Transition metal-metal oxide hybrids as versatile materials for hydrogen storage

Metal atom located on metal oxide (MMO) is a promising material with various applications such as hydrogen storage. As one of the metal oxides, niobium oxide (NbO) presents fascinating properties that make it a possibly applicable in MMOs. Here, we investigated the feasibility of transition metal-NbO hybrids as MMO materials for application in the hydrogen storage technology. In this respect, the hydrogen adsorption of transition metals (Fe, Ni, Cu, Pd, Ag, and Pt) decorated on the NbO nanocluster has been explored using density functional theory calculations. We found that the adsorption energy of the H₂ molecule on the NbO adsorbent is remarkably increased by locating the transition metals on the NbO metal oxide. Our results reveal that the transition metals decorated on the NbO nanocluster can act as active sites for hydrogen adsorption. Among the studied transition metals, Pt shows the highest hydrogen capacity up to 6.52 wt%.     

On the improvement of signal repeatability in laser-induced air plasmas

The relatively low repeatability of laser-induced breakdown spectroscopy (LIBS) severely hinders its wide commercialization. In the present work, we investigate the optimization of LIBS system for repeatability improvement for both signal generation (plasma evolution) and signal collection. Timeintegrated spectra and images were obtained under different laser energies and focal lengths to investigate the optimum configuration for stable plasmas and repeatable signals. Using our experimental setup, the optimum conditions were found to be a laser energy of 250 mJ and a focus length of 100 mm. A stable and homogeneous plasma with the largest hot core area in the optimum condition yielded the most stable LIBS signal. Time-resolved images showed that the rebounding processes through the air plasma evolution caused the relative standard deviation (RSD) to increase with laser energies of > 250 mJ. In addition, the emission collection was improved by using a concave spherical mirror. The line intensities doubled as their RSDs decreased by approximately 25%. When the signal generation and collection were optimized simultaneously, the pulse-to-pulse RSDs were reduced to approximately 3% for O(I), N(I), and H(I) lines, which are better than the RSDs reported for solid samples and showed great potential for LIBS quantitative analysis by gasifying the solid or liquid samples.     

(AlB2)m (m=1~6) 团簇的几何结构和电子性质

采用密度泛函理论(DFT)中的B3LYP方法得到了(AlB2)m团簇的平衡几何结构. 计算并分析了基态掺杂团簇的平均结合能、电离势、能隙和前线分子轨道. 结果表明：掺杂团簇(AlB2)m (m=1~6)整体上具有较高化学活性,(AlB2)5团簇具有金属特征. Al原子总是向团簇外围扩散并且以配位数较少的方式与主团簇结合,团簇表现出以AlB2分子为基元生长的迹象. B-Al键长大于B-B键长. 电荷总是从Al原子转移到B原子. (AlB2)m团簇中B原子的2p轨道在成键中起主要作用,并使(AlB2)m团簇趋于形成离域π键.     

Segregation of Pt at clean surfaces of (Pt, Ni)3Al

Experimental evidence for surface segregation of Pt at (1 1 1) surfaces of ternary (Pt, Ni)₃Al alloys is presented, based upon Auger electron spectroscopy, low energy ion scattering, and angle-resolved X-ray photoelectron spectroscopy. Density functional calculations in the dilute limit confirm that Pt segregation is energetically favored.     

Ab initio simulation of the BaZrO3 (0 0 1) surface structure

Electronic and atomic structures of different terminations of the (0 0 1) non-polar orientation of BaZrO₃ surfaces have been studied using first-principles calculations. We found that surface energies at both possible surface terminations, BaO and ZrO₂, were very close. The (0 0 1)-BaO and (0 0 1)-ZrO₂ terminated surfaces have bandgap values smaller than that of a bulk BaZrO₃ crystal. In addition, the relative surface stability has been analyzed as a function of chemical environment.     

KLL Auger resonant Raman transition in metallic Cu and Ni

High energy resolution KL₂₃L₂₃ Auger spectra of polycrystalline Cu and Ni were measured using photon energies up to about 50 eV above the K-absorption edge and down to 5 eV (Cu KLL) and 4 eV (Ni KLL) below threshold. The spectra show strong satellite structures varying considerably as a function of the photon energy. In the sub-threshold region the linear dispersion of the diagram line energy positions and a distortion of the line shape as a function of photon energy, attributable to the Auger resonant Raman process, is clearly observed, indicating the one-step nature of the Auger emission. These changes in the resonant spectra are interpreted using a simple model based on resonant scattering theory in combination with partial density of states obtained from cluster molecular orbital (DV-Xα) calculations.     

Many body effects in the electronic and optical properties of the (1 1 1) surface of diamond

The (1 1 1) face is the cleavage surface of diamond. Understanding its properties is very important for the growing technological interest on the chemistry of diamond surfaces. Within DFT the most stable reconstruction is the Pandey chain model, the atoms on the chain being neither buckled nor dimerised. However this geometry gives rise to a semi metallic band structure in contrast with experimental findings that show the presence of a gap ranging from 0.5 to 2 eV. Here we show that the same equilibrium geometry and thus the same metallic band structure is found relaxing the surface using screened exchange (sX) or Hartree-Fock (HF) functionals. We will discuss in detail how breaking the equivalence of the atoms on the chain affects the band structure and we will show that a buckling would yield a semiconducting surface, but is energetically unfavorable. A semiconducting character can be restored, within the equilibrium geometry, if quasiparticle corrections are carefully included within an iterative GW scheme. The result of the theoretical reflectance anisotropy spectra (RAS) at a DFT-RPA level are also presented and discussed. As expected, a strong anisotropy signal is found at low energies due to transitions between surface states inside the fundamental gap.     

NO dissociation pathways on Rh(1 0 0), (1 1 0), and (1 1 1) surfaces: A comparative density functional theory study

The chemisorption and dissociation pathways of NO on the Rh(1 0 0), (1 1 0), and (1 1 1) surfaces are studied by the plane-wave density functional theory (DFT) with CASTEP program. In addition, the electronic and geometrical effects that affect the NO dissociation reactions have been investigated in detail. The calculation results are presented as following: The effective activation energies of the best NO dissociation pathways on the Rh(1 0 0), the Rh(1 1 0), and the Rh(1 1 1) are 0.63, 0.66 and 1.77 eV, respectively. The activity of the Rh planes for NO dissociation is in the order of Rh(1 0 0) ≈ Rh(1 1 0) > Rh(1 1 1). The low dissociation barrier for Rh(1 0 0) and Rh(1 1 0) is associated with the existence of a lying-down NO structure which acts as a precursor for dissociation. By Mulliken population analysis and structure analysis, both electronic and geometrical effects are found to affect the NO dissociation reactions, but the geometrical effect exceed the electronic. The energy decomposition scheme has been used to provide further insight into the NO dissociation reactions. Based on the calculations, the interaction energy between N and O in the transition state on the Rh(1 1 1) is found much larger than that on the Rh(1 0 0) and the Rh(1 1 0). The major differences of should originate from the variation of the bonding competition effect.     

Chemisorption of 1,1-dichloroethene on the Si(1 1 1)-7 × 7 surface

Chemisorption of 1,1-dichloroethene (Cl₂CCH₂) to a Si(1 1 1)-7 × 7 surface was studied by means of X-ray photoelectron spectroscopy using synchrotron radiation, recording chlorine 2p and carbon 1s spectra. For carbon 1s, spectral assignment of the chemisorbed species is based on quantum chemical calculations of chemical shifts in model compounds.The results confirm the identity of covalently bonded 1-chlorovinyl (-CClCH₂) and vinylidene (CCH₂) adspecies. Upon chemisorption at room temperature it was found that about one-third of the molecules break one C-Cl bond while about two-thirds of the adsorbates break two C-Cl bonds. We do not, however, find evidence for isomerization of CCH₂ to di-bonded vinylene (-CHCH-).     

A density functional theory study on the adsorption and dissociation of N2O on Cu2O(1 1 1) surface

Density functional theory has been employed to investigate the adsorption and the dissociation of an N₂O at different sites on perfect and defective Cu₂O(1 1 1) surfaces. The calculations are performed on periodic systems using slab model. The Lewis acid site, Cu_CUS, and Lewis base site, O_SUF are considered for adsorption. Adsorption energies and the energies of the dissociation reaction N₂O → N₂ + O(s) at different sites are calculated. The calculations show that adsorption of N₂O is more favorable on Cu_CUS adsorption site energetically. Cu_CUS site exhibits a very high activity. The Cu_CUS-N₂O reaction is exothermic with a reaction energy of 77.45 kJ mol⁻¹ and an activation energy of 88.82 kJ mol⁻¹, whereas the O_SUF-N₂O reaction is endothermic with a reaction energy of 205.21 kJ mol⁻¹ and an activation energy of 256.19 kJ mol⁻¹. The calculations for defective surface indicate that O vacancy cannot obviously improve the catalytic activity of Cu₂O.