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Novel photocatalysts of fly ash cenospheres supported BiOBr hierarchical microspheres with high photocatalytic performance
Journal of Alloys and Compounds 615 (2014) 929–932
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Letter
Novel photocatalysts of fly ash cenospheres supported BiOBr hierarchical microspheres with high photocatalytic performance Li Lin a,b, Manhong Huang a,⇑, Liping Long b, Donghui Chen a,c,⇑ a
School of Environmental Science and Engineering, Donghua University, Shanghai 201620, China School of Chemical and Environmental Engineering, Hunan City University, Yiyang 413000, China c Shanghai Institute of Technology, Shanghai 200235, China b
a r t i c l e
i n f o
Article history: Received 14 April 2014 Received in revised form 14 June 2014 Accepted 16 June 2014 Available online 24 June 2014 Keywords: BiOBr Fly-ash cenospheres Photocatalytic Visible light irradiation
a b s t r a c t A novel fly-ash cenospheres (FACs) supported BiOBr hierarchical microspheres were successfully prepared via a facile one-pot solvothermal method. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), UV–visible diffuse reflectance spectroscopy (DRS) and N2 adsorption–desorption isotherms. The results indicated that polyvinylpyrrolidone (PVP) facilitated the loading of BiOBr hierarchical microspheres. Based on the photodegradation tests under visible light irradiation (k > 420 nm), the photocatalytic property of PVP/BiOBr/FACs photocatalysts was superior to that of BiOBr/FACs. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Heterogeneous photocatalytic technique has been considered as an environmentally friendly technique for the elimination of organic contaminants, and has attracted much attention over the past 30 years [1–3]. As the most attractive photocatalyst, TiO2 can only be activated by UV light irradiation (accounting for just 4% of the solar irradiation) which hinders its practical application [4]. To exploit new effective photocatalysts, various kinds of semiconductor materials, such as C–N co-doped TiO2 [5], CeO2/TiO2 [6] and Europium (III) and niobium (V) co-doped TiO2 [7] composites, have been developed. It is worthwhile noting that these particles catalyst will greatly restrict device applications because of their poor usage of light, hard recycle and low removal efficiency for the surface pollutant in sea or lake, such as the floating oil and algae. In Recent years, bismuth oxybromide (BiOBr) and its compounds, have been found to exhibit remarkable photocatalytic activity under visible light irradiation [8,9]. In particular, owing to their improved catalytic capacity and a faster adsorption property, much effort has been devoted into the design and fabrication of three-dimensional (3D) BiOBr with hierarchical structures by virtue of their broad application in environmental ⇑ Corresponding authors. Address: School of Environmental Science and Engineering, Donghua University, Shanghai 201620, China. Tel.: +86 13817007038 (D. Chen), +86 2167792546 (M. Huang). E-mail addresses: [email protected] (M. Huang), [email protected] (D. Chen). http://dx.doi.org/10.1016/j.jallcom.2014.06.088 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
remediation [10–12]. FACs, as a reuse solid waste from coal-firing power plants, have been reported as supports to coat TiO2 and BiVO4 film due to their low cost, nontoxicity, chemical and physical stability, low thermal conductivities and hollow framework [13,14]. Thus, it is reasonable to expect that 3D hierarchical BiOBr supported by FACs may also exhibit unique properties to solve previous problem. This inspired us to explore efficient approaches to the fabrication of 3D hierarchical BiOBr/FACs especially with unique properties. However, to the best of our knowledge, there have been few reports on the relative study. Herein, for the first time, we report a facile and one-pot solvothermal method to fabricate 3D hierarchical BiOBr/FACs. As one of a class of dye pollutants, rhodamine B have been selected as representative pollutant targets for degradation, because it is usually difficult to be removed from dye-containing wastewater by biological oxidation and physical chemical treatments [15]. It was found that PVP not only contributed to formation of flower-like BiOBr, but also benefited to their loading over the surface of FACs. Moreover, The photocatalytic activity of the as-obtained novel photocatalysts was investigated by the removal of RhB aqueous solution under visible light irradiation (k P 420 nm).
2. Experimental In a typical synthesis, Bi(NO3)35H2O (2.8 mmol) and polyvinylpyrrolidone (PVP) (0.15 g) were dissolved completely in 50 mL of ethylene glycol (EG) with magnetic stirring at room temperature (25 °C). Then, 1.4 mmol of NaBr and 2.0 g FACs were added into the sol and stirred for 30 min. Subsequently, the mixture
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was transferred into a 100 mL Teflon-lined stainless steel autoclave and maintained at 160 °C for 12 h. After the autoclave was cooled to room temperature naturally, the precipitate was washed with absolute ethanol and then dried at 65 °C for 6 h. The phase purity and crystal structure of as-prepared samples were characterized by X-ray diffractometer (XRD, Rigaku, D/max-2550 diffractometer), scanning electron microscopy (SEM, Quanta-250, field emission scanning electron microscope), energy dispersive X-ray spectroscopy (EDS, OXFORD ISIS, London, England) and UV–vis diffuse reflection spectra (DRS, Shimadzu, UV-3100 UV–vis spectrophotometer). Nitrogen adsorption–desorption isotherms were conducted by a Quantachrome NOVA 4000e sorption instrument at 77 K after the sample had been degassed in a flow of N2 at 100 °C overnight. The BET method was used to calculate the specific surface area (ABET). The weight ratio of Bi element in BiOBr/FACs and PVP/BiOBr/FACs was measured by inductively coupled plasma atomic emission spectrometer (Leeman Prodigy, USA). The photocatalytic experiments were carried out in a photochemical reactor (XPA-VII, Nanjing Xujiang Machineelectronic Plant, China), equipped with a 500 W Xe lamp combined with a 420 nm cut-off filter as the light source, which was about 14 cm from the liquid surface of the suspensions of RhB. In each experiment, 0.1 g of photocatalyst was added into 50 mL of RhB solution with a concentration of 15 mg/L. Before irradiation, the suspension was stirred for 30 min in the dark to reach adsorption–desorption equilibrium. At given time intervals, approximately 2 mL of the suspension was filtered to remove the photocatalyst particles and the clarified solution was analyzed by UV–vis spectroscopy (UV-1800, Shimadzu, Japan).
3. Results and discussion 3.1. XRD patterns The XRD spectra of the pure BiOBr powder, FACs, BiOBr/FACs and PVP/BiOBr/FACs composite are presented in Fig. 1. It can be clearly seen that all the inflection peaks can be readily indexed into the tetragonal phase BiOBr of (JCPDS No. 09-0393) for pure BiOBr powder, BiOBr/FACs and PVP/BiOBr/FACs. The results indicated that pure BiOBr powder was coated on the surface of FACs. In addition, compared with the diffraction peaks of FACs carriers, some peaks of SiO2, Fe2O3 and Al2O3 were all clearly seen for BiOBr/FACs and PVP/BiOBr/FACs, because the FACs are alumino-silicate based ceramic particles and the main components of FACs are siliceous oxide, ferreous oxide and aluminous oxide [16]. The intensity of these diffraction peak was weakened or disappeared with the BiOBr loaded on the surface of FACs. Simultaneously, although possessed the consistent characteristic, the inflection peaks intensity of PVP/BiOBr/FACs increased a bit relative to that of BiOBr/FACs. This result suggests that the adding of PVP was beneficial for the loading of BiOBr over FACs, which may be favorable for the photocatalytic ability. 3.2. SEM and EDS observation The SEM images of the pure BiOBr powder, FACs, BiOBr/FACs and PVP/BiOBr/FACs composite are shown in Fig. 2. From Fig. 2a
and d, it can be clearly seen that the FACs exhibit an essentially spherical shape with diameter of 130 lm and relatively uniform smooth-faced. Fig. 2b and e presents the SEM images of the BiOBr/FACs at low and high magnifications, respectively. It can be seen that few BiOBr microspheres are coated on the FAC surface and the diameter of these microspheres is irregular. In contrast, Fig. 2c and f present the surface micrograph of the PVP/BiOBr /FACs sample at low and high magnifications. Different from BiOBr/FACs, the FACs surface were loaded with a great deal of BiOBr microspheres. Additionally, all these loaded BiOBr microspheres consisted of interwoven nano-petals and appeared as threedimensional hierarchical microspheres with diameters ranging from 1 to 3 lm. These results illustrated that the addition of PVP contributed to the fabrication of hierarchical BiOBr over the surface of FACs in the solvothermal process. These microspheres consist of large numbers of irregular nano-petals with smooth surfaces. The rough surface of the microspheres confer high specific surface area, surface-to-volume ratio and abundant transport paths for small organic molecules, which are considered towards photocatalysis [17]. Fig. 2g and h shows the representative EDS spectra of the loaded BiOBr and the supported materials FACs for a single PVP/ BiOBr /FAC, respectively. The EDS analysis illustrated that the major constituents for the supported materials were Fe, O, Si and Al. The peaks of Bi, Br and O mainly generated by BiOBr microsphere. These results were consistent with the XRD data (Fig. 1). 3.3. DRS analysis The UV–vis diffuse reflectance spectra of FACs, BiOBr, BiOBr/ FACs and PVP/BiOBr/FACs are shown in Fig. 3. The results indicated that BiOBr/FACs and PVP/BiOBr/FACs exhibit a red-shift and stronger optical absorption in the visible light region of 450–650 nm than BiOBr. This observed red-shift could be attributed to a secondary phase and surface defect and the similar phenomenon was also observed for BiVO4/FACs [14,18]. The band gap energy of assynthesized samples could be calculated by the following formula [19,20]:
ahc ¼ Aðhc EgÞn=2
ð1Þ
where a, c, Eg, and A are the absorption coefficient, light frequency, band gap, and a constant, respectively. n depends on the characteristics of the transition in a semiconductor, including direct transitions (n = 1) or indirect transitions (n = 4). As previous reports indicated that BiOX (X = Br, I) was an indirect band gap material [21], the band gap energy could be estimated from a plot of (ahc)1/2 vs. the photon energy (hc). The x-axis intercept of the tangent to the plot approached the band gap energy of the sample. The band gap value of BiOBr was estimated to be 2.60 eV. The calculated result also demonstrated that BiOBr/FACs and PVP/BiOBr/FACs have the identical band gap values of 2.32 eV. This narrower energy band gap is beneficial for photocatalytic degradation under visible light irradiation, which is critical for the solar energy application. Therefore, the two kind of supported photocatalysts had potential application as visible light photocatalysts. 3.4. N2 adsorption–desorption isotherms
Fig. 1. XRD patterns of the samples.
The nitrogen adsorption–desorption isotherms and pore size distribution of the BiOBr/FACs and PVP/ BiOBr/FACs were further investigated. As can be seen in Fig. 4, these materials displayed a type-IV isotherms with distinct hysteresis loops observed in the range of 0.2–1.0P/P0, which signifies mesoporous materials [22,23]. The pores size distribution data shows that a majority of the pores are in the size range of 50–150 nm. The BET surface areas of the BiOBr/FACs and PVP/BiOBr/FACs are 3.25 m2/g and
L. Lin et al. / Journal of Alloys and Compounds 615 (2014) 929–932
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Fig. 2. The SEM images of (a and d) the FACs; (b and e) BiOBr/FACs; (c and f) PVP/BiOBr/FACs; The EDS spectra of (g) the supported materials FACs and (h) the loaded BiOBr.
7.83 m2/g, respectively. According to the relative literature [24], PVP can regulate the morphology of BiOBr and control the thickness and distance of nanosheets which lead to the enhancement of BET surface area, therefore the larger BET surface area of PVP/ BiOBr/FACs could be obtained. The result suggests higher adsorption ability and more reactive sites of PVP/BiOBr/FACs than that of BiOBr/FACs. 3.5. Photocatalytic evaluation
Fig. 3. The UV–vis diffuse reflection spectra of FACs, BiOBr, BiOBr/FACs and PVP/ BiOBr/FACs.
Visible light photocatalytic activities of the as-prepared photocatalysts on the degradation of RhB were further investigated in Fig. 5. Based on Fig. 5a, the results indicated that the direct photolysis, adsorption and photocatalytic degradation over pure FACs of RhB were negligible. However, after 30 min of dark equilibration, the percentage of RhB adsorbed on the surface of the BiOBr/FACs and PVP/BiOBr/FACs reached to 36% and 50%, respectively. This demonstrated that PVP/BiOBr/FACs composites possessed more outstanding adsorption ability than BiOBr/FACs, which basically agrees with the previous nitrogen adsorption–desorption isotherms analysis results. Subsequently, the removal efficiency of
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RhB increased under the visible light irradiation and more than 87% and 69% of RhB had been removed by PVP/BiOBr/FACs and BiOBr/FACs, respectively. This high efficiency degradation over PVP/BiOBr/FACs may be attributed to the excellent absorption ability and more reactive sites (as Fig. 4) which are the critical factors in heterogeneous photocatalytic reaction [25,26]. In addition, the weight ratio of Bi element was detected to be 10% and 15.6% which
corresponds to BiOBr/FACs and PVP/BiOBr/FACs. According to the result, the photocatalytic activity of the corresponding mass BiOBr was also compared with PVP/BiOBr/FACs. As can be seen from Fig. 5a, though BiOBr photocatalytic degradation rate increased faster than the latter, the removal efficiency of RhB reached only 83% after 80 min visible light irradiation. Fig. 5b illustrates the temporal evolution of the spectral changes of RhB degradation mediated by PVP/BiOBr/FACs composite. The RhB was obviously degraded in 80 min and a blue-shift of the main absorption peak (k = 554 nm) occurred which corresponds to de-ethylation of RhB due to attack by the reactive oxygen species on the N-ethyl group [27]. These above facts revealed that the as-obtained PVP/BiOBr/ FACs possesses superior photocatalytic degradation activity for some organic contaminant. 4. Conclusions A novel BiOBr/FACs photocatalysts with high visible light photocatalytic ability have been successfully synthesized through a facile one-pot solvothermal approach. Based on the observations, More flower-like BiOBr hierarchical microspheres were loaded over the surface of FACs by the aid of PVP. More importantly, the novel composite microspheres, which show excellent photocatalytic activity under visible light irradiation, may have potential application in the remove of floating organic pollutant.
Fig. 4. Typical nitrogen adsorption–desorption isotherms of PVP/BiOBr/FACs and BiOBr/FACs. The inset is the corresponding pore-size distribution.
Acknowledgements The authors would like to thank the National Natural Science Foundation of China (No. 21007010); Hu Nan province Ministry of Transportation scientific research project (No. 200908, 201105) and Ministry of Transport science and technology program (No. 2010353343290). References
Fig. 5. The absorbance–time curves of RhB in the presences of various catalysts (a), the UV–vis absorption spectra of RhB (b).
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Novel photocatalysts of fly ash cenospheres supported BiOBr hierarchical microspheres with high photocatalytic performance Li Lin a,b, Manhong Huang a,⇑, Liping Long b, Donghui Chen a,c,⇑ a
School of Environmental Science and Engineering, Donghua University, Shanghai 201620, China School of Chemical and Environmental Engineering, Hunan City University, Yiyang 413000, China c Shanghai Institute of Technology, Shanghai 200235, China b
a r t i c l e
i n f o
Article history: Received 14 April 2014 Received in revised form 14 June 2014 Accepted 16 June 2014 Available online 24 June 2014 Keywords: BiOBr Fly-ash cenospheres Photocatalytic Visible light irradiation
a b s t r a c t A novel fly-ash cenospheres (FACs) supported BiOBr hierarchical microspheres were successfully prepared via a facile one-pot solvothermal method. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), UV–visible diffuse reflectance spectroscopy (DRS) and N2 adsorption–desorption isotherms. The results indicated that polyvinylpyrrolidone (PVP) facilitated the loading of BiOBr hierarchical microspheres. Based on the photodegradation tests under visible light irradiation (k > 420 nm), the photocatalytic property of PVP/BiOBr/FACs photocatalysts was superior to that of BiOBr/FACs. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Heterogeneous photocatalytic technique has been considered as an environmentally friendly technique for the elimination of organic contaminants, and has attracted much attention over the past 30 years [1–3]. As the most attractive photocatalyst, TiO2 can only be activated by UV light irradiation (accounting for just 4% of the solar irradiation) which hinders its practical application [4]. To exploit new effective photocatalysts, various kinds of semiconductor materials, such as C–N co-doped TiO2 [5], CeO2/TiO2 [6] and Europium (III) and niobium (V) co-doped TiO2 [7] composites, have been developed. It is worthwhile noting that these particles catalyst will greatly restrict device applications because of their poor usage of light, hard recycle and low removal efficiency for the surface pollutant in sea or lake, such as the floating oil and algae. In Recent years, bismuth oxybromide (BiOBr) and its compounds, have been found to exhibit remarkable photocatalytic activity under visible light irradiation [8,9]. In particular, owing to their improved catalytic capacity and a faster adsorption property, much effort has been devoted into the design and fabrication of three-dimensional (3D) BiOBr with hierarchical structures by virtue of their broad application in environmental ⇑ Corresponding authors. Address: School of Environmental Science and Engineering, Donghua University, Shanghai 201620, China. Tel.: +86 13817007038 (D. Chen), +86 2167792546 (M. Huang). E-mail addresses: [email protected] (M. Huang), [email protected] (D. Chen). http://dx.doi.org/10.1016/j.jallcom.2014.06.088 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
remediation [10–12]. FACs, as a reuse solid waste from coal-firing power plants, have been reported as supports to coat TiO2 and BiVO4 film due to their low cost, nontoxicity, chemical and physical stability, low thermal conductivities and hollow framework [13,14]. Thus, it is reasonable to expect that 3D hierarchical BiOBr supported by FACs may also exhibit unique properties to solve previous problem. This inspired us to explore efficient approaches to the fabrication of 3D hierarchical BiOBr/FACs especially with unique properties. However, to the best of our knowledge, there have been few reports on the relative study. Herein, for the first time, we report a facile and one-pot solvothermal method to fabricate 3D hierarchical BiOBr/FACs. As one of a class of dye pollutants, rhodamine B have been selected as representative pollutant targets for degradation, because it is usually difficult to be removed from dye-containing wastewater by biological oxidation and physical chemical treatments [15]. It was found that PVP not only contributed to formation of flower-like BiOBr, but also benefited to their loading over the surface of FACs. Moreover, The photocatalytic activity of the as-obtained novel photocatalysts was investigated by the removal of RhB aqueous solution under visible light irradiation (k P 420 nm).
2. Experimental In a typical synthesis, Bi(NO3)35H2O (2.8 mmol) and polyvinylpyrrolidone (PVP) (0.15 g) were dissolved completely in 50 mL of ethylene glycol (EG) with magnetic stirring at room temperature (25 °C). Then, 1.4 mmol of NaBr and 2.0 g FACs were added into the sol and stirred for 30 min. Subsequently, the mixture
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was transferred into a 100 mL Teflon-lined stainless steel autoclave and maintained at 160 °C for 12 h. After the autoclave was cooled to room temperature naturally, the precipitate was washed with absolute ethanol and then dried at 65 °C for 6 h. The phase purity and crystal structure of as-prepared samples were characterized by X-ray diffractometer (XRD, Rigaku, D/max-2550 diffractometer), scanning electron microscopy (SEM, Quanta-250, field emission scanning electron microscope), energy dispersive X-ray spectroscopy (EDS, OXFORD ISIS, London, England) and UV–vis diffuse reflection spectra (DRS, Shimadzu, UV-3100 UV–vis spectrophotometer). Nitrogen adsorption–desorption isotherms were conducted by a Quantachrome NOVA 4000e sorption instrument at 77 K after the sample had been degassed in a flow of N2 at 100 °C overnight. The BET method was used to calculate the specific surface area (ABET). The weight ratio of Bi element in BiOBr/FACs and PVP/BiOBr/FACs was measured by inductively coupled plasma atomic emission spectrometer (Leeman Prodigy, USA). The photocatalytic experiments were carried out in a photochemical reactor (XPA-VII, Nanjing Xujiang Machineelectronic Plant, China), equipped with a 500 W Xe lamp combined with a 420 nm cut-off filter as the light source, which was about 14 cm from the liquid surface of the suspensions of RhB. In each experiment, 0.1 g of photocatalyst was added into 50 mL of RhB solution with a concentration of 15 mg/L. Before irradiation, the suspension was stirred for 30 min in the dark to reach adsorption–desorption equilibrium. At given time intervals, approximately 2 mL of the suspension was filtered to remove the photocatalyst particles and the clarified solution was analyzed by UV–vis spectroscopy (UV-1800, Shimadzu, Japan).
3. Results and discussion 3.1. XRD patterns The XRD spectra of the pure BiOBr powder, FACs, BiOBr/FACs and PVP/BiOBr/FACs composite are presented in Fig. 1. It can be clearly seen that all the inflection peaks can be readily indexed into the tetragonal phase BiOBr of (JCPDS No. 09-0393) for pure BiOBr powder, BiOBr/FACs and PVP/BiOBr/FACs. The results indicated that pure BiOBr powder was coated on the surface of FACs. In addition, compared with the diffraction peaks of FACs carriers, some peaks of SiO2, Fe2O3 and Al2O3 were all clearly seen for BiOBr/FACs and PVP/BiOBr/FACs, because the FACs are alumino-silicate based ceramic particles and the main components of FACs are siliceous oxide, ferreous oxide and aluminous oxide [16]. The intensity of these diffraction peak was weakened or disappeared with the BiOBr loaded on the surface of FACs. Simultaneously, although possessed the consistent characteristic, the inflection peaks intensity of PVP/BiOBr/FACs increased a bit relative to that of BiOBr/FACs. This result suggests that the adding of PVP was beneficial for the loading of BiOBr over FACs, which may be favorable for the photocatalytic ability. 3.2. SEM and EDS observation The SEM images of the pure BiOBr powder, FACs, BiOBr/FACs and PVP/BiOBr/FACs composite are shown in Fig. 2. From Fig. 2a
and d, it can be clearly seen that the FACs exhibit an essentially spherical shape with diameter of 130 lm and relatively uniform smooth-faced. Fig. 2b and e presents the SEM images of the BiOBr/FACs at low and high magnifications, respectively. It can be seen that few BiOBr microspheres are coated on the FAC surface and the diameter of these microspheres is irregular. In contrast, Fig. 2c and f present the surface micrograph of the PVP/BiOBr /FACs sample at low and high magnifications. Different from BiOBr/FACs, the FACs surface were loaded with a great deal of BiOBr microspheres. Additionally, all these loaded BiOBr microspheres consisted of interwoven nano-petals and appeared as threedimensional hierarchical microspheres with diameters ranging from 1 to 3 lm. These results illustrated that the addition of PVP contributed to the fabrication of hierarchical BiOBr over the surface of FACs in the solvothermal process. These microspheres consist of large numbers of irregular nano-petals with smooth surfaces. The rough surface of the microspheres confer high specific surface area, surface-to-volume ratio and abundant transport paths for small organic molecules, which are considered towards photocatalysis [17]. Fig. 2g and h shows the representative EDS spectra of the loaded BiOBr and the supported materials FACs for a single PVP/ BiOBr /FAC, respectively. The EDS analysis illustrated that the major constituents for the supported materials were Fe, O, Si and Al. The peaks of Bi, Br and O mainly generated by BiOBr microsphere. These results were consistent with the XRD data (Fig. 1). 3.3. DRS analysis The UV–vis diffuse reflectance spectra of FACs, BiOBr, BiOBr/ FACs and PVP/BiOBr/FACs are shown in Fig. 3. The results indicated that BiOBr/FACs and PVP/BiOBr/FACs exhibit a red-shift and stronger optical absorption in the visible light region of 450–650 nm than BiOBr. This observed red-shift could be attributed to a secondary phase and surface defect and the similar phenomenon was also observed for BiVO4/FACs [14,18]. The band gap energy of assynthesized samples could be calculated by the following formula [19,20]:
ahc ¼ Aðhc EgÞn=2
ð1Þ
where a, c, Eg, and A are the absorption coefficient, light frequency, band gap, and a constant, respectively. n depends on the characteristics of the transition in a semiconductor, including direct transitions (n = 1) or indirect transitions (n = 4). As previous reports indicated that BiOX (X = Br, I) was an indirect band gap material [21], the band gap energy could be estimated from a plot of (ahc)1/2 vs. the photon energy (hc). The x-axis intercept of the tangent to the plot approached the band gap energy of the sample. The band gap value of BiOBr was estimated to be 2.60 eV. The calculated result also demonstrated that BiOBr/FACs and PVP/BiOBr/FACs have the identical band gap values of 2.32 eV. This narrower energy band gap is beneficial for photocatalytic degradation under visible light irradiation, which is critical for the solar energy application. Therefore, the two kind of supported photocatalysts had potential application as visible light photocatalysts. 3.4. N2 adsorption–desorption isotherms
Fig. 1. XRD patterns of the samples.
The nitrogen adsorption–desorption isotherms and pore size distribution of the BiOBr/FACs and PVP/ BiOBr/FACs were further investigated. As can be seen in Fig. 4, these materials displayed a type-IV isotherms with distinct hysteresis loops observed in the range of 0.2–1.0P/P0, which signifies mesoporous materials [22,23]. The pores size distribution data shows that a majority of the pores are in the size range of 50–150 nm. The BET surface areas of the BiOBr/FACs and PVP/BiOBr/FACs are 3.25 m2/g and
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Fig. 2. The SEM images of (a and d) the FACs; (b and e) BiOBr/FACs; (c and f) PVP/BiOBr/FACs; The EDS spectra of (g) the supported materials FACs and (h) the loaded BiOBr.
7.83 m2/g, respectively. According to the relative literature [24], PVP can regulate the morphology of BiOBr and control the thickness and distance of nanosheets which lead to the enhancement of BET surface area, therefore the larger BET surface area of PVP/ BiOBr/FACs could be obtained. The result suggests higher adsorption ability and more reactive sites of PVP/BiOBr/FACs than that of BiOBr/FACs. 3.5. Photocatalytic evaluation
Fig. 3. The UV–vis diffuse reflection spectra of FACs, BiOBr, BiOBr/FACs and PVP/ BiOBr/FACs.
Visible light photocatalytic activities of the as-prepared photocatalysts on the degradation of RhB were further investigated in Fig. 5. Based on Fig. 5a, the results indicated that the direct photolysis, adsorption and photocatalytic degradation over pure FACs of RhB were negligible. However, after 30 min of dark equilibration, the percentage of RhB adsorbed on the surface of the BiOBr/FACs and PVP/BiOBr/FACs reached to 36% and 50%, respectively. This demonstrated that PVP/BiOBr/FACs composites possessed more outstanding adsorption ability than BiOBr/FACs, which basically agrees with the previous nitrogen adsorption–desorption isotherms analysis results. Subsequently, the removal efficiency of
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RhB increased under the visible light irradiation and more than 87% and 69% of RhB had been removed by PVP/BiOBr/FACs and BiOBr/FACs, respectively. This high efficiency degradation over PVP/BiOBr/FACs may be attributed to the excellent absorption ability and more reactive sites (as Fig. 4) which are the critical factors in heterogeneous photocatalytic reaction [25,26]. In addition, the weight ratio of Bi element was detected to be 10% and 15.6% which
corresponds to BiOBr/FACs and PVP/BiOBr/FACs. According to the result, the photocatalytic activity of the corresponding mass BiOBr was also compared with PVP/BiOBr/FACs. As can be seen from Fig. 5a, though BiOBr photocatalytic degradation rate increased faster than the latter, the removal efficiency of RhB reached only 83% after 80 min visible light irradiation. Fig. 5b illustrates the temporal evolution of the spectral changes of RhB degradation mediated by PVP/BiOBr/FACs composite. The RhB was obviously degraded in 80 min and a blue-shift of the main absorption peak (k = 554 nm) occurred which corresponds to de-ethylation of RhB due to attack by the reactive oxygen species on the N-ethyl group [27]. These above facts revealed that the as-obtained PVP/BiOBr/ FACs possesses superior photocatalytic degradation activity for some organic contaminant. 4. Conclusions A novel BiOBr/FACs photocatalysts with high visible light photocatalytic ability have been successfully synthesized through a facile one-pot solvothermal approach. Based on the observations, More flower-like BiOBr hierarchical microspheres were loaded over the surface of FACs by the aid of PVP. More importantly, the novel composite microspheres, which show excellent photocatalytic activity under visible light irradiation, may have potential application in the remove of floating organic pollutant.
Fig. 4. Typical nitrogen adsorption–desorption isotherms of PVP/BiOBr/FACs and BiOBr/FACs. The inset is the corresponding pore-size distribution.
Acknowledgements The authors would like to thank the National Natural Science Foundation of China (No. 21007010); Hu Nan province Ministry of Transportation scientific research project (No. 200908, 201105) and Ministry of Transport science and technology program (No. 2010353343290). References
Fig. 5. The absorbance–time curves of RhB in the presences of various catalysts (a), the UV–vis absorption spectra of RhB (b).
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