RESUMO
Whether in organic synthesis or solar energy conversion, light can be a powerful reagent in chemical reactions and introduce new opportunities for synthetic control including duration, intensity, interval, and energy of irradiation. Here, we report the use of a molecular photosensitizer as a reducing agent in metallic nanoparticle syntheses. Using this approach, we report three key findings. (1) Nanoparticles produced by photocatalytic reduction form via a continuous nucleation mechanism, as opposed to burst and burst-like nucleation processes typically observed in metal nanoparticle syntheses. (2) Because nucleation is continuous, as long as the solution is irradiated (and there remains excess reagents in solution), nanoparticle nucleation can be turned on and off by controlling the timing and duration of irradiation, with no observable particle growth. (3) This synthetic method extends to the formation of bimetallic nanoparticles, which we show also form via a continuous nucleation pathway, and follow predicted patterns of metal incorporation as a function of the magnitude of the difference between the reduction potentials of the two metals. Taken together, these results establish a versatile synthetic method for the formation of multimetallic nanoparticles using visible light.
RESUMO
Online analysis of ultrafine (<100 nm diameter) particles was performed by sending the aerosol through a condensation growth chamber (CGC) to create micrometer-size aqueous droplets that were subsequently analyzed by mass spectrometry with droplet-assisted ionization (DAI). Three experiments are reported which illustrate key performance characteristics of the method and give insight into the ion formation process: size-selected cortisone particles, size-selected secondary organic aerosol (SOA) particles, and freshly nucleated SOA under atmospherically relevant conditions. In each case, SOA was produced by α-pinene ozonolysis. For size-selected cortisone particles between 30 and 90 nm diameter and SOA particles between 30 and 70 nm, the ion signal intensity was found to be approximately independent of particle size. This observation is attributed to the formation of aqueous droplets in the CGC whose size distribution is independent of the original particle size. A consequence of this behavior is that the sensitivity of molecular detection increases as the particle size decreases, and the method is particularly well suited for new particle formation studies under atmospherically relevant conditions. This aspect of the CGC-DAI method was illustrated by the online analysis of freshly nucleated SOA samples with median diameters, number concentrations, and mass concentrations on the order of 25 nm, 104 cm-3, 0.2 µg m-3, respectively. Mass spectra of freshly nucleated SOA could be explained by condensation of highly oxidized molecules (HOMs) that subsequently reacted in the particle phase. Size-selected SOA showed increasing oligomerization with increasing particle size, which is consistent with established particle growth mechanisms.
