Rh Nanowires Research Articles

Surface effects on the mechanical properties of rhodium nanowires by molecular dynamics simulations

This study comprehensively investigates the mechanical properties of rhodium (Rh) nanowires using molecular dynamics simulations in which the interactions were expressed by embedded atom method potentials and interprets their relation with the deformation process. We have observed that the stress-strain behavior of Rh nanowires is radically different from the bulk material. As the size increased, the Elastic Modulus of nanowires gradually decreases and saturates to reach bulk value. It is revealed that the mechanical response of thicker nanowires than the critical value of 2.8 nm size is characterized by the core atoms and the surface effects lose their dominance due to the stability of the core atoms. The high strain rates delay yielding and neck formation and can be used to design the strengthened thin Rh nanowires.

Replicating the Defect Structures on Ultrathin Rh Nanowires with Pt to Achieve Superior Electrocatalytic Activity toward Ethanol Oxidation

AbstractMetal nanostructures with an ultrathin Pt skin and abundant surface defects are attractive for electrocatalytic applications owing to the increased utilization efficiency of Pt atoms and the presence of highly reactive sites. This paper reports a conformal, layer‐by‐layer deposition of Pt atoms on defective Rh nanowires for the faithful replication of surface defects (i.e., grain boundaries) on the Rh nanowires. The thickness of the Pt shell can be controlled from one monolayer up to 5.3 atomic layers. This series of Rh@PtnL (n = 1–5.3) core–sheath nanowires show greatly enhanced activity and durability in catalyzing the ethanol oxidation reaction in an acidic medium. Among others, the Rh @ Pt3.5L nanowires show the greatest mass activity (809 mA mg−1Pt) and specific activity (1.18 mA cm−2) after loaded on carbon support, which are 3.7 and 3.4 times those of the commercial Pt/C, respectively. In situ Fourier transform infrared spectroscopy studies indicate an enhanced interaction between the outermost Pt layer and the Rh nanowire can promote CC bond cleavage for complete oxidation of ethanol to CO2 while depress the dehydrogenation of ethanol to acetic acid. As the Pt shell thickness is increased, the selectivity for the CO2 pathway decreases while that for acetic acid is increased.

Ultrathin wavy Rh nanowires as highly effective electrocatalysts for methanol oxidation reaction with ultrahigh ECSA

Direct methanol fuel cells (DMFCs) have received tremendous research interests because of the facile storage of liquid methanol vs. hydrogen. However, the DMFC today is severely plagued by the poor kinetics and rather high overpotential in methanol oxidation reaction (MOR). Here we report the investigation of the ultrathin Rh wavy nanowires as a highly effective MOR electrocatalyst. We show that ultrathin wavy Rh nanowires can be robustly synthesized with 2–3 nm diameters. Electrochemical studies show a current peak at the potential of 0.61 V vs. reversible hydrogen electrode (RHE), considerably lower than that of Pt based catalysts (~ 0.8–0.9 V vs. RHE). Importantly, with ultrathin diameters and favorable charge transport, the Rh nanowires catalysts exhibit an ultrahigh electrochemically active surface area determined from CO-stripping (ECSACO) of 144.2 m2/g, far exceeding that of the commercial Rh black samples (20 m2/g). Together, the Rh nanowire catalysts deliver a mass activity of 722 mA/mg at 0.61 V, considerably higher than many previously reported electrocatalysts at the same potential. The chronoamperometry studies also demonstrate good stability and CO-tolerance compared with the Rh black control sample, making ultrathin Rh wavy nanowires an attractive electrocatalyst for MOR.

Pt and Pt–Rh nanowires supported on carbon and SnO2:Sb nanoparticles for ethanol electrochemical oxidation in acidic media

Pt and Pt–Rh nanowires were successfully synthesized by chemical reduction with formic acid without any surfactants and were applied for the study of ethanol electrochemical oxidation in acidic media. The synthesis process lasted 72 h at room temperature and the nanowires were obtained with 40 wt% metal loading with respect to the carbon support. In addition, mixtures of carbon- and antimony-doped tin oxide were prepared and used as support in order to improve the catalytic activity and stability of the catalysts. Electrochemical activity was studied by cyclic voltammetry, chronoamperometry, and derivative voltammetry and the stability was tested by electrolysis at 0.6 V for 22 h. Physical characterization was carried out by X-ray diffraction, field emission gun scanning electron microscopy, and transmission electron microscopy. All Pt–Rh nanowires show higher activity than the commercial Pt/C catalyst towards ethanol oxidation. This feature is due to the fact that rhodium facilitates the cleavage of the C–C bond of the alcohol, resulting in higher oxidation efficiency. Physical characterization confirms the formation of nanowires that are widely dispersed on the supports, demonstrating the feasibility of the developed methodology for synthesis of Pt–Rh nanowires.

Electrodeposition of Rhodium Nanowires Arrays and Their Morphology-Dependent Hydrogen Evolution Activity.

This work reports on the electrodeposition of rhodium (Rh) nanowires with a controlled surface morphology synthesized using an anodic aluminum oxide (AAO) template. Vertically aligned Rh nanowires with a smooth and coarse morphology were successfully deposited by adjusting the electrode potential and the concentration of precursor ions and by involving a complexing reagent in the electrolyte solution. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses were used to follow the morphological evolution of Rh nanowires. As a heterogeneous electrocatalyst for hydrogen evolution reactions (HER), the coarse Rh nanowire array exhibited an enhanced catalytic performance respect to smooth ones due to the larger surface area to mass ratio and the higher density of catalytically active defects, as evidenced by voltammetric measurements and TEM. Results suggest that the morphology of metallic nanomaterials could be readily engineered by electrodeposition. The controlled electrodeposition offers great potential for the development of an effective synthesis tool for heterogeneous catalysts with a superior performance for wide applications.

Arrays of rhodium nanowires based on anodic alumina: Preparation and electrocatalytic activity for nitrate reduction

Rhodium nanowires with an average diameter of 55nm and a length of 1.5–11μm have been prepared by electrodeposition in the pores of anodic aluminium oxide (AAO) membranes which were fabricated by two-step anodization technique. Rh/AAO nanocomposites have been investigated by the combination of instrumental and electrochemical methods. The electrochemical surface area of Rh nanowires is about one order of magnitude higher than the geometrical one that is beneficial for catalytic applications. The efficiency of both isolated Rh nanowires and Rh/AAO nanocomposites towards nitrate electrochemical reduction in acidic medium has been demonstrated. The preliminary annealing of AAO templates allows us to obtain nanocomposites that are suitable for long-term catalytic applications in aggressive medium.

Electronic structure of bimetallic Ni–Rh nanowires

The electronic structure of a bare Rh(553) surface and of a Ni-decorated Rh(553) surface has been investigated by angle-resolved UV photoelectron spectroscopy and density functional theory calculations. The self-assembly of Ni adatoms leads to the decoration of the steps of the Rh(553) surface with monoatomic Ni rows under suitable kinetic conditions, thus forming a regular array of pseudomorphic bimetallic Ni–Rh nanowires. The electronic structure of the clean Rh(553) surface has been compared to the one of the flat Rh(111) surface, and additional surface states localized at the step edges due to the lower coordination of the step atoms have been detected. The Ni wires are weakly hybridized with the Rh substrate states and are characterized by only weakly dispersing states. This leads to a strong narrowing of the d-band, which is argued to be the origin of the observed high chemical reactivity of the Ni–Rh nanowires.

Growth and Oxidation of Ni Nanostructures on Stepped Rh Surfaces

The growth and oxidation of Ni nanostructures on stepped Rh(15 15 13) and (553) surfaces, both vicinals of Rh(111), have been studied by scanning tunneling microscopy (STM), LEED, core level spectroscopies with use of synchrotron radiation (XPS and NEXAFS), and work function measurements. At low coverages, quasi-one-dimensional Ni rows decorate the Rh step edges. With increasing coverages, the Ni adlayers grow layer-by-layer in a restricted step-flow growth mode, pseudomorphically up to ∼2 monolayer (ML). Relaxation to the Ni bulk lattice occurs in the range 2−10 ML. The oxidation of the low-coverage Ni nanowires creates ordered quasi-one-dimensional Ni−O structures at the Rh step edges. In the Ni coverage range up to 2 ML, uniaxially ordered (2 × 1) and (6 × 1) oxide structures are observed. On thicker films, local NiO (100)-type and (111)-type reconstructions are both observed in the STM. The NEXAFS measurements indicate that the low-dimensional Ni−O nanophases are distinctly different from bulk-type NiO, and three chemically different oxygen species are identified in the O 1s XPS core level spectra. The chemical reactivity of the Ni−Rh surfaces toward oxygen seems to be highest for the one-dimensional Ni−Rh nanowires, where the Ni atoms are coupled directly with the Rh step atoms.

Stable field emission from arrays of vertically aligned free-standing metallic nanowires

We present a fully elaborated process to grow arrays of metallic nanowires with controlledgeometry and density, based on electrochemical filling of nanopores in track-etchedtemplates. Nanowire growth is performed at room temperature, atmospheric pressureand is compatible with low cost fabrication and large surfaces. This techniqueoffers an excellent control of the orientation, shape and nanowires density. It isapplied to fabricate field emission arrays with a good control of the emissionsite density. We have prepared Co, Ni, Cu and Rh nanowires with a height of3 µm, a diameter of 80 nmand a density of ∼107 cm−2. The electron field emission measurements and total energy distributions show that theas-grown nanowires exhibit a complex behaviour, first with emission activation under highfield, followed by unstable emission. A model taking into account the effect of anoxide layer covering the nanowire surface is developed to explain this particularfield emission behaviour. Finally, we present an in situ cleaning procedure by ionbombardment that collectively removes this oxide layer, leading to a stable andreproducible emission behaviour. After treatment, the emission current density is∼1 mA cm−2 fora 30 V µm−1 applied electric field.

Magnetic properties of Co–Rh and Ni–Rh nanowires

We have calculated the magnetic properties of pure Ni, Co and Rh, and alloyed Co–Rh and Ni–Rh free-standing nanowires by an ab initio method. We have found that the pure Co and Ni wires present an enhanced magnetic moment with respect to their bulk values, and we have obtained that a magnetic order appears for pure Rh wires. For concentrations up to 50%Rh, in the alloyed Ni–Rh linear chains there is an enhancement of the total magnetic moment with respect to the pure nanowires, and in the case of Co–Rh the alloying with Rh enhances the Co magnetic moment. In both systems we obtain very high Rh magnetic moments.

Preparation and catalysis of Pt and Rh nanowires and particles in FSM-16

Pt and Rh nanowires and/or particles were prepared in mesopores of FSM-16, and their characterization and catalytic performances were investigated. Irradiation of UV–Vis light to FSM-16 (pore size 2.7 nm) impregnated with H 2PtCl 6 in the presence of water and 2-propanol (or methanol) vapors led to the formation of Pt nanowires (diameter 2.5 nm, length 50–300 nm) in the mesopores of FSM-16. In contrast, H 2-reduction of H 2PtCl 6/FSM-16 at 673 K for 2 h resulted in the formation of Pt nanoparticles in FSM-16. The XANES and XPS studies show that the Pt wires and particles are slightly electron-deficient, implying the interaction with the internal surface of mesopore. At the initial stage of the formation of Pt wires, it is found that small Pt nanoparticles are formed in the mesopores. Then Pt ions migrate and are reduced on the surface of the small particles, thus resulting in the formation of the Pt nanowires. Rh nanoparticles were prepared in FSM-16 by calcination of RhCl 3/FSM-16 at 673 K and subsequent H 2-reduction at 473 K. Furthermore, nanowires (diameter 2.5 nm, length 10–50 nm) were formed in FSM-16 by the similar photoreduction of the FSM-16 co-impregnated with H 2PtCl 6 and RhCl 3. In hydrogenolysis of butane, Pt nanowire/FSM-16 exhibits higher catalytic activities than Pt nanoparticle/FSM-16, while Pt–Rh nanowire/FSM-16 shows high activity of isomerization to isobutane.