Ji Hyeon Kim

A recyclable, recoverable, and reformable hydrogel-based smart photocatalyst

We present a novel hydrogel-based photocatalyst that contains TiO2 nanoparticles (NPs) uniformly dispersed in an agarose hydrogel matrix. The preparation of the TiO2/agarose hybrid gel photocatalyst was based on the gelation of agarose in the presence of well-dispersed TiO2 NPs in hot water. TiO2 NPs were homogeneously distributed in the agarose gel as characterized by Fourier transform infrared spectroscopy (FT-IR) analysis. It was found that the size, uniformity, and concentration of the hybrid gel as well as the contents of constituent ingredients have significant effects on the photocatalytic activity. The smaller and uniform size of the hybrid gel at an appropriate concentration exhibited superior photocatalytic performance in both photodegradation of methylene blue (MB) under UV light and TiO2 leakage. In the moderate concentration range of TiO2 and agarose, the degradation rate of MB increased upon increasing the TiO2 content or decreasing the agarose concentration. Under the optimized conditions, our hybrid gel showed excellent recycling performance over repeated use. Furthermore, we demonstrate the additional excellent features of our hybrid gel, which are: i) regeneration of pure TiO2 NPs and ii) thermal reconstruction of the hybrid gel. During the recycling, the TiO2 NPs initially immobilized in the hydrogel could be recovered through the programmed heating and separation techniques. Also, our hybrid gel could be easily reshaped into a new hydrogel with a desired architecture in terms of its size and shape. These unprecedented properties make our hybrid gel a smart and cost-effective new promising material for use in practical waste/water treatment.

Electrochemical Performance of Sn/SnO/Ni3Sn Composite Anodes for Lithium-Ion Batteries

We have synthesized a novel composite material Sn/SnO/Ni₃Sn via galvanic replacement reaction between Sn and Ni2+ ions in triethylene glycol medium and at high temperature. The reaction time affected structure, particle size, and composition of Sn/SnO/Ni₃Sn composites, which were analyzed by high resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. The active component (Sn) reacts with Li+ ions while inactive component (Ni) acts as a metal framework to support the electrochemically active Sn, a buffer to reduce volume change during cycling, and the electron conductor. Among electrodes, the Sn/SnO/Ni₃Sn-6h electrode demonstrated stable cycling and reversible capacity of 246 mAh g-1 even after 300 cycles owing to the advantages from the unique hybrid structure.

Comparative Study of Mechanically Milled MoS2 and MoSe2 in Graphite Matrix as Anode Materials for High-Performance Lithium-Ion Batteries

Nanocomposites of MoS2/graphite and MoSe2/graphite were formed from two-dimensional materials (MoS2 and MoSe2) and graphite using a one-step ball-milling method (high energy mechanical milling, HEMM). As anode materials for lithium-ion batteries (LIBs), these nanocomposites showed higher specific capacity and greater stability during long cyclic operation compared to their pure counterparts (MoS2 and MoSe2). X-ray diffraction and transmission electron microscopy revealed that graphite nanoflakes were effectively exfoliated and covered MoS2 or MoSe2 layers to form homogeneous nanostructures via HEMM. As a result, the electrochemical performances of both MoS2/graphite and MoSe2/graphite were excellent; the specific capacities were as high as 684.8 (MoS2/graphite) and 787.3 mAh g-1 (MoSe2/graphite) after 100 cycles. Also, when compared with MoS2/graphite, the MoSe2/graphite nanocomposite showed higher specific capacity and better rate capability performance due to larger interlayer spacing, leading to fast and facile movement of Liions. Overall, we demonstrate that homogeneous nanocomposites between similar layered materials (MoS2, MoSe2 and graphite) can be easily synthesized via one-step HEMM, which can be used as excellent anode materials for LIBs.