DOI:
10.1039/C4RA11383G
(Paper)
RSC Adv., 2015,
5, 43473-43479
BiVO4/MIL-101 composite having the synergistically enhanced visible light photocatalytic activity
Received
28th September 2014
, Accepted 24th April 2015
First published on 27th April 2015
Abstract
In this study, a novel composite containing a Cr based metal–organic framework (MOF), MIL-101 and BiVO4 was successfully synthesized by hydrothermal method, and was fully characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, UV-vis diffuse reflectance absorption spectra and photoluminescence emission spectra. Moreover, the photocatalytic activities of BiVO4/MIL-101 composite and the pure materials were evaluated by measuring the degradation of Rhodamine B under visible light. The results show that the composite exhibits better photocatalytic activities than pure materials, which can be ascribed to the big adsorption capacity of MIL-101 and the enhanced separation of photogenerated charge carriers from assembly of MIL-101 on BiVO4. The synergistic effect between BiVO4 and MIL-101 was evaluated by the proposed synergistic factor, which is bigger than 1 for various ratios of BiVO4 to MIL-101, suggesting there exists positive interactions between pure materials for enhancing photocatalytic activities.
1. Introduction
In recent years, metal–organic frameworks (MOFs) have gained widespread attention due to their considerable physical and chemical properties.1,2 MOFs are an emerging class of highly porous materials, built from organic linkers and inorganic metal nodes, which can exhibit extremely high surface areas, as well as tunable pore size and functionality, and can act as hosts for a variety of guest molecules.3 To date, many studies concerning MOFs research adsorption capacity for gas, such as H2, N2, O2 and CO2 and organic dyes in aqueous solution.4 Although photocatalysis as an energy saving and environment friendly technique has been extensively investigated in removing various organic pollutants in water, few studies cover the photocatalytic activity of MOFs. On the other hand, BiVO4 has attracted more and more people's interest, due to its relatively narrow band gap energy (<2.5 eV) and high photocatalytic activity in evolution of H2 (ref. 5 and 6) and O2 (ref. 7–9) as well as degradation of organic contaminants.10,11 It is well known that several factors, such as crystal structure, oxygen vacancy density, morphology, surface area, pore structure and band gap energy can influence the performance of a photocatalyst. MIL-101, as one extensively investigated MOF, reveals certain photocatalytic activity, but its big band gap energy and easy recombination of the photogenerated electrons (e−) and holes (h+) can inhibit the photocatalytic efficiency. Therefore, great efforts have been made to expand the photo-absorption range and facilitate the separation of the photo-induced carriers. Coupling of two semiconducting oxides with variable band gaps has been proven to be a feasible approach to reduce the recombination probability of photogenerated electrons and holes, and thus has been widely employed to improve the photocatalytic activity.12 Various composite semiconductors, like CuCr2O4/TiO2, BiVO4/Bi2O3, AgBr/WO3, SnO2/ZnO, BiOI/TiO2 and Bi2S3/BiVO4 have been demonstrated to be efficient in separating electron and hole pairs.13–17
In this work, it is the first time to combine MIL-101 with BiVO4 to prepare composite via coordination reaction induced self-assembly process, which is a common method to construct MOFs containing composites. The as-obtained BiVO4/MIL-101 composite has been explored for the degradation of Rhodamine B (RhB) using visible light, and shows superior photocatalytic activity as compared with pure materials and physical mixture. This is due to the synergistic effect of big adsorption capacity of MIL-101 for RhB and the enhanced separation of the photo-induced carriers by compounding.
2. Experimental section
2.1. Synthesis of BiVO4
All of the chemicals including NaOH, HF, ammonium metavanadate, chromium nitrate nanohydrate, bismuth nitrate pentahydrate, terephthalic acid (TPA) and Rhodamine B (RhB) purchased from Sinopharm Chemical Reagent Company were of analytical grade, and used as received without further purification.
BiVO4 was synthesized by a hydrothermal method as described previously.18 In a typical synthesis, 5 mmol (2.425 g) of Bi(NO3)3·5H2O was added into 80 mL of distilled water under mild stirring, and a white suspension was formed immediately due to the hydrolysis of Bi(NO3)3. Then, 5 mmol (0.585 g) of NH4VO3 was added into the white suspension, and an orange suspension was formed. Then, its pH value was adjusted to 5–6 by adding NaOH, and the suspension was stirred for 1 h, resulting in a light yellow colloid solution. After that, the colloid solution was loaded into a Teflon-lined autoclave, which was kept heating in an oven of 160 °C for 12 h, and then naturally cooled to room temperature. The resulted yellow powder was collected, washed with distilled water and ethanol several times to remove ions and possible remnants, and dried for further characterization.
2.2. Synthesis of BiVO4/MIL-101
To prepare the composite, firstly 0.100 g BiVO4, 2.000 g Cr(NO3)3·9H2O and 0.820 g TPA were respectively put into 8 mL deionized water with stirring. Then the suspension of BiVO4 was ultrasonic for 0.5 h. The three solutions were mixed, to which was added 7 drops of HF. After 1 h stirring, the suspension was placed in a Teflon-lined autoclave bomb and kept in an oven at 220 °C for 24 h. After the reaction, the autoclave was cooled to room temperature, and the light green-colored products were collected by filtration. To remove the unreacted TPA, the as-synthesized composite was further purified by treatment with N,N-dimethylformamide (20 mL) following a method described previously.19 The purified composite was dried at 80 °C for 3 h, dissolved into 20 mL CHCl3 for 24 h to remove the adsorbed N,N-dimethylformamide, then separated and dried. Other composites with different ratios of BiVO4 to MIL-101 can be prepared under the same condition via changing the mass of BiVO4 from 0.100 g to 0.050 g, 0.300 g and 0.500 g, and the resulted composites are named as 0.1BM, 0.05BM, 0.3BM and 0.5BM, respectively.
For comparison, the physical mixtures were also prepared by simply mixing pure BiVO4 and MIL-101 with the same ratios as those determined by the following thermogravimetric analysis for the composites.
2.3. Material characterization
The crystallographic structure was characterized by X-ray diffraction (XRD, Bruker advance-D8 power diffractometer with Cu Kα radiation, λ = 0.154178 nm at 40 kV and 100 mA flux). The morphology of materials were examined by transmission electron microscopy (TEM: JEM-2100F, Japan). The valence band X-ray photoelectron spectroscopy (XPS) was conducted using an ESCALAB250 spectrometer. Photoluminescence (PL) emission spectra were measured on a PL measurement system (Fluorolog Tau-3) with the excitation wavelength of 320 nm. The optical properties were investigated using a UV-visible spectrophotometer in a wavelength range of 300 to 900 nm. Thermogravimetric analysis (TGA) was carried out on a Perkin-Elmer Diamond TG thermal analyzer at a rate of 20 K min−1 in atmosphere.
2.4. Photocatalytic experiments
Photocatalytic properties of the as-prepared materials were evaluated by the degradation of RhB in aqueous solution. A 75 W energy-saving lamp (λ < 380 nm was filtered out by a cut off filter, JLT70T830, Philips) was used as light source to provide visible light irradiation. In each experiment, 40 mg of the material was dispersed in 100 mL of RhB solution (initial concentration, CO = 10 mg L−1), and the mixture was stirred for about 3.5 h in dark to attain equilibrium adsorption on the catalyst surface. Then the solution was exposed to visible light irradiation under magnetic stirring. After each 0.5 h, 5 mL of the mixture was withdrawn, and the catalyst was separated by centrifugation. Photocatalytic activity was evaluated by monitoring the RhB decolorization at the maximum absorption around λ = 553 nm with a UV-vis spectrophotometer. For comparison, we also explored the photocatalytic activity of colorless benzoic acid by the as-prepared materials.
3. Results and discussion
3.1. XRD analysis
Fig. 1 shows the XRD patterns of MIL-101, BiVO4 and BiVO4/MIL-101. For BiVO4, the peaks centered at 2θ = 18.98, 28.94, 30.58 and 35.22° match well with those of the monoclinic BiVO4 indicating that the BiVO4 sample has a monoclinic scheelite type crystal structure.20,21 For MIL-101, all of the diffraction peaks were also in good agreement with those of the standard sample.22 BiVO4/MIL-101 displays the characteristic peaks of both MIL-101 and BiVO4, and no new diffraction peaks appear, revealing the coexistence of BiVO4 and MIL-101. Besides, the intensity of diffraction peaks of the composite has no change as compared to that of BiVO4 and MIL-101, illustrating the stability of crystal structures for MIL-101 and BiVO4 in the composite.
 |
| Fig. 1 XRD patterns of MIL-101, BiVO4 and BiVO4/MIL-101. | |
3.2. TEM analysis
The morphology of the as-prepared samples was characterized by TEM. Fig. 2 shows low and high resolution TEM images of BiVO4 and BiVO4/MIL-101. As shown in Fig. 2(a) and (b), BiVO4 has schistose shape with the lattice spacing of 0.32 nm as reported previously.23 Fig. 2(c) and (d) show the images of BiVO4/MIL-101. After the rotation 90° of Fig. 2(d), we can obtain the mapping as displayed in Fig. 3, from which, it can be seen that V, Bi, as well as Cr element have the same distribution shape, indicating that MIL-101 covers the whole range. In Fig. 3(d), different concentration distribution of O element results from overlapping of O element in BiVO4 and MIL-101. As indicated in Fig. 2 and 3, MIL-101 was successfully assembled on BiVO4 with dark and bright parts representing BiVO4 and MIL-101, respectively.
 |
| Fig. 2 TEM images of BiVO4 and BiVO4/MIL-101: (a) BiVO4, inset is a complete plate-like particle of BiVO4; (b) HRTEM image of BiVO4; (c) and (d) BiVO4/MIL-101. | |
 |
| Fig. 3 Distribution of V, Cr, Bi and O elements for Fig. 2(d). | |
3.3. UV-vis diffuse reflectance absorption spectra and XPS analysis
The energy band structure feature of a semiconductor is considered as a key factor in determining its photocatalytic property.24 Some studies have indicated that MOFs can be used as photocatalyst to decompose water.25 Fig. 4(a) shows the UV-vis diffuse reflectance absorption spectra of BiVO4 and MIL-101, as indicated, the absorption edge of BiVO4 is located in the visible light region at about 500 nm, which agrees well with the reported value,26 whereas MIL-101 displays strong absorption in the UV region and weak absorption in the visible light region. The band gap of BiVO4 and MIL-101 can be calculated by the following equation:
where α, h, ν, Eg and A each represent the absorption coefficients, Plank constant, light frequency, band gap and a constant, and the calculated results are 2.51 eV and 3.68 eV for BiVO4 and MIL-101, respectively. Some researches27,28 reveal that conduction band of BiVO4 is at 0.3 eV, thus its valence band is at 2.81 eV. Fig. 4(b) shows the valence band of MIL-101 is at 2.09 eV. As a result, the conduction band of MIL-101 is at −1.59 eV which is higher than that of BiVO4.
 |
| Fig. 4 (a) UV-vis diffuse reflectance spectra of BiVO4 and MIL-101; (b) valence band XPS figure of MIL-101. | |
3.4. PL analysis
The photoluminescence (PL) emission spectra can be regarded as a direct approach to understand the separation efficiency of photogenerated carriers.29,30 Fig. 5 shows PL spectra of BiVO4, MIL-101 and BiVO4/MIL-101 composite upon excitation at 320 nm. It is obvious that emission intensities of the composite and BiVO4 are much weaker than that of MIL-101. Possible explanation is that the combination of BiVO4 and MIL-101 efficaciously separated the photogenerated electrons and holes.
 |
| Fig. 5 PL emission spectra for BiVO4, MIL-101 and BiVO4/MIL-101. | |
3.5. TGA and photocatalytic activities
In order to determine the mass ratio of BiVO4 to MIL-101 in different composites, TGA was carried out as shown in Fig. 6, from which residual quantity can be obtained for MIL-101, BiVO4, 0.05BM, 0.1BM, 0.3BM and 0.5BM. Then the BiVO4 to MIL-101 mass ratio in composites can be calculated by the following formula: |
mBiVO4β + mMIL-101γ = mBiVO4/MIL-101α
| (1) |
|
mBiVO4 + mMIL-101 = mBiVO4/MIL-101
| (2) |
|
 | (3) |
where
is mass ratio of BiVO4 to MIL-101 in the different composites, and β, α and γ are residue percentages of BiVO4, different composites and MIL-101, respectively. According to the formula (3), BiVO4 to MIL-101 mass ratios in 0.05BM, 0.1BM, 0.3BM and 0.5BM are 1
:
37.20, 1
:
11.45, 1
:
2.02 and 1
:
1.28, respectively.
 |
| Fig. 6 TGA and residual quantity of MIL-101, BiVO4, 0.05BM, 0.1BM, 0.3BM and 0.5BM. | |
Physical mixtures consisting of BiVO4 and MIL-101 with the same mass ratios as in the composites are named as 0.05PBM, 0.1PBM, 0.3PBM and 0.5PBM, and their photocatalytic properties were also evaluated for degradation of RhB under the same conditions as those for the composites. The photocatalytic degradation process follows the pseudo first-order kinetics, and the value of the rate constant k can be calculated by the following formula:31,32
where
CO is the initial concentration of RhB, and
C is that at time
t.
Fig. 7 shows the photocatalytic activities of different catalysts in the degradation of RhB and the fitted rate constant
κ by the formula
(4). As indicated in
Fig. 7(a), most of the composites exhibit better photocatalytic activity than pure materials. This means there exists certain synergistic interaction between MIL-101 and BiVO
4. However, the BiVO
4/MIL-101 ratio should be appropriate. As shown in
Fig. 7(a), for very high BiVO
4/MIL-101 ratio, due to imperfect crystallization of MIL-101, the composite has too small surface to adsorb RhB, thus decreasing photocatalytic activity, whereas for very low BiVO
4/MIL-101 ratio, the reduced heterojunction interfaces will result in rapid recombination of photogenerated electron–hole pairs, and the photocatalytic activity will also decrease.
Fig. 7(b) shows the photocatalytic kinetics of the physical mixtures which decreases monotonously with the amount of BiVO
4 increasing. In other words, there does not exist obvious synergistic interactions between pure materials in the physical mixtures. Furthermore all of the composites display better photocatalytic activity than the physical mixtures. For the composites, the assembly of MIL-101 on BiVO
4 enhanced the transfer and separation of photogenerated charge carriers through the interface of pure materials as evidenced by above PL analysis.
 |
| Fig. 7 Photocatalytic kinetics of (a) MIL-101, BiVO4 RhB and composites and (b) physical mixtures. | |
To quantitatively evaluate the synergistic effect, the synergistic factor sκ is proposed and calculated using the following formula:
|
 | (5) |
where
κ,
κBiVO4 and
κMIL-101 are the fitted reaction rate constants of the composites, BiVO
4 and MIL-101, respectively. The synergistic factors of the composites are 1.730, 1.150, 1.754 and 1.239 for 0.05, 0.1, 0.3 and 0.5BM, respectively, and all are bigger than 1, suggesting there exists positive interactions between pure materials for enhancing photocatalytic activities.
3.6. Photocatalytic mechanism of BiVO4/MIL-101 composites
It is widely known that photocatalytic degradation of pollutants under UV or visible light illumination takes place via the oxidization or reduction of toxic organic molecules with active species (e.g., ˙OH, ˙OOH, and O2−) induced by the photogenerated electrons (e−) and holes (h+) on photocatalyst surfaces. As shown in Fig. 8(a), there exists synergistic photocatalytic effect for the composites. BiVO4 has a narrowing band gap33–35 which is smaller than that of MIL-101.25 Therefore, under visible light irradiation, due to optical sensibilization, the photogenerated electrons produced by RhB rapidly transfer to the conduction band (CB) of BiVO4. At the same time, h+ produced by BiVO4 transfer to the valence band of MIL-101, resulting in effective separation of the photogenerated electrons and holes. Meanwhile, the electrons located on the CB of BiVO4 can react with O2 adsorbed on the surface of the composite to produce ˙O2− which creates ˙OOH after protonation. The ˙OH radicals can be produced from the trapped electron after formation of the HOO˙ radical36 and further decompose RhB. h+ on the surface of MIL-101 can directly decompose RhB adsorbed by MIL-101 or react with H2O to yield ˙OH to decompose RhB in the aqueous solution. Besides, the as-obtained BiVO4/MIL-101 composite and pure BiVO4 have been explored for the degradation of colorless benzoic acid using visible light. From Fig. 8(b), it can be seen that both the composite and pure BiVO4 can degrade colorless benzoic acid, and the composite has higher photocatalytic activity. Moreover, the reason why benzoic acid can be degraded by BiVO4 is that BiVO4 itself can produce photogenerated electrons and holes under visible light. The photocatalytic mechanism of degradation of benzoic acid by BiVO4/MIL-101 composite is similar to that of RhB. But the main difference is that photogenerated electrons for degradation of benzoic acid is only from BiVO4, but not at the same time from optical sensibilization of dyes like degradation of RhB. As a result, the amount of photogenerated electrons is less for degradation of benzoic acid, and the photocatalytic activity of benzoic acid is lower than that of RhB. Due to big adsorption capacity of MIL-101 and fast separation of photogenerated charge carrier between interface of BiVO4 and MIL-101, the degradation of RhB is enhanced, and the adsorption equilibrium of BiVO4/MIL-101 composite for RhB is broken, leading to rapid migration of RhB from aqueous solution to the surface of the composite. In this way, RhB is photodegraded more efficiently on the composites than on the pure materials.
 |
| Fig. 8 (a) Schematic diagram of separation of electron–hole pairs over BiVO4/MIL-101 composite under visible light; (b) degradation of benzoic acid by BiVO4 and BiVO4/MIL-101 composite. | |
4. Conclusions
In summary, BiVO4/MIL-101 composite photocatalysts have been successfully synthesized by hydrothermal method. The results of photocatalytic experiments indicate that BiVO4/MIL-101 composites exhibit enhanced photocatalytic activities for the degradation of RhB under visible light irradiation as compared with pure materials. The synergistic effect is due to the big adsorption of MIL-101 and the enhanced separation of photogenerated charge carrier between the interface of BiVO4 and MIL-101.
Acknowledgements
This work was supported by the Anhui Provincial Natural Science Foundation (no. 1508085MB28), and the National Natural Science Foundation of China (Grant 51372062).
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