Photodegradation of methyl orange using magnetically recoverable AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] photocatalyst under visible light.
As a promising way to meet the challenges of environmental pollution, photocatalysis has attracted considerable interest over the past few decades [1-4]. With the shortage of energy sources becoming severe, significant efforts have now been directed toward the exploitation of highly efficient visible light responsible photocatalysts which can potentially utilize solar energy [5-8]. Very recently, [Ag.sub.3]P[O.sub.4] has been put forward as a novel photocatalyst with excellent oxidative capability for the purification of water under visible light irradiation, which thus inspired great enthusiasm [9-13]. It seems to be a promising material for efficient photodecomposition of organic contaminants. Nevertheless, it should be noted that, in the present [Ag.sub.3]P[O.sub.4] photocatalytic system, [Ag.sub.3]P[O.sub.4] is prone to be photochemically decomposed to Ag if no sacrificial reagent is involved , which inevitably becomes a main obstacle for [Ag.sub.3]P[O.sub.4] in practical application.
Recent reports indicated that epitaxial growth of an AgX (X = Br, I) nanoshell on the surface of [Ag.sub.3]P[O.sub.4] could greatly enhance the chemical stability and activity of [Ag.sub.3]P[O.sub.4] . For instance, Bi et al. introduced AgX (X = Cl, Br, I) for the modification of [Ag.sub.3]P[O.sub.4] by a simple in situ ion-exchange method and revealed the enhanced photocatalytic properties and stability . Cao et al. successfully synthesized AgBr/[Ag.sub.3]P[O.sub.4] as highly efficient and stable photocatalyst . This is mainlybecause AgX and [Ag.sub.3]P[O.sub.4] have matching band potentials, which could promote the transfer and separation of photo excited carriers through their heterojunctions. Other researchers also confirmed the enhancement in AgBr-based composites . Thus, combining [Ag.sub.3]P[O.sub.4] with AgX is a more promising and fascinating visible light response photocatalyst than pure [Ag.sub.3]P[O.sub.4].
For nano- or microsized photocatalysts, another problem that restrains their application is how to effectively separate the used photocatalysts from the mixed system in a simple way [18, 19]. Immobilizing catalysts on magnetic substrates by feasible methods is proven to be an effective approach for removing and recycling particles [20-23]. Moreover, [Fe.sub.3][O.sub.4] has excellent conductivity. Thus, [Fe.sub.3][O.sub.4] could act as an electron-transfer channel and acceptor, which would suppress the photogenerated carrier recombination . Therefore, given the magnetic separation ability and conducting properties of [Fe.sub.3][O.sub.4], it can be foreseen that fabrication of AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] heterostructures could combine the advantages of activity of AgBr@[Ag.sub.3] P[O.sub.4] with the merit of easy separation due to the incorporation of [Fe.sub.3][O.sub.4].
Nowadays, toxic organic dyes and their effluents are among the largest groups of water pollutants. The removal of these nonbiodegradable dye molecules from the environment is a crucial ecological problem, for their toxicity and potential carcinogenicity. To solve such pollution, the methyl orange (MO), which is a typical azo dye for textile industry, is chosen as the targeted pollutant in this paper. Herein, we prepared a novel magnetically separable AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] composite via a simple deposition-precipitation approach. The catalysts can be easily recovered by applying an external magnetic field. Furthermore, we demonstrate that this composite favors the separation of electron-hole pairs and exhibits the enhancement of stability and activity in the photocatalytic decomposition of MO under visible light.
2.1. Materials. All chemicals were of analytical grade and used as received without purification. Nano [Fe.sub.3][O.sub.4] (particle size <50 nm) was purchased from Sigma-Aldrich.
2.2. Sample Preparation. Firstly, the [Fe.sub.3][O.sub.4] nanoparticles were dispersed in distilled water (20 mL, 7.5 mM) and then added to the AgN[O.sub.3] solution (10 mL, 0.1 M). The solution was sonicated for 10 min. Subsequently, [Na.sub.2] HP[O.sub.4] aqueous solution (5 mL, 0.5 mM) was added dropwise to the above suspension. After sonicating for 10 min, a definite concentration of NaBr solution was added slowly into the above mixture. The theoretical molar percentage of added Br/original P was controlled to be 80%. The reaction was allowed to proceed for 10 min under sonication. Finally, the obtained precipitate was separated by an external magnetic field, washed with deionized water for several times, and then dried in a vacuum oven at 60[degrees]C for 12 h. The final sample was labeled as AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4].
For comparison, pure [Ag.sub.3]P[O.sub.4] particles were prepared by a simple precipitation method according to the previous study . [Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] and AgBr@[Ag.sub.3]P[O.sub.4] were also prepared by the same conditions by replacing the NaBr or [Fe.sub.3][O.sub.4] solution with water.
2.3. Characterization. For XRD studies, the samples were recorded on X'Pert Pro PANalytical automatic diffractometer, using Cu-Ka radiation ([lambda] = 0.154 nm) in the [theta] range of 10[degrees]-80[degrees]. TEM images were taken on a JEM-1200 (JEOL) microscope with an acceleration voltage of 80 kV. The UVVis diffuse reflectance spectra in the range of 230-700 nm were recorded on a Pgeneral TU-1901 PC spectrometer, using BaS[O.sub.4] as a standard.
2.4. Photocatalytic Tests. The photocatalytic activity of the sample was evaluated by photodegradation of MO at room temperature. Briefly, 60 mg of photocatalyst was added to an aqueous solution of MO (100 mL, 20 mg/L). The suspension was mechanically stirred for 45 min in dark conditions to reach complete adsorption-desorption equilibrium. Then, it was irradiated with a 150 w Xe lamp with a 400 nm light filter. During the illumination, at given time intervals, about 3 mL aliquots were sampled, magnetically separated, and centrifuged at 10,000 rpm for 5 min to remove the remaining particles. The concentrations of MO were analyzed on a UV-Vis spectrophotometer at 461 nm.
Additionally, the recycling experiments were performed for three consecutive cycles to test the stability and reusability of the as-prepared AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] composite. After each cycle, the photocatalyst was separated by an external magnetic field, washed thoroughly with deionized water, and then dried at 60[degrees] C for the next test.
3. Results and Discussion
3.1. Structural Characterization. XRD was used to investigate the different crystalline structures of the as-prepared photocatalysts. As shown in Figure 1(a), all the characteristic diffraction peaks can be readily indexed as the different crystalline planes of [Ag.sub.3]P[O.sub.4] (JCPDS, card number 06-0505). From Figure 1(b), the diffraction peaks can be well indexed to magnetite [Fe.sub.3][O.sub.4] (JCPDS, card number 19-0629). For the pattern of AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] (Figure 1(c)), besides the peaks of [Ag.sub.3]P[O.sub.4] and [Fe.sub.3][O.sub.4], the diffraction peaks of AgBr at 26.6[degrees], 30.9[degrees], 44.3[degrees], and 64.4[degrees] corresponding to the (111), (200), (220), and (400) have also been detected, confirming that AgBr have been formed on the [Ag.sub.3]P[O.sub.4] surface after reaction with NaBr. The diffraction peaks of [Fe.sub.3][O.sub.4] at 35.5[degrees], 43.2[degrees], and 62.8[degrees] correspond to the (311), (400), and (440). However, as shown in Figures 1(b) and 1(c), the diffraction peaks from [Fe.sub.3][O.sub.4] turn weaker in the as-prepared AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] composite due to the low content of [Fe.sub.3][O.sub.4]. These observations indicate the successful synthesis of AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] heterostructure.
The morphological and microstructural details of the AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] composite were then examined by TEM measurement. As shown in Figure 2(a), the [Fe.sub.3] [O.sub.4] exhibits regular spherical shape with diameter of about 2040 nm. Figure 2(b) reveals that the [Ag.sub.3]P[O.sub.4] possess an irregularly spherical morphology with diameter of 100-500 nm. Some big particles can be attributed to the agglomeration of small particles. In the case of AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] hybrid, as can be seen from Figures 2(c) and 2(d) in different magnification, it is evident that, alongside the [Ag.sub.3] P[O.sub.4], the [Fe.sub.3] [O.sub.4] nanoparticles are firmly anchored. This suggests a good combination between [Ag.sub.3]P[O.sub.4] and [Fe.sub.3][O.sub.4] particles. Unfortunately, we failed to obtain TEM images of the AgBr@[Ag.sub.3] P[O.sub.4]/[Fe.sub.3] [O.sub.4] samples, because AgBr nanoshells were easily destroyed by the high-energy electron beam during the measurements, as Wang et al. reported .
Figure 3 shows the UV-Vis diffuse reflectance spectra of [Ag.sub.3]P[O.sub.4], [Fe.sub.3][O.sub.4], and the related complex photocatalysts. Pure [Ag.sub.3] P[O.sub.4] shows a sharp fundamental absorption edge at about 520 nm, in accordance with the previous observation . In contrast to pure [Ag.sub.3]P[O.sub.4], the absorption of AgBr@[Ag.sub.3] P[O.sub.4]/[Fe.sub.3] [O.sub.4] sample toward the visible light region is remarkably enhanced. It could be mainly attributed to the introduction of [Fe.sub.3] [O.sub.4] nanoparticles, which is a well-performing light harvesting material as we can see in Figure 3.
3.2. Photocatalytic Performance. The photocatalytic activity of the as-prepared AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] was evaluated by the degradation of MO under visible light irradiation. Figure 4 gives the absorption spectra of an aqueous solution of MO exposed to visible irradiation for various time periods. In the reaction process, the color of the MO solution gradually diminished (as the inset shows), and the typical absorption peak at 461 nm disappeared after 15 min, indicating that the chromophoric structure of the dye was completely destroyed assisted by AgBr@[Ag.sub.3] P[O.sub.4]/[Fe.sub.3] [O.sub.4].
For comparison, the photodegradation of MO was also performed with photolysis, pure [Ag.sub.3] P[O.sub.4], [Fe.sub.3] [O.sub.4], [Ag.sub.3] P[O.sub.4]/ [Fe.sub.3] [O.sub.4], and AgBr@[Ag.sub.3] P[O.sub.4].
As can be seen from Figure 5, negligible degradation was detected under photolysis or using [Fe.sub.3] [O.sub.4] as photocatalyst. Similar to the previous reports, the pure [Ag.sub.3] P[O.sub.4] sample reveals a nice photodegradation performance under visible light (47.7% in 15 min). For comparison, after epitaxial growth of AgBr nanoshell on the surface of [Ag.sub.3] P[O.sub.4], the AgBr@[Ag.sub.3] P[O.sub.4] show much higher photocatalytic activity for the degradation of MO dye (94% in 15 min). This is mainly due to the effective coupling where the conduction band and valence band potentials of AgBr semiconductor are more negative than that of [Ag.sub.3] P[O.sub.4], which could promote the transfer and separation of photoexcited electron-hole pairs . In addition, the combination of [Fe.sub.3][O.sub.4] with [Ag.sub.3]P[O.sub.4] also achieved good degradation efficiency (87.3% in 15 min). As Xi et al. explained, because of the excellent conductivity, the charge transport is improved after introduction of [Fe.sub.3][O.sub.4] into the composite, which would enhance the separation of electron-hole pairs . Furthermore, just as the experimental results confirmed, once integrating the conductivity of [Fe.sub.3] [O.sub.4] and the structural match of AgBr with [Ag.sub.3] P[O.sub.4] particles, the AgBr@[Ag.sub.3] P[O.sub.4]/[Fe.sub.3] [O.sub.4] exhibits the highest photocatalytic efficiency.
3.3. Stability and Recyclability of AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4]. The stability of a photocatalyst is one of the most important parameters for its application. As our previous study demonstrated , [Ag.sub.3]P[O.sub.4] is quite unstable at repeated use. However, as Figure 6(a) presents, the MO solution is quickly bleached after every MO decomposition experiment, and photocatalyst ternary AgBr@[Ag.sub.3] P[O.sub.4]/[Fe.sub.3] [O.sub.4] is stable enough during the three repeated experiments without exhibiting any obvious loss of photocatalytic activity. Besides, the magnetic separation ability of the photocatalyst is impressive. As shown in Figure 6(b), the as-prepared AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] can be conveniently collected from the solution by applying an external magnetic field within 3 min. This desirable property is what other conventional powder photocatalysts lack. Therefore, the as-prepared AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] composite can work as an effective photocatalyst for pollutant degradation with good stability and recoverability.
3.4. Involved Active Species in the Photocatalysis. In order to investigate the photocatalytic degradation mechanism of AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4], it is necessary to verify the active species involved in the photocatalysis. Generally, photoinduced active species including [h.sup.+], * OH radicals, and *[O.sub.2.sup.-] are expected to be involved in the photocatalytic process. Herein, i-PrOH was added to the reaction system as an * OH scavenger, EDTA-[Na.sub.2] was introduced as a scavenger of [h.sup.+], and BQ was adopted to quench *[O.sub.2.sup.-] .
Figure 7 shows that, in the presence of EDTA, the photodegradation of MO was drastically inhibited with the degradation efficiency less than 5%. However, the employment of i-PrOH in the same photocatalytic system made a minor change caused in the photocatalytic degradation of MO. Furthermore, when the *[O.sub.2.sup.-] radical scavenger (BQ) was introduced, an evident decreasing photocatalytic activity of the AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] composite was observed. These results indicate that active species [h.sup.+] and *[O.sub.2.sup.-] contribute most to the photocatalytic system, and the presence of * OH radicals is considered to be of less importance to the reaction. Thus, we can anticipate the possible mechanism for the photocatalytic degradation of MO by AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] composites. Under visible light irradiation, [Ag.sub.3]P[O.sub.4] and AgBr can be simultaneously excited to form electron-hole ([h.sup.+]) pairs. As is known, AgBr and [Ag.sub.3]P[O.sub.4] have matching band potentials; the photoinduced electrons can transfer from the CB bottom of AgBr to that of [Ag.sub.3]P[O.sub.4], further migrate to [Fe.sub.3][O.sub.4] particles, and react with the adsorbed oxygen molecule to yield *[O.sub.2.sup.-]. At the same time, the holes also move in the opposite direction from the VB top of [Ag.sub.3]P[O.sub.4] to that of AgBr. The separated [h.sup.+] then mainly participate in the degradation of MO by direct oxidation, which would be together with *[O.sub.2.sup.-]. However, a small number of [h.sup.+] can still react with water to produce *OH radicals to degrade MO.
In summary, we reported an investigation on the preparation and photocatalytic activity of a novel magnetically recoverable AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] hybrid. Because of the magnetism of [Fe.sub.3][O.sub.4] and the matching band between AgBr and [Ag.sub.3]P[O.sub.4], the as-synthesized AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] nanoparticles exhibited efficient photocatalytic activity, good stability, and recyclability toward decomposition of MO under visible light irradiation. In addition, the quenching effects of different scavengers proved that reactive [h.sup.+] and *[O.sub.2.sup.-] played the major role for the MO degradation. We expected that this kind of magnetically separable AgBr@[Ag.sub.3]P[O.sub.4]/[Fe.sub.3][O.sub.4] composite would provide new insight for the design and fabrication of high performance photocatalysts toward environmental protection.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors genuinely appreciate the financial support of this work from Major Science and Technology Projects Focus on Social Development Projects of Zhejiang Province (2010C03003 and 2012C03004-1).
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Zhen Wang, Lu Yin, Ziwen Chen, Guowang Zhou, and Huixiang Shi
Department of Environmental Engineering, Zhejiang University, Yu Hang Tang Road, Hangzhou, Zhejiang 310058, China
Correspondence should be addressed to Huixiang Shi; email@example.com
Received 7 February 2014; Accepted 26 February 2014; Published 1 April 2014
Academic Editor: Haiqiang Wang
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|Title Annotation:||Research Article|
|Author:||Wang, Zhen; Yin, Lu; Chen, Ziwen; Zhou, Guowang; Shi, Huixiang|
|Publication:||Journal of Nanomaterials|
|Date:||Jan 1, 2014|
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