Eun Hee Cha

CoSe2 and NiSe2 Nanocrystals as Superior Bifunctional Catalysts for Electrochemical and Photoelectrochemical Water Splitting

Catalysts for oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) are central to key renewable energy technologies, including fuel cells and water splitting. Despite tremendous effort, the development of low-cost electrode catalysts with high activity remains a great challenge. In this study, we report the synthesis of CoSe2 and NiSe2 nanocrystals (NCs) as excellent bifunctional catalysts for simultaneous generation of H2 and O2 in water-splitting reactions. NiSe2 NCs exhibit superior electrocatalytic efficiency in OER, with a Tafel slope (b) of 38 mV dec(-1) (in 1 M KOH), and HER, with b = 44 mV dec(-1) (in 0.5 M H2SO4). In comparison, CoSe2 NCs are less efficient for OER (b = 50 mV dec(-1)), but more efficient for HER (b = 40 mV dec(-1)). It was found that CoSe2 NCs contained more metallic metal ions than NiSe2, which could be responsible for their improved performance in HER. Robust evidence for surface oxidation suggests that the surface oxide layers are the actual active sites for OER, and that CoSe2 (or NiSe2) under the surface act as good conductive layers. The higher catalytic activity of NiSe2 is attributed to their oxide layers being more active than those of CoSe2. Furthermore, we fabricated a Si-based photoanode by depositing NiSe2 NCs onto an n-type Si nanowire array, which showed efficient photoelectrochemical water oxidation with a low onset potential (0.7 V versus reversible hydrogen electrode) and high durability. The remarkable catalytic activity, low cost, and scalability of NiSe2 make it a promising candidate for practical water-splitting solar cells.

Tetragonal phase germanium nanocrystals in lithium ion batteries.

Various germanium-based nanostructures have recently demonstrated outstanding lithium ion storage ability and are being considered as the most promising candidates to substitute current carbonaceous anodes of lithium ion batteries. However, there is limited understanding of their structure and phase evolution during discharge/charge cycles. Furthermore, the theoretical model of lithium insertion still remains a challenging issue. Herein, we performed comparative studies on the cycle-dependent lithiation/delithiation processes of germanium (Ge), germanium sulfide (GeS), and germanium oxide (GeO2) nanocrystals (NCs). We synthesized the NCs using a convenient gas phase laser photolysis reaction and attained an excellent reversible capacity: 1100-1220 mAh/g after 100 cycles. Remarkably, metastable tetragonal (ST12) phase Ge NCs were constantly produced upon lithiation and became the dominant phase after a few cycles, completely replacing the original phase. The crystalline ST12 phase persisted through 100 cycles. First-principles calculations on polymorphic lithium-intercalated structures proposed that the ST12 phase Ge12Lix structures at x ≥ 4 become more thermodynamically stable than the cubic phase Ge8Lix structures with the same stoichiometry. The production and persistence of the ST12 phase can be attributed to a stronger binding interaction of the lithium atoms compared to the cubic phase, which enhanced the cycling performance.

Germanium sulfide(II and IV) nanoparticles for enhanced performance of lithium ion batteries

Germanium sulfide (GeS and GeS2) nanoparticles were synthesized by novel gas-phase laser photolysis and subsequent thermal annealing. They showed excellent cycling performance for lithium ion batteries, with a maximum capacity of 1010 mA h g(-1) after 100 cycles. Metastable tetragonal phase Ge nanoparticles were suggested as active materials for a reversible lithium insertion-extraction process.

Germanium–tin alloy nanocrystals for high-performance lithium ion batteries

Germanium-tin (Ge(1-x)Sn(x)) alloy nanocrystals were synthesized using a gas-phase laser photolysis reaction of tetramethyl germanium and tetramethyl tin. A composition tuning was achieved using the partial pressure of precursors in a closed reactor. For x < 0.1, cubic phase alloy nanocrystals were exclusively produced without separation of the tetragonal phase Sn metal. In the range of x = 0.1-0.4, unique Ge(1-x)Sn(x)-Sn alloy-metal hetero-junction nanocrystals were synthesized, where the Sn metal domain becomes dominant with x. Thin graphitic carbon layers usually sheathed the nanocrystals. We investigated the composition-dependent electrochemical properties of these nanocrystals as anode materials of lithium ion batteries. Incorporation of Sn (x = 0.05) significantly increased the capacities (1010 mA h g(-1) after 50 cycles) and rate capabilities, which promises excellent electrode materials for the development of high-performance lithium batteries.

Zn2GeO4 and Zn2SnO4 nanowires for high-capacity lithium- and sodium-ion batteries

Germanium (Ge) and tin (Sn) are considered to be the most promising alternatives to commercial carbon materials in lithium- and sodium-ion batteries. High-purity zinc germanium oxide (Zn2GeO4) and zinc tin oxide (Zn2SnO4) nanowires were synthesized using a hydrothermal method, and their electrochemical properties as anode materials in lithium- and sodium-ion batteries were comparatively investigated. The nanowires had a uniform morphology and consisted of single-crystalline rhombohedral (Zn2GeO4) and cubic (Zn2SnO4) phases. For lithium ion batteries, Zn2GeO4 and Zn2SnO4 showed an excellent cycling performance, with a capacity of 1220 and 983 mA h g−1 after 100 cycles, respectively. Their high capacities are attributed to a combination of the alloy formation reaction of Zn and Ge (or Sn) with Li, and the conversion reactions: ZnO + 2Li+ + 2e− ↔ Zn + Li2O and GeO2 (or SnO2) + 4Li+ + 4e− ↔ Ge (or Sn) + 2Li2O. For the first time, we examined the cycling performance of Zn2GeO4 and Zn2SnO4 in sodium ion batteries; their capacities were 342 mA h g−1 and 306 mA h g−1 after 100 cycles, respectively. The capacity of Zn2SnO4 is much higher than the theoretical capacity (100 mA h g−1), while that of Zn2SnO4 is close to the theoretical capacity (320 mA h g−1). We suggest a contribution of the conversion reaction in increasing the capacities, which is similar to the case of lithium ion batteries. The present systematic comparison between the lithiation and sodiation will provide valuable information for the development of high-performance lithium- and sodium-ion batteries.

Nb2O5 nanowire photoanode sensitized by a composition-tuned CdSxSe1−x shell

Nb2O5 nanowires (NWs) were grown by the thermal oxidation of Nb foils. The thermal chemical vapor transport of CdS/CdSe powders produces Nb2O5–CdSxSe1−x core–shell NWs with complete composition tuning. In the alloy composition, CdSe-like and CdS-like phases exist in the outer and inner regions of the shell, respectively, forming unique multi-shell structures. As x increases, the thickness of the CdSe-like outer shell decreases, while that of the CdS-like inner shell increases. We fabricated photoelectrochemical (PEC) cells using the as-grown Nb2O5–CdSxSe1−x NWs and measured their photocurrents and hydrogen generation rates under AM 1.5G irradiation. The PEC cells showed excellent photoconversion efficiency, which is comparable to that of TiO2–CdSxSe1−x NWs. It indicates that the present Nb2O5 NW promises an excellent photoanode of solar cell devices. The multi-shell structures increase the PEC performance by producing novel band alignment for efficient electron–hole separation following enhanced visible-range photon absorption.

Surface-Modified Ta3N5 Nanocrystals with Boron for Enhanced Visible-Light-Driven Photoelectrochemical Water Splitting

Photocatalysts for water splitting are the core of renewable energy technologies, such as hydrogen fuel cells. The development of photoelectrode materials with high efficiency and low corrosivity has great challenges. In this study, we report new strategy to improve performance of tantalum nitride (Ta3N5) nanocrystals as promising photoanode materials for visible-light-driven photoelectrochemical (PEC) water splitting cells. The surface of Ta3N5 nanocrystals was modified with boron whose content was controlled, with up to 30% substitution of Ta. X-ray photoelectron spectroscopy revealed that boron was mainly incorporated into the surface oxide layers of the Ta3N5 nanocrystals. The surface modification with boron increases significantly the solar energy conversion efficiency of the water-splitting PEC cells by shifting the onset potential cathodically and increasing the photocurrents. It reduces the interfacial charge-transfer resistance and increases the electrical conductivity, which could cause the higher photocurrents at lower potential. The onset potential shift of the PEC cell with the boron incorporation can be attributed to the negative shift of the flat band potential. We suggest that the boron-modified surface acts as a protection layer for the Ta3N5 nanocrystals, by catalyzing effectively the water splitting reaction.

Phase-and Size-Controlled Synthesis of CdSe/ZnS Nanoparticles Using Ionic Liquid

Ionic liquids are room-temperature molten salts, composed of organic mostly of organic ions that may undergo almost unlimited structural variation. We approach the new aspects of ionic liquids in applications where the semiconductor nanoparticles used as sensitizers of solar cells. We studied the effects of ionic liquids as capping ligand and/or solvent, on the morphology and phase of the CdSe/ZnS nanoparticles. Colloidal CdSe/ZnS nanoparticles were synthesized using a series of imidazolium ionic liquids; 1-R-3-methylimidazolium bis(trifluoromethylsulfo- nyl)imide ((RMIM)(TFSI)), where R = ethyl ((EMIM)), butyl ((BMIM)), hexyl ((HMIM)), octyl ((OMIM)). The average size of nanoparticles was 8~9 nm, and both zinc-blende and wurtzite phase was produced. We also synthesized the nanoparticles using a mixture of trihexyltetrade- cylphosphonium bis(trifluoromethylsulfonyl)imide ((P6,6,6,14)(TFSI)) and octadecene (ODE). The CdSe/ZnS nanoparticles have a smaller size (5.5 nm) than that synthesized using imidazolium, and with a controlled phase from zinc-blende to wurtzite by increasing the volume ratio of

Liquid crystal dimers having vary oxyethylene flexible spacers

ABSTRACT In this article, we are prepared that the liquid crystal dimers have aromatic-ester type mesogenic units or aromatic-Schiff base type mesogonic units and confirmed by 1H-NMR spectrometry. The mesomorphic and optical properties of the resultant dimers were studied by differential scanning calorimetry and polarizing optical microscopy.

Comparative Cycling Performance of Zn 2 GeO 4 and Zn 2 SnO 4 Nanowires as Anodes of Lithium- and Sodium Ion Batteries

High-yield zinc germanium oxide () and zinc tin oxide () nanowires were synthesized using a hydrothermal method. We investigated the electrochemical properties of these and nanowires as anode materials of lithium ion battery and sodium ion battery. The and nanowires showed excellent cycling performance of the lithium ion battery, with a maximum capacity of 1021 mAh/g and 692 mAh/g after 50 cycles, respectively, with a high Coulomb efficiency of 98 %. For the first time, we examined the cycling performance of and nanowires for sodium ion batteries. The maximum capacity is 168 mAh/g and 200 mAh/g after 50 cycles, respectively, with a high Coulomb efficiency of 97%. These nanowires are expected as promising electrode materials for the development of high-performance lithium ion batteries as well as sodium ion batteries.