 O 6 N·H 2 O, K 2 Ca 2 Ta 3 O 9 N·2H 2 O, and K 2 LaTa 2 O 6 N·1.6H 2 O have been found to be stable in aqueous media. − Herein, we report the preparation of a stable Ruddlesden–Popper phase layered oxynitride, Na 2 Ca 2 Ta 3 O 9 N, with triple-layer perovskite units in each layer and the metal oxynitride 2D crystal followed by the monolayer delamination of this material. The replacement site of nitrogen was predicted by density functional theory (DFT) calculations and Raman spectra.…”