Wenlei Zhu

Monodisperse Au Nanoparticles for Selective Electrocatalytic Reduction of CO2 to CO

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 selective electrocatalytic reduction of CO2 to CO.

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

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.

Core/Shell Au/CuPt Nanoparticles and Their Dual Electrocatalysis for Both Reduction and Oxidation Reactions

We report a facile synthesis of monodisperse core/shell 5/1.5 nm Au/CuPt nanoparticles by coreduction of platinum acetylacetonate and copper acetylacetonate in the presence of 5 nm Au nanoparticles. The CuPt alloy effect and core/shell interactions make these Au/CuPt nanoparticles a promising catalyst for both oxygen reduction reaction and methanol oxidation reaction in 0.1 M HClO4 solution. Their specific (mass) reduction and oxidation activities reach 2.72 mA/cm(2) (1500 mA/mg Pt) at 0.9 V and 0.755 mA/cm(2) (441 mA/mg Pt) at 0.8 V (vs reversible hydrogen electrode), respectively. Our studies show that the existence of the Au nanoparticle core not only minimizes the Pt usage but also improves the stability of the Au/CuPt catalyst for fuel cell reactions. The results suggest that the core/shell design is indeed effective for optimizing nanoparticle catalysis. The same concept may be extended to other multimetallic nanoparticle systems, making it possible to tune nanoparticle catalysis for many different chemical reactions.

New Approach to Fully Ordered fct-FePt Nanoparticles for Much Enhanced Electrocatalysis in Acid

Fully ordered face-centered tetragonal (fct) FePt nanoparticles (NPs) are synthesized by thermal annealing of the MgO-coated dumbbell-like FePt-Fe3O4 NPs followed by acid washing to remove MgO. These fct-FePt NPs show strong ferromagnetism with room temperature coercivity reaching 33 kOe. They serve as a robust electrocatalyst for the oxygen reduction reaction (ORR) in 0.1 M HClO4 and hydrogen evolution reaction (HER) in 0.5 M H2SO4 with much enhanced activity (the most active fct-structured alloy NP catalyst ever reported) and stability (no obvious Fe loss and NP degradation after 20 000 cycles between 0.6 and 1.0 V (vs RHE)). Our work demonstrates a reliable approach to FePt NPs with much improved fct-ordering and catalytic efficiency for ORR and HER.

A Sensitive H2O2 Assay Based on Dumbbell‐like PtPd‐Fe3O4 Nanoparticles

Dumbbell-like Pt(48)Pd(52)-Fe(3)O(4) nanoparticles are synthesized and functionalized with oleylamine-polyethyleneglycol to serve as an efficient catalyst for H(2)O(2) reduction and tetramethylbenzidine (TMB) oxidation in biological solutions. The Pt(48)Pd(52)-Fe(3)O(4)/TMB kit is even more active than the natural enzyme for H(2)O(2) detection with a detection limit reaching 2 μM, and is successfully used to quantitatively monitor the extracellular H(2)O(2) generated by neutrophils.

A New Core/Shell NiAu/Au Nanoparticle Catalyst with Pt-like Activity for Hydrogen Evolution Reaction.

We report a general approach to NiAu alloy nanoparticles (NPs) by co-reduction of Ni(acac)2 (acac = acetylacetonate) and HAuCl4·3H2O at 220 °C in the presence of oleylamine and oleic acid. Subject to potential cycling between 0.6 and 1.0 V (vs reversible hydrogen electrode) in 0.5 M H2SO4, the NiAu NPs are transformed into core/shell NiAu/Au NPs that show much enhanced catalysis for hydrogen evolution reaction (HER) with Pt-like activity and much robust durability. The first-principles calculations suggest that the high activity arises from the formation of Au sites with low coordination numbers around the shell. Our synthesis is not limited to NiAu but can be extended to FeAu and CoAu as well, providing a general approach to MAu/Au NPs as a class of new catalyst superior to Pt for water splitting and hydrogen generation.

Nanocatalyst superior to Pt for oxygen reduction reactions: the case of core/shell Ag(Au)/CuPd nanoparticles.

Controlling the electronic structure and surface strain of a nanoparticle catalyst has become an important strategy to tune and to optimize its catalytic efficiency for a chemical reaction. Using density functional theory (DFT) calculations, we predicted that core/shell M/CuPd (M = Ag, Au) NPs with a 0.8 or 1.2 nm CuPd2 shell have similar but optimal surface strain and composition and may surpass Pt in catalyzing oxygen reduction reactions. We synthesized monodisperse M/CuPd NPs by the coreduction of palladium acetylacetonate and copper acetylacetonate in the presence of Ag (or Au) nanoparticles with controlled shell thicknesses of 0.4, 0.75, and 1.1 nm and CuPd compositions and evaluated their catalysis for the oxygen reduction reaction in 0.1 M KOH solution. As predicted, our Ag/Cu37Pd63 and Au/Cu40Pd60 catalysts with 0.75 and 1.1 nm shells were more efficient catalysts than the commercial Pt catalyst (Fuel Cells Store), with their mass activity reaching 0.20 A/mg of noble metal at -0.1 V vs Ag/AgCl (4 M KCl); this was over 3 times higher than that (0.06 A/mg Pt) from the commercial Pt. These Ag(Au)/CuPd nanoparticles are promising non-Pt catalysts for oxygen reduction reactions.

Tuning Sn-Catalysis for Electrochemical Reduction of CO2 to CO via the Core/Shell Cu/SnO2 Structure

Tin (Sn) is known to be a good catalyst for electrochemical reduction of CO2 to formate in 0.5 M KHCO3. But when a thin layer of SnO2 is coated over Cu nanoparticles, the reduction becomes Sn-thickness dependent: the thicker (1.8 nm) shell shows Sn-like activity to generate formate whereas the thinner (0.8 nm) shell is selective to the formation of CO with the conversion Faradaic efficiency (FE) reaching 93% at -0.7 V (vs reversible hydrogen electrode (RHE)). Theoretical calculations suggest that the 0.8 nm SnO2 shell likely alloys with trace of Cu, causing the SnO2 lattice to be uniaxially compressed and favors the production of CO over formate. The report demonstrates a new strategy to tune NP catalyst selectivity for the electrochemical reduction of CO2 via the tunable core/shell structure.

Self-Illuminating 64Cu-Doped CdSe/ZnS Nanocrystals for in Vivo Tumor Imaging

Construction of self-illuminating semiconducting nanocrystals, also called quantum dots (QDs), has attracted much attention recently due to their potential as highly sensitive optical probes for biological imaging applications. Here we prepared a self-illuminating QD system by doping positron-emitting radionuclide 64Cu into CdSe/ZnS core/shell QDs via a cation-exchange reaction. The 64Cu-doped CdSe/ZnS QDs exhibit efficient Cerenkov resonance energy transfer (CRET). The signal of 64Cu can accurately reflect the biodistribution of the QDs during circulation with no dissociation of 64Cu from the nanoparticles. We also explored this system for in vivo tumor imaging. This nanoprobe showed high tumor-targeting ability in a U87MG glioblastoma xenograft model (12.7% ID/g at 17 h time point) and feasibility for in vivo luminescence imaging of tumor in the absence of excitation light. The availability of these self-illuminating integrated QDs provides an accurate and convenient tool for in vivo tumor imaging and detection.

Monodisperse MPt (M = Fe, Co, Ni, Cu, Zn) Nanoparticles Prepared from a Facile Oleylamine Reduction of Metal Salts

We report a simple, yet general, approach to monodisperse MPt (M = Fe, Co, Ni, Cu, Zn) nanoparticles (NPs) by coreduction of M(acac)2 and Pt(acac)2 (acac = acetylacetonate) with oleylamine at 300 °C. In the current reaction condition, oleylamine serves as the reducing agent, surfactant, and solvent. As an example, we describe in details the synthesis of 9.5 nm CoPt NPs with their compositions controlled from Co37Pt63 to Co69Pt31. These NPs show composition-dependent structural and magnetic properties. The unique oleylamine reduction process makes it possible to prepare MPt NPs with their physical properties and surface chemistry better rationalized for magnetic or catalytic applications.