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

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 A) and Sb–Cl (2.924 A) in Cs4CuSb2Cl12 than the uniform Cl–Sb bond (2.69 A) 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.