Ternary Hybrid Nanoparticle Isomers: Directing the Nucleation of Ag on Pt–Fe₃O₄ Using a Solid-State Protecting Group

Colloidal hybrid nanoparticles are an important class of materials that incorporate multiple nanoparticles into a single system through solid-state interfaces, which can result in multifunctionality and the emergence of synergistic properties not found in the individual components. These hybrid structures are typically produced using seeded-growth methods, where preformed nanoparticles serve as seeds onto which additional domains are added through subsequent reactions. For hybrid nanoparticles that contain more than two domains, multiple configurations with distinct connectivities and functionalities are possible, and these can be considered as nanoparticle analogues of molecular isomers. However, accessing one isomer relative to others in the same hybrid nanoparticle system is challenging, particularly when the formation of a target isomer is disfavored relative to more stable or synthetically accessible configurations. Here, we show that an iron oxide shell installed onto the Pt domain of Pt–Fe₃O₄ hybrid nanoparticles serves as a solid-state protecting group that can direct the nucleation of a third domain to an otherwise disfavored site. Under traditional conditions, Ag nucleates exclusively onto the Pt domain of Pt–Fe₃O₄ heterodimers, resulting in the formation of the Ag–Pt–Fe₃O₄ heterotrimer isomer. When the Pt surface is covered with an iron oxide protecting group, the nucleation of Ag is redirected onto the Fe₃O₄ domain, producing the distinct and otherwise inaccessible Pt–Fe₃O₄–Ag isomer. Similar results are obtained for the Au–Pt–Fe₃O₄ system, where formation of the favored Au–Pt–Fe₃O₄ configuration is blocked by the iron oxide protecting group. The thickness of the iron oxide shell that protects the Pt domain can be systematically tuned by adjusting the ratio of oleic acid to iron pentacarbonyl during the synthesis of the Pt–Fe₃O₄ heterodimers, and this insight is important for controllably implementing the protecting group chemistry

Synthetic Deconvolution of Interfaces and Materials Components in Hybrid Nanoparticles

Interfaces between nanoscale solids can facilitate coupling between dissimilar materials, leading to emergent and synergistic properties as well as mix-and-match multifunctionality. Seeded-growth methods, whereby one material is grown directly off of the surface of another, can lead to the formation of hybrid nanoparticles containing such solid-state heterojunctions. Successfully applying seeded-growth methods to the synthesis of hybrid nanoparticles, however, requires a precise balance of competing reaction variables, which limits the scope of materials that can be routinely incorporated into them and, accordingly, the types of achievable interfaces. Here, we describe an alternate pathway that overcomes key limitations of seeded-growth methods by synthetically deconvoluting the formation of particle–particle interfaces and the incorporation of desired materials components. Readily accessible hybrid nanoparticles can be rationally modified using sequential anion and cation exchange reactions which transform them into derivative products that contain different constituent materials but retain the preprogrammed morphologies and interfaces. Using Pt–MnO heterodimers as a synthetic entryway, we demonstrate the synthesis of seven different derivative Pt–<i>M</i>_<i>a</i><i>X</i>_<i>y</i> (<i>M</i> = metal, <i>X</i> = chalcogen) heterodimers through integrated ion exchange pathways. We also demonstrate that tunable domain sizes are retained during complete multistep material transformations and that partial exchanges can be used to introduce morphologically sophisticated features into hybrid nanoparticles, including core@shell components. The sequential ion exchange reactions can also be applied to different domains of three-component hybrid nanoparticles, demonstrated for the transformation of Fe₃O₄–Pt–MnO into FeS_<i>x</i>–Pt–Cu₂S. Finally, the hybrid nanoparticle ion exchange reactions proceed with retention of anion sublattice structure across the particle–particle interfaces, demonstrating that crystallographic orientation and domain alignment are preserved. Collectively, these results demonstrate the wide-ranging applicability of sequential ion exchange reactions to controllably access a diverse library of colloidal hybrid nanoparticles across a wide range of materials, morphologies, and interfaces

Ag₂Se to KAg₃Se₂: Suppressing Order–Disorder Transitions via Reduced Dimensionality

We report an order–disorder phase transition in the 2D semiconductor KAg₃Se₂, which is a dimensionally reduced derivative of 3D Ag₂Se. At ∼695 K, the room temperature β-phase (CsAg₃S₂ structure type, monoclinic space group C2/<i>m</i>) transforms to the high temperature α-phase (new structure type, hexagonal space group <i>R</i>3̅<i>m</i>, <i>a</i> = 4.5638(5) Å, <i>c</i> = 25.4109(6) Å), as revealed by in situ temperature-dependent X-ray diffraction. Significant Ag⁺ ion disorder accompanies the phase transition, which resembles the low temperature (∼400 K) superionic transition in the 3D parent compound. Ultralow thermal conductivity of ∼0.4 W m^–1 K^–1 was measured in the “ordered” β-phase, suggesting anharmonic Ag motion efficiently impedes phonon transport even without extensive disordering. The optical and electronic properties of β-KAg₃Se₂ are modified as expected in the context of the dimensional reduction framework. UV–vis spectroscopy shows an optical band gap of ∼1 eV that is indirect in nature as confirmed by electronic structure calculations. Electronic transport measurements on β-KAg₃Se₂ yielded <i>n</i>-type behavior with a high electron mobility of ∼400 cm² V^–1 s^–1 at 300 K due to a highly disperse conduction band. Our results thus imply that dimensional reduction may be used as a design strategy to frustrate order–disorder phenomena while retaining desirable electronic and thermal properties

Ag₂Se to KAg₃Se₂: Suppressing Order–Disorder Transitions via Reduced Dimensionality

We report an order–disorder phase transition in the 2D semiconductor KAg₃Se₂, which is a dimensionally reduced derivative of 3D Ag₂Se. At ∼695 K, the room temperature β-phase (CsAg₃S₂ structure type, monoclinic space group C2/<i>m</i>) transforms to the high temperature α-phase (new structure type, hexagonal space group <i>R</i>3̅<i>m</i>, <i>a</i> = 4.5638(5) Å, <i>c</i> = 25.4109(6) Å), as revealed by in situ temperature-dependent X-ray diffraction. Significant Ag⁺ ion disorder accompanies the phase transition, which resembles the low temperature (∼400 K) superionic transition in the 3D parent compound. Ultralow thermal conductivity of ∼0.4 W m^–1 K^–1 was measured in the “ordered” β-phase, suggesting anharmonic Ag motion efficiently impedes phonon transport even without extensive disordering. The optical and electronic properties of β-KAg₃Se₂ are modified as expected in the context of the dimensional reduction framework. UV–vis spectroscopy shows an optical band gap of ∼1 eV that is indirect in nature as confirmed by electronic structure calculations. Electronic transport measurements on β-KAg₃Se₂ yielded <i>n</i>-type behavior with a high electron mobility of ∼400 cm² V^–1 s^–1 at 300 K due to a highly disperse conduction band. Our results thus imply that dimensional reduction may be used as a design strategy to frustrate order–disorder phenomena while retaining desirable electronic and thermal properties