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Er3+ co-doped BiOCl sheets as efficient visible-light-driven photocatalysts
Materials Letters 179 (2016) 154–157
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of Yb3 þ /Er3 þ co-doped BiOCl sheets as efficient visible-light-driven photocatalysts Nuo Yu a,1, Yan Chen a,1, Wenhui Zhang a, Mei Wen a, Lisha Zhang b,n, Zhigang Chen a,n a State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, PR China b College of Environmental Science and Engineering, Donghua University, Shanghai 201620, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 April 2016 Received in revised form 4 May 2016 Accepted 11 May 2016 Available online 12 May 2016
Bismuth oxyhalide (BiOCl) has been demonstrated to be a new and excellent photocatalyst. To further improve its photocatalytic activity, we prepared BiOCl samples doped with different amount of Er3 þ and/ or Yb3 þ ions by a simple hydrothermal method. These doped BiOCl samples exhibit sheet-shaped structure, and their thickness goes down from 140 nm to 80 nm with the increase of the Ln doping amount from 0 to 5%. Among these doped samples, BiOCl sheets co-doped with 2.0% Yb3 þ /0.5% Er3 þ (BOC-2.5%) exhibits the highest degradation efficiency of 99.5% for Rhodamine B in 20 min under visiblelight illumination, which is 2.8 times the efficiency of undoped BiOCl. Especially, BOC-2.5% still retains the photocatalytic activity of 80% after four cycling test, indicating a good stability. These results confirm that Yb3 þ /Er3 þ co-doped BiOCl is an efficient and stable visible-light-driven photocatalyst. & 2016 Elsevier B.V. All rights reserved.
Keywords: BiOCl Nanocrystalline materials Semiconductors Doping Rare earth Photocatalysis
1. Introduction Harmful organic pollutants in water pose great serious threat to environmental sustainability, and one of efficient methods for eliminating organic pollutants is the semiconductor photocatalysis technology [1]. The key of photocatalysis technology is to develop photocatalysts with high efficiency [2,3]. TiO2 nanomaterials have been demonstrated to be one of the most excellent photocatalysts. However, the wide bandgap energy (3.2 eV) limits its photocatalytic activity in UV region which occupies only 4% of the solar light spectrum [4]. To extend the utilization of solar light, it is necessary to develop effective visible-light-driven photocatalysts, such as WO3, CdS, C3N4 and bismuth-based materials [4]. Recently, bismuth-based photocatalysts have been received tremendous attention due to its unique layered structure that helps the separation of photo-generated electron-hole pairs [5]. Among bismuth-based photocatalysts, bismuth oxyhalide (BiOCl) has been well-known as an effective photocatalyst for decomposing pollutants under UV or visible light illumination [5,6]. To further improve its visible-light-driven photocatalytic activity, various strategies have been developed, such as, by forming heterjuctions (Bi2S3/BiOCl [7] and Bi/BiOCl [8]), hierarchical nanostructures [9] and sheets with special-crystal-facets exposure [10]. n
Corresponding authors. E-mail addresses: [email protected] (L. Zhang), [email protected] (Z. Chen). 1 These authors contributed equally to the work.
http://dx.doi.org/10.1016/j.matlet.2016.05.071 0167-577X/& 2016 Elsevier B.V. All rights reserved.
Recently, lanthanides ions (Ln-ions) doping has been successfully applied to enhance photocatalytic performance of various conventional photocatalysts through a process of up-conversion luminescence [11–13]. For example, Yb3 þ /Er3 þ co-doped BiVO4 exhibits higher photocatalytic activity than the undoped materials [11]. Since BiOCl also can act as an efficient host for up-conversion luminescence [14], it can be expected that Ln-ions doping may improve the photocatalytic activity of BiOCl. To the best of our knowledge, there is no such report about Ln-ions doped BiOCl photocatalyst. Herein, we prepared BiOCl samples doped with different amount of Er3 þ and/or Yb3 þ ions through a facile hydrothermal route, and investigated their photocatalytic activity toward the degradation of Rhodamine B (RhB) under the visible light illumination.
2. Experimental All of the chemicals were used as received from Sinopharm Chemical Reagent Co., Ltd (China). Ln-ions doped BiOCl samples were prepared by a modified hydrothermal route [10]. Typically, Bi(NO3)3 5H2O (2 mmol), Ln(NO3)3 (Ln¼0.01 mmol Erþ 0.04 mmol Yb) and KCl (2.1 mmol) were sequentially dissolved in the de-ionized water (30 mL) under continuous stirring, and pH value of the solution was adjusted to be around 6 by dropping NaOH solution (1.0 M). After being agitated for about 30 min, the resulting solution was transferred to a 50 mL
N. Yu et al. / Materials Letters 179 (2016) 154–157
autoclave, and hydrothermally treated at 160 °C for 24 h. The system was cooled down to room temperature naturally, and the resulting precipitates were collected and washed with ethanol and de-ionized water thoroughly, and finally dried at 60 °C in air. This BiOCl product with Ln/Bi precursor ratio of 2.5% was denoted as BOC-2.5%. For comparison, BOC-0 (pure BiOCl without the addition of Ln(NO3)3), BOC-0.5% (0.01 mmol Er(NO3)3), BOC-1.25% (0.005 mmol Er þ0.02 mmol Yb), and BOC-5% (0.02 mmol Er þ0.08 mmol Yb) were also prepared by the similar method. The morphology of samples was investigated by a field emission-scanning electron microscopy (FE-SEM, Hitachi S-4800) and a transmission electron microscopy (TEM, JEOL JEM-2100F) equipped with energy dispersive spectroscopy (EDS). The phase was determined by XRD analysis (Bruker D4 X-ray diffractometer). UV– vis diffuse reflection spectra were recorded on a Shimadzu UV3100 spectrophotometer using an integrating sphere accessory. Photocatalytic activity of samples was evaluated by degrading of Rhodamine B (RhB) according to our previous method [15]. A 500 W Xe lamp with a UV cut-off filter (λ 4400 nm) was applied as the visible-light source. In each experiment, 20 mg of photocatalyst was added into RhB aqueous solution (100 mL, 10 5 mol L 1). Prior to illumination, the suspension was magnetically stirred in the darkness for one hour to obtain the adsorption–desorption equilibrium between the photocatalyst and RhB solution. During the illumination process, 3 mL of suspension was taken out at given time intervals and centrifuged to remove the remaining solids. UV–vis absorption spectra of the solutions were measured by Shimadzu UV-2550 UV–vis–NIR spectrophotometer, and then RhB concentration was calculated by analyzing the photoabsorption intensity at wavelength of 553 nm.
3. Results and discussion Ln-ions doping has been successfully applied to enhance photocatalytic performance of some photocatalysts (BiVO4 [11], Bi2WO6 [12], and Bi2MoO6 [13]) through the good up-converting
155
luminescence. In order to improve the photocatalytic activity of BiOCl, we prepared Er3 þ doped BiOCl and Yb3 þ /Er3 þ co-doped BiOCl samples. The effect of Ln-ions doping on the morphology of samples was firstly investigated (Fig. 1a–c and Fig. S1 in the Supporting information). Obviously, the pure BiOCl is composed of large-scale sheet-shaped structure with width of 1 4 mm and average thickness of about 140 nm (Fig. 1a). With the addition of 0.5 mol% of Er(NO3)3, there is no apparent change in morphology except the thickness of the sheets decreases slightly (Fig. 1b). By further increasing the amount of Ln(NO3)3 precursor (Yb3 þ /Er3 þ ) from 0.5 mol% to 1.25 5%, the sheets with the width of 2 mm can be observed, and their average thickness decreases to 80 nm (Fig. 1c and S1). The decrease in the width and thickness may result from the presence of excess Ln(NO3)3 precursors that influence the morphology of sheets. Further information of BOC2.5% can be obtained from TEM image (Fig. 1d), it shows welldefined sheet-shaped structure with the width of 1–4 mm, which agrees well with SEM image (Fig. 1c). HR-TEM image (Fig. 1e) demonstrates high crystallinity of sheets with an interplanar lattice spacing of about 2.68 Å calibrated from FFT pattern, corresponding to the (102) planes of the tetragonal BiOCl. To verify the existence of Ln ions in BOC-2.5%, EDS pattern has been performed. There are strong Bi, O and Cl element signals as well as weak Yb and Er element signals (the atomic ratio of Er/Yb/Bi is measured to be 0.3/ 1.4/100), and the additional signals (Cu and C elements) should be traced from carbon-coated copper grid (Fig. 1f). Subsequently, the phase of the obtained samples was investigated by XRD (Fig. 2a). All samples have high crystallinity, and the diffraction peaks are well indexed to tetragonal BiOCl (JCPDS no. 06-0249) without any additional peak, indicating the absence of impurities. Furthermore, Rietveld analysis confirms that the cell volume of doped samples (BOC-0.5%, BOC-1.25%, BOC-2.5%, and BOC-5%) are respectively determined to be 111.49, 111.09, 111.03, and 111.02 Å3, which are slightly smaller than that (111.61 Å3) of BOC-0 (Table S1). The progressive decrease in cell volume should be derived from the substitution of the larger Bi3 þ ion (radii: 117 pm) by smaller Yb3 þ /Er3 þ ions (radii: 101 pm/103 pm) [16],
Fig. 1. SEM images of BOC-0 (a), BOC-0.5% (b) and BOC-2.5% (c); TEM image (d), HR-TEM image (e, inset image shows the FFT pattern) and EDS pattern (f) of BOC-2.5%.
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N. Yu et al. / Materials Letters 179 (2016) 154–157
Fig. 2. XRD patterns (a) and UV–vis diffuse reflectance spectra (b) of pure and doped BOC samples.
confirming the successful incorporation of Yb3 þ /Er3 þ ions in BiOCl matrix. The optical properties were also measured by means of UV–vis diffuse reflectance spectroscopy. As shown in Fig. 2b, all BiOCl samples exhibit the typical UV light response with absorption edge around 380 nm. Surprisingly, there are three newly weak absorption bands centered at 488 nm, 522 nm and 655 nm, which are respectively assigned to the transitions from the 4I15/2 ground state to 4F7/2, 2H11/2, and 4F9/2 states of Er3 þ [14,16]. In addition, the optical bandgap energy (Eg) of different BiOCl samples is calculated by using (αhν)1/2 versus hν plots of the UV-absorption edge (Table S1). The Eg values are determined to be 3.40 eV for BOC-0.5% and BOC-1.25%, 3.39 eV for BOC-2.5%, and 3.37 eV for BOC-5.0%, which are slightly lower than that (3.42 eV) of pure BiOCl. The decrease in Eg after doping might be attributed to the charge transfer transition between Ln-ions intra-4f electrons and BiOCl matrix, resulting in the localized energy levels to form defect bands to narrow the bandgap [13,17]. Similar observations on the additional absorption bands and decreased bandgap energy have been also illustrated in a number of Ln-ions doped materials, indicating the absorption of more energy of solar spectrum [11,12]. Subsequently, the photocatalytic performances of BiOCl samples as well as P25 powder were evaluated by the degradation of RhB solution under visible-light illumination (Fig. 3a). The blank test indicates that the degradation of RhB is extremely slow without photocatalyst. When commercial available P25 powder or
pure BiOCl is used, the degradation efficiency of RhB increases to 20.5% or 35.7% after 20 min of illumination. With BOC-0.5% or BOC-1.25% as the photocatalysts, the degradation efficiency of RhB is determined to be 47.5% and 48.0% at 20 min. Importantly, when BOC-2.5% is used, 99.4% of RhB can be eliminated, indicating the highest photodegradation efficiency. These results indicate that Ln-ions doping significantly improve the photocatalytic activity of BiOCl. There are chiefly two reasons. One is that Er3 þ or Yb3 þ /Er3 þ ions can up-convert Near-IR light into visible-light and UV light which can be absorbed by semiconductor BiOCl matrix, creating photogenerated electron-hole pairs. The other is that the combination rate of electron–hole pairs can be efficiently prevented by Ln ions in BiOCl lattice [11–13,16]. It should be noted that if we further increase the amount of Ln precursor to 5.0%, the degradation efficiency declines to 73.1% at 20 min, indicating that excessive amount of Ln ions does not benefit for photocatalytic activity possibly due to the quenching effect [18]. At last, the stability of BOC-2.5% sample was measured by using a recycling test in which each cycle lasted 25 min (Fig. 3b). The photocatalytic activity of BOC-2.5% sample does not significantly decrease, and it still remains about 80% after four cycles. Thus, BOC-2.5% sample is capable of efficient photocatalytic activity and good stability under visible-light illumination, leading to its great potential for solving environmental problems related to organic pollutants.
Fig. 3. (a) The degradation efficiency of RhB as a function of irradiation time by different photocatalysts under visible-light illumination. (b) Cycling photocatalytic test of BOC-2.5%.
N. Yu et al. / Materials Letters 179 (2016) 154–157
4. Conclusions Yb3 þ /Er3 þ co-doped BiOCl sheets with different Ln-ions doping amount have been prepared through a simple hydrothermal treatment. Under visible-light illumination, these Yb3 þ /Er3 þ codoped BiOCl samples show high photodegradation efficiency for RhB in comparison with pure BiOCl and Er doped BiOCl samples. Especially, the BOC-2.5% nanosheets exhibit the highest photocatalytic activity and good stability. Furthermore, our experiment provides some insight into the design of novel photocatalysts through rare earth doping.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant nos. 21477019, 51272299, and 51473033), project of the Shanghai Committee of Science and Technology (13JC1400300), the Fundamental Research Funds for the Central Universities, and DHU Distinguished Young Professor Program.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2016.05.071.
157
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of Yb3 þ /Er3 þ co-doped BiOCl sheets as efficient visible-light-driven photocatalysts Nuo Yu a,1, Yan Chen a,1, Wenhui Zhang a, Mei Wen a, Lisha Zhang b,n, Zhigang Chen a,n a State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, PR China b College of Environmental Science and Engineering, Donghua University, Shanghai 201620, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 April 2016 Received in revised form 4 May 2016 Accepted 11 May 2016 Available online 12 May 2016
Bismuth oxyhalide (BiOCl) has been demonstrated to be a new and excellent photocatalyst. To further improve its photocatalytic activity, we prepared BiOCl samples doped with different amount of Er3 þ and/ or Yb3 þ ions by a simple hydrothermal method. These doped BiOCl samples exhibit sheet-shaped structure, and their thickness goes down from 140 nm to 80 nm with the increase of the Ln doping amount from 0 to 5%. Among these doped samples, BiOCl sheets co-doped with 2.0% Yb3 þ /0.5% Er3 þ (BOC-2.5%) exhibits the highest degradation efficiency of 99.5% for Rhodamine B in 20 min under visiblelight illumination, which is 2.8 times the efficiency of undoped BiOCl. Especially, BOC-2.5% still retains the photocatalytic activity of 80% after four cycling test, indicating a good stability. These results confirm that Yb3 þ /Er3 þ co-doped BiOCl is an efficient and stable visible-light-driven photocatalyst. & 2016 Elsevier B.V. All rights reserved.
Keywords: BiOCl Nanocrystalline materials Semiconductors Doping Rare earth Photocatalysis
1. Introduction Harmful organic pollutants in water pose great serious threat to environmental sustainability, and one of efficient methods for eliminating organic pollutants is the semiconductor photocatalysis technology [1]. The key of photocatalysis technology is to develop photocatalysts with high efficiency [2,3]. TiO2 nanomaterials have been demonstrated to be one of the most excellent photocatalysts. However, the wide bandgap energy (3.2 eV) limits its photocatalytic activity in UV region which occupies only 4% of the solar light spectrum [4]. To extend the utilization of solar light, it is necessary to develop effective visible-light-driven photocatalysts, such as WO3, CdS, C3N4 and bismuth-based materials [4]. Recently, bismuth-based photocatalysts have been received tremendous attention due to its unique layered structure that helps the separation of photo-generated electron-hole pairs [5]. Among bismuth-based photocatalysts, bismuth oxyhalide (BiOCl) has been well-known as an effective photocatalyst for decomposing pollutants under UV or visible light illumination [5,6]. To further improve its visible-light-driven photocatalytic activity, various strategies have been developed, such as, by forming heterjuctions (Bi2S3/BiOCl [7] and Bi/BiOCl [8]), hierarchical nanostructures [9] and sheets with special-crystal-facets exposure [10]. n
Corresponding authors. E-mail addresses: [email protected] (L. Zhang), [email protected] (Z. Chen). 1 These authors contributed equally to the work.
http://dx.doi.org/10.1016/j.matlet.2016.05.071 0167-577X/& 2016 Elsevier B.V. All rights reserved.
Recently, lanthanides ions (Ln-ions) doping has been successfully applied to enhance photocatalytic performance of various conventional photocatalysts through a process of up-conversion luminescence [11–13]. For example, Yb3 þ /Er3 þ co-doped BiVO4 exhibits higher photocatalytic activity than the undoped materials [11]. Since BiOCl also can act as an efficient host for up-conversion luminescence [14], it can be expected that Ln-ions doping may improve the photocatalytic activity of BiOCl. To the best of our knowledge, there is no such report about Ln-ions doped BiOCl photocatalyst. Herein, we prepared BiOCl samples doped with different amount of Er3 þ and/or Yb3 þ ions through a facile hydrothermal route, and investigated their photocatalytic activity toward the degradation of Rhodamine B (RhB) under the visible light illumination.
2. Experimental All of the chemicals were used as received from Sinopharm Chemical Reagent Co., Ltd (China). Ln-ions doped BiOCl samples were prepared by a modified hydrothermal route [10]. Typically, Bi(NO3)3 5H2O (2 mmol), Ln(NO3)3 (Ln¼0.01 mmol Erþ 0.04 mmol Yb) and KCl (2.1 mmol) were sequentially dissolved in the de-ionized water (30 mL) under continuous stirring, and pH value of the solution was adjusted to be around 6 by dropping NaOH solution (1.0 M). After being agitated for about 30 min, the resulting solution was transferred to a 50 mL
N. Yu et al. / Materials Letters 179 (2016) 154–157
autoclave, and hydrothermally treated at 160 °C for 24 h. The system was cooled down to room temperature naturally, and the resulting precipitates were collected and washed with ethanol and de-ionized water thoroughly, and finally dried at 60 °C in air. This BiOCl product with Ln/Bi precursor ratio of 2.5% was denoted as BOC-2.5%. For comparison, BOC-0 (pure BiOCl without the addition of Ln(NO3)3), BOC-0.5% (0.01 mmol Er(NO3)3), BOC-1.25% (0.005 mmol Er þ0.02 mmol Yb), and BOC-5% (0.02 mmol Er þ0.08 mmol Yb) were also prepared by the similar method. The morphology of samples was investigated by a field emission-scanning electron microscopy (FE-SEM, Hitachi S-4800) and a transmission electron microscopy (TEM, JEOL JEM-2100F) equipped with energy dispersive spectroscopy (EDS). The phase was determined by XRD analysis (Bruker D4 X-ray diffractometer). UV– vis diffuse reflection spectra were recorded on a Shimadzu UV3100 spectrophotometer using an integrating sphere accessory. Photocatalytic activity of samples was evaluated by degrading of Rhodamine B (RhB) according to our previous method [15]. A 500 W Xe lamp with a UV cut-off filter (λ 4400 nm) was applied as the visible-light source. In each experiment, 20 mg of photocatalyst was added into RhB aqueous solution (100 mL, 10 5 mol L 1). Prior to illumination, the suspension was magnetically stirred in the darkness for one hour to obtain the adsorption–desorption equilibrium between the photocatalyst and RhB solution. During the illumination process, 3 mL of suspension was taken out at given time intervals and centrifuged to remove the remaining solids. UV–vis absorption spectra of the solutions were measured by Shimadzu UV-2550 UV–vis–NIR spectrophotometer, and then RhB concentration was calculated by analyzing the photoabsorption intensity at wavelength of 553 nm.
3. Results and discussion Ln-ions doping has been successfully applied to enhance photocatalytic performance of some photocatalysts (BiVO4 [11], Bi2WO6 [12], and Bi2MoO6 [13]) through the good up-converting
155
luminescence. In order to improve the photocatalytic activity of BiOCl, we prepared Er3 þ doped BiOCl and Yb3 þ /Er3 þ co-doped BiOCl samples. The effect of Ln-ions doping on the morphology of samples was firstly investigated (Fig. 1a–c and Fig. S1 in the Supporting information). Obviously, the pure BiOCl is composed of large-scale sheet-shaped structure with width of 1 4 mm and average thickness of about 140 nm (Fig. 1a). With the addition of 0.5 mol% of Er(NO3)3, there is no apparent change in morphology except the thickness of the sheets decreases slightly (Fig. 1b). By further increasing the amount of Ln(NO3)3 precursor (Yb3 þ /Er3 þ ) from 0.5 mol% to 1.25 5%, the sheets with the width of 2 mm can be observed, and their average thickness decreases to 80 nm (Fig. 1c and S1). The decrease in the width and thickness may result from the presence of excess Ln(NO3)3 precursors that influence the morphology of sheets. Further information of BOC2.5% can be obtained from TEM image (Fig. 1d), it shows welldefined sheet-shaped structure with the width of 1–4 mm, which agrees well with SEM image (Fig. 1c). HR-TEM image (Fig. 1e) demonstrates high crystallinity of sheets with an interplanar lattice spacing of about 2.68 Å calibrated from FFT pattern, corresponding to the (102) planes of the tetragonal BiOCl. To verify the existence of Ln ions in BOC-2.5%, EDS pattern has been performed. There are strong Bi, O and Cl element signals as well as weak Yb and Er element signals (the atomic ratio of Er/Yb/Bi is measured to be 0.3/ 1.4/100), and the additional signals (Cu and C elements) should be traced from carbon-coated copper grid (Fig. 1f). Subsequently, the phase of the obtained samples was investigated by XRD (Fig. 2a). All samples have high crystallinity, and the diffraction peaks are well indexed to tetragonal BiOCl (JCPDS no. 06-0249) without any additional peak, indicating the absence of impurities. Furthermore, Rietveld analysis confirms that the cell volume of doped samples (BOC-0.5%, BOC-1.25%, BOC-2.5%, and BOC-5%) are respectively determined to be 111.49, 111.09, 111.03, and 111.02 Å3, which are slightly smaller than that (111.61 Å3) of BOC-0 (Table S1). The progressive decrease in cell volume should be derived from the substitution of the larger Bi3 þ ion (radii: 117 pm) by smaller Yb3 þ /Er3 þ ions (radii: 101 pm/103 pm) [16],
Fig. 1. SEM images of BOC-0 (a), BOC-0.5% (b) and BOC-2.5% (c); TEM image (d), HR-TEM image (e, inset image shows the FFT pattern) and EDS pattern (f) of BOC-2.5%.
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N. Yu et al. / Materials Letters 179 (2016) 154–157
Fig. 2. XRD patterns (a) and UV–vis diffuse reflectance spectra (b) of pure and doped BOC samples.
confirming the successful incorporation of Yb3 þ /Er3 þ ions in BiOCl matrix. The optical properties were also measured by means of UV–vis diffuse reflectance spectroscopy. As shown in Fig. 2b, all BiOCl samples exhibit the typical UV light response with absorption edge around 380 nm. Surprisingly, there are three newly weak absorption bands centered at 488 nm, 522 nm and 655 nm, which are respectively assigned to the transitions from the 4I15/2 ground state to 4F7/2, 2H11/2, and 4F9/2 states of Er3 þ [14,16]. In addition, the optical bandgap energy (Eg) of different BiOCl samples is calculated by using (αhν)1/2 versus hν plots of the UV-absorption edge (Table S1). The Eg values are determined to be 3.40 eV for BOC-0.5% and BOC-1.25%, 3.39 eV for BOC-2.5%, and 3.37 eV for BOC-5.0%, which are slightly lower than that (3.42 eV) of pure BiOCl. The decrease in Eg after doping might be attributed to the charge transfer transition between Ln-ions intra-4f electrons and BiOCl matrix, resulting in the localized energy levels to form defect bands to narrow the bandgap [13,17]. Similar observations on the additional absorption bands and decreased bandgap energy have been also illustrated in a number of Ln-ions doped materials, indicating the absorption of more energy of solar spectrum [11,12]. Subsequently, the photocatalytic performances of BiOCl samples as well as P25 powder were evaluated by the degradation of RhB solution under visible-light illumination (Fig. 3a). The blank test indicates that the degradation of RhB is extremely slow without photocatalyst. When commercial available P25 powder or
pure BiOCl is used, the degradation efficiency of RhB increases to 20.5% or 35.7% after 20 min of illumination. With BOC-0.5% or BOC-1.25% as the photocatalysts, the degradation efficiency of RhB is determined to be 47.5% and 48.0% at 20 min. Importantly, when BOC-2.5% is used, 99.4% of RhB can be eliminated, indicating the highest photodegradation efficiency. These results indicate that Ln-ions doping significantly improve the photocatalytic activity of BiOCl. There are chiefly two reasons. One is that Er3 þ or Yb3 þ /Er3 þ ions can up-convert Near-IR light into visible-light and UV light which can be absorbed by semiconductor BiOCl matrix, creating photogenerated electron-hole pairs. The other is that the combination rate of electron–hole pairs can be efficiently prevented by Ln ions in BiOCl lattice [11–13,16]. It should be noted that if we further increase the amount of Ln precursor to 5.0%, the degradation efficiency declines to 73.1% at 20 min, indicating that excessive amount of Ln ions does not benefit for photocatalytic activity possibly due to the quenching effect [18]. At last, the stability of BOC-2.5% sample was measured by using a recycling test in which each cycle lasted 25 min (Fig. 3b). The photocatalytic activity of BOC-2.5% sample does not significantly decrease, and it still remains about 80% after four cycles. Thus, BOC-2.5% sample is capable of efficient photocatalytic activity and good stability under visible-light illumination, leading to its great potential for solving environmental problems related to organic pollutants.
Fig. 3. (a) The degradation efficiency of RhB as a function of irradiation time by different photocatalysts under visible-light illumination. (b) Cycling photocatalytic test of BOC-2.5%.
N. Yu et al. / Materials Letters 179 (2016) 154–157
4. Conclusions Yb3 þ /Er3 þ co-doped BiOCl sheets with different Ln-ions doping amount have been prepared through a simple hydrothermal treatment. Under visible-light illumination, these Yb3 þ /Er3 þ codoped BiOCl samples show high photodegradation efficiency for RhB in comparison with pure BiOCl and Er doped BiOCl samples. Especially, the BOC-2.5% nanosheets exhibit the highest photocatalytic activity and good stability. Furthermore, our experiment provides some insight into the design of novel photocatalysts through rare earth doping.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant nos. 21477019, 51272299, and 51473033), project of the Shanghai Committee of Science and Technology (13JC1400300), the Fundamental Research Funds for the Central Universities, and DHU Distinguished Young Professor Program.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2016.05.071.
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