Sol–gel synthesis of highly pure α-Al2O3 nano-rods from a new class of precursors of salicylaldehyde-modified aluminum(iii) isopropoxide. Crystal and molecular structure of [Al(OC6H4CHO)3]

Abstract

Reactions of aluminum isopropoxide with salicylaldehyde in 1 : 1, 1 : 2 and 1 : 3 molar ratios in anhydrous benzene yield complexes of the type [(OPrⁱ)_3−nAl(OC₆H₄CHO)_n] {where n = 1(1); n = 2 (2); n = 3 (3)}. All the products are yellow solids and are soluble in common organic solvents. They were characterized by elemental analysis, ESI-mass spectrometry, FT-IR and ¹H, ¹³C and ²⁷Al NMR studies. The ESI-mass spectral studies indicate dimeric nature for (1) and (2) and monomeric nature for compound (3). Crystal and molecular structures of [Al(OC₆H₄CHO)₃] (3) suggest that salicylaldehyde ligands bind to the metal in a side-on dihapto η²-(O,O) manner, leading to the formation of a hexa-coordinated environment around the aluminum atom. Powder XRD, SEM image, and EDX analysis appear to indicate formation of nano-sized rods for precursor (3). Sol–gel hydrolysis of all the precursors Al(OPrⁱ)₃, (1), (2) and (3) followed by sintering at 1100 °C yielded α-Al₂O₃ (a), (b), (c) and (d), respectively. The powder X-ray diffraction patterns and the SEM images of all the oxides exhibit nano-sized microcrystalline morphology for (a) and mixed platelike nano-rod morphology for (b), (c) and (d). The EDX and TEM studies of (d) also corroborate the formation of α-Al₂O₃. The IR spectral studies of all the oxides indicate the formation of pure α-alumina, a versatile ceramic oxide known for exhibiting a wide variety of applications in engineering and biomedical areas.