Accepted Manuscript A bismuth based layer structured organic-inorganic hybrid material with enhanced photocatalytic activity Yuanyuan Liu, Guanzhi Wang, Juncai Dong, Yang An, Baibiao Huang, Xiaoyan Qin, Xiaoyang Zhang, Ying Dai PII: DOI: Reference:
S0021-9797(16)30090-X http://dx.doi.org/10.1016/j.jcis.2016.02.010 YJCIS 21072
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
15 January 2016 1 February 2016 2 February 2016
Please cite this article as: Y. Liu, G. Wang, J. Dong, Y. An, B. Huang, X. Qin, X. Zhang, Y. Dai, A bismuth based layer structured organic-inorganic hybrid material with enhanced photocatalytic activity, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.02.010
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A bismuth based layer structured organic-inorganic hybrid material with enhanced photocatalytic activity YuanyuanLiu,a Guanzhi Wang,a Juncai Dong,c YangAn,a Baibiao Huang,*a XiaoyanQin,a Xiaoyang Zhang a and Ying Dai b a
State key of crystal materials, Shandong University, 250100, P.R.China
b
School of physics, Shandong University, 250100, P.R.China
c
Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China.
Corresponding Author Tel: +86-531-8836-6324. E-mail: [email protected]
Abstract: A bismuth-based organic-inorganic hybrid material with layered structure (BiO-BTCIE) was synthesized by taking advantage of an ion exchange reaction. The structure of BiO-BTCIE was examined by XRD, EXAFS, FT-IR and TG/DTA etc. By replacing the HCOO- with BTC anions in the Bi2 O22+ interlayer, the Bi2O22+ layer is distorted as revealed by the EXAFS, which lead to a longer life time of the photogenerated charge carriers and a higher photocatalytic activity of BiO-BTCIE (more than 10 times).
1. Introduction Renewable
energy
substitutes
and
environmental
decontamination
using
semiconductor based photocatalysis have attracted intensive research interest because they are economic and ecologically safe options for solving energy and pollution problems. Bismuth-based semiconductors accounts for a very large proportion of the various photocatalysts, and most of them display layered structure, such as Bi2 WO6, BiVO4, BiOX (X=Cl, Br, I) and Bi2O2CO3, BiOIO3 [1-14] etc. Bismuth based organic inorganic hybrid materials, combining features of organic molecular and bismuth ions, are interesting systems due to their manifold properties ranging from photoluminescence, medicine to catalysis, photocatalysis, gas sorption and high dielectric constant materials. [15-23] Nevertheless, few bismuth based organic inorganic hybrid materials were reported to display significant photocatalytic activity. [19-20] Bi3+ cations are known for their stereoactive lone pair leading to interesting and versatile coordination geometries. The coordination number is irregular ranging from 3 to 10, [24-25] making the preparation of bismuth-based organic-inorganic hybrid materials time consuming and uneconomic. In addition, distorted BiOx polyhedral are usually observed for bismuth based organic inorganic materials. [26] The lone pair distortion has a great influence on the separation of photo-generated charge carriers. For example, monoclinic BiVO4 displays higher photocatalytic activity than the tetragonal phase due to the distortion of BiO8 dodecahedron in the monoclinic phase. [27-28] However, as far as we know, no investigation was reported about the effect of BiOx polyhydra on the photocatalytic activity. In this paper, a new bismuth based organic inorganic hybrid material has been synthesized via a facile ion exchange reaction. The HCOO- ions in BiOHCOO is replaced by trimesic acid anions (BTC), and the as-prepared BiO-BTCIE remains the layered structure of BiOHCOO, except a distortion of the Bi2O22+ layer. Photogenerated charge carriers in BiO-BTCIE show higher photocatalytic activity and longer life time. The distortion of the Bi2O22+ layer is assigned to the origin of the
improved photocatalytic activity. 2. Experimental Section All chemicals used were of analytical grade, and were used without further purification. Synthesis of BiOHCOO, Bi2O2[NC5H3(CO2)2], and BiO-BTCIE. BiOHCOO
and
Bi2O2[NC5H3(CO2)2]
(the
organic
linker
is
3,5-pyridinedicarboxylate) were synthesized according to the literature. [29-30] Synthesis of BiO-BTCIE: BiOHCOO was used as the starting material and the details are as follows. BiOHCOO and H3BTC in a molar ratio of 1:9 were added in a mixture of 5 ml DMF and 15 ml methanol. After being stirred for 0.5h, the mixed solutions were sealed in a 100 ml Teflon-lined autoclave, maintained at 150 °C for 48 h and cooled down to room temperature. The resultant solid was washed by DMF, water, ethanol and then dried in air at 60 °C. Synthesis of BiO-BTCIE using Bi2O2[NC5H3(CO2)2] as the starting material was the same except that BiOHCOO is replaced with Bi2O2[NC5H3(CO2)2]. Characterization The X-ray power diffraction (XRD) data of the as-prepared samples was collected on an X-ray powder diffraction (Bruker AXS D8). The Bi L3-edge extended X-ray absorption fine structure (EXAFS) spectra were collected in transmission mode at beamline 1W2B of the Beijing Synchrotron Radiation Facility in China. A fixed-exit Si(111) double crystal monochromator was used. The incident and transmit X-ray beam were measured by ionization chambers filled with 25% Ar and 75% N 2. The EXAFS raw data were then background-subtracted, normalized and Fourier transformed
by the
standard
procedures with the
IFEFFIT
package.[31]
Thermogravimetric-differential thermal analysis (TG/DTA) was carried out using a Diamond TG/DTA analyzer under air atmosphere with a heating rate of 5 °C/min. Fourier transform infrared (FT-IR) spectra was performed on a Bruker ALPHA-T spectrometer using KBr pellets. The UV-vis diffuse reflectance spectra (DRS) were obtained by a Shimadzu UV-2550 recording spectrophotometer with BaSO4 as the reference. Steady state photoluminesce (PL) spectra were recorded by a high
sensitivity fluorometer (Edinburgh FS920). The pump source is an Opolette HE 355 II tunable laser system with a laser pulse width of 5 ns and repetition rate of 20Hz. The time resolved PL spectra were conducted on Edinburgh FLS920 PL. The decay curve was fitted by using a biexponential decay function to obtain deconvolution of the instrument response function. The average life time was calculated by using the following equation: 〈τav 〉 = a1τ1 + a2τ2, (τ1 and τ2 are the life time, a1 and a2 are normalized pre-exponential factors). For BiO-BTCIE, the biexponential decay fitting suggests the lifetime values are 2.04ns (76.97%) and 8.69 ns (23.03%). Photocatalytic reaction The photocatalytic degradation of Rhodamine B (RhB) was carried out at room temperature. Before photoirradiation, a solution of 50 mg catalyst dispersed in 50 mL of RhB solution (20 mg L-1) was stirred for 2 h to reach adsorption/desorption equilibrium. A 300W Xe arc lamp (PLS-SXE300, Beijing Trusttech Co. Ltd) was used as the light source and the concentration of RhB was measured by UV/visible spectrophotometer (UV-2550, Shimadzu). The examination of active species was the same as above, except that 1mmol benzoquinone (BQ) or tert butanol (TBA) was added into the RhB solution. Photocatalytic oxygen evolution from water was performed in a top-irradiation vessel which is connected to a glass-enclosed gas circulation system. In the procedure, 30 mg
catalyst and 30 mg AgNO3 were mixed in 30 mL aqueous solution with
constant stirring. The reaction temperature was kept at 5℃. The amount of O2 evolution was determined by a gas chromatograph (Techcomp GC7890Ⅱ).
3. Results and discussion In our previous work, we reported a bismuth based MOFs (Bi-BTC), which possesses H3BTC as the organic linker and a unique {Bi2O14} unit acts as metal dots, and H3BTC acts as organic linker. The as-prepared Bi-BTC exhibits high photocatalytic activity of O2 evolution from water. [20] A further investigation using BiOHCOO as the starting material found a material which displays totally different XRD patterns from Bi-BTC (Fig. 1), and the materials was denoted as BiO-BTCIE.
BiOHCOO is an important sillenite oxide possessing layered structure (Bi2O22+ layer and HCOO- layer). Anion exchange reaction between HCOO- and BTC is most likely to occur, which is a common phenomenon for these bismuth based sillenite compounds. [32-35]
intensity
BiO-BTCIE
Bi-BTC
BiOHCOO 10
15
20
25
30
35
40
45
50
2 theta (degree) Fig. 1. XRD patterns of the as-prepared BiOHCOO, Bi-BTC, and BiO-BTCIE.
The following experiments further confirm the anion exchange reaction mentioned above.
When
the
starting
material
was
changed
from
BiOHCOO
to
Bi2O2[NC5H3(CO2)2], almost identical XRD patterns to BiO-BTCIE is observed (Fig. 2). The little difference of intensity may be caused by the preferential growth of a certain facet. The structure of Bi2O2[NC5H3(CO2)2] was reported previously [30], which also displays a layered structure consisting of Bi2O22+ layer and 3,5-pyridinedicarboxylate (pydc) anions layer.
Bi2O2[NC5H3(CO2)2]
intensity
BiO-BTCIE obtained from BiOHCOO
BiO-BTCIE obtained from Bi2O2[NC5H3(CO2)2]
10
15
20
25
30
35
40
45
50
2 theta (degree) Fig. 2. XRD patterns of the as-prepared Bi2O2[NC5 H3(CO2)2] and BiO-BTCIE obtained from BiOHCOO and Bi2O2[NC5H3(CO2)2], respectively.
EXAFS results of the Bi L3-edge confirm that BiO-BTCIE remains the layered structure of BiOHCOO. EXAFS is a useful technique to investigate the local atomic arrangements in the samples.[36] To gain quantitative insights into the structures of the Bi2O22+ layer, a least-squares curve fitting analysis was carried out for the first coordination shell. While the amplitude reduction factor (S02) was fixed, the energy shift (ΔE0), the path length R, coordination number (CN), and Debye-Waller factors σ2 were left as free parameters. The best-fitted results were shown in Fig. 3. For BiOHCOO, the obtained Bi-O bond length is 2.176(8) Å, the Bi-O coordination number is 3.9(4), and the Debye–Waller factors σ2 is 8.5(10)×10-3 Å2. For BiO-BTCIE, however, the Bi-O bond length splits into two sub-shells located at 2.110(25) and 2.622(22) Å, with coordination numbers of 1.3(4) and 3.0(8) and Debye–Waller factors σ2 of 8.0(22)×10-3 and 9.9(34)×10-3 Å2, respectively. Those results indicate that BiO-BTCIE remains the layered structure of BiOHCOO after the ion exchange reaction. The coordination number of Bi in BiO-BTCIE does not change, i.e. 4, though the BiO4 tetrahedron in the Bi2O22+ layer is distorted, which may be induced by the
larger size of BTC compared with HCOO ions.
Fig. 3.
Bi L3-edge EXAFS signals for (a)
BiOHCOO and (b) BiO-BTCIE. Experimental data: red dotted line, best fit results: black solid line.
FT-IR spectra were further investigated to obtain some structural information of BiO-BTCIE. The FT-IR spectra are shown in Fig. 4. For comparison, the FT-IR spectra of BiOHCOO and H3BTC are also presented. For BiO-BTCIE, new peaks appear at the 3000-3100 cm-1 region, which are the fingerprints of the C-H bond of benzene ring. [37] The peaks at 1545 cm-1 and 1380 cm-1 due to the C=O stretching vibration of HCOO- become weak, and two strong peaks at 1619cm-1 and 1365 cm-1 are observed, which are assigned to the asymmetric and symmetric vibrations of carboxylate anions in BTC anions, respectively. In addition, the two peaks shift to shorter wavenumber compared to those of H3BTC (1720cm-1 and 1403 cm-1), suggesting the coordination of the Ocoo- of BTC anions with bismuth. Besides that,
the symmetrical stretching vibration of the Bi-O bond shifts from 576 cm-1 for BiOHCOO to 520 cm-1 for BiO-BTCIE, which further supports the bonding of Bi cation and BTC anions. [38] Meanwhile, the absence of the strong and broad peak of BiO-BTCIE at about 2500~3200 cm-1 suggests the deprotonation of carboxylate groups, further indicating that the carboxylate groups of BTC are deprotonated to coordinate with bismuth.
Fig. 4.The FT-IR spectra of (a) BiOHCOO, (b) BiO-BTCIE and (c) H3BTC.
TG/DTA results suggest that BiOHCOO and BiO-BTCIE displays different thermal degradation behavior (Fig. 5). BiOHCOO shows two obvious steps of mass loss before 370℃, and the total weight loss is 14.8% (calculated value 13.7%). The final decomposition product is Bi2O3. The higher value of experimental weight loss may be due to the adsorbed solvent molecules on the surface. Obvious difference is observed between BiO-BTCIE and BiOHCOO. It can be clearly seen that the major weight loss (40.18%) occurs at higher temperature, i.e. after 370 ℃, which is due to the decomposition of the intercalated BTC anions. The weight loss before 370℃ may be due to the adsorbed solvents and the decomposition of the Bi2O22+ layer. Besides the larger weight loss, BiO-BTCIE displays a much stronger endothermic peak at about
400 °C and no other distinct endothermic peaks are distinguished, while BiOHCOO displays a much smaller endothermic peak at about 255 °C. 1.0
0.6 0.4 100
200
300
400
500
1.0 0.9 0.8 100
200
300
400
500
o temperature ( C)
heat flow (mW)
0.8
weight (w/w0)
60 40 20 0 -20 -40 -60 600 700 800 10 BiOHCOO 0 -10 -20 -30 -40 -50 -60 600 700 800
BiO-BTCIE
Fig. 5. TG/DTA curves of BiOHCOO and BiO-BTCIE.
DRS and PL spectra were further measured to investigate the photophysical properties of BiOHCOO and BiO-BTCIE. As can be seen from Fig. 6a, the onset of the absorption edge of BiOHCOO locates at around 370nm, with a band gap of 3.20eV, which is consistent with the reported results. [29] H3BTC introduction does not obviously change the absorption. The steady-state PL spectra of BiO-BTCIE and BiOHCOO are shown in Fig. 6b. BiOHCOO exhibits an intense broad emission at 510 nm, and a weak emission at 434nm. Compared with BiOHCOO, BiO-BTCIE displays an obvious blue shift and the strong emission locates at 474 nm. The blue shift is due to the changed coordination environment of Bi2O22+ from HCOO- to BTC anions, [25] which is well consistent with the above results.
1.0
a 0.8
BiO-BTCIE
Abs.
0.6
BiOHCOO
0.4 0.2 0.0 200
300
400
500
600
700
800
wavelength (nm)
1.0 b
BiO-BTCIE
intensity
0.8
BiOHCOO
0.6 0.4 0.2 0.0 400
450
500
550
600
650
700
wavelength(nm)
Fig. 6. The (a) UV-vis DRS and (b) normalized PL spectra of BiOHCOO, and BiO-BTCIE (excited at 300nm).
The photocatalytic activity of the as-prepared samples was evaluated using RhB as the target molecules. As can be seen in Fig. 7a, BiO-BTCIE shows much higher photocatalytic activity than BiOHCOO. After 60 min UV-vis light irradiation, about 93.6% of RhB molecules are decolorized by BiO-BTCIE, while only 21.3% over BiOHCOO. The rate of the degradation reaction can be described by a pseudo first-order kinetic according to the Langmuir–Hinshelwood model (Fig. 7b). The equation is expressed as ln(C0/Ct) = kt, where C0 is the initial RhB concentration, Ct is
the RhB concentration at time t, and k is the apparent first-order rate constant. The values of k for BiO-BTCIE and BiOHCOO are determined to be 4.75×10-2 min-1 and 3.97×10-3 min-1, respectively. Obviously, the rate constant is improved to be as high as more than 10 times after BTC anions are introduced into the interlayer of Bi2O22+.
a
1.0
C/C0
0.8 0.6 0.4 0.2
BiO-BTCIE BiOHCOO
0.0 0
10
20
30
40
50
60
time(min)
3.5
b BiO-BTCIE
ln(C0/Ct)(X100 min-1)
3.0
R2=0.987
BiOHCOO R2=0.990
2.5 2.0 1.5 1.0 0.5 0.0 -0.5 0
10
20
30
40
50
60
time (min)
1.0 Blank TBA BQ
C/C0
0.8 0.6 0.4 0.2 0.0 0
10
20
30
40
50
60
Irradiation time (min)
Fig. 7. (a) Photocatalytic degradation of RhB, (b) Pseudo first-order kinetic linear
fitting over BiO-BTCIE and BiOHCOO, and (c) The effects of BQ and TBA on the rate of RhB decolorization.
To investigate the active species during the RhB degradation over BiO-BTCIE, the effects of BQ and TBA on the degradation rate were investigated. It is well known that BQ is a very effective trap for .O2- and TBA is a very effective trap for .OH. From Fig. 7c, it can be seen that BQ leads to a marked suppression of the degradation rate of RhB, while TBA has nearly no effect on the degradation rate. The results suggest that .O2- plays an important role in the photocatalytic degradation of RhB. It is well known that RhB displays a self-sensitization effect. Therefore, photocatalytic O2 evolution from water over BiO-BTCIE was further carried out. The O2 evolution rate is about 753 μL h-1. The result of O2 evolution from water further confirms the photocatalytic activity of BiO-BTCIE. The results of time resolved PL spectra are consistent with the photocatalytic activity of BiO-BTCIE. As can be obviously seen from Fig. 8, BiO-BTCIE decays much more slowly than BiOHCOO. BiO-BTCIE exhibits an average life time of 3.6 ns, while the excited state of BiOHCOO decays so fast that it beyond the equipment detection limits (100 ps). The much longer lifetime of BiO-BTCIE suggests that the photogenerated charge carriers can exist longer before they return to the ground state, which result in the efficient electron-hole separation and therefore improved photocatalytic activity. As discussed above, the distorted Bi2O22+ layered (revealed by the EXAFS) is regarded as the essential reason for the longer life time of photogenerated charge carriers and higher photocatalytic activity of BiO-BTCIE.
BiO-BTCIE
intensity
1000
BiOHCOO
100
10
1 0
20
40
60
80
time(ns)
Fig. 8. Time resolved PL spectra for BiO-BTCIE and BiOHCOO detected at 510 and 472 nm, respectively. The excitation source is a 377.8 nm laser. The red curve represents the instrument response time.
4. Conclusion Based on the ion exchange reaction, a bismuth-based organic-inorganic hybrid material BiO-BTCIE with layered structure was successfully fabricated. The replacement of HCOO- ions by BTC ions is confirmed by XRD, EXAFS, FT-IR and TG/DTA spectra. Introduction of BTC into the Bi2O22+ interlayer does not obviously change the layered structure, except a distortion of Bi2O22+ layer. The distortion of Bi2O22+ layer is believed to be responsible for the higher photocatalytic activity and longer life time of photogenerated charge carriers. Further investigation is underway to obtain single crystal and the precise crystal structure of BiO-BTCIE, especially the configuration of BTC ion in the Bi2O22+interlayer. Based on the crystal structure, the electronic structure of BiO-BTCIE will be analyzed.
Acknowledgment This work was financially supported by the National Basic Research Program of China (the 973 Program, No. 2013CB632401), the National Natural Science Foundation of China (No. 21573135, 21333006, 21007031, 11374190 and 51321091), the Shandong Province Natural Science Foundation (ZR2014JL008) and Taishan Scholar Foundation of Shandong Province, China.
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RimJeon,
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Graphical abstract
BiO-BTCIE is obtained via the ion exchange reaction, i.e. the HCOO- in BiOHCOO is changed to be BTC anions. The obtained BiO-BTCIE remains the layered structure of BiOHCOO, except a distortion of the Bi2O22+ layer. The distortion is assigned to the origin of the enhanced photocatalytic activity and longer life time of photogenerated charge carriers of BiO-BTCIE.
S0021-9797(16)30090-X http://dx.doi.org/10.1016/j.jcis.2016.02.010 YJCIS 21072
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
15 January 2016 1 February 2016 2 February 2016
Please cite this article as: Y. Liu, G. Wang, J. Dong, Y. An, B. Huang, X. Qin, X. Zhang, Y. Dai, A bismuth based layer structured organic-inorganic hybrid material with enhanced photocatalytic activity, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.02.010
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A bismuth based layer structured organic-inorganic hybrid material with enhanced photocatalytic activity YuanyuanLiu,a Guanzhi Wang,a Juncai Dong,c YangAn,a Baibiao Huang,*a XiaoyanQin,a Xiaoyang Zhang a and Ying Dai b a
State key of crystal materials, Shandong University, 250100, P.R.China
b
School of physics, Shandong University, 250100, P.R.China
c
Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China.
Corresponding Author Tel: +86-531-8836-6324. E-mail: [email protected]
Abstract: A bismuth-based organic-inorganic hybrid material with layered structure (BiO-BTCIE) was synthesized by taking advantage of an ion exchange reaction. The structure of BiO-BTCIE was examined by XRD, EXAFS, FT-IR and TG/DTA etc. By replacing the HCOO- with BTC anions in the Bi2 O22+ interlayer, the Bi2O22+ layer is distorted as revealed by the EXAFS, which lead to a longer life time of the photogenerated charge carriers and a higher photocatalytic activity of BiO-BTCIE (more than 10 times).
1. Introduction Renewable
energy
substitutes
and
environmental
decontamination
using
semiconductor based photocatalysis have attracted intensive research interest because they are economic and ecologically safe options for solving energy and pollution problems. Bismuth-based semiconductors accounts for a very large proportion of the various photocatalysts, and most of them display layered structure, such as Bi2 WO6, BiVO4, BiOX (X=Cl, Br, I) and Bi2O2CO3, BiOIO3 [1-14] etc. Bismuth based organic inorganic hybrid materials, combining features of organic molecular and bismuth ions, are interesting systems due to their manifold properties ranging from photoluminescence, medicine to catalysis, photocatalysis, gas sorption and high dielectric constant materials. [15-23] Nevertheless, few bismuth based organic inorganic hybrid materials were reported to display significant photocatalytic activity. [19-20] Bi3+ cations are known for their stereoactive lone pair leading to interesting and versatile coordination geometries. The coordination number is irregular ranging from 3 to 10, [24-25] making the preparation of bismuth-based organic-inorganic hybrid materials time consuming and uneconomic. In addition, distorted BiOx polyhedral are usually observed for bismuth based organic inorganic materials. [26] The lone pair distortion has a great influence on the separation of photo-generated charge carriers. For example, monoclinic BiVO4 displays higher photocatalytic activity than the tetragonal phase due to the distortion of BiO8 dodecahedron in the monoclinic phase. [27-28] However, as far as we know, no investigation was reported about the effect of BiOx polyhydra on the photocatalytic activity. In this paper, a new bismuth based organic inorganic hybrid material has been synthesized via a facile ion exchange reaction. The HCOO- ions in BiOHCOO is replaced by trimesic acid anions (BTC), and the as-prepared BiO-BTCIE remains the layered structure of BiOHCOO, except a distortion of the Bi2O22+ layer. Photogenerated charge carriers in BiO-BTCIE show higher photocatalytic activity and longer life time. The distortion of the Bi2O22+ layer is assigned to the origin of the
improved photocatalytic activity. 2. Experimental Section All chemicals used were of analytical grade, and were used without further purification. Synthesis of BiOHCOO, Bi2O2[NC5H3(CO2)2], and BiO-BTCIE. BiOHCOO
and
Bi2O2[NC5H3(CO2)2]
(the
organic
linker
is
3,5-pyridinedicarboxylate) were synthesized according to the literature. [29-30] Synthesis of BiO-BTCIE: BiOHCOO was used as the starting material and the details are as follows. BiOHCOO and H3BTC in a molar ratio of 1:9 were added in a mixture of 5 ml DMF and 15 ml methanol. After being stirred for 0.5h, the mixed solutions were sealed in a 100 ml Teflon-lined autoclave, maintained at 150 °C for 48 h and cooled down to room temperature. The resultant solid was washed by DMF, water, ethanol and then dried in air at 60 °C. Synthesis of BiO-BTCIE using Bi2O2[NC5H3(CO2)2] as the starting material was the same except that BiOHCOO is replaced with Bi2O2[NC5H3(CO2)2]. Characterization The X-ray power diffraction (XRD) data of the as-prepared samples was collected on an X-ray powder diffraction (Bruker AXS D8). The Bi L3-edge extended X-ray absorption fine structure (EXAFS) spectra were collected in transmission mode at beamline 1W2B of the Beijing Synchrotron Radiation Facility in China. A fixed-exit Si(111) double crystal monochromator was used. The incident and transmit X-ray beam were measured by ionization chambers filled with 25% Ar and 75% N 2. The EXAFS raw data were then background-subtracted, normalized and Fourier transformed
by the
standard
procedures with the
IFEFFIT
package.[31]
Thermogravimetric-differential thermal analysis (TG/DTA) was carried out using a Diamond TG/DTA analyzer under air atmosphere with a heating rate of 5 °C/min. Fourier transform infrared (FT-IR) spectra was performed on a Bruker ALPHA-T spectrometer using KBr pellets. The UV-vis diffuse reflectance spectra (DRS) were obtained by a Shimadzu UV-2550 recording spectrophotometer with BaSO4 as the reference. Steady state photoluminesce (PL) spectra were recorded by a high
sensitivity fluorometer (Edinburgh FS920). The pump source is an Opolette HE 355 II tunable laser system with a laser pulse width of 5 ns and repetition rate of 20Hz. The time resolved PL spectra were conducted on Edinburgh FLS920 PL. The decay curve was fitted by using a biexponential decay function to obtain deconvolution of the instrument response function. The average life time was calculated by using the following equation: 〈τav 〉 = a1τ1 + a2τ2, (τ1 and τ2 are the life time, a1 and a2 are normalized pre-exponential factors). For BiO-BTCIE, the biexponential decay fitting suggests the lifetime values are 2.04ns (76.97%) and 8.69 ns (23.03%). Photocatalytic reaction The photocatalytic degradation of Rhodamine B (RhB) was carried out at room temperature. Before photoirradiation, a solution of 50 mg catalyst dispersed in 50 mL of RhB solution (20 mg L-1) was stirred for 2 h to reach adsorption/desorption equilibrium. A 300W Xe arc lamp (PLS-SXE300, Beijing Trusttech Co. Ltd) was used as the light source and the concentration of RhB was measured by UV/visible spectrophotometer (UV-2550, Shimadzu). The examination of active species was the same as above, except that 1mmol benzoquinone (BQ) or tert butanol (TBA) was added into the RhB solution. Photocatalytic oxygen evolution from water was performed in a top-irradiation vessel which is connected to a glass-enclosed gas circulation system. In the procedure, 30 mg
catalyst and 30 mg AgNO3 were mixed in 30 mL aqueous solution with
constant stirring. The reaction temperature was kept at 5℃. The amount of O2 evolution was determined by a gas chromatograph (Techcomp GC7890Ⅱ).
3. Results and discussion In our previous work, we reported a bismuth based MOFs (Bi-BTC), which possesses H3BTC as the organic linker and a unique {Bi2O14} unit acts as metal dots, and H3BTC acts as organic linker. The as-prepared Bi-BTC exhibits high photocatalytic activity of O2 evolution from water. [20] A further investigation using BiOHCOO as the starting material found a material which displays totally different XRD patterns from Bi-BTC (Fig. 1), and the materials was denoted as BiO-BTCIE.
BiOHCOO is an important sillenite oxide possessing layered structure (Bi2O22+ layer and HCOO- layer). Anion exchange reaction between HCOO- and BTC is most likely to occur, which is a common phenomenon for these bismuth based sillenite compounds. [32-35]
intensity
BiO-BTCIE
Bi-BTC
BiOHCOO 10
15
20
25
30
35
40
45
50
2 theta (degree) Fig. 1. XRD patterns of the as-prepared BiOHCOO, Bi-BTC, and BiO-BTCIE.
The following experiments further confirm the anion exchange reaction mentioned above.
When
the
starting
material
was
changed
from
BiOHCOO
to
Bi2O2[NC5H3(CO2)2], almost identical XRD patterns to BiO-BTCIE is observed (Fig. 2). The little difference of intensity may be caused by the preferential growth of a certain facet. The structure of Bi2O2[NC5H3(CO2)2] was reported previously [30], which also displays a layered structure consisting of Bi2O22+ layer and 3,5-pyridinedicarboxylate (pydc) anions layer.
Bi2O2[NC5H3(CO2)2]
intensity
BiO-BTCIE obtained from BiOHCOO
BiO-BTCIE obtained from Bi2O2[NC5H3(CO2)2]
10
15
20
25
30
35
40
45
50
2 theta (degree) Fig. 2. XRD patterns of the as-prepared Bi2O2[NC5 H3(CO2)2] and BiO-BTCIE obtained from BiOHCOO and Bi2O2[NC5H3(CO2)2], respectively.
EXAFS results of the Bi L3-edge confirm that BiO-BTCIE remains the layered structure of BiOHCOO. EXAFS is a useful technique to investigate the local atomic arrangements in the samples.[36] To gain quantitative insights into the structures of the Bi2O22+ layer, a least-squares curve fitting analysis was carried out for the first coordination shell. While the amplitude reduction factor (S02) was fixed, the energy shift (ΔE0), the path length R, coordination number (CN), and Debye-Waller factors σ2 were left as free parameters. The best-fitted results were shown in Fig. 3. For BiOHCOO, the obtained Bi-O bond length is 2.176(8) Å, the Bi-O coordination number is 3.9(4), and the Debye–Waller factors σ2 is 8.5(10)×10-3 Å2. For BiO-BTCIE, however, the Bi-O bond length splits into two sub-shells located at 2.110(25) and 2.622(22) Å, with coordination numbers of 1.3(4) and 3.0(8) and Debye–Waller factors σ2 of 8.0(22)×10-3 and 9.9(34)×10-3 Å2, respectively. Those results indicate that BiO-BTCIE remains the layered structure of BiOHCOO after the ion exchange reaction. The coordination number of Bi in BiO-BTCIE does not change, i.e. 4, though the BiO4 tetrahedron in the Bi2O22+ layer is distorted, which may be induced by the
larger size of BTC compared with HCOO ions.
Fig. 3.
Bi L3-edge EXAFS signals for (a)
BiOHCOO and (b) BiO-BTCIE. Experimental data: red dotted line, best fit results: black solid line.
FT-IR spectra were further investigated to obtain some structural information of BiO-BTCIE. The FT-IR spectra are shown in Fig. 4. For comparison, the FT-IR spectra of BiOHCOO and H3BTC are also presented. For BiO-BTCIE, new peaks appear at the 3000-3100 cm-1 region, which are the fingerprints of the C-H bond of benzene ring. [37] The peaks at 1545 cm-1 and 1380 cm-1 due to the C=O stretching vibration of HCOO- become weak, and two strong peaks at 1619cm-1 and 1365 cm-1 are observed, which are assigned to the asymmetric and symmetric vibrations of carboxylate anions in BTC anions, respectively. In addition, the two peaks shift to shorter wavenumber compared to those of H3BTC (1720cm-1 and 1403 cm-1), suggesting the coordination of the Ocoo- of BTC anions with bismuth. Besides that,
the symmetrical stretching vibration of the Bi-O bond shifts from 576 cm-1 for BiOHCOO to 520 cm-1 for BiO-BTCIE, which further supports the bonding of Bi cation and BTC anions. [38] Meanwhile, the absence of the strong and broad peak of BiO-BTCIE at about 2500~3200 cm-1 suggests the deprotonation of carboxylate groups, further indicating that the carboxylate groups of BTC are deprotonated to coordinate with bismuth.
Fig. 4.The FT-IR spectra of (a) BiOHCOO, (b) BiO-BTCIE and (c) H3BTC.
TG/DTA results suggest that BiOHCOO and BiO-BTCIE displays different thermal degradation behavior (Fig. 5). BiOHCOO shows two obvious steps of mass loss before 370℃, and the total weight loss is 14.8% (calculated value 13.7%). The final decomposition product is Bi2O3. The higher value of experimental weight loss may be due to the adsorbed solvent molecules on the surface. Obvious difference is observed between BiO-BTCIE and BiOHCOO. It can be clearly seen that the major weight loss (40.18%) occurs at higher temperature, i.e. after 370 ℃, which is due to the decomposition of the intercalated BTC anions. The weight loss before 370℃ may be due to the adsorbed solvents and the decomposition of the Bi2O22+ layer. Besides the larger weight loss, BiO-BTCIE displays a much stronger endothermic peak at about
400 °C and no other distinct endothermic peaks are distinguished, while BiOHCOO displays a much smaller endothermic peak at about 255 °C. 1.0
0.6 0.4 100
200
300
400
500
1.0 0.9 0.8 100
200
300
400
500
o temperature ( C)
heat flow (mW)
0.8
weight (w/w0)
60 40 20 0 -20 -40 -60 600 700 800 10 BiOHCOO 0 -10 -20 -30 -40 -50 -60 600 700 800
BiO-BTCIE
Fig. 5. TG/DTA curves of BiOHCOO and BiO-BTCIE.
DRS and PL spectra were further measured to investigate the photophysical properties of BiOHCOO and BiO-BTCIE. As can be seen from Fig. 6a, the onset of the absorption edge of BiOHCOO locates at around 370nm, with a band gap of 3.20eV, which is consistent with the reported results. [29] H3BTC introduction does not obviously change the absorption. The steady-state PL spectra of BiO-BTCIE and BiOHCOO are shown in Fig. 6b. BiOHCOO exhibits an intense broad emission at 510 nm, and a weak emission at 434nm. Compared with BiOHCOO, BiO-BTCIE displays an obvious blue shift and the strong emission locates at 474 nm. The blue shift is due to the changed coordination environment of Bi2O22+ from HCOO- to BTC anions, [25] which is well consistent with the above results.
1.0
a 0.8
BiO-BTCIE
Abs.
0.6
BiOHCOO
0.4 0.2 0.0 200
300
400
500
600
700
800
wavelength (nm)
1.0 b
BiO-BTCIE
intensity
0.8
BiOHCOO
0.6 0.4 0.2 0.0 400
450
500
550
600
650
700
wavelength(nm)
Fig. 6. The (a) UV-vis DRS and (b) normalized PL spectra of BiOHCOO, and BiO-BTCIE (excited at 300nm).
The photocatalytic activity of the as-prepared samples was evaluated using RhB as the target molecules. As can be seen in Fig. 7a, BiO-BTCIE shows much higher photocatalytic activity than BiOHCOO. After 60 min UV-vis light irradiation, about 93.6% of RhB molecules are decolorized by BiO-BTCIE, while only 21.3% over BiOHCOO. The rate of the degradation reaction can be described by a pseudo first-order kinetic according to the Langmuir–Hinshelwood model (Fig. 7b). The equation is expressed as ln(C0/Ct) = kt, where C0 is the initial RhB concentration, Ct is
the RhB concentration at time t, and k is the apparent first-order rate constant. The values of k for BiO-BTCIE and BiOHCOO are determined to be 4.75×10-2 min-1 and 3.97×10-3 min-1, respectively. Obviously, the rate constant is improved to be as high as more than 10 times after BTC anions are introduced into the interlayer of Bi2O22+.
a
1.0
C/C0
0.8 0.6 0.4 0.2
BiO-BTCIE BiOHCOO
0.0 0
10
20
30
40
50
60
time(min)
3.5
b BiO-BTCIE
ln(C0/Ct)(X100 min-1)
3.0
R2=0.987
BiOHCOO R2=0.990
2.5 2.0 1.5 1.0 0.5 0.0 -0.5 0
10
20
30
40
50
60
time (min)
1.0 Blank TBA BQ
C/C0
0.8 0.6 0.4 0.2 0.0 0
10
20
30
40
50
60
Irradiation time (min)
Fig. 7. (a) Photocatalytic degradation of RhB, (b) Pseudo first-order kinetic linear
fitting over BiO-BTCIE and BiOHCOO, and (c) The effects of BQ and TBA on the rate of RhB decolorization.
To investigate the active species during the RhB degradation over BiO-BTCIE, the effects of BQ and TBA on the degradation rate were investigated. It is well known that BQ is a very effective trap for .O2- and TBA is a very effective trap for .OH. From Fig. 7c, it can be seen that BQ leads to a marked suppression of the degradation rate of RhB, while TBA has nearly no effect on the degradation rate. The results suggest that .O2- plays an important role in the photocatalytic degradation of RhB. It is well known that RhB displays a self-sensitization effect. Therefore, photocatalytic O2 evolution from water over BiO-BTCIE was further carried out. The O2 evolution rate is about 753 μL h-1. The result of O2 evolution from water further confirms the photocatalytic activity of BiO-BTCIE. The results of time resolved PL spectra are consistent with the photocatalytic activity of BiO-BTCIE. As can be obviously seen from Fig. 8, BiO-BTCIE decays much more slowly than BiOHCOO. BiO-BTCIE exhibits an average life time of 3.6 ns, while the excited state of BiOHCOO decays so fast that it beyond the equipment detection limits (100 ps). The much longer lifetime of BiO-BTCIE suggests that the photogenerated charge carriers can exist longer before they return to the ground state, which result in the efficient electron-hole separation and therefore improved photocatalytic activity. As discussed above, the distorted Bi2O22+ layered (revealed by the EXAFS) is regarded as the essential reason for the longer life time of photogenerated charge carriers and higher photocatalytic activity of BiO-BTCIE.
BiO-BTCIE
intensity
1000
BiOHCOO
100
10
1 0
20
40
60
80
time(ns)
Fig. 8. Time resolved PL spectra for BiO-BTCIE and BiOHCOO detected at 510 and 472 nm, respectively. The excitation source is a 377.8 nm laser. The red curve represents the instrument response time.
4. Conclusion Based on the ion exchange reaction, a bismuth-based organic-inorganic hybrid material BiO-BTCIE with layered structure was successfully fabricated. The replacement of HCOO- ions by BTC ions is confirmed by XRD, EXAFS, FT-IR and TG/DTA spectra. Introduction of BTC into the Bi2O22+ interlayer does not obviously change the layered structure, except a distortion of Bi2O22+ layer. The distortion of Bi2O22+ layer is believed to be responsible for the higher photocatalytic activity and longer life time of photogenerated charge carriers. Further investigation is underway to obtain single crystal and the precise crystal structure of BiO-BTCIE, especially the configuration of BTC ion in the Bi2O22+interlayer. Based on the crystal structure, the electronic structure of BiO-BTCIE will be analyzed.
Acknowledgment This work was financially supported by the National Basic Research Program of China (the 973 Program, No. 2013CB632401), the National Natural Science Foundation of China (No. 21573135, 21333006, 21007031, 11374190 and 51321091), the Shandong Province Natural Science Foundation (ZR2014JL008) and Taishan Scholar Foundation of Shandong Province, China.
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Graphical abstract
BiO-BTCIE is obtained via the ion exchange reaction, i.e. the HCOO- in BiOHCOO is changed to be BTC anions. The obtained BiO-BTCIE remains the layered structure of BiOHCOO, except a distortion of the Bi2O22+ layer. The distortion is assigned to the origin of the enhanced photocatalytic activity and longer life time of photogenerated charge carriers of BiO-BTCIE.
