PNNL

The Science

Precisely synthesizing complex material structures requires understanding and controlling nucleation and growth. Non-classical nucleation pathways represent an avenue to reaching new material configurations. Researchers studied how electric forces affect the precipitation and growth of hydroxide films using a range of divalent and trivalent cations. They found that all the studied trivalent cations formed nanostructured films through a non-classical microphase separation mechanism, while the divalent cations formed continuous films through classical mechanisms. By tuning the pH, the team observed a new intermediate state with the trivalent cations. These observations were supported by a theoretical model constructed to explore the electronic forces underpinning this new nucleation and growth mechanism.

The Impact

Controlled precipitation is key to achieving precision materials synthesis. By discovering that microphase separation provides a novel alternative to classical nucleation and growth and showing how the new pathway can be accessed by varying the ion type, pH, and substrate properties, this work will facilitate the development of new strategies for precisely controlled synthesis of 2D and 3D nanostructured materials.

Summary

Understanding the mechanisms of crystal nucleation and growth is essential for achieving the precision synthesis of materials. Nucleation at interfaces is commonly understood via classical theories of heterogenous nucleation, but previous work has shown that a non-classical precipitation mechanism known as microphase separation can be active at interfaces. Researchers investigated the nucleation mechanisms for hydroxide films of both divalent and trivalent cations and developed new physical models to explain the observed distinct formation mechanisms. 

In a series of in situ atomic force microscopy (AFM) experiments, researchers mapped out the pH-dependent precipitation mechanisms for the cations. Notably, all trivalent cations produced nanostructured films via microphase separation, while all the divalent cations formed continuous films via classical nucleation and growth. In the lower pH range, the team observed a new intermediate state consisting of adsorbed trivalent ions with nanometer-scale spatial correlations, designated as ordered ion networks. Complementary streaming potential measurements show that the microphase separation pathway of the trivalent cations is accompanied by a significant diagnostic spike in ζ-potential that is not detected in the case of divalent ions. Nanoscale force mapping with 3D-AFM further demonstrated the occurrence of strong electrostatic forces around the islands of trivalent hydroxides. These results all point to interfacial charging as a key driver of microphase separation.

A Monte Carlo adsorption model was constructed to explore how the long-range repulsive forces that emerge from interfacial charging compete against the short-range forces that drive ion adsorption and clustering. The resulting charge-frustrated model predicted both the assembly of ions into ordered networks and the subsequent evolution of these clusters into microphase separated films. The model was used to produce phase diagrams and explain how key physical parameters determine whether precipitation occurs via microphase separation or classical nucleation and growth.

Research Contact

Ben Legg, Pacific Northwest National Laboratory, Benjamin.Legg@pnnl.gov 

Jim De Yoreo, Pacific Northwest National Laboratory, James.DeYoreo@pnnl.gov 

Funding

This work was primarily supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Division of Materials Science and Engineering (MSE), Synthesis and Processing Science Program at Pacific Northwest National Laboratory (PNNL) through FWP-67554. PNNL is a multiprogram national laboratory operated for the DOE by Battelle under contract number DE-AC05-76RL01830. Initial development of the charge-frustrated Monte Carlo model to predict the ion configurations at mica interfaces was supported by DOE BES MSE under FWP-77246 at PNNL. Streaming potential apparatus measurements for AlCl₃ solutions were supported by the DOE BES Geosciences Program under award numbers DE-SC-00021389 and DE-AC02-05CH11231 at the University of California, Riverside. Streaming potential apparatus measurements for MgCl₂ and other electrolyte solutions were supported by the DOE BES Chemical Sciences, Geosciences, and Biosciences Division, Separation Science program, through FWP-81462 at PNNL. BES-FWP-67554: to M.Z., B.A.L., B.A.H., P.J.P., C.J.M., and J.J.D.Y. BES-FWP-77246: to M.Z., B.A.L., and J.J.D.Y. BES-DE-SC-00021389: to Y.Z., R.K.Y., M.I., and Y.M. BES-FWP-81462: to S.T. and V.P.