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[211]||MoS2 [1100] when cosputtered at room temperature. However, they are metastable disordered (or random) alloys. Upon annealing at 400 degrees C, we find evidence for ordering of PtCu. This ordered intermetallic is hexagonal and adopts the orientation PtCu (0001)||MoS2 (0001); PtCu[1100]||MoS2 [1100]. The ordered intermetallic PdIn forms at room temperature with the orientation PdIn (111)||MoS2 (0001); PdIn[110]||MoS2 [1100]; however, it is a textured film with a wide mosaic spread, even after annealing at 400 degrees C. The ordered PtAl2 intermetallic phase does not grow epitaxially at room temperature; however, upon annealing at 400 degrees 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). However, the presence of an order-disorder transition on the phase diagram, as found for PdCu and PtCu intermetallics, can sometimes promote the formation of a metastable disordered phase at room temperature despite epitaxy on MoS2.