Enhancement of Catalytic Degradation of Rhodamine B under Sunlight with Au Loading TiO2 Nanotube Arrays

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Procedia Environmental Sciences 18 (2013) 620 – 624

2013 International Symposium on Environmental Science and Technology (2013 ISEST)

Enhancement of catalytic degradation of rhodamine B under sunlight with Au loading TiO 2 nanotube arrays Shuo Sun, Chunchang Chen, Jie Sun , Qiaoli Peng, Kangle Lü, Kejian Deng Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission and Ministry of Education, SouthCentral University for Nationalities, Wuhan 430074, China.

Abstract In this paper, gold nanoparticles on highly ordered TiO2 nanotube arrays (Au/TNTA) were prepared via electrochemical anodic oxidation method followed by photodeposition. The prepared samples were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS). Photocatalytic degradation of Rhodamine B(RhB) under simulated sunlight irradiation were carried out. The experiment results showed that the photocatalytic activity of TNTA was greatly enhanced after loading of gold. Both hydroxyl radicals (·OH) and holes are involved in the degradation of RhB. © The Authors. Authors. Published Published by byElsevier ElsevierB.V. B.V. © 2013 2013 The peer-review under responsibility of Beijing Institute of Technology. Selection and and/or peer-review under responsibility of Beijing Institute of Technology. Keywords:TiO2; highly ordered nanotube arrays; rhodamine B; Au; photocatalytic

1. Introduction Titanium oxide (TiO2) has attracted increasing attention due to its low operation temperature, nontoxic, inexpensive, and relatively high chemical stability[1]. The separation of TiO2 nanoparticles from solution is time-consuming, which hinders itspractical application. TiO2 nanotube arrays (TNTA) is expected to be a promising photocatalyst to overcome such drawbacks and it has been proven in a variety of applications including dye-sensitized solar cells, contamination removal, and photodegradation [2, 3]. However, the application of TiO2 nanotube arrays in photocatalytic processes is limited by its high rate recombination of photogenerated electron-hole pairs. Many efforts such as transition metal cations doping[4], nonmetal anions doping[5], and surface modification with noble metal[3, 6-8]have been made to depress the recombination of photogenerated electron-hole pairs in photocatalytic processes. The

Corresponding author, Jie Sun, Tel.: +86-13476035879; Fax: +86-27-67842752. E-mail address: [email protected].

1878-0296 © 2013 The Authors. Published by Elsevier B.V. Selection and peer-review under responsibility of Beijing Institute of Technology. doi:10.1016/j.proenv.2013.04.085

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research concerning Au nanoparticles on TiO2 nanotube arrays to improve the photocatalytic activity was seldom reported[9]. The activity of compound catalyst was affected by the preparation conditions and the size of gold particles. There are several methods to load gold nanoparticles on TiO 2, including depositionprecipitation, electrodeposition, physical evaporation and so on. However, it is hard to control the dispersion of gold nanoparticles using the methods mentioned above. In this paper, we report a convenient synthetic strategy for the fabrication of monodispered golden nanopartciles on the surface of TNTA (Au/TNTA) via photoreduction and calcination procedure. The photocatalytic activity of the catalysts were measured by photodegradation of dyes (RhB) under Xenon lamp irradiation.The influence factors and the mechanism of photocatalytic process were preliminary investigated in our work, and the composites showing a quite high photocatalytic activity[10]. 2. Experimental TNTA was formed on the Ti surface by electrochemical anodic oxidation method[11]. Au/TNTA was prepared by photodepositon of gold on the surface of Au using HAuCl4·3H2O as gold source. The composite catalyst can be prepared as the following steps˖(1) the as-prepared TNTA was immerged in 50 ml 1×10-5 M HAuCl4·3H2O (in ethanol) solution and stirred vigorously at room temperature for 3 h in dark to reach a complete absorption equilibrium. (2) the solution was irradiated at a 300 W Mercury lamp (wavelength 365 nm) with magnetic stirring for 4 h, then washed twice with absolute ethanol. (3) the sample was calcined again in air at 250 °C for 2 h in order to improve adhesion of the Au nanoparticles on the nanotube morphology with heating rate of 5 °C/min and cooling naturally. Au/TNTA photocatalysts with Au contents of about 0.1 wt%, 0.4 wt % and 0.8 wt% were obtained. The prepared samples were characterized by TEM, XRD, XPS, DRS. The photocatalytic activity of the catalysts was tested by degradation RhB in a glass reactor under the irradiation of a 36W Xenon lamp (simulated sunlight).The production of ·OH on the surface of the UV-illuminated TiO2 samples was detected by a photoluminescence method using coumarin as a probe molecule. 3. Results and Discussion 3.1 Characterization The TEM images of TNTA and Au/TNTA samples are shown in Fig. 1. It can be seen that the diameter of TNTA is approximately 100 nm. Comparing samples before and after the measurements, no visible loss of nanoparticles could be found, indicating that both types of nanoparticles adhere well on the TNTA surface. Fig. 2 shows XRD patterns of TNTA and Au/TNTA. It is apparent that all samples show anatase structures. However, the peak corresponding to metallic Au(200) plane at 2θ=44.4° could not be seen for all Au/TNTA samples, which may be due to low Au content. Relative intensity (a.u.)

1.6wt% 0.8wt% 0.4wt% 0.1wt% 0wt% Ti Rutile Anatase Anatase 10

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Fig. 1. TEM images of (a) TNTA and (b) Au(0.4 wt%)/TNTA.

Fig. 2. XRD spectra of different gold content of Au/TNTA.

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The high resolution XPS spectra of TNTA and Au/ TNTA are shown in Fig. 3. The binding energies of Au4f7/2 at 83.45ev and Au4f5/2 at 87.1ev (in Fig. 3 c) indicate that Au species are present with zero oxidative state on the surface of TNTA. We also summarize that spectra of Ti2p reveals double peaks which correspond to Ti2p3/2 and Ti2p1/2 and the spectra of O1s reveal single peak. Compared to TNTA the Ti2p3/2 binding energy of Au/ TNTA is shifted from 458.05 to 458.7ev (in Fig. 3 a) . In the meantime, the O1s binding energy of Au/ TNTA is shifted from 529.451 to 529.951ev (in Fig. 3b) when compared with TNTA, which can be attributed to the conduction band electron of TiO2 transfer to the Au nanoparticles, resulting in a decrease in the outer electron cloud density of Ti and O elements. Moreover, the photoinduced electron-hole is well separated and improved quantum efficiency of photocatalysis [12]. O1s

Au/TiO -NTA 2

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Fig. 3. XPS spectra of Ti 2p, O 1s of TNTA and Au/TNTA, Au 4f of Au/TNTA.

3.2. Effect of Au content on the RhB degradation From the UV–vis (DRS), it can be seen that Au/TNTA showed obvious absorption in 400-600 nm region(a shoulder @ 530 nm) compared to the pure TNTA (Fig. 4) . The Au loading obviously influences visible light absorption of TNTA. Comparing with TNTA, the absorption spectra of all Au/TNTA samples show an enhanced absorption in the visible light region. Also, the samples show a stronger absorption with increasing of Au content. The enhancement of visible light absorption peak based on localized surface plasmon resonance (LSPR), which is important for the visible-light-responsible photocatalyst. In the current investigation, the plot of ln(Ct/Co) versus time gave a straight line, which indicated that the degradation of RhB followed first-order kinetics: ln(Ct/Co)= - k t. Blank experiment revealed that the degradation of RhB under Xenon lamp illumination was negligible in the absence of TNTA. When the Au amount was 0.1 wt%, the enhancement on the catalytic activity of Au/TNTA was negligible. However, the photoactivity of Au/TNTA was improved obviously when the Au content increases from 0.1 wt% to 0.4 wt%. The degradation rate constant of RhB using Au (0.4 wt %) /TNTA as photocatalyst was 0.3672 h-1, which is 1.4 times higher than using pure TNTA (0.2634 h-1). However, when further increase in the Au content, the catalytic activity gradually decreased (inset of Fig. 5). Au/TNTA with an Au content of 1.6 wt%, showed similar photocatalytic activity to that of prestine TNAT. This was due to the fact that too much gold nanoparticles would hind the light-absorbance [13]. Repeat experiments showed that the composite of Au/TNTA photocatalyst was very stable, which can be reused for ten times without obvious losing the photocatalytic activity. 3.3. Detection of ROS and Mechanism discussion To detect ·OH during the degradation process, fluorescence technique was used to detect ·OH under Xenon lamp illuminated. The existence of photoexcited electron injection is proved by the fluorescence emission spectra, which gives a useful functional and conformational probe of reaction center.

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Photocatalysts generate electrons and holes after being activated by light, and recombination of some electrons and holes can release energy in the form of fluorescenceemission. Lower fluorescenceemission intensity implies lower electron–hole recombination rate. For TNTA and Au(0.4 wt%)/TNTA, the intensity @ 450 nm increased with irradiation time, indicating that the production of ·OH (Fig. 6). The insert displays a comparison of the fluorescence emission spectra between TNTA and Au(0.4 wt%)/TNTA. The result shows a considerable fluorescence decreased (or quenching), which indicates that the transfer of photogenerated electrons from TNTA to Au. To study the degradation mechanism of RhB on Au/TNTA, EDTA-Na and t-BuOH were used as holes and ·OH radicals scavengers, respectively. As shown in Fig. 7, when 500 equivalents of EDTA-2Na or tBuOH versus RhB were added into the reaction system, the degradation of RhB was obviously hindered, reflecting that both ·OH and holes played main roles in RhB degradation. 1.0

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Wavelength Fig. 4. DRS spectra of different gold content TNTA and Au/TNTA.

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Fig. 5. UV-Vis spectra changes of RhB (10PM, pH3) in presence of Au(0.4wt%)/ TNTA and the inset is the related rates. 0.4

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Fig. 6. Fuorensence spectrum of coumarin caputrure ·OH with Au(0.4wt%)/TNTA.

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Fig. 7. The kinetic constants of hydroxyl and hole quenching experiment of TNTA and Au(0.4wt%)/TNTA.

4. Conclusion In summary, gold nanoparticles on highly ordered TiO2 nanotube arrays (Au/TNTA) were prepared by electrochemical anodic oxidation and photodeposition method. The photocatalytic activity of TNTA was

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greatly enhanced after Au deposition with an optimum content of 0.4wt%. Both holes and ·OH were the main ROS in RhB degradation. This work displayed the potential application of Au/TNTA in wastewater treatment under sunlight irradiation. Acknowledgements This work was supported by the National Natural Science Foundation of China (20807057), Distinguished Young Foundation of Hubei Province (2011CDA106) and Technology Centre Project of Wuhan City (201160638173). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Su R et al. Oxidative degradation of dye pollutants over a broad pH range using hydrogen peroxide catalyzed by FePz(dtnCl2)4. Chemosphere. 2009;77:1146-1151. Lv KL et al. Preparation of thermally stable anatase TiO2 photocatalyst from TiOF2 precursor and its photocatalytic activity. Journal of Alloys and Compounds. 2011;509:4557–4562. Sun J et al. Photocatalytic degradation pathway for azo dye in TiO2 /UV/O3 system: hydroxyl radical versus hole. J. Mol. Catal. A. 2013;367:31-37. Peerakiatkhajorn P et al. Novel Photocatalytic Ag/TiO2 Thin Film on Polyvinyl Chloride for Gaseous BTEX Treatment. Mater Sci. Forum. 2012;712:133-145. Lv KL et al. (Bi, C and N) codoped TiO 2 nanoparticles. J. Hazard. Mater. 2009;161:396–401. Yang LX et al. Fabrication and catalytic properties of Co–Ag–Pt nanoparticle-decorated titania nanotube arrays. J. Phys. Chem.C. 2007;111: 8214–8217. Macak JM et al. Efficient oxygen reduction on layers of ordered TiO2 nanotubes loaded with Au nanoparticles. Electrochem. Commun. 2007;9:1783–1787. Song YY et al. Amphiphilic TiO2 Nanotube Arrays: An Actively Controllable Drug Delivery System. J. Am. Chem. Soc. 2009;131:4230–4232. Paramasivam I et al. Photocatalytic activity of TiO 2 nanotube layers loaded with Ag and Au nanoparticles. Electrochem. Commun. 2008;10:71-75. Shankar K et al. Highly Efficient Solar Cells using TiO2 Nanotube Arrays Sensitized with a Donor-Antenna Dye. Nano Lett. 2008;8:1654-1659. Zhuang HF et al. Some Critical Structure Factors of Titanium Oxide Nanotube Array in Its Photocatalytic Activity. Environ. Sci. Technol. 2007;41:4735-4740. Liu Y et al. TiO2 Nanoflakes Modified with Gold Nanoparticles as Photocatalysts with High Activity and Durability under near UV Irradiation. J. Phys. Chem. C. 2010;114:1641-1645. IlievV et al. Enhancement of photocatalytic oxidation of oxalic acid by gold modified WO3/TiO2 photocatalysts under UV and visible light irradiation. J. Mol. Catal. A. 2010;327:51-57.