Controlled Surface Chemistry for the Directed Attachment of Copper(I) Sulfide Nanocrystals

E demand on our traditional energy resources makes harnessing underutilized alternative energy sources all the more necessary and desirable to the scientific community. Colloidal copper(I) sulfide nanocrystals (NCs) are an attractive photovoltaic device component due to their solar absorption characteristics; Cu2S is a p-type semiconductor with a bandgap reported from 1.1 to 1.4 eV, and an absorption coefficient of 10 cm−1. Cu2S is also a parent material to ternary and quaternary copper sulfides such as CuInS2, which has emissive defects that can be used for LED lighting, biomedical applications and solar concentrating. The light-absorbing characteristics of copper(I) sulfide can be further enhanced by morphologies that increase the absorption cross section of the nanocrystals. Manipulating reaction mixture additives can generate highly faceted Cu2S nanocrystals, and careful stabilizer control has been used previously to make Cu2S nanoribbons from nanocrystal assemblies in aqueous media. Sadtler et al. were first to synthesize Cu2S nanorods through cation exchange with CdS nanorods, a process that lacks atom economy and generates a cadmium byproduct. Additionally, the cation exchange process likely leaves the Cu2S doped with Cd ions. Kruszynska et al. discovered that substoichiometric Cu2−xS nanorods could be grown from nucleations when using tert-DDT as a ligand and sulfur source, but the fully stoichiometric Cu2S was not achieved. Further developments to prepare rods structures without cadmium and of controlled stoichiometry are needed. The oriented attachment of Cu2S seed particles into larger single crystalline structures offers a means to obtain nanostructures with morphologies, sizes and compositions that may not be possible with traditional monomer-based growth. Oriented attachment of PbSe performed by Cho and later by Koh produced nanowires, nanorings, and nanorods from single-particle building blocks. Oriented attachment has also been used to create single-crystalline chains of TiO2 crystals, CdTe nanowires, and nanorods of ZnO, CdS and Ag2S. 14 The process occurs due to dipole interactions between particles. Hexagonal-like crystal structures (as shown in ZnO, CdS and Ag2S) inherently have a dipole due to crystal structure anisotropy, yet control over the surface chemistry was essential to orchestrate the directed attachment in many of these cases. The room temperature crystal structure of stoichiometric Cu2S (low chalcocite) and substoichiometric crystal structure djurleite (Cu1.96S) consist of copper atoms arranged around a distorted hexagonally close-packed sulfur sublattice (approximated here for simplicity by the hexagonal high chalcocite structure). Provided the surface chemistry can be effectively controlled, this structural anisotropy leaves the door open for the oriented attachment of Cu2S. Our group’s research has shown that Cu2S nanocrystals synthesized with alkanethiols acting as both the ligands and sulfur sources result in the ligand alkanethiolate sulfur atoms becoming integrated into the crystal lattice. These “crystalbound” thiolate ligands have sulfur atoms that sit in highcoordination-number sites, and are resistant to ligand exchange and photooxidative disulfide formation. Robust ligand binding in Cu2S nanocrystals has enabled the selective cleavage of ligands at higher temperatures and deposition of metal on exposed edge sites, resulting in nanoscale inorganic cages. Higher synthesis temperatures result in cleavage of the surface ligands through a breaking of the C−S bond. The KolnyOlesiak group observed that rods could be grown in the direction of crystallographic anisotropy (corresponding to the c-axis of chalcocite) using t-DDT as the sulfur source, suggesting that the c-facet can be selectively activated. This highly defined surface chemistry where ligands are not labile and are cleaved at specific temperatures, is an opportunity to develop oriented attachment-based syntheses for Cu2S. Here we present a new synthetic route to Cu2S nanorods in which copper sulfide NCs with crystal-bound ligands are joined through the mechanism of oriented attachment in the presence of 1,2-hexadecanediol (HDD) (Figure S1). Often shown in nanocrystal literature as a reducing agent, evidence discussed herein suggests that HDD functions as a nanocrystal stabilizer necessary to the oriented attachment process. It was found that control of the concentration of HDD was critical to successful directed attachment of nanocrystals in nanorod syntheses, and that the nanorod product had surface chemistry and a crystal structure similar to the constituent NCs. Prior to the nanorod synthesis, monodisperse quasi-spherical seed nanocrystals with crystal-bound thiol ligands were prepared using a modified literature synthesis. Briefly, Cu(acac)2, 1-dodecanethiol (DDT) and dioctyl ether (DOE) were heated under inert atmosphere to 215 °C for 1 h. The 12−14 nm seeds were then transferred into a glovebox and purified. For the nanorod synthesis, ∼3.7 × 10 NCs were then heated in 3 mL of DOE with HDD (50 mg, 193 μmol) for 1 h and subsequently washed with polar solvents to remove unreacted precursors. The seed NCs had a native crystalbound DDT surface density of 3.7 molecules/nm as