- Email: [email protected]
Bi2O3 heterojunction for efficient visible-light-driven photocatalysis
Materials Letters 185 (2016) 189–192
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of novel Au/FeVO4/Bi2O3 heterojunction for efficient visiblelight-driven photocatalysis Xiao Liu, Yong Kang n, Dian Luo School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300350 China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 July 2016 Received in revised form 23 August 2016 Accepted 27 August 2016 Available online 29 August 2016
In this paper, a novel Au/FeVO4/Bi2O3 photocatalyst was successfully synthesized by the precipitation and photodeposition method. The obtained photocatalysts were characterized by several techniques, such as X-ray powder diffraction, transmission electron microscopy, UV–vis absorption spectra and photoluminescence. The transmission electron microscopy showed that the Au nanoparticles were finally dispersed on the surface of FeVO4/Bi2O3 heterojunction. The enhanced absorption in visible light region and the efficient photogenerated electron-hole pairs separation were achieved after loading of Au and FeVO4 particles on Bi2O3 photocatalysts, as revealed by UV–vis absorption spectra and photoluminescence measurement. The photocatalytic examination of the photocatalyst was carried out through decomposition of malachite green under visible-light irradiation. The Au/FeVO4/Bi2O3 photocatalyst displayed superior photocatalytic activity for degradation of malachite green. Additionally, a possible catalytic mechanism for Au/FeVO4/Bi2O3 photocatalyst was proposed. & 2016 Elsevier B.V. All rights reserved.
Keywords: Au/FeVO4/Bi2O3 Heterojunction Nanoparticles Photocatalyst Semiconductors
1. Introduction It is well-known that water pollution is one of the critical issues in the world [1]. The semiconductor photocatalysts have attracted more interest for solving water pollution crisis by the utilization of solar energy [2]. Considering that about 50% of the sunlight is visible light, the visible-light-driven photocatalysts have been studied by a lot of researchers. Among these semiconductors, bismuth oxide (Bi2O3) has received much attention because of its unique chemical stability, non-toxicity, low cost, environmentally friendly and well catalytic activity [3]. However, single Bi2O3 shows a low photocatalytic efficiency since it has the fast recombination of photogenerated electron-hole pairs. Using semiconductors or metals compound with Bi2O3 can inhibit the separation rate of photogenerated electron-hole pairs in order to enhance the photocatalytic activity. Noble-metal, typically Au, can extend light absorption in the visible light region due to the localized surface plasmon resonance and retard recombination of photogenerated electron-hole pairs, which has beneficial efforts on improvement of photocatalytic activity [4]. For example, Lin et al. [5] loaded Bi2O3 with Au by depositionprecipitation method and the results showed the highest photocatalytic activity was reached by 1.0 wt% Au/Bi2O3. Besides, some efforts like construction heterojunction have been made for n
Corresponding author. E-mail address: [email protected] (Y. Kang).
http://dx.doi.org/10.1016/j.matlet.2016.08.141 0167-577X/& 2016 Elsevier B.V. All rights reserved.
improvement of the photocatalytic activity, such as SrTiO3/Bi2O3 [6] and Bi2O3/NaBiO3 [7]. In recent years, FeVO4 catalysts were used to treat various organic pollutants, such as removal of phenol from industrial waste water. In addition, as a new semiconductor, FeVO4 with a direct band gap of about 2.05 eV can compound with Bi2O3 for forming their heterojunction, resulting in more efficient separation of the photoinduced charges [8]. Based on the above analysis, we used both Au and FeVO4 nanoparticles to modify Bi2O3 to further improve the separation efficiency of electron-hole pairs. The photocatalytic ability of the novel Au/FeVO4/Bi2O3 sample was investigated by degradation of malachite green (MG) under visible light irradiation and the possible catalytic mechanism of Au/FeVO4/Bi2O3 photocatalyst was also proposed in detail.
2. Experimental The FeVO4 and Bi2O3 nanoparticles were prepared according to the method which was reported by our previous study [9]. For incorporating FeVO4 to Bi2O3 nanoparticles, 0.5 g Bi2O3 precursor and 0.5 g FeVO4 precursor were mixed in mortar and ground for 30 min. Then the above mixture was washed with deionized water and ethanol for three times, and dried at 80 °C for 24 h before calcined at 600 °C for 4 h. Then, Au nanoparticles were obtained on the surface of FeVO4/Bi2O3 heterojunction by photodeposition technique. 100 mg FeVO4/Bi2O3 heterojunction powders were suspended in 100 mL ethanol under ultrasonic dispersion for
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20 min. Then 1045 uL HAuCl4 solution (10 mg/mL) was added into the suspension. After that, the suspension was irradiated with a 500 W Xe lamp for 60 min under magnetic stirring. Finally, the resultant Au/FeVO4/Bi2O3 were centrifuged, washed with water four times, and dried at 60 °C for 6 h. The proportion of the content of Au, Bi2O3 and FeVO4 in the Au/FeVO4/Bi2O3 composite was 1:10:10. X-ray diffraction (XRD) measurement was characterized by Rigaku D/max 2500 X-ray diffractometer with Cu-Kα radiation. Photoluminescence (PL) spectroscopy was conducted with a fluorescence spectrometer (Jobin Yvon Fluorolog 3-21) under 350 nm excitation wavelength and the light source is a 450 W Xe lamp. The UV–vis absorption spectrum was recorded on an UV–vis spectrometer (PerkinElmer Lambda 750S). Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) analyses were performed with a Tecnai G2 F20
electron microscopy. The photocatalytic activity of Au/FeVO4/Bi2O3 photocatalyst was evaluated by degradation of MG dye under visible-light irradiation. 50 mg catalysts were suspended in 100 mL MG solution (25 mg/L). Prior to irradiation, the mixture was magnetically stirred for 60 min to reach the adsorption-desorption equilibrium between the photocatalysts and dyes in dark. The visible-light was obtained from a 500 W Xe lamp with a cut off filter (λ Z400 nm). 3 mL of solution was taken out in certain intervals and centrifuged during the process.
3. Results and discussion The XRD patterns of the samples were shown in Fig. 1. The Bi2O3 sample (Fig. 1(a)) showed the intense diffraction peaks at
Fig. 1. (a) XRD pattern of Bi2O3 (b) XRD pattern of FeVO4 (c) XRD pattern of FeVO4/Bi2O3 (d) XRD pattern of Au/FeVO4/Bi2O3 (e) TEM, (f)(g) HRTEM and (h) EDX of Au/FeVO4/ Bi2O3.
X. Liu et al. / Materials Letters 185 (2016) 189–192
27.04° and 33.1° corresponded to the (120) and (020) crystal planes of Bi2O3 (PDF #41-1449), respectively. The FeVO4 sample (Fig. 1(b)) presented that all the peaks could be indexed to the FeVO4 (PDF #38-1372). The results suggested that both Bi2O3 and FeVO4 had a high crystallinity. The calcined FeVO4/Bi2O3 composites (Fig. 1(c)) exhibited a co-existence of Bi2O3 (PDF #41-1449) and FeVO4 (PDF #38-1372), indicating the presence of Bi2O3 and FeVO4 particles. As shown in Fig. 1(d), the XRD pattern of Au/ FeVO4/Bi2O3 was very similar to that of FeVO4/Bi2O3 and the weak peak appeared around 38.18° suggested that the Au phase with the plane (111) was formed in the Au/FeVO4/Bi2O3 photocatalysts. In addition, no characteristic peaks of impurities were detected, demonstrating that the Au/FeVO4/Bi2O3 composites were composed of Au, FeVO4 and Bi2O3 successfully. The morphology of Au/FeVO4/ Bi2O3 was further explored using TEM and HRTEM. As observed in the TEM image (Fig. 1(e)), the Au nanoparticles were deposited over the surface of the FeVO4/Bi2O3 photocatalysts. The HRTEM image (Fig. 1(f)) showed the well-defined lattice spacings measured to be 0.408 and 0.333 nm, which corresponded to the spacings of the α-Bi2O3 (020) plane and the FeVO4 (210) plane, respectively. The result was in good agreement with the XRD results and further confirmed that α-Bi2O3 and FeVO4 were closely integrated. From the HRTEM image (Fig. 1(g)), the lattice fringe of 0.236 nm was consistent with the (111) crystal face of Au nanoparticle. These results suggested that the well interfacial contact existed in the Au/FeVO4/Bi2O3 samples, which was beneficial for electrons migration, thereby improving the charge separation. The EDX characterization (Fig. 1(h)) proved that Au nanoparticle existed in the sample. The UV–vis absorption spectra of the heterojunction samples were illustrated in Fig. 2(a). Compared with pure FeVO4, Bi2O3 and FeVO4/Bi2O3, Au/FeVO4/Bi2O3 catalysts exhibited clearly increased photoabsorption and red-shifted of the absorption edge in the visible light region, suggesting that it could efficiently utilize the visible light for contaminant decomposition under visible-light irradiation. The enhanced absorption ability of Au/FeVO4/Bi2O3 was mainly due to the surface plasmon resonance of Au nanoparticles and the enhanced absorption in the visible region was beneficial to improve their photocatalytic activities under visible light irradiation [4]. The estimated band gap energies are 2.86, 2.26, 2.08 and 1.88 eV for Bi2O3, FeVO4, FeVO4/Bi2O3 and Au/ FeVO4/Bi2O3, respectively. The photoluminescence spectra reveal the charge carriers migration, transfer, trapping and recombination in semiconductors. The emission spectrum of FeVO4 and Bi2O3 emitted an emission peak centered at the wavelength positions of 450 nm and 440 nm, respectively (Fig. 2(b)). It was noteworthy that FeVO4/Bi2O3 catalyst had a remarkably weaker emission, indicating the recombination of photogenerated electron-hole pairs was inhibited by the heterojunction structure effectively. However, the PL emission intensities of the Au/FeVO4/Bi2O3 sample further decreased after loading Au nanoparticles, suggesting that the efficient charge separation was attributed to photogenerated electrons transferred from the conduction band of Bi2O3 and FeVO4 to Au nanoparticles. Therefore, the photocatalytic ability of Au/ FeVO4/Bi2O3 catalyst could be effectively improved resulting from the inhibition of electrons and holes. As shown in Fig. 3(a), the photocatalytic performance of the prepared samples was investigated by degradation of MG dye. It could be seen that only about 57% and 65% MG degradation were reached in the presence of pure FeVO4 and Bi2O3, respectively. By contrast, nearly 92% of MG had been decomposed over the Au/ FeVO4/Bi2O3 photocatalysts, which showed the higher photocatalytic activity than the FeVO4/Bi2O3 composite. Cheng et al. reported that the Bi2O3/NaBiO3 catalysts could degrade 94% of MG (10 mg/L) [7]. However, Au/FeVO4/Bi2O3 photocatalysts presented better catalytic performance than the Bi2O3/NaBiO3 catalysts.
191
Fig. 2. UV–vis absorption spectra (a) (the inset shows (Ahν)2 versus hν plots of samples) and PL emission spectra (b) of different photocatalysts.
Because the enhancement of photocatalytic activity by Au/FeVO4/ Bi2O3 photocatalysts could be ascribed to the following two factors. On the one hand, the heterojunction structure formed between FeVO4 and Bi2O3 could effective separate the photogenerated carriers and the doping of Au nanoparticles on the FeVO4/Bi2O3 surfaces further facilitated the inhibition of electronhole pairs recombination. On the other hand, the enhanced visible light absorption by the modification of Au nanoparticles on the FeVO4/Bi2O3 promoted production of photoinduced electrons and holes, which led to higher photocatalytic activity for degradation of MG. A possible mechanism for photocatalytic reaction of the Au/ FeVO4/Bi2O3 had been represented in Fig. 3(b). Both FeVO4 and Bi2O3 semiconductors could be excited by the visible-light and the electron-hole pairs generated under the visible light irradiation. Simultaneously, the photogenerated electrons from the conduction band (CB) of FeVO4 could transfer to that of Bi2O3 while the holes could migrate from the valence band (VB) of Bi2O3 to that of FeVO4. Since the Fermi level of Au (Ef ¼ þ0.45 V vs. NHE) was lower than the CB of FeVO4 ( 0.58 V vs. NHE) and Bi2O3 (þ0.33 V vs. NHE), the excited electrons could inject from the CB of FeVO4 and Bi2O3 to Au nanoparticles and then were trapped by O2 to form O2 species, which could be further produced H2O2 by reacting with two protons [5]. Meanwhile, the left holes could adsorb water molecule to form hydroxyl radicals ( OH), which was responsible for photocatalytic reaction. Therefore, the Au/FeVO4/
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electron-hole pairs and exhibited superior activity for degradation of organic pollutants.
4. Conclusions In summary, the Au/FeVO4/Bi2O3 photocatalyst was successfully synthesized via the precipitation and photodeposition method. The results showed that the Au/FeVO4/Bi2O3 composites exhibited the highest photocatalytic activity for degradation of MG under visible light irradiation. The enhanced photocatalytic performance could be mainly attributed to the excellent visible light absorption ability and decreased recombination of the electron-hole pairs. In general, the Au/FeVO4/Bi2O3 heterojunction was an excellent photocatalyst which could be expected to apply in water pollution purification.
References [1] A.A. Al-Kahtani, M.F. Abou Taleb, J. Hazard. Mater. 309 (2016) 10–19. [2] G.L. He, Y.H. Zhong, M.J. Chen, X. Li, Y.P. Fang, Y.H. Xu, J. Mol. Catal. A – Chem. 423 (2016) 70–76. [3] Y. Huang, Y. Zheng, H. Zhu, J.J. Wang, J. Solid State Chem. 239 (2016) 274–281. [4] J. Jiang, J. Yu, S. Cao, J. Colloid Interface Sci. 461 (2016) 56–63. [5] H.Y. Jiang, K. Cheng, J. Lin, Phys. Chem. Chem. Phys. 14 (2012) 12114–12121. [6] Z.L. Zhang, J.C. Zhu, S.J. Li, Y.L. Mao, J. Solid State Chem. 211 (2014) 120–123. [7] L. Cheng, Y. Kang, Mater. Lett. 97 (2013) 125–128. [8] S.K. Biswas, J.O. Baeg, Int. J. Hydrog. 38 (2013) 14451–14457. [9] X. Liu, Y. Kang, Mater. Lett. 164 (2016) 229–231.
Fig. 3. Degradation of MG by different photocatalysts (a) and Mechanism for the possible charge separation process by Au/FeVO4/Bi2O3 composites (b).
Bi2O3 structure could efficiently separate the photogenerated
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of novel Au/FeVO4/Bi2O3 heterojunction for efficient visiblelight-driven photocatalysis Xiao Liu, Yong Kang n, Dian Luo School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300350 China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 July 2016 Received in revised form 23 August 2016 Accepted 27 August 2016 Available online 29 August 2016
In this paper, a novel Au/FeVO4/Bi2O3 photocatalyst was successfully synthesized by the precipitation and photodeposition method. The obtained photocatalysts were characterized by several techniques, such as X-ray powder diffraction, transmission electron microscopy, UV–vis absorption spectra and photoluminescence. The transmission electron microscopy showed that the Au nanoparticles were finally dispersed on the surface of FeVO4/Bi2O3 heterojunction. The enhanced absorption in visible light region and the efficient photogenerated electron-hole pairs separation were achieved after loading of Au and FeVO4 particles on Bi2O3 photocatalysts, as revealed by UV–vis absorption spectra and photoluminescence measurement. The photocatalytic examination of the photocatalyst was carried out through decomposition of malachite green under visible-light irradiation. The Au/FeVO4/Bi2O3 photocatalyst displayed superior photocatalytic activity for degradation of malachite green. Additionally, a possible catalytic mechanism for Au/FeVO4/Bi2O3 photocatalyst was proposed. & 2016 Elsevier B.V. All rights reserved.
Keywords: Au/FeVO4/Bi2O3 Heterojunction Nanoparticles Photocatalyst Semiconductors
1. Introduction It is well-known that water pollution is one of the critical issues in the world [1]. The semiconductor photocatalysts have attracted more interest for solving water pollution crisis by the utilization of solar energy [2]. Considering that about 50% of the sunlight is visible light, the visible-light-driven photocatalysts have been studied by a lot of researchers. Among these semiconductors, bismuth oxide (Bi2O3) has received much attention because of its unique chemical stability, non-toxicity, low cost, environmentally friendly and well catalytic activity [3]. However, single Bi2O3 shows a low photocatalytic efficiency since it has the fast recombination of photogenerated electron-hole pairs. Using semiconductors or metals compound with Bi2O3 can inhibit the separation rate of photogenerated electron-hole pairs in order to enhance the photocatalytic activity. Noble-metal, typically Au, can extend light absorption in the visible light region due to the localized surface plasmon resonance and retard recombination of photogenerated electron-hole pairs, which has beneficial efforts on improvement of photocatalytic activity [4]. For example, Lin et al. [5] loaded Bi2O3 with Au by depositionprecipitation method and the results showed the highest photocatalytic activity was reached by 1.0 wt% Au/Bi2O3. Besides, some efforts like construction heterojunction have been made for n
Corresponding author. E-mail address: [email protected] (Y. Kang).
http://dx.doi.org/10.1016/j.matlet.2016.08.141 0167-577X/& 2016 Elsevier B.V. All rights reserved.
improvement of the photocatalytic activity, such as SrTiO3/Bi2O3 [6] and Bi2O3/NaBiO3 [7]. In recent years, FeVO4 catalysts were used to treat various organic pollutants, such as removal of phenol from industrial waste water. In addition, as a new semiconductor, FeVO4 with a direct band gap of about 2.05 eV can compound with Bi2O3 for forming their heterojunction, resulting in more efficient separation of the photoinduced charges [8]. Based on the above analysis, we used both Au and FeVO4 nanoparticles to modify Bi2O3 to further improve the separation efficiency of electron-hole pairs. The photocatalytic ability of the novel Au/FeVO4/Bi2O3 sample was investigated by degradation of malachite green (MG) under visible light irradiation and the possible catalytic mechanism of Au/FeVO4/Bi2O3 photocatalyst was also proposed in detail.
2. Experimental The FeVO4 and Bi2O3 nanoparticles were prepared according to the method which was reported by our previous study [9]. For incorporating FeVO4 to Bi2O3 nanoparticles, 0.5 g Bi2O3 precursor and 0.5 g FeVO4 precursor were mixed in mortar and ground for 30 min. Then the above mixture was washed with deionized water and ethanol for three times, and dried at 80 °C for 24 h before calcined at 600 °C for 4 h. Then, Au nanoparticles were obtained on the surface of FeVO4/Bi2O3 heterojunction by photodeposition technique. 100 mg FeVO4/Bi2O3 heterojunction powders were suspended in 100 mL ethanol under ultrasonic dispersion for
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X. Liu et al. / Materials Letters 185 (2016) 189–192
20 min. Then 1045 uL HAuCl4 solution (10 mg/mL) was added into the suspension. After that, the suspension was irradiated with a 500 W Xe lamp for 60 min under magnetic stirring. Finally, the resultant Au/FeVO4/Bi2O3 were centrifuged, washed with water four times, and dried at 60 °C for 6 h. The proportion of the content of Au, Bi2O3 and FeVO4 in the Au/FeVO4/Bi2O3 composite was 1:10:10. X-ray diffraction (XRD) measurement was characterized by Rigaku D/max 2500 X-ray diffractometer with Cu-Kα radiation. Photoluminescence (PL) spectroscopy was conducted with a fluorescence spectrometer (Jobin Yvon Fluorolog 3-21) under 350 nm excitation wavelength and the light source is a 450 W Xe lamp. The UV–vis absorption spectrum was recorded on an UV–vis spectrometer (PerkinElmer Lambda 750S). Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) analyses were performed with a Tecnai G2 F20
electron microscopy. The photocatalytic activity of Au/FeVO4/Bi2O3 photocatalyst was evaluated by degradation of MG dye under visible-light irradiation. 50 mg catalysts were suspended in 100 mL MG solution (25 mg/L). Prior to irradiation, the mixture was magnetically stirred for 60 min to reach the adsorption-desorption equilibrium between the photocatalysts and dyes in dark. The visible-light was obtained from a 500 W Xe lamp with a cut off filter (λ Z400 nm). 3 mL of solution was taken out in certain intervals and centrifuged during the process.
3. Results and discussion The XRD patterns of the samples were shown in Fig. 1. The Bi2O3 sample (Fig. 1(a)) showed the intense diffraction peaks at
Fig. 1. (a) XRD pattern of Bi2O3 (b) XRD pattern of FeVO4 (c) XRD pattern of FeVO4/Bi2O3 (d) XRD pattern of Au/FeVO4/Bi2O3 (e) TEM, (f)(g) HRTEM and (h) EDX of Au/FeVO4/ Bi2O3.
X. Liu et al. / Materials Letters 185 (2016) 189–192
27.04° and 33.1° corresponded to the (120) and (020) crystal planes of Bi2O3 (PDF #41-1449), respectively. The FeVO4 sample (Fig. 1(b)) presented that all the peaks could be indexed to the FeVO4 (PDF #38-1372). The results suggested that both Bi2O3 and FeVO4 had a high crystallinity. The calcined FeVO4/Bi2O3 composites (Fig. 1(c)) exhibited a co-existence of Bi2O3 (PDF #41-1449) and FeVO4 (PDF #38-1372), indicating the presence of Bi2O3 and FeVO4 particles. As shown in Fig. 1(d), the XRD pattern of Au/ FeVO4/Bi2O3 was very similar to that of FeVO4/Bi2O3 and the weak peak appeared around 38.18° suggested that the Au phase with the plane (111) was formed in the Au/FeVO4/Bi2O3 photocatalysts. In addition, no characteristic peaks of impurities were detected, demonstrating that the Au/FeVO4/Bi2O3 composites were composed of Au, FeVO4 and Bi2O3 successfully. The morphology of Au/FeVO4/ Bi2O3 was further explored using TEM and HRTEM. As observed in the TEM image (Fig. 1(e)), the Au nanoparticles were deposited over the surface of the FeVO4/Bi2O3 photocatalysts. The HRTEM image (Fig. 1(f)) showed the well-defined lattice spacings measured to be 0.408 and 0.333 nm, which corresponded to the spacings of the α-Bi2O3 (020) plane and the FeVO4 (210) plane, respectively. The result was in good agreement with the XRD results and further confirmed that α-Bi2O3 and FeVO4 were closely integrated. From the HRTEM image (Fig. 1(g)), the lattice fringe of 0.236 nm was consistent with the (111) crystal face of Au nanoparticle. These results suggested that the well interfacial contact existed in the Au/FeVO4/Bi2O3 samples, which was beneficial for electrons migration, thereby improving the charge separation. The EDX characterization (Fig. 1(h)) proved that Au nanoparticle existed in the sample. The UV–vis absorption spectra of the heterojunction samples were illustrated in Fig. 2(a). Compared with pure FeVO4, Bi2O3 and FeVO4/Bi2O3, Au/FeVO4/Bi2O3 catalysts exhibited clearly increased photoabsorption and red-shifted of the absorption edge in the visible light region, suggesting that it could efficiently utilize the visible light for contaminant decomposition under visible-light irradiation. The enhanced absorption ability of Au/FeVO4/Bi2O3 was mainly due to the surface plasmon resonance of Au nanoparticles and the enhanced absorption in the visible region was beneficial to improve their photocatalytic activities under visible light irradiation [4]. The estimated band gap energies are 2.86, 2.26, 2.08 and 1.88 eV for Bi2O3, FeVO4, FeVO4/Bi2O3 and Au/ FeVO4/Bi2O3, respectively. The photoluminescence spectra reveal the charge carriers migration, transfer, trapping and recombination in semiconductors. The emission spectrum of FeVO4 and Bi2O3 emitted an emission peak centered at the wavelength positions of 450 nm and 440 nm, respectively (Fig. 2(b)). It was noteworthy that FeVO4/Bi2O3 catalyst had a remarkably weaker emission, indicating the recombination of photogenerated electron-hole pairs was inhibited by the heterojunction structure effectively. However, the PL emission intensities of the Au/FeVO4/Bi2O3 sample further decreased after loading Au nanoparticles, suggesting that the efficient charge separation was attributed to photogenerated electrons transferred from the conduction band of Bi2O3 and FeVO4 to Au nanoparticles. Therefore, the photocatalytic ability of Au/ FeVO4/Bi2O3 catalyst could be effectively improved resulting from the inhibition of electrons and holes. As shown in Fig. 3(a), the photocatalytic performance of the prepared samples was investigated by degradation of MG dye. It could be seen that only about 57% and 65% MG degradation were reached in the presence of pure FeVO4 and Bi2O3, respectively. By contrast, nearly 92% of MG had been decomposed over the Au/ FeVO4/Bi2O3 photocatalysts, which showed the higher photocatalytic activity than the FeVO4/Bi2O3 composite. Cheng et al. reported that the Bi2O3/NaBiO3 catalysts could degrade 94% of MG (10 mg/L) [7]. However, Au/FeVO4/Bi2O3 photocatalysts presented better catalytic performance than the Bi2O3/NaBiO3 catalysts.
191
Fig. 2. UV–vis absorption spectra (a) (the inset shows (Ahν)2 versus hν plots of samples) and PL emission spectra (b) of different photocatalysts.
Because the enhancement of photocatalytic activity by Au/FeVO4/ Bi2O3 photocatalysts could be ascribed to the following two factors. On the one hand, the heterojunction structure formed between FeVO4 and Bi2O3 could effective separate the photogenerated carriers and the doping of Au nanoparticles on the FeVO4/Bi2O3 surfaces further facilitated the inhibition of electronhole pairs recombination. On the other hand, the enhanced visible light absorption by the modification of Au nanoparticles on the FeVO4/Bi2O3 promoted production of photoinduced electrons and holes, which led to higher photocatalytic activity for degradation of MG. A possible mechanism for photocatalytic reaction of the Au/ FeVO4/Bi2O3 had been represented in Fig. 3(b). Both FeVO4 and Bi2O3 semiconductors could be excited by the visible-light and the electron-hole pairs generated under the visible light irradiation. Simultaneously, the photogenerated electrons from the conduction band (CB) of FeVO4 could transfer to that of Bi2O3 while the holes could migrate from the valence band (VB) of Bi2O3 to that of FeVO4. Since the Fermi level of Au (Ef ¼ þ0.45 V vs. NHE) was lower than the CB of FeVO4 ( 0.58 V vs. NHE) and Bi2O3 (þ0.33 V vs. NHE), the excited electrons could inject from the CB of FeVO4 and Bi2O3 to Au nanoparticles and then were trapped by O2 to form O2 species, which could be further produced H2O2 by reacting with two protons [5]. Meanwhile, the left holes could adsorb water molecule to form hydroxyl radicals ( OH), which was responsible for photocatalytic reaction. Therefore, the Au/FeVO4/
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electron-hole pairs and exhibited superior activity for degradation of organic pollutants.
4. Conclusions In summary, the Au/FeVO4/Bi2O3 photocatalyst was successfully synthesized via the precipitation and photodeposition method. The results showed that the Au/FeVO4/Bi2O3 composites exhibited the highest photocatalytic activity for degradation of MG under visible light irradiation. The enhanced photocatalytic performance could be mainly attributed to the excellent visible light absorption ability and decreased recombination of the electron-hole pairs. In general, the Au/FeVO4/Bi2O3 heterojunction was an excellent photocatalyst which could be expected to apply in water pollution purification.
References [1] A.A. Al-Kahtani, M.F. Abou Taleb, J. Hazard. Mater. 309 (2016) 10–19. [2] G.L. He, Y.H. Zhong, M.J. Chen, X. Li, Y.P. Fang, Y.H. Xu, J. Mol. Catal. A – Chem. 423 (2016) 70–76. [3] Y. Huang, Y. Zheng, H. Zhu, J.J. Wang, J. Solid State Chem. 239 (2016) 274–281. [4] J. Jiang, J. Yu, S. Cao, J. Colloid Interface Sci. 461 (2016) 56–63. [5] H.Y. Jiang, K. Cheng, J. Lin, Phys. Chem. Chem. Phys. 14 (2012) 12114–12121. [6] Z.L. Zhang, J.C. Zhu, S.J. Li, Y.L. Mao, J. Solid State Chem. 211 (2014) 120–123. [7] L. Cheng, Y. Kang, Mater. Lett. 97 (2013) 125–128. [8] S.K. Biswas, J.O. Baeg, Int. J. Hydrog. 38 (2013) 14451–14457. [9] X. Liu, Y. Kang, Mater. Lett. 164 (2016) 229–231.
Fig. 3. Degradation of MG by different photocatalysts (a) and Mechanism for the possible charge separation process by Au/FeVO4/Bi2O3 composites (b).
Bi2O3 structure could efficiently separate the photogenerated