Enhanced Photocatalytic Performance in Chromium(VI) Reduction and Dye Degradation using Sn3O4/SnS2 Nanocomposite and Mechanism Insight

7 Detection of residual organic and inorganic species in water bodies including drinking water 8 has led to developing strategies for their removal. Here we report a very efficient method of 9 photoreduction of Cr (VI) and photodegradation of methylene blue dye in aqueous medium 10 using Z-scheme heterojunction based Sn3O4/SnS2 solar photocatalyst. The photocatalyst is 11 synthesized by hydrothermal route and it is thoroughly characterized in terms of its structural, 12 compositional, morphological and optical properties. About 100 % of Cr (VI) reduction in 60 13 min and 99.6 % of methylene blue degradation in 90 min is achieve under sunlight exposure 14 at a photocatalytic rate of 0.066 min and 0.043 min, respectively. The total organic carbon 15 estimation of the post-degradation reaction medium corresponded to 85.1 % (MB) 16 mineralization. The photocatalytic degradation is attributed to in-situ generation of reactive 17 oxygen species (ROS) e.g., superoxide radicals, hydroxide radicals, and the role of ROS 18 towards reduction and degradation of Cr (VI) and MB respectively, is confirmed from ROS 19 scavenging studies. The dye degradation mechanism has been discussed by analyzing the 20 degradation products via UPLC-Q-Tof-MS. The photocatalytic degradation of methylene 21 blue by Sn3O4/SnS2 nanocomposites is significantly enhanced as compared to SnS2 22 photocatalyst, attributed to Z-scheme heterojunction and the charge carrier mobility. 23

In view of this, advanced oxidation process (AOP) is considered to be more efficient, 50 particularly for treating organic pollutants as it tends to mineralize the pollutants (Bethi et al. concern here is the stability of the sulphide component. Considering these conditions, we 77 present here a novel Z-type heterojunction photocatalyst comprising a low band gap SnS2 as a 78 base material in combination with Sn3O4 which is a large band gap materials. The 79 photocatalyst, represented as Sn3O4/SnS2 nanocomposites, has been thoroughly characterized 80 and was studied sunlight mediated photoreduction of hexavalent chromium Cr (VI) and 81 photocatalytic degradation of organic dye in aqueous medium. Hexavalent Chromium Cr (VI) 82 and methylene blue has been chosen as a model pollutants, as it has been detected in 83 ecosystem comprising river water, ground water, sediments and soils, and in drinking water  It is a two-step process, where Sn3O4 nanoflakes were first prepared via hydrothermal 100 route. In a typical process, 4 mmol of Stannous chloride dihydrate was dissolved in 12.5 mL 101 deionized water than 12.5 mmol tri-sodium citrate dihydrate was added and stirred until a 102 transparent solution was obtained, then 0.1 g of sodium hydroxide dissolved in 12.5 mL D.I 103 water, was added to the above solution dropwise and stir for 1 h and finally transfer the 104 solution into a 50 mL Teflon jar and heated at 180 °C for 12 h in a microprocessor-based 105 temperature-controlled furnace. Finally, the autoclave was cooled to room temperature. The

Characterization's techniques 121
The as-synthesized batches of Sn3O4/SnS2 NCs with different Sn3O4 weight ratios, 122 together with those of SnS2 nanoparticles, Sn3O4 nanoparticles are characterized by powder 123 X-ray Diffraction, X-ray photoelectron spectroscopy, field emission scanning electron 124 microscopy, high resolution transmission electron microscopy, Brunauer-Emmett-Teller 125 (BET) specific surface area technique, photoluminescence spectroscopy, electrochemical 126 impedance spectroscopy and by diffused reflectance spectroscopy. The details of these 127 techniques and sample preparation are given as Supporting Information (Section S1).

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The degradation intermediates were identified by UPLC (ACQUITY I) coupled to 129 QToF (XeVO G2-XS QToF). Mobile phase A was aqueous formic acid solution (0.1%, v/v), 130 and mobile phase B was acetonitrile. The gradient solvent was at a flow rate of 0.3 mL/min. beakers. This will be referred to as reaction assembly, which was subjected to slow 142 continuous stirring (less than 100 rpm) for 60 min in dark condition to establish adsorption-   The chromium solution with different pH was adjusted by 0.2 M H2SO4 or NaOH. 0.07Sn3O4/SnS2 NCs was assessed by monitoring photocatalytic reduction of Cr (VI) for five 170 successive cycles. At the end of each cycle, the used photocatalyst was recovered, dried, and 171 reused for the next cycle. After completing five cycles of photocatalytic reduction, the 172 stability of the photocatalyst was studied by XRD and FESEM.

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The powder X-ray diffraction (XRD) method was used to investigate the crystalline  (Table-184 S1). The intensity of the diffraction peaks of Sn3O4 increased for the batches of and SnS2 were discuss in the supporting information (Section S3).

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The morphology of Sn3O4/SnS2 NCs, along with their respective components e.g.,

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SnS2 nanoparticles and Sn3O4 nanoflakes were analysed by electron microscopy (given as

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The proposed structure of 0.07Sn3O4/SnS2 was further corroborated by compositional 213 analysis of tin (Sn), oxygen (O) and sulphur elements by XPS measurement (Fig. 3a). The 214 binding energy peaks of Sn 3d shows two peaks at 486.88 eV and 495.29 eV, which are 215 assigned to Sn 3d5/2 and Sn 3d3/2, respectively, suggesting the presence of Sn 4+ in 216 0.07Sn3O4/SnS2 (Fig. 3b).The binding energy peaks of S 2p displays a two strong peak at 217 161.72 eV and 162.85 eV corresponding to S 2p3/2 and S 2p1/2, respectively, which was 218 attributed to the S 2state (Fig. 3c). The O1s binding energy peak is deconvoluted into two peaks at 532.03 eV and 531.13 eV (Fig. 3d). The peaks corresponded to O-Sn 4+ bond and O-220 Sn 2+ , respectively, which are relevant to the proposed structural constituent of 221 0.07Sn3O4/SnS2 NCs. Therefore, the XPS results are consistent with the XRD and HRTEM 222 findings shown above. This further provides more evidence that the obtained 0.07Sn3O4/SnS2 223 composite consists of SnS2 and Sn3O4.

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The specific surface area of pristine SnS2 and 0.07Sn3O4/SnS2 NCs were analysed by 225 N2 adsorption-desorption isotherms and information about its porous nature was obtained 226 from the BJH pore size distribution plot (Table S2). The BET plot of pure SnS2 and  (Table S2). A narrow pore size distribution between 2-4 231 nm is obtained (inset of Fig. 4), which suggested mesoporous nature of the 0.07Sn3O4/SnS2 232 NCs. The mesoporous nature is also corroborated from the Type-IV isotherm plot (Peng et al.   degradation followed pseudo-first order kinetic model (Fig. 6d). The corresponding 338 photocatalytic degradation rate for 0.07Sn3O4/SnS2 (k2= 0.043 min -1 ), which is 5.37 times 339 higher than the pristine SnS2 (k= 0.008 min -1 ) and 10.75 times higher than Sn3O4 (k=0.004 sunlight irradiation is given as Supporting Information (Fig.S3b). It can be seen that the intensity of the characteristic absorption peak of MB at about 665 nm decreased gradually 344 and it nearly disappeared after 90 min.

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The degree of mineralization of MB dye was estimated from total organic content 346 (TOC) in the respective residual solutions treated with 0.07Sn3O4/SnS2 NCs after 347 photocatalytic degradation for 90 minutes sunlight exposure. Expectedly, these values were 348 high then the TOC removal by pristine SnS2 and Sn3O4 (supporting information Fig. S6) Table S3). The concentrations of the leached metal ions in the 377 reaction medium were determined from the respective calibration plots (Fig. S7). These 378 results suggested excellent chemical stability of 0.07Sn3O4/SnS2 NCs as photocatalyst.

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The photo-stability of 0.07Sn3O4/SnS2 NCs as photocatalyst for Cr (VI) 380 photoreduction is reflected from the re-usability of the photocatalyst for at least five  This is well supported by the radical trapping studies (Fig. 9c), and the photogenerated charge 467 carrier transfer process can be described as follows: Consequently conclusion can be drawn that the photo-degradation by Sn3O4/SnS2 477 heterojunctions followed a direct Z-scheme mechanism, which not only promotes the 478 separation efficiency of photogenerated electrons and holes but also shows a strong redox 479 ability for antibiotic degradation.