Synthesis, Internal Structure, and Formation Mechanism of Monodisperse Tin Sulfide Nanoplatelets

Tin sulfide nanoparticles have a great potential for use in a broad range of applications related to solar energy conversion (photovoltaics, photocatalysis), electrochemical energy storage, and thermoelectrics. The development of chemical synthesis methods allowing for the precise control of size, shape, composition, and crystalline phase is essential. We present a novel approach giving access to monodisperse square SnS nanoplatelets, whose dimensions can be adjusted in the range of 4-15 nm (thickness) and 15-100 nm (edge length). Their growth occurs via controlled assembly of initially formed polyhedral seed nanoparticles, which themselves originate from an intermediate tetrachlorotin-oleate complex. The SnS nanoplatelets crystallize in the α-SnS orthorhombic herzenbergite structure (space group Pnma) with no evidence of secondary phases. Electron tomography, high angle annular dark field scanning transmission electron microscopy and electron diffraction combined with image simulations evidence the presence of ordered Sn vacancy rich (100) planes within the SnS nanoplatelets, in accordance with their slightly S-rich composition observed. When using elemental sulfur instead of thioacetamide as the sulfur source, the same reaction yields small (2-3 nm) spherical SnS2 nanoparticles, which crystallize in the berndtite 4H crystallographic phase (space group P3m1). They exhibit quantum confinement (E(g) = 2.8 eV vs 2.2 eV in the bulk) and room temperature photoluminescence. By means of electrochemical measurements we determined their electron affinity EA = -4.8 eV, indicating the possibility to use them as a substitute for CdS (EA = -4.6 eV) in the buffer layer of thin film solar cells.