[show abstract][hide abstract] ABSTRACT: Hierarchical hollow spheres of Fe2 O3 @polyaniline are fabricated by template-free synthesis of iron oxides followed by a post in- and exterior construction. A combination of large surface area with porous structure, fast ion/electron transport, and mechanical integrity renders this material attractive as lithium ion anode, showing superior rate capability and cycling performance.
[show abstract][hide abstract] ABSTRACT: We report on the synthesis of two types of Ag nanoparticles (NPs) and the influence of adsorbed surfactant size on the NP surface for surface-enhanced Raman scattering (SERS) signals. Both particles were of similar size and morphology but were covered by surfactants of different sizes; one surfactant was sodium citrate (molecular weight: 258) and the other was sodium polyacrylate (molecular weight: 2100). For SERS measurement, 4-mecapobenzoic acid and 4-naphthalene thiol as Raman-active dyes were immobilized on the surface of each AgNP. The signals from Raman-active dyes on AgNPs covered with citrate displayed 10 times higher intensity than those from polyacrylate-stabilized AgNPs. Elemental analysis (EA) revealed that the average weight percentage of sulfur is 0.94 wt% and 0.12 wt% for citrate-stabilized and polyacrylate-stabilized AgNPs, respectively. The sulfur content difference was attributed to the size of the existing surfactant influencing the ligand exchange by steric hindrance and subsequently the amount of sulfur content of the particles. These experimental results suggest that the size of initial surfactant should be taken into account when synthesizing a metal particle for enhancing SERS signal.
Journal of Nanoscience and Nanotechnology 08/2013; 13(8):5840-3. · 1.15 Impact Factor
[show abstract][hide abstract] ABSTRACT: Fast Li transport inside the Si electrode of Li-ion batteries is retarded at the electrode surface. We investigated the intercalation of a Li atom into the surface and subsurface layers of Ge and Sn using density functional calculations. In the Ge and Sn surfaces, the Li atom diffuses about four and twelve times faster, respectively, along the 〈100〉 direction than along the 〈111〉 direction. The retardation of Li transport is negligible in Ge surfaces, but some retardation occurs in Sn surfaces. The rate-limiting step resulting in retardation of Li transport in Sn is the subsurface diffusion in both the Sn(100) and Sn(111) surfaces. Our calculations suggest that while the Li transport efficiency in the Ge electrode remains unchanged regardless of the electrode size, transport in the Sn electrode could deteriorate due to the effect of surface retardation as the surface-to-volume ratio of the electrode increases.
Physical Chemistry Chemical Physics 07/2013; · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: A series of glycolide derivatives, glycolide (GL00), 3-methyl glycolide (GL01), 3,6-dimethyl glycolide (GL11) and 3,3,6,6-tetramethyl glycolide (GL22), are examined as SEI-forming electrolyte additives for LiCoO 2 /graphite lithium-ion batteries (LIBs). The novel additives are also compared with vinylene carbonate (VC), the currently most popular SEI-forming additive in LIBs. Through both density functional calculations and electrochemical experiments, it is confirmed that the number of methyl groups systematically affects anodic stability and the SEI-forming reaction of the glycolide additives. The cyclability of the LiCoO 2 /graphite cell is improved in the following order: VC $ GL00 $ GL01 > GL11 > GL22. With elevated temperature (90 C) storage, however, the Al-pouch cells with VC and GL00 suffer from severe swelling, whereas the cells with GL01 and GL11 show reasonable dimensional stabilities. Both the good cyclability and the excellent thermal stability of GL01 make it the most appropriate SEI additive for the present LiCoO 2 /graphite chemistry.
Journal of Materials Chemistry 02/2013; 22:21003. · 5.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Development of nanostructured materials with enhanced redox reaction capabilities is important for achieving high energy and power densities in energy storage systems. Here, we demonstrate that the nanohybridization of ionic liquids (ILs, 1-butyl-3-methylimidazolium tetrafluoroborate) and cobalt hydroxide (Co(OH)</sub>2) through ionothermal synthesis leads to a rapid and reversible redox reaction. The as-synthesized IL-Co(OH)</sub>2 has a favorable, tailored morphology with a large surface area of 400.4 m</sup>2/g and a mesopore size of 4.8 nm. In particular, the IL-Co(OH)</sub>2-based electrode exhibits improvement in electrochemical characteristics compared with bare Co(OH)</sub>2, showing a high specific capacitance of 859 F/g at 1 A/g, high-rate capability (~95% retention at 30 A/g), and excellent cycling performance (~96% retention over 1000 cycles). AC impedance analysis demonstrates that the introduction of ILs on Co(OH)</sub>2 facilitates ion transport and charge transfer: IL-Co(OH)2 shows a higher ion diffusion coefficient (1.06 × 10</sup>-11 cm</sup>2/s) and lower charge transfer resistance (1.53 Ω) than those of bare Co(OH)</sub>2 (2.55 × 10</sup>-12 cm</sup>2/s and 2.59 Ω). Our density functional theory (DFT) calculations reveal that the IL molecules, consisting of anion and cation groups, enable easier hydrogen desorption/adsorption process, i.e., a more favorable redox reaction on the Co(OH)</sub>2 surface.
[show abstract][hide abstract] ABSTRACT: The volume expansion of silicon is the most important feature for electrochemical operations of high capacity Si anodes in lithium ion batteries. Recently, the unexpected anisotropic volume expansion of Si during lithiation has been experimentally observed, but its atomic-level origin is still unclear. By employing first-principles molecular dynamics simulations, herein, we report that the interfacial energy at the phase boundary of amorphous Li(x)Si/crystalline Si plays a very critical role in lithium diffusion and thus volume expansion. While the interface formation turns out to be favorable at x = 3.4 for all of the (100), (110), and (111) orientations, the interfacial energy for the (110) interface is the smallest, which is indeed linked to the preferential volume expansion along the ⟨110⟩ direction because the preferred (110) interface would promote lithiation behind the interface. Utilizing the structural characteristic of the Si(110) surface, local Li density at the (110) interface is especially high reaching Li(5.5)Si. Our atomic-level calculations enlighten the importance of the interfacial energy in the volume expansion of Si and offer an explanation for the previously unsolved perspective.
[show abstract][hide abstract] ABSTRACT: In a joint study involving electrochemical experiments and density functional calculations, we determined the oxidized and reduced structures of ethylene carbonate (EC), vinylene carbonate (VC), N-methyl-ε-caprolactam (Me-CL), and N-acetyl-ε-caprolactam (Ac-CL). This study reveals that the four molecules maintain their ring structures under the one-electron oxidation condition. Me-CL and Ac-CL have linear chain forms, whereas EC and VC still have ring-structures under the one-electron reduction condition. We suggest that such a collabora-tive study, including both experimentation and theory, is a simple and practical method for determining the structures of oxidized and reduced molecules.
[show abstract][hide abstract] ABSTRACT: We have studied the electrochemical and thermodynamic stability of Au(25)(SR)(18)(-), Au(38)(SR)(24), and Au(102)(SR)(44), R = CH(3), C(6)H(13), CH(2)CH(2)Ph, Ph, PhF, and PhCOOH, in order to examine ligand effects on the stability of thiol-stabilized gold nanoclusters, Au(m)(SR)(n). Aliphatic thiols, in general, have higher electrochemical and thermodynamic stability than aromatic thiols, and the -SCH(2)CH(2)Ph thiol is particularly appealing because of its high electrochemical and thermodynamic stability. The stabilization of Au(m) by nSR for Au(m)(SR)(n) can be rationalized by the stabilization of an Au atom by an SR for the simple molecule AuSR, regardless of interligand interaction and system size and shape. Thiol moieties play a strong role in the electron oxidation and reduction of Au(m)(SR)(n). Accounting for the characteristics of thiol ligands is essential for understanding the electronic and thermodynamic stability of thiol-stabilized gold nanoclusters.
[show abstract][hide abstract] ABSTRACT: A two-dimensional fluorinated fullerene (C(60)F(36)) superstructure has been successfully formed on Au(111) and was investigated using scanning tunneling microscopy (STM) and density functional theory calculations. Although there exist three isomers (C(3), C(1), and T) in our molecular source, STM images of the molecules in the well-ordered region all appear identical, with 3-fold symmetry. This observation together with the differences in the calculated lowest unoccupied molecular orbital (LUMO) distribution among the three isomers suggests that a well-ordered monolayer consists of only the C(3) isomer. Because of the strong electron-accepting ability of C(60)F(36), the adsorption orientation can be explained by localized distribution of its LUMO, where partial electron transfer from Au(111) occurs. Intermolecular C-F···π electrostatic interactions are the other important factor in the formation of the superstructure, which determines the lateral orientation of C(60)F(36) molecules on Au(111). On the basis of scanning tunneling spectra obtained inside the superstructure, we found that the LUMO is located at 1.0 eV above the Fermi level (E(F)), while the highest occupied molecular orbital (HOMO) is at 4.6 eV below the E(F). This large energy gap with the very deep HOMO as well as uniform electronic structure in the molecular layer implies a potential for application of C(60)F(36) to an electron transport layer in organic electronic devices.
[show abstract][hide abstract] ABSTRACT: Hierarchical mesoporous γ-MnO2 nanospheres with high crystallinity were prepared by a template-free self-assembly under applied ultrasonic waves, resulting in a solid spherical nanostructure with stacking of nanosized worm-like particles. This approach based on ultrasound exposure reduces the particle size and enhances the crystallinity of γ-MnO2, leading to a large effective area for the active domains and a stable structural arrangement of the active materials. The applicability of γ-MnO2 to high-power lithium batteries is demonstrated, based on improved capacity, excellent rate capability, and stable cyclability, which stem from the outstanding physical properties of the hierarchical mesoporous nanostructure.
[show abstract][hide abstract] ABSTRACT: Fast Li transport in battery electrodes is essential to meeting the demanding requirements for a high-rate capability anode. We studied the intercalation of a Li atom into the surface and subsurface layers of Si(100) and Si(111) using density functional calculations with a slab representation of the surfaces. We suggest that the Li atom migrates on the Si surfaces and is subsequently inserted into the inside for both Si(100) and Si(111). The rate-determining steps are the surface incorporation and subsurface diffusion in Si(100) and Si(111), respectively. Our diffusion rate calculations reveal that, once the Li atom is incorporated into the Si surface, Li diffuses faster by at least two orders of magnitude along the <100> direction than along the <111> direction. The importance of careful treatment of the slab thickness for the study of impurity insertion into subsurface layers is also stressed.
Physical Chemistry Chemical Physics 12/2011; 13(48):21282-7. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: Thermal decomposition of Co(acac)(3) and Cu(acac)(2) in benzylamine leads to the formation of  directed Cu-doped h-CoO nanorods, which are very stable in an aqueous solution. The formation mechanism of the  directed Cu-doped h-CoO nanorods is fully elucidated by using first-principles calculations, demonstrating that Cu-doping not only changes the growth direction but also enhances the stability of the nanorods significantly. Evaluation of the electrochemical performance of Cu-doped h-CoO nanorods shows high initial Coulombic efficiency and ultrahigh capacity with excellent cycling performance, indicating their suitability as an anode material for next generation lithium-ion batteries.
[show abstract][hide abstract] ABSTRACT: We performed density functional calculations to examine the intermolecular self-interaction of metal tetraauride MAu(4) (M=Ti, Zr, and Hf) clusters. We found that the metal auride clusters have strong dimeric interactions (2.8-3.1 eV) and are similar to the metal hydride analogues with respect to structure and bonding nature. Similarly to (MH(4) )(2) , the (μ-Au)(3) C(s) structures with three three-center two-electron (3c-2e) bonds were found to be the most stable. Natural orbital analysis showed that greater than 96 % of the Au 6s orbital contributes to the 3c-2e bonds, and this predominant s orbital is responsible for the similarity between metal aurides and metal hydrides (>99 % H 1s). The favorable orbital interaction between occupied Au 6s and unoccupied metal d orbitals leads to a stronger dimeric interaction for MAu(4) -MAu(4) than the interaction for MH(4) -MH(4) . There is a strong relationship between the dimeric interaction energy and the chemical hardness of its monomer for (MAu(4) )(2) and (MH(4) )(2) .
Chemistry - An Asian Journal 03/2011; 6(3):868-72. · 4.57 Impact Factor
[show abstract][hide abstract] ABSTRACT: We investigated the structure and stability of several aluminum hydride complexes to understand the essence of "superatom chemistry" and to gain a right perspective on the ligand (L)-stabilized metal (M) clusters. We successfully interpret the structure and stability using molecular orbital analysis, which clearly shows the failure of an electron-shell closing model (or a superatom model) to explain it. The structure and stability of Al(m)H(n) are closely associated with the molecular orbital stabilization owing to the effective orbital overlap between Al(m) (M(m)) and nH (nL). The importance of retaining the electronic structural integrity of M(m) in M(m)L(n)-within an electron-shell closing model-has been underestimated or even disregarded, and this has created the current controversies in the scientific community.
Journal of the American Chemical Society 03/2011; 133(15):6090-5. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Calculations are made of the lowest unoccupied molecular orbital (LUMO), chemical hardness (η), dipole moment (μ), and binding energy with a Li+ ion for 32 organic molecules that are electrolyte additives for solid electrolyte interphase (SEI) formation in lithium-ion batteries (LIBs). The results confirm that both the LUMO and η values are critical indicators of suitable SEI formation. The μ values of the additives are generally smaller than those of widely used solvents in LIBs. It is found that a low Li-ion binding affinity may be an important characteristic for SEI-forming additives. Li+ binding affinity is proposed as a factor in the computational screening process used to obtain promising additives.
[show abstract][hide abstract] ABSTRACT: A new artificial photosynthetic triad array, a fullerene-triosmium cluster/zinc-porphyrin/boron-dipyrrin complex (1, Os(3)C(60)/ZnP/Bodipy), has been prepared by decarbonylation of Os(3)(CO)(8)(CN(CH(2))(3)Si(OEt)(3))(mu(3)-eta(2):eta(2):eta(2)-C(60)) (6) with Me(3)NO/MeCN and subsequent reaction with the isocyanide ligand CNZnP/Bodipy (5) containing zinc porphyrin (ZnP) and boron dipyrrin (Bodipy) moieties. Triad 1 has been characterized by various spectroscopic methods (MS, NMR, IR, UV/Vis, photoluminescence, and transient absorption spectroscopy). The electrochemical properties of 1 in chlorobenzene (CB) have been examined by cyclic voltammetry; the general feature of the cyclic voltammogram of 1 is nine reversible one-electron redox couples, that is, the sum of those of 5 and 6. DFT has been applied to study the molecular and electronic structures of 1. On the basis of fluorescence-lifetime measurements and transient absorption spectroscopic data, 1 undergoes an efficient energy transfer from Bodipy to ZnP and a fast electron transfer from ZnP to C(60); the detailed kinetics involved in both events have been elucidated. The SAM of triad 1 (1/ITO; ITO=indium-tin oxide) has been prepared by immersion of an ITO electrode in a CB solution of 1 and diazabicyclo-octane (2:1 equiv), and characterized by UV/Vis absorption spectroscopy, water contact angle, X-ray photoelectron spectroscopy, and cyclic voltammetry. The photoelectrochemical properties of 1/ITO have been investigated by a standard three-electrode system in the presence of an ascorbic acid sacrificial electron donor. The quantum yield of the photoelectrochemical cell has been estimated to be 29 % based on the number of photons absorbed by the chromophores. Our triad 1 is unique when compared to previously reported photoinduced electron-transfer arrays, in that C(60) is linked by pi bonding with little perturbation of the C(60) electron delocalization.
Chemistry - A European Journal. 05/2010; 16(19):5586-99.
[show abstract][hide abstract] ABSTRACT: Alkali metal amides may exist in solution, the solid phase, and even the gas phase. Based on a theoretical model of a Li3N system which adsorbs and desorbs two hydrogen molecules, we examine the possible pathways of the Li3N+2H2↔LiNH2+2LiH reversible reaction. The dehydrogenation process can be separated into two-step reactions, Li2NH+LiH→Li3N+H2 (−9.5 kcal/mol exothermic) and LiNH2+LiH→Li2NH+H2 (+0.7 kcal/mol endothermic). Along the reaction pathway, two intermediates and a transition state for each reaction were found in our ab initio molecular orbital calculations at the MP2 and CCSD(T) levels of theory. A total of two H2 molecules can be stored and released at normal temperature and pressure if there are means to substantially raise the energy of the two stable intermediates. Reaction energy profiles from our calculations support the much higher temperature of the first step reaction in experiment.
[show abstract][hide abstract] ABSTRACT: A recent landmark achievement (Jadzinsky et al. Science 2007, 318, 430) reported the successful crystallization and X-ray measurements of para-mercaptobenzoic acid (p-MBA)-protected gold nanoparticles containing 102 gold atoms and 44 p-MBAs. To gain more insight into the Au102(SR)44 nanoclusters, we have performed density functional calculations for Au102(SMe)44 and its neighboring systems. The exceptional stability of Au102(SR)44 can be attributed to three coexisting factors: effective staple-motif formation, high stability against dissociation, and a large HOMO−LUMO gap.
The Journal of Physical Chemistry C 01/2010; 114(17):7548-7552. · 4.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: Why are they magical? Mixed-metal clusters have very different electronic properties from simple metal clusters. The large MO splitting and reordering of Al clusters by Cs doping render the jellium description invalid and are responsible for the magic nature of the highly symmetric Al(12)Cs(-) and Al(11)Cs(2) (-) clusters (see picture).