Investigation of metal doped mesoporous TiO2 nanostructures for environmental remediation


 In the recent past, metal oxides have attracted the researchers because of their applications in energy and environmental application domains. In the present work, hydrothermal technique used to prepared the Sn doped TiO2 nanoparticles and the effect of Sn concentration has been investigated. The structural, morphological, surface composition, optical and photocatalytic behavior were studied. XRD pattern revealed that doping of Sn makes easy transformation of rutile from anatase phase at lower temperature, providing tetragonal structure of rutile of TiO2. TEM analysis showed the formation of nanoparticles with spherical like morphology with good crystallinity. UV-Vis spectra, it is observed that optical absorption edge gets red shifted upon Sn doping and the band gap is found to be 2.6 eV. The photocatalytic activity of the synthesized nanoparticles has been investigated under visible light irradiation. Experimental results suggested that 0.5 wt % of Sn doped nanoparticles have shown to exhibit improved photocatalytic properties.


Introduction
Water is a precious resource and without it life is not possible on earth. Water is getting polluted day by day due to dyes, organic and inorganic pollutants have become one of the environmental problems which can cause extreme harmful effects to human organisms and appropriate environments [1].
Various methods such as membrane separation, ltration, distillation, adsorption, biological treatment and photodegradation have been developed for the degradation of pollutants through wastewater treatment [2][3][4]. Among these various methods, photodegradation is found to be the most effective method because of its operational simplicity, complete mineralization and low cost [5][6][7]. Semiconductor metal oxides such ZnO, CuO, WO 3 , NiO, TiO 2 , SnO 2 etc., have been widely used owing to its intense catalytic activity and stability. Among these various semiconductors metal oxides utilized, TiO 2 is found to have unique advantages as photocatalysts due to inertness, stability, low operational temperature ,less toxic, signi cantly low energy consumption and low cost [8]. However, there are few drawbacks for TiO 2 materials to limit their practical application such as wide band gap, high recombination rate and weak separation of charge carriers. These can be avoided by sensitization, doping and coupling of semiconductors [9]. The doping with metal ions and their oxides is one of the most effective methods to extend the absorption in the visible region as well as to enhance the e ciency.
Tin oxide (SnO 2 ) having a wide band gap (3.8 eV) which exhibits novel properties like unique optical, electronic and catalytic properties. The electronic, ionic and crystallographic properties of SnO 2 are similar to TiO 2 and when coupled with TiO 2 they can easily form substitutional solid solution which not only reduces the band gap but also can provoke in suppression of the charge carrier recombination rate directing to the enhancement of visible light activity [10]. After Sn doping, optical absorption edge shifted to longer wavelength(visible region) [11]. Also Sn can inhibit rutile phase transformation from anatase phase and decrease particle size to enhance the e ciency [12]. Due to large ionic radius of Sn are deposited on the surface of TiO 2 and increases surface area, active site and photocatalytic e ciency [13].
Lin et.al, pridicted that the e ciency nearly 15 times greater than that of anatase TiO 2 while doping Sn [14]. Tu et.al, reported that TIO 2 nanotube arrays have been exihibit optimum dopant amount of Sn is found to be 5.6 wt % [15]. In the present investigation, an attempt has been made to synthesize Sn (up to 0.9 wt %) doped TiO 2 nanoparticles by hydrothermal method and we found optimum dopant amount is 0.5 wt %. The photocatalytic degradation e ciency of the synthesized nanoparticles against Methylene Blue in aqueous solution was studied. It was observed that the increase in Sn doping on TiO 2 represents increase the photocatalytic performance under visible light illumination.

Preparation of Sn doped TiO 2 nanostructures:
The sample were prepared by the hydrothermal method. Titanium tetraisopropoxide and Tin tetrachloride pentahydrate were used as a precursor of Ti and Sn, ammonia as surfactant and ethanol as a solvent. 5 mL of Titanium tetraisopropoxide was dissolved in 100 mL of ethanol at room temperature under vigorous stirring for 1h to get complete dissolution. De nite mass of Tin tetrachloride pentahydrates such as 0.1, 0.3, 0.5, 0.7 and 0.9 wt% was dissolved in the exceeding solution and then blend was stirred for 2 h. Then, ammonia was added to the mixture, until solution reaches the pH = 7. Finally, the mixture was placed in a 100 ml of Te on-lined steel autoclave and maintained 180 ̊ C for 24 h. The obtained precipitates washed several times with ethanol and with deionized water by centrifugation and then dried at 80 ̊ C for 3 h. The dried samples were annealed at 600 ̊ C for 5 h. The resulting samples labeled as ST1 for 0.1% Sn, ST2 for 0.3% Sn, ST3 for 0.5% Sn, ST4 for 0.7% Sn, ST5 for 0.9% Sn and are stored in airtight container for further studies.

Characterization
An X' Pert PRO (PANalaytical diffractometer using Kα (λ = 1.5405 A) radiation at a scan rate of 0.02 ̊ / swere used to characterize the crystal structure and phase of the synthesized products. FEI Quanta FEG200 Field Emission Scanning Electron Microscope (FESEM) used to record the images of the prepared samples.. Transmission Electron Microscopy (TEM) micrograph were recorded using a JEOL JEM 2100F microscopeat an accelerating voltage of 200 kV. Functional group analysis was carried out by Bruker IFS 88 Fourier Transform InFrared (FTIR) spectrometer equipped with a MCT cryodetector. Optical absorption studies were done by shimadzu UV-2600 spectrophotometer. X-ray Photoelectron Spectra (XPS) were measured by Shimadzu ESCA 3400. Raman spectra were done with JASCO NR-1800 spectrometer.
Photoluminescence spectra recorded with Model S4 PIONEER BRUKER spectrometer.

Photocatalytic measurement
Sn doped TiO 2 nanoparticles were tested against methylene blue (MB) dye in aqueous solution for photocatalytic activities. The degradation experiment was studied by implementingimmersion type photo reactor with container have quartz tube. Photocatalytic activity was carried out using 150 W (Heber scienti c suppliers) tungsten lamp at wavelength greater than 400 nm and the irradiation intensity measured by lux meter was 23000 1ux. The MB dye solution (100 mL; 3 ppm) were prepared. UV-visible spectrometer used to record the absorption spectrum shows the maximum absorption intensity at 664 nm Then, 10 mg of synthesized sample was added to the MB dye solution and the solution wascontinuously stirred for 10 min in dark condition to analyze adsorption and desorption equilibrium between catalystand MB dye. The visible light used to irradiated with stable aqueous dye solution for 10 min and at every 10 min interval, the UV absorption spectra was recorded and continued upto 30 min at the end of the cycle, the solution was centrifuged to separate TiO 2 particles. The following Eq. (1) used to calculate thephotodegradation percentage of MB [16].
1 where C 0 is the initial concentration of the dye solution and C t is the concentration of the dye solution at every 30 min interval during the photocatalytic reaction.

Results And Discussion
The crystal structure and phase of the Sn doped TiO 2 nanoparticles (ST1, ST2, ST3, ST4, ST5) were determined by XRD ( Fig. 1(a) [19]. It was interesting to found that TiO 2 and SnO 2 oxides be the possession of the same crystal symmetry (Tetragonal) [20]. The sample ST5 slightly shift to the lower angle (0.79°) when compared with ST1.
Page 5/15 21,22]. When the Sn content increases Sn 4+ (0.0071 nm) occupy the lattice position of Ti 4+ (0.0068 nm) due to large ionic radius. As a result, the interplanar distance increases from 0.3200 nm to 0.3290 nm calculated from Braggs equation. This con rms the size increases after Sn doping. Moreover, in all the samples, Sn peak is not appeared in the XRD pattern. It is concluded that the Sn 4+ ion has been successfully incorporated into the crystal lattice site of Ti 4+ [23,24]. Scherrer formula used to calculate the crystallite size for predominant peak which corresponds to the plane (110). For ST1 sample, it was found to be 18.5 nm. Meanwhile, the amount of Sn in the synthesized samples increases the crystallite size from 13.8 nm to 9 nm decreases for the sample ST1 to ST3 which was con rmed by other researchers [25]. The crystallite sizes decrease gradually upto 0.5 wt% Sn doping and then increases for 0.7 wt% and 0.9 wt% of Sn due to ionic radius and enhanced agglomeration. Substitution of large sized Sn 4+ in Ti 4+ to the introduction of oxygen vacancies as a result in an increase photocatalytic e ciency. In general, the larger the surface area, smaller crystallite size to induce the separation of charge carriers and photocatalytic e ciency. Figure 1(c) shows the particle size with different concentration. The calculated XRD parameters of the prepared samples speci ed in Table 1.  [26]. The peak at 237 cm − 1 and 320 cm − 1 is strong second-order or disorder induced Raman scattering [27,28], which is a result of multi-phonon processes. The peak at 143 cm − 1 and 320 cm − 1 disappear due to higher concentration of Sn. Which was good agreement with XRD results. Moreover, all the samples preserved the rutile structure which suggest that Sn dopant are substitutionally incorporated in TiO 2 lattice [29]. When concentration of Sn increases in TiO 2 lattice the peak shift in Eg mode from 443 cm − 1 to 430 cm − 1 and A 1g mode from 610 cm − 1 to 628 cm − 1 were observed. The shift in raman peak occur due to high lattice defect such as oxygen vacancies can be created by doping of Sn ions [30,31].
The morphological analyses of Sn doped TiO 2 nanoparticles were performed by FESEM, TEM and HRTEM as shown in Fig. 3. The FESEM images for the sample ST1 [3(1(a))] and sample ST2 [3(2(a))] shows the mesoporous assembly, composed of rough spherical nanaoparticles, which are clustered and agglomerated. The particle size as measured through image is found to be 18 − 13 nm. The increasing content of Sn in uences the particle size and cluster formation as depicted in [ Fig. 3(3(a))]. It seen that for higher concentration of Sn the cluster formation gets distorted and the particle size from 9 nm to 15 nm increased due to larger ionic radius of Sn as shown in [ Fig. 3(4(a))] and [ Fig. 3(5(a))] [32,33,34]. TEM images of the samples shown in [ Fig. 3(1(b))] to [ Fig. 3(5(b))] reveals that the particles are agglomerated with size of about 8 nm 20 nm [35,36]. From the HRTEM images [ Fig. 3(1(c))] to [ Fig. 3(5(c))], it apparent that the particle consists of uniform and highly crystalline. The uniform fringes with lattice spacing of 0.32 nm corresponding to (110) rutile phase is observed. The lattice fringes spacing increase from 0.3200 nm to 0.3290 nm and shows considerable waviness. The expansion of fringes is caused by the defects possibly due to electric stress that may exist from Sn doping in TiO 2 matrix [37]. From the above morphological analysis, possible growth mechanism explained as shown in Pattern I.
FTIR spectra for the samples were recorded and are used to identify the functional groups and chemical bonding in TiO 2 and SnO 2 nanoparticles in the range of 400 cm − 1 to 4000 cm − 1 The peak at 2339 cm − 1 can be assigned to C-H vibrational band due to source of titanium. [38]. XPS spectra are presented in Fig.6. Figure 6( [49,50]. For Sn 3d (Figure 6(b)) the peak at 487.1eV and 495.5 eV was assigned for Sn 3d 5/2 and Sn 3d 3/2 respectively, and the splitting of the 3d doublet of Sn is 8.4 eV, and also small shift (0.2 eV) observed. This indicated a valence state of Sn 4+ [51,52]. XPS spectra of O 1s level shown in Figures 6 (c (1) ) -6 (c (5) ). It can clearly observe that O 1s consists of peak at 530.8 eV which are produced by lattice oxygen (O L ) in Ti-O-Ti bonds [53,54] and 532.1 eV can be attributed to the hydroxyl group (O H ) [55].
The recombination probability for prepared photocatalyst was characterized by photoluminescence spectroscopy. Figure 7 shows that 385 nm 400 nm were assigned to the band to band PL emission which was created by incident light energy which is approximately equal to that of band gap of the rutile phases of TiO 2 respectively [56,57]. The excitonic peak at 436 nm is assigned to defect in TiO 2 nanoparticles [58].
The PL peak located at the 526 nm is related with the shallow trap on surface oxygen vacancies of TiO 2 [59]. It has been clearly found that the PL intensity decreases from sample ST1 to ST3 with increasing Sn content in TiO 2 because of generated electron-hole pair can affect the charge transfer between the interface of Sn and TiO 2 resulting recombination rate is reduced. The decreasing charge carrier recombination rate is suitable for increasing photocatalytic activity. The PL spectra demonstrate a slight red shift. Which is in good agreement with UV spectra [60]. As the concentration of Sn increases, the PL intensity increases for samples ST4 and ST5, because the trap electrons increase the recombination rate, decreases the photocatalytic activity.

Photocatalytic activities of Sn doped TiO 2 nanoparticles:
The photocatalytic behavior of the synthesized nanoparticles was studied against methylene blue (MB) under visible light irradiation and the results are displayed in Fig. 8. Figure 8  However, there exists an optimal amount of dopant. Here we report optimal value of dopant is 0.5%. Below the optimal value of Sn dopant, the photocatalytic activity increases, but higher than optimal value the photocatalytic activity decreases due to average distance between the trap sites decreases [62].
The degradation percentage is calculated to be 93.5 % for STT1, 87.9 % for ST2, 95.3 % for ST3,93.1 % for ST4 and 86.4 % for ST5 respectively. The comparison was made between the photocatalytic decomposition performed in this work and other reported work as shown in Table 3. where C 0 is the initial concentration of the dye solution and C t is the concentration of the dye solution at every 10 min and K obs is the apparent rate constant. The values for the degradation rate constant are shown in Table 2.

The Plausible photocatalytic mechanism of Sn doped TiO 2 nanoparticles
The Plausible photocatalytic mechanism of prepared nanoparticles towards the degradation of MB under visible light is demonstrate in Fig. 9. The Fermi energy level of TiO 2 is grater than that of SnO 2 due to its smaller work function so electron transfer occurs from TiO 2 to SnO 2 and hole transfer from SnO 2 to TiO 2 [64]. When sample is irradiated with visible light the electron in the valance band (VB) get excited to the conduction band (CB Semiconductor h + + OH − → OH • (6) Semiconductor h + + Dye → Oxidation products (7) O 2 + Semiconductor e − → • O 2 − (8)

Conclusion
In the current work, Sn doped TiO 2 nanoparticles prepared by hydrothermal method. From the XRD results the diffraction angle slightly shift to lower angle which con rms the Sn present in the TiO 2 lattice. Optical spectra con rm the decreases in energy band gap as the Sn content increases. These parameters direct to the conclusion, an appropriate amount of Sn to enhance the e ciency. The optimum dopant amount of Sn is found to 0.5% in our experiment. It was concluded that Sn doped TiO 2 nanoparticle (ST3) shows the highest degradation e ciency (95.3%). The novelty of this work was by using less quantity of photocatalyst we obtained better photocatalytic activity when compare to other reported work.