Browsing by Subject "Nanocrystals--Synthesis"

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Aspects of colloidal nanocrystals: patterning, catalysis and doping
(2005) Stowell, Cynthia Ann; Korgel, Brian Allan
Colloidal nanocrystals have many advantages over those synthesized by other means due to the flexibility not only in synthesis conditions but in post-synthesis assembly. Three aspects of colloidal nanocrystals that demonstrate this versatility were studied: the self-assembled patterning of nanocrystals into arrays through the use of fluid dynamics, the catalytic properties of nanocrystals as a function of ligand type and reaction cycle, and the doping of III-V semiconductor nanocrystals with magnetic atoms during colloidal synthesis. While much work has been done on the thermodynamically-driven formation of monodisperse or bi-modal nanocrystal superlattices, another option exists for nanocrystal self-assembly: formations driven by fluid dynamics. Hexagonal networks of gold nanocrystals were observed after drop casting gold nanocrystals in chloroform on different substrates. The honeycomb-shaped structures were calculated to be created by surface tension driven (Marangoni) convection. The honeycomb networks have a lattice parameter of 4.3 µm and their formation is highly dependant not only particle size and size distribution but concentration of particles within solution. A new synthesis for iridium nanocrystals was developed. The iridium particles could be synthesized with any of five stabilizing molecules. Taking advantage of this fact, the effect of capping ligands on the catalysis of 1-decene hydrogenation was studied. Ligands that stabilized the iridium well prevented hydrogenation while “weak” capping ligands allowed the iridium nanocrystals to reach turnover rates as high as 270 s-1. Recycling the catalytic particles also affected the activity, as the turnover frequency increased with each cycle until the particle began to agglomerate and fall out of solution. MnxIn1-xAs and MnxIn1-xP nanocrystals ranging from 2 to 10 nm in diameter were synthesized with up to xMn=0.025 for InMnAs and xMn=0.11 for InMnP. Surface exchange and magnetic measurements confirmed that much of the dopant resides in the nanocrystal core and modifies the magnetic properties of the host material through antiferromagnetic superexchange interactions. The effective Bohr magnetons of Mn in the synthesized InMnAs nanocrystals ranged from 2.2 to 3.7 µ B Mn atom, and from 3.4 to 5.1 µ B Mn atom for InMnP, values below the theoretical value of 5.9 µ B Mn atom. This result is attributed to antisite defects and interstitial doping.
Design of novel catalysts by infusion of presynthesized nanocrystals into mesoporous supports
(2008-08) Gupta, Gaurav, Ph. D.; Johnston, Keith P., 1955-
Traditionally, supported metal catalysts have been synthesized by reduction of precursors directly over the support. In these techniques, it is challenging to control the metal cluster size, composition and crystal structure. Herein, we have developed a novel approach to design catalysts with controlled morphologies by infusing presynthesized nanocrystals into the supports. High surface area mesoporous materials, including graphitic carbons, have been utilized for obtaining a high degree of metal dispersion to enhance catalyst stabilities and activities. Gold and iridium nanocrystals have been infused in mesoporous silica with loadings up to 2 wt % using supercritical CO₂ as an antisolvent in toluene to enhance the van der Waals interactions between nanocrystals and the silica. The iridium catalysts show high catalytic activity and do not require high temperature annealing for ligand removal, as ligands bind weakly to the iridium surface. To further enhance metal loadings to >10 % in the catalysts, short-ranged interactions between the metal nanocrystals and the support are further strengthened with weakly binding ligands to expose more of the metal surface to the support. For pre-synthesized FePt nanocrystals, coated with oleic acid and oleylamine ligands, high loadings >10 wt % in mesoporous silica are achieved, without using CO₂. The strong metal-support interactions favor FePt adsorption on the support and also enhance stability against sintering at high temperatures. High resistance to sintering favors formation of the FePt intermetallic crystal structure with <4 nm size upon thermal annealing at 700 °C. The fundamental understanding of the metal-support interactions gained from these studies is then utilized in the design of highly stable Pt and Pt-Cu electrocatalysts with controlled size, composition and alloy structure supported on graphitized mesoporous carbons for oxygen reduction. The resistance of the graphitic carbons to oxidation coupled with strong metal-support interactions mitigate nanoparticle isolation from the support, nanoparticle coalescence, Pt dissolution and subsequent Ostwald ripening and thus enhance catalyst stability. The control of the Pt nanocrystal morphology with high concentrations of highly active (111) surface leads to 25% higher activities than commercial Pt catalysts. Furthermore, the catalyst activities obtained for Pt-Cu catalysts are 4-fold higher than Pt catalysts due to strained Pt shell generated from electrochemical dealloying of copper from the nanoparticle surface.
Nanocrystal stabilization, synthesis and assembly using supercritical fluids
(2003) Shah, Parag Suresh; Johnston, Keith P., 1955-; Korgel, Brian Allan
Supercritical and compressed solvents provide a unique medium for nanocrystal synthesis and assembly as their tunable solvation strength and favorable wetting characteristics have the potential to overcome current processing limitations. Here we examine nanocrystal dispersibility, separation, synthesis and organization with compressed solvents. Gold and silver nanocrystals were dispersed in carbon dioxide and ethane by using the appropriate capping ligands. Larger nanocrystals, which exhibit stronger core attractions, required better solvent conditions (higher densities) than smaller nanocrystals in order to form a dispersion. Lowering the solvent density precipitated the largest nanocrystals demonstrating density tunable colloidal separations in supercritical fluids. Silver, iridium and platinum nanocrystals were synthesized in supercritical CO2 by reducing a miscible organometallic precursor. By reducing the precursor in the presence of a thiol, particle growth was quenched and the nanocrystals could be collected, cleaned and redispersed in compatible solvents. Tuning solvent density and ligand type allowed the nanocrystal growth mechanism to be controlled from a mix of coagulation and condensation at conditions of strong steric stabilization, leading to small monodisperse particles, to coagulation at poor stabilization conditions, leading to large polydisperse particles. Superlattice formation was examined by assembling gold nanocrystals from liquid carbon dioxide. The resulting structures varied from disorganized liquids at fast evaporation rates to hexatic states with highly ordered regions at slower evaporation rates. Comparison with a computer simulated reference state showed that the crystallization kinetics were slower than diffusion limited, likely due to ensemble rearrangement during the late stages of assembly. Finally, gold and indium manganese arsenide nanocrystal dispersions were drop-cast from volatile solvents under humid conditions to form macroporous nanocrystal thin films. The porous structures were templated by condensed water droplets as a result of solvent evaporation. Prevention of droplet coalescence by interfacially active nanocrystals, which adsorbed onto the surface of the droplets, led to the formation of highly ordered pore structures.