Seed Crystal Phase Governs Interfacial Coherence and Strain in Janus Cu2-xS/CdS Heteronanorods

The atomic coherence of heterojunction interfaces critically governs charge transport but remains difficult to control in cation-mobile copper chalcogenides. Here, we identify the crystal phase of Cu2-xS seeds as a decisive and previously underappreciated parameter that governs interfacial coherence and strain in Janus Cu2-xS/CdS heteronanorods. By selectively employing hexagonal (covellite) and cubic (digenite) Cu2-xS seeds, heteronanorods with distinctly different interfacial structures are obtained. Atomic-resolution transmission electron microscopy combined with geometric phase analysis quantitatively reveals that the digenite seed forms a highly coherent interface with CdS, featuring a reduced lattice mismatch (~5.1% versus ~7.4%) and a markedly localized strain field with a halved maximum lattice strain (~4% versus ~8%). First-principles calculations further show that this coherent interface possesses a more negative formation energy and enhanced electronic coupling, giving rise to a stronger built-in electric field across the heterojunction. As a consequence of the improved interfacial coherence, the digenite-seeded heteronanorods exhibit more efficient charge separation and reduced interfacial resistance, which is reflected in their superior visible-light-driven photocatalytic hydrogen evolution performance. This work establishes seed crystal phase as a fundamental design parameter for regulating interfacial nanostructure and strain in heterostructures, providing a general physical guideline for minimizing interfacial losses in heteronanojunctions.