Lattice Contraction and Surface Stress of fcc Nanocrystals

Green light-responsive photoelectrochemical sensing nanoplatform based on copper cobaltite nanorods for ultrasensitive detection of furazolidone antibiotic residue in food samples

In this research, the preparation of copper cobaltite (CuCo2O4) nanorods and its potential application in photoelectrochemical sensing platform towards ultrasensitive detection of furazolidone are reported. X-ray diffraction, Raman spectra, scanning electron microscopy, and UV-visible spectroscopy have been performed to confirm the formation, morphology, phase composition, and optical properties of CuCo2O4 synthesized by a microwave-assisted hydrothermal method. The electrochemical characteristic parameters were calculated via electrochemical impedance spectroscopy, cyclic voltammetry, differential pulse voltammetry, and chronoamperometry techniques in the absence and presence of laser light irradiation. The CuCo2O4-based photoelectrochemical sensing platform with laser light irradiation exhibited outstanding electrochemical performance compared to without laser light irradiation with sensitivity for furazolidone detection of 1.11 μA μM-1 cm-2 within the linear ranges of 0.25 to 200 μM, and detection limit of 0.03 μM, due to CuCo2O4 nanorods having a narrow energy gap, a low recombination ratio of electron-hole pairs, and multiple valence states (Co2+/Co3+ and Cu2+/Cu3+) structure. In addition, the proposed CuCo2O4-based photoelectrochemical sensor with light assistance showed good repeatability, anti-interfering capability, long-term stability, and real applicability in honey and milk samples.

Lattice variation as a function of concentration and grain size in MgO-NiO solid solution system

The study aimed to understand how changes in crystal's size affect the lattice parameters and crystal structure of Mg1-xNixO solid solution for six X values ranging from x = 0 to x = 1. Mg1-xNixO was synthesized via two different wet-chemical techniques: the sol-gel and the microwave hydrothermal method, both followed by calcination at different temperatures of 673, 873, 1073, 1273 and 1473 K. As annealing caused grain growth, the varied temperature range allowed to examine a wide range of grain sizes. The lattice parameters and x values were determined from XRD (X-ray diffraction) peak positions and intensities respectively. The grain size was evaluated by XRD line profile analysis and supported by SEM (scanning electron microscope) observations. At the temperatures of 673 and 873 K grain size was in the nanometric range and from 1073 K and above grain size was in the micrometric range. A non-monotonic lattice variation versus grain size was found for each concentration. When grain size decreased there was a slight contraction, however for grain size in the nanometric range there was a severe lattice expansion. Both lattice parameter changes were explained by two effects acting together: contraction due to surface stress and expansion due to weakening of the ionic bonding at nanocrystalline particles. In this current research study, the lattice parameter was mapped in two dimensions: concentration and grain size. The findings of this study provided valuable insights into the lattice variation in the MgO-NiO solid solution system.

A Critical Look at Colloid Generation, Stability, and Transport in Redox-Dynamic Environments: Challenges and Perspectives

Colloid generation, stability, and transport are important processes that can significantly influence the fate and transport of nutrients and contaminants in environmental systems. Here, we critically review the existing literature on colloids in redox-dynamic environments and summarize the current state of knowledge regarding the mechanisms of colloid generation and the chemical controls over colloidal behavior in such environments. We also identify critical gaps, such as the lack of universally accepted cross-discipline definition and modeling infrastructure that hamper an in-depth understanding of colloid generation, behavior, and transport potential. We propose to go beyond a size-based operational definition of colloids and consider the functional differences between colloids and dissolved species. We argue that to predict colloidal transport in redox-dynamic environments, more empirical data are needed to parametrize and validate models. We propose that colloids are critical components of element budgets in redox-dynamic systems and must urgently be considered in field as well as lab experiments and reactive transport models. We intend to bring further clarity and openness in reporting colloidal measurements and fate to improve consistency. Additionally, we suggest a methodological toolbox for examining impacts of redox dynamics on colloids in field and lab experiments.

MnFe2O4@SiO2@CeO2 core–shell nanostructures for applications in water remediation†

Removal of dye pollutants from wastewater is among the most important emerging needs in environmental science and engineering. The main objective of our work is to develop new magnetic core–shell nanostructures and explore their use for potential removal of pollutants from water using an external magnetic field. Herein, we have prepared magnetic core–shell nanoparticles that demonstrated excellent dye pollutant adsorbent properties. These nanoparticles are composed of a manganese ferrite magnetic core coated with silica, to protect the core and enable further functionalisation, then finally coated with ceria, which is shown to be an effective adsorbent. The magnetic core–shell nanostructures have been synthesized by a modification of solvothermal synthesis. The nanoparticles were fully characterised at each stage of the synthesis by powder X-ray diffraction (pXRD), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM) and Fourier transform infrared spectroscopy (FTIR). These particles were found to be effective in removing methylene blue (MB) dye from water, which was validated by UV-visible (UV-vis) spectroscopy. These particles can be quickly removed from solution using a permanent magnet and then can be recycled after being placed in the furnace at 400 °C to burn off any organic residues. The particles were found to retain their ability to adsorb the pollutant after several cycles and TEM images of the particles after several cycles showed no change in the morphology. This research demonstrated the capacity of magnetic core–shell nanostructures to be used for water remediation.

Magnetic core–shell nanoparticles have been synthesised, characterised and used for methylene blue dye removal from water. They can be magnetically extracted and show no reduction in efficiency after several cycles.

Size-Dependent Photobleaching Mechanism and Kinetics Induced by Nanosecond Laser Pulses in Colloidal Semiconductor Quantum Dots

An experimental-theoretical approach is proposed to investigate the size-dependent photobleaching of colloidal semiconductor quantum dots (QDs) excited by a nanosecond pulsed laser. In the experimental background, the ground-state absorption and photoluminescence (PL) spectra of chemically prepared QDs are monitored over an excitation time at distinct laser irradiances. The magnitude of photobleaching in the QD solution is quantified by the decay rate of the PL signal as a function of the excitation time and the laser power. A theoretical spectroscopy model is then used to estimate the particle size distribution (PSD) in colloidal solution from the absorption data generated at different laser powers. The resulting evolution of the PSD of the QD ensemble under irradiation is analyzed in terms of classical crystallization theories dealing with the formation, growth, and dissolution of colloidal particles in a supersaturated medium. The QD response to laser irradiation is also interpreted by a simple mechanical model that correlates the photoinduced hydrostatic strain at the solid/liquid interface and the predicted variation of the mean particle size. The reported experimental and theoretical methods are used to completely elucidate the basic physico-chemical processes responsible for the laser-induced photobleaching kinetics of glutathione-capped CdTe aqueous QDs with very small mean sizes. For this purpose, we synthesized a series of colloidal QD samples with mean particle diameters ranging from 1.95 to 2.68 nm. Our results indicate that a faster photobleaching rate occurs in QD samples with smaller sizes in which particle dissolution under laser irradiation is predominant. On the other hand, the photobleaching rate becomes slower in samples with larger dot sizes, possibly due to the formation of core/shell structures in solution via thermal degradation of thiol ligands either during the chemical synthesis or as a consequence of the subsequent interaction with the excitation laser.

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

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

Transformation Pathway from CdSe Nanoplatelets with Absorption Doublets at 373/393 nm to Nanoplatelets at 434/460 nm

For those colloidal semiconductor CdSe nanospecies that exhibit sharp optical absorption doublets, different explanations have appeared in the literature regarding their morphological nature and formation, with no consensus reached. Here, we discuss the transformation pathway in two types of CdSe nanoplatelets (NPLs), from NPL-393 to NPL-460, exhibiting absorption doublets at 373/393 and 433/460 nm, respectively. Synchrotron-based small/wide-angle X-ray scattering (SAXS/WAXS) was performed to monitor the in situ transformation associated with the temperature. Combining the results of SAXS/WAXS, optical spectroscopy, and transmission electron microscopy, we propose that the transformation pathway experiences corresponding magic-sized clusters (MSCs), which display similar optical properties but with zero-dimensional structure. From stacked NPL-393 to stacked NPL-460, the transformation goes through sequentially individual NPL-393, MSC-393, MSC-460, and individual NPL-460 at their corresponding characteristic temperature. The present findings provide compelling evidence that both MSCs and their assembled NPLs exhibit similar optical absorption.

Comments on “Size dependence of the lattice parameters of carbon supported platinum nanoparticles: X-ray diffraction analysis and theoretical considerations,” RSC Adv., 2014, , 35959–35965

Leontyev and colleagues presented the results of an experiment and of its theoretical consequences.

BaGdF5 Nanophosphors Doped with Different Concentrations of Eu3+ for Application in X-ray Photodynamic Therapy

X-ray photodynamic therapy (XPDT) has been recently considered as an efficient alternative to conventional radiotherapy of malignant tissues. Nanocomposites for XPDT typically consist of two components—a nanophosphor which re-emits X-rays into visible light that in turn is absorbed by the second component, a photosensitizer, for further generation of reactive oxygen species. In this study, BaGdF₅ nanophosphors doped with different Eu:Gd ratios in the range from 0.01 to 0.50 were synthesized by the microwave route. According to transmission electron microscopy (TEM), the average size of nanophosphors was ~12 nm. Furthermore, different coatings with amorphous SiO₂ and citrates were systematically studied. Micro-CT imaging demonstrated superior X-ray attenuation and sufficient contrast in the liver and the spleen after intravenous injection of citric acid-coated nanoparticles. In case of the SiO₂ surface, post-treatment core–shell morphology was verified via TEM and the possibility of tunable shell size was reported. Nitrogen adsorption/desorption analysis revealed mesoporous SiO₂ formation characterized by the slit-shaped type of pores that should be accessible for methylene blue photosensitizer molecules. It was shown that SiO₂ coating subsequently facilitates methylene blue conjugation and results in the formation of the BaGdF₅: 10% Eu³⁺@SiO₂@MB nanocomposite as a promising candidate for application in XPDT.

Defect engineering of oxide perovskites for catalysis and energy storage: synthesis of chemistry and materials science

Oxide perovskites have emerged as an important class of materials with important applications in many technological areas, particularly thermocatalysis, electrocatalysis, photocatalysis, and energy storage. However, their implementation faces numerous challenges that are familiar to the chemist and materials scientist. The present work surveys the state-of-the-art by integrating these two viewpoints, focusing on the critical role that defect engineering plays in the design, fabrication, modification, and application of these materials. An extensive review of experimental and simulation studies of the synthesis and performance of oxide perovskites and devices containing these materials is coupled with exposition of the fundamental and applied aspects of defect equilibria. The aim of this approach is to elucidate how these issues can be integrated in order to shed light on the interpretation of the data and what trajectories are suggested by them. This critical examination has revealed a number of areas in which the review can provide a greater understanding. These include considerations of (1) the nature and formation of solid solutions, (2) site filling and stoichiometry, (3) the rationale for the design of defective oxide perovskites, and (4) the complex mechanisms of charge compensation and charge transfer. The review concludes with some proposed strategies to address the challenges in the future development of oxide perovskites and their applications.

Gas Phase Glycerol Valorization over Ceria Nanostructures with Well-Defined Morphologies

Glycerol solutions were vaporized and reacted over ceria catalysts with different morphologies to investigate the relationship of product distribution to the surface facets exposed, particularly, the yield of bio-renewable methanol. Ceria was prepared with cubic, rodlike, and polyhedral morphologies via hydrothermal synthesis by altering the concentration of the precipitating agent or synthesis temperature. Glycerol conversion was found to be low over the ceria with a cubic morphology, and this was ascribed to both a low surface area and relatively high acidity. Density functional theory calculations also showed that the (100) surface is likely to be hydroxylated under reaction conditions which could limit the availability of basic sites. Methanol space-time-yields over the polyhedral ceria samples were more than four times that for the cubic material at 400 °C, where 201 g of methanol was produced per hour per kilogram of the catalyst. Under comparable glycerol conversions, we show that the rodlike and polyhedral catalysts produce a major intermediate to methanol, hydroxyacetone (HA), with a selectivity of ca. 45%, but that over the cubic sample, this was found to be 15%. This equates to a 13-fold increase in the space-time-yield of HA over the polyhedral samples compared to the cubes at 320 °C. The implications of this difference are discussed with respect to the reaction mechanism, suggesting that a different mechanism dominates over the cubic catalysts to that for rodlike and polyhedral catalysts. The strong association between exposed surface facets of ceria to high methanol yields is an important consideration for future catalyst design in this area.

Atomic Layer Deposition Seeded Growth of Rutile SnO2 Nanowires on Versatile Conducting Substrates

Extended and oriented rutile nanowires (NWs) hold great promise for numerous applications because of their various tunable physicochemical properties in air and/or solution media, but their direct synthesis on a wide range of conducting substrates remains a significant challenge. Their device performance is governed by relevant NW geometries that cannot be fully controlled to date by varying bulk synthetic conditions. Herein, orientation engineering of rutile SnO₂ NWs on a variety of conducting substrates by atomic layer deposition (ALD) seeding has been investigated. The seeded growth controls the nucleation event of the NW, and thicknesses and crystallographic properties of seed layers are the key parameters toward tuning the NW characteristics. The seed layers on carbon cloth produce NWs with highly enhanced electrochemically active surface area, which would show efficient electrochemical CO₂ reduction. In addition, the hierarchical architecture resulted from the seeded growth of NWs on SnO₂ nanosheets allows thin layers of BiVO₄, forming a heterojunction photoanode, which shows a record charge separation efficiency of 96.6% and a charge-transfer efficiency of 90.2% at 1.23 V versus the reversible hydrogen electrode among, to date, the reported BiVO₄-based photoanodes for water oxidation. Our study illustrates that such a versatile interfacial engineering effort by the ALD technique would be promising for further wide range of practical applications.

Stress-Induced Structural Transformations in Au Nanocrystals

Nanocrystals can exist in multiply twinned structures like icosahedron or single crystalline structures like cuboctahedron. Transformations between these structures can proceed through diffusion or displacive motion. Experimental studies on nanocrystal structural transformations have focused on high-temperature diffusion-mediated processes. Limited experimental evidence of displacive motion exists. We report structural transformation of 6 nm Au nanocrystals under nonhydrostatic pressure of 7.7 GPa in a diamond anvil cell that is driven by displacive motion. X-ray diffraction and transmission electron microscopy were used to detect the structural transformation from multiply twinned to single crystalline. Single crystalline nanocrystals were recovered after unloading, then quickly reverted to the multiply twinned state after dispersion in toluene. The dynamics of recovery was captured using TEM which showed surface recrystallization and rapid twin boundary motion. Molecular dynamics simulations showed that twin boundaries are unstable due to defects nucleated from the interior of the nanocrystal.

Temperature reversible synergistic formation of cerium oxyhydride and Au hydride: a combined XAS and XPDF study

In situ studies on the physical and chemical properties of Au in inverse ceria alumina supported catalysts have been conducted between 295 and 623 K using high energy resolved fluorescence detection X-ray absorption near edge spectroscopy and X-ray total scattering. Precise structural information is extracted on the metallic Au phase present in a 0.85 wt% Au containing inverse ceria alumina catalyst (ceria/Au/alumina). Herein evidence for the formation of an Au hydride species at elevated temperature is presented. Through modelling of total scattering data to extract the thermal properties of Au using Grüneisen theory of volumetric thermal expansion it proposed that the Au Hydride formation occurs synergistally with the formation of a cerium oxyhydride. The temperature reversible nature, whilst remaining in a reducing atmosphere, demonstrates the activation of hydrogen without consumption of oxygen from the supporting ceria lattice.

Gold nanowire growth through stacking fault mechanism by oleylamine-mediated synthesis

Tadpole-like gold nanowires were obtained by the oleylamine-mediated synthesis presenting an unusual mixture of fcc and hcp phases. Structural analyses were performed to understand their structure and growth using aberration-corrected high-resolution scanning transmission electron microscopy and electron diffraction at the tail region of tadpoles showing that the anisotropic shape occurred due to stacking fault defects. Stacking faults and twin defects are responsible for the hcp phase inferring a defect dependent growth. The stacking fault model used in X-ray diffraction (XRD) refinement resulted in 60% of hcp stacking sequences. Temperature-dependent XRD analyses showed that the faults become unstable around 120 °C, and it is completely converted to the fcc phase at 230 °C. We attribute the nanowire formation to a stacking fault mechanism of growth that begins in the later stage of nanoparticle growth. The UV-Vis spectrum presented two localized surface plasmon resonance bands at 500 nm and from 800 nm extending to near-infrared, associated with transverse and longitudinal modes, respectively. A surprising ferromagnetic behavior is also observed with a blocking temperature near 300 K.

Determination of the melting temperature of spherical nanoparticles in dilute solution as a function of their radius by exclusively using the small-angle X-ray scattering technique

In this investigation the dependence on radius of the melting temperature of dilute sets of spherical nanocrystals with wide radius distributions was determined by a novel procedure exclusively using the results of small-angle X-ray scattering (SAXS) measurements. This procedure is based on the sensitivity of the SAXS function to small and rather sharp variations in the size and electron density of nanocrystals at their melting temperature. The input for this procedure is a set of experimental SAXS intensity functions at selected q values for varying sample temperatures. In practice, the sample is heated from a minimum temperature, lower than the melting temperature of the smallest nanocrystals, up to a temperature higher than the melting temperature of the largest nanocrystals. The SAXS intensity is recorded in situ at different temperatures during the heating process. This novel procedure was applied to three samples composed of dilute sets of spherical Bi nanocrystals with wide radius distributions embedded in a sodium borate glass. The function relating the melting temperature of Bi nanocrystals with their radius – determined by using the procedure proposed here – agrees very well with the results reported in previous experimental studies using different methods. The results reported here also evidence the predicted size-dependent contraction of Bi nanocrystals induced by the large surface-to-volume ratio of small nanocrystals and an additional size-independent compressive stress caused by the solid glass matrix in which liquid Bi nanodroplets are initially formed. This last effect is a consequence of the increase in the volume of Bi nanoparticles upon crystallization and also of differences in the thermal expansion coefficients of the crystalline phase of Bi and the glass matrix. This additional stress leads to a depression of about 10 K in the melting temperature of the Bi nanocrystals confined in the glass. The procedure described here also allowed the determination of the specific masses and thermal expansion coefficients of Bi nanoparticles in both liquid and crystalline phases.

Probing Dopant Locations in Silicon Nanocrystals via High Energy X-ray Diffraction and Reverse Monte Carlo Simulation

Understanding the locations of dopant atoms in ensembles of nanocrystals is crucial to controlling the dopants' function. While electron microscopy and atom probe tomography methods allow investigation of dopant location for small numbers of nanocrystals, assessing large ensembles has remained a challenge. Here, we are using high energy X-ray diffraction (HE-XRD) and structure reconstruction via reverse Monte Carlo simulation to characterize nanocrystal structure and dopant locations in ensembles of highly boron and phosphorus doped silicon nanocrystals (Si NCs). These plasma-synthesized NCs are a particularly intriguing test system for such an investigation, as elemental analysis suggests that Si NCs can be "hyperdoped" beyond the thermodynamic solubility limit in bulk silicon. Yet, free carrier densities derived from local surface plasmon resonances suggest that only a fraction of dopants are active. We demonstrate that the structural characteristics garnered from HE-XRD and structure reconstruction elucidate dopant location and doping efficacy for doped Si NCs from an atomic-scale perspective.

Nanoscopic Porous Iridium/Iridium Dioxide Superstructures (15 nm): Synthesis and Thermal Conversion by In Situ Transmission Electron Microscopy

Porous particle superstructures of about 15 nm diameter, consisting of ultrasmall nanoparticles of iridium and iridium dioxide, are prepared through the reduction of sodium hexachloridoiridate(+IV) with sodium citrate/sodium borohydride in water. The water-dispersible porous particles contain about 20 wt % poly(N-vinylpyrrolidone) (PVP), which was added for colloidal stabilization. High-resolution transmission electron microscopy confirms the presence of both iridium and iridium dioxide primary particles (1-2 nm) in each porous superstructure. The internal porosity (≈58 vol%) is demonstrated by electron tomography. In situ transmission electron microscopy up to 1000 °C under oxygen, nitrogen, argon/hydrogen (all at 1 bar), and vacuum shows that the porous particles undergo sintering and subsequent compaction upon heating, a process that starts at around 250 °C and is completed at around 800 °C. Finally, well-crystalline iridium dioxide is obtained under all four environments. The catalytic activity of the as-prepared porous superstructures in electrochemical water splitting (oxygen evolution reaction; OER) is reduced considerably upon heating owing to sintering of the pores and loss of internal surface area.

Size, dimensionality and composition effects on the Debye temperature of nanocrystals

As an important property for reflecting the binding forces between atoms, the Debye temperature of nanocrystals can be tuned by size, dimensionality and composition. In order to understand how these factors influence the Debye temperature, in this contribution, a new nanothermodynamic model without any adjustable parameter was established by considering the surface stress and bond number simultaneously. As expected, the Debye temperature decreases with a decrease in size if the dimensionality is given, while the size effect on nanowires is stronger than that on thin films and weaker than that on nanoparticles. It is also found that the Debye temperature of nanoalloys decreases with the increase of the component with smaller cohesive energy for the same size and dimensionality. The validity of the model is proved by the good consistency between the model predictions and experimental and computer simulation results.

Nanomaterials: Electrochemical Properties and Application in Sensors

The unique properties of nanoparticles make them an extremely valuable modifying material, being used in electrochemical sensors. The features of nanoparticles affect the kinetics and thermodynamics of electrode processes of both nanoparticles and redox reactions occurring on their surface. The paper describes theoretical background and experimental studies of these processes. During the transition from macro- to micro- and nanostructures, the analytical characteristics of sensors modify. These features of metal nanoparticles are related to their size and energy effects, which affects the analytical characteristics of developed sensors. Modification of the macroelectrode with nanoparticles and other nanomaterials reduces the detection limit and improves the degree of sensitivity and selectivity of measurements. The use of nanoparticles as transducers, catalytic constituents, parts of electrochemical sensors for antioxidant detection, adsorbents, analyte transporters, and labels in electrochemical immunosensors and signal-generating elements is described.

A Reduction in Particle Size Generally Causes Body-Centered-Cubic Metals to Expand but Face-Centered-Cubic Metals to Contract

From a careful analysis of existing data as well as new measurements, we show that the size dependence of the lattice parameters in metal nanoparticles with face-centered cubic (fcc) and body-centered cubic (bcc) symmetries display opposite trends: nanoparticles with fcc structure generally contract with decreasing particle size, while those with bcc structure expand. We present a microscopic explanation for this apparently puzzling behavior based on first-principles simulations. Our results, obtained from a comparison of density functional theory calculations with experimental data, indicate that the nanoparticles are capped by a surface monolayer of oxygen atoms, which is routinely detected by surface-sensitive techniques. The bcc- and fcc-based nanoparticles respond in contrasting fashion to the presence of the oxygen capping layer, and this dictates whether the corresponding lattice parameter would increase or decrease with size reduction. The metal-oxygen bonds at the surface, being shorter and stronger than typical metal-metal bonds, pull the surface metal atoms outward. This outward movement of surface atoms influences the core regions to a larger extent in the relatively open bcc geometry, producing a rather large overall expansion of the cluster, compared to the bulk. In case of fcc clusters, on the other hand, the outward movement of surface metal atoms does not percolate too far inside, resulting in either a smaller net expansion or contraction of the cluster depending on the extent of surface oxygen coverage. Our study therefore provides a convincing physicochemical basis for the correlation between the underlying geometry and the nature of change of the lattice parameters under size reduction.

A Review of Carbon-Composited Materials as Air-Electrode Bifunctional Electrocatalysts for Metal–Air Batteries

Metal–air batteries (MABs), particularly rechargeable MABs, have gained renewed interests as a potential energy storage/conversion solution due to their high specific energy, low cost, and safety. The development of MABs has, however, been considerably hampered by its relatively low rate capability and its lack of efficient and stable air catalysts in which the former stems mainly from the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and the latter stems from the corrosion/oxidation of carbon materials in the presence of oxygen and high electrode potentials. In this review, various carbon-composited bifunctional electrocatalysts are reviewed to summarize progresses in the enhancement of ORR/OER and durability induced by the synergistic effects between carbon and other component(s). Catalyst mechanisms of the reaction processes and associated performance enhancements as well as technical challenges hindering commercialization are also analyzed. To facilitate further research and development, several research directions for overcoming these challenges are also proposed.

Mechanochemically prepared SrFCl nanophosphor co-doped with Yb3+ and Er3+ for detecting ionizing radiation by upconversion luminescence

We report a novel method for detecting ionizing radiation by employing the phenomenon of upconversion luminescence. Nanocrystalline SrFCl:Yb³⁺/Er³⁺ was prepared by ball-milling and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The photoluminescence properties of nanocrystalline SrFCl:Yb³⁺, SrFCl:Er³⁺ and SrFCl:Yb³⁺/Er³⁺ before and after X-irradiation were investigated. The results demonstrate that both Yb³⁺ and Er³⁺ ions in the SrFCl host are reduced to their divalent state upon X-ray exposure. Under 980 nm infrared excitation, SrFCl:Yb³⁺/Er³⁺ nanocrystals displayed efficient upconversion luminescence. The upconversion luminescence intensity gradually decreased with increasing X-irradiation in a double exponential fashion with rate constants of k₁ = 0.08 Gy^-1 and k₂ = 0.01 Gy^-1. In comparison with other X-ray storage phosphors, the present system shows a much higher stability of stored information since it is not subject to photobleaching in the read-out process. This is the first report on detecting ionizing radiation by upconversion luminescence, with the potential for improved read-out performance over traditional storage phosphors. Possible applications of the present phosphor include bioimaging and in vivo cell-level X-ray dose monitoring.

Particle size effects on structural and optical properties of BaF2 nanoparticles

Barium fluoride (BaF₂) nanoparticles (NPs) with different sizes were produced through a hydrothermal microwave method (HTMW). We have found that microstructural strain is induced by the surface stress in the nanoparticles.

Nanoscopic Thermodynamics

Conventional thermodynamics for bulk substances encounters challenges when one considers materials on the nanometer scale. Quantities such as entropy, enthalpy, free energy, melting temperature, ordering temperature, Debye temperature, and specific heat no longer remain constant but change with the crystal dimension, size, and morphology. Often, one phenomenon is associated with a variety of theories from different perspectives. Still, a model that can reconcile the size and shape dependence of the thermal properties of the nanoscaled substances remains one of the goals of nanoscience and nanotechnology. This Account highlights the nanoscopic thermodynamics for nanoparticles, nanowires, and nanofilms, with particular emphasis on the bond energy model. The central idea is that the atomic cohesive energy determines the thermodynamic performance of a substance and the cohesive energy varies with the atomic coordination environment. It is the cohesive energy difference between the core and the shell that dictates the nanoscopic thermodynamics. This bond energy model rationalizes the following: (i) how the surface dangling bonds depress the melting temperature, entropy, and enthalpy; (ii) how the order-disorder transition of the nanoparticles depends on particle size and how their stability may vary when they are embedded in an appropriate matrix; (iii) predictions of the existence of face-centered cubic structures of Ti, Zr, and Hf at small size; (iv) how two elements that are immiscible in the bulk can form an alloy on the nanoscale, where the critical size can be predicted. The model has enabled us to reproduce the size and shape dependence of a number of physical properties, such as melting temperature, melting entropy, melting enthalpy, ordering temperature, Gibbs free energy, and formation heat, among others, for materials such as Pd, Au, Ag, Cu, Ni, Sn, Pb, In, Bi, Al, Ti, Zr, Hf, In-Al, Ag-Ni, Co-Pt, Cu-Ag, Cu-Ni, Au-Ni, Ag-Pt, and Au-Pt on the nanometer scale. Furthermore, this model predicts the phenomena of the thermal stability of metal particles on graphene, the superheating of embedded nanoparticles, the order-disorder transition of nanoalloys, the size-temperature phase diagram for low-dimensional solids and the alloying ability on the nanoscale. Extensions of this model may lead to the design of new functional nanomaterials.

Temperature-driven directional coalescence of silver nanoparticles

Silver nanoparticles were synthesized with a chemical reduction method in the presence of polyvinylpyrrolidone as stabilizing agent. The thermal stability behavior of the silver nanoparticles was studied in the temperature range from 25 to 700°C. Thermal gravimetric analysis was used to measure the weight loss of the silver nanoparticles. Scanning electron microscopy and high-resolution transmission electron microscopy were used to observe the morphology and the change in shape of the silver nanoparticles. In situ temperature-dependent small-angle X-ray scattering was used to detect the increase in particle size with temperature. In situ temperature-dependent X-ray diffraction was used to characterize the increase in nanocrystal size and the thermal expansion coefficient. The results demonstrate that sequential slow and fast Ostward ripening are the main methods of nanoparticle growth at lower temperatures (<500°C), whereas successive random and directional coalescences are the main methods of nanoparticle growth at higher temperatures (>500°C). A four-stage model can be used to describe the whole sintering process. The thermal expansion coefficient (2.8 × 10(-5) K(-1)) of silver nanoparticles is about 30% larger than that of bulk silver. To our knowledge, the temperature-driven directional coalescence of silver nanocrystals is reported for the first time. Two possible mechanisms of directional coalescence have been proposed. This study is of importance not only in terms of its fundamental academic interest but also in terms of the thermal stability of silver nanoparticles.

Nanoparticle shape, thermodynamics and kinetics

Nanoparticles can be beautiful, as in stained glass windows, or they can be ugly as in wear and corrosion debris from implants. We estimate that there will be about 70,000 papers in 2015 with nanoparticles as a keyword, but only one in thirteen uses the nanoparticle shape as an additional keyword and research focus, and only one in two hundred has thermodynamics. Methods for synthesizing nanoparticles have exploded over the last decade, but our understanding of how and why they take their forms has not progressed as fast. This topical review attempts to take a critical snapshot of the current understanding, focusing more on methods to predict than a purely synthetic or descriptive approach. We look at models and themes which are largely independent of the exact synthetic method whether it is deposition, gas-phase condensation, solution based or hydrothermal synthesis. Elements are old dating back to the beginning of the 20th century-some of the pioneering models developed then are still relevant today. Others are newer, a merging of older concepts such as kinetic-Wulff constructions with methods to understand minimum energy shapes for particles with twins. Overall we find that while there are still many unknowns, the broad framework of understanding and predicting the structure of nanoparticles via diverse Wulff constructions, either thermodynamic, local minima or kinetic has been exceedingly successful. However, the field is still developing and there remain many unknowns and new avenues for research, a few of these being suggested towards the end of the review.

Optimal metal domain size for photocatalysis with hybrid semiconductor-metal nanorods

Semiconductor-metal hybrid nanostructures offer a highly controllable platform for light-induced charge separation, with direct relevance for their implementation in photocatalysis. Advances in the synthesis allow for control over the size, shape and morphology, providing tunability of the optical and electronic properties. A critical determining factor of the photocatalytic cycle is the metal domain characteristics and in particular its size, a subject that lacks deep understanding. Here, using a well-defined model system of cadmium sulfide-gold nanorods, we address the effect of the gold tip size on the photocatalytic function, including the charge transfer dynamics and hydrogen production efficiency. A combination of transient absorption, hydrogen evolution kinetics and theoretical modelling reveal a non-monotonic behaviour with size of the gold tip, leading to an optimal metal domain size for the most efficient photocatalysis. We show that this results from the size-dependent interplay of the metal domain charging, the relative band-alignments, and the resulting kinetics.

The efficiency of photocatalysis on semiconductor-metal hybrid nanostructures can be influenced by myriad factors. Here, through both experimental and theoretical efforts, the authors elucidate the influence of metal domain size upon performance of such structures, allowing future rational design.

Surface Tension Effects on the Reactivity of Metal Nanoparticles

We present calculated adsorption energies of oxygen on gold and platinum clusters with up to 923 atoms (3 nm diameter) using density functional theory. We find that surface tension of the clusters induces a compression, which weakens the bonding of adsorbates compared with the bonding on extended surfaces. The effect is largest for close-packed surfaces and almost nonexistent on the more reactive steps and edges. The effect is largest at low coverage and decreases, even changing sign, at higher coverages where the strain changes from compressive to tensile. Quantum size effects also influence adsorption energies but only below a critical size of 1.5 nm for platinum and 2.5 nm for gold. We develop a model to describe the strain-induced size effects on adsorption energies, which is able to describe the influence of surface structure, adsorbate, metal, and coverage.

Critical size for the β- to α-transformation in tin nanoparticles after lithium insertion and extraction

The formation of the α-Sn phase in Sn/SnO_x core/shell nanoparticles after lithium insertion and extraction was investigated for the first time and a critical size for the transformation was determined.

The structural phase transition process of free-standing monoclinic vanadium dioxide micron-sized rods: temperature-dependent Raman study

Micro-sized rods show a lower phase transition temperature than nano-sized rods, and this is interpreted on the basis of nucleating defects.

Grain size effect on magnetic and phase transition features in one-dimensional S = 1/2 Heisenberg spin chain molecular crystals

Magnetic and thermal behaviors and the phase transition nature are strongly influenced by grain size in one-dimensional S = 1/2 molecular spin systems.

The Strongest Particle: Size-Dependent Elastic Strength and Debye Temperature of PbS Nanocrystals

We investigated the elastic compressibility of PbS nanocrystals (NCs) pressurized in a diamond anvil cell and simultaneously probed the structure using synchrotron-based X-ray diffraction. The compressibility of PbS NCs exhibits bimodal size dependence. The elastic modulus of small NCs increases with increasing diameter and peaks near a particle diameter of approximately 7 nm. For large NCs the elastic modulus decreases toward the bulk value with increasing NC diameter. We explain the bimodal size-dependence of the elastic modulus in terms of a core-shell model based on distinct elasticity of the crystal near the surface and in the core of the particle. We combined insights into the size-dependent elasticity and lattice spacing to determine the Debye temperature of PbS NCs as a function of particle diameter. Understanding the size-dependent elasticity of defect-free colloidal NCs provides new insights into their crystal structure and mechanical properties.

Size dependence of the lattice parameters of carbon supported platinum nanoparticles: X-ray diffraction analysis and theoretical considerations

The size effect of Pt/C nanoparticles with diameters ranging from 2 to 28 nm have been studied using X-ray diffraction.

Noble-transition metal nanoparticle breathing in a reactive gas atmosphere

In situ high-energy X-ray diffraction coupled to atomic pair distribution function analysis is used to obtain fundamental insight into the effect of the reactive gas environment on the atomic-scale structure of metallic particles less than 10 nm in size. To substantiate our recent discovery we investigate a wide range of noble-transition metal nanoparticles and confirm that they expand and contract radially when treated in oxidizing (O2) and reducing (H2) atmospheres, respectively. The results are confirmed by supplementary XAFS experiments. Using computer simulations guided by the experimental diffraction data we quantify the effect in terms of both relative lattice strain and absolute atomic displacements. In particular, we show that the effect leads to a small percent of extra surface strain corresponding to several tenths of Ångström displacements of the atoms at the outmost layer of the particles. The effect then gradually decays to zero within 4 atomic layers inside the particles. We also show that, reminiscent of a breathing type structural transformation, the effect is reproducible and reversible. We argue that because of its significance and widespread occurrence the effect should be taken into account in nanoparticle research.

Enhanced cooperative interactions at the nanoscale in spin-crossover materials with a first-order phase transition

We analyzed the size effect on a first-order spin transition governed by elastic interactions. This study was performed in the framework of a nonextensive thermodynamic core-shell model. When decreasing the particle size, differences in surface energies between the two phases lead to the shrinking of the thermal hysteresis width, the lowering of the transition temperature, and the increase of residual fractions at low temperature, in good agreement with recent experimental observations on spin transition nanomaterials. On the other hand, a modification of the particle-matrix interface may allow for the existence of the hysteresis loop even at very low sizes. In addition, an unexpected reopening of the hysteresis, when the size decreases, is also possible due to the hardening of the nanoparticles at very small sizes, which we deduced from the size dependence of the Debye temperature of a series of coordination nanoparticles.

Gold nanolayer and nanocluster coatings induced by heat treatment and evaporation technique

The paper is focused on the preparation and surface characterization of gold coatings and nanostructures deposited on glass substrate. Different approaches for the layer preparation were applied. The gold was deposited on the glass with (i) room temperature, (ii) glass heated to 300°C, and (iii) the room temperature-deposited glass which was consequently annealed to 300°C. The sheet resistance and concentration of free carriers were determined by the van der Pauw method. Surface morphology was characterized using an atomic force microscopy. The optical properties of gold nanostructures were measured by UV-vis spectroscopy. The evaporation technique combined with simultaneous heating of the glass leads to change of the sheet resistance, surface roughness, and optical properties of gold nanostructures. The electrically continuous layers are formed for significantly higher thickness (18 nm), if the substrate is heated during evaporation process. The annealing process influences both the structure and optical properties of gold nanostructures. The elevated temperature of glass during evaporation amplifies the peak of plasmon resonance in the structures, the surface morphology being significantly altered.

Surface lattice-engineered bimetallic nanoparticles and their catalytic properties

When nanoparticles become small (ca. <5 nm), surface stress becomes significant and generates strain that results in a change of surface structures. In this regard, the surface lattice of nanoparticles can be engineered to create strains or other structural changes with atomic positions away from the normal lattice points. Such changes impact the electronic and catalytic properties of nanoparticles. Recently, several groups have reported the change of catalytic and electrocatalytic properties of bimetallic nanoparticles. In this tutorial review, we discuss the principles related to lattice strain and other distorted structures, and the catalytic properties of bimetallic nanostructures.