Structure and Electron Transport at Metal Atomic Junctions Doped with Dichloroethylene

The front cover artwork was provided by the group of Prof. Nishino, Tokyo Institute of Technology. The image depicts the investigation of the structure and electron transport of the Au, Ag, Cu, Ni, Fe, and Pd atomic junctions doped with dichloroethylene. Read the full text of the Article at 10.1002/cphc.201900988 .     

Extractive concentration of platinum-group elements and their determination by atomic-absorption spectrophotometry

The extraction of Pt, Pd, Ir, Rh, Ru, Ag, Au, Co, Cu, Ni and Fe with n-octylaniline has been investigated. Noble metals are extracted 10(3)-10(4) times better than Cu, Ni, Co and Fe. A method of determination of Pt, Pd, Ir, Rh and Ru is proposed. They are first separated from Cu, Ni, Co and Fe by means of extraction (and then determined, in either the aqueous or organic phase, by atomic-absorption spectrophotometry. The atomic absorption of platinum metals (with the exception of Pd) is affected by other elements of the platinum group and by non-noble metals. La(NO(3))(3) and Nd(NO(3))(3) lower the limit of detection for Pt, Rh, Ir and Ru and inhibit the effect of Co, Cu, Ni, Fe, Bi, Zn, Na, etc. on their determination. Lanthanum and neodymium chlorides and sulphates produce a similar effect but only on the determination of Pt and Rh. The coefficient of variation of the determination, in both phases, is within 2-6.8%.     

Extending the capability of STM break junction for conductance measurement of atomic-size nanowires: an electrochemical strategy

The STM break junction (STM-BJ) and mechanically controllable break junction (MCBJ) are the two most widely applied techniques to fabricate atomic-size nanowires for conductance measurement. However, the drawbacks of the mechanical crashing between the two electrodes of the same material in these techniques hamper its capability of application in view of the variety of metals as well as the environment to perform the measurements. In this paper, we present an electrochemical strategy for STM-BJ by establishing a chemically well-defined metallic contact through a jump-to-contact mechanism between the tip and substrate of dissimilar metals, wherein the tip is in situ and electrochemically deposited with a thin film of a foreign metal of interest. The feasibility of the approach is demonstrated by taking Cu as a model system, followed by generalizing to Pd and Fe for which the conductance has been found otherwise difficult to measure at room temperature. The preferential point-contact conductance at 1, 0.9, and 0.86 G0 was measured for Cu, Pd, and Fe, respectively. The strategy present in this work not only extends the capability of STM-BJ to create a variety of metal nanowires including magnetic nanowires for further investigations but also provides opportunities to construct metal-molecule-metal junctions with a variety of choices of metals in the junctions.     

The relationship between adsorption energies of methyl on metals and the metallic electronic properties: a first-principles DFT study

A theoretical study of CH3 adsorbed on the (111) surface of some transition and noble metal surfaces M (M = Cu, Ni, Rh, Pt, Pd, Ag, Au) and on the Fe(100) is presented. We find that the hollow site is preferred more than the top one for Fe, Ni, Rh, and Cu, but it is the other way for Pt, Pd, Au, and Ag. In addition, a good linear relationship was observed between the chemisorption energy and d-band center for Group VIII metals or the square of the coupling matrix element for Group IB metals at the hollow site. Interestingly, with a detailed comparison of the adsorption energies at the top and hollow sites, we find that the adsorption energies among each group are very similar on the top site, which supports the theoretical model of Hammer and Norskov that the coupling between the HOMO of adsorbate and sp states of the metal is dominant and almost equal, and that the second coupling to the d-band contributes less but reflects the change of the adsorption energy. It confirms that the coupling to the d band comprises two opposite factors, that is, the d-band center was attractive and the square of the coupling matrix element was repulsive, such that the contributions from the two factors can counteract each other at the top site.     

Synthesis and characterization of volatile, thermally stable, reactive transition metal amidinates

A series of homoleptic metal amidinates of the general type [M(R-R'AMD)(n)](x) (R = (i)Pr, (t)Bu, R' = Me, (t)Bu) has been prepared and structurally characterized for the transition metals Ti, V, Mn, Fe, Co, Ni, Cu, Ag, and La. In oxidation state 3, monomeric structures were found for the metals Ti(III), V(III), and La(III). Bridging structures were observed for the metals in oxidation state 1. Cu(I) and Ag(I) are held in bridged dimers, and Ag(I) also formed a trimer that cocrystallized with the dimer. Metals in oxidation state 2 occurred in either monomeric or dimeric form. Metals with smaller ionic radii (Co, Ni) were monomeric. Larger metals (Fe, Mn) gave monomeric structures only with the bulkier tert-butyl-substituted amidinates, while the less bulky isopropyl-substituted amidinates formed dimers. The new compounds were found to have properties well-suited for use as precursors for atomic layer deposition (ALD) of thin films. They have high volatility, high thermal stability, and high and properly self-limited reactivity with molecular hydrogen, depositing pure metals, or water vapor, depositing metal oxides.     

Preconcentration of trace elements by partial dissolution of the matrix—multielement preconcentration from manganese

Trace elements such as Ag, Au, Bi, Cd, Co, Cu, Fe, Ga, In, Ni, Pb, Pd and Tl, can be preconcentrated with recoveries of better than 95% from high-purity manganese if the sample is coated with a thin layer of mercury before its dissolution in HCl to a small residue. For determination of the trace elements, the residue is completely dissolved in aqua regia. After separation of the mercury by reductive precipitation, Bi, Cd, Co, Cu, Fe, Ga, In, Ni, Pb, and Tl are determined by flame a.a.s. ('injection method'). Ag, Au and Pd are determined in the mercury-containing solution. The relative standard deviation was usually about 5%; Cu, Fe and Pb were proved to be inhomogeneously distributed in the sample (electrolytically produced manganese), and so the standard deviations were considerably greater. The detection limits for the different elements were between 0.6 and 0.004 ppm, depending on the sensitivity of their a.a.s. determination. The theoretical basis of this preconcentration method and its applicability to multielement analysis of different high-purity metals are discussed.     

Intrinsic stability and hydrogen affinity of pure and bimetallic nanowires

A density functional theory study of the intrinsic stability of pure and bimetallic wires is presented. Several bimetallic combinations forming one-atom thick wires are studied. An explanation for the experimental instability of Cu wires in contrast to the stability of Au and Ag wires is given, which relies on the higher surface energy of the former. All the possible intercalations between Ni, Pd, Pt, Cu, Ag, and Au are studied. The bimetallic wires AuCu and AuAg were found to be the most stable ones. The reactivity of the latter two systems is also examined using hydrogen adsorption as a microscopic probe. It was found that at the inter-metal interface, up to second neighbors, Cu and Ag become more reactive and Au becomes more inert than the corresponding pure wires. These results are explained within the d-band model.     

基于电化学方法研究以铜和银为电极的对苯二甲酸单分子结电导

利用基于电化学跳跃接触的扫描隧道显微镜裂结法(ECSTM-BJ), 通过现场形成金属电极, 对以Cu和Ag为电极的对苯二甲酸单分子结电导进行了测量. 研究结果表明: 利用该方法对所有数据直接线性统计即可得到很好结果; 两种电极下都存在两套高和低电导值, 其中以Cu为电极的单分子结电导高低值分别为11.5和4.0 nS, 而以Ag为电极的单分子结电导分别为10.3和3.8 nS, 高值都约为低值的3倍, 且以Cu为电极的单分子结电导要略大于以Ag为电极的电导, 可归结于电极和分子的耦合不同造成的. 与同样条件下测量得到的烷基链羧酸单分子结电导只存在一套值相比,对苯二甲酸表现出两套电导值, 反应了分子内主链对分子结电导的影响.     

Contact resistance in metal-molecule-metal junctions based on aliphatic SAMs: effects of surface linker and metal work function

Using conducting probe atomic force microscopy (CP-AFM), we have formed molecular tunnel junctions consisting of alkanethiols and alkane isonitrile self-assembled monolayers sandwiched between gold, platinum, silver, and palladium contacts. We have measured the resistance of these junctions at low bias (dV/dI |V=0) as a function of alkane chain length. Extrapolation to zero chain length gives the contact resistance, R0 . R0 is strongly dependent on the type of metal used for the contacts and decreases with increasing metal work function; that is, R0,Ag > R0,Au > R0,Pd > R0,Pt. R0 is approximately 10% smaller for Au junctions with isonitrile versus thiol surface linkers. We conclude that the Fermi level of the junction lies much closer to the HOMO than to the LUMO.     

Adsorption von Spuren von Insecticiden aus Wasser an Polyäthylen

Trace elements such as Bi, Cu, Ni, Pb, Ag, Au, and Pd that are present in high-purity cadmium can be enriched, if the cadmium is coated with a thin layer of mercury before dissolution in dilute nitric acid up to a small residue. For determination of the trace elements, the residue is completely dissolved in nitric acid. Bi, Cu, Ni, and Pb are determined after separation of the mercury; Ag, Au, and Pd are determined in the solution containing the mercury. The determination is made by atomic absorption spectroscopy. The following amounts can be determined (in ppm): Cu 0.02; Ag 0.05; Au 0.1; Bi, Ni, Pb, Pd 0.2.     

Retention and elution of precious metals on cyano-modified solid phase microparticle sorbent

We report on a study on the retention and elution of ions of the precious metals Au, Ir, Pd, Pt, Rh and Ru, sometimes in the presence of ions of the nonprecious elements Co, Cu, Fe, Ni, Pb and Te. A commercial cyano-modified microparticle-based solid phase was used as a sorbent and hydrochloric acid in various concentrations for sample solution and elution. Only Au and Pd (in the form of their chloro complexes) were retained, and Au is found to have a much higher affinity for the sorbent compared to Pd. In addition to the affinity of the metal towards the functional groups, the retention behavior of the precious metals seems to be mainly influenced by steric factors of their respective chloro complexes. Elution with 7.5 M HCl does not require the addition of organic eluent as required in other cases. The method can separate Au or both Au and Pd, from hydrochloric acid solutions containing ions of other precious metals or nonprecious elements. The sorbent was applied to recover gold from a mineral digest containing large amounts of metals such as Fe, Pb, Te, Cu, Ni and Zn.

First-principles study of superabundant vacancy formation in metal hydrides

Recent experiments have established the generality of superabundant vacancies (SAV) formation in metal hydrides. Aiming to elucidate this intriguing phenomenon and to clarify previous interpretations, we employ density-functional theory to investigate atomic mechanisms of SAV formation in fcc hydrides of Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au. We have found that upon H insertion, vacancy formation energies reduce substantially. This is consistent with experimental suggestions. We demonstrate that the entropy effect, which has been proposed to explain SAV formation, is not the main cause. Instead, it is the drastic change of electronic structure induced by the H in the SAV hydrides, which is to a large extent responsible. Interesting trends in systems investigated are also found: ideal hydrides of 5d metals and noble metals are unstable compared to the corresponding pure metals, but the SAV hydrides are more stable than the corresponding ideal hydrides, whereas opposite results exist in the cases of Ni, Rh, and Pd. These trends of stabilities of the SAV hydrides are discussed in detail and a general understanding for SAV formation is provided. Finally, we propose an alternative reaction pathway to generate a SAV hydride from a metal alloy.     

Preconcentration of microamounts of elements in natural waters with 8-mercaptoquinoline and bis(8-quinolyl) disulphide for their atomic-absorption determination

A method has been developed for the group concentration of microamounts of metals (Fe, Co, Ni, Mn, Cu, Zn, Cd, Hg, Pb, Bi, Sb, Mo, W, V, Cr, Ga, In, Sn, Ag, Au, Pd, Pt) in the form of their 8-mercaptoquinolinates co-precipitated on bis(8-quinolyl) disulphide as collector, the latter being an oxidation product of 8-mercaptoquinoline. The collector is formed during the co-precipitation process, which is the reason for its high co-precipitating power. The elements thus concentrated are determined by atomic-absorption spectrometry after dissolution of the precipitate in dilute nitric acid.     

Redox-driven conductance switching via filament formation and dissolution in carbon/molecule/TiO2/Ag molecular electronic junctions

Carbon/molecule/TiO2/Au molecular electronic junctions show robust conductance switching, in which a metastable high conductance state may be induced by a voltage pulse which results in redox reactions in the molecular and TiO2 layers. When Ag is substituted for Au as the "top contact", dramatically different current/voltage curves and switching behavior result. When the carbon substrate is biased negative, an apparent breakdown occurs, leading to a high conductance state which is stable for at least several hours. Upon scanning to positive bias, the conductance returns to a low state, and the cycle may be repeated hundreds of times. Similar effects are observed when Cu is substituted for Au and for three different molecular layers as well as "control" junctions of the type carbon/TiO2/Ag/Au. The polarity of the "switching" is reversed when the Ag layer is between the carbon and molecular layers, and the conductance change is suppressed at low temperature. Pulse experiments show very erratic transitions between high and low conductivity states, particularly near the switching threshold. The results are consistent with a switching mechanism based on Ag or Cu oxidation, transport of their ions through the TiO2, and reduction at the carbon to form a metal filament.     

Synthesis and dispersion of isolated high aspect ratio gold nanowires

We report the assembly properties of high density and high aspect ratio metal nanowire arrays (Au, Cu and Ag with diameters ranging from 40 to 250 nm) after release from the anodic alumina oxide (AAO) templates. Individual Ag and Cu nanowires were observed following release from the template, however, in the case of gold nanowires, the dispersion was dependent on size and aspect ratio. 40-100 nm gold wires aggregated to form bundles or disordered mats. We show that a simple cyanide-mediated release from the AAO template, results in isolated dispersion of wires even for the smallest wire diameters. Possible stabilising mechanisms for observed tendency of nanowires dispersion are discussed.     

Synthesis and characterization of silica gel microspheres encapsulated by salicyclic acid functionalized polystyrene and its adsorption of transition metal ions from aqueous solutions

The present study was undertaken to develop a novel adsorbent for heavy metal ions, and this paper presents the synthesis and characterization of a composite material-silica gel microspheres encapsulated by salicyclic acid functionalized polystyrene (SG-PS-azo-SA) with a core-shell structure. SG-PS-azo-SA was used to investigate the adsorption of Mn(II), Co(II), Ni(II), Fe(III), Hg(II), Zn(II), Cd(II), Cr(VI), Pd(II), Cu(II), Ag(I), and Au(III) from aqueous solutions. The results revealed that SG-PS-azo-SA has better adsorption capacity for Cu(II), Ag(I) and Au(III). Langmuir and Freundlich isotherm models were applied to analyze the experimental data, the best interpretation for the experimental data was given by the Langmuir isotherm equation with the maximum adsorption capacity for Cu(II), Ag(I), and Au(III) at 1.288 mmol g⁻¹, 1.850 mmol g⁻¹ and 1.613 mmol g^t-1, respectively. Thus, silica gel encapsulated by salicyclic acid functionalized polystyrene (SG-PS-azo-SA) is favorable and useful for the removal of Cu(II), Ag(I) and Au(III) metal ions.     

Quantitative analysis of precious metals in automotive shredder residue/combustion residue via EDX fluorescence spectrometry

The recovery of precious metals from automotive shredder residue (ASR) dust/combustion residue is an option that is not usually considered due to the lack of available information. Therefore, before any disposal or recovery application can be considered, it is necessary to determine the significance of the levels and distribution of precious metal in ASR dust/ASR combustion residue. In the present study, quantitative analysis of precious metals (Pt, Pd, Au, Ag and Cu) in the ASR residue samples was performed using energy dispersive X-ray (EDX) fluorescence spectrometer. With the fundamental parameter (FP) method, the X-ray intensity is obtained and the quantitative analysis is performed using theoretical calculation. This method is very effective for quantitative analysis of unknown samples without standard samples. Further, in order to analyse the precious metal distribution within the ASR combustion residues, the microstructural characterisation and elemental mapping were also carried out with the aid of field emission scanning election microscopy combined with electron dispersive spectroscopy (FE-SEM EDS). Significant amount of Pt, Pd, Au, Ag and Cu element concentrations in the ASR residue were identified. Total precious (Pt, Pd, Au, Ag and Cu) metals obtainable values are representing about 12.23 wt% from its initial ASR dust/combustion residues. Considering their relevant concentrations, these metals should be properly recovered for recycling purposes before to dispose or landfill.     

IB金属对Ni/SiO2催化剂乙炔选择性加氢反应性能的影响

乙烯是合成聚乙烯的原料,其主要来源是石油裂解气,其中少量的乙炔杂质会严重毒化生产聚乙烯的催化剂,因此需要将其去除.对于乙炔选择加氢反应,传统工业上使用的是Pd基催化剂,尽管其乙炔转化率很高,但对乙烯的选择性很低.我们前期的研究发现,IB族金属(Au,Ag和Cu)与Pd形成的合金单原子催化剂可以有效地提高乙烯的选择性.作为与Pd同组的非贵金属,Ni催化剂在多种催化加氢反应中显示出优异活性,而在乙炔选择加氢反应中,Ni是否能够替代贵金属Pd尚无定论.本文系统地研究了IB金属对Ni/SiO2催化剂乙炔选择性加氢性能的影响.与Pd/SiO2催化剂不同,单金属Ni/SiO2催化剂在低温下不具有活性.将IB金属添加到Ni/SiO2催化剂中,可以显著提高其催化活性以及对乙烯的选择性.其中,AuNix/SiO2和CuNix/SiO2催化剂的催化活性随还原温度升高而提高,而AgNix/SiO2催化剂对预处理温度不敏感.通过调变IB/Ni原子比和还原温度优化了催化剂的催化性能,发现优化后的三种催化剂(CuNi0.125/SiO2、AgNi0.5/SiO2和AuNi0.5/SiO2)的活性和选择性随反应温度升高表现出相似的变化趋势.催化稳定性考察结果显示,CuNi0.125/SiO2催化剂表现出最高选择性和稳定性;尽管AuNi0.5/SiO2的初始活性最高,但是稳定性最低.采用XRD、TPR和微量吸附量热等表征手段对不同IB金属对Ni基催化剂性质的影响进行了系统考察.以Cu-Nix/SiO2催化剂为例,H2-TPR测试结果表明,Cu-Ni双金属纳米颗粒的形成使得还原温度低于相应的单金属催化剂,表明铜和镍之间存在明显的相互作用.此外,通过TPR获得的CuNix/SiO2催化剂上的氢气消耗量与理论耗氢量相吻合,表明在还原处理的过程中双金属催化剂中的CuO和NiO可以被完全还原.乙炔的微量吸附量热结果表明,在CuNi0.125/SiO2,AgNi0.5/SiO2,AuNi0.5/SiO2和Ni0.5/SiO2催化剂上的初始吸附热分别为187,196,304和103 kJ/mol,即它们的初始乙炔吸附强度顺序为AuNi0.5/SiO2>AgNi0.5/SiO2>CuNi0.125/SiO2>Ni0.5/SiO2.该结果与三者的初始催化活性顺序一致,表明IB金属的加入可以增强乙炔在催化剂表面的吸附,从而提高催化活性.     

Benzoxazolylacrolein derivatives as ligands for extraction atomic absorption analysis

The extraction of chelate and ion-association complexes formed by various metal ions and two new organic reagents (derivatives of benzoxazolylacrolein) has been investigated in order to evaluate the possibilities for analytical application. The composition of the extracted species and their conditional extraction constants are given. Two simple extraction procedures for flame atomic absorption analysis of high purity lead salts have been developed. The proposed methods permit determination of (a) 1 × 10^–5% Co, Fe and Ni and 5 × 10^–6% Cu and Zn and (b) 1 × 10^–5% Pd and 5 × 10^–6% Ag and Au, with relative standard deviations in the range 4–8%.     

Solvent extraction of metal xanthates

The solvent extraction of metal xanthates is reviewed with emphasis on the characteristics and analytical uses of ethyl xanthate complexes. Elements reviewed include As, Sb, Bi, Cd, Co, Cu. Cr, Ga, Au, In, Fe, Pb, Mn, Hg, Mo, Ni, Pd, Pt, Re, Ag, Se, Te, Tc, Tl, Sn, U, V and Zn.