ARTICLE pubs.acs.org/JPCC
Novel Sieve-Like SnO2/TiO2 Nanotubes with Integrated Photoelectrocatalysis: Fabrication and Application for Efficient Toxicity Elimination of Nitrophenol Wastewater Shouning Chai, Guohua Zhao,* Peiqiang Li, Yanzhu Lei, Ya-nan Zhang, and Dongming Li Department of Chemistry, Tongji University, Shanghai 200092, China ABSTRACT:
A novel electrode with excellent photocatalytic (PC) and electrocatalytic (EC) performances was prepared by assembling sieve-like macroporous Sb-doped SnO2 (Mp-SnO2) film on vertically aligned TiO2 nanotubes (TiO2NTs) through a block copolymer softtemplate method. The pore size of Mp-SnO2 ranges from 150 to 400 nm. The construction of the macropore can increase the specific surface area and provide more active sites. The band gap of the Mp-SnO2/TiO2NTs is 2.93 eV, and it presents outstanding light absorption and photoelectrochemical properties. Under the light irradiation of 365 nm, a high photoelectric conversion efficiency of 35.2% can be obtained on Mp-SnO2/TiO2NTs, 3.1 times higher than that on TiO2NTs. Compared with traditional SnO2/Ti electrodes, the Mp-SnO2/TiO2NTs displays smaller electrochemical impedance, a larger electrochemical surface absorption volume, and lower reaction activation energy. The integrated photoelectrocatalytic (PEC) oxidation of p-nitrophenol wastewater on Mp-SnO2/TiO2NTs is investigated. Because of the remarkable synergistic effect between PC and EC performances of Mp-SnO2/TiO2NTs, toxic intermediates are easily incinerated, resulting in relatively low accumulated concentrations. In 4 h, the p-nitrophenol and TOC removal reaches 98% and 91%, respectively, and the toxicity of the wastewater vanishes. The mechanism of synergistic PEC degradation of refractory pollutants is also proposed. This study provides a distinctive integrated photoelectric material and a promising technique for treatment of highly concentrated refractory wastewater effluent.
1. INTRODUCTION Titanium dioxide is a powerful and robust photocatalyst for the degradation of various aquatic organic pollutants and has received considerable attention for development as a potential technology for water treatment. Over the past decades, various methods have been used to fabricate TiO2 nanotubes (TiO2NTs), including solgel,1 template-assisted, 2 seeded growth,3 and hydrothermal processes.4,5 The structures demonstrating the most remarkable properties are highly ordered nanotube arrays made by anodization of titanium, the dimension of which can be precisely controlled.69 Various TiO2NTs with a uniform diameter, wall thickness, and length can be obtained by simply tailoring electrochemical conditions.1013 Because of characteristics of high surface-to-volume ratio and large surface energy,14,15 these highly ordered and vertically oriented TiO2NTs prepared by anodization display excellent photocatalytic (PC) properties, far exceeding those of traditional TiO2 films.16,17 Nevertheless, in practical applications, there are many problems. For example, the intrinsic band gap of TiO2 (3.22 eV) limits its absorption in the ultraviolet part of the r 2011 American Chemical Society
solar spectrum,18 and the rapid recombination of photoinduced electrons and holes greatly lowers the quantum efficiency, resulting in low photogenerated current, so that highly concentrated refractory contaminants, such as aromatic organics and pesticides, could not effectively degrade by single TiO2NTs PC oxidation. Moreover, this technique is inapplicable to industrial sewage with high colority and concentration because the wastewater shields the light irradiation to TiO2NTs. Therefore, it is necessary to have effective solutions to assist TiO2NTs in enhancing PC activity and improving degradation efficiency. Electrocatalytic (EC) oxidation is considered a facile and environmentally friendly process to clean toxic and refractory contaminations with the advantages of strong oxidation ability, simplicity of operation, and high efficiency.1923 Thus, combining photocatalyst TiO2NTs to an electrocatalyst with remarkable electrochemical activity is a promising strategy. Sb-doped SnO2, Received: June 3, 2011 Revised: July 19, 2011 Published: August 10, 2011 18261
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Scheme 1. Schematic Illustration for the Growth of the Sieve-like Mp-SnO2/TiO2NTs Electrode
with excellent EC activity, high oxygen evolution potential, and excellent stability, is a candidate, which has been widely applied to organic pollutant disposal.21,24 Pure SnO2 is a good light transparent material and is usually used as the light transparent electrode in the spectral electroanalysis. However, undoped pure SnO2 is nonconductive, so Sb is always doped in for enhancing its conductivity. Although the Sb dopant is favorable for SnO2 conductivity, a trace of Sb has a cutoff effect on light. If the Sb-doped SnO2 film is directly loaded on the TiO2NTs surface by simple methods, such as dipping or spin-coating, the UV absorption of TiO2NTs would decreases inevitably.25 To solve this problem, a novel construction is proposed in this study by designing a two-dimensional (2D) sieve-like macroporous Sb-doped SnO2 film (Mp-SnO2) assembled on the TiO2NTs (Mp-SnO2/TiO2NTs). The light can easily transmit through these macropores and directly irradiate the TiO2NTs so that this composite electrode with excellent PC and EC performances may be superior in the integrated photoelectrocatalytic (PEC) oxidation of refractory contaminations. Herein, for the first time, a novel sieve-like macroporous Sbdoped SnO2/TiO2NTs with outstanding PC and EC performances is constructed. The detailed investigations on its PC and EC properties and the oxidation performance are carried out. This composite electrode is used to decrease the toxicity of highconcentration refractory organic wastewater by integrated PEC synergistic oxidation. p-Nitrophenol (PNP) is chosen as the model pollutant and tested in this study, which is recognized as a refractory, hazardous, and priority toxic pollutant by the U.S. Environmental Protection Agency,26,27 and the nitro group of which increases its resistance to traditional biological treatment techniques. The biodegradability and the acute toxicity of the degraded samples are investigated. The mechanisms of toxicity evolution, kinetics, and evolution of intermediates on the electrode are discussed.
2. EXPERIMENTAL SECTION 2.1. Preparation of the Sieve-like Mp-SnO2/TiO2NTs. TiO2NTs was prepared by the electrochemical anodic oxidation method according to the literature.24 The preparation of the SnO2 precursor is as follows: 6.0 g of styryl phenol polyoxyethylene ether (SPPE; number average molecular weight, Mn = 1622; relative molecular mass distribution width, D = 1.10) was
dissolved in 3.0 g of water, forming solution A. A 3.0 g portion of SnCl2 3 2H2O and 0.15 g of SbCl3 were dissolved in 3.0 g of 18 wt % hydrochloric acid, forming solution B. Solution C was obtained by mixing solutions A and B. The TiO2NTs was put into the buffer bottle, and solution C was added after vacuuming to 6 102 Pa. The TiO2NTs was immersed in solution C for 5 min, took out and aged at 40 °C for 24 h, and dried at 100 °C for 30 min. After that, the prepared electrode was placed in the oven at 300 °C for 30 min, then pyrolyzed at 500 °C for 1 h, naturally cooled to room temperature, and rinsed with ethanol and distilled water, so the sieve-like Mp-SnO2/TiO2NTs electrode was prepared (Scheme 1). The SnO2/TiO2NTs and SnO2/Ti electrodes were prepared by the same procedure on a TiO2NTs and a titanium foil, respectively, but without SPPE added. 2.2. Material Characterization. The morphology of the prepared electrodes was observed using scanning electron microscopy (EFEG-SEM, model Quanta 200 FEG, manufacturer FEI), and their crystalline structure was characterized by X-ray diffraction (XRD, model D/max2550VB3+/PC, manufacturer Rigaku). UVvis diffusive reflectance spectra (DRS) were obtained using a JASCO V-550 UVvis spectrometer. All electrochemical measurements were carried out in a three-electrode cell system of the CHI 660 electrochemical workstation. An SCE served as the reference and Pt wire as the counter electrodes. Cyclic voltammetry (CV) was used to test electrochemical properties of electrodes, and the sweep speed was 50 mV s1. Electrochemical impedance spectroscopy (EIS) was used to determine the conductivity of the catalysts, with the frequency range from 1 105 to 1 103 Hz, and the amplitude was 5 mV. The electrolyte was 0.05 M [Fe(CN)63]/[Fe(CN)64] solution. Photocurrent density was measured in 1 M KOH. A 300 W UV lamp (center wavelength, 365 nm; light intensity, 3 mW cm2) was used as the UV light source. The photoelectric conversion efficiency (η) was calculated according to the literature.28 The electrode activation energy (Ea) was measured according to the anodic polarization curves of electrodes at different temperatures in the 0.1 M H2SO4.29 The Ea value is determined in accordance with the law of Arrenhinus: lg i ¼ 18262
Ea þ Const 2:303RT
ð1Þ
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2.3. Integrated PEC Oxidation of PNP and Analysis. PEC oxidation of PNP was carried out in a cylindrical single-compartment cell equipped with a magnetic stirrer and a jacketed cooler to maintain a constant temperature (25 ( 2 °C). In the EC and PEC degradation experiments, the Mp-SnO2/TiO2NTs, or SnO2/Ti, work as the anode, the working area was 4.5 cm2, a Ti sheet with the same area was used as the cathode, and the gap between the electrodes was 1 cm. A 300 W UV lamp was the light source. A 100 mL sample of 200 mg L1 PNP in 0.1 M Na2SO4 was electrolyzed in the cell. The current density was controlled at 20 mA cm2. In the comparative PC degradation experiments, the Mp-SnO2/TiO2NTs and TiO2NTs work as the photoelectrode, and the effective irradiation area was also 4.5 cm2. The sample was monitored and analyzed as a function of time. The evolution of PNP and its aromatic and carboxylic acidic intermediates was measured by HPLC (Agilent HP1100), and the samples were detected and quantified by AQ-C18 (4.6 100 mm, 5 μm) and selected UV detector at λ = 230 nm. A 40:60 (v/v) methanol/phosphate buffer (pH = 2.3) was employed as the mobile phase at the flow rate of 0.8 mL min1. The total organic carbon (TOC) content was measured with a TOC analyzer (TOC-Vcpn, Shimadzu, Japan). BOD5 (biochemical oxygen demand in a five-day test period at 20 °C) was quantified by the dilution method.30 The BOD5/TOC ratio was calculated to evaluate the biodegradability of the wastewater samples during degradation. Toxicity measurements were carried out in triplicate by the Microtox toxicity text, with a standard bioassay (ISO 11348-3, 1998), which is a luminescent marine bacterium Vibrio fisheri (Photobacterium phosphoreum).31,32 A Microtox M500 Analyzer (Azur Environmental) was employed. To prevent a pH effect, the pH of each sample has to be adjusted into the range using a NaOH solution (0.05 M). Moreover, all aqueous samples and standards were adjusted with 2% NaCl for optimal reagent performance and were incubated at 15 °C for 15 min in an incubator. Generally, the value of EC50 is used to quantify the acute toxicity of a compound. It is defined as the effective nominal concentration (mg L1) of the compound that reduces the intensity of light emission by 50% in a 15 min contact time. Because it is impossible to define an EC50 value for a mixture of toxic compounds, a relative toxicity index, previously defined in the literatures,33,34 was used in this work to compare the toxicity of different samples obtained in the electrolyses. The relative toxicity index is calculated from the equation Relative Toxicity Index ¼
dilution factor for sample at time t dilution factor for sample at time t ¼ 0
ð2Þ where the dilution factor is defined as the dilution required to obtain a toxicity factor of 0.5 (decrease in one-half of the light emission of the luminescent bacterium) and t is the degradation time.
3. RESULTS AND DISCUSSION 3.1. Surface Microtopography and Crystal Structure of the Novel Electrode. In the preparation experiment, micelles first
form in the surfactant solution (Scheme 1b), then spontaneously form the advanced homogeneous lyotropic liquid crystal (LC) with a columnar structure when the macromolecule block copolymer surfactant of SPPE reaches a certain concentration (Scheme 1c). The LC is a special orderly arranged structure,
Figure 1. SEM images: (A) Top view and (B) side view of the MpSnO2/TiO2NTs. Inset: SEM image of TiO2NTs. (C) XRD patterns of as-synthesized (a) Mp-SnO2/TiO2NTs, (b) SnO2/Ti, and (c) TiO2NTs electrodes.
with a high viscosity and transparent appearance. With LC as the template, the interaction between the tinantimony ions and the LC molecules is strong (Scheme 1d); then the controlled growth of SnO2 on the TiO2NTs (Scheme 1e) is realized (Scheme 1fh). Figure 1A,B shows the SEM images of the sieve-like Mp-SnO2/TiO2NTs. The electrode has two layers: the upper SnO2 layer and the lower TiO2NTs base layer. The top view (Figure 1A) displays that SnO2 grows to an orderly 2D film with macropores. The macropores are close to each other, with circular orifices and diameters from 150 to 400 nm, which are formed with the removal of the columnar liquid crystal template. The wide distribution of these diameters is attributed to the surfactant, which is a mixture of certain width molecular weight copolymers. The holewalls with upper and lower permeation are linked to each other, and TiO2NTs can be seen through the macropores (Figure 1B). Beneath the SnO2, TiO2NTs are highly 18263
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Figure 2. (A) UVvis DRS of Mp-SnO2/TiO2NTs, SnO2/TiO2NTs, and TiO2NTs. (B) Schematic illustration of the band-gap energy and charge separation of the Mp-SnO2/TiO2NTs electrode. (C) Photocurrent density versus bias potential (vs SCE) at 365 nm in 1 M KOH solution: (a) TiO2NTs, dark; (a0 ) TiO2NTs, light; (b) Mp-SnO2/TiO2 NTs, dark; (b0 ) Mp-SnO2/TiO2NTs, light.
ordered and compactly arranged (inset image in Figure 1A), and the tubes are uniform in length and vertically align with the diameter of 5090 nm, obviously smaller than that of the SnO2 macropores. Compared with the granular SnO2 film prepared in our previous study,24 the sieve-like SnO2 film is very smooth. The reason is that the interactions between the surfactant and inorganic molecules (mainly electrostatic forces and hydrogen
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bonding) are induced, when the surfactant solution and the tinantimony solution are mixed. The inorganic molecules will grow along the alignment direction of the LC template and form an ordered membrane structure.35 The smooth surface of the macroporous SnO2 also indicates that the particles are small and uniformly distributed. The crystal structure of electrodes was characterized by X-ray diffraction analysis (XRD) and is shown in Figure 1C. SnO2 is in the rutile phase and TiO2 is in the composite phase, mainly anatase, which is in good agreement with standard data of the JCPDS card. The diffraction peaks observed at 2θ = 25.5° and 37.9° are assigned to the (101) and (004) planes of anatase TiO2, and peaks corresponding to 2θ = 26.6, 33.9, and 51.8° are assigned to the (110), (101), and (211) planes of SnO2, respectively. The peak positions agree well with the reflections of SnO2, indicating a tetragonal rutile structure. Compared with that of SnO2/Ti, the SnO2 diffraction peak of Mp-SnO2/TiO2NTs is clearly enhanced. The SnO2 particle size is calculated by the Scherrer formula (D = kλ/β cos θ), which is 14.2 nm on the surface of Mp-SnO2/TiO2NTs and is small for metal oxide particles. The SnO2 particle size of SnO2/Ti prepared by the traditional dipping method is about 100 nm, and that prepared on the TiO2NTs by solgel with silicone surfactant is 20 nm.36 Therefore, SnO2 prepared by the LC soft template not only exhibits a sieve-like porous structure but also has smaller particle sizes and better dispersion. The assembly of SnO2 by the LC soft template has another important advantage, a high loading amount, since its electrochemical activity is closely related to the loading amount of electrocatalyst. With the advantages of a three-dimensional microstructure, large specific surface area, and space utilization of TiO2NTs, the Sb-doped SnO2 loading amount can reach 27.3 g m2 with a one-time assembly for using the LC soft template, which is much higher than that on the SnO2/Ti electrode, 8.5 g m2. This is due to that once the surfactant is aggregated into the LC zone, the system becomes viscous because of the strong interaction between LC internal molecules, so a high loading amount is obtained. 3.2. Photochemical Property. The optical absorption property of a catalyst is an important prerequisite for photocatalysis. It is generally believed that the red shift of the absorption band edge and the increase in absorption intensity can improve the PC activity. Figure 2A shows the UVvisible diffuse reflectance spectroscopy (UVvis DRS) of the three catalysts, including TiO2NTs, the SnO2/TiO2NTs electrode prepared by the traditional solgel method, and the sieve-like Mp-SnO2/TiO2NTs with the block polymer LC template method. The absorption band edge of TiO2NTs is 385 nm, and its ultraviolet absorption band is continuous in the range of 250300 nm. However, its absorptive capacity for the long-wavelength ultraviolet declines sharply in the range of 300370 nm. The absorption band of the two SnO2/TiO2NTs catalysts exhibits a distinct red shift with SnO2 assembled to the TiO2NTs. Whether the macroporous structure or the coating structure uniformly spreads into or outside the TiO2NTs, the absorption band edge is about 424 nm, and the ultraviolet absorption band shifts to 300370 nm. Compared with the TiO2NTs, two SnO2/TiO2NTs have a better absorptive intensity in the long-wavelength range above 370 nm. The band gap (Eg) of TiO2NTs is 3.22 eV according to Eg = 1240/λ.36 The values of Eg for both SnO2/TiO2NTs are 2.93 eV. Because the value of Eg for the semiconductor is significantly narrower, it is easier to be excited, which can be explained by the band gap matching between the SnO2 and TiO2NTs (Figure 2B). It is reverse doping between SnO2 and TiO2, 18264
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Figure 3. (A) Electrochemical impedance spectroscopy and (B) cyclic voltammetry of (a) Mp-SnO2/TiO2NTs, (b) SnO2/Ti, and (c) TiO2NTs electrodes in 5 mM [Fe(CN)6]3/[Fe(CN)6]4 solution.
because TiO2 has a lower Eg (3.22 eV) than SnO2 (3.88 eV). When it is irradiated, TiO2 is excited as a sensitizer to generate electrons and holes.37 Because the valence band and conduction band of TiO2 are lower than those of SnO2, the excited electron transits to the SnO2 layer, which changes the bottom of the conduction band of TiO2, reducing its Eg. The holes spread correspondingly to the TiO2 layer. The more holes reach the surface of TiO2, generating an oxidation reaction, the more electrons will be delivered to SnO2. SnO2 is equivalent to an excellent electric conductor, which transfers electrons to the surface, inhibits the recombination of photogenerated electrons and holes, and improves the photoelectric separation efficiency.38 It helps to improve the PC performance. Compared with the ultraviolet absorption (250370 nm) of TiO2NTs, that of the traditional SnO2/TiO2NTs electrode declines with Sb doped. The defect can be overcome when the coating SnO2 film is constructed to a 2D sieve-like macroporous structure. The MpSnO2/TiO2NTs displays the best light absorption characteristics in the whole region of light wavelength. This is attributed to the special constructed microstructure. The vertical thickness of the SnO2 coating is about 100 nm, and the horizontal direction has a 2D nanoscale pore structure (Figure 1B), which has a good light transparent effect so that the light can pass through these holes and directly irradiate on the surface of TiO2 so as to enhance the light absorption. The PC performance and photoelectric conversion efficiency (η) of the resulting materials were further investigated by linearsweep photovoltammetry under UV-light (365 nm) irradiation.
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As shown in Figure 2C, the dark current density on the MpSnO2/TiO2NTs is 0.2 mA cm2, whereas that on the TiO2NTs is only 0.01 mA cm2, demonstrating that the conductivity of Mp-SnO2/TiO2NTs is increased by the assembling of the Sbdoped SnO2 electrocatalyst, which agrees with the alternating current (ac) impedance results (Figure 3A). The saturated photocurrent density of Mp-SnO2/TiO2NTs (1.2 mA cm2) is about 1.7 times that of TiO2NTs (0.7 mA cm2). The value of η of the Mp-SnO2/TiO2NTs increases to 35.2%, which is 4.1 times that of the TiO2NTs 8.5%. The enhanced photoelectric conversion efficiency (η) on the Mp-SnO2/TiO2NTs is attributed to its highlight response in the UV light region and the implanting of the Sb-doped SnO2 electrocatalyst, which may contribute to the generation of photocurrent, because the doping of SnO2 and TiO2 facilitates the separation of photogenerated electrons and holes.39 3.3. Electrochemical Property. Figure 3A shows the EIS of Mp-SnO2/TiO2NTs, SnO2/Ti, and TiO2NTs. TiO2NTs exhibits poor conductivity with an electrochemical impedance of about 104 Ω. With a macroporous SnO2 film on the TiO2NTs, the electrochemical impedance of the composite electrode reduces to 170 Ω, only about a quarter of that of SnO2/Ti (600 Ω), which may be ascribed to the higher loading amount per unit area (27.3 g m2) and better dispersion of the electrocatalyst SnO2 on the TiO2NTs than on Ti. To further explore EC performance, the behavior of the [Fe(CN)6]3/[Fe(CN)6]4 redox couple on the electrode is also studied. Figure 3B shows well-defined anodic and cathodic peaks on the Mp-SnO2/ TiO2NTs and SnO2/Ti electrodes. The cyclic voltammogram for the Mp-SnO2/TiO2NTs displays an anodic peak potential Epa at 0.25 V and a cathodic peak potential Epc at 0.15 V, and their corresponding currents Ipa and Ipc are 5.0 and 4.3 mA cm2, respectively. Compared with that of SnO2/Ti, the potential difference of the Mp-SnO2/TiO2NTs between the anodic and the cathodic peaks, ΔEp, reduces from 0.39 to 0.1 V, which means that the reversibility of the electrode is better. The negative shift of the anodic peak potential and positive shift of the cathodic peak potential of the cyclic voltammogram on the Mp-SnO2/TiO2NTs indicates that the redox reaction of the [Fe(CN)6]3/[Fe(CN)6]4 redox couple needs less energy. Therefore, it can be expected that this macroporous structure is beneficial to the electrochemical performance of Mp-SnO2/ TiO2NTs. The activation energy (Ea) is another factor to evaluate the EC oxidation capability of electrodes, which is obtained by measuring the anodic polarization curves in 0.1 M H2SO4. Ea values of Mp-SnO2/TiO2NTs, SnO2/Ti, and TiO2NTs electrodes are 27.3, 32.2, and 38.0 kJ mol1, respectively. The Mp-SnO2/ TiO2NTs electrode has the lowest Ea needed to generate an electrochemical reaction, whereas the TiO2NTs electrode needs the highest, which suggests that the Mp-SnO2/TiO2NTs has better EC performance. The electrochemical adsorption (G) of PNP on the electrode surface is calculated according to a chronocoulometric method.40 More pollutant absorbed on the electrode surface leads to more reaction volume for electrochemical oxidation. Under the same conditions, G is 2.07 109 mol cm2 on the Mp-SnO2/ TiO2NTs, almost 4.3 times higher than the value of 3.91 1010 mol cm2 on the SnO2/Ti. Therefore, the microstructure of macroporous SnO2 is helpful to increasing the surface adsorption volume of the pollutant and accordingly promotes the reaction rate of the pollutant. 18265
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Figure 4. (A) PNP concentration as a function of time. Inset: the relationship between the natural logarithm concentration of PNP and the time. (B) TOC removal with degradation time. (C) Biodegradability evolution of PNP wastewater in different processes.
3.4. Integrated Photochemical and Electrochemical Oxidation for PNP. 3.4.1. High Removal Efficiency and Improvement of Biodegradability. As a refractory pollutant, the high toxicity and
aromatic structure of PNP may inhibit the microbial activity and make the biodegradation difficult. For comparison, PC, EC, and PEC oxidations were applied to decompose the aromatic structure and reduce the toxicity of PNP, enhancing its biodegradability. Figure 4A shows the removal efficiencies of PNP under five different catalytic conditions. For 4 h, in a single PC oxidation
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process, on pure TiO2NTs, the removal of PNP is only 12.5%, and the value increases to 17% on Mp-SnO2/TiO2NTs, indicating that the PC oxidation capacity of Mp-SnO2/TiO2NTs is better than that of pure TiO2NTs, which is consistent with the results of the photoelectric conversion efficiency above. In the PEC oxidation process, with the combined effects of PC and EC oxidations after 4 h, the removal of PNP is 98% by Mp-SnO2/TiO2NTs, much higher than that of Mp-SnO2/TiO2NTs EC (75.5%) and SnO2/ Ti EC (65%) processes. These five processes follow the apparent pseudo-first-order reaction, and the reaction rate in PEC oxidation on the Mp-SnO2/TiO2NTs is the highest (Figure 4A, inset). The apparent first-order kinetic constants (k) of PNP degradation on Mp-SnO2/TiO2NTs is 1.05 h1 in the PEC process, which is 2.9 times that of a single EC and 21 times that of a single PC process. Figure 4B shows that the TOC removal with Mp-SnO2/TiO2NTs PEC (91% at 4 h) is also the most efficient technique in the five processes. The TOC decreases to 10 mg L1 in 4 h, suggesting an almost complete mineralization of PNP by PEC oxidation on MpSnO2/TiO2NTs. When the evolution of PNP and TOC in the PEC process is compared with that in the single PC and EC processes on MpSnO2/TiO2NTs, it can be observed that the degradation effect in the PEC process is better than the sum of that in PC and EC processes, indicating a synergistic effect of photocatalysis and electrocatalysis. In a single Mp-SnO2/TiO2NTs PC process, the absorbed PNP and intermediates on the electrode surface may reduce UV light absorption and passivate its active sites, decreasing the degradation efficiency. In the photoelectro-syenergistic catalytic process, the EC oxidation timely assists the removal of intermediate products, so the absorption of UV light is enhanced and promotes the PC oxidation effects. At the same time, the holes and some free radicals generated in PC oxidation also help to remove intermediates and reduce the electrocatalyst poisoning. Therefore, the Mp-SnO2/TiO2NTs electrode presents better PC and EC performances simultaneously. To evaluate the biodegradation evolution of PNP, the proportion of the biodegradable pollutants in the total amount of pollutants (BOD5/TOC) is calculated.4143 The greater the ratio of BOD5/TOC, the higher the biodegradability of the wastewater is. The original BOD5 and TOC of the 200 mg L1 PNP solution are 3.46 and 108 mg L1, respectively, giving a low BOD5/TOC ratio of 0.032, suggesting the poor biodegradability. The biodegradation can be dramatically improved by an integrated PC and EC oxidation treatment using the Mp-SnO2/ TiO2NTs. Figure 4C shows that the maximum biodegradability index BOD5/TOC rapidly reaches 1.05 in 2 h in the Mp-SnO2/ TiO2NTs PEC process so that the wastewater may be treated with the biological technique hereafter. It is interesting that the BOD5/TOC ratio at the early stage is even slightly lower than the initial value of the original PNP solution. It indicates that the byproduct formed during the initial steps of reaction may be more toxic to microorganisms. After reaching the maximum of 1.05, the index declines, because the accumulated intermediates in the wastewater are further mineralized to CO2 and H2O, reducing the amount of bioavailable substances. Although the trend of biodegradability in other processes is similar to that in the Mp-SnO2/TiO2NTs PEC oxidation process, the BOD5/ TOC ratio just reaches the maximum of 0.58 in 2.5 h and 0.75 in 3 h in SnO2/Ti EC and Mp-SnO2/TiO2NTs EC processes, respectively. The BOD5/TOC ratio changes little in the single Mp-SnO2/TiO2NTs PC and TiO2NTs PC processes, so these two processes are useless for this high-concentration PNP. 18266
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Figure 5. Evolutions of (A) the toxicity and (BF) intermediates in different PNP degradation processes as a function of time.
3.4.2. Evolutions of Toxicity and Intermediate Products. The above results indicate that the 2D macroporous structure of MpSnO2/TiO2NTs improves the EC and PC performances and exhibits superior PEC activity for the degradation of PNP. The acute toxicity of the PNP degradation in the wastewater is further evaluated. The original PNP wastewater is highly toxic (PNP EC50 = 8.8 mg L1), resulting in a low BOD5. The relative toxicity index is defined as 1.0. Although the PEC performance of Mp-SnO2/TiO2NTs is prominent, some intermediates with different toxicities are produced and accumulated inevitably during the degradation process, varying the toxicity of the PNP wastewater. These intermediates include aromatics with high toxicity, such as hydroquinone and benzoquinone, and the main carboxylic acids with low toxicity, such as maleic acid, fumaric acid, and oxalic acid. The order of (from high to low) the acute toxicity determined by the luminescent marine bacteria for the main intermediate products is as follows: benzoquinone (EC50 < 0.01 mg L1), hydroquinone (EC50 = 0.04 mg L1), PNP, maleic acid (EC50 = 250 mg L1), fumaric acid (EC50 = 260 mg L1), and oxalic acid (EC50 > 450 mg L1).44,45 During the PNP degradation, the toxicity of degraded samples is mainly decided by those highly toxic intermediates, such as hydroquinone, benzoquinone, and other undetected substances. As shown in Figure 5A, in the Mp-SnO2/TiO2NTs EC process, as PNP is rapidly oxidized to hydroquinone and benzoquinone, the toxicity of the samples sharply increases to 3.5 times that of the original simulated wastewater in 1.5 h. In 4 h, these toxic intermediates are gradually decomposed to small molecule carboxylic acids, so the toxicity decreases to 30% of the initial sample. In the MpSnO2/TiO2NTs PEC process, owing to the excellent synergistic PEC effect, the Mp-SnO2/TiO2NTs exhibits a strong mineralization ability compared with its single EC and PC oxidation. Accordingly, the toxic intermediates are rarely accumulated and the toxicity in the whole process is lower than that in other processes. In 1 h, the toxicity index reaches the maximum value,
which is 2.6 times that in the initial sample. Furthermore, because the mineralization rate of PNP is fast, the sample is basically nontoxic after 4 h. In the SnO2/Ti EC process, the wastewater shows higher toxicity than in other processes comparatively. The relative toxicity index rapidly increases to the maximum 4.7 and finally decreases to 3.5. In Mp-SnO2/TiO2NTs PC and TiO2NTs PC processes, their evolution trend is similar. Those highly toxic intermediates accumulate on the electrode surface since they cannot be decomposed timely and hinder the further oxidation process, so the toxicity of the wastewater increases slowly all the while. These results demonstrate that, for some aromatic pollutants, the overall toxicity of the wastewater should be evaluated in whole treatment process to prevent the generation of more toxic intermediates and thereby avoid more serious pollution. To further study the relationship between acute toxicity and intermediate products, the concentrations of several main intermediates are determined as a function of time (Figure 5BF). In single Mp-SnO2/TiO2NTs PC and TiO2NTs PC processes, the concentrations of hydroquinone and benzoquinone slowly increase to 1.3 and 1.4 mg L1, and 0.9 and 1.0 mg L1 in 4 h, respectively, whereas in the SnO2/Ti EC, Mp-SnO2/TiO2NTs EC, and Mp-SnO2/TiO2NTs PEC processes, the maximum concentration (Cmax) of hydroquinone is 6.9 mg L1at 2.5 h, 5.4 mg L1 at 1.5 h, and 3.8 mg L1 at 1 h, respectively. The accumulated concentrations of benzoquinone in five processes are all less than 4 mg L1. Although the concentrations of hydroquinone and benzoquinone are low, it is the key factor that determines the toxicity of wastewater. Thus, the trend of toxicity evolution is similar to that of these two aromatics. It is notable that the concentrations of fumaric acid and maleic acid are lower than 8 mg L1 in five processes. Moreover, Cmax of oxalic acid reaches 96 mg L1 at 3 h in SnO2/Ti EC, 78 mg L1 at 2.5 h in Mp-SnO2/TiO2NTs EC, and only 55 mg L1 at 2 h in the Mp-SnO2/TiO2NTs PEC process. In Mp-SnO2/TiO2NTs 18267
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The Journal of Physical Chemistry C PC and TiO2NTs PC processes, concentrations of the three carboxylic acid intermediates increase continuously. For the MpSnO2/TiO2NTs PEC process, the maximum concentrations of intermediates are lower than that in other processes, the times for these intermediates to reach their maximum concentrations are shorter, and these highly toxic intermediates can be decomposed easily with the PEC oxidation, resulting in the lowest detected toxicity. The above results further reveal that the electrochemical performance of the Mp-SnO2/TiO2NTs toward the intermediates is superior to that of the SnO2/Ti, which is attributed to the small particle size and the sieve-like macroporous structure of SnO2 in favor of the intermediates' adsorption. Moreover, when the integrated PC and EC oxidation reactions occur simultaneously on the Mp-SnO2/TiO2NTs surface, the catalytic oxidation efficiency is higher than the summation of its single PC and EC oxidation processes due to the synergistic effect. The light irradiation makes the active sites on the electrode surface fully exposed, which overcomes the shortcoming that the active sites are easy to be inactive without the light irradiation and improves the electrochemical current efficiency. At the same time, the good conductivity of the electrode not only promotes the separation of the photogenerated electronhole and increases the photoelectric efficiency, but also makes the PC oxidation less blocked because the species can be easily eliminated by EC oxidation.
4. CONCLUSION A sieve-like Mp-SnO2/TiO2NTs electrode is successfully constructed by assembling Sb-doped SnO2 on the vertically aligned TiO2NTs using a facile liquid crystal soft template method. SEM confirms that the diameter of macropores is between 150 and 400 nm, and the prepared Mp-SnO2/TiO2NTs shows strong absorption in the wavelength of less than 424 nm due to its intrinsic band-gap energy reduction. The Mp-SnO2/ TiO2NTs simultaneously possesses superior PC and EC performances. It displays excellent PEC synergistic oxidation ability in decreasing the toxicity of PNP. After 4 h, the PNP and TOC removal reaches 98% and 91%, respectively. The relationship of the relative toxicity evolution and the main intermediates is also discussed. This study provides some new insight into the design and fabrication of advanced integrated catalysts with remarkable PEC activity for the degradation of highly concentrated refractory toxic organic pollutants. ’ AUTHOR INFORMATION Corresponding Author
*Phone: (86)-21-65988570-8244. Fax: (86)-21-65982287. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported jointly by the National Natural Science Foundation of China (Project Nos. 21077077, 20877058) and the 863 Program (Project No. 2008AA06Z329) from the Ministry of Science. ’ REFERENCES (1) Jung, J. H.; Kobayashi, H.; van Bommel, K. J. C.; Shinkai, S.; Shimizu, T. Chem. Mater. 2002, 14, 1445–1447. (2) Lakshmi, B. B.; Dorhout, P. K.; Martin, C. R. Chem. Mater. 1997, 9, 857–862.
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