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.     

Composition-Controllable AuPt Alloy Catalysts for Electrooxidation of Formic Acid

AuPt alloy catalysts with various compositions have been successfully prepared simply by one-step co-reduction of Au and Pt precursors involving sodium citrate as stabilizer and reductant. XRD, TEM and EDX element mapping analysis confirmed that the resulting AuPt nanoparticles are single-phase alloys rather than random mixtures of tiny Au and Pt particles. Compared with Pt/C, alloying Au with Pt can effectively alter the kinetic process of formic acid oxidation, reducing the generation of CO-like intermediates. Au₈₁Pt₁₉ displays superior electrocatalytic activity and durability, ~11 times in the mass activity better than commercial Pt/C and may be of practical significance for the commercialization of direct formic acid fuel cell.

     

Binary clusters AuPt and Au6Pt: structure and reactivity within density functional theory

Within density functional theory with the general gradient approximation for the exchange and correlation, the bimetallic clusters AuPt and Au(6)Pt have been studied for their structure and reactivity. The bond strength of AuPt lies between those of Au(2) and Pt(2), and it is closer to that of Au(2). The Pt atom is the reactive center in both AuPt and AuPt(+) according to electronic structure analysis. AuPt(+) is more stable than AuPt. Au(6)Pt prefers electronic states with low multiplicity. The most stable conformation of Au(6)Pt is a singlet and has quasi-planar hexagonal frame with Pt lying at the hexagonal center. The doping of Pt in Au cluster enhances the chemical regioselectivity of the Au cluster. The Pt atom essentially serves as electron donor and the Au atoms bonded to the Pt atom acts as electron acceptor in Au(6)Pt. The lowest triplet of edge-capped rhombus Au(6)Pt clusters is readily accessible with very small singlet-triplet energy gap (0.32 eV). O(2) prefers to adsorb on Au and CO prefers to adsorb on Pt. O(2) and CO have stronger adsorption on AuPt than they do on Au(6)Pt. CO has a much stronger adsorption on AuPt bimetallic cluster than O(2) does. The adsorption of CO on Pt modifies the geometry of AuPt bimetallic clusters.     

Characterization of carbon-supported AuPt nanoparticles for electrocatalytic methanol oxidation reaction

In view of the recent finding that the bimetallic AuPt nanoparticles prepared by molecular-capping-based colloidal synthesis and subsequent assembly on carbon black support and thermal activation treatment exhibit alloy properties, which is in sharp contrast to the bimetallic miscibility gap known for the bulk counterparts in a wide composition range, there is a clear need to assess the electrocatalytic properties of the catalysts prepared with different bimetallic composition and different thermal treatment temperatures. This paper reports recent results of such an investigation of the electrocatalytic methanol oxidation reaction (MOR) activities of the carbon-supported AuPt nanoparticle catalysts with different bimetallic composition and thermal treatment temperatures. Au(m)Pt(100)(-)(m) nanoparticles of 2-3 nm core sizes with different atomic compositions ranging from 10% to 90% Au (m = 10 approximately 90) have been synthesized by controlling the feeding of the metal precursors used in the synthesis. The electrocatalytic MOR activities of the carbon-supported AuPt bimetallic catalysts were characterized in alkaline electrolytes. The catalysts with 65% to 85% Au and treated at 500 degrees C were found to exhibit maximum electrocatalytic activities in the alkaline electrolytes. The findings, together with a comparison with some well-documented catalysts as well as recent experimental and theoretical modeling results, have revealed important insights into the participation of CO(ad) and OH(ad) on Au sites in the catalytic reaction of Pt in the AuPt alloys with approximately 75% Au. The insights are useful for understanding the correlation of the bifunctional electrocatalytic activity of the bimetallic nanoparticle catalysts with the bimetallic composition and the thermal treatment temperatures.     

Removal of residual metal catalysts with iron/iron oxide nanoparticles from coordinating environments

The application of Fe@FexOy nanoparticles was examined for the sequestration of catalytic metal impurities from organic reaction products. An X-ray photoelectron spectroscopy (XPS) study of the recovered particles confirmed Fe@FexOy sequestered Co2+, Cu2+, Ni2+, RhX+, Pd2+, Ag+, and Pt4+ by coordination of the metal ion to the iron oxide surfaces and followed by subsequent reduction of the surface-bonded ions to their metallic state. Fe@FexOy metal sequestration was found to be effective for catalyst impurities in the absence of strongly coordinating environments but was inhibited by the presence of phosphines. Sequestration of phosphine-coordinated metal impurities was achieved through the addition of either cysteamine or 3-mercaptopropionic acid to the Fe@FexOy during sequestration. This approach was applied to model syntheses using Grubbs' Catalyst (first generation), Pd(PPh3)4, Pd2(dba)3, and Wilkinson's Catalyst (RhCl(PPh3)3).     

Electrooxidation of Several Organic Compounds on Simply Prepared Metallic Nanoparticles: A Comparative Study

The electrooxidation of several fuel compounds was studied using metallic nanoparticles of Au, Pd, Pt, AuPd and AuPt synthesized by direct electrodeposition by applying a constant potential of ‐200 mV (vs. Ag/AgCl) to pencil graphite in an acidic medium. Scanning electron microscope (SEM) images and X‐ray diffraction (XRD) data show that monometallic (Au, Pd and Pt) and alloys of bimetallic nanoparticles of AuPd and AuPt have been formed. The catalytic performance of the prepared electrodes was investigated in a neutral medium (100 mM phosphate buffer, pH 7) by cyclic voltammetry. Amongst all fuels, the highest current densities were obtained by the electrooxidation of formic acid (ca. 9.8 mA cm^?2) and formaldehyde (ca. 9.7 mA cm^?2) on the AuPt catalyst.     

Electrogenerated Chemiluminescence Imaging of Electrocatalysis at a Single Au‐Pt Janus Nanoparticle

Noble metal nanoparticles are promising catalysts in electrochemical reactions, while understanding the relationship between the structure and reactivity of the particles is important to achieve higher efficiency of electrocatalysis, and promote the development of single‐molecule electrochemistry. Electrogenerated chemiluminescence (ECL) was employed to image the catalytic oxidation of luminophore at single Au, Pt, and Au‐Pt Janus nanoparticles. Compared to the monometal nanoparticles, the Janus particle structure exhibited enhanced ECL intensity and stability, indicating better catalytic efficiency. On the basis of the experimental results and digital simulation, it was concluded that a concentration difference arose at the asymmetric bimetallic interface according to different heterogeneous electron‐transfer rate constants at Au and Pt. The fluid slip around the Janus particle enhanced local redox reactions and protected the particle surface from passivation.     

A simple one-step preparation of high utilization AuPt nanoparticles supported on MWCNTs for methanol oxidation in alkaline medium

A simple one-step preparation of gold–platinum electrocatalysts supported on multi-walled carbon nanotubes (MWCNTs) with high utilization is reported. A low Pt loading series of bimetallic AuPt/MWCNTs catalysts were prepared by the improved ethylene glycol reduction method, and then they were compared in terms of the electrocatalytic activity for methanol oxidation using cyclic voltammetry (CV) and chronoamperometry in alkaline solutions. The structure of AuPt/MWCNTs was characterized by the transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS). The results showed high Pt utilization, uniform AuPt nanoparticles size and good electrocatalytic activity for methanol electro-oxidation. The effect of Au/Pt mass ratio on electrocatalytic activity was also investigated by CV and chronoamperometry. The highest peak current density, lowest onset potential and best anti-poisoning effect for methanol electro-oxidation appeared at the Au/Pt/MWCNTs mass ratio of 2:4:32.     

Promoted aerobic oxidation of benzyl alcohol on CNT supported platinum by iron oxide

The catalytic activity of Pt/CNT for benzyl alcohol aerobic oxidation was remarkably improved by decorating iron oxide on Pt nanoparticles, and electrochemical measurements evidenced the enhanced activation of oxygen and benzyl alcohol at the FeO(x)/Pt interface.     

Preparation and characterization of metal-chitosan nanocomposites

Various metal-chitosan nanocomposites were synthesized, including silver (Ag), gold (Au), platinum (Pt), and palladium (Pd) in aqueous solutions. Metal nanoparticles were formed by reduction of corresponding metal salts with NaBH4 in the presence of chitosan. And chitosan molecules adsorbing onto the surface of as-prepared metal nanoparticles formed the corresponding metal-chitosan nanocomposites. Transmission electron microscopy (TEM) images and UV-vis spectra of the nanocomposites revealed the presence of metal nanoparticles. Comparison of all the resulting particles size, it shows that silver nanoparticles are much larger than others (Au, Pt and Pd). In addition, the difference in particles size leads to develop different morphologies in the films cast from prepared metal-chitosan nanocomposites. Polarized optical microscopy (POM) images show a batonet-like structure for Ag-chitosan nanocomposites film, while for the films cast from other metal (Au, Pt, and Pd)-chitosan nanocomposites, some branched-like structures with a few differences among them were observed under POM observation.     

Bovine serum albumin stabilized iron oxide and gold bimetallic heterodimers: Synthesis,characterization and Stereological study

In this time researchers make a great efforts to develop new hybrid nanoparticles for medical and pharmaceutical applications. Fe₃O₄‐Au hybrid heterodimers have been prepared with superior properties for various claims. Unfortunately, Fe₃O₄‐Au heterodimers are not stable in the physiological medium. In this study, we employed the albumin macromolecules as a stabilizer of Fe₃O₄‐Au hybrid nanoparticles (noted as Fe₃O₄‐Au‐BSA hybrid nanoparticles). After characterization of synthesized nanoparticles by FTIR, UV–Vis, TEM, DLS, DSC, VSM and XRD techniques, the in vitro and in vivo biocompatibility of these nanoparticles were also evaluated. We encountered with an amazing result which confirmed nanoparticles could be stabilized by linking the BSA on the surface of Fe₃O₄‐Au heterodimers. Also, intravenous injection of Fe₃O₄‐Au‐BSA hybrid nanoparticles up to 400 mg/kg to Balb C mice show that these nanoparticles were non‐toxic. The biocompatibility and stereological study had been performed for more than 30 days after nanoparticles administration, using hystomorphometric analysis. Remarkably, to the best of our knowledge, it was the first time the biocompatibility and biodegradability of Fe₃O₄‐Au were studied and evaluated by stereological technique. Further promotion and biomedical usage of this type of hybrid nanoparticles are underway in our laboratory.     

Adsorption of poly(vinyl formamide-co-vinyl amine) (PVFA-co-PVAm) polymers on zinc, zinc oxide, iron, and iron oxide surfaces

The adsorption of poly(vinyl formamide) (PVFA) and the statistic copolymers poly(vinyl formamide-co-vinyl amine) (PVFA-co-PVAm) onto zinc and iron metal particles as well as their oxides was investigated. The adsorbates were characterized by means of XPS, DRIFT spectroscopy, wet chemical analysis, and solvatochromic probes. Dicyano-bis-(1,10-phenanthroline)-iron(II) (1), 3-(4-amino-3-methylphenyl)-7-phenyl-benzo-[1,2-b:4,5-b']difuran-2,6-dione (2), and 4-tert-butyl-2-(dicyano-methylene)-5-[4-(diethylamino)-benzylidene]-Δ(3)-thiazoline (3) as solvatochromic probes were coadsorbed onto zinc oxide to measure various effects of surface polarity. The experimental findings showed that the adsorption mechanism of PVFA and PVFA-co-PVAm strongly depends on the degree of hydrolysis of PVFA and pH values and also on the kind of metal or metal oxide surfaces that were employed as adsorbents. The adsorption mechanism of PVFA/PVFA-co-PVAm onto zinc oxide and iron oxide surfaces is mainly affected by electrostatic interactions. Particularly in the region of pH 5, the adsorption of PVFA/PVFA-co-PVAm onto zinc and iron metal particles is additionally influenced by redox processes, dissolution, and complexation reactions.     

Self-assembled ultra-thin coatings of octadecyltrichlorosilane (OTS) formed at the surface of iron oxide nanoparticles

In a series of experiments, we coated iron oxide nanoparticles, which were originally stabilized with lauric acid, with a polymer layer of Octadecyltrichlorosilane (OTS). Characterization of the different coated nanoparticles was accomplished by Static and Dynamic Light Scattering, acoustic spectroscopy, and Atomic Force Microscopy. In various experiments, we systematically investigated the effect of different parameters such as the OTS concentration and iron oxide content on the particle size of the coated nanoparticles. It was recognized that the size of the coated nanoparticles mainly depend on the concentration of OTS (C _OTS) measured with respect to the concentration of the iron oxide particles (C _mag.). Below a well-defined threshold value of C _OTS /C _mag, we did not observe any adsorption of OTS on the surface of iron oxide nanoparticles. The particle size of OTS-coated iron oxide nanoparticles increased rapidly at concentration ratios above the threshold concentration and reached a typical plateau value for long periods of time.     

AuPt core–shell nanocatalysts with bulk Pt activity

In the presented work, the evaluation of an unsupported AuPt core–shell catalyst for the oxygen reduction reaction is introduced. Applying only basic chemicals in an upscalable synthesis route, it is demonstrated that uniform, flat, and complete Pt layers around a spherical Au core are obtained. The electrocatalytic measurements show that the surface area specific activity of the AuPt core–shell catalyst towards the important oxygen reduction reaction equals the one of polycrystalline bulk Pt. To our knowledge, this is the first time that the unfavorable particle size effect of Pt nanoparticles could be by-passed for a nanoscale catalyst.     

Templated nucleation of hybrid iron oxide nanoparticles on polysaccharide nanogels

Novel organic–inorganic hybrid nanoparticles consisting of polymer–hydrogel nanoparticles (nanogels) and iron oxide were developed for potential biomedical applications. Hybrid nanoparticles were prepared by a simple procedure using polysaccharide nanogels as a reactive site for iron oxide formation. The hybrid nanoparticles have a narrow size distribution with a diameter of approximately 30 nm and show high colloidal stability. These nanohybrid particles could be used as a contrast medium for magnetic resonance imaging or for magnetic hyperthermia therapy.     

Synthesis of iron oxide nanoparticles using chitosan and starch templates

Natural polymers like chitosan and starch have been employed as templates for the preparation of iron oxide nanoparticles. The templates offer selective binding sites for Fe(II) under aqueous conditions. Controlled drying and subsequent removal of the template backbone enables the synthesis of spatially separated iron oxide nanoparticles. The crystalline character of the iron oxide and near narrow particle size distribution pattern have been confirmed through powder XRD, Photon Correlation Spectroscopy, and TEM measurements. The crystallite sizes of the particles were found to be 26–35 nm irrespective of the nature of the template.     

Electrochemistry of conductive polymers 39. Contacts between conducting polymers and noble metal nanoparticles studied by current-sensing atomic force microscopy

Electrical properties of contacts formed between conducting polymers and noble metal nanoparticles have been examined using current-sensing atomic force microscopy (CS-AFM). Contacts formed between electrochemically prepared pi-conjugated polymer films such as polypyrrole (PPy), poly(3-methylthiophene) (P3MeT), as well as poly(3,4-ethylenedioxythiophene) (PEDOT) and noble metal nanoparticles including platinum (Pt), gold (Au), and silver (Ag) have been examined. The Pt nanoparticles were electrochemically deposited on a pre-coated PPy film surface by reducing a platinum precursor (PtCl62-) at a constant potential. Both current and scanning electron microscopic images of the film showed the presence of Pt islands. The Au and Ag nanoparticles were dispersed on the P3MeT and PEDOT film surfaces simply by dipping the polymer films into colloid solutions containing Au or Ag particles for specified periods (5 to approximately 10 min). The deposition of Au or Ag particles resulted from either their physical adsorption or chemical bonding between particles and the polymer surface depending on the polymer. When compared with PPy, P3MeT and PEDOT showed a stronger binding to Au or Ag nanoparticles when dipped in their colloidal solutions for the same period. This indicates that Au and Ag particles are predominantly linked with the sulfur atoms via chemical bonding. Of the two, PEDOT was more conductive at the sites where the particles are connected to the polymer. It appears that PEDOT has better aligned sulfur atoms on the surface and is strongly bonded to Au and Ag nanoparticles due to their strong affinity to gold and silver. The current-voltage curves obtained at the metal islands demonstrate that the contacts between these metal islands and polymers are ohmic.     

A Single‐Atom Manipulation Approach for Synthesis of Atomically Mixed Nanoalloys as Efficient Catalysts

Synthesis of well‐defined atomically mixed alloy nanoparticles on desired substrates is an ultimate goal for their practical application. Herein we report a general approach for preparing atomically mixed AuPt, AuPd, PtPd, AuPtPd NAs(nanoalloys) through single‐atom level manipulation. By utilizing the ubiquitous tendency of aggregation of single atoms into nanoparticles at elevated temperatures, we have synthesized nanoalloys on a solid solvent with CeO₂ as a carrier and transition‐metal single atoms as an intermediate state. The supported nanoalloys/CeO₂ with ultra‐low noble metal content (containing 0.2 wt % Au and 0.2 wt % Pt) exhibit enhanced catalytic performance towards complete CO oxidation at room temperature and remarkable thermostability. This work provides a general strategy for facile and rapid synthesis of well‐defined atomically mixed nanoalloys that can be applied for a range of emerging techniques.     

A Single-Atom Manipulation Approach for Synthesis of Atomically Mixed Nanoalloys as Efficient Catalysts

Synthesis of well-defined atomically mixed alloy nanoparticles on desired substrates is an ultimate goal for their practical application. Herein we report a general approach for preparing atomically mixed AuPt, AuPd, PtPd, AuPtPd NAs(nanoalloys) through single-atom level manipulation. By utilizing the ubiquitous tendency of aggregation of single atoms into nanoparticles at elevated temperatures, we have synthesized nanoalloys on a solid solvent with CeO₂ as a carrier and transition-metal single atoms as an intermediate state. The supported nanoalloys/CeO₂ with ultra-low noble metal content (containing 0.2 wt % Au and 0.2 wt % Pt) exhibit enhanced catalytic performance towards complete CO oxidation at room temperature and remarkable thermostability. This work provides a general strategy for facile and rapid synthesis of well-defined atomically mixed nanoalloys that can be applied for a range of emerging techniques.     

A highly efficient phase transfer method for preparing alkylamine-stabilized Ru, Pt, and Au nanoparticles

A highly efficient phase-transfer method was developed to prepare alkylamine-stabilized nanoparticles of several noble metals. This method involved first mixing the metal hydrosols and an ethanol solution of dodecylamine and then extracting the dodecylamine-stabilized metal nanoparticles into toluene. The efficiency of this phase-transfer method was nearly 100%. Alkylamine-stabilized Ru, Pt, and Au nanoparticles 3.45, 4.33, and 7.89 nm in diameter, respectively, could be prepared this way. The self-assembly of dodecylamine-stabilized Pt and Au particles was also detected by transmission electron microscopy (TEM).