Single-Crystal Intermetallic Catalysts Using MoS₂ as a Growth Template

Intermetallic alloy phases and especially ordered intermetallic phases offer interesting chemical and physical properties. For example, many ordered intermetallics are attractive catalysts including Pt- and Pd-bearing phases. To better understand and exploit these materials, single crystals offer advantages. Here, we demonstrate the preparation of highly oriented or epitaxial thin films of the intermetallics PdCu, PtCu, PdIn, and PtAl₂ on single-crystal MoS₂. We find that cubic PdCu and PtCu films orient on MoS₂ with the epitaxial relationship PdCu (111)||MoS₂ (0001); PdCu[2̅11]||MoS₂ [11̅00] when cosputtered at room temperature. However, they are metastable disordered (or random) alloys. Upon annealing at 400 °C, we find evidence for ordering of PtCu. This ordered intermetallic is hexagonal and adopts the orientation PtCu (0001)||MoS₂ (0001); PtCu[11̅00]||MoS₂ [11̅00]. The ordered intermetallic PdIn forms at room temperature with the orientation PdIn (111)||MoS₂ (0001); PdIn[11̅0]||MoS₂ [11̅00]; however, it is a textured film with a wide mosaic spread, even after annealing at 400 °C. The ordered PtAl₂ intermetallic phase does not grow epitaxially at room temperature; however, upon annealing at 400 °C, we observe the PtAl₂(100) face on MoS₂(0001) rather than the initially expected PtAl₂(111) face. The criteria we used previously to predict low-temperature epitaxy of elemental metals remain reliable (high adatom mobility, lack of reactivity with MoS₂, and matching symmetry with the basal plane of MoS₂).