Synthesis and characterization of Sn-doped TiO2 nanoparticles and the evaluation of their Photocatalytic performance under Vis-lights

INTRODUCTION 
Titanium dioxide nanoparticles are known to be predominant semiconductors [1]. Production of sensors [2], biocide agents [3,4], hydrogen [5], photo-catalysts [6,7], and water evaporation reduction [8] are just a few of the applications of TiO2. These particles are non-toxic, chemically stable, low-cost, and also have good optical qualities. When TiO2 nanoparticles are excited under light irradiations, they can hydrolyze organic molecules in an aqueous solution. Absorption of light which has an energy equal to or greater than the band gap (3.2 eV) causes the excitation of electrons from the valence band to the conduction band of the semiconductor [9]. Hydroxyl radicals, which are powerful oxidants of organic molecules, are formed as a result of this. Unfortunately, considering that titanium dioxide has a large band gap energy (3.2 eV), it can only be excited by absorbing UV light. As a result, many researchers have aimed to improve TiO2’s photocatalytic effectiveness under visible light, regarding that the light produced by the sun is significantly far better accessible than UV [10]. Doping the aforementioned semiconductor with metals such as Cu [11], Sn [12,13], Fe [14], Ag [15], Zn [16] and non-metals such as N [17], C [18], S [19], and co-doping [20] is one of the most successful methods of overcoming this issue. TiO2 nanoparticles could also be doped with tin to boost their photocatalytic capabilities. This metal can increase the number of hydroxyl radicals generated, which affects photocatalytic activity. Furthermore, using tin for doping is favorable due to the similar ionic structure of Sn4+ and Ti4+ particles (0.690 and 0.605, respectively), implying that Ti4+ can be replaced by Sn4+ in TiO2 crystal structure [7]. Many researchers have studied the enhancement of photocatalytic activity after doping TiO2 with Sn ions [7,10,12,13]. Rangel-Vázquez et al. (2015) used the sol-gel process using titanium and tin alkoxides to make Sn-doped TiO2 with a band gap of 3.16 eV. They reported the presence of brookite phase in their samples in addition to anatase [7]. Arpac et al. (2007) prepared Sn-doped TiO2 particles using a hydrothermal process. The resulting particles were thoroughly in the anatase phase with a purity of 84.09% [10]. In some recent studies, heating of TiO2 nanoparticles in the presence of suitable metal salts has been used for doping various metals such as zinc, silver, copper, and iron [21,6-14,11]. Although the resulting doped nanoparticles had acceptable catalytic activity in the presence of visible light, however, higher activities are desirable for utilization in field applications. To the best of our knowledge, there are no reports focusing on the thermal doping of TiO2 in the presence of tin salts. The thermal process would be much simpler, cheaper, and faster than other methods and would produce a high-purity product. Thus, we presented here a heat treatment method to prepare Sn-doped TiO2 nanoparticles. In addition, the photocatalytic activity produced catalysts were examined for the degradation of methyl orange in aqueous solutions under visible lights, and the results were compared with the photocatalytic activity of pure anatase.

MATERIAL AND METHODS
All chemicals, including methyl orange, TiO2 powder, and SnCl2 were purchased from Merck Co. 

Synthesis of Sn-doped TiO2
Different amounts of SnCl2 (3, 5, and 7 wt%) and TiO2 were thoroughly mixed and blended. Each mixture was labeled regarding the containing amount of SnCl2 (3%, 5%, and 7%). The mixture was heated in a 700 ᵒC furnace for 60 minutes and then the resulting powder was completely washed with distilled water. The nanoparticles were then dried in an oven at 25 ᵒC.

Characterization
LEO 1430VP instrument was used for scanning electron microscopy (SEM) and elemental dispersive X-ray spectroscopy (EDX) (LEO 1430VP, Germany). The powder X-ray diffraction analysis was done using a PW 1050 diffractometer (Philips, The Netherlands) with a Ni filter and Cu Kα (λ=1.54 Å) radiation. The average crystallite size of TiO2 nanoparticles was calculated according to Scherrer’s equation. A spectrophotometer was used to capture UV-Vis diffuse reflectance spectroscopy (DRS UV-Vis) in the wavelength range of 200–800 nm (Scinco S4100, S. Korea).

Photocatalytic activity
The photocatalytic activity of nanoparticles was investigated by the degradation of methyl orange. The amount of 50 ml solution with a concentration of 30 ppm was used for experiments. Firstly, nanoparticles (1 g catalyst/L solution) were added to methyl orange solution and stirred for 15 minutes in darkness. This was due to the stabilization of absorption and desorption between the organic dye molecules and the nanoparticles. Later on, the colloidal solution was settled under irradiation of a 90 W halogen lamp (Philips, Netherlands) at room temperature (Fig. 1). Subsequently, the photocatalyst solution was centrifuged and the degradation rate was immediately measured by a UV-visible spectrophotometer the rate was calculated using the following equation:

Degradation efficiency (%) = () × 100         (1)

Where A0 represents the initial absorption of the dye solution and A denotes the initial absorption after irradiation.

RESULTS AND DISCUSSION
Characterization
Fig. 2 is demonstrating SEM images of pure TiO2 and sample 7%. The accumulated spherical-shaped particles of relatively the same size are seen in a pure sample (Fig. 2-a). As can be seen, this state is almost preserved in sample 7% (Fig. 2-b). Accordingly, it can be deduced that the used synthesis method has not had any significant effect on the morphology of TiO2 nanoparticles. However, the sample 7% particles are smaller than TiO2 particles with regards to size (Fig 1-B). A literature survey indicates that similar uniform and spherical-shaped morphologies have been reported in the case of iron-silver doped TiO2 [4,14]. 
Fig. 3 shows the EDX spectra of prepared samples. Considering the chemical compositions, pure TiO2 and Sn-doped TiO2 compositions are given in Table 1. Given clear evidence, Sn-doped samples have experienced an increase in tin content. Moreover, the table reveals the presence of elemental Sn and TiO2 compounds.
The XRD analysis was used to the advantage of crystalline phase observations. The results are given in Fige 4. It is proved that all the peaks which are around 25.8ᵒ, 37.4ᵒ, 48.9ᵒ, 54.7ᵒ and 55.9ᵒ present in XRD patterns of pure TiO2 are consistent with anatase (1 0 1), (0 0 4), (2 0 0), (1 0 5) and (2 1 1) crystalline planes [14,21]. Like pure nanoparticles, samples 3, 5, and 7 only contain anatase peaks. Consequently, it could be said that the used doping method has no significant effect on the crystalline structure of TiO2 nanoparticles. Also, there has been no evidence showing the presence of either rutile or brookite in the XRD patterns. This is in agreement with the results of studies on doping with iron, copper, and silver prepared in a similar way to this study [4,14].  Similarly, no pattern regarding the tin-related phase was witnessed. Sn ions were evenly distributed throughout the anatase crystallite of TiO2, according to the predominant chemical state of Sn4+ [10]. On the other hand, the combined ion in the crystalline structure of TiO2 was Sn4+.
In Table 2, the crystalline sizes for anatase as a function of Sn4+ content are shown. It can be seen that, by increasing the amount of Sn4+, there is a significant reduction in the crystallite size of the anatase from 9.2 nm for a pure sample to 5.8 nm for a sample of 7%. In a six-coordination, like that present in anatase structures, the ionic radii of Ti4+ and Sn4+ are 0.605 and 0.69 Å, respectively. This difference which is around 14% in size, should be introducing a perturbation in the crystal structure of anatase, in a way that inhibits the growth of the crystallites [7,10]. 
The absorption of UV-Visible irradiations for pure TiO2 and Sn-doped nanoparticles is given in Figure 5. Photocatalytic absorption of visible light drastically increased after doping TiO2 with Sn. A dramatic shift in the red light region (380-500 nm) is also observed. This can be attributed to the transitions called “charge transfer” between the titanium dioxide conduction band and Sn4+ doped electrons. As Sn-doped titanium dioxide photocatalytic activity is extended in the visible light region, there is a possible enhancement in this characteristic of TiO2. The band gap energy of samples was calculated by using the equation: Eg (eV) = 1240/wavelength in nm [22]. Eg values for Sn-doped nanoparticles are 3.14, 2.70, 2.90, and 2.68 eV where the value is 3.2 eV for pure TiO2. Notably, the addition of Sn has slightly modified the optical behaviors of the above-mentioned by shifting it towards the red region. The calculated band gap values are proved to be far less than similar studies [10, 13].

Photocatalytic performance
The photocatalytic activities of the as-prepared photocatalysts were evaluated by degrading methylene orange under visible light irradiation. The photocatalyst was added as 1 g catalyst/L solution. Figure 6 illustrates the degradation results; It is revealed that the photo-activity of Sn-doped nanoparticles is higher than pure TiO2 under visible light irradiation. Degradation and photo-decolorization of methyl orange dye were determined after 120 minutes for each sample and values of 83%, 85%, and 90% were reported for samples 3%, 5%, and 7%. It is observed that the addition of Sn ions has enhanced the photo-activity of the nanoparticle. 
There are various reasons for the increase in photocatalytic performance such as the smaller size of Sn-doped TiO2 nanoparticles. On the other hand, the substitution of Sn4+ ions by Ti4+ ions in TiO2 produces a distortion in the network which leads to structural defects on the surface, resulting in the presence of unsaturated surface cations and surface hydroxyl groups. The unsaturated sites favor the adsorption of the organic molecule on the surface defects of nanoparticles, where the molecule is oxidized by the photogenerated holes in the valence band. With the increase of the specific surface area in tin-doped- TiO2 samples, an increase in the number of unsaturated sites and an enhancement in the photocatalytic activity should be expected. However, the activities of the sample 3% and 5% do not go far from that shown by the sample 7%. Furthermore, it was reported that Sn4+ incorporation leads to an increase in surface oxygen vacancies on TiO2. However, high oxygen vacancies in samples 3% and 5% can act as recombination centers and inhibit photoactivity increment [7,10].
A comparison between the photocatalytic performance of the above nanoparticles and some other results reported for other metal-doped TiO2 (which were produced using the same heat treatment method) revealed that the Sn-doped TiO2 has higher efficiency and capability towards dye degradation [11,14]. For instance, it has been reported that Cu-doped TiO2 nanoparticles could remove less than 80% of methyl orange at a dye concentration of 30 ppm [11]. The insufficient dye degradation was attributed to the phase transformation (Anatase → Rutile) and nanoparticle agglomeration during the doping treatment. Also, another study showed that the ability of Fe-doped TiO2 nanoparticles in photocatalytic degradation of methyl orange at the concentration of 25 ppm was about 70%.

CONCLUSION
Pure TiO2 and Sn-doped TiO2 nanoparticles were produced by heating TiO2 nanoparticles in the presence of SnCl2 (3, 5, and 7 percent w/w) at 700 °C for 60 minutes. TiO2 nanoparticles of both pure and Sn-doped TiO2 are relatively spherical-shaped particles with relatively the same size. In both the pure and Sn-doped TiO2 nanoparticles, the anatase was the only crystalline structure. By increasing the amount of dopant, there is a reduction in the crystallite size of the anatase from 9.2 nm for a pure sample to 5.8 nm for a sample of 7%. Photocatalytic absorption of visible light drastically increased after doping TiO2 with Sn. After 120 minutes, the degradation rates of samples 3%, 5%, and 7% that underwent methyl orange photo-decomposition were 85%, 85%, and 90%, respectively. The superior catalytic activities of the catalyst were attributed to the structural and consequently band gap changes in anatase by the incorporation of Sn ions into the TiO2 lattice. Compared to pure TiO2, which has a band gap of 3.2 eV, the band gap values for doped TiO2 ranged from 3.14 to 2.68 eV. It was concluded that the produced Sn-doped TiO2 nanoparticles are an efficient catalyst for methyl orange degradation. The proposed heat treatment method is fast and straightforward and can be used to control the anatase structure and maximize its dye removal capability.

CONFLICT OF INTEREST
The authors declare no conflict of interest.