Bond Length Contraction Research Articles

Role of gold nanoclusters supported on TiO2(110) model catalyst in CO oxidation reaction

It was reported previously that O adatoms adsorbed dissociatively on the five-fold Ti rows of rutile TiO2(110) made the surface O-rich and reacted with CO molecules to form CO2. An electronic charge transfer taking place from gold nanoclusters to the O-rich TiO2(110) support played a crucial role to enhance the catalytic activity [Mitsuhara et al., J. Chem. Phys. 136, 124303 (2012)]. In this study, the authors have further accumulated experimental data for the CO oxidation reaction enhanced by gold nanoclusters on the TiO2(110) surface. Based on the results obtained here and previously, the authors propose an “interface dipole model,” which explains the strong activity of Au nanoclusters supported on O-rich TiO2(110) in CO oxidation reaction. Simultaneously, the authors also discuss the cationic cluster model proposed by Wang and Hammer [Phys. Rev. Lett. 97, 136107 (2006)] and the d-band model predicted by Hammer and Nørskov [Adv. Catal. 45, 71 (2000)]. The latter is, in particular, widely accepted to explain the activities of heterogeneous catalysts. Contrary to the d-band model, our ab initio calculations demonstrate that the d-band center for Au nanoclusters moves apart from the Fermi level with decreasing the cluster size and this is due to contraction of the Au-Au bond length.

Measuring Surface Atom Bond Length Contraction in Au and Pt Nanoparticles Using High-Precision STEM Imaging

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

CdS nanopowder and nanofilm: Simultaneous synthesis and structural analysis

Chemical bath deposition technique has been used for simultaneous synthesis of CdS nanopowder and nanofilm. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) studies show a predominant α-CdS structure. Structural parameters show a random lattice disorder and bond length contraction leading to high density of nanopowder and nanofilm in comparison to standard CdS. The columnar structure growth of the nanofilm devoid of grain boundaries may be explored for window layers.

On structure and bonding of lanthanoid trifluorides LnF3 (Ln = La to Lu)

The trends in the series of lanthanoid (lanthanide) trifluoride molecules LnF3 (Ln = La to Lu) are governed by the valence-active Ln(4f,5d,5p,6s) shells. The series is investigated by quasi-relativistic density functional theory at both the scalar and spin-orbit-coupled levels. Integrating many of the previous experimental and theoretical deductions, we obtain the following comprehensive picture: (1) The comparatively small Ln-F bond length contraction of 14 pm from La to Lu is rather smooth but weakly modulated by spin-orbit coupling. (2) From La to Lu the floppy structure becomes more quasi-planar. (3) The heterolytic LnF bond energies (⅓LnF3→⅓Ln(3+) + F(-)) at the spin-orbit averaged level increase smoothly from 15.3 to 16.3 eV for La to Lu, only the 'divalent' lanthanoids Eu and Yb are outliers with 0.2 eV higher bond energies. (4) The homolytic LnF bond energies (⅓LnF3→⅓Ln + F) however show an overall W-shaped double-periodicity with maxima for LaF3, GdF3 and LuF3, decreasing from La to Eu and from Gd to Yb, the large individual variations being caused by different spin-orbit coupling and Coulomb interaction effects in Ln(0) and LnF3. (5) The Ln-F interaction is basically ionic (increasing with decreasing ionic radii) with some dative Ln(3+)← F(-) bonding. (6) The latter is of the Ln(5d)-F(2p) type with a rather constant bond order from La to Lu, with small Ln(5p) and very small Ln(4f) semi-core contributions decreasing from La to Lu. All these trends are rationalized.

Photohole-oxidation-assisted anchoring of ultra-small Ru clusters onto TiO2 with excellent catalytic activity and stability

Ultra-small metal clusters with good activity and stability are of great significance for their practical applications in catalysis and materials science. Here we report a photohole-oxidation-assisted approach for anchoring ultra-small Ru clusters (∼1.5 nm) with an extremely high density (∼1017 m−2) onto TiO2 support. The resulting clusters have good thermal stability and exhibit excellent long-term catalytic activity for the hydrogenation of CO2 to methane (methanation). The anchoring process involves the oxidation of Ru3+ in solution by photogenerated holes on the TiO2 surface to give tiny RuO2 species (<0.8 nm) immobilized on the surface, followed by a H2 reduction step to produce Ru0 clusters. Aberration-corrected high-resolution transmission electron microscopy (Cs-HRTEM) observations identify the Ru–Ru bond length contraction at the metal surface (relative to the interior of the particle) as well as bond length changes in the defect region at the metal–support interface. Density functional theory (DFT) calculations further demonstrate that the ultra-small Ru clusters are well stabilized and tightly anchored onto the TiO2 substrate via Ru–O covalent bonding in the defect region of the metal–support interface. The high-dispersion of ultra-small Ru clusters as well as the strong chemical bonding at the interface account for their surprisingly high catalytic reactivity and excellent thermal/reaction stability. This synthetic method may open up a new way to fabricate thermally stable ultra-small metal clusters for practical industrial applications in catalysis.

Tuning the electronic and optical properties of hydrogen-terminated Si nanocluster by uniaxial compression

The structural, electronic, and optical properties of hydrogen-terminated Si nanocluster (Si66H64) with a diameter of 1.3 nm under uniaxial compression have been investigated by means of density functional theory calculations. The structural deformation of silicon nanoparticle under axial strain manifests as reduction of cluster symmetry, contraction of bond length, and broadening of bond angle distribution. Such strain-induced distortion modifies the highest occupied molecular orbital (HOMO) the lowest unoccupied molecular orbital (LUMO) eigenvalues, HOMO–LUMO gap, and isosurfaces of HOMO and LUMO wavefunctions, that is, the HOMO–LUMO gap diminishes as strain increases and isosurface of HOMO and LUMO wavefunctions redistributes along the strain orientation. Moreover, uniaxial compression has a strong influence on the optical absorption spectra of the Si66H64 cluster. With increasing strain, the onset of absorption spectra red shifts. Interestingly, the strain-tunable photoluminescence in Si nanoparticle (Si66H64) can cover a broad spectrum (i.e., from visible light to ultraviolet), implying an exciting possibility for optical devices.

All-electron scalar relativistic calculation of the adsorption of NCO species onto small copper clusters

An all-electron scalar relativistic calculation of CunNCO (n = 1–13) clusters has been made using density functional theory with the generalized gradient approximation at BLYP level. In all CunNCO clusters, the NCO species prefered to occupy the single-fold coordination site and the small copper cluster tended to bond with the nitric. The Cun structures were only distorted slightly and the NCO species retained linear structure. The N-C bond-length contraction and C-O bond-length elongation were observed clearly. The reactivity enhancement of NCO species toward CO2 was obvious. But, no reactivity enhancement of NCO to form N2 related to the N-C bond strength could be observed. It seems that the NCO species is more favorably adsorbed by odd-numbered small copper clusters, relatively. Some discrepancies between our work and earlier works were found which may be explained in terms of the scalar relativistic effect. Further studies to focus on the reactivity enhancement of NCO to form N2 are clearly in order.

Stability of the Bulk Phase of Layered ZnO

Recently a novel phase of ZnO has been synthesized which is analogous to α-boron nitride, although more three dimensional, and consists of planar hexagonal sheets of ZnO. Examining the dynamic stability of the structure, we find unstable phonon modes over a considerable part of the Brillouin zone. Local-density approximation (LDA) and generalized gradient approximation level calculations have usually been able to predict the structural stability of s-p bonded systems. The failure in the present case is a surprise and is traced to the self-interaction error which incorrectly locates the localized Zn d states in the valence band of ZnO. Correcting for this with a Hubbard-like U on the Zn d states, the optimized structure is predicted to be stable. This highlights the fact that the large bond length contraction that one finds in going from sp(3)- to sp(2)-type bonding results in an increased necessity to correct for self-interaction errors.

First-principles study on MRh12 (M = Rh, Fe, Co, AND Ni) clusters

In this paper, a first-principles study on the stability, electronic and magnetic properties of MRh12 (M = Rh, Fe, Co and Ni) clusters is performed. By optimizing the geometrical structure, we find that MRh12 clusters change from a perfect icosahedron to a distorted structure and have an obvious bond length contraction as compared with the corresponding bulk phase; FeRh12, CoRh12, and NiRh12 clusters are more energetically stable than the RhRh12 cluster. The effect of the impurity M on the density of states, valence band width, HOMO and LUMO for MRh12 clusters is not significant, but when the central Rh atom is substituted with M, the magnetic moment of MRh12 reduces dramatically. The Mulliken population analysis indicates that there are more charge transfers from other orbitals to Rh4d and M3d orbitals, and the spd hybrid effect in d orbitals of MRh12 clusters is stronger than that in the RhRh12 cluster. this situation means that the unpaired d electrons have more chance to be paired, and the magnetic moments of MRh12 clusters can be reduced reasonably.

Electronicd-band properties of gold nanoclusters grown on amorphous carbon

The electronic d-band properties are important factors for the emerging catalytic activity of Au nanoclusters of sub-5-nm size. We analyzed the d-band properties of Au nanoclusters grown on amorphous carbon supports by photoelectron spectroscopy using synchrotron-radiation light coupled with high-resolution ion scattering spectrometry which enables us to estimate the size and shape of Au nanoclusters. The d-band width (W{sub d}), d-band center position (E{sub d}), and apparent 5d{sub 3/2}-d{sub 5/2} spin-orbit splitting (E{sub SO}) were determined as a function of a number of Au atoms per cluster (n{sub A}) and an average coordination number (n{sub C}) in a wide range (11<n{sub A}<1600). The W{sub d} and E{sub SO} values decrease steeply with decreasing n{sub A} below {approx}150 owing to band narrowing which is caused by hybridization of fewer wave functions of the valence electrons. However, E{sub d} shifts to the higher binding energy side with decreasing cluster size. The rapid movement of E{sub d} is attributed to the dynamic final-state effect, which results in higher binding energy shifts of core and valence states due to a positive hole created after photoelectron emission. We have estimated the contribution from the final-state effect and derived the approximated initial-state spectra. Modified data, however,more » still show a slight movement of the d-band center away from the Fermi level (E{sub F}) although the E{sub d} values for Au nanoclusters are closer to E{sub F} compared to the bulk value. This behavior is ascribed to the contraction of average Au-Au bond length with decreasing cluster size.« less

Octahedral complexes of the series of actinides hexafluorides AnF 6

Non-relativistic DFT (PW91, PBE, PB86) geometry optimizations followed by relativistic ZORA single point energy calculations on the neutral hexafluoride complexes of the series of actinides U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr in octahedral symmetry are presented. Bond lengths are in good agreement with available experimental data. Actinide bond length contraction as a function of the atomic number of the central metal atom of up to ≈ 4 % is observed. An approximate inverse relationship is predicted for the bonding energies as a function of the atomic number of the central cation at the two component ZORA//DFT level; pure DFT bonding energies do not exhibit the same pattern.

Characterization of a Tricationic Trigonal Bipyramidal Iron(IV) Cyanide Complex, with a Very High Reduction Potential, and Its Iron(II) and Iron(III) Congeners

Currently, there are only a handful of synthetic S = 2 oxoiron(IV) complexes. These serve as models for the high-spin (S = 2) oxoiron(IV) species that have been postulated, and confirmed in several cases, as key intermediates in the catalytic cycles of a variety of nonheme oxygen activating enzymes. The trigonal bipyramidal complex [Fe(IV)(O)(TMG(3)tren)](2+) (1) was both the first S = 2 oxoiron(IV) model complex to be generated in high yield and the first to be crystallographically characterized. In this study, we demonstrate that the TMG(3)tren ligand is also capable of supporting a tricationic cyanoiron(IV) unit, [Fe(IV)(CN)(TMG(3)tren)](3+) (4). This complex was generated by electrolytic oxidation of the high-spin (S = 2) iron(II) complex [Fe(II)(CN)(TMG(3)tren)](+) (2), via the S = 5/2 complex [Fe(III)(CN)(TMG(3)tren)](2+) (3), the progress of which was conveniently monitored by using UV-vis spectroscopy to follow the growth of bathochromically shifting ligand-to-metal charge transfer (LMCT) bands. A combination of X-ray absorption spectroscopy (XAS), Mössbauer and NMR spectroscopies was used to establish that 4 has a S = 0 iron(IV) center. Consistent with its diamagnetic iron(IV) ground state, extended X-ray absorption fine structure (EXAFS) analysis of 4 indicated a significant contraction of the iron-donor atom bond lengths, relative to those of the crystallographically characterized complexes 2 and 3. Notably, 4 has an Fe(IV/III) reduction potential of ∼1.4 V vs Fc(+/o), the highest value yet observed for a monoiron complex. The relatively high stability of 4 (t(1/2) in CD(3)CN solution containing 0.1 M KPF(6) at 25 °C ≈ 15 min), as reflected by its high-yield accumulation via slow bulk electrolysis and amenability to (13)C NMR at -40 °C, highlights the ability of the sterically protecting, highly basic peralkylguanidyl donors of the TMG(3)tren ligand to support highly charged high-valent complexes.

Au–Au bond length expansion in CTAB stabilized gold nanoparticles

The modifications of the electronic structure and the structural order with decreasing particle size are of great interest for the explanation of catalytic properties of gold nanoparticles. One of the issues is the predicted Au–Au bond length contraction with particle size decrease. For the purpose of the study, X-ray absorption spectroscopy (XAFS) has been chosen as a probe, since it provides information about both, the local coordination and the electronic structure of the absorbing element. We present an X-ray absorption spectroscopy study of gold nanoparticles of different sizes. The XAFS spectra of gold nanoparticles of 1.7, 5.7, 6.4 and 13.8 nm in radius, dispersed in liquid cetyl trimethylammonium bromide (CTAB) solution have been measured at the Au L 3-edge. The extended X-ray absorption fine structure (EXAFS) functions of the measured spectra were analyzed employing ab initio calculations. It was found that the average Au–Au distance is larger by 4.2(3) pm than this was found in bulk gold and there is no size-dependent trend recognizable concerning this value for the different particle radii examined.

Existence of both blue-shifting hydrogen bond and Lewis acid–base interaction in the complexes of carbonyls and thiocarbonyls with carbon dioxide

In this study, 16 gas phase complexes of the pairs of XCHZ and CO(2) (X = F, Cl, Br; Z = O, S) have been identified. Interaction energies calculated at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ level including both BSSE and ZPE corrections range from -5.6 to -10.5 kJ mol(-1) for XCHOCO(2) and from -5.7 to -9.1 kJ mol(-1) for XCHS···CO(2). Substitution of one H atom by one halogen in formaldehyde and thioformaldehyde reduces the interaction energy of XCHZ···CO(2), while a CH(3) substitution increases the interaction energy of both CH(3)CHO···CO(2) and CH(3)CHS···CO(2). NBO and AIM analyses also point out that the strength of Lewis acid-base interactions decreases going from >C1=S3···C6 to >C1=O3C6 and to >C1-X4···C6. This result suggests the higher capacity of solubility of thiocarbonyl compounds in scCO(2), providing an enormous potential application for designing CO(2)-philic materials based on the >C=S functional group in competition with >C=O. The Lewis acid-base interaction of the types >C=S···C, >C-Cl···C and >C-Br···C is demonstrated for the first time. The contribution of the hydrogen bonding interaction to the total interaction energy is larger for XCHS···CO(2) than for XCHO···CO(2). Upon complexation, a contraction of the C1-H2 bond length and a blue shift of its stretching frequency have been observed, as compared to the isolated monomer, indicating the existence of a blue-shifting hydrogen bond in all complexes examined. Calculated results also lend further support for the viewpoint that when acting as proton donor, a C-H bond having a weaker polarization will induce a stronger distance contraction and frequency blue shift upon complexation, and vice versa.

Bond length contraction in gold nanoparticles

The structure of nanoparticles typically differs from its bulk counterpart. Predominantly, the structures of gold nanoparticles have been under exceedingly intense discussion since the discovery of their high catalytic activity. We found an increasing bond length contraction with decreasing particle size for citrate-stabilized gold nanoparticles in aqueous solution as determined by in situ extended X-ray absorption fine structure (EXAFS) spectroscopy. Particle sizes and size distributions were determined by small-angle X-ray scattering. The analysis of the obtained EXAFS spectra employing ab initio calculations reveals that the Au-Au bond length undergoes a contraction of 2 pm for nanoparticles with a radius of 2.9 nm. NIST reference material RM 8011 gold nanoparticles with a radius of 4.4 nm exhibit a smaller contraction of approximately 1 pm. Finally, gold atoms in RM 8013 particles with a radius of 25.7 nm show distances of 288 pm--identical to the distance in gold foil--and exhibits bulk-like properties. The observed bond length contraction of gold nanoparticles in solution is significantly smaller than previously reported for gold nanoparticle deposited on surfaces, which is up to 15 pm. This indicates that the bond length contraction effect of "free" and "surface-immobilized" nanoparticles differ fundamentally. Such difference could be essential for the understanding of nanoparticle-supported catalysis.

Theoretical analysis of the (HNO)2, (HNO···HNS), and (HNS)2 dimers — A case of red and blue shifts of N–H stretching frequency

The hydrogen-bonded interactions in the simple (HNZ)2 dimers, with Z = O and S, were investigated using quantum chemical calculations with the second-order Møller–Plesset perturbation (MP2), coupled-cluster with single, double (CCSD), and triple excitations (CCSD(T)) methods in conjunction with the 6-311++G(2d,2p), aug-cc-pVDZ, and aug-cc-pVTZ basis sets. Six-membered cyclic structures were found to be stable complexes for the dimers (HNO)2, (HNS)2, and (HNO–HNS). The pair (HNS)2 has the largest complexation energy (–11 kJ/mol), and (HNO)2 the smallest one (–9 kJ/mol). A bond length contraction and a frequency blue shift of the N–H bond simultaneously occur upon hydrogen bond formation of the N–H···S type, which has rarely been observed before. The stronger the intramolecular hyperconjugation and the lower the polarization of the X–H bond involved as proton donor in the hydrogen bond, the more predominant is the formation of a blue-shifting hydrogen bond.

Preparation and photoluminescence properties of Ge nanocrystals embedded in SiO 2 matrices with Ge–GeO x core–shell structure

The core/shell structure of Ge nanocrystals embedded in SiO 2 matrix were prepared using the pulsed laser deposition (PLD) method with post-deposition annealing. X-ray diffractometry and Fullprof computer program revealed a bond length contraction of the Ge nanocrystals core in an annealed sample. This contraction was attributed to the compressive stress from the GeO x shell. The enhancement of photoluminescence was observed in the annealed sample, which is due to the Ge/O-related defects that existed in the interface between the Ge core and the GeO x shell.

Remarkable Blue Shifts of C−H and N−H Stretching Frequencies in the Interaction of Monosubstituted Formaldehyde and Thioformaldehyde with Nitrosyl Hydride

Weak interactions of monosubstituted formaldehydes and thioformaldehydes with nitrosyl hydride were investigated by using ab initio MO calculations at the MP2/aug-cc-pVTZ level. Thirty two equilibrium structures having different complex forms were located on the corresponding potential energy surfaces (all having C(s) symmetry). Obtained binding energies, which include both ZPE and BSSE corrections, range from 7 to 14 kJ x mol(-1) and 6 to 12 kJ x mol(-1) for complexes of substituted formaldehydes and thioformaldehydes, respectively. In each geometrical structure, the (XCHO,HNO) complex is consistently more stable than the (XCHS,HNO) complex. The H-bond strength significantly increases when one H atom is replaced by a methyl group in both formaldehyde and thioformaldehyde. When replacing H by a halogen atom, the binding energy tends to decrease. It is remarkable that all the C-H and N-H bonds are shortened upon complexation, resulting in an increase of their stretching frequencies. Furthermore, the blue shifts are consistently observed for the interacting N-H bonds in N-H...X, Z, with X = F, Cl, Br, and Z = O, S; such contraction of a covalent N-H bond is extremely rare. In addition, the N-H bond length contraction and its frequency blue shift in the N-H...S complex have been revealed for the first time.

Bond length contraction in cobalt nanoislands on Cu(001) analyzed by surface x-ray diffraction

We present a combined surface x-ray diffraction SXRD and theoretical analysis of the geometric structure of nanometer sized Co islands deposited on Cu001 at 170 K. Two-dimensional nanoislands consisting of only 20–40 atoms are characterized by a 4%–8% contraction of the interatomic distances as compared to the bulk 2.51 A. This strongly exceeds “usual” lattice relaxations normally seen for surfaces. The SXRD analysis is based on the analysis of the registry of the Co adatoms relative to the 1 1 surface unit cell of the Cu001 substrate crystal. Static displacements of the Co atoms of 0.18 A root mean square out of the equilibrium hollow sites are observed which are in agreement with predictions based on molecular-dynamics calculations.

SYNCHROTRON RADIATION CHARACTERIZATION OF COBALT NANOPARTICLES FORMED BY ION IMPLANTATION

Ion implantation of Co into silica layers and subsequent thermal annealing were used to form Co nanoparticles. Structural characterization was performed using X-ray Absorption Spectroscopy and Small Angle X-ray Scattering to determine, respectively, the local atomic structure and size distribution of the Co nanoparticles as a function of annealing temperature. For decreasing nanoparticle size, a bond-length contraction, an increase in Debye–Waller factor and a reduction in coordination number were observed. The bond-length contraction was related to the capillary pressure in the nanoparticles while an increase in Debye–Waller factor reflected an increase in disorder attributed to the high surface-area-to-volume ratio of small nanoparticles. X-ray Absorption Near Edge Structure spectroscopy revealed that the annealing conditions had a significant effect on the nanoparticle crystal structure and oxidised Co fraction.