Enhanced Photocatalytic Activity of Vanadium-Doped SnO2 Nanoparticles in Rhodamine B Degradation
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.
Nowadays, industries continually release hazardous toxic substances such as organic contaminants into water resources . 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 . 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.
Heterogenous photocatalysts, namely, zinc oxide (ZnO) [3–5], titanium dioxide (TiO2) [5–8], tungsten trioxide (WO3) , ferric oxide (Fe2O3) [10, 11], copper oxide (Cu2O) , 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 , chemical and mechanical stability, and high mobility of electrons . 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 , and photocatalytic processes .
Thus, SnO2 NPs have been synthesized using several techniques such as thermal evaporation , polyol method [28–30], chemical precipitation , sol-gel method , 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 , chromium , iron , magnesium , and vanadium , aiming to extend the photo response of SnO2 from the ultraviolet to the visible region . 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 .
Thus, nanostructured semiconductors can efficiently degrade different organic pollutants under UV light irradiation . 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 , TiO2/SnO2 , Fe2O3/SnO2 , and V2O5/SnO2 . In literature, there are several works reported on investigating the vanadium-doped SnO2 nanoparticles SnO2: V NPs  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).
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 . 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.
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.
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.
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 . 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 . Peaks observed at 1050 cm−1 and 1360 cm−1 were ascribed to C-O and C-H stretching vibrations, respectively . 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 .
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 . 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) . The band-gap energy of undoped and SnO2: V NPs was determined using the Tauc–Lorentz expression :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 . For SnO2, it possesses direct transition, so the exponent n was chosen 1/2.
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.
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 .
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 . These results are consistent with those reported in [56, 69].
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 . 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].
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 . 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  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.
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].
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 . 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.
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.
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.
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.
This work was funded by the Ministry of Higher Education and Scientific Research of Tunisia.
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