Khaled Saoud

Vapor-phase synthesis of metallic and intermetallic nanoparticles and nanowires: Magnetic and catalytic properties*

In this paper, we present several examples of the vapor-phase synthesis of intermetallic and alloy nanoparticles and nanowires, and investigate their magnetic and catalytic properties. In the first example, we report the vapor-phase synthesis of intermetallic aluminide nanoparticles. Specifically, FeAl and NiAl nanoparticles were synthesized via laser vaporization controlled condensation (LVCC) from their bulk powders. The NiAl nanoparticles were found to be paramagnetic at room temperature, with a blocking temperature of approximately 15 K. The FeAl nanoparticles displayed room-temperature ferromagnetism. In the second example, we report the vapor-phase synthesis of cobalt oxide nanoparticle catalysts for low-temperature CO oxidation. The incorporation of Au and Pd nanoparticles into the cobalt oxide support leads to significantly improved catalytic activity and stability of the binary catalyst systems. Finally, we report the synthesis of nanowires of Ge, Mg, Pd, and Pt using the vapor-liquid-solid (VLS) method where the vapor-phase growth of the wire is catalyzed using a proper metal catalyst present in the liquid phase.

STM studies of 1-D noble metal growth on silicon

Our scanning tunneling microscopy (STM) studies show that noble metals (Ag, Au) form a wide variety of 1-D structures on the high-index Si(5 5 12) surface. At coverages below 0.25 monolayer (ML), both metals grow as overlayer rows with an inter-row spacing of approximately 5 nm. At higher coverages and annealing temperatures, the underlying Si reconstruction is removed, but periodic row structures persist. Au can also induce faceting to nearby planes, e.g. (7 7 15) and (2 2 5), at temperatures above 500 degrees C. For all coverages and annealing temperatures studied here (0.02-1 ML, 450-800 degrees C), the Si(5 5 12) template initiates 1-D growth of the deposited noble metals.

Shortened aerogel fabrication times using an ethanol–water azeotrope as a gelation and drying solvent

Native and cross-linked aerogel monoliths were fabricated in a few hours using a technique that does not require solvent exchange prior to supercritical drying. Native oxide alcogels were synthesized by alkoxide hydrolysis–condensation using an ethanol–water azeotrope mixture as the gelation solvent. Cross-linked alcogels were synthesized by replacing part of the gelation solvent with a monomer, followed by visible light photopolymerization for which Eosin Y was used as an initiator and a tertiary amine used as a coinitiator. After aging for 2 hours, the alcogels were removed from the molds, placed in a pressure vessel, and dried using a supercritical ethanol–water azeotrope mixture. Starting from the sol, dried aerogels could be fabricated in about 6 hours. Most importantly, since solvent exchange was not required, native oxide and polymer cross-linked aerogels could be fabricated at the same rate. A systematic study was carried out to confirm that monoliths produced with our technique had density, surface area and Young’s modulus comparable to those of aerogels produced following more conventional pathways, such as supercritical CO2 drying. We synthesized samples using base- and acid-catalyzed chemistries, varied alkoxide concentration and, for cross-linked aerogels, monomer concentration. Depending on alkoxide concentration, native oxide aerogels had densities between about 0.06 and 0.17 g cm−3 and surface areas between about 300 and 500 m2 g−1. Depending on monomer type and concentration, cross-linked monoliths had a modulus between about 10 and 400 MPa, a density between 0.25 and 0.5 g cm−3 and a surface area between 150 m2 g−1 and 350 m2 g−1. Shrinkage was about 5% for base-catalyzed synthesis, about 20% for acid-catalyzed synthesis and about 10% for cross-linked monoliths. Infrared and Raman spectroscopies, solid state NMR and thermogravimetric analysis confirmed that drying in a supercritical ethanol–water azeotrope did not significantly affect the cross-linking polymer used to produce mechanically strong aerogels.

Fabrication of native silica, cross-linked, and hybrid aerogel monoliths with customized geometries

In this work, a previously developed (White et al 2015 J. Mater. Chem. A 3 762) rapid synthesis approach is used to fabricate native and cross-linked aerogel monoliths with customized geometries. This technique does not require solvent exchange, therefore fabrication times do not depend on part size. To prove this, parts with a smallest dimension of approximately 3.6 cm were fabricated within the same time scale as that of small cylinders with a diameter of 7 mm. In addition, monoliths with customized geometries exhibiting physical detail on the order of 1 mm were produced to demonstrate the versatility of this technique. Furthermore, hybrid materials consisting of native silica aerogel integrated with selected regions of polymer cross-linking were produced. The cross-linked regions allow for adhesion to other surfaces or labeling while the majority of the material retains the physical characteristics of a native silica aerogel. The physical and thermal properties of all aerogel components were examined. All aerogel materials produced in this work exhibited characteristics that were within the range of aerogel materials produced using more conventional methods. For native silica materials, this includes densities in the range of 0.03–0.116 g cm−3, surface areas between 342–799 m2 g−1, mode pore sizes in the range of 30–39 nm, and thermal conductivities in the range of 0.020–0.026 W m−1 K−1. For cross-linked aerogel materials, densities ranged between 0.154–0.340 g cm−3, surface areas were between 291–388 m2 g−1, mode pore sizes were in the range of 29–41 nm, and thermal conductivities were in range of 0.038–0.066 W m−1 K−1.

Au-Induced Faceting of Si(5 5 12)

The growth of Au on the stable, high-index Si(5 5 12) surface has been studied using scanning tunneling microscopy (STM). At very low coverages and moderate annealing temperatures (≤0.1 ML, 400–500°C), Au appears to decorate the underlying Si rows and form an array of rows that maintains the underlying (5 5 12) periodicity of 5.4 nm. For higher annealing temperatures and coverages, however, Au causes faceting to a number of nearby planes. The two primary facets formed at lower (∼0.15 ML) and higher (0.5–2 ML) coverages are the (337) and (225) planes, which are tilted 0.7° down [towards (111)] and 1.1° up from (5 5 12), respectively. Both orientations are in fact subunits of the (5 5 12) unit cell, so their presence is not surprising. In addition to these facets, two types of sawtooth morphologies composed of planes oriented further from (5 5 12) are found at very high annealing temperatures (800–900°C). These include (113)+(7 7 15) planes at very low coverage (0.05 ml) and (113)+(5 5 11) planes at higher coverage (∼1 ML), where (113) is tilted up by 5.3° and (7 7 15) and (5 5 11) are tilted down by 2.9° and 2.2°, respectively. Au adsorption on Si(5 5 12) therefore results in the formation of five possible facet planes: (113), (225), (337), (5 5 11), and (7 7 15).

Laser induced instantaneous gelation: aerogels for 3D printing

We present the synthesis of polymer cross-linked silica alcogels in a matter of seconds by illuminating the solution of TEOS, hexanedioldiacrylate, Eosin Y and amine with a laser beam (λ = 532 nm). The heat of polymerization triggers gelation instantly. We demonstrate printing of 3D letters on different substrates using this technique. The physical properties of the 3D printed samples are comparable to conventionally prepared cross-linked silica aerogels, i.e., shrinkage (10.4%), density (0.56 g cm−3), Youngs modulus (81.3 MPa) and BET surface area (155.3 m2 g−1). An important aspect of this rapid gelation is that it allows 3D printing/lithography of reinforced silica aerogel material without using a mould.

Microwave Assisted Preparation of Calcium Hydroxide and Barium Hydroxide Nanoparticles and Their Application for Conservation of Cultural Heritage

In this paper, we present the microwave-assisted synthesis and characterization of alkaline nanomaterials for the preservation of cultural heritage. Calcium and barium hydroxide nanomaterials suspended in aqueous as well as alcoholic medium were applied on different samples to check their effectiveness. The effect of treatment was measured using pH value and alkaline reserve test. The alkaline reserve test results indicate improvement in the alkaline buffer with significant reduction in acidity of the papers and improvement in pH value of the surfaces. Ethanol was found as best solvent on the basis of suspension of nanoparticles, pH value and ease of application through spray gun. Accelerated ageing test revealed that the reinforcement effect of these nanomaterials persisted throughout the ageing. All the results converged in individuating these nanometric particles as an innovative, completely compatible, and efficient material for the consolidation of old and new paper surfaces.

An investigation of physico-chemical properties of a new polyimide–silica composites

In the present study, a novel diamine 1,4-bis[4-(hydrazonomethyl)phenoxy]butane (4-BHPB) has been successfully synthesized by a facile method. 4-BHPB was reacted with pyromellitic dianhydride (PMDA) to derive a novel polyimide (PI). Highly compatibilized PI–SiO2 nanocomposites were tailored using synthesized PI matrix and modified silica nanoparticles. The compatibility between organic–inorganic (O–I) components was remarkably improved by charge transfer complex (CTC) formed between PI chains and modified silica nanoparticles. PI chains have electron donor and acceptor groups (diamine and dianhydride portion, respectively). These groups were generated in silica nanoparticles by the organic modification through 2,6-bis(3-(triethoxysilyl)propyl)pyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone (M-SiO2), which in turn, was prepared reacting PMDA with 3-aminopropyltriethoxysilane. Sol–gel method was used for in situ synthesis of silica nanoparticles from a mixture of M-SiO2 and TEOS. The enhanced compatibility between O–I phases through CTC formation furnished PI–SiO2 nanocomposites (designated as OI-M) with improved thermal stability, hydrophobicity, and surface smoothness. For the comparison of properties, another series of PI–SiO2 composites (OI-UM) were prepared dispersing unmodified silica micro-particles into PI matrix. OI-UM hybrid system does not contain CTC between PI matrix and silica particles. The structure of the monomer, PI, and organically modified silica network was analyzed by FTIR, and NMR spectroscopy. Thermogravimetric analysis, FE-SEM, contact angle measurement, and AFM were used to study thermal and morphological properties of the synthesized PI–SiO2 hybrids.

Nanoparticles in Astrochemistry: Synthesis and Characterization of Meteorite Dust Nanoparticles

Interstellar dust particles (IDPs) constitute most of the solid matter in the universe. Large quantities of IDPs are also present in the Solar System and fall on Earth. IDPs are also of interest as they can catalyze astrochemical reactions and prebiotic synthesis, and their organic contents are believed to have contributed to the origins of life. Their chemical composition is similar to carbonaceous chondrite comets, asteroids and meteorites. The IDPs are microporous web‐like aggregates of 10–100 nm phyllosilicate particles with morphologies similar to particles produced by the Laser Vaporization Controlled Condensation (LVCC) method. IDPs are available only as microscopic samples, and simulated IDPs are needed to study their chemical and catalytic effects. To produce such simulated IDPs, we formed nanoparticles from carbonaceous chondrite meteorites by LVCC processing. The compositions, morphologies, particle size distribution, FTIR spectra, and catalytic properties of the meteorite‐based nanoparticles w...

Fabrication of strong and ultra-lightweight silica-based aerogel materials with tailored properties

Cross-linked silica aerogels are promising, strong, lightweight materials for photolithographic applications. The work presented here details the preparation of ultra-lightweight aerogel materials with tailored properties through the appropriate combination of silica and methacrylate polymer using laser-induced rapid photogelation fabrication technique. For fabrication, an ethanolic solution of hexanediol diacrylate, tetraorthosilicate, Eosin Y and a tertiary amine was prepared. The amounts of reactants were varied to prepare different compositions of aerogel monoliths. The solution was irradiated with a green beam from a low power laser source. The samples, after drying in supercritical ethanol, were characterized using FTIR, BET, SEM, TGA, and a mechanical testing instrument. FTIR data suggests that neither low nor high silica content has an effect on the reactivity of acrylate functionalities during polymer formation. SEM micrographs reveal that variation in silica or polymer content does not produce any phase-separated structures. Instead, uniformly distributed nano-sized polymer–silica structures were obtained for all compositions. Our results suggest that a variety of combinations of mechanical and other properties (such as densities, surface areas, pore sizes, and pore volumes) can be produced through appropriate combination for diverse applications. All these findings provide convincing evidence that the variation of silica and/or polymer content can be used to fabricate aerogels with a variety of properties, which have the depth needed for use in laser-based 3D printing technology of simple or complex structures with nearly any dimensions.