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Enhanced Photoelectrocatalytic Activity of a Novel Bi2O3-BiPO4 Composite Electrode for the Degradation of Refractory Pollutants under Visible Light Irradiation Yanqing Cong, Juan Wang, Huan Jin, Xiao Feng, Qi Wang, Yun Ji, and Yi Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04591 • Publication Date (Web): 18 Jan 2016 Downloaded from http://pubs.acs.org on January 25, 2016

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Industrial & Engineering Chemistry Research

Enhanced Photoelectrocatalytic Activity of a Novel Bi2O3-BiPO4 Composite Electrode for the Degradation of Refractory Pollutants under Visible Light Irradiation Yanqing Cong,*,† Juan Wang,† Huan Jin,† Xiao Feng,‡ Qi Wang,† Yun Ji,† and Yi Zhang*,† †

College of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou

310018, China ‡

Department of Environmental Engineering, North China University of Water Resources and Electric

Power, Zhengzhou 450011, China

Corresponding Author: Yi Zhang, Ph.D Associate Professor Department of Environmental Engineering Zhejiang Gongshang University Hangzhou 310018, PR China

Tel: +86-571-28008211 Fax: +86-571-28008215 Email: [email protected]; [email protected].

ACS Paragon Plus Environment

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ABSTRACT: A novel Bi2O3-BiPO4 composite electrode was successfully synthesized by the method of electrophoretic deposition, electrodeposition and calcination in a proper sequence. The prepared electrodes were characterized by X-ray diffractometer (XRD) and scanning electronic microscopy (SEM). Linear sweep voltammetry (LSV) and photocurrent decay curves measurement indicated that the Bi2O3-BiPO4 composite electrode exhibited better photoelectrochemical (PEC) activity than either pure Bi2O3 or BiPO4 electrode. Incident photon to current conversion efficiency (IPCE) and Electrochemical impedance spectra (EIS) measurements revealed that the composite electrode significantly improved the efficiency of charge transfer and decreased the recombination of photogenerated charges. Furthermore, the composite electrode also displayed higher efficiency and stability in the PEC degradation of organic pollutants. The p-n type junction structure of Bi2O3-BiPO4 composite electrode was most likely responsible for the enhanced PEC activity.


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1. INTRODUCTION Photoelectrocatalytic (PEC) technique has been regarded as one of the most promising methods for water treatment and energy conversion.1-3 The PEC process used the photocatalyst films immobilized on the conductive electrode and could avoid the secondary pollution problems of the powdered photocatalysts in the photocatalytic (PC) process. In PEC process, the bias potential applied on the working electrode further promotes the separation of photogenerated electron-hole pairs and increases the efficiency of photocatalysts.4-6 Among various photocatalysts, TiO2 was considered to be the most attractive photocatalyst for its large availability, nontoxicity, high chemical stability and economical excellence.7,8 However, it can only be excited by UV light which accounts for ∼4% of solar spectrum due to its wide band gap (3.2 eV),9 and the efficiency for the separation of photogenerated electron-hole pairs was also low. Therefore, the development of efficient and practical photocatalysts still remains a great challenge.10 Especially, it is of significant importance to develop visible light-responsive materials for more efficient utilization of solar energy. Recently, the Bi (III)-based photocatalysts have attracted special concerns as a new type of promising photocatalysts due to their good stability and photocatalytic activity in degrading organic pollutants or water oxidation.11,12 Bi2O3 is an important p-type metal oxide semiconductor with a direct band gap of 2.8 eV which can absorb partial visible light. Its conduction band (CB) and valence band (VB) edges were reported to be +0.33 and +3.13 V (vs NHE), respectively.13 Fornasiero et al. reported that the photon induced formation of Bi2O4−x at the surface of -Bi2O3 under UV or natural sunlight irradiation significantly altered the photocatalytic activity of -Bi2O3 and showed excellent performance for the mineralization of 2-chloro and 2-nitrophenol in the visible region of sunlight.11,14 It was also reported that Bi2O3 could be coupled with BiOI or BiOCl to form heterojunctioned composite photocatalyst and has been proved to be an efficient visible light responsive photocatalyst.15,16 However, its photocatalytic activity is still low under visible light irradiation, especially for the degradation of refractory pollutants, since the oxidation ability of photocatalysts is one of the most important factors to


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determine the removal efficiency of organic pollutants in water treatment. Thus, it needed to further improve the photocatalytic activity of Bi2O3 for practical application. To simultaneously obtain the higher oxidation ability and stronger visible light activity, an effective strategy is to couple a narrow band gap semiconductor with a broad one since the narrow band gap semiconductor can absorb visible light and the broad one usually has good oxidation performance. BiPO4, a new broad band gap semiconductor, exhibits excellent photocatalytic activity in degradation of methylene blue (MB) under ultraviolet light irradiation.17 The CB and VB edge potentials of BiPO4 were estimated to be -0.65 and +3.2 V, respectively. Both the high position of VB and high separation efficiency of electron-hole pairs owing to the inductive effect of PO43- were considered as the main reason for the high photocatalytic oxidation activity. Moreover, BiPO4 is an n-type photocatalyst. Theoretically, a p-n heterojunction will generate when p-type Bi2O3 and n-type BiPO4 combine together, which would improve the separation of electron-hole pairs due to the existence of an internal electricfield.10,13 However, there are few reports on Bi2O3-BiPO4 composite photocatalyst. Especially, to the best of our knowledge, a Bi2O3-BiPO4 film electrode has never been reported in the PEC process. In this study, a novel Bi2O3-BiPO4 composite photoelectrode was fabricated by the method of electrophoretic deposition, electrodeposition and calcination. The obtained Bi2O3-BiPO4 composite electrode exhibited much higher PEC oxidation ability and stronger visible light absorbance than the pure BiPO4 and Bi2O3 electrode, respectively. The PEC properties were studied to explore the mechanism of the synergistic effect of the heterojunction electrode and to evaluate its potential application in the degradation of organic pollutants. 2. EXPERIMENTAL METHODS 2.1. Chemicals. All were used as purchased without further treatment. Bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O, ≥99.0%), Potassium iodide (KI, ≥99.0%), Nitric acid (HNO3, 65.0% ~ 68.0%), Ethylene glycol (C2H6O2, ≥99.5%) were purchased from Chengdu Kelong Chemical Reagent Co., Ltd. (Sichuan, China). Ammonium dihydrogen phosphate (NH4H2PO4, ≥99.0%) and p-


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Benzoquinone (C6H4O2, 99.0%) were obtained from Shanghai Zhangyun Chemical Co., Ltd. (Shanghai, China) and Hangzhou Lanbo Industrial Co., Ltd. (Zhejiang, China), respectively. Acetone (CH3COCH3, ≥99.5%) was obtained from Hangzhou Chemical Reagent Co., Ltd. (Zhejiang, China). 2.2. Preparation of photocatalysts Synthesis of BiPO4 film. The precursor powder of BiPO4 was prepared through a series of methods including precipitation, thermal treatment, and hand-grinding in a sequence. Briefly, an ethylene glycol solution (5 mL) of Bi(NO3)3·5H2O (2 mM) and an equal molar of NH4H2PO4 were dissolved in deionized water (65 mL) and magnetically stirred to form a white homogeneous precipitate, which was exposed in air at room temperature for 12 h. Then the suspension was filtrated by centrifugation, rinsed with deionized water and dried at 60 °C for 6 h. The obtained powder was annealed at 900 °C for 2 h, with a 10 °C per minute ramping rate. After annealing, the required BiPO4 powder was achieved by a process of hand-grinding for 90 min.18 The electrophoretic deposition (EPD) technique was used to obtain BiPO4 film (Figure 1b). 32 mg of BiPO4 and 8 mg of KI were dissolved in acetone (25 ml), sonicated for 5 minutes and magnetically stirred for 20 minutes to form a stable suspension. The BiPO4 powder was deposited onto a clean F-doped tin oxide (FTO, Nippon Sheet Glass, Japan) substrate (1 cm × 1 cm) by EPD technique at 30 V for 5 minutes and another clean FTO glass served as the counter electrode.19 Electrodeposition of BiPO4-Bi2O3 or pure Bi2O3 film. The depositions were conducted in a standard three-electrode configuration using the as-prepared BiPO4 electrode (for the electrodeposition of BiPO4-Bi2O3 film, Figure 1c) or a clean FTO glass (for the pure Bi2O3 film) as working electrode. The counter and reference electrodes were platinum and Ag/AgCl electrode, respectively. A potentiostat (CH Instruments, model 660D) was used for electrodeposition. The precursor solution was prepared by dissolving Bi(NO3)3·5H2O (40 mM) in an aqueous solution of KI (400 mM). The pH of the resulting solution was adjusted to 1.75 by HNO3. Then p-benzoquinone (50 mM) was added to the above mixed solution to obtain the plating solution. The deposition proceeded at a constant potential of -0.1 V (vs.


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Ag/AgCl) for 10 min at room temperature.20 After deposition, the electrode was thoroughly washed with deionized water, dried under nitrogen gas, followed by calcining in air at 500 °C for 3 h. Thus, the Bi2O3-BiPO4 film was obtained. 2.3 Characterization. The crystalline phases of the as-prepared samples were determined using an X-ray diffraction (XRD) with a Phillips PANalytical X’PERT diffractometer with monochromatic Cu-K radiation at 40 kV and 40 mA. Scanning electron microscope (SEM) was performed using a S-4700 (II) (Hitachi) to investigate the morphology of samples. The optical property of the films was measured by an UV-Vis diffuse reflectance spectrophotometer (Pgeneral, TU1901). All the spectra were recorded using integrating sphere assembly in 200-700 nm wavelength range. 2.4. PEC measurements. PEC experiments were carried out in a three-electrode configuration as mentioned in the preparation process. The photocatalytic electrodes were irradiated with light from a Xe lamp fitted with a cut-off filter to achieve visible light and the light intensity was ca. 100 mW cm-2. All films were back-illuminated through the FTO glass. Photocurrents were measured in 0.1 M Na2SO4 with 0.1 M Na2SO3 aqueous solution (pH 9.3). To measure the incident monochromatic photon-to-current conversion efficiency (IPCE), the Xe lamp fitted with different monochromatic filters (400, 430, 450, 475, 500, 550 nm) combined with a power meter (model FZ-A) was used. Electrochemical impedance spectra (EIS) were measured in 0.2 M Na2SO4 aqueous solution under dark and bright (visible light, λ > 420 nm) conditions with a frequency range of 100 kHz - 0.01 Hz and a scan rate of 5 mV s-1. 2.5. PEC degradation of organic pollutants. Phenol (10 mg/L) and rhodamine B (RhB) (1*10-5 M) dye were used as the model pollutants to evaluate the PEC degradation efficiency of the as-prepared Bi-based photoelectrodes. In the PEC degradation cell, the effective area of the working electrode was 1 cm × 1 cm. The applied potential is 2.0 V. The counter electrode is Ti with an active area of 2.5 cm × 2 cm. The Xe lamp with a 420 nm cutoff filter was used as visible light source. The pH of the electrolyte solution was adjusted to 3 with H2SO4 or NaOH solution, using 0.2 M Na2SO4 as the electrolyte. Before initiating the reactions under illumination, the mixed solutions containing organic pollutants reacted


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with catalyst electrode for 30 min under dark condition to make sure that an adsorption/desorption equilibrium was established. The concentrations of phenol and its degradation intermediates were analyzed by a high performance liquid chromatography (HPLC, Agilent 1200) equipped with a Diamonsil C18 reversed phase column (150 mm × 4.6 mm, 5 μm). The analyses were performed with a UV detector at a wavelength of 254 nm. The eluent consisted of 30:70 (v:v) methanol/purified water. The concentrations of RhB and its degradation intermediates in the solution were recorded via a spectrophotometer (Unico UV-2102PC Spectrophotometer). 3. RESULTS AND DISCUSSION 3.1. Characterization of BiPO4, Bi2O3, and Bi2O3-BiPO4 films. Figure 2 shows the X-ray diffraction patterns of Bi2O3, BiPO4 and Bi2O3-BiPO4 films. The high intensity peaks at 2θ of 26.6, 33.8, 37.9, 51.6, 61.7 and 65.6°in all samples are assigned to the FTO substrate, which were marked with “▼”. In the Bi2O3 film, peaks of 12.2, 29.6, 42.0, and 52.1°were observed corresponding to (1 1 0), (1 1 1), (2 1 1), and (2 2 1) planes for the cubic phase of Bi2O3, namely γ-Bi2O3 (JCPDS 03-065-3319).21 The main polymorphs of Bi2O3 were reported to include α-, β-, and γ- phases, among which γ-Bi2O3 exhibited the best photocatalytic performance.22 In addition, it can be easily observed that there existed three small peaks of 24.3, 26.5 and 29.4°in the XRD pattern of Bi2O3, which represent the tetragonal phase of Bi2O2.33 (JCPDS 00-27-0051). The Bi2O2.33 of non-stoichiometry was most likely generated during the calcination process. And the peak of 14.5°in the XRD pattern of Bi2O3 was identified as the tetragonal phase of Bi2O2.75 (PDF 27-0049). In BiPO4 film, 2θ of 14.6, 21.9, 25.3, 28.4, and 29.3°were assigned to (1 0 0), (1 0 1), (1 1 1), (0 0 2), and (0 2 1) planes of monoclinic BiPO4 (JCPDS 00-0150767). When Bi2O3 deposited on the BiPO4, weaker BiPO4 peak was observed in the composite films since BiPO4 was covered by Bi2O3. The peak of 18.4°in the XRD pattern of Bi2O3-BiPO4 was Bi(PO3)3 (PDF 45-1370). For the Bi2O3-BiPO4 composites, it can be observed that most diffraction peaks appear in the range of 25 - 40°(see Figure S1). New peak at 2θ of 31.3° in the Bi2O3-BiPO4 composites was assigned to (1 0 2) plane of monoclinic BiPO4 (JCPDS 00-015-0767). The main part of Bi2O3 was still


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γ-Bi2O3. New peaks of 27.2 and 27.6°in the XRD pattern represented monoclinic Bi 2O3 called α-Bi2O3 (PDF 02-0498). It indicated that α-Bi2O3 and γ-Bi2O3 mixed phases were formed in Bi2O3-BiPO4 heterostructure. This mixed phase structure could promote the spatial charge separation and hindered recombination of photogenerated electron-hole pairs across the phase interfaces, which was similar to the combination of anatase TiO2 and rutile TiO2.23 3.2. UV-vis diffuse reflectance spectra (DRS). Further investigation on the optical behaviour of the as-prepared films was also performed by UV-vis DRS (Figure 3). The absorption edge of BiPO4 occurs at about 322 nm in correspondence with its band gap energy.17 For Bi2O3 film, the light absorption started at around 500 nm (band gap ~2.5 eV), which was larger than 475 nm (band gap 2.61 eV for γ-Bi2O3) reported in the literature.24 The presence of non-stoichiometric Bi2O2.33 and Bi2O2.75 (Bi2O4−x, see Figure 2) shifted the band gap of γ-Bi2O3 to slightly lower value of 2.5 eV.11, 24 In the composite film, the onset of light absorption was similar to Bi2O3, and the intensity of light absorption of the Bi2O3-BiPO4 composite was greatly enhanced compared to BiPO4 in the whole wavelength range of 322 nm - 700 nm, which indicated that Bi2O3 is mainly responsible for absorbing visible light. The percentage of Bi2O3/BiPO4 is associated with the electrodeposition time. The longer the electrodeposition time of Bi2O3 is, the higher the percentage of Bi2O3/BiPO4 is. The effect of different Bi2O3/BiPO4 ratios on the optical properties of the composite films was shown in Figure S2. The UVvis absorption properties of Bi2O3-BiPO4 composite were enhanced with increased percentage of Bi2O3/BiPO4. The enhanced absorption of Bi2O3-BiPO4 composite film was attributed to increased percentage of Bi2O3 that has smaller band gap energy. In addition, some UV-vis absorbance peaks could come from film scattering for light due to thickness. 3.3. SEM analyses. The scanning electron microscopy (SEM) is utilized to characterize the particle size and morphology of the samples. As shown in Figure 4a, the pure BiPO4 film was made up of irregular shaped aggregates with sizes ranging from 50 nm to 600 nm. The surface of BiPO4 film exhibited large grains while small ones interspersed in the space among the large grains or adhered to


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their surface. In Figure 4b, the pure Bi2O3 film shows a porous morphology taking on a 3D crystal structure with the small grain sizes between 100 nm - 250 nm. When Bi2O3 was deposited onto the BiPO4 film, the composite also shows irregularly high porosity with the aperture sizes below 100 nm (Figure 4c). The contact between Bi2O3 and BiPO4 in Bi2O3-BiPO4 composite is particle with particle. Moreover, the small grains in composite present a reticular distribution on the FTO substrate or adhering to the surface of large grains. This reticular distribution closely connected the large and small grains, which enhanced the charge carrier transfer between Bi2O3 and BiPO4. Note that the Bi2O3 film was deposited on FTO, while Bi2O3 for composite films grew on the surface of BiPO4 layer. Obviously, the coexistence of Bi2O3 and BiPO4 significantly affected their morphology and the changed morphology probably affected the performance of the composite catalyst. Figure 4d is the energy dispersive X-ray spectrum (EDS) for the Bi2O3-BiPO4 composite. The EDS analysis demonstrated that besides Bi and O peaks, P peaks coming from the BiPO4 were also observed. The C and K peaks probably come from the processes of sample preparation and treatment of characterization. EDS analysis further confirmed that the Bi2O3-BiPO4 composite was composed of Bi2O3 and BiPO4. 3.4. PEC activity. Linear sweep voltammetry (LSV) was used to study the PEC performance of Bi2O3-BiPO4 composite films. Figure 5 shows the LSVs of BiPO4, Bi2O3 and Bi2O3-BiPO4 films in 0.1 M Na2SO4 with 0.1 M Na2SO3 aqueous solution (pH 9.3) under chopped visible ( λ > 420 nm) and UVvis light irradiation. The BiPO4 film shows almost no photocurrent in aqueous solution. However, the Bi2O3-BiPO4 composite exhibited a significant increase in photocurrent response relative to Bi2O3 film. The photocurrent response of the Bi2O3-BiPO4 composite was ca. 3 times higher under visible light irradiation and ca. 3.2 times higher under UV-visible light irradiation than that of Bi2O3 at 0.35 V (vs. Ag/AgCl), respectively. It indicated that a synergistic effect really existed between Bi2O3 and BiPO4, which is consistent with the behaviour of UV-Vis spectra of Bi2O3-BiPO4 composite film. The photocurrent densities as a function of applied potential for different Bi2O3-BiPO4 composite


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films which prepared at different electrodeposition time were also tested. As illustrated in Figure S3 (see the Supporting Information), the photocurrent density of Bi2O3-BiPO4 was slightly enhanced after electrodeposition for 1 min, and the value increased with prolonged electrodeposition time. As a result, the highest photocurrent density was obtained when the electrodeposition time reached by 10 min, and then photocurrent densities reduced as the electrodeposition time increased. Thus, the best electrodeposition time was 10 min. It indicated that there was an optimal film thickness at 10 min electrodeposition which achieves a harmonisation between light absorbance and photogenerated electron-hole separation. When the electrodeposition time was less than 10 min, the film was too thin and couldn’t absorb enough light. However, when the electrodeposition time was more than 10 min, the film thickness was too thick which might cause the recombination of photogenerated electron-hole pairs inside the film, leading to decreased photocatalytic activity. The thicknesses of BiPO4, Bi2O3 and Bi2O3BiPO4 with optimal photocatalytic activity prepared at optimized electrodeposition time of 10 min were measured to be 325 nm, 369 nm and 706 nm, respectively (Figure S4). 3.5. Nyquist Plots. The electrochemical impedance spectroscopy (EIS) measurements were also conducted to study the kinetics of enhanced photocatalytic activity and charge transfer in the composite system. In this work, the EIS of the as-prepared films was measured in 0.2 M Na2SO4 aqueous solution with a frequency range of 100 kHz - 0.01 Hz with different irradiation conditions. The EIS spectra were presented in the form of Nyquist diagram. A smaller arc radius of the Nyquist plots implies a better transfer efficiency and enhanced separation of photogenerated electron-hole pairs.25,26 As shown in Figure 6, it is obvious that the arc radius of Bi2O3-BiPO4 composite electrode is always the smallest one no matter under dark or bright conditions. These results indicated that the formation of this novel Bi2O3BiPO4 heterojunction structure significantly enhanced transfer and separation efficiency of electron-hole pairs in the composite film. This would be further demonstrated in the incident photon-to-current conversion efficiency (IPCE) spectra in the next section. 3.6. IPCE. Figure 7 shows the IPCE spectra of BiPO4, Bi2O3 and Bi2O3-BiPO4 films measured at


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0.3V (vs. Ag/AgCl) in 0.1 M Na2SO4 with 0.1 M Na2SO3 aqueous solution. The IPCE is defined by Eq. (1): IPCE (%) = 1240 × (iph / λPin) ×100

(1)

where iph is the photocurrent (mA), λ is the wavelength (nm) of incident radiation, and Pin is the incident light power intensity on the semiconductor electrode at the selected wave length (mW).27 In the IPCE spectrum (Figure 7), BiPO4 showed no photooxidation current in the range of 400-600 nm. This was because the optical absorption for BiPO4 started around 322 nm and the band gap energy was about 3.85 eV. The BiPO4 material just presented photocatalytic activity in aqueous medium under UV light irradiation.17 On the other hand, both the Bi2O3 and Bi2O3-BiPO4 composite films showed rising IPCE from around 450 nm, indicating the IPCE for Bi2O3-BiPO4 composite was more likely originated from the absorption by the Bi2O3 film in the range from 400 to 600 nm. Furthermore, the Bi2O3-BiPO4 composite showed significantly improved photoactivity compared to Bi2O3 film in the entire range of light irradiation, illustrating a better charge transfer characteristic at the interface of composite. According to Figure 6 and Figure 7, Bi2O3-BiPO4 composite exhibited remarkable synergistic effect in the photocatalytic performance by combining two semiconductors. 3.7. PEC degradation. To conduct a further study on the PEC performances of Bi2O3-BiPO4 composite, a series of PEC degradation experiments using phenol as model pollutant have been completed. As shown in Figure 8, the phenol degradation rate is in direct proportion to the reaction time and the PEC efficiency of the Bi2O3-BiPO4 composite exhibit a great increase compared with Bi2O3 films. The obtained experimental data were found to fit approximately a pseudo-first-order kinetic model by the linear transforms ln (C0/C) = kt (k is the kinetic constant).28 The corresponding kinetics constants, k, and regression coefficients, R2, of phenol degradation by various processes are listed in Table S1. Obviously, the kinetic constant of Bi2O3-BiPO4 composite is larger than that of Bi2O3, which further confirms the high PEC activity of Bi2O3-BiPO4 composite. Figure S5 shows the typical HPLC chromatograms of phenol degradation during the PEC process. The main intermediates of phenol


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degradation were benzoquinone and hydroquinone. Hydroquinone was easily oxidized to benzoquinone. Benzoquinone was one of the most toxic intermediates. Benzoquinone concentration reached the maximum after 60 min treatment and then decreased with prolonged PEC reaction time on Bi2O3-BiPO4 composite (Figure S6). It can be seen that as the reaction proceeded, the benzoquinone intermediate will be further oxidized to organic acids. The possible mechanism of phenol degradation was shown in Figure S7. Phenol was firstly oxidized to benzoquinone and hydroquinone. Then benzoquinone was converted to organic acids under photoelectrocatalytic oxidation, and partially turned out to be CO2 and H2O.29 RhB dye was also degraded under the same condition to better understand the PEC performance of the Bi2O3-BiPO4 composite and the results are shown in Table S2. The Bi2O3-BiPO4 composite exhibited excellent PEC activity under visible light irradiation. The UV-Vis spectral changes during the PEC degradation of RhB on Bi2O3-BiPO4 were showed in Figure S8. The maximum characteristic absorption peaks of RhB located at 552 nm, which decreased gradually with prolonged PEC reaction time. Negibile shift of the maximum absorbance was observed, indicating facile cleavage of the whole conjugated chromophore structure of RhB on the Bi2O3-BiPO4 photoanode.30 3.8. Reuse of Bi2O3-BiPO4 composite films. The stability of Bi2O3-BiPO4 composite films in PEC degradation of pollutants is an important factor for practical application. Five cyclic runs were conducted to identify the stability of Bi2O3-BiPO4 on phenol degradation and the results are shown in Figure 9. After five cycles, the degradation rate of phenol almost remained unchanged. This indicated that Bi2O3-BiPO4 composite films exhibit the high stability during the PEC process. It has been reported that Bi2O3 could be deactivated by formation of (BiO)2CO3 by interaction of the oxide with carbon dioxide.31,32 Fornasiero et al. found that the partial transformation of Bi2O3 into Bi2O4-x could protect the Bi2O3 materials and the formation of ZnO-Bi2O3 or NiO-Bi2O3 heterostructure significantly reduced this undesirable effect.31,33 We measured the XRD patterns of Bi2O3-BiPO4 composite films after five cycles. No observable peaks assigned to (BiO)2CO3 was detected. It indicated that the formation of (BiO)2CO3


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was a slow process on the surface of Bi2O3-BiPO4 composite films and the deactivation of photocatalyst was significantly reduced by the formation of heterostructure between p-type Bi2O3 and n-type BiPO4. In addition, different from Bi2O3 powders reported in literatures,31-33 this work used the Bi2O3-BiPO4 composite films in photoelectrochemical process. The applied potential might also be involved in the decomposition of (BiO)2CO3 or in the inhibition of its formation. 3.9. Possible mechanism. To study the behaviour of the charge recombination in a semiconductor electrode, the transient of photocurrent was employed.34,35 It can be determined by Eq. (2): D = (I(t) - I(st)) / (I(in) - I(st))

(2)

where D is the transient photocurrent, I(t) is the photocurrent at time t, I(in) is the initial photocurrent at t = 0, and I(st) is the steady-state photocurrent. t is the transient time constant. Figure 10 shows the photocurrent transient of the BiPO4, Bi2O3 and Bi2O3-BiPO4 composite films at a constant potential under visible light irradiation (λ > 420 nm). Symbols represent the value of lnD and the lines express fitting results by making use of an equivalent circuit to exhibit the photocurrent transient responses of different photoanodes biased at 0.35 V (vs. Ag/AgCl). For a specific value of lnD, the larger value of t the material has, the slower the recombination process is formed in this photoelectrode.36 When lnD is -1, value of t for Bi2O3-BiPO4 is 10.0 s, while value of t for BiPO4 and Bi2O3 is 1.0 s and 6.5 s, respectively. It is easy to observe that the value of t is the largest in Bi2O3BiPO4 film in the whole range of the applied bias, which reflects the slowest recombination process in Bi2O3-BiPO4 film. This high separation efficiency may attribute to the p-n heterojunction formed in the p-Bi2O3/nBiPO4 composite. Figure 11 shows the schematic diagram of the p-n type Bi2O3-BiPO4 heterojunction. The pure p-type Bi2O3 (ECB = 0.33 V and EVB = 3.13 V versus NHE) and n-type BiPO4 (ECB = -0.65 V and EVB = 3.2 V versus NHE) exhibit the nested band structure before contact (Figure 11b). When Bi2O3 and BiPO4 are contacted, the Fermi level (EF) of p-type Bi2O3 moves up, yet that of n-type BiPO4 descends until an equilibrium state of Fermi level is formed. Correspondingly, the whole energy band of


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Bi2O3 and BiPO4 move up and down in accordance with each EF at the sometime (Figure 11c). Thus, an inner electric field is established.10,13 Finally, the bottom of CB of Bi2O3 is higher than that of BiPO4. Bi2O3 can be excited by visible light and produce photogenerated electron-hole pairs. The photogenerated electrons on the CB of Bi2O3 can directly transfer to that of BiPO4 and photo-generated holes remaining on the VB of Bi2O3 would react with the pollutants adsorbed on the surface of the photocatalyst. This process makes charge separation more efficient and reduces the probability of photogenerated electron-hole recombination on the Bi2O3-BiPO4 interface. Under the applied bias, the electron was transferred to the external circuit, and thus PEC activity was significantly enhanced. In addition, the superior PEC performance of the Bi2O3-BiPO4 composite may be ascribed to the high porosity structure of the p-n type Bi2O3-BiPO4 heterojunction. 4. CONCLUSIONS The Bi2O3-BiPO4 composite electrode has been successfully synthesized by a series of electrophoretic deposition, electrodeposition and calcination methods in a proper sequence. After introduction of BiPO4, the photocurrent density of the Bi2O3-BiPO4 composite increased by ca. 3 times relative to the pure Bi2O3 film under visible light irradiation. The formation of Bi2O3-BiPO4 heterojunction structure significantly enhanced the transfer and separation efficiency of photogenerated electron-hole pairs in the composite film. Furthermore, the composite film displayed higher efficiency and stability in the PEC degradation of phenol and RhB. The results demonstrate that synthetic p-n type Bi2O3-BiPO4 heterojunction is a promising visible light responsive composite material for environment remediation. SUPPORTING INFORMATION Table S1-S2 and Figures S1-S8. ACKNOWLEDGEMENT This work was financially supported by the Zhejiang Provincial Natural Science Foundation of China (LY14E080002, LY14B070002, and R5100266), the National Science Foundation of China


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(21477114, 21576237), the Young Academic Leaders Project of Zhejiang Province (PD2013170) and Graduate Innovation Foundation of Zhejiaang Gongshang University (3100XJ1514159).


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REFERENCES (1) Kim, D.; Sakimoto, K. K.; Hong, D. C.; Yang, P. D. Artificial photosynthesis for sustainable fuel and chemical production. Angew. Chem. Int. Ed. 2015, 54, 3259. (2) Kim, T. W.; Choi, K. S. Nanoporous BiVO4 photoanodes with dual-layer oxygen evolution catalysts for solar water splitting. Science 2014, 343, 990. (3) Du, Y.; Feng, Y. J.; Qu, Y. P.; Liu, J.; Ren, N. Q.; Liu, H. Electricity generation and pollutant degradation using a novel biocathode coupled photoelectrochemical cell. Environ. Sci. Technol. 2014, 48, 7634. (4) Li, X. Z.; Liu, H. L.; Yue, P. T.; Sun, Y. P. Photoelectrocatalytic oxidation of rose bengal in aqueous solution using a Ti/TiO2 mesh electrode. Environ. Sci. Technol. 2000, 34, 4401. (5) Shang, J.; Zhao, F. W.; Zhu, T.; Wang, Q.; Song, H.; Zhang, Y. C. Electric-agitation-enhanced photodegradation of rhodamine B over planar photoelectrocatalytic devices using a TiO 2 nanosized layer. Appl. Catal. B-Environ. 2010, 96, 185. (6) Liu, Z.; Zhang, X.; Nishimoto, S.; Jin, M.; Tryk, D. A.; Murakami, T.; Fujishima, A. Highly ordered TiO2 nanotube arrays with controllable length for photoelectrocatalytic degradation of phenol. J. Phys. Chem. C 2008, 112, 253. (7) Li, C. H.; Koenigsmann, C.; Ding, W. D.; Rudshteyn, B.; Yang, K. R.; Regan, K. P.; Konezny, S. J.; Batista, V. S.; Brudvig, G. W.; Schmuttenmaer, C. A.; Kim, J. H. Facet-dependent photoelectrochemical performance of TiO2 nanostructures: an experimental and computational study. J. Am. Chem. Soc. 2015, 137, 1520. (8) Fujishima, A.; Zhang, X. T.; Tryk, D. A. TiO2 photocatalysis and related surface phenomena. Surf. Sci. Rep. 2008, 63, 515. (9) Chen, X. B.; Shen, S. H.; Guo, L. J.; Mao, S. S. Semiconductor-based photocatalytic hydrogen generation. Chem. Rev. 2010, 110, 6503. (10) Seabold, J. A.; Choi, K. S. Efficient and stable photo-oxidation of water by a bismuth vanadate


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(31) Hameed, A.; Gombac, V.; Montini, T.; Felisari, L.; Fornasiero, P. Photocatalytic Activity of Zinc Modified Bi2O3. Chem. Phys. Lett. 2009, 483, 254. (32) Eberl, J.; Kisch, H. Visible light photo-oxidations in the presence of -Bi2O3. Photochem. Photobiol. Sci. 2008, 7, 1400. (33) Hameed, A.; Gombac, V.; Montini, T.; Graziani, M.; Fornasiero, P. Synthesis, Characterization and Photocatalytic Activity of NiO- Bi2O3 Nanocomposites. Chem. Phys. Lett. 2009, 472, 212. (34) Hagfeldt, A.; Lindstrom, H.; Sodergren, S.; Lindquist, S. E. Photoelectrochemical studies of colloidal TiO2 films: The effect of oxygen studied by photocurrent transients. J. Electroanal. Chem. 1995, 381, 39. (35) Tafalla, D.; Salvador, P.; Benito, R. M. Kinetic approach to the photocurrent transients in water photoelectrolysis at n - TiO2 Electrodes: II. analysis of the photocurrent - time dependence. J. Electrochem. Soc. 1990, 137, 1810. (36) Ng, Y. H.; Iwase, A.; Kudo, A.; Amal, R. Reducing graphene oxide on a visible-light BiVO4 photocatalyst for an enhanced photoelectrochemical water splitting. J. Phys. Chem. Lett. 2010, 1, 2607.


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Figure Captions: Figure 1. The schematic of preparation process for Bi2O3-BiPO4 film. Figure 2. X-Ray diffraction patterns of BiPO4, Bi2O3 and Bi2O3-BiPO4 films. Figure 3. UV-vis DRS of BiPO4, Bi2O3 and the Bi2O3-BiPO4 composite films. Figure 4. Scanning electron microscopy images of (a) BiPO4 film, (b) Bi2O3 film, and (c) Bi2O3-BiPO4 composite film. (d) EDS data for cross-section of Bi2O3-BiPO4 composite. Figure 5. Linear sweep voltammograms of BiPO4, Bi2O3 and Bi2O3-BiPO4 composite films in 0.1 M Na2SO4 with 0.1 M Na2SO3 aqueous solution (pH 9.3) under chopped (a) visible and (b) UV-vis light irradiation. Scan rate: 10 mV s-1. Light intensity: 100 mW cm-2. Figure 6. Electrochemical impedance spectra (EIS) of prepared films. (a) BiPO4, Bi2O3 and Bi2O3BiPO4 composite films under dark conditions. (b) BiPO4, Bi2O3 and Bi2O3-BiPO4 composite films under bright (visible light, λ > 420 nm) conditions. Scan rate: 5 mV s-1 Light intensity: 100 mW cm-2 Figure 7. Incident photon to current conversion efficiency (IPCE) of BiPO4, Bi2O3 and Bi2O3-BiPO4 composite films. The IPCE was measured at 0.3V (vs. Ag/AgCl) in 0.1 M Na2SO4 with 0.1 M Na2SO3 aqueous Na2SO4 solution. Light intensity: 100 mW cm-2. Figure 8. PEC degradation rate of phenol for Bi2O3 and Bi2O3-BiPO4 composite films under visible light irradiation. Phenol concentration: 10mg/L, 25mL. The voltage value: 2 V. Electrolyte solution: 0.2 M Na2SO4. Figure 9. Cyclic utilization of Bi2O3-BiPO4 composite films in PEC degradation of phenol under visible light irradiation. Phenol concentration: 10mg/L, 25mL. The voltage value: 2 V. Electrolyte solution: 0.2 M Na2SO4. Figure 10. Normalized plots of the photocurrent-time dependence for BiPO4, Bi2O3 and Bi2O3-BiPO4 composite films. Inset represents a typical photocurrent transient response curve at a constant potential. Figure 11. Mechanism of charge transfer and electron transport between p-type Bi2O3 and n-type BiPO4 under visible light irradiation. ACS Paragon Plus Environment

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Figure 1


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Figure 2


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Figure 3


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(a) (a)

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(b)

600nm

250nm

(d)

(c) (c) Bi2O 3

BiPO 4

Figure 4

BiPO4 Bi2O3 600nm

540nm


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Figure 6


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Figure 8


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Figure 10


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