Synthesis, characterization and solar photocatalytic performance of In2O3-decorated Bi2O3

Materials Science in Semiconductor Processing 16 (2013) 1808–1812

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Synthesis, characterization and solar photocatalytic performance of In2O3-decorated Bi2O3 Jun Zeng, Jianzhang Li n, Junbo Zhong, Shengtian Huang, Wangling Shi, Jinjin He Key Laboratory of Green Catalysis of the Higher Education Institutes of Sichuan, College of Chemistry and Pharmaceutical Engineering, Sichuan University of Science and Engineering, Zigong 643000, P.R. China

a r t i c l e in f o

abstract

Available online 27 July 2013

In2O3/Bi2O3 composite photocatalysts with different In2O3 content were prepared using a pore impregnation method with Bi2O3 as the substrate. The composite photocatalysts exhibit enhanced photocatalytic activity compared to Bi2O3 for the degradation of aqueous methyl orange (MO) solution under solar irradiation. The samples were characterized in terms of BET surface area, X-ray diffraction (XRD), UV/Vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and surface photovoltage (SPV). On the basis of the results, a mechanism is proposed to account for the enhanced photocatalytic activity of these In2O3/Bi2O3 composites. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Photocatalyst Bi2O3 Photocatalytic performance Mechanism

1. Introduction The development of efficient visible-light-driven photocatalysts has attracted worldwide attention [1,2]. Among the visible light photocatalysts available, Bi2O3, with a bandgap of 2.6–2.8 eV [3], has been recognized as suitable for pollutant degradation [4–8]. However, it has been reported that the photocatalytic performance of Bi2O3 is usually not satisfactory because photoinduced electrons and holes tend to rapidly decay through recombination [6], which significantly restricts practical applications of Bi2O3 in photocatalysis. Therefore, effective approaches are required to improve the charge separation efficiency and photocatalytic activity of Bi2O3. The synthesis of composite materials is an area of growing interest for improving the charge transfer efficiency of photocatalysts. Composite semiconductor materials comprise heterostructures constructed from two semiconductors with matching band potentials. Thus, photogenerated electrons and/or holes can transfer from one semiconductor to the other, which can greatly inhibit electron–hole recombination, increase the charge carrier lifetime, and improve the photocatalytic efficiency [9–11]. To date, many Bi2O3-based n

Corresponding author. Tel./fax: +86 813 5505601. E-mail address: [email protected] (J. Li).

1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.06.020

composites have been prepared to extend the light response range and promote photoinduced charge carrier separation compared to single photocatalysts [3,6,12–16]. In2O3, an important transparent semiconductor with a wide bandgap (3.6 eV), has been widely applied in many industries [17–19]. However, the photocatalytic activity of In2O3/Bi2O3 composites under solar irradiation has been seldom reported. The aim of this study was to enhance the solar photocatalytic activity of Bi2O3 by decorating it with In2O3. The effects of In2O3 loading on the structure, surface texture, Bi 4f binding energy, and photoinduced charge separation efficiency were investigated in relation to the solar photocatalytic activity of In2O3/Bi2O3. The photocatalytic activity was evaluated in terms of decolorization of aqueous methyl orange (MO) solution under solar irradiation. 2. Experimental 2.1. Photocatalyst preparation All reagents (A.R. grade) were purchased from Cheng du Ke long Chemical Reagents Factory and used as received. Deionized water was used throughout the study. Bi2O3 was prepared by a parallel flow precipitation

J. Zeng et al. / Materials Science in Semiconductor Processing 16 (2013) 1808–1812

2.2. Photocatalyst characterization Specific surface area (SBET) measurements were performed on a SSA-4200 automatic surface analyzer (Builder, China). X-Ray diffraction (XRD) patterns were recorded on a DX-2600 X-ray diffractometer equipped with a graphite monochromator using Cu Kα (λ ¼0.15406 nm) radiation. The X-ray tube was operated at 40 kV and 25 mA. UV/Vis diffuse reflectance spectroscopy (DRS) was performed on a TU-1907 spectrometer using barium sulfate as the reference. X-Ray photoelectron spectroscopy (XPS) measurements were performed on an XSAM 800 instrument using Mg Kα radiation at 12 kV and 12 mA. XPS spectra were referenced to the C 1s peak (binding energy 284.80 eV). Surface photovoltage (SPV) measurements were carried out according to a procedure described in the literature [20]. 2.3. Photocatalytic activity The photocatalytic activity was evaluated for 100-mg samples in terms of decolorization of 100 mL of MO solution (10 mg L–1) in an open volumetric flask under solar irradiation. The pH of the MO solution was adjusted to 7.0 using HClO4 (0.1 mol L–1) and NaOH (0.1 mol L–1) solutions. After 15 h, the samples were removed and centrifuged (6000 rpm) to separate the photocatalyst for analysis. The MO concentration was measured on a 756 PC spectrophotometer at 460 nm and analyzed using the Lambert–Beer law. The effects of a series of scavengers on MO conversion over 2% In were investigated in an SGY-II photochemical reactor (Kai Feng HXSCI Science Instrument Factory, China). The irradiation source was a 500-W high-pressure mercury lamp with maximum emission at 365 nm. The lamp was encapsulated in a cooling quartz jacket and positioned in the middle of the reactor. Quartz test tubes were located around the lamp at a distance of 10 cm. For an initial MO concentration of 10 mg L–1, 50 mg of 2% In and different scavengers were added to 50 mL of MO solution. The reaction mixture was continuously aerated by a pump to provide oxygen and to aid in mixing of the reaction solution. The decolorization reaction was

performed at room temperature. The pH of the reaction solution was 7.0. After 1 h, samples were removed and centrifuged (6000 rpm) to separate the photocatalyst for analysis. All data reported are mean values for three parallel determinations.

3. Results and discussion According to the results in Table 1, the highest SBET was obtained for In loading of 2 mol% and then decreased for 3 mol%. SBET for 2% In is more than 1.5 times that for pure Bi2O3. The results suggest that a low In2O3 content can increase SBET for the photocatalyst, while a high content may lead to In2O3 nucleation on the Bi2O3 surface owing to the limited SBET of Bi2O3, which decreases the composite SBET. A high SBET can provide a large number of active sites, which is beneficial for photocatalytic performance. XRD patterns for the photocatalysts are shown in Fig. 1. The diffraction peaks for pure Bi2O3 demonstrate that Bi2O3 calcined at 773 K is composed of α and β phases. The characteristic 2θ peak at  27.451 corresponds to α-Bi2O3 phase (PDF No. 41-1449) and that at  28.01 to β-Bi2O3 (PDF No. 74-1374). For the composite photocatalysts, only the β phase was observed, suggesting the presence of strong interactions between In2O3 and Bi2O3. The mechanism involved needs to be investigated in more detail in the future. Furthermore, no In2O3 XRD peaks were detected in the 2θ range 201–601. Plausible explanations are as follows: (1) In2O3 is highly dispersed in the support matrix; and (2) the percentage In2O3 content is relatively low. However, the specific mechanism is not clear and requires further investigation in the future. Fig. 2 shows DRS spectra for the photocatalysts. Owing to partial spectral overlap, only spectra for pure Bi2O3 and 2% In are presented. The DRS spectra for these compounds Table 1 Specific surface area of the photocatalysts. Photocatalyst

0% In

1% In

2% In

3% In

SBET (m2/g)

4.9

5.8

7.5

6.4

20000

16000

Intensity (a.u.)

method using Bi(NO3)3
5H2O with aqueous NH3
H2O and (NH4)2CO3. Bi(NO3)3
5H2O (25.56 g) was completely dissolved in 200 mL of dilute HNO3 solution (15 mL of 65 wt.% HNO3+185 mL of H2O). NH3
H2O and (NH4)2CO3 were dissolved in deionized water to a concentration of 3:3 mol L  1. The two solutions were added to the same reactor using a peristaltic pump and were agitated vigorously. The pH of the precipitation reaction was maintained at 8.5 by adjusting the flow velocity of the two solutions. The precipitate was filtered, washed, spray dried, and then heated in air for 2 h at 773 K in a muffle furnace. In2O3/Bi2O3 composites with different In2O3 contents were prepared according to a pore impregnation method using In(NO3)3 solution and then heated at 573 K for 2 h. Samples with different molar amounts of impregnated In (0%, 1%, 2%, and 3%) are denoted as 0% In (pure Bi2O3), 1% In, 2% In, and 3% In, respectively.

1809

3% In

12000

2% In

8000

1% In α

4000

α 0 25

26

β

α 27

28

β

0% In 29

2 Theta (degree) Fig. 1. XRD patterns for the photocatalysts.

30

31

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100

2% In 0% In

3%In Intensity (a.u.)

Reflectance (%)

80

442.0

5000

60

40

2% In

4000

441.9 2%In

3000

441.8 2000

20

1%In

0 300

1000 400

500

600

700

800

453

450

447

444

441

438

Binding energy (eV)

Wavelength (nm)

Fig. 5. High-resolution XPS In 3d spectra for the surface of the photocatalysts. Fig. 2. UV/Vis diffuse reflectance spectra for the photocatalysts.

Bi4f

Intensity (a.u)

20000 16000

O1s Bi4d

Bi4p 1%In

Bi5d

C1s

Bi5p

In3d

12000

Bi4f 8000 4000

Bi4p O1s Bi4d 0%In

Bi5d

C1s

Bi5p 0 1000

800

600

400

200

0

Binding energy (eV) Fig. 3. XPS survey spectra for the surface of two photocatalysts.

18000

158.7

Intensity (a.u.)

15000

3%In

12000

158.6 2%In

9000

158.7

6000

1%In

3000

0%In

158.5

0 168

166

164

162

160

158

156

154

Binding energy (eV) Fig. 4. High-resolution XPS Bi 4f spectra for the surface of the photocatalysts.

are very similar, demonstrating that differences in photocatalytic performance are not due to the response to light. XPS measurements were carried out to determine the surface chemical composition of Bi2O3 and In2O3/Bi2O3 and the valence states of various species. As shown in Fig. 3, the XPS spectra for Bi2O3 and 1% In reveal characteristic peaks for Bi, O, and C. In was detected in the XPS spectrum for 2% In. Fig. 4 shows high-resolution XPS spectra for the

Bi 4f7/2 region measured on the surface of Bi2O3 and In2O3/ Bi2O3 composites. For Bi2O3, the peak located at 158.5 eV is assigned to Bi 4f7/2, indicating a normal state of Bi3+. However, for In2O3/Bi2O3 composites, the peaks for Bi 4f7/2 are shifted to higher values compared to Bi2O3, implying a strong interaction between Bi2O3 and In2O3. This Bi 4f7/2 shift means that the binding energy and chemical environment for Bi have changed. The 3d5/2 XPS spectrum has a characteristic peak centered at binding energy of 444.2 eV In2O3 [21], compared to 443.6 eV for metallic indium. The absence of a peak at 443.6 eV excludes the existence of metallic indium, confirming that indium in In2O3/Bi2O3 exists only in the oxide state. However, as shown in Fig. 5, the 3d5/2 peak for 1% In, 2% In, and 3% In is shifted to values lower than 444.2 eV, confirming that an interaction occurs between In2O3 and Bi2O3. This is in good agreement with the XRD results. To identify the reactive species, the effects of different scavengers on MO decolorization were investigated. The scavengers used were isopropanol (IPA) for dOH [22], ammonium oxalate (AO) for h+ [23], and benzoquinone – (BQ) for dO2 [24]. Quenching can inhibit the photocatalytic decolorization of MO and the MO conversion will therefore be lower. The higher the reduction in MO conversion by a scavenger, the more important is the role the played by the corresponding reactive species in photocatalytic MO decolorization. The effects of a series of scavengers on photocatalytic MO decolorization are shown in Fig. 6. The MO decolorization efficiency rapidly decreased from 40.4% to – 10.5% on addition of BQ, indicating that dO2 is the main active species in the photocatalytic decolorization process. When IPA and AO were added, the MO decolorization efficiency decreased to 27.7% and 18.2%, respectively, indicating that dOH and h+ play a secondary role in the photocatalytic oxidation of MO. The SPV responses of Bi2O3 and In2O3/Bi2O3 composites are shown in Fig. 7. There is an obvious Bi2O3 peak at  370 nm, which is attributed to electronic transitions from the valence band to conduction band according to DRS data and the Bi2O3 energy band structure. However, the SPV response peaks for 1% In, 2% In, and 3% In are shifted to lower wavelength; this is attributed to the presence In2O3, which has a bandgap of 3.6 eV. As shown

J. Zeng et al. / Materials Science in Semiconductor Processing 16 (2013) 1808–1812

75

40.4

40

69.1 58.2

Decolorization (%)

Decolorization(%)

60 30

27.7

18.2

20

10.5

1811

46.7 45

37.3 30

10 15

0

Blank

IPA

AO

0

BQ

0%In

Scavenger

2%In

3%In

Photocatalyst

Fig. 6. Effects of a series of scavengers on MO conversion over 2% In. Illumination time, 60 min; 100 mL of aqueous MO solution (10 mg L  1); scavenger dose, 0.2 mmol L  1.

Fig. 8. Removal of MO by photolysis for 15 h. The photocatalyst dose was 1 g L  1 and the MO concentration was 10 mg L  1 (100 mL). The pH of the MO solution was adjusted to 7.0 using HClO4 and NaOH solution.

2%In

0.012

Photovoltage (mV)

1%In

0.009

3%In 0.006

0.003

1%In Fig. 9. Schematic diagram of the photoexcited electron–hole separation process.

0%In

0.000 300

330

360

390

420

450

Wavelength (nm) Fig. 7. SPV response of the photocatalysts.

in Fig. 7, the In2O3/Bi2O3 composites have a stronger SPV response than pure Bi2O3. In general, a strong SPV response corresponds to a high separation rate for photoinduced charge carriers [25,26]. The SPV results therefore indicate that 2% In has the highest and Bi2O3 has the lowest charge separation rate. The number of photoinduced charge carriers has an important influence on photocatalytic performance: the higher the number of carriers, the better is the photocatalyst [27]. Higher charge separation can enhance the photocatalytic performance; this finding agrees well with the photocatalytic performance results. The photocatalytic activity of Bi2O3 and In2O3/Bi2O3 is shown in Fig.8. All the In2O3/Bi2O3 composites exhibit better solar photocatalytic activity than Bi2O3 and 2% In has the best photocatalytic performance among all the samples. The results suggest that In2O3 on the Bi2O3 surface can greatly enhance its solar photocatalytic activity and that there is an optimum In2O3 loading. Factors such as the specific surface area, structure, adsorption capacity, Bi 4f7/2 energy shift, and charge separation influence the photocatalytic activity. Overall, the best photocatalytic activity was observed for 2% In and the worst for Bi2O3.

On the basis of the above results, a mechanism can be proposed to explain the enhanced solar photocatalytic properties of In2O3/Bi2O3 composites. The conduction band of In2O3 is more positive than that of Bi2O3 and could act as a sink for photogenerated electrons. Thus, photoinduced electrons on the Bi2O3 surface could transfer to In2O3 via interfaces. Similarly, photoinduced holes on the In2O3 surface could migrate to Bi2O3, as shown in Fig. 9. Therefore, there would be a greater number of electrons on the In2O3 surface and holes on the Bi2O3surface, resulting in enhanced separation efficiency for photogenerated charges, which would have a positive effect on the photocatalytic performance. The improved photocatalytic activity of In2O3/ Bi2O3 can be attributed to the greater specific surface area, the shift in Bi4f7/2 energy, and the high separation efficiency for photoinduced charge carriers.

4. Conclusions In2O3/Bi2O3 composites with different In2O3 content were successfully prepared. Photocatalytic activity results demonstrate that In2O3/Bi2O3 composites exhibit higher solar photocatalytic performance than Bi2O3 for MO degradation. This improved photocatalytic activity is attributed to the greater specific surface area, the Bi 4f7/2 shift, and the high separation efficiency of photoinduced charge

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carriers. Modifying Bi2O3 with In2O3 is an effective method for improving its solar photocatalytic activity.

Acknowledgments This project was supported financially by the Science and Technology Department of Sichuan Province (No. 2013JY0080), the Education Department of Sichuan Province (No. 11ZA27 and 10ZA140), Research Fund Projects of Sichuan University of Science and Engineering (No. 2011PY04 and 2012PY05), the Construct Program of the Discipline in Sichuan University of Science and Engineering, and the Opening Project of the Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (No. LZJ1202). References [1] A. Kudo, K. Omori, H. Kato, J. Am. Chem. Soc. 121 (1999) 11459–11467. [2] X.C. Wang, J.C. Yu, Y.L. Chen, L. Wu, X.Z. Fu, Environ. Sci. Technol. 40 (2006) 2369–2374. [3] S.Y. Chai, Y.J. Kim, M.H. Jung, A.K. Chakraborty, D. Jung, W.I. Lee, J. Catal. 262 (2009) 144–149. [4] J. Eberl, H. Kisch, Photochem. Photobiol. Sci. 7 (2008) 1400–1406. [5] H. Cheng, B. Huang, J. Lu, Z. Wang, B. Xu, X. Qin, X. Zhang, Y. Dai, Phys. Chem. Chem. Phys. 12 (2010) 15468–15475. [6] H.Y. Jiang, K. Cheng, J. Lin, Phys. Chem. Chem. Phys. 14 (2012) 12114–12121. [7] Z. Ai, Y. Huang, S. Lee, L. Zhang, J. Alloy. Compd. 509 (2011) 2044–2049.

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