Chinese Journal of Catalysis 36 (2015) 2219 2228 催化学报 2015 年第 36 卷第 12 期 www.chxb.cn available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/chnjc Article (Special Issue on Photocatalysis) Facile synthesis of Ag2O TiO2/sepiolite composites with enhanced visible light photocatalytic properties Yu Du a, Dandan Tang a, Gaoke Zhang a,b, *, Xiaoyong Wu a a School of Resources and Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, Hubei, China b State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, Hubei, China A R T I C L E I N F O A B S T R A C T Article history: Received 17 August 2015 Accepted 11 November 2015 Published 20 December 2015 Keywords: Silver oxide Titanium dioxide Sepiolite Heterostructure Photocatalysis Acid red G Photocatalytic mechanism Ag2O TiO2/sepiolite heterostructure composites were synthesized by a simple two step method at low temperatures (100 450 C). Acid red G aqueous solution and gaseous formaldehyde were chosen as model organic pollutants to evaluate the photocatalytic performance of the as prepared composites. The results showed that the Ag2O TiO2/sepiolite exhibited enhanced photocatalytic activity over pure Ag2O TiO2, TiO2/sepiolite, and Ag2O/sepiolite under visible light irradiation (λ > 420 nm). The excellent photocatalytic efficiency of these composites can be ascribed to the synergistic effect between the heterojunction and the porous structure of the clay layers, which induced high adsorption and efficient charge separation. In addition, the active species involved in the degradation reaction have been investigated by photoluminescence spectroscopy and quenching experiments. A possible photocatalytic degradation mechanism of acid red G dye by the Ag2O TiO2/sepiolite composite is also discussed. 2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. 1. Introduction Photocatalysis technology is considered a promising method for air and sewage purification because it is efficient, economical, and environmentally viable [1 6]. Titanium dioxide (TiO2) has also received increased attention because of its nontoxicity, high chemical stability, and low cost [7 9]. However, TiO2 can only exhibit photocatalytic activity under UV light. Furthermore, its poor adsorption capacity and high recombination rate of photogenerated electron hole pairs hinder its practical application [10 12]. To resolve this problem, various methods, such as chemical modification [13], metallization [14], and the development of novel heterostructure semiconductors [15,16], have been employed to enhance the charge separation efficiency. For example, Tang et al. [17] reported that the Ag/AlO2/TiO2 heterojunction presented higher photocatalytic activity for decomposing formaldehyde under sunlight irradiation than did pure TiO2 and AgAlO2. This was attributed to the heterojunction providing a matching energy band structure. Lee et al. [18] found that TiO2/CuO nanofiber exhibited high photocatalytic properties for degrading and cleaning the organics produced from dye wastewater, which can be ascribed to the high quantum efficiency. In addition, researchers have found that silver oxide (Ag2O) can act as a novel visible light driven semiconductor material because of its unique band gap, good sensitivity to light, facile preparation, and inexpensiveness, meaning that this could be an excellent charge separation promoter and built in acceptor [19 21]. Ag particles are also used as a cleaning agent to remove bacteria from contaminated water [22 24]. Theoretically, the conduction band (CB) of Ag2O is more negative than the corresponding band of TiO2, and the valence band (VB) of TiO2 * Corresponding author. Tel: +86 27 87651816; Fax: +86 27 87887445; E mail: gkzhang@whut.edu.cn This work was supported by the National Key Technology R&D Program of China (2012BAJ25B02 03). DOI: 10.1016/S1872 2067(15)61015 4 http://www.sciencedirect.com/science/journal/18722067 Chin. J. Catal., Vol. 36, No. 12, December 2015
2220 Yu Du et al. / Chinese Journal of Catalysis 36 (2015) 2219 2228 is more positive than that of Ag2O [25]. Their matching band gaps and positions might favor the formation of Ag2O TiO2 heterostructures, which expands the photoabsorption range and facilitates the separation of photogenerated electrons and hole pairs [26 31]. However, their application is also limited by weak adsorption and the difficulty in separating them from wastewater. Sepiolite is a microfibrous clay mineral consisting of a 2:1 layered structure built by two tetrahedral silica sheets with a central magnesia sheet [32]. Its peculiar structural tunnels lead to interesting surface properties and high absorption capacity [33]. Zhang et al. [34] studied bicrystalline TiO2 by supporting it on porous sepiolite clay for the photocatalytic degradation of gaseous formaldehyde, wherein the TiO2/sepiolite showed a higher photocatalytic activity than commercial Degussa P 25 or bare TiO2. Moreover, the layered structure of sepiolite exhibited higher physicochemical stability. Thus, sepiolite may also be a good support for the Ag2O TiO2 heterojunction catalyst, which can improve the catalyst adsorption capacity and catalyst separation, and its recycling from wastewater. In the present work, heterostructure Ag2O TiO2/sepiolite composites were synthesized by a simple two step method. Their photocatalytic activities were investigated by the photocatalytic degradation of acid red G (ARG) aqueous solution and gaseous formaldehyde under visible light irradiation (λ > 420 nm). The structural features of the composites were analyzed through systematic characterization, and the visible light photocatalytic mechanism of the Ag2O TiO2/sepiolite composites is discussed in detail. 2. Experimental 2.1. Catalyst prepartation The sepiolite sample used in this study was from Hunan Province, China. All chemical reagents were of analytical grade and used without further purification. The synthesis of Ag2O TiO2/sepiolite composite comprises two main processes. TiO2 sol was prepared by an acid catalyzed sol gel method from a TiCl4 precursor (Sinopharm Chemical Reagent Co., Ltd., China). TiCl4 (8 ml) was added gradually to a HCl solution (22 wt%) under continuous stirring for 0.5 h at 25 C, and then aged for 6 h to obtain a transparent TiO2 sol. Then, the TiO2 sol was added dropwise into sepiolite powder saturated with deionized water (1 wt%) under vigorous stirring for 0.5 h at 70 C, before being aged for 12 h to obtain individual TiO2 loadings of 30 wt% on the sepiolite support (30%TiO2/sepiolite). The resulting mixed suspensions were centrifuged at 5000 r/min several times and then washed with deionized water to neutralize the supernatants until no Cl was detected by AgNO3 solution. After removing all excessive chloride, the samples were dried at 70 C for 5 h. Ag2O TiO2/sepiolite composites were synthesized via an impregnation method. TiO2/sepiolite powder (1.00 g) was dispersed in 18.5 ml of AgNO3 solution (5 mol/l) to produce a suspension (Ag/powders = 10 wt%). After stirring in the dark for 2 h, the resulting suspension was dried at 60 C and then calcined at 100, 200, 350, and 450 C for 30 min under ambient conditions. The resulting samples were stored in the dark in the form of fine white powders (< 400 C), in which the Ag2O phase may exist on the surface of the TiO2/sepiolite composites. The color of the Ag2O TiO2/sepiolite composites changed to gray by increasing the calcination temperature to 450 C, owing to the decomposition of Ag2O to metallic Ag [25]. For comparison and to investigate the effect of Ag2O particles on the photocatalytic performance of sepiolite clay, pure Ag2O TiO2 and Ag2O/sepiolite composites were also prepared following the same procedure to provide 10%Ag2O 30%TiO2/ sepiolite (200 C). 2.2. Catalyst characterization X ray diffraction (XRD, Rigaku D/MAX RB diffractometer, operating at 40 kv and 50 ma with Cu Kα radiation, λ = 0.15406 nm) was used to determine the structure and crystallinity of the as prepared samples. X ray photoelectron spectroscopy (XPS) data were collected using an ESCALAB II XPS system operating in hybrid mode, with a monochromatic Mg Kα source and a charge neutralizer. All binding energies obtained by the XPS spectral analysis used the C 1s peak at 284.5 ev as a reference. The morphologies of the samples were observed by scanning electronic microscopy (SEM, JSM5610LV). The absorption edge of the samples was measured by an ultraviolet visible (UV vis) spectrophotometer (UV 2550, Shimadzu), for which the raw sepiolite and TiO2 supported catalyst samples were used as the reflectance standard. A N2 adsorption desorption isotherm was obtained at liquid nitrogen temperature ( 196 C) using a Quantachrome AUTOSORB 1 nitrogen adsorption apparatus. The specific surface area was determined by the multi point BET method and the pore sizes were measured by the BJH method of adsorption. The Fourier transform infrared (FT IR) spectra of the chemical bonds on the surface of the samples were obtained using a Thermo Nicolet Nexus spectrometer. Photoluminescence (PL) spectra were recorded via a fluorescence spectrophotometer (RF 5301PC, Shimadzu) with an excitation wavelength of 312 nm, which is capable of detecting the hydroxyl radical ( OH) during the photocatalytic degradation process. 2.3. Photocatalytic activity measurements The photocatalytic degradation experiments of ARG dye and gaseous formaldehyde were performed under visible light irradiation at room temperature. A desired amount (0.15 g) of the as prepared catalyst was homogeneously suspended in the ARG dye aqueous solution (100 mg/l) under continuous stirring for 30 min in a dark environment to establish the adsorption desorption equilibrium. The reaction was started by direct exposure to visible light irradiation (300 W Dy lamp) with a 420 nm cutoff filter. At given time intervals of illumination, small aliquots of the stirred suspension were drawn and centrifuged to remove the photocatalyst. The supernatant liquor of the ARG dye was analyzed by UV vis spectrophotometry (UV751GD, China) at its maximum absorption wavelength
Yu Du et al. / Chinese Journal of Catalysis 36 (2015) 2219 2228 2221 (λmax) of 505 nm. Gaseous formaldehyde degradation was conducted in a closed stainless steel photoreactor with an inner volume of 0.8 L. The as prepared catalyst, with a dosage of 0.15 g, was dispersed in a dish of diameter 70 mm. Then the sample was set into the gas photoreactor, and formaldehyde solution (2 μl) was injected into the photoreactor by microsyringe. The reactor was blown by a fan for the formaldehyde to slowly volatilize until the initial concentration reached equilibrium. After several minutes, a 300 W Dy lamp was turned on to irradiate the photoreactor with a 420 nm cutoff filter. A 1412 photoacoustic field gas monitor (Innova AirTech Instruments, Denmark) was applied to detect the concentration of gaseous formaldehyde, and the mineralization products (CO2 and H2O) were also monitored. The tests were performed at room temperature (25 C). 3. Results and discussion 3.1. Photocatalyst characterization XRD was used to investigate the crystal structure of the catalyst particles. Fig. 1(a) shows the XRD patterns over a scan range of 5 to 70 for the sepiolite, TiO2/sepiolite, and Ag2O TiO2/sepiolite composites obtained at different temperatures. It is found that all samples show the main phase of sepiolite, for which the diffraction peaks at 2θ = 8.78, 12.2, 24.8, and 26.6 are indexed to the (110), (130), (231), and (080) planes, respectively, of the pristine sepiolite (JCPDS 26 1226). Meanwhile, the intensities of the (130), (231), and (080) peaks of the sepiolite gradually decreased by increasing the calcination temperature, which indicates that the layered structure of sepiolite was, to some extent, destroyed [35,36]. In addition, the diffraction peaks at 2θ = 25.3, 48.1, and 55.1 are indexed to the (101), (200), and (211) planes, respectively, of the anatase phase of TiO2 (JCPDS 83 2243), whereas the peaks at 2θ = 27.4 and 36.0 are indexed to the (110) and (101) planes, respectively, of the rutile phase of TiO2 (JCPDS 78 2485) [37]. There are commonly two naturally occurring phases of titania (rutile and anatase) when the calcination temperature is above 400 C [33], and the thermal stability of the anatase crystalline phase for Ag2O TiO2/sepiolite composites can likely be attributed to the stabilizing effect of the silica that exists in the sepiolite framework [32]. In Fig. 1(a), the peaks corresponding to the Ag2O phase are not clearly detectable. To further demonstrate the phase structures of the Ag on the surface of the sepiolite, the weak diffraction peaks of Ag2O at 2θ = 32.7 (JCPDS 12 0793) and metallic Ag at 2θ = 44.3 (JCPDS 04 0783) were further detected, as shown in Fig. 1(b) and (c). The samples obtained at 200 and 350 C were found to contain the Ag2O phase (Fig. 1(b)). Metallic Ag was observed as the calcination temperature was increased to 450 C (Fig. 1(c)), which can be attributed to the decomposition of Ag2O [38]. In this study, TiO2 could exist as a mixture of nanosized anatase and rutile in the composites, and the formation of the Ag2O phase could be inhibited by controlling the calcination temperature below 450 C. XPS was employed to analyze the chemical state of the elements on the surface of the Ag2O TiO2/sepiolite composites, and the results are shown in Fig. 2(a). In Fig. 2(b), the Ag 3d region shows XPS peaks with two individual peaks at 368.3 and 374.3 ev, which are assigned to the Ag 3d5/2 and Ag 3d3/2 peaks in Ag2O phase [39,40], respectively. In Fig. 2(c), the Ti 2p region exhibits two individual peaks at 458.7 and 464.3 ev, which indicate that Ti 4+ of TiO2 is the major Ti species in the Ag2O TiO2/sepiolite composites [41 43]. In addition, it can be seen that the binding energy values of the Ti 2p region are slightly increased, which is attributed to the interaction of Ag ions with the TiO2 surface [44,45]. In Fig. 2(d), the main peak of the O 1s region with a binding energy of 530.1 ev is assigned to the oxygen in the TiO2 crystal lattice [38,39,43], while the peak at 532.4 ev is assigned to the metal OH bonds [46]. The formation of silver oxide is attributed to the Ag + ions absorbing oxygen ions originating from the TiO2 metal crystal lattice or from dispersed OH in the clay [47]. The morphologies of the sepiolite, TiO2/sepiolite, and Ag2O TiO2/sepiolite composites were characterized by SEM. In Fig. 3(a), the sepiolite exhibits a smooth and flat surface, while the TiO2/sepiolite composite presents a relatively rough and loose knit section (Fig. 3(b)). From Fig. 3(c), it can be seen that a large number of the TiO2 and Ag2O particles assembled on the S A SS R R A S: Sepiolite A: Anatase R: Rutile A A (a) (1) (2) (3) (4) Ag 2O (b) (5) (4) Ag (6) (5) (4) (c) (5) (6) (1) (3) 10 20 30 40 50 60 70 2 /( o ) 30 31 32 33 34 2 /( o ) 43.5 44.0 44.5 45.0 2 /( o ) Fig. 1. (a) XRD patterns of sepiolite (1), TiO2/sepiolite (2), Ag2O TiO2/sepiolite (100 C) (3), Ag2O TiO2/sepiolite (200 C) (4), Ag2O TiO2/sepiolite (350 C) (5), and Ag2O TiO2/sepiolite (450 C) (6); The corresponding enlarged diffraction areas from 30 to 34 (b) and 43.5 to 45 (c) for some typical samples.
2222 Yu Du et al. / Chinese Journal of Catalysis 36 (2015) 2219 2228 (a) O 1s Survey (b) 368.3 Ag 3d Ti 2p Ag 3d C 1s Si 2p 374.3 1200 1000 800 600 400 200 0 Binding energy (ev) 380 376 372 368 364 Binding energy (ev) (c) 458.7 Ti 2p (d) 532.4 530.1 O 1s 464.3 Intersity (a.u.) 468 464 460 456 452 538 536 534 532 530 528 526 524 Binding energy (ev) Binding energy (ev) Fig. 2. XPS spectrum of Ag2O TiO2/sepiolite composites (a) and high resolution XPS spectra of Ag 3d region (b), Ti 2p region (c), and O 1s region (d). (a) (b) (c) Fig. 3. SEM images of raw sepiolite (a), TiO2/sepiolite (b), and Ag2O TiO2/sepiolite composites (c). surface of the sepiolite, and their shapes are quite irregular. The morphology of the sepiolite clay shows no obvious difference, while the layered structures of the mineral congeries were partially destroyed when the TiO2 nanoparticles were introduced into the sepiolite by the sol gel process. HRTEM was used to further investigate the phase structure of the Ag2O TiO2/sepiolite composite. Fig. 4(a) and (b) show typical TEM images of the Ag2O TiO2/sepiolite composite with a layered structure, which is consistent with the SEM observations. Fig. 4(c) and (d) show HRTEM images recorded from the white framed areas indicated in Fig. 4(b), in which it can be seen that two crystals of Ag2O and TiO2 are tightly interconnected, and the lattice fringe spacing of 0.272 nm (Fig. 4(c)) corresponds to the (111) crystallographic plane of Ag2O. The observed lattice spacings of 0.351 and 0.324 nm (Fig. 4(d)) correspond to the (101) plane of the anatase and the (110) plane of the rutile phase, respectively. The HRTEM images further confirm that heterostructures of Ag2O and TiO2 have been formed in the composites [48]. The N2 adsorption desorption isotherm and pore size distribution curve of the Ag2O TiO2/sepiolite composite (200 C) are shown in Fig. 5. The BET surface area, pore volume, and average pore size of the different samples are summarized in Table 1. The surface area (60.39 m 2 /g) of the Ag2O TiO2/sepiolite composite prepared at 200 C is five times higher than that of the raw sepiolite (12.21 m 2 /g). Taking into account the calcination temperature, Ag2O TiO2/sepiolite composites show a higher thermal stability and a loss of specific surface area of between 15% and 33% [49]. The N2 adsorption desorption isotherm of the composite seems to be type IV, according to the IUPAC classification, which demonstrates the presence of mesopores (2 50 nm) [26]. The shape of the hysteresis loops is of type H3, which is the main feature of a multiporous structure [32,50]. The continuously increased adsorption branch at relative pressure (p/p0 = 1 1.0) indicates capillary condensation of N2 molecules inside the different pore sized structure (micropores and mesoporous) [50 52]. Typical FT IR spectra of sepiolite and the Ag2O TiO2/ sepio
Yu Du et al. / Chinese Journal of Catalysis 36 (2015) 2219 2228 2223 (a) 100 nm (c) (b) 20 nm (d) Table 1 Specific surface areas and pore parameters of sepiolite, TiO2/sepiolite, Ag2O TiO2, and the Ag2O TiO2/sepiolite composites prepared at different temperatures. Sample BET surface area (m 2 /g) Pore volume (cm 3 /g) Average pore size (nm) Sepiolite 12.21 6 69.46 TiO2/sepiolite 63.21 0.10 37.60 Ag2O TiO2 3.13 1 42.19 Ag2O TiO2/sepiolite (100 C) 51.21 9 71.29 Ag2O TiO2/sepiolite (200 C) 60.39 0.10 43.66 Ag2O TiO2/sepiolite (350 C) 40.44 7 47.74 Ag2O TiO2/sepiolite (450 C) 44.99 7 90.60 Ag 2 O 5 nm TiO 2 Ag 2 O(111) 0.272 nm 5 nm r-tio 2 0.351 nm a-tio 2 0.324 nm Fig. 4. TEM (a,b) and HRTEM (c,d) images of the Ag2O TiO2/sepiolite composite prepared at 200 C. Volume (cm 3 /g, STP) 80 0.10 70 60 50 40 30 20 10 0 DV(d) (cm 3 nm g ) Dv(d) (cm -3 nm -1 g -1 ) 8 6 4 2 0 0 10 10 20 20 30 30 40 40 Pore diameter (nm) Pore diameter (nm) 0.2 0.4 0.6 0.8 1.0 Relative pressure (p/p 0) Fig. 5. N2 adsorption desorption isotherm of the Ag2O TiO2/sepiolite composite prepared at 200 C. The inset is the corresponding pore size distribution. lite composite before and after photocatalytic degradation of ARG dye are illustrated in Fig. 6. The bands observed at 3668 cm 1 can be attributed to the stretching vibrations of hydroxyl groups (Mg3OH) and water molecules bound to the octahedral sheets of Mg ions in the sepiolite clay [53 55]. The bands between 3435 and 1637 cm 1 can be assigned to the O H stretching mode and the H O H bending mode from the interbedded water molecule of the clay [37]. The bands at 1114 and 1033 cm 1 are due to the stretching of Si O in the Si O Si groups of the tetrahedral sheet, and the peak at 637 cm 1 is assigned to OH bending vibrations [56]. As reported, the Si O Al (octahedral) bending vibrations are present at 469 cm 1 [57]. As shown in Fig. 6(b), the disappearance of the band at 430 cm 1 is assigned to Si O Si bending vibrations [54], and the shift of the bands at 3424 and 1631 cm 1 might be caused by changes to the chemical bond vibration for TiO2 particles inserted into the layer of sepiolite or the bonding of Ti OH [58], indicating that the layered structure of sepiolite has been destroyed during the preparation process. Moreover, the characteristic bands of Si O in the Si O Si groups of the tetrahedral sheets (around 1114, 1033, and 469 cm 1 ) still exist, and the new peak at 908 cm 1 is caused by a Ti O H stretching vibration. In addition, the intensities of the vibration modes at 637 1033 cm 1 are decreased, which indicates that the organic matter on the clay was removed in the calcination process. The FT IR spectrum of the recovered composite does not show the characteristic peak of ARG dye, which confirms that other than that which was adsorbed by the composite, the ARG was degraded completely by the photocatalysis. Meanwhile, the result also indicates that the Ag2O TiO2/sepiolite composite has good stability. The UV vis diffuse reflectance spectra of sepiolite, TiO2, Ag2O, TiO2/sepiolite, and Ag2O TiO2/sepiolite composite are shown in Fig. 7. The absorption edge of the Ag2O TiO2/sepiolite composite tends to red shift compared with that of the TiO2/sepiolite sample. The band gaps of pure TiO2 and pure Ag2O are calculated to be 2.9 and 1.0 ev, respectively. The Ag2O TiO2/sepiolite composite exhibited enhanced photoabsorption from the UV light region to the visible light region in the range 370 430 nm. The results indicate that the Ag2O na Transmittance 3668 3435 3668 3424 (1) (2) (3) 1637 1631 1635 1114 1033 1114 908 633 1033 469 3668 3446 1114 913 1008 633 469 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm 1 ) 430 637 469 Fig. 6. FT IR spectra of sepiolite (1) and the Ag2O TiO2/sepiolite composite before (2) and after (3) photocatalytic reaction.
2224 Yu Du et al. / Chinese Journal of Catalysis 36 (2015) 2219 2228 Absorbance (a.u.) 2.1 1.8 1.5 1.2 0.9 0.6 0.3 Sepiolite TiO 2/sepiolite Ag 2O-TiO 2/sepiolite TiO 2 Ag 2O 1.5 TiO 2 1.0 Ag 2O 0.5 1.0 2.9 0 1 2 3 4 5 6 hv (ev) 300 400 500 600 700 800 Wavelength (nm) noparticles loaded onto the TiO2/sepiolite have a suitable band gap for photocatalytic decomposition of organic pollutants in the visible light region. 3.2. Photocatalytic activity of Ag2O TiO2/sepiolite composite Ahv 0.5 / ev 0.5 Fig. 7. UV vis diffuse reflectance spectra of sepiolite, TiO2, Ag2O, TiO2/sepiolite, and Ag2O TiO2/sepiolite composite. The photocatalytic activities of the Ag2O TiO2/sepiolite composites were evaluated by photocatalytic degradation of ARG dye solution and gaseous formaldehyde under visible light irradiation. As shown in Fig. 8, it is clear that the TiO2/sepiolite composites showed little photocatalytic performance. However, after coating Ag2O nanoparticles, the Ag2O TiO2/sepiolite composites showed an obviously enhanced photocatalytic activity and ARG was completely decomposed with an increase in irradiation time. Specifically, when the calcination temperature is 200 C, the obtained Ag2O TiO2/sepiolite composite shows superior photocatalytic activity and the concentration of ARG was decreased by 98% after 40 min, whereas only about 24% of ARG could be removed over the TiO2/sepiolite after 40 min irradiation. Furthermore, the photocatalytic degradation of the ARG solution followed a pseudo first order reaction rate [59]. Thus, the rate constant (k, min 1 ) of the ARG decomposition over Ag2O TiO2/sepiolite composites can be estimated by 3.0 2.5 2.0 k (min 1 ) 0.12 0.10 8 6 4 2 0 100 200 350 Calcination temperature ( o C) 450 Fig. 9. Comparison of apparent rate constants for the Ag2O TiO2/ sepiolite catalysts calcined at different temperatures for the degradation of ARG solution. ln(c0/c) = kt. Fig. 9 shows the comparison of the apparent rate constants of the Ag2O TiO2/sepiolite composites at different calcined temperatures for the degradation of ARG solution. The k values for the ARG decomposition by the catalysts calcined at 100, 200, 350, and 450 C are 64, 0.104, 48, and 20 min 1, respectively. Fig. 10 shows the photocatalytic activities of the samples with different amounts of Ag2O under visible light irradiation. It is clearly shown that the samples with a content of Ag2O from 2% to 10% exhibited enhanced photocatalytic activity. However, when the content of Ag2O was further increased to 15%, no further enhancement of ARG degradation can be observed. Hence, when the content of Ag2O was increased to 10%, a heterostructure of Ag2O and TiO2 was formed on the surface of sepiolite, which made the photocatalytic process more efficient. Fig. 11 displays the photocatalytic degradation of the ARG solution by the different photocatalysts. It is clear that the Ag2O TiO2/sepiolite composite exhibited the highest photocatalytic activity of the systems studied. ARG is almost completely degraded after 40 min irradiation over Ag2O TiO2/ sepiolite under visible light, whereas only about 7% of ARG could be removed over P 25, 11% over pure sepiolite, 24% over TiO2/sepiolite, 62% over TiO2 Ag2O, and 87% over Ag2O/ sepiolite after 40 min irradiation. Therefore, it is possible that a steady system is formed between Ag2O and TiO2 nanoparticles Ct/Co 1.0 0.8 0.6 0.4 0.2 Light off TiO 2/sepiolite 100 o C 200 o C 350 o C 450 o C -20-10 0 10 20 30 40 Irradiation time (min) Fig. 8. Photocatalytic degradation of ARG by TiO2/sepiolite and the Ag2O TiO2/sepiolite catalysts obtained at different temperatures under visible light irradiation. Ct/Co 1.0 0.8 0.6 0.4 0.2 0% Ag 2O 2% Ag 2O 5% Ag 2O 10% Ag 2O 15% Ag 2O Light off Light on -20-10 0 10 20 30 40 Irradiation time (min) Fig. 10. Visible light photocatalytic degradation of ARG by Ag2O TiO2/ sepiolite catalysts with different Ag2O contents.
Yu Du et al. / Chinese Journal of Catalysis 36 (2015) 2219 2228 2225 Ct/Co 1.0 0.8 0.6 0.4 0.2 Light off P-25 Sepiolite TiO2/sepiolite Ag2O/sepiolite Ag2O-TiO2 Ag2O-TiO2/sepiolite Light on -30-20 -10 0 10 20 30 40 Irradiation time (min) Fig. 11. Visible light photocatalytic degradation of ARG solution by different samples. Formaldehyde (ppm) 260 240 Formaldehyde (with catalyst) 220 200 CO 2 (with catalyst) 180 160 140 120 CO 2 (without catalyst) 100 0 60 120 180 240 300 Time (min) 160 140 120 100 Fig. 12. Visible light photocatalytic degradation of formaldehyde with and without Ag2O TiO2/sepiolite catalyst. 80 60 40 20 0 CO2 (ppm) based on multiphase heterostructures, which exist in this coordination to improve the photocatalytic activities of the composites. Formaldehyde, which is a typical volatile organic compound, was selected to further evaluate the photocatalytic properties of the Ag2O TiO2/sepiolite composites. Fig. 12 shows the photocatalytic degradation of gaseous formaldehyde under visible light irradiation. The concentration of formaldehyde in air was reduced and the decomposition product of CO2 gas was gradually increased with an increase in the irradiation time. Meanwhile, Fig. 12 also shows the results of a blank contrast experiment for gas formaldehyde exposed to visible light without catalyst, implying that the catalyst was critical for the degradation of gaseous formaldehyde to occur. These results suggest that the Ag2O TiO2/sepiolite composite also exhibits relatively high photocatalytic activity for the degradation of gaseous formaldehyde under visible light illumination. 3.3. Photocatalytic mechanism Photoinduced active species ( OH, h +, and O2 ) are very important for the mineralization of organic contaminants. Therefore, the terephthalic acid photoluminescence (TAPL) technique was conducted to detect the formation of OH. Scavengers were also employed to determine the oxidative species during the photocatalytic degradation process. As shown in Fig. 13(a), the intensity of the peaks increased slightly with an increase in the illumination time, indicating that OH was not the dominant species for the degradation of ARG solution. From Fig. 13(b), OH radicals trapping experiments (IPA) further determine that the OH was not the main active species in the photocatalytic process. KI was used as the scavenger of h + and OH, sodium oxalate (Na2C2O4) as the scavenger of h +, and benzoquinone (BQ) as the scavenger of O2 [60]. It can be seen that the KI and Na2C2O4 had significant effects on the photocatalytic activity of the Ag2O TiO2/sepiolite composite, which indicates that the h + is one of the main active species in the photocatalytic system. In the presence of BQ, the degradation efficiency of Ag2O TiO2/ sepiolite largely decreased, which suggests that O2 is also one of the major active species in the photocatalytic system. O2 could be generated by the adsorbed oxygen molecule on the surface of the catalyst capturing active electrons [26]. Thus, oxidative h + /O2 radicals could play an important role in the photocatalytic solution under visible light irradiation and in the photocatalytic ARG degradation ability of the composite capturing the reactive species h +, OH, and O2. 40 (a) 1.0 (b) Fluorescence intensity (a.u.) 30 20 10 50 min 40 min 30 min 20 min 10 min 0 min 0 360 390 420 450 480 510 540 570 Wavelength (nm) Ct/C0 0.8 0.6 0.4 0.2 KI Light off Na2C2O4 BQ IPA No quenchers -20-10 0 10 20 30 40 Irradiation time (min) Fig. 13. (a) OH trapping PL spectra of terephthalic acid (TA) solution in the presence of the Ag2O TiO2/sepiolite composite; (b) Photocatalytic degradation of ARG over the Ag2O TiO2/sepiolite with and without the quenchers.
2226 Yu Du et al. / Chinese Journal of Catalysis 36 (2015) 2219 2228 Organic dye Products Visible light O 2 The enhanced photocatalytic mechanism of Ag2O TiO2/ sepiolite heterostructure is presented in Fig. 14. The high photocatalytic activity of the Ag2O TiO2/sepiolite composite can be attributed to Ag2O TiO2 heterostructure being in the composite and the porous structure of sepiolite. Under visible light irradiation, Ag2O nanoparticles can be excited to h + and e because of their narrow band gap. Meanwhile, Ag2O has excessive negative charges that can easily adsorb onto positively charged TiO2 nanoparticles [48,61,62]. In this case, the photogenerated electrons on the CB of Ag2O can transfer into the CB of TiO2 and the photogenerated holes gather in the VB of Ag2O. Finally, the migration of photogenerated carriers is promoted because of the difference in band positions between those in the Ag2O TiO2 heterojunction. Thereafter, the photogenerated electrons on the surface of Ag2O can transfer into the CB of the TiO2 that has reacted with molecular oxygen to produce O2, and the holes generated in Ag2O directly oxidize the adsorbed dyes or participate in the reaction to generate other radical species that oxidize the organic compounds. Thus, the series of oxidation reduction reactions caused by the heterostructure of Ag2O TiO2 can restrain the fast recombination of photoinduced electron hole pairs effectively. Meanwhile, the porous structure of the sepiolite clay provides numerous nucleation sites for the formation of Ag2O TiO2 heterostructure. This synergistic effect leads to the enhancement in photocatalytic activity [63]. 4. Conclusions O 2 E g = 2.9 ev e - e - e - e - CB h + h + h + h + VB TiO 2 e - e - e - e - CB E g = 1.0 ev h + h + h + h + Novel Ag2O TiO2/sepiolite composites with heterostructure were synthesized by a simple two step method. The as prepared Ag2O TiO2/sepiolite composites exhibited enhanced visible light photocatalytic activity for the degradation of ARG. This enhancement may be ascribed to the porous structure of sepiolite and the high quantum efficiency of Ag2O TiO2 heterostructure. Under visible light irradiation, Ag2O nanoparticles as a visible light active component enhanced the Ag2O TiO2 heterostructure photocatalytic activity via synergetic effects on the electron hole separation and efficient electron transmission at the Ag2O TiO2 interface. Sepiolite clay as the VB Ag 2 O Products Visible light Organic dye Fig. 14. Proposed photocatalytic mechanism of Ag2O TiO2/sepiolite catalysts for the degradation of ARG solution under irradiation of visible light. substrate for Ag2O TiO2 heterostructure could provide more active sites and enhance the adsorption properties. In addition, the h + and O2 radicals could be the main active species during the photo oxidation process. 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2228 Yu Du et al. / Chinese Journal of Catalysis 36 (2015) 2219 2228 以酸性红 G 为模拟污染废水, 研究了焙烧温度和 Ag 2 O 负载量等制备条件对所制催化剂可见光催化性能的影响. 结果表明, 在可见光照射下, 焙烧温度为 200 C, Ag 2 O 负载量为 10% 条件下制备的复合光催化剂对酸性红 G 的降解率为 98%, 与 Ag 2 O-TiO 2 Ag 2 O/ 海泡石和 TiO 2 / 海泡石等复合物相比, Ag 2 O-TiO 2 / 海泡石复合光催化剂展现了优异的可见光催化性能. 此外, Ag 2 O-TiO 2 / 海泡石复合光催化剂同样能够在可见光条件下有效降解常见室内空气污染物甲醛, 进一步证实了催化剂优异的光催化性能. 化学荧光法和活性物种捕获实验表明, 复合光催化剂降解有机污染物的活性基团主要是光生空穴和超氧自由基. 催化剂能带结构分析表明, Ag 2 O 和 TiO 2 具有相匹配的能带结构, 两者复合有利于光生载流子分离和迁移, 增强催化剂光催化活性. 海泡石作为光催化剂载体能够有效固载光催化成分, 增加光催化剂有效表面积和活性位, 有利于提高复合光催化剂的吸附性能和回收利用率. 关键词 : 氧化银 ; 二氧化钛 ; 海泡石 ; 异质结 ; 可见光催化 ; 酸性红 G; 光催化机理 收稿日期 : 2015-08-17. 接受日期 : 2015-11-11. 出版日期 : 2015-12-20. * 通讯联系人. 电话 : (027)87651816; 传真 : (027)87887445; 电子信箱 : gkzhang@whut.edu.cn 基金来源 : 国家科技支撑计划 (2012BAJ25B02-03). 本文的英文电子版由 Elsevier 出版社在 ScienceDirect 上出版 (http://www.sciencedirect.com/science/journal/18722067).