BiVO4/BiPO4复合物的制备及可见光催化性能

尹延峰　周　锋　詹　溯　杨一凡　刘昱君(大连海事大学，大连　116026)

BiVO4/BiPO4复合物的制备及可见光催化性能

尹延峰周锋＊詹溯杨一凡刘昱君
(大连海事大学，大连116026)

采用水热法合成出具有不同V、P物质的量之比的BiVO4/BiPO4复合物。nV/nP分别为：0.1/9.9、0.5/9.5、1/9、3/7、5/5。采用XRD、FE-SEM、EDS、拉曼、可见光光度计、漫反射以及电化学等测试手段对BiVO4/BiPO4复合物进行表征。在可见光条件下降解亚甲基蓝来评价BiVO4/BiPO4复合物的光催化活性。结果显示，当nV/nP<3/7的时候，BiVO4/BiPO4复合物的光催化活性随着BiVO4含量的增加而增加，当nV/nP=3/7的时候，复合物具有最佳的光催化性能，反应速率常数k为0.005 1 min-1，是纯BiPO4的23.2倍。BiVO4/BiPO4复合物的光催化机制主要是由于BiVO4的加入,提高了电子-空穴的分离率,进而提高了光催化活性。

水热法；BiVO4；BiPO4；光催化活性；电子-空穴对

0　Introduction

Since the new century, environmental pollution became one of the toughest issues, and human beings need to face it. Photocatalysts can replace the traditional pollutant control technology because of full use of sunlight, complete degradation without secondary pollution and other advantages. At present,TiO2was the most widely studied photocatalysts. Because of their outstanding photocatalytic activity[1-4], TiO2-based photocatalysts had been proven to be one of the promising photocatalysts. But TiO2-based photocatalysts were hard to overcome the issue of high recombination rates of photogenerated electron-hole pairs. Therefore, how to carry out efficient photocatalytic materials had become an important issue of photocatalysts.

Currently, inorganic bismuth compounds (eg BiOX[5], BiVO4[6-8], Bi2WO6[9-10], Bi2MoO6[11-12]and BiPO4[13-14]) were widespread concerned because of their high electron-holes separation rate. BiPO4was a new type of photocatalysts. BiPO4had been reported that had excellent photocatalytic activity under UV light than TiO2(P25) for the degradation of methylene blue (MB)[15]. Zhu et al. reported PO43-was favorable for the separation of photo-induced e-/h+and PO43-can improve the photocatalytic activity of BiPO4[16]. Due to the wide band gap, BiPO4had no visible light response. It must be the largest hindrance for the further applications of BiPO4. Thus, it was an important mission to broaden the visible light absorption region of BiPO4. Up to now, for obtaining visible light induced, much work had been done for BiPO4, including C3N4/BiPO4[17], AgPO4/BiPO4[18-19], BiOI/BiPO4[20], and Bi2MoO6/BiPO4[21]. BiVO4had a narrow gap-band energy (～2.4 eV) and had been considered to be one of the most promising photocatalysts. BiVO4was usually selected as a sensitizer photocatalyst because of its high visiblelight response. BiVO4could degrade pollutants and evolve H2and O2under visible light (λ>420 nm). However, as stated previously, the poor separation of photoinduced e-/h+had a restriction on the photocatalytic activity of pure BiVO4. Therefore, we envisaged that constructing BiVO4/BiPO4heterostructured photocatalysts[22-23], which could be a promising method to improve the photocatalytic performance and broaden the visible light absorption region of BiPO4. However, until now, there were few reports about BiVO4/BiPO4composites photocatalysts.

In this study, BiVO4/BiPO4composites photocatalysts were synthesized by hydrothermal method. The photocatalytic activities of BiVO4/BiPO4composites were evaluated by the degradation of methylene blue (MB) under visible light (λ>420 nm). Besides, detailed photocatalytic mechanism of the BiVO4/BiPO4Composites had been discussed.

1　Experimental

1.1Experimental drugs and equipment required for the experiment

All chemicals were analytical purity and were used without further purification. Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O) was obtained from Tianjin Kermel Chemical Reagent Co., Ltd. Risodium phosphate dodecahydrate (Na3PO4·12H2O) was obtained from Shenyang Federal Reagent Factory and ammonium metavanadate (NaVO3) was obtained from Sinopharm Chemical Reagent Co., Ltd. Methylene blue (C6H18ClN3S·3H2O) was obtained from Tianjin Bodi Chemical Co., Ltd. Deionized water was used in all experiments.

The purity and crystallinity of pure BiPO4, pure BiVO4and BiVO4/BiPO4composites were characterized by X-ray diffraction (XRD) on Rigaku DMAX-Ultima+diffractometer with Cu Kα radiation (λ=0.154 06 nm). Raman spectrum was excited with the 514 nm line of an Ar+laser at an incident power of 20 mW. The morphologies of the samples were examined by a field emission scanning electron microscope (FE-SEM) with SUPRA 55 SAPPHHIRE. UV-Vis diffuse reflectance spectroscopy(DRS) measurements were measured using a TU-1901 UV-Vis spectrophotometer equipped with an integrating sphere attachment. The analysis range was from 200 to 800 nm, and BaSO4was used as a reflectance standard. Electrochemical experiments were performed in a flat cell having 0.1 mol·L-1Na2SO4solution by a remote controlled potentiostat/galvanostat (VMP3 EG&G Princeton Research).

1.2Preparation of photocatalysts

The BiVO4/BiPO4composites with different BiVO4contents were synthesized by hydrothermal method. In a typical process, the precursor solution was prepared by dissolving 2 mmol Bi(NO3)3·5H2O with 0.02 mmol NaVO3, 1.98 mmol Na3PO4·12H2O; 0.1 mmol NaVO3,1.9 mmol Na3PO4·12H2O; 0.2 mmol NaVO3, 1.8 mmol Na3PO4·12H2O; 0.6 mmol NaVO3, 1.4 mmol Na3PO4· 12H2O and 1 mmol NaVO3, 1 mmol Na3PO4·12H2O, respectively. Then the precursor solution was putted in 35 mL of distilled water. After stirring for 30 min, the resultant precursor solution was transferred into a 50 mL teflon-lined stainless steel autoclave. The autoclave was sealed and heated to 170℃for 24 h and allowed to cool down to room temperature naturally. The precipitate was washed with absolute ethanol and distilled water for many times, respectively, and dried at 70℃in air. In order to facilitate the expression, the composite ratios were named for nV/nP=0.1/9.9, 0.5/9.5, 1/9, 3/7, 5/5 as 0.1VP, 0.5VP, 1VP, 3VP, 5VP, respectively. For comparison, pure BiPO4sample was synthesized by adopting the method. BiPO4was synthesized by 2 mmol Bi(NO3)3·5H2O and 2 mmol Na3PO4·12H2O. The reaction process could be simply expressed as shown in Fig.1.

Fig.1 Reaction process and the crystal structure of BiVO4and BiPO4

1.3Photocatalytic activity tests

To carry out the photocatalytic activity of pure BiPO4and BiVO4/BiPO4composites, the sample of 50 mg was suspended in a MB dye aqueous solution (100 mL, 10-5mg·L-1). After stirring for 30 min to reach an equilibrium adsorption state in the dark, the solution was irradiated with a 500 W Xe arc lamp. The lamp provided visible light (λ>420 nm) with a cut off filter. At given time intervals, the solution(4 mL) was sampled and centrifuged. Then, the filtrates were analyzed by recording variations of the absorption band maximum (664 nm) in a UV-Vis spectra of MB by using a TU-1901 UV-Vis spectrophotometer.

The degradation efficiency was calculated as follows[24]:

Where C0was the absorbance of original methylene blue (MB) solution and C was the absorbance of the methylene blue(MB) solution after visible light irradiation for 180 min. According to the Langmuir-Hinshelwood kinetics model, the photocatalytic process of methylene blue (MB) could be expressed as the following apparent pseudo-first-order kinetics equation:

Where k was the apparent pseudo-first-order rate constant, C0was the original methylene blue (MB) concentration and C was methylene blue (MB) concentration in aqueous solution at time.

2　Results and discussion

2.1Photocatalytic activity

The photocatalytic activities of the BiVO4/BiPO4samples were measured on the degradation of methylene blue (MB) in deionized water under visible light irradiation (λ>420 nm) in Fig.2. It can be seen that pure BiPO4had less visible light photocatalytic activity for methylene blue (MB) degradation, due to the wide band gap(300 nm) of BiPO4. After the depositing of BiVO4, BiPO4can degrade methylene blue (MB) under visible light, which showed that BiVO4was a good visible light sensitizer to BiPO4. The efficient visible light absorption abilities of BiVO4/BiPO4composites ensured that the BiVO4/BiPO4composites generated sufficient electron-hole pairs under visible irradiation. In particularly, 3VP displayed the best photocatalytic activity. Fig.2a showed the degradation efficiency of BiVO4/BiPO4composites and the rate constant k. It could be seen that 3VP could degrade 60.2% methylene blue (MB) by 3 h illumination. It was calculated that 3VP possessed the maximal k value of 0.005 1 min-1whichwas 23.2 times of the pure BiPO4in Fig.2b.

As reported in the previous literature[25-29], generally, there was an optimal ratio of the two components in composite photocatalysts. When the component ratio of BiVO4/BiPO4composites was changed that not only mainly affected the number of effective heterojunctions and also influenced the separation efficiency of BiVO4/BiPO4composites. In case of the optimal content of 3VP, the most appropriate BiVO4/BiPO4heterojunction was formed. The BiVO4/BiPO4heterojunction could facilitate the high efficient separation of photoinduced electrons and holes, and endow the BiVO4/BiPO4composite with higher photocatalytic activity under visible light irradiation (λ>420 nm).

Fig.2 (a) Photodegradation efficiencies of MB as a function of irradiation time for different samples; (b) Rate constant k of MB degradation for the as-prepared samples

2.2Structural characterization

The purity and crystallinity of the BiVO4/BiPO4composites were characterized by XRD. The Fig.3 showed the XRD patterns of the as-prepared BiPO4, BiVO4and BiVO4/BiPO4composites. The BiVO4/BiPO4composites exhibited a coexistence of both BiPO4and BiVO4phase. All the peaks for the samples were readily indexed to the monoclinic structure of BiPO4(JCPDS No.15-0767). As it could be seen in the pattern of BiVO4sample, the diffraction peaks could be perfectly indexed to BiVO4phase (JCPDS No.21-0121). For the BiVO4/BiPO4composites, all diffraction peaks of BiPO4were clearly observed, indicating that the solvothermal did not influence the crystal structure of BiPO4. When the nV/nPwas 3/7, the strong characteristic diffraction peaks of sample BiPO4and monoclinic BiVO4were simultaneously found. With an increasing amount of BiVO4, more BiVO4diffraction peaks appeared.

Fig.3 XRD patterns of the as-prepared samples

To investigate the chemical bonding of the BiVO4/BiPO4composites, Raman spectra wereobtained and shown in Fig.4. The 3VP was selected for the study. In the Raman spectra, the observed intense band at 206 cm-1corresponded to the Bi-O stretching vibration[30]. The band at 825 cm-1could be assigned to the symmetric vibration of V-O and the band at 323 cm-1could be assigned to the asymmetric stretching of VO43-[31]. The v2 vibration of the PO43-occurred at 362 cm-1[30].

Fig.4 Raman spectrum of the as-prepared 3VP composite

Fig.5 FE-SEM images of the as-prepared samples

Combined with XRD results, all the evidences revealed the coexistence of both BiPO4and BiVO4phase.

2.3Morphological analysis

The morphology and microstructure of the BiVO4/BiPO4composites were characterized by FE-SEM. The FE-SEM images of the as-synthesized samples were given in Fig.5. The FE-SEM image (Fig.5a) of pure BiPO4showed that pure BiPO4had regular nanorods and had a clean surface. The nanorods had a length about 700 nm. After checking the relevant literature, pure BiVO4exhibited an irregular decahedron shape[6-8]. Many irregular particles or particle aggregates of BiVO4were observed to adhere to BiPO4(Fig.5b, c, d, e, f). In Fig.5, it can be seen that with the increasing of the content of BiVO4, the bulk morphology of the composite were increased, and the shape nanorods of the composite were decreased. When the nV/nPwas 3/7, the composite had the best performance.

To determine the exact ratio of nV/nP, EDS was carried out to further identify the elemental composition of 0.1VP, 0.5VP, 1VP, 3VP and 5VP in Table 1. For example, the EDS pattern of the 3VP clearly indicated that, besides the V, Bi and Odiffraction peaks corresponding to BiVO4, the P, Bi and O diffraction peaks coming from BiPO4were also observed, confirming that the samples were composed of both BiVO4and BiPO4. Meanwhile the molar ratio of nV/nPwas 0.439, which was matched the ratio of the value of 3/7. The exact ratio of nV/nPwas more or less the same as the one calculated from the preparation process.

Table 1 Characterization of the ratio of V/P

2.4Optical characterization

The optical absorption properties played a critical role in determining the photocatalytic performance of BiVO4/BiPO4composites. The optical properties of pure BiPO4and 3VP were measured by UVVis diffuse reflectance spectra (DRS) in Fig.6. It could be clearly seen that BiPO4could merely respond to the UV light. The absorption band edge of BiPO4was around 300 nm. After the depositing of BiVO4, the light absorption of 3VP was significantly broadened to the visible light range around 460 nm. Compared with the pure BiPO4, 3VP photocatalyst showed a notable red-shift in the the absorption edge. This phenomenon may be due to the interaction between BiVO4and BiPO4, which subsequently resulted in a higher photocatalytic activity under visible light irradiation.

2.5Electrochemical analysis

Electrochemical impedance spectra (EIS) measurements were conducted to investigate the separation efficiency of the photoinduced charge carriers and the charge transfer resistance. Fig.7 showed the EIS Nyquist plots of 3VP and pure BiPO4. It was known that when the diameter for arc radius was smaller, the charge transfer efficiency was higher[32]. The diameter for arc radius of 3VP lighting was smaller than that of without lighting, which indicated a decrease in the charge-transfer resistance and leaded an effective electronhole pair separation. The radius of 3VP was smaller than that of pure BiPO4of lighting, implying that the charge transfer efficiency of 3VP was higher than that of pure BiPO4. Therefore, it could be concluded that the existence of BiVO4could accelerate the separation efficiency of photogenerated carriers of BiVO4/BiPO4composites.

Fig.6 UV-Vis DRS of the as-prepared samples: pure BiPO4, 3VP

Fig.7 Nyquist plots for pure BiPO4, 3VP composite

2.6Photocatalytic mechanism of BiVO4/BiPO4composites

The enhancement of photocatalytic activity of BiVO4/BiPO4composites was mainly due to the higher separation efficiency induced by the hybrid effect of BiVO4and BiPO4. A proposed schematic mechanism of the BiVO4/BiPO4composites was shown in Fig.8. Through experiments, it was known that BiPO4had no or less visible light photocatalytic activity for MB degradation, which means that the electrons at the valence band (VB) of BiPO4could not inject into the conduction band(CB) of BiPO4under visible-light irradiation. After the depositing of BiVO4, at thebeginning of the reaction, photogenerated electronhole pairs were formed on BiVO4, under visible light irradiation (λ>420 nm). The electrons at the VB of BiVO4, not only could inject into the CB of BiVO4, but also the CB of BiPO4. The electrons injected into the CB of BiPO4not only revealed that BiPO4took part in the degradation of MB reaction under visible light irradiation and also leaded to a much reduced electron-hole recombination and improved the photocatalytic efficiency of the BiVO4/BiPO4composites for MB degradation. The whole process was described as follows:

Fig.8 Schematic diagram of the separation and transfer of photogenerated charges in the BiVO4/BiPO4composites under visible light irradiation

3　Conclusions

By the hydrothermal method, a series of BiVO4/BiPO4were synthesized with different nV/nP. UV-Vis diffuse reflectance spectra could demonstrate that all the composites exhibited broad absorption in the visible region. The optimal nV/nPwas 3/7. The k value was 0.005 1 min-1which was 23.2 times of the pure BiPO4. The heterojunction structure of BiVO4/BiPO4facilitated the efficient separation of photogenerated electron-hole pairs, greatly improving the photocatalytic efficiency of BiPO4. The synthesized of BiVO4/BiPO4composites provided a guideline for BiPO4transferred to visible light, increasing the utilization of sunlight.

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Preparation of BiVO4/BiPO4Composites With Enhanced Visible-Light-Driven Photocatalytic Properties

YIN Yan-Feng ZHOU Feng＊ZHAN Su YANG Yi-Fan LIU Yu-Jun
(Department of Materials Science and Engineering, Dalian Maritime University, Dalian, Liaoning 116026, China)

BiVO4/BiPO4composites with different nV/nPmolar ratios were synthesized by a simple one-pot hydrothermal method. The nV/nPmolar ratios were 0.1/9.9, 0.5/9.5, 1/9, 3/7, and 5/5, respectively. The BiVO4/BiPO4composites were characterized by X-ray diffraction, field emission scanning electron microscope, energydispersive spectroscopy, Raman spectrum, UV-Vis spectrophotometer, UV-Vis diffuse reflectance spectroscopy, and electrochemical impedance spectra. The photocatalytic activities of BiVO4/BiPO4composites were evaluated by the degradation of methylene blue (MB) under visible light irradiation (λ>420 nm). When the ratios of the BiVO4/BiPO4composites were less than 3/7, the photocatalytic activities of BiVO4/BiPO4composites were enhanced with an increasing amount of BiVO4.The result showed that the BiVO4/BiPO4composite ratio for nV/nP= 3/7 possessed the highest photocatalytic activity. The BiVO4/BiPO4composite ratio for nV/nP=3/7 possessed the maximal k value of 0.005 1 min-1. It is 23.2 times of the pure BiPO4. The photocatalytic mechanism of the BiVO4/BiPO4composites could be mainly ascribed to the existence of BiVO4which could accelerate the separation and migration efficiency of photogenerated carriers.

hydrothermal; BiVO4; BiPO4; photocatalytic activity; electron-hole pairs

TB321

A

1001-4861(2016)03-0483-08

10.11862/CJIC.2016.065

2015-07-22。收修改稿日期：2016-01-13。

国家自然科学基金(No.21276036)，交通运输部建设科技项目(No.2014328204050)和中央高校基本科研业务费(No.3132015085)资助项目。＊通信联系人。E-mail：zhoufeng99@mails.tsinghua.edu.cn