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Synthesis of Cu2Mo6S8 powders and thin films from intermediate oxides prepared by polymeric precursor method
Solid State Sciences 14 (2012) 719e724
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Synthesis of Cu2Mo6S8 powders and thin films from intermediate oxides prepared by polymeric precursor method S. Boursicot, V. Bouquet*, I. Péron, T. Guizouarn, M. Potel, M. Guilloux-Viry Sciences Chimiques de Rennes, UMR 6226 CNRS, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 July 2011 Received in revised form 31 January 2012 Accepted 18 March 2012 Available online 30 March 2012
Powders and thin films of the copper molybdenum sulfide Cu2Mo6S8 were synthesized from intermediate oxides prepared by polymeric precursor method based on Pechini process. In the case of the thin films, deposition was performed onto R-plane sapphire single crystal by spin coating. The influence of temperature and duration of the 3 step heat treatment cycle (calcination, sulfurization and reduction) were investigated to optimize the synthesis conditions. The first step of calcination under air atmosphere performed for 3 h at 450 C and 400 C is suitable to obtain the intermediate oxides powders and thin films, respectively. The sulfurization treatment at 600 C for 2 h under H2S/H2 gas flow followed by reduction at 650 C for 4 h under H2 gas flow allowed to obtain Cu2Mo6S8 in powder or thin film form. In the last case, a multilayer process led to dense and homogeneous films. Moreover, the insertion and superconducting behaviour of the final powders allowed to validate the Cu2Mo6S8 synthesis by this moderate temperature process. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Chevrel phases CuxMo6S8 Sulfides Insertion material Polymeric precursors Thin films
1. Introduction The Chevrel phases (called hereafter CP) of general formula MxMo6X8 where M is a cation and X is a chalcogen (S, Se or Te) represent a large variety of compounds intensively studied for their unusual properties such as superconductivity and high ionic mobility [1e4]. The channels formed by the 3D arrangement of the Mo6X8 clusters receive the metallic cations and according to the nature, the size and oxidation degree of these ones, the CP present different electronic and magnetic properties [5]. As an example, the lead molybdenum sulfide PbMo6S8 was intensively studied for its superconducting properties to be used in coils at high magnetic field [6,7]. The CP with small cations such as Ni2Mo6S8 and LixMo6S8 were studied for heterogeneous catalysis [8] and electrodes material for secondary battery [9] due to their structure favourable for intercalation/de-intercalation reactions. Another example is the copper molybdenum sulfide CuxMo6S8 which presents both superconducting and insertion properties [10,11]. The synthesis of these compounds can be performed by solid state reaction from the elements or binaries (for example, Mo, MoS2 and Cu or CuS are used to prepare Cu2Mo6S8) but it requires high temperature (900e1100 C) and long time of reaction [12]. Ternary
* Corresponding author. Tel.: þ33 2 23 23 56 56; fax: þ33 2 23 23 67 99. E-mail address: [email protected] (V. Bouquet). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2012.03.013
metal sulfides can be prepared by sulfurization of ternary oxide precursors [13] but it requires also high temperatures and the preparation of Chevrel phases by this method is unusual, not to say not reported. Some authors synthesized CP phases from precursors prepared by soft chemistry which allowed a decrease of the temperature synthesis (800 C) and the obtaining of powders with micrometric grains [14,15]. More recently, E. Lancry et al. [16] reported, for large scale production, the preparation of Cu2Mo6S8 by reaction (starting materials: CuS, MoS2, Mo) at 850 C for 60 h in a molten salt media. Thin film growth was also reported by different methods such as sputtering and pulsed laser deposition [17e20]. According to the applications, the synthesis in thin film form is indeed attractive in terms of cost, miniaturization and integration. However, it is not easy to obtain CP thin films by sputtering or evaporation methods due to difficulties to control the stoichiometry, related to the volatility of sulphur under vacuum at the high temperature required for in-situ crystallization [17e19,21]. N. Lemée et al. [20] showed that the Pulsed Laser Deposition could be used to synthesize CuxMo6S8 thin films under selected deposition conditions. In this present work, we propose to synthesize the copper molybdenum sulfide Cu2Mo6S8 (called hereafter CMS) from intermediate oxides prepared by an alternative chemical route derived from the polymeric precursor method based on Pechini process [22]. This method generally used to synthesize polycation
S. Boursicot et al. / Solid State Sciences 14 (2012) 719e724
CuMoO4 241
MoO 211 3
MoO 020 3
MoO 201 3 MoO 013 3
MoO 003 3
MoO 110 3
CuMoO 210 4
CuMoO 302 4
CuMoO4 201
CuMoO4 202
MoO 012 3
Powder
Heat treatments : - calcination - sulfurization - reduction
MoO 002 3
Precursor film
MoO 101 3
Viscosity adjustment + « spin coating »
CuMoO 101 4
Homogeneous resin Heat treatments : - pre-treatment - calcination - sulfurization - reduction
MoO3 001
+ Ethylene glycol (T ~ 80 °C)
MoO 011 3
(NH4)6Mo7O24, 4H2O + H2O + citric acid (T ~ 80 °C)
Intensity (arb.units)
Cu(NO3)2, 2.5H2O + H2O + citric acid (T ~ 80 °C)
CuMoO4 212
720
Crystallized film Fig. 1. Flowchart of the synthesis of Cu2Mo6S8 powders and thin films by polymeric precursor method.
10
20
30
40
50
2θ (degrees) Fig. 2. XRD pattern of powder calcined at 450 C e 3 h.
oxides [23,24] is a low temperature process, compared to solid state reactions, and offers the possibility to prepare powders as well as thin films [25e29]. In addition, the oxides prepared by this process generally present higher reactivity than those obtained by conventional ceramic route, which could be interesting in the case of subsequent heat treatments necessary to synthesize sulfides. Metallic cations are chelated by a hydrocarboxylic acid, such as citric acid, in aqueous media. The addition of a glycol, such as ethylene glycol, leads to polymerization under stirring at 80e90 C. The precursor solution, where the cations are uniformly distributed in the organic matrix, can be directly heat treated to synthesize ultrafine powders or can be deposited on a substrate by dip or spin coating after viscosity adjustment. Similarly to powders, precursor films have then to be submitted to heat treatment to synthesize the desired phase. This chemical route is particularly attractive because it allows to work in aqueous media and the phase stoichiometry can be controlled with high precision. Moreover, this process of low temperature and low cost (low cost precursors and very simple deposition equipment in the case of thin films) appears compatible with future developments in industry. As previously mentioned, this process is routinely used for oxides and very few works are reported in literature for non oxide synthesis. R.G. Freitas et al. [30] prepared Pt thin film electrodes on Ti substrates using the Pechini process. The metallic film was obtained from H2PtCl6 precursor at temperature as low as 300 C for 10 min under static air atmosphere. L. Baca et al. [31] succeeded in TiB2 powder synthesis from TiCl4 and B2O3 metallic precursors heated at 1200 C for 2 h under argon atmosphere with formation of TiC and TiO2 by-products. A.M. Stux et al. [32] used also the Pechini process to synthesize Mo2C and Ni6Mo6C powders at 850 C and 800 C, respectively, for 2 h. To obtain these carbides, from Moand Ni-acetate precursors, treatment under H2 gas flow was performed. The preparation of Mo-Cu composite powders was reported by Ming Zhao et al. [33] using (NH4)6Mo7O24$4H20 and Cu(NO3)2,3H2O metal precursors. In that case, no ethylene glycol was added in the cation citric solutions. A Mo e 30 wt%Cu composite was thus obtained after calcination at 500 C following by a two step reduction treatment under hydrogen gas at 500 C and 700 C. The aim of this present work was to synthesize also a non oxide compound, the copper molybdenum sulfide Cu2Mo6S8 (CMS) in powder and thin film form, from intermediate oxides prepared by the Pechini process. It clearly appears from the literature mentioned above that the heat treatment of the resin and of the
precursor film (temperature, duration, atmosphere) will be the crucial point to obtain a ternary sulfide by this chemical route. 2. Experimental The flowchart of Fig. 1 summarizes the whole process we used for CMS powders and thin films synthesis. In a first step, two solutions were prepared with the two cations precursors: ammonium heptamolybdate (NH4)6Mo7O24$4H20 (Fluka, p.a. >99%) and copper nitrate Cu(NO3)2$2.5H2O (Aldrich 98%), respectively. Each cation precursor was added to an aqueous citric acid solution under stirring and heating at 80e90 C. The citric acid:cation ratio was fixed to 3:1 (in mol). This ratio, usually reported in the literature, leads to homogeneous and dense films, as shown by M. Liu et al. [25]. Indeed, a low ratio (<1) leads to cracks whereas a high ration (>3) decreases the film density [25]. In a second step, the two cation citric solutions were mixed in stoichiometric amounts (Cu:Mo ¼ 2:6) and ethylene glycol was added with a mass ratio citric acid:ethylene glycol equal to 40:60. This ratio is not so critical for thin films but strongly influences the final agglomerate morphology of powders [25,26]. The 40:60 mass ratio is often reported in literature both for powders and thin films synthesis [27e29]. After stirring and heating at 80e90 C, a blue translucent polymeric precursor solution (called hereafter resin) was obtained, from which powders and thin films were then prepared. In the case of thin films, the resin viscosity was first adjusted to 30 cP and films were deposited onto R-plane sapphire (Al2O3) single crystal substrates by spin coating. The precursor resin and thin films were then submitted to a 3 step heat treatment cycle: calcination, sulfurization and reduction. For each step, various temperatures and durations were investigated in order to optimize the synthesis conditions. The calcination treatment aims to eliminate the organic matter and also to synthesize the intermediate oxides. The calcination was performed under air atmosphere from 350 C to 550 C for 2 h or 3 h. Note that for powders, a pre-treatment at 300 C was previously performed for a more efficient elimination of the organic matter. The second step was the sulfurization performed under 9% H2S/H2 gas flow from 400 C to 600 C for 30 min, 1 h or 2 h. The resulting mixture of sulfides was then submitted to a reduction under H2 gas flow from 550 C to 700 C for 4 h or 8 h. For the 3 step heat treatment, heating and cooling rates of 5 C/min were used for both powders and thin films synthesis.
S. Boursicot et al. / Solid State Sciences 14 (2012) 719e724
721
MoO 220
MoS 006 Cu S 220
MoS 103
MoO 211 or 020
MoS 100
MoO 011 Cu S 111
Intensity (arb. units)
After each step of the heat treatment cycle, the samples were characterized by X-ray diffraction (XRD) with a INEL CPS 120 diffractometer, using the Cu Ka1 radiation (l ¼ 1.5406 Å). In the case of thin films, the microstructure was observed with a JSM 6310F field emission scanning microscope (FE-SEM). Energy dispersive spectroscopy analyses (EDS) were also performed on some powders after the 3 step heat treatment cycle in order to determine the ultimate composition. In order to validate the synthesis process, insertion and superconducting behaviour were tested. Chemical de-intercalation was first carried out on 40 mg of CMS powder immersed during 24 h in hydrochloric acid solution (5 ml HCl 12 N þ 5 ml distilled water). Re-intercalation has been achieved on previously de-intercalated powder by preparing a pellet from a mixture of copper with the de-intercalated powder placed in a silica tube. This tube was then sealed under primary vacuum and heat treated at 450 C for 12 h. The superconducting behaviour of the final powders was also investigated by low temperature magnetic measurements using
MoS 002
Fig. 3. FE-SEM images of films calcined at (a) 450 C e 3 h (1 layer), (b) 400 C e 3 h (1 layer) and (c) 400 C e 3 h after each deposition (2 layers).
600°C/2h
400°C/2h 10
20
30
40
50
2θ (degrees) Fig. 4. XRD patterns of powders after sulfurization for 2 h at 400 C and 600 C (calcination was previously performed at 450 C e 3 h).
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Fig. 5. FE-SEM images performed for two magnifications on a 1-layer film after sulfurization at 400 C for 2 h (calcination was previously performed at 450 C e 3 h).
3.1.1. Calcination heat treatment Calcination heat treatments were performed on powders in different conditions under air atmosphere: 2 h at 350 C, 450 C and
3.1.2. Sulfurization heat treatment The powders calcined at 450 C for 3 h were then submitted to the sulfurization treatment under 9% H2S/H2 gas flow at 400 C for
*
125
232
015 223
Sulfurization conditions 321
131 303 312
122
104 220
211
003
021
110 012
Intensity (arb. units)
100
*
3.1. Optimization of the heat treatment cycle
500 C and 3 h at 450 C. All samples presented the same XRD patterns revealing the presence of the expected oxides CuMoO4 and MoO3. As illustration, the XRD pattern of the powder calcined at 450 C for 3 h is displayed in Fig. 2. Although no significant difference was observed by XRD on the different samples, only the treatments at 450 C for 3 h and 500 C for 2 h led to homogeneous green-yellow coloured powders. These results may be due to more efficient removal of the organic matter when increasing the duration or temperature of calcination. Consequently, all powders were then systematically calcined at 450 C for 3 h to study the subsequent steps of the thermal cycle, namely the sulfurization and reduction. In the case of thin films, we observed that the calcination temperature could be decreased to 400 C due to the smaller amount of matter. Moreover, this annealing temperature led to a more homogeneous and denser microstructure than that obtained at 450 C, as shown by the FE-SEM observations (Fig. 3a and b). However, the coverage of the substrate surface is not complete (Fig. 3b) but it can be improved by the deposition of a second layer (Fig. 3c). In this case, the 2-layer film presents a total average thickness estimated around 350 nm after the 3 step heat treatment cycle (calcination, sulfurization and reduction). The addition of subsequent layers increases the density while maintaining a homogeneous surface. The total thickness is not proportional to the number of the layers due to this densification, as already observed by Q. Simon et al. [29] in KTa1xNbxO3 thin films prepared by the same chemical process.
400°C/2h
*
+
500°C/2h
600°C/2h 10
20
30
40
50
2θ (degrees) Fig. 6. XRD patterns of powders after reduction at 650 C e 4 h for different temperatures of sulfurization for 2 h (calcination was previously performed at 450 C e 3 h). Peaks marked with * and þ are related to Mo and MoS2, respectively.
a Superconducting Quantum Interference Device (SQUID) (MPMSXL Quantum Design). 3. Results and discussion
Fig. 7. FE-SEM images performed for two magnifications on a 2-layer film after reduction at 650 C e 4 h (calcination at 400 C e 3 h was performed after each deposition. Sulfurization at 400 C e 2 h and reduction at 650 C e 4 h were performed only after the second deposition).
S. Boursicot et al. / Solid State Sciences 14 (2012) 719e724
015
232 321
303
125
131
312
*
104 220
211
003
232
321
024
122 104 220
211
021 003 202
20
30
40
125
321
312
232
015
131 303
104 220
211
021
a 10
003
012
110
122
223
100
110
b
131 303 015 312 214 223
101
c
021
110 012
122
223
100
In order to validate the CMS synthesis, insertion and superconducting behaviours, which are the signature of this material, were investigated. The insertion/de-insertion process in Chevrel phases is an oxydo-reduction reaction via electron/ion transfer process [11]:
+
3.1.3. Reduction heat treatment The first investigations on the reduction treatment under H2 gas flow, from 550 C to 700 C for 4 h or 8 h, were performed on powders calcined at 450 C for 3 h and sulfurized at 400 C for 2 h. The results showed that a reduction temperature inferior to 600 C was not sufficient to obtain the CMS phase, even increasing the treatment duration to 8 h. On the contrary, a duration of 4 h at 600 C, 650 C and 700 C was enough to synthesize the copper molybdenum sulfide. However, the reduction temperature strongly influenced the presence of secondary phases: MoO2, already observed after sulfurization, was still present after reduction at 600 C but its amount was strongly decreased at 650 C whereas at 700 C, a large amount of Mo was obtained, originated from CMS decomposition. Then, we studied the influence of the sulfurization temperature on the CMS formation for a reduction treatment fixed to 650 C for 4 h. As shown in Fig. 6, the presence and the amount of Mo after reduction were strongly dependant of the sulfurization temperature. It seems that for powders, a sulfurization at 600 C e 2 h followed by a reduction at 650 C e 4 h led to the best results, i.e. the formation of the CMS phase with no or a very small amount of secondary phases. The EDS analyses performed on several areas of various pellets treated with the optimized conditions led to average values of Cu:Mo:S ratio equal to 2 0.1:6 0.2:8 0.3. The other investigated treatments led to non stoichiometric powders (typically excess of Mo, S deficiency) that is in agreement with the presence of secondary phases revealed by XRD. In the case of the thin films, the CMS phase was also obtained with the optimized thermal treatment conditions (calcination at 400 C for 3 h, sulfurization at 600 C for 2 h and reduction at 650 C for 4 h). Since Cu2Mo6S8 presents a rhombohedral structure similarly to sapphire, this substrate is favourable for crystalline growth of such films. An epitaxial growth could even be expected on R-sapphire, as reported by Lemée et al. who obtained Cu2Mo6S8 epitaxial films grown by pulsed laser deposition under specific deposition conditions [20]. In our case, polycrystalline films with no preferential orientation were obtained that can be explained by the process which includes intermediate phases. The films presented a homogeneous and dense microstructure, in particular when 2 layers were deposited, as shown in Fig. 7. In this case, the calcination was performed after each deposition but sulfurization and reduction were carried out only after the second deposition.
3.2. Chemical and physical behaviour
counts (arb. units)
different durations (30 min, 1 h and 2 h) and at different temperatures (400 C, 450 C, 500 C and 600 C) for 2 h. The XRD patterns revealed the presence of the expected Cu2S and MoS2 sulfides (even for 30 min at 400 C) which amounts increased with the treatment duration. However, in all samples the presence of MoO2 was also observed which amount was strongly influenced by the sulfurization temperature. As example, Fig. 4 illustrates the XRD patterns obtained for powders after sulfurization for 2 h at 400 C and 600 C. As it can be observed, the amount of MoO2 decreased with temperature increase. The influence of the sulfurization temperature and consequently of the MoO2 amount will be discussed in the next section devoted to the reduction treatment. The sulfurization treatment performed on thin films led to similar XRD results to those obtained on powders (presence of the same phases). The observation of the microstructure by FE-SEM revealed no meaningful change of the film morphology before and after the treatment of sulfurization. Fig. 5 illustrates the microstructure of a 1-layer film calcined at 450 C for 3 h and treated under H2S/H2 gas flow at 400 C for 2 h. The grain size and shape, homogeneity and density of the film are quite similar to those observed after calcination (see Fig. 3a).
723
50
2θ (degrees) Fig. 8. XRD patterns of powders after (a) the 3 step heat treatment cycle, (b) deintercalation and (c) Cu re-intercalation. Peaks marked with * and þ are related to Mo and MoS2, respectively. Peaks of fig (a) and fig (c) were indexed as Cu2Mo6S8. Peaks of fig (b) were indexed as Mo6S8.
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properties of insertion and superconductivity that are the signature of the CMS phase. Moreover, dense and homogeneous films were also achieved using suitable heat treatment cycle and a multilayer process. Finally, it can be pointed out that the polymeric precursor method gives access to thin films as well as powders synthesis from a unique precursor solution and allows to obtain CMS phase from intermediate oxides with a drastic decrease of temperature and duration of the heat treatments compared to standard solid state process.
0.000000
M (emu)
-0.000025
-0.000050
-0.000075
-0.000100
-0.000125
Acknowledgements 2
4
6
8
10
12
temperature (K) Fig. 9. Magnetization versus temperature, performed under 0.01 T, obtained for CMS powders.
Mo6S8 þ xne þ xMnþ / MxMo6S8 The de-intercalation/re-intercalation of the CMS powders were followed by XRD (Fig. 8a, b and c). After the de-intercalation reaction, the resulting powder presented the typical pattern of Mo6S8 (Fig. 8b), proving the success of the copper de-insertion. As it can be observed in Fig. 8c, the process was reversible (re-intercalation of copper) since the CMS phase was again observed after reaction of the de-intercalated powder placed with a small amount of metallic copper in a silica sealed tube at 450 C for 12 h. It can be underlined that the most interesting properties of Cu2Mo6S8 are the intercalation ones. In fact, compared to other compounds, the Chevrel phases including Cu2Mo6S8 allow a fast and reversible insertion of various cations at room temperature [4]. The magnetic measurements performed at low temperatures on pellets showed the superconducting behaviour of the synthesized CMS powders (Fig. 9). Moreover, the value obtained for the critical temperature Tc (onset around 9 K) is consistent with the composition Cu2Mo6S8. Indeed, it is well known that Tc varies in function of the x value in CuxMo6S8 phases [34]. 4. Conclusion The copper molybdenum sulfide Cu2Mo6S8 was synthesized in powder and thin film form from intermediate oxides prepared by the polymeric precursor method based on Pechini process. For each step of the heat treatment cycle (calcination, sulfurization and reduction) various temperatures and durations were investigated in order to emphasize the influence of these different treatments on the final product and to optimize the synthesis conditions. The first step of calcination (where the oxides CuMoO4 and MoO3 are formed) can be performed for 3 h at 450 C and 400 C for powders and thin films, respectively. The results obtained for the sulfurization and reduction clearly showed that these treatments were strongly interdependent. For instance, a sulfurization treatment under H2S/H2 for 2 h at 600 C led to the formation of the expected sulfides Cu2S and MoS2 with less amount of MoO2 compared to a treatment at 400 C. The subsequent reduction under H2 at 650 C for 4 h allowed to obtain the CMS phase with no or very small amount of secondary phases provided that the sulfurization is performed at 600 C. The chemical and physical characteristics investigation on these powders revealed the
SEM observations and EDS analyses were performed at CMEBA at University of Rennes 1.
References [1] R. Chevrel, M. Sergent, J. Prigent, J. Solid State Chem. 3 (1971) 515e519. [2] Ø. Fischer, J. Appl. Phys. 16 (1978) 1e28. [3] M. Potel, P. Gougeon, R. Chevrel, M. Sergent, Rev. Chim. Min. 21 (1984) 509e536. [4] E. Levi, G. Gershinsky, D. Aurbach, O. Isnard, G. Ceder, Chem. Mater. 21 (2009) 1390e1399. [5] Ø. Fischer, M.B. Maple (Eds.), Superconductivity in Ternary Compounds I, Topics in Current Physics, Springer-Verlag, Heidelberg, 1982. [6] V. Bouquet, S. Even-Boudjada, L. Burel, R. Chevrel, M. Sergent, P. Genevey, H. Massat, P. Dubots, M. Decroux, B. Seeber, Ø. Fischer, Phys. C 235 (2) (1994) 769e770. [7] N. Cheggour, M. Decroux, A. Gupta, Ø. Fischer, J.A.A.J. Perenboom, V. Bouquet, M. Sergent, R. Chevrel, J. Appl. Phys. 81 (9) (1997) 6277e6284. [8] V. Harel-Michaud, G. Pesnel-Leroux, L. Burel, R. Chevrel, C. Geantet, M. Cattenot, M. Vrinat, J. Alloys Compd. 317e318 (2001) 195e200. [9] M.D. Levi, H. Gizbar, E. Lancry, Y. Gofer, E. Levi, D. Aurbach, J. Electroanal. Chem. 569 (2004) 211e223. [10] Z. Kaidi, C. Boulanger, J.M. Lecuire, N. Lemée, M. Guilloux-Viry, A. Perrin, Solid State Sci. 1 (1999) 623e635. [11] S. Seghir, C. Boulanger, S. Diliberto, J.M. Lecuire, M. Potel, O. MerdrignacConanec, Electrochem. Commun. 10 (2008) 1505e1508. [12] R. Chevrel, M. Sergent, J. Prigent, Mater. Res. Bull. 9 (1974) 1487e1498. [13] B.W. Eichhorn, in: Kenneth D. Karlin (Ed.), Progress in Inorganic Chemistry, vol. 42, Intersciences Publication, 1994 pp. 139e237. [14] S. Even-Boudjada, L. Burel, V. Bouquet, R. Chevrel, M. Sergent, J.C. Jegaden, J. Cors, M. Decroux, Mater. Sci. Forum 205 (1994) 152e153. [15] P. Rabiller, M. Rabiller-Baudry, S. Even-Boudjada, L. Burel, R. Chevrel, M. Sergent, M. Decroux, J. Cors, J.L. Maufras, Mater. Res. Bull. 29 (1994) 567e574. [16] E. Lancry, E. Levi, A. Mitelman, S. Malovany, D. Aurbach, J. Solid State Chem. 179 (2006) 1879e1882. [17] R. Ohtaki, B.R. Zhao, H.L. Luo, J. Low Temp. Phys. 54 (1984) 119e127. [18] G.B. Hertel, T.P. Orlando, J.M. Tarascon, Phys. B 135 (1985) 168e171. [19] Y. Quéré, A. Perrin, R. Horyn, M. Sergent, Mat. Lett. 3 (1985) 340e343. [20] N. Lemée, M. Guilloux-Viry, A. Perrin, F. Lalu, J. Lesueur, Mater. Sci. Eng. B 72 (2000) 47e55. [21] R.J. Webb, A.M. Goldman, J.H. Kang, J. Maps, M.F. Schimdt, IEEE Trans. Magn. MAG 21 (1985) 835e838. [22] M. Pechini, US Patent 3330697, 1967. [23] H.U. Anderson, M.J. Pennel, J.P. Guha, Adv. Ceram. 21 (1987) 91e98. [24] P.A. Lessing, Ceram. Bull. 68 (5) (1989) 1002e1007. [25] M. Liu, D. Wang, J. Mater. Res. 10 (12) (1995) 3210e3221. [26] L.W. Tai, P.A. Lessing, J. Mater. Res. 7 (2) (1992) 502e510. [27] S.M. Zanetti, E.R. Leite, E. Longo, J.A. Varela, Mater. Lett. 31 (1997) 173e178. [28] V. Bouquet, E.R. Leite, E. Longo, J.A. Varela, J. Mater. Res. 14 (7) (1999) 3115e3121. [29] Q. Simon, V. Bouquet, A. Perrin, M. Guilloux-Viry, Solid State Sci. 11 (1) (2009) 91e95. [30] R.G. Freitas, R.T.S. Oliveira, M.C. Santos, L.O.S. Bulhões, E.C. Pereira, Mater. Lett. 60 (2006) 1906e1910. [31] L. Baca, N. Stelzer, J. Eur. Ceram. Soc. 28 (2008) 907e911. [32] A.M. Stux, C. Laberty-Robert, K.E. Swider-Lyons, J. Solid State Chem. 181 (2008) 2741e2747. [33] M. Zhao, J. Wang, W. Liu, M. Zhou, J. Phys. Conf. Ser. 188 (2009) 012020. [34] R. Flukiger, A. Junod, R. Baillif, P. Spitzli, A. Treyvaud, A. Paoli, H. Devantay, J. Muller, Solid State Commun. 23 (1977) 699e702.
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Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Synthesis of Cu2Mo6S8 powders and thin films from intermediate oxides prepared by polymeric precursor method S. Boursicot, V. Bouquet*, I. Péron, T. Guizouarn, M. Potel, M. Guilloux-Viry Sciences Chimiques de Rennes, UMR 6226 CNRS, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 July 2011 Received in revised form 31 January 2012 Accepted 18 March 2012 Available online 30 March 2012
Powders and thin films of the copper molybdenum sulfide Cu2Mo6S8 were synthesized from intermediate oxides prepared by polymeric precursor method based on Pechini process. In the case of the thin films, deposition was performed onto R-plane sapphire single crystal by spin coating. The influence of temperature and duration of the 3 step heat treatment cycle (calcination, sulfurization and reduction) were investigated to optimize the synthesis conditions. The first step of calcination under air atmosphere performed for 3 h at 450 C and 400 C is suitable to obtain the intermediate oxides powders and thin films, respectively. The sulfurization treatment at 600 C for 2 h under H2S/H2 gas flow followed by reduction at 650 C for 4 h under H2 gas flow allowed to obtain Cu2Mo6S8 in powder or thin film form. In the last case, a multilayer process led to dense and homogeneous films. Moreover, the insertion and superconducting behaviour of the final powders allowed to validate the Cu2Mo6S8 synthesis by this moderate temperature process. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Chevrel phases CuxMo6S8 Sulfides Insertion material Polymeric precursors Thin films
1. Introduction The Chevrel phases (called hereafter CP) of general formula MxMo6X8 where M is a cation and X is a chalcogen (S, Se or Te) represent a large variety of compounds intensively studied for their unusual properties such as superconductivity and high ionic mobility [1e4]. The channels formed by the 3D arrangement of the Mo6X8 clusters receive the metallic cations and according to the nature, the size and oxidation degree of these ones, the CP present different electronic and magnetic properties [5]. As an example, the lead molybdenum sulfide PbMo6S8 was intensively studied for its superconducting properties to be used in coils at high magnetic field [6,7]. The CP with small cations such as Ni2Mo6S8 and LixMo6S8 were studied for heterogeneous catalysis [8] and electrodes material for secondary battery [9] due to their structure favourable for intercalation/de-intercalation reactions. Another example is the copper molybdenum sulfide CuxMo6S8 which presents both superconducting and insertion properties [10,11]. The synthesis of these compounds can be performed by solid state reaction from the elements or binaries (for example, Mo, MoS2 and Cu or CuS are used to prepare Cu2Mo6S8) but it requires high temperature (900e1100 C) and long time of reaction [12]. Ternary
* Corresponding author. Tel.: þ33 2 23 23 56 56; fax: þ33 2 23 23 67 99. E-mail address: [email protected] (V. Bouquet). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2012.03.013
metal sulfides can be prepared by sulfurization of ternary oxide precursors [13] but it requires also high temperatures and the preparation of Chevrel phases by this method is unusual, not to say not reported. Some authors synthesized CP phases from precursors prepared by soft chemistry which allowed a decrease of the temperature synthesis (800 C) and the obtaining of powders with micrometric grains [14,15]. More recently, E. Lancry et al. [16] reported, for large scale production, the preparation of Cu2Mo6S8 by reaction (starting materials: CuS, MoS2, Mo) at 850 C for 60 h in a molten salt media. Thin film growth was also reported by different methods such as sputtering and pulsed laser deposition [17e20]. According to the applications, the synthesis in thin film form is indeed attractive in terms of cost, miniaturization and integration. However, it is not easy to obtain CP thin films by sputtering or evaporation methods due to difficulties to control the stoichiometry, related to the volatility of sulphur under vacuum at the high temperature required for in-situ crystallization [17e19,21]. N. Lemée et al. [20] showed that the Pulsed Laser Deposition could be used to synthesize CuxMo6S8 thin films under selected deposition conditions. In this present work, we propose to synthesize the copper molybdenum sulfide Cu2Mo6S8 (called hereafter CMS) from intermediate oxides prepared by an alternative chemical route derived from the polymeric precursor method based on Pechini process [22]. This method generally used to synthesize polycation
S. Boursicot et al. / Solid State Sciences 14 (2012) 719e724
CuMoO4 241
MoO 211 3
MoO 020 3
MoO 201 3 MoO 013 3
MoO 003 3
MoO 110 3
CuMoO 210 4
CuMoO 302 4
CuMoO4 201
CuMoO4 202
MoO 012 3
Powder
Heat treatments : - calcination - sulfurization - reduction
MoO 002 3
Precursor film
MoO 101 3
Viscosity adjustment + « spin coating »
CuMoO 101 4
Homogeneous resin Heat treatments : - pre-treatment - calcination - sulfurization - reduction
MoO3 001
+ Ethylene glycol (T ~ 80 °C)
MoO 011 3
(NH4)6Mo7O24, 4H2O + H2O + citric acid (T ~ 80 °C)
Intensity (arb.units)
Cu(NO3)2, 2.5H2O + H2O + citric acid (T ~ 80 °C)
CuMoO4 212
720
Crystallized film Fig. 1. Flowchart of the synthesis of Cu2Mo6S8 powders and thin films by polymeric precursor method.
10
20
30
40
50
2θ (degrees) Fig. 2. XRD pattern of powder calcined at 450 C e 3 h.
oxides [23,24] is a low temperature process, compared to solid state reactions, and offers the possibility to prepare powders as well as thin films [25e29]. In addition, the oxides prepared by this process generally present higher reactivity than those obtained by conventional ceramic route, which could be interesting in the case of subsequent heat treatments necessary to synthesize sulfides. Metallic cations are chelated by a hydrocarboxylic acid, such as citric acid, in aqueous media. The addition of a glycol, such as ethylene glycol, leads to polymerization under stirring at 80e90 C. The precursor solution, where the cations are uniformly distributed in the organic matrix, can be directly heat treated to synthesize ultrafine powders or can be deposited on a substrate by dip or spin coating after viscosity adjustment. Similarly to powders, precursor films have then to be submitted to heat treatment to synthesize the desired phase. This chemical route is particularly attractive because it allows to work in aqueous media and the phase stoichiometry can be controlled with high precision. Moreover, this process of low temperature and low cost (low cost precursors and very simple deposition equipment in the case of thin films) appears compatible with future developments in industry. As previously mentioned, this process is routinely used for oxides and very few works are reported in literature for non oxide synthesis. R.G. Freitas et al. [30] prepared Pt thin film electrodes on Ti substrates using the Pechini process. The metallic film was obtained from H2PtCl6 precursor at temperature as low as 300 C for 10 min under static air atmosphere. L. Baca et al. [31] succeeded in TiB2 powder synthesis from TiCl4 and B2O3 metallic precursors heated at 1200 C for 2 h under argon atmosphere with formation of TiC and TiO2 by-products. A.M. Stux et al. [32] used also the Pechini process to synthesize Mo2C and Ni6Mo6C powders at 850 C and 800 C, respectively, for 2 h. To obtain these carbides, from Moand Ni-acetate precursors, treatment under H2 gas flow was performed. The preparation of Mo-Cu composite powders was reported by Ming Zhao et al. [33] using (NH4)6Mo7O24$4H20 and Cu(NO3)2,3H2O metal precursors. In that case, no ethylene glycol was added in the cation citric solutions. A Mo e 30 wt%Cu composite was thus obtained after calcination at 500 C following by a two step reduction treatment under hydrogen gas at 500 C and 700 C. The aim of this present work was to synthesize also a non oxide compound, the copper molybdenum sulfide Cu2Mo6S8 (CMS) in powder and thin film form, from intermediate oxides prepared by the Pechini process. It clearly appears from the literature mentioned above that the heat treatment of the resin and of the
precursor film (temperature, duration, atmosphere) will be the crucial point to obtain a ternary sulfide by this chemical route. 2. Experimental The flowchart of Fig. 1 summarizes the whole process we used for CMS powders and thin films synthesis. In a first step, two solutions were prepared with the two cations precursors: ammonium heptamolybdate (NH4)6Mo7O24$4H20 (Fluka, p.a. >99%) and copper nitrate Cu(NO3)2$2.5H2O (Aldrich 98%), respectively. Each cation precursor was added to an aqueous citric acid solution under stirring and heating at 80e90 C. The citric acid:cation ratio was fixed to 3:1 (in mol). This ratio, usually reported in the literature, leads to homogeneous and dense films, as shown by M. Liu et al. [25]. Indeed, a low ratio (<1) leads to cracks whereas a high ration (>3) decreases the film density [25]. In a second step, the two cation citric solutions were mixed in stoichiometric amounts (Cu:Mo ¼ 2:6) and ethylene glycol was added with a mass ratio citric acid:ethylene glycol equal to 40:60. This ratio is not so critical for thin films but strongly influences the final agglomerate morphology of powders [25,26]. The 40:60 mass ratio is often reported in literature both for powders and thin films synthesis [27e29]. After stirring and heating at 80e90 C, a blue translucent polymeric precursor solution (called hereafter resin) was obtained, from which powders and thin films were then prepared. In the case of thin films, the resin viscosity was first adjusted to 30 cP and films were deposited onto R-plane sapphire (Al2O3) single crystal substrates by spin coating. The precursor resin and thin films were then submitted to a 3 step heat treatment cycle: calcination, sulfurization and reduction. For each step, various temperatures and durations were investigated in order to optimize the synthesis conditions. The calcination treatment aims to eliminate the organic matter and also to synthesize the intermediate oxides. The calcination was performed under air atmosphere from 350 C to 550 C for 2 h or 3 h. Note that for powders, a pre-treatment at 300 C was previously performed for a more efficient elimination of the organic matter. The second step was the sulfurization performed under 9% H2S/H2 gas flow from 400 C to 600 C for 30 min, 1 h or 2 h. The resulting mixture of sulfides was then submitted to a reduction under H2 gas flow from 550 C to 700 C for 4 h or 8 h. For the 3 step heat treatment, heating and cooling rates of 5 C/min were used for both powders and thin films synthesis.
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721
MoO 220
MoS 006 Cu S 220
MoS 103
MoO 211 or 020
MoS 100
MoO 011 Cu S 111
Intensity (arb. units)
After each step of the heat treatment cycle, the samples were characterized by X-ray diffraction (XRD) with a INEL CPS 120 diffractometer, using the Cu Ka1 radiation (l ¼ 1.5406 Å). In the case of thin films, the microstructure was observed with a JSM 6310F field emission scanning microscope (FE-SEM). Energy dispersive spectroscopy analyses (EDS) were also performed on some powders after the 3 step heat treatment cycle in order to determine the ultimate composition. In order to validate the synthesis process, insertion and superconducting behaviour were tested. Chemical de-intercalation was first carried out on 40 mg of CMS powder immersed during 24 h in hydrochloric acid solution (5 ml HCl 12 N þ 5 ml distilled water). Re-intercalation has been achieved on previously de-intercalated powder by preparing a pellet from a mixture of copper with the de-intercalated powder placed in a silica tube. This tube was then sealed under primary vacuum and heat treated at 450 C for 12 h. The superconducting behaviour of the final powders was also investigated by low temperature magnetic measurements using
MoS 002
Fig. 3. FE-SEM images of films calcined at (a) 450 C e 3 h (1 layer), (b) 400 C e 3 h (1 layer) and (c) 400 C e 3 h after each deposition (2 layers).
600°C/2h
400°C/2h 10
20
30
40
50
2θ (degrees) Fig. 4. XRD patterns of powders after sulfurization for 2 h at 400 C and 600 C (calcination was previously performed at 450 C e 3 h).
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Fig. 5. FE-SEM images performed for two magnifications on a 1-layer film after sulfurization at 400 C for 2 h (calcination was previously performed at 450 C e 3 h).
3.1.1. Calcination heat treatment Calcination heat treatments were performed on powders in different conditions under air atmosphere: 2 h at 350 C, 450 C and
3.1.2. Sulfurization heat treatment The powders calcined at 450 C for 3 h were then submitted to the sulfurization treatment under 9% H2S/H2 gas flow at 400 C for
*
125
232
015 223
Sulfurization conditions 321
131 303 312
122
104 220
211
003
021
110 012
Intensity (arb. units)
100
*
3.1. Optimization of the heat treatment cycle
500 C and 3 h at 450 C. All samples presented the same XRD patterns revealing the presence of the expected oxides CuMoO4 and MoO3. As illustration, the XRD pattern of the powder calcined at 450 C for 3 h is displayed in Fig. 2. Although no significant difference was observed by XRD on the different samples, only the treatments at 450 C for 3 h and 500 C for 2 h led to homogeneous green-yellow coloured powders. These results may be due to more efficient removal of the organic matter when increasing the duration or temperature of calcination. Consequently, all powders were then systematically calcined at 450 C for 3 h to study the subsequent steps of the thermal cycle, namely the sulfurization and reduction. In the case of thin films, we observed that the calcination temperature could be decreased to 400 C due to the smaller amount of matter. Moreover, this annealing temperature led to a more homogeneous and denser microstructure than that obtained at 450 C, as shown by the FE-SEM observations (Fig. 3a and b). However, the coverage of the substrate surface is not complete (Fig. 3b) but it can be improved by the deposition of a second layer (Fig. 3c). In this case, the 2-layer film presents a total average thickness estimated around 350 nm after the 3 step heat treatment cycle (calcination, sulfurization and reduction). The addition of subsequent layers increases the density while maintaining a homogeneous surface. The total thickness is not proportional to the number of the layers due to this densification, as already observed by Q. Simon et al. [29] in KTa1xNbxO3 thin films prepared by the same chemical process.
400°C/2h
*
+
500°C/2h
600°C/2h 10
20
30
40
50
2θ (degrees) Fig. 6. XRD patterns of powders after reduction at 650 C e 4 h for different temperatures of sulfurization for 2 h (calcination was previously performed at 450 C e 3 h). Peaks marked with * and þ are related to Mo and MoS2, respectively.
a Superconducting Quantum Interference Device (SQUID) (MPMSXL Quantum Design). 3. Results and discussion
Fig. 7. FE-SEM images performed for two magnifications on a 2-layer film after reduction at 650 C e 4 h (calcination at 400 C e 3 h was performed after each deposition. Sulfurization at 400 C e 2 h and reduction at 650 C e 4 h were performed only after the second deposition).
S. Boursicot et al. / Solid State Sciences 14 (2012) 719e724
015
232 321
303
125
131
312
*
104 220
211
003
232
321
024
122 104 220
211
021 003 202
20
30
40
125
321
312
232
015
131 303
104 220
211
021
a 10
003
012
110
122
223
100
110
b
131 303 015 312 214 223
101
c
021
110 012
122
223
100
In order to validate the CMS synthesis, insertion and superconducting behaviours, which are the signature of this material, were investigated. The insertion/de-insertion process in Chevrel phases is an oxydo-reduction reaction via electron/ion transfer process [11]:
+
3.1.3. Reduction heat treatment The first investigations on the reduction treatment under H2 gas flow, from 550 C to 700 C for 4 h or 8 h, were performed on powders calcined at 450 C for 3 h and sulfurized at 400 C for 2 h. The results showed that a reduction temperature inferior to 600 C was not sufficient to obtain the CMS phase, even increasing the treatment duration to 8 h. On the contrary, a duration of 4 h at 600 C, 650 C and 700 C was enough to synthesize the copper molybdenum sulfide. However, the reduction temperature strongly influenced the presence of secondary phases: MoO2, already observed after sulfurization, was still present after reduction at 600 C but its amount was strongly decreased at 650 C whereas at 700 C, a large amount of Mo was obtained, originated from CMS decomposition. Then, we studied the influence of the sulfurization temperature on the CMS formation for a reduction treatment fixed to 650 C for 4 h. As shown in Fig. 6, the presence and the amount of Mo after reduction were strongly dependant of the sulfurization temperature. It seems that for powders, a sulfurization at 600 C e 2 h followed by a reduction at 650 C e 4 h led to the best results, i.e. the formation of the CMS phase with no or a very small amount of secondary phases. The EDS analyses performed on several areas of various pellets treated with the optimized conditions led to average values of Cu:Mo:S ratio equal to 2 0.1:6 0.2:8 0.3. The other investigated treatments led to non stoichiometric powders (typically excess of Mo, S deficiency) that is in agreement with the presence of secondary phases revealed by XRD. In the case of the thin films, the CMS phase was also obtained with the optimized thermal treatment conditions (calcination at 400 C for 3 h, sulfurization at 600 C for 2 h and reduction at 650 C for 4 h). Since Cu2Mo6S8 presents a rhombohedral structure similarly to sapphire, this substrate is favourable for crystalline growth of such films. An epitaxial growth could even be expected on R-sapphire, as reported by Lemée et al. who obtained Cu2Mo6S8 epitaxial films grown by pulsed laser deposition under specific deposition conditions [20]. In our case, polycrystalline films with no preferential orientation were obtained that can be explained by the process which includes intermediate phases. The films presented a homogeneous and dense microstructure, in particular when 2 layers were deposited, as shown in Fig. 7. In this case, the calcination was performed after each deposition but sulfurization and reduction were carried out only after the second deposition.
3.2. Chemical and physical behaviour
counts (arb. units)
different durations (30 min, 1 h and 2 h) and at different temperatures (400 C, 450 C, 500 C and 600 C) for 2 h. The XRD patterns revealed the presence of the expected Cu2S and MoS2 sulfides (even for 30 min at 400 C) which amounts increased with the treatment duration. However, in all samples the presence of MoO2 was also observed which amount was strongly influenced by the sulfurization temperature. As example, Fig. 4 illustrates the XRD patterns obtained for powders after sulfurization for 2 h at 400 C and 600 C. As it can be observed, the amount of MoO2 decreased with temperature increase. The influence of the sulfurization temperature and consequently of the MoO2 amount will be discussed in the next section devoted to the reduction treatment. The sulfurization treatment performed on thin films led to similar XRD results to those obtained on powders (presence of the same phases). The observation of the microstructure by FE-SEM revealed no meaningful change of the film morphology before and after the treatment of sulfurization. Fig. 5 illustrates the microstructure of a 1-layer film calcined at 450 C for 3 h and treated under H2S/H2 gas flow at 400 C for 2 h. The grain size and shape, homogeneity and density of the film are quite similar to those observed after calcination (see Fig. 3a).
723
50
2θ (degrees) Fig. 8. XRD patterns of powders after (a) the 3 step heat treatment cycle, (b) deintercalation and (c) Cu re-intercalation. Peaks marked with * and þ are related to Mo and MoS2, respectively. Peaks of fig (a) and fig (c) were indexed as Cu2Mo6S8. Peaks of fig (b) were indexed as Mo6S8.
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properties of insertion and superconductivity that are the signature of the CMS phase. Moreover, dense and homogeneous films were also achieved using suitable heat treatment cycle and a multilayer process. Finally, it can be pointed out that the polymeric precursor method gives access to thin films as well as powders synthesis from a unique precursor solution and allows to obtain CMS phase from intermediate oxides with a drastic decrease of temperature and duration of the heat treatments compared to standard solid state process.
0.000000
M (emu)
-0.000025
-0.000050
-0.000075
-0.000100
-0.000125
Acknowledgements 2
4
6
8
10
12
temperature (K) Fig. 9. Magnetization versus temperature, performed under 0.01 T, obtained for CMS powders.
Mo6S8 þ xne þ xMnþ / MxMo6S8 The de-intercalation/re-intercalation of the CMS powders were followed by XRD (Fig. 8a, b and c). After the de-intercalation reaction, the resulting powder presented the typical pattern of Mo6S8 (Fig. 8b), proving the success of the copper de-insertion. As it can be observed in Fig. 8c, the process was reversible (re-intercalation of copper) since the CMS phase was again observed after reaction of the de-intercalated powder placed with a small amount of metallic copper in a silica sealed tube at 450 C for 12 h. It can be underlined that the most interesting properties of Cu2Mo6S8 are the intercalation ones. In fact, compared to other compounds, the Chevrel phases including Cu2Mo6S8 allow a fast and reversible insertion of various cations at room temperature [4]. The magnetic measurements performed at low temperatures on pellets showed the superconducting behaviour of the synthesized CMS powders (Fig. 9). Moreover, the value obtained for the critical temperature Tc (onset around 9 K) is consistent with the composition Cu2Mo6S8. Indeed, it is well known that Tc varies in function of the x value in CuxMo6S8 phases [34]. 4. Conclusion The copper molybdenum sulfide Cu2Mo6S8 was synthesized in powder and thin film form from intermediate oxides prepared by the polymeric precursor method based on Pechini process. For each step of the heat treatment cycle (calcination, sulfurization and reduction) various temperatures and durations were investigated in order to emphasize the influence of these different treatments on the final product and to optimize the synthesis conditions. The first step of calcination (where the oxides CuMoO4 and MoO3 are formed) can be performed for 3 h at 450 C and 400 C for powders and thin films, respectively. The results obtained for the sulfurization and reduction clearly showed that these treatments were strongly interdependent. For instance, a sulfurization treatment under H2S/H2 for 2 h at 600 C led to the formation of the expected sulfides Cu2S and MoS2 with less amount of MoO2 compared to a treatment at 400 C. The subsequent reduction under H2 at 650 C for 4 h allowed to obtain the CMS phase with no or very small amount of secondary phases provided that the sulfurization is performed at 600 C. The chemical and physical characteristics investigation on these powders revealed the
SEM observations and EDS analyses were performed at CMEBA at University of Rennes 1.
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