No abstract

The formation of CO2 in the gas phase and on a polyaromatic hydrocarbon surface (coronene) via three possible pathways is investigated with density functional theory. Calculations show that the coronene surface catalyses the formation of CO2 on model grain surfaces. The addition of 3O to CO is activated by 2530 K in the gas phase. This barrier is lowered by 253 K for the Eley–Rideal mechanism and 952 K for the hot‐atom mechanism on the surface of coronene. Alternative pathways for the formation of CO2 are the addition of 3O to the HCO radical, followed by dissociation of the HCO2 intermediate. The O + HCO addition is barrierless in the gas phase and on the surface and is more than sufficiently exothermic to subsequently cleave the H–C bond. The third mechanism, OH + CO addition followed by H removal from the energized HOCO intermediate, has a gas‐phase exit barrier that is 1160 K lower than the entrance barrier. On the coronene surface, however, both barriers are almost equal. Because the HOCO intermediate can also be stabilized by energy dissipation to the surface, it is anticipated that for the surface reaction the adsorbed HOCO could be a long‐lived intermediate. In this case, the stabilized HOCO intermediate could react, in a barrierless manner, with a hydrogen atom to form H2+ CO2, HCO2H, or H2O + CO.

The interactions of charge stabilised gold nanoparticles with cationic and anionic dyes are reported. The nanoparticles were synthesised by the Turkevich citrate reduction method. It was found that when a solution of thiazine dye is titrated against gold citrate hydrosol, at a critical concentration of dye there is an enhanced maximum absorption in the dye. The extinction coefficient is increased up to ten-fold. This enhancement was observed for a number of cationic thiazine dyes, of which methylene blue and toluidine blue are established light-activated antimicrobial agents. The same enhancement was not observed for anionic, acidic dyes such as rose bengal which showed no communication with the gold nanoparticles and showed UV-visible titration experiments with well formed isosbestic points. By studying the interaction of the dye and gold nanoparticles under conditions of different ionic strength and by using a zetasizer and TEM to measure the gold nanoparticle size it was demonstrated that the cause of enhancement was not due to nanoparticle aggregation. It is proposed that thiazine cationic dyes coordinate around a gold nanoparticle and give significantly enhanced UV-visible absorptions.

Gold nanoparticle (Au NP) solutions were synthesised by the Turkevich reduction method and stored in either the light or dark. All solutions were monitored daily using UV-visible absorption spectroscopy and displayed surface plasmon resonance (SPR), typical of Au nanoparticle colloids. An increase in SPR intensity, a narrowing of the SPR peak as well as a gradual shift towards lower wavenumbers over time indicated a decrease in average nanoparticle diameter and a more mono-dispersed particle size. After two weeks no further changes were observable by UV-visible absorption spectroscopy. A series of high resolution transmission electron micrographs (HRTEM) taken over the evolution period confirmed that the plasmon resonance shifts correlated to a decrease in nanoparticle size. A systematic size decrease in nanoparticle size was also observed for solutions even after centrifugation to remove the excess un-reacted citrate and auric acid. This indicated that the size evolution was independent of further excess reactant chemistries and charge stabilities. The gold nanoparticle evolution followed an inverse Ostwald type growth, by which the size of the NPs decreases, in effect a digestive ripening. The aging process provides a reliable route to fairly mono-dispersed gold nanoparticles of ca. 11.5-12.5 nm in size via the Turkevich method.

A new straightforward method for the synthesis of gold nanoparticles from addition of cyclohexanone to aqueous solutions of auric acid at room temperature is presented. By understanding this process we have discovered a new organic chemistry transformation reaction for converting cyclic ketones to a-chloro ketones and a mechanism for the nanoparticle formation. Contrary to conventional gold nanoparticle syntheses, the reaction "self-initiates" at room temperature and forms an increasingly red solution over z60 minutes. By studying the gold colloid's formation using transmission electron microscopy it was observed that large dendritic (63 AE 21 nm diameter) structures made of clustered particles (6 AE 1 nm) were initially formed. These dendritic particles then compacted into an array of denser shapes that slowly increase in size until the reaction is complete. The most prominent shapes observed were spheres (43 AE 7 nm); other shapes included dodecahedra (39 AE 10 nm) triangular (z50 nm in height) and hexagonal (z70 nm wide) nanoplates. The solution was stable to precipitation for over 3 months. During this period the nanoplate structures substantially increased in size (triangular z 250 nm, hexagonal z 320 nm) whereas other structures showed no further growth. X-ray diffraction studies demonstrated that the gold nanoparticles were crystalline. The formation of the 2-chlorocyclohexanone by-product was observed in solution phase 1 H & 13 C NMR, gas phase chromatography and IR spectroscopy. A mechanism is presented to account for this by-product and the reduction of auric acid to gold.