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Solvothermal synthesis of BiOCl flower-like hierarchical structures with high photocatalytic activity
Catalysis Communications 51 (2014) 1–4
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Solvothermal synthesis of BiOCl flower-like hierarchical structures with high photocatalytic activity Dongfeng Sun a, Junping Li b, Zhihai Feng b, Lei He a, Bin Zhao a, Tingyu Wang a, Ruixing Li a,⁎, Shu Yin c, Tsugio Sato c a b c
Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China Aerospace Research Institute of Materials & Processing Technology, No. 1 Nan Da Hong Men Road, Fengtai District, Beijing 100076, China Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 9 January 2014 Received in revised form 3 March 2014 Accepted 4 March 2014 Available online 12 March 2014 Keywords: BiOCl Hierarchical structures Morphology Solvothermal Photocatalytic
a b s t r a c t Flower-like BiOCl hierarchical structures were successfully synthesized in the presence of citric acid through a solvothermal process using methanol as a solvent. The citric acid played significant roles, which functioned as both a chelating regent and a structure-directing agent. The as-synthesized flower-like BiOCl particles were assembled by numerous BiOCl nanosheets with a thickness of ca. 15–20 nm. Importantly, such a flower-like BiOCl morphology exhibited a higher photocatalytic activity than that of commercial TiO2 in degradation of methyl orange under either UV light or simulated sunlight illumination. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Bismuth-based oxyhalides, BiOX (X = Cl, Br or I), have recently attracted a great deal of attention from the scientific community because of their potential application in photocatalysts, ferroelectric materials, ionic conduction [1–4], etc. Among these BiOX catalysts, BiOCl has attracted more and more interest in the photocatalytic degradation organics due to its unique and excellent electrical and optical properties. BiOCl is known to be a tetragonal layered structure consisting of [Cl–Bi–O–Bi–Cl] sheets stacked together by nonbonding interactions through the Cl atoms along the c-xis. The strong internal static electric fields perpendicular to the Cl layer and the bismuth oxide-based fluorite-like layer enable the effective separation of the photo-generated electron–hole pairs, and these results in a high photocatalytic performance [5–7]. Up to now, several methods have been reported for the preparation of BiOCl with various morphologies such as nanoplates [8,9], nanobelts [10], nanosheets [11–13], nanowires [14], microspheres [15,16], flower-like hierarchical structures [17,18], etc. It has been reported that photocatalysts with special three-dimensional (3D) morphologies could not only enhance the absorbability to increase the photo⁎ Corresponding author at: School of Materials Science and Engineering, Beihang University (formerly Beijing University of Aeronautics and Astronautics), Xueyuanlu No. 37, Beijing 100191, China. Tel./fax: +86 10 8231 6500. E-mail address: [email protected] (R. Li).
http://dx.doi.org/10.1016/j.catcom.2014.03.004 1566-7367/© 2014 Elsevier B.V. All rights reserved.
absorption efficiency but also reduce the recombination opportunities of the photo-generated electron–hole pairs, thus they could transfer to the surface rapidly to degrade the organic molecules. Recently, 3D BiOCl hierarchical structures have been successfully synthesized via solvothermal method. In these studies, ethylene glycol or glycerol was used as solvent and structure directing agent [19–22]. Citric acid (CA) can strongly complexes metal ions and significantly alters the surface properties. It has been widely used as chelating agent in sol–gel, hydrothermal and solvothermal routes to prepare nanoparticles [23–25]. In this work, flower-like BiOCl hierarchical structures were obtained through a solvothermal synthesis assisted with CA in methanol mediated conditions. It was found that as-synthesized flower-like BiOCl particles exhibited higher photocatalytic activity than that of commercial TiO2 (P25) under either UV light or simulated sunlight illumination. 2. Experimental 2.1. Photocatalyst preparation Flower-like BiOCl particles were synthesized by a facile solvothermal method assisted with CA in methanol mediated conditions. All starting materials were purchased from commercial sources (analytical grade) and used without further purified. In a typical procedure, 0.005 mol of Bi(NO3) · 5H2O was dissolved in 30 mL of methanol, being followed by dissolution of 0.005 mol KCl and 0.005 mol CA (molar ratio of CA/Bi3 + = 1) into the above solution. After further stirring for
2
D. Sun et al. / Catalysis Communications 51 (2014) 1–4
0.5 h, the mixture was transferred into 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 150 °C for 6 h, and then cooled to room temperature. The precipitate was washed with distilled water and ethanol, and then dried under vacuum at 60 °C for 12 h. For comparison, plate-like BiOCl particles were also synthesized under the same conditions without the presence of CA. 2.2. Photocatalyst characterization See Appendix A. 2.3. Photocatalytic activity measurements See Appendix A. 3. Results and discussion Fig. 1 shows XRD patterns of the samples synthesized solvothermally at 150 °C for 12 h. All the identified peaks can be assigned to the tetragonal structure of BiOCl (JCPDS No. 06-0249). The major XRD diffraction peaks at 2θ = 12.05°, 25.94°, 32.48° and 33.66° correspond to the (001), (101), (110) and (102) planes, respectively. No other crystalline impurities were detected, demonstrating that the products are composed of a single phase BiOCl. Compared to the sample without CA (Fig. 1(a)), it is notable that a ratio of the diffraction intensity of (110)/(001) planes for the one with CA (Fig. 1(b)) is much higher, which is attributed to the formation of the sample without CA building units oriented along the [001] orientation [20]. Moreover, the (110) diffraction peak of the sample with CA is the strongest one, while the strongest peak of tetragonal phase BiOCl in the literature is the (101), indicating crystalline anisotropic growth of BiOCl is along the (110) plane in the present of CA. A further explanation will follow in the results of morphology. Fig. 2 shows the SEM images of the samples synthesized solvothermally at 150 °C for 12 h. As shown in Fig. 2(a), the product obtained in the absence of CA consists of a large number of irregular nanoplates. It clearly exhibits that flower-like BiOCl hierarchical structures were obtained in the present of CA (Fig. 2(b)). As shown from the high-magnification SEM images (Fig. 2(c) and (d)), unique flower-like morphology with an average diameter of 2 μm, assembled by numerous BiOCl nanosheets with a thickness ca. 15–20 nm is observed. Furthermore, although these nanosheets are not highly closepacked, the flower-like BiOCl hierarchical structures can maintain their integrity after vigorous ultrasonic treatment for 0.5 h, indicating the structural stability of the product. To understand the as-synthesized samples with various morphologies in more detail, CA dependent
experiments were conducted; it is found that the amount of CA plays an important role in controlling the morphologies of the samples. As shown in Fig. S1(a) and (b), when the amount of CA is less than CA/Bi3 + = 1, the samples are composed of irregular hierarchical structures. Alternatively, the sample obtained at CA/Bi 3 + = 2 displays a different flowery morphology, although it is also assembled by numerous nanosheets, as shown in Fig. S1(c) and (d). Comparing with the sample shown in Fig. 2(b)–(d), these nanosheets display larger thickness and are packed more densely. The morphology and structure of the BiOCl samples were further characterized by TEM, HRTEM and SAED. It is clearly observed that well-defined plate-like BiOCl particles (see Fig. 3(a)) are 150–300 nm in width and 20–30 nm in thickness. Fig. 3(b), an HRTEM of Fig. 3(a) indicates that a BiOCl plate is composed of aggregated BiOCl nanoparticles with a diameter of ca. 10 nm. Meanwhile, the clear lattice stripe indicates that these building blocks are well crystallized. And the distance of the lattice spacing is 0.736 nm, which is consistent with the (001) plane of tetragonal phase BiOCl, in good agreement with the result of XRD. Fig. 3(c) and (d) show the representative TEM images of the flower-like BiOCl hierarchical structures. It is demonstrated that the product is sheet-packed flowery structure, which is in agreement with that revealed by the SEM images shown in Fig. 2(b)–(d). The corresponding SAED pattern (inset in Fig. 3(d)) reveals that flower-like BiOCl particles are highly crystalline with characteristic (101), (110) and (102) reflections, which agrees with the XRD result. On the basis of the above experimental observations, it is found that CA exerted a remarkable level of control on the growth of BiOCl crystals and was responsible for the morphology and structure. It is known that CA possesses three carboxylic acid and one hydroxyl functional groups, providing chelating ability [26]. Bi3+ can combine with CA via chelating effect to produce [C6O7H5]3 −Bi3 + (Eq. (1)) [9,16], which reduce the concentration of Bi3 + in the reaction solution. Thus, the nucleation rate is certainly modified by the presence of CA. Moreover, during the growth process, CA can be selectively adsorbed on the facets of BiOCl crystals and dramatically inhibits their further growth [22,27]. Such a preferential adsorption can effectively restrict or promote the growth along specific directions, leading to dramatic modifications on the final shape of the crystals [28]. That is to say, the important roles of CA in the synthesis flower-like BiOCl hierarchical structures are attributed to its chelating property and preferential adsorption onto certain crystal facets. They should both influence the nucleation and growth of BiOCl and finally permit a precise tuning of the final crystal morphology. In this study, these effects give rise to both the crystalline anisotropic growth of BiOCl and the decrease of nanosheet thickness. Referring to Figs. 1(b) and 2(b)–(d), the diffraction peaks of the flower-like BiOCl particles are broad and weak, as well as the change of strongest peak comparing to the case of without CA. All these results are attributable to the decrease of crystal size and anisotropic growth resulted from the addition of CA [10]. Bi
Fig. 1. XRD patterns of samples (a) without CA and (b) with CA synthesized solvothermally at 150 °C for 6 h.
3þ
þ C6 O7 H8 →½C6 O7 H5
3−
Bi
3þ
þ 3H
þ
ð1Þ
Fig. 4(a) and (b) show the adsorption and photocatalytic efficiency of the as-prepared samples for the degradation of methyl orange (MO) under UV light and simulated sunlight illumination, respectively. For comparison, a blank experiment and an experiment with P25 were also performed under the same conditions. As seen from Fig. 4, the blank tests confirm that MO only slightly was degraded under UV light or simulated sunlight illumination in the absence of photocatalyst, indicating that the photolysis can be ignored. Compared with P25 and plate-like BiOCl particles, the flower-like BiOCl particles exhibited the highest adsorption capacity and photocatalytic efficiency. The excellent photocatalytic activity of the flower-like BiOCl particles may be explained by the following two aspects. On the one hand, BiOCl with layered structures has good photocatalytic activity. That is, the layered BiOCl structure can provide sufficient space to polarize
D. Sun et al. / Catalysis Communications 51 (2014) 1–4
3
Fig. 2. SEM images of (a) plate-like BiOCl and (b)–(d) flower-like BiOCl synthesized solvothermally at 150 °C for 6 h.
the related atoms and orbitals, which can effectively separate the photo-generated electron–hole pairs, thereby enhanced the photocatalytic activity of BiOCl [7]. On the other hand, it has been reported that large specific surface area helps to increase the photocatalytic reaction sites and promotes the efficiency of the electron–hole separation [29]. The BET surface areas of P25, plate-like and flower-like
BiOCl particles are 50, 16.88 and 53.35 m2 g− 1 , respectively. Although the BET surface area of P25 is similar to flower-like BiOCl particles, the adsorption capacity of P25 is lower than that BiOCl. This distinction for flower-like BiOCl particles may be related to the reactive of {110} facets. Since the MO molecules in the aqueous solution are anodic, they tend to be adsorbed on the surfaces which are
Fig. 3. BiOCl samples of (a) TEM image of plate-like, (b) HRTEM image of marked in (a), (c) and (d) TEM images of flower-like BiOCl and corresponding SAED pattern (inset) of marked area in (d) synthesized solvothermally at 150 °C for 6 h.
4
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flower-like BiOCl morphology was along the (110) plane, which facilitated the adsorption of MO molecules. The synthesized flower-like BiOCl particles showed higher photocatalytic activity than that of P25 and plate-like BiOCl particles in the degradation of MO under either UV light or simulated sunlight illumination. The enhancement of the photocatalytic activity is attributed to its large specific surface area and special structure. Acknowledgments The authors appreciate the financial support from the National Science Foundation of China (NSFC51372006) and the Start-Up Fund for High-End Returned Overseas Talents, Ministry of Human Resources and Social Security, China (Renshetinghan 2010, No. 411). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.03.004. References
Fig. 4. Photocatalytic degradation of MO with different samples under (a) UV light and (b) simulated sunlight illumination.
positively charged. With regard to BiOCl expose faces of {110}, the existence of surface atomic structure with exposed positively charged [Bi–Cl] layers could facilitate the adsorption of MO molecules to BiOCl [21]. In addition, the flower-like BiOCl hierarchical structure was found to be beneficial in increasing its surface area, which could benefit the adsorption of MO and the subsequent photocatalytic reactions. The chemical and physical stability of photocatalysts during photocatalytic reactions is significant in view of its applications. The photostability of flower-like BiOCl particles was evaluated through recycling experiments for the photocatalytic degradation of MO under UV light illumination. As shown in Fig. S2, no significant change in the photocatalytic activity was observed after five recycles. Furthermore, Fig. S3 shows the XRD patterns of flower-like BiOCl particles before and after the photocatalytic reaction, which reveals that the phase and structure remained intact. These results suggest that the obtained flower-like BiOCl hierarchical structures were stable during the photocatalytic reaction. 4. Conclusions In summary, nanosheet-assembled BiOCl with flower-like hierarchical structures was successfully synthesized by a solvothermal process in the presence of CA using methanol as a solvent. It was found that CA played a crucial role which functioned as both chelating regent and structure-directing agent. The crystalline anisotropic growth of
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Solvothermal synthesis of BiOCl flower-like hierarchical structures with high photocatalytic activity Dongfeng Sun a, Junping Li b, Zhihai Feng b, Lei He a, Bin Zhao a, Tingyu Wang a, Ruixing Li a,⁎, Shu Yin c, Tsugio Sato c a b c
Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China Aerospace Research Institute of Materials & Processing Technology, No. 1 Nan Da Hong Men Road, Fengtai District, Beijing 100076, China Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 9 January 2014 Received in revised form 3 March 2014 Accepted 4 March 2014 Available online 12 March 2014 Keywords: BiOCl Hierarchical structures Morphology Solvothermal Photocatalytic
a b s t r a c t Flower-like BiOCl hierarchical structures were successfully synthesized in the presence of citric acid through a solvothermal process using methanol as a solvent. The citric acid played significant roles, which functioned as both a chelating regent and a structure-directing agent. The as-synthesized flower-like BiOCl particles were assembled by numerous BiOCl nanosheets with a thickness of ca. 15–20 nm. Importantly, such a flower-like BiOCl morphology exhibited a higher photocatalytic activity than that of commercial TiO2 in degradation of methyl orange under either UV light or simulated sunlight illumination. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Bismuth-based oxyhalides, BiOX (X = Cl, Br or I), have recently attracted a great deal of attention from the scientific community because of their potential application in photocatalysts, ferroelectric materials, ionic conduction [1–4], etc. Among these BiOX catalysts, BiOCl has attracted more and more interest in the photocatalytic degradation organics due to its unique and excellent electrical and optical properties. BiOCl is known to be a tetragonal layered structure consisting of [Cl–Bi–O–Bi–Cl] sheets stacked together by nonbonding interactions through the Cl atoms along the c-xis. The strong internal static electric fields perpendicular to the Cl layer and the bismuth oxide-based fluorite-like layer enable the effective separation of the photo-generated electron–hole pairs, and these results in a high photocatalytic performance [5–7]. Up to now, several methods have been reported for the preparation of BiOCl with various morphologies such as nanoplates [8,9], nanobelts [10], nanosheets [11–13], nanowires [14], microspheres [15,16], flower-like hierarchical structures [17,18], etc. It has been reported that photocatalysts with special three-dimensional (3D) morphologies could not only enhance the absorbability to increase the photo⁎ Corresponding author at: School of Materials Science and Engineering, Beihang University (formerly Beijing University of Aeronautics and Astronautics), Xueyuanlu No. 37, Beijing 100191, China. Tel./fax: +86 10 8231 6500. E-mail address: [email protected] (R. Li).
http://dx.doi.org/10.1016/j.catcom.2014.03.004 1566-7367/© 2014 Elsevier B.V. All rights reserved.
absorption efficiency but also reduce the recombination opportunities of the photo-generated electron–hole pairs, thus they could transfer to the surface rapidly to degrade the organic molecules. Recently, 3D BiOCl hierarchical structures have been successfully synthesized via solvothermal method. In these studies, ethylene glycol or glycerol was used as solvent and structure directing agent [19–22]. Citric acid (CA) can strongly complexes metal ions and significantly alters the surface properties. It has been widely used as chelating agent in sol–gel, hydrothermal and solvothermal routes to prepare nanoparticles [23–25]. In this work, flower-like BiOCl hierarchical structures were obtained through a solvothermal synthesis assisted with CA in methanol mediated conditions. It was found that as-synthesized flower-like BiOCl particles exhibited higher photocatalytic activity than that of commercial TiO2 (P25) under either UV light or simulated sunlight illumination. 2. Experimental 2.1. Photocatalyst preparation Flower-like BiOCl particles were synthesized by a facile solvothermal method assisted with CA in methanol mediated conditions. All starting materials were purchased from commercial sources (analytical grade) and used without further purified. In a typical procedure, 0.005 mol of Bi(NO3) · 5H2O was dissolved in 30 mL of methanol, being followed by dissolution of 0.005 mol KCl and 0.005 mol CA (molar ratio of CA/Bi3 + = 1) into the above solution. After further stirring for
2
D. Sun et al. / Catalysis Communications 51 (2014) 1–4
0.5 h, the mixture was transferred into 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 150 °C for 6 h, and then cooled to room temperature. The precipitate was washed with distilled water and ethanol, and then dried under vacuum at 60 °C for 12 h. For comparison, plate-like BiOCl particles were also synthesized under the same conditions without the presence of CA. 2.2. Photocatalyst characterization See Appendix A. 2.3. Photocatalytic activity measurements See Appendix A. 3. Results and discussion Fig. 1 shows XRD patterns of the samples synthesized solvothermally at 150 °C for 12 h. All the identified peaks can be assigned to the tetragonal structure of BiOCl (JCPDS No. 06-0249). The major XRD diffraction peaks at 2θ = 12.05°, 25.94°, 32.48° and 33.66° correspond to the (001), (101), (110) and (102) planes, respectively. No other crystalline impurities were detected, demonstrating that the products are composed of a single phase BiOCl. Compared to the sample without CA (Fig. 1(a)), it is notable that a ratio of the diffraction intensity of (110)/(001) planes for the one with CA (Fig. 1(b)) is much higher, which is attributed to the formation of the sample without CA building units oriented along the [001] orientation [20]. Moreover, the (110) diffraction peak of the sample with CA is the strongest one, while the strongest peak of tetragonal phase BiOCl in the literature is the (101), indicating crystalline anisotropic growth of BiOCl is along the (110) plane in the present of CA. A further explanation will follow in the results of morphology. Fig. 2 shows the SEM images of the samples synthesized solvothermally at 150 °C for 12 h. As shown in Fig. 2(a), the product obtained in the absence of CA consists of a large number of irregular nanoplates. It clearly exhibits that flower-like BiOCl hierarchical structures were obtained in the present of CA (Fig. 2(b)). As shown from the high-magnification SEM images (Fig. 2(c) and (d)), unique flower-like morphology with an average diameter of 2 μm, assembled by numerous BiOCl nanosheets with a thickness ca. 15–20 nm is observed. Furthermore, although these nanosheets are not highly closepacked, the flower-like BiOCl hierarchical structures can maintain their integrity after vigorous ultrasonic treatment for 0.5 h, indicating the structural stability of the product. To understand the as-synthesized samples with various morphologies in more detail, CA dependent
experiments were conducted; it is found that the amount of CA plays an important role in controlling the morphologies of the samples. As shown in Fig. S1(a) and (b), when the amount of CA is less than CA/Bi3 + = 1, the samples are composed of irregular hierarchical structures. Alternatively, the sample obtained at CA/Bi 3 + = 2 displays a different flowery morphology, although it is also assembled by numerous nanosheets, as shown in Fig. S1(c) and (d). Comparing with the sample shown in Fig. 2(b)–(d), these nanosheets display larger thickness and are packed more densely. The morphology and structure of the BiOCl samples were further characterized by TEM, HRTEM and SAED. It is clearly observed that well-defined plate-like BiOCl particles (see Fig. 3(a)) are 150–300 nm in width and 20–30 nm in thickness. Fig. 3(b), an HRTEM of Fig. 3(a) indicates that a BiOCl plate is composed of aggregated BiOCl nanoparticles with a diameter of ca. 10 nm. Meanwhile, the clear lattice stripe indicates that these building blocks are well crystallized. And the distance of the lattice spacing is 0.736 nm, which is consistent with the (001) plane of tetragonal phase BiOCl, in good agreement with the result of XRD. Fig. 3(c) and (d) show the representative TEM images of the flower-like BiOCl hierarchical structures. It is demonstrated that the product is sheet-packed flowery structure, which is in agreement with that revealed by the SEM images shown in Fig. 2(b)–(d). The corresponding SAED pattern (inset in Fig. 3(d)) reveals that flower-like BiOCl particles are highly crystalline with characteristic (101), (110) and (102) reflections, which agrees with the XRD result. On the basis of the above experimental observations, it is found that CA exerted a remarkable level of control on the growth of BiOCl crystals and was responsible for the morphology and structure. It is known that CA possesses three carboxylic acid and one hydroxyl functional groups, providing chelating ability [26]. Bi3+ can combine with CA via chelating effect to produce [C6O7H5]3 −Bi3 + (Eq. (1)) [9,16], which reduce the concentration of Bi3 + in the reaction solution. Thus, the nucleation rate is certainly modified by the presence of CA. Moreover, during the growth process, CA can be selectively adsorbed on the facets of BiOCl crystals and dramatically inhibits their further growth [22,27]. Such a preferential adsorption can effectively restrict or promote the growth along specific directions, leading to dramatic modifications on the final shape of the crystals [28]. That is to say, the important roles of CA in the synthesis flower-like BiOCl hierarchical structures are attributed to its chelating property and preferential adsorption onto certain crystal facets. They should both influence the nucleation and growth of BiOCl and finally permit a precise tuning of the final crystal morphology. In this study, these effects give rise to both the crystalline anisotropic growth of BiOCl and the decrease of nanosheet thickness. Referring to Figs. 1(b) and 2(b)–(d), the diffraction peaks of the flower-like BiOCl particles are broad and weak, as well as the change of strongest peak comparing to the case of without CA. All these results are attributable to the decrease of crystal size and anisotropic growth resulted from the addition of CA [10]. Bi
Fig. 1. XRD patterns of samples (a) without CA and (b) with CA synthesized solvothermally at 150 °C for 6 h.
3þ
þ C6 O7 H8 →½C6 O7 H5
3−
Bi
3þ
þ 3H
þ
ð1Þ
Fig. 4(a) and (b) show the adsorption and photocatalytic efficiency of the as-prepared samples for the degradation of methyl orange (MO) under UV light and simulated sunlight illumination, respectively. For comparison, a blank experiment and an experiment with P25 were also performed under the same conditions. As seen from Fig. 4, the blank tests confirm that MO only slightly was degraded under UV light or simulated sunlight illumination in the absence of photocatalyst, indicating that the photolysis can be ignored. Compared with P25 and plate-like BiOCl particles, the flower-like BiOCl particles exhibited the highest adsorption capacity and photocatalytic efficiency. The excellent photocatalytic activity of the flower-like BiOCl particles may be explained by the following two aspects. On the one hand, BiOCl with layered structures has good photocatalytic activity. That is, the layered BiOCl structure can provide sufficient space to polarize
D. Sun et al. / Catalysis Communications 51 (2014) 1–4
3
Fig. 2. SEM images of (a) plate-like BiOCl and (b)–(d) flower-like BiOCl synthesized solvothermally at 150 °C for 6 h.
the related atoms and orbitals, which can effectively separate the photo-generated electron–hole pairs, thereby enhanced the photocatalytic activity of BiOCl [7]. On the other hand, it has been reported that large specific surface area helps to increase the photocatalytic reaction sites and promotes the efficiency of the electron–hole separation [29]. The BET surface areas of P25, plate-like and flower-like
BiOCl particles are 50, 16.88 and 53.35 m2 g− 1 , respectively. Although the BET surface area of P25 is similar to flower-like BiOCl particles, the adsorption capacity of P25 is lower than that BiOCl. This distinction for flower-like BiOCl particles may be related to the reactive of {110} facets. Since the MO molecules in the aqueous solution are anodic, they tend to be adsorbed on the surfaces which are
Fig. 3. BiOCl samples of (a) TEM image of plate-like, (b) HRTEM image of marked in (a), (c) and (d) TEM images of flower-like BiOCl and corresponding SAED pattern (inset) of marked area in (d) synthesized solvothermally at 150 °C for 6 h.
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flower-like BiOCl morphology was along the (110) plane, which facilitated the adsorption of MO molecules. The synthesized flower-like BiOCl particles showed higher photocatalytic activity than that of P25 and plate-like BiOCl particles in the degradation of MO under either UV light or simulated sunlight illumination. The enhancement of the photocatalytic activity is attributed to its large specific surface area and special structure. Acknowledgments The authors appreciate the financial support from the National Science Foundation of China (NSFC51372006) and the Start-Up Fund for High-End Returned Overseas Talents, Ministry of Human Resources and Social Security, China (Renshetinghan 2010, No. 411). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.03.004. References
Fig. 4. Photocatalytic degradation of MO with different samples under (a) UV light and (b) simulated sunlight illumination.
positively charged. With regard to BiOCl expose faces of {110}, the existence of surface atomic structure with exposed positively charged [Bi–Cl] layers could facilitate the adsorption of MO molecules to BiOCl [21]. In addition, the flower-like BiOCl hierarchical structure was found to be beneficial in increasing its surface area, which could benefit the adsorption of MO and the subsequent photocatalytic reactions. The chemical and physical stability of photocatalysts during photocatalytic reactions is significant in view of its applications. The photostability of flower-like BiOCl particles was evaluated through recycling experiments for the photocatalytic degradation of MO under UV light illumination. As shown in Fig. S2, no significant change in the photocatalytic activity was observed after five recycles. Furthermore, Fig. S3 shows the XRD patterns of flower-like BiOCl particles before and after the photocatalytic reaction, which reveals that the phase and structure remained intact. These results suggest that the obtained flower-like BiOCl hierarchical structures were stable during the photocatalytic reaction. 4. Conclusions In summary, nanosheet-assembled BiOCl with flower-like hierarchical structures was successfully synthesized by a solvothermal process in the presence of CA using methanol as a solvent. It was found that CA played a crucial role which functioned as both chelating regent and structure-directing agent. The crystalline anisotropic growth of
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