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Synthesis and characterization of nano-sized nickel(II), copper(I) and zinc(II) oxide nanoparticles
Materials Science and Engineering A338 (2002) 70 /75 www.elsevier.com/locate/msea
Synthesis and characterization of nano-sized nickel(II), copper(I) and zinc(II) oxide nanoparticles S. Illy-Cherrey a, O. Tillement a, J.M. Dubois a, F. Massicot a,b, Y. Fort b, J. Ghanbaja c, S. Be´gin-Colin a,* a
Laboratoire de Science et Ge´nie des Mate´riaux Me´talliques, UMR CNRS 7584, INPL, Ecole des Mines, Parc de Saurupt, F-54042 Nancy cedex, France b Synthe`se Organique et Re´activite´, UMR 7565, UHP-Nancy I, F-54506 Vandoeuvre-les-Nancy cedex, France c Service Commun de Microscopie Electronique par Transmission UHP-Nancy I, F-54506 Vandoeuvre-les-Nancy cedex, France Received 23 July 2001; received in revised form 3 January 2002
Abstract Ultrafine, equiaxed and monodisperse oxide particles with an average grain diameter in the range of 1 /10 nm have been prepared by a two-step chemical approach: the chemical reduction of metallic salts by activated sodium hydride in tetrahydrofuran solvent, followed by oxidation of the metallic species with small amounts of O2 /N2 gas. Such particles are easily, quantitatively and reproducibly prepared and are stable on storage. The average crystallite sizes and the agglomeration of particles were estimated from dark-field transmission electron micrographs. The nature of the chemical bonding was studied by electron-energy-loss spectroscopy and structural information were obtained using selected area electron diffraction patterns. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanoparticles; Oxides; Chemical synthesis; Electron-energy-loss spectroscopy characterization
1. Introduction The fabrication of assemblies of perfect nanometer scale crystallites identically replicated in unlimited quantities is the ultimate challenge of materials research with outstanding fundamental and potential technological prospects [1]. At nanometer size, crystallites are influenced by the presence of significant numbers of surface atoms, by the quantum confinement of the electronic states [2,3] and have novel properties compared with their corresponding bulk phases [4,5]. Among the various nanomaterials, oxide nanoparticles have attracted increasing technological and industrial interest. This interest has mainly to do with their properties (optical, magnetic, electrical, and catalytic properties) associated with general characteristics such
* Corresponding author. Tel.: /33-383-58-4285; fax: /33-383-576300. E-mail address: [email protected] (S. Be´gin-Colin).
as mechanical hardness, thermal stability or chemical passivity [6]. Nickel(II) oxide present very high electrochromic performance [7], copper(I) oxide is known to exhibit characteristic optical properties at relatively low temperature due to an exciton absorption [8] and its interaction with various phonon modes [9,10] and zinc(II) oxide has mainly applications in luminescent devices, photocatalysis and photoelectrochemistry [11 / 14]. Oxide nanoparticles can be prepared in several ways, e.g. by chemical vapor deposition [15], laser vaporization [16], hydrothermal technique [17], flame pyrolysis [18], precipitation from supersaturated aqueous solution [19]. Such methods usually lead to oxide nanoparticles with relatively large size and it appears very difficult to produce particles with sizes in the range of 1/10 nm with relatively good monodispersity. Oxide nanoparticles in the less than 10 nm range are mainly produced by precipitation in a liquid medium [20 /25]. In this paper, our purpose is to propose an original chemical approach for the synthesis of useful amounts
0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 2 ) 0 0 0 5 7 - 6
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of oxide nanoparticles with size between 1 and 10 nm. The synthesis method is based on a two-step organometallic process: the reduction of metallic salts by activated alkaline hydride followed by an oxidation of the metal colloid solution.
2. Experimental details Reduction of metal ions in organic solvents by use of activated alkaline hydride is an interesting method for producing nano-scale metal particles. This organicphase processing yields nanoparticles of a wide range of metallic species with substantial control of particles size, morphologies and agglomeration. We have recently shown that stable nonagglomerated metal nanoparticles such as Ni can be prepared at low temperature by reducing metallic salts with activated alkaline hydride (NaH, t-BuONa) in tetrahydrofuran (THF) solvent [26]. These particles with size in the range of 1/4 nm, present very high specific areas and are well known to be easily oxidizable. Thus, by adding small amounts of O2 /N2 gas, metals may be transformed into their oxide forms and this original process for synthesizing nanometals could be applied to the preparation of a wide variety of oxide nanoparticles. We present in this paper the synthesis and the characterization of three different transition metal oxides: NiO, ZnO and Cu2O. Nickel acetylacetonate, copper acetylacetonate, zinc acetylacetonate, sodium hydride and other chemical products were available commercially (Aldrich products). THF solvent was dried and distilled before use. 2.1. Preparation of nano-sized oxides Nickel(II) acetylacetonate (2.56 g, 10 mmol) and sodium hydride (1.55 g, 40 mmol) were suspended under argon in 30 ml of THF solvent. A 10 ml suspension of tBuOH (1.48 g, 20 mmol in THF) was added slowly to the solution at 63 8C. The solution quasi immediately became black and some effervescence was observed. A stoichiometric H2 disengagement evolution could be measured. Thus in a first step, the nickel(II) metallic salts were reduced by activated hydride ions to form nickel in a zero-valent oxidation state. For the second step, nickel oxide was obtained by adding 660 ml of O2 / N2 gas in metallic suspension. The formation reactions are as follows: 4NaH2t-BuOH 0 2(NaH; t-BuONa)2H2 2(NaH; t-BuONa)NiX2 0 Ni ¡ 2NaX2t-BuONaH2 (0:5O2 ) NiO ¡ 0
2NaX2t-BuONa X CH3 COCHCOCH3
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For the preparation of the nano-sized copper(I) and zinc(II) oxide, the procedures were analogous to the preceding one. Concerning copper(I) oxide, only 330 ml of O2 /N2 gas was added using a syringe. It is noteworthy that for the synthesis of each oxide the same procedure may be conducted with lithium hydride (LiH) in place of NaH and with metallic salts such as chloride metallic salts (X /Cl) or acetate metallic salts (X /CH3COO) in place of acetylacetonate (X /CH3COCHCOCH3). The time of preparation is then longer. Moreover, it can be noted that acetylacetonate compounds are very easily soluble in ethereal solvent at low temperature and that they may be only reduced with NaH without t-BuOH. With this approach, larger agglomerated particles have been obtained. Thus, the presence of an activated sodium hydride (NaH, t-BuONa) is an essential condition to obtain dispersed metallic or oxide particles with size in the range 1/10 nm. The final product consist of oxides nanoparticles dispersed in organic solvent.
2.2. Characterization Characterization of nickel(II), copper(I), and Zinc(II) oxide nanoparticles was performed using electron-energy-loss spectroscopy (EELS) combined with transmission electron microscopy (TEM). This association has advantage compared with other spectroscopies [27,28]. It provides spectra recorded from a typical nanometer scale area of the specimen which can moreover be identified by imaging and diffraction modes and a rather wide energy range can be covered within one spectrum, displaying edges relevant to the different elements locally present in the sample. Consequently, a quantitative chemical analysis can be performed on such reduced areas and the electron energy-loss near-edge (ELNES) on core loss spectra can be analyzed, reflecting the nature of chemical bonds as well as the local coordination around the excited atoms. For the electron microscopy studies, a few drops of the suspension were supported on carbon coated aluminum grids and were studied using a Philips CM20 instrument operating at 200 kV with an unsaturated LaB6 cathode. EELS spectra were acquired using a GATAN 666 parallel electron energy-loss spectrometer controlled by the GATAN EL/P software. Morphology and crystallite size of the particles were estimated by TEM dark-field imaging. The average grain diameter number D of metallic particles was estimated from ak ni Di /ai ni with ni the number of particles having a diameter Di . Structural information were obtained using selectedarea electron diffraction (SAED) patterns.
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3. Results and discussion With t-BuOH, sodium hydride constitutes an activated complex (NaH, t-BuONa) partially soluble in THF, which has considerable reductive power. Anhydrous acetylacetonate metallic salts are completely soluble in ethereal solvent at low temperature. Thus, the reaction of acetylacetonate salt with activated alkaline hydride leads usually and quickly to the reduction of the transition metal cation to the metallic state. Furthermore, the low temperature of the reaction favors the formation and the stabilization of a nanocrystalline metallic phase. It is important to keep in mind that the smaller the particles are, the larger the portion of their constituent atoms located at the surface. For example, in a 1.5 nm diameter nickel particles (rNi /0.125 nm), approximately 75% of the atoms are located at the surface. In solution, with O2 /N2 gas atmosphere and without excess of sodium hydride, particles with size in this range are unstable and the metals are nearly immediately transformed to oxides. Fig. 1 shows typical dark-field transmission electron micrographs with the corresponding distributions of grain size of a nickel sample before (1a) and after oxidation with O2 /N2 gas (1b) and the SAED pattern of NiO sample (1b). Nearly, spherical crystals in the range of 1 /4 nm can be observed for Ni and NiO nanoparticles. Compared with Ni metal, no coalescence can be detected for the corresponding oxide particles. Fig. 2 shows the nickel L2,3 edge spectrum after background subtraction of Ni and NiO samples produced after chemical reduction. It consists of two white lines L3 and L2 due to the transition from 2p3/2 and 2p1/2 core states to 3d unoccupied states localized on the excited nickel ions. The white-line intensity relative to the slowly decreasing tail increases dramatically in the oxide with respect to the metal. The intensity after the L3 peak falls almost to the background level. The Ni L3 peaks appear at 854 eV and the intensity ratio I (L3/L2) is estimated to be 3.62 and 4.46 for Ni and NiO, respectively, after integration of the intensity beneath the peaks above background level. As observed in the literature, L3-to-L2 ratio of the metal was found to be lower than that of the oxide in our samples [29]. Leapman and co-workers have first emphasized the I (L3/L2) ratio measured for some 3d transition metal and their oxides [30]. Sparrow et al. have systematically integrated the I (L3/L2) ratio for several series of transition metal oxides. They have found that the I (L3/L2) ratio is related to the occupancy of the 3d orbitals on the metal ions, i.e. the ratio is maximum for the d5 configuration and decreases towards the d0 or d10 configuration [31]. Observations of the nickel SAED pattern show that although reflections of cubic face centered (fcc) nickel oxide (d111 / 0.241 nm, d200 /0.209 nm, d220 /0.147 nm, d311 /0.126 nm, and d222 /0.120 nm) are present, the lattice
Fig. 1. Dark-field transmission electron micrographs with the corresponding distributions of grain sizes for a nickel sample (a) before and (b) after oxidation with O2 /N2 gas and (b) selected area electron diffraction pattern of NiO particles.
parameter of this fcc NiO structure is aNiO /0.4179/ 0.002 nm which agrees well with the known bulk values (aNiO /0.41769 nm [32]). Fig. 3 shows typical dark-field transmission electron micrographs and the corresponding selected area electron diffraction patterns of a copper sample after chemical reduction and oxidation with O2 /N2 gas. The copper particle size distribution exhibits roughly a gaussian distribution between 2 and 18 nm. The average grain diameter Dcu was estimated to be 10 nm. The SAED pattern shows the cubic spot rings assigned to the typical copper(I) oxide, Cu2O crystal (d110 /0.302 nm, d111 /0.246 nm, d200 /0.213 nm, d220 /0.151 nm, and
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Fig. 4. Copper oxide L2,3 edge spectrum.
d311 /0.129 nm) with lattice parameters corresponding to a 0.426 9/0.002 nm which agrees with the known bulk values (aCu2O /0.42696 nm [33]). The equilibrium bulk phase of unalloyed copper oxide is the normal monoclinic CuO phase. To our knowledge, there are few
studies related to the preparation of copper(I) oxide particles [34 /36]. Fig. 4 shows the copper oxide L2,3 edge spectrum after background subtraction. The chemical shift of the L3 edge was measured to 931.3 eV and the intensity ratio I (L3/L2) estimated to be 2.78 after integration of the intensity beneath the peak above background level. Compared with Cu2 spectrum, an additional peak at 934 eV can be pointed out. This peak is characteristic of Cu species [37]. Fig. 5 shows typical dark-field transmission electron micrograph and the corresponding selected area electron diffraction pattern of a zinc sample after chemical reduction and oxidation with O2 /N2 gas. Finely dispersed zinc particles exhibit a size distribution from 0.8 to 8 nm. The average grain diameter number DZn is comparable to DNi and was estimated to be 2 nm. The SAED pattern shows the hexagonal rings corresponding to Zinc(II) oxide (d100 /0.281 nm, d002 /0.260 nm, d101 /0.247 nm, d102 0.191 nm, d110 /0.162 nm, and d103 /0.147 nm). The lattice parameters of the ZnO hcp structure are thus a /b /0.3249/0.002 nm, c /0.5209/ 0.002 nm which agrees well with the known bulk values
Fig. 3. Dark-field transmission electron micrograph and corresponding SAED pattern of a Cu2O sample.
Fig. 5. Dark-field transmission electron micrograph and corresponding SAED pattern of ZnO sample.
Fig. 2. Nickel L2,3 edge spectrum of Ni and NiO samples produced after chemical reduction.
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References
Fig. 6. Zinc oxide L2,3 edge spectrum.
(aZnO /0.349 nm, cZnO /0.5205 nm [38]). Fig. 6 shows the zinc oxide L2,3 edge spectrum after background subtraction. The chemical shift of the L3 edge was measured to 1021 eV. No intense sharp white lines can be observed, corresponding to a filled 3d band. Only steps are present corresponding mainly to transitions to the relatively flat region of the density of states with some d character.
4. Conclusion Stable suspensions of nickel(II), copper(I), zinc(II) oxide clusters with average sizes in the range of 1 /10 nm were synthesized by a two-step chemical process at low temperature: the reduction of metallic salts, followed by the oxidation of metallic species with O2 /N2 gas. After the oxidation step, no significant coalescence phenomenon can be detected. Nickel(II) and zinc(II) oxides have grain sizes in the range 1 /4 and 0.8 /8 nm, respectively, with an average grain size similar to that of the initial metallic species (around 2 nm). The copper(I) oxide particles display grain sizes in the range 2/18 nm with an average grain diameter of 10 nm. The quantitative and reproducible chemical approach in organic solvent allows the control of the mean particle size, morphology and stability in organic solvent. Using a relatively simple apparatus, it can be applied at low temperature to the synthesis of numerous finely and stable oxides-simple or mixed-of transition metals which are crucial for industrial applications.
Acknowledgements The authors acknowledge ANVAR, for supporting this work under contract number A 97 08 075 LATTC.
[1] H. Fendler Janos, Nanoparticles and Nanostructured Films, Preparation, Characterization and Applications, Wiley/VCH, Weinheim, 1998. [2] P. Jena, S.N. Khanna, B.K. Rao, Proceedings of Science and Technology of Atomically Engineered Material, World Scientific, London, 1996. [3] Y. Volokitin, J. Sinzig, L.J. de Jongh, G. Schmid, M.N. Vargaftik, I.I. Moiseev, Nature 384 (1996) 621. [4] P.J. Reynolds, On Cluster and Clustering, Elsevier, Amsterdam, 1993. [5] J. Shi, S. Gider, K. Babcock, D.D. Awschalom, Science 271 (1996) 937. [6] J.P. Jolivet, De la solution a` l’oxyde, condensation des cations en solution aqueuse, Savoirs Actuels, Intereditions, CNRS Editions, Paris, 1994. [7] X. Chen, X. Hu, J. Feng, Nanostruct. Mater. 6 (1995) 309. [8] M. Hayashi, K. Katsuki, J. Phys. Soc. Jpn. 7 (1952) 599. [9] R.A. Forman, W.S. Brower, H.S. Parker, Phys. Lett. A36 (1971) 395. [10] A. Campaan, H.Z. Cummins, Phys. Rev. B23 (1972) 597. [11] D.W. Bahnemann, C. Kormann, M.R. Hofmann, J. Phys. Chem. 91 (1987) 3789. [12] V. Koch, A. Fojtik, H. Weller, A. Henglein, Chem. Phys. Lett. 122 (1985) 507. [13] M. Haase, H. Weller, A. Henglein, J. Phys. Chem. 92 (1988) 482. [14] L. Spanhel, M.A. Anderson, J. Am. Chem. Soc. 113 (1991) 2826. [15] D. Shigehito, K. Akamatsu, T. Yano, M. Mizuhata, A. Kajinami, J. Mater. Chem. 818 (1998) 1865. [16] M.S.El. Shall, D. Graiver, U. Pernisz, M.J. Baraton, Nanostruct. Mater. 6 (1995) 297. [17] C. Wang, Y. Hu, G. Yan, Zhao, Nanostruct. Mater. 7 (1996) 421. [18] C. Herard, P. Bowen, J. Lemaıˆtre, J. Dutta, Nanostruct. Mater. 6 (1995) 316. [19] G.W. Kriechbaum, P. Kleinschmit, Adv. Mater. 10 (1989) 330. [20] D. Gallaghar, W. Heady, J. Racy, R. Bhargava, J. Mater. Res. 10 (1994) 870. [21] K.C. Song, Y. Kang, Mater. Lett. 42 (2000) 283. [22] J.J. Zhu, Z.H. Lu, S.T. Aruna, D. Aurbach, A. Gedanken, Chem. Mater. 12 (2000) 2557. [23] H. Schmidt, Appl. Organomet. Chem. 15 (2001) 331. [24] C.L. Carnes, K.J. Klabunde, Langmuir 16 (2000) 3764. [25] G. Wakefield, H.A. Keron, P.J. Dobson, J.L. Hutchison, J. Phys. Chem. Solids 60 (1999) 503. [26] S. Illy, O. Tillement, F. Machizaud, J.M. Dubois, F. Massicot, Y. Fort, J. Ghanbaja, Philos. Mag. A79 (1999) 1021. [27] R.F. Egerton, EELS in the Electron Microscope, Plenum, New York, 1986. [28] C. Colliex, in: R. Barer, V.E. Cosslett (Eds.), Advances in Optical and Electron Microscopy, Academic Press, New York, 1984, p. 9. [29] W.G. Waddington, P. Rez, I.P. Grant, C.J. Humphreys, Phys. Rev. B34 (1986) 1467. [30] R.D. Leapman, L.A. Grunes, P.L. Fejes, Phys. Rev. B26 (1982) 614. [31] T.G. Sparrow, B.G. Williams, C.N.R. Rao, J.M. Thomas, Chem. Phys. Lett. 108 (1984) 547. [32] Powder diffraction file, JCPDS N8 4-835, ISSN 1084 /3116, ICDD, 1998. [33] Powder diffraction file, JCPDS N8 5-667, ISSN 1084 /3116, ICDD, 1988 [34] R.J. Aranjo, J. Butty, N. Peyghambarian, App. Phys. Lett. 68 (1996) 584.
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[37] M. Grioni, J.B. Goedkoop, R. Schoorl, F.M. de Groot, J.C. Fuggle, F. Scha¨fers, E.E. Koch, G. Rossi, J.M. Estera, R.C. Karnatak, Phys. Rev. B39 (1989) 1541. [38] Powder diffraction file, JCPDS N8 5-664, ISSN 1084-3116, ICDD, 1998.
Synthesis and characterization of nano-sized nickel(II), copper(I) and zinc(II) oxide nanoparticles S. Illy-Cherrey a, O. Tillement a, J.M. Dubois a, F. Massicot a,b, Y. Fort b, J. Ghanbaja c, S. Be´gin-Colin a,* a
Laboratoire de Science et Ge´nie des Mate´riaux Me´talliques, UMR CNRS 7584, INPL, Ecole des Mines, Parc de Saurupt, F-54042 Nancy cedex, France b Synthe`se Organique et Re´activite´, UMR 7565, UHP-Nancy I, F-54506 Vandoeuvre-les-Nancy cedex, France c Service Commun de Microscopie Electronique par Transmission UHP-Nancy I, F-54506 Vandoeuvre-les-Nancy cedex, France Received 23 July 2001; received in revised form 3 January 2002
Abstract Ultrafine, equiaxed and monodisperse oxide particles with an average grain diameter in the range of 1 /10 nm have been prepared by a two-step chemical approach: the chemical reduction of metallic salts by activated sodium hydride in tetrahydrofuran solvent, followed by oxidation of the metallic species with small amounts of O2 /N2 gas. Such particles are easily, quantitatively and reproducibly prepared and are stable on storage. The average crystallite sizes and the agglomeration of particles were estimated from dark-field transmission electron micrographs. The nature of the chemical bonding was studied by electron-energy-loss spectroscopy and structural information were obtained using selected area electron diffraction patterns. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanoparticles; Oxides; Chemical synthesis; Electron-energy-loss spectroscopy characterization
1. Introduction The fabrication of assemblies of perfect nanometer scale crystallites identically replicated in unlimited quantities is the ultimate challenge of materials research with outstanding fundamental and potential technological prospects [1]. At nanometer size, crystallites are influenced by the presence of significant numbers of surface atoms, by the quantum confinement of the electronic states [2,3] and have novel properties compared with their corresponding bulk phases [4,5]. Among the various nanomaterials, oxide nanoparticles have attracted increasing technological and industrial interest. This interest has mainly to do with their properties (optical, magnetic, electrical, and catalytic properties) associated with general characteristics such
* Corresponding author. Tel.: /33-383-58-4285; fax: /33-383-576300. E-mail address: [email protected] (S. Be´gin-Colin).
as mechanical hardness, thermal stability or chemical passivity [6]. Nickel(II) oxide present very high electrochromic performance [7], copper(I) oxide is known to exhibit characteristic optical properties at relatively low temperature due to an exciton absorption [8] and its interaction with various phonon modes [9,10] and zinc(II) oxide has mainly applications in luminescent devices, photocatalysis and photoelectrochemistry [11 / 14]. Oxide nanoparticles can be prepared in several ways, e.g. by chemical vapor deposition [15], laser vaporization [16], hydrothermal technique [17], flame pyrolysis [18], precipitation from supersaturated aqueous solution [19]. Such methods usually lead to oxide nanoparticles with relatively large size and it appears very difficult to produce particles with sizes in the range of 1/10 nm with relatively good monodispersity. Oxide nanoparticles in the less than 10 nm range are mainly produced by precipitation in a liquid medium [20 /25]. In this paper, our purpose is to propose an original chemical approach for the synthesis of useful amounts
0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 2 ) 0 0 0 5 7 - 6
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of oxide nanoparticles with size between 1 and 10 nm. The synthesis method is based on a two-step organometallic process: the reduction of metallic salts by activated alkaline hydride followed by an oxidation of the metal colloid solution.
2. Experimental details Reduction of metal ions in organic solvents by use of activated alkaline hydride is an interesting method for producing nano-scale metal particles. This organicphase processing yields nanoparticles of a wide range of metallic species with substantial control of particles size, morphologies and agglomeration. We have recently shown that stable nonagglomerated metal nanoparticles such as Ni can be prepared at low temperature by reducing metallic salts with activated alkaline hydride (NaH, t-BuONa) in tetrahydrofuran (THF) solvent [26]. These particles with size in the range of 1/4 nm, present very high specific areas and are well known to be easily oxidizable. Thus, by adding small amounts of O2 /N2 gas, metals may be transformed into their oxide forms and this original process for synthesizing nanometals could be applied to the preparation of a wide variety of oxide nanoparticles. We present in this paper the synthesis and the characterization of three different transition metal oxides: NiO, ZnO and Cu2O. Nickel acetylacetonate, copper acetylacetonate, zinc acetylacetonate, sodium hydride and other chemical products were available commercially (Aldrich products). THF solvent was dried and distilled before use. 2.1. Preparation of nano-sized oxides Nickel(II) acetylacetonate (2.56 g, 10 mmol) and sodium hydride (1.55 g, 40 mmol) were suspended under argon in 30 ml of THF solvent. A 10 ml suspension of tBuOH (1.48 g, 20 mmol in THF) was added slowly to the solution at 63 8C. The solution quasi immediately became black and some effervescence was observed. A stoichiometric H2 disengagement evolution could be measured. Thus in a first step, the nickel(II) metallic salts were reduced by activated hydride ions to form nickel in a zero-valent oxidation state. For the second step, nickel oxide was obtained by adding 660 ml of O2 / N2 gas in metallic suspension. The formation reactions are as follows: 4NaH2t-BuOH 0 2(NaH; t-BuONa)2H2 2(NaH; t-BuONa)NiX2 0 Ni ¡ 2NaX2t-BuONaH2 (0:5O2 ) NiO ¡ 0
2NaX2t-BuONa X CH3 COCHCOCH3
71
For the preparation of the nano-sized copper(I) and zinc(II) oxide, the procedures were analogous to the preceding one. Concerning copper(I) oxide, only 330 ml of O2 /N2 gas was added using a syringe. It is noteworthy that for the synthesis of each oxide the same procedure may be conducted with lithium hydride (LiH) in place of NaH and with metallic salts such as chloride metallic salts (X /Cl) or acetate metallic salts (X /CH3COO) in place of acetylacetonate (X /CH3COCHCOCH3). The time of preparation is then longer. Moreover, it can be noted that acetylacetonate compounds are very easily soluble in ethereal solvent at low temperature and that they may be only reduced with NaH without t-BuOH. With this approach, larger agglomerated particles have been obtained. Thus, the presence of an activated sodium hydride (NaH, t-BuONa) is an essential condition to obtain dispersed metallic or oxide particles with size in the range 1/10 nm. The final product consist of oxides nanoparticles dispersed in organic solvent.
2.2. Characterization Characterization of nickel(II), copper(I), and Zinc(II) oxide nanoparticles was performed using electron-energy-loss spectroscopy (EELS) combined with transmission electron microscopy (TEM). This association has advantage compared with other spectroscopies [27,28]. It provides spectra recorded from a typical nanometer scale area of the specimen which can moreover be identified by imaging and diffraction modes and a rather wide energy range can be covered within one spectrum, displaying edges relevant to the different elements locally present in the sample. Consequently, a quantitative chemical analysis can be performed on such reduced areas and the electron energy-loss near-edge (ELNES) on core loss spectra can be analyzed, reflecting the nature of chemical bonds as well as the local coordination around the excited atoms. For the electron microscopy studies, a few drops of the suspension were supported on carbon coated aluminum grids and were studied using a Philips CM20 instrument operating at 200 kV with an unsaturated LaB6 cathode. EELS spectra were acquired using a GATAN 666 parallel electron energy-loss spectrometer controlled by the GATAN EL/P software. Morphology and crystallite size of the particles were estimated by TEM dark-field imaging. The average grain diameter number D of metallic particles was estimated from ak ni Di /ai ni with ni the number of particles having a diameter Di . Structural information were obtained using selectedarea electron diffraction (SAED) patterns.
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3. Results and discussion With t-BuOH, sodium hydride constitutes an activated complex (NaH, t-BuONa) partially soluble in THF, which has considerable reductive power. Anhydrous acetylacetonate metallic salts are completely soluble in ethereal solvent at low temperature. Thus, the reaction of acetylacetonate salt with activated alkaline hydride leads usually and quickly to the reduction of the transition metal cation to the metallic state. Furthermore, the low temperature of the reaction favors the formation and the stabilization of a nanocrystalline metallic phase. It is important to keep in mind that the smaller the particles are, the larger the portion of their constituent atoms located at the surface. For example, in a 1.5 nm diameter nickel particles (rNi /0.125 nm), approximately 75% of the atoms are located at the surface. In solution, with O2 /N2 gas atmosphere and without excess of sodium hydride, particles with size in this range are unstable and the metals are nearly immediately transformed to oxides. Fig. 1 shows typical dark-field transmission electron micrographs with the corresponding distributions of grain size of a nickel sample before (1a) and after oxidation with O2 /N2 gas (1b) and the SAED pattern of NiO sample (1b). Nearly, spherical crystals in the range of 1 /4 nm can be observed for Ni and NiO nanoparticles. Compared with Ni metal, no coalescence can be detected for the corresponding oxide particles. Fig. 2 shows the nickel L2,3 edge spectrum after background subtraction of Ni and NiO samples produced after chemical reduction. It consists of two white lines L3 and L2 due to the transition from 2p3/2 and 2p1/2 core states to 3d unoccupied states localized on the excited nickel ions. The white-line intensity relative to the slowly decreasing tail increases dramatically in the oxide with respect to the metal. The intensity after the L3 peak falls almost to the background level. The Ni L3 peaks appear at 854 eV and the intensity ratio I (L3/L2) is estimated to be 3.62 and 4.46 for Ni and NiO, respectively, after integration of the intensity beneath the peaks above background level. As observed in the literature, L3-to-L2 ratio of the metal was found to be lower than that of the oxide in our samples [29]. Leapman and co-workers have first emphasized the I (L3/L2) ratio measured for some 3d transition metal and their oxides [30]. Sparrow et al. have systematically integrated the I (L3/L2) ratio for several series of transition metal oxides. They have found that the I (L3/L2) ratio is related to the occupancy of the 3d orbitals on the metal ions, i.e. the ratio is maximum for the d5 configuration and decreases towards the d0 or d10 configuration [31]. Observations of the nickel SAED pattern show that although reflections of cubic face centered (fcc) nickel oxide (d111 / 0.241 nm, d200 /0.209 nm, d220 /0.147 nm, d311 /0.126 nm, and d222 /0.120 nm) are present, the lattice
Fig. 1. Dark-field transmission electron micrographs with the corresponding distributions of grain sizes for a nickel sample (a) before and (b) after oxidation with O2 /N2 gas and (b) selected area electron diffraction pattern of NiO particles.
parameter of this fcc NiO structure is aNiO /0.4179/ 0.002 nm which agrees well with the known bulk values (aNiO /0.41769 nm [32]). Fig. 3 shows typical dark-field transmission electron micrographs and the corresponding selected area electron diffraction patterns of a copper sample after chemical reduction and oxidation with O2 /N2 gas. The copper particle size distribution exhibits roughly a gaussian distribution between 2 and 18 nm. The average grain diameter Dcu was estimated to be 10 nm. The SAED pattern shows the cubic spot rings assigned to the typical copper(I) oxide, Cu2O crystal (d110 /0.302 nm, d111 /0.246 nm, d200 /0.213 nm, d220 /0.151 nm, and
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Fig. 4. Copper oxide L2,3 edge spectrum.
d311 /0.129 nm) with lattice parameters corresponding to a 0.426 9/0.002 nm which agrees with the known bulk values (aCu2O /0.42696 nm [33]). The equilibrium bulk phase of unalloyed copper oxide is the normal monoclinic CuO phase. To our knowledge, there are few
studies related to the preparation of copper(I) oxide particles [34 /36]. Fig. 4 shows the copper oxide L2,3 edge spectrum after background subtraction. The chemical shift of the L3 edge was measured to 931.3 eV and the intensity ratio I (L3/L2) estimated to be 2.78 after integration of the intensity beneath the peak above background level. Compared with Cu2 spectrum, an additional peak at 934 eV can be pointed out. This peak is characteristic of Cu species [37]. Fig. 5 shows typical dark-field transmission electron micrograph and the corresponding selected area electron diffraction pattern of a zinc sample after chemical reduction and oxidation with O2 /N2 gas. Finely dispersed zinc particles exhibit a size distribution from 0.8 to 8 nm. The average grain diameter number DZn is comparable to DNi and was estimated to be 2 nm. The SAED pattern shows the hexagonal rings corresponding to Zinc(II) oxide (d100 /0.281 nm, d002 /0.260 nm, d101 /0.247 nm, d102 0.191 nm, d110 /0.162 nm, and d103 /0.147 nm). The lattice parameters of the ZnO hcp structure are thus a /b /0.3249/0.002 nm, c /0.5209/ 0.002 nm which agrees well with the known bulk values
Fig. 3. Dark-field transmission electron micrograph and corresponding SAED pattern of a Cu2O sample.
Fig. 5. Dark-field transmission electron micrograph and corresponding SAED pattern of ZnO sample.
Fig. 2. Nickel L2,3 edge spectrum of Ni and NiO samples produced after chemical reduction.
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References
Fig. 6. Zinc oxide L2,3 edge spectrum.
(aZnO /0.349 nm, cZnO /0.5205 nm [38]). Fig. 6 shows the zinc oxide L2,3 edge spectrum after background subtraction. The chemical shift of the L3 edge was measured to 1021 eV. No intense sharp white lines can be observed, corresponding to a filled 3d band. Only steps are present corresponding mainly to transitions to the relatively flat region of the density of states with some d character.
4. Conclusion Stable suspensions of nickel(II), copper(I), zinc(II) oxide clusters with average sizes in the range of 1 /10 nm were synthesized by a two-step chemical process at low temperature: the reduction of metallic salts, followed by the oxidation of metallic species with O2 /N2 gas. After the oxidation step, no significant coalescence phenomenon can be detected. Nickel(II) and zinc(II) oxides have grain sizes in the range 1 /4 and 0.8 /8 nm, respectively, with an average grain size similar to that of the initial metallic species (around 2 nm). The copper(I) oxide particles display grain sizes in the range 2/18 nm with an average grain diameter of 10 nm. The quantitative and reproducible chemical approach in organic solvent allows the control of the mean particle size, morphology and stability in organic solvent. Using a relatively simple apparatus, it can be applied at low temperature to the synthesis of numerous finely and stable oxides-simple or mixed-of transition metals which are crucial for industrial applications.
Acknowledgements The authors acknowledge ANVAR, for supporting this work under contract number A 97 08 075 LATTC.
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