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Na3Bi(PO4)2 heterogeneous nanostructures with enhanced visible-light responsive photocatalytic activity
Materials Letters 242 (2019) 39–41
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Preparation of novel Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures with enhanced visible-light responsive photocatalytic activity Hongdan Xue, Ke Wang ⇑, Yongqing Bai, Fei He, Haijun Yang, Fuli Wang, Pu Liu Department of Mathematics and Physics, Hebei Institute of Architecture and Civil Engineering, Zhangjiakou 075000, China
a r t i c l e
i n f o
Article history: Received 21 November 2018 Received in revised form 6 January 2019 Accepted 15 January 2019 Available online 23 January 2019 Keywords: Semiconductors Ag2O/Na3Bi(PO4)2 Microstructure Photocatalytic activity
a b s t r a c t Novel Na3Bi(PO4)2 nanostructures were synthesized for the first time by a facile hydrothermal reaction. Ag2O nanoparticles were deposited on the surface of Na3Bi(PO4)2 nanostructures by the chemical precipitation method. The photocatalytic performance of the pure Ag2O nanoparticles, Na3Bi(PO4)2 nanostructures and Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures were evaluated by photocatalytic decolorization of Rhodamine B (RhB) solution under visible-light irradiation. Compared with the pure Na3Bi(PO4)2 nanostructures, the Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures exhibited an obviously enhanced photocatalytic activity. And the photocatalytic stability of the Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures is much higher than that of the Ag2O nanoparticles. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the semiconductor-based photocatalysts with wide potential applications in environmental purification and solar energy conversion have drawn a widely public attention. And the preparation of a new visible-light-driven photocatalyst has become an area of growing interest as the visible light accounts for a large proportion of the solar spectrum. Bismuth containing compounds is a new type of photocatalytic materials due to their incomparable oxidation ability. Therefore, a variety of bismuth containing compounds such as BiOX (X = Cl, Br, I) [1], BiVO4 [2], Bi2O2CO3 [3], etc. have been synthesized. However, the photocatalytic performance under sunlight is poor with respect to Degussa P25 TiO2. Ag2O with a direct band gap of 1.46 eV is a kind of p-type semiconductors. And this band gap closes to the ideal value that required for photocatalytic applications depending on the charge carrier-transfer mechanism. On the other hand, Ag2O shows many excellent properties such as high efficiency, controllability, and facile synthesis processes. Thus, it may be a good idea to fabricate p-n heterojunction photocatalyst using Ag2O to improve the photocatalytic activity of bismuth containing compounds by reducing the charge recombination on semiconductor [4–8]. In the present work, novel Na3Bi(PO4)2 nanostructures were synthesized by a facile hydrothermal reaction. Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures were prepared with these Na3Bi ⇑ Corresponding author. E-mail address: [email protected] (K. Wang). https://doi.org/10.1016/j.matlet.2019.01.094 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
(PO4)2 as substrate materials. The Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures showed an obviously enhanced photocatalytic activity as compared with the pure Na3Bi(PO4)2. The photocatalytic stability of Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures is much higher than that of the Ag2O nanoparticles. This work demonstrates that the Na3Bi(PO4)2 three-dimensional structure is an excellent architecture for loading of Ag2O to build a highly efficient heterojunction photocatalyst.
2. Experimental Preparation: AR-grade reagents were used in our experiment. Na2HPO412H2O (14.33 g, 40 mmol) was put into 80 mL deionized water, then NaBiO32H2O powder (0.79 g, 2.5 mmol) was dispersed into above solution. The resultant mixture was put into 100 mL Teflon-lined stainless-steel autoclave and maintained at 170 °C in an oven for 24 h. The as-prepared sample was labeled as S-0. Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures labeled as S-1 were prepared by a facile solution precipitation method [8]. Ag2O nanoparticles (labeled as S-2) were also prepared by the above solution precipitation method. Characterization: The crystal structures were recorded on an Xray diffractometer (D/Max-Ultima IV, Rigaku, Japan). The microstructures were observed by a field emission scanning electron microscope (FE-SEM, SUPRA55 SAPPHIRE, ZEISS, Germany) and a transmission electron microscope (TEM, JEM-1200EX, JEOL, Japan). The UV–vis diffuse reflectance spectra were analyzed on a UV–vis spectrophotometer (Cary 5000, Varian, USA).
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H. Xue et al. / Materials Letters 242 (2019) 39–41
RhB was used to evaluate the visible-light photocatalytic performance of the samples. In a typical process, 100 mg of the samples were added to 100 mL of RhB solution with a concentration of 10 mg/L, and stirred in the dark for 30 min. Then the solution was irradiated with visible light from a 300 W Xe lamp. After centrifugation of the photocatalyst, the degradation of RhB solution was measured. In order to investigate the reusability of the photocatalyst, the sample was separated again by centrifugation, and dispersed in fresh RhB solution with the same concentration as the first cycle. And other experimental parameters remain unchanged in the next cycle.
3. Results and discussion Fig. 1 shows the XRD diffraction peaks of the sample S-0, which match well with the monoclinic structure of Na3Bi(PO4)2 (JCPDS 41-0178), indicating the pure Na3Bi(PO4)2 could be obtained by
Fig. 1. XRD patterns of the samples.
this facile hydrothermal method. It can also be seen in Fig. 1 that the diffraction peaks of the sample S-2 can be indexed to the cubic structure of pure Ag2O (JCPDS 76-1393). All the XRD diffraction peaks of the sample S-1 are consistent with sample S-0, this is because the amount of Ag2O is very low and the main diffraction peaks of cubic structure Ag2O are overlapped with Na3Bi(PO4)2. However, several peaks at 32.6°, 37.8° and 54.5° (marked with circles) are broaden obviously, which are corresponding to (1 1 1), (2 0 0) and (2 2 0) facets of Ag2O respectively, implying that the heterogeneous materials of Ag2O/Na3Bi(PO4)2 were indeed synthesized by this simple precipitation method at room temperature. As shown in Fig. 2a, the sample S-0 consisting of nanosheets exhibits three-dimensional structure. Hence, it may be a good substrate to form heterogeneous materials. Obviously the as-prepared sample S-1 (Fig. 2b) has two phases, the phase with a similar morphology to sample S-0 is Na3Bi(PO4)2, and the small nanoparticles on the hierarchical nanostructures are Ag2O. In order to further verify the composition of the nanoparticles, TEM image of sample S-1 is presented in Fig. 2c. It is obvious that some nanoparticles are firmly assembled on the surface of hierarchical nanostructures, which is consistent with the SEM results. The HRTEM image shown in Fig. 2d displayed the lattice spacing of the nanoparticle is 0.24 nm, corresponding well to the (2 0 0) plane of the cubic structure Ag2O. These results further confirm the formation of Ag2O/ Na3Bi(PO4)2 materials. And this is because the Ag+ in the solution may be adsorbed on the surface of Na3Bi(PO4)2 according to Fajans rules, with adding NaOH the Ag2O/Na3Bi(PO4)2 heterogeneous nanostructure materials were obtained finally. Fig. 3a shows the UV–vis diffuse reflectance spectra of the samples. It is found that the absorption edges of the samples S-0 and S2 locate at approximately 325 and 800 nm, respectively. It can also be observed that there are obvious red-shift and dramatic enhancement of visible-light absorption over sample S-1. This result indicates that the sample S-2 can serve as an effective visible-light sensitized material for the sample S-0. In order to investigate the band gaps of the samples, the following equation ahv = A(hv Eg)n/2 is used. In the equation, a, hv, Eg and A are the absorption coefficient, the photonic energy, band gap, and a constant, respectively. For direct and indirect gap semiconduct,
Fig. 2. SEM and TEM images of the samples S-0 (a) and S-1 (b, c, d).
H. Xue et al. / Materials Letters 242 (2019) 39–41
41
Fig. 3. UV–vis absorption spectra (a), band gaps (b), photocatalytic activities (c) of the samples and reusability (d) of the samples S-0, S-1.
n is 1 and 4, respectively. Both Ag2O and Na3Bi(PO4)2 are indirect gap semiconducts, the value of n is 4. The Eg can be estimated by extrapolating the straight line to the x-axis, and the Eg of the samples S-0 and S-2 is 4.31 and 1.48 eV (as shown in Fig. 3b). The Eg of the sample S-1 is found to be 3.87 and 2.45 eV. So the Ag2O/Na3Bi (PO4)2 with narrow Eg and enhanced visible-light response ability could be prepared by loading Ag2O on the Na3Bi(PO4)2 nanostructures. As shown in Fig. 3c, a low degradation efficiency of RhB over sample S-0 could be observed, which may due to the low absorption ability in visible light region. The degradation efficiency over sample S-1 has a marked improvement as compared with that over sample S-0. Therefore, the Ag2O that induced into the photocatalyst system plays a crucial role in improving the photocatalytic activity of Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures. It can be seen from Fig. 3d, the photocatalytic activity of Ag2O decreases remarkably after the first cycle, and this is because Ag2O may be reduced into Ag by photogenerated electrons. Although the photocatalytic performance of the Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures is slightly lower than that of the Ag2O nanoparticles, the photocatalytic stability of the Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures is much higher. This should be attributed to the effective interfacial charge transfer resulted from the fabrication of a heterojunction [9]. 4. Conclusions For the first time, Na3Bi(PO4)2 nanostructures were synthesized successfully by a facile hydrothermal method. Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures were also prepared by the chemical precipitation method. The as-prepared Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures have an enhanced visible light
photocatalytic performance as compared with the pure Na3Bi (PO4)2 nanostructures. Although the photocatalytic performance of the Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures is slightly lower than that of the Ag2O nanoparticles, the photocatalytic stability of the Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures is much higher. The Na3Bi(PO4)2 nanostructures prepared by this method may be further extended to synthesize a variety of Bicontaining heterogeneous nanostructure photocatalysts with different morphologies for great potential applications. Conflict of interest None. Acknowledgement This work was supported by the Science and Technology Support Program of Hebei Province (No. 15211414). References [1] J. Li, H. Li, G.M. Zhan, L.Z. Zhang, Acc. Chem. Res. 50 (2017) 112–121. [2] Y. He, R.H. Yuan, M.K.H. Leung, Mater. Lett. 236 (2019) 394–397. [3] H.D. Xue, F.L. Wang, Q.J. Bai, H.K. Sun, J. Qu, P. Liu, K. Wang, Mater. Lett. 219 (2018) 148–151. [4] P. Li, H.Q. Fan, Y. Cai, Sens. Actuator B-Chem. 185 (2013) 110–116. [5] M.C. Zhang, H.Q. Fan, N. Zhao, H.J. Peng, X.H. Ren, W.J. Wang, H. Li, G.Y. Chen, Y. N. Zhu, X.B. Jiang, P. Wu, Chem. Eng. J. 347 (2018) 291–300. [6] H.L. Tian, H.Q. Fan, J.W. Ma, L.T. Ma, G.Z. Dong, Electrochim. Acta 247 (2017) 787–794. [7] L.T. Ma, H.Q. Fan, K. Fu, S.H. Lei, Q.Z. Hu, H.T. Huang, G.P. He, ACS Sustainable Chem. Eng. 5 (2017) 7093–7103. [8] G. Liu, G.H. Wang, Z.H. Hu, Y.R. Su, L. Zhao, Appl. Surf. Sci. 465 (2019) 902–910. [9] C.L. Yu, G. Li, S. Kumar, K. Yang, R.C. Jin, Adv. Mater. 26 (2014) 892–898.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Preparation of novel Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures with enhanced visible-light responsive photocatalytic activity Hongdan Xue, Ke Wang ⇑, Yongqing Bai, Fei He, Haijun Yang, Fuli Wang, Pu Liu Department of Mathematics and Physics, Hebei Institute of Architecture and Civil Engineering, Zhangjiakou 075000, China
a r t i c l e
i n f o
Article history: Received 21 November 2018 Received in revised form 6 January 2019 Accepted 15 January 2019 Available online 23 January 2019 Keywords: Semiconductors Ag2O/Na3Bi(PO4)2 Microstructure Photocatalytic activity
a b s t r a c t Novel Na3Bi(PO4)2 nanostructures were synthesized for the first time by a facile hydrothermal reaction. Ag2O nanoparticles were deposited on the surface of Na3Bi(PO4)2 nanostructures by the chemical precipitation method. The photocatalytic performance of the pure Ag2O nanoparticles, Na3Bi(PO4)2 nanostructures and Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures were evaluated by photocatalytic decolorization of Rhodamine B (RhB) solution under visible-light irradiation. Compared with the pure Na3Bi(PO4)2 nanostructures, the Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures exhibited an obviously enhanced photocatalytic activity. And the photocatalytic stability of the Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures is much higher than that of the Ag2O nanoparticles. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the semiconductor-based photocatalysts with wide potential applications in environmental purification and solar energy conversion have drawn a widely public attention. And the preparation of a new visible-light-driven photocatalyst has become an area of growing interest as the visible light accounts for a large proportion of the solar spectrum. Bismuth containing compounds is a new type of photocatalytic materials due to their incomparable oxidation ability. Therefore, a variety of bismuth containing compounds such as BiOX (X = Cl, Br, I) [1], BiVO4 [2], Bi2O2CO3 [3], etc. have been synthesized. However, the photocatalytic performance under sunlight is poor with respect to Degussa P25 TiO2. Ag2O with a direct band gap of 1.46 eV is a kind of p-type semiconductors. And this band gap closes to the ideal value that required for photocatalytic applications depending on the charge carrier-transfer mechanism. On the other hand, Ag2O shows many excellent properties such as high efficiency, controllability, and facile synthesis processes. Thus, it may be a good idea to fabricate p-n heterojunction photocatalyst using Ag2O to improve the photocatalytic activity of bismuth containing compounds by reducing the charge recombination on semiconductor [4–8]. In the present work, novel Na3Bi(PO4)2 nanostructures were synthesized by a facile hydrothermal reaction. Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures were prepared with these Na3Bi ⇑ Corresponding author. E-mail address: [email protected] (K. Wang). https://doi.org/10.1016/j.matlet.2019.01.094 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
(PO4)2 as substrate materials. The Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures showed an obviously enhanced photocatalytic activity as compared with the pure Na3Bi(PO4)2. The photocatalytic stability of Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures is much higher than that of the Ag2O nanoparticles. This work demonstrates that the Na3Bi(PO4)2 three-dimensional structure is an excellent architecture for loading of Ag2O to build a highly efficient heterojunction photocatalyst.
2. Experimental Preparation: AR-grade reagents were used in our experiment. Na2HPO412H2O (14.33 g, 40 mmol) was put into 80 mL deionized water, then NaBiO32H2O powder (0.79 g, 2.5 mmol) was dispersed into above solution. The resultant mixture was put into 100 mL Teflon-lined stainless-steel autoclave and maintained at 170 °C in an oven for 24 h. The as-prepared sample was labeled as S-0. Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures labeled as S-1 were prepared by a facile solution precipitation method [8]. Ag2O nanoparticles (labeled as S-2) were also prepared by the above solution precipitation method. Characterization: The crystal structures were recorded on an Xray diffractometer (D/Max-Ultima IV, Rigaku, Japan). The microstructures were observed by a field emission scanning electron microscope (FE-SEM, SUPRA55 SAPPHIRE, ZEISS, Germany) and a transmission electron microscope (TEM, JEM-1200EX, JEOL, Japan). The UV–vis diffuse reflectance spectra were analyzed on a UV–vis spectrophotometer (Cary 5000, Varian, USA).
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H. Xue et al. / Materials Letters 242 (2019) 39–41
RhB was used to evaluate the visible-light photocatalytic performance of the samples. In a typical process, 100 mg of the samples were added to 100 mL of RhB solution with a concentration of 10 mg/L, and stirred in the dark for 30 min. Then the solution was irradiated with visible light from a 300 W Xe lamp. After centrifugation of the photocatalyst, the degradation of RhB solution was measured. In order to investigate the reusability of the photocatalyst, the sample was separated again by centrifugation, and dispersed in fresh RhB solution with the same concentration as the first cycle. And other experimental parameters remain unchanged in the next cycle.
3. Results and discussion Fig. 1 shows the XRD diffraction peaks of the sample S-0, which match well with the monoclinic structure of Na3Bi(PO4)2 (JCPDS 41-0178), indicating the pure Na3Bi(PO4)2 could be obtained by
Fig. 1. XRD patterns of the samples.
this facile hydrothermal method. It can also be seen in Fig. 1 that the diffraction peaks of the sample S-2 can be indexed to the cubic structure of pure Ag2O (JCPDS 76-1393). All the XRD diffraction peaks of the sample S-1 are consistent with sample S-0, this is because the amount of Ag2O is very low and the main diffraction peaks of cubic structure Ag2O are overlapped with Na3Bi(PO4)2. However, several peaks at 32.6°, 37.8° and 54.5° (marked with circles) are broaden obviously, which are corresponding to (1 1 1), (2 0 0) and (2 2 0) facets of Ag2O respectively, implying that the heterogeneous materials of Ag2O/Na3Bi(PO4)2 were indeed synthesized by this simple precipitation method at room temperature. As shown in Fig. 2a, the sample S-0 consisting of nanosheets exhibits three-dimensional structure. Hence, it may be a good substrate to form heterogeneous materials. Obviously the as-prepared sample S-1 (Fig. 2b) has two phases, the phase with a similar morphology to sample S-0 is Na3Bi(PO4)2, and the small nanoparticles on the hierarchical nanostructures are Ag2O. In order to further verify the composition of the nanoparticles, TEM image of sample S-1 is presented in Fig. 2c. It is obvious that some nanoparticles are firmly assembled on the surface of hierarchical nanostructures, which is consistent with the SEM results. The HRTEM image shown in Fig. 2d displayed the lattice spacing of the nanoparticle is 0.24 nm, corresponding well to the (2 0 0) plane of the cubic structure Ag2O. These results further confirm the formation of Ag2O/ Na3Bi(PO4)2 materials. And this is because the Ag+ in the solution may be adsorbed on the surface of Na3Bi(PO4)2 according to Fajans rules, with adding NaOH the Ag2O/Na3Bi(PO4)2 heterogeneous nanostructure materials were obtained finally. Fig. 3a shows the UV–vis diffuse reflectance spectra of the samples. It is found that the absorption edges of the samples S-0 and S2 locate at approximately 325 and 800 nm, respectively. It can also be observed that there are obvious red-shift and dramatic enhancement of visible-light absorption over sample S-1. This result indicates that the sample S-2 can serve as an effective visible-light sensitized material for the sample S-0. In order to investigate the band gaps of the samples, the following equation ahv = A(hv Eg)n/2 is used. In the equation, a, hv, Eg and A are the absorption coefficient, the photonic energy, band gap, and a constant, respectively. For direct and indirect gap semiconduct,
Fig. 2. SEM and TEM images of the samples S-0 (a) and S-1 (b, c, d).
H. Xue et al. / Materials Letters 242 (2019) 39–41
41
Fig. 3. UV–vis absorption spectra (a), band gaps (b), photocatalytic activities (c) of the samples and reusability (d) of the samples S-0, S-1.
n is 1 and 4, respectively. Both Ag2O and Na3Bi(PO4)2 are indirect gap semiconducts, the value of n is 4. The Eg can be estimated by extrapolating the straight line to the x-axis, and the Eg of the samples S-0 and S-2 is 4.31 and 1.48 eV (as shown in Fig. 3b). The Eg of the sample S-1 is found to be 3.87 and 2.45 eV. So the Ag2O/Na3Bi (PO4)2 with narrow Eg and enhanced visible-light response ability could be prepared by loading Ag2O on the Na3Bi(PO4)2 nanostructures. As shown in Fig. 3c, a low degradation efficiency of RhB over sample S-0 could be observed, which may due to the low absorption ability in visible light region. The degradation efficiency over sample S-1 has a marked improvement as compared with that over sample S-0. Therefore, the Ag2O that induced into the photocatalyst system plays a crucial role in improving the photocatalytic activity of Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures. It can be seen from Fig. 3d, the photocatalytic activity of Ag2O decreases remarkably after the first cycle, and this is because Ag2O may be reduced into Ag by photogenerated electrons. Although the photocatalytic performance of the Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures is slightly lower than that of the Ag2O nanoparticles, the photocatalytic stability of the Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures is much higher. This should be attributed to the effective interfacial charge transfer resulted from the fabrication of a heterojunction [9]. 4. Conclusions For the first time, Na3Bi(PO4)2 nanostructures were synthesized successfully by a facile hydrothermal method. Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures were also prepared by the chemical precipitation method. The as-prepared Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures have an enhanced visible light
photocatalytic performance as compared with the pure Na3Bi (PO4)2 nanostructures. Although the photocatalytic performance of the Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures is slightly lower than that of the Ag2O nanoparticles, the photocatalytic stability of the Ag2O/Na3Bi(PO4)2 heterogeneous nanostructures is much higher. The Na3Bi(PO4)2 nanostructures prepared by this method may be further extended to synthesize a variety of Bicontaining heterogeneous nanostructure photocatalysts with different morphologies for great potential applications. Conflict of interest None. Acknowledgement This work was supported by the Science and Technology Support Program of Hebei Province (No. 15211414). References [1] J. Li, H. Li, G.M. Zhan, L.Z. Zhang, Acc. Chem. Res. 50 (2017) 112–121. [2] Y. He, R.H. Yuan, M.K.H. Leung, Mater. Lett. 236 (2019) 394–397. [3] H.D. Xue, F.L. Wang, Q.J. Bai, H.K. Sun, J. Qu, P. Liu, K. Wang, Mater. Lett. 219 (2018) 148–151. [4] P. Li, H.Q. Fan, Y. Cai, Sens. Actuator B-Chem. 185 (2013) 110–116. [5] M.C. Zhang, H.Q. Fan, N. Zhao, H.J. Peng, X.H. Ren, W.J. Wang, H. Li, G.Y. Chen, Y. N. Zhu, X.B. Jiang, P. Wu, Chem. Eng. J. 347 (2018) 291–300. [6] H.L. Tian, H.Q. Fan, J.W. Ma, L.T. Ma, G.Z. Dong, Electrochim. Acta 247 (2017) 787–794. [7] L.T. Ma, H.Q. Fan, K. Fu, S.H. Lei, Q.Z. Hu, H.T. Huang, G.P. He, ACS Sustainable Chem. Eng. 5 (2017) 7093–7103. [8] G. Liu, G.H. Wang, Z.H. Hu, Y.R. Su, L. Zhao, Appl. Surf. Sci. 465 (2019) 902–910. [9] C.L. Yu, G. Li, S. Kumar, K. Yang, R.C. Jin, Adv. Mater. 26 (2014) 892–898.