Heterointerface Effect on Two-Step Nucleation Mechanism of Bi Particles.

The nucleation and crystallization of Bi particles on two matrices, crystalline bismuth sulfide (c-Bi2S3) and amorphized bismuth titanium oxide (a-Bi12TiO20), were studied by using in situ transmission electron microscopy (TEM) analysis. The atomic structures of the Bi particles were monitored by acquiring high-resolution TEM images in real time. The Bi particles were grown on c-Bi2S3 and a-Bi12TiO20 via a two-step nucleation mechanism; dense liquid clusters were clearly observed at the initial stage of nucleation, and the coalescence of clusters was frequently observed during the growth. However, the nucleation and crystallization behaviors of Bi particles were governed by the matrix; in particular, the evolution of their morphology and atomic structure was confined on c-Bi2S3 but free from matrix effects on a-Bi12TiO20. The matrix effect on the two-step nucleation mechanism was demonstrated from a thermodynamic point of view.

Accelerated decomposition of Bi2S3 nanorods in water under an electron beam: a liquid phase transmission electron microscopy study.

Evaluating the stability of semiconductor photocatalysts is critical in the development of efficient catalysts. The morphological and microstructural behaviors of nanorod-shaped Bi2S3 semiconductors in aqueous solution were studied using a liquid cell transmission electron microscopy (TEM) technique. The rapid decomposition of Bi2S3 in water was observed under electron beam irradiation during TEM. Rounded bright spots due to a reduction in thickness were observed on the Bi2S3 nanorods at the initial stage of the decomposition, and rounded dark particles appeared outside of the nanorods in the solution, continuing the decomposition. This was confirmed by analyzing the atomic structure of the newly formed small particles, which consisted of an orthorhombic Bi2S3 phase. The stability-related decomposition of the Bi2S3 nanorods was demonstrated by considering the reduction and oxidation potentials of Bi2S3 in an aqueous solution. The effect of water radiolysis by the incident electron during TEM observations on the decomposition process was also determined by considering the time-dependent concentration behavior of the chemical species. Our study therefore reflects a novel route to evaluate the stabilities of semiconductor photocatalysts, which could ultimately solve a range of energy and environmental pollution problems.

Synthesis of Hollow Iron Oxide Nanospheres and Their Application to Gas Sensors.

Hollow nanomaterials have attracted great interest because of their many applications in catalysis, nanoreactors, drug delivery systems, for lubrication and in gas sensors. Here, carbon sphere templates were prepared from glucose under hydrothermal conditions to facilitate the synthesis of hollow Fe2O3 nanospheres. Thermal decomposition of an iron precursor in benzylalcohol with the carbon spheres resulted in the deposition of Fe3O4 nanoparticles on the carbon sphere templates. The nanoparticles on the carbon surface naturally agglomerate and form a dense oxide shell during the calcination step, which produces typical Fe2O3 hollow structures. The gas sensing performance of the hollow Fe2O3 nanospheres was investigated at an operating temperature of 300 °C. The hollow Fe2O3 nanospheres showed high sensitivity (R = 10.766 at 1 ppm formaldehyde) with a linear response to formaldehyde gas concentration in the range of 0.8~2.4 ppm, and good selectivity to formaldehyde gas in volatile organic compounds, compared to commercial Fe2O3 nanoparticles.

Engineering Reaction Kinetics by Tailoring the Metal Tips of Metal-Semiconductor Nanodumbbells.

Semiconductor-metal hybrid nanostructures are one of the best model catalysts for understanding photocatalytic hydrogen generation. To investigate the optimal structure of metal cocatalysts, metal-CdSe-metal nanodumbbells were synthesized with three distinct sets of metal tips, Pt-CdSe-Pt, Au-CdSe-Au, and Au-CdSe-Pt. Photoelectrochemical responses and transient absorption spectra showed that the competition between the charge recombination at the metal-CdSe interface and the water reduction on the metal surface is a detrimental factor for the apparent hydrogen evolution rate. For instance, a large recombination rate (krec) at the Pt-CdSe interface limits the quantum yield of hydrogen generation despite a superior water reduction rate (kWR) on the Pt surface. To suppress the recombination process, Pt was selectively deposited onto the Au tips of Au-CdSe-Au nanodumbbells in which the krec was diminished at the Au-CdSe interface, and the large kWR was maintained on the Pt surface. As a result, the optimal structure of the Pt-coated Au-CdSe-Au nanodumbbells reached a quantum yield of 4.84%. These findings successfully demonstrate that the rational design of a metal cocatalyst and metal-semiconductor interface can additionally enhance the catalytic performance of the photochemical hydrogen generation reactions.

Improved photoelectrochemical water oxidation by the WO3/CuWO4 composite with a manganese phosphate electrocatalyst.

We describe a composite of the n-type semiconductors for the photoelectrochemical oxygen evolution reaction (OER). A simple drop-casting technique of mixed precursors and a one-step annealing process were used in the synthesis of the WO3/CuWO4 composite. The composite showed improved photocurrent for water oxidation compared to either of the two compounds individually. We discuss possible electron-hole separation mechanisms in two semiconductors comprising a primary photon-absorbing semiconductor of CuWO4 with a secondary semiconductor of WO3. When the WO3/CuWO4 composite is simultaneously irradiated, the photogenerated hole from the WO3 valence band transfers to CuWO4, which results in an enhanced charge separation of CuWO4. Furthermore, the OER catalytic activity of manganese phosphate (MnPO) was compared to manganese oxide nanoparticles (Mn2O3) by electrochemical measurements, showing that the manganese phosphate was more efficient for the OER reaction. To investigate the effect of catalysts on semiconductors, manganese phosphate was deposited on the WO3/CuWO4 composite. The result demonstrates the promise of manganese phosphate for improving the photocurrent as well as the stability of the WO3/CuWO4 composite.

Unlocking the Potential of Bi2S3-Derived Bi Nanoplates: Enhanced Catalytic Activity and Selectivity in Electrochemical and Photoelectrochemical CO2 Reduction to Formate.

Various electrocatalysts are extensively examined for their ability to selectively produce desired products by electrochemical CO2 reduction reaction (CO2RR). However, an efficient CO2RR electrocatalyst doesn't ensure an effective co-catalyst on the semiconductor surface for photoelectrochemical CO2RR. Herein, Bi2S3 nanorods are synthesized and electrochemically reduced to Bi nanoplates that adhere to the substrates for application in the electrochemical and photoelectrochemical CO2RR. Compared with commercial-Bi, the Bi2S3-derived Bi (S-Bi) nanoplates on carbon paper exhibit superior electrocatalytic activity and selectivity for formate (HCOO-) in the electrochemical CO2RR, achieving a Faradaic efficiency exceeding 93%, with minimal H2 production over a wide potential range. This highly selective S-Bi catalyst is being employed on the Si photocathode to investigate the behavior of electrocatalysts during photoelectrochemical CO2RR. The strong adhesion of the S-Bi nanoplates to the Si nanowire substrate and their unique catalytic properties afford exceptional activity and selectivity for HCOO- under simulated solar irradiation. The selectivity observed in electrochemical CO2RR using the S-Bi catalyst correlates with that seen in the photoelectrochemical CO2RR system. Combined pulsed potential methods and theoretical analyses reveal stabilization of the OCHO* intermediate on the S-Bi catalyst under specific conditions, which is critical for developing efficient catalysts for CO2-to-HCOO- conversion.

Designed synthesis of well-defined Pd@Pt core-shell nanoparticles with controlled shell thickness as efficient oxygen reduction electrocatalysts.

Improving the electrocatalytic activity and durability of Pt-based catalysts with low Pt content toward the oxygen reduction reaction (ORR) is one of the main challenges in advancing the performance of polymer electrolyte membrane fuel cells (PEMFCs). Herein, a designed synthesis of well-defined Pd@Pt core-shell nanoparticles (NPs) with a controlled Pt shell thickness of 0.4-1.2 nm by a facile wet chemical method and their electrocatalytic performances for ORR as a function of shell thickness are reported. Pd@Pt NPs with predetermined structural parameters were prepared by in situ heteroepitaxial growth of Pt on as-synthesized 6 nm Pd NPs without any sacrificial layers and intermediate workup processes, and thus the synthetic procedure for the production of Pd@Pt NPs with well-defined sizes and shell thicknesses is greatly simplified. The Pt shell thickness could be precisely controlled by adjusting the molar ratio of Pt to Pd. The ORR performance of the Pd@Pt NPs strongly depended on the thickness of their Pt shells. The Pd@Pt NPs with 0.94 nm Pt shells exhibited enhanced specific activity and higher durability compared to other Pd@Pt NPs and commercial Pt/C catalysts. Testing Pd@Pt NPs with 0.94 nm Pt shells in a membrane electrode assembly revealed a single-cell performance comparable with that of the Pt/C catalyst despite their lower Pt content, that is the present NP catalysts can facilitate low-cost and high-efficient applications of PEMFCs.

New crystal structure: synthesis and characterization of hexagonal wurtzite MnO.

Most transition metal oxides have a cubic rocksalt crystal structure, but ZnO and CoO are the only stable transition metal oxides known to possess a hexagonal structure. Unprecedented hexagonal wurtzite MnO has been prepared by thermal decomposition of Mn(acac)(2) on a carbon template. Structural characterization has been carried out by TEM, SAED, and a Rietveld analysis using XRD. The experimental and theoretical magnetic results indicate magnetic ordering of the hexagonal wurtzite MnO. Density functional calculations have been performed in order to understand the electronic and piezoelectric properties of the newly synthesized hexagonal wurtzite MnO.

Comprehensive design of carbon-encapsulated Fe3O4 nanocrystals and their lithium storage properties.

Controlling the bulk and surface structure of metal oxide nanostructures is crucial to obtain superior electronic and electrochemical properties. However, the synthetic or post-treatment techniques for preparing such structures, especially those with complex configurations, still remains a challenge. Herein, we report a completely novel approach-an amorphous carbon coating on the surface coupled with a controlled metal oxidation state in the bulk-via a simple glucose treatment. The bulk and surface structures of iron oxides are controlled by the carbothermal reaction associated with the decomposition of glucose. These novel configurations of iron oxides possess an amorphous carbon layer and ferrous state with high electronic conductivity, which definitely enhances their electrochemical properties compared to pristine iron oxides. Our findings provide an effective solution for the synthesis of complex metal oxide nanostructures, which can pave the way to further expand the electronic or electrochemical applications of metal oxides.

Redox-Transition from Irreversible to Reversible Vitamin C by Pore Confinement in Microporous Carbon Network.

Enhancement of redox-reversibility in electroactive species has been studied because of fundamental interest and their importance for energy storage systems. Various electroactive molecules suffer from redox-irreversible behavior, and this is a critical reason for their exclusion as redox electrolytes in energy storage systems. In this article, we fully demonstrated that ascorbic acid (ASC), which is an abundant but redox-irreversible molecule, can become redox-reversible when it is confined in microporous carbon regimes. From a theoretical perspective, redox-reversibility in an electrochemical reaction coupled with an irreversible chemical process can be greatly enhanced due to kinetic acceleration toward the inverse direction of the chemical reaction by accumulation of products in the nanoconfined regime. However, the kinetic acceleration in a nanoconfined domain shows limitations for enhancing the redox-reversibility, which indicates that stabilization of the species undergoing an irreversible chemical process is another important factor for redox-reversibility enhancement. The origin of nanoporous confinement of ASC and its enhanced redox-reversibility was rationalized by molecular dynamics simulations. We found that ASC-clusters of a fully protonated ASC and its conjugated base formed inside carbon pores, which would be a main driving force for its confinement in microporous carbon networks. Lastly, we demonstrated a prototype energy storage device using redox-reversible ASC in microporous carbon as the half electrode, which shows the feasibility of ASC as a possible redox electrolyte in an aqueous energy storage system.

Unraveling the Simultaneous Enhancement of Selectivity and Durability on Single-Crystalline Gold Particles for Electrochemical CO2 Reduction.

Electrochemical carbon dioxide reduction is a mild and eco-friendly approach for CO2 mitigation and producing value-added products. For selective electrochemical CO2 reduction, single-crystalline Au particles (octahedron, truncated-octahedron, and sphere) are synthesized by consecutive growth and chemical etching using a polydiallyldimethylammonium chloride (polyDDA) surfactant, and are surface-functionalized. Monodisperse, single-crystalline Au nanoparticles provide an ideal platform for evaluating the Au surface as a CO2 reduction catalyst. The polyDDA-Au cathode affords high catalytic activity for CO production, with >90% Faradaic efficiency over a wide potential range between -0.4 and -1.0 V versus RHE, along with high durability owing to the consecutive interaction between dimethylammonium and chloride on the Au surface. The influence of polyDDA on the Au particles, and the origins of the enhanced selectivity and stability are fully investigated using theoretical studies. Chemically adsorbed polyDDA is consecutively affected the initial adsorption of CO2 and the stability of the *CO2 , *COOH, and *CO intermediates during continuous CO2 reduction reaction. The polyDDA functionalization is extended to improving the CO Faradaic efficiency of other metal catalysts such as Ag and Zn, indicating its broad applicability for CO2 reduction.

Understanding and Improving Photocatalytic Activity of Pd-Loaded BiVO4 Microspheres: Application to Visible Light-Induced Suzuki-Miyaura Coupling Reaction.

The effective utilization of visible light is required for exploiting photocatalytic reactions in indoor and outdoor environments. In this study, Pd-supported BiVO4 microspheres (Pd-BiVO4) were prepared for visible light-induced photocatalytic reactions. Under irradiation with a white light-emitting diode, the obtained Pd-BiVO4 composite exhibited considerably improved catalytic activity for the decomposition of an organic dye compared with other BiVO4 catalysts. The Pd-BiVO4 composite was also effective for catalytic organic transformation via the visible light-induced Suzuki-Miyaura coupling reaction. The photogenerated electrons in the conduction band of BiVO4 flowed to the Pd nanoparticles and amplified cross-coupling reaction. The influence of the crystal structure and grain size of BiVO4 and the role of the deposited Pd nanoparticles were fully investigated to elucidate the visible light activity of the catalyst. This system highlights the possibility of an indoor light source with low energy density for sustainable organic transformations.

Electrochemical Generation of Mesopores and Residual Oxygen for the Enhanced Activity of Silver Electrocatalysts.

The development of stable and efficient electrocatalysts is of key importance for the establishment of a sustainable society. The activity of a metal electrocatalyst is determined by its electrochemically active surface area and intrinsic activity, which can be increased using highly porous structures and heteroatomic doping, respectively. Herein, we propose a general strategy of generating mesopores and residual oxygen in metal electrocatalysts by reduction of metastable metal oxides using Ag2O3 electrodeposited onto carbon paper as a model system and demonstrating that the obtained multipurpose porous Ag electrocatalyst has high activity for the electroreduction of O2 and CO2. The presence of mesopores and residual oxygen is confirmed by electrochemical and spectroscopic techniques, and quantum mechanical simulations prove the importance of residual oxygen for electrocatalytic activity enhancement. Thus, the adopted strategy is concluded to allow the synthesis of highly active metal catalysts with controlled mesoporosity and residual oxygen content.

Current research on single-entity electrochemistry for soft nanoparticle detection: Introduction to detection methods and applications.

In recent years, rapid progress in the field of single-entity electrochemistry (SEE) has opened a novel exploratory area in the field of analytical and electrochemistry. SEE is a method of studying the behavior of particles at the single particle level, which yields important information on the diffusion coefficient, individual particle size, size distribution, catalytic activity, collision frequency, and internal substances of the particles. Various types of particles have been studied through SEE. Among them, this review focuses on the results of analyzing soft particles (cells, proteins, viruses, liposomes, enzymes, vesicles, emulsion droplets, micelles, carbonaceous nanomaterials, and others). We introduce the various electrochemical techniques used in SEE and discuss important concepts and equations that must be known to apply SEE. We also introduce the results of recent research on several important soft particles. Finally, the potential applications in areas such as sensors, materials, catalysts, energy, and biomedicine, as well as the trends of related works are described. In the future, research on soft particle analysis using SEE is expected to increase rapidly, and this review is expected to be a valuable reference for researchers in the field.

Comprehensive Study of the Growth Mechanism and Photoelectrochemical Activity of a BiVO4/Bi2S3 Nanowire Composite.

A BiVO4/Bi2S3 composite comprising Bi2S3 nanowires on top of a BiVO4 film was prepared via hydrothermal reaction. Because additional Bi3+ ions were not delivered during the reaction, BiVO4 served as the Bi3+ ion source for the development of Bi2S3. A detailed growth mechanism of the nanowire was elucidated by an analysis of the concentration gradient of Bi3+ and S2- ions during the reaction. The in situ growth was followed by the etching of BiVO4 to Bi3+ and VO43- ions and regrowth to Bi2S3, which resulted in the rapid evolution of nanowires on the BiVO4 substrate. The fabricated BiVO4/Bi2S3NW composite exhibited an improved photoelectrochemical activity compared to other Bi2S3 samples. The improved efficiency was mainly attributed to both improved charge separation and effective adhesion obtained by the in situ growth.

Understanding Reaction Kinetics by Tailoring Metal Co-catalysts of the BiVO4 Photocatalyst.

In this study, the reaction mechanisms of metal-semiconductor composites used as photocatalysts were demonstrated by first preparing bismuth vanadate (BiVO4) and then performing photodeposition of metal nanoparticles. The photocatalytic activity of metal-BiVO4 (M-BiVO4, where M = Pt, Au, Ag) composites were evaluated through dye decomposition under UV-vis irradiation. The photocatalytic efficiency was significantly enhanced after Pt deposition as compared to other M-BiVO4 composites. The size or shape of BiVO4 was not the main factor for the efficiency of Pt-BiVO4. However, a deposited Pt co-catalyst was essential for the photocatalytic decomposition of dye on the BiVO4 surface. Radical scavengers were employed to elucidate the reaction mechanism during the photocatalytic reaction with the Pt-BiVO4 composite. This study provides details on the reaction mechanism of the photocatalytic reaction on Pt at the BiVO4 surface under solar irradiation.

In Situ Growth of the Bi2S3 Nanowire Array on the Bi2MoO6 Film for an Improved Photoelectrochemical Performance.

A single-crystalline Bi2S3 nanowire array (Bi2S3NWA) is synthesized by an in situ hydrothermal reaction on the surface of a Bi2MoO6 film. As no additional source of Bi3+ is provided during the process, the Bi2MoO6 layer acts as the Bi3+ source for the synthesis of Bi2S3 nanowires. The fabricated Bi2MoO6/Bi2S3NWA electrode exhibited an increased photoelectrochemical (PEC) sulfite oxidation activity, which is attributed mainly to the effective interface obtained by the in situ hydrothermal growth, compared to other Bi2S3 electrodes. The generated electron from the Bi2S3 conduction band rapidly transfers to that of Bi2MoO6, yielding an enhanced electron separation of Bi2S3. Furthermore, the single-crystalline Bi2S3 nanowire can provide a fast electron pathway to Bi2MoO6 through its single domain, which also contributes to the improved PEC activity.

Facile Synthesis of BiVO4 for Visible-Light-Induced C-C Bond Cleavage of Alkenes to Generate Carbonyls.

BiVO4 crystals synthesized by an ultrasonic-assisted method (Sono-BiVO4 ) showed improved efficiency as a heterogeneous photocatalyst under visible-light irradiation. Sono-BiVO4 was successfully used for the C-C bond cleavage of alkenes to generate carbonyl compounds. Styrene derivatives were converted into carbonyl compounds in the presence of Sono-BiVO4 under highly sustainable conditions requiring only natural sources, that is, molecular oxygen, visible light, and water at room temperature. Additionally, Sono-BiVO4 could be easily separated from the reaction mixture and reused.

Understanding metal-enhanced fluorescence and structural properties in Au@Ag core-shell nanocubes.

Au@Ag core-shell structures have received particular interest due to their localized surface plasmon resonance properties and great potential as oxygen reduction reaction catalysts and building blocks for self-assembly. In this study, Au@Ag core-shell nanocubes (Au@AgNCs) were fabricated in a facile manner via stepwise Ag reduction on Au nanoparticles (AuNPs). The size of the Au@AgNCs and their optical properties can be simply modulated by changing the Ag shell thickness. Structural characterization has been carried out by TEM, SAED, and XRD. The metal-induced fluorescence properties of probe molecules near the Au@AgNCs were measured during sedimentation of the Au@AgNCs. The unique ring-like building block of Au@AgNCs has dual optical functions as a fluorescence quencher or fluorescence enhancement medium depending on the assembled regions.

Highly stable mesoporous silica nanospheres embedded with FeCo/graphitic shell nanocrystals as magnetically recyclable multifunctional adsorbents for wastewater treatment.

Highly stable and magnetically separable mesoporous silica nanospheres (MSNs) embedded with 4.6 ± 0.8 nm FeCo/graphitic carbon shell nanocrystals (FeCo/GC NCs@MSNs) were synthesized by thermal decomposition of metal precursors in MSNs and subsequent methane CVD. The FeCo/GC NCs@MSNs had a high specific surface area (442 m2 g-1), large pore volume (0.65 cm3 g-1), and tunable size (65 nm, 130 nm, and 270 nm). Despite the low magnetic metal content (8.35 wt%), the FeCo/GC NCs@MSNs had a sufficiently high saturation magnetization (17.1 emu g-1). This is due to the superior magnetic properties of the FeCo/GC NCs, which also enable fast magnetic separation of the nanospheres. The graphitic carbon shell on the FeCo NCs not only protects the alloy core against oxidation and acid etching in 35% HCl(aq), but also facilitates non-covalent, hydrophobic interactions with the hydrocarbon chains of organic dyes such as methyl orange and methylene blue. Surface functionalization of the FeCo/GC NCs@MSNs with thiol groups provides efficient capacity for binding with Hg2+ ions. We have shown that the thiol-functionalized FeCo/GC NCs@MSNs (FeCo/GC NCs@MSNs-SH) work as multifunctional adsorbents for organic dyes (target organic pollutants) and Hg2+ ions (target inorganic pollutant). We also demonstrated that the FeCo/GC NCs@MSNs-SH are excellent recyclable adsorbents for methyl orange.