Role of Akaganeite (β-FeOOH) in the Growth of Hematite (α-Fe 2 O 3 ) in an Inorganic Silica Hydrogel Emily Asenath-Smith † and Lara A. Estroff* ,†,‡ † Department of Materials Science and Engineering, and ‡ Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, United States *S Supporting Information ABSTRACT: The use of an inorganic hydrogel as a means to modulate the hierarchical architectures of oxide compounds requires an understanding of the effect of the matrix on intermediate phases. In this work, we report on the crystallization of akaganeite (β-FeOOH), both within a silica hydrogel and from aqueous solution, with a focus on understanding the chemical effects of pH, [Fe 3+ ], and [Cl − ], in concert with the physical effects of the silica hydrogel, on the ultimate formation of hematite (α-Fe 2 O 3 ). A distinct physical consequence of the hydrogel crystallization microenvironment is the stabilization of akaganeite as three-dimensional assemblies, in contrast to the discrete rods that form in solution. Chemically, we find that [Fe 3+ ]affects the size of akaganeite crystals, while [H + ] determines the aspect ratio. We also identify that crystal splitting is correlated to high [Cl − ]. In addition, we demonstrate that planar, branched aggregates of akaganeite rods are favored at high [H + ] and are associated with a pathway to hematite that proceeds through the goethite polymorph (α-FeOOH). With these results, we highlight the physical and chemical variables of the crystallization microenvironment that dictate the structural features of akaganeite crystals and their corresponding hematite forms. ■ INTRODUCTION The growth of inorganic crystals within hydrogels has shown potential to control the architectures of functional oxide materials; 1−3 however, the role of intermediate oxyhydroxide phases in defining the final oxide morphology is less clearly understood. Research on the growth of ionic crystals within organic hydrogels has identified both chemical and physical characteristics of the hydrogel as key variables for influencing the structure and composition of the crystals. 4,5 While such aspects of organic matrix-mediated crystallization are well understood at ambient conditions, the variables associated with the growth of oxide compounds within inorganic hydrogels under hydrothermal conditions requires further investigation. In addition to the obvious complexity introduced by elevated temperatures, crystallization of the target oxide phase is often complicated by the existence of intermediate oxyhydroxide phases. 6−8 In some cases, the intermediate phases serve as templates for the nucleation of the oxide phase, thereby playing a role in defining the final structure. 9 An understanding of the physical and chemical effects of an inorganic hydrogel under hydrothermal conditions on initial oxyhydroxide phases will inform the development of this method for the design of new oxide materials with tightly regulated structural features. Using hematite (α-Fe 2 O 3 ) as a model oxide system, we have previously reported the modulation of the hierarchical structure of hematite by growth in an inorganic silica hydrogel. 1,10 Specifically, quasi-spheres of hematite formed in the hydrogel, as compared to the pseudocubes that formed under equivalent conditions in solution. The previous work focused on understanding the chemical effects of dissolved silica in the crystallization microenvironment in influencing the hematite structure across length scales. The chemical role of silica was able to explain the net orientation of the hematite lattice and the elongated nanodomain structure within the non-idiomor- phic quasi-spheres; however, the radial, stacked internal mesoscale structure could not be fully attributed to chemically dissolved silica alone. The iron oxyhydroxide phase akaganeite (β-FeOOH) was present as an intermediate in this work and has been implicated with a role as a structural template to hematite growth from aqueous solution. 9 With this background, we designed an investigation of the akaganeite phase to elucidate the chemical and physical role played by the silica hydrogel environment in modulating the growth and assembly of this intermediate iron oxyhydroxide phase. Hematite Morphology and Synthesis. Hematite, with the hexagonal space group R3̅c, can be formed in a range of non-idiomorphic forms with mosaic internal structures that preserve the net orientation of the hematite lattice on the microscale. For example, pseudocubes of hematite, formed by acidic hydrolysis of iron salts, 11 show a [001] orientation of the hematite lattice that is normal to two parallel faces. 10 Hematite spindles, which exhibit a [001] orientation along their long axis, Received: April 7, 2015 Revised: May 14, 2015 Article pubs.acs.org/crystal © XXXX American Chemical Society A DOI: 10.1021/acs.cgd.5b00475 Cryst. Growth Des. XXXX, XXX, XXX−XXX