Single-crystalline Platinum Research Articles

Crystalline Facet-Directed Generation Engineering of Ultrathin Platinum Nanodendrites.

In this work, we successfully prepare two-dimensional ultrathin single-crystalline platinum nanodendrites (PtNDs) with precisely controlled generation (size) through a surfactant-directed solution-phase synthesis. The amphiphilic surfactant of C22H45-N+(CH3)2CH2COOH (Br-) acts as the structure-directing template and facet-capping agent simultaneously to kinetically engineer in-the-plane epitaxial growth of Pt nanocrystals along selectively exposed {111} facets into ultrathin PtNDs. A novel formation mechanism defined as crystalline facet-directed step-by-step in-the-plane epitaxial growth, similar to the synthesis of organic dendrimers, is proposed on the basis of the nanostructure and crystalline evolution of PtNDs. The generation growth process is readily extended to precisely engineer the generation of PtNDs (from 0 to 25) and can also be utilized to grow other noble metal NDs (e.g., PdNDs and AuNDs) and core-shell Pt-Pd NDs. Because of the structural advantages, ultrathin PtNDs exhibit enhanced electrocatalytic performance toward hydrogen evolution reaction.

Intermediate stages of electrochemical oxidation of single-crystalline platinum revealed by in situ Raman spectroscopy.

Understanding the atomistic details of how platinum surfaces are oxidized under electrochemical conditions is of importance for many electrochemical devices such as fuel cells and electrolysers. Here we use in situ shell-isolated nanoparticle-enhanced Raman spectroscopy to identify the intermediate stages of the electrochemical oxidation of Pt(111) and Pt(100) single crystals in perchloric acid. Density functional theory calculations were carried out to assist in assigning the experimental Raman bands by simulating the vibrational frequencies of possible intermediates and products. The perchlorate anion is suggested to interact with hydroxyl phase formed on the surface. Peroxo-like and superoxo-like two-dimensional (2D) surface oxides and amorphous 3D α-PtO2 are sequentially formed during the anodic polarization. Our measurements elucidate the process of the electrochemical oxidation of platinum single crystals by providing evidence for the structure-sensitive formation of a 2D platinum-(su)peroxide phase. These results may contribute towards a fundamental understanding of the mechanism of degradation of platinum electrocatalysts.

Direct estimation of surface pressures by hydrogen adsorbates on platinum surfaces in perchloric acid

The surface pressure solely by hydrogen species at polycrystalline and single crystalline platinum is calculated from a modified Gibbs-Lippmann Equation taking the contributions of electrode potential and hydrogen coverage in perchloric acid. Hydrogen surface pressures at reference surface coverages are rather low for Pt(100) and Pt(110), i.e., 20–35μNcm−1, but 5-fold larger for Pt(111) due the formation of long range water-hydrogen layers on trigonal sites. Surface tensions also follow the same tendencies. On polycrystalline platinum they are sensibly inferior to those obtained on single crystals but similar in magnitude for both types of hydrogens indicating that the presence of water in weakly bound hydrogen responds to a slight interaction (not bound to platinum atoms).Free energy of adsorption for hydrogen ad-atoms on polycrystalline platinum is reported here at experimentally found reference coverages, i.e. −29 and −14kJmol−1 for both kinds of hydrogens in perchloric acid. The interaction/heterogeneity factor of the Frumkin/Temkin isotherm, which are similar (ca. 11–12kJmol−1) for both types of hydrogens, again verifies that water interaction is not straight to platinum atoms. On single crystalline surfaces, the most favorable value of adsorption energy was found on Pt(100) and the lowest on Pt(111). The three single crystals exhibit positive interaction/heterogeneity factors (from 11 to 18kJmol−1) being the greatest on Pt(110) as a consequence of being the most disordered surface.

3-Dimensionally self-assembled single crystalline platinum nanostructures on few-layer graphene as an efficient oxygen reduction electrocatalyst

Here, we report for the first time the synthesis of a 3-D self-assembled single crystalline platinum nanostructure directly on the graphene surface (PtNAGE) without using any harmful structural directing agents. A slow reduction method is used to prepare the desired platinum morphology. Initial formation of platinum nanoparticles and their homogenous dispersion on the surface of graphene have been observed 10 h after the commencement of the reduction using formic acid as the reducing agent. From these initially deposited seed particles, the growth starts on the {111} facets along the direction and the nanostructure formation is completed within 72 h of the commencement of the reaction. The individual assembly has a diameter of ∼80 nm. PtNAGE shows superior electrocatalytic activity towards oxygen reduction compared to graphene supported platinum (PtGE) and commercial carbon supported platinum (PtC) catalysts. PtNAGE is less vulnerable to strong hydroxyl adsorption compared to PtC and PtGE. Specific activity and mass activity of the catalyst are high compared to PtC by a factor of 6.50 and 1.80, respectively, and 4.00 and 3.05, respectively, compared to PtGE. The limiting current density of PtNAGE is 1.28 and 1.20 times higher than PtGE and PtC, respectively. Kinetic analysis of PtNAGE shows that the oxygen reduction reaction follows first order kinetics involving a four electron transfer mechanism with the direct formation of water. In addition to this, it has been observed that PtNAGE also prevents surface area degradation better than the commercial platinised carbon under potential induced conditions.

Iodine Ions Mediated Formation of Monomorphic Single-Crystalline Platinum Nanoflowers

Well-defined and strikingly monomorphic single-crystalline Pt nanoflowers were successfully synthesized through the addition of a large amount of iodine ions into polyol process (5 mM H2PtCl6, 30 mM KI, and 50 mM PVP in ethylene glycol solution at 160 °C). The detailed structures of the Pt nanoflowers were studied with high-resolution TEM, indicating that high-quality production of the Pt nanoflowers could be obtained when the KI concentration was increased to six times of H2PtCl6. The size of Pt nanoflowers could be tuned by changing the concentration of H2PtCl6 with the constant Pt/I ratio (1:6). The formation process of the nanoflowers was investigated by the UV–vis and EXAFS spectroscopic studies, demonstrating that the iodine ions played a key role in inducing the formation of the single-crystalline Pt nanoflowers. After the addition of iodine ions into the polyol synthesis, the Pt–I complex was formed and reduced by different kinetics compared with that of H2PtCl6 to induce the overgrowth of Pt nano...

Electrochemical characterization of a porous Pt nanoparticle “Nanocube-Mosaic” prepared by a modified polyol method with HCl addition

Porous and single crystalline platinum (Pt) nanoparticles (NPs) have been successfully synthesized by reduction of H2PtCl6·6H2O and then investigated by optical spectroscopy and transmission electron microscopy. H2PtCl6·6H2O was reduced using ethylene glycol in the presence of polyvinylpyrrolidone under highly acidic conditions (pH < 1) to form single crystalline Pt particles about 5 nm in size. These particles were then stacked via {100} facets, forming 50-nm length porous nanocubes with a mosaic structure. The porous Pt NPs exhibited excellent catalytic properties for methanol oxidation. In particular, the electrochemical surface area was ∼63 m2/g, five times higher than that for non-porous Pt NPs prepared using a conventional method. We suggest that the high catalytic activity of porous Pt NPs is due to a combination of the crystalline structure having exposed {100} facets and a porous morphology. Open image in new window

A novel electroless method for the deposition of single-crystalline platinum nanoparticle films on an organic solid matrix in the presence of gold single crystals

Single-crystalline platinum nanoparticles (Pt-NPs) were directly deposited on and inside an organic solid matrix by a novel electroless galvanic method. The reducing agent was a solid phase confined with both a reducing agent and a stabilizer combined with initially deposited minute gold single crystals (Au-SCs) on the surface (Au-SM). The oxidizing agent (K2PtCl4) was in the aqueous solution phase. The reaction was carried out at the solid–liquid interface system at 80 °C and pH 1. The deposited Pt-NPs formed a uniform silver-grey conductive coating on the top of the Au-SM when the concentration of K2PtCl4 in the solution was 50 mM. The film consisted of single-crystalline Pt-NPs nearly identical in shape and size (∼250 nm). The initially deposited minute Au-SCs by the electroless deposition process were found to catalyze the formation of Pt-NPs having fcc crystal structures with dominating {111} crystal facets. The formation of Pt-NPs was not observed in the absence of initially deposited Au-SCs on the solid matrix. This method was also applicable for the deposition of deep ruby-red, optically transparent and mechanically stable Pt-NPs on and inside the Au-SM for lower concentrations of K2PtCl4 (1 to 5 mM) in the solution phase. This characteristic color was due to the strong surface plasmon absorption of Pt-NPs in the visible range. The Pt atoms initially deposited on the surface and diffused through the entire Au-SM to the opposite side that was not in contact with the oxidizing agent during the period of the reaction. The Pt-NPs on the solution-side of the Au-SM were partially naked and stabilized only from the bottom side, i.e., the upper sides of these nanocrystals were free from the stabilizer coating. The Au-SCs immobilized thiol-based epoxy resin was used as the Au-SM. The proposed Au-SM method was also applicable for the formation of ruby-red iron, palladium and beautiful, rainbow-like colorful silver nanoparticles on the Au-SM surface.

Oxidation of carbon monoxide on poly-oriented and single-crystalline platinum electrodes over a wide range of pH

The oxidative stripping of pre-adsorbed carbon monoxide has been studied on poly-oriented platinum, and on Pt(111), Pt(100) and Pt(110) single-crystal electrodes in phosphate buffer solutions as a function of pH, both stripping voltammetry and chronoamperometry. It was found that the stripping peak potential has a tendency to decrease as a function of pH until a pH of ca. 10–11, which is ascribed to a weaker adsorption of phosphate on platinum with increasing pH. Above a pH of ca. 11, the stripping peak appears to stay constant or increase, depending on the surface structure. We hypothesize that this may be due to the fact that above a pH of ca. 10–11, the main product of carbon monoxide oxidation is carbonate, which may be block active sites on the surface. By comparison with the stripping on the single-crystalline Pt, the stripping voltammetry on the poly-oriented Pt electrode appears as a convolution of the oxidation on the different facets. A similar conclusion is drawn for the chronoamperometry experiments, and it is suggested that this may be the main reason for the asymmetrical transients observed on the poly-oriented Pt electrodes, as well as on other strongly heterogeneous Pt electrodes that have been studied in the literature.

Platinum-Centered Yolk−Shell Nanostructure Formation by Sacrificial Nickel Spacers

We have synthesized Pt@silica/nickel phyllosilicate and Pt@silica yolk-shell nanostructures from NiPt@silica core-shell particles by simple chemical treatments. Silica coating of the NiPt alloy nanoparticles via the microemulsion method yielded spherical NiPt@silica core-shell nanoparticles with an average core diameter of 6.5 nm. Under a reflux condition in water, the core-shell structure transformed into Pt@silica yolk-shell nanoparticles with branched nickel phyllosilicate, which exhibited high surface area and large pore volume. The addition of hydrochloric acid selectively etched the nickel component from the NiPt cores and yielded Pt@silica yolk-shell nanoparticles with single-crystalline platinum cores. The average diameter of the metal cores was reduced to 4.5 nm. In both cases, the nickel components behaved as sacrificial spacers and successfully formed a vacancy between the metal cores and the silica hollow shells.

Synthesis of Porous Single-Crystalline Platinum Nanocubes Composed of Nanoparticles

Single-crystalline platinum nanocubes with porous morphology were synthesized for the first time by using ethylene glycol, HCl, and polyvinylpyrrolidone as the reducing agents of H2PtCl6. The morphology and size distribution of the Pt particles formed were studied with a high-resolution transmission electron microscope and selected-area electron diffraction pattern. By controlling the material concentrations and reaction temperature and period, Pt single crystals about 5 nm in size were formed in the first stage of the reduction process that had {100} facets, which were stacked one on top of the other, forming porous nanocubes 20-80 nm in length. The synthesized Pt nanocubes exhibited enhanced catalytic activity for methanol oxidation.

Preparation of single-crystalline platinum nanowires with small diameters under mild conditions

We report a convenient method to synthesize single-crystalline platinum nanowires with high aspect ratio of ca. 2.0 nm diameter by sophisticated and precise control of Pt(0) nuclei and their growth.

Synthesis of Single‐Crystalline Platinum Nanorods within a Soft Crystalline Surfactant–PtII Complex

Single-crystalline platinum nanorods, monodisperse in diameter, are synthesized through a simple process at room temperature, in cetyltrimethyl ammonium bromide (CTAB) solution. The complexation of the CTA+ surfactant ion with tetrachloroplatinate in the presence of hexanol leads to the formation of a precipitate with a lamellar crystalline structure. The reduction of Pt(II) metal ions to Pt(0) is carried out using gamma radiolysis. Transmission electron microscopy (TEM) observations of the nanoparticles extracted from the solution, three weeks after radiolysis, revealed single-crystalline Pt nanorods, monodisperse in diameter (3-4 nm) and 20-60 nm long. By following the shape of the nanorods at various stages of the growth, it was found that the single-crystalline nanorods grow by coalescence of spherical seeds 3-4 nm in diameter. This suggests an aggregative mechanism similar to that recently observed for silver particles in solution.

Single-Crystalline Platinum Nanosheets from Nonionic Surfactant 2-D Self-Assemblies at Solid/Aqueous Solution Interfaces

Single-crystalline platinum nanosheets have been prepared via a new methodology based on the chemical reduction of a platinum salt (H2PtCl6) with hydrazine at a graphite/solution interface, using polyoxyethylene (20) sorbitan monostearate (Tween 60) based self-assembly (hemicylindrical micelle) templates. The platinum nanosheets with a uniform thickness of as thin as 3.5 +/- 1 nm are surface-smooth and continuous over relatively large length scales of micrometer sizes. In striking contrast to the Tween 60 based system, no Pt nanosheets are obtained with nonaethylene glycol monododecyl ether (C12EO9) and polyoxyethylene (23) dodecyl ether (C12EO23). No Pt nanosheets are also obtainable with a laterally homogeneous layer of Tween 60 formed at the silica/solution interface. These results indicate that surfactant Tween 60 molecules with a triple polyoxyethylene structure, as well as their hemicylindrical micelle templates, play an essential role for the formation of the Pt nanosheets. It is also suggested that the interfacially directed growth of Pt metals within the aqueous shells of the Tween 60 hemicylindrical micelles induces the thin Pt crystals as thick as the aqueous shells. The present approach could be extended to prepare a wide range of novel nanostructures of noble metals, using various micelle-like self-assemblies at interfaces.

Inhibition of the underpotential deposition of copper on single crystalline platinum surfaces

The process of underpotential deposition (UPD) of copper on Pt(111), Pt(100) and Pt(110) single crystalline surfaces changes upon the addition of inhibitors such as thiourea and dithiadecyldisodiumsulphonate to the electrolyte. The specific nature of cyclic voltammograms disappears and a new pattern with similar features for all Pt surfaces is generated. This holds also for polycrystalline Pt electrodes subjected to the same procedure. A scheme of the UPD layer that forms on inhibited surfaces is suggested.

Photoelectron emission from poly- and single-crystalline platinum and gold electrodes using pulsed ultraviolet laser irradiation

Photoelectron emission from poly- and single-crystalline platinum and gold electrodes into an electrolyte was studied using a pulsed excimer laser, operating with KrF (λ = 248 nm = 5 eV). The 5/2 power law can be applied to the plot of the photocharge Q versus the electrode potential. The values for the threshold energy determined from these plots were higher (mean value 4.0 eV) than the values established in the literaure (3.15 eV). Anodic photocurrents were observed for gold in acid electrolytes above 0.5 V vs. SHE, but below the oxide layer formation, which also showed a linear dependence of the photocurrents to the power of 0.4 on the electrode potential. A threshold energy of 5.0 eV was determined.

Volume and electrical resistivity change due to interstitial atoms in platinum

Lattice parameter and residual electrical resistivity were measured simultaneously for single crystalline platinum foils after irradiation with 3 MeV electrons at 4.5K. The defect specific ratio of both property changes was found. It remained constant through all recovery stages. The specific volume change due to a Frenkel pair was ( Delta nu / Omega FPt=1.6+or-0.1 and that due to an interstitial was ( Delta nu / Omega )1Pt=2.0+or-0.2.

Kirkendall effect and diffusion in the gold-platinum system—I: The Kirkendall effect

The Kirkendall effect has been studied in the gold-platinum system, one of the rare systems with a miscibility gap in the solid state. The effect has been measured as a function of time and temperature in a great number of “sandwich” couples. These couples consisted of pure gold and pure platinum, polycrystalline and singlecrystalline, and of pure gold and a gold-rich alloy with 73.5 at.% gold. It could be established, as a consequence of the obtained accuracy of the measurements, that the difference in magnitude of the effect in couples with polycrystalline platinum and in couples with singlecrystalline platinum has a physical significance. It has been interpreted in terms of an extra diffusion transport along the grain boundaries in polycrystalline platinum. Besides the Kirkendall effect the phenomena associated with this effect, such as porosity, dimensional and structural changes, has been studied as well as the phase-boundary displacement.