Lead (Pb) based perovskites with the general formula ABX₃ have attracted significant attention in providing desirable optoelectronic devices, such as solar cells, light-emitting diodes (LEDs), and photocatalysis. However, the high toxicity and low stability of Pb are a big concern for health and the environment, as well as commercialization, leading to extensive research to replace lead with less toxic elements. Recent works have been exhibiting tin (Sn²⁺) as a promising alternative due to comparable ionic radius and in a group electronic configuration to Pb⁺. Nevertheless, Sn²⁺ is prone to oxidation, easily transforming into Sn⁴⁺ in ambient conditions, which creates a destabilized perovskite structure and significantly reduces its functional performance. To overcome this issue, this work explores an alternative strategy by inserting the Sn²⁺ octahedron into a layered double perovskite (LDP) with the formula Cs₄M(II)M(III)₂Cl₁₂. In this LDP structure, Sn²⁺ is placed on the M(II) site, while the M(III) site is occupied by different trivalent metal cations such as In³⁺ or Sb³⁺. The two M(III) octahedrons in a unit cell serve as top and bottom protective layers, effectively suppressing the oxidation sensitivity of Sn²⁺ by minimizing the contact with any degradation factors, such as oxygen and water molecules leads to the formation of Sn⁴⁺.

Our x-ray diffraction (XRD) and elemental analysis confirm the successful formation of LDP Cs₄M(II)M(III)₂Cl₁₂ nanocrystals (NCs) via the modified hot injection method, showing well-defined and stable crystal structures. The as-synthesized perovskite NCs demonstrate a morphology of hexagonal nanoplatelets with diverse size distributions based on the selected M (III) cations. Specifically, In-based nanoplatelets show an average size of 41.4 nm, while Sb cases exhibit average sizes of 58.8 nm. 

Compared to conventional lead-based perovskite, these Sn-based LDP nanocrystals reveal significant enhancements in the structural stability. The long-term stability test upon the tracking of XRD patterns reveals that these NCs could retain their original structural integrity for more than 300 days, a remarkable improvement over Pb or other Sn(II)-based perovskite, which latter case normally degrades even within hours. In addition, the optical stability of the LDP NCs, as regularly measured by UV-visible absorption spectroscopy, maintained the original feature for over 40 days, indicating a hint of their potential for stable optoelectronic applications.

Besides, this nanomaterial demonstrates tunable electronic properties by substituting distinct trivalent metal cations at the M(III) site, which means the bandgap could be adjusted. For instance, the presence of In³⁺ shows a relatively wide bandgap at 3.54 eV, indicating a good fit for a wide scale of bandgap semiconductors. Interestingly, once Sb³⁺ is placed at the M(III) site, the bandgap could be narrowed to 1.89 eV. This characteristic of tunable bandgap is considered a good platform for customizing the material’s properties for corresponding needs. We further evaluated the photoelectrochemical performance through chronoamperometry (I-t curves) under 1 sun illumination. The samples exhibited rapid photo responses upon light switching on/off, with steady-state photocurrent densities of 35.31 µA cm⁻² and 49.66 µA cm⁻², respectively.

In summary, this work demonstrates a novel Sn(II)-based LDP NCs with different trivalent metal ions at the M(III) site, showing significantly enhanced structural stability and tunable optical properties, all which make them highly versatile for a wide range of application in future, such as solar cells, light-emitting devices, and photocatalysis.