Cation-exchange-induced phase transformations have recently been employed for the synthesis of a variety of nanomaterials. In this study, we induced a cation-exchange reaction by adding Bi-oleate to PbS quantum dots (QDs) and synthesized uniform Pb1–x Bi x S y QDs and nanorods (NRs) with a diameter of approximately 3 nm and length of 40 nm (aspect ratio 1:5), respectively. Colloidal QDs are promising materials for thermoelectric applications owing to their simple production process, easy dimensional control, low thermal conductivity, and high Seebeck coefficient. We studied the thermoelectric properties of Pb1–x Bi x S y QDs and NRs synthesized through cation exchange. The Pb1–x Bi x S y NRs exhibited high values of the Seebeck coefficient (−181.31 μV/K at 450 K) and power factor (3.14 μW/cm·K2 at 650 K), and the electrical conductivity dramatically increased from 0.91 to 14.88 S/cm with an increase in temperature.

We present the phase transformation from colloidal Cs 3 Cu 2 Cl 5 nanocrystals to CsMCl (M = Zn, Bi, Cd) by cation exchange reaction. Cs 2 ZnCl 4 , Cs 3 BiCl 6 , and CsCdCl 3 were successfully synthesized, and the feasibility of phase transformations was demonstrated using density functional theory calculations, which revealed the high thermodynamic stability of the three structures.The results indicate that these structures can be synthetically prepared. The difference in reactivity between Zn, Bi, and Cd cations, which was verified by changing the reaction temperatures, was demonstrated using chemical softness calculations considering the interactions between Cl − and three cations. Additionally, for each cation exchange reaction, thermodynamic stability, estimated in terms of the formation energy, contributed to reactivity. The Cs 2 ZnCl 4 structure required the mildest reaction condition (i.e., 110 °C). As a reverse reaction, Cu cations were added to solutions of Cs 2 ZnCl 4 , Cs 3 BiCl 6 , and CsCdCl 3 , and CsCu 2 Cl 3 was obtained instead of Cs 3 Cu 2 Cl 5 . The mechanism was not cation exchange, and transmission electron microscopy data showed that nanoparticles were used as precursors for forming CsCu 2 Cl 3 particles.

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Scheme 1. Schematic diagram of high chalcocite, bornite, and talnakhite CuFeS nanocrystals exhibiting semiconducting to metallic conduction via cation exchange reactions.

We synthesized colloidal cesium metal halide CsMX (M = Fe, Co, Ni; X = Cl, Br) nanoparticles (NPs) and assessed their crystal stability by density functional theory (DFT) calculations. We successfully synthesized Cs3FeCl5, Cs3FeBr5, Cs3CoCl5, Cs3CoBr5, CsNiCl3, and CsNiBr3 NPs. CsMX NPs with Fe and Co exhibited Cs3M1X5 and Cs2M1X4 structures depending on the reaction conditions; however, CsNiX NPs exhibited only the CsNiX3 structure. The differences in structural stability by central metal ions were explained using spin-polarized DFT calculations. The analysis revealed tetragonal Cs3M1X5 and orthorhombic Cs2M1X4 structures to have similar thermodynamic stabilities in the case of Fe and Co, whereas the hexagonal CsMX3 structure in the case of Ni was the most stable. Moreover, the calculation results were the same as the experimental results. In particular, cobalt-related Cs3CoBr5 NPs easily developed into Cs2CoCl4 nanorods with an increase in temperature.