Photodegradation of methyl violet 6B and methylene blue using tin-oxide nanoparticles (synthesized via a green route)

Accepted Manuscript Title: Photodegradation of Methyl Violet 6B and Methylene blue using tin-oxide nanoparticles (synthesized via a green route) Author: Archita Bhattacharjee M. Ahmaruzzaman Th. Babita Devi Jayashree Nath PII: DOI: Reference:

S1010-6030(15)30280-X http://dx.doi.org/doi:10.1016/j.jphotochem.2016.03.032 JPC 10187

To appear in:

Journal of Photochemistry and Photobiology A: Chemistry

Received date: Revised date: Accepted date:

29-11-2015 28-3-2016 30-3-2016

Please cite this article as: Archita Bhattacharjee, M.Ahmaruzzaman, Th.Babita Devi, Jayashree Nath, Photodegradation of Methyl Violet 6B and Methylene blue using tinoxide nanoparticles (synthesized via a green route), Journal of Photochemistry and Photobiology A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2016.03.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Photodegradation of Methyl Violet 6B and Methylene blue using tin-oxide nanoparticles (synthesized via a green route) Archita Bhattacharjee, M. Ahmaruzzaman*, Th. Babita Devi, Jayashree Nath Department of Chemistry National Institute of Technology, Silchar-788010, Assam, India *Corresponding author: [email protected]; Telephone/Fax Number: +913842-242915/+913842224797  

HIGHLIGHTS:  A fast, facile, green method was developed for the synthesis of SnO2 NPs using water and glycerol.  This method results in the formation of spherical SnO2 NPs of 8-30 nm within 5 min.  The sizes of SnO2 NPs can be tuned by controlling the volume of glycerol, which acts as a good complexing and capping agent.  The optical band gap energy of synthesized SnO2 NPs showed a clear blue shift from 3.78-4.17eV with a decrease in grain size.  SnO2 NPs act as an efficient photocatalyst in the degradation of methylene blue and methyl violet 6B, under direct sunlight.

ABSTRACT Green synthesis of tin dioxide nanoparticles were developed by microwave heating method using 1:1, 1:2 and 1:3 volumetric ratio of water and glycerol, wherein glycerol acts as a good complexing as well as capping agent. This method resulted in the formation of spherical SnO2 nanoparticles with an average diameter ~8-30 nm. The synthesized SnO2 NPs were characterized by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and Fourier transformed infrared spectroscopy (FT-IR). The optical properties were investigated using UV-visible spectroscopy. The photocatalytic activity of synthesized SnO2 NPs was evaluated for the degradation of two different toxic dyes namely, Methyl Violet 6B and Methylene blue dye under direct sunlight. 1  

Keywords: SnO2 nanoparticles, microwave heating, glycerol, photocatalyst, methyl violet 6B, methylene blue. 1. Introduction Tin dioxide (SnO2) is an n-type semiconductor with a wide band gap of 3.6 eV [1]. SnO2 with a tetragonal crystal structure, is the most intensively explored metal oxide due to its potential applications in catalysis, gas sensors, dye-based solar cells, transparent conducting electrodes, rechargeable lithium batteries, etc. [29]. A variety of methods were employed for the synthesis of SnO2 nanoparticles [10-13]. The majority of the process needs longer reaction time, utilization of poisonous chemicals and critical reaction conditions. However, large-scale synthesis of crystalline SnO2 nanoparticles by a green and facile method is still a challenging job. In this paper, we reported a green synthesis of SnO2 nanoparticles by microwave heating method using glycerol, wherein glycerol acts as a good complexing as well as capping agent. Dyes constitute a major class of organic compound having huge applications in our daily life. They are used in textile industries, dyeing, printing, cosmetics etc. However, most of the dyes are toxic in nature. The effluents coming out from these industries contaminate water system thereby causing water pollution. This poses a threat to water bodies and our ecosystem. Hence, complete removal of dye from industrial waste water is vital for reducing water pollution. In this study, methyl violet 6B (MV6B) and methylene blue (MB) dyes are selected. Methyl violet 6B is a water soluble dye, used in textile industries, paper dyeing, paints and printing ink. MV6B is also used as a disinfectant and is found very poisonous to animals. MV6B is carcinogenic in nature. Methylene blue is also a water soluble dye used as a colorant in textile industries. It is toxic and causes anemia, bladder irritation and gastrointestinal problems. Therefore, the removal of such dye effluents from water is very necessary for avoiding adverse health hazards and also to prevent our ecosystem. In recent years, there are various techniques developed for the removal of such dye pollutants from water, however most of them were associated with certain flaws [14]. Herein, we reported the 2  

sundriven photocatalytic degradation of the dyes namely methyl violet 6B and methylene blue in presence of synthesized SnO2 nanoparticles, acting as catalyst. This method does not cause secondary pollution and therefore proves to be an effective method for the removal of toxic dyes from waste water. Several metal oxide semiconductors, such as SnO2, TiO2, ZnO, NiO, V2O5, etc have been used as photocatalyst for the degradation of organic pollutants in water [4, 15-19]. Among them, SnO2 is found to an efficient photocatalyst due to its large reactivity of surface, immense absorption capacity of light radiation and huge number of active sites [15].

2. Experimental 2.1. Materials The reagents, stannous chloride dihydrate (SnCl2.2H2O), glycerol, methyl violet 6B and methylene blue were of analytical grade (AR) and purchased from Merck. The reaction was carried out in LG Microwave oven MG-557B (Input: 230V-50Hz; Reoutput: 900W; Microwave: 1350W; Frequency: 2450MHz). Double distilled water was used for the synthesis of SnO2 nanoparticles.  2.2. Synthesis of SnO2 nanoparticles In this present work, microwave heating method was employed for the synthesis of SnO2 nanoparticles. Three different sets of reactions were carried out by dissolving 0.3505 g of SnCl2.2H2O in 100 ml distilled water. To these three different sets, 1:1, 1:2 and 1:3 volumetric ratio of distilled water and glycerol were added drop wise with constant stirring. The reaction mixtures were then kept in a microwave oven and irradiated with sixty 10s shots. This method results in the formation of white precipitate. The obtained precipitate was centrifuged and washed three to four times with double distilled water. The white product was finally calcined at 400oC for 2h and then collected for characterization. The SnO2 nanoparticles

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obtained for different ratios of water and glycerol mixtures were marked as S1 (1:1), S2 (1:2) and S3 (1:3) respectively. 2.3. Characterization of SnO2 nanoparticles SnO2 nanoparticles were characterized by powder X-ray diffraction (XRD) method using Phillips X’Pert PRO diffractometer with Cu-Kα radiation of wavelength 1.5418 Å. The size, morphology and diffracted ring pattern of SnO2 nanoparticles were determined by JEM-2100 Transmission Electron Microscope. The Infrared spectra were recorded in the wave number range from 400–4000cm-1 by using Bruker Hyperion 3000 FTIR spectrometer. UV-visible absorption spectra of the synthesized SnO2 nanoparticles were recorded using Cary 100 BIO UV–visible spectrophotometer equipped with 1cm quartz cell.  The degradation products were identified by LC-MS (410 Prostar Binary LC with 500 MS IT PDA Detectors). 2.4. Photocatalytic activity of synthesized SnO2nanoparticles: The photocatalytic activity of synthesized SnO2 nanoparticles were evaluated by the degradation of two different organic dyes namely, methyl violet 6B (MV6B) and methylene blue (MB). To examine the photocatalytic activity, 10 mg of synthesized SnO2 photocatalyst was dispersed in 200 ml of 10-4 M aqueous solution of MV6B and MB dye. Both the solutions were then exposed to sunlight irradiation. Before exposing to direct sunlight, both the solutions were also kept in dark for next 30 min to achieve the adsorption/desorption equilibrium. The photodegradation of the dyes were carried out on a sunny day between 10 a.m-2 p.m when there were minimum fluctuations in solar intensity. The experiment was carried out in Silchar city in the month of May (outside temperature 35–30oC) on a sunny day when the average solar radiation was 4.29 kwh/m2/day. At regular interval of time, 5 ml of the each suspension was withdrawn and centrifuged immediately. The progress of the reaction was monitored by using UV–visible spectrophotometer. The degradation products were identified using LC-MS analysis. 3. Results and Discussion 4  

3.1. FT-IR studies: Fig. S1 (Supplementary Information) represented the FTIR spectra of synthesized S1, S2 and S3 nanoparticles. The assignment of FTIR bands of S1, S2 and S3 nanoparticles were recapitulated in Table 1. The peak around 600cm-1 was due to Sn-O-Sn stretching mode of surface bridging oxide formed by the condensation of adjacent hydroxyl groups [10, 11, 20, 2]. FTIR spectra were recorded in order to detect the formation of SnO2 and also to perceive the presence of glycerol as capping agent. The band around 3415cm1

and 1628cm-1 were attributed to O-H and C=O stretching vibration. This indicated the presence of glycerol

molecules on the surface of SnO2 which act as a capping agent. Hence, the formation of SnO2 was confirmed from the FTIR spectra. 3.2. XRD studies: The XRD patterns of SnO2 nanoparticles (S1, S2 and S3) were represented in Fig. 1 (a, b and c) respectively. The XRD pattern was recorded in order to investigate the crystal structure, purity and crystalline nature of synthesized SnO2 nanoparticles. The peak positions observed at 2θ values corresponded to (110), (101), (200), (211), (220), (002), (310), (301), (202) and (321) planes, respectively. The XRD pattern clearly reflected the tetragonal crystal structure of SnO2 nanoparticles (JCPDS 41-1445) [21, 22]. No characteristic peaks were found for other impurities which indicated the purity of the synthesized SnO2 nanoparticles. Hence, from the XRD pattern it was confirmed that SnO2 nanoparticles with tetragonal crystal structure were formed at a temperature 400oC with different volumetric ratio of water and glycerol. From the XRD pattern, it was also evident that the peaks became gradually sharper and the full width half maximum (FWHM) were reduced with an increase in the ratio of glycerol in the mixture. This indicated that the particle size and crystallinity of SnO2 nanoparticles increases with an increase in the ratio of glycerol. The average crystallite size of S1, S2 and S3 NPs calculated using Debye–Scherrer equation were found to be 9, 14 and 30 nm, respectively. This shows that the crystalline size of SnO2 nanoparticles increases with 5  

an increase in the glycerol volume. Hence, from the above studies it is evident that the size of SnO2 nanoparticles can be tuned by controlling the volume of glycerol. 3.3. Electron Microscopic Analysis: The detailed observations of electron microscopic analysis for the synthesized SnO2 NPs (S1, S2 and S3) are shown in Fig. 2, 3 and 4. The morphology and the size distribution of S1 NPs formed at 400oC can be depicted from the TEM and HRTEM images. Fig 2(a, b) represented the TEM and HRTEM microphotograph of S1 NPs. Fig. 2(a) showed the formation of spherical SnO2 nanoparticles with an average diameter of 8-10 nm. The spacing between adjacent lattice fringes calculated from the HRTEM image (Fig. 2b; inset of Fig. 2a) was found to be 0.26nm which corresponds to (101) lattice plane. The SAED pattern (Fig. 2c) depicted the presence of concentric diffraction rings which indicated the crystalline nature of SnO2 nanoparticles. The lattice spacings were calculated from Fig. 2(c) and found to be 0.34, 0.28, 0.24, 0.178 and 0.16 nm which corresponded to the lattice plane (110), (101), (200), (211) and (220), respectively. The lattice plane obtained from the SAED pattern matches well with the tetragonal crystal structure of SnO2 [21, 22]. Fig. 3(a, c) represented the TEM image and SAED pattern of S2 nanoparticles. The microstructure of the nanoparticles was investigated by the HRTEM image (Fig. 3b; inset of Fig. 3a). Fig. 3(a) showed the formation of spherical SnO2 nanoparticles having an average size of 13-17 nm. From the HRTEM image (Fig. 3b), the lattice spacing was found to be 0.23 nm which corresponded to (200) plane. The lattice spacings were calculated from the SAED pattern (Fig. 3c) and this corresponded to (110), (101), (200) and (211) lattice plane respectively. The SAED pattern clearly reflected the tetragonal crystal structure of SnO2 [21, 22]. The morphology and the size distribution of S3 NPs formed at 400oC can be depicted from the TEM and HRTEM images (Fig. 4a, 4b). The TEM image (Fig. 4a) showed the formation of spherical nanoparticles 6  

having an average diameter of ~ 30 nm. The spacing between lattice fringes calculated from the HRTEM image (Fig. 4b; inset of Fig. 4a) was found to be 0.174 nm which corresponded to (211) lattice plane. The SAED pattern (Fig. 4c) showed concentric diffraction rings which revealed the crystalline nature of SnO2 nanoparticles. The lattice spacing were calculated from the SAED pattern (Fig. 4c) and it corresponded to (110), (101), (200) and (211) lattice plane respectively. The lattice plane obtained from the SAED pattern matches well with the tetragonal crystal structure of SnO2 [21, 22]. From the TEM images and SAED pattern, it was evident that the SnO2 nanoparticles formed at 400oC with different volumetric ratio of water and glycerol were spherical and crystalline in nature with an average particle size ranging from ~8-30 nm. Hence, it can be stated that with an increase in the ratio of glycerol in the mixture, the grain size of the nanocrystallites started to grow, thereby exhibiting a high degree of crystallinity. At higher volume of glycerol, the neighboring particles agglomerate thereby increasing the grain size of SnO2 nanoparticles. The synthesized SnO2 nanoparticles also possess tetragonal crystal structure which was in resemblance with the XRD pattern. 3.4. Optical Properties: UV–visible spectra were recorded in order to explore the optical properties of synthesized SnO2 NPs. Fig. S2a, S3a and S4a (Supplementary Information) represented the absorption spectra of S1, S2 and S3 NPs respectively. From the Fig S2(a), it was evident that broad peaks of negligible intensity started to originate around 250-260 nm and 220nm after calcination at 400oC. From Fig. S3(a) and S4(a) it was apparent that the intensity of these broad peaks started to increase with an increase in the volumetric ratio of glycerol. Absorption spectra were recorded also to obtain the band gad energy of synthesized SnO2 NPs. The band gap energy (Eg) of the synthesized SnO2 nanoparticles can be calculated from the absorption spectra using Tauc plot. For semiconductor nanoparticles, following equation has been used to relate absorption coefficient with incident photon energy: 7  

α(ν)hν =K (hν-Eg)n ----- (1) where Eg is the band gap energy, hν is the incident photon energy, K is a constant, α(ν) is absorption coefficient which can be defined by the Beer–Lambert’s law as follows: α(ν) = 2:303 Aρ/cl, where A is the absorbance, ρ is the density of the SnO2 nanoparticles, c is the concentration and l is the path length. The exponent ‘n’ in Eq. (1) depends on the type of the transition and ‘n’ have values 1/2, 2, 3/2, 3 for allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively. In case of SnO2 nanoparticles the value of n is 1/2 for allowed direct transition. Therefore, by plotting (ahν)2 versus hν and extrapolating the curve to zero absorption coefficient band gap energy (Eg) can be determined using Eq. (1) [23, 22]. Fig. S2(b), S3(b) and S4(b) represented graphically the plot of (αhν)2 versus incident photon energy (hν) for S1, S2 and S3 NPs formed at 400oC. By extrapolating the curve to zero absorption coefficient, band gap energy was calculated using Eq. (1) and it was found to be 4.17, 4.00 and 3.78 eV for S1, S2 and S3 NPs, respectively. In case of semiconductors, band gap energy depends on the particle size. Hence, there is a decrease in the band gap energy with an increase in grain size of the synthesized SnO2 NPs. Table 2 represented the summary of results obtained from absorption spectra of synthesized SnO2 NPs. 3.5. Role of glycerol in the synthesis of SnO2 NPs: The glycerol content in the reaction system was decisive for the particle size. The TEM image of the sample showed that the particles agglomerate when the glycerol content was increased to some extent. The formation mechanism of SnO2 particles in glycerol was shown in Scheme 1. In glycerol solution, SnCl2 was surrounded and protected by glycerol molecules and Cl- was replaced by glycerol molecules [24]. When the system was irradiated with microwaves, the water molecules might attack the Sn-OCH2CH(OH)CH2OH bonds as a result of replacing the glycerol molecules to form Sn(OH)2 and finally decomposed into SnO and then was further oxidized to SnO2. The plausible mechanism for the formation of nanoparticles can be visualized as (Scheme 1): 8  

3.6. Photocatalytic activity of synthesized SnO2 NPs: The photocatalytic activity of synthesized SnO2 NPs was evaluated by carrying out the degradation of Methyl violet 6B (MV6B) and Methylene Blue (MB) dye under solar irradiation. The degradation of dyes depends on the band gap energy of the photocatalyst. Under solar irradiation, photocatalyst possessing lower band gap energy is more preferable. Therefore, the SnO2 nanoparticles possessing band gap energy of 3.78 eV (S3) was utilized as photocatalyst for the degradation of dyes namely MV6B and MB. The degradation of MV6B and MB passes through various intermediate stages. For example, on sunlight irradiation, MB undergoes degradation to leuco MB in the first step, which is prior to mineralization. Hence, on solar irradiation, various chemical reactions take place which are explained vividly in section 3.7, 3.8 and 3.9. The progress of the photodegradation reaction was also monitored by recording the changes in the absorption spectra of MV6B and MB solution at regular interval of time. Fig. 5(a) and 5(b) represented the absorption spectra for the photocatalytic degradation of MV6B and MB dye respectively using SnO2 NPs as photocatalyst under direct sunlight. The UV-visible spectra of the MV6B and MB showed a strong absorption band at 580 and 663 nm respectively. It was evident that addition of SnO2 nanoparticles lead to a decrease in the absorption band with time. The intensity of the bands gradually decreased with an increase in irradiation time in both cases. The absorption band at 580 and 663 nm for MV6B and MB dye disappeared completely within 270 min and 240 min respectively. The color of both the solution also faded away which indicated complete destruction of the chromophoric structure of dye. The photodegradation reaction follows pseudo first order kinetics and the rate constant of the reaction can be obtained from the linear plot of lnAt versus irradiation time, t [25]. The value of rate constant (k) was found to be 0.7x10-2 min-1 and 1.07x10-2 min-1 for MV6B and MB dye, respectively (Fig. S5a and S6a; Supplementary Information). It was also evident that 96.2% and 96% of MV6B and MB degraded

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photochemically within 270 min and 240 min, respectively using SnO2 NPs as photocatalyst (Fig. S5b and S6b; Supplementary Information). Simultaneously, to confirm the photocatalytic activity of synthesized SnO2 NPs, a control experiment was carried out where, MV6B and MB dye solutions were irradiated with sunlight in the absence of SnO2 NPs. Both the dye does undergo negligible degradation (about 5-6 %) within 240 min. Similarly, when both the dye solution (MV6B and MB) were treated with SnO2 NPs in the absence of sunlight i.e., in dark, the dyes undergo negligible degradation. This confirmed that the dyes namely MV6B and MB undergo photocatalysis under solar radiation. 3.7. Mechanism for the degradation of MV6B and MB dyes: The semiconductor photocatalyst (SnO2 nanoparticles) having wide band gap energy cannot absorb the visible light directly from solar irradiation. However, light absorption in the visible range can be extended by photosensitization [26]. Since, the dye molecules were sensitive to visible light, hence dye molecules enhance the visible light absorption of sensitized photocatalysts by injecting electrons from the lowest unoccupied molecular orbital (LUMO) of excited dye into the conduction band (CB) of the photocatalyst [26]. The electrons of the dye molecules get excited into singlet and triplet state which was further followed by the electron injection from the excited dye molecule to the conduction band of the photocatalyst. This lead to the formation of cationic dye radicals (dye•+). The electron injected to the conduction band of the dye-sensitized photocatalyst (i.e., SnO2 nanoparticles) reacts with the oxygen adsorbed from air to form oxidizing species (O2•-, HOO• and •OH radicals) which oxidize the dye pollutants. The semiconductor photocatalyst act as electron-transfer mediator and carry out the photooxidation of the dyes successfully under solar irradiation [27, 28]. Hence, SnO2 nanoparticles act as dye-sensitized photocatalyst for the visible light degradation of both the dyes. The probable mechanism for the degradation of the dyes viz., MV6B and MB using dye-sensitized SnO2 photocatalyst can be depicted as follows [27, 28]: 10  

Dye + hν → 1Dye. + 3Dye. Dye* or 3Dye. + SnO2 → Dye.+ + SnO2 (eCB-) SnO2 (eCB-) + O2 → O2.- + SnO2 O2.- + H+ → OOH+ OOH+ + O2.- + H+ → O2 +H2O2 H2O2 + O2.- → OH. + OH- + O2 Dye.+ + OH. → Degradation products Dye.+ + O2.- → Degradation products Dye.+ + O2 → Degradation products Scheme 2 visualizes the degradation pathway obeyed by the dye molecules (viz., MV6B and MB) on irradiation with sunlight in presence of dye-sensitized SnO2 photocatalyst. 3.8. Identification of degradation products of Methyl violet 6B dye MS fragmentation pattern of the degradation product obtained during the degradation of MV6B dye was shown in Figure S7(a) (Supplementary Information). The degradation products formed during the photodegradation of MV6B under direct sunlight were identified by the mass fragmentation pattern obtained from LC-MS and the mechanism for the formation of the degradation products were schematically depicted in Scheme 3. The OH. radical formed during the degradation process initiated the degradation of MV6B dye. The OH. radical act as an oxidizing agent and attack the methyl group to form aldehyde. The aldehyde group oxidizes to carboxylic acid group which decarboxylates into CO2. All the methyl groups present in the ring were demethylated by OH. radical thereby leading to the formation of p-rosaniline which was also identified in the mass fragmentation pattern (m/z= 288). The OH. radicals further attack the demethylated compound and 11  

lead to the decomposition of the dye into various fragments as detected in the mass fragmentation pattern (m/z= 213, 137, 93, 94).

3.9. Identification of degradation products of Methylene blue dye: The degradation products generated during the degradation of MB dye was analyzed by LC-MS and from the mass spectra the fragmented ions were identified. Figure S7(b) (Supplementary Information) represented the MS spectra of the degradation products obtained during the degradation of MB dye. The identified fragmented products analyzed from the mass spectra were represented in Scheme 4 and the pathway for the formation of the degradation products was also depicted in Scheme 4. On sunlight irradiation, MB undergoes degradation to leuco MB in the first step, detected in the MS fragmentation pattern at m/z= 284, which is prior to mineralization. During the photodegradation process, the photo-generated holes (h+) and the OH. radicals act as oxidizing agent. The OH. radical generated during photodegradation process attack the C-S+=C functional group of MB to form C-S(=O)-C thereby making its oxidizing state to pass from -2 to 0, which was identified from the MS fragmentation pattern obtained at m/z= 301. The sulphoxide group was again attacked by the OH. radical to produce sulphone (not detected in MS) wherein the oxidation state of sulphur increases from 0 to +5 and also lead to the dissociation of the ring. The sulphone was then attacked by third OH. radical to produce sulphonic acid (oxidation state= +6) which was identified from MS pattern showing m/z value at 201. The attack of OH. radical again lead to the release of SO42- ions. This reacts again with OH. radical to form phenolic compound (detected in MS fragmentation pattern). The methyl group present in the ring was also attacked by OH. radical to form corresponding aldehyde which on further oxidation give rise to the corresponding acid which further undergoes decarboxylation (detected in MS fragmentation pattern). 12  

The dimethyl phenyl amino group (detected in MS; m/z= 136) was also attacked by OH. radical producing an aldehyde which was detected in the MS fragmentation pattern. The aldehyde was further oxidized into acid which decarboxylates into CO2. The amino group present in the aromatic ring can be substituted by the OH. radical to form phenolic compound (detected in MS fragmentation pattern). The amino group released can form ammonium ions which can be further oxidized to nitrate. 4. Conclusion In this article, a facile and green synthesis of SnO2 NPs was developed by microwave heating method using different volumetric ratio of water and glycerol. Glycerol acts as a good complexing as well as capping agent in the synthesis of SnO2 NPs. The TEM, HRTEM images and SAED pattern showed the formation of well crystalline, spherical SnO2 NPs with an average particle size of ~8-30 nm. From the TEM images, it was evident that with an increase in the volumetric ratio of glycerol, the average grain size of SnO2 NPs increases. At higher volume of glycerol, the neighboring particles agglomerate thereby increasing the grain size of SnO2 nanoparticles. The SAED pattern reflects the tetragonal crystal structure of SnO2. The XRD pattern also confirms the tetragonal rutile structure of SnO2 NPs. The synthesized SnO2 NPs showed excellent dye-sensitized visible light photocatalytic activity for the degradation of methyl violet 6B and methylene blue dyes. Acknowledgement: We, the authors, express our heartfelt thanks and gratitude to the Director, NIT Silchar and TEQIP-II for providing lab facilities and scholarship. Our special thanks are extended to NEHU, IIT Bombay for providing TEM, IR data. References: 1.

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Figures: 1. Figure 1. XRD spectra of synthesized SnO2 QDs (a) S1, (b) S2 and (c) S3. 2. Figure 2. (a) TEM microphotograph of synthesized SnO2 NPs (S1), (b) HRTEM image (inset of Fig. 2a) of synthesized SnO2 NPs (S1), (c) SAED pattern of synthesized SnO2 NPs (S1). 3. Figure 3. (a) TEM microphotograph of synthesized SnO2 NPs (S2), (b) HRTEM image (inset of Fig. 3a) synthesized SnO2 NPs (S2), (c) SAED pattern of synthesized SnO2 NPs (S2). 4. Figure 4. (a) TEM microphotograph of synthesized SnO2 NPs (S3), (b) HRTEM image (inset of Fig. 4a) synthesized SnO2 NPs (S3), (c) SAED pattern of synthesized SnO2 NPs (S3). 5. Figure 5. (a) Photodegradation of Methyl violet 6B (MV6B) dye by solar irradiation using synthesized dye-sensitized SnO2 photocatalyst, (b) Photodegradation of Methylene blue (MB) dye by solar irradiation using synthesized dye-sensitized SnO2 photocatalyst.

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70

80

Wavelength (nm)

 

Figure 1. XRD pattern of (a) S1, (b) S2 and (c) S3 NPs.

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Figure 2. (a) TEM microphotograph of synthesized SnO2 NPs (S1), (b) HRTEM image of synthesized SnO2 NPs (S1) (inset of fig. 2a), (c) SAED pattern of synthesized SnO2 NPs (S1).

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Figure 3. (a) TEM microphotograph of synthesized SnO2 NPs (S2), (b) HRTEM images synthesized SnO2 NPs (S2) (inset of fig. 3a), (c) SAED pattern of synthesized SnO2 NPs (S2).

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Figure 4 (a) TEM microphotograph of synthesized SnO2 NPs (S3), (b) HRTEM images synthesized SnO2 NPs (S3) (inset of fig. 4a), (c) SAED pattern of synthesized SnO2 NPs (S3).

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0.5 (a) Methyl violet 6B (MV6B)

1.2 0 min 15 min 30 min 45 min 60 min 90 min 120 min 150 min 180 min 210 min 240 min 270 min

Absorbance (a.u.)

0.3

1.0

20 min

0.2

0.8

30 min 40 min

0.6

50 min 60 min 90 min

0.4

120 min 180 min

0.1

0.2 0.0 400

0 min 10 min

Absorbance (a.u.)

0.4

(b) Methylene Blue (MB)

240 min

0.0 500

600

700

400

Wavelength (nm)

500

600

700

Wavelength (nm)

 

Figure 5. (a)Photodegradation of Methyl violet 6B (MV6B) dye by solar irradiation using synthesized dyesensitized SnO2 photocatalyst, (b) Photodegradation of Methylene blue (MB) dye by solar irradiation using synthesized dye-sensitized SnO2 photocatalyst.

20  

 

Scheme 1. Formation of SnO2 NPs

Scheme 2. Probable mechanistic pathway for the degradation of the dyes using dye-sensitized SnO2 photocatalyst under direct sunlight.

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N(CH3)2

N(CH3)2

H3C H3C N

N

NH

m/z= 358

H3C

CH3

m/z= 344

H3C

N(CH3)2

NH2

N(CH3)2

H3C H2N

m/z= 316

N

NH2

m/z= 330

H

H H

N

N

H

CH3

H 2N H2N

m/z= 302

NH2

m/z= 288

NH2

22  

NH2

H

N

H

H2N

m/z= 93 OH

H2N

NH2

m/z= 288

H2N

O

NH2

O OH

H2N

H2N

m/z= 137

H2N

m/z=213

NH2

NH2

HO

m/z= 94

m/z= 93

Scheme 3. Photodegradation pathway of MV6B dye and identification of the degradation products.

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N CH3

H 3C N

S

N

m/z= 284

H 3C

CH3

H N CH3

H 3C N

S

N CH3

H 3C

O

m/z= 301 H2 N CH3

H3 C N

SO3H

m/z= 201

H3 C

CH3

m/z= 136

HO

O

O

N

HO CH3

HC

HOC N

N

SO3H

SO3H

CH

N

N

CH3

m/z= 215

H3 C

H 3C

O

CH3

m/z= 151

m/z= 137 HO HO

H N

H

SO3H

H3 C

COH

N

m/z= 201

HC N

O

H

m/z= 123

SO3H

N

CH3

CH3

O

m/z= 187

HO HO H H H

H

N N

H

N N

SO3H

m/z= 173

SO3H

m/z= 137

HOC

CO H

CH

O O

O HO

H N

m/z= 109

OH

m/z= 94

Scheme 4. Photodegradation pathway of MB dye and identification of degradation products

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H

Table 1. Assignment of FTIR bands of synthesized SnO2 nanoparticles SnO2 NPs

Precursor

FT-IR bands 3415 cm-1 (νO-H str.)

S1

SnCl2 + (Water + Glycerol) (Ratio =1:1)

600 cm-1 (νSn–O–Sn str.) 1628 cm-1 (νC=O str.) 3415 cm-1 (νO-H str.)

S2

SnCl2 + (Water + Glycerol) (Ratio =1:2)

600 cm-1 (νSn–O–Sn str.) 1628 cm-1 (νC=O str.) 3415 cm-1 (νO-H str.)

S3

SnCl2 + (Water + Glycerol) (Ratio =1:3)

600 cm-1 (νSn–O–Sn str.) 1628 cm-1 (νC=O str.)

 

Table 2. Summary of results obtained from absorption spectra of synthesized SnO2 NPs.

SnO2 NPs

Precursor

Band gap energy (eV)

S1

SnCl2 + (Water + Glycerol) (Ratio =1:1)

4.17

S2

SnCl2 + (Water + Glycerol) (Ratio =1:2)

4.00

S3

SnCl2 + (Water + Glycerol) (Ratio =1:3)

3.78

 

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