Hung-Lung Chou

Surface-enhanced Raman scattering ( SERS ) from Au:Ag bimetallic nanoparticles : the effect of the molecular probe

Surface-enhanced Raman scattering (SERS) from molecular probes adsorbed on Au:Ag bimetallic nanoparticles with various compositions was investigated. Au:Ag bimetallic nanoparticles (NPs), with the diameters between 3–5 nm, were prepared and characterized by HRTEM and UV-Vis absorption. Their SERS properties were examined by using four different probe molecules, and compared with NPs made of pure Au or Ag. It is found that the SERS property of the alloy NPs is not only dependent on the Au:Ag ratio of the bimetallic NPs, but also on the chemical nature of the SERS probe. For the two positively charged SERS probes, oxazine 720 (Oxa) and Nile Blue A (NBA), the alloy NPs with higher Au content provided the largest SERS signal. However, for the probes 4-hydroxythiophenol (HTP) and thiophenol (TP), the best SERS performance was obtained for the highest Ag ratio. DFT calculations indicated a charge-transfer between Au and Ag atoms in the alloys, creating positively charged domains rich in Ag atom, and negatively charge regions dominated by Au atoms. It is proposed that the probe-specific enhancement is related to the selective binding of probe molecules to the partially charged surface domains in the alloys. Our results suggest that SERS substrate optimizations based on bimetallic nanoparticles should consider the nature of the probes and the electronic-induced effects from the alloys.

Robust non-carbon Ti0.7Ru0.3O2 support with co-catalytic functionality for Pt: enhances catalytic activity and durability for fuel cells

Multifunctional binary metal oxide (Ti0.7Ru0.3O2), a novel functionalised co-catalytic support for Pt, is synthesized in a simple one-step hydrothermal process at low temperature. In practical applications Ti0.7Ru0.3O2 offers both excellent improvements in electrocatalytic activity and durability over commercial carbon supported Pt and PtRu catalysts for direct methanol fuel cell (DMFC), while at the molecular level it provides advantages in terms of its high surface area, and the strong interactions between Pt and the co-catalytic support. The Ti0.7Ru0.3O2 support acts as a co-catalyst supporting Pt activity, due to the high proton conductivity of hydrated Ti0.7Ru0.3O2 which underlies a ‘bifunctional mechanism’ and the synergistic effect between Pt and Ti0.7Ru0.3O2, modifying the electronic nature of the metal particles as well, which additionally enhances CO-tolerance, the catalytic activity and durability for methanol and hydrogen oxidation. Additionally, Ti0.7Ru0.3O2 can be fabricated as a much thinner catalyst layer resulting in improving mass transport kinetics, giving a broad scope for its wider application in other fuel cells, as demonstrated here by its application in a direct methanol fuel cell (DMFC) and polymer electrolyte membrane fuel cell (PEMFC) and can also be extended to other areas such as catalytic biosensor technology.

Kinetically Controlled Autocatalytic Chemical Process for Bulk Production of Bimetallic Core–Shell Structured Nanoparticles

Although bimetallic core@shell structured nanoparticles (NPs) are achieving prominence due to their multifunctionalities and exceptional catalytic, magnetic, thermal, and optical properties, the rationale underlying their design remains unclear. Here we report a kinetically controlled autocatalytic chemical process, adaptable for use as a general protocol for the fabrication of bimetallic core@shell structured NPs, in which a sacrificial Cu ultrathin layer is autocatalytically deposited on a dimensionally stable noble-metal core under kinetically controlled conditions, which is then displaced to form an active ultrathin metal-layered shell by redox-transmetalation. Unlike thermodynamically controlled under-potential deposition processes, this general strategy allows for the scaling-up of production of high-quality core-shell structured NPs, without the need for any additional reducing agents and/or electrochemical treatments, some examples being Pd@Pt, Pt@Pd, Ir@Pt, and Ir@Pd. Having immediate and obvious commercial potential, Pd@Pt NPs have been systematically characterized by in situ X-ray absorption, electrochemical-FTIR, transmission electron microscopy, and electrochemical techniques, both during synthesis and subsequently during testing in one particularly important catalytic reaction, namely, the oxygen reduction reaction, which is pivotal in fuel cell operation. It was found that the bimetallic Pd@Pt NPs exhibited a significantly enhanced electrocatalytic activity, with respect to this reaction, in comparison with their monometallic counterparts.

Controlled Synthesis of CdSe Quantum Dots by a Microwave-Enhanced Process: A Green Approach for Mass Production

A method that does not employ hot-injection techniques has been developed for the size-tunable synthesis of high-quality CdSe quantum dots (QDs) with zinc blende structure. In this environmentally benign synthetic route, which uses less toxic precursors, solvents, and capping ligands, CdSe QDs that absorb visible light are obtained. The size of the as-prepared CdSe QDs and thus their optical properties can be manipulated by changing the microwave reaction conditions. The QDs were characterized by XRD, TEM, UV/Vis, FTIR, time-resolved fluorescence spectroscopy, and fluorescence spectrophotometry. In this approach, the reaction is conducted in open air and at a much lower temperature than in hot-injection techniques. The use of microwaves in this process allows for a highly reproducible and effective synthesis protocol that is fully adaptable for mass production and can be easily employed to synthesize a variety of semiconductor QDs with the desired properties. Possible applications of the CdSe QDs were assessed by deposition on TiO(2) films.

Relating Structural Aspects of Bimetallic Pt3Cr1/C Nanoparticles to Their Electrocatalytic Activity, Stability, and Selectivity in the Oxygen Reduction Reaction

Two methods were used to prepare bimetallic Pt(3)Cr(1)/C nanocatalysts with similar composition but different alloying extent (structure). We investigated how these differences in alloying extent affect the catalytic activity, stability and selectivity in the oxygen reduction reaction (ORR). One method, based on slow thermal decomposition of the Cr precursor at a rate that matches that of chemical reduction of the Pt precursor, allows fine control of the composition of the Pt(3)Cr(1)/C alloy, whereas the second approach, using the ethylene glycol method, results in considerable deviation (>25 %) from the projected composition. Consequently, these two methods lead to variations in the alloying extent that strongly influence the Pt d-band vacancy and the Pt electroactive surface area (Pt ESCA). This relationship was systematically evaluated by transmission electron microscopy, X-ray absorption near edge structure spectroscopy, and electrochemical analysis. The ORR activity depends on two effects that nullify each other, namely, the number of active Pt sites and their activity. The Pt-site activity is more dominant in governing the ORR activity. The selectivity of the nanocatalyst towards the ORR and the competitive methanol oxidation reaction (MOR) depend on these two effects acting in cooperation to give enhanced ORR activity with suppressed MOR. The number of active Pt sites is associated with the Pt ESCA value, while Pt-site activity is associated with the alloying extent and Pt d-band vacancy (electronic) effects. The presence of Cr atoms in Pt(3)Cr(1)/C enhances stability during electrochemical treatment. Overall, the Pt(3)Cr(1)/C catalyst prepared by controlled-composition synthesis was shown to be superior in ORR activity, selectivity and stability owing to its favorable alloying extent, Pt d-band vacancy, and Pt ESCA.

Combined Experimental and Computational Studies of a Na2 Ni1-x Cux Fe(CN)6 Cathode with Tunable Potential for Aqueous Rechargeable Sodium-Ion Batteries.

Herein, potential-tunable Na2 Ni1-x Cux Fe(CN)6 nanoparticles with three-dimensional frameworks and large interstitial spaces were synthesized as alternative cathode materials for aqueous sodium-ion batteries by controlling the molar ratio of Ni(II) to Cu(II) at ambient temperature. The influence of the value of x on the crystalline structure, lattice parameters, electrochemical properties, and charge transfer of the resultant compound was explored by using powder X-ray diffractometry, density functional theory, cyclic voltammetry, galvanostatic charge-discharge techniques, and Bader charge analysis. Of the various formulations investigated, that with x=0.25 delivered the highest reversible capacity, superior rate capability, and outstanding cycling performance. These attributes are ascribed to its unique face-centered cubic structure for facile sodium-ion insertion/extraction and the strong interactions between Cu and N atoms, which promote structural stability.

Interactions between silver nanoparticles and polyvinyl alcohol nanofibers

The interaction of polyvinylalcohol (PVA) nanofibers with silver (Ag) nanoparticles (mean diameter 8nm) has been modeled using density functional theory (DFT) calculations. The physical adsorption of PVA through the hydroxyl group, to the Ag, and its corresponding molecular orientation was compared with experimental results obtained from surface-enhanced Raman scattering (SERS) studies of the same material. A good agreement was found between the computational model of the vibrational spectrum of the adsorbate and the experimentally observed SERS. In general, aliphatic capping molecules are used to passivate the surface of Ag55 nanocrystals (55 = atomic number of Ag). In this study, a DFT simulation was employed to show binding energies and electron contour map analyses of Ag55 with PVA. Here we show that the PVA interacts with the Ag nanoparticles surface, through the OH group, thereby contributing significantly to the increase in SERS activity.

Relating the Composition of PtxRu100−x/C Nanoparticles to Their Structural Aspects and Electrocatalytic Activities in the Methanol Oxidation Reaction

A controlled composition-based method--that is, the microwave-assisted ethylene glycol (MEG) method--was successfully developed to prepare bimetallic Pt(x)Ru(100-x)/C nanoparticles (NPs) with different alloy compositions. This study highlights the impact of the variation in alloy composition of Pt(x)Ru(100-x)/C NPs on their alloying extent (structure) and subsequently their catalytic activity towards the methanol oxidation reaction (MOR). The alloying extent of these Pt(x)Ru(100-x)/C NPs has a strong influence on their Pt d-band vacancy and Pt electroactive surface area (Pt ECSA); this relationship was systematically evaluated by using X-ray absorption (XAS), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), density functional theory (DFT) calculations, and electrochemical analyses. The MOR activity depends on two effects that act in cooperation, namely, the number of active Pt sites and their activity. Here the number of active Pt sites is associated with the Pt ECSA value, whereas the Pt-site activity is associated with the alloying extent and Pt d-band vacancy (electronic) effects. Among the Pt(x)Ru(100-x)/C NPs with various Pt:Ru atomic ratios (x = 25, 50, and 75), the Pt(75)Ru(25)/C NPs were shown to be superior in MOR activity on account of their favorable alloying extent, Pt d-band vacancy, and Pt ECSA. This short study brings new insight into probing the synergistic effect on the surface reactivity of the Pt(x)Ru(100-x)/C NPs, and possibly other bimetallic Pt-based alloy NPs.

Investigation of formation mechanism of Pt(111) nanoparticle layers grown on Ru(0001) core.

A layer growth mechanism of Pt-Ru bimetallic nanoparticles has been proposed with supporting experiments and calculations by density functional theory (DFT). Elongated Pt atoms on Ru nanoparticles were synthesized via a two-step route, and their structural details were obtained by high-resolution transmission electron microscopy. Because of the intrinsic mismatch of lattice spacing between the two elements, such an unusual growth was analyzed with the DFT simulations to explore the mystery of the growth mechanism. Pt atoms would rearrange the packing order and adjust the Pt-Pt atomic distance, and so do the Ru nanoparticles in order to achieve the optimal energy status of the bimetallic system. The resultant Pt(111) layers could stack on top of the Ru(0001) core more tightly by fitting the pockets left between the Ru atoms. The findings give insight into the formation mechanism of the nanosized Pt-Ru bimetallic catalyst and pave the way for designing bimetallic catalysts with tailored properties at the atomic level.

Trimetallic (Aurod-Pdshell-Ptcluster) Catalyst Used as Amperometric Hydrogen Peroxide Sensor

Bimetallic nanostructured core-shell structures are commonly used as catalysts in a wide variety of reactions. We surmised that the addition of an additional metal would potentially allow catalytic tailoring with the possibility of an increase in activity. Here a tri-metallic catalytic structure, consisting of clustered catalytic Pt on the surface of a Pd shell supported on a rod shaped Au core was fabricated. The significance of the additional metallic component is shown by comparative electrochemically active surface area (ECSA) analysis results for the trimetallic Aurod-Pdshell-Ptcluster, bimetallic Aurod-Ptcluster and monometallic JM-Pt (used as a reference), which have respective ECSA values (cm2/mgPt) of 1883.0, 1371.7 and 879. The potential utility of the trimetallic catalysts was shown in a hydrogen peroxide sensing protocol, which showed the catalyst to have a sensitivity of 604 ìA/mMcm2 within a linear range of 0.0013–6.191 mM.