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Preparation of Ag@AgI-intercalated K4Nb6O17 composite and enhanced photocatalytic degradation of Rhodamine B under visible light
Catalysis Communications 36 (2013) 71–74
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Preparation of Ag@AgI-intercalated K4Nb6O17 composite and enhanced photocatalytic degradation of Rhodamine B under visible light Wenquan Cui ⁎, Huan Wang, Yinghua Liang ⁎, Li Liu, Bingxu Han College of Chemical Engineering, Hebei United University, Tangshan, 063009, PR China
a r t i c l e
i n f o
Article history: Received 27 January 2013 Received in revised form 7 March 2013 Accepted 8 March 2013 Available online 17 March 2013 Keywords: Ag@AgI nanoparticles Intercalation K4Nb6O17 Plasmonic photocatalysis
a b s t r a c t In this study, a novel plasmonic photocatalyst, Ag@AgI intercalated layered niobate, was synthesized via a microwave-assisted ion-exchange method. The composite materials prepared were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray fluorescence spectrometry (XRF), Brunauer–Emmett–Teller (BET) and ultraviolet–visible diffuse reflection spectra (UV–vis). The as-prepared plasmonic photocatalyst exhibited an enhanced and stable photocatalytic performance for the degradation of Rhodamine B (RhB) and up to 83% of RhB was degraded in 40 min under visible light irradiation. The mechanism of separation of the photo-generated electrons and holes at the K4Nb6O17/Ag@AgI composite was discussed. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Ag@AgX, (X = Cl, Br, I) structures as efficient visible light photocatalysts have recently attracted much attention due to the incorporated surface plasmon resonance which can greatly increase the utilization of visible light. Recent reports revealed that the recombination of oxide semiconductors (such as WO3 [1], TiO2 [2], Al2O3 [3]) with plasmonic Ag@AgX photocatalysts can greatly increase the photocatalytic efficiency of the resulting bulk photocatalysts under visible light. Hu et al. [4] prepared AgI/TiO2 by the deposition-precipitation method, and the photocatalyst showed high efficiency for the degradation of azodyes under visible light irradiation and was also found to be highly effective in killing Escherichia coli and Staphylococcus aureus [5]. However, the size and shape of Ag nanoparticles (NPs), one of the key features affecting the performance of plasmonic photocatalysts [6], could not be welladdressed via the surface modification methods. K4Nb6O17 known as a layered compound has attracted much interest due to its distinctive layered structure, which can suppress the recombination of photo-induced electrons and holes under UV light irradiation with the intercalation of nano-semiconductor particles [7]. However, it cannot exhibit any photocatalytic activity under visible light irradiation due to its relatively large band gap of 3.2 eV [8], which restricts its use in practical large-scale applications. Herein, we explore Ag@AgI intercalated K4Nb6O17 photocatalyst synthesized under the assistance of microwave irradiation. The photocatalytic activities of the samples were investigated for the degradation
⁎ Corresponding author. Tel.: +86 315 2592014. E-mail addresses: [email protected], [email protected] (Y. Liang). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.03.011
of RhB using visible light irradiation. And, a possible mechanism for the reaction was proposed. 2. Experimental 2.1. Catalyst synthesis All of the reagents were analytical grade and used without further purification. The synthesis of K4Nb6O17 has been reported in our previous work [9] as well as H + intercalated K4Nb6O17 (H4Nb6O17) and butylamine intercalated K4Nb6O17 ((C4H9NH3)4Nb6O17). And thus the layered space of K4Nb6O17 was expanded by butylamine. After that, the (C4H9NH3)4Nb6O17 sample was reacted with 7.5 g CH3COOAg in 500 mL deionized water under microwave irradiation at 80 °C for 2.5 h, and the Ag+ intercalated K4Nb6O17 sample was obtained. According to the previous report [10], acetates (such as CH3COOCd) were used as precursors in the intercalation procedure with (C4H9NH3)4Nb6O17 due to the effect of acid-base between the acetate and organic amines. The obtained Ag+ intercalated K4Nb6O17 powder (designated as K4Nb6O17/Ag+) was added to 0.1 mol/L KI, and stirred for 1.0 h at room temperature in the dark, the above mixture was then irradiated with a 250 W metal halide lamp and then the K4Nb6O17/Ag@AgI was obtained. 2.2. Characterization The as-prepared samples were characterized by powder X-ray diffraction (XRD) using a Rigaku D/MAX2500PC diffractometer with Cu Kα radiation, with an operating voltage of 40 kV and an operating current of 100 mA. Scanning electron microscopy (SEM) images were
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2.3. Photocatalytic evaluation The photocatalytic activities of the samples were estimated by the degradation of RhB under visible light at room temperature, using a 250 W metal halide lamp with a UV filter (λ > 400 nm). In a typical procedure, 0.25 g photocatalysts was dispersed in 250 mL RhB solution (10 mg/L). Prior to irradiation, the suspensions were kept in the dark for 30 min to obtain equilibrium absorption between the photocatalysts and the dye. During the irradiation process, 3 mL suspension was sampled every 10 min and then centrifuged to separate the photocatalyst particles. The supernatant was then removed and analyzed by a UV–vis spectrophotometer. 3. Results and discussion 3.1. Catalyst characterization
Fig. 1. XRD patterns of (a) K4Nb6O17, (b) H4Nb6O17, (c) (C4H9NH3)4Nb6O17, (d) K4Nb6O17/ AgI, (e) K4Nb6O17/Ag@AgI.
observed by a Hitachi s-4800 SEM microscope. Transmission electron microscopy (TEM) was obtained using a JEM-2010 microscope operating at acceleration voltage of 200 kV. The chemical compositions of the samples were tested by an energy dispersive X-ray detector (EDX, Thermo Noran 7) and an X-ray fluorescence spectrometer (XRF, Rigaku, ZSX Promusll). The Brunauer–Emmett–Teller analysis (BET) measurements were performed on a JW-BK nitrogen adsorption apparatus at 77.3 K. UV–vis DRS spectra were recorded with a UV–vis spectrometer (Puxi, UV1901).
The XRD patterns of the as-prepared samples are given in Fig. 1. Based on Fig. 1a, the XRD peaks of K4Nb6O17 could be readily indexed to a pure phase of orthorhombic K4Nb6O17, according to JCPDS (21-1295). The main peak appeared at 10.76°, corresponding to the 040 crystal plane, indicating that the d value was 0.838 nm. The width of the layered space was calculated to be 0.278 nm by 4subtracting the layered thicknesses of the Nb6O17 (0.56 nm) [11] from the d040 value. Curves b and c show the XRD pattern of H4Nb6O17 and (C4H9NH3)4Nb6O17, respectively. The results evidenced the successful entrance of the H+ and CH3(CH2)3NH2 into the interlayer of K4Nb6O17 according to our previous study of CdS intercalated K4Nb6O17 [9]. Curves d and e show the XRD patterns of K4Nb6O17/AgI and K4Nb6O17/Ag@AgI, and the 040 crystals were shifted to 7.05° and 8.34°, respectively, compared to the 040 crystal face of (C4H9NH3)4Nb6O17, indicating
Fig. 2. SEM images of photocatalysts ((a) K4Nb6O17; (b) K4Nb6O17/Ag@AgI; (c) corresponding EDX pattern for (b)); (d) TEM image of photocatalyst K4Nb6O17/Ag@AgI.
W. Cui et al. / Catalysis Communications 36 (2013) 71–74 Table 1 XRF results of the samples. Molar ratio
K4Nb6O17/Ag@AgI
K4Nb6O17/AgI
K4Nb6O17
Ag@AgI
K:Nb Ag:I:Nb
29.90:100 191.85:100:378.03
29.40:100 113.34:100:347.09
72.39:100 –
– 161.20:100:0
Table 2 BET surface area and pore structure of the samples. Sample
SBET (m2/g)
Pore size (nm)
Pore volume (cm3/g)
K4Nb6O17 AgI Ag@AgI K4Nb6O17/AgI K4Nb6O17/Ag@AgI
0.473 0.082 0.152 1.652 2.201
3.818 4.265 4.222 3.818 3.049
0.001 0 0 0.008 0.008
Fig. 3. UV–vis spectra of the samples. (a) K4Nb6O17; (b) K4Nb6O17/AgI; (c) K4Nb6O17/Ag@ AgI; (d) AgI; (e) Ag@AgI.
an increase of the layered space. The 040 peak of K4Nb6O17/Ag@AgI at 8.34° was larger than that of K4Nb6O17/AgI, which could be explained by the photoreduction of AgI under visible light irradiation and the resulting reduced Ag 0 NPs were uniformly dispersed on the surface of AgI. As a result, the layered space of K4Nb6O17/Ag@AgI was narrower than that of K4Nb6O17/AgI. These results confirmed the fact that Ag@AgI was successfully intercalated into the interlayer
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space of K4Nb6O17. In Fig. 1d and e, the XRD patterns of K4Nb6O17/AgI and K4Nb6O17/Ag@AgI were composed of AgI phase, corresponding to JCPDS (09-0374). However, the diffraction peaks attributed to Ag could not be easily observed in Fig. 1e, and this was presumably due to the incorporation at very low contents, small particle size, and high dispersion in the interlaminar space of K4Nb6O17 [3]. SEM images of K4Nb6O17 and K4Nb6O17/Ag@AgI are shown in Fig. 2a and b. As shown, the layered structure of K4Nb6O17/Ag@AgI is not greatly damaged and the width of layered space is larger than that of pure K4Nb6O17 upon the intercalation of Ag@AgI. In the EDX spectrum (Fig. 2c), peaks associated with Ag, I, K, Nb and O were observed. Nb, and O result from K4Nb6O17, and Ag and I result from AgI, respectively. Besides, the elemental content of the samples determined by XRF analysis is given in Table 1. As shown, the K/Nb molar ratio of K4Nb6O17/Ag@AgI was found to be much lower than that of K4Nb6O17. This indicated that most of K + was replaced by H+, while a small amount of K+ still existed in the layered compound. Furthermore, the Ag/I molar ratio of K4Nb6O17/Ag@AgI was 191.85:100 and was higher than that of K4Nb6O17/AgI, indicating that partial AgI had been changed to be Ag0 NPs during the preparation process. The Ag NPs with an average diameter of about 5–10 nm are uniformly dispersed in the layered nanostructure as shown in Fig. 2d, and gave rise to a high photocatalytic activity under visible light irradiation. Additionally, it can be observed from the TEM image that there are some Ag particles with a diameter of about 50–100 nm, and these may have been due to Ag NPs agglomerating on the surface of AgI. Table 2 summarizes the physical properties of the samples. As shown, the BET surface area and pore volume of K4Nb6O17 increased with the intercalation of AgI and Ag@AgI, which can provide abundant active sites for the adsorption of RhB and enhance the catalytic activity on the RhB degradation. The UV–vis absorption spectra of the samples are shown in Fig. 3. As shown, the AgI (Fig. 3d) and Ag@AgI (Fig. 3e) possessed good visible light absorption due to the visible light response for AgI and the SPR effect of Ag 0 NPs formed on the surface of AgI, respectively. The absorption of K4Nb6O17 (Fig. 3b and c) was enhanced ranging from 450 nm to 750 nm with the intercalation of AgI and Ag@AgI, and subsequently gave rise to a higher photocatalytic activity under visible light. 3.2. Photocatalyst activity The photocatalytic activities for the degradation of RhB of the samples were evaluated under visible light. As shown in Fig. 4A, the K4Nb6O17/Ag@AgI photocatalyst exhibited an excellent photocatalytic activity under visible light irradiation, and almost 83% of RhB was degraded after irradiation for 40 min, while the concentration of RhB
Fig. 4. (A) Comparison of photocatalytic activity of as-prepared samples for the photocatalytic decomposition of RhB; (B) repeated degradation of K4Nb6O17/Ag@AgI under visible light irradiation for five cycles.
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improved. The electrons at the interface of Ag NPs and K4Nb6O17 transferred to molecular oxygen which was adsorbed on the surface of the catalyst to generate superoxide radicals (O2∙ −) [14], and induced the degradation of RhB. Meanwhile, the reactive holes located on the metallic Ag could attack organic pollutants adsorbed on the surface of the photocatalyst, as well. In summary, the high activity of the K4Nb6O17/Ag@AgI observed can be attributed to the synergistic effect of surface plasmon resonance (SPR) exhibited by Ag NPs on the surface of AgI, photocatalytic action by AgI itself, as well as the electronhole pairs separation derived from the matching band potentials between Ag@AgI and K4Nb6O17. 4. Conclusions
Fig. 5. Schematic diagram for the charge separation mechanism under visible light irradiation.
was unchanged in the absence of photocatalyst and for the dark reaction. The pure K4Nb6O17 showed no photocatalytic activity due to its wide band gap of 3.2 eV. For comparison, the photocatalytic activity of K4Nb6O17/AgI and an equivalent amount of Ag@AgI (calculated based on the equivalent Ag@AgI content in K4Nb6O17/Ag@AgI) were also studied under the same conditions. The Ag@AgI catalyst exhibited a much slower degradation rate than that of K4Nb6O17/Ag@AgI and it can be seen from Fig. 4A that only 66% of RhB was degraded for the K4Nb6O17/AgI catalyst. The above results presented in Fig. 4A indicated that the Ag@AgI intercalated K4Nb6O17 exhibited high photocatalytic activity when irradiated under visible light due to the existence of Ag@AgI and the associated SPR effect. In order to examine the stability of the photocatalyst, the catalysis process was repeated five times with the same photocatalyst. It should be noted that the high performance of K4Nb6O17/Ag@AgI was effectively maintained about 78% of RhB degradation in 40 min after 5 cyclic experiments, as shown in Fig. 4B, which confirms the excellent stability of the catalyst. The results indicate that the visible light photocatalytic activity of K4Nb6O17/Ag@AgI particles is efficiently stable. 3.3. Proposed mechanisms The band structure of Ag@AgI intercalated K4Nb6O17 was determined and depicted in Fig. 5. The mechanism of photocatalytic action of the K4Nb6O17/Ag@AgI could be explained as follows: under visible light irradiation, a visible light photon could be absorbed by a silver NP, generating a hole and an electron in the Ag NPs such that the electrons were transferred to the interface of Ag NPs and K4Nb6O17 [12], until the Fermi level of metallic Ag and K4Nb6O17 tended to equality. Besides, the photogenerated holes in the VB of AgI could easily flow into metallic Ag faster than the rate of electron-hole recombination between the VB and CB of AgI [13]. As a result, the photogenerated electron-hole pairs were separated and the quantum yield was efficiently
Ag@AgI intercalated K4Nb6O17 photocatalysts were successfully synthesized by an ion-exchange method using a microwave-assisted synthesis. The plasmonic photocatalyst K4Nb6O17/Ag@AgI exhibited excellent photocatalytic activity for the degradation of RhB under visible light irradiation, and almost 83% RhB was degraded in 40 min under the optimized conditions. The enhanced activity could be explained by the surface plasmon resonance of Ag NPs formed on the surface of AgI, as well as the visible light driven AgI and the electron-hole pair separation derived from the matching band potentials between Ag@AgI and K4Nb6O17. The as-prepared K4Nb6O17/Ag@AgI has a potential application in the degradation of organic pollutants in wastewater. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant nos. 50972037, 51172063, 51202056), and the Natural Science Foundation of Hebei Province, China (grant no. E2012401070). References [1] B.W. Ma, J.F. Guo, W.L. Dai, K.N. Fan, Applied Catalysis B: Environmental 123–124 (2012) 193. [2] M.R. Elahifard, S. Rahimnejad, S. Haghighi, M.R. Gholami, Journal of the American Chemical Society 129 (2007) 9552. [3] C. Hu, T.W. Peng, X.X. Hu, Y.L. Nie, X.F. Zhou, J.H. Qu, H. He, Journal of the American Chemical Society 132 (2010) 857. [4] C. Hu, X.F. Hu, L.S. Wang, J.H. Qu, A.M. Wang, Environmental Science and Technology 40 (2006) 7903. [5] C. Hu, J. Guo, J.H. Qu, X.X. Hu, Langmuir 23 (2007) 4982. [6] M. El-Sayed, Accounts of Chemical Research 34 (2011) 257. [7] S. Tawkaew, Y. Fujishiro, S. Yin, T. Stato, Colloids and Surfaces A 179 (2001) 139. [8] T. Nalato, H. Edakubo, T. Shimomura, Microporous and Mesoporous Materials 123 (2009) 280. [9] W.Q. Cui, Y.F. Liu, L. Liu, J.S. Hu, Y.H. Liang, Applied Catalysis A: General 417–418 (2012) 111. [10] L.L. Zhang, W.G. Zhang, Journal of Materials Science 41 (12) (2006) 3917. [11] S. Uchida, Y. Yamamoto, Y. Fujishiro, A. Watanabe, O. Ito, T. Sato, Journal of the Chemical Society, Faraday Transactions 93 (1997) 3229. [12] X.F. Zhou, C. Hu, X.X. Hu, T.W. Peng, J.H. Qu, Journal of Physical Chemistry C 114 (2010) 2746. [13] H. Tada, T. Mitsu, T. Kiyonaga, T. Akita, K. Tanaka, Nature Materials 5 (2006) 782. [14] J. Cao, B.Y. Xu, B.D. Luo, H.L. Lin, S.F. Chen, Applied Surface Science 257 (2011) 7083.
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Preparation of Ag@AgI-intercalated K4Nb6O17 composite and enhanced photocatalytic degradation of Rhodamine B under visible light Wenquan Cui ⁎, Huan Wang, Yinghua Liang ⁎, Li Liu, Bingxu Han College of Chemical Engineering, Hebei United University, Tangshan, 063009, PR China
a r t i c l e
i n f o
Article history: Received 27 January 2013 Received in revised form 7 March 2013 Accepted 8 March 2013 Available online 17 March 2013 Keywords: Ag@AgI nanoparticles Intercalation K4Nb6O17 Plasmonic photocatalysis
a b s t r a c t In this study, a novel plasmonic photocatalyst, Ag@AgI intercalated layered niobate, was synthesized via a microwave-assisted ion-exchange method. The composite materials prepared were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray fluorescence spectrometry (XRF), Brunauer–Emmett–Teller (BET) and ultraviolet–visible diffuse reflection spectra (UV–vis). The as-prepared plasmonic photocatalyst exhibited an enhanced and stable photocatalytic performance for the degradation of Rhodamine B (RhB) and up to 83% of RhB was degraded in 40 min under visible light irradiation. The mechanism of separation of the photo-generated electrons and holes at the K4Nb6O17/Ag@AgI composite was discussed. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Ag@AgX, (X = Cl, Br, I) structures as efficient visible light photocatalysts have recently attracted much attention due to the incorporated surface plasmon resonance which can greatly increase the utilization of visible light. Recent reports revealed that the recombination of oxide semiconductors (such as WO3 [1], TiO2 [2], Al2O3 [3]) with plasmonic Ag@AgX photocatalysts can greatly increase the photocatalytic efficiency of the resulting bulk photocatalysts under visible light. Hu et al. [4] prepared AgI/TiO2 by the deposition-precipitation method, and the photocatalyst showed high efficiency for the degradation of azodyes under visible light irradiation and was also found to be highly effective in killing Escherichia coli and Staphylococcus aureus [5]. However, the size and shape of Ag nanoparticles (NPs), one of the key features affecting the performance of plasmonic photocatalysts [6], could not be welladdressed via the surface modification methods. K4Nb6O17 known as a layered compound has attracted much interest due to its distinctive layered structure, which can suppress the recombination of photo-induced electrons and holes under UV light irradiation with the intercalation of nano-semiconductor particles [7]. However, it cannot exhibit any photocatalytic activity under visible light irradiation due to its relatively large band gap of 3.2 eV [8], which restricts its use in practical large-scale applications. Herein, we explore Ag@AgI intercalated K4Nb6O17 photocatalyst synthesized under the assistance of microwave irradiation. The photocatalytic activities of the samples were investigated for the degradation
⁎ Corresponding author. Tel.: +86 315 2592014. E-mail addresses: [email protected], [email protected] (Y. Liang). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.03.011
of RhB using visible light irradiation. And, a possible mechanism for the reaction was proposed. 2. Experimental 2.1. Catalyst synthesis All of the reagents were analytical grade and used without further purification. The synthesis of K4Nb6O17 has been reported in our previous work [9] as well as H + intercalated K4Nb6O17 (H4Nb6O17) and butylamine intercalated K4Nb6O17 ((C4H9NH3)4Nb6O17). And thus the layered space of K4Nb6O17 was expanded by butylamine. After that, the (C4H9NH3)4Nb6O17 sample was reacted with 7.5 g CH3COOAg in 500 mL deionized water under microwave irradiation at 80 °C for 2.5 h, and the Ag+ intercalated K4Nb6O17 sample was obtained. According to the previous report [10], acetates (such as CH3COOCd) were used as precursors in the intercalation procedure with (C4H9NH3)4Nb6O17 due to the effect of acid-base between the acetate and organic amines. The obtained Ag+ intercalated K4Nb6O17 powder (designated as K4Nb6O17/Ag+) was added to 0.1 mol/L KI, and stirred for 1.0 h at room temperature in the dark, the above mixture was then irradiated with a 250 W metal halide lamp and then the K4Nb6O17/Ag@AgI was obtained. 2.2. Characterization The as-prepared samples were characterized by powder X-ray diffraction (XRD) using a Rigaku D/MAX2500PC diffractometer with Cu Kα radiation, with an operating voltage of 40 kV and an operating current of 100 mA. Scanning electron microscopy (SEM) images were
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2.3. Photocatalytic evaluation The photocatalytic activities of the samples were estimated by the degradation of RhB under visible light at room temperature, using a 250 W metal halide lamp with a UV filter (λ > 400 nm). In a typical procedure, 0.25 g photocatalysts was dispersed in 250 mL RhB solution (10 mg/L). Prior to irradiation, the suspensions were kept in the dark for 30 min to obtain equilibrium absorption between the photocatalysts and the dye. During the irradiation process, 3 mL suspension was sampled every 10 min and then centrifuged to separate the photocatalyst particles. The supernatant was then removed and analyzed by a UV–vis spectrophotometer. 3. Results and discussion 3.1. Catalyst characterization
Fig. 1. XRD patterns of (a) K4Nb6O17, (b) H4Nb6O17, (c) (C4H9NH3)4Nb6O17, (d) K4Nb6O17/ AgI, (e) K4Nb6O17/Ag@AgI.
observed by a Hitachi s-4800 SEM microscope. Transmission electron microscopy (TEM) was obtained using a JEM-2010 microscope operating at acceleration voltage of 200 kV. The chemical compositions of the samples were tested by an energy dispersive X-ray detector (EDX, Thermo Noran 7) and an X-ray fluorescence spectrometer (XRF, Rigaku, ZSX Promusll). The Brunauer–Emmett–Teller analysis (BET) measurements were performed on a JW-BK nitrogen adsorption apparatus at 77.3 K. UV–vis DRS spectra were recorded with a UV–vis spectrometer (Puxi, UV1901).
The XRD patterns of the as-prepared samples are given in Fig. 1. Based on Fig. 1a, the XRD peaks of K4Nb6O17 could be readily indexed to a pure phase of orthorhombic K4Nb6O17, according to JCPDS (21-1295). The main peak appeared at 10.76°, corresponding to the 040 crystal plane, indicating that the d value was 0.838 nm. The width of the layered space was calculated to be 0.278 nm by 4subtracting the layered thicknesses of the Nb6O17 (0.56 nm) [11] from the d040 value. Curves b and c show the XRD pattern of H4Nb6O17 and (C4H9NH3)4Nb6O17, respectively. The results evidenced the successful entrance of the H+ and CH3(CH2)3NH2 into the interlayer of K4Nb6O17 according to our previous study of CdS intercalated K4Nb6O17 [9]. Curves d and e show the XRD patterns of K4Nb6O17/AgI and K4Nb6O17/Ag@AgI, and the 040 crystals were shifted to 7.05° and 8.34°, respectively, compared to the 040 crystal face of (C4H9NH3)4Nb6O17, indicating
Fig. 2. SEM images of photocatalysts ((a) K4Nb6O17; (b) K4Nb6O17/Ag@AgI; (c) corresponding EDX pattern for (b)); (d) TEM image of photocatalyst K4Nb6O17/Ag@AgI.
W. Cui et al. / Catalysis Communications 36 (2013) 71–74 Table 1 XRF results of the samples. Molar ratio
K4Nb6O17/Ag@AgI
K4Nb6O17/AgI
K4Nb6O17
Ag@AgI
K:Nb Ag:I:Nb
29.90:100 191.85:100:378.03
29.40:100 113.34:100:347.09
72.39:100 –
– 161.20:100:0
Table 2 BET surface area and pore structure of the samples. Sample
SBET (m2/g)
Pore size (nm)
Pore volume (cm3/g)
K4Nb6O17 AgI Ag@AgI K4Nb6O17/AgI K4Nb6O17/Ag@AgI
0.473 0.082 0.152 1.652 2.201
3.818 4.265 4.222 3.818 3.049
0.001 0 0 0.008 0.008
Fig. 3. UV–vis spectra of the samples. (a) K4Nb6O17; (b) K4Nb6O17/AgI; (c) K4Nb6O17/Ag@ AgI; (d) AgI; (e) Ag@AgI.
an increase of the layered space. The 040 peak of K4Nb6O17/Ag@AgI at 8.34° was larger than that of K4Nb6O17/AgI, which could be explained by the photoreduction of AgI under visible light irradiation and the resulting reduced Ag 0 NPs were uniformly dispersed on the surface of AgI. As a result, the layered space of K4Nb6O17/Ag@AgI was narrower than that of K4Nb6O17/AgI. These results confirmed the fact that Ag@AgI was successfully intercalated into the interlayer
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space of K4Nb6O17. In Fig. 1d and e, the XRD patterns of K4Nb6O17/AgI and K4Nb6O17/Ag@AgI were composed of AgI phase, corresponding to JCPDS (09-0374). However, the diffraction peaks attributed to Ag could not be easily observed in Fig. 1e, and this was presumably due to the incorporation at very low contents, small particle size, and high dispersion in the interlaminar space of K4Nb6O17 [3]. SEM images of K4Nb6O17 and K4Nb6O17/Ag@AgI are shown in Fig. 2a and b. As shown, the layered structure of K4Nb6O17/Ag@AgI is not greatly damaged and the width of layered space is larger than that of pure K4Nb6O17 upon the intercalation of Ag@AgI. In the EDX spectrum (Fig. 2c), peaks associated with Ag, I, K, Nb and O were observed. Nb, and O result from K4Nb6O17, and Ag and I result from AgI, respectively. Besides, the elemental content of the samples determined by XRF analysis is given in Table 1. As shown, the K/Nb molar ratio of K4Nb6O17/Ag@AgI was found to be much lower than that of K4Nb6O17. This indicated that most of K + was replaced by H+, while a small amount of K+ still existed in the layered compound. Furthermore, the Ag/I molar ratio of K4Nb6O17/Ag@AgI was 191.85:100 and was higher than that of K4Nb6O17/AgI, indicating that partial AgI had been changed to be Ag0 NPs during the preparation process. The Ag NPs with an average diameter of about 5–10 nm are uniformly dispersed in the layered nanostructure as shown in Fig. 2d, and gave rise to a high photocatalytic activity under visible light irradiation. Additionally, it can be observed from the TEM image that there are some Ag particles with a diameter of about 50–100 nm, and these may have been due to Ag NPs agglomerating on the surface of AgI. Table 2 summarizes the physical properties of the samples. As shown, the BET surface area and pore volume of K4Nb6O17 increased with the intercalation of AgI and Ag@AgI, which can provide abundant active sites for the adsorption of RhB and enhance the catalytic activity on the RhB degradation. The UV–vis absorption spectra of the samples are shown in Fig. 3. As shown, the AgI (Fig. 3d) and Ag@AgI (Fig. 3e) possessed good visible light absorption due to the visible light response for AgI and the SPR effect of Ag 0 NPs formed on the surface of AgI, respectively. The absorption of K4Nb6O17 (Fig. 3b and c) was enhanced ranging from 450 nm to 750 nm with the intercalation of AgI and Ag@AgI, and subsequently gave rise to a higher photocatalytic activity under visible light. 3.2. Photocatalyst activity The photocatalytic activities for the degradation of RhB of the samples were evaluated under visible light. As shown in Fig. 4A, the K4Nb6O17/Ag@AgI photocatalyst exhibited an excellent photocatalytic activity under visible light irradiation, and almost 83% of RhB was degraded after irradiation for 40 min, while the concentration of RhB
Fig. 4. (A) Comparison of photocatalytic activity of as-prepared samples for the photocatalytic decomposition of RhB; (B) repeated degradation of K4Nb6O17/Ag@AgI under visible light irradiation for five cycles.
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improved. The electrons at the interface of Ag NPs and K4Nb6O17 transferred to molecular oxygen which was adsorbed on the surface of the catalyst to generate superoxide radicals (O2∙ −) [14], and induced the degradation of RhB. Meanwhile, the reactive holes located on the metallic Ag could attack organic pollutants adsorbed on the surface of the photocatalyst, as well. In summary, the high activity of the K4Nb6O17/Ag@AgI observed can be attributed to the synergistic effect of surface plasmon resonance (SPR) exhibited by Ag NPs on the surface of AgI, photocatalytic action by AgI itself, as well as the electronhole pairs separation derived from the matching band potentials between Ag@AgI and K4Nb6O17. 4. Conclusions
Fig. 5. Schematic diagram for the charge separation mechanism under visible light irradiation.
was unchanged in the absence of photocatalyst and for the dark reaction. The pure K4Nb6O17 showed no photocatalytic activity due to its wide band gap of 3.2 eV. For comparison, the photocatalytic activity of K4Nb6O17/AgI and an equivalent amount of Ag@AgI (calculated based on the equivalent Ag@AgI content in K4Nb6O17/Ag@AgI) were also studied under the same conditions. The Ag@AgI catalyst exhibited a much slower degradation rate than that of K4Nb6O17/Ag@AgI and it can be seen from Fig. 4A that only 66% of RhB was degraded for the K4Nb6O17/AgI catalyst. The above results presented in Fig. 4A indicated that the Ag@AgI intercalated K4Nb6O17 exhibited high photocatalytic activity when irradiated under visible light due to the existence of Ag@AgI and the associated SPR effect. In order to examine the stability of the photocatalyst, the catalysis process was repeated five times with the same photocatalyst. It should be noted that the high performance of K4Nb6O17/Ag@AgI was effectively maintained about 78% of RhB degradation in 40 min after 5 cyclic experiments, as shown in Fig. 4B, which confirms the excellent stability of the catalyst. The results indicate that the visible light photocatalytic activity of K4Nb6O17/Ag@AgI particles is efficiently stable. 3.3. Proposed mechanisms The band structure of Ag@AgI intercalated K4Nb6O17 was determined and depicted in Fig. 5. The mechanism of photocatalytic action of the K4Nb6O17/Ag@AgI could be explained as follows: under visible light irradiation, a visible light photon could be absorbed by a silver NP, generating a hole and an electron in the Ag NPs such that the electrons were transferred to the interface of Ag NPs and K4Nb6O17 [12], until the Fermi level of metallic Ag and K4Nb6O17 tended to equality. Besides, the photogenerated holes in the VB of AgI could easily flow into metallic Ag faster than the rate of electron-hole recombination between the VB and CB of AgI [13]. As a result, the photogenerated electron-hole pairs were separated and the quantum yield was efficiently
Ag@AgI intercalated K4Nb6O17 photocatalysts were successfully synthesized by an ion-exchange method using a microwave-assisted synthesis. The plasmonic photocatalyst K4Nb6O17/Ag@AgI exhibited excellent photocatalytic activity for the degradation of RhB under visible light irradiation, and almost 83% RhB was degraded in 40 min under the optimized conditions. The enhanced activity could be explained by the surface plasmon resonance of Ag NPs formed on the surface of AgI, as well as the visible light driven AgI and the electron-hole pair separation derived from the matching band potentials between Ag@AgI and K4Nb6O17. The as-prepared K4Nb6O17/Ag@AgI has a potential application in the degradation of organic pollutants in wastewater. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant nos. 50972037, 51172063, 51202056), and the Natural Science Foundation of Hebei Province, China (grant no. E2012401070). References [1] B.W. Ma, J.F. Guo, W.L. Dai, K.N. Fan, Applied Catalysis B: Environmental 123–124 (2012) 193. [2] M.R. Elahifard, S. Rahimnejad, S. Haghighi, M.R. Gholami, Journal of the American Chemical Society 129 (2007) 9552. [3] C. Hu, T.W. Peng, X.X. Hu, Y.L. Nie, X.F. Zhou, J.H. Qu, H. He, Journal of the American Chemical Society 132 (2010) 857. [4] C. Hu, X.F. Hu, L.S. Wang, J.H. Qu, A.M. Wang, Environmental Science and Technology 40 (2006) 7903. [5] C. Hu, J. Guo, J.H. Qu, X.X. Hu, Langmuir 23 (2007) 4982. [6] M. El-Sayed, Accounts of Chemical Research 34 (2011) 257. [7] S. Tawkaew, Y. Fujishiro, S. Yin, T. Stato, Colloids and Surfaces A 179 (2001) 139. [8] T. Nalato, H. Edakubo, T. Shimomura, Microporous and Mesoporous Materials 123 (2009) 280. [9] W.Q. Cui, Y.F. Liu, L. Liu, J.S. Hu, Y.H. Liang, Applied Catalysis A: General 417–418 (2012) 111. [10] L.L. Zhang, W.G. Zhang, Journal of Materials Science 41 (12) (2006) 3917. [11] S. Uchida, Y. Yamamoto, Y. Fujishiro, A. Watanabe, O. Ito, T. Sato, Journal of the Chemical Society, Faraday Transactions 93 (1997) 3229. [12] X.F. Zhou, C. Hu, X.X. Hu, T.W. Peng, J.H. Qu, Journal of Physical Chemistry C 114 (2010) 2746. [13] H. Tada, T. Mitsu, T. Kiyonaga, T. Akita, K. Tanaka, Nature Materials 5 (2006) 782. [14] J. Cao, B.Y. Xu, B.D. Luo, H.L. Lin, S.F. Chen, Applied Surface Science 257 (2011) 7083.