A microscopic examination of nanosized mixed Ni-Al hydroxide surface precipitates on pyrophyllite

The nature of Ni-hydroxide precipitates on pyrophyllite were re-examined by analytical electron microscopy (AEM), high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), powder X-ray diffraction (PXRD) and X-ray absorption fine structure XAFS. Analytical analysis of precipitates shows that the precipitate contains about 20% Al. High resolution TEM imaging showed that the precipitate was amorphous and PXRD failed to find any crystalline peaks associated with crystalline Ni-Al layered double-hydroxide (LDH) or α-Ni(OH)2. These results confirm the conclusion from XAFS spectroscopic data that Al co-precipitates with Ni on Al-rich substrates to form Ni-Al hydroxide (LDH) surface precipitates. However, the HRTEM data clarifies that although the bonding environment of the precipitate is like that of Ni-Al LDH, no long range ordering of the structure exists. The study illustrates the need for TEM observations to complement XAFS data and the potential importance of amorphous materials in environmental settings.