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An emerging class of materials that are hybrid in nature is propelling a technological revolution in energy, touching many fundamental aspects of energy-generation, storage, and conservation. Hybrid materials combine classical inorganic and organic components to yield materials that manifest new functionalities unattainable in traditional composites or other related multicomponent materials, which have additive function only. This Research News article highlights the exciting materials design innovations that hybrid materials enable, with an eye toward energy-relevant applications involving charge, heat, and mass transport.

The effect of pore morphology on capillary condensation and evaporation in nanoporous silicon is studied experimentally. A variety of cooperative and local effects are observed in tailored nanopores with well-defined regions by directly probing gas adsorption in each region using optical interferometry. All observations are ascribed to the ability of the nanopore region to access the gas reservoir directly and the nucleation of liquid bridges at local heterogeneities within the nanopore region. These assumptions, consistent with recent simulations, can be extended to any real nanoporous system.

Hydrogen is a promising alternative energy carrier that can potentially facilitate the transition from fossil fuels to sources of clean energy because of its prominent advantages such as high energy density (142 MJ kg(-1); ref. 1), great variety of potential sources (for example water, biomass, organic matter), light weight, and low environmental impact (water is the sole combustion product). However, there remains a challenge to produce a material capable of simultaneously optimizing two conflicting criteria--absorbing hydrogen strongly enough to form a stable thermodynamic state, but weakly enough to release it on-demand with a small temperature rise. Many materials under development, including metal-organic frameworks, nanoporous polymers, and other carbon-based materials, physisorb only a small amount of hydrogen (typically 1-2 wt%) at room temperature. Metal hydrides were traditionally thought to be unsuitable materials because of their high bond formation enthalpies (for example MgH(2) has a ΔHf~75 kJ mol(-1)), thus requiring unacceptably high release temperatures resulting in low energy efficiency. However, recent theoretical calculations and metal-catalysed thin-film studies have shown that microstructuring of these materials can enhance the kinetics by decreasing diffusion path lengths for hydrogen and decreasing the required thickness of the poorly permeable hydride layer that forms during absorption. Here, we report the synthesis of an air-stable composite material that consists of metallic Mg nanocrystals (NCs) in a gas-barrier polymer matrix that enables both the storage of a high density of hydrogen (up to 6 wt% of Mg, 4 wt% for the composite) and rapid kinetics (loading in

Remote detection of reactive analytes using optical fi lms constructed from electrochemically prepared porous Si-based photonic crystals is demonstrated. Porous Si samples are prepared to contain either surface oxide or surface Si-H species, and analyte detection is based on irreversible reactions with HF (aq) or Cl 2(g) analytes, respectively. HF dissolves silicon oxide from the porous matrix, causing an irreversible blue-shift in the resonance peak of the photonic crystal. Cl 2 reacts with the native Si-H species present on the surface of as-etched porous Si to generate reactive silicon halides that evaporate from the surface and/or react with air to convert to silicon oxide. Either Cl 2 -related process reduces the net refractive index of the material that is detected as a blue shift in the spectrum. With suffi cient analyte concentrations or exposure times, the spectral blue shifts are visible to the unaided eye. A portion of the porous nanostructure is fiwith inert polystyrene, which acts as an internal spectral reference. The polymer fi ducial protects that portion of the sensor from attack by the corrosive analytes. Refl ectance spectra from both the polymer-fi lled and the unfi lled, reactive porous layers are acquired simultaneously. The fi ducial marker also allows elimination of artifacts associated with shifts of the resonance peak upon changing the angle of incidence of the optical probe. Theoretical angle-resolved spectra (transfer matrix method) show a good match with the experimental data. Hightemperature air or room-temperature ozone oxidation reactions are used to prepare the HF-reactive surface, and it is found that the ozone oxidation reaction produces a greater sensitivity to HF (LLOD of 0.1% HF in water).

Sensing ofthe volatile organic compounds (VOCs) isopropyl alcohol (IPA) and heptane in air using sub-millimeter porous silicon-based sensor elements is demonstrated in the concentration range 50-800 ppm. The sensor elements are prepared as one-dimensional photonic crystals (rugate filters) by programmed electrochemical etch of p ++ silicon, and analyte sensing is achieved by measurement of the wavelength shift of the photonic resonance. The sensors are studied as a function of surface chemistry: ozone oxidation, thermal oxidation, hydrosilylation (1-dodecene), electrochemical methylation, reaction with dicholorodimethylsilane and thermal carbonization with acetylene. The thermally oxidized and the dichlorodimethylsilane-modified materials show the greatest stability under atmospheric conditions. Optical microsensors are prepared by attachment of the porous Si layer to the distal end of optical fibers. The acetylated porous Si microsensor displays a greater response to heptane than to IPA, whereas the other chemical modifications display a greater response to IPA than to heptane. The thermal oxide sensor displays a strong response to water vapor, while the acetylated material shows a relatively weak response. The results suggest that a combination of optical fiber sensors with different surface chemistries can be used to classify VOC analytes. Application of the miniature sensors to the detection of VOC breakthrough in a full-scale activated carbon respirator cartridge simulator is demonstrated.

One-dimensional photonic crystals constructed from multilayered stacks of porous Si are used as sensors for gas-phase volatile organic compounds (VOCs). The ability of a double-stack structure to provide compensation for drift due to changing relative humidity (RH) is investigated. In this approach, two separate photonic crystals (dielectric stacks) are etched into a crystalline Si substrate, one on top of the other. The top stack is chemically modified to be hydrophobic (by hydrosilylation with dodecene) and the bottom stack is made hydrophilic (by hydrosilylation with undecylenic acid). It is shown that the optical spectrum of the double-stack structure provides an effective means to discriminate VOCs from water vapor. In this approach, shifts in the peak frequencies from both photonic crystals are measured simultaneously. Because the two stacks respond differently to water and to VOC, the effect of changing humidity can be nulled by calculating the weighted difference between the two peak frequencies. Reliable determination of the concentration of VOC vapor in nitrogen over a range of RH values (25%

Hydron exchange reaction rates, k(exch)M(-1) s(-1), using methanolic sodium methoxide are compared with gas-phase acidities, Delta G(Acid)(0) kcal/mol, for four 9-YPhenylfluorenes-9-H-i, seven (YC6H4CH)-H-i(CF3)(2), seven YC6H4-(CHClCF3)-H-i, and (C6F5H)-H-i. Fourteen of the fluorinated benzylic compounds and pentafluorobenzene result in near unity experimental hydrogen isotope effects that suggest substantial amounts of internal return associated with the exchange process. Although the reactions of 9-phenylfluorene have experimental isotope effects that appear to be normal in value, they do not obey the Swain-Schaad relationship. This suggests that they occur with small amounts of internal return. The entropies of activation, Delta S-double dagger, are +12 to +14eu, for the benzylic compounds and different significantly from those for the 9-YPhenylfluorenes, Delta S-double dagger of -8 to - 12 eu. The Delta S-double dagger similar to 1 eu for the reactions of pentafluorobenzene falls between the other compounds. Density functional calculations using B3LYP/6-31+G(d,p) are reported for the reactions of CH3O-(HOCH3)(3) with C6F5H, C6H5CH(CF3)(2), C6H5CHClCF3, and 9-phenylfluorene. Copyright (c) 2006 John Wiley & Sons, Ltd.

Demand for pragmatic alternatives to carbon-intensive fossil fuels is growing more strident. Hydrogen represents an ideal zero-carbon clean energy carrier with high energy density. For hydrogen fuel to compete with alternatives, safe and high capacity storage materials that are readily cycled are imperative. Here, development of such a material, comprised of nickel-doped Mg nanocrystals encapsulated by molecular-sieving reduced graphene oxide (rGO) layers, is reported. While most work on advanced hydrogen storage composites to date endeavor to explore either nanosizing or addition of carbon materials as secondary additives individually, methods to enable both are pioneered: “dual-channel” doping combines the benefits of two different modalities of enhancement. Specifically, both external (rGO strain) and internal (Ni doping) mechanisms are used to efficiently promote both hydriding and dehydriding processes of Mg nanocrystals, simultaneously achieving high hydrogen storage capacity (6.5 wt% in the total composite) and excellent kinetics while maintaining robustness. Furthermore, hydrogen uptake is remarkably accomplished at room temperature and also under 1 bar—as observed during in situ measurements—which is a substantial advance for a reversible metal hydride material. The realization of three complementary functional components in one material breaks new ground in metal hydrides and makes solid-state materials viable candidates for hydrogen-fueled applications.

wafer that is pre-shaped with periodic grooves. The grooves are prepared by fi rst microcontact printing a parallel line of oligomeric polydimethylsiloxane and then electrochemically etching the patterned wafer. The etch proceeds anisotropically, with the resist-covered regions etching more slowly than the resist-free regions. The grooved substrate is then etched using a sinusoidal current density waveform, generating a porosity-modulated photonic crystal (rugate fi lter) that is conformal with the grooves. This porous multilayer is then removed, resulting in a freestanding nanostructure with a corrugated topological modulation in the x‐y plane and a rugated porosity modulation in the z-direction. In addition to the resonant photonic refl ectance signature from the porosity-modulated rugate fi lter (along the z direction), the structures display optical diffraction in transmission from the x‐y plane due to the spatially modulated grooves. The silicon wafer that remains after removal of the porous multilayer still contains a rippled surface, allowing the process to be repeated without additional microcontact printing. Six generations of freestanding, three-dimensional diffraction gratings are produced from a single wafer and only one initial patterning step.

We analyze the geometric effects of two different carbon fillers on the enhancement of the thermal conductivity of carbon-epoxy composites. This study compares the thermal properties of composites containing graphite powder (2-dimensional) and carbon nanofibers (1-dimensional) incorporated in an industrial epoxy. Calculations using the generalized effective medium model were also used to examine the effect of the geometry and aspect ratio of the carbon filler. Experiments show that at a filler volume fraction loading of 0.10, the effective thermal conductivity of the composites was improved up to eightfold for carbon nanofiber and threefold for graphite in comparison to the neat epoxy. The superior performance of the carbon nanofiber composite is due to the larger aspect ratio of nanofiber which allows greater overlap between neighboring particles. However, this greater overlap also results in the composite becoming prohibitively viscous at low filler volume fractions. In graphite composite at the maximum filler volume fraction of 0.3, the resulting thermal conductivity improvement was 14-fold over the neat epoxy. Calculations indicated that the improved thermal conductivity was primarily due to the filler particle geometry. Additionally, calculations suggest the wider distribution of graphite particle aspect ratio could have a positive influence on enhancing composite thermal conductivity.

One-dimensional photonic crystals (rugate filters) constructed from porous silicon were modified by the chemical hydrosilylation of terminal alkenes (decyl, 10-carboxydecyl, and 10-hydroxydecyl) in the presence of a concentration gradient of diazonium salt initiators. The concentration gradient was generated by vertically orienting the Si wafer containing the porous Si layer in an alkene solution and then introducing the diazonium salt at the bottom edge of the wafer. Slow diffusion of the salt led to a varying density of grafted alkene across the surface of the porous layer. The modified surfaces were end-capped with methyl groups by electrochemical grafting to impart improved stability and greater hydrophobicity. The surface modified with 10-carboxydecyl species was ionized by deprotonation of the carboxy groups to increase the hydrophilicity of this porous silicon surface. The pore-wall modification gradients were characterized using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS). The more hydrophilic portion of the gradient changes color when water infiltrates the porous nanostructure because of a shift in the stop band of the photonic crystal. The more hydrophobic portion of the gradient excludes water, although mixtures of water and ethanol will infiltrate this region, depending on the concentration of ethanol in the mixture. A simple visual sensor for small quantities of ethanol in water, capable of detecting ethanol concentrations of between 0 and 8% with a resolution of 1% is demonstrated.

Thin film porous silicon layers have been constructed in which the level of chemical modification to the pore walls is altered in a controlled gradient across the material. The gradient modification within such a nanoporous material represents a significant advance over gradients imposed across a flat surface. Gradients of methyl, pentyl acetate, and decyl groups are formed via electrochemical attachment of organohalides with an asymmetric electrode arrangement. The stability and hydrophobicity of the latter two systems have been improved through postprocess “end-capping” of the porous silicon with methyl groups. Two-dimensional mapping transmission FTIR microspectrophotometry and ATR-FTIR have been employed to characterize these new materials. Cleaving the surface-attached pentyl acetate groups to 5-hydroxypentyl groups leads to materials that can act as efficient visual indicators of the ethanol concentration in water over the range 1−10 vol %.

The role of encapsulating polymers in nanocrystalline Mg air stability and hydrogen storage density was studied for a series of composites varying in both % Mg and polymer identity. In these materials, the Mg nanocrystals are completely dependent on the polymer for air stability. Remarkably, both air stability and hydrogen sorption capacity of poly(methyl methacrylate) composites were enhanced by reducing the amount of polymer. Composites consisting of 65 wt% Mg absorbed 6.95 wt% hydrogen and showed little oxidation after 3 months air exposure even after enduring the volume expansion induced by hydrogen sorption, whereas composites with 33.2 wt% Mg absorbed just 4.86 wt% hydrogen and were completely oxidized upon air exposure after hydrogen sorption. This surprising synergistic enhancement in stability and storage density is attributed to an increase in the tortuosity of the paths of gas molecules and increased interfacial structure-templating regions, which scale with % Mg loading and lead to nanoparticle entanglements, hindering polymer chain motion.

The carbon dioxide storage capacity of magnesium oxide (MgO) particles was examined as a function of particle size, shape, and surface area. Two types of MgO nanocrystals (5 nm spheres and 23 nm disks) were synthesized and compared against commercially available MgO (325 mesh/44 μm and 40 mesh/420 μm). The surface area of the four types of particles was determined by N2 gas adsorption. Carbon dioxide capture was measured at 60 °C and 600 °C using thermogravimetric analysis, with results indicating enhanced CO2 capacity correlating with increased surface area.

Metal hydrides, with their inherently high gravimetric and volumetric densities, present a compelling platform for hydrogen storage for mobile applications. However, several fundamental barriers have persistently impeded technological progress. This perspective provides an overview of the current hurdles plaguing metal hydride technology and the novel approaches recently adopted that may potentially surmount these challenges. In particular, nanocomposites, a homogenous matrix of two or more components synergistically integrated for enhanced material performance, is emerging as a new and promising class of material for hydrogen storage. This perspective highlights the potential of nanocomposites, specifically magnesium nanocomposites, for hydrogen storage. First, the existing challenges of metal hydrides are reviewed, followed by the progress achieved thus far by metal hydride size reduction to the nanoscale, and incorporation in a matrix material. Lastly, a novel nanocomposite synthesized by confining magnesium nanocrystals within a gas-selective polymer matrix is highlighted and the potential for improvement is discussed. This metal-polymer nanocomposite holds great promise as a general approach for future work on hydrogen-storage composites, as it simultaneously provides air-stability, high hydrogen storage density, and rapid hydrogenation kinetics.

Gas adsorption and capillary condensation of organic vapors are studied by optical interferometry, using anodized nanoporous alumina films with controlled geometry (cylindrical pores with diameters in the range of 10-60 nm). The optical response of the film is optimized with respect to the geometric parameters of the pores, for potential performance as a gas sensor device. The average thickness of the adsorbed film at low relative pressures is not affected by the pore size. Capillary evaporation of the liquid from the nanopores occurs at the liquid-vapor equilibrium described by the classical Kelvin equation with a hemispherical meniscus. Due to the almost complete wetting, we can quantitatively describe the condensation for isopropanol using the Cohan model with a cylindrical meniscus in the Kelvin equation. This model describes the observed hysteresis and allows us to use the adsorption branch of the isotherm to calculate the pore size distribution of the sample in good agreement with independent structural measurements. The condensation for toluene lacks reproducibility due to incomplete surface wetting. This exemplifies the relevant role of the fluid-solid (van der Waals) interactions in the hysteretic behavior of capillary condensation. (Some figures in this article are in colour only in the electronic version)

Gas adsorption and liquid desorption of a number of organic vapors in anodized nanoporous alumina, with controlled geometry (cylindrical pore diameters from 10 to 60 nm), are studied using optical interferometry. The narrow-diameter distribution of disconnected pores allows checking the validity of the (long-predicted but not experimentally verified) Kelvin equation without any adjustable parameters, modeling or other assumptions. Evaporation occurs at liquid-vapor equilibrium according to this equation, whereas condensation occurs from metastable states of the vapor phase by nucleation, enhanced by surface defects inside the nanopores. This produces hysteresis, in qualitative agreement with theoretical models and simulations that use Van der Waals interactions between the fluid and the pore surface. The reproducibility of the hysteresis depends on the strength of these interactions, which play an important role in the dynamics of capillary condensation.