2009 Vol. 22, No. 4

Letter
Content
2009, 22(4): 1-0. doi: 10.1088/1674-0068/22/4/1-0
Article
The growth and thermal stability of Au clusters on a partially-reduced rutile TiO2(110)-1×1 surface were investigated by high-resolution photoelectron spectroscopy using synchrotron- radiation-light. The valence-band photoelectron spectroscopy results demonstrate that the Ti3+3d feature attenuates quickly with the initial deposition of Au clusters, implying that Au clusters nucleate at the oxygen vacancy sites. The Au4f core-level photoelectron spectroscopy results directly prove the existence of charge transfer from oxygen vacancies to Au clusters. The thermal stability of Au clusters on the partially-reduced and stoichiometric TiO2(110) surfaces was also comparatively investigated by the annealing experiments. With the same film thickness, Au clusters are more thermally stable on the partially-reduced TiO2(110) surface than on the stoichiometric TiO2(110) surface. Meanwhile, large Au nanoparticles are more thermally stable than fine Au nanoparticles.
Resonance Raman spectra of aggregated meso-tetra(4-pyridinium)porphyrin diacid (H8TPyP6+)were studied with excitation near the exciton absorption bands of 470 nm.The UV-Vis absorption and resonance light scattering spectra of H8TPyP6+ monomers and aggregates were also measured. The observed Raman bands of monomeric and aggregated H8TPyP6+ were assigned on the basis of the observed deuteration shifts and by comparing with the Raman spectra of analogous porphyrin diacids. Aggregation causes moderate downshifts(2~6 cm-1) for high-frequency modes involving the in-plane CC/CN stretches of the porphyrin core and a dramatic upshift (12 cm-1) for the out-of-plane saddling mode of the porphyrin ring. The structural changes induced by aggregation and the possible hydrogen bonding interaction between the H8TPyP6+ molecules in the aggregate are discussed based on the spectral observations.
Nitrogen hydrate samples were synthesized using liquid nitrogen and powder ice at 16 MPa and 253 K. Confocal laser Raman spectroscopy was used to investigate the characteristics of nitrogen clathrate hydrates. The results show that the Raman peaks of N-N and O-H stretching vibration in nitrogen hydrates are observed at 2322.4 and 3092.1 cm-1, respectively, which are very similar to those in natural air clathrate hydrates. For comparison, we measured the Raman peaks of N-N stretching vibration both in liquid nitrogen and nitrogen molecules saturated water, which appear at 2326.6 and 2325.0 cm-1, respectively. The Raman spectroscopic observations on the dissociation process suggest that nitrogen molecules occupy both the large and small cages in nitrogen clathrate hydrates. However, only one Raman peak is observed for N-N stretching vibration because the difference of the environment of nitrogen molecules between large and small cages is too small to be differentiated by Raman spectroscopy.
The feed forward loop (FFL), wherein a gene X can regulate target gene Z alone or cooperatively with gene Y, is one of the most important motifs in gene regulatory networks. Gene expression often involves a small number of reactant molecules and thus internal molecular fluctuation is considerable. Here we studied how an FFL responds to small external signal inputs at gene X, with particular attention paid to the fluctuation resonance (FR) phenomenon of gene Z. We found that for all coherent FFLs, where the sign of the direct regulation path from X to Z is the same as the overall sign of the indirect path via Y, the FR shows a regular single peak, while for incoherent FFLs, the FR exhibits distinct bimodal shapes. The results indicate that one could use small external signals to help identify the regulatory structure of an unknown FFL in complex gene networks.
A correlation equation between the UV absorption wavenumbers of 1,4-disubstituted benzenes and the excited-state substituent constant was obtained. For 80 sorts of 1,4-disubstituted benzenes, the correlation coefficient was 0.9805, and the standard deviation was only 672.27 cm-1. The results imply that the excited-state substituent constant can be used productively for research on UV energy of 1,4-disubstituted benzenes. The present method provides a new avenue to study the UV absorption spectra of aromatic systems with the excited-state substituent constant, and it is helpful to understand the effect of substituent electrostatic effects on the chemical and physical properties of conjugated compounds with multiple substituents in excited state.
Low-lying electronic states of the lutetium dimer (Lu2) were studied based on density functional theory (DFT) using ten different density functionals together with three different relativistic effective core pseudopotentials (RECPs). Relative state energies, equilibrium bond lengths, vibrational frequencies, and ground-state dissociation energies were evaluated.It was found that the ground state is a triplet state irrespective of the type of functional and RECP used. This result is in contrast with a previous DFT calculation which gave a singlet ground state for Lu2. By comparing with the high-level ab initio and available experimental results, it is evident that the hybrid-GGA functionals combined with the Stuttgart small-core RECP yield the best overall agreement for the properties under study. The effects of Hartree-Fock exchange in B3LYP functional on the calculated bond length and dissociation energy of the ground state were examined, and rationalized in terms of 5d participation in Lu-Lu covalent bonding.
Boron is an element that has ability to build strong and highly directional bonds with boron itself. As a result, boron atoms form diverse structural motifs, ultimately can yield distinct nano structures, such as planar, quasi-planar, convex, cage, open-cage, tubular, spherical, ring, dome-like, shell, capsule, and so on, i.e., it can take almost any shape. Therefore, a deep understanding of the physical and chemical properties becomes important in boron cluster chemistry. Electronic and geometric structures, total and binding energies, harmonic frequencies, point symmetries, charge distributions, dipole moments, chemical bondings and the highest occupied molecular orbital-owest unoccupied molecular orbital energy gaps of neutral Bn (n=13~20) clusters have been investigated by density functional theory (DFT),B3LYP with 6-311++G(d,p) basis set. Furthermore, the first and the second energy differences are used to obtain the most stable sizes. We have observed that almost all physical properties are size dependent, and double-ring tubular form of B20 has the highest binding energy per atom. The icosahedral structure with an inside atom is found as impossible as a stable structure for the size thirteen. This structure transforms to an open-cage form. The structural transition from two-dimensional to three-dimensional is found at the size of 20 and consistent with the literature. The calculated charges by the Mulliken analysis show that there is a symmetry pattern with respect to the x-z and y-z planes for the charge distributions. The unusual planar stability of the boron clusters may be explained by the delocalized Π and α bonding characteristic together with the existence of the multicentered bonding. The results have been compared to available studies in the literature.
Electronic structure and optical properties of neutral and charged low band gap alternating copoly fluorenes (Green 1, which is based on alternating repeating units consisting of alkyl-substituted fluorene and a thiophene-[1,2,5]thiadiazolo-[3,4]quinoxaline-thiophene (T-TDQ-T) unit were investigated theoretically with time-dependent density functional theory (TD-DFT) method, and their excited state properties were further analyzed with 2D site and 3D cube representations. For neutral Green 1, the band gap, binding energy, exciton binding energy, and nuclear relaxation energy were obtained. The transition dipole moments of neutral and charged Green 1 are compared using 3D transition density, which reveals the orientation and strength of transition dipole moments. The charge redistribution of neutral and charged Green 1 upon excitation are displayed and compared with 3D charge difference density. The electron-hole coherences of neutral and charged Green 1 upon excitation are investigated with 2D site representation (transition density matrix). The excited state properties of neutral Green 1 calculated with TD-DFT method are compared with that calculated with ZINDO method, which reveals the importance of electron-electron interaction (in TD-DFT) in the excited state properties.
Density functional theory calculations were performed to study the structures and relative stability of the gadolinium complexes, Gd(H2O)n3+ (n=8,9), in vacuo and in aqueous solution. The polarizable continuum model with various radii for the solute cavity was used to study the relative stability in aqueous solution. The calculated molecular geometries for n=8 and 9 obtained in vacuo are consistent with those observed in experiments. It was found that while the nona-aqua complex is favored in the gas phase, in aqueous solution the octa-aqua conformation is preferred. This result, independent of the types of cavities employed, is in agreement with the experimental observation. The reliability of the present calculation was also addressed by comparing the calculated and experimental free energy of hydration, which revealed that the UA0, UAHF, and UAKS cavities are most appropriate when only the first solvation shell is treated explicitly.
We analyze for the first time the rules of breaking in an X-palindrome between human and chimpanzee.Results indicate that although the overall changes that occurred in the human X-palindrome are fewer than in the chimpanzee, mutations occurring between the left arm and right arm were nearly equivalent both in human and chimpanzee when compared with orangutan, which implies evolutionary synchronization.However, there are many moreA/T→G/C changes than G/C→A/T in a single arm, which would lead to an increasing trend in GC content and suggest that the composition is not at equilibrium. In addition,it is remarkable to find that there are much more asymmetrical nucleotide changes between the two arms of the human palindrome than that of the chimpanzee palindrome, and these mutations are prone to occur between bases with similar chemical structures. The symmetry seems higher in the chimpanzee palindrome than in the human X-palindrome.
The crystal structures and electronic transport properties of composites, xLa5/8Ca3/8MnO3:ErMnO3:(1-x)ErMnO3(x=0, 0.2, 0.4, 0.5, 0.6, 0.8, 1), where ErMnO3 is the insulating ferroelec-tric and La5/8Ca3/8MnO3 is the metallic ferromagnetic component, were studied. The magnetization of samples (x=0, 0.5, 1) were also measured as a function of temperature from 4 K to 300 K. The X-ray diffraction data show La5/8Ca3/8MnO3 and ErMnO3 are almost complete immiscible, originating from the significant difference in their crystal structures. All the composites show a metal-insulator transition when the molar fraction of xLa5/8Ca3/8MnO3 component x≥0.4 and the electronic transport behaviors follow the classical percolation theory model very well. Magnetization of the mixture with x=0.5 is unique and enhanced greatly compared with that of pure ErMnO3. Comprehensive analysis of the electronic transport and magnetic results suggests that this material system is a new kind of multiferroic with stronger magnetism in a wider temperature range compared with the single phase multiferroic ErMnO3.
Carbon nanotubes were coated with a layer of nickel-cobalt-phosphorus (Ni-Co-P) alloy with different compositions of Ni/Co through electroless plating. The effects of the concentration ratio of Co2+to Ni2+, bath temperature, and pH on deposition rate are discussed. The prepared carbon nanotubes covered with Ni-Co-P were characterized and analyzed by field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy dispersive spectroscopy, and a vibrating sample magnetometer. The results show that the deposition rate reached the maximum when the concentration ratio of Co2+to Ni2+ is 1 and the pH is 9; the deposition rate increases with the increase of bath temperature. The measurements of the magnetic properties of the obtained carbon nanotubes covered with Ni-Co-P indicate that the magnetic properties greatly depend on the concentration ratio of Co2+to Ni2+and the magnetic saturation reaches the maximum value when the Co2+to Ni2+ ratio is 1. In addition, there are two peaks in the coercivity curve at Co2+to Ni2+ ratios of 1/2 and 4/1, while the two peaks in the magnetic conductivity curve are located atCo2+to Ni2+ ratios of 1/4 and 4/1.
A storage and emission functional material of [Ca24Al28O64]4+·(Cl-)3.80(O2-)0.10(C12A7-Cl-), was prepared by the solid-state reactions of CaCO3, γ-Al2O3, and CaCl2 in Cl2/Ar mixture atmosphere. The anionic species stored in the C12A7-Cl- material were dominated by Cl-, about (2.21±0.24)×1021 cm-3, accompanied by a small amount of O2-, O-,, and O2-, measured via ion chromatography, electron paramagnetic resonance, and raman spectra measurements. These results also corroborate identification of time-of-flight mass spectroscopy—the anionic species emitted from the C12A7-Cl- surface were dominated by the Cl- (about 90%) together with a small amount of O-and electrons. The structure and morphological alterations of the material were investigated via X-ray diffraction and field emission scanning electron microscope, respectively.
Titania-based composite catalysts were prepared through a sol-gel route employing multiwalled carbon nanotubes with different diameters. The materials were characterized using thermogravimetric analysis, nitrogen adsorption-desorption isotherm, powder X-ray diffraction, scanning electron microscopy, and diffuse reflectance UV-Vis absorption spectra. The application of the catalysts to photocatalytic degradation of phenol was tested under UV-Vis irradiation. A synergetic effect on phenol removal was observed in case of composite catalysts, which was evaluated in terms of apparent rate constant, total organic carbon removal and photonic efficiency.
A porous NiO/yttria-stabilized zirconia anode substrate for tubular solid oxide fuel cells was prepared by gel casting technique. Nano-scale samaria-doped ceria (SDC) particles were formed onto the anode substrate to modify the anode microstructure by the impregnation of solution of Sm(NO3)3 and Ce(NO3)3. Electrochemical impedance spectroscopy, current-voltage and current-powder curves of the cells were measured using an electrochemical workstation. Scanning electron microcopy was used to observe the microstructure. The results indicate that the stability of the performance of the cell operated on humidified methane can be significantly improved by incorporating the nano-structured SDC particles, compared with the unmodified cell. This verifies that the coated SDC electrodes are very effective in suppressing catalytic carbon formation by blocking methane from approaching the Ni, which is catalytically active towards methane pyrolysis. In addition, it was found that a small amount of deposited carbon is beneficial to the performance of the anode. The cell showed a peak power density of 225 mW/cm2 when it was fed with H2 fuel at 700 °C, but the power density increased to 400 mW/cm2 when the fuel was switched from hydrogen to methane at the same flow rate. Methane conversion achieved about 90%, measured by gas chromatogram with a 10.0 mL/min flow rate of fuel at 700°C. Although the carbon deposition was not suppressed absolutely, some deposited carbon was beneficial for performance improvement.
We demonstrate surface enhanced Raman scattering (SERS) detection of self-assembled nano silver film using a low-cost electrolysis strategy at a proper voltage and silver nitrate concentration in electrolyte. The concentration dependence of SERS from crystal violet (CV) molecules adsorbed to silver film was systematically studied. Importantly, the SERS surface enhancement factor of such nano silver film was 603, which was measured by a portable Raman spectrometer. The minimum concentration of detectable CV molecules can be as low as 10-11 mol/L. The nano silver film prepared by this electrolysis method is an active, stable, cost-effective, and reusable SERS substrate.
Morphological evolution of Pd nanoparticles was studied in a solution-phase synthesis using cetyltrimethylammonium bromide (CTAB) and CTAB/sodium citrate mixture as capping agents, respectively. The morphological diversity of Pd nanoparticles is the combined effect of different Pd twinned seeds formed in the nucleation stage and selectively enlarging one set of crystallographic facets in the growth stage, both of which can be affected by the con-centrations of CTAB. Through changing the concentrations of CTAB and sodium ascorbate,Pd nanoparticles with different shapes were obtained. When citrate ions were introduced to manipulate the nucleation and growth process, star-shaped icosahedra and nanorods with pentagram cross-sections were obtained. Pd nanoparticles with different shapes have quite different surface plasmon resonance and surface-enhanced Raman scattering properties.