Unexpected Multi-Step Transformation of AgCuS to AgAuS During Nanoparticle Cation Exchange.

Cation exchange reactions can modify the compositions of colloidal nanoparticles, providing easy access to compounds or nanoparticles that may not be accessible directly. The most common nanoparticle cation exchange reactions replace monovalent cations with divalent cations or vice versa, but some monovalent-to-monovalent exchanges have been reported. Here, we dissect the reaction of as-synthesized AgCuS nanocrystals with Au+ to form AgAuS, initially hypothesizing that Au+ could be selective for Cu+ (rather than for Ag+) based on a known Au+-for-Cu+ exchange and the stability of the targeted AgAuS product. Unexpectedly, we found this system and the putative cation exchange reaction to be much more complex than anticipated. First, the starting AgCuS nanoparticles, which match literature reports, are more accurately described as a hybrid of Ag and a variant of AgCuS that is structurally related to mckinstryite Ag5Cu3S4. Second, the initial reaction of Ag-AgCuS with Au+ results in a galvanic replacement to transform the Ag component to a AuyAg1-y alloy. Third, continued reaction with Au+ initiates cation exchange with Cu+ in AuyAg1-y-AgCuS to form AuyAg1-y-Ag3CuxAu1-xS2 and then AuyAg1-y-AgAuS, which is the final product. Crystal structure relationships among mckinstryite-type AgCuS, Ag3CuxAu1-xS2, and AgAuS help to rationalize the transformation pathway. These insights into the reaction of AgCuS with Au+ reveal the potential complexity of seemingly simple nanoparticle reactions and highlight the importance of thorough compositional, structural, and morphological characterization before, during, and after such reactions.

Introducing Porosity into Refractory Molybdenum Boride through Controlled Decomposition of a Metastable Mo-Al-B Precursor.

The high temperatures typically required to synthesize refractory compounds preclude the formation of high-energy morphological features, including nanoscopic pores that are beneficial for applications, such as catalysis, that require higher surface areas. Here, we demonstrate a low-temperature multistep pathway to engineer mesoporosity into a catalytic refractory material. Mesoporous molybdenum boride, α-MoB, forms through the controlled thermal decomposition of nanolaminate-containing sheets of the metastable MAB (metal-aluminum-boron) phase Mo2AlB2 and amorphous alumina. Upon heating, the Mo2AlB2 layers of the Mo2AlB2-AlOx nanolaminate, which is derived from MoAlB, begin to bridge and decompose, forming inclusions of alumina in a framework of α-MoB. The alumina can be dissolved in aqueous sodium hydroxide in an autoclave, forming α-MoB with empty and accessible pores. Statistical analysis of the morphologies and dimensions of the pores reveals a correlation with grain size, which relates to the pathway by which the alumina inclusions form. The transformation of Mo2AlB2 to α-MoB is topotactic due to crystal structure relationships, resulting in a high density of stacking faults that can be modeled to account for the observed experimental diffraction data. Porosity was validated by comparing surface areas and demonstrating catalytic viability for the hydrogen evolution reaction.

Expanded Tunability of Intraparticle Frameworks in Spherical Heterostructured Nanoparticles through Substoichiometric Partial Cation Exchange.

Partial cation exchange reactions provide a synthetic pathway for rationally constructing heterostructured nanoparticles that incorporate different materials at precise locations. Multiple sequential partial cation exchange reactions can produce libraries of exceptionally complex heterostructured nanoparticles, but the first partial exchange reaction is responsible for defining the intraparticle frameworks that persist throughout and help to direct subsequent exchanges. Here, we studied the partial cation exchange behavior of spherical nanoparticles of roxbyite copper sulfide, Cu1.8S, with substoichiometric amounts of Zn2+. We observed the formation of ZnS-Cu1.8S-ZnS sandwich spheres, which are already well known in this system, as well as ZnS-Cu1.8S Janus spheres and Cu1.8S-ZnS-Cu1.8S central band spheres, which have not been observed previously as significant subpopulations of samples. Aliquots taken during the formation of the heterostructured nanoparticles suggest that substoichiometric amounts of Zn2+ limit the number of sites per particle where exchange initiates and/or propagates, thereby helping to define intraparticle frameworks that are different from those observed using excess amounts of exchanging cations. We applied these insights from mixed-population samples to the higher-yield synthesis of ZnS-Cu1.8S Janus spheres, as well as the higher-order derivatives ZnS-(CdS-Cu1.8S), ZnS-(CdS-ZnS), and ZnS-(CdS-CoS), which have unique features relative to previously reported analogues. These results demonstrate how the diversity of intraparticle frameworks in spherical nanoparticles can be expanded to produce a broader range of downstream heterostructured products.