Amorphous Precursor Research Articles

One-pot synthesis of core-shell structured SSZ-13 zeolite using aluminum isopropoxide as an aluminum source

SSZ-13 zeolite with core-shell structure was successfully synthesized by a facile one-step hydrothermal synthesis method using aluminum isopropoxide as an aluminum source. The obtained samples were characterized by XRD, SEM and N2 adsorption/desorption techniques. According to the results, the synthesized SSZ-13 zeolite showed unequivocally the core–shell structure with sub-micron crystalline aggregated SSZ-13 shell formed by epitaxial growth. Moreover, the core-shell structured SSZ-13 possessed distinct pore structure compared with conventional SSZ-13 synthesized by using aluminum nitrate as aluminum source. SEM images demonstrated the formation process of the core-shell structure. Crystals with an spherical morphology were formed from the dissolved Si and Al species, and then amorphous gel precursors deposited on the surface of large SSZ-13 cores and converted into sub-micron SSZ-13 crystals.

A comparative study on emergent pollutants photo-assisted degradation using ruthenium modified titanate nanotubes and nanowires as catalysts

Several methods have been used to tailor nanomaterials structure and properties. Sometimes, slight changes in the structure outcomes expressive improvements in the optical and photocatalytic properties of semiconductor nanoparticles. In this context, the influence of the metal doping and the morphology on a catalyst performance was studied in this work. Here, ruthenium doped titanate nanotubes (RuTNT) were synthesised for the first time using an amorphous Ru-containing precursor. Afterwards, the photocatalytic performance of this sample was compared to the one obtained for ruthenium titanate nanowires (RuTNW), recently reported. Two samples, RuTNW and RuTNT, were produced using the same Ru-containing precursor but distinct hydrothermal methodologies. The powders were structural, morphological and optical characterized by X-ray diffraction and fluorescence, transmission electron microscopy, Raman, X-ray photoelectron and photoluminescence spectroscopies. Distinct variations on the structural and optical properties of the RuTNT and RuTNW nanoparticles, due to ruthenium incorporation were observed. Their potential use as photocatalysts was evaluated on the hydroxyl radical photo-assisted production. Both samples were catalytic for this reaction, presenting better performances than the pristine counterparts, being RuTNT the best photocatalyst. Subsequently, the degradation of two emergent pollutants, caffeine and sulfamethazine, was studied. RuTNT demonstrated to be better photocatalyst than RuTNW for caffeine but identical performances were obtained for sulfamethazine. For both catalysts, the degradation mechanism of the pollutants was explored through the identification and quantification of the intermediate compounds produced and several differences were found. This indicates the importance of the structural and morphological aspects of a material on its catalytic performance.

Bulk SmCo3 nanocrystalline magnets with magnetic anisotropy

In bulk SmCo3 nanocrystalline materials with small grain sizes of 10–20 nm, it is a challenge to realize a strong crystal texture that is crucial for them as high-performance permanent magnets. Here we fabricated an anisotropic bulk SmCo3 nanocrystalline magnet with a strong (00l) texture combined with a small grain size of 10–20 nm using a high-pressure thermal compression (HPTC) approach from amorphous precursors. This magnet had an obvious magnetic anisotropy with Mr///Mr⊥ = 1.53 and a maximum energy product of 4.2 MGOe (33.8 kJ/m3) which were obviously higher than those Mr///Mr⊥ = 1.01 and 1.9 MGOe (15.1 kJ/m3) of the isotropic magnets prepared using the same precursors. These anisotropic magnets showed a coercivity temperature coefficient of β = –0.31 %/°C and a remanence temperature coefficient of α = –0.14 %/°C. The deformation and annealing temperatures played important roles on the microstructures and magnetic properties of these anisotropic SmCo3 nanocrystalline magnets. These findings make an important step toward the fabrication of oriented bulk SmCo3 magnets for practical applications.

Shape-preserving amorphous-to-crystalline transformation of CaCO3 revealed by in situ TEM

Organisms use inorganic ions and macromolecules to regulate crystallization from amorphous precursors, endowing natural biominerals with complex morphologies and enhanced properties. The mechanisms by which modifiers enable these shape-preserving transformations are poorly understood. We used in situ liquid-phase transmission electron microscopy to follow the evolution from amorphous calcium carbonate to calcite in the presence of additives. A combination of contrast analysis and infrared spectroscopy shows that Mg ions, which are widely present in seawater and biological fluids, alter the transformation pathway in a concentration-dependent manner. The ions bring excess (structural) water into the amorphous bulk so that a direct transformation is triggered by dehydration in the absence of morphological changes. Molecular dynamics simulations suggest Mg-incorporated water induces structural fluctuations, allowing transformation without the need to nucleate a separate crystal. Thus, the obtained calcite retains the original morphology of the amorphous state, biomimetically achieving the morphological control of crystals seen in biominerals.

Crystallization of TiO2 polymorphs from RF-sputtered, amorphous thin-film precursors

Crystalline TiO2 films of anatase, brookite, and rutile are reproducibly made from amorphous precursors deposited by RF magnetron sputtering, producing large-area, single phase films of uniform thickness. Sputtered amorphous TiO2 precursor thin films follow the general behavior observed for amorphous precursor thin films generated by pulsed laser deposition, namely, that oxygen deficiency is necessary for the formation of brookite and rutile. We quantify the oxygen deficiency and correlate it with the long wavelength optical absorption. We find that the precursor deposition rate is also a contributing factor to phase selection and that brookite and rutile form from films deposited more rapidly and anatase from films deposited more slowly. Sputtered and pulsed laser deposited amorphous precursor films prepared with similar oxygen deficiency and similar thickness result in the same final state after annealing, but the rate for sputtered precursors is slower.

Biomineralization Precursor Carrier System Based on Carboxyl-Functionalized Large Pore Mesoporous Silica Nanoparticles.

Bone and teeth are derived from intrafibrillarly mineralized collagen fibrils as the second level of hierarchy. According to polymer-induced liquid-precursor process, using amorphous calcium phosphate precursor (ACP) is able to achieve intrafibrillar mineralization in the case of bone biomineral in vitro. Therefore, ACP precursors might be blended with any osteoconductive scaffold as a promising bone formation supplement for in-situ remineralization of collagens in bone. In this study, mesoporous silica nanoparticles with carboxyl-functionalized groups and ultra large-pores have been synthesized and used for the delivery of liquid like biomimetic precursors (ACP). The precursor delivery capacity of the nanoparticles was verified by the precursor release profile and successful mineralization of 2D and 3D collagen models. The nanoparticles could be completely degraded in 60 days and exhibited good biocompatibility as well. The successful translational strategy for biomineralization precursors showed that biomineralization precursor laden ultra large pore mesoporous silica possessed the potential as a versatile supplement in demineralized bone formation through the induction of intrafibrillar collagen mineralization.

Arsenic(III) biotransformation to tooeleite associated with the oxidation of Fe(II) via Acidithiobacillus ferrooxidans

Tooeleite (Fe6(AsO3)4(SO4)(OH)4·4H2O), the only known ferric arsenite sulfate bearing mineral, has great potential for arsenic remediation due to its structure favoring incorporation of As(III). Based on the natural attenuation of removing As(III) directly by the formation of tooeleite via microorganisms, an iron-oxidizing bacterial strain Acidithiobacillus ferrooxidans ATCC 23270 (At.ferrooxidans) was selected to facilitate the formation of tooeleite. The optimized condition for the biogenic tooeleite was obtained at pH of 2.0, 30 °C and an initial arsenic of 500 mg/L. The process of biological mineralization is accompanied by the removal of 95.4% arsenic. What’s more, biosynthetic tooeleite crystallization via a three-stage process was revealed using a combination of liquid and solid analyses (ICP-OES, XRD, XPS, FT-IR, SEM, STEM, particle distribution). The three stages included Fe2+ oxidation by At.ferrooxidans, Fe3+ hydrolysis and an initial Fe–As amorphous precursors formation, and finally transforming to tooeleite crystal. Moreover, RT-qPCR was used to reveal the relationship between functional gene expression of At.ferrooxidans and the mineral formation. The results showed the biogenic tooeleite exerts significant control on the geochemistry of arsenic contaminated systems.

Growth of Pr2Ir2O7 thin films using solid phase epitaxy

(111)-oriented pyrochlore Pr2Ir2O7 epitaxial thin films were successfully fabricated on yttria-stabilized zirconia (111) substrates by means of solid phase epitaxy, namely, the combination of pulsed laser deposition (PLD) of an amorphous precursor film at room temperature, followed by epitaxial crystallization in an ex situ post-annealing process in air. The Pr2Ir2O7 phase cannot be obtained in a conventional in situ PLD process because of severe Ir loss. The Pr2Ir2O7 thin films showed metallic electronic conductivity and the spontaneous Hall effect. These transport properties are similar to the ones reported for bulk single crystal samples, indicating that the thin films had high crystalline quality.

Atomistic Simulation and Characterization of Spinel Li1+xMn2O4 (0 ≤ x ≤ 1) Nanoparticles

Lithium-ion batteries, comprising nanoparticulate Ni–Mn–Co (NMC) cathodes that have been used to power electric vehicles, can be improved by blending NMC with Li–Mn–O (LMO). However, LMO undergoes a cubic to tetragonal phase change during charge cycling, which cracks and pulverizes the material, resulting in capacity fading. Structural characterization during the phase transition is the first step in mitigating capacity fading and can be challenging experimentally and computationally. Here, we use simulated amorphization and crystallization to generate atom-level models of the LMO nanoparticles. This simulation strategy does not require any structural information to be predetermined. Instead, the structures evolve “naturally” from amorphous precursors. Analysis of the model Li–Mn–O nanoparticles reveals that they comprise domains of defect-rich spinel, Mn3O4, layered Li2MnO3, and lithium-rich spinel Li1+xMn2–xO4 phases together with complex microstructural features. The discharge process was modeled by inserting surplus lithium atoms into the nanoparticles, resulting in structural changes, accompanied by a variety of constituent polymorphs. A transitional multigrained structure, between the cubic (Li1Mn2O4) and tetragonal (Li2Mn2O4) phases, is observed at Li1.75Mn2O4. We also find that microstructural features, such as microtwinning and intrinsic dopants, vacancies, etc., result in a network of Li transport pathways, enabling Li mobility in all three spatial directions.

Direct Observation of Amorphous Precursor Phases in the Nucleation of Protein-Metal-Organic Frameworks.

Protein-metal-organic frameworks (p-MOFs) are a prototypical example of how synthetic biological hybrid systems can be used to develop next-generation materials. Controlling p-MOF formation enables the design of hybrid materials with enhanced biological activity and high stability. However, such control is yet to be fully realized due to an insufficient understanding of the governing nucleation and growth mechanisms in p-MOF systems. The structural evolution of p-MOFs was probed by cryo-transmission electron microscopy, revealing nonclassical pathways via dissolution-recrystallization of highly hydrated amorphous particles and solid-state transformation of a protein-rich amorphous phase. On the basis of these data, we propose a general description of p-MOF crystallization which is best characterized by particle aggregation and colloidal theory for future synthetic strategies.

Ultrathin silicon oxide prepared by in-line plasma-assisted N2O oxidation (PANO) and the application for n-type polysilicon passivated contact

We develop a plasma-assisted nitrous-oxide (N2O) gas oxidation (PANO) method to prepare the ultrathin silicon oxide (SiOx) for polysilicon (poly-Si) passivated contact. The effects of preparation conditions, including the substrate temperature, processing time, and plasma power, are studied. Afterwards, we integrate the PANO SiOx into the polysilicon passivated contact and optimize the passivation and contact performances. Excellent surface passivation with the n-type poly-Si and PANO SiOx on the n-type c-Si wafer is achieved by 880 °C annealing, which shows competitive passivation quality to the one with NASO SiOx. Champion implied open-circuit voltage (iVoc) and single-sided recombination saturated current (J0) reach 730 mV and 4.3 fA/cm2 after crystallization; and they are further improved to 747 mV and 2.0 fA/cm2 (3 × 1015cm−3) after subsequent AlOx/SiNx hydrogenation. Using transmission electron microscopy (TEM), we find that the thickness of PANO SiOx ranges 1.1–2.4 nm and the controlled nitric acid oxidized SiOx (NAOS) ranges 1.3–1.8 nm. The contact resistivity (ρc) is typically <10 mΩ cm2 with the annealing temperature of >820 °C. Also, the crystallinity, phosphorous in-diffusion profile, and current-leaking density of the passivated contacts are investigated. In general, the PANO SiOx and in-situ doping amorphous silicon precursor can be fabricated in one PECVD system without additional equipment or transfer procedures, which is favorable for the high-efficiency, low-cost industrial manufacture.

X-ray photoelectron and ion scattering spectroscopic surface analyses of amorphous and crystalline calcium phosphate nanoparticles with different chemical histories.

Abundant hydration, diffusivity, and volatile phase composition render the surface of calcium phosphates (CPs) a complex dynamic region. Three CP powders were analyzed using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) with regard to their surface compositions and fine structures: hydroxyapatite (HAp), amorphous CP (ACP), and a CP formed under the same conditions as ACP but allowed to ripen into HAp (ACP → HAp). XPS analyses revealed that the Ca/P atomic ratio in the 2-10 nm-thick layers of the nanoparticle surface was somewhat lower than the stoichiometric ratio for all the three CPs. However, it was still lower for ACP and ACP → HAp than that for HAp, indicating the incongruent dissolution of all the surfaces and also greater instability and higher solubility of ACP and ACP → HAp as compared to those of HAp. Consequently, as indicated by both XPS and ISS, the amount of adventitious carbon bound to HAp was higher than that on ACP or ACP → HAp. The binding energies of the most intense XPS lines of all the three main atomic elements in HAp, i.e., O1s, P2p, and Ca2p downshifted for HAp as compared to those for ACP and ACP → HAp, supporting the premise of similarity in the surface structures between the two CP powders with a common amorphous precursor and kinetic path of formation. The ISS analysis, which investigated the 1-2 topmost atomic layers of the surface, indicated a higher level of heterogeneity of the oxygen states. This coincided with 40Ca accounting for over 80% atoms in this uppermost atomic layer of the surface of all three CPs. Ca-O bonds were particularly dominant in the topmost surface layer of ACP, where the Ca/P atomic ratio was an order of magnitude higher than that in ACP → HAp or HAp. A thermodynamic explanation and structural model of the surface accounting for the overabundance of Ca2+ in the topmost layer and the overall depletion of it elsewhere are provided in the discussion. Overall, the combined results of XPS and ISS analyses demonstrate a similarity between the surfaces of the two different forms of HAp as compared to that for ACP, but also a definite impression of the traces of their formation history on them.

Effect of latent heat during primary crystallization on the nanostructural formation process in nanocrystalline soft magnetic materials

Nanocrystalline soft magnetic materials are known to be prepared by primary crystallization of Fe-based amorphous precursors. Since the crystallization reaction is exothermic, the amorphous precursor may experience a temporary rise in its temperature relative to its surroundings during the process of nanocrystallization. Given the typical latent heat of primary crystallization (∼ 100 kJ/kg), this temperature rise may exceed hundreds of degrees if not adequately controlled and thus, lead to the formation of unwanted magnetically hard compounds. This effect is generally small for isolated ribbons annealed with a moderate heating rate. However, the recent adoption of high heating rates and short annealing times has caused the self-heating effect to become relevant even for small sample sizes. In this work, the effect of self-heating on the microstructure and magnetic properties of nanocrystalline Fe86B14 is investigated. It is found that magnetically hard Fe-B compounds cannot be avoided when annealing under vacuum in an infrared furnace with a heating rate ≥3 K/s due to the self-heating effect. However, the high thermal conductivity of the copper blocks used by the ultra-rapid annealing process can successfully prevent a self-heating induced temperature rise during annealing, avoiding unwanted compound formation. Finite Element Analysis is also used for predicting the extent of self-heating during infrared annealing.

Molecular level nucleation mechanisms of hierarchical MFI and MOR zeolite structures via non-stochastic pathways.

Understanding the chemical mechanism of crystal nucleation at the molecular level is crucial for the design of architectural structures of valuable materials in the future. In this study, it has been revealed that amorphous silicate precursors, which play a role in the nucleation processes of zeolitic frameworks, can be regularly fragmented in mass spectroscopy due to the hydroxyl functional groups in their molecular structures. In this way, by using the mass spectra acquired from LDI-TOF MS, the systematic evolution stages of a common 1D precursor converting to the 3D unit cells of MFI and MOR zeolite structures observed in the same reaction medium were constructed through a nucleation mechanism at the molecular level for the first time. Here we show a novel nucleation pathway that does not occur via stochastic assembly of atoms or distinct building blocks by molecular recognition. Each of the proposed nucleation mechanisms of these different frameworks carrying structural similarities is from different combinations of sequential self-attaching intramolecular covalent couplings of identical origin precursors. The dynamic molecular structure capable of forming finite building units of target frameworks during the nucleation process of this precursor, which is the polymerized form of simple 6-membered siloxane chains, has been arranged around structure directing agents before a hydrothermal reaction.

Transformation of amorphous precursor to crystalline carbonate: Insights from Mg isotopes in the dolomite-analogue mineral norsethite [BaMg(CO3)2

Crystallization from an amorphous precursor is an important pathway of carbonate precipitation in nature. However, the mechanistic details of the transformation from an amorphous phase to a crystalline phase of carbonates remain a topic of intense debate. Two competing mechanisms, including solid-state transition and coupled dissolution-reprecipitation, have been proposed to explain this transformation process. Magnesium is a common element in carbonate crystal lattices and its isotopes may provide unique insights into this problem. In this study, we investigated the transformation of the amorphous carbonate (AC) precursor for norsethite [BaMg(CO3)2], a dolomite analogue mineral, by in situ XRD analysis and isotope exchange experiments using a 25Mg enriched tracer coupled with high precision isotope analyses of δ26Mg and δ25Mg values for aqueous and solid phases. In situ XRD experiments revealed that the AC can transformed to crystalline norsethite at various temperatures (25 °C, 50 °C and 70 °C) and no intermediate mineral formed during the AC transformation process. 25Mg tracers indicated that near-complete Mg isotope exchange occurred in all exchange experiments during AC transformation. More importantly, after the AC transformation, the system showed surprising apparent non-mass dependent fractionation relationship, that the δ25Mg value of solid phase became greater than that of aqueous solution from a lower value, producing positive Δ25Mgsolid-aq fractionation, whereas the Δ26Mgsolid-aq fractionation remained negative. We numerically modeled the behavior of Mg isotopes (in both δ26Mg and δ25Mg) for the experimental system according to the two competing mechanisms of AC transformation. The modeling results suggest that the apparent non-mass dependent isotope behavior can only be explained by the coupled dissolution-reprecipitation process. Therefore, this study does not support the solid-state transition mechanism for AC transformation. Further, this study rigorously proves that norsethite can form by precipitation from aqueous solution without replacement, and implies that Mg2+ in aqueous solutions can be efficiently dehydrated and incorporated into a well ordered dolomite-group mineral (norsethite) under abiotic, low temperature conditions, thus providing new insights for understanding dolomite precipitation in nature.

Utilizing TiO2 amorphous precursors for polymorph selection: An in situ TEM study of phase formation and kinetics

AbstractSelective synthesis of metastable polymorphs requires a fundamental understanding of the complex energy landscapes in which these phases form. Recently, the development of in situ high temperature and controlled atmosphere transmission electron microscopy has enabled the direct observation of nucleation, growth, and phase transformations with near atomic resolution. In this work, we directly observe the crystallization behavior of amorphous TiO2 thin films grown under different pulsed laser deposition conditions and quantify the mechanisms behind metastable crystalline polymorph stabilization. Films deposited at 10 mTorr chamber oxygen pressure crystallize into nanocrystalline Anatase at 325°C, whereas films deposited at 2 mTorr crystallize into significantly larger needle‐like grains of Brookite and Anatase at 270°C. Increasing film deposition rate by a factor of 4 results in a 10× increase in the crystalline growth front velocity as well as a decrease in crystallization temperature from 270°C to 225°C. Engineering the amorphous precursor state through deposition conditions therefore provides routes to microstructure control and the accessibility of higher energy metastable phases.

Effect of Co Content on Microstructures and Magnetic Properties of Anisotropic Bulk TbCu7-type Sm-Co Nanocrystalline Magnets

The influences of Co content on the microstructures and magnetic properties of the anisotropic bulk TbCu7-type Sm-Co nanocrystalline magnets prepared by high-pressure thermal compression (HPTC) from amorphous SmCox precursors with 6.37 ≤ × ≤ 8.5 had been investigated. We found that with Co content x = 6.37, the TbCu7-type Sm-Co phase and a small amount of CaCu 5 -type Sm-Co crystallized from the precursors; from x = 6.7 to x = 8.5, only single TbCu 7 -type Sm-Co phase generated during the HPTC process. When x = 6.37, an obvious (00l) texture was formed, and with increasing Co content, the (00l) texture of the synthesized magnets showed a decreasing trend. With increasing Co content, the remanence B r and saturation magnetization M s of the nanostructure significantly increased while the coercivity H ci gradually decreased, and at x = 7.25, a peak value of the maximum energy product (BH) max = 19.7 MGOe was obtained. The detailed mechanism was also discussed.

Phase evolution during accelerated CO2 mineralization of brucite under concentrated CO2 and simulated flue gas conditions

Experimental work on the carbonation of brucite has been carried out in the temperature and pressure ranges of 50−120 °C and 1−10 bar respectively, using concentrated CO2 and simulated flue gas. At 120 °C hydromagnesite and trace amounts of magnesite were identified. Highly to semi-ordered dypingite like-phases and nesquehonite, coexisting with a possibly amorphous carbonate phase, were identified at 50 °C. The dehydration temperatures and chemical composition of these amorphous phases are very close to those of crystalline, stoichiometric nesquehonite. This probably amorphous phase nourishes the late formation of dypingite. This latter mineral gradually undergoes a cell shrinkage due to the partial loss of molecular waters, becoming structurally more ordered as the carbonation reaction proceeds. It remains unclear whether nesquehonite formed directly from brucite or from the crystallization of an amorphous precursor. However, nesquehonite is precursor of dypingite and/or the possibly amorphous phase. Only crystalline carbonate phases were observed at 120 °C. Concentrated CO2 experiments yielded the highest amounts (up to 37 wt.%) of CO2 sequestered at 10 bar and 16 h of reaction, reaching an almost complete carbonation of brucite (>98 %). On the other hand, flue gas experiments results showed higher amounts of CO2 sequestered per unit of CO2 partial pressure than with concentrated CO2.

Evolution of microstructures and magnetic properties in bulk Nd-Fe-Cu-B nanocomposites prepared by high pressure thermal deformation

Evolution of microstructures and magnetic properties in bulk Nd-Fe-Cu-B based nanocomposites prepared by high pressure thermal deformation from amorphous precursors under different thermomechanical histories were investigated. We found that two-step isothermal deformation was favorable to the formation of (00l) texture of Nd2Fe14B phase and the enhancement of energy product compared with isothermal deformation, calefactive & isothermal deformation and isothermal & calefactive deformation, where a much higher energy product (BH)max = 22.8 MGOe was obtained for the former nanocomposite than those 15.6 MGOe, 17.3 MGOe and 20.1 MGOe for the latters, respectively. For the nanocomposites prepared by two-step isothermal deformation, the (00l) texture degree and the energy product increased with the first-step deformation-time up to 5 s then decreased, yielding the maximum magnetic anisotropy and energy product at 5 s. The magnetic anisotropy and energy product increased and then decreased with the temperatures increase from 500 °C to 560 °C in the first-step deformation, and the same change occurs in the second-step temperatures from 600 °C to 720 °C. The maximum anisotropy and energy product were obtained at 530 °C and 650 °C, respectively. The detailed mechanisms for these evolutions have been discussed.

Amorphous liquid phase induced synthesis of boron nitride nanospheres for improving sintering property of h-BN/ZrO2 composites

Hexagonal boron nitride (BN) nanospheres with large quantities and high crystallinity have been successfully synthesized from BxCyNzOkHl precursors without any catalyst. The uniform size of the obtained BN nanospheres is about 20 nm. The structure, morphology and surface chemical property of the products were characterized using XRD, FTIR, SEM, HRTEM and XPS. The results show that BN nanospheres with good crystallinity are prepared by heat treatment of amorphous BxCyNzOkHl precursor at 1100 °C in flowing N2. The urea content has great influence on the crystallinity and morphology of BN nanospheres. The diameter and morphology of BN nanospheres dramatically vary at different temperature. Based on the amorphous liquid phase encapsulated induce model, the reaction mechanism and morphological evolution mechanism were described. The viscous liquid containing B and N amorphous phase plays a crucial role in the formation of BN nanospheres. The BN nanospheres improved the density of h-BN/ZrO2 composites from 94.43% to 97.02%.