- Email: [email protected]
Electrodeposition preparation of Ag nanoparticles loaded TiO2 nanotube arrays with enhanced photocatalytic performance
Applied Surface Science 288 (2014) 513–517
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Electrodeposition preparation of Ag nanoparticles loaded TiO2 nanotube arrays with enhanced photocatalytic performance夽 Xu Liu a,b , Zhongqing Liu b , Jinlin Lu a , Xuelian Wu b , Bo Xu b , Wei Chu b,∗ a b
School of Materials & Metallurgy, University of Science & Technology Liaoning, Anshan 114051, PR China College of Chemical Engineering, Sichuan University, Chengdu 610065, PR China
a r t i c l e
i n f o
Article history: Received 2 May 2013 Received in revised form 2 October 2013 Accepted 11 October 2013 Available online 19 October 2013 Keywords: TiO2 nanotube arrays Photocatalytic Ag nanoparticles Electrodeposition
a b s t r a c t Small silver (Ag) nanoparticles (NPs) loaded TiO2 nanotube arrays (TNAs) were prepared by a twostep method based on anodization method followed by an electrodeposited process. UV–visible diffuse reflectance spectroscopy (UV–vis) and photoluminescence emission spectroscopy (PL) demonstrated that the loaded Ag NPs significantly enhanced the light absorption of TiO2 nanotube arrays in the visible spectral range and improved the separation of photo-generated charge carriers. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) were also used for the characterization of surface morphology, elemental composition and microstructure of the pure TNAs and Ag NPs decorated TNAs. The results indicate that Ag NPs are uniformly distributed in the TNAs, the deposition process does not damage its ordered tubular structure and the loaded amounts and size of Ag NPs increase with the electrodeposition time. Moreover, the Ag NPs decorated TNAs largely enhanced the photocatalytic degradation of methyl orange (MO) under UV–visible light irradiation. Finally, A possible photocatalytic reaction mechanism of Ag NPs/TNAs has also been proposed. © 2013 Published by Elsevier B.V.
1. Introduction Vertically oriented and high ordered TiO2 nanotube arrays (TNAs) prepared on a Ti foils by the anodization method have drawn an increasing attention due to its promising and significant prospects in photocatalysis, biosensor, photovoltage and other applications [1–8]. The main advantages of the self-assembled TNAs are highly ordered nanostructure, high surface area and efficient unidirectional charge transport routes [9,10]. However, its high recombination rate of photogenerated electron and hole pairs results in low photocatalytic efficiency [11–13]. Additionally, its wide bandgap (3.0–3.2 eV) leads to its absorption only in UV light, which severely limits the applicability of TNAs in photovoltaic cells and pollutant degradation [11,14]. Recently, noble metal nanoparticles decorated TNAs have obtained increasing attention because that these nanoparticles could slow the recombination of photogenerated electron and hole pairs as electron trapper and thus enhance the photocatalytic activity of TNAs [8,15,16]. Among modified noble metals, metallic Ag is
夽 This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. ∗ Corresponding author. Tel.: +86 28 8540 3836; fax: +86 28 8546 1108. E-mail addresses: lzq [email protected] (Z. Liu), [email protected] (W. Chu). 0169-4332/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.apsusc.2013.10.062
one of the most suitable for industrial applications due to its relative low cost as compared to other noble metals, nontoxicity and special behavior of oxygen adsorption [13,17]. Moreover, Ag nanoparticles exhibit an improved response in the visible light region in the range of 300–1200 nm due to the effect of localized surface plasmin resonance (LSPR), generated by the collective oscillations of electrons near the surface [18,19]. These strengths make supporting Ag metal as an effective approach could decrease recombination rate of photogenerated charge carrier and extend the light absorption of TiO2 into the visible range [20,21]. In recent years, many studies have been focused on the photocatalytic properties of TiO2 nanoparticles and TiO2 films loading with Ag nanoparticles under UV and visible light illumination. Sun et al. [22] reported Ag nanoparticles loaded onto self-assembled TiO2 nanotubes arrays through a photochemical route. Liang et al. [23] adopted a simple wet reduction method to fabricated high dispersed Ag nanoparticles on TNAs. Xie et al. [20] investigated Ag nanoparticles loading on TNAs by a pulse current deposition technique. Wang et al. [21] studied that Ag nanoparticles loaded on TNAs were prepared by a successive ionic layer adsorption and reaction (SILAR) method, and proved to have enhanced photocatalytic activity under UV irradiation. Although Ag nanoparticles decorated TiO2 nanotube arrays (Ag-TNTs) have been widely studied in literature during the past 10 years, few reports has been found regarding the effect of Ag NPs with different uniform sizes on photoelectrochemical properties and visible photocatalytic activity of Ag/TNAs [24].
514
X. Liu et al. / Applied Surface Science 288 (2014) 513–517
In this paper, metallic Ag NPs loading on TiO2 nanotube arrays were prepared by a direct electrodeposition method and subsequently examined by SEM, EDX, XPS, TEM and DRS analysis. The effects of the Ag NPs with different sizes on the UV and visible photocatalytic properties of Ag/TNAs were investigated by degradation of methyl orange. The results showed a significant improvement of the photocatalytic activity for the Ag nanoparticles loaded TiO2 nanotube array in UV and visible light regions. 2. Experimental 2.1. Preparation of Ag nanoparticles loading on TiO2 nanotube arrays Highly orderly TiO2 nanotube arrays were grown from Ti sheets (99.6% purity) by electrochemical anodic oxidation at 60 V for 2 h in ethylene glycol system containing 0.3 wt% NH4 F and 2.0 vol% deionized water. After anodization, the samples were rinsed with deionized water and then air dried. Then the as-prepared electrodes were directly annealed at 450 ◦ C for 2 h in air. Loading of Ag NPs on the TNAs was prepared by a direct and facile electrodeposition. The synthesis process was carried out using a three-electrode system with the TNA as the working electrode (1 × 1 cm2 ), a Pt foil counter electrode (2 × 2 cm2 ), and a quasi-reference electrode made of Ag wire. Aqueous solution containing 0.01 M AgNO3 , 0.1 M NaClO4 , 0.6 M ethylenediamine, and 0.02 M thiopropionic acid was used as the electrolyte. The deposition was carried out at −0.6 V for 20 s, 60 s and 90 s, and the relative samples were denoted as Ag(20)/TNAs, Ag(60)/TNAs and Ag(90)/TNAs for comparison. After deposition, all the samples were washed with ethanol and dried in vacuum. 2.2. Characterization The morphology and element composition of samples were directly observed using FE-SEM (FEI Corporation) equipped with an energy dispersive X-ray spectroscopy (EDX) detector. The microstructure was taken with a FEI Tecnai G2 F20 transmission electron microscope (TEM) operated at an accelerating voltage of 300 kV. The elemental state of the samples was analyzed by X-ray photoelectron spectroscopy (XPS, Kratos XSAM-800, Al K␣ radiation). All the binding energies were referenced to the signal for
adventitious carbon at 284.8 eV. UV–vis DRS were conducted at room temperature on a PERSEE TU-1901 spectrophotometer. PL measurements were conducted at room temperature on an F-7000 FL spectrophotometer using a 300 nm excitation light. 2.3. Photocatalytic activity test The photocatalytic activity of the prepared samples was estimated by measuring the degradation rate of 30 mL methyl orange (MO) with an initial concentration of 10 mg L−1 . A 300 W high pressure mercury lamp ( = 254 nm) was taken as the UV light source and 300 W Xe lamp with a UV cutoff filter was taken as the simulated visible light source ( > 430 nm). The illumination intensity of light was measured as 100 mW/cm2 . The amounts of MO remained after photoirradiation were determined at 464 nm every 30 min by a Vis spectrophotometer. 3. Results and discussion The morphologies of the samples were characterized by FE-SEM. The FE-SEM image of the pure TiO2 nanotube arrays indicates that the average inner diameter of TNAs is ∼100 nm with a wall thickness of ∼15 nm (see Fig. 1a). The corresponding cross-sectional image in Fig. 1b shows that the nanotubes with entirely smooth walls possess vertically oriented and ordered nanostructure. Fig. 1d and e gives FE-SEM images of Ag NPs deposition on TNAs under a constant direct potential of −0.6 V for 20 s, high dispersed Ag nanoparticles are observed on the top of TiO2 nanotubes. In fact, uniform Ag nanoparticles are deposited on the whole tube wall, including inner side and outer side and deposition process does not destroy the ordered tubular structure. The EDX spectra (Fig. 1c and f) show that the pure TNAs are composed of Ti and O elements, and the Ag NPs loaded TNAs are composed of elements Ti, O and Ag. Ag NPs loading on TNAs electrodes were further investigated by TEM technology as shown in Fig. 2. The TEM technology gives further and direct evidence of the existence of Ag NPs on the TiO2 nanostructures. The images of the Ag NPs loading on TNAs indicate that the nanoparticles are not agglomerates, but individual. The clear lattice image indicates the high crystallinity of Ag NPs loading on the nanotubes. The lattice spacing of 0.235 nm could be attributed to (1 1 1) plane of Ag in the HRTEM. Another lattice
Fig. 1. SEM images and EDX spectra of TNAs (a–c) and Ag(20)/TNAs (d–f). (a, d) Top views, (b, e) cross-section views, (c, f) EDX spectra.
X. Liu et al. / Applied Surface Science 288 (2014) 513–517
515
Fig. 2. TEM images and Ag nanoparticles size distribution of Ag(20)/TNAs (a, d), Ag(60)/TNAs (b, e) and Ag(90)/TNAs (c, f). Inset is the HR-TEM images of Ag(20)/TNAs.
spacing of 0.243 nm was determined corresponding to the lattice spacing of the (1 0 3) plane of the anatase phase TiO2 [23], which is in good agreement with the XRD results (as shown in Fig. S1). Moreover, as the deposited time increase, the size and amount of Ag NPs on nanotubes increase. When the deposited time is kept for 20 s, 60 s and 90 s, the average size of Ag NPs on the nanotubes is ∼5.7 nm, ∼9.6 nm and ∼12.2 nm (Fig. 2d–f), respectively. The state and amount of Ag NPs on the TNAs can be also approved by XPS. Fig. 3 is Ag 3d core level XPS scan over smaller energy window at higher resolution. The Ag 3d5/2 peak appears at 368.3 eV and the Ag 3d3/2 peak is found at 374.3 eV, with the splitting of the 3d doublet is 6.0 eV, indicating that Ag mainly exists in metallic state on the sample of Ag-TNTs [20,23]. Moreover, the content of Ag element on samples is approximately 2 at.%, 6 at.% and 9 at.% for Ag(20)/TNAs, Ag(60)/TNAs and Ag(90)/TNAs, respectively. Fig. 4 gives the UV–vis absorption spectra of the pure TNAs and Ag NPs/TNAs electrodes. As shown in Fig. 4, the absorption edge at a wavelength lower than 380 nm can be ascribed to the intrinsic bandgap absorption of pure anatase TiO2 [25]. Two weak absorption peaks were observed at about 385 nm and 500 nm, which are attributed to the trapped holes and trapped electrons, due to the existent of the sub-band gap state of TNAs [22]. Compared with
bare pure TNAs, the Ag NPs loading on TNAs display a broad absorption peak centered at 470 nm due to the LSPR of Ag NPs [26]. LSPR intensity increases with increasing the deposited time due to the increase of the loaded amounts of Ag NPs on TNAs. Moreover, due to Ag NPs size-dependency, these absorption edges are red-shift as the nanoparticle size increases [22,27]. The PL spectra have been widely used to investigate the transfer behavior of the photogenerated electrons and holes in solid semiconductor materials, so that it can provide information on the charge separation and recombination of photogenerated charge carriers [28]. Fig. 5 shows the PL spectra of the pure and Ag NPs/TNAs with an excitation wavelength of 300 nm. Two broad and gentle emission bands were observed in the scanning range of 350–550 nm. The broad peak appeared at about 350–400 nm originate from the free exciton emission and the other emission band peaked at about 465 nm was ascribed to bound exciton emission, which is similar to study of the PL spectra of TiO2 nanotube arrays have also been reported by Tachikawa and Majima [29] and Mercado et al. [30]. The Ag NPs/TNAs samples show a weaker PL intensity compared to pure TNAs sample. This result could be ascribed to the existence of Ag NPs decorated on the TNAs, which act as electron trappers to inhibit the recombination of photogenerated
Fig. 3. The high resolution XPS spectrum for Ag 3d region of Ag(20)/TNAs (a), Ag(60)/TNAs (b) and Ag(90)/TNAs (c).
Fig. 4. UV–vis diffuse reflectance spectra of TNAs (a), Ag(20)/TNAs (b), Ag(60)/TNAs (c) and Ag(90)/TNAs (d).
516
X. Liu et al. / Applied Surface Science 288 (2014) 513–517
Fig. 5. PL spectra of TNAs (a), Ag(20)/TNAs (b), Ag(60)/TNAs (c) and Ag(90)/TNAs (d).
electron and hole and decrease the PL intensity [31]. Generally, the low PL intensity suggested the high separation rate of photogenerated electron–hole pairs, which results in the high photocatalytic activity. Therefore, the lower PL intensity indicates that the Ag NPs/TNAs could possess the higher photocatalytic activities [32]. The evaluation of the photocatalytic activity of pure TNAs and Ag NPs/TNAs is carried out by the degradation of MO solution under UV illumination. Fig. 6a displays photocatalytic degradation rates of MO for Ag NPs/TNAs electrodes with different deposited times under UV irradiation. It is obvious that the plot of ln(C0 /Ct ) versus irradiation time shows a straight line and slope of linear regression is equivalent to the apparent rate constant of pseudo-first-order k [22]. The k values show that degradation of MO increasing significantly with the increasing of the electrodeposited time from 20 s to 90 s, reaching a maximum at electrodeposited time of 60 s, and decreasing with the further increasing of the electrodeposited time. The maximum of k value for Ag NPs/TNAs (∼0.016 min−1 ) is 4 times higher than that of pure TNAs (∼0.004 min−1 ). The mechanism of higher photocatalytic degradation rate for Ag NPs loaded TNAs is outlined in Fig. 6b. Under UV irradiation, the electrons are promoted from the valence band of anatase TiO2 to the conduction band and the holes are left in the valence band. When loading Ag NPs on TiO2 , photogenerated electrons moved into metallic Ag NPs and accumulated on the surface of Ag NPs and finally formed Schottky barrier between Ag NPs and TNAs, due to the lower workfunction of TiO2 compared to metallic Ag [15]. Thus, the coupling Ag NPs efficiently enhance the separation rate of photogenerated
electrons and holes [33]. In the reaction process, the photogenerated electrons can transfer to the absorbed oxygen on Ag surface rapidly, form O2− active groups, and further produce • OH active groups through a series of reaction with H+ . The holes left in the valence band of the TiO2 could also react with H2 O to • OH radicals, which is available for the photooxidation process [20,22]. However, excessive Ag NPs coupled with TNAs result in a lower efficiency of photocatalytic reaction, which can be ascribed to the follow reasons, 1) the excess Ag NPs deposited on the surface of TNAs decrease charge carrier space distance via efficiently trapping photoelectrons and therefore increase recombination [21,34]; 2) the excess coupling with Ag NPs may cover active sites on the surface of TiO2 and inhibit the absorption of the UV light, thereby reducing photodegradation efficiency. These results are also agreement with the PL analysis [35]. Due to the existence of intense LSPR effect caused by Ag NPs, the photocatalytic activity of the pristine and Ag NPs/TNAs is also investigated under visible light irradiation. Fig. 7a illuminates photocatalytic degradation rates of MO for Ag NP/TNAs electrodes with different deposited times under visible light irradiation. After Ag NPs are loaded on TNAs, the photocatalytic degradation rate of MO significantly increases, and the apparent rate constant k increases from ∼3 × 10−4 to ∼3.6 × 10−3 min−1 with the increasing of the electrodeposited time from 20 s to 90 s. Based on the study reported by Chen et al., when the radius of the small Ag NPs is less than 30 nm, the incident visible light can be strongly absorbed by Ag NPs [18,24]. Then, the photogenerated electrons excited by decaying surface plasmon can occupy the empty states in the excited state of the Ag NPs [36]. These excited electrons migrate from the excited state of Ag NPs to the conduction band of TiO2 , which can leads to the enhanced photocatalytic activity of small Ag PNs decorated TNAs [37]. Therefore, the visible light photocatalytic activity of photoelectrodes mainly depend on the amount of small Ag NPs decorated TNAs [38]. Thus, in our case, the Ag NPs/TNAs sample with the longest deposited time result in the fastest degradation rate under visible light irradiation. Fig. 7b illustrates the LSPR effect induced photocatalytic reaction on the Ag NPs/TNAs system. When the Ag NPs/TNAs samples are irradiated by visible light ( > 430 nm), the Ag NPs are activated into excited state, a higher energy level than the conduction band of TiO2 . Then the photoexcited electrons migrate into the conductor band of TiO2 , while the photogenerated holes of Ag NPs react with the donor species in electrolyte. The electrons transferred into the TNAs react with adsorbed oxygen to O2− active groups, further to form • OH [20]. The results indicate that the size of the Ag NPs on the TNAs is influential in the photoelectrochemical properties of these materials. The ultrafine Ag NPs decorated TNAs largely enhanced the photocatalytic degradation
Fig. 6. Degradation rates of MO by various processes on TNAs and Ag/TNAs under UV irradiation (a) and schematic diagram of the interface charge-carrier transfer of photocatalysis for Ag/TNAs under UV irradiation (b).
X. Liu et al. / Applied Surface Science 288 (2014) 513–517
517
Fig. 7. Degradation rates of MO by various processes on TNAs and Ag/TNAs under visible light irradiation (a) and schematic diagram of the interface charge-carrier transfer of photocatalysis for Ag/TNAs under visible light irradiation (b).
of MO under UV light irradiation, because Ag NPs were applied to act as electron reservoirs to suppress the electron–hole recombination and more holes were available for the oxidation reactions. On the other hand, the LSPR effect of Ag NPs gives rise to increased photoelectrochemical properties for the modified TNAs in visible light region. Thus, the tunable photoelectrochemical properties of Ag/TNAs materials can be expected to have promising applications in photoelectrochemical degradation and other light harvesting devices. 4. Conclusions Highly orderly TiO2 nanotube arrays decorated with Ag nanoparticles have been prepared by a direct electrodeposition method. The dispersed Ag nanoparticles are uniformly distributed in the TiO2 nanotube arrays, helpful for higher separation efficiency of electrons and holes. Increasing the electrodeposition time can favor to increase the amounts and size of Ag nanoparticles loaded TiO2 nanotube arrays. The Ag nanoparticles modified TiO2 nanotube arrays largely enhanced the photocatalytic degradation of methyl orange under UV–visible light irradiation. Acknowledgments We are grateful to Jingguang G. Chen for critical reading and editing of this manuscript. This work is supported by the National Basic Research Program of China (973 Program, 2011CB201202) of Ministry of Science and Technology of China (MOST) and the 985 Project of Sichuan University, the National Natural Science Foundation of China (Grant No. 51203093). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc. 2013.10.062. References [1] [2] [3] [4]
J. Yu, G. Dai, B. Cheng, J. Phys. Chem. C 114 (2010) 19378. Y.F. Tu, S.Y. Huang, J.P. Sang, X.W. Zou, Mater. Res. Bull. 45 (2010) 224. J.H. Park, S. Kim, A.J. Bard, Nano Lett. 6 (2006) 24. Z. Liu, B. Pesic, K.S. Raja, R.R. Rangaraju, M. Misra, Int. J. Hydrogen Energy 34 (2009) 3250.
[5] O.K. Varghese, M. Paulose, C.A. Grimes, Nat. Nanotechnol. 4 (2009) 592. [6] D. Kuang, J.r.m. Brillet, P. Chen, M. Takata, S. Uchida, H. Miura, K. Sumioka, S.M. ¨ ACS Nano 2 (2008) 1113. Zakeeruddin, M. Gratzel, [7] Q. Zheng, B. Zhou, J. Bai, L. Li, Z. Jin, J. Zhang, J. Li, Y. Liu, W. Cai, X. Zhu, Adv. Mater. 20 (2008) 1044. [8] Y.P. Gao, P.F. Fang, F.T. Chen, Y. Liu, Z. Liu, D.H. Wang, Y.Q. Dai, Appl. Surf. Sci. 265 (2013) 796. [9] G.P. Dai, S.Q. Liu, Y. Liang, T.X. Luo, Appl. Surf. Sci. 264 (2013) 157. [10] H.B. He, A.P. Chen, H. Lv, H.J. Dong, M. Chang, C.Z. Li, Appl. Surf. Sci. 266 (2013) 126. [11] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [12] S. Kumar, A.G. Fedorov, J.L. Gole, Appl. Catal. B: Environ. 57 (2005) 93. [13] M.M. Viana, N.D.S. Mohallem, D.R. Miquita, K. Balzuweit, E. Silva-Pinto, Appl. Surf. Sci. 265 (2013) 130. [14] Y.C. Jia, P. Xiao, H.C. He, J.Y. Yao, F.L. Liu, Z.F. Wang, Y.H. Li, Appl. Surf. Sci. 258 (2012) 6627. [15] X. He, Y. Cai, H. Zhang, C. Liang, J. Mater. Chem. 21 (2011) 475. [16] X. Feng, J.D. Sloppy, T.J. LaTempa, M. Paulose, S. Komarneni, N. Bao, C.A. Grimes, J. Mater. Chem. 21 (2011) 13429. [17] Y.C. Liang, C.C. Wang, C.C. Kei, Y.C. Hsueh, W.-H. Cho, T.P. Perng, J. Phys. Chem. C 115 (2011) 9498. [18] K.M. Mayer, J.H. Hafner, Chem. Rev. 111 (2011) 3828. [19] M. Rycenga, C.M. Cobley, J. Zeng, W. Li, C.H. Moran, Q. Zhang, D. Qin, Y. Xia, Chem. Rev. 111 (2011) 3669. [20] K. Xie, L. Sun, C. Wang, Y. Lai, M. Wang, H. Chen, C. Lin, Electrochim. Acta 55 (2010) 7211. [21] Q. Wang, X. Yang, D. Liu, J. Zhao, J. Alloys Compd. 527 (2012) 106. [22] L. Sun, J. Li, C. Wang, S. Li, Y. Lai, H. Chen, C. Lin, J. Hazard. Mater. 171 (2009) 1045. [23] Y. Liang, Z. Cui, S. Zhu, Y. Liu, X. Yang, J. Catal. 278 (2011) 276. [24] K.S. Chen, X.R. Feng, R. Hu, Y.B. Li, K. Xie, Y. Li, H.S. Gu, J. Alloys Compd. 554 (2013) 72. [25] N.K. Allam, A.J. Poncheri, M.A. El-Sayed, ACS Nano 5 (2011) 5056. [26] J. Zeng, Y. Zheng, M. Rycenga, J. Tao, Z.-Y. Li, Q. Zhang, Y. Zhu, Y. Xia, J. Am. Chem. Soc. 132 (2010) 8552. [27] M. Hu, C. Novo, A. Funston, H. Wang, H. Staleva, S. Zou, P. Mulvaney, Y. Xia, G.V. Hartland, J. Mater. Chem. 18 (2008) 1949. [28] X. Wang, M. Blackford, K. Prince, R.A. Caruso, ACS Appl. Mater. Interfaces 4 (2012) 476. [29] T. Tachikawa, T. Majima, J. Am. Chem. Soc. 131 (2009) 8485. [30] C.C. Mercado, F.J. Knorr, J.L. McHale, S.M. Usmani, A.S. Ichimura, L.V. Saraf, J. Phys. Chem. C 116 (2012) 10796. [31] X. Li, Y. Hou, Q. Zhao, G. Chen, Langmuir 27 (2011) 3113. [32] A. Takai, P.V. Kamat, ACS Nano 5 (2011) 7369. [33] S.M. Gupta, M. Tripathi, Chin. Sci. Bull. 56 (2011) 1639. [34] M. Sadeghi, W. Liu, T. Zhang, P. Stavropoulos, B. Levy, J. Phys. Chem. 100 (1996) 19466. [35] S. Sakthivel, M. Shankar, M. Palanichamy, B. Arabindoo, D. Bahnemann, V. Murugesan, Water Res. 38 (2004) 3001. [36] S. Lal, S. Link, N.J. Halas, Nat. Photonics 1 (2007) 641. [37] S. Mubeen, G. Hernandez-Sosa, D. Moses, J. Lee, M. Moskovits, Nano Lett. 11 (2011) 5548. [38] J.L. Wu, F.C. Chen, Y.S. Hsiao, F.C. Chien, P.L. Chen, C.H. Kuo, M.H. Huang, C.S. Hsu, ACS Nano 5 (2011) 959.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Electrodeposition preparation of Ag nanoparticles loaded TiO2 nanotube arrays with enhanced photocatalytic performance夽 Xu Liu a,b , Zhongqing Liu b , Jinlin Lu a , Xuelian Wu b , Bo Xu b , Wei Chu b,∗ a b
School of Materials & Metallurgy, University of Science & Technology Liaoning, Anshan 114051, PR China College of Chemical Engineering, Sichuan University, Chengdu 610065, PR China
a r t i c l e
i n f o
Article history: Received 2 May 2013 Received in revised form 2 October 2013 Accepted 11 October 2013 Available online 19 October 2013 Keywords: TiO2 nanotube arrays Photocatalytic Ag nanoparticles Electrodeposition
a b s t r a c t Small silver (Ag) nanoparticles (NPs) loaded TiO2 nanotube arrays (TNAs) were prepared by a twostep method based on anodization method followed by an electrodeposited process. UV–visible diffuse reflectance spectroscopy (UV–vis) and photoluminescence emission spectroscopy (PL) demonstrated that the loaded Ag NPs significantly enhanced the light absorption of TiO2 nanotube arrays in the visible spectral range and improved the separation of photo-generated charge carriers. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) were also used for the characterization of surface morphology, elemental composition and microstructure of the pure TNAs and Ag NPs decorated TNAs. The results indicate that Ag NPs are uniformly distributed in the TNAs, the deposition process does not damage its ordered tubular structure and the loaded amounts and size of Ag NPs increase with the electrodeposition time. Moreover, the Ag NPs decorated TNAs largely enhanced the photocatalytic degradation of methyl orange (MO) under UV–visible light irradiation. Finally, A possible photocatalytic reaction mechanism of Ag NPs/TNAs has also been proposed. © 2013 Published by Elsevier B.V.
1. Introduction Vertically oriented and high ordered TiO2 nanotube arrays (TNAs) prepared on a Ti foils by the anodization method have drawn an increasing attention due to its promising and significant prospects in photocatalysis, biosensor, photovoltage and other applications [1–8]. The main advantages of the self-assembled TNAs are highly ordered nanostructure, high surface area and efficient unidirectional charge transport routes [9,10]. However, its high recombination rate of photogenerated electron and hole pairs results in low photocatalytic efficiency [11–13]. Additionally, its wide bandgap (3.0–3.2 eV) leads to its absorption only in UV light, which severely limits the applicability of TNAs in photovoltaic cells and pollutant degradation [11,14]. Recently, noble metal nanoparticles decorated TNAs have obtained increasing attention because that these nanoparticles could slow the recombination of photogenerated electron and hole pairs as electron trapper and thus enhance the photocatalytic activity of TNAs [8,15,16]. Among modified noble metals, metallic Ag is
夽 This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. ∗ Corresponding author. Tel.: +86 28 8540 3836; fax: +86 28 8546 1108. E-mail addresses: lzq [email protected] (Z. Liu), [email protected] (W. Chu). 0169-4332/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.apsusc.2013.10.062
one of the most suitable for industrial applications due to its relative low cost as compared to other noble metals, nontoxicity and special behavior of oxygen adsorption [13,17]. Moreover, Ag nanoparticles exhibit an improved response in the visible light region in the range of 300–1200 nm due to the effect of localized surface plasmin resonance (LSPR), generated by the collective oscillations of electrons near the surface [18,19]. These strengths make supporting Ag metal as an effective approach could decrease recombination rate of photogenerated charge carrier and extend the light absorption of TiO2 into the visible range [20,21]. In recent years, many studies have been focused on the photocatalytic properties of TiO2 nanoparticles and TiO2 films loading with Ag nanoparticles under UV and visible light illumination. Sun et al. [22] reported Ag nanoparticles loaded onto self-assembled TiO2 nanotubes arrays through a photochemical route. Liang et al. [23] adopted a simple wet reduction method to fabricated high dispersed Ag nanoparticles on TNAs. Xie et al. [20] investigated Ag nanoparticles loading on TNAs by a pulse current deposition technique. Wang et al. [21] studied that Ag nanoparticles loaded on TNAs were prepared by a successive ionic layer adsorption and reaction (SILAR) method, and proved to have enhanced photocatalytic activity under UV irradiation. Although Ag nanoparticles decorated TiO2 nanotube arrays (Ag-TNTs) have been widely studied in literature during the past 10 years, few reports has been found regarding the effect of Ag NPs with different uniform sizes on photoelectrochemical properties and visible photocatalytic activity of Ag/TNAs [24].
514
X. Liu et al. / Applied Surface Science 288 (2014) 513–517
In this paper, metallic Ag NPs loading on TiO2 nanotube arrays were prepared by a direct electrodeposition method and subsequently examined by SEM, EDX, XPS, TEM and DRS analysis. The effects of the Ag NPs with different sizes on the UV and visible photocatalytic properties of Ag/TNAs were investigated by degradation of methyl orange. The results showed a significant improvement of the photocatalytic activity for the Ag nanoparticles loaded TiO2 nanotube array in UV and visible light regions. 2. Experimental 2.1. Preparation of Ag nanoparticles loading on TiO2 nanotube arrays Highly orderly TiO2 nanotube arrays were grown from Ti sheets (99.6% purity) by electrochemical anodic oxidation at 60 V for 2 h in ethylene glycol system containing 0.3 wt% NH4 F and 2.0 vol% deionized water. After anodization, the samples were rinsed with deionized water and then air dried. Then the as-prepared electrodes were directly annealed at 450 ◦ C for 2 h in air. Loading of Ag NPs on the TNAs was prepared by a direct and facile electrodeposition. The synthesis process was carried out using a three-electrode system with the TNA as the working electrode (1 × 1 cm2 ), a Pt foil counter electrode (2 × 2 cm2 ), and a quasi-reference electrode made of Ag wire. Aqueous solution containing 0.01 M AgNO3 , 0.1 M NaClO4 , 0.6 M ethylenediamine, and 0.02 M thiopropionic acid was used as the electrolyte. The deposition was carried out at −0.6 V for 20 s, 60 s and 90 s, and the relative samples were denoted as Ag(20)/TNAs, Ag(60)/TNAs and Ag(90)/TNAs for comparison. After deposition, all the samples were washed with ethanol and dried in vacuum. 2.2. Characterization The morphology and element composition of samples were directly observed using FE-SEM (FEI Corporation) equipped with an energy dispersive X-ray spectroscopy (EDX) detector. The microstructure was taken with a FEI Tecnai G2 F20 transmission electron microscope (TEM) operated at an accelerating voltage of 300 kV. The elemental state of the samples was analyzed by X-ray photoelectron spectroscopy (XPS, Kratos XSAM-800, Al K␣ radiation). All the binding energies were referenced to the signal for
adventitious carbon at 284.8 eV. UV–vis DRS were conducted at room temperature on a PERSEE TU-1901 spectrophotometer. PL measurements were conducted at room temperature on an F-7000 FL spectrophotometer using a 300 nm excitation light. 2.3. Photocatalytic activity test The photocatalytic activity of the prepared samples was estimated by measuring the degradation rate of 30 mL methyl orange (MO) with an initial concentration of 10 mg L−1 . A 300 W high pressure mercury lamp ( = 254 nm) was taken as the UV light source and 300 W Xe lamp with a UV cutoff filter was taken as the simulated visible light source ( > 430 nm). The illumination intensity of light was measured as 100 mW/cm2 . The amounts of MO remained after photoirradiation were determined at 464 nm every 30 min by a Vis spectrophotometer. 3. Results and discussion The morphologies of the samples were characterized by FE-SEM. The FE-SEM image of the pure TiO2 nanotube arrays indicates that the average inner diameter of TNAs is ∼100 nm with a wall thickness of ∼15 nm (see Fig. 1a). The corresponding cross-sectional image in Fig. 1b shows that the nanotubes with entirely smooth walls possess vertically oriented and ordered nanostructure. Fig. 1d and e gives FE-SEM images of Ag NPs deposition on TNAs under a constant direct potential of −0.6 V for 20 s, high dispersed Ag nanoparticles are observed on the top of TiO2 nanotubes. In fact, uniform Ag nanoparticles are deposited on the whole tube wall, including inner side and outer side and deposition process does not destroy the ordered tubular structure. The EDX spectra (Fig. 1c and f) show that the pure TNAs are composed of Ti and O elements, and the Ag NPs loaded TNAs are composed of elements Ti, O and Ag. Ag NPs loading on TNAs electrodes were further investigated by TEM technology as shown in Fig. 2. The TEM technology gives further and direct evidence of the existence of Ag NPs on the TiO2 nanostructures. The images of the Ag NPs loading on TNAs indicate that the nanoparticles are not agglomerates, but individual. The clear lattice image indicates the high crystallinity of Ag NPs loading on the nanotubes. The lattice spacing of 0.235 nm could be attributed to (1 1 1) plane of Ag in the HRTEM. Another lattice
Fig. 1. SEM images and EDX spectra of TNAs (a–c) and Ag(20)/TNAs (d–f). (a, d) Top views, (b, e) cross-section views, (c, f) EDX spectra.
X. Liu et al. / Applied Surface Science 288 (2014) 513–517
515
Fig. 2. TEM images and Ag nanoparticles size distribution of Ag(20)/TNAs (a, d), Ag(60)/TNAs (b, e) and Ag(90)/TNAs (c, f). Inset is the HR-TEM images of Ag(20)/TNAs.
spacing of 0.243 nm was determined corresponding to the lattice spacing of the (1 0 3) plane of the anatase phase TiO2 [23], which is in good agreement with the XRD results (as shown in Fig. S1). Moreover, as the deposited time increase, the size and amount of Ag NPs on nanotubes increase. When the deposited time is kept for 20 s, 60 s and 90 s, the average size of Ag NPs on the nanotubes is ∼5.7 nm, ∼9.6 nm and ∼12.2 nm (Fig. 2d–f), respectively. The state and amount of Ag NPs on the TNAs can be also approved by XPS. Fig. 3 is Ag 3d core level XPS scan over smaller energy window at higher resolution. The Ag 3d5/2 peak appears at 368.3 eV and the Ag 3d3/2 peak is found at 374.3 eV, with the splitting of the 3d doublet is 6.0 eV, indicating that Ag mainly exists in metallic state on the sample of Ag-TNTs [20,23]. Moreover, the content of Ag element on samples is approximately 2 at.%, 6 at.% and 9 at.% for Ag(20)/TNAs, Ag(60)/TNAs and Ag(90)/TNAs, respectively. Fig. 4 gives the UV–vis absorption spectra of the pure TNAs and Ag NPs/TNAs electrodes. As shown in Fig. 4, the absorption edge at a wavelength lower than 380 nm can be ascribed to the intrinsic bandgap absorption of pure anatase TiO2 [25]. Two weak absorption peaks were observed at about 385 nm and 500 nm, which are attributed to the trapped holes and trapped electrons, due to the existent of the sub-band gap state of TNAs [22]. Compared with
bare pure TNAs, the Ag NPs loading on TNAs display a broad absorption peak centered at 470 nm due to the LSPR of Ag NPs [26]. LSPR intensity increases with increasing the deposited time due to the increase of the loaded amounts of Ag NPs on TNAs. Moreover, due to Ag NPs size-dependency, these absorption edges are red-shift as the nanoparticle size increases [22,27]. The PL spectra have been widely used to investigate the transfer behavior of the photogenerated electrons and holes in solid semiconductor materials, so that it can provide information on the charge separation and recombination of photogenerated charge carriers [28]. Fig. 5 shows the PL spectra of the pure and Ag NPs/TNAs with an excitation wavelength of 300 nm. Two broad and gentle emission bands were observed in the scanning range of 350–550 nm. The broad peak appeared at about 350–400 nm originate from the free exciton emission and the other emission band peaked at about 465 nm was ascribed to bound exciton emission, which is similar to study of the PL spectra of TiO2 nanotube arrays have also been reported by Tachikawa and Majima [29] and Mercado et al. [30]. The Ag NPs/TNAs samples show a weaker PL intensity compared to pure TNAs sample. This result could be ascribed to the existence of Ag NPs decorated on the TNAs, which act as electron trappers to inhibit the recombination of photogenerated
Fig. 3. The high resolution XPS spectrum for Ag 3d region of Ag(20)/TNAs (a), Ag(60)/TNAs (b) and Ag(90)/TNAs (c).
Fig. 4. UV–vis diffuse reflectance spectra of TNAs (a), Ag(20)/TNAs (b), Ag(60)/TNAs (c) and Ag(90)/TNAs (d).
516
X. Liu et al. / Applied Surface Science 288 (2014) 513–517
Fig. 5. PL spectra of TNAs (a), Ag(20)/TNAs (b), Ag(60)/TNAs (c) and Ag(90)/TNAs (d).
electron and hole and decrease the PL intensity [31]. Generally, the low PL intensity suggested the high separation rate of photogenerated electron–hole pairs, which results in the high photocatalytic activity. Therefore, the lower PL intensity indicates that the Ag NPs/TNAs could possess the higher photocatalytic activities [32]. The evaluation of the photocatalytic activity of pure TNAs and Ag NPs/TNAs is carried out by the degradation of MO solution under UV illumination. Fig. 6a displays photocatalytic degradation rates of MO for Ag NPs/TNAs electrodes with different deposited times under UV irradiation. It is obvious that the plot of ln(C0 /Ct ) versus irradiation time shows a straight line and slope of linear regression is equivalent to the apparent rate constant of pseudo-first-order k [22]. The k values show that degradation of MO increasing significantly with the increasing of the electrodeposited time from 20 s to 90 s, reaching a maximum at electrodeposited time of 60 s, and decreasing with the further increasing of the electrodeposited time. The maximum of k value for Ag NPs/TNAs (∼0.016 min−1 ) is 4 times higher than that of pure TNAs (∼0.004 min−1 ). The mechanism of higher photocatalytic degradation rate for Ag NPs loaded TNAs is outlined in Fig. 6b. Under UV irradiation, the electrons are promoted from the valence band of anatase TiO2 to the conduction band and the holes are left in the valence band. When loading Ag NPs on TiO2 , photogenerated electrons moved into metallic Ag NPs and accumulated on the surface of Ag NPs and finally formed Schottky barrier between Ag NPs and TNAs, due to the lower workfunction of TiO2 compared to metallic Ag [15]. Thus, the coupling Ag NPs efficiently enhance the separation rate of photogenerated
electrons and holes [33]. In the reaction process, the photogenerated electrons can transfer to the absorbed oxygen on Ag surface rapidly, form O2− active groups, and further produce • OH active groups through a series of reaction with H+ . The holes left in the valence band of the TiO2 could also react with H2 O to • OH radicals, which is available for the photooxidation process [20,22]. However, excessive Ag NPs coupled with TNAs result in a lower efficiency of photocatalytic reaction, which can be ascribed to the follow reasons, 1) the excess Ag NPs deposited on the surface of TNAs decrease charge carrier space distance via efficiently trapping photoelectrons and therefore increase recombination [21,34]; 2) the excess coupling with Ag NPs may cover active sites on the surface of TiO2 and inhibit the absorption of the UV light, thereby reducing photodegradation efficiency. These results are also agreement with the PL analysis [35]. Due to the existence of intense LSPR effect caused by Ag NPs, the photocatalytic activity of the pristine and Ag NPs/TNAs is also investigated under visible light irradiation. Fig. 7a illuminates photocatalytic degradation rates of MO for Ag NP/TNAs electrodes with different deposited times under visible light irradiation. After Ag NPs are loaded on TNAs, the photocatalytic degradation rate of MO significantly increases, and the apparent rate constant k increases from ∼3 × 10−4 to ∼3.6 × 10−3 min−1 with the increasing of the electrodeposited time from 20 s to 90 s. Based on the study reported by Chen et al., when the radius of the small Ag NPs is less than 30 nm, the incident visible light can be strongly absorbed by Ag NPs [18,24]. Then, the photogenerated electrons excited by decaying surface plasmon can occupy the empty states in the excited state of the Ag NPs [36]. These excited electrons migrate from the excited state of Ag NPs to the conduction band of TiO2 , which can leads to the enhanced photocatalytic activity of small Ag PNs decorated TNAs [37]. Therefore, the visible light photocatalytic activity of photoelectrodes mainly depend on the amount of small Ag NPs decorated TNAs [38]. Thus, in our case, the Ag NPs/TNAs sample with the longest deposited time result in the fastest degradation rate under visible light irradiation. Fig. 7b illustrates the LSPR effect induced photocatalytic reaction on the Ag NPs/TNAs system. When the Ag NPs/TNAs samples are irradiated by visible light ( > 430 nm), the Ag NPs are activated into excited state, a higher energy level than the conduction band of TiO2 . Then the photoexcited electrons migrate into the conductor band of TiO2 , while the photogenerated holes of Ag NPs react with the donor species in electrolyte. The electrons transferred into the TNAs react with adsorbed oxygen to O2− active groups, further to form • OH [20]. The results indicate that the size of the Ag NPs on the TNAs is influential in the photoelectrochemical properties of these materials. The ultrafine Ag NPs decorated TNAs largely enhanced the photocatalytic degradation
Fig. 6. Degradation rates of MO by various processes on TNAs and Ag/TNAs under UV irradiation (a) and schematic diagram of the interface charge-carrier transfer of photocatalysis for Ag/TNAs under UV irradiation (b).
X. Liu et al. / Applied Surface Science 288 (2014) 513–517
517
Fig. 7. Degradation rates of MO by various processes on TNAs and Ag/TNAs under visible light irradiation (a) and schematic diagram of the interface charge-carrier transfer of photocatalysis for Ag/TNAs under visible light irradiation (b).
of MO under UV light irradiation, because Ag NPs were applied to act as electron reservoirs to suppress the electron–hole recombination and more holes were available for the oxidation reactions. On the other hand, the LSPR effect of Ag NPs gives rise to increased photoelectrochemical properties for the modified TNAs in visible light region. Thus, the tunable photoelectrochemical properties of Ag/TNAs materials can be expected to have promising applications in photoelectrochemical degradation and other light harvesting devices. 4. Conclusions Highly orderly TiO2 nanotube arrays decorated with Ag nanoparticles have been prepared by a direct electrodeposition method. The dispersed Ag nanoparticles are uniformly distributed in the TiO2 nanotube arrays, helpful for higher separation efficiency of electrons and holes. Increasing the electrodeposition time can favor to increase the amounts and size of Ag nanoparticles loaded TiO2 nanotube arrays. The Ag nanoparticles modified TiO2 nanotube arrays largely enhanced the photocatalytic degradation of methyl orange under UV–visible light irradiation. Acknowledgments We are grateful to Jingguang G. Chen for critical reading and editing of this manuscript. This work is supported by the National Basic Research Program of China (973 Program, 2011CB201202) of Ministry of Science and Technology of China (MOST) and the 985 Project of Sichuan University, the National Natural Science Foundation of China (Grant No. 51203093). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc. 2013.10.062. References [1] [2] [3] [4]
J. Yu, G. Dai, B. Cheng, J. Phys. Chem. C 114 (2010) 19378. Y.F. Tu, S.Y. Huang, J.P. Sang, X.W. Zou, Mater. Res. Bull. 45 (2010) 224. J.H. Park, S. Kim, A.J. Bard, Nano Lett. 6 (2006) 24. Z. Liu, B. Pesic, K.S. Raja, R.R. Rangaraju, M. Misra, Int. J. Hydrogen Energy 34 (2009) 3250.
[5] O.K. Varghese, M. Paulose, C.A. Grimes, Nat. Nanotechnol. 4 (2009) 592. [6] D. Kuang, J.r.m. Brillet, P. Chen, M. Takata, S. Uchida, H. Miura, K. Sumioka, S.M. ¨ ACS Nano 2 (2008) 1113. Zakeeruddin, M. Gratzel, [7] Q. Zheng, B. Zhou, J. Bai, L. Li, Z. Jin, J. Zhang, J. Li, Y. Liu, W. Cai, X. Zhu, Adv. Mater. 20 (2008) 1044. [8] Y.P. Gao, P.F. Fang, F.T. Chen, Y. Liu, Z. Liu, D.H. Wang, Y.Q. Dai, Appl. Surf. Sci. 265 (2013) 796. [9] G.P. Dai, S.Q. Liu, Y. Liang, T.X. Luo, Appl. Surf. Sci. 264 (2013) 157. [10] H.B. He, A.P. Chen, H. Lv, H.J. Dong, M. Chang, C.Z. Li, Appl. Surf. Sci. 266 (2013) 126. [11] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [12] S. Kumar, A.G. Fedorov, J.L. Gole, Appl. Catal. B: Environ. 57 (2005) 93. [13] M.M. Viana, N.D.S. Mohallem, D.R. Miquita, K. Balzuweit, E. Silva-Pinto, Appl. Surf. Sci. 265 (2013) 130. [14] Y.C. Jia, P. Xiao, H.C. He, J.Y. Yao, F.L. Liu, Z.F. Wang, Y.H. Li, Appl. Surf. Sci. 258 (2012) 6627. [15] X. He, Y. Cai, H. Zhang, C. Liang, J. Mater. Chem. 21 (2011) 475. [16] X. Feng, J.D. Sloppy, T.J. LaTempa, M. Paulose, S. Komarneni, N. Bao, C.A. Grimes, J. Mater. Chem. 21 (2011) 13429. [17] Y.C. Liang, C.C. Wang, C.C. Kei, Y.C. Hsueh, W.-H. Cho, T.P. Perng, J. Phys. Chem. C 115 (2011) 9498. [18] K.M. Mayer, J.H. Hafner, Chem. Rev. 111 (2011) 3828. [19] M. Rycenga, C.M. Cobley, J. Zeng, W. Li, C.H. Moran, Q. Zhang, D. Qin, Y. Xia, Chem. Rev. 111 (2011) 3669. [20] K. Xie, L. Sun, C. Wang, Y. Lai, M. Wang, H. Chen, C. Lin, Electrochim. Acta 55 (2010) 7211. [21] Q. Wang, X. Yang, D. Liu, J. Zhao, J. Alloys Compd. 527 (2012) 106. [22] L. Sun, J. Li, C. Wang, S. Li, Y. Lai, H. Chen, C. Lin, J. Hazard. Mater. 171 (2009) 1045. [23] Y. Liang, Z. Cui, S. Zhu, Y. Liu, X. Yang, J. Catal. 278 (2011) 276. [24] K.S. Chen, X.R. Feng, R. Hu, Y.B. Li, K. Xie, Y. Li, H.S. Gu, J. Alloys Compd. 554 (2013) 72. [25] N.K. Allam, A.J. Poncheri, M.A. El-Sayed, ACS Nano 5 (2011) 5056. [26] J. Zeng, Y. Zheng, M. Rycenga, J. Tao, Z.-Y. Li, Q. Zhang, Y. Zhu, Y. Xia, J. Am. Chem. Soc. 132 (2010) 8552. [27] M. Hu, C. Novo, A. Funston, H. Wang, H. Staleva, S. Zou, P. Mulvaney, Y. Xia, G.V. Hartland, J. Mater. Chem. 18 (2008) 1949. [28] X. Wang, M. Blackford, K. Prince, R.A. Caruso, ACS Appl. Mater. Interfaces 4 (2012) 476. [29] T. Tachikawa, T. Majima, J. Am. Chem. Soc. 131 (2009) 8485. [30] C.C. Mercado, F.J. Knorr, J.L. McHale, S.M. Usmani, A.S. Ichimura, L.V. Saraf, J. Phys. Chem. C 116 (2012) 10796. [31] X. Li, Y. Hou, Q. Zhao, G. Chen, Langmuir 27 (2011) 3113. [32] A. Takai, P.V. Kamat, ACS Nano 5 (2011) 7369. [33] S.M. Gupta, M. Tripathi, Chin. Sci. Bull. 56 (2011) 1639. [34] M. Sadeghi, W. Liu, T. Zhang, P. Stavropoulos, B. Levy, J. Phys. Chem. 100 (1996) 19466. [35] S. Sakthivel, M. Shankar, M. Palanichamy, B. Arabindoo, D. Bahnemann, V. Murugesan, Water Res. 38 (2004) 3001. [36] S. Lal, S. Link, N.J. Halas, Nat. Photonics 1 (2007) 641. [37] S. Mubeen, G. Hernandez-Sosa, D. Moses, J. Lee, M. Moskovits, Nano Lett. 11 (2011) 5548. [38] J.L. Wu, F.C. Chen, Y.S. Hsiao, F.C. Chien, P.L. Chen, C.H. Kuo, M.H. Huang, C.S. Hsu, ACS Nano 5 (2011) 959.