Synthesis of SnO2 and Zn doped SnO2 Nanoparticles by Flame Oxidation Process for Photocatalytic degradation of Methylene Blue Dye

A very simple and rapid flame oxidation method is effectively used to synthesis pure Tin Oxide (SnO 2 ) and Zinc doped Tin Oxide (Zn:SnO 2 ) nanoparticles from the metallic Tin (Sn) and Zinc (Zn) powders for the photocatalytic degradation of Methylene blue (MB) dye and characterized to study their structural, optical, elemental and chemical properties. From the X-ray diffraction analysis (XRD) it indicates that the synthesized SnO 2 and Zn:SnO 2 nanoparticles have pure tetragonal and cubical phases respectively and their average size increases when Zn was doped with SnO 2 . Raman spectral studies confirms the various mode of vibrations and the crystal structure of the synthesized nanoparticles from the spectral peaks of Raman shifts. Purity, atomic percentage and chemical composition were analysed using Energy dispersive X-ray analysis (EDX). The band gap energy was increasing from 3.5 eV to 3.6 eV when doping of Zn with SnO 2 , which was revealed from the UV-visible spectroscopic analysis. Photoluminescence analysis (PL) confirms the red shifted emission for Zn:SnO 2 due to the oxygen deficiency. The CIE chromaticity(x,y) for SnO 2 and Zn:SnO 2 was calculated from the emission spectra and the cordinates represents blue and violet region respectively. Field Emission Scanning Electron Microscopy (FESEM) analysis shows that the pure SnO 2 nanoparticles have irregular, agglomerated, nanoflowered and nanoclustered formation whereas Zn:SnO 2 nanoparticles have more crystalline, cubical and nanoflakes structures. The photocatalytic activity was enhanced due to the presence of Zn in SnO 2 under UV light irradiation. The efficiency of MB degradation by SnO 2 and Zn:SnO 2 nanoparticles are above 80%, which proves to be an effective photocatalyst.

In the daytoday life, every countries economy depends upon the goods and products that have been manufactured from the industries. Although industrial development offers many advantages to society, but also few disadvantages are there with the environmental industries [1]. Mainly, industrial effluent reflects its impact on human life due to its presence in the ecosystem. Generally, most industries produce effluents while producing products, among which the textile industry is the one that generates dyes [2]. Dyes are one of the main pollutants that create environmental risks for all the living organisms in the earth. As these dyes have complex structure makes them more stable and the removal or degradation of these complex molecules from effluents are very difficult. Recent research has focused on the elimination of toxic organic pollutants in wastewater by physical, chemical and biological process but these processes have certain limitations [3]. It is recognized that the photocatalyst is an effective method for wastewater treatment that includes organic contaminants. In photocatalytic degrading process, the nanoparticles catalytic abilities are enhanced due to the presence of ultraviolet radiation, which acts like an stimulator for light induced redox reactions [4]. Owing to the presence of nanoparticles as photocatalyst, degradation of organic dyes has improved, because of this, there is a urge for new techniques to produce several nanomaterials with the required chemical, physical and electronic properties [5]. Among these, metal oxide semiconductor nanoparticles having large band gap were recently explored to a large extent due to their surprising variations in their properties when they are reduced to small size because of quantum size effect or quantum confinement [6]. Metal oxide nanoparticles have been synthesized for different applications, amidst them SnO2 one of the most significant, wide band gap (3.6 eV) [7], n-type semiconductor was examined for applications like gas sensors, LED, Solarcells, photocatalysts, spintronic devices, optoelectronic devices, supercapacitors and Lithium Ion batteries [8][9][10][11][12][13][14][15][16][17][18][19][20]. SnO2 an inorganic compound having large exciton binding energy of 130 meV, was selected abundantly due to their ability of doping with many dopants like Iron, Graphene, Manganese, Antimony, Iodine, Copper, Zinc, Indium, Silicon and Fluorine . When SnO2 was doped with any of these dopants its properties like optical, structural, chemical compositional, magnetic behaviour, photovoltaic properties, gas sensing abilities, electrical properties, electrochemical properties, etc., where changed to a large extent due to the presence of large oxygen deficiency . Among these dopants Zn, a d-group element in the periodic table shows enhanced crystallite structures, optical, electrical and magnetic properties [22]. Many methods have been reported for the synthesis of SnO2 nanostructures like hydrothermal, sol-gel, low temperature solution process, chemical precipitation, coprecipitation, chemical digestion method, microwave irradiation method, spray combustion, electrospinning, magneton co-sputtering, laser pyrolysis, gas phase technique and chemical spray pyrolysis  but most of these methods employed hazardous chemicals, took longer time to react and involve sophisticated instruments. Synthesis of SnO2 and Zn doped SnO2 nanoparticles by flame oxidation or flame synthesis method proves to be very significant, low cost, high purity, high quantity, single step process producing materials in nanodimension [6,8]. The majority of textile industries use azo based acidic dyes for binding dyes on fabrics, methylene blue(MB) a cationic dye used abundantly for textile industrial purposes [4]. In the present research work, we report, the photodegradation of model effluent MB dye in the presence of pure SnO2 and Zn:SnO2 nanparticles as photocatalysts under uv irradiation and their characterization.

Materials and methods 2.1. Synthesis of pure SnO2 and Zn: SnO2 nanoparticles
The schematic diagram of the flame synthesis method is shown in fig.1. In this method, both the oxygen and acetylene gases are mixed in equal proportion (50O2:50C2H2) and send through the nozzle from the oxygen and acetylene cylinders. Using regulator the flow of gases through the nozzle can be regulated to get bluish flame. A powder feeder with a stopper was used to supply the powders into the flame. A powder collector was placed over the reaction chamber about 25 cm from the nozzle to collect the deposited nanopowders. Metallic Sn powders of about 40 μm was filled in the powder feeder and directed towards the bluish flame under gravitation and the uniformity of the powder flow was regulated by the stopper. The metallic Sn powders gets melted in the high temperature flame and gets oxidised directly to form SnO2 nanoparticles and are deposited over the powder collector [6,8]. Similarly, both the metallic Sn and Zn micro powders mixed in the ratio of 40 g : 10 g are filled in the powder feeder and fed into the oxy-acetylene bluish flame under gravitational force. Both these metallic powders gets melted in the flame and oxidised to form Zn:SnO2 nanoparticles. Then both the deposited pure SnO2 and Zn:SnO2 nanoparticles were collected from the powder collector and characterized for further studies [8].

Photocatalytic degradation experiment
The photocatalytic degradation activity of pure SnO2 and Zn:SnO2 nanoparticles as catalysts on Methylene blue (MB) irradiated under UV radiation were evaluated by the decolorization of MB aqueous solution [2]. A 125 mW/cm 2 (40 W) UV lamp was used as the uv light source to induce the photocatalytic process. 10 mg of SnO2 and Zn:SnO2 nanoparticles were magnetically stirred with 50 ml of aqueous MB solution to obtain uniform dispersion and placed under dark for 2 hours to achieve adsorption-desorption equilibrium. The reaction mixture containing both the photocatalysts and MB dye were irradiated with the uv light for (30, 60, 90, 120, 150 and 180 min) regular intervals of time and the sample was tested by UV-visible spectrophotometer to measure the steady state absorption. The photocatalytic degradation efficiency(E) of MB was calculated by, (1) Where Ct is the concentration of MB solution with the photocatalysts (after irradiation of UV light for 't' time interval) and Co is the concentration of MB solution without catalysts [16].

Results and Discussion
Pure SnO2 and Zn:SnO2 nanoparticles synthesized via direct oxidation of the flames were characterized by XRD, Raman spectroscopy, EDX, UV visible spectroscopy, Photoluminescence and FESEM. X-ray Diffraction (XRD) was done to analyse their structural properties like particle size, crystalline nature and phases present in the synthesized nanoparticles [19]. Raman spectroscopy was used to confirm the Raman active vibrational modes of the synthesized nanoparticles within the range of 50-900 cm -1 [25]. Chemical composition and purity of SnO2 and Zn:SnO2 nanoparticles was confirmed by Energy Dispersive X-ray Analysis (EDX) [19]. UV visible spectrophotometer (JASCO V-770) in the measurement range of 200-900 nm was used to study their optical properties. Using PL spectrometer FP-8300, photoluminescence (PL) studies of both the synthesized nanoparticles was done at the excitation wavelengths of 340 nm and 410 nm to check the presence of any defects or vacancies [24]. Both the synthesized nanoparticles were examined by FESEM to verify the crystal structure and surface texture [6].

Structural analysis
XRD is one of the most vital characterization method to identify the average crystallite sizes, structure and phases present in the synthesized nanoparticles.  202) and (321) matches with the tetragonal structured SnO2 (JCPDS card No. 00-041-1445, space group: P42/mnm, group number:136) [25]. From the xrd pattern of Zn:SnO2 nanoparticles, there exists peaks representing the planes (111), (220), (311), (222), (400), (422), (511), (440), (531), (620), (533) and (711) which matches with the cubical phased Zn:SnO2 (JCPDS card No. 00-024-1470, space group:Fd-3m, group number:227). From the xrd pattern it was observed that because of the Zn dopant in SnO2 the intensity of the peaks were decreased considerably which indicates the clear bonding of Zn in the SnO2 lattice [19]. The average crystallite size was calculated for both the SnO2 and Zn:SnO2 nanoparticles and found to be 29 nm and 30 nm respectively. Debye-Scherrer formula was used to calculate the crystallite size, D= K λ/ βcos (2) Where, K is a constant(0.9), λ is the Cu-K (1.5418 Å) X-ray wavelength,  is the Diffracted angle in radians and β is the FWHM intensity in radians [22,33]. Increase in the average particle size confirms the doping of Zn in the SnO2 lattice, when the Zn is doped some of the  [33]. Lattice constants are found as a=4.74 Å and c=3.19 Å for SnO2 and as a=8.66 Å for Zn:SnO2. Lattice constant values are calculated using the formula, The change in lattice constant value was due to the addition of Zn in the SnO2 lattice which changes the tetragonal phased SnO2 to Cubical phased Zn:SnO2 [19,33].

Raman spectroscopic analysis
Raman spectroscopy is used to investigate the structural defects like stacking faults, oxygen vacancies, crystallinity and the size effects of nanoparticles [25]. There exists 18 vibrational modes from six unit cell atoms of SnO2, which are symbolized by Γ=A1g+A2g+B1g+B2g+Eg+2A2u+2B1u+4Eu. Among these, Raman active modes are B1g, B2g, A1g and Eg while Eu and A2u corresponding to infrared active region. Acoustic modes are one A2u and two Eu [16,20] while A2g and B1u represents silent mode. Fig.3 shows the Raman spectra of pure SnO2 and Zn:SnO2 nanoparticles at room temperature. In the Raman active mode, Sn atoms are at rest whereas Oxygen atoms vibrates. Eg mode vibrates along the same directions whereas A1g, B1g, and B2g vibration modes vibrates at 90 o to the c-axis. Out of these non-degenerate modes of vibrations, B1g mode vibration around the c-axis was due to the rotation of six octahedral oxygen atoms [16]. From fig.3, Raman spectrum peaks at 631, 678 and 696 cm −1 matches to the asymmetric stretching of SnO2 (A1g, A2u and B2g modes), which confirms the tetragonal crystal structure of synthesized SnO2 nanoparticles. In the mean while the peak located at 670 cm −1 of Raman spectrum is very sharp, which relates to the cubical crystal structure of Zn:SnO2 nanoparticles obtained due to the A2u longitudinal optical phonon vibration mode. The increase in A2u mode peak might be attributed to the incorporation of Zn ions in the SnO2 lattice which in turns create more oxygen vacancies and trapping centres [25].

Chemical analysis
One of the important method to confirm the elemental, chemical composition and purity of the flame synthesised SnO2 and Zn:SnO2 nanoparticles is the Energy Dispersive X-ray Analysis (EDX). From fig.4 (a), the EDX spectrum of SnO2 reveals the characteristic peaks of tin at 3 and 3.4 eV and for Oxygen at 0.5 eV. Similarly, from fig.4 (b) the EDX spectrum of Zn:SnO2 reveals the presence of peaks representing to tin at 3.4 eV, Oxygen at 0.5 eV and Zinc at 1, 8.6 and 9.6 eV. These spectra confirms the purity of the synthesized nanoparticles without any other impurities, which is one of the main advantage of the flame synthesis method. From the fig. 4(b) it can be observed that the intensity of the Sn peak is reduced due to the replacement of Sn ions by Zn ions. When Zn is doped there arises three characteristic peaks representing the presence of zinc in the SnO2 lattice [9,19]. Table (1) and (2) shows the weight and atomic percentages of elements that are present in both the synthesized nanomaterials. From the tables it was clear that the oxygen and Sn percentage was considerably decreased due to the addition of Zn dopant.

Optical studies
UV Visible absorption spectroscopy is the most effective way to analyze the various optical properties of the flame synthesized nanoparticles. Fig.5 (a) shows the absorbance versus wavelength peaks for pure SnO2 and Zn:SnO2 nanoparticles. Both SnO2 and Zn:SnO2 shows very high absorbance around 300 nm range and low absorbance above 400 nm. From these peaks, absorbance edge is red shifted when Zn doped with SnO2 due to the transfer of electrons from O -2p states of valence band to Sn 3d states of conduction band. The band gap values of the synthesized nanoparticles were found using the Tauc plot method, ( ℎν) 2 = A (hν-Eg) (5) Where, is the absorption coefficient, ℎν is the energy of photon, A is a constant and Eg is the band gap energy. Fig. 5(b) shows the Tauc plot graph for both SnO2 and Zn:SnO2 nanoparticles and using the extrapolation line at the linear region the band gap values are determined as 3.5 eV and 3.4 eV for SnO2 and Zn:SnO2 respectively. There is an increase in the band gap value which can be explained by the Burstein-Moss broadening effect caused by the addition of Zn dopant ions in the SnO2 lattice. Due to the Zn doping there creates new electron-hole pairs which in turns create new energy levels in the valence band and conduction band. The increase in band gap and corresponding shift is due to the strong quantum confinement effect of the Zn dopant [18,19,22,24].

Photoluminescence (PL) studies
In order to find any defects or vacancies that are present in the flame synthesized nanoparticles, photoluminescence is one of the best method. Fig.6 (a) shows the PL emission spectra of pure SnO2 and Zn:SnO2 nanoparticles, which are excited at 340 nm and 410 nm respectively. Due to the electron-hole recombination process the PL peaks arises. SnO2 shows high intensity blue emission peaks at 373.5 nm and low intensity red emission peak at 549 nm due to the presence of oxygen vacancy which are created because of the inhibition of electron-hole recombination. Whereas, Zn:SnO2 shows two low intensity red emission peaks at 582.5 nm and 687 nm. Due to the doping of Zn ions in the SnO2 lattice there arises new electron-hole pairs which creates many oxygen vacancy sites [19,22]. Fig. 6(b) is the CIE (1931) Chromaticity diagram of pure SnO2 and Zn:SnO2 nanoparticles. From the chromaticity diagram the coordinates for SnO2 was calculated and found as x=0.29093 and y=0.33514, which lies in the blue region [32]. When Zn doped with SnO2 the coordinates changed to x=0.21049 and y=0.14863, which lies in the violet region. The straight line in the fig.6 (b) indicates the change of coordinates and the color location of the SnO2 and Zn:SnO2.

Surface analysis
Field Emission Scanning Electron Microscopy (FESEM) is the apt characterization method to analyse the surface morphology and crystalline nature of SnO2 and Zn:SnO2 nanoparticles. Fig. 7 (a) and (b) are the FESEM images of flame synthesized pure SnO2 and Zn:SnO2 nanoparticles. During the synthesis of SnO2 nanoparticles, when the bulk metal Sn powders falls on the oxy-acetylene flame, due to the very high temperature of the flame Sn immediately melts and reacts with oxygen to form irregular, agglomerated, nanoflowered and nanoclustered SnO2 nanoparticles. When the Zn is doped the surface morphology of SnO2 changes drastically to nanocubes and nanoflakes of much improved crystalline structures. FESEM images clearly shows the change in morphology and crystalline nature of the nanoparticles. Due to the doping effect of Zn, there creates new structure formation and enhanced crystalline nature [20,25]. Fig.8 (a) and (b) shows the absorbance spectra of MB in the presence of synthesized photocatalysts under ultraviolet irradiation. Due to the degradation of MB dye, around 661 nm range the absorption intensity decreases linearly with the increase of irradiation time from 0-180 min. It was also noted that the color of the aqueous solution gradually diminishes as the time of uv irradiation increases [16]. Fig.8(a) shows the MB dye degradation in the presence of SnO2 nanoparticles, before irradiating the MB solution it was tested without any catalyst(blank test) and with the catalyst in dark which exhibits very low photolysis [2]. When the uv radiations falls on the surface of SnO2, it absorbs the radiation and induce electrons from the lower energy level (valence band) to the higher energy level (conduction band) which results in the formation of highly reactive hydroxyl (OH -) radicals and superoxide radicals (O 2-) [16]. Both these radicals plays a vital role in the degradation of MB dye solution.

Photocatalytic Degradation of MB
At the end of 180 min uv irradiation, the degradation efficiency of MB dye in the presence of SnO2 catalyst reaches 82%, which can be revealed from the decolorization of the MB solution. Similarly, from fig.8 (b) the photocatalytic degradation efficiency of the aqueous MB dye solution in the presence of Zn:SnO2 catalyst was found as 88%. It is evident from the fig.8 (a & b), Zn:SnO2 exhibited more photocatalytic degradation activities than pure SnO2. Kinetic plot for the MB degradation was shown in the fig. 9, which indicates the total degradation process follows a pseudo-first order reaction and the rate constant values were calculated as 0.9555 and 0.9383 for both the nanoparticles. Fig. 10 represents the graph plotted between Ct/Co versus time, which clearly indicates the degradation of MB dye by SnO2 and Zn:SnO2 nano-photocatalysts [2]. Accordingly, increase in surface area and band gap can be the reason for the enhancement of photocatalytic activity of the Zn:SnO2 catalysts. Inferred from the analysis, under UV irradiation both SnO2 and Zn:SnO2 shows higher catalytic activity due to the following reasons: (i) generation of more electron-hole pairs, (ii) transfer of photo generated electrons through the interface from CB of SnO2 to CB of Zn:SnO2 and (iii) transfer of photo-generated holes from VB of Zn:SnO2 to VB of SnO2 which results to the decrease of recombination between the photo-generated holes and electrons [4]. Moreover the electron-hole recombination was reduced due to the presence of Zn ions that are present in the Zn:SnO2 catalyst acts as electron traps and increase the formation of superoxide radicals [2]. These radicals induce the direct chemical reaction between the photocatalyst and the dye, which enhance the degradation of MB solution. Moreover, the increase in the photocatalytic degradation activity of both the SnO2 and Zn:SnO2 photocatalysts are due to the increase of interfacial charge transfer to the substrates and the prolonged life time of the charge carriers by the efficient charge separation process [4].

Conclusions
Pure SnO2 and Zn:SnO2 nanoparticles were successfully synthesized by flame synthesis method using the high temperature oxy-acetylene flame. Formation of tetragonal structured pure SnO2 and cubical phased Zn:SnO2 nanoparticles were confirmed from the XRD analysis. Due to the doping of Zn in SnO2 lattice the average crystalline size of the nanoparticles is slightly increased from 29 nm to 30 nm. Raman spectroscopy reveals the oxygen deficiency and the structural changes of the nanoparticles from the vibrational mode analysis. From the EDX analysis, atomic weight percentage and chemical composition confirms the purity of SnO2 and Zn:SnO2 nanoparticles without any other impurities. UV-visible absorption spectra confirms the Burstein-Moss broadening effect when Zn is doped with SnO2. Increase in band gap value from 3.5 eV to 3.6 eV was clearly evident from the Tauc plot graph. PL studies exhibit the maximum intensity at 373.5 nm and 549 nm for pure SnO2 and for Zn:SnO2 at 582.5 nm and 687 nm. Red shifted PL peaks of Zn:SnO2 confirms the presence of oxygen vacancy sites in the SnO2 lattice. The CIE chromaticity diagram clearly indicates the change in coordinates and the color location (region) of SnO2 and Zn:SnO2. From the FESEM analysis, it is clearly visible that irregular, agglomerated, nanoflowered and nanoclustered SnO2 nanoparticles changes to nanocubical and nanoflaked Zn:SnO2 nanoparticles with enhanced crystalline structure. MB degradation analysis shows the high performance of SnO2 and Zn:SnO2 nanoparticles as photocatalysts under UV light due to the formation of highly reactive (OH -) hydroxyl radicals and superoxide(O 2-) radicals. From these inferences, the optical properties of SnO2 can be improvised by Zn doping and these nanoparticles synthesized through the very rapid and single step flame oxidation process can be referred as one of the most promising and competent photocatalysts for high performance photocatalytic devices and other potential applications.