Binary Cu_2–xS Templates Direct the Formation of Quaternary Cu₂ZnSnS₄ (Kesterite, Wurtzite) Nanocrystals

Abstract

Kesterite Cu 2 ZnSnS 4 (k-CZTS) nanocrystals have received attention for their tunable optoelectronic properties, as well as the earth abundance of their constituent atoms. However, the phase-pure synthesis of these quaternary NCs is challenging due to their polymorphism, as well as the undesired formation of related binary and ternary impurities. A general synthetic route to tackle this complexity is to pass through intermediate template nanocrystals that direct subsequent cation exchange toward the desired quaternary crystalline phase, particularly those that are thermodynamically disfavored or otherwise synthetically challenging. Here, working within this model multinary system, we achieve control over the formation of three binary copper sulfide polymorphs, cubic digenite (Cu 1.8 S), hexagonal covellite (CuS), and monoclinic djurleite (Cu 1.94 S). Controlled experiments with Cu 0 seeds show that selected binary phases can be favored by the identity and stoichiometry of the sulfur precursor alone under otherwise comparable reaction conditions. We then demonstrate that the nature of the Cu 2−x S template dictates the final polymorph of the CZTS nanocrystal products. Through digenite, the cation exchange reaction readily yields the k-CZTS phase due to its highly similar anion sublattice. Covellite nanocrystals template the k-CZTS phase but via major structural rearrangement to digenite that requires elevated temperatures in the absence of a strong reducing agent. In contrast, we show that independently synthesized djurleite nanorods template the formation of the wurtzite polymorph (w-CZTS) but with prominent stacking faults in the final product. Applying this refined understanding to the standard one-pot syntheses of k-and w-CZTS nanocrystals, we identify that these reactions are each effectively templated by binary intermediates formed in situ, harnessing their properties to guide the overall synthesis of phase-pure quaternary materials. Our results provide tools for the careful development of tailored nanocrystal syntheses in complex polymorphic systems.

Chemical Considerations for Colloidal Nanocrystal Synthesis

“…We conclude that oleylamine is a two-electron reductant. Aldimine seems to be the general oxidation product of oleylamine as it has also been identified in the synthesis of Cu(0) and Pd(0) nanocrystals from metal acetylacetonate precursors. , …”

“…Aldimine seems to be the general oxidation product of oleylamine as it has also been identified in the synthesis of Cu(0) and Pd(0) nanocrystals from metal acetylacetonate precursors. 50,51 As a second part of our investigation, we focused on the source of nitride in copper nitride. 43 The nitrate anion also oxidizes the primary amine to a primary aldimine, albeit at slightly higher temperatures (above 220 °C).…”

Anion-Directed Synthesis of Core–Shell and Janus Hybrid Nanostructures

“…Au seeds for Au@CZTS are synthesized from their chloride precursor, resulting in chloride ions adsorbed onto the gold nanoparticle surface prior to their injection into the CZTS precursor solution. Due to the highest chalcophilicity of Cu, it is reasonable to assume that the nucleation of Cu 2– X S onto the metal nanoparticles should occur first for the CZTS shells, similarly to the growth of CZTS itself, followed by the incorporation of Zn and Sn into the Cu 2– X S lattice. ,, …”

“…Due to the highest chalcophilicity of Cu, it is reasonable to assume that the nucleation of Cu 2−X S onto the metal nanoparticles should occur first for the CZTS shells, similarly to the growth of CZTS itself, followed by the incorporation of Zn and Sn into the Cu 2−X S lattice. 32,59,60 The higher reactivity of the CuCl 2 precursor relative to that of Cu(OAc) 2 , combined with the surface chemistry of the seed, is proposed to lead to rapid adsorption of the copper (sulfide) precursor onto the seed surface. The rapid depletion of the copper precursor prevents it from becoming concentrated enough to undergo significant self-nucleation (with the sulfur precursor).…”

Tuning Crystal Plane Orientation in Multijunction and Hexagonal Single Crystalline CsPbBr3 Perovskite Disc Nanocrystals

“…The vertical axis mostly remained the polar one, and the atomic parameters for two horizontal axes remained the same, but different than the vertical one. The most common phase of such a nanostructure is wurtzite, and this was largely reported for different metal chalcogenide and oxide nanostructures. − However, for square platelets of halide perovskites, these remained similar to their cube nanocrystals in either cubic or orthorhombic phase. ,,, Reaction parameters such as the nature of ligands and reaction temperature typically control their formation and properties. ,,,, There is no such clear evidence of a classical mechanism of 2D crystal growth established for the formation of these nanocrystals. Hence, shape modulations of these 2D nanostructures or tuning their vertical/horizontal axes remained indeed challenging and needs advanced synthetic procedures.…”