Accessible fabrication of Bi₂MoO₆/BiOCl for effectively conducting thermally-responsive catalytic decontamination of model pollutants

Abstract

Bi₂MoO₆/BiOCl (BMB) with thermally-responsive catalytic properties was synthesized by a one-step hydrothermal approach under a low-temperature environment. The influence of initial concentration, reaction temperature, pH, and ions on the thermally-responsive catalytic degradation of methylene blue (MB) was systematically investigated. The experimental evidence shows that an excellent thermally-responsive catalytic activity on decomposing MB is found using BMB under low-heat excitation. Diverse ions introduced toward bare solutions could significantly improve the succeeding degradation effect. The degradation products from thermally-responsive catalytic reactions were identified by GC-MS, demonstrating that this pattern possesses a substantial ability to achieve the destruction of water pollutants. Recycling evaluation reveals that the total degradation rate of MB over BMB remains at a level of >70% even after three use cycles, implying that BMB could be reused without severe loss of activity. A rational mechanism responsible for deteriorating pollutants is also proposed. Accordingly, BMB triggered with heat may provide a viable alternative catalyst for the abatement of organic contaminants from future chemical industries.

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1. Introduction

Industrial plants have generated increasing amounts of wastewater, causing severe environmental problems in our modern industrialized society. In particular, organic pollutants derived from effluent, highly toxic for human beings, have become one of the most serious global environmental issues nowadays.^1–3 To date, the mitigation methods concerning dye substances have developed a variety of possible routes such as adsorption,⁴ sonolysis,⁵ Fenton-like reactions,⁶ biological degradations⁷ and photocatalysis.^8,9 Ultimately, more and more attention is focused on photocatalysis as the mainstream treatment method for hazardous organics, and may provide a promising approach to the implementation of the large-scale removal of persistent contaminants at different levels in waste water.^10,11 It is well-known that TiO₂ has been widely employed for photocatalytic applications owing to its cheapness, non-toxicity, and stability.^12,13 However, it has greatly limited the practical applications of solar energy due to its low visible-light response and high photogenerated charge-carrier combination.^14–16 In this case, by means of a deep investigation of photocatalysis, numerous improved techniques are explored for other novel efficient photocatalysts, for instance, Bi-based semiconductor materials, which have a unique layered structure and high chemical activity, have attracted great concern.¹⁷

Bismuth oxyhalides has a structure-dependent photocatalytic property that favors the transfer of electrons and holes generated inside the crystal surface so as to promote electron/hole separation.¹⁸ Nevertheless, BiOCl is a wide band-gap (E_g = 3.4 eV) semiconductor and can only absorb ultraviolet light, which accounts for less than 5% of solar energy, leading to a poor photocatalytic performance under visible-light irradiation.^19–21 Thus, a reasonable modification of BiOCl with a narrow band-gap may be an effective way to obtain the visible-light-induced photocatalytic activity. On the other hand, many kinds of heterojunction semiconductors (such as antisotype p–n, isotype n–n, and crystal-phase junctions)^22–26 have been extended to the field of photocatalysis for environmental decontamination. Among these, the bismuth-based photocatalyst, Bi₂MoO₆, is thought to be an ideal photocatalyst from the viewpoint of visible light. However, the high recombination rate of electron–hole pairs that exist in Bi₂MoO₆ badly hinders its application in pollutant decomposition in a way similar to that of TiO₂.²⁷

Any physical or chemical changes in materials are attributed to corresponding environmental factors such as light, heat, moisture, chemical condition and biological activity.^28,29 To overcome the shortcomings of photocatalytic property above, here we offer a thermally-responsive catalytic route under near room temperature instead of the light-driven excitation to make up the shortcomings. Inspired by the synthesis of many other metal oxide-based heterojunction composites, BiOCl heterojunction composites covered with amorphous Bi₂MoO₆ are highly desirable and anticipated as potential superior catalysts because of the high interfacial charge-transfer ability between those components. Since the overlapped energy band from BMB composite may make the electrons and holes more easily transfer and jump in favor of joining in the thermally-responsive catalytic reactions. Herein, we report on a facile one-step method to synthesize hierarchical BiOCl covered with amorphous Bi₂MoO₆ composites to achieve a possible treatment for organic dyes. To the best of our knowledge, there are a few reports on several photocatalysts, such as BiOCl/Bi₂MoO₆,³⁰ BiOCl/Bi₂WO₆,³¹ and AgCl/BiOCl,³² that possess superior photocatalytic activities in the abatement for contaminants. However, no information about a thermally-responsive catalytic degradation course using BiOCl covered with amorphous Bi₂MoO₆ has been reported previously. In detail, in this work we prepared the heat-sensitive Bi₂MoO₆/BiOCl (BMB) to accomplish the degradation of organics in the dark, demonstrating its remarkable thermally-responsive catalytic potential. Meanwhile, the effect of the existence of PO₄³⁻, CO₃²⁻, Cl⁻, SO₄²⁻ and NO₃⁻ on thermally-responsive catalytic degradation of methylene blue (MB) was also investigated, including the exploration of the main by-products after the degradation. In conclusion, it may be a very promising approach to deal with dye-contaminated wastewater and provide new insights into the degradation of water pollutants by means of an ambient-heat stimulus catalysis.

2. Experimental

2.1. Materials

Bismuth nitrate (Bi(NO₃)₃·5H₂O), ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O), hydrochloric acid (HCl), and methylene blue (MB) were purchased from Chengdu Kelong Chemical Reagent Factory, China. All reagents were analytical grade and use without further purification.

2.2. Synthesis of Bi₂MoO₆/BiOCl

Bi₂MoO₆/BiOCl (BMB) samples were synthesized using a one-step hydrothermal method. In a typical synthesis process, Bi(NO₃)₃·5H₂O (12.3 g) and (NH₄)₆Mo₇O₂₄·4H₂O (4.42 g) were firstly dissolved in 100 mL deionized water under magnetic stirring. After stirring for 10 min, HCl was added dropwise to adjust the pH of the above mixed solution to 7.2, and the solution became brown and was then transferred into a Teflon-lined stainless steel autoclave at 70 °C for 10 h. Subsequently, the autoclave was cooled to room temperature naturally. The products were washed several times by deionized water thoroughly, and dried at 60 °C for 10 h.

2.3. Adsorption course and thermally-responsive catalytic reactions

Considering the effect of physical adsorption on thermally-responsive catalytic reactions, the dark adsorption experiments were engaged until a constant value in terms of the absorbance of corresponding pollutants was achieved, and subsequently thermally-responsive catalytic processes were employed. Thermally-responsive catalytic activities of the samples were evaluated by thermodegradation of methylene blue (MB) in the dark in a slurry reactor containing 80 mg of catalyst and 80 mL of MB solution. At every 10 min interval, 4 mL of the suspension was periodically withdrawn and analyzed after centrifugation. The concentration of MB solutions was analyzed by recording the optical absorption intensity at 660 nm using a UV3900 UV-Vis spectrophotometer.

2.4. Measurements

Powder XRD patterns were collected with a PANalytical X'Pert Pro X-ray diffractometer using Cu Kα radiation (1.54 Å), and the working voltage was 40 kV. The morphologies and microstructures of the products were examined by a field emission scanning electron microscope (FSEM, Zeiss Ultra 55) equipped with an EDX and transmission electron microscope (TEM, Zeiss Libra 200 FE). The surface compositions of the samples were obtained by X-ray photoelectron spectroscopy (XPS, XSAM-800), using Al Kα radiation (hν = 1486.6 eV, Kratos). The optical spectra of all the samples were obtained using an ultraviolet visible (UV-Vis) spectrometer (UV3900, Hitachi Corporation, Japan). The products were acquired on a gas chromatograph-mass spectrometer (GC-MS, Varian 3900, GC-Saturn 2100T).

3. Results and discussion

3.1. Structural monitoring of thermocatalyst

3.1.1. Morphological observation. FESEM was used to investigate the morphology and particle size of as-prepared samples. Fig. 1 shows the extraordinary size, shape, morphology and organization of BMB. As shown in Fig. 1a and b, BMB is a spherical particle with a diameter of 1 μm. It can be seen that the microspheres are fabricated by hundreds of small and elongated strips with a length of about 100 nm. In brief, BMBs are composed of sphere-like BiOCl tightly aggregating Bi₂MoO₆ outside, which indicate strongly an intimate contact between the two phases above. Fig. 1c displays that BMB are micro-spheres, which reveal that the particle surface is covered uniformly with small portions of an amorphous phase of Bi₂MoO₆. The dispersing diffraction peaks in Fig. 1d is the presentation of BiOCl. The SAED pattern unravels that BiOCl is polycrystal without any doubt. Further, some tiny crystallites with lattice spacing (0.27 nm) corresponding to the (110) plane of BiOCl³³ are clearly observed in BMB particles (Fig. 1d). On the basis of the experimental results, we believe that the tiny crystallites are the original source of those trace Bragg peaks surviving in the XRD pattern, demonstrating that the particles belong to BiOCl covered with amorphous Bi₂MoO₆ once again.

Fig. 1 FESEM images of BMB (a and b), TEM image of BMB (c) and HRTEM image of BiOCl (inset: SAED pattern) (d).

3.1.2. Crystalline structure. The composition and crystallographic structure of BMB were determined by XRD. As shown in Fig. 2, all of the diffraction peaks of BiOCl were well matched with the tetragonal phase of the BiOCl phase (JCPDS card no. 06-0249, lattice constants a = b = 3.891 Å and c = 7.369 Å). The four intense diffraction peaks at 2θ = 25.8°, 32.5°, 33.3°, 46.6° and three minor peaks at 2θ = 23.9°, 40.8°, 49.6°, corresponding to the (101), (110), (102) and (200) along with the (002), (112) and (113) crystal planes, respectively. It should be noted that some trace Bragg peaks still survived in the pattern of BMB, while the positions of the peaks imply the presence of a Bi₂MoO₆ phase. Fig. 2 shows that the reflection indexed to Bi₂MoO₆ (JCPDS card no. 22-0113), but only as a broad hump (28–61°), was observed for the as-prepared BMB. The XRD results suggest that Bi₂MoO₆ is amorphous and some tiny crystallites in nanoscale may exist in the amorphous form. XRD patterns of BMB show a combination of BiOCl and Bi₂MoO₆, indicating the successful synthesis of BMB.

Fig. 2 XRD patterns of BMB.

3.1.3. Chemical composition. In order to reveal the essential composition of the BMB composite, the chemical states of as-synthesized samples were carefully checked by XPS. As depicted in Fig. 3a, this clearly indicates that the product is mainly composed of elements such as Bi, O, Cl and Mo. The Bi 4f, O 1s, Cl 2p, and Mo 3d are shown in Fig. 3 (panels b, c, d and e, respectively). The high-resolution XPS spectrum of the Bi 4f region is seen in Fig. 3b and the peaks centered at 158.4 and 163.6 eV can be assigned to Bi 4f_7/2 and Bi 4f_5/2, respectively, indicating that the Bi is in the form of Bi³⁺. It is worth pointing out that the Bi 4f_5/2 and Bi 4f_7/2 binding energies of BMB are not exactly the same as those of BiOCl or Bi₂MoO₆ (159.3 and 164.6 eV for BiOCl, 164.5 and 159.50 eV for pure Bi₂MoO₆). These results indicate the formation of an interfacial structure between BiOCl and Bi₂MoO₆, which leads to a local environment and electron density of Bi that is different from those of BiOCl or Bi₂MoO₆. The O 1s peak is displayed in Fig. 3c, which is fitted to the peak centering at 532.2 eV and mainly assigned to the oxygen in the prepared sample lattice. In Fig. 3d, the Cl 2p spectrum is resolved into two peaks at 198.5 and 200.1 eV, which are assigned to Cl 2p_3/2 and Cl 2p_1/2 for BiOCl, respectively.³⁴ Additionally, Fig. 3e shows the binding energies of Mo 3d_3/2 and Mo 3d_5/2 in the Bi₂MoO₆, which are located at 235.6 and 232.6 eV, respectively, suggesting that a six-valent oxidation state for Mo⁶⁺ exists in the Bi₂MoO₆. The peaks corresponding to Mo 3d in the XPS spectra (Fig. 3e) of the hierarchical BMB composites lightly shift (about 0.2 eV) toward a higher binding energy compared to pure Bi₂MoO₆ due to the strong interaction with BiOCl. This evidence matches up with the XRD data as well, further proving the coexistence of Bi₂MoO₆ and BiOCl.

Fig. 3 XPS spectra of BMB. (a) Full XPS spectrum; (b) Bi 4f; (c) O 1s; (d) Cl 2p; (e) Mo 3d.

3.2. Thermally-responsive catalytic degradation property

Before the degradation reactions, the dark adsorption tests were conducted for an adsorption–desorption equilibrium, which was strongly magnetically stirred for 30 min in the dark. Fig. 4 shows the adsorption capacity of BMB to MB in different MB concentrations. It is found that the adsorption capacity generally increases with the increasing concentration, and an adsorption content of around 10% concerning MB is adsorbed onto the BMB in a concentration of 80 mg L⁻¹ at most. This adsorption capability of BMB to the dye might be attributed to the BMB sphere, owing to the fine micro-structures. In other words, a moderate physical adsorption ability would be beneficial to drive the subsequent degradation effect.

Fig. 4 The adsorption capacity of BMB for MB in different substance concentrations.

3.2.1. Effect of concentration and temperature. At first, it is worthwhile to state that all the degradation tests were carried out in a dark environment throughout the trial, including the dark adsorption measurements. The evidence obtained shows that the separate catalysts (BiOCl or Bi₂MoO₆) hardly has a degradation of performance in the dark (Table 1). The degradation property is not reflected until BMB is add to the reaction system as seen in Fig. 5, strongly signifying the success of the thermally-responsive catalytic approach over BMB. Therefore, the significant drop in the detected absorbance could be attributed to the decomposition of water pollutants by a thermally-responsive catalytic action.

Table 1 The changes of C/C₀ of MB throughout the degradation process via separate catalysts (BiOCl or Bi₂MoO₆)

Catalyst	C/C0
	10 min	20 min	30 min	40 min	50 min	60 min
BiOCl	99.5	99.0	98.7	98.1	97.8	96.9
Bi2MoO6	99.7	99.3	98.8	98.3	97.9	97.5

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Fig. 5 Thermally-responsive catalytic degradation of MB using BMB. (a) Different concentrations at 25 °C; (b) different temperatures at 40 mg L⁻¹.

Thermally-responsive catalytic performances of BMB were investigated through degradation tests of MB. Fig. 5a shows the results for the degradation of MB solutions in different concentrations such as 40, 50, 60, 70 and 80 mg L⁻¹. It can be clearly seen that the removal rate reaches as high as 77.3, 74.3, 62.3, 59.4 and 58.3% after 60 min only in a dark environment, respectively, wherein the best degradation efficiency is gained at a concentration of 40 mg L⁻¹. The above phenomena may be attributed to the appropriate active sites on the BMB surface. Too high a concentration of MB solution would possibly compete with the thermocatalyst for the limited sites, and at the same time too low a concentration of MB would not provide enough sites with the degradation reactions. The rate difference seems to be owed to the increase in total surface area, and many more active sites are formed, which are available to the subsequent degradation reactions. Namely, there still exist many vacant active sites for thermally-responsive catalytic reactions. From Fig. 5b, the trends of various curves all show a dramatic increase with the degradation treatment continuing. The removal rate reaches up to 56.5, 64.1, 78.3, 68.2 and 72.7, at temperatures of 30, 35, 40, 45 and 50 °C, respectively, and the case of 40 °C presents the optimal efficiency compared with the other cases. The degradation effect is strengthened gradually from a general view. Because a plenty of active species could be produced upon the temperature excitation, which join in the degradation reactions, resulting in the improvement of thermally-responsive catalytic activity. However, a decrease of degradation rate was also monitored under the higher temperature condition in Fig. 5b, owing to an intensive collision between molecules in adsorbing MB. Specifically, MB attached to active sites is not fixed itself in reacting with BMB, partly because heat is transferred to MB rather than BMB, not exerting a thermal responsive. Additionally, the intermediates formed from degradation reactions may compete with MB for the restricted active sites in the higher temperature environment, partly losing the degradation capacity.

3.2.2. Effect of pH. As is well known, the solution pH plays an important role in the degradation of organic pollutants.³⁵ As shown in Fig. 6, after undergoing 60 min, degradation rates of 61.9, 66.9, 74.9, 84.3, 84.5 and 90.6% MB occur at pH 2.5, 3.4, 5.7, 7.1, 9.0 and 10.0, respectively. The results show that an alkaline atmosphere is better than an acid one in terms of the degradation efficiency of MB. The reason for this is that pH variation could improve the surface charge case of the catalysts.³⁶ Particularly, the positive charges of the cation make the adsorption process occur more readily at the negative sites on the catalyst that exists in the alkaline solution. On the contrary, the acid solution would naturally lead to less adsorption content, which is disadvantageous to the degradation of MB.

Fig. 6 Thermally-responsive catalytic degradation efficiency of MB (40 mg L⁻¹, 40 °C) by BMB under different pH.

3.2.3. Effect of different ions. Generally speaking, there are a few different ions in the actual wastewater; as a result we investigated the effect of some common anions on the thermally-responsive catalytic degradation of MB via BMB in a heterogeneous system, as shown in Fig. 7. The degradation rate of MB increases evidently under the existence of these ions and the optimal removal rate reaches up to as high as 99.9%. The affecting extent follows the sequence: PO₄³⁻ > CO₃²⁻ > Cl⁻ > SO₄²⁻ > NO₃⁻ > control. This consequence may be ascribed to the reduction of the recombination of electrons and holes over thermal excitation. For example, the presence of Cl⁻ could obviously accelerate the thermally-responsive catalytic rate owing to the occurrence of the reaction: h⁺ + Cl⁻ → Cl˙, which could reduce the recombination and prolong the lifetime of thermal-generated electrons. The function of these ions in the MB degradation course is concentrated upon the acceleration of quenching of hydroxyl radicals. We notice that the anions with a high adsorption ability, such as PO₄³⁻ (the specifically-adsorbed anions can make the surface negatively charged³⁷), have a greater impact on the thermally-responsive catalytic process in comparison with the less or non-specifically-adsorbed ones (SO₄²⁻ and NO₃⁻).

Fig. 7 Influence of different ions on thermally-responsive catalytic degradation of MB in solutions containing 80 mL MB solution and 80 mg BMB at 40 °C: (a) 0.1 mol L⁻¹ PO₄³⁻; (b) 0.1 mol L⁻¹ CO₃²⁻; (c) 0.1 mol L⁻¹ Cl⁻; (d) 0.1 mol L⁻¹ SO₄²⁻; (e) 0.1 mol L⁻¹ NO₃⁻; ​(f) the blank without any ions. (Inset: photographs of the dye solutions throughout the process).

3.2.4. Catalytic stability evaluation. To further demonstrate the recycling property of BMB, three-cycle tests were executed using the cycled BMB. Apparently, the degradation rate increases steadily with increasing time (Fig. 8) after the first use of BMB, which is up to >90%. The degradation activity for removing MB does not much decrease after the second use as shown in Fig. 8. Even if three cycling processes are employed, the degradation efficiency remains at a level of >70%, meaning that BMB could be reused for repeated cycles without a large loss of activity. Besides, it is necessary to mention that the drying course was not unavoidably implemented throughout the three cycles. In summary, this BMB catalyst may exhibit an appreciable suitability for practical industrial applications in the disposal for water pollutants in consideration of the evidence of its property stability and moderate recycling usage.

Fig. 8 Repetition assessment of BMB during experiencing three cycling uses.

3.3. Identification of degradation products

Potential products after undergoing an MB degradation process were measured using GC-MS and statistical analysis. The MB (30 mg) was placed in a 50 mL glass vial, tightly capped with polytetrafluoroethylene bottle caps, and then the degradation experiment was performed for 40 min at 40 °C in the dark to allow for the equilibration of the volatiles in the headspace. After the equilibration time, the bottle caps were pierced with a SPME needle and the fiber was exposed to the headspace for 30 min. Fig. 9 shows a representative GC-MS chromatogram of already degraded MB, which contain seven major products. Many intermediates could be formed as ˙OH is unable to exhibit a high degree of selectivity towards various functional groups, resulting in multiple oxido-reductive pathways. All of the products present smaller molecular weights compared with MB, and some small characteristic mass losses such as the R_t at 2.50, 8.20 and 14.28 min are observed from the MS fragments, suggesting oxygen insertion into the structure, and eventually leading to the formation of carboxyl, carbonyl, and alcohol groups. The SPME method is very sensitive and the degradation by-products of MB are very complex, and so merely the main products were analyzed in this work. Seven main compounds detected in the solution are presented in Table 2, which are methanedithione, toluene, 4-methylpent-4-en-2-ol, o-xylene, 3-methylhexanal, (E)-2,6-dimethylhept-3-ene and hex-1-en-3-ol, respectively. Finally, it is worthwhile noting that most of the accurate mass measurements are monitored with an error of less than 2 ppm, thus providing a high degree of certainty in the assignment of formulas.

Fig. 9 A representative chromatogram of oxidized samples obtained during 15 min by a solid-phase microextraction and gas chromatography-mass spectrometry (SPME-GCMS) method.

Table 2 GC-MS information on the major products after experiencing degradation reactions

Products	Rt (min)	Calculated mass	Structures	Molecular formula
Methanedithione	1.58	76	S=C=S	CS2
Toluene	3.21	92		C7H8
4-Methylpent-4-en-2-ol	2.50	100		C6H12O
o-Xylene	6.31	106		C8H10
3-Methylhexanal	8.20	114		C7H14O
(E)-2,6-Dimethylhept-3-ene	9.96	126		C9H18
Hex-1-en-3-ol	14.28	100		C6H12O

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3.4. Possible thermally-responsive catalytic mechanism

Upon the thermal activation, two semiconductors (BMB) with overlapping band potentials are considered to be suitable components for constructing an effective heterojunction, which could drive an efficient separation of thermoexcited electrons and holes and then promote the thermally-responsive catalytic activation.^38,39 To better understand the formation of BMB, the initial energy band structures of Bi₂MoO₆ and BiOCl are provided in Fig. 10. Meanwhile, the band positions of Bi₂MoO₆ and BiOCl are also calculated (Fig. 10) by the following empirical formulas (eqn (1) and (2)):

E_VB = X − E_e + 0.5E_g (1)

E_CB = E_Vb − E_g (2)

where E_VB is the valence band edge potential, E_CB is the conduction band edge potential, E_e is the energy of free electrons on the hydrogen scale (E_e = 4.5 eV), and E_g is the band gap energy of the semiconductor. X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. The X values for BiOCl and Bi₂MoO₆ are 6.36 and 6.31 eV, respectively. Particularly, the conduction band level (E_CB) of BiOCl is 0.26 eV, which is more positive than that of Bi₂MoO₆ (E_CB = −0.32 eV) and the valence band (E_VB) energy level of BiOCl (3.55 eV) is higher than that of Bi₂MoO₆ (2.6 eV).^40,41 Besides, the CB energy levels of BiOCl and Bi₂MoO₆ are mainly dominated by unoccupied Bi 6p states, and the identical energy level distribution of BMB exhibits high delocalized electron states. Due to the special band structure of the BMB composite, the thermogenerated electrons on Bi₂MoO₆ could inject more freely into the CB of BiOCl and subsequently be trapped by O₂ to produce O₂⁻. For this reason, a large number of holes from the VB of Bi₂MoO₆ would react with OH⁻ to produce ˙OH, making the electrons and holes separate.

Fig. 10 The schematic diagram of the band structures for the BMB composite.

4. Conclusions

A one-step hydrothermal approach was used to synthesize Bi₂MoO₆/BiOCl (BMB) under low-temperature conditions. The influence of initial concentration, reaction temperature, solution pH, and common ions on the thermally-responsive catalytic degradation property of methylene blue (MB) was studied in detail using XRD, FESEM, TEM, and XPS. The results obtained show that BMB exists in a spherical form and is covered with an amorphous Bi₂MoO₆ layer. An outstanding thermally-responsive catalytic activity on decomposing MB was measured in the presence of BMB in the dark at a relative low temperature, ultimately suggesting the optimal conditions (40 °C, 40 mg L⁻¹, and pH at 11). Intriguingly, it is found that the addition of PO₄³⁻, CO₃²⁻, Cl⁻, SO₄²⁻ and NO₃⁻ anions in MB solutions could harvest a better subsequent degradation effect. In addition, the degradation products from thermally-responsive catalytic MB reactions were analyzed by GC-MS, showing that the thermally-responsive catalytic pattern has the capacity to execute the destruction of organic dyes. Recycling tests reveal that the total degradation efficiency of MB using BMB still maintains a level of >70% even after three degradation cycles, so BMB may be recovered and reused without appreciable loss in the catalytic activity. A likely mechanism for the destruction of pollutants using BMB is also discussed. Thus, this BMB responsive to heat excitation may be very useful for the highly efficient degradation of water pollutants in future environmental remediation.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

The authors acknowledge the financial support from the Key Program of National Nuclear Facility Decommissioning and Radioactive Waste Treatment (Grant no. 2014ZG6101) and Hunan Provincial Innovation Foundation For Postgraduate (Grant no. CX2016B002).

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Footnote

† These authors contributed equally to this paper.

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