Atomic structure analysis of nanocrystalline Boehmite AlO(OH)

TL;DR

This study uses atomic pair distribution function analysis from powder diffraction data to elucidate the atomic structure and distortions in nanocrystalline Boehmite AlO(OH), revealing increased reactivity due to structural widening of hydrogen bonds.

Contribution

It demonstrates that atomic pair distribution function analysis can resolve atomic details and distortions in nanocrystalline Boehmite from in-house diffraction data, even with limited Qmax.

Findings

Boehmite nanocrystals have layered structures with ~2 nm distorted domains.

Hydrogen bonds are significantly widened (+13 pm), increasing reactivity.

Results align with previous Rietveld studies on size-dependent structural trends.

Abstract

Nanocrystalline n-AlO(OH) (Disperal P2(R) of Sasol) was investigated by means of the atomic pair distribution function (PDF). The PDF is derived from powder diffraction data, an ideally resolved PDF is obtained from a synchrotron source which provides a large maximal scattering vector length Qmax > 300 nm-1. Here, however, we were able to reveal atomic structure details of the about 4 nm particles from in-house diffraction data (Qmax = 130 nm-1): PDF least squares model refinements show that n-AlO(OH) is of the layered Boehmite structure type (oC16, Cmcm). But the structure is uniformly distorted in domains of ca. 2 nm size within the nano particles. The hydrogen bonds between the layers are widened up significantly by +13 pm, accounting for the higher reactivity when compared to microcrystalline Boehmite. Our results from only one "nanocrystallographic" experiment are consistent with a trend which was found via the Rietveld technique on a series of AlO(OH) of different crystallite size (Bokhimi et al., 2001). In addition, the PDF contains information on structural distortion as a function of (coherence) domain size within the nano particles.