Synthesis and crystallographic analysis of shape-controlled SnS nanocrystal photocatalysts: Evidence for a pseudotetragonal structural modification

Tin sulfide, SnS, is a narrow band gap semiconductor comprised of inexpensive, earth abundant, and environmentally benign elements that is emerging as an important material for a diverse range of applications in solar energy conversion, energy storage, and electronics. Relative to many comparable systems, much less is known about the factors that influence the synthesis or morphology-dependent properties of SnS nanostructures. Here, we report the synthesis of colloidal SnS cubes, spherical polyhedra, and sheets and demonstrate their activity for the photocatalytic degradation of methylene blue. We also study their morphology-dependent polymorphism using an in-depth crystallographic analysis that correlates high-resolution TEM data of individual nanocrystals with ensemble-based electron diffraction and powder XRD data. These studies reveal that the crystal structure adopted by the SnS cubes and spherical polyhedra is expanded along the a and b axes and contracted along c, converging on a pseudotetragonal cell that is distinct from that of orthorhombic α-SnS, the most stable polymorph. All of the peaks observed in powder XRD patterns that are often interpreted as originating from a mixture of metastable zincblende-type SnS and α-SnS can instead be accounted for by this single-phase pseudotetragonal modification, and this helps to rationalize discrepancies that exist between theoretical predictions of SnS polymorph stability and interpretations of experimental diffraction data. This same crystallographic analysis also indicates the morphologies of the nanocrystals and the facets by which they are bound, and it reveals that the SnS cubes form through selective overgrowth of spherical polyhedral seeds.