Fluorescent carbon dots (CDs) represent a promising eco-friendly next-generation phosphor. However, most CDs exhibit broad photoluminescence (PL) spectra [full width at half-maximum (fwhm) over 60 nm]; few works on CDs with sharp PL spectra (fwhm less than 40 nm) have been reported. In addition, their syntheses and color tuning require harsh conditions of high temperatures, long reaction times, and high pressures with catalysts. Here, we successfully prepared narrow-bandwidth emissive CDs (fwhm of 27−40 nm) from phloroglucinol in a glycol solvent of 1,2-pentanediol at temperatures as low as 180 °C for a reaction duration of as short as 6 h under ambient conditions without any catalysts via an open reaction system in which dehydration and condensation reactions among phloroglucinol molecules were enhanced. We shifted the emission peak from 463 to 511 nm by selecting seven kinds of solvents with different polarities, that is, emission colors could be tuned from blue to green by taking advantage of fluorescence solvatochromism. The CD-dispersed polymer films showed a similar solvatochromic behavior and sharp PL spectra, verifying the feasibility of applying the CDs to displays with a wide color gamut.
Environmentally friendly and visible fluorescent carbon dots (CDs) have received attention as alternatives to compound semiconductor quantum dots containing toxic elements. Hydrothermal reaction of water-soluble carbohydrates can provide an eco-friendly and facile synthesis approach to CDs. In this study, we have attempted to efficiently produce uniform CDs using a microwave heating system that can maintain the reaction solution at a constant temperature. We synthesized CDs from D-glucose through hydrothermal treatment at 200 • C for 5-60 min. From transmission electron microscopy, the particle size of the CDs increased in proportion to the hydrothermal duration. Furthermore, the absorbance and maximum photoluminescence intensity of the CDs aqueous dispersions (10 mg L −1 ) increased with an increase in hydrothermal duration. The maximum photoluminescence intensity of the dispersions decreased at pH < 4 and pH > 10; the absorbance at the excitation wavelength remained unchanged. These results indicate that the number of emission sites and absorption sites increased with an increase in the particle size and that emission sites were located near the surface of the obtained CDs.
Fluorescent carbon nanoparticles (carbon dots) can be synthesized through hydrothermal treatment of carbon materials in an autoclave. Carbon dots are expected to possess homogeneous properties by using a microwave heating system instead of an electric heating system. Here, we reports particulate and photoluminescent (PL) properties of the carbon dots synthesized by microwave-assisted autoclave treatment.
D(+)-glucose and potassium dihydrogen phosphate were dissolved in ultrapure water. The solution was purged with Ar gas and transferred into a Teflon vessel. The sealed Teflon vessel was heated at 200 °C using a microwave-assisted autoclave. The heating duration was changed from 5 to 60 min. The resulting dispersion was purified by centrifuging, dialyzing, and filtrating to yield a dispersion of carbon dots. The powder of carbon dots was obtained by freeze drying.
According to the TEM observation, the mean particle size of carbon dots increased proportionally to hydrothermal treatment duration. According to the XRD pattern, carbon dots were mainly amorphous. Raman spectra revealed that carbon dots had a framework of graphite. A broad band with its maximum at ~360 nm was observed in PL excitation spectra of dispersions. The PL peak shifted to longer wavelength with increasing excitation wavelength. Thus, the PL spectra contained broad bands with maxima at ~400–700 nm. Maximum PL intensity and absorbance at optimum excitation wavelength increased with increasing hydrothermal treatment duration. This is attributed to an increase in fluorescence-related sites inside grown carbon dots.
Graphene oxide (GO)-based membranes with high selectivity
and permeability
can reduce energy use and address environmental impact in chemical
separations. These materials are of particular interest to mitigate
the energy-intensive weak black liquor (WBL) concentration in the
kraft pulping process. WBL is a high pH (∼12) corrosive fluid
that includes polymers, other organic compounds, inorganic salts,
and high total solids that is processed at elevated temperature (up
to 95 °C). Herein, we focused on a covalently functionalized
graphene synthesized through a Johnson–Claisen rearrangement
of GO, providing ethyl ester groups attached to the basal plane via
robust carbon–carbon bonds, to endure the harsh separation
conditions. Amidation was achieved under mild conditions between the
saponified Claisen-rearranged graphene and linear diamines and this
functionalization improves the selectivity for separation of the large
organics included in WBL, enhances permeability, and produces a robust
enlarged interlayer spacing stabilized by cross-linking. We successfully
synthesized four types of water-dispersible graphene derivatives with
different interlayer spacings (up to 17.67 Å) and created membranes
on hydrophilic poly(ether sulfone) supports. The expanded interlayer
spacing and hydrophilic nature enhanced the selectivity for the exclusion
of large molecules (∼1 kDa) and gave high permeability. WBL
was concentrated with a stable rate and permeance using the graphene
derivative functionalized with poly(ethylene glycol) diamine. The
versatile amidated graphene demonstrated significant potential in
the production of membranes for WBL concentration, and the base methods
can be adapted to other chemical separations.
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