Mechanisms for iron oxide formation under hydrothermal conditions: an in situ total scattering study

MLstructureMining: a machine learning tool for structure identification from X-ray pair distribution functions

Synchrotron X-ray techniques are essential for studies of the intrinsic relationship between synthesis, structure, and properties of materials. Modern synchrotrons can produce up to 1 petabyte of data per day. Such amounts of data can speed up materials development, but also comes with a staggering growth in workload, as the data generated must be stored and analyzed. We present an approach for quickly identifying an atomic structure model from pair distribution function (PDF) data from (nano)crystalline materials. Our model, MLstructureMining, uses a tree-based machine learning (ML) classifier. MLstructureMining has been trained to classify chemical structures from a PDF and gives a top-3 accuracy of 99% on simulated PDFs not seen during training, with a total of 6062 possible classes. We also demonstrate that MLstructureMining can identify the chemical structure from experimental PDFs from nanoparticles of CoFe2O4 and CeO2, and we show how it can be used to treat an in situ PDF series collected during Bi2Fe4O9 formation. Additionally, we show how MLstructureMining can be used in combination with the well-known methods, principal component analysis (PCA) and non-negative matrix factorization (NMF) to analyze data from in situ experiments. MLstructureMining thus allows for real-time structure characterization by screening vast quantities of crystallographic information files in seconds.

From Gel to Crystal: Mechanism of HfO2 and ZrO2 Nanocrystal Synthesis in Benzyl Alcohol

Nonaqueous sol-gel syntheses have been used to make many types of metal oxide nanocrystals. According to the current paradigm, nonaqueous syntheses have slow kinetics, thus favoring the thermodynamic (crystalline) product. Here we investigate the synthesis of hafnium (and zirconium) oxide nanocrystals from the metal chloride in benzyl alcohol. We follow the transition from precursor to nanocrystal through a combination of rheology, EXAFS, NMR, TEM, and X-ray total scattering (PDF analysis). Upon dissolving the metal chloride precursor, the exchange of chloride ligands for benzylalkoxide liberates HCl. The latter catalyzes the etherification of benzyl alcohol, eliminating water. During the temperature ramp to the reaction temperature (220 °C), sufficient water is produced to turn the reaction mixture into a macroscopic gel. Rheological analysis shows a network consisting of strong interactions with temperature-dependent restructuring. After a few minutes at the reaction temperature, crystalline particles emerge from the gel, and nucleation and growth are complete after 30 min. In contrast, 4 h are required to obtain the highest isolated yield, which we attribute to the slow in situ formation of water (the extraction solvent). We used our mechanistic insights to optimize the synthesis, achieving high isolated yields with a reduced reaction time. Our results oppose the idea that nonaqueous sol-gel syntheses necessarily form crystalline products in one step, without a transient, amorphous gel state.

The Chemistry of Spinel Ferrite Nanoparticle Nucleation, Crystallization, and Growth

The nucleation, crystallization, and growth mechanisms of MnFe2O4, CoFe2O4, NiFe2O4, and ZnFe2O4 nanocrystallites prepared from coprecipitated transition metal (TM) hydroxide precursors treated at sub-, near-, and supercritical hydrothermal conditions have been studied by in situ X-ray total scattering (TS) with pair distribution function (PDF) analysis, and in situ synchrotron powder X-ray diffraction (PXRD) with Rietveld analysis. The in situ TS experiments were carried out on 0.6 M TM hydroxide precursors prepared from aqueous metal chloride solutions using 24.5% NH4OH as the precipitating base. The PDF analysis reveals equivalent nucleation processes for the four spinel ferrite compounds under the studied hydrothermal conditions, where the TMs form edge-sharing octahedrally coordinated hydroxide units (monomers/dimers and in some cases trimers) in the aqueous precursor, which upon hydrothermal treatment nucleate through linking by tetrahedrally coordinated TMs. The in situ PXRD experiments were carried out on 1.2 M TM hydroxide precursors prepared from aqueous metal nitrate solutions using 16 M NaOH as the precipitating base. The crystallization and growth of the nanocrystallites were found to progress via different processes depending on the specific TMs and synthesis temperatures. The PXRD data show that MnFe2O4 and CoFe2O4 nanocrystallites rapidly grow (typically <1 min) to equilibrium sizes of 20-25 nm and 10-12 nm, respectively, regardless of applied temperature in the 170-420 °C range, indicating limited possibility of targeted size control. However, varying the reaction time (0-30 min) and temperature (150-400 °C) allows different sizes to be obtained for NiFe2O4 (3-30 nm) and ZnFe2O4 (3-12 nm) nanocrystallites. The mechanisms controlling the crystallization and growth (nucleation, growth by diffusion, Ostwald ripening, etc.) were examined by qualitative analysis of the evolution in refined scale factor (proportional to extent of crystallization) and mean crystallite volume (proportional to extent of growth). Interestingly, lower kinetic barriers are observed for the formation of the mixed spinels (MnFe2O4 and CoFe2O4) compared to the inverse (NiFe2O4) and normal (ZnFe2O4) spinel structured compounds, suggesting that the energy barrier for formation may be lowered when the TMs have no site preference.

Influence of the Precursor Structure on the Formation of Tungsten Oxide Polymorphs

Understanding material nucleation processes is crucial for the development of synthesis pathways for tailormade materials. However, we currently have little knowledge of the influence of the precursor solution structure on the formation pathway of materials. We here use in situ total scattering to show how the precursor solution structure influences which crystal structure is formed during the hydrothermal synthesis of tungsten oxides. We investigate the synthesis of tungsten oxide from the two polyoxometalate salts, ammonium metatungstate, and ammonium paratungstate. In both cases, a hexagonal ammonium tungsten bronze (NH4)0.25WO3 is formed as the final product. If the precursor solution contains metatungstate clusters, this phase forms directly in the hydrothermal synthesis. However, if the paratungstate B cluster is present at the time of crystallization, a metastable intermediate phase in the form of a pyrochlore-type tungsten oxide, WO3·0.5H2O, initially forms. The pyrochlore structure then undergoes a phase transformation into the tungsten bronze phase. Our studies thus experimentally show that the precursor cluster structure present at the moment of crystallization directly influences the formed crystalline phase and suggests that the precursor structure just prior to crystallization can be used as a tool for targeting specific crystalline phases of interest.

Influence of organic ligands on the stoichiometry of magnetite nanoparticles

Magnetite, a ubiquitous mineral in natural systems, is of high interest for a variety of applications including environmental remediation, medicine, and catalysis. If the transformation of magnetite to maghemite through the oxidation of Fe2+ has been well documented, mechanisms involving dissolution processes of Fe2+ in aqueous solutions have been overlooked. Here, the effect of dissolved organic ligands (EDTA (ethylenediaminetetraacetic acid), acetic, lactic and citric acids) on Fe2+ solubility and on the stoichiometry (Fe(ii)/Fe(iii)) of magnetite-maghemite nanoparticles (∼10 nm) was investigated. These ligands were chosen because of their environmental relevance and because they are widely used as coating agents for nanotechnology applications. Results show an insignificant effect of 2 organic ligands (acetate and lactate) on the dissolution of Fe. By contrast, citrate and EDTA enhanced Fe solubility because of the formation of dissolved Fe(ii)- and Fe(iii)-ligand complexes. Both ligands selectively bound Fe(ii) over Fe(iii), but EDTA was much more selective than citrate. The combined effects of oxidation and H+- and ligand-promoted dissolution of Fe from magnetite were predicted using a magnetite-maghemite solid solution model, accounting for the formation of dissolved Fe(ii)- and Fe(iii)-ligand complexes. Therefore, these results show that citrate and EDTA (i) enhance Fe solubility in the presence of magnetite nanoparticles and (ii) modify magnetite stoichiometry, which affects its environmental behavior and its properties for nanotechnology applications.

Chemical Insights into the Formation of Colloidal Iridium Nanoparticles from In Situ X-ray Total Scattering: Influence of Precursors and Cations on the Reaction Pathway

Iridium nanoparticles are important catalysts for several chemical and energy conversion reactions. Studies of iridium nanoparticles have also been a key for the development of kinetic models of nanomaterial formation. However, compared to other metals such as gold or platinum, knowledge on the nature of prenucleation species and structural insights into the resultant nanoparticles are missing, especially for nanoparticles obtained from Ir_xCl_y precursors investigated here. We use in situ X-ray total scattering (TS) experiments with pair distribution function (PDF) analysis to study a simple, surfactant-free synthesis of colloidal iridium nanoparticles. The reaction is performed in methanol at 50 °C with only a base and an iridium salt as precursor. From different precursor salts─IrCl₃, IrCl₄, H₂IrCl₆, or Na₂IrCl₆─colloidal nanoparticles as small as Ir_∼55 are obtained as the final product. The nanoparticles do not show the bulk iridium face-centered cubic (fcc) structure but show decahedral and icosahedral structures. The formation route is highly dependent on the precursor salt used. Using IrCl₃ or IrCl₄, metallic iridium nanoparticles form rapidly from Ir_xCl_y^n- complexes, whereas using H₂IrCl₆ or Na₂IrCl₆, the iridium nanoparticle formation follows a sudden growth after an induction period and the brief appearance of a crystalline phase. With H₂IrCl₆, the formation of different Ir_n (n = 55, 55, 85, and 116) nanoparticles depends on the nature of the cation in the base (LiOH, NaOH, KOH, or CsOH, respectively) and larger particles are obtained with larger cations. As the particles grow, the nanoparticle structure changes from partly icosahedral to decahedral. The results show that the synthesis of iridium nanoparticles from Ir_xCl_y is a valuable iridium nanoparticle model system, which can provide new compositional and structural insights into iridium nanoparticle formation and growth.

A mixing-flow reactor for time-resolved reaction measurements distributed in space

Probing short-lived reaction species is challenging owing to the need for both high signal-to-noise ratio, which can require long measurement time, and fast time resolution. Here, a novel in situ sample environment is presented that decouples time resolution from measurement time by distributing reaction time over space for the reaction under flow. In the mixing-flow reactor, precursor solutions are mixed at a specific position along the flow path, where the reaction is initiated. As the reaction mixture flows within a reaction capillary, the reaction time increases with distance from the mixing point. A measurement can be taken at a specific distance from the mixing point for as long as is needed to accumulate good statistics without compromising the time resolution of the measurement. Applications of the mixing-flow reactor for pair distribution function measurements of the initial nuclei formed during the hydrolysis of Al3+ at high pH are shown.

Using small-angle scattering to guide functional magnetic nanoparticle design

Magnetic nanoparticles offer unique potential for various technological, biomedical, or environmental applications thanks to the size-, shape- and material-dependent tunability of their magnetic properties. To optimize particles for a specific application, it is crucial to interrelate their performance with their structural and magnetic properties. This review presents the advantages of small-angle X-ray and neutron scattering techniques for achieving a detailed multiscale characterization of magnetic nanoparticles and their ensembles in a mesoscopic size range from 1 to a few hundred nanometers with nanometer resolution. Both X-rays and neutrons allow the ensemble-averaged determination of structural properties, such as particle morphology or particle arrangement in multilayers and 3D assemblies. Additionally, the magnetic scattering contributions enable retrieving the internal magnetization profile of the nanoparticles as well as the inter-particle moment correlations caused by interactions within dense assemblies. Most measurements are used to determine the time-averaged ensemble properties, in addition advanced small-angle scattering techniques exist that allow accessing particle and spin dynamics on various timescales. In this review, we focus on conventional small-angle X-ray and neutron scattering (SAXS and SANS), X-ray and neutron reflectometry, gracing-incidence SAXS and SANS, X-ray resonant magnetic scattering, and neutron spin-echo spectroscopy techniques. For each technique, we provide a general overview, present the latest scientific results, and discuss its strengths as well as sample requirements. Finally, we give our perspectives on how future small-angle scattering experiments, especially in combination with micromagnetic simulations, could help to optimize the performance of magnetic nanoparticles for specific applications.

Towards a mechanistic understanding of the sol–gel syntheses of ternary carbides

The formation mechanism during the sol–gel synthesis of MAX phase Cr2GaC is unraveled using a combination of complementary techniques.

Formation and growth mechanism for niobium oxide nanoparticles: atomistic insight from in situ X-ray total scattering

Understanding the mechanisms for nanoparticle nucleation and growth is crucial for the development of tailormade nanomaterials. Here, we use X-ray total scattering and Pair Distribution Function analysis to follow the formation and growth of niobium oxide nanoparticles. We study the solvothermal synthesis from niobium chloride in benzyl alcohol, and through investigations of the influence of reaction temperature, a formation pathway can be suggested. Upon dissolution of niobium chloride in benzyl alcohol, octahedral [NbCl6-xOx] complexes form through exchange of chloride ligands. Heating of the solution results in polymerization, where larger clusters built from multiple edge-sharing [NbCl6-xOx] octahedra assemble. This leads to the formation of a nucleation cluster with the ReO3 type structure, which grows to form nanoparticles of the Wadsley-Roth type H-Nb2O5 structure, which in the bulk phase usually only forms at high temperature. Upon further growth, structural defects appear, and the presence of shear-planes in the structure appears highly dependent on nanoparticle size.

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.

Local and long-range atomic/magnetic structure of non-stoichiometric spinel iron oxide nanocrystallites

Spinel iron oxide nanoparticles of different mean sizes in the range 10-25 nm have been prepared by surfactant-free up-scalable near- and super-critical hydro-thermal synthesis pathways and characterized using a wide range of advanced structural characterization methods to provide a highly detailed structural description. The atomic structure is examined by combined Rietveld analysis of synchrotron powder X-ray diffraction (PXRD) data and time-of-flight neutron powder-diffraction (NPD) data. The local atomic ordering is further analysed by pair distribution function (PDF) analysis of both X-ray and neutron total-scattering data. It is observed that a non-stoichiometric structural model based on a tetragonal γ-Fe2O3 phase with vacancy ordering in the structure (space group P43212) yields the best fit to the PXRD and total-scattering data. Detailed peak-profile analysis reveals a shorter coherence length for the superstructure, which may be attributed to the vacancy-ordered domains being smaller than the size of the crystallites and/or the presence of anti-phase boundaries, faulting or other disorder effects. The intermediate stoichiometry between that of γ-Fe2O3 and Fe3O4 is confirmed by refinement of the Fe/O stoichiometry in the scattering data and quantitative analysis of Mössbauer spectra. The structural characterization is complemented by nano/micro-structural analysis using transmission electron microscopy (TEM), elemental mapping using scanning TEM, energy-dispersive X-ray spectroscopy and the measurement of macroscopic magnetic properties using vibrating sample magnetometry. Notably, no evidence is found of a Fe3O4/γ-Fe2O3 core-shell nanostructure being present, which had previously been suggested for non-stoichiometric spinel iron oxide nanoparticles. Finally, the study is concluded using the magnetic PDF (mPDF) method to model the neutron total-scattering data and determine the local magnetic ordering and magnetic domain sizes in the iron oxide nanoparticles. The mPDF data analysis reveals ferrimagnetic collinear ordering of the spins in the structure and the magnetic domain sizes to be ∼60-70% of the total nanoparticle sizes. The present study is the first in which mPDF analysis has been applied to magnetic nanoparticles, establishing a successful precedent for future studies of magnetic nanoparticles using this technique.

Unravelling the local structure of catalytic Fe-oxo clusters stabilized on the MOF-808 metal organic-framework

Stabilizing catalytic iron-oxo-clusters within nanoporous metal-organic frameworks (MOFs) is a powerful strategy to prepare new active materials for the degradation of toxic chemicals, such as bisphenol A. Herein, we combine pair distribution function analysis of total X-ray scattering data and X-ray absorption spectroscopy, with computational modelling to understand the local structural nature of added redox-active iron-oxo clusters bridging neighbouring zirconia-nodes within MOF-808.

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.

Self-Confined Nucleation of Iron Oxide Nanoparticles in a Nanostructured Amorphous Precursor

Crystallization from solution is commonly described by classical nucleation theory, although this ignores that crystals often form via disordered nanostructures. As an alternative, the classical theory remains widely used in a "multistep" variant, where the intermediate nanostructures merely introduce additional thermodynamic parameters. However, this variant still requires validation by experiments addressing indeed proper time and spatial scales (millisecond, nanometer). Here, we used in situ X-ray scattering to determine the mechanism of magnetite crystallization and, in particular, how nucleation propagates at the nanometer scale within amorphous precursors. We find that the self-confinement by an amorphous precursor slows down crystal growth by 2 orders of magnitude once the crystal size reaches the amorphous particle size (∼3 nm). Thus, not only the thermodynamic properties of transient amorphous nanostructures but also their spatial distribution determine crystal nucleation.

Why Does Bi2WO6 Visible-Light Photocatalyst Always Form as Nanoplatelets?

Bi₂WO₆ nanocrystals exhibit excellent photocatalytic properties in the visible range of the solar spectrum, and intense efforts are directed at designing effective synthesis processes with control of size, morphology, and hierarchical structure. All known hydrothermal syntheses produce either nanoplatelet morphology or hierarchical structures based on such primary entities. Here we investigate the nucleation and growth of Bi₂WO₆ nanocrystals under hydrothermal conditions using in situ X-ray total scattering (TS) and powder X-ray diffraction (PXRD) measurements. It is shown that the preferential growth of Bi₂WO₆ nanoplates is due to the presence of disordered layers of Bi₂O₂²⁺ molecular complexes in the precursor solution with an approximate length of 13 Å. These layers interact with tetrahedral WO₄^2- molecular units and eventually form the disordered cubic (Bi_0.933W_0.067)O_1.6) crystalline phase. When enough tungsten units are intertwined between Bi₂O₂²⁺ layers formation of Bi₂WO₆ pristine nanoplates takes place by necessary sideways addition of units in the ac plane. The experimentally observed formation mechanism suggests that the Bi/W atomic ratio must play a central role in the nucleation (assembly of initial crystal layers). Indeed, it is observed in separate continuous flow supercritical synthesis that for a stoichiometric (Bi/W = 2:1) precursor, a (Bi_0.933W_0.067)O_1.6) impurity phase is always observed together with the main Bi₂WO₆ product. Excess tungsten is required in the precursor to form phase-pure Bi₂WO₆ material. Thus, the present study also reports a fast, scalable, and green method for production of this highly attractive photocatalyst.

There's no place like real-space: elucidating size-dependent atomic structure of nanomaterials using pair distribution function analysis

The development of new functional materials builds on an understanding of the intricate relationship between material structure and properties, and structural characterization is a crucial part of materials chemistry. However, elucidating the atomic structure of nanomaterials remains a challenge using conventional diffraction techniques due to the lack of long-range atomic order. Over the past decade, Pair Distribution Function (PDF) analysis of X-ray or neutron total scattering data has become a mature and well-established method capable of giving insight into the atomic structure in nanomaterials. Here, we review the use of PDF analysis and modelling in characterization of a range of different nanomaterials that exhibit unique atomic structure compared to the corresponding bulk materials. A brief introduction to PDF analysis and modelling is given, followed by examples of how essential structural information can be extracted from PDFs using both model-free and advanced modelling methods. We put an emphasis on how the intuitive nature of the PDF can be used for understanding important structural motifs, and on the diversity of applications of PDF analysis to nanostructure problems.

Evolution of Atomic-Level Structure in Sub-10 Nanometer Iron Oxide Nanocrystals: Influence on Cation Occupancy and Growth Rates

Spinel iron oxide nanocrystals (NCs) have been reported to have atomic-level core and surface structural features that differ from those of the bulk material. Recent advances in a continuous growth synthesis of metal oxide NCs make it possible to prepare a series of NCs with subnanometer control of size with diameters below 10 nm that are well-suited for investigating size-dependent structure and reactivity. Here, we study the evolution of size-dependent structure in spinel iron oxide and determine how nanoscale structure influences the growth of NCs. We synthesized spinel iron oxide NCs via a continuous growth method that permits layer-by-layer control of size in order to monitor nanoscale structure over 16 core sizes between 3 and 10 nm. X-ray total scattering data were collected and analyzed with pair distribution function (PDF) analysis in order to refine quantitative structural features including cation occupancies that could be used to detect changes both in the oxidation state and the presence of tetrahedrally coordinated cation vacancies in the NCs. We find that the average iron oxidation state increases as core diameters decrease from 8 down to 3 nm. The trend in iron oxidation state can be explained by the oxidation of surface layers in the NCs. For samples exposed to air for several weeks, oxidation appears to cease when a volume equivalent to that of an ∼1.3 nm shell is converted to the more oxidized maghemite. The number of tetrahedrally coordinated cation vacancies also increases as the NC core size decreases. The correlation between the number of these vacancies and the faster growth for smaller NCs suggests that these reactive vacancies may be responsible for the rapid growth observed for nanocrystals with diameters smaller than 8 nm.

Local Structure and Magnetism of Fe2O3 Maghemite Nanocrystals: The Role of Crystal Dimension

Here we report on the impact of reducing the crystalline size on the structural and magnetic properties of γ-Fe2O3 maghemite nanoparticles. A set of polycrystalline specimens with crystallite size ranging from ~2 to ~50 nm was obtained combining microwave plasma synthesis and commercial samples. Crystallite size was derived by electron microscopy and synchrotron powder diffraction, which was used also to investigate the crystallographic structure. The local atomic structure was inquired combining pair distribution function (PDF) and X-ray absorption spectroscopy (XAS). PDF revealed that reducing the crystal dimension induces the depletion of the amount of Fe tetrahedral sites. XAS confirmed significant bond distance expansion and a loose Fe-Fe connectivity between octahedral and tetrahedral sites. Molecular dynamics revealed important surface effects, whose implementation in PDF reproduces the first shells of experimental curves. The structural disorder affects the magnetic properties more and more with decreasing the nanoparticle size. In particular, the saturation magnetization reduces, revealing a spin canting effect. Moreover, a large effective magnetic anisotropy is measured at low temperature together with an exchange bias effect, a behavior that we related to the existence of a highly disordered glassy magnetic phase.

Heterometallic trinuclear oxo-centered clusters as single-source precursors for synthesis of stoichiometric monodisperse transition metal ferrite nanocrystals

Solvothermal reactions of M_IIFe2III(μ₃-O)(μ₂-O₂CR)₆(H₂O)₃ (R = CF₃, M = Fe, Co, Ni, Cu, Zn) clusters produce monodisperse, phase-pure MFe₂O₄ nanocrystals.

Size Induced Structural Changes in Molybdenum Oxide Nanoparticles

Nanosizing of metal oxide particles is a common strategy for improving materials properties; however, small particles often take structures different from the bulk material. MoO₂ nanoparticles show a structure that is distinct from the bulk distorted rutile structure and which has not yet been determined. Here, we present a model for nanostructured MoO₂ obtained through detailed atomic pair distribution function analysis combined with high-resolution electron microscopy. Defects occur in the arrangement of [MoO₆] octahedra, in both large (40-100 nm) nanoparticles, where the overall distorted rutile structure is preserved, and in small nanoparticles (<5 nm), where a new nanostructure is formed. The study provides a piece in the puzzle of understanding the structure/properties relationship of molybdenum oxides and further our understanding of the origin of structural changes taking place upon nanosizing in oxide materials.

Laboratory setup for rapid in situ powder X-ray diffraction elucidating Ni particle formation in supercritical methanol

The design and function of a custom-made Soller slit for a laboratory 2D area detector is presented through a series of demonstration images and an in situ experiment following the formation of nickel particles in supercritical methanol. The in situ experiment is performed in a capillary sample environment, modified for a laboratory scale Rigaku Smartlab diffractometer, and with a temperature range of 300-1050 K. The formation of nickel particles was followed successfully using laboratory in situ X-ray powder diffraction with a time resolution in the order of 27 s. Observations from the area detector images showed the appearance of three distinct phases during the reaction: Ni₃(NO₃)₂(OH)₄, NiO, and Ni. The images were linearly integrated and analyzed using Rietveld refinement. A reaction mechanism is proposed based on an evaluation of the weight fractions and scattering factors as a function of reaction time.

The Role of the OH Group in Citric Acid in the Coordination with Fe3O4 Nanoparticles

The role of the C?OH group in citric acid (CA) in the molecular coordination with Fe₃O₄ nanoparticles (NPs) has been elusive for a long time. In this study, attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectral deconvolution and thermogravimetric analysis (TGA) have been used to quantitatively clarify its significance in CA adsorption and its corresponding conformation. The experimental results show that the coordination and the corresponding conformation are exclusively determined by COOH not C?OH at pH 3, where its adsorption behavior conforms to the Brunauer?Emmett?Teller (BET) multilayer model with a maximal monolayer coordination number of 2.1/nm². However, C?OH is involved in the coordination at pH 10, and CA conforms to the Langmuir monolayer model with 1.4/nm² as its maximal monolayer coordination number, which is more stable than the COOH-only coordination. Especially, the conformational transformation is observed for the first time at pH 3, where the CA molecules adjust their conformation upon elution to maximize the utilization of the available binding sites on Fe₃O₄ NPs. This finding deepens the understanding on the fundamental mechanism for the interaction between the C?OH and COOH groups containing the organic ligand and metal oxide.

The rise of the X-ray atomic pair distribution function method: a series of fortunate events

The atomic pair distribution function (PDF) technique is a powerful approach to gain quantitative insight into the structure of materials where the structural coherence extends only over a few nanometres. In this paper, I focus on PDF from synchrotron X-rays and describe what is the PDF and where it came from, as well as key moments on the journey that have contributed to its enormous recent growth and expanding impact in materials science today. Synchrotron X-ray sources played a starring role in this story. This article is part of the theme issue 'Fifty years of synchrotron science: achievements and opportunities'.

Structural Chemistry of Akdalaite, Al10O14(OH)2, the Isostructural Aluminum Analogue of Ferrihydrite

As part of an effort to characterize clusters and intermediate phases likely to be encountered along solution reaction pathways that produce iron and aluminum oxide-hydroxides from Fe and Al precursors, the complete structure of Al10O14(OH)2 (akdalaite) was determined from a combination of single-crystal X-ray diffraction (SC-XRD) data collected at 100 K to define the Al and O positions, and solid-state nuclear magnetic resonance (NMR) and neutron powder diffraction (NPD) data collected at room temperature (~300 K) to precisely determine the nature of hydrogen in the structure. Two different synthesis routes produced different crystal morphologies. Using an aluminum oxyhydroxide floc made from mixing AlCl3 and 0.48 M NaOH, the product had uniform needle morphology, while using nanocrystalline boehmite (Vista Chemical Company Catapal D alumina) as the starting material produced hexagonal plates. Akdalaite crystallizes in the space group P63mc with lattice parameters of a = 5.6244(3) Å and c = 8.8417(3) Å (SC-XRD) and a = 5.57610(2) Å and c = 8.77247(6) Å (NPD). The crystal structure features Al13O40 Keggin clusters. The structural chemistry of akdalaite is nonideal but broadly conforms to that of ferrihydrite, the nanomineral with which it is isostructural.

In Situ Synchrotron X-ray Characterization Shining Light on the Nucleation and Growth Kinetics of Colloidal Nanoparticles

Rational synthesis of colloidal nanoparticles with desirable properties relies on precise control over the nucleation and growth kinetics, which is still not well understood. The recent development of in situ high energy synchrotron X-ray techniques offers an excellent opportunity to quantitatively monitor the growth trajectories of colloidal nanoparticles in real time under real reaction conditions. The time-resolved, quantitative data of the growing colloidal nanoparticles are unique to reveal the mechanism of nanoparticle formation and determine the corresponding intrinsic kinetic parameters. This review discusses the kinetics of major steps of forming colloidal nanoparticles and the capability of in situ synchrotron X-ray techniques in studying the corresponding kinetics.

The hydrothermal processing of iron oxides from bacterial biofilm waste as new nanomaterials for broad applications

Iron oxides and their hydroxides have been studied and analysed with properties of their mutual transformations under different hydrothermal conditions being indicated. Amorphous bacteria nanowires produced from biofilm waste were investigated under the influence of pH at a fixed duration (20 h) and reaction temperature (200 °C). The morphology, structure, and particle size of the transformation of hematite (α-Fe2O3) was obtained and characterised with SEM, XRD, FTIR, and particle sizer. The optimal conditions for the complete conversion of amorphous iron oxide nanowires to crystalline α-Fe2O3 is under acidic conditions where the pH is 1. The flower-like α-Fe2O3 structures have photocatalytic activity and adsorbent properties for heavy metal ions. This one-pot synthesis approach to produce α-Fe2O3 at a low cost would be greatly applicable to the recycling process of biofilm waste in order to benefit the environment.

Approaching Ferrite-Based Exchange-Coupled Nanocomposites as Permanent Magnets

During the past decade, CoFe₂O₄ (hard)/Co-Fe alloy (soft) magnetic nanocomposites have been routinely prepared by partial reduction of CoFe₂O₄ nanoparticles. Monoxide (i.e., FeO or CoO) has often been detected as a byproduct of the reduction, although it remains unclear whether the formation of this phase occurs during the reduction itself or at a later stage. Here, a novel reaction cell was designed to monitor the reduction in situ using synchrotron powder X-ray diffraction (PXRD). Sequential Rietveld refinements of the in situ data yielded time-resolved information on the sample composition and confirmed that the monoxide is generated as an intermediate phase. The macroscopic magnetic properties of samples at different reduction stages were measured by means of vibrating sample magnetometry (VSM), revealing a magnetic softening with increasing soft phase content, which was too pronounced to be exclusively explained by the introduction of soft material in the system. The elemental compositions of the constituent phases were obtained from joint Rietveld refinements of ex situ high-resolution PXRD and neutron powder diffraction (NPD) data. It was found that the alloy has a tendency to emerge in a Co-rich form, inducing a Co deficiency on the remaining spinel phase, which can explain the early softening of the magnetic material.

Pitfalls and reproducibility ofin situsynchrotron powder X-ray diffraction studies of solvothermal nanoparticle formation

In situpowder X-ray diffraction (PXRD) is a powerful characterization tool owing to its ability to provide time-resolved information about phase composition, crystal structure and microstructure. The application of high-flux synchrotron X-ray beams and the development of custom-built reactors have facilitated second-scale time-resolved studies of nanocrystallite formation and growth during solvothermal synthesis. The short exposure times required for good time resolution limit the data quality, while the employed high-temperature–high-pressure reactors further complicate data acquisition and treatment. Based on experience gathered during ten years of conductingin situstudies of solvothermal reactions at a number of different synchrotrons, a compilation of useful advice for conductingin situPXRD experiments and data treatment is presented here. In addition, the reproducibility of the employed portablein situPXRD setup, experimental procedure and data analysis is evaluated. This evaluation is based on repeated measurements of an LaB6line-profile standard throughout 5 d of beamtime and on the repetition of ten identicalin situsynchrotron PXRD experiments on the hydrothermal formation of γ-Fe2O3nanocrystallites. The study reveals inconsistencies in the absolute structural and microstructural values extracted by Rietveld refinement and whole powder pattern modelling of thein situPXRD data, but also illustrates the robustness of trends and relative changes in the extracted parameters. From the data, estimates of the effective errors and reproducibility ofin situPXRD studies of solvothermal nanocrystallite formation are provided.

Early stage structural development of prototypical zeolitic imidazolate framework (ZIF) in solution

Given the wide-ranging potential applications of metal organic frameworks (MOFs), an emerging imperative is to understand their formation with atomic scale precision. This will aid in designing syntheses for next-generation MOFs with enhanced properties and functionalities. Major challenges are to characterize the early-stage seeds, and the pathways to framework growth, which require synthesis coupled with in situ structural characterization sensitive to nanoscale structures in solution. Here we report measurements of an in situ synthesis of a prototypical MOF, ZIF-8, utilizing synchrotron X-ray atomic pair distribution function (PDF) analysis optimized for sensitivity to dilute species, complemented by mass spectrometry, electron microscopy, and density functional theory calculations. We observe that despite rapid formation of the crystalline product, a high concentration of Zn(2-MeIm)₄ (2-MeIm = 2-methylimidazolate) initially forms and persists as stable clusters over long times. A secondary, amorphous phase also pervades during the synthesis, which has a structural similarity to the final ZIF-8 and may act as an intermediate to the final product.

Continuous Hydrothermal Synthesis of Inorganic Nanoparticles: Applications and Future Directions

Nanomaterials are at the leading edge of the emerging field of nanotechnology. Their unique and tunable size-dependent properties (in the range 1-100 nm) make these materials indispensable in many modern technological applications. In this Review, we summarize the state-of-art in the manufacture and applications of inorganic nanoparticles made using continuous hydrothermal flow synthesis (CHFS) processes. First, we introduce ideal requirements of any flow process for nanoceramics production, outline different approaches to CHFS, and introduce the pertinent properties of supercritical water and issues around mixing in flow, to generate nanoparticles. This Review then gives comprehensive coverage of the current application space for CHFS-made nanomaterials including optical, healthcare, electronics (including sensors, information, and communication technologies), catalysis, devices (including energy harvesting/conversion/fuels), and energy storage applications. Thereafter, topics of precursor chemistry and products, as well as materials or structures, are discussed (surface-functionalized hybrids, nanocomposites, nanograined coatings and monoliths, and metal-organic frameworks). Later, this Review focuses on some of the key apparatus innovations in the field, such as in situ flow/rapid heating systems (to investigate kinetics and mechanisms), approaches to high throughput flow syntheses (for nanomaterials discovery), as well as recent developments in scale-up of hydrothermal flow processes. Finally, this Review covers environmental considerations, future directions and capabilities, along with the conclusions and outlook.

Designed synthesis and surface engineering strategies of magnetic iron oxide nanoparticles for biomedical applications

Iron oxide nanoparticles (NPs) hold great promise for future biomedical applications because of their magnetic properties as well as other intrinsic properties such as low toxicity, colloidal stability, and surface engineering capability. Numerous related studies on iron oxide NPs have been conducted. Recent progress in nanochemistry has enabled fine control over the size, crystallinity, uniformity, and surface properties of iron oxide NPs. This review examines various synthetic approaches and surface engineering strategies for preparing naked and functional iron oxide NPs with different physicochemical properties. Growing interest in designed and surface-engineered iron oxide NPs with multifunctionalities was explored in in vitro/in vivo biomedical applications, focusing on their combined roles in bioseparation, as a biosensor, targeted-drug delivery, MR contrast agents, and magnetic fluid hyperthermia. This review outlines the limitations of extant surface engineering strategies and several developing strategies that may overcome these limitations. This study also details the promising future directions of this active research field.

Towards atomistic understanding of polymorphism in the solvothermal synthesis of ZrO2nanoparticles

Varying atomic short-range order is correlated with the ratio of the monoclinic (m) to tetragonal (t) phase in ZrO2nanoparticle formation by solvothermal methods. Reactions from Zr oxynitrate in supercritical methanol and Zr acetate in water (hydrothermal route) were studiedin situby X-ray total scattering. Irrespective of the Zr source and solvent, the structure of the precursor in solution consists of edge-shared tetramer chains. Upon heating, the nearest-neighbor Zr—O and Zr—Zr distances shorten initially while the medium-range connectivity is broken. Depending on the reaction conditions, the disordered intermediate transforms either rapidly intom-ZrO2, or more gradually into mixedm- andt-ZrO2with a concurrent increase of the shortest Zr—Zr distance. In the hydrothermal case, the structural similarity of the amorphous intermediate andm-ZrO2favors the formation of almost phase-purem-ZrO2nanoparticles with a size of 5 nm, considerably smaller than the often-cited critical size below which the tetragonal is assumed to be favoured. Pair distribution function analysis thus unravels ZrO2phase formation on the atomic scale and in this way provides a major step towards understanding polymorphism of ZrO2beyond empirical approaches.

The Molecular Mechanism of Iron(III) Oxide Nucleation

A molecular understanding of the formation of solid phases from solution would be beneficial for various scientific fields. However, nucleation pathways are still not fully understood, whereby the case of iron (oxyhydr)oxides poses a prime example. We show that in the prenucleation regime, thermodynamically stable solute species up to a few nanometers in size are observed, which meet the definition of prenucleation clusters. Nucleation then is not governed by a critical size, but rather by the dynamics of the clusters that are forming at the distinct nucleation stages, based on the chemistry of the linkages within the clusters. This resolves a longstanding debate in the field of iron oxide nucleation, and the results may generally apply to oxides forming via hydrolysis and condensation. The (molecular) understanding of the chemical basis of phase separation is paramount for, e.g., tailoring size, shape and structure of novel nanocrystalline materials.

The chemistry of ZnWO4 nanoparticle formation

The need for a new approach to describing nanoparticle nucleation and growth different from the classical models is highlighted. In and ex situ total scattering experiments combined with additional characterization techniques are used to unravel the chemistry dictating ZnWO4 formation.

The need for a change away from classical nucleation and growth models for the description of nanoparticle formation is highlighted. By the use of in situ total X-ray scattering experiments the transformation of an aqueous polyoxometalate precursor mixture to crystalline ZnWO₄ nanoparticles under hydrothermal conditions was followed. The precursor solution is shown to consist of specific Tourné-type sandwich complexes. The formation of pristine ZnWO₄ within seconds is understood on the basis of local restructuring and three-dimensional reordering preceding the emergence of long range order in ZnWO₄ nanoparticles. An observed temperature dependent trend in defect concentration can be rationalized based on the proposed formation mechanism. Following nucleation the individual crystallites were found to grow into prolate morphology with elongation along the unit cell c-direction. Extensive electron microscopy characterization provided evidence for particle growth by oriented attachment; a notion supported by sudden particle size increases observed in the in situ total scattering experiments. A simple continuous hydrothermal flow method was devised to synthesize highly crystalline monoclinic zinc tungstate (ZnWO₄) nanoparticles in large scale in less than one minute. The present results highlight the profound influence of structural similarities in local structure between reactants and final materials in determining the specific nucleation of nanostructures and thus explains the potential success of a given synthesis procedure in producing nanocrystals. It demonstrates the need for abolishing outdated nucleation models, which ignore subtle yet highly important system dependent differences in the chemistry of the forming nanocrystals.

Core–shell nanoparticles by silica coating of metal oxides in a dual-stage hydrothermal flow reactor

A green, fast, high-throughput, continuous-flow hydrothermal synthesis method is explored for preparation of silica coated nanoparticles with narrow size distribution.

A compact and low-weight sputtering unit for in situ investigations of thin film growth at synchrotron radiation beamlines

In this work, we report on a highly variable, compact, and light high-vacuum sputter deposition unit designed for in situ experiments using synchrotron radiation facilities. The chamber can be mounted at various synchrotron beamlines for scattering experiments in grazing incidence geometry. The sample position and the large exit window allow to perform x-ray experiments up to large q values. The sputtering unit is easy to mount on existing experimental setups and can be remote-controlled. In this paper, we describe in detail the design and the performance of the new sputtering chamber and present the installation of the apparatus at different 3rd generation light sources. Furthermore, we describe the different measurement options and present some selected results. The unit has been successfully commissioned and is now available for users at PETRA III at DESY.

In situ powder X-ray diffraction study of magnetic CoFe2O4 nanocrystallite synthesis

The evolution of size and size distribution during hydrothermal synthesis of nanocrystalline CoFe2O4 has been studied by in situ synchrotron powder X-ray diffraction (PXRD). Varying synthesis temperature or [OH(-)] concentration in the precursor proves to have no significant effect on the final volume-weighted nanocrystallite sizes (∼12 nm) of CoFe2O4. However, analysis by whole powder pattern modeling of the [OH(-)] concentration series reveals a substantial difference in the number-weighted size distributions when varying the amount of base used. Furthermore, changing the metal ion concentration prior to NaOH addition in the precursor preparation gives a handle to control the nanoparticle sizes (∼5-15 nm). All in situ experiments show almost instantaneous formation of the CoFe2O4 nanocrystallites, without significant growth or broadening of the size distribution after 60 s. Magnetic hysteresis curve measurements illustrate, how this facilitates the tailoring of materials with specific magnetic properties, as larger particles (∼15 nm) exhibit hard magnetic properties while the smaller particles (∼6-7 nm) are superparamagnetic.

Time-resolved in situ studies on the formation mechanism of iron oxide nanoparticles using combined fast-XANES and SAXS

The reaction of iron chlorides with an alkaline reagent is one of the most prominent methods for the synthesis of iron oxide nanoparticles.