Preparation and characterization of molybdenum and tungsten nitride nanoparticles obtained by thermolysis of molecular precursors

Abstract

Treatment of Mo(N^tBu)₂Cl₂ with [K(Ph₂pz)(THF)]₆ (pz = pyrazolyl) in tetrahydrofuran afforded Mo(N^tBu)₂(Ph₂pz)₂ (85%), while treatment of W(N^tBu)₂(NH^tBu)₂ with 3,5-diphenylpyrazole afforded W(N^tBu)₂(Ph₂pz)₂ (97%). The complexes M(N^tBu)₂(Ph₂pz)₂ were characterized completely by spectral and analytical data, and by an X-ray crystal structure determination for Mo(N^tBu)₂(Ph₂pz)₂. Thermolysis of M(N^tBu)₂(Ph₂pz)₂ at 800 °C under nitrogen afforded 2–3 nm metal nitride nanoparticles that were embedded in an amorphous carbon–oxygen matrix, as determined by X-ray powder diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. X-Ray photoelectron spectroscopy also revealed the presence of metal oxide phases, which were amorphous by X-ray powder diffraction. The nanoparticles prepared at 800 °C were insoluble. Thermolysis of M(N^tBu)₂(Ph₂pz)₂ at 425 °C afforded amorphous 2–3 nm nanoparticles, as determined by X-ray powder diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. X-Ray photoelectron spectroscopy suggested a molybdenum(IV) nitride and W₂N/WN, as well as MoO₃ and an oxidized tungsten nitride. Materials prepared at 425 °C were not embedded in a matrix, and were soluble in tetrahydrofuran. Infrared and NMR spectroscopy suggested the presence of surface organic fragments that contain alkyl-substituted phenyl groups. Such surface groups are most likely derived from decomposition of the heterocyclic ligands. Simultaneous differential thermal analysis/thermogravimetric analysis indicated that the amorphous nitride materials convert to crystalline nanoparticles consistent with M₂N phases between 600–700 °C, along with a weight loss that may correspond to dinitrogen evolution. The amorphous carbon–oxygen matrix also forms upon heating from 425 to 800 °C. This work provides the first description of tungsten nitride nanoparticles, as well as the first description of soluble group 4–6 nanoparticles.