Dan Meisel

Nanoparticles in advanced oxidation processes

Photocatalysis using semiconductor nanoparticles as an advanced oxidation technique (AOT) has been a focus of research by a number of groups during the last two decades. The photocatalytic approach has been adopted successfully to develop self-cleaning glasses and air purification systems. Enhancing the photoconversion efficiency, maximizing the rate of degradation, and extending the photoresponse of the semiconductor catalyst into the visible range still pose a major challenge. Recent developments that address these issues and new approaches that expand the scope of semiconductor and metal nanoparticles in AOTs and related applications are presented in this overview.

Surface-enhanced Raman scattering of colloid-stabilizer systems

Abstract Surface-enhanced Raman scattering (SERS) front silver and gold colloids stabilized by poly(vinyl alcohol) PVA, poly(ethylene glycol), carbowax, and poly(vinyl pyridine), PVP, has been observed. From the bands observed in the SERS of the PVA stabilized sols it is concluded that the PVA adsorbs on the metal through unhydrolyzed acetate groups. Carbowax adsorbs on the metal through the COC groups. PVP adsorbs through the pyridine π system which leads to a flat lying ring conformation on the metal.

Probing Silver Nanoparticles During Catalytic H2 Evolution

Employing silver nanoparticles from a recently developed synthesis [Evanoff, D. D.; Chumanov, G. J. Phys. Chem. B 2004, 108, 13948] and a well-studied probe molecule, p-aminothiophenol, we follow changes at the surface of the particles during the conditioning and eventually the catalytic production of hydrogen from water using strongly reducing radicals. Injection of electrons into the particles causes pronounced variations in the intensity of the surface enhanced Raman scattering (SERS) spectrum of the probe molecule. These spectral changes are caused by changes in the Fermi-level energy that are in turn caused by changes in the silver ion concentrations or in the pH, or by changes in electron density in the particle. This correlation highlights the effect of the chemical potential on the SERS enhancement at the end of the particles synthesis. The intensity of the SERS spectra increases in the presence of the silver ions when excitation at 514 nm is utilized. When the Ag(+) ions in the colloidal suspension are completely reduced by the radicals and the particles operate in the catalytic mode, the SERS spectrum is too weak to record, but it can reversibly be recovered upon the addition of Ag(+). The effect of pH on the SERS intensity is similar in nature to that of the silver ions but is complicated by the pKa of the aminothiol and the point of zero charge (pzc) of the particles. It is hypothesized that as the particles cross the pzc (around neutral pH) the electrostatic interaction between the protonated amine headgroup of the probe and the positively charged surface increases the density of probe molecules in the perpendicular orientation at the expense of a competing species. This conversion results in enhanced SERS signals and is observable during the preconditioning stage of the particles. Indeed, adsorption isotherms of the probe indicate the presence of two species. In analogous previous observations these two species have been attributed to perpendicular and flat adsorption orientations of the deprotonated probe molecule relative to the particle surface. However, preliminary density functional calculations on relevant prototypes raise the possibility that the two species may be the probe molecule and a cationic form produced by charge transfer in the ground state from the chemisorbed probe to the metal. These two forms of the probe have differing electronic structures and vibrational frequencies, with perhaps differing orientations relative to the surface. Whichever is the correct interpretation, a neutral molecule in a flat orientation or a radical cation, this species is easier to replace than the other in competitive adsorption by ethanethiol.

Mechanism of the catalytic hydrogen production by gold sols. Hydrogen/deuterium isotope effect studies

The H/D isotope effect on the catalytic production of hydrogen from water using either Cr/sup 2 +/ or (CH/sub 3/)/sub 2/COH radical as reductants was studied. The catalysts used were gold sols stabilized by either citrate or poly(vinyl sulfate). Similar isotope effects (in the range of 5 to 6.7) were measured for the various reductants and stabilizers. Comparison of this isotope effect to the one measured for the production of hydrogen on gold cathodes leads to the conclusion that similar mechanisms operate in all three systems. It was also observed that hydrogen abstraction from the relatively stable supports, when adsorbed on the colloid, occurs. This leads to the conclusion the hydrogen atoms adsorbed on the particles can either proceed to produced hydrogen molecules by the catalytic reaction or react via other competing pathways.

PHOTOPHYSICAL STUDIES OF PYRENE INCORPORATED IN NAFION MEMBRANES

Abstract— High concentrations of organic chromophores, in particular pyrene, were found to incorporate into the cluster network of water swollen Nation. The vibrational structure of the fluorescent molecule and its fluorescence lifetime were used to probe the location of pyrene in this structure. Strong heavy‐atom induced room temperature phosphorescence was observed from the chromophore in the water swollen membrane. Excimer formation in tm‐butanol swollen Nation was observed and was utilized to estimate the micro viscosity in this system.

Size control and properties of thiol capped CdS particles

Abstract The growth of CdS particles was radiolytically initiated by the release of HS − ions from a thiol, via its reaction with hydrated electrons, in Cd 2+ containing solutions. The size of particles thus obtained could be controlled by the concentration ratio of the solution components Cd 2+ /H 2 S/ RSH. The higher is the concentration of Cd 2+ and RSH at a given H 2 S concentration the smaller are the particles obtained. A mechanism to rationalize this size control is suggested to involve competition between complexation of thiolate ions with Cd 2+ ions at the surface of a core CdS particle and growth of the particles, primarily by ion-cluster addition. The formation of surface thiol-Cd 2+ complexes was indicated by conductivity measurements and by energy dispersive X-ray analysis of the particles. The particles thus obtained were found to be strongly fluorescent at sizes larger than ca 16A. This emission results from recombination of trapped charge carriers. Generation of electrons and holes, by flash photolysis, and injection of electrons into the particles leads to blue shift in the absorption spectra of the particles. This shift of the exciton band to higher energies is shown to result from screening effects on the exciton binding energy rather than from band filling effects.

Radiation-induced dissolution of colloidal manganese oxides☆

Abstract The reductive dissolution of several well-characterized suspensions of manganese oxides (α-, δ-, γ-MnO 2 , and Mn 5 O 8 ) under steady γ-irradiation at pH 3 has been studied. Radiolytically produced reducing radicals (e.g., (CH 3 ) 2 COH, CH 3 CHOH, HO 2 , and viologen radicals) can be quantitatively captured by the oxide suspension to release Mn 2+ ions into the solution. The radiolytically produced H 2 O 2 similarly leads to quantitative dissolution of the oxide. The rate-determining step in the dissolution process is slower than the diffusion-controlled limit. The stoichiometry of the dissolution process under complete scavenging of the radicals is determined by the oxidation state of the oxide ( x in MnO x ).

dc onductivity signals observed following pulsed photoionization of solutions of anthracene in dielectric liquids

Abstract dc conductivity in several hydrocarbon liquids containing anthraccne (2 × 10 −6 M) and higher concentrations of efficient electron scavengers is measured following an 18 ns pulse of 248 nm light. With cyclohexane as the solvent, an initial conductivity signal, the origin of which is uncertain, decays over ≈1μs and is initially more than an order of magnitude greater than the much longer lived free-ion signal. Strong effects of the solvent on this initial signal are observed.

Time-resolved EPR studies of the benzophenone—diphenyl kethyl radical system. Possible evidence for quartet—doublet intersystem crossing☆

Abstract The photoreduction of benzophenone by several substrates has been studied by time-resolved EPR. When cyclohexadiene, MTHF, and decane are employed as hydrogen donors to the excited triplet state of benzophenone the diphenyl ketyl radical which is produced exhibits emissive electron spin polarization with rise times on the order of a few microseconds. It is suggested that the experimental observations can be explained by intersystem crossing to a quartet state in the excited ketyl radical.

Electron transfer and dimerization of viologen radicals on colloidal TiO2

The reaction of several viologen radical cations with colloidal TiO2 particles of 70 A radius has been studied in detail. The electron-transfer reaction from methyl viologen radicals (MV+) to the TiO2 particles is controlled by the rate of the heterogeneous electron-transfer step. Protonation of the reduced particle follows the electron-transfer reaction in a temporally well separated reaction. Analysis of the subsequent equilibrium stage, but before protonation occurs, allows an estimate of the charge carrier density in the colloid. The effect of added Pt, either as a separate colloid or by photodeposition on the TiO2 particle, has also been studied. Kinetic analysis indicates that a mixture of Pt and TiO2 colloids results in adsorption of the Pt particle on the TiO2 colloid. However, the adsorbed Pt colloid reacts independently of the TiO2 particle.Heptyl viologen radicals (HV+) catalytically dimerize in the presence of TiO2 particles in a heterogeneous reaction between an absorbed HV+ and a dissolved radical. The competition between dimerization and hydrogen evolution can be directed towards the latter reaction either by reducing the pH or by loading the TiO2 particles with Pt. The unsymmetric C14MV+ viologen radicals aggregate even in the absence of any colloid. At low pH values and high Pt loadings they can, however, be directed towards hydrogen evolution.