Solution Synthesis of Monodisperse Indium Nanoparticles and Highly

Jul 22, 2010 - Monodisperse sub-10-nm In nanoparticles were synthesized in high .... isobutylamine (being a primary amine) is a stronger surfactant th...
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DOI: 10.1021/cg100857y

Solution Synthesis of Monodisperse Indium Nanoparticles and Highly Faceted Indium Polyhedra

2010, Vol. 10 3854–3858

Teck H. Lim Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, U.K.

Bridget Ingham Industrial Research Limited, P.O. Box 31-310, Lower Hutt 5040, New Zealand

Khadijah H. Kamarudin, Pablo G. Etchegoin, and Richard D. Tilley* MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand Received June 28, 2010; Revised Manuscript Received July 14, 2010

ABSTRACT: We report a simple and effective synthetic method to prepare monodisperse indium metal nanoparticles less than 10 nm in size via a simple lithium borohydride reduction method conducted in amine based solvents. The method has advantages over literature methods; in particular, use of highly reactive and costly organoindium precursors and small gold clusters is required for the heterogeneous growth of sub-10-nm indium nanoparticles. Highly monodisperse spherical nanoparticles produced in one pot in our system could be selectively passivated with commonly available surfactants, including oleylamine, trioctylphosphine and trioctylphosphine oxide. The amine solvents could be readily moved by evaporation under reduced pressure and the particles easily redispersed in common organic solvent in colloidal form. Additionally, highly faceted polyhedra of indium metal could also be produced under specific conditions.

Indium is a Group III element which in bulk metallic form has a melting point of 156 °C. Indium metal has been found to be superconducting1 (superconducting critical temperature 3.4 L K), is surface plasmon resonance (SPR) active,2 and has been used as a soldering material3 and solid state lubricant.4 Nanosized indium metal nanoparticles, as part of the rapid development in nanomaterials research, have been used as a catalyst to promote anisotropic growth of III-V and II-IV semiconductor nanowires and nanorods via a solution-liquid-solid (SLS) mechanism.5-11 Indium metal nanoparticles have been previously synthesized from a range of physical and chemical methods, inclusive of ultrasonication,4 laser ablation,12 dispersing molten indium into paraffin oil,13 and reduction with sodium metal,2 zinc powder,14 and alkalides and electrides.15 These methods, however, mainly produce indium nanoparticles several tens or hundreds of nanometers in diameter. In the year 2008, a sodium borohydride reduction method was reported for the preparation of polyvinylpyrrolidone (PVP) stabilized indium nanooctahedra and nanowires, with the smallest particle size being 80 nm when the reducing mixture solution of sodium borohydride and tetraethylene glycol was added at a rate of 1 drop per second (15 μL per drop).16 More recently, Hammarberg and Feldmann successfully devised a two-step synthesis to produce indium nanoparticles 10-15 nm in size in diethylene glycol using sodium borohydride as reducing agents. A phase transfer reaction was then carried out employing oleylamine as surfactant to transfer the indium nanoparticles into nonpolar dodecane and pentane.17 This method is certainly a significant and innovative improvement. The two-step nature of the synthesis, however, makes it more challenging in the context of scale-up production, in particular the efficiency of mixing two immiscible liquids at relatively large scale during the phase transfer step, which, in turn, would decide the final yield of

*Corresponding author. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 07/22/2010

indium nanoparticles available. A simple and effective one step synthesis and stabilization process with the capacities of producing sub-10-nm indium nanoparticles would be more suited for large scale production, for example, when integrated into a continuous flow reactor system, as has been demonstrated in the case 18 of CdSe quantum dots. Decomposition of an active organometallic precursor of indium(I) such as cyclopentadienyl indium (InC5H5), first reported by Yu et al., has been the only means to prepare small indium nanoparticles less than 10 nm in diameter.19,20 These sub10-nm indium nanoparticles have been used as the catalyst to grow III-V and II-IV semiconductor nanowires and nanorods, during which the diameters of the anisotropic nanostructures are largely determined by the diameters of the indium metal nanoparticles used (in the case where radial growth is insignificant). These nanostructures exhibit interesting physical properties different from those of their spherical nanoparticle counterparts, due to anisotropy in the quantum confinement of electron-hole pairs.5-11 The decomposition method described by Yu et al. relies on the methanol assisted decomposition of cyclopentadienylindium. In their system, particle size control was achieved by inducing heterogeneous growth of indium on small gold clusters [Au101(PPh3)21Cl5] (1.5 nm in diameter), as seeds, in conjunction with the use of a polymeric surfactant, poly(styrene0.86-co-vinylpyrrolidinone0.14).19 With Yu’s method, monodisperse indium metal nanoparticles less than 10 nm in size can be produced, but it does have several issues which render it less economical and practical for large scale production. Firstly, the reagents, cyclopentadienylindium; the gold clusters; and the surfactant, poly(styrene0.86-co-vinylpyrrolidinone0.14), are not readily commercially available and require inhouse preparation. Secondly, the best yield of the gold clusters that could be prepared after a laborious purification process was 26%.21 Owing to the fact that the total yield of the uniformly sized indium nanoparticles to be produced is dependent on the r 2010 American Chemical Society

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Figure 1. Bright field TEM images of indium nanoparticles prepared from reducing indium trichloride with lithium borohydride using OLA (a and b), TOP (c and d), and TOPO (e and f) as surfactant and isobutylamine as solvent. Spherical particles of uniform diameter could be observed for all three systems, with the average diameter being 7.8 ( 0.3 nm, 7.1 ( 0.5 nm, and 8.1 ( 1.1 nm, respectively. The inset in part b showed a high magnification image of an indium nanoparticle. No clear lattice fringes could be observed, and this was attributed to the particles being in a molten state as a result of interaction with the electron beam in the TEM.

total number of gold clusters used, a large scale preparation would be costly. Therefore, there is still the prospect of the development of a safer and more economical method which would allow the production of monodisperse indium metal nanoparticles less than 10 nm in size from readily available precursors and without the use of gold clusters. We report here for the first time a simple hydride reduction method conducted in an alkylamine solvent which allows a onepot synthesis of monodisperse sub-10-nm indium nanoparticles in high yield. As an extension of the simple method, we show that highly faceted indium microstructures could also be produced by controlling the choice of solvents, the amount of surfactants used, and the reaction temperature.

Scheme 1. Standard Electromotive Force (EMF) Calculations for Two Theoretical Electrochemical Reactionsa

a

The calculations were based on values given in ref 22.

Based on a thermodynamics argument from the perspective of electromotive force (EMF) theory, readily available reducing hydrides such as lithium aluminum hydride (LiAlH4) and lithium borohydride (LiBH4) should be able to reduce indium trichloride spontaneously to indium metal, as shown in Scheme 1.22 The reduction reaction mentioned above was carried out in two solvent systems: isobutylamine (S1) and N,N-diethylaniline

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(S2). Surfactants employed included oleylamine (OLA), trioctylphosphine (TOP), and trioctylphosphine oxide (TOPO). Indium metal nanoparticles prepared in both systems were purified and isolated from excess surfactant and solvent under an inert atmosphere to avoid oxidation prior to transmission electron microscopy (TEM) studies. TEM specimens were prepared from the purified samples in a N2 filled drybox. As shown in Figure 1, the reaction in system S1 at 70 °C produced monodisperse spherical indium nanoparticles. Particles produced with OLA (Figure 1a and b), TOP (Figure 1c and d), and TOPO (Figure 1e and f) have average diameters of 7.8 ( 0.3 nm,

Figure 2. EDS spectrum obtained from the TOPO passivated indium nanoparticles shown in Figure 1e and f. The spectrum indicated in average indium was the dominating species. The oxygen signal is attributed to trioctylphosphine oxide and the chlorine signal to lithium chloride residue. The copper signal was due to the TEM grid used and carbon from both the carbon film (a part of the TEM grid) and surfactant TOPO.

Lim et al. 7.1 ( 0.5 nm, and 8.1 ( 1.1 nm, respectively. Typical yields of small isolated surfactant passivated In NPs are ∼80%. Despite the small difference in average particle diameter and size distribution, the nanoparticles in general were similar in both size and shape. Particles passivated by TOPO appeared to have a stronger propensity to form a large thick assembly on a TEM grid than those passivated by OLA and TOP, which were observed to be more dispersed on TEM grids. No lattice fringes were observed for the nanoparticles at high magnification (Figure 1b inset). This observation is similar to that observed by both Yu et al. and Nedeljkovic et al. in which no sharp clear lattice fringes were observed in TEM for indium nanoparticles less than 10 nm in size.19,20 This can be attributed to either the nanoparticles being amorphous or melting of the nanoparticles impinged by an electron beam in the TEM. While bulk indium metal has a melting point of ∼156 °C, indium nanoparticles are expected to have a lower melting point as a result of melting point depression.23,24 It has been observed that an electron beam could introduce physical changes such as melting, phase change, and even coalescence of nanoparticles during TEM characterization.25,26 When subjected to XRD analysis in an inert atmosphere (Figure a in the Supporting Information), the indium nanoparticles we prepared gave a diffraction pattern which could be interpreted and matched to the XRD pattern of tetragonal indium. The relatively large full width half maxima (fwhm) was attributed to the presence of a small proportion of large indium nanoparticles present in an aged sample which can be filtered off easily with commonly used filter papers without altering significantly the overall particle yield. Energy dispersive X-ray spectrometry (EDS) studies of the indium nanoparticles revealed that indium was the dominating element (Figure 2). Small amounts of oxygen and chlorine were detected and can be attributed to TOPO and lithium chloride residue, respectively. When N,N-diethylaniline (orange in color) was used instead of isobutylamine, highly faceted polyhedral structures of indium, several hundreds of nanometers in size were produced at room temperature (Figure 3a-c). The reaction proceeded rapidly,

Figure 3. (a-c) Bright field TEM images of highly faceted indium polyhedra synthesized in N,N-diethylaniline with at least a 10-fold molar excess of TOPO to indium trichloride. (d and e) With a 1:1 ratio, more polydisperse and rounded nanoparticles were produced.

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Figure 4. XRD pattern of highly faceted indium polyhedra prepared in system S2, indicating tetragonal indium metal as the only crystalline phase present.

and a significant color change from orange to yellow to black and/or gray was observed. No lattice fringes could be observed for these polyhedral structures in TEM studies because these particles were too thick. An XRD study performed on these highly faceted polyhedral structures showed that the particles were highly crystalline and the XRD pattern (Figure 4) matched that of tetragonal indium (JCPDS 00-005-0642).27 In this synthetic system (S2), the amount of trioctylphosphine oxide must be kept in at least 10-fold excess (molar ratio of InCl3/TOPO g 1:10) to sustain/maintain the highly faceted features. With a much lower loading of trioctylphosphine oxide, particles of similar sizes are produced, but these lack the highly faceted features (Figure 3d and e). The formation of relatively monodisperse indium nanoparticles in system S1 was believed to be a result of a single nucleation event made possible by thermal treatment of the precursor mixtures in isobutylamine, which was stable for sometime at room temperature. This is supported by the observation of an abrupt color change of the solution (from colorless to yellow when the reaction temperature reached 70 °C). The color of the solution started to change rapidly from colorless to yellow, brown, and finally black after being heated at 70 °C for a period of 1 h. The choice of surfactants did not seem to have a significant effect on the average diameters of the particles. Particles passivated with OLA or TOP have a slightly narrower size distribution than those passivated by TOPO. The overall particle formation mechanism may involve the following: (1) Formation of an isobutylamine-borane complex and surfactant stabilized dichloro-indium-hydride (InCl2H) after treating indium trichloride with lithium borohydride in an isobutylamine/surfactant mixture at room temperature; (2) when heated to ∼70 °C, the surfactant-InCl2H species becomes thermally instable and is further reduced by remaining lithium borohydride and or isobutylamine-borane and decomposed into indium metal nanoparticles. A tetrahydrofuran stabilized dichloroindium-hydride (THF-InCl2H) has been previously synthesized by Takami et al. in the year 2003, and it is known to be stable at 25 °C, which allows it to be used as catalyst to reduce aryl iodides and bromides in the presence of triethylborane as radical initiator.28 The indium metal nanoparticles formed in isobutylamine were relative small and monodispersed in size while those produced in N,N-diethylaniline were relatively large and polydispersed in size. The differences in both size and size dispersity may be explained in that isobutylamine (being a primary amine) is a stronger surfactant than N,N-diethylaniline toward indium metal nano-

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particles and that the rate of metal particle formation was faster in N,N-diethylaniline than that in isobutylamine, evidenced by the rapid development of black coloration after the addition of lithium borohydride. The enhanced rate may be attributed to the formation of a borane-N,N-diethylaniline adduct in system S2, which from the literature is known to be a more reactive species than a borane-alkylamine adduct.29,30 To summarize, we have demonstrated for the first time that monodisperse spherical indium metal nanoparticles less than 10 nm in diameter could be readily prepared from indium trichoride and lithium borohydride. The method reported here is relatively facile and economically advantageous when compared to the best literature method currently available. This single step method has the potential to be integrated into a continuous flow reactor for the large scale production of indium nanoparticles. The indium nanoparticles produced in our system have great potential in being used as seeds for the VLS and SLS growth of semiconductor nanowires and nanorods, as we have demonstrated in a report elsewhere (see ref 31). We have also prepared highly faceted nanostructures of indium at room temperature which may have surface plasmon resonance (SPR) related applications. Acknowledgment. Portions of the work reported were undertaken on the Power Diffraction beamline at the Australian Synchrotron, Victoria, Australia. T.H.L. thanks the New Zealand International Doctoral Research Scholarship for funding. K.H.K. and R.D.T. thank the Malaysian Government for Ph.D. funding. Supporting Information Available: Details of experimental methods, including the synthesis and purification of indium metal nanoparticles and polyhedra, specifications of TEM and XRD instruments, and a synchrotron XRD pattern of nanoparticles. This material is available free of charge via the Internet at http:// pubs.acs.org.

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