- Email: [email protected]
Improvement of photocatalytic activity under solar light of BiVO4 microcrystals synthesized by surfactant-assisted hydrothermal method
Materials Science in Semiconductor Processing 27 (2014) 41–46
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Improvement of photocatalytic activity under solar light of BiVO4 microcrystals synthesized by surfactant-assisted hydrothermal method Jun Zeng n, Junbo Zhong, Jianzhang Li, Zhen Xiang, Xinlu Liu, Jiufu Chen 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
a r t i c l e i n f o
Keywords: Semiconductors BiVO4 Hydrothermal preparation Photocatalytic performance
abstract In this paper, we reported the obtention of bismuth vanadate (BiVO4) microcrystals synthesized by the surfactant-assisted hydrothermal method with sodium dodecylbenzenesulfonate (SDBS). The specific surface area, structure, morphology and the photoinduced charge separation efficiency of BiVO4 microcrystals were characterized by means of Brunauer–Emmett–Teller (BET) method, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and surface photovoltage spectroscopy (SPS), respectively. The results show that SDBS surfactant alters the surface parameters and the morphology, enhances the photo-induced charge separation efficiency of BiVO4 under solar light irradiation. The photocatalytic activity of catalysts for photocatalytic decolorization of Rhodamine B (RhB) aqueous solution under solar light irradiation was investigated. The results demonstrate that the photocatalytic activity for degradation of RhB relate to SDBS-BiVO4 microcrystals is 7.5 times that of the reference BiVO4 microcrystals and the underlying reason is suggested. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction The development of p-type semiconductor as photocatalyst related to bismuth vanadate (BiVO4) microcrystals attracted great attention of scientific community [1–3]. BiVO4 has three crystalline phases [4], which are tetragonal zircon, tetragonal scheelite and monoclinic scheelite, respectively. The band gap width of monoclinic BiVO4 is 2.4 eV, which can directly absorb solar light as energy, making this catalyst has been widely researched in recent years. However, the low separation efficiency of photo-generated
n
Corresponding author. Tel/fax: þ 86 813 5505601. E-mail address: [email protected] (J. Zeng).
http://dx.doi.org/10.1016/j.mssp.2014.06.014 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
carrier and low surface all limit the photocatalytic activity of BiVO4 crystals. Up to now, many synthesis methods such as sol–gel, solid-state, aqueous process, hydrolysis, sonochemical, and hydrothermal methods have been successfully applied to fabricate BiVO4 [5]. Among these synthesis approaches, hydrothermal method has remarkably attractive advantages: high purity, narrow size distribution, and low aggregation by adjusting the hydrothermal reaction conditions [6]. Sodium dodecylbenzenesulfonate (SDBS) is a neutral anionic surfactant. It is sensitive to water hardness, resistant to oxidation, foaming power, detergency high, and low cost, the synthesis process maturity, and therefore widely used. It is now generally accepted that adding surfactants into the preparation process will greatly
42
J. Zeng et al. / Materials Science in Semiconductor Processing 27 (2014) 41–46
influence the photocatalytic properties of materials [7]. The influence of surfactant on the preparation and photocatalytic activity TiO2 and ZnO have been reported [8,9]. However, the influence of SDBS on the fabrication and photocatalytic property of BiVO4 prepared by the hydrothermal method has been seldom concerned. Rhodamine B (RhB) is a water-soluble dye, rat studies illustrate that RhB can cause subcutaneous tissue sarcoma suspected to be a carcinogen [10]. Thus, safe removal of such a dye is the prime aim of our present research. In the present paper, RhB was chosen as the model dye. Therefore, in this paper, we reported the effect of SDBS anionic surfactant on fabrication and the solar light photocatalytic performance of BiVO4 microcrystals. Photocatalytic activity of BiVO4 microcrystals prepared was evaluated by degradation of RhB aqueous solution. 2. Experimental details 2.1. Surfactant-assisted hydrothermal synthesis of BiVO4 nanocrystals
about 10 7 Pa. A Mg Kα X-ray source was used. The XPS spectrum was referenced to the C1s peak (Eb ¼284.8 eV) resulting from adventitious hydrocarbon (i.e. from the XPS instrument itself) present on the sample surface. The surface photovoltage spectroscopy (SPS) measurements of the samples were carried out according to the procedure described in reference [11]. The level of OH radicals was detected according to the procedure given in reference with some modification [12]. 50 mg of photocatalyst was suspended in 50 mL of an aqueous solution containing 20 mM NaOH and 6 mM terephthalic acid (TA). The solution was stirred in the dark for 40 min before exposure to UV light. After illumination with a 500 W high-pressure mercury lamp for 20 min, the suspension was centrifuged, and the supernatant was sampled for analysis by recording the fluorescence signal of the generated 2-hydroxyterephthalic acid (TAOH) on a fluorescence spectrometer (Cary Eclips, Agilent, USA). The wavelength of the excitation light was 312 nm, and the scanning speed was 600 nmmin 1. The widths of the excitation slit and the emission slit were both 5 nm.
All chemicals (analytical grade reagents) were supplied from Chengdu Kelong Chemical Reagent Factory and used as received. All the studies were done using deionized water. In a typical synthesis, 8.50 g Bi(NO3)3 5H2O was dissolved in nitric solution (HNO3:H2O ¼1:8), 0.50 g SDBS was added to above-solution, forming solution A. 2.02 g NH4VO3 was dissolved in 50 mL water at 80 1C, resulting in solution B. A and B were mixed together to obtain a yellow suspension solution. pH of the suspension solution was adjusted to 7 using NH3 H2O and further stirred for 1 h, then the resulting precursor suspension was transferred into a Teflon-lined stainless steel autoclave with a capacity of 200 mL and maintained at 180 1C for 24 h, and subsequently cooled to room temperature naturally. The precipitate was collected by centrifugation, washed with water and ethanol for several times to remove ions and then dried in a vacuum oven at 80 1C for 10 h. BiVO4 was also prepared as the same procedure mentioned above without the presence of SDBS.
The photocatalytic activity of photocatalyst (50 mg) was evaluated by decolorization of 50 mL RhB aqueous solution (the concentration is 10 mg L 1) in volumetric flask (without bottle cork) and the mixture was stirred in the dark for 30 min in order to achieve adsorptiondesorption equilibrium between dye and photocatalyst, then the RhB aqueous solution was irradiated under solar light irradiation. The pH value of RhB aqueous solution was 7 adjusted with HClO4 (0.1 mol L 1) and sodium hydroxide (0.1 mol L 1) solution. At regular intervals, the samples were removed and centrifuged (5000 rpm) to separate the photocatalyst for analysis. The concentration of RhB was measured by a GBC UV/Vis 916 spectrophotometer at 554 nm and analyzed by Lambert-Beer law. All reported data were the average values of three parallel determinations.
2.2. Characterizations
3.1. Characterization of photocatalysts
The Brunauer–Emmett–Teller (BET) specific surface area and pore size measurement were performed on a SSA-4200 automatic surface analyzer (Builder, China). XRD patterns were recorded on a DX-2600 X-ray diffractometer using Cu Kα (λ ¼0.15406 nm) radiation equipped with a graphite monochromator. The X-ray tube was operated at 40 kV and 25 mA. SEM images were taken with a JSM7500F scanning electron microscope, using an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) images were obtained using a FEI F2 instrument at an accelerating voltage of 200 kV. The ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy was performed on a spectrometer (TU-1901) using barium sulfate as the reference. X-ray photoelection spectroscopy (XPS) measurement was carried out on a spectrometer (XSAM-800, KRATOS Co.) equipped with two ultrahigh-vacuum chambers, the pressure in the chambers during experiment was
The specific surface parameters of the photocatalysts are shown in Table 1. As shown in Table 1, the specific surface of SDBS-BiVO4 is 1.46 times than that of BiVO4. The pore volume increases from 0.0021 cc/g for BiVO4 to 0.0035 cc/g for SDBS-BiVO4, the pore size drops from 11.1 Å to 9.0 Å, accordingly. It is widely accepted that the photocatalytic process is mainly related to the adsorption and desorption of molecules on the surface of the catalyst. The surface active sites can be benefited from larger BET
2.3. Photocatalytic experiment
3. Results and discussion
Table 1 Specific surface parameter of photocatalysts. Photocatalyst
SBET (m2/g)
Pore volume(cc/g)
Pore size(Å)
BiVO4 SDBS-BiVO4
3.7 5.4
0.0021 0.0035
11.1 9.0
J. Zeng et al. / Materials Science in Semiconductor Processing 27 (2014) 41–46
surface area, thus the adsorbed reactive species have more chance to react with adsorbed organic pollutant [13], which can promote the photocatalytic performance. This result is consistent with the results of photocatalytic activity measurements. Powder X-ray diffraction (XRD) patterns of BiVO4 and SDBS-BiVO4 are shown in Fig. 1. Clear characteristic peaks with 2θ at 18.91, 28.91, 30.51, 35.21, 39.81 and 53.21 are observed, which are indexed to the standard card (JCPDS No.83-1700). These diffraction peaks indicate that the two samples are monoclinic scheelite structure with high crystallinity. The results show that, SDBS-BiVO4 and BiVO4 have the same diffraction peaks, and no other substances detected. However, the two samples are different in width of peaks; the peaks of SDBS-BiVO4 are wider than that of BiVO4, so the crystal size is smaller than BiVO4. This result can be further confirmed by the results of TEM results. The SEM and TEM images of photocatalysts prepared are shown in Fig. 2. Fig. 2(a and b) are the SEM images of the BiVO4 and SDBS-BiVO4 samples, indicating that they are composed of particles with the size ranging from 50 to 300 nm, and they are both irregular lump, pores are seldom. SDBS-BiVO4 samples are composed of even smaller primary nanocrystals. Fig. 2 c and d show that the BiVO4 particles are generally ca. 200 nm in size and the SDBS-BiVO4 particles are generally ca. 50 nm in size. And it is clearly seen that these SDBS-BiVO4 particles are composed of many primary crystals, which are confirmed by a higher magnification TEM image for SDBS-BiVO4 (Fig. 2(e)). So the average particles size of SDBS-BiVO4 is smaller than that of BiVO4; the small particles size can increase the specific surface area. SDBS is a kind of surfactant, which can reduce the surface tension of the solution and make the precursors of BiVO4 highly dispersive. Morphology of photocatalyst plays an important role in influencing the photocatalytic activity, which can be further confirmed by the result of photocatalytic activity. This result illustrates that the decreased crystal size leads to the BET surface area increase, which fits well with the result of BET surface area. As shown in Fig. 3, SDBS-BiVO4 enlarges the light absorption range. The indirect bandgap of the asprepared samples was measured by fitting to a plot of (Ahv)2 versus hv, where A is absorbance. The results are shown in Fig. 4. The indirect bandgap for SDBS-BiVO4 and BiVO4 is 2.23 eV and 2.38 eV, respectively. The results
Fig. 1. XRD patterns of photocatalysts.
43
indicate that the addition of SBDS improves the material absorption in the visible light range in the process of material preparation. It is now commonly recognized that the energy band structure feature of a semiconductor can greatly influence the photocatalytic activity. Nanocrystalline semiconductor under illumination with light of energy equal to or larger than the band gap produces electron– hole pairs, electrons in the conduction band (CB) and holes in the valence bands (VB). Moreover, the photo-induced electrons on oxygen vacancy states cannot recombine easily with photo-induced holes as the oxygen vacancies are active electron traps [14,15]. Active electron traps are positive factors in photocatalytic de-colorization of RhB. This result can be further confirmed by the results of SPS results. The X-ray photoelectron spectroscopy (XPS) was carried out to determine the surface chemical composition of the BiVO4 and SDBS-BiVO4 samples, and the valence states of various species are also present therein. As shown in Fig. 5, two XPS spectra reveal characteristic peaks for Bi, V, O and C. On SDBS-BiVO4 XPS spectrum no S was detected, indicating that SDBS was removed by washing. Fig. 6 shows the high-resolution XPS spectrum of the Bi 4f region on the surfaces of the two photocatalysts. Compared with BiVO4, the peaks for Bi 4f of SDBS-BiVO4 have no obvious difference. Fig. 7 shows the high resolution XPS spectra of the O1s region taken on the surface of the BiVO4 and SDBS-BiVO4. The O1s can be fit by their curves appearing at 529.9 and 531.6 eV, which can be attributed to V–O (529.9 eV) and O–H (531.6 eV) components [16]. The curve-fitting results of O1s XPS spectra for two photocatalysts (Fig. 7) have been listed in Table 2, where ri (%) represents the ratio of each kind of contribution to the total of the two kinds of oxygen contributions. As indicated in Table 2, the hydroxyl content on SDBS-BiVO4 is greater than on BiVO4. Usually, the increase of hydroxyl content on the surface of BiVO4 based photocatalyst is beneficial to the enhancement of photocatalytic activity [17,18]. The OH radicals produced during the photocatalytic process were detected and shown in Fig. 8. It is clear that an obvious PL signal at ca. 425 nm is observed, demonstrating that dOH radials are formed in the photocatalytic oxidation process. Furthermore, the PL intensity of SDBSBiVO4 is higher than that of BiVO4. Hydroxyl radical is an extremely strong, nonselective oxidant (E0 ¼ þ3.06 V) [19], which leads to the partial or complete mineralization of organic pollutants presenting at or near the surface of photocatalyst. High formation rate of dOH radicals is beneficial to the photocatalytic de-colorization of RhB. As shown in Fig. 9, BiVO4 and SDBS-BiVO4 display obvious SPS response from 300 nm to 540 nm. The SDBSBiVO4 shows stronger peaks than that of BiVO4. In general, the strong SPS response corresponds to the high separation rate of photo-induced charge carriers on the basis of the SPS principle [20,21]. The strong SPS signal in the visible region indicates that SDBS-BiVO4 has relative rich trapping states, namely, SDBS-BiVO4 has rich surface states. Surface net charge promotes the flow of electrons and holes in the opposite direction. This result demonstrates that the generated electron–hole pairs can be
44
J. Zeng et al. / Materials Science in Semiconductor Processing 27 (2014) 41–46
a
c
BiVO4
BiVO4
e
b
SDBS-BiVO4
d
SDBS-BiVO4
SDBS-BiVO4
Fig. 2. SEM (a and b) and TEM (c, d and e) images of photocatalysts.
Fig. 3. UV–vis diffuse reflectance spectra of photocatalysts.
Fig. 4. Relationship between (Ahv)2 and hv.
J. Zeng et al. / Materials Science in Semiconductor Processing 27 (2014) 41–46
separated effectively under the irradiation of visible light. Higher charge separation rate in the visible region is beneficial to the solar light driven photocatalytic performance. This result agrees well with the result of photocatalytic activity.
3.2. Photocatalytic activity
45
Table 2 Curve-fitting results of the high resolution XPS spectra for the O1s region. Photocatalysts
BiVO4 SDBS-BiVO4
O1s(V–O)
O1s (O–H)
Eb, eV
Ri, %
Eb, eV
Ri, %
529.9 529.9
90.44 81.17
531.6 531.6
9.56 18.83
Note: ri (%) represents the ratio Ai/ΣAi (Ai is the area of each peak).
The photocatalytic activity of BiVO4 and SDBS-BiVO4 is shown in Fig. 10. As shown in Fig. 10, the order of photocatalytic performance is SDBS-BiVO4 4BiVO4.
Fig. 8. PL spectral changes of different photocatalysts in TA solution. Fig. 5. XPS survey spectrum for the surface of two photocatalysts.
Fig. 6. High resolution Bi 4f region XPS spectra of samples BiVO4 and SDBS- BiVO4.
Fig. 9. Surface photovoltage response of the photocatalysts.
Fig. 7. High resolution XPS spectra of the O1s region on the surface of two photocatalysts.
46
J. Zeng et al. / Materials Science in Semiconductor Processing 27 (2014) 41–46
constant for RhB over BiVO4 and SDBS-BiVO4 is 0.0032 h 1 and 0.0241 h 1, respectively. The results show that the photocatalytic activity of SDBS-BiVO4 is 7.5 times that of the reference BiVO4.
Acknowledgments
Fig. 10. Variation of RhB concentration with time.
And by the blank test, light without a catalyst almost does not make RhB fade. The apparent decolorization rate constant for RhB over BiVO4 and SDBS-BiVO4 is 0.0032 h 1 and 0.0241 h 1, respectively. The results show that the photocatalytic activity of SDBS-BiVO4 is 7.5 times that of the reference BiVO4. Photocatalytic activity can be influenced by many factors in which specific surface area, the hydroxyl radical and charge separation efficiency are three key factors [22]. As mentioned before, SDBS-BiVO4 has larger specific surface area and small crystal size, hydroxyl radical higher and strong photovoltage response which can enhance the photocatalytic activity of the material. 4. Conclusions In summary, crystalline BiVO4 microcrystals were successfully obtained by surfactant-assisted hydrothermal method. SDBS-BiVO4 has high photocatalytic performance under sun light irradiation. XRD results revealed that the crystals were of the monoclinic scheelite type. Adding SDBS into the synthetic system did not change the crystal structure of the photocatalysts. BET results showed that the specific surface of SDBS-BiVO4 is 1.46 times than that of BiVO4. SEM and TEM images revealed that the average particles size of SDBS-BiVO4 is smaller than that of BiVO4. The band gap shifts from 2.38 eV to 2.23 eV by adding SDBS into the synthetic system. XPS and PL results all confirmed that the hydroxyl content on SDBS-BiVO4 was greater than on BiVO4. SPS results demonstrated that photo-generated carrier separation efficiency of BiVO4 was promoted. Adding SDBS into the synthetic system was the optimum condition for the preparation of BiVO4 photocatalysts characterized by the decolorization of RhB. The apparent decolorization rate
This project was supported financially by the program of Science and Technology Department of Sichuan province (No.2013JY0080), Research Fund Projects of Sichuan University of Science and Engineering (No.2011KY09), Construct Program of the Discipline in Sichuan University of Science, the Project of Zigong city (2013H02) and Engineering, and the Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (No.LYJ1101). References [1] A. Kudo, Int. J. Hydrog. Energy 31 (2006) 197–202. [2] B.P. Xie, H.X. Zhang, P.X. Cai, R.L. Qiu, Y. Xiong, Chemosphere 63 (2006) 956–963. [3] L. Zhang, D.R. Chen, X.L. Jiao, J. Phys. Chem. B 110 (2006) 2668–2673. [4] A. Kudo, K. Omori, H. Kato, J. Am. Chem. Soc. 121 (1999) 11459–11467. [5] S.S. Dunkle, R.J. Helmich, K.S. Suslick, J. Phys. Chem. C 113 (2009) 11980–11983. [6] A. Iwase, A. Kudo, J. Mater. Chem. 20 (2010) 7536–7542. [7] J.B. Zhong, J.Z. Li, F.M. Feng, Y. Lu, J. Zeng, W. Hu, Z. Tang, J. Mol. Catal. A: Chem. 357 (2012) 101–105. [8] J.B. Zhong, J.Z. Li, F.M. Feng, S.T. Huang, J. Zeng, Mater. Lett. 100 (2013) 195–197. [9] J.B. Zhong, J.Z. Li, Z.H. Xiao, W. Hu, X.B. Zhou, X.W. Zheng, Mater. Lett. 91 (2013) 301–303. [10] J.C. Mirsalis, C.K. Tyson, K.L. Steinmetz, E.K. Loh, C.M. Hamilton, J.P. Bakke, J.W. Spalding, Environ. Mol. Mutagen. 14 (1989) 155–164. [11] Q.D. Zhao, D.J. Wang, L.L. Peng, Y.H. Lin, M. Yang, T.F. Xie, Chem. Phys. Lett. 434 (2007) 96–100. [12] L.A. Gu, J.Y. Wang, H. Cheng, Y.Z. Zhao, L.F. Liu, X.J. Han, ACS Appl. Mater. Interfaces 5 (2013) 3085–3093. [13] H. Huang, D. Ye, X. Guan, Catal. Today 139 (2008) 43–48. [14] L.S. Cavalcante, J.C. Sczancoski, N.C. Batista, E. Longo, J.A. Varela, M.O. Orlandi, Adv. Powder Technol. 24 (2013) 344–353. [15] L.S. Cavalcante, F.M.C. Batista, M.A.P. Almeida, A.C. Rabelo, I.C. Nogueira, N.C. Batista, J.A. Varela, M.R.M.C. Santos, E. Longo, M. S. Li, RSC Adv. 2 (2012) 6438–6454. [16] J.C. Yu, J.G. Yu, J.C. Zhao, Appl. Catal. B: Environ. 36 (2002) 31–43. [17] J.G. Yu, X.J. Zhao, Mater. Res. Bull. 36 (2001) 97–107. [18] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [19] L. Mohapatra, K. Parida, M. Satpathy, J. Phys. Chem. C 116 (2012) 13063–13070. [20] L. Kronik, Y. Shapira, Surf. Sci. Rep. 37 (1999) 1–206. [21] L.Q. Jing, J. Wang, Y. Qu, Y. Luan, Appl. Surf. Sci. 256 (2009) 657–663. [22] H.M. Fan, D.J. Wang, L.L. Wang, H.Y. Li, P. Wang, T.F. Jiang, T.F. Xie, Appl. Surf. Sci. 257 (2011) 7758–7762.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Improvement of photocatalytic activity under solar light of BiVO4 microcrystals synthesized by surfactant-assisted hydrothermal method Jun Zeng n, Junbo Zhong, Jianzhang Li, Zhen Xiang, Xinlu Liu, Jiufu Chen 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
a r t i c l e i n f o
Keywords: Semiconductors BiVO4 Hydrothermal preparation Photocatalytic performance
abstract In this paper, we reported the obtention of bismuth vanadate (BiVO4) microcrystals synthesized by the surfactant-assisted hydrothermal method with sodium dodecylbenzenesulfonate (SDBS). The specific surface area, structure, morphology and the photoinduced charge separation efficiency of BiVO4 microcrystals were characterized by means of Brunauer–Emmett–Teller (BET) method, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and surface photovoltage spectroscopy (SPS), respectively. The results show that SDBS surfactant alters the surface parameters and the morphology, enhances the photo-induced charge separation efficiency of BiVO4 under solar light irradiation. The photocatalytic activity of catalysts for photocatalytic decolorization of Rhodamine B (RhB) aqueous solution under solar light irradiation was investigated. The results demonstrate that the photocatalytic activity for degradation of RhB relate to SDBS-BiVO4 microcrystals is 7.5 times that of the reference BiVO4 microcrystals and the underlying reason is suggested. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction The development of p-type semiconductor as photocatalyst related to bismuth vanadate (BiVO4) microcrystals attracted great attention of scientific community [1–3]. BiVO4 has three crystalline phases [4], which are tetragonal zircon, tetragonal scheelite and monoclinic scheelite, respectively. The band gap width of monoclinic BiVO4 is 2.4 eV, which can directly absorb solar light as energy, making this catalyst has been widely researched in recent years. However, the low separation efficiency of photo-generated
n
Corresponding author. Tel/fax: þ 86 813 5505601. E-mail address: [email protected] (J. Zeng).
http://dx.doi.org/10.1016/j.mssp.2014.06.014 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
carrier and low surface all limit the photocatalytic activity of BiVO4 crystals. Up to now, many synthesis methods such as sol–gel, solid-state, aqueous process, hydrolysis, sonochemical, and hydrothermal methods have been successfully applied to fabricate BiVO4 [5]. Among these synthesis approaches, hydrothermal method has remarkably attractive advantages: high purity, narrow size distribution, and low aggregation by adjusting the hydrothermal reaction conditions [6]. Sodium dodecylbenzenesulfonate (SDBS) is a neutral anionic surfactant. It is sensitive to water hardness, resistant to oxidation, foaming power, detergency high, and low cost, the synthesis process maturity, and therefore widely used. It is now generally accepted that adding surfactants into the preparation process will greatly
42
J. Zeng et al. / Materials Science in Semiconductor Processing 27 (2014) 41–46
influence the photocatalytic properties of materials [7]. The influence of surfactant on the preparation and photocatalytic activity TiO2 and ZnO have been reported [8,9]. However, the influence of SDBS on the fabrication and photocatalytic property of BiVO4 prepared by the hydrothermal method has been seldom concerned. Rhodamine B (RhB) is a water-soluble dye, rat studies illustrate that RhB can cause subcutaneous tissue sarcoma suspected to be a carcinogen [10]. Thus, safe removal of such a dye is the prime aim of our present research. In the present paper, RhB was chosen as the model dye. Therefore, in this paper, we reported the effect of SDBS anionic surfactant on fabrication and the solar light photocatalytic performance of BiVO4 microcrystals. Photocatalytic activity of BiVO4 microcrystals prepared was evaluated by degradation of RhB aqueous solution. 2. Experimental details 2.1. Surfactant-assisted hydrothermal synthesis of BiVO4 nanocrystals
about 10 7 Pa. A Mg Kα X-ray source was used. The XPS spectrum was referenced to the C1s peak (Eb ¼284.8 eV) resulting from adventitious hydrocarbon (i.e. from the XPS instrument itself) present on the sample surface. The surface photovoltage spectroscopy (SPS) measurements of the samples were carried out according to the procedure described in reference [11]. The level of OH radicals was detected according to the procedure given in reference with some modification [12]. 50 mg of photocatalyst was suspended in 50 mL of an aqueous solution containing 20 mM NaOH and 6 mM terephthalic acid (TA). The solution was stirred in the dark for 40 min before exposure to UV light. After illumination with a 500 W high-pressure mercury lamp for 20 min, the suspension was centrifuged, and the supernatant was sampled for analysis by recording the fluorescence signal of the generated 2-hydroxyterephthalic acid (TAOH) on a fluorescence spectrometer (Cary Eclips, Agilent, USA). The wavelength of the excitation light was 312 nm, and the scanning speed was 600 nmmin 1. The widths of the excitation slit and the emission slit were both 5 nm.
All chemicals (analytical grade reagents) were supplied from Chengdu Kelong Chemical Reagent Factory and used as received. All the studies were done using deionized water. In a typical synthesis, 8.50 g Bi(NO3)3 5H2O was dissolved in nitric solution (HNO3:H2O ¼1:8), 0.50 g SDBS was added to above-solution, forming solution A. 2.02 g NH4VO3 was dissolved in 50 mL water at 80 1C, resulting in solution B. A and B were mixed together to obtain a yellow suspension solution. pH of the suspension solution was adjusted to 7 using NH3 H2O and further stirred for 1 h, then the resulting precursor suspension was transferred into a Teflon-lined stainless steel autoclave with a capacity of 200 mL and maintained at 180 1C for 24 h, and subsequently cooled to room temperature naturally. The precipitate was collected by centrifugation, washed with water and ethanol for several times to remove ions and then dried in a vacuum oven at 80 1C for 10 h. BiVO4 was also prepared as the same procedure mentioned above without the presence of SDBS.
The photocatalytic activity of photocatalyst (50 mg) was evaluated by decolorization of 50 mL RhB aqueous solution (the concentration is 10 mg L 1) in volumetric flask (without bottle cork) and the mixture was stirred in the dark for 30 min in order to achieve adsorptiondesorption equilibrium between dye and photocatalyst, then the RhB aqueous solution was irradiated under solar light irradiation. The pH value of RhB aqueous solution was 7 adjusted with HClO4 (0.1 mol L 1) and sodium hydroxide (0.1 mol L 1) solution. At regular intervals, the samples were removed and centrifuged (5000 rpm) to separate the photocatalyst for analysis. The concentration of RhB was measured by a GBC UV/Vis 916 spectrophotometer at 554 nm and analyzed by Lambert-Beer law. All reported data were the average values of three parallel determinations.
2.2. Characterizations
3.1. Characterization of photocatalysts
The Brunauer–Emmett–Teller (BET) specific surface area and pore size measurement were performed on a SSA-4200 automatic surface analyzer (Builder, China). XRD patterns were recorded on a DX-2600 X-ray diffractometer using Cu Kα (λ ¼0.15406 nm) radiation equipped with a graphite monochromator. The X-ray tube was operated at 40 kV and 25 mA. SEM images were taken with a JSM7500F scanning electron microscope, using an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) images were obtained using a FEI F2 instrument at an accelerating voltage of 200 kV. The ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy was performed on a spectrometer (TU-1901) using barium sulfate as the reference. X-ray photoelection spectroscopy (XPS) measurement was carried out on a spectrometer (XSAM-800, KRATOS Co.) equipped with two ultrahigh-vacuum chambers, the pressure in the chambers during experiment was
The specific surface parameters of the photocatalysts are shown in Table 1. As shown in Table 1, the specific surface of SDBS-BiVO4 is 1.46 times than that of BiVO4. The pore volume increases from 0.0021 cc/g for BiVO4 to 0.0035 cc/g for SDBS-BiVO4, the pore size drops from 11.1 Å to 9.0 Å, accordingly. It is widely accepted that the photocatalytic process is mainly related to the adsorption and desorption of molecules on the surface of the catalyst. The surface active sites can be benefited from larger BET
2.3. Photocatalytic experiment
3. Results and discussion
Table 1 Specific surface parameter of photocatalysts. Photocatalyst
SBET (m2/g)
Pore volume(cc/g)
Pore size(Å)
BiVO4 SDBS-BiVO4
3.7 5.4
0.0021 0.0035
11.1 9.0
J. Zeng et al. / Materials Science in Semiconductor Processing 27 (2014) 41–46
surface area, thus the adsorbed reactive species have more chance to react with adsorbed organic pollutant [13], which can promote the photocatalytic performance. This result is consistent with the results of photocatalytic activity measurements. Powder X-ray diffraction (XRD) patterns of BiVO4 and SDBS-BiVO4 are shown in Fig. 1. Clear characteristic peaks with 2θ at 18.91, 28.91, 30.51, 35.21, 39.81 and 53.21 are observed, which are indexed to the standard card (JCPDS No.83-1700). These diffraction peaks indicate that the two samples are monoclinic scheelite structure with high crystallinity. The results show that, SDBS-BiVO4 and BiVO4 have the same diffraction peaks, and no other substances detected. However, the two samples are different in width of peaks; the peaks of SDBS-BiVO4 are wider than that of BiVO4, so the crystal size is smaller than BiVO4. This result can be further confirmed by the results of TEM results. The SEM and TEM images of photocatalysts prepared are shown in Fig. 2. Fig. 2(a and b) are the SEM images of the BiVO4 and SDBS-BiVO4 samples, indicating that they are composed of particles with the size ranging from 50 to 300 nm, and they are both irregular lump, pores are seldom. SDBS-BiVO4 samples are composed of even smaller primary nanocrystals. Fig. 2 c and d show that the BiVO4 particles are generally ca. 200 nm in size and the SDBS-BiVO4 particles are generally ca. 50 nm in size. And it is clearly seen that these SDBS-BiVO4 particles are composed of many primary crystals, which are confirmed by a higher magnification TEM image for SDBS-BiVO4 (Fig. 2(e)). So the average particles size of SDBS-BiVO4 is smaller than that of BiVO4; the small particles size can increase the specific surface area. SDBS is a kind of surfactant, which can reduce the surface tension of the solution and make the precursors of BiVO4 highly dispersive. Morphology of photocatalyst plays an important role in influencing the photocatalytic activity, which can be further confirmed by the result of photocatalytic activity. This result illustrates that the decreased crystal size leads to the BET surface area increase, which fits well with the result of BET surface area. As shown in Fig. 3, SDBS-BiVO4 enlarges the light absorption range. The indirect bandgap of the asprepared samples was measured by fitting to a plot of (Ahv)2 versus hv, where A is absorbance. The results are shown in Fig. 4. The indirect bandgap for SDBS-BiVO4 and BiVO4 is 2.23 eV and 2.38 eV, respectively. The results
Fig. 1. XRD patterns of photocatalysts.
43
indicate that the addition of SBDS improves the material absorption in the visible light range in the process of material preparation. It is now commonly recognized that the energy band structure feature of a semiconductor can greatly influence the photocatalytic activity. Nanocrystalline semiconductor under illumination with light of energy equal to or larger than the band gap produces electron– hole pairs, electrons in the conduction band (CB) and holes in the valence bands (VB). Moreover, the photo-induced electrons on oxygen vacancy states cannot recombine easily with photo-induced holes as the oxygen vacancies are active electron traps [14,15]. Active electron traps are positive factors in photocatalytic de-colorization of RhB. This result can be further confirmed by the results of SPS results. The X-ray photoelectron spectroscopy (XPS) was carried out to determine the surface chemical composition of the BiVO4 and SDBS-BiVO4 samples, and the valence states of various species are also present therein. As shown in Fig. 5, two XPS spectra reveal characteristic peaks for Bi, V, O and C. On SDBS-BiVO4 XPS spectrum no S was detected, indicating that SDBS was removed by washing. Fig. 6 shows the high-resolution XPS spectrum of the Bi 4f region on the surfaces of the two photocatalysts. Compared with BiVO4, the peaks for Bi 4f of SDBS-BiVO4 have no obvious difference. Fig. 7 shows the high resolution XPS spectra of the O1s region taken on the surface of the BiVO4 and SDBS-BiVO4. The O1s can be fit by their curves appearing at 529.9 and 531.6 eV, which can be attributed to V–O (529.9 eV) and O–H (531.6 eV) components [16]. The curve-fitting results of O1s XPS spectra for two photocatalysts (Fig. 7) have been listed in Table 2, where ri (%) represents the ratio of each kind of contribution to the total of the two kinds of oxygen contributions. As indicated in Table 2, the hydroxyl content on SDBS-BiVO4 is greater than on BiVO4. Usually, the increase of hydroxyl content on the surface of BiVO4 based photocatalyst is beneficial to the enhancement of photocatalytic activity [17,18]. The OH radicals produced during the photocatalytic process were detected and shown in Fig. 8. It is clear that an obvious PL signal at ca. 425 nm is observed, demonstrating that dOH radials are formed in the photocatalytic oxidation process. Furthermore, the PL intensity of SDBSBiVO4 is higher than that of BiVO4. Hydroxyl radical is an extremely strong, nonselective oxidant (E0 ¼ þ3.06 V) [19], which leads to the partial or complete mineralization of organic pollutants presenting at or near the surface of photocatalyst. High formation rate of dOH radicals is beneficial to the photocatalytic de-colorization of RhB. As shown in Fig. 9, BiVO4 and SDBS-BiVO4 display obvious SPS response from 300 nm to 540 nm. The SDBSBiVO4 shows stronger peaks than that of BiVO4. In general, the strong SPS response corresponds to the high separation rate of photo-induced charge carriers on the basis of the SPS principle [20,21]. The strong SPS signal in the visible region indicates that SDBS-BiVO4 has relative rich trapping states, namely, SDBS-BiVO4 has rich surface states. Surface net charge promotes the flow of electrons and holes in the opposite direction. This result demonstrates that the generated electron–hole pairs can be
44
J. Zeng et al. / Materials Science in Semiconductor Processing 27 (2014) 41–46
a
c
BiVO4
BiVO4
e
b
SDBS-BiVO4
d
SDBS-BiVO4
SDBS-BiVO4
Fig. 2. SEM (a and b) and TEM (c, d and e) images of photocatalysts.
Fig. 3. UV–vis diffuse reflectance spectra of photocatalysts.
Fig. 4. Relationship between (Ahv)2 and hv.
J. Zeng et al. / Materials Science in Semiconductor Processing 27 (2014) 41–46
separated effectively under the irradiation of visible light. Higher charge separation rate in the visible region is beneficial to the solar light driven photocatalytic performance. This result agrees well with the result of photocatalytic activity.
3.2. Photocatalytic activity
45
Table 2 Curve-fitting results of the high resolution XPS spectra for the O1s region. Photocatalysts
BiVO4 SDBS-BiVO4
O1s(V–O)
O1s (O–H)
Eb, eV
Ri, %
Eb, eV
Ri, %
529.9 529.9
90.44 81.17
531.6 531.6
9.56 18.83
Note: ri (%) represents the ratio Ai/ΣAi (Ai is the area of each peak).
The photocatalytic activity of BiVO4 and SDBS-BiVO4 is shown in Fig. 10. As shown in Fig. 10, the order of photocatalytic performance is SDBS-BiVO4 4BiVO4.
Fig. 8. PL spectral changes of different photocatalysts in TA solution. Fig. 5. XPS survey spectrum for the surface of two photocatalysts.
Fig. 6. High resolution Bi 4f region XPS spectra of samples BiVO4 and SDBS- BiVO4.
Fig. 9. Surface photovoltage response of the photocatalysts.
Fig. 7. High resolution XPS spectra of the O1s region on the surface of two photocatalysts.
46
J. Zeng et al. / Materials Science in Semiconductor Processing 27 (2014) 41–46
constant for RhB over BiVO4 and SDBS-BiVO4 is 0.0032 h 1 and 0.0241 h 1, respectively. The results show that the photocatalytic activity of SDBS-BiVO4 is 7.5 times that of the reference BiVO4.
Acknowledgments
Fig. 10. Variation of RhB concentration with time.
And by the blank test, light without a catalyst almost does not make RhB fade. The apparent decolorization rate constant for RhB over BiVO4 and SDBS-BiVO4 is 0.0032 h 1 and 0.0241 h 1, respectively. The results show that the photocatalytic activity of SDBS-BiVO4 is 7.5 times that of the reference BiVO4. Photocatalytic activity can be influenced by many factors in which specific surface area, the hydroxyl radical and charge separation efficiency are three key factors [22]. As mentioned before, SDBS-BiVO4 has larger specific surface area and small crystal size, hydroxyl radical higher and strong photovoltage response which can enhance the photocatalytic activity of the material. 4. Conclusions In summary, crystalline BiVO4 microcrystals were successfully obtained by surfactant-assisted hydrothermal method. SDBS-BiVO4 has high photocatalytic performance under sun light irradiation. XRD results revealed that the crystals were of the monoclinic scheelite type. Adding SDBS into the synthetic system did not change the crystal structure of the photocatalysts. BET results showed that the specific surface of SDBS-BiVO4 is 1.46 times than that of BiVO4. SEM and TEM images revealed that the average particles size of SDBS-BiVO4 is smaller than that of BiVO4. The band gap shifts from 2.38 eV to 2.23 eV by adding SDBS into the synthetic system. XPS and PL results all confirmed that the hydroxyl content on SDBS-BiVO4 was greater than on BiVO4. SPS results demonstrated that photo-generated carrier separation efficiency of BiVO4 was promoted. Adding SDBS into the synthetic system was the optimum condition for the preparation of BiVO4 photocatalysts characterized by the decolorization of RhB. The apparent decolorization rate
This project was supported financially by the program of Science and Technology Department of Sichuan province (No.2013JY0080), Research Fund Projects of Sichuan University of Science and Engineering (No.2011KY09), Construct Program of the Discipline in Sichuan University of Science, the Project of Zigong city (2013H02) and Engineering, and the Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (No.LYJ1101). References [1] A. Kudo, Int. J. Hydrog. Energy 31 (2006) 197–202. [2] B.P. Xie, H.X. Zhang, P.X. Cai, R.L. Qiu, Y. Xiong, Chemosphere 63 (2006) 956–963. [3] L. Zhang, D.R. Chen, X.L. Jiao, J. Phys. Chem. B 110 (2006) 2668–2673. [4] A. Kudo, K. Omori, H. Kato, J. Am. Chem. Soc. 121 (1999) 11459–11467. [5] S.S. Dunkle, R.J. Helmich, K.S. Suslick, J. Phys. Chem. C 113 (2009) 11980–11983. [6] A. Iwase, A. Kudo, J. Mater. Chem. 20 (2010) 7536–7542. [7] J.B. Zhong, J.Z. Li, F.M. Feng, Y. Lu, J. Zeng, W. Hu, Z. Tang, J. Mol. Catal. A: Chem. 357 (2012) 101–105. [8] J.B. Zhong, J.Z. Li, F.M. Feng, S.T. Huang, J. Zeng, Mater. Lett. 100 (2013) 195–197. [9] J.B. Zhong, J.Z. Li, Z.H. Xiao, W. Hu, X.B. Zhou, X.W. Zheng, Mater. Lett. 91 (2013) 301–303. [10] J.C. Mirsalis, C.K. Tyson, K.L. Steinmetz, E.K. Loh, C.M. Hamilton, J.P. Bakke, J.W. Spalding, Environ. Mol. Mutagen. 14 (1989) 155–164. [11] Q.D. Zhao, D.J. Wang, L.L. Peng, Y.H. Lin, M. Yang, T.F. Xie, Chem. Phys. Lett. 434 (2007) 96–100. [12] L.A. Gu, J.Y. Wang, H. Cheng, Y.Z. Zhao, L.F. Liu, X.J. Han, ACS Appl. Mater. Interfaces 5 (2013) 3085–3093. [13] H. Huang, D. Ye, X. Guan, Catal. Today 139 (2008) 43–48. [14] L.S. Cavalcante, J.C. Sczancoski, N.C. Batista, E. Longo, J.A. Varela, M.O. Orlandi, Adv. Powder Technol. 24 (2013) 344–353. [15] L.S. Cavalcante, F.M.C. Batista, M.A.P. Almeida, A.C. Rabelo, I.C. Nogueira, N.C. Batista, J.A. Varela, M.R.M.C. Santos, E. Longo, M. S. Li, RSC Adv. 2 (2012) 6438–6454. [16] J.C. Yu, J.G. Yu, J.C. Zhao, Appl. Catal. B: Environ. 36 (2002) 31–43. [17] J.G. Yu, X.J. Zhao, Mater. Res. Bull. 36 (2001) 97–107. [18] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [19] L. Mohapatra, K. Parida, M. Satpathy, J. Phys. Chem. C 116 (2012) 13063–13070. [20] L. Kronik, Y. Shapira, Surf. Sci. Rep. 37 (1999) 1–206. [21] L.Q. Jing, J. Wang, Y. Qu, Y. Luan, Appl. Surf. Sci. 256 (2009) 657–663. [22] H.M. Fan, D.J. Wang, L.L. Wang, H.Y. Li, P. Wang, T.F. Jiang, T.F. Xie, Appl. Surf. Sci. 257 (2011) 7758–7762.