Geniece L. Hallett-Tapley

Photochemical Norrish type I reaction as a tool for metal nanoparticle synthesis: importance of proton coupled electron transfer

The Norrish type I photocleavage is an excellent source of strongly reducing free radicals that can be used to convert soluble metal ions into their atomic state that proceed to form nanoparticles. Proton coupled electron transfer (PCeT) is a useful tool to interpret the mechanism for metal ion reduction, a process that in these systems involves multisite PCeT, with proton and electron having separate receiving substrates.

Tuning plasmon transitions and their applications in organic photochemistry

The ketone-photoinduced formation of Au, Ag, and Cu nanoparticles from their corresponding ions in solution has been carried out using benzoin photoinitiators. Ketones are good photosensitizers for nanoparticle synthesis not because of the energy they can absorb or deliver, but rather because of the reducing free radicals they can generate. Efficient photochemical nanoparticle generation thus requires a careful selection of substrates and experimental conditions such that free radical generation occurs with high quantum efficiency, where metal ion precursors do not inhibit radical formation. A key consideration to achieve nanoparticle synthesis with short exposure times is to minimize excited-state quenching by metal ions. Applications of nanostructures in catalysis require control of the nanoparticle characteristics, such as dimension, morphology, and surface properties. Part of this article describes the strategies to modify photochemically prepared particles. Finally, we illustrate some of the nanoparticle applications that interest us, with some emphasis on plasmon-mediated processes.

Synthesis, acid properties and catalysis by niobium oxide nanostructured materials

Several forms of niobium oxide were prepared, including nanostructured mesoporous materials, and their acidity properties were comprehensively investigated and compared with commercially available materials. The composites were characterized by a variety of techniques, including XRD, TEM, N2 adsorption and Hammett acid indicator studies. The acidity of the niobium oxide derivatives was also investigated by the ability of the materials to successfully promote the halochromic ring-opening of an oxazine-coumarin probe that was specifically designed for use in fluorescence imaging studies. The ring-opening reaction was easily monitored using UV-visible, fluorescence and NMR spectroscopy. Single molecule microscopy was employed to gain a more in-depth understanding of the niobium oxide acid catalysis pathway. Using this technique, the rate of niobium oxide mediated protonation was estimated to be 1.8 × 10−13 mol m−2 s−1. Single molecule analysis was also used to obtain a detailed map of Bronsted acid sites on the niobium oxide surface. The active sites, located by multiple blinking events, do not seem to be localized on any area of the material, but rather randomly distributed throughout the solid state surface. As the reaction proceeds, the sites with the highest acidity and accessibility are gradually consumed, making the next tier of acid sites available for reaction. The phenomenon was more closely characterized by using time lapsed reactivity maps.

Photochemical synthesis and characterization of novel samarium oxide nanoparticles: toward a heterogeneous Brønsted acid catalyst

Samarium oxide nanoparticles (Sm2O3NP) were prepared photochemically for the first time. Characterization shows spherical, polydisperse Sm2O3NP stabilized by 4-HEBA, a substituted benzoic acid. The Sm2O3NP also possess Bronsted acidity. This new material may prove to be a potent heterogeneous acid catalyst.

Mechanistic insights into the Nb2O5 and niobium phosphate catalyzed in situ condensation of a fluorescent halochromic assembly

Solid niobium oxides (Nb2O5·nH2O) and niobium phosphate were used as heterogeneous acid catalysts to promote the condensation between a switchable oxazine and a fluorescent coumarin in an aprotic solvent. The catalysts were found to promote the generation of an active methylene from the enamine-based portion of the oxazine, which was followed by a nucleophilic attack on the aldehyde functionality of the coumarin reagent. In the resulting system, the emission of the conjugated fluorophore can be observed at 670 nm and, thus, the processes occurring at the catalyst surface can be monitored in real time by total internal reflection fluorescence microscopy (TIRFM).

Electron transfer from the benzophenone triplet excited state directs the photochemical synthesis of gold nanoparticles

The rarely recognized electron donating ability of the benzophenone triplet excited state provides an unusual route for the photochemical synthesis of gold nanoparticles.

Single component photoacid/photobase generators: potential applications in double patterning photolithography

193 nm light as an excitation source for resist patterning is limited due to the inability to achieve pitch division much below limits of λ/2. Current techniques are examining the use of a variety of photochemical manipulations as a means to extend lithographic patterning to small, more defined images. Double patterning, or dual tone lithography, has recently garnered considerable interest due to the potential of patterning two features within one excitation exposure. In this contribution, single component carbamate photoacid/photobase (PAG/PBG) generators are studied as potential substrates for implementing dual tone lithography. At lower exposure powers, only the acid is generated and complete film dissolution was observed, while at higher 193 nm laser powers, photobase activation resulted in little to no film dissolution. Ideally, at intermediate laser doses, both the photoacid and photobase are activated giving rise to the desired double patterning. The energy required to initiate dual tone characteristics was found to be easily adjusted using the additional of amine quenchers or via manipulation of the PAG/PBG concentration. Film thickness measurements were used to determine the energies required for both photoacid and photobase activation, while laser flash photolysis and NMR spectroscopy studies were used in an attempt to understand the PAG/PBG activation mechanism.

Size-controlled photochemical synthesis of niobium nanoparticles

The size of photochemically-prepared niobium nanoparticles (NbNP) can be controlled by varying the concentration of the photoinitiator in the reaction mixture. The particles, which may be metallic in nature, are readily oxidized upon air exposure to form stable niobium(v) oxide nanoparticles (NbONP) that act as strong Brønsted acids.

Exploiting the photocatalytic activity of gold nanoparticle-functionalized niobium oxide perovskites in nitroarene reductions

Novel gold nanoparticle-doped niobium perovskites are synthesized using a chemical reduction technique, affording supported nanoparticles on the order of 8–10 nm. The UVA-initiated photocatalytic activity of the nanocomposites was studied using nitroarene reduction as a probe reaction which yielded the corresponding aminoarenes in as little as 3 hours with yields up to 92%. Analyses of kinetic data, using Hammett parameters and computational methods, show that electron-withdrawing groups accelerate the photocatalytic process and suggest that electron transfer from the gold nanoparticle surface to the nitro group is an integral part of the rate-limiting step of this reaction. The linearity of the Hammett plot shows that no change in the photocatalytic mechanism can be expected upon variation of the para-aryl substitution. Investigation of the reaction mechanism using CH3OH-d4 illustrates that CH3OH is the most likely source of protons, as both electrons and protons are required for successful photoreduction. Moderate recyclability of the heterogeneous nanomaterial over three catalytic cycles is observed.

AuNP@TiO2 Catalyzed Peroxidation of Ethyl- and n-Propylbenzene: Exploring the Interaction Between Radical Species and the Nanoparticle Surface

Ethyl- and n- propylbenzene peroxidation has been studied in the presence of supported gold nanoparticles on TiO2(AuNP@TiO2). The decomposition products obtained from Fenton-induced cumene hydroperoxide decomposition, as well as the oxidation products of ethyl- and n- propylbenzene, implicate the participation of reactive oxygen species adsorbed on the AuNP surface. This surface intermediate is believed to be a fundamental participant in hydrogen abstraction reactions required to facilitate the formation of the observed carbonyl derived product.