Preparation of an Fe-doped visible-light-response TiO2 film electrode and its photoelectrocatalytic activity

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Preparation of an Fe-doped visible-light-response TiO2 film electrode and its photoelectrocatalytic activity

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Wenwei Tang a,∗ , Jin Xia a , Xiaoying Chen a , Gong Jiemin a , Xinping Zeng b,∗ a b

Department of Chemistry, Tongji University, Shanghai 200092, China School of Life Sciences and Technology, Tongji University, Shanghai 200092, China

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Article history: Received 7 December 2013 Received in revised form 17 March 2014 Accepted 28 April 2014 Available online xxx

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Keywords: Fe-doped TiO2 film electrode Visible-light response Photoelectrocatalysis Methyl orange Rhodamine B

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1. Introduction

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Fe-doped TiO2 film electrodes were prepared by sol–gel and dip-coating methods, and their photoelectrocatalytic properties were investigated under both ultraviolet and visible light through degradation of methyl orange and Rhodamine B. The results showed that the Fe-doped TiO2 film electrodes mainly consisted of anatase TiO2 . The Fe-doping effectively restrained the grain growth of TiO2 and the phase transformation of rutile. The modified film substantially extended the photo response from 452 nm to 604 nm and band gap decreased to 2.05 eV. The photocatalytic performance of the Fe-doped TiO2 film electrode was enhanced. The response current of 1% Fe-doped TiO2 film electrode was 30.3 ␮A while TiO2 film electrode had no response under visible light. The decolorization rate of RhB by 1% Fe-doped TiO2 film electrode was 22% higher than that of undoped electrode, which indicated that the extended responsive wavelength range greatly expanded the application potential of the modified electrode. © 2014 Published by Elsevier B.V.

Visible-light responsive photocatalytic technology has attracted widely attention because it can make full use of solar energy and eliminate undesired chemical substances for environmental conservation [1,2]. Titanium dioxide (TiO2 ), as a semiconductor with a wide band gap, is one of the most promising photocatalytic materials in the area of environmental protection because of its superior properties in photo-conversion and photocatalysis [3,4]. In addition, the advantages of TiO2 include operation simplicity, moderate reaction conditions, high degradation efficiency, easy automation and no secondary pollution [5]. However, the dominant light absorption of TiO2 is in the UV range due to its wide band gap, which greatly limits its industrial application as there is only 4% UV light in sunlight. Meanwhile, its high recombination rate of electron–hole (e–h) pairs also leads to a low number of active free radicals and photocatalytic efficiency [6]. Efforts have been made over the past decades to develop nano-TiO2 photocatalysts. Yang et al. [7] prepared a TiO2 /carbon membrane via a sol–gel process, coating TiO2 in a reactor used as an electrocatalyst. Wu et al. [8] immobilized TiO2 onto a titanium

∗ Corresponding authors. Tel.: +86 02165983366; fax: +86 02165983366. E-mail addresses: [email protected] (W. Tang), [email protected] (X. Zeng).

electrode to conduct methyl tert-butyl ether degradation in aqueous solution. To substitute the UV light with visible light or sunlight, doping metals or non-metal elements are promising approaches to increase the photocatalytic activity. Peng et al. [9] discovered that S-doped TiO2 films exhibited excellent photocatalytic activity comparing to pure TiO2 films because of their surface microstructure. Liu et al. [10] prepared N–doped TiO2 nanotube arrays through electrochemical anode oxidation using N2 –plasma etc. as material, which resulted in a significant enhancement of the photocatalytic activity under visible light. Zhang et al. [11] found that non-calcined Fe-doped TiO2 film caused higher photocatalytic degradation of MO, which could be described as a pseudo-first order reaction. Ostovari et al. [12] applied an atmospheric pressure chemical vapor deposition (APCVD) method to prepare Fe-doped TiO2 consisted of anatase and rutile phases. It showed a considerable enhancement in the wettability of the ferromagnetic nanostructure when applying a magnetic field under UV and visible light. Su et al. [13] investigated the nitrogen and Fe co-doping TiO2 , which showed a narrow band gap and significantly improved photocatalytic activity under visible irradiation. Meanwhile, the density functional theory (DFT) calculations indicated that the co-doping of nitrogen and Fe induced the formation of new states between the valence band and the conduction band. Efforts also have been made to develop nano-TiO2 photoelectrocatalytic (PEC) oxidation. Compared to the traditional photocatalysis, an external bias potential is helpful for restricting the recombination of electron/hole pairs via an external circuit,

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consequently improving the photocatalytic efficiency [14]. Doping is a popular method, by this way, the band gap of TiO2 narrowed and the photocatalytic activity was improved [15,16]. In the present study, the sol–gel method was used to prepare the Fe-doped TiO2 film electrodes. Their photoelectrocatalytic properties were investigated under UV and visible light irradiation using MO and RhB as model pollutants, and the results showed improved photocatalytic properties and the application potential of the modified electrode. 2. Materials and methods 2.1. Preparation of Fe-doped TiO2 film electrodes

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Methyl orange (MO) and Rhodamine B (RhB) were purchased from Shanghai Dyeing Plant (Shanghai, China), and all chemicals were of analytical purity. Ti sheet was chosen as the Ti substrate (20 mm × 30 mm × 1 mm) from Shanxi Ti Company (Shanxi, China). The Ti substrate was polished, ultrasonically cleaned in detergent and acetone, and detarnished under acid etching followed by washing with distilled water, and finally sealed under anhydrous ethanol [17]. Fe-doped TiO2 was synthesized using a sol–gel method. In a typical synthesis, three components (A–C) were respectively prepared at first. For component A, 5 mL isopropanol was added into 10 mL tetrabutyl titanate with 3 mL acetylacetone at ambient temperature, and stirred for 30 min. Component B consisted of 2 mL distilled water and 2.5 mL isopropanol, a thimbleful of ethanolamine, which served as a regulator, and different amounts of ferric nitrate according to nFe/nTi = 0, 0.1%, 0.2%, 0.5%, 1.0% and 2.0% (0, 0.0119, 0.0237, 0.0594, 0.1187 and 0.2374 g, respectively). Component C was obtained by dissolving an appropriate amount of polyethylene glycol in 2.5 mL isopropanol to avoid agglomeration of particles and increase the hydrophilicity. Then component B was added dropwise into component C under vigorous stirring and followed by dropwise addition of component A. The mixture was stirred for another 30 min and aging overnight to obtain the soliquid. The Fe-doped TiO2 film electrode was prepared by the dipcoating method due to its low cost and controllability. Ti sheet was cleaned and vertically dipped into the soliquid for 5 min, then elevated at a certain speed and dried under an infrared lamp for 10 min to form a gel. The film electrode was annealed at 500 ◦ C for 30 min in a muffle furnace after 5 times of dip-coatings to guarantee the coating layer oxidized completely. Finally, when 15 layers were coated onto the electrode, the last annealing time was prolonged to 2 h to acquire a modified anatase TiO2 film electrode.

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2.2. Test methods of the Fe-doped TiO2 film electrode

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X-ray diffraction (XRD) patterns were obtained from a Bruker Foucs D8 X-ray diffractometer with Cu K␣ radiation in a 2 range of 10–70◦ . The Raman spectra were collected using Renishaw Invia confocal microscopic Raman spectroscopy. Its optical efficiency was more than 30%, the spectral resolution was 1 cm−1 , the laser wavelength was 514 nm, and Raman shift range for tests was 100–1000 cm−1 . The UV–vis diffuse reflectance spectra were measured in the wavelength range from 300 to 800 nm using an Agilent 8453 UV–vis spectrometer. The morphologies of the Fe-doped TiO2 film electrodes were observed by a Hitachi S-4800 scanning electron microscope. An Autolab model PGSTAT30 electrochemical workstation was used to test the electrochemical performances of the electrodes.

The prepared electrode was used as the working electrode (20 mm × 30 mm), the platinum electrode was used as the counter electrode (4 mm × 15 mm × 15 mm), and a saturated calomel electrode (SCE) was used as the reference electrode. All the photocatalytic and electrocatalytic tests were performed in 60 mL 0.5 mol/L Na2 SO4 solution. The equilibrium time for chronocoulometry (C–C) test was 2 s, time interval was 0.1 s, the potential duration was 0.25 V s−1 , the bias voltage applied to the working electrode was 1 V under the UV pulse and 2 V under the visible light pulse. For the electrochemical impedance spectroscopy (EIS) test, the frequency range was 10,000–0.1 Hz and the electric potential of the amplitude was 50 mV. 2.3. Photocatalytic activity test

=

1−

 C

C0

× 100%

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For the UV photocatalytic activity test, a 56 W Phillips UV lamp was used as the UV light source with a main emission at 365 nm, and the objective degradation pollutant was 60 mL 30 mg/L MO solution; For the electrocatalytic activity test, 1 V external bias potential was applied between the working electrode and the counter electrode; For the UV photoelectric activity test, a 56 W Phillips UV lamp was used as the UV light source and 1 V external bias potential was applied. For the visible photoelectric activity test, a 300 W Xenon arc lamp was used as the visible light source with UV cut off (wavelength < 400 nm), 2 V external bias potential was applied between the working electrode and the electrode and the objective degradation pollutant was 60 mL 10 mg/L RhB solution. The whole degradation process was conducted in a beaker containing different objective degradation pollutants and connected to a condensate pipe with continuing cooling water. The degradation was operated under UV and visible light for 3 h, respectively. A UV–vis spectrometer was used for whole wavelength scanning. The maximum absorptive wavelength of MO was 465 nm and that of RhB was 553 nm. The concentration after degradation was determined by an Agilent 8453 UV–vis spectrometer. The calculation of the decolorization rate for MO and RhB was:



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(1)

 is the decolorization rate of dye (%); C0 is the initial concentration (mg/L); C is the final concentration (mg/L).

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3. Results and discussions

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3.1. Characterization of the Fe-doped TiO2 film electrode

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3.1.1. XRD The XRD patterns of the TiO2 film electrode samples were shown in Fig. 1, of which the molar ratios between Fe and Ti were 0, 0.1:100, 0.2:100, 0.5:100, 1.0:100 and 2.0:100. The pure TiO2 film electrode exhibited the characteristic peaks of anatase (major peaks: 25.6◦ , 37.8◦ and 48.3◦ ) and rutile (major peaks: 27.4◦ and 35.5◦ ). However, other crystalline phases, such as Fe2 O3 , were not observed, which revealed that the iron ions might substitute titanium in the TiO2 matrix at high temperature calcinations or locate interstitially to form a Fe-TiO2 solid solution due to the similar ionic radius of Fe3+ and Ti4+ [18,19]. Fe-doping did not change the anatase phase of TiO2 but inhibited the formation of rutile TiO2 with an increase of the Fe contents, which was in accordance with a previous study [20]. According to the Scherrer equation, D = ks /ˇ cos , where D is the crystal size, ks is the Scherrer constant,  is the X-ray wavelength, ˇ is the full width at half maximum and  is the diffraction angle. The calculation in Table 1 indicated that Fe doping had the

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Fig. 2. Raman spectrum of the TiO2 electrodes with different Fe contents. Fig. 1. XRD patterns of the TiO2 electrodes with different Fe contents.

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effect of refining the grain size; the minimum crystal size was 17.9 nm when nFe:nTi = 1.0:100. Whether the Fe-doping was done by the way of replacing or embedding the space, it would cause TiO2 lattice corresponding micro strain, accompanied with change of crystallinity. Data in Table 1 showed that certain amount of Fe-doping effectively restrained the grain growth of TiO2 . When nFe:nTi was 1%, the grain size was the smallest with only 17.9 nm while the undoped grain was 22.6 nm. Smaller grains can increase the nano-TiO2 surface area, increase the surface oxygen vacancies and defects concentration, which is conducive to the separation of photo-electrons and holes. However excess doped (nFe:nTi = 2%) may make the lattice distortion serious, the number of defects decrease, the oxygen holes less and the grain size increase. 3.1.2. Raman spectra The Raman spectra with increasing amounts of doping iron were shown in Fig. 2. Five characteristic bands at 144, 196, 397, 517 and 639 cm−1 were the fundamental vibration modes of anatase TiO2 . Because Fe doping led to lattice distortions and decrease in the degree of crystallinity, the peak intensity of the anatase vibration mode decreased [21].

Fig. 3. UV–vis spectrum of TiO2 powders with different Fe contents.

3.1.3. Diffuse reflectance spectra (DRS) The prepared sol solutions under different molar ratios of Fe and Ti were dried for 10 h at 80 ◦ C and annealed at 500 ◦ C for 2 h in a muffle furnace to obtain TiO2 powder with different amounts of iron doping. The optical diffuse reflectance spectra of the different percentages of Fe-doped TiO2 are shown in Fig. 3. Compared with the relatively purer TiO2 sample, the Fe-doped one had considerably large red shift in its absorption spectrum, which effectively widened the photo corresponding area of TiO2 . With the increasing amount of Fe-doping, the trend of such expansion became more significant, while the photo corresponding area was widened from 452 nm to 604 nm. The energy gap was calculated by the following formula (Eg = hc/ = 1240/, Eg is the width of the forbidden band of the

semiconductor, h is Planck’s constant of 6.626 × 10−34 J s. c is the speed of light of 3 × 108 m/s.  is the absorption wavelength (nm), which can be given by the coordinate intersection of the tangent line of absorption edge and the X axis), and the results were shown in Table 2. The energy gap fell from 2.74 eV to 2.05 eV after TiO2 was doped with Fe at nFe:nTi = 2:100. The absorption spectra red shift and energy gap decrease can be explained as following: during the process of calcinations, the crystal lattice of TiO2 was distorted and produced large amount of defects, the Fe3+ produced by the collapse of chemical bond might scattered into the distorted crystal lattice of TiO2 , the energy level of Fe2+ /Fe3+ was slightly lower than that of conduction band of TiO2 , while the energy level of Fe3+ /Fe4+ was a bit higher than that of the valence band of TiO2 , so a new doping level is introduced. Fedoping increases the absorption of visible light, which might be due to the transition of photo-electrons from the new doped energy level (Fe3+ /Fe4+ ) to the conduction band of TiO2 after absorb energy [22]. Therefore, Fe-doping decreased the energy gap, enlarged the

Table 1 Grain size of TiO2 with different Fe contents.

Table 2 Energy gap of TiO2 with different Fe contents. nFe:nTi

nFe:nTi

Grain size (nm)

0

0.1%

0.2%

0.5%

1%

2%

22.6

19.1

19.3

19.3

17.9

19.7

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0

0.1%

0.2%

0.5%

1%

2%

2.74

2.64

2.46

2.35

2.20

2.05

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Fig. 4. SEM image of Fe-doped TiO2 electrode (nFe:nTi = 1.0:100): (A) ×2k, (B) ×100k.

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absorption wavelength range and expanded the optical response region.

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3.1.4. SEM The detailed structures of the Fe-doped TiO2 electrodes were observed by SEM. A typical ‘island structure’ was found at low magnification because the film wrinkles led to the aggregation of particles under high temperature annealing (Fig. 4A). At a high magnification (Fig. 4B), the TiO2 nanoparticles showed close and uniform arrangement on the Ti sheet with no cracks or spaces.

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3.2. Photoelectrochemical property

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3.2.1. Photoelectrochemical property under UV light The efficiencies of photo-induced charge separation of the pure TiO2 and the Fe-doped TiO2 film electrodes were evaluated by measuring the photocurrent responses under UV light at an applied potential of 1 V versus SCE. Their photocurrent responses increased and decreased periodically when exposed to the light source at an interval of 50 s. From Fig. 5(A) we can observe that the photocurrent of the Fedoped TiO2 film was lower than that of the pure TiO2 under UV light irradiation. Moreover, the UV-caused photocurrent response decreased along with the increase of the Fe-doping amount. Doping iron was not conducive to improve the UV photocatalytic properties in this preparation technique. The probable causes were as follows: (1) electron transfer occurred among the ferric iron, and the valence band and the conduction band of TiO2 ; (2) due to the electron–hole trap effect, Fe O Ti bonds formed in the lattice which led to a change of the electron density and a difference in the electronegativity; (3) the pure TiO2 contained a small amount of rutile that was beneficial to the absorbance of UV light [23]. EIS was applied to analyze the electron transport properties of the Fe-TiO2 film electrode, which is an effective tool for probing the features of surface-modified electrodes. As shown in the EIS Nyquist plots of the pure TiO2 and the Fe-doped TiO2 film electrodes (Fig. 6A and B), the impedance arc radii under UV irradiation were much smaller than those in the dark, which implied an improved charge carrier separation under UV irradiation [24,25]. In addition, the arc radius for the pure TiO2 film electrode was smaller than that of the Fe-doped TiO2 film electrode under both dark and UV irradiation. The results further indicated that the pure TiO2 film electrode had a higher separation efficiency of the photogenerated electron–hole pairs and faster charge-transfer than the Fe-doped TiO2 film electrode under dark and UV irradiation on the solid–liquid interface. The arc radii were inversely proportional to

the electron transfer rate constants. In other words, a smaller arc radius indicated a larger electron transfer rate constant and thus a faster photocatalytic reaction rate [26,27]. Therefore, the pure TiO2 film electrode had the best UV photoelectric properties. 3.2.2. Photoelectrochemical property under visible light The photocurrent response was also measured under visible light to evaluate the photo-induced charge separation efficiency of the pure TiO2 and the Fe-doped TiO2 film electrodes (Fig. 5B). The pure TiO2 film electrodes had no photocurrent response to visible

Fig. 5. I–t curves of the TiO2 electrodes with different Fe contents: (A) under UV light, (B) under visible light.

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Fig. 6. EIS of TiO2 electrodes with different Fe contents: (A) nFe:nTi = 0 under UV light (B) nFe:nTi = 0.5% under UV light (C) nFe:nTi = 0 and nFe:nTi = 1.0% under visible light.

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light; nevertheless, the Fe-doped TiO2 film electrodes showed various degrees of responses. The photocurrent enhanced from 6.1 ␮A to 30.3 ␮A as the Fe-doping amount increased, but the growth trend declined from 30.3 ␮A to 17.1 ␮A. The iron doping realized visible light response, and the Fe to Ti molar ratios in the range of 1–100 provided the best photoelectric properties. The reason of improvement the visible light performance of TiO2 film electrode by doping Fe can be analyzed from the energy level in solid-state physics. When Fe3+ substitutes the position of Ti4+ in crystal lattice of TiO2 , electric charge is uneven distributed. In order to even electric charge, the oxygen hole which is strapped by TiO2 must be formed in near space, addition level of which is at the bottom of the forbidden band. Only little energy is needed to capture the electrons in conduction band, which contributes in the separation of electrons and holes, and finally enhances the catalytic ability of visible light. Another explanation is that Fe3+ moves into crystal body of TiO2 , and becomes electron trap as well as holetrap. When Fe is over doped, Fe3+ will become the complex center of photo-electrons and holes, which depresses the catalytic ability of visible light. As shown in the Nyquist plots, the impedance arc radii of the Fe-doped TiO2 film electrodes were smaller under visible light compared with the pure TiO2 film electrodes (Fig. 6C). The results further indicated that the Fe-doped TiO2 film electrode displayed a higher separation efficiency of photo-generated electron–hole pairs and a faster charge-transfer than the pure TiO2 film electrode under visible light on the solid–liquid interface. Therefore, the Fedoped TiO2 film electrode had the best visible-light photoelectric properties.

3.3. Photocatalytic activity

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3.3.1. Photocatalytic degradation under UV light Photoelectrocatalytic, photocatalytic and electrocatalytic degradation of MO in the presence of Fe-doped TiO2 film electrodes under UV light were evaluated by UV–visible spectroscopy. The pure TiO2 electrodes showed greater photocatalytic activity than the Fe-doped TiO2 electrodes, which might be attributed to the mixture of anatase and rutile forms (Fig. 7A). The decolorization efficiency of the photoelectrocatalysis reached 85% at 3 h. The degradation efficiency decreased along with the increased dopingiron amounts. The degradation efficiency of the pure photocatalysis and electrocatalysis by the pure TiO2 electrodes were only 38% and 19%, respectively (Fig. 7B and C). Meanwhile, the degradation efficiency also decreased when the doping-iron amounts increased. These results suggested that photoelectrocatalysis had a synergistic interaction with photocatalysis and electrocatalysis. Reaction kinetic models for the photocatalytic degradation are often expressed in terms of Langmuir–Hinshelwood (L–H) [28,29] as below: r=

− dCt kKCt = 1 + KCt dt

(2)

where r is the degradation rate, Ct is the concentration of the objective degradation pollutant at the time of t (mg/L), C0 is the initial concentration, k is the Langmuir rate constant and K is the absorption coefficient of the reagent on the catalyst. From Eq. (2), it can be learned that at very high concentrations of reactants, C is in a linear relationship with t, and it is a zero order

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Fig. 7. Catalytic degradation of MO and RhB by Fe-doped TiO2 electrodes: (A) photoelectrocatalytic degradation of MO under UV light, (B) photocatalytic degradation of MO under UV light, (C) electrocatalytic degradation of MO and (D) photoelectrocatalytic degradation of RhB under visible light.

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reaction. At very low reactant concentrations, if there is a linear relationship between ln(C0 /Ct ) and t, the reaction is a first-order reaction; if there is a linear relationship between (1/Ct − 1/C0 ) and t, it is a two order reaction. Therefore, the degradation rate constant could be obtained by plotting ln(C0 /Ct ) ∼ t and (1/Ct − 1/C0 ) ∼ t, and performing the kinetics fitting. In this study, it was found that the decolorization reaction of MO under UV light closely matched a pseudo-second-order reaction because R2 (correlation coefficient) of the pseudo-second-order reaction (0.9768–0.9984) was higher than that of the pseudo-first-order reaction (0.9481–0.9884) (Table 3). The maximum reaction rate constant at 0.1964 belonged to the pure TiO2 electrode. 3.3.2. The photoelectrocatalytic degradation under visible light The Fe-doped TiO2 electrodes showed high photocatalytic activity under visible light; the decolorization rate of RhB by the 1%

Table 3 Kinetic parameters of Fe-doped TiO2 on the photoelectrocatalytic decoloration of methyl orange under UV light. nFe:nTi 0 0.1:100 0.2:100 0.5:100 1:100 2:100

Pseudo-first-order reaction 2

ln(C0 /Ct ) = 0.6513t (R = 0.9525) ln(C0 /Ct ) = 0.4208t (R2 = 0.9653) ln(C0 /Ct ) = 0.3350t (R2 = 0.9481) ln(C0 /Ct ) = 0.2501t (R2 = 0.9829) ln(C0 /Ct ) = 0.2239t (R2 = 0.9665) ln(C0 /Ct ) = 0.1966t (R2 = 0.9884)

Pseudo-second-order reaction 1/Ct − 1/C0 = 0.1964t (R = 0.9768) 1/Ct − 1/C0 = 0.0868t (R2 = 0.9814) 1/Ct − 1/C0 = 0.0563t (R2 = 0.9894) 1/Ct − 1/C0 = 0.0384t (R2 = 0.9984) 1/Ct − 1/C0 = 0.0329t (R2 = 0.9869) 1/Ct − 1/C0 = 0.0276t (R2 = 0.9939) 2

Table 4 Kinetic parameters of Fe-doped TiO2 on the photoelectrocatalytic decoloration of Rhodamine B under visible light. nFe:nTi

Pseudo-first-order reaction

Pseudo-second-order reaction

0 0.1:100 0.2:100 0.5:100 1:100 2:100

ln(C0 /Ct ) = 0.0294t (R2 = 0.9901) ln(C0 /Ct ) = 0.0371t (R2 = 0.9847) ln(C0 /Ct ) = 0.0492t (R2 = 0.9758) ln(C0 /Ct ) = 0.0714t (R2 = 0.9606) ln(C0 /Ct ) = 0.0870t (R2 = 0.9925) ln(C0 /Ct ) = 0.0564t (R2 = 0.9964)

1/Ct − 1/C0 = 0.0030t (R2 = 0.9885) 1/Ct − 1/C0 = 0.0039t (R2 = 0.9880) 1/Ct − 1/C0 = 0.0052t (R2 = 0.8695) 1/Ct − 1/C0 = 0.0079t (R2 = 0.9728) 1/Ct − 1/C0 = 0.0129t (R2 = 0.9519) 1/Ct − 1/C0 = 0.0064t (R2 = 0.9941)

Fe-doped TiO2 film electrode was 22% higher than the undoped electrode. The decolorization reaction of RhB under visible light was a pseudo-first-order reaction (Fig. 7D and Table 4). These results indicated that modified TiO2 film electrodes achieved photoresponse of visible light, and got well decolorization of RhB. Thus Fe-doped TiO2 film electrode would have a great potential. 4. Conclusions The Fe-doped TiO2 film electrode was successfully prepared by Fe-doping, which restrained the grain growth of TiO2 and the phase transformation of rutile, and the electrode mainly consisted of anatase. The photo response of the modified electrode extended from 452 nm to 604 nm, and the band gap decreased to 2.05 eV. The decolorization rate of MO reached 85% under UV light. The photocatalytic performance of the Fe-doped electrode was improved under visible light, though it was compromised under UV light.

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The response current of the 1% Fe-doped TiO2 film electrode was 30.3 ␮A, whereas the TiO2 film electrode had no response under visible light. The decolorization rate of RhB by the 1% Fe-doped TiO2 film electrode was 22% higher than the undoped electrode under visible light, and the reaction was a pseudo-first-order reaction. The extended responsive wavelength range greatly expanded the application potential of the modified electrode, and the preparation method seemed of great value for TiO2 film electrode modification. Acknowledgements

This work was supported by the National Natural Science FounQ3 dation of China (No. 21277098), the Natural Science Foundation of 387 Shanghai (No. 10ZR1432500), the Shanghai Municipal Commission 388 of Economy and Informatization (No. 20122846) and Central Lab389 oratory of Department of Chemistry and School of Life Science and 390 Technology at Tongji University. 391 Q2 386

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References [1] D. Li, H. Haneda, S. Hishita, et al., Mater. Sci. Eng. B 117 (2005) 67. [2] K.Q. Tan, H.R. Zhang, C.F. Xie, et al., Catal. Commun. 11 (2010) 331.

[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

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Y.H. Shi, H.M. Meng, D.B. Sun, et al., J. Funct. Mater. 38 (2007) 2696. X.W. Li, P.P. Lu, K.F. Yao, et al., J. Inorg. Mater. 27 (2012) 1153. N.C. Meng, J. Bo, W.K. Christopher, et al., Water Res. 44 (2010) 2997. H.M. Yang, K. Zhang, R.R. Shi, et al., J. Alloys Compd. 413 (2006) 302. Y. Yang, H. Wang, J.X. Li, et al., Environ. Sci. Technol. 46 (2012) 6815. T.N. Wu, T.C. Pan, L.C. Chen, Electrochim. Acta 86 (2012) 170. X.L. Peng, M.M. Yao, F. Li, et al., Particul. Sci. Technol. 30 (2012) 81. X. Liu, Z.Q. Liu, J. Zheng, et al., J. Alloys Compd. 509 (2011) 9970. Y.R. Zhang, Q. Li, Solid State Sci. 16 (2013) 16. F. Ostovari, Y. Abdi, Phys. J. Appl. Phys. 58 (2012) 30402. Y.L. Su, Y.T. Xiao, Y. Li, et al., Mater. Chem. Phys. 126 (2011) 761. X. Li, D.T. Wang, J.F. Chen, X. Tao, Electrochim. Acta 80 (2012) 126. S. Pany, K.M. Parida, B. Naik, RSC Adv. 3 (2013) 4976. Y.Y. Liu, Y. Li, W.Z. Li, S. Han, C.J. Liu, Appl. Surf. Sci. 258 (2012) 5038. K. Archana, R.S. York, S. Vaidyanathan, Environ. Sci. Technol. 43 (2009) 3260. J.A. Wang, R. Limas-Ballesteros, J. Phys. Chem. B 105 (2001) 9692. C.A. Castro-López, A. Centeno, S.A. Giraldo, Catal. Today 157 (2010) 119. C.Y.W. Lin, D. Channei, P. Koshy, et al., Ceram. Int. 38 (2012) 3943. D.A.H. Hanaor, C.C. Sorrell, J. Mater. Sci. 46 (2011) 855. P. Vijayan, C. Mahendiran, C. Suresh, et al., Catal. Today 141 (2009) 220. W.C. Hung, Y.C. Chen, H. Chu, et al., Appl. Surf. Sci. 255 (2008) 2205. Y.K. Lai, H.F. Zhuang, K.P. Xie, et al., N. J. Chem. 34 (2010) 1335. Q. Wu, J.J. Ouyang, K.P. Xie, et al., J. Hazard. Mater. 199 (2012) 410. P.J. French, P.T.J. Gennissen, P.M. Sarro, Sens. Actuators A: Phys. 62 (1997) 652. J.M. Lopez Villegas, J. Samitier, J. Bausells, et al., J. Micromech. Microeng. 7 (1997) 162. [28] M.H. Khedr, K.S.A. Halim, N.K. Soliman, Mater. Lett. 63 (2009) 598. [29] C.L. Chen, X.L. Li, D.L. Zhao, et al., Colloids Surf. 302 (2007) 449.

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