AFM images crystallisation in real-time
Top image: High temperature liquid cell attached to the atomic force microscope: the cell is equipped with inlet/outlet ports for liquid injection and a heating element that regulates temperatures as high as 300ºC.
US-based researchers have developed an atomic force microscope to image, in situ, crystal growth at extreme temperatures and in acidic conditions.
The new technique allowed Professor Jeffrey Rimer and Alexandra Lupulescu, from the Department of Chemical and Biomolecular Engineering, University of Houston to view crystal growth on the surface of zeolite crystals, pinpointing previously unconfirmed growth mechanisms.
Jeff Rimer confirms that zeolite crystal growth takes place via two mechanisms.
While researchers typically examine zeolite growth by removing crystals from natural synthesis and analysing changes in their physical properties, this makes understanding the fundamental mechanism of zeolite growth a real challenge.
For more than two decades, researchers theorised that nanoparticles, known to be present in zeolite growth solutions, played a role in the growth, but couldn't gather any direct evidence.
And while most crystals grow through classical means - the addition of atoms or molecules to the crystal - the gradual consumption of nanoparticles during zeolite crystallisation had suggested a non-classical growth mechanism was also taking place.
With this in mind, Rimer and Lupulescu turned to in situ AFM to monitor the growth of a face of silicate-1 zeolite under realistic synthesis conditions.
Teaming up with Asylum Research, they designed a liquid cell that would allow them to use AFM to image zeolite surface growth at much higher temperatures.
They also developed a new software suite that accounts for lateral cantilever drift while using AFM to continuously image the surface of zeolite crystals.
This so-called “drift correlation” software automatically accounts for lateral drift by shifting the view so the same surface area is being imaged every time, allowing researchers to scan zeolite surfaces for up to 48 hours.
"We used and AFM scanner with a feedback controller and drift correlation software... to achieve small drift rates of around 5nm an hour, corresponding to less than 0.3%/hour loss of the imaging area," he says.
Analysis of the AFM images also took into account changes in tip geometry due to silica deposition. And all images were collected in tapping mode to minimise damage to new crystal growth.
The researchers discovered that during zeolite growth, both classical and non-classical growth were taking place, involving the addition of silica molecules as well as the direct attachment of nanoparticles.
"We have shown that a complex set of dynamics takes place,” says Rimer. “In doing so, we have revealed that there are multiple pathways in the growth mechanism, which solves a problem that has been debated for nearly 25 years.”
Rimer and Lupulescu hope the time-resolved AFM method will now be extended to a broader class of zeolites as well as other materials including metal oxides, minerals and metal-organic frameworks.
Research is published in Science.