Kuramite Cu₃SnS₄ and Mohite Cu₂SnS₃ Nanoplatelet Synthesis Using Covellite CuS Templates with Sn(II) and Sn(IV) Sources

Abstract

Colloidal synthesis of copper tin sulfide (CTS) nanocrystals has been of great interest due to their potential for use in solution-processed photovoltaics containing only earth-abundant, low-toxicity elements. Postsynthetic incorporation of Sn into copper sulfide is an effective means of producing CTS, and Sn content can be controlled to some extent by the amount of Sn precursor provided. However, the oxidation (valence) state of Sn before and after doping and the effectiveness of Sn(II) vs Sn(IV) precursors remain topics of significant debate. Here, we demonstrate a step-growth method for preparing monodisperse copper tin sulfide (CTS) nanoplatelets (NPls). We show that Sn2+ ions, but not Sn4+ ions, can convert covellite to Cu3SnxS4 NPls (x ≤ 1) without addition of any reducing or copper-extracting agent. In this case Sn2+ reduces the disulfide bond in covellite and the crystal structure evolves from covellite to kuramite. When dodecanethiol (DDT) is added prior to Sn, only Sn4+ ions, and not Sn2+ ions, are incorporated, ultimately producing mohite Cu2SnS3. Here, DDT can reduce the disulfide bonds and extract Cu from the lattice. When djurleite Cu2–xS was used as the template, only Sn4+, and not Sn2+, could be incorporated without use of DDT, and the extent of Sn incorporation was very limited, perhaps only filling Cu vacancies and not displacing any Cu from the lattice. Together with previous studies of CTS preparation from Cu2–xS using Sn4+ and DDT, these results provide a flexible array of methods of achieving a desired final CTS composition, crystal structure, and morphology. The ability to produce two different crystal phases of CTS, with corresponding differences in band gap and other properties, from the same covellite template while retaining the template size and morphology could be particularly valuable.