Nada M. Dimitrijevic

Colloidal semiconductors as photocatalysts for solar energy conversion

Considerable attention has been drawn in recent years to develop ultrafine semiconductor particles which exhibit excellent photocatalytic properties. Size quantization effects, nonlinear optical properties and enhanced photoredox properties make quantized colloidal semiconductors potentially useful in the design of photoelectrochemical systems. Photochemical and photophysical processes that directly influence the photocatalytic properties of the semiconductor colloids and the factors that limit the charge transfer at the semiconductor/electrolyte interface are discussed here. Applications of semiconductor particulate systems in the conversion and storage of solar energy are also described.

The important role of tetrahedral Ti4+ sites in the phase transformation and photocatalytic activity of TiO2 nanocomposites.

Highly photoactive, tetrahedral Ti4+ sites can be created, other than in zeolite cavities and on silica substrate, in mixed-phase TiO2 nanocomposites. The tetrahedral Ti4+ species was shown to be an intermediate formed during the thermally driven phase transformation from anatase to rutile.

Dynamics of Localized Charges in Dopamine-Modified TiO2 and their Effect on the Formation of Reactive Oxygen Species

Modification of TiO(2) nanoparticles with dopamine enables harvesting of visible light and promotes spatial separation of charges. The formation of reactive oxygen species (OH, (1)O(2), O(2)(-), HO(2), H(2)O(2)) upon illumination of TiO(2)/dopamine was studied using complementary spin-trap EPR and radical-induced fluorescence techniques. The localization of holes on dopamine suppresses oxidation of adsorbed water molecules at the surface of nanoparticles, and thus formation of OH radicals. At the same time, dopamine does not affect electronic properties of photogenerated electrons and their reaction with dissolved oxygen to produce superoxide anions. Superoxide anions are proposed to generate singlet oxygen through dismutation reaction, resulting in a low yield of (1)O(2) detected.

Primary photochemical events in CdS semiconductor colloids as probed by picosecond laser flash photolysis

Abstract A picosecond laser flash photolysis study has been carried out to elucidate the primary photochemical events of CdS semiconductor colloids dispersed in acetonitrile. Excitation of CdS colloids with a 355 nm laser pulse (pulse width 18 ps) led to the formation of S −• ( k = 5 × 10 8 s −1 ) and this was suppressed when hole scavengers such as I − and triethanolamine were added to the colloidal suspension. The reduction of viologen compound and the oxidation of diethyldithiocarbamic acid were investigated to elucidate the interfacial charge transfer processes occurring in the subnanosecond time domain; the reduction and oxidation reactions occurred within 18 and 250 ps, respectively.

Size quantization of colloidal semiconductor particles in silicate glasses

Abstract A method for incorporating quantized particles of colloidal semiconductors in transparent silicate glasses has been developed. The absorption spectra of the particles in the glasses are identical to those of the starting colloidal aqueous solutions. The luminescence intensity of colloidal semiconductors in silicate glasses is much higher than that for colloids in liquid solutions. This is attributed to passivation of surface recombination sites by the silicate polymer.

Reaction of trichloromethyl and trichloromethylperoxyl radicals with C60. A pulse radiolysis study

Abstract Both trichloromethyl ( CCl 3 ) and trichloromethylperoxyl (CCl 3 OO ) radicals react with C 60 . The bimolecular rate constants are (2.7 ± 0.5) × 10 8 mol −1 dm 3 s −1 and 2 × 10 6 mol −1 dm 3 s −1 , respectively. The transient absorption spectra of the radical adducts are presented.

Titanium Dioxide Nanoparticles in Advanced Imaging and Nanotherapeutics

Semiconductor photocatalysis using nanoparticulate TiO(2) has proven to be a promising technology for use in catalytic reactions, in the cleanup of water contaminated with hazardous industrial by-products, and in nanocrystalline solar cells as a photoactive material. Metal oxide semiconductor colloids are of considerable interest because of their photocatalytic properties. The coordination sphere of the surface metal atoms is incomplete and thus traps light-induced charges, but also exhibits high affinity for oxygen-containing ligands and gives the opportunity for chemical modification. We use enediol linkers, such as dopamine and its analogs, to bridge the semiconductors to biomolecules such as DNA or proteins. Nanobio hybrids that combine the physical robustness and chemical reactivity of nanoscale metal oxides with the molecular recognition and selectivity of biomolecules were developed. Control of chemical processes within living cells was achieved using TiO(2) nanocomposites in order to develop new tools for advanced nanotherapeutics. Here, we describe general experimental approaches for synthesis and characterization of high crystallinity, water soluble 5 nm TiO(2) particles and their nanobio composites, methods of cellular sample preparation for advanced Synchrotron-based imaging of nanoparticles in single cell X-ray fluorescence, and a detailed experimental setup for application of the high-performance TiO(2)-based nanobio photocatalyst for targeted lysis of cancerous or other disordered cells.

Picosecond Charge Transfer Processes in Ultrasmall CdS and CdSe Semiconductor Particles

The kinetic and mechanistic details of the chemical events associated with charge trapping and the interfacial charge transfer processes in colloidal CdS and CdSe systems are elucidated with picosecond transient absorption spectroscopy

Photoinitiated charge separation in a hybrid titanium dioxide metalloporphyrin peptide material

In natural systems, electron flow is mediated by proteins that spatially organize donor and acceptor molecules with great precision. Achieving this guided, directional flow of information is a desirable feature in photovoltaic media. Here, we design self-assembled peptide materials that organize multiple electronic components capable of performing photoinduced charge separation. Two peptides, c16-AHL3K3-CO2H and c16-AHL3K9-CO2H, self-assemble into fibres and provide a scaffold capable of binding a metalloporphyrin via histidine axial ligation and mineralize titanium dioxide (TiO2) on the lysine-rich surface of the resulting fibrous structures. Electron paramagnetic resonance studies of this self-assembled material under continuous light excitation demonstrate charge separation induced by excitation of the metalloporphyrin and mediated by the peptide assembly structure. This approach to dye-sensitized semiconducting materials offers a means to spatially control the dye molecule with respect to the semiconducting material through careful, strategic peptide design.

Charge-transfer reactions of C60 in surfactant-based complex fluid media

Abstract Surfactant-based complex fluid media enables the investigation of charge-transfer reactions between a hydrophobic C 60 molecule and hydrophilic solutes. The formation of a ground state charge-transfer complex between C 60 and ethylamine was demonstrated in an ionic surfactant/water/oil microemulsion, the value of the complexation of K = 1.1 M −1 was determined. The one-electron reduction of fullerene occurs in reaction with the excess electrons stored on nanometer-sized metal(Ag) or quantized semiconductor (TiO 2 ) particles in microemulsions.