Advances in Condensed Matter Physics
Volume 2019 (2019), Article ID 2157428, 11 pages
https://doi.org/10.1155/2019/2157428

Research Article

Enhanced Photocatalytic Activity of Vanadium-Doped SnO₂ Nanoparticles in Rhodamine B Degradation

H. Letifi,1,2 Y. Litaiem,² D. Dridi,2,3 S. Ammar,1,2 and R. Chtourou²

¹Unité de Recherche Electrochimie, Matériaux et Environnement (UREME), Faculté des Sciences de Gabès, Université de Gabès, Cité Erriadh, 6072 Gabes, Tunisia
²Laboratory of Nanomaterials and Systems for Renewable Energies (LaNSER), Research and Technology Center of Energy, Techno-Park Borj-Cedria, Hammam-Lif, Tunis, Tunisia
³Université de Carthage, Faculté des Sciences de Bizerte, Tunis, Tunisia

Correspondence should be addressed to Y. Litaiem; yousralitaiem@gmail.com

Received 27 April 2019; Revised 15 June 2019; Accepted 25 July 2019; Published 9 October 2019

Academic Editor: Golam M. Bhuiyan

Copyright © 2019 H. Letifi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

In this paper, we have reported a novel photocatalytic study of vanadium-doped SnO₂ nanoparticles (SnO₂: V NPs) in rhodamine B degradation. These NPs have been prepared with vanadium concentrations varying from 0% to 4% via the coprecipitation method. Structural, morphological, and optical properties of the prepared nanoparticles have been investigated by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscope (TEM), and UV-Vis and photoluminescence (PL) spectroscopy. Structural properties showed that both undoped and SnO₂: V NPs exhibited the tetragonal structure, and the average crystal size has been decreased from 20 nm to 10 nm with the increasing doping level of vanadium. Optical studies showed that the absorption edge of SnO₂: V NPs showed a redshift with the increasing vanadium concentration. This redshift leads to the decrease in the optical band gap from 3.25 eV to 2.55 eV. A quenching in luminescence intensity has been observed in SnO₂: V NPs, as compared to the undoped sample. Rhodamine B dye (RhB) has been used to study the photocatalytic degradation of all synthesized NPs. As compared to undoped SnO₂ NPs, the photocatalytic activity of SnO₂: V NPs has been improved. RhB dye was considerably degraded by 95% within 150 min over on the SnO₂: V NPs.

1. Introduction

Nowadays, industries continually release hazardous toxic substances such as organic contaminants into water resources [1]. Therefore, a global effort has been paid to overcome this environmental issue. Photocatalysis plays an important role in the disinfection of water by removing these biological and chemical pollutants from water. In a photocatalytic reaction, an electron-hole pair was generated with the use of a photocatalyst which further creates free radicals. These free radicals break down the aromatic structure of dye molecules [2]. Photocatalysts are activated with light in a number of competing processes that schematically are presented in Figure 1. Steps (1) and (2) show electronic processes of e⁻ and h⁺ active centers on the photocatalyst surface. These processes have to compete with deactivation processes (3) and (4), leading to e⁻ and h⁺ recombination. The increase of the distance toward the surface of the active centers of photogenerated electrons and holes also increases the probability of their recombination. The increase in the photocatalyst surface enhances the catalyst activity because the number of active centers depends on the surface area. In order to hinder the recombination process and to increase the photocatalytic activity, it is important that the semiconductor particles must be small and well-crystallized.

Figure 1: Schematic diagram showing the charge carrier dynamic upon irradiation of a semiconductor. (1) Electron transport to the surface; (2) hole transport to the surface; (3) surface recombination; (4) bulk recombination; (5) electron transfer to an acceptor molecule; (6) hole trapping by a donor molecule.

Heterogenous photocatalysts, namely, zinc oxide (ZnO) [3–5], titanium dioxide (TiO₂) [5–8], tungsten trioxide (WO₃) [9], ferric oxide (Fe₂O₃) [10, 11], copper oxide (Cu₂O) [12], and tin dioxide (SnO₂) [13–18], have gained huge interest for environmental applications such as water disinfection, hazardous remediation, and water purification.

Among all mentioned photocatalysts, SnO₂ appears as a promised candidate, thanks to its outstanding and versatile properties such as its strong chemical and physical interaction with absorbed species, high transparency in the visible region, strong thermal stability in air, low operating temperature, wide band gap energy equal to 3.6 [19], chemical and mechanical stability, and high mobility of electrons [20]. These important properties are crucial to make SnO₂ NPs as a practical candidate for various applications, namely, gas sensing [21, 22], optoelectronic devices [23, 24], dye-based solar cells [25], and photocatalytic processes [26].

Thus, SnO₂ NPs have been synthesized using several techniques such as thermal evaporation [27], polyol method [28–30], chemical precipitation [31], sol-gel method [32], hydrothermal method [33, 34], and coprecipitation methods [35, 36]. Among these various methods, the coprecipitation method was chosen in this study, thanks to its important advantages including its low fabrication cost, simplicity, high purity, good reproducibility, and low operation temperature.

So far, several attempts have been conducted on doping SnO₂ NPs with transition metals such as cobalt [37, 38], nickel [39], chromium [40], iron [41], magnesium [42], and vanadium [43], aiming to extend the photo response of SnO₂ from the ultraviolet to the visible region [44]. Moreover, adding amounts of these dopant species can efficiently extend the absorption edge of SnO₂ NPs into the visible region which increases the photocatalytic activity. In contrast, high doping content can create electron-hole recombination sites, and the wide energy band gap hinders this photoactivation [45].

Thus, nanostructured semiconductors can efficiently degrade different organic pollutants under UV light irradiation [46]. The decomposition of organic dyes such as methylene blue (MB) and methyl orange [47–49] in aqueous suspension is used as a probe reaction to investigate the catalytic activity of metal oxide nanomaterials. Many studies have been devoted to the use of nanocomposite or heterojunction catalysts such as ZnO/SnO₂ [50], TiO₂/SnO₂ [46], Fe₂O₃/SnO₂ [51], and V₂O₅/SnO₂ [52]. In literature, there are several works reported on investigating the vanadium-doped SnO₂ nanoparticles SnO₂: V NPs [53] by studying the effect of doping concentration on chemical, physical, and structural properties of nanoparticles of much importance for technological applications. However, few reports have been conducted on evaluating the photocatalytic activity of SnO₂: V NPs. Thus, SnO₂: V NPs are more active as catalysts than pure SnO₂ [54–57]. All these studies have proved the enhanced photocatalytic activity of SnO₂ NPs after doping with vanadium (Table 1).

Table 1: Photocatalytic activity of SnO₂: V NPs using different methods.

Method
Crystallite size (nm)
Dye
Photocatalytic efficiency
Ref.

SnO₂: V NPs
Sol-gel
3
Methylene blue and rhodamine B
5% (mol%) showed better photocatalytic activity
[53]

SnO₂: V NPs
Combustion synthesis method
4.52

0.05% (mol%) showed better photocatalytic activity
[55]

Rhodamine B

SnO₂: V NPs
Polyol method
12.1
Methylene blue
10% (mol%) showed better photocatalytic activity
[56]

In this paper, undoped and SnO₂: V NPs have been prepared via the coprecipitation method. Then, a detailed discussion has been conducted on studying the structural, morphological, optical, and photoluminescence properties of SnO₂: V NPs. Finally, the photocatalytic activity of the synthesized samples was investigated in rhodamine B degradation under UV light irradiation.

2. Experimental Details

2.1. Sample Preparation

Synthesis of undoped and SnO₂: V NPs was carried by the coprecipitation method. All of the used materials including tin chloride dihydrate (SnCl₂. , 99.99%) and ammonium metavanadate (NH₄VO₃, 99.99%) have been used in research grade (Merck Co.). Deionized (DI) water has been used for washing precipitants.

A typical and sample preparation procedure consists first to dissolve 4.23 g of tin chloride dihydrate (SnCl₂, 99.99%) in 150 ml deionized water. After that, various amounts of ammonium metavanadate (NH₄VO₃, 99.99%) corresponding to 0, 1, 2, and 4 (mol%) will be added with vigorous agitation stirring for 1 h at a temperature of 80°C. pH solution was adjusted by adding ammonia aqueous solution (28%) to reach a value of about 11 for 4 hours. Centrifugation was used to separate the obtained precipitates. The latter was washed many times with ethanol (C₂H₅OH). Finally, the resulting products were dried 12 hours at 80°C and then were calcined for 4 hours at 600°C.

The photocatalytic activities of undoped and SnO₂: V NPs were evaluated for the degradation of rhodamine B dye (RhB) in aqueous solution under a UV light irradiation. The reaction system includes a 9 W UV Philips Bulb lamp emitting a UV light ( = 365 nm) with a measured intensity of 20 W/m². In a typical experiment, 50 mg of catalyst was dispersed into 50 mL of aqueous solution of RhB solution. The suspension was stirred for about 30 min at 25°C in dark in order to obtain the adsorption-desorption equilibrium between RhB dye and the photocatalyst. The solution was continuously stirred during the experiments. At given irradiation time intervals, 4 mL of the suspension was collected and then centrifuged (6000 rpm, 10 min) to separate the photocatalyst particles. Finally, under constant continuous stirring, the photodegradation process was investigated under UV light irradiation.

2.2. Characterization Techniques

To evaluate crystalline structure and grain size of the all synthesized samples, the X-ray diffraction (XRD) measurements were conducted through using a Panalytical X’Pert Pro diffractometer with the Cu Kα radiation source ( = 1.54056 Å). Morphology studies and grain size distribution of the all nanoparticles were recorded with the transmission electron microscopy (TEM, JEM-200CX). FTIR spectra were recorded by means of thermo Nicolet 670 Nexus spectrophotometer (FTIR) spectrometer. The optical properties were attained using UV-visible spectrophotometer (Shimadzu UV 3101). Photoluminescence (PL) spectra were recorded using Shimadzu RF-5301 spectrophotometer with an excitation source wavelength of 266 nm at room temperature and in the spectral range between 300 and 900 nm. The absorption spectra were monitored with PerkinElmer Lambda UV/Vis 950 spectrophotometer using the quartz cuvette in the range 400 and 800 nm during the photodegradation process.

3. Results and Discussion

3.1. Structural Properties

Figure 2 illustrates the XRD spectra of vanadium-doped SnO₂ nanoparticles. As shown in Figure 2, the peaks of all the prepared samples are well indexed according the standard rutile-like, i.e., cassiterite, crystalline structure (JCPDS card no 41-1445). There are no traces neither of vanadium nor of vanadium oxide that can be observed, which illustrates the good crystallinity of the nanoparticles [57]. As compared to the pure SnO₂ NPs, a continuous shift towards higher angle values along with a gradual reduction in the diffraction peak intensity and an expansion of the peak width was observed with the increasing vanadium content.

Figure 2: X-ray diffraction of pure SnO₂ and SnO₂: V NPs.

The average crystallite size and the lattice parameters (a = b and c) for all synthesized samples have been estimated using the Debye–Scherer formula (equation (1)) and equation (2), respectively [58, 59]:

where is the crystalline size (nm), k is a constant related to the crystallite shape (0.9), the X-ray wavelength  = 1.54056 Å, is the scattering angle of the reflection identified in the spectra (in radians), is the peak full width at half maximum (FWHM) having the highest intensity (110), d is the interreticular distance, and hkl are the Miller indices. The results are summarized in Table 2.

Table 2: Crystallographic data (a and c parameters and volume of the tetragonal unit cell) and average crystallite from XRD of pure SnO₂ and SnO₂: V NPs.

Sample
Position of 2 (°)
Hkl
dhkl
a = b (Å)
c (Å)
Average crystallite size (nm) (XRD)
Average crystallite size (nm) (TEM)

Pure SnO₂
26.54
110
3.354
4.743
3.207
19.8
21.2

33.68
101
2.657
—
—

51.69
211
1.766
—
—

SnO₂: V 1% NPs
26.57
110
3.350
4.736
3.199
17.7
—

33.76
101
2.651
—
—

51.77
210
1.764
—
—

SnO₂: V 2% NPs
26.6
110
3.347
4.733
3.195
16.8
—

33.81
101
2.648
—
—

51.84
211
1.761
—
—

SnO₂: V 4% NPs
26.67
110
3.34
4.723
3.182
10.3
11.2

33.92
101
2.639
—
—

51.94
211
1.758
—
—

Consequently, the crystallite sizes have decreased with the increase of vanadium doping concentrations from 19.8 nm for undoped SnO₂ NPs to 10.3 nm for SnO₂: V 4% NPs. Therefore, the density of the SnO₂: V NPs nucleation centers has improved [57, 60]. Moreover, the decrease in lattice constants a and c can be justified by the integration of V ions into the SnO₂ lattice and the substitution of Sn sites by vanadium in the form of Vx+ (x = 3, 4, 5), namely, with V⁴⁺ (rV⁴⁺ = 0.63 Å) or V⁵⁺ (rV⁵⁺ = 0.59 Å) ions since their ionic radii are smaller than that of Sn⁴⁺ (rSn⁴⁺ = 0.69 Å) [54, 55].

3.2. Morphological Properties

The TEM images of the pure and SnO₂: V 4% NPs are shown in Figures 3(a) and 3(b), respectively. It can be seen from Figure 3 that the individual particles were nearly spherical in the size in the range of 10 nm–23 nm. The morphologies of the prepared nanoparticles confirm that the grain sizes decrease with the vanadium incorporation into SnO₂ NPs. This result is in well agreement with XRD results. However, a small agglomeration was observed.

(a)

(b)

Figure 3: TEM images of (a) pure SnO₂ and (b) SnO₂: V NPs.

3.3. FTIR Analysis

Figure 4 shows the FTIR spectra of pure SnO₂ and SnO₂: V NPs. Generally, the stretching vibrations of Sn-O have been detected between 300 cm⁻¹ and 800 cm⁻¹ [61]. In our case, the peak observed at 736 cm⁻¹ was assigned to the vibration of the Sn-O-Sn bond in SnO₂ lattice. After vanadium incorporation into SnO₂ NPs, a significant new peak located at 935 cm⁻¹ for the 2% and 4% vanadium doping contents appeared. The occurrence of this new phase at 935 cm⁻¹ may be ascribed to the V-O-Sn bond which can be resulted from the formation of vanadium oxide species. This last result was in agreement with those obtained in [62]. Peaks observed at 1050 cm⁻¹ and 1360 cm⁻¹ were ascribed to C-O and C-H stretching vibrations, respectively [57]. The hydroxyl groups were observed at 1630 cm⁻¹ which is due to the bending vibration of coordinated as well as Sn-OH [28, 63]. The basic stretching vibrations of hydroxyl groups (free or bonded) were observed as a broad absorption peak at 3600 cm⁻¹. From this figure, we have noticed a blue shift in distinctive peaks originated from the vanadium incorporation into SnO₂ host lattice [61].

Figure 4: FTIR spectra of pure SnO₂ and SnO₂: V NPs calcined at 600°C.

3.4. Optical Properties

Figure 5 shows the absorption spectra of undoped and SnO₂: V NPs at different vanadium doping concentrations. It is obvious from Figure 5 that a sharp absorption edge occurred at around 360 nm. It can be assigned to the intrinsic band gap absorption of SnO₂ [64]. The incorporation of vanadium ions into SnO₂ nanoparticles extends the absorption edge to the visible region. Furthermore, the increase of vanadium doping concentration leads to a substantial red-shift of SnO₂: V NPs absorption. This behavior can be explained by the charge transfer process from the valence band (VB) of SnO₂ to the t2g energy level of vanadium which is located just below the conduction band (CB) [56]. The band-gap energy of undoped and SnO₂: V NPs was determined using the Tauc–Lorentz expression [65]:

where represents the absorption coefficient, is the photon energy ( is Planck’s constant and is the light frequency), is a constant, is an optical band gap, and n is the parameter according to the nature of the semiconductor [66]. For SnO₂, it possesses direct transition, so the exponent n was chosen 1/2.

Figure 5: UV-Vis DRS of pure SnO₂ and SnO₂: V NPs with different V doping contents.

The band gap of the undoped and SnO₂: V NPs (Figure 6) was determined by extrapolating the linear portion of versus . The calculated band gap values of all synthesized samples are listed in Table 3. It can be clearly seen that the band gap values decrease from 3.3 eV for pure SnO₂ NPs to 2.6 eV for SnO₂: V 4% NPs.

Figure 6: Plots of versus hν of pure SnO₂ and SnO₂: V NPs.

Table 3: Band gap energy and average crystallite size of pure SnO₂ and SnO₂: V NPs.

Samples
Gap energy (eV)
Average crystallite size (nm) (XRD)

Pure SnO₂
3.25
19.8

SnO₂ :V 1%NPs
2.80
17.7

SnO₂ :V 2%NPs
2.63
16.8

SnO₂ :V 4%NPs
2.55
10.3

A significant decrease in values was observed with the increasing vanadium doping content. The reduce in band gap energy was due from an sp-d exchange among d electrons of the vanadium ions substituted on the tin sites and the s and p band electrons of the SnO₂ matrix [56].

3.5. Photoluminescence Analysis

Figure 7 displays the room temperature PL emission spectra of the SnO₂: V NPs. PL spectra of SnO₂: V NPs exhibited a strong band centered at around 436 nm. The band emission around 436 nm can be attributed to the near-band edge emission (NBE) coming from the holes in the valence band and radiative recombination of electrons in the conduction band [40, 67]. The PL intensity of SnO₂: V NPs has been decreased with increasing vanadium doping content into SnO₂ nanoparticles. This decrease was attributed to the vanadium doping effect. This enhances the nonradiative recombination of the excited electrons [68]. These results are consistent with those reported in [56, 69].

Figure 7: Photoluminescence spectra of pure SnO₂ and SnO₂: V NPs.

3.6. Photocatalytic Activity

The photocatalytic activity of the SnO₂: V NPs photocatalysts was carried out by degradation of rhodamine B dye under UV light irradiation by varying the exposure time from 0 to 90 min, which is given in Figure 8(a). It can be seen from Figure 8(a) that the intensity of the characteristic peak of RhB dye at around 554 nm was slightly diminished by increasing the exposure time from 0 to 90 min. The optimum time needed for the degradation of RhB dye was about 1 h 30 min. It was noticeable from Figure 8(b) that, after vanadium incorporation into SnO₂ NPs, the photocatalytic activity has been enhanced as compared to undoped photocatalyst. The highest photocatalytic activity was obtained with the SnO₂: V 4% NPs photocatalyst. This result was in agreement with both of the obtained UV-Vis and PL results. The progressive decrease of PL intensity along with band gap values after vanadium incorporation into SnO₂ NPs leads to improve the electron-hole charge separation rate [70]. The highest photocatalytic performance obtained with SnO₂: V 4% NPs resulted from the enhanced charge separation rate at this proper 4% vanadium doping content. These results are similar to those reported in such research works [56, 61, 70–72].

(a)

(b)

Figure 8: (a) Time-dependent UV-Vis absorbance spectra; (b) photo degradation rate of the rhodamine B pollutant under UV light and light irradiation time.

Figure 9 displays the plot of versus irradiation time of SnO₂: V 4% NPs by varying the catalyst amount from 0.1 g to 2 g. A noticed improvement in the degradation rate was achieved by modifying the catalyst quantity from 0.1 g/L to 1.2 g/L. The degradation rate has increased from 51% to 95% which can be attributed to the availability of more active sites on the catalyst surface for adsorption of the dye molecules [73]. Further increase in the catalyst amount up to 2 g/L leads to a slight decrease in the photocatalytic rate from 95% to 85%. This can be ascribed to a reduced light penetration due to the turbidity of the solution [74] or even to the agglomeration of SnO₂ nanoparticles in the reaction system and a reduction in active sites on the catalyst surface, as suggested by B. Babu et al. in [75–77]. Hence, the optimum degradation rate above 95% after 150 minutes of irradiation was achieved by a fixed catalyst quantity equal to 1.2 g/L.

Figure 9: Plot of versus irradiation time of SnO₂: V 4% NPs by varying the catalyst amount from 0.1 g to 2 g.

Afterwards, in order to enhance the RhB degradation, hydrogen peroxide () was added using 1.2 g which is the optimal amount of SnO₂: V 4% catalyst. The degradation of the RhB dye by varying the molar ratio R of with respect to (RhB)₀ from 0 to 30 is shown in Figure 10. It is readily seen that the presence of inhibited the photocatalytic process with a molar ratio R lying between 20 and 30, while a positive impact of was noticed for R values in the range between 4 and 10. Additional increase of R beyond 10 contributes to a reduce in the RhB degradation efficiency. It can be related to the increase in the concentration of radical concentration in the SnO₂: V NPs surface and added to that also to the valence band hole (h⁺) oxidative potential leading to organic molecules degradation, as recently reported in [78–80].

Figure 10: Degradation of the RhB dye for SnO₂: V 4% NPs by varying the molar ratio R of from 0 to 30.

However, for high concentrations of hydrogen peroxide , the following reactions took place:

It can be noticed from these two reactions that high concentration of has a negative effect on photodegradation kinetics at the one hand. At another hand, these two reactions yield both of radicals () and () which are necessary for photodegradation of organic molecules.

Indeed, the reaction of electrons with dissolved oxygen molecules to give superoxide radical anions (), yielding hydroperoxyl radicals () on protonation and finally radicals, will be more efficient for the photodegradation [80]. The reactions occurred during this mechanism are as follows:

Aiming to confirm the efficiency of SnO₂: V photocatalyst in the treatment of dangerous pollutants, the mineralization of organic molecules at a fixed molar ratio about 10 was evaluated from the total organic carbon (TOC) to be total 100% after 150 min, as shown in Figure 11.

Figure 11: Bar diagram for the % degradation of rhodamine B dye (TOC) in the presence of pure SnO₂ and SnO₂: V NPs.

The photocatalytic stability of SnO₂: V NPs was investigated by the recycle degradation test of rhodamine B. The used photocatalysts were washed, dried, and reused in each cycle. As shown in Figure 12, the concentration of RhB varies with respect to time at five cycles. During the repeated experiments, SnO₂: V photocatalysts remain stable enough. It can be concluded that the photocatalysts conserved the same original structure after 5 cycles, and the cycle stability of samples is a result of the stable nanostructure.

Figure 12: Photocatalytic stability of the SnO₂: V 4% NPs sample by the recycle degradation test of rhodamine B with five cycles.

4. Conclusion

In this present work, SnO₂: V NPs have been successfully synthesized via the coprecipitation method. XRD patterns showed that the synthesized samples have rutile phases, and the crystallite structure was tetragonal. Lattice parameters values (a = b and c) decreased with the increasing vanadium content, indicating the incorporation of vanadium ions in SnO₂ lattice. Crystallite sizes of the SnO₂: V NPs nanoparticles were obtained from XRD. The same crystallite sizes of SnO₂: V NPs were estimated by TEM. Vanadium incorporation into SnO₂ NPs was proved by FTIR analyses. A red-shift was noticed from UV-Vis spectra of SnO2 : V NPs which can be assigned to a decrease in band gaps from 3.25 eV for pure SnO₂ NPS to 2.55 eV SnO₂: V 4% NPs. Furthermore, the PL intensity increased with the addition of the vanadium amount. The photocatalytic degradation was carried out by using the RhB dye under UV light irradiation of all synthesized SnO₂: V NPs. RhB dye was considerably degraded by 95% within 150 min over on the SnO₂: V NPs. SnO₂: V 4% NPs have the smallest crystallite size estimated from XRD and TEM analyses, the highest band gap energy, and highest photocatalytic activity in RhB degradation.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

All authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This work was funded by the Ministry of Higher Education and Scientific Research of Tunisia.

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