TiO2 and Sn/TiO2 nanoparticles were successfully synthesized by sol-gel method. The resulting nanoparticles were characterized by XRD, TEM, SEM, UV-Vis reflectance spectroscopy, and BET analysis methods. The effects of Sn-doping on the crystal structure, surface area, adsorption properties, pore size distribution, and optical absorption properties of the catalysts were investigated. The effect of different Sn content on the amount of hydroxyl radical was discussed by using salicylic acid as probe molecule. The photocatalytic activity of samples was tested by photocatalytic mineralization of amoxicillin trihydrate (AMOX) as a model pollutant. Sn/TiO2 nanoparticles exhibited high photocatalytic activity during the mineralization of AMOX under UV light due to increase in the generated hydroxyl radicals, band gap energy, specific surface area, and decrease in the crystallite size. The kinetic of the mineralization of AMOX can be explained in terms of the Langmuir-Hinshelwood model. The values of the adsorption equilibrium constant (
Recently, there has been an increasing concern, particularly in highly developed countries, about penetration of pharmaceuticals into the environment and related risks [
Several alternatives to destroy these kinds of compounds have been considered in recent studies in the literature. These include reverse osmosis, adsorption on activated carbons, or advanced oxidation technologies, such as Fenton reaction, ozonation, and peroxidation combined with UV light [
Photocatalytic oxidation of some antibiotics such as Lincomycin, tetracycline, oxolinic acid, and fluoroquinolone has been reported [
In this work, preparation and characterization of pure and Sn/TiO2 nanoparticles is reported. These nanosized catalysts were characterized by the techniques such as XRD, SEM, TEM, DRS, and BET analysis methods. The photocatalytic activity of the prepared nanoparticles has been studied on the mineralization of AMOX, as a prototype molecule. Chemical structure and characteristics of AMOX have been given in Table
Chemical structure and characteristics of AMOX.
Characteristics | |
IUPAC name | (2 |
Molecular formula | C16H19N3O5S3H2O |
Molecular weight | 419.45 |
Appearance | White or almost white powder |
Melting point | 152–156°C |
Structure |
Titanium n-butoxide (TBOT, Ti(OC4H9)4), ethanol with absolute grade, and tin (IV) chloride were used without any further purification. All chemicals used in this study were analytical grade and purchased from Merck (Germany).
According to [
Powder X-ray diffraction (XRD) was used for identification of crystalline phases and estimation of the crystallite size. The X-ray diffraction (XRD) patterns were recorded on a Siemens/D5000 X-ray diffractometer with Cu K
Mineralization of AMOX under UV light was used as a model reaction to evaluate the photocatalytic activity of prepared samples. Photocatalytic activity measurements were carried out at atmospheric pressure in a batch quartz reactor [
All the concentration profiles can be correlated to irradiation time by the following exponential function with good agreement:
Therefore, the photocatalytic mineralization of AMOX is pseudo-first-order reaction, and its kinetics may also be expressed as
In this equation, TOC0 and TOC are the antibiotic concentrations (mg L−1) at times 0 and
Based on the XRD spectra, the crystalline phases could be categorized into two primary components, an anatase (
In this equation,
Phase content and crystallite size of prepared nanoparticles.
Sample | Phase structure | Crystalline size (nm) |
---|---|---|
TiO2 | ||
1 mol% Sn | ||
1.5 mol% Sn | ||
3 mol% Sn |
XRD patterns of (a) TiO2, (b) 1 mol% Sn/TiO2, (c) 1.5 mol% Sn/TiO2, (d) 3 mol% Sn/TiO2.
The SEM image of 1.5 mol% Sn/TiO2 is shown in Figure
SEM image of 1.5 mol% Sn/TiO2 nanoparticles.
Figure
TEM images of (a) TiO2, (b) 1.5 mol% Sn/TiO2 nanoparticles.
Figure
Adsorption-desorption isotherms of TiO2 and 1.5 mol% Sn/TiO2.
Figure
Pore diameter distribution of 1.5 mol% Sn/TiO2.
The specific surface area, pore volume, and pore size of TiO2 and 1.5 mol% Sn/TiO2 nanoparticles are presented in Table
BET data for TiO2 and 1.5 mol% Sn/TiO2.
Sample | BET surface area (m2 g−1) | Total pore volume (cm3 g−1) | Mean pore diameter (nm) |
---|---|---|---|
TiO2 | 47.03 | 0.112 | 9.7 |
1.5 mol% Sn/TiO2 | 80.03 | 0.178 | 8.9 |
To investigate the optical absorption properties of synthesized samples, diffuse reflectance spectra (DRS) were analyzed. Sn-doping obviously affects light absorption characteristics of TiO2 as shown in Figure
UV-Vis absorption spectra of (a) TiO2, (b) 1 mol% Sn/TiO2, (c) 1.5 mol% Sn/TiO2, (d) 3 mol% Sn/TiO2.
The direct band-gap energy can be estimated from a plot of
It is well understood that hydroxyl radical is generated upon proper photon illumination to photocatalyst. The hydroxyl radical is a powerful oxidizing species, having potential oxidation of approximately 2.8 volt (versus NHE), which may lead to complete mineralization of pollutants. Generally, the greater the formation rate of *OH radicals is, the higher separation efficiency of electron-hole pairs is achieved. So, the photocatalytic activity is a positive correlation to the formation rate of *OH radicals, namely, a faster formation rate of *OH radicals leads to a higher photo-catalytic activity [
Hydroxyl radical amounts on Sn/TiO2 nanoparticles with different Sn content.
The results of mineralization of AMOX using Sn/TiO2 nanoparticles are illustrated in Figure
Influence of Sn content on the photocatalytic activity of samples under UV irradiation: (a) 0.0% Sn/TiO2, (b) 0.5 mol% Sn/TiO2, (c) 1 mol% Sn/TiO2, (d) 1.5 mol% Sn/TiO2, (e) 2 mol% Sn/TiO2 and (f) 3 mol% Sn/TiO2.
(i) TiO2 is a photoactive semiconductor that when illuminated with photon energy equal or greater than its band gap energy, the following reaction took place on the surface of the photo-catalyst:
SnO2 and TiO2 are both wide band gap semiconductors. Although the band gap of SnO2 (3.8 eV) is wider than that of TiO2 (3.1 V), the Fermi lever of SnO2 is lower than that of TiO2. It means that the photo-generated electrons may easily transfer from TiO2 to SnO2, but not to recombine with the photo-generated holes on the surface of TiO2 immediately. Consequently, more and more holes are present on the surface and take part in the reactions of oxidizing OH− and H2O into hydroxyl radicals. Hydroxyl radicals would be finally responsible for the degradation of pollutants into H2O and CO2 [
(ii) A small amount of metal ions can act as a photo-generated hole and a photo-generated electron trap and inhibit the hole-electron recombination:
The trapped electron may thus be readily transferred to oxygen molecule to form a superoxide radical anion (
Sn/TiO2 nanoparticles show higher band-gap energy than TiO2 nanoparticles, which not only suppressed the electron-hole recombination but also generated more *OH radicals [
(iii) The superior degradation efficiency of Sn/TiO2 nanoparticles could be attributed to a larger surface area compared to another photocatalyst. A large surface area may be an important factor in certain photocatalytic mineralization reactions, as a large amount of adsorbed organic molecules promotes the reaction rate [
(iv) Particle size is another important parameter influencing photocatalytic efficiency, since the electron-hole recombination rate may depend on the particle size. It is well known that in the nanometer-size range, physical and chemical properties of semiconductors are modified (compared with bulk). Small variations in particle diameters lead to great modifications in the surface/bulk ratio, thus influencing the recombination rates of volume and surface electrons and holes [
Moreover, a decrease in the activity is expected when the content of Sn becomes too large. The detrimental effect of tin on TiO2 photoactivity has several reasons.
(v) The amount of photoinduced electrons accumulating instantly at the SnO2 conduction band is too much due to excess SnO2 so that some photoelectrons can indirectly recombine with holes [
(vi) An excess amount of Sn dopant can produce the recombination center of photoinduced electron and hole pairs. Recombination of e--h+ pairs reduces the rate of photocatalytic mineralization [
(vii) Excessive coverage of TiO2 catalyst limits the amount of light reaching to the TiO2 surface, reducing the number of photogenerated e--h+ pairs and lowering consequently the TiO2 photoactivity [
(viii) Doped metal may occupy the active sites on the TiO2 surface for the desired photocatalytic reactions causing the TiO2 to lose its activity [
(ix) The probability of the hole capture is increased by the large number of tin particles at high tin dopings, which decrease the probability of holes reacting with adsorbed species at the TiO2 surface [
Based on the results, the optimum content of Sn-doped TiO2 for treatment of AMOX in aqueous solution is 1.5 mol%.
Photocatalytic mineralization of AMOX may depend on the catalyst amount. In order to investigate the effect of catalyst amount, experiments were carried out by varying the amount of photocatalyst from 100 to 600 mg L−1, and the mineralization profile is shown in Figure
Effect of Sn/TiO2 amount on AMOX mineralization.
In order to investigate the efficiency of Sn/TiO2 nanoparticles in mineralization of AMOX in the real water, 20 mg L−1 of antibiotic was added into a real water sample (carbonate hardness: 94 mg L−1 CaCO3, sulphate concentration: 175.1 mg L−1
Investigation of the efficiency of 1.5 mol% Sn/TiO2 in mineralization of AMOX from real water.
These ions may also block the active sites on the Sn/TiO2 surface thus deactivating the catalysts towards AMOX and intermediate molecules. Although the generated radical anions have been shown to be an oxidant itself, but its oxidation potential is less than that of the hydroxyl radicals.
The effect of varying AMOX initial concentration was studied in the range of 10–40 mg L−1. Figure
Determination of the pseudo-first-order kinetic rate constants,
By applying a least square regression analysis, the values of
Effect of initial AMOX concentration in the photocatalytic reaction.
Table
Pseudo-first-order kinetic rate constants in photocatalytic experiments with different initial concentration of AMOX.
400 | 40 | 0.068 | 14.5 | 0.997 |
400 | 30 | 0.09 | 11.1 | 0.999 |
400 | 20 | 0.143 | 6.95 | 0.994 |
400 | 10 | 0.25 | 3.98 | 0.991 |
Several reports have established that the heterogeneous photo-oxidation rate fits well to the classic Langmuir-Hinshelwood (L-H) mechanism [
Determination of the adsorption equilibrium constant,
TiO2 and Sn/TiO2 nanoparticles could be prepared by sol-gel method using titanium n-butoxide and tin (IV) chloride as precursors. The XRD results showed that the crystallite size greatly decreased due to Sn-doping but an increase in surface area, pore volume, and band gap energy was observed. Nitrogen adsorption-desorption isotherms showed that the adsorption ability was enhanced owing to Sn-doping. The absorbance of Sn/TiO2 nanoparticles was shifted toward shorter wavelength than TiO2 nanoparticles. The effect of Sn dopant on the photoinduced charge property was estimated by measuring hydroxyl radicals using salicylic acid as probe molecule. The photocatalytic efficiency for AMOX decomposition was remarkably enhanced owing to Sn-doping, and 1.5 mol% Sn/TiO2 sample had the highest photocatalytic activity due to increase in the generated hydroxyl radicals, band gap energy, specific surface area, and decrease in the crystal size. Langmuir-Hinshelwood kinetic model provided a good fit to the photocatalytic mineralization of AMOX, used in this study. This study confirms the potentialities of heterogeneous photocatalysis to decontaminate wastewaters containing organic pollutants.