Goethite and Hematite Nucleation and Growth from Ferrihydrite: Effects of Oxyanion Surface Complexes.

The presence of oxyanions, such as nitrate (NO3-) and phosphate (PO43-), regulates the nucleation and growth of goethite (Gt) and hematite (Hm) during the transformation of ferrihydrite (Fh). Our previous studies showed that oxyanion surface complexes control the rate and pathway of Fh transformation to Gt and Hm. However, how oxyanion surface complexes control the mechanism of Gt and Hm nucleation and growth during the Fh transformation is still unclear. We used synchrotron scattering methods and cryogenic transmission electron microscopy to investigate the effects of NO3- outer-sphere complexes and PO43- inner-sphere complexes on the mechanism of Gt and Hm formation from Fh. Our TEM results indicated that Gt particles form through a two-step model in which Fh particles first transform to Gt nanoparticles and then crystallographically align and grow to larger particles by oriented attachment (OA). In contrast, for the formation of Hm, imaging shows that Fh particles first aggregate and then transform to Hm through interface nucleation. This is consistent with our X-ray scattering results, which demonstrate that NO3- outer-sphere and PO43- inner-sphere complexes promote the formation of Gt and Hm, respectively. These results have implications for understanding the coupled interactions of oxyanions and iron oxy-hydroxides in Earth-surface environments.

Multivariate regression analysis of factors regulating the formation of synthetic aluminosilicate nanoparticles.

Interest is growing in nanoparticles made of earth abundant materials, like alumino(silicate) minerals. Their applications are expanding to include catalysis, carbon sequestration reactions, and medical applications. It remains unclear, however, what factors control their formation and abundance during laboratory synthesis or on a larger industrial scale. This work investigates the complex system of physicochemical conditions that influence the formation of nanosized alumino(silicate) minerals. Samples were synthesized and analyzed by powder X-ray diffraction, in situ and ex situ small angle X-ray scattering, and transmission electron microscopy. Regression analyses combined with linear combination fitting of powder diffraction patterns was used to model the influence of different synthesis conditions including concentration, hydrolysis ratio and rate, and Al : Si elemental ratio on the particle size of the initial precipitate and on the phase abundances of the final products. These models show that hydrolysis ratio has the strongest control on the overall phase composition, while the starting reagent concentration also plays a vital role. For imogolite nanotubes, we determine that increasing concentration, and relatively high or low hydrolysis limit nanotube production. A strong relationship is also observed between the distribution of nanostructured phases and the size of precursor particles. The confidences were >99% for all linear regression models and explained up to 85% of the data variance in the case of imogolite. Additionally, the models consistently predict resulting data from other experimental studies. These results demonstrate the use of an approach to understand complex chemical systems with competing influences and provide insight into the formation of several nanosized alumino(silicate) phases.

Geometrically frustrated interactions drive structural complexity in amorphous calcium carbonate.

Amorphous calcium carbonate is an important precursor for biomineralization in marine organisms. Key outstanding problems include understanding the structure of amorphous calcium carbonate and rationalizing its metastability as an amorphous phase. Here we report high-quality atomistic models of amorphous calcium carbonate generated using state-of-the-art interatomic potentials to help guide fits to X-ray total scattering data. Exploiting a recently developed inversion approach, we extract from these models the effective Ca⋯Ca interaction potential governing the structure. This potential contains minima at two competing distances, corresponding to the two different ways that carbonate ions bridge Ca2+-ion pairs. We reveal an unexpected mapping to the Lennard-Jones-Gauss model normally studied in the context of computational soft matter. The empirical model parameters for amorphous calcium carbonate take values known to promote structural complexity. We thus show that both the complex structure and its resilience to crystallization are actually encoded in the geometrically frustrated effective interactions between Ca2+ ions.

Operando characterization and regulation of metal dissolution and redeposition dynamics near battery electrode surface.

Mn dissolution has been a long-standing, ubiquitous issue that negatively impacts the performance of Mn-based battery materials. Mn dissolution involves complex chemical and structural transformations at the electrode-electrolyte interface. The continuously evolving electrode-electrolyte interface has posed great challenges for characterizing the dynamic interfacial process and quantitatively establishing the correlation with battery performance. In this study, we visualize and quantify the temporally and spatially resolved Mn dissolution/redeposition (D/R) dynamics of electrochemically operating Mn-containing cathodes. The particle-level and electrode-level analyses reveal that the D/R dynamics is associated with distinct interfacial degradation mechanisms at different states of charge. Our results statistically differentiate the contributions of surface reconstruction and Jahn-Teller distortion to the Mn dissolution at different operating voltages. Introducing sulfonated polymers (Nafion) into composite electrodes can modulate the D/R dynamics by trapping the dissolved Mn species and rapidly establishing local Mn D/R equilibrium. This work represents an inaugural effort to pinpoint the chemical and structural transformations responsible for Mn dissolution via an operando synchrotron study and develops an effective method to regulate Mn interfacial dynamics for improving battery performance.

Size and Strain of Zinc Sulfide Nanoparticles Altered by Interaction with Organic Molecules.

Nanosized zinc sulfides (nano-ZnS) have size-dependent and tunable physical and chemical properties that make them useful for a variety of technological applications. For example, structural changes, especially caused by strain, are pronounced in nano-ZnS < 5 nm in size, the size range typical of incidental nano-ZnS that form in the environment. Previous research has shown how natural organic matter impacts the physical properties of nano-ZnS but was mostly focused on their aggregation state. However, the specific organic molecules and the type of functional groups that are most important for controlling the nano-ZnS size and strain remain unclear. This study examined the size-dependent strain of nano-ZnS synthesized in the presence of serine, cysteine, glutathione, histidine, and acetate. Synchrotron total scattering pair distribution function analysis was used to determine the average crystallite size and strain. Among the different organic molecules tested, those containing a thiol group were shown to affect the particle size and size-induced strain most strongly when added during synthesis but significantly reduced the particle strain when added to as-formed nano-ZnS. The same effects are useful to understand the properties and behavior of natural nano-ZnS formed as products of microbial activity, for example, in reducing environments, or of incidental nano-ZnS formed in organic wastes.

Oxyanion Surface Complexes Control the Kinetics and Pathway of Ferrihydrite Transformation to Goethite and Hematite.

The rate and pathway of ferrihydrite (Fh) transformation at oxic conditions to more stable products is controlled largely by temperature, pH, and the presence of other ions in the system such as nitrate (NO3-), sulfate (SO42-), and arsenate (AsO43-). Although the mechanism of Fh transformation and oxyanion complexation have been separately studied, the effect of surface complex type and strength on the rate and pathway remains only partly understood. We have developed a kinetic model that describes the effects of surface complex type and strength on Fh transformation to goethite (Gt) and hematite (Hm). Two sets of oxyanion-adsorbed Fh samples were prepared, nonbuffered and buffered, aged at 70 ± 1.5 °C, and then characterized using synchrotron X-ray scattering methods and wet chemical analysis. Kinetic modeling showed a significant decrease in the rate of Fh transformation for oxyanion surface complexes dominated by strong inner-sphere (SO42- and AsO43-) versus weak outer-sphere (NO3-) bonding and the control. The results also showed that the Fh transformation pathway is influenced by the type of surface complex such that with increasing strength of bonding, a smaller fraction of Gt forms compared with Hm. These findings are important for understanding and predicting the role of Fh in controlling the transport and fate of metal and metalloid oxyanions in natural and applied systems.

The Structure of Natural Biogenic Iron (Oxyhydr)oxides Formed in Circumneutral pH Environments.

Biogenic iron (Fe) (oxyhydr)oxides (BIOS) partially control the cycling of organic matter, nutrients, and pollutants in soils and water via sorption and redox reactions. Although recent studies have shown that the structure of BIOS resembles that of two-line ferrihydrite (2LFh), we lack detailed knowledge of the BIOS local coordination environment and structure required to understand the drivers of BIOS reactivity in redox active environments. Therefore, we used a combination of microscopy, scattering, and spectroscopic methods to elucidate the structure of BIOS sampled from a groundwater seep in North Carolina and compare them to 2LFh. We also simulated the effects of wet-dry cycles by varying sample preparation (e.g., freezing, flash freezing with freeze drying, freezing with freeze drying and oven drying). In general, the results show that both the long- and short-range ordering in BIOS are structurally distinct and notably more disordered than 2LFh. Our structure analysis, which utilized Fe K-edge X-ray absorption spectroscopy, Mössbauer spectroscopy, X-ray diffraction, and pair distribution function analyses, showed that the BIOS samples were more poorly ordered than 2LFh and intimately mixed with organic matter. Furthermore, pair distribution function analyses resulted in coherent scattering domains for the BIOS samples ranging from 12-18 Å, smaller than those of 2LFh (21-27 Å), consistent with reduced ordering. Additionally, Fe L-edge XAS indicated that the local coordination environment of 2LFh samples consisted of minor amounts of tetrahedral Fe(III), whereas BIOS were dominated by octahedral Fe(III), consistent with depletion of the sites due to small domain size and incorporation of impurities (e.g., organic C, Al, Si, P). Within sample sets, the frozen freeze dried and oven dried sample preparation increased the crystallinity of the 2LFh samples when compared to the frozen treatment, whereas the BIOS samples remained more poorly crystalline under all sample preparations. This research shows that BIOS formed in circumneutral pH waters are poorly ordered and more environmentally stable than 2LFh.

Preferred orientations of garnet porphyroblasts reveal previously cryptic templating during nucleation.

Electron back scattered diffraction data of garnet crystals from the Nelson Aureole, British Columbia and the Mosher's Island formation, Nova Scotia, reveals that 22 garnet crystals are all oriented with one of three crystal directions parallel to the trace of the foliation plane in thin section. Structural models suggest that these relationships are due to preferential garnet nucleation onto muscovite, with the alignment of repeating rows of Al octahedra and Si tetrahedra in each leading to inheritance of garnet orientation from the muscovite. These results highlight that epitaxial nucleation may be a prevalent process by which porphyroblast minerals nucleate during metamorphism and carry implications for the role that non-classic nucleation pathways play in the crystallization of metamorphic minerals, the distribution of porphyroblasts in metamorphic rocks, and, in cases in which nucleation is the rate limiting step for crystallization, the energetics of metamorphic reactions.

Oligo(l-glutamic acids) in Calcium Phosphate Precipitation: Chain Length Effect.

The understanding of calcium phosphate precipitation is of major interest in different fields of science, including medicine, biomaterials, and physical chemistry. The presence of additive biomacromolecules has been known to influence various stages of the precipitation process from nucleation to crystal growth. In the current work, well-defined sequences of short, negatively charged peptides, oligo(l-glutamic acids), were utilized as a model, inspired by contiguous sequences of acidic amino acids in natural biomineralization proteins. The precipitate morphology and phases, the element time profile in solution and in the precipitates, as well as the kinetics during the precipitation process were analyzed to explain the effect of these short peptides on calcium phosphate precipitation. The results show that peptides can delay the phase transformation of an amorphous precursor phase to hydroxyapatite and that there is an optimal chain length for this effect at a given concentration of peptide. This study is the first part of a two-part series and is followed by a subsequent work to reveal the mechanism by which these short peptides influence the calcium phosphate precipitation.

Oligo(l-glutamic acids) in Calcium Phosphate Precipitation: Mechanism of Delayed Phase Transformation.

Proteins and their mimics that contain negatively charged sequences are important in natural and biomimetic mineralization. The mechanism by which these sequences affect calcium phosphate mineralization is not well understood. Here, peptides containing different numbers of repeat units of contiguous glutamic acid residues, oligo(l-glutamic acid)n (n = 3, 7, 8, 10), were investigated with regards to the mechanism in delaying the crystallization of amorphous calcium phosphate (ACP) while holding the amount of carboxylic acid groups in solution constant. Increasing peptide chain length increases the stability of ACP at a certain total amount of carboxylic acid groups in solution. This effect is shown to be due to stronger binding as well as binding to more calcium ions per peptide by the longer oligopeptides compared to the shorter ones. It is proposed that these associations delay the structural rearrangement of calcium ions and the dehydration of ACP, which are required for the crystallization of hydroxyapatite. The initial nucleation and the local structure of ACP, however, do not vary with chain length. This second part of a two-part series provides an improved mechanistic understanding of how organic additives, especially those with contiguous acidic amino acid sequences, modulate the kinetics of calcium phosphate precipitation and phase transformation.

Nanocomposite structure of two-line ferrihydrite powder from total scattering.

Ferrihydrite is one of the most important iron-containing minerals on Earth. Yet determination of its atomic-scale structure has been frustrated by its intrinsically poor crystallinity. The key difficulty is that physically-different models can appear consistent with the same experimental data. Using X-ray total scattering and a nancomposite reverse Monte Carlo approach, we evaluate the two principal contending models-one a multi-phase system without tetrahedral iron(III), and the other a single phase with tetrahedral iron(III). Our methodology is unique in considering explicitly the complex nanocomposite structure the material adopts: namely, crystalline domains embedded in a poorly-ordered matrix. The multi-phase model requires unphysical structural rearrangements to fit the data, whereas the single-phase model accounts for the data straightforwardly. Hence the latter provides the more accurate description of the short- and intermediate-range order of ferrihydrite. We discuss how this approach might allow experiment-driven (in)validation of complex models for important nanostructured phases beyond ferrihydrite.

Targeted Surface Doping with Reversible Local Environment Improves Oxygen Stability at the Electrochemical Interfaces of Nickel-Rich Cathode Materials.

Elemental doping represents a prominent strategy to improve interfacial chemistry in battery materials. Manipulating the dopant spatial distribution and understanding the dynamic evolution of the dopants at the atomic scale can inform better design of the doping chemistry for batteries. In this work, we create a targeted hierarchical distribution of Ti4+, a popular doping element for oxide cathode materials, in LiNi0.8Mn0.1Co0.1O2 primary particles. We apply multiscale synchrotron/electron spectroscopy and imaging techniques as well as theoretical calculations to investigate the dynamic evolution of the doping chemical environment. The Ti4+ dopant is fully incorporated into the TMO6 octahedral coordination and is targeted to be enriched at the surface. Ti4+ in the TMO6 octahedral coordination increases the TM-O bond length and reduces the covalency between (Ni, Mn, Co) and O. The excellent reversibility of Ti4+ chemical environment gives rise to superior oxygen reversibility at the cathode-electrolyte interphase and in the bulk particles, leading to improved stability in capacity, energy, and voltage. Our work directly probes the chemical environment of doping elements and helps rationalize the doping strategy for high-voltage layered cathodes.

A new method for in situ structural investigations of nano-sized amorphous and crystalline materials using mixed-flow reactors.

Structural investigations of amorphous and nanocrystalline phases forming in solution are historically challenging. Few methods are capable of in situ atomic structural analysis and rigorous control of the system. A mixed-flow reactor (MFR) is used for total X-ray scattering experiments to examine the short- and long-range structure of phases in situ with pair distribution function (PDF) analysis. The adaptable experimental setup enables data collection for a range of different system chemistries, initial supersaturations and residence times. The age of the sample during analysis is controlled by adjusting the flow rate. Faster rates allow for younger samples to be examined, but if flow is too fast not enough data are acquired to average out excess signal noise. Slower flow rates form older samples, but at very slow speeds particles settle and block flow, clogging the system. Proper background collection and subtraction is critical for data optimization. Overall, this MFR method is an ideal scheme for analyzing the in situ structures of phases that form during crystal growth in solution. As a proof of concept, high-resolution total X-ray scattering data of amorphous and crystalline calcium phosphates and amorphous calcium carbonate were collected for PDF analysis.

Mechanism and Kinetics of Hydrogen Peroxide Decomposition on Platinum Nanocatalysts.

The decomposition of H2O2 to H2O and O2 catalyzed by platinum nanocatalysts controls the energy yield of several energy conversion technologies, such as hydrogen fuel cells. However, the reaction mechanism and rate-limiting step of this reaction have been unsolved for more than 100 years. We determined both the reaction mechanism and rate-limiting step by studying the effect of different reaction conditions, nanoparticle size, and surface composition on the rates of H2O2 decomposition by three platinum nanocatalysts with average particle sizes of 3, 11, and 22 nm. Rate models indicate that the reaction pathway of H2O2 decomposition is similar for all three nanocatalysts. Larger particle size correlates with lower activation energy and enhanced catalytic activity, explained by a smaller work function for larger platinum particles, which favors chemisorption of oxygen onto platinum to form Pt(O). Our experiments also showed that incorporation of oxygen at the nanocatalyst surface results in a faster reaction rate because the rate-limiting step is skipped in the first cycle of reaction. Taken together, these results indicate that the reaction proceeds in two cyclic steps and that step 1 is the rate-limiting step. Step 1: Pt + H2 O2 → H2 O + Pt( O). Step 2: Pt( O) + H2 O2 → Pt + O2 + H2 O. Overall: 2 H2 O2 → O2 + 2 H2 O. Establishing relationships between the properties of commercial nanocatalysts and their catalytic activity, as we have done here for platinum in the decomposition of H2O2, opens the possibility of improving the performance of nanocatalysts used in applications. This study also demonstrates the advantage of combining detailed characterization and systematic reactivity experiments to understand property-behavior relationships.

Accelerated Evolution of Surface Chemistry Determined by Temperature and Cycling History in Nickel-Rich Layered Cathode Materials.

Nickel-rich layered cathode materials have the potential to enable cheaper and higher energy lithium ion batteries. However, these materials face major challenges (e.g., surface reconstruction, microcracking, potential oxygen evolution) that can hinder the safety and cycle life of lithium ion batteries. Many studies of nickel-rich materials have focused on ways to improve performance. Understanding the effects of temperature and cycling on the chemical and structural transformations is essential to assess the performance and suitability of these materials for practical battery applications. The present study is focused on the spectroscopic analysis of surface changes within a strong performing LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode material. We found that surface chemical and structural transformations (e.g., gradient metal reduction, oxygen loss, reconstruction, dissolution) occurred quicker and deeper than expected at higher temperatures. Even at lower temperatures, the degradation occurred rapidly and eventually matched the degradation at high temperatures. Despite these transformations, our performance results showed that a better performing nickel-rich NMC is possible. Establishing relationships between the atomic, structural, chemical, and physical properties of cathode materials and their behavior during cycling, as we have done here for NMC811, opens the possibility of developing lithium ion batteries with higher performance and longer life. Finally, our study also suggests that a separate, systematic, and elaborate study of surface chemistry is necessary for each NMC composition and electrolyte environment.

Environmental exposure to TiO2 nanomaterials incorporated in building material.

Nanomaterials are increasingly being used to improve the properties and functions of common building materials. A new type of self-cleaning cement incorporating TiO2 nanomaterials (TiO2-NMs) with photocatalytic properties is now marketed. This promising cement might provide air pollution-reducing properties but its environmental impact must be validated. During cement use and aging, an altered surface layer is formed that exhibits increased porosity. The surface layer thickness alteration and porosity increase with the cement degradation rate. The hardened cement paste leaching behavior has been fully documented, but the fate of incorporated TiO2-NMs and their state during/after potential release is currently unknown. In this study, photocatalytic cement pastes with increasing initial porosity were leached at a lab-scale to produce a range of degradation rates concerning the altered layer porosity and thickness. No dissolved Ti was released during leaching, only particulate TiO2-NM release was detected. The extent of release from this batch test simulating accelerated worst-case scenario was limited and ranged from 18.7 ± 2.1 to 33.5 ± 5.1 mg of Ti/m2 of cement after 168 h of leaching. TiO2-NMs released into neutral aquatic media (simulate pH of surface water) were not associated or coated by cement minerals. The TiO2-NM release mechanism is suspected to start from freeing of TiO2-NMs in the altered layer pore network due to partial cement paste dissolution followed by diffusion into the bulk pore solution to the surface. The extent of TiO2-NM release was not solely related to the cement degradation rate.

CRYSTAL GROWTH. Crystallization by particle attachment in synthetic, biogenic, and geologic environments.

Field and laboratory observations show that crystals commonly form by the addition and attachment of particles that range from multi-ion complexes to fully formed nanoparticles. The particles involved in these nonclassical pathways to crystallization are diverse, in contrast to classical models that consider only the addition of monomeric chemical species. We review progress toward understanding crystal growth by particle-attachment processes and show that multiple pathways result from the interplay of free-energy landscapes and reaction dynamics. Much remains unknown about the fundamental aspects, particularly the relationships between solution structure, interfacial forces, and particle motion. Developing a predictive description that connects molecular details to ensemble behavior will require revisiting long-standing interpretations of crystal formation in synthetic systems, biominerals, and patterns of mineralization in natural environments.

Uranium incorporation into aluminum-substituted ferrihydrite during iron(ii)-induced transformation.

Uranium retention processes (adsorption, precipitation, and incorporation into host minerals) exert strong controls on U mobility in the environment, and understanding U retention is therefore crucial for predicting the migration of U within surface and groundwater. Uranium can be incorporated into Fe (hydr)oxides during Fe(ii)-induced transformation of ferrihydrite to goethite. However, ferrihydrite seldom exists as a pure phase within soils or sediments, and structural impurities such as Al alter its reactivity. The presence of Al in ferrihydrite, for example, decreases the rate of transformation to goethite, and thus may impact the retention pathway, or extent of retention, of U. Here, we investigate the extent and pathways of U(vi) retention on Al-ferrihydrite during Fe(ii)-induced transformation. Ferrihydrite containing 0%, 1%, 5%, 10%, and 20% Al was reacted with 10 µM U and 300 µM Fe(ii) in the presence of 0 mM and 4 mM Ca(2+) and 3.8 mM carbonate at pH 7.0. Solid reaction products were characterized using U L3-edge EXAFS spectroscopy to differentiate between adsorbed U and U incorporated into the goethite lattice. Uranium incorporation into Al-ferrihydrite declined from ∼70% of solid-phase U at 0% and 1% Al to ∼30% of solid phase U at 20% Al content. The decrease in U incorporation with increasing Al concentration was due to two main factors: (1) decreased transformation of ferrihydrite to goethite; and, (2) a decrease of the goethite lattice with increasing Al, making the lattice less compatible with large U atoms. However, uranium incorporation can occur even with an Al-substituted ferrihydrite precursor in the presence or absence of Ca(2+). The process of U incorporation into Al-goethite may therefore be a potential long-term sink of U in subsurface environments where Al-substituted iron oxides are common, albeit at lower levels of incorporation with increasing Al content.

Constraints on Precipitation of the Ferrous Arsenite Solid HFe(AsO).

Formation of Fe(II)-As(III) solids is suspected to limit dissolved As concentrations in anaerobic environments. Iron(II) precipitates enriched in As(III) have been observed after microbial reduction of As(V)-loaded lepidocrocite (γ-FeOOH) and symplesite (Fe(II)(As(V)O)]·8HO) and upon abiotic reaction of Fe(II) with As(III). However, the conditions favorable for Fe(II)-As(III) precipitation and the long-term stability (relative to dissolution) of this phase are unknown. Here we examine the composition, local structure, and solubility of an Fe(II)-As(III) precipitate to determine environments where such a solid may form and persist. We reveal that the Fe(II)-As(III) precipitate has a composition of HFe(AsO) and a log of 34 for the dissolution reaction defined as: HFe(AsO) + 8H = 4Fe + 5HAsO. Extended X-ray absorption fine structure spectroscopic analysis of HFe(AsO) shows that the molecular environment of Fe is dominated by edge-sharing octahedra within an Fe(OH) sheet and that As is dominated by corner-sharing AsO pyramids, which are consistent with previously published structures of As(III)-rich Fe(II) solids. The HFe(AsO) solid has a pH-dependent solubility and requires millimolar concentrations of dissolved Fe(II) and As(III) to precipitate at pH <7.5. By contrast, alkaline conditions are more conducive to formation of HFe(AsO); however, a high concentration of Fe(II) is required, which is unusual under alkaline conditions.

Application of total X-ray scattering methods and pair distribution function analysis for study of structure of biominerals.

Total X-ray scattering and pair distribution function (PDF) analysis, using a high-energy synchrotron source, allow direct study of the short- and intermediate-range structure that distinguish amorphous, structurally disordered, and nanocrystalline biominerals. For such samples in which diffuse scatter is a significant component, care must be taken in the experimental procedures to optimize data quality and extract the useful signal necessary to calculate the PDF. General methods are described for data collection and processing, including commonly used software programs. Methods for analysis and interpretation of PDFs are presented, including direct real-space refinement and reverse Monte Carlo methods. Greater application of PDFs to amorphous and poorly crystallized biominerals will provide new insight into structure, especially over length scales that are not probed by other techniques. The rapid data collection available at synchrotron facilities also allows in situ kinetic studies of reactions involving biominerals.