In this research, the nano-structured SnO2 photocatalyst was successfully synthesized using the solvothermal method. Moreover, its photocatalytic activity for the degradation of methyl orange under simulated solar light irradiation was investigated and compared with ZnO photocatalytic activity prepared via a simple precipitation route. To determine the physical, chemical and optical characteristics of prepared photocatalysts, XRD, FESEM, EDX-dot mapping, BET-BJH, DRS and FTIR were used. The BET-BJH analysis demonstrated that the synthesized nanophotocatalysts have mesopores structures. According to the DRS analysis, the ability of the light absorption in the visible and violet regions by synthesized nanophotocartalysts was proved. Also, the results revealed that SnO2 owning to its proper optical and structural properties had the highest efficiency (91.6%) and reaction rate (k=0.037 min-1) on the photo-degradation of methyl orange from water. The enhanced photocatalytic performance of SnO2 was ascribed to the suitable light absorption capability and effective production of charge carriers. Furthermore, the reaction mechanisms were suggested for the photo-degradation of methyl orange over ZnO and SnO2 photocatalysts.

Introduction

One of the major environmental challenges is water pollution. Dyes are considered one of the most important and abundant pollutants of water resources. Organic dyes such as acid orange 7, methylene blue, methyl orange, and so on are the materials that can enter water sources through industrial pollutants. Advanced oxidation processes (AOPs) hold great promise in removing organic contaminants. Among the AOP processes, photocatalytic oxidation has been demonstrated as an efficient technology for the distracting organic dyes. In the photocatalytic purification method, electron-hole pairs are generated by light photons which also lead to strong oxidants such as hydroxyl radicals •)OH) and holes. During the past forty years, various photocatalysts have been used in water treatment. TiO2 and ZnO are widely used for environmental applications due to their advantages such as nontoxicity, cheapness, and physical and chemical stability. On the other hand, the SnO2 semiconductor with the unique band gap and properly layered structure can degrade organic pollutants from waste water under sunlight irradiation. Therefore, according to the existing research gap in the field of photocatalytic performance of SnO2, the nanostructured SnO2 photocatalyst was synthesized by the solvothermal method, and its photocatalytic performance was compared with the synthesized ZnO for the degradation of methyl orange pollutant.

Materials and methods
Firstly, ZnO was prepared by using the precipitation method. In a typical procedure, a weighted amount of Zn(NO3)2.6H2O was dissolved in deionized water under stirring. Afterward, 1 M sodium hydroxide was dropped into the prepared solution to adjust the pH value to 10. After mixing the solution for 2 hours at room temperature, the suspension was filtered and washed with distilled water and dried at 110 °C for 12 h.
Moreover, the SnO2 photocatalyst was synthesized via the solvothermal method. A desired value of SnF2 was dissolved in the solution of deionized water and ethanol. In a second step for obtaining the form of SnO2, hydrochloric acid was poured dropwise into the prepared solution until the pH reached 1under mixing conditions for 30 min. Then, the solution was heated at 180 °C for 6 h in a stainless-steel autoclave. After cooling to ambient temperature, the resulting product was washed with distilled water and dried at 110 °C for 12 h.

Results and discussion
The results from the XRD and FESEM analyses demonstrated the production of highly crystallized ZnO nanopowder with more agglomerated particles in comparison with SnO2. Moreover, the surface of the SnO2 nanophotocatalyst had a uniform distribution of particles. EDX dot-mapping illustrated the uniform distribution of all elements for SnO2 which could be effective for photocatalytic activity. The SnO2 nanophotocatalyst also had a higher surface area in comparison with ZnO, resulting in efficient photocatalytic performance due to providing more active sites for degradation. The band gap values of ZnO and SnO2 were found to be 3.14 and 2.85 eV, respectively. The obtained observations of the FTIR spectrum were in agreement with the results of XRD analysis and confirmed the formation of crystalline structures of ZnO and SnO2 nanophotocatalysts. The photocatalytic activity of samples was evaluated for degradation of 10 mg/L MO under simulated solar light irradiation. The degradation efficiency showed that the high photocatalytic degradation of MO was achieved by SnO2 after 90 min. This observation can be attributed to the superior structural and optical properties of SnO2.

Conclusion
One of the main challenges of photocatalytic performance is to achieve a photocatalyst with high activity and suitable properties under sunlight irradiation. For this purpose, the nanophotocatalysts of ZnO and SnO2 were successfully synthesized and their photocatalytic performance for the removal of MO was evaluated under simulated sunlight irradiation. The results of XRD, FESEM, EDX dot-mapping and BET-BJH showed that using the solvothermal method for SnO2 synthesis led to a high specific surface area, uniform distribution of elements and particles and low surface roughness. Also, DRS analysis indicated an increment in the range of light absorption for SnO2. The nanophotocatalysts of SnO2 represented higher photocatalytic efficiency (91.6%) after 75 min. The existence of suitable photocatalytic properties for SnO2 semiconductor including narrow band gap, improved positions of conduction and valence bands, low recombination rate of charge carriers and the accessibility of active sites caused the superior photocatalytic performance for this sample.
he photocatalytic process is carried out to generate highly potent oxidizing species, electron-hole, on the surface of semiconductor photocatalysts. Under simulated sunlight irradiation (315 nm < λ), the energy of electrons in the valence band is elevated to the conduction band, which is at a higher energy level. This results in the formation of positive holes in the valence band, and each of the created centers can initiate a series of chain reactions on the surface of photocatalysts. Both the valence and conduction bands of ZnO nanophotocatalyst are higher than these energy levels for the SnO2 nanophotocatalyst. Based on the presented results, it seems that ZnO and SnO2 nanophotocatalysts use almost similar mechanisms to remove methyl orange pollutant. However, the results of the performance evaluation of these nanophotocatalysts showed that they have different efficiencies in removing the methyl orange color pollutant. This event can be attributed to the diverse electronic positions of the valence and conduction layers for ZnO and SnO2 nanophotocatalysts, different energy gaps responsible for creating and separating electrons and holes, the ability to absorb higher wavelength limits, the number of active sites available, and the lower surface roughness for SnO2 nanophotocatalysts.