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Recent Advances in the Synthesis and Main Applications of Metallic Nanoalloys Blanca M. Mu~noz-Flores, Boris I. Kharisov,* Víctor M. Jimenez-Perez, Perla Elizondo Martínez, and Susana T. Lopez Facultad de Ciencias Químicas, Universidad Autonoma de Nuevo Leon, Av. Pedro de Alba s/n, C. P. 66451, San Nicolas de los Garza, N. L., Mexico ABSTRACT: Syntheses of nanoalloys using diverse experimental methods are reviewed. The main techniques include microwave heating, laser-assisted methods, chemical vapor deposition, and electrochemical methods, as well as a series of chemical reduction and decomposition routes. The nanoalloys possess a series of useful properties, such as magnetic, semiconducting, field-emission materials, that are being applied in the catalytic processes and the creation of optical, electronic, and magnetic materials.
1. INTRODUCTION The research field of nanometals in the last decades is rapidly developing, because of the fact that physical and chemical properties of the metal nanoparticles differ considerably from those of both bulk metal and isolated atoms.1,2 Metallic and bimetallic nanoparticles possess unique properties, allowing them to be explored for a variety of applications, including nanocatalysis particularly for efficient selective catalysts, sensors, optical markers, and filters, among many other applications. The nanoalloys area as a part of the “nanometals” topic, refers to metallic clusters composed of two or more metal elements and their physical and chemical properties, are defined not only by their size and stoichiometry (atomic arrangement), but also challenging their composition.39 Really, the nanoalloys field first appeared in the 19th century, a long time before the beginning of nanotechnology: the oldest topic in nanoscience is the size-dependent optical properties of Au and Ag colloids or nanoparticles, which was first studied scientifically by Michael Faraday in 1857.10 Recently, the area of nanoalloys is very extensive; however, a predominant number of reports correspond to nickel and noble metals (for example, AuRe11 or CoPt12), because of their catalytic applications. Uniform nanoalloys should be formally distinguished from coreshell metallic nanoparticles by containing two or more metals and by being built of a nucleus/kernel (for example, metallic iron) and a covering shell (for instance, Au); however, sometimes these two terms are not strictly separated in different publications. Computer simulations that have been carried out13 in order to study the mechanism of formation of nanoparticles synthesized in microemulsions have allowed the main idea for obtaining nanoalloys or coreshell nanoparticles to be expressed as follows: a coreshell structure is obtained when the reduction rates of both metals are very different; meanwhile, a nanoalloy is obtained if both reaction rates are similar. As well as for bulk alloys, a very wide range of combinations and compositions are possible also for nanoalloys. Bimetallic nanoalloys (AmBn) can be generated with, more or less, controlled size (m þ n) and composition (m/n). The cluster structures and degree of A-B segregation or mixing may depend r 2011 American Chemical Society
on the method and conditions of cluster generation (type of cluster source, temperature, pressure, etc.).5 Several methods based on physical and chemical approaches have been developed for the synthesis of controlled size and shape nanostructures including nanoalloys, such as chemical reduction,14,15 thermal decomposition16 (in particular, thermal destruction of bimetallic carbonyl precursors17), electrochemical synthesis,7,18,19 microwave synthesis,20,21 and molecular beams.22,23 Recently, the chemistry of nanoalloys has been widely investigated, because of their interesting and useful properties and applications in catalysis, photonics, electronic devices, among many others.24,25 The applications of nanoparticle alloys are expected to enhance many fields of advanced materials and relevant technology, particularly in the areas mentioned above, as well as chemical and biological sensors, optoelectronics, and others.26,27 The present review is dedicated to recent achievements in the synthesis and main applications of nanoalloys in diverse areas of chemistry and chemical technology, omitting coreshell metallic nanoparticles.
2. THEORETICAL CALCULATIONS Nanoalloys have been the subject of systematic theoretical investigations that reveal their stable structures and compositions in order to associate these data with their potential applications. Thus, a systematic ab initio study of the structures and magnetic properties of Mo4xFex clusters (x = 13) was reported.28 In a related report,29 ab initio calculations of the structure and electronic density of states (DOS) of the perfect coreshell Ag27Cu7 nanoalloy attested to its D5h symmetry and confirmed that it has only six nonequivalent atoms (two Cu and four Ag). Also, the global structural optimizations for NiAl nanoalloy clusters at different compounds were investigated using particle swarm optimization combined with a simulated annealing method.30 Some stable structures were described for NixAlx (x = 18), Ni3xAlx (x = 14), and NixAl3x (x = 14) nanoalloy clusters. In Received: January 25, 2011 Accepted: May 16, 2011 Revised: May 13, 2011 Published: May 16, 2011 7705
dx.doi.org/10.1021/ie200177d | Ind. Eng. Chem. Res. 2011, 50, 7705–7721
Industrial & Engineering Chemistry Research
Figure 1. (Top row) Chiral structure of Ag107Cu85. (Middle row). Achiral pentaIh structure of Ag90Cu56. (Bottom row) Achiral pentaIh structure of Ag102Cu75. From left to right: for all structures, the first and second snapshots are side views (with the second showing the structure of the inner Cu core), and the third depiction is a top view. Reproduced with permission from ref 31. Copyright 2010, American Chemical Society, Washington, DC.
addition, a specific class of nanomaterials (coreshell nanoalloys with a Cu, Ni, or Co core and a chiral Ag or Au shell of monoatomic thickness) possessing the highest degree of chiral symmetry, the chiral icosahedral symmetry, was predicted31 by a combination of global optimization searches and first-principle calculations. High-symmetry chiral nanoalloys associated strong energetic stability with potential for applications in optics, catalysis, and magnetism. Figure 1 shows an example of such compounds.
3. MAIN METHODS FOR THE SYNTHESIS OF NANOALLOYS 3.1. Microwave Synthesis. 3.1.1. Microwave Heating. The fabrication of nanoalloys via the method of microwave heating (irradiation) (MWI) is a facile and fast method that has been developed to prepare a wide variety of pure metallic and bimetallic alloy nanoparticles with controlled size and shape.14 The MWI approach provides simple and fast routes to the synthesis of nanomaterials, since no high temperatures or high pressures are needed. Furthermore, MWI is particularly useful for controlled large-scale synthesis, because it minimizes the thermal gradient effects.15,32 As a result of the difference in the solvent and reactant dielectric constant, selective dielectric heating can provide significant enhancement in reaction rates. The most important advantage of microwave dielectric heating over convective heating is that the reactants can be added at room temperature (or slightly higher temperatures). In addition, nanoparticle size can be tuned by varying the concentration of the precursor and the MWI duration and the shape of formed nanostructures is controlled by varying the concentration and composition of the ligating solvents. Among recent reports, Abdelsayed described the synthesis and characterization of bimetallic alloys of the Au, Pt, Pd, Rh, Ag, Cu,
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Figure 2. (a) TEM images of bimetallic nanoalloys prepared by the MWI method. (b) Digital photographs of metallic and bimetallic nanocrystals dispersed in toluene solutions. Reproduced with permission from ref 20. Copyright 2009, American Chemical Society, Washington, DC.
and Ni;20 the MWI approach was established as a general procedure for the synthesis of a variety of high-quality crystalline bimetallic nanoalloys with controlled size and shape. Figure 2a shows TEM images of several examples of bimetallic nanoalloys. Figure 2b shows digital photographs of metallic nanocrystals and bimetallic nanoalloys dispersed in toluene solutions. Different colors of the nanoalloys, in comparison to the individual metals, are clearly visible in all cases. The same method can be used to synthesize bimetallic nanoalloys supported on ceria nanoparticles as nanocatalysts. Thus, it was revealed that the CuPd, CuRh, and AuPd supported nanoalloys exhibited high activity for CO oxidation. Figure 3a shows the XRD patterns of the PtAu and PtRh nanoalloys, and Figure 3b shows the XRD patterns of the CuAu and CuPt nanoalloys. Comparing the XRD pattern of the nanoalloy to the patterns of the individual metals, it is clear that the diffraction peaks of the nanoalloy are located between the corresponding peaks of the individual metals. This suggests the formation of a solid solution corresponding to the specific nanoalloy. The nanoalloy patterns also show no evidence of any pure metallic peaks, which indicates that binary nucleation has been the major nucleation process involved in the formation of these alloys. Lai et al.33 synthesized PtCo binary alloys by MWI of aqueous solutions containing Pt and Co precursors, and a significant enhancement in activity for oxygen reduction reaction (ORR) was observed for compositions such as Pt50Co50 as a result of a higher degree of alloying between Pt and Co. In addition, the relationship between the variations in alloying extent and Pt d-band vacancies in PtCo/C electrocatalysts was established,21 which was tunable with Pt and Co composition and had a strong impact on the catalytic activity for the ORR. It was found that the catalytic activity of Pt1Co1/C for ORR was higher when compared to that of Pt/C, Pt1Co3/C, and Pt3Co1/ C, because of its higher alloying extent, smaller particle size, and proper composition. 7706
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Industrial & Engineering Chemistry Research
Figure 3. Comparison of the XRD patterns of nanoalloys (a) PtAu and PtRh and (b) AuCu and PtCu, with the patterns of the individual metal nanocrystals prepared via the MWI method. Reproduced with permission from ref 20. Copyright 2009, American Chemical Society, Washington, DC.
A new approach, based on MWI, has been developed for the work reported by El-Shall et al.34 This method is a facile and effective approach that is based on MWI for the incorporation of a variety of metallic and bimetallic nanoparticle catalysts within the highly porous coordination polymer MIL-101 (a chromiumbased MOF with the molecular formula Cr3F(H2O)2O[(O2C)C6H4(CO2)]3 3 nH2O, where n ≈ 25). The authors used two methods for the incorporation of the metal nanoparticles. In the first method (method A), MIL-101 suspended in water was treated with the appropriate amount of metal nitrate, M(NO3)2 (M = Pd, Cu), under stirring, followed by the addition of hydrazine hydrate under MWI. In the second method (method B), solution infiltration (incipient wetness impregnation) was used before the hydrazine reduction in microwave synthesis. Method A results in higher loadings of the metal nanoparticles (as determined by inductively coupled plasma (ICP) analysis), which have a tendency to disperse on the external surface of the MIL (in addition to within the pores). The solution infiltration method results in smaller loading percentages, with most of the nanoparticles incorporated within the bulk phase of MIL crystallites. In the case of Pd nanoparticles formed via method B, it was clearly indicated that the number of nanoparticles formed on the external surfaces of the MIL crystals was significantly reduced. In addition, a general review on the control of size and shape of nanoparticles fabricated via microwave synthesis should be recommended for researchers working in this area.35
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3.1.2. Microwave Plasma Treatment. In this method, highfrequency microwaves are applied through a resonator to generate plasma. Microwave plasma sources are used increasingly for thin-film deposition.36 In particular, plasmas produced by resonant absorption at the electron cyclotron frequency, in the so-called ECR (electron-cyclotron resonance) devices, offer many desirable characteristics, including high plasma density, low-pressure operation, efficient gas utilization, and uniform plasma with a high degree of ionization and decomposition of gases.37 The microwave power (2.45 GHz, 1.3 kW maximum) is provided by a homemade source using a conventional magnetron from commercial microwave ovens.38 A schematic illustration of the two different elemental apparatus for the production of plasma via microwave synthesis39,40 is shown in Figure 4. A series of carbon-supported monometallic (Fe, Co) and bimetallic (CoMo) materials (average metal particle diameters of
