Synthesis of a Mesoporous SnO2 Catalyst Support and the Effect of Its Pore Size on the Performance of Polymer Electrolyte Fuel Cells.

The aim of this study was to clarify the effectiveness and challenges of applying mesoporous tin oxide (SnO2)-based supports for Pt catalysts in the cathodes of polymer electrolyte fuel cells (PEFCs) to simultaneously achieve high performance and high durability. Recently, the focus of PEFC application in automobiles has shifted to heavy-duty vehicles (HDVs), which require high durability, high energy-conversion efficiency, and high power density. It has been reported that employing mesoporous carbon supports improves the initial performance by mitigating catalyst poisoning caused by sulfonic acid groups of the ionomer as well as by reducing the oxygen transport resistance through the Pt/ionomer interface. However, carbon materials in the cathode can degrade oxidatively during long-term operation, and more stable materials are desired. In this study, we synthesized connected mesoporous Sb-doped tin oxides (CMSbTOs) with controlled mesopore sizes in the range of 4-11 nm and tested their performance and durability as cathode catalyst supports. The CMSbTO supports exhibited higher fuel cell performance at a pore size of 7.3 nm than the solid-core SnO2-based, solid-core carbon, and mesoporous carbon supports under dry conditions, which can be attributed to the mitigation of the formation of the Pt/ionomer interface and the better proton conductivity within the mesopores even at the low-humidity conditions. In addition, the CMSbTO supports exhibited high durability under oxidative conditions. These results demonstrate the promising applicability of mesoporous tin oxide supports in PEFCs for HDVs. The remaining challenges, including the requirements for improving performance under wet conditions and stability under reductive conditions, are also discussed.

Synthesis of Cu Nanoparticles Using Copper Carbonate as Cu Source Toward Versatile Applications.

Cu nanoparticles (NPs) coated with polyvinylpyrrolidone (PVP) were fabricated by polyol method using copper carbonate as a raw material. To increase the reaction temperature, glycol multimers such as diethylene glycol, triethylene glycol, or tetraethylene glycol were examined as a solvent. With increasing degree of multimerization, average diameter of Cu NPs decreased. The synthesis of Cu NPs was further investigated by changing reaction temperature, the amount and molecular weight of PVP in triethylene glycol as a solvent. Average diameter and standard deviation of Cu NPs were found to be highly dependent on those factors. As a result, fine Cu NPs ranging from 28 to 67 nm in average size with narrow size distribution (standard deviation: 16-28%) were obtained. The obtained Cu NPs were applied to a nanofluid, which showed higher thermal conductivity than the theoretical value. The antibacterial activity of Cu NPs was also demonstrated, and found to have strong antibacterial activity.

Silicon deposition in nanopores using a liquid precursor.

Techniques for depositing silicon into nanosized spaces are vital for the further scaling down of next-generation devices in the semiconductor industry. In this study, we filled silicon into 3.5-nm-diameter nanopores with an aspect ratio of 70 by exploiting thermodynamic behaviour based on the van der Waals energy of vaporized cyclopentasilane (CPS). We originally synthesized CPS as a liquid precursor for semiconducting silicon. Here we used CPS as a gas source in thermal chemical vapour deposition under atmospheric pressure because vaporized CPS can fill nanopores spontaneously. Our estimation of the free energy of CPS based on Lifshitz van der Waals theory clarified the filling mechanism, where CPS vapour in the nanopores readily undergoes capillary condensation because of its large molar volume compared to those of other vapours such as water, toluene, silane, and disilane. Consequently, a liquid-specific feature was observed during the deposition process; specifically, condensed CPS penetrated into the nanopores spontaneously via capillary force. The CPS that filled the nanopores was then transformed into solid silicon by thermal decomposition at 400 °C. The developed method is expected to be used as a nanoscale silicon filling technology, which is critical for the fabrication of future quantum scale silicon devices.

Novel method to incorporate Si into monodispersed mesoporous carbon spheres.

Liquid silicon precursor is used as a silicon source and very simple and easy method for the incorporation of Si into mesoporous carbon spheres is presented. By using capillary condensation, the liquid precursor, Cyclopentasilane, penetrates into mesopores of carbon spheres homogeneously and subsequent heating brings the decomposition of the precursor and the formation of silicon inside meso-channels of carbon even though the decomposition is done much higher than the boiling point of the precursor. The homogeneous distribution of silicon is verified by EDX mapping of the composite as well as SEM observation of the calcined one. More than 45wt% of Si can be incorporated into mesopores by just one operation. The Si@mesoporous carbon composite works as an anode for a Lithium ion battery.

In situ green synthesis of fluorescent monodispersed mesoporous silica spheres/poly(p-phenylenevinylene) composites.

A facile one-pot synthesis for the composite materials fabricated from conjugated polymer, poly(p-phenylenevinylene) (PPV), and monodispersed mesoporous silica spheres (MMSS) is demonstrated. Composite materials having superior photoluminescence properties are easily obtained using ethylene glycol as a reaction solvent in which PPV monomers are effectively exchanged with cationic surfactants in MMSS and subsequently polymerized in the solution. The method can prevent serious reduction of photoluminescence properties which occurs inevitably during thermal treatment (200°C) to polymerize PPV. In our method, the temperature of 100°C is enough to obtain the fully polymerized PPV, which is confirmed in Fourier transform infrared (FT-IR) spectrum. Reaction mechanism is verified through direct observation of its distinguishable color changes in the reaction solution and the measurement of surface electrical potential (ζ-potential). The obtained results strongly support that PPV chains are impregnated within mesopores in isolated condition, leading to high fluorescence quantum yield (nearly 80%). Compared to the conventional route, this method reduces multistep synthesis to one-step and eliminates high temperature and high vacuum process, leading to the facile eco-friendly procedure.

Hollow Metal-Incorporated Monodispersed Mesoporous Silica Spheres.

Expansion of mesopores and concomitant incorporation of metal species into mesoporous silica particles was achieved through the hydrothermal treatment of monodispersed mesoporous silica spheres in an aqueous metal salt solution. Mesopore size was increased to 24 nm from 2.2 nm while retaining monodispersed spherical shape. Surprisingly, hollow spheres were produced when Mg salt was used. Incorporation of metal species led to drastic improvement in adsorption performance. The amount of Rhodamine B adsorbed was increased by 27 times.

Self-assembly of monodisperse starburst carbon spheres into hierarchically organized nanostructured supercapacitor electrodes.

We report a three-dimensional (3D) porous carbon electrode containing both nanoscale and microscale porosity, which has been hierarchically organized to provide efficient ion and electron transport. The electrode organization is provided via the colloidal self-assembly of monodisperse starburst carbon spheres (MSCSs). The periodic close-packing of the MSCSs provides continuous pores inside the 3D structure that facilitate ion and electron transport (electrode electrical conductivity ∼0.35 S m(-1)), and the internal meso- and micropores of the MSCS provide a good specific capacitance. The capacitance of the 3D-ordered porous MSCS electrode is ∼58 F g(-1) at 0.58 A g(-1), 48% larger than that of disordered MSCS electrode at the same rate. At 1 A g(-1) the capacitance of the ordered electrode is 57 F g(-1) (95% of the 0.24 A g(-1) value), which is 64% greater than the capacitance of the disordered electrode at the same rate. The ordered electrode preserves 95% of its initial capacitance after 4000 charging/discharging cycles.

Anisotropic lattice expansion of three-dimensional colloidal crystals and its impact on hypersonic phonon band gaps.

We report anisotropic expansion of self-assembled colloidal polystyrene-poly(dimethylsiloxane) crystals and its impact on the phonon band structure at hypersonic frequencies. The structural expansion was achieved by a multistep infiltration-polymerization process. Such a process expands the interplanar lattice distance 17% after 8 cycles whereas the in-plane distance remains unaffected. The variation of hypersonic phonon band structure induced by the anisotropic lattice expansion was recorded by Brillouin measurements. In the sample before expansion, a phononic band gap between 3.7 and 4.4 GHz is observed; after 17% structural expansion, the gap is shifted to a lower frequency between 3.5 and 4.0 GHz. This study offers a facile approach to control the macroscopic structure of colloidal crystals with great potential in designing tunable phononic devices.

Investigation of the stability of Platinum nanoparticles incorporated in mesoporous silica with different pore sizes.

The effect of the pore size of mesoporous silica on the stability of Pt nanoparticles (NPs) has been investigated. TEM observation and XRD measurement were conducted in situ for Pt loaded mesoporous silica with different mesopore sizes. It turns out that smaller pores are more effective to stabilize Pt NPs below 600 °C. However, aggregation of Pt NPs on the surface of particles is not fully suppressed more than 1000 °C in ambient atmosphere even though smaller mesopore size is applied. The type of precursor does not affect the stability of Pt NPs.

Monodispersed mesoporous silica spheres with various mesopore symmetries.

Monodispersed mesoporous silica spheres (MMSS) with different mesopore symmetries, such as hexagonal, cubic, or the mixture of hexagonal/cubic, are synthesized changing synthesis conditions. It seems that the direction of mesopores is retained through the particle in MMSS with cubic symmetry. In the case of hexagonal/cubic mixed symmetry, cubic structure is observed at the center of the particle, while hexagonal structure is observed near the surface. It is assumed that cubic structure forms at early stage of the particle growth and hexagonal symmetry forms at the later stage, leading to the formation of cubic core/hexagonal shell structure.

Highly monodispersed tin oxide/mesoporous starbust carbon composite as high-performance Li-ion battery anode.

The widespread commercialization of today's plug-in hybrid and all electric vehicles will rely on improved lithium batteries with higher energy density, greater power, and durability.To take advantage of the high density of SnO2 anodes for Li ion batteries, we achieved a smart design of monodispersed SnO2/MSCS composite with very high content of SnO2 by a simple infiltration procedure. The synergistic effects of the unique nanoarchitecture of MSCS and the ultrafine size of SnO2 nanoparticle endowed the composite with superior electrochemical performance. Because of the high density of the composite resulting from its monodispersed submicrometer spherical morphology, an exceptionally high reversible lithium storage capacity (both gravimetric and volumetric), very close to the theoretical capacity (1491 mA h/g), can be achieved with good cyclability (capacity retention of 92.5% after 15 cycles). The SnO2/MSCS composite anode exhibited a high reversible average capacity of about 1200 mAh/g over 30 cycles at a current of 80 mAh/g, which corresponds to about 1440 mAh/cm(3) (practical volumetric capacity). In addition, a Coulombic efficiency close to 100% was achieved, and less than 25% first irreversible capacity loss was observed.

Photoinduced electron transfer between the anionic porphyrins and viologens in titania nanosheets and monodisperse mesoporous silica hybrid films.

Photoinduced electron transfer between an anionic porphyrin derivative (tetrakis(p-carboxyphenyl)porphyrin; H(2)TCPP(4-)) and an electron accepting methyl viologen (MV(2+)) was investigated in two different nanoscale configurations, i.e., layered titania nanosheet (TNS) photocatalysts and ammonium-functionalized monodisperse mesoporous silica (AMMSS) particles. Cationic MV(2+) intercalated within the TNS interlayers while anionic H(2)TCPP(4-) was accommodated within AMMSS nanocavities to form (MV(2+)-TNS)/(H(2)TCPP(4-)-AMMSS) hybrid films. Upon irradiation with UV light and excitation of the TNS in the (MV(2+)-TNS)/(H(2)TCPP(4-)-AMMSS) hybrid films, the consumption of H(2)TCPP(4-) and the formation of a one-electron reduced MV(2+) (MV(+·)) were simultaneously observed. No consumption of H(2)TCPP(4-) was observed when an electrically insulating poly(styrene) (PS) was also introduced at the interface. These results suggest that photoinduced electron transfer occurred at the interface between the TNS and the AMMSS.

Directional study of the optical properties of Tb3+- and Eu3+-doped nanoparticles embedded in silica photonic crystals.

The effects of the stop band (SB) in colloidal photonic crystals composed of silica spheres containing Eu(3+)- and Tb(3+)-doped yttria nanoparticles are analysed. Reflection and transmission spectra indicate movement of the stop band, due to the 111 series of planes, towards shorter wavelengths with increasing angle of observation. The profile of the emission spectra is modified by the presence of the SB depending on the angle of measurement. Such a modification is more effective for a narrow emission band and it is thus more evident in the case of Tb(3+) than Eu(3+). An angular effect is also observed in the lifetime, which presents two maxima and one minimum. In the case of Tb(3+) the maxima are at observation angles of 35 and 50 degrees, and the minimum at 45 degrees. We attribute this behaviour to penetration of the excitation beam at 475 nm modulated by the stop band. The ions excited in this way emit from different depths in the crystal, and therefore their lifetime will be affected differently by the same stop band, depending on the thickness of the crystal that must be crossed. Eu(3+) shows a similar but less pronounced effect for two reasons: first, the main stop band (due to the 111 planes) is not effective at the excitation wavelength of 392 nm; second, the broadness of the Eu(3+) emission is comparable to the width of the SB, and a decrease in the transition rate at the wavelength of the SB maximum is compensated by an increase at the sides of the SB.

Artificial black opal fabricated from nanoporous carbon spheres.

A nanocasting method via chemical vapor deposition of acetonitrile was successfully employed to fabricate porous carbon colloidal crystal using colloidal crystal from monodispersed mesoporous silica spheres (MMSS) as a sacrificial scaffold. The mesostructure as well as periodic arrays within (111) plane of MMSS were replicated for the carbon colloidal crystal (black opal) with the length scale in the centimeter range. Brilliant iridescent colors were clearly observed for the first time on the black carbon colloidal crystal fabricated from porous carbon spheres, and they changed dramatically in accordance with the observation angle, like natural black opals. Reflection spectra measurements based on 2D surface diffraction and Bragg diffraction in the mirror mode were conducted for the fabricated carbon periodic arrays. The periodicity in the (111) plane as well as in the direction perpendicular to the (111) plane of the colloidal crystal was evaluated by comparing the results obtained from these two measurements. It was found that the periodicity in the direction perpendicular to the (111) surface is not high for the obtained black carbon opal. On the other hand, the relationship between the incident angles and the peak wavelengths of the reflection spectra, collected in the condition where the incident light and the reflected light pass through in the same direction, is governed by an approximation based on 2D surface diffraction. The results imply that the origin of the iridescent colors on the fabricated black carbon opal is derived from the periodicity not in the direction perpendicular to the (111) plane but within the (111) plane.

New strategy using glycol-modified silane to synthesize monodispersed mesoporous silica spheres applicable to colloidal photonic crystals.

A new approach focusing on the reactivity of a silica source was developed for the particle size control of monodispersed mesoporous silica spheres (MMSSs). A glycol-modified silanes, tetrakis(2-hydroxyethyl) orthosilicate was chosen as the silica source and successfully applied in the surfactant-templated synthesis of MMSSs in a very dilute alkaline alcohol-water mixture. Due to its higher hydrolysis rate compared with tetraalkyl orthosilicates, it took less time for the primary particles to come out, resulting in the formation of small particles with diameters falling in the low submicrometer range. The resultant spheres possessed a well-ordered mesoporous structure, which was typically MCM-41-type hexagonal. The MCM-48-type cubic spheres could also be obtained by changing the reaction condition. The monodispersity and particle size of the spheres were precisely controlled by the adjustment of the solvent composition in the methanol-ethanol-water system. Furthermore, ionic colloidal crystals exhibiting a well-defined stop band in the visible light region could be fabricated from the resultant MMSSs for the first time. This work encourages the further utilization of MMSSs in photonics as well as in catalysis or biochemistry.

Nanoscale control over phase separation in conjugated polymer blends using mesoporous silica spheres.

A method of preparing blended conjugated polymer microparticles using mesoporous silica spheres is described. Poly(3,4-ethylenedioxythiophene) (PEDOT) was blended with poly(furfuryl alcohol) (PFA) by a sequential infiltration-polymerization approach. The materials were evaluated by both scanning and transmission electron microscopy and are shown to retain the overall spherical structure of the silica template. The filling of the mesopores and the polymer distribution within individual particles were determined by a combination of energy-dispersive X-ray microanalysis, X-ray photoelectron spectroscopy, and nitrogen adsorption. The results suggest that when PEDOT is added to the silica host, followed by PFA, the phase separation of the two immiscible polymers is constrained by the dimensions of the silica mesopores, ensuring nanoscale contact between the two phases. The silica template can be removed by etching with 25% hydrofluoric acid, leaving behind a blended polymer microparticle. The etched microparticles exhibit macroporous morphologies different from that of pure PEDOT particles prepared by a similar route. The blended microparticles also appear to undergo limited phase separation; no evidence for distinct polymer domains was observed. Conductivity measurements indicate that the blended particles are above the percolation threshold and support the conclusion that the phase domains are extremely small. Importantly, when PFA is added to the host first, followed by PEDOT, there is a striking difference to the final composition and morphology of the particles. This reversal of the blending order results in a more amorphous, phase-separated material. These results demonstrate the preparation of conjugated polymer blends with engineered nanoscale phase separation and may allow for future improvements in organic device architecture and performance.

Manipulation of the spontaneous emission in mesoporous synthetic opals impregnated with fluorescent guests.

The spontaneous emission of light from light-emitting materials adsorbed within the ordered pores of monodispersed mesoporous silica spheres (MMSS) has been investigated. By taking advantage of the ordered starburst pores of MMSS, we can provide a simple strategy for fabricating synthetic opals consisting of homogeneous individual building blocks in which fluorescent guests are uniformly and stably impregnated. In this study, tris(8-hydroxyquinolinato)aluminum(III) (Alq(3)) and Rhodamine B (Rh B) are selected as the fluorescent guests. The former has a wider emission band than the reflection spectrum of MMSS synthetic opals, whereas the emission band of the latter is considerably narrower than the reflection spectrum of the opals. The spontaneous emissions of these functionalized synthetic opals are clearly influenced by the stop band governed by the Bragg equation. In the case of the Alq(3)-MMSS conjugate, the shape of the Alq(3) emission spectrum varies in accordance with the shift in the stop band. The emission of the Rh B-MMSS conjugate is noticeably narrowed, and its intensity is enhanced when the excitation intensity is increased. These results are well explained by an inhibition of spontaneous emission caused by a reduction in the density of optical states within the stop band. The results of this study indicate that MMSS synthetic opals are promising for use in novel optical applications in which the spontaneous emission can be manipulated.

Supercapacitive properties of PEDOT and carbon colloidal microspheres.

The synthesis and characterization of a new PEDOT-carbon composite prepared using a microporous carbon template are described. The electrochemical behavior of this composite, as well as that of three other colloidal materials-PEDOT-silica, PEDOT, and microporous carbon particles-is investigated with respect to their suitability as electrode materials in supercapacitors. This was accomplished by a combination of cyclic voltammetry and galvanostatic charge/discharge cycles. It was found that the PEDOT-silica composite had the lowest specific capacitance of the four materials (ca. 60 F g(-1)) and also the worst retention of the capacitance at high scan rates. In the case of pure PEDOT, microporous carbon, or PEDOT-carbon microspheres, the specific capacitances of the materials were dramatically higher (C(M) = 115, 109, and 106 F g(-1), respectively). These values are higher than those of either unstructured electropolymerized PEDOT or commercially available high-surface-area carbon. The pure PEDOT materials retained this high capacitive behavior even at faster scan rates, although the capacitance of the carbon and PEDOT-carbon microspheres dropped substantially. These results are interpreted in the context of the local microstructure of the individual colloidal particles, as well as the overall film morphology. The morphologies of both the individual particles and the electrode films were investigated by field-emission scanning electron microscopy. Due to the monodisperse nature of the microspheres, films composed of these materials necessarily possess an interconnected network of interstitial pores that allow for facile ionic diffusion. This allows for more penetration of the conjugated polymer by the ionic electrolyte and therefore higher capacitances relative to the bulk materials. These results demonstrate the feasibility of utilizing monodisperse colloidal microparticles containing conjugated polymers as electrode materials for high-energy and high-power-density supercapacitors.

Influence of surface morphology on the colloidal and electronic behavior of conjugated polymer-silica microspheres.

A template approach to the synthesis of a series of conjugated polymer-mesoporous silica composite microspheres is described. Poly(3,4-ethylenedioxythiophene) (PEDOT), poly(thiophene), and poly( N-methylpyrrole) composites were prepared. The surface morphology of the samples was analyzed by scanning electron microscopy, and it was found that well-defined, monodisperse colloidal materials could only be prepared when the monomer is insoluble in the polymerization medium. The filling of the mesopores was systematically varied from 0% to 100%, and powder X-ray diffraction and nitrogen adsorption studies were used to confirm the pore filling. Thermogravimetric analysis shows that the polymer loading tracks the monomer loading in an asymptotic fashion. Conductivity measurements show that the conductivity of the PEDOT materials is relatively constant at high polymer loadings but decreases exponentially at low loadings. Measurements of the electrophoretic mobility were made in order to explain this behavior. These data suggest that, at high polymer loadings, the particle surface is characteristic of the polymer, while at low polymer loadings it is characteristic of the silica host. These results identify important design criteria for the template synthesis of a variety of new colloidal materials. Importantly, these optimized parameters may open the door to the preparation of colloids and colloidal crystals of previously unprocessable materials.

Optical response of mesoporous synthetic opals to the adsorption of chemical species.

We have demonstrated the fabrication of a colloidal crystalline array (synthetic opal) from monodispersed mesoporous silica spheres (MMSS) and the control of its optical response simply by changing the amount of benzene vapor adsorbed into the pores of MMSS. It was revealed that the refractive index of the colloidal crystal of MMSS showed an 11.7% increase by taking advantage of benzene adsorption, and thereby, the structural color changed reversibly. We also conducted the same measurement on silica spheres without mesopores and observed no change in the refractive index or the structural color. This optical response gives rise to the possibility of using MMSS colloidal crystals not only for controlling light reflection but also as sensing devices based on color change due to vapor adsorption. We have also incorporated an organic dye, the porphyrin derivative alpha,beta,chi,delta,-tetrakis(1-methylpyridinium-4-yl)porphyrin rho-toluenesulfonate (TMPyP), into the pores of MMSS. By adopting an electrophoretic deposition process in ethanol, periodic arrays fabricated from TMPyP-MMSS conjugates with absolute zeta-potentials near zero were obtained. The Bragg diffraction peak of the colloidal crystalline array shifted to longer wavelengths due to an increase in the refractive index with increasing amounts of TMPyP adsorbed in the pores. The current work demonstrates the new possibility of creating colloidal crystals from MMSS with mesopores filled with various kinds of adsorbates to control the optical response effectively.