Fabrication of hybrids based on graphene and metal nanoparticles by in situ and self-assembled methods

In this work, we developed two novel strategies to attach metal nanoparticles (Au and Ag) to the surface of graphene nanosheets, in which graphene oxide was first modified by the linking molecule (3-mercaptopropyl)triethoxysilane and then subjected to different treatments including in situ and self-assembled techniques. The synthesis processes and the resulting hybrids were investigated by ultraviolet-visible measurements, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. It was found that both approaches could effectively immobilize metal nanoparticles onto a graphene surface, and that better distribution and size control of metal nanoparticles were obtained by the self-assembled method. Moreover, we prepared poly(vinylidene fluoride)/graphene-Ag nanocomposites by a solution blending method. The AC conductivity of the resulting nanocomposites could be increased significantly when the loading amount of graphene-Ag was only 2 wt%. We expect that such graphene-metal nanoparticle hybrids may be potentially useful in composite reinforcement, sensors, and electronic devices.     

Stresses and adhesion of thin metallic coatings on oxide substrates

The growth of thin metallic coatings on oxide substrates prepared by vapour deposition of gold or copper on alumina or magnesia was investigated by transmission electron microscopy. The stresses developed during the growth were followed by the bending method. The evolution of stresses is related to the growth mechanisms. The stresses are compressive for discontinuous deposits and change to tensile as the deposit becomes continuous. The compressive stresses are interpreted as due to the increase of the free energy of the metal–oxide system during the growth of metal particles. The tensile stresses result from the deposit lattice mismatch with the substrate. During the growth partial recrystallization and annealing of defects occur which modify the tensile behaviour of continuous coatings. The adhesion of deposits (Au and Cu) on oxide substrates (Al₂O₃ and MgO) was evaluated by the pull-off method as a function of deposition temperature. Au/Al₂O₃, Cu/Al₂O₃ and Au/MgO systems exhibit identical behaviour, while Cu/MgO shows a large dependence on deposition temperature compared to the others. This behaviour is interpreted as due to the formation of chemical bonds between the metal (Cu) and the oxide (MgO), leading to an interfacial oxide.     

Strain analysis of noble metal islands grown on multiwalled carbon nanotubes

We analyze the local microscopic deformation left by Au, Ag and Cu islands on carbon nanotube walls observed by transmission electron microscopy. We employ finite-element simulations within continuum elasticity theory to demonstrate that the observed deformation of the tube walls is the result of elastic strain energy relaxation induced by the nucleation of noble metal clusters on the graphene lattice. We find that the magnitude of the tube deformation is strictly correlated to the lattice mismatch of the metal/carbon system under study. In this context, the tube deformation under the island is a proof of the pseudomorphical character of the system, even in the case of high lattice misfit (～15%) as for Au and Ag.     

Alkylamine capped metal nanoparticle "inks" for printable SERS substrates, electronics and broadband photodetectors

We report a facile and general method for the preparation of alkylamine capped metal (Au and Ag) nanoparticle "ink" with high solubility. Using these metal nanoparticle "inks", we have demonstrated their applications for large scale fabrication of highly efficient surface enhanced Raman scattering (SERS) substrates by a facile solution processing method. These SERS substrates can detect analytes down to a few nM. The flexible plastic SERS substrates have also been demonstrated. The annealing temperature dependent conductivity of the nanoparticle films indicated a transition temperature above which high conductivity was achieved. The transition temperature could be tailored to the plastic compatible temperatures by using proper alkylamine as the capping agent. The ultrafast electron relaxation studies of the nanoparticle films demonstrated that faster electron relaxation was observed at higher annealing temperatures due to stronger electronic coupling between the nanoparticles. The applications of these highly concentrated alkylamine capped metal nanoparticle inks for the printable electronics were demonstrated by printing the oleylamine capped gold nanoparticles ink as source and drain for the graphene field effect transistor. Furthermore, the broadband photoresponse properties of the Au and Ag nanoparticle films have been demonstrated by using visible and near-infrared lasers. These investigations demonstrate that these nanoparticle "inks" are promising for applications in printable SERS substrates, electronics, and broadband photoresponse devices.     

Reduced graphene oxide–gold nanoparticle membrane for water purification

The reduced graphene oxide–gold nanoparticle (rGO–Au NP) membranes are prepared by vacuum filtration method. The sizes of the Au NPs on the surface of the rGO are about 8–10 nm, and the lattice spacing of Au NPs is 0.0241 nm, which is relative to the cubic lattice of the gold crystal. The layer-by-layer stacking structure of rGO–Au NP membrane can be observed clearly by field emission scanning electron microscopy. The water flux of the rGO–Au NP membrane is as high as 204.1 L m^?2 h^?1 bar^?1, and its retention for Rhodamine B (RhB) is as high as 99.79%.     

Polyvinylpyrrolidone adhesion layer for increased uniformity and optical transmittance of silica nanoparticle antireflective coatings

The uniformity of silica nanoparticle antireflective coatings deposited from aqueous solutions on glass substrates is limited by the high surface tension and low evaporation rate of water. In this work, thin films of polyvinylpyrrolidone (PVP) were utilized as an adhesion layer to increase the uniformity and optical transmittance of silica nanoparticle coatings. The increase in adhesive force caused by the presence of the PVP layer was measured using atomic force microscopy (AFM). The micro- and nanoscale uniformities of silica nanoparticle films with and without PVP adhesion layers were characterized using scanning electron microscopy and AFM. It was found that a thin PVP adhesion layer provides the adhesion required to form uniform films of silica nanoparticles. Solar weighted transmittance of 97.6% over a wavelength range of 330–1000 nm was achieved with soda-lime glass substrates coated on both sides.     

Improved electron field emission from metal grafted graphene composites

Metal nanoparticle grafted graphene films (GFs) form a new composite for electron field emission devices. The GFs were deposited on Ni coated Si wafers by microwave plasma enhanced chemical vapour deposition. Graphene-based composites using Ti, Pd, Ag and Au were formed by thermal evaporation. The surface morphology and microstructure were probed by scanning and high resolution transmission electron microscopy. Improvement in the electron field emission and reduction in the turn on and threshold fields were observed in metal grafted GFs as compared to those from pristine films. It was found that among the grafted metals, Ti adsorption contributed more efficiently in enhancing the electron field emission properties by lowering its work function. The enhanced electron field emission characteristics were analyzed using the density functional theory calculations for metal grafted graphene ribbon. Our results indicate increased density of states near the Fermi level for metal grafted graphene ribbon which is responsible for the improvement in electron field emission. We suggest that grafting of metal nanoparticles on GFs could be exploited for the development of efficient field emitters.     

In situ formation of noble metal nanoparticles on multiwalled carbon nanotubes and its implication in metal–nanotube interactions

A simple method to decorate multiwalled carbon nanotubes (MWCNTs) with Au, Ag and Cu nanoparticles is illustrated. The method consists in directly depositing the selected metals by thermal evaporation on the carbon nanotubes. Comparative measurements carried out on samples that differ in the quantity and type of the deposited metal, reveal that isolated discrete particles form on the nanotube outer wall for all three metals. The CNT-based composites have been investigated by scanning and transmission electron microscopy to determine the size, shape and distribution of the nanoparticles. The results indicate that the quantity of evaporated metal only affects the nanoparticle size and not the average particle density. Particle composition was determined by X-ray photoelectron spectroscopy study. The results are discussed in terms of metal nanoparticle–tube interactions, an important issue for the fundamental and practical applications of similar MWCNT based composites.     

Hollow mesoporous silica spheres supported Ag and Ag–Au catalyzed reduction of 4-nitrobenzo-15-crown

Hollow mesoporous silica (HMS) spheres of size within the range 120–220 nm have been prepared using propanol–water solvent as template and cetyltrimethylammonium bromide (CTAB) as stabilizer. HMS supported silver and silver–gold catalysts were prepared by impregnating metal nanoparticles on HMS and were characterized by ultraviolet–visible spectroscopy (UV–vis), dynamic light scattering (DLS), optical microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), inductive coupled plasma optical emission spectroscopy (ICP-OES) and N₂ adsorption–desorption. The reduction of 4-nitrobenzo-15-crown (4-NB-15-C) was compared using HMS supported silver and silver–gold nanocatalysts varying experimental parameters. Bimetallic Ag–Au/HMS nanocatalysts was found to be more active than monometallic Ag/HMS nanocatalyst.     

UV Curable Acrylate Nanocomposites: Properties and Applications

Transparent nanocomposites were prepared from nano-sized silica and radiation curable acrylates. To improve the embedding of silica nanoparticles within the acrylate matrix the filler surface was modified by trialkoxysilanes. Instead of an expected monomeric silane grafting polysiloxane structures were anchored on the nanoparticle surface due to acid catalyzed pre-hydrolysis/condensation of trialkoxysilanes. The polysiloxanes structures covering the silica surface were characterized by temperature-programmed oxidation, infrared and multinuclear MAS NMR spectroscopy. MALDI-TOF mass spectroscopy revealed the formation of polysiloxane oligomers with more than 20 monomeric silane units. Ladder-like polysiloxane chains have been proposed and atomic force microscopy were used to visualize the structure of surface-anchored organosilanes. These ladder-like structures are assumed to build up a short range interpenetrating network with polyacrylate chains during UV or EB curing.Due to the organophilic modification of silica nanoparticles reinforced acrylate formulations can contain up to 50 wt.-% nanofiller maintaining satisfactory rheological properties. These formulations can be used as coatings on substrates such as polymer films, paper, metal, wood, engineered wood, etc. After UV/EB curing nanoparticle reinforced polyacrylate coatings are obtained which show markedly improved properties as compared to neat polymers, e.g., increased microhardness and modulus, improved scratch and abrasion resistance, higher gas barriers and temperature resistance. Due to the nano-sized silica filler the cured coatings remain transparent, hazeless and glossy.On a pilot scale unit acrylate nanocomposite coatings are manufactured for roll coating, curtain coating and spray applications. Basic properties of acrylate nanocomposite coatings are described.     

Au/Pd core-shell nanoparticles with varied hollow Au cores for enhanced formic acid oxidation

A facile method has been developed to synthesize Au/Pd core-shell nanoparticles via galvanic replacement of Cu by Pd on hollow Au nanospheres. The unique nanoparticles were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, ultraviolet–visible spectroscopy, and electrochemical measurements. When the concentration of the Au solution was decreased, grain size of the polycrystalline hollow Au nanospheres was reduced, and the structures became highly porous. After the Pd shell formed on these Au nanospheres, the morphology and structure of the Au/Pd nanoparticles varied and hence significantly affected the catalytic properties. The Au/Pd nanoparticles synthesized with reduced Au concentrations showed higher formic acid oxidation activity (0.93 mA cm^-2 at 0.3 V) than the commercial Pd black (0.85 mA cm^-2 at 0.3 V), suggesting a promising candidate as fuel cell catalysts. In addition, the Au/Pd nanoparticles displayed lower CO-stripping potential, improved stability, and higher durability compared to the Pd black due to their unique core-shell structures tuned by Au core morphologies.     

Silica Shell Thickness-Dependent Fluorescence Properties of SiO2@Ag@SiO2@QDs Nanocomposites

Silica shell coatings, which constitute important technology for nanoparticle (NP) developments, are utilized in many applications. The silica shell’s thickness greatly affects distance-dependent optical properties, such as metal-enhanced fluorescence (MEF) and fluorescence quenching in plasmonic nanocomposites. However, the precise control of silica-shell thicknesses has been mainly conducted on single metal NPs, and rarely on complex nanocomposites. In this study, silica shell-coated Ag nanoparticle-assembled silica nanoparticles (SiO₂@Ag@SiO₂), with finely controlled silica shell thicknesses (4 nm to 38 nm), were prepared, and quantum dots (QDs) were introduced onto SiO₂@Ag@SiO₂. The dominant effect between plasmonic quenching and MEF was defined depending on the thickness of the silica shell between Ag and QDs. When the distance between Ag NPs to QDs was less than ~10 nm, SiO₂@Ag@SiO₂@QDs showed weaker fluorescence intensities than SiO₂@QD (without metal) due to the quenching effect. On the other hand, when the distance between Ag NPs to QDs was from 10 nm to 14 nm, the fluorescence intensity of SiO₂@Ag@SiO₂@QD was stronger than SiO₂@QDs due to MEF. The results provide background knowledge for controlling the thickness of silica shells in metal-containing nanocomposites and facilitate the development of potential applications utilizing the optimal plasmonic phenomenon.     

A simple plasma reduction for synthesis of Au and Pd nanoparticles at room temperature☆

A simple and fast plasma reduction method is developed for synthesis of Au and Pd metal nanoparticles. The scanning electron microscopy (SEM) analysis indicates a formation of aggregates of Au and Pd nanoparticles with branched structure. The transmission electron microscopy (TEM) image shows that the inclusive nanopar-ticles are al about 5 nm in size. Compared to conventional hydrogen reduction method, plasma method inhibits the agglomeration of metal particles. The room temperature operation is very helpful to limit the nanoparticle size. Most interestingly, plasma reduction produces more flattened metal particles. This plasma reduction does not require the use of any hazardous reducing chemicals, showing the great potential for the fabrication of noble metal nanoparticles.     

A surface-enhanced Raman spectroscopy study of thin graphene sheets functionalized with gold and silver nanostructures by seed-mediated growth

We describe a simple method for decorating graphene (1–5 layers) with Au and Ag nanostructures (nanoparticles, nanorods, and nanoplates). We deposit graphene electrostatically from highly-oriented pyrolytic graphite onto Si/SiO₂ surfaces functionalized with (aminopropyl)trimethoxysilane and grow the metal nanostructures by a seed-mediated growth method from hexanethiolate-coated Au monolayer-protected cluster “seeds” that are attached to graphene by hydrophobic interactions. Scanning electron microscopy reveals the selective growth of Au or Ag nanostructures on the graphene surface. In the case of Au, the low pH 2.8 growth solution causes etching of the graphene and formation of scroll-like structures. For Ag, the high pH 9.3 solution does not seem to affect the graphene. Raman spectroscopy is consistent with the graphene morphology and reveals that the presence of Au and Ag nanostructures increases the Raman scattering from the graphene by a factor of about 45 and 150, respectively. This work demonstrates a simple method for decorating graphene with noble metal nanostructures that may have interesting optical, electronic, and chemical properties for applications in nanoelectronics, sensing, and catalysis.     

Hollow mesoporous silica capsules loaded with copper,silver, and zinc oxide nanoclusters for sustained antibacterial efficacy

Intensive and overuse of antibiotics during the last years has triggered a distinct rise in antibiotic resistance worldwide. In addition to the newly developed antimicrobials, there is a high demand for alternative treatment options against persistent bacterial infections. The biocidal impact of metal ions like copper (Cu²⁺), silver (Ag⁺), and zinc (Zn²⁺), also known as the oligodynamic effect has been used for ages to kill or inhibit the growth of microorganisms and to employ long-term prevention strategies against their biological antagonists. Herein, we report on the synthesis of Cu, Ag, and Zn metal and corresponding oxide nanoparticles immobilized on hollow mesoporous silica capsules (HMSCs) obtained by a hard-template assisted sol-gel synthesis followed by reduction of appropriate metal salts in the presence of HMSCs. Compartmentalization of nanosized metal and oxide clusters in Ag@HMSCs, Cu@HMSCs, and ZnO@HMSCs particles prevented their agglomeration and offered high release kinetics of metal ions between 2.0 and 3.7 mM during 24 h, as monitored by UV-vis analyses. The distribution and morphology of pristine and metal functionalized HMSCs were evaluated by transmission electron microscopy analysis revealing the successful synthesis of Ag, Cu, and ZnO nanoparticles supported on HMSCs. X-ray photoelectron spectroscopy revealed that mainly Cu(II), Ag(0), and Zn(II) species were present in the modified HMSCs. In addition to the surface attachment of preformed metal (Ag and Cu) and metal oxide (ZnO) cluster, nucleation of metal nanoparticles inside the void of HMSCs provided an internal reservoir which allowed for a time-dependent release of metal ions through slower dissolution rates leading to a long-term and sustained bacterial inhibition over several hours. The high antimicrobial efficiency of Ag@HMSCs, Cu@HMSCs, and ZnO@HMSCs particles was investigated toward both Gram-positive (Bacillus subtilis) and Gram-negative (Escherichia coli) bacteria by INT assays showing a complete growth inhibition for both bacteria types after 24 h. While Ag@HMSCs and Cu@HMSCs showed a higher susceptibility against Gram-negative bacteria, ZnO@HMSCs showed a higher susceptibility against Gram-positive bacteria. This demonstrates the promise of metal-loaded capsules as antibacterial delivery vehicles with dual-mode time-release profiles being potential alternatives for antibiotic drugs.     

Bi-Functional Silica Nanoparticles Doped with Iron Oxide and CdTe Prepared by a Facile Method

Cadmium telluride (CdTe) and iron oxide nanoparticles doped silica nanospheres were prepared by a multistep method. Iron oxide nanoparticles were first coated with silica and then modified with amino group. Thereafter, CdTe nanoparticles were assembled on the particle surfaces by their strong interaction with amino group. Finally, an outer silica shell was deposited. The final products were characterized by X-ray powder diffraction, transmission electron microscopy, vibration sample magnetometer, photoluminescence spectra, Fourier transform infrared spectra (FT-IR), and fluorescent microscopy. The characterization results showed that the final nanomaterial possessed a saturation magnetization of about 5.8 emu g⁻¹ and an emission peak at 588 nm when the excitation wavelength fixed at 380 nm.     

Controllable preparation and heavy‐metal‐ion adsorption of lignosulfonate‐polypyrrole composite nanosorbent

Lignosulfonate‐polypyrrole (LS‐PPY) composite nanospheres were prepared facilely via an in situ polymerization of pyrrole monomers in the presence of lignosulfonate as a dispersant and ammonium persulfate as an oxidant. The LS‐PPY composite was characterized with Fourier Transform infrared spectroscopy (FTIR), thermogravimetric analysis, wide‐angle X‐ray diffraction (XRD), scanning electron microscopy (SEM), field‐emission SEM, and transmission electron microscopy. Uniform LS‐PPY solid composite nanospheres with an average diameter of 154 nm were obtained. The LS‐PPY composite nanospheres were applied to adsorption of Ag(I) and Pb(II) ions from aqueous solutions. Maximum adsorption capacities of Ag(I) and Pb(II) were up to 759.3 mg g⁻¹ and 207.5 mg g⁻¹, respectively. Furthermore, the silver ions can be reduced to metallic silver nanowires through a redox reaction between the LS‐PPY composite nanospheres and the silver ions. A productive no‐template route to fabrication of LS‐PPY composite nanospheres with controllable size and heavy‐metal‐ion adsorption ability was achieved. POLYM. COMPOS., 36:1546–1556, 2015. © 2014 Society of Plastics Engineers     

Synthesis and characterization of polyurethane/reduced graphene oxide composite deposited on steel

There has been an ongoing effort by the coatings industry to improve surface properties in order to increase corrosion and wear resistances, as well as other material properties. In this work, we report a methodology for producing nanocomposite films of polyurethane and graphene oxide and polyurethane and reduced graphene oxide. The coatings were applied on steel. The nanocomposites coatings were characterized by optical microscopy, scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, contact angle measurements, and electrochemical impedance spectroscopy. Corrosion tests reveal that the use of reduced graphene oxide increases corrosion resistance when compared with the use of graphene oxide as filler.     

A simple route for preparing Au/mesoporous silica yolk/shell particles for Au-catalyzed reactions

We present a simple route to prepare mesoporous hollow silica particles containing an Au core, i.e., yolk/shell particles, by sol-gel and selective etching processes. Using tetraethoxysilane as a silica precursor, zinc acetate as a base catalyst, and cetyltrimethylammonium chloride as a soft template in the presence of Au nanoparticles, double-layered mesoporous shells were produced in one step. Elemental analysis showed that the inner shell consists of zinc silicate and the outer shell is pure silica. Au/mesoporous silica yolk/shell nanoparticles were obtained by selective etching of the zinc oxide phase with citrate buffer. The particle structure and composition were studied by transmission electron microscopy with energy disperse spectroscopy, UV-vis spectroscopy, X-ray diffraction, and nitrogen sorption experiments. Formation of double shells on the Au core in a single step was explained by a difference in the formation rates of the silica and zinc silicate phases. Au/mesoporous yolk/shell particles showed a high catalytic activity for reduction of 4-nitrophenol.     

Exfoliated graphene sheets decorated with metal/metal oxide nanoparticles: Simple preparation from cation exchanged graphite oxide

We produced carbon hybrid materials of graphene sheets decorated with metal or metal oxide nanoparticles of gold, silver, copper, cobalt, or nickel from cation exchanged graphite oxide. Measurements using powder X-ray diffraction, transmission electron microscopy, and X-ray absorption spectra revealed that the Au and Ag in the materials (Au–Gr and Ag–Gr) existed on graphene sheets as metal nanoparticles, whereas Cu and Co in the materials (Cu–Gr and Co–Gr) existed as a metal oxide. Most Ni particles in Ni–Gr were metal, but the surfaces of large particles were partly oxidized, producing a core–shell structure. The Ag–Gr sample showed a catalytic activity for the oxygen reduction reaction in 1.0 M KOH aq. under an oxygen atmosphere. Ag–Gr is superior as a cathode in alkaline fuel cells, which should not be disturbed by the methanol cross-over problem from the anode. We established an effective approach to prepare a series of graphene-nanoparticle composite materials using heat treatment.