Enhanced photoelectrocatalytic performance of polyoxometalate-titania nanocomposite photoanode

Applied Catalysis B: Environmental 76 (2007) 15–23 www.elsevier.com/locate/apcatb

Enhanced photoelectrocatalytic performance of polyoxometalate-titania nanocomposite photoanode Yibing Xie a,b,*, Limin Zhou a,*, Haitao Huang c a

Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong b State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, China c Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong Received 13 March 2006; received in revised form 27 April 2007; accepted 1 May 2007 Available online 5 May 2007

Abstract Polyoxophosphotungstate-incorporated titania/titanium nanocomposite (POPTA-TiO2/Ti) has been developed as a photo-anode for photoelectro-catalytic application. Highly-ordered anatase TiO2 nanotube array with a vertical orientation is synthesized by a controlled anodization plus a post-calcination. Hybrid incorporation is followed by embedding POPTA into TiO2 tubule channels. The morphological characteristics, crystal behavior and chemical structure of POPTA-TiO2 are examined by field emission scanning electron microscopy, X-ray diffraction (XRD), thermogravimetry analysis and Fourier transformed infra-red spectroscopy. Electrochemical properties and photocurrent responses have been investigated by linear sweep voltammetry. Photo-catalytic and photo-electro-catalytic applications for recalcitrant organic pollutant degradation have been investigated to examine photo-electrochemical efficiency and effectiveness of POPTA-TiO2/Ti nanocomposite photo-anode. Electrogeneration hydrogen pyroxide (H2O2)-assisted TiO2 photo-electro-catalysis has been also conducted by using dual functional electrodes of a nanotubular POPTA-TiO2/Ti photo-anode and a vitreous carbon cathode for enhanced photo-electrochemical performance. # 2007 Elsevier B.V. All rights reserved. Keywords: Nanocomposite photo-anode; Photo-electro-catalytic; Polyoxophosphotungstate; Titania nanotube array

1. Introduction In the area of the advanced oxidation processes, both titania (TiO2) and polyoxometalate (POM) photo-catalysis has been intensively investigated for the oxidized and even mineralized degradation of various recalcitrant organic pollutants through in situ generated reactive radicals [1–3]. The same general photochemical characteristics are involved in TiO2 and POM photo-catalysis. For example, UV excitation of POM induces a ligand (oxygen) to metal charge transfer (LMCT) with promoting an electron transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), which can be considered as a similar process analogous to the band-gap excitation from valence band (VB) to conduction band (CB) in the TiO2 lattices. In particular, both of them can undergo stepwise multi-electron redox reactions,

* Corresponding author. Tel.: +852 27666663; fax: +852 23654703. E-mail addresses: [email protected] (Y. Xie), [email protected] (L. Zhou). 0926-3373/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2007.05.006

while their chemical structure remains intact. A comparison of photo-catalytic oxidation by POM (such as phosphotungstate, silicotungstate and phosphomolybdate) and TiO2 has been reported [4]. POM immobilization on solid supports (such as zeolite, silica and polymer) has also been investigated to improve photo-catalytic oxidation efficiency as well as to conduct a recyclable photo-catalysis [5,6]. Especially for TiO2 photo-catalysis in presence of POM, TiO2 photo-electrons in the excited state can be extracted and transferred to POM in the ground state and consequently retard the fast charge pair recombination because POM can act as an efficient electrons trapper to form a comparatively stable reduction product of POM (e) [7–9]. So, POM modification can improve its inherent photo-catalytic reactivity of TiO2. Some research works had also proved that electronically coupled POM-TiO2 could act well as a composite photo-catalyst for an effective degradation of various organic pollutants [10]. POM, as an ideal quantum size cluster, mostly conducts a homogeneous reaction in aqueous solution. However, TiO2, as a bulk semi-conductor material, only conducts a heterogeneous reaction. The approach of simple mixture of POM with TiO2 can not realize the

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effective recycling photo-catalysis for practical application since the good solubility of POM will cause a fast dissolving loss till to form a homogeneous POM reaction solution. Multidimensional structure configuration (such as microporous or mesoporous TiO2) by self-organization, chemical template or electrochemical synthesis routes could promote its photocatalytic activity. Moreover, nanotubular TiO2 has exhibited a better photo-electrochemical performance due to its novel nano-architecture. The strategy of POM encapsulation in TiO2 nanotubes becomes accessible to construct a novel POM-TiO2 hybrid photo-catalyst, which keeps both a high photo-catalytic activity and physicochemical stability. On the other hand, the approaches of applying external positive potential on TiO2 or adding peroxides in reactants solution can improve total photo-electrochemical reactivity due to the promotion role of electron–hole pair separation. In most photo-electro-catalytic system, TiO2 electrode is used as a photo-anode while inert platinum (Pt) electrode is ordinarily used as a cathode. The role of the cathode beyond a counter electrode is usually disregarded. Some further advantages of utilizing the cathode for generating useful oxidant species of hydrogen peroxide (H2O2) within the same photo-reactor can be realized, if the counter electrode is used as a functional cathode. H2O2, as a sacrificial oxidant, which can effectively capture the photo-induced TiO2 CB electrons to form the hydroxyl radicals (OH), has been proven to be beneficial for improving TiO2 photo-catalytic efficiency [11]. In fact, H2O2 can be well generated by an electrochemical reduction method on various carbon electrodes, such as graphite or reticulated vitreous carbon (RVC) materials [12]. In this study, the POM-incorporated TiO2 nanocomposite is constructed, in which TiO2 nanotube array acts as an electrode matrix and polyoxophosphotungstate (POPTA) is used as a Keggin-type POM for hybrid modification. A soft-chemical synthesis method will be used to embed POPTA in TiO2 tubule channels to prevent the dissolving loss of POPTA. In addition, the electro-generation H2O2 can be introduced into the photoelectro-catalysis system by using an oxygen-diffused carbon electrode as another functional cathode. Bisphenol A, as one of typical endocrine disrupting chemicals, is used to evaluate photo-catalytic and photo-electro-catalytic reactivity of POPTA-TiO2/Ti nanocomposite electrode. The full investigation about photo-electrochemical properties of POPTA-TiO2 will provide us a better understanding of interactive oxidation reactivity through the mediated electron transfer process. 2. Experimental 2.1. Preparation of nanocomposite electrode A rectangle titanium (Ti) foil (size 8  50 mm, thickness 140 mm, purity >99.6%) is ultrasonically cleaned in alcohol and acetone solution for 20 min and then chemically polished in the concentrated HF–HNO3 solution for 15 s to form a fresh smooth surface. This Ti foil is used as an anode and Pt foil (size 10 mm  40 mm, purity 99.99%) as a cathode in an electrolysis set-up. The distance between Ti foil and Pt foil is fixed at 25 mm.

A direct-current power supply with a programmable function is used to control experimental current and voltage in this electrochemical process. A galvanostatic-potentiodynamic anodization is initially performed in aqueous H3PO4 (0.5 M)–HF (0.15 M) solution to form an ultra thin oxide layer at a constant current density of 1.2 mA cm2 along with a continuous increase of anode-to-cathode voltage till to 20 V. The following potentiostatic-galvanodynamic anodization is maintained at 20 V till to an ultimate current density of 2.5 mA cm2. The whole anodization process lasts for about 40 min. This TiO2/Ti electrode is adequately washed with deionized water and dried in an oven at 378 K for 24 h. It is finally calcined at 723 K for 2 h and cooled to room temperature. To couple POPTA with TiO2, initial TiO2/Ti matrix is firstly immersed in a cationic surfactant of 0.1 M n-hexadecyltrimethylammonium chloride (CTAC) aqueous solution for 60 min and washed with deionized water to form a positively charged tubule channels. Then, this pre-treated TiO2/Ti is put into a Teflon vessel with H3PW12O40 (0.2 M)–HF (0.01 M)– HClO4 (0.1 M) aqueous solution. This vessel is sealed and placed in a stainless-steel autoclave to keep at 388 K for 8 h. Finally, the obtained electrode is rinsed adequately with distilled water and ethanol, dried and calcined at 673 K for 1 h. 2.2. Characterization and analysis To investigate surface morphology, field emission scanning electron microscopy (FESEM, JEOL JSM-6335F) is used to observe pore size, pore distribution and layer thickness. To determine the crystallization behavior, X-ray diffraction (XRD) experiments are carried out on an X-ray diffractometer fitted with a graphite monochromator (Philips PW3020). To identify elements composition, an energy dispersive X-ray (EDX, Oxford ISIS 310) analysis is carried out. To determine chemical structure of POPTA-TiO2, Fourier transformed infra-red (FTIR) spectroscopy measurement has been carried out on FTIR Spectrometer (Magna-IR 760 ESP, Nicolet). Thermogravimetry (TG) analysis has also performed on a thermogravimetric apparatus (Setsys24 Evolution TGA-DTA/DSC, Setaram Engineering, France) with a temperature range from 25 to 700 8C at the heating rate of 10 8C min1 in an air atmosphere. BPA concentration is determined by a high-performance liquid chromatography (Spectrasystem high performance liquid chromatography (HPLC), Finnigan Corp.), which includes an Atlantis d-C18 reversed phase column (column size 150  4.6 mm, filling beads diameter 5 mm), a high-pressure pump (P4000), a UV diode array detector (6000LP), and an auto sampler (AS3000). A mobile phase of acetonitrile/water (70/30, v/v) is employed with a flow rate of 0.8 ml min1. The pH value is measured by an Orion Research pH/ISE meter (ORION, EA 940) equipped with an Orion composed electrode. 2.3. Photo-electrochemical experiments All photo-electrochemical measurements and applications are conducted in a specially designed cylindrical quartz cell with a three-electrode configuration and an external UVA light

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source installation (main wavelength of 365 nm and radiation intensity of 0.68 mW cm2). The working electrode potential and current are controlled by an electrochemical workstation (CHI660B, CH Instruments Inc. USA). The photocurrent and polarization current response are measured in 0.01 M (Na2SO4) solution under UVA illumination. The photo-electro-catalytic degradation experiments are carried out in the undivided quartz cell with a constant anodic potential model. This threeelectrode system involves POPTA-TiO2/Ti (effective electrode size 40 mm  8 mm  0.14 mm) as a working photo-anode along with UVA irradiation, a RVC sheet or Pt foil cathode as a counter electrode and a saturated calomel reference electrode (SCE). The photo-catalytic reaction is also performed in the same system only without applying any external electric potential on POPTA-TiO2/Ti electrode. The pure oxygen is continuously diffused into the reaction solution at a flow rate of 30 mL min1 to supply the dissolved oxygen. 2,2–Bis(4hydroxyphenyl)propane (Bisphenol A, BPA) is used as a model pollutant with an initial concentration of 11.2 mg L1 and 0.01 M Na2SO4 is used as a supporting electrolyte. To avoid any hydrolysis of polyoxophosphotungstate in alkaline environment, BPA solution will be adjusted to pH 3.0 with HClO4. In view of physical adsorption of BPA on electrode surface, all electrodes, prior to reaction, have been immersed in reaction solution for 120 min to reach adsorption/desorption equilibrium. The samples are collected from the reaction solution at regular intervals to determine the BPA concentration by using a chromatographic analysis method. 3. Results and discussion 3.1. Morphology and microstructure characterization Surface morphology of TiO2/Ti and POPTA-TiO2/Ti has been investigated and the field emission SEM and crosssectional SEM images are shown in Fig. 1. Regarding TiO2/Ti electrode, the highly-ordered and vertically-oriented TiO2 nanotube array has been well formed by the controlled anodization of Ti metal. Each pore mouth is open on the top layer due to preferential corrosion dissolution by HF, while the bottom of the nanotubes is closed by presence of an oxide barrier layer. The mean size of the inner diameter is about 60 nm and its wall thickness is about 15 nm. Concerning the POPTA incorporation with TiO2, the negatively charged phosphotungstate conducts the oriented adsorption and hydrothermal reaction to form polyoxophosphotungstate aggregation of [(PW12O40)n]3n in individual tubule because its pore-wall inner surface has been positively charged through pre-treatment of CTAC surfactant. Actually, this polyoxo-anion of POPTA can be regarded as an inorganic polymer, which is formed in the nanotube array by polycondensation process of phosphotungstic anion. As a result, POPTA can be well encapsulated in TiO2 tubule channels. The cross-sectional SEM shows the average thickness of POPTA-TiO2 composite is about 520 nm, which is very close to the initial thickness of TiO2 tubules. It means that most POPTA is embedded inside nanotubes rather than on the external surface of TiO2 layer for POPTA-TiO2/Ti.

Fig. 1. FESEM images of nanotubular (A) TiO2/Ti electrode, (B) POPTA-TiO2/ Ti electrode and (C) cross-sectional SEM image of POPTA-TiO2/Ti electrode.

X-ray diffraction measurement has been conducted to investigate the crystal structure and their patterns are shown in Fig. 2. Both TiO2 and POPTA-TiO2 exhibit the regular anatase crystal phase, which is ascribed to the appearance of characteristic diffraction peaks at 2u = 25.48 for hkl (1 0 1) and 2u = 48.18 for hkl (2 0 0) planes. Actually, the low-voltage anodization process at 20 V only contributes the formation of

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0.52 keV. Concerning POPTA-TiO2 nanocomposite, besides all above peaks, additional new peaks are also present in its EDX spectrum. The peaks at 1.14, 1.82, 2.15, 8.38, 9.62, and 9.89 keV are corresponding to W and the peak at 1.89 keV is ascribed to P element. Approximate composition ratio of W/Ti elements in POPTA-TiO2 can be estimated by calculating the integral area of their characteristic peaks. The result indicates that the doping amount of POPTA in TiO2 nanotube array is about 19.7 wt%. So, cationic surfactant-induced adsorption and hydrothermal process could well achieve the hybrid incorporation to form an encapsulation-structured POPTA-TiO2 nanocomposite. 3.2. Photo-electrochemical response Fig. 2. XRD patterns of TiO2/Ti and POPTA-TiO2/Ti electrodes.

amorphous TiO2 due to its low anodic potential far below the critical sparking discharge voltage of about 110 V. The subsequent calcination treatment at 723 K can promote TiO2 phase transmission from amorphous to anatase. In addition, POPTA-TiO2 exhibits not only characteristic diffraction peaks of anatase titania, but also other new diffraction peaks at 2u = 23.88 and 33.68, which is ascribed to the formation of polyox-oanion complex compound of partially Ti-substituted Keggin phosphotungstate [PTi2W10O40]7 by high-temperature solid phase reaction [13]. Energy dispersive X-ray measurement has been carried out to confirm the hybrid incorporation effect of POPTA-TiO2. The corresponding characteristic peaks have been identified to figure out the elemental composition of this complex metal oxide layer (see Fig. 3). EDX spectrum of pure TiO2 shows that strong Ka and Kb peaks from Ti element appear at 4.51 and 4.92 keV, while moderate Ka peak of O element appears at

Fig. 3. Energy dispersive X-ray spectrum of (A) TiO2/Ti and (B) POPTA-TiO2/ Ti electrodes.

The photo-electrochemical properties have been investigated by linear sweep voltammetry experiments in a standard three-electrode assembly, which consists of POPTA-TiO2/Ti as a working electrode, Pt foil as a counter electrode, SCE as a reference electrode and 0.01 M Na2SO4 as an electrolyte solution (see Fig. 4). Under dark condition, the anodic polarization current of POPTA-TiO2/Ti and pure TiO2/Ti is maintained at almost 0 mA when the working potential is scanned below 1.5 V in a positive potential scanning mode. Then, the anodic current begins to generate along with the further increase of anodic potential. The critical potential threshold is about 1.6 V for POPTA-TiO2/Ti electrode, which is ascribed to water oxidation reaction. This reaction is obviously intensified over the anodic potential of 2.8 V. Subsequently, the sharp rising of anodic current means dissolution corrosion reaction of TiO2 semi-conductor begins to occur when anodic potential is above 3.0 V, which can cause the breakdown of TiO2. The anodic oxidation potential of oxygen evolution is much more positive than the theoretical value of the electrolytic reaction (2H2O  4e ! O2 + 4H+, E0 = 0.987V versus SCE), which is due to the high over-potential effect. Additionally in a negative potential scanning mode, the cathodic polarization process has also been promoted for hydrogen generation on

Fig. 4. Linear sweep voltammetry curves of TiO2/Ti and POPTA-TiO2/Ti electrodes under dark and UV light illumination.

Y. Xie et al. / Applied Catalysis B: Environmental 76 (2007) 15–23

POPTA-TiO2/Ti electrode in comparison with TiO2/Ti. Both POPTA-TiO2/Ti and TiO2/Ti exhibit a similar anodic voltampere property, which indicates the electrochemical behavior of POPTA-TiO2 still acts as a typical n-type semi-conductor as well as pure TiO2. Under UV illumination, the corresponding positive polarization current intensity is much higher than that under dark condition when the same bias potential is applied on either POPTA-TiO2/Ti or TiO2/Ti. It means that anodic photoelectrons induced by UV excitation mainly contribute to enhancing the photocurrent in a positive potential scanning mode. In particular, the polarized photocurrent intensity of POPTA-TiO2/Ti is much higher than that of TiO2/Ti under UV illumination. So, the electron transfer process driven by a positive bias is much more feasible on the POPTA-TiO2/Ti electrode interface, which can ultimately promote the electro– hole pair separation during photo-catalytic and photo-electrocatalytic processes. To exert photo-electrochemical efficiency to the utmost, a constant potential of +3.0 V will be applied on reactive photo-anodes in this study, which is the permissive safe anodic potential to avoid the breakdown of POPTA-TiO2 semiconductor and meanwhile contribute a high working cathodic current. 3.3. Structural stability analysis The thermogravimetric analysis of POPTA-TiO2 has been performed to investigate the thermal stability of POPTA in the process of coupling modification and heating treatment. TG curves of POPTA crystal and POPTA-TiO2 nanocomposite are shown in Fig. 5. Concerning the TG curve of pure POPTA, the initial steep decreasing stage below 140 8C is due to quick loss of the weakly physisorbed water molecules ðH2 O    H3 PW12 O40  12H2 O    H2 O    ! H3 PW12 O40  12H2 O þ 2H2 OÞ: The second decreasing stage in an interval of 200– 330 8C is ascribed to the dehydration process of the hydrated water molecules ðH3 PW12 O40  12H2 O ! H3 PW12 O40  nH2 O þ ð12  nÞH2 OÞ: Finally, a slow degressive stage appears at a higher temperature range from 340 to 570 8C. In this period, the hydrated water molecules have been mostly removed and the dehydration rearrangements of tetrahedral coordination of P-WO6 have also occurred due to further lose of molecule water of intrinsic phosphotungstates, which

Fig. 5. TG curves of (a) pure POPTA crystal and (b) POPTA-TiO2/Ti electrode.

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ultimately cause the complete breakdown of Keggin unit of original POPTA ðH3 PW12 O40 ! PW12 O38:5 þ 1:5H2 Þ [14]. In view of the TG curve of POPTA-TiO2, the initial quick loss of the physisorbed water occurs below 100 8C. Then, the dehydration reaction of H3PW12O4012H2O is followed till about 400 8C. In this stage, the Keggin structure is still retained although the crystal cell parameters have a fine modification. With further increasing temperature till to 620 8C, the Keggin unit begins to be rearranged and ultimately destroyed. This TG experiment analysis indicates the thermal stability of POPTA can be improved through its encapsulation modification inside TiO2 nanotubes. So, the chemical structure of POPTA in POPTA-TiO2 nanocomposite can be mostly remained in the course of heating process below 400 8C. Although the calcination at high temperature will cause a molecular destruction of POPTA in a certain degree, it is still a necessary post-processing step to effectively enhance the coupling strength between POPTA and TiO2 with the purpose of avoiding possible dissolution loss of POPTA during the photoelectrochemical application. In this study, the post-calcination of 400 8C has been applied for this POPTA-TiO2/Ti electrode. Fourier transformed infra-red spectroscopy analysis has also been carried out to examine chemical structure stability in the photo-electro-catalytic process. Fig. 6 shows FTIR spectra of the original, post-calcinated, and photo-electro-catalyzed POPTA-TiO2. The characteristic absorption peaks locate at nas (P = Oa) = 1070 cm1, nas (W = Ot) = 953 cm1, nas (W– Oc–W) = 877 cm1 and nas (W–Oe–W) = 745 cm1, respectively, which can be ascribed to Keggin unit of POPTA (Ot, terminal oxygen; Oa, center tetrahedral oxygen; Oc, bridged oxygen of two octahedral sharing a corner; and Oe, bridged oxygen sharing an edge) [15]. In view of the POPTA-TiO2 with heating at 400 8C, the characteristic absorption peaks are well preserved although the absorption intensities have somewhat declined in comparison with original sample due to partial dehydration reaction (see curve a and b). By comparing the FTIR spectra of POPTA-TiO2/Ti before and after photoelectro-catalytic reaction, the less change of characteristic peak shape and intensity means that POPTA can mostly keep its intrinsically molecular framework structure (see curve b and c).

Fig. 6. FTIR spectra of POPTA-TiO2/Ti electrodes with different treatment.

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Therefore, the approach of embedding POPTA into TiO2 nanotube array can maintain its primary Keggin structure during the thermal treatment and photo-electro-catalytic processes which can mostly uphold the high catalytic reactivity of POPTA-TiO2. 3.4. Photo-electrochemical application To investigate the photo-electrochemical reactivity of POPTA-TiO2/Ti nanocomposite electrode, several sets of photo-catalytic and photo-electro-catalytic reactions are carried out for BPA degradation. The heterogeneous photo-catalytic reaction is supposed to follow the law of the pseudo first-order kinetics, the experimental data can be simply fitted by the firstorder reaction model formula of c = c0 ekt and the corresponding apparent kinetic constant k can be obtained for comparison of BPA degradation efficiency.

Fig. 8. Temporal HPLC chromatograms of BPA degradation products by POPTA-TiO2 photo-catalysis.

3.4.1. Photo-catalytic activity The photo-catalytic degradation curves are shown in Fig. 7. After photo-catalysis for 180 min, BPA degradation ratio achieves to 88.9% for POPTA-TiO2, which is higher than 80.1% for pure TiO2. Comparatively, the degradation ratio achieves to 98.0% by pure POPTA photo-catalysis. The corresponding reaction rate constants are 0.0081, 0.0124, 0.0269 min1 for TiO2, POPTA-TiO2 and POPTA, respectively. It means that POPTA homogeneous photo-catalysis shows higher reaction effectiveness than TiO2 and even POPTA-TiO2 heterogeneous photo-catalysis for BPA degradation. However, such a photo-catalytic efficiency mostly depends on the concentration of homogeneous POPTA catalyst. Additionally, BPA degradation by pure photolysis can be neglected in TiO2 or POPTA photo-catalytic process since pure UV photolytic reaction without any photo-catalyst only results in BPA removal ratio of 2.9%. So, incorporation modification of POPTA is very helpful to promote the photo-catalytic activity of TiO2 nanotube array. Fig. 8 shows HPLC chromatograms of BPA degradation products by POPTA-TiO2 photo-catalysis. The retention time

(RT) of BPA is easily ascertained as 4.43 min according to the standard characteristic HPLC peak. Along with photo-catalytic process, this chromatographic peak area of BPA is gradually decreasing. Meanwhile, other chromatographic peaks begin to generate and their peak areas vary accordingly. After photocatalysis for 180 min, HPLC chromatogram of final BPA degradation product demonstrates five peaks, whose retention time locates at (A) 4.43, (B) 4.07, (C) 3.85, (D) 3.33, and (E) 2.17 min, respectively. The corresponding UV absorption spectra of these intermediates are shown in Fig. 9. On the base of the standard HPLC chromatographic peaks and UV spectra, the product A is obviously identified as the residual BPA. The products B, C and D belong to phenolic derivatives because all of them have the same characteristic UV absorption peaks at 196, 225, and 284 nm, whose spectra are very similar to the phenol. However, the main intermediate E should be ascribed as alkyl carboxyl compound due to its strong UV absorption band at 200 nm and the characteristic chromatogram peak at about RT 2.18 min, which is the same as that of the oxalic acid. The production of multi-component intermediates

Fig. 7. Temporal BPA degradation in TiO2 and POPTA-TiO2 photo-catalytic reaction systems.

Fig. 9. UV absorption spectra of BPA degradation products by POPTA-TiO2 photo-catalysis for 180 min.

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Fig. 10. Temporal BPA degradation in TiO2 and POPTA-TiO2 photo-electrocatalytic reaction systems.

also means POPTA-TiO2 photo-catalytic process results in more destruction of BPA molecule structure rather than complete mineralization in this oxidation process. 3.4.2. Photo-electro-catalytic activity Fig. 10 shows BPA degradation curves in photo-electrocatalytic processes by applying external positive potential of +3.0 V versus SCE on TiO2/Ti or POPTA-TiO2/Ti photoanodes. Direct electrolytic oxidation is a weak process for BPA degradation since BPA removal ratios achieve to 5.58% for TiO2/Ti and 6.98% for POPTA-TiO2/Ti without UV illumination (see curve a and b). Either pure electrolysis or photolysis only plays a very insignificant role in photo-electro-catalytic processes. Concerning TiO2/Ti photo-electro-catalysis for 180 min, BPA degradation ratio achieves to 82.7% and reaction rate constant is 0.0116 min1. In view of POPTA-TiO2/Ti, BPA has been almost completely removed and the rate constant is 0.0184 min1 (see curve c and d). So, photo-electro-catalysis exhibits higher reaction efficiency than sole photo-catalysis for both TiO2/Ti and POPTA-TiO2/Ti. The photo-electrochemical reactivity has also been improved for POPTA-TiO2/Ti electrode. In the following experiments, a RVC electrode, instead of Pt electrode, is applied as another functional cathode to generate H2O2 simultaneously. As a result, interactive photoelectro-catalysis has caused complete degradation of BPA even after 120 min for both TiO2 and POPTA-TiO2. Corresponding rate constants are 0.0224 and 0.0315 min1, respectively (see curve e and f). Such a reaction system involves H2O2–TiO2 interactive oxidation and electroassisted photo-catalysis, which well contribute to enhancing the overall degradation efficiency for both POPTA-TiO2/Ti and TiO2/Ti photo-anodes. It is noted the corresponding RVC cathode current is merely 0.062 mA and ultimate H2O2 accumulation concentration achieves to only 0.027 mM when the photo-anode potential is constant at +3.0 V. The total electro-generation H2O2 amount is still beyond the optimal consumption in this H2O2-assisted photo-electro-catalytic system. However, in situ continuous electro-generation of

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Fig. 11. HPLC chromatograms of BPA degradation products by POPTA-TiO2 photo-electro-catalysis for 180 min.

H2O2 can greatly benefit to improve TiO2 photo-catalytic reactivity although its concentration is still at a very low level. Therefore, for such an integrative reaction system by applying two functional electrodes of POPTA-TiO2/Ti and RVC, the distinctive enhancement of total photo-electrochemical reaction efficiency mainly depends on POPTA incorporation along with the assistance of electro-generated H2O2 for POPTA-TiO2/Ti photo-anode. Fig. 11 shows HPLC chromatograms of the final BPA degradation products by POPTA-TiO2/Ti photo-electro-catalysis. Besides the residual BPA (RT = 4.38 min), the main intermediate product has been identified as oxalic acid (RT = 2.15 min) along with other degradation compounds (RT = 4.17, 3.82, and 3.27 min). The curves b and c exhibit the similar characteristic peaks except for their peak heights and peak areas in their HPLC chromatograms. It means the same kinds of intermediate products have been produced by POPTATiO2 photo-catalysis and photo-electro-catalysis. However, the corresponding concentration of individual degradation compound differs apparently. Regarding the electro-generation H2O2-assisted POPTA-TiO2 photo-electro-catalysis, BPA chromatogram peak has completely disappeared and the chromatogram peak of oxalic acid has also decreased drastically only except for the presence of the intermediate of phenolic derivate (RT = 3.27 min). In this photo-electrocatalytic system, the promoted mineralization efficiency has been achieved for BPA degradation, which is mostly ascribed to the interactive oxidation. 3.5. Reaction mechanisms Both heterogeneous TiO2 and homogeneous POPTA catalysts can conduct photo-catalytic reaction for oxidative degradation of organic compounds. The essential improvement of photo-catalytic reactivity ultimately relies on an effective separation of the photo-generated electron–hole pair. Corresponding photo-catalytic reaction mechanisms are illustrated as follows. All referred redox potentials are volts versus normal hydrogen electrode (NHE) at pH 7.

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3.5.1. POPTA homogeneous photo-catalysis Photonic activation could occur in the case of light energy higher than the band gap of POPTA (3.54 eV), which lead to intra-molecular ligand-to-metal charge transfer (O(2p ! W(5d)) W(5d)) and the formation of the photo-excited state species (POPTA*). The photo-electron transfer process is presented as follows:

3.5.3. H2O2-assisted TiO2 photo-electro-catalysis For electro-generation H2O2-assisted TiO2 photo-electrocatalytic process, some key reactions are summarized as follows [17].

POPTA þ hy ! POPTA $ POPTAðhþ þ e Þ

Extracting electrons on TiO2 photo-anode

(1)

Photoactivation of TiO2 : TiO2 þ hy ! hþ þ eCB  ðTiO2 Þ (5)

: eCB  ðTiO2 Þ ! anodic current þ





þ

POPTAðh þ e Þ þ H2 O ! POPTAðe Þ þ OH þ H

(2)

POPTAðhþ þ e Þ þ S ! POPTAðe Þ þ SOX

(3)

(6)

Electro-generation H2 O2 on RVC cathode : O2 þ 2Hþ þ 2e ! H2 O2

(7)

where S is the organic substrate and Sox is the oxidized product.

Scavenging electrons : H2 O2 þ eCB  ðTiO2 Þ !  OH þ OH (8)

POPTAðe Þ þ O2 ! POPTA þ O2 

Radical generation : H2 O þ hþ !  OH þ Hþ

(4)

Hydroxyl radicals are generated by photo-holes reaction with surface hydroxyls groups, which are strong and unselective oxidant species for oxidative degradation and even mineralization of organic substrates. Additionally, according to the redox potential of POPTA* (E0(PW12O403*/ PW12O404) = + 3.76 V), high oxidative capability of POPTA* would also contribute to a direct oxidation reaction for BPA degradation [16]. 3.5.2. POPTA-TiO2 heterogeneous photo-catalysis In view of the redox potentials of TiO2 and POPTA (E0(TiO2(VB)) = +2.7 V; E0(TiO2(CB)) = 0.5 V; E0(PW12 O403/4) = + 0.22 V), the electron transfer from the TiO2(VB) to the POPTA(LUMO) is in principle allowed to occur according to their individual energy levels under UV joint photo-excitation of electronically coupled TiO2-POPTA. POPTA both in ground state and in photo-excited state could serve well as an electron shuttle to mediate electron transfer from TiO2 conduction band to electron acceptor of di-oxygen molecule, which retards TiO2 electron–hole pair recombination. So, the electronic guest–host structure of POPTA-TiO2 greatly contributes charge pair separation. The promoted photocatalysis activity of POPTA-TiO2 nanocomposite can be consequently achieved in the presence of an electron acceptor (di-molecular oxygen) and an electron donor (organic compound). Redox potentials and intermolecular electron transfer processes are schematically illustrated in Fig. 12.

(9)

Photodegradation reaction : BPA þ OH=hþ ! degraded=mineralized products

(10)

Since H2O2 can act as a much better electron trapper than dioxygen molecule, the promotive production of hydroxyl radicals through scavenging TiO2 photo-electrons by electrogeneration H2O2 will play an important role to improve photoelectro-catalytic reactivity of TiO2 as well as POPTA-TiO2 nanocomposite. 4. Conclusions Polyoxophosphotungstate-incorporated titania nanotube array of POPTA-TiO2 has been synthesized as a nanocomposite photo-anode for the photo-electro-catalytic application. The encapsulation of POPTA in TiO2 tubule channels can be well applied to construct a hybrid photo-catalyst with a high activity, chemical stability and re-usability. POPTA-TiO2/Ti exhibits much higher photo-catalytic and photo-electro-catalytic efficiency for BPA degradation than pure TiO2/Ti. Furthermore, electro-generation H2O2-assisted POPTA-TiO2/Ti photo-electro-catalytic system can achieve a high performance of photodegradation and photo-mineralization by applying an oxygendiffused RVC electrode as another functional cathode. Such an integrative photo-electrochemical application of nanotubular POPTA-TiO2/Ti nanocomposite photo-anode would provide a promising perspective in the advanced oxidation area. References

Fig. 12. POPTA-TiO2 photo-electrochemical reaction mechanism.

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