Atomic force microscopy of synthetic imogolite

Abstract

The morphology of imogolite has been substantially studied using transmission electron microscopy (TEM) under ultra-high vacuum. However, the surface feature of imogolite at a nanometer scale under ambient conditions remains to be uncovered. The surface feature and the particle size of synthetic imogolite were investigated using contact- and tapping-mode atomic force microscopy (AFM) in the present study. The imogolite was synthesized at an initial Si concentration of 1.6 mmol l −1, a Si/Al molar ratio of 0.5, and an OH/Al molar ratio of 2.0. The X-ray diffractogram and infrared (IR) spectrum of the precipitates indicate that imogolite was the dominant reaction product. The in situ surface features of imogolite were revealed for the first time by using AFM under ambient conditions. The morphological features of synthetic imogolite were more clearly observed by using the tapping-mode AFM rather than the contact-mode AFM. Compared with the contact-mode AFM, the tapping-mode AFM required a shorter sample-drying period. Use of the contact-mode AFM tips, which have low aspect ratios, resulted in enlarging the observed diameter values of imogolite threads, which were subjected to the optimal ultrasonification (at 150 W for 5 s), by 21% apparently due to tip-sample convolution. The imogolite, which was subjected to the optimal ultrasonification (at 150 W for 5 s) that resulted in the least alteration of imogolite particles, appears in the tapping-mode AFM three-dimensional images as curved threads, varying in diameter from 40.2 to 95.5 nm (standard error<±0.1 nm for each thread). The tapping-mode AFM is a powerful and reliable technique to investigate the morphological features of imogolite and its related surface chemistry in soil environments.