Two-Dimensional WS2@Nitrogen-Doped Graphite for High-Performance Lithium Ion Batteries: Experiments and Molecular Dynamics Simulations.

As promising candidates for anode materials in lithium ion batteries (LIB), two-dimensional tungsten disulfide (WS2) and WS2@(N-doped) graphite composites were synthesized, and their electrochemical properties were comprehensibly studied in conjunction with calculations. The WS2 nanosheets, WS2@graphite, and WS2@N-doped graphite (N-graphite) exhibit outstanding cycling performance with capacities of 633, 780, and 963 mA h g-1, respectively. To understand their lithium storage mechanism, first-principles calculations involving a series of ab initio NVT- NPT molecular dynamics simulations were conducted. The calculated discharge curves for amorphous phase are well matched with the experimental ones, and the capacities reach 620, 743, and 915 mA h g-1 for WS2, WS2@graphite, and WS2@N-graphite, respectively. The large capacities of the two composites can be attributed to the tendency of W and Li atoms to interact with graphite, suppressing the formation of W metal clusters. In the case of WS2@N-graphite, vigorous amorphization of the N-graphite enhances the interaction of W and Li atoms with the fragmented N-graphite in such a way that unfavorable Li-W repulsion is avoided at very early stage of lithiation. As a result, the volume expansion in WS2@graphite and WS2@N-graphite is calculated to be remarkably small (only 6 and 44%, respectively, versus 150% for WS2). Therefore WS2@(N-)graphite composites are expected to be almost free of mechanical pulverization after repeated cycles, which makes them promising and excellent candidates for high-performance LIBs.

Arsenic for high-capacity lithium- and sodium-ion batteries.

We report arsenic (As) as a promising alternative to graphite anode materials in lithium- and sodium-ion batteries (LIBs and SIBs). The electrochemical properties of the As/carbon nanocomposite for both LIBs and SIBs were investigated using experimental and theoretical approaches. The LIBs showed excellent cycling performance, with a reversible capacity of 1306 mA h g-1 (after 100 cycles), which is much higher than that of Li3As (1072 mA h g-1). In the corresponding SIBs, the measured reversible capacity was 750 mA h g-1 (after 200 cycles), which is lower than that of Na3As. Extensive first-principles calculations were performed employing a structure prediction method for crystalline LixAs and NaxAs (x = 1-6) phases, as well as ab initio molecular dynamics simulations for their amorphous phases. In good agreement with the experimental LIB data, our calculations successfully predict the discharge capacity versus voltage curves, showing that the capacity of the amorphous phase reaches up to that of Li4As. In contrast, the SIB exhibited difficulty in reaching the predicted capacity (x = 3.5), probably due to significant volume expansion. Comparison of the theoretical discharge curves with the experimental data provides valuable information for the development of high-performance LIBs and SIBs.

Schottky nanocontact of one-dimensional semiconductor nanostructures probed by using conductive atomic force microscopy.

To develop the advanced electronic devices, the surface/interface of each component must be carefully considered. Here, we investigate the electrical properties of metal-semiconductor nanoscale junction using conductive atomic force microscopy (C-AFM). Single-crystalline CdS, CdSe, and ZnO one-dimensional nanostructures are synthesized via chemical vapor transport, and individual nanobelts (or nanowires) are used to fabricate nanojunction electrodes. The current-voltage (I -V) curves are obtained by placing a C-AFM metal (PtIr) tip as a movable contact on the nanobelt (or nanowire), and often exhibit a resistive switching behavior that is rationalized by the Schottky (high resistance state) and ohmic (low resistance state) contacts between the metal and semiconductor. We obtain the Schottky barrier height and the ideality factor through fitting analysis of the I-V curves. The present nanojunction devices exhibit a lower Schottky barrier height and a higher ideality factor than those of the bulk materials, which is consistent with the findings of previous works on nanostructures. It is shown that C-AFM is a powerful tool for characterization of the Schottky contact of conducting channels between semiconductor nanostructures and metal electrodes.

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.

FeP and FeP2 nanowires for efficient electrocatalytic hydrogen evolution reaction.

We report achieving of efficient electrocatalytic hydrogen evolution reaction using composition-controlled iron phosphide (FeP and FeP2) nanowires. The respective Tafel slopes of nanowire arrays were 39 and 37 mV dec(-1) in 0.5 M H2SO4, and 75 and 67 mV dec(-1) in 1 M KOH. The richer P composition produced excellent electrocatalytic efficiency.

Laser-induced graphitization of colloidal nanodiamonds for excellent oxygen reduction reaction.

Graphitized nanodiamonds were conveniently prepared by the laser irradiation of colloidal solution using various solvents. The nanodiamonds were converted into a fully graphitized onion-like structure, which became a cage-like mesoporous structure by the degradation of graphitic layers. Alcohols, acetone, and acetonitrile are more efficient solvents for the graphitization compared to water and hydrocarbons. Therefore the number and morphology of the graphitic layers can be simply controlled by the solvent and laser-irradiation duration. We suggest a graphitization model, in which the photocatalytic oxidation of the solvent accelerates the graphitization of nanodiamonds. The graphitized nanodiamonds were easily doped with the nitrogen and sulfur atoms in a controlled manner. In particular, the spherical graphitic layers were preferentially doped with the pyrrolic nitrogen that enhances remarkably electrocatalytic activity for the oxygen reduction reaction.

Phase evolution of tin nanocrystals in lithium ion batteries.

Sn-based nanostructures have emerged as promising alternative materials for commercial lithium-graphite anodes in lithium ion batteries (LIBs). However, there is limited information on their phase evolution during the discharge/charge cycles. In the present work, we comparatively investigated how the phases of Sn, tin sulfide (SnS), and tin oxide (SnO2) nanocrystals (NCs) changed during repeated lithiation/delithiation processes. All NCs were synthesized by a convenient gas-phase photolysis of tetramethyl tin. They showed excellent cycling performance with reversible capacities of 700 mAh/g for Sn, 880 mAh/g for SnS, and 540 mAh/g for SnO2 after 70 cycles. Tetragonal-phase Sn (ß-Sn) was produced upon lithiation of SnS and SnO2 NCs. Remarkably, a cubic phase of diamond-type Sn (α-Sn) coexisting with ß-Sn was produced by lithiation for all NCs. As the cycle number increased, α-Sn became the dominant phase. First-principles calculations of the Li intercalation energy of α-Sn (Sn8) and ß-Sn (Sn4) indicate that Sn4Li(x) (x ≤ 3) is thermodynamically more stable than Sn8Li(x) (x ≤ 6) when both have the same composition. α-Sn maintains its crystalline form, while ß-Sn becomes amorphous upon lithiation. Based on these results, we suggest that once α-Sn is produced, it can retain its crystallinity over the repeated cycles, contributing to the excellent cycling performance.

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-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.

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.

Charge-selective surface-enhanced Raman scattering using silver and gold nanoparticles deposited on silicon-carbon core-shell nanowires.

The deposition of silver (Ag) or gold (Au) nanoparticles (NPs) on vertically aligned silicon-carbon (Si-C) core-shell nanowires (NWs) produces sensitive substrates for surface-enhanced Raman spectroscopy (SERS). The undoped and 30% nitrogen (N)-doped graphitic layers of the C shell (avg thickness of 20 nm) induce a higher sensitivity toward negatively (-) and positively (+) charged dye molecules, respectively, showing remarkable charge selectivity. The Ag NPs exhibit higher charge selectivity than the Au NPs. The Ag NPs deposited on p- and n-type Si NWs also exhibit (-) and (+) charge selectivity, respectively, which is higher than that of the Au NPs. The X-ray photoelectron spectroscopy analysis indicates that the N-doped graphitic layers donate more electrons to the metal NPs than the undoped ones. More distinct electron transfer occurs to the Ag NPs than to the Au NPs. First principles calculations of the graphene-metal adducts suggest that the large electron transfer capacity of the N-doped graphitic layers is due to the formation of a N→Ag coordinate bond involving the lone pair electrons of the N atoms. We propose that the more (-) charged NPs on the N-doped graphitic layers prefer the adsorption of (+) charged dyes, enhancing the SERS intensity. The charge selectivity of the Si NW substrates can also be rationalized by the greater electron transfer from the n-type Si to the metal NPs.