Excellent Photocatalytic Degradation of Indigo Dye using Low Cost Chemical Route Grown Highly Luminescent SnO2 Decorated Polystyrene Nanocomposites

Irrational use of dye is a challenge for our environment specifically for clean water. Highly luminescent SnO 2 decorated Polystyrene nanocomposites developed as an effective solution for it. The low cost chemical synthesis of highly luminescent tin oxide decorated Polystyrene (SnO 2 -PS) polymer nanocomposites using recyclable expended polystyrene waste has been reported. Sol gel grown tin oxide nanoparticles, thoroughly dissolved in dissolved in toluene were used with recyclable expended polystyrene waste. The composites were grown either on glass substrates or developed as flexible self sustaining layers and characterized by optical, structural & morphological characterizations. X ray diffractograms of SnO 2 -PS polymer nanocomposites exhibit crystalline behavior with tetragonal structure of SnO 2 . Accumulation of SnO 2 particles on the surface with increasing concentration, in the form of spherical structures is observed in AFM micrographs. Hollow vertical chain like growth is also observed. Absorption edge shift towards higher wavelength results in decrease in band gap with increasing concentration. The Photoluminescence (PL) spectra for higher SnO 2 shows a significant peak peaks in visible spectra at about 425 nm. SnO 2 decorated Polystyrene nanocomposites synthesized using recyclable expended polystyrene waste opens a new scope in flexible optoelectronic applications with visible region photoluminescence.


Introduction
Modernization caused an irrational use of dyes, and resulted in severe environment thereats. As a result, degradation of dyes have now become an urgent area of research [1]. Recently Nano photocatalysts showed a new promising way to address this challange. Despite of their improved properties, the small size of the nano composites also come up with issues like difficulty in their separation, lack of process for reuse, and also a possible risks to ecosystems by the potential release of the nanoparticles into the environment [2,3]. An efficient approach to overcome the above drawbacks is to immobilize the nanosized particles into a host material of larger size namely in a polymer nanocomposite. Polystyrene (PS) has shown its best candidature for such nanocomposites.
Bekri-abbes et al. [4] has reported converting polystyrene waste into cation exchange resin. Achilias et al. [5] has reported sulfonation recycling to convert polymer waste into useful material. Maharana et al. [6] also reported the gaseous and liquid form of recycled polystyrene depending upon temperature and reaction catalyst. Solubility and stability of polystyrene wastage in recycling process studied by Garcia et al. [7]. Instead of recycling the PS, development of Semiconductor PZ nano composits for various application is also realized recently. In this regard, Chae et al, [8] reported decrease in tensile strength of polystyrene composites on increasing concentration of ZnO Nanoparticles PS. Soon Ma et al. [9] also reported resistivity of ZnO/PS Nanocomposite decreases as the amount of ZnO increases in the composites. In exploring other properties Zhong et al. [10] reported about change in magnetization property of fe3O4 in Fe3O4/PS nanocomposites. G. Nenna et al. [11] exhibited that the optical properties of ZnO/PS and showed that theses nanocomposites can improve the light extraction in Organic Light-Emitting Diode. Ali et al. [12] has prepared Nanocomposite films of Fe3O4/PS having good thermal stability and supermagnetic behavior. It is used in chemical sensor and supercapacitor. Qianqian Ding et al. [13] reported that PS/Cu Nanocomposites with enhanced Raman scattering performance. Kassaee et al, and polystyrene [14,15] have been studied that polystyrene as an organic material for the synthesis of composites with inorganic materials like PS-fe3O4, PS-MnO2, PS-ZnO, PS-ZrO2, many other polystyrene based compounds are given in Polystyrene: synthesis, characteristics, and applications. In other applications Deborah Ruziwa et al. [16] reported that sulphonated waste polystyrene to remove the heavy metal ions like Zn 2+ and Pb +2 , However to the best of our knowledge very less on PS/SnO2 nanocomposites have been reported.

Experimental details
SnO2 decorated Polystyrene nanocomposites were synthesized using recyclable expended polysterene waste. Starting materials were AR grade Stannous chloride (SnCl2) Toluene, Acetone, obtained by Ranbaxy and used without further purification. Thoroughly cleaned soda lime glass was used as substrates for the synthesis. The whole process of synthesis has been divided in the following steps:

1.
Preparation of PS solution 2. Preparation of SnO2 Nanoparticles

3.
Formation of PS-SnO2 Nanocomposites films on glass and self-sustained layers 4. Formation of PS-SnO2 films.

Step 1. Preparation of PS solution
Thermocol waste (a form of PS) which is used as packaging material, collected and thoroughly cleaned with distilled water and then sonicated to remove dust, crushed into small pieces and finally dried for 12 hours at room temperature. The pieces were added in 60 ml toluene and stirred magnetically for 10 minutes. The Thermocol get dissolved and resulted in transparent solution.

Step 2. Preparation of SnO2 Nanoparticles and their solutions
SnO2 Nanoparticles were obtained using sol gel route using the method already been reported [23].
The different weights of obtained SnO2 particles (A1: 1g, A2: 3g and A3: 5g) were added in 20 ml of acetone and stirred continuously for one an hour after which it was kept in ultasonicator at 30 0 C for another hour.

Step3. Formation of PS-SnO2 nanocomposites films on glass and self-substrate
SnO2 dissolved in Acetone (A1, A2, A3), were added in 20 ml of PS solution separately and stirred for another 10 minutes finally spread on glass substrates to get films using doctors blade method.

Step4. Formation of PS-SnO2 films.
For film deposition, thoroughly cleaned Soda glass slides having dimension 10mm x10mm were used as substrate. PS-SnO2 solution of various ratios were dropped on these substrates and spread evenly using doctor blade method and then dried at room temperature. After several hours the films are self-etched from the glass substrate.

Experimental setup and procedure
Photocatalytic experiments were performed in a photo reactor using our own designed setup.

Characterization
All the samples were analyzed by structural, morphological and optical characterizations. X-ray diffractograms were obtained for the angle in the range 10 o to 60 o using Bruker D8 Advanced XRD with using Cu k target. AFM micrographs were obtained by using Tapping mode IUAC Indore. PL, UV-Vis spectra were carried by Perkin Elmer Lambda-25 and Photoluminescence (PL) spectra obtained by Perkin Elmer LS-55.

Results and discussion
PS-SnO2 nancomposites grown on glass or self sustained films were analyzed using optical, structural and morphological characterizations.

Structural Studies
X-ray diffraction pattern of PS, PS/SnO2 flexible self-sustained films grown on glass substrate are shown in Fig. 2 (a) and 2(b). The average nano-crystallite size (D) was calculated using the Scherrer formula, = 0.9 ⁄ ……….. [32] Where λ is the X-ray wavelength, θ is the Bragg diffraction angle, and β is the FWHM of the XRD peak appearing at the diffraction angle θ. The crystalline sizes were estimated from the Scherer's

Optical Studies
Absorption spectra of SnO2 of different ratios were obtained shown in Fig. 6(a) . SnO2 particles show good transmission in visible region, however overall absorption increases with increasing concentration of SnO2 in PS. The (αhν) 2 vs hν plot were obtained and shown shown in Fig.6b  The mechanism of dye degradation is as follows: Ground state dye molecules absorb radiation and get promoted to singlet excited state from which it will undergo intersystem crossover to triplet state. Valence band electrons in tin oxide will absorb radiation and get promoted to conduction band thus generating hole. The valence electron will react with the dissolved oxygen molecule in the solution to generate radical. Radicals react with dye molecule to generate dye superoxide ion thus degrading into carbon dioxide and water molecules. On the other hand, holes present in the valence band will react with hydroxyl ions to generate hydroxylfree radical which attacks dye to generate degraded products. Steps