X-ray diffraction and IR spectrum for activated surface hydrolysis of Al metal into AlO(OH)·αH2O nanocrystals in a new monoclinic crystal structure

Article abstract of DOI:10.1006/jssc.2000.9034

Activated spontaneous surface hydrolysis of nascent Al metal in distilled water at room temperature results in AlO(OH)·αH₂O (boehmite) nanocrystals of an average 35 nm diameter. The nanocrystals are separated by 2 to 5 nm with a fairly sharp size distribution in the electron micrograph. Their X-ray diffraction is analyzed by assuming a monoclinic crystal structure with lattice parameters a=0.866 nm, b=0.506 nm, c=0.983 nm, and β=94.56°. It is a new structure of boehmite in comparison to the orthorhombic bulk structure. It involves 3.3 times the lattice volume in an orthorhombic lattice enclosed in a smaller surface area, S=3.7×10³ m²/g, as compared to 4.5×10³ m²/g in it. This structure forms in a fast Al surface hydrolysis reaction in this example because it undergoes a fast release of its total surface energy as a function of its fast growth. The molecular vibrational structure of AlO(OH)·αH₂O nanocrystals is analyzed from their IR spectrum which has three characteristic bandgroups at 200-1200 cm^-1, 1300-2500 cm^-1, and 2800-4000 cm^-1. A considerably improved value of the O-H group stretching frequency by as much as 360 cm^-1 relative to the bulk value appears for a proton localized structure in the nanocrystals.