Organometallic-metallic-cyclotriphosphazene mixtures: Solid state method for metallic nanoparticle growth

We review a recent general solid state method to obtain metallic, metal oxide and phosphate nanoparticles and crystals by pyrolysis at 800 °C using organometallic derivatives of cyclo and polyphosphazene precursors containing diverse organometallic fragments linked to polymeric or oligomeric phosphazenes. When the preparation of the molecular precursor is not possible or results in low yield, an alternative method using solid state mixtures of the type MLn/N₃P₃[O₂C1₂H₈]n, where MLn can be a single metallic salt, and a coordination compound or an organometallic, is possible. For AuCl(PPh₃)/[NP(O₂C₁₂H₈)]_n mixtures, single crystal cubic Au nanoparticles form, whose morphology, crystal shape, size and distribution strongly depends on deposition quantity and the mixture molar ratio. Nanoparticles as small as 3.5 nm are observed if the mixture is prepared in a crucible and varied geometries of microcrystals found when the mixture was deposited on Si or SiO2 wafers, including single-crystal gold fullerene structures. Extension to Ag, Pd and Re-containing precursor mixtures such as Ag(PPh₃)(CF₃SO₃)/[NP(O₂C₁₂H₈)]₃, PdCl₂/N₃P₃[O₂C₁₂H₈]₃, and KReO₄/N₃P₃[O₂C₁₂H₈]₃ allows microcrystal formation during pyrolysis. A thermally induced phase demixing mechanism describes the evolution of the crystal growth, aided microphase separation of the polymer mixture. This microphase demixing is shown to be an overarching mechanism involved in the nano to micro scale growth of crystals. A probable mechanism of the atomic and molecular-level chemistry is also proposed based on decomposition of the macromolecular polymeric, trimer and oligomeric precursors for the initial stages.