Photoluminescence Properties of Polythiophene/Tin Oxide (PTh/SnO2) Polymer Nanocomposites

In the present work, we have synthesized, polythiophene/tin oxide (PTh/SnO2) polymer nanocomposites through an in-situ chemical polymerization of thiophene monomers using anhydrous FeCl3 as an oxidant. The structural, thermal and optical characterization of PTh/SnO2 nanocomposites were investigated using X-ray diffraction, field emission scanning electron microscopy, thermogravimetric analyzer and fluorescence emission spectroscopy. XRD spectra show the formation of pure polythiophene and incorporation of SnO2 nanofiller into the polythiophene matrix. FESEM images depict the formation of irregular rod and chain like structures with incorporation of SnO2 are noticed. TGA results reveal that the thermal stability of polymer nanocomposites is higher than that of pure polythiophene due to the strong interaction between nanofiller and polymer matrix. Fluorescence emission spectra show the emission intensity of polymer nanocomposites decreases as the concentration of SnO2 nanofiller increases due to the variation of rate of electron-hole recombination and conjugation length of polymer chain. Our results of fluorescence emission analyses suggest the PTh/SnO2 polymer nanocomposites could be a potential material for photonic devices.


INTRODUCTION
In the last decade, polymer nanocomposites, in particular polymer embedded with metal oxide nanoparticles as nanofiller have attracted a great deal of research interest due to their remarkable properties and wide variety of applications.They find applications in organic solar cells, rechargeable batteries, sensors, light-emitting diodes, supercapacitor and many more [1][2][3][4].Among the vast family of conducting polymers, polythiophene (PTh) is a unique polymer with high thermal stability, excellent electrical conductivity with tunable optical properties [5][6][7].Properties of polymer nanocomposites, strongly depend on composition, structure and nanofiller concentration.Further, optical properties are sensitive to nature of the matrix, type of nanofiller and morphology of nanocomposites.
Among the metal oxides, SnO2 is a unique filler material to polythiphene matrix due to the wide band gap, high optical transparency and good optoelectronic properties [8,9].In literature, there exist a few reports on polythiophene nanocomposites with different kind of nanofillers.Thakur et al [10] have fabricated PTh/TiO2 polymer nanocomposites via in-situ polymerization method using anhydrous iron(III) chloride as an oxidant and they observed the shift of absorption band towards higher wavelength as concentration of TiO2 increases.Using thiophene monomer in presence of FeCl3 as an oxidizing material, the synthesis of graphene oxide-copper oxide-polythiophene (Go-CuO-PTh) hybrid nanocomposites via an oxidative polymerization route was reported by Kalyani et al [11].They observed the high tendency of absorption in UV region and the shift of UV absorption peak towards lower wavelength of Go-CuO-PTh hybrid nanocomposites upon addition of GO-CuO nanofiller.Husain et al [12] have synthesized PTh/SnO2 nanocomposites via an in-situ chemical polymerization route, In their optical studies, they noticed the shift of absorption edge towards higher wavelength as concentration of SnO2nanofiller increases.Making use of thiophene monomer and ammonium persulphate as an oxidant, PTh/ZnO nanocomposites were prepared through in-situ oxidative polymerization technique by Bulla et al [13].They noticed the blue shift in photoluminescence (PL) peak and increase in PL intensity of nanocomposites as content of ZnO increases.However, a very little attention is given to optical properties of PTh/SnO 2 polymer nanocomposites.Moreover, PTh is a good fluorescent polymer with high electrical conductivity and SnO2 is a supportive filler material.Hence in this work we have synthesized PTh/SnO2 polymer nanocomposites via an in-situ polymerization of thiophene monomer and the effect of nanofiller concentration on photoluminescence properties of PTh/SnO2 polymer nanocomposites is investigated. .

Materials and method
Thiophene monomers (SD-Fine Chemical), anhydrous iron(III) chloride (Molychem), tin sulphate (Molychem), sodium hydroxide and acetonitrile (SD-Fine Chemical) are of analytical grade and used without further purification.SnO2 nanoparticles were synthesized through a chemical co-precipitation method [14].For synthesis of nanocomposites, 0.0125mol (1ml) of thiophene monomer is dissolved in 20 ml of acetonitrile.Iron chloride (FeCl3) solution is obtained by dissolving 0.0625mol (10.13 gm) of anhydrous iron(III) chloride in 50 ml of acetonitrile.The known quantity of SnO2 nanoparticles -5% and 10% (See table 1) were dispersed in 15 ml of acetonitrile and added into thiophene solution drop wise under constant stirring and continued it for 2 hours for proper adsorption of thiophene on SnO2 surface.Thereafter, the solution of FeCl3 is added drop wise to the above composite solution.During the addition of FeCl3 solution, grey colored solution changes into dark brown and kept it for 24 hours to completion of chemical reaction.Finally, a dark brown colored product was formed.Then, the product was filtered out and washed using methanol and de-ionized water to remove the unreacted FeCl3.Finally, the obtained product was dried at 80 o C in a hot air oven for an hour.Hereafter, we name the SnO2 nanoparticles as P0 and pure, 5 % and 10% SnO2 embedded polythiophene as P1, P2 and P3 respectively.

Characterization techniques
The crystalline properties of samples are studied through X-ray diffractometer (Rigaku, Ultima-IV) with Cukα radiation of wavelength 1.5406Å.Surface structure analysis of samples were made using JEOL, JSM-7100F field emission scanning electron microscopy.DST Q600, thermogravimertic analyzer was used to check the thermal stability of synthesized samples.Using HORIBA, Fluromax-4 spectroflurometer we have recorded the fluorescence emission spectra of samples.

RESULTS AND DISCUSSION
In figure 1, graphs (a-d) show the X-ray diffraction (XRD) spectra of P0, P1, P2 and P3 samples, respectively.In graph (a), we show the XRD spectrum of sample P0 (SnO2 nanoparticles) and all diffraction peaks are well assigned to the tetragonal SnO2 phase (JCPDS data 41-1445) [15,16].XRD pattern of P1 sample (Fig. b), exhibits the broad peak around at 25 o which confirms the formation of polythiophene [17].From XRD spectrum of samples P2 and P3 (Fig. c-d), it is clearly observed that the diffraction peaks corresponding to both polythiophene (broad peak between 25 o -35 o ) and SnO2 nanofiller.Furthermore, the intensity of diffraction peaks related to SnO2 increases as concentration of SnO2 nanofiller increases, which confirms the successful incorporation of SnO2 nanofiller into polythiophene matrix.The crystallite size, D and micro strain, ε are calculated by using relations.Here D is the crystallite size, K is the shape factor, λ is the wavelength X-ray beam, β is the FWHM of diffraction peak, and θ is the diffraction angle.The estimated value of microstrain and particle size of polymer nanocomposites is summarized in the table 2 .The fractional change in value of particle size and microstrain with an increasing concentration of SnO2 nanofiller is because of the strong intermolecular interaction between polythiophene and SnO2 nanofiller.The obtained value of microstrain well matches with the findings by Das and co-workers for PCz/SnO2 polymer nanocomposites [18].
Figure 1 represents temperature variation of electronic thermal conductivity, κe for the monolayer HfSe2.Curves a and b represent, respectively contributions from acoustic phonons and charged impurities.Curve 1 represents the variation of overall κe.It is found that for T<10K , the contribution from the charged impurities is significant for the parameters considered .With the increasing the temperature, we find that the charged impurities contributions become increasingly dominant in limiting κe throughout the temperature range.Further, our calculations show that transverse acoustic phonons are more dominant than that of longitudinal phonons.Figure 1(a-d).XRD pattern of samples P0, P1, P2 and P3 respectively.
In figure 2, images (a-d) display the FESEM micrograph of samples P0, P1, P2 and P3 , respectively.Image (a) shows the irregular shaped and highly agglomerated SnO2 nanoparticles are formed.From image (b) we noticed the formation of chain and some rodlike structures.The formation of rod like and highly agglomerated particles is noticed from images (c-d).This is due to the branching and changes in conjugation length of polymer chain.
Thermal response curve of P0, P1, P2 and P3, samples is given in figure 3(a-d), respectively.The initial weight loss for all samples in the temperature range of 50 o C-200 o C due to the evaporation of surface absorbed water molecules [19,20].TGA result of sample P0 shows that the weight loss begins at 342 o C and continuously decreases until degrades completely at 747 o C due to the decomposition of the material [21,22].From TGA curve of P1 (curve-b), the weight loss starts at 221 o C, which is then rapidly decreases and completely degrades at 656 o C due to the decomposition of polythiophene chain [23,24].The main weight loss of polymer nanocomposites (curves c-d) occurs at around 234 o C due to the degradation of unreacted functional groups of polymers and completely degrades at 684 o C. It is clearly noticed that the degradation temperature of polymer nanocomposites shifted towards higher temperature as content of nanofiller increases.This may be due to the strong intermolecular interaction between sulphur atom of polythiophene and oxygen atom of SnO2 nanofiller.
Figure 3(a-d).TGA curve of samples P0, P1, P2 and P3 respectively.Fluorescence emission spectra of PTh/SnO2 polymer nanocomposites are presented in figure 4. Curves (a-d) respectively, represent the emission intensity of P0, P1, P2 and P3 samples with fixed excitation wavelength kept at 300 nm.The maximum emission peak of sample P0 (curve a) is at 410nm, which is due to the direct recombination of electron-hole pairs [25].The emission spectrum of samples P1, P2 and P3 (curves b-d) show the peak located around at 430nm due to the emission of side chain of polythiophene [26,27].It can be seen that the slight blue shift of PL peak and the nature of emission peak of polymer nanocomposites is almost same because of the intermolecular energy transfer of the excitons from the conjugated side chain to the main chain [28,29].With an increasing SnO2 content, the emission intensity reduces due to the strong interaction of SnO2 nanofiller with polythiophene chain.The formation of non-fluorescent material reduces the rate of electron-hole recombination and absorption of defect related energy level causes the decrease in PL intensity [30,31].Similar behavior was observed by Tripathi et al [32] for PTh/Al2O3 polymer nanocomposites.The significant variation of FL intensity makes PTh/SnO2 polymer nanocomposite as a potential candidate for optical devices.

Conclusions
Using anhydrous iron (III) chloride as an oxidant, polythiophene/tin oxide (PTh/SnO2) polymer nanocomposites were synthesized via an in-situ polymerization method.XRD, FESEM, TGA and fluorescence emission spectroscopy are used to characterize the synthesized samples.The formation of pure polythiophene and incorporation of SnO2nanofiller into polythiophene matrix was confirmed by XRD analysis.The rod shaped and chain like structures are noticed from FESEM images.From TGA analysis, it is clearly noticed that thermal stability of polymer nanocomposites is greater than that of pure polymer.Photoluminescence studies of PTh/SnO2 polymer nanocomposites shows the variation intensity with an increasing concentration of SnO2 nanofiller is because of change in rate of exciton recombination and branching of polymer chain upon addition of nanofiller.

Table 1 .
Stoichiometric calculations for preparation of polymer nanocomposites.

Table 2 .
Estimated value of the crystalline size and micro strain