Infrared spectral study of molecular vibrations in amorphous, nanocrystalline and AlO(OH) • αH2O bulk crystals

Amorphous, nanocrystalline, and bulk AlO(OH) . xH(2)O crystals have six fundamental modes (FM) of vibration in a nonlinear AlO(OH) molecular structure. Most of them appear in groups of four IR and Raman bands. Their positions and relative intensities differ significantly in three specimens. The nanocrystals (monoclinic structure with z = 8 molecules per unit cell) have four OH stretching bands at values enhanced by up to 360 cm(-1) at 3120, 3450, 3560 cm(-1) in comparison to those in bulk crystals or amorphous specimens. The first two bands are broad, bandwidth Delta nu (1/2) similar to 200 to 350 cm(-1), while the other two are sharp, Delta nu (1/2) less than or equal to 90 cm(-1). The sharp bands shift to 3525 and 3595 cm(-1) after heating the sample at 100 degreesC. They no longer appear after heating at 300 or 500 degreesC for 2 h (the specimen decomposes to Al2O3)(,) leaving behind only two bands at 3100 and 3400 cm(-1). A Delta nu (1/2) value of 500 cm(-1) appears in the 3400 cm(-1) in a delocalized distribution of H atoms. Two bands also occur at 3098 and 3300 cm(-1) in bulk crystals (orthorhombic structure with z = 4) or at 2990 and 3515 cm(-1) in an amorphous sample. More than one bands appear in a FM vibration in occurrence of sample in more than one conformers. The amorphous sample has approximately the same conformer structure as the bulk crystals. An amorphous surface structure exists in nanocrystals with a group of three bands at similar to 1420, 1510 and 1635 cm(-1) in an interconnected network structure. It encapsulates the nanocrystals in an amorphous shell. Its volume fraction, 33% estimated from the integrated intensity in three bands, determines 2.2 nm thickness in the shell in spherical shape of nanocrystals in 35 nm diameter. (C) 2001 Elsevier Science B.V. All rights reserved.