TiO2-Based Nanomaterials: Design, Synthesis, and ApplicationsView this Special Issue
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Wei Guan, Fangying Ji, Zhigang Xie, Rongan Li, Nan Mei, "Preparation and Photocatalytic Performance of Nano-TiO2 Codoped with Iron III and Lanthanum III", Journal of Nanomaterials, vol. 2015, Article ID 869821, 13 pages, 2015. https://doi.org/10.1155/2015/869821
Preparation and Photocatalytic Performance of Nano-TiO2 Codoped with Iron III and Lanthanum III
Nanoscale titanium dioxide (nano-TiO2) was modified via metal doping to improve its photocatalytic activity and utilization of visible light. Nano-TiO2 doped with iron III (Fe3+) only, lanthanum III (La3+) only, and both Fe3+/La3+ was prepared using the sol-gel method. The photocatalytic activities of the three forms of doped nano-TiO2 were evaluated. Metal codoping limited crystal growth of crystal, and the sol-gel method was shown to be an effective technique for doping the lattice of TiO2 with Fe3+ and La3+. Codoping of nano-TiO2 with the tombarthite metal mixture had a synergistic effect of the photocatalytic performance, with the codoped nano-TiO2 exhibiting a performance greater than the sum of those of the single-doped nano-TiO2 samples. Kinetic studies showed that the photodegradation reaction of methyl orange by nano-TiO2 follows the Langmuir-Hinshelwood first order mechanism.
Global environmental pollution and energy shortages are becoming increasingly serious problems [1, 2]. The control of environmental pollution has become a major and urgent topic of concern. In 1972, Fujishima and Honda published the first article in Nature declaring that the semiconductor titanium dioxide crystal electrode has the ability to photocatalytically split water to produce hydrogen [3, 4]. This discovery signaled the beginning of heterogeneous photocatalysis research in the area of semiconductors. Photocatalysis technology, as a representative of green chemistry, is widely applied in many areas, such as wastewater treatment, air purification, and solar energy transfer and storage [5–7].
Nanoscale titanium dioxide (nano-TiO2) has become a preferred material for these applications due to its high catalytic activity and stable chemical properties and because it is cheap and nontoxic [8–10]. However, there are some disadvantages of using nano-TiO2, such as the high recombination rate of photoproduced electron-hole pairs, low quantum efficiency, and poor photocatalytic performance [9–12]. Nano-TiO2 can only use the ultraviolet portion of the solar spectrum range (only 3–5% of the total range) due to a wide band gap (3.2 eV), which leads to low effective utilization of sunlight [13–15]. Researchers have used a variety of methods to modify nano-TiO2, including noble metal modification, compound semiconductor, dye sensitization, metal ion doping, and others [16–19]. With these modification methods, the recombination rate of photogenerated electron-hole pairs of nano-TiO2 photocatalyst is decreased, and the photocatalysis efficiency and range of visible light that generates a response are increased [20–23]. Our research group found that modification by codoping with two elements can increase the visible light photocatalytic activity of nano-TiO2.
The main aim of the present study was to increase the visible light photocatalytic activity of nano-TiO2. The originality and significance of this study are described as follows.(1)We prepared nano-TiO2 photocatalysts by doping of nano-TiO2 powder with iron III ions (Fe3+), lanthanum III ions (La3+), or both via the sol-gel method.(2)The microstructure and chemical composition of the prepared nano-TiO2 photocatalysts were analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), fluorescence spectroscopy (FS), and UV-Visible absorption spectroscopy (UV-Vis). Furthermore, the catalytic mechanism was revealed in the present study.(3)The effects of various parameters, such as the roasting temperature, roasting time, catalyst dosage, initial pH value, doping amount, and doping type on the photocatalytic activity were investigated. In conclusion, a new mechanism for metal doping of nano-TiO2 was proposed.
2. Experimental Section
2.1. Preparation of Nano-TiO2 Photocatalysts
The preparation process for nano-TiO2 photocatalysts is shown in Figure 1. First, 100 mL absolute ethanol and 5 mL glacial acetic acid were added to a 250 mL beaker. After magnetic stirring for 30 min, the pH value of the solution was adjusted to 2 using nitric acid. The obtained mixture was designated solution A. Then, 15 mL ethanol and different doses (doping percentage is described as mole ratio) of modifier (Fe3+ and/or La3+) were added to a separate beaker to obtain solution B. Solution B was then added to solution A to obtain solution C. Then, 20 mL tetrabutyl titanate was added to the mixed solution C, followed by the addition of 5 mL distilled water. This solution was then stirred for 4 h. Sol TiO2 was obtained after sealing the container for 2 days. The sol TiO2 gradually formed nano-TiO2 upon drying, grinding, and heat treatment.
2.2. Characterization of Photocatalysts
The phase of the as-prepared samples was analyzed using a Shimazu XRD-6000 X-ray diffractometer with a copper target (Cu Kα, nm), a voltage of 40.0 KV, and a current of 30.0 mA.
Surface charge analysis was conducted using a British Kratos XPSAM800 multifunctional surface analysis electron spectrometer, with an Al target (1486.6 ev) X-ray gun operating under 12 kv × 15 ma power. The analysis chamber background vacuum was 2 × 10−7 Pa, adopting FAT working style. The spectrometer was operated with Cu2P3/2 (932.67 ev), Ag3d5 (368.30 ev), and Au4f7/2 (84.00 ev) prototype correction, and data were corrected using carbon pollution Cls (284.8 ev).
The compounds produced in photocatalyzed reactions were identified using UV-Vis absorption spectroscopic analysis. This study used a Shimadzu UV-Vis 2550 spectrophotometer (integrating sphere method) for fixed UV-Vis spectroscopy. A fixed amount of photocatalyst powder was placed in a quartz ware, using standard BaSO4 powder as a reference. Spectra were collected with a sweep rate of 1800 nm/min and a scanning range of 300–650 nm.
The molecular structures of reaction produces were analyzed by fluorescence spectroscopy (FS). This study used solid form testing with a fixed fluorescence intensity at a certain incident light wavelength (275 nm), and the results were combined with the experimentally determined photocatalytic activity to analyze the fluorescent light characteristics of the photocatalysts.
2.3. Evaluation of Photocatalyst Activity
2.3.1. Target Compounds
This study used methyl orange (chemical name, dimethylamino azo benzene sulfonic acid sodium), which is produced by nitriding aminobenzene sulfonic acid via N, N-dimethylaniline coupling, as the target compound for assessing photocatalyst activity. The molecular formula of methyl orange is C14H14O3N3SNa. Its molecular weight is 327.34 Da, and its molecular structural contains a benzene group and N and S heteroatoms as shown in Scheme 1 .
Its molecular structure has certain representativeness. Remediation of methyl orange is a widespread problem, because this material is widely used as an industrial dye and is harmful to the aquatic environment. Methyl orange shows obvious absorption of visible light, as its absorbance and concentration have a linear relationship within a certain range, according to the Lambert-Beer law. In the continual degradation process of methyl orange, the maximum absorption wavelength has been at near 465 nm, almost without deviation. The UV-Vis absorption spectra of methyl orange solution undergoing degradation with TiO2 photocatalyst is shown in Figure 1.
2.3.2. Photocatalytic Reaction Experiment
In a typical photocatalytic experiment, with a 30-W UV lamp and 35-W xenon lamp as light sources, 100 mL methyl orange solution (10 mg/L) and photocatalyst were added to five 300 mL beakers to form separate mixed suspensions. Prior to illumination, these mixed suspensions were stirred using a magnetic stirrer for 30 min. Then, the mixed suspensions were illuminated (the distance from the liquid level to the UV lamp was 10 cm) for 180 min. The mixtures obtained after illumination were separated by centrifugation for 20 min.
2.3.3. Photocatalytic Activity Experiment
The photocatalytic activity of the as-prepared TiO2 samples was evaluated according to the decolorization rate of methyl orange solution. First, 100 mL methyl orange solution and a specified amount of TiO2 photocatalyst were placed in a homemade photocatalytic reaction container. After 30 min of magnetic stirring, the reaction mixture was illuminated using the UV lamp and xenon lamp. By measuring the absorbance of the solution at the maximum absorption wavelength of methyl orange ( nm), the decoloring rate can be calculated as follows: where is the decoloring rate, is the initial absorbance before illumination, and is the absorbance after illumination time, .
3. Results and Discussion
3.1. Phase Distribution, Particle Size, and Lattice Distortion of the Prepared Nano-TiO2 Photocatalysts
Figure 2 shows the XRD spectra for the different types of nano-TiO2 photocatalyst treated at 500°C for 2 h. Compared to the standard X-ray spectrum of TiO2, it can be seen that the nano-TiO2 powders and Fe3+/La3+ co-doped nano-TiO2 powders were anatase phase. As shown in Figure 2, the peak shapes of (101), (004), and (200) crystal plane diffraction was sharp, indicating that anatase phase had completely developed. Due to the small amounts of iron and lanthanide used for doping, no metal oxide diffraction peaks were observed corresponding to Fe3+ or La3+. According to previous reports, in the La3+-doped nano-TiO2, La3+ ions on the surface of nano-TiO2 are oxidized and form a single layer of lanthanide oxide, which is difficult to detect by XRD. According to the solid physical band theory, in the nano-TiO2 crystal, the ionic radius of Ti4+ is 0.074 nm, and the ionic radius of Fe3+ is 0.069 nm. Thus, Fe3+ can easily spread into the nano-TiO2 lattice and replace Ti4+ in the nano-TiO2 lattice. The ionic sizes of Fe3+ and Ti4+ ions are different, leading to nano-scale TiO2 crystal lattice deformation. Upon La3+ doping onto nano-TiO2, La3+ replaces the lattice Ti4+. The ionic radius of La3+ is 0.115 nm, which is larger than that of Ti4+. Thus, the substitution of La3+ for Ti4+ will cause distortion and inflation of the nano-TiO2 crystal lattice, which will improve the photocatalytic activity of the material.
The size of the nanoscale grain obtained using this formula, that is, the first particle size of oriented crystal growth, cannot reflect particle agglomeration. The calculation results are shown in Table 1.
The calculation results show that the average particle size of Fe3+/La3+ codoped nano-TiO2 was lower than that of pure TiO2. The particle size of 0.01% Fe3+/0.6% La3+-doped TiO2 was the smallest among those tested (6.1 nm). According to the results of photocatalytic degradation of methyl orange solution, the photocatalytic activity of 0.01% Fe3+/0.6% La3+-doped TiO2 was the best among the photocatalysts tested. The average grain size of tombarthite-doped TiO2 was smaller than that of pure TiO2, indicating that the mixture of tombarthite ions inhibited the growth of the nanocrystalline phase. The average grain size of nano-TiO2 codoped with tombarthite ions and transition metal ions was smaller, indicating that doping improved this inhibition.
Doping with metal ions will affect the phase transition temperature, grain size, and other parameters and cause lattice distortion. Fe3+ partly replaced lattice Ti4+, inevitably causing oxygen defects, and the existence of oxygen vacancies is thought to promote grain growth of the rutile phase. Therefore, Fe3+ doping has a beneficial effect on the transformation of nano-TiO2 from anatase to rutile type. Doping with La3+ can inhibit the transformation of TiO2 from anatase to rutile and thereby increase the content of the highly photocatalytic anatase phase, causing the grain size of nano-TiO2 to decrease and thus the quantization effect to increase. Therefore, the synergistic effect of codoping with Fe3+ and La3+ makes the photocatalyst activity higher than the sum of the activities with single ion doping. According to Figure 2, upon doping with a small amount of Fe3+ and La3+, the diffraction peaks of nano-TiO2 shift towards the low angle direction, indicating that the diffraction peaks of Fe3+/La3+ codoped nano-TiO2 catalyst are wider than those of pure TiO2. Compared with pure nano-TiO2, the particle size of doped nano-TiO2 was reduced. This is because a certain amount Fe3+ and La3+ penetrates the nano-TiO2 crystal lattice, restricting the transfer and rearrangement of Ti and O ions, inhibiting the growth of nano-TiO2 crystals and decreasing the particle size.
3.2. Elemental Analysis of the Prepared Nano-TiO2 Photocatalyst
Figures 3(a), 8(b), and 8(c) show the XPS spectra of different elements in 0.01% Fe3+/0.6% La3+ codoped nano-TiO2, 0.01% Fe3+ doped TiO2, and 0.6% La3+ doped TiO2, respectively. Figure 3(d) shows XPS spectrum of different elements in TiO2. According to the high-resolution XPS patterns of Ti2p in these four spectra, there are two characteristic peaks of the same type at 463 eV and 458 eV. These two peaks correspond to the absorption peaks of Ti2p1/2 and Ti2p2/3 in anatase type TiO2, respectively, indicating that elemental Ti exists in the form of Ti4+ and the titanium oxide bond is stable. In Fe3+ doped nano-TiO2, the combining capacities of Ti2p2/3 and Ti2p1/2 are 458.28 eV and 463.90 eV, respectively. In 0.6% La3+ doped nano-TiO2, the combining capacities of Ti2p2/3 and Ti2p1/2 are 458.20 eV and 463.82 eV, respectively. In 0.01% Fe3+/0.6% La3+ doped nano-TiO2, the combining capacities of Ti2p2/3 and Ti2p1/2 are 458.25 eV and 463.88 eV, respectively. The values of combining capacity of these samples are different from those of pure TiO2 (combining capacities of Ti2p2/3 and Ti2p1/2 equal to 458.74 eV and 463.31 eV, resp.). This is due to doping with Fe3+ and La3+. In the Fe3+/La3+ codoped TiO2, these two peaks shift 0.5 eV toward the higher energy direction, indicating that the effective positive charge of Ti was increased. Upon doping with elemental Fe and La, on the surface or in the lattice of nano-TiO2, electronic redistribution occurs and leads to a decrease in the Ti outer electron density, a reduction in the shielding effect, and an increase in the electron binding energy. These effects are beneficial for increasing photocatalytic activity. The binding energy difference between catalyst Ti2p and O1s is 71.3 eV, which indicates that Ti in the three prepared catalysts is in the tetravalent form (TiO2).
According to Figures 3(a)–3(c), the peaks at 529.6–529.8 eV in the O1s high-resolution XPS patterns are mostly related to Ti, and the surface hydroxyl or oxygen in oxide defects is the key. Hydroxyl groups on the surface of the catalyst are considered to be an important factor affecting photocatalytic activity. A hydroxyl group on the nano-TiO2 catalyst surface can capture light and generate an ·OH free radical, which has strong oxidation ability. The ·OH free radical is the main strong oxidizer in the photocatalytic reaction. Therefore, as the hydroxyl content on the surface of nano-TiO2 catalyst increases, the surface becomes more conducive to the generation of ·OH free radicals and the quantization efficiency is further improved, thereby effectively improving the catalytic activity of the nano-TiO2 catalyst.
According to Figures 3(a) and 8(b), in the Fe2p high-resolution XPS patterns for 0.01% Fe3+ doped nano-TiO2 and 0.01% Fe3+/0.6% La3+ doped nano-TiO2, Fe2p peaks appear at 710.68 eV and 710.98 eV. This is trivalent iron, indicating that iron doped on TiO2 is in the form of Fe2O3. In addition, as shown in Figure 3(c), no Fe2p peak appears in the TiO2 XPS spectrum, indicating that elemental Fe exists only in crystalline Fe3+/La3+ doped nano-TiO2. Elemental Fe in the three prepared catalysts is in the tetravalent form.
According to Figures 3(a) and 3(c), in the La3d high-resolution XPS patterns of 0.6% La3+ doped nano-TiO2 and 0.01% Fe3+/0.6% La3+ doped nano-TiO2, La3d peaks appear at 835.75 eV and 836.20 eV. In Figure 3(d), no La3d peaks appear in the TiO2 XPS spectrum at these positions, confirming that elemental La was present only in La3+ doped nano-TiO2 and Fe3+/La3+ codoped nano-TiO2 powders. The difference in the binding energies of La3+ doped nano-TiO2 and Fe3+/La3+ codoped nano-TiO2 is 0.5 eV. This suggests that La3+ doping changed the electronic distribution on the nano-TiO2 surface or lattice, thus improving photocatalytic performance. In the La3d spectrum, two peaks appear for La3d3/2 and La3d5/2. According to previous reports, La exists in the form of La2O3 [25, 26]. Thus, La3+ ions did not enter into the lattice of TiO2. This is because the ionic radius of La3+ ions is bigger than that of Ti4+ ions, and thus, La3+ ions cannot enter into the lattice of TiO2.
3.3. FS Analysis of the Prepared Nano-TiO2 Photocatalyst
Figure 4 shows the fluorescence spectra of TiO2 and the three prepared nano-TiO2 photocatalysts. According to Figure 4(a)(A–D), the fluorescence spectra for Fe3+/La3+ codoped nano-TiO2 and pure TiO2 have a fluorescence peak at 417 nm. The intensity of this peak for Fe3+/La3+ codoped nano-TiO2 is lower than that in the spectra for nano-TiO2 doped with either metal or pure TiO2. Combined with the experimental results for methyl orange solution decolorization, this decrease in fluorescence intensity indicates a reduced recombination rate of photo-produced electron-hole pairs, and thus, an increased photocatalytic activity. The above results show that the improvement in visible light catalytic activity is due to the reduction of the light carrier recombination rate by doping.
Figure 4(b) shows the fluorescence spectra of nano-TiO2 doped with different amounts of La3+. Figure 4(c) shows the fluorescence spectra of nano-TiO2 doped with different amounts of Fe3+. Figure 4(d) shows the fluorescence spectra of nano-TiO2 doped with different amounts of Fe3+ and La3+. According to these spectra, TiO2 shows a strong peak at 417 nm, and the position of this peak is not affected by doping of the nano-TiO2 with any amount of Fe3+ and/or La3+. The intensity of this peak is weaker in the spectra for Fe3+/La3+ codoped nano-TiO2. La and Fe exist in the form of La2O3 and Fe2O3, respectively, and these metal oxides can function as agents to capture photo-produced electrons. After capture of a photo-produced electron, it is difficult for the electron to recombine with a hole. Together with the results of the methyl orange decolorization experiments, these results showing that the fluorescence intensity of codoped samples is smaller indicate that their photocatalytic activity is better. Codoping with Fe3+ and La3+ reduces the recombination rate of photo-produced electron-hole pair and improves the quantum efficiency, thus leading to improvement in the photocatalytic efficiency.
3.4. UV-Vis Analysis
Figure 5(a) shows UV-Vis absorption spectra of nano-TiO2 doped with different amounts of Fe3+. Relative to the absorption spectra of pure TiO2, the absorption band edge of Fe3+ doped nano-TiO2 shows an obvious red-shift. The obviously enhanced absorption strength in the visible area is beneficial for improving the utilization of sunlight and the photocatalytic efficiency. The main reason for this improvement is that the radius of Fe3+ (0.064 nm) is similar to that of Ti4+ (0.068 nm), and thus, Fe3+ can replace some Ti4+ in the lattice and create lattice defects. Impurity level formed in nano-TiO2 band gap, and the energy of semiconductor optical electronic transiting to guide reduced, smaller energy photoproduction electronic can also transit, so the spectrum redshift, light response range extended.
Figure 5(b) shows the UV-Vis absorption spectra of nano-TiO2 doped with different amounts of La3+. The absorption sideband of La3+ doped nano-TiO2 moved towards the longer wavelengths, and the absorption rate of light increased. This is because the radius of La3+ is 0.115 nm, which is larger than that of Ti4+ (0.068 nm). Thus, La3+ has difficultly entering the lattice of nano-TiO2. When the quantity of doped La was very small, La covered the surface of nano-TiO2 mainly in its oxide form, hindering grain growth. This led to the particles of La-doped nano-TiO2 being smaller than those of nondoped nano-TiO2. The tombarthite elements covering the surface of nano-TiO2 particles can absorb light over a wide range and transfer the energy to the nano-TiO2, thus improving the photocatalytic reactivity.
Figure 6 shows the UV-Vis absorption spectra for single- and codoped nano-TiO2 photocatalysts. The absorption bands for these nano-TiO2 catalysts shifted to the visible light region at varying degrees. For catalyst doped with La3+ and Fe3+ individually, the absorption edge moved 35 nm and 41 nm toward the visible light region, respectively. That for nano-TiO2 photocatalyst codoped with 0.01% Fe3+ and 0.6% La3+ moved 49 nm toward the visible light region. Therefore, the absorption sideband of Fe/La codoped nano-TiO2 red-shifted more than that for the Fe3+ or La3+ single-doped nano-TiO2, and the absorption of visible light by the codoped catalyst is stronger than that by the single-doped catalysts. This indicated that the codoping with both elements has a synergistic effect. The main reasons are as follows: the 3d orbital of Fe3+ is above the valence band of nano-TiO2. Electrons on the 3d orbital can absorb 415 nm visible light and transit it to nano-TiO2 to create Fe4+, and thus, Fe3+ acts as an electron trap. The vacant 5d orbital of La3+ serves as a good electron transfer orbital. This orbital can be used to transfer the photo-produced electrons in the TiO2 photocatalytic reaction, and thus, La3+ also acts as an electron trap. Therefore, codoping with Fe3+ and La3+ inhibited recombination of photo-produced electrons and holes, and thereby improved the quantum efficiency of photo-production.
3.5. Influence of Doping Amount on the Photocatalytic Activity of Nano-TiO2
3.5.1. Influence of Doping with Fe3+ or La3+ on the Photocatalytic Activity of Nano-TiO2
Figure 7(a) demonstrates the influence of Fe3+ doping concentration on the photocatalytic activity of nano-TiO2. According to Figure 7(a), the optimum doping amount of Fe3+ is 0.01%, which gives methyl orange decolorization rates of 93.5% with 3 h of UV illumination (versus 56.88% for pure TiO2) and 29.8% with 5 h of visible light illumination (versus 4.2% for pure TiO2). Doping with Fe3+ causes the nano-TiO2 to not only be able to capture electrons, but also to capture holes and the carrier is easily released. Thus, doping with Fe3+ can increase the photocatalytic activity of the nano-TiO2 catalyst. Doping with a small amount of Fe3+ can reduce the recombination rate of electrons and holes and enhance the photocatalytic activity of nano-TiO2 in the visible region, by improving the visible light utilization efficiency. At a low doping concentration, Fe3+ can play a dual role as an electron and a hole trap, and thereby improve the photocatalytic activity of the catalyst. At a high doping concentration, Fe3+ can reduce the quantum efficiency of photo-produced electrons and holes, leading to a decrease in the photocatalytic activity of the catalyst. This also can explain the influence of the doping amount on the photocatalytic activity via the process of capturing electrons and holes crossing the barrier. The recombination rate depends on the distance, , of separation between the electron and hole :where is the recombination rate constant, is the capture carrier hydrogen-like wave equation, and is the distance of separation between the electron and hole.
According to the formula above, when the doping concentration is less than the optimum value, the semiconductor does not have enough traps to catch carriers. When the doping concentration is larger than the optimum value, due to the reduction in the average distance between the electrons and traps, the recombination rate grows exponentially as the doping concentration is increased. Thus, use of the optimum doping amount of transition metal ions is critical.
Figure 7(b) shows the influence of La3+ doping concentration on the photocatalytic activity of nano-TiO2. As shown in Figure 7(b), doping with tombarthite element La3+ improved the photocatalytic activity of nano-TiO2. The methyl orange degradation rate under visible light illumination is greatly improved by La doping. The methyl orange decolorization rate increased as the doping amount of La3+ increased. The highest photocatalytic activity was observed for a doping concentration of La3+ of 0.6%. The methyl orange decolorization rate was 88.1% with 3 h of UV irradiation (pure TiO2) and 27.4% with 5 h of visible light irradiation (versus 4.2% for pure TiO2). With greater doping amounts, the photocatalytic activity did not continue to increase, but instead decreased. Doping with La ions increased catalytic activity, because tombarthite elements can produce electron configuration, polycrystalline type, and thermal stability. Doping with the appropriate amount of a tombarthite element has a positive role in improving the crystal type and photocatalytic properties of nano-TiO2. Because the La3+ radius is 0.106 nm, which is different from that of Ti4+ (0.068 nm), doping with La ions caused an increase in oxygen vacancy and defects on the surface of the nano-TiO2, effectively inhibiting the nano-TiO2 photo-production of electron-hole pairs, thereby improving the photocatalytic activity. However, too much tombarthite element also may cause a free electron transfer center to become a free electron recombination center and increase the photo-production of electron-hole pairs, thus reducing the photocatalytic activity.
The f orbital of tombarthite elements can have a coordination effect with the degradation substrate, and doping with a certain amount of La ions can effectively separate the nano-TiO2 photo-produced electrons and holes, generating many active groups with strong oxidizing ability involved in the photocatalytic oxidation reduction reaction, thereby improving the photocatalytic activity of the catalyst. However, when the doping amount exceeds a certain concentration, too much tombarthite metal ion deposition on the surface of nano-TiO2 hinders electron and hole transfer from the surface of the catalyst. Thus, tombarthite metal ions on the surface of the nano-TiO2 become charge carrier recombination centers, resulting in a decrease in catalytic activity.
3.6. The Influence of Fe3+/La3+ Codoping on Nano-TiO2 Photocatalytic Activity
Figure 8 shows the influence of Fe3+/La3+ codoping on the photocatalytic activity of nano-TiO2 under UV illumination, and Figure 9(a) shows the effect of Fe3+/La3+ codoping on the photocatalytic activity of nano-TiO2 under visible light illumination. According to the results shown in these figures, the catalytic activity of codoped nano-TiO2 is higher than that of catalyst doped with either Fe3+ or La3+. The 0.01% Fe3+ and 0.6% La3+ codoped nano-TiO2 possessed the highest photocatalytic activity. After 3 h of UV irradiation, the decolorization rate of methyl orange for 0.01% Fe3+ and 0.6% La3+ codoped nano-TiO2 was 99.8%. After 5 h of visible light irradiation, the decolorization rate of methyl orange for 0.01% Fe3+ and 0.6% La3+ codoped nano-TiO2 was 40.7%. Both of these rates are greatly improved over those achieved by pure TiO2. Doping with transition metal Fe3+ ions alone did not hinder the modification of tombarthite ions, but worked together with tombarthite La3+ ions to further improve the activity of the photocatalyst.
The experimental results show that there are optimum doping amounts for both Fe3+ and La3+. A high concentration of doping ions can reduce the photocatalytic activity. Under the conditions of high concentrations, neither Fe3+ nor La3+ can effectively penetrate the crystal lattice of nano-TiO2, and therefore, these ions gather on the surface of crystals. An excessive of doping ions can catch large numbers of electrons and holes, reduce the quantum efficiency, and reduce the activity of catalysts. For low doping concentration, an increase in the doping ion concentration can improve the optical carrier separation effect. Therefore, because the thickness of the space between electrons and the surface of nano-TiO2 decreases with an increasing amount of doping tombarthite element, when the optimum concentration of doping metal is reached, the distance between the electrons and the surface is equal to the penetration depth of incident light into the solid and photoproduction of electrons and holes is achieved by optimal light irradiation, benefiting the photocatalytic reaction. The combined effects of Fe3+ and La3+ upon codoping of nano-TiO2 photocatalyst promoted the optimum separation of photoproduced electrons and holes and thus improved the photocatalytic activity of the photocatalyst.
3.7. Kinetics of the Photocatalytic Activity of Codoped Nano-TiO2
For a heterogeneous photocatalytic system, such as the nano-TiO2 photocatalytic system, the reaction rate of photocatalytic oxidation can be described by the Langmuir-Hinshelwood dynamics equation as follows :where is the concentration of reactant, is the activity constant, and is the adsorption equilibrium constant of the reaction. Integration of (3) gives
When the concentration is small, (4) can be transformed into where is the apparent rate constant and is a constant.
The kinetics for the degradation of methyl orange by the different prepared nano-TiO2 photocatalysts were investigated in the present study. The relationship between ( is the initial concentration and is the concentration at time ) and photocatalysis time is shown in Figure 9(b). The fitting of the data for the photocatalytic degradation of methyl orange with a first-order kinetic curve is shown in Table 2.
According to Table 2, the value for the fitted straight line is far less than 0.01, indicating that and are significantly linearly correlated. As shown in Figure 9, under visible light irradiation, the degradation of methyl orange by different doped nano-TiO2 catalysts is well described by first-order reaction kinetics. The high correlation coefficients indicate that this model can be used to describe this photodegradation reaction.
In the present study, nano-TiO2 powder photocatalyst was prepared and modified via a sol-gel method by doping with either Fe3+ or La3+ individually or codoping with both Fe3+ and La3+. Codoping of nano-TiO2 photocatalysts with both Fe3+ and La3+ resulted in better catalytic performance than that achieved by doping with either Fe3+ or La3+, as well as better inhibition of nanocrystal growth and better refinement of grain size. Doping with tombarthite ions can effectively inhibit the shift of nano-TiO2 from anatase to rutile. La3+ doping changed the nano-TiO2 surface or lattice electron distribution. The sol-gel method can be used to effectively dope the lattice of nano-TiO2 with Fe3+ and La3+. Compared with catalyst doped with only Fe3+ or La3+, the light absorption intensity of Fe3+/La3+ codoped nano-TiO2 photocatalyst was stronger. This is because the absorption band edge redshifted obviously, and the spectral response range was extended into the visible light region, increasing the utilization of visible light. Fe3+/La3+ codoped nano-TiO2 photocatalyst showed superior photocatalytic performance compared to the single-doped samples. Because nano-TiO2 codoped with two elements can achieve higher catalytic activity under visible light, this approach increases the potential utility of nano-TiO2 photocatalyst materials in important environmental purification processes.
Conflict of Interests
The authors declare no conflict of interests.
This research is financially supported by the Scientific Research Foundation of Chongqing University of Arts and Sciences (R2014CH08), the Science and Technology Project from Chongqing (cstc2014jcyjA20023), the National Training Programs of Innovation and Entrepreneurship for Undergraduates (201410642003), and the Chongqing Training Programs of Innovation and Entrepreneurship for Undergraduates (201410642008).
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