Similar Atomic Size Research Articles

Theoretical insights on the effect of alloying with Co in the mechanism of graphene growth on a Cu[sbnd]Co (1 1 1) catalyst

Alloying Cu catalyst with transition metals with high C solubility is an effective technique for growing graphene with a precisely controlled number of layers. Co is a transition metal with high C solubility and is suitable for alloying with Cu catalysts due to its similar atomic size. In this study, we elucidate the mechanism of graphene growth on a CuCo(111) catalyst by evaluating the energetics, thermodynamics, and kinetics of potential C source species, CHi (i = 0, 1, 2, 3), using a first-principles method. Alloying Cu with Co atoms upshifts the d-band center, which induces a robust p-d hybridization between the CuCo(111) catalyst and the C source species. From relative population calculations, it is found that the C monomer on the catalyst subsurface always becomes dominant in the range of growth conditions studied. Therefore, the C monomer in the catalyst subsurface controls the segregation growth mechanism. In addition, it is found that the diffusion of C monomer into the CuCo(111) catalyst subsurface is an exothermic process with a low activation energy. These results enable us to suggest a novel alternating alloy catalyst for growing few-layer graphene with a precisely controlled number of layers.

Self-adaptively electronic transfer of ternary Ni3Ga0.8In0.2/SiO2 alloy catalysts toward enhancing selectivity hydrogenation

Non-noble metal-based alloy catalysts are extensively utilized in diverse applications. Nevertheless, finding a suitable preparation strategy to precisely moderate the catalytic active site remains a substantial challenge and subsequently improves the atomic utilization and reaction efficiency. Herein, the non-noble trimetallic alloy catalysts (Ni3Ga0.8In0.2/SiO2) were fabricated by using the strategy of incorporating doping (Indium as doping metal) and modulating the Ni3Ga bimetallic alloy in which to precisely tune the electronic structure of catalysts. The dispersion of alloy nanoparticles on the support was investigated by HRTEM, alongside the valence and elemental distribution of the components by EDS, XRD, and XPS, demonstrating the successful alloying of the composite. Moreover, other metal-doped Ni-Ga-M/SiO2 (M=Cu, Pt) catalysts and Ni-Ga alloy were prepared and evaluated catalytic performance by the model reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The experimental results revealed that suitable metal doping enhanced the reaction rate significantly, with the optimal effect of Indium among the different doping metals. Particular synergies arise from the homogeneous metals' similar atomic size and electronic structure. Theoretical calculations by Density Function Theory (DFT) have revealed that the doping of Indium resulted in electron transfer at the active interface of the catalyst, the gain of additional electrons by Ni, and the creation of electron vacancies at the active interface as a result of the electron deficiency of Ga and In, which enables the optimization of the catalytic reaction. In addition, the strategy can be extended to synthesize other trimetallic nanoparticle catalysts, which is expected to provide a new avenue for precise optimization and preparation of highly active non-precious metal catalysts.

The effect of Ni or Co additions on the structure of Zr60Cu30Al10 bulk metallic glass revealed by high-energy synchrotron radiation

The effect of substituting Cu by elemental additions of Ni or Co on the atomic structure of the Zr60Cu30Al10 ternary bulk metallic glass (BMG) is studied using high-energy synchrotron radiation X-ray diffraction. Analyses of the structural features in reciprocal and real space using the structure factors S(Q) and pair-distribution functions (PDF) point to an increase in the structural disorder for the Ni- or Co-bearing quaternary alloys. This is consistent with the “confusion principle” since upon alloying the initially nearly identical atomic sizes of Cu, Ni and Co diversify due to local electronic interactions. In real space, the disordering is manifested by a reduced deviation from the average particle density visible in the nearest-neighbour (NN) atomic shell structure over the complete short- and medium-range order region. Despite their similar atomic size, enthalpies of mixing with the main alloy elements and apparent disordering of the structure, the additions of Ni or Co have different effects on thermal stability of the ternary “mother” alloy.

Role of chemical disorder on radiation-induced defect production and damage evolution in NiFeCoCr

Understanding chemical disorder in many concentrated solid solution alloys (CSAs) at the levels of electrons and atoms has attracted increasing attention as a path forward to reveal and identify underlying mechanisms for extraordinary mechanical properties and improved radiation tolerance. Single-phase NiFeCoCr CSA is a common base for many high-entropy alloys (HEAs) that have shown improved mechanical strength and radiation tolerance. In this study, defect production and damage evolution in NiFeCoCr under ion irradiation at room temperature to dose over 20 dpa are determined using ion channeling technique along both <100> and <110> directions utilizing multiple probing beam energies. The results obtained from the multi-axial and multi-energy channeling analysis are compared with those previously obtained for Ni crystals irradiated under similar conditions. The influence of chemical complexity on defect production and clustering at early-stage under room temperature irradiation up to dose of 1 dpa is discussed based on positron annihilation spectroscopy results. Defect structure evaluation in Ni and NiFeCoCr is also discussed based on transmission electron microscopy results over a prolonged irradiation at both room and elevated temperatures. Compared with chemically complex NiFeCoCr, larger dislocation loops thus less lattice strain are expected to form in pure Ni. Moreover, the role of chemical disorder in this CSA is also investigated based on ab initio calculations using large supercells. To understand the impact of chemical complexity effect on defect structure evolution, this integrated research effort attempts to link the relatively large charge redistribution due to difference in valence electron counts resulting from alloying different 3d transition metal elements, moderate lattice distortion arising from similar adaptable atomic size, and notable suppressed or delayed damage evolution in NiFeCoCr.

An X-ray absorption spectroscopic study of the Fe(II)-induced transformation of Cr(VI)-substituted schwertmannite

The environmental chemistry of Cr is of widespread interest due to the hazardous nature of Cr(VI). Because of similar atomic size and charge, CrVIO42- can substitute for SO42- within schwertmannite - an Fe(III) oxyhydroxysulfate mineral that occurs widely in acidic and sulfate-rich systems. The presence of aqueous Fe(II) can induce transformation of schwertmannite to more stable Fe(III) phases (e.g. goethite) which may potentially impact the behaviour of co-associated Cr(VI). Here, we investigate the Fe(II)-induced transformation of Cr(VI)-substituted schwertmannite as a function of pH (4−8) and the degree of Cr(VI) substitution (0.16–13 mol% CrVIO42--for-SO42- substitution). Iron K-edge EXAFS spectroscopy revealed that higher levels of Cr(VI) substitution inhibited Fe(II)-induced schwertmannite transformation. Chromium K-edge XANES spectroscopy indicated that this outcome could be partly attributed to consumption of Fe(II) by reaction with Cr(VI), and the resulting formation of a passivating Cr(III)-Fe(III) hydroxide phase which stabilizes schwertmannite at greater levels of Cr(VI) substitution and at higher pH while also decreasing further reduction of structural Cr(VI). Overall, this study enriches our understanding of interactions between hazardous Cr(VI) and schwertmannite in environmental and engineered systems.

Short-range order controlling atomic dynamics in Y-based metallic glasses

Secondary \ensuremath{\beta} relaxation is considered a key issue in glassy materials, while its structural origin is still not well understood. In this paper, we report pronounced and unpronounced \ensuremath{\beta}-relaxation behaviors in ${\mathrm{Y}}_{60}{\mathrm{Ni}}_{16}{\mathrm{Al}}_{24}$ and ${\mathrm{Y}}_{60}{\mathrm{Fe}}_{16}{\mathrm{Al}}_{24}$ metallic glasses (MGs) obtained from dynamical mechanical analyses, respectively, despite the same composition and similar atomic sizes of Ni and Fe atoms. We elucidate that the \ensuremath{\beta} relaxation in ${\mathrm{Y}}_{60}{\mathrm{Ni}}_{16}{\mathrm{Al}}_{24}$ and ${\mathrm{Y}}_{60}{\mathrm{Fe}}_{16}{\mathrm{Al}}_{24}$ MGs mainly depends on the vibration of small Ni and Fe atoms using synchrotron x-ray techniques and theoretical calculations, respectively. From the x-ray absorption fine structure and Voronoi tessellation statistics, it is found that the dominant local structure of mobile atoms changes from $\ensuremath{\langle}0,3,6,0\ensuremath{\rangle}$ centered by Ni atoms into $\ensuremath{\langle}0,2,8,0\ensuremath{\rangle}$ centered by Fe atoms. More large Y atoms in shells promote the local vibration of center Ni atoms. In contrast, with more Y atoms replaced by Fe atoms, the vibration of center Fe atoms gets significantly slowed down, thus leading to an unpronounced \ensuremath{\beta} relaxation. Our results indicate that the atomic dynamics of mobile atoms in MGs are not only related to their surrounding local geometry but also chemical constitutions, i.e., the short-range order. In this paper, we provide an idea for understanding the structural origin of \ensuremath{\beta}-relaxation behaviors in MGs and other glassy materials from their local atomic environments and dynamics point of view.

Different lattice distortion effects on the tensile properties of Ni-W dilute solutions and CrFeNi and CoCrFeMnNi concentrated solutions

The lattice distortion of a solute primarily occurs because its atomic size and chemical bonding are different from those of neighboring atoms. The lattice distortion effects in conventional and high-entropy alloys are different; however, a detailed investigation on these effects has yet to be conducted. To fill this research gap, this study produced face-centered cubic-structured dilute solutions (Ni, Ni–2 at.% W, and Ni–4 at.% W) and concentrated solutions (equiatomic CrFeNi and CoCrFeMnNi) and compared their tensile properties. For the two W-containing alloys, lattice distortion occurred only around the large and strong W atoms. However, for the two concentrated solutions, which had a similar interelement atomic size and shear modulus to the aforementioned alloys, lattice distortion occurred at all lattice sites. These two types of lattice distortion had significantly different effects on tensile properties. The strength and ductility of the alloys with a high concentration of distorted lattice points were higher than those of the alloys with a low concentration of distorted lattice points, although the alloys with a low concentration of distorted lattice points had a larger nominal atomic size difference and shear modulus difference. The mechanisms underlying the evolution of different mechanical properties under different types of lattice distortion were examined for the dilute and concentrated alloys. Moreover, the universal solid solution strengthening mechanism was observed.

Different Lattice Distortion Effects on the Tensile Properties of Ni-W Dilute Solutions and CrFeNi and CoCrFeMnNi Concentrated Solutions

The lattice distortion from a solute is mainly due to atomic size and chemical bonding differences with neighboring atoms. The lattice distortion effects in conventional alloys and high-entropy alloys are known to be different but lack of clarification. In this study, FCC-structured dilute solid solutions of Ni, Ni–2at.%W, and Ni–4at.%W and concentrated solutions of equiatomic CrFeNi and CoCrFeMnNi metals were designed to compare their tensile properties. The lattice distortion of both W-containing alloys was only around the large and strong W atoms but that of the two concentrated solutions with similar inter-element atomic size and shear modulus prevailed at all lattice sites. Results showed that the trends in both types of lattice distortion were significantly different on tensile properties. The combination with strength and ductility from high concentration of distorted lattice points was superior over that from low concentration ones even the latter had larger nominal atomic size difference and shear modulus difference. The evolution mechanisms on different mechanical properties from dilute alloys to concentrated alloys were elucidated and clarified under the large contrast of lattice distortion. The universal solid solution strengthening mechanism had been established.

Electronic structure and optical properties of Ge- and F-doped α-Ga2O3: First-principles investigations* *Project supported by the National Natural Science Foundation of China (Grant No. 51302215) and the Natural Science Basic Research Program of Shaanxi Province, China (Grant Nos. 2018JQ6084 and 2019JQ-860).

The prospect of α-Ga2O3 in optical and electrical devices application is fascinating. In order to obtain better performance, Ge and F elements with similar electronegativity and atomic size are selected as dopants. Based on density functional theory (DFT), we systematically research the electronic structure and optical properties of doped α-Ga2O3 by GGA+U calculation method. The results show that Ge atoms and F atoms are effective n-type dopants. For Ge-doped α-Ga2O3, it is probably obtained under O-poor conditions. However, for F-doped α-Ga2O3, it is probably obtained under O-rich conditions. The doping system of F element is more stable due to the lower formation energy. In this investigation, it is found that two kinds of doping can reduce the α-Ga2O3 band gap and improve the conductivity. What is more, it is observed that the absorption edge after doping has a blue shift and causes certain absorption effect on the visible region. Through the whole scale of comparison, Ge doping is more suitable for the application of transmittance materials, yet F doping is more appropriate for the application of deep ultraviolet devices. We expect that our research can provide guidance and reference for preparation of α-Ga2O3 thin films and photoelectric devices.

Anomalous behavior of glass-forming ability and mechanical response in a series of equiatomic binary to denary metallic glasses

Herein, we systematically investigated the impact of configuration entropy (CE) on the glass-forming ability (GFA) and mechanical response in a series of equiatomic binary Cu50Zr50 to denary (CuNiBeCoFe)50(ZrTiHfTaNb)50 metallic glasses (MGs) with similar atomic size difference and enthalpy of mixing. Interestingly, the senary (CuNiBe)50(ZrTiHf)50 MG with a medium CE of 1.79R exhibits the maximum GFA among the MGs, which means that higher CE by itself is not a sufficient condition for higher GFA, although it should be a major design parameter according to the confusion principle. The mechanical response analysis was comprehensively performed using nanoindentation test including statistical analysis of pop-in event to elucidate deformation dynamics of shear-avalanches, and the analysis result was compared with the intensive structure data obtained by high energy X-ray scattering analysis. The nanohardness and the Young's modulus (E) in MG with higher CE are shown to outwardly increase which is dominantly due to increased 3-atom connections of cluster polyhedra as well as lower fragility. However, the severe local structural irregularity and the compositional complexity in MG with higher CE promotes the manifestation of relatively chaotic behavior as well as the loss of MG's property inheritance, which results in the unexpected local softening of MG and ultimately modulating the response towards ductile deformation. Thus, the denary MG with the highest CE of 2.30R exhibits an abnormally low incremental rate in the cut-off values of strain burst size as well as measured E. Consequently, it can be concluded that the CE could be one of the crucial factors in designing an MG to alter its characteristics towards achieving desirable properties such as high GFA and enhanced plasticity.

Revealing the factors influencing grain boundary segregation of P, As in Si: Insights from first-principles

Phosphorous (P) and arsenic (As) segregation at grain boundaries (GBs) usually deteriorate the electrical performance of n-type doped Si-wafers. In order to probe the factors influencing P and As segregation behaviors along Si GBs, a systematic investigation upon the interactions between P, As atoms and a series of coincidence site lattice Si GBs, including Σ3 {111}, Σ9 {221}, Σ27 {552}, Σ3 {112}, was carried out via first-principles calculations. It is revealed that the segregation behaviors of P, As along different GBs are different, which is dependent on both GB characteristics, i.e. the intrinsic lattice distortion, number of dangling/extra bonds, deep levels in the bandgap of density of states, and also the impurity properties. It reveals that smaller P atoms tend to segregate to the compressed atomic sites in GBs, by which the intrinsic GB lattice distortion is reduced. However, GB lattice distortion hardly induce As segregation owing to the similar atomic size between As and Si atoms. Calculations also indicate that under-coordinated core sites with dangling bond along GBs are remarkably attractive for P and As segregation, as a result of the nature of group-V elements to be three-coordinated rather than four-coordinated. Furthermore, GBs having deep levels in the bandgap of density of states, produced either by dangling or extra bonds, show a strikingly high attractive strength for both P and As segregation. The present work is supposed to provide important insights on substitutional impurity segregation along GBs in multi-crystalline Si in the atomic and electronic level.

Phase evolution and stability of nanocrystalline CoCrFeNi and CoCrFeMnNi high entropy alloys

High entropy alloys (HEAs) have emerged as promising class of materials having equiatomic or near equiatomic multicomponent configurations. Nanostructured HEAs have added a new facet to the development of these alloys, showing remarkable strength and functional properties. The present work examined the phase evolution and thermal stability of nanocrystalline CoCrFeNi and CoCrFeMnNi HEAs prepared through mechanical alloying (MA) followed by spark plasma sintering (SPS). After MA, both the alloys showed single phase FCC structure with minor fractions of tungsten carbide arising due to contamination from milling media. After SPS, the major phase remained as FCC in both the alloys along with Cr7C3 evolution. Phase stability of CoCrFeNi and CoCrFeMnNi HEAs, (MA powders and SPS pellets) were investigated in the temperature range 1073–1373 K up to 96 h. Formation of Cr7C3, concomitant with Cr-depletion in FCC matrix, was observed on heat treatment of MA powders. SPS alloys retained their mixture of FCC + Cr7C3 on thermal exposure with minimal change in the fraction of carbides. The presence of single phase field in their respective phase diagrams, similar atomic sizes of constituents and non-equilibrium nature of MA combined to lend a highly stable FCC phase in the nanocrystalline HEAs studied.

Efficient planar heterojunction perovskite solar cells employing a solution-processed Zn-doped NiOX hole transport layer

Optimal interface of planar heterojunction perovskite solar cells determines efficient charge transport and collection for achieving high power conversion efficiency. Nickel oxide (NiOX) is one of the most well-known hole transport materials for perovskite solar cells because of its wide band gap, p-type transport characteristic, good chemical stability and high optical transparency. However, pristine NiOX possesses unsatisfactory electrical properties such as high surface trap density or low electrical conductivity which could deteriorate the device performance. We thus adopted Zn as a dopant to employ the Zn-doped NiOX as a hole transport layer in the planar heterojunction PSCs as considering that not only Zn has a similar atomic size with Ni but also Zn forms highly crystalline oxide materials. As a result, 5% Zn-doped devices achieved a power conversion efficiency up to 13.72% with improvements of a VOC by 4.0%, a JSC by 16.7% and a fill factor by 8.9% as compared to the un-doped NiOX counterpart.

Substitution of Zn in Earth‐Abundant Cu2ZnSn(S,Se)4 based thin film solar cells – A status review

Cu2ZnSnS4 (CZTS) and Cu2ZnSn(S,Se)4 (CZTSSe) are the most promising quaternary earth abundant photo-absorber materials for thin film solar cells, with reported power conversion efficiencies (PCE) of more than 12%. In CZTS and CZTSSe based solar cells, similar atomic sizes of Cu and Zn is thought to be the main reason for CuZn and ZnCu antisite defect formation, resulted in severe potential fluctuations and tail states that can be inhibited by an incorporation of alternative bigger atoms such as Cd and Mn. In addition, the large open-circuit voltage deficit observed in CZTS and CZTSe device because of existence of Cu and Zn cation disorder in kesterite crystal. Recently, in the development of CZTS based thin film solar cells the reduction of antisite defects by cation substitution has received considerable attention. This also opens up the possibility to explore materials with similar crystal structures and band gaps as CZTS/CZTSSe such as Cu2XSnS4 (where X = Mn, Cd, Fe, Co, Ni and Ba). In this review, the effect of substituting other metals for Zn is discussed, providing a way to alter defects and tail states in the absorber material of CZTS/CZTSSe thin film solar cells.

On the controllability of phase formation in rapid solidification of high entropy alloys

The demonstration of high entropy alloys (HEAs), or more generally multi-principal-element alloys (MPEAs), which display a greater resistance to softening at elevated temperatures and embrittlement at cryogenic temperatures, has offered an accessible alternative alloying process for materials scientists and engineers. Although solidification is a fundamental process in synthesis of alloys which strongly affects their microstructure and properties, a firm understanding of this process in HEAs is scarce. Here, using molecular dynamics (MD) simulations, we study the rapid solidification of the multi-principal-element CoFeNiPd alloy as a prototypical single-phase HEA. Our simulations reveal an intense competition between fcc and bcc structures during homogenous nucleation in this alloy. According to our numerical analyses, the crystalline fcc phase is thermodynamically the most stable structure in this alloy, which is in agreement with the experimental observations. We show that formation of the fcc structure can be facilitated through heterogeneous nucleation by including a pre-existing fcc seed in the liquid phase. Interestingly, we illustrate that altering the crystal structure of the seed from fcc to bcc leads to obtaining a single phase bcc alloy. Our detailed analyses confirm that the tendency of HEA systems to undergo crystallization (rather than amorphisation) at fast cooling rates is owing to the neutral chemical affinity and similar atomic size of their constitutive elements. Our findings suggest that rapid solidification accompanied with heterogeneous (e.g. seed-assisted) nucleation could be a promising processing route to control the microstructure and properties of the HEAs.

Unique Role of Refractory Ta Alloying in Enhancing the Figure of Merit of NbFeSb Thermoelectric Materials

AbstractNbFeSb‐based half‐Heusler alloys have been recently identified as promising high‐temperature thermoelectric materials with a figure of merit zT &gt; 1, but their thermal conductivity is still relatively high. Alloying Ta at the Nb site would be highly desirable because the large mass fluctuation between them could effectively scatter phonons and reduce the lattice thermal conductivity. However, practically it is a great challenge due to the high melting point of refractory Ta. Here, the successful synthesis of Ta‐alloyed (Nb1−xTax)0.8Ti0.2FeSb (x = 0 – 0.4) solid solutions with significantly reduced thermal conductivity by levitation melting is reported. Because of the similar atomic sizes and chemistry of Nb and Ta, the solid solutions exhibit almost unaltered electrical properties. As a result, an overall zT enhancement from 300 to 1200 K is realized in the single‐phase Ta‐alloyed solid solutions, and the compounds with x = 0.36 and 0.4 reach a maximum zT of 1.6 at 1200 K. This work also highlights that the isoelectronic substitution by atoms with similar size and chemical nature but large mass difference should reduce the lattice thermal conductivity but maintain good electrical properties in thermoelectric materials, which can be a guide for optimizing the figure of merit by alloying.

A petrological assessment of diamond as a recorder of the mantle nitrogen cycle

Nitrogen is fundamental to the evolution of Earth and the life it supports, but for reasons poorly understood, it is cosmochemically the most depleted of the volatile elements. The largest reservoir in the bulk silicate Earth is the mantle, and knowledge of its nitrogen geochemistry is biased, because ≥90% of the mantle nitrogen database comes from diamonds. However, it is not clear to what extent diamonds record the nitrogen characteristics of the fluids/melts from which they precipitate. There is ongoing debate regarding the fundamental concept of nitrogen compatibility in diamond, and empirical global data sets reveal trends indicative of nitrogen being both compatible (fibrous diamonds) and incompatible (non-fibrous monocrystalline diamonds). A more significant and widely overlooked aspect of this assessment is that nitrogen is initially incorporated into the diamond lattice as single nitrogen atoms. However, this form of nitrogen is highly unstable in the mantle, where nitrogen occurs as molecular forms like N2 or NH4+, both of which are incompatible in the diamond lattice. A review of the available data shows that in classic terms, nitrogen is the most common substitutional impurity found in natural diamonds because it is of very similar atomic size and charge to carbon. However, the speciation of nitrogen, and how these different species disassociate during diamond formation to create transient monatomic nitrogen, are the factors governing nitrogen abundance in diamonds. This suggests the counter-intuitive notion that a nitrogen-free (Type II) diamond could grow from a N-rich media that is simply not undergoing reactions that liberate monatomic N. In contrast, a nitrogen-bearing (Type I) diamond could grow from a fluid with a lower N abundance, in which reactions are occurring to generate (unstable) N atoms during diamond formation. This implies that diamond’s relevance to nitrogen abundance in the mantle is far more complicated than currently understood. Therefore, further petrological investigations are required to enable accurate interpretations of what nitrogen data from mantle diamonds can tell us about the deep nitrogen budget and cycle.

Pdpt Alloy Nanocubes As Electrocatalysts for Oxygen Reduction Reaction in Acid Media

Platinum-based catalysts are the best electrocatalysts for oxygen reduction reaction (ORR) in fuel cells.1 However, because of the high price and limited supply of Pt, numerous researches have focused on finding a way to reduce its amount in the catalysts. One way to decrease Pt loading is to replace it partially by other metals. Pd is a promising substitute due to the similar properties to the Pt (same group of the periodic table, similar atomic size and crystalline structure).2 In this work, PdPt alloy nanocubes with different metal ratios were synthesized in the presence of polyvinylpyrrolidone.3 The surface morphology of the PdPt samples was characterized by transmission electron microscopy (TEM). TEM images showed that PdPt nanoparticles were cubic-shaped and the average size of the cubes was about 8–10 nm. TEM image for sample Pd54Pt46 is shown in Fig. 1a. Electrochemical measurements were performed to explore the electrocatalytic properties of the PdPt alloy nanocubes toward the ORR in 0.5 M H2SO4 using the rotating disk electrode (RDE) method. CO stripping and cyclic voltammetry (CV) were used for cleaning and characterizing the surface of catalysts. The CV responses showed improvement in surface cleanness after CO oxidation compared with those CVs initially measured. Comparison of polarization curves received from RDE measurements at 1900 rpm are presented in Fig. 1b. The half-wave potentials (E 1/2) for O2 reduction on PdPt alloys were found to be higher than that of Pd nanocubes and were comparable with that of Pt nanocubes. The onset potential for Pd36Pt64 was the highest. The RDE data were analyzed using the Koutecky-Levich (K-L) equation and from the slopes the number of transferred electrons (n) was found. The value of n was close to four for all the catalysts studied. For better comparison of the kinetic parameters of all the electrodes the Tafel plots were constructed from the RDE data (Fig. 1c). Two regions with distinct slope values were observed. In the low overpotential region the slope value was close to -60 mV dec‒ 1 and at high overpotentials the Tafel slope value was over -130 mV dec‒ 1. All the alloyed catalysts showed enhanced electrocatalytic activity for ORR as compared to the monometallic cubic Pd nanoparticles. From the alloyed catalysts Pd36Pt64 exhibited the highest specific activity, which was only slightly lower than that of cubic Pt nanoparticles. Work is in progress to study the electroreduction of oxygen on PdPt alloy nanocubes in 0.1 M KOH solution using the RDE method. Single-waved polarization curves with well-defined current plateaus were observed. The E 1/2 values for PdPt alloys in alkaline solution were comparable. The K-L analysis showed that O2 reduction on these alloys proceeds via 4-electron pathway in 0.1 M KOH. The Tafel analysis revealed that the mechanism of O2 reduction on PdPt alloy nanocubes in alkaline media is similar to that in acid media. The PdPt alloy nanocubes exhibit good electrocatalytic activity for the electroreduction of oxygen in both electrolytes. This indicates a great promise of these materials as cathode catalysts for low-temperature fuel cells.

Influence of B, Al and Nb addition on the glass forming ability and stability of mechanically alloyed Zr-Ni amorphous alloys

We investigated the influence of minor additions of B, Al and Nb that have representative atomic sizes on the glass forming ability (GFA) and stability of Zr-Ni amorphous alloys during mechanical alloying. The results show that the minor addition of B, Al or Nb does not shorten the initial time of the full amorphization reaction or improve the glass forming ability of the Zr-Ni alloys at a low rotation speed. However, B addition can effectively improve the mechanical stability of the amorphous phase against mechanically induced crystallization. Furthermore, the amorphous phase gradually transforms into a metastable fcc-phase with increasing milling time. The addition of Al and Nb that have similar atomic sizes has a similar effect on the GFA and the mechanical stability of the Zr-Ni amorphous phase. Moreover, Al and Nb addition can alter the crystallization behavior and improve the thermal stability of the Zr-Ni amorphous phase.

Atomic size and chemical effects on the local order ofZr2M(M=Co, Ni, Cu, and Ag) binary liquids

First-principles molecular dynamics simulations are performed to investigate the atomic size and chemical effects on the short-range order (SRO) in superheated and undercooled Zr-based metallic liquids, ${\text{Zr}}_{2}M$ ($M=\text{Co}$, Ni, Cu, and Ag). We demonstrate that the local atomic structures in liquids are quite sensitive to the atomic size ratio and the electronic interactions between component elements. The large negative heats of mixing for $\text{Zr-}M$ do not favor icosahedral SRO in these binary liquids, contrary to the common belief. Full icosahedral structure units are few in the superheated liquids, although the number of icosahedral clusters increases upon undercooling. Comparing ${\text{Zr}}_{2}\text{Co}$, ${\text{Zr}}_{2}\text{Ni}$, and ${\text{Zr}}_{2}\text{Cu}$, all of which have very similar atomic size ratios, we find that the degree of local icosahedral order increases with decreasing interaction strength between the $d$ electrons in Zr-Co, Zr-Ni, and Zr-Cu. A comparison of ${\text{Zr}}_{2}\text{Cu}$ and ${\text{Zr}}_{2}\text{Ag}$ alloys shows that the degree of icosahedral order increases much more in ${\text{Zr}}_{2}\text{Ag}$ than in ${\text{Zr}}_{2}\text{Cu}$ with decreasing temperature. The difference in atomic sizes of Cu and Ag may account for the subtle discrepancy in the evolution of short-range ordering in undercooled ${\text{Zr}}_{2}\text{Cu}$ and ${\text{Zr}}_{2}\text{Ag}$ liquids.