ABSTRACT
A detailed atomic force microscopy study has been performed on the open-framework, microporous material silicalite. Emphasis has been placed on determining the effect of supersaturation on the crystal growth process. The relative rates of fundamental crystal growth processes can be substantially altered by tuning the supersaturation. In this manner, it is possible to, for instance, switch on and off surface nucleation while retaining terrace spreading. This offers a potential mechanism by which it might be possible to control important crystal aspects such as defect density and intergrowths.
ABSTRACT
Future applications of nanoporous materials will be in opto-electronic devices, magnetic and chemical sensors, shape-selective and bio-catalysis, structural materials and nuclear waste management. Crucially, in all such applications, an understanding of crystal growth to the same depth as has been achieved in semiconductor technology is needed. Therefore, defects, intergrowths, dopants and isomorphous substitution must be controlled, and crystal habit and size (e.g. single crystal films) must be fabricated with precision. These goals elude the community because of lack of understanding of crystal growth processes. Modern microscopy techniques including AFM, ultra-high resolution SEM and HREM coupled with theoretical calculations are beginning to reveal the details of these growth processes yielding the important thermodynamic data crucial to effect synthetic control such as: controlling defects; controlling intergrowths: introducing chirality; modifying surface access; altering diffusion pathways; controlling crystal habit; synthesising templated materials cheaply in order to render them economically viable; controlling crystal size for instance as single crystal films. In this paper we will discuss recent results including: the details of surface alteration processes in nanoporous materials, measured in situ, under different chemical environments and the ability to switch processes on and off by the control of growth conditions. Further we illustrate an approach to theoretically model the crystal growth in such complex systems which ultimately delivers activation energies for fundamental growth processes.
ABSTRACT
Atomic force microscopy has been used to image the various facets of two morphologically distinct samples of silicalite. The smaller (20 microm) sample A crystals show 1 nm high radial growth terraces. The larger (240 microm) sample B crystals show growth terraces 1 to 2 orders of magnitude higher than the terraces on sample A with growth edges parallel to the crystallographic axes. Moreover, the terraces on the (010) face are significantly higher than the terraces on the (100) face - inconsistent with the previously proposed 90 degrees intergrowth structure. Sample A highlights that under certain synthetic conditions, silicalite grows in a manner akin to zeolites Y and A, via the deposition of layers comprising, in the case of silicalite, pentasil chains. It is probable that the rate of terrace advance is identical on the (010) and (100) faces, and it is the rate of terrace nucleation that dictates the overall growth rate of each facet and hence the relative size expressed in the final crystal morphology. Analysis of the growth terraces of sample B and detailed consideration of the structures of both MFI, and a closely related material MEL, lead to the proposal of a generalized growth mechanism for silicalite including the incorporation of defects within the structure. These defects are thought to be responsible for both the relative and the absolute terrace heights observed and may also explain the hourglass phenomenon observed by optical microscopy. The implications of this growth mechanism, supported by results of infrared microscopy, generate a new dimension to the continuing debate on the existence of intergrowths within one of the most important structures relevant to zeolite catalysis.
