Synthesis and Investigation of Structural, Microstructural and Optical Properties of Iron-doped Sno2 Nanoparticles : Application in Photocatalysis

Iron-doped tin dioxide nanoparticles Sn 1-x Fe x O 2 (0% ≤ x ≤ 10% ), were prepared by coprecipitation method. X-ray diffraction (XRD) patterns indicate that Sn 1-x Fe x O 2 nanoparticles (NPs) crystallize in the tetragonal rutile-like structure. Scanning Electron Microscope (SEM) observations did not show more modification of the SnO 2 morphology with Fe addition. All the results are consistent with the fact that Fe is strongly soluble in SnO 2 host. The optical band gap energy decreased from 3.65 to 3.30 eV with increasing the iron doping concentration in the solution. Finally, the photocatalytic efficiency of Fe 0.05 Sn 0.95 O 2 nanoparticles (NPs) was examined for the degradation of MB in aqueous solution under UV irradiation. It was found that smaller bandgap of Fe doped SnO 2 photocatalyst resulted in a prominent increase in photocatalytic activity of nanoparticles against Methylene Blue (MB) under UV irradiaion.

Metallic oxides and in particular tin oxides are excellent examples for accessing new or improved properties. Tin oxide nanoparticles offer attractive magnetic, electrical and optical properties [4] as well as high chemical reactivity [5][6][7], giving them a wide range of fields of application such as biomedical, treatment of water, catalysis, gas sensors, lithium-ion accumulators [8,9].
Tin oxide is a transparent conductive oxide of type n with a energy band gap of 3.6 eV, The structure of tin dioxide called cassiterite crystallizes in the tetragonal rutile type system represented with the parameters: a = b = 4.737 Å and c = 3.186 Å [10]. The elementary cell contains six atoms, four oxygen atoms and two tin atoms. Its space group is P4 / mnm [11].
Each tin ion Sn 4 + is at the center of an octahedron formed by six oxygen ions O 2-, as long as each oxygen ion O 2is at the center of an isosceles triangle formed by three ions of tinSn4 +.
cation Sn 4 + and the anion O 2have the value 0.071 and 0.14 nm [12]. The oxygen vacancies formed by the transfer of an oxygen atom from a site to a normal site in the gaseous state, make it possible to obtain a semiconductor of this type. Indeed, the oxygen gap thus created has two electrons, it is then said to be neutral which can be yielded under the effect of temperature.
There is then simple or double ionization thereof. The released electrons can attach to Sn 4 + tin atoms. They then become Sn 2 + and behave like electron donors [13,14]. It is arguably the most widely used compared to other oxides in the area of air pollution monitoring and the detection of toxic gases. [15] In fact, it has electrical properties linked to the remarkable surface adsorption. Tin oxide is also known for its catalytic properties, in particular it facilitates the decomposition of many hydrocarbons.
The doping of SnO2 by suitable elements such as Mn, Fe, Co, Al, Mg ... etc, can improve its optical, electrical and magnetic characteristics and accelerate the race for its practical applications [16].
The addition of transition metals in the photocatalyst influences the trapping of electrons, slows down the speed of recombination of electron / hole pairs, increases the efficiency of charge transfer. It also influences the optical gap of the photocalyser by creating the energy levels inside the forbidden band which helps us to extend the spectrum of adsorption towards visible light and to take advantage of solar radiation.
Recently Fe-doped SnO2 has attracted attention as a candidate for diluted magnetic semiconductors (DMS) [17,18]. It has the advantage of a lower surface potential barrier than metals and a high catalytic activity, which can be very vital in the field of electron emission [19].
In this paper, Fe doped SnO2 nanoparticles are synthesized by coprecipitation method for photocatalytic application in the treatment of water purification. The objective that we set ourselves in this work is to study the influence of doping on the structural, microstructural, and morphological properties of the of synthesized nanostructures and their photo-catalytic performance was investigated with the degradation of Methylene Blue (MB) dye 2 Experimental Procedure
Initially, 0.1M SnCl4.5H2O was dissolved in 100 ml double distilled water and maintained under agitation for 10 min. 0.3M NaOH solution was prepared separately by dissolving appropriate amount of NaOH in 50 ml double distilled water. The prepared NaOH solution was added drop wise into the initial aqueous solution under constant stirring up to 2hrs at room temperature to reach the pH value of 12. The formed gelatinous white precipitates were filtered and then washed seven times using double distilled water and ethanol to eliminate the impurities and chlorine ions from precipitates. Dry for overnight at room temperature and dried in oven at 100 ⁰C for six hours. The dried white precipitates then annealed at 600 ⁰C for two hours followed by grinding to get fine particles. The same procedure is repeated for the remaining samples synthesized with nominal compositions of Sn1-xFexO2 (x= 0.00, 0.02, 0.05and 0.10). These were used for various characterization techniques [26][27][28]

Characterization Techniques
The crystal structure of Fe-doped SnO2 nanoparticles was determined by X-ray diffractometer (MiniFlex600) with Cu-Kα radiation, 0.15418 nm). The surface morphology of the prepared nanoparticles was studied using a scanning electron microscopy (TESCAN_VEGA3) equipped with energy dispersive X-ray spectroscopy (EDS) for chemical analysis. The infrared absorption (FTIR) spectra were recorded to examine the structure by using Thermo-Nicolet equipment in the 4000-400 cm -1 region. The absorbance property was studied using Parkin Elmer UV-VIS-NIR Lambda 19 spectrophotometer in the 190-1800 nm spectral range.

Photocatalytic experiments
The photocatalytic activity of Sn1-xFexO2 [x= 0.00,0.02,0.05 and 0.10] nanocomposites was In the scan range from 10° to 90°, both the samples show the four major peaks (100), (101) , (200) and (211), which correspond well with that of cassiterite phase (JCPDS file number 01-071-0652). No secondary peak characteristic of ferric oxide phase appear for all Fe-doped samples was observed within the detection limit of used XRD, indicating that Fe ions successfully occupy lattice sites rather than interstitial sites of SnO2 which possess a high solubility [29], and the good atomic-level blending of all constituent elements through the coprecipitation method. This important solubility of iron in SnO2 has also been recorded for many other trivalent ions, such as Ga and In [30,31].
However, the addition of Fe influences the SnO2, XRD pattern either in intensity or in broadening of peaks, suggesting that the crystallinity and the particle size are modified by iron doping.
In Sn0.98Fe0.02O2 sample, the maximum intensity peak characterizing the structure rutile corresponding to (110) , (101), (200) and (211)plane which decreasing with doping level. The reduced diffraction intensity with doping may be due to impurities that oppose the growth of SnO2. It is also due to the movement of Sn 4+ ions in the interstitial sites and also to an increase in the amorphous phase and disorder. A decrease in the intensity of the diffracted peaks with the increase of dopant level was also reported in sol-gel synthesized Fe doped SnO2 [32].
Diffraction analysis is a very important tool for studying the crystal growth of a nanoparticules.
It allows us to calculate the size of crystallites which can play an important role in the physical properties (electrical, optical,…) of materials.
The grain sizes were evaluated from the width at mid-height (in 2θ) of the diffraction peak using Scherrer's formula [33]: Where: λ = 1.54056Å, θ is the angle of incidence.
From relation (1) the Values a and c are calculated as follows:  [35]), this substitution of Sn 4+ by Fe 3+ ions is very possible. It is probable that the relative 3+ oxidation state of the iron ions, which is already contained in the precursor used (FeCl3-6H2O), is mainly kept in the SnO2 matrix [36,37]. It is clear that the addition of Fe blocked the progression of the SnO2 crystal grains. Note that the decrease in unit cell volume with the rate doping confirms the insertion of Fe 3+ in SnO2 lattice which can be considered as a greater distortion of the cell.
The lattice strain, it iusually means the inhomogeneous local strain in the material which gives rise to x-ray peak broadening. It is only one factor causing peak broadening. The other factors are small crystallite size and presence of lattice defects.
The determination of the stresses produced in the nanoparticles due to lattice distortion and impurities was carried out using the Stoke Wilson formula [38]: Pure SnO2 shows a strain of 0.17×10 -3 while with a doping (Fe) content of 2%, it increases to 0.3×10 -3 , 0.45×10 -3 with 5% doping, and 0.50×10 -3 with 10% doping. The positive signs of the stress values correspond to tensile stress. These strain variants may occur due to the deformation generated in the samples at the time of thermal expansion in different directions. In addition, the crystallite size is reversed proportionally to the deformation, which implies that a change in the crystallite size will cause a variability of the deformation in the sample.
The dislocation density value (δ) represents the number of dislocation lines per unit volume of the crystal, which is the size of the defects in a crystal. In other words, the dislocation density value will show the degree of crystallinity of the nanoparticle profile.
The result of the dislocation density (δnp) of the Sn1-xFexO2[x=0, 2, 5 and 10 at%] nanoparticles is 0.444×10 -3 nm -2 , 1.306×10 -3 nm -2 , 2.929×10 -3 nm -2 and 3.631×10 -3 nm -2 respectively . Based on the results of this calculation is known that the dislocation density (δnp) of the nanoparticles obtained in this study is increase with increasing of rate doping wich indicates that SnO2:Fe nanoparticles have been produced had a low degree of crystallinity because of presence of impurty (Fe). which confirmed the presence of Fe 3+ ions in the SnO2 lattice.

Morphological properties: SEM
Scanning Electron Microscope (SEM) observations have also been carried out. on all synthesized nanocatalysts in order to evaluate the evolution of crystallite morphology and size.
SEM characterizations were performed at two magnifications for all synthesized pure SnO2 and Fe-doped powders. The images in Figure 2, show the presence of nanoparticle agglomerates consisting of crystallites of mainly quasi-spherical shape with a relatively small and homogeneous particle size distribution.

EDS characterization
The EDS spectra of the chemical composition of our Fe-doped SnO2 layers are as follows The stoichiometric of the Sn1-xFexO2 nanostructure is measured exclusively from the Fe and Sn signals. According to the following relation (Eq. (5)) [39,40]: Where x represents the Fe atom content and r the ratio of the Sn and Fe EDS signals (see Table   2).

FTIR Spectroscopy
The study by FTIR spectroscopy performed on the different samples (in the wavelength range from 400 to 4000 cm-1) is presented in figure 5,

UV-Vis-NIR spectroscopy
The synthesized nanoparticles were analyzed by UV-Vis-NIR spectroscopy in the 300-700nm wavelength region. Fig. 6 shows the optical absorbance spectra of SnO2, Sn1-xFexO2[x=0, 2, 5 and 10 at%] nanoparticles. The absorption value decreases with doping and minimizes at Fe = 10%. During Fe substitution, the absorption edge is shifted to the higher wavelength side. The nanoparticle absorption is expected to be related to various factors such as particle size, band gap, defects and impurity centers.
The higher absorption intensity at Fe = 5% is due to the distortion caused by the Fe ions in the Sn02 lattice. In addition, the higher absorption in the lower wavelength region (300-350 nm) is accompanied by a higher absorption in the visible region. This higher UV absorption at Fe = 5% is the result of the improved transition of electrons from the valence band to the conduction band and a decrease in the size of the crystallites (size effect) is noted in this case.
The Optical Transmittance Spectra of Fe-doped SnO2 nanoparticles from 300 nm to 700 nm are shown in Fig. 7. It can be seen that the sample doped with Fe = 5% has a lower transmittance value and the sample doped with Fe = 10% has a higher transmittance value. A higher optical absorption or lower transmittance may be due to the optical transitions from the 3d occupied band to the 4s-4p band of Fe point defects.
On the nanoscale, the band gap increases with the reduction of the size of the nanoparticles due to the quantum confinement effect, which generates discrete energy levels in the valence band and the conduction band. As the energy band gap increases, the restriction of electron movement increases. This shifts the absorption peak to lower wavelengths, i.e. to the blue region.
The direct optical band gap (Eg) of Sn1-xFexO2 nanoparticles can be determined from the absorption coefficient (α) and photon energy (hν) by the following relation [42]: Where A is a constant, Eg is optical band gap of the material and the exponent n depends upon the direct/indirect allowed transition. In the present case, n is taken as ½ because of allowed direct transition. The curves have been plotted as αhν Vs hν for all the samples as exposed in The absorption spectra show a continuous red shift with increasing Fe content.The band gap energy decreases and the absorption edge increases with Fe content, is interpreted to incorporation of Fe3+ into SnO2 NPs [44,45]. It was assigned to the charge transfer transitions between the d-electrons of Fe3+ and the conduction or valence band of SnO2 [44] and to the the reduction of the size of the nanoparticles

Photocatalytic activity
The evolution of the UV spectra of solutions irradiated from BM (8×10 -3 M) at 250 nm in the presence of Sn0.95Fe0.05O2 photocatalyst as a function of the irradiation time ( Figure. 9) shows essentially a decrease of the optical density at the absorption maximum of MB at 664 nm.
Images of MB samples irradiated in the presence of the Sn0.95Fe0.05O2 photocatalyst after 3 hours clearly show the progressive discoloration of the pollutant compared to the initial nonirradiated sample. The rate in Fig.10 represents the ratio between the amount of reagent processed and the initial amount, using the following equation: Where A0 is the initial absorbance of the dye, At the absorbance of the dye in solution at time t Figure .11 shows that irradiation leads to a progressive discoloration of the MB. A reduction of about 65% is observed after 3hours of irradiation, the kinetics of the degradation reaction of methylene blue in Solution, and he calculated kinetic parameters of MB degradation are shown in Table 3, according to the equation [46].
While the unit of (pseudo-) order rate constant k is the inverse of the unit of time used (min -1 ), X is the amplitude of the process, E is the endpoint, both of them have the same units as the measured quantity A. The photodegradation was linear (R 2 = 0.9513), following an apparent kinetics of order 1 with an apparent speed constant of 0.0358 min -1 .
Several factors influence the photocatalytic activity of Sn0.95Fe0.05O2 fine nanopowder, namely, the morphology of the nanoparticles, as well as the porosity of the nanoparticles. In addition, it has been reported that increasing the specific surface area by reducing the size of SnO2 crystallites also increases the efficiency of photocatalysis [47], which could provide more active sites for BM molecules and thus promote the efficiency of electron-hole separation.
The specific surface area of the samples was obtained using equation (14) [48]: S: the specific surface area of the nanoarticules.
D: the average size of crystals.
ρ : the density of the material.
The increase in photocatalytic activity of fine nanopowder Sn0.95Fe0.05O2 may be due to the increase in the number of crystal defects. In other terms, the network distortion (0,45×10 -3 ) and dislocation density (2,929 ×10 -3 nm -2 ) relative to the to undoped samples resulted in a reduction in particle size (18.47nm). This led to an increase in the specific surface area (46.74 m 2 .g -1 ,

Conclusion
In this work, nanoparticles of photosensitive SnO2, Sn1-xFexO2 [x=2. 5 and 10 at%] were synthesized, characterized and then used in photocatlytic applications. The nanomaterials, pure tin dioxide and iron-doped tin dioxide at different concentrations, were synthesized by a simple and classical "co-precipitation" method. The aim is to obtain materials with controlled properties, especially with regard to crystallite size, morphology and especially optical properties which play a very important role in photocatlytic applications.
All the X-ray diffraction spectra of our samples showed a polycrystalline growth and all peaks   (D), c/a ratio, micro-strain (ε), dislocation (ẟ), specific surface area (S) and the lattice parameters "a" and "c") of different Sn1-xFexO2 nanoparticles. Table 2 Dispersion parameters of the Sn1-xFexO2 nanoparticles extracted by fitting the experimental data. Table 3. Pseudo-first-order kinetic parameters of MB degradation.       is the absorbance at time t, and k is the rate constant of photocatalysis.               Transmittance spectra of of Sn1-xFeXO2 [x=0, 2, 5 and 10 at%] nanoparticles.

Figure 9
The effect of Sn0.95Fe0.05O2 nanoparticles on the absorption spectra of MB solution for different reaction time under UV irradiation.

Figure 10
Degradation rate of Sn0.95Fe0.05O2 nanoparticles in MB degradation Figure 11 Decolorization kinetics of MB aqueous solutions by Sn0.95Fe0.05O2 nanoparticles without UV irradiation. The nanoparticles were prepared by using co-precipitation method. The photocatalytic process at UV-irradiation follows rst-order kinetics according to the equation A=X*exp(-k*t)+E (cercles) Experimental data point and (solid lines) tted curve (solid lines). Here A0 is the initial absorbance of the dye solution, A(t) is the absorbance at time t, and k is the rate constant of photocatalysis.