We report selective electrocatalytic reduction of carbon dioxide to carbon monoxide on gold nanoparticles (NPs) in 0.5 M KHCO3 at 25 °C. Among monodisperse 4, 6, 8, and 10 nm NPs tested, the 8 nm Au NPs show the maximum Faradaic efficiency (FE) (up to 90% at −0.67 V vs reversible hydrogen electrode, RHE). Density functional theory calculations suggest that more edge sites (active for CO evolution) than corner sites (active for the competitive H2 evolution reaction) on the Au NP surface facilitates the stabilization of the reduction intermediates, such as COOH*, and the formation of CO. This mechanism is further supported by the fact that Au NPs embedded in a matrix of butyl-3-methylimidazolium hexafluorophosphate for more efficient COOH* stabilization exhibit even higher reaction activity (3 A/g mass activity) and selectivity (97% FE) at −0.52 V (vs RHE). The work demonstrates the great potentials of using monodisperse Au NPs to optimize the available reaction intermediate binding sites for efficient and ...

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Monodisperse Au nanoparticles for selective electrocatalytic reduction of CO2 to CO.

In this communication, we show that ultrathin Au nanowires (NWs) with dominant edge sites on their surface are active and selective for electrochemical reduction of CO2 to CO. We first develop a facile seed-mediated growth method to synthesize these ultrathin (2 nm wide) Au NWs in high yield (95%) by reducing HAuCl4 in the presence of 2 nm Au nanoparticles (NPs). These NWs catalyze CO2 reduction to CO in aqueous 0.5 M KHCO3 at an onset potential of −0.2 V (vs reversible hydrogen electrode). At −0.35 V, the reduction Faradaic efficiency (FE) reaches 94% (mass activity 1.84 A/g Au) and stays at this level for 6 h without any noticeable activity change. Density functional theory (DFT) calculations suggest that the excellent catalytic performance of these Au NWs is attributed both to their high mass density of reactive edge sites (≥16%) and to the weak CO binding on these sites. These ultrathin Au NWs are the most efficient nanocatalyst ever reported for electrochemical reduction of CO2 to CO.

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Active and Selective Conversion of CO2 to CO on Ultrathin Au Nanowires

Despite intense research in past decades, the lack of high-performance catalysts for fuel cell reactions remains a challenge in realizing fuel cell technologies for transportation applications. Here we report a facile strategy for synthesizing hierarchical platinum-cobalt nanowires with high-index, platinum-rich facets and ordered intermetallic structure. These structural features enable unprecedented performance for the oxygen reduction and alcohol oxidation reactions. The specific/mass activities of the platinum-cobalt nanowires for oxygen reduction reaction are 39.6/33.7 times higher than commercial Pt/C catalyst, respectively. Density functional theory simulations reveal that the active threefold hollow sites on the platinum-rich high-index facets provide an additional factor in enhancing oxygen reduction reaction activities. The nanowires are stable in the electrochemical conditions and also thermally stable. This work may represent a key step towards scalable production of high-performance platinum-based nanowires for applications in catalysis and energy conversion.

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Surface engineering of hierarchical platinum-cobalt nanowires for efficient electrocatalysis

The search for improved heterogeneous catalysts is an important but difficult task. Scaling relations between the adsorption energies of reaction intermediates greatly facilitate the computational design of catalysts. However, this methodology does not currently incorporate structure sensitivity and hence cannot describe adequately the overall activity of realistic catalyst particles and extended surfaces with several facets, edges and apices. Here, we generalize scaling relations by examining twelve different low-index, stepped and kinked surfaces of nine transition metals. This allows us to quantify the effect of the adsorption-site geometry on these relations, ensures a full prediction of their parameters, and helps in identifying intrinsic thermodynamic restrictions to the performance of catalysts. The resulting fully predictable, structure-sensitive scaling relations are a step towards the long-sought rational design of multifaceted catalytic particles. Such a design can now target not only the chemical nature of active materials but also the actual geometry of their active sites.

Introducing structural sensitivity into adsorption–energy scaling relations by means of coordination numbers

Platinum is a prominent catalyst for a multiplicity of reactions because of its high activity and stability. As Pt nanoparticles are normally used to maximize catalyst utilization and to minimize catalyst loading, it is important to rationalize and predict catalytic activity trends in nanoparticles in simple terms, while being able to compare these trends with those of extended surfaces. The trends in the adsorption energies of small oxygen- and hydrogen-containing adsorbates on Pt nanoparticles of various sizes and on extended surfaces were analyzed through DFT calculations by making use of the generalized coordination numbers of the surface sites. This simple and predictive descriptor links the geometric arrangement of a surface to its adsorption properties. It generates linear adsorption-energy trends, captures finite-size effects, and provides more accurate descriptions than d-band centers and usual coordination numbers. Unlike electronic-structure descriptors, which require knowledge of the densities of states, it is calculated manually. Finally, it was shown that an approximate equivalence exists between generalized coordination numbers and d-band centers.

Fast Prediction of Adsorption Properties for Platinum Nanocatalysts with Generalized Coordination Numbers

Gold is known to become significantly more catalytically active as its particle size is reduced, and other catalysts are also known to exhibit finite-size effects. To understand the trends related to finite-size effects, we have used density functional theory to study adsorption of representative adsorbates, CO and O, on the late transition metals Co, Ni, Cu, Ir, Pd, Ag, Rh, Pt and Au. We studied adsorption energies and geometries on 13-atom clusters and compared them to the fcc(111) and fcc(211) crystal facets. In all cases, adsorbates were found to bind significantly more strongly to the 13-atom clusters than to the extended surfaces. The binding strength of both adsorbates were found to correlate very strongly with the average coordination number of the metal atoms to which the adsorbate binds, indicating that the finite-size effects in bonding are not specific to gold.

Finite-Size Effects in O and CO Adsorption for the Late Transition Metals

The interplay of viscous and elastic stresses is relevant to a number of flow problems involving slender elastic fibers. These range from the swimming of microorganisms to the transport of pulp fibers in processing flow as well as from nanotube and nanocarpet applications to semi-flexible polymer behavior. In some applications, slender fibers are attached to walls where they experience externally applied flows. In this paper, we focus on the model problem of a wall mounted filament in a (compressive) extensional flow and characterize the flow-induced bending and buckling of the fiber. Using a combination of stability analysis and numerical simulations (with the latter based on a discretized beam model), we show that, for a critical value of the ratio between viscous and elastic forces, the filament is susceptible to bending and buckling instabilities at supercritical bifurcation points.

Buckling transitions of an elastic filament in a viscous stagnation point flow

In the present work, we have extended a slip-link based model, originally proposed by Masubuchi et al. [J. Chem. Phys. 115, 4387 (2001)], to simulate planar extensional flow. The salient features of the extended model are (i) an unrestricted simulation time for planar extensional flow using Kraynik–Reinelt periodic boundary conditions, (ii) an implicit time stepping scheme to enhance numerical stability, and (iii) a mesh-free Lagrangian method for the evaluation of the osmotic force. With the new code, we have simulated both the static and dynamic responses of monodisperse linear worm-like and inverse Langevin chains and compared the simulations results with the experimental data reported by Bach et al. [J. Rheol. 47, 429 (2003)]. The simulations were run for monodisperse linear chains including up to 50 entanglements. The model is able to reasonably capture the rheological behavior of entangled systems in the nonlinear range. Based on our simulations, we also present an alternative explanation for the dy...

Slip-link simulations of entangled polymers in planar extensional flow: Disentanglement modified extensional thinning

We present computer simulations of entangled finitely extensible wormlike chains, including real space chain coupling, in shearing flows. Extending existing simulation techniques, we examine the mi...

Slip-Link Simulations of Entangled, Finitely Extensible, Wormlike Chains in Shear Flow

DNA has been recently explored as a powerful tool for developing molecular scaffolds for making reproducible and reliable metal contacts to single organic semiconducting molecules. A critical step in the process of exploiting DNA-organic molecule-DNA (DOD) array structures is the controlled tethering and stretching of DNA molecules. Here we report the development of reproducible surface chemistry for tethering DNA molecules at tunable density and demonstrate shear flow processing as a rationally controlled approach for stretching/aligning DNA molecules of various lengths. Through enzymatic cleavage of λ-phage DNA to yield a series of DNA chains of various lengths from 17.3 μm down to 4.2 μm, we have investigated the flow/extension behavior of these tethered DNA molecules under different flow strengths in the flow-gradient plane. We compared Brownian dynamic simulations for the flow dynamics of tethered λ-DNA in shear, and found our flow-gradient plane experimental results matched well with our bead-spring simulations. The shear flow processing demonstrated in our studies represents a controllable approach for tethering and stretching DNA molecules of various lengths. Together with further metallization of DNA chains within DOD structures, this bottom-up approach can potentially enable efficient and reliable fabrication of large-scale nanoelectronic devices based on single organic molecules, therefore opening opportunities in both fundamental understanding of charge transport at the single molecular level and many exciting applications for ever-shrinking molecular circuits.

Amit Kushwaha

Papers

Finite-Size Effects in O and CO Adsorption for the Late Transition Metals

Buckling transitions of an elastic filament in a viscous stagnation point flow

Slip-link simulations of entangled polymers in planar extensional flow: Disentanglement modified extensional thinning

Slip-Link Simulations of Entangled, Finitely Extensible, Wormlike Chains in Shear Flow

The shear flow processing of controlled DNA tethering and stretching for organic molecular electronics.