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

Abstract

Metal oxynitride two-dimensional (2D) crystals prepared by the delamination of layered compounds have attracted attention as photocatalysts, electrode catalysts, and electrolyte materials. However, the preparation of such materials is a significant challenge because of the difficulty related to producing chemically stable layered oxynitrides. In this study, oxynitride 2D crystals having triple perovskite units (A site: Ca, B site: Ta) were successfully prepared via the perfect delamination of the chemically stable Ruddlesden–Popper phase layered oxynitrides, Na2Ca2Ta3O9N. The layered compound was found to be stable during proton exchange in 6 M HNO3 and a subsequent delamination process, and the resulting oxynitride 2D crystals (herein [Ca2Ta3O9.5N0.5]−1.5) were observed to have a yellow coloration along with a band gap of 2.58 eV. The [Ca2Ta3O9.5N0.5]−1.5 2D crystals modified with RhCrO x and CoO x cocatalysts restacked with alkali ions exhibited photocatalytic activity for H2 and O2 evolution from pure water under UV light and visible light illumination. A transparent, free-standing oxynitride 2D-crystal film having a thickness of around 1 μm was prepared from the 2D-crystal suspension. This film was perfectly oriented in the [001] direction, which resulted in flexibility, mechanical strength, and airtightness due to face-to-face lamination of the 2D crystals. The proton conductivity of the oxynitride 2D-crystal film was around 10–3–10–4 S/cm at 25–50 °C (RH: 90–100%). The cell exhibited a power density of 9 mW/cm2 at 25 mA/cm2. This type of free-standing, flexible oxynitride film is expected to provide a new possibility and to enable the use of oxynitride materials in various applications.

Stress-Induced Photoluminescence Change of Monolayer Nanosheet Prepared by Delamination of Aurivillius-Phase Layered Perovskite

“…In previous literature, the Raman peak around 800 cm −1 was shown to correspond to one of the out-of-plane vibration modes of perovskite nanosheet layer in a layered perovskite. 27,42 Figure S17 shows another example of the comparison of Raman peaks at 700−850 cm −1 between the concave and convex sides of the free-standing BCTO nanosheet membrane. The peak also corresponded to the out-of-plane vibration of the perovskite layer and shifted slightly to higher wavenumbers for the convex side.…”

“…Several previous authors reported that free-standing membranes obtained by restacking monolayer nanosheets by spin-coating or vacuum filtration techniques showed excellent flexibility. − Figure shows the cross-sectional SEM images and top-view photographs for the free-standing membranes fabricated by the vacuum filtration technique using BCTO, BSTO, and BNTO nanosheet dispersions. The SEM images showed the stacking structure of the membranes, which reflected face-to-face restacking of the nanosheets.…”

“…Moreover, the fragile nature of ceramics has greatly hampered developments in the MCL field using inorganic materials. On the other hand, in previous literature, nanosheet Nb 6 O 17 has shown great potential in applying monolayer transition metal oxide nanosheets in flexible ceramic devices by forming a nanotube phase with tube diameter of ∼20 nm and length of ∼500 nm with no fractures from the starting nanosheet phase by a self-scrolling process. − Therefore, Bi 3+ -substituted monolayer perovskite oxide nanosheets obtained from Aurivillius-phase layered perovskites are some of the best candidates to overcome the difficulties of developing an MCL material from ceramics because of the outstanding flexibility derived from their ultimate thinness. − However, due to the strong electrostatic interactions between the high-charge-density anionic nanosheet layers and interlayer cations, delamination of Aurivillius-phase layered perovskites has not been as successful as with other layered perovskites such as KLaNb 2 O 7 , KCa 2 Nb 3 O 10 , and CsSr 2 Nb 3 O 10 .…”

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

“…These sheets exhibited photocatalytic water splitting activity and were also able to transition to a free-standing film. 25 The present work demonstrates the interlayer modification and single layer exfoliation of the Ruddlesden–Popper oxynitride K 2 LaTa 2 O 6 N with the aim of obtaining high photocatalytic activity. 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.…”

Photocatalytic Water Splitting Using Layered Zirconium Chloride Nitride

“…In particular, a mixed anion strategy to adjust the valence band by the preparation of semiconductors that include oxygen and other anion species has been investigated . To date, oxynitrides (O–N system), − oxysulfides (O–S system), − and oxyhalides (O–Cl, −Br, −I systems) , such as TaON, GaN:ZnO, Y 2 Ti 2 O 5 S 2 , Bi 4 NbO 8 Cl, and BiOBr have been reported. In these photocatalysts, anions such as N 3– , S 2– , and Cl – contribute to the formation of the orbitals at the top of the valence band, which results in a smaller bandgap than the corresponding oxides.…”