Growing crystals by attaching particles
Crystals grow in a number a ways, including pathways involving the assembly of other particles and multi-ion complexes. De Yoreo
et al.
review the mounting ...evidence for these nonclassical pathways from new observational and computational techniques, and the thermodynamic basis for these growth mechanisms. Developing predictive models for these crystal growth and nucleation pathways will improve materials synthesis strategies. These approaches will also improve fundamental understanding of natural processes such as biomineralization and trace element cycling in aquatic ecosystems.
Science
, this issue
10.1126/science.aaa6760
Materials nucleate and grow by the assembly of small particles and multi-ion complexes.
BACKGROUND
Numerous lines of evidence challenge the traditional interpretations of how crystals nucleate and grow in synthetic and natural systems. In contrast to the monomer-by-monomer addition described in classical models, crystallization by addition of particles, ranging from multi-ion complexes to fully formed nanocrystals, is now recognized as a common phenomenon. This diverse set of pathways results from the complexity of both the free-energy landscapes and the reaction dynamics that govern particle formation and interaction.
Whereas experimental observations clearly demonstrate crystallization by particle attachment (CPA), many fundamental aspects remain unknown—particularly the interplay of solution structure, interfacial forces, and particle motion. Thus, a predictive description that connects molecular details to ensemble behavior is lacking. As that description develops, long-standing interpretations of crystal formation patterns in synthetic systems and natural environments must be revisited.
Here, we describe the current understanding of CPA, examine some of the nonclassical thermodynamic and dynamic mechanisms known to give rise to experimentally observed pathways, and highlight the challenges to our understanding of these mechanisms. We also explore the factors determining when particle-attachment pathways dominate growth and discuss their implications for interpreting natural crystallization and controlling nanomaterials synthesis.
ADVANCES
CPA has been observed or inferred in a wide range of synthetic systems—including oxide, metallic, and semiconductor nanoparticles; and zeolites, organic systems, macromolecules, and common biomineral phases formed biomimetically. CPA in natural environments also occurs in geologic and biological minerals. The species identified as being responsible for growth vary widely and include multi-ion complexes, oligomeric clusters, crystalline or amorphous nanoparticles, and monomer-rich liquid droplets.
Particle-based pathways exceed the scope of classical theories, which assume that a new phase appears via monomer-by-monomer addition to an isolated cluster. Theoretical studies have attempted to identify the forces that drive CPA, as well as the thermodynamic basis for appearance of the constituent particles. However, neither a qualitative consensus nor a comprehensive theory has emerged. Nonetheless, concepts from phase transition theory and colloidal physics provide many of the basic features needed for a qualitative framework. There is a free-energy landscape across which assembly takes place and that determines the thermodynamic preference for particle structure, shape, and size distribution. Dynamic processes, including particle diffusion and relaxation, determine whether the growth process follows this preference or another, kinetically controlled pathway.
OUTLOOK
Although observations of CPA in synthetic systems are reported for diverse mineral compositions, efforts to establish the scope of CPA in natural environments have only recently begun. Particle-based mineral formation may have particular importance for biogeochemical cycling of nutrients and metals in aquatic systems, as well as for environmental remediation. CPA is poised to provide a better understanding of biomineral formation with a physical basis for the origins of some compositions, isotopic signatures, and morphologies. It may also explain enigmatic textures and patterns found in carbonate mineral deposits that record Earth’s transition from an inorganic to a biological world.
A predictive understanding of CPA, which is believed to dominate solution-based growth of important semiconductor, oxide, and metallic nanomaterials, promises advances in nanomaterials design and synthesis for diverse applications. With a mechanism-based understanding, CPA processes can be exploited to produce hierarchical structures that retain the size-dependent attributes of their nanoscale building blocks and create materials with enhanced or novel physical and chemical properties.
Major gaps in our understanding of CPA.
Particle attachment is influenced by the structure of solvent and ions at solid-solution interfaces and in confined regions of solution between solid surfaces. The details of solution and solid structure create the forces that drive particle motion. However, as the particles move, the local structure and corresponding forces change, taking the particles from a regime of long-range to short-range interactions and eventually leading to particle-attachment events.
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.
The solubility of Ag NPs can affect their toxicity and persistence in the environment. We measured the solubility of organic-coated silver nanoparticles (Ag NPs) having particle diameters ranging ...from 5 to 80 nm that were synthesized using various methods, and with different organic polymer coatings including poly(vinylpyrrolidone) and gum arabic. The size and morphology of Ag NPs were characterized by transmission electron microscopy (TEM). X-ray absorption fine structure (XAFS) spectroscopy and synchrotron-based total X-ray scattering and pair distribution function (PDF) analysis were used to determine the local structure around Ag and evaluate changes in crystal lattice parameters and structure as a function of NP size. Ag NP solubility dispersed in 1 mM NaHCO3 at pH 8 was found to be well correlated with particle size based on the distribution of measured TEM sizes as predicted by the modified Kelvin equation. Solubility of Ag NPs was not affected by the synthesis method and coating as much as by their size. Based on the modified Kelvin equation, the surface tension of Ag NPs was found to be ∼1 J/m2, which is expected for bulk fcc (face centered cubic) silver. Analysis of XAFS, X-ray scattering, and PDFs confirm that the lattice parameter, a, of the fcc crystal structure of Ag NPs did not change with particle size for Ag NPs as small as 6 nm, indicating the absence of lattice strain. These results are consistent with the finding that Ag NP solubility can be estimated based on TEM-derived particle size using the modified Kelvin equation for particles in the size range of 5–40 nm in diameter.
Despite the increasing use of silver nanoparticles (Ag-NPs) in nanotechnology and their toxicity to invertebrates, the transformations and fate of Ag-NPs in the environment are poorly understood. ...This work focuses on the sulfidation processes of PVP-coated Ag-NPs, one of the most likely corrosion phenomena that may happen in the environment. The sulfur to Ag-NPs ratio was varied in order to control the extent of Ag-NPs transformation to silver sulfide (Ag2S). A combination of synchrotron-based X-ray Diffraction (XRD) and Extended X-ray Absorption Fine Structure spectroscopy shows the increasing formation of Ag2S with an increasing sulfur to Ag-NPs ratio. TEM observations show that Ag2S forms nanobridges between the Ag-NPs leading to chain-like structures. In addition, sulfidation strongly affects surface properties of the Ag-NPs in terms of surface charge and dissolution rate. Both may affect the reactivity, transport, and toxicity of Ag-NPs in soils. In particular, the decrease of dissolution rate as a function of sulfide exposure may strongly limit Ag-NPs toxicity since released Ag+ ions are known to be a major factor in the toxicity of Ag-NPs.
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 + H 2 O 2 → H 2 O + Pt ( O ). Step 2: Pt(O) + H 2 O 2 → Pt + O 2 + H 2 O. Overall: 2H 2 O 2 → O 2 + 2H 2 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.
Structure of Ferrihydrite, a Nanocrystalline Material Michel, F. Marc; Ehm, Lars; Antao, Sytle M ...
Science (American Association for the Advancement of Science),
06/2007, Volume:
316, Issue:
5832
Journal Article
Peer reviewed
Despite the ubiquity of ferrihydrite in natural sediments and its importance as an industrial sorbent, the nanocrystallinity of this iron oxyhydroxide has hampered accurate structure determination by ...traditional methods that rely on long-range order. We uncovered the atomic arrangement by real-space modeling of the pair distribution function (PDF) derived from direct Fourier transformation of the total x-ray scattering. The PDF for ferrihydrite synthesized with the use of different routes is consistent with a single phase (hexagonal space group P6₃mc; a = ~5.95 angstroms, c = ~9.06 angstroms). In its ideal form, this structure contains 20% tetrahedrally and 80% octahedrally coordinated iron and has a basic structural motif closely related to the Baker-Figgis δ-Keggin cluster. Real-space fitting indicates structural relaxation with decreasing particle size and also suggests that second-order effects such as internal strain, stacking faults, and particle shape contribute to the PDFs.
The effects of alloying concentration on the aqueous corrosion behavior of aluminum-manganese-molybdenum (Al-Mn-Mo) alloys with 8–20 at% Mn and 0–30 at% Mo were investigated by experiments and ...atomistic simulations. The pitting potential of Al-Mn-Mo increased with Mo% from − 0.16–0.49 V. The passive film thickness depended on the total alloy concentration, while its compactness and defect density on the individual ones. Specifically, Al80Mn8Mo12 exhibited higher corrosion resistance than Al80Mn20 due to the formation of a more compact and less defective passive film, as explained by the roles Mo played in both the substrate and the passive film.
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•Mo enhanced the aqueous corrosion resistance and pitting potential of Al-Mn-Mo.•Passive layer thickness depends on the total alloy composition of Mn and Mo.•Selective surface dissolution of Mo increased free volume at metal/oxide interface.•Simulation showed barrier characteristics of oxide increased with free volume in Al.
River systems connect the terrestrial biosphere, the atmosphere and the ocean in the global carbon cycle. A recent estimate suggests that up to 3 petagrams of carbon per year could be emitted as ...carbon dioxide (CO2) from global inland waters, offsetting the carbon uptake by terrestrial ecosystems. It is generally assumed that inland waters emit carbon that has been previously fixed upstream by land plant photosynthesis, then transferred to soils, and subsequently transported downstream in run-off. But at the scale of entire drainage basins, the lateral carbon fluxes carried by small rivers upstream do not account for all of the CO2 emitted from inundated areas downstream. Three-quarters of the world's flooded land consists of temporary wetlands, but the contribution of these productive ecosystems to the inland water carbon budget has been largely overlooked. Here we show that wetlands pump large amounts of atmospheric CO2 into river waters in the floodplains of the central Amazon. Flooded forests and floating vegetation export large amounts of carbon to river waters and the dissolved CO2 can be transported dozens to hundreds of kilometres downstream before being emitted. We estimate that Amazonian wetlands export half of their gross primary production to river waters as dissolved CO2 and organic carbon, compared with only a few per cent of gross primary production exported in upland (not flooded) ecosystems. Moreover, we suggest that wetland carbon export is potentially large enough to account for at least the 0.21 petagrams of carbon emitted per year as CO2 from the central Amazon River and its floodplains. Global carbon budgets should explicitly address temporary or vegetated flooded areas, because these ecosystems combine high aerial primary production with large, fast carbon export, potentially supporting a substantial fraction of CO2 evasion from inland waters.
•Mn enhanced the aqueous corrosion resistance of Al when alloyed in solid solution.•A thinner passive layer with closer to stoichiometry composition was formed on Al-Mn.•Selective surface dissolution ...of Mn increased free volume at metal/oxide interface.•Simulation showed barrier characteristics of oxide increased with free volume in Al.
The effects of manganese on the aqueous corrosion of aluminum-manganese alloys were investigated by experiments and atomistic simulations. Electrochemical measurements, x-ray photoelectron spectroscopy, and atom probe tomography analysis indicate that manganese addition enhanced the corrosion resistance of aluminum without participating in the surface oxidation. The selective dissolution of manganese was believed to increase the free volume at the metal/oxide interface to facilitate the formation of a denser, thinner oxide layer with closer to stoichiometry composition. Atomistic simulations showed that the oxide layer compactness increased, and defect density decreased with increasing free volume content in Al, resulting in enhanced barrier characteristics.
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 (SO4 2–), and arsenate (AsO4 3–). 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 (SO4 2– and AsO4 3–) 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 properties of amorphous calcium carbonate provide the foundation for understanding mineralization of calcified skeletons but inconsistencies between studies suggest the physical picture is ...incomplete. This study quantifies the metastable solubility and structure of ACC produced under a series of chemical conditions. Using complementary in situ methods that quantify ACC structure (in situ PDF analysis) in parallel with chemical composition (TGA, ICP-OES), we find two short-range structures are produced that also show distinct morphological differences. Conditions with high carbonate ion concentrations stabilize a Ca-rich ACC with short-range order that is independent of Mg content. In contrast, low carbonate solutions favor Mg-rich ACC with mixed Ca- and Mg-short range order. Measurements of solubility product determined from supersaturated and undersaturated conditions find the distinctive structures are related to solution carbonate activity while solubility is primarily determined by the magnesium content of the solid. Bulk solubility is a composite that concurs with established values. The findings demonstrate the type of ACC that forms is tuned by Mg2+ and CO3−2− concentration and suggest ACC structure, not bulk composition, is a reliable indicator of intermediate phase. This chemical framework also provides a basis for interpreting apparent differences between observations of ACC in synthetic and natural systems that reconciles previous structural studies.
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