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Cite this: Chem. Commun., 2014, 50, 1454 Received 25th October 2013, Accepted 28th November 2013

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Facile and scalable synthesis of Ti5Si3 nanoparticles in molten salts for metal-matrix nanocomposites† Marc Estruga,a Steven N. Girard,a Qi Ding,a Lianyi Chen,b Xiaochun Lib and Song Jin*a

DOI: 10.1039/c3cc48168a www.rsc.org/chemcomm

We report a novel synthesis of Ti5Si3 nanoparticles (NPs) via the magnesio-reduction of TiO2 NPs and SiO2 in eutectic LiCl–KCl molten salts at 700 8C. The Ti5Si3 particle size (B25 nm) is confined to the nanoscale due to the partial dissolution of Mg and silica in the molten salts and a subsequent heterogeneous reduction on the surface of the TiO2 NPs.

Metal silicides are a broad family of refractory intermetallic compounds between transition metals and silicon.1 Depending on the metal and their composition, metal silicides can exhibit metallic or semiconducting and/or magnetic behaviour, therefore silicide nanomaterials have a variety of technological applications, such as solar energy conversion,1a thermoelectrics,2 catalysis,3 spintronics,4 microelectronics,1b etc. Another potential industrial application of silicide nanomaterials would be as reinforcement elements in metal-matrix nanocomposites (MMNCs), which are hybrid materials consisting of nanoscale ceramic reinforcements distributed throughout a continuous metallic matrix.5 The mechanical, electrical, and thermal properties of MMNCs can be beneficially tuned by controlling the size and distribution of the nanoscale phase. The chemical compatibility with the matrix and physical properties of the nanoscale reinforcement material are critical to achieve good wetting, distribution and capture.6 For example, metal borides have shown much better wetting and dispersion behaviour in Al than more traditional ceramics such as oxides, carbides or nitrides, owing to their thermodynamic stability in molten metal and higher electrical conductivity.7 The demonstration of enhanced strengthening in MMNCs with borides has increased attention on using concomitantly electrically conductive and thermally robust nanomaterials to improve strengthening. Nanomaterials of refractory a

Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin, 53706, USA. E-mail: [email protected]; Tel: +1-608-262-1562 b Department of Mechanical Engineering, University of Wisconsin-Madison, 1513 University Avenue, Madison, Wisconsin, 53706, USA † Electronic supplementary information (ESI) available: Details of materials and reagents, Ti5Si3 NP synthetic procedure and supplementary SEM micrographs. See DOI: 10.1039/c3cc48168a

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metallic silicides, such as Ti5Si3, can be excellent candidate reinforcement materials for MMNCs, due to their unique combination of properties: relatively low density,8 excellent corrosion and oxidation resistance,9 high hardness and stiffness,10 high melting point and high conductivity. Specifically, Ti5Si3 is the most conductive titanium silicide, with an electrical resistivity as low as 15 mO cm,11 only one order of magnitude larger than Cu. Ti5Si3 has a high (congruent) melting point of 2122 1C and is thermodynamically stable across a wide temperature range.12 Simple and scalable synthesis of Ti5Si3 nanomaterials will be vital for their technological applications, including in MMNCs. It is challenging to synthesize nanostructured refractory silicides, such as Ti5Si3, at a large scale. The high melting points of the silicides and slow diffusion of silicon often demand high temperature synthesis in the vapour phase, which has been the most common route to synthesize silicide nanowires so far.1a Indeed, nanowires of Ti5Si3 and other metal-rich silicides (M5Si3)13 have been grown via chemical vapour deposition (CVD),14 although it is difficult to produce the large quantity of Ti5Si3 nanomaterials needed for MMNCs. Solution synthesis of silicide nanomaterials of late transition metals, such as iron and copper silicides, in high boiling point organic solvents has been recently reported,15 but the higher reactivity of early transition metal Ti will likely make conventional solution-based synthesis of Ti5Si3 much more difficult. Therefore, alternative synthetic methods of utilizing molten salts as solvents (or flux) present an exciting and scalable means to produce nanomaterials of a wide range of refractory silicides. The use of molten salt fluxes is a traditional solid-state chemistry synthesis technique. Recently, several classes of nanomaterials have been synthesized using molten salt solvents, including nanostructured Si, Ge, metal borides, nitrides and carbides,16 using both chemical and electrochemical reduction routes from their respective higher oxidation state oxides. The advantages of molten salt synthesis, as compared to a vapour phase process, include enhanced control of the solubility of the precursors, low vapour pressure at elevated temperatures, large electrochemical stability, and the promise of potential industrial scalability. (Note that the industrial production of aluminium metal is accomplished by electrochemical

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reduction in molten salts.) Also, when compared to classic solidstate reactions, molten salt syntheses allow better kinetic reaction control and afford particles with a smaller size.17 There have been reports of titanium silicide phases synthesized in molten salt fluxes via electrochemical reduction, but the particle size is large (around 1 mm) and the Ti–Si products suffer from phase impurities.18 In this work, we report for the first time the synthesis of Ti5Si3 NPs via the magnesio-reduction of TiO2 NPs and SiO2 in eutectic LiCl–KCl solutions at 700 1C, study their formation mechanism and investigate the wetting behaviour of Ti5Si3 NPs in molten Al for MMNC applications. For a typical Ti5Si3 autoclave synthesis (see ESI† for details), Mg powder, TiO2 NPs, SiO2 NPs (20 : 5 : 3 molar ratio) and a pre-dried LiCl–KCl eutectic mixture (45/55 wt%) were manually ground in a mortar for 15 min. The mixture was then sealed into a home-built stainless steel autoclave inside an Ar-filled glovebox and heated up to 700 1C for 2 h in a box furnace. Following the reaction, the autoclave was allowed to cool down to room temperature and the eutectic salt was dissolved in water. The solid product was then washed using a 2 M HCl solution and water to remove the residual MgO by-product. After drying overnight at 60 1C, the Ti5Si3 NP product was recovered as a fine black powder. Powder X-ray diffraction (PXRD) confirmed the perfect match (Fig. 1a) with hexagonal Ti5Si3 (PDF 29-1362; space group P63/mcm, crystal structure in Fig. S1, ESI†). Scherrer analysis of the most intense peak, corresponding to the (211) plane, suggests an average crystallite size of 23 nm. Scanning electron microscopy (SEM) images of the Ti5Si3 NPs dispersed on a Si/SiO2 substrate revealed that the as-prepared NPs are homogeneous, with particle size clearly at the nanoscale (inset of Fig. 1a). Transmission electron microscopy (TEM) images revealed an average particle size of 25  5 nm (Fig. 1b) and highresolution TEM/FFT lattice spacing confirmed the NPs to be comprised exclusively of Ti5Si3 (Fig. S2, ESI†). Moreover, energydispersive X-ray spectroscopy (EDXS) confirmed that MgO was successfully removed during the washing procedure, showing Ti, Si and only residual O (5.6 at% Ti; 4.5 at% Si; 10.5 at% O; C and Cu signals are attributable to the TEM grid). The overall Ti5Si3 formation should proceed via the reduction of titania and silica by metallic Mg (reaction 1): 5TiO2 + 3SiO2 + 16Mg - Ti5Si3 + 16MgO

(1)

To further study the Ti5Si3 formation mechanism, an analogous reaction was conducted in the absence of the LiCl–KCl salt melt. In this case, the amount of Mg available to reduce the TiO2 and SiO2 would be much higher, since the Mg metal is a liquid at the

Fig. 1 PXRD (a), SEM (inset) and TEM (b) characterization of the Ti5Si3 NPs. The PXRD pattern matches the standard pattern of Ti5Si3 (PDF-29-1362).

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Fig. 2 PXRD (a), SEM (b) and TEM (inset) of the Ti5Si3 NPs prepared in absence of molten salts.

processing temperature. These higher supersaturation conditions led to the formation of the Ti5Si3 product with larger particle size, resulting in sharper diffraction peaks in the PXRD pattern (Fig. 2a). Indeed, Scherrer analysis confirmed the larger crystallite size to be 67 nm. The larger particle size was also verified by SEM and TEM, which also revealed a much wider particle size distribution, ranging from 30 to >200 nm (Fig. 2b). According to these results, the Ti5Si3 formation occurs via the reduction of metal oxides by the electrons that come from dissolved Mg, solvated in the eutectic salt (reaction 2).16a It is well known that many metals are partially soluble in molten salts through a corrosion reaction that yields the corresponding metal cations and free electrons in the melt.19 As electrons are consumed during the reduction of titanium oxide and silicon oxide, more Mg is dissolved, resulting in a dynamic equilibrium between solvated Mg2+/electrons and zero-valent Mg metal (see eqn (2)). As a result, this dynamic system provides the low supersaturation conditions that restrict the Ti5Si3 particle growth, leading to smaller NPs. Additional experiments using different Mg : LiCl–KCl salt melt ratios (10–18 wt%) revealed that the Ti5Si3 crystallinity and NP size were not significantly different (Fig. S3, ESI†), demonstrating that the supersaturation was limited due to the slow Mg dissolution process in the salt melt.16a Mg(s) ! Mg2+(diss) + 2e

(diss)

(2)

To better elucidate the reduction of the silica, experiments using both nano and microsized SiO2 particles were conducted. Surprisingly, we found that both amorphous SiO2 NPs (B12 nm in diameter) and micron-scale crystalline SiO2 particles (B40 mm) resulted in basically pure Ti5Si3 NPs (Fig. 3a and b, respectively). The trace amounts of C49-TiSi2 and TiSi impurity phases are attributable to incomplete reagent mixing during the grinding process. Simultaneously, the Ti5Si3 NP size is also preserved at B25 nm (Fig. S4, ESI†). This result suggests that the silica reduction occurs via a dissolution mechanism; since the nanoscale Ti5Si3 phase obtained did not depend on the initial silica particle size, we propose that the SiO2 precursors, either nano- or microscale, become solvated in the molten salts to facilitate the reaction to form a nanoscale phase-pure product. Conversely, if the reduction were to occur at the TiO2–SiO2–Mg interface, we would expect that product particle sizes would have correlated with the size of SiO2 particles used, as well as an expected decrease in phase control due to limited diffusion and demixing.16c To further understand this silica dissolution mechanism, a control experiment was conducted by mixing MgO and SiO2 nanopowders in a eutectic LiCl–KCl solution at 700 1C for 2 h,

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Fig. 4 Cross-section SEM micrographs (a, inset), EDXS mapping (b) and schematics (c) of the Al–Ti5Si3 nanocomposite powder.

Fig. 3 PXRD of the Ti5Si3 NPs obtained using different SiO2 and Si sources: 12 nm SiO2 NPs (a), B40 mm SiO2 microparticles (b), and B100 nm Si NPs (c).

which resulted in the formation of Mg2SiO4, as confirmed by PXRD (Fig. S5, ESI†). Thus, we propose that the trace amount of MgO present in the Mg powder dissolves in the salt melt (reaction 3),20 to further react with the solid silica particles (either nano or micro), yielding soluble magnesium silicates (reaction 4). An analogous SiO2 dissolution mechanism in molten CaCl2 has been recently reported, in this case via the formation of CaSiO3, to facilitate the reactions.21 Moreover, when Si NPs (B100 nm in diameter) instead of SiO2 were used as the reagent, similar Ti5Si3 phase and NP size were obtained (Fig. 3c). In this case, we believe that the native silicon oxide on the surface of Si NPs, as well as latent oxides arising from impurities in the salt or Mg, could result in a similar dissolution process as in reaction 3, also leading to the formation of soluble silicates via the same dissolution–precipitation mechanism. MgO(s) ! Mg2+(diss) + 2O2

(diss)

(3)

- SiO44

(diss)

(4)

SiO2(s) + 2O2

(diss)

In contrast, the reduction of TiO2 depends heavily on the initial titania size, suggesting that it occurs via a template mechanism. While nanosized TiO2 starting materials resulted in complete Ti4+ reduction and pure Ti5Si3 NPs, microsized titania particles yielded a mixture of partially reduced TiO with mixed titanium silicide phases (Fig. S6, ESI†). These results suggest that the reduction of TiO2, as the last step of Ti5Si3 formation, is an interfacial heterogeneous process (reaction 5), which is enhanced by increasing the available surface area using nanoscale TiO2 and ensuring a Ti-rich environment that favours the Ti5Si3 phase. 5TiO2(s) + 3SiO44

(diss)

+ 32e

(diss)

- Ti5Si3(s) + 22O2

(diss)

(5) In order to demonstrate the potential application of the as-prepared Ti5Si3 NPs for MMNCs, dispersion experiments in an immiscible Al–NaCl/KCl system were conducted to obtain Al–Ti5Si3 nanocomposite powders (see ESI† for details). The interfacial interaction between the Ti5Si3 NPs and the molten Al will govern their

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wetting and incorporation into the metal, which are critical for the successful preparation of MMNCs by solidification processing. Indeed, we found that the surface of the as-prepared Ti5Si3 NPs was covered by an amorphous TixSiy layer by HRTEM (Fig. S7, ESI†), most likely formed by partial dissolution of Ti5Si3 during the MgO washing step using HCl. This layer showed poor wetting with Al droplets, and the Ti5Si3 NPs were found to form clusters separated from the Al droplets (Fig. S8, ESI†). However, if more diluted HCl (0.1 M) is used during washing, the amorphous shell could be avoided (Fig. S9, ESI†) and the Ti5Si3 NPs showed excellent wetting behaviour with metal and were directly embedded inside the Al droplets (Fig. 4), promoting homogeneous dispersion as shown by the EDXS mapping, which shows promise for the preparation of Al–Ti5Si3 nanocomposites. In summary, the magnesio-reduction of TiO2 NPs and SiO2 particles in eutectic LiCl–KCl melts at 700 1C results in the formation of Ti5Si3 NPs with an average particle size of 25  5 nm. The silica reduction occurs via a dissolution mechanism forming silicates, while the smaller TiO2 particle size promotes the complete Ti4+ reduction and Ti5Si3 NP formation on the titania surface via a heterogeneous template mechanism. The limited solubility of Mg powder in the molten salts maintains low supersaturation conditions, thus allowing the control of crystal growth at the nanoscale. The resulting Ti5Si3 NPs possess excellent wetting and dispersion properties in molten Al when an amorphous layer on their surface was avoided.

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Chem. Commun., 2014, 50, 1454--1457 | 1457

Facile and scalable synthesis of Ti5Si3 nanoparticles in molten salts for metal-matrix nanocomposites.

We report a novel synthesis of Ti5Si3 nanoparticles (NPs) via the magnesio-reduction of TiO2 NPs and SiO2 in eutectic LiCl-KCl molten salts at 700 °C...
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