X-Ray Diffraction and IR Spectrum for Activated Surface Hydrolysis of Al Metal into AlO(OH)·ALPHAH2O Nanocrystals in a New Monoclinic Crystal Structure

Activated spontaneous surface hydrolysis of nascent Al metal in distilled water at room temperature results in AlO(OH)·H2O (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 =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)·H2O 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.