Mechanisms for species-selective oriented crystal growth at organic templates
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Paul Zschack and Evguenia Karapetrova Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439
Jianming Bai Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Pulak Duttaa) Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208 (Received 13 March 2007; accepted 1 June 2007)
Langmuir monolayers floating on supersaturated aqueous subphases can act as templates for the growth of oriented inorganic films—a “bioinspired” nucleation process. We have performed in situ grazing incidence x-ray diffraction studies of the selective nucleation of BaClF and BaF2 under fatty acid monolayers. The arrangement of the fatty acid headgroups, the monolayer charge, and ion-specific effects all play important roles in selecting the inorganic species. When the monolayer is in a neutral state, both BaClF and BaF2 nucleate at the interface and are well aligned, but when the monolayer headgroup is deprotonated, only oriented BaF2 grows at the interface. We also observe an enhanced alignment of BaF2 crystals during growth from highly supersaturated solutions, presumably due to reorganization of preformed crystals at the organic template. These results show that a delicate interplay between multiple factors governs the oriented growth of inorganic films at organic templates.
I. INTRODUCTION
Langmuir monolayers floating on supersaturated aqueous subphases are known to be species-selective and crystal-face-selective templates for the growth of inorganic crystals.1,2 The processes that led to selective crystal nucleation at an organic surface are complex and depend on the state of the monolayer headgroups (charged or neutral),3 the relative geometric arrangements of the organic molecules and the ions in the crystal structure,4,5 the type of ions constituting the inorganic lattice,6 and in certain cases on subphase conditions such as degree of supersaturation.7 For example, by tuning the charge of an amine monolayer, exclusive nucleation of barium fluoride crystals can be promoted in a system where barium nitrate could also grow.3 Crystals with similar geometric arrangements of ions, such as CdCO3 and CaCO3 (calcite), orient along different directions when grown under the same monolayers.6 A shift between vaterite and cal-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0347 J. Mater. Res., Vol. 22, No. 10, Oct 2007
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cite (two crystal forms of calcium carbonate) can be attained simply by changing the subphase supersaturation.7 Studies of crystal growth performed under monolayers of similar structures but different headgroup characteristics can help us differentiate between the roles played by geometric and electrostatic interactions between the monolayer and inorganic ions in selective nucleation at the organic–inorganic interface. To perform such a study we chose the BaF2/BaClF/Ba2ClF3 system (all necessary ions are present in the solution and all three species nucleate in a
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