Catalysis Science & Technology

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Jan 17, 2016 - Ag3PO4 is considered to be an ideal candidate for water photooxidation and organic contaminant decomposition un- der visible light due to its ...

Catalysis Science & Technology PAPER

Cite this: Catal. Sci. Technol., 2016, 6, 4116

Ag3PO4 immobilized on hydroxy-metal pillared montmorillonite for the visible light driven degradation of acid red 18† Tianyuan Xu,abc Runliang Zhu,*ab Jianxi Zhu,ab Xiaoliang Liang,ab Yun Liu,d Yin Xud and Hongping Heab This work reports the facile fabrication of Ag3PO4/Fe–Al/Mt and Ag3PO4/Al/Mt by loading Ag3PO4 on hydroxy-iron–aluminum pillared montmorillonite (Fe–Al/Mt) and hydroxy-aluminum pillared montmorillonite (Al/Mt). The structural characteristics of the resulting materials were studied with XRD, SEM-EDS, XPS, ICP, nitrogen adsorption–desorption isotherms, and UV-vis diffuse reflectance spectra; the photocatalytic activity of the obtained catalysts was tested using acid red 18 (AR18) as a model contaminant under visible light irradiation. The obtained results illustrate that Ag3PO4 of a high dispersity and smaller size was successfully loaded on hydroxy-metal pillared montmorillonite. The photocatalytic activity and structural stability of the three synthesized catalysts were in the order Ag3PO4/Fe–Al/Mt > Ag3PO4/Al/Mt > Ag3PO4. An efficiency of 98.5% was achieved for AR18 degradation by Ag3PO4/Fe–Al/Mt after recycling seven times, while only 54.9% was achieved for Ag3PO4. The superoxide radical anion (O2˙−) was confirmed to be the dominant reactive species in all the three degradation systems, and the Ag3PO4/Fe–Al/Mt system formed

Received 9th December 2015, Accepted 17th January 2016 DOI: 10.1039/c5cy02129d

the largest amount of O2˙−. Except for the larger specific surface area and smaller particle size, the high removal efficiency of AR18, remarkable O2˙− generation performance, and good stability of Ag3PO4/Fe–Al/Mt could be attributed to the presence of Fe3+ as well, which can act as an electron acceptor for photoinduced electrons from Ag3PO4 during the photocatalytic process and then inhibit the transformation of

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Ag+ into metallic Ag.

1. Introduction Ag3PO4 is considered to be an ideal candidate for water photooxidation and organic contaminant decomposition under visible light due to its appropriate band gap position, nontoxicity, and high quantum yield.1–6 However, pure Ag3PO4 may decompose to metallic Ag during the catalytic degradation of organic contaminants and generally has a small specific surface area (SSA).4,7 Accordingly, structural stability and the photocatalytic activity deteriorate gradually,

a

CAS Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 511 Kehua Street, Guangzhou 510640, China. E-mail: [email protected]; Fax: +86 2085297603; Tel: +86 2085297603 b Guangdong Provincial Key Laboratory of Mineral Physics and Materials, 511 Kehua Street, Guangzhou 510640, China c University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China d Xiangtan University, Hunan, 411105, China † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c5cy02129d

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which are the main hindrances for the practical application of Ag3PO4 as an efficient and recyclable photocatalyst. Some studies have shown that coupling Ag3PO4 with other semiconductors or metals to form composite photocatalysts can largely increase its stability in resisting recombination of photogenerated and electron–hole pairs.8–12 Yu et al.13 synthesized an FeIJIII)/Ag3PO4 composite photocatalyst by loading Fe3+ on Ag3PO4, in which the Fe3+ worked as an active site for the following oxygen reduction to reduce the recombination rate of photogenerated electrons and holes. Similarly, other support materials such as carbon quantum dots14 and graphene oxide15–19 in the composite also can act as an electron acceptor to suppress the charge recombination and prevent the photocorrosion of Ag3PO4, which then can ultimately enhance the photocatalytic activity and stability of Ag3PO4. Moreover, increasing the SSA of catalysts generally also can enhance their photocatalytic reactivity; thus, immobilizing catalysts on porous supports has quite often been used to enhance the photocatalytic activity of catalysts.7 Because of their low cost and relatively large SSA, clays and clay minerals such as montmorillonite (Mt),20 attapulgite,21 sepiolite,22 and

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layered double hydroxides23 have been used as supports to load Ag3PO4, and the resulting composites generally have a larger SSA and enhanced photocatalytic properties as compared with pure Ag3PO4. Hydroxy-metal pillared Mts such as hydroxy-iron– aluminum pillared Mt (Fe–Al/Mt), due to their large SSA, lowcost, and high adsorption capabilities, have been used as an adsorbent for oxyanions and metal cations.24 Previous work has shown that hydroxy-metal pillared montmorillonites can serve as efficient adsorbents to simultaneously uptake heavy metal cations (e.g., Cd2+, Cu2+) and phosphate from water.25,26 As such, hydroxy-metal pillared montmorillonite may effectively load Ag3PO4. On the other hand, considering that some metals (e.g., Fe3+) can act as electron acceptors, the corresponding pillared Mt may be used as a support material to suppress the charge recombination of Ag3PO4. Hence, we expect that some hydroxy-metal pillared Mts may be an ideal support and at the same time can act as an electron acceptor for Ag3PO4 to suppress charge recombination, and the resulting composites with a high SSA can effectively degrade organic pollutants under visible light irradiation and have better stability. In the present work, hydroxy-aluminum pillared Mt (Al/ Mt) and Fe–Al/Mt were selected as hydroxyl-metal pillared Mts and used as support materials to load Ag3PO4, with the purpose of synthesizing new materials (i.e., Ag3PO4/Al/Mt and Ag3PO4/Fe–Al/Mt) with high photocatalytic activity and good stability. The microstructure of the resulting materials and their photocatalytic activity in the decolorization of acid red 18 (AR18) were studied. Moreover, the stability of the resulting composites and the active oxidative species in these catalytic degradation systems were further examined. The results of this work show that Ag3PO4/Fe–Al/Mt has a better catalytic activity and stability than both Ag3PO4/Al/Mt and pure Ag3PO4.

2. Experimental section 2.1 Materials Sodium exchanged Mt from Anji County, China, was used as a starting material.27 AR18 was purchased from National Medicine Group Chemical Reagent Co., Ltd. (PRC). Potassium superoxide (KO2) and 4-chloro-7-nitrobenzo-2-oxa-1,3diazole (NBD-Cl) were obtained from Aladdin (PRC). Other chemicals were obtained from the Guangzhou Chemical Reagent Factory (Guangzhou, PRC). All the chemicals used were of analytical grade and were used without further purification. 2.2 Preparation of catalyst A hydroxy-aluminum solution was prepared by slowly adding 0.2 mol L−1 Na2CO3 solution to 0.2 mol L−1 AlIJNO3)3 solution under magnetic stirring at 60 °C until the molar ratio of OH−/Al3+ reached 2.4.24 Then, the solution was aged for 24 h. To prepare hydroxy-iron–aluminum solutions, 0.2 mol L−1 AlIJNO3)3 solution was mixed with 0.2 mol L−1 FeIJNO3)3

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solution until the molar ratio of Fe3+/(Fe3+ + Al3+) reached the desired value (0.1, 0.2, 0.4, and 0.7). After that, 0.2 mol L−1 Na2CO3 was slowly added to the mixed solution under magnetic stirring at 60 °C, until the molar ratio of OH−/(Fe3+ + Al3+) reached 1.2; which was followed by ageing for 24 h. Then, the obtained solutions were added to a 2 wt% (4 g in 250 mL of water) Mt dispersion under stirring for 24 h. The final oligomeric cations/Mt ratio was equal to 10 mmol g−1. After stirring for 24 h at 60 °C, the products were centrifuged and washed. The resulting materials were denoted as Al/Mt and Fe–Al/Mt. After that, the Al/Mt and Fe–Al/Mt materials were each dispersed in 500 mL of water, and then were slowly mixed with 500 mL of 4.5 × 10−2 mol L−1 NaH2PO4 solution under magnetic stirring. After stirring for 24 h, the obtained materials were denoted as P/Al/Mt and P/Fe–Al/Mt, respectively. Finally, 500 mL of 0.135 mol L−1 AgNO3 was slowly added to the P/Al/ Mt and P/Fe–Al/Mt suspensions, and then the pH value of the suspensions was adjusted to 7 by slowly adding NH3·H2O. After stirring for 4 h, the product was centrifuged and washed with deionized water several times. Depending on the amount of additional Fe, the finally obtained materials were denoted as Ag3PO4/Al/Mt and Ag3PO4/Fe–Al/Mt (0.1, 0.2, 0.4, and 0.7), respectively. For comparison purposes, pure Ag3PO4 was prepared as well. All of the obtained samples were freeze-dried at −40 °C, and pulverized to pass through a 200 mesh sieve. According to the investigation on the effect of Fe amount on the photocatalytic performance of Ag3PO4/Fe–Al/Mt, the Ag3PO4/Fe–Al/Mt (0.2) photocatalyst showed the highest photocatalytic activity (Fig. S1†). Thus, the optimal photocatalyst was Ag3PO4/Fe–Al/Mt (2.7%), in this study, which is referred to as Ag3PO4/Fe–Al/Mt, and its structural characteristics and photocatalytic reactivity are studied in detail in this article.

2.3 Characterization X-ray diffraction (XRD) patterns of the prepared samples were acquired using a Bruker D8 ADVANCE X-ray diffractometer. The measurements were performed at 40 kV and 40 mA with Cu Kα irradiation, and a 2θ range between 1° and 80° was recorded with a scanning speed of 2° min−1. X-ray photoelectron spectroscopy (XPS) analyses were carried out using a Thermo Fisher Scientific K-Alpha spectrometer. To compensate for charging effects, the binding energies were corrected by referencing C 1s to 284.8 eV. Morphologies of the samples were characterized using a Carl Zeiss SUPRA55SAPPHIR scanning electron microscope (SEM) with an Oxford Inca250 X-Max20 energy dispersive X-ray spectrometer (EDS). The chemical compositions of the samples were determined using a PerkinElmer Optima 2000DV inductively coupled plasma optical emission spectrometer (ICP-OES). UV-vis diffuse reflectance spectra were measured using a Shimadzu UV-2550 double-beam digital spectrophotometer

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equipped with the conventional components of a reflectance spectrometer, and BaSO4 was used as a reference. Nitrogen adsorption–desorption isotherms were measured using a Micromeritics ASAP 2020M instrument. Before the adsorption tests, the samples were outgassed for 12 h at 30 °C. The multiple-point Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface areas of the samples.

2.4 Photocatalytic tests and analytical methods The photocatalytic activity of the obtained samples was determined by the degradation of AR18 under various conditions. The experiments were conducted using a photochemical reaction instrument (BL-GHX-V, Shanghai Depai Biotech. Co. Ltd., China). To simulate the visible spectrum, a 400 W halogen lamp was applied as the light source with a cutoff filter to filter out light of wavelengths below 420 nm, and the filter was positioned inside a cylindrical Pyrex vessel surrounded by a circulating water jacket. In the photocatalytic activity evaluation experiments, 50 mg of catalyst was added to 50 mL of AR18 (6.5 × 10−5 mol L−1) solution. All experiments were carried out under constant stirring to ensure good dispersion of the catalysts. During the photolysis process, samples were collected at desired intervals from the solution, which was followed by centrifugation to separate solid from liquid before measurement. The AR18 concentration was quantified by its absorbance at a wavelength of 509 nm using a spectrophotometer (759S, Shanghai JingHua Instrument Co. Ltd., China).

2.5 Analysis of reactive species Free radical capture experiments were used to ascertain the reactive species in the photodegradation process of AR18. Isopropyl alcohol (i-PrOH), sodium azide (NaN3), benzoquinone (BQ), and ammonium oxalate (AO) were chosen as a hydroxyl radical (OH˙) scavenger, singlet oxygen (1O2) scavenger, superoxide iron radical (O2˙−) scavenger, and h+ scavenger, respectively.28–31 The detailed free radical capture processes were similar to the photocatalytic experiments. In order to quantitatively determine the concentration of O2˙−, NBD-Cl (200 μmol L−1) was used as a fluorescent probe. An identification of the NBD-Cl product (reaction product of O2˙− and NBD-Cl)32,33 was conducted by recording a fluorescence emission spectrum (excited at 470 nm) using a F-4500 fluorescence spectrometer (Hitachi, Japan) at 550 nm. The NBD-Cl concentration was measured by its absorbance at a wavelength of 342 nm using a spectrophotometer.

Catalysis Science & Technology

3. Results and discussion 3.1 Structural characterization results The XRD patterns of the samples (Fig. 1) reveal that the basal spacing values of Mt, Al/Mt and Fe–Al/Mt are 1.25, 1.73, and 1.63 nm, respectively, indicating the intercalation of hydroxymetal cations into the interlayers of Mt. The diffraction peaks of Ag3PO4 (JCPDS no. 06-0505) can be distinguished in the XRD patterns of the pure Ag3PO4, Ag3PO4/Al/Mt, and Ag3PO4/ Fe–Al/Mt composites. The characteristic reflections of Mt at 19.8° and 28.6° (2θ) can be observed for both Ag3PO4/Al/Mt and Ag3PO4/Fe–Al/Mt, though the (001) reflection of Mt disappears, probably because of the large loading amount of Ag3PO4 on Al/Mt and Fe–Al/Mt. Morphologies of the as-prepared materials were characterized using SEM. Both Al/Mt and Fe–Al/Mt display lamellar structures (Fig. S2†). The SEM image of the pure Ag3PO4 (Fig. 2, a1) shows a large particle of mixed morphology (such as cubic, tetrahedral, sphere-like) with a smooth surface. When Al/Mt and Fe–Al/Mt dispersions were introduced into the synthetic system, the diameter of Ag3PO4 gradually decreased, which was much less than that of pure Ag3PO4 (Fig. 2, b1 and c1). This phenomenon indicates an obvious tailoring effect of Al/Mt and Fe–Al/Mt relative to the size of the Ag3PO4 particles on the composites. This is probably because negatively charged phosphate binds directly to the surface of positively charged Al/Mt and Fe–Al/Mt via electrostatic interaction. Then, Ag+ combines with phosphate to form Ag3PO4, which to some extent hinders the generation of Ag3PO4 seed particles and controls the growth of the Ag3PO4 particles.25,34 In addition, the EDS patterns of these samples (Fig. 2, a2–c2) show that Al, Si, P, Ag, and O are the prevalent components of Ag3PO4/Al/Mt. On the other hand, Al, Si, Fe, P, Ag, and O

2.6 Photocatalytic stability experiments Ag3PO4/Fe–Al/Mt, Ag3PO4/Al/Mt and pure Ag3PO4 were used repeatedly to degrade AR18 to evaluate their photocatalytic stabilities. The experimental process was similar to the above photocatalytic experiments. After reaction in each run, the photocatalyst was collected by centrifugation for the next use.

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Fig. 1 The XRD patterns of samples.

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Fig. 2 SEM images and EDS patterns of samples (a: Ag3PO4, b: Ag3PO4/Al/Mt, and c: Ag3PO4/Fe–Al/Mt).

are the main components of Ag3PO4/Fe–Al/Mt. The results confirm the successful loading of Ag3PO4 on Al/Mt and Fe–Al/ Mt. EDS mapping was selected for further analysis of the elemental distribution of the three samples. As shown in Fig. S3,† O, P, and Ag elements of Ag3PO4, Si, Mg, Al, O, P, and Ag elements of Ag3PO4/Al/Mt, and Si, Mg, Al, Fe, O, P, and Ag elements of Ag3PO4/Fe–Al/Mt were well distributed in the respective regions. ICP was carried out to investigate the precise silver contents of the samples (Table 1). The Ag3PO4 content of Ag3PO4/ Al/Mt was 43.2 wt%, higher than that of Ag3PO4/Fe–Al/Mt (40.7 wt%). The UV-vis diffuse reflectance spectra (Fig. 3) show that Ag3PO4, Ag3PO4/Al/Mt, and Ag3PO4/Fe–Al/Mt all have a significant absorption in the visible light region,

indicating that these materials can be potential visible light driven photocatalysts.

Table 1 The structural characteristics of various samples

Samples

Ag (wt%)

P (wt%)

SSA (m2 g−1)

Mt Ag3PO4 Al/Mt Fe–Al/Mt Ag3PO4/Al/Mt Ag3PO4/Fe–Al/Mt

— — — — 33.4 31.5

— — — — 4.2 3.4

16.5 1.2 117.1 114.9 20.4 35.3

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Fig. 3 UV-vis diffuse reflectance spectra of samples.

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3.2 Decolorization of AR18 The decolorization efficiency of AR18 under various conditions was examined (Fig. 4). With only visible light, the decolorization of AR18 was just about 5.0%, indicating that AR18 can resist visible light. Without any visible light, Ag3PO4, Ag3PO4/Al/Mt, and Ag3PO4/Fe–Al/Mt showed insignificant decolorization of AR18, implying that the three materials have a poor adsorption capacity for AR18. 14.0% and 38.4% of AR18 was adsorbed on Al/Mt and Fe–Al/Mt in the dark, respectively, and 14.7% and 41.5% of AR18 was removed by these catalysts under visible light irradiation, implying that Al/Mt and Fe–Al/Mt have weak photocatalytic ability for AR18 degradation. Under visible light irradiation, Ag3PO4/Al/Mt and Ag3PO4/Fe–Al/Mt exhibit better photocatalytic activity than pure Ag3PO4. Among them, Ag3PO4/Fe–Al/Mt displayed the best photocatalytic activity; and more specifically, AR18 could be completely decomposed in 100 min under visible light irradiation. According to the ICP results, the actual Ag3PO4 concentrations in the Ag3PO4/Al/Mt and Ag3PO4/Fe–Al/Mt systems were 0.434 and 0.407 g L−1 respectively in the AR18 decolorization process, which are much lower than that in the pure Ag3PO4 system (1 g L−1). Based on the above results, it can be concluded that under the same reaction conditions, the photocatalytic activity of the photocatalysts was in the order Ag3PO4/Fe–Al/Mt > Ag3PO4/Al/Mt > Ag3PO4. These results clearly demonstrate that introducing Fe into a Ag3PO4/ Al/Mt photocatalyst system can greatly enhance its photocatalytic activity. 3.3 Photocatalytic stability of samples The photocatalytic stabilities of pure Ag3PO4, Ag3PO4/Al/Mt, and Ag3PO4/Fe–Al/Mt were investigated by repeating the degradation of AR18 for seven cycles (Fig. 5). The results show

Fig. 4 Decolorization of acid red 18 under different conditions.

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Fig. 5 Decolorization of acid red 18 after seven recycling runs.

that Ag3PO4/Fe–Al/Mt was stable enough during the recycling experiments to not exhibit any significant loss of photocatalytic activity after recycling seven times, and the degradation efficiency of AR18 remained about 98.5%, which is much higher than that of the pure Ag3PO4 (54.9%). The XRD spectra of the recycled Ag3PO4, Ag3PO4/Al/Mt, and Ag3PO4/Fe–Al/Mt after three cycles of the AR18 photodegradation experiment were recorded to evaluate their structural stability (Fig. 6). The X-ray reflection of metallic Ag appears in the pattern of pure Ag3PO4, indicating that partial reduction of Ag3PO4 into metallic Ag particles occurs during

Fig. 6 The XRD patterns of samples after three recycling runs.

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the AR18 degradation process. A weak characteristic reflection belonging to metallic Ag can be observed in the pattern of Ag3PO4/Al/Mt but not in the pattern of Ag3PO4/Fe–Al/Mt, which implies that Fe greatly enhances the stability of Ag3PO4 in Ag3PO4/Fe–Al/Mt. In addition, the morphology and composition of the samples after three cycles of the AR18 photodegradation experiment were characterized using SEM (Fig. S4,† a1–c1) with EDS (Fig. S4,† b1–c1). The results show that the morphologies of Ag3PO4/Fe–Al/Mt and Ag3PO4/Al/Mt did not change significantly; however, it is obvious that Ag3PO4 has a rough surface, maybe due to the formation of metallic Ag particles. The EDS results show that the molar ratios of Ag/P in Ag3PO4/Al/Mt and Ag3PO4 are 5.9 and 4.1, respectively, manifesting the formation of metallic Ag particles in those two samples, which is in good agreement with the results obtained by XRD. Meanwhile, Fe 2p3/2 XPS spectra from Ag3PO4/Fe–Al/Mt after the reaction (Fig. 7) show two peaks with binding energies of 709.3 and 711.0 eV, which can be assigned to Fe2+ and Fe3+, respectively.35,36 The presence of Fe2+ proves that Fe3+ can accept photo-induced electrons from Ag3PO4 and be reduced to Fe2+. Accordingly, Ag3PO4/Fe–Al/Mt is more stable than pure Ag3PO4 and Ag3PO4/Al/Mt.

(1O2 scavenger) and AO (h+ scavenger) were employed, evident inhibition was observed in the photocatalytic degradation of AR18. Furthermore, when an O2˙− radical scavenger (BQ) was introduced, the photodegradation of AR18 was drastically inhibited, with a negligible degradation efficiency. These results indicate that the active oxidative species O2˙− contributed most to all of the photocatalytic systems. The formation of O2˙− was monitored during the photocatalytic degradation of NBD-Cl. Obviously, the catalytic degradation of NBD-Cl by Ag3PO4/Al/Mt and Ag3PO4/Fe–Al/Mt shows a higher efficiency than that by pure Ag3PO4 (Fig. 8a). Meanwhile, Fig. 8b clearly indicates that the formation of O2˙− in the presence of Ag3PO4/Fe–Al/Mt was the highest, followed by Ag3PO4/Al/Mt when compared with Ag3PO4. Additionally, during the first 40 min (60 min for Ag3PO4), the O2˙− formation rate increased and then decreased, and this tendency was the same as that observed by Ikhlaq.37 This may be due to the reaction of the NBD-Cl product with the catalysts and other oxidative species present in these systems. The high O2˙− concentration contributes to the high photocatalytic activity of Ag3PO4/Fe–Al/Mt for AR18 degradation.

3.4 Analysis of active oxidative species

The above results indicate that Ag3PO4/Fe–Al/Mt not only possesses excellent visible light photocatalytic activity for the degradation of AR18, but also exhibits superior stability. The possible reasons for the enhanced photocatalytic activity and stability of Ag3PO4/Fe–Al/Mt may originate from two aspects. The first one may be the high dispersity and smaller size of Ag3PO4 in Fe–Al/Mt. BET data in Table 1 show that the SSA of Ag3PO4/Fe–Al/Mt (35.3 m2 g−1) is higher than that of Ag3PO4/ Al/Mt (20.4 m2 g−1), and only 1.2 m2 g−1 higher than that of Ag3PO4. In addition, SEM results imply that the particle sizes of Ag3PO4/Fe–Al/Mt and Ag3PO4/Al/Mt are smaller than pure Ag3PO4. The other aspect may be attributed to the Fe–Al/Mt support, which can result in the efficient separation of photogenerated electron–hole pairs by Ag3PO4 and high O2˙− generation from the reduction of dissolved oxygen. When Ag3PO4 is irradiated by visible light, electron–hole pairs are generated. In the absence of electron acceptors, the transformation of Ag+ into metallic Ag usually takes place during the photocatalytic process. Some previous studies have found that the presence of metallic Ag nanoparticles can increase light adsorption and act as a charge transfer channel to increase the photocatalytic efficiency.2,12,13 However, in our study, too many metallic Ag nanoparticles means that not only the structure is destroyed but also the content of Ag3PO4 decreases, which sequentially is against the photocatalytic degradation of AR18 and stability of Ag3PO4. The presence of Fe3+ in Ag3PO4/Fe–Al/Mt (the redox potential of the Fe3+/Fe2+ pair is 0.77 V),38 means that it can be an electron acceptor during the photocatalytic process to inhibit the

AR18 degradation by Ag3PO4, Ag3PO4/Al/Mt, and Ag3PO4/ Fe–Al/Mt decreased in all the reaction systems as scavengers of the reactive species were added, and the scavengers showed a similar inhibitory effect on AR18 degradation (Fig. S5†). In the presence of i-PrOH (OH˙ scavenger), the decolorization of AR18 was slightly inhibited. However, when NaN3

Fig. 7 The Fe 2p3/2 XPS spectra from Ag3PO4/Fe–Al/Mt before and after reaction.

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3.5 Possible mechanism for the enhanced photocatalytic activity

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Fig. 8 Removal of NBD-Cl (a) and the formation of superoxide radical anion in the process (b).

transformation of Ag+ into Ag (the redox potential of the Ag3PO4/Ag pair is 0.45 V).6,39 According to the results of XRD, SEM-EDS, and XPS from Ag3PO4/Fe–Al/Mt after reaction, Fe3+ can accept photo-induced electrons from Ag3PO4 and be reduced to Fe2+, and then inhibit the transformation of Ag+ into metallic Ag. In summary, Ag3PO4/Fe–Al/Mt is more stable, and more effective in the photodegradation of AR18 than pure Ag3PO4 and Ag3PO4/Al/Mt. From the demonstration above and literature information,40–45 a simple mechanism for Ag3PO4/Fe–Al/Mt as a catalyst in the photoassisted degradation of AR18 is proposed (Fig. 9). First, Ag3PO4 is irradiated by visible light, and electron–hole pairs are generated (eqn (1)). The e− in the CB may facilitate the multiple electron reduction reaction of O2 to form O2˙− (eqn (2)) and H2O2 (eqn (3)), and the h+ in the VB may react with H2O and O2˙−, producing ˙OH (eqn (4)) and O21 (eqn (5)), respectively. When Fe3+ is present, it can accept the photo-induced electrons from Ag3PO4 and be reduced to Fe2+ (eqn (6)), at the same time; Fe2+ can trap the h+ and turn into Fe3+ (eqn (7)). Moreover, the reaction of hydrogen peroxide with Fe3+ is referred to as a Fenton-like reaction (eqn (8)– (10)). Furthermore, free radicals (such as ˙OH, HO2˙, O21, O2˙− or h+) attack AR18, giving rise to reaction intermediates such as adjacent ring structures. With continuous oxidation, the reaction intermediates are finally mineralized into CO2 and H2O.

Fig. 9 Possible photocatalytic mechanism.

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Ag3PO4 + hν (visible light) → e−+h+

(1)

O2 + e− → O2˙−

(2)

O2 + 2H+ + 2e− → H2O2

(3)

H2O + h+ → OH˙ + H+

(4)

O2 ˙ − + h + → O 2 1

(5)

Fe3+ + e− → Fe2+

(6)

Fe2+ + h+ → Fe3+

(7)

Fe2+ + H2O2 → Fe3+ + OH˙ + OH−

(8)

Fe3+ + H2O2 ↔ Fe−OOH2+ + H+

(9)

Fe−OOH2+ → Fe2+ + HO2˙

(10)

HO2˙ → O2˙− + H+

(11)

4. Conclusion Ag3PO4/Fe–Al/Mt and Ag3PO4/Al/Mt composites were synthesized successfully by using Fe–Al/Mt and Al/Mt as host materials to load Ag3PO4, respectively. The obtained Ag3PO4/Fe–Al/ Mt exhibited a much higher catalytic activity and better structural stability than Ag3PO4/Al/Mt and pure Ag3PO4 in degradating AR18 under visible light irradiation. In the photocatalytic process, O2˙− contributed most to the photocatalytic system, followed by 1O2 and OH˙. Interestingly, Fe3+ in Ag3PO4/Fe–Al/Mt could accept photo-induced electrons from Ag3PO4 and be reduced to Fe2+, which subsequently could reduce electron–hole recombination and hinder the

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reduction of Ag+ into metallic Ag particles. As a result, Fe–Al/ Mt as a host material to load Ag3PO4 can improve the photocatalytic performance and structural stability of Ag3PO4.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (41572031, 41322014, 21177104), “One Hundred Talents Program” of CAS (KZZDEW-TZ-10), National Youth Top-notch Talent Support Program Newton Advanced Fellowship (NA150190), Guangdong Provincial Youth Top-notch Talent Support Program (2014TQ01Z249), and CAS/SAFEA International Partnership Program for Creative Research Teams (20140491534).

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