Mechanism study of Single-Step synthesis of Fe(core)@Pt(shell) nanoparticles by sonochemistry.

Transition metal (TM) core-platinum (Pt) shell nanoparticles (TM@Pt NPs) are attracting a great deal of attention as highly active and durable oxygen reduction reaction (ORR) electrocatalysts of fuel cells and metal-air batteries. However, most of the reported synthesis methods of TM@Pt NPs are multistep in nature, a significant disadvantage for real applications. In this regard, our group has reported a single-step method to synthesize TM@Pt NPs for TM = Mn, Fe, Co, and Ni by using sonochemistry, namely the UPS (ultrasound-assisted polyol synthesis) method. Previously, we proposed the mechanism of the formation of these TM@Pt NPs by UPS method, but rather in a rough sense. Some details are missing and the optimal conditions have not been established. In the present work, we performed detailed studies on the formation mechanism of UPS reaction by using Fe@Pt NPs as the model system. Effects of synthesis parameters such as the nature of metal precursor, conditions of ultrasound, and temperature profile as a function of reaction time were assessed, along with the analyses of intermediates during the UPS reaction. As results, we verified our previously proposed mechanism that, under appropriate conditions, Fe core is formed through the cavitation and implosion of the solvent, induced by the ultrasound, and the Pt shell is formed by the chemical reaction between Fe core and Pt reagent, independent from the direct effect of ultrasound. In addition, we established the optimal conditions to obtain a high purity Fe@Pt NPs in a high yield (>90% based on Pt), which may enable the increase of synthesis scale of Fe@Pt NPs, a necessary step for the real application of TM@Pt NPs.

Facile synthesis of Nafion-supported Pt nanoparticles with ultra-low loading as a high-performance electrocatalyst for hydrogen evolution reaction.

x%Pt-Naf-CV (Pt-Nafion-Cyclic Voltammetry) catalysts with homogeneously distributed platinum nanoparticles and ultra-low Pt loading are successfully synthesized by using a facile potential cycling approach. The as-synthesized 0.8%Pt-Naf-CV catalyst exhibits an enhanced electrocatalytic activity for hydrogen evolution reaction (HER) in 0.5 M H2SO4 solution, which obtains a low overpotential of 34 mV at 10 mA cm-2. The linear sweep voltammetry (LSV) curve of 0.8%Pt-Naf-CV catalyst is almost consistent with that of commercial Pt/C. However, the 0.8%Pt-Naf-CV catalyst displays a more excellent stability and durability in comparison with commercial Pt/C. Besides, the Pt loading of Pt/C (Pt-10 wt%) is about 10 times that of 0.8%Pt-Naf-CV catalyst. The improved electrocatalytic performances are derived from the synergistic effects of Pt and Nafion. The Nafion plays a significant role as a dispersant, carrier and structure directing agent on the morphology and size of the Pt catalyst. This result contributes a promising method to enhance the catalytic activity and reduce the amount of Pt.

CO2 absorption and desorption characteristics of MgO-based absorbent promoted by triple eutectic alkali carbonate.

Carbon capture and sequestration is emerging as a promising technology to mitigate the greenhouse effect by reducing CO2 emissions. Of a number of metal oxides applied as CO2 absorbents, MgO is a potential material that can operate in a relatively low elevated temperature range (200-500 °C), namely, intermediate-temperatures. In the present research, we investigated the characteristics of CO2 absorption and desorption on MgO-based absorbents promoted by molten alkali metal carbonate that has a melting point of 397 °C (eutectic molar ratio of Li2CO3 : Na2CO3 : K2CO3 = 0.435 : 0.315 : 0.250). These absorbents absorb CO2 in two steps with the first step being very fast and large in capacity in comparison with other related absorbents in the literature and the second step being much slower than the first one. The overall capacity can be as large as 77% MgO conversion, much larger than other carbonate-promoted MgO absorbents in the literature. The fast first step of CO2 absorption is associated with the reaction of highly basic sites on the MgO surface formed through the interaction between the carbonates and MgO upon pre-treatment. Detailed analyses via in situ XRD revealed that MgCO3 and a new phase, probably a double carbonate between Mg and alkali ions, are formed as the carbonation products. A detailed mechanism is proposed based on the experimental data, which highlights the unique properties of the molten alkali carbonate as a dissolution medium for CO2 and MgO and even the product MgCO3.

Ternary core-shell PdM@Pt (M = Mn and Fe) nanoparticle electrocatalysts with enhanced ORR catalytic properties.

In this work, we introduce composition-tunable core-shell-like PdM@Pt (M = Mn and Fe) nanoparticles (NPs) on carbon support (PdM@Pt/C) synthesized by one-pot sonochemical reactions using high-intensity ultrasonic probe (150 W, 20 kHz, with 13 mm solid probe) and investigate their electrocatalytic performance for oxygen reduction reaction (ORR). The core-shell-like structure of the NPs are evidenced by the elemental distribution maps obtained by energy dispersive X-ray spectroscopy equipped on scanning transmission electron microscopy. Based on the characterization data, PdM@Pt NPs were synthesized with variable elemental compositions (Pd49Fe21@Pt30, Pd17Fe31@Pt52, Pd46Mn6@Pt48 and Pd15Mn5@Pt80). All PdM@Pt samples are composed of large (~10 nm) and small (~3 nm) NPs, the large ones appear to be aggregates of the smaller ones, and the proportion of the larger NPs increases with the Pd content, which can be explained with the known mechanisms of sonochemical reactions of related systems. Electrochemical analyses on samples show that the ORR mass activity of PdM@Pt/C is 3-fold (normalized by Pt) and 1.7-fold (normalized by platinum group metal (PGM)) higher than those of Pt/C (commercial). All PdM@Pt/C sample show superior durability with the electrochemical surface area (ECSA) change of -4.4-+12.0% and half-wave potential change (ΔE1/2) of 8-14 mV after 10 k cycles accelerated stress test (AST) to Pt/C with ECSA change of -25.6% and ΔE1/2 of 19 mV.

A thermosensitive fluorescent Eu-based metal-organic framework and its polyether sulfone composite film as a thermal sensor.

Compound 1, [Eu(NDC)(H2O)Cl] (H2NDC = 2,6-Naphthalene dicarboxylic acid), with thermosensitive fluorescence was fabricated into composite films with polyether sulfone, and the one with 40% loading of 1 showed highly sensitive and repeatable thermal sensor properties in an alarm device.

Peptide-Programmable Nanoparticle Superstructures with Tailored Electrocatalytic Activity.

Biomaterials derived via programmable supramolecular protein assembly provide a viable means of constructing precisely defined structures. Here, we present programmed superstructures of AuPt nanoparticles (NPs) on carbon nanotubes (CNTs) that exhibit distinct electrocatalytic activities with respect to the nanoparticle positions via rationally modulated peptide-mediated assembly. De novo designed peptides assemble into six-helix bundles along the CNT axis to form a suprahelical structure. Surface cysteine residues of the peptides create AuPt-specific nucleation site, which allow for precise positioning of NPs onto helical geometries, as confirmed by 3-D reconstruction using electron tomography. The electrocatalytic model system, i.e., AuPt for oxygen reduction, yields electrochemical response signals that reflect the controlled arrangement of NPs in the intended assemblies. Our design approach can be expanded to versatile fields to build sophisticated functional assemblies.

Facile sonochemical synthesis of amorphous NiFe-(oxy)hydroxide nanoparticles as superior electrocatalysts for oxygen evolution reaction.

In this work, we present facile synthesis of amorphous Ni/Fe mixed (oxy)hydroxide (NiFe(H)) nanoparticles (NPs) and their electrocatalytic performance for oxygen evolution reaction (OER) in alkaline media. a-NiFe(H) NPs have received lots of attention as OER electrocatalysts with many desirable properties. By using a simple sonochemical route, we prepared amorphous Ni and Fe-alkoxide (NiFe(A)) NPs whose composition can be controlled in the entire composition range (Ni100-xFex, 0≤x≤1). These samples are composed of extremely small NiFe(A) NPs with Ni and Fe atoms homogeneously distributed. NiFe(A) NPs are readily converted into corresponding electrocatalytically active NiFe(H) NP by a simple electrochemical treatment. Electrochemical analysis data show that the OER activity of amorphous NiFe(H) samples follows the volcano-type trend when plotted against the Fe content. Ni70Fe30(H) sample showed the lowest overpotential of 292mV at 10mAcm-2geo and the lowest Tafel slope of 30.4mVdec-1, outperforming IrOx/C (326mV, 41.7mVdec-1). Our samples are highly durable based on the chronopotentiometry data at the current density of 10mAcm-2geo for 2h which show that Ni70Fe30 sample maintains the steady-state potential, contrary to the time-varying IrOx/C.

Ensemble averaged structure-function relationship for nanocrystals: effective superparamagnetic Fe clusters with catalytically active Pt skin.

Practical applications require the production and usage of metallic nanocrystals (NCs) in large ensembles. Besides, due to their cluster-bulk solid duality, metallic NCs exhibit a large degree of structural diversity. This poses the question as to what atomic-scale basis is to be used when the structure-function relationship for metallic NCs is to be quantified precisely. We address the question by studying bi-functional Fe core-Pt skin type NCs optimized for practical applications. In particular, the cluster-like Fe core and skin-like Pt surface of the NCs exhibit superparamagnetic properties and a superb catalytic activity for the oxygen reduction reaction, respectively. We determine the atomic-scale structure of the NCs by non-traditional resonant high-energy X-ray diffraction coupled to atomic pair distribution function analysis. Using the experimental structure data we explain the observed magnetic and catalytic behavior of the NCs in a quantitative manner. Thus we demonstrate that NC ensemble-averaged 3D positions of atoms obtained by advanced X-ray scattering techniques are a very proper basis for not only establishing but also quantifying the structure-function relationship for the increasingly complex metallic NCs explored for practical applications.

Characteristics of NaNO3-Promoted CdO as a Midtemperature CO2 Absorbent.

In this study, we explored the reaction system CdO(s) + CO2(g) ⇄ CdCO3(s) as a model system for CO2 capture agent in the intermediate temperature range of 300-400 °C. While pure CdO does not react with CO2 at all up to 500 °C, CdO mixed with an appropriate amount of NaNO3 (optimal molar ratio NaNO3/CdO = 0.14) greatly enhances the conversion of CdO into CdCO3 up to ∼80% (5.68 mmol/g). These NaNO3-promoted CdO absorbents can undergo many cycles of absorption and desorption by temperature swing between 300 and 370 °C under a 100% CO2 condition. Details of how NaNO3 promotes the CO2 absorption of CdO have been delineated through various techniques using thermogravimetry, coupled with X-ray diffraction and electron microscopy. On the basis of the observed data, we propose a mechanism of CO2 absorption and desorption of NaNO3-promoted CdO. The absorption proceeds through a sequence of events of CO2 adsorption on the CdO surface covered by NaNO3, dissolution of so-formed CdCO3, and precipitation of CdCO3 particles in the NaNO3 medium. The desorption occurs through the decomposition of CdCO3 in the dissolved state in the NaNO3 medium where CdO nanoparticles are formed dispersed in the NaNO3 medium. The CdO nanoparticles are aggregated into micrometer-large particles with smooth surfaces and regular shapes.

Idiopathic Avascular Necrosis of First Metatarsal Head in a Pediatric Patient.

Idiopathic avascular necrosis of the first metatarsal head rarely occurs in pediatrics. The present case of avascular necrosis of the first metatarsal head occurred in a 13-year-old male who came to the clinic with a 9-month history of pain in the first metatarsophalangeal joint. Conservative treatment had been applied for 9 months, but the pain had not been relieved. Therefore, surgical treatment, including decompression and debridement, was performed in the first metatarsal head of the patient. After 6 months of follow-up monitoring, full range of motion of the first metatarsophalangeal joint was observed, and the pain had disappeared. No any other complications had developed during 18 months of follow-up monitoring.

Mechanisms of absorption and desorption of CO2 by molten NaNO3-promoted MgO.

In order to realize carbon capture and sequestration (CCS), a technology proposed to circumvent the global warming problem while maintaining the present level of economic activity, the development of efficient carbon-capturing agents is of prime importance. In addition to the prevailing amine-based agents that operate at temperatures lower than 200 °C, agents that can operate at higher temperatures are being considered to reduce the cost of CCS. For the mid-temperature (200-500 °C) operation, alkali nitrate-promoted MgO is a promising candidate; whose detailed reaction mechanisms are not yet fully understood, however. In the present study, we have performed a comprehensive investigation on the mechanisms of CO2 absorption and desorption of NaNO3-promoted MgO. Highly efficient CO2 absorbents were obtained by decomposing Mg5(CO3)4(OH)2·4H2O with NaNO3 intimately mixed with it. Our collective data, including isothermal CO2 uptake curves, MgO solubility in molten NaNO3, and observations on the reaction of MgO wafers with CO2, indicate that the absorption takes place in the molten NaNO3 medium in which both CO2 and MgO are dissolved. MgCO3 is formed inside the molten promoter through the nucleation and growth steps. The decomposition of MgCO3 back to MgO, that is desorption of CO2, is also facilitated by molten NaNO3, which we attribute to the decreased relative stability of MgCO3 with respect to MgO when in contact with molten NaNO3. The relative affinity of molten nitrate to MgO and MgCO3 was estimated by measuring the 'contact angles' of nitrate on them. Implications of our findings for the real applications of alkali nitrate-promoted MgO absorbents with numerous repeated cycles of absorption and desorption of CO2 are discussed.

Self-arrangement of nanoparticles toward crystalline metal oxides with high surface areas and tunable 3D mesopores.

We demonstrate a new design concept where the interaction between silica nanoparticles (about 1.5 nm in diameter) with titania nanoparticles (anatase, about 4 nm or 6 nm in diameter) guides a successful formation of mesoporous titania with crystalline walls and controllable porosity. At an appropriate solution pH (~1.5, depending on the deprotonation tendencies of two types of nanoparticles), the smaller silica nanoparticles, which attach to the surface of the larger titania nanoparticles and provide a portion of inactive surface and reactive surface of titania nanoparticles, dictate the direction and the degree of condensation of the titania nanoparticles, resulting in a porous 3D framework. Further crystallization by a hydrothermal treatment and subsequent removal of silica nanoparticles result in a mesoporous titania with highly crystalline walls and tunable mesopore sizes. A simple control of the Si/Ti ratio verified the versatility of the present method through the successful control of mean pore diameter in the range of 2-35 nm and specific surface area in the ranges of 180-250 m(2) g(-1). The present synthesis method is successfully extended to other metal oxides, their mixed oxides and analogues with different particle sizes, regarding as a general method for mesoporous metal (or mixed metal) oxides.

Multi-component electrocatalyst for low-temperature fuel cells synthesized via sonochemical reactions.

This review presents recent advances in multi-component electrocatalysts for low-temperature fuel cells (FCs) synthesized via sonochemical reactions. As a feasible approach to develop novel electrocatalysts that can overcome the many problems of the prevailing Pt electrocatalysts, Pt- or Pd-based alloy and core-shell M@Pt nanoparticles (NPs) have been pursued. Synthesizing NPs with desirable properties often turn out to be challenging. Sonochemistry generates extreme conditions via acoustic cavitation, which have been utilized in the syntheses of various Pt and Pd NPs and Pt- and Pd-based alloy NPs. Especially, it has been reported that several M@Pt core-shell NPs can be synthesized by sonochemistry, which is hard to achieve by other methods. The principles of sonochemistry are presented with examples. Also alloy NPs and core-shell NPs synthesized by sonochemistry and those by other methods are compared.

Hematite thin films with various nanoscopic morphologies through control of self-assembly structures.

Hematite (α-Fe2O3) thin films with various nanostructures were synthesized through self-assembly between iron oxide hydroxide particles, generated by hydrolysis and condensation of Fe(NO3)3 · 6H2O, and a Pluronic triblock copolymer (F127, (EO)106(PO)70(EO)106, EO = ethylene oxide, PO = propylene oxide), followed by calcination. The self-assembly structure can be tuned by introducing water in a controlled manner through the control of the humidity level in the surrounding of the as-cast films during aging stage. For the given Fe(NO3)3 · 6H2O:F127 ratio, there appear to be three different thermodynamically stable self-assembly structures depending on the water content in the film material, which correspond to mesoporous, spherical micellar, and rod-like micellar structures after removal of F127. Coupled with the thermodynamic driving forces, the kinetics of the irreversible reactions of coalescence of iron oxide hydroxide particles into larger ones induce diverse nanostructures of the resultant films. The length scale of so-obtained nanostructures ranges from 6 nm to a few hundred nanometers. In addition to water content, the effects of other experimental parameters such as aging temperature, spin rate during spin coating, type of substrate, and type of iron reagent were investigated.

Direct hydrogenation of biomass-derived butyric acid to n-butanol over a ruthenium-tin bimetallic catalyst.

Catalytic hydrogenation of organic carboxylic acids and their esters, for example, cellulosic ethanol from fermentation of acetic acid and hydrogenation of ethyl acetate is a promising possibility for future biorefinery concepts. A hybrid conversion process based on selective hydrogenation of butyric acid combined with fermentation of glucose has been developed for producing biobutanol. ZnO-supported Ru-Sn bimetallic catalysts exhibits unprecedentedly superior performance in the vapor-phase hydrogenation of biomass-derived butyric acid to n-butanol (>98% yield) for 3500 h without deactivation.

Ultra-high capacitance hematite thin films with controlled nanoscopic morphologies.

We synthesized α-Fe2O3 (hematite) thin films with two different nanoscopic morphologies through self-assembly between a Fe-precursor and a Pluronic tri-block copolymer (F127) followed by aging and calcination. Relative humidity (RH) during the aging step of the spin-coated films was found to be critical in determining the morphologies. A network structure of nanowires ∼6 nm in diameter formed when the RH was 75%. The resulting nanowire hematite thin film (NW) had 150-250 nm-sized macropores. When the RH was 0%, a mesoporous hematite thin film (MP) with a wormlike pore structure and a pore size of ∼9 nm formed. Investigation of the electrochemical properties of these films revealed that they had very high specific capacitances of 365.7 and 283.2 F g(-1) for NWs and MPs, respectively, at a current density of 3 A g(-1) in a 0.5 M Na2SO3 electrolyte. Both of these capacitance values are considerably higher than those previously reported for hematite-based electrodes. We attributed this to the high porosity of the thin films, which enables ready access of electrolyte ions to the electrode surfaces, and their ultra-thin size, comparable to that of the depletion layer, allowing the low conductivity of hematite to be overcome. The higher capacitance of NWs than MPs is likely due to the accelerated electron transport through the crystalline nanowires in NWs.

Uniform growth of high-quality oxide thin films on graphene using a CdSe quantum dot array seeding layer.

Graphene displays outstanding properties as an electrode and a semiconducting channel material for transistors; however, the weak interfacial bond between graphene and an inorganic oxide material-based insulator presents a major constraint on these applications. Here, we report a new approach to improving the interface between the two materials using a CdSe quantum dot (QD)-based seeding layer in an inorganic material-graphene junction. CdSe QDs were electrochemically grown on graphene without degrading the properties of the graphene layer. The graphene structure was then used as the electrode in an oxide semiconductor by depositing a zinc oxide thin film onto the graphene coated with a QD seed layer (QD/G). The zinc oxide film adhered strongly to the graphene layer and provided a low contact resistance. A high-k dielectric layer in the form of an HfO2 film, which is an essential element in the fabrication of high-performance graphene-based field effect transistors, was also uniformly formed on the QD/G sheet using atomic layer deposition. The resulting transistors provided a relatively good performance, yielding hole and electron mobilities of 2600 and 2000 cm(2)/V·s.

One-step sonochemical syntheses of Ni@Pt core-shell nanoparticles with controlled shape and shell thickness for fuel cell electrocatalyst.

We demonstrate a facile one-step method to synthesize Ni@Pt core-shell nanoparticles (NPs) with a control over the shape and the Pt-shell thickness of the NPs. By adjusting the relative reactivity of the Pt and Ni reagents in ultrasound-assisted polyol reactions, two Ni@Pt NP samples of the same composition (Ni/Pt=1) and size (3-4 nm) but with different particle shape (octahedral vs. truncated octahedral) and different Pt-shell thicknesses (1-2 vs. 2-3 monolayer) are obtained. The control is achieved by using different Ni reagents, Ni(acac)2 (acac=acetylacetonate) and Ni(hfac)2 (hfac=hexafluoroacetylacetonate). A reaction mechanism that can explain all of the observations is proposed. The Ni@Pt NPs show up to threefold higher mass activity than pure Pt NPs in oxygen reduction reaction. Between the two Ni@Pt NP samples, the one composed of octahedral NPs with the thicker Pt-shell has higher activity than the other.

Rational syntheses of core-shell Fex@Pt nanoparticles for the study of electrocatalytic oxygen reduction reaction.

We report on the syntheses of core-shell Fex@Pt (x=0.4-1.2) nanoparticles (NPs) with Pt-shell thickness systematically controlled while the overall particle size is constant. The syntheses were achieved via one-pot ultrasound-assisted polyol synthesis (UPS) reactions. Fe1.2@Pt showed a record-breaking high core-element content (55 at%) of core-shell NPs. Based on observations from a series of control experiments, we propose a mechanism of the NPs' formation that enables control of shell thickness in UPS reactions. Fex@Pt NPs showed drastic enhancements in mass and specific activity for oxygen reduction reaction (ORR) and significantly enhanced durability compared to commercial Pt NPs. Fex@Pt with a 1 (monolayer) ML Pt shell showed the highest activity. The ab initio density functional theory calculations on the binding energies of oxygen species on the surfaces of Fex@Pt NPs showed that the 1 ML case is most favourable for the ORR, and in good agreement with the experimental results.

Investigation of porosity and heterojunction effects of a mesoporous hematite electrode on photoelectrochemical water splitting.

In this paper, we report the porosity and heterojunction effects of hematite (α-Fe2O3) on the photoelectrochemical (PEC) water splitting properties. The worm-like mesoporous hematite thin films (MHFs) with a pore size of ~9 nm and a wall thickness of ~5 nm were successfully obtained through the self-assembly process. MHFs formed on FTO showed much better PEC properties than those of nonporous hematite thin films (NP-HF) owing to the suppression of charge recombination. The PEC data of MHFs under front and back illumination conditions indicated that the porous structure allows the diffusion of electrolyte deep inside the MHF increasing the number of holes to be utilized in the water oxidation reaction. A heterojunction structure was formed by introducing a thin layer of SnO2 (~15 nm in thickness) between the MHF and FTO for a dramatically enhanced PEC response, which is attributed to the efficient electron transfer. Our spectroscopic and electrochemical data show that the SnO2 layer functions as an efficient electron transmitter, but does not affect the recombination kinetics of MHFs.