Haitao Zhang

Ammonium sulfide ((NH₄)₂S) exhibits high reactivity as a sulfide reagent in anion exchange reactions that transform CoO to cobalt sulfide nanoparticles (NPs). The faster diffusion of Co²⁺ and O²⁻ than the incoming S²⁻ during the anion exchange causes a significant expansion of the NP voids. The low temperature (70 °C) anion exchange reaction produces amorphous cobalt sulfide NPs with Co : S ratio of ca. 3 : 4, which are converted into crystalline NPs with a major phase of cubic Co₃S₄ by annealing at high temperature in an organic solution.

A rational synthetic method is developed to produce monodisperse metal sulfide nanocrystals (NCs) in organic nonpolar solutions by using (NH₄)₂S as a sulfide precursor. (NH₄)₂S is stabilized in an organic primary amine solution and exhibits high reactivity toward metal complexes. This novel technique exhibits wide applicability for organic phase metal sulfide NC synthesis: a large variety of monodisperse NCs have been synthesized, including Cu₂S, CdS, SnS, ZnS, MnS, Ag₂S, and Bi₂S₃. The stoichiometric reactions between (NH₄)₂S and metal salts afford high conversion yields, and large-scale production of monodisperse NCs (more than 30 g) can be synthesized in a single reaction. The high reactivity of (NH₄)₂S enables low temperature (2S and Bi₂S₃ quantum dots.

We report the discovery of unintentional phosphorus (P) doping when tri-n-octylphosphine (TOP) ligands are used in Ni nanoparticle synthesis, which is the most common method for monodisperse Ni nanoparticle synthesis. The nanoparticles appear pure face-centered cubic (fcc) Ni in X-ray diffraction despite the surprisingly high level (5 atomic %) of P. We find that the P doping follows a direct relationship with increased reaction time and temperature and that the P doping can be estimated with the degree of lattice expansion shown from a peak shift in the XRD spectrum. Through EXAFS modeling and density-functional (DFT) calculations of defect formation energies we find that the P atoms are preferentially located on the fcc lattice as substitutional dopants with oxidation state of zero. Magnetic and catalytic properties are shown to be greatly affected by this doping; DFT calculations show magnetization losses in the Ni system, as well as in Fe and Co systems. These findings are likely relevant for other metal syntheses that employ phosphine ligands.

A novel method is reported to create inorganically connected nanocrystal (NC) assemblies for both II–VI and IV–VI semiconductors by removing surfactant ligands using (NH₄)₂S. This surface modification process differs from ligand exchange methods in that no new surfactant ligands are introduced and the post-treated NC surfaces are nearly bare. The detailed mechanism study shows that the high reactivity between (NH₄)₂S and metal–surfactant ligand complexes enables the complete removal of surfactant ligands in seconds and converts the NC metal-rich shells into metal sulfides. The post-treated NCs are connected through metal–sulfide bonding and form a larger NCs film assembly, while still maintaining quantum confinement. Such “connected but confined” NC assemblies are promising new materials for electronic and optoelectronic devices.

We report the structural evolution and the diffusion processes which occur during the phase transformation of nanoparticles (NPs), ε-Co to Co₂P to CoP, from a reaction with tri-n-octylphosphine (TOP). Extended X-ray absorption fine structure (EXAFS) investigations were used to elucidate the changes in the local structure of cobalt atoms which occur as the chemical transformation progresses. The lack of long-range order, spread in interatomic distances, and overall increase in mean-square disorder compared with bulk structure reveal the decrease in the NP’s structural order compared with bulk structure, which contributes to their deviation from bulk-like behavior. Results from EXAFS show both the Co₂P and CoP phases contain excess Co. Results from EXAFS, transmission electron microscopy, X-ray diffraction, and density functional theory calculations reveal that the inward diffusion of phosphorus is more favorable at the beginning of the transformation from ε-Co to Co₂P by forming an amorphous Co-P shell, while retaining a crystalline cobalt core. When the major phase of the sample turns to Co₂P, the diffusion processes reverse and cobalt atom out-diffusion is favored, leaving a hollow void, characteristic of the nanoscale Kirkendall effect. For the transformation from Co₂P to CoP theory predicts an outward diffusion of cobalt while the anion lattice remains intact. In real samples, however, the Co-rich nanoparticles continue Kirkendall hollowing. Knowledge about the transformation method and structural properties provides a means to tailor the synthesis and composition of the NPs to facilitate their use in applications.

Hexafluoroacetylacetonates that contain lead and divalent first-row transition metals, PbM(hfac)₄ (M = Ni (1), Co (2), Mn (3), Fe (4), and Zn (5)), have been synthesized. Their heterometallic structures are held together by strong Lewis acid−base interactions between metal atoms and diketonate ligands acting in chelating−bridging fashion. Compounds 1−5 are highly volatile and decompose below 350 °C. Fluorinated heterometallic β-diketonates have been used for the first time as volatile single-source precursors for the preparation of mixed-metal fluorides. Complex fluorides of composition Pb₂MF₆ have been obtained by decomposition of 1−5 in a two-zone furnace under low-pressure nitrogen flow. Lead−transition metal fluorides conform to orthorhombically distorted Aurivillius-type structure with layers of corner-sharing [MF₆] octahedra separated by α-PbO-type (Pb₂F₂) blocks. Pb₂NiF₆ and Pb₂CoF₆ were found to exhibit magnetic ordering below 80 and 43 K, respectively. The ordering is antiferromagnetic, with a weak, uncompensated moment due to the canting of spins. The Pb₂MF₆ fluorides represent a new class of prospective magnetoelectric materials combining transition metals and lone-pair main-group cations.

A new method to produce hyperbranched Co₂P nanocrystals that are uniform in size, shape, and symmetry was developed. In this reaction tri-n-octylphosphine oxide (TOPO) was used as both a solvent and a phosphorus source. The reaction exhibits a novel monomer-saturation-dependent tunability between Co metal nanoparticle (NP) and Co₂P NP products. The morphology of Co₂P can be controlled from sheaflike structures to hexagonal symmetric structures by varying the concentration of the surfactant. This unique product differs significantly from other reported hyperbranched nanocrystals in that the highly anisotropic shapes can be stabilized as the majority shape (>84%). This is the first known use of TOPO as a reagent as well as a coordinating background solvent in NP synthesis.

No evidence was found for the existence of a previously reported mononuclear square-planar form of unsolvated cobalt(II) acetylacetonate, Co(acac)₂, in all samples that have been obtained by using a variety of preparative techniques and crystallization conditions. It was confirmed that the structure of tetramer Co₄(acac)₈, reported back in 1964 by Cotton and Elder, is correct, the synthesis is reproducible, and the bulk material corresponds to single-crystal data. Additionally, the title compound can be isolated in tetranuclear form by reducing cobalt(III) acetylacetonate with cobalt metal in solvent-free conditions or by crystallization from a hexanes solution. At the same time, from noncoordinating halogenated solvents, Co(acac)₂ crystallizes as a trinuclear core molecule, in which all Co atoms also exhibit octahedral coordination. From coordinating solvents such as ethanol, cobalt(II) acetylacetonate was found to appear in the form of its bis-adduct Co(acac)₂(EtOH)₂. On the basis of observations made in this work and the details presented in the original paper, we suggest that the reported mononuclear structure of square-planar acetylacetonate should likely contain copper instead of cobalt.