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Molybdenum nitride nanoparticles — high-resolution transmission electron microscopy study
Materials Letters 61 (2007) 4763 – 4765 www.elsevier.com/locate/matlet
Molybdenum nitride nanoparticles — high-resolution transmission electron microscopy study J. Chaudhuri a,⁎, L. Nyakiti a , R. Lee a , Y. Ma a , P. Li b , Q.L. Cui c , L.H. Shen c a
b
Department of Mechanical Engineering, Texas Tech University, Lubbock, TX 79409-1021, United States Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, United States c National Laboratory of Superhard Materials, Jilin University, Changchun 130012, China Received 22 December 2006; accepted 4 March 2007 Available online 14 March 2007
Abstract The structure and size of molybdenum nitride nanoparticles were investigated using high-resolution transmission electron microscopy (HRTEM). Typical sizes of the particles were between 3 and 5 nm and they were mostly clustered together. High-resolution lattice imaging shows that the particles are single crystalline in nature and defect free. Two different phases of molybdenum nitride, γ-Mo2N (cubic) and δ-MoN (hexagonal) were identified. In addition, body centered cubic molybdenum phase was also present. It is anticipated that molybdenum is formed because of insufficient N2 supply or slow reaction rate. A mixture of γ-Mo2N and δ-MoN suggests the existence of a temperature gradient in the chamber leading to formation of γ-Mo2N at lower temperature (500 to 700 °C) and δ-MoN at higher temperature (850 °C). © 2007 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Nanoparticles; Molybdenum nitride nanoparticles; Crystal structure; High-resolution electron microscopy; dc arc charge system
1. Introduction Transition metal nitrides constitute an important class of interstitial compounds with exceptional chemical stability, high strength, high hardness, high melting point and good electrical conductivity [1]. For example, tungsten and molybdenum nitrides are good barriers against diffusion of copper in microelectronic circuits [2]. They can also be used in electrodes for thin film capacitors and field effect transistors. Moreover, transition metal nitrides have long been known to be an effective catalyst for various organic reactions such as hydrodesulfurization (HDS), in nitrogen fixation reactions (converting molecular nitrogen to ammonia) and in reducing carbon monoxide to methane gas [3]. Molybdenum nitrides have also attracted much attention as a superconducting material. Molybdenum forms several nitrides including γ-Mo2N (face centered cubic), β-
⁎ Corresponding author. Tel.: +1 806 742 3563x224; fax: +1 806 742 3540. E-mail address: [email protected] (J. Chaudhuri). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.03.022
Fig. 1. Schematic representation of the dc arc discharge system.
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J. Chaudhuri et al. / Materials Letters 61 (2007) 4763–4765
resolution field emission analytic transmission electron microscopy (TEM) and scanning TEM (STEM) with 200 kV and 0.23 nm point resolution in the Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM. The particles were separated by diluting them in methyl
Fig. 2. TEM image of the Mo2N nanoparticles.
MoN (tetragonal), and δ-MoN (hexagonal) with all three phases being superconducting [4–9]. The reduction of grain size to nanometer range will lead to the change in physical and chemical properties of a material. Thus nanocrystalline materials are of great interest currently and have been expected to lead discoveries to new materials with novel performances [10]. A previous paper reported a new synthesis method for the preparation of nanocrystalline molybdenum nitride [11]. X-ray diffraction data suggested the formation of γ-Mo2N nanoparticles. In this paper we report high-resolution transmission electron microscopy study of the molybdenum nitride nanoparticles to reveal the structure and size of these nanoparticles for the first time. 2. Experimental The details of the experimental set up can be found in the previous paper [11]. The synthesis of Mo2N was performed with the direct current arc discharge method. As is shown in Fig. 1, a rod of molybdenum (99.99% purity) was used as both cathode and anode material. On the anode side, the molybdenum rod was tightly set in the hole on a rod of graphite, which was enclosed in a water-cooled crucible. The reaction chamber was first purged by argon gas. Then nitrogen (99.999% purity) was introduced to reach 10 kPa in the system. A direct current (150 A, 40 V) was then applied and the arc was ignited. The reaction was kept for 20 min, and the product was deactivated for 12 h under nitrogen environment at 80 kPa before collection. The synthesized product was characterized using JEOL 2010F, i.e., a highFig. 3. (a) and (b) High-resolution transmission electron micrographs with inserted fast Fourier transform (FFT) showing that individual nanoparticles are single crystals with sizes 3 to 5 nm and without any defects. The upper and lower FFTs in (a) are from BCC Mo and FCC Mo2N, respectively. The FFT in (b) is from a hexagonal MoN particle. (c) Inverse fast Fourier transform (IFFT) of (b) using a lattice mask. The interplanar spacing of different particles has been clearly identified in all three figures (d111 of FCC or γ-Mo2N varies between 0.23 and 0.257 nm, d110 of Mo varies between 0.207 and 0.218 nm, and d10.0 of hexagonal or δ-MoN is 0.283 nm).
J. Chaudhuri et al. / Materials Letters 61 (2007) 4763–4765
4765
Table 1 Interplanar spacing, lattice parameters and crystal structure of nanoparticles
ber leading to the formation of γ-Mo2N at lower temperature (500 to 700 °C) and δ-MoN at higher temperature (850 °C) [4,13].
Interplanar Crystallographic Lattice Bulk lattice % Crystal spacing planes parameter parameter difference a structure (nm) (nm) (nm)
4. Conclusions
0.207– 0.218 0.23– 0.257 0.283
Mo (110) γ-Mo2N (111) δ-MoN (1000)
0.293– 0.308 0.398– 0.445 0.283 (a)
0.315
6.9–2.2
Mo BCC
0.416– 0.419 0.286 (a)
4.7–6.6
γ-Mo2N FCC δ-MoN Hex
1.0
a
% difference between the measured lattice spacing in the nanoparticles as compared to the bulk lattice parameters.
alcohol and using ultrasonic agitation. All the image analyses were performed using a GATAN digital micrograph software [12].
High-resolution transmission electron microscopy study of molybdenum nitride nanoparticles was performed to identify structure and size of these particles. The particles were mostly clustered together and their typical size was between 3 and 5 nm. The particles are single crystalline in nature and defect free as evidenced by high-resolution lattice imaging technique. The nanoparticles consisted of mostly two different phases: body centered cubic molybdenum and face centered cubic molybdenum nitride. A few particles of molybdenum nitride with hexagonal phase were also found. Acknowledgement
3. Results and discussion Low magnification TEM image of Mo2N nanoparticles is shown in Fig. 2. It can be seen that the particles are mostly clustered together. Fig. 3(a) and (b) shows high-resolution lattice images from some of the particles with corresponding diffraction patterns superimposed. Fig. 3 (c) shows the inverse fast Fourier transforms (IFFT) of masked FFT of HRTEM images of particles shown in Fig. 3(b). Fig. 3(a), (b) and (c) indicates that the nanoparticles are single crystals without any defects present and the typical size of the particles varies between 3 and 5 nm. From Fig. 3(a), (b) and (c), the interplanar spacing of the particles can be categorized in three different groups as shown in Table 1. These are 0.207–0.218, 0.23–0.257 and 0.283 nm. The corresponding crystal structures are identified as Mo body centered cubic (BCC), γ-Mo2N face centered cubic (FCC) and δ-MoN hexagonal structure. These δMoN particles could not be identified by the X-ray diffraction technique [10]. HRTEM results confirmed that the nanoparticles synthesized mostly consists of molybdenum and γ-Mo2N (FCC) nanoparticles with a few hexagonal δ-MoN. The lattice parameter of molybdenum particles is of slightly smaller size compared to bulk lattice parameter (percent difference 6.9–2.2%) probably due to the application of high pressure, whereas the lattice parameters of cubic (percent difference 4.7–6.6%) and hexagonal molybdenum nitrides (percent difference 1.0%) are close to the bulk lattice parameter values. It is anticipated that molybdenum is formed because of insufficient N2 supply or slow reaction rate. A mixture of γ-Mo2N and δ-MoN particles suggests the existence of a temperature gradient in the cham-
This work was supported by the NSF grant # DMR0515858. HRTEM work was performed in the Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
[11] [12] [13]
N. Masatishi, K. Masaru, T. Hiroyuki, O. Shinzo, Chem. Lett. 6 (2000) 702. J.S. Becker, R.G. Gordon, Appl. Phys. Lett. 82 (2003) 2239. R. Kojima, K.-I. Aika, Appl. Catal., A Gen. 218 (2001) 121. B.T. Mathias, J.K. Hulm, Phys. Rev. 87 (1952) 799. K. Inumaru, K. Baba, S. Yamanaka, Chem. Mater. 17 (2005) 5935. A. Bezinge, K. Yvon, J. Muller, Solid State Commun. 63 (1987) 141. C.L. Bull, P.F. McMillan, E. Soignard, K. Leinen Weber, J. Solid state Chem. 177 (2004) 1488. H. Ihara, Y. Kimura, K. Sebzaki, H. Kezuka, M. Hirabayashi, Phys. Rev., B 31 (1985) 3177. http://emaps.mrl.uiuc.edu/. P. Alivisatos, P.F. Barbara, A.W. Castleman, J. Chang, D.A. Dixon, M.L. Klein, G.L. McLendon, J.S. Miller, M.A. Ratner, P.J. Rossky, S.I. Stupp, M.E. Thompson, Adv. Mater. 10 (1999) 1297. L.H. Shen, Q.L. Cui, J. Zhang, X.F. Li, Q. Zhou, G.T. Zou, Chin. Phys. Lett. 22 (2005) 3192. http://www.gatan.com/. T. Nakajima, T. Shirasaki, J. Electrochem. Soc. 144 (1997) 2096.
Molybdenum nitride nanoparticles — high-resolution transmission electron microscopy study J. Chaudhuri a,⁎, L. Nyakiti a , R. Lee a , Y. Ma a , P. Li b , Q.L. Cui c , L.H. Shen c a
b
Department of Mechanical Engineering, Texas Tech University, Lubbock, TX 79409-1021, United States Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, United States c National Laboratory of Superhard Materials, Jilin University, Changchun 130012, China Received 22 December 2006; accepted 4 March 2007 Available online 14 March 2007
Abstract The structure and size of molybdenum nitride nanoparticles were investigated using high-resolution transmission electron microscopy (HRTEM). Typical sizes of the particles were between 3 and 5 nm and they were mostly clustered together. High-resolution lattice imaging shows that the particles are single crystalline in nature and defect free. Two different phases of molybdenum nitride, γ-Mo2N (cubic) and δ-MoN (hexagonal) were identified. In addition, body centered cubic molybdenum phase was also present. It is anticipated that molybdenum is formed because of insufficient N2 supply or slow reaction rate. A mixture of γ-Mo2N and δ-MoN suggests the existence of a temperature gradient in the chamber leading to formation of γ-Mo2N at lower temperature (500 to 700 °C) and δ-MoN at higher temperature (850 °C). © 2007 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Nanoparticles; Molybdenum nitride nanoparticles; Crystal structure; High-resolution electron microscopy; dc arc charge system
1. Introduction Transition metal nitrides constitute an important class of interstitial compounds with exceptional chemical stability, high strength, high hardness, high melting point and good electrical conductivity [1]. For example, tungsten and molybdenum nitrides are good barriers against diffusion of copper in microelectronic circuits [2]. They can also be used in electrodes for thin film capacitors and field effect transistors. Moreover, transition metal nitrides have long been known to be an effective catalyst for various organic reactions such as hydrodesulfurization (HDS), in nitrogen fixation reactions (converting molecular nitrogen to ammonia) and in reducing carbon monoxide to methane gas [3]. Molybdenum nitrides have also attracted much attention as a superconducting material. Molybdenum forms several nitrides including γ-Mo2N (face centered cubic), β-
⁎ Corresponding author. Tel.: +1 806 742 3563x224; fax: +1 806 742 3540. E-mail address: [email protected] (J. Chaudhuri). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.03.022
Fig. 1. Schematic representation of the dc arc discharge system.
4764
J. Chaudhuri et al. / Materials Letters 61 (2007) 4763–4765
resolution field emission analytic transmission electron microscopy (TEM) and scanning TEM (STEM) with 200 kV and 0.23 nm point resolution in the Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM. The particles were separated by diluting them in methyl
Fig. 2. TEM image of the Mo2N nanoparticles.
MoN (tetragonal), and δ-MoN (hexagonal) with all three phases being superconducting [4–9]. The reduction of grain size to nanometer range will lead to the change in physical and chemical properties of a material. Thus nanocrystalline materials are of great interest currently and have been expected to lead discoveries to new materials with novel performances [10]. A previous paper reported a new synthesis method for the preparation of nanocrystalline molybdenum nitride [11]. X-ray diffraction data suggested the formation of γ-Mo2N nanoparticles. In this paper we report high-resolution transmission electron microscopy study of the molybdenum nitride nanoparticles to reveal the structure and size of these nanoparticles for the first time. 2. Experimental The details of the experimental set up can be found in the previous paper [11]. The synthesis of Mo2N was performed with the direct current arc discharge method. As is shown in Fig. 1, a rod of molybdenum (99.99% purity) was used as both cathode and anode material. On the anode side, the molybdenum rod was tightly set in the hole on a rod of graphite, which was enclosed in a water-cooled crucible. The reaction chamber was first purged by argon gas. Then nitrogen (99.999% purity) was introduced to reach 10 kPa in the system. A direct current (150 A, 40 V) was then applied and the arc was ignited. The reaction was kept for 20 min, and the product was deactivated for 12 h under nitrogen environment at 80 kPa before collection. The synthesized product was characterized using JEOL 2010F, i.e., a highFig. 3. (a) and (b) High-resolution transmission electron micrographs with inserted fast Fourier transform (FFT) showing that individual nanoparticles are single crystals with sizes 3 to 5 nm and without any defects. The upper and lower FFTs in (a) are from BCC Mo and FCC Mo2N, respectively. The FFT in (b) is from a hexagonal MoN particle. (c) Inverse fast Fourier transform (IFFT) of (b) using a lattice mask. The interplanar spacing of different particles has been clearly identified in all three figures (d111 of FCC or γ-Mo2N varies between 0.23 and 0.257 nm, d110 of Mo varies between 0.207 and 0.218 nm, and d10.0 of hexagonal or δ-MoN is 0.283 nm).
J. Chaudhuri et al. / Materials Letters 61 (2007) 4763–4765
4765
Table 1 Interplanar spacing, lattice parameters and crystal structure of nanoparticles
ber leading to the formation of γ-Mo2N at lower temperature (500 to 700 °C) and δ-MoN at higher temperature (850 °C) [4,13].
Interplanar Crystallographic Lattice Bulk lattice % Crystal spacing planes parameter parameter difference a structure (nm) (nm) (nm)
4. Conclusions
0.207– 0.218 0.23– 0.257 0.283
Mo (110) γ-Mo2N (111) δ-MoN (1000)
0.293– 0.308 0.398– 0.445 0.283 (a)
0.315
6.9–2.2
Mo BCC
0.416– 0.419 0.286 (a)
4.7–6.6
γ-Mo2N FCC δ-MoN Hex
1.0
a
% difference between the measured lattice spacing in the nanoparticles as compared to the bulk lattice parameters.
alcohol and using ultrasonic agitation. All the image analyses were performed using a GATAN digital micrograph software [12].
High-resolution transmission electron microscopy study of molybdenum nitride nanoparticles was performed to identify structure and size of these particles. The particles were mostly clustered together and their typical size was between 3 and 5 nm. The particles are single crystalline in nature and defect free as evidenced by high-resolution lattice imaging technique. The nanoparticles consisted of mostly two different phases: body centered cubic molybdenum and face centered cubic molybdenum nitride. A few particles of molybdenum nitride with hexagonal phase were also found. Acknowledgement
3. Results and discussion Low magnification TEM image of Mo2N nanoparticles is shown in Fig. 2. It can be seen that the particles are mostly clustered together. Fig. 3(a) and (b) shows high-resolution lattice images from some of the particles with corresponding diffraction patterns superimposed. Fig. 3 (c) shows the inverse fast Fourier transforms (IFFT) of masked FFT of HRTEM images of particles shown in Fig. 3(b). Fig. 3(a), (b) and (c) indicates that the nanoparticles are single crystals without any defects present and the typical size of the particles varies between 3 and 5 nm. From Fig. 3(a), (b) and (c), the interplanar spacing of the particles can be categorized in three different groups as shown in Table 1. These are 0.207–0.218, 0.23–0.257 and 0.283 nm. The corresponding crystal structures are identified as Mo body centered cubic (BCC), γ-Mo2N face centered cubic (FCC) and δ-MoN hexagonal structure. These δMoN particles could not be identified by the X-ray diffraction technique [10]. HRTEM results confirmed that the nanoparticles synthesized mostly consists of molybdenum and γ-Mo2N (FCC) nanoparticles with a few hexagonal δ-MoN. The lattice parameter of molybdenum particles is of slightly smaller size compared to bulk lattice parameter (percent difference 6.9–2.2%) probably due to the application of high pressure, whereas the lattice parameters of cubic (percent difference 4.7–6.6%) and hexagonal molybdenum nitrides (percent difference 1.0%) are close to the bulk lattice parameter values. It is anticipated that molybdenum is formed because of insufficient N2 supply or slow reaction rate. A mixture of γ-Mo2N and δ-MoN particles suggests the existence of a temperature gradient in the cham-
This work was supported by the NSF grant # DMR0515858. HRTEM work was performed in the Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
[11] [12] [13]
N. Masatishi, K. Masaru, T. Hiroyuki, O. Shinzo, Chem. Lett. 6 (2000) 702. J.S. Becker, R.G. Gordon, Appl. Phys. Lett. 82 (2003) 2239. R. Kojima, K.-I. Aika, Appl. Catal., A Gen. 218 (2001) 121. B.T. Mathias, J.K. Hulm, Phys. Rev. 87 (1952) 799. K. Inumaru, K. Baba, S. Yamanaka, Chem. Mater. 17 (2005) 5935. A. Bezinge, K. Yvon, J. Muller, Solid State Commun. 63 (1987) 141. C.L. Bull, P.F. McMillan, E. Soignard, K. Leinen Weber, J. Solid state Chem. 177 (2004) 1488. H. Ihara, Y. Kimura, K. Sebzaki, H. Kezuka, M. Hirabayashi, Phys. Rev., B 31 (1985) 3177. http://emaps.mrl.uiuc.edu/. P. Alivisatos, P.F. Barbara, A.W. Castleman, J. Chang, D.A. Dixon, M.L. Klein, G.L. McLendon, J.S. Miller, M.A. Ratner, P.J. Rossky, S.I. Stupp, M.E. Thompson, Adv. Mater. 10 (1999) 1297. L.H. Shen, Q.L. Cui, J. Zhang, X.F. Li, Q. Zhou, G.T. Zou, Chin. Phys. Lett. 22 (2005) 3192. http://www.gatan.com/. T. Nakajima, T. Shirasaki, J. Electrochem. Soc. 144 (1997) 2096.