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Structure and magnetic properties of Cr(N)–β-Cr2N nanoparticles prepared by arc-discharge
Journal of Alloys and Compounds 425 (2006) 4–9
Structure and magnetic properties of Cr(N)–-Cr2N nanoparticles prepared by arc-discharge W.J. Feng ∗ , D. Li, W.F. Li, S. Ma, Y.B. Li, D.K. Xiong, W.S. Zhang, Z.D. Zhang Shenyang National Laboratory for Materials Science, Institute of Metal Research, International Centre for Materials Physics, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China Received 6 December 2005; received in revised form 6 January 2006; accepted 10 January 2006 Available online 20 February 2006
Abstract Cr(N)–-Cr2 N nanoparticles were prepared by arc-discharge process in a mixture of argon, hydrogen and nitrogen gases. The nanoparticles are composed of the two components, i.e. Cr(N) solid–solution nanoparticles (polyhedral shape) and -Cr2 N nitrogen-deficiency nanoparticles (cuboidal shape). Both the nanoparticles share a similar shell-core structure: the shell consists of chromium oxide Cr2 (CrO4 )3 or Cr O (20 at% Cr), and the complex of Cr and N O, as well as the core of Cr(N) solid–solution or -Cr2 N. Moreover, the nanoparticles have a blocking temperature of 29 K and a narrow size distribution. The weak ferromagnetism can be interpreted in terms of the presence of non-compensated surface spins. For the field-cooled (FC) magnetization with the field of 0.01 T, a perfect Curie–Weiss fit of the data above 50 K indicates paramagnetic characteristics of the nanoparticles. This is supported by the hysteresis loops at 5 and 295 K. The paramagnetic Curie temperature of −124 K suggests the type of dominant interaction is antiferromagnetic. © 2006 Published by Elsevier B.V. Keywords: Arc-discharge; Nanoparticles; Blocking temperature; Curie–Weiss law
1. Introduction Nano-scaled materials have attracted considerable interest because these materials are suitable for the investigation of some fundamentally physical behaviors under reduced dimensionality, as well as they have a number of novel promising applications [1–4]. Among these materials, magnetic nanocapsules/nanoparticles are of interest for their unique shell-core structures and remarkable properties. It is well known that the nanoparticles are easy to be oxidized, due to a large ratio of surface to volume. A number of materials, for instance, carbon [5], silica [6], aluminum oxide [7], boron oxide [8], and boron nitride [9], etc. have been employed as the encapsulating ones of the nanoparticles. For well-coated nanoparticles, the role of encapsulating materials is to prevent the core component, such as Fe, Co, and rare-earth nanoparticles, etc. from oxidation. On the other hand, chromium nitride
∗
Corresponding author. Tel.: +86 24 83978751 E-mail address: [email protected] (W.J. Feng).
0925-8388/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jallcom.2006.01.019
(CrN) with the cubic rock-salt structure has been known as a technologically important material because of its hardness, excellent resistance to corrosion, oxidation, and wear [10,11]. In fact, CrN/Cr2 N has been successfully used as a protective coating appliance, e.g., wear and corrosion-resistant coating for various materials, tools, machine parts and so on [12,13]. To date, however, there have been few studies on the nanoparticles of chromium nitride (CrN/Cr2 N). It may shed some lights on the study of enlarging the family of nanocapsules/nanoparticles. In our previous work [14–16], a process of arc-discharge was developed to fabricate magnetic nanocapsules/nanoparticles with different types of shell-core structures by changing either atmosphere or anode material. In the present paper, Cr(N)–-Cr2 N nanoparticles are prepared by arc-discharging in the mixture of Ar, H2 , and N2 gases. The microstructure and magnetic properties of the Cr(N)–-Cr2 N nanoparticles are characterized by means of X-ray-diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), and magnetic measurements, respectively.
W.J. Feng et al. / Journal of Alloys and Compounds 425 (2006) 4–9
2. Experimental procedures The process of arc-discharge was similar to that developed elsewhere [8,14]. The anode was bulk Cr of 99.5%, while a tungsten needle of 2 mm in diameter served as the corresponding cathode. Argon (20 000 Pa) was introduced into an evacuated chamber (7 × 10−3 Pa) before a potential in the range of 10–20 V was applied between the cathode and the anode. When the arc became stable with a current of 60 A, a mixture of N2 (8000 Pa) and H2 (1000 Pa), served as a reactant gas and a source of hydrogen plasma, respectively, was introduced into the chamber. During the experimental process, the current was maintained at 60 A, while the potential varied in a range of 10–20 V. Deposits on the watercooled wall of the chamber were collected for experiments. XRD spectra were recorded at room temperature in a D/max-␥A diffrac˚ radiation and a pyrolytic monochromator. tometer with Cu K␣ (λ = 1.54056 A) The morphology and the structure of the deposits were observed by a JEOL 2010EX TEM operating at 200 kV. Surface compositions were investigated by XPS (Witchford, UK), while Al K␣ line was used as an X-ray source emitting at 1486.8 eV. The experimental conditions of each cross-section were kept the same, and only time for etching changed. The etching rate was determined by calibrations on typical materials. The etching acted approximately the same on alumina as it did on the nanoparticles in the present system. The magnetic properties of the sample were measured by a superconducting quantum interference device (SQUID, Quantum Design) magnetometer. The procedures for the zerofield-cooled (ZFC) and field-cooled (FC) magnetization measurements were as follows: For ZFC, the sample was first cooled without any magnetic field from room temperature to 5 K, then the magnetization was measured under a dc magnetic field in the warming process. For FC, the magnetization was measured while the sample was cooled in the same dc magnetic field from 350 to 5 K.
3. Results and discussion Fig. 1 shows XRD pattern of the as-prepared nanoparticles, from which, two phases, i.e. cubic Cr and hexagonal -Cr2 N, could be indexed. However, it should be noticed that the XRD peaks of Cr shift to lower angles while those of -Cr2 N to the opposite direction, which indicates a lattice expansion of the former, as well as lattice shrinkage of the latter. According to the (2 1 1) diffraction peak, the calculated lattice parameter of ˚ which is bigger the as-prepared cubic Cr component is 2.887 A, ˚ than 2.881 A of the raw Cr powders. This difference suggests
Fig. 1. XRD spectrum of Cr(N)–-Cr2 N nanoparticles. For comparison, the corresponding XRD patterns of the Cr raw powders and the standard -Cr2 N phase (Card number: 35-0803) are also represented below the experimental data.
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that some nitrogen atoms may enter as the solid–solution atoms into the body-centered cubic (BCC) Cr lattice. On the other hand, according to the (3 0 0) and (1 1 3) diffraction peaks of the corresponding -Cr2 N component, the calculated lattice param˚ and c = 4.448 A, ˚ eters of -Cr2 N component are a = 4.776 A ˚ which are both smaller than the standard values (a = 4.811 A and ˚ It seems to be indicative of nitrogen vacancies for c = 4.484 A). the -Cr2 N component. In addition, from the half-width of XRD peaks of the two components, the particle sizes are estimated as 20–40 nm and 10–30 nm for Cr(N) solid–solution nanoparticles and -Cr2 N nanoparticles, respectively. The typical morphology of the Cr(N)–-Cr2 N nanoparticles is represented in a TEM micrograph (Fig. 2(a)). Obviously, the nanoparticles are composed of some polyhedral particles, with a size range from 10 to 50 nm, and other cuboidal ones, with a smaller size. The size of the particles is basically coincident with the results estimated from the XRD pattern. Fig. 2(b) and (c) represent the TEM images in a larger magnification of the -Cr2 N nanoparticles and the Cr(N) solid–solution nanoparticles, respectively, which are supported by two selected area electron diffractions (SAED) shown in their respective insets. The electron diffraction pattern (see the inset of Fig. 2(b)) is mainly composed of concentric diffuse rings, again suggesting the nanocrystalline nature of the particles. All the rings are indexed out, which are well coincident with some of the XRD peaks corresponding to the hexagonal -Cr2 N phase. The SAED patterns in the inset of Fig. 2(c) are also well consistent with the XRD patterns correspondent to the cubic Cr(N) solid–solution phase. Furthermore, from both the HRTEM images, one can find there is a thin coating layer around the corresponding nanoparticles. The layer can be tentatively ascribed to chromium oxide, because an oxide layer easily forms on the surface of the nanoparticles when they are exposed to air [17,18]. The reason why we have not found any traces of peaks of chromium oxides in the XRD patterns is that the oxides layer is too thin to give any detectable XRD peaks. To obtain more information on the surface and inner chemistry, we investigate our sample with XPS. Full-scan XPS results (not shown here) have verified the presence of the Cr, N, O and C elements. Obviously, the occurrence of C element is owing to carbon dioxide of air. Fig. 3(a)–(c) represent the XPS spectra of the Cr 2p, N 1s, and O 1s electrons with the etching time of 0, 30, 90, and 210 s (corresponding to etching depths of 0, 0.9, 2.7, and 6.3 nm, respectively). For different etching depths, binding energy’s (BE) intensities are changed. This expresses that the distributions of these atoms are different on each cross section [19]. The curves of Cr 2p BEs, shown in Fig. 3(a), give the distribution of the peak shapes and positions. The inset of Fig. 3(a) represents the fitting curves of the etching time 210 s for Cr 2p 574.7 eV (Cr) and 577.6 eV (Cr O (20 at% Cr) surface non-bombarded). It should be noted that the intensity of Cr 2p 574.7 eV (Cr) becomes stronger, while that of Cr 2p 577.6 eV (Cr O (20 at% Cr) surface non-bombarded) becomes weaker, as a function of the etching time. This suggests that the Cr atoms are inside the nanoparticles, whereas chromium oxides on the surfaces of the nanoparticles. Furthermore, according
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W.J. Feng et al. / Journal of Alloys and Compounds 425 (2006) 4–9
Fig. 2. TEM images of (a) the overall; (b) the -Cr2 N; and (c) the Cr(N) solid–solution nanoparticles. The insets in (b) and (c) show the corresponding selected area electron diffractions of -Cr2 N nanoparticles and Cr(N) solid–solution nanoparticles, respectively.
to the fitting curves, the intensity of metallic Cr 2p peak is higher than that of the oxidized chromium, which means that the percentage of metallic Cr is higher than that of chromium oxides. According to the fitting curves of the original surfaces (see the inset of Fig. 3(b)), the main N peaks are the N 1s at
397.4 eV for Cr2 N, and at 400.2 eV for N O. But for the other curves of N 1s BEs, the absence of the peak corresponding to N O indicates that the N O can only exist around the surfaces of the nanoparticles. For the curves of O 1s BEs (shown in Fig. 3(c)), the peak shapes and the position (530.9 eV
W.J. Feng et al. / Journal of Alloys and Compounds 425 (2006) 4–9
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Fig. 3. XPS survey patterns of the binding energy for (a) Cr 2p; (b) N 1s; and (c) O 1s electrons on the surfaces with different etching depths of the Cr(N)–-Cr2 N nanoparticles. The inset of (a) shows the experimental curve (the solid line) for etching time 210 s and the fitting curves (the dotted line) of (1) 574.7 eV (Cr); (2) 577.6 eV (Cr O (20 at% Cr) surface non-bombarded). The inset of (b) represents N 1s survey curve (the solid line) for original surfaces and the fitting curves of (1) 397.4 eV (Cr2 N); (2) 400.2 eV (N O).
for Cr2 (CrO4 )3 ) are almost unchangeable, which means that chromium oxide, i.e. Cr2 (CrO4 )3 , exists in this nanoparticles. Moreover, as the etching time is prolonged, the intensity of O 1s BEs the peak becomes weaker. This also indicates that Cr2 (CrO4 )3 can appear on the surfaces of the nanoparticles. From the XRD, TEM, SAED and XPS analyses above, we conclude that the sample is composed of two phases, i.e. nitrogen-deficiency -Cr2 N (cuboidal shape) and Cr(N) solid–solution (polyhedral shape). Furthermore, both the nanoparticles share the similar shell-core structure: the shell consists of chromium oxide Cr2 (CrO4 )3 or Cr O (20 at% Cr), and the complex of Cr and N O, as well as the core of Cr(N) solid–solution or -Cr2 N. In previous work, Zhang et al. [16], Kimoto and Nishida [20] prepared Cr nanoparticles with cubic shape. These nanoparticles are obviously different from the
present Cr(N) solid–solution ones with polyhedral shape. For their nanoparticles, the shell is mainly Cr2 O3 , which is obviously different from the shell of our nanoparticles. Therefore, different shell should have different changes of surface energy, which leads to their different shapes. Furthermore, the formation of the nanoparticles is perhaps concerned with two different zones of the arc: the center and the annular one near to the surface, of the arc just above the upper surface of the anode material. For the center zone, the Cr atoms, evaporated from the anode, will have comparatively adequate time to combine with the [N], dissociated from N2 by the applied potential. Therefore, according to the Cr N binary phase diagram, -Cr2 N nanoparticles are produced. On the other hand, for the annular zone, the Cr atoms have only very short time to absorb [N], as a consequence, Cr(N) solid–solution nanoparti-
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W.J. Feng et al. / Journal of Alloys and Compounds 425 (2006) 4–9
Fig. 4. Temperature dependences of the zero-field-cooled and field-cooled magnetizations at a field of 0.01 T, and field-cooled magnetization at the magnetic field of 1 T, of the Cr(N)–-Cr2 N nanoparticles. Inset: temperature dependence of the reciprocal magnetization 1/M measured in the FC process with the field of 0.01 T, and the line shows that 1/M is fitted to a Curie–Weiss relation for T > 50 K.
cles are formed. After their exposure to air, oxygen from the air combines spontaneously with the surface atoms of the nanoparticles because of their large ratio of surface to volume, and their unique shell-core structures are formed. The temperature dependences of the ZFC and FC magnetizations (defined as MZFC and MFC , respectively) of Cr(N)–-Cr2 N nanoparticles with a dc magnetic field of 0.01 T, are plotted in Fig. 4. The temperature dependence of MFC recorded with a dc magnetic field of 1 T is also shown for comparison. The inset in this figure shows the temperature dependence of the reciprocal magnetization 1/M, as measured in the FC process with the field of 0.01 T. As temperature is lowered from room temperature, a very sharp cusp at about 29 K is observed in the MZFC curve with a dc magnetic field of 0.01 T, while a gradual increase of MFC appears, especially, below 50 K. Namely, an increasingly strong irreversibility is observed between MZFC and MFC, with the onset of about 300 K. For MFC with the magnetic field of 1 T, it exhibits a behavior similar to that of MFC with the dc magnetic field of 0.01 T. As is well known, the blocking temperature TB (the temperature above which one particle has enough time, within the observation time, to reverse its moments to the applied field [21]) is the temperature correspondent to the maximum of MZFC , or around 29 K. While bulk Cr is an antiferrromagnet with a N´eel temperature TN of 311 K [22], Cr nanoparticles exhibit mainly antiferromagnetic (AFM) properties, in addition to a weak ferromagnetic (WFM) component [16]. According to N´eel [23], WFM behaviors from very fine antiferromagnetic particles can be attributed to the uncompensated spins on the surfaces of the fine particles, which was also verified for a lot of AFM nanoparticles such as MnO [24], Co3 O4 [25] and Cr2 O3 [16] ones, etc. Moreover, -Cr2 N shows Pauli paramagnetic down to 1.8 K [26]. Therefore, it is thought that the WFM component arises from the uncompensated spins of the surfaces on Cr(N) solid–solution
Fig. 5. Hysteresis loops of Cr(N)–-Cr2 N nanoparticles measured at 5 and 295 K. The inset shows the two loops in a larger magnification.
nanoparticles, and perhaps their unique shells. Furthermore, it is interesting to note that no any cusp appears on the MZFC –T curve in the previous work, in spite of the Cr nanoparticles exhibiting WFM [16]. The appearance of the blocking temperature for the present nanoparticles may be ascribed mainly to the narrower distribution of the Cr(N) solid–solution nanoparticles than that of Cr nanoparticles prepared in the previous work [16]. Furthermore, this also suggests that the weak ferromagnetism in the former should be stronger than that in the latter. In addition, there is no any distinctive signature of magnetic transition around 311 K, the N´eel temperature of bulk Cr. For antiferromagnetic nanoparticles, the WFM behavior, arising from uncompensated surface spins, can easily dominate the antiferromagnetic contribution, leading to the disappearance of the characteristics of the AFM component. Besides, the inset in Fig. 4 shows that the Curie–Weiss law, i.e. χ = C/T − Δ, fits well for the data above 50 K of MFC with the dc magnetic field of 0.01 T (see the line in the inset), with the paramagnetic Curie temperature Δ = −124 K. The almost linear temperature dependence above 50 K of the reciprocal magnetization is indicative of the paramagnetic behavior for the nanoparticles above the blocking temperature. Besides, the negative paramagnetic Curie temperature signifies the domination of the antiferromagnetic interaction. Fig. 5 shows the hysteresis loops of the Cr(N)–-Cr2 N nanoparticles at 5 and 295 K. The coercive field at 295 K is about 0.007 T, while that at 5 K is about 0.07 T. This also indicates that the former is associated with the paramagnetic behavior whereas the latter is in a slow relaxation process. Besides, the magnetic hysteresis at 295 K is observed only at a very low field, while at 5 K the same property can be observed in a rather higher field range, which indicates that weak ferromagnetic component is gradually enhanced with lowering temperature.
W.J. Feng et al. / Journal of Alloys and Compounds 425 (2006) 4–9
4. Conclusion The Cr(N)–-Cr2 N nanoparticles, produced by arc-discharge process in a mixture of argon, hydrogen, and nitrogen gases, consist of two components, Cr(N) solid–solution nanoparticles and -Cr2 N nanoparticles. The Cr(N) solid–solution nanoparticles exhibit a polyhedral shape, while the -Cr2 N nanoparticles display cuboidal one. Both the nanoparticles have the similar shell-core structure: the shell consists of chromium oxide Cr2 (CrO4 )3 or Cr O (20 at% Cr), and the complex of Cr and N O, as well as the core of Cr(N) solid–solution or -Cr2 N. The nanoparticles have a blocking temperature of 29 K and a narrow size distribution, and their weak ferromagnetic component is ascribed to the uncompensated spins on the surfaces. For the field-cooled magnetization with the field of 0.01 T, the data above 50 K are well fitted by the Curie–Weiss law, which indicates the sample is paramagnetic above the blocking temperature. This is supported by the hysteresis loops at 5 and 295 K. The paramagnetic Curie temperature of −124 K suggests that the type of dominant interaction is antiferromagnetic. Acknowledgment This work has been supported by the National Natural Science Foundation of China under Grant Nos. 50332020 and 50331030. References [1] S. Iijima, Nature (London) 354 (1991) 56. [2] S. Iijima, T. Ichihashi, Nature (London) 363 (1993) 603. [3] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, Nature (London) 318 (1985) 162. [4] Z.X. Tang, C.M. Sorensen, K.J. Klabunde, G.C. Hadjipanayis, Phys. Rev. Lett. 67 (1991) 3602.
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[5] R.S. Ruoff, D.C. Lorents, B. Chan, R. Malhotra, S. Subramoney, Science 259 (1993) 346. [6] K.L. Klug, V.P. Dravid, D.L. Johnson, J. Mater. Res. 18 (2003) 988. [7] D.Y. Geng, Z.D. Zhang, W.S. Zhang, P.Z. Si, X.G. Zhao, W. Liu, K.Y. Hu, Z.X. Jin, X.P. Song, Scripta Mater. 48 (2003) 593. [8] Z.D. Zhang, J.L. Yu, J.G. Zheng, I. Skorvanek, J. Kovac, X.L. Dong, Z.J. Li, S.R. Jin, H.C. Yang, Z.J. Guo, W. Liu, X.G. Zhao, Phys. Rev. B 64 (2001) 024404. [9] P.Z. Si, M. Zhang, C.Y. You, D.Y. Geng, J.H. Du, X.G. Zhao, X.L. Ma, Z.D. Zhang, J. Mater. Sci. 38 (2003) 689. [10] R.M. Ren, Z.G. Yang, L.L. Shaw, Nanostruct. Mater. 11 (1) (1999) 25. [11] T. Aizawa, H. Kuwahara, M. Tamura, J. Am. Ceram. Soc. 85 (1) (2002) 81. [12] U. Bardi, S.P. Chenakin, F. Ghezzi, C. Giolli, A. Goruppa, A. Lavacchi, E. Miorin, C. Pagura, A. Tolstogouzov, Appl. Surf. Sci. 252 (2005) 1339. [13] J.C. Avelar-Batista, E. Spain, J. Housden, G.G. Fuentes, R. Rebol´e, R. Rodriguez, F. Montal´a, L.J. Carreras, T.J. Tate, Thin Solid Films 491 (2005) 177. [14] X.L. Dong, Z.D. Zhang, Y.C. Chuang, S.R. Jin, Phys. Rev. B 60 (1999) 3017. [15] X.L. Dong, Z.D. Zhang, S.R. Jin, B.K. Kim, J. Appl. Phys. 86 (1999) 6701. [16] W.S. Zhang, E. Br¨uck, Z.D. Zhang, O. Tegus, W.F. Li, P.Z. Si, D.Y. Geng, K.H.J. Buschow, Physica B 358 (2005) 332. [17] S. Li, S. Silvers, M.S. El-shall, J. Phys. Chem. B 101 (1997) 1794. [18] Y.B. Pithawalla, M.S. El-Shall, S.C. Deevi, K.V. Rao, Phys. Rev. B 64 (2001) 132408. [19] D.Y. Geng, W.Y. Park, J.H. Yu, C.J. Choi, Z.D. Zhang, J. Mater. Res. 20 (9) (2005) 2534. [20] K. Kimoto, I. Nishida, J. Phys. Soc. Jpn. 22 (1967) 744. [21] P.Z. Si, E. Br¨uck, Z.D. Zhang, O. Tegus, K.H.J. Buschow, W.S. Zhang, J.C.P. Klaasse, F.R. de Boer, Physica B 353 (2004) 1. [22] E. Fawcett, Rev. Mod. Phys. 60 (1988) 209. [23] L. N´eel, in: C. Dewitt, B. Dreyfus, P.G. de Gennes (Eds.), Low Temperature Physics, Gordon and Breach, New York, 1962, p. 413. [24] Y.Q. Chang, D.P. Yu, Z. Wang, Y. Long, H.Z. Zhang, R.C. Ye, J. Cryst. Growth 281 (2005) 678. [25] Y. Ichiyanagi, Y. Kimishima, S. Yamada, J. Magn. Magn. Mater. 272–276 (2004) e1245. [26] M. Mekata, H. Yoshimura, H. Takaki, J. Phys. Soc. Jpn 33 (1972) 62.
Structure and magnetic properties of Cr(N)–-Cr2N nanoparticles prepared by arc-discharge W.J. Feng ∗ , D. Li, W.F. Li, S. Ma, Y.B. Li, D.K. Xiong, W.S. Zhang, Z.D. Zhang Shenyang National Laboratory for Materials Science, Institute of Metal Research, International Centre for Materials Physics, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China Received 6 December 2005; received in revised form 6 January 2006; accepted 10 January 2006 Available online 20 February 2006
Abstract Cr(N)–-Cr2 N nanoparticles were prepared by arc-discharge process in a mixture of argon, hydrogen and nitrogen gases. The nanoparticles are composed of the two components, i.e. Cr(N) solid–solution nanoparticles (polyhedral shape) and -Cr2 N nitrogen-deficiency nanoparticles (cuboidal shape). Both the nanoparticles share a similar shell-core structure: the shell consists of chromium oxide Cr2 (CrO4 )3 or Cr O (20 at% Cr), and the complex of Cr and N O, as well as the core of Cr(N) solid–solution or -Cr2 N. Moreover, the nanoparticles have a blocking temperature of 29 K and a narrow size distribution. The weak ferromagnetism can be interpreted in terms of the presence of non-compensated surface spins. For the field-cooled (FC) magnetization with the field of 0.01 T, a perfect Curie–Weiss fit of the data above 50 K indicates paramagnetic characteristics of the nanoparticles. This is supported by the hysteresis loops at 5 and 295 K. The paramagnetic Curie temperature of −124 K suggests the type of dominant interaction is antiferromagnetic. © 2006 Published by Elsevier B.V. Keywords: Arc-discharge; Nanoparticles; Blocking temperature; Curie–Weiss law
1. Introduction Nano-scaled materials have attracted considerable interest because these materials are suitable for the investigation of some fundamentally physical behaviors under reduced dimensionality, as well as they have a number of novel promising applications [1–4]. Among these materials, magnetic nanocapsules/nanoparticles are of interest for their unique shell-core structures and remarkable properties. It is well known that the nanoparticles are easy to be oxidized, due to a large ratio of surface to volume. A number of materials, for instance, carbon [5], silica [6], aluminum oxide [7], boron oxide [8], and boron nitride [9], etc. have been employed as the encapsulating ones of the nanoparticles. For well-coated nanoparticles, the role of encapsulating materials is to prevent the core component, such as Fe, Co, and rare-earth nanoparticles, etc. from oxidation. On the other hand, chromium nitride
∗
Corresponding author. Tel.: +86 24 83978751 E-mail address: [email protected] (W.J. Feng).
0925-8388/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jallcom.2006.01.019
(CrN) with the cubic rock-salt structure has been known as a technologically important material because of its hardness, excellent resistance to corrosion, oxidation, and wear [10,11]. In fact, CrN/Cr2 N has been successfully used as a protective coating appliance, e.g., wear and corrosion-resistant coating for various materials, tools, machine parts and so on [12,13]. To date, however, there have been few studies on the nanoparticles of chromium nitride (CrN/Cr2 N). It may shed some lights on the study of enlarging the family of nanocapsules/nanoparticles. In our previous work [14–16], a process of arc-discharge was developed to fabricate magnetic nanocapsules/nanoparticles with different types of shell-core structures by changing either atmosphere or anode material. In the present paper, Cr(N)–-Cr2 N nanoparticles are prepared by arc-discharging in the mixture of Ar, H2 , and N2 gases. The microstructure and magnetic properties of the Cr(N)–-Cr2 N nanoparticles are characterized by means of X-ray-diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), and magnetic measurements, respectively.
W.J. Feng et al. / Journal of Alloys and Compounds 425 (2006) 4–9
2. Experimental procedures The process of arc-discharge was similar to that developed elsewhere [8,14]. The anode was bulk Cr of 99.5%, while a tungsten needle of 2 mm in diameter served as the corresponding cathode. Argon (20 000 Pa) was introduced into an evacuated chamber (7 × 10−3 Pa) before a potential in the range of 10–20 V was applied between the cathode and the anode. When the arc became stable with a current of 60 A, a mixture of N2 (8000 Pa) and H2 (1000 Pa), served as a reactant gas and a source of hydrogen plasma, respectively, was introduced into the chamber. During the experimental process, the current was maintained at 60 A, while the potential varied in a range of 10–20 V. Deposits on the watercooled wall of the chamber were collected for experiments. XRD spectra were recorded at room temperature in a D/max-␥A diffrac˚ radiation and a pyrolytic monochromator. tometer with Cu K␣ (λ = 1.54056 A) The morphology and the structure of the deposits were observed by a JEOL 2010EX TEM operating at 200 kV. Surface compositions were investigated by XPS (Witchford, UK), while Al K␣ line was used as an X-ray source emitting at 1486.8 eV. The experimental conditions of each cross-section were kept the same, and only time for etching changed. The etching rate was determined by calibrations on typical materials. The etching acted approximately the same on alumina as it did on the nanoparticles in the present system. The magnetic properties of the sample were measured by a superconducting quantum interference device (SQUID, Quantum Design) magnetometer. The procedures for the zerofield-cooled (ZFC) and field-cooled (FC) magnetization measurements were as follows: For ZFC, the sample was first cooled without any magnetic field from room temperature to 5 K, then the magnetization was measured under a dc magnetic field in the warming process. For FC, the magnetization was measured while the sample was cooled in the same dc magnetic field from 350 to 5 K.
3. Results and discussion Fig. 1 shows XRD pattern of the as-prepared nanoparticles, from which, two phases, i.e. cubic Cr and hexagonal -Cr2 N, could be indexed. However, it should be noticed that the XRD peaks of Cr shift to lower angles while those of -Cr2 N to the opposite direction, which indicates a lattice expansion of the former, as well as lattice shrinkage of the latter. According to the (2 1 1) diffraction peak, the calculated lattice parameter of ˚ which is bigger the as-prepared cubic Cr component is 2.887 A, ˚ than 2.881 A of the raw Cr powders. This difference suggests
Fig. 1. XRD spectrum of Cr(N)–-Cr2 N nanoparticles. For comparison, the corresponding XRD patterns of the Cr raw powders and the standard -Cr2 N phase (Card number: 35-0803) are also represented below the experimental data.
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that some nitrogen atoms may enter as the solid–solution atoms into the body-centered cubic (BCC) Cr lattice. On the other hand, according to the (3 0 0) and (1 1 3) diffraction peaks of the corresponding -Cr2 N component, the calculated lattice param˚ and c = 4.448 A, ˚ eters of -Cr2 N component are a = 4.776 A ˚ which are both smaller than the standard values (a = 4.811 A and ˚ It seems to be indicative of nitrogen vacancies for c = 4.484 A). the -Cr2 N component. In addition, from the half-width of XRD peaks of the two components, the particle sizes are estimated as 20–40 nm and 10–30 nm for Cr(N) solid–solution nanoparticles and -Cr2 N nanoparticles, respectively. The typical morphology of the Cr(N)–-Cr2 N nanoparticles is represented in a TEM micrograph (Fig. 2(a)). Obviously, the nanoparticles are composed of some polyhedral particles, with a size range from 10 to 50 nm, and other cuboidal ones, with a smaller size. The size of the particles is basically coincident with the results estimated from the XRD pattern. Fig. 2(b) and (c) represent the TEM images in a larger magnification of the -Cr2 N nanoparticles and the Cr(N) solid–solution nanoparticles, respectively, which are supported by two selected area electron diffractions (SAED) shown in their respective insets. The electron diffraction pattern (see the inset of Fig. 2(b)) is mainly composed of concentric diffuse rings, again suggesting the nanocrystalline nature of the particles. All the rings are indexed out, which are well coincident with some of the XRD peaks corresponding to the hexagonal -Cr2 N phase. The SAED patterns in the inset of Fig. 2(c) are also well consistent with the XRD patterns correspondent to the cubic Cr(N) solid–solution phase. Furthermore, from both the HRTEM images, one can find there is a thin coating layer around the corresponding nanoparticles. The layer can be tentatively ascribed to chromium oxide, because an oxide layer easily forms on the surface of the nanoparticles when they are exposed to air [17,18]. The reason why we have not found any traces of peaks of chromium oxides in the XRD patterns is that the oxides layer is too thin to give any detectable XRD peaks. To obtain more information on the surface and inner chemistry, we investigate our sample with XPS. Full-scan XPS results (not shown here) have verified the presence of the Cr, N, O and C elements. Obviously, the occurrence of C element is owing to carbon dioxide of air. Fig. 3(a)–(c) represent the XPS spectra of the Cr 2p, N 1s, and O 1s electrons with the etching time of 0, 30, 90, and 210 s (corresponding to etching depths of 0, 0.9, 2.7, and 6.3 nm, respectively). For different etching depths, binding energy’s (BE) intensities are changed. This expresses that the distributions of these atoms are different on each cross section [19]. The curves of Cr 2p BEs, shown in Fig. 3(a), give the distribution of the peak shapes and positions. The inset of Fig. 3(a) represents the fitting curves of the etching time 210 s for Cr 2p 574.7 eV (Cr) and 577.6 eV (Cr O (20 at% Cr) surface non-bombarded). It should be noted that the intensity of Cr 2p 574.7 eV (Cr) becomes stronger, while that of Cr 2p 577.6 eV (Cr O (20 at% Cr) surface non-bombarded) becomes weaker, as a function of the etching time. This suggests that the Cr atoms are inside the nanoparticles, whereas chromium oxides on the surfaces of the nanoparticles. Furthermore, according
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W.J. Feng et al. / Journal of Alloys and Compounds 425 (2006) 4–9
Fig. 2. TEM images of (a) the overall; (b) the -Cr2 N; and (c) the Cr(N) solid–solution nanoparticles. The insets in (b) and (c) show the corresponding selected area electron diffractions of -Cr2 N nanoparticles and Cr(N) solid–solution nanoparticles, respectively.
to the fitting curves, the intensity of metallic Cr 2p peak is higher than that of the oxidized chromium, which means that the percentage of metallic Cr is higher than that of chromium oxides. According to the fitting curves of the original surfaces (see the inset of Fig. 3(b)), the main N peaks are the N 1s at
397.4 eV for Cr2 N, and at 400.2 eV for N O. But for the other curves of N 1s BEs, the absence of the peak corresponding to N O indicates that the N O can only exist around the surfaces of the nanoparticles. For the curves of O 1s BEs (shown in Fig. 3(c)), the peak shapes and the position (530.9 eV
W.J. Feng et al. / Journal of Alloys and Compounds 425 (2006) 4–9
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Fig. 3. XPS survey patterns of the binding energy for (a) Cr 2p; (b) N 1s; and (c) O 1s electrons on the surfaces with different etching depths of the Cr(N)–-Cr2 N nanoparticles. The inset of (a) shows the experimental curve (the solid line) for etching time 210 s and the fitting curves (the dotted line) of (1) 574.7 eV (Cr); (2) 577.6 eV (Cr O (20 at% Cr) surface non-bombarded). The inset of (b) represents N 1s survey curve (the solid line) for original surfaces and the fitting curves of (1) 397.4 eV (Cr2 N); (2) 400.2 eV (N O).
for Cr2 (CrO4 )3 ) are almost unchangeable, which means that chromium oxide, i.e. Cr2 (CrO4 )3 , exists in this nanoparticles. Moreover, as the etching time is prolonged, the intensity of O 1s BEs the peak becomes weaker. This also indicates that Cr2 (CrO4 )3 can appear on the surfaces of the nanoparticles. From the XRD, TEM, SAED and XPS analyses above, we conclude that the sample is composed of two phases, i.e. nitrogen-deficiency -Cr2 N (cuboidal shape) and Cr(N) solid–solution (polyhedral shape). Furthermore, both the nanoparticles share the similar shell-core structure: the shell consists of chromium oxide Cr2 (CrO4 )3 or Cr O (20 at% Cr), and the complex of Cr and N O, as well as the core of Cr(N) solid–solution or -Cr2 N. In previous work, Zhang et al. [16], Kimoto and Nishida [20] prepared Cr nanoparticles with cubic shape. These nanoparticles are obviously different from the
present Cr(N) solid–solution ones with polyhedral shape. For their nanoparticles, the shell is mainly Cr2 O3 , which is obviously different from the shell of our nanoparticles. Therefore, different shell should have different changes of surface energy, which leads to their different shapes. Furthermore, the formation of the nanoparticles is perhaps concerned with two different zones of the arc: the center and the annular one near to the surface, of the arc just above the upper surface of the anode material. For the center zone, the Cr atoms, evaporated from the anode, will have comparatively adequate time to combine with the [N], dissociated from N2 by the applied potential. Therefore, according to the Cr N binary phase diagram, -Cr2 N nanoparticles are produced. On the other hand, for the annular zone, the Cr atoms have only very short time to absorb [N], as a consequence, Cr(N) solid–solution nanoparti-
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Fig. 4. Temperature dependences of the zero-field-cooled and field-cooled magnetizations at a field of 0.01 T, and field-cooled magnetization at the magnetic field of 1 T, of the Cr(N)–-Cr2 N nanoparticles. Inset: temperature dependence of the reciprocal magnetization 1/M measured in the FC process with the field of 0.01 T, and the line shows that 1/M is fitted to a Curie–Weiss relation for T > 50 K.
cles are formed. After their exposure to air, oxygen from the air combines spontaneously with the surface atoms of the nanoparticles because of their large ratio of surface to volume, and their unique shell-core structures are formed. The temperature dependences of the ZFC and FC magnetizations (defined as MZFC and MFC , respectively) of Cr(N)–-Cr2 N nanoparticles with a dc magnetic field of 0.01 T, are plotted in Fig. 4. The temperature dependence of MFC recorded with a dc magnetic field of 1 T is also shown for comparison. The inset in this figure shows the temperature dependence of the reciprocal magnetization 1/M, as measured in the FC process with the field of 0.01 T. As temperature is lowered from room temperature, a very sharp cusp at about 29 K is observed in the MZFC curve with a dc magnetic field of 0.01 T, while a gradual increase of MFC appears, especially, below 50 K. Namely, an increasingly strong irreversibility is observed between MZFC and MFC, with the onset of about 300 K. For MFC with the magnetic field of 1 T, it exhibits a behavior similar to that of MFC with the dc magnetic field of 0.01 T. As is well known, the blocking temperature TB (the temperature above which one particle has enough time, within the observation time, to reverse its moments to the applied field [21]) is the temperature correspondent to the maximum of MZFC , or around 29 K. While bulk Cr is an antiferrromagnet with a N´eel temperature TN of 311 K [22], Cr nanoparticles exhibit mainly antiferromagnetic (AFM) properties, in addition to a weak ferromagnetic (WFM) component [16]. According to N´eel [23], WFM behaviors from very fine antiferromagnetic particles can be attributed to the uncompensated spins on the surfaces of the fine particles, which was also verified for a lot of AFM nanoparticles such as MnO [24], Co3 O4 [25] and Cr2 O3 [16] ones, etc. Moreover, -Cr2 N shows Pauli paramagnetic down to 1.8 K [26]. Therefore, it is thought that the WFM component arises from the uncompensated spins of the surfaces on Cr(N) solid–solution
Fig. 5. Hysteresis loops of Cr(N)–-Cr2 N nanoparticles measured at 5 and 295 K. The inset shows the two loops in a larger magnification.
nanoparticles, and perhaps their unique shells. Furthermore, it is interesting to note that no any cusp appears on the MZFC –T curve in the previous work, in spite of the Cr nanoparticles exhibiting WFM [16]. The appearance of the blocking temperature for the present nanoparticles may be ascribed mainly to the narrower distribution of the Cr(N) solid–solution nanoparticles than that of Cr nanoparticles prepared in the previous work [16]. Furthermore, this also suggests that the weak ferromagnetism in the former should be stronger than that in the latter. In addition, there is no any distinctive signature of magnetic transition around 311 K, the N´eel temperature of bulk Cr. For antiferromagnetic nanoparticles, the WFM behavior, arising from uncompensated surface spins, can easily dominate the antiferromagnetic contribution, leading to the disappearance of the characteristics of the AFM component. Besides, the inset in Fig. 4 shows that the Curie–Weiss law, i.e. χ = C/T − Δ, fits well for the data above 50 K of MFC with the dc magnetic field of 0.01 T (see the line in the inset), with the paramagnetic Curie temperature Δ = −124 K. The almost linear temperature dependence above 50 K of the reciprocal magnetization is indicative of the paramagnetic behavior for the nanoparticles above the blocking temperature. Besides, the negative paramagnetic Curie temperature signifies the domination of the antiferromagnetic interaction. Fig. 5 shows the hysteresis loops of the Cr(N)–-Cr2 N nanoparticles at 5 and 295 K. The coercive field at 295 K is about 0.007 T, while that at 5 K is about 0.07 T. This also indicates that the former is associated with the paramagnetic behavior whereas the latter is in a slow relaxation process. Besides, the magnetic hysteresis at 295 K is observed only at a very low field, while at 5 K the same property can be observed in a rather higher field range, which indicates that weak ferromagnetic component is gradually enhanced with lowering temperature.
W.J. Feng et al. / Journal of Alloys and Compounds 425 (2006) 4–9
4. Conclusion The Cr(N)–-Cr2 N nanoparticles, produced by arc-discharge process in a mixture of argon, hydrogen, and nitrogen gases, consist of two components, Cr(N) solid–solution nanoparticles and -Cr2 N nanoparticles. The Cr(N) solid–solution nanoparticles exhibit a polyhedral shape, while the -Cr2 N nanoparticles display cuboidal one. Both the nanoparticles have the similar shell-core structure: the shell consists of chromium oxide Cr2 (CrO4 )3 or Cr O (20 at% Cr), and the complex of Cr and N O, as well as the core of Cr(N) solid–solution or -Cr2 N. The nanoparticles have a blocking temperature of 29 K and a narrow size distribution, and their weak ferromagnetic component is ascribed to the uncompensated spins on the surfaces. For the field-cooled magnetization with the field of 0.01 T, the data above 50 K are well fitted by the Curie–Weiss law, which indicates the sample is paramagnetic above the blocking temperature. This is supported by the hysteresis loops at 5 and 295 K. The paramagnetic Curie temperature of −124 K suggests that the type of dominant interaction is antiferromagnetic. Acknowledgment This work has been supported by the National Natural Science Foundation of China under Grant Nos. 50332020 and 50331030. References [1] S. Iijima, Nature (London) 354 (1991) 56. [2] S. Iijima, T. Ichihashi, Nature (London) 363 (1993) 603. [3] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, Nature (London) 318 (1985) 162. [4] Z.X. Tang, C.M. Sorensen, K.J. Klabunde, G.C. Hadjipanayis, Phys. Rev. Lett. 67 (1991) 3602.
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