Planar Sites Research Articles

Site-controlled VLS growth of planar nanowires: yield and mechanism.

The recently emerged selective lateral epitaxy of semiconductor planar nanowires (NWs) via the vapor-liquid-solid (VLS) mechanism has redefined the long-standing symbolic image of VLS NW growth. The in-plane geometry and self-aligned nature make these planar NWs completely compatible with large scale manufacturing of NW-based integrated nanoelectronics. Here, we report on the realization of perfectly site-controlled growth of GaAs planar NW arrays with unity yield using lithographically defined gold (Au) seed dots. The growth rate of the planar NWs is found to decrease with the NW width at fixed spacing, which is consistent with the conventional VLS model based on the Gibbs-Thomson effect. It is found that in general, the planar and out-of-plane NW growth modes are both present. The yield of planar NWs decreases as their lateral dimension shrinks, and 100% yield of planar NWs can be achieved at moderate V/III ratios. Based on a study of the shape of seed particles, it is proposed that the adhesion between the liquid-phase seed particle and the substrate surface is important in determining the choice of growth mode. These studies represent advances in the fundamental understanding of the VLS planar NW growth mechanism and in the precise control of the planar NW site, density, width, and length for practical applications. In addition, high quality planar InAs NWs on GaAs (100) substrates is realized, verifying that the planar VLS growth mode can be extended to heteroepitaxy.

Fluctuation induced conductivity analysis of Mn doped Cu0.5Tl0.5Ba2Ca2Cu3−xMnxO10−δ (x = 0, 0.1, 0.15) superconductors

A series of superconducting samples of type Cu0.5Tl0.5Ba2Ca2Cu3−xMnxO10−δ (x = 0, 0.1, 0.15) are prepared by using solid state reaction method. The electrical resistivity ρ(T) was measured as a function of temperature. The effect of doped Mn-atoms on the superconducting carriers has been investigated by carrying out fluctuation induced conductivity analyses of conductivity data of Cu0.5Tl0.5Ba2Ca2Cu3−xMnxO10−δ (x = 0, 0.1, 0.15) superconducting samples by Aslamazov-Larkin and Lawrence-Doniach models. The width of both two dimensional and three dimensional conductivity regimes are enhanced with doping of Mn at Cu site. The doping of Mn at the CuO2 planar sites increases the inter-layer coupling and coherence length ξc along the c-axis. A decrease in Bc0(T), Bc1(T), Jc(0) and increase in the values of κ, λp.d is observed. The value of the order-parameter is suppressed from |ψ|2 = 1 in the neighborhood of spin carrying doped Mn atoms, which is compensated by increased penetration of magnetic field lines, consequently resulting into increased penetration depth λp.d with increased Mn-doping.

Key factors to understand in-situ behavior of Cs in Callovo–Oxfordian clay-rock (France)

Understanding the behavior of 137Cs and 135Cs in soils and geological formations is of considerable interest in the context of nuclear accidents and nuclear waste repositories. Although the clay fraction is known to be responsible for sorption, there are still unanswered questions raised by the literature data concerning (i) the reversibility of the sorption process(es), (ii) the validity of the additivity rule (the overall distribution coefficient (Kd) for a radionuclide on a mixture of minerals is predicted from the distribution coefficients measured on individual minerals) and (iii) the validity of model transposition from dispersed systems to consolidated/intact systems. Because of these uncertainties, the validity of sorption models at equilibrium under in-situ conditions and for very long-term interaction is still pending. These different issues are studied in the present work for the Callovo–Oxfordian (COx) clay-rich rock Formation, which is under investigation in France as a geological barrier for a long-term nuclear waste repository. The work is based on sorption data measured on thirteen samples of different mineralogy taken from five different boreholes at several depths within the COx sedimentary layer. To our knowledge, it is the most extended Cs sorption dataset that has been published for a single clay formation in term of (i) sample locations (and thus natural variability), (ii) sorption conditions (powder dispersed in suspension, compacted powders and intact samples) and (iii) equilibration time (from one week to five years). Moreover, for the first time ever, radioactive Cs sorption results were compared to the natural distribution of non-radioactive Cs isotopes between pore water and the solid phase. The experimental system appeared to be in chemical equilibrium as much as can be expected for an ion-exchange reaction. More particularly, no kinetically-controlled process leading to partial Cs irreversibility was observed, in contrary to what was found in the literature for soils. This difference in behavior may be related to the difference in the illite studied, i.e. a soil-type illite which would be more altered than a sedimentary formation-type illite. No decrease in site capacity was observed between dispersed and intact/compacted states. A model based on exchange reactions with cations interacting with illite (frayed edge, type-II and planar sites) and mixed layer illite–smectite (I/S) (planar sites) using parameters published in the literature enabled the Kd variation to be described as a function of Cs concentration, the mineralogy of the samples, the change in water composition and the temperature (22–80°C). Our study clearly demonstrates that no frayed edge sites should be considered on the illite fraction of I/S, thus emphasizing the difference of sorption properties between an I/S mixed layer mineral and a corresponding mechanical mix of illite and smectite minerals. The robustness of the model was confirmed by data analysis describing the behavior of naturally-occurring Cs in the formation thereby demonstrating the effectiveness of the Cs sorption processes in a very long-time period prospective. Lastly, the model was used to predict the sorption of trace concentrations of Cs in the COx Formation on the time-scale relevant for nuclear waste disposal performance assessment. As expected, the retention was significant with Kd values ranging from 100 to 2000L/kg whatever the conditions that were probed and a simulation covering a period of over 105years could show that the COx Formation is an efficient barrier to prevent Cs transport from the storage facility to the surrounding environment.

Modeling of cesium sorption on biotite using cation exchange selectivity coefficients

Abstract For the modeling of cesium sorption on biotite, samples of natural biotite separated from gneissic rocks were converted into monoionic potassium, sodium, and calcium forms, and sorption isotherms for Cs/K, Cs/Na and Cs/Ca exchange were determined at pH 6 and 8 in E-4–E-8 M Cs solutions. Selectivity coefficients for Cs/K, Cs/Na, and Cs/Ca ion exchange reactions were calculated from the isotherm data, using the Gaines-Thomas convention. At Cs loadings below 1% of the total ion exchange capacity, the overall selectivity coefficient for Cs/Ca exchange was approximately five and seven orders of magnitude higher than those for Cs/Na and Cs/K exchange, respectively. Based on the selectivity coefficients, the ion exchange isotherms were modeled with the U.S. Geological Survey PhreeqC program, assuming three different types of ion exchange site: sites on the basal planes on biotite crystal surfaces with 95% site abundance, probable interlayer sites on crystal edges [frayed edge sites (FESs)] (0.02%) and third-type sites (5%), the physical background of which is unclear. Of these three types, the FES sites were superior in Cs selectivity, while the planar sites exhibited the lowest selectivity, and the third-type sites had selectivity between these two. The functionality of the model was successfully verified by modeling the Cs sorption isotherms on crushed mica gneiss rock in saline groundwater. Determination of the exchangeable ions K, Na, Ca, and Cs on the basal plane and edge surfaces by scanning electron microscopy-energy-dispersive x-ray spectroscopy (SEM-EDX) supports the results of modeling: edge sites highly prefer Cs ions and also Ca and Na ions but not K ions.

An EXAFS study on the effects of natural organic matter and the expandability of clay minerals on cesium adsorption and mobility

The relationship between cesium (Cs) adsorption on clay minerals with various expandabilities and Cs mobility in environment was investigated using sequential extraction, batch adsorption, X-ray diffraction (XRD), generalized adsorption model (GAM), and Cs LIII-edge extended X-ray absorption fine structure (EXAFS) analyses with molecular simulations using the density functional theory (DFT). In particular, the difference between the affinities of illite (non-expansion) and vermiculite (intermediate expansion) for Cs and the effect of humic acid (HA) addition on the Cs/clay mineral system were highlighted in this study. These two factors affect Cs mobility and bioavailability in surface soil and sediments.The batch adsorption results showed that Cs adsorption was inhibited to some extent in the ternary clay+HA+Cs system because of (i) the blocked access of Cs to the frayed edge site (FES) and type II site [inner–sphere (IS) complex in GAM] by HA, and (ii) the reduced availability of the interlayer site in vermiculite. EXAFS analysis further confirmed that the adsorbed Cs in clay minerals was drastically changed by the sequential addition of HA. In addition, the dominant IS complex in the illite+Cs and illite+Cs+HA systems (in which HA was added after Cs adsorption on illite) can be converted to the outer–sphere (OS) complex largely in the illite+HA+Cs system (in which HA was added prior to Cs adsorption). These results are consistent with the sequential extraction and GAM results.The IS complex of dehydrated Cs+ mainly formed at the FES and interlayer site on illite (non-expansion) without resulting in any illite structural changes. However, on vermiculite (intermediate expansion), the dehydrated Cs+ can be adsorbed as an IS complex associated with the siloxane group of the di-trigonal cavity in the tetrahedral SiO4 sheet. This adsorption is accompanied by collapse of the layer, which can be easily coated by HA molecules to prevent Cs fixation. However, a nearly complete OS complex was observed at the planar site of montmorillonite (large expansion). These processes were confirmed by sequential extraction, batch adsorption, XRD, and EXAFS, which clearly showed that Cs mobility in soil highly depends on clay mineral expandability, natural organic matter (NOM), and the coupling of both effects. The atomic-scale information given by EXAFS is consistent with the distribution data from adsorption experiments, GAM, sequential extraction, and DFT. These results can be used as a basis for a clearer understanding of Cs behavior in natural systems.

Probing “ambivalent” snug-fit sites in the KcsA potassium channel using three-dimensional reference interaction site model (3D-RISM) theory

Abstract The potassium channel is highly selective for K+ over Na+, and the mechanism underlying this selectivity remains unclear. We show the three-dimensional distribution functions (3D-DFs) of small cations (Li+, Na+, and K+) and the free energy profile of ions inside the open selectivity filter (SF) of the KcsA channel. Our previous results [S. Phongphanphanee, N. Yoshida, S. Oiki, F. Hirata. Abstract Book of 5th International Symposium on Molecular Science of Fluctuations toward Biological Functions, P062 (2012)] indicate that the 3D-DF for K+ exhibits distinct peaks at the sites formed by the eight carbonyl oxygen atoms belonging to the surrounding peptide-backbone and residues (the cage site). Li+ has sharp distributions in the 3D-DF at the center of a quadruplex composed of four carbonyl oxygen atoms (the plane site). Na+ has a rather diffuse distribution throughout the SF region with peaks both in the plane and in cage sites. The results provide microscopic evidence of the phenomenological findings that Li+ and Na+ are not excluded from the SF region and that the binding affinity alone does not cause the ion selectivity of KcsA. In the present study, with an ion placed explicitly along the pore axis, the free energy profiles of the ions inside the SF were calculated; from these profiles we suggest a new mechanism for selective K+ permeation. According to the model, a K+ ion must overcome a free energy barrier that is approximately half that of Na+ to exit from either of the SF mouths due to the existence of an intermediate local minimum along the route for climbing the barriers.

Structural investigation of the oxide-ion electrolyte with SrMO3(M = Si/Ge) structure

A neutron-diffraction study of the oxide-ion solid electrolytes Sr1−xNaxSiO3−0.5x (x = 0.2 and 0.4) and Sr0.8K0.2Ge1−ySiyO2.9 (y = 0.0 and 0.5) reveals that there are no interstitial oxygen atoms in the structures; the oxygen vacancies are more concentrated in the planar oxygen sites (O3 and O5) of corner-sharing tetrahedral units of the M3O9 (M = Si/Ge) complexes. From thermogravimetric analysis (TGA), the K-substituted samples lose weight above 100 °C and are hygroscopic at room temperature; the oxygen vacancies are concentrated at the in-plane O3 and O5 sites and those in the terminal O2 or O4 are responsible for a 2D oxide-ion conductivity between the M3O9 complexes. The Na-substituted samples lose little weight by 800 °C and are not hygroscopic; the oxygen vacancies are located at all oxygen atom positions, being more pronounced at the in-plane O3 and O5; those in the terminal oxygen sites give an excellent oxide-ion conductivity. Moreover the high temperature NPD data for Sr0.6Na0.4SiO2.8 disclose that the vacancies become more randomly dispersed above 400 °C to give a smaller activation energy above 550 °C for vacancy transfer between Si3O9−0.5x complexes.

Ionic strength and pH dependent multi-site sorption of Cs onto a micaceous aquifer sediment

Caesium-137 (t1/2=30years) is a common contaminant at nuclear legacy sites. Often the mobility of 137Cs in the environment is governed by its sorption to charged sites within the sediment. To this end it is important to understand the sorption behaviour of caesium across a wide range of environmental conditions. This work investigates the effect of varying solution composition (pH and competing ions) on the sorption of caesium to micaceous aquifer sediment across a large concentration range (1.0×10−11 – 1.0×10−1molL−1 Cs+). Experimental results show that Cs+ exhibits three distinct sorption behaviours at three different concentration ranges. At very low concentrations<1.0×10−6molL−1 Cs+ sorption was unaffected by competition with Na+ or H+ but significantly reduced in high ionic strength K+ solution. Secondly between 1×10−6 and 1.0×10−3molL−1 Cs+ is strongly sorbed in a neutral pH, low ionic strength background but sorption is significantly reduced in solutions with either a high concentration of Na+ or K+ ions or low pH. At high concentrations>1.0×10−3molL−1 Cs+ sorption is reduced in all systems due to saturation of the sediment’s sorption capacity. A multi-site cation exchange model was used to interpret the sorption behaviour. From this it was determined that at low concentrations Cs+ sorbs to the illite frayed edge sites only in competition with K+ ions. However, once the frayed edge sites are saturated the Cs+ sorbs to the Type II and Planar sites in competition with K+, Na+ and H+ ions. Therefore sorption of Cs+ at concentrations>1.0×10−6molL−1 is significantly reduced in both high ionic strength and low pH solutions. This is a significant result with regard to predicting the migration of 137Cs+ in acidic or high ionic strength groundwaters.

Highly confined ions store charge more efficiently in supercapacitors

Liquids exhibit specific properties when they are adsorbed in nanoporous structures. This is particularly true in the context of supercapacitors, for which an anomalous increase in performance has been observed for nanoporous electrodes. This enhancement has been traditionally attributed in experimental studies to the effect of confinement of the ions from the electrolyte inside sub-nanometre pores, which is accompanied by their partial desolvation. Here we perform molecular dynamics simulations of realistic supercapacitors and show that this picture is correct at the microscopic scale. We provide a detailed analysis of the various environments experienced by the ions. We pick out four different adsorption types, and we, respectively, label them as edge, planar, hollow and pocket sites upon increase of the coordination of the molecular species by carbon atoms from the electrode. We show that both the desolvation and the local charge stored on the electrode increase with the degree of confinement.

Modeling cesium retention onto Na-, K- and Ca-smectite: Effects of ionic strength, exchange and competing cations on the determination of selectivity coefficients

Cesium (137Cs) retention onto three homoionic smectites (Na-, K- and Ca-smectite), obtained from natural Spanish FEBEX bentonite, was studied. Special emphasis was given to the analysis of non-linear sorption behaviour and the dependence of selectivity on the ionic strength.A very large set of experimental sorption data was generated from sorption tests under a wide range of pHs (2–11), ionic strengths (10−3 to 100M), and radionuclide concentrations (10−10 to 10−3M). The aqueous phase, in contact with the clay, was analysed to quantify the effects of the presence of trace aqueous ions on Cs retention.For all the exchanged clays, Cs sorption was non-linear and a two-site exchange model approach was adopted to interpret and model sorption data. Highly selective sites for Cs sorption (Type 1 sites, T1), resembling those present in micaceous materials, with very low capacity but controlling uptake of Cs at low concentration, were observed. The logarithm value of selectivity coefficients determined for Cs+ in respect to Na+, K+ and Ca2+ in these sites is: LogNaCsKSEL(T1)=7.59±0.15,LogKCsKSEL(T1)=5.15±0.15 and LogCaCsKSEL(T1)=14.41±0.17, respectively.The exchange sites at the surface of smectite sheets (planar sites), with a capacity approximately equivalent to the cation exchange capacity (CEC) of the clay, constitute the second type of sorption sites (Type 2 sites, T2). The logarithm of the selectivity coefficients determined for Cs+ with respect to Na+, K+ and Ca2+ is: LogNaCsKSEL(T2)=1.68±0.15,LogKCsKSEL(T2)=1.16±0.15LogCaCsKSEL(T2)=3.02± 0.15, respectively.The analysis of the dependence of sorption values on the ionic strength clearly indicated that for a correct interpretation of data, competition effects of trace ions in solution must be always accounted for.Data obtained in this work and performed analyses are basic to explain the behaviour of raw FEBEX bentonite, and other smectite-based clay materials, under more complex experimental conditions.

Topochemical Reduction of the Ruddlesden–Popper Phases Sr2Fe0.5Ru0.5O4 and Sr3(Fe0.5Ru0.5)2O7

Reaction of the Ruddlesden-Popper phases Sr2Fe(0.5)Ru(0.5)O4 and Sr3(Fe(0.5)Ru(0.5))2O7 with CaH2 results in the topochemical deintercalation of oxide ions from these materials and the formation of samples with average compositions of Sr2Fe(0.5)Ru(0.5)O(3.35) and Sr3(Fe(0.5)Ru(0.5))2O(5.68), respectively. Diffraction data reveal that both the n = 1 and n = 2 samples consist of two-phase mixtures of reduced phases with subtly different oxygen contents. The separation of samples into two phases upon reduction is discussed on the basis of a short-range inhomogeneous distribution of iron and ruthenium in the starting materials. X-ray absorption data and Mössbauer spectra reveal the reduced samples contain an Fe(3+) and Ru(2+/3+) oxidation state combination, which is unexpected considering the Fe(3+)/Fe(2+) and Ru(3+)/Ru(2+) redox potentials, suggesting that the local coordination geometry of the transition metal sites helps to stabilize the Ru(2+) centers. Fitted Mössbauer spectra of both the n = 1 and n = 2 samples are consistent with the presence of Fe(3+) cations in square planar coordination sites. Magnetization data of both materials are consistent with spin glass-like behavior.

Magnetic-field-enhanced spin freezing on the verge of charge ordering in YBa2Cu3O6.45

Using 63Cu NMR, we establish that the enhancement of spin order by a magnetic field H in YBa2Cu3O6.45 arises from a competition with superconductivity because the effect occurs for H perpendicular, but not parallel, to the CuO2 planes, and it persists up to field values comparable to Hc2. We also find that the spin-freezing has a glassy nature and that the frozen state onsets at a temperature which is independent of the magnitude of H. These results, together with the presence of a competing charge-ordering instability at nearby doping levels, are strikingly parallel to those previously obtained in La-214. This suggests a universal interpretation of magnetic field effects in underdoped cuprates where the enhancement of spin order by the field may not be the primary phenomenon but rather a byproduct of the competition between superconductivity and charge order. Low-energy spin fluctuations are manifested up to relatively high temperatures where they partially mask the signature of the pseudogap in 1/T1 data of planar Cu sites.

Spin glass to superconducting phase transformation by oxidation of a molybdo-cuprate: Mo0.3Cu0.7Sr2TmCu2Oy

A detailed study of the structure and properties for the as-prepared and oxygen annealed Mo0.3Cu0.7Sr2TmCu2Oy material is reported. The Cu/Mo cationic distribution is established using a combination of x-ray/neutron powder diffraction refinement. The chemical substitution of the Mo ions for the Cu ions in the CuYSr2Cu2O7−δ structure is found to occur in both of the copper sites for the as-prepared sample. Interestingly, no trace of Mo substitution in the copper plane site is found to occur after oxygenation. The as-prepared Mo0.3Cu0.7Sr2TmCu2Oy material is found to be a spin glass (SG) system and explained on the basis of the cluster-by-cluster freezing model. On the other hand, the oxygen annealed material is superconducting (SC) (TSC,onset = 31 K). A peak has been observed in the critical current density plot and can be explained on the basis of field induced pins. The influence of oxygen annealing in the structure and properties of this material are presented and discussed. This seems to be the first case of a SG–SC transformation following an oxidation reaction in cuprates.

Selectivity between Sodium, Calcium and Potassium Ions in Bacterial Sodium and Calcium Channels

The families of voltage gated sodium and calcium channels are responsible for the upstroke of action potentials and converting electrical signals into cellular responses. The ability for these channels to be able to transport only the desired ion species across the membrane is critical to their function. The recent publication of a structure of a bacterial voltage gated sodium channel has opened the door for the mechanisms of selectivity in both these channel families to be investigated.Using molecular dynamics simulations and free energy calculations we are able to show how a range of bacterial sodium and calcium selective channels are able to differentiate between ion species. The selectivity of NavAB and NaChBac for sodium over potassium is shown to result from the inability of potassium to fit through the narrowest portion of the channel with the preferred solvation geometry. In contrast, rejection of calcium happens slightly further into the channel where the square planar coordination sites more aptly fit sodium ions. The gradual mutation of NavAB into the calcium selective channel CaChBac progressively makes the high field strength sites more favourable for calcium to the point where it out competes sodium ions.

Fluctuation induced conductivity analyses of Cd doped Cu0.5Tl0.5Ba2Ca2Cu3−yCdyO10−δ (y=0, 0.5, 1.0, 1.5) superconductors

Abstract Fluctuation induced conductivity (FIC) analyses in the critical fluctuations region (cr), three dimensional (3D), two dimensional (2D) and zero dimensional (0D) region are reported for Cd-doped (Cu0.5Tl0.5)Ba2Ca3(Cu3−yCdy)O10−δ (y=0, 0.5, 1.0, 1.5) superconductors. The coherence length along c-axis, ξc(0), Fermi velocity, VF, and Fermi energy of the carriers, EF, were calculated from such analyses. Using the cross-over temperature (TG) of critical to 3D regime the Ginzberg number, NG, is determined. By using NG, thermodynamic critical field, Bc(0), the lower critical field, Bc1(0), and critical current density, Jc(0), are extracted. It was found from these analyses that widths of critical and 3D regimes are shrunken with the increased doping of Cd in the final compound. Also ξc(0), VF and the coupling constant J are suppressed with increased Cd doping. The decrease in important superconductivity parameters is suggested to be arising due to anharmonic oscillations induced by the heavier Cd atoms doped at Cu planar sites which in turn suppress the density of the desired phonons required for optimum superconducting properties. In these analyses we found that the critical magnetic fields (Bc(0), Bc1(0)) and Jc(0) increase with increased Cd concentration. The most likely reason for the improvement of magnetic properties is increased population of spin-less Cd atoms from which the magnetic field lines are not deflected and behave like pinning centers.

Improved eIF4E Binding Peptides by Phage Display Guided Design: Plasticity of Interacting Surfaces Yield Collective Effects

Eukaryotic initiation factor (eIF)4E is over-expressed in many types of cancer such as breast, head and neck, and lung. A consequence of increased levels of eIF4E is the preferential translation of pro-tumorigenic proteins (e.g. c-Myc and vascular endothelial growth factor) and as a result is regarded as a potential therapeutic target. In this work a novel phage display peptide has been isolated against eIF4E. From the phage sequence two amino acids were delineated which improved binding when substituted into the eIF4G1 sequence. Neither of these substitutions were involved in direct interactions with eIF4E and acted either via optimization of the helical capping motif or restricting the conformational flexibility of the peptide. In contrast, substitutions of the remaining phage derived amino acids into the eIF4G1 sequence disrupted binding of the peptide to eIF4E. Interestingly when some of these disruptive substitutions were combined with key mutations from the phage peptide, they lead to improved affinities. Atomistic computer simulations revealed that the phage and the eIF4G1 derivative peptide sequences differ subtly in their interaction sites on eIF4E. This raises the issue, especially in the context of planar interaction sites such as those exhibited by eIF4E, that given the intricate plasticity of protein surfaces, the construction of structure-activity relationships should account for the possibility of significant movement in the spatial positioning of the peptide binding interface, including significant librational motions of the peptide.

Targeted syntheses of homo- and heterotrinuclear complexes involving MII–NiII–MII (M = Ni, Cu, and Pd) nonlinear core: Structure, spectroscopy, magnetic and redox studies

Homo- and heterotrinuclear complexes [LNi{M(Ln)}2](ClO4)2·H2O involving NiIIMII2 nonlinear cores (M=Ni, Cu, and Pd) (1–6) have been synthesized by a single-pot reaction when the oximato metal complexes [MLn(H2O)]ClO4 (HLn are tridentate oxime ligands), prepared in situ in methanol are allowed to react with the precursor nickel(II) complex [LNi(H2O)2] (H2L=N,N′-dimethyl-N,N′-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine). Single crystal X-ray diffraction analysis, and ESI-MS spectroscopy have been used to establish their identities which involve an octahedral Ni(II) site flunked by two metal-oximate moieties, each in a square planar environment. The electronic and molecular structures of these compounds are interesting due to a synergistic bonding mechanism operative through the deprotonated oxime and the phenolate oxygen atoms via the metal centers. Thus the weak spin-forbidden 3A2→1E transition (ε, 4–8mol−1cm2), characteristic of the octahedral Ni(II) center in Ni(II)–Ni(II)2 (1, 2) and Ni(II)–Pd(II)2 complexes (5, 6), gains intensity by ca. 25-fold in the spin-coupled Ni(II)–Cu(II)2 complexes (3, 4) in which the metal centers are connected antiferromagnetically through a moderately strong coupling (JCu-Ni/kB=−119.6(6)K). In the presence of excess pyridine (5×10−2M), the trinickel compound (1) in acetonitrile displays a pair of quasi-reversible oxidations at E1/2=0.48 and 0.69V (vs. Ag/AgCl reference at a high scan speed, 1000–3000mVs−1) due to Ni(II/III) oxidations at the square planar nickel sites. Coordinated oxime helps in stabilizing these Ni centers in +3 oxidation state in the time-scale of cyclic voltammetry possibly through the formation of octahedral geometry with the added pyridine as axial ligands. In the cathodic range, all the three Ni centers undergo Ni(II/I) reductions at E1/2=−1.17, −1.35 and −1.62V, the most cathodic one being due to the octahedral site.

Critical regime and suppression of the pseudo-gap in Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ superconductors via excess conductivity analyses

We have doped Zn at the Cu planar sites in Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ (y=0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0) superconductors, which decrease the population of small spins of Cu atoms and most likely suppress the anti-ferromagnetically aligned spins in the inner-CuO2 planes (IP). For the observation of such effects, fluctuation induced conductivity (FIC) analyses of the above mentioned samples have been carried out using the Aslamazov–Larkin (AL) theory in the critical regime closer to the transition temperature (Tc) and the Lawrence–Doniach (LD) model closer to Tc and above. The idea behind these analyses is the fact that Cooper pair formation starts well above Tc. The influence of the Cooper-pairs on the normal electrons is determined by the Maki-Thompson (MT) model. From these analyses we have determined the coherence length along the c-axis (ξc(0)), the inter-plane coupling constant (J), the phase relaxation time of the carriers (τφ), the Fermi velocity (VF) of the carriers and the energy required to break apart the electrons pairs (Epb). Using the Ginzburg number (NG) and the Ginzburg–Landau (GL) equations, the thermodynamic critical magnetic field (Bc(0)), the lower critical field (Bc1(0)), the upper critical field (Bc2(0)), the critical current density (Jc(0)) and the penetration depth (λp.d) are also calculated. Four fluctuation regions above Tc are observed for all of the samples, namely the critical (cr), three dimensional (3D), two dimensional (2D) and zero dimensional (0D) fluctuation regions. The values of Bc(0), Bc1(0) and Jc(0) are increased while the values of E and λp.d are suppressed with increasing Zn content at Cu sites. The intersection of 2D LD, and 0D exponents determines a cross-over of 2D LD & MT contributions. Associated with the width of 3D AL/LD regimes, the crossover temperature T3D-2D is shifted to higher values with enhanced Zn doping. It is most likely that decreased density of small spins of Cu atoms induced by increased Zn doping, may suppress the spin scatterings that consequently result in higher values of T3D-2D, critical fields (Bc(0), Bc1(0)), Jc(0) and lower values of Epb and λp.d in the final compound. The decreased density of small spins of Cu atoms in the CuO2/ZnO2 planes is viewed in terms of suppression of the spin gap and, hence, the pseudo-gap in Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ (y=0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0) superconductors.

Dynamics of direct H2O2 synthesis from H2 and O2 on a Pd nano-particle catalyst protected with polyvinylpyrrolidone

The kinetics of the H2–O2 reaction was studied on Pd–PVP (polyvinylpyrrolidone) nano-particles in water. With H+ and Br−, Pd–PVP catalyzed the H2O2 synthesis. H2 and O2 partial pressure dependencies revealed that the H2O2 formation followed the Langmuir–Hinshelwood mechanism and that the H2O2 selectivity varied greatly with pH2, which was quite different from the results using Pd/C. In the absence of Br−, the H2–O2 reaction primarily yielded H2O, and the reaction rate was very high and proportional to pH2. Br− adsorption measurements suggested that the proportion of highly unsaturated sites on Pd–PVP was much higher than on Pd/C. In the presence of H+ and Br−, the most unsaturated sites such as a corner site of the Pd particles would be blocked by the HBr adsorption and inert, and the major reaction would proceed on moderately unsaturated sites such as an edge site, which stabilize intermediates more than the least unsaturated sites such as a plane site.

Anthraquinone Monosulfonate Adsorbed on Graphite Shows Two Very Different Rates of Electron Transfer: Surface Heterogeneity Due to Basal and Edge Plane Sites

Graphitic electrodes find widespread use throughout electrochemistry; understanding their fundamental electrochemical properties is imperative. It is widely thought that graphite edge plane sites exhibit faster rates of electron transfer as compared to basal plane sites. Hitherto the different rates of electron transfer at the edge and basal sites have been inferred indirectly using diffusional systems. To avoid possible complications we alternatively study a surface-bound system to simplify the interpretation. The voltammetric response of graphitic-surface-bound anthraquinone monosulfonate (AQMS) with varying pH, reveals two distinct voltammetric responses, ascribed as being due to the basal and edge plane sites; where the pK(a) s associated with the reduced anthraquinone are found to differ for the two sites. Through modelling of the system based upon a "scheme of squares" mechanism it is possible to conclude that both the thermodynamics and kinetics of the species differ in the two environments in which the rate of electron transfer at the basal plane site is shown to be 2-3 orders of magnitude slower than that of the edge plane site. This work provides the first example of a voltammetric response which is purely due to electron transfer at a basal plane site. Further, we believe this is the first time a full "scheme of squares" model has been used for the quantitative analysis of a diffusionless 2H(+)2e(-) system.