Johanna Scheck1 and Denis Gebauer1
Despite the widespread occurrence of iron(III) (oxyhydr)oxides and their numerous applications in industry, no general pathway for their precipitation mechanism has been established. An understanding of the key steps and the occurring intermediate phases would be highly interesting, as it would enable controlled syntheses of materials with desired properties. In this article we present how the precipitation mechanism and the underlying chemical and physical chemical processes can be elucidated, and used for the development of synthetic strategies.
Iron(III) (oxyhydr)oxides exist in a variety of phases, differing in their crystallinity, shape, composition, physical, and chemical properties. As a result, the possible applications of iron(III) (oxyhydr)oxides in various fields of industry are very diverse. They are used as pigments, sorbents, catalysts, and as coating for protection from corrosion, just to name a few . In order to control the phase identity, the potential introduction of guest ions, crystallinity, shape and therefore the properties of the resulting particles, the onset of phase separation needs to be understood. Control of the precipitation mechanism is the key to control the precipitation product. Another field where knowledge of the precipitation pathway would be beneficial is the recycling of acid mine drainage. Here, the precipitation or iron(III) oxides is used for the removal of heavy metal ions via co-precipitation. As much as the process of iron(III) oxide precipitation constitutes a central point in many industrial applications, the mechanisms underlying the pathway are not understood.
Classical Nucleation Theory (CNT)
The initial phase separation event, e.g. nucleation, can follow different mechanisms. Most systems are described using Classical Nucleation Theory, CNT. This framework was established in the 1920s and divides the formation of nuclei and their subsequent growth to a bulk material into three stages; the pre-critical, the critical and the post-critical regime. In the pre-critical regime, prior to phase separation, small nuclei form upon statistical addition of single ions.
These pre-critical nuclei are already structured like the final crystal lattice and are therefore assumed to possess the same properties. Owing to their small size of only a few atoms, they exhibit an extremely high surface to bulk ratio. The high surface area is energetically unfavorable and cannot be balanced by bulk energy, due to the small size of the nuclei. Due to this, the pre-critical nuclei are thermodynamically unstable and dissolve again, and thus exist in a very small population. This scenario changes with size, however. With increasing radius of the nucleus, the surface energy grows with r2, while the bulk energy increases at a faster rate with r3. The radius at which the favorable bulk energy starts to balance the unfavorable surface energy is called the critical radius and defines the so-called critical nucleus. It is a metastable state, i.e., both dissolution and growth result in more stable states. Consequently, post-critical nuclei possessing a radius larger than the critical radius grow without limit.
Non-classical PNC Pathway
CNT has been challenged repeatedly in the recent years, since experimental observations made in many systems display a significant discrepancy with the predictions of this theory. One major issue of CNT is the assumption of bulk properties for pre-critical nuclei, as material properties change significantly when particle sizes reach the nanometre-regime. In contrast to the notions of CNT, it was shown that prior to phase separation, a significant population of thermodynamically stable clusters can be present, so-called pre-nucleation clusters (PNCs) . They are highly dynamic and exhibit a high water content. Due to these characteristics, PNCs are conceived of as solutes, where an interface with the surrounding solution does not exist, because there is no difference in dynamics between the clusters and the solution. The phase separation event in the PNC pathway is then not governed by a critical size, but rather by the development of an interface due to a distinct decrease in the cluster dynamics. Thereby, the clusters become nanodroplets that aggregate in order to decrease their thermodynamically unfavorable interfacial surface area.
When it comes to the elucidation of nucleation mechanisms, especially the very early stages prior to the precipitation event are of high interest. Prenucleation species represent the primary stage to the forming crystal. Their investigation, however, is challenging due to their small size and high reactivity. Another difficulty arises from the fact that an isolation of the early compounds is likely to induce changes in their structure. To overcome these problems a special titration experiment (fig. 1) was designed for the preparation of reaction solutions that contain the early stages. By mixing very dilute solutions, controllable, reproducible and homogenous reaction conditions can be provided. In situ analytical techniques, i.e., techniques that allow an investigation of the components without requiring isolation from the solution, were applied for detailed characterizations of the distinct nucleation stages; the special titration experiment enabled access to the different stages of iron(III) (oxyhydr)oxide precipitation, especially the pre-nucleation regime. Moreover, it can be modified easily, allowing studies on different systems, leading to different iron(III) (oxyhydr)oxide phases, such as akaganéite or ferrihydrite [3,4].
It was shown that the precipitation of iron(III) (oxyhydr)oxides follows the non-classical PNC pathway . This means that very dynamic, solute, stable, low-density iron(III) clusters, PNCs, are present in the early stage of the precipitation (fig. 2). The chemical reactions that govern and direct the phase separation event were identified. The formation of the PNCs proceeds via olation reactions, i.e., the bridging of iron(III) centres with hydroxo bridges. This reaction is reversible and the reaction is in (metastable) chemical equilibrium at this stage. The properties of the reaction solution change significantly with the onset of another chemical reaction, oxolation. During oxolation, more stable and more rigid oxo-bridges are formed. As a result, the clusters become denser and much less dynamic, and develop an interface with the surrounding solution. Thus, the onset of oxolation within olation PNCs marks the onset of phase separation. Particles precipitate and aggregate and further oxolation processes take place resulting in the final structures.
The key findings for directed syntheses are that the materials originate from PNCs and that cluster transformation from solutes to a solid phase determines their appearance. This can be illustrated by means of the akaganéite system. Akaganéite is an iron(III) oxyhydroxide exhibiting a very specific tunneled structure. The tunnels are filled with and stabilized by chloride ions. The mechanism by which chloride ions are introduced into the iron(III) phase were debated. Using the above described experiment, it was shown that they bind already in the pre-nucleation stage to PNCs . They compete with hydroxo-ligands and are thus incorporated into the olation clusters. Upon the onset of phase separation, i.e., oxolation, the chloride ions are released from the iron(III) centers. However, due to their presence not only on the surface but also within the clusters, complete expulsion is not possible. This leads to a microphase separation within the material and to the formation of the characteristic tunneled crystal structure.
The knowledge of the precipitation pathway can also be used to design novel materials, employing the experimental setup designed to investigate the iron(III) (oxyhydr)oxide system, monitoring the formation of the material. By adding the polymer poly(aspartic acid) to the iron(III) system, a novel material, consisting of nanoscopic, polydisperse spheres can be obtained (fig. 3). Again, the PNCs play a key role in the formation of this material. While the polymer shows no interaction with unhydrolyzed iron(III) ions, it does significantly affect the fate of the PNCs. Due to their interaction with the polymer, and the resulting close proximity of the relevant atoms in the PNC-polymer hybrid structure, the oxolation reaction is facilitated. The resulting composite material contains both organic and inorganic compounds and is formed by the strong interaction of the PNCs with the polymer. It exhibits different properties than both the polymeric and the inorganic compounds and might serve in the future for interesting applications.
These studies serve as excellent examples for how the understanding of precipitation pathways can be utilized to not only understand and improve the formation of known materials but also for the directed synthesis of interesting, new materials. As the experimental setup can be used for the investigations of different systems, future studies will be used to understand the formation of specific materials and improve the synthesis protocols in order to optimize the reaction products with respect to their application.
1 University of Konstanz, Department of Chemistry, Physical Chemistry, Konstanz, Germany
University of Konstanz
Department of Chemistry
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