Review pubs.acs.org/cm
Oleylamine in Nanoparticle Synthesis Stefanos Mourdikoudis*,† and Luis M. Liz-Marzán*,†,‡,§ †
Departamento de Química Física, Universidade de Vigo, 36310 Vigo, Spain BioNanoPlasmonics Laboratory, CIC biomaGUNE, Paseo de Miramón 182, 20009 Donostia - San Sebastián, Spain § Ikerbasque, Basque Foundation for Science, 48011 Bilbao, Spain ‡
ABSTRACT: Wet chemistry in organic solvents has proven highly efficient for the preparation of several types of metallic, metal-oxide, and semiconductor nanostructures. This Short Review focuses on the use of oleylamine (OAm) as a versatile reagent for the synthesis of various nanoparticle systems. We describe the ability of OAm to act as a surfactant, solvent, and reducing agent, as a function of other synthesis parameters. We also discuss the specific role of OAm either alone or in combination with other reactants, to form nanostructures using a variety of organic or inorganic compounds as precursors. In certain cases OAm can form complex compounds with the metal ions of the corresponding precursor, leading to metastable compounds that can act as secondary precursors and thus be decomposed in a controlled way to yield nanoparticles. We also point out that OAmstabilized particles can often be dispersed in different organic solvents yielding solutions with enhanced colloidal stability over long times and the potential to find applications in a number of different fields. KEYWORDS: alkylamines, precursor complex, nonaqueous, ligand, capping agent, shape control, oleic acid
1. INTRODUCTION Nanoscale materials can display physical and chemical properties that are different from those of their bulk counterparts. For this reason, design and optimization of synthetic methods for the production of nanoparticles have been intensively investigated during the last few decades. As a result, important progress has been achieved on the synthesis of nanomaterials with tailored composition, size, shape, and crystalline structure. Such nanostructures can find applications in a wide range of domains. Nevertheless, the scale-up of the successful laboratory-adapted protocols to a more industrial level is often not straightforward, and there is still a strong need for the development of simple and inexpensive protocols for large-scale synthesis of high quality nanoparticles. Examples of wet-chemical routes for the synthesis of nanoparticles are the seeded-growth method, the polyol process, and the DMF-mediated reduction. In the two latter approaches, diethyleneglycol (DEG) and dimethylformamide (DMF) are used as solvents that can also reduce selected salts to produce nanoparticles in the presence of surfactants or polymers.1 Very often, the compound used as metal source cannot be decomposed at room temperature, even in the presence of strong reducing agents. However, thermolytic reduction at higher temperatures can be achieved, in principle, in two ways: (i) using a heating mantle or an oil bath to increase the temperature of the reaction mixture (“heat up” approach) and (ii) through the so-called “hot-injection” technique, where a “cold” solution of precursor molecules is rapidly injected into a hot coordinating alkyl solvent. Wang and Li have recently reported an improved hot-injection method which involves the simultaneous use of octadecylamine (ODA) © 2013 American Chemical Society
as a solvent, surfactant, and reducing agent, for the controlled preparation of a wide range of noble metal, metal oxide, and bimetallic nanocrystals.2 Oleylamine (OAm) is another long-chain primary alkylamine, such as ODA or hexadecylamine (HDA), which can act as electron donor at elevated temperatures. Interestingly, commercial OAm has a much lower cost than commonly used pure alkylamines, though some concerns regarding purity and reproducibility have also been raised. Moreover, OAm is liquid at room temperature, which may simplify the washing procedures that follow the chemical synthesis of nanoparticles. However, care should be taken during handling, since OAm can be corrosive to the skin, as indicated in the corresponding material safety data sheet. The molecular structure of OAm is depicted in Scheme 1.3 The double bond (CC) in the middle of the molecule is another special feature of the compound under discussion. In particular, although ODA and OAm exhibit similar basicity and affinity to metals through their NH2 functional groups, the resulting morphology and crystallinity of the produced nanoparticles can be significantly different. An example is the formation of Au decahedra in the presence of either OAm or ODA, where the lack of the CC bond in ODA was suggested to limit its coordination with AuCl, thus yielding modified shapes.4 In the context of understanding the bonding mode of OAm with nanoparticle surfaces, FTIR spectroscopy has become a primary characterization tool. Figure 1 shows the FTIR spectrum of pure oleylamine,5 while Received: January 7, 2013 Revised: February 11, 2013 Published: February 12, 2013 1465
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synthesis of heterostructures comprising a metal and a metal oxide with combined properties. The preparation of certain kinds of nanostructures containing rare-earth elements is also presented. Finally, Section 7 presents several examples of largescale NP synthesis.
Scheme 1. Chemical Structure of Oleylamine (CH3(CH2)7CHCH(CH2)8NH2)a
a
2. OAM IN THE SYNTHESIS OF MAGNETIC NPS Oleylamine is widely used in the synthesis of nanostructures comprising at least one magnetic element. The high boiling point of OAm (≈350 °C) allows the possibility to employ strong heating conditions if necessary. OAm can not only act as a solvent for many organic and inorganic compounds, but it can also become a surfactant and even a mild reducing agent. Such properties are certainly correlated to the specific nature of the target nanomaterial and to the reaction conditions. For example, in the presence of stronger reducing agents, the role of OAm is limited to act as a surfactant and/or solvent. As a first example of the use of OAm in the synthesis of monometallic magnetic nanoparticles, we refer to the work by Nam et al.,7 where fcc-Co hollow nanoparallelepipeds were prepared by thermolysis of the fcc-CoO solid counterparts in neat OAm at 290 °C. Plausible reaction pathways for such transformation are proposed in Scheme 2. Pure OAm was also
Reprinted with permission from ref 3. Copyright 2011 Elsevier B.V.
Table 1 summarizes the assignment of its characteristic FTIR absorption bands.5,6
Scheme 2. Proposed Reaction Pathwaysa,b
Figure 1. FTIR spectrum of pure oleylamine in the 1000−3500 cm−1 region. Reprinted with permission from ref 5. Copyright 2003 Elsevier B.V.
Table 1. Infrared Vibrational Assignments for the OAm Molecule5,6 vibrational modesa
frequency (cm−1)
νas(NH2) and νs(NH2) δ(CH) νas(CH) and νs(CH) δ(CC) δ(NH2) δ(CH3) δ(CN) δ(CC)
3376, 3300 3006 2922, 2854 1647 1593, 795 1465 1071 722
a
Reprinted with permission from ref 7. Copyright 2008 WILEY-VCH. (a) Reduction of fcc CoO to fcc Co by oleylamine with formation of heptadecene and heptadecadiene; (b) conversion of oleylamine into octadecene and octadecadiene by fcc Co.
νs = symmetric stretching vibration; νas = asymmetric stretching vibration; δ = bending vibration.
b
In the following sections we review the role of oleylamine on the synthesis of various types of nanoparticle systems. Section 2 is devoted to nanostructures that contain at least one magnetic element (Fe, Co, Ni, etc.). Noble “nonplasmonic” nanomaterials (e.g., Pd- and Pt-based) are discussed in Section 3, while plasmonic nanostructures (Ag, Au, Cu) are presented in Section 4. Section 5 discusses the use of OAm for the synthesis of semiconductor nanomaterials (for example, sulfur-based systems). We show in Section 6 some examples the OAm-based
used to produce polypod-like structures using Co(OAc)2 as a metal source.8 Chaudret and co-workers used H2-mediated reduction to obtain hcp-Co nanorods using a complex of Co with cyclooctadiene and cyclooctatetraene (Co(COD)(COT)) as precursor in anisole, with an oleic acid (OAc)−alkylamine surfactant mixture. They reported that OAm produced nanorods with lower regularity and less opportunities for long-range organization compared to the use of ODA, which was related to the “cis” configuration and buckled molecular
a
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chain of the OAm molecule.9 In a more comprehensive study, the same group managed to produce hcp-Co nanowires (NWs) with micrometer scale length and 7−9 nm diameter, using a selected oleic acid/OAm concentration ratio.10 Iron is another monometallic magnetic system of large interest. OAm has been reported to complex iron pentacarbonyl, forming Fe(CO)x-OAm (x < 5), which can be used as nanoparticle precursor.11 The thermal decomposition of this complex precursor resulted in the formation of spherical iron nanoparticles with high saturation magnetization and sizes ranging between 2 and 10 nm. Farrell et al. have described the synthesis of iron/iron oxide nanoparticles using OAm/OAc surfactants via both heterogeneous and homogeneous nucleation strategies.12 Slight modifications of the above protocols resulted in the production of Fe@FexOy@FePt and Fe@FexOy@Pt coated particles.13 Sun and colleagues employed OAm as the only surfactant in the preparation of controllably oxidized Fe/Fe3O4 core shell nanoparticles.14 Such particles could be transferred into water upon surface modification, by replacing the OAm capping ligand with a dopamine-based surfactant. The third room-temperature ferromagnetic element, Ni, has also been synthesized in nanoscale sizes with the help of OAm. Highly disordered Ni nanoparticles with a size range of 8−16 nm were produced by decomposition of nickel acetylacetonate (Ni(acac)2), in the presence of OAm as ligand, together with OAc and trioctylphosphine (TOP).15 Carenco et al. investigated the role of the binary ligand system comprising OAm and TOP for the synthesis of monodisperse, size tunable (ca. 2 to 30 nm) Ni nanoparticles, by the thermal decomposition of Ni(acac)2.16 OAm served as the main reducing agent while TOP provided a tunable surface stabilization through coordination on the Ni(0) surface. The simple route of combining Ni(acac)2 with OAm at high temperatures was also studied by Zhang et al. through three different independent processes: direct thermolysis, seed-assisted growth, and hot injection. The product contained single-crystalline fcc Ni particles (20−60 nm) with narrow size distributions.17 On the other hand, applying a strong reducing agent such as borane tributylamine (BTB) resulted in the production of small (≈3 nm) Ni nanoparticles using Ni(acac)2 as precursor and OAm as solvent and cosurfactant, together with OAc.18 Those particles showed good catalytic behavior for hydrogen release from the hydrolysis of ammonia−borane at ambient conditions. The thermolytic reduction of Ni(acac)2 in alkylamines (including OAm) illustrated the possibility of tuning the crystalline structure of the nanoparticles. In principle, higher reaction temperatures (e.g., >240 °C) favored the formation of an hcp phase, while lower temperatures yielded fcc Ni particles.19 The particle size distribution and morphology were further controlled by introducing additional surfactants (OAc and TOP). The same group improved their approach by using a TOPO/TOP surfactant mixture for better size control. The control over the crystalline phase was achieved by adjusting not only the reaction temperature but also the amine concentration and heating rate.20 A similar fcc-hcp structure tuning was reported when Ni acetate was used as nickel source, while 1-adamantanecarboxylic acid (ACA) and TOPO facilitated size control in the range of 5−120 nm.21 This interesting ability to tailor the crystal structure of Ni nanoparticles has continued to draw scientific attention for further investigation, and significantly different magnetic
properties have been reported for fcc- and hcp-structured Ni particles.22 The introduction of atoms as C or H in the Ni crystal lattice has also been achieved using OAm as the reaction medium. More specifically, the addition of hydrazine helped toward the formation of NiHx nanoparticles.23 These particles possess a larger lattice constant compared to pure Ni, and their magnetic characterization revealed both ferromagnetic and paramagnetic features, indicating the existence of two compositional phases. Besides, Schaefer et al. showed that heating a mixture of Ni(acac) 2 with OAm and octadecene allowed Ni 3 C 1−x nanoparticles to be acquired with increasing carbon content as the reaction time was prolonged. The insights from this work helped to experimentally rationalize the discrepancies in lattice constants and magnetic properties that were previously reported for hcp-Ni.24 A relevant work by Goto et al. on the thermolysis of nickel acetylacetonate in OAm is in agreement with the existence of hexagonal nickel carbide. These authors proposed a formation mechanism for such hcp Ni3C nanoparticles:25 metallic Ni NPs are first formed by the thermal decomposition of the acetylacetonate and reduction of Ni(II) by OAm, and then carbidization into hexagonal nickel carbide occurs, with formation of cubic nickel carbide as an intermediate product. The carbidization of the fcc-Ni progresses by exposure to the CO gas emitted during acac decomposition above ≈240 °C. Hyeon’s group reported the synthesis of highly monodisperse Ni and NiO nanoparticles through the thermal decomposition of Ni−OAm complexes. The authors explained that the Ni−OAm complex was prepared by reacting Ni(acac)2 with OAm at moderate temperature. The resulting solution was later injected into TPP (triphenylphosphine), followed by stronger heating conditions. Finally the particles could be readily dispersed in nonpolar solvents such as hexane and toluene and presented the ability to self-assemble into superlattices with long-range order via controlled solvent evaporation. In addition, these particles were active catalysts for a Suzuki coupling reaction.26 The Ni(acac)2−OAm system has been used for the synthesis and thorough characterization of Ni/NiO core−shell nanoparticles,27,28 while the combination of the traditional chemical synthesis with microwave irradiation for the control of size, shape, and shell thickness has also been reported.29 OAm has also served as a reaction medium for the synthesis of Ni2P−Ni core−shell nanoparticles.30 In this case, Ni(0) NPs were synthesized in a first step, followed by the addition of P4 and further in situ thermal treatment. A detailed study on the role of OAm concentration during the synthesis of nickel phosphides was reported by Muthuswamy et al. These researchers prepared hollow Ni12P5 and Ni2P particles as well as solid Ni 2 P NPs. It was shown that increasing OAm concentration favored the formation of the Ni12P5 phase rather than the Ni2P one.31 Moreover, it was shown that an increase in OAm concentration was systematically related to a tunable modification of void size in hollow Ni12P5 particles formed at low P:Ni rations. Metal Oxide Magnetic Nanostructures. OAm has been employed successfully with a “triple role” (solvent, surfactant, reductant) in the synthesis of hexagonal and cubic CoO nanocrystals, using Co(acac)3 as precursor.32 Taking into account that the reactions were carried out under inert atmosphere, the oxygen in CoO is assumed to originate from the acac ligand of the precursor. 1467
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Size- and shape-controlled FeO nanoparticles (Figure 2) were prepared by reductive decomposition of Fe(acac)3 in an
Figure 3. Self-assembled multilayer pattern of octahedral Fe3O4 nanoparticles. TEM (a) and SEM (b−d) images of a self-assembled Fe3O4 NPs multilayer. Reprinted with permission from ref 39. Copyright 2009 Royal Society of Chemistry.
Figure 2. TEM images of FeO nanoparticles: (a) 14-nm spheres and (b) 32-nm and (c) 53-nm truncated octahedra. (d) SEM image of 100nm truncated octahedra. The sizes refer to average lengths of one side in the projected images. (reproduced from ref 33 with permission from Wiley-VCH).
Such nanomaterials presented size-dependent but rather unusual magnetic properties.40 Moreover, MnO nanospheres and nanorods were produced by tuning the temperature in a reaction involving the preparation of a Mn−OAm complex, which was subsequently injected into preheated TOP.41 Bimetallic Magnetic Nanoparticles. Magnetic nanostructures comprising at least two metals, of which at least one is a magnetic element, have been successfully synthesized using OAm as a reaction medium. Chaubey et al. employed OAm as a cosurfactant, together with OAc, in the synthesis of 20 nm FeCo nanoparticles with high saturation magnetization, using Fe(acac)3 and Co(acac)2 as precursors.42 Polycrystalline FeCo nanoparticles with a random orientation were obtained using the above surfactant mixture with Co(COD)(COT) and Fe(CO)5 as metal sources.43 OAm was also used as surfactant for growing Fe on preformed Co NPs by Fe(CO) 5 decomposition at 180 °C. The further thermal treatment of the as-synthesized Co/Fe core−shells at 250 °C yielded FeCo alloy NPs.44 Ternary superparamagnetic alloys with tunable composition (FexCoyPt100−x−y) were also prepared using the OAc/OAm surfactant mixture.45 In addition, anisotropic cobalt−iron phosphide nanocrystals with controlled composition and magnetic response were produced using the Co and Fe oleates, in the presence of OAm and TOP.46 Superparamagnetic Co50Ni50 and ferromagnetic Co80Ni20 nanoparticles were produced using OAm in a triple ligand mixture together with OAc and TOP. It was suggested that the metal−acetylacetonate precursors were first decomposed at 200 °C to form their TOP complexes, which were later decomposed at 240 °C to produce Co and Ni nuclei.47 Ni− Co nanoparticles with a Ni-rich core and a Co-rich shell were produced by mixing and heating Ni−OAm and Co−OAm complex precursors under microwave irradiation (Figure 4). Those precursors were initially prepared by stirring and heating at intermediate temperatures mixtures of Co(II) formate dihydrate or Ni(II) acetate tetrahydrate with OAm.48
OAc/OAm mixture.33 The binding difference between oleate and OAm on the crystal planes was claimed to determine the final morphology of the particles. The same argument was used to explain the growth of γ-Fe2O3 tetrapods using a ternary surfactant mixture (OAc, OAm, and hexadecanediol).34 Fe3O4 (magnetite) nanocrystals in the range of 7−10 nm were also produced using OAm as reducing agent, stabilizer ,and cosolvent with benzyl ether, in a facile heating protocol using iron(III) acetylacetonate as metal source.35 Suitable ratios between OAm, OAc, and hexadecanediol resulted in ca. 4 nm Fe3O4 nanoparticles, which readily formed self-assembled monolayers and multilayers upon evaporation of hexane.36 This approach was extended to the synthesis of other ferrite nanocrystals with the general formula MFe2O4 (M = Fe, Co, Mn).37 It was shown that the simultaneous use of OAc and OAm was necessary for the desired particle formation, whereas OAc alone would lead to a viscous red-brown product which was difficult to purify and characterize. On the other hand, OAm alone did produce iron oxide nanoparticles but in a very low yield. Regarding more anisotropic shapes, star-like cubes and flower-like magnetite nanoparticles were synthesized by the pyrolysis method using surfactants with different hydrocarbon structures such as OAm on the one hand and adamantaneamine, adamantanecarboxylic acid, and trioctylamine on the other hand.38 It was considered that the linear structure of OAm should attack the electropositive carbonyl carbon of an intermediate iron complex more easily than the bulky adamantyl groups of adamantaneamine (or the corresponding bulky groups of trioctylamine), thus influencing the particle growth mode. Additionally, the molar ratio of OAm to iron oleate has proven to be crucial for the formation of octahedral ferromagnetic Fe3O4 NPs (Figure 3).39 Other magnetic oxide NPs such as MnO and Mn3O4 have been prepared by prolonged heating of slurries of OAm with Mn(acac)2 in the absence or presence of water, respectively. 1468
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also resulted in the production of spherical FePd nanoparticles when starting from iron pentacarbonyl and Pd(acac)2.51 FePt nanowires and nanorods were obtained in a simple protocol reported by Sun and co-workers, where OAm was used alone or diluted with octadecene in a heated solution containing Pt(acac)2 and Fe(CO)5. It was proposed that possibly OAm self-organized into an elongated reverse-micellelike structure within which the FePt nuclei were formed.52 The same researcher was involved in a pioneering, highly influential work on the formation of monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices using OAm and OAc as stabilizers.53 Modifications of the latter protocol (absence of hexadecanediol, sequential addition of surfactants) allowed the preparation of more faceted FePt NPs with an increased size up to 9 nm and the possibility to coat with a Fe3O4 shell if an excess of iron precursor was used.54 For example, it has been reported that if OAm was added first, sphere-like FePt nanoparticles were obtained, possibly through the formation of a stable Pt-NH2 complex. On the other hand, if OAc was added in a first stage before the addition of OAm, ≈7 nm FePt nanocubes were obtained.55 In the OAm/OAc surfactant pair, Fe tends to bind to −OOC while Pt binds to the −NH2 group. This dual ligand system has been introduced as the optimum for the preparation of extended ordered layers of monodisperse FePt nanoparticles, in comparison, for instance, to ligand pairs such as octanoic acid/octylamine, oleic acid/ODA, or octadecanoic acid/ODA.56 Advanced realtime TEM imaging has provided important knowledge toward the understanding of the mechanism of one-dimensional colloidal nanocrystal growth during the formation of FePt3 nanorods in a pentadecane/OAm system in the presence or absence of OAc.57 It has been shown that the OAm/OAc binary surfactant system can be replaced by PEI (polyethyleneimine) via ligand exchange to form a PEI/FePt NPs multilayer assembly. Such an assembly might be suitable for ultrahigh-density data storage media, as its thermal annealing transformed the initial chemically disordered fcc phase into the chemically ordered tetragonal fct phase, rendering FePt nanoparticles with the desirable high magnetocrystalline anisotropy and room temperature ferromagnetism.58,59 Such slightly reductive annealing treatment (e.g., under a flow of Ar(95%)/H2(5%)) has also been described to result in the formation of relatively large (≈17 nm) fct-FePt NPs, using Pt@ Fe2O3 core−shell NPs prepared in the presence of OAm/ OAc.60 The synthesis of biocompatible FePt@Au nanoparticles also required a ligand exchange approach. In this case the OAm/OAc pair was successfully replaced by mercaptoundecanoic acid (MUA), rendering the particles water-dispersible while the gold coating would allow further functionalization with different biomolecules and provide heating resistance, which is necessary for hyperthermia applications.61 The synthesis of Ni-based bimetallic magnetic NPs was also carried out in the presence of OAm. Irregular-shaped NiFe NPs were formed using OAm in the triple (solvent, reductant, surfactant) role, during the decomposition of Fe(acac)3 and Ni(acac) 2 . The morphology was better controlled by introducing cosurfactants such as dodecanediol or TOP.62 Nickel− and platinum−acetylacetonates were reduced into spherical NiPt NPs using the OAm/OAc binary surfactant in benzyl ether.63 NiPt3 nanopolyhedra were produced via the tungsten-mediated reduction of Ni(acac)2 and Pt(acac)2 in the presence of OAm/OAc. The authors explained that OAm played the role of a reducing agent but it could also stabilize
Figure 4. HAADF-STEM image of Ni50Co50 NPs (a). Elemental maps of Co (b), Ni (c), and O (d) in the area shown in (a). Reprinted with permission from ref 48. Copyright 2011 American Chemical Society.
Interesting ferromagnetic CoPt polypod-like nanostructures were obtained by direct thermolytic reduction of Pt(acac)2 and Co(OAc)2 in hot OAm (Figure 5). In this case oleylamine
Figure 5. Bright field TEM micrographs of CoPt nanopolypods prepared in oleylamine. Inset: Electron diffraction pattern. Reproduced with permission from ref 49. Copyright 2005 American Chemical Society.
played the role of high boiling point coordinating solvent, as well as reducing and capping agent. It was proposed that OAm might act as a ligand to form stable complexes with Pt2+ while heating induced the thermolytic reduction of such Pt complexes to the metallic state, and the obtained small Pt seeds could reduce Co2+ to Co(0), thereby generating CoPt nanoparticles.49 A similar process, but replacing Co−acetate with Co2(CO)8, resulted in the formation of either nanowires or flower-like CoPt structures, depending on the specific temperature and concentrations of the employed reactants.50 Another cobalt precursor, Co(CO)3NO, was applied for the formation of spherical CoPt nanoparticles using OAm as a cosurfactant with OAc and 1,2-hexadecanediol as reductant. This approach 1469
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used the H2-mediated reduction approach for the preparation of Pt nanostructures with complex shapes such as cubic dendrites and fivefold stars (Scheme 4), by reacting H2PtCl6
Pt3Ni {111} by lowering the surface energy of Pt3Ni {111} facets, thus inducing an octahedral morphology.64 Ni−Cu alloy nanoplates with either hexagonal or triangular shape were produced using OAm as a solvent, reductant, and cosurfactant with TOP.65 The final shape of those particles was determined by the overall reducing environment of the reaction (temperature, absence or presence of benzyl ether as cosolvent Scheme 3). Moreover, the OAm/OAc ligand pair facilitated the
Scheme 4. General Overview of the Versatile Synthesis of Pt0 NPs in OAma,b
Scheme 3. Schematic Illustration for the Formation of Triangular and Hexagonal Ni−Cu Alloy Nanoplatesa
a
Reprinted with permission from ref 75. Copyright 2012 Wiley-VCH. The shape could be controlled by the platinum concentration and the nature of the seeds.
b
with OAm at selected temperatures, Pt concentrations, and dihydrogen pressures.75 In another approach, in situ formed Au nanoparticles during the reaction of PtCl2 in OAm in the presence of AuCl3 triggered the nucleation and anisotropic growth of Pt nanoflowers, nanopolyhedra, and other similar structures.76 However, gold nanocrystals were rather inactive toward the seeded growth of Pt nanocrystals in dichlorobenzene, in the presence of OAm and hexadecanediol. Cobalt traces were used as surfactant-independent means of shape control, leading to the formation of various anisotropic Pt nanocrystal morphologies, as reported by Puntes and colleagues.77 Besides, Tilley and co-workers published several works on the reduction of platinum acetylacetonate in OAm under hydrogen atmosphere. They managed to produce shapes such as octapods and cubes, showing, for example, that the initial Pt concentration can affect the reaction kinetics and the final shape.78−80 The platinum precursor−tungsten system formed by introduction of W(CO)6 in the reaction mixture served as a “buffer” facilitating also the growth of Pt and Pt3Co nanocubes, in the presence of OAm/OAc.81 The binary surfactant system was in a 4:1 volume ratio, and both compounds acted as solvents and capping agents. The resulting nanocubes showed good catalytic activity for the electrooxidation of methanol. Besides, OAm served as a surfactant during the preparation of Pt and PtCo nanocrystals, where the influence of competing reducing agents toward size and shape control was examined.82 Pt-on-Pd bimetallic nanostructures were also prepared in the presence of OAm. This protocol involved a two-step procedure, and the final material was tested for electrocatalytic oxygen reduction.83 In addition, various Pt alloys (Pt−M, M = Pd, Ni, Fe, Co) with controlled shape and composition were prepared using the CO gas reduction approach and OAm as one of the surfactants.84 PtAu nanoparticles are also interesting catalysts. It has been suggested that, in the presence of butyllithium, a strong reductant, OAm may be deprotonated to form an amide, and the amine-to-amide ratio might be important for PtAu NWs formation and subsequent branching.85 In a rather simple protocol, the sole use of OAm was reported to reduce a mixture of HAuCl4 and H2PtCl6 at 160 °C, resulting in the formation of PtAu NPs.86 The seed-mediated method was employed for the preparation of heterogeneous Pt−Au nanostructures, where the role of OAm had to do mainly with the initial reduction of HAuCl4 to form the gold seeds.87,88
a
Reprinted with permission from ref 65. Copyright 2012 Royal Society of Chemistry.
synthesis of Mn−Pt alloy NPs with spherical or cubic shape in ether solvents, which displayed interesting magnetic and electrocatalytic properties, respectively.66,67
3. NOBLE (NONPLASMONIC) METAL NANOPARTICLES Pt-Based Nanostructures. Nanostructures based on metals such as palladium and platinum have attracted interest for various applications, such as catalysis and electrocatalysis. OAm was used as a surfactant together with OAc for the synthesis of faceted Pt nanoparticles in the 3−7 nm size range. These particles were active for the catalytic reduction of O2.68 The OAm/OAc combination helped also toward the production of ≈8 nm Pt nanocubes, in the presence of trace amounts of Fe(CO)5 in octadecene.69 Unlike the preparation of FePt nanocubes, in which the addition sequence of OAc and OAm was important,55 the synthesis of these Pt nanocubes did not require such sequential surfactant addition. Wu et al. demonstrated the crucial role of carbon monoxide (CO) as a reductant and stabilizer of the Pt(100) surface for the formation of Pt nanocubes in OAm, even in the absence of oleic acid.70 The ability of CO to influence the reaction kinetics was extended also for the production of Pt nanocubes, Pd NPs, and Au NWs in the presence of OAm.71 The thermolytic reduction of Pt(NH3)2Cl2 in different reaction media as OAm, HDA, and oleyl alcohol resulted in different anisotropic structures. In that case, the nature of the selected solvent (including its capping and reducing capabilities) seemed to play a major role in directing the morphology of the final structure.72 The use of a suitable environment to burst nucleation, using morpholine borane and N-methyl-pyrrolidone, yielded 10−12 nm Pt nanocubes within one minute of reaction, where OAm served as solvent and surfactant.73 Control of the reaction kinetics in a relatively simple protocol involving the heating of Pt(acac)2 in OAm was shown to yield either spherical or branched Pt nanostructures, depending on the reaction temperature (higher temperatures favored more isotropic shapes).74 Lacroix et al. 1470
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The role of the OAm/OAc mixture on the compositiondependent formation of PtAg NWs via the oriented attachment mechanism on the {111} surface was proposed by Peng et al.89 OAm was also necessary for the formation of PtCu nanorod catalysts, presumably due to its tendency to stabilize the (100) facets during growth.90 Furthermore, high-quality PtCu nanocube catalysts for formic acid oxidation were synthesized thanks to a synergistic stabilizing effect offered by a mixture of OAm and tetraoctylammonium bromide.91 Other Noble Metal (Nonplasmonic) Nanostructures. OAm acted as a solvent, surfactant, and coreductant of Pd(acac)2 (together with BTB) for the production of 4.5 nm Pd NPs, which were evaluated for catalytic formic acid oxidation.92 An excess of OAm/OAc with respect to the Pd(OAc)2 concentration led to the synthesis of 3.0 nm Pd NPs via the reducing ability of tert-butylamine borane (TBAB).93 On the other hand, OAm was found to form intermediate complexes expressed as [Pd(acac)x(OAm)y] in the presence of formaldehyde, and the formation of these intermediates allowed a good kinetic control of the synthesis with subsequent shape control over the final products.94 The role of OAm was also demonstrated upon the study of the effect of the local ligand environment on the nucleation and growth of NPs in a two-component reagent mixture (Pd precursorsystematically variedand OAm).95 The combination of OAm with alkylammonium alkylcarbamate (AAAC) allowed the manipulation of the final Pd morphology from spheres to tetrahedra and polypods.96 Metin et al. reported that OAm not only stabilizes Pd(0) nanoclusters but in addition does not have any deleterious effect on their catalytic activity for the dehydrogenation of ammonia−borane.97 Tilley and colleagues employed the H2-mediated reduction for the preparation of Pd nanostructures, when the introduction of OAc as a cosurfactant with OAm favored the formation of highly branched morphologies, while OAm alone yielded rather isotropic shapes.98,99 On the other hand, the simultaneous use of OAm with TOP has also been reported to provide monodisperse Pd NPs, both using Pd(acac)2100 and Na2PdCl4 as precursors.101 Rh and Ir NPs were also prepared in OAm as a reaction medium. OAm-capped rhodium nanotetrahedra showed excellent catalytic activity for the hydrogenation of anthracene.102 On the contrary, the insertion of OAc (together with OAm) for the synthesis of iridium particles rather quenched their catalytic activity for the hydrogenation of 1-decene.103
Figure 6. TEM images at different magnifications of Au NWs prepared by OAm reduction (a, b) and HRTEM image (c) showing their singlecrystalline structure. Reproduced with permission from ref 104. Copyright 2008 American Chemical Society.
had previously reported the synthesis of single-crystalline thin gold wires, but the method was rather complicated as it involved a variety of reagents, high temperature, and intermediate steps.111 By lowering the precursor concentration in the latter protocol, spherical Au particles were obtained.112 These NPs displayed strong broad-band optical-limiting properties for both femtosecond and nanosecond laser pulses. On the other hand, thicker Au NWs (d ≈ 9 nm) were produced by adding oleic acid in the HAuCl4/OAm mixture in a 1:1 volume ratio with respect to OAm.113 If chloroauric acid was dissolved in water prior to addition of excess OAm, 11−13 nm NPs were finally isolated, as shown by Polavarapu and Xu.114 Size tuning in the 1−10 nm range was achieved by using the burst nucleation method with TBAB as reductant in the precursor/OAm mixture and tetralin as solvent. These particles were highly active for CO oxidation.115 Moreover, the addition of OAc in the presence of a different precursor, Au(ac)3, in OAm led to the formation of 6.7 nm Au NPs.116 Although OAm-capped Au NPs are hydrophobic, they can be made hydrophilic through a ligand-exchange procedure that involved replacing OAm with 3-mercaptopropionic acid.117 Silver nanoparticles were also obtained by using OAm as a reducing agent for AgNO3 and stabilizer in the presence of a high-boiling-point solvent, liquid paraffin.118 Size- and shapecontrolled Cu nanoparticles were prepared with the OAm/OAc surfactant mixture in the presence of hexadecanediol as reductant and copper acetylacetonate as metal precursor.119 Besides, Hyeon and colleagues have shown that Cu(acac)2 could be thermally decomposed in neat OAm to yield uniform 15 nm Cu particles which readily formed a Cu2O oxide shell upon air exposure. These Cu@Cu2O core−shell NPs were good catalysts for Ullmann type coupling reactions of aryl chlorides.120 AuxCuy alloy nanoparticles were also produced by using excess OAm in the presence of hexadecanediol at 160 °C, employing HAuCl4 and Cu(acac)2 as metal sources. These particles were envisaged not only for optical but also for catalytic applications (e.g., electrochemical reduction of CO2).121 Wang et al. used OAm as reductant/surfactant, in the presence of a small amount of octadecene and chloroauric acid/silver nitrate metal precursors at moderate temperature, 120 °C. The weak bonding of OAm allowed its easy removal
4. PLASMONIC NANOSTRUCTURES Nanoparticle systems based on gold, silver, and copper have been successfully prepared by using oleylamine. A relatively simple general protocol that involves the use of only two reagents (HAuCl4 and OAm) has been simultaneously reported by several groups104−106 for the synthesis of long, ultrathin Au NWs (Figure 6). The main idea behind the formation mechanism of the Au NWs was that mesostructures of Au+− OAm complex were initially formed, serving as growth templates that govern one-dimensional growth in the nanoscale. In certain cases, Ag or Fe NPs were inserted in the reaction mixture to boost the reduction stage or act as growth templates.107−109 The addition of triisopropylsilane (TIPS) as highly effective reductant in the binary reactant mixture accelerated the reaction rate and improved the yield of ultrathin gold nanowires at room temperature within a few hours. These NWs demonstrated an intriguing application in surfaceenhanced Raman scattering (SERS).110 Halder and Ravishankar 1471
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from the surface of the particles, rendering “clean” enough AuAg NPs that are catalytically active for CO oxidation, as shown by the corresponding evaluation tests.122
5. NANOSCALE SEMICONDUCTORS PREPARED WITH OLEYLAMINE Colloidal semiconductor nanocrystals can also be synthesized in the presence of OAm. For example, uniformly sized Ag2S nanocrystals were prepared by pyrolysis using AgNO3 and S powder as precursors and OAm as solvent, surfactant, and reductant. These particles were proposed for application as SERS substrates.123 A similar heat-up protocol using OAm was employed for the synthesis of CdS nanorods and CdSe nanoparticles.124 Highly luminescent CdSe NPs were produced using OAm as a coligand whose concentration affected the reaction kinetics and the final particle size.125 Elemental sulfur and copper acetylacetonate were thermally treated in OAm to obtain monodisperse hexagonal Cu2S nanoplates. These materials were considered as potential solutions for application in fields such as solar cells, photoelectric devices, and colloidal photonic crystals.126 The combination of Pb(OAc)2, dodecanethiol, and OAm has been recently reported to yield PbS nanocrystals with polyhedral or truncated-cubic shape depending on the reaction temperature.127 PbS NPs were also prepared by mixing and heating initially prepared PbCl2− OAm and sulfur−OAm solutions.128 PbTe nanocubes were prepared by mixing OAm/OAc surfactants with Pd−acetate, followed by hot-injection of trioctylphosphine telluride at 200 °C.129 ZnS nanostructures with different morphologies were also obtained by heating a mixture of OAm, zinc stearate, and sulfur precursor at 280 °C. The kind of precursor used (sulfur powder, thiourea, dodecanethiol) determined the final particles morphology (nanorods, dot-shaped, and quasi-cubic-shaped, respectively).130 Reagents such as TOP, octenoic acid, and dichlorobenzene were added for the production of high-quality Ni3S4 and CuS nanocrystals with OAm as the reaction medium.131 Hyeon’s group reported a generalized strategy for the synthesis of semiconducting metal sulfide NPs (PbS, ZnS, CdS, and MnS) with various sizes and shapes. This procedure initially involved the formation of a metal−oleylamine complex, by dissolution of metal chloride with OAm at an intermediate temperature, and the final nanomaterials were obtained after injecting elemental sulfur and further heating.132 Thomson et al. used NMR techniques to study the sulfur−oleylamine interactions during the synthesis of sulfide NPs. They showed that thioamides can be also used as S precursors due to their rapid kinetics.133 Copper-based quaternary chalcogenides have recently drawn interest as low-cost alternatives to conventional absorber materials in photovoltaics. Shavel et al.134 prepared Cu2ZnSnS4 nanoparticles with controlled composition by reacting metal− amino complexes with S in a continuous flow reactor at 300− 330 °C (Figure 7). The complexes were formed by dissolving metal precursors in an OAm/octadecene mixture. OAm was also employed for the production of Cu2ZnSn(SxSe1−x)4,135 CuInS2,136 and CuInSe2137 and other Cu-based selenide NPs.138 Germanium NWs139 and NPs140 were produced in the presence of OAm. In the latter case oleylamine initially served for the formation of a secondary complex precursor after mixing with GeCl4. Additionally, In2O3 nanoparticles were produced by thermal decomposition of In(acac)3 in OAm under inert atmosphere. The oxygen in the nanoparticles
Figure 7. (A) Scheme of the flow reactor setup and image of a 1 g pellet made of CZTS nanoparticles. (B) TEM micrograph of cleaned CZTS nanoparticles prepared inside the flow reactor at 300 °C at a flow rate of 2.0 mL/min. The inset shows a HRTEM image of a CZTS nanocrystal and the corresponding SAED pattern. Reproduced with permission from ref 134. Copyright 2012 American Chemical Society.
composition was claimed to originate from the acetylacetonate ligand of the precursor.141 The combination of OAm, OAc, and trimethylamine N-oxide in hexadecane was considered essential for the formation of monodisperse indium oxide NPs upon decomposition of indium acetate.142 Selishcheva et al. recently reported that the addition of copper ions in the reaction solution can help toward the shape tuning of the obtained In2O3 nanostructures.143 NMR data indicate that OAm is converted to oleylamide during nanoparticle formation: 2In(CH3COO)3 + 6R − NH 2 = 6R − NH − CO − CH3 + In2O3 + 3H 2O
On the other hand, ZnO nanostructures with several shapes such as nanorods, nanotetrahedrons, and nanosquamas were produced via an effective aminolytic reaction of zinc carboxylates with OAm in coordinating or noncoordinating solvents. These nanostructures displayed interesting optical properties such as sharp band-edge emission or broad deep-trap emission, depending on the shape and the state of their structural defects.144
6. HETEROSTRUCTURES AND RARE EARTH-BASED NPS A variety of heterostructured nanomaterials composed of a metal (Au, Ag, Pt, or Ni) and Fe3O4 or MnO were synthesized by thermal decomposition of mixtures of metal−oleate complexes (for the oxide component) and metal−OAm complexes (for the metallic component).145 Ag−FexOy hybrid nanoparticles were obtained by reducing Ag+ ions by OAm in the presence of Fe/FexOy nanoparticles. These structures exhibited both plasmonic and superparamagnetic properties, thus being candidate materials, for example, to be used as both SERS substrates and contrast agents for magnetic resonance imaging (MRI) and optical imaging.146 OAm also served as a surfactant and reducing agent of HAuCl4 to coat Fe3O4 NPs with Au shells.147 In the approach by Yu et al.,148 Au NPs were 1472
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first formed, followed by decomposition of Fe(CO)5 on their surface and subsequent oxidation, to yield Au−Fe3O4 dumbbell-like nanoparticles. In this case OAm/OAc were used as surfactants and octadecene was the high-boiling-point solvent. Platinum was attached on the surface of iron oxide NPs with the help of OAm as surfactant and dodecanediol as the reducing agent of K2PtCl4.149 In addition, FePt@MnO nanoheterostructures were prepared by heating manganese oleate with FePt NPs in benzyl ether containing OAm/OAc surfactants.150 Surface-modified rare-earth compound nanocrystals have attracted interest due to their unique optical properties and promising applications in, for example, UV shielding, luminescent displays, optical communication, biochemical probes, and medical diagnostics. The role of the OAm/OAc surfactants on the synthesis of anisotropic La2O3, Pr2O3, Nd2O3, Eu2O3, and other rare-earth oxide nanocrystals has been discussed by Si et al.151 Ceria (CeO2) nanoparticles were prepared by decomposing cerium(III) nitrate hexahydrate using OAm surfactant. Adding oleic acid as a cosurfactant with OAm led to the production of anisotropic wire- and tadpole-shaped nanocrystals.152 The OAm/OAc surfactant pair was also employed for the formation of gadolinium oxide (Gd2O3) NPs in octadecene.153 Moreover, the effect of the concentration of OAm on the size of NaYF4:2% Er3+,20%Yb3+ particles has been studied by Ostrowski et al.154 The cooperative effect of the OAm/OAc surfactant pair toward the synthesis of monodisperse NaLa(MoO4)2 bipyramids has been discussed in detail by Bu et al.155 In fact, Cao and colleagues have shown that these two commonly used surfactants can be condensed together to form N-(cis-9-octadecenyl)oleamide (OOA) during the synthesis of uranium dioxide (UO2) NPs. Their results claim to provide unambiguous evidence that it is the OOA not the simple mixture of OAc and OAmthat plays the major role in controlling the formation of UO2 nanocrystals.156 The above binary ligand pair was also used for the formation of ferromagnetic SmCo5 NPs,157 as well as one-dimensional W18O49 and V2O5 nanostructures.158
h in air,161 whereas Bi2Te3 nanoplates were prepared by reaction between bismuth thiolate and tri-n-octylphosphine telluride in OAm. The subsequent fabrication of n-type nanostructured thermoelectric materials was implemented through the sintering of surfactant-removed Bi2Te3 nanoplates using a spark plasma sintering process.162 Hiramatsu and Osterloh reported almost a decade ago a simple large-scale synthesis of nearly monodisperse Au and Ag nanoparticles using only three reagents (tetrachloroauric acid or silver acetate, OAm, and a solvent).163 The synthesis of about 10 g of Ag3PO4 NPs with precise size control ranging from 8 to 16 nm has been achieved by a room-temperature reaction of silver nitrate, oleylamine, and H3PO4 in a toluene/ ethanol solvent mixture. These NPs exhibited high visible light photocatalytic activity.164 Water-dispersible cubic ceria nanocrystals were produced by a simple large-scale sol−gel reaction of cerium salt in the presence of OAm.165 The use of microwave heating was proposed as a scalable approach for the synthesis of rare earth oxides (M2O3, M = Pr, Nd, Sm, Eu, Gd, Tb, Dy) with various shapes, as it provides the advantage of avoiding thermal gradient effects. Typical reaction conditions included the combination of metal acetate or acetylacetonate precursors with the OAm/OAc surfactant mixture.166 Fe/Fe3O4 core/shell NPs prepared following the protocol described by Peng et al.14 were employed by Jaeger and coworkers for the fabrication of large-scale freestanding nanoparticle monolayer membranes via a drying-mediated selfassembly process. These freely suspended layers showed remarkable mechanical properties with Young’s moduli of the order of several GPa, regardless of membrane size.167 Moreover, Shi et al. reported that monodisperse magnetite nanocrystals can be successfully prepared in large quantities via a facile synthetic procedure based on the pyrolysis of Fe(acac)3 in the presence of OAm/OAc surfactants.168 The presence of an organic OAm layer was reported as the key point to serve as a carbon source for the large-scale preparation of graphitic carbon-coated FeCo NPs via the reductive thermal conversion of nanometric Prussian blue analogues (PBAs).169 Finally, it has to be noted that OAm is reported, in many cases, to improve the colloidal stability of various types of nanostructures, for example, including dispersions of nickelcoated single walled carbon nanotubes (SWNTs) in THF or toluene. In the above example, the π electrons in the SWNTs interact with the hydrophobic part of the OAm molecule, and the amine groups interact with the nickel atoms. Amine−amine interactions between OAm molecules lead to formation of micelles, which help to form a network in the solvent. Such NiSWNT materials are aimed to be used as fillers in composite materials, for the enhancement of electrical conductivity and optimized performance in lightning-strike protection of aircrafts.170
7. OLEYLAMINE FOR LARGE-SCALE NANOPARTICLE SYNTHESIS The significance of OAm as reagent of choice in nanoparticle synthesis, not only from the academic point of view but also regarding technological exploitation, is better illustrated by the separate presentation of some characteristic examples of its use in large-scale NP synthesis. Large-scale preparation is usually achieved at the expense of compromising NPs quality.125 However, some of the synthetic protocols presented in the former sections are readily adaptable for large-scale production, which may even reach gram-scale quantities.93,110,114,126,134,152 Du et al. have recently reported a general method for the largescale synthesis of metal sulfide nanocrystals (CuS nanosheets, ZnS NWs, Bi2S3 NWs, and Sb2S3 NWs). For example, ultrathin CuS nanosheets were prepared from an intermediate lamellar complex formed via the reaction of CuCl with OAm and octylamine, followed by the addition of S powder at moderate temperatures.159 Large-scale superlattices of wurtzite CuInS2 nanocrystals were obtained by solvent evaporation using copper nitrate, indium nitrate, dodecanethiol, OAc, and OAm. The band gap of these CuInS2 NPs was measured to be 1.63 eV, which is optimal for solar cell applications.160 High-quality Bi2S3 nanorods were successfully synthesized in large scale by reacting Bi[S2P−(OC8H17)2]3 with OAm at 140−160 °C for 5
8. SUMMARY AND OUTLOOK This Short Review illustrated all the important aspects of the suitability and the versatility of oleylamine as the reaction medium for the chemical synthesis of a broad range of nanoscale materials. We showed the different and combined roles of OAm in such syntheses, which demonstrate its suitability as solvent, surfactant, and reducing agent. Other advantages of the use of oleylamine, such as its liquid state at room temperature, easy removal via centrifugation, high boiling point, low cost, and tendency to form metal−OAm complexes at intermediate temperatures, so that it can be controllably 1473
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decomposed to produce nanoparticles, were clearly explained. The composition, size, and shape of the obtained nanocrystals can be tuned by careful choice of additional reaction parameters, depending on the system under study. Concerning commercially available OAm, a future goal might be the improvement of its purity, as up to now OAm is not available at purities higher than 85−90% (or 70%, for the technical grade reagent). A decrease in the level of impurities could possibly ensure an even better reproducibility and facilitate scaling the synthetic reaction protocols where OAm is involved up to industrial scale, provided that its cost would remain reasonably low. In this way, the number of applications for the OAmcapped nanomaterials, though already remarkable, might be greatly increased.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (S.M.); llizmarzan@cicbiomagune. es (L.M.L.-M.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was implemented within the framework of the Action “Supporting Postdoctoral Researchers” of the Operational Program “Education and Lifelong Learning” (Action’s Beneficiary: General Secretariat for Research and Technology of Greece) and is cofinanced by the European Social Fund (ESF) and the Greek State [Project code PE4(1546)]. L.M.L.M. acknowledges funding from the European Research Council (ERC Advanced Grant 267867, PLASMAQUO). S.M. thanks L. Polavarapu for suggesting the idea to write this Short Review for a compound he is also familiar with.
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Chemistry of Materials
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Chemistry of Materials
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