Journal of Nanomaterials
Volume 2016 (2016), Article ID 8182190, 11 pages
http://dx.doi.org/10.1155/2016/8182190

Research Article

Synthesis of N/Fe Comodified TiO₂ Loaded on Bentonite for Enhanced Photocatalytic Activity under UV-Vis Light

Xi Cao,¹ Chen Liu,² Yandi Hu,³ Wenli Yang,³ and Jiawei Chen¹

¹State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China
²Environmental Protection and Energy Saving Center, China Waterborne Transport Research Institute, Beijing 100088, China
³Department of Civil & Environmental Engineering, University of Houston, Houston, TX 77204-4003, USA

Received 7 December 2015; Accepted 21 January 2016

Academic Editor: Daniela Predoi

Copyright © 2016 Xi Cao 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.

To improve the efficiency of TiO₂ as a photocatalyst for contaminant degradation, a novel nanocomposite catalyst of (N, Fe) modified TiO₂ nanoparticles loaded on bentonite (B-N/Fe-TiO₂) was successfully prepared for the first time by sol-gel method. The synthesized B-N/Fe-TiO₂ catalyst composites were characterized by multiple techniques, including scanning electron microscope (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), X-ray fluorescence (XRF), nitrogen adsorption/desorption, UV-Vis diffuse reflectance spectra (DRS), and electron paramagnetic resonance (EPR). The results showed that bentonite significantly enhanced the dispersion of TiO₂ nanoparticles and increased the specific surface area of the catalysts. Compared with nondoped TiO₂, single element doped TiO₂, or unloaded TiO₂ nanoparticles, B-N/Fe-TiO₂ had the highest absorption in UV-visible region. The photocatalytic activity of B-N/Fe-TiO₂ was also the highest, based on the degradation of methyl blue (MB) at room temperature under UV and visible light irradiation. In particular, the synthesized B-N/Fe-TiO₂ showed much greater photocatalytic efficiency than N/Fe-TiO₂ under visible light, the newly synthesized B-N/Fe-TiO₂ is going to significantly increase the photocatalytic efficiency of the catalyst using sun light.

1. Introduction

Environmental applications of titanium dioxide (TiO₂) have received a lot of recent interests from researchers. Due to its fine electronic properties, high photocatalytic activity, chemical stability, and low costs, TiO₂ is commonly used as a photocatalyst for the degradation of various pollutants [1, 2]. For the photocatalytic degradation, first, TiO₂ absorbs the solar light to boost its electrons from the valence band to the conduction band, thus forming electron-hole pairs (e⁻-h⁺). Then, the holes, as oxidants on TiO₂ surface, can absorb H₂O and hydroxide ions and oxidize them to hydroxyl radicals, which are the main oxidizing agents for pollutant degradation [3, 4]. Despite the great potential of TiO₂ for environmental treatment, there are mainly three disadvantages that have impeded their large-scale application. First, due to its band structure, TiO₂ absorbs UV light which only accounts for 3–5% of sunlight [5]. Second, the high rate of electron-hole recombination in TiO₂ system leads to low photocatalytic efficiency [6]. Third, the fast agglomeration of TiO₂ nanoparticles causes the formation of larger particles resulting in lower catalytic efficiency [7]. To overcome these three difficulties, researchers have attempted to modify TiO₂ with three specific strategies.

First, doping design has been applied to extend the absorption band edge of TiO₂ from UV region to visible light region [8–12]. For example, TiO₂ has been doped with nitrogen, and the modified catalyst showed higher photocatalytic activity [13]. Regarding the mechanisms, Asahi suggested that level could mix with , which narrowed the band gap of the catalyst and extended its photocatalytic activity into the visible region [14, 15], while some other researchers suggested that N doping in TiO₂ (N-TiO₂) can create a midgap state serving as an electron acceptor or donor in the band gap of TiO₂, causing its increased photocatalytic activity under visible light [16–18].

Second, the photocatalytic efficiency also depends on the competition between the electron-hole recombination rate and the surface charge carrier transfer rate. It was reported that small amount of Fe³⁺ ions can act as the traps for the photogenerated electrons and holes, resulting in the inhibition of electron-hole recombination. Therefore, Fe³⁺ has been doped in TiO₂ (Fe-TiO₂) to enhance the photocatalytic activity of TiO₂ [19–21].

Third, to prevent the agglomeration of TiO₂ nanoparticles, TiO₂ nanoparticles have been dispersed into the interlayers of clay minerals. Among the clay minerals, bentonite is widely used owing to economic concerns. The TiO₂ nanoparticles loaded on bentonite (B-TiO₂) showed increased specific surface area, thermal stability, cations exchange ability, and photocatalytic efficiency for pollutant degradation [22–25].

It is promising to combine the modification strategies for increasing the photocatalytic efficiency of TiO₂ nanoparticles. For example, TiO₂ nanoparticles have been comodified as N/Fe-TiO₂ (doped with both N and Fe) and B-Fe-TiO₂ (doped with Fe and loaded on bentonite), both of which showed improved photocatalytic activities compared to nondoped TiO₂, single element doped TiO₂, or unloaded TiO₂ nanoparticles [26–29]. A recent study synthesized that N/Fe-TiO₂ film on the bentonite surface by microwave power, loading TiO₂ nanoparticles on the bentonite surface by sol-gel method, has never been studied yet [30]. Sol-gel method serves as a common way to produce TiO₂ particles, since it has several advancements, such as producing nanosized crystallized powder of high purity at relatively low temperature, possibility of stoichiometry controlling process, and production of homogeneous materials [31]. Therefore, it is necessary to synthesize (Fe/N) comodified TiO₂ nanoparticles loaded on bentonite (B-N/Fe-TiO₂) by sol-gel method. Furthermore, there is no report on the B-N/Fe-TiO₂ photocatalytic efficiency difference under UV and visible lights for the degradation of a model pollutant methyl blue (MB). In this study, we synthesized and fully characterized B-N/Fe-TiO₂, and we measured its photocatalytic efficiency for MB degradation under UV and visible lights, and the mechanisms of its enhanced performance were also elucidated.

2. Experimental

2.1. Materials

Tetrabutyl titanate (TBOT), ethanol, and ferric chloride (FeCl₃·6H₂O) were purchased from Sinopharm Chemical Reagents Company (China). Urea and acetic acid were purchased from Beijing Chemical Works. All the chemicals were of analytical grade and used without further purification. Type P25 TiO₂ was purchased from Evonik Degussa Company (Germany) with a grain size of 20 nm. Methyl blue (MB, reagent grade, Sinopharm Chemical Reagents Company) was selected as the probe compound for the photocatalytic degradation reactions. Na-bentonite was purchased from Sinopharm Chemical Reagents Company (China).

2.2. Synthesis of Modified TiO₂ Catalysts

Pure TiO₂ and modified TiO₂ were synthesized by a sol-gel process. First, an “A” solution was made by mixing 20 mL tetrabbutyl titanate, 60 mL absolute ethanol, and 2 mL acetic acid, while a “B” solution was made by mixing 0.6 mL HCl and 10 mL ultrapure water (18.3 MΩ cm). For the case of N and/or Fe modified TiO₂ synthesis, urea and/or ferric chloride were added to the “B” solutions. The molar ratio of Ti/N was 1 : 2 and the Fe ratio was 5 w%. Then, the modified “B” solution, containing urea and/or ferric chloride, was added dropwise to “A” solution under stirring vigorously, and pure TiO₂ and N and/or Fe modified TiO₂ were synthesized. For the synthesis of bentonite supported catalysts (B-TiO₂, B-N-TiO₂, B-Fe-TiO₂, and B-N/Fe-TiO₂), 1 g Na-bentonite was added into the mixtures of “A” and modified “B” solutions, and the solutions were stirred continuously for 2 hours. Finally, the solutions were let stand in room temperature for 12 hours, dried at 80°C for 24 hours, and calcined at 500°C for 3 hours with a heating rate of 5°C/min from room temperature. After synthesis, the calcined samples were ground and sieved, and fine powders smaller than 200 mesh size were used in the experiments.

2.3. Catalyst Characterization

Multiple techniques were employed to characterize the morphology, composition, and structure of the synthesized TiO₂ catalyst composites. The synthesized samples were coated with platinum, and their morphologies were observed using scanning electron microscopy (SEM, JSM-6460LV, Zeiss MERLIN VP Compact, Germany). The elemental compositions, in terms of weight and atomic percentages, of the catalyst composites were determined using energy dispersive spectrometry (EDS). In addition, the weight percentages of TiO₂ in the synthesized composites were measured by X-ray fluorescence spectrometry (XRF, XRF-1800, Japan). To characterize the structure of the catalyst composites, X-ray diffraction (XRD, Rigaku D/Max 2500, Japan) and Fourier transform infrared spectroscopy (FTIR, PerkinElmer) measurements were conducted.

Furthermore, specific properties (such as specific surface area, UV-Vis light absorption and and generation), which control the catalytic efficiency of the catalyst composites, were also characterized. The specific surface areas of the synthesized composites were measured using an Autosorb IQ2 nitrogen adsorption/desorption apparatus (Quantachrome, USA). The UV-Vis absorption spectra of the catalyst composites were measured by TU-1901 spectrophotometer. Electron paramagnetic resonance (EPR) spectra were acquired with a Bruker E580 CW spectrometer at 295 K, operating with a microwave frequency of 9.7 GHz. Both materials were weighted 15 mg. was captured by DMPO in water and was captured by DMPO in methanol. The UV and visible light were at 320–400 nm and 420–800 nm, respectively.

2.4. Photocatalytic Degradation of MB

Stirred batch experiments of MB degradation were performed using TiO₂, B-TiO₂, N-TiO₂, B/N-TiO₂, Fe-TiO₂, B-Fe-TiO₂, N/Fe-TiO₂, and B-N/Fe-TiO₂. UV light irradiation and visible light irradiation were achieved using a 250 W high-pressure Hg lamp and xenon lamp with UV cut-off filter, respectively. Under the UV or visible light irradiation conditions, the lamp was hanged in a dark box and kept at about 20 cm above the reacting liquid.

Stirred batch experiments were conducted in 10 mg/L MB solution with 1 g/L catalyst compositions added. Prior to irradiation, the solution with suspending catalyst composites was stirred in dark for 30 min to attain the adsorption-desorption equilibrium for MB and dissolved oxygen (DO) on the surface of the catalyst composites. Then, with continuous stirring, the solutions were illuminated with UV for 120 minutes or visible light for 200 minutes. During the degradation reactions, at given time intervals, 2 mL solution containing suspensions were sampled from the batch and were filtered through a Millipore filter (pore size, 0.22 μm) immediately to separate the catalyst compositions from the solution. Finally, the residual MB concentrations in solution were determined by a UV-Vis spectrometer (Shimadzu UV-1750).

3. Results and Discussion

3.1. Characterization of Successfully Synthesized B-N/Fe-TiO₂

In order to prove that the synthesized catalysts have been successfully modified with N/Fe doping and loaded on bentonite, multiple characterization techniques were employed, including FTIR, XRD, SEM-EDS, and XRF. First, FTIR analysis in the range of 4000 cm⁻¹ to 400 cm⁻¹ was performed on TiO₂, N-TiO₂, Fe-TiO₂, and B-N/Fe-TiO₂ (Figure 1). As shown in Figure 1(a), for B-N/Fe-TiO₂, it has absorption peaks at 3442 cm⁻¹, 1630 cm⁻¹, 1088 cm⁻¹, 1040 cm⁻¹, 930 cm⁻¹, and 542 cm⁻¹. The absorption peaks at 3442 cm⁻¹ and 1630 cm⁻¹ (Figure 1(a)) were, respectively, assigned to the stretching vibration and the bending vibration of OH, due to the adsorbed water in the samples surface [32, 33]. The high adsorption at 1040 cm⁻¹ was attributed to the asymmetric stretching of Si-O-Si bonds (Figure 1(a)) [7, 34], which belongs to SiO₂ as the main composition of bentonite, indicating that the catalysts have been successfully loaded on bentonite. The absorption band at 400 cm⁻¹–600 cm⁻¹ was attributed to the stretching vibration of Ti-O bond [33, 35]. The adsorption at 930 cm⁻¹ was attributed to Ti-O-Si which formed during calcining process [36, 37]. The absorption peak at about 1088 cm⁻¹ was attributed to Ti-O-N [38], indicating the successful doping of the catalyst with N. Finally, the peak at 542 cm⁻¹ was attributed to Fe-O [34, 39, 40], indicating the successful doping of the catalyst with Fe (i.e., successful substitution of Fe atom with Ti). While no absorption peak for Fe-N was observed, indicating that at sites where nitrogen atoms substitute oxygen atoms, Fe would not substitute the Ti bonded to nitrogen.

Figure 1: FTIR spectra of different catalysts: (a) B-N/Fe-TiO₂, (b) Fe-TiO₂, (c) N-TiO₂, and (d) TiO₂.

XRD measurements of TiO₂, N/Fe-TiO₂, and B-N/Fe-TiO₂ were also conducted (Figure 2). For B-N/Fe-TiO₂ (Figure 2(c)), XRD diffraction peaks at 2θ = 25.3°, 38.6°, 48.0°, 53.9°, and 55.1° were measured, which reflected the anatase TiO₂ (101), (112), (200), (105), and (211) planes, respectively [5]. The XRD patterns indicated that pure anatase phase was synthesized using the sol-gel method in this study. Compared with N/Fe-TiO₂, B-N/Fe-TiO₂ showed additional small peaks around 2θ = 25°, which is consistent with the characteristic diffraction peaks of bentonite (Figure 2(c)). The XRD patterns indicated the successful loading of the modified TiO₂ catalyst on bentonite. Furthermore, for pure TiO₂ photocatalyst, the characteristic diffraction peaks of TiO₂ were very sharp (Figure 2(a)), indicating that the synthesized TiO₂ was well crystallized, while, for the modified catalyst compositions of N/Fe-TiO₂ and B-N/Fe-TiO₂ (Figures 2(b) and 2(c)), the characteristic diffraction peaks of TiO₂ were much broader and the peak intensities were much lower. Such peak broadening indicated the successful doping of N and Fe: due to different radii of N and O, Fe and Ti, doping TiO₂ with N (i.e., nitrogen atoms substitute oxygen sites in TiO₂) or Fe (i.e., Fe can enter TiO₂ lattice by substituting Ti) can lead to TiO₂ lattice distortion, resulting in the observed broadening of the TiO₂ diffraction peaks [41–43]. According to Scherrer formula calculation [7, 44], the average particle sizes of TiO₂, N/Fe-TiO₂, and B-N/Fe-TiO₂ were around 25.1 nm, 11.2 nm, and 9.5 nm, respectively. It indicated that N or Fe doping and bentonite loading could inhibit the growth of TiO₂. In addition, XRD pattern of B-N/Fe-TiO₂ (Figure 2(c)) showed the characteristic peaks of Fe₂O₃. It is possible that Fe₂O₃ were generated during calcination of Fe³⁺ doped TiO₂ gel. Future studies of the potential calcination effect on Fe₂O₃ formation can be performed; however, it is not the focus of the present study.

Figure 2: XRD spectra of different catalysts: (a) TiO₂, (b) N/Fe-TiO₂, and (c) B-N/Fe-TiO₂.

To further confirm the successful doping of N and Fe in TiO₂ and their loading onto bentonite, the morphologies of the modified catalyst composites were observed by SEM (Figure 3), and the elemental compositions of the synthesized B-N/Fe-TiO₂ catalyst composites were also measured by EDS (Figure 4). In the absence of bentonite, the catalysts nanoparticles of TiO₂, N-TiO₂, Fe-TiO₂, and N/Fe-TiO₂ tended to aggregation, whereas, for synthesized B-TiO₂, B-N-TiO₂, B-Fe-TiO₂, and B-N/Fe-TiO₂, TiO₂ nanoparticles were observed to attach onto the dispersed lamellar bentonite and the aggregation was decreased in the presence of bentonite.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Figure 3: SEM images of synthesized catalyst composites (a) TiO₂, (b) B-TiO₂, (c) N-TiO₂, (d) B-N-TiO₂, (e) Fe-TiO₂, (f) B- Fe-TiO₂, (g) N/Fe-TiO₂, and (h) B-N/Fe-TiO₂.

Figure 4: EDS spectra for B-N/Fe-TiO₂.

EDS measurement of B-N/Fe-TiO₂ (Figure 4) showed the presence of Ti, confirming the loading of TiO₂ in the catalyst composite. Also, the presence of Si, Al, Na, Ca, and Mg elements indicated the presence of Na-bentonite in the synthesized B-N/Fe-TiO₂ catalyst composite. Moreover, the presence of Fe elements demonstrated the successful doping of Fe.

In addition, the weight percentages of TiO₂ in different catalyst composites were quantified using XRF. As shown in Table 1, the weight percentages of TiO₂ in N-TiO₂, Fe-TiO₂, and N/Fe-TiO₂ were 99.8%, 91.9%, and 92.6%, respectively. The results showed that N doping contributed to only 0.2% weight of the catalyst composites, which was consistent with the fact that N in the composites was below the detection limit of EDS, while Fe₂O₃ which formed in Fe³⁺ doping contributed to ~5.5–8% of the total weight. For the catalysts loaded on bentonite, the weight percentages of TiO₂ in the catalyst composites were much lower, being 85.7%, 86.5%, 82.1%, and 84.8% for B-TiO₂, B-N-TiO₂, B-Fe-TiO₂, and B-N/Fe-TiO₂, respectively, indicating that bentonite contributed to ~9–14% of total weight of the catalyst composites.

Table 1: Specific surface areas and TiO₂ weight percentages (wt%) of the catalyst composites.

Catalysts
P25
TiO₂
B-TiO₂
N-TiO₂
Fe-TiO₂
N/Fe-TiO₂
B-N-TiO₂
B-Fe-TiO₂
B-N/Fe-TiO₂

Specific surface area (m²/g)
55.25
45.01
60.91
53.14
64.17
66.04
65.99
77.23
91.60

TiO₂ wt%
100
100
85.7
99.8
91.9
92.6
86.5
82.1
84.8

In summary, FTIR, XRD, SEM-EDS, and XRF were employed to characterize the newly synthesized B-N/Fe-TiO₂ catalyst composites, and consistent results were obtained. All measurements confirmed the successful N/Fe doping in TiO₂ and their successful loading onto bentonite. Very small amount of N (0.2% weight) was doped in the catalyst, and Fe doping (~7-8%) and bentonite loading (~8–14%) contributed significantly to the total mass of the catalyst composites.

3.2. Highest Specific Surface Area of Newly Synthesized B-N/Fe-TiO₂

As specific surface area is an important parameter determining the reactivity of catalyst, here, the specific surface area of the synthesized TiO₂ catalyst composites was measured (Table 1). First, the effects of N and/or Fe doping on the specific surface area of the catalyst composites were investigated. As shown in Table 1, the specific surface areas of TiO₂, N-TiO₂, Fe-TiO₂, and N/Fe-TiO₂ were 45.01, 53.13, 64.17, and 66.04 m²/g, respectively. The results showed that doping N and Fe slightly increased the specific surface area of TiO₂ catalyst composite. As discussed in Section 3.1, N and/or Fe doping in TiO₂ can lower the crystallinity of the TiO₂, which may result in more specific surface area available for N₂ adsorption.

Also, the effects of bentonite loading on the specific surface area of the catalyst composites were investigated. The specific surface areas of B-TiO₂, B-N-TiO₂, B-Fe-TiO₂, and B-N/Fe-TiO₂ were 60.91, 65.99, 77.23, and 91.60 m²/g, respectively. The measurements showed that the presence of bentonite dramatically increased the specific surface area of the TiO₂ catalyst composites. Since natural bentonite has lamellar structure, it has large interlayer surface area for TiO₂ attachment. The TiO₂ attached onto bentonite particles can be stabilized on bentonite surfaces, which can avoid the aggregation of TiO₂ nanoparticles.

3.3. Highest UV-Vis Light Absorption of Synthesized B-N/Fe-TiO₂

Generally in natural photodegradation processes photocatalyst can absorb the ultraviolet (UV) and/or visible light from natural sunlight and utilize the energy for the redox degradation reactions. To characterize the absorption of UV and visible light by the synthesized TiO₂ composites, UV-Vis diffuse reflectance spectra of the photocatalyst composites TiO₂, P25, N-TiO₂, Fe-TiO₂, B-TiO₂, B-N-TiO₂, B-Fe-TiO₂, N/Fe-TiO₂, and B-N/Fe-TiO₂ were measured (Figure 5). For synthesized TiO₂ (Figure 5(i)) and the purchased P25 type TiO₂ (Figure 5(h)), significant absorption was observed for light with wavelength shorter than 390 nm. This is the intrinsic absorption edge of TiO₂ for the electron transfer from to , corresponding with the valence band to conduction band transition of TiO₂ [7, 45]. For these TiO₂ composites without Fe and/or N doping, there was almost no light absorption in the visible light region, with wavelength ranging from 400 to 800 nm.

Figure 5: UV-Vis diffuse reflection spectra of different catalysts.

Since bentonite can absorb visible light, B-TiO₂ had more absorption of visible light than bare TiO₂. For TiO₂ composites doped with Fe³⁺ (Figures 5(a), 5(b), 5(c), and 5(d)), they showed significant enhancement in visible light absorption. This is because Fe³⁺ doping or Fe₂O₃ could form a dopant energy level within the band gap of TiO₂. It was studied that Fe₂O₃ has small band gap ( ev) and can absorb and utilize about 40% of the incident solar spectra [46]. N doping (Figures 5(a), 5(b), 5(e), and 5(f)) also increased the adsorption in visible light region. Our observation here was consistent with previous studies, which showed that N doping into the lattice of TiO₂ can shift the absorption edge of TiO₂ into the visible light range and exhibit increased absorption between 400 nm and 600 nm [47]. This is because nitrogen doping can form the localized midgap states above the top of the valence band of TiO₂ reducing the band gap of TiO₂. To further conform the N or Fe doping effects, the formula (where is the band gap and is the absorption edge shown in Figure 5) was used to calculate the catalyst’s band gap. The band gaps of TiO₂, N-TiO₂, Fe-TiO₂, N/Fe-TiO₂, and B-N/Fe-TiO₂ were calculated to be 3.26, 2.95, 2.34, 2.17, and 2.03 eV, respectively. The results showed that B-N/Fe-TiO₂ had the smallest band gap and its electrons could be easily excited from the valence band to the conduction band under visible light irradiation.

In conclusion, N and/or Fe doping reduced the band gap of the TiO₂ catalyst composites, therefore extending the light absorption of the modified catalysts from UV to the visible light region. In addition, the comodification with N and Fe showed synergistic effect in reducing the band gap of the TiO₂ catalyst composites. Meanwhile, bentonite can absorb visible light as well. Therefore, compared with all TiO₂ catalyst composites (Figure 5), the newly synthesized B-N/Fe-TiO₂ (Figure 5(a)) showed the highest light absorption in the UV-Vis region.

3.4. and Generation under UV-Vis

As hydroxyl radicals and super oxygen ions play key roles in the pollutant degradation process, the generation of free radicals ( and ) during photoirradiation process (shown in (1)–(3)) was investigated. The EPR spectra of the same weight of pure TiO₂, N/Fe-TiO₂, and B-N/Fe-TiO₂ with UV (Figure 6) and visible irradiation (Figure 7) were obtained. The intensity of the signal represented the relative amount of and produced by the photocatalyst. The constants of were ,  mT, and the constants of were ,  mT,  mT, and  mT.

(a)

(b)

Figure 6: EPR spectra of the (a) and (b) radicals formed upon UV irradiation with catalysts: (A) TiO₂, (B) N/Fe-TiO₂, and (C) B-N/Fe-TiO₂.

(a)

(b)

Figure 7: EPR spectra of the (a) and (b) radicals formed upon visible irradiation with catalysts: (A) TiO₂, (B) N/Fe-TiO₂, and (C) B-N/Fe-TiO₂.

Compared with TiO₂ system, the radicals of and in modified TiO₂ were significantly generated under UV-Vis light. Under UV irradiation, the intensities of and were about 1810 and 380 in N/Fe-TiO₂ system, respectively, while and in B-N/Fe-TiO₂ system were about 1740 and 460, respectively. The amount of produced by N/Fe-TiO₂ was slightly higher than that produced by B-N/Fe-TiO₂, while the amount of produced by N/Fe-TiO₂ was slightly lower than the later one. As the strong cations exchange ability of bentonite, the surface of B-N/Fe-TiO₂ had more electrons which can produce more . Under visible irradiation, the amounts of and generated by N/Fe-TiO₂ were about 830 and 180; the amount of and generated by B-N/Fe-TiO₂ were about 880 and 260, respectively. The free radicals produced by B-N/Fe-TiO₂ were both higher than N/Fe-TiO₂ under visible light. It resulted from the highest visible light absorption of B-N/Fe-TiO₂, and it could form more electron-hole pairs and more free radicals in visible region.

3.5. Highest MB Photodegradation Efficiency of Synthesized B-N/Fe-TiO₂

To compare the photocatalytic efficiency of different catalyst composites, the degradation of MB was conducted with the addition of 1 g/L of different catalyst composites (bentonite, TiO₂, B-TiO₂, N-TiO₂, Fe-TiO₂, N/Fe-TiO₂, B-N-TiO₂, B-Fe-TiO₂, and B-N/Fe-TiO₂). The residual MB concentrations in solution after reaction under UV (Figure 8(a)) and visible light irradiation (Figure 8(b)) for different time intervals were measured. Figure 8 showed that bentonite has slight influence on the degradation of MB. Without catalyst TiO₂, MB did not degrade under UV and visible light. With the addition of different catalyst composites, various degradation kinetics were observed (Figure 8). Under UV light irradiation, B-N/Fe-TiO₂ showed the highest photocatalytic activity for MB degradation with 100% MB removal in 50 minutes. For N/Fe-TiO₂, the slower process with 100% MB removal was used in 80 minutes. The photocatalytic efficiency of MB removal by TiO₂, B-TiO₂, N-TiO₂, Fe-TiO₂, B-N-TiO₂, and B-Fe-TiO₂ within 120 minutes was 56.7%, 45.1%, 94.7%, 69.2%, 89.2%, and 76.5%, respectively. Comparably, under visible light, B-N/Fe-TiO₂ also performed the fastest degradation of 100% MB in 180 minutes. The rates of MB degradation by TiO₂, B-TiO₂, N-TiO₂, Fe-TiO₂, B-N-TiO₂, B-Fe-TiO₂, and N/Fe-TiO₂ were 15.2%, 19.1%, 31.1%, 37.8%, 27.8%, 41.8%, and 85.5%, respectively. The results indicated that the newly synthesized B-N/Fe-TiO₂ showed the fastest photocatalyst efficiency for MB degradation under both UV and visible lights. As discussed earlier, B-N/Fe-TiO₂ had the highest specific surface area and the highest UV-Vis adsorption, both of which can contribute to its fastest photocatalyst efficiency for MB degradation.

(a) UV light

(b) Visible light

Figure 8: Residual concentrations of MB in solution after reaction for different time intervals and in the absence and the presence of TiO₂, B-TiO₂, N-TiO₂, Fe-TiO₂, N/Fe-TiO₂, B-N-TiO₂, B-Fe-TiO₂, and B-N/Fe-TiO₂ under UV (a) and visible (b) light irradiation.

The photocatalytic efficiency improvement for the B-N/Fe-TiO₂ compared to N/Fe-TiO₂ under visible light (Figure 8(b)) was more significant than under UV light (Figure 8(a)) irradiation. This is because bentonite can adsorb visible light and B-N/Fe-TiO₂ can produce more free radicals under visible light. In terms of energy, sunlight at Earth’s surface is about 52–55% infrared, 42-43% visible, and 3–5% UV. Considering the much smaller energy percentage of UV (3–5%) than visible light (42-43%) on Earth’s surface, the newly synthesized B-N/Fe-TiO₂ is going to significantly increase the photocatalytic efficiency of the catalyst using sunlight.

4. Conclusion

In this study, bentonite supported (N/Fe) comodified TiO₂ nanoparticles composite (B-N/Fe-TiO₂) was successfully synthesized by sol-gel method. The novel photocatalyst B-N/Fe-TiO₂ could extend the UV-Vis light working range and enhance the degradation of MB in water. B-N/Fe-TiO₂ enhanced photoactivity attributes to its larger surface area and higher UV and visible light adsorption. In particular, the newly synthesized B-N/Fe-TiO₂ showed significantly increased photocatalytic efficiency for contaminant degradation under visible light, making it a good photocatalyst for water remediation under sunlight.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This study was supported by National Natural Science Foundation of China (41472232, 41272061), Fundamental Research Funds for the Central Universities, Open Program of State Key Laboratory of Biogeology and Environmental Geology (GBL21404), and Prospective Basic Research Project of China Waterborne Transport Research Institute (WTI61421).

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