Bismuth tungstate incorporated zirconium metal–organic framework composite with enhanced visible-light photocatalytic performance

Abstract

Metal–organic frameworks (MOFs) have many outstanding properties that make them candidate materials for the development of high performance catalysts. Visible-light promoted photocatalysis for the degradation of organic pollutants is a promising direction in the field of water treatment. However, studies applying MOFs as photocatalyst for water treatment are very limited. In this study, UiO-66, a zirconium based MOF, was incorporated with bismuth tungstate (Bi₂WO₆) by a simple hydrothermal method for the development of visible-light photocatalysts. The Bi₂WO₆/UiO-66 composite not only exhibited enhanced photocatalytic activity for the degradation of Rhodamine B (RhB) under visible-light irradiation, but also showed good catalyst stability. In the recycled dye degradation experiments, most activity of the composite was reserved, and the structure and morphology of the composite did not vary after the experiment of dye degradation, either. The photocatalytic activity of Bi₂WO₆/UiO-66 composites with varying Bi : Zr molar ratios were investigated and the optimum Bi₂WO₆ content was found. Also, by introducing different scavengers to compete for the active species involved in the degradation process, the mechanism of the photocatalytic degradation of RhB by the Bi₂WO₆/UiO-66 composite was studied.

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1. Introduction

Since the last decade, a great number of studies has focused on metal–organic frameworks (MOFs), a class of porous solid materials in which inorganic metal (or metal-containing cluster) nodes are bridged by organic linkers to form uniform framework structures.^1–4 Due to their outstanding properties, such as high specific surface area, tunable pore size, great chemical variety, and relatively good thermostability, MOFs have been widely applied in the areas of separation, gas storage, catalysis, chemical sensors, biomedicine, and luminescence.^5–13 Furthermore, recent studies show that certain MOFs exhibit semiconductor behavior,^14,15 meaning that such MOFs can facilitate charge transfer¹⁶ or directly harvest light irradiation.^17,18 Hence, MOFs could be a promising candidate material for the application in photocatalysis. Since its first description by Lillerud's group, UiO-66, a zirconium based MOF, has drawn extensive attention. Besides the outstanding properties of MOFs mentioned earlier, UiO-66 also possesses higher thermostability and chemical stability compared to other types of MOFs.¹⁹ For example, UiO-66 has been extensively used for gas storage, adsorption and hydrogen generation.^10,20–22 Moreover, it was reported that UiO-66 has excellent structural stability in water.²³ The stability of UiO-66 can even be preserved after introduction of missing-linker defects,²⁴ or incorporation of active functional groups.²⁵ In addition, UiO-66 has been proven to be a semiconductor material.^10,26 Therefore, UiO-66 can be regarded as a promising candidate for the development of heterogeneous photocatalysts for water treatment. Photocatalysts with activity in the visible-light range are attractive due to the more efficient utilization of the solar energy.^27,28 Although the relatively wide band gap of UiO-66 may limit its optical absorption in the visible-light range, certain strategies can be used to solve this problem and to prepare UiO-66 based visible-light photocatalysts. These strategies include modifying the organic linker with functional groups,²⁹ or incorporating UiO-66 together with narrow band gap semiconductors.³⁰ Even so, to the best of our knowledge, studies on UiO-66 based visible-light photocatalysts for water treatment are very limited. For instance, Wu's group used an amino-functionalized UiO-66 anchored with palladium nanoparticles as photocatalyst to degrade methyl orange and methylene blue.³¹ However, Cr(VI) had to be introduced into the system in order to promote the degradation process, which is obviously unpractical in water treatment. Therefore, it is challenging yet highly necessary to develop a UiO-66 based photocatalyst that can efficiently degrade organic pollutants under visible-light irradiation.

As a narrow band gap semiconductor material, bismuth tungstate (Bi₂WO₆) has been intensively used for the degradation of organic contaminants under visible-light irradiation.^32,33 Currently, the major strategies to improve the photocatalytic activity of Bi₂WO₆ are synthesizing Bi₂WO₆ with specially designed morphology,^34,35 and incorporating it with another semiconductor.^36,37 Nevertheless, photocatalysts based on Bi₂WO₆/MOF composites have not been reported yet. Due to the superior properties of Bi₂WO₆ and UiO-66, the composite is expected to exhibit enhanced visible-light photocatalytic activity and good stability.

Herein, we report a Bi₂WO₆ incorporated UiO-66 composite as photocatalyst for the application of highly efficient water treatment. This Bi₂WO₆/UiO-66 composite is prepared by a simple hydrothermal method, and good interfacial interaction is formed between the two semiconductor materials based on the ingeniously designed procedure. The activity of the Bi₂WO₆/UiO-66 composite is evaluated for the degradation of Rhodamine B (RhB) under visible-light irradiation as test reaction, and the mechanism of the photocatalysis is also investigated.

2. Experimental

2.1. Synthesis of UiO-66

UiO-66 was synthesized following the previously reported method.¹⁹ Typically, ZrCl₄ (0.053 g, 0.227 mmol) and 1,4-benzenedicarboxylic acid (H₂BDC, 0.034 g, 0.227 mmol) were dissolved in N,N′-dimethylformamide (DMF, 24.9 g, 340 mmol) at room temperature. The obtained mixture was then sealed in an autoclave and heated in an oven at 120 °C for 24 h. The resulting white solid was collected by filtration.

2.2. Preparation of Bi₂WO₆/UiO-66 composites

A series of Bi₂WO₆/UiO-66 composites with varying Bi₂WO₆ contents were prepared based on the molar ratio of Bi : Zr. The formula of UiO-66 is reported as Zr₂₄O₁₂₀C₁₉₂H₉₆.¹⁹ To prepare a Bi₂WO₆/UiO-66 composite with a Bi : Zr molar ratio of 1 : 1 (denoted as BWO/UiO-66-1), 30.4 mg (0.092 mmol) Na₂WO₄·2H₂O was dissolved in 10 mL deionized water by vigorous stirring. 50 mg (7.7 × 10⁻³ mmol) UiO-66 was then added, and the resulting mixture was kept stirring for 1 h. Thereafter, 89.3 mg (0.184 mmol) Bi(NO₃)₃·5H₂O was dissolved in 10 mL DMF, and this Bi(NO₃)₃ DMF solution was added to the UiO-66 prepared as described in step 1. The reaction precursor was then stirred for another 30 min, and transferred to a 25 mL autoclave. Subsequently, the autoclave was sealed and heated in an oven at 120 °C for 12 h. Finally, the solid product was collected by filtration, and washed several times with deionized water. Other composites with Bi : Zr molar ratios of 0.1 : 1 (denoted as BWO/UiO-66-0.1), 0.5 : 1 (denoted as BWO/UiO-66-0.5), 2 : 1 (denoted as BWO/UiO-66-2) were also prepared through a similar procedure by adjusting the amount of the precursors for Bi₂WO₆. Pristine Bi₂WO₆ was also synthesized by the same procedure, excluding the addition of UiO-66.

2.3. Characterization

Powder XRD data was collected on a Bruker-AXS D8 DISCOVER with GADDS Powder X-ray diffractometer. The Cu Kα line (λ = 1.5406 Å) was used as the radiation source. The product morphology was characterized by electron microscopy (JEOL JSM-6701F FESEM, and JEOL JSM-3010 TEM with integrated Oxford INCA EDX system). UV-Vis diffuse reflectance spectra were recorded by a Shimadzu UV-2450 UV/VIS spectrometer with an ISR-240A Integrating Sphere Attachment, with BaSO₄ as reference. N₂ adsorption–desorption isotherms were collected with a Micromeritics Tristar 3000 at liquid nitrogen temperature. The specific surface areas were evaluated with the Brunauer–Emmett–Teller (BET) method, and the pore size distributions were calculated by the Barrett–Joyner–Halenda (BJH) equation. The total pore volumes were obtained at P/P₀ = 0.99.

2.4. Evaluation of the visible-light photocatalytic activity of Bi₂WO₆/UiO-66

The visible-light photocatalytic activity of the Bi₂WO₆/UiO-66 composite was evaluated by measuring the absorbance of the RhB solution at 554 nm during the process of dye degradation. In a typical degradation experiment with BWO/UiO-66-1, 15 mg BWO/UiO-66-1 was added to 30 mL RhB aqueous solution (0.03 mM) in a 50 mL round bottom flask. The mixture was then stirred thoroughly with a magnetic stirrer in the dark for 1 h to reach complete adsorption equilibrium. Thereafter, the suspension was irradiated by a 500 W halogen lamp. A 420 nm cutoff filter (Newport, 65CGA-420) and a water filter were placed between the dye degradation system and the light source to eliminate the UV and infrared irradiation. The irradiation intensity in the center of the flask was measured to be about 38 mW cm⁻² by an Ophir Nova II power/energy meter. During the entire degradation process, the solids were kept in suspension by magnetic stirring and air was continuously bubbled through the reaction mixture at the rate of 10 mL min⁻¹ to maintain a constant dissolved oxygen content. At certain time intervals, 0.8 mL aliquots were sampled and centrifuged. The dye concentration of the clear supernatant was then measured by a Shimadzu UV-1700 UV/VIS spectrometer. In the degradation experiments with BWO/UiO-66-0.1, BWO/UiO-66-0.5, BWO/UiO-66-2, and the control experiment in which pristine UiO-66 and Bi₂WO₆ were mechanically mixed, all conditions were the same as those in the experiment with BWO/UiO-66-1; and the amount of pristine UiO-66 or Bi₂WO₆ was equal to the actual amount of UiO-66 or Bi₂WO₆ contained in BWO/UiO-66-1. The blank experiment was also handled under the same conditions, but no catalyst was added.

2.5. Investigation of the photocatalytic mechanism

To investigate the mechanism of photocatalytic degradation of RhB by the Bi₂WO₆/UiO-66 composite, isopropyl alcohol (IPA), benzoquinone (BQ), and EDTA were introduced as scavengers for hydroxyl radicals (HO˙), superoxide radicals (O₂⁻˙), and holes (h⁺), respectively.^38–40 IPA, BQ or EDTA was added to the RhB solution to reach their final concentrations of 500 mM, 1 mM or 1 mM before the addition of Bi₂WO₆/UiO-66-1. All other conditions remained the same as in the photodegradation experiment mentioned in Section 2.4. In the experiment with N₂ purging, air was replaced by N₂ to bubble into the suspension at the same rate with other conditions unchanged. The HO˙ formed on the surface of the BWO/UiO-66-1 composite was detected by the photoluminescence technique, in which terephthalic acid was used as a probe molecule.⁴¹ The detection process was similar to the evaluation of photocatalytic activity, except that RhB solution was replaced by 5 × 10⁻⁴ M terephthalic acid dissolved in 30 mL of 2 × 10⁻³ M NaOH aqueous solution. During light irradiation, 0.8 mL reaction solution was sampled every 10 minutes to measure the fluorescence intensity at 425 nm (Shimadzu RF-5301PC fluorescence spectrometer; excitation wavelength 315 nm).

3. Results and discussion

3.1. Material characterization

To confirm the composition of Bi₂WO₆/UiO-66, samples with different Bi₂WO₆ contents were characterized by XRD. Fig. 1a shows the XRD pattern of the pristine UiO-66 sample. The pattern is consistent with that reported in the literature,¹⁹ indicating the successful synthesis of UiO-66. After incorporation of Bi₂WO₆, most of the peaks characteristic for UiO-66 can still be clearly identified in all composites, suggesting that the UiO-66 remained intact after the incorporation of Bi₂WO₆ during the hydrothermal synthesis process (Fig. 1b and S1a†). This should be due to the superior stability of UiO-66 as mentioned before, which further confirms that UiO-66 is one of the most suitable porous materials to incorporate with Bi₂WO₆. In addition, all of the Bi₂WO₆/UiO-66 composites display good crystallinity and show sharp diffraction peaks corresponding to Bi₂WO₆ (JCPDS: 73-1126). The result implies that crystallized Bi₂WO₆ was successfully obtained in the current synthetic conditions. Also, with the increase in Bi₂WO₆ content in the composite, the diffraction peaks of UiO-66 weakened, while the peaks belonging to Bi₂WO₆ became correspondingly more intense. This could be due to the suppression of the diffraction peaks of UiO-66 by those of Bi₂WO₆, which become dominant in the composites with the increase in Bi : Zr ratio. Pristine Bi₂WO₆ was also synthesized under the same experimental conditions as that of BWO/UiO-66-1, excluding the addition of UiO-66. As shown in Fig. S1b,† pristine Bi₂WO₆ was also successfully synthesized. Besides, the width of the diffraction peaks of pristine Bi₂WO₆ was slightly broader than that of BWO/UiO-66-1, indicating a smaller crystallite size of Bi₂WO₆ in the pristine sample.

Fig. 1 XRD patterns of (a) UiO-66, (b) BWO/UiO-66-1, and BWO/UiO-66-1 after RhB degradation experiment (denoted as BWO/UiO-66-1-A). In the pattern of BWO/UiO-66-1, the peaks belonging to UiO-66 were indicated by arrows.

Fig. 2a shows the SEM image of pristine UiO-66. The UiO-66 nanoparticles exhibit nanocube morphology, and the size of the nanoparticles is less than 100 nm. However, the exact dimension of these nanocubes cannot be identified from the SEM image. Fig. 2b and c show the SEM images of BWO/UiO-66-1, from which the UiO-66 nanoparticles can be clearly distinguished. This result is also in line with that of XRD characterization, suggesting that the structure of UiO-66 was conserved in the obtained Bi₂WO₆/UiO-66 composites. It can be easily observed that there are many nanoplates inserted among the UiO-66 nanoparticles, and the dimension of these nanoplates is larger than a few hundred nanometers. It is believed that these nanoplates are Bi₂WO₆, which was further confirmed by EDX. These Bi₂WO₆ nanoplates are uniformly dispersed in the UiO-66 nanoparticles, and it is expected that such a pattern might facilitate the interaction between Bi₂WO₆ and UiO-66. A Bi₂WO₆ crystalline structure with nanoplate morphology was also reported by other researchers.^42–44 However, the SEM images of pristine Bi₂WO₆ (Fig. S2†) showed that instead of nanoplates, only nanoparticles were obtained.

Fig. 2 SEM images of (a) pristine UiO-66, (b and c) BWO/UiO-66-1, and (d) BWO/UiO-66-1 after RhB degradation experiment (scale bars are 500 nm).

The morphology of pristine UiO-66, Bi₂WO₆ and BWO/UiO-66-1 was also characterized by TEM. As shown in Fig. 3a, the UiO-66 nanoparticles display uniform nanocube morphology, which is in agreement with the result of SEM characterization. The size of these UiO-66 nanocubes is around 50 nm. After the incorporation of Bi₂WO₆ (Fig. 3b and c), the UiO-66 nanocubes can still be clearly identified, in agreement with the results of XRD and SEM. Noticeably, the nanoplate structure of Bi₂WO₆ is also observed in the TEM images of the BWO/UiO-66-1 sample. Additionally, small spot EDX was used to analyze the composition of the nanocubes and nanoplates (Fig. S3†). Only the metal element Zr was found in the EDX result of the nanocubes, demonstrating that these nanocubes should be UiO-66. In addition, the nanoplates should be consisted of Bi₂WO₆, proved by the dominant metal elements Bi and W observed in the EDX result of the nanoplates. Moreover, the atomic ratio of Bi : W roughly equals 2 : 1, consistent with the formula of Bi₂WO₆. Interestingly, the morphology of the pristine Bi₂WO₆ is small nanoparticles (Fig. S4†), which is quite different from that of the Bi₂WO₆ component in the BWO/UiO-66-1. These nanoparticles are relatively uniform, and the size is less than 20 nm. It is proposed that the existence of UiO-66 affects the growth process of the Bi₂WO₆ crystals, meaning that the Bi₂WO₆ crystals may selectively grow in certain directions in the presence of UiO-66, leading to the formation of the nanoplate morphology. In contrast, in the absence of UiO-66, it is much more likely for Bi₂WO₆ to grow unrestricted and form the nanoparticle morphology.

Fig. 3 TEM images of (a) pristine UiO-66, (b and c) BWO/UiO-66-1, and (d) BWO/UiO-66-1 after RhB degradation experiment (scale bars are 100 nm).

The specific surface areas of the Bi₂WO₆/UiO-66 composites, and pristine UiO-66 and Bi₂WO₆ were evaluated by N₂ adsorption. The pore size distributions of these samples were calculated by the BJH equation. The results are listed in Table 1, and the figures of adsorption–desorption isotherms and pore size distributions are shown in Fig. S5.† The specific surface area of the pristine UiO-66 obtained in this study is 808 m² g⁻¹. Although this value is slightly lower than some model examples, such as 1069 m² g⁻¹,²⁶ and 1110 m² g⁻¹,⁴⁵ it is comparable the values cited by other studies, e.g., 700 m² g⁻¹,⁴⁶ or 850 m² g⁻¹.²⁰ Note that no moderator, such as acetic acid,^24,46 was added during the synthesis of UiO-66 in this study, and the synthesis conditions were not optimized, either. Incorporation of Bi₂WO₆, leads to a decrease of the specific surface areas of the composites with increasing Bi₂WO₆ content. The specific surface area of the pristine Bi₂WO₆ is relatively low, less than 50 m² g⁻¹. The total pore volumes of the composites were also lower after introduction of Bi₂WO₆.

Table 1 Comparison of the properties (i.e., Bi : Zr ratio, specific surface area and total pore volume) and photocatalytic activity (reaction rate constant) of different catalyst samples and blank experiment

Sample	Bi : Zr Ratio^a	Specific surface area^b/m^2 g^−1	Total pore volume^c/cm^3 g^−1	Reaction rate constant (k)^d/min^−1
a The Bi : Zr ratios were molar ratios, and were determined based on the reaction precursor.b The specific surface areas were evaluated by the BET method.c The total pore volumes were obtained at P/P0 = 0.99.d The reaction rate constants (k) were calculated based on a pseudo-first-order kinetic model.e The reaction rate constant of the control experiment was obtained from the mixture of pristine UiO-66 and Bi2WO6, and the amount of UiO-66 or Bi2WO6 was equal to the actual amount of that contained in BWO/UiO-66-1.f In the blank degradation experiment, no catalyst was added.
BWO/UiO-66-0.1	0.1 : 1	583.9	0.75	0.0090
BWO/UiO-66-0.5	0.5 : 1	363.6	0.55	0.0159
BWO/UiO-66-1	1 : 1	275.1	0.39	0.0226
BWO/UiO-66-2	2 : 1	198.3	0.28	0.0104
Pristine UiO-66	—	807.7	0.75	0.0062^e
Pristine Bi2WO6	—	48.3	0.37
Blank^f	—	—	—	0.0001

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The optical absorption of different Bi₂WO₆/UiO-66 composites, and pristine UiO-66 and Bi₂WO₆ samples was measured by UV-Vis. As illustrated in Fig. 4, UiO-66 is transparent in the wavelength range from 350 to 800 nm, which is in line with the result that UiO-66 only absorbed UV-light.²⁶ Similar to the pristine Bi₂WO₆, all of the Bi₂WO₆/UiO-66 composites have an absorption edge at about 450 nm, demonstrating that these composites all possess some visible-light absorption ability. Therefore, these Bi₂WO₆/UiO-66 composites should be capable of utilizing visible-light energy.

Fig. 4 UV-Vis diffuse reflectance spectra of pristine UiO-66, pristine Bi₂WO₆, and Bi₂WO₆/UiO-66 composites with different Bi₂WO₆ contents.

3.2. Photocatalytic activity

The photocatalytic activities of the Bi₂WO₆/UiO-66 composites were evaluated by using them for the degradation of RhB in aqueous solution under visible-light irradiation. In the control experiment, RhB was degraded in the presence of a mechanical mixture of pristine UiO-66 and Bi₂WO₆ (where the amount of UiO-66 and Bi₂WO₆ was equal to the amount used in the BWO/UiO-66-1 composite). The blank experiment (i.e., without catalyst) was performed under the same conditions as in the other RhB degradation experiments. As illustrated in Fig. 5a, the BWO/UiO-66-1 sample exhibits the fastest degradation rate, and RhB was almost completely degraded in 180 min. In the control experiment, although the pristine UiO-66 and Bi₂WO₆ mixture show some activity, the degradation rate is the lower compared to those of the composites. To quantify the photocatalytic activities of these samples, the reaction rate constants (k) for RhB degradation were calculated, assuming that the reactions follow a pseudo-first-order kinetic model (Table 1 and Fig. 5b).⁴⁷ By comparing the reaction rate constants, it is clearly seen that the photocatalytic activity of the Bi₂WO₆/UiO-66 composite increases when the Bi : Zr ratio is enhanced from 0.1 : 1 to 1 : 1. The BWO/UiO-66-1 sample with Bi : Zr ratio equaling 1 : 1 shows the highest photocatalytic activity. However, when the Bi : Zr ratio is increased further to 2 : 1, the photocatalytic activity of the composite drops drastically, and is almost the same as that of the composite with Bi : Zr ratio of 0.1 : 1. The pristine UiO-66 and Bi₂WO₆ mixture in the control experiment exhibits the lowest photocatalytic activity of all the photocatalyst samples prepared for this study. This suggests that there is a cooperative synergism between the Bi₂WO₆ and UiO-66 in the composites. This good interaction may be attributed to the ingeniously designed preparation strategy, which will be discussed below. In the control experiment with mechanical mixing, it is expected that no such good interaction could form. Therefore, compared to those of the specially prepared composites, the photocatalytic activity of the sample in the control experiment is much lower. Also note that the photolysis of RhB in the absence of any catalyst is negligible (Fig. 5a, blank).

Fig. 5 (a) Photocatalytic degradation of RhB in the presence and absence (blank) of different catalysts (BWO/UiO-66-0.1, BWO/UiO-66-0.5, BWO/UiO-66-1, and BWO/UiO-66-2) under visible-light irradiation. For the control experiment (mechanical mixture of pristine UiO-66 and pristine Bi₂WO₆), the amount of pristine UiO-66 or Bi₂WO₆ was equal to the actual amount of that in BWO/UiO-66-1. (b) Comparison of the reaction rate constant (k) in the presence of different catalysts (assuming that the reactions follow the pseudo-first-order kinetic model).

3.3. Photocatalyst stability

The structure of BWO/UiO-66-1 was characterized before and after 180 min of RhB degradation to assess its stability under the current experimental conditions. As shown in Fig. 1b, no apparent variation in the XRD patterns of BWO/UiO-66-1 is observed before and after RhB degradation. The comparison of the SEM images of BWO/UiO-66-1 before (Fig. 2b and c) and after (Fig. 2d) RhB degradation also indicates that there is no observable morphology change. Furthermore, the same conclusion can also be obtained based on comparison of the TEM figures of the same sample before (Fig. 3b and c) and after (Fig. 3d) the degradation experiment. The stability and reusability of BWO/UiO-66-1 were further studied by reusing the composite for four cycles of dye degradation experiments. As illustrated in Fig. S6,† the activity of the catalyst decreased in the second cycle. A possible explanation is that part of the degradation products might be adsorbed or trapped in the pores of the composite. These stuck molecules would reduce the accessible surface area of the catalyst, leading to the decline of its activity. Fortunately, most surface area of the composite was still accessible after the first degradation experiment, since more than 80% of the dye was degraded in the second degradation cycle. Furthermore, no obvious loss of photocatalytic stability can be observed in the third and fourth cycles, which implies the excellent stability of the Bi₂WO₆/UiO-66 photocatalyst. The superior stability of UiO-66 and the excellent stability of Bi₂WO₆ both contribute to the stability of this Bi₂WO₆/UiO-66 composite. The stability of Bi₂WO₆ under various degradation conditions was also reported by other researchers.^33,34 However, it is worth to mention that the good interaction between Bi₂WO₆ and UiO-66 in this composite might be another important factor contributing to the stability of the photocatalyst. To achieve this effect, highly crystalline UiO-66 was first obtained, and the order of addition of the reagents is important. During the preparation process of the Bi₂WO₆/UiO-66 composites, UiO-66 was first dispersed in the aqueous solution of Na₂WO₄. Since the Zr₆O₄(OH)₄ octahedra (red clusters shown in Fig. S7†) are positively charged, it is foreseen that an electrostatic interaction will occur between these octahedra and the negatively charged WO₄²⁻ anions.²⁶ Therefore, the WO₄²⁻ anions accumulate around the UiO-66 framework structure, and then react with the next reagent (i.e., Bi₂O₂²⁺ cations),^34,42 resulting in the incorporation of Bi₂WO₆ to the UiO-66 framework structure during the synthesis process.

3.4. Photocatalytic mechanism

To understand the photocatalytic mechanism of this Bi₂WO₆/UiO-66 photocatalyst system for the degradation of RhB, the potential roles of HO˙, O₂⁻˙, and h⁺ during the degradation process were investigated. These are the three species mostly responsible for initiating the photocatalytic oxidation.^48–51 To identify the contributions of these different species, IPA, BQ, and EDTA were introduced in the degradation process separately to attempt to trap HO˙, O₂⁻˙, and h⁺, respectively.^38–40 As shown in Fig. 6, the introduction of IPA, a scavenger of HO˙, does not cause any significant change in the RhB degradation rate. This implies that HO˙ is not an important active species for the photocatalytic process. To further confirm this result, the HO˙ generated on the surface of the photocatalyst was detected by a photoluminescence technique, in which terephthalic acid was used as a probe molecule.⁴¹ As illustrated in Fig. S8,† during the 30 min monitoring process, no obvious change in photoluminescence intensity is observed, demonstrating that almost no HO˙ was formed by the Bi₂WO₆/UiO-66 photocatalyst under visible-light irradiation. This further supports the conclusion that HO˙ radicals do not dominate the photodegradation of RhB by the Bi₂WO₆/UiO-66 photocatalyst. However, the introduction of BQ obviously decreases the degradation rate of RhB (Fig. 6). Since BQ is able to quench O₂⁻˙, this result suggests that O₂⁻˙ should be one of the major contributors to the decomposition of RhB. Considering that the dissolved O₂ is the crucial reagent to form O₂⁻˙, N₂ was purged instead of air in a control experiment to study its effect on the change in degradation rate. As shown in Fig. 6, N₂ purging also has a negative effect on the degradation rate, and the result is very similar to that of the experiment in the presence of BQ. Thus, it is established that O₂⁻˙ is an important active species for RhB degradation in the current Bi₂WO₆/UiO-66 system. Nevertheless, BQ addition and N₂ purging could only partially suppress the degradation reaction, implying that RhB was also decomposed by other routes. To understand the mechanism further, EDTA was added to the reaction system to trap h⁺. Only very limited degradation of RhB was observed in the presence of EDTA. Since the addition of EDTA accelerates the surface recombination of e⁻ and h⁺, this effect also decreases e⁻ generation in the conduction band, resulting in the suppression of the generation of O₂⁻˙. Hence, by cutting off the formation of O₂⁻˙ and h⁺ at the same time, the degradation of RhB was almost totally restrained. Therefore, in the Bi₂WO₆/UiO-66 photocatalyst system, RhB can also be oxidized by the photogenerated h⁺. The result that O₂⁻˙ and h⁺ are the two main active species for the Bi₂WO₆ based photocatalyst system was also confirmed by other research groups.^33,52

Fig. 6 Effects of different scavengers and N₂ purging on the degradation of RhB in the presence of BWO/UiO-66-1 under visible-light irradiation.

According to the reported properties of UiO-66 and Bi₂WO₆, a possible photocatalytic mechanism of the Bi₂WO₆/UiO-66 photocatalyst is proposed and shown in Scheme 1.^26,53,54 Based on this mechanism, Bi₂WO₆/UiO-66 may promote the separation of the photogenerated electron–hole pairs, which explains the enhanced photocatalytic activity. Note that the photogenerated e⁻ in the conduction band of UiO-66 is negative enough to react with dissolved O₂ to form O₂⁻˙. Meanwhile, although the h⁺ generated on the valence band of Bi₂WO₆ is not positive enough to react with H₂O to form HO˙, they can still directly oxidize RhB. This proposed photocatalytic mechanism also supports the conclusion of the investigation of active species.

Scheme 1 Proposed mechanism of photocatalytic degradation of RhB by the Bi₂WO₆/UiO-66 composite under visible-light irradiation.

4. Conclusion

In summary, UiO-66 was incorporated with Bi₂WO₆ by a simple hydrothermal method. The developed Bi₂WO₆/UiO-66 composite exhibited enhanced photocatalytic activity for the degradation of RhB under visible-light irradiation, compared to the mechanical mixture of the individual UiO-66 and Bi₂WO₆. It is proposed that the enhancement of the activity is due to the good interaction between Bi₂WO₆ and UiO-66, by which the photogenerated electron–hole pairs can be efficiently separated. Furthermore, the ingenious preparation strategy is the critical step to achieve the intimate combination of Bi₂WO₆ and UiO-66. In addition, by adjusting the Bi₂WO₆ content in the Bi₂WO₆/UiO-66 composite, a molar ratio of Bi : Zr equaling 1 : 1 was found to be optimum for the composite to achieve the highest photocatalytic activity. This Bi₂WO₆/UiO-66 photocatalyst also exhibited good stability, since no obvious changes in the structure and morphology were observed after a complete degradation experiment and most of the activity was reserved in the long-term degradation. The mechanism of photocatalytic degradation of RhB by the Bi₂WO₆/UiO-66 composite was also investigated, and the results implied that O₂⁻˙ and h⁺ were two main active species involving in the degradation process of RhB. Last but not least, this study should open the opportunities to the development of various MOFs based visible-light photocatalysts for water treatment in future.

Acknowledgements

We acknowledge the financial support from the Singapore-Peking-Oxford Research Enterprise (SPORE), COY-15-EWI-RCFSA/N197-1.

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Footnote

† Electronic supplementary information (ESI) available: Additional XRD figures, SEM and TEM images, EDX data, N₂ adsorption–desorption isotherms and pore size distribution figures, four cycles degradation data, crystal structural illustration of UiO-66, photoluminescence spectra for HO˙ detection. See DOI: 10.1039/c4ra13000f

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