The top-down synthesis of single-layered Cs4CuSb2Cl12 halide perovskite nanocrystals for photoelectrochemical application

Abstract

The development of an all-inorganic lead-free perovskite nanocrystal is of crucial importance to solve the instability and lead toxicity of organic–inorganic lead hybrid perovskites. Herein, single-layered Cs₄CuSb₂Cl₁₂ nanocrystals (NCs) with a narrow band gap of 1.6 eV were prepared for the first time via an ultrasonic exfoliation technique. This powerful top-down method was further generalized to synthesize a class of lead-free perovskite (Cs₃Bi₂X₉ and Cs₃Sb₂X₉) NCs. The experimental and theoretical studies revealed that not only inter-layer van der Waals forces but also in-plane chemical bonds played a critical role in the exfoliation process. Specifically, smaller uniform-sized NCs were observed for Cs₄CuSb₂Cl₁₂ (∼3 nm) as compared to those for Cs₃Sb₂Cl₉ (∼20 nm) although both Cs₄CuSb₂Cl₁₂ and Cs₃Sb₂Cl₉ exhibited a similar exfoliation energy (∼0.310 J m⁻²). This can be ascribed to the weaker in-plane chemical bonds of Cu–Cl (2.808 Å) and Sb–Cl (2.924 Å) in Cs₄CuSb₂Cl₁₂ than the uniform Cl–Sb bond (2.69 Å) in Cs₃Sb₂Cl₉ that allow for an easier exfoliation process. In addition, exfoliation of the Cs₄CuSb₂Cl₁₂ microcrystal into NCs results in an indirect-to-direct bandgap transition and a reduced electron effective mass, which provides a rapid and steady photoelectrochemical response, demonstrating that Cs₄CuSb₂Cl₁₂ NCs are a promising candidate for optoelectronic applications.