BiOI heterostructures with enhanced solar-driven photocatalytic activity

Current Applied Physics 16 (2016) 240e244

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Current Applied Physics journal homepage: www.elsevier.com/locate/cap

Charge separation properties of (BiO)2CO3/BiOI heterostructures with enhanced solar-driven photocatalytic activity Yujun Si a, Jianzhang Li a, **, Junbo Zhong a, *, Jun Zeng a, Shengtian Huang a, Wei Yuan a, Minjiao Li a, b, Jie Ding a a

Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, College of Chemistry and Pharmaceutical Engineering, Sichuan University of Science and Engineering, Zigong, 643000, PR China Sichuan Provincial Academician (Expert) Workstation, Sichuan University of Science and Engineering, Zigong, 643000, PR China

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 September 2015 Received in revised form 5 December 2015 Accepted 8 December 2015 Available online 12 December 2015

(BiO)2CO3/BiOI heterojunctions were prepared in-situ by a pore impregnating method using HI aqueous solution. The specific surface area, structure, morphology, and charge separation properties of the heterostructures prepared were characterized by BrunauereEmmetteTeller (BET) method, X-ray diffraction (XRD), scanning electron microscopy (SEM), and surface photovoltage (SPV) spectroscopy, respectively. The active species were investigated using scavengers. The photocatalytic activities of (BiO)2CO3/BiOI heterojunctions towards discoloration of rhodamine B (RhB) under simulated sun light irradiation were evaluated. Benefiting from enhanced charge separation efficiency, the composite photocatalysts exhibit higher photocatalytic performance than the pure BiOI. Finally, the possible reason was discussed. © 2015 Elsevier B.V. All rights reserved.

Keywords: Photocatalyst (BiO)2CO3 BiOI Photocatalytic performance Charge separation

1. Introduction Recently, bismuth-containing photocatalysts have prompted increasing research interest due to their relatively high photocatalytic performance, the typical photocatalysts are BiVO4 [1], Bi2WO6 [2], BiOX (X ¼ Cl, Br, I) [3e5], Bi12SiO20 [6], Bi2MoO6 [7] and so on. Among these bismuth-containing commands, (BiO)2CO3 (ntype semiconductor) is suitable as a photocatalyst owing to its outstanding properties [8e10]. (BiO)2CO3 has a layered structure with the plane of the CO2 3 group orthogonal to the plane of the [Bi2O2]2þ layer [11]. In fact, (BiO)2CO3 has been successfully applied to decompose lots of organic pollutants, however, the practical application of (BiO)2CO3 under sunlight irradiation is limited by its wider band gap (2.87e3.58 eV) [12,13]. To settle this issue, tremendous research efforts have been focused on improving the photocatalytic performance of (BiO)2CO3 under solar illumination, such as doping [14e16], preparation of (BiO)2CO3 with unique morphology [17e19], and heterojunctions construction [20e22].

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (J. Zhong). http://dx.doi.org/10.1016/j.cap.2015.12.008 1567-1739/© 2015 Elsevier B.V. All rights reserved.

Construction of heterostructures can facilitate charge transfer and separation, resulting in relatively high photocatalytic activity. As a p-type semiconductor, BiOI has a layered structure similar to (BiO)2CO3, [Bi2O2]2þ slabs are interleaved by double slabs of iodine atoms. The bandgap of BiOI is approximately 1.88 eV [23]. Coupling BiOI with (BiO)2CO3 has received special interest due to the similarity of their layered structures. (BiO)2CO3/BiOI heterostructures have been prepared by many groups [24e26]. Cao and co-workers fabricated BiOI/(BiO)2CO3 and (BiO)2CO3/BiOI heterojunctions by etching (BiO)2CO3 precursor with hydroiodic acid (HI) solution [24,25], their results showed that BiOI/(BiO)2CO3 and (BiO)2CO3/BiOI heterojunctions had been formed in the composites and the photocatalytic activity could be greatly promoted benefiting from the high separation of photo-induced charges due to the presence of heterojunctions. Ou et al. prepared (BiO)2CO3/ BiOI solid solutions by chemical precipitation method at room temperature [26], the (BiO)2(CO3)x(I2)1x solid solutions displayed efficient photocatalytic activity and high durability toward photooxidation of NO in air. The surface photovoltage (SPV) method is an effective approach to characterize semiconductors, providing important information about optical and charges transport properties of different regions in the material under study [27]. Generally speaking, the SPV response relies on the light absorption, charge separation, and

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charge transport properties of the materials. However, few attentions have been paid on the charge separation properties (charge separation rate and charge properties) of (BiO)2CO3/BiOI heterostructures. The information on charge separation properties of (BiO)2CO3/BiOI heterostructures can provide important understanding for investigating the effect of coupling BiOI with (BiO)2CO3 on the catalytic performance of photocatalysts. Thus, the intent of this paper is to study the charge separation properties of (BiO)2CO3/ BiOI heterostructures. In this work, (BiO)2CO3/BiOI heterojunctions were prepared insitu by HI acid etching using a pore impregnating method. The effects of charge separation rate on the photocatalytic performance were investigated and the charge properties were elucidated. The results can provide new insights into the charge separation properties of (BiO)2CO3/BiOI heterojunctions.

with a Model Zolix UOM-1S illuminometer made in China. The photocatalytic performance of (BiO)2CO3/BiOI photocatalysts was investigated by the discoloration of rhodamine B (RhB) aqueous solution under simulated sun light irradiation. In typical photocatalytic experiment, 50 mg of photocatalyst was added into 50 mL of RhB aqueous solution (10 mg L1). The light source was a 500 W Xe lamp (simulated sun light) and the initial pH of RhB solution was 7.0. After 30 min, the suspension was centrifuged to remove the photocatalyst particles for analysis. To investigate the active species during the photocatalytic reaction, isopropanol (IPA), benzoquinone (BQ) and ammonium oxalate (AO) were added into the RhB solution to detect radicals (OH), superoxide radicals ($O2  ) and holes (hþ), respectively, the procedure was the same as the photocatalytic experiments mentioned above.

2. Experimental section

3. Results and discussion

All chemicals with analytical grade were obtained from Chengdu Kelong Chemical Reagent Factory and used without further purification. (BiO)2CO3 was prepared by parallel flow precipitation method using Bi(NO3)3$5H2O and with Na2CO3 aqueous solution. 20 g Bi(NO3)3$5H2O was dissolved in 200 mL dilute HNO3 solution (20 mL HNO3 (65 wt%) þ 180 mL H2O). The concentration of Na2CO3 was 3 mol/L. The pH value of precipitation reaction was around 8.0 by controlling the flow velocity of two solutions. The white precipitate was filtered, washed with deionized water many times. The precipitate was dispersed in absolute ethanol and then dried in air at 333 K overnight. (BiO)2CO3/BiOI heterostructures with different molar ratio of (BiO)2CO3/BiOI were prepared by a pore impregnating method using HI aqueous solution. To eliminate the effects of volume variation of HI solution on the preparation of (BiO)2CO3/BiOI heterostructures, the pore impregnating method was followed using the procedure as described in literature [28] with some modification. In a typical procedure, 5 g of as-prepared (BiO)2CO3 was put into a beaker, water was added and made (BiO)2CO3 to be humidified (no water was observed on the surface of (BiO)2CO3) assisted by ultrasonication, the volume of the water is the water pore volume of 5 g (BiO)2CO3. 5 g (BiO)2CO3 was added into HI aqueous solution with different concentration (the volume of HI solution is equal to the water pore volume of 5 g (BiO)2CO3) and ultrasonic dispersed for 20 min. Compared to the procedure as described in literature [28], in the present paper, no desired HI solution was dissolved into water whose volume is the water pore volume of 5 g (BiO)2CO3 to obtain HI aqueous solution with different concentration, thus the procedure is more controllable. The molar ratio of (BiO)2CO3/BiOI is1/0 (pure (BiO)2CO3), 2/1, 3/4, 1/3, 1/8 and 0/2 (pure BiOI) by adjusting the concentration of HI aqueous solution. The mixture was kept in a static condition for 1 h, dried in 353 K for 1 h, and then dried in 373 K overnight. The samples with different molar ratios of (BiO)2CO3/BiOI were labeled as 1/0, 2/1, 3/4, 1/3, 1/8 and 0/2, respectively. The BrunauereEmmetteTeller (BET) specific surface area was analyzed by N2 adsorption in a SSA-4200 automatic surface analyzer. The X-ray diffractometer (XRD, DX-2600) was applied to study the crystal structure of the samples. Scanning electron microscopy (SEM) images were performed on a JSM-7500F scanning electron microscope operating at 5 kV. UVeVis diffuse reflectance spectra (DRS) were carried out on a spectrometer (TU-1907) using BaSO4 as a reference. SPV measurements were recorded according to the procedure described in reference [29]. The powder samples were sandwiched between two ITO glass electrodes, and the change of surface potential barrier between in the presence of light and in the dark is the SPV signal. The raw SPV data were normalized

Table 1 shows the specific surface area of photocatalysts. Considering the measurement error, the specific surface area has no obvious difference, thus the specific surface area will not be the main factor for the enhanced photocatalytic activity of (BiO)2CO3/ BiOI heterojunctions. Fig. 1 shows the XRD patterns of the photocatalysts. All the peaks of 1/0 (pure (BiO)2CO3) accord well with the standard tetragonal phase (JCPDS No. 41-1488) and 0/2 (pure BiOI) can be indexed to tetragonal phase (JCPDS No. 73-2062). For 2/1, 3/4, 1/3 and 1/8 samples, the presence of (BiO)2CO3 and BiOI phases was detected. The intensities of diffraction peaks of BiOI gradually increase, while the intensities of diffraction peaks of (BiO)2CO3 decrease with the concentration of HI increasing, which demonstrates that (BiO)2CO3 transforms into BiOI gradually. The results fit well with the results reported by Cao and co-workers [24]. The diffuse reflectance spectra of composites partially overlap, thus only the spectra of 1/0, 2/1 and 0/2 are shown in Fig. 2. As the amount of BiOI increasing, the absorbance of composites increases and appears red-shift, which is due to the presence of BiOI in the composites, since BiOI is a narrow-band semiconductor. The

Table 1 Specific surface area of photocatalysts. Catalysts SBET (m2/g)

1/0 7.0

2/1 8.1

3/4 8.1

1/3 7.9

1/8 7.0

0/2 6.0

30000 0/2 1/8

Intensity (a.u.)

20000

1/3 3/4 10000 2/1 1/0 0 10

15

20

25

30

35

2 Theta (degree) Fig. 1. XRD patterns of photocatalysts.

40

45

242

Y. Si et al. / Current Applied Physics 16 (2016) 240e244

Absorbance (a.u.)

1.0

1/0 2/1 0/2

0.8 0.6 0.4 0.2 0.0

681 365 300

671 400

500

600

700

800

Wavelength (nm) Fig. 2. UVeVis diffuse reflectance spectra of photocatalysts.

bandgap of photocatalysts can be calculated using the equation Eg ¼ 1240/l. The band gap of 1/0, 2/1 and 0/2 is about 3.40, 1.85 and 1.82 eV, respectively. Compared to the pure BiOI, the wide bandgap of (BiO)2CO3 indicates that valence band/conduction band of (BiO)2CO3 sample is more positive/negative than that of BiOI. So, the electrons and holes on the surfaces of (BiO)2CO3 have stronger redox ability than that of composites and the pure BiOI [30]. Similarly, the electrons and holes on the surfaces of 2/1 have

stronger redox ability than that of other composites and the pure BiOI. The SEM images of photocatalysts are shown in Fig. 3. Interestingly, all photocatalysts exhibit irregular lump and plate-like morphologies, and pores were seldom, resulting in low specific surface area, which is in good agreement with the results shown in Table 1. The results here suggest that the SEM image of BiOI is determined by the SEM of precursor, which may be due to the similar layered structure of BiOI and (BiO)2CO3. The SPV responses of samples are shown in Fig. 4. (BiO)2CO3 displays obvious SPV response from 300 to 600 nm, the SPV peak is around 330 nm, which is assigned to the wide bandgap of (BiO)2CO3. Moreover, (BiO)2CO3 exhibits SPV response in visible light region, demonstrating that (BiO)2CO3 has rich surface states, however, the underlying reason is unclear, remaining for further study. BiOI has strong SPV response from 300 to 600 nm. However, (BiO)2CO3/BiOI composites hold a similar SPV response range as BiOI, indicating that the photovoltage responses of composites in the visible region mainly result from BiOI. Furthermore, the SPV response intensities of composites are not the simple addition of the pure (BiO)2CO3 and BiOI. The SPV responses of composites increase as the loading of BiOI is increasing, suggesting the electrons of exciton pairs generated from BiOI are pushed toward (BiO)2CO3 [24]. Thus, the exciton pairs in (BiO)2CO3/BiOI heterojunctions can be effectively separated. Among all the samples, the 1/3 sample has the strongest SPV response, and then the SPV response decreases as the loading of BiOI increasing. According to the principle of SPV, a

Fig. 3. SEM image of photocatalysts;(A)1/0;(B)2/1;(C)0/2.

Y. Si et al. / Current Applied Physics 16 (2016) 240e244

1/0 2/1 3/4 1/3 1/8 0/2

Photovoltage (mV)

4 3 2 1

*50

0 300

400

500

600

Wavelength (nm) Fig. 4. SPV responses of photocatalysts, the SPV value of 1/0 is 50 times of the pristine.

stronger SPV response intensity means high separation efficiency of photo-induced charges [27]. The SPV results demonstrate that the 1/3 sample possesses higher separation efficiency of photo-induced charges than others. It is suggested that there is an interfacial electric field in (BiO)2CO3/BiOI composites due to the formation of BiOI during HI acid etching of (BiO)2CO3. The interfacial electric field may play a dominant role in the separation of photo-induced charges [31]. Light can reach the interface of (BiO)2CO3 and BiOI, so (BiO)2CO3/BiOI composites have higher SPV response than the pure (BiO)2CO3 and BiOI because the interfacial electric field enhances the separation of photo-induced charges. Compared to the other composites, the 2/1 sample has a smaller amount of BiOI resulting in a relative weaker absorption in the visible light, so there was a weaker SPV response intensity. When the molar ratio of (BiO)2CO3 and BiOI is 1/8, photons reached to the interface decreases due to the agglomeration of the BiOI particles on the surfaces of (BiO)2CO3, increasing the migration distance of photo-induced charges [31]. So the SPV response of 1/8 is weaker than the intensity of 1/3. Usually, higher separation efficiency is beneficial to form more active species during the photocatalytic reaction, resulting in a higher photocatalytic performance. Fig. 5 shows the phase degree of photocatalysts. The phase values of all photocatalysts are positive, indicating that the photoinduced electrons transfer to the top electrode from which light is incident [32]. For 2/1 and 0/2, the phase value is gradually reduced

with respect to 180 from 400 to 600 nm in the phase spectra to general trend, which is similar to the phase characteristics of the ptype Si wafer from sub band gap to super band gap energy reported in ref [33]. For the pure (BiO)2CO3, from 450 to 600 nm, the complex phase value indicates that some factors that control the separation and transport of photo-generated charges. Construction of (BiO)2CO3/BiOI heterostructures cannot change the electronic transfer property, though (BiO)2CO3 and BiOI are different type semiconductor. The photolysis of RhB aqueous solution (10 mgL1) under simulated sun light illumination without photocatalyst after 30 min can be totally ignored; the adsorption of RhB on different photocatalysts after 30 min in a dark and catalytic performance of photocatalysts are shown in Fig. 6. The results show that all (BiO)2CO3/ BiOI heterostructures hold better photocatalytic performance than the pure BiOI under simulated solar light irradiation. Among these six photocatalysts, 2/1 exhibits the best photocatalytic performance. Construction of (BiO)2CO3/BiOI composites can greatly enhance the photocatalytic activity of (BiO)2CO3 and BiOI, when the molar ratio of (BiO)2CO3 and BiOI is 2/1, the composite possesses the best photocatalytic activity under simulated sun light irradiation. To detect the reactive species during the photocatalytic process, the effects of three scavengers on the photocatalytic decolorization of RhB aqueous solution were investigated and the results are shown in Fig. 7. The photocatalytic decolorization of RhB over 2/1 sample decreases remarkably from 81.3% to 34.9% after adding BQ, indicating that $O2  is the main active species during the photocatalytic process. When AO and IPA were added, the photocatalytic decolorization efficiency of RhB drops to 62.6% and 73.3%, respectively, which demonstrates that OH and hþ play the secondary important role in decolorization of RhB. During the photocatalysis process, so many factors can influence the photocatalytic activity, such as specific surface area, morphology, the redox ability of electron and hole, the separation rate of photo-generated charges and so on. The photocatalytic activity is the synergy of all the factors mentioned above. In this work, based on all the observations, the increased photocatalytic activity of 2/1 is assigned to the improved photo-induced charge separation and relative strong redox ability of electron and hole, these two main factors together decide the relative high photocatalytic activity of (BiO)2CO3/BiOI heterojunctions with molar ratio of 2/1.

90

Decolorization(%)

1/0 2/1 0/2

160

Phase (degree)

Adsorption Activity

75

170

150 140

60 45 30

130

15

120

0

110

243

300

350

400

450

500

550

600

Wavelength (nm) Fig. 5. Phase spectra of the as-prepared photocatalysts.

650

1/0

2/1

3/4

1/3

1/8

0/2

Photocatalyst Fig. 6. Catalytic activity of photocatalysts; the concentration of rhodamine B was 10 mg L1, the initial pH of rhodamine B was 7.0, the concentration of photocatalysts was 1 g L1; the irradiation time was 30 min.

244

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90

81.3

Decolorization (%)

[7]

73.3

75

62.6

[8]

60 [9]

45 34.9

[10]

30 [11]

15 0

[12]

Blank

IPA

AO

BQ

Scavenger Fig. 7. Effects of three scavengers on the photocatalytic decolorization of rhodamine B over 2/1 (Illumination time ¼ 30 min, Concentration of rhodamine B was 10 mg L1, Scavenger dosage ¼ 0.2 mmol/L).

[13]

[14]

[15]

[16]

4. Conclusions In this work, (BiO)2CO3/BiOI heterojunctions were prepared insitu by a pore impregnating method using HI aqueous solution. Acid etching of (BiO)2CO3 by HI aqueous solution results in the formation of heterojunctions, which induces the red-shift of absorbance, and increases the separation rate of charge carriers. When the molar ratio of (BiO)2CO3 and BiOI is 2/1, the composite possesses the best photocatalytic activity under simulated sun light irradiation. The enhanced photocatalytic activity of (BiO)2CO3/BiOI heterostructures can be attributed to the improved separation rate of photo-induced charges and relative strong redox ability of electron and hole on the surface. Acid etching is an effective and simple approach to promote the photocatalytic performance of (BiO)2CO3 and BiOI. Acknowledgments This project was supported financially by the Opening Project of Jiangsu Key Laboratory for Environment Functional Materials (No. SJHG1307), Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (No. LZJ14202, LZY1101) and Sichuan Provincial Academician (Expert) Workstation (No. 2015YSGZZ03).

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

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