Environ. Sci. Technol. 2010, 44, 5098–5103
Electrochemically Assisted Photocatalytic Degradation of 4-Chlorophenol by ZnFe2O4-Modified TiO2 Nanotube Array Electrode under Visible Light Irradiation Y A N G H O U , † X I N Y O N G L I , * ,†,‡ QIDONG ZHAO,† XIE QUAN,† AND G U O H U A C H E N * ,‡ Key Laboratory of Industrial Ecology and Environmental Engineering (MOE) and State Key Laboratory of Fine Chemical, School of Environmental Science and Technology, Dalian University of Technology, Dalian, 116024, China, and Department of Chemical and Biomolecular Engineering, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong
Received January 10, 2010. Revised manuscript received May 24, 2010. Accepted May 24, 2010.
A well-aligned ZnFe2O4/TiO2 composite nanotube array (ZnFe2O4/ TiO2-NTs) electrode with visible-light activity was successfully prepared using a two-step electrochemical process of anodization and a novel cathodic electrodeposition method followed by annealing. The ZnFe2O4 nanoparticles were highly dispersed inside the TiO2-NTs but minimized at the tube entrances. The structure and optical properties of the TiO2 nanotubes and the derived composites have been well characterized. The composites displayed a strong photo response in the visible region and low recombination rate of the electron-hole pairs. In addition, the synthesized ZnFe2O4/TiO2-NTs electrode showed much higher photocurrent density in the visible region than pure TiO2-NTs electrode. The dramatically enhanced electrochemically assisted photocatalytic activity of the composite electrode was evaluated in the decomposition of 4-chlorophenol and dichloroacetate under visible light irradiation (420 nm < λ < 600 nm). The improved photoelectrocatalytic (PEC) activity is derived from the synergetic effect between ZnFe2O4 and TiO2, which promoted the migration efficiency of photogenerated carriers at the interface of the composite and enhanced the efficiency of photon harvesting in the visible region. The degradation of 4-chlorophenol was monitored by measuring Cl- concentrations and analyzing reaction intermediates by highperformance liquid chromatography-mass spectroscopy (HPLC-MS).
Introduction Among the top priority pollutants, chlorophenols represent an important class of environmental water pollutants (1). * Address correspondence to either author. Tel: +852-2358-7138 (G. Chen); +86-411-8470-7733 (X. Li). Fax: +852-2358-0054 (G. Chen); +86-411-8470-8084 (X. Li). E-mail:
[email protected] (G. Chen);
[email protected] (X. Li). † Dalian University of Technology. ‡ Hong Kong University of Science & Technology. 5098
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Most of those recalcitrant pollutants are considered toxic or potentially carcinogenic and mutagenic to mammalian as well as aquatic life, so they are listed among the priority pollutants by U.S. EPA (2). In particular, 4-chlorophenol (4CP), a representative of this class, generated as a byproduct in petrochemical, paper making, plastic, and insecticidal industries, can have serious effects on human health and on the environment (3). The conventional biological or physicalchemical treatment processes are slow or nondestructive in eliminating some of the recalcitrant compounds (4), but photocatalytic degradation, such as photocatalysis involving titanium dioxide (TiO2), has received considerable attention because it is environmentally friendly, capable of complete mineralization, low cost, nontoxic, and easily available (5). In particular, regular TiO2-NTs by electrochemical anodization (6) have been used in many fields, such as gas sensing (7), catalysis (8), solar cell (9), electrochromic devices (10), and photo cleavage of water (11). The highly ordered TiO2-NTs exhibit good performance when used as a photoelectrode, due to the unique nanostructure that facilitates the separation of the photoexcited charges and results in higher charge collection efficiency (12). In addition, the aligned nanotube structure favors absorption of radiation by reducing the loss of light reflection, since photons that enter the nanotubes are less likely to escape due to multiple radiations scattering by the walls (13). However, one severe disadvantage of this semiconductor material is the large band gap of 3.2 eV (for anatase), which limits its photoresponse to the ultraviolet (UV) region. Unfortunately, only a very small fraction (2-3%) of the solar spectrum falls in the UV region (14). To make full use of solar energy, many attempts have been made to sensitize titania to utilize the much larger visible region, such as through the deposition of transition metals (15) and doping with nonmetal atoms (16). Although the above modifications could partly improve the photocatalytic activity of TiO2, some key problems remain unresolved, for example, doped materials suffer from thermal instability, photo corrosion, lattice distortion, and an increase in the carrier-recombination probability (17). One of the promising strategies to overcome this drawback is to couple TiO2 with other narrow band gap semiconductors capable of harvesting the photons in the visible range (18). Coupling TiO2 with these semiconductors is found to be effective in enhancing its visible light activity due to the fact that the photogenerated electrons from the conduction band of the narrow band gap semiconductor could be injected into the conduction bands of TiO2, resulting in the photocatalytic reaction. Recently, an inorganic semiconductor with a relatively narrow band gap, ZnFe2O4 (1.9 eV), has attracted attention in the conversion of solar energy, photocatalysis, and photochemical hydrogen production from water, because of its visible-light response and good photochemical stability (19). Yuan et al. (20) demonstrated the high efficiency of the ZnFe2O4/TiO2 nanocomposite for the photodecomposition of phenol. Xu et al. (21) prepared zinc ferrite-doped TiO2 photocatalyst using liquid catalytic phase transformation and the sol-gel method. They found that TiO2 doped with ZnFe2O4 had improved photocatalytic activity under visible light irradiation. Yin and co-workers (22) reported that ZnFe2O4 and TiO2 nanoparticles could be prepared using the coprecipitation method. The examined photoelectrochemical properties, such as photocurrent, were strongly influenced by the ZnFe2O4 sensitizer. However, the energy conversion efficiency was relatively low and it was troublesome to separate and recycle the composite powder from the reaction 10.1021/es100004u
2010 American Chemical Society
Published on Web 06/07/2010
system. The combination of TiO2-NTs and ZnFe2O4 nanoparticles may provide a better system to overcome the above problems. In this study, a well-aligned, visible-light-active ZnFe2O4/ TiO2-NTs electrode was successfully prepared using first a two-step electrochemical process of anodization, followed by a novel cathodic electrodeposition. The composite material was annealed before characterization or further usage. The ZnFe2O4 nanoparticles were well deposited and dispersed inside the TiO2-NTs but minimized at the tube entrances. Thus, pore clogging was prevented. The resultant ZnFe2O4/ TiO2-NTs electrode exhibited dramatically enhanced photoelectrocatalytic (PEC) activity and excellent photostability toward degrading 4-CP under visible light irradiation. In addition, the intermediates and photodegraded products were also analyzed in detail by using the HPLC-MS technique to propose a possible degradation pathway.
Experimental Section Preparation of ZnFe2O4-Modified TiO2 Nanotube Array Electrode. All chemicals were analytical grade reagents (4chlorophenol, benzoquinone, hydroxyhydroquinone, sodium dichloroacetate, etc.) and used without further treatment. Electrolyte was freshly prepared from deionized water. The highly ordered TiO2-NTs were synthesized by anodic oxidation in a HF electrolyte, similar to that described previously (23). ZnFe2O4 nanoparticles were deposited into the crystallized TiO2 nanotubes using a novel cathodic electrodeposition followed by annealing. A conventional three-electrode setup in an undivided cell was used for the cathodic deposition, with TiO2-NTs electrode (with an effective area of 4 cm2) as the working electrode, Pt foil as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. First, the TiO2-NTs electrode was soaked in a mixture solution containing 0.05 M zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O) and 0.1 M ferric nitrate nonahydrate (Fe(NO3)3 · 9H2O) for 20 min under ultrasonication. Then, the TiO2-NTs electrode was transferred into a new medium that contained only an inert supporting electrolyte (0.1 M Na2SO4). The potentiostatic DC electrodeposition was carried out with a potential of -0.8 V for 20 min and the temperature of the electrolyte was maintained at 85 °C. After the electrodeposition in this medium, Zn and Fe nanoparticles were deposited into pores of the TiO2 while minimizing deposition at the tube entrances (Zn-Fe/TiO2-NTs). A desired deposition amount of Zn and Fe in the pores was obtained after 10 repetitions of the ultrasonification and deposition procedure. After electrodeposition, the material was electrochemically oxidized with a potential of 1.6 V vs the SCE for 2 min at room temperature using another alkaline electrolytic bath prepared by dissolving 1 M KOH in distilled water. This time the electrode with the electrodeposited material (Zn-Fe/TiO2-NTs) was connected as the anode and Pt foil was the cathode (1 cm separation). After this electrochemical oxidization, the electrodeposited material (Zn-Fe/TiO2-NTs) was converted into the corresponding oxides (a precursor (ZnO-Fe2O3/TiO2-NTs) of the ZnFe2O4/ TiO2-NTs electrode). This precursor was heated at 773 K for 120 min, after which the temperature was raised at 2 °C min-1, until finally the ZnFe2O4/TiO2-NTs electrode was formed. Characterization. The characterization of the ZnFe2O4/ TiO2-NTs electrode included environmental scanning electron microscopy (ESEM), X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), high-resolution transmission electron microscopy (HRTEM), UV-vis diffuse reflectance spectroscopy (UV-DRS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and Kelvin probe (KP) based surface photovoltage (SPV) measurements. Detailed information can be found in the Supporting Information.
FIGURE 1. (a) Top-view ESEM images of the TiO2-NTs. The inset shows the images of TiO2-NTs at high magnification. (b) Side-view ESEM images of the TiO2-NTs. (c) Top-view ESEM images of the ZnFe2O4/TiO2-NTs. The inset shows the images of ZnFe2O4/TiO2-NTs at high magnification. (d) Corresponding HRTEM image of the ZnFe2O4/TiO2-NTs. Photoelectrochemical Measurements. Photoelectrochemical measurements were performed using a conventional three-electrode cell system and a CHI 760c (CHI Co., USA) electrochemical workstation (Scheme S1 in the Supporting Information). The ZnFe2O4/TiO2-NTs electrode was employed as the working electrode. Meanwhile, a saturated calomel electrode and a platinum electrode served as the reference and counter electrode, respectively. All the potentials were referred to the SCE unless otherwise stated. The working electrode was irradiated with visible light (420 nm < λ < 600 nm) through a UV-cutoff filter and an IR-cutoff filter (Shanghai Seagull Colored Optical Glass Co., Ltd.) from a high-pressure xenon short arc lamp (a Phillips 500 W Xe lamp). The incident light intensity (I0) of the visible light was 33 mW cm-2, which was measured with a radiometer (Photoelectric Instrument Factory Beijing Normal University, model FZ-A). The electrochemical impedance spectroscopy (EIS) was carried out in the frequency range of 10-2 to 105 Hz with an ac voltage amplitude of 10 mV at a dc bias of 0.3 V (vs the SCE) in a 0.01 M Na2SO4 solution. PEC Activity Test. The PEC oxidation of 4-CP was carried out in a round-bottom quartz reactor. All the experiments were performed with magnetic stirring, using 0.01 M Na2SO4 as the electrolyte. The initial concentration of the 4-CP aqueous solution was 20 mg L-1 during the experiment. (See the Supporting Information for detailed information on the experimental procedure and the product analysis). Prior to the start of light experiments, dark (adsorption) experiments were carried out for 20 min under continuous stirring until sufficient adsorption of the 4-CP onto the surface of the catalyst (Figure S1).
Results and Discussion Characterization of Photocatalysts. ESEM and HRTEM images of TiO2-NTs were taken before and after modification with ZnFe2O4 nanoparticles. From the top-view (Figure 1a) and the cross sectional (Figure 1b) examination of the TiO2-NTs, it can be observed that the diameter of the highly ordered, well aligned TiO2 nanotubes is about 100 nm, the wall thickness is around 20 nm, and the average tube length is approximately 700 nm. After electrodeposition modification on the TiO2-NTs, the tube walls became decorated with VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. X-ray diffraction patterns of (a) Ti substrate, (b) TiO2-NTs, and (c) ZnFe2O4/TiO2-NTs. aggregates of fine ZnFe2O4 nanoparticles that partly penetrated into the TiO2-NTs pores (Figure 1c). The average diameter of the nanoparticles is about 20 nm. The nanotubular structure of TiO2 maintained its integrity without significant morphological changes. In some areas, clusters of ZnFe2O4 nanoparticles were formed in the interior of the tube openings. Further study by HRTEM (Figure 1d) shows that the ZnFe2O4/TiO2-NTs is structurally uniform with a lattice fringe of 0.351 nm corresponding to the 〈101〉 plane of anatase TiO2. The observed lattice spacing values of 0.487 and 0.254 nm in the nanotubes correspond to the 〈111〉 and 〈311〉 planes of ZnFe2O4, respectively. These results also confirm that ZnFe2O4 nanoparticles have been successfully assembled into the TiO2-NTs. The XRD spectrum of anodically etched Ti foil confirms the structure of the TiO2 nanotubes to be anatase identified mainly by the 〈101〉 reflection (Figure 2, curve b). The peak of the 〈101〉 crystal face at 2θ ) 25.2° indicates a fine preferential growth of the TiO2-NTs in the 〈101〉 direction. Besides TiO2 and Ti diffraction peaks, also presented in Figure 2, curve c, are ZnFe2O4 peaks (JCPDS file 79-1150). These peaks reveal that the ZnFe2O4 particles actually have the spinel structure which is in agreement with that revealed by the HRTEM image. The composition and structure of the TiO2 nanotubes and ZnFe2O4 nanoparticles were also characterized by EDX and Raman (Figure S2 and Figure S3), which further confirm that the ZnFe2O4/TiO2-NTs were composed of TiO2 nanotubes and ZnFe2O4 nanoparticles. In addition, the UV-vis absorption spectra of ZnFe2O4/TiO2-NTs (with an absorption edge at 587 nm) and the unmodified TiO2-NTs are shown in Figure S4. The results indicate that the absorption of visible light by ZnFe2O4/TiO2-NTs is clearly more than that of TiO2-NTs, which is due to the contribution of ZnFe2O4. The KP-based SPV response of the TiO2-NTs was apparently enhanced after the loading of ZnFe2O4 nanoparticles and the photo response successfully extended from the UV to the visible light region, which is in agreement with the results of DRS (Figure S5). Furthermore, the XPS results also demonstrate the existence of Ti4+ [Ti (2p) peaks at 464.8 and 459.2 eV] (24), Zn2+ [Zn (2p) peaks at 1044.4 and 1022.8 eV] (25), and Fe3+ [Fe (2p) peaks at 710.6 and 724.4 eV] (26) in the ZnFe2O4/TiO2-NTs (Figure S6). Photoelectrochemical Measurement. As shown in Figure S7, the ZnFe2O4/TiO2-NTs electrode has a strong instant photoresponse to the visible light irradiation. The shortcircuit photocurrent density of the ZnFe2O4/TiO2-NTs electrode is as great as 4.9 times that of the TiO2-NTs electrode. This demonstrates that the generation of visiblelight photocurrent is ascribed to the modification of ZnFe2O4 nanoparticles for the TiO2-NTs electrode. To further investigate the influence of ZnFe2O4 modification on the photoelectric property, the EIS technique was employed to study the solid/electrolyte interfaces of the 5100
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FIGURE 3. Decontamination of 4-CP solutions by electrochemical process, direct photolysis, photocatalysis, and photoelectrocatalysis, respectively, with the ZnFe2O4/TiO2-NTs electrode under visible light irradiation. Inset: Decontamination of DCA solutions by electrochemical process, direct photolysis, photocatalysis, and photoelectrocatalysis, respectively, with the ZnFe2O4/TiO2-NTs electrode under visible light irradiation (I0 ) 33 mW cm-2, 0.6 V vs SCE bias potential). TiO2-NTs electrode and the ZnFe2O4/TiO2-NTs electrode. Figure S8 shows the EIS responses of the two electrodes under dark and visible light irradiation. The radius of the semicircle on the EIS Nyquist plots reflects the reaction rate occurring at the surface of electrode. In the dark, both electrodes showed a pronounced arc (semicircle portion) at higher frequencies in the EIS plane, the diameter of which corresponded to the electron transfer resistance controlling the kinetics at the electrode interface (27). Significant change in the EIS spectra was observed for the TiO2-NTs electrode following ZnFe2O4 modification. In fact, deposition of the ZnFe2O4 nanoparticles on the TiO2-NTs electrode resulted in a decrease in the semicircle diameter, which justified an analogous decrease in the electron-transfer resistance and indicated an increase in the charge-transfer paths from the ZnFe2O4/TiO2-NTs electrode to the Ti substrate. However, under visible light irradiation (420 nm < λ < 600 nm), the arc radius on the EIS Nyquist plots of ZnFe2O4/TiO2-NTs electrode was smaller than that of TiO2-NTs electrode, which suggested that a more effective separation of photogenerated electron-hole pairs and faster interfacial charge transfer occurred on the ZnFe2O4/TiO2-NTs electrode (28) (a heterojunction might have been established between the TiO2 nanotubes and ZnFe2O4 nanoparticles, which could promote separation of photogenerated charge carriers, Supporting Information Figure S9). PEC Degradation of 4-CP. To investigate the PEC activity of the ZnFe2O4/TiO2-NTs electrode, several PEC experiments were carried out for 4-CP degradation under visible light irradiation (33 mW cm-2). The 4-CP removals in the various degradation processes, that is, the PEC process, the photocatalytic (PC) process, the electrochemical process (EC), and the direct photolysis (DP), are presented in Figure 3. It is obvious that the PEC process provides the most powerful way to degrade the 4-CP in aqueous solution. The 59.6% of 4-CP removal was obtained after 120 min, while only 22% of the 4-CP removal was obtained in PC process with the same illumination time. In addition, when DP was used as the control experiment in the PEC experiment, the degradation efficiency of 4-CP reached 7.5%. The removal with EC was insignificant, which proves that the 4-CP is stable in this process. Additionally, the degradation of 4-CP in the PEC process was higher than the summation of the individual PC and EC methods. That is to say, there was cooperative interaction between PC and EC (29, 30), confirming that the ZnFe2O4/TiO2-NTs electrode had higher performance in the PEC process than in either the photocatalytic or electro-
FIGURE 4. Concentration variations of 4-CP treated using PEC technique with TiO2 electrode (Hoffmann), TiO2-NTs electrode, ZnFe2O4/TiO2 (Hoffmann) electrode, and ZnFe2O4/TiO2-NTs electrode under visible light illumination (I0 ) 33 mW cm-2, 0.6 V vs SCE bias potential). catalytic process alone. The experimental data of Figure 3 were found to fit approximately a pseudo-first-order kinetic model by the linear transforms ln(C0/Ct)/f(t) ) kt (k is rate constant). The values of k and regression coefficient (R2) are listed in Table S1. The experimental results show that the reaction rate of 4-CP degradation in the PEC using the ZnFe2O4/TiO2-NTs electrode was the fastest among all the reactions. To have an overview of the degradation efficiency of 4-CP in the present research, we indirectly compared our results with those reported by Hoffmann et al. (31), as their experiments could be carried out under conditions identical (man-made design) to ours. Clearly, the degradation efficiency of 4-CP of the ZnFe2O4/TiO2-NTs electrode was about 1.73 or 17.54 times of that of TiO2 electrode (Hoffmann) under UV or visible light irradiation, respectively (Figure S10 and Figure 4). The higher degradation efficiency of 4-CP in the present work demonstrates the advantage of the ZnFe2O4/TiO2-NTs electrode over the conventional TiO2 electrode, which is mainly attributed to its large specific surface area (nanotubes), lower levels of electron-hole recombination, and the modification of ZnFe2O4. In addition, to further evaluate and compare the PEC performance of the TiO2-NTs electrode and ZnFe2O4/ TiO2-NTs electrode in the elimination of aqueous contaminants, the decomposition of dichloroacetate (DCA) was also studied (inset of Figure 3 and Figure S11). The TiO2-NTs electrode was ineffective but the DCA showed a very high decomposition rate for the ZnFe2O4/TiO2-NTs electrode under visible light irradiation. The corresponding photonic efficiencies for 4-CP and DCA degradation are given in Tables S2 and S3. PEC Mechanism Discussion. According to the above results, it is evident that the interaction between ZnFe2O4 and TiO2 was responsible for the efficient generation and separation process under visible light excitation. Scheme S2 is an illustration of the photoelectron transfer mechanism for the ZnFe2O4/TiO2-NTs electrode under visible light irradiation. As shown in Scheme S2b, ZnFe2O4 with narrow band gap energy (1.9 eV) could be easily excited by visible light with energy less than 2.95 eV (λ > 420 nm) and induced the generation of electron-hole pairs (20). Since the conduction band of TiO2 lay more positive than that of the ZnFe2O4 conduction band, electron injection was expected from the photoexcited ZnFe2O4 nanoparticles into the TiO2 conduction band. Hence, the photoelectrons were collected from ZnFe2O4 and transferred across the interface of the heterostructure to the surface of the TiO2 nanotubes. The electrons then traveled along the TiO2 nanotubes, passed through the interface between TiO2 and Ti to the external circuit under the external electrostatic field, leaving the
photogenerated holes in the valence band of ZnFe2O4. In such a way, the photoinduced electron-hole pairs were effectively separated. Meanwhile, some photogenerated holes reacted directly with the surface-adsorbed 4-CP molecules to produce 4-CP•+ (Scheme S2a). Further, the reactive 4-CP•+ radical transformed into degradation products (32). However, others reacted with H2O, producing hydroxyl radicals (OH•), which further hydroxylated and oxidized organic compounds into H2O and CO2. Furthermore, the suitable positions of the redox level of 4-CP molecule legitimated the process of charge transfer, which led to the degradation of 4-CP. According to the above analysis, the relevant reactions at the composite semiconductors surface can be expressed as follows: ZnFe2O4 + hν f ZnFe2O4(e + h) f ZnFe2O4 + heat (1) ZnFe2O4(e) + TiO2 f ZnFe2O4 + TiO2(e)
(2)
TiO2(e) + external electrostatic field f external circuit (3) ZnFe2O4(h) + 4 - CP f 4 - CP•+ f degradation products (4) ZnFe2O4(h) + H2O(Red) f H+ + OH•(Ox)
(5)
OH• + 4 - CP f degradation products
(6)
These results demonstrate that the ZnFe2O4/TiO2-NTs electrode behaves as an efficient material in utilizing solar energy for the photo decomposition of pollutants. TOC and Cl- Analyses. The decrease of TOC and the formation of Cl- in the PEC degradation of 4-CP using the ZnFe2O4/TiO2-NTs electrode were also observed. The results are shown in Figure S12. The TOC removal efficiency was remarkably slower than that of 4-CP. About 71.5% of TOC still remained after 120 min of visible light irradiation while 59.6% of 4-CP was dechlorinated. The concentration of Clincreased to reach 94.5% (3.11 mg L-1) of the theoretical quantity (3.29 mg L-1). It could be concluded from this result that there might have been transient organic intermediates present in the PEC system. Determination of PEC Degradation Intermediates. Degradation intermediates of 4-CP during the PEC process of 5 h were thus monitored by HPLC-MS. The results are displayed in Figure S13 and Figure 5. Additionally, under visible light irradiation for 5 h, the degradation efficiency of 4-CP in the PEC process was 56.9% and 100% for the ZnFe2O4/TiO2 electrode and the ZnFe2O4/TiO2-NTs electrode, respectively, while it was only 13.5% for the TiO2-NTs electrode (Figure S14). Figure S13 displays the high-performance liquid chromatogram of 4-CP PEC degradation using the ZnFe2O4/ TiO2-NTs electrode after 5 h under visible light irradiation and the corresponding mass spectra of peaks a, b, c, d, and e. According to the HPLC-MS analysis (Figure 5), the peak a with retention time of 4.7 min (m/z ) 127) is attributed to the initial 4-CP, and the other peaks appearing at 2.2 (peak b, m/z ) 110), 2.6 (peak d, m/z ) 108), 3.5 (peak c, m/z ) 144), and 4.0 (peak e, m/z ) 126) min were found to be hydroquinone (HQ), benzoquinone (BQ), 4-chlorocatechol (4-CC), and hydroxyhydroquinone (HHQ), respectively. A possible pathway of 4-CP degradation is proposed in Scheme 1. First, 4-CP is found to hydroxylate to HQ and 4-CC. Subsequently, HQ and 4-CC is dehydrogenated and hydroxylated to BQ and HHQ, respectively. The oxidation of BQ and HHQ, after ring cleavage, leads to the formation of aliphatic carboxylic acids, which are finally degraded into CO2 and H2O. The findings confirm the results reported by VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Total ion chromatogram and mass spectra of peaks a, b, c, d, and e obtained from the 4-CP photodegradation over ZnFe2O4/TiO2-NTs electrode after 5 h under visible light irradiation.
SCHEME 1. Schematic Reaction Pathways of 4-CP Degradation
Mills et al. (33) for 4-chlorophenol photomineralization on a thin film covered with Degussa P25 TiO2 under UV light irradiation and by Bahnemann et al. (34) for the photocatalytic degradation of 4-chlorophenol in aerated aqueous titanium dioxide suspensions under UV light irradiation. Stability of ZnFe2O4/TiO2-NTs Electrode. Figure S15 shows the time profile of eight repeated experiments of 4-CP PEC degradation, using the ZnFe2O4/TiO2-NTs electrode under the same experimental conditions. The results clearly 5102
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show a good reproducibility with the degradation rate of 95% retained even after eight repeated experiments.
Acknowledgments This work was supported financially by the National Nature Science Foundation of China (20877013, 20837001), the National High Technology Research and Development Program of China (863 Program) (2007AA061402), the Major State Basic Research Development Program of China (973
Program) (2007CB613306), and the Ph.D. Program Foundation of Ministry of Education of China (20070141060). We thank Mr. Tin Ka Ping for awarding a fellowship to X.Y. Li. G. Chen thanks Changjiang Scholar (Chair) Fellowship from Ministry of Education, China.
Supporting Information Available Auxiliary information on experimental procedures, analytical method, characterization of samples, adsorption curves of 4-CP, UV-vis spectrum of 4-CP, PC mechanism graph, variation of the photocurrent density vs time, I-V, ESR signal, stability, photonic efficiency, emission spectrum from a Xenon arc lamp, PEC degradation of 4-CP, DCA, BQ, and HHQ on TiO2-NTs and the ZnFe2O4/ TiO2-NTs electrode under visible light or simulated sunlight irradiation, PEC degradation of 4-CP as affected by bias potential for the ZnFe2O4/TiO2-NTs electrode under visible light irradiation, the effect of a different number of repetitions, TOC removal, determination of PEC degradation intermediates, the formation of Cl-, and HPLC data. This material is available free of charge via the Internet at http://pubs.acs.org.
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