In-situ synthesis of nanocrystalline TiC powders, nanorods, and nanosheets in molten salt by disproportionation reaction of Ti(II) species

1. Introduction

Titanium carbide (TiC), a kind of transition metal carbides, has attracted extensive attention due to numerous advantages such as ultra-high melting point (3200 °C), high hardness (32 GPa), low density (4.93 g cm^-3), high chemical and thermal stability, high electrical and thermal conductivity, and high wear and corrosion resistance [[1], [2], [3]]. Therefore, it has been widely used in cutting tools, polishing pastes, grinding wheels, and aerospace materials [[4], [5], [6], [7]]. In particular, nano-sized TiC powder with high specific surface area attracts considerable attention due to diverse applications such as catalyst supports [[8], [9], [10]] and electrodes [[11],[12]]. In addition, mechanical property and corrosion resistance of the matrix are significantly enhanced by adding nano-sized TiC particles [[13], [14], [15]].

Therefore, it is imperative to produce nano-sized TiC with high purity and dispersibility on a large scale. In the last few years, titanium carbide/carbon composites also show promising applications because of their advanced mechanical and functional properties [[16],[17]]. Among the variety of titanium carbide, one-dimensional TiC nanorods show good application prospects in electronics and quantum devices, and drug delivery due to their shape and properties [18]. Additionally, TiC nanorods and two-dimensional TiC nanosheets will be promising reinforcements for next-generation materials [[19], [20], [21]].

In fact, numerous methods have already been developed to synthesize TiC powders such as carbothermal reduction [[22], [23], [24]], mechanical alloying (MA) [[25],[26]], mechanically induced self-propagating reaction (MSR) [[27],[28]], self-propagating high temperature synthesis (SHS) [29], metallic thermal reduction [[30],[31]], and chemical vapor deposition (CVD) [32]. However, for carbothermal reduction, high temperature (above 1700 °C) and long duration (ca. 10-24 h) are needed and it is difficult to prepare high purity nano-sized TiC powder. Although fine TiC powder has been obtained by MA and MSR methods, impurities from mill wear are inevitably involved and needed to be removed. In the case of SHS, it suffers from some limitations such as incomplete reaction, explosive nature of reaction and lack of opportunity to control the process [33]. For metallic thermal reduction, high quality TiC nanocrystalline has also been obtained, but the expensive active metal limits large-scale production of TiC powders. Recently, several new synthetic routes have already been developed to synthesize nanocrystalline TiC powders such as sonochemical synthesis [34], mechanochemical synthesis [35], microwave synthesis [36], and thermal plasma [[37],[38]]. However, challenges still exist in achieving low cost and high performance nano-sized TiC. Therefore, new routes for synthesis of TiC nanomaterials are still crucially needed.

Recently, it is reported that molten salt method has been used to synthesize several functional materials [[39], [40], [41], [42]] due to obvious advantages such as simple synthesis route, relatively low reaction temperature, and simple equipment. In fact, TiC coatings have also been synthesized by molten salt method [[43],[44]]. However, TiC nanopowders, nanorods, and nanosheets synthesized by molten salt method have not yet been literally reported and mechanism of TiC formation in molten salt is still not clear.

In this study, a simple method for large-scale synthesis of nanocrystalline TiC powders, nanorods, and nanosheets by using mixture of Ti powder and acetylene black, multiwalled carbon nanotubes (MWCNTs), and graphene as precursors, respectively, in molten salt was presented. Effects of temperature and duration on TiC formation were also investigated. Furthermore, mechanism of TiC formation in molten salt was firstly investigated by electrochemical method.

2. Experimental

A eutectic NaCl-KCl (supplied by Tianjin Kemiou Chemical Reagent Co., Ltd., analytical grade ≥ 99.5%) molten salt with molar ratio of 1:1 was used as liquid reaction medium. The salt was firstly dried at 350 °C for 36 h to remove residual moisture. For the synthesis of TiC nanomaterials, a 1.5 g mixture composed of Ti powder (supplied by Beijing Xing Rong Yuan Technology Co., Ltd., 325 mesh, purity > 99.8%) and carbon sources, including acetylene black (supplied by Tianjin Day First Century Chemical Industry Co., Ltd., 30-45 nm), MWCNTs (supplied by Shenzhen Nanotech Port Co., Ltd., diameter 20-40 nm, length <2 μm, purity >97%), and graphene (supplied by Deyang Carbonene Technology Co., Ltd., average layer 5-6, mean thickness <3 nm, lamellar size 5-15 μm), with molar ratio of 1:1 were used as precursors. Precursors and the dried salt with mass ratio of 1:7 were firstly mixed and then placed in an alumina crucible covered with a lid. After that, the crucible was placed in the bottom of a sealed chamber, and subsequently heated to target temperature at a rate of 5 °C min^-1 by a well furnace and held for different time. After completion of prior thermal exposures, the furnace was naturally cooled down to room temperature and obtained powder with solidified salt inside the crucible were rinsed with boiling distilled water several times to remove the salt. Finally, the resulting powder was dried at 120 °C for 2 h. For electrochemical measurement, linear scan voltammetry (LSV) was performed with Gamry Interface 1010B Potentiostat/Galvanostat electrochemical workstation by using a three-electrode electrochemical cell, in which a molybdenum wire (supplied by Rare Metallic Co., Ltd., purity > 99.9%) was used as a working electrode, a graphite rod as a counter electrode and a platinum wire electrode as a pseudo-reference electrode. A dried eutectic NaCl-KCl containing 0.1 mol L^-1 Ti powder was used as electrolyte to investigate dissolution behavior of Ti powder at 800 °C for different time. All experiments mentioned above were performed under an argon (purity > 99.999%) atmosphere at a flow rate of 30 sccm.

The identification of phase constitution of the product was carried out by X-ray diffraction (XRD, PANalytical X’Pertpro) with Cu target Kα radiation. Field-emission scanning electron microscope (FESEM, Inspect F50, FEI Co., Hillsboro, OR, USA) coupled with energy dispersive X-ray spectrometer (EDX, Oxford) were used to investigate morphology and elemental composition of the powder. Particle size was calculated by software of SEM image analysis (Nano Measurer System, 1.2). High resolution transmission electron microscopy (FEI Tecnai G² F20) and selected-area electron diffraction (SAED) were used to characterize morphology and microstructure of the obtained nanomaterials.

3. Results and discussion

3.1. Effect of temperature on TiC formation

XRD patterns of samples obtained by using equimolar ratio of Ti powder and acetylene black as precursors in NaCl-KCl molten salt at different temperatures for 2 h are presented in Fig. 1. Result shows that TiC initially forms at 750 °C in NaCl-KCl molten salt as shown in Fig. 1(a). Whereas, some unreacted metallic Ti and a small quantity of oxidation products of the unreacted Ti powders, such as Ti₆O, TiO₂, and TiOCl₂, are also detected. Absence of unreacted acetylene black in the sample is due to rinsing of boiling water. However, with increasing temperature to 800 °C as shown in Fig. 1(b), intensity of diffraction peaks indexed to unreacted Ti and Ti₆O decreases obviously and diffraction peaks indexed to TiO₂ and TiOCl₂ disappear, which suggests that more desired TiC forms with increasing temperature. As temperature is raised to 850 or 900 °C, diffraction peaks indexed to Ti and Ti₆O disappear completely and only five obvious diffraction peaks indexed to TiC (ICDD card No. 65-8417) are obtained as shown in Fig. 1(c) and (d). This suggests that pure TiC powder is obtained at 850 or 900 °C for 2 h. These results suggest that temperature of TiC preparation by this method is quite lower than that prepared by direct chemical reaction of element Ti and C. From the above analysis, it is concluded that the TiC formation is dramatically influenced by reaction temperature and a higher temperature is more beneficial for TiC formation.

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Fig. 1.   XRD patterns of samples obtained by using equimolar ratio of Ti powder and acetylene black as precursors in NaCl-KCl molten salt at different temperatures for 2 h.

3.2. Effect of reaction time on TiC formation

The effect of reaction time on synthesis of TiC powder at 800 °C is also studied by interrupted time-series experiments and corresponding XRD patterns of products are shown in Fig. 2. Result shows that the product is mainly composed of desired TiC, some unreacted metallic Ti and small quantity of oxidation products of unreacted Ti powders, such as Ti₂O and TiOCl₂, after 1 h, as shown in Fig. 2(a). However, with increasing the duration to 2 h, the relative intensity of diffraction peaks indexed to Ti and Ti₂O decreases while that of TiC increases, as shown in Fig. 2(b). As the duration further increases to 3 h or 4 h, all the observed diffraction peaks are well indexed to TiC, as shown in Fig. 2(c) and (d), indicating that pure TiC is also obtained at 800 °C for more than 3 h. Therefore, it is concluded that TiC formation is also influenced by reaction time and a longer duration is more beneficial for TiC preparation.

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Fig. 2.   XRD patterns of samples obtained by using equimolar ratio of Ti powder and acetylene black as precursors in NaCl-KCl molten salt at 800 °C for different times.

3.3. Characterization of the prepared TiC powder

The typical morphology and chemical composition of TiC powder obtained by using equimolar ratio of Ti powder and acetylene black as precursors in NaCl-KCl molten salt at 850 °C for 2 h are characterized by FE-SEM coupled with EDX as shown in Fig. 3. Spherical TiC particles are obtained as shown in Fig. 3(a) and (b). Additionally, a statistical particle size distribution was also analyzed by SEM image analysis software, and mean particle size is estimated to be ˜50 nm as shown in the inset of Fig. 3(b). This is in agreement with particle size of acetylene black precursor, indicating that TiC particle may be formed via template-growth. However, the image displays a slight agglomeration of single particles, which is due to attractive van der Waals force and electrostatic forces between nanoparticles. Additionally, corresponding elemental mapping analysis reveals that titanium and carbon are uniformly distributed in the sample, as shown in Fig. 3(c) and (d), respectively.

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Fig. 3.   (a, b) FE-SEM images of the sample obtained by using equimolar ratio of Ti powder and acetylene black as precursors in NaCl-KCl molten salt at 850 °C for 2 h; the inset shows a statistical particle size distribution in Fig. 3(b); (c, d) the corresponding elemental mappings of titanium and carbon of area A in Fig. 3(a).

A typical TEM image and corresponding SAED pattern of TiC powder obtained by using equimolar ratio of Ti powder and acetylene black in NaCl-KCl molten salt at 850 °C for 2 h are shown in Fig. 4. Result indicates that TiC powder is fine-grained with particle size of about 50 nm in diameter. From SAED pattern shown in Fig. 4(b), d values corresponding to diffraction rings (from inner to outer) are 0.250, 0.216, 0.153, 0.130, and 0.125 nm, which are indexed to planes of (111), (200), (220), (311), and (222) of cubic TiC, respectively. TEM image of a single TiC particle is also shown in Fig. 4(c). Result indicates that particle size is less than 30 nm. This is in agreement with the value calculated by Debye-Scherrer’s equation to be about 20 nm based on XRD data for peak width of the low angle (111) reflection in Fig. 1(c). In addition, from obvious lattice fringes of area A in Fig. 4(c), the interplanar spacing is measured as 0.153 nm, which represents stripe image of (220) plane of cubic TiC, as shown in Fig. 4(d).

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Fig. 4.   (a) TEM image and (b) the corresponding SAED pattern of TiC powder obtained by using equimolar ratio of Ti and acetylene black in NaCl-KCl molten salt at 850 °C for 2 h; (c) TEM image of a single TiC nanocrystalline and (d) HRTEM image of area A in Fig. 4(c).

3.4. Mechanism of TiC formation

In this work, TiC powder with particle size of about 50 nm is successfully prepared by mixing equimolar ratio of Ti powder and acetylene black without ball milling in NaCl-KCl molten salt at 800-900 °C for 2-3 h. Particle size of obtained TiC powder and acetylene black is approximately equivalent, indicating that TiC is formed via template-growth in which acetylene black acts as template. However, it is impossible that nano-sized TiC can be obtained by direct reaction between coarse grain Ti powder (with particle size of about 45 μm) and acetylene black (with particle size of 30-45 nm) in molten salt. Therefore, an essential assumption is that titanium species may firstly form in the molten salt and then react with acetylene black to form TiC nanoparticles. Therefore, valence states of titanium species dissolved in the molten salt should be firstly investigated.

LSV curves of a molybdenum electrode in NaCl-KCl molten salt without and with 0.1 mol L^-1 Ti powder at 800 °C are recorded at various time as shown in Fig. 5. Result reveals that no reduction peak except a current increasing of alkali metal deposition was observed when a molybdenum electrode was immersed in NaCl-KCl molten salt without Ti powder. However, when Ti powder was added into NaCl-KCl molten salt, two reduction peaks (R₁, R₂) besides the current increasing of the deposition of alkali metals are observed. In addition, current of two peaks firstly increases with reaction time and then remains unchanged as reaction time reaches 60 min, which means that concentration of titanium species in molten salt increases with increasing reaction time until dissolution of Ti powder reaches equilibrium. According to our review of literature [[45],[46]], reduction peak R₁ at -1.0 V vs. Pt is attributed to reduction of Ti(III) species to Ti(II) species and peak R₂ at -1.35 V vs. Pt is attributed to reduction of Ti(II) species to Ti metal, respectively. Moreover, it reveals that Ti(II) species also exists in the molten salt because gas bubbles are observed when the salt is sucked out with a quartz tube and rinsed with hot water [47]. Actually, partial dissolution of Ti in molten KCl or NaCl salts to form TiCl₃ and TiCl₂, which have also been observed by Kreye and Kellogg [48] and Straumanis et al [49]. Therefore, both Ti(III) and Ti(II) species exist when Ti powder is added into NaCl-KCl molten salt and concentration of Ti(III) and Ti(II) species increases with increasing reaction time until dissolution of Ti powder reaches equilibrium.

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Fig. 5.   LSV curves of a molybdenum electrode (S = 0.163 cm²) in NaCl-KCl molten salt without and with 0.1 mol L^-1 Ti powder at 800 °C for different times at scan rate of 300 mV s^-1.

LSV curves of a molybdenum electrode in NaCl-KCl molten salt containing 0.1 mol L^-1 Ti powder at different scan rates are also obtained as shown in Fig. 6(a). Two peaks R₁ and R₂ attributed to reduction of Ti(III) species to Ti(II) species and the reduction of Ti(II) species to Ti metal, respectively, are also observed at different scan rates, and peak current increases with increasing scan rate. The relationship between peak potential and logarithm of scan rate is also obtained and shown in Fig. 6(b). Result indicates that both of peak potentials of R₁ and R₂ vary linearly with logarithm of scan rate. Accordingly, both of electrochemical reductions of Ti(III) species and Ti(II) species in the molten salt are irreversible.

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Fig. 6.   LSV curves (a) of a molybdenum electrode (S = 0.163 cm²) in NaCl-KCl molten salt containing 0.1 mol L^-1 Ti powder at different scan rates after Ti powder was added for 1 h at 800 °C; (b) the relationship between cathodic peak potential and logarithm of scan rate.

From the above analysis, mechanism of TiC formation in NaCl-KCl molten salt is demonstrated in Fig. 7 and details are described as follows: when Ti powder and acetylene black are well mixed in the molten salt, a small quantity of Ti powder is dissolved to form Ti(II) and Ti(III) species. Then, Ti(II) species transport to surface of acetylene black which acts as template and floats on top of the molten salt. Finally, TiC will be in-situ synthesized by reaction between Ti atoms, which come from disproportionation reaction of Ti(II) species in molten salt, and C atoms on surface of acetylene black. Furthermore, result of the effect of temperature on TiC formation in section 3.1 also suggests that TiC is in-situ synthesized by disproportionation reaction of Ti(II) species, because disproportionation reaction is dramatically influenced by ambient temperature and a higher temperature is preferable for higher reaction rate of disproportionation [50]. Therefore, disproportionation reaction of Ti(II) species on surface of acetylene black is summarized and shown in Eq. (1).

$3Ti(II)+C\to TiC+2Ti(III)$ (1)

$2Ti(III)+Ti\to 3Ti(II)$ (2)

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Fig. 7.   Schematic diagram of TiC formation in NaCl-KCl molten salt by using equimolar ratio of Ti powder and acetylene black as precursors.

Eq. (1) indicates that the equilibrium shifts to right due to formation of TiC, and the produced and the dissolved Ti(III) species may reacts with Ti powder to produce Ti(II) species until Ti powder burns out, as shown in Eq. (2). Hence, total reaction for TiC formation is presented in Eq. (3),

$Ti+C\to TiC$ (3)

The above analysis is in agreement with previous hypothesis that TiC formation is a solid-liquid reaction between acetylene black and Ti(II) species. Therefore, this high-efficiency solid-liquid reaction is very suitable for mass production of TiC nanopowders.

3.5. Preparation of TiC nanorods and nanosheets

TiC formation in molten salt is considered as a disproportionation reaction of titanium species on carbon source used as template, herein TiC nanorods are also prepared by disproportionation reaction of Ti(II) species on template of MWCNTs in NaCl-KCl molten salt at 850 °C for 2 h. The corresponding XRD diffractogram of the sample is presented in Fig. 8(a). Result shows that pure TiC is obtained at 850 °C for 2 h because only five obvious diffraction peaks indexed to TiC (ICDD card No. 65-8417) are obtained. Typical TEM images of TiC nanorods are shown in Fig. 8(b)-(d). Results indicate that TiC nanorods are made of interconnected TiC nanocrystalline of around 20-30 nm in diameter as shown in Fig. 8(d). The TiC nanorod is also characterized by HRTEM images as shown in Fig. 8(e) and (f). The interplanar spacing of obvious lattice fringes is measured as 0.216 nm, which represents the stripe image of (200) plane of TiC cubic phase. This further confirms the formation of TiC nanorods.

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Fig. 8.   (a) XRD diffractogram and (b, c, d) TEM images of TiC nanorods prepared by using equimolar ratio of Ti powder and MWCNTs as precursors in NaCl-KCl molten salt at 850 °C for 2 h; (e, f) HRTEM images of TiC nanoparticle; the inset shows the corresponding SAED pattern of area A in Fig. 8(e).

Furthermore, TiC nanosheets are also in-situ synthesized by disproportionation reaction of Ti(II) species on surface of graphene in NaCl-KCl molten salt at 850 °C for 2 h. XRD diffractogram of the product is presented in Fig. 9(a). Only five obvious diffraction peaks indexed to TiC (ICDD card No. 65-8417) are obtained, suggesting that formed materials are pure TiC. Morphology of the product is also displayed in Fig. 9(b), which further confirms that TiC formation is a disproportionation reaction of Ti(II) species on graphene template. TEM images of an individual TiC nanosheet are shown in Fig. 9(c) and (d), and the inset in Fig. 9(d) shows corresponding SAED pattern. From SAED pattern, diffraction rings from inner to outer are indexed to planes of (111), (200), (220), (311), and (222) of cubic TiC, respectively. In addition, the result indicates that TiC nanosheet is made of homogeneous TiC nanocrystalline with size of about 20 nm as shown in Fig. 9(d). HRTEM image of a TiC nanoparticle is also displayed in Fig. 9(e) and (f). The interplanar spacing is measured as 0.153 nm, which implies the stripe image of (220) plane of cubic TiC as shown in Fig. 9(f).

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Fig. 9.   (a) XRD diffractogram and (b) SEM image of TiC nanosheets prepared by using equimolar ratio of Ti powder and graphene as precursors in NaCl-KCl molten salt at 850 °C for 2 h; (c) TEM image of an individual TiC nanosheet; (d) high magnification TEM image of area A in Fig. 9(c); the inset shows the corresponding SAED pattern; (e) and (f) HRTEM images of a TiC nanocrystalline; the inset shows the corresponding SAED pattern of area B in Fig. 9(e).

4. Conclusion

Nanocrystalline TiC powders, nanorods, and nanosheets have been in-situ synthesized by using equimolar ratio of Ti powder and acetylene black, MWCNTs, and graphene, respectively, as precursors in NaCl-KCl molten salt at 800-900 °C for 2-3 h, and higher temperature and longer duration are more beneficial for TiC formation. The mechanism of nanocrystalline TiC formation is described as follows. Firstly, a small quantity of Ti powder is dissolved to form Ti(II) and Ti(III) species. Then, Ti(II) species transports to surface of carbon source which floats on top of the molten salt. Finally, nanocrystalline TiC are in-situ synthesized by reaction between Ti atoms, which come from disproportionation reaction of Ti(II) species in molten salt, and C atoms on surface of carbon sources by following reaction, 3Ti(II) + C → TiC + 2Ti(III). Meanwhile, the produced and the dissolved Ti(III) species reacts with Ti powder to produce Ti(II) species until Ti powder is burned out. Therefore, other nano-sized ceramics, e.g. ZrC, VC, and TaC with similar structures to TiC produced here, can also be synthesized by the present disproportionation reaction method in molten salt.

Acknowledgement

This work was supported financially by the National Natural Science Foundation of China (Nos. 51671204 and 51501205).