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Abstract
Nanogold is widely used in many areas of physics and chemistry due to its environment-sensitive plasmon resonance absorption. The immobilization of gold nanoparticles in highly porous silica aerogel offers an attractive alternative to liquid gold solutions as they show a mechanically stable structure, are permeable to gases, and can even be used at elevated temperatures. We have found that the commercially available citrate-stabilized 10 nm gold nanoparticles may suffer from aggregation prior to or under the base-catalyzed gelation process of tetramethoxy silane. In the wet gels, Au particles increased in size, changed shape, and demonstrated the loss of plasmon resonance absorption, due to the formation of larger aggregates. We have studied a range of water-miscible organic solvents, stabilizing agents, and the gelation conditions to minimize changes from occurring in the aerogel setting and the supercritical drying process. It has been found that atmospheric carbon dioxide has a significant effect on aggregation, and it cannot be entirely excluded under normal synthetic conditions. Methanol resulted in an increase in the particle size only, while dimethyl sulfoxide, dimethylformamide, and urea changed the shape of nanoparticles to rod-like shapes, and diols led to an increase in both size and shape. However, using the polymeric stabilizer poly(vinyl pyrrolidone) efficiently prevented the aggregation of the particles, even in the presence of high concentrations of carbon dioxide, and allowed the production of nanoAu containing silica aerogels in a single step, without the modification of technology.
Highlights
When dispersed down to the few nanometer sizes, gold is not a “noble” metal anymore
Gold nanoparticles are most commonly synthesized from tetrachloroauric acid HAuCl4 under a reductive environment that is provided by reducing agents like citric acid or NaBH4 [9,10,11,12]
10 nm Au were used as guest a change of the originalofred wasred observed in several experiments nanoparticles were particles, used as guest particles, a change thecolor original color was observed in several either during the hydrolysis or in the aging period, and the result was a grey-colored aerogel at the experiments either during the hydrolysis or in the aging period, and the result was a grey-colored end of the process (Figure aerogel at the end of the process (Figure 1)
Summary
Nanogold particles (AuNPs) were extensively studied in the last two decades in several fields of science including physics, chemistry, biology, and medicine, leading to a sort of science “gold rush” [1] Due to their high atomic number, electric properties, small size, and functionalizable surface, several practical applications have been found for the gold nanoclusters. Gels 2018, 4, x FOR PEER REVIEW recently, liquid phase laser ablation is used to provide gold nanoparticles in a chloride-free environment [13]. Act asGold heterogeneous-phase carbon-carbon formation, coupling, isomerization nanoparticles that catalysts a high number syntheticroles chemical reactionsimaging like hydrogenation, selective oxidations, are largerinthan. They can be made virtually material that can be gelled gels arefrequently prepared from The they most used They and can known aerogels are silica aerogels,that synthesized i.e.,[30,31,32].
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