Hydrogen Storage in Carbon-Based Nanostructured Materials

Carbon nanostructures represent a revolution in science and hold the potential for a large range of applications because of their interesting electrical, mechanical, and optical properties. Multiwall carbon nanotubes and carbon nanofibers of herringbone formation were grown by chemical vapor deposition on different catalysts from a number of hydrocarbon sources. After the total or particle removal of the catalyst system, the carbon nanostructures were analyzed for hydrogen uptake. Six samples of nanofibers grown on a Pd-based catalyst system (with a surface area of 425–455 m²/g) were controlled oxidized in air, such that they had different ratios of Pd/C varying from 0.05 to 0.9 mole ratio. The hydrogen uptake experiments were performed volumetrically in a Sievert-type installation and showed that the quantity of desorbed hydrogen (for pressure intervals ranging from 1 to 100 bars) by the carbon nanostructures free of any metal catalyst particles was between 0.04 and 0.33% by weight. For the samples of nanofibers that contained Pd in various Pd/C ratios, palladium revealed catalytic properties and supplied atomic hydrogen at the Pd/C interface by dissociating the H₂ molecules. The results show a direct correlation between the Pd/C ratio and the quantity of hydrogen absorbed by these samples. A saturation value of about 1.5 wt.% was reached for a high ratio of about 1:1 of Pd/C. The multiwall carbon nanotubes grown on a Fe:Co:CaCO₃ catalytic system and purified by acid cleaning and air oxidation showed a hydrogen uptake value of 0.1 to 0.2 wt.%.     

Synthesis of CdS-Au2S-Au hybrid dendritic nanostructures

The hybrid CdS-Au₂S-Au dendritic nanocrystals were synthesized in toluene solution at 70 °C. UV-vis and photoluminescence (PL) spectra recorded the optical properties of hybrid nanostructures, which showed an obvious blue shift relative to the absorption peak of CdS dendritic nanocrystals. The initial CdS dendritic nanocrystals exhibited band gap and trap state emission, both of which were quenched by Au parts. Analysis of the hybrid nanostructures by XRD shows the presence of appreciable amounts of Au₂S, indicating that the chemical process involving cation exchanges between Au⁺ ions and Cd²⁺ ions was found.     

Formation of novel assembled silver nanostructures from polyglycol solution

This paper described a simple and mild chemical reduction approach to prepare novel silver nanostructures with different morphologies. Dendritic silver nanostructure was obtained by a fast reduction reaction using hydrazine as a reducing agent in aqueous solution of polyglycol, while both the zigzag and linear Ag nanostructures were slowly assembled using polyglycol as a reducing agent. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and field emission scanning electron microscopy (FE-SEM) were used to characterize the obtained silver nanostructures. Fourier transform infrared absorption (FT-IR) spectra were recorded to show that there exists a certain coordination of the oxygen atoms in the polyglycol with Ag⁺ ions in aqueous solution of the AgNO₃/polyglycol. Furthermore, the examination of the morphologies of the products obtained at different stages of the reaction of Ag⁺ ions with polyglycol revealed that such a coordination is of utmost importance for the formation of the silver nanostructures, namely polyglycol provided lots of active sites for the coordination, nucleation, growth and serves as backbones for directing the assembly of the metal particles formed. The formation mechanism of the dendritic silver nanostructure was called a coordination–reduction–nucleation–growth–fractal growth process. The strong surface plasmon absorption bands at 470 nm for the zigzag silver and at 405 nm for the dendritic silver were found.     

Controlled synthesis of concave tetrahedral palladium nanocrystals by reducing Pd(acac)2 with carbon monoxide

CO reducing strategy to control the morphologies of palladium nanocrystals was investigated. By using CO as a reducing agent, uniform and well-defined concave tetrahedral Pd nanocrystals with a mean size of about 55 ± 2 nm were readily synthesized with Pd(acac)₂ as a precursor and PVP as a stabilizer. The structures of the as-prepared Pd nanocrystals were characterized by transmission electron microscopy (TEM), X-ray powder diffraction (XRD), ultraviolet–visible (UV–vis) absorption spectroscopy and electrochemical measurements. The results demonstrated that CO was the most essential for the formation of the concave tetrahedral Pd nanostructures. The morphologies and sizes of the final products can be well controlled by adjusting the flow rate of CO. The most appropriate CO flow rate, temperature and time for the formation of the ideal concave tetrahedral Pd nanocrystals was 0.033 mL s^?1, 100 °C and 3 h, respectively.     

Facile preparation of TiO2 nanostructures by direct annealing of the Ti foil

TiO₂ nanostructures have been fabricated by direct annealing of the Ti foil with Pd catalyst. The Pd catalysts play an important role in reducing sintering temperature for the synthesis of TiO₂ nanostructures. The morphologies can be varied between the nanowires (NWs) and nanobelts (NBs) by adjusting the amount of Pd and annealing temperature. It shows that the vapor liquid solid and vapor solid may be the most suitable mechanism for the TiO₂ nanostructure growth. Experimental results suggest that this method is an effective, simple and inexpensive route for the preparation of TiO₂ nanostructures with high quality and high yield.     

Fabrication of dumbbell-like CdTe/Au nanohybrids

Dumbbell-like CdTe/Au nanohybrids were synthesized by assembly of CdTe quantum dots with the assistance of AuCl₄⁻ in aqueous solution. The products were characterized by TEM and SEM techniques. The images reveal that dumbbell-like nanostructures with uniform size were well formed. The dumbbell-like nanostructures were further characterized by HRTEM and EDX spectrum. The results indicate that the as-prepared dumbbell-like nanostructures were composed of CdTe quantum dots and Au nanoparticles. The effect of HAuCl₄ concentration on the morphology of the products was also investigated, which shows that the morphology of the products evolved from sheaf-like nanostructures to rod-like nanostructures and finally dumbbell-like nanostructures as the HAuCl₄ concentration increased. Based on the above results, a possible mechanism for the formation of dumbbell-like CdTe/Au nanohybrids is proposed.     

Controlled microstructure and production of TiO2 nanostructures by Ar and Ar–H2 atmosphere

Titanium dioxide nanostructures have been synthesised by annealing the Pd loaded titanium foil at 680°C under the atmosphere of Ar or Ar–H₂ gas mixture. The microstructure and production of titanium dioxide are found to depend critically on the concentration of Pd catalyst and the Ar–H₂ atmosphere. The catalyst of Pd and the introduction of 5%H₂ gas are the key factor for the formation of the long (about 100?μm in the average) and smooth titanium dioxide nanowires at low temperature. Scanning electron microscope and X-ray diffraction show that they are all rutile phase. The further result shows that the titanium dioxide nanowires possess good crystallinity and high surface photovoltage response.     

Synthesis, morphology and optical properties of multi-pods Au/FeO(OH) and Au/Fe2O3 nanostructures

Multi-pods Au/FeOOH nanostructures were synthesized by a hydrothermal treatment of an aqueous solution of mixed micellar formed by gold nanoparticles, hexadecyltrimethyl ammonium bromide (CTAB), and (NH₄)₃[FeF₆] at 160 °C for 48 h and sequential calcined at 290 °C for 1.5 h, resulting in the formation of multi-pods Au/Fe₂O₃ nanostructures. The as-obtained products were characterized by powder X-ray diffraction, transmission electron microscopy, selected area electron diffraction, field emission scanning electron microscopy, and UV-vis spectroscopy. Surface plasmon resonance band of gold nanoparticles was observed in the multi-pods Au/FeOOH nanostructures. However, a similar behavior was not seen with multi-pods Au/Fe₂O₃ nanostructures. The critical role of F⁻ ions and CTAB molecules in the formation of FeO(OH) multipods and the probable mechanism of the formation of multi-pods Au/FeOOH and Au/Fe₂O₃ nanostructures were discussed.     

Palladium infiltration in high surface area microporous silica nanoparticles

Metallic Pd clusters were embedded into a host matrix of microporous SiO₂ nanoparticles via a solution reduction of Pd(NO₃)₂ by hydrazine hydrate. The infiltration of 33 wt.% Pd leads to a 13% porosity loss of SiO₂ nanoparticles, which demonstrated an initial surface area of 748 m²/g. The presence of Pd in the pores was demonstrated by EDS spectroscopy and by X-ray diffraction. The metallic guest species presumably reside in the accessible micropores with an estimated size about 1.3 nm. Hydrogen uptake was measured for Pd-infiltrated SiO₂ nanoparticles. A possible mechanism for the formation of composite nanoparticles is proposed based on electrostatic interaction between Pd²⁺ and SiO₂ nanoparticles.     

Nucleation and growth mechanisms for Pd-Pt bimetallic nanodendrites and their electrocatalytic properties

In a seed-mediated synthesis, nanocrystal growth is often described by assuming the absence of homogeneous nucleation in the solution. Here we provide new insights into the nucleation and growth mechanisms underlying the formation of bimetallic nanodendrites that are characterized by a dense array of Pt branches anchored to a Pd nanocrystal core. These nanostructures can be easily prepared by a one-step, seeded growth method that involves the reduction of K₂PtCl₄ by L-ascorbic acid in the presence of 9-nm truncated octahedral Pd seeds in an aqueous solution. Transmission electron microscopy (TEM) and high-resolution TEM analyses revealed that both homogeneous and heterogeneous nucleation of Pt occurred at the very early stages of the synthesis and the Pt branches grew through oriented attachment of small Pt particles that had been formed via homogeneous nucleation. These new findings contradict the generally accepted mechanism for seeded growth that only involves heterogeneous nucleation and simple growth via atomic addition. We have also investigated the electrocatalytic properties of the Pd-Pt nanodendrites for the oxygen reduction and formic acid oxidation reactions by conducting a comparative study with foam-like Pt nanostructures prepared in the absence of Pd seeds under otherwise identical conditions.     

Hydrothermal synthesis of LaPO4:Ce,Tb@LaPO4 core/shell nanostructures with enhanced thermal stability

LaPO₄:Ce³⁺,Tb³⁺ (LAP) nanorods were prepared by hydrothermal method, and LaPO₄:Ce³⁺,Tb³⁺@LaPO₄ core/shell nanostructures were formed by hydrothermal growth of LaPO₄ nanocoating onto the LAP nanorods. Oxidation behavior of the LAP nanorods and core/shell nanostructures was systematically presented by investigating change of their photoluminescence intensities with different durations of exposure to air at different temperatures. The results revealed that the LAP nanorods had severe loss of photoluminescence due to the oxidation of Ce³⁺ and Tb³⁺ to their respective tetravalent ions. In contrast, the photoluminescence properties of the core/shell nanostructures are more stable than those of the LAP nanorods with regard to thermal stress under aerobic conditions due to the surface protection from the LaPO₄ nanocoating. Therefore, formation of core/shell nanostructure may be a alternative route for photoluminescence stable LAP nanophosphors.     

Graphene-induced Pd nanodendrites: A high performance hybrid nanoelectrocatalyst

A facile and green approach has been developed for the in situ synthesis of hybrid nanomaterials based on dendrite-shaped Pd nanostructures supported on graphene （RG）. The as-synthesized hybrid nanomaterials （RG-PdnDs） have been thoroughly characterized by high resolution transmission electron microscopy, X-ray photoelectron spectroscop）~ atomic force microscop）~ Raman spectroscopy and electrochemical techniques. The mechanism of formation of such dendrite- shaped Pd nanostructures on the graphene support has been elucidated using transmission electron microscopy （TEM） measurements. The RG induces the formation of, and plays a decisive role in shaping, the dendrite morphology of Pd nanostructures on its surface. Cyclic voltammetry and chronoamperometry techniques have been employed to evaluate the electrochemical performance of RG-PdnDs towards oxidation of methanol. The electrochemical （EC） activities of RG-PdnDs are compared with graphene-supported spherical-shaped Pd nanostructures, Pd nanodendrites alone and a commercial available Pd/C counterpart. The combined effect of the graphene support and the dendrite morphology of RG-PdnDs triggers the high electrocatalytic activity and results in robust tolerance to CO poisoning.     

Small Adsorbate-Assisted Shape Control of Pd and Pt Nanocrystals

The shape control of noble metal nanocrystals is crucial to their optical properties and catalysis applications. In this Progress Report, the recent progress of shape-controlled synthesis of Pd and Pt nanostructures assisted by small adsorbates is summarized. The use of small strong adsorbates (e.g., I⁻, CO, amines) makes it possible to fabricate Pd and Pt nanostructures with not only well-defined surface structure but also morphologies that have not been achieved by other synthetic strategies. The roles of small adsorbates in shape control of Pd and Pt nanocrystals are discussed in the Report. Also presented in the Report are unique optical and catalytic properties of several Pd and Pt nanostructures (e.g., ultrathin Pd nanosheets, concave Pt octapod, concave Pd tetrahedra), as well as their bioapplications, to demonstrate the power of using small strong adsorbates in the shape control of Pt and Pd nanostructures.     

Impact of reaction temperature,stirring and cosolvent on the solvothermal synthesis of anatase TiO2 and TiO2/titanate hybrid nanostructures: Elucidating the growth mechanism

One-dimensional anatase TiO₂ and hybrid TiO₂/titanate nanostructures are synthesized by a simple low temperature solvothermal route followed by the Na⁺/H⁺ ion-exchange and final calcination process. We investigated the impact of reaction temperature, stirring conditions and cosolvent on the morphologies of the as-prepared nanostructures. Nanotubes and nanorods are formed in alkaline solution, while nanorods/nanowires and nanoporous nanoribbons are formed in alkaline water–ethanol and alkaline water–ethylene glycol mixed solvents, respectively. X-ray diffraction, Raman scattering and high-resolution transmission electron microscopy studies are employed to identify the structure and phase composition. The formation of different morphologies of the as-synthesized nanostructures is investigated by field emission scanning electron microscopy and transmission electron microscopy. The growth mechanism and reaction process of the as-prepared nanostructures are explained based on the experimental observations. The photoluminescence, optical absorption and the tuning of band gap of the prepared samples are also studied. This work will be valuable for understanding the growth mechanism of various nanostructured TiO₂ and to explore the commercial applications of nanoporous nanoribbons of TiO₂.     

Fabrication of Pd–DNA and Pd–CNT hybrid nanostructures for hydrogen sensors

This review reports fabrication methods for ordered metallic nanostructures such as nanowires and nanoparticles based on deoxyribonucleic acid (DNA) templates. The phosphate groups in DNA are negatively charged; consequently, the DNA conformation may mineralize metals, e.g., palladium (Pd) at a relatively high metal concentration. We successfully form unique spherically shaped moss-like hybrid Pd nanoparticles using the small compacted globular state of DNA by controlling the reductive reaction. Pd can absorb hydrogen to become PdH_x, and hydrogen storage increases the electrical resistance and volume of Pd materials. Hence, the use of this material is attracting growing interest as a reliable, cheap, ultracompact, and safe hydrogen sensor. Pd–DNA hybrid nanoparticles can be used as highly sensitive hydrogen sensors, which exhibit a switch response that depends on the volume expansion in a cyclic atmosphere exchange. This paper also shows the fabrications of Pd–carbon nanotube (CNT) hybrid nanostructures.     

Facile fabrication of 3D dendritic gold nanostructures with an AuSn alloy by square wave potential pulse

Multifunctional three dimensional (3D) dendritic gold (DG) nanostructures have been synthesized facilely by square wave potential pulse (SWPP) with an Au₄₅Sn₅₅ (wt.%) alloy in an electrolyte of NaOH or H₂SO₄. The dendrite formation involves combined actions of selective removal of Sn, oxidation-reduction cycles (ORCs) of gold and intensive hydrogen evolution, wherein the nanopores from dealloying and the movement of hydrogen bubbles facilitate the dendrite formation by rearranging the newly produced Au atoms from the ORCs. An alkaline medium is preferable and the dendritic structure can be obtained as quickly as in 100 s. Some applications have been demonstrated with the prepared DG including electrocatalytic oxidation of glucose, surface-enhanced Raman scattering, and superhydrophobicity.     

Dendritic and epitaxial growths of ZnO particle-rod and rod-rod nanostructures with quasi-one-dimensional and two-dimensional configurations

Quasi-one-dimensional and two-dimensional ZnO nanostructures have been fabricated through thermal evaporation approach. The microstructures of the ZnO nanostructures have been studied using scanning electron microscopy and high-resolution electron microscopy. Quasi-one-dimensional ZnO nanostructures are formed by dendritic growths of ZnO nanoparticles from the stem nanorods surfaces, forming particle-rod nanostructures. While epitaxial growths of branch nanorods from the stem nanorods configure two-dimensional ZnO nanostructures. The epitaxial growth orientation relationship can be described as [2? 110]_R1 || [2? 110]_R2 and (0001) _R1 || (011?0)_R2. The growth mechanism of the quasi-one-dimensional and two-dimensional ZnO nanostructures has been discussed.     

Influence of palladium on gas-sensing performance of magnesium ferrite nanoparticles

Commercial ferrites with high densities are mostly used in the electromagnetic devices, which require high temperature synthesis. In this article the gas-sensing characteristics of pure and Pd-doped MgFe₂O₄ powder has been discussed. The synthesis has been carried out by using a simple molten salt method. This method facilitates rapid synthesis at comparatively lower temperature enabling formation of nanostructures, suitable for the gas-sensing application. Various physicochemical techniques have been used for the characterization of samples. X-ray diffraction analysis confirmed the single-phase formation of pure and Pd-doped MgFe₂O₄ having crystallite size 15–20 nm. Pure MgFe₂O₄ showed highest responses towards liquid petroleum gas (LPG) at 350 °C while, on doping with Pd the highest response shifted towards lower operating temperature of ～200 °C. Pure MgFe₂O₄ exhibited some response towards 200 ppm of LPG which markedly increased on doping of palladium (Pd). The probable mechanism is proposed to explain the selective response towards LPG.     

Electrospinning preparation of LaOBr:Tb3+ nanostructures and their photoluminescence properties

Tb³⁺-doped LaOBr nanostructures including nanofibers, nanobelts, and hollow nanofibers were synthesized for the first time via calcinating the electrospun polyvinyl pyrrolidone/[La(NO₃)₃ + Tb(NO₃)₃ + NH₄Br] composites. X-ray diffraction analysis revealed that LaOBr:Tb³⁺ nanostructures are tetragonal in structure with space group of P4/nmm. The morphologies and sizes of LaOBr:Tb³⁺ nanostructures were investigated using scanning electron microscope and transmission electron microscope. Under the excitation of 254-nm ultraviolet light, LaOBr:Tb³⁺ nanostructures exhibit the green emissions of predominant peak at 543 nm, which is ascribed to ⁵D₄ → ⁷F₅ transition of Tb³⁺ ions. It is found that the optimum doping concentration of Tb³⁺ ions in the LaOBr:Tb³⁺ nanofibers is 3 %. Interestingly, we found that the luminescence intensity of hollow nanofibers is obviously greater than that of nanofibers and nanobelts for LaOBr:Tb³⁺ under the same measuring conditions. Moreover, the luminescence of LaOBr:Tb³⁺ nanostructures are located in the green region in Commission Internationale de L’Eclairage chromaticity coordinates diagram. The formation mechanisms of LaOBr:Tb³⁺ nanofibers, nanobelts, and hollow nanofibers were also proposed. LaOBr:Tb³⁺ nanostructures are promising nanomaterials for applications in the fields of light display systems and optoelectronic devices.     

CO and NH3 sensor properties and paramagnetic centers of nanocrystalline SnO2 modified by Pd and Ru

Nanocrystalline gas sensitive materials based on tin dioxide modified by Pd or Ru were synthesized via wet chemical route. The interaction of modified materials with CO and ammonia was studied by in situ DC-conductivity measurements and ex situ EPR spectroscopy. Modification by Pd yields the material highly sensitive to CO in low temperature region, while Ru-modified SnO₂ is very sensitive to NH₃ at raised temperature. We have detected that O₂⁻ and OH radicals are the main spin centers in unmodified nanocrystalline tin dioxide. The modification of tin dioxide by Pd and Ru is accompanied by formation of new spin centers in the samples: Pd^+ 3 and Ru^+ 3. The concentration of these paramagnetic species on the materials interacting with CO and ammonia gases decreased because of their transition to the diamagnetic state Pd^+ 2, Pd⁰ and Ru^+ 4, respectively.