Ficus carica mediated synthesis of simulated solar light driven nano-sized zinc stannate photocatalyst for the degradation of methylene blue


 The synthesis of zinc stannate nanocomposite (Zn2SnO4 NC) was carried out using an environment friendly process that included the use of Ficus carica (F. carica) leaves extract as a capping and reducing agent. X-ray diffraction (XRD) analysis was used to analyze the structural and crystallographic parameters, and the crystallite was discovered to have cubic geometry. The elemental composition of the studied Zn2SnO4 NC was investigated using energy dispersive X-ray (EDX), which revealed that it was extremely pure. The band gap (3.12 eV) was calculated through Tauc plot using diffuse reflectance (DRS) data where the functional groups were explored through Fourier transform infrared (FTIR) spectroscopy. Prior to the photocatalytic reaction, some preliminary experiments were performed, which proposed that pH 9 is suitable for the mineralization of methylene blue (MB) (10 ppm) in the presence of 20 mg of Zn2SnO4 NC and simulated solar light. The 96 % of MB was degraded in 80 min with the degradation rate of 0.038 min-1.


Introduction
The environmental pollution are mainly attributed to the diversifying urbanization and industrialization. Water and soil pollution is caused by the industrial effluents that have been discharged into environment, having negative impacts on all living organism [1]. The high concentration of chemicals, dyes and heavy metals are released into water bodies from dyeing industries polluting the ground and surface water and make it unfit for drinking [2]. Water adulteration via dye discharge from several industries (food processing, paints, cosmetics, textile dyeing, paper making) has captivated major attention because of risk to ecosystem and public health [3]. The making of synthetic dyes is above 7 lac tons/year and greater than 15% of this synthetic colorant is expelled into water per annum and are proved to be mutagenic, xenobiotic and carcinogenic to living organisms [4]. Dyes effluents from the industries like cosmetic, food, dye synthesis, pulp mill, electroplating, textile, paper, and printing are caused water pollution.
The xenobiotic properties and complicated aromatic structure of dyes form them arduous to mineralize. A few of the organic dyes and their related products have mutagenic and carcinogenic effects on humans. There is an urgent need to treat the dyes before their release into the water bodies [5]. Methylene blue (MB), a cationic dye and has many applications including silk, wood, and dying cotton. However MB is not believed to be extremely noxious but it can produce some detrimental results such as quadriplegia, jaundice, cyanosis, shock, diarrhea, increased heart rate, vomiting, and tissue deathin human being. Consecutively, MB containing wastewater should be managed properly before letting out into the water bodies [6].
Various biological, chemical, and physical techniques are used for the elimination of unwanted hazardous compounds from contaminated water comprising membrane separation, advanced oxidation, coagulation and adsorption, precipitation, reverse osmosis, and membrane processes [7]. The methods reported to eliminate the dyes from aqueous solution were of physical and chemical nature but these methods are highly expensive and less efficient for the dye degradation. The stability of these dyes is maintained by using chemicals which are toxic in nature and thus add to the toxicity of the environment [8]. Among these, photocatalysis is one of the most important processes. Photocatalysis is widely used for many applications in treatment of wastewater and manifested a fair approach for the abolition of inorganic and organic pollutants including organic dyes, oil leakage, and pesticides that cannot be degraded in natural forms.
Nano photocatalysis is a versatile oxidation process, utilized in the eradication of very minute amount of pollutant from air and water streams. It is more efficacious than conventional methods due to the large surface area-to-volume ratio, uniform and controlled particle size, structure, composition. Among nanophotocatalysts, nanocomposites are preferred because they are efficient, economic and easy to synthesize [9]. The binary composites oxides render the electronhole recombination process thus increasing the photocatalytic activity. Zinc stannate is a tertiary oxide semiconductor having band gap (∼3.6 eV) and termed as zinc tin oxide (ZTO) [10]. The Zn2SnO4 depict wide-band semiconductor oxides with the conductivity of n-type, (transparent within the visible light region), they are optimistic for applications in the fields of gas sensors, lithium-ion batteries, solar cells, transparent conductors, lead-free ferroelectrics and photo catalysis [11]. Depending on the molar ratio of the primary components (Sn, Zn and O), this material exists in two states: ZnSnO3 of the perovskite-type structure and Zn2SnO4 of the spineltype structure [12]. Different approaches are used for the preparation of zinc stannate nanocomposite including thermal evaporation calcinations, sol-gel synthesis, mechanical grinding, hydrothermal, and ion-exchange methods. However, these methods not user friendly due to the wide use of toxic chemical reagents and expensive instruments(Baruah sand Dutta 2011). The green method is one of the most effective, safe and low cost process and is a possible alternate of the conventional methods [14].
The present study was conducted to mineralize MB from aqueous solution on illumination of simulated solar light irradiation using Zn2SnO4 NC as photocatalyst. The Zn2SnO4 NC was synthesized via ecofriendly and economic process using F. carica leaves extract which belongs to the family Moraceae [15]. The physicochemical properties was analyzed through FTIR, DRS, EDX, XRD, and SEM techniques. The photocatalytic degradation of MB was examined under the effects of initial concentration, pH and catalyst dose. The stability of the Zn2SnO4 NC as photocatalyst was also evaluated for several stage degradation process.

Materials
Analytical-grade chemicals were used in this research work. The zinc chloride dihydrate, tin chloride dihydrate, sodium hydroxide, and methylene blue were given by Sigma Aldrich and were used without further purification. All of the requisite solutions were made with deionized water. The botany department classified the F. carica leaves that were collected from the chemistry department's lawn.

Preparation of Extract
The F. carica leaves were picked, washed in deionized water to remove dust, and then dried in the shade. To make the extract, 50 g of dried leaves were placed in an airtight jar with 1000 mL boiled and deionized water and left for 24 hours. The crude extract was then purified and centrifuged for 10 minutes at 4000 rpm, with the upper layer being saved for future experiments.

Synthesis of Zn2SnO4 NC
For the fabrication of Zn2SnO4 NC, the calculated amount (1.05 g) of ZnCl2.2H2O was solvated in 50 mL deionized water and 20 mL of the prepared plant extract was introduced. The pH of the reaction mixture was fixed at 10 by dropwise addition 0.1 molar solution of NaOH and was then stirred (250 rpm) and heated (50 °C) for 30 min. The white gel formed was aged at room temperature for 6 h. At the same time, the SnCl2.2H2O (0.95 g) was solvated in 50 mL deionized water and 20 mL of the plant extract was introduced and stirred (250 rpm) and heated (50 °C) for 30 min and white gel formed that was aged for 6 h. Afterward, both the gel were mixed with constant stirring (300 °C) and heating (50 °C) for 5 h at room temperature and was then aged for 24 h. The final product was repeatedly washed with deionized water and was oven dried at 150 °C and stored in air tight polyethylene bottle.

Characterization
The different physicochemical techniques were utilized for the inspection of the structural and surface properties of the synthesized Zn2SnO4 NC. The XRD model Panalytical X-Pert Pro was utilized to analyze the crystalline nature of powder. The XRD quantification was made of 20° to 80° in 2θ range and Debye-Scherrer equation was employed for determining the average crystallite size. Morphology was studied by using SEM model JEOL 5910 (Japan). The band gap was determined from data obtained by running DRS (lambda 950) in the 400-1000 nm range using Tauc's plot. The material's elemental analysis was performed on an EDX model INCA200 (UK) operating in the 1-20 keV scale. The FTIR spectrum was recorded using a Nicolet 6700(USA) spectrometer in the range of 4000-400 cm -1 [16].

Photocatalytic activity
During the photocatalytic removal of MB, the photocatalytic behavior of the assynthesized Zn2SnO4 NC was examined. To begin, a 10 ppm MB solution was made in deionized water, and 50 mL of that solution was poured into a reaction vessel, along with 20 mg of Zn2SnO4 NC. The reaction mixture was mixed in the dark for 30 min to obtain the adsorptiondesorption equilibrium. The solution was then subjected to artificial solar radiation. To extract the catalyst, the 3 mL sample was centrifuged at 4000 rpm for 4 min after a fixed time interval (in min). On a spectrophotometric analysis of the centrifuged sample using a double beam spectrophotometer, the absorbance limit was recorded as a function of time (Thermo Spectronic UV 500).

Physicochemical study
The diffractogram shown in Fig. 1  (Position for Fig.1) The EDX spectrum of Zn2SnO4 NC shown as inset in Fig. 1 (Position for Fig. 2) The electronic state of the Zn2SnO4 NC was calculated using the transmittance spectrum, which indicates that all of the samples are translucent over a broad wavelength range. The band gap energies were determined using the Tauc relation (eq.1), where B is constant, hv is the light intensity, is the absorption coefficient, and the exponent n is depending on the type of transition: direct, forbidden direct, indirect, or forbidden indirect, and can have values of 1/2, 2, 3/2, or 3 respectively [17].

(Position for Fig. 3)
The band gap energies were calculated for Zn2SnO4 NC from Tauc plots by joining sharp rising portion with horizontal axis of the (hvlnT) 2 against hv is 3.12 eV [18,19]. The band gap energy for Zn2SnO4 NC is divergent from that of SnO2 NPs and ZnO NPs suggesting that the production of new species and all the deduced band gap energies are in accordance with the reported data [20][21][22][23][24][25]. In the FTIR spectrum of Zn2SnO4 NC (Fig.4), (Position for Fig.4)

3.2.Photocatalytic study
The photocatalytic potency of Zn2SnO4 NC was analyzed to degrade MB with the catalyst concentration of 20mg on irradiating simulated solar light. The visual photodegradation of MB was observed due to the fading of color with the increase in irradiation time. The eq. 4 was used to determine the percentage degradation (Fig. 5), that inferred 96% of the dye was mineralized in

Dosage study
The 10 ppm MB was exposed different quantities of the Zn2SnO4 NC (5, 10, 15, 20, 25 and 30 mg) to check the affect of catalyst dosage on the catalytic mineralization process. From the experiment Fig. 6(A), it was inferred that the degradation of MB increases with increasing the dose of catalyst up to 20 mg while further elevation in the dose reduced the degradation process.
This decrease in the degradation is attributed to the phenomenon of deactivation of catalyst.
When large amount of catalyst is introduced to the solution collision ensues between the activated and the ground state catalyst that leads to the deactivation of the activated molecules and consequently degradation decreases [28].

Initial concentration study
In the present work, 5, 10, 20, 30 and 40 ppm MB solutions were unveiled to the Zn2SnO4 NC to scrutinize the effect of dye concentration on the photocatalytic mineralization. The results shows

pH study
The pH of the solution is the key parameter that significantly affects surface properties of catalyst. The pH study on photocatalytic degradation of MB was evaluated in pH range of 3 to 11 using 10 ppm dye solution and exposed 20 mg of Zn2SnO4 NC as shown in Fig. 6(C). The upshots inferred that the percentage degradation of MB was maximum at pH 9 as compared to other studied pH. MB being a cationic dye shows maximum degradation in the alkaline medium and maximum degradation is exhibited at pH 9 and the activity decreased with further increased.
The enhanced activity at pH is attributed to formation of hydroxyl ions that are accountable for the production of hydroxyl radicals. The low degradation at pH 11 is might be due to the dissociation of Zn2SnO4 NC, thus optimum pH for the degradation of MB is 9 [29].

Reusability study
The stability of the Zn2SnO4 NPs in term of its reusability was analyzed against the five-fold degradation of the MB. The experiment was performed by the addition of fresh MB solution by degrading the previous one under same conditions for five times. The Fig. 6(d) shows that there is a negligible reduction in the efficiency of the Zn2SnO4catalystafter three cycles and decrease on the efficiency of the catalyst that is ascribed to the surface coverage of the catalyst.
Overlooking to the photocatalytic efficiency, the Zn2SnO4NCis the most appropriate catalyst to be used for degradation of organic pollutants under simulated solar light and it can be effectively used for various stage reactions.

Photocatalytic mechanism
Photocatalysis is a kind of reaction that proceeds on the expense of energy that is equal or higher than the band gap of the catalyst. The electrons move from the valance band to the conduction band on irradiating of semiconductor with a light source. After effect equal number of holes are engendered in valence band. All types of microbial, organic, and inorganic contaminants owing to their redox potential are degrading via photo-generated holes and electrons. They react with adsorbed electron acceptors and electron donors by migrating to the surface to produce hydroxyl radicals, hydrogen peroxides, superoxide radical anions. The hydroxyl radical reacts with aqueous solution resultantly producing innocuous compounds. Under same reaction conditions, the organic pollutants are thoroughly oxidized to halide ions, H2O and CO2 on negligible production of displeasing by-products [9]. In the present study, the reaction mixture was irradiated with the simulated solar light and the electrons of both the oxides get excited to conduction and the holes are generated in the valance band. The electrons and the holes are adequately separated as the holes get assembled in the VB of ZnO while they were generated in the VB of SnO2 similarly, electrons get shifted to the CB of SnO2 from the CB of ZnO. Later the • OH and O2 -• are produced by the reaction of holes (h + ) and electrons (e -) with the water and absorbed oxygen respectively. The O2 -• free radical serve as an additional source of producing • OH radicals which then lead to the degradation of the MB to non-toxic material [16].

Conclusion
The manipulation of plant materials for the fabrication of nanomaterials is the most effective,