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n-TNA ternary heterojunction electrode for tetracycline degradation
Accepted Manuscript Title: Photoeletrocatalytic Activity of an n-ZnO/p-Cu2 O/n-TNA Ternary Heterojunction Electrode for Tetracycline Degradation Author: Jinhua Li Shubin Lv Yanbiao Liu Jing Bai Baoxue Zhou Xiaofang Hu PII: DOI: Reference:
S0304-3894(13)00646-8 http://dx.doi.org/doi:10.1016/j.jhazmat.2013.09.002 HAZMAT 15382
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
4-7-2013 29-8-2013 3-9-2013
Please cite this article as: J. Li, S. Lv, Y. Liu, J. Bai, B. Zhou, X. Hu, Photoeletrocatalytic Activity of an n-ZnO/p-Cu2 O/n-TNA Ternary Heterojunction Electrode for Tetracycline Degradation, Journal of Hazardous Materials (2013), http://dx.doi.org/10.1016/j.jhazmat.2013.09.002 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.
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Photoeletrocatalytic Activity of an n-ZnO/p-Cu2O/n-TNA Ternary Heterojunction Electrode
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for Tetracycline Degradation
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Jinhua Li, Shubin Lv, Yanbiao Liu, Jing Bai, Baoxue Zhou, Xiaofang Hu
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School of Environmental Science and Engineering, Shanghai Jiao Tong University No.800
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Dongchuan Rd, Shanghai 200240, China
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Corresponding author: Baoxue Zhou, Tel/Fax: +86-21-54747351;
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E-mail: [email protected]
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Photoeletrocatalytic Activity of an n-ZnO/p-Cu2O/n-TNA Ternary Heterojunction Electrode
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for Tetracycline Degradation
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Jinhua Li, Shubin Lv, Yanbiao Liu, Jing Bai, Baoxue Zhou*, Xiaofang Hu
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School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240,
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China
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Abstract
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In
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(n-ZnO/p-Cu2O/n-TNA) nanophotocatalyst with a sandwich-like nanostructure was constructed and
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applied for the photoelectrocatalytic (PEC) degradation of typical PPCPs, tetracycline (TC). The
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ternary heterojunction n-ZnO/p-Cu2O/n-TNA was obtained by depositing Cu2O on the surface of
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TNA via sonoelectrochemical deposition (SED) and subsequently building a layer of ZnO ontop the
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p-Cu2O/n-TNA surface through hydrothermal synthesis. After being deposited by the Cu2O, the
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absorption-band edge of the p-Cu2O/n-TNA was obviously red-shifted to the visible region (to
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505nm), and the band gap was reduced from its original 3.20 eV to 2.46 eV. The band gap
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absorption edge of the ternary n-ZnO/p-Cu2O/n-TNA is similar to that of p-Cu2O/n-TN and extends
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the visible spectrum absorption to 510 nm, corresponding to an Eg value of about 2.43 eV. Under
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illumination of visible light, the photocurrent density of the ternary heterojunction
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n-ZnO/p-Cu2O/n-TNA electrode at 0.5V (vs. Ag/AgCl) was more than 106 times as high as that of
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the pure TNAs electrode, 3.6 times as high as that of the binary heterojunction p-Cu2O/n-TNA
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electrode. The degradation of TC indicated that the ternary heterojunction n-ZnO/p-Cu2O/n-TNA
study,
a
novel
ternary
heterojunction
n-ZnO/p-Cu2O/n-TiO2
nanotube
arrays
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Corresponding author: Baoxue Zhou, Tel/Fax: +86-21-54747351 *
Abbreviations: TC, tetracycline; CB, conduction band; VB, valence band; TNA, TiO2 nanotube arrays;.PEC, photoelectrocatalytic; SED, sonoelectrochemical deposition method
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electrode maintained a very high photoelectrocatalytic activity and excellent stability and reliability.
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Such kind of ternary heterojunction electrode material has a broad application prospect not only in
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pollution control but also in many other fields.
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Key words: Ternary heterojunction; ZnO; Cu2O; TNA; Photoeletrocatalytic oxidation
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1. Introduction
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Photocatalysis is a promising technique in environmental purification and solar energy
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conversion [1]. Compared with conventional treatment processes, it has advantages of environmental
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friendliness, energy-saving property, high efficiency and non-secondary pollution [2–6]. Thus, the
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fabrication of a high-performance semiconductor photocatalyst is a key issue in the photocatalytic
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technology. However, the common semiconductor materials, such as TiO2 or ZnO, can only absorb
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UV light because of their wide band gaps [7], which greatly limit the usage of solar light.
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The coupled semiconductor heterojunction is an effective method to improve their
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photoelectrocatalytic (PEC) efficiency [8]. Due to the different energy levels between the conduction
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bands (CB) of the semiconductors, the photoexcited electrons can be quickly passed from one
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semiconductor to a neighboring semiconductor so as to accelerate electron-hole pair separation and
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improve PEC efficiency.
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In the past several years, the coupling of n-ZnO or n-TiO2 with other metal oxides or sulfides
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such as, n-WO3, n-CdS, n-Fe2O3, p-SnO2, p-Cu2O and so on, have been reported [9–17]. Cuprous
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oxide, Cu2O, is an attractive p-type oxide for photoelectrochemical hydrogen production with a
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direct band gap of 2.0 eV and a light-to-hydrogen conversion efficiency of 18% based on the AM 1.5
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spectrums [10, 18-20]. In recent years, some researchers reported the binary p-n heterojunctions by
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coupling of p-Cu2O with n-TiO2 or n-ZnO [20-25]. In these work, an enhancement of the charge 3 Page 3 of 28
separation was achieved since the CB of Cu2O (-1.16V) is narrow than that of TiO2 (-0.1V) or ZnO
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(-0.2V). For example, Bessekhouad et al [20] observed the Cu2O/TiO2 composite oxides via the
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mixture of Cu2O (particle size < 5nm) with commercial P25 TiO2 exhibited a superior activity for the
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degradation of Orange II under visible irradiation. Xu et al [21] prepared the ZnO/Cu2O compound
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photocatalysts by “soak-deoxidize-airoxidation” and found that the photocatalytic properties of
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ZnO/Cu2O compound were improved greatly compared with pure ZnO. When the ZnO/Cu2O
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compound was used as photocatalyst, the degradation of MO reached 73% after 180 min of reaction.
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In spite of some advantages by coupling Cu2O with TiO2 or ZnO, The main drawback for the use of
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Cu2O as a photocathode is its poor stability in aqueous solutions, because the redox potentials for the
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reduction and oxidation of monovalent copper oxide lie within the band gap [22-25]. Thus, how to
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protect the photocatalyst Cu2O against photocorrosion is an interesting issue.
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Based on the above analysis and our previous work, a novel ternary heterojunction
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n-ZnO/p-Cu2O/n-TNA photocatalyst with a sandwich-like nanostructure is proposed. The ternary
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heterojunction n-ZnO/p-Cu2O/n-TNA material has the following advantage: first, the CB of p-Cu2O
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is more negative than that of n-ZnO and n-TiO2, which favors the charge separation and transfer.
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Second, the TNA as the substrate of ternary heterojunction is already a very good photocatalyst and
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can maintain the high PEC activity in the UV region. Third, the coupling of the high PEC active
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TNA with Cu2O having narrow band gap extends the visible spectrum absorption and favors the
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charge separation and transfer. Fourth, the coupling of n-ZnO with binary heterojunction
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p-Cu2O/n-TNA protects the Cu2O from photocorrosion and exhibits desirable stability.
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Herein, the n-ZnO/p-Cu2O/n-TNA was prepared by depositing Cu2O on the TNAs via SED and
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then building a layer of n-ZnO on the p-Cu2O/n-TNA surface through hydrothermal synthesis. The 4 Page 4 of 28
detailed synthesis process, characterization, and photoelectrochemical property testing for this
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composite catalyst were also discussed. In addition, the n-ZnO/p-Cu2O/n-TNA electrode was applied
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for the degradation of PPCPs TC to evaluate the PEC performance. TC is widely used in aquiculture
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and live stocking, but the waste TC is usually discharged along with waste water, which causes
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serious ecological pollution.
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2. Experimental
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2.1. Materials
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Pieces of titanium (Ti) sheets (purity > 99.99%) were obtained from Kurumi works, Japan. TC
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(C22H24N2O8·HCl), hydrofluoric acid, sodium sulfate, sodium hydroxide, and acetone were
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purchased obtained from Shanghai Chemical Reagent Company (China) without further purification.
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All solutions were prepared with doubly distilled deionized water.
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2.2 The preparation of electrodes
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2.2.1 Preparation of TNA
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TNA was synthesized via the anodization of pretreated Ti in 0.5 % HF aqueous (HF-H2O) [24].
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Electrochemical setup consisted of a two-electrode configuration with a platinum foil as the cathode
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and a DC power supply (Tradex, MPS 305) was used to drive the anodization. After anodization at
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20V for 30 min at room temperature, the as-prepared TNA samples were crystallized by annealing in
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air atmosphere for 3 h at 450 °C with heating and cooling rates of 1º/min.
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2.2.2 Preparation of p-Cu2O/n-TNA
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The p-Cu2O/n-TNA was prepared by the sonoelectrochemical deposition method (SED) [22].
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The electrolyte solution contained 0.1 M sodium acetate and 0.02 M cupric acetate. The TNA worked
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as the work electrode, a platinum foil was counter electrode and a saturated Ag/AgCl was reference 5 Page 5 of 28
electrode. After the deposition at -2.5v for 30 min, the samples were immediately removed from the
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electrolyte and then rinsed with DI water. The prepared p-Cu2O/n-TNA electrodes were annealed at
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200 °C for 60 min in nitrogen atmosphere.
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2.2.3 Preparation of n-ZnO/p-Cu2O/n-TNA
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The n-ZnO/p-Cu2O/n-TNA was synthesized by the hydrothermal method. First, the 0.15 M ZnO
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coating precursor solution was prepared by dissolving 5 mM Zn(CH3COO)2 to ethanol and stirring
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for 0.5 h. And then the precursor solution were added to the p-Cu2O/n-TNA substrate, spun at 100
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rpm for 3 s, and then spun at 3000 rpm for 10 s. Such a cycle was repeated 5 times. Then the
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substrate were immersed into the 0.02 M Zn (NO3)2 and (CH2)6N4 growing precursor solution at
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90 °C for a certain time. Finally, the resultant films were washed with deionized water to remove any
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residual materials and dried at room temperature.
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2.3 The characteristics of electrodes
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The phase of products was identified by X-ray powder diffraction (XRD, Shimadzu), using Cu
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KR (λ>0.15406 nm) radiation at 50 kV and 50 mA at a scanning rate of 8°/min in the 2θ range from
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10 to 60° at room temperature. TEM images were collected by using a field emission scanning
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electron microscopy (FESEM FEI-Sirion200). UV–visible absorption spectra of the samples were
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recorded on a photospectrometer (TU-1901, Pgeneral, China). Electrochemical impedance
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spectroscopy (EIS) was performed to determine the conductivity of the electrodes, with the
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frequency ranging from 10-3 to 105 Hz, and the potential was 1V. The electrolyte was 0.005 M
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[Fe(CN)6 3-]/[Fe(CN)6 4-] solution.
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Photoelectrochemical properties are measured in a rectangular shaped quartz reactor
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(20mm×30mm×50mm). The setup mainly consists of external irradiation source, three-electrode 6 Page 6 of 28
configuration and data recording system. A computer controlled electrochemical station (CH Ins.,
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CHI660, USA) was used to apply the bias potential and record the photocurrent. The as-prepared
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samples worked as the work electrode, a platinum foil was counter electrode and a saturated
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Ag/AgCl was reference electrode in a solution containing 0.2 M Na2SO4. An external Xenon lamp
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with AM 1.5 (a radiation intensity of 100 mW cm-2) is used as light source. The visible light was
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obtained by a glass filter that allowed wavelengths between 380 and 800 nm to be passed through.
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2.4 PEC degradation experiment
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The PEC degradation experiment of TC setup under visible light and solar light is the same as
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PEC setup. The degradation experiment conditions were as follows: illumination area 10mm×10 mm,
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and 0.5V (vs. Ag/AgCl) of electric bias, pH 5.5, 0.2 M Na2SO4 as electrolyte. The initial
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concentration of TC solution was 20 mg L−1 and the reaction solution was 20 mL. During the
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degradation experiments, the stirring was done by the magnetic stirring with a small magnetic stirrer.
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The reaction solution (~3mL) was quickly withdrawn at given reaction intervals, and was quickly
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returned to the reactor after being analyzed with a spectrophotometer (UV759, Shanghai) at 352 nm.
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3. Results and discussions
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3.1 Surface microtopography and structure of the electrodes
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Figure 1 a–d depicts the formation of the ternary n-ZnO/p-Cu2O /n-TNA electrodes featured in this
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work. First, highly ordered TiO2 nanotube arrays were synthesized via an anodization process
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following calcination at 450°C to transfer amorphous TiO2 into anatase (Fig.1b). Second, Cu2O was
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deposited on the TNAs by electrochemical deposition (Fig.1c). Third, ZnO nanowires were
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overlayed on the top of the p-Cu2O/n-TNA by a hydrothermal method (Fig.1d).
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In order to confirm the composite electrode, FE-SEM was conducted to investigate the 7 Page 7 of 28
morphology change of electrode. It can be seen from Fig. 2a that highly ordered TNAs were
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fabricated after a typical anodization in HF-H2O electrolyte. The typical pore size is ranging from
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80-100 nm with an average interstice of 10 nm. The electrochemical deposited Cu2O were shown in
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Fig. 2b. The Cu2O nanoparticles had uniform size and were mostly octahedral structure. Although
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most of Cu2O nanoparticles were found on the top of the nanotubes due to their narrow pore size,
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considerable amount of them also were embedded in the tube mouth. This could offer the comparable
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contact region between Cu2O nanoparticles and TNAs so as to improve the electro-hole pair
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separation. Fig. 2c presents the morphology of ZnO nanowires after hydrothermal growth. It is
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apparent that ZnO nanowires have a high degree of orientation with the c-axis vertical to the
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substrate surface. The cross-section further demonstrates the ternary structure of the composite
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electrode as shown in Fig. 2d. As expected, ZnO nanowires grown on the top of the TNAs
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representing an obvious two-tier architecture, whereas the Cu2O nanoparticles should be wrapped by
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the TNAs and ZnO NWs. It also demonstrated that the ZnO nanowires have fully covered the surface
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of electrode that can protect the Cu2O particles from phototcorrosion.
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Fig. 3 corresponds to the X-ray diffraction (XRD) patterns of pure TNA, p-Cu2O/n-TNA,
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n-ZnO/p-Cu2O/n-TNA respectively. It can be seen from pattern (curve a) that the sample possesses
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characteristic peaks at 25.3°(101), 38.1° (004), and 48.2° (200) for the anatase phase. After coupling
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with Cu2O, the sample clearly shows additional characteristic peaks of Cu2O at 29°, 37° (curve b).
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The ternary heterojunction n-ZnO/p-Cu2O/n-TNA was approved by the new dominant diffraction
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peak (curve c), which indicates corresponding to the ZnO nanowires to be a high degree of
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orientation with the c-axis vertical to the substrate surface.
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Fig. 4a gives the X-ray photoelectron spectroscopy (XPS) survey scan over a large energy range 8 Page 8 of 28
at low resolution of n-ZnO/p- Cu2O/n-TNA material, which represents Ti, Cu, Zn and O. Fig. 4b
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shows the Zn 2p core level XPS scans, at higher resolution over smaller energy windows. As can be
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seen, the Zn 2p core level XPS spectrum has two sharp peaks at1022.5 eV (Zn 2p3/2) and 1045.6
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eV(Zn 2p1/2), which is in good agreement with the reported values for ZnO.
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3.2 The PEC activity of electrodes
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The UV/Vis diffuse reflection spectra (DRS) of TNA, p-Cu2O/n-TNA, n-ZnO/p-Cu2O/n-TNA
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are given in Fig. 5. It is evident that the pure TNA electrode has no obvious absorption of visible
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light (curve a). The absorption band is 375 nm and band gap is about 3.20 eV (curve a), according to
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the equation: Eg= 1240/λg. As shown in curve b, the absorption edge of p-Cu2O/n-TNA is observed
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at the wavelength 505 nm, corresponding to a band gap of 2.48eV, indicating that the deposition of
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Cu2O for TNA extend the absorption range to the visible region. As can be seen from curve c, the
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band gap absorption edge of the ternary n-ZnO/p-Cu2O/n-TNA is similar to that of p-Cu2O/n-TN
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and extends the visible spectrum absorption (to 510 nm), corresponding to an Eg value of about 2.43
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eV. Under illumination of visible light, the slight red-shifted of the band gap absorption and the
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change in Eg may be ascribed to the growth ZnO on p-Cu2O/n-TNA, which can protect the Cu2O
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from photocorrosion and facilitate kinetic separation of photogenerated charges. The results suggest
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that the coupling of n-ZnO with binary heterojunction p-Cu2O/n-TNA could be more efficient in
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solar light harvest.
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Further investigation was carried out to compare the current density generated by the TNA
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electrode, p-Cu2O/n-TNA electrode, and n-ZnO/p-Cu2O/n-TNA electrode under visible light (Fig. 6).
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The dark current densities of these three electrodes are found to be negligible. However, the pure
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TNA also shows no obvious light response in the visible light region, indicating no electron/hole pair 9 Page 9 of 28
was induced by visible light. After the chemically assembled sensitization by Cu2O, The
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p-Cu2O/n-TNA electrode increases the photoelectrocatalytic activity under visible light. In addition,
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the n-ZnO/p-Cu2O/n-TNA electrode exhibits the higher visible light response. At an applied potential
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of 0.5 V, the current density is 0.83mA cm-2, which is 106 times as high as the pure TNA electrode
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and 3.6 times the p-Cu2O/n-TNA electrode.
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Fig. 7 shows a schematic illustration of the band-gap energy and the charge separation of the
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ternary n-ZnO/p-Cu2O/n-TNA electrode. The CB of Cu2O is more negative than that of TiO2 and
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ZnO. Therefore, the photoelectron transfer may occur from the CB of Cu2O towards the CB of ZnO,
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then to that of TiO2, while the photogenerated holes transfer may occur in the opposite direction.
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Between the bands of TiO2 and Cu2O, ZnO acts as the beneficial role of the intermediate stage that
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would greatly promote the hole electron transfer, thus reducing the chance of hole–electron
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recombination. Therefore, the ternary n-ZnO/p-Cu2O/n-TNA has the higher photocatalytic
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properties than the pure TNA and binary p-Cu2O/n-TNA.
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EIS is a high effective method for probing the photoelectrochemical characterization of
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surface-modified electrodes. In this study, EIS was carried out to investigate the changes of electron
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transfer resistance that aroused from every surface modification step as shown in Fig. 8. The pure
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TNA electrode exhibits poor conductivity with an EIS of about 1000 Ω. After deposited with Cu2O
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(curve b), the EIS of p-Cu2O/n-TNA electrode shows a small semicircle (250Ω), only about a quarter
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of that of pure TNA, which suggests the successful deposition of Cu2O on the TNA electrode. The
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EIS of the ternary n-ZnO/p-Cu2O/n-TNA is similar to that of p-Cu2O/n-TNA (curve c), which may
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be attributed to the better transfer and dispersion due to the TNA surface coating with Cu2O and
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ZnO.
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3.4 PEC degradation of TC under visible light and solar light PEC
degradations
of
TC
using
TNA
electrode,
p-Cu2O/n-TNA
electrode
and
n-ZnO/p-Cu2O/n-TNA electrode under visible light and solar light were present in Fig. 9a and Fig.
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9b, respectively.
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As given in Fig. 9a, within 3 h, the removal percentage of TC on pure TNA electrode is
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obviously much slower than that of p-Cu2O/n-TNA electrode and n-ZnO/p-Cu2O/n-TNA electrode.
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The pure TNA exhibit no obvious light response in the visible-light region. A rapid increase in the
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removal percentage of TC can be obtained on p-Cu2O/n-TNA electrode, where the removal
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percentage reaches 75 %. However, the n-ZnO/p-Cu2O/n-TNA also shows the highest
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photoelectrocatalytic activity, with a removal percentage of 85 % after 3 h degradation. Further
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investigations were carried out under solar light and the results were present in Fig. 9b. Comparing
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the degradations data under visible light, an obvious increase in the removal percentage of TC by
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pure TNA electrode can be observed. The n-ZnO/p-Cu2O/n-TNA also shows the highest
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photoelectrocatalytic activity, with a removal percentage of 90% after 3 h degradation, indicating an
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excellent photoelectrocatalytic activity in both the visible light and solar light of ternary
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n-ZnO/p-Cu2O/n-TNA. In addition, the terminal pH of reaction solution degraded by ternary
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n-ZnO/p-Cu2O/n-TNA electrode was also determined. The terminal pH value of reaction solution
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under visible light is 5.89 and under solar light is 5.84, respectively, which are close to the initial pH
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value of 5.5. The small change of pH value may be ascribed to the presence of some small molecular
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compounds during the degradation, which act as the buffer solution to keep the relative stable pH
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value of reaction solution.
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After first being deposited by the Cu2O and then coated by ZnO, the ternary heterojuncrion
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n-ZnO/p-Cu2O/n-TNA electrode has a broad-wave response in the visible light region, indicating that
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the separation of photogenerated holes and electron was promoted. This charge separation prevented
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the photoinduced electron and the hole from recombination and consequently leads to higher
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degradation efficiency. The results indicate that the n-ZnO/p-Cu2O/n-TNA photocatalyst is optimal
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for the oxidation of organic pollutants.
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3.4 The stability of three electrodes 10
shows
the
stability
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TNA
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p-Cu2O/n-TNA
electrode
and
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n-ZnO/p-Cu2O/n-TNA electrode for treatment of TC under solar light. As can be seen from Fig. 9,
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the results of the 4 repeated experiments for degradation of TC in 3 h shows that the degradation
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efficiencies using TNA electrode is stable with an average degradation efficiency of 72.5 ± 1.2 %.
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This indicates that TNAs electrode processes excellent stability and reliability. However, the
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removal percentage is 85 % at the beginning and decreases quickly to 75 % after 4 repeated
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experiments for degradation of TC using p-Cu2O/n-TNA electrode, which is due to the
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photocorrosion
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n-ZnO/p-Cu2O/n-TNA electrode decreased slightly with the repeated times, removal percentage of
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TC still reaches 85 % after 4 repeated experiments, indicating higher stability after depositing the
250
ZnO on the surface of p-Cu2O/n-TNA. The higher removal percentage and stability might be
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explained by the following two reasons. One is that the coupled n-ZnO/p-Cu2O/n-TNA photoanode
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might facilitate kinetic separation of photogenerated charges and decreases the recombination rate
253
within the electrode. Another is that the ZnO coating on p-Cu2O/n-TNA electrode can protect Cu2O
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from photocorrosion.
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of
Cu2O
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Although
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removal
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TC
using
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4. Conclusions In summary, a novel n-ZnO/p-Cu2O /n-TNA material with higher photocatalytic efficiency and
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stability was designed and applied to degrade TC. The coupled n-ZnO/p-Cu2O/n-TNA photoanode
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might facilitate kinetic separation of photogenerated charges and decreases the recombination rate
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within the electrode. Moreover, the ZnO coating on p-Cu2O/n-TNA electrode can protect Cu2O from
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photocorrosion. The ternary heterojunction n-ZnO/p-Cu2O/n-TNA electrode offers higher
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photocatalytic efficiency during the PEC process, and thus obtains much higher degradation
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efficiency in comparison to the pure TNA and binary p-Cu2O/n-TNA electrode. The results also
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show that the ternary heterojunction n-ZnO/p-Cu2O/n-TNA electrode possesses excellent stability
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and reliability during the serial experiments. Since the Cu2O was protected against photocorrosion in
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solution by coating a layer of n-ZnO ontop the p-Cu2O/n-TNA surface. The enhanced
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photoelectrochemical properties and mechanical stability of the composite electrode material make it
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a highly efficient photoanode material for various potential applications.
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Acknowledgement
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The authors would like to acknowledge the National Nature Science Foundation of China (No.
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21207088,
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(No.20110073110029), and the Shanghai Basic Research Key Project (11JC1406200), the State Key
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Development Program for Basic Research of China (Grant No. 2009CB220004) for financial support,
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and Instrumental analysis center of Shanghai Jiao Tong University for materials characterization.
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Reference
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[1] Y. H. Xu, D. H. Liang, M. L. Liu, D. Z. Liu, Preparation and characterization of Cu2O–TiO2:
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Efficient photocatalytic degradation of methylene blue, Mater. Res. Bull. 43(2008)3474–3482.
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21177085),
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method
using
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highly
ordered
TiO2 nanotube
array,
Water
Res.
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Figure captions Figure 1 Schematic illustration of the structure of the ternary n-ZnO/p-Cu2O/n-TNA Figure 2 FE-SEM images of the electrodes: (a) TNA electrode after anodization for 30min; (b) after
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Cu2O sonoelectrochemical deposition for 5 min; (c) and after ZnO hydrothermal deposition for 4 h
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Figure 3 XRD patterns of: (a) TNA; (b) p-Cu2O/n-TNA; (c)n-ZnO/p-Cu2O/n-TNA
of n-ZnO/p-Cu2O/n-TNA electrode UV/Vis
diffuse
reflection
spectra
(DRS)
of
TNA,
p-Cu2O/n-TNA
and
an
Figure5
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Figure 4 (a) XPS survey scan for n-ZnO/p-Cu2O/n-TNA electrode; (b) Zn 2p XPS core level spectra
n-ZnO/p-Cu2O/n-TNA
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Figure 6 Photocurrent density of TNA electrode, p-Cu2O/n-TNA electrode, n-ZnO/p-Cu2O/n-TNA electrode
n-ZnO/p-Cu2O/n-TNA
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Figure 7 Schematic illustration of the band gap energy and charge separation of the ternary
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Figure 8 Electrochemical impedance spectroscopy of three electrodes: (a) TNA; (b) p-Cu2O/n-TNA; (c)n-ZnO/p-Cu2O/n-TNA
Figure 9 (a) PEC degradation of solutions of TC by TNA electrode, p-Cu2O/n-TNA electrode, n-ZnO/p-Cu2O/n-TNA electrode under visible light; (b) PEC degradation of solutions of TC by TNA electrode, p-Cu2O/n-TNA electrode, n-ZnO/p-Cu2O/n-TNA electrode under solar light Figure 10 Durability of by TNA electrode, p-Cu2O/n-TNA electrode, n-ZnO/p-Cu2O/n-TNA electrode for degradation of TC
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Highlights The TNA as the substrate exhibits better charge separation and transfer in UV region. The coupling of TNA with Cu2O extends the visible spectrum absorption.
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The coupling of ZnO with Cu2O/TNA protects the Cu2O from photocorrosion .
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S0304-3894(13)00646-8 http://dx.doi.org/doi:10.1016/j.jhazmat.2013.09.002 HAZMAT 15382
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
4-7-2013 29-8-2013 3-9-2013
Please cite this article as: J. Li, S. Lv, Y. Liu, J. Bai, B. Zhou, X. Hu, Photoeletrocatalytic Activity of an n-ZnO/p-Cu2 O/n-TNA Ternary Heterojunction Electrode for Tetracycline Degradation, Journal of Hazardous Materials (2013), http://dx.doi.org/10.1016/j.jhazmat.2013.09.002 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.
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Photoeletrocatalytic Activity of an n-ZnO/p-Cu2O/n-TNA Ternary Heterojunction Electrode
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for Tetracycline Degradation
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Jinhua Li, Shubin Lv, Yanbiao Liu, Jing Bai, Baoxue Zhou, Xiaofang Hu
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School of Environmental Science and Engineering, Shanghai Jiao Tong University No.800
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Dongchuan Rd, Shanghai 200240, China
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Corresponding author: Baoxue Zhou, Tel/Fax: +86-21-54747351;
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E-mail: [email protected]
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Photoeletrocatalytic Activity of an n-ZnO/p-Cu2O/n-TNA Ternary Heterojunction Electrode
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for Tetracycline Degradation
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Jinhua Li, Shubin Lv, Yanbiao Liu, Jing Bai, Baoxue Zhou*, Xiaofang Hu
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School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240,
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China
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Abstract
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In
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(n-ZnO/p-Cu2O/n-TNA) nanophotocatalyst with a sandwich-like nanostructure was constructed and
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applied for the photoelectrocatalytic (PEC) degradation of typical PPCPs, tetracycline (TC). The
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ternary heterojunction n-ZnO/p-Cu2O/n-TNA was obtained by depositing Cu2O on the surface of
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TNA via sonoelectrochemical deposition (SED) and subsequently building a layer of ZnO ontop the
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p-Cu2O/n-TNA surface through hydrothermal synthesis. After being deposited by the Cu2O, the
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absorption-band edge of the p-Cu2O/n-TNA was obviously red-shifted to the visible region (to
29
505nm), and the band gap was reduced from its original 3.20 eV to 2.46 eV. The band gap
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absorption edge of the ternary n-ZnO/p-Cu2O/n-TNA is similar to that of p-Cu2O/n-TN and extends
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the visible spectrum absorption to 510 nm, corresponding to an Eg value of about 2.43 eV. Under
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illumination of visible light, the photocurrent density of the ternary heterojunction
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n-ZnO/p-Cu2O/n-TNA electrode at 0.5V (vs. Ag/AgCl) was more than 106 times as high as that of
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the pure TNAs electrode, 3.6 times as high as that of the binary heterojunction p-Cu2O/n-TNA
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electrode. The degradation of TC indicated that the ternary heterojunction n-ZnO/p-Cu2O/n-TNA
study,
a
novel
ternary
heterojunction
n-ZnO/p-Cu2O/n-TiO2
nanotube
arrays
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Corresponding author: Baoxue Zhou, Tel/Fax: +86-21-54747351 *
Abbreviations: TC, tetracycline; CB, conduction band; VB, valence band; TNA, TiO2 nanotube arrays;.PEC, photoelectrocatalytic; SED, sonoelectrochemical deposition method
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electrode maintained a very high photoelectrocatalytic activity and excellent stability and reliability.
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Such kind of ternary heterojunction electrode material has a broad application prospect not only in
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pollution control but also in many other fields.
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Key words: Ternary heterojunction; ZnO; Cu2O; TNA; Photoeletrocatalytic oxidation
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1. Introduction
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Photocatalysis is a promising technique in environmental purification and solar energy
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conversion [1]. Compared with conventional treatment processes, it has advantages of environmental
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friendliness, energy-saving property, high efficiency and non-secondary pollution [2–6]. Thus, the
44
fabrication of a high-performance semiconductor photocatalyst is a key issue in the photocatalytic
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technology. However, the common semiconductor materials, such as TiO2 or ZnO, can only absorb
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UV light because of their wide band gaps [7], which greatly limit the usage of solar light.
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The coupled semiconductor heterojunction is an effective method to improve their
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photoelectrocatalytic (PEC) efficiency [8]. Due to the different energy levels between the conduction
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bands (CB) of the semiconductors, the photoexcited electrons can be quickly passed from one
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semiconductor to a neighboring semiconductor so as to accelerate electron-hole pair separation and
51
improve PEC efficiency.
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In the past several years, the coupling of n-ZnO or n-TiO2 with other metal oxides or sulfides
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such as, n-WO3, n-CdS, n-Fe2O3, p-SnO2, p-Cu2O and so on, have been reported [9–17]. Cuprous
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oxide, Cu2O, is an attractive p-type oxide for photoelectrochemical hydrogen production with a
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direct band gap of 2.0 eV and a light-to-hydrogen conversion efficiency of 18% based on the AM 1.5
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spectrums [10, 18-20]. In recent years, some researchers reported the binary p-n heterojunctions by
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coupling of p-Cu2O with n-TiO2 or n-ZnO [20-25]. In these work, an enhancement of the charge 3 Page 3 of 28
separation was achieved since the CB of Cu2O (-1.16V) is narrow than that of TiO2 (-0.1V) or ZnO
59
(-0.2V). For example, Bessekhouad et al [20] observed the Cu2O/TiO2 composite oxides via the
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mixture of Cu2O (particle size < 5nm) with commercial P25 TiO2 exhibited a superior activity for the
61
degradation of Orange II under visible irradiation. Xu et al [21] prepared the ZnO/Cu2O compound
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photocatalysts by “soak-deoxidize-airoxidation” and found that the photocatalytic properties of
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ZnO/Cu2O compound were improved greatly compared with pure ZnO. When the ZnO/Cu2O
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compound was used as photocatalyst, the degradation of MO reached 73% after 180 min of reaction.
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In spite of some advantages by coupling Cu2O with TiO2 or ZnO, The main drawback for the use of
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Cu2O as a photocathode is its poor stability in aqueous solutions, because the redox potentials for the
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reduction and oxidation of monovalent copper oxide lie within the band gap [22-25]. Thus, how to
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protect the photocatalyst Cu2O against photocorrosion is an interesting issue.
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Based on the above analysis and our previous work, a novel ternary heterojunction
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n-ZnO/p-Cu2O/n-TNA photocatalyst with a sandwich-like nanostructure is proposed. The ternary
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heterojunction n-ZnO/p-Cu2O/n-TNA material has the following advantage: first, the CB of p-Cu2O
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is more negative than that of n-ZnO and n-TiO2, which favors the charge separation and transfer.
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Second, the TNA as the substrate of ternary heterojunction is already a very good photocatalyst and
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can maintain the high PEC activity in the UV region. Third, the coupling of the high PEC active
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TNA with Cu2O having narrow band gap extends the visible spectrum absorption and favors the
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charge separation and transfer. Fourth, the coupling of n-ZnO with binary heterojunction
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p-Cu2O/n-TNA protects the Cu2O from photocorrosion and exhibits desirable stability.
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Herein, the n-ZnO/p-Cu2O/n-TNA was prepared by depositing Cu2O on the TNAs via SED and
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then building a layer of n-ZnO on the p-Cu2O/n-TNA surface through hydrothermal synthesis. The 4 Page 4 of 28
detailed synthesis process, characterization, and photoelectrochemical property testing for this
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composite catalyst were also discussed. In addition, the n-ZnO/p-Cu2O/n-TNA electrode was applied
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for the degradation of PPCPs TC to evaluate the PEC performance. TC is widely used in aquiculture
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and live stocking, but the waste TC is usually discharged along with waste water, which causes
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serious ecological pollution.
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2. Experimental
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2.1. Materials
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Pieces of titanium (Ti) sheets (purity > 99.99%) were obtained from Kurumi works, Japan. TC
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(C22H24N2O8·HCl), hydrofluoric acid, sodium sulfate, sodium hydroxide, and acetone were
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purchased obtained from Shanghai Chemical Reagent Company (China) without further purification.
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All solutions were prepared with doubly distilled deionized water.
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2.2 The preparation of electrodes
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2.2.1 Preparation of TNA
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TNA was synthesized via the anodization of pretreated Ti in 0.5 % HF aqueous (HF-H2O) [24].
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Electrochemical setup consisted of a two-electrode configuration with a platinum foil as the cathode
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and a DC power supply (Tradex, MPS 305) was used to drive the anodization. After anodization at
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20V for 30 min at room temperature, the as-prepared TNA samples were crystallized by annealing in
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air atmosphere for 3 h at 450 °C with heating and cooling rates of 1º/min.
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2.2.2 Preparation of p-Cu2O/n-TNA
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The p-Cu2O/n-TNA was prepared by the sonoelectrochemical deposition method (SED) [22].
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The electrolyte solution contained 0.1 M sodium acetate and 0.02 M cupric acetate. The TNA worked
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as the work electrode, a platinum foil was counter electrode and a saturated Ag/AgCl was reference 5 Page 5 of 28
electrode. After the deposition at -2.5v for 30 min, the samples were immediately removed from the
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electrolyte and then rinsed with DI water. The prepared p-Cu2O/n-TNA electrodes were annealed at
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200 °C for 60 min in nitrogen atmosphere.
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2.2.3 Preparation of n-ZnO/p-Cu2O/n-TNA
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The n-ZnO/p-Cu2O/n-TNA was synthesized by the hydrothermal method. First, the 0.15 M ZnO
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coating precursor solution was prepared by dissolving 5 mM Zn(CH3COO)2 to ethanol and stirring
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for 0.5 h. And then the precursor solution were added to the p-Cu2O/n-TNA substrate, spun at 100
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rpm for 3 s, and then spun at 3000 rpm for 10 s. Such a cycle was repeated 5 times. Then the
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substrate were immersed into the 0.02 M Zn (NO3)2 and (CH2)6N4 growing precursor solution at
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90 °C for a certain time. Finally, the resultant films were washed with deionized water to remove any
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residual materials and dried at room temperature.
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2.3 The characteristics of electrodes
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The phase of products was identified by X-ray powder diffraction (XRD, Shimadzu), using Cu
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KR (λ>0.15406 nm) radiation at 50 kV and 50 mA at a scanning rate of 8°/min in the 2θ range from
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10 to 60° at room temperature. TEM images were collected by using a field emission scanning
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electron microscopy (FESEM FEI-Sirion200). UV–visible absorption spectra of the samples were
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recorded on a photospectrometer (TU-1901, Pgeneral, China). Electrochemical impedance
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spectroscopy (EIS) was performed to determine the conductivity of the electrodes, with the
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frequency ranging from 10-3 to 105 Hz, and the potential was 1V. The electrolyte was 0.005 M
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[Fe(CN)6 3-]/[Fe(CN)6 4-] solution.
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Photoelectrochemical properties are measured in a rectangular shaped quartz reactor
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(20mm×30mm×50mm). The setup mainly consists of external irradiation source, three-electrode 6 Page 6 of 28
configuration and data recording system. A computer controlled electrochemical station (CH Ins.,
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CHI660, USA) was used to apply the bias potential and record the photocurrent. The as-prepared
126
samples worked as the work electrode, a platinum foil was counter electrode and a saturated
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Ag/AgCl was reference electrode in a solution containing 0.2 M Na2SO4. An external Xenon lamp
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with AM 1.5 (a radiation intensity of 100 mW cm-2) is used as light source. The visible light was
129
obtained by a glass filter that allowed wavelengths between 380 and 800 nm to be passed through.
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2.4 PEC degradation experiment
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The PEC degradation experiment of TC setup under visible light and solar light is the same as
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PEC setup. The degradation experiment conditions were as follows: illumination area 10mm×10 mm,
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and 0.5V (vs. Ag/AgCl) of electric bias, pH 5.5, 0.2 M Na2SO4 as electrolyte. The initial
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concentration of TC solution was 20 mg L−1 and the reaction solution was 20 mL. During the
135
degradation experiments, the stirring was done by the magnetic stirring with a small magnetic stirrer.
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The reaction solution (~3mL) was quickly withdrawn at given reaction intervals, and was quickly
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returned to the reactor after being analyzed with a spectrophotometer (UV759, Shanghai) at 352 nm.
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3. Results and discussions
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3.1 Surface microtopography and structure of the electrodes
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Figure 1 a–d depicts the formation of the ternary n-ZnO/p-Cu2O /n-TNA electrodes featured in this
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work. First, highly ordered TiO2 nanotube arrays were synthesized via an anodization process
142
following calcination at 450°C to transfer amorphous TiO2 into anatase (Fig.1b). Second, Cu2O was
143
deposited on the TNAs by electrochemical deposition (Fig.1c). Third, ZnO nanowires were
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overlayed on the top of the p-Cu2O/n-TNA by a hydrothermal method (Fig.1d).
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In order to confirm the composite electrode, FE-SEM was conducted to investigate the 7 Page 7 of 28
morphology change of electrode. It can be seen from Fig. 2a that highly ordered TNAs were
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fabricated after a typical anodization in HF-H2O electrolyte. The typical pore size is ranging from
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80-100 nm with an average interstice of 10 nm. The electrochemical deposited Cu2O were shown in
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Fig. 2b. The Cu2O nanoparticles had uniform size and were mostly octahedral structure. Although
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most of Cu2O nanoparticles were found on the top of the nanotubes due to their narrow pore size,
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considerable amount of them also were embedded in the tube mouth. This could offer the comparable
152
contact region between Cu2O nanoparticles and TNAs so as to improve the electro-hole pair
153
separation. Fig. 2c presents the morphology of ZnO nanowires after hydrothermal growth. It is
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apparent that ZnO nanowires have a high degree of orientation with the c-axis vertical to the
155
substrate surface. The cross-section further demonstrates the ternary structure of the composite
156
electrode as shown in Fig. 2d. As expected, ZnO nanowires grown on the top of the TNAs
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representing an obvious two-tier architecture, whereas the Cu2O nanoparticles should be wrapped by
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the TNAs and ZnO NWs. It also demonstrated that the ZnO nanowires have fully covered the surface
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of electrode that can protect the Cu2O particles from phototcorrosion.
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Fig. 3 corresponds to the X-ray diffraction (XRD) patterns of pure TNA, p-Cu2O/n-TNA,
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n-ZnO/p-Cu2O/n-TNA respectively. It can be seen from pattern (curve a) that the sample possesses
162
characteristic peaks at 25.3°(101), 38.1° (004), and 48.2° (200) for the anatase phase. After coupling
163
with Cu2O, the sample clearly shows additional characteristic peaks of Cu2O at 29°, 37° (curve b).
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The ternary heterojunction n-ZnO/p-Cu2O/n-TNA was approved by the new dominant diffraction
165
peak (curve c), which indicates corresponding to the ZnO nanowires to be a high degree of
166
orientation with the c-axis vertical to the substrate surface.
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Fig. 4a gives the X-ray photoelectron spectroscopy (XPS) survey scan over a large energy range 8 Page 8 of 28
at low resolution of n-ZnO/p- Cu2O/n-TNA material, which represents Ti, Cu, Zn and O. Fig. 4b
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shows the Zn 2p core level XPS scans, at higher resolution over smaller energy windows. As can be
170
seen, the Zn 2p core level XPS spectrum has two sharp peaks at1022.5 eV (Zn 2p3/2) and 1045.6
171
eV(Zn 2p1/2), which is in good agreement with the reported values for ZnO.
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3.2 The PEC activity of electrodes
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The UV/Vis diffuse reflection spectra (DRS) of TNA, p-Cu2O/n-TNA, n-ZnO/p-Cu2O/n-TNA
174
are given in Fig. 5. It is evident that the pure TNA electrode has no obvious absorption of visible
175
light (curve a). The absorption band is 375 nm and band gap is about 3.20 eV (curve a), according to
176
the equation: Eg= 1240/λg. As shown in curve b, the absorption edge of p-Cu2O/n-TNA is observed
177
at the wavelength 505 nm, corresponding to a band gap of 2.48eV, indicating that the deposition of
178
Cu2O for TNA extend the absorption range to the visible region. As can be seen from curve c, the
179
band gap absorption edge of the ternary n-ZnO/p-Cu2O/n-TNA is similar to that of p-Cu2O/n-TN
180
and extends the visible spectrum absorption (to 510 nm), corresponding to an Eg value of about 2.43
181
eV. Under illumination of visible light, the slight red-shifted of the band gap absorption and the
182
change in Eg may be ascribed to the growth ZnO on p-Cu2O/n-TNA, which can protect the Cu2O
183
from photocorrosion and facilitate kinetic separation of photogenerated charges. The results suggest
184
that the coupling of n-ZnO with binary heterojunction p-Cu2O/n-TNA could be more efficient in
185
solar light harvest.
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Further investigation was carried out to compare the current density generated by the TNA
187
electrode, p-Cu2O/n-TNA electrode, and n-ZnO/p-Cu2O/n-TNA electrode under visible light (Fig. 6).
188
The dark current densities of these three electrodes are found to be negligible. However, the pure
189
TNA also shows no obvious light response in the visible light region, indicating no electron/hole pair 9 Page 9 of 28
was induced by visible light. After the chemically assembled sensitization by Cu2O, The
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p-Cu2O/n-TNA electrode increases the photoelectrocatalytic activity under visible light. In addition,
192
the n-ZnO/p-Cu2O/n-TNA electrode exhibits the higher visible light response. At an applied potential
193
of 0.5 V, the current density is 0.83mA cm-2, which is 106 times as high as the pure TNA electrode
194
and 3.6 times the p-Cu2O/n-TNA electrode.
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Fig. 7 shows a schematic illustration of the band-gap energy and the charge separation of the
196
ternary n-ZnO/p-Cu2O/n-TNA electrode. The CB of Cu2O is more negative than that of TiO2 and
197
ZnO. Therefore, the photoelectron transfer may occur from the CB of Cu2O towards the CB of ZnO,
198
then to that of TiO2, while the photogenerated holes transfer may occur in the opposite direction.
199
Between the bands of TiO2 and Cu2O, ZnO acts as the beneficial role of the intermediate stage that
200
would greatly promote the hole electron transfer, thus reducing the chance of hole–electron
201
recombination. Therefore, the ternary n-ZnO/p-Cu2O/n-TNA has the higher photocatalytic
202
properties than the pure TNA and binary p-Cu2O/n-TNA.
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EIS is a high effective method for probing the photoelectrochemical characterization of
204
surface-modified electrodes. In this study, EIS was carried out to investigate the changes of electron
205
transfer resistance that aroused from every surface modification step as shown in Fig. 8. The pure
206
TNA electrode exhibits poor conductivity with an EIS of about 1000 Ω. After deposited with Cu2O
207
(curve b), the EIS of p-Cu2O/n-TNA electrode shows a small semicircle (250Ω), only about a quarter
208
of that of pure TNA, which suggests the successful deposition of Cu2O on the TNA electrode. The
209
EIS of the ternary n-ZnO/p-Cu2O/n-TNA is similar to that of p-Cu2O/n-TNA (curve c), which may
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be attributed to the better transfer and dispersion due to the TNA surface coating with Cu2O and
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ZnO.
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212 213
3.4 PEC degradation of TC under visible light and solar light PEC
degradations
of
TC
using
TNA
electrode,
p-Cu2O/n-TNA
electrode
and
n-ZnO/p-Cu2O/n-TNA electrode under visible light and solar light were present in Fig. 9a and Fig.
215
9b, respectively.
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As given in Fig. 9a, within 3 h, the removal percentage of TC on pure TNA electrode is
217
obviously much slower than that of p-Cu2O/n-TNA electrode and n-ZnO/p-Cu2O/n-TNA electrode.
218
The pure TNA exhibit no obvious light response in the visible-light region. A rapid increase in the
219
removal percentage of TC can be obtained on p-Cu2O/n-TNA electrode, where the removal
220
percentage reaches 75 %. However, the n-ZnO/p-Cu2O/n-TNA also shows the highest
221
photoelectrocatalytic activity, with a removal percentage of 85 % after 3 h degradation. Further
222
investigations were carried out under solar light and the results were present in Fig. 9b. Comparing
223
the degradations data under visible light, an obvious increase in the removal percentage of TC by
224
pure TNA electrode can be observed. The n-ZnO/p-Cu2O/n-TNA also shows the highest
225
photoelectrocatalytic activity, with a removal percentage of 90% after 3 h degradation, indicating an
226
excellent photoelectrocatalytic activity in both the visible light and solar light of ternary
227
n-ZnO/p-Cu2O/n-TNA. In addition, the terminal pH of reaction solution degraded by ternary
228
n-ZnO/p-Cu2O/n-TNA electrode was also determined. The terminal pH value of reaction solution
229
under visible light is 5.89 and under solar light is 5.84, respectively, which are close to the initial pH
230
value of 5.5. The small change of pH value may be ascribed to the presence of some small molecular
231
compounds during the degradation, which act as the buffer solution to keep the relative stable pH
232
value of reaction solution.
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After first being deposited by the Cu2O and then coated by ZnO, the ternary heterojuncrion
234
n-ZnO/p-Cu2O/n-TNA electrode has a broad-wave response in the visible light region, indicating that
235
the separation of photogenerated holes and electron was promoted. This charge separation prevented
236
the photoinduced electron and the hole from recombination and consequently leads to higher
237
degradation efficiency. The results indicate that the n-ZnO/p-Cu2O/n-TNA photocatalyst is optimal
238
for the oxidation of organic pollutants.
239
3.4 The stability of three electrodes 10
shows
the
stability
cr of
TNA
us
Fig.
electrode,
p-Cu2O/n-TNA
electrode
and
an
240
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n-ZnO/p-Cu2O/n-TNA electrode for treatment of TC under solar light. As can be seen from Fig. 9,
242
the results of the 4 repeated experiments for degradation of TC in 3 h shows that the degradation
243
efficiencies using TNA electrode is stable with an average degradation efficiency of 72.5 ± 1.2 %.
244
This indicates that TNAs electrode processes excellent stability and reliability. However, the
245
removal percentage is 85 % at the beginning and decreases quickly to 75 % after 4 repeated
246
experiments for degradation of TC using p-Cu2O/n-TNA electrode, which is due to the
247
photocorrosion
248
n-ZnO/p-Cu2O/n-TNA electrode decreased slightly with the repeated times, removal percentage of
249
TC still reaches 85 % after 4 repeated experiments, indicating higher stability after depositing the
250
ZnO on the surface of p-Cu2O/n-TNA. The higher removal percentage and stability might be
251
explained by the following two reasons. One is that the coupled n-ZnO/p-Cu2O/n-TNA photoanode
252
might facilitate kinetic separation of photogenerated charges and decreases the recombination rate
253
within the electrode. Another is that the ZnO coating on p-Cu2O/n-TNA electrode can protect Cu2O
254
from photocorrosion.
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of
Cu2O
particles.
Although
the
removal
percentage
of
TC
using
12 Page 12 of 28
255
4. Conclusions In summary, a novel n-ZnO/p-Cu2O /n-TNA material with higher photocatalytic efficiency and
257
stability was designed and applied to degrade TC. The coupled n-ZnO/p-Cu2O/n-TNA photoanode
258
might facilitate kinetic separation of photogenerated charges and decreases the recombination rate
259
within the electrode. Moreover, the ZnO coating on p-Cu2O/n-TNA electrode can protect Cu2O from
260
photocorrosion. The ternary heterojunction n-ZnO/p-Cu2O/n-TNA electrode offers higher
261
photocatalytic efficiency during the PEC process, and thus obtains much higher degradation
262
efficiency in comparison to the pure TNA and binary p-Cu2O/n-TNA electrode. The results also
263
show that the ternary heterojunction n-ZnO/p-Cu2O/n-TNA electrode possesses excellent stability
264
and reliability during the serial experiments. Since the Cu2O was protected against photocorrosion in
265
solution by coating a layer of n-ZnO ontop the p-Cu2O/n-TNA surface. The enhanced
266
photoelectrochemical properties and mechanical stability of the composite electrode material make it
267
a highly efficient photoanode material for various potential applications.
268
Acknowledgement
269
The authors would like to acknowledge the National Nature Science Foundation of China (No.
270
21207088,
271
(No.20110073110029), and the Shanghai Basic Research Key Project (11JC1406200), the State Key
272
Development Program for Basic Research of China (Grant No. 2009CB220004) for financial support,
273
and Instrumental analysis center of Shanghai Jiao Tong University for materials characterization.
274
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method
using
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highly
ordered
TiO2 nanotube
array,
Water
Res.
16 Page 16 of 28
Figure captions Figure 1 Schematic illustration of the structure of the ternary n-ZnO/p-Cu2O/n-TNA Figure 2 FE-SEM images of the electrodes: (a) TNA electrode after anodization for 30min; (b) after
ip t
Cu2O sonoelectrochemical deposition for 5 min; (c) and after ZnO hydrothermal deposition for 4 h
cr
Figure 3 XRD patterns of: (a) TNA; (b) p-Cu2O/n-TNA; (c)n-ZnO/p-Cu2O/n-TNA
of n-ZnO/p-Cu2O/n-TNA electrode UV/Vis
diffuse
reflection
spectra
(DRS)
of
TNA,
p-Cu2O/n-TNA
and
an
Figure5
us
Figure 4 (a) XPS survey scan for n-ZnO/p-Cu2O/n-TNA electrode; (b) Zn 2p XPS core level spectra
n-ZnO/p-Cu2O/n-TNA
M
Figure 6 Photocurrent density of TNA electrode, p-Cu2O/n-TNA electrode, n-ZnO/p-Cu2O/n-TNA electrode
n-ZnO/p-Cu2O/n-TNA
te
d
Figure 7 Schematic illustration of the band gap energy and charge separation of the ternary
Ac ce p
Figure 8 Electrochemical impedance spectroscopy of three electrodes: (a) TNA; (b) p-Cu2O/n-TNA; (c)n-ZnO/p-Cu2O/n-TNA
Figure 9 (a) PEC degradation of solutions of TC by TNA electrode, p-Cu2O/n-TNA electrode, n-ZnO/p-Cu2O/n-TNA electrode under visible light; (b) PEC degradation of solutions of TC by TNA electrode, p-Cu2O/n-TNA electrode, n-ZnO/p-Cu2O/n-TNA electrode under solar light Figure 10 Durability of by TNA electrode, p-Cu2O/n-TNA electrode, n-ZnO/p-Cu2O/n-TNA electrode for degradation of TC
17 Page 17 of 28
Highlights The TNA as the substrate exhibits better charge separation and transfer in UV region. The coupling of TNA with Cu2O extends the visible spectrum absorption.
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The coupling of ZnO with Cu2O/TNA protects the Cu2O from photocorrosion .
18 Page 18 of 28
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