M. Samy El-Shall

Microwave synthesis of graphene sheets supporting metal nanocrystals in aqueous and organic media

We have developed a facile and scalable chemical reduction method assisted by microwave irradiation for the synthesis of chemically converted graphene sheets and metal nanoparticles dispersed on the graphene sheets. The method allows rapid chemical reduction of exfoliated graphite oxide (GO) using a variety of reducing agents in either aqueous or organic media. It also allows the simultaneous reduction of GO and a variety of metal salts thus resulting in the dispersion of metallic and bimetallic nanoparticles supported on the large surface area of the thermally stable 2D graphene sheets.

Metallic and bimetallic nanocatalysts incorporated into highly porous coordination polymer MIL-101

This paper reports the development of a facile, general and effective approach, based on microwave irradiation (MWI), for the incorporation of a variety of metallic and bimetallic nanoparticle catalysts within the highly porous coordination polymer MIL-101. The current approach is based on the simultaneous activation of the pores of MIL-101 and the rapid chemical reduction of metal precursors using MWI in the presence of a reducing agent. Small Pd, Cu and Pd–Cu nanoparticles of 2–3 nm are incorporated within the pores and larger particles of 4–6 nm are supported on the surface of the MIL-101 crystals. TEM images reveal that the loading of the particles using MWI is uniform across the MIL crystals. The observed catalytic activities toward CO oxidation of the Pd nanocatalysts supported on the highly porous MIL-101 polymer are significantly higher than any other reported metal clusters supported on metal–organic frameworks. The observed high activity is attributed to the small metal nanoparticles imbedded within the pores of the MIL crystals. The activity of the small embedded particles is higher than those supported on the surface. This allows the use of small metal loadings for efficient low temperature CO oxidation. These results should allow optimization of a new class of nanocatalysts incorporated within the highly porous MIL-101. These materials are promising environmentally relevant catalyst systems.

Ultrasmall Gold Nanoparticles Anchored to Graphene and Enhanced Photothermal Effects by Laser Irradiation of Gold Nanostructures in Graphene Oxide Solutions

In this work we demonstrate the coupling of the photothermal effects of gold nanostructures of controlled size and shape with graphene oxide nanosheets dispersed in water. The enhanced photothermal effects can be tuned by controlling the shape and size of the gold nanostructures, which result in a remarkable increase in the heating efficiency of the laser-induced size reduction of gold nanostructures. The Raman spectra of the Au-graphene nanosheets provide direct evidence for the presence of more structural defects in the graphene lattice induced by laser irradiation of graphene oxide nanosheets in the presence of Au nanostructures. The large surface areas of the laser-reduced graphene oxide nanosheets with multiple defect sites and vacancies provide efficient nucleation sites for the ultrasmall gold nanoparticles with diameters of 2-4 nm to be anchored to the graphene surface. This defect filling mechanism decreases the mobility of the ultrasmall gold nanoparticles and, thus, stabilizes the particles against the Ostwald ripening process, which leads to a broad size distribution of the laser-size-reduced gold nanoparticles. The Au nanostructures/graphene oxide solutions and the ultrasmall gold-graphene nanocomposites are proposed as promising materials for photothermal therapy and for the efficient conversion of solar energy into usable heat for a variety of thermal, thermochemical, and thermomechanical applications.

Highly efficient electron field emission from graphene oxide sheets supported by nickel nanotip arrays.

Electron field emission is a quantum tunneling phenomenon whereby electrons are emitted from a solid surface due to a strong electric field. Graphene and its derivatives are expected to be efficient field emitters due to their unique geometry and electrical properties. So far, electron field emission has only been achieved from the edges of graphene and graphene oxide sheets. We have supported graphene oxide sheets on nickel nanotip arrays to produce a high density of sharp protrusions within the sheets and then applied electric fields perpendicular to the sheets. Highly efficient and stable field emission with low turn-on fields was observed for these graphene oxide sheets, because the protrusions appear to locally enhance the electric field and dramatically increase field emission. Our simple and robust approach provides prospects for the development of practical electron sources and advanced devices based on graphene and graphene oxide field emitters.

Growth mechanism of anisotropic gold nanocrystals via microwave synthesis: formation of dioleamide by gold nanocatalysis.

A facile and fast one-pot microwave irradiation method has been developed to prepare different shapes of gold nanoparticles capped with a mixture of oleylamine and oleic acid. The size, shape, and morphology of the nanocrystals could be tailored by varying the ratio of oleylamine to oleic acid, the microwave time, and the concentration of the gold ions. These effects are directly reflected in the surface plasmon resonance properties of the resulting nanocrystals in the visible and near-infrared regions. Pure amine leads to the formation of only spherical particles. Introducing oleic acid increases the growth rate and enhances the formation of anisotropic shapes. Experimental evidence and new insights on the reaction mechanism confirm the formation of dioleamide from the reaction of oleic acid and oleylamine catalyzed by the gold nanocrystals. In the absence of gold nanoparticles, the conventional synthesis of dioleamide requires 12 h of reaction time at 120 degrees C. New insights on the reaction mechanism indicate that excess oleic acid enhances the formation of hexagons and more anisotropic shapes of the gold nanocrystals.

P-Type Nitrogen-Doped ZnO Nanostructures with Controlled Shape and Doping Level by Facile Microwave Synthesis

We report herein the development of a facile microwave irradiation (MWI) method for the synthesis of high-quality N-doped ZnO nanostructures with controlled morphology and doping level. We present two different approaches for the MWI-assisted synthesis of N-doped ZnO nanostructures. In the first approach, N-doping of Zn-poor ZnO prepared using zinc peroxide (ZnO2) as a precursor is carried out under MWI in the presence of urea as a nitrogen source and oleylamine (OAm) as a capping agent for the shape control of the resulting N-doped ZnO nanostructures. Our approach utilizes the MWI process for the decomposition of ZnO2, where the rapid transfer of energy directly to ZnO2 can cause an instantaneous internal temperature rise and, thus, the activation energy for the ZnO2 decomposition is essentially decreased as compared to the decomposition under conductive heating. In the second synthesis method, a one-step synthesis of N-doped ZnO nanostructures is achieved by the rapid decomposition of zinc acetate in a mixture of urea and OAm under MWI. We demonstrate, for the first time, that MWI decomposition of zinc acetate in a mixture of OAm and urea results in the formation of N-doped nanostructures with controlled shape and N-doping level. We report a direct correlation between the intensity of the Raman scattering bands in N-doped ZnO and the concentration of urea used in the synthesis. Electrochemical measurements demonstrate the successful synthesis of stable p-type N-doped ZnO nanostructures using the one-step MWI synthesis and, therefore, allow us to investigate, for the first time, the relationship between the doping level and morphology of the ZnO nanostructures. The results provide strong evidence for the control of the electrical behavior and the nanostructured shapes of ZnO nanoparticles using the facile MWI synthesis method developed in this work.

Enhanced photocatalytic activity of ZnO–graphene nanocomposites prepared by microwave synthesis

This work reports a simple one-step synthesis of ZnO nanopyramids supported on reduced graphene oxide (RGO) nanosheets using microwave irradiation (MWI) of zinc acetate and GO in the presence of a mixture of oleic acid and oleylamine. The rapid decomposition of zinc acetate by MWI in the presence of the mixture of oleic acid and oleylamine results in the formation of hexagonal ZnO nanopyramids. GO has a high affinity for absorbing MWI, which results in a high local heating effect around the GO nanosheets and facilitates the reduction of GO by the oleylamine. The RGO nanosheets act as heterogeneous surface sites for the nucleation and growth of the ZnO nanopyramids. Using ligand exchange, the ZnO–RGO nanocomposites can be dispersed in an aqueous medium, thus allowing their use as photocatalysts for the degradation of the malachite green dye in water. The ZnO–RGO nanocomposites show enhanced photocatalytic activity for the degradation of the dye over the unsupported ZnO nanopyramids. The enhanced activity is attributed to efficient charge transfer of the photogenerated electrons in the conduction band of ZnO to graphene. This enhances the oxidative pathway of the holes generated in the valence band of ZnO which can effectively lead to the degradation and mineralization of the malachite green. The ZnO nanopyramids supported on RGO could have improved performance in other photocatalytic reactions and also in solar energy conversion.

Synthesis and characterization of nanoscale zinc oxide particles: I. laser vaporization / condensation technique

Abstract A method which combines laser vaporization of metal targets with controlled condensation in a diffusion cloud chamber is used to synthesize nanoscale metal oxide and metal carbide particles (10 – 20 nm). In this work we present the results for the synthesis and characterization of ZnO as an example of metal oxide nanoparticles. In part II (this issue), we present the results for silicon carbide nanoparticles synthesized using the same method described here.

Synergetic catalysis of palladium nanoparticles encaged within amine-functionalized UiO-66 in the hydrodeoxygenation of vanillin in water

Ultrasmall palladium nanoparticles (1.5–2.5 nm) encapsulated in metal–organic frameworks (MOFs) have been prepared by introducing a palladium precursor into a highly porous and hydrothermally stable amine-functionalized UiO-66 (NH2-UiO-66) via a direct anionic exchange and subsequent H2 reduction. The prepared Pd@NH2-UiO-66 catalyst was then applied in the hydrodeoxygenation of vanillin (a typical model compound of lignin) at a low H2 pressure in aqueous media. Excellent catalytic results (100% conversion of vanillin with exclusive selectivity for 2-methoxy-4-methylphenol) could be achieved over the developed 2.0 wt% Pd@NH2-UiO-66 catalyst under mild conditions. Furthermore, the catalytic activity and selectivity were not affected after six reaction cycles indicating excellent stability and reproducibility of this catalyst system. It was found that the presence of free amine groups in the framework of NH2-UiO-66 plays a key role in the formation of uniform, well-dispersed and leaching resistant palladium nanoparticles within the MOF host. Moreover, the developed Pd@NH2-UiO-66 exhibits a novel synergetic catalysis in the hydrodeoxygenation due to the cooperation between the well-dispersed metallic Pd sites and the amine-functionalized MOF support, in which Pd offers hydrogenation activity and the MOF support facilitates hydrogenolysis of the intermediate vanillin alcohol to the 2-methoxy-4-methylphenol product.

Synthesis and characterization of photochromic molybdenum and tungsten oxide nanoparticles

Abstract Weblike aggregates of coalesced MoO3 and WO3 nanoparticles have been prepared by a laser vaporization-controlled condensation technique. The nanoparticles show a very strong photochromic effect: the color change is one order of magnitude faster than thai observed for the corresponding bulk material. The photochromic effect is also observed upon exposure of the MoO3 nanoparticles to normal room light.