A number of synthetic methodologies and applications of carbon quantum dots (CQDs) have been reported since they were first discovered nearly two decades ago. Unlike metal‐based or ...semiconductor‐based (e. g., metal chalcogenides) quantum dots (MSQDs), CQDs have the unique feature of being prepared through a variety of synthetic protocols, which are typically understood from considerations of reaction models and photoluminescence mechanisms. Consequently, this brief review article describes quantum dots, in general, and CQDs, in particular, from various viewpoints: (i) their definition, (ii) their photophysical properties, and (iii) the superiority of CQDs over MSQDs. Where possible, comparisons are made between CQDs and MSQDs. First, however, the review begins with a general brief description of quantum dots (QDs) as nanomaterials (sizes≤10 nm), followed by a short description of MSQDs and CQDs. Described subsequently are the various top‐down and bottom‐up approaches to synthesize CQDs followed by their distinctive photophysical properties (emission spectra; quantum yields, Φs).
Carbon quantum dots (CQDs) synthesized in four different solvents or mixed solvents with 1,3,6‐trinitropyrene as the carbon precursor: blue‐emitting (DMF/H2O), green‐emitting (EtOH/CH3CO2H), yellow‐emitting (EtOH), and red‐emitting (DMF). Photograph shows the photoluminescent CQDs dispersed in toluene under UV light, and normalized emission spectra with bands at 460, 517, 581, and 620 nm, respectively.
•The generating mechanism of hot spot was examined in the microwave's electric and magnetic fields.•The formation of hot spots was simulated by an electromagnetic field analysis and subsequently ...evidenced experimentally.•No formation of byproduct occurred in the organic synthesis under all microwave irradiation conditions.
Hot spots are generated when carrying out the heterogeneous Suzuki–Miyaura cross coupling reaction for the synthesis of 4-methylbiphenyl in toluene solvent in the presence of Pd/AC catalyst (AC: activated carbon; see for example parts I–IV11For part I, see Ref. 8; for part II, see Ref. 9; for part III, see Ref. 2; and for part IV, see Ref. 10.). Controlling these hot spots could render the microwave-assisted catalyzed reaction more effective. Accordingly, the present article examines the mechanism by which the hot spots are generated through particle aggregation observed by means of a high-speed camera; the influence of particle size was also examined. Moreover, the formation of hot spots within the spatial gap between two AC particles was simulated by an electromagnetic field analysis and subsequently evidenced experimentally. The heterogeneous Suzuki–Miyaura coupling reaction for the synthesis of 4-methylbiphenyl in toluene solvent in the presence of activated carbon (AC; no Pd) under microwave irradiation has been re-visited to ascertain what the effect of the reagents might be as to whether or not hot spots are formed. The presence of the reagents used in the synthesis of 4-methylbiphenyl did cause a firm connectivity between the activated carbon particles, which changed with the directions of the electric field and the magnetic field. The relationship between the generation of by-products and the formation of hot spots has also been considered in the synthesis of 4-methylbiphenyl in toluene solvent catalyzed by Pd/AC.
This study used controlled microwaves to elucidate the response of adhesive components to microwaves and examined the advantages of microwave radiation in curing epoxy adhesives. Curing of adhesives ...with microwaves proceeded very rapidly, even though each component of the adhesive was not efficiently heated by the microwaves. The reason the adhesive cured rapidly is that microwave heating was enhanced by the electrically charged (ionic) intermediates produced by the curing reaction. In contrast, the cured adhesive displayed lower microwave absorption and lower heating efficiency, suggesting that the cured adhesive stopped heating even if it continued to be exposed to microwaves. This is a definite advantage in the curing of adhesives with microwaves, as, for example, adhesives dropped onto polystyrene could be cured using microwave heating without degrading the polystyrene base substrate.
Organic reactions driven by microwaves have been subjected for several years to some enigmatic phenomenon referred to as the microwave effect, an effect often mentioned in microwave chemistry but ...seldom understood. We identify this microwave effect as an electromagnetic wave effect that influences many chemical reactions. In this article, we demonstrate its existence using three different types of microwave generators with dissimilar oscillation characteristics. We show that this effect is operative in photocatalyzed TiO
reactions; it negatively influences electro-conductive catalyzed reactions, and yet has but a negligible effect on organic syntheses. The relationship between this electromagnetic wave effect and chemical reactions is elucidated from such energetic considerations as the photon energy and the reactions' activation energies.
The origin of the specific effect of microwaves on chemical reactions (the microwave effect) was investigated by examining the effect of microwaves on small groups of molecules such as clusters. The ...origin of the effect was verified by introducing 2.45 GHz microwaves into a system equipped with a supersonic molecular jet and a special microwave feedthrough to record the fluorescence excitation spectrum of molecules. The carrier gas was bubbled through water and introduced into a phenol-filled sample holder to generate phenol-water clusters. Subsequently, it was confirmed that exposure of the phenol-water clusters contained in the molecular jet ejected from the pulse valve to microwave radiation increased the fluorescence derived from the phenol monomer. This is considered to occur because the phenol-water clusters in the molecular jet absorb microwaves and collapse, thereby increasing the abundance of phenol monomers. This result suggests that microwaves affect not only bulk systems but also small groups of molecules, and that local selective heating, which is one of the causes of the microwave effect, may occur.
The focus of this article rests on our discovery that a water-soluble polymer could be cross-linked to form a gel using a novel Green Chemistry gelation method: the microwave-induced in-liquid-plasma ...(MILP) method that requires neither a cross-linking agent nor an initiator as are required in the conventional chemical method. For instance, the water-soluble polyvinyl pyrrolidone (PVP) polymer was gelled by MILP plasma irradiation within a few minutes without using toxic cross-linking agents and initiators. As well, the hydrophobic dimethylpolysiloxane macromolecule was dispersed in aqueous media to a colloidal sol, which could then also be easily gelled under MILP irradiation conditions within a few minutes, in comparison to the conventional method that often requires several hours to days for gelation to occur in the presence of cross-linking agents and initiators. The viscosity of the MILP silicone gel was greater than a similar gel formed by the conventional method. In contrast, the viscosity of the MILP-formed PVP gel was lower than the viscosity of the PVP gel obtained from the conventional method. Gels were characterized by
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C-NMR spectrometry, FT-IR spectroscopy, SEM microscopy, viscosity measurements, and dynamic light scattering for particle size distributions. Plausible mechanistic stages for the two gelation occurrences have been inferred as involving the synergistic effects from microwaves, together with the sound waves (cavitation microbubbles), heat, UV and &z.rad;OH radicals resulting from the microwave-generated in-liquid-plasma.
The discovery of a water-soluble polymer that cross-links to form a gel using a novel green gelation method: the microwave-induced in-liquid-plasma method that requires neither a cross-linking agent nor an initiator as are required in the conventional chemical method.
The microwave-induced acceleration of photocatalytic reactions was discovered serendipitously in the late 1990s. The activity of photocatalysts is enhanced significantly by both microwave radiation ...and UV light. Particularly relevant, other than as a heat source, was the enigmatic phenomenon of the non-thermal effect(s) of the microwave radiation that facilitated photocatalyzed reactions, as evidenced when examining various model contaminants in aqueous media. Results led to an examination of the possible mechanism(s) of the microwave effect(s). In the present article we contend that the microwaves' non-thermal effect(s) is an important factor in the enhancement of TiO2-photoassisted reactions involving the decomposition of organic pollutants in model wastewaters by an integrated (coupled) microwave-/UV-illumination method (UV/MW). Moreover, such coupling of no less than two irradiation methods led to the fabrication and ultimate investigation of microwave discharged electrodeless lamps (MDELs) as optimal light sources; their use is also described. The review focuses on the enhanced activity of photocatalytic reactions when subjected to microwave radiation and concentrates on the authors' research of the past few years.
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