Grain growth of CuO nanocrystal activated by high energy ball milling

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Physica B 389 (2007) 135–139 www.elsevier.com/locate/physb

Grain growth of CuO nanocrystal activated by high energy ball milling A.E. Bianchia,b,, S.J. Stewartb, G. Puntea,b, R. Vin˜aa, T.S. Plivelicc, I.L. Torrianic,d a

LANADI, Departamento de Fı´sica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CC67, 1900 La Plata, Argentina b IFLP, Departamento de Fı´sica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CC67, 1900 La Plata, Argentina c Laborato´rio Nacional de Luz Sı´ncrotron (LNLS), Campinas, SP, Brazil d Instituto de Fı´sica (IFGW)-UNICAMP, Campinas, SP, Brazil

Abstract X-ray Diffraction (XRD), small-angle X-ray scattering, scanning electron microscopy and energy dispersive X-ray Analysis were used to investigate the effect of controlled high energy ball milling (HEBM) on the average volume weighted crystallite size, /DSV and weighted average microstrain, /eS, of nanostructures of CuO prepared by solid state reaction. The starting material, S0, consists of almost strain free nanocrystals of monoclinic CuO with /DSV E 20nm , as determined by XRD data Rietveld analysis. It was found that after an initial decrease of /DSV and increase of /eS, the values of these parameters go through a steady-state stage followed by an increase of an order of magnitude in /DS after a period of only 120 m of HEBM. According to the results here presented, the presence of small amounts of contaminants in the starting material can have an influence on the kinetics of crystal growth in HEBM CuO. r 2006 Elsevier B.V. All rights reserved. PACS: 61.10.Eq; 61.82.Rx; 68.37.Hk; 81.20.Wk Keywords: High energy ball milling; CuO; Nanoparticles; SAXS

1. Introduction High energy ball milling (HEBM) is a cheap and multipurpose method frequently employed to produce nanocrystalline materials that are characterized by crystallite sizes less than 100 nm [1]. The unique microstructure of these materials has shown to provide them with exceptional properties. As a consequence, they have been attracting wide attention in materials research. Many investigations performed in the last several years have addressed the characteristics of the grain sizes obtained from HEBM [1–5]. Oleszak and Shingu [2] found out that in metals the crystallite size decreases with milling time, leading to a minimum value. Other authors [3, 4] have tried to correlate minimum grain size with different material characteristics like melting temperature, crystal structure class, hardness and dislocation activity. There is a clear consensus on Corresponding author. LANADI, Departamento de Fı´ sica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CC67, 1900 La Plata, Argentina. Tel.: 54 221 4246062; fax: 54 221 4252006. E-mail address: bianchi@venus.fisica.unlp.edu.ar (A.E. Bianchi).

0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.07.040

milling as a means of achieving nanoscale refining of all solid elements and it is known that there exists a minimum grain size that can be reached, but the reasons for this practical limit are still a matter of study. Low milling temperature has been shown to help in the production of smaller grain sizes ([1, 6] and references therein). On the other hand, it has been demonstrated that HEBM brittle compounds, with initial average crystallite size below 10 nm, increases crystallite size ([7] and references therein). Karagedov and Lyakhov ([8] and references therein) observed that grinding of inorganic oxides as a function of grinding intensity or grinding time leads to a minimum crystallite size. Further increase in the energy of the powder treatment (or increase in the duration of treatment) would cause an increase in particle size, these authors do not state the brittleness of the studied samples. In an attempt to contribute to the understanding of the processes that are at play during HEBM of different compounds we have prepared and submitted to HEBM nanocystals of CuO. This compound was chosen because of the intrinsic interest of the cupric oxide nanocrystals in different fields (like magnetism, catalysis and gas sensing). The kinetics of

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crystallite growth was studied as a function of grinding time, tm. From previous studies we knew that CuO is less brittle than hematite and that the evolution of mean crystallite size and average strain as a function of tm are different [9,10]. In this work the HEBM process was performed at room temperature (r.t.) and designed to avoid excess of temperature within the vial. The results are presented and discussed below. 2. Experimental The nanocrystalline CuO sample used as starting material, S0, was synthesized by one-step solid state reaction at r.t. following Xu et al. [11]. Samples Si, i ¼ 14 were produced by HEBM of S0, in a vibratory horizontal miller (Retsch) with stainless-steel vial and ball (mass-to-powder ratio 10:1), for different periods of time: 15 min, S1; 30 min, S2; 60 min, S3, and 120 min, S4. To avoid large temperature rise within the vial, which could bias the results, the process was performed in cycles consisting of 15 min of milling followed by 15 min of rest. The XRD, shown in Fig. 1, were obtained in a Phillips PW-1710 diffractometer. Data were collected in the 201p2yp1201 range, in 0.021 step scans with counting rates no less than 15 s per step, using monochromatic CuKa radiation. Smallangle X-ray scattering (SAXS) experiments were performed

at the D11A SAXS workstation of the Brazilian Synchrotron Light Source (LNLS), Campinas, Brazil [12]. The samples were placed in 0.3 mm thick cells with 30 mm mica windows. The exposures were made using a vacuum chamber directly connected to the beam path. The wavelength used was 1.757 A˚. The range of q values detected in the experiments was 0.015pqp0.5 A˚1. All measurements were made at r.t., using a linear position sensitive detector. SAXS data treatment was performed using the program TRAT1D [13], which corrects the intensity values for sample absorption, background contribution and detector response. Possible sample contamination was controlled by energy dispersive spectrometry (EDS) in a Philips 505 scanning electron microscope (SEM), working at 20 kV, equipped with an ultrathin window EDAX detector (S-UTW). 3. Results and discussion Rietveld analysis of the XRD data was performed using the program Fullprof [14], the initial model for CuO was that proposed by A˚sbrink and Norrby [15] from singlecrystal results. Modified pseudo-Voigt functions were found to provide the best fit for the line profiles. The structural parameters of the crystallites in all the samples studied were obtained and are presented in Table 1. Fig. 2 shows a typical Rietveld analysis fit. The existence of superimposed reflections in the diffraction patterns, does not lead to a single valued peak deconvolution of XRD data. Then, the average volume-weighted crystallite size /DSv and the weighted average strain /eS were determined from the Rietveld refinement using Desai and Young approximation [16]. All SAXS curves in Fig. 3a, except the one corresponding to S4 (Fig. 3b), exhibit a power law behavior: I(q) ¼ Kqa, with 3pap4, for a range of q of approximately one decade. This behavior of the scattered intensity can be attributed to the surface fractal nature of the nanoparticle system. At large q values the curves follow Porod’s law [17] as can be seen in Fig. 3a, indicating a sharp interface for the scattering particles. Table 2 shows the evolution of /DSV, /eS and a values as a function of HEBM time. As observed when milling coarse commercial powders [18] there is a decrease of /DSV and a values and an increase of /eS value, after 15 min of milling. Further milling times (samples S2 and S3) cause a fluctuation of the 3 magnitudes around S1 values.

Table 1 Cell parameters for all samples, obtained after Rietveld refinement a (A˚)

V (A˚3)

Fig. 1. XRD patterns for as-synthesized sample, S0, and for samples obtained by HEBM S0, S1–4.

S0 S1 S2 S3 S4

8.365 81.479 81.484 81.473 81.323

(1) (1) (1) (1) (1)

4.6902 4.6815 4.6809 4.6816 4.8067

(1) (3) (3) (3) (3)

b (A˚)

c (A˚)

b (1)

3.4221 (1) 3.4325 (2) 3.4334 (2) 3.4328 (2) 3.42850 (2)

5.1381 (2) 5.1369 (2) 5.1365 (4) 5.1361 (3) 5.13599 (3)

99.39 99.23 99.23 99.24 99.36

(1) (1) (1) (1) (1)

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Fig. 2. Typical Rietveld refinement analysis for S3.

100

100 S1

Intensity (Arb.unit.)

S0 α = 3.38 10–4

0.01

α = 3.38 10–4 0.01

0.1

0.1

100

100

S3

S2

α = 3.38

α = 3.38 10–4 0.01

0.1

10–4 0.01

0.1

q (Å–1)

Intensity (Arb.unit.)

104

S4 101

10–2 10–1 q (Å–1) Fig. 3. SAXS curves (a) from as-synthesized sample and for samples S1–3, (b) from sample obtained after 120 min of HEBM, S4.

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However, after 120 min of milling a sudden change in that trend is observed. Indeed, S4 shows a mean crystallite size of about one order of magnitude larger than that of S3, a smaller /eS value, similar to that of S0, and a SAXS curve that departs from the power law behavior. EDAX data show no contamination in all the samples but S4. In this sample a small quantity of Cl, 1.0(1)%, was detected. The morphology of the as-prepared particles and of the S1 sample at different resolutions can be seen from SEM pictures exhibited in Fig. 4. In our previous studies on HEBM-treated coarse commercial CuO [18] we have shown that an increase in tm induces /DSV and a reduction and /eS enhancement, and that these parameters reached stable values for 360otmp540 min. The values of the 3 magnitudes have been found to depend on the quantity of powder treated. The observed changes in /DSV and /eS seems to be consistent with the milling process model proposed by Fecht et al. [19]. According to the aforementioned authors, who first described the nanocrystallization by mechanical attrition, there are 3 stages during the mechanical grinding: the localization of Table 2 Average volume-weighted crystallite size, /DSV, weighted average strain, /eS and exponent of the power exponential, a, for all samples

S0 S1 S2 S3 S4

tm (m)

/DSV (nm)

/eS

0 15 30 66 120

21 14 15 14 137

0.007 0.012 0.012 0.012 0.008

(1) (1) (1) (1) (3)

a (1) (1) (1) (1) (1)

3.385 3.298 3.261 3.234 —

(3) (4) (5) (7)

deformations in shear bands, the formation of cell structures (or subgrains), within the nanoscale range, with dense dislocation arrays forming into subgrain boundaries and the appearance of high-angle grain boundaries. Recent results [20] on cryomilled Zn would indicate that highangle nanocrystalline grains can also be formed by a dynamic recrystallization route, which takes place when the dislocation density due to strain hardening reaches a critical level, rather than in the 3 stages proposed by Fecht et al. According to those cryomilling results, grain growth might be expected, and has been detected in HEBM nanocrystals of brittle materials [7]. In the present study we not only observed CuO increase in crystallite size raise but also found that the grinding time necessary to reach a /DSV value 10 times that of the minimum was rather short, less than 120 m. In addition the kinetics of the grain growth as a function tm differs from that of ZnS, hematite and other oxides reported in the literature [7,8]. The presence of Cl in S4 along with the change in the SAXS intensity curve going from S3 to S4, lead us to investigate the possibility of a second phase segregation helping the fast grain size increase and strain reduction. Very low intensity peaks in the XRD pattern of sample S4, could be attributed to the most intense peaks of atacamite, Cu2Cl (OH)3. The lack of Cl detection in samples S0–3, probably was due to a very low level of contamination that makes the signal undistinguishable from the background. Nanocrystalline samples prepared previously from solid state reaction proved to be strain free [18]. S0 contamination with Cl could have also been the cause of the small

Fig. 4. SEM for S0 and S1.

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average microstrain detected in this sample (Table 1). The discrepancies found between present and previous [21] results can be explained as due to the presence of a second proto-phase in S0–3 and to a second phase segregation in S4. Taking into account Cl contamination, it is, then, concluded that this impurity could have induced faster crystallite growth as a function of tm producing results which are different from those previously reported [21]. Acknowledgments Thanks are due to the financing agencies CAPES, SeTCIP, CNPq, CLAF and CONICET as well as the Brazilian National Synchrotron Laboratory (LNLS). AEB thanks UNLP for a research fellowship. References [1] C.C. Koch, Rev. Adv. Mater. Sci. (2003) 91. [2] D. Oleszak, P.H. Shingu, J. Appl. Phys. 79 (6) (1996) 2975. [3] J. Eckert, J.C. Holzer, C.E. Kill III, W.L. Johnson, J. Mater. Res. 7 (1992) 1751. [4] C.C. Koch, Nanostruct. Mater. 9 (1997) 13. [5] F.A. Mohamed, Y. Xun, Mater. Sci. Eng. A 354 (2003) 133. [6] D.B. Witkin, E.J. Lavernia, Prog. Mater. Sci. 51 (2006) 1.

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[7] S.J. Stewart, M.J. Tueros, G. Cernicchiaro, R.B Scorzelli, Solid State Commun. 129 (2004) 347. [8] G.R. Karagedov, N.Z. Lyakhov, KONA 21 (2003) 76. [9] S.J. Stewart, R.A. Borzi, G. Punte, R.C. Mercader, F. Garcı´ a, J. Phys. Condens. Matter 13 (2001) 1743. [10] S.J. Stewart, R.A. Borzi, E.D. Cabanillas, G. Punte, R.C. Mercader, J. Magn. Magn. Mater. 260 (2003) 447. [11] J.F. Xu, et al., J. Solid State Chem. 147 (1999) 516. [12] G. Kellermann, F. Vicentin, E. Tamura, M. Rocha, H. Tolentino, A. Barbosa, A. Craievich, I. Torriani, J Appl. Crystallogr. 30 (1997) 880. [13] C.L.P. Oliveira, TRAT1D —computer program for SAXS data treatment, LNLS Technical Manual MT 01/2003, Campinas, Brazil, 2003. [14] J. Rodriguez-Carvajal, M.T. Fernandez-Diaz, J.L. Martinez, J. Phys.: Condens. Matter 3 (1991) 3215. [15] S. A˚sbrink, L.J. Norrby, Acta Crystallogr. Sect. B 26 (1970) 8. [16] R.A. Young, The Rietvel Method, Oxford University Press, Oxford, 1993. [17] O. Glatter, O. Kratky, Small-Angle X-Ray Scattering, Academic Press, New York, 1982. [18] A.E. Bianchi, S. Stewart, G. Punte, T.S. Plivelic, I.L.Torriani, Activity Report No. LNLS 2001, 2002, pp. 301–302. [19] H.J. Fecht, E. Hellstern, Z. Fu, W.L. Johnson, Metall. Trans. A 21 (1990) 2333. [20] X. Zhang, H. Wang, R.O. Scattergood, J. Narayan, C.C. Koch, Acta. Mater. 50 (2002) 3995. [21] Ana. E. Bianchi, S.J. Stewart, G. Punte, T.S. Plivelic, I.L. Torriani, Activity Report No. LNLS 2002, 2003, 131.