In situ atomic force microscopy of zeolite A dissolution.

In the present study, the {100} surface of zeolite A was exposed to a range of solutions and the response was monitored in real-time by means of atomic force microscopy (AFM). The zeolite dissolves by a well-defined layer process that is characterised by uncorrelated dissolution of units that are structurally unconnected and terrace retreat when building units are inter-connected. This process was observed to be coupled with the formation of nano-squares that are stabilized at the zeolite surface for a period before complete dissolution. Theoretical work suggests that three terminating structures are central to understanding the dissolution mechanism. Stripping the surface of the secondary building unit, the single 4-ring, is predicted to be a rate-determining step in dissolution, but this process occurs by removing monomeric rather than oligomeric units.

Controlling relative fundamental crystal growth rates in silicalite: AFM observation.

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.

Crystal growth in nanoporous framework materials.

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.

Differentiating fundamental structural units during the dissolution of zeolite A.

In situ atomic force microscopy (AFM) is used to differentiate temporally both structure and mechanism in the removal of fundamental structural units during the dissolution of zeolite A.

Atomic force microscopy study of the molecular sieve MnAPO-50.

Atomic force microscopy (AFM) imaging of MnAPO-50 reveals multiply-nucleated, elliptical terraces, oriented in registry with the facet edges with step heights ranging from one to six template repeat distances on the [100] facets and terraces with step heights ranging from one to thirty three times the c unit cell parameter on the [001] facets.

Silicalite crystal growth investigated by atomic force microscopy.

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.

Fundamental Zeolite Crystal Growth Rates from Simulation of Atomic Force Micrographs.

An accurate model of the surface growth of one of the most important industrial zeolites, zeolite A, has been created. Comparison of the simulation with experimental data in the form of atomic force micrographs highlights the non-diffusion-limited nature of zeolite growth and provides the first ever quantification of fundamental crystal growth processes in zeolites.