Enhanced Photocatalytic Performance for the BiPO4–x Nanorod

Aug 14, 2013 - However, after vacuum treatment at 260 °C for 3 h, the XRD peak of BiPO4–x at 2θ = 27.1° (200) moves to a big angle slightly, indi...
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Enhanced Photocatalytic Performance for the BiPO4−x Nanorod Induced by Surface Oxygen Vacancy Yanhui Lv, Yanyan Zhu, and Yongfa Zhu* Department of Chemistry, Beijing Key Laboratory for Analytical Methods and Instrumentation, Tsinghua University, Haidian, Beijing 100084, PR China S Supporting Information *

ABSTRACT: The BiPO4−x nanorod with surface oxygen vacancy was fabricated via vacuum deoxidation. The concentration and kind of oxygen vacancy could be controlled by tuning the deoxidation temperature and time in vacuum. The photocatalytic activity depended on the concentration and kind of surface oxygen vacancy, and the optimum photocatalytic activity and photocurrent of the BiPO4−x nanorod was about 1.5 and 2.5 times as high as that of pure BiPO4, respectively. Besides, the photocatalytic response wave range of the BiPO4−x nanorod has been expanded to more than 365 nm. The enhancement of photocatalytic activity is attributed to the high separation efficiency of photoinduced electron−hole pairs due to the broadening of the valence band (VB) induced by surface oxygen-vacancy states, and the extending of photoresponse is considered to be the narrowing of energy band gap resulting from the rise of the valence band maximum (VBM).

1. INTRODUCTION Photocatalysis technology has attracted enormous interest in organic wastewater treatment for environmental remediation because it is an inexpensive and convenient method that can totally decompose dye molecules into inorganic small molecules.1−7 Heterogeneous photocatalysts afford great potential for converting photon energy into chemical energy and for decomposing organic pollutants.8−10 Zhu and coauthors reported that BiPO4 as one oxyacid salt photocatalyst exhibited more attractive activity on dye degradation than that of P25 because of its exceptional optical properties, nontoxicity, low cost, and higher catalytic efficiency.11−13 However, the band gap of BiPO4 is wide, about 4.1 eV, and the light response range narrow (λ < 300 nm), and the separation efficiency of photoinduced electron−hole pairs may be further improved. It will be very meaningful if a new method can be found to further improve the photocatalytic activity and expand the range of photoresponse. It has been reported that the photoabsorption performance of materials can be enhanced or expanded by introducing oxygen defects.14−18 Especially, surface oxygen defects can serve as photoinduced charge traps as well as adsorption sites where the charge transfer to adsorbed species can prevent the electron−hole recombination, which is conductive to the improvement of photocatalytic activity,18−20 and the photoresponse range can be expanded by introducing surface oxygen vacancies, due to the narrowing of band gap induced by surface oxygen-vacancy states. Oxygen vacancy properties have been deeply surveyed in simple semiconductors, such as TiO2,17−20 ZnO,21−23 and Fe2O3;24 however, complicated oxyacid salt with a wide band gap has seldom been reported. In this work, we attempt to enhance the © XXXX American Chemical Society

photocatalytic activity and expand the photoresponse of BiPO4 by introducing surface oxygen vacancies. Here, BiPO4−x with surface oxygen vacancy is first fabricated by a facile soft-chemical approach, vacuum deoxidation. After surface oxygen vacancies were introduced on vacancy BiPO4−x, its photocatalytic activity and photocurrent were improved about 1.5 and 2.5 times, respectively (λ = 254 nm). Although no visible photoactivity was observed for vacancy BiPO4−x, its photoactivity was generated under 365 nm light irradiation. As is well-known, the bandgap of monoclinic BiPO4 is about 4.1 eV (about 300 nm), which cannot be excited by λ = 365 nm UV light. So the photoresponse of vacancy BiPO4−x was expanded, induced by surface oxygen vacancies. The reduced total organic carbon (TOC) of methylene blue (MB) solution by vacancy BiPO4−x after 25 min degradation is 85%, indicating that vacancy BiPO4−x has a good capability of mineralization. In addition, the degradations of the other three kinds of organic pollutions were also investigated. Vacancy BiPO4−x shows higher activity on all of them than that of BiPO4, indicating that the high activity of BiPO4−x may be general and essential for environmental application.

2. EXPRIMENTAL SECTION 2.1. Materials Preparation. The BiPO4 nanorod (7.63 m2/g) was synthesized through the reported method by our groups previously.25 All chemicals used were analytic grade reagents without further purification. Vacuum deoxidation Received: June 6, 2013 Revised: August 10, 2013

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Figure 1. UV photocatalytic activity of BiPO4 untreated and vacuum treated with (A) various temperatures for 3 h and (B) various times at 140 °C on the degradation of MB. Insets: degradation rate constant k of BiPO4 and vacancy BiPO4−x photocatalysts (λ = 254 nm). (C) Photocurrents of BiPO4 and vacancy BiPO4−x (treated for 3 h at 130, 140, 160, and 260 °C, respectively) electrodes, under UV light (λ = 254 nm). (D) TOC removal plots of MB over BiPO4 and vacancy BiPO4−x (treated at 140 °C for 3 h), under UV light (λ = 254 nm).

ethanol to produce a slurry, which was then dip-coated onto a 2 cm × 4 cm indium−tin oxide (ITO) glass electrode. Electrodes were exposed to UV light for 12 h to remove ethanol and subsequently calcined at 120 °C for 10 h. All investigated electrodes were of similar thickness (0.8−1.0 μm). 2.2. Characterizations. A high-resolution transmission electron microscope (HR-TEM, JEM 2010F) was operated at an accelerating voltage of 200 kV. The electron paramagnetic resonance (EPR) measurement of photocatalyst powder was carried out using an Endor spectrometer (JEOL ES-ED3X) at 77 K. The g factor was obtained by taking the signal of manganese. The electron spin resonance (ESR) signals of radicals spin-trapped by spin-trap reagent DMPO (5,5′dimethyl-1-pirroline-N-oxide) (Sigma Chemical Co.) in water were examined on a Bruker model ESR JES-FA200 spectrometer equipped with a quanta-Ray Nd:YAG laser system as the irradiation source (λ = 270−410 nm, 10 Hz). To minimize experimental errors, the same type of quartz capillary tube was used for all ESR measurements. The EPR (or ESR) spectrometer was coupled to a computer for data acquisition and instrument control. Magnetic parameters of the radicals detected were obtained from direct measurements of magnetic field and microwave frequency. The room-temperature photoluminescence (PL) spectra of BiPO4 and vacancy BiPO4−x samples were investigated utilizing the Perkin-Elmer LS55 spectrophotometer equipped with a xenon (Xe) lamp with an excitation wavelength of 270 nm. The photocurrent and electrochemical impedance spectroscopy (EIS) were measured on an electrochemical system (CHI-660B, China). UV light 254 nm was obtained from an 11 W germicidal lamp

treated BiPO4 samples were prepared as follows: (1) the temperature-programmed deoxidizing (TPD) measurement using helium (He) gas was performed in a specially designed quartz tube with 0.023 g of the BiPO4 nanorod sample. The tube was put in a cylindrical electric furnace. The temperature of the furnace was controlled by a programmable regulator with the thermocouple. A thermal conductivity detector (TCD) was used to detect He consumption during the vacuum treatment process. The BiPO4 sample was pretreated by nitrogen (N2) gas from room temperature to 100 °C at a temperature ramping rate of 5 °C·min−1 for 1.5 h. Then, it cooled naturally to room temperature in a N2 atmosphere. Afterward, He gas was introduced into the homemade quartz tube equipped with the BiPO4 sample, and the gas flow rate was 25 mL·min−1 accompanied by the temperature gradually increasing to 600 °C for 5 min. (2) According to the TPD graph, the deoxidation temperature range was 110−160 °C at 10 °C intervals, and 260 °C was also chosen; the time was 1, 2, 3, 4, and 5 h. The vacuum deoxidation process was performed to prepare BiPO4 samples for the photocatalytic degradation reaction. The asprepared BiPO4 nanorod was put into a self-made quartz tube and then placed into a furnace connected with a vacuum pump and a program heating device. Pressure in the vacuum deoxidation process was about 1× 10−3∼10−4 Torr. The temperature was increased from room temperature to the designed temperature at an increasing rate of 10 °C·min−1, and the time was kept for 1, 2, 3, 4, and 5 h, respectively. Finally, the samples were cooled naturally to room temperature. BiPO4 and BiPO4−x electrodes were prepared as follows: 7 mg of as-prepared photocatalyst was suspended in 1 mL of B

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Figure 2. (A) Photocatalytic degradation of MB over BiPO4 and vacancy BiPO4−x photocatalysts, under UV light (λ = 365 nm); the inset shows degradation rate constant k of BiPO4 and vacancy BiPO4−x photocatalysts. (B) IPCE of pure BiPO4 and vacancy BiPO4−x electrodes.

with the increase of vacuum temperature and time; when the temperature reaches to 140 °C, time is 3 h, and vacancy BiPO4−x displays the highest photocatalytic activity. The apparent rate constant k is 0.300 min−1, and it is about 1.5 times as high as that of pure BiPO4. Further increasing the temperature or prolonging the time, however, the degradation rate decreases. Even after 260 °C 3 h vacuum deoxidation, the photoactivity of vacancy BiPO4−x is slightly lower than that of pure BiPO4. From the inset graph of Figure 1A and B, it can be seen that the temperature and time in the process of vacuum deoxidation both greatly influence the photocatalytic activity of vacancy BiPO4−x photocatalysts. To further understand the mineralization property of the as-prepared photocatalyst, the removal of TOC in the UV photodegradation of MB over BiPO4 and vacancy BiPO4−x (treated at 140 °C for 3 h, the following is the same, except special directions) photocatalysts was investigated (see Figure 1C). It can be found that the TOC removal percentage is about 63% and 85% for BiPO4 and vacancy BiPO4−x after 25 min photocatalytic reaction, respectively. This indicates that the mineralization property of vacancy BiPO4−x is evidently enhanced. The photocurrent responses of BiPO4 and BiPO4−x after deposition on ITO electrodes, under UV light (λ = 254 nm), are provided in Figure 1D. It can be seen that BiPO4−x electrodes after vacuum deoxidation at 130, 140, 160, and 260 °C for 3 h show different photocurrent density. The photocurrent of BiPO4−x samples gradually enhanced with the increase of vacuum temperature: when the temperature reaches 140 °C, vacancy BiPO4−x displays the highest photocurrent, which is about 2.5 times as high as that of pure BiPO4; however, the photocurrent of BiPO4−x (vacuum treated at 260 °C for 3 h) is lower than that of pure BiPO4 (only about 2/3 of pure BiPO4). The enhancement/decrease of photocurrent demonstrates that the increase/decrease of photoinduced carriers transport rate and the boost/inhibition of photogenerated electron−hole pair separation leads to the enhancement/decrease of photocatalytic activity, which is controlled by the concentration and kind of oxygen vacancy. This result is in accord with the photocatalytic activity of the as-prepared samples. To check the extending of photoresponse range, under UV (λ = 365 nm) light, the photocatalytic activity of BiPO4 and vacancy BiPO4−x on degradation of MB was investigated, as shown in Figure 2A. As reported previously, the bandgap of monoclinic BiPO4 is about 4.1 eV, and its absorption edge is no more than 300 nm.26 So, under λ = 365 nm light, pure BiPO4

(Institute for Electric Light Sources, Beijing). A standard threeelectrode cell with ITO/BiPO4 as a working electrode, a platinum wire as a counter electrode, and a standard calomel electrode (SCE) as reference electrode were used in photoelectric studies. 0.1 M Na2SO4 was used as the electrolyte solution. Potentials were given with reference to the SCE. The photoelectric responses of the photocatalysts as light on and off were measured at 0.0 V. Total organic carbon analyzer (TOCVwp, Shimadzu, Japan) was employed for mineralization degree analysis of MB solutions. Ultraviolet−visible diffuse reflectance spectroscopy (UV-DRS) was performed in Hitachi U-3010, and BaSO4 was used as reference. 2.3. Photocatalytic Evaluations. The photocatalytic activities of the as-prepared samples were evaluated by the decomposition of methylene blue (MB) in solution under UV light. The UV light source was obtained by a 15 W UV germicidal lamp (λ = 254 nm), and the average light intensity was 0.82 mW·cm−2. An amount of 25 mg of photocatalyst was added into prepared 50 mL of 1 × 10−5 M MB aqueous solution. Before the light irradiation, the suspensions were first ultrasonic dispersed in dark for 15 min and then magnetically stirred for 15 min to reach the absorption−desorption equilibrium. At given time intervals, 3 mL aliquots were sampled and centrifuged to remove the photocatalyst particles. Synchronously, the filtrates of MB solutions at different conditions were analyzed by recording variations of the maximum absorption peak 663 nm in the UV−vis spectra using a Hitachi U-3010 UV−vis spectrophotometer. A 365 nm UV source was obtained from a 300 W high-pressure mercury lamp with a 365 nm cutoff filter. The method was similar with the 254 nm UV degradation mentioned above. Further intermediates of phenol over vacancy BiPO4−x were determined by HPLC analysis with a UV detector at 270 nm. The mobile phase was methanol and water (6:4), and the flow rate was 1 mL/min. The concentration of phenol was 5 mg/L.

3. RESULTS AND DISCUSSION 3.1. Enhancement of Photocatalytic Activity and Photocurrent. Figure 1A and B shows the UV photocatalytic activities of pure BiPO4 and vacuum deoxidation BiPO4−x treated with various temperatures for 3 h and various times at 140 °C on the degradation of MB (λ = 254 nm). The degradation process is fitted to pseudo-first-order kinetics, and the value of the rate constant k is equal to the corresponding slope of the fitting line. After vacuum treatment, the photocatalytic activities of BiPO4−x samples gradually enhanced C

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Figure 3. (A) UV photocatalytic degradation on MB five times in the presence of vacancy BiPO4−x, λ = 254 nm. (B) UV photocatalytic activities of fresh and after storage 10 months vacancy BiPO4−x on the degradation of MB, compared with BiPO4. (C) Selectivity on the photocatalytic degradation of MO, RhB, and phenol in the presence of BiPO4 and vacancy BiPO4−x. (D) HPLC of intermediates at different irradiation intervals on phenol over BiPO4 and vacancy BiPO4−x, respectively.

3.2. Stability, Selectivity, and Intermediates. To survey the stability of photodegradation, reaction and storage of reduced vacancy BiPO4−x, UV photocatalytic degradation on MB with five recycles by vacancy BiPO4−x, and UV photocatalytic activities of fresh and after storage for 10 months vacancy BiPO4−x on the degradation of MB, compared with pure BiPO4, were examined. From Figure 3A, the photocatalyst vacancy BiPO4−x exhibits almost no significant loss of activity, confirming that vacancy BiPO4−x is stable during the photocatalytic oxidation of pollutant molecules. As seen from Figure 3B, the UV photocatalytic activity of vacancy BiPO4−x after storage for 10 months only decreases slightly (k = 0.286 min−1), compared with that of fresh reduced ZnO1−x (k = 0.300 min−1), but it is still much higher than that of pure BiPO4 photocatalyst (k = 0.154 min−1). These indicate that vacancy BiPO4−x has a good stability on both photocatalytic degradation and storage. It is well-known that photoinduced holes are almost nonselective, and they will directly react with various organic pollutants owing to their strong oxidation ability. In fact, the photodegradation of organic dyes and transparent organic pollutant phenol by BiPO4 and vacancy BiPO4−x samples (Figure 3C) confirms this conclusion. It can be seen that the degradation activity of vacancy BiPO4−x on anionic dye (rhodamine B (RhB)), cationic dye (methyl orange (MO)), and benzene ring compounds (phenol) is obviously higher than that of pure BiPO4, which suggests that the high activity of vacancy BiPO4−x is not due to the special combination of the

shows almost no photocatalytic activity. However, vacancy BiPO4−x reveals a significant photoactivity, and its reaction rate constant k reaches to 0.012 min−1, which is about 10 times that of pure BiPO4 (the inset of Figure 2A). Incident photon-tocurrent conversion efficiency (IPCE) is a better parameter to characterize the photoconversion efficiency of different photoanodes because it is independent from the light sources and filters used in the measurement .17,27,28 IPCE measurements were performed on an as-prepared electrode in a two-electrode cell, as shown in Figure 2B. ITO/BiPO4 samples or standard electrode served as a working electrode and a platinum wire as the counter electrode, and 0.1 M Na2SO4 was used as the electrolyte solution. The irradiation source was a 75 W xenon lamp powered by a high precision constant current source coupled to a f4 matched monochromator with 1200 lines/in. gratings. IPCEs at each incident wavelength (at intervals of 5 nm) were calculated from the equation IPCE(λ) = (1240Iph)/λP0

In this equation, Iph is the incident photocurrent density in mA· cm−2; λ is the wavelength of incident radiation in nm; and P0 is the photon flux in mW·cm−2. In comparison to pure BiPO4, vacancy BiPO4−x exhibits significantly enhanced photoactivity from 300 to 380 nm. Although the IPCEmax of vacancy BiPO4−x is only about 5.5% at 340 nm, it is direct evidence for illustrating the expanding of photoresponse range of vacancy BiPO4−x with surface oxygen vacancy. D

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Figure 4. (A) O2-TPD profile of the BiPO4 nanorod. Inset: the ball-and-stick model of BiPO4 coordination. (B) UV−vis diffuse reflectance spectra and the color of BiPO4 and vacancy BiPO4−x. (C), (D) HR-TEM images of BiPO4 and vacancy BiPO4−x. (E) The PL spectra of BiPO4 and vacancy BiPO4−x, at wavelength 270 nm excitation. (F) Situ EPR spectra of BiPO4 and vacancy BiPO4−x at 77 K.

catalysts to some specific substrates but due to the strong oxidation reactions of photoinduced holes. Therefore, the high photocatalytic activity of vacancy BiPO4−x may be general and essential for environmental application. To demonstrate the change and reveal some details of the reaction process of pure BiPO4 and vacancy BiPO4−x, photodegradation intermediates distribution of phenol at intervals during the photocatalytic process were monitored by HPLC, and the results are displayed in Figure 3D. The intermediates at different retention time as references were shown in Table S1 (Supporting Information). At 5.3 min the product is phenol, and based on steric effect, those at about 2.7 min are 4,4′-dihydroxybiphenyl and 4-phenoxyphenol.29,30

Others are intermediates, HQ (2. Five min), P-BQ (3.1 min), and catechol (3.8 min), respectively.31−34 The peak of phenol disappears more quickly in vacancy BiPO4−x than that in pure BiPO4. Furthermore, peaks attributed to degradation intermediates increase and then decrease in the vacancy BiPO4−x system, while in pure BiPO4 there was hardly any reduction. This indicates that the mineralization property on phenol of vacancy BiPO4−x is also evidently enhanced, compared with pure BiPO4. 3.3. Formation of Surface Oxygen Vacancy. To investigate the deoxidation process, temperature-programmed deoxidation (TPD) was performed on the BiPO4 nanorod, as shown in Figure 4A. The O2-TPD profile of BiPO4 exhibits E

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Figure 5. DMPO spin-trapping ESR spectra of (A) BiPO4 and (B) vacancy BiPO4−x photocatalysts in water; photogenerated carrier trapping in the system of photodegradation of MB by (C) BiPO4 and (D) vacancy BiPO4−x, under UV light (λ = 254 nm), respectively.

several little broad peaks, which gradually enhanced with the rise of temperature in the 120−580 °C range. As seen from the inset of Figure 4A, the bond length between the oxygen (O) atom and phosphorus (P) atom or bismuth (Bi) atom is not equivalent in the BiPO4 sample. On the basis of the difference of the bond length (representing bond energy) between each O atom and P or Bi, O atoms may be removed first from surface to bulk, and the order is from O1, O2, O4, to O3.35 Finally, the PO43− is totally detached from BiPO4, generating oxygen vacancies with different number and kind. These can be confirmed from the X-ray diffraction (XRD) patterns of pure BiPO4 and BiPO4−x samples (Figure S1, Supporting Information). No phase transformation or any impurity is observed for BiPO4 after vacuum deoxidation at 140 °C for 3 h, so here most oxygen atoms are removed from the BiPO4 surface, generating surface oxygen vacancies. However, after vacuum treatment at 260 °C for 3 h, the XRD peak of BiPO4−x at 2θ = 27.1° (200) moves to a big angle slightly, indicating that the lattice spacing d decreases, which may be a result of the loss of lattice oxygen and the formation of bulk oxygen vacancy, and after 524.5 °C 3 h vacuum treatment, BiPO4 loses PO43‑ and totally becomes metal Bi (Figure S1A and B, Supporting Information). UV−vis diffuse reflectance spectra (UV-DRS) of BiPO4 and vacancy BiPO4−x photocatalysts are shown in Figure 4B. BiPO4 shows the characteristic spectrum with its fundamental absorption sharp edge at about 300 nm. The absorbance edge of vacancy BiPO4−x photocatalysts exhibits only a little redshift from 300 to 305 nm. However, the absorbance of vacancy BiPO4−x was enhanced in the range of 300−800 nm, which may be induced by oxygen vacancies. The color of BiPO4

samples changes from white to light gray by vacuum deoxidation as shown in the inset of Figure 4B. Further study by HR-TEM (Figure 4C and D) showed that the BiPO4 samples both are structurally uniform with a lattice spacing of about 0.328 nm corresponding to the (200) plane. Differently, pure BiPO4 displays perfect lattice features, but the edge of vacancy BiPO4−x particles becomes disordered (thickness about 1.5 nm), which indicates the surface structure is damaged and maybe surface oxygen vacancies are formed. To further confirm the existence of oxygen vacancies, in situ electron paramagnetic resonance (EPR) of BiPO4 samples was performed at 77 K. It can provide a sensitive and direct method to monitor various behaviors of the presence of oxygen defects.36 The intensity of the EPR signal at g ∼ 2.001 of vacancy BiPO4−x is much higher than that of pure BiPO4 under the same conditions, as seen from Figure 4E. As reported previously, the peak at g ∼ 2.001−2.004 can be attributed to netural oxygen vacancies about the surface.37−39 Otherwise, two other peaks at g ∼ 2.038 correspond to chemisorbed oxygen O2−,40,41 and g ∼ 1.967 is controversial and is assigned to shallow donors or singly ionized oxygen vacancy (Vo+).41,42 However, these indicate that other defects may be produced on BiPO4, besides those that surface oxygen vacancy mainly generates. In addition, it was reported previously that surface oxygen vacancy is positively related with the photoluminescence spectra (PL) signal in ZnO and TiO2.18,22 From the photoluminescence (PL) spectra of BiPO4 and vacancy BiPO4−x at wavelength 270 nm excitation in Figure 4F, it can be seen that the PL peak of vacancy BiPO4−x is obviously enhanced in the range of 300−520 nm, which might be induced F

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by surface oxygen vacancy.43,44 The larger the surface oxygenvacancy concentration, the stronger the PL signal.18,21,22 Therefore, under the same conditions, the strengthening of the PL spectrum of vacancy BiPO4−x may be attributed to the formation of surface oxygen vacancies. 3.4. Mechanism of Enhancement of Photocatalytic Activity. The photocatalytic mechanism can be studied by an electron spin resonance (ESR) spin-trap technique and trapping experiments of radicals and holes.9,45,46 DMPO (5,5dimethyl-1-pyrroline N-oxide) is a nitrone spin trap generally used for trapping radicals due to the generation of some stable radicals, DMPO-hydroxyl radical (•OH) and DMPO-superoxide radical (•O2−). The ESR spin-trap technique was employed to monitor the reactive oxygen species generated during the irradiation of the pure BiPO4 and vacancy BiPO4−x system with DMPO in water, and the results were shown in Figure 5A and B. Both the signals of the DMPO-hydroxyl radical (•OH) and the DMPO-superoxide radical (•O2−) could be observed when pure BiPO4 and vacancy BiPO4−x suspension were irradiated for 3 min by a Quanta-Ray Nd:YAG pulsed laser system (λ = 270−410 nm, 10 Hz). However, the intensity of •OH radicals is higher than that of •O2− radicals. Therefore, •OH radicals play a more important role than •O2− radicals in the photocatalysis process (•OH > •O2−). Furthermore, the holes and hydroxyl radicals during the photodegradation of MB over pure BiPO4 and vacancy BiPO4−x were also investigated with the addition of EDTA2Na (holes scavenger)47 and t-BuOH (hydroxyl radicals scavenger),48 as shown in Figure 5C and D. In the BiPO4 system, the addition of 0.5 mM t-BuOH as a hydroxyl radials scavenger only caused a small decrease of the photocatalytic degradation of MB, while the photodegradation of MB could be greatly prevented with the addition of a scavenger for holes (EDTA-2Na), suggesting that the photoinduced holes are the main oxidative species of the BiPO4 system and that hydroxyl radicals play an assistant role. From Figure 5D, in the vacancy BiPO4−x system, the photocatalytic degradation of MB is also greatly suppressed by the addition of EDTA-2Na, and the main oxidative species is the same as that as in the BiPO4 system. These results indicate that the photocatalytic degradation mechanism of vacancy BiPO4−x on MB is not changed (holes > •OH > •O2−), compared with that of the pure BiPO4 photocatalyst. As discussed above, the degradation mechanism of vacancy BiPO4−x is not changed, and particle size and the crystal phase structure (Figure S1, Supporting Information) are not evidently changed. Also, the limited adsorptivity enhancement (Figure S2, Supporting Information) is not the major factor of the enhancement of the photocatalytic activity of the vacancy BiPO4−x photocatalyst. As is well-known, the influence of separation efficiency of photoinduced electron−hole pairs is a crucial factor for the enhancement of photocatalytic activity. The charge separation efficiency can be investigated by the typical electrochemical impedance spectra (EIS) (presented as Nyquist plots). From Figure 6, the EIS Nynquist plots of BiPO4 and vacancy BiPO4−x electrodes show that the arc radius of the vacancy BiPO4−x electrode is smaller than that of the BiPO4 electrode, with and without UV-light irradiation. The smaller the arc radius of an EIS Nynquist plot, the smaller the electric resistance of the electrode and the higher the efficiency of charge separation, so the photocatalytic activity of vacancy BiPO4−x improves. Therefore, the significant enhancement of UV photocatalytic activity is mainly attributed to the increase of

Figure 6. Electrochemical impedance spectroscopy (EIS) Nynquist plots of BiPO4 and vacancy BiPO4−x electrodes with and without UVlight irradiation.

the separation efficiency of photoinduced electron−hole pairs due to the broadening of the valence band (VB) width induced by surface oxygen-vacancy states. On the basis of the results above, a proposed schematic diagram for the separation of electron−hole pairs and the photocatalytic reaction process of the vacancy BiPO4−x photocatalyst is shown in Figure 7. Surface oxygen vacancy is

Figure 7. Schematic diagram for the separation of electron−hole pairs and photocatalytic process over the vacancy BiPO4−x photocatalyst.

a shallow defect, which may be near the conduction band minimum (CBM) or above the valence band maximum (VBM). It was reported previously by Zhao and coauthors that the band gap of the PO4 salt LiTi2(PO4)3 significantly reduced due to the formation of oxygen-vacancy states above the VB, under poor oxygen conditions.35 Our experiments also showed that for vacancy BiPO4−x not only its UV photocatalytic performance distinctly increased but also the photoresponse range was expanded, compared with pure BiPO4. The enhancement of the UV photoactivity is attributed to the broadening of the VB width due to the rising of the VBM induced by surface oxygen-vacancy states, which is above and partly overlapping with the VB of BiPO4. This is conducive to increase the transport rate of photoinduced carriers, resulting in more efficient charge separation and reducing the probability of photoinduced electron−hole recombination, then leading to an G

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obviously enhanced photoactivity. Owing to the rising of the VBM, the energy band gap of vacancy BiPO4−x is reduced, and the photoresponse range of vacancy BiPO4−x can be expanded (from less than 300 nm for pure BiPO4 to more than 365 nm for vacancy BiPO4−x), although not to the visible light region. This is because the band gap of pure BiPO4 is wide (about 4.1 eV), and the rising of the VBM caused by a shallow DOS of surface oxygen vacancies is not high enough to be in the visible light region (about 2.9 eV). Nevertheless, the photoresponse range of vacancy BiPO4−x is still expanded by surface oxygen vacancies. In addition, the introduction of surface oxygen vacancy only caused the small rising of the VBM and the small narrowing of band gap for BiPO4 and hardly any other change of the energy band structure, so the degradation mechanism was the same as that of pure BiPO4, excepting the enhancement of photoactivity and the expanding of photoresponse range. Furthermore, the concentration and kind of oxygen vacancy, controlled by tuning the temperature and time in vacuum, greatly influence the photocatalytic activity/photocurrent of BiPO4−x samples. When the concentration of oxygen vacancies is low (low temperature or time), the photocatalytic activity/ photocurrent only improves slightly and enhances gradually with the increase of the concentration of surface oxygen vacancy; when the concentration is too high (high temperature or time), bulk oxygen vacancy can easily generate in the deep of the forbidden band, which is the recombination center of electron−hole pairs and will result in the decrease of photocatalytic activity/photocurrent, as shown in Figure 1A and C. Therefore, only high oxygen-vacancy concentration controlled on the surface of BiPO4−x and almost no bulk oxygen vacancy generated can obtain the highest photocatalytic activity of vacancy BiPO4−x. So the concentration and kind of oxygen vacancy controlled by tunning vacuum deoxidation temperature and time play an important role in improving the photocatalytic activity

AUTHOR INFORMATION

Corresponding Author

*Fax: (+86)10-6278-7601. Tel.: (+86)10-6278-3586. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by National Basic Research Program of China(973 Program)(2013CB632403), National High Technology Research and Development Program of China (2012AA062701) and Chinese National Science Foundation (20925725 and 21373121).

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4. CONCLUSIONS BiPO4−x photocatalysts with surface oxygen vacancies were prepared via a facile economic vacuum deoxidation method. After vacuum treatment, the BiPO4−x photocatalyst possessed significant enhancement of photocatalytic activity, and its photoresponse range can be extented visibly; however, the photocatalytic process is not changed, and the main active species are still photoinduced holes. The distinct enhancement of UV activity is attributed to high separation efficiency of photoinduced electron−hole pairs caused by the broadening of the VB width induced by surface oxygen-vacancy states, and the expanding of photoresponse range originates from the narrow energy band gap due to the rise of valence band maximum (VBM). This facile and effective approach is proposed to develop a new type of optical materials and to be extended for other wide bandgap materials.

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S Supporting Information *

XRD, adsorption curves in dark of pure BiPO4 and BiPO4−x, HPLC on MB, and intermediate distribution of phenol by pure BiPO4 and BiPO4−x. This material is available free of charge via the Internet at http://pubs.acs.org. H

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