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Anodic fabrication of advanced titania nanotubes photocatalysts for photoelectrocatalysis decolorization of Orange G dye
Chemosphere 144 (2016) 2462e2468
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Anodic fabrication of advanced titania nanotubes photocatalysts for photoelectrocatalysis decolorization of Orange G dye Yaju Juang a, 1, Yijin Liu a, 1, Ervin Nurhayati a, 1, Nguyen Thi Thuy a, Chihpin Huang a, *, Chi-Chang Hu b, ** a b
Institute of Environmental Engineering, National Chiao Tung University, Hsinchu, Taiwan Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Successful fabrication of wellarranged TNTs photo-anode via anodizing. Application of the TNTs in photoelectrocatalysis (PEC) for dye wastewater treatment. The interrelationship of TNTs preparation condition, properties and PEC performance is examined. There is an optimum tube length-todiameter ratio to produce highest photocurrent. Synergistic effect of photocatalytic and electrochemical process increases PEC efficiency.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 August 2015 Received in revised form 8 November 2015 Accepted 9 November 2015 Available online xxx
Titania nanotubes (TNTs) were fabricated on Ti mesh substrates by the anodizing technique. The effects of preparation variables, such as anodizing voltage, time and calcination temperature on the textural characteristics and photocatalytic activity of TNTs were investigated. The surface morphology, crystalline phase, and chemical composition were analyzed using field emission-scanning electron microscopy and X-ray diffraction. The photo-electrochemical properties of TNTs were examined by voltammetry. The TNTs were tested as a photoanode for advanced oxidation processes, such as photocatalytic, electrocatalytic, and photoelectrocatalytic decolorization of Orange G dye. The well-arranged TNTs electrode prepared in this work showed a high photocurrent density of 101 mA cm2 at an optimum length-todiameter aspect ratio of 31.2. In dye decolorization tests, the electrochemical photocatalytic system using TNTs as the photoanode achieved total decolorization and 64% mineralization under extended reaction time. These results show that TNTs prepared by this method is greatly stable in prolonged use and suitable as a photoanode in the photocatalytic/photoelectrocatalytic treatments of dye wastewater. © 2015 Elsevier Ltd. All rights reserved.
Handling Editor: E. Brillas Keywords: Anodizing Orange G Titania nanotube Photoanode Photoelectrocatalysis
* Corresponding author. Institute of Environmental Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 300, Taiwan. ** Corresponding author. Department of Chemical Engineering, National Tsing Hua University, 101, Section 2 Kuangfu Road, Hsinchu 300, Taiwan. E-mail addresses: [email protected] (C. Huang), [email protected] (C.-C. Hu). 1 Co-first authors. http://dx.doi.org/10.1016/j.chemosphere.2015.11.029 0045-6535/© 2015 Elsevier Ltd. All rights reserved.
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1. Introduction Since the discovery of TiO2 application for photocatalytic water splitting (Akira and Honda, 1972), TiO2-based photocatalysis has been extensively studied for water and wastewater treatment applications because of its nontoxicity, photochemical stability, and low cost (Carp et al., 2004). For such applications the photocatalytic (PC) activity of TiO2 is mainly determined by its ability to create electron/hole pairs for generating free radicals, such as hydroxyl, a species essential for advanced oxidation processes (AOPs) in wastewater treatment (Sood and Gouma, 2013). Photoelectrocatalytic (PEC) oxidation, which drives the photogenerated electrons to the counter electrode through potential biases via an external circuit to reduce the recombination of electrons and holes, has been found to be more efficient than PC oxidation (Vinodgopal et al., 1993; Byrne and Eggins, 1998; Calvo et al., 2001; Sene et al., 2003). In PEC systems, a TiO2 nanoparticles film electrode is usually used as the photoanode. Unfortunately, the structural disorder of nanoparticle system increases the scattering of free electrons and therefore reduces electron mobility (Peng et al., 2003; Paulose et al., 2006). Other than the recombination rate of photo-generated charge carriers, the surface area provided by titania also strongly influences its performance (Sood and Gouma, 2013). Several studies have shown that TiO2 thin films grown directly on a Ti substrate by anodizingdhighly ordered and perpendicularly aligned TiO2 nanotubes (TNTs)dsuccessfully reduces the resistance to electron transfer and avoids the recombination of photo-generated electron/ hole pairs, while at the same time overcoming the weak adherence issue of TiO2 on a substrate (Quan et al., 2005; Su et al., 2008) and provide a large surface area that results in a large adsorption capacity of organic pollutants (Liu et al., 2009). Recently, anodic fabrication techniques have successfully created TNTs with ultrahigh aspect ratio of length-to-diameter, from ~1500 (Wang and Lin, 2008) to ~10,000 (Paulose et al., 2007). Nevertheless, the different application of TNTs might require different aspect ratio, which means the higher the aspect ratio not necessarily the better the TNTs. Herein, we report fabrication of TNTs with lower aspect ratio, but have been optimized specifically for the application of dye wastewater treatment in this study. Although TNT arrays have been widely applied for the photoelectrochemical processes (Quan et al., 2005; Paulose et al., 2006; Chen et al., 2008; Liu et al., 2009), the interrelationship of preparation conditions, microstructures, and the photocatalytic/photoelectrocatalytic performances is still not clear. The objective of this study is to control the microstructure (morphology and crystalline phase) of TNTs by varying the preparation conditions, such as anodizing voltage and duration, and calcination temperature. The correlation between microstructure and photocatalytic/photoelectrocatalytic performance are then established. The most promising TNTs electrode is then used to degrade Orange G dye under various AOPs systems in order to determine the best condition for the wastewater treatment. Orange G dye which has a complex molecular structure is chosen to simulate highly persistent organic dye wastewater that cannot be easily degraded using conventional treatment. 2. Methods 2.1. Preparation of TNTs electrode The preparation of TNTs array electrodes consisted of three steps: substrate pretreatment, anodizing and post treatment. In pretreatment, 50 mm 30 mm pieces of Ti mesh of mesh size
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1 mm 2 mm, was degreased by sonication in a neutral cleaning agent for 15 min, followed by heating in 6 M HCl at 90 C for 30 min to remove oxide impurities from its surface. The etched substrate were then rinsed with de-ionized (DI) water and cleaned in a sonication bath for 30 min and subsequently dried in an oven at 100 C for 30 min. Self-arranged TNT array electrodes were fabricated by anodizing the pretreated Ti mesh substrates at 20 C with a platinum plate (50 mm 20 mm) serving as the cathode, using a DC power supply (Major Science MP-500V). Anodizing was carried out at various constant potentials of 20, 30, 40, and 50 V for 40 and 60 min duration. The anodizing electrolyte consisted of ethylene glycol (EG) as the solvent, with 2 vol% DI water and 0.3 wt% NH4F. The post-treatment step involved surface cleaning and calcination. Surface cleaning was done to remove the disordered and amorphous structures from the surface of TNT arrays by sonication in DI water for 1 min. To induce crystallization, the samples were subjected to calcination at 350, 450, 550, 650 and 750 C for 2 h, at a heating rate of 2 C min1 under air flow. All chemicals used in this study were of analytical grade. 2.2. Characterization of TNTs electrodes The surface morphology of TNT arrays was determined by field emission scanning electron microscopy (FESEM, JEOL JSM-6700F). To evaluate the crystalline structure of TNT arrays, XRD analysis was obtained using a Rigaku TTRAX III. The photo-electrocatalytic activities of fabricated TNTs electrodes were examined using voltammetry performed by an electrochemical analysis work station (AutoLab Potensionstat/ Galvanostat, PG Stat 302N) in a three-electrode single-cell system with 0.1 M Na2SO4. A platinum plate (40 mm 20 mm) and an Ag/ AgCl electrode served as counter and reference electrodes, respectively. The photocurrents of TNTs electrodes were obtained by linear sweep voltammetry (LSV) at a scan rate of 10 mV s1 from 0 to 1.5 V with and without UV (24 W) illumination. Cyclic voltammetry (CV) measurement was conducted with the same setup applying scan rate of 10 mV s1 from 0 to 2.5 V for 100 cycles. 2.3. Orange G dye degradation The performance of TNT electrodes for the catalytic degradation of Orange G dye was evaluated using several AOP methods: photocatalysis (PC), electrocatalysis (EC), and photoelectrocatalysis (PEC) in a 180-min reaction period with an ambient air flow into the solution at 200 mL min1. A 0.5 mM Orange G solution was prepared by dissolving commercial Orange G powders (Sigma) in DI water and adjusting the pH to 3 with H2SO4 (Sigma). A photolysis experiment was conducted by illuminating this solution with UV light to evaluate the natural photo-degradation of Orange G. PC experiments were conducted by immersing the TNT array electrodes with UV light irradiation. EC oxidation reaction was conducted by employing a TNT array anode and a graphite cathode under a constant current density without UV light irradiation, and PEC degradation of the Orange G dye was carried out in a cell with a TNT array photoanode and a graphite cathode under a constant current with the UV light irradiation. UV light intensity was 1 mW cm2; the current applied to the cell was 0.5 mA; the geometric area of the TNT array electrode and the graphite cathode were 12 cm2. The reactor setup is shown in Fig. SM-1. In order to avoid interference of physical adsorption of dyes on the performances of AOPs, both electrodes were immersed in the solution for 30 min to reach adsorption saturation before each experiment. The dye solutions were periodically sampled and analyzed by spectrophotometer (Metertech SP-8001). Individual peak intensities obtained at 230, 248, 330 nm were used to indicate the
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presence carboxyl group, benzene ring and naphthalene ring, respectively. Decolorization of Orange G resulting from the N ¼ N breakage was indicated by the intensity of absorbance at 478 nm. TOC degradation was monitored with a total organic carbon analyzer which uses oxidative combustion (Shimadzu TOC-L Series). The Orange G concentration was measured by high performance liquid chromatography (HPLC) (Waters 2695 separation module), coupled with a photo diode detector (detection wavelength: 480 nm, Waters 2996 PDA), with a Waters XBridge™ column (C18, 4.6 mm 250 mm inner diameter, 5 mm beads). The mobile phase setting was adapted from reference (Bokare et al., 2008). 3. Results and discussion 3.1. Effect of anodizing parameters on TNTs microstructure and photocurrent The microstructure, i.e. pore diameter and length, of TNTs can be controlled by varying the anodizing conditions such as applied voltage and time (Bauer et al., 2006; Raja et al., 2007; Yasuda et al., 2007; Chen et al., 2008; Macak et al., 2008; Kojima et al., 2012; Regonini et al., 2012). Fig. 1 clearly shows that tube diameter and length were increased by increasing the voltage and anodizing time. These images provide well-defined view; the boundary between Ti substrate and TNTs can be easily distinguished from the cross-section photos while the thickness and pore diameter are very uniform. Therefore, the average pore diameter of TNT arrays prepared under various conditions can be estimated from these images, as presented in Fig. 2. The relationship of the tube diameter and length with the anodizing conditions is shown in Fig. 2a. From curves 1 and 2, the average diameter of TNTs increases approximately linearly with increasing voltage. At the specified anodizing voltages the average pore diameters obtained at 60 min are larger than that at 40 min. Curves 3 and 4 show that the TNTs length is also proportional to the voltage at both anodizing times. The tube can also be easily elongated by increasing the anodizing time under a specified voltage. At an anodizing duration of 40 min, increasing the voltage from 20 to 50 V enlarges the average diameter from 37.9 to 89.3 nm and increases the tube length from 0.87 to 3.16 mm. Similarly, under an anodizing time of 60 min, the average diameter and length of TNTs increase from 45.5 to 98.6 nm and from 1.34 to 4.66 mm when the voltage is increased from 20 to 50 V, respectively. Pore diameter and length of TNTs are thus easily controlled by varying the
Fig. 2. (a) (1,2) Diameter and (3,4) length of TNTs against the anodizing voltage for (1,3) 40 min and (2,4) 60 min. (b) Photocurrent against the aspect ratio (L/D) of TNTs prepared under various anodizing conditions.
Fig. 1. FESEM images of top and side views of TNTs fabricated at anodizing (voltage, time): (a) (20 V, 40 min), (b) (30 V, 40 min), (c) (40 V, 40 min), (d) (50 V, 40 min), (e) (30 V, 60 min), and (f) (50 V, 60 min).
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anodizing voltage and time. Similar phenomenon for the dependence of tube diameter and length on the anodizing voltage and time has been reported in the literature (Bauer et al., 2006; Macak et al., 2008; Liu et al., 2012). However, in this case the diameter of TNTs generally depends more on the voltage of anodizing than the anodizing time. For example, increasing the anodizing voltage from 30 to 50 V at an anodizing time of 40 min results in an increase in the tube length from 1.43 to 3.16 mm; tube length reached only 2.04 mm under an applied voltage of 30 V for 60 min. This is possibly a result of quasi-steady state condition, where after a certain time the formation and dissolution of TiO2 occurred at similar rate. The microstructure of TNTs is related to the photocurrent produced, because it will affect the rate at which electron/hole pairs are generated (Yu and Wang, 2010). Fig. 2b plots photocurrent against the L/D (length/diameter) aspect ratio of TNTs prepared under different anodizing condition. With an anodizing time of 40 min, the increase in the voltage from 20 to 50 V increases the L/D from 23.0 to 35.4. Interestingly, the photocurrent also steadily increases from 53.5 to 94.6 mA. Similarly, increasing the applied voltage from 20 to 50 V increases the L/D from 29.5 to 49.8 at an anodizing time of 60 min. However, the photocurrent of this series of TNTs reached maximum (101 mA) at 30 V, after which the photocurrent kept decreasing with increasing applied voltage, regardless any further increase in the L/D. Based on the above data, there are two linear dependence of the photocurrent on the L/D: when the L/D is less than/equal to 31.2, the photocurrent increases nearly linearly proportional to the increase of L/D and a maximum photocurrent is obtained; on the other hand, as the L/D becomes larger than 31.2, the photocurrent linearly decreases with increasing the L/D aspect ratio, although the slope (absolute value) of this downward trend is obviously less than that of the upward trend. The exact reasons for the above phenomena are still unclear. Photocurrent is generally proportional to the amount of TNTs on the photo-electrode because it should be linear to the lights absorbed by TNT arrays, leading to the more electron/hole pair generation. However, the longer a tube is, the longer transport pathway of the charge carrier is, which results in the greater chance of electron trapping. This effect will extend the residence time of photo-generated electrons in the film before being collected on the current collector (i.e., Ti under the TNTs) (Zhu et al., 2007), leading to a lower photocurrent. Other than that, disorder and defects in the longer TNTs increase the chance of electrons-holes recombination, which would extend the electron transport pathway, leading to a lower photocurrent (Zhu et al., 2007). From these results can be concluded that there is an optimal value of the L/D aspect ratio (31.2 as determined in this work) that will generate the maximum photocurrent. 3.2. Effects of the calcination temperature on TNTs properties TNTs prepared by anodizing typically exhibit an amorphous structure (Macak et al., 2007); thermal annealing is usually employed to convert it into crystalline anatase or rutile phases (Macak et al., 2008). Upon annealing, transformation in crystalline structure and a change in the morphology of the as-prepared TNTs takes place at the same time. The transformation of crystalline structure of TNTs via annealing was determined by XRD analysis (Fig. 3) and its influences on the morphology and integrity of the tubular structure was verified with FESEM images (Fig. 4). XRD pattern of as-prepared TNTs (Fig. 3, pattern 1) shows only the diffraction peaks with 2q values at 35.09, 38.43, 40.17, 53.01, and 70.66 corresponding to facets (100), (002), (101), (102) and (103) of hexagonal titanium, respectively, indicating the amorphous
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Fig. 3. XRD patterns of (1) as-prepared TNTs, and those with calcination at (2) 350, (3) 450, (4) 550, (5) 650, and (6) 750 C.
structure of TNTs. This amorphous as-prepared TNTs film consists of highly ordered and well-aligned nanotube arrays perpendicularly stand on the Ti substrate (Fig. 4a). However, after calcination at 350 and 450 C (see patterns 2 and 3 Fig. 3), the amorphous TNTs were crystallized to form the anatase phase with diffraction peak centered at 25.3 (101). Interestingly, these TNTs retain the original morphology, showing no obvious variance in diameter and good integrity of both tubular wall and tube-substrate interface. As the annealing temperature reaches 550 C, a minor but distinct rutile (110) peak located at 27.5 appears (pattern 4 Fig. 3). Clearly, the TNTs show an anatase-rutile bi-phase structure. At the same time, a change at the TNT-substrate interface occurs (see the cross-section image in Fig. 4b). When the calcination temperature increases to 650 C, the rutile phase becomes the dominant structure (pattern 5 Fig. 3), and shrinkage of the whole tube became obvious, while the tubular structure was still visible (Fig. 4c). By further increasing the temperature to 750 C, the anatase-rutile biphase structure is converted into pure rutile (Fig. 3 pattern 6) and the tubular structure collapsed completely (Fig. 4d). A model has been proposed (Yu and Wang, 2010) to explain the anatase-torutile phase transformation and morphology architecture evolution of TNTs, which is strongly linked to crystal nucleation activation energy (Zhang and Banfield, 2011). 3.3. Photocurrent and photocatalytic activity of annealed TNTs The photocurrent generated by the electrodes was determined by photocurrent-voltage curves using LSV (Fig. 5a). The photocurrent of curves 4e6 increase sharply to the quasi-saturated value (the plateau) when the potential biases are equal to or more than ca. 0.3 V. This indicates that the photo-generated electron/hole pairs can be effectively separated by applying a low potential bias. The order of TNTs with decreasing the photocurrent obtained at 0.4 V is: TNT-550 (0.094 mA) z TNT-450 (0.092 mA) > TNT-350 (0.08 mA) > TNT-650 (0.021 mA) > TNT-as-prepared (0.007 mA). The results of cyclic voltammetry measurement for 100 cycles to examine the stability of TNTs show that TNTs calcined at 550 C has high electro-stability (Fig. SM-2). Comparing Fig. 5a and b, Orange G decolorization efficiency by PC process generally follows the trend of the TNT photocurrent in the plateau region. This also goes to show that the photocurrent measured from the LSV curves is a good index of photocatalytic activity of TNTs. Similar results have been found for Degussa P25
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Fig. 4. FESEM images of top and cross-section view of (a) as-prepared TNTs, and those calcined at (b) 550, (c) 650, and (d) 750 C.
TiO2 in our previous work (Lin et al., 2013), confirming the reliability of photo-electrochemical evaluation. 3.4. TNTs applications for Orange G decolorization in various AOPs The TNTs electrode was applied to various AOPs for Orange G dye decolorization and the results are shown in Fig. 6. When Orange G solutions were illuminated with UV for 180 min, only 1% decolorization resulted (curve 1 Fig. 6a), confirming that photolysis was negligible and that the very persistent characteristics of this dye can only be degraded by strong oxidants. Decolorization via photocatalytic process suggests that the recombination rate of charge carrier on the TNTs electrode is too fast to produce a sufficient amount of hydroxyl radicals to degrade the Orange G dye (8%, curve 2 Fig. 6a). Moreover, in earlier work (Lin et al., 2013) we found that Orange G dye in the solution inhibits UV absorption on TiO2. Fig. 6a shows that a higher decolorization rate of Orange G is obtained in the electrochemical decolorization process (curve 3) than the photocatalytic one (curve 2). This phenomenon is attributed to three effects. First, dye decolorization/degradation via direct electron transfer that likely occurs under a high positive potential (Xie and Li, 2006). Second, highly oxidative electroactive species, other than hydroxyl radicals, capable of decomposing Orange G are probably produced on the anode at high potentials. Third, to balance the anodic currents, H2O2 can be generated on the graphite cathode as per Eq. (1), leading to additional oxidation of dye molecules by hydroxyl radicals generated by the reaction between H2O2 and superoxide radicals (Eq. (2)). All these effects result in a higher decolorization efficiency of ca. 28% by electrochemical degradation compared to a photocatalytic system.
O2 þ2Hþ þ 2e/H2 O2
(1)
H2 O2 þ O2 / OHþ OH þ O2
(2)
Based on these results, combining photocatalytic and electrochemical degradation processes, i.e. a PEC process, is expected to increase the decolorization rate of Orange G (Fig. 6a, curve 4). The decolorization rate of 60% attained is in fact higher than the sum of dye decolorization via the decolorization processes produced by photocatalytic and electrochemical process individually. This synergetic effect of the PEC process is a result of several additional
factors. First, the H2O2 generated from the graphite cathode decomposes to produce OH under UV irradiation (Eq. (3)). Further, the potential bias on the photo-anode significantly hinders the recombination rate of electron/hole pairs on the photo-anode (Eq. (4)), and photo-generated electrons migrate to the graphite cathode for H2O2 production, which avoids the scavenging effect on the hydroxyl radicals created on the photo-generated holes (Eq. (5)). Garcia-Segura et al. (2013) suggested that this photo-generated electron will also react with H2O2 to produce hydroxyl radicals (Eq. (6)), thus further increase the PEC activity.
H2 O2 þ hv/2 OH
(3)
TiO2 þ hv/eCB þhþ VB ðon TiO2 Þ/TiO2 þ heat
(4)
eCB þ OH/OH
(5)
eCB þH2 O2 / OH þ OH
(6)
Based on above results, the potential bias and its corresponding current in a PEC process play an important role in Orange G decolorization. The effect of applied current on the decolorization rate of Orange G via the PEC process for a 180-min degradation test are shown in Fig. 6b. Note that the Orange G decolorization percentages of 25%, 35%, 52%, and 60% are generally proportional to the applied current of 0.3, 0.5, 0.8, and 1.0 mA, with corresponding electrode potentials of 0.15, 0.80, 1.70 and 2.60 V vs. Ag/AgCl, respectively. The pH of final solutions is not significantly affected by the applied current after the 180-min test, varying between 3.08 and 3.12, very close to the initial pH of 3. A separate LSV to measure photocurrent without Orange G dye presence (data not shown) shows that TNTs can provide 390 mA under UV illumination. It means an applied current of 0.3 mA (potential of 0.15 V vs. Ag/AgCl) should not promote the electron/hole separation significantly. The potential bias on the photo-anode becomes more positive with the increase of current, significantly enhancing the separation of electron/hole pairs and the generation of OH, leading to higher rates of Orange G decolorization. At 1.0 mA, the electrode potential of the TNT array electrode reaches 2.6 V (vs. Ag/AgCl), a potential high enough for both direct and indirect oxidation of Orange G (Xie and Li, 2006), which is expected to be the practical optimum applied current.
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Fig. 5. (a) LSV curves of (1) background current, and photocurrents of (2) as-prepared TNTs, and those calcined at: (3) 650, (4) 350, (5) 450, and (6) 550 C. (b) Photocatalytic activity (Orange G decolorization) of TNTs annealed at: (1) 350, (2) 450, (3) 550, and (4) 650 C.
To evaluate the long-term performance of these electrodes, they were tested for Orange G dye decolorization for 24 h, typical results are given in Fig. 7. Curve 1 in Fig. 7a shows the decolorization of above 90% at 8 h. The relative absorbance of phenol (curve 4) and carboxyl (curve 2) are consistently above the other intermediates because both structures are likely to be in the final form of intermediates before mineralized to CO2. More than 80% of all of intermediates have been removed after 24-h reaction. Extending the reaction time thus significantly enhances degradation efficiency, and that the TNTs electrode fabricated in this study is suitable for the PEC application, and is stable even under the extended reaction times. Fig. 7b shows an orange G concentration of less than 10% after 5h PEC degradation, and was undetectable after a degradation time of longer than 10 h. Total mineralization of Orange G was evaluated by measuring the solution TOC. Theoretically, the TOC value of 0.1 M Orange G is 19.2 mg L1, but the TOC measurement only showed 14.8 mg L1 indicated that the complex structure of Orange G was difficult to be combusted completely during TOC analysis.
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Fig. 6. (a) Decolorization of Orange G by (1) photolysis, (2) photocatalytic (PC), (3) electrocatalytic (EC), and (d) photoelectrocatalytic (PEC) processes. The UV radiation intensity is 1 mW cm2 at 365 nm and the applied current is 1 mA. (b) Decolorization of Orange G in a PEC system under (1) 0.3, (2) 0.5, (3) 0.8, and (4) 1.0 mA.
The slight increase and then gradual decrease of TOC during the initial PEC degradation resulted from the initial selective degradation process with accumulation of various intermediates that reacting differently during TOC analysis. The rate of TOC removal becomes higher when all Orange G molecules degraded into intermediates after about 10 h, suggesting that OH generated via the PEC process initially shows the selective oxidation of Orange G (the cleavage of azo p-conjugation structure) leading to a low rate of initial TOC removal. The oxidative degradation proceeds easier once the complex structure of the original Orange G dye has been broken down. As a result, the final TOC removal is 64% after a 24-h PEC treatment. 4. Conclusions The well-ordered TNT arrays were successfully fabricated on a Ti mesh substrate by a simple anodizing method. This shows promising potential for electrochemical-photocatalytic wastewater
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References
Fig. 7. Degradation of Orange G in an PEC system under 1.0 mA for 24 h in terms of (a) UV/Vis absorbance for (1) Orange G (478 nm), (2) carboxyl (230 nm), (3) benzene ring (248 nm), (4) phenol (280 nm), and (5) naphthalene ring (330 nm); and (b) HPLC/PAD analyses for (1) TOC and (2) Orange G concentration.
treatment because of its high photocatalytic activity and stability as a photo-anode in various AOPs processes. The diameter and length of TNT arrays are easily controlled by adjusting the voltage and time of anodizing, while crystalline structure and crystal phase composition are easily tuned by varying the calcination temperature. Both the morphology and crystallinity of TNTs strongly affect their photocatalytic activity, where an optimum L/D aspect ratio and biphase TiO2 resulted in the highest photocurrent. A synergistic effect in Orange G decomposition has been obtained when photocatalytic and electrochemical processes were combined into a PEC process. Acknowledgment This research was financially supported by Ministry of Economic Affairs: 101-EC-17-A-08-S1-208, Taiwan, R.O.C., project number 101-EC-17-A-08-S1-208 and Ministry of Science & Technology, MOST 104-3113-E-194-002. All the material surface analysis was supported by Ministry of Science and Technology: MOST 104-3113E-194-002 Instrument Center, R.O.C. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.chemosphere.2015.11.029.
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Anodic fabrication of advanced titania nanotubes photocatalysts for photoelectrocatalysis decolorization of Orange G dye Yaju Juang a, 1, Yijin Liu a, 1, Ervin Nurhayati a, 1, Nguyen Thi Thuy a, Chihpin Huang a, *, Chi-Chang Hu b, ** a b
Institute of Environmental Engineering, National Chiao Tung University, Hsinchu, Taiwan Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Successful fabrication of wellarranged TNTs photo-anode via anodizing. Application of the TNTs in photoelectrocatalysis (PEC) for dye wastewater treatment. The interrelationship of TNTs preparation condition, properties and PEC performance is examined. There is an optimum tube length-todiameter ratio to produce highest photocurrent. Synergistic effect of photocatalytic and electrochemical process increases PEC efficiency.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 August 2015 Received in revised form 8 November 2015 Accepted 9 November 2015 Available online xxx
Titania nanotubes (TNTs) were fabricated on Ti mesh substrates by the anodizing technique. The effects of preparation variables, such as anodizing voltage, time and calcination temperature on the textural characteristics and photocatalytic activity of TNTs were investigated. The surface morphology, crystalline phase, and chemical composition were analyzed using field emission-scanning electron microscopy and X-ray diffraction. The photo-electrochemical properties of TNTs were examined by voltammetry. The TNTs were tested as a photoanode for advanced oxidation processes, such as photocatalytic, electrocatalytic, and photoelectrocatalytic decolorization of Orange G dye. The well-arranged TNTs electrode prepared in this work showed a high photocurrent density of 101 mA cm2 at an optimum length-todiameter aspect ratio of 31.2. In dye decolorization tests, the electrochemical photocatalytic system using TNTs as the photoanode achieved total decolorization and 64% mineralization under extended reaction time. These results show that TNTs prepared by this method is greatly stable in prolonged use and suitable as a photoanode in the photocatalytic/photoelectrocatalytic treatments of dye wastewater. © 2015 Elsevier Ltd. All rights reserved.
Handling Editor: E. Brillas Keywords: Anodizing Orange G Titania nanotube Photoanode Photoelectrocatalysis
* Corresponding author. Institute of Environmental Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 300, Taiwan. ** Corresponding author. Department of Chemical Engineering, National Tsing Hua University, 101, Section 2 Kuangfu Road, Hsinchu 300, Taiwan. E-mail addresses: [email protected] (C. Huang), [email protected] (C.-C. Hu). 1 Co-first authors. http://dx.doi.org/10.1016/j.chemosphere.2015.11.029 0045-6535/© 2015 Elsevier Ltd. All rights reserved.
Y. Juang et al. / Chemosphere 144 (2016) 2462e2468
1. Introduction Since the discovery of TiO2 application for photocatalytic water splitting (Akira and Honda, 1972), TiO2-based photocatalysis has been extensively studied for water and wastewater treatment applications because of its nontoxicity, photochemical stability, and low cost (Carp et al., 2004). For such applications the photocatalytic (PC) activity of TiO2 is mainly determined by its ability to create electron/hole pairs for generating free radicals, such as hydroxyl, a species essential for advanced oxidation processes (AOPs) in wastewater treatment (Sood and Gouma, 2013). Photoelectrocatalytic (PEC) oxidation, which drives the photogenerated electrons to the counter electrode through potential biases via an external circuit to reduce the recombination of electrons and holes, has been found to be more efficient than PC oxidation (Vinodgopal et al., 1993; Byrne and Eggins, 1998; Calvo et al., 2001; Sene et al., 2003). In PEC systems, a TiO2 nanoparticles film electrode is usually used as the photoanode. Unfortunately, the structural disorder of nanoparticle system increases the scattering of free electrons and therefore reduces electron mobility (Peng et al., 2003; Paulose et al., 2006). Other than the recombination rate of photo-generated charge carriers, the surface area provided by titania also strongly influences its performance (Sood and Gouma, 2013). Several studies have shown that TiO2 thin films grown directly on a Ti substrate by anodizingdhighly ordered and perpendicularly aligned TiO2 nanotubes (TNTs)dsuccessfully reduces the resistance to electron transfer and avoids the recombination of photo-generated electron/ hole pairs, while at the same time overcoming the weak adherence issue of TiO2 on a substrate (Quan et al., 2005; Su et al., 2008) and provide a large surface area that results in a large adsorption capacity of organic pollutants (Liu et al., 2009). Recently, anodic fabrication techniques have successfully created TNTs with ultrahigh aspect ratio of length-to-diameter, from ~1500 (Wang and Lin, 2008) to ~10,000 (Paulose et al., 2007). Nevertheless, the different application of TNTs might require different aspect ratio, which means the higher the aspect ratio not necessarily the better the TNTs. Herein, we report fabrication of TNTs with lower aspect ratio, but have been optimized specifically for the application of dye wastewater treatment in this study. Although TNT arrays have been widely applied for the photoelectrochemical processes (Quan et al., 2005; Paulose et al., 2006; Chen et al., 2008; Liu et al., 2009), the interrelationship of preparation conditions, microstructures, and the photocatalytic/photoelectrocatalytic performances is still not clear. The objective of this study is to control the microstructure (morphology and crystalline phase) of TNTs by varying the preparation conditions, such as anodizing voltage and duration, and calcination temperature. The correlation between microstructure and photocatalytic/photoelectrocatalytic performance are then established. The most promising TNTs electrode is then used to degrade Orange G dye under various AOPs systems in order to determine the best condition for the wastewater treatment. Orange G dye which has a complex molecular structure is chosen to simulate highly persistent organic dye wastewater that cannot be easily degraded using conventional treatment. 2. Methods 2.1. Preparation of TNTs electrode The preparation of TNTs array electrodes consisted of three steps: substrate pretreatment, anodizing and post treatment. In pretreatment, 50 mm 30 mm pieces of Ti mesh of mesh size
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1 mm 2 mm, was degreased by sonication in a neutral cleaning agent for 15 min, followed by heating in 6 M HCl at 90 C for 30 min to remove oxide impurities from its surface. The etched substrate were then rinsed with de-ionized (DI) water and cleaned in a sonication bath for 30 min and subsequently dried in an oven at 100 C for 30 min. Self-arranged TNT array electrodes were fabricated by anodizing the pretreated Ti mesh substrates at 20 C with a platinum plate (50 mm 20 mm) serving as the cathode, using a DC power supply (Major Science MP-500V). Anodizing was carried out at various constant potentials of 20, 30, 40, and 50 V for 40 and 60 min duration. The anodizing electrolyte consisted of ethylene glycol (EG) as the solvent, with 2 vol% DI water and 0.3 wt% NH4F. The post-treatment step involved surface cleaning and calcination. Surface cleaning was done to remove the disordered and amorphous structures from the surface of TNT arrays by sonication in DI water for 1 min. To induce crystallization, the samples were subjected to calcination at 350, 450, 550, 650 and 750 C for 2 h, at a heating rate of 2 C min1 under air flow. All chemicals used in this study were of analytical grade. 2.2. Characterization of TNTs electrodes The surface morphology of TNT arrays was determined by field emission scanning electron microscopy (FESEM, JEOL JSM-6700F). To evaluate the crystalline structure of TNT arrays, XRD analysis was obtained using a Rigaku TTRAX III. The photo-electrocatalytic activities of fabricated TNTs electrodes were examined using voltammetry performed by an electrochemical analysis work station (AutoLab Potensionstat/ Galvanostat, PG Stat 302N) in a three-electrode single-cell system with 0.1 M Na2SO4. A platinum plate (40 mm 20 mm) and an Ag/ AgCl electrode served as counter and reference electrodes, respectively. The photocurrents of TNTs electrodes were obtained by linear sweep voltammetry (LSV) at a scan rate of 10 mV s1 from 0 to 1.5 V with and without UV (24 W) illumination. Cyclic voltammetry (CV) measurement was conducted with the same setup applying scan rate of 10 mV s1 from 0 to 2.5 V for 100 cycles. 2.3. Orange G dye degradation The performance of TNT electrodes for the catalytic degradation of Orange G dye was evaluated using several AOP methods: photocatalysis (PC), electrocatalysis (EC), and photoelectrocatalysis (PEC) in a 180-min reaction period with an ambient air flow into the solution at 200 mL min1. A 0.5 mM Orange G solution was prepared by dissolving commercial Orange G powders (Sigma) in DI water and adjusting the pH to 3 with H2SO4 (Sigma). A photolysis experiment was conducted by illuminating this solution with UV light to evaluate the natural photo-degradation of Orange G. PC experiments were conducted by immersing the TNT array electrodes with UV light irradiation. EC oxidation reaction was conducted by employing a TNT array anode and a graphite cathode under a constant current density without UV light irradiation, and PEC degradation of the Orange G dye was carried out in a cell with a TNT array photoanode and a graphite cathode under a constant current with the UV light irradiation. UV light intensity was 1 mW cm2; the current applied to the cell was 0.5 mA; the geometric area of the TNT array electrode and the graphite cathode were 12 cm2. The reactor setup is shown in Fig. SM-1. In order to avoid interference of physical adsorption of dyes on the performances of AOPs, both electrodes were immersed in the solution for 30 min to reach adsorption saturation before each experiment. The dye solutions were periodically sampled and analyzed by spectrophotometer (Metertech SP-8001). Individual peak intensities obtained at 230, 248, 330 nm were used to indicate the
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presence carboxyl group, benzene ring and naphthalene ring, respectively. Decolorization of Orange G resulting from the N ¼ N breakage was indicated by the intensity of absorbance at 478 nm. TOC degradation was monitored with a total organic carbon analyzer which uses oxidative combustion (Shimadzu TOC-L Series). The Orange G concentration was measured by high performance liquid chromatography (HPLC) (Waters 2695 separation module), coupled with a photo diode detector (detection wavelength: 480 nm, Waters 2996 PDA), with a Waters XBridge™ column (C18, 4.6 mm 250 mm inner diameter, 5 mm beads). The mobile phase setting was adapted from reference (Bokare et al., 2008). 3. Results and discussion 3.1. Effect of anodizing parameters on TNTs microstructure and photocurrent The microstructure, i.e. pore diameter and length, of TNTs can be controlled by varying the anodizing conditions such as applied voltage and time (Bauer et al., 2006; Raja et al., 2007; Yasuda et al., 2007; Chen et al., 2008; Macak et al., 2008; Kojima et al., 2012; Regonini et al., 2012). Fig. 1 clearly shows that tube diameter and length were increased by increasing the voltage and anodizing time. These images provide well-defined view; the boundary between Ti substrate and TNTs can be easily distinguished from the cross-section photos while the thickness and pore diameter are very uniform. Therefore, the average pore diameter of TNT arrays prepared under various conditions can be estimated from these images, as presented in Fig. 2. The relationship of the tube diameter and length with the anodizing conditions is shown in Fig. 2a. From curves 1 and 2, the average diameter of TNTs increases approximately linearly with increasing voltage. At the specified anodizing voltages the average pore diameters obtained at 60 min are larger than that at 40 min. Curves 3 and 4 show that the TNTs length is also proportional to the voltage at both anodizing times. The tube can also be easily elongated by increasing the anodizing time under a specified voltage. At an anodizing duration of 40 min, increasing the voltage from 20 to 50 V enlarges the average diameter from 37.9 to 89.3 nm and increases the tube length from 0.87 to 3.16 mm. Similarly, under an anodizing time of 60 min, the average diameter and length of TNTs increase from 45.5 to 98.6 nm and from 1.34 to 4.66 mm when the voltage is increased from 20 to 50 V, respectively. Pore diameter and length of TNTs are thus easily controlled by varying the
Fig. 2. (a) (1,2) Diameter and (3,4) length of TNTs against the anodizing voltage for (1,3) 40 min and (2,4) 60 min. (b) Photocurrent against the aspect ratio (L/D) of TNTs prepared under various anodizing conditions.
Fig. 1. FESEM images of top and side views of TNTs fabricated at anodizing (voltage, time): (a) (20 V, 40 min), (b) (30 V, 40 min), (c) (40 V, 40 min), (d) (50 V, 40 min), (e) (30 V, 60 min), and (f) (50 V, 60 min).
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anodizing voltage and time. Similar phenomenon for the dependence of tube diameter and length on the anodizing voltage and time has been reported in the literature (Bauer et al., 2006; Macak et al., 2008; Liu et al., 2012). However, in this case the diameter of TNTs generally depends more on the voltage of anodizing than the anodizing time. For example, increasing the anodizing voltage from 30 to 50 V at an anodizing time of 40 min results in an increase in the tube length from 1.43 to 3.16 mm; tube length reached only 2.04 mm under an applied voltage of 30 V for 60 min. This is possibly a result of quasi-steady state condition, where after a certain time the formation and dissolution of TiO2 occurred at similar rate. The microstructure of TNTs is related to the photocurrent produced, because it will affect the rate at which electron/hole pairs are generated (Yu and Wang, 2010). Fig. 2b plots photocurrent against the L/D (length/diameter) aspect ratio of TNTs prepared under different anodizing condition. With an anodizing time of 40 min, the increase in the voltage from 20 to 50 V increases the L/D from 23.0 to 35.4. Interestingly, the photocurrent also steadily increases from 53.5 to 94.6 mA. Similarly, increasing the applied voltage from 20 to 50 V increases the L/D from 29.5 to 49.8 at an anodizing time of 60 min. However, the photocurrent of this series of TNTs reached maximum (101 mA) at 30 V, after which the photocurrent kept decreasing with increasing applied voltage, regardless any further increase in the L/D. Based on the above data, there are two linear dependence of the photocurrent on the L/D: when the L/D is less than/equal to 31.2, the photocurrent increases nearly linearly proportional to the increase of L/D and a maximum photocurrent is obtained; on the other hand, as the L/D becomes larger than 31.2, the photocurrent linearly decreases with increasing the L/D aspect ratio, although the slope (absolute value) of this downward trend is obviously less than that of the upward trend. The exact reasons for the above phenomena are still unclear. Photocurrent is generally proportional to the amount of TNTs on the photo-electrode because it should be linear to the lights absorbed by TNT arrays, leading to the more electron/hole pair generation. However, the longer a tube is, the longer transport pathway of the charge carrier is, which results in the greater chance of electron trapping. This effect will extend the residence time of photo-generated electrons in the film before being collected on the current collector (i.e., Ti under the TNTs) (Zhu et al., 2007), leading to a lower photocurrent. Other than that, disorder and defects in the longer TNTs increase the chance of electrons-holes recombination, which would extend the electron transport pathway, leading to a lower photocurrent (Zhu et al., 2007). From these results can be concluded that there is an optimal value of the L/D aspect ratio (31.2 as determined in this work) that will generate the maximum photocurrent. 3.2. Effects of the calcination temperature on TNTs properties TNTs prepared by anodizing typically exhibit an amorphous structure (Macak et al., 2007); thermal annealing is usually employed to convert it into crystalline anatase or rutile phases (Macak et al., 2008). Upon annealing, transformation in crystalline structure and a change in the morphology of the as-prepared TNTs takes place at the same time. The transformation of crystalline structure of TNTs via annealing was determined by XRD analysis (Fig. 3) and its influences on the morphology and integrity of the tubular structure was verified with FESEM images (Fig. 4). XRD pattern of as-prepared TNTs (Fig. 3, pattern 1) shows only the diffraction peaks with 2q values at 35.09, 38.43, 40.17, 53.01, and 70.66 corresponding to facets (100), (002), (101), (102) and (103) of hexagonal titanium, respectively, indicating the amorphous
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Fig. 3. XRD patterns of (1) as-prepared TNTs, and those with calcination at (2) 350, (3) 450, (4) 550, (5) 650, and (6) 750 C.
structure of TNTs. This amorphous as-prepared TNTs film consists of highly ordered and well-aligned nanotube arrays perpendicularly stand on the Ti substrate (Fig. 4a). However, after calcination at 350 and 450 C (see patterns 2 and 3 Fig. 3), the amorphous TNTs were crystallized to form the anatase phase with diffraction peak centered at 25.3 (101). Interestingly, these TNTs retain the original morphology, showing no obvious variance in diameter and good integrity of both tubular wall and tube-substrate interface. As the annealing temperature reaches 550 C, a minor but distinct rutile (110) peak located at 27.5 appears (pattern 4 Fig. 3). Clearly, the TNTs show an anatase-rutile bi-phase structure. At the same time, a change at the TNT-substrate interface occurs (see the cross-section image in Fig. 4b). When the calcination temperature increases to 650 C, the rutile phase becomes the dominant structure (pattern 5 Fig. 3), and shrinkage of the whole tube became obvious, while the tubular structure was still visible (Fig. 4c). By further increasing the temperature to 750 C, the anatase-rutile biphase structure is converted into pure rutile (Fig. 3 pattern 6) and the tubular structure collapsed completely (Fig. 4d). A model has been proposed (Yu and Wang, 2010) to explain the anatase-torutile phase transformation and morphology architecture evolution of TNTs, which is strongly linked to crystal nucleation activation energy (Zhang and Banfield, 2011). 3.3. Photocurrent and photocatalytic activity of annealed TNTs The photocurrent generated by the electrodes was determined by photocurrent-voltage curves using LSV (Fig. 5a). The photocurrent of curves 4e6 increase sharply to the quasi-saturated value (the plateau) when the potential biases are equal to or more than ca. 0.3 V. This indicates that the photo-generated electron/hole pairs can be effectively separated by applying a low potential bias. The order of TNTs with decreasing the photocurrent obtained at 0.4 V is: TNT-550 (0.094 mA) z TNT-450 (0.092 mA) > TNT-350 (0.08 mA) > TNT-650 (0.021 mA) > TNT-as-prepared (0.007 mA). The results of cyclic voltammetry measurement for 100 cycles to examine the stability of TNTs show that TNTs calcined at 550 C has high electro-stability (Fig. SM-2). Comparing Fig. 5a and b, Orange G decolorization efficiency by PC process generally follows the trend of the TNT photocurrent in the plateau region. This also goes to show that the photocurrent measured from the LSV curves is a good index of photocatalytic activity of TNTs. Similar results have been found for Degussa P25
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Fig. 4. FESEM images of top and cross-section view of (a) as-prepared TNTs, and those calcined at (b) 550, (c) 650, and (d) 750 C.
TiO2 in our previous work (Lin et al., 2013), confirming the reliability of photo-electrochemical evaluation. 3.4. TNTs applications for Orange G decolorization in various AOPs The TNTs electrode was applied to various AOPs for Orange G dye decolorization and the results are shown in Fig. 6. When Orange G solutions were illuminated with UV for 180 min, only 1% decolorization resulted (curve 1 Fig. 6a), confirming that photolysis was negligible and that the very persistent characteristics of this dye can only be degraded by strong oxidants. Decolorization via photocatalytic process suggests that the recombination rate of charge carrier on the TNTs electrode is too fast to produce a sufficient amount of hydroxyl radicals to degrade the Orange G dye (8%, curve 2 Fig. 6a). Moreover, in earlier work (Lin et al., 2013) we found that Orange G dye in the solution inhibits UV absorption on TiO2. Fig. 6a shows that a higher decolorization rate of Orange G is obtained in the electrochemical decolorization process (curve 3) than the photocatalytic one (curve 2). This phenomenon is attributed to three effects. First, dye decolorization/degradation via direct electron transfer that likely occurs under a high positive potential (Xie and Li, 2006). Second, highly oxidative electroactive species, other than hydroxyl radicals, capable of decomposing Orange G are probably produced on the anode at high potentials. Third, to balance the anodic currents, H2O2 can be generated on the graphite cathode as per Eq. (1), leading to additional oxidation of dye molecules by hydroxyl radicals generated by the reaction between H2O2 and superoxide radicals (Eq. (2)). All these effects result in a higher decolorization efficiency of ca. 28% by electrochemical degradation compared to a photocatalytic system.
O2 þ2Hþ þ 2e/H2 O2
(1)
H2 O2 þ O2 / OHþ OH þ O2
(2)
Based on these results, combining photocatalytic and electrochemical degradation processes, i.e. a PEC process, is expected to increase the decolorization rate of Orange G (Fig. 6a, curve 4). The decolorization rate of 60% attained is in fact higher than the sum of dye decolorization via the decolorization processes produced by photocatalytic and electrochemical process individually. This synergetic effect of the PEC process is a result of several additional
factors. First, the H2O2 generated from the graphite cathode decomposes to produce OH under UV irradiation (Eq. (3)). Further, the potential bias on the photo-anode significantly hinders the recombination rate of electron/hole pairs on the photo-anode (Eq. (4)), and photo-generated electrons migrate to the graphite cathode for H2O2 production, which avoids the scavenging effect on the hydroxyl radicals created on the photo-generated holes (Eq. (5)). Garcia-Segura et al. (2013) suggested that this photo-generated electron will also react with H2O2 to produce hydroxyl radicals (Eq. (6)), thus further increase the PEC activity.
H2 O2 þ hv/2 OH
(3)
TiO2 þ hv/eCB þhþ VB ðon TiO2 Þ/TiO2 þ heat
(4)
eCB þ OH/OH
(5)
eCB þH2 O2 / OH þ OH
(6)
Based on above results, the potential bias and its corresponding current in a PEC process play an important role in Orange G decolorization. The effect of applied current on the decolorization rate of Orange G via the PEC process for a 180-min degradation test are shown in Fig. 6b. Note that the Orange G decolorization percentages of 25%, 35%, 52%, and 60% are generally proportional to the applied current of 0.3, 0.5, 0.8, and 1.0 mA, with corresponding electrode potentials of 0.15, 0.80, 1.70 and 2.60 V vs. Ag/AgCl, respectively. The pH of final solutions is not significantly affected by the applied current after the 180-min test, varying between 3.08 and 3.12, very close to the initial pH of 3. A separate LSV to measure photocurrent without Orange G dye presence (data not shown) shows that TNTs can provide 390 mA under UV illumination. It means an applied current of 0.3 mA (potential of 0.15 V vs. Ag/AgCl) should not promote the electron/hole separation significantly. The potential bias on the photo-anode becomes more positive with the increase of current, significantly enhancing the separation of electron/hole pairs and the generation of OH, leading to higher rates of Orange G decolorization. At 1.0 mA, the electrode potential of the TNT array electrode reaches 2.6 V (vs. Ag/AgCl), a potential high enough for both direct and indirect oxidation of Orange G (Xie and Li, 2006), which is expected to be the practical optimum applied current.
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Fig. 5. (a) LSV curves of (1) background current, and photocurrents of (2) as-prepared TNTs, and those calcined at: (3) 650, (4) 350, (5) 450, and (6) 550 C. (b) Photocatalytic activity (Orange G decolorization) of TNTs annealed at: (1) 350, (2) 450, (3) 550, and (4) 650 C.
To evaluate the long-term performance of these electrodes, they were tested for Orange G dye decolorization for 24 h, typical results are given in Fig. 7. Curve 1 in Fig. 7a shows the decolorization of above 90% at 8 h. The relative absorbance of phenol (curve 4) and carboxyl (curve 2) are consistently above the other intermediates because both structures are likely to be in the final form of intermediates before mineralized to CO2. More than 80% of all of intermediates have been removed after 24-h reaction. Extending the reaction time thus significantly enhances degradation efficiency, and that the TNTs electrode fabricated in this study is suitable for the PEC application, and is stable even under the extended reaction times. Fig. 7b shows an orange G concentration of less than 10% after 5h PEC degradation, and was undetectable after a degradation time of longer than 10 h. Total mineralization of Orange G was evaluated by measuring the solution TOC. Theoretically, the TOC value of 0.1 M Orange G is 19.2 mg L1, but the TOC measurement only showed 14.8 mg L1 indicated that the complex structure of Orange G was difficult to be combusted completely during TOC analysis.
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Fig. 6. (a) Decolorization of Orange G by (1) photolysis, (2) photocatalytic (PC), (3) electrocatalytic (EC), and (d) photoelectrocatalytic (PEC) processes. The UV radiation intensity is 1 mW cm2 at 365 nm and the applied current is 1 mA. (b) Decolorization of Orange G in a PEC system under (1) 0.3, (2) 0.5, (3) 0.8, and (4) 1.0 mA.
The slight increase and then gradual decrease of TOC during the initial PEC degradation resulted from the initial selective degradation process with accumulation of various intermediates that reacting differently during TOC analysis. The rate of TOC removal becomes higher when all Orange G molecules degraded into intermediates after about 10 h, suggesting that OH generated via the PEC process initially shows the selective oxidation of Orange G (the cleavage of azo p-conjugation structure) leading to a low rate of initial TOC removal. The oxidative degradation proceeds easier once the complex structure of the original Orange G dye has been broken down. As a result, the final TOC removal is 64% after a 24-h PEC treatment. 4. Conclusions The well-ordered TNT arrays were successfully fabricated on a Ti mesh substrate by a simple anodizing method. This shows promising potential for electrochemical-photocatalytic wastewater
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References
Fig. 7. Degradation of Orange G in an PEC system under 1.0 mA for 24 h in terms of (a) UV/Vis absorbance for (1) Orange G (478 nm), (2) carboxyl (230 nm), (3) benzene ring (248 nm), (4) phenol (280 nm), and (5) naphthalene ring (330 nm); and (b) HPLC/PAD analyses for (1) TOC and (2) Orange G concentration.
treatment because of its high photocatalytic activity and stability as a photo-anode in various AOPs processes. The diameter and length of TNT arrays are easily controlled by adjusting the voltage and time of anodizing, while crystalline structure and crystal phase composition are easily tuned by varying the calcination temperature. Both the morphology and crystallinity of TNTs strongly affect their photocatalytic activity, where an optimum L/D aspect ratio and biphase TiO2 resulted in the highest photocurrent. A synergistic effect in Orange G decomposition has been obtained when photocatalytic and electrochemical processes were combined into a PEC process. Acknowledgment This research was financially supported by Ministry of Economic Affairs: 101-EC-17-A-08-S1-208, Taiwan, R.O.C., project number 101-EC-17-A-08-S1-208 and Ministry of Science & Technology, MOST 104-3113-E-194-002. All the material surface analysis was supported by Ministry of Science and Technology: MOST 104-3113E-194-002 Instrument Center, R.O.C. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.chemosphere.2015.11.029.
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