Synthesis and Structural Characterization of a Family of Modified Hafnium tert-Butoxide for Use as Precursors to Hafnia Nanoparticles

Abstract

A series of modified, hafnium tert-butoxide ([Hf(OBu(t))4]) compounds (1-26) were crystallographically characterized, and representative species were then used to produce HfO2nanoparticles. This systematically varied family of [Hf(OR)4] compounds was developed from the reaction of [Hf(OBu(t))4] with a series of (i) Lewis basic solvents, tetrahydrofuran, pyridine, or 1-methylimidazole; (ii) simple phenols, HOC6H4(R)-2 or HOC6H3(R)2-2,6 where R = CH3, CH(CH3)2, or C(CH3)3; and (iii) complex polydentate alcohols, tetrahydrofuran methanol (H-OTHF), pyridinecarbinol (H-OPy), and tris(hydroxymethylethane) (THME-H3). The solvent-modified products were crystallographically characterized as [Hf(OBu(t))4(solv)n] (1-3). The phenoxide (OAr)-exchanged [Hf(OBu(t))4] products isolated from toluene were characterized as dimeric [Hf(OAr)n(OBu(t))4-n]2 (4 and 5) or [Hf(μ-OH)(OAr)3(HOBu(t))]2 (6 and 7) for the less sterically demanding OAr ligands and [Hf(OAr)n(OBu(t))4-n(HOBu(t))] (8 and 9) monomers for the larger OAr ligands. When Lewis basic solvents were employed, solvated monomers of varied OAr substitutions were observed as [Hf(OAr)n(OBu(t))4-n(solv)x], where solv = THF (10, 11, and 13-15) and py (16 and 19-21). The nuclearities of the remaining complex polydentate alcohol derivatives ranged from monomers (24, OPy) to dimers (22, OTHF; 23, OPy) to tetramers (25 and 26, THME). On the basis of their nuclearities, select members of this family of [Hf(OR)4] compounds (monomer, [Hf(OBu(t))4], 8; dimer, 19a, 22; tetramer, 25) were used to determine the validity of using [Hf(OR)4] precursors for the production of hafnia (HfO2) nanoparticles under solvothermal (oleylamine/oleic acid) conditions. After a 650 °C thermal treatment, the resulting powder X-ray diffraction pattern for each powder was found to be consistent with HfO2 (PDF 00-040-1173), and after a 1000 °C treatment, larger particles of HfO2 (PDF 00-043-1017) were reported. Transmission electron microscopy images confirmed that nanomaterials had formed. Because identical processing conditions had been employed for each HfO2 nanomaterial, the morphological variations observed in this study may be attributed to the individual precursors ("precursor structure affect").