ARTICLE pubs.acs.org/crystal
TiC Nanorods Derived from Cotton Fibers: Chloride-Assisted VLS Growth, Structure, and Mechanical Properties Xinyong Tao,† Jun Du,† Yingchao Yang,‡ Yiping Li,† Yang Xia,† Yongping Gan,† Hui Huang,† Wenkui Zhang,*,† and Xiaodong Li*,‡ † ‡
College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou, 310014, China Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, South Carolina 29208, United States ABSTRACT: In this work, single-crystalline TiC nanorods (NRs) were successfully synthesized via a simple, convenient, and cost-effective biotemplate method. Use of natural nanoporous cotton fibers as both the carbon source and the template for formation of catalyst particles significantly simplifies the synthesis process of TiC NRs. On the basis of the structural, morphological, and elemental analyses, a chloride-assisted vaporliquidsolid growth mechanism and the corresponding growth model were proposed. The activation energy for the TiC NRs was calculated to be 259 kJ/mol. From in situ nanoscale three-point bending measurements, we also determined the Young’s modulus of TiC NRs, which is an important parameter for practical applications. The measured Young’s modulus of TiC NRs is in the range from 394 to 468 GPa with an average value of 432 ( 22 GPa.
1. INTRODUCTION Transition metal carbides (TMC) have elevated melting points, excellent mechanical properties, high resistance to nonoxidizing acids, and good electrical and thermal conductivities. These properties make TMC very attractive for numerous applications in high-temperature-resistant composites, coatings, cutting tools, superconducting devices, energy storage electrodes, and catalysis.18 Titanium carbide (TiC) is one representative example of TMC with extreme hardness (2835 GPa), low density (4.93 gcm3), high melting point (3067 °C), and high Young’s modulus (300480 GPa).9,10 Recently, one-dimensional (1D) TiC nanostructures, such as nanowires, nanorods (NRs), nanofibers, nanowhiskers, and nanotubes, have stimulated significant interest in the fields of catalysis and materials science due to their unique physical and chemical properties.1020 To date, several techniques including carbon nanotube (CNT) confined reaction,12,14,18 chloride-assisted carbothermal reduction,10,21 and chemical vapor deposition1517 have been developed to synthesize 1D TiC nanostructures. To synthesize 1D TiC nanostructures, a carbon source is essential. For all these methods, engineering materials such as CNTs, hydrocarbon gases, activated carbon, and carbon black were usually selected as the carbon source.1021 These conventional synthesis methods required long reaction times, and a stepwise synthetic procedure was usually adopted: metal catalytic nanoparticles were first synthesized and subsequently added to the precursors as the catalysts.10,16,17 Cotton has been used by mankind for at least 7000 years. It is the oldest and most commonly used nanoporous material, which is structurally constructed from polysaccharide chains arranged into amorphous and crystalline regions.22 Today, the world production of cotton is approximately 20 million tons per annum, mainly for r 2011 American Chemical Society
clothing, paper, and medical uses. In this work, we report a new and facile biotemplate method to synthesize TiC NRs using a commercial cotton T-shirt as both the carbon source and the template. Compared with the well-established synthesis techniques for growing TiC nanostructures, the mechanical properties of 1D TiC nanostructures are still lacking in the literature. This limits their practical applications in fabricating TiC-based nanocomposites and functional nanodevices. Here, we also report the mechanical properties of individual TiC NRs measured using the in situ atomic force microscopy (AFM) three-point bending technique. It was found that the TiC NRs exhibit a high Young’s modulus of 394468 GPa and should find significant applications as reinforcements for composites and as structural/functional building blocks for nanoelectromechanical systems (NEMS).
2. EXPERIMENTAL SECTION In a typical experiment to prepare TiC NRs, 0.15 g of Ni(NO3)2 3 6H2O, 0.29 g of NaCl, and 0.8 g of TiO2 powders were dissolved into 50 mL of ethanol to form a NiTiCl emulsion under ultrasound irradiation. Then, a piece of T-shirt with a weight of 1.5 g was cut and dipped in the NiTiCl emulsion. After stirring for 0.5 h, the cotton textile was dried at 85 °C for 30 min in a preheated oven and finally cured at 110 °C for 2 h. The nickel-, chlorine-, and titanium-loaded cotton textile was placed in a sealed graphite crucible and then heated at 11501350 °C with 350 sccm continuous flow of argon. After 13 h calcination, the argon flow was terminated and the furnace was turned off and allowed to cool naturally to room temperature. Received: May 11, 2011 Revised: July 15, 2011 Published: August 16, 2011 4422
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Figure 1. TiC NRs synthesized using a cotton T-shirt as both the template and the carbon source. (a) Digital camera image of a commercial cotton T-shirt. (b) Piece of final textile. (c) Low-magnification SEM image showing the surface morphology of the final textile obtained after 1 h calcination at 1200 °C. (d) High-magnification SEM image of NR arrays along the cotton fiber radial direction. The catalyst particles can be found on the tips of the NRs in the inset of d. (e) XRD pattern of the final textile.
Figure 2. Microstructural characterization of TiC NRs. (a and b) TEM images of TiC NRs. (c and e) Typical [110] and [100] zone axis HRTEM images and the corresponding FFT patterns (insets) of a TiC NR, respectively. (d and f) Corresponding structural model projected along the [110] and [100] zone axis, respectively. (g) Structural model of the TiC NR, showing the growth direction along the [001] axis. The phase purity and crystalline structure of the samples were characterized by X-ray diffraction (XRD) using an X’Pert Pro diffractometer with a step size of 0.02 for Cu Kα radiation (λ = 1.5418 Å). The microstructure and morphology were observed by a scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscope (TEM, FEI Tecnai G2 F30) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. A drop of an ethanol solution containing the TiC NRs was placed on a standard AFM reference sample (Veeco Metrology Group) with uniform trenches with a width of 1 μm and depth of 200 nm. In order to avoid NR sliding during the bending tests, both ends of the NR, which bridged the trench, were clamped by 20 min electron beam-induced deposition (EBID) of carbonaceous materials (paraffin). A Veeco Dimension 3100 AFM instrument was used to perform three-point bending tests by directly indenting the center of a suspended NR that bridged the channel with a tapping-mode silicon AFM tip. A total of 15 bending tests were performed to obtain the Young’s modulus.
3. RESULTS AND DISCUSSION A digital camera image of the 100% cotton T-shirt used in this study is shown in Figure 1a. There are a large number of hydroxyl
(OH) groups on the outer edge of the polysaccharide chains. These negatively charged OH groups make cotton absorb large quantities of liquids, particularly water. Cotton can absorb the solution with Ni(NO3) 3 6H2O, NaCl, and TiO2 nanoparticles. After calculation of the cotton T-shirt with the absorbed precursors, light gray textile (Figure 1b) was obtained. A typical lowmagnification SEM image of the final textile is shown in Figure 1c. Abundant straight TiC NRs grew radially on the entire length of the carbon microfiber (Figure 1c). A high-magnification SEM image (Figure 1d) shows that the NRs have a diameter ranging from 80 to 200 nm and length varying from 1 to 3 μm. A catalyst nanoparticle was observed at the tip of each NR (inset in Figure 1d), indicating the top growth mechanism of these NRs. Most of the catalyst nanoparticles are conical and have a diameter distribution of 90220 nm. Figure 1e shows the XRD pattern of the prepared TiC NR textile. Five typical diffraction peaks are observed in the XRD patterns, which can be indexed to the (111), (200), (220), (311), and (222) reflections of the cubic TiC (Fm3m). The lattice parameter a is calculated to be 0.4326 nm, which is in good agreement with the standard lattice parameter of TiC (JCPDS 650242). No titanium oxides, TiOxCy, and Ti metal were detected, 4423
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Crystal Growth & Design suggesting TiO2 was fully converted to TiC. The weak peak at around 25.6° can be indexed as (002) of carbon, indicating that carbon is superfluous due to incomplete pyrolysis of cotton cellulose. Excessive carbon must be favorable for full conversion of titanium oxides into TiC. The products were further characterized by TEM. Figure 2a shows the low-magnification bright-field TEM image of the TiC NRs, in which clean and flat surfaces can be observed. Figure 2c is a typical high-resolution TEM (HRTEM) image taken from the NR in Figure 2b, displaying the atomic structure of the TiC crystal projected along the [110] zone axis direction. No structural defects and surface disorder were observed in the TiC NR (Figure 2c). The inset in Figure 2c is the corresponding fast Fourier transform (FFT) pattern, which is consistent with the [110] zone axis diffraction pattern for an fcc single crystal. Figure 2d shows the corresponding structural model of the fcccubic TiC projected along the [110] direction. Figure 2e shows another HRTEM image taken along the [100] zone axis from the TiC NR, which is obtained after tilting the same NR about 45° using a double-tilt TEM holder. The corresponding FFT pattern (inset in Figure 2e) is consistent with the fcc [100] zone axis diffraction pattern. The spacing between two adjacent lattice fringes is 0.22 nm (Figure 2c and 2e), consisting with the {002} planes of cubic TiC (JCPDS 65-0242). The HRTEM images (Figure 2c and 2e) are in good agreement with the schematic structural model (Figure 2d and 2f). On the basis of these HRTEM and FFT results, a structural model (Figure 2g) is proposed to illustrate growth of the single-crystalline TiC NR along the [001] direction. Scanning transmission electron microscopy (STEM) combined with EDS line scan analysis was employed to determine the chemical composition of the NR and the attached catalyst particle. Figure 3a is a typical STEM image of a single TiC NR
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with a catalyst particle on the tip. For growth of 1D nanostructures, there are usually two kinds of growth mechanisms: vapor liquidsolid (VLS) growth and vaporsolid (VS) growth.23 A catalyst agent is need for the VLS growth mechanism, while the VS mechanism is used to explain the growth of nanostructures from direct condensation of the precursor vapor. The catalyst is necessary for synthesis of the TiC NR, which is proved by their presence at the apex of the NR (see Figure 3a). Therefore, we believe that the growth mechanism in the present case is VLS rather than VS. The diameter of the catalyst nanoparticle is larger than the diameter of the main body of the TiC NR. Figure 3b, 3c, and 3d shows the line scans of C, Ti, and Ni across the interface between the NR and the catalyst particle, which were measured simultaneously (for position of the line scan, see Figure 3a). It is generally accepted that the driving force for the VLS growth of 1D nanostructure is more concentration gradient than the temperature gradient, that is, the concentration of Ti and C in the head area should be higher than other areas. However, our EDS results showed that the concentration of Ti and C gradually decreased toward the head of the catalyst particle (Figure 3b and 3c), conflicting with the normal viewpoint. We believe that there must be a concentration change for the catalyst particle due to the huge temperature drop from 1200 °C to room temperature. The line scan of Ni (Figure 3d) reveals that Ni is nearly completely located in the catalyst particle. Ni has been proven to be a highly efficient catalyst for carbon nanotubes, and accordingly, Ni (l) is known to solvate Ti and C.24 Ti can solvate Ni to form a NiTi binary eutectic alloy with a low melting point (1117 °C). Our experiments showed that only a few short TiC NRs can be obtained below 1150 °C. Abundant nanorods with a larger diameter and longer length can be synthesized with increasing growth temperature and growth time. It was also found that the TiC NR growth rate followed an Arrhenius law within the temperature range of 11501350 °C. From the Arrhenius equation, the activation energy (Ea) can be expressed as Ea ¼ RT lnðk=AÞ
Figure 3. (a) STEM image of a single TiC NR with a catalyst particle on the tip. (b, c, and d) Results of the STEM-EDS line scans of C, Ti, and Ni across the interface between the NR and the catalyst particle (indicated by the red line in a), respectively.
ð1Þ
where A is the frequency factor for the reaction, R is the universal gas constant, T is the temperature (in Kelvin), and k is the growth rate constant. Ea for growth of TiC NRs was calculated to be 259 kJ/mol, which is similar to the Ea of 269273 kJ/mol for TiC films.25 The catalyst nanoparticle plays a key role during VLS growth of NRs. In conventional synthesis methods for growing 1D TiC nanostructures, a stepwise synthetic procedure was usually adopted. Metal catalytic nanoparticles were first synthesized and subsequently added to the precursors as catalysts.10,16,17 However, here the Ni catalyst nanoparticles were synthesized in situ on the surface of the textile during calcination of the cotton fibers with absorbed Ni precursors. Use of natural nanoporous cotton fibers as both the carbon source and the template for formation of
Figure 4. Cellulose structure with intermolecular distances. 4424
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Crystal Growth & Design catalyst particles significantly simplifies the synthesis process of TiC NRs. Cotton is a remarkably pure fiber material with a cellulose content of greater than 95 wt % in the dry fiber. In the fiber, 42 vol % is cellulose in amorphous regions and 58 vol % in crystalline regions. The crystallites are approximately 57 nm wide and 912 nm long with amorphous pores of 15 nm width between them.22 Figure 4 shows the cellulose structure with intermolecular distances. There are a large number of OH groups on the outer edge of the polysaccharide chains (Figure 4). These negatively charged OH groups make cotton absorb large quantities of Ni2+ ions. It is believed that the nanoporous cotton fibers act as the template for formation of the Ni catalyst nanoparticles. During VLS growth of the TiC NRs, there must be gas-phase transport of C and Ti to the Ni catalyst particle. During pyrolysis (eq 2), cotton cellulose undergoes different reactions to form three general classes of compounds: (1) noncondensable gases, (2) condensable vapors that can be condensed into a liquid product (tar), and (3) solid carbon fiber that consists of elemental carbon.26 Secondary cracking of tars (eq 3) will occur under high-temperature calcination (1300 °C), which is believed to be a significant pathway for production of CO, H2, and several light hydrocarbon gases such as CH4, C2H4, C2H6, and C3H6.27,28
Figure 5. Cross-sectional schematics show the formation mechanism of TiC NRs. (a) Cotton fiber absorbed abundant nickel nitrate and titanium sources as well as other precursors. (b) Catalyst nanoparticles are formed on the surface of the fiber. (c) TiC NRs grow radially on a carbon fiber via a top growth mechanism.
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These hydrocarbon gases resulting from pyrolysis of cellulose can serve as the carbon source. ðC6 H10 O5 Þn f CH4 ðgÞ þ COðgÞ þ C2 H2 ðgÞ þ C2 H6 ðgÞ þ C3 H6 ðgÞ þ CO2 ðgÞ þ H2 OðgÞ þ H2 ðgÞ þ tarsðlÞ þ charðsÞ
ð2Þ
tars f CH4 ðgÞ þ Cx Hy ðgÞ þ C2 H5 OHðgÞ þ CH3 OHðgÞ þ CH2 OðgÞ þ CH3 CHOðgÞ þ H2 OðgÞ þ H2 ðgÞ þ COðgÞ þ CO2 ðgÞ
ð3Þ
In our experiments, we also found that NaCl played a critical role in the growth of TiC NRs. No TiC NRs were obtained without introducing chlorides. Gaseous chloride species and such as TiOCl2 (g), TiCl4 (g), and TiCl3 (g) can be formed at the reaction temperature and must be responsible for transport of Ti. On the basis of our EDS and TEM results, the following reactions are expected to occur during growth of TiC NRs Tiy Clx ðgÞ þ 2CðgÞ þ NiðlÞ f NiðTi, CÞðlÞ þ yClðgÞ ð4Þ NiðTi, CÞðlÞ f TiCðsÞ þ NiðlÞ
ð5Þ
All these reactions were not consecutive but probably occurred simultaneously during the VLS growth of TiC NRs. Figure 5 shows cross-sectional schematics, illustrating the formation mechanism of TiC NRs. Cotton fiber absorbed abundant nickel nitrate, titanium sources, and other precursors (Figure 5a). Ni catalyst nanoparticles can be produced on the surface of the fiber during calcination, which can absorb C and Ti source gases (eq 4 and Figure 5b). When the catalyst droplet Ni (Ti, C) supersaturates with Ti and C, the Ti NR may precipitate and grow up via a top growth mechanism (eq 5 and Figure 5c). Compared with the well-established synthesis techniques for growing 1D TiC nanostructures, their mechanical properties are
Figure 6. In situ AFM three-point bending test on a single TiC NR. (a) Schematic of an EBID-fixed TiC NR in a three-point bending test with an AFM tip. (b) SEM and (c) AFM images of a fixed TiC NR suspended over the trench. (d) Representative bending FZ curves for a NR on substrate (for calibration) and a suspended NR. 4425
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still lacking in the literature. This limits their practical applications in constructing TiC-based nanocomposites and functional nanodevices. In situ AFM three-point bending tests were performed directly on individual TiC NRs to obtain their mechanical properties (Figure 6a). To avoid sliding during the bending tests, both ends of the NR, which bridged the trench, were clamped by EBID of carbon. Figure 6b shows a representative SEM image of an EBID-fixed NR. Figure 6c is the corresponding AFM image. Figure 6d shows representative forcepiezo position (FZ) curves for a TiC NR on substrate (for calibration) and a suspended TiC NR. The FZ curves exhibit a strong linear relationship up to 1400 nN. On the basis of the assumption that the NR follows the linear elastic theory of an isotropic material, the elastic modulus of the NR, En, can be calculated from29 En ¼
Kn L 3 192I
ð6Þ
where I is the moment of inertia and L is the suspended length of the NR. The measured Young’s modulus of TiC NRs ranges from 394 to 468 GPa, and the average modulus is 432 ( 22 GPa, which is close to nearly stoichiometric TiC ceramics (436 ( 26 GPa for TiC0.98; 400 GPa for TiC 0.84; 449 GPa for TiC0.91),30 higher than the reported value of 200300 GPa for magnetron-sputtered TiC films.31 Cubic TiC crystallizes in the NaCl-type structure (B1, space group Fm3m), and it has been proven that cubic TiC generally exhibits nonstoichiometry over a wide range of C:Ti ratios, TimCn (n/m = 0.51.0) without obvious change in the crystal structure.9 The mechanical properties of TMC are usually correlated with vacancies in the carbon sublattice. Previously reported Young’s modulus values for bulk TiC exhibit large variations (up to 50%). The stoichiometric deviation must contribute to the variations in elastic modulus. Thus, it is difficult to connect the NR’s mechanical property with the nanosize effect and surface stress effect.
4. CONCLUSIONS Single-crystalline TiC NRs on the surface of carbon microfibers were successfully synthesized via a simple, convenient, and costeffective biotemplate method. Use of natural nanoporous cotton fibers as both the carbon source and the template for formation of catalyst particles significantly simplifies the synthesis process of TiC NRs. On the basis of the structural, morphological, and elemental analyses, a chloride-assisted VLS growth mechanism and the corresponding growth model were proposed. The Ea for growth of TiC NRs was calculated to be 259 kJ/mol, which is similar to the Ea of bulk TiC films. From in situ AFM three-point bending measurements we determined the Young’s modulus of TiC NRs, which is an important parameter for practical applications. The measured Young’s modulus of TiC NRs ranges from 394 to 468 GPa, and the average modulus is 432 ( 22 GPa. The high-modulus TiC NRs hold great promise as reinforcements for composites and as structural/functional building blocks for NEMS. ’ AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected] (W.Z.);
[email protected] (X.L.).
’ ACKNOWLEDGMENT This work was supported by the NSFC (51002238 and 51172205), NSF (CMMI-0968843, CMMI-0824728, and CMMI-0653651),
Zhejiang Provincial NSF of China (Y4090420), Qianjiang Talent Project (2010R10029), ‘Qianjiang Scholars’ program, and project sponsored by the Project-sponsored by SRF for ROCS (2010609), State Education Ministry.
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