Chemical equilibria and nanocrystal synthesis in high temperature supercritical solutions
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In supercritical water (SCW), reaction equilibrium constants may differ by several orders of magnitude from room temperature values due to changes in the solvent dielectric constant. While these changes have been well characterized for ionic acid-base reactions, non-ionic polar reaction equilibria in SCW have received little attention. Here, we have shown for several oxidation states of NOx species that polarity differences from reactants to products result in significant changes in equilibrium constants that scale with density. In addition, linear extrapolation of these equilibrium constants to zero density results in values which are very close to independent gas phase values. Nitrate reaction equilibria provide the framework for the synthesis of copper nanocrystals in SCW. Aqueous copper nitrate (Cu(NO3)2) solutions when taken to supercritical conditions, hydrolyze to form large copper (II) oxide particles. Because of the low dielectric environment, SCW is a suitable solvent to employ organic capping ligands to control and stabilize the synthesis of nanocrystals. The presence of 1-hexanethiol results in reduction of copper (II) and produces copper nanocrystals approx. 7 nm in diameter. A proposed mechanism for sterically stabilized nanocrystal growth in SCW describes competing pathways of hydrolysis to large oxidized copper particles versus ligand exchange and arrested growth by thiols to produce small monodisperse Cu nanoparticles. A new synthetic method was developed to produce organic-monolayer passivated silicon and germanium nanocrystals in solvents under supercritical conditions. By thermally degrading organosilane or organogermane precursors at high temperatures and pressures, sterically-stabilized nanocrystals could be obtained using octanol as a capping ligand. During the reaction, octanol binds to the nanocrystal surface through an alkoxide linkage to provide steric stabilization through the hydrocarbon chain. The absorbance and luminescence spectra of the nanocrystals exhibit significant size-dependent blue shifts in optical properties from bulk luminescence due to quantum confinement effects. High luminescence quantum yields are observed. The smallest nanocrystals exhibit discrete optical transitions, characteristic of quantum confinement effects for crystalline nanocrystals with a narrow size distribution.