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.     

Thermodynamics and Kinetics of Prenucleation Clusters,Classical and Non‐Classical Nucleation

Recent observations of prenucleation species and multi‐stage crystal nucleation processes challenge the long‐established view on the thermodynamics of crystal formation. Here, we review and generalize extensions to classical nucleation theory. Going beyond the conventional implementation as has been used for more than a century now, nucleation inhibitors, precursor clusters and non‐classical nucleation processes are rationalized as well by analogous concepts based on competing interface and bulk energy terms. This is illustrated by recent examples of species formed prior to/instead of crystal nucleation and multi‐step nucleation processes. Much of the discussed insights were obtained from molecular simulation using advanced sampling techniques, briefly summarized herein for both nucleation‐controlled and diffusion‐controlled aggregate formation.     

Controlled Aqueous Growth of Hematite Nanoplate Arrays Directly on Transparent Conductive Substrates and Their Photoelectrochemical Properties

Two‐dimensional (2D) hematite nanoplate arrays were synthesized directly on fluorine‐doped tin oxide (FTO)‐coated glass by using a facile and novel hydrothermal method. High‐temperature annealing retained the morphology of the nanoplate arrays while simultaneously introducing porosity. The thickness and length of the nanoplates could be tailored by adjusting the precursor composition. Photoelectrochemical (PEC) measurements showed that the photocurrent generated with bare hematite nanoplate photoelectrode under backside illumination was about four times of that under frontside illumination in the entire bias range used, which suggested that slow electron transport was a limiting factor for its PEC performance. Upon Sn doping and Co‐Pi co‐catalyst addition, the photocurrent increased significantly owing to the enhancement of electron conductivity and oxidation kinetics. Electrochemical impedance spectroscopy (EIS) measurements were conducted to understand the surface properties of the nanoplate arrays. Since this strategy is simple, cost‐effective, and highly reproducible, it provides exciting opportunities for the large‐scale growth of porous 2D metal oxide photoelectrodes for a variety of photoelectrochemical and photocatalytic applications.     

Utilization of Hematite Particles for Economical Removal of o-xylene in a High-Temperature Gas-Solid Reactor

To establish a novel approach for VOCs resource utilization, coupled o-xylene oxidation and hematite reduction was investigated in this study in a high-temperature gas-solid reactor in the temperature range 300–700 °C. As the o-xylene-containing inert gas (N₂) stream traveled through the hematite particle bed, its reaction behavior was determined in programmed heating and constant temperature modes. Consequently, the effect of bed temperature, flow rate and o-xylene inlet concentration on both o-xylene removal performance and degree of hematite reduction was studied. The raw hematite and solid products were analyzed by TGA, XRF, XRD and SEM-EDS. The results showed that a temperature above 300 °C was required to completely eliminate o-xylene by hematite, and both o-xylene removal capacity and degree of hematite reduction at 5% breakthrough points enhanced on increasing the temperature and decreasing the flow rate. The increment in temperature from 300 °C to 700 °C led to a gradual reduction of Fe₂O₃ to Fe₃O₄, FeO and metallic iron. Thus, this study provides a novel, economic and promising technology for treating the VOC pollutants.     

Particle formation: Clusters and nuclei

The study of fine particles is a growing area of interest in both the academic and industrial communities. Through investigation of the formation and properties of clusters one can elucidate mechanisms of nucleation and investigate the changing properties of matter as it transcends from gas-like through surface dominated and ultimately to bulk state behavior. This paper provides an overview of the importance of the field and shows examples of how research on small clusters is providing insight into diverse phenomena such as nucleation, changing electronic properties with cluster size, and concomitant influences on reactions.     

The Mechanism of the Thermal Transformation From Goethite to Hematite

Synthetic pigments of goethite (Bayferrox^R) of different particle size were investigated by DTA, IR, DSC, TG and X-ray diffraction measurements. It follows that a so-called ‘hydrohematite’ described in the literature does not exist as a discrete intermediate during the dehydration course from goethite to hematite. Instead we observed a dependence of the dehydration mechanism on the particle size. Transformation enthalpies and activation energies for the dehydration process will be given. A plausible dehydration mechanism, which is compatible with our DTA/DSC results, is deduced from TEM investigations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Acid Treatment Enables Suppression of Electron–Hole Recombination in Hematite for Photoelectrochemical Water Splitting

We report a strategy for efficient suppression of electron–hole recombination in hematite photoanodes. Acid‐treated hematite showed a substantially enhanced photocurrent density compared to untreated samples. Electrochemical impedance spectroscopy studies revealed that the enhanced photocurrent is partly due to improved efficiency of charge separation. Transient absorption spectroscopic studies coupled to electrochemical measurements indicate that, in addition to improved bulk electrochemical properties, acid‐treated hematite has significantly decreased surface electron–hole recombination losses owing to a greater yield of the trapped photoelectrons being extracted to the external circuit.     

Sub-nanometer,Ultrafine α-Fe2O3 Sheets Realized by Controlled Crystallization Kinetics for Stable,High-Performance Energy Storage

The development of energy devices based on iron oxides/hydroxides is largely hindered by their poor conductivity and large volume changes, especially with regard to specific capacitance and cycle stability. Herein, superior capacitance (1575 F g⁻¹ at 1.25 A g⁻¹) and high rate performance (955 F g⁻¹ at 25 A g⁻¹) were realized by synthesizing sub-nanometer, ultrafine α-Fe₂O₃ sheets loaded on graphene (SU-Fe₂O₃-rGO). An assembled asymmetric supercapacitor showed outstanding cycle stability (106 % retention after 30 000 cycles). This excellent performance arises from the unique structural characteristics of the α-Fe₂O₃ sheets, which not only enrich electrochemically reactive sites, but also largely eliminate the volume changes after long-term charge/discharge cycling. The synthesis of SU-Fe₂O₃-rGO critically depends on control of the crystallization kinetics during growth. A controlled heterogeneous nucleation mechanism results in the formation of atomically thin α-Fe₂O₃ sheets on graphene rather than large particles in solvent, as clarified by theoretical calculations. This strategy paves a new way to synthesizing atomically thin transition metal oxide sheets and low-cost, eco-friendly iron-based energy storage.     

One-pot aqueous solution syntheses of iron oxide nanostructures with controlled crystal phases through a microbial-mineralization-inspired approach

Iron-oxidizing bacteria produce trivalent iron oxides with the controlled crystal phases outside of their cells. Herein we have synthesized iron oxides with controlled oxidation states and crystal phases through a microbial-mineralization-inspired approach in an aqueous solution at low temperature. Trivalent iron oxides, such as lepidocrocite, ferrihydrite, goethite, and hematite, are selectively obtained from an aqueous solution containing divalent iron ions below 90 °C. The presence of a chelating agent facilitates the control of the oxidation states through the ligand-controlled approach because the precipitation of the divalent iron species is inhibited by the complexation between divalent iron ion and a chelating agent. The control of the crystal phases is achieved by the tuning of the synthetic conditions, such as the initial pH, the concentration of a chelating agent, and the reaction temperature. Furthermore, the resultant iron oxides have hierarchically organized structures consisting of nanoscale objects. The microbial-mineralization-inspired approach by using a chelating agent has potentials for the further morphological control of iron oxides and the further application to aqueous-solution syntheses of other metal oxides.     

A High-Efficiency Hematite Photoanode with Enhanced Bonding Energy Around Fe Atoms

Hematite nanoarrays are important photoanode materials. However, they suffer from serious problems of charge transfer and surface states; in particular, the surface states hinder the increase in photocurrent. A previous strategy to suppress the surface state is the deposition of an Fe-free metal oxide overlayer. Herein, from the viewpoint of atomic bonding energy, it is found that the strength of bonding around Fe atoms in the hematite is the key to suppressing the surface states. By treating the surface of hematite with Se and NaBH₄, the Fe₂O₃ transforms to a double-layer nanostructure. In the outer layer, the Fe−O bonding is reinforced and the Fe−Se bonding is even stronger. Therefore, the surface states are inhibited and the increase in the photocurrent density becomes much faster. Besides, the treatment constructs a nanoscale p–n junction to promote the charge transfer. Improvements are achieved in onset potential (0.25 V shift) and in photocurrent density (5.8 times). This work pinpoints the key to suppressing the surface states and preparing a high-efficiency hematite nanoarray, and deepens our understanding of hematite photoanodes.     

Laser Control of Zeolite Nucleation

Precursor solutions for the synthesis of zeolites are irradiated by means of a Nd‐YAG laser. These solutions are subsequently submitted to a hydrothermal treatment and the results analyzed by X‐ray diffraction and electron microscopy. Laser irradiation promotes the formation of silica nanoparticles that nucleate into zeolite (silicalite‐1), following a hydrothermal treatment. The average crystal size (in the 0.6–3.6 μm range) of the zeolite exponentially decreases as a function of laser irradiation time. In addition, a longer irradiation time results in a narrower crystal size distribution.     

3D Study of the Morphology and Dynamics of Zeolite Nucleation

The principle aspects and constraints of the dynamics and kinetics of zeolite nucleation in hydrogel systems are analyzed on the basis of a model Na‐rich aluminosilicate system. A detailed time‐series EMT‐type zeolite crystallization study in the model hydrogel system was performed to elucidate the topological and temporal aspects of zeolite nucleation. A comprehensive set of analytical tools and methods was employed to analyze the gel evolution and complement the primary methods of transmission electron microscopy (TEM) and nuclear magnetic resonance (NMR) spectroscopy. TEM tomography reveals that the initial gel particles exhibit a core–shell structure. Zeolite nucleation is topologically limited to this shell structure and the kinetics of nucleation is controlled by the shell integrity. The induction period extends to the moment when the shell is consumed and the bulk solution can react with the core of the gel particles. These new findings, in particular the importance of the gel particle shell in zeolite nucleation, can be used to control the growth process and properties of zeolites formed in hydrogels.     

High-pressure Nucleation of Low-Density Polymorphs**

New polymorphs β and γ of bis-3-nitrophenyl disulphide, crystallized above 0.3 GPa, are less dense than the ambient-pressure polymorph α. This counterintuitive density relation results from the high-entropy nucleation and subsequent kinetic crystallization. The work performed by pressure compensates the high entropy and temperature product, substantiated in varied conformers and increased chemical potential. Pressure-increased viscosity promotes the kinetic polymorphs, in accordance with empirical Ostwald's rule of stages. It contrasts to mechanochemical techniques, favouring high-density polymorphs.     

Nucleation,growth, and fractal dimensions of icosahedral and polyicosahedral clusters

In this paper, the concept offractal geometry was applied to the growth of icosahedral and polyicosahedral clusters. Fractal dimensions were calculated for high-frequency icosahedral casters, vertex-sharing polyicosahedral clusters, and linked polyicosahedral clusters. These cluster growth pathways were compared with the fractal growth mechanisms of colloidal particles. Close similarities were found despite the tremendous differences in particle sizes and the forces governing their nucleation and growth processes.Dedicated to Prof. Lawrence F. Dahl on his 65th birthday; based in part on a lecture presented by BKT at the Dahl symposium held at the University of Wisconsin, Madison, Wisconsin on September 17, 1994.     

Liquid Metastable Precursors of Ibuprofen as Aqueous Nucleation Intermediates

The nucleation mechanism of crystals of small organic molecules, postulated based on computer simulations, still lacks experimental evidence. In this study we designed an experimental approach to monitor the early stages of the crystallization of ibuprofen as a model system for small organic molecules. Ibuprofen undergoes liquid–liquid phase separation prior to nucleation. The binodal and spinodal limits of the corresponding liquid–liquid miscibility gap were analyzed and confirmed. An increase in viscosity sustains the kinetic stability of the dense liquid intermediate. Since the distances between ibuprofen molecules within the dense liquid phase are similar to those in the crystal forms, this dense liquid phase is identified as a precursor phase in the nucleation of ibuprofen, in which densification is followed by generation of structural order. This discovery may make it possible to enrich poorly soluble pharmaceuticals beyond classical solubility limitations in aqueous environments.     

Nucleation and Growth of CeF3 and NaCeF4 Nanocrystals

Studying growth : The diffusion‐controlled kinetic (DCK) model and the surface chemical thermodynamics (SCT) model have been successfully applied to interpret the nucleation and growth mechanism of CeF₃ (see TEM images) and NaCeF₄ nanocrystals, and may generally give light to the size‐control and morphology prediction of rare‐earth fluorides at the nanoscale level.

     

Nonclassical nucleation towards separation and recycling science: Iron and aluminium (Oxy)(hydr)oxides

Separation, analysis and recycling technologies are of high interest for our modern societies, where colloidal iron and aluminium (hydr)oxides have important applications. However, there are significant gaps in the fundamental understanding of how these phases form in real systems. Classical nucleation theory cannot account for many experimental observations, and there is a dichotomy between the chemistry of hydrolysing/condensating systems and the physical notion of supersaturation. Reviewing parts of the established and recent literature, we demonstrate that concepts of nonclassical nucleation pathways can overcome these issues. This broader, chemistry-based conceptual framework has a high potential for advancing current applications, and developing new strategies towards separation, analysis and recycling applications, which seem to be urgently required for the future.