Oxidation Does Not (Always) Kill Reactivity of Transition Metals: Solution-Phase Conversion of Nanoscale Transition Metal Oxides to Phosphides and Sulfides

Unexpected reactivity on the part of oxide nanoparticles that enables their transformation into phosphides or sulfides by solution-phase reaction with trioctylphosphine (TOP) or sulfur, respectively, at temperatures of ≤370 °C is reported. Impressively, single-phase phosphide products are produced, in some cases with controlled anisotropy and narrow polydispersity. The generality of the approach is demonstrated for Ni, Fe, and Co, and while manganese oxides are not sufficiently reactive toward TOP to form phosphides, they do yield MnS upon reaction with sulfur. The reactivity can be attributed to the small size of the precursor particles, since attempts to convert bulk oxides or even particles with sizes approaching 50 nm were unsuccessful. Overall, the use of oxide nanoparticles, which are easily accessed via reaction of inexpensive salts with air, in lieu of organometallic reagents (e.g., metal carbonyls), which may or may not be transformed into metal nanoparticles, greatly simplifies the production of nanoscale phosphides and sulfides. The precursor nanoparticles can easily be produced in large quantities and stored in the solid state without concern that “oxidation” will limit their reactivity

Synthetic Levers Enabling Independent Control of Phase, Size, and Morphology in Nickel Phosphide Nanoparticles

Simultaneous control of phase, size, and morphology in nanoscale nickel phosphides is reported. Phase-pure samples of discrete nanoparticles of Ni12P5 and Ni2P in hollow and solid morphologies can be prepared in a range of sizes (10−32 nm) by tuning key interdependent synthetic levers (P:Ni precursor ratio, temperature, time, oleylamine quantity). Size and morphology are controlled by the P:Ni ratio in the synthesis of the precursor particles, with large, hollow particles formed at low P:Ni and small, solid particles formed at high P:Ni. The P:Ni ratio also impacts the phase at the crystallization temperature (300−350 °C), with metal-rich Ni12P5 generated at low P:Ni and Ni2P at high P:Ni. Moreover, the product phase formed can be decoupled from the initial precursor ratio by the addition of more “P” at the crystallization temperature. This enables formation of hollow particles (favored by low P:Ni) of Ni2P (favored by high P:Ni). Increasing temperature and time also favor formation of Ni2P, by generating more reactive P and providing sufficient time for conversion to the thermodynamic product. Finally, increasing oleylamine concentration allows Ni12P5 to be obtained under high P:Ni precursor ratios that favor solid particle formation. Oleylamine concentration also acts to “tune” the size of the voids in particles formed at low P:Ni ratios, enabling access to Ni12P5 particles with different void sizes. This approach enables an unprecedented level of control over phase and morphology of nickel phosphide nanoparticles, paving the way for systematic investigation of the impact of these parameters on hydrodesulfurization activities of nickel phosphides

Control of Phase in Phosphide Nanoparticles Produced by Metal Nanoparticle Transformation: Fe₂P and FeP

The transformation of Fe nanoparticles by trioctylphosphine (TOP) to phase-pure samples of either Fe2P or FeP is reported. Fe nanoparticles were synthesized by the decomposition of Fe(CO)5 in a mixture of octadecene and oleylamine at 200 °C and were subsequently reacted with TOP at temperatures in the region of 350−385 °C to yield iron phosphide nanoparticles. Shorter reaction times favored an iron-rich product (Fe2P), and longer reaction times favored a phosphorus-rich product (FeP). The reaction temperature was also a crucial factor in determining the phase of the final product, with higher temperatures favoring FeP and lower temperatures Fe2P. We also observe the formation of hollow structures in both FeP spherical nanoparticles and Fe2P nanorods, which can be attributed to the nanoscale Kirkendall effect. Magnetic measurements conducted on phase-pure samples suggest that ∼8 × 70 nm Fe2P rods are ferromagnetic with a Curie temperature between 215 and 220 K and exhibit a blocking temperature of 179 K, whereas FeP is metamagnetic with a Néel temperature of ∼120 K. These data agree with the inherent properties of bulk-phase samples and attest to the phase purity that can be achieved by this method

Facile Synthesis of Germanium Nanoparticles with Size Control: Microwave <i>versus</i> Conventional Heating

A facile size-controlled synthesis (microwave/conventional) of <i>quasi</i>-spherical germanium nanoparticles is reported. Oleylamine serves as a solvent, a binding ligand, and a reducing agent in the synthesis. Reactions were carried out with microwave-assisted heating, and the results have been compared with those produced by conventional heating. Germanium iodides (GeI₄, GeI₂) were used as the Ge precursor, and size control in the range of 4–11 nm was achieved by controlling the ratio of Ge⁴⁺/Ge²⁺ in the precursor mix. Longer reaction times and higher temperatures were also observed to have an effect on the nanoparticle size distribution. Microwave heating resulted in crystalline nanoparticles at lower temperatures than conventional resistive heating because of the ability of germanium iodides to convert electromagnetic radiation directly to heat. The reported approach for germanium nanoparticle preparation avoids the use of strong reducing agents (LiAlH₄, <i>n</i>-BuLi, NaBH₄) and HF for etching and, thus, can be considered simple, safe, and amenable to industrial-level scaleup. The as-prepared nanoparticles are a stable dispersion (hexane or toluene) for weeks when stored under an inert atmosphere (N₂/Ar). The stability of the colloidal dispersion was observed to be dependent on the nanoparticle size, with smaller nanoparticles exhibiting longer stability. On exposure to ambient conditions, oxidation occurs over a period of time and results in slow precipitation of the nanoparticles. The nanoparticles have been characterized by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), and spectroscopic techniques (UV-Vis-NIR, FTIR, Raman)

Sacrificial Silver Nanoparticles: Reducing GeI₂ To Form Hollow Germanium Nanoparticles by Electroless Deposition

Herein we report the electroless deposition of Ge onto sacrificial Ag nanoparticle (NP) templates to form hollow Ge NPs. The formation of AgI is a necessary component for this reaction. Through a systematic study of surface passivating ligands, we determined that tri-<i>n</i>-octylphosphine is necessary to facilitate the formation of hollow Ge NPs by acting as a transport agent for GeI₂ and the oxidized Ag⁺ cation (i.e., AgI product). Annular dark-field (ADF) scanning transmission electron microscopy (STEM) imaging of incomplete reactions revealed Ag/Ge core/shell NPs; in contrast, completed reactions displayed hollow Ge NPs with pinholes which is consistent with the known method for dissolution of the nanotemplate. Characterization of the hollow Ge NPs was performed by transmission electron microscopy, ADF-STEM, energy-dispersive X-ray spectroscopy, UV–vis spectrophotometry, and Raman spectroscopy. The galvanic replacement reaction of Ag with GeI₂ offers a versatile method for controlling the structure of Ge nanomaterials

Development of Iron-Doped Silicon Nanoparticles As Bimodal Imaging Agents

We demonstrate the synthesis of water-soluble allylamine-terminated Fe-doped Si (Si_<i>x</i>Fe) nanoparticles as bimodal agents for optical and magnetic imaging. The preparation involves the synthesis of a single-source iron-containing precursor, Na₄Si₄ with <i>x</i>% Fe (<i>x</i> = 1, 5, 10), and its subsequent reaction with NH₄Br to produce hydrogen-terminated Si_<i>x</i>Fe nanoparticles. The hydrogen-capped nanoparticles are further terminated with allylamine <i>via</i> thermal hydrosilylation. Transmission electron microscopy indicates that the average particle diameter is ∼3.0 ± 1.0 nm. The Si_5Fe nanoparticles show strong photoluminescence quantum yield in water (∼10%) with significant <i>T</i>₂ contrast (<i>r</i>₂<i>/r</i>₁ value of 4.31). Electron paramagnetic resonance and Mössbauer spectroscopies indicate that iron in the nanoparticles is in the +3 oxidation state. Analysis of cytotoxicity using the resazurin assay on HepG2 liver cells indicates that the particles have minimal toxicity

Rational Design of Nickel Phosphide Hydrodesulfurization Catalysts: Controlling Particle Size and Preventing Sintering

The size-dependent catalytic activity of Ni2P for hydrodesulfurization (HDS) remains unstudied because the traditional temperature programmed reduction (TPR) method used in catalyst preparation results in highly polydisperse Ni2P particles. The ability to control the Ni2P particle size in the range 5–20 nm by varying the quantity of oleylamine in solution-phase arrested precipitation reactions is reported. Particles were introduced to a high surface area silica support (Cab-O-Sil, M-7D grade, 200 m2/g) via incipient wetness, and HDS activity was probed against dibenzothiophene (DBT). All samples were less active than TPR prepared materials, and the smallest particles were the least active, contrary to expectation. This is attributed in part to particle sintering under HDS conditions. Sintering occurs independently of wt% loading of catalyst, time, incipient wetness procedure, and ionic additives, at all temperatures greater than 200 °C. Sintering is minimized by encapsulation of Ni2P nanoparticles in a mesoporous silica shell, achieved by sol–gel silica formation around Ni2P-containing surfactant liquid crystal assemblies and subsequent calcination, resulting in a doubling of HDS activity

Halogen-Induced Crystallinity and Size Tuning of Microwave Synthesized Germanium Nanocrystals

The reduction of Ge halides in oleylamine (OAm) provides a simple, yet effective high-yield synthetic route to germanium nanocrystals (NCs). Significant advances based on this approach include size control of Ge NCs, Bi doping of Ge NCs, and synthesis of Ge1–xSnx alloys. It has been shown that the size of Ge NCs can be controlled by the ratio of Ge2+/Ge4+ in the reaction. Here, we show that finer control of absolute size and crystallinity can be achieved by the addition of molecular iodine (I2) and bromine (Br2) to germanium­(II) iodide (GeI2). We also show the presence of a Ge–amine–iodide complex and production of hydrogen and ammonia gases as side products of the reduction reaction. All reactions were carried out by microwave-assisted heating at 250 °C for 30 min. I2 and Br2 are shown to oxidize GeI2 to GeI4 in situ, providing good control over size and crystallinity. The kinetics of Br2 oxidation of GeI2 is slightly different, but both I2 and Br2 provide size control of the Ge NCs. The samples are highly crystalline as indicated by powder X-ray diffraction, selected area electron diffraction, transmission electron microscopy and Raman spectroscopy. Although both I2 and Br2 improve the crystallinity of the Ge NCs, I2 provides overall higher crystallinity in the NCs compared to Br2. Absorption (UV–vis–NIR) spectroscopy is consistent with quantum confinement for Ge NCs. The solutions of I2, GeI2, and colloidal Ge NCs were investigated with Fourier transform infrared and 1H NMR spectroscopies and showed no evidence for imine or nitrile formation. The hydrogen on the amine in OAm is shifted downfield with increasing amounts of I2, consistent with a more acidic ammonium species. Hydrogen and ammonia gases were detected after the reaction by gas chromatography and high-resolution mass spectrometry. The presence of a Ge–amine–iodide complex was also confirmed with no evidence for a hydrazine-like species. These results provide an efficient fine-tuning of size and crystallinity of Ge NCs using halogens in addition to the mixed-valence precursor synthetic protocol previously reported and demonstrate the formation of hydrogen as a reducing agent in OAm

Thiol-Capped Germanium Nanocrystals: Preparation and Evidence for Quantum Size Effects

Applications of Ge nanocrystals (NCs) are limited by the stability and air reactivity of the Ge surface. In order to promote stability and increase the diversity of ligand functionalization of Ge NCs, the preparation of thiol-passivated Ge NCs via a ligand exchange process was investigated. Herein a successful replacement of oleylamine ligands on the surface of Ge NCs with dodecanethiol is reported. The successful ligand exchange was monitored by FTIR and NMR spectroscopy and it was found that dodecanethiol provided a better surface coverage, leading to stable Ge NC dispersions. Dodecanethiol capping also enabled band gap determination of the NCs by surface photovoltage (SPV) spectroscopy. The SPV measurements indicated an efficient charge separation in the ligand-exchanged Ge NCs. On the other hand, oleylamine-terminated Ge NCs of similar sizes exhibited a very small photovoltage, indicating a poorly passivated surface

Thermochemistry, Morphology, and Optical Characterization of Germanium Allotropes

A thermochemical study of three germanium allotropes by differential scanning calorimetry (DSC) and oxidative high-temperature drop solution calorimetry with sodium molybdate as the solvent is described. Two allotropes, microcrystalline <i>allo</i>-Ge (<i>m-allo</i>-Ge) and 4<i>H</i>-Ge, have been prepared by topotactic deintercalation of Li₇Ge₁₂ with methanol (<i>m-allo</i>-Ge) and subsequent annealing at 250 °C (4<i>H</i>-Ge). Transition enthalpies determined by differential scanning calorimetry amount to 4.96(5) ± 0.59 kJ/mol (<i>m-allo</i>-Ge) and 1.46 ± 0.55 kJ/mol (4<i>H</i>-Ge). From high-temperature drop solution calorimetry, they are energetically less stable by 2.71 ± 2.79 kJ/mol (<i>m-allo</i>-Ge) and 5.76 ± 5.12 kJ/mol (4<i>H</i>-Ge) than α-Ge, which is the stable form of germanium under ambient conditions. These data are in agreement with DSC, as well as with the previous quantum chemical calculations. The morphology of the <i>m-allo</i>-Ge and 4<i>H</i>-Ge crystallites was investigated by a combination of scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. Even though the crystal structures of <i>m-allo</i>-Ge and 4<i>H</i>-Ge cannot be considered as truly layered, these phases retain the crystalline morphology of the layered precursor Li₇Ge₁₂. Investigation by diffuse reflectance infrared Fourier transform spectroscopy and UV–vis diffuse reflectance measurements reveal band gaps in agreement with quantum chemical calculations