Synthesis and Characterization of Nanometric Pure Phase SnO 2 Obtained from Pyrolysis of Diorganotin ( IV ) Derivatives of Macrocycles

Thermal decomposition of diorganotin(IV) derivatives of macrocycles of general formula, R2Sn(L1) and R2Sn(L2) (where R = n-butyl (1/4), methyl (2/5), and phenyl (3/6); H2L1 = 5,12-dioxa-7,14-dimethyl-1,4,8,11-tetraazacyclotetradeca-1,8-diene and H2L2 = 6,14-dioxa-8,16-dimethyl-1,5,9,13-tetraazacyclotetradeca-1,9-diene), provides a simple route to prepare nanometric SnO2 particles. X-ray line broadening shows that the particle size varies in the range of 36–57 nm. The particle size of SnO2 obtained by pyrolysis of 3 and 5 is in the range of ∼5–20 nm as determined by transmission electron microscope (TEM). The surface morphology of SnO2 particles was determined by scanning electron microscopy (SEM). Mathematical analysis of thermogravimetric analysis (TGA) data shows that the first step of decomposition of compound 4 follows first-order kinetics. The energy of activation (E∗), preexponential factor (A), entropy of activation (S∗), free energy of activation (G∗), and enthalpy of activation (H∗) of the first step of decomposition have also been calculated. Me2Sn(L2) and Ph2Sn(L1) are the best precursors among the studied diorganotin(IV) derivatives of macrocycles for the production of nanometric SnO2.


Introduction
In recent years nanometric SnO 2 is of current interest because of its semiconducting, optical, and electronic properties.In addition to these, tin(IV) oxide possesses potential applications such as catalytic supports [1,2], transparent conducting electrodes [3], and gas sensors [4,5].SnO 2 is preferred over other metal oxides in gas sensors because of its high sensitivity and selectivity for different gases (e.g., H 2 ) in mixture [6].Nanometric SnO 2 has different properties from bulk crystals; therefore, much attention has been addressed to synthesis and characterization of such materials.To produce nanometric SnO 2 , a variety of chemical and physical methods [7][8][9][10][11] have been reported in the literature.
Tetraazamacrocycles and their derivatives have drawn special attention because of their applications in various fields such as analytical, industry, medicinal, and biological [12][13][14][15][16].A thorough survey of literature reveals that only a few attempts have been made to study the thermal stability of metal complexes of tetraazamacrocycles [17].However, a considerable attention has been given to the thermal decomposition of organometallic compounds in the last few years because they decompose at low temperature producing metallic oxides/sulfides and metallic particles.A thorough survey of literature reveals that limited studies have been carried out to prepare nanometric SnO 2 by the pyrolysis of single source organotin precursors [18][19][20].Further, it is important to mention that there is only a single reference recently reported by us in which some organotin-macrocyclic complexes are used as single source precursors for preparation of nanometric SnO 2 through their pyrolysis route [21].It is, therefore, worth investigating to explore the best precursors which would produce pure phase, nanosized SnO 2 on thermal decomposition.

Experimental Section
The details of synthesis and characterization of these organotin(IV)-macrocyclic complexes are similar to those reported previously [22].Thermal measurements thermogravimetric (TG), differential thermal analysis (DTA) and derivative thermogravimetric (DTG), infrared (IR), and Xray diffraction pattern (XRD) of residues were recorded on the same instruments as reported previously [22].Nanometric SnO 2 was prepared by the thermal decomposition of organotin(IV) precursors in a tube furnace under similar experimental conditions up to the formation temperature of SnO 2 as determined by TG analysis.The surface morphology of SnO 2 particles was studied by using scanning electron microscope (SEM) and FESEM-EDX.The SEM images were recorded on a LEO 435 VP electron microscope and FESEM-EDX on FEI Quanta 200 FEG.The transmission electron photographs of SnO 2 particles obtained from Ph 2 Sn(L1) and Me 2 Sn(L2) were recorded on a FEI TECNAI 20 G2STWIN, at Institute Instrumentation Center, Indian Institute of Technology Roorkee, Roorkee.To record the SEM and FESEM with EDX analysis, SnO 2 particles were previously agitated ultrasonically for 30 min in dichloromethane to separate out nanoparticles from the algometric form.After agitation, the sample was placed on the glass slit with the help of a capillary, which was made conducting by silver gel and after drying the solvent, the sample was covered with gold thin layer.For TEM studies, the agitated particles under suspension were taken on the carbon grid.After drying the sample the TEM photographs were taken and recorded.
n-Bu 2 Sn(L2), decomposed in two or three steps in the temperature range 100-1000 • C yielding SnO 2 in the temperature range 252-600 • C, which was confirmed by the XRD analysis and by the presence of ν(Sn-O) at 620 ± 5 cm −1 .
where λ = 1.5418Å is the wavelength of the incident beam, ΔW is the full width at half maximum in radians of the highest intensity line, and θ is the Bragge's angle.
The SEM image of the residue (SnO 2 ) obtained by thermal decomposition of n-Bu 2 Sn(L1) at 500-525 • C is shown in Figure 5.The residues obtained by the pyrolysis of diorganotin(IV) derivatives of macrocycles were agitated ultrasonically for 30 min.In all the cases the SEM images of the residues (SnO 2 ) showed uniform grain size with almost spherical shape.The grain size measured by SEM is in the range of ∼20-200 nm in diameter.FESEM image with EDX analysis of the residue obtained by pyrolysis of Me 2 Sn(L2) is represented in Figure 6.These images depict the formation of almost spherical particles in all of the cases.EDX analyses of these particles in SEM micrographs at various locations marked with the sign "+" show them to consist of Sn and O with SnO 2 composition.
The size of SnO 2 particles obtained by pyrolysis of Ph 2 Sn(L1) and Me 2 Sn(L2) measured by TEM is in the range   of ∼5-20 nm (Figures 7 and 8).There are many particles and holes about 5-20 nm size.Many connected grains form a random net work, in which various nanosized holes appear (Figure 8).Such structure is named as nanosponge [1].This microstructure leads to a very high rate of interface and surface.There are a large number of nanometric grains in Figure 7, which have well-ordered lattices.The electron diffraction pattern of selected area (inset in Figures 7 and  8) appeared to be typical polycrystalline diffraction rings.According to the diameters of the rings, the spacings are 0.34,  TG curves of n-Bu 2 Sn(L2) show that the first step of the complex exhibits a characteristic, well-defined, and nonoverlapping pattern.Two different methods were used to evaluate kinetic data from the TG traces.The following kinetic data are applicable only for the first step of decomposition of the complexes, as the other steps are very slow and do not follow the first-order kinetics; therefore, kinetic parameters for those steps have not been evaluated.The order of decomposition of the n-Bu 2 Sn(L2) complex is obtained by using Horowitz and Metzger equation [24].The value of Cs is 0.391.Therefore, the calculated order shows that the decomposition follows first-order kinetics.The values of energy of activation (E * ) and preexponential factor (A) have been calculated using two different methods, namely, Horowitz and Metzger [24] and Coats and Redfern [25] methods.Following equations were employed to calculate the entropy of activation (2), the enthalpy of activation (3), and free energy of activation (4): where h and k are the Planck's and Boltzmann constants, respectively.Kinetic parameters are given in Table 2.The negative values of S * indicate that the decomposition reaction is slow.The plot of Horowitz and Metzger method of n-Bu 2 Sn(L2) (4) is presented in Figure 9(a), whereas that of Coats and Redfern method is presented in Figure 9(b).

Conclusions
The residues obtained from thermal decomposition of R 2 Sn(L1) and R 2 Sn(L2) are SnO 2 as confirmed previously [22] by XRD analysis and infrared spectral studies.The crystal average size of SnO 2 calculated by Scherrer equation is in the range of 36-57 nm, whereas the size measured by      SEM and TEM is in the range of ∼20-200 nm and ∼5-20 nm, respectively, in diameter.Me 2 Sn(L2) and Ph 2 Sn(L1) are the best precursors among the studied diorganotin(IV) derivatives of macrocycles for the production of nanosized tetragonal SnO 2 .The kinetic parameters of the first-step decomposition of n-Bu 2 Sn(L2) indicate that its decomposition is slow.

Figure 7 :
Figure 7: TEM images of SnO 2 particles obtained by pyrolysis of Ph 2 Sn(L1) with electron diffraction pattern.

Figure 8 :
Figure 8: TEM images of SnO 2 particles obtained by pyrolysis of Me 2 Sn(L2) with electron diffraction pattern.

Table 1 :
Comparison of average size of the SnO 2 particles obtained by XRD, SEM, and TEM.