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TiO2 hybrid films for photoelectrocatalytic degradation of methyl orange
Accepted Manuscript Fabrication of Au nanoparticle/TiO2 hybrid films for photoelectrocatalytic degradation of methyl orange Chong Fu, Mingji Li, Hongji Li, Cuiping Li, Xiao guo Wu, Baohe Yang PII:
S0925-8388(16)32872-9
DOI:
10.1016/j.jallcom.2016.09.119
Reference:
JALCOM 38953
To appear in:
Journal of Alloys and Compounds
Received Date: 22 July 2016 Revised Date:
4 September 2016
Accepted Date: 11 September 2016
Please cite this article as: C. Fu, M. Li, H. Li, C. Li, X.g. Wu, B. Yang, Fabrication of Au nanoparticle/ TiO2 hybrid films for photoelectrocatalytic degradation of methyl orange, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.09.119. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Fabrication of Au nanoparticle/TiO2 hybrid films for photoelectrocatalytic degradation of methyl orange Chong Fu a, Mingji Li a,*, Hongji Li b,*, Cuiping Li a, Xiao guo Wu, Baohe Yang a Tianjin Key Laboratory of Film Electronic and Communicate Devices, School of Electronics
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a
Information Engineering, Tianjin University of Technology, Tianjin 300384, P. R. China b
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of
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Chemistry & Chemical Engineering, Tianjin University of Technology, Tianjin 300384, P. R. China
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*Corresponding authors:
E-mall: [email protected] (Mingji Li); [email protected] (Hongji Li)
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Tel.: +86 022 60215346.
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ACCEPTED MANUSCRIPT Abstract Au nanoparticles and TiO2 nanotube hybrid materials were prepared electrochemically. The Ti wire was used to form TiO2 nanotube arrays via anodization and subsequent annealing process. The Au
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nanoparticles were then electrodeposited in chloroauric acid solution to form Au/TiO2 hybrid layers. These two steps were repeated to prepare Au/TiO2, TiO2/Au/TiO2 and Au/ TiO2/Au/TiO2 hybrid materials, and the preparation mechanism is discussed. The Au/TiO2 hybrid materials also exhibited
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different photoelectrocatalytic activities for decomposing methyl orange. The application of the Au/
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TiO2/Au/TiO2 hybrid material as a photo-anode has been simultaneously examined toward electrochemical-oxidation, photo-oxidation, and photoelectro-oxidation of methyl orange. The photoelectrocatalytic mechanism of Au/TiO2/Au/TiO2 hybrid material and its degradation reaction
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mechanism for methyl orange were evaluated.
Keywords: Au nanoparticle; TiO2 nanotube; Electrochemical method; Photoelectrocatalytic
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activities; Methyl orange
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ACCEPTED MANUSCRIPT 1. Introduction Semiconductor TiO2 photo-degrades various harmful organics under UV-light [1-3]. However, its intrinsic property implies a large forbidden bandwidth of 3.2 eV that reduced its photocatalytic
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efficiency. Thus, the quantum yield is lower because of the recombination of electron-hole pairs in the catalysis process. To improve the performance of the photo-degradation process, researchers have incorporated electro-catalytic degradation processes with TiO2 nanotubes as a photo-electrode [4-10].
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The TiO2 nanotubes as a photocatalyst have high electron mobility and larger specific surface area
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[11-13]; as an electrode, they offer a direct pathway for charge transport that enhances electro-catalytic activity [14-16]. Photoelectrocatalysis (PEC) [17, 18] generates ·OH radicals via the combination of semiconductor electrodes with a light irradiation source. When semiconductor electrodes are irradiated with light, the electrons are promoted from the valence band to the
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conduction band, which generates highly oxidative holes (h+) in the valence band. This produces OH radicals on the electrode surface due to water oxidation. The incorporation of metal nanoparticles such as Pt, Au, Cu and Ag on the TiO2 surface
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improves the photoelectrocatalytic performance because it minimizes electron/hole recombination
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and amplifies visible light absorption [19-23]. The coupling of Au nanoparticles and TiO2 nanotubes possessing different redox energy levels for the bands increases the lifetime of the charge carriers. This enhances the efficiency of interfacial charge transfer [24, 25]. Many papers have discussed the photo-catalytic (PC) process at the Au/TiO2 nanotubes under UV light illumination [26]. The design of an optimized interface structure between the Au nanoparticles and the TiO2 nanotubes is a critical scientific question in increasing the photoelectrocatalytic behavior of the Au/TiO2 nanotube electrode. 3
ACCEPTED MANUSCRIPT The interface structures of the Au nanoparticles and TiO2 nanotubes include the following steps: nanoparticles attach to the nanotube wall [27], the nanoparticles moves into the interior of the nanotube [26, 28], the nanoparticles are distributed on top of the nanotubes [27, 29], and
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nanoparticles are buried in the bottom of the nanotubes. We investigated in detail the synergistic catalytic effect of Au nanoparticles and TiO2 nanotubes. We found that the composite and interface structure between them had the best photocatalytic and electrochemical oxidation results to create
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optimal photo-electrode products.
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Here, we synthesized TiO2 nanotubes decorated with Au nanoparticles (Au/ TiO2) via a two-step electrochemical processes. Their photoelectrocatalytic behavior in the degradation of methyl orange (MO) dye was investigated. MO is a representative organic dye used extensively in the printing, textile, and photographic industries [30-32]. To reveal the photoelectrocatalysis mechanism of the
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Au/TiO2 hybrid structures, we compared the electrochemical oxidation (EC), PC, and PEC as a tool
2. Experimental
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to degrade MO. A PEC degradation path of MO on the Au/TiO2 hybrid structure was proposed.
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2.1 Preparation of Au nanoparticle/TiO2 nanotube hybrid structures The Au/TiO2 hybrid structures were constructed on the Ti wires using electrochemical methods. First, the TiO2 nanotube array layer was prepared with anodization of the Ti wire. The Ti wires with a diameter of 1.5 mm and a length of 6 cm were used as the anode. The details are described in our previous studies [33]. Au nanoparticles were electrodeposited on TiO2 nanotubes using 0.02 mM hydrogen tetrachloroaurate solution via a multi-potential step technology. The fabrication parameters of the Au/TiO2 hybrid structures are summarized in Table 1. 4
ACCEPTED MANUSCRIPT Table 1 x 2.2 Degradation of MO The photoelectrocatalytic activities of the as-prepared Au/TiO2 hybrid materials were assessed
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via degradation of MO. A 350 W mercury lamp and IT6720 auto range DC power supply were used as the UV light source (λ=365 nm) and power source (current: 0.03 A, electrode area: 706.5 mm-2), respectively. Prior to degradation, the Au/TiO2 hybrid materials as photo-electrodes were inserted
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into a quartz cup containing MO solution (initial concentration: 5.2 mg L-1, pH 6.7) via a magnetic
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stirrer. A 0.1 M Na2SO4 solution was used as the supporting electrolyte. The lamp and power supply were simultaneously or separately turned on. The change in MO concentration during treatment was analyzed with UV spectrophotometry at MO’s characteristic wavelength. The absorption intensities before and after degradation as well as the degradation ratio of MO can be calculated as follows:
c A = c0 A0 (A0 − A )
× 100
A0
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D(%) =
(1)
(2)
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where D is the degradation ratio, c0 and A0 are the concentration and absorbance of the initial MO,
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respectively, and c and A are the concentration and measured absorption intensity after degradation for a known time, respectively. 2.3 Characterization
Field-emission scanning electron microscopy (FESEM) images were obtained using a MERLIN Compact instrument (Carl Zeiss, Germany) equipped with an energy-dispersive X-ray spectrum (EDS) operated at an accelerating voltage of 5 kV. The range of elements analyzed was 4Be to 94Pu. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained 5
ACCEPTED MANUSCRIPT using a JEM-2100 electron microscope (JEOL, Japan) with an acceleration voltage of 120 and 200 kV, respectively. The sample was prepared by depositing a drop of the ultrasonic suspension in ethanol on a copper grid coated with a carbon film. The X-ray diffraction (XRD) patterns were
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obtained using XRD (Rigaku D/max-2500/PC, Japan) with Cu Kα radiation (40 kV, 40 mA, λ=0.15406). The data were collected from 10o to 100o (2θ) with a resolution step size of 0.1o s-1. UV-visible (UV-vis) absorption spectroscopy was performed on a U-3900 spectrophotometer
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(Hitachi, Japan). Photoelectrochemical measurements used a conventional three electrode system
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including a quartz cell with an electrochemical station. A 350 W mercury lamp (λ=365 nm) was used as the UV source. A bias potential of -1.2 V vs. SCE was applied on the photoanode for the photocurrent test under on-off light conditions with an amperometric i-t curve method. 3. Results and discussion
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An Au/TiO2/Au/TiO2 electrode was fabricated via an electrochemical anodization and an electrodeposition method [33, 34] (Fig. 1). The Au nanoparticles were deposited on the TiO2 nanotubes in HAuCl4 solution, and a multi-potential step technique was used to control the size and
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distribution of the Au particles. The electro-ionization process of the [AuCl4]- ions can be expressed as [AuCl4 ] ↔ Au 3 + + 4Cl − , and the electrochemical reduction process of the Au can be depicted
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as Au 3 + + 3e − ↔ Au .
According to Le Chatelier’s principle, the ionization balance will shift to the right side as Au3+ is consumed in the reduction reaction. This could induce diffusion of [AuCl4]- ions from bulk solution onto the work electrode surface. The concentration of [AuCl4]- ions at the solid-liquid interface was supplemented during applied positive potential in the electrodeposition process. This reduced the influence of the concentration polarization. There is a subsequent reduction of [AuCl4]- under 6
ACCEPTED MANUSCRIPT negative potential. This promotes the formation of nanocrystals. The Au/TiO2/Au/TiO2 electrode as the final product was again prepared via alternating anodization and electrodeposition. During the secondary anodic oxidation, the array structures of the TiO2 nanotubes does not form because the Au
Fig. 1. x
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nanoparticles interfere with the electrical signal and direction of propagation.
The representative SEM images shown in Fig. 2 were taken during different electrochemical
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processes and comprise electrodeposition of Au nanoparticles and anodization of the Ti layer. The
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anodic oxidation products are tubular, hollow, and neatly aligned (Fig. 2a). The diameter of the nanotubes is approximately 80-100 nm, and the thickness of the tube wall is about 5-10 nm. The electrodeposition forms Au nanoparticles that deposit on the nanotube surface. A change in the coloration of the TiO2 nanotube electrode before and after electrodeposition confirms the formation
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of deposits on the TiO2 nanotube surface (Fig. 1). The Au nanoparticles appear as islands distributed along the surface of the TiO2 nanotube layer after the first deposition of Au (Fig. 2b). When the Au/TiO2 electrode is anodized, the Au nanoparticles were not observed on the nanotube wall.
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Meanwhile, the nanotubes on the surface layer become disordered; the nanotubes appear longer (Fig.
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2c). The electrodeposition treatment was performed again for the TiO2/Au/TiO2 electrode. The Au nanoparticles are uniformly distributed along the wall of the TiO2 nanotube (Fig. 2 d). Fig. 2. x
Fig. 3 shows TEM images of Au/TiO2 hybrid structures. Fig. 3a illustrated that the TiO2 nanotubes have diameters of around 80-100 nm and nanotube walls of around 10 nm. The 5 to 10 nm diameter nanoparticles were deposited along the TiO2 nanotubes and in the pores of the nanotubes (Fig. 3a). The HRTEM image of the Au/TiO2 nanotube reveals interplanar spacing values of 0.313 7
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(101)
spacing of TiO2 anatase and the d
(111)
spacing of the
metallic Au, respectively. The microstructure analysis reveals that the Au/TiO2 hybrid structures could be successfully fabricated via two-step electrochemical treatments.
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Fig. 3. x To identify the crystal phase of the TiO2 nanotubes and Au nanoparticles, the XRD result were considered in light of the data in Fig. 4a. All samples clearly show the TiO2 anatase phase (JCPDS
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No: 21-1272) and the Ti metal phase (JCPDS No: 44-1294). The XRD patterns of the Au/TiO2 and
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Au/TiO2/Au/TiO2 samples had no significant characteristic peaks, but the peaks of Ti and TiO2 were significantly weakened. This also indicates the presence of other phases. Au nanoparticles are visible by SEM or TEM analysis in Fig. 2 and Fig. 3. To further verify the presence of Au phase, Fig. 4b shows the EDX spectrum of the Au/TiO2/Au/TiO2 samples. This
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indicates the presence of Ti, O and Au. These results are evidence of successful Au deposition on the surface of the TiO2 nanotubes.
Fig. 4. x
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To study the photoelectric catalytic activity of the TiO2 nanotubes, Au/TiO2, TiO2/Au/TiO2, and
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Au/TiO2/Au/TiO2 samples, MO was degraded using these photo-electrodes (Fig. 5). After degradation for 30 min, the concentration of MO decreased. The percent degradation of MO on TiO2 nanotubes, Au/TiO2, TiO2/Au/TiO2, and Au/ TiO2/Au/TiO2 were 39.2%, 17.0%, 39.2%, and 68.1%, respectively. The interaction between the Au nanoparticles and TiO2 nanotubes was an important factor that affected the degradation rate. The higher catalytic activity of Au/TiO2/Au/TiO2 hybrid structures as catalysts is due to the following three significant aspects: (1) The Au has a smaller particle size. When the particle size decreases, the surface area to volume ratio increases—this 8
ACCEPTED MANUSCRIPT enhances the electron-hole separation[29]. (2) The morphology of the catalysts plays an important role in enhancing the final degradation efficiency similar to the literature[35]. (3) The degradation rate of MO was significantly improved because gold nanoparticles have an electro-catalytic
37]. Fig. 5. x
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oxidation function for organic and biological species. This has been shown in electrochemistry [36,
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The PC, EC, and PEC degradation MO processes were performed in an aqueous solution via the
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Au/TiO2/Au/TiO2 electrode. Fig. 6(a-c) reveals that the maximum absorption peak of MO (464 nm) decreases rapidly and completely decolorizes within different times in the presence of the Au/ TiO2/Au/TiO2 hybrid structure under different degradation methods. Fig. 6d shows the UV-vis curves after degradation for 30 min. The MO was not degraded via PC. This was attributed to the low
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quantum efficiency and poor UV absorption of Au. The UV-light applied on the system degradation could not produce sufficient electron-hole pairs on the anode surface to oxidize MO molecules. There was only a 21% MO removal rate during EC. This indicated that the bias potential applied on
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the Au/TiO2/Au/TiO2 electrode produced oxidative radicals on the electrode surface. This is needed
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to oxidize MO molecules. A much higher MO removal rate (68.1%) was achieved using the bias between the Au/TiO2/Au/TiO2 electrode and the Pt wire electrode, to the case of the PEC process. There is a linear relationship between the C/C0 and the light irradiation time (Fig. 6e), the Ln (C/C0) plot and light irradiation time (Fig. 6f). This indicates that the photoelectro-reduction of Au/ TiO2/Au/TiO2 hybrid film and degradation of MO followed a pseudo-first-order kinetic feature, which can be described as follows:
ln(
C ) = −kt C0
(3)[38] 9
ACCEPTED MANUSCRIPT where C is MO concentration at time t, C0 is the initial MO concentration, and k is the first-order apparent rate constant. The rate constants were determined to be 0.000196 h-1 for the EC process and 0.0003305 h-1 for PEC. Additional bias current between the two electrodes could induce the
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migration of photo-excited electrons to the Pt wire electrode through the outer circuit. This would prevent recombination of electrons and the holes and improve the catalytic activity and rate constant. The PC, EC, and PEC fade times for MO were 200, 150, and 90 min; the corresponding percent
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degradations of the Au/TiO2/Au/TiO2 hybrid structure were 72.5%, 81.6%, and 83.0%, respectively
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(Fig. 7). Fig. 6. X Fig. 7. X
A possible mechanism for the PEC process of MO degradation using Au/TiO2/Au/TiO2 electrode
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is proposed in Fig. 8. The conduction band (CB) and valence band (VB) of the TiO2 are -0.29 eV and 2.91 eV, respectively, while the work function of Au is about 5.1 eV. When TiO2 was exposed to ultraviolet light, both a photo-generated hole (h+) and electron (e-) were formed. The electrons were
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captured in electrolyte by the electro-catalytic oxidation on Au nanoparticles. A Schottky barrier was
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formed at the interfaces between the Au nanoparticles and the TiO2 nanotubes due to the large work function of Au. Thus, the electrons could be easily transferred to the CB of the TiO2 nanotubes. The high separation rate of electron-hole pairs makes it easier to form OH· radicals on the surface of Au/TiO2/Au/TiO2 that interact with MO. This enhances the photo-degradation activity. There was also a bias voltage applied between the Au/TiO2/Au/TiO2 electrode and Pt electrode—the electrons transferred along the TiO2 nanotube arrays to the Ti wire and eventually reached the Pt electrode through the outer circuit. The transferred electrons were further reduced to the absorbed oxygen (O2) 10
ACCEPTED MANUSCRIPT on the Pt electrode to form superoxide anions (O2-). The activated O2- further produced hydroxyl radicals (·OH) following a series of reactions with H+ that were responsible for the degradation of MO. The –N=N- double bond in the azo dyes is the chromophoric group for color [39], and this
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conjugated structure will disconnect when it reacts with ·OH [40-42]. This results in discoloration. The reaction intermediates of MO were further oxidized into a series of aromatic metabolites that are
degradation [43, 44].
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Fig. 8. x
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finally mineralized to CO2 and H2O. This suggests that ·OH plays a major role in benzene
4. Conclusions
An Au/TiO2/Au/TiO2 photo-electrode was successfully fabricated by simple electrochemical ways. The PEC degradation of MO was also investigated to test the use of a Au/TiO2/Au/TiO2
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photo-electrode. The major conclusions were: (1) The Au/TiO2/Au/TiO2 photo-electrode was composed of the TiO2 nanotube array layer and a randomly distributed TiO2 nanotube layer with highly-dispersed Au nanoparticles. These demonstrated a high photocurrent response between the
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Au/TiO2/Au/TiO2 photo-electrode and the Pt electrode. (2) The Au/TiO2/Au/TiO2 composite
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electrode had good degradation and mineralization capability of MO. The MO removal rate at PEC degradation process was higher than the EC and PC degradation processes. The synergy effect between TiO2 nanotubes and Au nanoparticles facilitated the charge separation and transfer. This led to high-efficiency PEC activity in the Au/TiO2/Au/TiO2 composite electrode. We also concluded that the MO degradation in PEC was due to ·OH radials produced in the two electrodes. Acknowledgements This work was supported by the National key R&D program (No. 2016YFB0402700), the 11
ACCEPTED MANUSCRIPT National Nature Science Foundation of China (Nos. 61301045, 61401306 and 61504096), the Natural Science Foundation of Tianjin City (Nos. 13JCZDJC36000, 15JCYBJC24000, 16JCYBJC16300), the Excellent Young Teachers Program of Tianjin, and the Youth Top-notch
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Talents Program of Tianjin.
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ACCEPTED MANUSCRIPT References [1] D. Yañez, S. Guerrero, I. Lieberwirth, M.T. Ulloa, T. Gomez, F.M. Rabagliati, P.A. Zapata, Photocatalytic inhibition of bacteria by TiO2 nanotubes-doped polyethylene composites, Appl. Catal.
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A 489 (2015) 255-261. [2] R. Zouzelka, Y. Kusumawati, M. Remzova, J. Rathousky, T. Pauporté, Photocatalytic activity of porous multiwalled carbon nanotube-TiO2 composite layers for pollutant degradation, J. Hazard.
SC
Mater. 317 (2016) 52-59.
M AN U
[3] Q. Wang, H. Jiang, S. Zang, J. Li, Q. Wang, Gd, C, N and P quaternary doped anatase-TiO2 nano-photocatalyst for enhanced photocatalytic degradation of 4-chlorophenol under simulated sunlight irradiation, J. Alloys Compd. 586 (2014) 411-419.
[4] C.Y. Zhai, M.S. Zhu, D. Bin, H.W. Wang, Y.K. Du, C.Y. Wang, P. Yang, Visible-Light-Assisted
TE D
Electrocatalytic Oxidation of Methanol Using Reduced Graphene Oxide Modified Pt Nanoflowers-TiO2 Nanotube Arrays, ACS Appl. Mater. Interfaces 6 (2014) 17753-17761. [5] S.Y. Yang, W. Choi, H. Park, TiO2 Nanotube Array Photoelectrocatalyst and Ni-Sb-SnO2
EP
Electrocatalyst Bifacial Electrodes: A New Type of Bifunctional Hybrid Platform for Water
AC C
Treatment, ACS Appl. Mater. Interfaces 7 (2015) 1907-1914. [6] M. Mollavali, C. Falamaki, S. Rohani, Preparation of multiple-doped TiO2 nanotube arrays with nitrogen, carbon and nickel with enhanced visible light photoelectrochemical activity via single-step anodization, Int. J. Hydrogen Energy 40 (2015) 12239-12252. [7] M. Radecka, A. Wnuk, A. Trenczek-Zajac, K. Schneider, K. Zakrzewska, TiO2/SnO2 nanotubes for hydrogen generation by photoelectrochemical water splitting, Int. J. Hydrogen Energy 40 (2015) 841-851. 13
ACCEPTED MANUSCRIPT [8] Q. Zhang, N. Bao, X. Zhu, D. Ma, Y. Xin, Preparation and photocatalytic properties of graphene/TiO2 nanotube arrays photoelectrodes, J. Alloys Compd. 618 (2015) 761-767. [9] J. Wu, C.L. Tang, H. Xu, W. Yan, Enhanced photoelectrochemical performance of PbS sensitized
RI PT
Sb-SnO2/TiO2 nanotube arrays electrode under visible light illumination, J. Alloys Compd. 633 (2015) 83-91.
[10] Q. Luo, Q. Cai, X. Li, X. Chen, Characterization and photocatalytic activity of large-area single
M AN U
seed layers, J. Alloys Compd. 597 (2014) 101-109.
SC
crystalline anatase TiO2 nanotube films hydrothermal synthesized on Plasma electrolytic oxidation
[11] J. Podporska-Carroll, E. Panaitescu, B. Quilty, L.L. Wang, L. Menon, S.C. Pillai, Antimicrobial properties of highly efficient photocatalytic TiO2 nanotubes, Appl. Catal. B 176 (2015) 70-75. [12] C.R. Xue, T. Yonezawa, M.T. Nguyen, X. Lu, Cladding Layer on Well-Defined Double-Wall
TE D
TiO2 Nanotubes, Langmuir 31 (2015) 1575-1580.
[13] J. Su, L. Zhu, P. Geng, G. Chen, Self-assembly graphitic carbon nitride quantum dots anchored on TiO2 nanotube arrays: An efficient heterojunction for pollutants degradation under solar light, J.
EP
Hazard. Mater. 316 (2016) 159-168.
AC C
[14] N. Mojumder, S. Sarker, S.A. Abbas, Z. Tian, V. Subramanian, Photoassisted Enhancement of the Electrocatalytic Oxidation of Formic Acid on Platinized TiO2 Nanotubes, ACS Appl. Mater. Interfaces 6 (2014) 5585-5594. [15] M. Sobaszek, K. Siuzdak, M. Sawczak, J. Ryl, R. Bogdanowicz, Fabrication and characterization of composite TiO2 nanotubes/boron-doped diamond electrodes towards enhanced supercapacitors, Thin Solid Films 601 (2016) 35-40. [16] M. Salari, S.H. Aboutalebi, A.T. Chidembo, K. Konstantinov, H.K. Liu, Surface engineering of 14
ACCEPTED MANUSCRIPT self-assembled TiO2 nanotube arrays: A practical route towards energy storage applications, J. Alloys Compd. 586 (2014) 197-201. [17] S. Dosta, M. Robotti, S. Garcia-Segura, E. Brillas, I.G. Cano, J.M. Guilemany, Influence of
RI PT
atmospheric plasma spraying on the solar photoelectro-catalytic properties of TiO2 coatings, Appl. Catal. B 189 (2016) 151-159.
[18] E. Brillas, C.A. Martínez-Huitle, Decontamination of wastewaters containing synthetic organic
SC
dyes by electrochemical methods. An updated review, Appl. Catal. B 166-167 (2015) 603-643.
M AN U
[19] L.C. Almeida, B.F. Silva, M.V.B. Zanoni, Photoelectrocatalytic/photoelectro-Fenton coupling system using a nanostructured photoanode for the oxidation of a textile dye: Kinetics study and oxidation pathway, Chemosphere 136 (2015) 63-71.
[20] B.Y. Yang, D.W. He, W.S. Wang, Z.L. Zhuo, Y.S. Wang, Gold-plasmon enhanced photocatalytic
TE D
performance of anatase titania nanotubes under visible-light irradiation, Mater. Res. Bull. 74 (2016) 278-283.
[21] L. Xing, J.B. Jia, Y.Z. Wang, B.L. Zhang, S.J. Dong, Pt modified TiO2 nanotubes electrode:
AC C
(2010) 12169-12173.
EP
Preparation and electrocatalytic application for methanol oxidation, Int. J. Hydrogen Energy 35
[22] L. Wu, F. Li, Y. Xu, J.W. Zhang, D. Zhang, G. Li, H. Li, Plasmon-induced photoelectrocatalytic activity of Au nanoparticles enhanced TiO2 nanotube arrays electrodes for environmental remediation, Appl. Catal. B 164 (2015) 217-224. [23] S.S. Zhang, B.Y. Peng, S.Y. Yang, H.G. Wang, H. Yu, Y.P. Fang, F. Peng, Non-noble metal copper nanoparticles-decorated TiO2 nanotube arrays with plasmon-enhanced photocatalytic hydrogen evolution under visible light, Int. J. Hydrogen Energy 40 (2015) 303-310. 15
ACCEPTED MANUSCRIPT [24] S. Luo, Y. Xiao, L. Yang, C. Liu, F. Su, Y. Li, Q. Cai, G. Zeng, Simultaneous detoxification of hexavalent chromium and acid orange 7 by a novel Au/TiO2 heterojunction composite nanotube arrays, Sep. Purif. Technol. 79 (2011) 85-91.
RI PT
[25] S. Chen, M. Malig, M. Tian, A.C. Chen, Electrocatalytic Activity of PtAu Nanoparticles Deposited on TiO2 Nanotubes, J. Phys. Chem. C 116 (2012) 3298-3304.
[26] N.T. Nguyen, M. Altomare, J. Yoo, P. Schmuki, Efficient Photocatalytic H-2 Evolution:
SC
Controlled Dewetting-Dealloying to Fabricate Site-Selective High-Activity Nanoporous Au Particles
M AN U
on Highly Ordered TiO2 Nanotube Arrays, Adv. Mater. 27 (2015) 3208-3215.
[27] P. Zhang, J.L. Guo, P. Zhao, B.L. Zhu, W.P. Huang, S.M. Zhang, Promoting effects of lanthanum on the catalytic activity of Au/TiO2 nanotubes for CO oxidation, RSC Adv. 5 (2015) 11989-11995.
TE D
[28] X. Fang-Xing, S.F. Hung, J.W. Miao, H.Y. Wang, H.B. Yang, B. Liu, Metal-Cluster-Decorated TiO2 Nanotube Arrays: A Composite Heterostructure toward Versatile Photocatalytic and Photoelectrochemical Applications, Small 11 (2015) 554-567.
EP
[29] Z. Xu, Y. Lin, M. Yin, H. Zhang, C. Cheng, L. Lu, X. Xue, H.J. Fan, X. Chen, D. Li,
AC C
Understanding the enhancement mechanisms of surface plasmon-mediated photoelectrochemical electrodes: A case study on Au nanoparticle decorated TiO2 nanotubes, Adv. Mater. Interfaces 2 (2015) 1500169.
[30] D. Ljubas, G. Smoljanic, H. Juretic, Degradation of Methyl Orange and Congo Red dyes by using TiO2 nanoparticles activated by the solar and the solar-like radiation, J. Environ. Manage. 161 (2015) 83-91. [31] S. Kuriakose, V. Choudhary, B. Satpati, S. Mohapatra, Facile synthesis of Ag-ZnO hybrid 16
ACCEPTED MANUSCRIPT nanospindles for highly efficient photocatalytic degradation of methyl orange, Phys. Chem. Chem. Phys. 16 (2014) 17560-17568. [32] R. Kumar, G. Kumar, M.S. Akhtar, A. Umar, Sonophotocatalytic degradation of methyl orange
RI PT
using ZnO nano-aggregates, J. Alloys Compd. 629 (2015) 167-172. [33] Z. Shao, H. Li, M. Li, C. Li, C. Qu, B. Yang, Fabrication of polyaniline nanowire/TiO2 nanotube array electrode for supercapacitors, Energy 87 (2015) 578-585.
SC
[34] M. Nischk, P. Mazierski, M. Gazda, A. Zaleska, Ordered TiO2 nanotubes: The effect of
M AN U
preparation parameters on the photocatalytic activity in air purification process, Appl. Catal. B 144 (2014) 674-685.
[35] Z.L. He, W.X. Que, J. Chen, X.T. Yin, Y.C. He, J.B. Ren, Photocatalytic Degradation of Methyl Orange over Nitrogen-Fluorine Codoped TiO2 Nanobelts Prepared by Solvothermal Synthesis, ACS
TE D
Appl. Mater. Interfaces 4 (2012) 6815-6825.
[36] F. Arduini, C. Zanardi, S. Cinti, F. Terzi, D. Moscone, G. Palleschi, R. Seeber, Effective electrochemical sensor based on screen-printed electrodes modified with a carbon black-Au
EP
nanoparticles composite, Sens. Actuators, B 212 (2015) 536-543.
AC C
[37] C. Wang, R. Yuan, Y. Chai, F. Hu, Simultaneous determination of hydroquinone, catechol, resorcinol
and
nitrite
using
gold
nanoparticles
loaded
on
poly-3-amino-5-mercapto-1,2,4-triazole-MWNTs film modified electrode, Anal. Methods 4 (2012) 1626-1628.
[38] A. Tayyebi, M. Outokesh, M. Tayebi, A. Shafikhani, S.S. Sengor, ZnO quantum dots-graphene composites: Formation mechanism and enhanced photocatalytic activity for degradation of methyl orange dye, J. Alloys Compd. 663 (2016) 738-749. 17
ACCEPTED MANUSCRIPT [39] Y.Y. Sha, I. Mathew, Q.Z. Cui, M. Clay, F. Gao, X.J. Zhang, Z.Y. Gu, Rapid degradation of azo dye methyl orange using hollow cobalt nanoparticles, Chemosphere 144 (2016) 1530-1535. [40] L. Gomathi Devi, S. Girish Kumar, K. Mohan Reddy, C. Munikrishnappa, Photo degradation of
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Methyl Orange an azo dye by Advanced Fenton Process using zero valent metallic iron: Influence of various reaction parameters and its degradation mechanism, J. Hazard. Mater. 164 (2009) 459-467. [41] H.M. Yang, J.T. Liang, L. Zhang, Z.H. Liang, Electrochemical Oxidation Degradation of Methyl
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Orange Wastewater by Nb/PbO2 Electrode, Int. J. Electrochem. Sci. 11 (2016) 1121-1134.
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[42] J. Guo, D. Jiang, Y. Wu, P. Zhou, Y. Lan, Degradation of methyl orange by ZnO assisted with silica gel, J. Hazard. Mater. 194 (2011) 290-296.
[43] T. Soltani, B.-K. Lee, Novel and facile synthesis of Ba-doped BiFeO3 nanoparticles and enhancement of their magnetic and photocatalytic activities for complete degradation of benzene in
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aqueous solution, J. Hazard. Mater. 316 (2016) 122-133.
[44] H. Lee, Y.K. Park, S.J. Kim, B.H. Kim, H.S. Yoon, S.C. Jung, Rapid degradation of methyl
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(2016) 205-210.
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orange using hybrid advanced oxidation process and its synergistic effect, J. Ind. Eng. Chem. 35
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ACCEPTED MANUSCRIPT Table captions
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Table 1-The fabrication parameters of different Au/TiO2 hybrid structures.
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ACCEPTED MANUSCRIPT Figure captions Fig. 1. Schematic process for synthesizing the Au/TiO2/Au/TiO2 electrode. Fig. 2. SEM images of (a) TiO2 nanotube array, (b) Au nanoparticles/TiO2 nanotubes, (c)
cross-sectional SEM images. Fig. 3. TEM and HRTEM images of Au/TiO2 hybrid structures.
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TiO2/Au/TiO2, and (d) Au/ TiO2/Au/TiO2 hybrid structures. The insets are corresponding
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Fig. 4. (a) The XRD patterns of TiO2 nanotubes, Au/TiO2, TiO2/Au/TiO2, and Au/TiO2/Au/TiO2. (b)
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EDX spectrum of the Au/ TiO2/Au/TiO2.
Fig. 5. UV-vis spectra of the PEC degradation of MO catalyzed by TiO2 nanotubes, Au/TiO2, TiO2/Au/TiO2, and Au/TiO2/Au/TiO2 during 30 min of reaction time.
Fig. 6. Time-dependent UV-vis spectra profiles changes of the (a) PC, (b) EC, and (c) PEC
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degradation of 5 mg/L MO catalyzed using the Au/TiO2/Au/TiO2 hybrid structure. (d) UV-vis spectra of PC, EC, and PEC degradation of 5 mg/L MO over 30 min. (e) The degradation of MO by EC, PC, and PEC under UV-light irradiation via Au/TiO2/Au/TiO2 electrode. (f) Corresponding dependence
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of Ln (C0/C) on UV-light irradiation time for MO degradation.
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Fig. 7. Percent degradation and fade time of 5 mg/L MO by PC, EC, and PEC. Fig. 8. Schematic diagram of the charge carrier transfer of photoelectrocatalysis for Au/TiO2/Au/TiO2 electrode and the degradation mechanism of MO.
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ACCEPTED MANUSCRIPT Table 1 The fabrication parameters of different Au/TiO2 hybrid structures. Sample
Anodization Potential /V
Time
[(NH4)F] -1
/h
/ (g L )
Annealing
Electrodeposition of Au nanoparticles
T
Time
Step potential,
Time
[HAuCl4]
/h
step time
/min
/ mM
o
/C
/ V, s 19.9
3
9.75
540
3
-
-
-
Au/TiO2
19.9
3
9.75
540
3
2,1;-2,5
150
0.02
TiO2/Au/TiO2
19.9
3
9.75
540
3
2,1;-2,5
150
0.02
19.9
3
9.75
540
3
19.9
3
9.75
540
3
2,1;-2,5
150
0.02
19.9
3
9.75
540
3
2,1;-2,5
150
0.02
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Au/TiO2/Au/TiO2
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TiO2
1
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Fig. 1. Schematic process for synthesizing the Au/TiO2/Au/TiO2 electrode.
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ACCEPTED MANUSCRIPT Fig. 2. SEM images of (a) TiO2 nanotube array, (b) Au nanoparticles/TiO2 nanotubes, (c) TiO2/Au/TiO2, and (d) Au/ TiO2/Au/TiO2 hybrid structures. The insets are corresponding
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cross-sectional SEM images.
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Fig. 3. TEM and HRTEM images of Au/TiO2 hybrid structures.
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ACCEPTED MANUSCRIPT Fig. 4. (a) The XRD patterns of TiO2 nanotubes, Au/TiO2, TiO2/Au/TiO2, and Au/TiO2/Au/TiO2. (b)
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EDX spectrum of the Au/ TiO2/Au/TiO2.
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ACCEPTED MANUSCRIPT Fig. 5. UV-vis spectra of the PEC degradation of MO catalyzed by TiO2 nanotubes, Au/TiO2,
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TiO2/Au/TiO2, and Au/TiO2/Au/TiO2 during 30 min of reaction time.
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ACCEPTED MANUSCRIPT Fig. 6. Time-dependent UV-vis spectra profiles changes of the (a) PC, (b) EC, and (c) PEC degradation of 5 mg/L MO catalyzed using the Au/TiO2/Au/TiO2 hybrid structure. (d) UV-vis spectra of PC, EC, and PEC degradation of 5 mg/L MO over 30 min. (e) The degradation of MO by EC, PC,
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of Ln (C0/C) on UV-light irradiation time for MO degradation.
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and PEC under UV-light irradiation via Au/TiO2/Au/TiO2 electrode. (f) Corresponding dependence
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Fig. 7. Percent degradation and fade time of 5 mg/L MO by PC, EC, and PEC.
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ACCEPTED MANUSCRIPT Fig. 8. Schematic diagram of the charge carrier transfer of photoelectrocatalysis for
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Au/TiO2/Au/TiO2 electrode and the degradation mechanism of MO.
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ACCEPTED MANUSCRIPT Highlights 1. Au/TiO2 hybrid layers on Ti wire were prepared by electrochemical methods. 2. TiO2 nanotubes were decorated with the maximum numbers of Au nanoparticles.
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3. The formation of the Au/TiO2/Au/TiO2 hybrid layer was proposed. 4. We investigated the synergistic catalytic effect of Au NPs and TiO2 nanotubes.
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5. The hybrid layer shows good photoelectrocatalytic activity for MO degradation.
S0925-8388(16)32872-9
DOI:
10.1016/j.jallcom.2016.09.119
Reference:
JALCOM 38953
To appear in:
Journal of Alloys and Compounds
Received Date: 22 July 2016 Revised Date:
4 September 2016
Accepted Date: 11 September 2016
Please cite this article as: C. Fu, M. Li, H. Li, C. Li, X.g. Wu, B. Yang, Fabrication of Au nanoparticle/ TiO2 hybrid films for photoelectrocatalytic degradation of methyl orange, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.09.119. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Fabrication of Au nanoparticle/TiO2 hybrid films for photoelectrocatalytic degradation of methyl orange Chong Fu a, Mingji Li a,*, Hongji Li b,*, Cuiping Li a, Xiao guo Wu, Baohe Yang a Tianjin Key Laboratory of Film Electronic and Communicate Devices, School of Electronics
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a
Information Engineering, Tianjin University of Technology, Tianjin 300384, P. R. China b
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of
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Chemistry & Chemical Engineering, Tianjin University of Technology, Tianjin 300384, P. R. China
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*Corresponding authors:
E-mall: [email protected] (Mingji Li); [email protected] (Hongji Li)
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Tel.: +86 022 60215346.
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ACCEPTED MANUSCRIPT Abstract Au nanoparticles and TiO2 nanotube hybrid materials were prepared electrochemically. The Ti wire was used to form TiO2 nanotube arrays via anodization and subsequent annealing process. The Au
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nanoparticles were then electrodeposited in chloroauric acid solution to form Au/TiO2 hybrid layers. These two steps were repeated to prepare Au/TiO2, TiO2/Au/TiO2 and Au/ TiO2/Au/TiO2 hybrid materials, and the preparation mechanism is discussed. The Au/TiO2 hybrid materials also exhibited
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different photoelectrocatalytic activities for decomposing methyl orange. The application of the Au/
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TiO2/Au/TiO2 hybrid material as a photo-anode has been simultaneously examined toward electrochemical-oxidation, photo-oxidation, and photoelectro-oxidation of methyl orange. The photoelectrocatalytic mechanism of Au/TiO2/Au/TiO2 hybrid material and its degradation reaction
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mechanism for methyl orange were evaluated.
Keywords: Au nanoparticle; TiO2 nanotube; Electrochemical method; Photoelectrocatalytic
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activities; Methyl orange
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ACCEPTED MANUSCRIPT 1. Introduction Semiconductor TiO2 photo-degrades various harmful organics under UV-light [1-3]. However, its intrinsic property implies a large forbidden bandwidth of 3.2 eV that reduced its photocatalytic
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efficiency. Thus, the quantum yield is lower because of the recombination of electron-hole pairs in the catalysis process. To improve the performance of the photo-degradation process, researchers have incorporated electro-catalytic degradation processes with TiO2 nanotubes as a photo-electrode [4-10].
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The TiO2 nanotubes as a photocatalyst have high electron mobility and larger specific surface area
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[11-13]; as an electrode, they offer a direct pathway for charge transport that enhances electro-catalytic activity [14-16]. Photoelectrocatalysis (PEC) [17, 18] generates ·OH radicals via the combination of semiconductor electrodes with a light irradiation source. When semiconductor electrodes are irradiated with light, the electrons are promoted from the valence band to the
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conduction band, which generates highly oxidative holes (h+) in the valence band. This produces OH radicals on the electrode surface due to water oxidation. The incorporation of metal nanoparticles such as Pt, Au, Cu and Ag on the TiO2 surface
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improves the photoelectrocatalytic performance because it minimizes electron/hole recombination
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and amplifies visible light absorption [19-23]. The coupling of Au nanoparticles and TiO2 nanotubes possessing different redox energy levels for the bands increases the lifetime of the charge carriers. This enhances the efficiency of interfacial charge transfer [24, 25]. Many papers have discussed the photo-catalytic (PC) process at the Au/TiO2 nanotubes under UV light illumination [26]. The design of an optimized interface structure between the Au nanoparticles and the TiO2 nanotubes is a critical scientific question in increasing the photoelectrocatalytic behavior of the Au/TiO2 nanotube electrode. 3
ACCEPTED MANUSCRIPT The interface structures of the Au nanoparticles and TiO2 nanotubes include the following steps: nanoparticles attach to the nanotube wall [27], the nanoparticles moves into the interior of the nanotube [26, 28], the nanoparticles are distributed on top of the nanotubes [27, 29], and
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nanoparticles are buried in the bottom of the nanotubes. We investigated in detail the synergistic catalytic effect of Au nanoparticles and TiO2 nanotubes. We found that the composite and interface structure between them had the best photocatalytic and electrochemical oxidation results to create
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optimal photo-electrode products.
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Here, we synthesized TiO2 nanotubes decorated with Au nanoparticles (Au/ TiO2) via a two-step electrochemical processes. Their photoelectrocatalytic behavior in the degradation of methyl orange (MO) dye was investigated. MO is a representative organic dye used extensively in the printing, textile, and photographic industries [30-32]. To reveal the photoelectrocatalysis mechanism of the
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Au/TiO2 hybrid structures, we compared the electrochemical oxidation (EC), PC, and PEC as a tool
2. Experimental
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to degrade MO. A PEC degradation path of MO on the Au/TiO2 hybrid structure was proposed.
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2.1 Preparation of Au nanoparticle/TiO2 nanotube hybrid structures The Au/TiO2 hybrid structures were constructed on the Ti wires using electrochemical methods. First, the TiO2 nanotube array layer was prepared with anodization of the Ti wire. The Ti wires with a diameter of 1.5 mm and a length of 6 cm were used as the anode. The details are described in our previous studies [33]. Au nanoparticles were electrodeposited on TiO2 nanotubes using 0.02 mM hydrogen tetrachloroaurate solution via a multi-potential step technology. The fabrication parameters of the Au/TiO2 hybrid structures are summarized in Table 1. 4
ACCEPTED MANUSCRIPT Table 1 x 2.2 Degradation of MO The photoelectrocatalytic activities of the as-prepared Au/TiO2 hybrid materials were assessed
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via degradation of MO. A 350 W mercury lamp and IT6720 auto range DC power supply were used as the UV light source (λ=365 nm) and power source (current: 0.03 A, electrode area: 706.5 mm-2), respectively. Prior to degradation, the Au/TiO2 hybrid materials as photo-electrodes were inserted
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into a quartz cup containing MO solution (initial concentration: 5.2 mg L-1, pH 6.7) via a magnetic
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stirrer. A 0.1 M Na2SO4 solution was used as the supporting electrolyte. The lamp and power supply were simultaneously or separately turned on. The change in MO concentration during treatment was analyzed with UV spectrophotometry at MO’s characteristic wavelength. The absorption intensities before and after degradation as well as the degradation ratio of MO can be calculated as follows:
c A = c0 A0 (A0 − A )
× 100
A0
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D(%) =
(1)
(2)
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where D is the degradation ratio, c0 and A0 are the concentration and absorbance of the initial MO,
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respectively, and c and A are the concentration and measured absorption intensity after degradation for a known time, respectively. 2.3 Characterization
Field-emission scanning electron microscopy (FESEM) images were obtained using a MERLIN Compact instrument (Carl Zeiss, Germany) equipped with an energy-dispersive X-ray spectrum (EDS) operated at an accelerating voltage of 5 kV. The range of elements analyzed was 4Be to 94Pu. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained 5
ACCEPTED MANUSCRIPT using a JEM-2100 electron microscope (JEOL, Japan) with an acceleration voltage of 120 and 200 kV, respectively. The sample was prepared by depositing a drop of the ultrasonic suspension in ethanol on a copper grid coated with a carbon film. The X-ray diffraction (XRD) patterns were
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obtained using XRD (Rigaku D/max-2500/PC, Japan) with Cu Kα radiation (40 kV, 40 mA, λ=0.15406). The data were collected from 10o to 100o (2θ) with a resolution step size of 0.1o s-1. UV-visible (UV-vis) absorption spectroscopy was performed on a U-3900 spectrophotometer
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(Hitachi, Japan). Photoelectrochemical measurements used a conventional three electrode system
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including a quartz cell with an electrochemical station. A 350 W mercury lamp (λ=365 nm) was used as the UV source. A bias potential of -1.2 V vs. SCE was applied on the photoanode for the photocurrent test under on-off light conditions with an amperometric i-t curve method. 3. Results and discussion
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An Au/TiO2/Au/TiO2 electrode was fabricated via an electrochemical anodization and an electrodeposition method [33, 34] (Fig. 1). The Au nanoparticles were deposited on the TiO2 nanotubes in HAuCl4 solution, and a multi-potential step technique was used to control the size and
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distribution of the Au particles. The electro-ionization process of the [AuCl4]- ions can be expressed as [AuCl4 ] ↔ Au 3 + + 4Cl − , and the electrochemical reduction process of the Au can be depicted
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as Au 3 + + 3e − ↔ Au .
According to Le Chatelier’s principle, the ionization balance will shift to the right side as Au3+ is consumed in the reduction reaction. This could induce diffusion of [AuCl4]- ions from bulk solution onto the work electrode surface. The concentration of [AuCl4]- ions at the solid-liquid interface was supplemented during applied positive potential in the electrodeposition process. This reduced the influence of the concentration polarization. There is a subsequent reduction of [AuCl4]- under 6
ACCEPTED MANUSCRIPT negative potential. This promotes the formation of nanocrystals. The Au/TiO2/Au/TiO2 electrode as the final product was again prepared via alternating anodization and electrodeposition. During the secondary anodic oxidation, the array structures of the TiO2 nanotubes does not form because the Au
Fig. 1. x
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nanoparticles interfere with the electrical signal and direction of propagation.
The representative SEM images shown in Fig. 2 were taken during different electrochemical
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processes and comprise electrodeposition of Au nanoparticles and anodization of the Ti layer. The
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anodic oxidation products are tubular, hollow, and neatly aligned (Fig. 2a). The diameter of the nanotubes is approximately 80-100 nm, and the thickness of the tube wall is about 5-10 nm. The electrodeposition forms Au nanoparticles that deposit on the nanotube surface. A change in the coloration of the TiO2 nanotube electrode before and after electrodeposition confirms the formation
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of deposits on the TiO2 nanotube surface (Fig. 1). The Au nanoparticles appear as islands distributed along the surface of the TiO2 nanotube layer after the first deposition of Au (Fig. 2b). When the Au/TiO2 electrode is anodized, the Au nanoparticles were not observed on the nanotube wall.
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Meanwhile, the nanotubes on the surface layer become disordered; the nanotubes appear longer (Fig.
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2c). The electrodeposition treatment was performed again for the TiO2/Au/TiO2 electrode. The Au nanoparticles are uniformly distributed along the wall of the TiO2 nanotube (Fig. 2 d). Fig. 2. x
Fig. 3 shows TEM images of Au/TiO2 hybrid structures. Fig. 3a illustrated that the TiO2 nanotubes have diameters of around 80-100 nm and nanotube walls of around 10 nm. The 5 to 10 nm diameter nanoparticles were deposited along the TiO2 nanotubes and in the pores of the nanotubes (Fig. 3a). The HRTEM image of the Au/TiO2 nanotube reveals interplanar spacing values of 0.313 7
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(101)
spacing of TiO2 anatase and the d
(111)
spacing of the
metallic Au, respectively. The microstructure analysis reveals that the Au/TiO2 hybrid structures could be successfully fabricated via two-step electrochemical treatments.
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Fig. 3. x To identify the crystal phase of the TiO2 nanotubes and Au nanoparticles, the XRD result were considered in light of the data in Fig. 4a. All samples clearly show the TiO2 anatase phase (JCPDS
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No: 21-1272) and the Ti metal phase (JCPDS No: 44-1294). The XRD patterns of the Au/TiO2 and
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Au/TiO2/Au/TiO2 samples had no significant characteristic peaks, but the peaks of Ti and TiO2 were significantly weakened. This also indicates the presence of other phases. Au nanoparticles are visible by SEM or TEM analysis in Fig. 2 and Fig. 3. To further verify the presence of Au phase, Fig. 4b shows the EDX spectrum of the Au/TiO2/Au/TiO2 samples. This
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indicates the presence of Ti, O and Au. These results are evidence of successful Au deposition on the surface of the TiO2 nanotubes.
Fig. 4. x
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To study the photoelectric catalytic activity of the TiO2 nanotubes, Au/TiO2, TiO2/Au/TiO2, and
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Au/TiO2/Au/TiO2 samples, MO was degraded using these photo-electrodes (Fig. 5). After degradation for 30 min, the concentration of MO decreased. The percent degradation of MO on TiO2 nanotubes, Au/TiO2, TiO2/Au/TiO2, and Au/ TiO2/Au/TiO2 were 39.2%, 17.0%, 39.2%, and 68.1%, respectively. The interaction between the Au nanoparticles and TiO2 nanotubes was an important factor that affected the degradation rate. The higher catalytic activity of Au/TiO2/Au/TiO2 hybrid structures as catalysts is due to the following three significant aspects: (1) The Au has a smaller particle size. When the particle size decreases, the surface area to volume ratio increases—this 8
ACCEPTED MANUSCRIPT enhances the electron-hole separation[29]. (2) The morphology of the catalysts plays an important role in enhancing the final degradation efficiency similar to the literature[35]. (3) The degradation rate of MO was significantly improved because gold nanoparticles have an electro-catalytic
37]. Fig. 5. x
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oxidation function for organic and biological species. This has been shown in electrochemistry [36,
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The PC, EC, and PEC degradation MO processes were performed in an aqueous solution via the
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Au/TiO2/Au/TiO2 electrode. Fig. 6(a-c) reveals that the maximum absorption peak of MO (464 nm) decreases rapidly and completely decolorizes within different times in the presence of the Au/ TiO2/Au/TiO2 hybrid structure under different degradation methods. Fig. 6d shows the UV-vis curves after degradation for 30 min. The MO was not degraded via PC. This was attributed to the low
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quantum efficiency and poor UV absorption of Au. The UV-light applied on the system degradation could not produce sufficient electron-hole pairs on the anode surface to oxidize MO molecules. There was only a 21% MO removal rate during EC. This indicated that the bias potential applied on
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the Au/TiO2/Au/TiO2 electrode produced oxidative radicals on the electrode surface. This is needed
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to oxidize MO molecules. A much higher MO removal rate (68.1%) was achieved using the bias between the Au/TiO2/Au/TiO2 electrode and the Pt wire electrode, to the case of the PEC process. There is a linear relationship between the C/C0 and the light irradiation time (Fig. 6e), the Ln (C/C0) plot and light irradiation time (Fig. 6f). This indicates that the photoelectro-reduction of Au/ TiO2/Au/TiO2 hybrid film and degradation of MO followed a pseudo-first-order kinetic feature, which can be described as follows:
ln(
C ) = −kt C0
(3)[38] 9
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migration of photo-excited electrons to the Pt wire electrode through the outer circuit. This would prevent recombination of electrons and the holes and improve the catalytic activity and rate constant. The PC, EC, and PEC fade times for MO were 200, 150, and 90 min; the corresponding percent
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degradations of the Au/TiO2/Au/TiO2 hybrid structure were 72.5%, 81.6%, and 83.0%, respectively
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(Fig. 7). Fig. 6. X Fig. 7. X
A possible mechanism for the PEC process of MO degradation using Au/TiO2/Au/TiO2 electrode
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is proposed in Fig. 8. The conduction band (CB) and valence band (VB) of the TiO2 are -0.29 eV and 2.91 eV, respectively, while the work function of Au is about 5.1 eV. When TiO2 was exposed to ultraviolet light, both a photo-generated hole (h+) and electron (e-) were formed. The electrons were
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captured in electrolyte by the electro-catalytic oxidation on Au nanoparticles. A Schottky barrier was
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formed at the interfaces between the Au nanoparticles and the TiO2 nanotubes due to the large work function of Au. Thus, the electrons could be easily transferred to the CB of the TiO2 nanotubes. The high separation rate of electron-hole pairs makes it easier to form OH· radicals on the surface of Au/TiO2/Au/TiO2 that interact with MO. This enhances the photo-degradation activity. There was also a bias voltage applied between the Au/TiO2/Au/TiO2 electrode and Pt electrode—the electrons transferred along the TiO2 nanotube arrays to the Ti wire and eventually reached the Pt electrode through the outer circuit. The transferred electrons were further reduced to the absorbed oxygen (O2) 10
ACCEPTED MANUSCRIPT on the Pt electrode to form superoxide anions (O2-). The activated O2- further produced hydroxyl radicals (·OH) following a series of reactions with H+ that were responsible for the degradation of MO. The –N=N- double bond in the azo dyes is the chromophoric group for color [39], and this
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conjugated structure will disconnect when it reacts with ·OH [40-42]. This results in discoloration. The reaction intermediates of MO were further oxidized into a series of aromatic metabolites that are
degradation [43, 44].
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Fig. 8. x
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finally mineralized to CO2 and H2O. This suggests that ·OH plays a major role in benzene
4. Conclusions
An Au/TiO2/Au/TiO2 photo-electrode was successfully fabricated by simple electrochemical ways. The PEC degradation of MO was also investigated to test the use of a Au/TiO2/Au/TiO2
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photo-electrode. The major conclusions were: (1) The Au/TiO2/Au/TiO2 photo-electrode was composed of the TiO2 nanotube array layer and a randomly distributed TiO2 nanotube layer with highly-dispersed Au nanoparticles. These demonstrated a high photocurrent response between the
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Au/TiO2/Au/TiO2 photo-electrode and the Pt electrode. (2) The Au/TiO2/Au/TiO2 composite
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electrode had good degradation and mineralization capability of MO. The MO removal rate at PEC degradation process was higher than the EC and PC degradation processes. The synergy effect between TiO2 nanotubes and Au nanoparticles facilitated the charge separation and transfer. This led to high-efficiency PEC activity in the Au/TiO2/Au/TiO2 composite electrode. We also concluded that the MO degradation in PEC was due to ·OH radials produced in the two electrodes. Acknowledgements This work was supported by the National key R&D program (No. 2016YFB0402700), the 11
ACCEPTED MANUSCRIPT National Nature Science Foundation of China (Nos. 61301045, 61401306 and 61504096), the Natural Science Foundation of Tianjin City (Nos. 13JCZDJC36000, 15JCYBJC24000, 16JCYBJC16300), the Excellent Young Teachers Program of Tianjin, and the Youth Top-notch
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Talents Program of Tianjin.
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ACCEPTED MANUSCRIPT References [1] D. Yañez, S. Guerrero, I. Lieberwirth, M.T. Ulloa, T. Gomez, F.M. Rabagliati, P.A. Zapata, Photocatalytic inhibition of bacteria by TiO2 nanotubes-doped polyethylene composites, Appl. Catal.
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A 489 (2015) 255-261. [2] R. Zouzelka, Y. Kusumawati, M. Remzova, J. Rathousky, T. Pauporté, Photocatalytic activity of porous multiwalled carbon nanotube-TiO2 composite layers for pollutant degradation, J. Hazard.
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Mater. 317 (2016) 52-59.
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[3] Q. Wang, H. Jiang, S. Zang, J. Li, Q. Wang, Gd, C, N and P quaternary doped anatase-TiO2 nano-photocatalyst for enhanced photocatalytic degradation of 4-chlorophenol under simulated sunlight irradiation, J. Alloys Compd. 586 (2014) 411-419.
[4] C.Y. Zhai, M.S. Zhu, D. Bin, H.W. Wang, Y.K. Du, C.Y. Wang, P. Yang, Visible-Light-Assisted
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Electrocatalytic Oxidation of Methanol Using Reduced Graphene Oxide Modified Pt Nanoflowers-TiO2 Nanotube Arrays, ACS Appl. Mater. Interfaces 6 (2014) 17753-17761. [5] S.Y. Yang, W. Choi, H. Park, TiO2 Nanotube Array Photoelectrocatalyst and Ni-Sb-SnO2
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Electrocatalyst Bifacial Electrodes: A New Type of Bifunctional Hybrid Platform for Water
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Treatment, ACS Appl. Mater. Interfaces 7 (2015) 1907-1914. [6] M. Mollavali, C. Falamaki, S. Rohani, Preparation of multiple-doped TiO2 nanotube arrays with nitrogen, carbon and nickel with enhanced visible light photoelectrochemical activity via single-step anodization, Int. J. Hydrogen Energy 40 (2015) 12239-12252. [7] M. Radecka, A. Wnuk, A. Trenczek-Zajac, K. Schneider, K. Zakrzewska, TiO2/SnO2 nanotubes for hydrogen generation by photoelectrochemical water splitting, Int. J. Hydrogen Energy 40 (2015) 841-851. 13
ACCEPTED MANUSCRIPT [8] Q. Zhang, N. Bao, X. Zhu, D. Ma, Y. Xin, Preparation and photocatalytic properties of graphene/TiO2 nanotube arrays photoelectrodes, J. Alloys Compd. 618 (2015) 761-767. [9] J. Wu, C.L. Tang, H. Xu, W. Yan, Enhanced photoelectrochemical performance of PbS sensitized
RI PT
Sb-SnO2/TiO2 nanotube arrays electrode under visible light illumination, J. Alloys Compd. 633 (2015) 83-91.
[10] Q. Luo, Q. Cai, X. Li, X. Chen, Characterization and photocatalytic activity of large-area single
M AN U
seed layers, J. Alloys Compd. 597 (2014) 101-109.
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crystalline anatase TiO2 nanotube films hydrothermal synthesized on Plasma electrolytic oxidation
[11] J. Podporska-Carroll, E. Panaitescu, B. Quilty, L.L. Wang, L. Menon, S.C. Pillai, Antimicrobial properties of highly efficient photocatalytic TiO2 nanotubes, Appl. Catal. B 176 (2015) 70-75. [12] C.R. Xue, T. Yonezawa, M.T. Nguyen, X. Lu, Cladding Layer on Well-Defined Double-Wall
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TiO2 Nanotubes, Langmuir 31 (2015) 1575-1580.
[13] J. Su, L. Zhu, P. Geng, G. Chen, Self-assembly graphitic carbon nitride quantum dots anchored on TiO2 nanotube arrays: An efficient heterojunction for pollutants degradation under solar light, J.
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Hazard. Mater. 316 (2016) 159-168.
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[14] N. Mojumder, S. Sarker, S.A. Abbas, Z. Tian, V. Subramanian, Photoassisted Enhancement of the Electrocatalytic Oxidation of Formic Acid on Platinized TiO2 Nanotubes, ACS Appl. Mater. Interfaces 6 (2014) 5585-5594. [15] M. Sobaszek, K. Siuzdak, M. Sawczak, J. Ryl, R. Bogdanowicz, Fabrication and characterization of composite TiO2 nanotubes/boron-doped diamond electrodes towards enhanced supercapacitors, Thin Solid Films 601 (2016) 35-40. [16] M. Salari, S.H. Aboutalebi, A.T. Chidembo, K. Konstantinov, H.K. Liu, Surface engineering of 14
ACCEPTED MANUSCRIPT self-assembled TiO2 nanotube arrays: A practical route towards energy storage applications, J. Alloys Compd. 586 (2014) 197-201. [17] S. Dosta, M. Robotti, S. Garcia-Segura, E. Brillas, I.G. Cano, J.M. Guilemany, Influence of
RI PT
atmospheric plasma spraying on the solar photoelectro-catalytic properties of TiO2 coatings, Appl. Catal. B 189 (2016) 151-159.
[18] E. Brillas, C.A. Martínez-Huitle, Decontamination of wastewaters containing synthetic organic
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dyes by electrochemical methods. An updated review, Appl. Catal. B 166-167 (2015) 603-643.
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[19] L.C. Almeida, B.F. Silva, M.V.B. Zanoni, Photoelectrocatalytic/photoelectro-Fenton coupling system using a nanostructured photoanode for the oxidation of a textile dye: Kinetics study and oxidation pathway, Chemosphere 136 (2015) 63-71.
[20] B.Y. Yang, D.W. He, W.S. Wang, Z.L. Zhuo, Y.S. Wang, Gold-plasmon enhanced photocatalytic
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performance of anatase titania nanotubes under visible-light irradiation, Mater. Res. Bull. 74 (2016) 278-283.
[21] L. Xing, J.B. Jia, Y.Z. Wang, B.L. Zhang, S.J. Dong, Pt modified TiO2 nanotubes electrode:
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(2010) 12169-12173.
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Preparation and electrocatalytic application for methanol oxidation, Int. J. Hydrogen Energy 35
[22] L. Wu, F. Li, Y. Xu, J.W. Zhang, D. Zhang, G. Li, H. Li, Plasmon-induced photoelectrocatalytic activity of Au nanoparticles enhanced TiO2 nanotube arrays electrodes for environmental remediation, Appl. Catal. B 164 (2015) 217-224. [23] S.S. Zhang, B.Y. Peng, S.Y. Yang, H.G. Wang, H. Yu, Y.P. Fang, F. Peng, Non-noble metal copper nanoparticles-decorated TiO2 nanotube arrays with plasmon-enhanced photocatalytic hydrogen evolution under visible light, Int. J. Hydrogen Energy 40 (2015) 303-310. 15
ACCEPTED MANUSCRIPT [24] S. Luo, Y. Xiao, L. Yang, C. Liu, F. Su, Y. Li, Q. Cai, G. Zeng, Simultaneous detoxification of hexavalent chromium and acid orange 7 by a novel Au/TiO2 heterojunction composite nanotube arrays, Sep. Purif. Technol. 79 (2011) 85-91.
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[25] S. Chen, M. Malig, M. Tian, A.C. Chen, Electrocatalytic Activity of PtAu Nanoparticles Deposited on TiO2 Nanotubes, J. Phys. Chem. C 116 (2012) 3298-3304.
[26] N.T. Nguyen, M. Altomare, J. Yoo, P. Schmuki, Efficient Photocatalytic H-2 Evolution:
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Controlled Dewetting-Dealloying to Fabricate Site-Selective High-Activity Nanoporous Au Particles
M AN U
on Highly Ordered TiO2 Nanotube Arrays, Adv. Mater. 27 (2015) 3208-3215.
[27] P. Zhang, J.L. Guo, P. Zhao, B.L. Zhu, W.P. Huang, S.M. Zhang, Promoting effects of lanthanum on the catalytic activity of Au/TiO2 nanotubes for CO oxidation, RSC Adv. 5 (2015) 11989-11995.
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[28] X. Fang-Xing, S.F. Hung, J.W. Miao, H.Y. Wang, H.B. Yang, B. Liu, Metal-Cluster-Decorated TiO2 Nanotube Arrays: A Composite Heterostructure toward Versatile Photocatalytic and Photoelectrochemical Applications, Small 11 (2015) 554-567.
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[29] Z. Xu, Y. Lin, M. Yin, H. Zhang, C. Cheng, L. Lu, X. Xue, H.J. Fan, X. Chen, D. Li,
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Understanding the enhancement mechanisms of surface plasmon-mediated photoelectrochemical electrodes: A case study on Au nanoparticle decorated TiO2 nanotubes, Adv. Mater. Interfaces 2 (2015) 1500169.
[30] D. Ljubas, G. Smoljanic, H. Juretic, Degradation of Methyl Orange and Congo Red dyes by using TiO2 nanoparticles activated by the solar and the solar-like radiation, J. Environ. Manage. 161 (2015) 83-91. [31] S. Kuriakose, V. Choudhary, B. Satpati, S. Mohapatra, Facile synthesis of Ag-ZnO hybrid 16
ACCEPTED MANUSCRIPT nanospindles for highly efficient photocatalytic degradation of methyl orange, Phys. Chem. Chem. Phys. 16 (2014) 17560-17568. [32] R. Kumar, G. Kumar, M.S. Akhtar, A. Umar, Sonophotocatalytic degradation of methyl orange
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using ZnO nano-aggregates, J. Alloys Compd. 629 (2015) 167-172. [33] Z. Shao, H. Li, M. Li, C. Li, C. Qu, B. Yang, Fabrication of polyaniline nanowire/TiO2 nanotube array electrode for supercapacitors, Energy 87 (2015) 578-585.
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[34] M. Nischk, P. Mazierski, M. Gazda, A. Zaleska, Ordered TiO2 nanotubes: The effect of
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preparation parameters on the photocatalytic activity in air purification process, Appl. Catal. B 144 (2014) 674-685.
[35] Z.L. He, W.X. Que, J. Chen, X.T. Yin, Y.C. He, J.B. Ren, Photocatalytic Degradation of Methyl Orange over Nitrogen-Fluorine Codoped TiO2 Nanobelts Prepared by Solvothermal Synthesis, ACS
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Appl. Mater. Interfaces 4 (2012) 6815-6825.
[36] F. Arduini, C. Zanardi, S. Cinti, F. Terzi, D. Moscone, G. Palleschi, R. Seeber, Effective electrochemical sensor based on screen-printed electrodes modified with a carbon black-Au
EP
nanoparticles composite, Sens. Actuators, B 212 (2015) 536-543.
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[37] C. Wang, R. Yuan, Y. Chai, F. Hu, Simultaneous determination of hydroquinone, catechol, resorcinol
and
nitrite
using
gold
nanoparticles
loaded
on
poly-3-amino-5-mercapto-1,2,4-triazole-MWNTs film modified electrode, Anal. Methods 4 (2012) 1626-1628.
[38] A. Tayyebi, M. Outokesh, M. Tayebi, A. Shafikhani, S.S. Sengor, ZnO quantum dots-graphene composites: Formation mechanism and enhanced photocatalytic activity for degradation of methyl orange dye, J. Alloys Compd. 663 (2016) 738-749. 17
ACCEPTED MANUSCRIPT [39] Y.Y. Sha, I. Mathew, Q.Z. Cui, M. Clay, F. Gao, X.J. Zhang, Z.Y. Gu, Rapid degradation of azo dye methyl orange using hollow cobalt nanoparticles, Chemosphere 144 (2016) 1530-1535. [40] L. Gomathi Devi, S. Girish Kumar, K. Mohan Reddy, C. Munikrishnappa, Photo degradation of
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Methyl Orange an azo dye by Advanced Fenton Process using zero valent metallic iron: Influence of various reaction parameters and its degradation mechanism, J. Hazard. Mater. 164 (2009) 459-467. [41] H.M. Yang, J.T. Liang, L. Zhang, Z.H. Liang, Electrochemical Oxidation Degradation of Methyl
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Orange Wastewater by Nb/PbO2 Electrode, Int. J. Electrochem. Sci. 11 (2016) 1121-1134.
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[42] J. Guo, D. Jiang, Y. Wu, P. Zhou, Y. Lan, Degradation of methyl orange by ZnO assisted with silica gel, J. Hazard. Mater. 194 (2011) 290-296.
[43] T. Soltani, B.-K. Lee, Novel and facile synthesis of Ba-doped BiFeO3 nanoparticles and enhancement of their magnetic and photocatalytic activities for complete degradation of benzene in
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aqueous solution, J. Hazard. Mater. 316 (2016) 122-133.
[44] H. Lee, Y.K. Park, S.J. Kim, B.H. Kim, H.S. Yoon, S.C. Jung, Rapid degradation of methyl
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(2016) 205-210.
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orange using hybrid advanced oxidation process and its synergistic effect, J. Ind. Eng. Chem. 35
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ACCEPTED MANUSCRIPT Table captions
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Table 1-The fabrication parameters of different Au/TiO2 hybrid structures.
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ACCEPTED MANUSCRIPT Figure captions Fig. 1. Schematic process for synthesizing the Au/TiO2/Au/TiO2 electrode. Fig. 2. SEM images of (a) TiO2 nanotube array, (b) Au nanoparticles/TiO2 nanotubes, (c)
cross-sectional SEM images. Fig. 3. TEM and HRTEM images of Au/TiO2 hybrid structures.
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TiO2/Au/TiO2, and (d) Au/ TiO2/Au/TiO2 hybrid structures. The insets are corresponding
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Fig. 4. (a) The XRD patterns of TiO2 nanotubes, Au/TiO2, TiO2/Au/TiO2, and Au/TiO2/Au/TiO2. (b)
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EDX spectrum of the Au/ TiO2/Au/TiO2.
Fig. 5. UV-vis spectra of the PEC degradation of MO catalyzed by TiO2 nanotubes, Au/TiO2, TiO2/Au/TiO2, and Au/TiO2/Au/TiO2 during 30 min of reaction time.
Fig. 6. Time-dependent UV-vis spectra profiles changes of the (a) PC, (b) EC, and (c) PEC
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degradation of 5 mg/L MO catalyzed using the Au/TiO2/Au/TiO2 hybrid structure. (d) UV-vis spectra of PC, EC, and PEC degradation of 5 mg/L MO over 30 min. (e) The degradation of MO by EC, PC, and PEC under UV-light irradiation via Au/TiO2/Au/TiO2 electrode. (f) Corresponding dependence
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Fig. 7. Percent degradation and fade time of 5 mg/L MO by PC, EC, and PEC. Fig. 8. Schematic diagram of the charge carrier transfer of photoelectrocatalysis for Au/TiO2/Au/TiO2 electrode and the degradation mechanism of MO.
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ACCEPTED MANUSCRIPT Table 1 The fabrication parameters of different Au/TiO2 hybrid structures. Sample
Anodization Potential /V
Time
[(NH4)F] -1
/h
/ (g L )
Annealing
Electrodeposition of Au nanoparticles
T
Time
Step potential,
Time
[HAuCl4]
/h
step time
/min
/ mM
o
/C
/ V, s 19.9
3
9.75
540
3
-
-
-
Au/TiO2
19.9
3
9.75
540
3
2,1;-2,5
150
0.02
TiO2/Au/TiO2
19.9
3
9.75
540
3
2,1;-2,5
150
0.02
19.9
3
9.75
540
3
19.9
3
9.75
540
3
2,1;-2,5
150
0.02
19.9
3
9.75
540
3
2,1;-2,5
150
0.02
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Au/TiO2/Au/TiO2
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TiO2
1
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Fig. 1. Schematic process for synthesizing the Au/TiO2/Au/TiO2 electrode.
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ACCEPTED MANUSCRIPT Fig. 2. SEM images of (a) TiO2 nanotube array, (b) Au nanoparticles/TiO2 nanotubes, (c) TiO2/Au/TiO2, and (d) Au/ TiO2/Au/TiO2 hybrid structures. The insets are corresponding
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cross-sectional SEM images.
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Fig. 3. TEM and HRTEM images of Au/TiO2 hybrid structures.
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EDX spectrum of the Au/ TiO2/Au/TiO2.
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TiO2/Au/TiO2, and Au/TiO2/Au/TiO2 during 30 min of reaction time.
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ACCEPTED MANUSCRIPT Fig. 6. Time-dependent UV-vis spectra profiles changes of the (a) PC, (b) EC, and (c) PEC degradation of 5 mg/L MO catalyzed using the Au/TiO2/Au/TiO2 hybrid structure. (d) UV-vis spectra of PC, EC, and PEC degradation of 5 mg/L MO over 30 min. (e) The degradation of MO by EC, PC,
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of Ln (C0/C) on UV-light irradiation time for MO degradation.
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and PEC under UV-light irradiation via Au/TiO2/Au/TiO2 electrode. (f) Corresponding dependence
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Fig. 7. Percent degradation and fade time of 5 mg/L MO by PC, EC, and PEC.
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ACCEPTED MANUSCRIPT Fig. 8. Schematic diagram of the charge carrier transfer of photoelectrocatalysis for
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Au/TiO2/Au/TiO2 electrode and the degradation mechanism of MO.
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ACCEPTED MANUSCRIPT Highlights 1. Au/TiO2 hybrid layers on Ti wire were prepared by electrochemical methods. 2. TiO2 nanotubes were decorated with the maximum numbers of Au nanoparticles.
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3. The formation of the Au/TiO2/Au/TiO2 hybrid layer was proposed. 4. We investigated the synergistic catalytic effect of Au NPs and TiO2 nanotubes.
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5. The hybrid layer shows good photoelectrocatalytic activity for MO degradation.