TiO2 photocatalysts with engineered facets for enhanced degradation of bisphenol A through persulfate activation

Journal Pre-proof Polyoxometalates/TiO2 photocatalysts with engineered facets for enhanced degradation of bisphenol A through persulfate activation Qingwen Tang, Xiaoqiang An, Huachun Lan, Huijuan Liu, Jiuhui Qu

PII:

S0926-3373(19)31140-3

DOI:

https://doi.org/10.1016/j.apcatb.2019.118394

Reference:

APCATB 118394

To appear in:

Applied Catalysis B: Environmental

Received Date:

12 July 2019

Revised Date:

25 October 2019

Accepted Date:

6 November 2019

Please cite this article as: Tang Q, An X, Lan H, Liu H, Qu J, Polyoxometalates/TiO2 photocatalysts with engineered facets for enhanced degradation of bisphenol A through persulfate activation, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118394

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Polyoxometalates/TiO2 photocatalysts with engineered facets for enhanced degradation of bisphenol A through persulfate activation Qingwen Tanga,b, Xiaoqiang Ana, Huachun Lana,*, Huijuan Liua, Jiuhui Qua a

Center for Water and Ecology, State Key Joint Laboratory of Environment Simulation and

Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China b

College of Environmental Science and Engineering, Guilin university of technology, Guilin

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541006, China.

Highlights

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Graphical Abstract

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 Facet engineered Fe-POMs/TiO2 was used as photocatalysts for persulfate

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activation.

 Polyoxometalates provide active sites for charge separation and activation reaction.

 The catalysts exhibited significantly improved activity for bisphenol A degradation.  SO4-• derived from persulfate activation played the dominant role in the

degradation.  We provide a new way to design facet-engineered catalysts for synegistic oxidation.

Abstract: Photocatalysis-assisted persulfate oxidation is an attractive technology for water decontamination, due to the enhanced reaction kinetics and non-independence of pH.

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Understanding the microstructure-dependent activity of TiO2-based photocatalysts for persulfate activation is challenging but vital. Herein, the strategy of facet engineering was used to improve the efficiency of catalysts for bisphenol A (BPA) degradation. The surface modification of 001-faceted TiO2 by Fe-containing polyoxometalates (Fe-POMs) not only

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improved the separation of charge carriers, but also provided active sites for persulfate activation. The synergistic effects resulted in the 10- and 33-times higher activity than single

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photocatalysis and photoactivated persulfate, respectively. Fe-POMs/001-TiO2 with exposed high-energy facets even presented 2.1- and 3.6-fold increased efficiency than 101-faceted and

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irregular samples. This work presents a new opportunity to boost the efficiency of photocatalytic-persulfate oxidation for water purification by engineering the exposed facets of

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catalysts.

Keyword: Titanium Dioxide, polyoxometalates, facet engineering, persulfate activation,

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bisphenol A

1. Introduction

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Persulfate-based advanced oxidation processes have attracted tremendous attentions for the

degradation of recalcitrant organic contaminants, owing to the long half-life, high redox potential (E=2.6 V vs. NHE) and good selectivity of SO4-• [1]. According to previous studies, the activated persulfate exhibits maximum oxidizing capacity under neutral conditions. Therefore, it outperforms the traditional H2O2-involed Fenton process for water treatment, which was limited by the poor selectivity and non-independence of pH [2]. To generate reactive sulfate radicals for water decontamination, numerous activators have been employed to activate

persulfate, such as transition metal catalyst, heat, electricity and ultrasound energies [3,4]. Among these strategies, photocatalytic activation of persulfate has emerged as an environmentally-friendly and energy conservation approach towards contaminants degradation, which combines the advantages of heterogeneous catalysis and low-cost solar energy [5]. Mechanism investigations have revealed the synergistic effect between photocatalysis and persulfate activation. On one hand, the transfer of electrons from photocatalysts to persulfate initiates the one-electron reduction for SO4−• production. On the other hand, the consumption of photoelectrons also enhances the separation efficiency of charge carriers in photocatalysts,

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resulting in the improved oxidation capability [6,7]. Currently, the key issue of this heterogeneous process is exploring catalysts with high activity for both photocatalytic oxidation and persulfate activation.

Much efforts have been devoted to activate persulfate by photo-induced electrons in TiO2-

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based photocatalysts. The synergistic effect between sulfate radicals and photocarriers exhibited pronounced effect on degrading organic contaminants in water [8-11]. For example,

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Grilla et al. studied the activation of sodium persulfate by P25 TiO2 in a pilot scale reactor for degrading trimethoprim (TMP) [9]. Reduced graphene oxide was also coupled with Ti3+ and

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oxygen vacancy self-doped TiO2 to enhance the charge separation for persulfate activation [10]. Yang et al. reported that D35-TiO2/g-C3N4/PS photocatalytic system has a strong non-selective

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oxidative ability to remove micropollutants [11]. However, most of these studies focus on irregular TiO2 photocatalysts. With respect to the photocatalytic process, activity of a semiconductor is strongly dependent on the structure. In particular, previous studies have shown

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that exposed facets of TiO2 governed its geometrical and electronic structure, considerably affecting its adsorptive property, catalytic activity and stability [12,13]. Numerous studies are

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engaged in improving photocatalytic performance of TiO2 through facet engineering. For example, theoretical calculation demonstrated that 001-faceted TiO2 is composed of 100% 5fold-coordinated titanium atoms (Ti5c), compared to the only 50% Ti5c on the common 101 surface [14]. This unique feature results in the much higher surface energy of 001-TiO2 (0.90 Jm-2) than thermodynamically stable {101} surface (0.44 J m-2). It means that TiO2 with high percentages of exposed {001} facets should be a more advantageous protocol for the synergistic reaction than conventional TiO2 with mixed crystal facets [15]. Unfortunately, the impact of

facet engineering on the activation of persulfate by TiO2-based photocatalysts is still an open question. In order to explore more efficient synergistic reactions for pollutant elimination, it is a critical point to deliberate the applicability of facet-engineered photocatalysts for activating persulfate. Pursuant to this hypothesis, TiO2 nanosheets with dominantly exposed {001} facets were fabricated by hydrothermal method, which were further modified by iron-containing polyoxometalates (Fe-POMs). When used as photocatalysts to activate persulfate, these POM nanoclusters not only improved the separation of charge carriers in TiO2, but also acted as

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molecular activator to generate oxidative radicals. Benefited from the synergistic effects between faceted TiO2 and Fe-POMs, composite photocatalysts with high-energy {001} facets exhibited superior activity for bisphenol A(BPA) degradation than {101}-faceted and irregular samples. This work provides new insights into the design of facet-dependent advanced

2. Experimental

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2.1. Fabrication of faceted TiO2 photocatalysts

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oxidation processes for synergistic environmental remediation.

A facile hydrothermal method was used to fabricate TiO2 photocatalysts with exposed {001}

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facets as high-energy surface. Typically, 1.5 mL of hydrofluoric acid was added into 12.5 mL of titanium (IV) butoxide and stirred for 0.5 hour. The solution was transferred into a Teflon-

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lined autoclave with a capacity of 50 mL. The hydrothermal reaction was carried out at 200℃ for 24 h. The precipitate was separated by high-speed centrifugation and further treated by 0.1 M NaOH solution for 12 h, in order to remove the residual fluorine ions. After fully rinsing and

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drying, white TiO2 powders with dominant (001) facets (001-TiO2) were obtained. For comparison, TiO2 with dominant (101) facets was also fabricated and the corresponding sample

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was denoted as 101-TiO2.

2.2. Fabrication of polyoxometalates/TiO2 photocatalysts Fe-POM/001-TiO2 composites were fabricated by a conventional impregnation method.

Impregnation solution was prepared by dissolving a certain amount of FeCl3 and H3PW12O40·xH2O into 100 ml of water. The pH value was then adjusted to 1.0 by HCl solution. 0.1 g of TiO2 was added into as-formed solution and vigorously agitated for 12 hours. As shown in Fig. S1, under appropriate reaction temperature, complexation interaction between FeCl3 and

H3PW12O40·xH2O resulted in the formation of crystalline Fe-POM particles on the surface of TiO2 [16,17]. The precipitate was washed and dried to obtain Fe-POM/001-TiO2 photocatalysts. 2.3. Characterization X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku RINT 2100, at a voltage of 40 Kv. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB MKII spectrometer with a Mg Kα excitation source (hν = 1253.6 eV). The morphology of products was observed by a field-emission scanning electron microscope (FE-SEM, SIGMA) and a high-resolution transmission electron microscope (HR-TEM, JEOL-2100F). UV−vis

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diffusion reflectance spectra (DRS) were acquired on a UV−vis−NIR spectrophotometer (Cary 5000). A Bruker E500 spectrometer was used to carry out the electron spin resonance (ESR) analysis. The steady-state fluorescence and time-resolved fluorescence spectra were collected by a fluorescence spectrometer (FLS-980, Edinburgh Instruments Ltd.).

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2.4. Catalytic degradation of bisphenol A photocatalysts

BPA, a representative endocrine disrupting chemical, was used as a model pollutant to

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evaluate the catalytic activity of polyoxometalates/TiO2 composites. For the photocatalytic activation of persulfate, 50 mg of catalysts was added into the solution of BPA with the

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concentration of 50 mg/L. The suspension was magnetically stirred for 1.5 h in the dark to achieve the adsorption/desorption equilibrium. Photocatalytic reaction was carried out in a 100

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mL beaker placed in a water-cooling cell. After the addition of a certain amount of K2S2O8 (500 mg/L), the solution was irradiated by a xenon lamp (CEL-S500, 300 W). The light intensity was determined to be 200mw/cm2. At given time intervals, 2 ml liquids were collected and

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filtered by 0.45 μm filtration membrane for analysis. The BPA concentration was analyzed by a high-performance liquid chromatogram (HPLC) (Agilent, USA). Methanol/water solution

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with the ratio of 75:25 was used as flow phase. The flow rate was 1 ml/Min. The removal ratio of pollutant molecules was calculated by C/C0, where C was the concentration of pollutant at each irradiated time interval and C0 was the initial concentration of pollutant solution. 2.5. Photoelectrochemical measurements Electrical impedance spectroscopy and Mott-Schottky measurements were performed at an electrochemical workstation (Gamry, Interface 1000). The electrode was prepared by the dropcasting method. Platinum wire and Ag/AgCl electrode were used as counter electrode and

reference electrode, respectively. 0.5 M Na2SO4 was used as electrolyte. 3. Results and discussion Phase structure of TiO2-based catalysts before and after Fe-POMs modification was investigated by XRD. As shown in Fig. 1a, all of the diffraction peaks were indexed to the anatase phase (JCPDS card No. 21-1272) [18]. No characteristic peaks associated with FePOMs were detected in the composite, indicating the high dispersion of polyoxometalate molecules on the surface of TiO2. Morphology observation showed that pristine sample was composed of uniform nanosheets with square outlines and smooth surface, which was

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consistent with that of TiO2 with exposed high-energy {001} facets (Fig. 1b). Based on the HRTEM image, the average side size and thickness of these nanosheets were determined to be 90 nm and 5 nm, respectively (Fig. 1c). The clear fringe spacing of 0.235 nm was consist with the {001} lattice plane of anatase TiO2 (Fig. S2a). The subsequent surface modification by Fe-

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POMs showed no obvious influence on the main structure of TiO2 nanosheets (Fig. S2b). Differently, black spots with the size smaller than 1 nm could be easily observed on the surface

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of nanosheets (Fig. 1d). To confirm the formation of Fe-POM heterogeneous particles on the surface of TiO2, composite sample with higher amount of Fe-POM was fabricated and

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characterized by HR-TEM. As shown in Fig. S3, abundant nanoparticles with the size of 1-2 nm were uniformly distributed on the surface of TiO2 nanosheets [19,20]. Based on X-ray

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energy dispersive spectroscopy analysis, these highly dispersed spots were determined to be Fe-POMs clusters, thus suggesting the successful formation of faceted Fe-POM/001-TiO2

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composites [21].

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Fig. 1. (a) XRD patterns of Fe-POM/001-TiO2, 001-TiO2 and Fe-POM; (b) SEM image of {001}-faceted TiO2; HR-TEM images of (c) 001-TiO2 and (d) Fe-POM/001-TiO2.

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XPS was employed to investigate the electronic structure of as-prepared photocatalysts. Compared to blank TiO2, the appearance of Fe, W and P signals in Fig. S4, confirmed the

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component elements in Fe-POM/001-TiO2. The calculated atomic ratio of P and W indicated the transformation of Keggin structured [PW12O40]3− [22,23]. Fig. 2a displayed the high-

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resolution Fe 2p spectra of Fe-POMs and Fe-POM/001-TiO2. The characteristic doublet peaks located at 710.9 eV and 724.0 eV could be indexed to Fe 2p3/2 and Fe 2p1/2 of Fe3+ in Fe-POMs [24]. The formation of heterostructured interface exhibited obvious impact on the electronic

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structure of photocatalysts. As shown in Fig. 2b, surface modification of TiO2 by Fe-POMs resulted in the remarkable shift of Ti 2p peaks toward higher binding energies. It suggested the

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electron transfer from TiO2 to polyoxometalates molecules caused by the strong interfacial interactions [25]. Meanwhile, significantly increased paramagnetic signal at g=2.002 was detected in the ESR measurement, a sensitive technique for detecting surface defects in nanocatalysts (Fig. 2c). Thus, surface modification of TiO2 led to the formation of abundant oxygen vacancies, which should be favorable for the migration of charge carriers [26]. The electronic structure of Fe-POMs before and after junction formation was thereafter investigated. In Fig. 2d, two asymmetrical peaks were easily observed in the high resolution W 4f spectrum

of Fe-POM/001-TiO2. This was different from the symmetrical peaks of blank Fe-POM nanoparticles (Fig. S5). After convolution using Gaussian distribution, the two strong peaks located at 36.0 and 37.8 eV were ascribed to W 4f5/2 and W 4f7/2 of W6+. The appearance of XPS signals at 35.6 and 37.4 eV suggested the formation of W5+. It further evidenced the role of Fe-POM as electron acceptor for charge separation, which was consistent with our previous

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studies [17].

Fig. 2. (a) Fe 2p spectra of Fe-POMs and Fe-POM/001-TiO2; (b) Ti 2p spectra of 001-TiO2 and

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Fe-POM/001-TiO2; (c) ESR spectra of 001-TiO2 and Fe-POM/001-TiO2; (d) W 4f spectrum of Fe-POM/001-TiO2.

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The influence of surface modification on the light absorption ability of TiO2 was investigated in Fig. S6a. Based on the transformed Kubelka–Munk plots (Fig. S6b), the band gap of 001TiO2 and Fe-POM was estimated to be 3.05 and 2.20 eV, respectively. The integration of TiO2 with Fe-POM resulted in the increased optical absorption in the visible light region, with a redsifted absorption edge. Accordingly, the color of powders changed from white to light yellow (inset of Fig. S6a). To demonstrate the potential of facet-engineered catalysts for advanced oxidation

applications, Fe-POM/001-TiO2 with dominantly exposed {001} facets was used as photocatalysts for persulfate activation. Fig. 3a showed the time course of the percentage of BPA removal under different conditions. Bare persulfate exhibited very poor oxidation capability for BPA degradation, even after the activation by light irradiation. Only 10% pollutants could be removed by the persulfate oxidation process. For photocatalysis process, Fe-POM/001-TiO2 presented moderate activity for BPA photodegradation under simulated solar light. More significantly, the simultaneous addition of Fe-POM/001-TiO2 and persulfate dramatically improved the oxidation efficiency. It seems that their decomposition kinetic

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behaviors obeyed the pseudo-first-order kinetics based on Langmuir-Hinshelwood model [27]. It was noted that the reaction rate constant (k) of synergistic system was almost 33- and 10times higher than single persulfate and photocatalysis processes (Fig. S7). Control experiments indicated that there was a synergistic effect between Fe-related species and light irradiation, as

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the addition of Fe-POM was favorable for the light activation of persulfate (Fig. 3b). However, blank Fe-POM exhibited ignorable photoactivity for degrading organic pollutants, such as BPA

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and 5-Sulfosalicylic acid. According to Fig. S8, the photocurrent of Fe-POM was equivalent to the background signal of FTO substrate. Thus, Fe-POM could hardly generate or separate

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electrons and holes for photooxidation reactions. The superior performance of Fe-POM/TiO2 composite should be ascribed to the improved charge separation mediated by surface Fe-related

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species. As depicted in Fig. 3c, upon increasing the content of Fe-POMs from 1.5% to 15%, the reaction rate constant gradually increased and reached the highest value of 0.195 for 15% FePOM/001-TiO2. Further increase the proportion of Fe-POMs content led to the deteriorated

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performance of composite catalysts. It indicated that appropriate surface modification by FePOMs was beneficial for the interfacial separation of photo-generated charge carriers, but

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excess amounts of Fe-POMs might shield the reactive sites of TiO2 photocatalysts (Fig. S9a). To further demonstrate the advantages of our strategy, 101-TiO2, 010-TiO2 and benchmark P25 were used to construct polyoxometalate-hybridized photocatalysts with different exposed facets (Fig. 3d). The degradation rate of BPA over Fe-POM/001-TiO2 was 2.1- and 3.6-times higher than that of Fe-POM/101-TiO2 and Fe-POM/P25-TiO2, respectively. As can be seen from Fig. S9b, the activity of Fe-POM/010-TiO2 was comparable to that of Fe-POM/001-TiO2. This was understandable that 010-TiO2 possessed much higher surface area (102.7 m²/g), which could

facilitate the surface reaction (Fig. S10a). These results well evidenced the potential of faceted

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TiO2 for the photocatalytic activation of persulfate (Fig. 3d).

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Fig. 3. (a) Time course of BPA degradation through different oxidation technologies; (b) Synergistic degradation of BPA by different types of catalysts; (c) Effect of POM content on the efficiency of synergistic reactions; (d) A comparison of BPA degradation by POM-

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hybridized TiO2 with different exposed facets.

Aniline was further used as a typical pollutant to evaluation the photoactivity of Fe-

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POM/001-TiO2. As presented in Fig. S11a, persulfate-involved degradation of aniline was obviously accelerated by 001-TiO2 in the presence of light irradiation. The removal rate of Fe-

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POM/001-TiO2 was 2.4 times higher than that of pristine 001-TiO2. It confirmed the contribution of surface modification to the extraction of charge carriers for persulfate activation. The effect of Fe3+ and POM on the degradation of BPA was evaluated by directly adding these soluble ions into the reaction solution. As shown in Fig. S11b, POM ions could slightly enhance the photocatalytic performance of TiO2. In comparison, the addition of Fe3+ significantly improved the efficiency of BPA degradation. This was reasonable that the diffusion of Fe3+ in homogeneous solution was orders-of-magnitude faster than the heterogeneous system.

Nevertheless, the usage of Fe3+ suffered from the problem of iron leaching, thus at a disadvantage. The influence of surface area on the photocatalytic performance was evaluated by analyzing the N2 adsorption-desorption isotherms of different TiO2-based samples. As can be seen from Fig. S10b, 001-TiO2 (63.7 m²/g) exhibited a little higher BET surface area than 101-TiO2 (48.9 m²/g), due to the exposure of high-energy surface. The surface area of Fe-POM was 5.4 m²/g. As a consequence of junction formation, a slight increase of surface area was observed for FePOMs modified 001-TiO2 (74.7 m²/g). Thus, the change of surface area should not be the main

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reason for the enhanced photoactivity. Thereafter, active species trapping experiments were carried out to gain fundamental insights into the degradation mechanism. For persulfatemediated reactions, ethanol was usually used as quenching agent for both SO4-• and •OH, because of the similar reaction rate constants. Tert-butyl alcohol (TBA), benzo quinone (BQ)

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and EDTA-2Na were typical scavengers for •OH, •O2- and holes, respectively [28]. As showed in Fig. 4a, the oxidation degradation of BPA was not affected by the addition of EDTA-2Na.

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The presence of TBA led to the slight decrease of removal ratio, while BQ further inhibited the reaction efficiency. However, the degradation of BPA was drastically quenched by ethanol. It

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was thereby concluded that SO4-• generated by persulfate activation and •O2- generated by lightinduced electrons were the dominant reactive species for BPA degradation.

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ESR measurements were conducted to validate the above deduction, using DMPO as the radical spin trapping agent. As illustrated in Fig. 4b, there was no definable ESR peaks for FePOM/001-TiO2 under dark condition. In contrast, the quartet lines with peak intensity of 1:1:1:1

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could be easily detected upon irradiation. It indicated the strong capacity of Fe-POM/001-TiO2 for •O2- generation. The addition of persulfate led to the emergence of DMPO-•OH adducts

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even under dark condition (Fig. 4c). This was in consistent with literatures, which suggested the oxidation ability of persulfate [29]. With the co-existence of persulfate and light irradiation, strong ESR signal corresponding to DMPO-SO4-• adducts was observed, indicating the efficient activation of persulfate by photocatalytic reaction [30]. Taken together, Fe-POM/001-TiO2 photocatalysts with dominantly exposed {001} facets indeed exhibited strong ability for generating reactive SO4-• and •O2- radicals, which ultimately contributed to the catalytic degradation of BPA.

Besides catalytic activity for pollutant removal, the stability of Fe-POM/001-TiO2 photocatalyst is another important issue for its practical application. ICP measurements were carried out to evaluate the leaching of Fe3+ into the solution. As shown in Table S1, the concentration of Fe3+ in the solution was always in low level during catalytic reaction. The value of 0.03 mg/L is much lower than the initial concentration (75 mg/L), indicating that lightinduced charge carriers could effectively inhibit the leaching of Fe from Fe-POM. A cycling test for BPA degradation was further carried out under light irradiation mediated by persulfate activation. In Fig. 4d, our catalysts exhibited good photoactivity during four consecutive cycles,

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although a slight decrease of removal rate was observed. To clarify the possible reason, the influence of surface adsorption on the degradation reaction was investigated. According to Fig. S12, the amount of BPA molecules adsorbed onto recovered Fe-POM/001-TiO2 gradually decreased. Note that the leaching of iron was fairly low, this performance deterioration should

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ascribe to the obstruction of adsorption by surface residuals and the mass loss during catalyst

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recovering.

Fig. 4. (a) Degradation of BPA over Fe-POM/001-TiO2 in the presence of different scavengers; (b) ESR spectra of DMPO-•O2- adducts in the presence of Fe-POM/001-TiO2 with and without

light irradiation; (c) ESR spectra of DMPO-•OH and DMPO-SO4-• adducts in the presence of Fe-POM/001-TiO2 with and without light irradiation; (d) Repeated test of BPA degradation with Fe-POM/001-TiO2 in the presence of persulfate and light irradiation. The fundamental contribution of facet engineering to the synergistic reactions were investigated. In general, crystal facets with more undercoordinated atoms and high surface energy usually exhibit better activity for photocatalytic reactions. Inherently, 001-TiO2 should exhibit superior activity for persulfate activation than 101-faceted and irregular samples, as the ratio of 5-fold-coordinated titanium atoms in 001-TiO2 was 100% [16,31]. With respect to

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composite photocatalysts, our recent studies have revealed that the interfacial behavior of charge carriers in TiO2-based heterostructures was highly dependent with the exposed facets [32,33]. Based on the valence band XPS spectra (Fig. 5a), 001-TiO2 possessed relatively higher valence band (VB) and conduction band (CBM) positions. Therefore, excitation of 001-TiO2

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resulted in the formation of abundant high-energy electrons in the CB. As a consequence of junction formation, there high-energy electrons could easily migrate from the CB of 001-TiO2

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to Fe-POM molecules. In principle, this unique charge transfer was more favorable for the separation of charge carriers than 101- and P25-based heterostructures. Electrochemical

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measurements were thereafter conducted to investigate the interfacial behaviors of charge carriers. In Fig. 5b, the slope of Mott-Schottky plot was remarkably decreased after the loading

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of Fe-POMs. Combined with the EPR measurements, it was deduced that junction-induced oxygen vacancies could effectively decrease the interfacial resistance of charge transfer and increase the donor density in TiO2 photocatalysts (Fig. S13) [17,34].

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It is reasonably believed that these defects could essentially decrease the interfacial resistance for charge transfer, which was well reflected by the significantly decreased radius of

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Nyquist plot (Fig. 5c). Profited from the improved charge transfer behavIours, Fe-POM/001TiO2 with high energy surface showed much stronger surface photovoltage response, therefore exhibited superior performance for persulfate activation (Fig. 5d) [26].

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Fig. 5. (a) VB XPS spectra of 001-TiO2 and 101-TiO2; (b)Mott-Schottky plots, (c) EIS spectra and (d) surface photovoltage spectra of TiO2 and Fe-POM/001-TiO2.

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Based on the above results, a hypothetic schematic illustration of synergistic catalytic mechanism was presented in Fig. 6. Firstly, abundant electrons and holes were generated caused by the photoexcitation of facet-engineered TiO2. Benefitted from the highly reactive surface

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and POM-enhanced charge separation, accumulated electrons on Fe-POM clusters efficiently reduced oxygen molecules into •O2- radicals (Equation 1), while hole oxidized water into •OH

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(Equation 2). Secondly, after the addition of persulfate, Fe-POMs could act as charge shuttle to facilitate the transfer of electrons from TiO2 to persulfate. These extracted electrons (Equation

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3), together with •O2- radicals (Equation 4), could further reduce persulfate and produce SO4-• radicals with high oxidative capabilities. Thirdly, the significant role of Fe-POMs as activator further contributed to the persulfate-involved degradation. Based on the ignorable color change and undetected photocurrent, the excitation of PW12O403- into heteropoly blue could be excluded. Thus, it seems that Fe-POMs participated in the interfacial electron transfer via Fe3+/Fe2+ redox cycle [35]. As a consequence of accepting electrons from TiO2, Fe (II)-related species were in-situ generated in Fe-POMs (Equation 5) [36]. This process provided additional

reactive sites for persulfate activation to generate SO4-•, and the synergistic process ensured the redox cycling of Fe (II)/Fe (III) (Equation 6). Finally, SO4-• generated by persulfate activation and •O2- generated by photocatalysis decomposed BPA molecules into small molecules

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(Equation 7).

Fig.6. Schematic illustration of the proposed reaction mechanism for BPA removal.

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Finally, degradation products were identified to clarify the possible reaction pathway. TOC measurements suggested the formation of reaction intermediates during the degradation of BPA

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(Fig. S14). The possible formation of reaction intermediates was studied by GC-MS analysis [37]. As can be seen from Fig. S15 and Table S2, 4-hydroxyacetophenone, hydroquinone and

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1,4-benzoquinone were determined to be the dominant intermediates in our experiments. Based on the previous reports, the possible degradation pathway of BPA was proposed and shown in Fig. S16. In general, active radicals generated by photocatalysis and activated persulfate could

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attack the carbon atoms between two benzene rings of BPA. This resulted in the formation of 4-isopropenylphenol and phenol as main intermediates. Then, 4-isopropenylphenol was

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oxidized into 4-hydroxyacetophenone, and phenol was transformed into hydroquinone. The ring-opening reactions resulted in the further mineralization of intermediates into small

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inorganic molecules [38-41].

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

4. Conclusions In summary, this study provides a proof-in-concept demonstration of the facet engineering of TiO2-based photocatalysts for activating persulfate. We revealed that Fe-POMs modification facilitated the formation of oxygen vacancies in 001-faceted TiO2, which contributed to the efficient separation of photocarriers. By virtue of the interfacial charge transfer and POMs-

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involved active sites, heterostructured catalysts exhibited superior activity for producing SO4-• radicals by persulfate activation. Compared to 101-faceted and irregular samples, 001-TiO2/FePOMs showed 2.1- and 3.6-fold higher apparent rate constants for BPA degradation. This work

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provides a promising strategy of designing nanostructured catalysts with engineered facets for the synergistic and efficient removal of environmental pollutants.

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Acknowledgments

This work was supported by the National Natural Science Foundation of China (51722811,

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51978372) and National Key R&D Program of China (Grant No. 2016YFC0400502). References

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