X-Ray Diffraction and IR Spectrum for Activated Surface Hydrolysis of Al Metal into AlO(OH)·αH2O Nanocrystals in a New Monoclinic Crystal Structure
✍ Scribed by S. Rana; S. Ram
- Publisher
- Elsevier Science
- Year
- 2001
- Tongue
- English
- Weight
- 309 KB
- Volume
- 157
- Category
- Article
- ISSN
- 0022-4596
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✦ Synopsis
Activated spontaneous surface hydrolysis of nascent Al metal in distilled water at room temperature results in AlO(OH) ' H 2 O (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 di4raction is analyzed by assuming a monoclinic crystal structure with lattice parameters a ؍ 0.866 nm, b ؍ 0.506 nm, c ؍ 0.983 nm, and ؍ 94.563. 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 ؋ 10 3 m 2 / g, as compared to 4.5 ؋ 10 3 m 2 / 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) ' H 2 O 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.