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Surface-doping of Cr2O3 particles by the 119Sn Mössbauer probe
Pergamon
Materials Research Bulletin 35 (2000) 629 – 635
Surface-doping of Cr2O3 particles by the 119Sn Mo¨ssbauer probe Influence of the gaseous environment on the dopant distribution I.S. Bezverkhya, M.I. Afanasova, M. Danotb,*, P.B. Fabritchnyia a
Department of Chemistry, M.V. Lomonosov Moscow State University, 119899 Moscow V-234, Russia Laboratoire de Chimie des Solides, Institut des Mate´riaux Jean Rouxel, UMR 6502, CNRS-Universite´ de Nantes, 2, rue de la Houssinie`re, BP 32229, 44322 Nantes Cedex, France
b
(Refereed) Received 29 April 1999; accepted 24 May 1999
Abstract Chromium sesquioxide with 119Sn(II) dopant cations located just at the solid-gas interface (in the topmost cationic layer of the crystallites) has previously been synthesized by thermal treatment, under hydrogen flow, of Sn(IV) bulk-doped hydrated amorphous Cr(III) oxide. After exposure of the resulting Sn(II)/Cr2O3 material to various reactive gases, the probe environment was studied using Mo¨ssbauer spectroscopy. The present work reports synthesis of Sn(II)/Cr2O3 under argon atmosphere, instead of hydrogen, which could modify the probe behavior in reactivity experiments. In addition, it shows that migration of the dopant from the bulk towards the surface of the crystallites depends upon the ability of the synthesis atmosphere to act as an electron donor for the oxide matrix. After exposure of these Sn(II)/Cr2O3 samples to chlorine, the 119Sn probe environment is found to be the same as for Sn(II)/Cr2O3 prepared under hydrogen, which excludes HCl involvement in the surface rearrangement process evidenced by previous Mo¨ssbauer investigations. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; A. Surfaces; C. Mo¨ssbauer spectroscopy; D. Surface properties
*Corresponding author. Tel.: ⫹33-2-40-37-39.25; fax: ⫹33-2-40-37-39-95. E-mail address: [email protected] (M. Danot). 0025-5408/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 0 ) 0 0 2 5 0 - 6
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1. Introduction It has previously been shown [1] that polycrystalline Cr2O3 samples doped with small amounts of tin can allow Mo¨ssbauer absorption spectroscopy to be used as a surface technique, for instance, to study processes occurring on the surface of the oxide matrix exposed to various reactive atmospheres. Such an unusual application of Mo¨ssbauer spectroscopy is based on the ability of the 119Sn probe to be located immediately on the surface of the Cr2O3 crystallites [2,3]. This particular distribution of the dopant was first realized by annealing, under hydrogen flow, Sn(IV) bulk-doped hydrated Cr(III) oxide. This treatment results in reduction of bulk Sn(IV) to surface Sn(II), the surface location of divalent tin being related to the stabilization effect of the stereochemically active lone pair. Moreover, in the case of Cr2O3 and other oxide matrices with the corundum structure, the occurrence of the dopant in the divalent state has been shown [1] to attest unambiguously its surface location. Hydrogen used upon annealing permits preservation of these characteristic Sn(II) ions from oxidation after cooling the reactor to room temperature. However, the presence of hydrogen in the reactor is not desirable for further study of the probe environment after exposure to reactive atmospheres, since hydrogen could take part in the solid– gas interaction [4]. For this reason, we considered it would be worthwhile to synthesize similar Sn(II)/Cr2O3 samples, replacing hydrogen with an inert gas such as argon, for instance. 2. Experimental The precursors we used were hydrated oxides of Cr(III) and Sn(IV) obtained by coprecipitation methods, according to refs. 2 and 5. An acetic solution containing Sn(IV) (enriched to 92% in the 119 isotope) and Cr(III) in 3:1000 atomic ratio was added under stirring to aqueous ammonia. The resulting precipitate was filtered, washed, and dried in air, first at room temperature, then at 200°C. It has been shown [2] that, at this stage, the Sn(IV) dopant is homogeneously distributed in the particles of amorphous Cr2O3:Sn(IV) 䡠 xH2O (which will be referred to as the A precursor). Subsequent thermal treatments under various flowing gas were carried out in a quartz reactor equipped with a thin-wall measurement cell for in situ Mo¨ssbauer experiments. At the end of annealing at the chosen temperature, before cooling, the gas flow was stopped and the reactor isolated by shutting its valves. The Mo¨ssbauer measurements were performed with a constant acceleration spectrometer, using a Ca119mSnO3 source at 295 K. At sample temperatures lower than 308 K, the Ne´el point of the Cr2O3 matrix, the 119Sn Mo¨ssbauer spectra show Zeeman splitting due to hyperfine field transfer from the neighboring Cr(III) cations to the probe. In order to facilitate analysis of the valence state of the dopant, the measurements were performed in the paramagnetic region (at 320 K). 3. Results and discussion 3.1. Preparation of Sn(II)Cr2O3 Several routes [2,6,7] can be used for obtention of Sn(II)/Cr2O3 under hydrogen flow.
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3.1.1. Annealing of the A precursor In this case, a 10-hour treatment at 900°C leads to formation of a Cr2O3 polycrystalline powder with totality of the tin dopant in the (II) oxidation state and located on surface sites [2]. This procedure is referred to hereafter as procedure 1. 3.1.2. Annealing of Sn(IV)/Cr2O3 obtained by air oxidation of Sn(II)/Cr2O3 Air exposure of Sn(II)/Cr2O3 (prepared, for instance, according to the preceding procedure) results in formation of Sn(IV)/Cr2O3, the superficial location of the probe being retained. For these Sn(IV) species already located on surface sites, reduction back to the (II) oxidation state is then much easier (3 h at 300°C) [6]. This procedure is referred to hereafter as procedure 2. 3.1.3. Annealing of Sn(IV) bulk-doped crystalline Cr2O3 If calcinated in air (10 h at 900°C), the A precursor transforms into crystalline Cr2O3 with the Sn(IV) dopant homogeneously distributed within the particles (this material will be denoted Cr2O3:Sn(IV)). Hydrogen treatment of Cr2O3:Sn(IV) under conditions similar to those used in procedure 1 produces only partial reduction to Sn(II) [7]. Total reduction and surface location were found to be achieved after annealing for 20 h at 900°C. This procedure is referred to hereafter as procedure 3. 3.2. Experiments with argon After preparation of Sn(II)/Cr2O3 under hydrogen according to procedure 1, for instance, and cooling to room temperature, admission of argon flow into the reactor quickly leads to oxidation of tin to the (IV) state. This shows that surface Sn(II) can be readily oxidized by oxygen traces contained in flowing argon. For this reason, total displacement of hydrogen by argon flow, which requires important argon volume, cannot be performed at room temperature without change of the initial state of at least part of the dopant species. We tested the possibility of using argon instead of hydrogen in procedure 2. A 3-hour annealing of Sn(IV)/Cr2O3 under argon flow at 300°C did not produce any change. The treatment temperature was then increased to 900°C. In situ 119Sn Mo¨ssbauer spectra (Fig. 1) show that tin after 3 h at this temperature has effectively been reduced to Sn(II): the isomer shift and quadrupole splitting (␦ ⫽ 2.75 mm/s and ⌬ ⫽ 2.10 mm/s) are similar to those for Sn(II)/Cr2O3 synthesized under hydrogen according to procedure 1 [8]. (These values of ␦ and ⌬, in fact, represent mean values. The spectrum in Fig. 1b as well as that of the compound obtained under hydrogen according to procedure 1 contain two doublets, which reflects for Sn(II) the existence of two sites with slightly different environments. For more details, see ref. 8.) This result led us to think that such a prolonged treatment under argon flow could be efficient for displacement of hydrogen (involved in procedure 1 prior to procedure 2 carried out under argon). However, we decided to test a synthesis route excluding any hydrogen contact in the sample history. For this purpose, the A precursor was directly annealed under argon, instead of hydrogen, according to procedure 1 (10 h at 900°C). The Mo¨ssbauer spectrum of the obtained compound was identical to that presented in Fig. 1b, meaning that Sn(II)/Cr2O3 can be synthesized without recourse to hydrogen.
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Fig. 1. Mo¨ssbauer spectra of 119Sn surface doped crystalline Cr2O3 (Sn/Cr2O3). Tin, initially in the (IV) oxidation state (a), is reduced to the divalent state (b) after a 3-hour annealing at 900°C under argon flow. Tmeas ⫽ 320 K (paramagnetic region, TN ⫽ 308 K).
In both of these experiments under argon flow, the only way to account for reduction of Sn(IV) to Sn(II) is to consider that, at the reaction temperature, oxygen anions are oxidized due to the equilibrium: Sn共 ⫹ IV兲 ⫹ O共 ⫺ II兲 7 Sn共 ⫹ II兲 ⫹ 1/ 2 O 2 Resulting dioxygen is dragged out of the reactor by the argon flow, the equilibrium being therefore constantly displaced towards the right. Once the reactor valves have been shut, the oxygen traces confined in the reactor are not sufficient to reoxidize Sn(II) upon cooling so that the high temperature state remains stable at room temperature. Finally, it should be noted that transposition of procedure 3 (starting from bulk-doped crystalline Cr2O3, i.e., Cr2O3: Sn(IV)) with replacement of hydrogen by argon (20 h at 900°C) was unsuccessful: no Sn(II) contribution could be detected in the Mo¨ssbauer spectra. This result was surprising, since argon use was found efficient for procedures 1 and 2. This point will be discussed later. 3.2.1. Surface stabilization by Sn(II) In addition to realization of hydrogen-free synthesis, the experiments described above present in fact more general interest. First of all, it must be emphasized that the success of syntheses performed under argon flow was anything but foreseeable since, in its own oxide SnO, Sn(II) is known [9] to disproportionate upon annealing at about 400°C in inert atmosphere, according to 4SnO 3 Sn ⫹ Sn 3O 4 The fact that similar disproportionation does not occur under argon with Sn(II)/Cr2O3 thus reflects stabilization of divalent tin, bonded to the same oxygen ligand, due to its location on
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the surface of the Cr2O3 grains. This stabilization also explains that, after treatment of the A precursor at 900°C under hydrogen, Sn is effectively found in the (II) oxidation state, instead of being reduced to metallic tin. As stated in ref. 10, this stabilization effect can be related to cationic sites with reduced coordination number, which progressively appear, due to surface dehydroxylation, when the temperature is increased. Such sites can hardly accommodate Cr(III), which requires octahedral coordination due to its d3 configuration. On the contrary, they are suitable for being occupied by Sn(II), whose stereochemically active lone pair can behave as a supplementary ligand. Occupancy of superficial sites by Sn(II) thus diminishes the number of Cr(III) with unsaturated coordination, which consequently reduces the surface energy. 3.2.2. Tin migration from the bulk towards the surface of the Cr2O3 crystallites In the case of procedure 3 (starting from crystalline Cr2O3:Sn(IV)) carried out under hydrogen at 900°C, Sn(IV) species can reach the surface due to diffusion and readily be reduced by hydrogen to Sn(II). Because of their much larger ionic radius (RSn(II) ⫽ 0.93 Å, RSn(IV) ⫽ 0.71 Å [11]), the Sn(II) which are formed cannot diffuse (through octahedral voids) back to the bulk of the crystallites. Disappearance of Sn(IV) at the surface results in a concentration gradient which favors their migration from the bulk and occupation of low coordination surface sites by the Sn(II) formed. On the contrary, as indicated above, such a behavior is not observed under argon atmosphere: Sn(IV) reduction does not occur. It can be assumed that this difference is related to the chemical properties of the two gases, with only hydrogen being able to act as an electron donor during annealing. This idea is corroborated by the result of annealing Cr2O3:Sn(IV) under carbon monoxide flow. In the presence of this electron donor, accommodation of tin in surface sites is observed. Considering the very different sizes of the various gaseous agents involved, these results show that tin distribution cannot be related to diffusion rate of the gases in the solid and that, essentially, interface phenomena are involved. Cr2O3 is a p-type semi-conductor [12]. For air-annealed samples, presence of Cr(IV) has been evidenced near the grain surface [13]. Drastic decrease of the conductivity observed in reducing conditions [14] can thus be related to decrease of the Cr(IV) amount due to reaction with the gaseous environment. It can thus be thought that, in the case of Sn(IV) bulk-doped crystalline Cr2O3, presence of Cr(IV) can locally limit the acceptable amount of the isovalent dopant, Sn(IV), in the corresponding region of the particles. Sn(IV) solubility being very low (s ⬍ 1 at% [15]), even in the bulk of Cr2O3 particles, where the Cr(IV) amount is necessarily lower than near the surface, the presence of Cr(IV) can inhibit Sn(IV) migration. In syntheses starting from Cr2O3:Sn(IV) (procedure 3), hydrogen and carbon monoxide can reduce the Cr(IV) species, which allows tin migration towards the surface. Argon cannot reduce the Cr(IV) species and, thus, migration does not occur. The result is completely different when the same inert gas is used for treatment of the amorphous A precursor (procedure 1), since reduction of the totality of tin is observed. We have noted that, during annealing of this A precursor at 900°C, passage of tin to the (II) oxidation state occurs before crystallization of the Cr2O3 matrix. This means that, in this case, the dopant is already located on surface sites of the grains when crystallization occurs, and that, being divalent, it remains in such positions during particle growth.
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3.2.3. Reaction with chlorine of Sn(II)/Cr2O3 prepared under argon Sn(II)/Cr2O3 prepared under hydrogen flow had been exposed to chlorine atmosphere at room temperature, and resulting transformations of the probe environment studied by Mo¨ssbauer spectroscopy [4]. In order to estimate the possible impact of hydrogen presence, similar experiments have now been carried out with Sn(II)/Cr2O3 prepared under argon flow. No significant difference appears in the two experiment results: instability of mixed (O2⫺, Cl⫺) anionic environments is still evidenced and a similar amount of Sn(IV) involved in the (SnCl6)2⫺ species is still observed. This means that the interface rearrangement previously described was not due to interfering reactions related to HCl formation.
4. Conclusion Location of divalent tin at the surface of crystalline Cr2O3 particles can be achieved under inert atmosphere by two synthesis routes, first, by annealing under argon amorphous hydrated chromium(III) oxide doped with Sn(IV) by coprecipitation, and second, by annealing under argon a sample with tin already accommodated in superficial sites but in the (IV) oxidation state. In both cases, the dopant distribution is the same as in samples prepared under hydrogen. However, if the starting material is Sn(IV) bulk-doped crystalline Cr2O3, in contrast to hydrogen treatment, argon treatment does not allow Sn(II) to be obtained. This difference can be related to the decreased Sn(IV) solubility in superficial layers, due to the presence of isovalent Cr(IV) whose amount depends upon the reducing properties of the gaseous environment. Even if the hydrogen-free synthesis does not affect the final environment of the dopant after exposure to chlorine, this new synthesis route is of interest, since it will allow for study of the reactivity of Sn(II)/Cr2O3 in the presence of stronger oxidizing agents (fluorine or its derivatives) which would unavoidably react with residual hydrogen.
Acknowledgments The described research was supported by the Russian Foundation for Basic Research (Grant N 97-03-331179a).
References [1] M.I. Afanasov, P.B. Fabritchnyi, Rossiiskii Khim Zh (Russ J Chem) 40 (1996) 54. [2] P.B. Fabritchnyi, A.N. Protsky, V.P. Gorkov, T. M. Duc, G. Demazeau, P. Hagenmuller, Sov Phys JETP 54 (1981) 608. [3] M. Danot, M.I. Afanasov, I.S. Bezverkhy, P.B. Fabritchnyi, J. Rouxel, Solid State Commun 91 (1994) 675. [4] P.B. Fabritchnyi, M.I. Afanasov, I.S. Bezverkhy, M. Danot, J. Rouxel, Solid State Commun 102 (1997) 749. [5] P.B. Fabritchnyi, A.N. Protsky, G. Demazeau, P. Hagenmuller, Mater Res Bull 19 (1981) 429. [6] M. Danot, M.I. Afanasov, A.A. Ryabchikov, S. Maingaud, P.B. Fabritchnyi, J. Rouxel, J Solid State Chem 132 (1997) 284.
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[7] P.B. Fabritchnyi, N.R. Sudakova, V.P. Gorkov, V.V. Berentsveig, G. Demazeau, P. Hagenmuller, Mater Res Bull 28 (1993) 179. [8] M.I. Afanasov, M. Danot, A.A. Ryabchikov, S. Maingaud, P.B. Fabritchnyi, J. Rouxel, Mater Res Bull 31 (1996) 465. [9] H. Spandau, E.J. Kohlmeyer, Z Anorg Allg Chem 254 (1947) 65. [10] P.B. Fabritchnyi, Zh Vsesoyuznogo Khim Obchestva im D.I. Mendeleeva 30 (1985) 143. [11] R.D. Shannon, C.T. Prewitt, Acta Crystallogr B25 (1969) 925. [12] R.F. Huang, A.K. Agarwal, H.U. Anderson, J Am Ceram Soc 67 (1984) 146. [13] S.R. Morrison, The Chemical Physics of Surfaces, Plenum Press, New York, 1977. [14] D. Baresel, W. Gellert, W. Sarholz, P. Scharner, Sensors Actuators 6 (1984) 35. [15] K.P. Mitrofanov, P.B. Fabritchnyi, E.V. Lamykin, A.M. Babeshkin, L.P. Fefilatiev, Vest Mos Univ ser. Fizika-Astronomiya 16 (1975) 742.
Materials Research Bulletin 35 (2000) 629 – 635
Surface-doping of Cr2O3 particles by the 119Sn Mo¨ssbauer probe Influence of the gaseous environment on the dopant distribution I.S. Bezverkhya, M.I. Afanasova, M. Danotb,*, P.B. Fabritchnyia a
Department of Chemistry, M.V. Lomonosov Moscow State University, 119899 Moscow V-234, Russia Laboratoire de Chimie des Solides, Institut des Mate´riaux Jean Rouxel, UMR 6502, CNRS-Universite´ de Nantes, 2, rue de la Houssinie`re, BP 32229, 44322 Nantes Cedex, France
b
(Refereed) Received 29 April 1999; accepted 24 May 1999
Abstract Chromium sesquioxide with 119Sn(II) dopant cations located just at the solid-gas interface (in the topmost cationic layer of the crystallites) has previously been synthesized by thermal treatment, under hydrogen flow, of Sn(IV) bulk-doped hydrated amorphous Cr(III) oxide. After exposure of the resulting Sn(II)/Cr2O3 material to various reactive gases, the probe environment was studied using Mo¨ssbauer spectroscopy. The present work reports synthesis of Sn(II)/Cr2O3 under argon atmosphere, instead of hydrogen, which could modify the probe behavior in reactivity experiments. In addition, it shows that migration of the dopant from the bulk towards the surface of the crystallites depends upon the ability of the synthesis atmosphere to act as an electron donor for the oxide matrix. After exposure of these Sn(II)/Cr2O3 samples to chlorine, the 119Sn probe environment is found to be the same as for Sn(II)/Cr2O3 prepared under hydrogen, which excludes HCl involvement in the surface rearrangement process evidenced by previous Mo¨ssbauer investigations. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; A. Surfaces; C. Mo¨ssbauer spectroscopy; D. Surface properties
*Corresponding author. Tel.: ⫹33-2-40-37-39.25; fax: ⫹33-2-40-37-39-95. E-mail address: [email protected] (M. Danot). 0025-5408/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 0 ) 0 0 2 5 0 - 6
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1. Introduction It has previously been shown [1] that polycrystalline Cr2O3 samples doped with small amounts of tin can allow Mo¨ssbauer absorption spectroscopy to be used as a surface technique, for instance, to study processes occurring on the surface of the oxide matrix exposed to various reactive atmospheres. Such an unusual application of Mo¨ssbauer spectroscopy is based on the ability of the 119Sn probe to be located immediately on the surface of the Cr2O3 crystallites [2,3]. This particular distribution of the dopant was first realized by annealing, under hydrogen flow, Sn(IV) bulk-doped hydrated Cr(III) oxide. This treatment results in reduction of bulk Sn(IV) to surface Sn(II), the surface location of divalent tin being related to the stabilization effect of the stereochemically active lone pair. Moreover, in the case of Cr2O3 and other oxide matrices with the corundum structure, the occurrence of the dopant in the divalent state has been shown [1] to attest unambiguously its surface location. Hydrogen used upon annealing permits preservation of these characteristic Sn(II) ions from oxidation after cooling the reactor to room temperature. However, the presence of hydrogen in the reactor is not desirable for further study of the probe environment after exposure to reactive atmospheres, since hydrogen could take part in the solid– gas interaction [4]. For this reason, we considered it would be worthwhile to synthesize similar Sn(II)/Cr2O3 samples, replacing hydrogen with an inert gas such as argon, for instance. 2. Experimental The precursors we used were hydrated oxides of Cr(III) and Sn(IV) obtained by coprecipitation methods, according to refs. 2 and 5. An acetic solution containing Sn(IV) (enriched to 92% in the 119 isotope) and Cr(III) in 3:1000 atomic ratio was added under stirring to aqueous ammonia. The resulting precipitate was filtered, washed, and dried in air, first at room temperature, then at 200°C. It has been shown [2] that, at this stage, the Sn(IV) dopant is homogeneously distributed in the particles of amorphous Cr2O3:Sn(IV) 䡠 xH2O (which will be referred to as the A precursor). Subsequent thermal treatments under various flowing gas were carried out in a quartz reactor equipped with a thin-wall measurement cell for in situ Mo¨ssbauer experiments. At the end of annealing at the chosen temperature, before cooling, the gas flow was stopped and the reactor isolated by shutting its valves. The Mo¨ssbauer measurements were performed with a constant acceleration spectrometer, using a Ca119mSnO3 source at 295 K. At sample temperatures lower than 308 K, the Ne´el point of the Cr2O3 matrix, the 119Sn Mo¨ssbauer spectra show Zeeman splitting due to hyperfine field transfer from the neighboring Cr(III) cations to the probe. In order to facilitate analysis of the valence state of the dopant, the measurements were performed in the paramagnetic region (at 320 K). 3. Results and discussion 3.1. Preparation of Sn(II)Cr2O3 Several routes [2,6,7] can be used for obtention of Sn(II)/Cr2O3 under hydrogen flow.
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3.1.1. Annealing of the A precursor In this case, a 10-hour treatment at 900°C leads to formation of a Cr2O3 polycrystalline powder with totality of the tin dopant in the (II) oxidation state and located on surface sites [2]. This procedure is referred to hereafter as procedure 1. 3.1.2. Annealing of Sn(IV)/Cr2O3 obtained by air oxidation of Sn(II)/Cr2O3 Air exposure of Sn(II)/Cr2O3 (prepared, for instance, according to the preceding procedure) results in formation of Sn(IV)/Cr2O3, the superficial location of the probe being retained. For these Sn(IV) species already located on surface sites, reduction back to the (II) oxidation state is then much easier (3 h at 300°C) [6]. This procedure is referred to hereafter as procedure 2. 3.1.3. Annealing of Sn(IV) bulk-doped crystalline Cr2O3 If calcinated in air (10 h at 900°C), the A precursor transforms into crystalline Cr2O3 with the Sn(IV) dopant homogeneously distributed within the particles (this material will be denoted Cr2O3:Sn(IV)). Hydrogen treatment of Cr2O3:Sn(IV) under conditions similar to those used in procedure 1 produces only partial reduction to Sn(II) [7]. Total reduction and surface location were found to be achieved after annealing for 20 h at 900°C. This procedure is referred to hereafter as procedure 3. 3.2. Experiments with argon After preparation of Sn(II)/Cr2O3 under hydrogen according to procedure 1, for instance, and cooling to room temperature, admission of argon flow into the reactor quickly leads to oxidation of tin to the (IV) state. This shows that surface Sn(II) can be readily oxidized by oxygen traces contained in flowing argon. For this reason, total displacement of hydrogen by argon flow, which requires important argon volume, cannot be performed at room temperature without change of the initial state of at least part of the dopant species. We tested the possibility of using argon instead of hydrogen in procedure 2. A 3-hour annealing of Sn(IV)/Cr2O3 under argon flow at 300°C did not produce any change. The treatment temperature was then increased to 900°C. In situ 119Sn Mo¨ssbauer spectra (Fig. 1) show that tin after 3 h at this temperature has effectively been reduced to Sn(II): the isomer shift and quadrupole splitting (␦ ⫽ 2.75 mm/s and ⌬ ⫽ 2.10 mm/s) are similar to those for Sn(II)/Cr2O3 synthesized under hydrogen according to procedure 1 [8]. (These values of ␦ and ⌬, in fact, represent mean values. The spectrum in Fig. 1b as well as that of the compound obtained under hydrogen according to procedure 1 contain two doublets, which reflects for Sn(II) the existence of two sites with slightly different environments. For more details, see ref. 8.) This result led us to think that such a prolonged treatment under argon flow could be efficient for displacement of hydrogen (involved in procedure 1 prior to procedure 2 carried out under argon). However, we decided to test a synthesis route excluding any hydrogen contact in the sample history. For this purpose, the A precursor was directly annealed under argon, instead of hydrogen, according to procedure 1 (10 h at 900°C). The Mo¨ssbauer spectrum of the obtained compound was identical to that presented in Fig. 1b, meaning that Sn(II)/Cr2O3 can be synthesized without recourse to hydrogen.
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Fig. 1. Mo¨ssbauer spectra of 119Sn surface doped crystalline Cr2O3 (Sn/Cr2O3). Tin, initially in the (IV) oxidation state (a), is reduced to the divalent state (b) after a 3-hour annealing at 900°C under argon flow. Tmeas ⫽ 320 K (paramagnetic region, TN ⫽ 308 K).
In both of these experiments under argon flow, the only way to account for reduction of Sn(IV) to Sn(II) is to consider that, at the reaction temperature, oxygen anions are oxidized due to the equilibrium: Sn共 ⫹ IV兲 ⫹ O共 ⫺ II兲 7 Sn共 ⫹ II兲 ⫹ 1/ 2 O 2 Resulting dioxygen is dragged out of the reactor by the argon flow, the equilibrium being therefore constantly displaced towards the right. Once the reactor valves have been shut, the oxygen traces confined in the reactor are not sufficient to reoxidize Sn(II) upon cooling so that the high temperature state remains stable at room temperature. Finally, it should be noted that transposition of procedure 3 (starting from bulk-doped crystalline Cr2O3, i.e., Cr2O3: Sn(IV)) with replacement of hydrogen by argon (20 h at 900°C) was unsuccessful: no Sn(II) contribution could be detected in the Mo¨ssbauer spectra. This result was surprising, since argon use was found efficient for procedures 1 and 2. This point will be discussed later. 3.2.1. Surface stabilization by Sn(II) In addition to realization of hydrogen-free synthesis, the experiments described above present in fact more general interest. First of all, it must be emphasized that the success of syntheses performed under argon flow was anything but foreseeable since, in its own oxide SnO, Sn(II) is known [9] to disproportionate upon annealing at about 400°C in inert atmosphere, according to 4SnO 3 Sn ⫹ Sn 3O 4 The fact that similar disproportionation does not occur under argon with Sn(II)/Cr2O3 thus reflects stabilization of divalent tin, bonded to the same oxygen ligand, due to its location on
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the surface of the Cr2O3 grains. This stabilization also explains that, after treatment of the A precursor at 900°C under hydrogen, Sn is effectively found in the (II) oxidation state, instead of being reduced to metallic tin. As stated in ref. 10, this stabilization effect can be related to cationic sites with reduced coordination number, which progressively appear, due to surface dehydroxylation, when the temperature is increased. Such sites can hardly accommodate Cr(III), which requires octahedral coordination due to its d3 configuration. On the contrary, they are suitable for being occupied by Sn(II), whose stereochemically active lone pair can behave as a supplementary ligand. Occupancy of superficial sites by Sn(II) thus diminishes the number of Cr(III) with unsaturated coordination, which consequently reduces the surface energy. 3.2.2. Tin migration from the bulk towards the surface of the Cr2O3 crystallites In the case of procedure 3 (starting from crystalline Cr2O3:Sn(IV)) carried out under hydrogen at 900°C, Sn(IV) species can reach the surface due to diffusion and readily be reduced by hydrogen to Sn(II). Because of their much larger ionic radius (RSn(II) ⫽ 0.93 Å, RSn(IV) ⫽ 0.71 Å [11]), the Sn(II) which are formed cannot diffuse (through octahedral voids) back to the bulk of the crystallites. Disappearance of Sn(IV) at the surface results in a concentration gradient which favors their migration from the bulk and occupation of low coordination surface sites by the Sn(II) formed. On the contrary, as indicated above, such a behavior is not observed under argon atmosphere: Sn(IV) reduction does not occur. It can be assumed that this difference is related to the chemical properties of the two gases, with only hydrogen being able to act as an electron donor during annealing. This idea is corroborated by the result of annealing Cr2O3:Sn(IV) under carbon monoxide flow. In the presence of this electron donor, accommodation of tin in surface sites is observed. Considering the very different sizes of the various gaseous agents involved, these results show that tin distribution cannot be related to diffusion rate of the gases in the solid and that, essentially, interface phenomena are involved. Cr2O3 is a p-type semi-conductor [12]. For air-annealed samples, presence of Cr(IV) has been evidenced near the grain surface [13]. Drastic decrease of the conductivity observed in reducing conditions [14] can thus be related to decrease of the Cr(IV) amount due to reaction with the gaseous environment. It can thus be thought that, in the case of Sn(IV) bulk-doped crystalline Cr2O3, presence of Cr(IV) can locally limit the acceptable amount of the isovalent dopant, Sn(IV), in the corresponding region of the particles. Sn(IV) solubility being very low (s ⬍ 1 at% [15]), even in the bulk of Cr2O3 particles, where the Cr(IV) amount is necessarily lower than near the surface, the presence of Cr(IV) can inhibit Sn(IV) migration. In syntheses starting from Cr2O3:Sn(IV) (procedure 3), hydrogen and carbon monoxide can reduce the Cr(IV) species, which allows tin migration towards the surface. Argon cannot reduce the Cr(IV) species and, thus, migration does not occur. The result is completely different when the same inert gas is used for treatment of the amorphous A precursor (procedure 1), since reduction of the totality of tin is observed. We have noted that, during annealing of this A precursor at 900°C, passage of tin to the (II) oxidation state occurs before crystallization of the Cr2O3 matrix. This means that, in this case, the dopant is already located on surface sites of the grains when crystallization occurs, and that, being divalent, it remains in such positions during particle growth.
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3.2.3. Reaction with chlorine of Sn(II)/Cr2O3 prepared under argon Sn(II)/Cr2O3 prepared under hydrogen flow had been exposed to chlorine atmosphere at room temperature, and resulting transformations of the probe environment studied by Mo¨ssbauer spectroscopy [4]. In order to estimate the possible impact of hydrogen presence, similar experiments have now been carried out with Sn(II)/Cr2O3 prepared under argon flow. No significant difference appears in the two experiment results: instability of mixed (O2⫺, Cl⫺) anionic environments is still evidenced and a similar amount of Sn(IV) involved in the (SnCl6)2⫺ species is still observed. This means that the interface rearrangement previously described was not due to interfering reactions related to HCl formation.
4. Conclusion Location of divalent tin at the surface of crystalline Cr2O3 particles can be achieved under inert atmosphere by two synthesis routes, first, by annealing under argon amorphous hydrated chromium(III) oxide doped with Sn(IV) by coprecipitation, and second, by annealing under argon a sample with tin already accommodated in superficial sites but in the (IV) oxidation state. In both cases, the dopant distribution is the same as in samples prepared under hydrogen. However, if the starting material is Sn(IV) bulk-doped crystalline Cr2O3, in contrast to hydrogen treatment, argon treatment does not allow Sn(II) to be obtained. This difference can be related to the decreased Sn(IV) solubility in superficial layers, due to the presence of isovalent Cr(IV) whose amount depends upon the reducing properties of the gaseous environment. Even if the hydrogen-free synthesis does not affect the final environment of the dopant after exposure to chlorine, this new synthesis route is of interest, since it will allow for study of the reactivity of Sn(II)/Cr2O3 in the presence of stronger oxidizing agents (fluorine or its derivatives) which would unavoidably react with residual hydrogen.
Acknowledgments The described research was supported by the Russian Foundation for Basic Research (Grant N 97-03-331179a).
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