Construction and High Performance of a Novel Modified Boron-Doped

Mar 5, 2010 - 1. Introduction. In recent years, a boron-doped diamond (BDD) film electrode .... dissolved in the mixed solution of 9.0 mL of water and...
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5906

J. Phys. Chem. C 2010, 114, 5906–5913

Construction and High Performance of a Novel Modified Boron-Doped Diamond Film Electrode Endowed with Superior Electrocatalysis Guohua Zhao,* Peiqiang Li, Fuqiao Nong, Mingfang Li, Junxia Gao, and Dongming Li Department of Chemistry, Tongji UniVersity, Shanghai, China ReceiVed: September 25, 2009; ReVised Manuscript ReceiVed: February 10, 2010

A modified boron-doped diamond (BDD) electrode was prepared by evenly assembling Sb-doped SnO2 nanoparticles on the BDD surface, which possesses excellent electrocatalytic performance and is more suitable to degrade the pollutants. The growth of Sb-doped SnO2-NPs was controlled using micelles of the block copolymer surfactant and homogeneous precipitation approaches. SEM and HRTEM confirmed that BDD still can be fully exposed after modification of Sb-doped SnO2-NPs. The prepared Sb-doped SnO2-NPs/ BDD electrode maintained high oxygen evolution potential (2.3 V vs SCE); meanwhile, its conductivity is greatly improved when the resistance decreases to 1.2 from 60.8 kΩ and the reaction activation energy also reduces from 8.02 to 4.93 kJ mol-1, owing to the excellent electrocatalytic performance. Moreover, Sbdoped SnO2-NPs/BDD has higher oxidation ability. The reaction rate constant of 2,4-D on the Sb-doped SnO2-NPs/BDD is 2 times and mineralization current efficiency at 30 min on the Sb-doped SnO2-NPs/ BDD is 1.6 times that on the BDD. The time for complete removal of the total organic carbon (TOC) is 240 min on the Sb-doped SnO2-NPs/BDD electrode, while it is beyond 360 min on the BDD. Energy consumption on the BDD is 1.3 times that on the Sb-doped SnO2-NPs/BDD for the TOC total removal. The generation and further oxidation of the intermediates further approved the high efficiency of the Sb-doped SnO2-NPs/ BDD. 1. Introduction In recent years, a boron-doped diamond (BDD) film electrode has been the focus in wastewater treatment because of its outstanding properties such as high oxygen evolution potential, greater amounts of hydroxyl radicals, high chemical and electrochemical stability, inert surface, antipollution, pollution recovery ability, and so on.1-11 From the viewpoint of electrochemical oxidation, a good performance electrode should have three characteristics: good conductivity, high oxygen evolution potential, and excellent electrocatalytic properties. However, BDD only has high oxygen evolution potential, accompanied with large resistance, low electrocatalytic activity, and difficult surface reconstruction. Increasing the boron-doped concentration will lead to high conductivity, but it will not improve the electrocatalytic of BDD.12-14 To overcome these shortcomings, many efforts have been made to modify the surface of the BDD.15-19 As is known, the diamond surface is in a sp3 structure with high stability, which makes it difficult to restructure the surface. The reported modification methods mainly include the covalent bonding, adsorption, polymer thin-film, combination, and so on.15-17,19 For example, the growth of cobalt nuclei on a BDD electrode was prepared under potentiostatic control.15 Au nanoparticles modified onto the BDD surface are prepared by pretreating the BDD surface with allylamine. Researchers have successfully modified the BDD surface with different catalytic species in different morphologies and the resultant electrode just can be used for electroanalysis. For example, the Au-coated BDD electrode was used to test inorganic arsenic,20 and the Pt/BDD electrode was used to perform the methanol electrooxidation.21 However, it should be noted that those modification methods * To whom correspondence should be addressed. Phone: (86)-2165981180. Fax: (86)-21-65982287. E-mail: [email protected].

make the BDD surface fully covered by the modified catalytic species and, accordingly, not suitable for the exposure of the BDD base. Thus, the obtained material is mainly to show the performance of the modified species at the expense of its inherent advantages, especially the high oxygen evolution potential of BDD.22 The lower oxygen evolution potential will lead to the side reaction of oxygen evolution, resulting in a significant reduction of the current efficiency. Thus, through such previous modification, it is not suitable for the electrochemical oxidation of pollutants. Based on this, it is of great significance to obtain an anode simultaneously with high oxygen evolution potential and excellent electrocatalytic ability. To the best of our knowledge, such kind of ideal electrode is rarely reported. Our research interest also lies in this. It has been demonstrated that Sb-doped SnO2 is of good electrocatalytic activity among so many catalyst species,23-25 which is especially suitable for the electrocatalytic oxidation of organic pollutants. A previous study showed that Sb-doped SnO2 loaded on titanium substrate has a higher oxidation efficiency for phenol, quinone, aromatic compounds, and nitrogen pollutants, the toxic intermediates that can be fast and completely oxidized.23,24 Thus, it has higher current efficiency than Pt and traditional DSA electrodes. In addition, Sb-doped SnO2 itself has higher oxygen evolution potential (1.8 V, vs SCE).23,25 Thus, some organic compounds which are hard to be oxidized can also be mineralized on this electrode. On the basis of these advantages, Sb-doped SnO2 was selected to modify the BDD. Meanwhile, the crucial technical problem to fully keep the inherent characteristics of BDD is that the modified species could not form a dense membrane to fully block the BDD surface. The study emphatically proceeded from the design of electrode’s microstructure. The growth of Sb-doped SnO2 was

10.1021/jp909248w  2010 American Chemical Society Published on Web 03/05/2010

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SCHEME 1: Schematic Illustration for the Growth of Sb-Doped SnO2-NPs/BDD Electrode

controlled to form small size nanoparticles using micelles of the block copolymer surfactant and homogeneous precipiation approaches. The Sb-doped SnO2 nanoparticles (Sb-doped SnO2-NPs) were highly dispersed onto the BDD surface to prepare the BDD and Sb-doped SnO2-NPs hybrid electrode (Sb-doped SnO2-NPs/BDD). The Sb-doped SnO2-NPs can act as the active sites for the electrocatalytic reaction, which greatly improved the poor electrocatalytic property of the BDD. Moreover, this method could make the BDD fully expose after modifying, so it almost did not reduce the high oxygen evolution potential. In the preparation process, the BDD surface appears terminal oxygen after the pretreatment, the surfactant polar group and the terminal oxygen can bind together in virtue of the surfactant function, which facilitates the modification of catalytic species to the BDD surface. The novel Sb-doped SnO2-NPs/BDD electrode constructed in this research simultaneously has excellent electrocatalytic performance and high oxygen evolution potential. The detailed investigation of its physical and chemical properties and the oxidation performance were carried out. This modified BDD electrode is successfully used in electrochemical oxidation for the target contamination 2,4-dichlorophenoxyacetic acid (2,4D), which is a phenoxy carboxylic acid herbicide widely used in crop weeding and lawn maintenance in large amounts and for a long period of time.26,27 It has mutagenicity and teratogenicity. Addtionally, it is difficult to be biochemically degraded because the chlorine-containing metabolic intermediates are of high toxicity.28-30 In the further study, the high efficient oxidation mechanism of the Sb-doped SnO2-NPs/BDD is elaborated from the generation and further oxidation of the intermediates. This study provides a new idea for exploring a more efficient anode with high catalytic oxidation degradation ability as an alteration for the traditional BDD. 2. Experimental Section Preparation of Sb-Doped SnO2-NPs/BDD Electrode. The preparation of precursor is as follows: 5.0 g of styrene phenol polyoxyethylene ether (abbreviated as SPPE, number average j n ) 1622, relative molecular mass distribumolecular weight M tion width D ) 1.10), 2.24 g SnCl2 · 2H2O and 0.11 g SbCl3 is dissolved in the mixed solution of 9.0 mL of water and 20 mL of ethanol, then 0.9 g hexamethylenetetramine was slowly added to the above solution, which was mixed fully, and white precipitate was produced after intense mixing (Scheme 1A).

SnO2 was assembled to the BDD (BDD is made by chemical vapor deposition on a conductive monocrystalline silicon substrate with the doping of 1300 ppm boron; the thickness of the diamond film is 2-3 µm and the thickness of the obtained BDD is about 1 mm) surface: the BDD surface was rinsed by aqua regia (Scheme 1b), and then the prepared precursor solution was spin coated to the surface of the BDD at 3000 rpm on the even plastic machine (Scheme 1c). The prepared electrode was placed in the oven at 300 °C for 20 min (Scheme 1d), naturally cooled to room temperature, and rinsed with ethanol and distilled water, followed by spin-coating again, drying, and calcining. Finally, the experiment was repeated twice and slowly warmed to 450 °C pyrolysis oxidation for 1 h. Then the Sb-doped SnO2-NPs/BDD hybrid electrode was obtained (Scheme 1e; heating and cooling rates both are 1 °C/min). Characterization of the Surface Structure and Physicochemical Properties. The morphology of the electrode was characterized by the field emission environmental scanning electron microscopy (EFEG-SEM, Model Quanta 200 FEG, manufacturer FEI). The crystal structure of the electrode was characterized by X-ray diffraction analysis (XRD, Model D/max2550 VB3+/PC, manufacturer Rigaku). The hydrophilic property was determined by an interfacial tension measuring instrument (model POWEREACH JC2000A, manufacturer Zhongchen Shanghai). High-resolution transmission electron micrograph (HRTEM) images were obtained using a JEOL 2010F microscope operating at an accelerating voltage of 200 kV. The samples for HRTEM were prepared by dispersing the final powders in ethanol; this suspension was then dropped on carbon-copper grids. X-ray photoelectron spectroscopy (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 properties were measured in the threeelectrode system of CHI660C electrochemical workstation. BDD and Sb-doped SnO2-NPs/BDD electrodes work as the anode, platinum plate as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode. The oxygen evolution potential was tested in 1 M H2SO4 solution. Cyclic voltammetry (CV) was used to test the electrode’s electrochemical properties, and the sweep speed was 50 mV s-1. Electrochemical impedance spectroscopy (EIS) was used to determine the conductivity of electrode. The electrocatalytic oxidation capacity for 2,4-D was

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determined by the curve of current density versus time (i∼t) under 3.0 V in the 0.1 M Na2SO4 solution. The electrode activation energy (Ea) was measured according to the anodic polarization curves of electrodes at different temperature in 0.1 M H2SO4. Ea was determined in accordance with the law of Arrenhinus:31

lg i )

Ea + const 2.303RT

(1)

It is a linear relation between lg i and 1/T, and Ea of the electrode can be determined by the slope of the straight line. Electrocatalytic Degradation Experiment. The electrocatalytic degradation experiment was carried out in a round reaction pool in which water is recycled to maintain the reaction temperature at 25 °C. BDD and Sb-doped SnO2-NPs/BDD electrodes work as the anode, titanium as the cathode with an area of 1.2 cm2, and the intergap of 1.0 cm. The current density is 20 mA cm-2, and 0.1 M Na2SO4 containing 100 mg L-1 2,4-D was used as the simulated wastewater. Concentrations of 2,4-D and the intermediates are tested by HPLC (Varian 3900 HPLC). The samples were detected and quantified by Ultiate TM AQ-C18 (4.6 × 150 mm, 5 µm) and selected UV detector at λ ) 230 nm. Methanol/phosphate buffer (40:60 (v/v); pH ) 2.3) was employed as the mobile phase at the flow rate of 0.8 mL min-1. TOC of the system was determined using the total organic carbon analyzer (TOC, TOC-Vcpn, Shimadzu). Because 2,4-D (C8H6Cl2O3) can be completely electrochemically oxidized to CO2 and H2O, the whole reaction process can be expressed as

C8H6Cl2O3 + 13H2O f 8CO2 + 32H+ + 30e- + 2Cl(2) In the given time t, 2,4-D mineralization current efficiency (MCE) can be expressed as follows32

MCE )

∆(TOC)exp × 100% ∆(TOC)theor

(3)

In the formula, ∆(TOC)exp is the TOC experimental change values in the t degradation time and ∆(TOC)theor is the TOC theoretical change values corresponding to the same time. The theoretical value is calculated by the following formula32

∆(TOC)theor

I×t × nc × M × 103 ne × F ) mg L-1 V

(4)

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 transfer in the eq 1, 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 30 and 8, respectively. All the experiments in our work repeat three times, and the result is the average value of the three parallel experiments.

Figure 1. (a) SEM of BDD, (b) SEM of Sb-doped SnO2-NPs/BDD, (c) HRTEM of Sb-doped SnO2-NPs, and (d) amplified HRTEM of Sb-doped SnO2-NPs, with the inset showing the selected-area electron diffraction.

3. Results and Discussion Surface Microtopography and the Crystal Structure of the Electrode. Figure 1a showed the SEM of BDD. BDD was polycrystalline thin film. And the BDD surface was smoothing, clear edges, dense growing, and showing step form (Figure 1a). The phase of BDD mainly was (110) and (100) crystal plane (Figure S1 in the Supporting Information). The average crystallite size was micrometer grade. Figure 1b showed the SEM of Sb-doped SnO2-NPs/BDD. The image showed that the granular Sb-doped SnO2-NPs were regularly dispersed on the very smooth surfaces of BDD, and the particles are homogeneous and small. HRTEM investigations give further insight into the structural features of Sb-doped SnO2-NPs. Figure 1c shows an overview image and illustrates that the particles are partially aggregated but otherwise quite uniform in size and shape. The well-developed lattice fringes are randomly oriented (Figure 1d), which underlines the high crystallinity of the sample and the random orientation of the NPs with respect to one another. The grain boundaries are clearly visible on these HRTEM images, confirming a particle size predominantly in the range of 6.0-8.0 nm, which is small for the oxide particle, and Sb-doped SnO2-NPs make every edge and plane of the BDD fully exposed. It means that the special novel electrode will provide two reaction surfaces, the BDD surface and the Sb-doped SnO2-NPs surface, simultaneously. The selected-area electron-diffraction (SAED) pattern of Sb-doped SnO2-NPs (Figure 1d, inset) shows the characteristic diffraction rings, which correspond to the reflections (1,1,0), (1,0,1), (2,0,0), (2,1,1), and (3,0,1) of the SnO2 tetragonal rutile structure. The Debye-Scherrer rings clearly indicate the polycrystalline nature of the powder. XRD analysis was used to further determine the characteristics of catalyst (Figure S1 in the Supporting Information). Three major wide peaks were observed, 2θ ) 26.6, 33.9, and 44.0, and the phase was (110), (101), and (111) crystal planes. The results demonstrated that SnO2 is the tetragonal rutile phase, Sb could not be found from XRD (the reason is that the Sb5+ ionic radius is 0.78 Å, which is smaller than Sn4+ ionic radius of 0.83 Å), and Sb is doped in the SnO2 unit cell and appears in a form of solid solution.33 This kind of SnO2 has good

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Figure 2. (a) XPS survey scan of the Sb-doped SnO2-NPs; (b) Sn 3d XPS spectra; (c) Sb 3d XPS spectra.

electrocatalytic activity, and the diffraction peak intensity of BDD was similar before and after modifying Sb-doped SnO2-NPs. It showed that the BDD film can be fully exposed after modifying. XPS analysis was used to investigate the chemical states and the surface atomic ratios of Sn and Sb contents in the as-prepared samples. The binding energies in the XPS spectra presented in Figure 2 were calibrated by contaminant carbon (C1s ) 284.6 eV), and XPS spectra (Figure 2a) confirmed the high chemical purity of the Sb-doped SnO2-NPs, which consisted solely of Sn, Sb, and O. The XPS spectrum shows two peaks of Sn3d5/2 and Sn3d3/2 at 486.7 and 495.2 eV with clearer symmetry (Figure 2b), and they are assigned to the inlattice points in tin oxide. The peak separations of 8.5 eV between these two peaks is in agreement with the energy reported for SnO2. The values correspond to the 3d binding energy of Sn(IV) ions (indexed Standard ESCA Spectra of the Elements and Line Energy Information, F Co., U.S.A.). The valence state of Sb was determined to be +5 by the XPS analysis, as the banding energy of Sb3d5/2 and Sn3d3/2 is around 530.9 and 540.2 eV (Figure 2c), in agreement with other researchers.34 Calculations of the area of the Sn3d5/2 and Sb3d5/2 emission lines (10.4 for Sn3d5/2 and 0.5 for Sb3d5/2) resulted in an atomic Sb/Sn ratio of 20.8, which is slightly larger than the composition in the starting recipes. Sb-doped SnO2-NPs can be well-controlled growth on the BDD surface because surfactant and homogeneous precipitation approaches were used in the preparation process. OH- formed in the hydrolysis of precipitator, hexamethylenetetramine, at a certain temperature will reacted with the antimony-tin salt and generated hydroxide precipitation. Hexamethylenetetramine effectively prevented the local supersaturation of the precipitant, and prevented the reactions of homogeneous and heterogeneous nucleation, which will result in asymmetry of the particle size distribution. At the same time, in the precipitation formation process, the block copolymer surfactant SPPE formed the micelles. Micelles effectively protected the occurrence of the agglomeration, and the size of the precipitation particles can be well-controlled. Therefore, the uniform and small size of oxide particles can be obtained easily. The polarity of the surfactant vulnerably combined with terminal oxygen of the BDD. Therefore, it is easier to modify the catalytic species to the BDD surface. Electrochemical Performance. For BDD electrode, the most concerned about is whether the BDD electrode can obtain excellent electrocatalytic property. Electrocatalytic properties of the electrodes were studied through the curves of the current density versus time. Figure 3a is the current versus time curves of the two electrodes in the solution at the applied potential of

Figure 3. (a) Current density change curves with time in the Na2SO4 solution at 3.0 V; (b) polarization curves of BDD and Sb-doped SnO2-NPs/BDD electrodes in 1 M H2SO4 solution.

3.0 V (higher than the oxygen evolution potential). It can be seen that the current density is improved to 14.3 mA cm-2 from 11.6 mA cm-2 when Sb-doped SnO2-NPs modified to the BDD, which indicated that the conductivity property of the Sb-doped SnO2-NPs/BDD was improved. The current densities of the BDD and Sb-doped SnO2-NPs/BDD electrodes were both improved after adding 2,4-D into the solution, and the current density of BDD increased to 21.3 mA cm-2, an increase of 9.7 mA cm-2. The current density of Sb-doped SnO2-NPs/BDD increased to 26.6 mA cm-2, an increase of 12.3 mA cm-2 (Table 1). The increase of the current density value was attributed to

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TABLE 1: Performance Comparison between the BDD and the Sb-Doped SnO2-NPs/BDD

BDD current density increase (mA cm-2) Ea (kJ mol-1) rate constant (s-1) C( · OH)180 min (µM) 2,4-D removal120 min (%) TOC removal120 min (%) MCE30 min (%) E(energy consumption) (kWh m-3)

9.7 8.02 0.019 14.5 86.5 60.0 12.6 4.2

Sb-doped SnO2-NPs/ BDD

(Sb-doped SnO2-NPs/ BDD)/BDD

12.3

1.3

4.93 0.036 19.5 98.5 82.2 19.8 3.3

0.6 1.9 1.4 1.2 1.4 1.6 0.7

the 2,4-D electrocatalytic oxidation on the electrode, and the current density increased more obviously after Sb-doped SnO2-NPs modified to the BDD surface. It showed that the modification is beneficial to the improvement of the electrocatalytic performance. To the best of our knowledge, we speculated that the improvement of the electrocatalytic performance is contributed to the microstructure and surface chemical properties of the Sb-doped SnO2-NPs, which is a high active electrocatalyst. And the electrode reaction activation energy (Ea) is obtained through measuring the various eletrodes’ anodic polarization curves in the sulfuric acid solution. Ea of BDD and Sb-doped SnO2-NPs/BDD electrodes are 8.02 and 4.93 kJ mol-1, respectively. The former is 1.6 times that of the latter (Table 1). It can be seen that Sb-doped SnO2-NPs/BDD has the lower Ea needed to generate an electrochemical reaction, while the BDD is higher. It effectively shows that Sb-doped SnO2-NPs/BDD has excellent electrocatalytic performance. When the two electrodes’ cyclic voltammetry in the Fe(CN)63-/ Fe(CN)64- solution were compared (Figure S2 in the Supporting Information), it can be seen that there were no redox peaks on the nonmodified BDD electrode and it behaved inert. There were noticeable redox peaks on the modified BDD electrode, which were at 0.8 and -0.5 V, respectively. It showed that the electrocatalytic performance of the BDD was greatly improved after the modification of Sb-doped SnO2-NPs onto it. The results further confirmed the above conclusion. Therefore, Sbdoped SnO2-NPs/BDD electrode overcome the shortcoming of BDD with inert electrocatalytic property and obtained excellent electrocatalytic property after modifying the Sb-doped SnO2-NPs to the inert BDD surface. It is suitable to be used to electrocatalytically degradate pollutants. It is equally important that the electrode has high oxygen evolution potential, which can obtain high electrochemical oxidation performance. Figure 3b is polarization curves of the electrodes in 1 M H2SO4 solution. The oxygen evolution potential of BDD is 2.4 V, which is very high among the anodes. After Sb-doped SnO2-NPs modification, Sb-doped SnO2-NPs/ BDD maintained the oxygen evolution potential of 2.3 V. High oxygen evolution potential means high electrochemical oxidation efficiency. The reason is that it has the competitive reaction of oxygen evolution when the organic matter was oxidized on the anode. If the oxidation potential of organic compounds is higher than the oxygen evolution potential, there was no electrochemical oxidation of pollutants before oxygen evolution potential and the anode current efficiency should be very low. The oxygen evolution reaction will occur when the potential reaches that of the oxidation current, and the oxidation current will rapidly increase. Therefore, the current efficiency of the organic matter oxidation degradation will greatly decrease and the energy consumption will increase due to the oxygen evolution. It can

TABLE 2: Electrochemical Properties of BDD and Sb-Doped SnO2-NPs/BDD BDD Sb-doped SnO2-NPs/ BDD

Rp (kΩ)

Rs (Ω)

C (F)

Eoxygen (V)

60.8 1.2

217.6 13.0

8.599 × 10-7 3.184 × 10-6

2.4 2.3

be concluded that, the higher oxygen evolution potential the electrode is, the more difficult the oxygen evolves, the probability of the organic oxidation will be greater, and the electrochemical efficiency will be higher. Sb-doped SnO2-NPs/ BDD has a higher oxygen evolution potential, which means that Sb-doped SnO2-NPs/BDD has higher oxidation efficiency and lower energy consumption in the electrochemical oxidation course. Besides, we conducted a more detailed study on the electrochemical impedance spectroscopy (EIS) of the electrode: the EIS can reflect the resistance of the electrode itself (Rp), the resistance between the electrode and the electrolyte (Rs), and the performance of the double-layer between the electrode surface and the electrolyte (C). The study gives us good help in understanding the electrochemical properties of the electrode. The electrochemical parameters are presented in Table 2. With the poor conductivity, Rp and Rs of BDD are high: 60.8 kΩ (Figure S3 in the Supporting Information). The conductivity of the Sb-doped SnO2-NPs/BDD is greatly improved: Rp is 1.2 kΩ and Rs is about 13 Ω after assembling the Sb-doped SnO2-NPs on the BDD surface (Figure S3 in the Supporting Information), which is 1/50 and 1/17 times that of BDD, respectively. The decrease of Rp is closely related to the species modified to the BDD surface and the shape of the species. From the SEM image we can see that Sb-doped SnO2-NPs, with a small particle size and evenly dispersed on BDD surface, will help to improve the electrochemical performance of the electrode. The decrease of the Rs may be related to the surface hydrophilic property. The contact angle between water and BDD is 65°, while water can well spread on the Sb-doped SnO2-NPs/ BDD surface, the contact angle of which is 18.2° (Figure S4 in the Supporting Information). Sb-doped SnO2-NPs make the surface more hydrophilic, declining the value of Rs. The surface characteristics of the Sb-doped SnO2-NPs/BDD also affect another important electrode property, the double-layer capacitance. The interface double-layer plays an important role of porter. Higher double-layer capacitance leads to charge transfer in a shorter time. The double-layer capacitance of Sb-doped SnO2-NPs/BDD is 3 times higher than that of the BDD (Table 2), which indicates that the prepared anode may have higher adsorption and desorption rates in oxidation reaction, and it promoted the dynamics process of electrode reaction. Enhancement of the Electrochemical Oxidation for 2,4D. 2,4-D degradation has important environmental significance,35-37 so we selected the 2,4-D as the target pollutant. The degradation effect of 2,4-D in the electrocatalytic oxidation process is emphasized and compared on the Sb-doped SnO2/Ti, BDD and Sb-doped SnO2-NPs/BDD. The detailed discussions included the removal of 2,4-D and the evolution of TOC, MCE, and energy consumption. Figure 4a shows the evolution of the removal of initial 2,4D. 2,4-D is completely converted on the Sb-doped SnO2-NPs/ BDD at 120 min. The time of complete conversion on BDD is 240 min, and it still has 32.0 mg L-1 unconverted at 300 min on the Sb-doped SnO2/Ti. The kinetics of 2,4-D conversion is further analyzed on the three electrodes. Good linear plots are only obtained when the fit is to a pseudo-first-order reaction.

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Figure 4. (a) 2,4-D concentration changes with time on the Sb-doped SnO2/Ti, BDD, and Sb-doped SnO2-NPs/BDD electrodes. The inset is the apparent rate constant changes with time; (b) TOC removal changes with time on the Sb-doped SnO2/Ti, BDD, and Sb-doped SnO2-NPs/BDD electrodes; (c) Energy consumption changes with TOC removal rate on the Sb-doped SnO2/Ti, BDD, and Sb-doped SnO2-NPs/BDD electrodes; and (d) MCE changes with time on the Sb-doped SnO2/Ti, BDD, and Sb-doped SnO2-NPs/BDD electrodes.

The kinetics analysis is depicted in the inset map of Figure 4a. The pseudo-first-order rate constants (k) of 2,4-D on Sb-doped SnO2/Ti, BDD, and Sb-doped SnO2-NPs/BDD electrodes are 0.005, 0.019, and 0.036 s-1, respectively. The k on the Sb-doped SnO2-NPs/BDD is higher, which is about 1.9 and 7.2 times on the BDD and Sb-doped SnO2/Ti electrode, respectively (Table 2). So, 2,4-D is removed more quickly on the Sb-doped SnO2-NPs/BDD, demonstrating the highest electrocatalytic oxidation ability for the 2,4-D. Figure 4b shows the variation of TOC removal of 2,4-D. TOC removal on the Sb-doped SnO2-NPs/BDD electrode is also higher than that on BDD and Sb-doped SnO2/Ti electrodes under the same experimental conditions. TOC removal is 96.7% on the BDD electrode at 360 min, while the value reaches 97.9% on the Sb-doped SnO2-NPs/BDD electrode at 240 min, and it can be seen from the TOC removal at 120 min, TOC removal rate is 82.2, 60.0, and 39% on the Sb-doped SnO2-NPs/BDD, BDD, and Sb-doped SnO2/Ti electrodes, respectively. The value on the Sb-doped SnO2-NPs/BDD electrode is 1.4 times that on the BDD electrode (Table 2). This indicates that the electrochemical oxidation degradation ability of Sb-doped SnO2-NPs/BDD electrode is much higher than that of the BDD electrode, which leads to the mineralization of pollutants with higher efficiency after modifying. In addition, Sb-doped SnO2NPs/BDD also can reach the mineralization of pollutants with less energy consumption. The energy consumption on the Sbdoped SnO2-NPs/BDD electrode is 3.3 kW m-3 to reach TOC removal of 96.6%, and the values are 4.2 and 5.7 kW m-3 on the BDD and Sb-doped SnO2/Ti electrodes (Figure 4c). The energy consumption on the BDD is 1.3 times that on the Sbdoped SnO2-NPs/BDD electrode. Sb-doped SnO2-NPs/BDD electrode has the superior mineralization ability with less energy

consumption. The conclusion is accordance with the lower resistance and Ea of Sb-doped SnO2-NPs/BDD electrode. The superior mineralization ability of the Sb-doped SnO2-NPs/ BDD electrode can be further illustrated from the evolution of MCE. Figure 4d shows the evolution of MCE on the three electrodes. MCE all decreased with electrolysis time on the three electrodes. However, MCE at 30 min on the Sb-doped SnO2NPs/BDD is 1.6 and 2.4 times that on the BDD and Sb-doped SnO2/Ti (Table 2). This indicates that BDD modified with Sbdoped SnO2-NPs has higher oxygen evolution potential and excellent electrocatalytic ability, which is consistent with the above analyzed conclusion. The detailed reasons can be explained as follows: BDD has poor electrocatalytic ability in the electrochemical oxidation, and the mineralization of the pollutants is just in virtue of the high oxygen evolution potential of the BDD. Modified with Sb-doped SnO2-NPs, it is equivalent to increase the active sites of the electrocatalysis on the electrode surface. The concentration of •OH generated on the electrodes under 20 mA cm-2 is used to analyze the electrocatalytic capacity of the electrode. The amount of •OH generated at 180 min on Sb-doped SnO2-NPs/BDD is 1.4 times that on BDD (Table 2), so the electrocatalytic oxidation capacity for organics on Sb-doped SnO2-NPs/BDD is higher than that on BDD, and the activation energy of the electrocatalytic oxidation reaction is decreased, leading to the electrocatalytic oxidation being carried out more easily. Moreover, Sb-doped SnO2-NPs/ BDD electrode still possesses high oxygen evolution potential, so the electrocatalytic oxidation ability of this electrode is improved. In the experiment, the stability of the Sb-doped SnO2-NPs/ BDD is also investigated by comparing the electrode’s property changes before and after repeated degradation of 2,4-D 15 times.

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Figure 5. Aromatic intermediates of catechol and 2,4-dichlorophenol concentration change with time.

First, the 2,4-D concentration change between the first degradation and the fifteenth degradation is compared; after the fifteenth degradation of 2,4-D, the degradation result is almost the same as the first degradation (Figure S5 in the Supporting Information). The difference between a one time degradation and the fifteenth time degradation is no more than 5%, which indicated that it is a stable contact between Sb-doped SnO2-NPs and BDD. Furthermore, XRD analysis was used to further determine the stability of Sb-doped SnO2-NPs/BDD, and it can be seen that the characteristic peak of SnO2 has not change (Figure S6 in the Supporting Information), which shows that Sb-doped SnO2-NPs did not break off from the BDD surface. Also, SEM of Sb-doped SnO2-NPs/BDD after the fifteenth degradation of 2,4-D is the same as the initial state (Figure S7 in the Supporting Information), and the Sb-doped SnO2-NPs evenly disperse on the BDD surface and do not break off from the BDD surface. The information above indicated that the combination between Sb-doped SnO2-NPs and BDD is stable and Sb-doped SnO2NPs/BDD will have persistent catalytic ability. Dynamics of the Degradation Reaction and the Evolution of Intermediates. The intermediates were analyzed by HPLC during the electrochemical oxidation of 2,4-D. The intermediates are the same on the BDD, Sb-doped SnO2/Ti, and Sb-doped SnO2-NPs/BDD electrodes. Aromatic intermediates are 2,4-dichlorophenol (tr ) 10.23 min) and hydroquinone (tr ) 2.88 min). Carboxylic acids are fumaric acid (tr ) 2.32 min), maleic acid (tr ) 2.46 min), and oxalic acid (tr ) 2.17 min). The solution pH varied from 6.5 to 6.3 in the whole degradation process and it will not affect the kinetic behavior. 2,4-Dichlorophenol and hydroquinone are identified because 2,4-D is attacked by hydroxyl radicals generated on the electrode surface, which leads to the aromatic intermediates produced. The evolution of the aromatic intermediates is shown in Figure 5. The two aromatic intermediate concentrations increase at first

Figure 6. Carboxylic acid intermediates of maleic, fumaric, and oxalic acid concentration changes with time.

and then decrease. The maximum concentration of 2,4-dichlorophenol on Sb-doped SnO2/Ti, BDD, and Sb-doped SnO2-NPs/ BDD electrodes is 7.0, 6.8, and 5.3 mg L-1, respectively. For hydroquinone, the value is 1.7, 1.5, and 1.3 mg L-1, respectively. So the maximum concentration of aromatic intermediates is lower on Sb-doped SnO2-NPs/BDD than that on Sb-doped SnO2/Ti and BDD. Moreover, the time to reach the maximum concentration and further oxidation to mineralization is shorter for Sb-doped SnO2-NPs/BDD. These indicate that Sb-doped SnO2-NPs/BDD has higher electrochemical oxidation degradation ability for aromatic intermediates. The carboxylic intermediates are also measured by HPLC. They include fumaric, maleic and oxalic acids (Figure 6). Fumaric and maleic acids come from the destruction of the aromatic intermediates, whereas oxalic acid is formed from the oxidation of fumaric and maleic acids, then oxalic acid is mineralized to CO2 and H2O. Oxalic acid concentration is higher than that of fumaric and maleic acids. This indicates that fumaric and maleic acids are easy to be oxidized by hydroxyl radicals. However, oxalic acid is degraded slowly which leads to the

Boron-Doped Diamond Film Electrode higher concentration in the solution. The maximum concentrations of fumaric, maleic, and oxalic acids on BDD are 4.1, 2.9, and 19.1 mg L-1, respectively and the values on Sb-doped SnO2/ Ti are 4.3, 3.0, and 19.5 mg L-1, respectively, while the values on Sb-doped SnO2-NPs/BDD are 3.8, 2.6, and 16.1 mg L-1, respectively. They are lower than that on Sb-doped SnO2/Ti and BDD. Moreover, the time to reach the maximum concentration and further oxidation to mineralization is also shorter for Sb-doped SnO2-NPs/BDD. These indicate that Sb-doped SnO2-NPs/BDD also has higher electrochemical oxidation degradation ability for carboxylic intermediates. The electrochemical oxidation degradation ability for aromatic and carboxylic intermediates is enhanced on Sb-doped SnO2NPs/BDD. This is consistent with the superior property of this electrode and its efficient degradation effect, which is closely related to the property and structure of the species on the electrode surface. As we know, BDD has high oxygen evolution potential and electro-oxidation ability. However, the poor conductivity and electrocatalytic ability inhibit the development of BDD. Sb-doped SnO2-NPs with high electrocatalytic ability are assembled to the BDD surface. Thus, the novel Sb-doped SnO2-NPs/BDD not only has high oxygen evolution potential, but also has excellent electrocatalytic performance because more active sites are produced on the electrode surface. The amount of the hydroxyl radical concentration can prove this, and it can be seen from Figure S8 (in the Supporting Information) that the hydroxyl radical on the Sb-doped SnO2-NPs/BDD is much larger than that on the BDD. Thus, Sb-doped SnO2-NPs/BDD behaves superior in electrocatalytic oxidation ability for the pollutant. 4. Conclusion The modified BDD electrode was prepared using micelles of the block copolymer surfactant, and homogeneous precipitation approaches the growth of SnO2 nanoparticles, which can be well controlled in the micelles. SEM confirmed that BDD can be fully exposed after modification. The anode material is a novel electrode with a special structure. This kind of structure and modification endowed the original inert BDD with a superior electrocatalytic oxidation property. The modified BDD electrode almost did not change the high oxygen evolution potential of BDD (2.4 V vs SCE). Moreover, Sb-doped SnO2-NPs/BDD has high electrochemical oxidation ability for the 2,4-D pollutants, which can be timely and efficiently removed on the Sbdoped SnO2-NPs/BDD. The generation and further oxidation of the intermediates further approved the high efficiency of the Sb-doped SnO2-NPs/BDD. It provides a new idea for exploring the electrode material with high electrochemical oxidation degradation ability, and it is significant for the research and application of the advanced oxidation degradation of biorefractory pollutants. Acknowledgment. This work was supported jointly by the National Natural Science Foundation P.R. China (Project No. 20877058), 863 Program (Project No. 2008AA06Z329) from the Ministry of Science, and Nanometer Science Foundation of Shanghai (Project No. 0852 nm01200). Supporting Information Available: Additional supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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