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 HfO 2nanoparticles. 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, HOC 6H 4(R)-2 or HOC 6H 3(R) 2-2,6 where R = CH 3, CH(CH 3) 2, or C(CH 3) 3; and (iii) complex polydentate alcohols, tetrahydrofuran methanol (H-OTHF), pyridinecarbinol (H-OPy), and tris(hydroxymethylethane) (THME-H 3). 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 (HfO 2) 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 HfO 2 (PDF 00-040-1173), and after a 1000 °C treatment, larger particles of HfO 2 (PDF 00-043-1017) were reported. Transmission electron microscopy images confirmed that nanomaterials had formed. Because identical processing conditions had been employed for each HfO 2 nanomaterial, the morphological variations observed in this study may be attributed to the individual precursors ("precursor structure affect"). © 2012 American Chemical Society.