Yujin Choi

Exfoliation of black phosphorus in ionic liquids

We report the characterization and formation of sonication-assisted liquid phase exfoliation of bulk black phosphorus (BP) crystals with the incorporation of two representative ionic liquids (ILs) ([Emim][Tf2N] and [Bmim][Tf2N]) as green dispersing media was attempted, which resulted in stable dispersion of multi-layer BP flakes with unsuspected high oxidation resistance and chemical/structural integrity due to the presence of IL layer on top of BP flakes. There are two unveiled issues for the generation of BP dispersion in ILs. First, thin films of BP flakes can be simply prepared through our approach. Because self-oxidation of BP in ambient condition can be significantly minimized in ILs, vacuum filtration step can be adopted to produce BP thin films in ambient condition. Second, the binding of IL molecules on BP flakes has been firstly demonstrated by the time-of-flight secondary ion mass spectrometry characterization. In addition to the exploitation of ILs as the green solvents with less environmental harmfulness, IL-based exfoliation of BP might be easily scalable because harsh control of atmospheric oxygen and moisture is unnecessary in this approach.

Highly Efficient Visible Blue-Emitting Black Phosphorus Quantum Dot: Mussel-Inspired Surface Functionalization for Bioapplications

The preparation of blue-emitting black phosphorus quantum dots (BPQDs) is based on the liquid-phase exfoliation of bulk BP. We report the synthesis of soluble BPQDs showing a strong visible blue-light emission. Highly fluorescent (photoluminescence quantum yield of ≈5% with the maximum emission (λmax) at ≈437 nm) and dispersible BPQDs in various organic solvents are first prepared by simple ultrasonication of BP crystals in chloroform in the ambient atmosphere. Furthermore, simple mussel-inspired surface functionalization of BPQDs with catechol-grafted poly(ethylene glycol) in basic buffer afforded water-soluble blue-emitting BPQDs showing long-term fluorescence stability, very low cytotoxicity, and excellent fluorescence live cell imaging capability.

Simple Microwave-Assisted Synthesis of Amphiphilic Carbon Quantum Dots from A3/B2 Polyamidation Monomer Set

Highly fluorescent and amphiphilic carbon quantum dots (CQDs) were prepared by microwave-assisted pyrolysis of citric acid and 4,7,10-trioxa-1,13-tridecanediamine (TTDDA), which functioned as an A3 and B2 polyamidation type monomer set. Gram quantities of fluorescent CQDs were easily obtained within 5 min of microwave heating using a household microwave oven. Because of the dual role of TTDDA, both as a constituting monomer and as a surface passivation agent, TTDDA-based CQDs showed a high fluorescence quantum yield of 29% and amphiphilic solubility in various polar and nonpolar solvents. These properties enable the wide application of TTDDA-based CQDs as nontoxic bioimaging agents, nanofillers for polymer composites, and down-converting layers for enhancing the efficiency of Si solar cells.

Mechanochemical synthesis of fluorescent carbon dots from cellulose powders

A novel mechanochemical method was firstly developed to synthesize carbon nanodots (CNDs) or carbon nano-onions (CNOs) through high-pressure homogenization of cellulose powders as naturally abundant resource depending on the treatment times. While CNDs (less than 5 nm in size) showed spherical and amorphous morphology, CNOs (10-50 nm in size) presented polyhedral shape, and onion-like outer lattice structure, graphene-like interlattice spacing of 0.36 nm. CNOs showed blue emissions, moderate dispersibility in aqueous media, and high cell viability, which enables efficient fluorescence imaging of cellular media.

Visible-light-driven photocatalysis with dopamine-derivatized titanium dioxide/N-doped carbon core/shell nanoparticles

Titanium dioxide/N-doped carbon core/shell nanoparticles enabling efficient visible-light-driven photocatalytic degradation of rhodamine B, considered a model compound for water-soluble environmental pollutants, were successfully prepared by the carbonization of dopamine-grafted TiO2 nanoparticles. These precursor nanoparticles were prepared via simple ligand-to-metal charge transfer (LMCT) between TiO2 nanoparticles and dopamine. Owing to the incorporation of Ti–O–C chelating bonds and the subsequent narrowing of the optical band gap, the dopamine-derivatized photocatalyst demonstrated enhanced activity compared with that of commercial photocatalysts and promoted the photocatalytic degradation of rhodamine B under both UV light and visible light. This LMCT-mediated incorporation of thin amorphous N-doped carbon shells onto the surface of semiconducting photocatalysts may be widely applicable for the generation of novel and robust hybrid materials with enhanced photocatalytic activities for many applications.