attapulgite nanocomposite

Applied Clay Science 67–68 (2012) 11–17

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Research paper

Preparation and photocatalytic properties of visible light driven Ag\AgBr/attapulgite nanocomposite Yanqing Yang, Gaoke Zhang ⁎ School of Resources and Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China

a r t i c l e

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Article history: Received 20 March 2012 Received in revised form 26 June 2012 Accepted 27 June 2012 Available online 30 August 2012 Keywords: Ag\AgBr/attapulgite Photocatalyst Visible light Rhodamine B Photoactive radicals

a b s t r a c t An efficient visible light photocatalyst of Ag\AgBr/attapulgite nanocomposite was prepared via the reaction between Ag+ and Br- ions in the presence of attapulgite and then reducing partial Ag+ ions to Ag0 species via the light-induced chemical reduction. Characterization methods, such as X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV–visible diffused reflectance spectroscopy (UV–vis DRS) were employed to study the phase structure, chemical state and optical properties of the nanocomposite. The Ag\AgBr/attapulgite nanocomposite exhibited an efficient photocatalytic activity for the degradation of Rhodamine B (RhB) aqueous solution under visible light radiation (λ > 400 nm). In addition, the photoactive radical species involved in the degradation reaction have been investigated by using the HO‐trapping photoluminescence (PL) spectra and quenching experiments. A possible photodegradation mechanism of RhB dye by the Ag\AgBr/attapulgite nanocomposite was postulated. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Environmental contaminations caused by wastewater become a serious problem. Dye wastewater especially resulting from textile industries is hard to treat, as is toxic and mostly nonbiodegradable. Traditional methods of wastewater treatment such as coagulating sedimentation, electro-coagulation, filtration and adsorption, are ineffective for removing the dyes and have several disadvantages (Chen and Ray, 2001). As such, it is important to find new effective approaches to treat wastewater and to decrease environmental pollution. Photocatalysis technologies as promising methods of water and air cleaning have attracted extensive attentions in recent years. Among various photocatalytic materials, considerable efforts were focused on the development of TiO2 due to its low cost, chemical stability and nontoxicity (Leghari et al., 2011; Xu et al., 1999; Zeng et al., 2011). However, its photocatalytic activity requires ultraviolet light (λ b 400 nm), which only accounts for a small fraction of the solar spectrum (b4%). In order to extend the absorption band-edge of TiO2 from UV to visible light region, some kinds of surface modification methods have been explored, such as metal doping, nonmetal doping and ion-implanting (Choi et al., 1994; Khan et al., 2002; Sakthivel and Kisch, 2003; Zhao et al., 2004). Despite extensive efforts, improving the photocatalytic activity of TiO2 has met only a limited success. Therefore, it is critical and promising to exploit new type of photocatalysts with high photocatalytic performance under visible light radiation. ⁎ Corresponding author. Tel.: +86 27 87651816; fax: +86 27 87887445. E-mail address: [email protected] (G. Zhang). 0169-1317/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2012.06.013

Silver bromide, which has a bandgap of 2.6 eV, is a photo-sensitive compound extensively used as a source material in photographic films. Recently, there have been reports on the use of AgBr as a photoactive component in several catalysts. AgBr dispersed on Al-MCM-41 showed high visible light activity for the decomposition of acetaldehyde in gas phase (Rodrigues et al., 2005). Moreover, noble metal nanoparticles have been applied as active components for the preparation of various efficient visible light photocatalyst due to their unique localized surface plasmon resonance (SPR) (Pourahmad et al., 2010; Wang et al., 2011a). In particular, Ag nanoparticles show efficient plasmon resonance in the visible region, which have been investigated in contact with different semiconductors. Hu et al. (2006a) found that the Ag/AgBr/TiO2 catalyst has an efficient destruction of pathogenic bacteria under visible light irradiation, and their evidence indicates that AgBr is the visible-light active component of the catalyst and that Ag0 species on the surface of the catalyst is probably contributing to enhancing the electron–hole separation and interfacial charge transfer. On the other hand, clay minerals supported photocatalysts have been widely investigated. The natural clay minerals are available at low cost and high chemical and mechanical stability. They are particularly attractive because they possess large surface areas, which is beneficial for organic compounds to reach and leave the active sites on the surface. Attapulgite, a hydrated magnesium aluminum silicate mineral [Si8(Al,Mg,Fe)5O20(OH)2(H2O)4·4H2O] with commonly a lath or fibrous morphology, is cheap, chemically inert and environmentally friendly (Melo et al., 2002). Attapulgite consists of two double chains of the pyroxene-type (SiO3)2− like amphibole (Si4O11)6− running parallel to the fiber axis (Frost et al., 1998). The unique morphology and structure of attapulgite provide potential for diverse applications, such as catalyst

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supports and environmental adsorbents (Bouna, et al., 2011; Chen and Wang, 2007; Pan et al., 2010; Zhao, et al., 2007). In this paper, we have investigated the synthesis of the Ag\AgBr/ attapulgite nanocomposite by photoreducing the AgBr/attapulgite nanocomposite prepared by a precipitation method. As the support for the as-prepared photocatalyst, attapulgite may facilitate the adsorption of organic contaminants, the separation and recovery of catalyst. The photocatalytic activity of the Ag\AgBr/attapulgite nanocomposite was investigated by the photocatalytic decomposition of azo dyes under visible light irradiation (λ > 400 nm). Terephthalic acid photoluminescence probing technique (TA-PL) was used to detect the formation of the HO• radicals in the process of photodegradation, and a visible light induced decomposition mechanism was suggested.

examined with transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images by a JEM 2100F electron microscope operated at an accelerating voltage of 200 kV. The optical properties of the samples were studied by the UV–visible diffuse reflectance spectroscopy (DRS) using a UV–vis spectrometer (UV2550, Shimadzu, Japan) in the range of 190 to 800 nm, in which BaSO4 was used as the reflectance standard material. X-ray photoelectron spectroscopy (XPS) analysis was carried out by using a Thermo VG Multilab 2000 spectrometer (UK) with a monochromatic Al Kα source. All binding energies were referred to the C 1 s peak at 284.63 eV of the surface adventitious carbon and revised. The HO· trapping fluorescence spectra were taken on a fluorescence spectrophotometer (Shimadzu RF-5300PC). 2.4. Evaluation of photocatalytic activity

2. Experimental section 2.1. Materials The attapulgite (CEC: 25–50 meq/100 g) was obtained from Jiangsu, P. R. China. Silver nitrate (AgNO3, SCRC, China) and sodium bromide (NaBr, SCRC, China), which were purchased from Sinopharm Chemical Reagent Co., Ltd., China, and were of analytical grade without further purification. Distilled water was used in the whole experiment. 2.2. Preparation The Ag\AgBr/attapulgite nanocomposite was prepared by a precipitation–photoreduction method, which was similar to the preparation method reported by Yu et al. (2009) and Wang et al. (2011b). Typically, the attapulgite powders were immersed in distilled water, and the dispersion was sonicated for 30 min. Then, 4 mmol of silver nitrate (AgNO3) was added to the dispersion (the ratio of Ag+ to clay mineral was 2.0 mmol/g) and the mixture was stirred magnetically for 30 min under ambient temperature. Sodium bromide (NaBr) aqueous solution with a concentration of 0.05 mol/L was slowly added to the mixture and then magnetically stirred for 6 h. The product was washed with water and dried at 70°C to obtain the AgBr/attapulgite nanocomposite. Finally, the AgBr/attapulgite nanocomposite was mixed with distilled water and was irradiated by visible light (λ > 400 nm) for 30 min to reduce partly adsorbed Ag+ to Ag0. Then the precipitate was collected and dried in air. As a reference, Ag\AgBr photocatalyst was prepared by a similar method without the attapulgite. 2.3. Characterization Powder X-ray diffraction of the as-synthesized powders were obtained on a D/MAX-RB X-ray diffractometer (Rigaku, Japan) equipped with Cu Kα radiation (λ = 1.5406 Å) and recorded with 2θ ranging from 2.5° to 70°, while the accelerating voltage and the applied current were held at 40 kV and 50 mA, respectively. Morphologies and microstructures of the Ag\AgBr/attapulgite nanocomposite were further

The photocatalytic performance of the as-prepared photocatalyst was evaluated by decomposing rhodamine (RhB) under visible light irradiation. A 300 W Dy lamp was used as the light source with a 400 nm cutoff filter to ensure complete removal of radiation below 400 nm. The photodegradation experiments were carried out with the sample powders (150 mg) suspended in RhB (100 mL, 5 mg/L) with constant stirring. Prior to irradiation, the dispersions were magnetically stirred in the dark for 20 min to disperse the photocatalyst sufficiently. At given irradiation time intervals, about 6 mL dispersions were collected and centrifuged to remove the particles. Then the absorption UV–vis spectrum of the centrifugated solution was recorded using an UV–visible spectrophotometer (Unico UV-2102PC). For comparison, the photocatalytic degradation of RhB by the Ag\AgBr catalyst was performed using the same procedure as above. The dosage of Ag\AgBr catalyst was equivalent to the AgBr content in the 0.15 g Ag\AgBr/attapulgite sample (the AgBr content in the nanocomposite was 2 mmol/g as described in the preparation section). 3. Results and discussion 3.1. Formation mechanism The attapulgite is an ideal template for preparing nano-structured materials in terms of its cheap and unique structure. In this paper, we used attapulgite as a substrate to prepare the Ag\AgBr/attapulgite nanocomposite by a facile method, as shown in Fig. 1. The attapulgite carries negative charges resulting from the isomorphic substitution of Si 4+ by Al 3+ which is interacting with the positively charged Ag + ions through electrostatic interactions. On the other hand, there are abundant active groups (such as OH and Si\O) on the surface of attapulgite which are favorable to form stable chemical bonding with Ag + ions and subsequently grown to AgBr particles via added NaBr aqueous solution. Then, partly Ag + ions on the surface of AgBr/attapulgite nanocomposite were reduced to Ag 0 under visible light irradiation to form the Ag\AgBr/attapulgite nanocomposite.

Fig. 1. Schematic illustration of the synthesis procedure of the Ag\AgBr/attapulgite nanocomposite.

Y. Yang, G. Zhang / Applied Clay Science 67–68 (2012) 11–17

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Fig. 4. Ag 3d XPS spectra of (a) the AgBr/attapulgite nanocomposite and (b) the Ag\AgBr/ attapulgite nanocomposite.

Fig. 2. XRD patterns of (a) attapulgite, (b) the Ag\AgBr/attapulgite nanocomposite.

3.3. TEM images 3.2. XRD analysis In order to confirm the structure of the prepared Ag\AgBr/ attapulgite nanocomposite, powder XRD studies were carried out. Fig. 2 shows the XRD patterns of the attapulgite and the Ag\AgBr/attapulgite nanocomposite. From Fig. 2a, the typical diffraction peaks at 2θ=8.7°, 13.6°, 20.2° and 26.7° were observed, which were in agreement with the primary diffraction of the (110), (200), (040), and (400) planes of the attapulgite, respectively. The (110) diffraction peak is attributed to the basal plane of the attapulgite structure (Cao et al., 1996). The (200) and (040) peaks represent the Si\O\Si layers in the clay mineral (Cao et al., 1996). The characteristic peaks of attapulgite are also observed in the Ag\AgBr/attapulgite nanocomposite, which suggested that the precipitation–photoreduction process did not destroy the characteristic structures of attapulgite. As seen in Fig. 2b, the peaks with 2θ values of 30.9°, 44.3°, 55.1° and 64.5° are corresponded to (200), (220), (222) and (400) crystal planes of cubic AgBr (JCPDF No.79-0149), respectively. No obvious diffraction peaks of metallic Ag 0 were found in Fig. 2b, which may result from its low content, and well dispersion on the surface of AgBr/attapulgite.

To further obtain the microscopic morphology and structure information, the TEM analysis of the Ag\AgBr/attapulgite nanocomposite has been performed. From Fig. 3a, it is clearly observed that some dark particles with diameters in the range of 50–100 nm are dispersed on the fibrous surface of attapulgite, which may correspond to the Ag\AgBr particles. The corresponding HRTEM of the nanocomposite, as shown in Fig. 3b, shows that the determined lattice spacing of 0.204 nm match with the (200) crystallographic planes of Ag0. However, it should be noted that the crystal lattice of AgBr cannot be found by HRTEM analysis, probably owing to its instability under the irradiation of the high energy electron beam. In view of the AgBr phase in Fig. 2b and Ag particles in Fig.3b, it is clear that the prepared nanocomposite can be referred to the Ag\AgBr particles which formed on the attapulgite surface. 3.4. XPS analysis The chemical status of the AgBr/attapulgite and the Ag\AgBr/ attapulgite nanocomposite were analyzed by XPS. Fig. 4a–b shows the high resolution XPS spectra of Ag 3d of the AgBr/attapulgite and

Fig. 3. TEM image (a) and HRTEM image (b) of the Ag\AgBr/attapulgite nanocomposite.

Y. Yang, G. Zhang / Applied Clay Science 67–68 (2012) 11–17

the Ag\AgBr/attapulgite nanocomposite, respectively. As shown in Fig. 4, the binding energies of Ag 3d shift to the higher binding energy after irradiated by visible light (λ > 400 nm). The changes in the binding energy indicate the metal Ag 0 generated after photoreduction (Moulder et al., 1992). This is also confirmed by the UV–vis DRS spectra. 3.5. UV–vis DRS analysis The UV–vis diffuse reflectance spectra of the attapulgite, the AgBr/ attapulgite and the Ag\AgBr/attapulgite nanocomposite were presented in Fig. 5. The attapulgite and the AgBr/attapulgite nanocomposite have strong absorption in the UV region, but do not strongly absorb in the visible region. In contrast, the Ag\AgBr/attapulgite nanocomposite has much stronger absorption in the visible region. And the Ag\AgBr/ attapulgite nanocomposite has a plasmon resonance absorption band around 450–700 nm. According to the previous results, the absorption shoulder around 450–700 nm can be attributed to the localized surface plasmon resonance of Ag nanoparticles (Wang et al., 2008). This also indicates that the Ag\AgBr/attapulgite nanocomposite contains metal Ag0 due to photochemical decomposition or photocatalytic reduction of the photoirradiated nanocomposite (Yu et al., 2009). As the results of the UV–vis DRS, the Ag\AgBr/attapulgite nanocomposite can be expected to have an excellent catalytic activity to the organic contaminants. 3.6. Photodegradation activity under visible light irradiation The photocatalytic activity of the samples was evaluated by the degradation of RhB aqueous solution, which is a kind of organic dye that is chemically stable and difficult to be degraded. The RhB dye contains two N-ethyl groups at each side of the xanthene ring (seen in the inset of Fig. 6), which is stable in aqueous solution. Fig. 6 shows the changes in the absorption spectra of RhB aqueous solution exposed to visible light for various times in the presence of Ag\AgBr/attapulgite. A progressive absorption decrease at wavelength of 554 nm accompanied by a shift of this absorption band towards shorter wavelength is observed for RhB aqueous solution under visible light illumination. The blue shift in the maximum absorbance has proved to be derived from the formation of a series of N-deethylated intermediates of RhB (Chen et al., 2004; Hu et al., 2006b). The specific peaks of the conjugated chromophore structure (shown in the absorption peak at 554 nm) and the N-de-ethylated intermediates become smoother rapidly during the degradation processes, which indicated that the Ag\AgBr/attapulgite

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Wavelength (nm) Fig. 6. UV–vis spectra changes of RhB solution during the photocatalytic degradation by the as-prepared photocatalyst under visible light illumination.

nanocomposite exhibited excellent photocatalytic activity for RhB degradation under visible light irradiation. Fig. 7 displays the degradation of RhB solution under different conditions. C/C0 in the maximum absorption band of the RhB solution at different irradiation time was used to analyze the photocatalytic activity of the Ag\AgBr/attapulgite nanocomposite. As shown in Fig. 7, a blank test in the absence of the photocatalyst under visible light irradiation shows that the photolysis of RhB was negligible. The adsorption test shows that the Ag\AgBr/attapulgite nanocomposite and attapulgite has a high adsorption capacity to the RhB solution, which indicates that the attapulgite facilitates the adsorption of RhB on the prepared nanocomposite due to electrostatic interactions in RhB degradation experiment (Zhang et al., 2012). The high adsorption capacity is in favor of the photocatalytic decomposition of organic contaminants. As a comparison, the photodegradation of RhB with Ag\AgBr was also performed. 100% of RhB was removed over the Ag\AgBr/attapulgite nanocomposite within 10 min, whereas only 69% of the dye was degraded for bare Ag\AgBr catalyst during the same time. It indicates that the Ag\AgBr/attapulgite nanocomposite has the excellent photocatalytic activity under visible light irradiation. To further investigate the photocatalytic stability of the as-prepared nanocomposite, the circulating runs in the photodegradation of RhB in the presence of Ag\AgBr/attapulgite under visible light irradiation

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Wavelength (nm) Fig. 5. UV–vis diffuse reflectance spectra of the Ag\AgBr/attapulgite nanocomposite, the AgBr/attapulgite nanocomposite and attapulgite.

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Y. Yang, G. Zhang / Applied Clay Science 67–68 (2012) 11–17

(λ >400 nm) was checked. The photocatalytic activity did not exhibit obviously decreased after five recycles for the degradation of RhB. It indicates that the Ag\AgBr/attapulgite nanocomposite is stable during the RhB degradation. 3.7. Visible light photodegradation mechanism Previous studies have shown that the radical species such as hydroxyl radicals (HO•) and superoxide radical anions (O2• –) are most important factors affecting the photodegradation activity. Therefore, to investigate the photo-induced active species with strong oxidation power for degradation of RhB over the Ag\AgBr/attapulgite nanocomposite, the corresponding experiments were employed to determine the photogenerated radical species that played important roles in the photodegradation process. Terephthalic acid photoluminescence probing technique (TA-PL), a highly sensitive and simple method, has been widely used in detection of HO• radicals (He et al., 2009; Hirakawa and Nosaka, 2002; Huang et al., 2009). 2-hydroxylterephthalic acid (TAOH), which is generated when terephthalic acid captures the HO• radicals (as shown in Eq. (1)), performs a strong fluorescence at around 426 nm on the excitation of its own 312 nm absorption band (Ishibashi et al., 2000a). HO• þ TA → TAOH

ð1Þ

Thus, HO• radicals can be detected indirectly by monitoring the fluorescence intensity changes in a basic solution of TA. Experimental procedures are similar to the measurement of the photocatalytic activity except that RhB aqueous solution was replaced by the TA aqueous solution. As shown in Fig. 8, an increase in photoluminescence (PL) intensity at 426 nm is observed with increasing visible light irradiation time. However, no PL signal is observed in the absence of visible light irradiation. This suggests that the fluorescence is from the chemical reactions between the TA and HO• formed during the photocatalytic reactions (Ishibashi et al., 2000b; Xiao et al., 2008). The experiments confirm that the hydroxyl radicals are active species and participate in photocatalytic reactions. 1,4-Benzoquinone (BQ) is a quencher of the superoxide radical anions (O2• –) via fast electron transfer according to Eq. (2) generating benzoquinone radicals (Bandara and Kiwi, 1999; Li et al., 2009). And the BQ radicals formation would inhibit the participation of O2• – in the contaminant decomposition. –

–

BQ þ O2 • →BQ • þ O2

ð2Þ

To further investigate the effect of superoxides in the photodegradation process, the comparison experiments were performed between the degradation curves of photocatalyst/contaminants dispersions with or without BQ into the initial solution. As shown in Fig. 9, the photodegradation of RhB over the Ag\AgBr/attapulgite nanocomposite was suppressed to a certain degree after 2 mmol/L BQ was added to the reaction system. The experimental result suggests that the superoxide may also participate in the oxidative degradation of RhB. A possible mechanism was proposed for the dye photodegradation over the Ag\AgBr/attapulgite nanocomposite (as shown in Fig. 10), which is similar to the mechanism proposed by Zhou et al. (2010) and Cheng et al. (2011). AgBr and Ag0 can be excited by visible light due to the band gap structure and surface plasmon resonance, respectively. As seen in Fig. 10, electrons are promoted from the valence band to the conduction band of the photocatalyst to give electron–hole pairs. The photogenerated electrons excited to the conduction band of AgBr were trapped by Ag0 formed on AgBr particles and separated from the holes in the valence bands of AgBr. The Ag0 species on the surface of Ag\AgBr/attapulgite could act as electron traps to facilitate the separation of photogenerated electron–hole pairs. These electrons would react with O2 absorbed on the surface of catalyst to generate superoxide radical anions (O2• –). Meanwhile, the separated holes would generate HO• radicals (Li et al., 2008; Zhou et al., 2010) or Br0 (Cao et al., 2011; Wang et al., 2009). These formed radicals or Br 0 are a powerful oxidizing agent and attack organic pollutants presented at or near the surface of Ag\AgBr/attapulgite. In summary, AgBr is a visible light active component of the nanocomposite and Ag0 species probably contributing to enhancing the electron-holes separation. As the support of the Ag\AgBr, the attapulgite could increase the adsorption of the organic contaminants on the as-prepared nanocomposite, thus improve the degradation activity. 4. Conclusions Novel visible light driven plasmonic photocatalyst Ag\AgBr/ attapulgite can be prepared by deposited Ag\AgBr particles on the surface of attapulgite by a precipitation-photoreduction method. The UV–vis spectra indicate that the nanocomposite has a strongly absorption in the visible range due to the plasmon resonance of Ag0. The prepared Ag\AgBr/attapulgite nanocomposite exhibited a much higher photocatalytic activity than Ag\AgBr for the degradation of RhB aqueous solution. The high photocatalytic performance of Ag\AgBr/ attapulgite nanocomposite may be considered due to the high adsorption

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Time (min) Fig. 9. Photocatalytic degradation of RhB solution over the Ag\AgBr/attapulgite nanocomposite with or without the quencher under visible light irradiation.

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Fig. 10. Proposed schematic illustrations of the Ag\AgBr/attapulgite photocatalytic reaction process under visible light irradiation.

ability of attapulgite and the localized surface plasmon resonance of Ag particles. The active species measurements demonstrate that the degradation of RhB over the Ag\AgBr/attapulgite nanocomposite under visible-light irradiation is mainly via O2•– and HO• oxidation mechanism. Considering the high photocatalytic activity the Ag\AgBr/attapulgite nanocomposite, it may be a promising efficient nanocomposite for environmental purification.

Acknowledgment This work was supported by Program of Wuhan Subject Chief Scientist (201150530147), National Basic Research Program of China (973 Program) 2007CB613302, the Wuhan Technologies R & D Program and the Fundamental Research Funds for the Central Universities.

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