Homogeneous Nanoparticles of Multimetallic Phosphides via Precursor Tuning: Ternary and Quaternary M₂P Phases (M = Fe, Co, Ni)

Abstract

Transition metal phosphides (TMPs) are a highly investigated class of nanomaterials due to their unique magnetic and catalytic properties. Although robust and reproducible synthetic routes to narrow polydispersity monometallic phosphide nanoparticles (M2P; M = Fe, Co, Ni) have been established, the preparation of multimetallic nanoparticle phases (M2–x M′ x P; M, M′ = Fe, Co, Ni) remains a significant challenge. Colloidal syntheses employ zero-valent metal carbonyl or multivalent acetylacetonate salt precursors in combination with trioctylphosphine as the source of phosphorus, oleylamine as the reducing agent, and additional solvents such as octadecene or octyl ether as “noncoordinating” cosolvents. Understanding how these different metal precursors behave in identical reaction environments is critical to assessing the role the relative reactivity of the metal precursor plays in synthesizing complex, homogeneous multimetallic TMP phases. In this study, phosphorus incorporation as a function of temperature and time was evaluated to probe how the relative rate of phosphidation of organometallic carbonyl and acetylacetonate salt precursors influences the homogeneous formation of bimetallic phosphide phases (M2–x M′ x P; M, M′ = Fe, Co, Ni). From the relative rate of phosphidation studies, we found that where reactivity with TOP for the various metal precursors differs significantly, prealloying steps are necessary to isolate the desired bimetallic phosphide phase. These insights were then translated to establish streamlined synthetic protocols for the formation of new trimetallic Fe2–x–y Ni x Co y P phases.

Controlling Phase in Colloidal Synthesis

“…The choice of metal precursor also influences phase formation v ia different reactivities of the metal or the attached anions. The Brock group found different reactivity in Fe, Ni, and Co precursors when forming M 2 P particles (at 300 °C) . They used metal acetylacetonate salts and metal carbonyls with trioctylphosphine, oleylamine, and octadecene to explore monometallic and multimetallic transition metal phosphides.…”

“…They also found that rate of phosphidation was much slower for Fe­(CO) 5 than for Co 2 (CO) 8 and Ni­(acac) 2 , overall showing metal precursor reactivity should be taken into account when synthesizing metal phosphides. When forming trimetallic phosphides, the order addition of the metals was key to success because of the different amorphous intermediates that formed . While the Brock group focused on reactivity of different metals, precursors of the same metal also show varying reactivity.…”

Colloidal Nanoparticles of High Entropy Materials: Capabilities, Challenges, and Opportunities in Synthesis and Characterization

“…Similarly, colloidal high entropy metal phosphide nanoparticles have not yet been reported, although progress has been made in targeting three-metal colloidal phosphide nanoparticles. 50 In general, high entropy chalcogenide and phosphide nanoparticles have been slower to emerge than alloy systems, despite the frequency with which single-metal chalcogenide and phosphide colloidal nanoparticles are synthesized. To make metal sulfide and phosphide nanoparticles, one must react metal salts and/or complexes with chalcogen or phosphide reagents.…”

“…Solvothermal methods have been reported to produce a limited number of high entropy metal sulfide nanoparticles, − but to our knowledge, high entropy metal selenide and telluride nanoparticles have not yet been reported using colloidal synthesis techniques. Similarly, colloidal high entropy metal phosphide nanoparticles have not yet been reported, although progress has been made in targeting three-metal colloidal phosphide nanoparticles . In general, high entropy chalcogenide and phosphide nanoparticles have been slower to emerge than alloy systems, despite the frequency with which single-metal chalcogenide and phosphide colloidal nanoparticles are synthesized.…”

Compositional dependence of hydrodeoxygenation pathway selectivity for Ni2−xRhxP nanoparticle catalysts

“…Traditional synthesis methods for TMPs via impregnation of metal and phosphorus salts on high surface area metal-oxide supports followed by high temperature reduction can lead to broad particle size distributions, overreduction of metal salts to metal NPs, non-uniform phosphorus incorporation, and excess phosphate species that do not participate in TMP formation. 24,25 Solution synthesis routes, on the other hand, can enable control over composition, particle morphology, and crystal structure, 22,[26][27][28][29][30][31][32] providing model systems to understand the role of the second metal. Our group has previously established solution synthesis approaches to obtain Ni 2 P, Rh 2 P, and a variety of alloyed Ni 2 P NPs via the thermolysis of low-valent metal triphenylphosphine complexes.…”