Preparation of V2O5 and SnO2 Nanoparticles and Their Application as Pollutant Removal

*Corresponding Author: Rashed Taleb Rasheed 100010@uotechnology.edu.iq Abstract Vanadium pentoxide and Tin dioxide nanoparticles were prepared by hydrothermal methods to remove methylene blue (MB) and bromothymol blue (BTB) dyes). First the prepared nanoparticles were heated at different temperature (90 C and 500 C). Then these nanoparticles were characterized by different methods such as UV/Visible, FT-IR Spectroscopy, X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM), and Scanning Electron Microscopy (SEM). Our results explain that vanadium pentoxide nanoparticles appeared high remove activity against both types of organic dyes (MB and BTB) compared to tin dioxide nanoparticles. Furthermore,vanadium pentoxide heating at 90 C has grain size (86.02 nm) more than that of Tin dioxide (48.02 nm) at same temperature. The activity of vanadium pentoxide is more than that of tin dioxide.may be related to high ionic potential (0.073), compared with Tin dioxide (0.048).


Introduction
Water is the essential element of life and it is a precious resource for human civilization. Clean water is a major global challenge in the twenty-first century. Thus, global climate change highlights the already unequal distribution of freshwater [1]. Clean water is water that is free from toxic substances, chemicals, and pathogens. Dyes are considered one of the most critical organic pollutants that cause water pollution and cause many diseases. The adsorption technique, is chemical adsorption, biological adsorption and ion exchange [2][3][4].

Experimental Procedure 2.1 Preparation of Vanadium Pentoxide Nanoparticles (V2O5 NPS)
Vanadium pentoxide was prepared by the hydrothermal (reflex) method by dissolving 2 grams of ammonium metavanadate (NH4VO3) in a mixed solution of distilled water and ethanol in a ratio of a ratio (7:3) with add 0.5 of cetyltrimethylammonium bromide (CTAB) and drops of nitric acid(HNO3) until it reaches the pH of 2.5, heat the solution for 6 hours, then washed several washes after the reaction ends, dried at 90 °C and annealed at 500°C for 2 hours [8,9].

Preparation of Tin Dioxide Nanoparticles (SnO2 NPS)
Tin dioxide was prepared using the hydrothermal method (autoclave) by dissolving two grams of Tin tetrachloride (SnCl4) in distilled water, adding to it 0.92 g of urea and 0.5 g of cetyltrimethylammonium bromide (CTAB) placing it with an autoclave for 12 hours at 200 and washed several washes and dried at 90 and annealed at 500 for two hours [10].

The adsorption of methylene blue (MB) / or bromothymol blue (BTB) dyes.
We performed two experiments to verify the adsorption of two organic dyes (methylene blue (MB) and bromothymol blue (BTB)). 0.1 g of each nanoparticle (V2O5 and SnO2) was added to 25 ml (20 mg/l) of the dye solutions (MB or BTB) in concentration (4x10 -5 M, 64x10 -5 M) respectively. The first experiment was to adsorption of methylene blue (MB) dye onto nanoparticles. The second experiment was to adsorption of bromothymol blue (BTB) dye onto nanoparticles. In these two experiments, we used our nanoparticles after heating at 90 o C and 500 o C. The removal percentages (R%) of MB / or BTB dyes onto nanoparticles were calculated through the equation (7) [8].

(R%) = [(Co − Ct) /Co] × 100 % (7)
Where; Co indicates the initial concentration of the dye mg/L, and Ct indicates the final concentration of the dye mg/ L.
The adsorption capacity (qt) of the MB/ or BTB dyes onto nanoparticles was calculated through equation (8).

qt = (Co − Ct) V/ W (8)
Where; W indicates weight in grams of nanoparticles, V volume of dye solution in millilitres.

Optical properties of the nanoparticles solutions.
The optical properties of the prepared nanoparticle solutions (10 -5 M( were done by dissolved the nanoparticle in Dimethylsulfoxide (DMSO) solvent. The plot of optical transmission curves against the wavelength for nanoparticles at different temperatures (90 °C and 500 °C,) from the region between 200 nm to 800 nm is explained in figure (1). The spectrum of vanadium pentoxide ( figure (1-a)) shows the transmittance edge is shifted to a lower wavelength (blue shift) from 480 nm to 320 nm with an increase of annealing temperatures. We can produce the value of the energy gap (the space between HOMO and LUMO) according to the following experiment equation: Energy gap (eV) = 1240/λmax (9) Where 1240 is a factor that converts nm to eV, λmax is the first maximum transmittance in nm. Therefore, the energy gap of V2O5.nH2O equal to 2.583 eV and that annealing at 500 o C (V2O5) equal to 3.875 eV respectively. These results are agreed with reference [11] which found equal to 2.63 eV and 3.48 eV at temperature 90 o C and 500 o C respectively..
The tin dioxide nanoparticles show a decrease in the transmittance edge (blue shift), from 347 nm to 339 nm, annealing temperatures, this may be relating to converting the as-prepared (tin hydroxide) nanoparticles to tin dioxide throw autoclave reaction. Therefore, the energy gap of as-prepared equal to 3.573 eV and that of annealing at 500 o C equal to 3.657 eV respectively, as in figure (1 -b). These results are agreed with reference [12] which is due to the fundamental absorption of light.

Fourier Transform Infrared Spectroscopy (FTIR) 3.2.1 FTIR Spectra for V2O5.nH2O and V2O5 nanoparticles
The FTIR spectrum of vanadium pentoxide nanoparticles (as-prepared) V2O5.nH2O that heated at 90 °C for 60 min explains in figure (2-a). The broadband at 3203 cm -1 relates to stretching vibration and the peak at 1610 cm -1 relate to bending of the O-H group in the water. The peaks at 1404 cm -1 , 1004 cm -1 , 964 cm -1 and 837 cm -1 corresponding to the bending vibration of C=C vibration of CTAB, vanadyl stretching of (δ V-O), V=O vibration [13], and V-O-V vibration respectively. The bands at 717 cm -1 and 626 cm -1 corresponding to the asymmetric and symmetric stretch around V-O-V. Figure (2 -b) shows V2O5 annealing at 500°C for 120 min, there are no peaks that appeared to relate to water in this sample. The peaks at 1016 cm -1 correspond to stretching vibration of V=O and that at 812 cm -1 relate to stretching of V-O-V and that at 648 cm -1 correspond to V-O stretching [14].

FTIR Spectra for SnO2 nanoparticles Figures (2-c and 2-d)
show the FTIR spectra of SnO2 as prepared and annealed at 500 • C. There are no high differences between both spectra, which means both compounds are the same (SnO2). These may relate to converting tin hydroxide (Sn(OH)4) to tin dioxide (SnO2) during the reaction proses in the autoclave, due to the high reaction temperature (200 o C). The broad peaks around 3400 cm -1 and 1631 cm -1 correspond to O-H bond's stretching and bending in water in water respectively. The broad peak at 544 cm -1 assigned to the Sn-O vibration and this broad peak improves to 544 cm -1 [15].

X-Ray Diffraction (XRD) 3.3.1 The X-Ray Diffraction for V2O5.nH2O and V2O5 nanoparticles
The results of the X-ray analysis of vanadium pentoxide prepared by the hydrothermal method (reflex), and heated at 90 °C, showed the main diffraction peaks at 2θ = (40.  [17,18]. The grain size of the nanoparticles has been estimated from the full width at half maximum (FWHM) of the diffraction peak using the Debye-Sherrer formula according to the equation: Where, (D) is crystallite size, (k) is the shape factor (≈ 0.9), ( λ ) is the x-ray wavelength (1.540 Å), (β) is the full width half maximum (FWHM) in radians, and (θ) is the Bragg's diffraction angle value [19]. Table (1) shows the average grain size for V2O5 annealed at 500 •C according to the Scherrer equation.

The X-Ray Diffraction for SnO2 nanoparticles
The X-ray results of heated SnO2 at 90 o C, is very similar to that of annealing at 500 o C with a slight change in intensity, this indicates that the formation of SnO2 in both situations. The most prominent of the main peaks that    (4). The distribution and accumulation of nanoparticles seem to large balls (figure 4-a) and convert to smaller when annealing at 500 o C ( figure 4-b). This may relate to the loss of water molecules from the sample, as explained in equation [9].
These results agree with the average grain size obtained from AFM analysis, which converts the average grain size from 86.02 nm (V2O5.nH2O) to 70.34nm of V2O5 when the heating increases from 90 o C to 500 o C respectively, as in table (2).
The distribution and accumulation of tin dioxide in nanoparticles like to smooth rods (figure 4-c) become large bubbles with an increase in size when annealing at 500 o C (figure 4-d). That is mean the product may be converted to tin dioxide nanoparticles during the preparation method (autoclave) and the grain size increases from 48.08 nm to 98.34 nm by increasing temperature as a result of improving the crystalline and the accumulation of the nanoparticles [21], as in table (2).

By Surface scanning Electron Microscopy (SEM)
The surface morphology magnification SEM images of the metal oxides (V2O5 and SnO2) nanoparticles annealing at temperature 500 °C for 120 min, with a magnification of 100 µm, are shown in figures (5). The surface morphology of V2O5 ( figure 5-a), has a high aggregate consisting of close rough leaves, and that for SnO2 (figure 5-b) seem to be nanoclusters.

Applications 4.1 The adsorptions of methylene blue (MB) dye onto nanoparticles.
Vanadium petoxide (V2O5.nH2O) nanoparticles have the highest value of removal percentage (R%) compare with tin dioxide (SnO2) nanoparticles, as in figure (6). Our results showed that methylene blue dye onto nanoparticles increased sharply during the first 10 min, it may be related to the availability of large bonding sites on the surface area of the nanoparticles. After that, very slow adsorption happens between 10 min to 60 min, this situation continues until the equilibrium happens. While the low increase in adsorption onto nanoparticles takes place (after 10 min), it relates to the aggregation or overlap of the adsorption sites [22]. The annealing at 500 o C is similar to the same sequences (to some extent) of the nanoparticles heating at 90 o C, except for the result at 50 min (for V2O5) which decreases in adsorption compared with the measurements between 10 min to 60 min. This decrease may relate to the dissociation of clusters of dye on the surface of the nanoparticles, as in figure (6). While the removal percentage (R%) for kerosene by V2O5 doping onto oxide multiwall carbon nanotube was 80 %. [5]. When the adsorption capacity (qt) of MB onto nanoparticles plot against contact time of nanoparticles (heating at 90 o C), figure (7), we found the adsorption capacities (qt) are precisely like the situations that happen in the removal percentage operation (at 90 o C), with the identical sequences. As well as the adsorption capacities (qt) of the samples heated at temperature 500 o C, show same the sequences of removal percentage. This similarity is attributed to using of the same volume of dye and the exact weight of nanoparticles in all experiments. Except for the result at 10 min of SnO2 nanoparticles which gave some increase in removal percentage (qt) compared with the measurements after that (20 min to 60 min), this increase may relate to providing large surface areas after that dissociation of clusters dyes on the surface of the nanoparticles. As in figure (7). While the adsorption capacity (qt) for kerosene by V2O5 doping onto oxide multiwall carbon nanotube was 10.2 [5]. When the adsorption capacity (qt) of BTB onto nanoparticles plot against contact time of nanoparticles (heating at 90 o C), figure (9). The adsorption capacity (qt) of nanoparticles is a similarity in shape compared to removal percentage (R %) at the same temperature (heating at 90 o C). The increase continues (in the V2O5 nanoparticles) to get the maximum adsorption capacities (qt) at 10 min, then began to decrease at 40 min after that the increase continues. The increase in adsorption capacities (qt), related to nanoparticles' surface will get some empty spaces to bond with dyes molecules. While the decrease relating to the dissociation of clusters dyes on the surface of the nanoparticles.
When the adsorption capacity (qt) of BTB onto nanoparticles plot against contact time of nanoparticles (heating at 500 o C), figure (9), we found the adsorption capacities (qt) are precisely like the situations that happen in the removal percentage operation (at 500 o C), with the identical sequences. This similarity is attributed to the use of the same volume of the dye and the exact weight of nanoparticles in all experiments. The highest adsorption capacities (qt) where to V2O5 nanoparticles. The maximum value of qt it was at 40 min, may be relate to providing large surface areas after that dissociation of clusters of the dyes on the surface of the nanoparticles. The ability of vanadium pentoxide to adsorb MB or BTB dyes is the largest. This may be related to the ionic potential (charge/radius) of the nanoparticles. The highest ionic potential appears to be vanadium pentoxide (0.073), and the second appears to be tin dioxide (0.048). These results agree with the explanation of removal percentage (R%) sequence and adsorption capacities (qt) of MB or BTB dyes onto nanoparticles in both situations (90 o C and 500 o C). Therefore, we can consider that the ionic potential may be the critically affected agent in this operation.

Mechanisms to Remove MB and BTB Dyes 5.1 Mechanisms to Remove Methylene blue (MB)
The mechanism to remove Mb from aqueous solution by nanoparticles can be occurredby two methods: first method by the interaction between the positive charge on the sulphur atom of the MB and the partial negative charge on the oxygen atom of the nanoparticle to form an ionic bond. The second method is the interaction between alone pair electrons (as base) on the nitrogen atom of the MB and metal ion (as acid) in the nanoparticles to form a coordination bond [22]. Therefore there is two bonds (ionic and covalent) formation between MB and nanoparticles, as in figure (10) [23].

Figure (10):
The suggested mechanism to remove methylene blue (MB) by nanoparticles.

Mechanisms to Remove Bromothymol blue (BTB).
The mechanism to remove BTB from aqueous solution by the nanoparticles can occur through the interaction between the partially positive charges on the metal in the nanoparticle, with a negative charge in the BTB to form an ionic bond be explained in figure (11).
According to these mechanisms, MB can bond with nanoparticles from two sides (sulphur and nitrogen), while BTB can bond with nanoparticles only by one side (phenoxide). Therefore, we can conclude why the removal of nanoparticles by MB is higher than that of BTB.

Conclusion
The conclusion can be summarized in the following points: 1. We successfully prepared some nanoparticles (V2O5.nH2O and SnO2) and their oxides (V2O5 and SnO2) by different methods. 2. After heating temperature at 90 o C SnO2 has the lowest average grain size (48.08) nm. 3. After heating temperature at 500 o C, V2O5 has the lowest average grain size (70.34) nm, 4. Both V2O5.nH2O and V2O5 have the highest removal percentage (R%) and adsorption capacity (qt) against MB and BTB dyes (as a pollutant), after heating temperatures (90 o C and 500 o C), and the activity is more when heating at 500 o C compared to other nanoparticles. 5. The highest values of R% and qt for vanadium pentoxide nanoparticles may be relate to the highest ionic potential (0.073). Moreover, we conclude the adsorption may be depend on this factor and do not depend on the average grain size or surface area.