Structure-Shifting Intermediates During Nanoparticle Cation Exchange for the Retrosynthetic Construction of Intraparticle Heterophase Homojunctions

Heterophase homojunctions, which connect two compounds having different crystal structures but the same compositions, are important components of many nanoscale photocatalysts and electronic device systems because they integrate two electronically distinct materials with minimal lattice mismatch. Making heterophase homojunctions in high yield is challenging and has largely been limited to post-processing, aggregation, and modulated growth techniques. As a result, heterophase homojunctions within colloidal nanoparticles are rare, despite their potentially beneficial electronic characteristics. Here, we demonstrate a retrosynthetic pathway for rationally incorporating heterophase homojunctions into colloidal nanoparticles. Our approach leverages a structure-shifting intermediate that is applied during nanoparticle cation exchange. Specifically, partial cation exchange reactions of roxbyite Cu 1.8 S nanorods, which exhibit a distorted hexagonal close-packed (hcp) structure, with Ni 2+ produce regions of Ni 9 S 8 that have a distorted cubic close-packed (ccp) structure. The resulting hcp-Cu 1.8 S/ccp-Ni 9 S 8 nanorods interface crystallographically aligned hcp and ccp regions and provide a synthetic entryway, through additional series of cation exchange reactions, to form derivative nanorods that maintain the hcp/ccp junction while further modifying composition. Using this approach, we demonstrate multistep retrosynthetic pathways to two distinct metal sulfide heterophase homojunctions, roxbyite-Cu 1.8 S (hcp)/digenite-Cu 1.8 S (ccp) and wurtzite-CdS (hcp)/zincblende-CdS (ccp). The CdS nanorods that incorporate a heterophase homojunction exhibit a single band gap that is intermediate in energy between those of the two individual phases. The ability to design synthetic pathways to heterophase homojunctions in colloidal nanoparticles is important for achieving synergistic and enhanced electronic and optical properties in nanoscale semiconductor systems.