MWCNTs photocatalysts for simultaneous Cr(VI) reduction and orange II degradation under visible light irradiation

Applied Surface Science 353 (2015) 939–948

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Highly effective and stable Ag3 PO4 –WO3 /MWCNTs photocatalysts for simultaneous Cr(VI) reduction and orange II degradation under visible light irradiation Li Cai a,b,∗ , Xiaoli Xiong a , Ninggang Liang a , Qiyi Long a a b

College of Chemistry and Material Science, Sichuan Normal University, Chengdu 610068, China Key Laboratory of Special Waste Water Treatment, Sichuan Province Higher Education System, Chengdu 610068, China

a r t i c l e

i n f o

Article history: Received 17 April 2015 Received in revised form 12 June 2015 Accepted 5 July 2015 Available online 14 July 2015 Keywords: Ag3 PO4 –WO3 MWCNTs Simultaneous Cr(VI) reduction and orange II degradation Visible light

a b s t r a c t A series of high-performance photocatalysts of Ag3 PO4 –WO3 /multi-walled carbon nanotubes (MWCNTs) were fabricated through a deposition–precipitation method. Their structures and physical properties were characterized by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscope (TEM), specific surface analyzer, X-ray diffraction (XRD), UV–vis absorption spectra, photoluminescence spectra (PL) and X-ray photo-electron spectroscopy (XPS). SEM, EDS and TEM analyses verified that Ag3 PO4 –WO3 /MWCNTs composites have been successfully prepared. PL analysis illustrated that Ag3 PO4 –WO3 /MWCNTs have the lower emission peak intensities, compared with Ag3 PO4 and WO3 . By using simultaneous decontaminations of Cr(VI) and orange II as model reactions, the photocatalytic efficiencies of Ag3 PO4 , WO3 and Ag3 PO4 –WO3 /MWCNTs were evaluated. The reaction results showed that Ag3 PO4 –WO3 /MWCNTs have strong photocatalytic activities. The effect of Ag3 PO4 :WO3 ratio on the photocatalytic activity was systemically studied. The catalyst AWM7/3 was found to exhibit the highest photocatalytic activity and excellent chemical stability in repeated and long-term applications. The improvement of photocatalytic activity and stability was mainly attributed to the highly effective separation of photo-generated electron–hole pairs and special transfer pathway of electrons and holes in Ag3 PO4 –WO3 /MWCNTs composites. Therefore, the prepared Ag3 PO4 –WO3 /MWCNTs could act as a high-performance catalyst for the simultaneous decontaminations of Cr(VI) and orange II, and also suggested the promising applications. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The coexistence of heavy metal ion (e.g., Cr(VI), Hg(II), Pb(II)) and organic matter (e.g., azo dyes) in wastewater is a common phenomenon [1]. These pollutants are discharged together in industrial processes such as wood preserving, metal finishing, petroleum refining, leather tanning and finishing, paint and ink formulation, and manufacturing of automobile parts [2]. Among the heavy metal ions, Cr(VI) is one of the most dangerous heavy metals if its concentration is more than the minimum concentration (0.05 mg L−1 ) allowed by the Department of Environment (DOE) [3]. Unlike Cr(VI), Cr(III) is 100 times less toxic than Cr(VI) [4] and its allowable standard limits (0.20 mg L−1 for standard A and 0.10 mg L−1 for standard B) are higher than that of Cr(VI) [3]. Thus if the hazardous

∗ Corresponding author at: College of Chemistry and Material Science, Sichuan Normal University, Chengdu 610068, China. E-mail addresses: [email protected], [email protected] (L. Cai). http://dx.doi.org/10.1016/j.apsusc.2015.07.028 0169-4332/© 2015 Elsevier B.V. All rights reserved.

Cr(VI) can be transformed to the less toxic Cr(III) form, then the pollution problem might be substantially solved [5]. Dyes are generally water-soluble organic colorants, gradually being increased owing to the tremendous increase of industrialization and requirements of human beings for color [6]. But about 15% of the total world production of dyes is lost during the dyeing process and is released [7]. This release of these colored wastewaters in the ecosystem is a dramatic source of non-esthetic pollution, eutrophication and perturbations in the aquatic life [8]. So, an effective treatment is necessary before it is discharged into water body around in case of severe pollution [9]. Many measures including physical methods (as adsorption or reverse osmosis) [10], biological methods (as biodegradation) [11] and chemical methods (as chlorination or ozonation) [12] have been studied to treat Cr(VI) or dyes wastewater. Among them, photocatalysis is found to be one of most effective ways to simultaneously reduce Cr(VI) and degrade organic dye from wastewater [13]. Over the past few decades, the semiconductor photocatalysts, such as TiO2 and ZnO, have attracted much attention originating

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from their promising applications in environment purification and solar energy conversion [14]. Nevertheless, for these wide band gap semiconductor photocatalysts, the main disadvantages of the low quantum yields and lack of visible light utilization greatly limit their performance and large-scale application [15]. Therefore, the synthesis of photocatalysts with the high catalytic activity and efficient visible light photoresponse is still an attractive challenge. Recently, a breakthrough on visible-light-driven photocatalysts was made by Yi et al. [16], who reported the use of Ag3 PO4 as an excellent visible-light-driven photocatalyst for the oxidation of water and degradation of organic contaminants. Bi et al. [17] subsequently reported that Ag3 PO4 can achieve a quantum yield of up to 90% at wavelengths longer than 420 nm, while most photocatalysts show a comparatively low quantum yield (about 20%). Nevertheless, it was also found that Ag3 PO4 always exhibits low structural stability and is easily corroded by the photo-generated electrons (Ag3 PO4 + 3e− → 3Ag0 + PO3− 4 ), which prevent its wide use in environment and energy regions [18]. Fortunately, a few attempts have been made to solve such a problem by surface modification and composite of Ag3 PO4 with different materials, such as Ag3 PO4 –TiO2 [19], Ag3 PO4 –AgX (X = Cl, Br and I) [20], Ag3 PO4 –BiPO4 [21], Ag3 PO4 –g-C3 N4 [22], and Ag3 PO4 –In(OH)3 [23]. These Ag3 PO4 composite photocatalysts exhibit higher photocatalytic activities than that of pure Ag3 PO4 . The redox potentials of conduction band (CB) and valence band (VB) of TiO2 , AgX, BiPO4 , gC3 N4 and In(OH)3 are more negative than those of Ag3 PO4 . It means that under light irradiation the photo-generated electrons could move from the CB of combinational materials to that of Ag3 PO4 , and the photo-generated holes in the VB of Ag3 PO4 could migrate to that of combinational materials, which would effectively avoid the recombination of the electron–hole pairs and promote their photocatalytic activities. However, enriched electrons on CB of Ag3 PO4 would be captured by Ag+ ions, which would accelerate production of metallic Ag and photo-corrosion of Ag3 PO4 . Tungsten trioxide (WO3 ), as an n-type semiconductor material with an indirect band gap of 2.7 eV, is chemically stable in acid media and cannot be further oxidized [24]. WO3 nanoparticles have been widely used in the field of photocatalysis [25]. Especially, the redox potentials (CB and VB) of WO3 are more positive than those of Ag3 PO4 [26]. The transfer processes of photo-generated electrons and holes between Ag3 PO4 and WO3 have been described by Yu [26] and Liu et al. [27]. It implies that under light irradiation the photo-generated electrons could move from the CB of Ag3 PO4 to that of WO3 , and the photo-generated holes in the VB of WO3 could migrate to that of Ag3 PO4 , which avoid the recombination of the electron–hole pairs and accumulation of photo-generated electrons on CB of Ag3 PO4 , and thus increase the stability of Ag3 PO4 . Therefore, WO3 emerges as a promising combinational candidate. In addition, carbon nanotubes (CNTs), as a new type of nanocarbon, possess unique hollow-layered structure and superb conductive property (electronic conductivity: 102 s cm−1 ) [28]. They have displayed excellent performance of photoelectricity and attracted considerable attention [29]. Some composite materials, such as CdS/CNTs [30], ZnO/CNTs [31] and TiO2 /CNTs [32] exhibited dramatically enhanced photocatalytic activities and structural stabilities. Therefore, it gives us a great source of inspiration to synthesize a novel Ag3 PO4 –WO3 /multi-walled carbon nanotubes (MWCNTs) photocatalyst, which has the high photocatalytic activity and excellent long-term stability. In this paper, we reported that a series of novel Ag3 PO4 –WO3 /MWCNTs photocatalysts were prepared and characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscope (TEM), specific surface analyzer, X-ray diffraction (XRD), UV–vis absorption spectra, photoluminescence spectra (PL) and X-ray photo-electron spectroscopy (XPS). Simultaneous

decontaminations of Cr(VI) and orange II were chosen as probe reactions to estimate the photocatalytic activities and structural stabilities of Ag3 PO4 –WO3 /MWCNTs under visible light irradiation. 2. Experimental 2.1. Materials Multi-walled carbon nanotubes (MWCNTs >95%) with a mean outside diameter of 20–40 nm and length of 20–30 ␮m were purchased from Alpha Nano Technology Co., Ltd. (Chengdu, China). All reagents except MWCNTs used in this study were of analytical grade without further purification and supplied by Kelong Chemical Reagent Ltd. (Chengdu, China). 2.2. Purification of MWCNTs and preparation of WO3 /MWCNTs MWCNTs were purified by the method reported previously [33]. In brief, 1.0 g of MWCNTs were placed in a flask containing 100 mL of HNO3 by stirring and refluxing at 120 ◦ C for 4 h. After washing with deionized water and filtration, the purified MWCNTs were dried at 80 ◦ C under vacuum for 10 h and stored in a desiccator for further use. WO3 /MWCNTs were synthesized by a simple hydrothermal process. Sodium tungstate dehydrate (Na2 WO4 ·2H2 O, 0.3299 g) was dissolved in distilled water (15 mL), then MWCNTs (0.1 g) were ultrasonically dispersed in the Na2 WO4 solution. Afterwards, nitric acid (10 mL) was added dropwise to the above solution under vigorous magnetic stirring at room temperature. After stirring for about 20 min, the mixture was transferred into a 50 mL Teflon-line stainless-steel autoclave. The autoclave was sealed and maintained at 180 ◦ C for 12 h and then cooled to room temperature naturally. The gray product was collected after centrifuging, washing for several times with distilled water and absolute ethanol, and drying at 60 ◦ C for 12 h, then calcined at 500 ◦ C under vacuum for 2 h. WO3 was also synthesized as the above procedure without adding MWCNTs. 2.3. Preparation of Ag3 PO4 –WO3 /MWCNTs The Ag3 PO4 –WO3 /MWCNTs photocatalysts with different mass ratios of Ag3 PO4 to WO3 were prepared by a deposition–precipitation process. Firstly, WO3 /MWCNTs were dispersed in distilled water, and then the required amount of AgNO3 solution (0.10 M) and Na2 HPO4 solution (0.15 M) were successively dropped into this suspension under stirring. The reaction was conducted at 25 ◦ C with constant stirring for 4 h in the dark. Then the obtained precipitate was washed with distilled water for several times and dried in vacuum at 60 ◦ C for 12 h. According to the mass ratio of Ag3 PO4 to WO3 , the as-synthesized Ag3 PO4 –WO3 /MWCNTs photocatalysts were marked as AWM5/5 , AWM6/4 , AWM7/3 , AWM8/2 and AWM9/1 , respectively. In additional, to prove the advantage of MWCNTs in Ag3 PO4 –WO3 /MWCNTs photocatalysts, Ag3 PO4 –WO3 (the mass ratio of Ag3 PO4 to WO3 : 7:3) was synthesized as the above procedure without adding MWCNTs and marked as AW7/3 . For comparison, Ag3 PO4 was also synthesized as follows: 20 mL of 0.10 M AgNO3 was added dropwise to 4.5 mL of 0.15 M Na2 HPO4 aqueous solution, and then reacted by magnetic stirring for 4 h in the dark. Subsequent processing of generated yellow precipitate of Ag3 PO4 was the same as the procedure of Ag3 PO4 –WO3 /MWCNTs. 2.4. Characterizations A JEOL Ltd. JSM-5900LV scanning electron microscope (SEM) was used to characterize the surface morphologies of Ag3 PO4 ,

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WO3 , MWCNTS and Ag3 PO4 –WO3 /MWCNTs, while energydispersive X-ray spectroscopic (EDS) study was performed to characterize the chemical composition of AWM7/3 . Transmission electron microscopy (TEM) images of AWM7/3 were collected on a Hitachi H-800 transmission electron microscope. The Brunauer–Emmett–Teller (BET) specific surface areas (SBET ) of Ag3 PO4 –WO3 /MWCNTs were determined by using the nitrogen adsorption approach (Micromeritics ASAP 2010, nitrogen absorption apparatus). XRD (Philips X’ Pert Pro-MPD) studies were performed to identify the formation of crystal phase by using Cu K X-radiation ( = 0.154 nm). The UV–vis absorption spectra were recorded using UV–vis–NIR spectrophotometer with an integrating sphere (DUV-3700, Shimadzu, Japan), which BaSO4 was used as a reference. The photoluminescence spectra (PL) of samples were recorded with a PE LS 55 spectrofluorophotometer with an excitation wavelength at 250 nm. X-ray photo-electron spectroscopy (XPS) measurements were performed on a PHI5000 Versa Probe electron spectrometer using Al K␣ radiation (ULVAC-PHI, Japan) to identify the elemental chemical states. 2.5. Photocatalytic decontaminations of Cr(VI) and orange II Photocatalytic decontaminations of Cr(VI) and orange II were conducted in a self-designed cylindrical reactor (1 L) which was made of quartz glass. Visible light source was a 150 W halogen lamp and a glass optical filter was inserted to cut off short wavelength components ( < 420 nm). The reactor was well aerated and stirred with a magnetic stirrer to ensure sufficient mixing. The reaction suspensions were prepared by adding the photocatalyst (0.5 g) into 500 mL aqueous solutions containing Cr(VI) (50 mg L−1 ) and orange II (35 mg L−1 ), followed by adjusting pH to pH 2.0. Prior to irradiation, the suspensions were first sonicated for 10 min to eliminate aggregate, and then magnetically stirred in the dark for 20 min to ensure the establishment of adsorption–desorption equilibrium of the substances on the catalyst surface. The reaction was started by switching on the light and 5 mL of reaction solution was sampled at an indicated interval for Cr(VI) and orange II analyses. The Cr(VI) concentration in the supernatant solution was determined colorimetrically at 540 nm using diphenylcarbazide as a color agent. The concentration of orange II was determined through its absorption maximum band (484 nm) using a UV–vis spectrophotometer (UV-2501PC Shimadzu) and the total organic carbon (TOC) was measured by a TOC analyzer (Takmar Dohrmamn Apollo 9000). The reduction conversion (%) of Cr(VI), and the degradation conversion (%) and mineralization conversion (%) of orange II were used to characterize the photocatalytic efficiency. 3. Result and discussion 3.1. SEM, EDS, TEM and surface area analyses The surface morphologies and microstructural details of the as-synthesized photocatalysts were examined by SEM, EDS and TEM measurements, and the results are shown in Figs. 1 and 2. It could be seen from Fig. 1a,b that the obtained Ag3 PO4 is sphericallike with a particle size of 100–200 nm, while WO3 is composed of a large number of nanoplates which are about 100–170 nm in side length and 30–50 nm in thickness. Fig. 1c shows the SEM image of MWCNTs. From Fig. 1c, MWCNTs are of highly ordered structure with approximately 20–40 nm in diameter. SEM image of the WO3 /MWCNTs is presented in Fig. 1d. It could be found from Fig. 1d that WO3 particles are densely dispersed on MWCNTs. For the Ag3 PO4 –WO3 /MWCNTs samples, as shown in Fig. 1e–i, the spherical-like Ag3 PO4 particles with a diameter of 100–200 nm are

Fig. 1. SEM images of (a) Ag3 PO4 , (b) WO3 , (c) MWCNTs, (d) WO3 /MWCNTs, (e) AWM5/5 , (f) AWM6/4 , (g) AWM7/3 , (h) AWM8/2 , (i) AWM9/1 and (j) EDS image of AWM7/3 .

dispersed on the surface of plate-like WO3 and tubular-like MWCNTs. With growing the amount of Ag3 PO4 in the composite samples, more and more Ag3 PO4 particles exist on the surface of WO3 and

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Fig. 2. (a) TEM and (b) HRTEM images of AWM7/3 .

Table 1 Surface areas of Ag3 PO4 –WO3 /MWCNTs. BET surface area (m2 g−1 )

AWM5/5 AWM6/4 AWM7/3 AWM8/2 AWM9/1

43 42 40 38 35

MWCNTs. Moreover, EDS pattern indicates that the AWM7/3 contain the elements of Ag, P, W, O and C (as shown in Fig. 1j). Fig. 2a presents TEM image of the AWM7/3 . It could be seen that the tubular materials are MWCNTs, and the dark regions are due to the Ag3 PO4 and WO3 . A representative high resolution TEM (HRTEM) image is shown in Fig. 2b. The two types of lattice fringes are clearly visible with a spacing of about 0.264 and 0.37 nm, which correspond to the (2 1 0) lattice spacing of Ag3 PO4 and (2 0 0) lattice spacing of WO3 , respectively [34,35]. Therefore, the above analyses verify that the Ag3 PO4 –WO3 /MWCNTs composite have been successfully prepared. In additional, a lattice fringe with a spacing of 0.231 nm, corresponding to the (1 1 1) crystalline plane of metallic Ag [27], is also showed in Fig. 2b. It suggests that the metallic Ag could be generated on the surface of Ag3 PO4 during the TEM analysis. The surface areas of Ag3 PO4 –WO3 /MWCNTs were further analyzed by means of N2 adsorption techniques. The results are summarized in Table 1. The surface areas of Ag3 PO4 –WO3 /MWCNTs are between 43 and 35 m2 g−1 , and slightly decrease with increasing the amount of Ag3 PO4 . 3.2. XRD analysis In order to determine the crystal phase composition of photocatalysts, the XRD analysis was carried out. Fig. 3 shows the XRD patterns of the as-synthesized Ag3 PO4 –WO3 /MWCNTs composites. For comparison, the XRD patterns for pure Ag3 PO4 , WO3 and MWCNTs are also given. For the pattern of pure Ag3 PO4 , all of the diffraction peaks could be clearly identified to the cubic phase of Ag3 PO4 (JCPDS card No. 06-0505) [36]. Fig. 3a shows nine characteristic diffraction peaks of Ag3 PO4 with 2 values at approximately 21◦ , 29◦ , 33◦ , 37◦ , 47◦ , 52◦ , 55◦ , 57◦ and 62◦ belonging to (1 1 0), (2 0 0), (2 1 0), (2 1 1), (3 1 0), (2 2 2), (3 2 0), (3 2 1) and (4 0 0) crystal planes, respectively [36]. For the pure WO3 , as shown in Fig. 3b, the diffraction peaks of (0 0 1), (0 2 0), (2 0 0), (1 2 0), (1 1 1), (2 0 1),

(100)

(h) (g) Intensity (a.u.)

Sample

(002)

(f) (e)

(200)

(d)

(020) (001) (110)

(201) (220) (120)(111)

(200)

(c) (221)

(211)

(420) (320) (310) (222) (321) (400)

(210) 20

30

40

50

(b) (a)

60

2 theta(degree) Fig. 3. XRD patterns of photocatalysts: (a) Ag3 PO4 , (b) WO3 , (c) AWM5/5 , (d) AWM6/4 , (e) AWM7/3 , (f) AWM8/2 , (g) AWM9/1 , (h) MWCNTs.

(2 2 0), (2 2 1) and (4 2 0) can be attributed to the orthorhombic phase WO3 (JCPDS No. 20-1324) [34]. The XRD patterns of the Ag3 PO4 –WO3 /MWCNTs composites are presented in Fig. 3c–g. The major diffraction peaks of Ag3 PO4 and WO3 are clearly observed, and the diffraction peak (2 0 1) of WO3 and the strongest diffraction peak (2 1 0) of Ag3 PO4 overlap each other. With increasing the amount of Ag3 PO4 , the intensities of Ag3 PO4 diffraction peaks improve remarkably, while the intensities of WO3 diffraction peaks gradually decrease, even disappear. There are no additional diffraction peaks observed in the XRD patterns of Ag3 PO4 –WO3 /MWCNTs composites. Fig. 3h shows the XRD pattern of MWCNTs, indicating that the MWCNTs clearly display a primary peak at around 26◦ , which can be indexed to the reflection (0 0 2) of graphite, and the next peak at 42◦ corresponding to the (100) plane of graphite [37]. Nevertheless, the apparent diffraction peaks of MWCNTs are not discerned in the patterns of the Ag3 PO4 –WO3 /MWCNTs composites, which is due to the trace amount of the loaded MWCNTs. 3.3. UV–vis absorption spectra analysis UV–vis absorption spectra were carried out to investigate the optical properties of the samples. Fig. 4 depicts the UV–vis absorption spectra of Ag3 PO4 , WO3 , MWCNTs and Ag3 PO4 –WO3 /

L. Cai et al. / Applied Surface Science 353 (2015) 939–948

1.0

Absorbance (a.u)

transition of WO3 [40]. For the Ag3 PO4 , the emission centered at around 460 nm arises from the charge transfer between the O2p orbital and the empty d orbital of Ag+ [41]. The PL spectra of Ag3 PO4 –WO3 /MWCNTs are similar to that of Ag3 PO4 , but their peak intensities are lower than those of pure Ag3 PO4 and WO3 at similar emission position. Especially, AWM7/3 exhibits the weakest PL intensity, indicating that AWM7/3 might have the lowest recombination rate of photo-generated electrons and holes. The one of main reasons might be ascribed to the matched energy band structures of Ag3 PO4 and WO3 . In addition, the other possible reason is that MWCNTs exhibit a large electron storage capacity (one electron for every 32 carbon atoms) [42]. The photo-generated electrons in CB of Ag3 PO4 and WO3 may quickly retransfer to the surface of the MWCNTs to participate in the photocatalytic reaction, thus decrease the probability of photo-generated electron–hole pairs recombination.

MWCNTs

0.8

AWM5/5 WO3

AWM6/4

0.6

0.4

AWM9/1

0.2

AWM8/2

AWM7/3

Ag3PO4 0.0 400

500

600

Wavelength (nm)

700

943

800

3.5. Visible light catalytic activities of the samples Fig. 4. UV–vis absorption spectra of Ag3 PO4 , WO3 , MWCNTs and Ag3 PO4 –WO3 / MWCNTs.

WO3 Ag3PO4 AWM5/5 Intensity (a.u)

AWM9/1

300

AWM6/4 AWM8/2 AWM7/3

350

400

450

500

550

600

650

Wavelength (nm) Fig. 5. PL spectra of Ag3 PO4 , WO3 , AWM5/5 , AWM6/4 , AWM7/3 , AWM8/2 , AWM9/1 .

MWCNTs. In Fig. 4, the absorption edge wavelength of WO3 is less than 460 nm, and indirect band gap is around 2.7 eV. Ag3 PO4 could absorb solar energy with a wavelength shorter than 530 nm, and indirect band gap is about 2.45 eV. The black MWCNTs show a broad absorbance in the visible range. As presented in Fig. 4, the absorption edges of Ag3 PO4 –WO3 /MWCNTs are gradually redshifted with the increasing Ag3 PO4 amount. 3.4. PL analysis Generally, in the photocatalytic process of semiconductors, the photocatalytic efficiency depends on the fate of photo-generated hole–electron pairs under light irradiation [38]. Recently, PL spectra have been widely used to investigate the efficiency of charge carrier trapping, migration, and transfer in efforts to understand the fate of electron–hole pairs in semiconductors [39]. Herein, PL emission spectra of Ag3 PO4 , WO3 , and Ag3 PO4 –WO3 /MWCNTs composites were recorded with an excitation wavelength at 250 nm, which is shown in Fig. 5. It is found that pure WO3 has five blue luminescence emission peaks at about 450, 467, 481, 491 and 550 nm. The strong emission peak at about 467 nm can be ascribed to the recombination of electron–hole pairs excited on the surface of WO3 [40]. Four minor emission peaks at about 450, 481, 491 and 550 nm correspond to the surface defects and indirect band–band

The photocatalytic activities of Ag3 PO4 , WO3 and Ag3 PO4 –WO3 /MWCNTs were evaluated by comparing the reduction conversions of Cr(VI) and degradation conversions of orange II under visible-light irradiation (as shown in Fig. 6a,b). Before irradiation, Cr(VI) and orange II mixed solutions with different photocatalysts were magnetically stirred in dark condition for 20 min to achieve the adsorption–desorption equilibrium. Adsorption tests show that 12–15% of Cr(VI) and 13–19% of orange II are adsorbed on Ag3 PO4 –WO3 /MWCNTs. The adsorption conversions are much higher than those of Ag3 PO4 (4% and 8%) and WO3 (2% and 5%). It is well known that prior to the degradation process, the pollutants must be adsorbed on photocatalysts [43]. For this reason, pollutant adsorption is of primary importance in the photocatalytic process and the higher adsorption suggests the higher degradation conversion [43]. The photolysis of Cr(VI) and orange II is not observed without photocatalyst under visible light irradiation for 30 min, indicating that Cr(VI) and orange II are stable and difficult to be decontaminated in the absence of photocatalyst. From Fig. 6a,b, it could be found that the Ag3 PO4 –WO3 /MWCNTs photocatalysts exhibit excellent activities for simultaneous Cr(VI) reduction and orange II degradation. Especially for AWM7/3 , the reduction conversion of Cr(VI) and degradation conversion of orange II are up to 98% and 99%, respectively. However, only 42% and 29% of Cr(VI), and 52% and 38% of orange II could be removed within the same time using pure Ag3 PO4 and WO3 as photocatalyst, respectively. For pure Ag3 PO4 , the slightly lower activity could be ascribed to the relatively higher recombination of electron–hole pairs. The significantly lower catalytic activity of WO3 is mainly attributed to the more positive CB potential of WO3 compared with that of the single-electron reduction of O2 to • O2 − [39], as well as its absorption edge (∼460 nm) which would lead to the less absorption of visible light [40]. Owing to transfer channels design of photo-generated electrons and holes between Ag3 PO4 and WO3 , the catalytic activity of AM7/3 (reduction conversions of Cr(VI): 56%, degradation conversions of orange II: 58%) is slightly higher those of pure Ag3 PO4 and WO3 , but still lower than those of Ag3 PO4 –WO3 /MWCNTs. These results indicate that MWCNTs display excellent performance of photoelectricity and could dramatically enhance photocatalytic activity. The notably enhanced photocatalytic performances of Ag3 PO4 –WO3 /MWCNTs are mainly due to the highly effective separation of photo-generated electron–hole pairs. In addition, one may note that among these Ag3 PO4 –WO3 /MWCNTs, AWM7/3 exhibits the highest catalytic activity for Cr(VI) reduction and orange II degradation. The possible reason is that there is more remarkable synergistic effect when the mass ratio of Ag3 PO4 :WO3 is 7:3.

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(b)

(a) Reduction conversion of Cr(IV)(%)

100

Degradation conversion of orange II(%)

without photocatalyst WO3 Ag3PO4 AW7/3

80

Light on

AWM5/5 AWM6/4

60

AWM7/3 AWM8/2

40

AWM9/1 Dark

20

0 -20

-10

without photocatalyst WO3

100

0

10

20

Ag3PO4

Light on

AW7/3

80

AWM5/5 AWM6/4

60

AWM7/3 AWM8/2 AWM9/1

40

Dark 20

0

30

-20

-10

(c) Mineralization conversion of orange II(%)

0

10

20

30

Time (min)

Time (min)

without photocatalyst WO3 Ag3PO4

70 60 50

AW7/3

AWM5/5

AWM6/4

AWM7/3

AWM8/2

AWM9/1

40 30 20 10 0 -10 0

5

10

15

20

25

30

Time (min) Fig. 6. The simultaneous (a) reduction conversions of Cr(VI), (b) degradation conversions and (c) mineralization conversions of orange II under visible light irradiation using as-prepared photocatalysts.

At the other hand, the photocatalytic studies of Cr(VI) and orange II alone were also carried out (as shown in Fig. S1). For AWM7/3 , the reduction conversion of Cr(VI) and degradation conversion of orange II alone are high up to 95% and 96%, respectively, which are also much higher than those of pure Ag3 PO4 (37% and 47%) and WO3 (24% and 32%). Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2015.07. 028 It has been widely reported that some degradation intermediates products are more toxic and carcinogenic than the parent organic matter [44]. Prior to wastewater discharge, if organic matters are mineralized by photocatalysts, it is highly desirable [33]. Although it is not easy to achieve a high mineralization conversion during the photocatalytic degradation, especially under visible light irradiation, 70% of the mineralization conversion of orange II is still obtained for 30 min over AWM7/3 . The mineralization conversion is much higher than those of pure Ag3 PO4 , WO3 and AW7/3 (as shown in Fig. 6c). It further demonstrates that the combination of Ag3 PO4 , WO3 and MWCNTs could dramatically improve photocatalytic performance.

3.6. Cycle experiment and stability Besides photocatalytic activity, the stability of photocatalysts is also very important for practical application. To compare the stability of the as-prepared photocatalysts, it conducted the repeatability experiments of Cr(VI) reduction and orange II degradation over Ag3 PO4 and AWM7/3 and the results are shown in Fig. 7. From Fig. 7, it can be seen that after visible light irradiation of 30 min, the reduction conversions of Cr(VI) have been degraded from 42% to 23%, and the degradation conversions of orange II have been degraded from 53% to 27% in five repeated cycles, indicating a loss of about 45–49% of the photocatalytic efficiency in the Ag3 PO4 . In contrast, Fig. 7 also shows that in the presence of AWM7/3 , the reduction conversions of Cr(VI) show a decrease of 98%, 95%, 90%, 85% and 83%, and the degradation conversions of orange II present a decrease of 99%, 96%, 93%, 89% and 88%, which the variations are about 11–15%. In addition, the used AWM7/3 shows the same color as the newly prepared catalyst, while the color of used Ag3 PO4 has been changed from yellow to black. These results suggest that the Ag3 PO4 –WO3 / MWCNTs appear to be more desirable in repeated and long-term applications.

L. Cai et al. / Applied Surface Science 353 (2015) 939–948

(a)

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(b)

Fig. 7. Long-term catalytic stability of Ag3 PO4 and AWM7/3 in repetitive experiments of (a) Cr(VI) reduction and (b) orange II degradation.

(a )

(b ) Ag(111) Ag(200)

Ag(220)

5 th

Ag(111) Ag(200)

3 rd Ag(111)

Intensity (a.u.)

Intensity (a.u.)

Ag(111)

5 th 3 rd

1 st

1 st 20

30

40

50

60

2 theta(degree)

20

30

40

50

60

2 theta(degree)

Fig. 8. XRD patterns of (a) Ag3 PO4 and (b) AWM7/3 after the first, third and fifth cycle experiments.

To evaluate the structural stability, the crystalline structures of Ag3 PO4 and AWM7/3 after the first, third and fifth cycle experiments were studied (as shown in Fig. 8). Fig. 8a displays that a diffraction peak (1 1 1) of metallic silver appears in the XRD pattern of Ag3 PO4 after the first cycle experiment and three obvious diffraction peaks (1 1 1), (2 0 0) and (2 2 0) of metallic silver appear in Ag3 PO4 after the fifth successive experimental run. In contrast, Fig. 8b shows that no evident crystalline structure changes could be observed in the XRD pattern of AWM7/3 after the first and third successive experimental runs. The one weak diffraction peak (1 1 1) of metallic silver appears in the XRD pattern of AWM7/3 after the fifth cycle experiment. These results clearly indicate that structural stability of AWM7/3 is significantly better than that of the Ag3 PO4 . The Ag chemical statuses of Ag3 PO4 and AWM7/3 after fifth cycle experiment were further investigated by XPS, as shown in Fig. 9. Fig. 9a,b show the Ag3d XPS spectra of used Ag3 PO4 and AWM7/3 . The peaks near 368 and 374 eV are attributed to Ag3d5/2 and Ag3d3/2 , respectively, each of which could be fitted to two separate peaks corresponding to Ag0 and Ag+ ions [21]. For Ag3 PO4 , as shown in Fig. 9a, the peaks at 374.0 and 367.9 eV could be attributed to Ag0 , and the peaks at 374.8 and 368.8 eV could be assigned to Ag+ ions [45]. Fig. 9b shows two peaks of AWM7/3 at 374.1 and 368.5 eV belonging to Ag0 , and other two peaks at 374.2 and 368.6 eV ascribing to Ag+ ions. The slight peak shifts are mainly

attributed to the interaction between WO3 and Ag3 PO4 . The calculated mole contents of Ag0 are 50.2% and 10.3% of total silver element on the surface of Ag3 PO4 and AWM7/3 after fifth cycle experiment, respectively. It indicates that much less metallic Ag is formed on the surface of AWM7/3 and photo-corrosion of Ag3 PO4 could be significantly hindered by introducing WO3 and MWCNTs. 3.7. Analysis of active species To investigate the mechanism of the photocatalytic degradation by Ag3 PO4 –WO3 /MWCNTs, the influences of active species such as hole (h+ ), hydroxyl radical (• OH) and superoxide radical (• O2 − ) in the photocatalytic process were explored. Different scavengers were employed individually to remove the corresponding active species so that the function of different active species in the degradation process is understood. The scavengers used in this study are potassium iodide (KI) for h+ , n-butyl alcohol for • OH and benzoquinone for • O2 − [46]. The results are shown in Fig. 10. The reduction conversion of Cr(VI) and degradation conversion of orange II are 98% and 99% without scavengers, respectively. When potassium iodide, n-butyl alcohol and benzoquinone are added, there are not significant changes in the reduction conversions of Cr(VI). The possible reason is that the Cr(VI) could be directly reduced to Cr(III) by photo-generated electrons [47], and the

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Fig. 9. Ag3d XPS spectra of (a) Ag3 PO4 and (b) AWM7/3 after fifth cycle experiment.

Fig. 10. Effects of scavengers on the (a) Cr(VI) reduction and (b) orange II degradation using AWM7/3 as a photocatalyst.

influences of h+ , • OH and • O2 − could be negligible. The degradation conversions of orange II are 21%, 92% and 95% with potassium iodide, n-butyl alcohol and benzoquinone, respectively, which indicates that hole is the main active specie in the degradation of orange II, and • OH and • O2 − can be ignored in the reaction.

3.8. The possible photocatalytic mechanism of Ag3 PO4 –WO3 /MWCNTs The photocatalytic mechanism is shown in Fig. 11. From Fig. 11, it could be seen that under the visible light irradiation, electrons in both Ag3 PO4 and WO3 would be excited from the VB to CB. The CB (+0.45 eV) and VB (+2.9 eV) of Ag3 PO4 are more negative than those of the WO3 (CB: +0.64 eV, VB: +3.34 eV) [26]. Therefore, the photo-generated electrons in the Ag3 PO4 could be easily transferred to the CB of WO3 , while the photo-generated holes in the WO3 could migrate to the VB of Ag3 PO4 . Thus the orange II adsorbed on Ag3 PO4 –WO3 /MWCNTs could be directly oxidized by the holes on Ag3 PO4 surface [27]. At the same time, due to the good conductivity of MWCNTs, the electrons in the CB of WO3 and Ag3 PO4 could also be easily transferred to the surface of MWCNTs to participate in the photocatalytic reaction. In other words, Ag3 PO4 –WO3 and MWCNTs act as an electron donor and an electron acceptor, respectively, in the Ag3 PO4 –WO3 /MWCNTs system [33], which accelerates the electrons migration rate, promotes the effective separation of photo-generated electron–hole pairs and improves photocatalytic performance of the catalyst.

Fig. 11. The proposed photocatalytic mechanism diagram of Ag3 PO4 –WO3 / MWCNTs.

This coincided with the results of PL analysis (as shown in Fig. 5) and photocatalytic activity analysis (as shown in Fig. 6). Furthermore, the electrons’ accumulation in the CB of Ag3 PO4 could be effectively avoided [48], and the decomposition and photo-corrosion of Ag3 PO4 could be significantly hindered, because photo-generated electrons in the Ag3 PO4 could be fleetly transferred to WO3 and MWCNTs, and consumed through a reduction reaction with Cr(VI) (Cr2 O7 2− + 14H+ + 6e− → 2Cr3+ + 7H2 O) [5]. Therefore, the Ag3 PO4

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could keep the original structure and Ag3 PO4 –WO3 /MWCNTs have outstanding stability (as shown in Figs. 7–9). 4. Conclusions In this study, a series of Ag3 PO4 –WO3 /MWCNTs photocatalysts were successfully prepared with different Ag3 PO4 :WO3 mass rations. Remarkably, the application in simultaneous decontaminations of Cr(VI) and orange II demonstrates that Ag3 PO4 –WO3 /MWCNTs photocatalysts have strong photocatalytic activity and excellent chemical stability in repeated and longterm applications. Among the series of Ag3 PO4 –WO3 /MWCNTs photocatalysts, AWM7/3 shows the highest catalytic activity, and the reduction conversion of Cr(VI) and degradation conversion of orange II are high up to 98% and 99% under visible light irradiation for 30 min, respectively. Acknowledgements This research was financially supported by National Science Foundation for Distinguished Young Scholars (Grant No.41102220), Scientific Research Fund of Sichuan Provincial Education Department, China (Grant No.15ZB0024 and 14ZA0027) and the Foundation of Key Laboratory of Special Waste Water Treatment, Sichuan Province Higher Education System (Grant No. SWWT2014-2). References [1] F. Hashemzadeh, A. Gaffarinejad, R. Rahimi, Porous p-NiO/n-Nb2 O5 nanocomposites prepared by an EISA route with enhanced photocatalytic activity in simultaneous Cr(VI) reduction and methyl orange decolorization under visible light irradiation, J. Hazard. Mater. 286 (2015) 64–74. [2] J.W. Patterson, Industrial Wastewater Treatment Technology, 2nd ed., Butterworth Publishers, Stoneham, MA, 1985. [3] A. Idris, E. Misran, N.M. Yusof, Photocatalytic reduction of Cr(VI) by PVA-alginate encapsulated Fe2 O3 magnetic beads using different types of illumination lamp and light, J. Ind. Eng. Chem. 18 (2012) 2151–2156. [4] J. Yoon, E. Shim, S. Bae, H. Joo, Application of immobilized nanotubular TiO2 electrode for photocatalytic hydrogen evolution: reduction of hexavalent chromium (Cr(VI)) in water, J. Hazard. Mater. 161 (2009) 1069–1074. [5] Q. Sun, H. Li, S.L. Zheng, Z.M. Sun, Characterizations of nano-TiO2 /diatomite composites and their photocatalytic reduction of aqueous Cr (VI), Appl. Surf. Sci. 311 (2014) 369–376. [6] S.V. Mohan, N.C. Rao, K.K. Prasad, J. Karthikeyan, Treatment of simulated Reactive Yellow 22 (Azo) dye effluents using Spirogyra species, Waste Manag. 22 (2002) 575–582. [7] A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard, J.M. Herrmann, Photocatalytic degradation pathway of methylene blue in water, Appl. Catal. B: Environ. 31 (2001) 145–157. [8] F.R. Araújo, J.G. Baptista, L. Marc, K.J. Ciuffia, E.J. Nassar, P.S. Calefi, M.A. Vicenteb, R. Trujillano, V. Rivesb, A. Gil, S. Korili, E.H. de Faria, Versatile heterogeneous dipicolinate complexes grafted into kaolinite: catalytic oxidation of hydrocarbons and degradation of dyes, Catal. Today 227 (2014) 105–115. [9] M.E. Olya, A. Pirkarami, Cost-effective photoelectrocatalytic treatment of dyes in a batch reactor equipped with solar cells, Sep. Purif. Technol. 118 (2013) 557–566. [10] M.T. Yagub, T.K. Sen, S. Afroze, H.M. Ang, Dye and its removal from aqueous solution by adsorption: a review, Adv. Colloid Interface Sci. 209 (2014) 172–184. [11] E.J.R. Almeida, C.R. Corso, Comparative study of toxicity of azo dye Procion Red MX-5B following biosorption and biodegradation treatments with the fungi Aspergillus niger and Aspergillus terreus, Chemosphere 112 (2014) 317–322. [12] T. Mano, S. Nishimoto, Y. Kameshima, M. Miyake, Water treatment efficacy of various metal oxide semiconductors for photocatalytic ozonation under UV and visible light irradiation, Chem. Eng. J. 264 (2015) 221–229. [13] H.L. Liu, Y. Zhou, H.Y. Huang, Y.Y. Feng, Phthalic acid modified TiO2 and enhanced photocatalytic reduction activity for Cr(VI) in aqueous solution, Desalination 278 (2011) 434–437. [14] É. Karácsonyi, L. Baiab, A. Dombi, V. Danciuc, K. Mogyorósi, L.C. Pop, G. Kovács, V. Cosoveanuc, A. Vulpoi, S. Simon, Zs. Pap, The photocatalytic activity of TiO2 /WO3 /noble metal (Au or Pt) nanoarchitectures obtained by selective photodeposition, Catal. Today 208 (2013) 19–27.

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