Photoelectrochemical degradation of dye wastewater on TiO2-coated titanium electrode prepared by electrophoretic deposition

Accepted Manuscript Photoelectrochemical degradation of dye wastewater on TiO2-coated titanium electrode prepared by electrophoretic deposition Ching-Fang Liu, C.P. Huang, Chi-Chang Hu, Yaju Juang, Chihpin Huang PII: DOI: Reference:

S1383-5866(16)30153-8 http://dx.doi.org/10.1016/j.seppur.2016.03.045 SEPPUR 12932

To appear in:

Separation and Purification Technology

Received Date: Accepted Date:

2 December 2015 26 March 2016

Please cite this article as: C-F. Liu, C.P. Huang, C-C. Hu, Y. Juang, C. Huang, Photoelectrochemical degradation of dye wastewater on TiO2-coated titanium electrode prepared by electrophoretic deposition, Separation and Purification Technology (2016), doi: http://dx.doi.org/10.1016/j.seppur.2016.03.045

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Photoelectrochemical degradation of dye wastewater on TiO2-coated titanium electrode prepared by electrophoretic deposition Ching-Fang Liua, C.P. Huangb, Chi-Chang Huc, Yaju Juanga, Chihpin Huang a* a

Institute of Environmental Engineering, National Chiao Tung University, Hsin-Chu 30010, TAIWAN b Department of Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, USA c Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 30013, TAIWAN

Submitted to

Separation and Purification Technology

*Corresponding Author:

Chihpin Huang, e-mail: [email protected]

1

Abstract The degradation of azo dye Orange G (OG) on TiO2/Ti electrode was studied using a photoelectrocatalytic system. TiO2/Ti electrode was prepared by electrophoretic deposition (EPD) process. Results of surface analyses, namely, X-ray diffraction (XRD), SEM, X-ray photoelectron spectroscopy (XPS), and linear sweep voltammetry (LSV), revealed that the optimal EPD condition was a deposition potential of 180 V for 1 min and annealed at 350oC. The TiO2/Ti electrode was used as photoanode and a graphite cathode for the degradation of OG. The coupling effect of photocatalytic and electrochemical oxidation processes was evaluated in terms of the decolorization of OG. The degradation of OG increased in the following increasing order: photocatalysis < electrochemical oxidation < photoelectrochemical degradation. A decolorization of 54.3% was achieved at an applied current density of 75 A cm-2. The degradation of Orange G follows the first-order kinetics in those three treatment processes, which suggests the participation of OH radicals in the decolorization reaction.

Keywords: Photoelectrocatalysis; Orange G; electrophoretic deposition; photocatalysis; electrochemical oxidation

2

1.0 Introduction The release of organic dyes in industrial waste streams is a current water pollution problem of developed and developing countries over the world. Azo dyes, constituted mainly of aromatic moieties and linked together by –N=N– bonding (a type of chromophore), represents the largest class of dye pollutants, particularly [1, 2]. The release of dyes to natural environment, mainly aquatic medium, can have toxic and carcinogenic effects toward the aquatic life. The wide variation of physical–chemical properties in each class of dyes and the use of chemicals such as dispersing agents, salts, and many others organics during the dyeing manufacturing make the treatment of dye industrial wastewater a complicated and challenging task. Effluents from dye industrial wastewater treatments were generally processed into sludge cakes and deposited in landfills [3, 4], which could have further long-range environmental risk due to the presence of the partially oxidized dyes. In recent years, advanced oxidation techniques including electrochemical advanced oxidation processes (EAOPs) [5-8] have gained much attention. EAOPs have been employed widely for the decontamination of wastewaters containing high-strength organic pollutants due to their versatility, high efficiency, and environmental compatibility [8]. The effectiveness of EAOPs is ascribed to the generation of hydroxyl 3

radical (OH•), which has a high redox potential that can react non-selectively with most organic compounds to produce CO2, water, and inorganic ions [9, 10]. Titanium dioxide (TiO2) as a photocatalyst, has been extensively studied in water purification and wastewater treatment, since almost all organic pollutants can be mineralized by the strong oxidizing power of the photogenerated holes [11-16]. However, this technique is insufficient for practical application due to the rapid recombination of active electrons and holes upon photoexcitation [17, 18]. To further improve the efficiency

of

photocatalytic

(PC)

oxidation

reactions,

heterogeneous

photoelectrochemical process (PEC) has become an attractive technique recently for the effective generation of OH radicals by the application of a bias DC potential that separates the electrons from holes upon production. In PEC, when TiO2 is irradiated with ultraviolet light, electrons are excited from the valence band ( band (

) to the conduction

), which generates highly oxidative holes (h+) in the valence band that produce

OH radicals on the electrode surface via water oxidation [19]. The PEC process has been successfully employed for oxidizing a broad list of hazardous chemicals such as herbicides [20], textile dyes [21], and pharmaceuticals [22]. In most PEC oxidation systems usually the photoanode was prepared by coating TiO2 thin film on a support such as conducting glass or metals. 4

In our study of the photoelectrochemical degradation of dye using a TiO 2 coated stainless steel electrode (TiO2/SS) [23], the TiO2/SS electrode was

prepared by

electrophoretic deposition (EPD) process (the mean of this sentence is too weak) . The PEC process over TiO2/SS electrode could achieve high decolorization efficiency compared to that of PC process. However our studies showed that the adhesion of TiO 2 on the stainless steel mesh substrate was relatively poor due to the substantial anodic currents that oxidized the substrates on TiO2-SS under positive polarization. Noteworthy, the above phenomena were not observed on TiO2 /Ti electrodes however, since the dark currents of the as-prepared TiO2/Ti electrode were extremely low. Among various fabrication methods, electrophoretic deposition (EPD) is the simplest one for the preparation, because of its relative low cost, as well as high manufacturing throughput [23-25]. Under a DC bias, charged colloidal particles were transported toward and deposited on the oppositely charged electrode surface to produce consolidated layers of coating. In this work, TiO2/Ti electrode was prepared using the EPD method in 2-propanol with zinc nitrate (Zn(NO3)2) electrolyte. The crystalline structure, morphology, and the amount of TiO2 deposited could be controlled by the annealing temperature and the 5

deposition time and voltage The photoelectrocatalytic activity of the TiO2/Ti electrode was tested for Orange G (OG) degradation in a PEC reactor against a graphite cathode. The performance of the TiO2/Ti electrode was evaluated in terms of color removal in PEC mode and compared with that of photocatalysis (PC) and electrochemical oxidation (ECO). 2.0 Experimental 2.1. Materials The azo acid dye orange G (OG) (pKa1 and pKa2:-7, pKa3: 11.5) of chemical reagent grade was purchased from Sigma Chemical Co. and used without further purification. Titanium dioxide powder (P25) with 80% anatase and 20% rutile, from the Degussa AG Company, had an average particle size and specific surface area of 30 nm and 50 m2 g-1, respectively. The electrode materials were Ti mesh (2 cm × 5 cm, 200 mesh), platinum sheet (2 cm × 5 cm), and graphite rod (2.5π cm × 5 cm), all from Sigma–Aldrich. 2.2. Preparation of TiO2/Ti mesh anode The TiO2/Ti electrode was prepared by coating P25 powders onto titanium mesh by electrophoretic deposition (EPD) procedure. Raw Ti mesh was treated in 6 M HCl at 90◦C for 30 min, fully rinsed with DI water, and then dried with a N2-airgun before EPD processing. The Ti mesh was placed vertically at the center of a 250-mL jacket cell, 6

facing two Pt sheets and then began deposit the P25 film. A 200-mL isopropanol solution containing 10−2 M of Zn(NO3)2 and 3.2 g L−1 of P25 was used as the deposition solution. Zn(NO3)2 was used to enhance the uniformity and adhesion of P25 on the Ti substrate. Electrophoretic deposition was conducted at 4◦C under different applied potential and deposition time. Table 1 shows details of EPD operation. The TiO2/Ti electrodes were then rinsed with DI water and annealed in air at various temperatures (200, 250, 350, 450, and 550oC) for 60 min.

Table 1 Electrophoretic deposition (EPD) parameters for the preparation of TiO2/Ti mesh electrode Sample

EPD time (min)

EPD potential (V)

TiO2/Ti (1,160)

1

160

TiO2/Ti (2,160)

2

160

TiO2/Ti (1,170)

1

170

TiO2/Ti (2,170)

2

170

0.5

180

TiO2/Ti (1,180)

1

180

TiO2/Ti (2,180)

2

180

TiO2/Ti (3,180)

3

180

TiO2/Ti (0.5,180)

2.3. Surface characterization of the TiO2/Ti electrode

7

Scanning electron microscopy (SEM) was used to study the surface morphology of the TiO2/Ti electrode. X-ray diffraction (XRD) measurement, carried out with Rigaku TTRAX III powder diffractometer (XRPD-WAG), was used to characterize the crystal phase composition of the TiO2/Ti electrode. A voltage of 40 V and a current of 30 mA were used to produce Cu Kα radiation at a wavelength of 1.5418 Å and scan rate of 4◦ min−1 (2θ from 20◦ to 80◦). Linear sweep voltammetry (LSV) was performed using a Potentionstat (AutoLab). 2.4 Hydrogen peroxide production To study the effect of H2O2 generation at different applied voltage for the PEC system, experiments with the Na2SO4 electrolyte at concentration of 0.1 M and pH 3 were carried out, using the TiO2/Ti electrode as the photoanode and a commercial graphite as the cathode under UV-A illumination. The concentration of H2O2 in the test solution was measured by the N,N-diethyl- p-phenylenediamine (DPD) method [26], respectively. 2.5 Degradation of OG To evaluate the reactivity of the photoelectrochemcial (PEC) reaction in the two-electrode system, several degradation experiments were carried out in an undivided cell with aeration. The major function of the graphite cathode was for the electrochemical generation of H2O2. The reaction solution contained 200 mL of azo dye OG at a 8

concentration of 0.1 mM, pH 3, and 0.1 M Na2SO4 of supporting electrolyte. The O2 flow was controlled at 200 mL min-1. Samples were collected at regular time intervals to determine the residual OG concentration. Experiments were carried out with a photoelectrochemical system operated in constant current mode. The total volume of the reactor was 200 mL. The anode was a titanium mesh coated with P25 powders (TiO2/Ti mesh). The graphite cathode was made of graphite and placed near the oxygen diffuser in the reactor. The surface area of the graphite was 44.17 cm2. The UV light source was a medium-pressure mercury lamp (Entela UVGL-25; power 8 W; average radiation intensity 0.33 mW cm-2 ; main wavelength 365 nm). The absorbance spectrum of the OG dye in aqueous solution was measured by Metertech SP-8001 Spectrophotometer.

3.0 Results and discussion 3.1. EPD deposition time and potential affected the surface morphology of the TiO2/Ti electrode Fig. 1 shows the mass of TiO2 deposited onto the Ti mesh as function of the EPD time. Clearly, the mass of TiO2 was monotonously increased from 0.23 to 1.1 mg cm2 9

when the EPD time was increased from 0.5 to 3 min. For the applied potential in EPD process was presented in Supporting Information (S. 1), the amount of TiO2 deposit increased with increase in applied potential. The successful deposition of TiO2 onto Ti mesh (the negative electrode) could be attributed to the positively charged TiO2 particles in the 2-propanol solution containing 10-2 M of Zn(NO3)2. The rate of TiO2 deposition was also a function of temperature and applied electric voltage. According to our previous preliminary results [23], the optimal temperature and applied electric field were 4oC and 180 V, respectively. Hamakers [27] calculated the amount of TiO2 deposited onto Ti mesh during EPD according to the following equation: (1) where w is the mass deposit,  is the electrophoretic mobility, E is the electric field strength, A is the surface area of the electrode, and C is the particle mass concentration in the suspension. From Eq. (1), it is seen that the amount of TiO2 deposit will be directly proportional to the EPD potential and time.

10

Mass (mg/cm2)

1.4 1.2 1.0 0.8 0.6 0.4 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Time (min) Fig. 1. TiO2 mass as function of deposition time. Experimental conditions: Deposition potential = 180 V; [TiO2] = 3.2 g/L; [Zn(NO3)2] = 10-2 M; Temperature = 4oC; Working distance = 2 cm.

The surface morphology of TiO2 on Ti mesh was examined by SEM. Fig. 2 (b-e) shows the SEM images of the TiO2 layer at deposition time between 0.5 to 3 min. The surface of Ti substrate was smooth initially. After deposition for 0.5 to 1 min, the TiO2 particles were uniformly coated on the surface of Ti substrate. Some micro-cracks (50 mm) were observed on the TiO2 layer when the deposition time was greater than 2 min. The length of micro-cracks was up to 100 mm long when the deposition time was 3 min. According to the linear elastic fracture mechanics, cracking behavior is dependent on the 11

layer thickness i.e., cracking of the TiO2 layers increased with the layer thickness [28]). Obviously, the TiO2 layer thickness increased with deposition time. Since the thickness and micro-cracks of TiO2 films might increase the resistance to charge-transfer of photo-excited electrons and enhance electrode corrosion, the deposition time was fixed at 1 min. The TiO2 film had a uniform coating layer free of micro-cracks at high applied potential (Fig.2 (f-h)), since the TiO2 layer thickness was not large enough to form cracks at the applied potential of 180 V.

(a)

200 m

(b)

50 m

(c)

200 m

200 m

50 m

(d)

200 m

50 m

12

50 m

(e)

200 m

(f)

50 m

50 m

(h)

(g)

200 m

200 m

50 m

200 m

50 m

Fig. 2. SEM images of TiO2 deposited on Ti mesh at different deposition time (b–e) and applied potential (f–h). (a) Ti mesh substrate; (b) 0.5 min; (c) 1 min; (d) 2 min; (e) 3 min; (f) 160 V; (g) 170 V; (h) 180 V. Experimental conditions: [TiO2] = 3.2 g/L; [Zn(NO3)2] = 10-2 M; Annealing temperature = 350oC; Working distance = 2 cm; Deposition time = 1 min; Deposition potential = 180 V. Insets are magnified images of the area marked by the white square.

3.2. Annealing temperature affected the crystallinity of the TiO2/Ti electrode Fig.3 shows the XRD pattern of the TiO2/Ti electrode. Result shows that the ratio of peak intensity of anatase to bulk Ti increased with annealing temperature, which indicated the presence of TiO2 film. In addition, the noise level of the XRD data decreased with increase in annealing temperature, which implied the increase of 13

crystalline TiO2. Anatase (A) was the predominant crystalline phase of TiO2 (P25), shown by the corresponding characteristic diffraction peaks at 25.4o and 48.1o for facets (101) and (200), respectively. For films annealed at ≤ 450oC, the most intense peak of anatase structure corresponding to the (101) facet appeared. Other peaks of the anatase structure could not be detected since they were smaller than the noise level. Annealing at 550oC displayed peaks of both anatase and rutile phase (110), but the intensity of the diffraction peaks corresponding to facets (101) and (200) in the anatase phase decreased significantly. The above observations clearly indicated that annealing at 550 oC for 1 h resulted in a phase transformation from anatase to rutile. In general, TiO2 exhibited a photocatalytic activity stronger than either pure anatase or rutile [29]. Thus, to achieve high photocatalytic activity and to improve the physical integrity of the TiO2 layer, all TiO2/Ti electrodes were annealed at 350◦C for 1 h.

14

o

25.4 A(101)

Relative intensity (A.U.)

A ─ anatase R ─ rutile

o

48.1 A(200)

o

27.4 R(110)

Ti

Ti

Ti

(f) (e) (d) (c) (b) (a)

20

30

40

50

2

60

70

80

o)

Fig. 3. XRD patterns of TiO2 deposited on Ti mesh substrate annealing at different temperature. (a) Ti at 350oC; (b) TiO2/Ti at 200oC; (c) TiO2/Ti at 250oC; (d) TiO2/Ti at 350oC; (e) TiO2/Ti at 450oC; (f) TiO2/Ti at 550oC.

Fig.4 shows the SEM images of the TiO2/Ti electrode at different annealing temperatures. The TiO2 layers on the Ti substrate showed no cracks when the annealing temperature was increased to 350oC. When the annealing temperature was increased to 450oC micro-cracks appeared on the TiO2 film surface. Moreover, the number of micro-cracks was increased when the annealing temperature reached 550oC. Dehydration 15

of surface hydroxo and the neck-to-neck binding among TiO2 particles played a major role on the interparticle connectivity via the formation Ti–O–Ti bridges during thermal treatment [30]. When the annealing temperature was increased, the internal energy of TiO2 particles was increased. According to the Griffith’s energy balance of fracture mechanics, the formation of cracks will increase with increasing annealing temperature [31].

(a)

200 m

(b)

50 m

200 m

(c)

200 m

50 m

(d)

50 m

200 m

16

50 m

(e)

200 m

(f)

50 m

200 m

50 m

Fig. 4. SEM images of TiO2 deposited Ti mesh at different annealing temperature. (a) without annealed (b) 200 oC, (c) 250oC, (d) 350oC, (e) 450oC, (f) 550oC. Experimental conditions: [TiO2] = 3.2 g/L; [Zn(NO3)2] = 10-2 M; deposition potential = 180 V; Working distance = 2 cm; deposition time = 1 min.; Insets are the enlarge image of the area marked in the square.

Anodic potential was an important factor affecting the photocurrent density, which transported the photoelectrons away from the TiO2/Ti electrode and reduced the hole and electron recombination. Fig. 5 shows the linear sweep voltammogram (LSV) of the TiO2/Ti electrode at different annealing temperatures with and without UV illumination. Results clearly showed the production of photocurrent under UV illumination. Moreover, the photocurrent increased with the increase of annealing temperature, especially when annealed at 550oC the photocurrent was significantly greater than that of electrodes prepared at other temperature, which implied that the TiO2 film crystallinity could affect the generation of photoinduced electrons due to the co-existence of anatase and rutile. However, the population of micro-cracks was significant when the annealing temperature 17

was over 450oC. Surface cracks will increase the electrical resistance in the photoelectrochemcial reaction. In order to achieve a high hotocatalytic activity and to improve the physical integrity of TiO2 layers, all TiO2/Ti electrodes were annealed at 350 ◦C in this study.

I ( A/cm2)

6

4

2

0 0.0

0.5

1.0

1.5

V (v vs. Ag/AgCl) Fig. 5. Linear sweep voltammetry (LSV) of TiO2/Ti electrodes at different annealing temperatures. Experimental conditions: [Na2SO4] = 0.1 M; pH = 3.0; Temperature = 25oC; Deposition potential = 180 V; Deposition time = 1 min. Lines: (Black) Dark; (Green) 200oC; (Red) 250oC; (Yellow) 350oC; (Blue) 450oC; (Fuchsia) 550oC.

XPS analysis was used to study the chemical composition of the TiO2 layer on the Ti mesh. The O 1s orbital of the Ti and the TiO2/Ti mesh were asymmetric, which indicated the presence of at least two kinds of chemical states (Fig. 6). Signal convolution 18

enabled the resolution of a peak that was correspondent to the lattice oxygen of TiO2 (OL), and another peak at higher binding energy, was attributed to the surface species such as Ti–OH and Ti–O–O, resulted from the chemisorbed water (OH) [32]. Table 2 lists the fitting results of the corresponding Ti, Zn and O orbitals and the relative atomic ratio of Zn to Ti and O to Ti. As seen from Table 2, the small amount of ZnO existed in the TiO2 layer relative to the Ti to O ratio of ca. 2 under the lower deposition potential condition. Our results agreed slightly with Benehkohal et al.[24, 25] who reported the co-deposition of TiO2-ZnO film during the preparation of TiO2 film on FTO-glass using EPD process, 2-propanol co-solvent, and Zn(NO3)2 additive. The Zn deposition in TiO2 occurred at low DC voltage via the formation of metallic zinc (Eq. 2). The connected Zn and zinc hydroxide worked as “nanoglue” for the TiO2 particles to form thin film on the Ti surface. Further annealing transformed the zinc hydroxide to ZnO (Eq. 3) [24, 25], which strengthened the TiO2 attachment on the Ti surface. In this work, the observed ZnO might be incorporated into the TiO2 layers via the following reactions: °

(2) (3)

19

OL

O 1s

Intensity (A.U.)

OH

180 V

170 V

160 V 536

534

532

530

528

526

Binding Energy (eV) Fig. 6. O 1s XPS spectra of TiO2/Ti electrode as function of deposition potential.

Table 2 Binding energies (eV) of Ti 2p, Zn 2p, O 1s, the TiOx content, and the Zn/Ti the molar ratio Ti EPD potential

Zn

O x/TiOxa

Zn/Tia

1022.804 530.66 532.19

1.975

0.021

170 V

459.45 465.137 1022.325 530.66 532.08

1.908

0.017

180 V

459.43

2.08

0.016

2p3/2 (eV)

2p1/2 (eV)

160 V

459.44

465.13

465.13

2p3/2 (eV)

(OL) (eV)

1022.71

530.66

a

(OH) (eV)

532.3

The XPS quantitative analysis was carried out according to the equation: . CA: concentration of element A (%); nA: atomic concentration of element A, ASFA: atomic sensitivity factor for element A; IA: intensity of the element A. 20

The TiO2/Ti electrode treated at different annealing temperature was tested for the degradation of Orange G (OG) (Supporting Information S.2). The aqueous solution of OG was stable under UV radiation in the absence of TiO2/Ti photoanode. The degradation of OG was assessed in terms of color change in the PEC system under UV light irradiation. The results showed that the maximum OG decolorization occurred at an annealed temperature of 350oC. Annealing at 550oC had the lowest degree of decolorization, though the photocurrent was higher than those electrodes prepared at other temperatures. The photocatalytic activity of the TiO2 layer was unstable because of existing cracks that resulted in an increase in electrical resistance and peeling off the TiO 2 layers from the Ti substrate during the photoelectrochemcial reaction.

3.3. Applied voltage and current density used in the preparation of TiO2/Ti electrode affected the photoelectrochemical performance Fig. 7 shows results of the performance of the electrode with respect to H2O2 generation as a function of applied potential. The H2O2 production increased with applied potential and reached a maximum concentration of 2.3 mg L-1 at 5.5 V, then deceased upon further increase in applied potential (Fig. 7a). Hydrogen peroxide was produced by 21

the reduction of O2 in a two-electron reduction (Eq. 4), whereas the reduction of O2 to water is four-electron reduction reaction (Eq. 5)[33]: °

(4)

°

(5)

It can be explained that when the applied voltage was below 5.5 V, the reduction of O2 followed the two-electron reduction. Thus, when the applied voltage was increased there was increase in H2O2. However, when the applied voltage was over 5.5 V, the reduction of O2 forming H2O via the four-electron reaction occurred. As a result, the reduction of O2 to H2O at 7.5 V was proceeded that to H2O2 at 6.5 V. Another reason was the oxygen evolution reaction might occur on the TiO 2 layer, which results in cracking TiO2 film on the Ti substrate. Therefore, the generation rate of photocurrent was decreased, causing the electron transport to the cathode. It was why the accumulation of H2O2 took place at applied potential smaller than 5.5 V. Fig. 7b shows the relationship between applied voltage and current change under UV light irradiation. The current was increased with increase in applied voltage then decreased when the applied voltage was greater than 5.5 V. Ti substrate corrosion due to the presence of oxygen gas occurred on the TiO2/Ti electrode surface, which resulted in the formation of micro-cracks on the TiO2 layer. 22

(a) 2.5

-1

[H2O2] (mg L )

2.0 1.5 1.0 0.5 0.0 2

3

4

5

6

7

8

V (v vs Ag/AgCl)

0.08

0.06

0.06

0.04

0.04

0.02

0.02

0.00

0.00

-0.02

2

3

4

5

6

7

8

2

0.08

Q, (C/cm )

2

I (mA/cm )

(b)

-0.02

V (v vs Ag/AgCl)

Fig. 7. Effect of applied voltage on (a) H2O2 generation and (b) current/charge (Symbols: (●) Charge density; (○) Current density). [Na2SO4] = 0.1 M; pH = 3; Reaction time = 60 min; O2 flow rate = 200 mL min-1 ; UV light = 365 nm. 23

Fig. 8 shows results of OG degradation as function of applied voltage. Results demonstrated that the remaining OG after 60 min was 96.4, 96.2, 95.2, 90.95, 91.2, and 92.8%, respectively, at the applied potentials of 2.5, 3.5, 4.5, 5.5, 6.5 and 7.5 V. The highest OG degradation occurred at applied voltage of 5.5 V. The effect of the applied potential on the OG degradation was consistent with that of photocurrent. The OG degradation efficiency was deceased when the applied voltage was increased to > 5.5 V, which also coincided with the largest current density of 75 A cm-2. To retain the photocatalytic activity of TiO2 and to improve the efficiency of OG degradation, an optimum current density of 75 A cm-2 was applied in all experiments.

0.98

[OG] (C/C0)

0.96

0.94

0.92

0.90

2

3

4

5

6

7

8

V ( v vs Ag/AgCl)

Fig. 8. Decolorization of orange G after 60 min reaction time by different applied voltages in 0.1 mM Orange G (pH 3.0) solution with 0.1 M Na2SO4. Reaction time was 60 min. 24

The degradation of OG was performed and compared in three different modes of reactions including, (i) photocatalysis (PC), (ii) electrochemical oxidation (EO), and (iii) photoelectrochemcial degradation (PEC). Fig.9 shows the results of OG degradation by the above methods. Results illustrated that the degradation of OG increased in the following increasing order: PC < EO < PEC. The OG degradation was 7% and 41.6% in 180 min, respectively, by the PC and EO processes. The PEC process had increased the OG degradation significantly to 54.3% in 180 min, which strongly suggested that photocatalysis and electrochemical could not generate adequate hydroxyl radicals for OG degradation. The degradation of OG (i.e., PC, EO and PEC) followed a first-order kinetics rate law (Supporting Information S.3). The rate constant (k1) was 4.1 10-4 min-1(R2 = 0.970), 3.0 10-3 min-1 (R2 = 0.997), and 4.3 10-3 min-1 (R2 = 0.999), for PC, EO, PEC, respectively. Table 3 compares results on the degradation of various pollutants using TiO2 type photoanode using PEC process by several recent authors. Xie and Li [34] reported a color removal of 20% after 5 h treatment of OG using the TiO2/Ti-Pt system. The color removal was improved from 20 to 50% after 5 h treatment using the reticulated vitreous carbon (RVC) that produced H2O2 in the PEC system. Liu et al. [35], Feleke et al. [37] 25

and Xin et al. [20] reported 50-60% oxidation efficiency for the treatment

of dye and

hazardous organic compounds. It must be noted that the organic chemicals studied by these authors had simpler structure than that of OG and were much amenable to degradation, therefore. Furthermore, the photoenergy used was one order of magnitude greater than that of the present study. Nonetheless, our results were in the same range as reported by these authors. Jiang et al. reported a 80% removal of methyl orange dye using the TiO2 nanotube/Ti (TNT/Ti) [36]. This relatively high performance on dye decolorization could be attributed to the relatively high energy intensity used. Note that the light energy used by Jiang et al. [36] was ten times that used in the present study. It is worthy of notice that Xie and Li [34] also used the TiO2(P25)/Ti electrode, prepared by electrochemical anodization. Our results were comparable with that of Xie and Li [34], except that our electrode was more stable than that prepared by anodization. Comparing with results of the previous study from our group, Lin et al. [23] reported 25.7% oxidation efficiency of methyl orang after 3 h of reaction. The result in this study, 58% degradation of orange Methyl, is better than that of 25.7%, which indicated the superiority of our electrode, prepared based on titanium substrate, was more stable and superiority than that prepared with stainless steel substrate which is vulnerable to corrosion during the reaction. 26

Table 3. PEC degradation of various pollutants using TiO2 photoanode

Electrodes Anode-cathode

TiO2/Ti - Pt TiO2/Ti - RVC

Anode area (cm2)

5

Light Pollutant / volume

0.1 mM OG (80 mL)

0.21 mM TNT/Ti - Pt

Wormhole-sha ped TiO2/Ti -

1.9

1

phenol (70 mL) 0.02 mM Alachlor

source/ Intensity (mW cm-2) UV light 365 nm/ 0.68 UV light 250-450 nm/ 3.1 Visible light 500 nm/

Pt

(30 mL)

TNT/Ti - Pt

1.05

0.05 mM MO

3.14

0.061 mM MO (40 mL)

LED 375 nm/ 2

20

0.14 mM OG

UV light 365 nm 0.66

20

0.1 mM OG (200 mL)

TiO2/Cu2O/tinoxide-coated glass (TCO) Pt TiO2/Stainless steel Graphite

TiO2/Ti Graphite

62.54 UV light 253 nm/ 10

Applied voltage (V)

27

Color removal (%)

Ref.

20 50

[34]

50

[35]

60

[20]

pH 6.2 0.71 (v.s. SCE)

0.6 (v.s. SCE)

1.5 (v.s. SCE)

0.01 M Na2SO4 5h 0.01 M Na2SO4 3.3 h 0.5 M Na2SO4 2h

0.5 (v.s. SCE)

0.05 M K2SO4 3h

80

[36]

1.0 (v.s. SCE)

pH 6.5 0.1 M KCl 3h

60

[37]

25.7

[23]

pH 3 1.0 (v.s. SCE)

UV light 365 nm 0.66

Exp. conditions

5.5

0.01 M Na2SO4 3h pH 3 0.1 M Na2SO4 3h

54

[OG] (C/C0)

1.0

0.8

0.6 PC EO PEC

0.4

0

50

100

150

200

Time (min) Fig. 9. Decolorization of orange G using photocatalysis(PC), electrochemical oxidation (EO), and photoelectrochemical (PEC) system. Experimental conditions: Anode: TiO2/Ti mesh (1,180); Cathode: Graphite; [OG] = 0.1 mM; [Na2SO4] = 0.1 M; pH = 3; reaction time = 60 min; O2 flow rate = 200 mL min-1 ; UV light = 365 nm. Symbols: (●)PC; (○)EO; (▼)PEC.

Fig. 10 presents the change of UV absorption spectra of the reacting solution at different irradiation times. The initial OG spectrum showed three characteristic bands in the visible region at λ = 478 nm, λ = 420 nm, and λ = 330 nm corresponding to the azo form of the dye[38], and two bands in the UV region at λ = 330 nm and λ = 215 nm, attributed to the benzoic and naphthalene rings, respectively. As the reaction proceeded, the intensity of the bands decreased significantly without the appearance of new absorption peaks. The visible band at 478 nm (rate constant = 4.3 10-3 min-1) 28

disappeared faster than that at 330 nm (rate constant = 8 10-4 min-1) and 248 nm (4 10-4 min-1). The azo peaks decreased when the band at 210 nm appeared, possibly due to the formation of intermediates (containing benzene and naphthalene rings). It was suggested that the first attack by OH radicals azo groups opened the N=N bonds and destructed the long conjugated π bondings and consequently causing decolorization[39]. The N=N bonds are weaker than the aromatic ring structures. The band at 210 nm appeared when the azo peaks diminished showing the presence of intermediates.

Absorbance (A.U.)

0 10 min 30 min 60 min 90 min 120 min 180 min

200

300

400

500

600

Wavelength (nm) Fig. 10. Decolorization of OG in the PEC processes as a function of time. Experimental conditions: Anode: TiO2/Ti mesh (1,180); Cathode: Graphite; [OG] = 0.1 mM; [Na2SO4] = 0.1 M; pH = 3; reaction time = 60 min; O2 flow rate = 200 mL min-1; UV light = 365 nm.

3.4. Degradation of Orange G by TiO2/Ti electrode took place upon the attack by 29

hydroxyl radicals in the solution phase

Based on above results and currently available information [8, 40-42], reaction pathways of OG degradation over TiO2/Ti photoanode can be proposed. At pH 3, HG2- is the major OG species [40]. The key features of the reaction scheme were: 1) the formation of OH radicals due to the anodic oxidation of water or hydroxide ion and the reaction of hydrogen peroxide and oxygen radicals at cathode; 2) hydroxyl radical attack as the main route for the degradation of HG2-; 3) hydroxyl radical oxidation of HG2taking place in the solution phase; 4) the slow rate of back reaction involving G2-․, a highly unstable and reactive intermediate. The detail of pertinent steps involved in the OG degradation in the PEC system is shown in Supporting Information (S4). The major steps include the adsorption of H2O or OH- on the electrode surface then direct electrochemical reduction converts H2O or OH- to hydroxyl radicals. Two cases of possible reaction schemes can be possible. Scheme I. Rate determining step occurred between hydroxyl radical and orange O in the solution phase: (6) (7) 30

By applying the steady-state approximation technique on OH․, O2․- in the bulk and species adsorbed on electrode surface (OH․, O2․-, H2O, and O2), one has the oxidation rate of HG2- based on Eq. 7 (Supporting Information S4): (8) If the

, also the generation rate of hydrogen

peroxide was large, i.e.,

, the oxidation rate

of HG2- can be simplified as: (9) or (10) Where of Eq. 10, one has:

. Integration

(11) By applying the minimum mean technique fitting the experimental results, the rate constants were calculate as the following:

Scheme II. Rate determining steps occurred between HG2- and OH radical on the anode surface. (12) (13) By applying the steady-state approximation technique on species adsorbed on 31

electrode surface (OH․, O2․-, H2O, and O2) and OH․, O2․- in the bulk, one has the oxidation rate of HG2- based on Eq.13 (Supporting Information S4): (14) If

, Eq. 14 can be simplified as: (15)

Where

.

Integration of Eq. 15, one has: (16)

The kinetics of dye degradation was fitted by Schemes I and II (Eq. 11 and 16) as shown in Fig.11. Results indicated that Scheme I, a first-order reaction, fitted better than Scheme II, which was a zero-order reaction. The fitting parameters, k1, k2 and k3, were obtained by the minimal residuals method. The fitted kinetic constants were: k1 = 4.91 10-5 min-1, k2 =5.93 10-3 min-1 and k3 = 5.71 10-3 min-1, for k1, k2 and k3, respectively. The rate constant (kods) of OG degradation was 4.42 10-3 min-1 which was closed to the 4.3 10-3 min-1 obtained from light absorbance measurements (see Fig. 9). (17)

32

1.1 1.0

[OG] (C/C0)

0.9 0.8 0.7 0.6 0.5 0.4 0.3

0

50

100

150

200

Time (min) Fig. 11. Initial OG photoelechemical degradation rate dependence on OG concentration. Points represents experimental results and line represents the fitting curves of the proposed model. Black line: Scheme I; Red line: Scheme II.

4. Conclusion TiO2 particles were coated on Ti mesh (TiO2/Ti) successfully by electrophoretic deposition method. The crack-free and stable TiO2/Ti film was prepared at an applied potential of 180 V for 1 min and annealed at 350oC for 1 h. The ZnO-TiO2 composite films were observed at lower EPD potential although the ZnO content in TiO2 layer was small. It has been demonstrated that the decolorization of OG increased in the order: PC < EO < PEC at lower pH value. The result indicated the first destruction of azo group (N=N) in OG structure by OH radicals; the aromatic rings are difficult to open. The results show that it is possible to generate OH radicals from photoassisted electrolysis of water by TiO2/Ti anode and production of hydrogen peroxide by oxygen reduction at 33

graphite cathode by controlling the applied current and dissolved oxygen content in solution.

Acknowledgement The financial support of this work by the Technology Development Program (101-EC-17-A-08-S1-208), Ministry of Economic Affairs of the Republic of China is gratefully acknowledged. References [1] P. Sarkar, H.R.D. De, J Mater Sci 39 (2004) 819-823. [2] T. Sauer, G. Cesconeto Neto, H.J. José, R.F.P.M. Moreira, Journal of Photochemistry and Photobiology A: Chemistry 149 (2002) 147-154. [3] D. Brown, H.R. Hitz, L. Schäfer, Chemosphere 10 (1981) 245-261. [4] R. Ganesh, G.D. Boardman, D. Michelsen, Water Research 28 (1994) 1367-1376. [5] A.G. Vlyssides, M. Loizidou, P.K. Karlis, A.A. Zorpas, D. Papaioannou, Journal of Hazardous Materials 70 (1999) 41-52. [6] D. Rajkumar, B.J. Song, J.G. Kim, Dyes and Pigments 72 (2007) 1-7. [7] J. Hastie, D. Bejan, M. Teutli-León, N.J. Bunce, Industrial & Engineering Chemistry Research 45 (2006) 4898-4904. [8] L.C. Almeida, S. Garcia-Segura, C. Arias, N. Bocchi, E. Brillas, Chemosphere 89 (2012) 751-758. [9] E. Brillas, I. Sirés, M.A. Oturan, Chemical Reviews 109 (2009) 6570-6631. [10] L.C. Almeida, S. Garcia-Segura, N. Bocchi, E. Brillas, Applied Catalysis B: Environmental 103 (2011) 21-30. [11] F. Kazuhito Hashimoto and Hiroshi Irie and Akira, Japanese Journal of Applied Physics 44 (2005) 8269. [12] L. Sun, T. An, S. Wan, G. Li, N. Bao, X. Hu, J. Fu, G. Sheng, Separation and Purification Technology 68 (2009) 83-89. [13] M.N. Chong, S. Lei, B. Jin, C. Saint, C.W.K. Chow, Separation and Purification Technology 67 (2009) 355-363. 34

[14] L. Zhang, T. Kanki, N. Sano, A. Toyoda, Separation and Purification Technology 31 (2003) 105-110. [15] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chemical Reviews 95 (1995) 69-96. [16] G. Centi, P. Ciambelli, S. Perathoner, P. Russo, Catalysis Today 75 (2002) 3-15. [17] N. Chandrasekharan, P.V. Kamat, The Journal of Physical Chemistry B 104 (2000) 10851-10857. [18] D.H. Kim, M.A. Anderson, Environmental Science & Technology 28 (1994) 479-483. [19] Y. Zhang, X. Xiong, Y. Han, X. Zhang, F. Shen, S. Deng, H. Xiao, X. Yang, G. Yang, H. Peng, Chemosphere 88 (2012) 145-154. [20] Y. Xin, H. Liu, L. Han, Y. Zhou, Journal of Hazardous Materials 192 (2011) 1812-1818. [21] L.C. Almeida, M.V.B. Zanoni, Journal of the Brazilian Chemical Society 25 (2014) 579-588. [22] X. Cheng, H. Liu, Q. Chen, J. Li, P. Wang, Electrochimica Acta 108 (2013) 203-210. [23] W.-C. Lin, C.-H. Chen, H.-Y. Tang, Y.-C. Hsiao, J.R. Pan, C.-C. Hu, C. Huang, Applied Catalysis B: Environmental 140–141 (2013) 32-41. [24] N.P. Benehkohal, G.P. Demopoulos, Journal of The Electrochemical Society 159 (2012) B602-B610. [25] N. Parsi Benehkohal, M.A. Gomez, R. Gauvin, G.P. Demopoulos, Electrochimica Acta 87 (2013) 169-179. [26] H. Bader, V. Sturzenegger, J. Hoigné, Water Research 22 (1988) 1109-1115. [27] H.C. Hamaker, Transactions of the Faraday Society 35 (1940) 279-287. [28] J.M. Heintz, O. Sudre, F.F. Lange, Journal of the American Ceramic Society 77 (1994) 787-791. [29] R.R. Bacsa, J. Kiwi, Applied Catalysis B: Environmental 16 (1998) 19-29. [30] T. Miyasaka, M. Ikegami, Y. Kijitori, Journal of The Electrochemical Society 154 (2007) A455-A461. [31] A.A. Griffith, Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 221 (1921) 163-198. [32] M.C. Biesinger, L.W.M. Lau, A.R. Gerson, R.S.C. Smart, Applied Surface Science 257 (2010) 887-898. [33] J.J. Lingane, Electroanalytical chemistry, [s.n.], New York [etc.], 1958. [34] Y.B. Xie, X.Z. Li, Materials Chemistry and Physics 95 (2006) 39-50. [35] Z. Liu, X. Zhang, S. Nishimoto, M. Jin, D.A. Tryk, T. Murakami, A. Fujishima, The 35

Journal of Physical Chemistry C 112 (2008) 253-259. [36] D. Jiang, T. Zhou, Q. Sun, Y. Yu, G. Shi, L. Jin, Chinese Journal of Chemistry 29 (2011) 2505-2510. [37] Z. Feleke, R. van de Krol, P.W. Appel, in: E. Yanful (Ed.), Appropriate Technologies for Environmental Protection in the Developing World, Springer Netherlands, 2009, pp. 181-196. [38] C. Galindo, P. Jacques, A. Kalt, Journal of Photochemistry and Photobiology A: Chemistry 130 (2000) 35-47. [39] N. Daneshvar, D. Salari, A.R. Khataee, Journal of Photochemistry and Photobiology A: Chemistry 157 (2003) 111-116. [40]H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard, J.-M. Herrmann, Applied Catalysis B: Environmental 39 (2002) 75-90. [41] L.C. Almeida, B.F. Silva, M.V.B. Zanoni, Chemosphere 136 (2015) 63-71. [42] J. Madhavan, F. Grieser, M. Ashokkumar, Ultrasonics Sonochemistry 17 (2010) 338-343.

36

Graphical abstract

37

Highlights 1. Photoelectrochemical degradation was effective in the treatment of dye wastewater. 2. TiO2/Ti electrode was prepared by electrophoretic deposition. 3. The photocatalytic and electrochemical reaction are synergistic in the degradation of OG 4. Graphite cathode can generate H2O2 to improve the PEC performance. 5. Degradation of OG was attacked by OH․ in the solution phase

38