Crystal growth in hydrogels is an extension of low temperature solution-based methods.  Incorporation of the hydrogel matrix during growth was seen initially seen as a drawback to this method, but later proved to be a means to form composite crystals, similar to biogenic minerals that contain hierarchically ordered levels of structure.  Translating these methods from organic-hydrogel, carbonate-mineral systems to inorganic-hydrogel, oxide-mineral systems is not a straightforward task, as most hydrogels lose their structure (melt) at the higher temperatures used for oxide synthesis.  In this work, we report the growth of iron (III) oxide, hematite, in an inorganic silica hydrogel under hydrothermal conditions.  The success of this approach stems from the parallel chemistries associated with the formation of each individual material (hematite in solution, silica as a hydrogel), and presents a means to form inorganic-inorganic composites with tunable hierarchical structures.

The well-known solution method for forming hematite pseudocubes (Fig. 1a) from iron (III) chloride under acidic conditions¹ was carried out within a silica hydrogel matrix.  The resulting spherical particles (Fig. 1b) are not simply polycrystalline particles, but mosaic crystals that contain crystallographically-oriented subunits.  Both the solution- and hydrogel-grown hematite particles (Fig. 1a&b, respectively) extinguish during rotation under cross-polarized light and diffract electrons as single crystals.  The size of the crystalline subunits contained within these particles was calculated from x-ray powder diffraction data, and confirmed visually using transmission electron microscopy (TEM) with etching studies.   The net relationship between the particle shape and the underlying hematite lattice was determined using high resolution TEM and selected area electron diffraction analysis on thin sections that were prepared using a focused ion beam.  The macroscopic change in shape of the hematite crystals upon growing in a hydrogel is correlated to a change in the nano-scale, hierarchical structure of the subunits contained within hematite particles; formation of hematite in a silica hydrogel matrix is associated with an increase in the aspect ratio of the subunits, which become elongated along [110] of the hematite lattice. 

The composite nature of the hydrogel grown particles is clearly demonstrated by their etching behavior.  Upon etching in acidic solution, where hematite is soluble, a porous framework remains (Fig. 1c).  Conversely, upon exposing the particles to basic media, where silica species have a higher solubility than hematite, layers of rods emerge from within a base-soluble matrix (Fig. 1d).  These results indicate the two-phase nature of the hematite particles formed in silica hydrogel; crystalline hematite within a porous, silica-based matrix.  The identity of the porous phase contained within the hydrogel grown hematite particles will be discussed in terms of chemical composition, crystallinity, and distribution within the interior of the particles with the goal of understanding of the molecular-level growth environment that leads to the levels of structure contained within hydrogel grown materials. 

Figure 1.  Scanning electron micrographs of hematite crystals formed under hydrothermal conditions in solution (a) and in a silica hydrogel matrix (b).  (c) Acid- and (d) base-etched, hydrogel-grown particles showing the composite nature of the hydrogel-grown particles.

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1. Hierarchically-structured inorganic-inorganic composite materials formed by crystal growth in hydrogels