High Oxygen Evolution Potential and Efficient ... - ACS Publications

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Distinctive Tin Dioxide Anode Fabricated by Pulse Electrodeposition: High Oxygen Evolution Potential and Efficient Electrochemical Degradation of Fluorobenzene Tao Wu, Guohua Zhao,* Yanzhu Lei, and Peiqiang Li Department of Chemistry, Tongji University, Shanghai 200092, China ABSTRACT: A distinctive Sb-doped SnO2 anode with a high oxygen evolution potential, 2.4 V vs the saturated calomel electrode, and a strong electrochemical oxidation ability was prepared on TiO2 nanotubes through the pulse electrodeposition method. Compared with the traditional Sb-doped SnO2 electrode prepared by the sol-gel method, the proposed SnO2 electrode has a higher crystallinity, a higher order degree of the atomic lattice, and a lower concentration of oxygen vacancies. The scanning electron microscopy image confirms that the surface of the electrode presents a three-dimensional structure consisting of Sb-doped SnO2 nanoparticles with a certain microspherical structure, which increases the specific area greatly and provides more active sites. The reaction activation energy also decreases from 11.67 kJ mol-1 for the traditional SnO2 electrode to 5.73 kJ mol-1. This SnO2 electrode is demonstrated to have a superior electrochemical oxidation ability for refractory fluorobenzene, which is extremely stable and cannot even be degraded effectively on a boron-doped diamond electrode with a strong oxidation capacity. The results also indicate that the distinctive SnO2 electrode has a higher apparent rate constant, total organic carbon removal, and mineralization current efficiency, which are 12, 2.6, and 3.3 times those of the traditional SnO2 electrode, respectively. The evolution of intermediates and the degradation mechanism of fluorobenzene were further discussed. This study provides a distinctive SnO2 anode for the effective electrochemical oxidation of refractory toxic organic pollutants.

1. INTRODUCTION Fluorobenzene (FB) is widely used in the pesticide, pharmaceutical, dyestuff, fluorocarbon surfactant, and fluorine plastic, etc. industries.1 A great deal of wastewater containing fluorobenzene is generated each year, resulting in seriously persistent pollution. Fluorobenzene can penetrate the human body through the skin and respiratory tract, irritating the eyes, skin, and respiratory tract. A long-term exposure to fluorobenzene may damage the liver, kidneys, lungs, and central nervous system.2,3 Fluorobenzene is considered bioinert,1 which makes it difficult to degrade by using general biological techniques. It is well-known that aromatic compounds with the structure of the benzene ring are very stable. Thus, the treatment of fluorobenzene by traditional chemical methods is also inconvenient. In recent years, electrochemical oxidation has been widely applied in wastewater treatment, which exhibits some significant advantages, such as versatility, high energy efficiency, easy handling, and environmental compatibility, especially for the degradation of toxic pollutants, which is difficult to deal with by biological techniques.4-7 However, the oxidation potentials of aromatic hydrocarbons are usually very high, e.g., 2.8 V (vs the saturated calomel electrode, SCE) for benzene, which make them very difficult to electrochemically oxidize directly by using common electrodes with a low oxygen evolution potential (OEP). Moreover, the fluorine atom has a small radius and a large r 2011 American Chemical Society

electronegativity. Thus, the length of the C-F bond is shorter and the bond energy (485 kJ mol-1) is much higher than those of the C-H bond.8,9 Meanwhile, the shielding effect of fluorine atoms makes the C-C bond much stronger and causes the stability of the binding sites with the fluorine atom to increase.1,10 Compared with benzene, fluorobenzene has higher thermal stability and chemical stability.11 Therefore, fluorobenzene is difficult to electrochemically oxidize directly. From the viewpoint of electrochemical oxidation and practical application, an electrode with good performance should have three characteristics: good conductivity, a high oxygen evolution potential,12,13 and an excellent electrochemical activity. A higher OEP can reduce the side reaction of oxygen evolution, leading to the improvement of the oxidation efficiency of contaminants and the reduction of energy consumption. The boron-doped diamond (BDD) film electrode has been widely used14-18 in recent years as an efficient anode material with a high OEP (2.4 V vs SCE). However, it possesses a larger resistance than noble metal and metal oxide electrodes, causing a higher cell voltage in degradation application.19-21 The BDD surface is chemically inert, and its electrocatalytic activity is low.22 In addition, the Received: October 22, 2010 Revised: January 9, 2011 Published: February 21, 2011 3888

dx.doi.org/10.1021/jp110149v | J. Phys. Chem. C 2011, 115, 3888–3898

The Journal of Physical Chemistry C

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Scheme 1. Schematic Illustration for the Preparation of the HOEP SnO2 Electrode

preparation of BDD is complicated and costly, especially for a large area, which also limits its application to some extent. The metal oxides have attracted attention widely as anode materials applied to the electrochemical oxidation of organic pollutants in wastewater. Among them, Sb-doped SnO2 is regarded as one of the most promising electrode materials for the degradation of refractory organic pollutants. It is well-known that pure SnO2 is an n-type semiconductor with a direct band gap of roughly 3.5 eV and cannot be used directly as an electrode material due to its low conductivity at normal temperature.23 The electrochemical properties of SnO2 can be enhanced by doping other metal ions such as Fe, F, Pt, and Ni, etc. Antimony is believed to perform the best among the doped elements.24 In addition, Sb at the proper doping level can improve the conductivity of SnO2 and has no significant influence on the oxygen evolution potential.23 Sb-doped SnO2 is prepared by a number of methods, including spray pyrolysis, the solution method, the vapor-phase transport method, and the sol-gel method, etc.25-31 It has a higher OEP, 1.8 V vs SCE, than other noble metal and metal oxide anodes such as Pt, RuO2, and IrO2, etc.24,32,33 Although the OEP of Sb-doped SnO2 is lower than that of BDD, it has a better electrochemical activity than BDD, especially toward phenol, benzoic acid, quinone, aromatic amines, ammonia, and other pollutants.34 Furthermore, the SnO2 electrode is prepared easily and inexpensively. The key point is to make a further improvement in the OEP of Sb-doped SnO2 and maintain its excellent electrochemical activity simultaneously, expecting an effective electrochemical oxidation of fluorobenzene. To the best of our knowledge, for the traditional Sb-doped SnO2 anode achieved on a Ti substrate by the sol-gel method, it is hard to obtain a higher OEP than 2.0 V vs SCE. Therefore, various attempts to improve the OEP of the traditional SnO2 electrode by microstructure design have been made. An intermediate layer with highly ordered and uniform TiO2 nanotube arrays prepared on a Ti substrate by anodic oxidation35-37 has

been constructed. With a high surface area and space utilization ratio, the TiO2 nanotubes (TiO2-NTs) could serve as an excellent microstructured tubular template which is conducive to the electrodeposition of SnO2 and the growth of SnO2 crystals. On this basis, a distinctive SnO2 electrode was constructed in this study by assembling SnO2 into TiO2-NTs. The pulse electrodeposition method was applied. Beforehand, TiO2 -NTs were partially reduced by the electrochemical method, and some copper was deposited at the bottom of the nanotubes, which could improve the combination between SnO2 and the Ti substrate and the conductivity of the electrode. Moreover, the intermediate layer was also favorable for the deposition of SnO2 with copper particles serving as seeds. Thus, a distinctive SnO2 electrode with a high oxygen evolution potential (HOEP), excellent electrical conductivity, and high electrochemical activity was obtained for the effective electrochemical oxidation of fluorobenzene. The microstructure of the distinctive SnO2 electrode was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The structure-effect relationship for the high OEP and electrochemical activity achieved on this distinctive SnO2 electrode was also elaborated. The oxidation mechanism of fluorobenzene, kinetics, and evolution of intermediates on the electrode were further discussed.

2. EXPERIMENTAL SECTION Preparation of the HOEP SnO2 Electrode. Titanium sheets were first mechanically polished with different abrasive papers and washed in twice-distilled water and acetone by ultrasonic washing. Prior to anodic oxidation, the titanium sheets were etched in 18% hydrochloric acid at 85 °C for 10 min. Anodization experiments were carried out at room temperature using a twoelectrode system (1 cm separation) using platinum foil as the 3889

dx.doi.org/10.1021/jp110149v |J. Phys. Chem. C 2011, 115, 3888–3898

The Journal of Physical Chemistry C counter electrode at the voltage of 20 V. The electrolyte consisted of sodium fluoride (0.8 wt %), sodium sulfate (1.6 wt %), polyethylene glycol (10 wt %), and twice-distilled water (87.6 wt %). After 3 h of anodic oxidation with stirring, the obtained TiO2-NTs, initially amorphous, were crystallized by annealing in an oxygen atmosphere for 1.5 h at 500 °C at both heating and cooling rates of 1 °C min-1. Partial reduction of the TiO2-NTs was carried out in 1 M (NH4)2SO4 at a potential of -1.5 V (vs SCE). A small amount of Cu was deposited at the bottom of the TiO2-NTs using a current pulsing method with a cathodic pulse (-70 mA, 10 ms), an anodic pulse (þ70 mA, 1 ms), and a relaxation time (0 mA, 1 s) in 1.5 M CuSO4 at 40 °C.38 The area of the electrode is 4.5 cm2. TiO2-NTs were vertically placed into a buffer bottle under vacuum conditions. The resulting vacuum-treated TiO2-NTs were immersed into the electrolyte containing 0.02 M SnCl2 3 H2O, 0.02 M SbCl3, and a certain concentration of hydrochloric acid for 5 min. After that, for SnO2 deposition, a current pulse deposition method with a cathodic pulse (45 mA, 10 ms), an anodic pulse (-45 mA, 1 ms), and a relaxation time (0 mA, 1 s) in 0.1 M Na2SO4 was applied at 40 °C for 15 min. In this step, Na2SO4 solution without Sn2þ and Sb3þ ions was used as an inert supporting electrolyte, which results in the deposition of a small quantity of SnO2 only within the pores by immediate deposition after immersion in this medium which could serve as seeds for the subsequent deposition. This procedure was repeated 2-3 times. Then the same method in the electrolyte containing 0.02 M SnCl2 3 H2O, 0.02 M SbCl3, and a certain concentration of hydrochloric acid was continued at 40 °C for 2 h. A 0.5 wt % ethoxyl(aminopropyl)trisiloxane solution was added to this mixture solution to lower the surface tension. The area of the electrode was 4.5 cm2. A schematic diagram for the design and preparation of the HOEP SnO2 electrode is shown in Scheme 1. The traditional SnO2 anode was prepared by the sol-gel technique according to the literature.39 The BDD electrode is made by chemical vapor deposition on a conductive monocrystalline silicon substrate with a doping of 1300 ppm boron (CSEM, Switzerland). The thickness of the obtained diamond film is about 1 μm. The conductivity is 1.1 10-4 S cm-1. Characterization. The morphology and crystal structure of the electrode were characterized by field emission environmental scanning electron microscopy (EFEG-SEM; model Quanta 200 FEG, manufacturer FEI) and XRD analysis (model D/max2550 VB3þ/PC, manufacturer Rigaku). XPS measurements were performed on a PHI 5000C ESCA system with a Mg KR source operated at 14.0 kV and 25 mA. All the binding energies were referenced to the C1s peak at 284.6 eV from the surface adventitious carbon. Electrochemical measurements were carried out in a conventional three-electrode cell at a CHI 660 electrochemical workstation (CHI, United States). An SCE served as the reference and Pt as the counter electrode. Polarization curves were measured to test the OEP of the electrodes, and the electrochemical impedances of the electrodes were measured by ac impedance curves. The electrochemical oxidation capacity for fluorobenzene was determined by the current density versus time (i-t) curve under 3.0 V in a 0.1 M Na2SO4 solution. The electrode activation energy (Ea) was measured according to the anodic polarization curves of electrodes at different temperatures in 0.1 M H2SO4, according to the law

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of Arrenhinus:40 log i ¼

Ea þ constant 2:303RT

ð1Þ

There is a linear relation between log i and 1/T, and Ea of the electrode can be determined by the slope of the linear fitting curve. Electrochemical Degradation and Analysis. The electrochemical degradation of fluorobenzene was carried out in a cylindrical single-compartment cell equipped with a magnetic stirrer and a jacketed cooler to maintain the reaction temperature at 25 °C. The HOEP SnO2, traditional SnO2, and BDD electrodes with an area of 6 cm2 served as the anodes, and a titanium foil with the same area was used as the cathode, with a gap between the electrodes of about 1 cm. A 100 mL volume of solution containing 50 mg L-1 fluorobenzene and 0.1 M Na2SO4 was used as the simulated wastewater. The current density was set at 20 mA cm-2. The samples at different times were analyzed. The total organic carbon (TOC) of the sample was determined using a TOC analyzer (TOC-Vcpn, Shimadzu). The intermediates formed during the anodic oxidation of fluorobenzene were monitored by high-performance liquid chromatography (HPLC; 1100, Agilent). An ultimate TMAQ-C18 column (5 μm, 4.6 100 mm) with a working wavelength of 210 nm was employed. In each experiment, phosphate buffer at pH 2.3 (cNaH2PO4 = 50 mmol L-1, cH3PO4 = 50 mmol L-1, VNaH2PO4: VH3PO4 = 1:2) was used as the mobile phase at a flow rate of 1 mL 3 min-1 and the injection volume was 20 μL. Prior to each analysis, the sample was filtered through a microporous filter (13 0.20 mm). The concentrations of the main intermediates were determined by comparing the area counts of the HPLC peaks at their retention times with those recorded in calibration analyses of aqueous solutions of the single compounds. The concentration of hydroxyl radicals (•OH) is determined with dimethyl sulfoxide (DMSO) trapping and HPLC according to the literature.41 The defluorinate rate was determined by measuring the concentration of F-. It was performed with a F- ion selective electrode (ISE). The first step was to make a calibration curve by recording the voltage values of standard concentrations of F-, from which the relationship between the concentration of F- and the voltage indicated by a pH meter could be obtained. On this basis, the concentration of F- in each sample could be determined. The qualitative determination of the intermediates was performed by GC/MS analysis. The GC/MS system consisted of a GC system (Varian cp3800 system) equipped with a WCOT fused silica series column (30 m 0.25 mm, film thickness 0.25 μm) interfaced directly to the mass spectrometer (Varian Saturn 2000). The GC column was operated at a temperature of 80 °C for 1 min, then increased to 250 °C at the rate of 15 °C min-1, and kept at that temperature for 15 min. The other experimental conditions were as follows: impact ionization 70 eV, helium as the carrier gas, injection temperature 280 °C, source temperature 80 °C. The whole reaction process of fluorobenzene (C6H5F) can be expressed as C6 H5 F þ 12H2 O f 6CO2 þ 29Hþ þ F- þ 28e-

ð2Þ

In the given time t, the mineralization current efficiency (MCE; %) of FB can be expressed as follows:42 3890



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Table 1. Parameters of the HOEP SnO2, Traditional SnO2, and BDD Electrodes HOEP SnO2

traditional SnO2

BDD

preparation method oxygen evolution potential (V vs SCE)

pulse electrodeposition 2.4

sol-gel 1.8

chemical vapor deposition 2.4 34500

electrochemical impedance(Ω)

190

380

cell voltagea (V)

6.2

7.3

9.3

current density without FB (mA cm-2)

13.3

11.5

10.8

current density with FB (mA cm-2)

20.5

14.1

15.2

current density increase (mA cm-2)

7.2

2.6

4.4

Ea (kJ mol-1)

5.73

11.67

8.02

The cell voltage was tested in 0.1 M Na2SO4 containing 50 mg L-1 FB, the gap between the electrodes was 1 cm, and the current density was 20 mA cm-2. a

MCE ¼

ΔðTOCÞexptl ΔðTOCÞtheor

100

ð3Þ

where Δ(TOC)exptl is the TOC experimental change in the t degradation time and Δ(TOC)theor (mg L-1) is the TOC theoretical change corresponding to the same time. The theoretical value is calculated by the following formula:42 ΔðTOCÞtheor

It nc M 103 ne F ¼ V

ð4Þ

where I is the current intensity (A), t is the electrolysis time (s), F is the Faraday constant (96485 C mol-1), ne is the number of electron transfers in eq 2, nc is the C number of the organic compounds in the degradation process, M is the atomic weight of C, M = 12 g mol-1, V is the volume corresponding to the degradation solution (L), and ne and nc are 28 and 6, respectively. The specific energy consumption (Ec, kW h m-3) is obtained as follows: Ec ¼

Ucell It 3600V

ð5Þ

where Ucell is the average cell voltage (V), I is the current (A), t is the electrolysis time (s), and V is the volume of the treated solution (dm3). All the experiments were repeated three times, and the result is the average value of the three parallel experiments.

3. RESULTS AND DISCUSSION High OEP and Electrochemical Activity of HOEP SnO2. Table 1 shows the OEP of the HOEP SnO2, traditional SnO2, and BDD electrodes in 0.1 M H2SO4 solution. The OEP of the HOEP SnO2 electrode is 2.4 V, which is much higher than that of the traditional SnO2 electrode and comparable with that of BDD. The high OEP of HOEP SnO2 is related to the surface composition and the oxygen vacancies. The XRD patterns of the HOEP SnO2 electrode prepared by the pulse electrodeposition method, the traditional SnO2 electrode prepared by the sol-gel method, and the SnO2/TiO2-NT electrode prepared by the sol-gel method are shown in Figure 1. SnO2 was shown to be in the rutile phase and TiO2 in the anatase phase, which is in good agreement with standard data of the JCPDS card. The diffraction peaks observed at 2θ = 25.5°, 37.9°, and 48.2° were assigned to the (101), (004), and (200) planes of anatase TiO2.,

and peaks corresponding to 2θ = 26.6°, 33.9°, and 51.8° were assigned to the (110), (101), and (211) planes of SnO2. The peak positions agreed well with the reflections of SnO2, indicating a tetragonal rutile structure. The average values of the lattice parameters can be calculated for different electrodes by Bragg’s formula43 according to the XRD patterns. The lattice parameters of SnO2 in HOEP SnO2 (a = b = 4.702 nm, c = 3.149 nm) and traditional SnO2 (a = b = 4.732 nm, c = 3.156 nm) are lower than those of the standard SnO2 (a = b = 4.738 nm, c = 3.187 nm). The intensities of the (110), (101), and (211) diffraction peaks for SnO2 of the traditional SnO2 electrode and the SnO2/TiO2-NT electrode have no significant difference, so there is no obvious preferred orientation of the grains. Contrastively, the intensity of the (110) diffraction peak for SnO2 of the HOEP SnO2 electrode is much stronger than that of (110) and (211), especially the faint (211), suggesting the highly preferred orientation along the (110) direction. XRD patterns of the three electrodes were compared. It can be seen that the intensities of the diffraction peaks for SnO2 of the traditional SnO2 and SnO2/TiO2-NT electrodes are weaker than that of HOEP SnO2, while the widths are broader, indicating that the growth of the SnO2 crystal is not complete and the grain size is smaller, so that the long-range ordered structure is difficult to form on the surface. Hence, the crystallinity of SnO2 on the traditional SnO2 electrode and the SnO2/TiO2-NT electrode is poor, and the degree of atomic lattice is low. Nevertheless, as mentioned above, the intensities of the diffraction peaks for SnO2 of HOEP SnO2 increase clearly and the widths become more narrow. Therefore, the HOEP SnO2 has excellent crystallinity, preferring the (110) orientation and highly ordered degree of atomic lattice, which are considered to be ascribed to the pulse electrodeposition. XPS measurements of the HOEP SnO2 and traditional SnO2 electrodes were carried out to analyze the surface compositions and chemical states of the electrodes. The binding energy values were measured with respect to the C1s peak of adventitious carbon (284.6 eV). Wide survey scans were recorded initially, followed by a detailed scanning of each element. The XPS investigations were focused mainly on the detailed analysis of O elements. Figure 2 shows the core level O1s spectra of the HOEP SnO2 (a) and the traditional SnO2 (b). They were fitted with the nonlinear least-squares fit program using GaussLorentzian peak shapes, and the deconvoluted values are given in Table 2. Two O1s peaks appeared after deconvolution. A lower binding energy component peak (1) with the BE values at about 530.0-530.3 eV and a higher component peak (2) at about 531.7-532.3 eV could be observed. The lower binding energy peak is assigned to the lattice oxygen species,41,43-46 and 3891



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Figure 2. XPS spectra of O1s on the surface of the HOEP SnO2 (a) and traditional SnO2 (b) electrodes.

Figure 1. XRD patterns of (a) the HOEP SnO2 electrode prepared by the pulse electrodeposition method, (b) the traditional SnO2 electrode prepared by the sol-gel method, and (c) the SnO2/TiO2-NT electrode prepared by the sol-gel method.

the higher binding energy peak may be ascribed to adsorbed hydroxyl oxygen species.42,45,47 The relative content (molar fraction) of different kinds of oxygen species can be estimated semiquantitatively by using the XPS peak area and sensitivity factor according to the following equation:48 ne1 Ie1 =Se1 ¼ ne2 Ie2 =Se2

ð6Þ

where n is the atomic number, e is an element, I is the XPS peak area, and S is the elemental sensitivity factor. In our case, both OL and Oab note the oxygen element. Therefore, the sensitivity

factors S1 and S2 are equivalent. Table 2 shows the XPS data of different chemical states of the O element on the surface of the HOEP SnO2 and traditional SnO2 electrodes. Compared with that for the traditional SnO2 electrode, the percentage of crystal lattice oxygen for the HOEP SnO2 electrode is lower. It is 67.45% for the HOEP SnO2 electrode and 74.48% for the traditional SnO2 electrode. Accordingly, the amount of adsorbed hydroxyl oxygen for the HOEP SnO2 electrode increases, from 25.52% for the traditional SnO2 electrode to 32.55% for the HOEP SnO2 electrode. In addition, the atomic number ratio of OL and Oad decreases from 2.92 for the traditional SnO2 electrode to 2.07 for the HOEP SnO2 electrode. These results are probably due to the higher degree of crystallinity and higher order degree of atomic lattice for HOEP SnO2 prepared by the pulse electrodeposition than for traditional SnO2, which results in less atomic defects in the SnO2 crystal. It is known that the predominant atomic defects in SnO2 crystal are oxygen vacancies.49 Hence, the concentration of oxygen vacancies for HOEP SnO2 is lower than that for traditional SnO2. It is considered that the high OEP of the HOEP SnO2 electrode is related to the production of lattice oxygen. The first common step for the oxygen evolution reaction on metal oxide electrodes is the discharge of water molecules on the metal oxide (MOx) surface to form adsorbed •OH, MOx(•OH).50,51 Then the adsorbed •OH may interact with the metal oxide anode, forming the so-called higher oxide MOxþ1. In this process, oxygen transfers from the •OH to the lattice of the metal oxide anode, becoming lattice oxygen. The oxygen evolution reactions of the SnO2 electrode are as follows:50 3892



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Table 2. XPS Data of Different Chemical States of the O Element on the Surface of the HOEP SnO2 and Traditional SnO2 Electrodes binding energy/eV O1s (OL)

O1s (Oad)

atomic number ratio of OL and Oad

OL content/%

Oad content/%

HOEP SnO2

530.3

532.3

2.07

67.45

32.55

traditional SnO2

530.0

531.7

2.92

74.48

25.52

sample

MOx þ H2 O f MOx ð• OHÞ þ Hþ þ e-

ð7Þ

MOx ð• OHÞ f MOx þ 1 þ Hþ þ e-

ð8Þ

MOx ð• OHÞ f MOx þ Hþ þ e- þ 1 =2 O2

ð9Þ

MOx þ 1 f 1 =2 O2 þ MOx

ð10Þ

The HOEP SnO2 electrode with a lower concentration of oxygen vacancies would provide less opportunities for oxygen atom transfer from adsorbed •OH to the crystal lattice and has a lower amount of crystal lattice oxygen.52 This means reaction 8 has less opportunity to occur to form MOxþ1; namely, there is less opportunity for oxygen atom transfer from adsorbed •OH to the crystal lattice. Contrastively, reaction 7 is preferred to occur to form more MOx(•OH). Oxygen evolution takes place through reactions 9 and 10. For the HOEP SnO2 electrode, reaction 9 occurs much more easily compared with reaction 10. This is also confirmed by the analysis of the XPS measurement. The amount of crystal lattice oxygen of the HOEP SnO2 electrode is less than that of the traditional SnO2 electrode, but the amount of adsorbed •OH is more than that of the traditional SnO2 electrode. Therefore, there are more MOx(•OH) molecules participating in reaction 9 and less MOxþ1 molecules participating in reaction 10. When reaction 9 mainly occurs on the electrode, it indicates a high oxygen evolution potential. Contrastively, when reaction 10 mainly occurs, the oxygen evolution potential is low.50,51 Thus, the oxygen evolution potential of the HOEP SnO2 electrode (as high as 2.4 V) is much higher than that of the traditional SnO2 electrode. The above results are further proved by the concentration of • OH generated on the electrodes (Figure 3). The concentration of •OH on HOEP SnO2 is higher than that on traditional SnO2, indicating that there are more adsorbed •OH radicals on HOEP SnO2. Therefore, on HOEP SnO2, more MOx(•OH) molecules participate in reaction 9 and the oxygen evolution potential is higher than that of the traditional SnO2. These results may also be related to the surface morphology of the electrodes. As shown in Figure 4a, the in situ grown TiO2NTs are uniform in size and vertically aligned with diameters ranging from 50 to 70 nm. The inset of Figure 4a shows that each nanotube is empty and looks like a “hollow bottle” with a smooth surface. Thus, those nanotubes could provide a large surface area and more free space for the deposition of SnO2. Figure 4b shows the top view of the traditional SnO2 electrode prepared by the sol-gel method. The nanoparticles arrange closely and disperse evenly on the surface of the Ti substrate, with sizes of about 2030 nm. The SEM image of the HOEP SnO2 electrode is shown in Figure 4c, indicating that the Sb-doped SnO2 nanoparticles display a certain microspherical structure and disperse regularly on the surface of the TiO2-NT substrate. The cross-sectional

Figure 3. Concentration evolution of hydroxyl radicals on the HOEP SnO2 and traditional SnO2 electrodes.

view of the HOEP SnO2 (inset of Figure 4c) further indicates that the SnO2 nanoparticles are introduced into the TiO2 tubes, and it also can be seen that the walls of the TiO2-NTs became rough due to the pulse electrodeposition of SnO2 nanoparticles. The predeposited Cu nanoparticles are difficult to observe at the bottoms of the nanotubes due to the very small deposition amount. However, the electron transfer resistance (Rct) obtained on the TiO2-NTs deposited only with Cu was 280 Ω, far less than 40 000 Ω on the TiO2-NTs, which is in accordance with ref 38. In addition, the above result indicates that there are Cu nanoparticles electrodeposited into the TiO2 tubes. The Sb-doped SnO2 nanoparticles are quite consistent in size and shape and are barely aggregated. A three-dimensional structure is constructed, which greatly increases the specific surface area and well improves the electrode surface conditions. Such a structure consequently enables the HOEP SnO2 electrode to possess the natural properties of TiO2-NTs and the coated tin oxide simultaneously, which is conducive to enhancing the performance of HOEP SnO2. Table 1 shows the electrochemical impedance spectroscopy (EIS) data of the HOEP SnO2, traditional SnO2, and BDD electrodes, which helps in gaining a better understanding of the electrochemical properties of the electrodes. Electron transfer resistance (Rct) values obtained on the three electrodes are 190, 380, and 34 500 Ω, respectively. The BDD electrode presents the largest Rct value due to its low conductivity, which makes it unsuitable for charge transfer between the electrode surface and the solution. The Rct value on the traditional SnO2 electrode is 380 Ω, far less than that of BDD, mainly due to the superior conductivity of Sb-doped SnO2. Compared with BDD and traditional SnO2, the Rct value is only 190 Ω on HOEP SnO2. The excellent conductivity of the HOEP SnO2 electrode may be due to the superior electrochemical activity of Sb-doped SnO2 and the conducting layer between the Ti substrate and the Sbdoped SnO2 electrocatalyst. The conducting layer consists of highly ordered and uniform TiO2 nanotube arrays with a poor 3893



Figure 4. SEM images of the TiO2-NT (a), traditional SnO2 (b), and HOEP SnO2 (c) electrodes. The insets are the corresponding crosssectional images.

conduction capacity (Rct of about 104 Ω). However, after reduction of the TiO2-NTs and deposition of a small amount of copper into the bottom of the TiO2-NTs, the conduction capacity of the layer is highly improved. Moreover, as mentioned above, the three-dimensional microstructure of the TiO2-NTs

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could provide a large specific surface and space utilization for the deposition of Sb-doped SnO2. Thus, the amount of the electrocatalysts on TiO2-NTs is more than that on the Ti substrate, which enhances the electrochemical properties of the electrode. Notably, the Sb-doped SnO2 can be electrodeposited into the TiO2-NTs, which is beneficial for contact with the conducting layer of the substrate. The conducting layer is greatly beneficial to increasing the conductivity of the HOEP SnO2 electrode because the introduced Cu nanoparticles on this layer can serve as seeds for further deposition of a large quantity of Sb-doped SnO2. Further concern focuses on the electrochemical property of the HOEP SnO2 electrode, which was studied through the change of the current density versus time. Table 1 shows the evolution of the current densities on the three electrodes at the applied potential of 3.0 V (higher than the OEP). It can be seen that the current densities of HOEP SnO2, traditional SnO2, and BDD are 13.3, 11.5, and 10.8 mA cm-2, respectively. The current density of HOEP SnO2 is the maximum, which is consistent with its minimum impedance. The current densities of HOEP SnO2, traditional SnO2, and BDD are all improved after addition of FB to the solution. On traditional SnO2, the current density increases to 14.1 mA cm-2, with a 22.6% increment. On BDD, the current density increases to 15.2 mA cm-2, with a 40.7% increment. Contrastively, on HOEP SnO2, the current density increases to 20.5 mA cm-2, with a much higher increment of 54.1%. The increase of the current density can be attributed to the electrochemical oxidation of FB on the electrodes. The current density increase on HOEP SnO2 is higher than that of traditional SnO2 and BDD, indicating its better electrochemical performance. The enhanced oxidation of HOEP SnO2 can be partially ascribed to the larger surface area of Sb-doped SnO2 in TiO2-NTs, which provides multiple nanostructures and more exposed active sites. It also should be noted that HOEP SnO2 has a higher degree of crystallinity than traditional SnO2, which results in a higher concentration of adsorbed •OH on the electrode surface as mentioned above. The large quantity of • OH also enhances the oxidation ability of HOEP SnO2. In addition, the electrode reaction activation energy (Ea) is obtained through measurement of anodic polarization curves of various electrodes in a sulfuric acid solution, and the values are shown in Table 1. Ea of HOEP SnO2 is 5.73 kJ mol-1, much lower than that of traditional SnO2 (11.67 kJ mol-1) and BDD (8.02 kJ mol-1), respectively. This also shows that HOEP SnO2 has excellent electrochemical performance. On the basis of the above results, it can be concluded that the better electrochemcal performance and enhanced oxidation ability of the HOEP SnO2 electrode are obtained due to the effects of TiO2-NTs and the pulse electrodeposition method. TiO2-NTs with a large surface area provide more free space for the deposition of Sb-SnO2. Meanwhile, pulse electrodeposition can result in a high dispersion of the electrocatalysts with multiple nanostructures and more exposed active sites, which possess better electrochemical properties and a higher oxidation capacity. In addition, a higher crystallinity and a higher order degree of atomic lattice can be obtained by pulse electrodeposition, which further lead to less oxygen vacancies and a higher concentration of •OH, providing a good oxidation efficiency for the organic pollutants. Besides, the electronic properties of Sbdoped SnO2 might also be modified by the nanotubular effect of TiO2-NTs, which still influence the electrochemical property of this electrode. 3894



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Figure 6. (a) MCE changes with time on the HOEP SnO2, traditional SnO2, and BDD electrodes. (b) Energy consumption changes with the TOC removal rate on the HOEP SnO2, traditional SnO2, and BDD electrodes.

Figure 5. (a) FB concentration changes with time on the HOEP SnO2, traditional SnO2, and BDD electrodes. The inset is the apparent rate constant changes with time. (b) F- concentration changes with time on the HOEP SnO2, traditional SnO2, and BDD electrodes. (c) TOC removal changes with time on the HOEP SnO2, traditional SnO2, and BDD electrodes.

Electrochemical Oxidation of FB. Figure 5a shows the concentration evolution of FB as a function of time. FB is almost converted on HOEP SnO2 at 2 h, with the removal of 99.8%. At the same time, the concentration of FB on BDD is 14.3 mg L-1, with the removal of 71.4%. There is still 29.6 mg L-1 FB unconverted on traditional SnO2, with the removal of only 59.2%. The reaction kinetics of FB on the three electrodes is represented in the inset of Figure 5a. It shows four linear curves, indicating an apparent pseudo-first-order reaction. The apparent rate constants (k) on BDD and traditional SnO2 are 1.05 10-4 and 5.57 10-5 s-1, respectively. The k on HOEP SnO2 reaches

6.73 10-4 s-1, which is 6.4 and 12 times that of BDD and traditional SnO2, respectively. The concentration evolution of F- is shown in Figure 5b. At 2 h, the concentration of F- on HOEP SnO2 reaches the maximum value, 0.446 mM, and then almost remains unchanged. On BDD and traditional SnO2, the concentrations are 0.145 and 0.065 mM, respectively, only 32.5% and 14.6% of that on HOEP SnO2. They increase with time. At 4 h, the concentrations of F- on BDD and traditional SnO2 remain 45.3% and 24.4% of that on HOEP SnO2. The TOC removal of FB on the three electrodes is shown in Figure 5c. The TOC removal on HOEP SnO2 is higher than that on traditional SnO2 and BDD during the whole process of degradation. At 3 h, the TOC removal is 90.6%, 60.1%, and 34.7% on HOEP SnO2, BDD, and traditional SnO2, respectively. The TOC removal on HOEP SnO2 is 1.5 times and 2.6 times that on BDD and traditional SnO2, indicating its higher electrochemical oxidation capacity. The MCE is another evaluation of the electrodes’ electrochemical degradation performance for the pollutants. As shown in Figure 6a, the MCE on HOEP SnO2 (16.2% at 1 h) is higher than that on BDD (8.03%) and traditional SnO2 (4.9%). At 4 h, the MCE values on HOEP SnO2, traditional SnO2, and BDD are 7.8%, 4.1%, and 5.6%, respectively; the value on HOEP SnO2 is 1.9 and 1.4 times that on traditional SnO2 and BDD. Therefore, the HOEP SnO2 has a higher efficiency in mineralizing pollutants. Figure 6b shows the energy consumption of different electrodes. It can be seen that the HOEP SnO2 consumes the lowest 3895



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Figure 7. Schematic degradation mechanism for FB electrochemical oxidation on the HOEP SnO2 electrode.

energy because of its lower cell voltage than BDD and traditional SnO2 (6.2, 9.3, and 7.3 V for HOEP SnO2, BDD, and traditional SnO2, respectively, at 20 mA cm-2) and stronger oxidation capacity for organic pollutants. The energy consumption of BDD is about 3.1 times that of HOEP SnO2 to reach TOC removal of 60%. Intermediate Evolution and Degradation Mechanism of FB. To determine the intermediate evolution during the electrochemical oxidation of FB, an electrolysis experiment was carried out by GC/MS and HPLC. The main aromatic intermediates of FB are p-fluorophenol, phenol, and benzoquinone, while the main carboxylic acid intermediates are maleic acid and oxalic acid. On the basis of these intermediates, the degradation mechanism of FB on HOEP SnO2 could be formulated (shown in Figure 7). The possible pathway mainly consists of three steps. First, FB is attacked by •OH; some forms p-fluorophenol, and some defluorinates to phenol. Then hydroxylation leads to the formation of catechol, resorcinol, and hydroquinone. Hydroquinone is subsequently dehydrogenated to benzoquinone. In the subsequent steps, the aromatic compounds undergo ring

Figure 8. Evolution of aromatic and carboxylic acid intermediate concentrations during FB electrochemical degradation.

cleavage, which leads to the formation of aliphatic acids, such as maleic acid, formic acid, and oxalic acid. The last step is the complete mineralization of these acids. The final products are 3896


The Journal of Physical Chemistry C carbon dioxide and water. However, we also consider that FB may not follow the aforementioned pathway strictly in the degradation process. It is possible that some reactions of different steps occurred simultaneously as parallel reactions. Furthermore, the possibility that FB was degraded through other pathways is not ruled out yet. For example, a small amount of FB may be directly converted into the final products CO2 and H2O. In addition, the degradation processes of FB on the traditional SnO2 and BDD electrodes were also studied. It is found that the main intermediates on the two electrodes are the same as that on the HOEP SnO2 electrode and the main possible pathways are similar to that of the HOEP SnO2 electrode. However, further research indicated that there existed a difference among the three electrodes, which was mainly reflected in the concentrations of intermediates accumulated on the electrodes. The concentration evolution of the intermediates on the three electrodes is further investigated (shown in Figure 8). The concentrations of phenol and benzoquinone increase at first and then decrease. The maximum concentration (Cmax) of phenol on HOEP SnO2, traditional SnO2 and BDD is 10.5, 8.1, and 7.6 mg L-1, respectively, while the accumulation concentrations of benzoquinone on the three electrodes are 2.5, 5.1, and 2.6 mg L-1, respectively. The concentrations of benzoquinone on HOEP SnO2 and BDD are especially low, due to their high oxidation capacities. The accumulated benzoquinone on the electrode can be quickly oxidized to carboxylic acid. This also shows that the time for thorough mineralization is shorter on HOEP SnO2. Cmax of maleic acid on HOEP SnO2 is 3.2 mg L-1, lower than that on traditional SnO2 (3.7 mg L-1) and BDD (3.5 mg L-1). For oxalic acid, Cmax is 9.4 mg L-1, also lower than that on traditional SnO2 and BDD, 12.4 and 11.6 mg L-1, respectively. Moreover, the time to reach Cmax and further mineralization is shorter for HOEP SnO2. These results indicate that HOEP SnO2 has a higher electrochemical degradation ability for aromatic intermediates.

4. CONCLUSION A distinctive SnO2 anode with a high oxygen evolution potential (2.4 V vs SCE) and a high electrochemical activity was constructed by pulse electrodeposition, assembling Sbdoped SnO2 into pretreated TiO2-NTs. XRD and XPS proved that the proposed SnO2 electrode had a higher crystallinity, a higher order degree of atomic lattice, and less oxygen vacancies compared with the traditional Sb-doped SnO2 electrode prepared by the sol-gel method, which resulted in the high oxygen evolution potential of this SnO2 electrode. Furthermore, the distinctive SnO2 electrode has a high electrochemical oxidation ability for the fluorobenzene pollutants, and the TOC removal at 3 h is 97.2%. The high efficiency of the distinctive SnO2 electrode was further confirmed by the generation and oxidation of the intermediates, and the degradation mechanism of fluorobenzene was further discussed. This research develops a new idea for investigating the electrode material with a high oxygen evolution potential and high electrochemical oxidation degradation ability for degradation of refractory toxic organic pollutants. ’ AUTHOR INFORMATION Corresponding Author

*Phone: (86)-21-65981180. Fax: (86)-21-65982287. E-mail: [email protected].

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’ ACKNOWLEDGMENT This work was supported jointly by the National Natural Science Foundation, People's Republic of China (Projects 20877058 and 21077077), and 863 Program (Project 2008AA06Z329) from the Ministry of Science and Technology of the People's Republic of China.

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