Shuhui Sun

A Highly Durable Platinum Nanocatalyst for Proton Exchange Membrane Fuel Cells: Multiarmed Starlike Nanowire Single Crystal

Despite significant recent advances, the long-term durability of Pt catalyst at the cathode is still being recognized as one of the key challenges that must be addressed before the commercialization of proton exchange membrane fuel cells (PEMFCs). 2] The loss of Pt electrochemical surface area (ECSA) over time, because of corrosion of the carbon support and Pt dissolution/aggregation/Oswald ripening, is considered one of the major contributors to the degradation of fuel cell performance. Up to now, highly dispersed Pt nanoparticles (NPs, 2–5 nm) on carbon supports are still being widely used as the state-of-the-art commercial catalysts, and most reported studies are focused on nanoparticles of Pt. However, Pt with nanosized particle morphologies has high surface energies, thereby inducing severe Oswald ripening and/or grain growth during fuel cell operation. One-dimensional (1D) nanostructures of Pt, such as nanowires (NWs) and nanotubes (NTs), have been demonstrated to overcome the drawbacks of NPs in fuel cells, owning to their unique 1D morphologies. Yan et al. reported that unsupported Pt nanotubes exhibit much enhanced catalytic activity and durability as fuel cell cathode catalyst. Sun et al. and Zhou et al. reported the improved oxygen reduction reaction (ORR) activities of Pt NWs at the cathode under fuel cell operating conditions. However, up to now, the durability of Pt NW-based electrocatalysts has never been reported in the literature. Here we describe a new approach to address, for the first time, both the activity and durability issues by using carbonsupported multiarmed starlike Pt nanowires (starlike PtNW/ C) as electrocatalysts. Interestingly, the durability can be further improved by eliminating the carbon support, that is, using unsupported Pt nanowires as the cathode catalyst. As a result of their unique 1D morphology, the starlike Pt nanowire electrocatalyst can provide various advantages. First, the interconnected network consists of multiarmed, star-shaped 1D NWs with arm lengths of tens of nanometers which makes the Pt less vulnerable to dissolution, Ostwald ripening, and aggregation during fuel cell operation compared to Pt nanoparticles. Second, this network structure reduces the number of embedded electrocatalyst sites in the micropores of the carbon supports relative to those in nanogrannular Pt. Third, the mass transfer within the electrode can be effectively facilitated by networking the anisotropic morphology. Carbon-supported multiarmed starlike platinum nanowires were synthesized by the chemical reduction of a Pt precursor with formic acid in aqueous solution at room temperature and under ambient atmosphere. No surfactant, which is usually harmful for catalytic activities, was used in the experiments. Figure 1A and B show the representative scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, respectively, of carbon-supported Pt nanowires at 40 wt % loading of Pt. It can be seen that the assynthesized Pt is nanostar-shaped, being composed of several short arms of Pt nanowires. The number of arms of each nanostar is found to vary ranging from several to over ten. Occasionally, single-armed nanowires standing on the carbon surface can also be observed. Diameter and length of the arms of starlike Pt nanowires are about 4 nm and 15 nm, respectively. More interestingly, from the connected atomic arrangement shown in the high-resolution TEM (HRTEM) images (see Figure S1 in the Supporting Information and the inset in Figure 1B), the nanostar is found to be a single crystal. The fast Fourier transform (FFT; see inset in Figure S1) of the original HRTEM image shows a dotted pattern, further proving that the nanostar is a single crystal. This indicates that the formation mechanism of the nanostar involves seeded growth rather than an aggregation of seeded particles or an assembly process of the nanowires. The X-ray diffraction (XRD) pattern (Figure S2) confirms that the carbon-supported Pt nanowires are crystallized in a face-centered-cubic (fcc) structure similar to bulk Pt, which is consistent with the HRTEM investigations. We believe that the growth of the multiarmed starlike PtNWs on carbon black supports follows a similar process to that for Pt NWs on other supports. Typically, Pt nuclei are first formed in solution through the reduction of H2PtCl6 by HCOOH, and they deposit on the surface of carbon spheres. The freshly formed nuclei act as the sites for further nucleation through the continual absorption and reduction of Pt(IV) ions leading to the formation of particle seeds. For fcc structures, the sequence of surface energies is g{111} < g{100} [*] Dr. S. Sun, Dr. G. Zhang, Dr. D. Geng, Y. Chen, R. Li, Prof. X. Sun Department of Mechanical and Materials Engineering The University of Western Ontario London, Ontario N6A 5B9 (Canada) Fax: (+ 1)519-661-3020 E-mail: xsun@eng.uwo.ca

Single-atom Catalysis Using Pt/Graphene Achieved through Atomic Layer Deposition

Platinum-nanoparticle-based catalysts are widely used in many important chemical processes and automobile industries. Downsizing catalyst nanoparticles to single atoms is highly desirable to maximize their use efficiency, however, very challenging. Here we report a practical synthesis for isolated single Pt atoms anchored to graphene nanosheet using the atomic layer deposition (ALD) technique. ALD offers the capability of precise control of catalyst size span from single atom, subnanometer cluster to nanoparticle. The single-atom catalysts exhibit significantly improved catalytic activity (up to 10 times) over that of the state-of-the-art commercial Pt/C catalyst. X-ray absorption fine structure (XAFS) analyses reveal that the low-coordination and partially unoccupied densities of states of 5d orbital of Pt atoms are responsible for the excellent performance. This work is anticipated to form the basis for the exploration of a next generation of highly efficient single-atom catalysts for various applications.

Raman scattering study of rutile SnO2 nanobelts synthesized by thermal evaporation of Sn powders

The Raman spectrum of single-crystalline rutile tin dioxide (SnO2) nanobelts synthesized by thermal evaporation of tin powders was studied. Three Raman shifts (474, 632, 774 cm−1) showed the typical feature of the rutile phase of the as-synthesized SnO2 nanobelts. It was found that two infrared (IR)-active modes (313 and 690 cm−1) appeared in Raman spectrum and some peaks were broadened.

Synthesis and optical properties of S-doped ZnO nanowires

S-doped ZnO nanowires with an average diameter of 80 nm and length up to several tens of micrometers were produced through a simple physical evaporation approach. The nanowires had a single-crystal hexagonal structure and grew along the [102] direction. Photoluminescence (PL) measurements showed that the doping of sulfur shifted the PL spectrum peak towards short wavelengths, and the doping quantity was found responsible for the different characteristics.

Direct Growth of Single‐Crystal Pt Nanowires on Sn@CNT Nanocable: 3D Electrodes for Highly Active Electrocatalysts

A newly designed and fabricated novel three dimensional (3D) nanocomposite composed of single-crystal Pt nanowires (PtNW) and a coaxial nanocable support consisting of a tin nanowire and a carbon nanotube (Sn@CNT) is reported. This nanocomposite is fabricated by the synthesis of Sn@CNT nanocables by means of a thermal evaporation method, followed by the direct growth with PtNWs through a facile aqueous solution approach at room temperature. Electrochemical measurements demonstrate that the PtNW--Sn@CNT 3D electrode exhibits enhanced electrocatalytic performance in oxygen reduction reaction (ORR) for polymer electrolyte membrane fuel cells (PEMFCs), methanol oxidation (MOR) for direct methanol fuel cells (DMFCs), and CO tolerance compared with commercial ETEK Pt/C catalyst made of Pt nanoparticles.

Electrochemical synthesis of copper nanowires

Large-scale copper nanowires have been fabricated by potentiostatic electrochemical deposition (ECD) of copper sulphate solution within the nanochannels of porous anodic alumina templates. Scanning electron microscopy, transmission electron microscopy, selected-area electron diffraction and x-ray diffraction techniques were used to characterize the copper nanowires obtained. It is found that the individual copper nanowires are dense and continuous, with uniform diameters (60 nm) along the entire lengths of the wires (30 µm). The single-crystal and polycrystal copper nanowires can be prepared by choosing suitable applied potentials in the copper ECD processes. Moreover, the formation of copper oxides in nanochannels is also discussed in detail. The investigation results reveal that a lower overpotential is necessary to fabricate copper nanowires with fine crystalline structures by the potentiostatic ECD technique.

Micro-Raman and infrared properties of SnO2 nanobelts synthesized from Sn and SiO2 powders

Rutile structured SnO2 nanobelts have been synthesized from the mixture of Sn powders and SiO2 nanoparticle powders. Each nanobelt has a uniform width of about several hundred nanometers and a thickness of about tens of nanometers along its entire length. Micro-Raman spectrum measurement on the SnO2 nanobelts shows that the first-order Raman A1g mode (632.9 cm−1) is very strong, and two weak Raman bands 498 and 694 cm−1 seem to correspond to infrared (IR)-active longitudinal optical (LO) and transverse optical (TO) of A2u modes. In addition, the IR spectrum of the SnO2 nanobelts shows the A2u (LO) (701.9 cm−1) and Eu (1) (TO) (634.5 cm−1) modes and one surface mode (565.2 cm−1). The IR-active bands in the Raman spectrum and the surface mode in IR spectrum, which may be due to the nanoscale morphology of the nanobelts.

Alumina nanowire arrays standing on a porous anodic alumina membrane

Alumina nanowire arrays standing on the surface of a porous anodic alumina membrane have been achieved by first forming a porous anodic alumina membrane with parallel Y-branched nanochannels by reducing the applied anodizing voltage by a factor of in the anodization process of high-purity Al foil, and then chemically etching the Y-branched nanochannel alumina membrane in an aqueous phosphoric acid solution. The novel nanostructures may be used for two-dimensional photonic bandgap structural materials.

Porous hollow α-Fe2O3@TiO2 core–shell nanospheres for superior lithium/sodium storage capability

Porous hollow α-Fe2O3@TiO2 core–shell nanospheres for use as anode materials in lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) have been successfully fabricated by a simple template-assisted method, which has been rarely reported before. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and N2 adsorption–desorption isotherms reveal that the as-prepared α-Fe2O3@TiO2 is composed of a hollow inner cavity and an outer shell with massive mesopores. This porous hollow structure is capable of buffering the large volume variation of α-Fe2O3 during cycling and preventing the electrode from pulverization and aggregation, as well as providing sufficiently large interstitial space within the crystallographic structure to host alkalis (Li and Na). As a consequence, this hybrid composite exhibits outstanding electrochemical properties, e.g., high specific capacity, excellent cyclability, satisfactory rate performance, and splendid initial coulombic efficiency for both LIBs and SIBs.

Y-branched Bi nanowires with metal–semiconductor junction behavior

Y-branched Bi nanowires (NWs) embedded in anodic aluminum oxide templates were synthesized by electrochemical deposition. Transmission electron microscope observations revealed that the “stem” and the “branches” of the Y-branched Bi NWs are about 80 and 50nm in diameter, respectively. Selected area electron diffraction studies showed that both the stem and the branches are single crystalline. Current–voltage measurement revealed that the parallel Y-branched Bi NWs have characteristics of conventional metal–semiconductor junctions. Our approach to produce one-dimensional metal–semiconductor junctions using Y-branched NWs consisting of only one kind of semimetal and without any external doping can be exploited to create metal–semiconductor junctions of other semimetals, which may find various applications in nanodevices.