the influence of the r group in the thermal stability of sn4r4o6 (r = methyl, n-butyl or phenyl)

The influence of the R group in the thermal stability ofSn4R4O6 (R ¼ methyl, n-butyl or phenyl)

A.G. Pereiraa, L.A.R. Batalhaa, A.O. Portoa,1, G.M. de Limaa,*,G.G. Silvaa, J.D. Ardissonb, H.G.L. Siebalda

aDepartamento de Quımica, ICEx, Universidade Federal de Minas Gerais (UFMG), Belo Horizonte, MG 31270-901, BrazilbLaboratorio de Fısica Aplicada, CDTN/CNEN Belo Horizonte, MG 31270-010, Brazil

Received 5 April 2003; received in revised form 7 August 2003; accepted 7 September 2003

Abstract

Pyrolysis experiments were carried out with Sn4R4O6 {R ¼ methyl (1), butyl (2) and phenyl (3)}. The thermal

behaviour of the organotin oxides was studied by Thermogravimetric Analysis (TG) with simultaneous

Differential Thermal Analysis (DTA) and the residues obtained after decomposition were characterised by X-ray

electron probe microanalysis (EPMA), Scanning Electron Microscopy (SEM), X-ray diffraction and 119Sn

Mossbauer spectroscopy. The results have shown that the methyl derivative is the best precursor, producing

nanoparticles of pure phase rutile-type tetragonal SnO2 in O2 with a yield up to 90%.

# 2003 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures; A. Inorganic compounds; A. Oxides; A. Organometallic compounds

1. Introduction

Tin (IV) oxide, SnO2, is a n-type semiconductor with a wide band gap (Egap ¼ 3:6 eV at 300 K) andpossess potential applications as catalyst support [1], transparent conducting electrode [2] and gassensor [3]. Nanocrystalline SnO2 has different properties from bulk crystal and much attention has beenaddressed to the synthesis and characterisation of such material. A great variety of chemical andphysical methods [4–6] has been used to produce this semiconductor.

The outcomes of an introductory work, reported recently by us [7] have accounted for the formationof nanometric SnO2 powders through the thermal decomposition of Sn3Bu6O3 and Sn4Bu4O6 (2) inspecific conditions (atmosphere, gas flux, temperature). Due to the technological importance of Sn(IV)

Materials Research Bulletin 38 (2003) 1805–1817

* Corresponding author. Tel.: þ55-3134995744; fax: þ55-3134995720.

E-mail addresses: [email protected] (A.O. Porto), [email protected] (G.M. de Lima).1 Co-corresponding author.

0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2003.09.002

oxide it is worth it to go further and investigate the influence of the R (R ¼ Me, Ph and n-Bu) ligand inthe thermal behaviour of organotin oxides, as well as the best precursor for preparing such material.Herein, we report the results of 119Sn Mossbauer Spectroscopy and X-ray electron probe microanalysis(EPMA) for the residues obtained by pyrolysis of compounds (1), (2) and (3) in air, O2 and N2. Thestudies were performed employing Thermogravimetric Analysis (TG) with simultaneous DifferentialThermal Analysis (DTA). The residues were characterised by X-ray EPMA, X-ray diffraction (XRD),Scanning Electron Microscopy (SEM) and 119Sn Mossbauer spectroscopy.

2. Experimental

2.1. Synthesis and characterisation of the organotin oxides

The Sn4R4O6 {R ¼ Me(1) and Ph(3)}, precursors, Fig. 1, were prepared and characterised employingthe same synthetic pathway described by us [7] with small changes from the literature procedure [8].

2.2. Thermal studies

Thermogravimetric Analysis with simultaneous DTA were carried out using a TA Instruments SDT2960 Simultaneous DTA-TGA equipment with a heating rate of 5 8C/min until 500 8C in air, oxygenand nitrogen atmospheres.

2.3. Thermal decomposition and residue characterisation

Compounds (1) and (3) were decomposed in oxygen, air and nitrogen atmospheres (gas fluxes,100 ml/min) in a quartz tube furnace using a heating rate of 5 8C/min until 500 8C.

X-ray diffraction patterns were collected with a Rigaku Geigerflex equipment by the use ofNi-filtered Cu Ka radiation (l ¼ 1:5418 A) and a graphite monochromator in the diffracted beam. Puresilicon was used as internal standard for angle calibration. The SnO2 crystallite average size (D) wascalculated by the Scherrer equation (Eq. (1)):

D ¼ lD

Wcosy (1)

Sn

O

O

O

Sn

Sn

Sn

O

O

O

R

R

R

R

Fig. 1. Chemical structure of Sn4R4O6 (R ¼ methyl, n-butyl and phenyl).

1806 A.G. Pereira et al. / Materials Research Bulletin 38 (2003) 1805–1817

100 200 300 400 50060

65

70

75

80

85

90

95

100

(3)

(2)

(1)

O2

Temperature / ºCTemperature / ºC

Temperature / ºC

100 200 300 400 50060

65

70

75

80

85

90

95

100

(3)

(2)

(1)

Air

100 200 300 400 50060

65

70

75

80

85

90

95

100

(3)

(2)(1)

N2

Wei

ght L

oss

/ %

Wei

ght L

oss

/ %

Wei

ght L

oss

/ %

Fig. 2. TG of Sn4Me4O6 (1), Sn4Bu4O6 (2) and Sn4Ph4O6 (3) in air, oxygen and nitrogen.

A.G

.P

ereiraet

al./M

ateria

lsR

esearch

Bu

lletin3

8(2

00

3)

18

05

–1

81

71

80

7

where l is the wavelenght of the incident beam, DW is the full width at half maximum in radians and yis the Bragg’s angle.

The SEM images were taken in a JEOL JSM-840A equipment and the EPMA was carried out in aJXA 89000 RL wavelenght/energy dispersive combined microanalyser and the samples were recoveredwith a thin film of Gold and Carbon, respectively, deposited by sputtering.

119Sn Mossbauer measurements were performed on a conventional apparatus with the samples atliquid N2 temperature and a CaSnO3 source kept at room temperature, in the residue obtained afterpyrolysis in order to identify the Sn oxidation state and number of different sites.

3. Results and discussion

The temperature of decomposition, TD, observed in the TG curves of (1), (2) and (3), Fig. 2, in air andoxygen are smaller than in nitrogen. Compound (3) experiences a sublimation process at very lowtemperatures (above 100 8C).

For (1) and (3) the decomposition occurs in a single step mechanism and for (2) a two step processwas observed in air and O2.

The initial weight values were fixed as 100% at 60 8C to disregard the initial loss of weight due to thepresence of solvent molecules. The weight residue (WR) and the theoretical weight residue (TW) wereobtained considering the total conversion of the precursors into SnO2, Table 1. For (1) and (2), the WRvalues obtained in oxygen and air are very close to the expected one, within the experimental error,indicating in this case, the formation of only SnO2. The discrepancy of about 10% for compound (1)and 6% for (2) in nitrogen between WR and TW values is probably due to the formation of a mixture ofSn oxides. On the contrary, for (3) the WR values obtained in all atmospheres agrees very well with theexpected value of TW. It certifies that compound (3) decomposes into SnO2 even in inert atmosphere.

The DTA curves, Fig. 3, showed that the decomposition process are all exothermic in air and oxygen.The decomposition of all precursors in N2 is an endothermic process and the peaks above 400 8C areassociated to the crystallisation of the samples.

EPMA results of the residues of (1), (2) and (3) showed only the presence of Sn and O, Fig. 4 (itshows only the results of (1) in oxygen since they are all similar). The carbon present in the sampleswas deposited for the EPMA analysis.

Typical XRD patterns of the residues of (1), (2) and (3) are shown in Fig. 5 as well as the pattern ofstandard tetragonal SnO2 (rutile-type structure). The main diffraction lines of tetragonal SnO2 {(1 1 0),

Table 1

Temperature of decomposition (TD) and amount of residual powder (WR) obtained by analysing the TG curves for (1), (2) and

(3) decomposed in air, O2 and N2

Compound TD (8C) WRa (%) TW (%)

Air O2 N2 Air O2 N2

Sn4Me4O6 (1) 313 328 371 89 92 85 95

Sn4Bu4O6 (2) 229 234 338 72 72 69 75

Sn4Ph4O6 (3) 333 322 351 65 66 68 68

The accuracy in TD is approximately �1 8C.a Experimental error 3%.


100 200 300 400 500

(3)

(2)

(1)

Air E

ndo

Temperature / 0C

100 200 300 400 500

(3)

(2)

(1)

O2

End

o

Temperature / 0C

100 200 300 400 500

(3)

(2)

(1)

N2E

ndo

Temperature / 0C

Fig. 3. DTA of Sn4Me4O6 (1), Sn4Bu4O6 (2) and Sn4Ph4O6 (3) in air, oxygen and nitrogen.

A.G

.P

ereiraet

al./M

ateria

lsR

esearch

Bu

lletin3

8(2

00

3)

18

05

–1

81

71

80

9

Fig. 4. EPMA of the residue of (1) in oxygen.

Fig. 5. XRD patterns of the residue of decomposition of Sn4Me4O6 (1), Sn4Bu4O6 (2) and Sn4Ph4O6 (3) obtained in air (a),

O2(b), N2(c) and as well as the standard SnO2 (d).


(1 0 1), (2 0 0), and (2 1 1)} were observed in all residues. The sharp and intense peaks obtained for (1)and (3) decomposed in oxygen indicate a high degree of crystallisation. Less intense and narrow peaks{(1 1 0) and (1 0 1)} were detected for the product of the decomposition of (1) in air and of (2) in air andoxygen, which is probably due to the worst quality of the SnO2 crystals. The XRD pattern of (1) and (3)

0,84

0,88

0,92

0,96

1,00

(a)

(b)

(c)

(d)

(e)

(f)

0,86

0,88

0,90

0,92

0,94

0,96

0,98

1,00

Rel

ativ

e T

rans

mis

sion

(%)

-6 -4 -2 0 2 4 6

0,84

0,88

0,92

0,96

1,00

Velocity (mm/s)

0 1 2 3

∆ (mm/s)(A) (B)

Fig. 6. 119Sn-Mossbauer spectra (A) and quadrupole distribution (B) of the residue of (1) decomposed in air (a, d), O2 (b, e)

and N2 (c, f) obtained at liquid nitrogen temperature.

A.G. Pereira et al. / Materials Research Bulletin 38 (2003) 1805–1817 1811

decomposed in nitrogen presented not only the main SnO2 diffraction lines but also three other lines{2 y ¼ 29.80, 33.27 and 50.63 degrees} revealing the formation of other Sn oxides. The outcome of theXRD results is that more crystalline SnO2 is achieved when compound (1) and (3) is decomposed in O2.

The (1 1 0) diffraction line was used to estimate the crystallite average size by Scherrer equation(Eq. (1)), Table 2. For the residues of (1), (2) and (3) the crystallite average size is bigger in oxygen

0,94

0,96

0,98

1,00

(a)

(b)

(c)

(d)

(e)

(f)

0,85

0,90

0,95

1,00

-6 -4 -2 0 2 4 60,88

0,92

0,96

1,00

Rel

ativ

e tr

ansm

issi

on

(%

)

Velocity (mm/s)

0,0 0,6 1,2 1,8 2,4

PD

eq

Deq (mm/s)(A) (B)

Fig. 7. 119Sn-Mossbauer spectra (A) and quadrupole distribution (B) of the residue of (2) in air (a, d), O2 (b, e) and N2 (c, f)

obtained at liquid nitrogen temperature.


than in air or nitrogen. The D values obtained in this case is probably the biggest due to the veryexothermic decomposition process in oxygen.

The 119Sn Mossbauer parameters, isomer shift (IS) and quadrupole splitting (QS) were obtained as themaximum values related to the curves of quadrupole distribution, Figs. 6–8 and Table 3. The IS values

0,90

0,95

1,00

(a)

(b)

(c)

(d)

(e)

(f)

0,8

0,9

1,0

Rel

ativ

e T

ran

smis

sio

n (

%)

-6 -4 -2 0 2 4 60,8

0,9

1,0

Velocity (mm/s)

0 1 2 3

∆(mm/s)(A) (B)

Fig. 8. 119Sn-Mossbauer spectra (A) and quadrupole distribution (B) of the residue of decomposition of (3) under air (a, d),

O2 (b, e) and N2 (c, f) obtained at liquid nitrogen temperature.


were consistent with the presence of Sn(IV) and correlated very well with (i) other parameters reportedin the literature [9] and (ii) SnO2 recorded as standard. The IS values obtained for the residues were verydifferent from the starting materials (0.90 mm/s Sn4Me6O4 (1), 0.86 mm/s for Sn4Bu6O4 (2) and0.98 mm/s for Sn4Ph6O4) indicating the complete decomposition of them into tin inorganic materials.

The Mossbauer data certify that precursors (1) and (2) have produced pure SnO2 by thermaldecomposition in air and O2. For compound (3) decomposed in O2 a small amount of Sn(II) oxide was

Table 2

Crystallite average size (D), full width at half maximum (DW) of the (1 0 0) diffraction peak calculated by the Scherrer

equation for the residues of (1), (2) and (3)

Residue Air Oxygen Nitrogen

D (nm) DW/2y (degree) D (nm) DW/2y (degree) D (nm) DW/2y (degree)

(1) 6.37 1.28 74.27 0.11 21.51 0.38

(2) 12.03 0.68 14.90 0.56 12.96 0.06

(3) 1.94 4.21 9.22 0.95 8.02 1.02

Table 3119Sn Mossbauer parameters, isomer shift (IS), quadrupole splitting (QS), area and width obtained at liquid nitrogen

temperature for residue of pyrolysis of (1), (2) and (3)

Residue Atmosphere Site IS (mm/s) QS (mm/s) Area (%) Width (mm/s)

(1) Air Sn(IV) 0.06 0.04 100a 0.90

0.04 1.94 0.90

O2 Sn(IV) 0.04 0.43 100a 0.90

0.03 2.08 0.90

N2 Sn(IV) 0.04 0.50 45a 0.90

0.04 2.30 0.90

Sn(II) 2.69 1.69 55 1.15

(2) Air Sn(IV) 0.04 0.59 100 0.90

O2 Sn(IV) 0.04 0.50 100a 0.90

0.06 1.72 0.90

N2 Sn(IV) 0.00 0.70 43a 0.90

0.05 1.64 0.90

Sn(II) 2.90 0.60 57 1.15

(3) Air Sn(IV) 0.06 0.52 100a 0.90

0.10 1.64 0.90

O2 Sn(IV) 0.05 0.46 96a 0.90

0.06 1.42 0.90

0.07 2.40 0.90

Sn(II) 2.65 1.86 4 0.90

N2 Sn(IV) 0.07 0.46 93a 0.90

0.07 1.46 0.90

0.08 2.50 0.90

Sn(II) 2.69 1.69 7 1.15

The experimental errors associated to IS, QS and width are, respectively, 0.04, 0.05 mm/s and 0.05%.a These values relates to the total area associated to Sn(IV) sites.


observed. A complicated pattern was obtained for the residue prepared in N2, revealing mixtures of Sn(IV) and Sn (II) oxides for all compounds. The IS are somewhat higher than for pure SnO2 (0.03 mm/s)which was recorded as standard, suggesting a small variation in the Sn–O bonding scheme in ourmaterials. The high IS (above 2 mm/s) values are characteristic of Sn(II).

The non-zero QS, showed a break of symmetry at the Sn centre, possibly by the existence of oxygenvacancies and it also explains the deviation in the IS parameters compared to the standard SnO2. These119Sn-Mossbauer results confirmed the results obtained by XRD and TG, which suggested theformation of mixtures of Sn-oxides for (1), (2) and (3) decomposed in N2.

The results of quadrupole distribution, performed in order to explain the broad line widths and also toallow more certainty in calculation of the Mossbauer parameters, suggested that not only mixtures ofSn(IV) and Sn(II) were formed but also different types of Sn(IV) oxides. Unfortunately it can notenable us to assign each of the oxides. The product obtained in N2 is probably a discrete mixture ofSnO2 and SnO, or furthermore Sn3O4 [10]. No sign of Sn metal was observed in the 119Sn-Mossbauerspectra.

The 119Sn-Mossbauer experiments proved to be very helpful in identifying more than one Snoxidation state in the residues obtained in nitrogen, especially in the case of (3), for which the XRD didnot provide high quality data due to the poor crystal quality of the samples.

Fig. 9. SEM images of the residue of (1) (a), (2) (b) and (3) (c) decomposed in O2.


The SEM results (Figs. 9 and 10) showed that the decomposition of (1), (2) and (3) in oxygenafforded powders with uniform grain size of approximately 2–75 nm. In nitrogen the fluffy powders didnot present well defined grains.

4. Conclusions

The results reported in here have shown that the organotin oxide Sn4Me4O6 (1) is the best precursorfor pure nanometric Sn(IV) oxide. The thermal decomposition of (1) in oxygen has produced veryuniform nanosized grains of the rutile-type tetragonal SnO2 in high yield. The 119Sn Mossbauerspectroscopy has presented a good correlation to the other methods and, consequently, proved to bea useful tool for understanding and achieving a reasonable interpretation of nanometric materials.

Fig. 10. SEM images of the residue of (1) (a), (2) (b) and (3) (c) decomposed in N2.


According to the parameters IS and QS pure Sn(IV) oxide was only obtained when compounds (1) and(2) were decomposed in air or O2. In spite of the lower decomposition temperature of Sn4Ph4O6, it hasproduced pure SnO2 only in air, however, in lower yield. In the other conditions SnO was alsogenerated by (3).

Acknowledgements

We would like to thank TWAS—Third World Academy of Science (Research Grant Agreement 00-229RG/CHE/LA) and CNPq-Brazil for the financial support. The present work was partially developed atthe Laboratorio de Microscopia Eletronica e Microanalise (LMA)—Fısica, Quımica, Geologia-UFMGand CDTN-CNEN financed by FAPEMIG (Project CEX 1074/95).

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the influence of the r group in the thermal stability of sn4r4o6 (r = methyl, n-butyl or phenyl)

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