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Materials Chemistry and Physics 144 (2014) 1e7

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Materials Chemistry and Physics

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Materials science communication

Initial approach for obtaining sintered composites fromheterobimetallic complexes containing SneFe bond

Jonas L. Neto a,*, Geraldo M. de Lima b, Arilza O. Porto b, José D. Ardisson c,Carlos C. Rezende a

aChemistry Department, Federal University of Lavras, 37200-000 Lavras, MG, BrazilbChemistry Department, Federal University of Minas Gerais, 31270-901 Belo Horizonte, MG, BrazilcCDTN/CNEN, 31270-901 Belo Horizonte, MG, Brazil

h i g h l i g h t s

* Corresponding author. Tel.: þ55 35 3829 2222; faE-mail address: neto.jl@dqi.ufla.br (J.L. Neto).

0254-0584/$ e see front matter � 2014 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2013.12.043

g r a p h i c a l a b s t r a c t

� We have used complexes containingSneFe bond as precursors for sin-tered materials.

� Sintered composites were obtainedat lower temperature.

� The composites were characterizedas syn cassiterite and a-hematite.

� A higher concentration of Sn and Feseems to affect the sintering process.

a r t i c l e i n f o

Article history:Received 3 April 2013Received in revised form21 November 2013Accepted 30 December 2013

Keywords:A. Composite materialsB. SinteringC. Thermogravimetric analysis (TGA)C. Powder diffractionD. Mossbauer effect

a b s t r a c t

Three heterobimetallic complexes, [FeCp(CO)2]2SnCl2, [FeCp(CO)2]SnCl3 and [FeCp(CO)2]2Sn(PDC) inwhich Cp ¼ cyclopentadienyl and PDC ¼ pyridine-2,6-dicarboxylate, were synthesized, characterized bynuclear magnetic resonance spectroscopy (1H, 13C and 119Sn NMR), infrared spectroscopy (IR), ther-mogravimetric experiments (TG/DT), and elemental analysis (C, H, and N). The complexes were thenused as precursors for an initial approach of obtaining sintered composites respectively designated as (1),(2) and (3). X-ray diffraction (XRD, powder method) and 119Sn and 57Fe Mössbauer results have showedthe presence of SnO2, syn cassiterite and Fe2O3, syn hematite as main constitutes of the materials.Electron probe X-ray microanalysis (EPMA) and scanning electron micrographs (SEM) have indicated theformation of a compact solid for samples (1) and (2) and the PDC ligand have contributed to the presenceof more well-formed and dispersed grains for (3).

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

The interest in tin oxide-based powders is mainly justified bytheir application as gas sensors and catalysts [1,2]. Different workshave been reported using a large number of oxides as sintering aids[3e6], and they have been found to control the electrical conduc-tivity of the composites. It has been reported that SnO2-based

x: þ55 35 3829 1271.

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materials can be obtained by means of mixed oxide routes [7],polymeric precursor routes [3,8], co-precipitation [9], and solegelmethods [10]. The main aspect involving the polymeric precursorroute is the advantage of obtaining a homogeneous distribution ofthe additives in the sintered composite using soluble salts of themetals coordinated to carboxylic acids as chelate agents. Thethermal treatments in the presence of a polyhydroxy alcohol resultin the desired materials [3].

The use of tin oxide ceramics is limited by the low densificationduring sintering due to the dominance of non-densifying

J.L. Neto et al. / Materials Chemistry and Physics 144 (2014) 1e72

mechanisms for mass transport [11]. The densification can be ob-tained by hot isostatic pressing (HIP) [12] or with the help of anadditive like MnO2, CuO, Li2CO3, ZnO, Nb2O5, Fe2O3 or Co2O3 [13e15]. In this context it is important to study the effects of the addi-tives in the synthesis processes of sintered tin-based materials.

Heterobimetallic compounds containing transition metaletinbonds are especially important in the field of catalysis [16e18] andhave been studied as possible precursors for obtaining mixed ox-ides and sintered composites. The use of these kinds of compoundsas precursor for sinteredmaterials has not been deeply investigatedalthough some references employing other precursors can be foundin the literature [3,19]. Some of us have dedicated to the synthesis ofnanostructured solids from metal complexes and the results seemto be promising [20,21].

Therefore this work represents the initial step in the evaluationof synthesizing sintered tin oxide-based materials straight fromheterobimetallic compounds containing SneFe bond and deter-mine the influence of the PDC ligand in the materials formation.The employed complexes were: [FeCp(CO)2]2SnCl2, [FeCp(CO)2]SnCl3 and [FeCp(CO)2]2Sn(PDC) inwhich Cp¼ cyclopentadienyl andPDC ¼ pyridine-2,6-dicarboxylate. The thermal treatment gener-ated the composites (1), (2) and (3), respectively.

2. Experimental

2.1. Synthesis and characterization of the precursors

Stannyl complexes were prepared according to the literatureprocedures [22,23]. The synthesis procedures were carried out in anatmosphere of dry nitrogen. Allmanipulationswere conducted usingSchlenk techniques, employing a vacuum/nitrogen line. Solventsweredistilled fromNasuspensionandkept inSchlenkflaskswithKorNa mirror or dry molecular sieves. Other compounds were obtainedfromMerk, Aldrich or Strem.NMRspectrawere recordedat400MHz,1H; 100.62 MHz, 13C{1H}; 50.29 MHz, 119Sn{1H} using a Bruker DPX-400 spectrometer equipped with an 89 mm wide-bore magnet. 1Hand 13C shifts are reported relative to SiMe4 and119Sn shifts relative toSnMe4. The infrared spectra were recorded with samples pressed asKBr pellets in a PerkineElmer 283B spectrometer in the range of4000e400 cm�1. Carbon, hydrogen and nitrogen analysis was per-formed in a PerkineElmer PE-2400 CHN-analysis using tin sample-tubes. Thermogravimetric experiments (TG/DT) were carried out ina Shimadzu TGA-50H equipment with a heating rate of 10 �C min�1

until 900 �C at a flow rate of 50 ml min�1 in N2 and air.

2.1.1. [FeCp(CO)2]2SnCl2A solution of [FeCp(CO)2]2 5.0 g (14.13 mmol) and SnCl2$2H2O

10.0 g (44.32 mmol) in methanol (250 mL) and ethyl acetate(25 mL), was refluxed for 20 h under constant stirring. The solutionwas allowed to cool to room temperature and the yellow crystalswere then filtered off and washed with methanol and ethyl ether.The compound was purified from a methanol solution containing afew drops of toluene.

2.1.2. [FeCp(CO)2]SnCl3A solution of [FeCp(CO)2]2Cl2 1.03 g (1.90 mmol) and SnCl4

0.33 mL (2.82 mmol) in toluene (30 mL), was heated (100e110 �C)for 1 h under constant stirring. The solution was hot-filtered. Thesolution was allowed to cool to room temperature and the yellowsolid was then filtered off, dissolved in dichloromethane andprecipitated with carbon tetrachloride.

2.1.3. [FeCp(CO)2]2Sn(PDC)A solution of [FeCp(CO)2]2Cl2 2.40 g (4.40 mmol) in ethanol

(200 mL) was added to 0.74 g (4.43 mmol) of H2PDC (2,6-

pyridinedicarboxylic acid) and 1.23 mL (8.86 mmol) of triethyl-amine in the same solvent (220 mL). The system was stirred atroom temperature for 24 h. The solvent was partially removed atreduced pressure and the bright yellow solid was filtered off andwashed with water, ethanol and ethyl ether. The yellow productwas purified from a methanol solution containing a few drops oftoluene.

2.2. Obtainment and characterization of the sintered composites

Pyrolysis experiments were carried out using a Lindberg/Blue Mprogrammable tube furnace with a quartz tube placed inside. Oneside of the quartz tube was connected to dry nitrogen, oxygen or airsources and the other side led into a liquid trap. The samples wereplaced in a Coors porcelain boat which had been dried in an oven at500 �C for 40 min and then cooled to room temperature. The ex-periments were carried out under room atmosphere and the ovenwas programmed with a temperature ramp of 5 �C min�1 and thenheld at the plateau temperature (900 �C) for 5 min. The compositeswere characterized by EPMA, XRD, TG/DTG, SEM, and 57Fe Möss-bauer microscopy.

X-ray powder diffraction (XRD) experiments were executed in aRigaku Geigerflex using Cu Ka radiation (l ¼ 1.54178�A). A scan rateof 4�min�1 in the 2q range from 4� to 100� was used. The scanningelectron micrographs (SEM) were recorded in a JEOL JSM-840Aequipment and the electron probe X-ray microanalyses (EPMA)were carried out in a JXA 89000 RL wavelength dispersive/energydispersive combined microanalyser. For the SEM images and X-raymicroanalyses, the samples were covered with a thin film of goldand carbon, respectively, deposited by sputtering.

Mössbauer measurements were obtained using a conventionalapparatus with 57Co/Rh source of g-radiation kept at room tem-perature. 57Fe Mössbauer data were obtained at room temperatureusing a-Fe0 as reference. 119Sn Mössbauer measurements wereperformed with the samples at liquid N2 temperature and CaSnO3.

3. Results and discussion

3.1. Characterization of the stannyl derivatives

All the heterobimetallic complexes containing SneFe bondwerefully characterized and the spectroscopic and analytical data werecomparable to those results found in the literature [22,23].

3.1.1. [FeCp(CO)2]2SnCl2Yield: 85% (6.5 g). MP: 168e169 �C. Infrared (KBr, cm�1): 3102

n(CeH); 2003, 1966 n(C^O); 628, 576 d(MeC^O); 505 n(MeC^O);456 n(MeCp). 1H NMR (CDCl3 400 MHz): d 5.05 (h5-C5H5). 13C{1H}NMR (CDCl3 100.62 MHz): d 211.4 (C^O); d 84.4 (h5-C5H5). 119Sn{1H} NMR (DMF, in D2O 149.21 MHz): d 567. Elemental analysis forC14H10O4Cl2Fe2Sn, calcd. (found): C% 30.9 (30.5); H% 1.85 (1.79).

3.1.2. [FeCp(CO)2]SnCl3Yield: 45% (0.85 g). MP: 226 �C (decomposition). Infrared (KBr,

cm�1): 3110 n(CeH); 2055, 1989 n(C^O); 612, 576 d(MeC^O); 497n(MeC^O); 444 n(MeCp). 1H NMR (CDCl3 400 MHz): d 5.24 (h5-C5H5). 13C{1H} NMR (CDCl3 100.62 MHz): d 208.1 (C^O); d 84.5 (h5-C5H5). 119Sn{1H} NMR (DMF, in D2O 149.21 MHz): d �122.5.Elemental analysis for C7H5O2Cl3FeSn, calcd. (found): C% 20.9(21.5);H% 1.25 (1.05).

3.1.3. [FeCp(CO)2]2Sn(PDC)Yield: 80% (2.25 g). MP: 266 �C (decomposition). Infrared (KBr,

cm�1): 3088 n(CeH); 1991, 1958 n(C^O); 1662 nasym(COO); 2296n(CC/CN); 1350 nsym(COO); 632, 583 d(MeC^O); 514 n(MeC^O);

Fig. 1. TG/DTG results at synthetic air atmosphere for the compounds [FeCp(CO)2]2SnCl2, [FeCp(CO)2]SnCl3 and [FeCp(CO)2]2Sn(PDC), precursors for composites (1), (2), and (3),respectively.

J.L. Neto et al. / Materials Chemistry and Physics 144 (2014) 1e7 3

458 n(MeCp). 1H NMR (CDCl3 400 MHz): d 8.59, 8.46 (C5H3N, Py);d 5,05 (h5-C5H5). 13C{1H} NMR (CDCl3 100.62 MHz): d 211.5 (C^O);d 163.4 (COO); d 146.8, 144.4, 126.9 (C5H3N, Py); d 83.1 (h5-C5H5).119Sn{1H} NMR (DMF, in D2O 149.21 MHz): d 189. Elemental anal-ysis for C21H13NO8Fe2Sn, calcd. (found): C% 39.5(38.9); H% 2.00(1.95), N% 2.20 (2.20).

3.2. Obtainment and characterization of the sintered composites

The TG/DTG experiments (Fig. 1) have allowed establishing thetemperature range in which the composites would be free oforganic materials, especially chloride anions (about 600 �C) since itcould be harmful to the desired characteristics of the solids. The TG/

Fig. 2. EPMA spectrum obtained for composite (1).

J.L. Neto et al. / Materials Chemistry and Physics 144 (2014) 1e74

DTG experiments for compounds (1) and (2) have presented threestages of weight loss. In both cases the thermal decompositionprocesses were observed in the temperature range of 160e500 �Ccorresponding to the loss of CO, Cl and C5H5 respectively [24,25].The analysis of the TG/DTG results for the compound (3) hasshowed a thermal decomposition process in two steps. The firstone, in the temperature range of 233e334 �C, corresponding to theloss of CO þ Cp fragments (calcd. 17.8%). The second step hasoccurred in the temperature range of 334e447 �C attributed to theloss of Cp þ PDC release (calcd. 36.1%) [24e26]. The results haveindicated that the precursors containing chlorine atoms havegenerated composites in percentage different from those expectedfor the formation of SnO2 and Fe2O3. The mechanisms of formationof the composites seem to take place by the liquid-phase sinteringprocess since the melting point or the beginning of the

Fig. 3. XRD patterns for the co

decomposition processes occurs until 300 �C for compounds (1), (2)and (3) [27]. But this observation is not conclusive and more ex-periments need to be done.

It has been related that the presence of iron can reduce thesintering temperature of SnO2 [5] and so the chosen temperature,as an initial experiment based on the beginning of the sinteringprocess described in the literature, was 900 �C [28,29]. The ob-tained materials presented a bright surface which can suggest thebeginning of a sintering process except for the composite (3) inwhich a porous and grayish surface was observed. The EPMA re-sults indicated the absence of chloride anions and the presence ofFe, Sn, and O as main elements in the composites as shown in Fig. 2.The detected carbon is originated from the coating procedures ofthe samples and not from non-decomposed organic materials oncethe microanalysis results indicated the absence of carbon in the

mposites (1), (2), and (3).

Fig. 4. 57Fe Mössbauer spectra for the samples at room temperature.

J.L. Neto et al. / Materials Chemistry and Physics 144 (2014) 1e7 5

materials obtained from the thermal experiments (results notshown). Compounds (2) and (3) have presented similar results (notshown in this work). Quantitative results were not obtained.

XRD analyses indicated the presence of SnO2 and Fe2O3 for allcomposites (Fig. 3). These results are in agreement with the EPMAsince we have observed just Sn, Fe, O and carbon (coating pro-cedures of the samples) for all the composites. The X-ray linesshowed considerable broadening, indicating the fine nature of theoxides. All patterns showed the typical reflections of SnO2, syncassiterite (JCPDS 041-1445) and Fe2O3, syn hematite (JCPDS 033-0664) [30]. The observed peaks at (110), (101), (200), (111), (211),

Fig. 5. 119Sn Mössbauer spectra for the samples at room temperature.

(220), (112), (301), and (210), (104), (110), (113), (024), (116), (122),(018), (214), (300) are in agreement with the formation of syncassiterite and syn hematite, respectively. Despite the possibility ofsolid solution formation based on iron and tin by preparing oxidesunder controlled conditions, it has not been clearly observed in thiswork.

The 119Sn and 57Fe Mössbauer data corroborated the XRD re-sults suggesting the formation of syn cassiterite and syn hematiterespectively (Figs. 4 and 5, Table 1). The 119Sn Mössbauer resultsshowed two sites with approximately 85 and 22% in area. Theisomer shifts for almost all composites are somewhat differentthan that for pure SnO2 (0.03 mm s�1) suggesting a variation ofthe tin dioxide environment. The non-zero quadrupole splittingindicated a break of symmetry at the tin center, possibly by theexistence of oxygen vacancies, according to the deviation of theobserved isomer shifts for the composites compared with pureSnO2 [31]. The 57Fe Mössbauer parameters are in agreement withthe XRD results and showed the formation of syn hematite(Table 1 and Fig. 5). Typical sextets for Fe2O3 with a relative area ofapproximately 94% and hyperfine fields of 51.8T were observed.Nevertheless, the presence of doublets with relative areas about6% can be seen for all materials, probably associated to the pres-ence of superparamagnetic hematite with smaller particle size[30,32]. The Mössbauer parameters obtained for the compositesare quite different from the values found in the literature for theprecursors [23].

The SEM images, Fig. 6, were obtained for each composite inorder to investigate the level of homogeneity and the presence ofoxide agglomerates. The surface of compound (3) presented a moregranular aspect probably due to the presence of the ligand PDCwhich could promote a better dispersion and production of well-formed grains during the thermal experiment [3]. For composites(1) and (2) more homogeneous surfaces were observed, possiblyindicating the onset of a sintering process. The material (1) pre-sented smaller particles size than (2) and a more effectivearrangement. A higher concentration of metals might havecontributed to the better sintering of (1) as well as the lower con-centration of chlorine atoms in the precursors [3,33].

4. Conclusions

Heterobimetallic complexes containing SneFe bond were usedas precursors for obtaining sintered materials. All the complexeswere characterized as the desired products and then submitted tothe thermal treatment. The thermal experiments were performedfor the precursors and the temperature was established at 900 �C inan attempted to obtain the sintered materials. The obtained

Table 157Fe and 119Sn Mössbauer parameters for the prepared materials.

Compound Metalcenter

d/mm s�1

(0.05)D/mm s�1

(0.05)2 3Q/mm s�1

(0.05)Bhf/T(0.3)

RA/%(1)

(1) Sn(IV) 0.00 0.57 e e 84Sn(IV) 0.06 2.03 e e 16Fe(III) 0.36 e �0.21 51.8 94Fe(III) 0.34 0.86 e e 6

(2) Sn(IV) 0.00 0.56 e e 86Sn(IV) 0.10 2.08 e e 14Fe(III) 0.36 e �0.21 51.8 94Fe(III) 0.35 0.80 e e 6

(3) Sn(IV) 0.00 0.54 e e 87Sn(IV) 0.03 2.18 e e 13Fe(III) 0.36 �0.21 51.8 94Fe(III) 0.34 0.73 e e 6

d ¼ isomer shift relative to CaSnO3 and aFe; 2 3Q ¼ quadrupole shift, D ¼ quadrupolesplitting, Bhf ¼ magnetic hyperfine field and RA ¼ relative subspectral area.

Fig. 6. SEM micrographs for the materials (1), (2) and (3).

J.L. Neto et al. / Materials Chemistry and Physics 144 (2014) 1e76

composites were characterized as solids constituted by SnO2, syncassiterite and Fe2O3, syn hematite based on EPMA, XRD and 119Snand 57Fe Mössbauer results. The surface analyses indicated that thecomposites (1) and (2) presented evidence of sintering processes,mainly in the first case. Compound (3) presented a surface con-taining agglomerates of tin and iron oxide. This behavior could beattributed to the presence of the ligand PDC which contributed tothe presence of well-formed grains during the thermal treatment.

Acknowledgments

The authors are grateful to CNPq and Fapemig for the financialsupport.

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