Amorphous Spheres Research Articles

Tunable, Functional Carbon Spheres Derived from Rapid Synthesis of Resorcinol-Formaldehyde Resins

In this article, the rapid synthesis of colloidal, spherical polymer resins via enhanced copolymerization and polycondensation of resorcinol with formaldehyde is presented. The ultrasound-mediated technique assembles perfectly spherical resins in less than 5 min due to generated active species and free radicals produced in an aqueous ammonia-ethanol-water solvent. In this report, numerous controlled experiments account for and support the important role of high intensity ultrasounds in the rapid cluster formation, condensation, and gelation process of resorcinol with formaldehyde in the presence of ammonia catalyst. After a controlled heat treatment process, amorphous carbon spheres are obtained from these spherical polymer resins. The effect of temperature (up to 1100 °C) on the structural evolution of these carbon spheres is meticulously studied which is lacking in the previous literature. The resorcinol-formaldehyde resins carbonized at 600 and 900 °C demonstrate BET surface areas of 592.4 m(2)/g and 952.5 m(2)/g with specific capacitances of 17.5, and 33.5 F/g (scan rate of 5 mV/s), respectively.

Dispersed-nanoparticle loading synthesis for monodisperse Au-titania composite particles and their crystallization for highly active UV and visible photocatalysts.

Submicrometer-sized amorphous titania spheres incorporating Au nanoparticles (NPs) were prepared in a one-pot synthesis consisting of a sol-gel reaction of titanium(IV) isopropoxide in the presence of chloroauric acid and a successive reduction with sodium borohydride in a mixed solvent of ethanol/acetonitrile. The synthesis was allowed to prepare monodisperse titania spheres that homogeneously incorporated Au NPs with sizes of ca. 7 nm. The Au NP-loaded titania spheres underwent different crystallization processes, including 500 °C calcination in air, high-temperature hydrothermal treatment (HHT), and/or low-temperature hydrothermal treatment (LHT). Photocatalytic experiments were conducted with the Au NP-loaded crystalline titania spheres under irradiation of UV and visible light. A combined process of LHT at 80 °C followed by calcination at 500 °C could effectively crystallize titania spheres maintaining the dispersion state of Au NPs, which led to photocatalytic activity higher than that of commercial P25 under UV irradiation. Under visible light irradiation, the Au NP-titania spheres prepared with a crystallization process of LHT at 80 °C for 6 h showed photocatalytic activity much higher than a commercial product of visible light photocatalyst. Structure analysis of the visible light photocatalysts indicates the importance of prevention of the Au NPs aggregation in the crystallization processes for enhancement of photocatalytic activity.

Civil Society and the Responsibility to Protect

While a vast and growing literature on various aspects of responsibility to protect (R2P) exists, little attention has yet been given to the role played by civil society in the development and implementation of this doctrine. This paper aims to fill this gap. After providing a typology of civil society organisations—categorising this amorphous sphere into advocacy, operational and indigenous groups—this paper examines the contribution of civil society to the discussion surrounding the development of R2P. In particular, civil society has debated three main issues: the pros and cons of building an international coalition to promote R2P, the possibility that great powers could hijack the doctrine and the opportunity of using force during R2P implementation. As a whole, this paper argues that not only advocacy, operational and indigenous civil society organisations hold different ideas about R2P and have different strategies for its implementation but, more importantly, their approaches, strategies and actions may end up undercutting one another. This argument is explored in the case of Darfur—the first crisis many analysts have described as an R2P situation.

Optimization of cellular solids for energy absorption

Analysis of equations governing specific energy absorption for cellular solids indicates that silicate glass-based materials should outperform other cellular solids, including metallic foams. Quasi-static compression tests of silicate glass cellular materials fabricated by thermally bonding hollow spheres above the glass transition temperature (Tg) experimentally supports the analysis. Materials with some of the highest energy-absorbing capacities in the literature (14.8 MJ m−3 or 26.5 kJ kg−1) are reported. Fabrication techniques are generalizable to any amorphous hollow spheres with thermal stability above Tg.

Colloidal and transparent opal-matrix nanocomposites filled with europium-doped silica sols

Three-dimensional ordered opal-matrix composites filled with europium-doped silica sols have been produced using methods of colloid chemistry. According to elemental analysis data, the Eu concentration in the nanocomposites was ∼30 ppm. A uniform europium distribution over the tetrahedral and octahedral pores of the 3D opal matrix was ensured by repeatedly filling the opal pores with silica sols doped with europium salts or europium oxide. Varying high-temperature annealing conditions, one can control the microstructure of 3D ordered nanocomposites, producing opaline and transparent photonic crystals. The microstructure of opal photonic crystals has the form of an ordered fcc lattice of amorphous silica spheres, whose tetrahedral and octahedral pores are filled with mesoporous europium-doped glass. Partial sintering of the silica spheres and mesoporous glass in transparent photonic crystals results in a periodic arrangement of quantum dots enriched in Eu-doped silica.

Coupled hydrothermal synthesis/hot pressing of PbSe/C@PbTe heterostructured composites with enhanced thermoelectric performance

PbTe coated amorphous carbon sphere core–shell structure (C@PbTe) was embedded into PbSe nanopowders and subsequently assembled into macroscopic composites by hot pressing. Thermoelectric properties of the samples with 0, 1, and 5wt% C@PbTe additions were examined between 300 and 550K. Enhanced Seebeck coefficient and lower thermal conductivity are obtained in the PbSe/1wt% C@PbTe sample. The maximum figure of merit ZT is 0.54 at 400K, showing a 13% improvement over that of the PbSe matrix. These findings facilitate the application of hydrothermally synthesized carbon spheres in thermoelectric materials and also provide a potential direction to enhance thermoelectric performance via constructing core–shell structures.

Characterization of light-absorbing carbon particles at three altitudes in East Asian outflow by transmission electron microscopy

Abstract. The morphology, microstructure, and composition of the submicron fraction of individual light-absorbing carbon (LAC) particles collected by research aircraft during the ACE-Asia (Asian Pacific Regional Aerosol Characterization Experiment) project above the Yellow Sea at altitudes of 120, 450 and 1500 m are investigated by transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). Two types of carbonaceous particles, small spherule soot with graphitic spherules and amorphous carbonaceous spheres (brown carbon), are found at all altitudes in high concentration. For soot particles, emphasis of the study is on the component subparticles (spherules). The nanoscopic structures of the small spherule soot show no significant difference at three altitudes although the size distribution of primary spherules showed that 70% of the total volume lies in the ranges 30–50, 50–85 and 30–50 nm, respectively. For the amorphous carbonaceous spheres, 70% of the total volume from three altitudes lies in the range 200–350, 160–470 and 150–320 nm, respectively. Within the size fraction studied (submicron, with most particles in the range 50 to 500 nm) the number concentration ratios of the amorphous carbonaceous spheres to primary spherules in soot at altitudes of 120, 450 and 1500 m are about 1, 1.5 and 10, respectively and their volume ratios are about 260, 50 and 1400. Lower relative concentrations of large spherule soot with intermediate graphitic structure were observed at 120 m. Also, low relative number concentrations of carbon cenospheres were observed at 120 and 1500 m. A key result of the study is that in vertically stratified outflow from East Asia, the character of LAC may have strong variance with altitude thus resulting in optical characteristics that vary with altitude. Also, apparent "aging" of LAC deduced from samples at multiple ground stations may instead reflect differences in the original carbon aerosols.

Pair distribution function analysis of X-ray diffraction from amorphous spheres in an asymmetric transmission geometry: application to a Zr58.5Cu15.6Ni12.8Al10.3Nb2.8glass

A method for the calculation of the pair distribution and structure functions from X-ray intensity data obtained with an area detector for an off-center incident X-ray beam on an amorphous sphere is presented. Error propagation for converting from the structure function to the pair distribution function is also described, including a summation series approach to treat the error from a high-qtruncation. A Zr58.5Cu15.6Ni12.8Al10.3Nb2.8glass (Vitreloy 106a) is used to demonstrate the techniques. In particular, the semi-analytical corrections presented to calculate the effects of secondary scatter within and asymmetric transmission through a spherical sample are verified.

Photodegradation by a Heterogeneous Mixture of Micro-Sized Anatase and Truncated Rhomboid Anatase Hollow Spheres

TiO2 has attracted immense attention because of its unique properties, which include excellent photostability, ecofriendliness, and low cost. It has a wide range of applications in photocatalysis, dye-sensitized solar cells (DSSCs), lithium-ion storage, and electrochromic devices. TiO2 is the most studied photocatalyst because of the high oxidation capacity of its holes. A general photocatalytic procedure involves three main processes: irradiation-induced formation of electrons and holes, bulk diffusion of electrons and holes to the surface, and a surface reaction of the photogenerated carriers with the pollutant. Increasing the light harvesting, reducing the charge recombination, and increasing the surface reactivity are three methods to improve photocatalytic efficiency. To increase light harvesting, TiO2 has been treated by using a photosensitive material or doped to extend its light absorbance into the visible-light region. However, such additional treatment also causes thermal or crystal instability and the trapping of carriers, which considerably reduces efficiency. However, Mie scattering by a submicron structure increases the average photon path-length, which markedly increases light harvesting. A hollow sphere structure with a strong scattering effect and high surface area was used to enhance photocatalytic activity. To reduce the recombination of electrons and holes in the photocatalytic process, heterogeneous structures have been extensively adopted against self-recombination. Generally, photoexcited electrons are transferred to the interface of the heterogeneous structure, which results in the efficient separation of electrons and holes. Commercial P25 (particles comprised of 75% anatase and 25% rutile) is an effective photocatalyst owing to the synergy between anatase and rutile. Additionally, our group has reported an excellent photocatalyst that is effective over a wide range of pH values, which was formed by mixing microand nano-sized anatases. Anatase of different sizes has different flat-band states. Mixing TiO2 particles with various band positions facilitates the transfer of electrons therein, which results in the separation of electrons and holes and greatly improves the photocatalytic performance. Additionally, the atomic arrangement and Ti coordination at the surface strongly influence the transfer of photoexcited carriers at the surface and the adsorption/desorption between the photocatalyst and the pollutant molecules. Numerous studies have reported that anatase TiO2 with exposed {001} facets exhibits excellent photocatalytic activity. This study attempts to elucidate the details of the photocatalytic reaction. The aforementioned three strategies are used to improve the photocatalytic efficiency. Highly uniform hollow anatase TiO2 submicron spheres with exposed {001} and {101} Highly uniform hollow spheres were assembled from truncated rhomboid anatase TiO2 nanoparticles (HS), which were synthesized from the hydrothermal treatment of a NaF solution by using amorphous TiO2 spheres as a precursor and template. The submicron hollow spheres with highly photocatalytic active surfaces exhibited excellent photocatalytic behavior. In this investigation, the factors that enhanced the photocatalytic performance were examined in terms of light harvesting, photoefficiency, and reaction activity. The photocatalytic performance was increased by a factor of approximately 1.5 for the scattering structure of submicron hollow spheres than that of nanofragments of submicron hollow sphere powder. The rate constant of anatase TiO2 with specific photoactive surfaces at {001} and {101} was approximately six times higher than that of randomly exposed TiO2 particles. Furthermore, a heterogeneous mixture of the submicron hollow spheres and micro-sized anatase TiO2 (HS@micro-sized TiO2) prohibited electron–hole recombination in the catalyst during the degradation reaction. The rate constant for the heterojunction system was 3.4 times greater than that of the individual constituents. The rate constant for HS@micro-sized TiO2 (k=0.175 min ) was 1.8 times larger than that of commercial P25 (k=0.099 min ). HS@micro-sized TiO2 is a promising photocatalytic material.

Alloying and dealloying of CuPt bimetallic nanocrystals

Bimetallic CuPt nanocrystals with size ranging from 3 to 30nm were synthesized in the presence of either hexadecylamine or poly(vinylpyrrolidone) as a capping agent. Different growth stages of CuPt nanoparticles prepared with hexadecylamine have been investigated and a non-classic mechanism governing the formation of the metal alloy was revealed. It was found that the precursor molecules aggregate into amorphous spheres at a very early stage, followed by surface multiple nucleation, formation and combination of crystalline islands to produce a core–shell structure with surface-to-core extension of the crystallization to achieve single crystals. CuPt nanocrystals synthesized with poly(vinylpyrrolidone) grew via the classic route. Dealloying treatment was applied on these CuPt nanoalloys to selectively remove Cu. Large particles (~30nm) with Cu-rich cores exhibited hollow structures after dealloying while 3nm particles remained solid, demonstrating that particle size and composition have a great influence on the final morphology of dealloyed particles.

Exact Theory of Dense Amorphous Hard Spheres in High Dimension. II. The High Density Regime and the Gardner Transition

We consider the theory of the glass phase and jamming of hard spheres in the large space dimension limit. Building upon the exact expression for the free-energy functional obtained previously, we find that the random first order transition (RFOT) scenario is realized here with two thermodynamic transitions: the usual Kauzmann point associated with entropy crisis and a further transition at higher pressures in which a glassy structure of microstates is developed within each amorphous state. This kind of glass-glass transition into a phase dominating the higher densities was described years ago by Elisabeth Gardner, and may well be a generic feature of RFOT. Microstates that are small excitations of an amorphous matrix-separated by low entropic or energetic barriers-thus emerge naturally, and modify the high pressure (or low temperature) limit of the thermodynamic functions.

In Situ TEM of Two-Phase Lithiation of Amorphous Silicon Nanospheres

To utilize high-capacity Si anodes in next-generation Li-ion batteries, the physical and chemical transformations during the Li-Si reaction must be better understood. Here, in situ transmission electron microscopy is used to observe the lithiation/delithiation of amorphous Si nanospheres; amorphous Si is an important anode material that has been less studied than crystalline Si. Unexpectedly, the experiments reveal that the first lithiation occurs via a two-phase mechanism, which is contrary to previous understanding and has important consequences for mechanical stress evolution during lithiation. On the basis of kinetics measurements, this behavior is suggested to be due to the rate-limiting effect of Si-Si bond breaking. In addition, the results show that amorphous Si has more favorable kinetics and fracture behavior when reacting with Li than does crystalline Si, making it advantageous to use in battery electrodes. Amorphous spheres up to 870 nm in diameter do not fracture upon lithiation; this is much larger than the 150 nm critical fracture diameter previously identified for crystalline Si spheres.

Mesoporous titanate-based cation exchanger for efficient removal of metal cations

A mesoporous titanate-based Na+-exchanger with high surface area and tunable pore size has been successfully synthesized by using NaOH-etched silica mesoporous microspheres as templates. Templating against porous silica microspheres followed by etching the templates with base produces amorphous sodium titanate spheres with controllable porosity. The resultant Na+ exchangers have BET surface areas as high as 386.47 m2 g−1 and average pore sizes tunable from ∼5 nm to 30 nm. Adsorption experiments indicate that the as-obtained Na+-exchangers possess excellent adsorption capacity (up to 2.90 mmol g−1) for ten metal cations in low concentration ranges without any selectivity due to fast ion exchange between Na+ and the target cations, while at high concentrations, the selectivity sequence follows the order of Hg2+ < Co2+ < Cd2+ < Cu2+ < Ni2+ < Mn2+ < Zn2+ < Sn4+ < Cr3+ < Fe3+.

Facile synthesis of hollow sphere amorphous MnO2: the formation mechanism, morphology and effect of a bivalent cation-containing electrolyte on its supercapacitive behavior

Nearly X-ray amorphous hollow sphere manganese oxides (hollow sphere MnO2) have been synthesized by a carboxylic acid-mediated system containing KMnO4 and Na2S2O4 under ambient conditions for supercapacitor applications. The product was characterized by powder XRD, Raman spectroscopy and thermal analysis. SEM and TEM were used to investigate the morphology of MnO2. The as-prepared MnO2 was X-ray amorphous and had particles in the size range 0.1–1 μm. A mechanism has been proposed for the formation of hollow sphere structures in the micro-emulsion medium. Upon annealing the sample at temperatures greater than 500 °C, the amorphous MnO2 transforms into Mn2O3. Cyclic voltammetry and galvanostatic charge–discharge cycling were used to evaluate the electrochemical performance. The initial discharge capacities were found to be 283 and 188 F g−1 in 0.1 M Ca(NO3)2 and 0.1 M Na2SO4, respectively, at a current density of 0.5 mA cm−2. The higher specific capacitance in the electrolyte with a bivalent cation is attributed to the reduction of two Mn4+ to Mn3+ by each of the bivalent cations present in the electrolyte.

Morphology-controlled synthesis of hematite hierarchical structures and their lithium storage performances

A series of hematite hierarchical structures, including amorphous hierarchical spheres with varied internal polycrystalline structures, single-crystalline nanoflakes and co-aligned micro-layered structures, have been fabricated by simply controlling the reaction time in a one-step hydrothermal procedure. In the early stages of the reaction, an Ostwald ripening process dominates the successive occurrence of single-shell, double-shell and yolk–shell spheres, which reoccur in reverse order under the effect of an H+ etching process on prolonging the reaction time. Afterwards, the oriented attachment mechanism takes charge of the formation of single-crystalline nanoflakes and co-aligned micro-layered structures at the expense of the previously formed spheres. It is found that the different adsorption configuration of the hydroxyl groups in different crystallographic planes is of fundamental importance to the morphology evolution from nanoflakes to micro-layered structures. The electrochemical measurements of these hierarchical structures show that their lithium storage properties are closely related to their crystallinity and morphology. The amorphous-based architectures exhibit quite similar electrochemical properties regardless of their morphologies, whereas the crystallized architectures present morphology-dependent electrochemical properties. Also, well-crystallized electrodes facilitate the access of lithium-ions and thus possess superior lithium storage performances, whether in the initial charge–discharge cycling or in the subsequent cyclic capacity retention.

Highly efficient photoluminescence of SiO2 and Ce–SiO2 microfibres and microspheres

Semiconductor nanocrystals and nanostructures have been extensively studied in the last few years due to their interesting optical and optoelectronic properties. Nevertheless, combining precise photoluminescence properties with controlled morphologies of SiO2 is a major hurdle for a broad range of basic research and technological applications. Here, we demonstrate that microemulsion droplet interfacial elasticity can be manipulated to induce definite morphologies associated with specific intrinsic and extrinsic photoluminescent defects in the silica matrix. Thus, under precise experimental conditions hollow crystalline and compact amorphous SiO2 spheres showing ultraviolet-photoluminescence and helicoidal fibrils of Ce-doped amorphous silica with violet-blue emissions are obtained. Overall, it is demonstrated that the combination of microemulsions and doping represents an easy strategy for the design of specific nanoscale structures with high efficiency photoluminescence. The detailed structural analysis provided in the present work is expected to be useful as accurate information on assessment of technological nanostructures.

Spontaneous assembly of DNA–amorphous calcium phosphate nanocomposite spheres for surface-mediated gene transfer

Submicrospheres composed of amorphous calcium phosphate (ACP) and DNA spontaneously assembled on a substrate via homogeneous calcium phosphate (CaP) nucleation followed by deposition in a highly supersaturated labile CaP solution. The nanocomposite layer of sphere assembly showed higher gene transfer efficiency than a continuous film nanocomposite layer previously fabricated via heterogeneous nucleation in a metastable supersaturated solution.

Stability of jammed packings I: the rigidity length scale

In 2005, Wyart et al. (Europhys. Lett., 72 (2005) 486) showed that the low frequency vibrational properties of jammed amorphous sphere packings can be understood in terms of a length scale, called l*, that diverges as the system becomes marginally unstable. Despite the tremendous success of this theory, it has been difficult to connect the counting argument that defines l* to other length scales that diverge near the jamming transition. We present an alternate derivation of l* based on the onset of rigidity. This phenomenological approach reveals the physical mechanism underlying the length scale and is relevant to a range of systems for which the original argument breaks down. It also allows us to present the first direct numerical measurement of l*.

Morphological diversity of Mn(iii) metalloporphyrin-based nano- and microsized CPAs assembled via kinetic and thermodynamic controls and their application in heterogeneous catalysis

A series of narrowly dispersed nano- and microsized CPAs has been prepared from a (porphyrin)Mn(III) biscarboxylic acid and Co(OAc)2. The morphologies have been diversified from amorphous spheres to crystalline cubes by varying reaction periods. In addition, their heterogeneous catalytic olefin oxidations are successfully demonstrated.

Surfactant-free sacrificial template synthesis of submicrometer-sized YVO4:Eu3+ hierarchical hollow spheres with tunable textual parameters and luminescent properties

For the first time, well-dispersed submicrometer-sized YVO(4):Eu(3+) hollow spheres were successfully synthesized though a surfactant-free method by employing Y(OH)CO(3):Eu(3+) colloidal spheres as a sacrificial template and NH(4)VO(3) as a vanadium source. The synthetic process mainly consists of two steps, i.e., hydrothermal reaction and acid erosion. By simply changing the amount of NH(4)VO(3) added, the textural parameters of the as-obtained hollow spheres, such as the inner diameter and shell-thickness, can be easily tuned. Moreover, double-shelled hollow spheres could also be obtained when the amount of NH(4)VO(3) was increased to a certain extent. Particularly, the amorphous colloidal spheres of the template could be completely consumed when the amount of NH(4)VO(3) was in large excess, giving rise to the direct formation of uniform hollow spheres without acid erosion. The possible formation process is discussed in detail. Under ultraviolet excitation, the obtained hollow YVO(4):Eu(3+) phosphors showed strong red emissions, and the Commission Internationale d'Eclairage (CIE) coordinates of the YVO(4):Eu(3+) phosphors were closely related with the textural parameters such as the inner diameter, shell-thickness and number of shells, indicating a size-dependent characteristic.