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Through a series of carefully executed experiments, we discovered the prevalence of anisotropic shell growth in many upconversion NaREF4 systems caused by a combination of factors: selective adsorption of ligands on the core surface due to the core crystal structure, ligand etching, and the lattice mismatch between core and shell components. This could lead to incomplete shell formation in core-shell nanocrystals under certain conditions. Shell growth is always faster in the a and b crystallographic directions than in the c direction. In the case of a larger lattice mismatch between the core and shell, shell growth only occurs in the a and b directions resulting in an oblong core-shell structure. These findings are useful for rationalizing shell-dependent emission properties, understanding the emission mechanisms in complex core-shell nanostructures, and for creating accurate models of core-shell designs for multifunctionality and optimal performance in applications.

The ethanol-mediated phase transfer protocol was extended herein to prepare alkylamine-stabilized nanoparticles of several noble metals by transferring them from aqueous environment into toluene. This method relies on the use of ethanol as a mediator to provide and maintain adequate contact between dodecylamine and metal nanoparticles during the transfer process and involves first mixing the metal hydrosols and an ethanol solution of dodecylamine and then extracting the dodecylamine-stabilized metal nanoparticles into toluene. Alkylamine-stabilized Ag, Pd, Rh, Ir and Os nanoparticles with 7.09, 3.45, 6.89, 2.42 and 4.52 nm in diameter, respectively, could be prepared this way. The self-assembly of dodecylamine-stabilized Ag and Rh nanoparticles was also detected by transmission electron microscopy (TEM).

The rational design of nonprecious-metal electrocatalysts with activities comparable to or greater than that of platinum is extremely valuable to the development of high energy density metal-air batteries. Herein, the two-step preparation of a highly active oxygen electrocatalyst based on ultrasmall cobalt nanoparticles stabilized in a nitrogen-doped graphene matrix is reported. The catalyst performs as well as the commercial Pt/C catalyst in the oxygen reduction reaction, and better than the Pt/C catalyst in the oxygen evolution reaction. This particular electrocatalyst could significantly lower the overpotentials of oxygen electrochemical reactions in aqueous lithium-air batteries to attain a round-trip efficiency of about 79.0 % at a current density of 0.1 mA cm−2, thereby surpassing the performance of the commercial Pt/C catalyst. The good performance may be attributed to strong metal-support interactions, maximized by a high dispersion of ultrasmall cobalt nanocrystals in a nitrogen-doped graphene matrix, which yields electrocatalytic properties greater than the sum of its parts. [ABSTRACT FROM AUTHOR]

As nanoparticle syntheses in aqueous and organic systems have their own merits and drawbacks, specific applications may call for the transfer of newly formed nanoparticles from a polar to a non-polar environment (or vice versa) after synthesis. This critical reviewfocuses on the application of phase transfer in nanoparticle synthesis, and features core–shell structures in bimetallic nanoparticles, replacement reactions in organic media, and catalytic properties of various nanostructures. It also describes the reversible organic and aqueous phase transfer of semiconductor and metallic nanoparticles for biological applications, and the use of phase transfer in depositing noble metals on semiconductor nanoparticles (258 references). [ABSTRACT FROM AUTHOR]

Abstract: Vulcan XC-72 carbon-supported Pt nanoparticles are prepared by a conventional route, i.e. by adding NaBH4 solution to a carbon slurry of a Pt precursor salt (Pt-1/C); and by a simple reversal of the mixing sequence in which the Pt precursor salt was added to a carbon slurry of NaBH4 (Pt-2/C). Transmission electron microscopy and X-ray photoelectron spectroscopy are used to obtain information on the particle size and size distribution, as well as on the surface oxidation state of the Pt nanoparticles. From cyclic and anodic CO-stripping voltammetric evaluation of catalyst activity for the methanol oxidation reaction (MOR) in acidic solution at room temperature, the Pt-2/C catalyst, which has none of the attributes generally associated with a good Pt catalyst (small particle size, narrow size-distribution, high metal dispersion, low Pt oxidation state), demonstrated higher specific activity and improved CO tolerance than the conventionally prepared Pt-1/C, which has all the common features of a good Pt catalyst. It is concluded that the higher activity of Pt-2/C is linked to its surface oxygen species, which is present in greater abundance and in a more accessible form for reaction with the strongly adsorbed CO-like intermediates. [Copyright &y& Elsevier]

Abstract: Carbon supported Pt nanoparticles were prepared in aqueous solutions by two methods: (1) room temperature NaBH4 reduction of H2PtCl6 with (Pt-1) or without (Pt-2) sodium citrate as the stabilizing agent and (2) microwave-assisted reduction of H2PtCl6 at boiling temperature using citrate as the reducing agent cum stabilizer (Pt-3 and Pt-4). The catalysts were characterized by X-ray diffraction, energy dispersive X-ray microanalysis and transmission electron microscopy, and their activities in room temperature methanol electrooxidation were evaluated by cyclic voltammetry and benchmarked against a commercial E-TEK catalyst with the same Pt loading. TEM showed smaller Pt particles if citrate was present in the synthesis, with average particle sizes of 4.0 and 2.2nm for Pt-1 and Pt-3, respectively. While the presence of citrate was generally beneficial to forming smaller and more uniform Pt nanoparticles, a large excess of citrate (H2Cyt−:PtCl6 2− =20:1) in the microwave synthesis had led to a decrease in catalytic activity. The microwave-synthesized Pt/C obtained using a smaller excess of citrate (H2Cyt−:PtCl6 2− =5:1) demonstrated the best mass activity, surpassing the performance of the commercial catalyst. In addition, a size effect was found in this study: with the decrease in the platinum particle size in the range of 2.2–4nm, the specific activity of Pt for room temperature methanol oxidation also decreased. [Copyright &y& Elsevier]

Abstract: A study was carried out to compare the binding affinities of 1-dodecanethiol and dodecylamine for small Pt nanoparticles. The results showed that the affinity of thiol for Pt nanoparticles increases with increasing particle size whereas the affinity of amine for Pt nanoparticles decreases. The divergence in binding affinities has resulted in differences in catalytic activities of thiol- and amine-protected Pt nanoparticles in the room temperature electro-oxidation of methanol (a fuel cell reaction). It was therefore possible for the larger Pt nanoparticles to be catalytically more active than the smaller Pt nanoparticles up to a certain critical size, before the surface area effects of nanoparticles become noticeable. [Copyright &y& Elsevier]

Abstract: The binding affinity of single stranded DNA (ssDNA) for Au nanoparticles was found to be strongly size dependent. The data clearly indicate that the interaction is not due to a charge screening effect. The smaller particles (5nm) showed the most pronounced effect and inhibited the hybridization of complementary DNA sequences adsorbed on the Au nanoparticle surface. An experimental procedure was subsequently developed to allow the assembly of 17 and 5nm gold nanoparticles using only single stranded DNA without the need for DNA hybridization. The assembly was confirmed by transmission electron microscopy (TEM). A potential pitfall in using DNA melting point analysis to infer metal nanoparticle assembly by DNA hybridization was also discovered during the development of this experimental procedure. [Copyright &y& Elsevier]

Abstract: The inhibition of DNA hybridization by small metal nanoparticles has been examined in detail. DNA melting point analysis showed that the oligonucleotides adsorb strongly and nonspecifically on small metal nanoparticles, inhibiting the hybridization of complementary DNA sequences in common buffered solutions. The nonspecific interaction is even strong enough to disrupt pre-existing hydrogen bonds in short double-stranded DNA. The nonspecific interaction could be weakened by increasing the particle size. As an example, a core-shell assisted method was used to successfully assemble Pt nanoparticles by DNA hybridization that could not be done otherwise. [Copyright &y& Elsevier]

Abstract: A K2BSPP (dipotassium bis(p-sulfonatophenyl)phenylphosphane dihydrate)-based method to sort and size refine Au and Pt nanoparticles has been developed. It makes use of K2BSPP to impart graduated stability to the nanoparticles in a number of NaCl solutions. The method offers a systematic approach to preparing metal nanoparticles of small diameters and a narrow size distribution from an arbitrary particle size distribution. TEM investigations confirmed the size refinement efficacy of this treatment method: in a typical experiment, the mean particle size and relative standard deviation of Au nanoparticles after three successive treatments were 5.18 and 0.055nm, respectively, down from the corresponding values of 8.22 and 0.26nm from the initial untreated nanoparticle solution. The K2BSPP-based size refinement procedure is a simple alternative to more complex chemical procedures and stringent process control that are currently required for the preparation of mono-dispersed systems. [Copyright &y& Elsevier]

Abstract: The difference between the transfer of citrate-stabilized Au and Ru nanoparticles from the aqueous phase to toluene was used to develop a simple experimental procedure that can positively identify the formation of bimetallic Au–Ru nanoparticles with the core–shell structure formed in the seed-mediated growth process. The procedure was independently verified by UV–vis spectroscopy, transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). The experimental results clearly showed that core–shell nanoparticles of Au–Ru were formed by the seed mediated growth method using Au nanoparticle seeds. The reverse order of using Ru nanoparticles as the seeds, on the contrary, could only produce a physical mixture of isolated nanoparticles of Ru and Au under the same experimental conditions. [Copyright &y& Elsevier]

Abstract: Carbon-supported PtRu nanoparticles were prepared by different methods that involve the simultaneous chemical reduction of H2PtCl6 and RuCl3 by NaBH4 at room temperature (PtRu-1), by ethanol under reflux (PtRu-2), and by the thermal decomposition of a single-source molecular precursor [(bipy)3Ru] (PtCl6) (PtRu-3). Transmission electron microscopy (TEM) examinations show that the mean diameter of the PtRu nanoparticles is lowest for PtRu-1 followed by PtRu-2 and PtRu-3. Measurements of electrocatalytic properties, however, reveal a different trend, namely: PtRu-3>PtRu-1>PtRu-2. This is attributed to the formation of a more homogenous alloy nanoparticle system from the thermolysis of the single-source molecular precursor. All three catalysts are more active than commercially available E-TEK (20wt.%) Pt catalyst. PtRu-3 also displays the highest tolerance to carbon monoxide. Heat treatment of PtRu-1 and PtRu-2 only marginally affects their electrocatalytic performance, whereas the co-reduction of H2PtCl6 and RuCl3 under alkaline conditions has more adverse outcomes. [Copyright &y& Elsevier]

Abstract: Carbon-supported Pt-Co and Pt catalysts are prepared by NaBH4 reduction of metal precursors. The particles size of Pt-Co varies with the pH used in the preparation, it is largest (12nm) in alkaline solution and smallest (3.7nm) in un-buffered solution. For the latter, X-ray photoelectron spectroscopy shows that Pt exists principally in the metallic state, whereas cobalt is mostly oxidized. The performance of the catalysts for methanol electro-oxidation in acidic media at room temperature is evaluated and compared with that of a Pt/C catalyst. Electrochemical measurements by cyclic voltammetry and chronoamperometry demonstrate consistently high catalytic activities and improved resistance to carbon monoxide for the Pt-Co catalysts, particularly for that prepared in un-buffered solution. [Copyright &y& Elsevier]

Pt and PtRu nanoparticles supported on HNO3-treated carbon nanotubes (CNTs) were prepared by microwave-assisted heating of polyol process and characterized by XRD and TEM. It was found that the metal nanoparticles were uniform in shape and size and well dispersed on the carbon nanotubes. The average metal diameters were about 3.0–3.1 nm for Pt and 2.8 nm for PtRu. [Copyright &y& Elsevier]

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). [Copyright &y& Elsevier]

An improved method to prepare alkylamine-stabilized Pt and Ru nanoparticles based on the original Brust’s procedure [J. Chem. Soc., Chem. Commun. (1994) 801] has been developed. The new method involves, firstly, mixing an aqueous solution of metal salts such as PtCl62-, PtCl42- or Ru3+ with an ethanol solution of dodecylamine; extracting the metal ions into a toluene layer; and finally reducing the metal ions to their zero valent states using NaBH4. Alkylamine-stabilized Pt nanoparticles prepared this way had a polyhedral or wormlike appearance, depending closely on the chemical nature of the metal precursor salts being used. On the contrary, dodecylamine-stabilized ruthenium nanoparticles were predominantly spherical. The particle formation and growth processes in the hydrocarbon layer could be influenced by the different ways dodecylamine was bound to H2PtCl6, K2PtCl4 or RuCl3 at the precursor stage. [Copyright &y& Elsevier]

Positively charged ruthenium nanoparticles were prepared by NaBH4 reduction at room temperature and at pH values lower than 4.9. The ruthenium nanoparticles were characterized by zeta potential measurement, TEM, XPS, and XRD. Particles with a mean diameter of 1.8 nm and a standard deviation of 0.40 nm could be obtained under the experimental conditions. The surface charge on the particles is believed to originate from hydrated proton adsorption. The positively charged ruthenium nanoparticles could be used as the starting material for further functionalization by PVP, ethylenediamine, and dodecylamine. [Copyright &y& Elsevier]

A novel method has been developed to prepare water-dispersible ethylenediamine (en)-stabilized ruthenium nanoparticles. The procedure involves the reduction of an en–RuCl3 complex by sodium borohydride. The Ru nanoparticles so prepared are fairly stable in water. TEM imaging shows a mean diameter of about 2.1 nm for the particles and a narrow particle size distribution. [Copyright &y& Elsevier]

PVP-protected SnO2 nanoparticles and SnO2–graphite nanocomposites were prepared by urea-mediated homogeneous hydrolysis of SnCl4. For unsupported SnO2 TEM examination showed particles in the range of 4–6 nm, and a narrow particle size distribution. The particles also dispersed very well on a graphite surface. The SnO2–graphite nanocomposites prepared as such combine the high specific capacity of Sn-based anodes and the cyclability of graphite, and are promising anode materials for Li ion battery applications. [Copyright &y& Elsevier]

Nanocomposites of carbon nanotubes (CNTs) with Sn2Sb alloy nanoparticles were prepared by KBH4 reduction of SnCl2 and SbCl3 precursors in the presence of CNTs. SEM and TEM examinations showed that most of the Sn–Sb alloy nanoparticles were present in high dispersion in the CNT web, while others were deposited directly on the outside surface of the carbon nanotubes. Constant current charge and discharge tests using the nanocomposites as Li+ storage compounds showed higher specific capacities than pristine CNTs and better cyclability than unsupported Sn2Sb particles. The first cycle de-lithiation capacity of 580 mAh/g from a CNT–56 wt%Sn2Sb nanocomposite was nevertheless reduced to 372 mAh/g after 80 deep charge and discharge cycles. The uniform dispersion of Sn2Sb alloy in the CNT web and on the surface of CNTs have substantially improved the usability of the Sn2Sb particles to the extent that the nanocomposites of CNTs and Sn2Sb may be considered as a candidate anode material for Li-ion batteries. [Copyright &y& Elsevier]

Intensity-dependent enhancement of saturable absorption in a film of PbS–Au4 nanohybrid composites has been observed by femtosecond time-resolved transient absorption measurement at 780 nm. The nonlinear absorption coefficient (α2) of saturable absorption in PbS–Au4 nanohybrid composites is found to be dependent on excitation irradiance and it is determined to be -2.9 cm/GW at 78 GW/cm2, an enhancement of nearly fourfold in comparison with that of pure PbS quantum dots (QDs). The enhancement is attributed to the excitation of surface plasmon by resonant energy transfer between PbS QDs and Au nanoparticles through Auger recombination. [ABSTRACT FROM AUTHOR]

Abstract: This work aims at enhancing Pt utilization in electrocatalysts by covering of preformed silica nanoparticles. Pt/C electrocatalysts were prepared by reductive deposition of Pt by citrate at moderate temperatures on silica nanoparticles with varying atomic silica to Pt ratios (1.7:1 and 3.3:1) to study the effects of silica to Pt ratio. Considerable voidages were created by inter-situated 10–20nm silica nanoparticles between support carbon particulates to facilitate mass transfer of reactants and products. This particular method of catalyst preparation increases the Pt metal utilization, and generates a large amount of accessible voidage in the interpenetrating particle network of carbon and silica to support the facile transport of reactants and products. Electrochemical hydrogen adsorption/desorption has shown an increase in electrochemically active surface area by this approach. Methanol electro-oxidation was used as a test reaction to evaluate the catalytic activity. It was found that the Pt catalyst modified with silica at silica:Pt=1.7:1 atomic ratio was more active than a catalyst prepared when silica to Pt ratio increased to 3.3:1. [Copyright &y& Elsevier]

Abstract: The dissociation of double-stranded DNA (dsDNA) by 5nm Au nanoparticles was observed through a series of DNA melting transition measurements. Experimental evidence implicates a strong non-specific interaction between the dsDNA and small Au nanoparticles as the cause. Subsequently the relative affinities of mononucleotides and polynucleotides for the 5nm Au nanoparticle were determined by measuring the stability of mono- or polynucleotide-stabilized 5nm Au nanoparticles in salt solutions of different concentrations as a function of time and temperature. The experimental data shows that for the mononucleotides, the affinity decreases in the following order: dA>dG>dC>dT. The order from the polynucleotides was however different, with the affinity decreasing as poly A∼poly C∼poly T>poly G. The lack of correlation between the two ranked orders indicates that the trend obtained from one cannot be used to infer the trend in the other, or vice versa. The evidence provided suggests that the persistence length of the oligonucleotides plays an important role, and must be considered alongside with the individual nucleotide binding strength to determine the overall interaction between the oligonucleotides and Au nanoparticles. [Copyright &y& Elsevier]

Abstract: A solution chemistry method for transforming polycrystalline Ag spherical particles into single crystalline triangular Ag nanoplates has been developed. The synthesis consists of three consecutive steps: (1) the synthesis of Ag nanospheres by NaBH4 reduction of AgNO3 in the presence of sodium citrate; (2) the conversion of citrate-stabilized Ag nanospheres into SDS (sodium dodecyl sulfate)-stabilized Ag nanospheres, and (3) the aging of the SDS-stabilized Ag nanospheres in 0.01 M NaCl solution. Our study indicates that the shape evolved through a Ag nanoparticle dissolution- and re-deposition process; and demonstrated the critical role of SDS in the process: SDS regulates the dynamics in the dissolved O2/Cl− etching of the Ag nanospheres and the reduction of the released Ag+ by citrate ions in the same solution. SDS also functions as a shape-directing agent to assimilate the Ag0 atoms into single crystalline triangular Ag nanoplates. A model for the shape conversion is also proposed which provides the clue for the synthesis of anisotropic Ag nanoparticles with other shapes (rods, wires, cubes, etc.). [Copyright &y& Elsevier]

Abstract: Platinum nanoparticles were prepared as catalysts supported on microporous amorphous carbon with and without a graphitic carbon shell. The electro-oxidation of methanol in acidic solutions at room temperature was used as a probe reaction to explore the effect of the carbon structure on catalysis. CO anodic stripping voltammetry and cyclic voltammetry both recorded enhanced performance for the catalyst supported on the carbon with a graphitic shell. Some rationalizations of the possible roles of the graphitic shell are provided. [Copyright &y& Elsevier]

Abstract: The reaction between silver nitrate and poly(N-vinyl-2-pyrrolidone) (PVP) in pyridine at ambient conditions could lead to the formation of spherical nanoparticles or quadrilateral and triangular silver nanoplates, depending on the silver-to-PVP ratio used. It is proposed that the spherical Ag nanoparticles, which were formed early in the reaction, were transformed into nanoplates through an Ostwald ripening process driven by the bridging flocculation of small spherical Ag nanoparticles. This unique and hitherto unreported shape evolution process was carefully followed by a combination of techniques, viz., UV–visible spectroscopy, TEM, and powder X-ray diffraction. [Copyright &y& Elsevier]

Abstract: Uniform platinum nanoparticles with average diameter of about 3.0nm and good dispersion on carbon nanotubes were prepared by microwave heated polyol process and characterized by EDX, XRD, TEM and XPS. TEM observations showed that microwave synthesized Pt nanoparticles had a sharp size distribution and highly dispersed on the surface of carbon nanotubes. Cyclic voltammetry demonstrated that the microwave synthesized Pt/carbon nanotubes catalysts exhibited higher catalytic activity for methanol electrooxidation at room temperature than a commercial Pt/C catalyst. The significant improvement in electrocatalytic activity was due to that the microwave synthesized Pt nanoparticles was uniform in size and well dispersed on the surface of carbon nanotubes. [Copyright &y& Elsevier]

Abstract: Platinum (Pt) and platinum–ruthenium (PtRu) nanoparticles supported on Vulcan XC-72 carbon and single-wall carbon nanotubes (SWCNT) are prepared by a microwave-assisted polyol process. The catalysts are characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The PtRu nanoparticles, which are uniformly dispersed on carbon, have diameters of 2–6nm. All the PtRu/C catalysts display the characteristic diffraction peaks of a face centred cubic Pt structure, excepting that the 2θ values are shifted to slightly higher values. The results from XPS analysis reveal that the catalysts contain mostly Pt(0) and Ru(0), with traces of Pt(II), Pt(IV) and Ru(IV). The electrooxidation of methanol is studied by cyclic voltammetry, linear sweep voltammetry, and chronoamperometry. Both PtRu/C catalysts have high and more durable electrocatalytic activities for methanol oxidation than a comparative Pt/C catalyst. Preliminary data from a single direct methanol fuel cell using the SWCNT supported PtRu alloy as the anode catalyst delivers high power density. [Copyright &y& Elsevier]

Abstract: Pt nanoparticles supported on Vulcan XC-72 carbon and carbon nanotubes (CNTs) were prepared by a microwave-assisted polyol process. The catalysts were characterized by TEM, XRD and XPS. The Pt nanoparticles, which were uniformly dispersed on carbon, were 2–6nm in diameter. The Pt/C catalysts prepared as such displayed the characteristic diffraction peaks of a Pt fcc structure. XPS analysis revealed that the catalysts contained mostly Pt(0), with traces of Pt(II) and Pt(IV). The electroreduction of oxygen was studied by cyclic voltammetry. It was found both Pt/C catalysts had high electrocatalytic activity in the oxygen reduction reaction. Test runs on a single stack proton exchange membrane (PEM) fuel cell showed that these electrocatalysts are very promising for fuel cell applications. [Copyright &y& Elsevier]