Sb-Doped SnO2 Electrode

Jan 20, 2009 - This Article puts forward a new idea to construct an electrode combining the advantages of a TiO2 nanotube photocatalyst and an excelle...
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J. Phys. Chem. C 2009, 113, 2375–2383

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Constructing Stake Structured TiO2-NTs/Sb-Doped SnO2 Electrode Simultaneously with High Electrocatalytic and Photocatalytic Performance for Complete Mineralization of Refractory Aromatic Acid Peiqiang Li, Guohua Zhao,* Xiao Cui, Yonggang Zhang, and Yiting Tang Department of Chemistry, Tongji UniVersity, Shanghai 200092, China ReceiVed: September 3, 2008; ReVised Manuscript ReceiVed: December 9, 2008

This Article puts forward a new idea to construct an electrode combining the advantages of a TiO2 nanotube photocatalyst and an excellent Sb-doped SnO2 electrocatalyst, which realized high electrocatalytic (EC) and photocatalytic (PC) oxidation efficiency at the same time. Under vacuum conditions, in virtue of the titanium oxide nanotubes (TiO2-NTs) as a template, well dispersed Sb-doped SnO2 (particle size 20 nm) was embedded into TiO2-NTs (diameter 60-90 nm and wall thickness 10-20 nm), resulting in the stake structured TiO2NTs/Sb-doped SnO2 electrode (TiO2-NTs/SnO2). The loading amount of Sb-doped SnO2 on a TiO2-NTs/SnO -2 2 electrode is 21.4 g m , which is increased by 2 times compared with the situation of direct loading Sbdoped SnO2 on the Ti substrate (Ti/SnO2). The crystal lattice parameter of SnO2 becomes smaller, and crystal lattice parameter of TiO2 is larger, so the combination between TiO2-NTs and Sb-doped SnO2 becomes more tight. Compared with the electrochemical properties of Ti/SnO2, the apparent rate constant of benzoic acid (BA) conversion (ks) on the TiO2-NTs/SnO 2 electrode is (1.44 ( 0.04) × 10-4 s-1 and that of Ti/SnO2 is (1.01 ( 0.03) × 10-4 s-1. Furthermore, its electrochemical stability and antideactivation properties are greatly improved, and the accelerated service lifetime of the TiO2-NTs/SnO2 electrode is increased by 1.0 time. The initial instantaneous current efficiency of the degradation of BA on the TiO2-NTs/SnO2 electrode is 26.8%, and that on the Ti/SnO2 electrode is 13.3%. Compared with the PC properties of TiO2-NTs, the band gap of TiO2-NTs/SnO2 decreases from 3.22 to 2.93 eV, and the photoconversion efficiency is raised to 26.1% from 8.2%. ks on TiO2-NTs/SnO2 is (0.82 ( 0.02) × 10-4 s-1, and that of TiO2-NTs is (0.41 ( 0.02) × 10-4 s-1. In the photoelectrocatalytic (PEC) aspect of BA, the current densities under 3.0 V increase by 4.0, 2.1, and 0.09 mA cm-2 on the TiO2-NTs/SnO2, Ti/SnO2, and TiO2-NTs electrodes, respectively. The initial instantaneous current efficiency of the TiO2-NTs/SnO2 electrode increases to 100%, which is much higher than 41.7% and 31.3% on Ti/SnO2 and TiO2-NTs electrodes, respectively. ks on TiO2-NTs/SnO2 is (5.26 ( 0.16) × 10-4 s-1, which is 3.2 times that of Ti/SnO2 and 4.8 times that of TiO2-NTs. The TiO2-NTs/SnO2 electrode has both excellent PC properties and excellent EC properties. After PEC degradation of BA on the electrode for 3.5 h, chemical oxygen demand (COD) removal is 100%. This research has enriched the PEC theory on the electrode’s microstructured interface and developed a new idea for exploring highly efficient PEC technology. 1. Introduction Photocatalytic (PC) oxidation and electrocatalytic (EC) oxidation, as the advanced oxidation technologies with distinguished catalytic characteristics, have different energy conversion forms. These years, photoelectrocatalytic (PEC) degradation technology that combines the advantages of the two technologies has been becoming the focus of research.1-10 The crucial technique in PEC is to prepare electrode materials which possess perfect optical and electrical catalytic properties simultaneously. Compared with TiO2 nanofilms, TiO2 nanotube arrays in situ grown on the surface of a pure Ti sheet by electrochemical anodic oxidation11-18 not only has a larger specific surface area and higher surface energy but also has a higher adsorption capacity and more active sites. Therefore, TiO2 nanotube arrays show higher PC efficiency than TiO2 nanofilms.1 Furthermore, in order to effectively separate photoproduced electrons-holes, the anodic bias voltage, which plays an electrically assisted photocatalysis role, is generally applied on the TiO2 electrode to improve the photoelectric conversion efficiencies.19-21 However, * Corresponding author. E-mail: g.zhao@ tongji.edu.cn. Telephone: +8621-65988570-8244. Fax: +86-21-65982287.

considering the low conductivity and poor electrocatalysis of TiO2 nanofilms and TiO2 nanotubes, they are not suitable to be used as the electrode material for electrochemical reaction. Moreover, the applied bias voltage is generally lower than the oxidation potentials of organic contaminants, which is not helpful for the electrode to perform EC degradation. As an excellent electrocatalyst, the oxygen evolution potential of Sb-doped SnO2 is as high as 1.8 V versus saturated calomel electrode (SCE),22,27,28 which can reduce the anodic oxygen evolution of water splitting. Therefore, the improvement of the oxidation efficiency of contaminants and the reduction of energy consumption can be obtained. Especially, it is suitable for the electrocatalytic oxidation degradation of organic contaminants including phenol, benzoquinone, aromatic compounds, and ammonia nitrogen contaminants.22-26 The mentionable thing documented is that toxic intermediates can be catalytically oxidized more rapidly and completely during the degradation process, and then a higher current efficiency can be obtained. Therefore, Sb-doped SnO2 is more suitable to serve as a highly efficient anode material than Pt, dimensional stable anode (DSA) electrodes.14,15,18 Additionally, the Sb-doped SnO2 electrocatalyst is easy to prepare with low cost. Owing to the above numerous

10.1021/jp8078106 CCC: $40.75  2009 American Chemical Society Published on Web 01/20/2009

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SCHEME 1: Schematic Illustration for the Growth of a TiO2-NTs/SnO2 Electrode

advantages possessed by the Sb-doped SnO2 electrode, application of the Sb-doped SnO2 electrode in environmental electrocatalysis has inspired great interest among environmental electrochemists. Sb-doped SnO2 was generally supported on the surface of a titanium substrate; however, the binding force between the titanium substrate and oxide active layer on the SnO2 electrode surface is so weak, resulting in the poor stability of the SnO2 electrode, which limits the practical application in electrochemical degradation. Researchers have tried to solve this problem by doping a noble metal in SnO2, but they face the decrease of the oxygen evolution potential and current density of the electrode.29,30 Therefore, a new method must be developed to improve the electrochemical stability of Sb-doped SnO2 and keep the excellent EC property and high oxygen evolution potential at the same time. On the basis of the understanding of the TiO2 photocatalyst and Sb-doped SnO2 electrocatalyst, a new idea was put forward to construct an electrode combining the advantages of the TiO2 nanotube photocatalyst and the excellent Sb-doped SnO2 electrocatalyst. High density, well-ordered, and uniform titanium oxide nanotubes are fabricated on the surface of a titanium substrate by electrochemical anodic oxidation, and then a SnO2 electrocatalyst capable of being penetrated by light source is implanted into TiO2 nanotubes. Thus, the stake structured TiO2 nanotube/SnO2 electrode is fabricated. High PC and EC efficiency is simultaneously realized on this electrode. As one of common organic contaminants in industrial waste water,31-33 benzoic acid (BA) is the secondary product of mellow, aldehyde and many complex organic pollutants. It is a kind of typical refractory oxidation organic contaminant owing to its high oxidation potential. Therefore, BA is selected as the target contaminant to carry on PEC oxidation degradation research. The separation efficiency of the photogenerated carrier of the resulting stake structured electrode in this research work was improved by the embedding of Sb-doped SnO2. The TiO2-NTs/ SnO2 electrode presents a higher photoelectric conversion efficiency than the TiO2-NTs electrode. Compared with the Ti/ SnO2 electrode, the loading amount of electrocatalyst on the TiO2-NTs/SnO 2 electrode was increased due to the high surface area and space utilization ratio of TiO2 nanotubes. Therefore, the antideactivation property of the electrocatalyst was improved and the higher electrochemical oxidation efficiency was obtained on the TiO2-NTs/SnO2 electrode. The PEC oxidation of BA on the TiO2-NTs/SnO2 electrode shows that BA was completely mineralized in a short time. This research work provides a new idea for exploring photoelectric integrated electrode materials

with high efficiency, which has important significance for the research and application on PEC oxidation of contaminants. 2. Experimental Section 2.1. Stake Structured TiO2-NTs/SnO 2 Electrode Preparation. The titanium sheets with a thickness of 1 mm and purity of 99.8% were first mechanically polished to a mirror image with 100# and 500# abrasive papers and washed in twicedistilled water and acetone by ultrasonic washing for 15 min, respectively. Prior to anodization experiments, the titanium sheets were etched in boiling 18 wt % hydrochloric acid for 10 min. All anodization experiments were carried out at room temperature using a two-electrode system with an interelectrode gap of 1 cm. A titanium sheet was used as the anode, and platinum foil was used as the counter electrode at the applied voltage of 20 V. The electrolyte consisted of 0.8 wt % NH4F, 1.6 wt % NaSO4, and 10 wt % PEG400. After 3 h anodization accompanied by stirring, they were rinsed with twice-distilled water and dried in a nitrogen stream. And then the initially amorphous TiO2-NTs were crystallized by annealing them in oxygen atmosphere for 1.5 h at 500 °C at both heating and cooling rates of 1 °C/min to obtain TiO2-NTs. The precursor consisted of 10 g of SnCl2 · H2O and 0.5 g SbCl3 using a mixture of 100 mL of ethanol and 10 mL of concentrated hydrochloric as solvents. Then 0.5 wt % ethoxyl aminopropyl trisiloxane solution was added into this mixture solution and aged for 8 h to obtain tin-antimony sol-gel. TiO2 nanotubes were vertically placed into a buffer bottle under vaccum conditions. The resulting vaccum treated TiO2 nanotubes were immersed into tin-antimony sol-gel for 5 min. After that, they were dried at 100 °C for 10 min and calcined in a furnace at 500 °C for 10 min. This procedure was repeated 10 times. Finally, the electrodes were annealed at 500 °C for 1 h to obtain TiO2-NTs/SnO2 electrodes. The Ti/SnO2 electrode was prepared by the same procedures except that the substrate was titanium foil. A schematic diagram of designing and preparing TiO2NTs/SnO2 electrode is shown in Scheme 1. 2.2. Characterization. The microstructure and morphology of the TiO2-NTs/SnO2 electrode, Ti/SnO2 electrode, and TiO2NTs electrode were characterized by X-ray diffraction (XRD, D/max2550VB3+/PC, Rigaku) using Cu KR radiation (0.15416 nm) and field-emission scanning electron microscopy (FE-SEM, Quanta200F, FEI). Their optical absorption property was measured using ultraviolet visible diffuse reflectance spectroscopy (UV-vis DRS, BWS002, BWtek). The amount of deposited Sb-doped SnO2 was determined by weighing the

Stake Structured TiO2-NTs/Sb-Doped SnO2 Electrode

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Figure 1. Electrode AC impedance spectroscopy in 5 mmol L-1 [Fe(CN)6]3-/[Fe(CN)6]4- solution: (a) TiO2-NTs/SnO2, (b) Ti/SnO2, and (c) TiO2-NTs.

electrode before and after the Sb-doped SnO2 deposition using a balance with the precision of 0.1 mg. 2.3. Electrochemical Property Test. Electrochemical measurements were performed on a CHI 660c electrochemical workstation (CHI Co.) using a conventional three-electrode cell system. The TiO2-NTs/SnO2 electrode, TiO2-NTs electrode, and Ti/SnO2 electrode were employed as working electrodes. A SCE served as the reference electrode and a Pt wire as the counter electrode. All the potentials were referred to SCE unless otherwise stated in this work. Electrochemical impedance spectrum (EIS) was tested in the electrolyte consisting 5 mM [Fe(CN)6]3-/[Fe(CN)6]4- by means of alternative current (AC) impedance with the frequency ranging from 1 × 105 to 1 × 10-4 Hz and an impedance amplitude of 5 mV. The dependence of current density on time was measured using a potentiostatic method in 0.1 M NaSO4 solution containing 140 mg L-1 BA. The accelerated life tests were performed by anodic polarization of the different electrodes at 100 mA cm-2 in a 0.1 M Na2SO4 solution. The anode potential was measured as a function of time, and the electrode was considered to be deactivated when the potential increased to 5 V from its initial value. 2.4. Photochemical Property Test. Photocurrent densities were measured on a CHI660c workstation with a TiO2-NTs/ SnO2 electrode, TiO2-NTs electrode, and Ti/SnO2 electrode as the photoanode, Pt foil as the counter electrode, and SCE as the reference electrode in 1.0 M KOH solution. A 15 W ultraviolet lamp (Shanghai Tianfu Lighting Co. China) was used as the UV light source to provide a light intensity of 9.0 mW cm-2. The photoconversion efficiency (η) from light energy to chemical energy in the presence of a bias potential is calculated as34

η (%) ) [(total power output electrical power input) ⁄ light power input] × 100 ) jp(Erev - |Eapp|) × 100 ⁄ I0 -2

(1)

where jp is the photocurrent density (mA cm ), jpErev is the total power output, jpEapp is the electrical power input, and I0 is the power density of the incident light (mW cm-2). Erev is the standard reversible potential (which is 1.23 V for the water splitting reaction at pH ) 0), and Eapp is the absolute value of the applied potential. which is obtained as Eapp ) Emeans - Eocp, where Emeas is the electrode potential of the working electrode at which jp was measured under illumination and Eocp is the applied potential at open circuit in the same electrolyte and under the same illumination.

Figure 2. SEM images of (a) TiO2-NTs electrode and (b) TiO2-NTs/ SnO 2 electrode.

2.5. PEC Oxidation Degradation of BA. The PEC oxidation of BA was carried out in a single circular electrochemical reaction cell which was externally connected to circulating water to keep the reaction at constant temperature of 25 °C. TiO2NTs/SnO2 electrode, TiO2-NTs electrode, and Ti/SnO2 electrode serving as the photoanode (electrode area 3.0 cm2) and Pt foil serving as the cathode were placed in parallel in the reactor with a separation of 1.0 cm, and a SCE served as the reference electrode. Simulant waste water consisted of 0.1 M Na2SO4 and 140 mg L-1 BA, and the volume of waste water was 50 mL. A 15 W ultraviolet lamp was used as the UV light source, providing a light intensity of 9.0 mW cm-2. The determination of the BA concentration was performed by HPLC (Agilent1100) with an AQ-C18 (5 µm, 4.6 × 100 mm) chromatographic column at 25 °C. The mobile phase was 1.0 mL min-1 methanol and water (v/v ) 30:70). The detection wavelength was 210 nm. Chemical oxygen demand (COD) was measured using dichromate method. The COD value was defined by O2 (mg L-1), so there were 4 mol electrons taking part in reaction when 1 mol O2 was consumed. According to the COD change value in dt time, the instantaneous current efficiency of the reaction of organic compounds on the electrode can be calculated as follows:35

ICE )

FV d(COD) 8000I dt

(2)

in which ICE is the instantaneous current efficiency (%), d(COD) is the COD variable quantity in dt time (mg L-1), t is

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TABLE 1: Parameters of the TiO2-NTs/SnO2, TiO2-NTs, and Ti/SnO2 Electrodes electrochemical impedance spectrum (kΩ) loading amount of Sb-doped SnO2 (g m-2) oxidation evolution potential (V) accelerated service lifetime (h) absorption band edge (nm) band gap (eV) photoconversion efficiency (%) current density (EC) (mA cm-2)a current density (EC + BA) (mA cm-2)a current density (PEC + BA) (mA cm-2)a a

TiO2-NTs/SnO2

Ti/SnO2

TiO2-NTs

0.59 21.4 1.80 45 424 2.93 26.1 14.70 16.99 21.04

3.85 9.3 1.64 22

31.19

12.40 13.90 15.90

385 3.22 8.2 0.007 0.01 0.11

Current density is tested in 0.1 M Na2SO4 solution under 3.0 V potential.

electrolysis time (s), I is current (A), V is electrolyte volume (L), and F is the Faraday constant (C mol-1). All the experiments in our work were repeated three times, and the result is the average value of the three parallel experiments. 3. Results and Discussion 3.1. Electrochemical Property. Figure 1 shows EIS of TiO2NTs/SnO2, TiO2-NTs, and Ti/SnO2 electrodes. Electron transfer resistance (Ret) obtained on the three electrodes are 0.59 kΩ (Figure 1a), 3.85 kΩ (Figure 1b), and 31.19 kΩ (Figure 1c), respectively. The TiO2-NTs electrode presents the largest Ret value due to the low conductivity of TiO2-NTs. The large Ret makes it unsuitable for charge transfer between the electrode surface and solution. Thus, the Faraday current between the TiO2-NTs electrode and [Fe(CN)6]3-/[Fe(CN)6]4- solution is the least (Figure S1 of the Supporting Information). The Ret on the Ti/SnO2 electrode is 0.1 times that of TiO2-NTs, mainly due to the superior conductivity of Sb-doped SnO2. Compared with TiO2-NTs and Ti/SnO2, the Ret is 0.59 kΩ on TiO2-NTs/ SnO2, and it decreases dramatically upon Sb-doped SnO2 electrocatalyst being embedded into TiO2-NTs. The excellent conductivity of the TiO2-NTs/SnO2 electrode may be due to the well dispersion of Sb-doped SnO2 electrocatalyst inside the TiO2-NTs. One of the advantages of the SnO2 electrode is the high oxygen evolution potential. As shown in Table 1, the oxygen evolution potential on the TiO2-NTs/SnO2 electrode is 1.80 V in 0.1 M Na2SO4, greater than that of 1.64 V on the Ti/SnO2 electrode (Figure S2 of the Supporting Information), indicating that the introduction of TiO2-NTs on the Ti substrate can increase the oxygen evolution potential of the traditional SnO2 electrode. Because the Sb-doped SnO2 can enter the inside of the TiO2-NTs electrode by the vacuum method, the Ti substrate and Sb-doped SnO2 can be conjugated more firmly and evenly, resulting in the increase of the TiO2-NTs/SnO2 electrode’s oxygen evolution potential. To test the stability of TiO2-NTs/SnO2 and Ti/SnO2 electrodes, the accelerated service lifetime experiment was performed. As seen from Table 1, the accelerated lifetime for Ti/ SnO2 electrode is only 22 ( 1 h. Under the same conditions, that for the TiO2-NTs/SnO2 electrode reached 45 ( 1 h, nearly 2 times that of the Ti/SnO2 electrode. The poor stability of the Ti/SnO2 electrode is due to the generation of random TiO2 during the pyrolysis process, which results in an uneven interface between the Ti substrate and tin-antimony oxide. Therefore, the oxide layer will exfoliate and lose catalytic activity when a strong current is imposed on it. Thus, uneven stressing on different sites occurred. The following reasons can explain the stability increase of the TiO2-NTs/SnO2 electrode.

One is that the stake structure comprising SnO2 and TiO2NTs on the electrode surface results in the increase of the service lifetime. As the SEM image of TiO2 nanotubes shows, the in situ grown TiO2 nanotubes are uniform in size and vertically aligned. The tube diameter ranges from 60 to 90 nm (Figure 2a), and the thickness of the tube wall is from 10 to 20 nm. The shapes of the tubes are diverse, such as round and elliptical. This kind of highly ordered TiO2 nanotubes present a large surface area and high specific volume. The Sb-doped SnO2 particle size is about 20 nm, smaller than the nanotube’s diameter, which ensures that the Sb-doped SnO2 particles can enter the nanotube and thus the even and ordered dispersion of Sb-doped SnO2 may be obtained. On the other hand, the tin-antimony oxide sol-gel with fine particle size and ultralow surface tension can be embedded into the inner part of TiO2 nanotubes. When TiO2 nanotubes are vacuumized to 6 × 10-2 Pa, the air in the nanotubes can be driven away, and then the tin-antimony oxide sol-gel can enter into the inside of the nanotubes unhindered. As is known, ethoxyl amino propyl trisiloxane with high surface activity arranging at the interface contains large amounts of methyl groups, through which the water surface tension can be decreased to 21 mN m-1. It can only be decreased to 30 mN m-1 by the traditional surfactant. Therefore, it is easy for trisiloxane to wet and spread on the low-energy hydrophobic interface. When trisiloxane is added into the tin-antimony oxide sol-gel, the sol-gel can totally spread on the TiO2-NTs and the contact angle is zero owing to the ultralow surface tension (Figure S3 of the Supporting Information). The second is that the loading amount increase results in the increase of the service lifetime. The loading amount of Sb-SnO2 on the TiO2-NTs/SnO2 electrode is improved to 21.4 ( 0.6 g m-2 which is 2.4 times of that 9.3 ( 0.3 g m-2 on the Ti/SnO2 electrode. And it results in the increase of the electrochemical active ingredient; thus, the anti-inactive properties of the TiO2NTs/SnO2 electrode are enchanced. The third is that it can be further confirmed by X-ray diffraction results. To further explore the essence of the TiO2NTs/SnO2 electrode’s stability, XRD was employed to analyze its microstructure. The XRD spectra of the TiO2-NTs, TiO2NTs/SnO2, and Ti/SnO2 electrodes are shown in Figure S4 of the Supporting Information. It can be seen that TiO2 with rutile and anatase phases is found, which is not surprising because at 500 °C rutile and anatase phases coexist. As seen from the XRD of the TiO2-NTs/SnO2 electrode and SnO2 electrode, only the peak positions which agree well with the reflections of SnO2 with a rutile type structure are present. Meanwhile, no peaks corresponding to antimony oxides are detected. Table 2 shows the average values of the lattice parameters (a ) b) obtained for different electrodes calculated by using Bragg’s formula. It

Stake Structured TiO2-NTs/Sb-Doped SnO2 Electrode

Figure 3. UV-vis DRS spectra of for TiO2-NTs/SnO2 and TiO2-NTs electrodes under UV light irradiation.

TABLE 2: Lattice Parameters of TiO2-NTs/SnO 2 and Ti/SnO2 Electrodes lattice parameter SnO2 TiO2

a)b c a)b c

TiO2-NTs/SnO2

Ti/SnO2

4.713 3.152 3.786 9.871

4.732 3.156

standard lattice parameter 4.738 3.187 3.785 9.514

is observed from Table 2 that the lattice parameters of TiO2NTs/SnO2 are all smaller than those of Ti/SnO2, which probably represents that the TiO2-NTs formed on the Ti substrate enable SnO2 and Sb2O5 to intermix more firmly. Furthermore, the Sb5+ ions (ionic radius 0.78 Å) are smaller than the Sn4+ ions (ionic radius 0.83 Å), so the two kinds of oxides appear in a form of solid solution.36 It is also noticed from Table 2 that the TiO2 lattice parameters in the TiO2-NTs/SnO2 electrode are larger than those in TiO2-NTs. The reason can be explained from the aspect of the ionic radius. The Ti4+ ions (ionic radius 0.68 Å), which are smaller than Sn4+ ions, are doped in the SnO2 unit cell. Conversely, the increase of the TiO2 lattice parameters indicates that Sn4+ enters into the TiO2 lattice. Therefore, it is assumed that SnO2 and TiO2 intermix mutually in their lattice. So, the TiO2-NTs can make the Sb-doped SnO2 and Ti substrate combine more tightly and firmly by the intermingling of the SnO2 and TiO2 lattice, which is also a main cause for the TiO2NTs/SnO2 electrode’s enhanced stability. Meanwhile, according to the Scherrer formula,37 the average crystal size of Sb-doped SnO2 measured by XRD is 20 nm on the TiO2-NTs/SnO2 electrode. Hence, the grown Sb-doped SnO2 oxides are small in size and dispersed in high degree (Figure 2b) due to the restriction of the TiO2-NTs structure, resulting in the great increase of active sites, which is helpful for the TiO2-NTs/SnO2 electrode to increase its service lifetime. 3.2. Photochemical Property. Figure 3 shows the UV-vis diffuse reflectance spectra (DRS) of the TiO2 nanotube arrays and TiO2-NTs/SnO2 electrode. In the UV light region, TiO2NTs present a continuous wide absorption band. The maximum absorption peak locates at 300 nm, the absorption band edge is at about 385 nm, and the band gap is about 3.22 eV according to Eg ) 1240/λg.38 For the TiO2-NTs/SnO2 electrode, the continuous UV absorption band is 80 nm wider (from 320 to 400 nm) than that of TiO2-NTs. In the λ < 320 nm region, the absorption intensity of the TiO2-NTs/SnO2 electrode is lower than that of the TiO2-NTs electrode owing to the doping of Sb (which is hard to be penetrated by UV light) on the electrode surface. In the λ > 320 nm region, TiO2-NTs/SnO2 electrode’s

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Figure 4. Variation of photocurrent density with bias potential (vs SCE) in 1 M KOH solution on TiO2-NTs/SnO2 and TiO2-NTs electrodes under UV light irradiation: (a) TiO2-NTs electrode, no light; (a′) TiO2NTs electrode, UV; (b) TiO2-NTs/SnO2 electrode, no light; and (b′) TiO2-NTs/SnO2 electrode, UV.

absorption intensity is obviously improved and even higher than that for TiO2-NTs, indicating that the TiO2-NTs/SnO2 electrode is more sensitive to the light source near the UV region than TiO2-NTs. Compared with TiO2-NTs, the absorption edge of TiO2-NTs/SnO2 is red-shifted to 424 nm with the band gap decreased to 2.93 eV, indicating that the impregnation of Sbdoped SnO2 exerts an impact on the Ti and O atomic chemical state in the TiO2 crystal lattice and causes the decrease of the band gap energy. Figure 4 shows the photocurrent density variations of TiO2NTs/SnO2 and TiO2-NTs electrodes with potential in 1 M KOH solution. The dark current density on the TiO2-NTs/SnO2 electrode is 0.4 mA cm-2, while that on TiO2-NTs is 0.02 mA cm-2, demonstrating that the conductivity of TiO2-NTs/SnO2 is increased by the implanting of Sb-doped SnO2 electrocatalyst, which agrees with the AC impedance results very well. The saturated photocurrent densities of TiO2-NTs/SnO2 (1.3 mA cm-2) is about 1.3 times as much as that of the nondoped one (0.99 mA cm-2). Moreover, the saturated photocurrent densities are obviously improved over the dark current densities. The results prove our assumption, that is, an elevated photocurrent density is obtained on the TiO2-NTs/SnO2 electrode for its higher light response in UV light region. Apart from the enhancement of light absorption, the implanting of Sb-doped SnO2 electrocatalyst might also contribute to the photocurrent produced because the doping of SnO2 and TiO2 could facilitate the separation of photogenerated electrons and holes. The above results are further supported by the data of photoconversion efficiency (Figure 5). The photoconversion efficiency of the TiO2-NTs/SnO2 electrode increases to 26.1%, which is 3.18 times as much as 8.2% of the TiO2-NTs. The result further confirms that the modification of TiO2-NTs by doping SnO2 intensified the photoconversion efficiency in the UV light region. Calculation analysis of electrons and holes separation is further carried out. Figure 6 is the analysis of the above photochemical experimental phenomenon. SnO2 and TiO2 intermixing in the stake structured TiO2-NTs/SnO2 electrode is a microstructure scale doping. The band gap of SnO2 (3.88 eV) is greater than that of TiO2 (3.22 eV), so it is reverse phase doping for SnO2 and TiO2 according to the energy band theory, which is suitable for the photogenerated separation of electrons and holes. Upon band gap excitation, as it is more sensitive, TiO2 is excited first, and electron/hole pairs are generated. The valence band of SnO2 (+3.67 V) is much higher than that of TiO2 (+2.87 V), and the conduction band (CB) edges of SnO2 and TiO2 are suitated at -0.34 and +0.07 V, respectively, versus

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Figure 5. Photoconversion efficiency as a function of applied potential (vs SCE) in 1 M KOH solution under UV light irradiation: (a) TiO2NTs and (b) TiO2-NTs/SnO2.

the normal hydrogen electrode.39-41 In terms of energetics, photogenerated electrons flow into the SnO2 intermediate layer, which correspondingly changes the bottom of the conduction band (BCB), decreasing the band gap of the TiO2 nanotubes. This fact can be confirmed by UV-vis measurement results, so it is favorable for the TiO2 to be more excited. While holes oppositely diffuse into the TiO2 nanotube peripheral layer, more holes reach the TiO2 nanotube surface, causing oxidation reaction. Hence, there would be more electrons transferring to SnO2, causing reduction reaction with O2 at the SnO2 surface (Figure 6b). Thus, the interfacial electron transfer from TiO2 nanotubes to SnO2 can effectively separate the electrons and holes, inhibiting the recombination of electron/hole pairs. Simultaneously, SnO2, as the “nano-stake”, disperses well into TiO2 nanotubes, having excellent conductivity. Just like numerous “nanoconducting wires”, those can timely and rapidly export the electrons out, which is favorable for separating the electrons and holes. Moreover, due to the template function of TiO2-NTs, the contact mode of the TiO2-NTs/SnO2 electrode is that TiO2NTs peripherally wrap the SnO2, while the Ti/SnO2 electrode is just SnO2 on the top of the TiO2 film, which is very different.42 The TiO2-NTs/SnO2 electrode improves the contact area, so electrons can be more rapidly and completely transferred from TiO2-NTs to SnO2. Numerous TiO2-NTs and SnO2 matrix combination is just as numerous separator of photogenerated electrons and holes, where the pairs are separated efficiently, avoiding the recombination of electrons and holes. Therefore, it improves photocoversion efficiency. 3.3. PEC Oxidation BA. The current density comparison during the photoelectrolysis of BA is shown in Figure 7. As can be seen, in 0.1 M Na2SO4 solution containing no BA, the current densities on three electrodes under the applied potentials of 1.4 and 3.0 V are sequenced as follows: TiO2-NTs/SnO2 > Ti/SnO2 > TiO2-NTs. The TiO2-NTs/SnO2 electrode is most suitable to be used as an electrode material. The current densities increase greatly under 3.0 V because the electrode was up to oxygen evolution potential. Adding 140 mg L-1 BA to 0.1 mol L-1 Na2SO4 (0.1M), at the potential of 1.4 V, which is lower than the oxidation potential of BA, the current densities increase by 0.027, 0.006, and 0.005 mA cm-2 on the TiO2-NTs/SnO2, Ti/SnO2, and TiO2-NTs electrodes, respectively. This shows that the TiO2-NTs/SnO2 electrode is good at indirect EC oxidation. At 3.0 V potential that is higher than the oxidation potential of BA, the current densities increase by 16.99, 13.90, and 0.01 mA cm-2, respectively. The current density increases greatly on TiO2-NTs/SnO2 and Ti/SnO2 electrodes comparing with the current density under 1.4 V. It shows that the TiO2-NTs/SnO2

Li et al. electrode has the best direct EC oxidation ability. And the EC oxidation ability is bad on the TiO2-NTs electrode according to this research. The current densities increase by 0.062, 0.010, and 0.009 mA cm-2, respectively, on the TiO2-NTs/SnO2, Ti/ SnO2, and TiO2-NTs electrodes under 1.4 V on adding UV light irradiation. It shows that the TiO2-NTs/SnO2 electrode is good at PC oxidation. The current densities increase by 21.04, 15.90, and 0.11 mA cm-2, respectively, on the TiO2-NTs/SnO2, Ti/ SnO2, and TiO2-NTs electrodes under 3.0 V on adding UV light irradiation. The TiO2-NTs/SnO2 electrode also has the best PC effect. The increasing of current density is greater on the TiO2NTs/SnO2 electrode under 3.0 V because of the good efficiency on separating electrons and holes. Therefore, more electrons and holes can transfer to the electrode surface to perform redox reaction. Meanwhile, the good PC efficiency can remove the mediate products fast, avoiding the passivation of electrochemical active sites. Figure 8a shows the evolution of BA with consumed time at three different catalytic conditions. The concentration of BA is 49.5, 64.7, and 74.0 mg L-1 on the TiO2-NTs/SnO2, Ti/SnO2, and TiO2-NTs electrodes, respectively, for 3.5 h in the PC process. BA is converted quickly to the intermediates, which is favorable to continue the PC process on the TiO2-NTs/SnO2 electrode. However, the concentration of BA is 24.0, 40.5, and 83.6 mg L-1 on the above three electrodes for 3.5 h in the EC process. It can be seen that the TiO2-NTs/SnO2 electrode is optimal for the degradation of BA. In the PEC process, BA is completely mineralized on the TiO2-NTs/SnO2 electrode, while there is 19.7 mg L-1 BA on the Ti/SnO2 electrode and 35.6 mg L-1 BA on the TiO2-NTs electrode. The above results show that the TiO2-NTs electrode is optimal for oxidation of BA in all processes including PC, EC, and PEC processes. The data for BA concentration decay were further analyzed by kinetic equations with different reaction orders. Good linear plots were only obtained when fitted to a pseudo-first-order reaction (Figure 8b). The apparent rate constant of BA degradation ks on TiO2NTs/SnO2 is (5.26 ( 0.16) × 10-4 s-1 in the PEC process (Table 3), which is 3.2 times that of Ti/SnO2 and 4.8 times that of TiO2-NTs; it also supports the conclusion. Additionally, the mineralization of BA in the PEC process is higher than the summation of the individual PC and EC methods (Figure 8a, inset). That is to say, there is coordinated interaction between PC and EC oxidation,43-45 and the good linear relationship between ln(CBA) and time (Figure 8b) just shows that the TiO2NTs/SnO2 electrode simultaneously has high photocatalytic and electrocatalytic performance rather than a single photocatalytic or electrocatalytic function. Generally, in the catalytic oxidation process, on one hand, BA and intermediates are adsorbed on the electrode which leads to the decrease of the adsorption of UV. On the other hand, the absorption of them on the electrode can result in the fouling of the electrocatalytic sites. The two aspects result in the drop of oxidation efficiency. Oppositely, the intermediates are removed on the electrode in the PC process, which can enhance the adsorption of UV and accelerate the PC process. Similarly, the hole and the catalytic radicals produced in the PC process are helpful to remove the intermediates, thus alleviating the fouling of the electrode. The enhancement of the EC process is realized. So, we can draw the conclusion that the mineralization of BA in the PEC process is higher than the combination of the PC process and the EC process. COD was studied when large amounts of BA were photoelctrocatalyzed. The COD removal of BA with time is shown in Figure 9 on different electrodes. The experiments were

Stake Structured TiO2-NTs/Sb-Doped SnO2 Electrode

J. Phys. Chem. C, Vol. 113, No. 6, 2009 2381

Figure 6. (a) Profile schematic diagram of photoelectron transfer mechanism for Sb-doped TiO2-NTs/SnO2 electrode under UV light irradiation. (b) Electron and hole transfer on the TiO2-NTs/SnO2 electrode surface.

Figure 7. Comparison of current density on TiO2-NTs/SnO2, Ti/SnO2, and TiO2-NTs electrodes to the injection of BA in 0.1 M Na2SO4 at 1.4 and 3.0 V potential.

conducted at a constant current density of 20 mA cm-2 for 3.5 h. In the individual PC process, 42.4% COD removal of BA on the TiO2-NTs/SnO2 electrode is higher than that on the Ti/SnO2 and TiO2-NTs electrodes corresponding to 32.8% and 26.7% respectively. It indicates that the PC of BA is enhanced when the SnO2 electrocatalyst is embedded into the TiO2-NTs. It has the same trend in the EC process. The highest COD removal of BA which is 65.8% is obtained on the TiO2-NTs/SnO2 electrode, and the others are 47.4% on the Ti/SnO2 and 22% on the TiO2-NTs electrode. It illustrates that the SnO2 electrocatalyst can be orderly and uniformly dispersed into the high ordered TiO2-NTs template, which leads to the enhancement of the EC ability of the electrode. COD removal of 100% is reached on the TiO2-NTs/SnO2 electrode during the PEC process, which indicates complete mineralization of BA and its intermediates. The TiO2-NTs/SnO2 electrode is optimal in the PEC process as the PC and EC processes are interacted positively. The results of COD deeply confirm TiO2-NTs/SnO2 as the integrative electrode with high PC and EC efficiency. Fitting of the COD variation curve allows one to obtain the change rate of COD over time (dCOD/dt) by integrating the fitting equation. The instantaneous current efficiency (ICE) can

Figure 8. (a) Variation of BA concentration with electrolysis time on different anodes and (b) variation of ln(CBA) with electrolysis time on different anodes (TiO2-NTs/SnO2, black line; Ti /SnO2, red line; and TiO2-NTs, blue line). (1) TiO2-NTs, EC; (2) TiO 2-NTs, PC; (3) Ti/ SnO2, PC; (4) TiO2-NTs/SnO2, PC; (5) Ti/SnO2, EC; (6) TiO2-NTs, PEC; (7) TiO2-NTs/SnO2, EC; (8) Ti/SnO2, PEC; (9) TiO2-NTs/SnO2, PEC; and (10) TiO2-NTs/SnO2, PEC theoretical value (summation of individual PC and EC processes).

then be obtained via eq 2. In the EC oxidation process of BA, the oxygen evolution reaction is the main side effect, so the current efficiency will decrease when the organic compound concentration decreases and the oxygen evolution reaction rate increases. Figures 9 and 10 show that the COD value decreased

2382 J. Phys. Chem. C, Vol. 113, No. 6, 2009

Li et al. 4. Conclusion

Figure 9. Variation of COD removal rate with electrolysis time during oxidation of BA for different anodes (TiO2-NTs/SnO2, black line; Ti /SnO2, red line; and TiO2-NTs, blue line): (1) TiO2-NTs, EC; (2) TiO2NTs, PC; (3) Ti/SnO2, PC; (4) TiO2-NTs/SnO2, PC; (5) Ti/SnO2, EC; (6) TiO2-NTs, PEC; (7) TiO2-NTs/SnO2, EC; (8) Ti/SnO2, PEC; (9) TiO2-NTs/SnO2, PEC; and (10) TiO2-NTs/SnO2, PEC theoretical value.

Under vacuum conditions, the stake structured TiO2-NTs/ SnO2 electrode is constructed by means of implanting a tin-antimony oxide sol-gel with super wetting and spreading properties into the TiO2-NTs. The electrode constructing method not only formed the stake structured electrode by making the TiO2-NTs as template but also obtained high PC and EC performance simultaneously on the TiO2-NTs/SnO2 electrode. And the conversion of BA during the PEC process is higher than the summation of that in the PC and EC processes; it realizes synergistic effects of electrocatalysis and photocatalysis on the TiO2-NTs/SnO2 electrode. After PEC degradation of BA on the electrode for 3.5 h, COD removal is 100%. It simultaneously realizes high EC and PC oxidation efficiency on the TiO2-NTs/SnO2 electrode. This research has enriched the PEC theory on the electrode’s microstructured interface and developed a new idea for exploring highly efficient PEC technology. Acknowledgment. This work was supported jointly by the National Nature Science Foundation P.R. China (Project Nos. 20877058and20577035),863Program(ProjectNo.2008AA06Z329) from the Ministry of Science, and Nanometer Science Foundation of Shanghai (Project No. 0852nm01200). Supporting Information Available: Cyclic voltammetry plot, polarization curves, image of tin-antimony sol-gel wetting and spreading on TiO2 nanotubes, and XRD spectra. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes

Figure 10. Variation of ICE with electrolysis time during electrochemical oxidation of BA.

TABLE 3: Parameters of EC, PC, and PEC BA -4

-1

EC

PC

PEC

TiO2-NTs

ks (10 s ) COD removal (%) ICE (%)

0.52 22.0 6.5

0.41 26.7

1.09 73.3 31.3

Ti/SnO2

ks (10-4 s-1) COD removal (%) ICE (%)

1.01 42.4 13.7

0.62 32.8

1.63 81.9 41.7

TiO2-NTs/SnO2

ks (10-4 s-1) COD removal (%) ICE (%)

1.44 65.8 26.4

0.82 47.4

5.26 100.0 100.0

with time, namely the organic compound concentration decreased. Meanwhile, the current efficiency was reduced. When there is no UV light irradiation, the original ICE of the TiO2NTs/SnO2 electrode is 26.4%, which is higher than that of the Ti/SnO2 electrode of 13.7% and the TiO2-NTs electrode. Under UV light irradiation conditions, the original ICE of the TiO2NTs/SnO2 electrode is 100%, which is much higher than 41.7% and 31.3% on the Ti/SnO2 and TiO2-NTs electrodes, respectively. It reveals that the TiO2-NTs/SnO2 electrode is more sensitive to UV light irradiation. Meanwhile, light irradiation accelerates the electrochemical reaction due to more electrochemical active sites on the electrode surface.

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