ARTICLE pubs.acs.org/JPCC
In Situ Induced Visible-Light Photoeletrocatalytic Activity from Molecular Oxygen on Carbon Aerogel-Supported TiO2 Yuning Jin, Guohua Zhao,* Meifen Wu, Yanzhu Lei, Mingfang Li, and Xueping Jin Department of Chemistry, Tongji University, 200092 Shanghai, China ABSTRACT: This study presents a stable, efficient visible-light photoelectrocatalytic method induced by molecular-oxygen assistance on a carbon aerogel-supported TiO2 (TiO2/CA) electrode, which combines the in situ surface synthesis of H2O2 and Ti-peroxide photocatalysis under visible light. Results reveal that the optical absorption edge for TiO2/CA, which is cathodic polarized under aerobic conditions, is red-shifted to 530 nm. Under visible light (λ > 420 nm) irradiation, the increment from dark current to photocurrent density obtained on TiO2/CA is 315 times that on TiO2/ITO. The mechanism of the molecularoxygen-induced visible light photoeletrocatalytic activity is proposed and further verified through investigating the hydroxyl radical evolution and monitoring the surface changes of photocatalyst by Raman spectra and diffuse reflection spectra (DRS). This method is further applied in the degradation of the Rhodamine 6G (Rh-6G) wastewater. The result shows that Rh-6G molecules are almost totally decomposed with high TOC removal by the in situ induced photoelectrocatalytic process on TiO2/CA (TiO2/CA,PE-O2), which is closely related to the degradation mechanism and pathway of the pollutant. It is found that the intermediate products detected in the TiO2/CA,PE-O2 process are less than those in the traditional photocatalytic degradation on TiO2/ITO.
1. INTRODUCTION Over the past decades, TiO2-induced photocatalysis has attracted intensive attention in water and wastewater treatment to eliminate toxic and recalcitrant organic compounds because it is nontoxic, relatively cheap, chemically stable within a wide pH range, and robust under UV light irradiation.13 However, the widespread use of TiO2 as an effective photocatalyst in practical applications is curbed by its optical property. TiO2 can only absorb UV light (λ < 387.5 nm) because of its broad band gap energy (3.2 eV).4,5 Unfortunately, UV light in sunlight accounts for only 35% of the overall energy of the sunlight in comparison to the visible region (∼45%), and artificial UV light is expensive. Therefore, alot of effort has been devoted to shifting the optical response of TiO2 from the UV to the visible spectral range to effectively utilize solar energy. Investigations in recent years have focused on broadening the absorption spectrum of TiO2 by doping techniques, coupled with narrow band gap materials (such as Fe2O3 and CdS) or with dye sensitization. However, composite photocatalysts generally suffer from corrosion or low efficiency problems.6,7 The direct realization of stable, efficient visible-light photocatalysis on a TiO2 surface is rarely reported. It has been reported that the oxidation reaction of organic compounds occurs under irradiation of visible light when TiO2 particles are used as a photocatalyst with the addition of H2O2 to form a TiO2/H2O2 suspension system.8,9 However, because conventional TiO2 possesses a low surface area (2150 m2 g1), previous studies usually applied a TiO2 powder suspension system to increase the amount and contact surface area of the photocatalyst,810 but the subsequent separation process of the TiO2 powder is complicated and uneconomical. In the meantime, r 2011 American Chemical Society
the transport, handling, and storage of H2O2 are potentially hazardous, because of its strong oxidizing nature and decomposition. Thus, the use of a TiO2/H2O2 suspension in practical applications is curbed. Moreover, the degradation capability of the TiO2/H2O2 suspension system is based on the •OH radicals from the decomposition of H2O2. The degradation rate would greatly decrease with the consumption of added H2O2 and surface peroxides. Therefore, the key to overcoming these problems is the highly efficient in situ generation of H2O2 on the catalyst surface. Herein, we report molecular oxygen-induced visible-light photoelectrocatalytic activity on a TiO2 surface without any doping or modification. By loading TiO2 on a carbon aerogel (CA) with high surface area and excellent electrochemical properties, a visible light response of TiO2/CA can be realized under simple aerobic conditions. The in situ visible-light photoelectrocatalytic activity is ascribed to the combinatory effect between the in situ surface synthesis of H2O2 from molecular oxygen and Ti-peroxide-induced photocatalysis. In this study, TiO2 is immobilized on CA substrate, and the following effects may be exhibited on the prepared electrode. The three-dimensional network structure of CA substrate can endow the TiO2 photocatalyst with high loading capacity and surface area, which are beneficial for the Ti-peroxide-induced photocatalysis. More importantly, the high surface area and electrochemical properties of TiO2/CA can significantly facilitate the in situ surface synthesis of H2O2 from molecular oxygen. Received: January 28, 2011 Revised: April 4, 2011 Published: May 03, 2011 9917
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The Journal of Physical Chemistry C H2O2 will be continuously supplemented to the catalyst surface to form titanium peroxide and produce •OH so that the oxidation of pollutants can be kept highly efficient. In addition, TiO2/CA is an excellent electrosorption material. The cathode potential used for H2O2 generation also acts as the electrosorption bias. With this effect, the generated H2O2 polar molecules are immediately enriched on the surface of photocatalyst, resulting in a high efficiency of H2O2 utilization and on-site formation of Tiperoxides. In photocatalytic processes, the reaction rate increases with the pollutant concentration on the catalyst surface. Electrosorption can keep a high local concentration of pollutant and intermediates on the catalyst surface, so the photocatalytic degradation and TOC removal rate can be greatly promoted. Therefore, the proposed visible-light photoelectrocatalytic method in this work is dependent on the structure and superior characteristics of TiO2/CA. In this study, the in situ surface synthesis of H2O2 and its interaction with TiO2/CA are investigated in detail by a photocurrent test, UVvis diffuse reflection spectra (DRS), Raman spectra, H2O2 yield evaluation, and hydroxyl radical evolution. This method is further applied in the degradation of Rh-6G wastewater. The removal ratio of Rh-6G and mineralization performance are characterized. A possible mechanism and degradation pathway are proposed based on the identification of intermediate products and compared with the traditional photocatalytic degradation on TiO2/ITO. This work provides a new method for photoelectrocatalysis with stable, high efficiency under visible light.
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5 mM [Fe(CN)6]3/[Fe(CN)6]4 by means of alternative current (AC) impedance with the frequency ranging from 1 105 to 1 104 Hz and an impedance amplitude of 5 mV. Raman spectra were studied (RM100 Confocal Raman Microscope, Renishaw) with argon ion laser excitation at λ0 = 514.5 nm and laser power of 4 mW focused on the sample by a 50 N-plan objective lens (Leica). The EDS spectrum was measured by field emission scanning electron microscopy (Hitachi-S4800) to assess the purity and elemental composition of the synergistic electrode. The amount of TiO2 loaded on the CA surface was determined from ignition loss in air at 800 °C by using comprehensive thermal analysis apparatus (STA 409 PC/4/H Luxx). 2.3. Generation of H2O2 and Hydroxyl Radical Test. The experiment for H2O2 generation was performed in an undivided cell of 100 mL capacity using constant magnetic stirring. The solution comprised Na2SO4 as supporting electrolyte, and its pH was adjusted to 2 using H2SO4 prior to electrolysis. O2 was pulsed with a flow rate of 200 mL min1 for 300 min, and the H2O2 was formed by the reduction of dissolved O2. Once the power to the electrochemical workstation was turned on, the cathode potential was adjusted to a suitable value. The concentration of H2O2 was determined spectrophotometrically using the potassium titanium(IV) oxalate method (402 nm).13 At certain time intervals, samples were analyzed and the current efficiency (CE) for H2O2 generation is defined. The current efficiency (CE) for H2O2 generation is defined as CE ¼
2. EXPERIMENTAL METHODS
ð1Þ
0
2.1. Photoelectrocatalytic Behavior of TiO2/CA. The carbon
aerogels (CA) were prepared by an ambient drying technique, which includes solgel formation, solvent exchange, ambient pressure drying, and pyrolysis. TiO2 was loaded by immersing CA in titanium tetrabutylate solgel and then calcined in argon atmosphere.11 Photocurrent density experiments were recorded using an electrochemical workstation (CHI760, CH Instruments Inc., Austin, TX) in a conventional three-electrode cell at 25 °C. TiO2/CA (2.5 4.2 cm2) was selected as the working electrode. Platinum foil was used as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The distance between the working electrode and counter electrode was 3 cm. A high-pressure xenon short arc lamp (PLS-SXE-300UV, Beijing Changtuo) served as the visible light source. The light passed through a glass filter, allowing wavelengths between 420 and 800 nm to be incident on the photocathode at a measured intensity of 100 mW cm2. The optical absorption characteristics of the photocatalyst are determined by ultraviolet and visible (UVvis) diffuse reflectance spectroscopy (UVvis DRS, Model BWS002, manufacturers BWtek). 2.2. Characterization Experiments. The crystalline structure of the CA and TiO2/CA electrode were characterized by X-ray diffraction (XRD) (D/max2550VB3þ/PC, Rigaku Co., Japan) using Cu KR radiation (at 40 kV, 30 mA over the 2θ range 1080°). The average TiO2/CA particle size calcined at different temperatures was evaluated from the DebyeScherrer formula D = 0.9λ/(β cos θ),12 where D is the average particle size, λ is the wavelength of the incident X-ray, θ is the corresponding Bragg angle, and β is the full-width at halfmaximum (fwhm) of the XRD peak. Electrochemical impedance spectrum (EIS) was tested in the electrolyte consisting
nFC½H2 O2 V 100% Z t Idt
where n is the number of electrons transferred for oxygen reduction to H2O2, F is the Faraday constant (96486 C mol1), C[H2O2] is the concentration of H2O2 (mol L1), V is the bulk volume (L), I is the applied current (A), and t is the time (s). The hydroxyl radicals were determined according to the literature,14 in which formaldehyde was generated quantitatively by the reaction between hydroxyl radicals and dimethyl sulfoxide (DMSO) and then reacted with 2,4-dinitrophenylhydrazine (DNPH) to form the corresponding hydrazone (HCHO DNPH) and analyzed by HPLC (Agilent HP 1100, Agilent Co., Santa Clara, CA). An Agilent Zorbax Eclipse XDB-C18 column (150 4.6 mm, particle size 5 μm) was used at room temperature and with a selected UV detector at λ = 355 nm. To perform the isocratic elution at a flow rate of 1.0 mL min1, a mixture of methanol and water (60:40, v/v) was used as the mobile phase. 2.4. Degradation and Analysis. The procedures for dye wastewater decolorization were very similar to the test of photocurrent densities, except that the solution used was Rh6G simulated wastewater (50 mg/L) and its pH value was adjusted to 2 using H2SO4. The cathode potential was held at 0.9 V. The concentration of Rh-6G was determined by UVvis spectrometer (Agilent 8453, Agilent Co.). GC/MS (Agilent 6890/5973N, Agilent Co.) and HPLC (Agilent 1100, Agilent Co.) were used in sample analysis. The Rh-6G concentration was determined by UVvis spectrometer (Agilent 8453) from the absorbance at the wavelength of 525 nm by using a calibration curve. The total organic carbon was determined by a TOC analyzer (TOC-Vcpn, Shimadzu Co., Japan). Intermediates were analyzed by HPLC (Agilent 1100, Agilent Co.) (Eclipse XDBC8, 4.6 150 mm, 5 um, water/methanol = 40/60 at 1 mL/min) 9918
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and GC/MS (Agilent 6890/5973N, Agilent Co.) (Hp-1, L = 30 m, Ø = 0.25 mm, e = 0.5 um). The column temperature program was 323 K (5 min), 323473 K (15 K min1, hold time: 5 min).
3. RESULTS AND DISCUSSION 3.1. Visible-Light Photoelectrocatalytic Activity of TiO2/ CA under Aerobic Conditions. The photocatalytic activity of
TiO2 is closely related to its crystal structure. Figure 1 shows the XRD patterns of TiO2/ITO and TiO2/CA at different calcination temperatures. In the XRD patterns of TiO2/CA, the diffraction peak at 25.5° corresponding to the typical (101) plane of anatase appears when sintered at 500 °C, indicating the formation of the anatase phase.15,16 With the increase of calcination temperature, diffraction peak at 25.5° can be well indexed. The TiO2 obtained at 600 °C represents a well-crystallized titanate. The reflection peaks of TiO2/CA anatase structure do not shift as the calcination temperature increases, while the intensity is gradually enhanced. When the calcination temperature increases to 700 °C, a diffraction peak appears at 54.3°, corresponding to the (110) plane of the rutile phase.17,18 After 2 h calcination at 700 °C, the (101) plane of anatase phase TiO2 is still observable, while the diffraction peaks of rutile phase at 27.4°, 36.1°, 41.3°, and 54.3° become more obvious, suggesting the coexistence of the two crystal phases. The anatase phase (101) disappears with the TiO2/CA sintered at 800 °C for 2 h, showing that the anatase phase is transformed to rutile phase completely. It is found that the half-bandwidth of TiO2/CA
Figure 1. XRD spectra of TiO2/CA and TiO2/ITO (inset).
calcined at 600 °C is the largest, indicating the smallest crystallite size of TiO2, which is about 20 nm calculated from the Scherrer equation for the (101) direction. It has been reported that the photocatalytic activity of TiO2 is closely related to its structure and size.19,20 The anatase crystallite structure and highly dispersed nanoparticles are obviously beneficial to the enhancement of photocatalytic activity. Therefore, the TiO2/CA electrode calcined at 600 °C with the anatase phase was used throughout the experiments. The inset of Figure 1 shows the XRD patterns of TiO2/ITO. The well-crystallized titanate on TiO2/ITO was obtained at 500 °C, while the crystallite phase was mostly converted to rutile phase at 600 °C. Thus, the TiO2/ITO electrode calcined at 500 °C was used in all the tests. The phase transformation from anatase to rutile phase for TiO2/ITO occurs at 500600 °C. Some studies reported that the calcination temperature of TiO2 should be 450550 °C.2123 However, in this work, the phase transition temperature of TiO2 loaded on CA was 600700 °C. This phenomenon may be attributed to the good dispersion of TiO2 on the porous network CA with high specific surface area, which improves the thermal stability of the anatase phase and increases the crystalline transition temperature of TiO2. Thus, the transformation of anatase to rutile phase becomes more difficult, and anatase is still the major phase of TiO2 at 600 °C. This result is consistent with the TiO2 loaded on active carbon.24,25 The current densities on TiO2/CA and TiO2/ ITO under aerobic or anaerobic conditions are shown in Table 1. For the TiO2/CA electrode, the anaerobic dark density is 5.6 mA cm2. Under visible-light illumination, it increases to 5.8 mA cm2, only 3% higher than the dark density, indicating that effective photocatalysis cannot be attained on TiO2/CA in the absence of O2. In contrast, the aerobic photocurrent density increases to 10.3 mA cm2, 83.9% higher than the dark density. Thus, O2 is necessary for the obvious photocurrent response of TiO2/CA under visible light. Moreover, the photocatalytic activity of immobilized TiO2 also depends on the properties of CA. For the traditional TiO2/ITO, the photocurrent response is much lower. The dark current density is only 0.093 mA cm2, and the photocurrent densities of TiO2/ITO are 0.097 mA cm2 and 0.095 mA cm2 in the solution under aerobic and anaerobic conditions, increased by 2.1% and 4.3%, respectively. The increment from dark current to photocurrent density on TiO2/CA is 315 times that on TiO2/ITO. In the meantime the dark current density of TiO2/CA is much higher than that of TiO2/ITO because of the outstanding electrochemical property of CA substrate.11 The measured physical resistance of ITOsupported TiO2 is 500 Ω cm1. However, with TiO2 loaded on CA, the conductivity and dispersion of TiO2 is greatly facilitated, and the measured resistance is only 3.5 Ω cm1. Figure 2A shows the UVvis DRS of TiO2/ITO and TiO2/ CA. In the UV light region, TiO2/ITO presents a continuous
Table 1. Photocurrent Response and Treatment Efficiencies of Various Processes reactive processes (a) TiO2/ITO, PE
Rh-6G removal, % 7.42
Rh-6G removal rate, 103 min1
TOC removal ratio, %
|Ilight Idark|, μA cm2
0.13
1.1
2
(b) TiO2/ITO, PE-O2
13.2
0.24
6.8
97
(c) TiO2/CA, P- O2
25.0
0.47
4.7
(d) TiO2/CA, E-O2 (e) TiO2/CA, PE
38.9 44.3
0.84 0.98
13.5 10.1
(f) CA, PE-O2
51.5
1.17
22.1
(g) TiO2/CA, PE-O2
90.3
3.61
83.3 9919
200 4700
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Figure 3. (A) The H2O2 yields and average current efficiencies (CE) of the in situ synthesis of H2O2. (B) Instantaneous concentration of H2O2 on TiO2/CA and CA as a function of reaction time (pH = 2, and E = 0.9 V).
Figure 2. (A) UVvis diffuses reflection spectra (DRS) of pure TiO2/ ITO and TiO2/CA. (B) Raman spectra variation of TiO2/CA and CA under aerobic or anaerobic conditions.
absorption band from 200 to 385 nm, and the band gap is about 3.22 eV. As for TiO2/CA, a slight red-shift of the band gap absorption edge is observed from 385 to 415 nm. This may be attributed to the good dispersion of TiO2 particles. The high surface area and porous structure can significantly facilitate the dispersion of TiO2 particles and endow the catalyst with low particle size (20 nm). Under 0.9 V cathode bias with O2, the band gap absorption edge of TiO2/CA is further expanded from 415 to 530 nm, suggesting that the photoabsorption range can be in situ sensitized to visible region by the aerobic electrochemical method. The red shift of band gap absorption edge may be ascribed to the surface reaction on TiO2/CA electrode. 3.2. Mechanism of in Situ Induced Visible-Light Photoelectrocatalysis from Molecular Oxygen. The above results demonstrate that potential bias, CA substrate, and aerobic conditions are necessary for visible-light photoelectrocatalytic activity. To monitor the surface change of TiO2/CA under aerobic electrochemical conditions, Raman spectra characterization is conducted (Figure 2B). For the CA electrode, there are two strong vibration peaks at 1335.3 and 1590.5 cm1
attributed to the graphite crystallites D and G, respectively.26 When a 0.9 V bias is applied, no change in the Raman spectra can be observed regardless of whether CA is placed in an aerobic or anaerobic atmosphere, revealing that no new phase is produced on the surface of unloaded CA. For the Raman spectra of the TiO2/CA electrode under bias potential, two peaks attributed to the graphite crystallites D and G are still observable. In addition, five strong vibration peaks are identified at 147 cm1 (Eg), 193 cm1 (Eg), 394 cm1 (B1g), 502 cm1 (A1g), and 615 cm1 (E1g),27 attributed to the Raman active modes (A1gþB1gþ3Eg) of anatase, respectively. Under aerobic conditions, new split peaks located in the range of 700950 cm1 are detected. It has been reported that different Ti-peroxide species possess wide and split Raman peaks in the range of 700 950 cm1.28,29 On the basis of these assignments, it is assumed that TiO2 surface peroxides can be in situ electrogenerated on TiO2/CA under aerobic conditions. As the reaction proceeds, the intensity of peaks attributed to surface peroxides gets stronger relative to those of TiO2. It is well-known that the Tiperoxides formed on the surface of TiO2 can be excited by visible light to produce electrons, which are transferred to the conduction band of TiO2 to generate visible-light-induced current.9 The red shift of absorption range and photocurrent response of TiO2/CA can be further confirmed by the in situ formation of peroxides. 9920
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The Journal of Physical Chemistry C It is revealed that the visible-light photoelectrocatalytic activity of TiO2/CA is closely related to the on-site formation of surface Ti-peroxides, which generally result from the chemisorption of H2O2 on the surface of TiO2 in an aqueous system. Thus, the formation of the Ti-peroxides may be attributed to the effective in situ surface synthesis of H2O2 on TiO2/CA. It has been reported that H2O2 can be generated from the electroreduction of O2 on cathode. Especially when high surface area carbon materials are applied as working electrodes, the H2O2 yield is usually remarkable. This phenomenon is usually applied in the studies of electro-Fenton.30 The electrode materials used in the electrosynthesis process of H2O2 are carbon-based cathodes such as graphite,31 carbon felt,32 or gas diffusion electrodes (GDE).33 Moreover, high H2O2 yield can be obtained on the cathodes with high surface area because the porous structure in the electrodes promotes gas mass-transport by acting as gas supplying channels.34 Therefore, the capacity of TiO2/CA for generation of H2O2 under aerobic conditions and different cathode potentials is investigated and compared with that of the conventional TiO2/ITO electrode. Figure 3A shows the in situ H2O2 yields and the corresponding current efficiencies (CE) of TiO2/CA and TiO2/ITO under different cathode voltages. The TiO2/CA electrode presents higher H2O2 yields and CE than TiO2/ITO in the whole investigated potential range, owning to the different properties of the substrate materials, such as the structure and characteristics. CA possesses high surface area (744 m2 g1) and excellent conductivity. Figure 4A shows EIS of TiO2/CA and CA electrodes. The electron transfer resistance (Ret) of CA is 7.75 Ω and the physical resistance is 2.5 Ω cm1. With TiO2 loaded, the electron transfer resistance and physical resistance of TiO2/CA are 10.01 Ω and 3.5 Ω cm1 respectively, and the conductivity of CA does not decrease significantly, so the TiO2/ CA electrode presents better conductivity. The properties of TiO2/CA can promote the in situ generation of H2O2. Furthermore, the EDS analysis of TiO2/CA (Figure 4B) reveals that plenty of C is on the surface of TiO2/CA electrode, suggesting the coexistence of CA and TiO2. The exposure of CA maintains the conductivity of TiO2/CA. This result indicates that no intense aggregation occurs during the loading of TiO2 film, while the TiO2 films formed on the flat surface (such as ITO and Ti) are generally conglomerated to a great mass. The exposure of CA on the electrode surface is verified with Raman spectra (Figure 2B). Therefore, most features of CA are maintained in TiO2/CA, which is beneficial for the H2O2 yield. The highest H2O2 yield is obtained under 0.9 V with a relatively high CE. Figure 3B shows the variation of H2O2 concentration with reaction time under 0.9 V. When CA is applied as the cathode with O2, the concentration of H2O2 steadily increases to 172 mg L1 in 300 min. Qiang et al. reported that the maximal H2O2 yield on graphite was not more than 80 mg L1.31 Choudhary et al. studied the H2O2 production on Pd/alumina, in which the optimal H2O2 yield was around 100 mg L1.35 Therefore, CA is an appropriate electrode material for electrochemical generation of H2O2. With TiO2 loaded, the surface area and exposure of CA slightly decrease. Because the three-dimensional network structure of CA greatly increases the loading amount, the calculated loading amount of TiO2 is as high as 15 wt % (the TG/DTA analysis of the TiO2/CA is shown in Figure 4C). The generated H2O2 molecules can be adsorbed on the surface of electrode, giving rise to the Ti-peroxides. Therefore, the detected H2O2
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Figure 4. (A) Electrode AC impedance spectroscopy in 5 mM [Fe(CN)6]3-/[Fe(CN)6]4- solution. (B) Energy dispersive spectroscopy (EDS) of TiO2/CA. (C) TG/DTA analysis of the TiO2/CA.
production is lower than CA, 78.7 mg L1 in 300 min. Besides, it is found that the evolution of H2O2 concentration on TiO2/ CA is different from that on CA. Unlike the rapid ascent of the in situ synthesized H2O2 on CA, the generation rate of H2O2 on TiO2/CA first increases slowly and then more rapidly with the increase of reaction time. The variation further confirms the surface reaction between in situ generated H2O2 and supported TiO2. During the early stage of aerobic electrochemical process on TiO2/CA, the H2O2 yield is low whereas the loading amount 9921
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Scheme 1. Possible Mechanism of the in Situ Induced Photoelectrocatalysis on TiO2/CA
of TiO2 on CA is high. Most of the H2O2 molecules are adsorbed on the electrode surface and react with TiO2, so the detected concentration of H2O2 increases slowly. As the reaction time proceeds, the H2O2 yield increases gradually, and when the reaction equilibrium between H2O2 and TiO2 is reached, the producing rate of H2O2 will quickly increase. The comparative study suggests that under anaerobic conditions, only a small amount of H2O2 can be generated and all of the H2O2 molecules are immediately adsorbed and combined with TiO2. Thus, only a small amount of H2O2 is detected. The above analysis about the surface reaction and characterization of TiO2/CA electrode indicates that the in situ induced visible-light photoelectrocatalysis from molecular oxygen is an indirect process combining the highly effective electrosynthesis of H2O2 from molecular O2 and the on-site formation of Ti-peroxides that induce the visible-light activity. The mechanism of the TiO2/CA,PE-O2 process can be concluded as follows. (1) Under 0.9 V potential and aerobic conditions, H2O2 is in situ synthesized from the molecular oxygen on TiO2/CA surface. Then the polar H2O2 molecules are rapidly adsorbed on the cathode by the electrosorption effect of TiO2/CA and react with the TiO2 nanoparticles, giving rise to the surface Ti-peroxides, extending the photoresponse to the visible region. (2) Under visible-light irradiation, the Ti-peroxides are excited. The excited surface complex injects an electron to the conduction band of the semiconductor and generates the conduction band electron ecb and Ti(IV)•OOH. Then the unstable Ti(IV)•OOH radical decomposes to the original TiO2.9 The electrons on the conduction band of TiO2 further initiate the decomposition of the adsorbed H2O2 molecules, generating hydroxyl radicals that possess strong oxidation ability (2.4 V vs NHE) and may further oxidize pollutants. The proposed mechanism is shown in Scheme 1. 3.3. In Situ Induced Photoelectrocatalytic Degradation of Rh-6G under Visible Light. In the electrophotocatalytic process, it is well-known that pollutants are degraded mainly by indirect electrochemical oxidation mediated by •OH generated on the electrode surface. The accumulated concentration of •OH in the solution is shown in Figure 6A. Curve b is the evolution of •OH on TiO2/CA with O2 and 0.9 V bias in the dark (denoted as TiO2/CA,E-O2). It shows that •OH radicals are difficult to be
produced on TiO2/CA in the absence of visible-light irradiation because the surface complex cannot be excited and inject electrons to induce the decomposition of H2O2. The concentration of •OH is only 0.91 μM at 3 h. When the potential and visible light are applied under anaerobic conditions (denoted as TiO2/CA,PE, curve d), •OH production is not efficient because the in situ generation of H2O2 is very weak without O2. Only a little H2O2 is produced and reacts with TiO2 to form a peroxide complex, so the response to visible light is very limited and the •OH concentration is very low. The yield in 3 h is 10.43 μM. Curve a shows the aerobic photoelectrocatalytic process without an electric field (denoted as TiO2/CA,P-O2). Little •OH can be detected in the solution because no H2O2 and surface complex are produced and the TiO2 on CA can not directly respond to visible light. In the presence of 0.9 V potential and O2, CA electrode is excellent to generate H2O2 in situ. However, without supported TiO2, the titanium peroxide-induced photocatalysis under visible light cannot be fulfilled, resulting in a low •OH concentration (denoted as CA,PE-O2, curve e). The TiO2/CA, PE-O2 process (curve f) exhibits the highest •OH concentration in 3 h (36.02 μM), confirming the significance of the in situ generation of H2O2, highly dispersed TiO2, and the visible light irradiation in the in situ induced photoelectrocatalysis. In addition, the traditional TiO2/ITO shows very poor ability for the generation of •OH under optimized conditions (denoted as TiO2/ITO,EP-O2, curve c). A comparative experiment is also conducted on the TiO2/CA with added H2O2 under visible light (denoted as TiO2/CA, PH2O2 inset in Figure 6A). In the process of TiO2/CA,PH2O2, the •OH concentration rapidly increases to 13.32 μM in the first 0.5 h and then decreases with time, because the added H2O2 is quickly consumed in the production of surface complex and •OH. Thus, the efficiency of photocatalysis cannot be maintained. In the TiO2/CA, PE-O2 process, H2O2 is continuously generated in situ on the catalyst surface so that the •OH yield increases gradually with the reaction time. This method is further applied in the degradation of the azo dye Rh-6G wastewater. The typical UVvis spectral variations of 50 mg L1 Rh-6G in different processes are inspected (Figure 5). The peaks at 245 and 346 nm are assigned to aromatic rings and other unsaturated groups of CdO in dye 9922
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Figure 5. UVvis spectra of Rh-6G as a function of reaction time on TiO2/ITO (A) and TiO2/CA (B) electrodes under visible-light irradiation at an Rh-6G concentration of 50 mg/L and a cathode potential of 0.9 V.
molecules, respectively. A more intensive peak at 525 nm is attributed to a conjugated chromophoric group, so the change in peak absorbance at λ = 525 nm is used to investigate the degradation of Rh-6G. Figure 6B is the variation of Rh-6G removal with reaction time. As shown by curve g, the TiO2/ CA,PE-O2 method presents the highest removal rate of Rh-6G; the removal ratio in 300 min is 90.3% and the TOC removal is 83.3%, indicating that the Rh-6G wastewater is effectively degraded by the in situ induced photoelectrocatalytic method on TiO2/CA. Curves e and c show the treatment performance of TiO2/CA,PE and TiO2/CA,P-O2. Without cathode potential or O2, H2O2 is hardly generated in the two processes to form a surface complex. Because TiO2/CA is not sensitive to visible light, the elimination of dye molecules is mainly attributed to the adsorption or electrosorption of TiO2/CA. In 300 min the Rh6G removals of TiO2/CA,PE and TiO2/CA,P-O2 are 44.3% and 25.0%, and the corresponding TOC (including the Rh-6G washed from the electrode surface) removals are only 10.1% and 4.7%, respectively. This result is similar to the CA,PE-O2 process, which also fails in the formation of the peroxide complex because of the absence of loaded TiO2. By the electrosorption of CA, 51.5% of Rh-6G is removed in 300 min, and the TOC removal is 22.1%. Curve d shows the treatment performance of the electrosorption process on TiO2/CA (TiO2/CA,E-O2). Unlike the above processes, plenty of titanium peroxide is produced under O2-enriched conditions on TiO2/CA, while
Figure 6. (A) Evolution of hydroxyl radical in different processes on CA, TiO2/CA, and TiO2/ITO. (B) Variation of Rh-6G removal ratio with reaction time (at initial pollutant concentration of 50 mg L1 and cathode potential of 0.9 V).
little •OH is generated without visible light irradiation because the surface complex cannot be excited, leading to low treating efficiency. The Rh-6G removal is 38.9% and TOC removal is only 13.5%. For the traditional TiO2/ITO electrode, the color removal in different processes is very poor (curves a and b). The Rh-6G removal in the TiO2/ITO,EP-O2 process in 300 min is 13.2%, and the TOC removal is only 6.8%. The dye and TOC removal in the same process on TiO2/CA are 6.8 and 12.5 times those on TiO2/ITO, respectively. The apparent rate constant of Rh-6G degradation ks is also investigated (Table 1). The rate constant of the TiO2/CA,PE-O2 process is 3.6 103 min1, which is 15.1 times that of the same process on TiO2/ITO and 3.7 times that of the anaerobic photoelectron process on TiO2/CA. The single photodegradation and eletrosorption function on TiO2/CA (curves c and d in Figure 6B) also presents a rate constant much lower than that on TiO2/CA,PE-O2, only 0.47 and 0.84 min1, respectively. Additionally, the removal ratio of Rh-6G in the TiO2/CA,PE-O2 process is higher than the summation of the TiO2/CA,P-O2 and TiO2/CA,E-O2 methods. The electrosorption 9923
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Figure 7. (A) Degradation pathway of Rh-6G by an in situ visible-light photoelectrocatalytic process on TiO2/CA. (B) Degradation pathway of Rh-6G by a photocatalytic process on TiO2/ITO.
effect of TiO2/CA can promote H2O2 yields and increase the local surface concentration of H2O2 and pollutants, facilitating the on-site formation of surface peroxides and degradation rate. At the same time, the in situ regeneration of electrode is fulfilled by the visible-light induced photocatalysis, so that the saturation adsorption is effectively hindered. Therefore, the treatment efficiency in the TiO2/CA,PE-O2 process is higher than the summation of the TiO2/CA,P-O2 and TiO2/CA,E-O2 methods because of the coordinated interaction. The degradation intermediates of Rh-6G in TiO2/CA,PE-O2 process were investigated and compared with the traditional UV-light photocatalysis on a conventional TiO2/ITO electrode. The intermediates generated during the degradation process were analyzed by GC/MS and HPLC, which were identified by comparison with commercial standards. Possible reaction pathways of these two processes are proposed and shown in Figure 7. Unlike the photocatalytic process on conventional TiO2/ITO, in which there are usually many organic intermediate products before mineralization, the TiO2/CA,PE-O2 process is found with much less organic intermediates. In the TiO2/CA,PE-O2 process (Figure 7A), with TiO2/CA used as a cathode under a low applied bias, electric charges are generated on the surface of the electrode to make TiO2/CA negatively charged. The pollutants with opposite charges are enriched on the surface of electrodes by Coulomb interactions. Because TiO2/CA possesses excellent electrosorption properties, the adsorption rate and capacity can be greatly increased. At the initial stage of the reaction, the Rh-6G molecules are positively charged in acidic aqueous solution by the ionization effect of diethylamine function and adsorbed on the TiO2/CA cathode through the positively charged diethylamine function. The first step probably involves an N-deethylation process. In the TiO2/CA,PE-O2 process, no other N-deethylated products, but only a small amount of C24H23N2O3, are
observed. Then, the conjugated structure of N-deethylated products is destructed by the attack of radicals and oxidized to some one-ring aromatic metabolites. At this stage, three aromatic metabolites are detected, i.e., catechol, phthalate, and 3-hydroxybenzoic acid. Then, the ring opening and mineralization proceed. By the further attack of radicals, the aromatic metabolites are ring-opened and some aliphatic acid molecules (such as succinic acid, oxalic acid, and malonate) are produced. Subsequently, these aliphatic acids are oxidized to water and carbon dioxide, presenting a high mineralization rate in the TiO2/CA,PE-O2 process. Figure 7B shows that many more organic intermediate products are produced in the degradation by traditional photocatalysis on conventional TiO2/ITO, compared with the TiO2/CA,PE-O2 process. With the destruction of conjugated structure, three other aromatic metabolites such as ophthalic anhydride, hydroquinonecarboxylic acid, and p-phthalic acid are produced. After the ring-opening process, the aliphatic acids produced in the photocatalytic process are also more than those in TiO2/CA,PE-O2 process. This may be attributed to the electrosorption enhancement and the enrichment effect of the TiO2/CA electrode. Because TiO2/CA is an ideal electrosorption electrode material, the electrosorption effect of TiO2/CA is fulfilled by applying a low potential bias. Some intermediates with low concentrations are enriched on the surface of the TiO2/ CA electrode by electrosorption. Then a high local concentration of these intermediates can be kept on the catalyst surface, increasing the reaction rate of the photocatalytic process. So these intermediates can be mineralized rapidly before they are transported to the bulk solution by phase-transfer, enhancing the photocatalytic degradation efficiency and TOC removal. This result indicates that the TiO2/CA,PE-O2 process is of high treatment efficiency with less organic intermediates and is more environmentally friendly. 9924
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4. CONCLUSIONS The TiO2/CA electrode, combining the advantages of a TiO2 photocatalyst and the electrosorption property of CA, was used in an in situ induced photoelectrocatalytic method. By loading TiO2 on CA substrate, the visible-light photoelectrocatalytic activity is realized in situ on a TiO2 surface under simple aerobic conditions. Compared with the traditional TiO2/ITO electrode, TiO2/CA exhibited much higher photoelectrocatalysis. In the proposed TiO2/CA,PE-O2 process, the high surface area and superior electrochemical properties of CA substrate promoted the in situ surface synthesis of H2O2 under aerobic conditions. The generated H2O2 interacted with the loaded TiO2, resulting in the on-site formation of Ti-peroxides. The band absorption edge was red-shifted to 530 nm, and the light adsorption spectrum was accordingly extended. Thus, the photocurrent density was highly enhanced. This method is further applied in the degradation of Rh-6G wastewater. Results show that Rh-6G is degraded effectively by the TiO2/CA,PE-O2 process. The removal ratio of Rh-6G is 90.3% in 300 min, and the TOC removal is 83.3%. The rate constant of the TiO2/CA,PE-O2 process is 15.1 times that of the same process on TiO2/ITO and 3.7 times that of the anaerobic photoelectron process on TiO2/CA. A possible mechanism and degradation pathway is proposed based on the identification of intermediate products and compared with the traditional photocatalytic degradation on TiO2/ITO. The TiO2/CA,PE-O2 process is found with much less intermediates than the traditional photocatalytic process on TiO2/ITO, indicating that the proposed method exhibits high treatment efficiency and low energy consumption. A primary insight into the structureeffect relationship of this process is obtained .The study is systematic, and it provides a new idea for exploring TiO2 photocatalysts under visible light, which is significant for the research and application of the photoelectrocatalytic oxidation of pollutants. ’ AUTHOR INFORMATION Corresponding Author
*Phone: (86)-21-65981180. Fax: (86)-21-65982287. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported jointly by the National Natural Science Foundation of China (20877058, 21077077) and 863 Program (2008AA06Z329) from the Ministry of Science. We cordially thank Prof. Dr. Dongming Li for her kind help and suggestions in the revision of the manuscript.
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