Low temperature co-pyrolysis of hexabenzylditinsulfide and selenium. an alternate route to Sn(SxSe1−x)

Materials Research Bulletin, Vol. 34, Nos. 14/15, pp. 2327–2332, 1999 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/99/$–see front matter

PII S0025-5408(00)00152-5

LOW TEMPERATURE CO-PYROLYSIS OF HEXABENZYLDITINSULFIDE AND SELENIUM. AN ALTERNATE ROUTE TO Sn(SxSe1ⴚx)

Philip Boudjouk*, Michael P. Remington, Jr., Dean J. Seidler, Bryan R. Jarabek, Dean G. Grier, Brian E. Very, Raquel L. Jarabek, and Gregory J. McCarthy Center for Main Group Chemistry, Department of Chemistry, North Dakota State University, Fargo, ND 58105, USA (Refereed) (Received February 17, 1999; Accepted February 19, 1999)

ABSTRACT Benzyl-substituted tin chalcogenides (Bn3Sn)2S (1) and (Bn3Sn)2Se (2) yield polycrystalline-phase pure SnS and SnSe in good ceramic yields when pyrolyzed with S and Se, respectively, at 275°C. Heating mixtures of (1) and elemental selenium produce solid solutions of the formula Sn(SxSe1⫺x). Combustion analysis showed less than 1% residual carbon in all ceramic products. This methodology allows the complete conversion of tin to tin chalcogenides and eliminates the need to synthesize organosulfur and organoselenium intermediates. © 2000 Elsevier Science Ltd KEYWORDS: A. chalcogenides, A. inorganic compounds, A. semiconductors, B. chemical synthesis, D. X-ray diffraction INTRODUCTION Tin sulfide (SnS) and tin selenide (SnSe) have many favorable properties that make them appealing for use as semiconductor materials. In their orthorhombic phases, these compounds are classified as narrow-bandgap semiconductors and may serve as efficient materials in photovoltaic applications. SnS, with an optical bandgap of 1.08 eV (very close to that of silicon), obtains high-conversion efficiency in photovoltaic devices [1]. SnS is also favorable in terms of cost, availability, toxicity, and stability [1c]. Studies of the photoelectric properties of SnSe reveal similar promise for energy conversion devices and in its polycrystalline

*To whom correspondence should be addressed. 2327

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form [2]. For example, polycrystalline films of SnSe have been found useful in memoryswitching devices [3]. A semiconducting material useful in bulk, polycrystalline form circumvents many difficulties in preparing epitaxial films directly from the elements. Structural studies of Sn(S,Se) solid solutions have shown obedience to Vegard’s law [4,5]. Other Group 14/16 ternary systems show similar behavior [6,7]. However, the electrical and optical properties of Sn(S,Se) were found to vary in an orderly, but nonlinear fashion, as a function of composition [4], making it feasible to tailor the physical, electrical, and optical properties of materials by varying the composition to obtain a specific, desired material [8]. We have developed a variety of approaches for synthesizing stoichiometric and nonstoichiometric semiconducting materials [5,9 –12]. Recently, we reported the pyrolysis of hexabenzylditin chalcogenides as a mild method of preparing SnS, SnSe, and Sn(S,Se) solid solutions [5,13]. While this route is attractive because the organotin chalcogenides (Bn 3Sn) 2E 3 SnE ⫹ Sn ⫹ Bn 2 (Bn ⫽ PhCH 2; E ⫽ S, Se) (Bn 3Sn) 2S ⫹ (Bn 3Sn) 2Se 3 Sn(S,Se) ⫹ Sn ⫹ Bn 2 are easy to prepare, convenient to handle, and produce the tin chalcogenides at low temperatures, it also generates tin metal as a major product. This is not only an inefficient use of the reagents, but also is problematic in purifying the target chalcogenide. We have remedied that deficiency by simply adding elemental chalcogens to the organotin chalcogenides and heating the mixtures at 275°C, well below the temperatures of ⬎1000°C normally required to prepare tin chalcogenides and their solid solutions [14]. EXPERIMENTAL All reactions and manipulations were performed in a fume hood in accordance with standard Schlenk techniques. (Bn3Sn)2S (1) and (Bn3Sn)2Se (2) were prepared as described previously [5]. Flow pyrolyses were performed using a Lindberg model 55035 programmable tube furnace, 36 cm long and 3 cm in diameter, with a 55 ⫻ 2.5 cm Vycor silica glass tube placed inside. One end of the tube was fitted with a one-hole septum connected to a dry nitrogen source and sealed with parafilm. Nitrogen flow was monitored at the exit of the tube by a mineral oil bubbler. The flow was set at approximately 50 mL/min and the tube was purged at this rate for at least 0.5 h before introduction of the sample. In a typical experiment, the amount of precursor (1) or (2) was weighed and placed in a Coors porcelain boat, which had been oven-dried and cooled under a stream of dry nitrogen in the tube furnace. A measured quantity of S or Se was placed in the boat on top of the precursor. Premixing of the reactants in a mortar and pestle did not result in significant differences in the composition of the products. The crucible containing sample, typically 500 to 800 mg, was placed in the tube at the center of the furnace. For all runs, the oven was programmed at a rate of 10°C/min until it reached 120°C, held at this temperature for 1 h to remove moisture, and then ramped up at a rate of 2.5°C/min to 275°C. This temperature was maintained for 10 h before allowing the oven to cool to room temperature. The volatile products were condensed near the exit of the tube using dry ice and collected by washing the tube with acetone and hexane. The solvent was removed in vacuo or under a stream of air and the remaining residue analyzed by 1H NMR. Yields were computed on samples weighed prior to characterization by X-ray powder diffraction (XRPD).

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TABLE 1 Pyrolyses of Hexabenzylditinchalcogenide and Chalcogens Ceramic yield (%)

Mass balance (%)

XRD

NMR of volatiles

E

Exp.

Theor.

S

34.8

35.6

72

phase pure SnS

Bn2:Bn2S2:Bn2S (6:3:1)

Se

41.2

42.5

80

phase pure SnSe

Bn2:Bn2Se (2:1)

Comments SnS: good crystallinity; traces of Sn and Sn oxides SnSe: good crystallinity; traces of Sn and tin oxides

X-ray powder patterns were recorded from hexane slurry samples mounted on glass slides, using a Philips automated diffractometer with a graphite diffracted beam monochromator and variable divergence slit, using Cu K␣ (␭ ⫽ 1.5418 Å) radiation. NIST 660 lanthanum hexaboride (LaB6) was used as an internal d-spacing standard. 1 H-NMR spectra were obtained on a JEOL GSX270 spectrometer at 270.17 MHz. Typical samples were prepared as 0.1 or 0.2 M solutions in CDCl3. Combustion analyses were performed by Galbraith Laboratories, Knoxville, TN. RESULTS AND DISCUSSION The results of the pyrolyses of hexabenzylditinchalcogenide and chalcogens (eq. 1) at 275°C for 10 h under a continuous flow of nitrogen at 1 atm are summarized in Table 1. (Bn 3Sn) 2E ⫹ E 3 SnE ⫹ Bn 2

(1)

The SnS and SnSe produced by these reactions were phase pure and polycrystalline, with unit-cell parameters in excellent agreement with those found in the PDF [15]. Elemental tin would not form if the precursor lost only bibenzyl (Bn2) (eq. 1). However, some of the chalcogen was removed from the system as BnxEx. This indicates a more complex decomposition mechanism than observed for the thermolysis of hexabenzylditinchalcogenides [5,13]. Small quantities of the tin oxides SnO and SnO2 were detected in the product mixture. Presumably they were formed from the oxidation of traces of elemental tin during handling. Solid solutions were prepared according to eq. 2, using the same conditions as above. (Bn 3Sn) 2S ⫹ Se 3 Sn(S,Se) ⫹ Bn 2

(2)

The compositions of the Sn(S,Se) solid solutions were determined by comparison of the unit cell to a linear interpolation between the end members. LaB6 was used as an internal d-spacing standard. Figure 1 shows the diffractograms of the end members and the 1:3, 1:1, and 3:1 solid solutions. Table 2 summarizes our results.

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FIG. 1 Diffratograms of the end members and solid solutions with A: (Bn3Sn)2Se ⫹ Se; B: (Bn3Sn)2S ⫹ 3Se; C: (Bn3Sn)2S ⫹ Se; D: 3(Bn3Sn)2S ⫹ Se; and E: (Bn3Sn)2S ⫹ S. All unlabeled peaks are due to Sn(S,Se). x ⫽ tin, * ⫽ LaB6 std., o ⫽ SnO, and s ⫽ Sn(S,Se)2. In all cases, reactions carried out under identical conditions gave reproducible percentages of Se substitution, within 5%. For example, the reaction of 1 mole of hexabenzylditinsulfide with 3 moles of Se gave values of 60.8 and 65% SnSe at 275°C for 10 h. It should be noted that longer reaction times (e.g., 275°C for 72 h) or reactions at higher temperatures and shorter reaction times (e.g., 450°C for 2 h) did not result in any significant increase in crystallinity or difference in the amount of Se substitution. Thus, 1:1 reactions at 275°C/10 h, 275°C/72 h, and 450°C/2 h gave values of 45.7, 46.9, and 42.8%, respectively, with all the resulting ceramic crystallinities being comparable. At higher concentrations of Se, oxidation was observed, in addition to the formation of the phase ⬃Sn(S0.37Se0.63). Thus, significant amounts of the Sn(IV)(S,Se) solid solutions were produced as a crystalline, ceramic impurity. TABLE 2 Solid Solutions of Sn(S,Se) from (Bn3Sn)2S and Elemental Selenium Reaction conditions (precursor:Se) 275°C, 10 h (1:1) 275°C, 10 h (3:1) 275°C, 10 h (1:3) a

% SnSea Theor.

Exp.

Impurities

Crystallinity

50 25 75

42 (⫾5) 22 (⫾5) 63 (⫾5)

traces of Sn and SnO2 major Sn and traces of SnO major Sn(S,Se)2

good good very poor

Percent SnSe dissolved in SnS.

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The ceramic yields and mass balances in these reactions were excellent. Analysis of the organic byproducts by 1H NMR revealed a majority of Bn2 and Bn2Se in a 10:1 ratio for the 1:1 reaction, 50:1 for the 3:1 reaction, and 5:1 for the 1:3 reaction. The formation of Bn2Se in the byproducts accounts for the discrepancy between the targeted and final stoichiometry of the ceramic product. As mentioned above, the product distribution of the organic residues indicates a rather complicated mechanism. Thus, it seems that a multipathway-condensed phase reaction is responsible for the depletion of E and the formation of tin-enriched ceramics. A reasonable mechanism could be chalcogen insertion into the Sn–C bond [16] followed by competing homolytic bond cleavage to form bibenzyl [5] and intra- or intermolecular nucleophilic displacement to form Bn2E [11]. In summary, we have found that elemental S and Se react with (1) and (2), respectively, to produce phase-pure, polycrystalline SnS and SnSe at low temperatures in a condensedphase pyrolysis. The reaction between (1) and elemental Se is an efficient source of Sn(SxSe1⫺x) solid solutions. This methodology avoids the synthesis and handling of organoselenium intermediates. Small amounts of BnxSey compounds were produced, indicating the presence of reaction pathways competitive with the dominant mechanism of homolytic cleavage of the benzyl group from tin. ACKNOWLEDGMENTS Financial support from the Department of Energy (Grant No. DE-FG02-91ER75674), the Office of Naval Research (grant no. NOO14-96-1 1271), and the National Science Foundation (grant no. OSR 9452892) is gratefully acknowledged. M.R. is thankful for a North Dakota State EPSCoR Doctoral Dissertation Fellowship. REFERENCES 1.

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