The Molecular Mechanism of Iron(III) Oxide Nucleation

Dimerization of Fe(III) Ion in an Aqueous Medium: Mechanistic Modelling and Effects of Ligands

Aqueous iron solutions generally undergo spontaneous hydrolysis followed by aggregation resulting in the precipitation of nanocrystalline oxyhydroxide minerals. The mechanism of nucleation of such multinuclear oxyhydroxide clusters are unclear due to limited experimental evidence. Here, we investigate the mechanistic pathway of dimerization of Fe(III) ions using density functional theory (DFT) in aqueous medium considering effects of other ligands. Two hydrolyzed monomeric Fe(III) ions in aqueous medium may react to form two closely related binuclear products, the μ-oxo and the dihydroxo Fe2 dimer. Our studies indicate that the water molecules in the second coordination sphere and those co-ordinated to the Fe(III) ion, both participate in the dimerization process. The proposed mechanism effectively explains the formation of dihydroxo and μ-oxo Fe2 dimers with interconversion possibilities, for the first time. Results show, with only water molecules present in the second co-ordination sphere, dihydroxo Fe2 dimer is the thermodynamically and kinetically favored product with a low activation free energy. We calculated the step-wise reaction free energies of dimerization in the presence of nitrate ions in the first and second coordination sphere of Fe(III) ion separately, which shows that with nitrate ions in the second co-ordination sphere, the μ-oxo Fe2 dimer is the kinetically favored product.

Observation of Transient Prenucleation Species of Calcium Carbonate by DNP-Enhanced NMR

Knowledge of the mechanism by which polymorphic inorganic species, such as carbonates, are formed is crucial to understand and guide the selective crystallization of end products. Recently it has been shown that a key step in the crystallization of calcium carbonate is the formation of intermediate species known as prenucleation clusters. However, the observation of these prenucleation clusters in solution is exceedingly challenging because of their short lifetime and low concentrations. Here, using dissolution DNP-enhanced NMR spectroscopy, we observe signals from prenucleation species of calcium carbonate from which the kinetics of formation and conversion are determined.

An Atomistic View on the Mechanism of Diatom Peptide-Guided Biomimetic Silica Formation

Deciphering nature's remarkable way of encoding functions in its biominerals holds the potential to enable the rational development of nature-inspired materials with tailored properties. However, the complex processes that convert solution-state precursors into solid biomaterials remain largely unknown. In this study, an unconventional approach is presented to characterize these precursors for the diatom-derived peptides R5 and synthetic Silaffin-1A1 (synSil-1A1). These molecules can form defined supramolecular assemblies in solution, which act as templates for solid silica structures. Using a tailored structural biology toolbox, the structure-function relationships of these self-assemblies are unveiled. NMR-derived constraints are employed to enable a recently developed fractal-cluster formalism and then reveal the architecture of the peptide assemblies in atomistic detail. Finally, by monitoring the self-assembly activities during silica formation at simultaneous high temporal and residue resolution using real-time spectroscopy, the mechanism is elucidated underlying template-driven silica formation. Thus, it is demonstrated how to exercise morphology control over bioinorganic solids by manipulating the template architectures. It is found that the morphology of the templates is translated into the shape of bioinorganic particles via a mechanism that includes silica nucleation on the solution-state complexes' surfaces followed by complete surface coating and particle precipitation.

pH-Driven Rotational Configuration of Keggin-Fe13 Clusters and Their Transformations

Keggin-Fe13 clusters are considered foundational building blocks or prenucleation precursors of ferrihydrite. Understanding the factors that influence the rotational configuration of these clusters, and their transformations in water, is vital for comprehending the formation mechanism of ferrihydrite. Here, we report syntheses and crystal structures of four lanthanide-iron-oxo clusters, namely, [Dy6Fe13(Gly)12(μ2-OH)6(μ3-OH)18(μ4-O)4(H2O)17]·13ClO4·19H2O (1), [Dy6Fe13(Gly)12(μ3-OH)24(μ4-O)4(H2O)18]·13ClO4·14H2O (2), [Pr8Fe34(Gly)24(μ3-OH)28(μ3-O)30(μ4-O)4(H2O)30]·6ClO4·20H2O (3), and [Pr6Fe13(Gly)12(μ3-OH)24(μ4-O)4(H2O)18]·13ClO4·22H2O (4, Gly = glycine). Single-crystal analyses reveal that 1 has a β-Keggin-Fe13 cluster, marking the first documented instance of such a cluster to date. Conversely, both 2 and 4 contain an α-Keggin-Fe13 cluster, while 3 is characterized by four hexavacant ε-Keggin-Fe13 clusters. Magnetic property investigations of 1 and 2 show that 2 exhibits ferromagnetic interactions, while 1 exhibits antiferromagnetic interactions. An exploration of the synthetic conditions for 1 and 2 indicates that a higher pH promotes the formation of α-Keggin-Fe13 clusters, while a lower pH favors β-Keggin-Fe13 clusters. A detailed analysis of the transition from 3 to 4 emphasizes that lacunary Keggin-Fe13 clusters can morph into Keggin-Fe13 clusters with a decrease in pH, accompanied by a significant change in their rotational configuration.

Organic Nucleation: Water Rearrangement Reveals the Pathway of Ibuprofen

The organic nucleation of the pharmaceutical ibuprofen is investigated, as triggered by the protonation of ibuprofen sodium salt at elevated pH. The growth and aggregation of nanoscale solution species by Analytical Ultracentrifugation and Molecular Dynamics (MD) simulations is tracked. Both approaches reveal solvated molecules, oligomers, and prenucleation clusters, their size as well as their hydration at different reaction stages. By combining surface-specific vibrational spectroscopy and MD simulations, water interacting with ibuprofen at the air-water interface during nucleation is probed. The results show the structure of water changes upon ibuprofen protonation in response to the charge neutralization. Remarkably, the water structure continues to evolve despite the saturation of protonated ibuprofen at the hydrophobic interface. This further water rearrangement is associated with the formation of larger aggregates of ibuprofen molecules at a late prenucleation stage. The nucleation of ibuprofen involves ibuprofen protonation and their hydrophobic assembly. The results highlight that these processes are accompanied by substantial water reorganization. The critical role of water is possibly relevant for organic nucleation in aqueous environments in general.

Structural Rearrangement Followed by Entrapment of Subnanometer Building Blocks of Iron Oxyhydroxide in Aqueous Iron Chloride Solutions

Iron oxyhydroxide, a natural nanophase of iron found in the environment, plays a crucial role in regulating surface and groundwater composition. Recent research proposes that within the nonclassical prenucleation cluster growth model, subnanometer-sized clusters (olation clusters/Fe13 δ-Keggin oxolation clusters) might act as the prenucleation clusters (PNCs) of ferrihydrite or iron oxyhydroxide solid phase. However, these clusters are difficult to characterize as they are only observable momentarily in low-pH, high-Fe concentration solutions before agglomerating into extended solids, keeping the controversy over the true nature of the PNCs alive. In this study, we introduce large quantities of zinc acetate salt (ZA) into iron chloride solutions at the olation-oxolation boundary (3.6 mM Fe3+ at pH ∼2.6). Remarkably, this manipulation is found to alter the structural arrangement of these subnanometer clusters before blocking them in isolation for hours, even at pH 6, where extended iron oxyhydroxide phases typically precipitate. On the other hand, controlled addition of ZA allows partial unblocking, leading to anisotropic agglomeration into cylindrical rod-like structures. Experimental techniques such as synchrotron-based small-angle X-ray scattering, X-ray absorption spectroscopy, high-resolution transmission electron microscopy (TEM), and cryo-TEM, along with density functional theory (DFT) calculations, reveal the nature of the structural rearrangement and the crucial role of Zn2+ ions in cluster stabilization.

Role of Water during the Early Stages of Iron Oxyhydroxide Formation by a Bacterial Iron Nucleator

Understanding the nucleation of iron oxides and the underlying hydrolysis of aqueous iron species is still challenging, and molecular-level insights into the orchestrated response of water, especially at the hydrolysis interface, are lacking. We follow iron(III) hydrolysis in the presence of a synthetic bacterial iron nucleator, which is a magnetosome membrane specific peptide, by using a constant pH titration technique. Three distinct hydrolysis regimes were identified. Interface-selective sum frequency generation (SFG) spectroscopy was used to probe the interfacial reaction and water in direct contact with the peptide. SFG data reveal that iron(III) species react quickly with interfacial peptides while continuously enhancing water alignment into the later stages of hydrolysis. The gradually aligning water molecules are associated with initially promoted (regimes I and II) and later suppressed (regime III) hydrolysis after the saturation of water alignment has occurred until regime II. These interfacial insights are crucial for understanding the early stage of iron oxide biomineralization.

Evidence for liquid-liquid phase separation during the early stages of Mg-struvite formation

The precipitation of struvite, a magnesium ammonium phosphate hexahydrate (MgNH4PO4 · 6H2O) mineral, from wastewater is a promising method for recovering phosphorous. While this process is commonly used in engineered environments, our understanding of the underlying mechanisms responsible for the formation of struvite crystals remains limited. Specifically, indirect evidence suggests the involvement of an amorphous precursor and the occurrence of multi-step processes in struvite formation, which would indicate non-classical paths of nucleation and crystallization. In this study, we use synchrotron-based in situ x-ray scattering complemented by cryogenic transmission electron microscopy to obtain new insights from the earliest stages of struvite formation. The holistic scattering data captured the structure of an entire assembly in a time-resolved manner. The structural features comprise the aqueous medium, the growing struvite crystals, and any potential heterogeneities or complex entities. By analysing the scattering data, we found that the onset of crystallization causes a perturbation in the structure of the surrounding aqueous medium. This perturbation is characterized by the occurrence and evolution of Ornstein-Zernike fluctuations on a scale of about 1 nm, suggesting a non-classical nature of the system. We interpret this phenomenon as a liquid-liquid phase separation, which gives rise to the formation of the amorphous precursor phase preceding actual crystal growth of struvite. Our microscopy results confirm that the formation of Mg-struvite includes a short-lived amorphous phase, lasting >10 s.

Curious Behavior of Fe3+-As3+ Chemical Interactions and Nucleation of Clusters in Aqueous Medium

The simultaneous presence of Fe³⁺ and As³⁺ ions in groundwater (higher ppb or lower ppm level concentrations at circumneutral pH) as well as in acid mine drainages (AMDs)/industrial wastewater (up to few thousand ppm concentration at strongly acidic pH) are quite common. Therefore, understanding the chemical interactions prevalent between Fe³⁺ and As³⁺ ions in aqueous medium leading to nucleation of ionic clusters/solids, followed by aggregation and growth, is of great environmental significance. In the present work, we attempt to probe the nucleation process of Fe³⁺-As³⁺ clusters in solutions of various concentrations and pHs (from AMD to groundwater-like) using a combination of experimental and theoretical techniques. Interestingly, our study reveals nucleation of primary FeAs clusters in nearly all of them independent of concentration or pH. Theoretical studies employed density functional theory (DFT) to predict the primary clusters as stable Fe₄As₄ units. The surprising resemblance of these clusters with known Fe³⁺-As³⁺ minerals at the local level was observed experimentally, which provides an important clue about solid-phase growth from a range of Fe³⁺-As³⁺ solutions. Our experimental findings are further supported by a stepwise reaction mechanism established from detailed DFT studies.

Isolation of dissolved organic matter from aqueous solution by precipitation with FeCl3: mechanisms and significance in environmental perspectives

Ferric ions can bind strongly with dissolved organic matter (DOM), including humic acids (HA), fulvic acids (FA), and protein-like substances, whereas isolation of Fe-DOM precipitates (Fe-DOM_P) and their biochemical characteristics remain unclear. In this work FeCl₃ was used to isolate DOM components from various sources, including river, lake, soil, cow dung, and standard tryptophan and tyrosine, through precipitation at pH 7.5-8.5. The Fe-DOM_P contribute to total DOM by approximately 38.6-93.8% of FA, 76.2% of HA and 25.0-30.4% of tryptophan and tyrosine, whilst fluorescence spectra allowed to monitor/discriminate the various DOM fractions in the samples. The relative intensity of the main infrared peaks such as 3406‒3383 cm^-1 (aromatic OH), 1689‒1635 cm^-1 (‒COOH), 1523-1504 cm^-1 (amide) and 1176-1033 cm^-1 (‒S=O) show either to decline or disappear in Fe‒DOM_P. These results suggest the occurrence of Fe bonds with various functional groups of DOM, indicating the formation of π-d electron bonding systems of different strengths in Fe‒DOM_P. The novel method used for isolation of Fe-DOM_P shows promising in opening a new frontier both at laboratory and industrial purposes. Furthermore, results obtained may provide a better understanding of metal-organic complexes involved in the regulation of the long-term stabilization/sequestration of DOM in soils and waters.

Nanostructured Poly(3,4-ethylenedioxythiophene) Coatings on Functionalized Glass for Energy Storage

Conducting polymers rise among some of the most promising transparent supercapacitor electrode materials due to high conductivity, environmental stability, light weight, and ease of synthesis. A major challenge for depositing conducting polymers on a glass substrate is the lack of molecular interactions between organic and inorganic moieties resulting in poor adhesion and low cycling stability of the electrode. We present a synthetic approach by covalently linking poly(3,4-ethylyenedioxythiophene) (PEDOT) and glass through Friedel-Crafts alkylation on a self-assembled diphenyldimethoxysilane monolayer. This method obviates the need for a conductive FTO or ITO coating, enabling the fabrication of current collector-free planar supercapacitor electrodes on any glass surface. The electrode produced from our vapor-phase synthesis is coated with a highly conductive nanofibrillar PEDOT film (sheet resistance 2.1 Ω/□) possessing a gravimetric capacitance of ∼200 F/g. Our PEDOT planar supercapacitor possesses outstanding stability (86% capacitance retention after 50,000 cycles). We also fabricate a proof-of-concept transparent tandem supercapacitor on PEDOT-coated glass using 3D-printed frames that supplies enough voltage and current to light up a blue light-emitting diode (LED).

Revealing the crystal phases of primary particles formed during the coprecipitation of iron oxides

The mechanistic investigation of the coprecipitation formation of iron oxides has been a long-standing challenge due to the rapid reaction kinetics and high complexity of iron hydrolysis reactions. Although a few studies have suggested that the coprecipitation of iron oxide nanoparticles follows a non-classic route through inter-particle attachment, the compositions of the primary particles remain undetermined. Herein, by using a specially designed gas/liquid mixed phase fluidic reactor we controlled the reaction time from 3 s to over 5 min, and successfully identified the concentration of different intermediate phases as a function of time. We suggest that the initial Fe³⁺ ions are hydrolyzed under the alkaline condition to give Fe(OH)₃, which then rapidly dehydrates to yield α-FeOOH. In the presence of Fe²⁺ ions, which could also act as the catalyst, α-FeOOH finally transforms to Fe₃O₄.

Sunlight powered degradation of pentoxifylline Cs0.5Li0.5FeO2 as a green reusable photocatalyst: Mechanism, kinetics and toxicity studies

The degradation of Pentoxifylline (PXF) was achieved successfully by green energy in a built-in solar photocatalytic system using hybrid LiCs ferrites (Li_0.5Cs_0.5FeO₂) as magnetically recoverable photocatalysts. Kinetics showed a first-order reaction rate with maximum PXF removal of 94.91% at mildly acidic pH; additionally, the ferromagnetic properties of catalyst allowed recovery and reuse multiple times, reducing costs and time in degradation processes. The degradation products were identified by HPLC-MS and allowed us to propose a thermodynamically feasible mechanism that was validated through DFT calculations. Additionally, toxicity studies have been performed in bacteria and yeast where high loadings of Cs showed to be harmful to Staphylococcus aureus (MIC≥ 4.0 mg/mL); Salmonella typhi (MIC≥ 8.0 mg/mL) and Candida albicans (MIC≥ 10.0 mg/mL). The presented setup shows effectiveness and robustness in a degradation process using alternative energy sources for the elimination of non-biodegradable pollutants.

Wettability of Magnetite Nanoparticles Guides Growth from Stabilized Amorphous Ferrihydrite

Crystal formation via amorphous precursors is a long-sought-after gateway to engineer nanoparticles with well-controlled size and morphology. Biomineralizing organisms, like magnetotactic bacteria, follow such a nonclassical crystallization pathway to produce magnetite nanoparticles with sophistication unmatched by synthetic efforts at ambient conditions. Here, using in situ small-angle X-ray scattering, we demonstrate how the addition of poly(arginine) in the synthetic formation of magnetite nanoparticles induces a biomineralization-reminiscent pathway. The addition of poly(arginine) stabilizes an amorphous ferrihydrite precursor, shifting the magnetite formation pathway from thermodynamic to kinetic control. Altering the energetic landscape of magnetite formation by catalyzing the pH-dependent precursor attachment, we tune magnetite nanoparticle size continuously, exceeding sizes observed in magnetotactic bacteria. This mechanistic shift we uncover here further allows for crystal morphology control by adjusting the pH-dependent interfacial interaction between liquidlike ferrihydrite and nascent magnetite nanoparticles, establishing a new strategy to control nanoparticle morphology. Synthesizing compact single crystals at wetting conditions and unique semicontinuous single-crystalline nanoparticles at dewetting conditions in combination with an improved control over magnetite crystallite size, we demonstrate the versatility of bio-inspired, kinetically controlled nanoparticle formation pathways.

Formation and Evolution of Nanoscale Calcium Phosphate Precursors under Biomimetic Conditions

Simulated body fluids (SBFs) that mimic human blood plasma are widely used media for in vitro studies in an extensive array of research fields, from biomineralization to surface and corrosion sciences. We show that these solutions undergo dynamic nanoscopic conformational rearrangements on the timescale of minutes to hours, even though they are commonly considered stable or metastable. In particular, we find and characterize nanoscale inhomogeneities made of calcium phosphate (CaP) aggregates that emerge from homogeneous SBFs within a few hours and evolve into prenucleation species (PNS) that act as precursors in CaP crystallization processes. These ionic clusters consist of ∼2 nm large spherical building units that can aggregate into suprastructures with sizes of over 200 nm. We show that the residence times of phosphate ions in the PNS depend critically on the total PNS surface. These findings are particularly relevant for understanding nonclassical crystallization phenomena, in which PNS are assumed to act as building blocks for the final crystal structure.

Shaping Magnetite by Hydroxyl Group Numbers of Small Molecules

Despite numerous reports on magnetite formation with the assistance of various additives, the role of hydroxyl group (-OH) numbers in small polyol molecules has not yet been understood well. We selected small molecules containing different -OH numbers, such as ethanol, ethylene glycol, propanetriol, butanetetrol, pentitol, hexanehexol, and cyclohexanehexol, as additives in coprecipitation. By increasing the -OH number in these small polyol molecules, the formation of crystallization was slowed, and the size and shape of magnetite were regulated as well possibly due to the changed complexation strength and the stability of the precursor. The increase in temperature and the Fe²⁺/Fe³⁺ ratio can reduce the complexation strength. The nucleation and growth of magnetite proceed possibly through the aggregation of polyol-stabilized amorphous complexes and two-line ferrihydrite with low crystallinity based on the -OH numbers, suggesting a nonclassical pathway. The as-prepared magnetite showed a r₂/r₁ ratio after in vitro MRI measurement as follows: Fe₃O₄@He-6OH rod < Fe₃O₄@Pr-3OH sheet < Fe₃O₄@Pe-5OH cube. The Fe₃O₄@He-6OH rod and Fe₃O₄@Pr-3OH sheet displayed T₁-T₂ dual modal contrast ability, while the Fe₃O₄@Pe-5OH cube can be T₂-dominated. This research provides a simple but an essential approach for designing MRI contrast agents.

Impact of Solution Chemistry on Growth and Structural Features of Mo-Substituted Spinel Iron Oxides

The effect of crystallizing solution chemistry on the chemistry of subsequently as-grown materials was investigated for Mo-substituted iron oxides prepared by thermally activated co-precipitation. In the presence of Mo ions, we find that varying the oxidation state of the iron precursor from Fe(II) to Fe(III) causes a progressive loss of atomic long-range order with the stabilization of 2-4 nm particles for the sample prepared with Fe(III). The oxidation state of the Fe precursor also affects the distribution of Fe and Mo cations within the spinel structure. Increasing the Fe precursor oxidation state gives decreased Fe-ion occupation and increased Mo-ion occupation of tetrahedral sites, as revealed by the extended X-ray absorption fine structure. The stabilization of Mo within tetrahedral sites appears to be unexpected, considering the octahedral preferred coordination number of Mo(VI). The analysis of the atomic structure of the sample prepared with Fe(III) indicates a local ordering of vacancies and that the occupation of tetrahedral sites by Mo induces a contraction of the interatomic distances within the polyhedra as compared to Fe atoms. Moreover, the occupancy of Mo into the thermodynamic site preference of a Mo dopant in Fe₂O₃ assessed by density functional theory calculations points to a stronger preference for Mo substitution at octahedral sites. Hence, we suggest that the synthetized compound is thermodynamically metastable, that is, kinetically trapped. Such a state is suggested to be a consequence of the tetrahedral site occupation by Mo ions. The population of these sites, known to be reactive sites enabling particle growth, is concomitant with the stabilization of very small particles. We confirmed our hypothesis by using a blank experiment without Mo ions, further supporting the impact of tetrahedral Mo ions on the growth of iron oxide nanoparticles. Our findings provide new insights into the relationships between the Fe-chemistry of the crystallizing solution and the structural features of the as-grown Mo-substituted Fe-oxide materials.

Hydroxyl, Fe2+, and Acidithiobacillus ferrooxidans Jointly Determined the Crystal Growth and Morphology of Schwertmannite in a Sulfate-Rich Acidic Environment

Schwertmannite, ubiquitously found in iron and sulfate-rich acid mine drainage, is generated via biological oxidation of ferrous ions by Acidithiobacillus ferrooxidans (A. ferrooxidans). However, little information on the mechanisms of biogenic schwertmannite formation and crystal growth is available. This study deliberately investigated the relationships among mineral morphology, solution chemistry, and phase transformation of schwertmannite in A. ferrooxidans-containing ferrous sulfate solutions. The formation of schwertmannite could be divided into three stages. In the first nucleation stage, crystallites are presented as nonaggregative or aggregative forms via a successive polymerization process. In the second stage, ellipsoidal aggregates, which are identified as ferrihydrite and/or schwertmannite, are formed. In the third stage, needles appear on the surface of ellipsoidal aggregates, which is caused by the phase transformation of ferrihydrite or schwertmannite to lepidocrocite and goethite through a Fe2+ (aq) catalysis-driven pathway. After three stages, a typical characteristic "hedgehog" morphology finally appears. In addition, A. ferrooxidans could significantly speed up the mineral transformation. Solution pH affects the morphology of schwertmannite by acid leaching. The experimental results also reveal that the formation of schwertmannite depend on the content of hydroxyl complexes or the transformation of the monomers to polymers, which are greatly affected by the solution pH.

Multiple Pathways in NaCl Homogeneous Crystal Nucleation

NaCl crystal nucleation from metastable solutions has long been considered to occur according to a single-step mechanism where the growth in the size and crystalline order of the emerging nuclei...

Mechanistic insight into biopolymer induced iron oxide mineralization through quantification of molecular bonding

Microbial production of iron (oxyhydr)oxides on polysaccharide rich biopolymers occurs on such a vast scale that it impacts the global iron cycle and has been responsible for major biogeochemical events. Yet the physiochemical controls these biopolymers exert on iron (oxyhydr)oxide formation are poorly understood. Here we used dynamic force spectroscopy to directly probe binding between complex, model and natural microbial polysaccharides and common iron (oxyhydr)oxides. Applying nucleation theory to our results demonstrates that if there is a strong attractive interaction between biopolymers and iron (oxyhydr)oxides, the biopolymers decrease the nucleation barriers, thus promoting mineral nucleation. These results are also supported by nucleation studies and density functional theory. Spectroscopic and thermogravimetric data provide insight into the subsequent growth dynamics and show that the degree and strength of water association with the polymers can explain the influence on iron (oxyhydr)oxide transformation rates. Combined, our results provide a mechanistic basis for understanding how polymer-mineral-water interactions alter iron (oxyhydr)oxides nucleation and growth dynamics and pave the way for an improved understanding of the consequences of polymer induced mineralization in natural systems.

Versatile Process for the Preparation of Nanocomposite Sorbents: Phosphorus and Arsenic Removal

Nanomaterials are being increasingly utilized for environmental remediation. The use of these materials, however, is greatly hindered due to challenges in material handling and deployment. Here we present a novel nanocomposite synthesis method based on the direct growth of nanoparticles on and within solid support materials, referred to as Crescoating. In this work, iron and copper nanoparticles have been grown on polyurethane support materials using this process and applied as sorbents for dissolved phosphorus and arsenic in water, respectively. These nanocomposite sorbents exhibit rapid sorption with saturation occurring in less than 5 min. The loading capacity is 104.8 mg PO₄^3- g^-1 and 254.4 mg As(III) g^-1 for the iron and copper nanocomposite sorbents respectively, which is up to four times higher than commercially available alternatives. In addition, phosphorus can be recovered from the iron nanocomposite sorbent. This coating by growth process produces nanocomposites that do not emit particles and has the capability to be scaled and applied to other nanoparticles for diverse pollutant sorption applications.

Chemical trigger toward phase separation in the aqueous Al(III) system revealed

Although Al(III) hydrolysis, condensation, and nucleation play pivotal roles in the synthesis of Al-based compounds and determine their chemical behavior, we still lack experimental evidence regarding the chemistry of nucleation from solution. Here, by combining advanced titration assays, high-resolution transmission electron microscopy (HR-TEM), and ²⁷Al-nuclear magnetic resonance spectroscopy, we show that highly dynamic solute prenucleation clusters (PNCs) are fundamental precursors of nanosolid formation. Chemical changes from olation to oxolation bridging within PNCs rely on the formation of tetrahedral AlO₄ in solution and trigger phase separation at low driving force (supersaturation). This does not include the formation of Keggin-Al₁₃ ions, at least during the earliest stages. The PNC pathway of the formation of Al(III) (oxy)(hydr)oxides offers new possibilities toward the development of strategies for controlling the entire crystallization process.

Assessing the Onset of Calcium Phosphate Nucleation by Hyperpolarized Real-Time NMR

We report an experimental approach for high-resolution real-time monitoring of transiently formed species occurring during the onset of precipitation of ionic solids from solution. This is made possible by real-time nuclear magnetic resonance (NMR) monitoring using dissolution dynamic nuclear polarization (D-DNP) to amplify signals of functional intermediates and is supported by turbidimetry, cryogenic electron microscopy, and solid-state NMR measurements. D-DNP can provide drastic signal improvements in NMR signal amplitudes, permitting dramatic reductions in acquisition times and thereby enabling us to probe fast interaction kinetics such as those underlying formation of prenucleation species (PNS) that precede solid–liquid phase separation. This experimental strategy allows for investigation of the formation of calcium phosphate (CaP)-based minerals by ³¹P NMR—a process of substantial industrial, geological, and biological interest. Thus far, many aspects of the mechanisms of CaP nucleation remain unclear due to the absence of experimental methods capable of accessing such processes on sufficiently short time scales. The approach reported here aims to address this by an improved characterization of the initial steps of CaP precipitation, permitting detection of PNS by NMR and determination of their formation rates, exchange dynamics, and sizes. Using D-DNP monitoring, we find that under our conditions (i) in the first 2 s after preparation of oversaturated calcium phosphate solutions, PNS with a hydrodynamic radius of R_h ≈ 1 nm is formed and (ii) following this rapid initial formation, the entire crystallization processes proceed on considerably longer time scales, requiring >20 s to form the final crystal phase.

Strategy to control magnetic coercivity by elucidating crystallization pathway-dependent microstructural evolution of magnetite mesocrystals

Mesocrystals are assemblies of smaller crystallites and have attracted attention because of their nonclassical crystallization pathway and emerging collective functionalities. Understanding the mesocrystal crystallization mechanism in chemical routes is essential for precise control of size and microstructure, which influence the function of mesocrystals. However, microstructure evolution from the nucleus stage through various crystallization pathways remains unclear. We propose a unified model on the basis of the observation of two crystallization pathways, with different ferric (oxyhydr)oxide polymorphs appearing as intermediates, producing microstructures of magnetite mesocrystal via different mechanisms. An understanding of the crystallization mechanism enables independent chemical control of the mesocrystal diameter and crystallite size, as manifested by a series of magnetic coercivity measurements. We successfully implement an experimental model system that exhibits a universal crystallite size effect on the magnetic coercivity of mesocrystals. These findings provide a general approach to controlling the microstructure through crystallization pathway selection, thus providing a strategy for controlling magnetic coercivity in magnetite systems.

Transient metastable intermediates play an important role in the crystallization process. Here, the authors unveil the microstructural changes in magnetite mesocrystals that depend on the intermediate polymorphism and the universal crystallite size effect on the magnetic coercivity of mesocrystals.

Recent experimental explorations of non-classical nucleation

This work provides a clearer picture for non-classical nucleation by revealing the presence of various intermediates using advanced characterization techniques.

Stabilizing Reactive Fe(III) Clusters by Freeze-Dry/Solvent-Exchange To Benchmark Iron Hydrolysis Pathways

Isolating Fe(III) clusters from water without stabilizing ligands is significantly challenged by the high acidity of Fe³⁺-bound water, leading to uncontrolled precipitation of iron oxyhydroxides. Here we demonstrate a freeze-drying solvent-exchange method that enabled the isolation of a metastable Fe(III) sulfate decameric cluster formulated [Fe₁₀O₂(SO₄)₁₂(OCH₃)₂]·14CH₃OH (Fe₁₀). Without stabilization by solvent-exchange, the aqueous species undergoes rapid conversion to the iron sulfate mineral schwertmannite. Monitoring the hydrolysis process from cluster intermediates to schertmannite by small-angle X-ray scattering, we observe the progression from Fe₁₀ to 37 Å soluble nanoparticles prior to the precipitation process. This condensation behavior of Fe₁₀ is further exploited to develop a simple laboratory synthesis of schwetmannite. In addition, we demonstrate the versatility of the freeze-drying solvent-exchange method by isolating Al(III), Zn(II), and Cd(II) substituted Fe(III) sulfate clusters. The freeze-drying solvent-exchange method provides a unique opportunity to isolate cluster intermediates and models to aid in our understanding of metal-ion hydrolysis processes in environmental, material science, and geological studies.

Unravelling the growth mechanism of the co-precipitation of iron oxide nanoparticles with the aid of synchrotron X-Ray diffraction in solution

Co-precipitation is the most ubiquitous method for forming iron oxide nanoparticles. For a typical co-precipitation synthesis, the pH of a ferrous and/or ferric ion solution is increased via the addition of a base. The latter can be added either slowly (a steady addition over either minutes or hours) or fast (a one-time addition) resulting in an abrupt increase in the pH. However, understanding the mechanism of particle formation is still lacking, which limits the reproducibility of the co-precipitation reaction due to intermediate phases still being present in the final product. In this work, we study in detail a co-precipitation synthesis with an abrupt increase in pH via the addition of sodium carbonate. Fast and reproducible mixing at defined precursor and base solution temperatures was achieved utilising a flow reactor. Transmission electron microscopy, electron diffraction and room temperature 57Fe Mössbauer spectroscopy showed a distinct transition from an amorphous ferrihydrite phase to a mixture of magnetite-maghemite (Fe3O4/γ-Fe2O3). Synchrotron X-ray diffraction revealed the initial formation of crystalline iron hydroxide carbonate (green rust) plates occurring before the Fe3O4/γ-Fe2O3 appeared. The ferrihydrite particles increase in size over time as the proportion of iron hydroxide carbonate plates are re-dissolved into solution, until the ferrihydrite particles crystallise into Fe3O4/γ-Fe2O3.

Colloidal FeIII , MnIII , CoIII , and CuII Hydroxides Stabilized by Starch as Catalysts of Water Oxidation Reaction with One Electron Oxidant Ru(bpy)3 3

Colloidal catalysts for oxidation of water to dioxygen, which are stable on storage and under the reaction conditions, are synthesized based on Co^III , Mn^III , Fe^III and Cu^II hydroxides. Stabilization of the colloids with dextrated starch allows the process of hydroxide ageing to be stopped at the stage of the formation of primary nuclei (ca. 2-3 nm from transmission electron microscopy data). Molecular mechanics and dynamic light scattering studies indicate a core-shell type structure of the catalysts, where the hydroxide core is stabilized by the molecular starch network (ca. 5-7 nm). The colloidal catalysts are highly efficient in oxidizing water with one electron oxidant Ru(bpy)₃ ³⁺ at pH 7 to 10. The influence of pH, catalyst concentration, and buffer nature on the oxygen yield is studied. The maximal yields are 72, 53, and 78 % over Fe-, Mn- and Co-containing catalysts, respectively, and turnover numbers are 7.8; 54 and 360, respectively. The Cu-containing catalyst is poorly effective to the water oxidation (the maximal yield is 28 % O₂ ). The synthesized catalysts are of interest for stopped-flow kinetic studies of the mechanism of the water oxidation and as precursors for anchoring nanosized hydroxides onto various supports in order to develop biomimetic systems for artificial photosynthesis.

From Solute, Fluidic and Particulate Precursors to Complex Organizations of Matter

The organization of matter from its constitutive units recruits intermediate states with distinctive degrees of self-association and molecular order. Existing as clusters, droplets, gels as well as amorphous and crystalline nanoparticles, these precursor forms have fundamental contributions towards the composition and structure of inorganic and organic architectures. In this personal account, we show that the transitions from atoms, molecules or ionic species to superstructures of higher order are intertwined with the interfaces and interactions of precursor and intermediate states. Structural organizations distributed across different length scales are explained by the multistep nature of nucleation and crystallization, which can be guided towards functional hybrid materials by the strategic application of additives, templates and reaction environments. Thus, the non-classical pathways for material formation and growth offer conceptual frameworks for elucidating, inducing and directing fascinating material organizations of biogenic and synthetic origins.

How Can Additives Control the Early Stages of Mineralisation?

The interactions between additives and mineral precursors and intermediates are at the heart of additive-controlled crystallisation, which is of high importance for various fields. In this commentary, we reflect on potential modes of additive control according to classical nucleation theory on one hand, and from the viewpoint of the so-called pre-nucleation cluster pathway on the other. This includes a brief review of the corresponding literature. While the roles of additives are discussed generally, i.e., without specific chemical or structural details, corresponding properties are outlined where possible. Altogether, our discussion illustrates that “non-classical” nucleation pathways promise an improved understanding of additive-controlled scenarios, which could be utilised in targeted applications in various fields, ranging from scale inhibition to materials chemistry.

The Role of Bi3+ in Promoting and Stabilizing Iron Oxo Clusters in Strong Acid

Metal oxo clusters and metal oxides assemble and precipitate from water in processes that depend on pH, temperature, and concentration. Other parameters that influence the structure, composition, and nuclearity of "molecular" and bulk metal oxides are poorly understood, and have thus not been exploited. Herein, we show that Bi³⁺ drives the formation of aqueous Fe³⁺ clusters, usurping the role of pH. We isolated and structurally characterized a Bi/Fe cluster, Fe₃ BiO₂ (CCl₃ COO)₈ (THF)(H₂ O)₂ , and demonstrated its conversion into an iron Keggin ion capped by six Bi³⁺ irons (Bi₆ Fe₁₃ ). The reaction pathway was documented by X-ray scattering and mass spectrometry. Opposing the expected trend, increased cluster nuclearity required a pH decrease instead of a pH increase. We attribute this anomalous behavior of Bi/Fe(aq) solutions to Bi³⁺ , which drives hydrolysis and condensation. Likewise, Bi³⁺ stabilizes metal oxo clusters and metal oxides in strongly acidic conditions, which is important in applications such as water oxidation for energy storage.

Structures and infrared spectra of calcium phosphate clusters by ab initio methods with implicit solvation models

Since the first detection of pre-nucleation clusters during the formation of calcium phosphate minerals, determining such clusters' compositions and structures has become crucial for understanding the early-stage nucleation of these minerals in solutions. In previous experimental studies, the composition and sizes of pre-nucleation clusters have been calculated, but their structural information has been difficult to determine because they are very small (<1 nm). In this study, we examined the structures and infrared spectra of small- and medium-sized calcium phosphate clusters using ab initio calculations combined with implicit solvation models. Adding solvent effects increased the possibility of the existence of alternative configurations of calcium phosphate clusters other than their compact configurations. The calcium atoms had a tendency to be located outside of the clusters to coordinate with water molecules in the aqueous environment. The computed infrared spectra of extended small calcium phosphate clusters captured some of the features measured in the in situ infrared spectra, which supports the network structures proposed by large-scale molecular dynamics studies and X-ray adsorption near-edge spectra. The relative stabilities of medium-sized Ca₉(PO₄)₆ clusters with respect to the stability of Posner's cluster in water were also reviewed. We found that in water, alternative structures with low symmetry or large dipole moments had lower energies than Posner's cluster.

In situ microscopy across scales for the characterization of crystal growth mechanisms: the case of europium oxalate

The development of targeted syntheses requires a better understanding of how production pathways affect the final product, but many ex situ techniques used for studying nanoparticle growth are unsuitable as standalone methods for identifying and characterizing growth mechanisms.

Alignment of Amorphous Iron Oxide Clusters: A Non-Classical Mechanism for Magnetite Formation

Despite numerous studies on the nucleation and crystallization of iron (oxyhydr)oxides, the roles of species developing during the early stages, especially primary clusters and intermediate amorphous particles, are still poorly understood. Herein, both ligand-free and ligand-protected amorphous iron oxide (AIO) clusters (<2 nm) were synthesized as precursors for magnetite formation. Thermal annealing can crystallize the clusters into magnetite particles, and AIO bulk phases with domains of pre-aligned clusters are found to be direct precursors to crystals, suggesting a non-classical aggregation-based pathway that differs from the reported oriented attachment or particle accretion mechanisms.

Crystallization kinetics of cerium oxide nanoparticles formed by spontaneous, room-temperature hydrolysis of cerium(iv) ammonium nitrate in light and heavy water

Ceria nanocrystals form tenfold more slowly in D₂O vs. H₂O, revealing a rate-determining proton transfer reaction and a non-classical crystallization mechanism.

Kinetic Pathways and Mechanisms of Two-Step Nucleation in Crystallization

Crystallizations often pass through multiple intermediate structures before reaching the final state, such as amorphous precursors, polymorphs, or denser liquid droplets. However, the atomistic pathways from these metastable phases to final crystals still remain unclear. Here, we investigated the structure evolution process from liquid to final crystals of homogeneous nucleation by atomic-scale simulations and analyzed the intrinsic mechanisms that influence the nucleation pathways. Three different pathways of two-step nucleation were found by visualizing the precursors' evolutions, and some new micromechanisms of two-step nucleation are revealed. We suggest that the solid bond fluctuations can trigger the formation of intermediate precursors, while the precursors' packing density dominates the structural transformation pathways from intermediate phases to crystals. These findings not only shed light on the mechanisms of nucleation but also provide guidance for future refinements of two-step nucleation theory.

Polyaspartic acid facilitates oxolation within iron(iii) oxide pre-nucleation clusters and drives the formation of organic-inorganic composites

The interplay between polymers and inorganic minerals during the formation of solids is crucial for biomineralization and bio-inspired materials, and advanced material properties can be achieved with organic-inorganic composites. By studying the reaction mechanisms, basic questions on organic-inorganic interactions and their role during material formation can be answered, enabling more target-oriented strategies in future synthetic approaches. Here, we present a comprehensive study on the hydrolysis of iron(iii) in the presence of polyaspartic acid. For the basic investigation of the formation mechanism, a titration assay was used, complemented by microscopic techniques. The polymer is shown to promote precipitation in partly hydrolyzed reaction solutions at the very early stages of the reaction by facilitating iron(iii) hydrolysis. In unhydrolyzed solutions, no significant interactions between the polymer and the inorganic solutes can be observed. We demonstrate that the hydrolysis promotion by the polymer can be understood by facilitating oxolation in olation iron(iii) pre-nucleation clusters. We propose that the adsorption of olation pre-nucleation clusters on the polymer chains and the resulting loss in dynamics and increased proximity of the reactants is the key to this effect. The resulting composite material obtained from the hydrolysis in the presence of the polymer was investigated with additional analytical techniques, namely, scanning and transmission electron microscopies, light microscopy, atomic force microscopy, zeta potential measurements, dynamic light scattering, and thermogravimetric analyses. It consists of elastic, polydisperse nanospheres, ca. 50-200 nm in diameter, and aggregates thereof, exhibiting a high polymer and water content.