Diselenide Dianion’s Dual Powers: PdSe2Polymorph Control and Pd3Se10Superatomic Crystal Creation

Abstract

There is a diverse family of palladium selenide compounds, including semiconducting orthorhombic PdSe2 (O-PdSe2) and monoclinic PdSe2 (M-PdSe2), which are unusual among transition-metal dichalcogenides as they are composed of diselenide dianions Se22–. Thus far, the solution syntheses of materials with Se–Se bonds typically require the in situ reduction of Se precursors. Here, we explore the use of electrochemically precise reactions between Na2Se2 and a Pd2+ source, Na2PdCl4, as a solution-phase route to selectively form different PdSe2 polymorphs in the absence of surfactants. By altering the reactant molar ratios and time, we map out a synthetic phase space diagram that shows how to create a wide variety of palladium selenide phases. With increasing Se22–/Pd2+ molar ratios, regions are identified where Pd17Se15, M-PdSe2, and O-PdSe2 exist as the dominant or exclusive thermodynamic product. Additionally, we discover Pd3Se10, a superatomic crystal composed of Pd6Se20 cube-shaped clusters held together by van der Waals forces, which forms as a kinetic product under short reaction times. In total, the use of the diselenide dianion precursor allows for the selective solution-phase synthesis of M-PdSe2 or O-PdSe2, as well as the discovery of a previously unreported palladium selenide phase.