Using Perovskite Oxynitrides to Develop New 2D Nanosheets

2D monolayer nanosheets composed of layered perovskite come with several attractive properties. However, it has been challenging to build them with tunable bandgaps in the visible region with no addition of oxygen defects.

In recent times, Japanese researchers could efficiently design chemically stable nanosheets from perovskite oxynitrides that possessed controllable bandgaps. Such nanosheets come with huge potential for future applications in electrocatalysts, photocatalysis, and other sustainable technologies.

Nanosheets, comprising the recognized material graphene, are materials that have nanoscale homogenous thicknesses, high crystallinity, and flat surfaces. Nanosheets have broad applications in photoluminescence, photocatalysis, and electronics.

Perovskites with semiconductor properties have gained focus in the scientific community as a potential material for developing two-dimensional (2D) monolayer nanosheets. Nevertheless, these nanosheets would need a bandgap pertaining to the energy of visible light to be advantageous, as this would identify when the semiconductor conducts electricity.

For scientists, the tunability of the bandgap has stayed a major difficulty because creating 2D nanosheets from perovskite with a tunable bandgap is challenging.

To address this issue, a research team from Kumamoto University, along with Professor Shintaro Ida from the Institute of Industrial Nanomaterials, agreed to concentrate on a group of perovskite materials called Ruddlesden–Popper (RP) phase layered perovskite oxynitrides.

The scientists could efficiently create 2D perovskite oxynitride nanosheets with a tunable bandgap with their new method. This study was published in Small.

Metal oxynitride semiconductor nanosheets containing oxygen, nitrogen, and a metal have not been researched much. Thin films made of these materials demonstrate functions superior to those of oxides. Thus, their synthesis will have a huge impact in this field. We synthesized nanosheets from RP-phase perovskite oxynitrides whose properties, such as its bandgap, are freely tunable.

Prof. Shintaro Ida, Study Corresponding Author, Institute of Industrial Nanomaterials, Kumamoto University

Firstly, the scientists utilized pristine Dion–Jacobson phase lanthanum niobium oxide (KLaNb₂O₇) as a precursor material. Then, they went on to include nitrogen through an approach known as nitridation. The scientists added nitrogen to the material at various temperatures ranging between 750 and 800 ℃.

This resulted in the formation of the RP-phase oxynitride derivative. After that, they could employ a two-step intercalation approach to exfoliate out lanthanum niobium oxynitride nanosheets with the formula LaNb₂O_7-xN_x (“x” being the quantity of nitrogen included in the perovskite).

When these nanosheets were tested, the scientists found that the material had a 1.6 nm homogenous thickness and showed various colors, ranging between white and yellow, based on the nitridation temperature. The nanosheets also possessed the attractive semiconductor property of having a tunable bandgap in the visible region, ranging from 2.03 to 2.63 eV, depending on the nitridation temperature.

Then, the team prepared a “superlattice” structure comprising alternating layers of the synthesized nanosheets and oxide (Ca₂Nb₃O₁₀) nanosheets. When the properties of this superlattice were tested, they discovered that it had excellent photocatalytic activity and superior proton conductivity.

The results of this study will open new possibilities for producing multiple superlattices by employing soft−chemical nano−architectonics based on 2D nanosheets. This will get us one step closer to a sustainable society, as these nanosheets would enable efficient splitting of water as a photocatalyst and also in creating more complex and better performing electronics.

Prof. Shintaro Ida, Study Corresponding Author, Institute of Industrial Nanomaterials, Kumamoto University

Journal Reference:

Hsu, C., et al. (2023). Bandgap Tunable Oxynitride LaNb₂O_7–xN_x Nanosheets. Small. doi.org/10.1002/smll.202206552.