Electrochemical Designing of Au/Pt Core Shell Nanoparticles as Nanostructured Catalyst with Tunable Activity for Oxygen Reduction

Au/Pt core shell nanoparticles (NPs) have been prepared via a layer‐by‐layer growth of Pt layers on Au NPs using underpotential deposition (UPD) redox replacement technique. A single UPD Cu monolayer replacement with Pt(II) yielded a uniform Pt film on Au NPs, and the shell thickness can be tuned by controlling the number of UPD redox replacement cycles. Oxygen reduction reaction (ORR) in air‐saturated 0.1 M H₂SO₄ was used to investigate the electrocatalytic behavior of the as‐prepared core shell NPs. Cyclic voltammograms of ORR show that the peak potentials shift positively from 0.32 V to 0.48 V with the number of Pt layers increasing from one to five, suggesting the electrocatalytic activity increases with increasing the thickness of Pt shell. The increase in electrocatalytic activity may originate mostly from the large decrease of electronic influence of Au cores on surface Pt atoms. Rotating ring‐disk electrode voltammetry and rotating disk electrode voltammetry demonstrate that ORR is mainly a four‐electron reduction on the as‐prepared modified electrode with 5 Pt layers and first charge transfer is the rate‐determining step.     

One‐Dimensional Carbon Nanotube/SnO2/Noble Metal Nanoparticle Hybrid Nanostructure: Synthesis,Characterization, and Electrochemical Sensing

Herein we report a facile and efficient method for self‐assembling noble‐metal nanoparticles (NPs) to the surface of SnO₂‐coated carbon nanotubes (CNT@SnO₂) to construct CNT@SnO₂/noble metal NP hybrids. By using SnCl₄ as the precursor of the SnO₂ shell on the surface of CNTs, the hydrolysis speed of SnCl₄ was slowed down in ethanol containing a trace amount of urea and water. The coaxial nanostructure of CNT@SnO₂ was confirmed by using X‐ray powder diffraction (XRD) and transmission electron microscopy (TEM). It was found that the coating layer of SnO₂ was homogeneous with the mean thickness of 8 nm. The CNT@SnO₂/noble‐metal NP hybrids were obtained by mixing noble‐metal NPs with as‐prepared CNT@SnO₂ coaxial nanocables by means of a self‐assembly strategy. With the amino group terminated, the CNT@SnO₂ coaxial nanocable can readily adsorb the as‐prepared noble‐metal NPs (Au, Ag, Au? Pt, and Au? Pd NPs). The presence of an amino group at the surface of SnO₂ was proved by use of X‐ray photoelectron spectroscopy (XPS). In addition, H₂O₂ sensing by amperometric methods could serve as detection models for investigating the electrocatalytic ability of as‐prepared hybrid materials. It was found that wide linear ranges and low detection limits were obtained by using the enzyme‐free CNT@SnO₂@Au? Pt modified electrode, which indicated the potential utilizations of the hybrid based on CNT@SnO₂ for electrochemical sensing.     

Designed Synthesis of Well‐Defined Pd@Pt Core–Shell Nanoparticles with Controlled Shell Thickness as Efficient Oxygen Reduction Electrocatalysts

Improving the electrocatalytic activity and durability of Pt‐based catalysts with low Pt content toward the oxygen reduction reaction (ORR) is one of the main challenges in advancing the performance of polymer electrolyte membrane fuel cells (PEMFCs). Herein, a designed synthesis of well‐defined Pd@Pt core–shell nanoparticles (NPs) with a controlled Pt shell thickness of 0.4–1.2 nm by a facile wet chemical method and their electrocatalytic performances for ORR as a function of shell thickness are reported. Pd@Pt NPs with predetermined structural parameters were prepared by in situ heteroepitaxial growth of Pt on as‐synthesized 6 nm Pd NPs without any sacrificial layers and intermediate workup processes, and thus the synthetic procedure for the production of Pd@Pt NPs with well‐defined sizes and shell thicknesses is greatly simplified. The Pt shell thickness could be precisely controlled by adjusting the molar ratio of Pt to Pd. The ORR performance of the Pd@Pt NPs strongly depended on the thickness of their Pt shells. The Pd@Pt NPs with 0.94 nm Pt shells exhibited enhanced specific activity and higher durability compared to other Pd@Pt NPs and commercial Pt/C catalysts. Testing Pd@Pt NPs with 0.94 nm Pt shells in a membrane electrode assembly revealed a single‐cell performance comparable with that of the Pt/C catalyst despite their lower Pt content, that is the present NP catalysts can facilitate low‐cost and high‐efficient applications of PEMFCs.     

Observation of Single Pt Nanoparticle Collisions: Enhanced Electrocatalytic Activity on a Pd Ultramicroelectrode

Single Pt nanoparticle (NP) collisions on an electrode surface were detected by using an electrocatalytic amplification method with a Pd ultramicroelectrode (UME). Pd is not a preferred material for UMEs for the detection of single Pt NP collisions, because Pd shows similar electrocatalytic activity compared with Pt for hydrazine oxidation, thus resulting in a high background current level. However, a Pt NP colliding on the Pd UME shows greatly enhanced activity compared with a Pt NP on an inert UME, such as a Au UME, which is usually used for the detection of single Pt NP collisions. The use of an electroactive UME material instead of an inert one facilitated the study of single‐NP activity on the various solid supports, which is important in many NP applications.     

Hyperbranched polyglycidol assisted green synthetic protocols for the preparation of multifunctional metal nanoparticles

Biocompatible hyperbranched polyglycidol (HBP) has been demonstrated to be an effective reducing and stabilizing agent for the synthesis of highly water-soluble monometallic (Au, Ag, Pt, Pd, and Ru) and bimetallic (Au/Pt, Au/Pd, and Au/Ru) nanoparticles (NPs), which provides a general and green protocol to fabricate metal NPs. The HBP-assisted reduction of metal ions follows an analogous polyol process. The reduction reaction rate increases sharply by increasing the temperature and the molecular weight of HBP. The size of NPs is controllable simply by changing the concentration of the metal precursor. High molecular weight HBP is favorable for the formation of NPs with uniform size and improved stability. By utilizing hydroxyl groups in the HBP-passivation layer of Au NPs, TiO(2)/Au, GeO(2)/Au, and SiO(2)/Au nanohybrids are also fabricated via sol-gel processes, which sets a typical example for the creation of versatile metal NPs/inorganic oxide hybrids based on the as-prepared multifunctional NPs.     

Observation of single metal nanoparticle collisions by open circuit (mixed) potential changes at an ultramicroelectrode

Single nanoparticle (NP) collisions were successfully observed by a potentiometric measurement. The open circuit potential (OCP) of a measuring Au ultramicroelectrode (UME) changes when Pt NPs collide with the UME in a hydrazine solution. The OCP change is related to the redox processes, the concentration of particles, particle size, and electrode size. Compared with the amperometric technique, this approach has several advantages: higher sensitivity, simpler apparatus, fewer problems with NP decomposition, and contamination.     

Self‐Assembled Au Nanoparticles as Substrates for Surface‐Enhanced Vibrational Spectroscopy: Optimization and Electrochemical Stability

Three‐dimensional nanostructured metallic substrates for enhanced vibrational spectroscopy are fabricated by self‐assembly. Nanostructures consisting of one to 20 depositions of 13 nm‐diameter Au nanoparticles (NPs) on Au films are prepared and characterized by means of AFM and UV/Vis reflection–absorption spectroscopy. Surface‐enhanced polarization modulation infrared reflection–absorption spectroscopy (PM‐IRRAS) is observed from Au NPs modified by the probe molecule 4‐hydroxythiophenol. The limitation of this kind of substrate for surface‐enhanced PM‐IRRAS is discussed. The surface‐enhanced Raman scattering (SERS) from the same probe molecule is also observed and the effect of the number of Au‐NP depositions on the SERS efficiency is studied. The SERS signal from the probe molecule maximizes after 11 Au‐NP depositions, and the absolute SERS intensities from different batches are reproducible within 20 %. In situ electrochemical SERS measurements show that these substrates are stable within the potential window between ?800 and +200 mV (vs. Ag/AgCl/sat. Cl^?).     

Advanced Catalytic Performance of Au–Pt Double‐Walled Nanotubes and Their Fabrication through Galvanic Replacement Reaction

Bimetallic tubular nanostructures have been the focus of intensive research as they have very interesting potential applications in various fields including catalysis and electronics. In this paper, we demonstrate a facile method for the fabrication of Au–Pt double‐walled nanotubes (Au–Pt DWNTs). The DWNTs are fabricated through the galvanic displacement reaction between Ag nanowires and various metal ions, and the Au–Pt DWNT catalysts exhibit high active catalytic performances toward both methanol electro‐oxidation and 4‐nitrophenol (4‐NP) reduction. First, they have a high electrochemically active surface area of 61.66 m² g^?1, which is close to the value of commercial Pt/C catalysts (64.76 m² g^?1), and the peak current density of Au–Pt DWNTs in methanol oxidation is recorded as 138.25 mA mg^?1, whereas those of Pt nanotubes, Au/Pt nanotubes (simple mixture), and commercial Pt/C are 24.12, 40.95, and120.65 mA mg^?1, respectively. The Au–Pt DWNTs show a markedly enhanced electrocatalytic activity for methanol oxidation compared with the other three catalysts. They also show an excellent catalytic performance in comparison with common Au nanotubes for 4‐nitrophenol (4‐NP) reduction. The attractive performance exhibited by these prepared Au–Pt DWNTs can be attributed to their unique structures, which make them promising candidates as high‐performance catalysts.     

Selective and Simultaneous Determination of Dihydroxybenzene Isomers Based on Green Synthesized Gold Nanoparticles Decorated Reduced Graphene Oxide

In the present study, we report the simultaneous electrochemical determination of hydroquinone (HQ), catechol (CC) and resorcinol (RC) at gold nanoparticles (Au‐NPs) decorated reduced graphene oxide (RGO) modified electrode. An enhanced and well defined peak current response with a better peak separation of HQ, CC and RC is observed at RGO/Au‐NPs composite than that of RGO and Au‐NPs modified electrodes. The fabricated modified electrode shows a wide linear response in the concentration range of 3–90 µM, 3–300 µM and 15–150 µM for HQ, CC and RC, respectively. The detection limit of HQ, CC and RC is found as 0.15 µM, 0.12 µM and 0.78 µM, respectively.     

Relative stability of icosahedral and cuboctahedral metallic nanoparticles

The size and form of metallic nanoparticles (NPs) significantly affects their adsorptive, chemical, and catalytic activity. One of the most interesting nanoscale size effects is the transition from icosahedral to octahedral forms with growth in the NP size. We compared the stability of icosahedral, decahedral and cuboctahedral NPs made from eight metals Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au using the local optimization of total energy, which was computed from the tight-binding second moment approximation and quantum Sutton–Chen potentials. The obtained results predicted that the icosahedral form would be most stable for Ni, and least stable for Au. For Rh, and especially for Ir, a strong dependency of the stability of the different forms on the NP size was revealed.     

A General and High‐Yield Galvanic Displacement Approach to Au?M (M=Au,Pd, and Pt) Core–Shell Nanostructures with Porous Shells and Enhanced Electrocatalytic Performances

In this work, we utilize the galvanic displacement synthesis and make it a general and efficient method for the preparation of Au? M (M=Au, Pd, and Pt) core–shell nanostructures with porous shells, which consist of multilayer nanoparticles. The method is generally applicable to the preparation of Au? Au, Au? Pd, and Au? Pt core–shell nanostructures with typical porous shells. Moreover, the Au? Au isomeric core–shell nanostructure is reported for the first time. The lower oxidation states of Au^I, Pd^II, and Pt^II are supposed to contribute to the formation of porous core–shell nanostructures instead of yolk‐shell nanostructures. The electrocatalytic ethanol oxidation and oxygen reduction reaction (ORR) performance of porous Au? Pd core–shell nanostructures are assessed as a typical example for the investigation of the advantages of the obtained core–shell nanostructures. As expected, the Au? Pd core–shell nanostructure indeed exhibits a significantly reduced overpotential (the peak potential is shifted in the positive direction by 44 mV and 32 mV), a much improved CO tolerance (I_f/I_b is 3.6 and 1.63 times higher), and an enhanced catalytic stability in comparison with Pd nanoparticles and Pt/C catalysts. Thus, porous Au? M (M=Au, Pd, and Pt) core–shell nanostructures may provide many opportunities in the fields of organic catalysis, direct alcohol fuel cells, surface‐enhanced Raman scattering, and so forth.     

Underpotential Deposition Preparation of Pt‐loading AuNPs/Reduced Graphene Oxide and Its Catalytic Detection of Catechol

Ultralow Pt‐loading Au nanoparticles have been fabricated on the surface of reduced graphene oxide (RGO) by using underpotential deposition (UPD) monolayer redox replacement process. The Pt/Au/RGO modified electrode exhibits an excellent electrocatalytic activity toward catechol and hydroquinone. Under the optimized condition, the separation of peak‐to‐peak between hydroquinone and catechol is 197 mV, which is wide enough to distinguish the isomers of benzenediol. Catechol is detected by the Pt/Au/RGO/GCE with a low detection limit in the presence of hydroquinone.     

Simultaneous Determination of Nitrophenol Isomers Based on β‐Cyclodextrin Functionalized Reduced Graphene Oxide

β‐CD modified reduced graphene oxide (RGO) sheets have been prepared and characterized by TEM, AFM, IR, EIS and CVs. In comparison with bare glass carbon electrode (GCE) and RGO modified GCE, CD‐RGO/GCE showed much higher peak currents to the reduction of nitrophenol isomers (NPs), attributed to the larger specific surface area of RGO and high quantities of host–guest recognition sites. Three pairs of redox peaks are observed on the CVs of CD‐RGO for p‐NP (0.3 V), o‐NP (?0.2 V) and m‐NP (0.05 V), separating well with each other. Under the optimized condition, the anodic peak currents were linear over ranges around 1–10 mg dm^?3 for p‐NP, 1–9 mg dm^?3 for o‐NP and 1–6 mg dm^?3 for m‐NP, with the detection limits of 0.05 mg dm^?3, 0.02 mg dm^?3 and 0.1 mg dm^?3, respectively. Thus, the CD‐RGO is expected to be a promising sensor material for detecting trace NPs in waste water.     

Preparation of Ag Nanowire @ Au Nanoparticle Hybrid Nanowires and their Influence on the Fluorescence Properties of P3HT

In this study, the galvanic displacement reaction between silver and AuCl₄⁻ was carried out to synthesize a series of silver nanowire (Ag NW) @ gold nanoparticle (Au NP) hybrid nanowires. The influence of Ag NW @ Au NP hybrid nanowires on the fluorescence properties of the poly (3‐hexylthiophene) (P3HT) was investigated. The particle sizes of Au NPs on the hybrid nanowires could be adjusted by varying the reaction time and the concentration of the HAuCl₄ solution. Furthermore, steady‐state fluorescence measurements showed that the fluorescence intensity of the P3HT films was higher on various Ag NW @ Au NP hybrid nanowires compared to that on a bare silicon substrate. This was due to the increase in the intensity of electromagnetic field by the localized surface plasmon resonances of Au NPs and surface plasmon polaritons of Ag NWs from the hybrid nanowires. The results were further confirmed by the Raman spectra of the P3HT films on different substrates.     

Engineering inorganic hybrid nanoparticles: tuning combination fashions of gold, platinum, and iron oxide

Multistep colloidal chemical routes were employed to synthesize Pt/Au, Pt/iron oxide (IO), and Au/Pt/IO hybrid nanoparticles (NPs). The starting templates, Pt NPs, were synthesized by controlling the decomposition kinetics of platinum acetylacetonate in oleylamine. The morphologies of binary metal Pt/Au hybrid NPs were modulated by controllable attachment of Au nanoscale domains to Pt templates. Similarly, Pt/IO and Au/Pt/IO hybrid NPs were fabricated by the controllable attachment of Fe to the Pt or Pt/Au template NPs. The noble metal domains of as-prepared hybrid NPs had face center cubic crystal structures and did not alloy, as verified by high resolution transmission electron microscopy and X-ray diffraction spectrometry. X-ray diffraction spectrometry study indicates that the IO domains in the as-prepared NPs have a spinel structure. UV-vis study of binary metal Pt/Au hybrid NPs revealed that they have a characteristic plasmon resonance around 525 nm, while dumbbell-like Au/Pt/IO NPs had a plasmon resonance around 600 nm. Furthermore, magnetism study of the binary Pt-IO NPs clearly indicated that the interfacial interactions between Pt and IO domains could result in a shift of the blocking temperature.     

Single Pt–Pd Bimetallic Nanoparticle Electrode: Controllable Fabrication and Unique Electrocatalytic Performance for the Methanol Oxidation Reaction

Understanding the electrocatalytic activity at single nanoparticles/nanoclusters level is extremely important. In this work, a method for the electro-deposition of single Pt–Pd nanoparticles (NPs) is described using a single nanopore electrode as a template. The electro-deposition process was investigated carefully and the results show that the process is controlled by diffusion and electro-crystallization process, simultaneously, and the glass sheath property around the nanopore has a large impact on the formation of single Pt–Pd NPs due to the “edge effect”. The prepared single Pt–Pd NPs exhibit excellent electrocatalytic activity in the methanol oxidation reaction, which can be used to screen electrocatalysts with high efficiency for utility in the energy field.     

Challenges in semiconductor single-entity photoelectrochemistry

It is challenging to study the single semiconductor nanocrystal electrochemistry and photoelectrochemistry. The photocatalytic processes, such as the oxidation of methanol and iodide, that result from the electron–hole pair formed within a nanoparticle (NP) allow the detection of discrete current transient events assigned to single entities. Photocatalytic current amplification allows detection of collisions between the semiconductor NPs and the ultramicroelectrode that produce current transient. Staircase responses and blips in the i vs. t response indicate that irreversible and reversible NP/electrode interactions result depending on the experimental conditions. Dye sensitization increases the photocurrent magnitude of ZnO and TiO₂ with respect to bare TiO₂ NPs. The microelectrodes used are Pt, TiO₂/Pt, TiO₂/Au, and F-doped SnO₂.     

Pd@Pt Core–Shell Nanoparticles with Branched Dandelion‐like Morphology as Highly Efficient Catalysts for Olefin Reduction

A facile synthesis based on the addition of ascorbic acid to a mixture of Na₂PdCl₄, K₂PtCl₆, and Pluronic P123 results in highly branched core–shell nanoparticles (NPs) with a micro–mesoporous dandelion‐like morphology comprising Pd core and Pt shell. The slow reduction kinetics associated with the use of ascorbic acid as a weak reductant and suitable Pd/Pt atomic ratio (1:1) play a principal role in the formation mechanism of such branched Pd@Pt core–shell NPs, which differs from the traditional seed‐mediated growth. The catalyst efficiently achieves the reduction of a variety of olefins in good to excellent yields. Importantly, higher catalytic efficiency of dandelion‐like Pd@Pt core–shell NPs was observed for the olefin reduction than commercially available Pt black, Pd NPs, and physically admixed Pt black and Pd NPs. This superior catalytic behavior is not only due to larger surface area and synergistic effects but also to the unique micro–mesoporous structure with significant contribution of mesopores with sizes of several tens of nanometers.     

Sensitive determination of dopamine in the presence of uric acid and ascorbic acid using TiO2 nanotubes modified with Pd, Pt and Au nanoparticles

A simple modified TiO(2) nanotubes electrode was fabricated by electrodeposition of Pd, Pt and Au nanoparticles. The TiO(2) nanotubes electrode was prepared using the anodizing method, followed by modifying Pd nanoparticles onto the tubes surface, offering a uniform conductive surface for electrodeposition of Pt and Au. The performance of the modified electrode was characterized by cyclic voltammetry and differential pulse voltammetry methods. The Au/Pt/Pd/TiO(2) NTs modified electrode represented a high sensitivity towards individual detection of dopamine as well as simultaneous detection of dopamine and uric acid using 0.1 M phosphate buffer solution (pH 7.00) as the base solution. In both case, electro-oxidation peak currents of dopamine were linearly related to accumulated concentration over a wide concentration range of 5.0 × 10(-8) to 3.0 × 10(-5) M. However in the same range of dopamine concentration, the sensitivity had a significant loss at Pt/Pd/TiO(2) NTs electrode, suggesting the necessity for Au nanoparticles in modified electrode. The limit of the detection was determined as 3 × 10(-8) M for dopamine at signal-to-noise ratio equal to 3. Furthermore, the Au/Pt/Pd/TiO(2) NTs modified electrode was able to distinguish the oxidation response of dopamine, uric acid and ascorbic acid in mixture solution of different acidity. It was shown that the modified electrode possessed a very good reproducibility and long-term stability. The method was also successfully applied for determination of DA in human urine samples with satisfactory results.