DOI: 10.1002/chem.201405047
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A Bismuth-Based Metal–Organic Framework as an Efficient Visible-Light-Driven Photocatalyst Guanzhi Wang,[a] Qilong Sun,[b] Yuanyuan Liu,*[a] Baibiao Huang,*[a] Ying Dai,[b] Xiaoyang Zhang,[a] and Xiaoyan Qin[a] vantage of Bi is that the coordination mode with organic ligand is geometrically flexible, which is beneficial for MOFs construction.[24] Therefore, bismuth-based materials are more appropriate candidates for photocatalysis, especially for waste water purification and harmful pollutant removal. Nevertheless, to date, little attention has been paid to bismuth-based MOFs in terms of photocatalysts. Herein, we report a highly efficient photocatalyst composed of Bi3 + and H2mna (2-mercaptonicotinic acid) under visiblelight irradiation, which was denoted as Bi-mna. As far as we know, this is the first example of bismuth-based MOFs exhibiting high photocatalytic activity. The powders of this MOF were synthesized by a solvothermal reaction, which contains Bi(NO3)3, H2mna, and KOH in DMF in 100 8C for three days. To determine the crystal structure and purity of the as-prepared Bi-mna, XRD pattern was measured (Supporting Information, Figure S1). All of the diffraction peaks are identical to the simulation result obtained from single crystal (orthorhombic phase, space group P212121).[25] No other impurities such as Bi2O3, Bi2S3, or metal Bi are found. The strong intensity of the peaks shows a good crystallinity of Bi-mna. As reported by Sun, there is a unique six-coordinate Bi3 + atom (Supporting Information, Figure S2), which coordinates with four mna2 ligands through S, N, and OCOO atoms. Furthermore, planar rings among C, N, S, and Bi are observed, in which delocalization of electrons takes place, and this is believed to be advantageous for electron transfer. Meanwhile, two distinct homo-chiral helices along the b axis and the c axis form two types of channels in the 3D framework (Supporting Information, Figure S3a). The left-handed channels are small, as the pyridine rings of the mna ligands point to the helical channels. And the righthanded channels are filled by the dimethylammonium cations (from the decomposition of DMF), which also make the channels small. Furthermore, the release of dimethylammonium ion would lead to the collapse of the whole framework (TG/DTA results; Supporting Information, Figure S4). The N2 adsorption– desorption isotherms at 77 K are shown in the Supporting Information, Figure S3b. The isotherms display a typical type IV curve and a hysteresis loop at medium relative pressure of the desorption branch. As expected, the BET surface area is determined to be 35.2 cm2 g 1, with a pore volume of 0.085 cm3 g 1, which is much smaller than typical MOFs owing to the unique structure of Bi-mna. XPS spectroscopy, which is known for its high surface sensitivity, was performed to investigate the purity of the as-prepared Bi-mna. The Bi 4d and S 2s orbitals were investigated
Abstract: A visible-light-responsive bismuth-based metal– organic framework (Bi-mna) is demonstrated to show good photoelectric and photocatalytic properties. Combining experimental and theoretical results, a ligand-toligand charge transfer (LLCT) process is found to be responsible for the high performance, which gives rise to a longer lifetime of photogenerated charge carriers. Our results suggest that bismuth-based MOFs could be promising candidates for the development of efficient visiblelight photocatalysts.
Metal–organic frameworks (MOFs), a class of crystalline-hybrid materials, have drawn unprecedented attention not only because of their unique properties, such as high specific surface areas, structural diversity, and tunable pore channels,[1] but also because of the resulting wide applications in selective gas separation and adsorption, gas storage, optical materials, and also catalysis.[2–9] Particularly, MOFs were used as photocatalysts over the past few years to produce hydrogen from water,[10] degrade organic pollutants,[11–13] and reduce CO2 into useful fuels.[14, 15] The first member of photocatalytic active MOFs is MOF-5, which consists of terephthalate and Zn4O clusters. Garcia et al. first reported the semiconductor behavior of MOF5 and confirmed its photocatalytic activity by degradation of phenol.[16] However, the framework of MOF-5 collapses when exposed to water. After that, a series of water stable MOFs with good photocatalytic performance were developed, such as MIL-125(Ti), NH2-MIL-125(Ti),[10, 17] UiO-66, and NH2-UiO66.[18, 19] As can be seen, the selection of metals is often confined to Ti and Zr. Therefore, searching for new photocatalytically active MOFs composed of other metals is necessary. Bismuth(III)-containing inorganic semiconductors show high performance in photocatalysis, such as BiVO4,[20] BiOX (X = Cl, Br, I),[21, 22] and Bi2O2CO3.[23] Furthermore, bismuth-based photocatalysts are of low toxicity and earth abundance. Another ad[a] G. Wang, Prof. Y. Liu, Prof. B. Huang, X. Zhang, X. Qin State Key Laboratory of Crystal Materials Shandong University, Jinan, 250100 (P.R. China) E-mail: [email protected] [email protected] [b] Q. Sun, Prof. Y. Dai School of Physics, Shandong University, Jinan, 250100 (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405047. Chem. Eur. J. 2014, 20, 1 – 5
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Communication (Supporting Information, Figure S5), considering the overlaps of binding energy between Bi 4f and S 2p. Binding energies at 442.0 eV and 466.0 eV are observed, which are assigned to Bi 4d5/2 and Bi 4d3/2, respectively. This result indicates that Bi forms the MOF Bi-mna in its + 3 valence state. No other peaks associated with metal Bi were detected and fitted.[26] Only one peak at 226.0 eV in S 2s region indicates the identical chemical environment of S in the as-prepared samples. Furthermore, this result suggests the existence of S2 in the final product. Furthermore, the TG/DTA results also exclude the existence of any impurities (see the Supporting Information for details of TG/DTA). Transient photocurrent responses were carried out to investigate the photoelectric properties of Bi-mna. As can be clearly seen from Figure 1 a, a strong response is observed under visible-light irradiation, which is attributable to Bi-mna, as no other impurities are observed. This result indicates that Bi-mna is able to generate electron–hole pairs upon light irradiation, and these photogenerated carriers can be efficiently separated. These results imply that Bi-mna can be used as a potential photocatalyst.
Electrochemical impedance spectroscopy (EIS) further confirms that the photogenerated carriers are able to be efficiently separated and transferred to the surface of Bi-mna. Figure 1 b shows EIS Nyquist plots of the Bi-mna electrode in the absence and presence of light irradiation. The Nyquist plot consists of one dominant semicircle, the diameter of which is related to charge-transfer resistance at Bi-mna/electrolyte interface. The diameter of the arc radius on the EIS Nyquist plot in the presence of visible light is smaller than that in the dark, indicating the efficient charge separation and transport.[27] Based on the above discussion, it is reasonable to expect that Bi-mna displays high photocatalytic activity. Decomposition of organic dyes Rhodamine B (RhB) and methylene blue (MB), common pollutants existing in industrial wastewater, over Bi-mna were carried out under visible-light irradiation, considering that Bi-mna displays visible light absorption (the UV/Vis diffuse reflectance spectra of the as-prepared Bi-mna is shown in the Supporting Information, Figure S6). The results are shown in Figure 2. It is obvious that Bi-mna shows much better performance than that of the commercial a-Bi2O3. More than 95 % RhB is decomposed within 2 h over Bimna, while less than 10 % is decomposed over commercial aBi2O3 (Figure 2 a). Bi-mna shows similar behavior for the photodegradation of MB (Supporting Information, Figure S7). MB is hardly degraded over commercial a-Bi2O3 ; on the contrary, the decomposition of MB over Bi-mna occurs smoothly and efficiently. Apart from dye degradation, O2 evolution from water over Bi-mna was further carried out, which is regarded as the key process of water splitting. As can be seen from Figure 2 b, Bi-mna exhibits high activity towards O2 generation under visible-light irradiation (l 420 nm), and the generation rate is about 216 mL h 1. When light was turned off, no oxygen increase was detected, confirming that the oxygen production is indeed driven by light irradiation. To find the reason for the high photocatalytic activity of Bimna, we first investigated the properties of the surrounding bonding type of the Bi3 + atom using ELF and the calculated electron localization function plot as shown in Figure 3 a. We mainly focus on the Bi S, Bi N, and Bi O2 bonds, which provide possible pathways for charge transfer from ligand to metal. The bonding type of Bi S is more akin to the C C bond, as they have to be nodded jointly and the value of ELF is close to the maximum. Therefore, the Bi S bond displays dominantly covalent character. As expected, the picture is similar to the bonds of Bi N and Bi O2, with a difference of small diminution in covalent component. Generally speaking, the more delocalized the electrons are in the covalent bond, the more conducive it is for charge transfer. We then checked the Fukui functions of the Bi-mna, which are well-known to provide a measurement of the change in chemical potential as the number of electrons changes.[28] Regions where F + (r) is large will stabilize the uptake of charge from electron donors, while regions with high F (r) will readily donate charge to electron acceptors. It can be seen from Figure 3 b that Fukui functions are highly localized around the ligands, while the site of Bi3 + has no distribution. This phenomenon demonstrates that the excited electrons should move
Figure 1. a) Transient photocurrent responses of Bi-mna with bias = 0 V (black line) and bias = 0.2 V (gray line). b) EIS Nyquist plots the Bi-mna electrode in the dark and under visible light (l 420 nm) illumination. The electrolyte solution was 0.2 m LiCl aqueous solution.
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Figure 2. a) Photodegradation of 100 mL of 20 mg L 1 RhB over 100 mg Bimna and commercial a-Bi2O3 irradiation in the presence of oxygen. b) Time course of photocatalytic O2 evolution from water with 50 mg Bi-mna mixed in 50 mL aqueous solution containing 100 mg AgNO3 as electron sacrificial agent. The light source is a 300W Xe arc lamp with a cutoff filter to provide visible light (l 420 nm).
from the S atom to the nonadjacent pyridine ring. According to this result, we can determine that the Bi atom acts as bridges between ligands in charge transfer. The results of total density of states (TDOS) and partial density of states (PDOS) of this system confirm the above discussions on the ELF and Fukui function issues. From Figure 3 c it can be seen that the upper valence band (VB) is mainly contributed by states of S atoms, which confirms the results of the Fukui functions that the sites of S atoms should be readily donate charge to electron acceptors. Furthermore, the states of Bi and S in the valence band (VB) distribute energetically in the similar range, which indicates they can effectively hybrid with each other and form typical covalent bond. This conclusion is consistent with the ELF results mentioned above. Obviously, the electrons will transfer from the bonding orbital of the Bi S to the antibonding orbital located in higher energy range in conduction band (CB) first. Subsequently, the excited electrons will flow into the orbitals of O atoms in the CB, which has a lower energy and is more stable. Finally, the charge transfer could be accomplished with the help of the Bi atoms. Based on these results, a ligand-to-ligand charge transfer (LLCT) process can be proposed to occur in Bi-mna. This is Chem. Eur. J. 2014, 20, 1 – 5
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Figure 3. a) Calculated electron localization function (ELF) plots for Bi-mna. b) Fukui function F (r) and F + (r) for Bi-mna. The isosurface value is 0.0015 e 3. Bi purple, C brown, O red, N gray, H pink. c) Calculated total density (TDOS) and partial density of states (PDOS) of Bi-mna.
different from the mostly accepted mechamism of MOFs, that is, ligand-to-metal charge transfer (LMCT),[19] which is most likely due to the closed shell character of Bi3 + (d10s2). 3
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Communication Photoluminescence (PL) spectroscopy, a powerful tool to study electron-transfer process,[29] confirms the results of theoretical calculation. Steady-state PL spectra were first investigated (Supporting Information, Figure S8). H2mna displays strong emissions at 475 and 530 nm; however, the maximum emissions of Bi-mna are blue-shifted to 430 and 475 nm. The observed blue-shift of the emissions may be caused by the variations of the energy gap of the frontier orbitals after the organic coordination with the metal centers. Time-resolved PL spectrum of Bi-mna (Figure 4) indicates an average life time of
Keywords: bismuth · metal–organic frameworks · photocatalysis · photoelectric properties · semiconducting materials [1] L. J. Murray, M. Dinca˘, J. R. Long, Chem. Soc. Rev. 2009, 38, 1294 – 1314. [2] O. M. Yaghi, C. E. Davis, G. M. Li, H. L. Li, J. Am. Chem. Soc. 1997, 119, 2861 – 2968. [3] J. C. Rowsell, O. M. Yaghi, Angew. Chem. Int. Ed. 2005, 44, 4670 – 4679; Angew. Chem. 2005, 117, 4748 – 4758. [4] O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi, J. Kim, Nature 2003, 423, 705 – 714. [5] H. K. Chae, D. Y. Siberio-Prez, J. Kim, Y. B. Go, M. Eddaoudi, A. J. Matzger, M. O’Keeffe, O. M. Yaghi, Nature 2004, 427, 523 – 527. [6] N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O’Keeffe, O. M. Yaghi, Science 2003, 300, 1127 – 1129. [7] H. Reinsch, M. A. Van der Veen, B. Gil, B. Marszalek, T. Verbiest, D. De Vos, N. Stock, Chem. Mater. 2013, 25, 17 – 26. [8] J. Y. Lee, O. K. Farha, J. Robert, K. A. Scheidt, S. T. Nguyen, J. T. Hupp, Chem. Soc. Rev. 2009, 38, 1450 – 1459. [9] A. Corma, H. Garca, F. X. Llabrs i Xamena, Chem. Rev. 2010, 110, 4606 – 4655. [10] Y. Horiuchi, T. Toyao, M. Saito, K. Mochizuki, M. Iwata, H. Higashimura, M. Anpo, M. Matsuoka, J. Phys. Chem. C 2012, 116, 20848 – 20853. [11] H. Yang, X.-W. He, F. Wang, Y. Kang, J. Zhang, J. Mater. Chem. 2012, 22, 21849 – 21851. [12] L. L. Wen, J. B. Zhao, K. L. Lv, Y. H. Wu, K. J. Deng, X. K. Leng, D. F. Li, Cryst. Growth Des. 2012, 12, 1603 – 1612. [13] J. L. Wang, C. Wang, W. B. Lin, ACS Catal. 2012, 2, 2630 – 2640. [14] Y. Y. Liu, Y. M. Yang, Q. L. Sun, Z. Y. Wang, B. B. Huang, Y. Dai, X. Y. Zhang, X. Y. Qin, ACS Appl. Mater. Interfaces 2013, 5, 7654 – 7658. [15] C. Wang, Z. G. Xie, K. E. deKrafft, W. B. Lin, J. Am. Chem. Soc. 2011, 133, 13445 – 13454. [16] F. X. Llabrs i Xamena, A. Corma, H. Garca, J. Phys. Chem. C 2007, 111, 80 – 85. [17] Y. H. Fu, D. R. Sun, Y. J. Chen, R. K. Huang, Z. X. Ding, X. Z. Fu, Z. H. Li, Angew. Chem. Int. Ed. 2012, 51, 3364 – 3367; Angew. Chem. 2012, 124, 3420 – 3423. [18] C. Gomes Silva, I. Luz, F. X. Llabrs i Xamena, A. Corma, H. Garca, Chem. Eur. J. 2010, 16, 11133 – 11138. [19] D. R. Sun, Y. H. Fu, W. J. Liu, L. Ye, D. K. Wang, L. Yang, X. Z. Fu, Z. H. Li, Chem. Eur. J. 2013, 19, 14279 – 14285. [20] Y. Y. Liu, B. B. Huang, Y. Dai, X. Y. Zhang, X. Y. Qin, M. H. Jiang, M.-H. Whangbo, Catal. Commun. 2009, 11, 210 – 213. [21] H. F. Cheng, B. B. Huang, Y. Dai, X. Y. Qin, X. Y. Zhang, Langmuir 2010, 26, 6618 – 6624. [22] Y. Y. Liu, W.-J. Son, J. B. Lu, B. B. Huang, Y. Dai, M.-H. Whangbo, Chem. Eur. J. 2011, 17, 9342 – 9349. [23] Y. Y. Liu, Z. Y. Wang, B. B. Huang, K. S. Yang, X. Y. Zhang, X. Y. Qin, Y. Dai, Appl. Surf. Sci. 2010, 257, 172 – 175. [24] A. Thirumurugan, A. K. Cheetham, Eur. J. Inorg. Chem. 2010, 24, 3823 – 3828. [25] Y. Q. Sun, S. Z. Ge, Q. Liu, J. C. Zhong, Y. P. Chen, CrystEngComm 2013, 15, 10188 – 10192. [26] D. F. Hou, X. L. Hu, P. Hu, W. Zhang, M. F. Zhang, Y. H. Huang, Nanoscale 2013, 5, 9764 – 9772. [27] Q. W. Huang, S. Q. Tian, D. W. Zeng, X. X. Wang, W. L. Song, Y. Y. Li, W. Xiao, C. S. Xie, ACS Catal. 2013, 3, 1477 – 1485. [28] W. Wei, Y. Dai, K. R. Lai, M. Guo, B. B. Huang, Chem. Phys. Lett. 2011, 510, 104 – 108. [29] D. Jarzab, F. Cordella, M. Lenes, F. B. Kooistra, P. W. M. Blom, J. C. Hummelen, M. A. Loi, J. Phys. Chem. B 2009, 113, 16513 – 16517.
Figure 4. Time-resolved PL spectra for H2mna and Bi-mna detected at 475 nm and 430 nm, respectively. The excitation source is a 377.8 nm laser. The red curve represents the instrument response rime.
1.1 ns, while that for H2mna is below the detection limit of the equipment (100 ps). The much longer lifetime is a result of LLCT process. A longer lifetime of the excited state of Bi-mna means the photogenerated charge carrier can go further before it decays back to the ground state, leading to more efficient electron–hole separation and thus higher photocatalytic activity. In summary, a visible-light-responsive bismuth–organic framework (Bi-mna) has been shown to exhibit good photoelectric and photocatalytic properties. According to theoretical calculations, a LLCT process is proposed to occur in Bi-mna. PL spectra suggests a much longer lifetime of photogenerated carriers compared with the organic ligand, which is believed to suppress electron–hole recombination and thus increase photocatalytic efficiency. Our results suggest that bismuth based MOFs could be promising candidates for the development of efficient visible light photocatalysts.
Acknowledgements This work was financially supported by a research grant from the National Basic Research Program of China (the 973 Program, No. 2013CB632401) and the National Natural Science Foundation of China (No. 21333006, 11374190, and 51021062).
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COMMUNICATION & Photocatalysis
A visible-light-responsive bismuthbased metal–organic framework (Bimna) is shown to exhibit good photoelectric and photocatalytic properties. Combined with the experimental and theoretical results (see electron localization function plot), a ligand-to-ligand charge transfer (LLCT) process is found to be responsible for the high performance. This process gives rise to a longer lifetime of the photogenerated charge carriers.
Chem. Eur. J. 2014, 20, 1 – 5
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G. Wang, Q. Sun, Y. Liu,* B. Huang,* Y. Dai, X. Zhang, X. Qin && – && A Bismuth-Based Metal–Organic Framework as an Efficient VisibleLight-Driven Photocatalyst
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A Bismuth-Based Metal–Organic Framework as an Efficient Visible-Light-Driven Photocatalyst Guanzhi Wang,[a] Qilong Sun,[b] Yuanyuan Liu,*[a] Baibiao Huang,*[a] Ying Dai,[b] Xiaoyang Zhang,[a] and Xiaoyan Qin[a] vantage of Bi is that the coordination mode with organic ligand is geometrically flexible, which is beneficial for MOFs construction.[24] Therefore, bismuth-based materials are more appropriate candidates for photocatalysis, especially for waste water purification and harmful pollutant removal. Nevertheless, to date, little attention has been paid to bismuth-based MOFs in terms of photocatalysts. Herein, we report a highly efficient photocatalyst composed of Bi3 + and H2mna (2-mercaptonicotinic acid) under visiblelight irradiation, which was denoted as Bi-mna. As far as we know, this is the first example of bismuth-based MOFs exhibiting high photocatalytic activity. The powders of this MOF were synthesized by a solvothermal reaction, which contains Bi(NO3)3, H2mna, and KOH in DMF in 100 8C for three days. To determine the crystal structure and purity of the as-prepared Bi-mna, XRD pattern was measured (Supporting Information, Figure S1). All of the diffraction peaks are identical to the simulation result obtained from single crystal (orthorhombic phase, space group P212121).[25] No other impurities such as Bi2O3, Bi2S3, or metal Bi are found. The strong intensity of the peaks shows a good crystallinity of Bi-mna. As reported by Sun, there is a unique six-coordinate Bi3 + atom (Supporting Information, Figure S2), which coordinates with four mna2 ligands through S, N, and OCOO atoms. Furthermore, planar rings among C, N, S, and Bi are observed, in which delocalization of electrons takes place, and this is believed to be advantageous for electron transfer. Meanwhile, two distinct homo-chiral helices along the b axis and the c axis form two types of channels in the 3D framework (Supporting Information, Figure S3a). The left-handed channels are small, as the pyridine rings of the mna ligands point to the helical channels. And the righthanded channels are filled by the dimethylammonium cations (from the decomposition of DMF), which also make the channels small. Furthermore, the release of dimethylammonium ion would lead to the collapse of the whole framework (TG/DTA results; Supporting Information, Figure S4). The N2 adsorption– desorption isotherms at 77 K are shown in the Supporting Information, Figure S3b. The isotherms display a typical type IV curve and a hysteresis loop at medium relative pressure of the desorption branch. As expected, the BET surface area is determined to be 35.2 cm2 g 1, with a pore volume of 0.085 cm3 g 1, which is much smaller than typical MOFs owing to the unique structure of Bi-mna. XPS spectroscopy, which is known for its high surface sensitivity, was performed to investigate the purity of the as-prepared Bi-mna. The Bi 4d and S 2s orbitals were investigated
Abstract: A visible-light-responsive bismuth-based metal– organic framework (Bi-mna) is demonstrated to show good photoelectric and photocatalytic properties. Combining experimental and theoretical results, a ligand-toligand charge transfer (LLCT) process is found to be responsible for the high performance, which gives rise to a longer lifetime of photogenerated charge carriers. Our results suggest that bismuth-based MOFs could be promising candidates for the development of efficient visiblelight photocatalysts.
Metal–organic frameworks (MOFs), a class of crystalline-hybrid materials, have drawn unprecedented attention not only because of their unique properties, such as high specific surface areas, structural diversity, and tunable pore channels,[1] but also because of the resulting wide applications in selective gas separation and adsorption, gas storage, optical materials, and also catalysis.[2–9] Particularly, MOFs were used as photocatalysts over the past few years to produce hydrogen from water,[10] degrade organic pollutants,[11–13] and reduce CO2 into useful fuels.[14, 15] The first member of photocatalytic active MOFs is MOF-5, which consists of terephthalate and Zn4O clusters. Garcia et al. first reported the semiconductor behavior of MOF5 and confirmed its photocatalytic activity by degradation of phenol.[16] However, the framework of MOF-5 collapses when exposed to water. After that, a series of water stable MOFs with good photocatalytic performance were developed, such as MIL-125(Ti), NH2-MIL-125(Ti),[10, 17] UiO-66, and NH2-UiO66.[18, 19] As can be seen, the selection of metals is often confined to Ti and Zr. Therefore, searching for new photocatalytically active MOFs composed of other metals is necessary. Bismuth(III)-containing inorganic semiconductors show high performance in photocatalysis, such as BiVO4,[20] BiOX (X = Cl, Br, I),[21, 22] and Bi2O2CO3.[23] Furthermore, bismuth-based photocatalysts are of low toxicity and earth abundance. Another ad[a] G. Wang, Prof. Y. Liu, Prof. B. Huang, X. Zhang, X. Qin State Key Laboratory of Crystal Materials Shandong University, Jinan, 250100 (P.R. China) E-mail: [email protected] [email protected] [b] Q. Sun, Prof. Y. Dai School of Physics, Shandong University, Jinan, 250100 (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405047. Chem. Eur. J. 2014, 20, 1 – 5
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Communication (Supporting Information, Figure S5), considering the overlaps of binding energy between Bi 4f and S 2p. Binding energies at 442.0 eV and 466.0 eV are observed, which are assigned to Bi 4d5/2 and Bi 4d3/2, respectively. This result indicates that Bi forms the MOF Bi-mna in its + 3 valence state. No other peaks associated with metal Bi were detected and fitted.[26] Only one peak at 226.0 eV in S 2s region indicates the identical chemical environment of S in the as-prepared samples. Furthermore, this result suggests the existence of S2 in the final product. Furthermore, the TG/DTA results also exclude the existence of any impurities (see the Supporting Information for details of TG/DTA). Transient photocurrent responses were carried out to investigate the photoelectric properties of Bi-mna. As can be clearly seen from Figure 1 a, a strong response is observed under visible-light irradiation, which is attributable to Bi-mna, as no other impurities are observed. This result indicates that Bi-mna is able to generate electron–hole pairs upon light irradiation, and these photogenerated carriers can be efficiently separated. These results imply that Bi-mna can be used as a potential photocatalyst.
Electrochemical impedance spectroscopy (EIS) further confirms that the photogenerated carriers are able to be efficiently separated and transferred to the surface of Bi-mna. Figure 1 b shows EIS Nyquist plots of the Bi-mna electrode in the absence and presence of light irradiation. The Nyquist plot consists of one dominant semicircle, the diameter of which is related to charge-transfer resistance at Bi-mna/electrolyte interface. The diameter of the arc radius on the EIS Nyquist plot in the presence of visible light is smaller than that in the dark, indicating the efficient charge separation and transport.[27] Based on the above discussion, it is reasonable to expect that Bi-mna displays high photocatalytic activity. Decomposition of organic dyes Rhodamine B (RhB) and methylene blue (MB), common pollutants existing in industrial wastewater, over Bi-mna were carried out under visible-light irradiation, considering that Bi-mna displays visible light absorption (the UV/Vis diffuse reflectance spectra of the as-prepared Bi-mna is shown in the Supporting Information, Figure S6). The results are shown in Figure 2. It is obvious that Bi-mna shows much better performance than that of the commercial a-Bi2O3. More than 95 % RhB is decomposed within 2 h over Bimna, while less than 10 % is decomposed over commercial aBi2O3 (Figure 2 a). Bi-mna shows similar behavior for the photodegradation of MB (Supporting Information, Figure S7). MB is hardly degraded over commercial a-Bi2O3 ; on the contrary, the decomposition of MB over Bi-mna occurs smoothly and efficiently. Apart from dye degradation, O2 evolution from water over Bi-mna was further carried out, which is regarded as the key process of water splitting. As can be seen from Figure 2 b, Bi-mna exhibits high activity towards O2 generation under visible-light irradiation (l 420 nm), and the generation rate is about 216 mL h 1. When light was turned off, no oxygen increase was detected, confirming that the oxygen production is indeed driven by light irradiation. To find the reason for the high photocatalytic activity of Bimna, we first investigated the properties of the surrounding bonding type of the Bi3 + atom using ELF and the calculated electron localization function plot as shown in Figure 3 a. We mainly focus on the Bi S, Bi N, and Bi O2 bonds, which provide possible pathways for charge transfer from ligand to metal. The bonding type of Bi S is more akin to the C C bond, as they have to be nodded jointly and the value of ELF is close to the maximum. Therefore, the Bi S bond displays dominantly covalent character. As expected, the picture is similar to the bonds of Bi N and Bi O2, with a difference of small diminution in covalent component. Generally speaking, the more delocalized the electrons are in the covalent bond, the more conducive it is for charge transfer. We then checked the Fukui functions of the Bi-mna, which are well-known to provide a measurement of the change in chemical potential as the number of electrons changes.[28] Regions where F + (r) is large will stabilize the uptake of charge from electron donors, while regions with high F (r) will readily donate charge to electron acceptors. It can be seen from Figure 3 b that Fukui functions are highly localized around the ligands, while the site of Bi3 + has no distribution. This phenomenon demonstrates that the excited electrons should move
Figure 1. a) Transient photocurrent responses of Bi-mna with bias = 0 V (black line) and bias = 0.2 V (gray line). b) EIS Nyquist plots the Bi-mna electrode in the dark and under visible light (l 420 nm) illumination. The electrolyte solution was 0.2 m LiCl aqueous solution.
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Figure 2. a) Photodegradation of 100 mL of 20 mg L 1 RhB over 100 mg Bimna and commercial a-Bi2O3 irradiation in the presence of oxygen. b) Time course of photocatalytic O2 evolution from water with 50 mg Bi-mna mixed in 50 mL aqueous solution containing 100 mg AgNO3 as electron sacrificial agent. The light source is a 300W Xe arc lamp with a cutoff filter to provide visible light (l 420 nm).
from the S atom to the nonadjacent pyridine ring. According to this result, we can determine that the Bi atom acts as bridges between ligands in charge transfer. The results of total density of states (TDOS) and partial density of states (PDOS) of this system confirm the above discussions on the ELF and Fukui function issues. From Figure 3 c it can be seen that the upper valence band (VB) is mainly contributed by states of S atoms, which confirms the results of the Fukui functions that the sites of S atoms should be readily donate charge to electron acceptors. Furthermore, the states of Bi and S in the valence band (VB) distribute energetically in the similar range, which indicates they can effectively hybrid with each other and form typical covalent bond. This conclusion is consistent with the ELF results mentioned above. Obviously, the electrons will transfer from the bonding orbital of the Bi S to the antibonding orbital located in higher energy range in conduction band (CB) first. Subsequently, the excited electrons will flow into the orbitals of O atoms in the CB, which has a lower energy and is more stable. Finally, the charge transfer could be accomplished with the help of the Bi atoms. Based on these results, a ligand-to-ligand charge transfer (LLCT) process can be proposed to occur in Bi-mna. This is Chem. Eur. J. 2014, 20, 1 – 5
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Figure 3. a) Calculated electron localization function (ELF) plots for Bi-mna. b) Fukui function F (r) and F + (r) for Bi-mna. The isosurface value is 0.0015 e 3. Bi purple, C brown, O red, N gray, H pink. c) Calculated total density (TDOS) and partial density of states (PDOS) of Bi-mna.
different from the mostly accepted mechamism of MOFs, that is, ligand-to-metal charge transfer (LMCT),[19] which is most likely due to the closed shell character of Bi3 + (d10s2). 3
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Communication Photoluminescence (PL) spectroscopy, a powerful tool to study electron-transfer process,[29] confirms the results of theoretical calculation. Steady-state PL spectra were first investigated (Supporting Information, Figure S8). H2mna displays strong emissions at 475 and 530 nm; however, the maximum emissions of Bi-mna are blue-shifted to 430 and 475 nm. The observed blue-shift of the emissions may be caused by the variations of the energy gap of the frontier orbitals after the organic coordination with the metal centers. Time-resolved PL spectrum of Bi-mna (Figure 4) indicates an average life time of
Keywords: bismuth · metal–organic frameworks · photocatalysis · photoelectric properties · semiconducting materials [1] L. J. Murray, M. Dinca˘, J. R. Long, Chem. Soc. Rev. 2009, 38, 1294 – 1314. [2] O. M. Yaghi, C. E. Davis, G. M. Li, H. L. Li, J. Am. Chem. Soc. 1997, 119, 2861 – 2968. [3] J. C. Rowsell, O. M. Yaghi, Angew. Chem. Int. Ed. 2005, 44, 4670 – 4679; Angew. Chem. 2005, 117, 4748 – 4758. [4] O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi, J. Kim, Nature 2003, 423, 705 – 714. [5] H. K. Chae, D. Y. Siberio-Prez, J. Kim, Y. B. Go, M. Eddaoudi, A. J. Matzger, M. O’Keeffe, O. M. Yaghi, Nature 2004, 427, 523 – 527. [6] N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O’Keeffe, O. M. Yaghi, Science 2003, 300, 1127 – 1129. [7] H. Reinsch, M. A. Van der Veen, B. Gil, B. Marszalek, T. Verbiest, D. De Vos, N. Stock, Chem. Mater. 2013, 25, 17 – 26. [8] J. Y. Lee, O. K. Farha, J. Robert, K. A. Scheidt, S. T. Nguyen, J. T. Hupp, Chem. Soc. Rev. 2009, 38, 1450 – 1459. [9] A. Corma, H. Garca, F. X. Llabrs i Xamena, Chem. Rev. 2010, 110, 4606 – 4655. [10] Y. Horiuchi, T. Toyao, M. Saito, K. Mochizuki, M. Iwata, H. Higashimura, M. Anpo, M. Matsuoka, J. Phys. Chem. C 2012, 116, 20848 – 20853. [11] H. Yang, X.-W. He, F. Wang, Y. Kang, J. Zhang, J. Mater. Chem. 2012, 22, 21849 – 21851. [12] L. L. Wen, J. B. Zhao, K. L. Lv, Y. H. Wu, K. J. Deng, X. K. Leng, D. F. Li, Cryst. Growth Des. 2012, 12, 1603 – 1612. [13] J. L. Wang, C. Wang, W. B. Lin, ACS Catal. 2012, 2, 2630 – 2640. [14] Y. Y. Liu, Y. M. Yang, Q. L. Sun, Z. Y. Wang, B. B. Huang, Y. Dai, X. Y. Zhang, X. Y. Qin, ACS Appl. Mater. Interfaces 2013, 5, 7654 – 7658. [15] C. Wang, Z. G. Xie, K. E. deKrafft, W. B. Lin, J. Am. Chem. Soc. 2011, 133, 13445 – 13454. [16] F. X. Llabrs i Xamena, A. Corma, H. Garca, J. Phys. Chem. C 2007, 111, 80 – 85. [17] Y. H. Fu, D. R. Sun, Y. J. Chen, R. K. Huang, Z. X. Ding, X. Z. Fu, Z. H. Li, Angew. Chem. Int. Ed. 2012, 51, 3364 – 3367; Angew. Chem. 2012, 124, 3420 – 3423. [18] C. Gomes Silva, I. Luz, F. X. Llabrs i Xamena, A. Corma, H. Garca, Chem. Eur. J. 2010, 16, 11133 – 11138. [19] D. R. Sun, Y. H. Fu, W. J. Liu, L. Ye, D. K. Wang, L. Yang, X. Z. Fu, Z. H. Li, Chem. Eur. J. 2013, 19, 14279 – 14285. [20] Y. Y. Liu, B. B. Huang, Y. Dai, X. Y. Zhang, X. Y. Qin, M. H. Jiang, M.-H. Whangbo, Catal. Commun. 2009, 11, 210 – 213. [21] H. F. Cheng, B. B. Huang, Y. Dai, X. Y. Qin, X. Y. Zhang, Langmuir 2010, 26, 6618 – 6624. [22] Y. Y. Liu, W.-J. Son, J. B. Lu, B. B. Huang, Y. Dai, M.-H. Whangbo, Chem. Eur. J. 2011, 17, 9342 – 9349. [23] Y. Y. Liu, Z. Y. Wang, B. B. Huang, K. S. Yang, X. Y. Zhang, X. Y. Qin, Y. Dai, Appl. Surf. Sci. 2010, 257, 172 – 175. [24] A. Thirumurugan, A. K. Cheetham, Eur. J. Inorg. Chem. 2010, 24, 3823 – 3828. [25] Y. Q. Sun, S. Z. Ge, Q. Liu, J. C. Zhong, Y. P. Chen, CrystEngComm 2013, 15, 10188 – 10192. [26] D. F. Hou, X. L. Hu, P. Hu, W. Zhang, M. F. Zhang, Y. H. Huang, Nanoscale 2013, 5, 9764 – 9772. [27] Q. W. Huang, S. Q. Tian, D. W. Zeng, X. X. Wang, W. L. Song, Y. Y. Li, W. Xiao, C. S. Xie, ACS Catal. 2013, 3, 1477 – 1485. [28] W. Wei, Y. Dai, K. R. Lai, M. Guo, B. B. Huang, Chem. Phys. Lett. 2011, 510, 104 – 108. [29] D. Jarzab, F. Cordella, M. Lenes, F. B. Kooistra, P. W. M. Blom, J. C. Hummelen, M. A. Loi, J. Phys. Chem. B 2009, 113, 16513 – 16517.
Figure 4. Time-resolved PL spectra for H2mna and Bi-mna detected at 475 nm and 430 nm, respectively. The excitation source is a 377.8 nm laser. The red curve represents the instrument response rime.
1.1 ns, while that for H2mna is below the detection limit of the equipment (100 ps). The much longer lifetime is a result of LLCT process. A longer lifetime of the excited state of Bi-mna means the photogenerated charge carrier can go further before it decays back to the ground state, leading to more efficient electron–hole separation and thus higher photocatalytic activity. In summary, a visible-light-responsive bismuth–organic framework (Bi-mna) has been shown to exhibit good photoelectric and photocatalytic properties. According to theoretical calculations, a LLCT process is proposed to occur in Bi-mna. PL spectra suggests a much longer lifetime of photogenerated carriers compared with the organic ligand, which is believed to suppress electron–hole recombination and thus increase photocatalytic efficiency. Our results suggest that bismuth based MOFs could be promising candidates for the development of efficient visible light photocatalysts.
Acknowledgements This work was financially supported by a research grant from the National Basic Research Program of China (the 973 Program, No. 2013CB632401) and the National Natural Science Foundation of China (No. 21333006, 11374190, and 51021062).
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Chem. Eur. J. 2014, 20, 1 – 5
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Received: August 29, 2014 Revised: November 18, 2014 Published online on && &&, 0000
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COMMUNICATION & Photocatalysis
A visible-light-responsive bismuthbased metal–organic framework (Bimna) is shown to exhibit good photoelectric and photocatalytic properties. Combined with the experimental and theoretical results (see electron localization function plot), a ligand-to-ligand charge transfer (LLCT) process is found to be responsible for the high performance. This process gives rise to a longer lifetime of the photogenerated charge carriers.
Chem. Eur. J. 2014, 20, 1 – 5
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G. Wang, Q. Sun, Y. Liu,* B. Huang,* Y. Dai, X. Zhang, X. Qin && – && A Bismuth-Based Metal–Organic Framework as an Efficient VisibleLight-Driven Photocatalyst
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